Perovskite BaTaO2N: From Materials Synthesis to Solar Water Splitting

Abstract Barium tantalum oxynitride (BaTaO2N), as a member of an emerging class of perovskite oxynitrides, is regarded as a promising inorganic material for solar water splitting because of its small band gap, visible light absorption, and suitable band edge potentials for overall water splitting in the absence of an external bias. However, BaTaO2N still exhibits poor water‐splitting performance that is susceptible to its synthetic history, surface states, recombination process, and instability. This review provides a comprehensive summary of previous progress, current advances, existing challenges, and future perspectives of BaTaO2N for solar water splitting. A particular emphasis is given to highlighting the principles of photoelectrochemical (PEC) water splitting, classic and emerging photocatalysts for oxygen evolution reactions, and the crystal and electronic structures, dielectric, ferroelectric, and piezoelectric properties, synthesis routes, and thin‐film fabrication of BaTaO2N. Various strategies to achieve enhanced water‐splitting performance of BaTaO2N, such as reducing the surface and bulk defect density, engineering the crystal facets, tailoring the particle morphology, size, and porosity, cation doping, creating the solid solutions, forming the heterostructures and heterojunctions, designing the photoelectrochemical cells, and loading suitable cocatalysts are discussed. Also, the avenues for further investigation and the prospects of using BaTaO2N in solar water splitting are presented.

To tackle the rising global energy demand and to reduce greenhouse gas emissions from the combustion of diminishing fossil fuels, the development of renewable energy is indispensable.Hydrogen is regarded as a potential zero-emission energy carrier with the highest gravimetric energy density (120 MJ kg −1 ) despite its low volumetric energy density (8 MJ L −1 ) [1] and it can play a vital role in the successful implementation of the Paris Agreement, [2] which is essential for the achievement of the 17 United Nations Sustainable Development Goals.However, the current global hydrogen production still heavily relies on fossil fuels (>95%), which is the cheapest option in most parts of the world. [3,4][7][8] Solar-driven water splitting proceeds over cocatalyst-assisted semiconductor according to three consecutive steps (Figure 1a): i) the absorption of photons with higher energy values to excite Figure 1.a) Working principle of the photoelectrochemical cell for water splitting using a photoanode and a photocathode.b) OER mechanism for acid (blue line) and alkaline (red line) conditions.The black and green lines indicate that the oxygen evolution involves the formation of a peroxide (M-OOH) intermediate and the direct reaction of two adjacent oxo (M-O) intermediates, respectively.Reproduced with permission. [16]Copyright 2017, The Royal Society of Chemistry.
electrons from the valence band to the conduction band, ii) the separation and transfer of photo-excited charge carriers (electrons and holes) from the bulk to the surface of the semiconductor, and iii) the initiation of water redox reactions by the involvement of photo-excited charge carriers [9] : Hydrogen evolution reaction (HER): 2H 2 O + 2e − → H 2 + 2OH − (alkaline media) Oxygen evolution reaction (OER): Particularly, photoelectrochemical (PEC) water splitting is a powerful yet complex process, where the redox potentials for the decomposition of water determine the required band gap energy and band-edge potentials for semiconductors to be used as photoanodes and photocathodes.However, to efficiently and sustainably split water into H 2 and O 2 , several key criteria must be met simultaneously: i) sufficient voltage must be generated upon irradiation to split water, ii) the bulk band gap must be small enough to absorb a significant portion of the solar spectrum, iii) for the unbiased operation of a PEC cell, the band edge potentials at the surfaces must straddle the hydrogen and oxygen redox potentials (the conduction band minimum must be more negative than E 0 (H + /H 2 ) = 0 V versus RHE at pH 0 and the valence band maximum must be more positive than E 0 (O 2 /H 2 O) = 1.23 V versus RHE at pH 0), iv) the system must be stable for a long period of reaction time, v) charge transfer from the semiconductor surface to the electrolyte must be facile to minimize energy losses, and vi) low-cost and earth-abundant elements must be used. [10]n addition to these important requirements, the photoelectrode materials must exhibit at least >10% solar-to-hydrogen (STH) conversion efficiency for their commercial viability. [11]The U.S. Department of Energy estimated the cost of green hydrogen produced by the PEC process to be US$5.7 kg −1 in 2020 (with a 20% STH efficiency) and to lower to US$2.1 kg −1 (with a 25% STH efficiency) in the more distant future. [12]It has also been suggested that the STH efficiency of 25% and the photoelectrode lifetime of 10 years are required for the PEC systems to be economically consistent with fossil fuel-based hydrogen production processes. [13]Recently, monolithic systems and integrated PEC modules under light concentration reached STH efficiencies as high as 17.12% [14] and 19%. [15]However, the high capital and operational costs significantly hamper their economic viability.To date, no cost-effective and highly efficient PEC systems have been developed to satisfy all those key criteria despite significant progress in this field.Thus, further studies are necessary to discover novel materials with unprecedented physicochemical and optoelectronic properties that can simultaneously meet those key criteria set for the design and application of the PEC system for green hydrogen production.
As one of the two key reactions of overall water splitting, the water oxidation reaction proceeds via the following general steps (Figure 1b) [16] : i) water dissociation and formation of surface-bonded OH ads , ii) the further oxidation of OH ads to O ads , and iii) the formation of OOH ads , which is a precursor for O 2 evolution [17,18] : Using first-principles periodic density functional theory (DFT) calculations, Man et al. [19] proposed that the binding energies of reaction intermediates (e.g., HO*, O*, and HOO*) are responsible for the origin of OER activity over select oxide surfaces.When the binding energy of oxygen is quite high, the OER overpotential is limited by the formation of HOO* species, and otherwise, the formation of HO* species is dominant.Therefore, various Figure 2. a) Crystal and band structures of TiO 2 and IPCE spectra of reduced TiO x nanotube arrays before and after Ar/H 2 treatment at different temperatures.Reproduced with permission. [30]Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA.b) Crystal and band structures of WO 3 and photocurrent-potential plots for a 2-μm-thick WO 3 electrode illuminated with AM 1.5G simulated sunlight: in 1 M aq.HClO 4 (curve A) and after the addition of 0.1 mol dm 0 −3 of methanol (curve B).Reproduced with permission. [36].Copyright 2001, American Chemical Society.c) Crystal and band structures of CuWO 4 and linear potential sweep curves of the doped CuWO 4 nanoflake photoanodes with different Mo doping concentrations.Reproduced with permission. [39]Copyright 2019, Elsevier.d) Crystal and band structures of Fe 2 O 3 and photoelectrochemical performance of Fe 2 O 3 , Ag/Fe 2 O 3 , and Co-Pi/Ag/Fe 2 O 3 photoelectrodes under chopped light illumination in 1 M NaOH (pH 13.6).Reproduced with permission. [47]Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA.e) Crystal and band structures of ZnFe 2 O 4 and linear scanning J-V curves of ZnFe 2 O 4 photoanodes in 1 M NaOH under intermittent 1 sun illumination (100 mW cm −2 ).Reproduced with permission. [51]Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA.f) Crystal and band structures of BiVO 4 and I-V characteristics of the optimized WO 3 -nanorods (green), WO 3 -nanorods/BiVO 4 (blue) and WO 3 -nanorods/BiVO 4 +CoPi (red) samples.Reproduced with permission. [56]Copyright 2015, Springer Nature AG & Co. KGaA.
strategies to stabilize HOO* species compared with HO* species were developed. [20]The suitable photoanodes must satisfy the expected conditions: ΔG(OH ads ) = C O = 1.23 eV, ΔG(O ads ) = 2 × C O = 2.46 eV, and ΔG(OOH ads ) = 3 × Co = 3.69 eV. [18]The water oxidation reaction requires the transfer of four electrons and four protons, which needs high overpotential, with the simultaneous formation of the O-O bond in comparison to the two-electrontransfer water reduction reaction. [21]Therefore, water oxidation is thermodynamically and kinetically more challenging than water reduction.As shown in Figure 1b, a sufficient amount of energy must be supplied in each step to drive the water oxidation reaction.Thus, it leads to high energy barriers and slow kinetics in addition to the uphill reaction thermodynamics. [22]A sluggish water oxidation reaction is the rate-determining step that governs the reaction rate of water splitting. [23]The water oxidation reaction is generally enhanced by modifying the photoanodes with oxygen evolution cocatalysts that promote the efficient charge separation of photo-excited electron-hole pairs. [24]ccording to the three basic stages of water splitting, the efficiency of the water oxidation reaction is mainly determined by the light absorption, the separation efficiency of photo-excited charge carriers, and the surface catalytic reaction [9] :  =  absorption ×  separation ×  reaction (9)   which can be controlled by modulating the physicochemical, optoelectronic, and surface properties of n-type semiconductors.Also, the valence band edge potentials of photocatalysts must be more positively positioned than that of E 0 (O 2 /H 2 O) and the water oxidation active sites must be sufficient on the photocatalyst surface to hinder the recombination of photo-excited electrons and holes. [25]

Oxide-Based Photocatalysts for PEC OER
Since the first successful demonstration of solar-induced unassisted water splitting over a TiO 2 photoanode by Honda and Fujishima, [26] a large number of studies have been conducted to maximize the STH efficiency of various heterogeneous oxidebased semiconductors but few have shown relatively outstanding water oxidation performance and stability.The overview of some n-type oxide semiconductors for PEC OER is shown in Figure 2 and Table 1. [27]iO 2 is one of the most widely studied oxide semiconductors because of its low cost, chemical stability, earth abundance, non-corrosiveness, and non-toxicity.TiO 2 typically exists in three crystalline structures: anatase (tetragonal, I4 1 /amd), rutile (tetragonal, P4 2 /mnm), and brookite (orthorhombic, Pbca).In addition to other factors, the STH efficiency of TiO 2 depends  [31, 56, 58]   on its polymorphs.The photocatalytic overall water splitting reaction can take place on rutile but hardly on anatase and brookite and becomes feasible for anatase and brookite only under prolonged UV light irradiation. [28]Recently, compact anatase TiO 2 layers fabricated on the FTO by introducing a Ti interlayer and suboxide TiO 2 nanotubes exhibited the highest incident photon-to-current efficiency (IPCE) value of 75% at 300 nm [29] and 340 nm (Figure 2a), [30] respectively.Although the anatase-TiO 2 polymorph has better electron mobility and conductivity in comparison to the rutile-TiO 2 and brookite-TiO 2 polymorphs, its theoretical STH efficiency and photocurrent density can only reach the maximum of 1.3% and 1.1 mA cm −2 , respectively, because of wide optical bandgap energy of 3.2 eV. [31]n addition to the low STH efficiency, a fast recombination rate of photo-excited charge carriers, a rapid backward reaction (recombination of H 2 and O 2 ), and a large overpotential for the HER also hamper the practical application of TiO 2 in solar water splitting despite considerable progress. [32,33]he monoclinic phase with space group P2 1 /n is the most stable polymorph of WO 3 at room temperature and has a perovskitelike structure without an A-site ion.Theoretical studies on the correlation between the crystal structure and the band gap of WO 3 revealed that the bandgap energy values of different WO 3 polymorphs decrease in the following order: monoclinic > orthorhombic > triclinic > tetragonal ≥ cubic. [34]Also, the structural modification by introducing the nitride ions into the lattice of the monoclinic WO 3 phase led to the reduction of the bandgap energy from 2.6 eV to 1.9 eV. [35]Highly transparent nanoporous WO 3 films fabricated by layer-by-layer deposition of a colloidal so-lution of tungstic acid and annealing exhibited a maximum IPCE of 75% and a photocurrent density of 2.4 mA cm −2 at 1.23 V versus RHE under simulated solar irradiation (Figure 2b). [36]However, the rapid recombination of photo-excited charge carriers, slow charge transfer at the WO 3 /electrolyte, restricted light absorption up to 450 nm, instability at pH > 5, and a theoretical STH efficiency of 4.8% and a photocurrent density of 3.9 mA cm −2 still limit its application. [31]o circumvent the drawbacks of WO 3 , triclinic CuWO 4 with a distorted wolframite crystal structure (space group P 1) offers a smaller bandgap energy and limits the formation of soluble tungstates due to the strong covalency in the metal oxo bonds, yielding higher photocurrent in neutral pH under visible light irradiation. [37]The CuWO 4 photoanodes exhibited photocurrent densities of ≈0.15-0.16mA cm −2 and 0.62 mA cm −2 at the thermodynamic potential for water oxidation (1.23 V versus RHE) under simulated solar irradiation (Figure 2c) [37][38][39] and an IPCE efficiency of >20%. [40]Despite its theoretical STH efficiency of ≈13% and photocurrent density of 10.7 mA cm −2 , the PEC performance of CuWO 4 is significantly hindered by its low light absorption coefficient and high bulk charge transfer resistance. [41,42]-Fe 2 O 3 with a trigonal structure (space group R 3c) has been intensively explored as a promising photoanode material for PEC water splitting due to its small bandgap energy, suitable valence band edge potential to thermodynamically drive the water oxidation reaction, excellent stability under alkaline conditions, earth abundance, and environmentally friendliness. [43,44]With the reported bandgap energy of 2.0-2.2eV, -Fe 2 O 3 has the potential to reach a theoretical maximum STH efficiency of 16.8% and a photocurrent density of 12.6 mA cm −2 , [45] which manifestly exceeds the STH benchmark efficiency of 10% required for practical applications.The -Fe 2 O 3 nanowires-based photoelectrode, which was fabricated by a chemical bath deposition method followed by hydrogen treatment and then loaded with ultrathin TiO 2 overlayer and CoPi cocatalyst, exhibited a stable photocurrent density of ≈ 6 mA cm −2 at 1.23 V versus RHE over 100 h under AM 1.5G simulated sunlight and an IPCE value of ≈18% at 420 nm owing to its excellent light absorption and bulk charge separation capacity, the enhanced electrical conductivity, and the decreased surface recombination. [46]A hematite nanosheets-based electrode modified by Ag and CoPi nanoparticles showed an IPCE value of ≈40% at 420 nm and an STH efficiency of ≈0.55% due to the improved light harvesting and the facilitated charge transfer by Ag nanoparticles and the reduction of surface/bulk recombination and the stabilization of the photoelectrode surface by CoPi cocatalyst (Figure 2d). [47]owever, several limitations, such as low absorption coefficient due to an indirect band gap, short excited-state lifetime (≈10 −12 s), short hole-diffusion length (2-4 nm), low charge carrier mobility (≈10 -2 to ≈10 -1 cm 2 V −1 s −1 ), poor electric conductivity, and poor OER kinetics still hinder achieving the practical maximum STH efficiency of -Fe 2 O 3 . [44,48]ubic spinel ZnFe 2 O 4 (space group Fd 3m) also received much interest due to its narrow bandgap energy (E g = 1.9 eV), high energy level conduction band minimum, outstanding photochemical stability, low cost, and magnetic recyclability. [49,50]Interestingly, ZnFe 2 O 4 with a relatively poor crystallinity but a higher spinel inversion degree (due to cation disorder) shows a superior efficiency in photo-excited charge carrier separation and an improved majority charge carrier transport compared to ZnFe 2 O 4 with higher crystallinity but a lower inversion degree. [51]The optimization of these factors and the addition of a nickel-iron cocatalyst overlayer resulted in a new benchmark photocurrent density of 1.0 mA cm −2 at 1.23 V versus RHE, and an IPCE value of about 25% was reached for ZnFe 2 O 4 nanorod photoanode (Figure 2e). [51]At potentials between 0.8 and 1.3 V versus RHE, ZnFe 2 O 4 can exhibit a considerably higher charge transfer efficiency due to a slower surface charge recombination rate. [52]Despite its maximum theoretical STH efficiency of about 20% and a photocurrent density of ≈11 mA cm −2 , [49,51] the efficiency of ZnFe 2 O 4 is substantially limited by a rapid surface and bulk recombination rate of photo-excited charge carriers and poor minority career.
BiVO 4 crystallizes in three different polymorphs: monoclinic scheelite, tetragonal scheelite, and tetragonal zircon.The monoclinic scheelite BiVO 4 (space group C2/c) is an n-type semiconductor with a direct bandgap energy of 2.4 eV and a valence band edge potential of ≈2.4 V versus RHE, which is sufficiently positive than E 0 (O 2 /H 2 O) = 1.23 V versus RHE. [53,54]Therefore, the monoclinic scheelite BiVO 4 shows the highest photocatalytic water oxidation activity among the three polymorphs. [55]A photocurrent density of 6.72 mA cm −2 at 1.23 V versus RHE and an IPCE of ≈90% were achieved by the combination of BiVO 4 with more conductive WO 3 nanorods in the form of core-shell heterojunction (Figure 2f). [56]The high recombination rate of photo-excited charge carriers was significantly reduced by creating a thin absorber layer of BiVO 4 , which was thinner than the carrier diffusion length.The tandem device constructed with a GaAs/InGaAsP solar cell exhibited an STH efficiency of 8.1%. [56]he charge separation efficiency of 97.1% and charge transfer efficiency of 90.1% at 1.23 V versus RHE were obtained by engineering the hierarchical nanoporosity of BiVO 4 , [57] and an applied bias photon-to-current conversion efficiency (ABPE) of 1.75% was found at a potential as low as 0.6 V versus RHE for nanoporous BiVO 4 photoanodes. [58]However, the water oxidation efficiency of BiVO 4 is still limited by a high electron-hole recombination rate, poor charge transport properties, a low carrier collection efficiency, inadequate water oxidation kinetics, and the bandgap energy limiting an AM 1.5G solar photocurrent density to 7.4 mA cm −2 and an STH efficiency to 9.1%. [31]espite their encouraging and relatively outstanding performance achieved so far, TiO 2 , WO 3 , CuWO 4 , Fe 2 O 3 , ZnFe 2 O 4 , and BiVO 4 alone are unlikely to satisfy the criteria set for ensuring the practical application in solar to chemical energy conversion.This paved the way for the development of novel materials, including mixed-anion compounds, for PEC applications.

