Nb2O5‐Based Photocatalysts

Abstract Photocatalysis is one potential solution to the energy and environmental crisis and greatly relies on the development of the catalysts. Niobium pentoxide (Nb2O5), a typically nontoxic metal oxide, is eco‐friendly and exhibits strong oxidation ability, and has attracted considerable attention from researchers. Furthermore, unique Lewis acid sites (LASs) and Brønsted acid sites (BASs) are observed on Nb2O5 prepared by different methods. Herein, the recent advances in the synthesis and application of Nb2O5‐based photocatalysts, including the pure Nb2O5, doped Nb2O5, metal species supported on Nb2O5, and other composited Nb2O5 catalysts, are summarized. An overview is provided for the role of size and crystalline phase, unsaturated Nb sites and oxygen vacancies, LASs and BASs, dopants and surface metal species, and heterojunction structure on the Nb2O5‐based catalysts in photocatalysis. Finally, the challenges are also presented, which are possibly overcome by integrating the synthetic methodology, developing novel photoelectric characterization techniques, and a profound understanding of the local structure of Nb2O5.


Introduction
Abundant fossil resources are utilized to fulfill the growing energy and chemical requirements. [1] However, carbon dioxide generated in these processes is inevitably released into the environment, accompanying global warming, ocean acidification, and a Hence, we provide an overview of the recent advances in different Nb 2 O 5 -based photocatalysts, including the synthesis, application, and relationship between the photoelectronic properties, surface structures, and activities. First, the physicochemical properties of Nb 2 O 5 are introduced. Then, reported Nb 2 O 5 -based photocatalysts are classified into two categories: i) pure Nb 2 O 5 catalysts with diverse morphology, and ii) Nb 2 O 5 catalysts with other species, such as metal species and other components. In the synthetic sections, we summarize the method and the role of treatment and additives in the control of morphology and structure. After that, we discuss the optical and catalytic properties of Nb 2 O 5 -based photocatalysts (Figure 2). Except for the generation, migration, and recombination of charge carriers, acidic properties and Nb-O-metal interface are also taken into account, which can affect the interaction between substrate molecules and catalyst, the product selectivity, and reaction rate. In the end, we give the summary and outlook of Nb 2 O 5 -based photocatalysts.

The Nature of Nb 2 O 5 2.1. Resource
The abundance of niobium on earth is 20 ppm. [8a] Niobium does not exist in a pure metallic form but is often found as mixtures with other metals in minerals, which are unevenly distributed around the globe. [10] Abundant columbic mines are mainly located in Brazil, Canada, and Nigeria. [8a] Nowadays, niobium compounds are widely utilized in superconducting, electronics, and catalytic industries, indicating that the recovery of niobium-containing solid waste is potential strategies to produce desired niobium oxide. [11] In the field of catalysis, niobium powders and Nb 2 O 5 obtained from the minerals are the raw material for the production of niobium chloride, niobium oxalate, ammonia niobium oxalate, niobium pentabutoxide, and other organic niobium salts, which can be utilized for further preparation of nanostructured Nb 2 O 5 due to the differences in acidity, alkalinity, and solubility. [4a,8a,12]

Physicochemical Properties
The Nb 2 O 5 is an n-type semiconductor. [13] The structure of Nb 2 O 5 depends on the preparation conditions. [14] Amorphous Nb 2 O 5 can be transformed into pseudohexagonal phase (TT-Nb 2 O 5 ), orthorhombic phase (T-Nb 2 O 5 ), and monoclinic phase (H-Nb 2 O 5 ) by increasing the temperature. [8a] Unique properties are observed in Nb 2 O 5 . Nb 2 O 5 has the bandgap energy (E g ) value of ≈3.0-3.4 eV, which is suitable for redox reaction in photocatalysis. [4a] As shown in Figure 3a, the excited electrons and holes on Nb 2 O 5 migrate to the surface under light irradiation and then interact with the substrate in the reduction process and oxidation process, respectively. Besides, LASs and BASs are observed on tetrahedral NbO 4 and octahedral NbO 6 units ( Figure 3b,c), respectively. [14a,b,15] The Nb 2 O 5 exhibits high acid strength and is utilized in the dehydration reaction, hydrolysis reaction, and hydrodeoxygenation reaction. [8a,16] Meanwhile, these structures can be distorted by decreasing the thickness of Nb 2 O 5 and reduction with the formation of Nb 2 O 5−x and NbO 2 . [15,17] In addition, H 2 O and hydroxyl groups on the Nb 2 O 5 surface can be removed by post-treatment, resulting in the improvement of absorption performance. [18] Except for the unique acidity and redox properties, Nb 2 O 5 is robust in organic acid solutions. This property implies that Nb 2 O 5 can be stable in acidic solutions derived from biomass and other acid products in photocatalysis. However, there are still some limitations of Nb 2 O 5 . An unavoidable trade-off is present between the restricted optical absorption and suitable photoredox ability of Nb 2 O 5 . In addition, the methods are still necessary for the large-scale production of specific morphology of Nb 2 O 5 -based photocatalysts. Thus, an overview is conducive to the systematic understanding and development of Nb 2 O 5 -based photocatalysts.
from layered niobates. [23] First, layered K 4 Nb 6 O 17 was synthesized from the solid reaction of Nb 2 O 5 and K 2 CO 3 under the calcination. Then, the scrolled H 4 Nb 6 O 17 was prepared via the exfoliation of K 4 Nb 6 O 17 with the assistance of acid and base. Finally, the Nb 2 O 5 nanotubes were obtained from H 4 Nb 6 O 17 by dehydration under 400-450°C. The heat treatment was a vital process for the transformation of nanosheets to nanotubes. [23] Besides, the Kirkendall effect was applied in the synthesis of H-Nb 2 O 5 nanotubes from nanorods by a two-step hydrothermal synthesis approach. [24] The TT-Nb 2 O 5 nanorod arrays grew on niobium foils in the first hydrothermal process. Due to the Kirkendall effect, the outside walls of TT-Nb 2 O 5 nanorods exhibited preferential nucleation and growth in the second hydrothermal process, which is accompanied by the migration of inside core composition and the formation of nanotubes. After that, a one-step hydrothermal method was reported. [25] The Nb 2 O 5 powders, hydrofluoric acid (HF), hydrogen peroxide, and Ti powders were introduced into the precursor. Hydrofluoric acid acted as an etching reagent to disperse Nb powders in solution. [25,26] In addition, the evolution of nanotubes was significantly affected by the concentration of F − ions. This phenomenon may be due to the fact that F − anions act as a structure-directing agent to control the crystal growth. [25,26] Similarly, T-Nb 2 O 5 nanotubes were obtained by the electrochemical method, which consisted of the anodization of Nb with ammonium fluoride. [27] However, toxic reagents (e.g., HF, NH 4 F, or H 2 O 2 ) are required in these processes.
Compared to the nanotubes, Nb 2 O 5 nanorods can be synthesized directly from Nb probes and foils by a calcination method at ≈1000°C. [28] Besides, the decomposition of niobium isopropoxide was controlled to prepare the Nb 2 O 5 nanorods in chemical vapor deposition (CVD) at 950°C. [12d] The synthesis methods of Nb 2 O 5 nanorods were reported at lower temperatures in other studies. For instance, TT-Nb 2 O 5 nanorods encased in carbon were obtained from the niobium ethoxide by calcination at 800°C in the 3 mL autoclave under nitrogen. [29] To remove the carbonaceous residues, the as-synthetic material was further treated at 500°C under air condition, leading to the formation of T-Nb 2 O 5 nanorods. [29] Besides, the topochemical method was developed, which is composed of the i) synthesis of specific morphology of niobates, ii) ion-exchange for removal of other metal ions on niobates, and iii) calcination for the phase transformation. [30] Typically, the KNb 3 O 8 nanowires were prepared by molten salts of Nb 2 O 5 and KCl under 800°C and treated with HNO 3 to produce H 3 ONb 3 O 8 nanorods, which were further calcinated to produce the H-Nb 2 O 5 nanorods. [30b] Similarly, CaNb 2 O 6 nanowires were also utilized to prepare the H 2 Nb 2 O 6 nanorods, which were further transformed to produce the T-Nb 2 O 5 nanorods. [31] Additionally, the solvothermal approach was reported in the catalyst preparation, following the calcination treatment to prepare the Nb 2 O 5 nanorods from amorphous Nb 2 O 5 ·nH 2 O. The additives, like alcohol, played a key role in the hydrothermal process. [32] If alcohols are present in the solution, the Nb 2 O 5 particles were observed. According to this phenomenon, TT-Nb 2 O 5 nanorods were synthesized by a one-step alcohothermal method, which is contributed to the growth direction of [001] for the 1D structure under enough reaction time, temperature, and concentration of NbCl 5 . [32] Likewise, TT-Nb 2 O 5 nanorods were obtained by introducing oleic acid and trioctylamine, isopropanol, benzyl alcohol (BA), and triethylamine in the hydrothermal process. [12a,33] In ad-dition, corrosive NH 4 F and H 2 O 2 , HF, cetyltrimethylammonium bromide, and ionic liquid are also effective additives. [34] Particularly, the rodlike structure was observed when the hydrothermal process was prolonged to 30 days without any additives. [35] Electrospinning was also utilized in the synthesis of Nb 2 O 5 nanorods. [36] A mixture of Nb(OEt) 5 , polyvinylpyrrolidone (PVP), acetic acid, and ethanol solution was prepared before the electrospinning. The complex of PVP and acetic acid in the solution acted as a template. [36] After electrospinning operation, the obtained material was treated at 550°C to produce the Nb 2 O 5 nanorods under air condition. [36] In these methods, calcination is generally utilized for the removal of carbonaceous impurities or the change in the crystallinity.
For the synthesis of Nb 2 O 5 nanowires, a thermal oxidation approach was reported from the linear Nb foils under 900-1000°C. [37] Similarly, Nb 2 O 5 nanowires can be obtained from the topochemical method. T-Nb 2 O 5 was synthesized from NaNbO 3 nanowires under 700°C. [38] H-Nb 2 O 5 nanowires were also prepared from Nb 3 O 7 (OH) nanorods by calcination at 450°C. [39] Besides, TT-Nb 2 O 5 nanowires can be prepared with the assistance of reflux. [40] The refluxing is a useful approach to synthesize metal oxide nanorods with suitable additives for crystal growth, like trioctylamine, which directly affects the pH, the hydrolysis, and deposition of the precursor. [40] When the precursor solution tends to be acidic, high crystallinity of Nb 2 O 5 nanowires is observed after calcination. [40] In addition, electrospinning was also reported in the preparation of T-Nb 2 O 5 nanowires. [41] Furthermore, Nb foil was treated with oxygen plasma to induce the growth of Nb 2 O 5 nanowires over the Nb foil. [42]

