Topological synthesis of crystalline Ag/T-Nb 2 O 5 nanobelts with enhanced solar photoelectrochemical properties for splitting water

The tetragonal phase Nb 2 O 5 (T-Nb 2 O 5 ) nanobelts with preferentially exposed the (010) facet have been successfully prepared for the first time by insitu topological reaction using the KNb 3 O 8 nanobelts as the precursor. Meanwhile, the transformation mechanism of the crystal structure and morphology from the layered KNb 3 O 8 nanobelt to the T-Nb 2 O 5 nanobelt was revealed in detail by using XRD, FE-SEM, TEM, HRTEM, SAED, UV-vis DRS, EDX, XPS, and Raman spectra. In addition, Ag deposited T-Nb 2 O 5 (Ag/T-Nb 2 O 5 ) nanobelts were prepared by photo-reduction reaction. The photoelectrochemical properties of the T-Nb 2 O 5 nanobelts and Ag/T-Nb 2 O 5 nanobelts as a photocatalyst were evaluated by splitting water under simulated sunlight. The results show that the photocurrent density generated by the prepared Ag/T-Nb 2 O 5 sample is 48.1 µ A ∙ cm − 2 , which is 2.3 times that of the T-Nb 2 O 5 sample, and the prepared tetragonal phase T-Nb 2 O 5 nanobelts produce a photocurrent value 1.5 times that of the contrast orthogonal phase O-Nb 2 O 5 nanosheets. It indicates that the (010) facet of the tetragonal phase Nb 2 O 5 is one of the highest photocatalytic active surfaces, and the visible light utilization efficiency of the T-Nb 2 O 5 nanobelt with Ag deposited on the surface has been significantly improved, which implies that it has potential applications in the field of photocatalysis.