Crystal and Electronic Structures of Perovskite BaTaO 2 N
Mixed-anion compounds, containing more than one anionic species in a single phase, are an emerging class of advanced materials with the potential to contribute to solar fuel production in the future. [59]Unlike single-anion compounds, mixedanion compounds exhibit diverse structures, chemical and physical properties, and new functionalities because of different anionic characteristics, such as ionic radii, valency, electronegativity, and polarizability. [60]Also, the combination of hetero-anions can realize the structures of compounds that cannot be generally stabilized by homo-anions.Especially, to develop photocatalytic materials that can efficiently function under visible light, anions less electronegative than oxygen can be simultaneously introduced.Having similar chemical, structural, and electronic properties, oxygen and nitrogen substitute each other in the anion site to form oxynitrides. [61] In oxynitrides, the nitride anions (N 3− ) having atomic orbitals with potential energy higher than O 2p atomic orbitals of the oxide anions (O 2− ) shift the valence band maximum upward without affecting the conduction band minimum, leading to the increased covalency of metal-anion bonds, the improved absorption property, and the decreased optical band gap (Figure 3a). [62,63]Also, most d 0 transition metal oxynitrides can absorb photons with absorption band edges in the range of 500-760 nm and have theoretical STH conversion efficiencies in the range of 8-32% and suitable conduction and valence band edge potentials straddling the proton reduction and water oxidation reaction potentials, respectively, and can drive the overall water splitting reaction (Figure 3b). [64]s a typical representative of the AB(O,N) 3 perovskites, BaTaO 2 N is regarded as one of the promising photocatalysts for solar water splitting due to its absorption of visible light up to 660 nm, small bandgap energy (E g = 1.9 eV), good stability under light irradiation in concentrated alkaline solutions, and nontoxicity. [65]Moreover, the conduction band minimum and valence band maximum of BaTaO 2 N are located at -0.4 V and 1.5 V versus NHE at pH 0, respectively, which should theoretically drive the water-splitting reaction in the absence of an external bias. [66]BaTaO 2 N can generate a photocurrent density of Reproduced with permission. [62]Copyright 2007, American Chemical Society.b) Bandgaps and energy diagrams of several types of typical (oxy)nitrides.Reproduced with permission. [64]Copyright 2021, WILEY-VCH Verlag GmbH & Co. KGaA.c) Crystal structure of BaTaO 2 N. [70] d) Space groups stemming from cis-and trans-anion ordering and octahedral tilting in mixed-anion perovskites.Reproduced with permission. [81]Copyright 2014, American Chemical Society.
about 18 mA•cm −2 under AM 1.5G simulated sunlight based on the assumption of an IPCE efficiency of 100% at < 660 nm. [67]ncidentally, the STH conversion efficiency of BaTaO 2 N is about 24%. [10]Its large dielectric constant can promote a facile separation of photo-excited charge carriers. [68]he BaTaO 2 N phase was first synthesized in 1986 by Marchand et al., [69] and its centrosymmetric cubic crystal structure with space group Pm 3m (no.221) and a random O/N distribution were determined by a powder time-of-flight neutron diffraction method (Figure 3c). [70]Ba is at the origin (0,0,0), Ta is at the cube center (1/2, 1/2, 1/2), and three anions (one nitrogen and two oxygen) are randomly distributed at the face centers.The Ba atom is 12-fold surrounded by anions with a Ba-(O,N) distance of 2.908 Å, and the Ta atom is octahedrally coordinated (Ta(O,N 6 ) with a Ta-(O,N) distance of 2.056 Å. [69,70] Although the conventional diffraction analyses using neutron [70] and Xray [68] radiations similarly confirmed an average cubic perovskite structure with Pm 3m symmetry, the extended X-ray absorption fine structure (EXAFS), [71] zone-axis electron diffraction (ED), [72] pair-distribution-function (PDF), [73] solid-state magic-angle spinning (MAS) NMR spectroscopy, [74] constant-wavelength neutron diffraction, [75] and X-ray absorption near edge structure (XANES) spectroscopy [76] analyses revealed short-range structural distor-tions around the Ta atoms and widely distributed Ta-O/N bond distances in BaTaO 2 N. First-principles investigation conducted by Xu and Jiang [77] found the existence of both short-range order in the intra-octahedron O/N occupation and randomness of interoctahedron O/N occupation, which is consistent with the cubic symmetry.Their study also revealed that the electronic properties of BaTaO 2 N do not strongly depend on the O/N configuration, whereas the dielectric properties indicate a stronger dependence on the O/N configuration.Since local structure relaxation is expected due to the different ionic radii and valency of the oxygen and nitrogen ions, the local symmetry of BaTaO 2 N was, however, not well defined.The DFT analyses suggested that the local symmetry in the crystal is lower, resulting in the different Ta-O and Ta-N bond lengths, and it predicted the possibility of either orthorhombic, tetragonal, or monoclinic symmetry without contradicting the macroscopic cubic description of the crystal structure of BaTaO 2 N with space group Pm 3m and found BaTaO 2 N to be most stable in the Pmc2 1 (no.26) structure type with an ordered anionic sublattice (Figure 3d). [71,78,79]In contrast to the bonding of all occupied Ta-O states in Pnma and I4/mcm, antibonding interactions close to the Fermi level were observed in Pm 3m.This describes the increased bulk moduli of the cubic model because the Fermi level is forced even deeper into the antibonding region whenever pressure is applied (Figure 4a). [78]n fact, multiple possible orderings make it difficult to conduct systematic investigations using DFT calculations with predetermined elemental arrangements.According to Kaneko et al., [80] anion ordering in large supercells within perovskite-type oxynitrides can be quickly predicted based on machine learning.Combined with the Metropolis Monte Carlo method, machine learning allows the exploration of the stable anion orderings of large supercells within perovskite-type oxynitrides without costly DFT calculations.
The BaTaO 2 N stoichiometry is locally maintained with each Ta atom surrounded by two N and four O atoms in two possible local N/O orderings in the TaO 4 N 2 octahedral structures: cis and trans configurations corresponding to the N−Ta−N bonds with 90°and 180°angles, respectively (Figure 4b). [73,79]Pair distribution function (PDF) analysis of the total neutron scattering reveals that although a long-range anion order is not present in BaTaO 2 N, the TaO 4 N 2 octahedra predominantly adopt a cis-configuration with small Ta displacements toward the N atoms on a local scale. [73,81,82]The cis-chains propagate in all three dimensions in BaTaO 2 N, retaining the average cubic crystal symmetry. [83]onetheless, the partial formation of a metastable trans-type configuration by applying strain engineering can increase the N occupancy at axial sites. [84]By applying an automated bonding analysis with Crystal Orbital Hamilton Populations (an orbitalbased technique), George et al. [85] correlated the total energy of the systems with the strongest covalent interaction (Ta-N bond) of BaTaO 2 N and observed anti-correlation with the bond energies of Ta-O, implying the importance of covalency of the Ta-N interactions in the anionic ordering of BaTaO 2 N. It was also found that the structure of BaTaO 2 N with a cis-type local symmetry is much more stable than the corresponding trans-type local symmetry. [78,79] study on the influence of the cation size on the local atomic structure revealed that cis-type N ordering and associated Ta displacements cooperate to stabilize local point charge dipole correlations among the TaO 4 N 2 octahedra in BaTaO 2 N. [86] Therefore, the Ta displacements significantly stabilize the structure of BaTaO 2 N, and the TaO 4 N 2 tilting is minor.In the case of the Sr cation, the TaO 4 N 2 octahedra are slightly tilted against each other, which is not observed for a larger Ba cation, promoting a stronger bonding in SrTaO 2 N than in BaTaO 2 N, and the occupied antibonding levels for the Ta−N bonds cannot be observed in BaTaO 2 N. [87] Compared with an anion-disordered cubic BaTaO 2 N (Pm 3m (no.221)), tetragonal BaTaO 2 N (P4/mmm (no.123)) shows a site preference for oxide anions in the two opposite corners (along the c-axis) of the TaO 4 N 2 octahedra rather than the four-square corners in the ab plane, leading to a distortion of the unit cell with the c-axis being slightly longer than the a-axis. [88]igure 4c,d shows a schematic representation of the band structure and DFT-calculated band structure of BaTaO 2 N. The top of the valence band (highest occupied molecular orbital -HOMO) shifts upward due to the presence of hybridized N 2p and O 2p orbitals, while the bottom of the conduction band (lowest unoccupied molecular orbital -LUMO), which is composed of empty Ta 5d orbitals, remains unaffected. [62,89]By comparing the three different crystal structures of BaTaO 2 N, Bettine et al. [90] found that the electronic and optical properties are strongly related to the TaO 4 N 2 octahedral configurations, whereas the bandgap energy is influenced by the internal electric fields (polar versus non-polar-trans-type orderings), which creates an asymmetry in the Ta-N bond lengths, and the minimum band gaps of the P4mm, I4/mmm, and Pmma structures were calculated to be 1.83 eV, 1.59 eV, and 1.49 eV, respectively.The density, separation, and transfer of charge carriers in oxynitrides depend on the nature and concentration of nonstoichiometric defects. [91]Recently, Kousika and Thomas [92] applied the Mott-Littleton (M-L) method, which is commonly used for defect studies in molecular static calculations, rather than density functional theory (DFT) and molecular dynamics (MD) calculations to calculate the defect and migration energies of oxygen vacancies in BaTaO 2 N. It was found that BaTaO 2 N has a low oxygen vacancy defect energy (ΔE M-L = 25.23 eV), which may reportedly improve its ionic conductivity, and the energy required for migration of oxygen for BaTaO 2 N is 2.42 eV.Although this study was mainly focused on calculating the defect and migration energies of oxygen vacancies, other prominent defects, such as nitrogen and cation vacancies, were not considered.

Dielectric, Ferroelectric, and Piezoelectric Properties of Perovskite BaTaO 2 N
Along with the carrier concentration, a dielectric constant of semiconductors is a key parameter in determining the space charge layer, which can be applied to achieve the effective separation of photo-excited charge carriers. [93]Therefore, the dielectric properties of transition metal oxynitrides have been of particular interest.The frequency dependence of the dielectric response of BaTaO 2 N was studied using infrared reflection spectroscopy, and no infrared-active polar soft mode, which is the origin of the ferroelectric property of oxides, was not observed, and the dielectric response was found not to reach very high values because the substitution of one nitrogen for oxygen cancels the tendency at displacement-type instability. [94]Despite its small octahedral tilt, BaTaO 2 N after the thermal treatment of the cold isostatically pressed sample at 1020 °C for 2 h exhibited an unexpectedly high bulk dielectric constant of ≈5000 at room temperature, which remained almost constant in the temperature range of 180-300 K (Figure 5a). [68]The high bulk dielectric constant was explained by the increased covalency of the Ta−N bonds, resulting in the off-center displacement of the Ta cation through a second-order Jahn−Teller distortion leading to a local polarization at each octahedron.Such high dielectric permittivity with a fairly weak temperature dependence was assumed to have a ferroelectric-like behavior despite the absence of experimental evidence for its ferroelectricity.Further, different types of structural analysis were involved to understand the origin of the dielectric property of BaTaO 2 N.For instance, by applying extended X-ray absorption fine-structure (EXAFS) spectroscopy, Ravel et al. [71] found that local structural distortions with very short correlation lengths are consistent with large dielectric permittivity of BaTaO 2 N.However, the difficulty with linking the dielectric property of BaTaO 2 N directly with the off-center displacement of the Ta cation is that such compositional disorder (and associated polar behavior) is presumably frozen up until high temperature and hence not available to switch sign under the action of an applied electric field. [72]Instead, Withers et al. [72] correlated the  [78] Copyright 2008, WILEY-VCH Verlag GmbH & Co. KGaA.b) Local cis and trans ordering of TaO 4 N 2 octahedron in BaTaO 2 N. c) Schematic representation of the band structure of BaTaO 2 N and d) DFT-calculated band structure of BaTaO 2 N. Reproduced with permission. [62]Copyright 2007, American Chemical Society.Reproduced with permission. [68]Copyright 2004 American Chemical Society.b) Displacements of atoms in BaTaO 2 N with opposite polarization.Reproduced with permission. [96]Copyright 2012, American Chemical Society.c) high-resolution TEM image of BaTaO 2 N/SrRuO 3 interface and temperature-dependent dielectric permittivity and dielectric loss of a 600-nm-thick BaTaO 2 N film.Reproduced with permission. [97]Copyright 2007, American Chemical Society.d) SEM image of a BaTaO 2 N compact and relative permittivity and dielectric loss of BaTaO 2 N 0.85 ceramics sintered at 1400 °C for 3 h.Reproduced with permission. [99]Copyright 2016, Elsevier.e) Digital photographs, SEM image, and dielectric properties of BaTaO 2 N ceramics sintered by spark plasma method.Reproduced with permission. [100]Copyright 2016, Elsevier.f) STEM and PFM images of BaTaO 2 N ceramics.Reproduced with permission. [102]opyright 2018, Elsevier.g) Polar nanoregions presenting the average Pm 3m cubic crystal lattice in which most of the O and N atoms are randomly distributed at 3c sites (i), polar nanoregions growing along the applied electric field (ii), and polarization saturated at applied electrical bias higher than ±60 V (iii).Reproduced with permission. [103]Copyright 2019, American Chemical Society.
relaxor-type ferroelectric behavior of BaTaO 2 N with the formation of structurally frustrated one-dimensional polar nanoregions (1D PNRs) along the ⟨001⟩ axis and suggested that the role of anion ordering is to set up random local strain fields suppressing transverse correlations of the inherent ⟨001⟩ chain dipoles and inhibiting the development of a long-range ordered ferroelectric state.Similarly, Ziani et al. [95] observed no paramount effect of anion ordering on the dielectric property of SrTaO 2 N based on a combined experimental and theoretical study.However, local anion ordering with cis-type TaO 4 N 2 was experimentally and theoretically confirmed for BaTaO 2 N. Using systematic first-principles calculations, Hinuma et al. [96] proposed a new mechanism that attributes the relaxor-type ferroelectric behavior of BaTaO 2 N to the presence of low-energy displacements having opposite polarization directions in the -Ta-N-coiled chain motif composed of the TaO 4 N 2 octahedra with a cis-type configuration (Figure 5b).By coupling the DFT calculations with the finite difference time domain (FDTD) simulations, Hafez et al. [89] unraveled that BaTaO 2 N has large dielectric constants in the [001] direction based on the calculated real and imaginary parts of the diagonal components of the dielectric tensor  xx ,  yy , and  zz , which make it a promising dielectric material for various applications.
The BaTaO 2 N thin films grown epitaxially on (100)-cut SrTiO 3 by pulsed laser deposition exhibited dielectric permittivities ranging from 200 to 240 with a slight frequency dependence (Figure 5c). [97]More than an order of magnitude difference noted in the dielectric permittivities of thin-film ( ≈ 220) and incompletely sintered ceramic ( ≈5000) samples of BaTaO 2 N was attributed to the formation of boundary-layer capacitors.Despite their differences in dielectric permittivity, both thin-film and ceramic BaTaO 2 N samples do not undergo a structural or electrical phase transition, indicating the characteristics of ferroelectrics and relaxors.The X-ray line broadening analysis indicated that BaTaO 2 N was formed with greater micro-strains than its oxide analog KTaO 3 due to the local atomic displacements, and the dielectric behavior of BaTaO 2 N was associated with the local polarization induced by geometry relaxation. [98]The highly insulating BaTaO 2 N 0.85 ceramic, which was obtained by Kikkawa and co-workers [99] via the synthesis of BaTaO 2 N by solid-state reaction in NH 3 at 1400 °C and further ammonolysis in the presence of BaCO 3 additive, with a relative density of 73.0% showed real relative dielectric constants of  r = 620 at 10 2 Hz and  r = 320 at 10 8 Hz with the dielectric loss of less than 0.1, and it was found that porosity was necessary to eliminate the electronic contribution (Figure 5d).To avoid the partial loss of nitrogen from the perovskite lattice, the same research group [100] applied a spark plasma sintering method with BaCN 2 additive to synthesize a stoichiometric and electrically insulating BaTaO 2 N ceramic with a relative density of 79.8%, which showed relative dielectric constants in the range of  r = 320-650 and dielectric loss values in the range of tan = 0.04-0.19 at room temperature (Figure 5e).Takeuchi et al. [101] fabricated a thick BaTaO 2 N film with a relative density of 89% using an electrophoretic deposition and high-temperature sintering with a B 2 O 3 additive.The nitrogen loss was fully recovered after annealing in NH 3 , and the dielectric constant of 320 and the low dielectric loss of 0.05 at 1 MHz were achieved after nitridation for 100 h.Using piezoresponse force microscopy (PFM), Kikkawa and co-workers also observed a piezoresponse for the polished slice of a dense BaTaO 2 N ceramic with a relative density of 93%, which increased up to 7 V and gradually decreased with time but persisted even after 150 min at 6 V (Figure 5f). [102]Interestingly, highly porous BaTaO 2 N showed no piezoresponse because its porous surface was hydrolyzed.A weak piezoresponse was observed for an electrically conductive slice obtained from the deep inside of a dense BaTaO 2 N ceramic, suggesting that such a difference is linked to the existence of ferroelectricity.Cubic crystals of BaTaO 2 N with a size of up to 3.1 μm, grown in molten BaCN 2 , exhibited a much more highly insulating behavior than its ceramic counterpart and a ferroelectric piezoresponse that is attributable to the polar nanoregions induced by the polar linkages between cistype TaO 4 N 2 octahedra (Figure 5g). [103]This polarization switching of the self-standing oxynitride perovskite without an electrical leakage was observed for the first time.According to Kikkawa and Masubuchi, [104] ororhombic non-centrosymmetric BaTaO 2 N with space group Pmc2 1 can exhibit a second-harmonic generat possibly have a ferroelectric domain structure.Despite the absence of polarization at room temperature due to the different arrangement directions of the Pmc2 1 domain, it can still exhibit ferroelectric polarization after its poling treatment under an electric field exceeding the coercive field.In this case, the ferroelectricity of BaTaO 2 N can be explained in a classical way. [105]he crystal and electronic structures of the BaTaO 2 N perovskite, presented in the previous two sections, provide valuable insight into its fundamental properties.By understanding its unique atomic arrangement and electronic configuration, a foundation for exploring the dielectric, ferroelectric, and piezoelectric properties of this material is established.In the next section, we delve into the synthesis techniques used to synthesize BaTaO 2 N perovskite, with the aim of taking advantage of its structural and electronic characteristics to achieve desirable functionalities.By combining knowledge of its crystal structure with the understanding of its dielectric, ferroelectric, and piezoelectric responses, we aim to further unlock the potential of BaTaO 2 N perovskite for various applications.

