In-Situ Synthesis of Nb 2 O 5 /g-C 3 N 4 Heterostructures as Highly Efﬁcient Photocatalysts for Molecular H 2 Evolution under Solar Illumination

: This work focuses on the synthesis of heterostructures with compatible band positions and a favourable surface area for the efﬁcient photocatalytic production of molecular hydrogen (H 2 ). In particular, 3-dimensional Nb 2 O 5 /g-C 3 N 4 heterostructures with suitable band positions and high surface area have been synthesized employing a hydrothermal method. The combination of a Nb 2 O 5 with a low charge carrier recombination rate and a g-C 3 N 4 exhibiting high visible light absorption resulted in remarkable photocatalytic activity under simulated solar irradiation in the presence of various hole scavengers (triethanolamine (TEOA) and methanol). The following aspects of the novel material have been studied systematically: the inﬂuence of different molar ratios of Nb 2 O 5 to g-C 3 N 4 on the heterostructure properties, the role of the employed hole scavengers, and the impact of the co-catalyst and the charge carrier densities affecting the band alignment. The separation/transfer efﬁciency of the photogenerated electron-hole pairs is found to increase signiﬁcantly as compared to that of pure Nb 2 O 5 and g-C 3 N 4 , respectively, with the highest molecular H 2 production of 110 mmol/g · h being obtained for 10 wt % of g-C 3 N 4 over Nb 2 O 5 as compared with that of g-C 3 N 4 (33.46 mmol/g · h) and Nb 2 O 5 (41.20 mmol/g · h). This enhanced photocatalytic activity is attributed to a sufﬁcient interfacial interaction thus favouring the fast photogeneration of electron-hole pairs at the Nb 2 O 5 /g-C 3 N 4 interface through a direct Z-scheme.