2D Nb 2 O 5 Catalysts
Nb 2 O 5 nanosheets are typical 2D materials, which are attractive due to their unique structure and electronic properties. Previously, Nb 2 O 5 nanosheets could be fabricated directly from the raw Nb 2 O 5 and NbCl 5 without any templates or organic polymers. [43] For instance, ≈3-5 nm T-Nb 2 O 5 nanosheets were produced from commercial NbO 2 particles in a solution containing ethanol and urea under 130°C for 30 days. [44] Similarly, TT-Nb 2 O 5 nanosheets were also synthesized from NbCl 5 in ethylenediamine solution by hydrothermal treatment and calcination. [45] In this process, the alkaline additive may be beneficial for the nanosheet evolution. [46] Especially, Wang's group reported the synthesis of the Nb 2 O 5 ·xH 2 O nanosheets from NbCl 5 by a one-step hydrothermal method and revealed the effect of additives. [47] They speculated that the alkaline NH 3 ·H 2 O may play a key role in nanosheet synthesis. To prove the opinion, they replaced the NH 3 ·H 2 O with other alkaline additives, like the NaOH, n-butyl amine, and t-butylammonium hydroxide (TBAOH). [47] Nanosheetlike morphology is only obtained using n-butyl amine and TBAOH, suggesting the vital role of the NH 4 + ions. The ions in the Nb 2 O 5 nanosheets, like NbO 4 3− , NbO 5 5− , and NbO 6 7− , exhibit negative charges, which show an electrostatic interaction with NH 4 + ions that act as a capping agent. This interaction can restrain the interlamination growth and avoid the formation of bulk Nb 2 O 5 . [47] In addition to NbO 2 and NbCl 5 , other 2D columbic compounds are also applied in the synthesis of Nb 2 O 5 nanosheets by the topochemical method (Figure 4a). For instance, 2D Nb m X n (X = Se and S) materials were utilized to prepare the Nb 2 O 5 nanosheets. [48] The Nb 4+ species in NbSe 2 oxidized to Nb 5+ ions with the formation of Se under calcination, leading to the generation of porous Nb 2 O 5 nanosheet. [48] Besides, the Nb 3 O 7 F nanosheets were produced by a hydrothermal approach and further calcinated to prepare the T-Nb 2 O 5 nanosheets. [49] Additionally, other niobates were also reported in the synthesis of Nb 2 O 5 nanosheets. [17c,50] [51] Besides, layered templates are applied in the synthesis of Nb 2 O 5 nanosheets (Figure 4b). The graphene oxide (GO), a typical 2D material, was introduced into the cyclohexane solution, following the addition of Nb(OC 4 H 9 ) 5 . [52] After the hydrothermal process, NbO x /GO nanosheets were observed. [52] Subsequently, ≈2-4 nm Nb 2 O 5 nanosheets were obtained by the removal of GO under calcination. [52] Moreover, when the calcination temperature was elevated to 750°C, holey T-Nb 2 O 5 nanosheets were observed by the utilization of GO. [53] Likewise, silica/graphene nanosheets were utilized to prepare T-Nb 2 O 5 nanosheets by the hydrolysis of NbCl 5 . [54] The silica in the template was removed by NaOH, maintaining the porous structure of Nb 2 O 5 . [54] These preparation methods of Nb 2 O 5 nanosheets can be divided into two classes. One kind of method is to synthesize layered intermediate that contains niobium to prepare nanosheets. Another one is to afford 2D templates that are niobium-free to induce the growth of Nb 2 O 5 nanosheets.
Furthermore, 2D ordered porous Nb 2 O 5 were prepared with the assistance of chain ligand. [55] In 1996, Ying's group put forward a method to synthesize the mesoporous Nb 2 O 5 molecular sieve with the assistance of ligands. [55] In this approach, Nb(OEt) 5 and tetradecylamine were utilized to produce ringed Nb 2 O 5 via hydrolysis, which is controlled precisely by the volume of water, hydrolysis temperature, and time. [55] Because of the interaction between the basic amine ligand and Nb 2 O 5 , the residual tetradecylamine molecules were further removed by strong acid HNO 3 /EtOH. [55] After that, block-copolymer/inorganic-salt methodology was developed. [56] The tetradecylamine can be replaced by poly (alkylene oxide) block copolymer, such as P-123. [56] Meanwhile, inorganic niobium salts, NbCl 5 , were also utilized as a precursor in an ethanol solution. [56] The ordered mesoporous Nb 2 O 5 was obtained by the formation of crown-ether-type complexes between alkylene oxide segments and inorganic ions through weak coordination bonds. [56] Especially, when inorganic Nb salts and aqueous solution were introduced simultaneously into the precursor with controlled hydrolysis, 3D mesoporous Nb 2 O 5 were observed. [57] In addition to the utilization of a single hydrophilic ligand, amphiphilic block copolymers were also developed to fabricate Nb 2 O 5 materials. The poly(ethylene-cobutylene)-b-poly(ethylene oxide) diblock copolymers were mixed with NbCl 5 in the ethanol solution, following the calcination to remove the polymers and obtain the 2D mesoporous Nb 2 O 5 . [58] The orientation of porous structure can be regulated by the changes in polymer hydrophilicity and hydrophobicity. [59] For instance, the polymers with different chain lengths, like www.advancedsciencenews.com www.advancedscience.com amphiphilic L64, P85, and P103, were effective in the synthesis of porous Nb 2 O 5 . [59]