High-Temperature Synthesis of BaTaO 2 N
In addition to its dielectric, ferroelectric, and piezoelectric properties discussed above, the optical property, [68] electrical conductivity, [68] CO 2 reduction, [106] solar water splitting, [107] photocatalytic degradation of organic water pollutants, [108] and photobiocatalytic H 2 evolution [109] of perovskite BaTaO 2 N were also explored.Among them, photocatalytic and photoelectrochemical water splitting has been of particular research interest in the past decades.The photocatalytic activity and photoelectrochemical performance of BaTaO 2 N are known to be susceptible to its synthetic history.In the last three decades, various synthetic approaches, including one-and two-step synthesis routes, have been developed to synthesize BaTaO 2 N. Perovskite BaTaO 2 N was first synthesized by a solid-state reaction using BaCO 3 and Ta 2 O 5 at 1000 °C for a prolonged period of time under an NH 3 flow by Marchand and co-workers (Figure 6a) [69,70,110,111] : In addition to its nitriding role, NH 3 also acts as a reducing agent, and the N 2 atmosphere is, therefore, essential to prevent the reduction of Ta 5+ .[114] Clarke et al. [112] explored the use of high-temperature conproportionation of binary oxides, nitrides, and oxynitrides under pure nitrogen gas in a radiofrequency induction furnace to limit the oxide content of the final products and synthesized BaTaO 2 N using TaON at 1500 °C for 3 h under 1 atm N 2 .This method allowed approaching the thermodynamic limit with regard to crystallographic O/N order/disorder and concluded that the O/N disorder is thermodynamically favored in the perovskite structures (e.g., BaTaO 2 N) at high temperatures, while the O/N order is favored in the lower dimensionality K 2 NiF 4 -type structures (e.g., Ba 2 TaO 3 N).Nie et al. [113] succeeded in the synthesis of phase-pure BaTaO 2 N with a homogeneous particle size distribution and a slightly off-stoichiometric composition (BaTaO 1.86 N 0.49 □ 0.65 ) at 1200 °C for 5 h under NH 3free atmosphere using BaCO 3 and TaN as starting materials.By involving Ta 3 N 5 , Sun et al. [114] further reduced the synthesis temperature and synthesized nearly single-phase BaTaO 2 N particles with high homogeneity at 850 °C for 10 h under the N 2 atmosphere.
BaTaO 2 N was also synthesized by a two-step method, where i) the corresponding oxide precursor was first synthesized and ii) then subjected to high-temperature ammonolysis under an NH 3 flow for a prolonged period of time (e.g., 20 h, 30 h, etc.). [67,115]The oxide-to-oxynitride conversion also leads to the formation of porous structures through the lattice condensation process caused by the partial replacement of O 2− with N 3− in the anionic network. [116]Unlike BaTaO 2 N, its crystalline oxide precursor does not have stoichiometry due to the larger Ba 2+ ions, and Ba-rich Ba 5 Ta 4 O 15 has been routinely used as the oxide precursor to synthesize BaTaO 2 N by a two-step method (Figure 6c). [67,117]Dong et al. [118] significantly reduced the defect density and increased the surface area of BaTaO 2 N by involving Ba-rich LiBa 4 Ta 3 O 12 grown by a KCl flux method as a precursor (Figure 6d).However, the high-temperature ammonolysis of a nonstoichiometric oxide leads to the formation of a byproduct (BaO), which must be removed after completing the hightemperature ammonolysis, along with BaTaO 2 N due to the exclusion of one of the five Ba atoms from the unit cell of BaTaO 2 N.
In order to avoid the formation of BaO, BaTaO 2 N was alternatively synthesized by the high-temperature ammonolysis of amorphous Ba 2 Ta 2 O 7 (Figure 6e), [65] BaTaO x synthesized by a polymerized complex method (Figure 6f), [115] and highly reactive BaTaO y synthesized by a hydrothermal method (Figure 6g) [119] with a stoichiometric Ba/Ta ratio of unity.
Generally, NH 3 dissociates into hydrogen and nitrogen at temperatures as low as 500 °C, [120] and the decomposition rate of NH 3 depends on the nature of the surfaces where the NH 3 molecules are adsorbed and the diffusion of NH 3 on the surface. [121,122]hen, the formed hydrogen converts into a molecular state and removes oxygen as water vapor, while nitrogen forms an "active" species to be incorporated into the oxide lattice.Nitrogen in a molecular state has negligible activity during ammonolysis at <1000 °C because of its high dissociation enthalpy (ΔH diss.= 995 kJ mol −1 at 1027 °C).As the temperature increases, the ammonolysis of the oxide precursors becomes more prevalent with a greater formation rate of inert N 2 .Therefore, the synthe-sis of oxynitrides is only feasible with nonequilibrium conditions and proper control of the gas-phase composition.Consequently, this forces a constant replenishing of NH 3 and active nitrogen through either recirculation or regulation of gas flow.Hence, Brophy et al. [123] studied the effects of key variables, including temperature, gas flow velocity, sample position, furnace configuration, etc., on the reaction rate and purity of BaTaO 2 N and found that the oxide precursors must be placed closer to the gas inlet to maximize the amount of active nitrogen and non-dissociated NH 3 .
The ammonolysis of the oxide precursors is the most commonly used method, where NH 3 acts as a reduction agent and a nitrogen source.The oxide-to-BaTaO 2 N conversion process takes place at a high temperature under an NH 3 flow for an extended Reproduced with permission. [124]Copyright 2021, American Chemical Society.b) TEM image, formation mechanism, and visible-light-driven photocatalytic reaction time courses of H 2 and O 2 evolution of cube-like BaTaO 2 N crystals.Reproduced with permission. [129]Copyright 2015, American Chemical Society.c) SEM image and schematic representation of photodeposited Pt nanoparticles on the {100} facets of BaTaO 2 N, densities of states of different energy levels for {100} and {110} facets of BaTaO 2 N, hydrogen evolution rates of BaTaO 2 N with coexposed {100} and {110} and only {100} facets and diagram of electron−hole transfer in {100} and {110} facet junction.Reproduced with permission. [131]Copyright 2019, American Chemical Society.TEM images, SAED patterns, and H 2 evolution rates of i) BaTaO 2 N (RbCl), ii) BaTaO 2 N (CsCl), and iii) BaTaO 2 N (BaCl 2 ), and BaTaO 2 N (BaCl 2 ).Reproduced with permission. [132]Copyright 2020, American Chemical Society.period of time, generating various bulk and surface defects.Such defects are known to act as the recombination hubs, hindering the efficient separation and transfer processes of photo-excited electrons and holes.In our previous works, we drastically reduced the synthesis time and defect density of BaTaO 2 N by applying an NH 3 -assisted flux method, where NH 3 was supplied by flowing from one end to the other of the horizontal tube furnace.In such a system, the transfer distance of active nitriding species (NH 2 , NH, N, etc.) becomes too far from a synthesis mixture even at a high gas flow rate, maximizing the generation of N 2 and H 2 before active nitriding species reach the surface of the synthesis mixture.Consequently, an insufficient supply of active nitriding species results in a higher defect density in BaTaO 2 N due to prolonged high-temperature ammonolysis.In our recent work, [124] we specifically localized an NH 3 delivery system just above the synthesis mixture using a small-diameter gas-supplying horizontal tube to reduce the defect density of BaTaO 2 N, which generally results from long high-temperature ammonolysis, by a fresh supply of more active nitriding species and non-dissociated NH 3 .As a result, single-phase BaTaO 2 N with low defect density and high crystallinity was synthesized at 950 °C for ≥6 and ≥4 h by solid-state reaction and flux method, respectively, indicating the advantage of flux method over solid-state reaction in this system (Figure 7a).
However, the ammonolysis of the oxide precursors at high temperatures for an extended period of time leads to a selfdecomposition.The anion vacancies act as recombination centers for photo-excited charge carriers and generate a large band bending at the solid-liquid interface, forming a Schottky-type barrier that hinders the prompt migration of electrons from the bulk to the surface reaction sites, which reduces the photocatalytic water splitting activity. [62]The photocatalytic activity can be enhanced through the synthesis of semiconductor materials with a large surface area, high dispersion, high crystallinity, low defect density, defined morphology, and specifically exposed facets, allowing to reduce the number of recombination sites and increasing the photocatalytically active sites.Particularly, one-and twodimensional crystals can significantly enhance photocatalytic activity by increasing the active sites on the surface and decreasing the travel distance of photo-excited charge carriers. [125]However, the morphology control and the nanostructure fabrication of BaTaO 2 N are still challenging.Highly crystalline photocatalysts are preferred because the density of defects, which act as trapping sites and recombination centers for photo-excited electrons and holes, can be reduced by improving the crystallinity.Also, the photocatalyst particles must be reasonably small (a few hundred nanometers) so that electrons and holes generated in the bulk of semiconductor material can more easily reach the surface.
The flux method (also known as the molten salt method) is one of the crystal growth techniques, allowing the growth of highly crystalline crystals free from thermal and mechanical constraints at lower temperatures, and supersaturation is the driving force for crystal growth.The flux growth is classified into three groups depending on the method of obtaining supersaturation: i) cooling the solution, ii) evaporating the flux, or iii) using a temperature gradient.[128] The flux method is not only beneficial for obtaining highly crystalline transition metal oxynitrides but also for improving the kinetics of the nitridation (Figure 6h-j).One of our previous studies on the influence of different types of fluxes, including KCl, KI, KF, MgCl 2 , CaCl 2 , SrCl 2 , BaCl 2 •2H 2 O, K 2 SO 4 , K 2 MoO 4 , and K 2 CO 3 , on the formation of BaTaO 2 N crystals revealed that the KCl flux could specifically induce the formation of highly crystalline cube-like crystals of BaTaO 2 N with an average size of 125 nm and exposed {100} and {110} facets (Figure 7b).According to the results obtained from time-and temperature-dependent experiments, the growth of cube-like BaTaO 2 N crystals had possibly the following reaction steps [129,130] : i) the decomposition of BaCO Although cube-like BaTaO 2 N crystals were still synthesized via the formation of intermediate oxide phases, the flux method significantly reduced the synthesis time to 10 h, and a molten KCl flux facilitated the growth of BaTaO 2 N crystals by creating a highly reactive environment because of its high mobility and volatility characteristics.In comparison to BaTaO 2 N modified with 1.5 wt% IrO 2 and 0.3 wt% Pt nanoparticles, [107] cubelike BaTaO 2 N crystals modified with Pt and CoO x nanoparticles showed relatively higher H 2 and O 2 evolution rates, respectively, due to the reduced defect density and higher crystallinity achieved by an NH 3 -assisted flux method using KCl (Figure 7b).
In comparison to the NaCl flux, which led to the formation of an almost perfect cubic structure with six isotropic {100} facets, the KCl flux resulted in a similar cubic-like structure with exposed smooth edges belonging to the {110} facets at the intersection of the {100} facets.It was revealed that the photo-excited electrons and holes separately transfer to the {100} and {110} facets of BaTaO 2 N because of the different energy levels of the {100} and {110} facets, enhancing the charge separation through the inter-crystalline charge separation between the two facets (Figure 7c). [131]The {100} facet-dominant BaTaO 2 N crystals were grown by a flux method using RbCl and CsCl, whereas the tetrahedral-shaped BaTaO 2 N crystals with exposed {111} and {100} facets were grown using the BaCl 2 •2H 2 O flux (Figure 7d). [132]As a result, the BaTaO 2 N crystals grown using the RbCl flux exhibited a significantly higher photocatalytic H 2 evolution rate in comparison to those grown using the CsCl and BaCl 2 •2H 2 O fluxes.The BaTaO 2 N crystals with differently exposed facets were formed because of changes in the electrostatic forces induced by the involved fluxes.The flux exhibits an ionic state at higher temperatures, in which the cations and anions are adsorbed on the surfaces of the precursor particles to form an electric double layer between the precursor particles and the molten salts.The Cl − anions exhibit larger negative adsorption energy than the alkali cations, resulting in more facile adsorption on the surface, which induces a negative surface charge on BaTaO 2 N.These surface negative charges are then neutralized by the adsorption of alkali cations on the surface.The strength of the repulsion force between the particles due to the surface negative charges is therefore determined by the amount of adsorbed alkali cations.However, the adsorption energy of the alkali cation decreases as the size of the alkali cation increases, implying that smaller cations can be adsorbed more easily on the BaTaO 2 N surface to decrease the repulsion force and promote the aggregation of BaTaO 2 N particles.As Cs + (0.18 nm) is slightly larger than Rb + (0.17 nm), the BaTaO 2 N particles grown in the CsCl flux have a greater repulsion force than those grown in the RbCl flux.The increased repulsion force suppresses the aggregation of the BaTaO 2 N particles, leading to the formation of additional edge steps in the BaTaO 2 N (CsCl) samples.In the case of the BaTaO 2 N (BaCl 2 ) samples, Ba 2+ (0.16 nm) is smaller than both Rb + (0.17 nm) and Cs + (0.18 nm), thereby resulting in the increased adsorption of Ba 2+ cations on the surfaces of the BaTaO 2 N particles.Since barium is one of the constituent elements of BaTaO 2 N, excess Ba 2+ adsorption on the surfaces of the growing BaTaO 2 N particles can affect the atomic arrangement of BaTaO 2 N, leading to the exposure of different crystal facets. [132]Due to its cubic crystal structure, BaTaO 2 N commonly crystallizes into a cubic shape even when Ba 5 Ta 4 O 15 with plate-like structures [129] is involved as an intermediate oxide phase.Nevertheless, because of a small lattice mismatch (0.7%) in the atomic arrangements of the Ba 5 Ta 4 O 15 (001) plane and the BaTaO 2 N (111) plane, the plate-like submicron-sized structures of BaTaO 2 N were directly synthesized by a KCl flux-mediated ammonolysis. [133]Similarly, Luo et al. [134] also synthesized platy BaTaO 2 N crystals with well-developed {111} facets via the simultaneous formation and transformation of Ba 5 Ta 4 O 15 using the K 2 CO 3 /KCl binary flux (Figure 8a).An amorphous layer is often formed on the surface of BaTaO 2 N during the high-temperature ammonolysis process, which promotes the recombination of photo-excited electrons and holes, thus reducing its performance.Therefore, the postsynthesis thermal treatment of BaTaO 2 N particles in an Ar or H 2 atmosphere and the necking treatment were found to be advantageous for enhancing its performance. [65,124,135]Although the flux  [111] direction (bottom), and schematic illustration of the formation mechanism of platy BaTaO 2 N crystals.Reproduced with permission. [134]Copyright 2020, American Chemical Society.b) Synthesis procedure and top-view SEM image of BaTaO 2 N film, current−potential curves of bare and CoPi-covered BaTaO 2 N electrodes, IPCE of CoPi/BaTaO 2 N measured at 1.23 V versus RHE, and H 2 and O 2 evolution from a CoPi/BaTaO 2 N photoanode at 1.23 V versus RHE in a 0.5 M potassium phosphate solution (pH 13) under AM 1.5G simulated sunlight.Reproduced with permission. [66]Copyright 2016, American Chemical Society.c) SEM images of BaTaO 2 N nanoparticle films with different Ba:Ta atomic ratios on Nb substrates, LSV curves of BaTaO 2 N photoanodes with NiCoFe-Bi co-catalyst in 1 M KOH (pH 13.6) under simulated AM 1.5G simulated sunlight, ABPE of the BaTaO 2 N photoanodes, and IPCE spectra of the BaTaO 2 N photoanodes at 1.23 V versus RHE and the corresponding integrated photocurrent over the standard AM 1.5G solar spectrum.Reproduced with permission. [148]Copyright 2022, Elsevier.
www.advancedscience.commethod is one of the promising synthesis routes for BaTaO 2 N with a lower defect density, the flux residue must be removed in order to reduce its negative impact on the performance of BaTaO 2 N.

