Boosting Charge Mediation in Ferroelectric BaTiO3−x‐Based Photoanode for Efficient and Stable Photoelectrochemical Water Oxidation

Oxygen evolution reaction (OER) is a bottleneck to photoelectrochemical (PEC) water splitting; however, there remains an impressive challenge for intrinsic charge transport for the development of integrated photoanodes. Herein, covalent triazine frameworks as conjugated molecules are grafted on the surfaces of ferroelectric BaTiO3−x (CTF/BTO) nanorod array, and then oxyhydroxide oxygen evolution cocatalyst (OEC) is constructed as an integrated photoanode. The OEC/CTF/BTO array not only achieves a high photocurrent density of 0.83 mA cm−2 at 1.23 V versus reversible hydrogen electrode (vs RHE) and low onset potential of ≈0.23 VRHE, but also optimizes outstanding stability. To disclose the origin, the enhanced PEC activity can be contributed to the integration of CTF and OEC, enhancing light‐harvesting capability, boosting charge carrier mediation, and promoting water oxidation kinetics through electrochemical analysis and density functional theory calculations. This study not only provides an alternative to accelerate charge transfer, but also paves the rational design and fabrication of integrated photoanodes for boosting PEC water splitting performance.

20] Among them, there is a promising solution to regulate the charge transfer and separation by the construction of an internal electric field.Especially, a polarization-induced internal electric field could be conducted by a ferroelectric material, such as PbTiO 3 , BaTiO 3 , BiMnO 3 , and Bi 2 FeCrO 6 , [21][22][23][24] promoting the migration of the photogenerated charges and the suppression of charge recombination by the electric polarization.[27][28][29][30] Nevertheless, poor charge transport and the intrinsic light absorption of BaTiO 3 limit the practical applications in PEC water splitting.Thus, it is desirable to explore efficient and stable BaTiO 3 photoanode.
[33][34] For example, thin films of depleted uranium oxide (U 3 O 8 ) in conjunction with hematite (α-Fe 2 O 3 ) overlayers realize a high-efficiency PEC water splitting process. [35]Sulfide heterojunction has been designed as a photoanode with an ultrahigh charge separation efficiency and a solar-to-hydrogen conversion efficiency. [36]Compared with inorganic semiconductors, different molecule materials, such as porphyrin and polyaniline, [37] nonconjugated polymer, [38] and covalent triazine-based polymers, [39] have been applied for the construction of heterostructure photoanode.Among them, covalent triazine frameworks (CTFs) have been considered as promising candidate for practical PEC application due to tunable electronic structures and donor-acceptor strategy, boosting the collection and transport of charge carrier. [40,41]Besides, the inherent chemical stability makes CTFs a qualified guardian against photocorrosion, striking a balance between efficiency and stability flawlessly. [42]nspired by the above analysis, it is significantly important to develop BaTiO 3 -based photoanode integrated with covalent triazine frameworks for boosting PEC performance.
Herein, covalent triazine frameworks as conjugated molecules are grafted on the surfaces of ferroelectric BaTiO 3Àx (CTF/BTO) nanorod array, followed by the integration of oxyhydroxide oxygen evolution cocatalyst (OEC) on the photoanode.The obtained OEC/CTF/BTO photoanode not only achieves a high photocurrent density of 0.83 mA cm À2 at 1.23 V versus reversible hydrogen electrode (vs RHE) and low onset potential of %0.23 V RHE , but also optimizes outstanding stability.To disclose the origin, the enhanced PEC activity can be contributed to the integration of CTF and OEC, enhancing light-harvesting capability, boosting charge carrier mediation, and promoting water oxidation kinetics through electrochemical analysis and density functional theory (DFT) calculations.