Introduction
Renewable energy sources are currently needed by our society to address the foreseable future energy crisis and growing environmental issues.The production of molecular H 2 through photoelectrochemical or photocatalytic water splitting is a viable replacement of fossil fuels [1].In the past, due to certain limitations of the most frequently employed photocatalyst, TiO 2, many other photocatalysts have been developed and explored for photocatalytic molecular H 2 evolution.Their significant limitations are the high recombination rate of photogenerated electron-hole pairs and unfavorable band edges, hence their low photocatalytic activity.For favorable band edges, CuO and Cu 2 O are thought of as good alternatives but their stability and effective light absorption present additional issues.Somehow, these issues have been solved by adopting the atomic layer deposition (ALD) technique and doping, etc. [2][3][4].The non-toxicity, facile synthesis, high visible light absorption and good physicochemical stability of g-C 3 N 4 have made it an amiable photocatalyst [5][6][7][8][9].However, the fast recombination rate of photogenerated charge carriers has reduced its charismatic effect.Some researchers proposed to solve this problem by forming heterojunctions with TiO 2 [10], WS 2 [11], BiOCl [12] and WO 3 [13,14].Still, there is considerable scope to develop other photocatalysts, with the particular focus being on facile synthesis, low toxicity, easy accessibility, and, most importantly, high photocatalytic activity [15].
Niobium pentoxide (Nb 2 O 5 ) is an n-type and wide bandgap (3.4 eV) semiconductor, which has been extensively investigated in recent years for electrochemical, photocatalytic, and energy storage applications [16,17].Its light absorption can be effectively shifted into the visible region by synthesizing composites with small bandgap materials.Noticeably, heterojunctions prepared with small amounts of Nb 2 O 5 have shown a significant improvement in the photocatalytic activity of TiO 2 [8], ZnO [18], and g-C 3 N 4 [7,9,19].To date, there are only a few reports regarding the synthesis of g-C 3 N 4 /Nb 2 O 5 heterojunctions and their photocatalytic properties.Y. Z. Hong et al. [19] and Q. Z. Huang et al. [20] have prepared g-C 3 N 4 /Nb 2 O 5 heterojunctions facing, however, considerable limitations concerning the control of the size and shape of Nb 2 O 5 , and resulting in a rather small specific surface area.Moreover, to the best of our knowledge, Nb 2 O 5 /g-C 3 N 4 heterostructures have so far not been explored for molecular H 2 evolution.
To increase photocatalytic activities, semiconductor-semiconductor heterostructures exhibiting lower recombination rates of photogenerated electron-hole pairs have been studied.Depending on the energetic situation, these heterostructures have been classified as type-II heterostructures and Z-scheme heterostructures, respectively (Figure 1; for a detailed discussion see the Supporting Information) [20][21][22].The direct Z-scheme system seems promising for overcoming the limitations associated with enhanced photocatalytic activity, due to the strong oxidation and reduction potential developed at different active sites [21].
As depicted in Figure 1, a conventional type-II heterostructure can easily be converted into a Z-scheme structure by controlling the Fermi level or the band potentials.Moreover, the Fermi level and band potentials can also be modulated to attain the Z-scheme by the addition of suitable hole scavengers such as triethanolamine (TEOA), which has been reported to exhibit a larger H 2 evolution rate than methanol [19,22].
Catalysts 2019, 9, 169 2 of 16 additional issues.Somehow, these issues have been solved by adopting the atomic layer deposition (ALD) technique and doping, etc. [2][3][4].The non-toxicity, facile synthesis, high visible light absorption and good physicochemical stability of g-C3N4 have made it an amiable photocatalyst [5][6][7][8][9].However, the fast recombination rate of photogenerated charge carriers has reduced its charismatic effect.Some researchers proposed to solve this problem by forming heterojunctions with TiO2 [10], WS2 [11], BiOCl [12] and WO3 [13,14].Still, there is considerable scope to develop other photocatalysts, with the particular focus being on facile synthesis, low toxicity, easy accessibility, and, most importantly, high photocatalytic activity [15].Niobium pentoxide (Nb2O5) is an n-type and wide bandgap (3.4 eV) semiconductor, which has been extensively investigated in recent years for electrochemical, photocatalytic, and energy storage applications [16,17].Its light absorption can be effectively shifted into the visible region by synthesizing composites with small bandgap materials.Noticeably, heterojunctions prepared with small amounts of Nb2O5 have shown a significant improvement in the photocatalytic activity of TiO2 [8], ZnO [18], and g-C3N4 [7,9,19].To date, there are only a few reports regarding the synthesis of g-C3N4/Nb2O5 heterojunctions and their photocatalytic properties.Y. Z. Hong et al. [19] and Q. Z. Huang et al. [20] have prepared g-C3N4/Nb2O5 heterojunctions facing, however, considerable limitations concerning the control of the size and shape of Nb2O5, and resulting in a rather small specific surface area.Moreover, to the best of our knowledge, Nb2O5/g-C3N4 heterostructures have so far not been explored for molecular H2 evolution.
To increase photocatalytic activities, semiconductor-semiconductor heterostructures exhibiting lower recombination rates of photogenerated electron-hole pairs have been studied.Depending on the energetic situation, these heterostructures have been classified as type-II heterostructures and Zscheme heterostructures, respectively (Figure 1; for a detailed discussion see the Supporting Information) [20][21][22].The direct Z-scheme system seems promising for overcoming the limitations associated with enhanced photocatalytic activity, due to the strong oxidation and reduction potential developed at different active sites [21].
As depicted in Figure 1, a conventional type-II heterostructure can easily be converted into a Zscheme structure by controlling the Fermi level or the band potentials.Moreover, the Fermi level and band potentials can also be modulated to attain the Z-scheme by the addition of suitable hole scavengers such as triethanolamine (TEOA), which has been reported to exhibit a larger H2 evolution rate than methanol [19,22].In the present work, Nb2O5/g-C3N4 Z-scheme heterojunctions were prepared considering their suitable band edges for photocatalytic H2 production, i.e., Nb2O5 (ECB = −0.69V, EVB = 2.32 V) and g-C3N4 (ECB = −1.68V, EVB = 0.88 V) vs NHE (pH = 7).In comparison to previous reports, we obtained a controlled shape of the heterojunction with a specific surface area as high as 227 m 2 g −1 .The prepared heterojunctions were tested for H2 evolution without and with deposited platinum (Pt) acting as a cocatalyst, thus faciliating the interfacial electron transfer.The role of hole scavengers in the overall mechanism of the hydrogen evolution reaction was also investigated.A significant increase in the H2 evolution rate was observed in the presence of TEOA compared with methanol.These findings have suggested a possible change in the photocatalytic mechanism to increase the evolution rate to such an extent.An excellent H2 production rate of 110 mmol/h•g was found by employing platinized In the present work, Nb 2 O 5 /g-C 3 N 4 Z-scheme heterojunctions were prepared considering their suitable band edges for photocatalytic H 2 production, i.e., Nb 2 O 5 (E CB = −0.69V, E VB = 2.32 V) and g-C 3 N 4 (E CB = −1.68V, E VB = 0.88 V) vs. NHE (pH = 7).In comparison to previous reports, we obtained a controlled shape of the heterojunction with a specific surface area as high as 227 m 2 g −1 .The prepared heterojunctions were tested for H 2 evolution without and with deposited platinum (Pt) acting as a cocatalyst, thus faciliating the interfacial electron transfer.The role of hole scavengers in the overall mechanism of the hydrogen evolution reaction was also investigated.A significant increase in the H 2 evolution rate was observed in the presence of TEOA compared with methanol.These findings have suggested a possible change in the photocatalytic mechanism to increase the evolution rate to such an extent.An excellent H 2 production rate of 110 mmol/h•g was found by employing platinized Nb 2 O 5 /g-C 3 N 4 as the photocatalyst and TEOA as the hole scavenger.Noticeably, the high molecular H 2 rate could also be associated with the tuning of shape, size and structures.The compact interfacial development between Nb 2 O 5 /g-C 3 N 4 also suggested that a Z-scheme system was formed, thus facilitating fast charge carrier separation and excellent photocatalytic performance as compared with simple physical mixing.The effect of different ratios of g-C 3 N 4 to Nb 2 O 5 has also been studied in detail.

