In situ constructed oxygen-vacancy-rich MoO_3−x/porous g-C₃N₄ heterojunction for synergistically enhanced photocatalytic H₂ evolution

Abstract

A simple method was developed for enhanced synergistic photocatalytic hydrogen evolution by in situ constructing of oxygen-vacancy-rich MoO_3−x/porous g-C₃N₄ heterojunctions. Introduction of a MoO_3−x precursor (Mo(OH)₆) solution into g-C₃N₄ nanosheets helped to form a porous structure, and nano-sized oxygen-vacancy-rich MoO_3−x in situ grew and formed a heterojunction with g-C₃N₄, favorable for charge separation and photocatalytic hydrogen evolution (HER). Optimizing the content of the MoO_3−x precursor in the composite leads to a maximum photocatalytic H₂ evolution rate of 4694.3 μmol g⁻¹ h⁻¹, which is approximately 4 times higher of that of pure g-C₃N₄ (1220.1 μmol g⁻¹ h⁻¹). The presence of oxygen vacancies (OVs) could give rise to electron-rich metal sites. High porosity induced more active sites on the pores' edges. Both synergistically enhanced the photocatalytic HER performance. Our study not only presented a facile method to form nano-sized heterojunctions, but also to introduce more active sites by high porosity and efficient charge separation from OVs.

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1. Introduction

Photocatalytic hydrogen evolution (HER) is considered to be one of the most promising ways to alleviate the environment and energy crisis.^1–6 However, exploring highly efficient and environmentally-friendly catalysts remains a challenge for the current photocatalysis field.⁷ The efficiency of photocatalytic HER lies in three main aspects: (i) the generation and separation of photo-generated charges, (ii) the migration distance of carriers, and (iii) the oxidation–reduction reaction on the surface of photocatalysts.⁸ Various strategies have been proposed to improve the photocatalytic efficiency, with a broad range of photocatalysts.⁷

The prevailing photocatalysts include metal oxides,⁹ metal sulfides,¹⁰ metal nitrides,¹¹ organometallic complexes,¹² and metal-free semiconductors.^13–17 Among the various photocatalysts, g-C₃N₄, a metal-free semiconductor, has attracted extensive attention in photocatalytic HER due to its outstanding characteristics, such as appropriate band edge, environmental friendliness, high thermal and chemical stability, facile fabrication and cost-effectiveness.^18,19 However, the photocatalytic HER efficiency of g-C₃N₄ is still far from satisfactory, due to the low specific surface area, limited active sites and fast recombination rate of photogenerated electron–hole (e⁻–h⁺) charges.²⁰ g-C₃N₄ nanosheets were reported previously with better performance than bulk g-C₃N₄ with increased surface area, higher photogenerated electron reduction potential, better electron transport capacity and longer lifetime etc. Our strategy has designed a more efficient nano-junction based on g-C₃N₄ nanosheets.