Synthesis and Characterization
To disclose the insights into the role of the heterojunction upon the charge transfer at atomic level, CTF/BaTiO 3Àx is chosen as an ideal material model for the analysis of electronic structures.To explore the charge redistribution at the CTF/BaTiO 3Àx interface and verify the rationality of the heterojunction, 3D charge density difference has been calculated by DFT calculations.Figure 1a,c reveal the electrons transfer from CTF to BaTiO 3Àx , which leads to the net charge accumulation and thus forms a built-in electric field across the heterostructure, confirming the significant contribution of CTF/BaTiO 3Àx heterojunction for a remarkable charge separation and transfer.After adsorbing water molecules, it can be observed that charge depletion occurs on the water surface and charge accumulation on the CTF surface, implying the electrons are mainly diverted from the water to the CTF (Figure 1b,d).Based on the abovementioned theoretical support, it is practicable to combine BaTiO 3Àx and CTF to construct an effective heterojunction for the enhanced photoelectrocatalytic process.Therefore, it is highly desirable to develop a convenient approach to produce the integrated CTF/BaTiO 3Àx photoanode.
For the construction of OEC/CTF/BaTiO 3Àx architectural arrays as an integrated configuration, a versatile strategy was designed as the optimization of PEC performance, which is schematically shown in Figure S1, Supporting Information.First of all, the pure BaTiO 3 nanorods array was grown on a FTO substrate by a two-step hydrothermal method, and the ferroelectricity was confirmed by P-E loop (Figure S2, Supporting Information). [43,44]After annealing under a hydrogen atmosphere, the BaTiO 3Àx was obtained.CTFs were designed by a typical process (Figure S3, Supporting Information). [42]The monomer was composed by triazine skeleton and three thiophene groups, which can be confirmed by 1 H NMR spectra, 13 C NMR spectra, and mass spectra (Figure S4, Supporting Information).Then, ultrathin CTF layers were fabricated via a simple electrodeposition method, leading to the formation of CTF/BaTiO 3Àx array.13a,e] The morphology and structure of BaTiO 3Àx -based photoanodes were explored by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).As demonstrated in Figure S5a, Supporting Information, in addition to the diffraction peaks indexed to SnO 2 (JCPDS PDF no.77-0452), five characteristic diffraction peaks of TiO 2 match well with rutile TiO 2 (JCPDS PDF no.02-0494).After fabricating BaTiO 3 by in situ transformation of TiO 2 arrays, XRD pattern presents seven characteristic peaks at 22.2°, 31.6°,38.9°, 45.4°, 51.0°, 56.3°, and 65.7°, which were attributed to (100), ( 110), ( 111), ( 200), ( 201), (211), and (220) planes of the tetragonal BaTiO 3 crystal (JCPDS PDF no.05-0626).Furthermore, no additional peaks are observed in Figure S5b, Supporting Information, for all BaTiO 3Àx -based photoanodes, confirming the amorphous state of CTF films and OEC films after electropolymerization.As exhibited in Figure 2a-c and S6-S7, Supporting Information, BaTiO 3Àx maintains the rod-like structure after the deposition of CTF and OEC layer.The energy-dispersive spectra (EDS) for OEC/CTF/BaTiO 3Àx (Figure 2d) show the homogeneous distribution of all elements (Ba, Ti, O, C, N, S, Ni, and Fe).It was proposed from TEM (Figure 2e) that the coated amorphism of CTF and OEC was anchored over the BaTiO 3Àx hosts, consistent with the results of XRD patterns and SEM images.Moreover, the TEM of depositing OEC on BaTiO 3Àx alone and CTF on BaTiO 3Àx alone (Figure S8, Supporting Information) are given to further confirm the amorphous morphology of CTF and OEC after the deposition.A typical TEM image, as presented in Figure 2f, indicates that the lattice fringe of 0.233 nm corresponds to the (111) plane of BaTiO 3 .Meanwhile, selected area electron diffraction (SAED) pattern displayed in Figure 2g further illustrates the good crystallinity of the BaTiO 3 .This observation concurs with the results from Figure 2f.The elemental mapping images (Figure S9 and S10, Supporting Information) intuitively demonstrated the homogeneous distribution of Ba, Ti, O, C, N, and S elements in CTF/BaTiO 3Àx arrays.As for CTF, optical picture is displayed in Figure S11, Supporting Information.Besides, SEM and TEM images showed the morphology and element composition of CTF monomer (Figure S12-S14, Supporting Information).These characterizations give an eloquent proof of the CTF and OEC thin films on the surface of BaTiO 3Àx nanorods array.
