ZnS microsphere/g-C₃N₄ nanocomposite photo-catalyst with greatly enhanced visible light performance for hydrogen evolution: synthesis and synergistic mechanism study

Abstract

In this work, ZnS microsphere/g-C₃N₄ nanocomposites with different ZnS contents were synthesized by a facile precipitation route and characterized by a variety of techniques. The TEM results confirmed that ZnS microspheres are deposited on the surface of g-C₃N₄ nanosheets with intimate contact in the nanocomposites. Such type-II heterostructures formed between ZnS and g-C₃N₄ can unexpectedly improve the photocatalytic H₂-production rate of g-C₃N₄ under visible light, which reaches an optimal value of up to 194 μmol h⁻¹ g⁻¹ at the ZnS content of 50 wt%. The synergistic effect of ZnS and g-C₃N₄ was proposed to be responsible for the efficient separation of the photogenerated charge carriers and, consequently, the enhancement of the visible light photocatalytic H₂ production activity of the nanocomposites. This work highlights that the integration of g-C₃N₄ and ZnS will be an opportunity to obtain a g-C₃N₄-based nanocomposite photocatalyst with enhanced photocatalytic H₂ production activity, and we hope our work may be helpful for the construction of photocatalysts with efficient visible light H₂-production activities.

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1. Introduction

Since the discovery by Honda and Fujishima of the photoelectron-chemical splitting of water over a TiO₂ electrode,¹ photocatalytic hydrogen production using semiconductor particles has been regarded as one of the most promising means of clean energy production from renewable sources.^2,3 In the past decades, various semiconductor photocatalysts have been developed for the production of H₂, including oxides, sulfides, oxynitrides and nanocomposites.^4–10 Graphite-like carbon nitride (g-C₃N₄), a promising metal-free semiconductor photocatalyst, has attracted increasing attention owing to its visible-light response, suitable band position and stable, low toxicity, and low cost characteristics.^7,11 However, the photocatalytic efficiency of pristine g-C₃N₄ is predominately constrained by the high recombination rate of its photogenerated electron–hole pairs. To restrain such undesirable recombination and achieve high activity, pristine g-C₃N₄ was often coupled with a variety of metal oxides or metal sulfides to construct binary or ternary hybrid photocatalysts with specially designed type-II heterostructures.^12–32 These formed type-II heterostructures could accelerate the spatially separation of photogenerated charge carriers, and consequentially increase the photocatalytic activity of g-C₃N₄. Notably, in some of these photocatalysts, g-C₃N₄ performs as an effective visible-light sensitizer for the coupled wide-gap semiconductor (such as TiO₂, ZnO and SrTiO₃), whilst the combined wide band gap semiconductor could help to separate the photogenerated charge carriers. Utilization of this “synergistic effect” has been proven to feasible way to access enhanced photocatalytic H₂ production activity for g-C₃N₄, but so far just very few examples were reported. Thus, combination of the g-C₃N₄ and wide-gap semiconductor to form hybrid photocatalyst for H₂ production is highly desirable and challenging.

From the thermodynamic point of view, in the g-C₃N₄-based type-II heterostructures for the photocatalytic H₂ production, the bottom level of the CB of the coupled semiconductor is generally more positive than that of g-C₃N₄, while at the same time this CB position should be more negative as much as possible than the redox potential of H⁺/H₂ (0 V vs. NHE).^33,34 When searching this semiconductor, we found that ZnS is in principle suitable. ZnS, with a bandgap of around 3.5 eV, is one of the most important metal sulfide materials in photocatalysis, due to the high CB position (−0.9 eV), good thermal stability, high electronic mobility and low toxicity.^35–37 However, as a single component, the high recombination ratios of photoinduced electron–hole pairs and poor response to visible light greatly limiting its practical applications.^38–41 It is expected that unite of g-C₃N₄ and ZnS will be an opportunity to obtain a novel g-C₃N₄-based composite photocatalyst with improved photocatalytic H₂ production activity.