Based on the review, various strategies have been proposed based on g-C₃N₄, including electronic structure modulation, crystal structure engineering, nanostructure and hetero-structure construction.^8,21–25 The construction of g-C₃N₄-based heterojunction can help the separation of charges at the interface, as a variety of g-C₃N₄-based heterojunctions has been reported, such as TiO₂/g-C₃N₄,^26,27 ZnO/g-C₃N₄,^28,29 WO₃/g-C₃N₄,³⁰ WS₂/g-C₃N₄,³¹ NiO/g-C₃N₄,³² ZnIn₂S₄/g-C₃N₄,³³ Zn_xCd_1−xIn₂S₄/g-C₃N₄,³⁴ Bi₂Se₃/g-C₃N₄,³⁵ MoO₃/1T-MoS₂/g-C₃N₄,³⁶ etc. MoO₃ has a large band gap of 3 eV and a high dielectric constant of 6–18, suitable for constructing heterojunction photocatalysis with g-C₃N₄.^37–39 Oxygen-vacancy-rich MoO_3−x was even more favourable. The synthesis of porous g-C₃N₄ and few-layered MoO₃ has been widely reported, however, the combination of porous g-C₃N₄ and oxygen-vacancy-rich MoO_3−x to construct MoO_3−x/g-C₃N₄ photocatalytic system remains a great challenge. The introduction of porous structure in g-C₃N₄ usually produces many photocatalytic active sites.⁴⁰ Xiao et al. introduced a bottom-up method for preparing porous few-layer g-C₃N₄ by a sequential molecule self-assembly, alcohol molecules intercalation, thermal-induced exfoliation and polycondensation process.⁴⁰ Moreover, conventional silicon dioxide (SiO₂) template or ammonium bicarbonate (NH₄HCO₃) has also been used as pore forming agent to prepare porous g-C₃N₄ in the process of polycondensation.^41,42 For the construction of oxygen vacancies, metal oxides with oxygen vacancies are usually obtained by using hydrogen thermal treatment,⁴³ high energy particle bombardment,⁴⁴ and heating metal oxides under vacuum or oxygen depleted conditions.⁴⁵ Although these methods have their own advantages, they are inevitably limited by high-energy input and complex post-processing. According to the literature, there are electrostatic composite and in situ construction method.^41,46 The former is simple to implement, but it is greatly affected by the pH of photocatalytic reaction. Therefore, it is urgent to develop a new method to integrate porous structure and oxygen vacancy into a heterojunction system.

Herein, we propose a salt assisted in situ growth method to construct nano-sized oxygen-vacancy-rich MoO_3−x/porous g-C₃N₄ heterojunction. The introduction of MoO_3−x precursor (Mo(OH)₆) into g-C₃N₄ not only helps to prepare porous nanosheets, but also in situ grown oxygen-vacancy-rich MoO_3−x can form an atomic-scale compact heterointerface with g-C₃N₄. In additional, the heterojunction between MoO_3−x and g-C₃N₄ was established, which synergically improved photocatalytic HER performance, including higher charge separation efficiency, shorter charge transport path, and more catalytic active sites. By HR-TEM, HAADF-STEM and AFM, the morphology of the nanosized MoO_3−x/porous g-C₃N₄ heterojunction was confirmed. XPS proved the oxygen-vacancy in MoO_3−x. As a result, the optimized MoO_3−x/g-C₃N₄ heterojunction produces H₂ at 4694.3 μmol g⁻¹ h⁻¹, approximately 4 times higher of that of pure g-C₃N₄ (1220.1 μmol g⁻¹ h⁻¹). Our strategy can be universal for constructing diverse heterojunction with OVs and porous characteristics.

2. Experimental sections

2.1. Materials and reagents

All chemical reagents and materials were purchased and used without further purification. Molybdenum powder (Mo, 98%), urea ((NH₂)₂CO, ≥98%) were purchased from Sigma. Sodium chloride (NaCl, AR), absolute ethanol (CH₃CH₂OH, AR), and triethanolamine (TEOA, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical grade and used without further purification.

2.2. Preparation of g-C₃N₄

g-C₃N₄ was synthesized by thermal condensation of urea directly. In detail, 10 g urea powder was placed in an alumina crucible and heated to 550 °C for 2 h in static air with a heating rate of 5 °C min⁻¹. The resulting yellow agglomerates were then collected and milled into powder for further synthesis and measurements.

2.3. Preparation of Mo(OH)₆ precursor

Typically, 0.1 g molybdenum (Mo) powder was dispersed in 10 mL ethanol with stirring for several minutes. Then 0.35 mL H₂O₂ (30%) solution was added into the Mo power suspension solution. After 18 h, the Mo oxide solution turned from grey to yellow and finally turned to blue.

2.4. Preparation of MoO₃

The 10 mL Mo(OH)₆ precursor was dried in a 60 °C vacuum oven for 30 min, then transferred to a tubular furnace and calcined at 400 °C for 2 h (5 °C min⁻¹, under nitrogen condition).