and so on. Therefore, it has very attractive application prospects, especially using solar energy to decompose water in order to produce hydrogen, which will become a valuable and important technology for sustainable development in the future. [1,2] Due to the different nature of semiconductor photocatalysts, their photocatalytic activity is also different. The energy level structure of a semiconductor has a close relationship with its band gap (Eg). Different semiconductors have different electronic energy level structures, and their valence band (VB) and conduction band (CB) positions are also different. As a result, different semiconductor photocatalysts have different photocatalytic activities. Even the same kind of photocatalyst shows different photocatalytic activities in the process of photocatalytic reaction due to its crystal type, crystallinity, exposed crystal facet, morphology, particle size and other properties. [3][4][5] Among them, inorganic semiconductor materials are the most widely used photocatalytic active materials. In order to improve the utilization rate of solar energy and separation efficiency of photoelectrons, a lot of researches focus on the lattice structure (including lattice distortion, defect modulation, etc.), surface/interface chemistry, electronic structure, and morphological structure of semiconductor, and it has been found that these inherent properties have a great influence on the photocatalytic activity of semiconductor. [6] This is very important to design and develop high efficient semiconductor photocatalytic materials.
In recent years, niobium oxide materials have been widely used in many fields, that is, catalysis, ferroelectricity, microelectronics, light refraction, nonlinear optics, piezoelectricity, and Li-ion batteries. [7][8][9][10][11][12][13][14] Among niobium oxide materials, the most typical type is niobium pentoxide (Nb 2 O 5 ), which is an important n-type semiconductor functional material. The most common crystalline phases are hexagonal Nb 2 O 5 , orthorhombic Nb 2 O 5 , monoclinic Nb 2 O 5 , and tetragonal Nb 2 O 5 . [15][16][17] Nb 2 O 5 has excellent optical properties, electrochemical response, chemical stability, corrosion resistance, and color changes. And it is widely used in many fields of optoelectronic devices, catalysts, high-performance dye-sensitized solar cells, [18][19][20] selective oxidation of organic compounds, [21][22][23][24][25][26][27][28] optical sensors, [29][30][31] biological sensor, [32] etc. Because the photocatalytic reaction takes place on the surface of the photocatalyst, the crystal structure on the surface of the catalyst has a great influence on its photocatalytic activity. However, in terms of photocatalysis, the research of niobium pentoxide is based on direct preparation or doping with various non-metals, metals, and forming heterojunctions to study its photocatalytic activity. For the different crystal phases of Nb 2 O 5 , in addition to the orthogonal phase and the hexagonal phase, the other crystal phases have relatively little research on photocatalysis. Since it is difficult to control the exposed crystal facets of the particles, there is very little research on the photocatalytic activity of each crystal facet. In order to obtain Nb 2 O 5 photocatalysts with high photocatalytic activity, it is necessary to study its crystal phase (such as tetragonal and monoclinic phase) and exposed crystal facets. It is expected to replace TiO 2 [33,34] and provide a reference for the development of new photocatalysts.
In recent years, we have developed a new hydrothermal topological synthesis process, that is, firstly synthesizing the oxylate precursor with layered structure, then conducting acid exchange, and then preparing the oxide exposed the specific crystal facet related to the morphology of the precursor through hydrothermal topological synthesis. First, we have prepared the anatase TiO 2 nanocrystals exposed dominantly (010) facet with high photocatalytic activity from titanate nanosheets precursors, [35][36][37] and the orthorhombic Nb 2 O 5 nanosheets exposed dominantly(010) facet with high photocatalytic activity from K 4 Nb 6 O 17 ⋅4.5H 2 O nanosheets precursors under hydrothermal condition. [38] Then, the monoclinic WO 3 nanosheets exposed dominantly the (100) facet have been prepared using H 2 WO 4 disk particles as precursors by an in situ hydrothermal topological reaction. [39] In this study, we first hydrothermally synthesized niobate KNb 3 O 8 nanobelts with layered structure and uniform particle size by adding sodium oleate (C 17 H 33 CO 2 Na). Then, the tetragonal phase Nb 2 O 5 (T-Nb 2 O 5 ) nanobelt-like crystals with exposed dominantly (010) facet was successfully prepared by in situ hydrothermal topological reaction. The morphology and crystal structure evolution mechanism to Nb 2 O 5 nanobelts from KNb 3 O 8 nanobelts are introduced in detail. The photocatalytic activity of the prepared nanomaterials can be improved by doping with metal impurities. Thus, crystalline Ag deposited T-Nb 2 O 5 (Ag/T-Nb 2 O 5 ) nanobelts were prepared by the in situ photocatalytic reduced reaction between the prepared Nb 2 O 5 nanobelts and AgNO 3 aqueous solution. The photoelectrochemical properties of the obtained Ag/T-Nb 2 O 5 nanobelts are evaluated by decomposing water into H 2 under simulated sunlight. We find that the prepared Ag/T-Nb 2 O 5 nanobelts have high solar photocatalytic activity and high photoelectric conversion efficiency, indicating that it has potential application value in the preparation of photocatalysts and clean energies.