Synthesis Procedure
Nb 2 O 5 (NBO) and Nb 2 O 5 /g-C 3 N 4 (NBCN) heterostructures were synthesized via a hydrothermal synthesis considering its possible principal advantages such as: (a) attaining porous structures with high surface areas; (b) reagents mixing at the atomic level, and (c) high reaction rates at a low reaction temperature due to the atomic mixing level [23][24][25].Thus, a suitable strategy to tune the shape, size, and structure of Nb 2 O 5 (NBO) and Nb 2 O 5 /g-C 3 N 4 (NBCN) heterostructures with a high specific surface area and a sufficient contact interface was employed here.
NBO was synthesized via the oxidant-peroxo method (OPM).A niobium salt (NbCl 5 ) was dissolved in diluted nitric acid (HNO 3 ) to avoid salt residuals.In a second step H 2 O 2 was added to the prepared solution to remove chloride ions by an oxidation-reduction process.The resulting yellow solution (pH = 0.5) indicated the presence of the water-soluble niobium peroxo-complex [Nb(O 2 ) 4 ] 3− (named as NPC).A possible reaction is provided in  Nb2O5/g-C3N4 as the photocatalyst and TEOA as the hole scavenger.Noticeably, the high molecular H2 rate could also be associated with the tuning of shape, size and structures.The compact interfacial development between Nb2O5/g-C3N4 also suggested that a Z-scheme system was formed, thus facilitating fast charge carrier separation and excellent photocatalytic performance as compared with simple physical mixing.The effect of different ratios of g-C3N4 to Nb2O5 has also been studied in detail.

Synthesis Procedure
Nb2O5 (NBO) and Nb2O5/g-C3N4 (NBCN) heterostructures were synthesized via a hydrothermal synthesis considering its possible principal advantages such as: (a) attaining porous structures with high surface areas; (b) reagents mixing at the atomic level, and (c) high reaction rates at a low reaction temperature due to the atomic mixing level [23][24][25].Thus, a suitable strategy to tune the shape, size, and structure of Nb2O5 (NBO) and Nb2O5/g-C3N4 (NBCN) heterostructures with a high specific surface area and a sufficient contact interface was employed here.
NBO was synthesized via the oxidant-peroxo method (OPM).A niobium salt (NbCl5) was dissolved in diluted nitric acid (HNO3) to avoid salt residuals.In a second step H2O2 was added to the prepared solution to remove chloride ions by an oxidation-reduction process.The resulting yellow solution (pH = 0.5) indicated the presence of the water-soluble niobium peroxo-complex [Nb(O2)4] 3− (named as NPC).A possible reaction is provided in Figure 2. The decomposition of H2O2 into molecular oxygen possibly accelerated the condensation reaction.Amorphous hierarchical spheres of Nb2O5 and Nb2O5•nH2O were obtained due to an excess amount of H2O2.Finally, annealing in the 200-500 °C range resulted in the formation of Nb2O5 with adequate composition and controlled morphology.For the preparation of the heterostructures, a g-C3N4 (GCN) suspension was prepared in deionised water by 1 h of continuous stirring.The prepared suspension was added to the solution of NPC (as described above) which changed the pH from 0.5 to 0.7.The pH change resulted in a positive surface charge on GCN.The developed electrostatic attraction between NPC and GCN favors the effective in-situ formation of heterojunctions with a controlled morphology [26].This procedure was employed to prepare other heterostructures with a different wt % of GCN (as described in the experimental sections).Heterostructures have also been prepared by physical mixing of NBO and g- For the preparation of the heterostructures, a g-C 3 N 4 (GCN) suspension was prepared in de-ionised water by 1 h of continuous stirring.The prepared suspension was added to the solution of NPC (as described above) which changed the pH from 0.5 to 0.7.The pH change resulted in a positive surface charge on GCN.The developed electrostatic attraction between NPC and GCN favors the effective in-situ formation of heterojunctions with a controlled morphology [26].This procedure was employed to prepare other heterostructures with a different wt % of GCN (as described in the experimental sections).Heterostructures have also been prepared by physical mixing of NBO and g-C 3 N 4 (the method is described in the supporting information).A schematic presentation of the synthetic procedure is given in Figure 3. C3N4 (the method is described in the supporting information).A schematic presentation of the synthetic procedure is given in Figure 3.The phases and the crystal structures of as-prepared NBO, GCN, and NBCN-X were analyzed by XRD (Figure 4).The XRD data obtained for a NBO sample before annealing corresponded to a mixture of niobic acid and amorphous NBO.By annealing at 200 °C the mixture converted to the