3D Nb 2 O 5 Catalysts
Generally, the porous structure is beneficial for the diffusion and transmission of substrates. [60] Although the porous Nb 2 O 5 , HY-340, is supplied from the CBMM (Brazil, one commercial company), many research groups are still devoted to designing and developing novel synthetic methods of 3D porous Nb 2 O 5 catalysts.
To date, 3D porous Nb 2 O 5 catalysts can be synthesized from the Nb foils, niobium salts, and raw Nb 2 O 5 . The Nb foils were irradiated within a constant flux of 100 eV He + ions under 500-950°C to prepare the porous Nb 2 O 5 . [61] With the increase of temperature, the pore diameter over Nb 2 O 5 was larger, which can be up to 230 nm. [61] In addition, Nb(OH) 5 was obtained by the anodization of Nb foil in ethylene glycol containing 4 vol% HF and 2 vol% H 2 O 2 . Then, mesoporous Nb 2 O 5 was obtained by the calcination of Nb(OH) 5 . [62] In the anodization process, the porous structure was controlled by the changes in voltage, electrolyte temperature, time, and solution. [17d,62,63] Particularly, a careful cleaning process is necessary to remove the impurity on the surface of Nb foils before anodization. [63d] Besides, the sol-gel approach was also reported. Nb(OC 2 H 5 ) 5 was hydrolyzed with the assistance of the NH 3 ·H 2 O solution and calcined at 300 and 650°C to TT-Nb 2 O 5 and T-Nb 2 O 5 , respectively. [64] Similarly, wormhole-like amorphous Nb 2 O 5 and hierarchically porous Nb 2 O 5 were prepared from the hydrolysis of Nb salts (NbCl 5 and Nb(OC 2 H 5 ) 5 ) by the addition of P-123 and surfactant (Brij 56), respectively. [20,65] Other uniform templates, like polystyrene spheres and zeolites, were also utilized to prepare the porous Nb 2 O 5 . For instance, polystyrene spheres were introduced into the Nb-citric complex solution via the reaction of Nb 2 O 5 with HF, NH 3 ·H 2 O, and citric acid. [66] The template was subsequently removed by the calcination with the formation of macroporous Nb 2 O 5 . [66] The macropores size of Nb 2 O 5 was greatly dependent on the diameter of the polystyrene spheres. Similarly, the ammonium niobate oxalate was deposited on the FDU-1, one type of zeolite, by an impregnation method. [67] Then, the FDU-1 is removed by a diluted NaOH solution. Furthermore, T-Nb 2 O 5 and TT-Nb 2 O 5 were obtained with the assistance of other porous carbon materials, such as cotton. [68] In these methods, the evolution of ordered porous structure mainly depended on the hydrolysis of niobium salt and the uniformity of templates. [57] Porous Nb 2 O 5 constituted by stacked particles was also reported. [69] For instance, T-Nb 2 O 5 can be prepared by direct calcination of Nb powders. [69] In addition, the hydrolysis of organic Nb salts was also applied in the synthesis of nanoparticles. Especially, Nb(OBu) 5 was dissolved in toluene with different amounts of water in an autoclave under 300°C for 2 h. [70] When the amount of water was up to 30 cm 3 , amorphous Nb 2 O 5 were transformed to TT-phase and grew from ≈5 to 30-60 nm, indicating that water was beneficial for the dissolutionrecrystallization process on the growth of Nb-O-Nb structure. [70] Instead of toluene, ethanol, triethylamine, and H 2 O 2 solution were also applied in the synthesis of Nb 2 O 5 particles. [71] Besides, the NbCl 5 and Nb-fluoro complex were used in the synthesis of H-Nb 2 O 5 and TT-Nb 2 O 5 particles, respectively. [72] In the hydrolysis process, structure-directing agents were introduced, such as the lauryl amine hydrochloride and F127. [73] The smaller particles of Nb 2 O 5 were observed with the increase of pH. [74] Furthermore, other methods were developed for the synthesis of Nb 2 O 5 particles. For instance, the supercritical-CO 2 -assisted approach was introduced into catalyst preparation. [75] The hydrolyzed mixture of NbCl 5 dissolved in ethanol solution and aged in the supercritical CO 2 under 80°C for 3 h. After the calcination under 200°C, amorphous Nb 2 O 5 particles with a high surface area (≈340 m 2 g −1 ) were obtained. [75] In addition, the ball-milling process was reported for the low-temperature synthesis. Mixed Nb 2 O 5 and Na 2 CO 3 were formed by the reaction of NbCl 5 and Na 2 CO 3 . [76] The molar ratio of NbCl 5 to Na 2 CO 3 and calcination temperature were controlled to inhibit the generation of unwanted niobates. [76] The additives, such as urea and melamine, acted as the fuel and template to fabricate TT-Nb 2 O 5 particles in the calcination process. [12b,77] Furthermore, the as-synthesized H-Nb 2 O 5 particles can be treated under laser pulses to prepare amorphous Nb 2 O 5 , T-Nb 2 O 5 , and TT-Nb 2 O 5 , realizing the reversible transformation of crystal structures. [78] Other morphologies of Nb 2 O 5 catalysts were also reported, such as the bulk, octahedron, hollow structure, and others. [79] The synthetic methods of these morphologies were partly similar to that of Nb 2 O 5 particles under different conditions. For instance, the Nb 2 O 5 particles can be further calcinated to prepare the bulk counterpart. [75] Besides, the resorcinol, formaldehyde, and ammonium niobate oxalate were utilized in the hydrothermal process for the synthesis of Nb 2 O 5 @polymer materials, which were further calcinated to remove the polymer and obtain the hollow Nb 2 O 5 microspheres. [80]

Doped Nb 2 O 5 Catalysts
Because of the wide bandgap of Nb 2 O 5 (≈3.0-3.4 eV), the strategies by introducing doped atoms are applied to enhance the optical absorption ability, which was confirmed by experiments and first-principles calculations. [81] The synthetic methodologies can be classified into two approaches: a) synthesis from the mixture of additives and columbic precursors, and b) post-treatment of as-synthesized Nb 2 O 5 catalysts ( Figure 5). For instance, ethanol and acetic acid acted as the carbon sources and the solvent to prepare the carbon-doped Nb 2 O 5 (C-Nb 2 O 5 ) in the solvothermal process ( Figure 5, path I). [82] Similarly, the niobium ethoxide and NbCl 5 were dispersed in a mixed solution of alcohol and nitrogenous additives in the solvothermal process, leading to the formation of nitrogen-doped Nb 2 O 5 (N-Nb 2 O 5 ). [33b,83] Besides, N-Nb 2 O 5 can be obtained from the calcination of niobium salts with urea, melamine, and ammonium chloride. [12b,84] A series of rareearth (Er, Eu, Pr, Tm, and Yb), Ag, Fe, Mo, Pd, Sr, W, Y, Zn, and Zr doped Nb 2 O 5 materials were also synthesized from the calcination of mixing the niobium salts with other metal additives. [77,85] Additionally, the alkali metal doped Nb 2 O 5 materials were also fabricated by the electrochemical approach. The Nb foils were oxidized at a pulsed current while the alkali metal ions in the electrolytes were feasible to interact with NbO x , resulting in the generation of M-Nb 2 O 5 (M = Li, Na, K, Rb, and Cs). [86] Moreover, as-synthesized Nb 2 O 5 can be further treated (Figure 5, path II). Mesoporous Nb 2 O 5 and citric acid were utilized to synthesize the C-Nb 2 O 5 under the calcination at 400°C. [73] The Nb 2 O 5 was treated with NH 3 gas to prepare N-Nb 2 O 5 under 400-600°C. [87] In addition, the N-Nb 2 O 5 was also synthesized from the calcination of porous Nb 2 O 5 with urea. [19,50f,88] If urea is replaced by thiourea, the generation of sulfur-doped Nb 2 O 5 (S-Nb 2 O 5 ) can be observed. [73] Furthermore, N, S codoped Nb 2 O 5 was obtained when the ratio of thiourea to Nb 2 O 5 increased from 0.37 to 1. [89] Other metal atoms, like Mo atoms, can be introduced into the Nb 2 O 5 lattices, which were synthesized from the hydrothermal process of ultrathin Nb 2 O 5 nanosheets and ammonium molybdate. [90]