Other Methods Applied for the Synthesis of Perovskite BaTaO 2 N
High-temperature synthesis of perovskite oxynitrides under an NH 3 flow for a prolonged period of time causes the reduction of transition metal cations to lower oxidation state species for charge compensation of the nitrogen vacancies.Those reduced species form a donor level just below the conduction band minimum, which may act as recombination centers for photo-excited electrons and holes, thus decreasing its performance.Accordingly, much effort has been made to reduce the defect density of transition metal oxynitrides by developing novel strategies. [136]articularly, low-temperature routes that use amide, urea, and azide as nitrogen sources instead of ammonia gas have been demonstrated to be alternative methods for the synthesis of transition metal oxynitrides with a lower defect density.The use of NaNH 2 with different NaNH 2 /Ta molar ratios as a flux-nitrogen source promoted the synthesis of BaTaO 2 N even at 493 K for 20 h in a tightly sealed autoclave, which is ≈500 K lower than the temperature applied generally to synthesize BaTaO 2 N. [137] However, the nature of an explosive reaction was not described in detail.The highest photocatalytic H 2 and O 2 evolution rates were observed for BaTaO 2 N synthesized with the NaNH 2 /Ta molar ratio of 3 because of its higher crystallinity and lower defect density associated with the reduced Ta 3+ species.Odahara et al. [138] applied a similar synthesis approach using Ba(OH) 2 , TaCl 5 , and NaNH 2 , which caused an explosive reaction and resulted in snuff powders as the products.Nevertheless, the addition of hexane reduced the risk of explosion during the mixing of starting materials and enabled to control of this exothermic reaction, leading to the formation of BaTaO 2 N: Ba(OH) 2 + TaCl 6 + 5NaNH 2 → BaTaO 2 N + 5NaCl + 4NH 3 (16)   Cordes et al. [139] synthesized BaTaO 2 N by an ammonothermal method at temperatures of 900 K and maximum pressures of up to 300 MPa in high-pressure custom-built autoclaves using Ta and Ba metals and NaN 3 and NaOH as mineralizers.Intermediates with amide, imide, and hydroxide groups were assumed to be the most likely reactive species in the formation reaction of BaTaO 2 N. Conventional ammonothermal synthesis of oxynitrides requires an alloy precursor prepared by arc melting, [140] which can lead to the volatilization of low-boiling-point components.Thus, a direct synthesis by a one-pot method is advantageous.Yoshimura and co-workers [141] successfully synthesized BaTaO 2 N by an ammonothermal method at >600 for 20 h under a pressure of 100 MPa using supercritical ammonia, NaOH as an oxygen source, and NaNH 2 as a basic mineralizer to improve the solubility of starting materials.However, perovskite oxynitrides are unstable during high-temperature synthesis and release a portion of their lattice nitrogen at >900 °C.Therefore, the use of nitrogen-rich starting materials to intermittently supply nitrogen to the reaction system can assist to avoid a partial nitrogen loss from the formed oxynitrides during the high-temperature synthesis.
Alkaline earth metal cyanamides are compounds containing alkaline earth metals and nitrogen together as their constituents like oxynitride perovskites.They are expected to melt at temperatures lower than the temperature for the nitrogen release from perovskite oxynitrides.Since the partial decomposition of SrTaO 2 N begins at 950 °C, which is ≈100 °C higher than the decomposition temperature of BaTaO 2 N, SrTaO 2 N was chosen to be partially soluble in the melt of metal cyanamide and recrystallizes in cooling.Therefore, the molten BaCN 2 partially dissolved SrTaO 2 N and formed a solid-solution precipitate Sr 1−x Ba x TaO 2 N from the melt upon cooling.The obtained products had a compositional gradient from a strontium-rich interior to a bariumrich exterior in their crystals. [142]Hosono et al. [143] found that t-BaNCN is more stable than r-BaNCN, and it melts at 1183 K, making it a promising flux for the crystal growth and sintering of various (oxy)nitrides.Alkaline earth metal carbodiimides are ionic crystals comprising metal cations and symmetric [N = C = N] 2− anions, which are different from asymmetric cyanamide anions ([N = C (three bonds N] 2− ).These compounds can act as fluxes for the synthesis of perovskite oxynitrides.Kikkawa and co-workers [103] synthesized small cubic crystals (3.1 μm) of perovskite BaTaO 2 N via the formation of a Ruddlesden−Poppertype oxynitride from the reaction between BaTaO 2 N powder and molten BaCN 2 flux.On the other hand, BaCN 2 was also used as a starting material along with Ta 2 O 5 to synthesize highly dispersed BaTaO 2 N particles with a size of 100-850 nm at ≈900 °C for 5 h because of the partial dissolution of BaTaO 2 N in the BaCN 2 melt. [144]Further, to prevent the thermal decomposition and the partial nitrogen loss, spark plasma sintering with (100 MPa) and without pressure was applied to rapidly synthesize BaTaO 2 N powder for 3 and 10 min using BaCN 2 and urea as additive and nitrogen sources, respectively. [100,145]The BaTaO 2 N grains were bonded through dissolution and precipitation at their surfaces in the BaCN 2 melt, and a minor Ba-rich compound, possibly Ruddlesden-Popper-type Ba 2 TaO 3 N, was identified at the boundaries of the BaTaO 2 N grains. [100]BaTaO 2 N synthesized by a pressureless spark plasma sintering technique exhibited a very high room-temperature relative permittivity up to 9550 with a dielectric loss down to 0.001 at 100 Hz due to its O/N ordering in the cis configuration and high purity (97.8%). [145]Gomathi et al. [146] also employed urea as a nitrogen source to synthesize BaTaO 2 N nanoparticles with a size of about 60 nm at 1226 K for 2 h.NH 3 is yielded upon urea decomposition at 523 K and can react with starting materials to form oxynitrides.By varying the amount of urea, the nitrogen stoichiometry can, therefore, be controlled since urea gives a nitrogen content close to the theoretical value.However, urea generally decomposes at a relatively low temperature that is far below the temperature required for the formation of perovskite oxynitrides and the ammonolysis of their oxide precursors.Therefore, it is still challenging to precisely control the synthesis process, purity, and stoichiometry of BaTaO 2 N. The presented synthesis approaches generally result in BaTaO 2 N powders, which need to be further deposited on substrates for PEC OER.The PEC-OER performance depends on the thickness, morphology, crystallinity, grain boundaries, etc. of the BaTaO 2 N films.

Fabrication of BaTaO 2 N films
The design of devices for practical applications typically requires high-quality thin films.The fabrication of thin films is an attractive approach for the synthesis of dense and highly crystalline perovskite oxynitrides.The epitaxial thin film of BaTaO 2 N was grown on a 100-cut SrTiO 3 single-crystal substrate with a conducting buffer layer of SrRuO 3 by a pulsed laser deposition technique at 760 °C in a mixed gas atmosphere of 100 mTorr N 2 /O 2 . [97]The dielectric permittivity of the fabricated thin film was much different from the value reported for incompletely sintered ceramics of BaTaO 2 N with considerable porosity (45%). [68]It was found that the epitaxial strain effect can cause a tetragonal distortion of the unit cell of perovskite BaTaO 2 N with a negligible volume change.The nanostructured thin films of BaTaO 2 N were formed on Ta substrates by the ammonolysis of hydrothermally grown Ba 5 Ta 4 O 15 nanosheet layers at 1000 °C for 2 h. [66]The fabricated BaTaO 2 N thin film deposited with a CoPi layer exhibited a photocurrent density of ≈0.75 mA cm −2 at 1.23 V versus RHE and generated O 2 for 5 h with a Faradaic efficiency of > 90% without significant deactivation under AM 1.5G simulated sunlight due to the high crystallinity of the formed film (Figure 8b).Although the branching structure of the BaTaO 2 N film could promote the transfer of photo-excited holes to the electrode surface by shortening the diffusion distance, the pores of the fabricated film would hamper the efficient transfer of photo-excited electrons to the back contact.Therefore, the photocurrent at low applied potential was not significantly enhanced.The BaTaO 2 N thin film was fabricated by the ammonolysis of the oxide precursor film, which was deposited on the alumina substrate by the dip coating of a gel made by a polymerizable complex method, at 950 °C for 15 h. [147]he BaTaO 2 N thin film was fabricated by depositing BaF 2 and Ta 2 O 5 on an Nb foil by a dual-source electron-beam deposition, followed by ammonolysis at 1273 K for 10 h under an NH 3 flow. [148]With increasing the Ba:Ta ratio, the size of nanoparticles in the BaTaO 2 N film increased and the shape of nanoparticles became well-defined, indicating the improved crystallinity of less densely packed BaTaO 2 N nanoparticles.The BaTaO 2 N nanoparticle photoanode achieved a photocurrent density of 4.7 mA cm −2 at 1.23 V versus RHE under AM 1.5G simulated sunlight and a maximum ABPE of 1.18% (Figure 8c).The BaTaO 2 N/Ta 2 N/Ta thin film was fabricated by depositing the TaN, Ta 3 N 5 , and BaCO 3 layers on the Ta metal substrate by radio frequency (FR) sputtering and heat treatment at 1173-1273 K for 0.5 to 1 h under N 2 atmosphere. [149]CoO x -deposited BaTaO 2 N/Ta 2 N/Ta electrodes produced a photocurrent density of 4.6 mA cm −2 at 1.23 V versus RHE and exhibited a 9% IPCE at 600 nm during water oxidation under AM 1.5G simulated sunlight due to the remarkable conductivity of Ta 2 N, promoting an efficient transfer of electrons between BaTaO 2 N and Ti layers, and both H 2 and O 2 were produced with a Faradaic efficiency of almost 100% (Figure 9a).In contrast, the layer of densely-packed polyhedral crystals of BaTaO 2 N was directly grown on the Ta metal substrate by a fluxcoating technique, and no intermediate TaO x byproduct layer was found, indicating high-quality crystal layers with interfaces. [150]ippert and co-workers [151] comparatively studied the photoelectrochemical performance of oxynitride, including BaTaO x N y , thin film-and particle-based photoelectrodes fabricated by conventional pulsed laser deposition (PLD) method and elec-trophoretic deposition, respectively.Interestingly, they found that the particle-based photoelectrodes could exhibit a higher photocurrent density due to the improved absorption properties and a larger electrochemical surface area (Figure 9b).The thin film-based photoelectrodes could surpass the particle-based photoelectrodes because of higher crystallinity and good electrical contact between grains, facilitating the separation and transfer of photo-excited charge carriers.The growth of crystalline and fully dense films on three-dimensional nanostructures was suggested to widen the electrochemical surface area of the films while preserving the bulk properties.One of the challenging issues of oxynitride-based semiconductors is physicochemical degradation at the interface with water, where the electrochemical reaction takes place.To understand the degradation mechanism and to find strategies to mitigate such detrimental effects, Pergolesi et al. [152] suggested that the study of solid-liquid interface can benefit enormously from the use of thin films with well-defined and atomically flat surfaces for synchrotronbased surface-sensitive X-Ray scattering methods and neutron reflectometry.Particularly, synchrotron radiation-based soft X-ray angle-resolved photoemission spectroscopy (SX-APRES) enables the probing of the k-resolved electronic structure in the subsurface region despite atmospheric surface contamination and provides the opportunity to explore materials, surfaces, and interfaces as well as their reactivity and evolution.The anion arrangement in oxynitrides significantly influences their optical and electronic properties.However, it is difficult to assess anion arrangement in thin films.Yamamoto et al. [153] demonstrated inverse photoelectron holography, which is an atomic-resolution holography technique, to directly measure the local structure around atoms of light elements.The obtained holograms of anions in the SrTaO 2 N thin films showed a remarkable difference between O and N holograms, revealing a trans-type anion ordering.The simulated hologram agrees well with the experimentally obtained data.To achieve high efficiency in BaTaO 2 N film, it is necessary to select an appropriate deposition technique and optimize the deposition parameters.

Control of Particle Morphology, Size, Porosity, and Surface Properties of BaTaO 2 N
Controlling the particle morphology, size, and dimension, porosity, and surface properties is essential for improving the watersplitting performance of BaTaO 2 N. Previously, the particle morphology, size, and porosity of BaTaO 2 N were modulated by the ammonolysis of the Ba 5 Ta 4 O 15 precursor, synthesized by a flux method using BaCl 2 , KCl, RbCl, CsCl, KCl+BaCl 2 , and K 2 SO 4 with different solute concentrations, with and without KCl flux. [117]The flux method enabled to control the morphology, size, and dimension of Ba 5 Ta 4 O 15 crystals.It was found that the ammonolysis of Ba 5 Ta 4 O 15 with the KCl flux could lead to the formation of the dense crystals of BaTaO 2 N, whereas the ammonolysis of Ba 5 Ta 4 O 15 without the KCl flux resulted in the porous structures of BaTaO 2 N (Figure 6i).Among the samples with different morphologies, sizes, and porosities, the BaTaO 2 N crystal structures obtained by a flux-free ammonolysis of Ba 5 Ta 4 O 15 crystals grown using CsCl at a 10 mol% solute concentration (sample C10) exhibited the highest photocurrent density of ≈3.11 mA cm −2 at 1.2 V versus RHE due to their average size of 1-2 μm  [149] Copyright 2020, WILEY-VCH Verlag GmbH & Co. KGaA.b) Schematic representation of thin filmand particle-based oxynitride photoanodes and their photoelectrochemical performance.Reproduced with permission. [151]Copyright 2019, American Chemical Society.and a porous surface.A further increase and a decrease in the particle size led to a reduction in the photocurrent density due to the existence of a long transfer distance to reach the surface for photo-excited charge carriers and the surface degradation by excess high-temperature nitridation, increasing the bulk and surface recombination rates, respectively.Within the first 3 hours of the photocatalytic reaction, BaTaO 2 N obtained by a flux-inclusive ammonolysis of Ba 5 Ta 4 O 15 crystals grown using BaCl 2 at a 10 mol% solute concentration (sample B10K) showed a higher O 2 evolution rate of 134.64 μmol h −1 (404 μmol) because of its larger particle size, predominantly exposed surfaces with a similar nature, high crystallinity, reduced surface defect density, and fewer grain boundaries (Figure 10a).
According to Anbalagan and Thomas, [154] the sintering of particles with a diameter of <10 nm requires no external sintering aids, including the addition of barium sources, and can lead to the formation of a cluster of tantalum and oxygen atoms at the interface of the BaTaO 2 N particles.Another study based on molecular dynamics on size and temperature-dependent specific heat capacity and diffusion constants of ultra-small BaTaO 2 N nanoparticles revealed that specific heat capacity increases with temperature, but with additional features that become pronounced with a size reduction due to the surface contribution and the formation of vacancies. [155]The diffusion constant decreases with increasing the size of BaTaO 2 N nanoparticles.These affect the defect chemistry, phase and interface stability, and sintering dynamics Reproduced with permission. [117]Copyright 2019, The Royal Society of Chemistry.b) Schematic representation of the synthesis of BaTaO x N y powders and laser fragmentation process, potentiodynamic scans for the BaTaO x N y -zp photoanodes in NaOH electrolyte under chopped light illumination, and photocurrent density and APCE values at 1.23 V versus RHE as a function of the number of passages applied for fragmentation.Reproduced with permission. [156]Copyright 2020, Elsevier.
of BaTaO 2 N nanoparticles.Haydous et al. [156] studied the effect of downstream laser fragmentation, a novel method to increase surface area, on the specific surface area and photoelectrochemical performance of barium tantalum oxynitride (Figure 10b).Despite the increase in surface area, the performance of BaTaO 2 N photoanodes did not improve significantly and the absorbed photonto-current conversion efficiency (APCE) decreased within the first fragmentation passages due mainly to the presence of grain boundaries in the fragmented BaTaO 2 N particles, which became recombination hubs for photo-excited electrons and holes.Also, the laser fragmentation resulted in the loss of N content and reduced crystallinity of BaTaO 2 N, which significantly affected the optical properties and photoelectrochemical performance.In general, the surface area becomes greater when the particle size is minimized.The reduced particle size may give a quantum size effect, resulting in a broader band gap.Therefore, a high degree of crystallinity is often preferred over a large surface area for uphill water splitting reaction because of the recombination of photoexcited charge carriers in the defects. [157]he water-splitting performance of photocatalysts is sensitive to the surface local structures.Recent studies on the effect of different exposed surfaces found that the BaTaO 2 N crystals with well-developed {111} facets [134] and co-exposed {100} and {110} facets [131] could exhibit a significantly enhanced photocatalytic activity for H 2 evolution in comparison to the BaTaO 2 N crystals with only {100} facets.Hence, a number of theoretical studies have been conducted to understand the impact of the surface local structures on various properties of BaTaO 2 N. The influence of different surface terminations on the electronic, optical, and photocatalytic properties of trans-and cis-BaTaO 2 N was studied using DFT calculations by Zhou et al. [158] The calculated work functions showed that the Ba-terminated surfaces have smaller work functions than the Ta-terminated ones.This indicates that the former is more favorable to promoting the separation of photo-excited charge carriers.The hydrogen evolution reaction proceeds more easily on the surfaces terminated with Ta, O, and N atoms, while the oxygen evolution reaction proceeds more readily on the surfaces terminated with N atoms than on the surfaces terminated with O atoms.In another work, [159] they also found that the water dissociation prefers to occur on the BaNbO 2 N surface terminated with N atoms than on the surface terminated with O atoms.Dissociative water adsorption on surfaces with the ( 100 Molecular dynamics (MD) simulation was involved to gain insights into the respective effect of doping with various aliovalent cations on the adsorption energy of water molecules and formed intermediates (H* for H 2 evolution and HO*, O*, and HOO* for O 2 evolution) on the predominant (110) surface of BaTaO 2 N (BTON) terminated with TaO 6 , TaN 6 , and TaO 4 N 2 octahedra (Figure 11a). [133]The kinetic V-type plots in H 2 evolution and the linear energy−performance correlations in O 2 evolution were observed using the relative kinetic analysis (ln[(r XBTON )/(r BTON )] versus -(E XBTON −E BTON )) plot.The experimental photocatalytic reaction rates were satisfactorily described using the adsorption energies of intermediates (H* for HER and HO* and O* for OER) estimated by MD calculations, rationalizing the effects of aliova-lent cation doping (Al 3+ , Ga 3+ , Mg 2+ , Sc 3+ , and Zr 4+ ) and surface chemical termination (TaO 6 , TaN 6 , and TaO 4 N 2 ) of BaTaO 2 N. A satisfactory match for all dopants was obtained for the BaTaO 2 N surface terminated with the TaO 6 octahedra.Nevertheless, the BaTaO 2 N:Zr surface terminated with the TaO 4 N 2 octahedra and the BaTaO 2 N:Mg surface terminated with the TaN 6 and TaO 4 N 2 octahedra showed trend deviations possibly due to simplifications in the MD simulation.
Strain engineering has also been demonstrated to be one of the effective methods in modulating the properties of perovskite oxynitrides.Castelli and co-workers [160] explored the effect of strain engineering on the OER for the different surface terminations of BaTaO 2 N and found that 1% tensile uniaxial strain in the [001] direction could lower the theoretical overpotential from 0.43 V to 0.37 V for the (100) facet TaO 2 N-terminated surface under (photo)electrochemical conditions (Figure 11b).Zhao et al. [161] investigated the effect of tensile and compressive strains on the electronic and optical properties of BaTaO 2 N based on the first-principles calculations.Their findings indicated that the optical properties of BaTaO 2 N were more sensitive to strains in the [100] direction than in the [010] direction, and a pronounced redshift of the absorption edge and the reduction of bandgap energy were observed under tensile strain (Figure 11c).Precise control of the morphology, dimension, particle size, porosity, and surface properties of BaTaO 2 N, as discussed in this section, lays the foundation for further advances in the properties and efficiency of BaTaO 2 N.

Cation Doping of BaTaO 2 N
Doping is one of the effective strategies to modulate the electronic structure, optical properties, electrical conductivity, charge density, charge mobility, and charge separation and transfer, leading to enhanced water-splitting efficiency. [9,157]Earlier studies have also confirmed that the defect density can be reduced by doping but at the expense of visible light absorption. [133,162]The physicochemical and photophysical properties were tuned and the surface local structure and anion ordering were tailored by an intentional introduction of foreign cations with different radii and valences into the A-site or B-site of BaTaO 2 N without altering its perovskite structure. [163]Particularly, dopants with different valences act as either electron donors (higher valency) or acceptors (lower valency) and change the carrier concentration.