Structural and Morphological Characterization
The phases and the crystal structures of as-prepared NBO, GCN, and NBCN-X were analyzed by XRD (Figure 4).The XRD data obtained for a NBO sample before annealing corresponded to a mixture of niobic acid and amorphous NBO.By annealing at 200 • C the mixture converted to the amorphous pseudohexagonal phase of Nb 2 O 5 (TT-NBO, JCPDS#30-0873).No significant change in the phase was observed for samples annealed at temperatures between 200 • C and 400 • C.However, annealing at 500 • C results in the transformation of the amorphous pseudohexagonal phase of NBO in to the pseudohexagonal phase (JCPDS#30-0873) of NBO.The corresponding XRD results are provided in the supporting information in Figure S1.In the XRD of GCN, two peaks are observed at 13.4 • and 27.0 • , which were associated with the (100) and (002) diffraction planes, respectively.The (100) distinct peak was due to the characteristic inter-planar staking of aromatic systems, while the (002) peak corresponded to the inter-layer structural packing [26].The XRDs of NBCN-X (X = 1-4) exhibited combinatory characteristic peaks of GCN and NBO.The characteristic peak of GCN (002) become stronger with an increasing amount of GCN, indicating a significant coupling between the GCN and NBO.The phases and the crystal structures of as-prepared NBO, GCN, and NBCN-X were analyzed by XRD (Figure 4).The XRD data obtained for a NBO sample before annealing corresponded to a mixture of niobic acid and amorphous NBO.By annealing at 200 °C the mixture converted to the amorphous pseudohexagonal phase of Nb2O5 (TT-NBO, JCPDS#30-0873).No significant change in the phase was observed for samples annealed at temperatures between 200 °C and 400 °C.However, The FTIR patterns further confirmed the development of compact heterostructure interfaces, as shown in Figure 5a.The amorphous pseudohexagonal phase of NBO possessed slightly distorted NbO 6 , NbO 7 , and NbO 8 groups and some highly distorted NbO 6 sites (a scheme is provided in Figure 5b).The peak at 665 cm −1 has been assigned to the symmetric stretching mode of Nb-O polyhedra (NbO 6 , NbO 7, and NbO 8 ) [27].The broad shoulder between 850 and 970 cm −1 has been assigned to the stretching of Nb=O groups.The removal of coordinated water further distorted the highly distorted NbO 6 octahedra, which led to the formation of Nb=O bonds [28].The peaks at 414 cm −1 and 459 cm −1 have been attributed to the symmetric stretching v s [Nb(O) 2 ] and the asymmetric stretching v a [Nb(O) 2 ], respectively.Their occurrence indicated the presence of a small amount of coordinated peroxide on the Nb(V) ions [29].With increasing calcination temperatures, the peak at 459 cm −1 first decreased from 200-400 • C and then disappeared entirely at 500 • C. The peaks at 3753 cm −1 and 1559 cm −1 have been attributed to the vibration of OH groups v(O-H) of adsorbed water molecules [30], which disappeared after increasing the calcination temperature (as shown in Figure S2).

Structural and Morphological Characterization
The FTIR spectrum of GCN shows numerous bands in the 1100-1630 cm −1 region corresponding to the typical stretching modes of GCN heterocycles.The FTIR spectra of the heterostructures NBCN-X(X = 1-4) exhibited characteristic peaks of NBO and GCN.However, for NBCN-1 and NBCN-2 the GCN band's peaks were found to be weaker than NBCN-3 and NBCN-4, due to the lower amount of GCN incorporated in the heterostructure.The results were consistent with the XRD results.Moreover, the sharp band at 810 cm −1 was associated with the tri-s-triazine forming the GCN structure.The broad band around 3163 cm −1 could be associated with the N-H or O-H bonds of the residual amino groups or absorbed H 2 O molecules [31,32].Detailed FTIR spectra are also provided in Table S1.
provided in the supporting information in Figure S1.In the XRD of GCN, two peaks are observed at 13.4° and 27.0°, which were associated with the (100) and (002) diffraction planes, respectively.The (100) distinct peak was due to the characteristic inter-planar staking of aromatic systems, while the (002) peak corresponded to the inter-layer structural packing [26].The XRDs of NBCN-X (X = 1-4) exhibited combinatory characteristic peaks of GCN and NBO.The characteristic peak of GCN (002) become stronger with an increasing amount of GCN, indicating a significant coupling between the GCN and NBO.The FTIR patterns further confirmed the development of compact heterostructure interfaces, as shown in Figure 5a.The amorphous pseudohexagonal phase of NBO possessed slightly distorted NbO6, NbO7, and NbO8 groups and some highly distorted NbO6 sites (a scheme is provided in Figure 5b).The peak at 665 cm −1 has been assigned to the symmetric stretching mode of Nb-O polyhedra (NbO6, NbO7, and NbO8) [27].The broad shoulder between 850 and 970 cm −1 has been assigned to the stretching of Nb=O groups.The removal of coordinated water further distorted the highly distorted NbO6 octahedra, which led to the formation of Nb=O bonds [28].The peaks at 414 cm −1 and 459 cm −1 have been attributed to the symmetric stretching vs[Nb(O)2] and the asymmetric stretching va[Nb(O)2], respectively.Their occurrence indicated the presence of a small amount of coordinated peroxide on the Nb(V) ions [29].With increasing calcination temperatures, the peak at 459 cm −1 first decreased from 200-400 °C and then disappeared entirely at 500 °C.The peaks at 3753 cm −1 and 1559 cm −1 have been attributed to the vibration of OH groups v(O-H) of adsorbed water molecules [30], which disappeared after increasing the calcination temperature (as shown in Figure S2).
The FTIR spectrum of GCN shows numerous bands in the 1100-1630 cm −1 region corresponding to the typical stretching modes of GCN heterocycles.The FTIR spectra of the heterostructures NBCN-X(X = 1-4) exhibited characteristic peaks of NBO and GCN.However, for NBCN-1 and NBCN-2 the GCN band's peaks were found to be weaker than NBCN-3 and NBCN-4, due to the lower amount of GCN incorporated in the heterostructure.The results were consistent with the XRD results.Moreover, the sharp band at 810 cm −1 was associated with the tri-s-triazine forming the GCN structure.The broad band around 3163 cm −1 could be associated with the N-H or O-H bonds of the residual amino groups or absorbed H2O molecules [31,32].Detailed FTIR spectra are also provided in Table S1.SEM and TEM images of as-synthesized photocatalysts are shown in Figure 6.The bulk of the GCN and the hierarchical nanospheres of NBO can be seen in Figure 6a,b, respectively, while the NBCN heterostructures are given in Figure 6c-f.A significant change in the morphology can be observed for the NBO and NBCN heterostructures.BET analysis helped to further analyze the change in the surface area of bare and heterostructure photocatalysts.The high surface area of NBO-BA gradually decreased with increasing the annealing temperature from 200 to 500 • C (see Table S2).A high annealing temperature favored pore coalescence due to the crystallisation of walls separating mesopores in their structures.The BET surface areas of NBCN-X (X = 1-4) annealed at 200 • C and are provided in Table S3.The specific surface area decreased gradually with the increase in the amount of GCN, due to the low surface area of GCN.
Catalysts 2019, 9, 169 6 of 16 SEM and TEM images of as-synthesized photocatalysts are shown in Figure 6.The bulk of the GCN and the hierarchical nanospheres of NBO can be seen in Figure 6a and b, respectively, while the NBCN heterostructures are given in Figure 6c-f.A significant change in the morphology can be observed for the NBO and NBCN heterostructures.BET analysis helped to further analyze the change in the surface area of bare and heterostructure photocatalysts.The high surface area of NBO-BA gradually decreased with increasing the annealing temperature from 200 to 500 °C (see Table S2).A high annealing temperature favored pore coalescence due to the crystallisation of walls separating mesopores in their structures.The BET surface areas of NBCN-X (X = 1-4) annealed at 200 °C and are provided in Table S3.The specific surface area decreased gradually with the increase in the amount of GCN, due to the low surface area of GCN.A TEM analysis for NBO and NBCN-2 was conducted, as shown in Figure 7.For NBO (Figure 7a), the lattice fringes had 0.39 nm d-spacing's corresponding to the (001) lattice plane of Nb 2 O 5 .A small number of lattice fringes were observed due to the amorphous nature of NBO.The SAED pattern (Figure 7a inset) indicated that the NBO was in the transforming state from the amorphous to the crystalline phase (consistent with XRD results).More TEM images at different resolutions are provided in Figure S3.For NBCN-2 (Figure 7b), lattice fringes for NBO and GCN were observed.However, due to a smaller amount of GCN, only a few lattice fringes for GCN were observed (0.31 nm d-spacing of (002) plane).The (001) plane of NBO was found to be compatible with the (002) plane of GCN, thus favoring the in-situ growth of NBO on the surface of GCN.A TEM analysis for NBO and NBCN-2 was conducted, as shown in Figure 7.For NBO (Figure 7a), the lattice fringes had 0.39 nm d-spacing's corresponding to the (001) lattice plane of Nb2O5.A small number of lattice fringes were observed due to the amorphous nature of NBO.The SAED pattern (Figure 7a inset) indicated that the NBO was in the transforming state from the amorphous to the crystalline phase (consistent with XRD results).More TEM images at different resolutions are provided in Figure S3.For NBCN-2 (Figure 7b), lattice fringes for NBO and GCN were observed.However, due to a smaller amount of GCN, only a few lattice fringes for GCN were observed (0.31