Metal Species Supported on Nb 2 O 5 (M/Nb 2 O 5 ) Catalysts
Diverse metals, metal oxides, and metal salts are utilized in the synthesis of M/Nb 2 O 5 . For instance, Pt and Ag 2 O powders were mixed with Nb 2 O 5 to prepare the Pt/Nb 2 O 5 and Ag/Nb 2 O 5 , respectively. [91] Accompanied by the development in nanotechnology, the size of metal particles was precisely controlled by the colloidal method, which can be further applied in the synthesis of M/Nb 2 O 5 . [48,92] For instance, ≈7.0 nm Pd nanoparticles were protected by the ligand, oleylamine, or oleic acid and introduced into the Nb 2 O 5 suspension with the assistance of hexanes. The ligands on the Pd species can be further removed by calcination. [92d] Metal salts were applied in the synthesis of M/Nb 2 O 5 by the wet chemistry methods. [93] The Au/Nb 2 O 5 , Ir/Nb 2 O 5 , Rh/Nb 2 O 5 , Ru/Nb 2 O 5 , Pd/Nb 2 O 5 , and Pt/Nb 2 O 5 were synthesized by the incipient wetness impregnation method. [93] The heteroatoms in the precursor, such as nitrogen and chlorine atoms, are reasonably removed by the calcination process. [93a] Whereas, the aggregation of metal species to nanoparticles was observed, ascribed to the heat treatment. The deposition-precipitation method was also developed to produce M/Nb 2 O 5 , following the calcination under lower temperature (≈300°C). [8c] In the preparation of catalysts, Au 3+ ions were deposited on the Nb 2 O 5 surface with the assistance of urea or ammonium hydroxide. The average diameter of Au nanoparticles was ≈5 nm after calcination. [8c] The H 2 can be replaced by NaBH 4 or hydrazine, which is an effective reductant for RuCl 3 and Pd(acac) 2 to Ru/Nb 2 O 5 and Pd/Nb 2 O 5 without heat treatment. [94] Instead of NaBH 4 and hydrazine, the reductive electrons can be directly generated from Nb 2 O 5 under UV light irradiation. [4a] The Au/Nb 2 O 5 , Pt/Nb 2 O 5 , and Pd/Nb 2 O 5 were fabricated by this approach. [95] Meanwhile, the photogenerated holes were captured by sacrificial agents, such as ethanol or isopropanol. [95] Besides, the electrostatic adsorption was available to prepare the highly dispersed metal nanoparticles supported on Nb 2 O 5 under room temperature, attributed to the difference in point of zero charges (PZCs) between the metal ions and Nb 2 O 5 at the same pH. Thus, Ag/Nb 2 O 5 can be synthesized via the interaction between the Ag(NH 3 ) 2 + and Nb 2 O 5 at high pH (>5). [96]

Composited Nb 2 O 5 Catalysts
The Nb 2 O 5 can be modified by other metal oxides, metal sulfides, metal carbides, carbon materials, carbon nitride (g-C 3 N 4 ), and black phosphorus (BP) to enhance its activity. [13,97] As-prepared Nb 2 O 5 , NbC 2 , Nb 3 O 7 F, and niobium salts were reported in the synthesis of composited Nb 2 O 5 catalysts (Figure 6). Different Nb salts were reported in the synthesis of composited Nb 2 O 5 photocatalysts ( Figure 6, path I). [98] The methods mentioned in the preparation of pure phase Nb 2 O 5 were also applied in the composited Nb 2 O 5 photocatalysts. For instance, the CVD method was utilized to fabricate Nb 2 O 5 /SiO 2 . [98] Besides, Nb precursors, like NbCl 5 , and other metal salts were cohydrolyzed and precipitated to prepare mixed metal oxide particles. [99] Especially, ≈25-51 nm Nb 2 O 5 nanocrystals were observed on the surface of ZnO nanorods, while ≈3-5 nm Nb 2 O 5 microspheres were fabricated on the g-C 3 N 4 . [99e,g] This phenomenon can be ascribed to P123, which were conducive to the stabilization and dispersion of Nb micelles in the synthesis of Nb 2 O 5 /g-C 3 N 4 . [99g] In addition, as-prepared Nb 2 O 5 can be ground directly with TiO 2 , SrNb 2 O 6 , Bi 2 O 3 , and GO to prepare the TiO 2 /Nb 2 O 5 , SrNb 2 O 6 /Nb 2 O 5 , and GO/Bi 2 O 3 /Nb 2 O 5 , respectively. [100] Further calcination was conducted to improve the interaction between the Nb 2 O 5 and other components ( Figure 6, path II). [101] Besides, as-prepared Nb 2 O 5 was also dispersed in solutions, such as isopropanol or tetrahydrofuran, to enhance the contact with the TiO 2 , BP, and C 60 . [97c,102] In addition, the metal salt precursors were introduced into Nb 2 O 5 suspension instead of assynthesized metal oxide or metal sulfide. [103] The ZnO/Nb 2 O 5 and CdS/Nb 2 O 5 were prepared from Zn(NO 3 ) 2 and CdCl 2 by the impregnation approach, respectively. [104] The metal precursors, like TiCl 4 , were hydrolyzed by the addition of NH 3 ·H 2 O and deposited on the Nb 2 O 5 to obtain the TiO 2 /Nb 2 O 5 .
[103a] Besides, CdS/Nb 2 O 5 /N-GO was obtained from Nb 2 O 5 by the deposition of CdS in the hydrothermal process. Especially, electrostatic adsorption was also reported to fabricate composited Nb 2 O 5 catalysts. [13,105] In principle, the PZC can be utilized to screen materials that exhibit positive or negative surface charges, which are opposite to that on the Nb 2 O 5 surface at the same pH value. Fortunately, SiO 2 and g-C 3 N 4 as the potential candidates were reported. [13,105] Positive charges originated from amino groups that were exposed on the g-C 3 N 4 surface in a pH range of 3-4. Meanwhile, the Nb 2 O 5 surface is electronegative, ascribed to the presence of surface hydroxyl groups. [13] Compared to the impregnation approach, this method is available to prepare highly dispersed components on Nb 2 O 5 , due to the adsorption equilibrium. Excess g-C 3 N 4 are possibly removed by washing and filtration, which differ from the drying treatment in the impregnation approach. [13] Furthermore, isolated species on Nb 2 O 5 may be obtained by this method with precise control of precursor concentration, pH, and temperature.
Additionally, as-synthesized NbC 2 and Nb 3 O 7 F were also utilized to prepare corresponding Nb 2 O 5 -based catalysts ( Figure 6, path III). The Nb 2 O 5 /C/Nb 2 C and Nb 3 O 7 F/Nb 2 O 5 photocatalysts were obtained by one-step calcination. [97d,106] The formation of Nb 3 O 7 F/Nb 2 O 5 was ascribed to the decomposition of Nb 3 O 7 F to Nb 2 O 5 when the temperature is higher than 400°C. [106] Similarly, the observed Nb 2 O 5 supported on Nb 2 C was due to the reaction between Nb 2 C and CO 2 under 850°C. [97d] Considering the thermal reaction, a series of composited photocatalysts with different Nb 2 O 5 fractions are feasibly prepared, using the Nb 2 C, NbN, NbSe 2 , NbS 2 , and Nb 3 O 7 F. [107]