Characterization of KNb 3 O 8 samples synthesized by hydrothermal method
In order to prepare KNb 3 O 8 nanocrystalline with layered structure, whose morphology and particle size meet the F I G U R E 1 XRD patterns of the samples obtained by hydrothermal treatment at 200 • C for 24 hours at different C 17 H 33 CO 2 Na/Nb 2 O 5 molar ratio requirements of the precursor, the synthesis conditions of KNb 3 O 8 nanocrystalline prepared by hydrothermal method were investigated. Figure 1 shows the XRD patterns of the samples obtained by hydrothermal treatment at 200 • C for 24 hours at different C 17 H 33 CO 2 Na/Nb 2 O 5 molar ratio. When the molar ratio of C 17 H 33 CO 2 Na/Nb 2 O 5 is 0, there is no diffraction peak of KNb 3 O 8 in the pattern, as shown in Figure 1A, indicating that there is no crystallization of KNb 3 O 8 in the obtained sample. As the molar ratio of C 17 H 33 CO 2 Na/Nb 2 O 5 increases, it is found that KNb 3 O 8 samples with good crystallinity can be obtained, as shown in Figure 1A to 1E. As shown in Figure 1C, when the molar ratio of C 17 H 33 CO 2 Na/Nb 2 O 5 is 0.06, all of the diffraction peaks observed are exactly consistent with those of the orthorhombic phase  Figure 2 shows the FE-SEM images of the above samples. When the molar ratio of C 17 H 33 CO 2 Na/Nb 2 O 5 is 0, the sample shows irregular granules as shown in Figure 2A. When the molar ratio is 0.03, some sheet-like morphology in the sample can be observed as shown Figure 2B. When the molar ratio is 0.06, the sample displays regular belt-like morphology with a size of 4∼8 µm in length, about 400 nm in width and about 20 nm in thickness ( Figure 2C). When the molar ratio of the two is 0.12, the length of the particles is about 10 µm, the width is about 600 nm, and the thickness is about 45 nm ( Figure 2D). The size of the sample particles has obviously grown, which indicates that the addition of sodium oleate is conducive to the formation of KNb 3 O 8 crystals. But when the molar ratio is 0.24, the FE-SEM images of the sample ( Figure 2E) shows not only the belt-like morphology of KNb 3 O 8 particles, but also the flocculent morphology, which is found to be residual organic sodium oleate by EDX analysis. We found that the KNb 3 O 8 nanoparticles gradually become longer, wider and thicker as the amount of sodium oleate increases and its morphology has also changed from the original short irregular rectangular nanosheets to slightly longer and regular nanobelts, finally to the longer strip-like nanobelts (see Figure 2B-2D). The above results indicate that the addition of an appropriate amount of sodium oleate is beneficial to the formation of the precursor KNb 3 O 8 . [40][41][42] However, excessive addition of sodium oleate not only made it difficult to clean, but also made the prepared KNb 3 O 8 nanoparticles larger, thus affecting the photocatalytic performance of the prepared oxide. In summary, C 17 H 33 CO 2 Na/Nb 2 O 5 molar ratio of 0.06 is the best choice for the preparation of pure orthogonal phase KNb 3 O 8 nanobelts with particle size meeting the requirements of the precursor. The synthesis time of the precursor KNb 3 O 8 nanobelts has been greatly reduced by compared with the hydrothermal synthesis method of KNb 3 O 8 reported. [43]

Characterization of protonated niobate H 3 ONb 3 O 8 sample
Proton niobate was obtained by the ion exchange reaction between the precursor KNb 3 O 8 nanobelts and HNO 3 solution (1.2 mol⋅L -1 ). The XRD patterns of niobate before and after acid exchange are shown in Figure 3. The peak position of the XRD pattern is exactly the same as that of the orthorhombic phase H 3 ONb 3 O 8 (JCPDS No. 44-0672, Pnnm (58), Z = 4, a = 9.183 Å, b = 22.47 Å, c = 3.775 Å, α = β = γ = 90 • ). The peak shape is high and sharp, indicating good crystallinity of the sample obtained after the acid exchanges. Compared with the XRD pattern of the KNb 3 O 8 sample before acid exchange at the bottom, the diffraction peak of (020) facet shift to low angle, indicating that the interlayer spacing of the sample H 3 ONb 3 O 8 increased from 1.06 to 1.12 nm. The EDX analysis result of chemical composition of the sample H 3 ONb 3 O 8 shows that there is almost no K element, which indicates that the K + ions in the sample KNb 3 [44,45] Figure 4D and 4E correspond to the HRTEM image and SAED pattern of the orthorhombic phase H 3 ONb 3 O 8 , respectively. The lattice fringe spacings of d = 0.91 and 0.37 nm correspond to the (100) and (001) facets of the orthorhombic phase H 3 ONb 3 O 8 crystal, respectively ( Figure 4D). According to Figure 4E, it is found that the diffraction spots with d = 0.91, 0.37 and 0.35 nm correspond to the (100), (001), and (101) facets of the orthorhombic phase H 3 ONb 3 O 8 , respectively. The above results show that the long axis direction and dominant exposed surface of the H 3 ONb 3 O 8 crystal obtained by H + exchange are consistent with those of the precursor KNb 3 O 8 crystal.
To determine the chemical composition of the samples, the XPS spectra of precursor KNb 3 O 8 sample and H 3 ONb 3 O 8 sample are showed in Figure S2. The C 1 second peak from the adventitious carbon with band energy of 284.8 eV, was used as an internal standard. In the XPS spectrum of the KNb 3 O 8 nanobelts measured, the peaks from K, Nb, C, and O elements are detected and shown in Figure S2a. The high resolution XPS spectrum of Nb 3d in Figure S2b can be assigned to a binding energy of Figure C is the SAED diffraction pattern of the corresponding sample 206.84 eV for Nb 3d 5/2 and 209.58 eV for Nb 3d 3/2 , which are identified as the binding energies of Nb 5+ . [46] The two binding energy peaks of 292.56 and 295.28 eV shown in Figure S2c, corresponding to K 2p 3/2 and K 2p 1/2 , respectively, can be identified as the binding energies of K + within the KNb 3 O 8 nanobelts. [47] Figure S2d shows the XPS measurement spectrum of the protonated niobate H 3 ONb 3 O 8 sample. It is found that the two binding energy peaks of K2p 3/2 and K2p 1/2 disappeared, again proving that the component K was completely replaced. The high resolution XPS spectrum of Nb 3d of the H 3 Figure S3. In the Raman spectrum of the KNb 3 O 8 nanobelts ( Figure S3a), the peaks observed at 950.9, 920.6, and 894.1 cm -1 are attributed to stretching modes of the short Nb-O bonds. [48,49] The peaks between 656-571 cm -1 can be assigned to stretching vibrations of the longer Nb-O bonds. [50] The peak at 451.2 cm -1 is assigned to Nb-O-Nb bending modes, and the bands at 321.5 and 351.7 cm -1 can be assigned to the E mode corresponding to deformation in the NbO 6 framework. [51] This indicates that the orthorhombic phase KNb 3 O 8 has two different types of octahedral. The first one has shorter Nb-O bonds causing a sharp deformation of such NbO 6 units. The other has longer Nb-O bonds and is slightly distorted. Compared with that of the layered KNb 3 O 8 , the Raman spectrum of the H 3 ONb 3 O 8 indicates that the Nb-O terminal bond was affected by the H + -exchanged reaction, resulting in the Raman energy band at 950.9 cm -1 becoming weaker and broader and a new weak band at 965.7 cm -1 arising at the same time ( Figure S3b). The hydrogen bonds, which were formed by the H + -exchange reaction, in the layered H 3 ONb 3 O 8 crystal structure, associated with the interlayer terminal oxygen atoms of NbO 6 to form Nb-O⋅⋅⋅H bonds. This is similar to the phenomenon appearing in the previous studies. [52]