Photocatalytic Evolution of Molecular H 2
A systematic study was conducted to signify the role of all possible parameters that could change the rate of generated molecular hydrogen (H 2 ).Initially, the photocatalytic formations of molecular H 2 for P25, GCN, NBO and NBCN-X (X = 1-4) were studied with and without Pt deposition in the presence of TEOA (a hole scavenger).The evolved H 2 is shown in Figure 8a,b, respectively.The total liberated amount in 7 h is also shown in Figure 8c.All heterostructures prepared generated more molecular H 2 than P25, NBO or GCN, both with and without Pt deposition.This increase in the H 2 formation rate with Pt deposition can be taken as evidence for the successful interfacial charge separation.Among all heterostructures, NBCN-2 showed the highest H 2 generation rate of 110 mmol/g•h, which could be associated with both its high surface area and suitable band positions.The evolution rates for other prepared composites: NBCN-1 = 103.27mmol/g•h, NBCN-3 = 77.88mmol/g•h, and NBCN-4 = 56.04mmol/g•h, are also higher than those for P25 = 45.46 mmol/g•h, GCN=33.46mmol/g•h or NBO= 41.20 mmol/g•h.Thus, the enhanced photocatalytic activity may have been caused by GCN loading acting as a visible light active material and as an efficient electron-hole separator for NBCN prepared heterostructures.However, this effect was only obvious provided that the optimised ratio of GCN and NBO was used.
For P25, the formation rate of molecular H 2 was increased in the first 4 h then a gradual decrease was observed.In comparison to bare materials the H 2 formation rate of P25 was larger than GCN and NBO, i.e., 45.46 mmol/g•h, but the rate was not stable after Pt deposition, as shown in Figure 8a,b, respectively.The H 2 formation rate was also smaller than all heterostructures prepared and the ~66% increase in molecular H 2 evolution rate was observed after Pt deposition, which was less than all heterostructures prepared.Moreover, the prepared composites showed more of a stable increase in the molecular H 2 formation rate than P25.The effect of methanol over platinized P25 could be studied through the Kandiel papers [33,34].mmol/g•h, which could be associated with both its high surface area and suitable band positions.The evolution rates for other prepared composites: NBCN-1 = 103.27mmol/g•h, NBCN-3 = 77.88mmol/g•h, and NBCN-4 = 56.04mmol/g•h, are also higher than those for P25 = 45.46 mmol/g•h, GCN=33.46mmol/g•h or NBO= 41.20 mmol/g•h.Thus, the enhanced photocatalytic activity may have been caused by GCN loading acting as a visible light active material and as an efficient electron-hole separator for NBCN prepared heterostructures.However, this effect was only obvious provided that the optimised ratio of GCN and NBO was used.For P25, the formation rate of molecular H2 was increased in the first 4h then a gradual decrease was observed.In comparison to bare materials the H2 formation rate of P25 was larger than GCN and NBO, i.e., 45.46 mmol/g•h, but the rate was not stable after Pt deposition, as shown in Figure 8a and b, respectively.The H2 formation rate was also smaller than all heterostructures prepared and the ~66% increase in molecular H2 evolution rate was observed after Pt deposition, which was less than all heterostructures prepared.Moreover, the prepared composites showed more of a stable increase Secondly, to understand the role of the scavenger, the NBCN-2 activity employing methanol as hole scavenger was also investigated with and without Pt deposition.The change in H 2 evolution with time and in comparison with TEOA is shown in Figure 8d.A total of 5.53 mmol/g•h and 9.3 mmol/g•h of H 2 were liberated without and with Pt deposition, respectively.This increase was just 25%, and was 60% with TEOA.Moreover, in the presence of methanol, the H 2 amount was 67% (without Pt) and 85% (with Pt) less than TEOA.In light of the above results, TEOA not only gave higher activity, but also favored interfacial charge separation after Pt deposition.The findings indicated a possible change in the photocatalytic scheme.NBCN-2 results have also been compared with the physically mixed compound (with the same composition) in the presence of methanol.The NBCN-2 photocatalyst and physically mixed photocatalyst with the same ratio of NBO and GCN were used.A drastic change in the evolution rate was observed, as shown in Figure 8d, and more clearly in Figure S4.The NBCN-2 molecular H 2 generation rate showed a 35% increase over the physically mixed photocatalyst.Thus, in-situ heterostructures have higher activity than physically mixed heterostructures, and this can be attributed to good intimate contact.The total generated amount of H 2 for all studied photocatalysts has been provided in Table S4.