Photodegradation of Pollutants
Nowadays, Nb 2 O 5 -based photocatalysts were widely reported in the photocatalytic photodegradation of pollutants ( Table 1). [108] The hydrocarbons and chlorinated hydrocarbons (  [77,92b,109] In these processes, the wavelength ( ) of light sources is vital in photocatalysis, due to the limited absorption edge of photocatalysts. [8e] For instance, pure Nb 2 O 5 exhibits large E g and is active under UV light irradiation ( < 400 nm). [4a] Accordingly, the black lamp, Xe lamp, and Hg lamp are available light sources. [108,110] Considering that ≈4% of the total solar spectrum is UV fraction, the strategies were developed to enhance the utilization efficiency of sunlight.  ation from other sources, including the fluorescent lamps, halide lamp, white LEDs, solar simulator, and sunlight. [77,99e,109b,111] In these processes, the ultraviolet filter can be utilized to eliminate the effect of UV light. Especially, the degradation of RhB was driven under visible light over pure Nb 2 O 5 without the limitation of its bandgap energy ( Table 1, No. 101). This process is ascribed to the dye-sensitized photocatalysis, in which the RhB molecules adsorbed on the Nb 2 O 5 surface are excited by 440 nm light irradiation. [109e,112] The electrons transfer from the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) of RhB molecules and inject into the Nb 2 O 5 conduction band, which induce the generation of active species for the succedent mineralization of the organic pollutant. [112] Furthermore, the photodegradation of textile wastewater, palm oil mill effluent, petrol station wastewater, and vinasse was also reported ( Table 1, Nos. [146][147][148][149][150][151][152][153][154][155]. [113] These results suggested the potential of Nb 2 O 5 -based photocatalysts in practical applications. In these processes, the efficiency of catalysts is important in photocatalytic performance. [5b] Generally, the degradation rate is a common criterion for the comparison of activity (Table 1). However, the degradation rate is related to the ratio of the moles of organic pollutants to the mass of catalyst, temperature, and intensity of the light source. For instance, a change of reaction temperature is beneficial to the separation of photogenerated electrons and holes. [114] Increasing the intensity of the light source can improve the number of incident photons to enhance the reaction rate. [115] Thus, it is a complicated and difficult process for the comparison of the activity results. The utilization of photogenerated electrons and holes over Nb 2 O 5 -based photocatalysts can be used as another one criterion for the comparison of their activity. The organic pollutants can be degraded by the superoxide anions (•O 2 − ), hydroxyl radicals (•OH), and photogenerated holes (h + ), corresponding to the path A, path B, and path D in Figure 7. [13,116] Therefore, the efficiency of electrons can be calculated by the ratio of products to the pollutants. However, the qualitative and quantitative analysis of obtained products is not always mentioned in the literature, leading to challenges in the UV light 25 61% (120 min) [ 163] (Continued)  [ 190] 141 Nb 2 O 5 /Bi 2 WO 6 Dibenzo-thiophene 5 W LED lamps r.t. 99% (120 min) [ 191] 142 Nb 2 O 5 Reactive blue 59 400 W Hg lamp n.m. 89% (150 min) [ 192] 143  [ 196] 154 Nb 2 O 5 Petrol station wastewater 250 W Hg lamp n.m. ≈35% (300 min) [ 197] 155 comparison of activity. These phenomena can be ascribed to the complex reaction mechanism and the difficulty in the product analysis. Further studies in this area are still necessary. Pure Nb 2 O 5 , N-Nb 2 O 5 , and composited Nb 2 O 5 photocatalysts were also applied in the removal of toxic Cr(VI) species (Table 1, Nos. [156][157][158][159][160][161][162][163]. [77,99c,117] Unlike the degradation of organic pollutants, the Cr(VI) species are reduced by the photogenerated electrons ( Figure 7, path C). Besides, the utilization of Nb 2 O 5based photocatalysts was also reported in the inactivation of bacteria (Table 1, Nos. [164][165][166]. The Staphylococcus aureus and Escherichia coli were inactivated by •O 2 − species that were generated from the reaction between O 2 and excited electrons on the Nb 2 O 5 surface (Figure 7, path A). [77] Considering the nontoxic demand for drugs, these results imply the potential application of Nb 2 O 5 -based photocatalysts in the medical field.

Photocatalytic H 2 and O 2 Evolution
Photocatalytic hydrogen evolution is the potential process to produce H 2 as a clean energy carrier. [5b,204] Generally, the excited electrons in semiconductor photocatalysts are utilized for the reduction of H + ions or H 2 O to H 2 . [5b] However, the reaction rate over Nb 2 O 5 is limited by several factors, including i) the high recombination efficiency of charge carriers, ii) the low reduction rate of catalytic protons to H 2 , iii) the insufficient absorption capacity of visible light, and so on. Thus, some strategies have been devel-oped to enhance the photocatalytic performance. [5b] For instance, sacrificial agents, like triethanolamine (TEOA), methanol, lactic acid, Na 2 S, and Na 2 SO 3 , were introduced into the reaction mixture to consume the holes on Nb 2 O 5 -based photocatalysts while the remaining electrons were still utilized for the production of H 2 ( Furthermore, the Nb 2 O 5 and Pt/Nb 2 O 5 were applied in the oxidation of water to O 2 (Table 2, Nos. 33-34). In this process, AgNO 3 acted as the sacrificial agent, which was reduced by the photogenerated electrons on Nb 2 O 5 -based photocatalysts. Especially, the apparent quantum yield (AQY) was mentioned in these processes (Table 2, Nos. 2-4), which is defined by the number of the reacted electrons to the number of incident photons. [206] The AQY can be a benchmark for comparison of efficiency in different photocatalytic systems. Recently, the AQY in the photocatalytic water splitting to H 2 and O 2 was up to ≈96% on Al-SrTiO 3 under 360 nm light irradiation. [207] This result is much higher than that reported on Nb 2 O 5 -based catalysts (Table 2, Nos. 2-4). Hence, the challenge and opportunity are still present in further improving the activity of Nb 2 O 5 -based photocatalysts.

Photoreduction of CO 2
CO 2 as a carbonaceous resource can be applied in the production of chemicals and fuels. [218] For instance, CO 2 can be reduced to one-carbon (C 1 ) molecules, like CO, HCOOH, HCHO, CH 3 OH, and CH 4 , and C 2+ products. [218] There are two typical reaction modes for photocatalytic reduction of CO 2 : solid-liquid interface reaction mode (mode I) and solid-vapor interface reaction mode (mode II). [219] In the first mode, the photocatalysts were introduced into an aqueous solution. Dissolved CO 2 in water can be reduced on the solid-liquid interface. For another one, CO 2 molecules were directly reduced on the solid-photocatalysts surface. Especially, two modes were both reported with the uti-lization of Nb 2 O 5 -based photocatalysts. [9b,220] The CO, HCOOH, CH 3 OH, CH 4 , and CH 3 COOH were observed in these reduction process ( Table 3). [9b,220] Because the dissolved CO 2 in water is limited in mode I that was widely reported, sacrificial agents, such as triethylamine, were added to improve the solubility of CO 2 in water and consumed the excited holes. Besides, the photoreduction of CO 2 can be occurred on amorphous Nb 2 O 5 without any additives ( Table 3, No. 3). Some possible reaction pathways were proposed as following Equations (1)-(8) [9a,219] CO 2 + 2H + + 2e − → HCOOH (1) Adv. Sci. 2021, 8, 2003156 www.advancedsciencenews.com www.advancedscience.com It is very important to underline some critical, analytical, and mechanistic aspects in the photocatalytic conversion of CO 2 . Over the past decade, it is known that carbon residues can be involved in photocatalytic water activation and CO 2 reduction. [221] This is particularly relevant for the correct evaluation of the rates of artificial photosynthesis using photocatalysts synthesized with carbon-containing precursors. For this reason, it has become more and more relevant to the use of 13 CO 2 to prove the mechanism of CO 2 reduction. In fact, the reaction products, often in trace levels, can derive also from light-induced desorption or reaction of carbonaceous impurities or residues from the synthesis in organic media that are not fully removed even by calcination. For instance, the CH 4 can be observed from the catalysts under light irradiation without CO 2 . [220c] After eliminating the effects of carbon residues, the experimental results are conducive to reveal the process of photocatalytic reduction of CO 2 . The photocatalytic efficiency can be evaluated by the AQY, which was not mentioned in these processes (Table 3). [9b,220] Besides, the selectivity of products is also important for catalytic performance. The C 1 products from CO 2 are important chemical intermediates and fuels. [218] High selectivity (>99%) of CO and CH 4 has been obtained, respectively (Table 3, Nos. 1-2). Although other acid products were observed in pure Nb 2 O 5 , [9d] the selectivity of HCOOH (35%) was competitive with that of CH 3 COOH (42%; Table 3, No.3). The formation of CH 3 COOH involved the C-C coupling reaction of •COOH radicals (Equations (6)-(8)). [9a] Unfortunately, the uncontrollable activity of •COOH radicals leads to the simultaneous generation of CH 3 COOH and HCOOH. [115] Besides, syngas that is vital in Fischer-Tropsch synthesis can be directly obtained from the reduction of CO 2 and H 2 evolution in photocatalysis. [222] To date, such processes are yet to be recognized over Nb 2 O 5 -based photocatalysts.