2.5. Preparation of MoO_3−x

The Mo(OH)₆ precursor was added into a certain amount of NaCl, stirred well, and dried on the heating mantle at 80 °C. Then, transferred to a quartz tube furnace, to anneal for 2 h at 280 °C (5 °C min⁻¹, under nitrogen condition). The sample was dissolved in deionized water, and then filtered, washed and dried to obtain solid powder.

2.6. Preparation of MoO_3−x/porous g-C₃N₄ nanosheets

A typical fabrication process as follows: 50 mg g-C₃N₄ was dissolved in 200 mL of anhydrous ethanol and dispersed it evenly by ultrasonic treatment. 200 g NaCl prepared beforehand was added to the solution, followed by stirring, and the ethanol was removed by placing it on a heating mantle at 80 °C, the obtained sample was annealed at 300 °C for 2 h in a quartz tube furnace (5 °C min⁻¹, under nitrogen condition). The Mo(OH)₆ precursor was added into the above g-C₃N₄/NaCl mixture, stirred well, and dried on the heating table at 80 °C. The dried powder was transferred to a quartz tube furnace, to anneal for 2 h at 280 °C (5 °C min⁻¹, under nitrogen condition). The obtained sample was dissolved in deionized water, filtered, washed and dried to obtain solid powder. 1%, 5%, 10%, 50% MoO_3−x/g-C₃N₄ samples were prepared by adjusting the mass ratio of Mo powder to g-C₃N₄. NaCl acts as a removable template, provides a place for the construction of heterojunction in situ, and plays a good role in dispersing g-C₃N₄ precursor, so that Mo(OH)₆ can better contact with g-C₃N₄. As an antisolvent, ethanol does not dissolve NaCl and can be quickly removed from the system under 80 °C heating, so it acts as a medium for mixing the two materials.

2.7. Characterization

The phase structures of samples were recorded by X-ray powder diffraction (XRD, BrokerAXS Germany). Chemical state analysis was performed by using X-ray photoelectron spectroscopy (XPS, ESCALab 250). FTIR spectrometer (Nicolet iS50) was applied to investigate the functional groups and chemical structure of samples. Scanning electron microscope (SEM, HITACHI SU8010 Hitachi Japan), transmission electron microscope (TEM, Tecnai G20) and atomic force microscope (AFM, BRUKER MultiMode 8) were conducted to confirm the morphology and thickness of samples. Ultraviolet-visible (UV-vis) spectrophotometer (Shimadzu instruments Ltd., Suzhou, UV-2600) and Spectrofluorometer (Edinburgh Instruments Ltd., FS5) measurements were performed to study optical properties of samples. Photocurrent response, EIS Nyquist plots and Mott–Schottky measurements were conducted to comprehend electrochemical properties with the help of CHI 760E electrochemical system (Chenhua, Shanghai, China).

2.8. Photoelectrochemical measurement

Photoelectrochemical experiments were carried out on the CHI 760E electrochemical workstation using a traditional three-electrode system. The Ag/AgCl electrode was used as the reference electrode, Pt electrode was used as the counter electrode, and photocatalyst/ITO was used as working electrode. The detailed preparation of working electrode is as follows: firstly, 10 mg catalyst was dispersed in 2 mL DI water, followed by another ultra-sonication for half an hour to form a uniform suspension. Secondly, the suspension was dropped onto the surface of the ITO glass. Finally, the ITO glass was placed under an infrared lamp for drying. 0.1 mol L⁻¹ sodium sulfate solution and a 300 W xenon lamp were used as electrolyte solution and light source respectively, in the photocurrent responses and electrochemical impedance spectra experiments. In Mott–Schottky measurement, the frequency is 1000 Hz, the potential range is −1 to 1 V, and the electrolyte solution is 0.5 mol L⁻¹ sodium sulfate solution.