To ascertain the elemental states and chemical composition, X-ray photoelectron spectra (XPS) were measured.With regard to Ba 3d and Ti 2p XPS spectra (Figure 3a,b and S15, Supporting Information), the positive shift in binding energies (BEs) comparing OEC/CTF/BaTiO 3Àx and BaTiO 3 demonstrated the intimate interaction of OEC/CTF/BaTiO 3Àx .15b-e] The peaks located at 284.8, 286.0, and 288.6 eV for C 1s XPS spectra (Figure 3d) corresponded to C═C&C─C, C─S, and C═N in the CTF. [45,46]With regard to N 1s XPS spectra (Figure 3e), the peak at 398.5 eV can be indexed to the existence of C─N═C in the CTF. [47]Likewise, the peaks at 164.1 (S 2p 3/2 ) and 164.9 (S 2p 1/2 ) eV can be fitted to C─S─C in the CTF. [48]In addition, Ni 2p and Fe 2p XPS spectra in Figure S17, Supporting Information, demonstrated the presence of OEC in OEC/CTF/BaTiO 3Àx . [49,50]

PEC Performance
The PEC measurements of ferroelectric BaTiO 3Àx -based photoanodes were performed in 1 M KOH (pH = 13.6)under AM1.5 G sun illumination (100 mW cm À2 ) in a standard three-electrode system, and the scanning speed of all LSV curves is 0.05 V s À1 .In the linear sweep voltammetry (LSV) curves in Figure 4a,b, OEC/CTF/BaTiO 3Àx photoanodes display the highest photocurrent density of 0.83 mA cm À2 at 1.23 V (vs RHE), 5.93 times and 3.61 times improved compared to that of the pristine BaTiO 3 (0.14 mA cm À2 ) and BaTiO 3Àx (0.23 mA cm À2 ), higher than those of the reported photoanodes (Table S1, Supporting Information).At the same time, the OEC/CTF/BaTiO 3Àx photoanodes also show a 0.1 V negative shift in onset potential, illustrating the cooperative effect of CTF and OEC layers on the porous BaTiO 3Àx photoanode.J-V curves of BaTiO 3Àx photoanodes with the tunable hydrogen reduction temperature are shown in Figure S18, Supporting Information.To explore the precise utilization consumption of CTF, CTF/BaTiO 3Àx photoanodes with different electropolymerization time of CTF presented the obvious change upon the photocurrent in Figure S19, Supporting Information, affirming the best electropolymerization time for the deposition of CTF layer.Moreover, it can be seen that the PEC performance of NiFeOOH/CTF/BaTiO 3Àx photoanodes is higher than those of NiOOH/CTF/BaTiO 3Àx and FeOOH/CTF/ BaTiO 3Àx photoanodes (Figure S20, Supporting Information).
To calculate the effect of monochromatic light upon the photocurrent density, the incident photon-to-current conversion efficiencies (IPCE) at 1.23 V (vs RHE) of all photoanodes are displayed in Figure 4c.The OEC/CTF/BaTiO 3Àx photoanode reaches a maximum IPCE value of 57% at 372 nm, whereas the bare BaTiO 3Àx only exhibits a low value of 18%.In Figure 4d, an applied bias photo-to-current efficiency (ABPE) of the integrated OEC/CTF/BaTiO 3Àx photoanodes achieves 0.42% at 0.57 V (vs RHE), which is markedly higher than that of pristine BaTiO 3Àx (0.12%), showing the efficient PEC conversion.To expound the origin of the excellent PEC performance, a series of tests on the electrochemical kinetics and charge transfer behavior were conducted.By use of UV-vis diffuse reflectance spectra (Figure S21, Supporting Information), the optical behavior of the electrodes was carried out, displaying the enhanced light absorption after the optimization coupling of CTF and BaTiO 3Àx .To explore the interfacial charge transfer, electrochemical impedance spectroscopy (EIS) measurements of BaTiO 3 , BaTiO 3Àx , CTF/BaTiO 3Àx , and OEC/CTF/BaTiO 3Àx photoanodes were performed at 0.4 V (vs RHE) in Figure 5a.The diameter of the arc for OEC/CTF/BaTiO 3Àx photoanode is smaller than those records for BaTiO 3 , BaTiO 3Àx , and CTF/BaTiO 3Àx photoanodes, demonstrating the smallest resistance and the fastest oxygen evolution