Here, we report the synthesis of ZnS microsphere/g-C₃N₄ (denoted as ZnS/g-C₃N₄) nanocomposite photocatalysts by a facile precipitation route. The ZnS/g-C₃N₄ nanocomposite exhibits distinctly enhanced H₂ production efficiency under visible light irradiation as compared to pure g-C₃N₄ and ZnS. Based on the experimental results, a possible synergistic mechanism for the enhanced photocatalytic H₂ evolution activity was proposed. This work for the first time shows a feasible example to enhance the photocatalytic H₂ production activity of g-C₃N₄ by constructing type-II heterostructures with wide band gap ZnS. Moreover, the use of g-C₃N₄ as a wide band gap semiconductor sensitiser candidate in the construction of visible-light photocatalytic H₂ production system was confirmed.

2. Experimental

2.1 Preparation of photocatalysts

All reagents were of analytical grade, purchased from Sinopharm Chemical Reagent Co., Ltd., China, and were used without further purification. The formation of ZnS microsphere/g-C₃N₄ composite involves a two-step process, and the schematic illustration of the formation process is described in Fig. 1. g-C₃N₄ was first prepared by thermal treatment of 10 g of urea in a crucible with a cover under ambient pressure in air, similar to reported methods. After dried at 80 °C for 24 h, the precursor was heated to 550 °C at a heating rate of 2.3 °C min⁻¹ in a tube furnace for 4 h in air. The resulted final light yellow powder were washed with nitric acid (0.1 mol L⁻¹), distilled water and absolute ethanol for three times, then dried at 60 °C for 12 h.

Fig. 1 Schematic illustration of the formation process of ZnS/g-C₃N₄ nanocomposites.

ZnS/g-C₃N₄ composite photocatalysts were synthesized by a facile precipitation method. Typically, 400 mL of a mixed solution of different amounts of g-C₃N₄, Zn(NO₃)₂ (0.05 M) and HNO₃ (0.01 M) was heated to 70 °C; then an appropriate amount of thioacetamide (C₂H₅NS, TAA) was added. After that, the solution was immersed in a 70 °C water bath for 5 h. Then the reaction was quickly transferred to an ice water bath for quenching to below 5 °C. The product was collected by centrifugation, washed with distilled water and absolute ethanol, and dried in an oven at 60 °C in air. According to this method, different weight ratios of the ZnS to g-C₃N₄ samples were obtained and labeled as 20% ZnS/g-C₃N₄, 30% ZnS/g-C₃N₄, 40% ZnS/g-C₃N₄, 50% ZnS/g-C₃N₄, 60% ZnS/g-C₃N₄, and 70% ZnS/g-C₃N₄, respectively. Pure ZnS was also synthesized for comparison purpose in the absence of g-C₃N₄. In order to compare the photocatalytic activity with ZnS/g-C₃N₄ composites, a mechanically (physically) mixed ZnS/g-C₃N₄ sample with ZnS to g-C₃N₄ weight ratio of 50% (donated to m-50% ZnS/g-C₃N₄) was prepared. A measured amount of as-prepared bulk g-C₃N₄ and ZnS added in 50 mL methanol. After ultrasonically treated for 30 min, the homogeneous suspension stirred at room temperature until the residual methanol was removed. Then the obtained solid were collected and dried in a vacuum oven at 100 °C for 24 h. To improve the photocatalytic H₂ evolution activity, 1 wt% Pt-dispersed photocatalysts were prepared by an incipient impregnation method using chloroplatinic acid as metal precursor.