Selective Transformation of Organic Molecules
Amines, aldehydes, and ketones are important organic intermediates for medicines and polymers. [223] The VB maximum of Nb 2 O 5 is up to ≈+2.50 V versus NHE (normal hydrogen electrode), implying its potential application in the oxidation and succedent transformation of organic molecules. [9a] The amines, alcohols, propene, cyclohexane, toluene, and ethylbenzene were selective oxidation to corresponding imines, aliphatic aldehydes, ketones, benzaldehyde, and acetophenone ( Table 4). Similar to the photocatalytic reduction of CO 2 (Section 4.3), there were also two typical reaction modes in the selective transformation of organic molecules. In the first mode, O 2 , solid photocatalysts, pure organic liquid, or the substrate dissolved in the solvent, like benzene and acetonitrile, were present in the system (Table 4, Nos. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. For another one, the mixture of O 2 , substrate, and solid photocatalysts were introduced into the reactor (Table 4, No. 17). The reaction rate of benzylamine observed on Nb 2 O 5 was higher than that of TiO 2 . [9c] Meanwhile, the selectivity of N-benzylidene benzylamine on Nb 2 O 5 is up to 98%. [9c] Besides, the selectivity of partial oxidation products was up to 97% after the deposition of Nb 2 O 5 on the TiO 2 surface under UV light irradiation. [97a] This may be attributed to that the amounts of photogenerated O 3 − species over the catalyst drastically decreased, which were estimated by electron spin resonance spectroscopy. [97a] Interestingly, primary alcohols oxidized to aldehydes without the generation of acid on Nb 2 O 5 under visible light irradiation. [224] A detailed relationship between product selectivity and structure of Nb 2 O 5based photocatalysts is summarized in the next section.
In addition, Pd/HNb 3 O 8 nanosheets were efficient in the reduction of aryl nitro-compounds to aniline (Table 4, No. 18). [50c] Moreover, Nb 2 O 5 catalysts were also applied in the photocatalytic coupling reaction. Xie's group demonstrated that the polyethylene was completely photodegraded on Nb 2 O 5 nanosheets while generated CO 2 was further reduced to CH 3 COOH (Table 4, No. 19). [9a] Possible reaction mechanism was provided, in which CO 2 was reduced to •COOH radicals, HCOOCOOH, and CH 3 COOH (Section 4.3, Equations (6)-(8)). [9a] The transformation of waste plastics to chemicals and fuel can be realized by this process. Although the yield of CH 3 COOH is limited, further designs in photocatalysts are possible to enhance its activity. In addition, the acid sites on Nb 2 O 5 play an important role in the coupling reaction. For instance, dimethoxymethane molecules were generated in the photooxidation methanol, indicating the coupling formaldehyde and methanol catalyzed by the BAS of Au/Nb 2 O 5 (  n.m. [227] 5 N b 2 O 5 1-pentanol Pentanal 500 W Hg lamp 50 ≈1.28 n.m. [228] 6 H N b 3

20)
. [225] The other example reported by Lei's group demonstrated that 2,5-diformylfuran (DFF) and H 2 are produced from 5hydroxymethylfurfural (HMF) and H 2 O on Nb 2 O 5 /ZnIn 2 S 4 , in which HMF acted as a sacrificial agent to consume the holes to improve the evolution of H 2 with the formation of DFF (Table 4, No. 21). [217] This strategy is available to improve the efficiency of photogenerated holes and electrons simultaneously. [7a] 5. The Structure-Activity Relationship

The Role of Size and Crystalline Phases
Previously, Nb 2 O 5 -based catalysts with high SSA can be efficient photocatalysts (Figure 8a-c). [70] In the degradation process of trichloroethylene, T-Nb 2 O 5 nanotubes showed higher activity than that of layered K 4 Nb 6 O 17 , which were ascribed to their higher crystallinity and specific surface area. [23c] After that, Zhang's group reported that H-Nb 2 O 5 nanorods exhibited higher photocurrent density than that of commercial counterpart, due to the positive effect of high SSA on the separation of photogenerated carriers. [205b] Besides, the thickness of Nb 2 O 5 -based catalysts may play an important role in photocatalysis. Tsang's group reported that a higher photocatalytic H 2 evolution rate was observed with smaller numbers of the layer on Nb 2 O 5 -based nanosheets. [12c,43] In Yu's results, the photocatalytic H 2 evolution rate was enhanced with a decrease in the wall thickness of porous Nb 2 O 5 . [20] These results imply a positive role of the thin-walled structure of Nb 2 O 5 in the separation of charge carriers. Although the crystal faces of Nb 2 O 5 , like (010), were revealed by the high-resolution transmission electron microscopy (HRTEM), the role of these structures is rarely reported in photocatalysis. [33a,234] Previously, Kudo's group afforded a possible explanation. [40b] In the photocatalytic H 2 evolution, higher activity of TT-Nb 2 O 5 nanowires was observed than that of bulk counterpart in their results. [40b] Meanwhile, the Pt particles were selectively distributed on the short-axis plane of the TT-Nb 2 O 5 nanowires in the photodeposition process. These results suggested that the photogenerated electrons moved along the nanowire growth direction while holes migrated to the nanowire sidewall. [40b] Although some Pt particles were also observed on other facets, Kudo's group described the mobility difference of charge carriers in the crystal growth direction. After that, the driving force can be ascribed to the formation of a built-in electric field between different facets, which is instructive to the separation of e − and h + . [235] In addition, Tsang's group reported that {001} facet on Nb 2 O 5 nanorods was active for photodegradation of methylene blue. [33a] This can be attributed to the strong Lewis acidity of Nb 2 O 5 nanorods, which is summarized in Section 5.3. [33a] Previously, TT-Nb 2 O 5 exhibited a higher SSA and reactivity than those of T-Nb 2 O 5 and H-Nb 2 O 5 . [9e, 70,135] Whereas, some research groups found that T-Nb 2 O 5 and H-Nb 2 O 5 showed higher activity than that of TT-Nb 2 O 5 in photodegradation of methylene permission. [240] Copyright 2015, American Chemical Society. h-j) Reproduced with permission. [239] Copyright 2017, American Chemical Society.
blue and selective oxidation of 5-hydroxymethylfurfural (Table 4, No. 9). [9e,135] For instance, the T-Nb 2 O 5 obtained by calcination at 800°C showed the highest photocurrent density than those of counterparts treated at 300 and 500°C. [9e] Except for the SSA, these results indicated other factors might play an important role in photocatalysis. Previously, the formation energy of oxygen vacancy is changed on different crystalline phases of metal oxide. [236] As revealed by the results of X-ray photoelectron spectra (XPS), a high concentration of oxygen vacancies was observed on H-Nb 2 O 5 . [135] These results suggest the positive effect of oxygen vacancy induced by phase transformation in photocatalysis. The detailed discussion of the role of oxygen vacancy is shown in Section 5.2.
As mentioned above, the photochemical characterizations of catalysts are necessary to reveal the distribution and migration of charge carriers. The photocurrent response, electrochemical impedance spectroscopy (EIS), and photoluminescence spectroscopy (PL) are developed to verify the separation of electrons and holes (Figure 8d-f). Besides, the theoretic calculation was utilized to study the transfer process of excited holes and electrons on metal oxide (Figure 8g). [237,240] Recently, the charge carriers can be directly detected by surface photovoltage microscopy (SPVM). [238] As shown in Figure 8h-j, a circular ring was observed on the differential spectrum of surface photovoltage over Au/TiO 2 , which is corresponding to the accumulation of excited holes in the interface (Au-O-Ti) under light irradiation. [239] In principle, these characterization techniques are universal and conducive to the profound understanding of the spatial distribution of charge carriers on Nb 2 O 5 .