The present results have also been compared with previous reports on GCN, NBO and NBCN in the presence of TEOA/Methanol.The comparison is provided in Table 1.A detailed analysis of the table has proven the significance of our work in the following ways: (a) higher molecular H 2 rate in less time, (b) less concentration of photocatalysts and (c) the synthesized heterostructures have a higher molecular H 2 rate than many of them, even without the Pt deposition.To estimate the electronic structure and possible interfacial band bending of materials, we conducted Mott-Schottky measurements (C −2 versus applied potential).The flat band/conduction band potentials (E FB /E CB ), intercepted for NBO and GCN, are −0.61V (−0.69) and −1.6 (−1.68)V vs. NHE (pH = 7), as shown in Figure 8a.The values have been converted to pH = 7 by using equations Equation (S1) and Equation (S2), respectively.By using the bandgap energy calculated through UV-vis-Absorption, the estimated valence band potentials (E VB ) are 2.32 V and 0.88 V for NBO and GCN at pH = 7, respectively (provided in Figure S5).Judging by the bandgap positions, typically a type-II mechanism should be followed, e.g., the photogenerated electrons in GCN could easily move to the NBO conduction band following the reduction there, and the photogenerated holes could easily move from the NBO valence band to the GCN valence band following the oxidation process.However, based on the recent report by Huang, Z.-F et al. [22], when TEOA is adsorbed on the surface the electron transfer is inverted as shown in Figure 1, and hence the Z-Scheme mechanism is followed.Following their results, the NBO electrons recombined with the holes of the GCN and consequently, reduction and oxidation reactions at the GCN and NBO occurred, respectively.The high production rate also favoured the direct Z-Scheme.
We estimated the charge carrier densities (N D ) to understand the interfacial band bending by using the Mott-Schottky slope (Figure 9).For a theoretical overview, the inclination angle of the M-S plot fo GCN was smaller than NBO.Since the relative dielectric constant (ε) was directly proportional to the inclination angle (θ) and inversely proportional to donor density (N D ) of the material.Hence, GCN should have a smaller dielectric constant and a higher donor density than NBO.In the literature, ε ~7-8 for GCN [32] and ε ~38 for pseudohexagonal NBO [45] has been reported.GCN ε is six times larger than NBO, so the N D for GCN should be around six times larger than NBO.Mathematically, N D has been calculated by using Equation (1) [46]: where, e 0 , ε 0 , ε and dV are electron charge, vacuum permittivity, material dielectric constant and Mott-Schottky slope, respectively.The approximated values for GCN N D ≈ 6.1 × 10 21 cm −3 and for NBO N D ≈ 7.9 × 10 20 cm −3 , have been calculated, i.e., NBO N D is seven times smaller than GCN, as expected.For the n-type, due to high donor density, the Fermi level lay close to the bottom of the conduction band.Eventually, upward band bending occurred due to the high electron density of the conduction band and low acceptor density of the valence band.On the other hand, for low donor density of n-type, the Fermi level lay close to the middle of the bandgap.Moreover, a downward band bending of the conduction band and the valence band occurred.Thus, following the concept, after irradiation, the equilibrium Fermi level (E F ) of NBO and GCN was lying close to the redox potential (−0.41 V) vs. NHE (pH = 7).However, to justify the Z-Scheme and high production rate, fast recombination at the interface and fast charge transfer at the proposed conduction and valence bands should be followed.
Electrochemical Impedance Spectroscopy (EIS) analysis helped to further estimate the recombination and charge transfer process at the working electrode and electrolyte interfaces.Nyquist plots for NBO, GCN and NBCN-2 in the frequency range of 1000 kHz-0.01Hz under UV-Vis light irradiation have been recorded (Figure 8b).The Randles circuit model [47] has been employed to describe the behavior of the electrode, as shown in the inset of Figure 8b.The arc radius has been associated with charge transfer resistance (R CT ) across the interface of working electrode/electrolyte, i.e., the small radius means efficient interfacial charge transfer/slow recombination rate of photogenerated electron/hole pairs.All three electrodes showed explicit arcs and the fitted values of R CT are: NBO = 342 kΩ, GCN = 348 kΩ and NBCN-2 = 159 kΩ.The lowest R CT for NBCN-2 has been recorded, which indicated that the composites have better efficiency of charge transfer than NBO and GCN.The NBO showed slightly better photoactivity than GCN.The large R CT for GCN indicated its poor charge transfer characteristics, which may be associated with a low charge transfer rate leading to the fast recombination of photogenerated electron/hole pairs and poor photoactivity.
Thus, there was two times reduced resistance for the NBCN-2 heterostructures than NBO and GCN, which favored the reduced charge-transfer barrier with an increase in charge carrier density, and hence the Z-scheme system is followed.The proposed energy diagram and mechanism is shown in Figure 10.The proposed mechanism has been associated with the compatible band positions of NBO and GCN, which favored the direct Z-scheme transfer of charges and thus a higher molecular H 2 production rate.That is, the 1 wt % platinizied NBCN heterostructure exhibits a high activity for H 2 generation of 110 mmol/g•h because the direct transfer and recombination of photogenerated electrons in NBO and holes in GCN could greatly extend the lifetime time of carriers.The negative shift of the flat band of NBCN heterojunction (provided in Figure S6) further confirms the proposed Z-scheme mechanism.