2.9. Photocatalytic H₂ evolution test

The photocatalytic H₂ evolution experiments were carried out in a 200 mL Pyrex reaction cell connected to a glass closed gas circulation system with vacuum (Labsolar-III (AG), Perfectlight Technology Co. Ltd, Beijing, P. R. China). In a photocatalytic reaction, 50 mg as-prepared photocatalyst powder was suspended in a glass cylindrical reactor, then, added to 90 mL deionized water, and stirred for a while. 10 mL triethanolamine (TEOA) was added in the above suspension as a sacrificial agent. The photocatalytic reaction was performed using a 300 W xenon lamp (λ = 320–780 nm). Before irradiation, the whole system maintains a vacuum to avoid contact with air. H₂PtCl₆ aqueous solution was added as the precursor for the co-catalyst Pt, which was in situ photo-reduced during the photocatalytic reaction (2 wt% Pt). The amount of H₂ produced during the photocatalytic reaction was determined by a gas chromatography (Techcomp, GC7900) (TCD detector, 5 Å molecular sieve column) for every one hour.

3. Results and discussion

3.1. Synthesis of nano-sized oxygen-vacancy-rich MoO_3−x/porous g-C₃N₄ heterojunction by salt assisted in situ growth method

We schematically illustrated the method in Fig. 1: firstly, g-C₃N₄ nanosheets were synthesized as the method reported and dispersed in anhydrous ethanol by ultrasonication, followed by adding NaCl powder. After the removal of ethanol by annealing at 80 °C, g-C₃N₄ nanosheets/NaCl was obtained (step 1). Secondly, the ethanol solution of Mo(OH)₆ as precursor was mixed with the prepared g-C₃N₄ nanosheets/NaCl, annealing at 280 °C in a CVD furnace to obtain MoO_3−x/g-C₃N₄/NaCl (step 2). Washing with deionized water several times to remove NaCl, the MoO_3−x/g-C₃N₄ heterojunction material was obtained (step 3). In addition, by changing the mass ratio of Mo powder to g-C₃N₄ (1%, 5%, 10%, and 50%), MoO_3−x/g-C₃N₄ samples with different Mo percentage were also prepared. The amorphous structure of prepared MoO_3−x/g-C₃N₄ nanosheets was confirmed by X-ray diffraction (XRD) analysis as shown in Fig. S1.† Fig. S2† shows the XRD pattern of pure α-MoO₃ fabricated from Mo(OH)₆ precursor.

Fig. 1 Schematic diagram of the fabrication processes of oxygen-vacancy-rich MoO_3−x/porous g-C₃N₄ heterojunction.

3.2. Morphological analysis

HAADF-STEM and TEM images revealed the abundant nano-sized pores in nanosheets with tens of nanometers diameter (Fig. 2a and b), consistent with SEM observation (Fig. S3†). MoO_3−x nanocrystals assembled on the surface of g-C₃N₄ nanosheets (Fig. 2c and S4†). For further confirmation, EDX elemental mapping of 5% MoO_3−x/g-C₃N₄ nanosheets indicated C, N, O, Mo elements distribution in the heterojunction (Fig. 2d–g). Atomic force microscopy (AFM) images showed the thicknesses of 5% MoO_3−x/g-C₃N₄ heterojunction around 3–5 nm and pore diameter around tens of nanometers, consistent with (Fig. 2h and S5†). Thus, the microscopic morphology of MoO_3−x/porous g-C₃N₄ heterojunction has been confirmed.

Fig. 2 (a) HAADF-STEM image. (b and c) TEM images. (d–g) EDX elemental maps of MoO_3−x/porous g-C₃N₄ heterojunction (C, N, O, Mo). (h) AFM images of BCN and (i) MoO_3−x/porous g-C₃N₄ heterojunction.