Characterization of the obtained T-Nb 2 O 5 nanobelts
The transformation temperature from H 3 ONb 3 O 8 into T-Nb 2 O 5 is related to the pH value of the reaction solution. The XRD patterns of the samples obtained by treating hydrothermally H 3 ONb 3 O 8 nanobelts suspension with different pH value are shown in Figure S4. According to the XRD analysis results of the samples obtained under different pH conditions, the temperature and pH dependences of the samples are summarized in Figure 5.  Figure 6D). The results indicate that the exposed facet of T-Nb 2 O 5 nanobelts dominantly is the (010) facet and the [001] direction is the long axis direction of the particles, which is the same as the precursor KNb 3 O 8 nanobelts.
The XPS measurement spectrum of the T-Nb 2 O 5 sample is shown in Figure S2f. The centers of the two double peaks in the high resolution XPS spectrum of Nb 3d are 207.4 eV (3d 5/2 ) and 210.14 eV (3d 3/2 ) ( Figure S2g). Both peaks are derived from the Nb 5+ state within T-Nb 2 O 5 nanobelt crystals. [54][55][56] The binding energy was reduced by 0.37 and 0.39 eV compared to the H 3 ONb 3 O 8 sample, respectively, which means that the Nb 5+ state in the sample become slightly weaker after the in-situ hydrothermal topological reaction.
For comparison, the O-Nb 2 O 5 nanosheet crystals with a regular rectangular morphology ( Figure S5) were used as a contrast in this study. The Raman spectra of T-Nb 2 O 5 sample and the contrast O-Nb 2 O 5 nanosheets are shown in Figure S6. It can be observed from Figure S6a [57] The Raman band at 261.12 cm -l probably arises from the T2u mode associated with the noncubic structures of the edge-shared octahedral. [48] Compared with the T-Nb 2 O 5 nanobelts, the Raman bands of the O-Nb 2 O 5 nanosheets became stronger, and the weak Raman band at 993.62 cm -1 disappear, and the Raman band at 626.43 cm -1 is shifted to 681.35 cm -1 , while the two bands in the low-wave-number region moved towards each other. These changes are due to their different crystal structures. [52]