Monovalent Cation Doping
Despite having ionic radii with sizes close to that of barium in the twelve-coordinated A-site of perovskite BaTaO 2 N, monovalent alkali-metal cations are prone to sublimation during high-temperature synthesis and ammonolysis under an NH 3 flow.Nevertheless, small amounts of lithium, [118] sodium, [164] and potassium [129] were found in BaTaO 2 N synthesized by the ammonolysis of LiBa 4 Ta 3 O 12 and a flux method using the KCl and RbCl fluxes, respectively.In the time course of the Z-scheme overall water splitting reaction over BaTaO 2 N synthesized by the ammonolysis of LiBa 4 Ta 3 O 12 , the evolution rates of H 2 and O 2 in the first hour were 3.1 and 1.55 μmol, respectively, confirming  [133] Copyright 2022, American Chemical Society.b) Volcano plot of the free-energy difference of (ΔG O* -ΔG OH* ) and the OER theoretical overpotential () for TaO 2 N-terminated (100) full O-covered surfaces.Gibbs free energy diagrams for the full O-covered TaO 2 N-terminated (100) surface with 4% compressive uniaxial strain in the [010] direction, 1% tensile uniaxial strain in the [001] direction, and 2% tensile uniaxial strain in the [001] direction.Reproduced with permission. [160]Copyright 2021, American Chemical Society.c) Effect of strain on band edges and band gaps of ATaO 2 N (A = Ca, Sr, and Ba) and their optical absorption under strain.Reproduced with permission. [161]Copyright 2019, Springer Nature AG & Co. KGaA.
the overall water splitting with the stoichiometric H 2 /O 2 ratio of 2:1 due to the efficient transfer of photo-excited charge carriers to the surface (Figure 12a). [118]A high degree of cocatalyst dispersion and intimate contact with the photocatalyst are important for improving photocatalytic activity.Interestingly, the addition of a small number of Na ions, as a promoter, to the Pt-loaded BaTaO 2 N enhanced the photocatalytic H 2 evolution (about 600 μmol within 5 h) under visible light irradiation, resulting in an apparent quantum yield for the H 2 evolution reaction of ≈1% at 420 nm, and during Z-scheme overall water splitting, a mixture of Na-containing Pt/BaTaO 2 N and H + -Cs + -modified PtO x /WO 3 evolved stoichiometric amounts of H 2 and O 2 from an aqueous NaI solution under visible light up to 12 h (Figure 12b). [164]The highest degree of H 2 evolution activity was obtained for BaTaO 2 N with 0.28 wt% Pt and 0.23 wt% Na.The presence of Na improved the dispersion and structural stability of the Pt cocatalyst, resulting in more efficient electron extraction from BaTaO 2 N, and no Na was incorporated into the BaTaO 2 N lattice.0.47 at% K was unintentionally introduced from the KCl flux into cube-like BaTaO 2 N crystals synthesized by an NH 3 -assisted direct flux method. [129]Although cube-like BaTaO 2 N crystals exhibited the evolution rates of 10.14 μmol H 2 and 22.18 μmol O 2 within 6 hours of the photocatalytic half-reactions, the influence of potassium on the photocatalytic activity of BaTaO 2 N was not Figure 12. a) Photocatalytic reaction time courses for H 2 evolution in the presence of methanol (i) and Z-scheme overall water splitting (ii) of BaTaO 2 N synthesized by flux method and solid-state reaction and iii) cathodoluminescence (CL) spectra of BTON-SSR (red), 0.5 wt% Pt-BTON-SSR (pink), BTON-Flux (black), and 0.5 wt% Pt-BTON-Flux (grey).Reproduced with permission. [118]Copyright 2017, The Royal Society of Chemistry.b) Photocatalytic reaction time courses for H 2 evolution over 0.28 wt% Pt/BaTaO 2 N with and without 0.23 wt% Na (i).Photocatalytic H 2 evolution rates over Na-containing 0.28 wt% Pt/BaTaO 2 N as a function of Na addition (ii).STEM images and particle size distributions of (iii) 0.23 wt% Na-containing and iv) Na-free 0.28 wt% Pt/BaTaO 2 N samples after hydrogen reduction at 523 K for 1 h.Effect of Pt loading amount (with constant Na ion level of 0.23 wt%) on photocatalytic reaction time courses for H 2 and O 2 evolution over a mixture of v) Na-containing and vi) Na-free 0.3 wt% Pt/BaTaO 2 N and H + -Cs + -0.5 wt% PtO x /WO 3 .Reproduced with permission. [164]Copyright 2021, The Royal Society of Chemistry.
investigated.The incorporation of Na + and Zn 2+ with similar ionic radii to control the crystal structure and composition of the precursor oxide (Na 1/4 Ba 3/4 )(Zn 1/4 Ta 3/4 )O 3 improved the OER activity of BaTaO 2 N (699 μmol h −1 ), with an apparent quantum yield of 11.9% at 420 nm (Figure 13a). [165]However, the exact structures of alkali metal species and their impacts on the local crystal structure, anion ordering, and photocatalytic water splitting activity of BaTaO 2 N were not studied in detail.Moon et al. [166] developed a novel synthetic strategy, which accomplishes cation intercalation with concomitant anion substitution, for developing new oxynitrides.The partial replacement of (Ta,N) in BaTaO 2 N by (Li,O) or (Na,O) modified its electronic structure.The compositional variation (BaLi 0.2 Ta 0.8 O 2.8 N 0.2 and BaNa 0.2 Ta 0.8 O 2.8 N 0.2 ) could control the lattice iconicity, optical band gap, and color, which also play an important role in solar water splitting (Figure 13b-d). [166,167]

Divalent Cation Doping
The effect of divalent cation doping on the photocatalytic activity of BaTaO 2 N was also explored.Particularly, Mg 2+ doping was found to be beneficial to shorten the Ta-O/N bonds or to strengthen the covalence bonding networks between Ta and O/N, which is favorable for inhibiting the formation of Ta 4+ -related defects and improving the charge separation (Figure 13e-g). [168]igure 13.a) Oxygen evolution rate for CoO x -loaded BaTaO 2 N obtained from ammonolysis of (Na 1/4 Ba 3/4 )(Zn 1/4 Ta 3/4 )O 3 blended with BaCO 3 and NaCl in aqueous AgNO 3 solution under visible light irradiation as a function of nitridation temperature (i) and action spectrum for oxygen evolution reaction using CoO x -loaded BaTaO 2 N ammonolyzed at 1223 K along with DRS of oxynitride (ii).Reproduced with permission. [165]Copyright 2020, The Royal Society of Chemistry.b) Diffuse-reflectance absorption spectra of SrTaO 2 N (ST), SrLi 0.2 Ta 0.  [167] Copyright 2015, American Chemical Society.e) UV-vis absorption spectra of BaTaO 2 N and BaTa 0.95 Mg 0.05 O 2+x N 1-y , f) Photocatalytic oxygen production of BaTaO 2 N and BaTa 0.95 Mg 0.05 O 2+x N 1-y under visible light illumination, and g) action spectra of BaTa 0.95 Mg 0.05 O 2+x N 1-y for O 2 production.Reproduced with permission. [168]Copyright 2020, Elsevier.h) Overall water splitting performance of IrO 2 /Cr 2 O 3 /Na-Rh/BaTaO 2 N:Mg (Ba/Ta/Mg = 2.5:1:0.1).Reaction time courses of gas evolution over IrO 2 /Cr 2 O 3 /Na-Rh/BaTaO 2 N:Mg from water under visible light and i) schematic representation of its mechanism.Reproduced with permission. [169]Copyright 2022, American Chemical Society.This is because hybridizations between Ta 5d and O/N 2p orbitals have major contributions to the conduction and valence bands of BaTaO 2 N near the Fermi level.As a result, an apparent quantum efficiency of 2.59% at 420 ± 20 nm was achieved for Mg 2+ -doped BaTaO 2 N. Also, a 5% Mg 2+ doping altered the electronic, optical, and surface properties and significantly enhanced the photocatalytic O 2 evolution reaction rate (503.6 μmol) of BaTaO 2 N crystals grown by a flux method. [133]The Mg doping along with Cr 2 O 3 /(Na)Rh and IrO 2 co-catalyst loading led to the one-step excitation overall water splitting with an apparent quantum yield of 0.08% at 420 nm and an STH conversion efficiency of 4 × 10 −4 % for BaTaO 2 N synthesized by an RbCl fluxassisted ammonolysis (Figure 13h,i). [169]Mg was found to promote a charge transfer to the loaded co-catalysts.Moon et al. [166] found that among various magnesium sources, such as oxide, halides, hydroxide, and carbonate, MgCl 2 was the most suitable one to successfully intercalate Mg 2+ into the layered oxide precursor Ba 5 Ta 4 O 15 , which was then ammonolyzed at 930 °C for 48 h to obtain BaMg 0.2 Ta 0.8 O 2.6 N 0.4 .By incorporating and increasing the amount of Ca 2+ in the BaTaO 2 N lattice, Xu and co-workers [162] observed the enlargement of optical bandgap energy and the decrease of defect density and nitrogen content.The formation of defects, such as Ta 4+ species, was effectively suppressed, the photocatalytic activity for water oxidation was significantly enhanced, and the stability against photocatalytic self-decomposition was largely improved upon partial substitution of Ca 2+ in BaTaO 2 N due to the efficient charge separation and longer lifetime of photo-excited electrons (Figure 14a).Nearly 3-fold enhancement in oxygen evolution was reached for BaCa 0.10 Ta 0.90 O 2.27 N 0.73 in comparison to pristine BaTaO 2 N, and an apparent quantum efficiency of ≈2.1% at 420 ± 20 nm was achieved for Ca 2+ -doped BaTaO 2 N. Castelli and co-workers [160] theoretically investigated the effect of divalent cation (Ca 2+ or Sr 2+ ) doping and strain engineering on the OER for differently terminated surfaces of BaTaO 2 N. The smallest theoretical overpotential (0.53 V) was obtained for Ca 2+ -doped TaON-terminated BaTaO 2 N (001) clean surface, with 4% tensile uniaxial strain in the [010] direction (Figure 11b).A simple crossover from the two-dimensional (2D) to the three-dimensional (3D) correlated disorder of O and N atoms on a cubic lattice was discovered within Ba 1-x Sr x TaO 2 N (0 ≤x ≤1) by Johnston et al. [170] The Ba 1-x Sr x TaO 2 N with cubic Pm 3m structure at x = 0-0.2 is consistent with a 3D distribution of disordered cis-chains, while neutron occupancies reveal that a long-range 2D confinement of anion chain layers is present across the tetragonal P4/mmm symmetry at x = 0.4-1.The local structures of ATaO 2 N (A = Ba, Sr, and Ca) were investigated by Page and co-workers [86] using a combination of experimental and theoretical approaches, including neutron total scattering, 3), 2 wt% CoO z was loaded as a cocatalyst and monochromic light was generated by filtering the output of lamp using bandpass filters.Reproduced with permission. [162]opyright 2018, Elsevier.b) UV-Vis absorption spectra of BTON and BZTON-15 h, Ta4f XPS spectra of BTON and BZTON samples nitrided for different time, comparison of visible-light-driven photocatalytic hydrogen evolution rates, and photocatalytic hydrogen evolution activities of BTON and BZTON-15 h as a function of reaction time.Reproduced with permission. [171]Copyright 2021, Elsevier.density functional theory (DFT), and ab initio molecular dynamics (AIMD) simulations.Their study shows that the local cis ordering and Ta off-centering can play decreasing roles in overall lattice stability, overshadowed by the stabilizing effects of octahedral tilting, by replacing Ba 2+ with Sr 2+ or Ca 2+ in BaTaO 2 N. The findings indicate that the anion order may have a larger influence on local dipoles and dipole ordering in such perovskite systems.Zn 2+ with an effective ionic radius of 0.68 Å, which is close to that of Ta (0.64 Å), was substituted in the crystal lattice site of Ta in BaTaO 2 N by the synthesis and ammonolysis of the Ba(Zn 1/3 Ta 2/3 )O 3 precursor. [171]The reduction and sublimation of Zn 2+ during the ammonolysis process and the effective charge balance by Zn 2+ resulted in an extremely low concentration of Ta 4+ -associated defects, efficient charge separation, and improved photocatalytic HER rate of about 65 μmol within 5 h (Figure 14b).

Trivalent Cation Doping
Trivalent cation (Al 3+ , Ga 3+ , and Sc 3+ ) doping was found to be useful for shifting the respective conduction band potential (E CB ) of BaTaO 2 N to more negative values. [133]The photocatalytic HER rate increased significantly from 6.59 μmol H 2 h −1 (pristine BaTaO 2 N) to 21.92, 10.27, and 12.12 μmol H 2 h −1 by doping 5% Al 3+ , Ga 3+ , and Sc 3+ , respectively, revealing the advantageous effect of trivalent dopants.In contrast, the photocatalytic OER rate decreased from 316.3 μmol (pristine BaTaO 2 N) to 252.5 and 188.0 μmol when 5% Ga 3+ and Al 3+ were substituted in BaTaO 2 N, whereas a 5% Sc 3+ dopant increased the photocatalytic OER rate within 5 h.Interestingly, 1% La 3+ doping resulted in the HER rate that is three times lower than that of pristine BaTaO 2 N. [172] Based on synchrotron X-ray powder diffraction analyses, Kim and Woodward [173] found that BaSc 0.05 Ta 0.95 O 2.1 N 0.9 has a simple cubic symmetry similar to BaTaO 2 N, whereas LaMg 1/3 Ta 2/3 O 2 N and LaMg 1/2 Ta 1/2 O 5/2 N 1/2 are isostructural to the oxide La 2 Mg(Mg 1/3 Ta 2/3 )O 6 (space group P21/n).The impedance spectroscopy analysis revealed an interesting capacitor geometry in BaSc 0.05 Ta 0.95 O 2.1 N 0.9 in which the semiconducting oxynitride grains are separated by the insulating secondary phases.Although the Rietveld refinement implies the presence of secondary phases like BaSc 2 O 4 , Sc 2 O 3 , or ScN, no impurity phases were detected in the synchrotron XRPD measurements.The diffusion of Sc-rich phase out of bulk to form amorphous shells surrounding crystalline BaTaO 2 N grains was proposed as a plausible model for such behavior.
Partial Al 3+ -Mg 2+ dual substitution (5%) was applied to engineer structural defects and to modulate optoelectronic and surface properties and photocatalytic activity of BaTaO 2 N. [174] The optical absorption edge of BaTaO 2 N shifted to shorter wavelengths after (co)substitution of Al 3+ and/or Mg 2+ for Ta 5+ , leading to the increase in the optical bandgap energy.This effect was more pronounced in the BaTaO 2 N samples with higher levels of Mg 2+ dopant because a large number of O 2− were substituted for N 3− to compensate charge balance.The initial photocatalytic reaction rates for O 2 and H 2 evolution confirmed the enhancement of the photocatalytic performance of BaTaO 2 N due to the dual substitution of Al 3+ and Mg 2+ for Ta 5+ (Figure 15a).Especially, Figure 15.a) UV-Vis diffuse reflectance spectra and b) reaction time courses for photocatalytic O 2 and H 2 evolution over BaTaO 2 N powders with no substituent (BTON1), 5% Al 3+ (BTON2), 5% Mg 2+ (BTON3), 2.5% Al 3+ + 2.5% Mg 2+ (BTON4), 3.5% Al 3+ + 1.5% Mg 2+ (BTON5), and 1.5% Al 3+ + 3.5% Mg 2+ (BTON6) loaded with CoO x and Pt nanoparticles under visible light irradiation.Reproduced with permission. [174]Copyright 2022, The Royal Society of Chemistry.Photocatalytic H 2 evolution rates of Na-Pt/BaTaO 2 N doped with various metal cations with an M/Ta ratio of 0. .Reproduced with permission. [172]Copyright 2023, The Royal Society of Chemistry.
BaTaO 2 N modified with 1.48% Al 3+ + 3.51% Mg 2+ generated the highest quantity of O 2 (178.66 μmol h −1 ) with an apparent quantum efficiency of 0.18% at 420 nm, while BaTaO 2 N modified with 3.47% Al 3+ + 1.52% Mg 2+ produced the highest quantity of H 2 (18.94 μmol h −1 ) with an apparent quantum efficiency of 0.64% at 420 nm.The enhancement achieved by partial Al 3+ -Mg 2+ dual substitution is related to the changes in the defect density, dynamics of charge carriers, electronic band structure, improvement in water and methanol adsorption, and a favorable shift in the band energy levels with respect to water reduction and oxidation potentials.

Tetravalent Cation Doping
Tetravalent cation (Zr 4+ and Ti 4+ ) doping lowered the photoelectrochemical performance of BaTaO 2 N photoanodes due to the decreased donor density and reduced titanium species (Ti 3+ ), which generated anion defects simultaneously, increasing the donor density but facilitated the recombination rate between photoexcited electrons and holes through the redox cycle. [175]The stabilization of the +4 oxidation state of titanium can be achieved by the inductive effect of doped rare-earth elements that share some electrons with the closest TM-O,N bond to enhance its covalency. [176]In contrast, 5% Zr-doped BaTaO 2 N showed a high photocatalytic activity in both O 2 (446.8μmol) and H 2 (80.4 μmol) evolution half-reactions within 5 h due to the altered potentials of the valence and conduction bands by Zr 4+ doping. [133]Intro-ducing Zr 4+ did not change the E CB value compared to that of pristine BaTaO 2 N presumably due to the compensation given by the change of valence band (E VB ) to more positive values.This is possible if it is considered that the incorporation of a dopant with a larger ionic radius (Zr 4+ ) leads to a greater substitution of O 2− for N 3− to compensate ionic charge imbalance, justifying the change in the E VB in the Zr-doped BaTaO 2 N. Recently, Li et al. [172] found that the incorporation of 1% Zr in BaTaO 2 N could enhance the photocatalytic H 2 evolution activity to the greatest extent (≈500 μmol h −1 ) because of the extension of the lifetime of electrons and the promotion of electron injection into the Na-Pt cocatalyst, while the higher doping levels reduced the activity.The Z-scheme overall water splitting (ZOWS) system based on Cr 2 O 3 /Na-Pt/BaTaO 2 N:Zr0.01 as the HEP, CoO x /Au/BiVO 4 as the OEP, and [Fe(CN) 6 ] 3−/4− as redox ions evolved H 2 and O 2 under visible light up to 520 nm, and its STH conversion efficiency reached 0.022% and the apparent quantum yields at 420 nm and 520 nm under monochromatic light were 1.5% and 0.2%, respectively (Figure 15b).