Methods
Niobium pentachloride NbCl 5 (99.9%,Sigma Aldrich, Munich, Germany), melamine C 3 H 6 N 6 (99.0%,Sigma Aldrich, Munich, Germany), hydrogen peroxide solution H 2 O 2 (30 wt %), nitric acid HNO 3 (60 wt %), Evonil Aeroxide TiO 2 P25, chloroplatinic acid hexahydrate H 2 PtCl 6 .6H 2 O (≥37.05%,Sigma Aldrich, Munich, Germany) and triethanolamine (HOCH 2 CH 2 ) 3 N (99.5%,Sigma Aldrich, Munich, Germany) were used without further purification.0.5 g of hygroscopic yellow powder of NbCl 5 was added into a mixture of 20 mL de-ionised water and 0.5 mL 3 .The solid dissolved immediately yielding a transparent solution.10 mL of aqueous H 2 O 2 solution was added drop-wise into a clear solution under vigorous stirring.The solution became light yellow, confirming the formation of the niobium-peroxo complex (NPC).This solution was diluted to 60 mL by further addition of de-ionised water.The prepared light-yellow solution was subesquently transferred into a 250 mL autoclave and maintained at 110 • C for 24 h in an oven.The synthesised white precipitate was centrifuged and washed several times with de-ionised water.Subsequently, the prepared powder was dried in an oven at 90 • C for 12 h (named as NBO-BA).The resultant NBO-BA samples were further annealed in a muffle furnace at different temperatures ranging from 200 • C to 500 • C for 2 h with a heating rate of 10 • C min −1 .In the present work, the photocatalyst annealed at 200 • C has been named as NBO, throughout.
3.1.2.Preparation of g-C 3 N 4 (GCN) 10 g of melamine were placed in a crucible with a cover lid on top and then annealed at 550 • C for 3 h in a muffle furnace employing a heating rate of 10 • C min −1 .After natural cooling, a yellow powder of bulk g-C 3 N 4 was obtained (named as GCN).The Nb 2 O 5 /g-C 3 N 4 heterostructures named as NBCN-X (where X = 1, 2, 3, 4) were prepared by using different amounts of GCN.Typically, 2.5 mg of NBO-BA were obtained by hydrothermal synthesis.Therefore, for the heterostructure preparations, the selected amount of GCN was added to the prepared solution of NBO-BA, i.e., X: 1 = 5 wt %, 2 = 10 wt %, 3 = 30 wt % and 4 = 60 wt %.The synthesis process was as follows: initially, GCN suspension was prepared in 20 mL of de-ionised water following the ultra-sonication for 30 min.Then, the prepared GCN suspension was added to a light-yellow solution of NPC under continuous stirring.The amount was adjusted to 60 mL by further addition of de-ionised water and stirring for 30 min.Choosing pH = 0.7 should assure the positive and negative surface charges have been developed over GCN and NPC, respectively, for favorable interface development.The prepared concentrated solution was subsequently transferred to a 250 mL autoclave and maintained at the 110 • C for 24 h in the oven.After washing and drying at 90 • C for 12 h, a light-yellow powder is obtained, which was further annealed at 200 • C for 2 h in a muffle furnace with a 10 • C min −1 ramp rate.

