In-Situ Synthesis of Nb₂O₅/g-C₃N₄ Heterostructures as Highly Efficient Photocatalysts for Molecular H₂ Evolution under Solar Illumination

Abstract

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 (H₂). In particular, 3-dimensional Nb₂O₅/g-C₃N₄ heterostructures with suitable band positions and high surface area have been synthesized employing a hydrothermal method. The combination of a Nb₂O₅ with a low charge carrier recombination rate and a g-C₃N₄ 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 Nb₂O₅ to g-C₃N₄ 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 Nb₂O₅ and g-C₃N₄, respectively, with the highest molecular H₂ production of 110 mmol/g·h being obtained for 10 wt % of g-C₃N₄ over Nb₂O₅ as compared with that of g-C₃N₄ (33.46 mmol/g·h) and Nb₂O₅ (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 Nb₂O₅/g-C₃N₄ interface through a direct Z-scheme.

1. 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 H₂ 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 H₂ 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 Cu₂O 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-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 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 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 (E_CB = −0.69 V, E_VB = 2.32 V) and g-C₃N₄ (E_CB = −1.68 V, 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 m² 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 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.

2. Results and Discussion

2.1. 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 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-C₃N₄ (the method is described in the supporting information). A schematic presentation of the synthetic procedure is given in Figure 3.

2.2. 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 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, 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 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, NbO_7, 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 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).

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.

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 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.

2.3. Photocatalytic Evolution of Molecular H₂

A systematic study was conducted to signify the role of all possible parameters that could change the rate of generated molecular hydrogen (H₂). Initially, the photocatalytic formations of molecular H₂ 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 H₂ 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 H₂ than P25, NBO or GCN, both with and without Pt deposition. This increase in the H₂ formation rate with Pt deposition can be taken as evidence for the successful interfacial charge separation. Among all heterostructures, NBCN-2 showed the highest H₂ 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 = 103.27 mmol/g·h, NBCN-3 = 77.88 mmol/g·h, and NBCN-4 = 56.04 mmol/g·h, are also higher than those for P25 = 45.46 mmol/g·h, GCN=33.46 mmol/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 4 h 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,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 in the molecular H₂ formation rate than P25. The effect of methanol over platinized P25 could be studied through the Kandiel papers [33,34].

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 H₂ 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 H₂ 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 H₂ 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 H₂ 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 H₂ 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 brief comparison of present molecular H₂ evolution rate employing g-C₃N₄ and/or Nb₂O₅.

Photocatalysts	Co-Catalyst	Scavenger	Reaction Conditions a	Light Source	H2 (µmol/h/g)	Ref
Nb2O5	Pt (1.0 wt %)	TEOA (10 Vol. %)	45 mL, 10 mg (7 h)	Simulated Solar Lamp	41,200	Present
g-C3N4					33,460
P25					45,460
Nb2O5/g-C3N4 (Best Composite)					110,000
Cu2O/g-C3N4-Mechanical Composite	Pt (1.0 wt %)	TEOA (10 Vol. %)	180 mL, 100 mg (11 h)	Visible Light	142	[35]
Cu2O (0.05 wt %)-g-C3N4-In-Situ Composite					241
g-C3N4					142
Pt-g-C3N4-TiO2	Pt (not defined)	TEOA (10 Vol. %)	100 mL, 100 mg	Visible Light (420 nm)	1240	[36]
g-C3N4-Pt-TiO2					1780
Urea-polymerized-g-C3N4	Pt (3.0 wt %)	TEA (10 Vol. %)	100 mL, 80 mg (8 h)	Simulated Solar Lamp	590	[37]
		nil	100 mL, 80 mg (25 h)		3.13
Zn-tri-PcNc sensitized g-C3N4	Pt (1.0 wt %)	TEOA (10 Vol. %)	10 mL H2O + 50 mL Ascorbic Acid, 10 mg (10 h)	Infrared 500 nm	12,500	[38]
g-C3N4	Pt (2.0 wt %)	TEOA (10 Vol. %)	100 mL, 100 mg	Visible Light (420 nm)	480	[39]
NiS/g-C3N4	NiS (1.5 mol %)				450
g-C3N4	Pt (3.0 wt %)	TEOA (10 Vol. %) + K2HPO4	270 mL, 50 mg	Visible Light (420 nm)	18,940	[40]
Nb2O5/g-C3N4 (Best Composite)	--	Methanol (10 Vol. %)	45 mL, 10 mg (7 h)	Simulated Solar Lamp	5530	Present
	Pt (1.0 wt %)				9322
Nb2O5/g-C3N4 (Physically Mixed)					851
Mesoporous Nb2O5	Pt (0.5 wt %)	Methanol (12.5 Vol. %)	400 mL, 10 mg (7 h)	Mercury Lamp (200–600 nm)	12,350	[41]
Mesoporous Nb2O5 nano particles	Pt (1.0 wt %)	Methanol (9.1 Vol. %)	220 mL, 20 mg (4 h)	Mercury Lamp Internal Irradiation	191	[42]
Mesoporous Nb2O5	--	Methanol (1M) + H2SO4 (1M)	200 mL, 200 mg (6 h)	White Light Source	1120	[43]
	Pt (2.0 wt %)				510
	CuO				1405
	NiO				800
Mesoporous Nb2O5	--	MeOH: H2O = (1:5)	550 mL, 200 mg (6 h)	UV Light (360 nm) Internal Irradiation	328	[44]
	Au (1.0 wt %)				2091
	Pt (1.0 wt %)				4647
	Cu (1.0 wt %)				1572
	NiO (1.0 wt %)				709
	C				8.5