3.3. The formation of porous g-C₃N₄ nanosheets

FT-IR was applied to analyse the chemical structures of g-C₃N₄ nanosheets and 5% MoO_3−x/g-C₃N₄ (Fig. 3a). A new vibration band at 2178 cm⁻¹ in MoO_3−x/g-C₃N₄ nanosheets, attributed to C≡N bond, indicated the deprotonation of the amino group (–NH₂) in g-C₃N₄ nanosheets.⁸ On the other side, NH₃ was released in the deprotonation process, and led to the formation of pores.

Fig. 3 (a) FTIR spectrum, (b) XPS survey spectra, (c) C 1s XPS profiles and (d) N 1s XPS profiles of g-C₃N₄ and MoO_3−x/g-C₃N₄ (5%) samples.

To further investigate the chemical states and elemental composition of the samples, XPS and elemental analysis measurements were performed, respectively. As shown Fig. 3b, the XPS spectrum of 5% MoO_3−x/g-C₃N₄ nanosheets exhibited C 1s and N 1s signals at the position same as that of g-C₃N₄ nanosheets. In Fig. 3c, the C 1s peak of g-C₃N₄ was fitted into two peaks at 284.8, and 288.0 eV, assigned to graphitic C–C, and sp²-hybridized carbon in N containing aromatic ring (N–C=N), respectively.⁴⁷ The C 1s peak of MoO_3−x/g-C₃N₄ (5%) nanosheets slightly shifted from 288.0 eV to 288.2 eV, attributed to the interaction between –NH₂ and HO–Mo(OH)₅.⁸ In Fig. 3d, the N 1s peak for g-C₃N₄ could be deconvoluted into four peaks at 398.7, 399.9, 401.0 and 404.4 eV, attributed to the sp²-bonded N in the tri-s-triazine units (C=N–C), bridging nitrogen atoms in N–(C)₃, nitrogen atoms bonded with hydrogen atoms (–NH_x) and the charging effects, respectively.⁴⁷ Compared with g-C₃N₄ nanosheets, MoO_3−x/g-C₃N₄ (5%) nanosheets showed obvious shift of N–(C)₃ peaks (from 399.9 eV to 399.6 eV), suggesting cyano-groups and consistent with the FTIR results.⁴⁸ In addition, the introduction of N defects can be supported by elemental analysis measurements as showed in Table S1,† supporting NH₃ releasing. The N/C atomic ratio for g-C₃N₄ is 1.32, close to the ideal g-C₃N₄ composition (N/C = 1.33),⁴⁹ while the N/C atomic ratios for MoO_3−x/g-C₃N₄ (5%) decrease to 1.02, suggesting the loss of lattice nitrogen in MoO_3−x/g-C₃N₄ (5%) due to the production of NH₃.

3.4. The formation of oxygen vacancies on MoO_3−x

As a reference, we synthesized MoO₃ by the reported method. We compared the XPS spectra of the MoO₃ and 5% MoO_3−x/g-C₃N₄ in Fig. 4a and b. For MoO₃, the O 1s peaks at 530.5, 532.2, and 533.0 eV, are due to Mo–O, oxygen vacancy, and –OH, respectively.⁵⁰ For MoO_3−x/g-C₃N₄ (5%) nanosheets, the O 1s peaks from Mo–O and –OH positively shifted 0.6 and 1.6 eV, respectively, attributed to the interaction between the –OH of Mo(OH)₆ and the –NH₂ of g-C₃N₄. In order to further obtain more details information of oxygen vacancy, the ratios of different O species of MoO₃ and MoO_3−x/g-C₃N₄ (5%) calculated by XPS, the analysis results are shown in Tables S2 and S3.† The concentration of oxygen vacancy increased from 21.84% to 48.67% in MoO₃ and MoO_3−x/g-C₃N₄ (5%), respectively. By the O 1s spectra comparison, the ratio of Mo–O decreases and the ratio of –OH increases, suggesting the unsaturation of Mo coordination. In MoO_3−x/g-C₃N₄ (5%) composite, the broadened doublets at 231.7 and 234.9 eV are ascribed to the electron binding energies of the Mo 3d_3/2 and Mo 3d_5/2 of Mo⁵⁺ (Fig. 3b). The existence of the Mo⁵⁺ may come from the oxygen vacancies and unsaturated coordination of Mo.^51,52

Fig. 4 (a) O 1s XPS and Mo 3d XPS profiles (b) of α-MoO₃ and MoO_3−x/g-C₃N₄ (5%) nanosheets.