2.2 Characterization

The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer) with Cu-Kα radiation (λ = 1.5406 Å). Infrared spectra were obtained on KBr pellets on a Nicolet NEXUS470 FTIR in the range of 4000–500 cm⁻¹. Field emission scanning electron microscopy (FESEM) measurement was performed by JSM-6700F instrument operated at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) was recorded on a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV. Surface analysis of the sample was examined by X-ray photoelectron spectroscopy (XPS) using a ESCA PHI500 spectrometer. The surface areas of the samples were measured by a TriStar II 3020-BET/BJH Surface Area. UV-vis diffuse reflectance spectra (DRS) were performed on a Shimadzu UV-2401 spectrophotometer equipped with spherical diffuse reflectance accessory. The photoluminescence (PL) spectra of the photocatalyst were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 370 nm. The electrochemical impedance spectroscopy (EIS) measurements were carried out in a conventional three-electrode, single-compartment quartz cell on an electrochemical station (CHI 660D).

2.3 Photocatalytic hydrogen production

The photocatalytic hydrogen production experiments were carried out in a 100 mL Pyrex flask connected to a closed gas circulation and evacuation system. Four low power UV-LEDs (3 W, 420 nm) (Shenzhen LAMPLIC Science Co. Ltd. China), used as light sources to trigger the photocatalytic reaction, were positioned 1 cm away from the reactor in four different directions. The focused intensity and areas on the flask for each UV-LED was ca. 80.0 mW cm⁻² and 1 cm², respectively. Typically, 50 mg of photocatalyst was dispersed by magnetic stirrer in 80 mL aqueous solution containing 20 mL methanol. Prior to irradiation, the suspension of the catalyst was dispersed by an ultrasonic bath for 10 min, and then purged with nitrogen for 40 min to completely remove the dissolved oxygen and ensure the reactor was in an anaerobic condition. A 0.4 mL gas was intermittently sampled through the septum, and the amount of H₂ was determined by gas chromatography (GC-14C, Shimadzu, Japan, TCD, with nitrogen as a carrier gas and 5 Å molecular sieve column).

3. Results and discussion

3.1 Structure and morphology

The XRD patterns of the pure g-C₃N₄, ZnS, and ZnS/g-C₃N₄ composites are shown in Fig. 2. The distinct diffraction peaks at 13.0° and 27.4° in the g-C₃N₄ sample, corresponding to the in-plane structural packing motif and interlayer stacking of aromatic segments, can be indexed as the (100) and (002) peaks for graphitic materials, respectively. For pure ZnS sample, all the peaks can be well indexed to the cubic phase (JCPDS 65-0309). The main diffraction peaks at 28.7°, 33.3°, 47.7°, 56.5°, 69.7°, and 77.0° correspond to the (111), (200), (220), (311), (400) and (331) crystal planes, respectively. All the ZnS/g-C₃N₄ samples exhibit diffraction peaks corresponding to both g-C₃N₄ and ZnS, while the characteristic peaks of g-C₃N₄ (27.4°) and ZnS (28.7°) were very close and overlap with each other. There are no other impure peaks can be observed, reflecting these composites consist of two phases.

Fig. 2 XRD patterns of (a) g-C₃N₄, (b) 20% ZnS/g-C₃N₄, (c) 30% ZnS/g-C₃N₄, (d) 40% ZnS/g-C₃N₄, (e) 50% ZnS/g-C₃N₄, (f) 60% ZnS/g-C₃N₄, (g) 70% ZnS/g-C₃N₄, and (h) ZnS.