The Role of Unsaturated Nb Sites and Oxygen Vacancies
The oxygen vacancies of Nb 2 O 5 play important roles in the absorption and activation of the substrate. [229,241] Previously, the unsaturated Nb sites were observed with the formation of oxygen vacancies, which were revealed by the results of electron paramagnetic resonance (EPR) and XPS. [229,241] The EPR signal at 2.003 is assigned to the oxygen vacancy. [229] After the adsorption of BA on HNb 3 O 8 nanosheets, the intensity of the signal at 2.003 was weakened. Meanwhile, the characteristic O-H and C-O bands of BA were shifted to the lower wavenumber in Fourier transform infrared spectra (FT-IR), indicating that the BA molecules were adsorbed on unsaturated Nb sites by the formation of C-O-Nb complex. [229] As a result, the optical absorption edge of BA/HNb 3 O 8 nanosheets was extended to visible light, indicating that this structure was beneficial for the migration of charge carriers. [229] Besides, the unsaturated Nb sites and oxygen vacancies are conducive to the separation of charge carriers. [50c] The HNb 3 O 8 nanosheets with abundant oxygen vacancies exhibited a faster reaction rate and higher separation efficiency of charge carriers than those of urchin-like Nb 2 O 5 and Nb 2 O 5 nanorods, as shown by the results of EIS, photocurrent response. [12c] Meanwhile, PL analysis and time-resolved fluorescence (TRF) results suggested that the oxygen vacancies were conducive to the separation of photogenerated holes and electrons. [77] In addition, the optical absorption ability of catalysts can be affected by the unsaturated Nb sites and oxygen vacancies. [17c,242] The HNb 3 O 8 nanosheets exhibited a wide bandgap (≈3.4 eV), corresponding to the absorption spectrum within 370 nm. [90] With the formation of oxygen vacancies, the bandgap of HNb 3 O 8 nanosheets was narrowed. [50f] Moreover, new energy states with a small bandgap (<0.5 eV) were observed with the increasing concentration of surface unsaturated Nb sites and oxygen vacancies. [17c] This electronic structure can be utilized for the absorption of the full solar spectrum (250-2500 nm). [17c] Thus, heat sourced from the absorption of infrared light over HNb 3 O 8 nanosheets contributed to enhancing reaction rate in kinetics. [17c] The inert C-H bonds of hydrocarbons can be activated by the holes generated at the valence band under UV light irradiation in the thermodynamics. [9a] These results shed light on the balance of electronic structure and concentration of oxygen vacancies for photocatalyst design and preparation.
Especially, the unsaturated Nb sites and oxygen vacancies can be formed without any sacrificial agents under UV light irradiation, indicating that light irradiation can be utilized for the modification of catalyst. [50c] Meanwhile, the concentration of oxygen vacancies was not constant and changed under irradiation. [50c] To reveal the structure of Nb 2 O 5 in situ under light irradiation, theoretical calculations in oxygen-vacancy formation energy on different facets and crystalline phases are necessary for the profound understanding of these phenomena. [9e]

The Role of LAS, BAS, and Acidity
There are consecutive tandem steps involved in photocatalytic processes: 1) light-harvesting on photocatalysts, 2) separation and migration of photogenerated holes and electrons, and 3) succedent surface redox reactions. [243] Particularly, the surface redox reactions and selectivity of products are related to the acid-base properties of photocatalysts. [243] Previously, propene was oxidized to propanal and acetaldehyde on Nb 2 O 5 /SiO 2 photocatalysts under light irradiation. [233] The selectivity of propanal was much different on the Nb 2 O 5 /SiO 2 catalysts. [105a] As revealed by results of x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS), tetrahedral NbO 4 and octahedral NbO 6 units were present on 0.66 and 4.6 wt% Nb 2 O 5 /SiO 2 , respectively. [233] Notably, the NbO 6 octahedra acted as the BAS. [14a] The LAS can be observed with the formation of NbO 5 and NbO 4 polyhedron. [15] The roles of acid sites are possibly revealed by these results. However, other products, like acetone and acrolein, were also observed, leading to the difficulty in the understanding of the relationship between BAS, LAS, and photooxidation processes. [105a] After that, the unique acidity of Nb 2 O 5 was found to be instructive to the adsorption-activation process in the photooxi-dation reaction. [14a,244] The deep oxidation products of alcohols were observed with increasing Lewis acidity of Nb 2 O 5 , which may be ascribed to the strong adsorption of aldehydes intermediates on LAS. [228] Besides, the Nb 2 O 5 -amide surface complex was generated by the adsorption of amines on Nb 2 O 5 . [245] This complex can be excited by light ( > 390 nm) with lower energy than that of Nb 2 O 5 . [245] Meanwhile, the yield and selectivity of benzylamine to N-benzylidene benzylamine over commercial Nb 2 O 5 were higher than those over TiO 2 under visible light irradiation. [9c] This is mainly due to the activation of amines adsorbed on Nb 2 O 5 via the ligand-to-metal charge transfer (LMCT) transition (Figure 9). [245] The electrons transferred from the N 2p orbitals of amides to Nb 4d orbitals of Nb 2 O 5 . [244] Then, the -C-H bonds were activated to form the imines. [245] This direct excitation of amines is beneficial to improve the selectivity of the product by inhibiting the generation of other oxygen-containing species, such as ozonide anion radicals (O 3− ) and hydroxyl radicals (HO•). [246] Thus, the undesired deep oxidation was restricted under visible light irradiation. [246] Similarly, alcohols were selectively oxidized to corresponding aldehydes without further oxidation to carboxylic acids. [224] When the surface isolated -OH groups on Nb 2 O 5 were partly removed by a vacuum heat treatment, inert aromatic hydrocarbons with relatively large ionization energy also can be transformed to aldehydes under visible light irradiation (Figure 9). [18] However, the active sites on Nb 2 O 5 are still unclear for the activation of the inert sp 3 C-H bond on toluene. [12b,18] Besides, Tsang's group proposed the adsorptionactivation process of dyes on Nb 2 O 5 . [33a] In their results, Nb 2 O 5 nanorods exhibited higher activity than that of ZnO in the photocatalytic degradation of methylene blue. [33a] After the introduction of a radical scavenger, the comparable activity of Nb 2 O 5 nanorods was observed, indicating that hydroxyl radicals (HO•) played a marginal role in the reaction. [33a] As revealed by FT-IR results, the number of methyl blue molecules adsorbed on Nb 2 O 5 nanorods was higher than that on ZnO, implying that the strong Lewis acidity of Nb 2 O 5 nanorods was beneficial for the adsorption-activation process of methylene blue. [33a] In addition, the acidity of Nb 2 O 5 is also associated with the selectivity of products in the photoreduction reaction. The photoreduction of CO 2 to CH 4 was dominated on HNb 3 O 8 nanosheets and SiO 2 -pillared HNb 3 O 8 that mainly exposed the BAS. [15,221b,c] The yield of CH 4 reached 2.9 µmol g cat.
−1 h −1 over SiO 2 -pillared HNb 3 O 8 , which is much higher than that over HNb 3 O 8 (0.47 µmol g cat. −1 h −1 ), implying the promoting effects of BAS derived from the dispersed HNb 3 O 8 on SiO 2 . [220c] Meanwhile, Ribeiro's group found a quite different tendency that improving the number of acid sites on Nb 2 O 5 can promote the photoreduction of CO 2 to CO, HCOOH, and CH 3 COOH. [9b] As shown in Equations (1)-(8) (Section 4.3), protons are vital in the reduction of CO 2 to CH 4 . However, the number of acid sites on different Nb 2 O 5 samples was measured by the ionicexchange and titration approach, leading to unclear amounts of BASs on the surface of catalysts. [9b] Meanwhile, the activation of CO 2 is related to the local structure of Nb-O-Nb. [247] These results lead to an ambiguous understanding of the photoreduction of CO 2 . Hence, further studies are still necessary to get insight into the relationship between the surface -OH groups, NbO x units, BAS, LAS, and product selectivity in the photoreduction of CO 2 . Figure 9. The schematic illustration of methylene blue, benzylamine, benzyl alcohol, and toluene adsorbed on Nb 2 O 5 . Adapted with permission. [230] Copyright 2009, American Chemical Society. Adapted with permission. [248] Copyright 2012, American Chemical Society.