kinetic of OEC/CTF/BaTiO 3Àx photoanode.Mott-Schottky (MS) curves (Figure 5b) present a positive slope for n-type semiconductor.Meanwhile, the charge carrier density (N D ) can be calculated based on MS plots, such as BaTiO 3 (5.4Â 10 18 ) < BaTiO 3Àx (7.5 Â 10 18 ) < CTF/BaTiO 3Àx (1.5 Â 10 19 ) < OEC/CTF/BaTiO 3Àx (1.8 Â 10 19 ), suggesting the synergistic effect of the CTF and OEC to enhance the charge carrier generation and refrain the electron-hole recombination.To gain in-depth insights of the carrier dynamics in the bulk, steady-state photoluminescence (PL) spectra and time-resolved PL (TRPL) decay spectra of all photoanodes were used.Notably, the OEC/CTF/BaTiO 3Àx exhibits the lowest intensity of PL (Figure 5c), indicating a relatively slow recombination rate of electron-hole pairs. [51]As shown in Figure 5d and Table S2, Supporting Information, the OEC/ CTF/BaTiO 3Àx photoanodes possess the longest carrier lifetime (23.02 ns) compared with BaTiO 3 (5.74ns), BaTiO 3Àx (8.91 ns), and CTF/BaTiO 3Àx (13.53 ns) from TRPL spectra.The above steady/transient spectra analysis manifests the preferable capability of OEC/CTF/BaTiO 3Àx photoanodes, indicating the suppressed electron-hole recombination and the enhanced carrier transfer efficiency in PEC system. [36,52]he stability of various photoanodes is a critical challenge for practical solar water splitting.As expected, OEC/CTF/BaTiO 3Àx photoanodes display excellent durability after 18 h continuous illumination (Figure 6a).In sharp contrast, the photocurrent density of bare BaTiO 3Àx photoanodes displays a rapid decay within several hours.Moreover, XPS and SEM tests were employed to characterize BaTiO 3Àx -based photoanodes after the PEC reaction (Figure S17 and S22, Supporting Information).It turns out that there is a little change upon the chemical structures and surface morphologies of OEC/CTF/BaTiO 3Àx , indicating that the as-prepared CTF layers can also be served as a significant protection media to suppress the anodic photocorrosion. [42]The yields of evolved hydrogen and oxygen were quantified by gas chromatography.From Figure 6b, the total amounts of hydrogen and oxygen are 30.85 and 15.38 μmol over 2 h with a high Faradaic efficiency of 95.2%, presenting the adequate utilization of charge carriers to generate the gases.To comprehensively expound the origin of the enhanced PEC performance of the integrated OEC/CTF/BaTiO 3Àx photoanodes, the schematic diagrams of the charge transfer process for BaTiO 3Àx and OEC/CTF/BaTiO 3Àx photoanodes are displayed according to the valence band (VB) and conduction band (CB) position of individual part from the experimental evidence (Figure 5b and S23-S24, Supporting Information) and the relative reports. [42,43]n detail, Figure S23 and S24a,b, Supporting Information, show the bandgap (E g ) of BaTiO 3Àx , OEC/CTF/BaTiO 3Àx , and CTF is 2.92, 2.79, and 2.34 eV, respectively.The CB of BaTiO 3Àx and OEC/CTF/BaTiO 3Àx which are obtained by the MS curves (Figure 5b) is 0.05 V and À0.03 V (vs RHE), so the VB of BaTiO 3Àx and OEC/CTF/BaTiO 3Àx is 2.97 and 2.76 V (vs RHE), respectively.As for CTF, ultraviolet photoelectron spectroscopy (UPS) reveals the VB position of CTF (Figure S24c,d, Supporting Information), and the VB was calculated to be 1.97 V (vs RHE).The CB was further derived to be À0.37 V (vs RHE).Under light irradiation and applied bias, the photogenerated holes of the pristine BaTiO 3Àx are captured by the surface state and recombined with the electrons, resulting in the limited PEC performance (Figure 6c).Nevertheless, the coordination of CTF and OEC films on BaTiO 3Àx photoanodes effectively improves charge transfer and separation and thus promotes the PEC performance (Figure 6d).