Transformation mechanism of morphology and crystal structure
The evolution mechanism of the crystal structure and morphology in the process of T-Nb 2 O 5 formed by acid exchange and dehydration reactions with KNb 3 O 8 as the precursor is shown in Figure 7. There are three Nb-O octahedrons in the structural unit existing in the KNb 3 O 8 crystal structure. In which, two Nb1-O octahedrons formed by Nb1 are connected by sharing an edge, and are connected sharing angle through the two vertexes on the same side with the two angles on one side of another Nb2-O octahedron formed by Nb2 to form a stacked 3Nb structural unit. The Nb2-O octahedron in each structural unit is connected respectively by sharing angle with the Nb1-O octahedron in the other two structural units, forming a long chain along the a-axis direction. Each chain is superposed and connected by sharing angle to form a two-dimensional layered skeleton structure along the c-axis. The layers are stacked in a mirror image and staggered along the b-axis direction, and the layer space is filled by K + to form a threedimensional structure.
In the H 3 ONb 3 O 8 crystal structure formed by acid exchange, the skeleton structure of the layer is still retained, but the layer spacing is increased slightly from 2.11 to 2.25 nm due to exchanging K + into the H 3 O + between the layers. Therefore, the morphology of H 3 ONb 3 O 8 crystals is still a strip particle with a main exposed surface of the (010) and a long axis direction of the [001].
In the process of hydrothermal treating the H 3 ONb 3 O 8 nanobelts, the T-Nb 2 O 5 is formed by a topological dehydration reaction between two adjacent columns in the same layer along the b-axis direction and the interlayer H 3 O + in the crystal structure of the H 3 ONb 3 O 8 . In the T-Nb 2 O 5 crystal structure, every 16 Nb-O octahedrons are connected by common angles to form a square parallel to the (001) facet and perpendicular to the (110) facet. The four corners of these squares are connected by sharing edges together to form the two-dimensional network structure, and the nets are interlaced and superposed along the [001] direction to form the three-dimensional structure of the T-Nb 2 O 5 . In the small square structure unit formed by 16 Nb-O octahedrons, Nb has three different connection ways. One is that an Nb-O octahedron is connected sharing angles with the four Nb-O octahedrons in the same layer; the second is that an Nb-O octahedron is connected sharing angles with the three Nb-O octahedrons in the same layer; and the third is that an Nb-O octahedron is connected by the common edge with another Nb-O octahedron in the same layer. It can be found that Nb-O octahedron in the H 3 ONb 3 O 8 is mainly connected in the form sharing edges, while Nb-O octahedron in the T-Nb 2 O 5 formed after the dehydration reaction is mainly connected in the form of common angles. However, these changes mainly occurred in the plane (010), while the octahedron's connection mode in caxis direction did not change. Therefore, its morphology still maintains the shape of belt-like particles with the long axis direction of the [001] and exposing mainly the (010) plane.