The Role of Dopant and Surface Metal Species
The photodegradation of pollutants was observed on doped Nb 2 O 5 and M/Nb 2 O 5 catalysts under visible light irradiation (Table 1). The optical absorption ability on doped Nb 2 O 5 catalysts was revealed by the experimental and theoretical analysis. For instance, an energy level sourced from N 2p orbitals was higher than that of the conduction band of O 2p states in pristine Nb 2 O 5 , leading to low bandgap energy of N-Nb 2 O 5 (≈2.61 eV). [19] In addition, the doping level formed by metal dopants was lower than that of the conduction band on pristine Nb 2 O 5 . [85j,249] Meanwhile, the corresponding energy levels are still competent in the generation of O 2 •− species for photodegradation. Furthermore, Nb 2 O 5 catalysts modified by surface species can be active under visible light irradiation. [171] Zhang's group reported the carbonate modified Nb 2 O 5 for photodegradation of RhB under visible light irradiation. [171] The E g of C-Nb 2 O 5 was increased to 3.06 eV after 500°C calcination, suggesting the carbonaceous species for the enhanced visible-light harvesting. [171] Similar phenomena were also observed in N modified and C, N comodified Nb 2 O 5 . [117,172,173] This can be ascribed to the transfer of electrons from surface NO x and CO x species to Nb 2 O 5 under irradiation, which is analogical of dye-sensitized photocatalysis. [173] For M/Nb 2 O 5 catalysts, the surface plasmon resonance (SPR) effect of metal species (e.g., Ag, Au, and Cu) is conducive to enhance the response to the visible light irradiation, ascribed to the match between the frequency of the incident light photons and the frequency of surface electrons on metal species. [113a,206,225] The catalysts structure and properties are also changed by the introduction of dopants on Nb 2 O 5 -based photocatalysts, including the SSA, concentration of oxygen vacancies, and acidity. For instance, the SSA of doped Nb 2 O 5 catalysts was higher than that of pristine counterpart. [82,85j] These phenomena may be ascribed to the lattice distortion and inhibition of crystal growth by heteroatoms. [82,85j] Besides, the concentration of oxygen va-cancies can be increased when Nb 2 O 5 catalysts were doped with N, Zr, Y, Zn, or Mo species. [77,90,184] These phenomena were also observed on other metal oxides, like Cu-CeO 2 . [250] As a result, the photocurrent density of N-Nb 2 O 5 was higher than that of the pristine one. [12b,19] The recombination efficiency of charge carriers is also changed with the concentration of oxygen vacancies, implying an optimal concentration of oxygen vacancies on the Nb 2 O 5 -based photocatalysts in photocatalysis. In the future, machine learning is a promising tool to predict the structure and performance of catalysts. [251] In addition, the acidity of Nb 2 O 5 -based photocatalysts is also influenced by the dopant and surface metal species. Wolski's group reported ≈40% selectivity of dimethoxymethane (DMM) from methanol on Au/Nb 2 O 5 while <5% selectivity of DMM on Nb 2 O 5 was observed under UV light irradiation. [225] The DMM is produced by the condensation reaction between the formaldehyde from the oxidation of methanol and adsorbed methanol molecules, implying the significantly decreased acidity of Nb 2 O 5 after the introduction of the Au species. [225] As revealed by FT-IR results, the number of LASs decreased faster than that of BASs. [225] Meanwhile, the numbers of BASs and LASs were distinct by different preparation approaches. [225] These may be ascribed to the formation of Nb 4+ species from the reduction of Nb 5+ in NbO 4 units by hydrogen spillover in the reducing atmosphere. [14b,252] Besides, the BAS is possibly neutralized by basic additives in the deposition-precipitation approach. [8c] Occasionally the adsorption of the substrate is enhanced by the dopant on Nb 2 O 5 -based photocatalysts. The adsorption of methylene blue molecules on Zr-Nb 2 O 5 is considerably stronger than that over the pristine counterpart. [77] Similarly, the intact interaction between methyl violet molecules and Mo-Nb 2 O 5 cluster was speculated, as revealed by the results of surface-enhanced Raman scattering (SERS) and first-principles calculation. [90] Furthermore, CO was obtained with a selectivity of 99.5% from the hydrogenation of gaseous CO 2 using the small Pd nanocrystals supported on Nb 2 O 5 . [253] Density functional theory (DFT) calculations suggested that the Pd(111) facets dominated on the larger nanoparticles were the most favorable sites for methanation of CO 2 . [253]

The Role of Formed Heterojunctions
Previously, Zheng's group synthesized an amorphous layer on TT-Nb 2 O 5 microfibers (HN-500). [254] As revealed by the results of VB XPS spectra, the edges of the maximum energy for and TT-Nb 2 O 5 were identical with that of the amorphous counterpart. [254] Meanwhile, the bandgap of amorphous-phase is ≈0.2 eV higher than that of TT-Nb 2 O 5 , indicating the formation of heterojunction on HN-500. [254] The electronic structure of heterojunction has an advantage in the separation of charge carriers Besides, short-range ordered Nb 2 O 5 can be dispersed on the surface of amorphous structure, leading to the formation of the interface between amorphous and ordered Nb 2 O 5 . [166] This may be conducive to the desorption of desired products, due to the distinction in acid strength on different phases. [14a] In addition, the spatial distribution of charge carriers can be observed in other Nb 2 O 5 -based photocatalysts. [255] The type II heterojunctions were obtained by modification of Nb 2 O 5 with other composites, such as TiO 2 , WO 3 , ZnO, CdS, C 3 N 4 , Ag 3 PO 4 , SrNb 2 O 6 , BiWO 4 , BiNb 5 O 14 , and so on. [99e,100b,101a,116,160,162,181,191,205a] For instance, electrons transfer from the CB of Nb 2 O 5 to the CB of BiWO 4 while the holes migrated from the VB of BiWO 4 to the VB of Nb 2 O 5 under UV light irradiation, leading to limited redox potentials of Nb 2 O 5 /BiWO 4 . [191] When Nb 2 O 5 was deposited on the ZnIn 2 S 4 , a Z-scheme heterojunction was observed. [217] Partial photogenerated holes and electrons were still present on the VB of Nb 2 O 5 and CB of ZnIn 2 S 4 , respectively. This electronic structure can maintain the oxidizing potential of Nb 2 O 5 and reducing the capacity of ZnIn 2 S 4 , respectively. [217] In Z-scheme heterojunction, partial electrons are transferred from the CB of Nb 2 O 5 to the VB of ZnIn 2 S 4 , which is attributed to photocatalytic performance. [217] Similar heterojunction structures were also observed on Er-Y 3 Al 5 O 12 @Nb 2 O 5 /Pt/In 2 O 3 composite catalyst. [214] Especially, the interface is present in composited Nb 2 O 5 catalysts. [227] Previously, Cu 2 O/Nb 2 O 5 exhibited higher activity than that of the pristine counterpart in the photooxidation of alcohol. [248,256] As revealed by FT-IR results, the adsorption of cyclohexanone over Cu (I) was weaker than that over Nb 5+ species in the photooxidation of cyclohexanol. [256a] Meanwhile, the oxidative dehydrogenation of alcohols was realized on Nb 5+ species, indicating the accelerated catalytic recycle on Nb-O-Cu (I) interface. [248] To this end, a series of Nb 2 O 5 -based catalysts have been widely reported in the photocatalytic process. However, few studies focus on the role of the catalyst interface in photocatalysis.

Summary and Outlook
This review summarized recent advances in the synthesis and application of Nb 2 O 5 -based photocatalysts. Especially, the reaction pathways in the reduction of CO 2 , oxidation of amines, alco-hols, and hydrocarbons are related to the acidity, generated oxygen species, and functional groups on Nb 2 O 5 -based photocatalysts under light irradiation. The understanding greatly relies on the studies in the local structure of Nb 2 O 5 , which is still attractive for researchers, especially in the field of photocatalysis. The universal characterization techniques and photoelectronic properties are the cornerstones to reveal the active sites of Nb 2 O 5based photocatalysts.
In the future, Nb 2 O 5 -based photocatalysts are still the potential candidates for the conversion of waste plastics and biomass that are abundant carbon resources. The activations of sp 3 C-H bonds and C-C bonds are feasible over Nb 2 O 5 that exhibits suitable photoredox potentials under light irradiation. Although the yield and selectivity of desired products are limited in the conversion of hydrocarbon and biomass, the reactivity of Nb 2 O 5 can be further improved by the design of catalyst structure and components. Besides, the local structure and corresponding acidity strength can be in situ controlled on Nb 2 O 5 under light irradiation. As a result, it is possible to realize the hydrolysis reaction, dehydration reaction, and hydrodeoxygenation reaction under mild conditions. Using the blueprint of photocatalysis, we can improve the conversion of waste plastics and renewable biomass.
Despite the great potential of Nb 2 O 5 -based photocatalysts, the exposed challenges and issues should be considered. The roles of NbO 4 and NbO 6 units are unclear in photocatalysis, due to the lack of spatiotemporal characterization technique. Apart from that, the studies are insufficient in the interface of Nb-O-metal, which can afford the profound understanding of the adsorption-desorption process and active sites in photocatalysis. Meanwhile, the development of the reactor is conducive to the practical application of the photocatalytic process. [257] Studies in these aspects are necessary for the rational design of Nb 2 O 5based photocatalysts. Moreover, novel processes and concepts are urgently required for the large-scale production of Nb 2 O 5 -based photocatalysts.