Pentavalent Cation Doping
Although BaNbO 2 N can absorb visible light up to 740 nm, a partial substitution of pentavalent niobium for Ta 5+ in BaTaO 2 N reduced the photocatalytic O 2 evolution reaction rate from 331.1 μmol to 46.8 μmol in the first 1 h of the photocatalytic reaction despite an insignificant change in the local density of states  [175] Copyright 2015, AIP Publishing LLC.
(Figure 16a,c). [177]This is attributed to the presence of reduced niobium species and anion defects because Nb 5+ is reduced more easily under a reducing NH 3 atmosphere than Ta 5+ because of the higher electronegativity of the former. [178]

Hexavalent Cation Doping
Hexavalent Mo and W species with the [Kr] 4d 5 5s 1 and [Xe] 4f 14 5d 4 6s 2 electronic configurations were partially substituted for Ta 5+ to control the donor density in the bulk for improving the performance of photoelectrochemical water splitting on porous BaTaO 2 N photoanodes under visible light irradiation (Figure 16d,e). [175]The Faradic efficiencies for H 2 and O 2 evolution were confirmed to be about 93% in the reaction.The partial substitution of Ta 5+ in BaTaO 2 N by Mo 6+ (up to 5%) was found to increase the donor density effectively, enhancing the photoelectrochemical performance.In contrast, the decreased photoelectrochemical performance in W-doped BaTaO 2 N was noted due to the facilitated recombination rate through the redox cycle between W 4+ and W 6+ species.
In general, the water-splitting performance may be decreased when transition metal cations with partly filled orbitals are doped in semiconductor-based photocatalysts.The doped elements can form either a donor or acceptor level in the forbidden band of semiconductors.Despite the improved visible-light absorption, dopants can also hinder a fast transfer of photo-excited charge carriers at the surface and in the bulk.Therefore, it is necessary to gain deeper insights into the effect of dopants on the local crystal structure, anion ordering, electronic structure, optical properties, electrical conductivity, charge density, charge mobility, charge separation and transfer, and solar water splitting performance of perovskite BaTaO 2 N.

Solid Solutions with BaTaO 2 N
Creating a solid solution with a wide-band gap oxide semiconductor is beneficial for enhancing the photocatalytic activity of various (oxy)nitrides. [179]The BaZrO 3 -BaTaO 2 N (0 ≤ Zr/Ta ≤ 0.1) solid solutions with band gaps of 1.7-1.8eV showed increasing rates of photocatalytic HER and OER under visible light irradiation above 660 nm with increasing the Zr/Ta ratio and the IPCE of the IrO 2 /TiO 2 /BaZrO 3 -BaTaO 2 N/FTO electrode for water oxidation was estimated to be ≈1.0%at 1.2 V versus RHE under 500 nm monochromatic light (Figure 17a). [180]This was the first report of a photocatalytic material that is capable of both reducing and oxidizing water even under irradiation above 660 nm.This is because the conduction band minimum and the valence band maximum of BaZrO 3 -BaTaO 2 N solid solution straddle the water-splitting potential.The apparent quantum yields of H 2 and O 2 evolution were about 0.06% and 0.03% at 420 nm, respectively.A relatively low apparent quantum yield for water oxidation was  SO 4 solution (pH 5.9) using IrO 2 -loaded TiO 2 /BaZrO 3 -BaTaO 2 N electrode.Reproduced with permission. [180]opyright 2012, Wiley-VCH Verlag GmbH & Co. KGaA.b) Dependence of conduction and valence band edge potentials for BaZrO 3 -BaTaO 2 N (Zr/Ta = 0.025) on pH of electrolyte.Reproduced with permission. [115]Copyright 2014, Elsevier.c) Reaction time courses of O 2 evolution on 1.5 wt% IrO 2loaded BaTaO 2 N and BaWO x N y -BaTaO 2 N (W/Ta = 0.005) under visible-light irradiation.Reproduced with permission. [181]Copyright 2013, Wiley-VCH Verlag GmbH & Co. KGaA.d) Reaction time course of O 2 evolution on 0.5 wt% IrO 2 -loaded (BaTaO 2 N) 0.99 (SrWO 2 N) 0.01 under visible-light irradiation, e) refined crystal structures with thermal ellipsoids and MEM electron-density distribution on the (100) planes with yellow equi-density surface at 0.5 Å −3 of (BaTaO 2 N) 1−x (SrWO 2 N) x and f) schematic density of states for i) intrinsic semiconductor BaTaO 2 N and ii) n-type semiconductor (BaTaO 2 N) 1−x (SrWO 2 N) x .Reproduced with permission. [182]Copyright 2017, The Royal Society of Chemistry.
obtained owing to a small energy difference (≈0.3 eV) between the water oxidation potential and the valence band maximum. [115]he dependence of the conduction and valence band edge potentials of the IrO 2 /TiO 2 /BaZrO 3 -BaTaO 2 N/FTO electrode was also studied as a function of pH.It was found that the positions of the conduction band minimum and valence band maximum of BaZrO 3 -BaTaO 2 N were dependent on pH, which were satisfactory for water oxidation at all pH values against H 2 evolution at the counter electrode (Figure 17b). [115]The BaWO x N y -BaTaO 2 N (0 < W/Ta < 0.05) [181] and (BaTaO 2 N) 1-x (SrWO 2 N) x (x = 0.01) [182] exhibited photocatalytic OER rates that were much higher than those of pristine BaTaO 2 N, SrWO 2 N, and BaZrO 3 -BaTaO 2 N.In the BaWO x N y -BaTaO 2 N and (BaTaO 2 N) 1-x (SrWO 2 N) x , the introduced W 5+ species formed a donor level just below the conduction band, promoting the n-type semiconducting nature of BaTaO 2 N (Figure 17c-f).This led to the upward band bending and increased the density of d electrons originating from pentavalent W species.The photo-excited holes in the valence band could easily migrate to the surface according to the upward band bending, enhancing the water oxidation activity.Because of an increased driving force for surface redox reactions and fewer defects in comparison to BaTaO 2 N, the BaZrO 3 -BaTaO 2 N for O 2 evolution.Reproduced with permission. [185]Copyright 2021, The Royal Society of Chemistry.
solid solution exhibited a six-to nine-fold improvement in photocatalytic non-sacrificial H 2 evolution from water under visible light irradiation, and an apparent quantum yield for the reaction was calculated to be about 0.6% at 420-440 nm. [183]Despite its small bandgap energy to drive water reduction and oxidation, the BaZrO 3 −BaTaO 2 N solid solution modified with Pt nanoparticles as water reduction promoters in combination with either PtO x /WO 3 or rutile-TiO 2 as an O 2 evolution photocatalyst in the presence of an IO 3 − /I − shuttle redox mediator exhibited a solardriven Z-scheme water splitting (Figure 18a). [184]On the other hand, it was difficult to achieve simultaneous H 2 and O 2 evolution in the presence of Fe 3+ /Fe 2+ redox couple due to the competitive oxidation of Fe 2+ .
Low photocatalytic activity is often associated with high defect density, structural distortions, and unsuitable band edge alignments that hamper the transfer of photo-excited charge carriers.In perovskite ATaO 2 N, structural characteristics (e.g., Ta-O/N distance, Ta-O/N-Ta angle, etc.) and optoelectronic properties (e.g., band structure, band gap, etc.) can be tuned by controlling the solid solution levels, which govern the photocatalytic performance.The La 1-x Ba x TaO 1+y N 2-y (0 ≤ x,y ≤ 1) solid solutions were synthesized by a high-temperature ammonolysis of oxide precursors obtained by a polymerized complex (PC) method. [185]The solid solutions exhibited higher photocatalytic activity for water oxidation than their parent compounds.Among them, the La 0.2 Ba 0.8 TaO 1+y N 2-y (x = 0.8) solid solution showed the highest photocatalytic activity for water oxidation with an apparent quantum efficiency of 3.91% at 420 ± 20 nm.This is because its conduction band minimum is positioned positively than that of LaTaON 2 and its valence band maximum is located negatively than that of BaTaO 2 N, resulting in a suitable overpotential for water oxidation and reduction (Figure 18b-f).
Various compounds containing transition-metal cations with d 0 electron configuration (e.g., Nb 5+ ) were explored to be active for solar water splitting.However, the lifetimes of the photoexcited holes become shorter once electrons from the conduction band or mid-gap states are available for recombination. [186]s mentioned earlier, Nb 5+ is reduced more easily than Ta 5+ under a high-temperature NH 3 atmosphere, and the photo-excited charge carriers in Nb-based oxynitrides may not have sufficient reactivity for solar water splitting in comparison to Ta-based counterparts. [178]Thus, in the BaNb 1−x Ta x O 2 N (x = 0.25, 0.50, 0.75) solid solutions, the increased concentration of Ta 5+ substituted for Nb 5+ led to the reduction of crystal size and background absorption intensity and the monotonic increase in the photocatalytic OER rate due to a more positively positioned valence band maximum and a lowered density of mid-gap states associated with defects. [177]he BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 solid solution was synthesized by the ammonolysis of BaNa 0.25 Ta 0.75 O 3 oxide precursor having a lattice-matched crystal structure with BaTaO 2 N and containing volatile Na. [187]During high-temperature ammonolysis, the lattice matching decreased the thermodynamic barrier of atomic diffusion rearrangement, and the sublimation of Na facilitated the direct phase transition of BaNa 0.25 Ta 0.75 O 3 to BaTaO 2 N.These processes significantly inhibited the formation of defects.Therefore, the BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 solid solution exhibited higher photocatalytic activity (54.62 μmol h −1 ) for H 2 evolution than pristine BaTaO 2 N (3.56 μmol h −1 ; Figure 19ac).In Z-scheme overall water splitting, Pt-modified BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 as the H 2 evolution photocatalyst and surfacetreated WO 3 as the O 2 evolution photocatalyst, and IO 3 − /I − as the electron mediator exhibited the highest photocatalytic activity, giving the H 2 and O 2 evolution rates of 14.46 and 6.68 μmol h −1 , respectively.No solid solutions between CaTaO 2 N and BaTaO 2 N Reproduced with permission. [187]Copyright 2022, American Association for the Advancement of Science.d) TEM and HRTEM images of Ba(0.3)-Ta 3 N 5 sample, e) Mott-Schottky plots and schematic representation of band structures of Ta 3 N 5 and BaTaO 2 N, and f) multiple cycles of Z-scheme overall water splitting with 0.5 wt% Pt/Ba(0.3)-Ta 3 N 5 and 0.45 wt% PtO x /WO 3 .Reproduced with permission. [192]Copyright 2017, The Royal Society of Chemistry.
were formed because of a large disparity in cation sizes, while complete solid solutions between SrTaO 2 N and BaTaO 2 N were obtained, changing the cubic Pm 3m BaTaO 2 N to the tetragonal I4/mcm SrTaO 2 N at about x = 0.5-0.6 in Ba x Sr 1-x TaO 2 N. [188] This is consistent with the earlier study on Ba x Sr 1-x TaO 2 N, conducted by Pors et al. [189] The AZr x Ta 1-x O 2+x N 1-x (A = Ca, Sr, Ba) solid solutions were synthesized using Zr-Ta xerogels and ACO 3 . [190]he unit cell parameters increased with x as the Ta 5+ ions were substituted by larger Zr 4+ ions.The CaZr x Ta 1-x O 2+x N 1-x and BaZr x Ta 1-x O 2+x N 1-x solid solutions were found to be orthorhombic and cubic for all x values, respectively, whereas the SrZr x Ta 1-x O 2+x N 1-x solid solutions were orthorhombic for x ≥ 0.60 and cubic for x ≤ 0.60.Both studies mainly emphasized the synthesis and structural analysis of solid solutions, and the photocatalytic performance of these solid solutions for solar water splitting was not investigated.

Heterostructures, Heterojunctions, and Photoelectrochemical Cells with BaTaO 2 N
The separation efficiency of photo-excited charge carriers can also be improved by forming the heterostructures without losing their light absorption ability.The following criteria must be met in order to develop the efficient heterostructures: i) the matching band-edge positions that thermodynamically allow a photocarrier transfer across their interfaces, ii) the maximized contact that allows multiple charge transfer pathways, and iii) the reduced structure mismatch across the interfaces that avoid nonstoichiometry and dangling bonds which may become trapping sites for photo-excited charge carriers. [191]he construction of a Ta 3 N 5 /BaTaO 2 N heterostructure by a one-pot synthesis method decreased the defect density, improved the spatial charge separation efficiency, and enhanced the relatively poor photocatalytic proton reduction activity of Ta 3 N 5 due to the formation of BaTaO 2 N on the surface of Ta 3 N 5 leading to surface passivation. [192]A visible-light-driven Z-scheme overall water splitting system constructed by using Ta 3 N 5 /BaTaO 2 N heterostructure, PtO x /WO 3 , and IO 3 − /I − pair as H 2 -and O 2evolving photocatalysts and redox mediator, respectively, showed an apparent quantum efficiency of 0.1% at 420 nm (Figure 19df).Earlier, Domen and co-workers [107] first achieved overall water splitting under visible light beyond 600 nm for the combination of Pt-BaTaO 2 N as the H 2 -evolving photocatalyst and Pt-WO 3 as the O 2 -evolving photocatalyst in the presence of IO 3 − /I − as a shuttle redox-mediator (Figure 20a).This demonstrated the potential of a two-step water-splitting system in a broader region of the visible spectrum.The Z-scheme overall water splitting was successfully achieved under visible light irradiation using barium-modified Ta 3 N 5 , PtO x /WO 3 , and IO 3 − /I − as the H 2 -and O 2 -evolving photocatalysts and a redox mediator, respectively.In the Ta 3 N 5 /BaTaO 2 N heterostructure composed of Ta 3 N 5 nanorods (1D) and BaTaO 2 N nanoparticles (0D), the Ta 3 N 5 nanorods could transfer electrons along the rod orientation direction, while the holes moved in the lateral direction.The intimate interface of the formed heterostructure was thought to stem from similar Ta-containing octahedron units of Ta 3 N 5 and BaTaO 2 N, which promoted a charge separation and enhanced the solar hydrogen production from water splitting one order of magnitude (Figure 20b). [193]The BaMg 1/3 Ta 2/3 O 3-x N y /Ta 3 N 5 heterostructure synthesized by a one-pot ammonolysis strategy exhibited superior charge separation and transfer ability and 20 times higher photocatalytic activity for proton reduction with respect to the corresponding counterparts, and the apparent quantum efficiency of Pt-loaded BaMg 1/3 Ta 2/3 O 3-x N y /Ta 3 N 5 (0.4) was measured to be 0.1% at 420 nm as the H 2 -evolving photocatalyst. [194]Higashi et al. [195] achieved overall water splitting into H 2 and O 2 under visible light with an apparent quantum efficiency of ≈0.1% at 420-440 nm by combining Pt-BaTaO 2 N and Pt-WO 3 in the presence of IO − 3 /I − as a shuttle redox mediator with (Figure 20c).Pihosh et al. [196] fabricated a core-shell heterojunction photoanode of Ta 3 N 5 -nanorods/BaTaO 2 N by combining glancing angle deposition and dip coating techniques.The heterojunction photoanode covered by a FeNiO x cocatalyst (Ta 3 N 5 -NRs/BaTaO 2 N/FeNiO x ) generated a stable photocurrent density of ≈4.5 mA cm −2 at 1.23 V versus RHE under AM 1.5G simulated sunlight and 34%−35% IPCEs in the broad range of 380-540 nm were achieved.The BaTaO 2 N shell on Ta 3 N 5 nanorods improved visible light harvesting, leading to the efficient generation and extraction of charge carriers and a stable evolution of stoichiometric O 2 (36.2 μmol h −1 cm −2 ) and H 2 (72.4 μmol h −1 cm −2 ) with Faradaic efficiencies near 96% (Figure 20d).Since Pt-modified BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 solid solution exhibited an enhanced photocatalytic activity for H 2 evolution, Luo et al. [187] further studied the Z-scheme overall water splitting by involving the BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 solid solution as the H 2 -evolving photocatalyst, surface-treated WO 3 as the O 2 -evolving photocatalyst, and IO 3 − /I − as a redox mediator under visible light irradiation.The system prepared with the BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 solid solution exhibited the highest photocatalytic activity with 14.46 μmol h −1 H 2 and 6.68 μmol h −1 O 2 evolution rates (Figure 19a).Castelli et al. [197] computationally investigated the band gaps and optical properties of functional perovskites composed of the layers of two cubic perovskite semiconductors (BaSnO 3 and BaTaO 2 N) and found that the different layers of perovskites can be used to design a direct band gap of functional perovskites between 2.3 and 1.2 eV.The stacking of BaSnO 3 and BaTaO 2 N layers was described as a type-II heterojunction, whereas the stacking of LaAlO 3 and LaTiO 2 N layers was suggested for designing a type-I heterojunction.
BaTaO 2 N was also involved as the O 2 -evolving photocatalyst in the photoelectrochemical (PEC) overall water splitting.The p/n PEC cell, prepared by connecting cobalt species-loaded BaTaO 2 N photoelectrodes and Pt-loaded Al-doped La 5 Ti 2 Cu 0.9 Ag 0.1 S 5 O 7 solid solution, enabled to accomplish an unassisted PEC water splitting at a Faradaic efficiency of unity and an STH conversion efficiency of ≈0.1% under irradiation of up to 710 nm (Figure 21a). [198]Later, the surface modification of BaTaO 2 N with Ir and Co species led to the increased photocurrent density (0.26 mA cm −2 ) and an STH conversion efficiency (0.14%) at 0.7 V versus RHE, which were three times higher than those obtained for BaTaO 2 N modified only with Co species (Figure 21b,c). [199]The PEC cell incorporating an Ir/Co-BaTaO 2 N photoanode and Ptloaded TiO 2 /CdS-modified Al-doped La 5 Ti 2 Cu 0.9 Ag 0.1 S 5 O 7 photocathode exhibited spontaneous overall water splitting with an STH conversion efficiency of 0.14% following a minute of AM 1.5G simulated sunlight.To achieve the targeted STH conversion efficiency, it is important to fabricate semi-transparent front photoanodes for PEC tandem devices.Recently, a semi-transparent three-dimensional macroporous CaTaO 2 N photoanode was successfully fabricated on a GaN/Al 2 O 3 substrate via a chemical route, which exhibited a high transmittance (>60%) in the wide solar spectrum, a photo-response onset at −0.3 V versus RHE under simulated solar illumination, and a plateau photocurrent density of 0.21 mA cm −2 at 0.4 V versus RH, which is 50-fold higher than that of particle-based CaTaO 2 N/GaN/Al 2 O 3 , due to its efficient charge carrier separation and the reduced diffusion distance for minority carriers. [200]Such strategies must be further optimized for the fabrication of semi-transparent BaTaO 2 N photoanodes that can be used for the design of PEC tandem systems.