Photoelectric performance of the obtained T-Nb 2 O 5 nanobelts and Ag/T-Nb 2 O 5 nanobelts
The visible light catalytic activity of the material can be remarkably improved by increasing the absorption of the material on the visible light. In order to increase the absorption of the T-Nb 2 O 5 nanobelts in the visible light range and improve its utilization of solar energy, Ag ions are in situ photo-reduced and deposited on the surface of the sample in AgNO 3 solution. Its formation mechanism is similar to that of Ag deposited on TiO 2 under irradiation of UV light in AgNO 3 solution. [58] The XPS and EDX of the Ag/T-Nb 2 O 5 sample are shown in Figure S7. Figure S7a shows the XPS measurement spectrum of the Ag/T-Nb 2 O 5 sample. The two double peaks in the Nb 3d high resolution XPS spectrum of the Ag/T-Nb 2 O 5 nanobelt are 207.33 eV (3d 5/2 ) and 210.08 eV (3d 3/2 ) respectively ( Figure S7b). Compared with the T-Nb 2 O 5 nanobelt, it has a negative shift of 0.06 eV but it still derived from the two peaks of Nb 5+ state. Two peaks located at 374.14 and 368.14 eV can be assigned to 3d 5/2 and 3d 3/2 of metallic Ag in the high resolution XPS spectrum of Ag 3d, respectively ( Figure S7c). But there is a negative shift of 0.16 eV relative to bulk Ag (368.3 eV for 3d 5/2 and 374.3 eV for 3d 3/2 ), suggesting interactions between the T-Nb 2 O 5 nanobelts and Ag nanoparticles. The Ag content in the Ag/T-Nb 2 O 5 nanobelts determined by XPS is 5.1%, which is basically consistent with the result (5.2%) of EDX analysis ( Figure S7d).
The contrast O-Nb 2 O 5 nanosheets with a regular rectangular shape was used for comparison. The visible light photoelectrochemical performance of the obtained T-Nb 2 O 5 and Ag/T-Nb 2 O 5 nanobelts as photocatalysts was evaluated by decomposing water to H 2 under simulated sunlight and measuring the accompanying photocurrent. The preliminary electrocatalytic activity of catalysts T-Nb 2 O 5 nanobelts, Ag/T-Nb 2 O 5 nanobelts and O-Nb 2 O 5 nanosheets was studied toward hydrogen evolution reaction by linear sweep voltammetry (LSV) [59] technique under the dark and the simulated sunlight (LED, 30 W, 380∼780 nm) illumination. In LSV measurement process, 0.5 mol•L -1 Na 2 SO 4 aqueous solution, Ag/AgCl reference electrode and 10 mV•s -1 scanning rate were used for hydrogen evolution. It can be seen from Figure 8A Figure 8B. Figure [38] The Kubelka-Munk function A = B•(hν − E g ) 2 /(hν) represents the relation between absorption coefficient (A) and incident photon energy (hν), where B and E g are the absorption constant and bandgap energy respectively. [60] The E g is estimated from the transformed Kubelka-Munk function versus the energy of light (inset in Figure 9) is not much different. The above results indicate that the photocatalytic activity of the T-Nb 2 O 5 is higher than that of the O-Nb 2 O 5 because the forbidden band width of the T-Nb 2 O 5 is narrower and has a larger specific surface area, which is beneficial to its absorption and utilization of sunlight. And compared with the Nb 2 O 5 samples reported in the literatures [18,27,38,61] (Table S1), the prepared T-Nb 2 O 5 sample is obviously advantage as a photocatalyst.
When Ag nanoparticles are deposited onto the surface of the Nb 2 O 5 nanobelt, a Schottky barrier forms between the Nb 2 O 5 and Ag particles because the Femi level of Ag is lower than that of Nb 2 O 5 , which drives the efrom the CB of the Nb 2 O 5 into the Ag nanoparticles. This electron transfer process can not only promote the separation of photogenerated e --h + pairs, but also inhibit the recombination. [62,63] Therefore, the deposition of Ag improves the visible photoelectric conversion efficiency of the T-Nb 2 O 5 sample.

Preparation of nanobelts KNb 3 O 8 and protonated niobate H 3 ONb 3 O 8
The preparation of KNb 3 O 8 sample using the hydrothermal method has two steps. First, 0.6 g of Nb 2 O 5 (99.9%, Aladdin America) and 20 mL of 3 mol⋅L -1 KOH (95%, Aladdin America) aqueous solution were placed in a Teflon-lined, sealed stainless-steel vessel with an inner volume of 100 mL, and then hydrothermally treated at 200 • C for 4 hours under stirring condition. In the second step, the solid sodium oleate was added to the resulting solution, and the pH of the solution was adjusted between 5 and 6 using a 3 mol⋅L -1 HCl solution under stirring condition. The obtained solution was added to the same autoclave with above, and hydrothermally treated at 200 • C for 24 hours under stirring condition to obtain the KNb 3 O 8 sample. The sample was centrifuged and washed with deionized water and ethanol, and dried at the room temperature. 0.6 g of KNb 3 O 8 sample was stirred in 100 mL 1.2 mol⋅L -1 of HNO 3 solution for 2 days to exchange K + in the layered structure with H 3 O + , and then the sample was washed with ethanol and deionized water. After the acid treatment was done twice, an H + -form layered protonated niobate H 3 ONb 3 O 8 sample was obtained.