The structure and morphology of the samples were characterized by SEM and TEM. The pure g-C₃N₄ sample, synthesized by the polymerization reaction of urea, exhibits a layered and sheet-like surface morphology (Fig. S1a and b†), while the pure ZnS sample is composed of a large number of well-defined hierarchical microspheres with diameters ranging from 200 to 600 nm (Fig. S1c and d†). Taking the 50% ZnS/g-C₃N₄ nanocomposite sample as an example, we examine the morphology of the resulting nanocomposite photocatalysts. As shown in Fig. 3a and b, hierarchical ZnS microspheres are well loaded on or attached with the g-C₃N₄ nanosheets. From the magnified TEM image shown in Fig. 3c, we can further found that ZnS microspheres are closely deposited on the surface of g-C₃N₄ nanosheets, indicating the intimate interfacial contact between the ZnS microspheres and g-C₃N₄ nanosheets is readily obtained by such a simple precipitation approach. This intimate interfacial contact can be further confirmed by the high-resolution TEM image (Fig. 3d). The lattice fringes can be clearly observed in the inset of Fig. 3d, suggesting the well-defined crystal structure of ZnS; and the lattice spacing of ca. 0.31 nm can be assigned to the (111) plane of ZnS. It is important to note that the intimate contact (marked by the red dashed line in Fig. 3d) of ZnS and g-C₃N₄ could favor the transfer of charge carriers in the composite, therefore improving the charge separation and photocatalytic efficiency. Similarly, such intimate contact can be also found in the other composite samples, which are composed of hierarchical ZnS microspheres and sheet-like g-C₃N₄, but just with the clear differences in the ZnS content (Fig. S2†).

Fig. 3 (a and b) SEM, (c) TEM and (d) HRTEM images of the 50% ZnS/g-C₃N₄ sample. Inset in (d) shows the enlarged HRTEM image.

To further confirm the existence of g-C₃N₄ in the composite sample, FT-IR spectroscopy is performed. It could also be clearly seen that the main characteristic peaks of g-C₃N₄ appeared in ZnS/g-C₃N₄ nanocomposite photocatalysts. As shown in Fig. 4a–g, the peaks at 1637 cm⁻¹ are attributable to C=N stretching vibration modes, while the 1252, 1323, 1407 and 1568 cm⁻¹ can be ascribed to aromatic C–N stretching vibration modes.⁴² The peak at 810 cm⁻¹ is related to characteristic breathing mode of s-triazine units.⁴³ The broad band centered at 3187 cm⁻¹ can be assigned to the stretching mode of the N–H bond.⁴⁴ In the composites, the characteristic peaks of g-C₃N₄ did not change after combination with ZnS. With regard to the pure ZnS synthesized in the presence of TAA, the peaks at 1383 and 1615 cm⁻¹ could be attributed to the TAA on the surface of ZnS.

Fig. 4 FTIR spectra of (a) pure g-C₃N₄, (b) 20% ZnS/g-C₃N₄, (c) 30% ZnS/g-C₃N₄, (d) 40% ZnS/g-C₃N₄, (e) 50% ZnS/g-C₃N₄, (f) 60% ZnS/g-C₃N₄, (g) 70% ZnS/g-C₃N₄, (h) pure ZnS.

XPS analyses were carried out on a typical 50% ZnS/g-C₃N₄ sample to investigate the chemical valence state of the elements. The high resolution XPS spectra of C 1s shows that there were two peaks located at 284.7 and 287.3 eV (Fig. 5a). The peak centered at 284.7 eV can be ascribed to graphitic or surface adventitious carbon which was usually observed on the XPS characterization of g-C₃N₄.⁴⁵ The peak centered at 287.3 eV can be assigned to sp²-hybridized carbon in the aromatic ring. In the N 1s spectrum (Fig. 5b), several binding energies can be separated. The main signal showed occurrence of C–N–C groups (398.0 eV), tertiary nitrogen N–(C)₃ groups (399.8 eV) and N–H groups (401.0 eV).⁴⁶ The peak at 404.2 eV could be related to the charging effects.⁴⁷ The peaks of S 2p_3/2 and S 2p_1/2, which were located at around 161.5 and 162.7 eV (Fig. 5c) were assigned to sulfur anions in the lattice of ZnS. Fig. 5d shows the peaks at 1021.8 and 1044.9 eV, which could be attributed to Zn 2p_3/2 and Zn 2p_1/2 binding energies of Zn²⁺ in ZnS/g-C₃N₄ nanocomposite. The XPS spectra of pure g-C₃N₄ and ZnS were shown in Fig. S3.† No obvious shift was observed in the Zn 2p and S 1s peak after the combination of ZnS, which indicates that there is no strong interfacial interaction between the ZnS and g-C₃N₄. Evidently, results from XRD, FT-IR and XPS revealed that the obtained composites contained g-C₃N₄ and ZnS.