Other Compounds in the Ba-Ta-O-N System
Various members of the quaternary Ba-Ta-O-N system were also synthesized, and their solar water-splitting performance was evaluated.For instance, Ba 2 TaO 3 N adopting the K 2 NiF 4 -type 3 wt% Pt-BaTaO 2 N and 0.5 wt% Pt-WO 3 under visible light.Reproduced with permission. [107]Copyright 2008, Elsevier.b) Cutoff wavelength dependence performance (blue circles) and UV-vis diffuse reflectance spectrum (black line) of the Ta 3 N 5 /BaTaO 2 N. Reproduced with permission. [193]Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.c) Reaction time courses of H 2 and O 2 evolution over a mixture of Pt-BaTaO 2 N and Pt-WO 3 under visible light irradiation.Reproduced with permission. [194]Copyright 2009, American Chemical Society.d) Schematic representation of charge separation and transfer and water oxidation of Ta 3 N 5 -NRs/BaTaO 2 N/FeNiO x , e) photocurrent density versus voltage curves of Ta 3 N 5 -NRs/FeNiO x and Ta 3 N 5 -NRs/BaTaO 2 N/FeNiO x photoanodes, f) conceptual model and corresponding energy band alignment of Ta 3 N 5 -NR and core−shell Ta 3 N 5 -NR/BaTaO 2 N photoanodes, g) solar light absorption along with the integrated current density, and h) IPCE spectra of Ta 3 N 5 -NRs/FeNiO x and Ta 3 N 5 -NRs/BaTaO 2 N/FeNiO x photoanodes.Reproduced with permission. [195]Copyright 2020, American Chemical Society.
structure with space group I4/mmm was synthesized by Clarke et al. [112] using TaON and BaO at 1500 °C under 1 atm N 2 .It was found that O/N ordering dictated by the competition between the cations of different electronegativity for the anions was favored in the K 2 NiF 4 -type structure, whereas O/N disorder was observed for perovskite structures.The (111)-layered Bsite deficient hexagonal perovskite Ba 5 Ta 4 O 15 was also doped with nitrogen. [201,202]The resulting Ba 5 Ta 4 O 15−x N x compounds showed a significantly enhanced visible light absorption due to the upward shift of the valence band maximum by N 2p states without affecting the conduction band minimum (Figure 21d-j).The unique layered structure provides intergallery spacings between the perovskite layers for the nitrogen dopant to diffuse easily, resulting in the uniform distribution of the nitrogen dopant.In the photocatalytic H 2 (495 μmol h −1 in the presence of ethanol [201] and 12 μmol h −1 in the presence of methanol [202] ) and O 2 (19.9 μmol h −1 [202] ) evolution reactions, the Ba 5 Ta 4 O 15−x N x compounds exhibited much higher photocatalytic activity with respect to pristine Ba 5 Ta 4 O 15 (Figure 21h-j).New Dion−Jacobson phase three-layer perovskite CsBa 2 Ta 3 O 10 crystals were synthesized by a conventional solid-state reaction, and the Ba 2 Ta 3 O 10-x N x nanosheets with lateral sizes ranging (2.32 cm 2 ) placed side by side.Reproduced with permission. [198]Copyright 2015, The Royal Society of Chemistry.b) I-E curves and c) I-t curves recorded at 0.7 V versus RHE for Co/BaTaO 2 N and Ir/Co/BaTaO 2 N photoanodes in 0.2 m K 2 HPO 4 aqueous solution (at pH 13 by KOH) under AM 1.5G simulated sunlight.Reproduced with permission. [199]Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA.d) Structural models, e) UV-Vis absorbance spectra, f) schematic representation of band structures, and g) H 2 production rates of Ba 5 Ta 4 O 15 and N-doped Ba 5 Ta 4 O 15 under simulated solar irradiation.Reproduced with permission. [201]Copyright 2011, American Chemical Society.h) UV-Vis diffuse reflectance spectra and digital photographs of Ba 5 Ta 4 O 15 (i) and Ba 5 Ta 4 O 15-x N x (ii) powders, i) reaction time course of H 2 evolution of 0.3 wt% Pt/Ba 5 Ta 4 O 15-x N x , and j) reaction time course of O 2 evolution of 1.0 wt% CoO x /Ba 5 Ta 4 O 15-x N x .Reproduced with permission. [202]Copyright 2013, The Royal Society of Chemistry.
from several hundred nanometers to a few micrometers and a thickness of about 2.3 nm were fabricated via the nitridationprotonation-intercalation-exfoliation processes of CsBa 2 Ta 3 O 10 (Figure 22a,b). [203]With increasing the ammonolysis time, the intensity of background absorption gradually became higher (Figure 22c).The two-dimensional Ba 2 Ta 3 O 10-x N x nanosheets can be applied to design novel systems for visible-light-driven water splitting.Ba 3 Ta 5 V O 14 N, which crystallizes isostructurally to Ba 3 Ta 4 V Ta IV O 15 , was synthesized by the ammonolysis of an amorphous ternary Ba-Ta-O phase under a mixture gas flow of NH 3 and O 2 at 850 °C for 24 h (Figure 22d,e). [204]The synthesized Ba 3 Ta 5 V O 14 N exhibited a light-yellow color, an optical bandgap energy of about 2.8 eV, and active surface sites for photocatalytic H 2 generation with (115 μmol h −1 in the presence of Rh nanoparticles) and without (≈100 μmol h −1 ) any cocatalyst (Figure 22f).
Using 2 m NaNO 2 solution as an optical filter showed no significant activity in H 2 generation because NaNO 2 completely blocked the light with energies >3.05 eV, resulting in a low absorption window.

Effect of Cocatalyst on Water Splitting Performance of BaTaO 2 N
[207] Pt nanoparticles have been mostly used as the HER-cocatalyst, whereas  [203] Copyright 2016, American Chemical Society.d) Unit cell of Ba 3 Ta 5 O 14 N and polyhedral representation of the crystal structure.Ta(O/N) 6 octahedra -green; Ba -black balls.e) Ta(O/N) 6 octahedra in Ba 3 Ta 5 O 14 N with the determined bond lengths (Å).f) Photocatalytic hydrogen evolution rates of Ba 3 Ta 5 O 14 N before and after reductive in situ photodeposition of 0.0125 wt% Rh.Reproduced with permission. [204]Copyright 2016, Elsevier.
CoO x , Co(OH) x , Co 3 O 4 , IrO 2 , NiO, FeO x , RhO x , etc. have enhanced the photocatalytic and photoelectrochemical OER rate.Okamoto et al. [208] successfully increased the photocatalytic O 2 evolution rate three-fold by annealing under an N 2 flow after loading the Co species on the surfaces of BaTaO 2 N crystals in comparison to that prepared by annealing under an NH 3 flow.The photocatalytic O 2 evolution rate was further improved by a factor of two, yielding an apparent quantum efficiency of 0.55% at 420 nm, by subsequent annealing under an H 2 atmosphere because the CoO x particles were localized on the surface of BaTaO 2 N (Figure 23a).An intimate contact formed between the CoO x cocatalyst and BaTaO 2 N enabled an efficient transfer of photo-excited electron-hole pairs.Stable and efficient photoelectrochemical water splitting was achieved using a BaTaO 2 N photoanode decorated with CoO microflowers, which not only effectively collected the holes from BaTaO 2 N but also protected BaTaO 2 N against photocorrosion. [209]The Faradaic efficiency of almost unity (99.2%) was recorded for OER, and the tips of the CoO microflowers were found to be the most active sites for OER (Figure 23b-f).Using a Co catalyst, a photoanode of particulate BaTaO 2 N fabricated by the particle transfer method using a Ta contact layer and a Ti conductor layer generated anodic photocurrent densities of 4.2 mA cm −2 at 1.2 V versus RHE under AM 1.5 G simulated sunlight and 25 mA cm −2 at 1.2 V versus RHE under visible light irradiation from 300 W Xe lamp (Figure 23gi). [67]The half-cell solar-to-hydrogen conversion efficiency (HC-STH) of the photoanode reached 0.7% at 1.0 V versus RHE, and the Faradaic efficiency for oxygen evolution was virtually 100% during the reaction for 6 h, indicating its robustness.A particulate BaTaO 2 N photoanode was modified with cobalt phosphate (CoPi)-loaded TiO 2 nanoparticles, resulting in an enhancement in the PEC-OER performance. [210]The TiO 2 nanoparticles functioned as transparent and conductive support with a high surface area to immobilize CoPi nanoparticles on the photoanode surface (Figure 24a).This led to improved reaction kinetics and increased electrochemically active surface area of the CoPi cocatalysts by a factor of 7.45.The addition of (Na)Rh/Cr 2 O 3 and IrO 2 as cocatalysts was essential to achieve a one-step excitation overall water splitting of BaTaO 2 N:Mg under visible light. [169]Without IrO 2 , the water oxidation reaction did not proceed on the bare BaTaO 2 N:Mg surface, and the net activity was reduced.In comparison to 1.5 wt% IrO 2 loaded on BaTaO 2 N in the previous study, [181] 0.3 wt% IrO 2 in this study was found to be suitable with 6 wt% Rh and 6 wt% Cr to drive a one-step excitation overall water splitting of BaTaO 2 N:Mg.Seo et al. [135] achieved a photocurrent density of 6.5 mA cm −2 at 1.23 V versus RHE for BaTaO 2 N, which was activated by annealing in Ar, loaded with Co(OH) x -FeO y cocatalyst during solar water splitting, corresponding to a maximum STH conversion efficiency of 1.4% at 0.88 V versus RHE (Figure 24b-e).The Na-Pt cocatalyst was involved along with Cr 2 O 3 , which was employed to suppress reverse reactions, over BaTaO 2 N:Zr0.01. [172]The Cr 2 O 3 /Na-Pt/BaTaO 2 N:Zr0.01 was almost inactive in aqueous solutions containing I − or Fe 2+ shuttle ions, while it exhibited a decent activity when [Fe(CN) 6 ] 4− Figure 23.a) Comparison of CoO x /BaTaO 2 N photocatalysts annealed at 773 K under N 2 flow and annealed at 1173 K under N 2 flow and subsequently under H 2 at 573 K. Reproduced with permission. [208]Copyright 2020, Elsevier.b) FESEM image of CoO@BaTaO 2 N, c) LSV of BaTaO 2 N and CoO@BaTaO 2 N photoanodes, d) wavelength dependence of IPCE of BaTaO 2 N and CoO@BaTaO 2 N photoanodes with UV-vis absorption spectrum of BaTaO 2 N, e) steady-state photocurrents of BaTaO 2 N and CoO@BaTaO 2 N photoanodes, and f) temporal gas evolution from the PEC cell with CoO@BaTaO 2 N as the photoanode and Pt foil as the cathode.Reproduced with permission. [209]Copyright 2021, The Royal Society of Chemistry.g) I−E curve of a Co/BaTaO 2 N/Ta/Ti photoelectrode under AM 1.5G simulated sunlight.A 0.2 m potassium phosphate aqueous solution adjusted to pH 13 by adding KOH was used as an electrolyte.h) Wavelength dependence of IPCE for Co/BaTaO 2 N/Ta/Ti electrodes and i) amounts of evolved hydrogen and oxygen of Co/BaTaO 2 N/Ta/Ti electrodes.A 0.2 m potassium phosphate aqueous solution (pH 13) and a CrO x -coated Pt mesh were used as the electrolyte and counter electrode, respectively.Reproduced with permission. [67]Copyright 2015, American Chemical Society.
was employed as the electron donor.The Z-scheme system based on Cr 2 O 3 /Na-Pt/BaTaO 2 N:Zr0.01 as the H 2 -evoling photocatalyst, CoO x /Au/BiVO 4 as the O 2 -evolving photocatalyst, and [Fe(CN) 6 ] 3−/4− as redox ions evolved H 2 and O 2 under visible light up to 520 nm, with the STH conversion efficiency of 0.022%.Compared with an individual loading, the combination of preloading of CoO x on the BaTaO 2 N particles and post-loading of RhO x was found to be highly efficient in improving both the photocurrent efficiency and stability under visible light irradiation and shifted the onset potential for water oxidation negatively (≈300 mV) (Figure 25a-c). [65]A stepwise loading of a Pt cocatalyst by impregnation-reduction and subsequent photodeposition remarkably enhanced the photocatalytic H 2 evolution activity of single-crystalline particulate BaTaO 2 N (Figure 25dh). [211]The photocatalytic H 2 evolution rate of BaTaO 2 N was dependent on the amount of deposited Pt nanoparticles, among which the deposition of 0.5 wt% Pt cocatalyst exhibited the optimal activity. [118]The sequential decoration method produced highly dispersed and uniformly sized Pt active sites firmly on BaTaO 2 N, enabling the rapid transfer of photo-excited electrons across the interface and active H 2 evolution reaction on the surface.As an outcome, Pt-loaded BaTaO 2 N exhibited an apparent  [210] Copyright 2021, AIP Publishing.b) SEM image of BaTaO 2 N powder, c) LSV data, d) chronoamperometry curve, e) reaction time courses of O 2 and H 2 generation, f) HC-STH energy conversion efficiency, and g) IPCE values of Co(OH) x −FeO y /BaTaO 2 N photoanode at different applied potentials as functions of wavelength, with the UV−Vis DRS spectrum of BaTaO 2 N powder.Reproduced with permission. [135]Copyright 2019, American Chemical Society.
quantum yield of 6.8 ± 0.5% at 420 nm for photocatalytic H 2 evolution from a sacrificial methanol aqueous solution, and an apparent quantum yield of 4.0% at 420 nm and an STH conversion efficiency of 0.24% in Z-scheme water splitting.Recently, onestep-excitation overall water splitting using pristine BaTaO 2 N, which was synthesized by the direct ammonolysis of a mixture of amorphous Ta 2 O 5 (Ta 2 O 5 •3H 2 O) nanoparticles and BaCO 3 , was achieved for the first time by modification with Rh (or Ru), Cr 2 O 3 , and IrO 2 as cocatalysts. [212]It was found that the photocatalytic activity of pristine BaTaO 2 N was influenced by the concentrations of the chosen cocatalysts, and the optimal amounts of Rh (or Ru), Cr 2 O 3 , and IrO 2 were determined to be 4 wt% (2 wt%), 1 wt%, and 0.3 wt%, respectively.The apparent quantum yield and STH conversion efficiency of Rh/Cr 2 O 3 /IrO 2 -loaded BaTaO 2 N reached 0.1% at 400 nm and 5 × 10 −4 %, respectively.Dual cocatalysts loaded by photodeposition can promote simultaneous oxidation and reduction reactions of photocatalysts.However, it is still challenging to photodeposit oxygen evolution cocatalysts because of weak driving forces for oxidation reaction.Recently, Kobayashi et al. [213] successfully loaded the FeO x cocatalyst onto a Mg-doped BaTaO 2 N photocatalyst by an oxidative photodeposition technique, and BaTaO 2 N coloaded with Pt and FeO x exhibited an apparent quantum yield of 1.2% at 420 nm during the oxygen evolution reaction.It is noteworthy that this photodeposition does not require a further heat treatment, which is advantageous for the design of novel cocatalyst-loaded photocatalysts that are prone to thermal decomposition at elevated temperature.

Photocatalytic and Photoelectrochemical Performance of BaTaO 2 N
The photocatalytic and photoelectrochemical performance of BaTaO 2 N for the water-splitting reaction is presented in the previous sections with respect to its synthesis methods, film fabri-cation, doping, solid solution, heterostructures, heterojunctions, and cocatalysts.This analysis also includes the effect of morphology, particle size, porosity, and surface chemistry.As a final objective, most of the studies seek to efficiently generate H 2 from water splitting using BaTaO 2 N under simulated AM 1.5G solar illumination.It is emphasized that all these strategies are based on the requirement that the valence band potential of semiconductor must be more positive than that required for the oxidation of water, while the conduction band potential must be more negative than that required for the reduction of water.The generated H 2 flux, apparent quantum yield (AQY), and rate constants are often used to compare the photocatalytic performance.Quantitative approaches toward the evaluation of the photocatalytic water splitting were previously recommended by Takanabe [93] and Kisch and Bahnemann. [214]n the realm of BaTaO 2 N-based photocatalysis, the photocatalytic performance has been reported to be closely dependent on the synthesis method, indicating a number of general aspects that allow exploiting the water decomposition reaction using BaTaO 2 N-based materials.Some of these general aspects include synthesis methods, [124,171] anion doping, [160] and cation doping [65,133,163] to improve overall photocatalytic performance, combination with other materials for solid-state Z-scheme photocatalytic water splitting, [118,172,187,192,194,211] and the use of co-catalysts to promote OER (Pt, [107,131,132,141,164,183,184,194] Rh, [169,172] CrO 3 [169,172,211] ) and HER (PtO x , [118,[192][193][194] CoO x , [117,129,137,165,168,177,185,207] and IrO x [115,181,211] ).Table 2 summarizes the photocatalytic performance of BaTaO 2 N-based photocatalysts.
The photoelectrochemical studies provide important information to the understanding of the performance of the photocatalysts in the water-splitting reaction.Since BaTaO 2 N is an n-type semiconductor, thin films of BaTaO 2 N can be fabricated on conductive substrates and used as photoanodes for     Reproduced with permission. [65]Copyright 2013, American Chemical Society.d) Schematic representation of sequential Pt cocatalyst deposition on BaTaO 2 N, e) photocatalytic H 2 evolution and f) apparent quantum yield of Pt-modified BaTaO 2 N (RbCl), and (g,h) reaction time courses of H 2 and O 2 evolution during Z-scheme water-splitting reaction using Pt(0.1% IMP + 0.2% PD)/BaTaO 2 N. Reproduced with permission. [211]Copyright 2021, Springer Nature AG & Co. KGaA.
The photoelectrochemical studies have enabled the resolution of trends in visible-light-assisted water oxidation based on BaTaO 2 N photoanodes, revealing an inverse relationship between recombination and charge transfer phenomena. [124]124] The deactivation process under open-circuit conditions highlights differences in the electronic properties of the modified BaTaO 2 N photocatalyst, leading to longer lifetimes of charge carriers for photo-electrodes.By utilizing co-catalysts and dopants, the photocurrent collection increases even at low    overpotentials.Studies on current transients under cycles of light and dark revealed interesting behavior in BaTaO 2 N-based materials, particularly regarding the balance between electron transfer and recombination.Generally, the current signal is higher during light exposure than in the dark, showing a transient behavior characterized by a current spike at the beginning of illumination.When the light is turned off, the photocurrent drops, exhibiting a negative overshoot.All these features are commonly observed at the semiconductor-electrolyte interfaces, where recombination plays a significant role. [215]Similar phenomena have been reported for various photoanodes, including TiN-modified Imma-LaTiO 2 N [216] and BaTaO 2 N/carbonaceous materials. [217]As a consequence of the photoelectrochemical behavior described, the carrier deactivation processes (recombination) limit the PEC response and can be improved by applying state-of-the-art approaches.