3.5. Photocatalytic H₂ evolution

Fig. 5a exhibits HER rates of the as-prepared samples all with 2 wt% Pt as co-catalyst and TEOA as hole-sacrificial agent. The 5% MoO_3−x/g-C₃N₄ heterojunction exhibits the highest HER activity as the optimized condition and lower or higher MoO_3−x percentage induced less HER activity. With the further increasing MoO_3−x, excess MoO_3−x has a tendency to self-aggregate, reducing effective interfacial area between MoO_3−x and g-C₃N₄ and the active sites. So, the overmounted MoO_3−x led to lower performance. Fig. 5b shows that the high HER stability of 5% MoO_3−x/g-C₃N₄ nanosheets, with no obvious decrease during 3 cycles. Fig. 5c compared the HER rate with different percentage of MoO_3−x in MoO_3−x/g-C₃N₄ nanosheets. The 5% MoO_3−x/g-C₃N₄ nanosheets exhibits the highest HER rate (4694.3 μmol g⁻¹ h⁻¹), about 4 times higher than pure g-C₃N₄ (1220.1 μmol g⁻¹ h⁻¹). Fig. 5d compared the HER rates of the pure g-C₃N₄ and 5% MoO_3−x/g-C₃N₄ nanosheets using lactic acid and TEOA as hole-sacrificial agents, respectively. MoO_3−x/g-C₃N₄ (5%) nanosheets exhibited higher HER rates in both systems of lactic acid and TEOA as hole-sacrificial agents. Photocatalytic hydrogen evolution rates of g-C₃N₄, 5% MoO₃/g-C₃N₄ and 5% MoO_3−x/g-C₃N₄ as shown in Fig. S6.† 5% MoO_3−x/g-C₃N₄ showed the best HER performance, which further proved the promoting effect of OVs in MoO_3−x/g-C₃N₄ photocatalyst on HER. We also compared the HER performance of similar photocatalysts reported in some recent references as shown in Table S4.†

Fig. 5 Photocatalytic H₂ evolution performance and stability test. (a) The photocatalytic H₂ evolution of MoO_3−x/g-C₃N₄ samples with 2 wt% Pt as co-catalyst. (b) Photocatalytic cycle stability test of 5% MoO_3−x/g-C₃N₄ with 2 wt% Pt as co-catalyst. (c) Optimum photocatalytic H₂ evolution efficiency of MoO_3−x/g-C₃N₄ photocatalysts with different contents of MoO_3−x. (d) The photocatalytic hydrogen evolution performance for g-C₃N₄ and 5% MoO_3−x/g-C₃N₄ with 2 wt% Pt as co-catalyst in different sacrificial agent systems (lactic acid and TEOA).