Preparation of tetragonal crystal T-Nb 2 O 5 nanobelts
15 mL of 2 mg•mL -1 H 3 ONb 3 O 8 suspension was placed in a Teflon-lined, sealed stainless-steel vessel with an inner volume of 50 mL. The pH value of the H 3 ONb 3 O 8 suspension was adjusted to 0.5-5.0 with a 3 mol•L -1 of HCl aqueous solution, and then hydrothermally treated at a desired temperature under stirring condition. After the hydrothermal treatment, the product was centrifuged and washed with deionized water and ethanol, finally dried at the room temperature.

Preparation of the Ag/T-Nb 2 O 5 nanobelts
The crystalline Ag/T-Nb 2 O 5 nanobelts are prepared by a photo-reduction process. 20 mg of the obtained T-Nb 2 O 5 nanobelt sample was dispersed to a 4 mL 0.01 mol L −1 AgNO 3 solution, and stirred for 10 minutes under light emitting diode light illumination (China, 30 W, 6000 K). Then, the product was washed with deionized water, centrifuged, and dried at room temperature.

Physical analysis
Powder X-ray diffraction (XRD) analysis of the samples was carried out on a Rigaku D/max-2200 PC X-ray diffractometer with Cu Kα (λ = 0.15418 nm) radiation. A field-emission scanning electron microscope (FE-SEM, HitachiSU8010) with an accelerating voltage of 5 kV were used to characterize the size and morphology of the sample particles. Raman-scattering data were collected by using Raman microscope (Invia, Renishaw), from a laser operating at 532 nm, 50 times objectives. The laser power at the sample was estimated to be 1% and the nominal laser spot size was 0.5 µm. A transmission electron microscope (TEM, FEI Tecnai G2 F20) and selected-area electron diffraction (SAED) were performed by using a system at 200 kV. The solid-state UV-vis diffuse reflectance spectra (DRS) of the samples were measured on a UV-vis spectrophotometer (Shimadzu UV-3600) equipped with an integrating sphere attachment and a standard BaSO 4 plate is used as 100% reflectance standard. The chemical composition of the sample is analyzed by using in situ energy dispersive X-ray spectroscopy (EDX) equipped in the FE-SEM system and in situ energy dispersive X-ray photoelectron spectroscopy (XPS), which uses a Nexsa multifunctional X-ray photoelectron spectroscopy system and Al-Kα radiation as the excitation source. The C 1 second peak from the adventitious carbon, with band energy of 284.8 eV, was used as an internal standard. The Mott-Schottky plots were collected at 5 KHz a Zahner XPOT electrochemical analyser. The Brunauer-Emmet-Teller (BET) specific surface area was measured by N 2 adsorption and desorption on a ASAP-2460 specific surface area tester instrument (Mike, USA). The pretreatments of the obtained catalysts were degassed under vacuum at 120 • C.

Measurement of the photocatalytic performance of the samples for splitting water
Photoelectrochemical measurements of the samples were carried out on a Zahner XPOT electrochemical analyser by using a three-electrode system consisting of a fluorinedoped tin oxide (FTO) glass covered with the sample as working electrode, the platinum sheet as the counter electrode, and an Ag/AgCl electrode (KCl saturated) as reference. The three electrodes were immersed in 0.5 mol⋅L -1 of Na 2 SO 4 electrolyte solution. The 5 mg sample was dispersed in 1 mL of deionized water to form a suspension, and then 0.25 mL suspension was dropped on clean FTO glass and dried at room temperature to form a working electrode with an area of about 1 cm 2 . The distance between the working electrode and the light source was kept at 10 cm. And the working electrode was irradiated with a light emitting diode flat downlight (LED, Wolike, China, 30 W, 380-780 nm) during the measurement. A contrast orthogonal phase Nb 2 O 5 (O-Nb 2 O 5 ) nanosheets was prepared according to the method in Reference. [38] The preparation method of the contrast O-Nb 2 O 5 electrode was the same as that of the sample electrode.

A C K N O W L E D G M E N T S
We gratefully acknowledge the Natural Science Foundation of China (No. 21173003).

D E C L A R AT I O N O F I N T E R E S T S TAT E M E N T
The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available in figshare at https://doi.org/10.6084/m9. figshare.14481960, reference number [14481960].