Conclusions and Perspectives
In this review, the basic principles of photoelectrochemical water splitting and the merits and demerits of oxide-based photocatalysts researched for photoelectrochemical oxygen evolution reactions were introduced.As one of the most intensively explored representatives of the transition metal (oxy)nitrides family, perovskite BaTaO 2 N has emerged as a promising photocatalyst for solar water splitting due to its capability to absorb visible light up to 660 nm, suitable band-edge potentials for overall water splitting in the absence of an external bias, and theoretical solar-to-hydrogen energy conversion efficiency of ≈24% under AM 1.5G simulated sunlight.Over the past three decades, the crystal structure, anion ordering, electronic structure, dielectric, ferroelectric, and piezoelectric properties of BaTaO 2 N were investigated.The apparent quantum yield, photocurrent density, incident photon-to-current conversion efficiency, and solar-tohydrogen conversion efficiency of BaTaO 2 N have been progres-sively improved.However, its solar-to-hydrogen conversion efficiency lags behind the benchmark efficiency required for practical application.This review presented various strategies applied for achieving enhanced solar water-splitting efficiency.Namely, high-and low-temperature synthesis techniques, advanced synthesis approaches, film fabrication, surface and bulk defect density controlling, crystal facet engineering, and particle morphology, size, and porosity tailoring were overviewed.Also, the impacts of cation doping (mono-, di-, tri-, tetra-, penta-, and hexavalent cations), creating the solid solutions, forming the heterostructures and heterojunctions, designing the photoelectrochemical cells, and loading suitable cocatalysts were discussed.Other members of the Ba-Ta-O-N system were highlighted for solar water splitting.Despite significant progress made in the past decades (Figure 26), several challenges still remain in the realm of BaTaO 2 N for solar water splitting.These challenges encompass the precise control of various factors, such as homogeneity (e.g., oxygen-nitrogen ratio), crystallinity, particle morphology/size, and defect density.Furthermore, there is a pressing need to identify a suitable deposition technique for BaTaO 2 N particles, which can preserve their physicochemical and optoelectronic properties.The ability to maintain a long-term water-splitting efficiency relies on a comprehensive understanding of surface properties and stability during photocatalytic and photoelectrochemical reactions.Scaling up the photoelectrochemical cells and photocatalytic reactors beyond the laboratory scale is yet another facet of the challenge.
Similar to other transition metal (oxy)nitrides, perovskite BaTaO 2 N is known to exhibit defects that have a substantial impact on its properties and water-splitting performance.Hence, it is imperative to gain a deep understanding of the defect chemistry, including anion and cation vacancies, within BaTaO 2 N.This is crucial for the development of high-efficiency BaTaO 2 N for future solar water-splitting applications.
Addressing these challenges with a rigorous approach involves understanding the intricate structure-property relationships and unraveling the nature of the photocatalyst-cocatalyst interface at the atomic level.Furthermore, the continued exploration of the underlying kinetics and mechanisms, using operando/in-situ characterization along with theoretical studies, may pave the way for further development of BaTaO 2 N as a promising material for solar water splitting.Also, in-depth photoelectrochemical studies can provide valuable information on the interplay between electron transfer dynamics and recombination phenomena.Machine learning and advanced synthesis techniques have enormous potential for the discovery of new photocatalytic materials within the Ba-Ta-O-N system.The optimization of photocatalystelectrocatalyst systems is also of utmost importance.Ongoing scientific efforts to integrate transition metal borides, carbides, nitrides, and chalcogenides in water oxidation as electrocatalysts/cocatalysts are very promising.
To achieve high solar-to-hydrogen (STH) conversion efficiency, the challenges related to the fabrication of semi-transparent photoanodes need to be addressed for the development of tandem systems.To overcome these limitations and scale up BaTaO 2 Nbased photoelectrochemical systems, it is crucial to directly observe Fermi level pinning or unpinning, mitigate photocorrosion and recombination processes, and minimize energy losses during PEC reactor design, construction, and operation.

Figure 4 .
Figure 4. a) Crystal Orbital Hamilton Populations (COHP) for the Ta-O interactions in three different polymorphs of BaTaO 2 N. Reproduced with permission. [78]Copyright 2008, WILEY-VCH Verlag GmbH & Co. KGaA.b) Local cis and trans ordering of TaO 4 N 2 octahedron in BaTaO 2 N. c) Schematic representation of the band structure of BaTaO 2 N and d) DFT-calculated band structure of BaTaO 2 N. Reproduced with permission.[62]Copyright 2007, American Chemical Society.

Figure 5 .
Figure5.a) Temperature-dependent dielectric permittivity and dielectric loss at 1 MHz and 1 kHz of BaTaO 2 N and SrTaO 2 N. Reproduced with permission.[68]Copyright 2004 American Chemical Society.b) Displacements of atoms in BaTaO 2 N with opposite polarization.Reproduced with permission.[96]Copyright 2012, American Chemical Society.c) high-resolution TEM image of BaTaO 2 N/SrRuO 3 interface and temperature-dependent dielectric permittivity and dielectric loss of a 600-nm-thick BaTaO 2 N film.Reproduced with permission.[97]Copyright 2007, American Chemical Society.d) SEM image of a BaTaO 2 N compact and relative permittivity and dielectric loss of BaTaO 2 N 0.85 ceramics sintered at 1400 °C for 3 h.Reproduced with permission.[99]Copyright 2016, Elsevier.e) Digital photographs, SEM image, and dielectric properties of BaTaO 2 N ceramics sintered by spark plasma method.Reproduced with permission.[100]Copyright 2016, Elsevier.f) STEM and PFM images of BaTaO 2 N ceramics.Reproduced with permission.[102]Copyright 2018, Elsevier.g) Polar nanoregions presenting the average Pm 3m cubic crystal lattice in which most of the O and N atoms are randomly distributed at 3c sites (i), polar nanoregions growing along the applied electric field (ii), and polarization saturated at applied electrical bias higher than ±60 V (iii).Reproduced with permission.[103]Copyright 2019, American Chemical Society.
3 and dissolution of BaO and Ta 2 O 5 in the KCl flux, ii) reactant diffusion through the molten KCl flux, iii) nucleation and growth of platelike BaTa 2 O 6 and Ba 5 Ta 4 O 15 crystals, iv) the dissolution of the BaTa 2 O 6 and Ba 5 Ta 4 O 15 crystals, and v) the crystallization and growth of cube-like BaTaO 2 N crystals under an NH 3 flow, as expressed by the following reactions:

Figure 8 .
Figure 8. a) TEM image of BaTaO 2 N grown using K 2 CO 3 −KCl binary flux with a molar ratio of 20/80 at 950 °C for 8 h, crystal structures of Ba 5 Ta 4 O 15 viewed from the [00 1] direction (top) and BaTaO 2 N viewed from the[111] direction (bottom), and schematic illustration of the formation mechanism of platy BaTaO 2 N crystals.Reproduced with permission.[134]Copyright 2020, American Chemical Society.b) Synthesis procedure and top-view SEM image of BaTaO 2 N film, current−potential curves of bare and CoPi-covered BaTaO 2 N electrodes, IPCE of CoPi/BaTaO 2 N measured at 1.23 V versus RHE, and H 2 and O 2 evolution from a CoPi/BaTaO 2 N photoanode at 1.23 V versus RHE in a 0.5 M potassium phosphate solution (pH 13) under AM 1.5G simulated sunlight.Reproduced with permission.[66]Copyright 2016, American Chemical Society.c) SEM images of BaTaO 2 N nanoparticle films with different Ba:Ta atomic ratios on Nb substrates, LSV curves of BaTaO 2 N photoanodes with NiCoFe-Bi co-catalyst in 1 M KOH (pH 13.6) under simulated AM 1.5G simulated sunlight, ABPE of the BaTaO 2 N photoanodes, and IPCE spectra of the BaTaO 2 N photoanodes at 1.23 V versus RHE and the corresponding integrated photocurrent over the standard AM 1.5G solar spectrum.Reproduced with permission.[148]Copyright 2022, Elsevier.

Figure 9 .
Figure 9. a) Schematic representation of thin-film fabrication, I-V curves of a BaTaO 2 N/Ta 2 N/Ta thin-film electrode, and H 2 and O 2 evolution from a CoO x -deposited BaTaO 2 N/Ta 2 N/Ta photoanode held at 1.0 V versus RHE in an aqueous 0.1 M potassium phosphate solution (pH 13) under AM 1.5G simulated sunlight.Reproduced with permission.[149]Copyright 2020, WILEY-VCH Verlag GmbH & Co. KGaA.b) Schematic representation of thin filmand particle-based oxynitride photoanodes and their photoelectrochemical performance.Reproduced with permission.[151]Copyright 2019, American Chemical Society.

Figure 10 .
Figure 10.a) SEM images of BaTaO 2 N crystal structures obtained by ammonolysis of the flux-grown Ba 5 Ta 4 O 15 crystals, grown using BaCl 2 and a 10 mol% sol.conc.(B10), CsCl and a 10 mol% sol.conc.(C10), KCl and a 1 mol% sol.conc.(K1), KCl and a 10 mol% sol.conc.(K10), KCl + BaCl 2 and a 10 mol% sol.conc.(KB10), K 2 SO 4 and a 50 mol% sol.conc.(KS50), and RbCl and a 10 mol% sol.conc.(R10), at 950 °C for 20 h with (bottom row) and without (top row) KCl flux, and the amount of photocatalytically evolved oxygen (first 3 h) and photocurrent density at 1.2 V versus RHE.Reproduced with permission.[117]Copyright 2019, The Royal Society of Chemistry.b) Schematic representation of the synthesis of BaTaO x N y powders and laser fragmentation process, potentiodynamic scans for the BaTaO x N y -zp photoanodes in NaOH electrolyte under chopped light illumination, and photocurrent density and APCE values at 1.23 V versus RHE as a function of the number of passages applied for fragmentation.Reproduced with permission.[156]Copyright 2020, Elsevier.

Figure 14 .
Figure 14.a) Digital photographs, UV-Vis absorption spectra, visible-light-driven photocatalytic oxygen evolution activity of BaCa x/3 Ta 1-x/3 O 2+y N 1-y powders, and action spectra (apparent quantum efficiency (AQE) versus excitation wavelength) of BaCa 0.10 Ta 0.90 O 2.27 N 0.73 (x = 0.3), 2 wt% CoO z was loaded as a cocatalyst and monochromic light was generated by filtering the output of lamp using bandpass filters.Reproduced with permission.[162]Copyright 2018, Elsevier.b) UV-Vis absorption spectra of BTON and BZTON-15 h, Ta4f XPS spectra of BTON and BZTON samples nitrided for different time, comparison of visible-light-driven photocatalytic hydrogen evolution rates, and photocatalytic hydrogen evolution activities of BTON and BZTON-15 h as a function of reaction time.Reproduced with permission.[171]Copyright 2021, Elsevier.

Figure 16 .
Figure 16.a) UV-Vis diffuse reflectance spectra of i) BaNbO 2 N, ii) BaNb 0.75 Ta 0.25 O 2 N, iii) BaNb 0.50 Ta 0.50 O 2 N, iv) BaNb 0.25 Ta 0.75 O 2 N, and v) BaTaO 2 N crystals.b) Reaction time courses of photocatalytic O 2 evolution, I-E curves, and wavelength dependence of IPCE of BaTaO 2 N crystals after flux growth (i), oxidation at 300 °C for 1 h and ammonolysis at 900 °C for 5 h (ii), and ammonolysis at 900 °C for 5 h (iii).Current density versus time of BaTaO 2 N crystals (iv).c) Local density of states (LDOS) of i) BaNbO 2 N, ii) BaNb 0.75 Ta 0.25 O 2 N, iii) BaNb 0.50 Ta 0.50 O 2 N, iv) BaNb 0.25 Ta 0.75 O 2 N, and v) BaTaO 2 N and partial electron density distribution around the valence band maximum and the conduction band minimum of BaNb 0.5 Ta 0.5 O 2 N. Reproduced with permission. [177]Copyright 2016, The Royal Society of Chemistry.d) Influence of the cation-doping on the oxidative photocurrent densities generated by the BaTaO 2 N/Ti and CoO y /BaTaO 2 N/Ti electrodes in an aqueous Na 2 SO 4 solution (pH 6) under visible light irradiation.e) Reaction time courses of H 2 and O 2 evolution in a two-electrode system composed of CoO y /BTON:Mo-5/Ti, CoO y /BTON:H 2 /Ti, or CoO y /BTON/Ti electrode and Pt-wire coated with Cr 2 O 3 in phosphate buffer solution (pH 8) under visible light irradiation.Reproduced with permission.[175]Copyright 2015, AIP Publishing LLC.

Figure 19 .
Figure 19.a) Reaction time courses of H 2 evolution of BaTaO 2 N and BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 , b) H 2 and O 2 evolution rates during Z-scheme overall water splitting reaction over BaTaO 2 N and BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 synthesized by varying ammonolysis times, and c) reaction time courses of H 2 and O 2 evolution during Z-scheme overall water splitting over BaTaO 2 N-BaNa 0.25 Ta 0.75 O 3 synthesized by ammonolysis for 10 h for the stability test.Reproduced with permission.[187]Copyright 2022, American Association for the Advancement of Science.d) TEM and HRTEM images of Ba(0.3)-Ta 3 N 5 sample, e) Mott-Schottky plots and schematic representation of band structures of Ta 3 N 5 and BaTaO 2 N, and f) multiple cycles of Z-scheme overall water splitting with 0.5 wt% Pt/Ba(0.3)-Ta 3 N 5 and 0.45 wt% PtO x /WO 3 .Reproduced with permission.[192]Copyright 2017, The Royal Society of Chemistry.

Figure 20 .
Figure20.a) Reaction time courses of H 2 and O 2 evolution over a mixture of 0.3 wt% Pt-BaTaO 2 N and 0.5 wt% Pt-WO 3 under visible light.Reproduced with permission.[107]Copyright 2008, Elsevier.b) Cutoff wavelength dependence performance (blue circles) and UV-vis diffuse reflectance spectrum (black line) of the Ta 3 N 5 /BaTaO 2 N. Reproduced with permission.[193]Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.c) Reaction time courses of H 2 and O 2 evolution over a mixture of Pt-BaTaO 2 N and Pt-WO 3 under visible light irradiation.Reproduced with permission.[194]Copyright 2009, American Chemical Society.d) Schematic representation of charge separation and transfer and water oxidation of Ta 3 N 5 -NRs/BaTaO 2 N/FeNiO x , e) photocurrent density versus voltage curves of Ta 3 N 5 -NRs/FeNiO x and Ta 3 N 5 -NRs/BaTaO 2 N/FeNiO x photoanodes, f) conceptual model and corresponding energy band alignment of Ta 3 N 5 -NR and core−shell Ta 3 N 5 -NR/BaTaO 2 N photoanodes, g) solar light absorption along with the integrated current density, and h) IPCE spectra of Ta 3 N 5 -NRs/FeNiO x and Ta 3 N 5 -NRs/BaTaO 2 N/FeNiO x photoanodes.Reproduced with permission.[195]Copyright 2020, American Chemical Society.

Figure 21 .
Figure 21.a) Unassisted PEC water splitting under visible light irradiation by a p/n PEC cell consisting of Pt/Al-LTC 0.9 A 0.1 (0.43 cm 2 ) and Co/BaTaO 2 N(2.32 cm 2 ) placed side by side.Reproduced with permission.[198]Copyright 2015, The Royal Society of Chemistry.b) I-E curves and c) I-t curves recorded at 0.7 V versus RHE for Co/BaTaO 2 N and Ir/Co/BaTaO 2 N photoanodes in 0.2 m K 2 HPO 4 aqueous solution (at pH 13 by KOH) under AM 1.5G simulated sunlight.Reproduced with permission.[199]Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA.d) Structural models, e) UV-Vis absorbance spectra, f) schematic representation of band structures, and g) H 2 production rates of Ba 5 Ta 4 O 15 and N-doped Ba 5 Ta 4 O 15 under simulated solar irradiation.Reproduced with permission.[201]Copyright 2011, American Chemical Society.h) UV-Vis diffuse reflectance spectra and digital photographs of Ba 5 Ta 4 O 15 (i) and Ba 5 Ta 4 O 15-x N x (ii) powders, i) reaction time course of H 2 evolution of 0.3 wt% Pt/Ba 5 Ta 4 O 15-x N x , and j) reaction time course of O 2 evolution of 1.0 wt% CoO x /Ba 5 Ta 4 O 15-x N x .Reproduced with permission.[202]Copyright 2013, The Royal Society of Chemistry.

Figure 24 .
Figure24.a) Schematic illustration of the assumed structures of CoPi/BaTaO 2 N and CoPi/TiO 2 /BaTaO 2 N. Reproduced with permission.[210]Copyright 2021, AIP Publishing.b) SEM image of BaTaO 2 N powder, c) LSV data, d) chronoamperometry curve, e) reaction time courses of O 2 and H 2 generation, f) HC-STH energy conversion efficiency, and g) IPCE values of Co(OH) x −FeO y /BaTaO 2 N photoanode at different applied potentials as functions of wavelength, with the UV−Vis DRS spectrum of BaTaO 2 N powder.Reproduced with permission.[135]Copyright 2019, American Chemical Society.

Table 1 .
Overview of some n-type oxide semiconductors for PEC OER.