Fig. 5 XPS spectra of (a) C 1s, (b) N 1s, (c) S 2p and (d) Zn 2p of the sample 50% ZnS/g-C₃N₄.

3.2 Optical absorption properties

The light absorption properties of the pristine ZnS, g-C₃N₄, and ZnS/g-C₃N₄ composites were characterized by UV-vis diffuse reflectance spectroscopy. The band gap energy of pristine ZnS and g-C₃N₄ can be calculated by the following formula: αhν = A(hν − E_g)^n/2, where α, h, ν, E_g, and A are the absorption coefficient, Planck constant, the light frequency, the band gap, and a constant, respectively.⁴⁸ Among them, n depends on the characteristics of the transition in a semiconductor (direct transition n = 1 and indirect transition n = 4). As shown in Fig. 6a, the pristine ZnS absorbs only ultraviolet light with wavelength shorter than around 365 nm, which can be assigned to a band gap of around 3.4 eV (Fig. 6b). The pristine g-C₃N₄ shows an absorption edge at about 455 nm (Fig. 6a), corresponding to band gap of 2.7 eV (Fig. 6b). After being coupled with g-C₃N₄, the optical absorption of the ZnS/g-C₃N₄ composites is enhanced distinctly in the visible light region, which allows for more efficient utilization of the solar spectrum to create photogenerated electrons and holes. The adsorption intensities increase gradually with the enhancement of the amount of g-C₃N₄ in the composites.

Fig. 6 (a) UV-vis diffuse reflectance spectra of pristine ZnS, g-C₃N₄, and the ZnS/g-C₃N₄ composites, (b) the band gaps (E_g) of pristine ZnS and g-C₃N₄.

3.3 Photocatalytic H₂-evolution activity and photostability

The H₂ evolution experiments were performed by taking 50 mL of a 25 vol% methanol solution and 0.05 g of powdered photocatalyst. Before the actual photocatalytic experiment was performed, control experiment was carried out by taking a pure methanol solution in the absence of either photocatalyst or irradiation. As there was no appreciable H₂ production was detected, it was confirmed that H₂ was produced by photocatalytic reaction. The influence of different ZnS contents in the ZnS/g-C₃N₄ composites on the H₂ evolution is presented in Fig. 7a. It can be found that both the pure g-C₃N₄ and ZnS show a negligible visible light H₂ evolution activity. As shown in Fig. 7a, the rate of H₂ evolution increases with increasing of ZnS amount in the composites, and can reach to about 194 μmol h⁻¹ g⁻¹ when the mass ratio of ZnS to g-C₃N₄ is increased to 0.5. With the further increasing of the content of ZnS, the photocatalytic activity of the nanocomposite samples decreases. But they still show enhanced photocatalytic activities than that of pure g-C₃N₄ and ZnS. As a control, we also test the H₂-production activity of the m-50% ZnS/g-C₃N₄ synthesized by the physically mixed method. Notably, we found that the photocatalytic H₂ efficiency of the m-50% ZnS/g-C₃N₄ is significantly lower than that of ZnS/g-C₃N₄ sample, which prepared by the precipitation method. This result conforms that the formation of intact interface structure between the ZnS and g-C₃N₄ is the crucial parameter to the high photocatalytic activity of the resulting nanocomposite samples. The stability and reusability of 50% ZnS/g-C₃N₄ nanocomposite were evaluated by the cycling H₂ evolution experiment, and the results are shown in Fig. 7b. The results show that the 50% ZnS/g-C₃N₄ nanocomposite photocatalysts do not display obvious decrease of photocatalytic H₂ production activity under visible light. Only insignificant loss in photocatalytic activity is observed, which might be partly caused by incomplete collection of the photocatalyst during each step. This indicates that the ZnS/g-C₃N₄ nanocomposite photocatalysts is sufficiently stable for photocatalytic H₂ production.