^a Reaction Conditions: volume of photocatalytic solution, amount of photocatalyst and total time for H₂ evolution.

. A detailed analysis of the table has proven the significance of our work in the following ways: (a) higher molecular H₂ rate in less time, (b) less concentration of photocatalysts and (c) the synthesized heterostructures have a higher molecular H₂ 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 −0.61 V (−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]:

$${\mathrm{N}}_{\mathrm{D}}=\frac{2}{e_0{\epsilon }_0\epsilon }{\left[\frac{d\left(1/C^2\right)}{dV}\right]}^{-1}$$

(1)

where, e₀, ε₀, ε and $\frac{d\left(1/C^2\right)}{dV}$ are electron charge, vacuum permittivity, material dielectric constant and Mott–Schottky slope, respectively. The approximated values for GCN N_D ≈ 6.1 × 10²¹ cm⁻³ and for NBO N_D ≈ 7.9 × 10²⁰ 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–0.01 Hz 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 H₂ production rate. That is, the 1 wt % platinizied NBCN heterostructure exhibits a high activity for H₂ 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.

3. Methods

Niobium pentachloride NbCl₅ (99.9%, Sigma Aldrich, Munich, Germany), melamine C₃H₆N₆ (99.0%, Sigma Aldrich, Munich, Germany), hydrogen peroxide solution H₂O₂ (30 wt %), nitric acid HNO₃ (60 wt %), Evonil Aeroxide TiO₂ P25, chloroplatinic acid hexahydrate H₂PtCl₆.6H₂O (≥37.05%, Sigma Aldrich, Munich, Germany) and triethanolamine (HOCH₂CH₂)₃N (99.5%, Sigma Aldrich, Munich, Germany) were used without further purification.

3.1. Material Synthesis

3.1.1. Preparation of Nb₂O₅ (NBO)

0.5 g of hygroscopic yellow powder of NbCl₅ was added into a mixture of 20 mL de-ionised water and 0.5 mL HNO₃. The solid dissolved immediately yielding a transparent solution. 10 mL of aqueous H₂O₂ 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-C₃N₄ (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-C₃N₄ was obtained (named as GCN).

3.1.3. Preparation of Nb₂O₅/g-C₃N₄ (NBCN)

The Nb₂O₅/g-C₃N₄ 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.

3.2. Photodeposition of Platinum (Pt)

The photo-deposition technique was applied to deposit 1 wt % platinum (Pt) on the samples. Hexachloroplatinic acid (H₂PtCl₆·6H₂O) was used as the Pt precursor and methanol as a reducing agent. The calculated amounts of H₂PtCl₆·6H₂O 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.

3.3. 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.

3.4. Photocatalytic Molecular Hydrogen (H₂) Formation

The prepared photocatalysts without and with Pt deposits were used to conduct photocatalytic molecular hydrogen (H₂) 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 H₂ 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).

3.5. 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 Na₂SO₄ 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.

4. Conclusions

Z-scheme Nb₂O₅/g-C₃N₄ heterostructures with excellent molecular H₂ production activity were prepared via in-situ hydrothermal syntheses. The prepared heterostructures exhibited excellent photocatalytic activity compared to individual g-C₃N₄ and Nb₂O₅ under simulated solar light illumination. The highest reported H₂ evolution rate was 110 mmol/g·h (7.7 mmol). We found that by increasing the amount of g-C₃N₄, the molecular H₂ production rate decreased, indicating more intimidating interface development does not favor photocatalytic reactions. However, the molecular H₂ 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 for a new dimension in photocatalytic studies of Nb₂O₅ and g-C₃N₄ nanocomposites for enhanced molecular H₂ production.