To disclose the origin of the enhanced PEC activity, DFT calculations were performed to investigate the mechanisms of superior OER for CTF/BaTiO 3Àx photoanodes.The models of BaTiO 3Àx and CTF/BaTiO 3Àx photoanodes were constructed   (Figure 7a,b and S25-S26, Supporting Information).There are three intermediates in four-step OER reactions to present Gibbs free energy profiles, such as *OH, *O, and *OOH (*stands for active centers of the catalyst). [53]As shown in Figure 7c, the conversion to *OOH for pristine BaTiO 3Àx with a big energy barrier (η) of 1.03 eV is the potential rate-determining step (RDS), confirming the weak binding between *OOH and Ti atom.Compared to pristine BaTiO 3Àx , it is noted that CTFs deposited on the surface of BaTiO 3Àx have an evident impact on the decrement of the energy barrier (Figure 7d), achieving the RDS of 0.84 eV from *O to *OOH at the equilibrium potential (U = 1.23 V).The smallest free energy barrier of CTF/BaTiO 3Àx demonstrates that the introduction of CTF is indispensable for accelerating electrochemical kinetics.

Conclusion
In summary, we have developed a universal coupling strategy to fabricate the CTF and OEC films on the surface of ferroelectric BaTiO 3Àx nanorods array for the synthesis of OEC/CTF/BTO photoanode.As a representative catalyst, the OEC/CTF/BTO array not only achieves a high photocurrent density of 0.83 mA cm À2 at 1.23 V (vs.RHE) and a low onset potential of %0.23 V (vs RHE), but also optimizes outstanding stability.Combining UV-vis absorption, electrochemical analysis, and DFT calculations, the superior visible PEC performance can be ascribed to the increased donor density and suppressed charge recombination.Furthermore, the CTF configuration on photoanodes can also be confirmed as a protection layer, promoting the stability to participate in real water redox reaction.This work not only provides the significant role of CTF in BaTiO 3Àxbased photoanodes, but also offers a general strategy toward the fabrication of organic/inorganic hybrid photoanodes toward high photoconversion efficiency for feasible PEC water splitting applications.

Figure 1 .
Figure 1.a) The charge density difference across the CTF/BaTiO 3Àx heterojunction.b) The charge density difference across the interface between the CTF/BaTiO 3Àx heterojunction adsorbed with water molecules.c) Planar averaged charge density difference Δρ (Δρ ¼ ρ junction -ρ BaTiO 3Àx -ρ CTF ) along the z-direction.d) Planar averaged charge density difference Δρ (Δρ ¼ ρ heterojunction -ρ BaTiO 3 Àx -ρ CTF ) along the z-direction adsorbed with water molecules.The yellow and cyan areas indicate electron accumulation and depletion, respectively.The red, cyan, green, blue, white, yellow, and gray atoms denote O, Ti, Ba, N, H, S, and C elements, respectively.

Figure 5 .
Figure 5. a) Electrochemical impedance spectroscopy curves.b) MS plots in the dark.c) Steady-state PL spectra.d) TRPL decay curves.

Figure 6 .
Figure 6.a) Photocurrent density stabilities of BaTiO 3Àx and OEC/CTF/BaTiO 3Àx photoanodes measured at 1.23 V (vs RHE) under AM1.5 G sunlight (100 mW cm À2 ).b) Plots of the theoretical charge number obtained from the J-t curves collected at 1.23 V (vs RHE) and the actual quantities of H 2 and O 2 evolution of the OEC/CTF/BaTiO 3Àx photoanode.c,d) Illustrations of the charge transfer process for BaTiO 3Àx and OEC/CTF/BaTiO 3Àx photoanodes.