Photodeposition of Platinum (Pt)
The photo-deposition technique was applied to deposit 1 wt % platinum (Pt) on the samples.Hexachloroplatinic acid (H 2 PtCl 6 •6H 2 O) was used as the Pt precursor and methanol as a reducing agent.The calculated amounts of H 2 PtCl 6 •6H 2 O and the photocatalyst were added to 10 vol/vol.% of aqueous methanol solution.The resulting suspension was transferred into a closed reactor and placed under UV-light for 12 h with continuous stirring.The suspension was washed several times with de-ionised water to remove non-deposited Pt and subsequently with ethanol.The obtained Pt loaded photocatalysts were dried in an oven for 12 h at 90 • C.

Material Characterization
A Bruker (D8 Advance) instrument using Cu Kα (α = 0.15406 nm) radiation was used to record the X-ray diffraction (XRD) data of the as-synthesized samples.Scanning electron microscopy (SEM, JEOL JSM-6700F, JEOL, Tokyo, Japan) with a LEI detector (Lower Secondary Electron Image) was employed to analyze the morphologies.Transmission electron microscopy (TEM), using an FEI Tecnai G2 F20 microscope operating at 200 kV was used to characterise the samples further.Micrographs were taken in bright field (BF) and in a selected area electron diffraction mode.A Varian Cary 100 Bio was used to measure the UV-vis absorption spectra.A Bruker Vertex 80v spectrophotometer (Bruker, Billerica, MA, USA) was used to measure the FTIR spectra from 4000 to 400 cm −1 in vacuum.

Photocatalytic Molecular Hydrogen (H 2 ) Formation
The prepared photocatalysts without and with Pt deposits were used to conduct photocatalytic molecular hydrogen (H 2 ) evolution reactions.In a typical experimental run, 0.01 g of the photocatalyst were added into 45 mL of de-ionised water (maintaining pH 5.6).Methanol or TEOA were added resulting in a suspension which contains 10 vol % of a hole scavenger.The suspension inside the photoreactor was thoroughly degassed for 30 min with Ar to remove air and then stirred for 30 min in the dark to establish the adsorption equilibrium.Afterwards, the photoreactor was placed under a Xenon light source (1000 W, 1.5G) suitable to simulate solar light for 7 h.The temperature was maintained constant by using a homemade cooling system.The evolved amounts of H 2 were measured every 1 h by using a gas chromatograph (Shimadzu 8A (Shimadzu, Kyoto, Japan) equipped with a TCD detector and a 5 Å molecular sieve packed column; Ar was used as the carrier gas).

Photoelectrochemical Measurements
The photoelectrochemical (PEC) measurements were conducted by using an electrochemical workstation (CHI-660B, CH Instruments, Inc., Austin, TX, USA) accompanying a ZAHNER PECC-2 reactor and 450W xenon lamp as a light source.Moreover, a standard three-compartment cell (consisting of a photo-/working electrode (WE), a Pt wire counter electrode (CE) and an Ag/AgCl reference electrode (RE)) with 0.2 M Na 2 SO 4 electrolyte solution (pH = 5.6) were used.The working electrode was prepared using a screen-printing method and then annealed at 400 • C for 2 h to remove organic chemicals.Mott-Schottky measurements were performed at a frequency range of 10-1000 Hz with 10 mV amplitude.Electrochemical impedance spectra were obtained under irradiation at open circuit voltage over a frequency range from 1000 Hz to 0.01 Hz, with an AC voltage at 250 mV vs. Ag/AgCl reference electrode.

Conclusions
Z-scheme Nb 2 O 5 /g-C 3 N 4 heterostructures with excellent molecular H 2 production activity were prepared via in-situ hydrothermal syntheses.The prepared heterostructures exhibited excellent photocatalytic activity compared to individual g-C 3 N 4 and Nb 2 O 5 under simulated solar light illumination.The highest reported H 2 evolution rate was 110 mmol/g•h (7.7 mmol).We found that by increasing the amount of g-C 3 N 4 , the molecular H 2 production rate decreased, indicating more intimidating interface development does not favor photocatalytic reactions.However, the molecular H 2 evolution rate for all prepared heterostructures was higher than many other semiconductors reported in the literature.Moreover, we justified our results by a reduced recombination rate, high charge carrier density and complemented band positions.For future work, we suggest that the various heterojunction materials possessing diverse structural morphology exhibiting a higher photocatalytic activity be prepared by the simple methodology described here.These studies will pave the way

Figure 1 .
Figure 1.Schematic illustration of Type-II and Z-Scheme systems and their interfacial band bending under bandgap irradiation. .

Figure 1 .
Figure 1.Schematic illustration of Type-II and Z-Scheme systems and their interfacial band bending under bandgap irradiation.

Figure 2 .
The decomposition of H 2 O 2 into molecular oxygen possibly accelerated the condensation reaction.Amorphous hierarchical spheres of Nb 2 O 5 and Nb 2 O 5 •nH 2 O were obtained due to an excess amount of H 2 O 2 .Finally, annealing in the 200-500 • C range resulted in the formation of Nb 2 O 5 with adequate composition and controlled morphology.

Figure 2 .
Figure 2. The proposed reaction mechanism for Nb2O5 formation.

Figure 2 .
Figure 2. The proposed reaction mechanism for Nb 2 O 5 formation.

Figure 5 .
Figure 5. (a) FTIR spectra of NBO, GCN and NBCN-2 and (b) Highly and slightly distorted NbO6 octahedra coexist in the structure.

Table 1 .
A brief comparison of present molecular H 2 evolution rate employing g-C 3 N 4 and/or Nb 2 O 5 .

5 /g-C 3 N 4 (Physically Mixed) 851 Mesoporous Nb 2 O 5
Reaction Conditions: volume of photocatalytic solution, amount of photocatalyst and total time for H 2 evolution. a