3.6. The electronic structure and photocatalytic mechanism

The optical absorption properties and band gap of the heterojunction were studied by UV-vis absorption spectroscopy. The g-C₃N₄ shows an absorption edge of approximately 435 nm (Fig. 6a), consistent with the ref. 53. The 5% MoO_3−x/g-C₃N₄ heterojunction exhibits a red-shift of absorption edge, given to the interfacial charge transfer (Fig. 6a). UV-vis absorption spectra of g-C₃N₄ and MoO_3−x is shown in Fig. S7,† the illustrations are their corresponding Tauc's plot. In Fig. 6b, the pure g-C₃N₄ nanosheets in ethanol exhibited a strong PL peak at 450 nm, while 5% MoO_3−x/g-C₃N₄ nanosheets with the same concentration of g-C₃N₄ nanosheets in ethanol showed much smaller PL peak at 450 nm. The PL peak at 350 nm was attributed to MoO_3−x (excitation wavelength: 300 nm). These results suggest that, in the heterojunction, the MoO_3−x absorbed the excitation energy and transferred some part to g-C₃N₄. In Fig. 6c, the 5% MoO_3−x/g-C₃N₄ exhibit higher photocurrent density than g-C₃N₄. Compared with pure g-C₃N₄, 5% MoO_3−x/g-C₃N₄ nanosheets have stronger photocurrent density as well as smaller arc radius (Fig. 6d), leading to lower resistance and favourable for charge separation. The corresponding optical band-gaps (E_g) were calculated to be 2.69 eV for g-C₃N₄ and 2.98 eV for MoO_3−x (Fig. S7a and b†). In Mott–Schottky plots (Fig. 6e), the extrapolated conduction band edge (E_CB) positions of g-C₃N₄ and MoO_3−x are −1.23 and −0.20 V (vs. Ag/AgCl, pH = 7) or −0.59 and +0.44 V (vs. NHE, pH = 0), respectively. Regarding E_g of g-C₃N₄ (2.69 eV) and MoO_3−x (2.98 eV) nanosheets, the valence band energy (E_VB) was calculated as 2.10 and 3.42 eV (vs. NHE, pH = 0) as shown in Fig. 6f. According to the step-scheme photocatalytic system,³⁰ the e⁻ on the CB of MoO_3−x would recombine with the h⁺ of VB g-C₃N₄ at the interface in the MoO_3−x/g-C₃N₄. Consequently, e⁻ accumulated in the CB of g-C₃N₄ and h⁺ accumulated in the VB of MoO_3−x. H⁺ was reduced to hydrogen by accumulated electrons at the CB of g-C₃N₄, while the TEOA was oxidized by the h⁺ accumulated at the VB of MoO_3−x.

Fig. 6 (a) UV-vis spectra of g-C₃N₄ and 5% MoO_3−x/g-C₃N₄. (b) PL emission spectra of g-C₃N₄ and 5% MoO_3−x/g-C₃N₄. (c) The transient photocurrent responses under dark and irradiated conditions. (d) The electrochemical impedance spectra of g-C₃N₄ and 5% MoO_3−x/g-C₃N₄ photocathodes under light. (e) Mott–Schottky plots of g-C₃N₄ and MoO_3−x. (f) The photocatalytic H₂ evolution mechanism of the MoO_3−x/g-C₃N₄ system under light irradiation (λ = 320–780 nm).

4. Conclusions

In this work, we have constructed oxygen-vacancy-rich MoO_3−x/porous g-C₃N₄ nanosheets heterojunction by in situ growth. The heterojunction photocatalysts under light (λ = 320–780 nm) accelerated photogenerated e⁻–h⁺ separation and hydrogen evolution reaction. This method is simple and feasible, which endows g-C₃N₄ nanosheets with highly porous structure and more active sites, and endows MoO_3−x with oxygen-vacancy-rich MoO_3−x and more efficient charge separation, synergistically enhancing the photocatalytic performance. It could be a general method to inorganic semiconductor heterojunction or even multi-junction for photocatalysts.

Author contributions

H. B. S. conceived the project. Y. F. P. designed and performed the experiments. H. B. S. and Y. F. P. analyzed and interpreted the experimental data. Y. F. P. wrote the paper. H. B. S., Y. W. and Z. L. revised the paper. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by Hubei Provincial Natural Science Foundation of China (2020CFA037), and Engineering Research Center of Nano-Geomaterials of Ministry of Education (NGM2019KF023). The authors appreciate the Analytical and Testing Center of China of University of Geosciences (CUG) and the facility support of the Engineering Research Center of Nano-Geomaterials of Ministry of Education, CUG.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra05620d

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