Fig. 7 (a) Photocatalytic activities of Pt-dispersed pure g-C₃N₄ (A), 20% ZnS/g-C₃N₄ (B), 30% ZnS/g-C₃N₄ (C), 40% ZnS/g-C₃N₄ (D), 50% ZnS/g-C₃N₄ (E), 60% ZnS/g-C₃N₄ (F), 70% ZnS/g-C₃N₄ (G), pure ZnS (H), and m-50% ZnS/g-C₃N₄ (I) for H₂ production under visible-light irradiation. (b) Cycle runs for the photocatalytic H₂ production over 50% ZnS/g-C₃N₄.

3.4 Possible mechanism of enhanced photocatalytic activity

It is generally accepted that photocatalytic activity is mainly governed by light-absorption ability, surface properties (including adsorption property, specific surface area, etc.), and photogenerated charge-separation efficiency. The H₂-production activity of the 50% ZnS/g-C₃N₄ was greatly enhanced relative to the pure ZnS and g-C₃N₄. The BET surface area of the pure g-C₃N₄, ZnS and 50% ZnS/g-C₃N₄ is 32.35, 3.30 and 13.40 m² g⁻¹ (Fig. S4†), indicating that the BET surface area was greatly reduced by the introduction of ZnS. Therefore, the enhanced photocatalytic activity of the samples has no direct relation with the BET surface areas. As discussed above, the ZnS loading did not significantly change the light-absorption property of g-C₃N₄, indicating that the light-adsorption property factor is also not crucial to the photocatalytic activity for ZnS/g-C₃N₄ nanocomposites. Therefore, the enhanced photocatalytic activities of ZnS/g-C₃N₄ nanocomposites might be mainly attributed to the synergic effect between ZnS and g-C₃N₄, which can greatly accelerate the separation of photogenerated carriers. The mechanism for the superior photocatalytic hydrogen evolution over the as-prepared ZnS/g-C₃N₄ nanocomposite under visible light irradiation was proposed in Fig. 8. Under visible light irradiation, the VB electrons of g-C₃N₄ are excited to the CB, creating holes in the VB. Normally, these photogenerated electrons and holes are easy to recombine, resulting in a low photocatalytic activity of g-C₃N₄ itself. However, in the ZnS/g-C₃N₄ system, due to the intimate contact of ZnS and g-C₃N₄, the CB electrons of g-C₃N₄ can be injected into the ZnS because the CB level of g-C₃N₄ is higher than that of ZnS. Therefore, the recombination process of the electron–hole is inhibited, leading to an enhanced photoactivity of the ZnS/g-C₃N₄ nanocomposites.

Fig. 8 Schematic diagram of electron–hole pairs separation and the possible reaction mechanism over ZnS/g-C₃N₄ nanocomposite photocatalyst under visible light irradiation.

To investigate the migration, transfer and recombination processes of photogenerated electron–hole pairs, PL spectra of the samples were recorded. Fig. 9a shows the photoluminescence spectra of the as-prepared samples activated at room temperatures and excitation wavelength λ_ex = 370 nm. The main emission peak centers at about 450 nm for the pure g-C₃N₄ sample, which can be ascribed to the band gap recombination of electron–hole pairs. Obviously, after the formation of ZnS/g-C₃N₄ composite, the PL intensity of the composite decreased rapidly, demonstrating the efficient charge separation of the composite. Therefore, it is reasonable to conclude that the enhanced photocatalytic activity could be attributed to the effective interfacial charge transfer between ZnS and g-C₃N₄. For the comparison purpose, we also examine the PL spectra of the m-50% ZnS/g-C₃N₄ sample. As expected, we found that the PL intensity of the m-50% ZnS/g-C₃N₄ sample is higher than that of 50% ZnS/g-C₃N₄ sample. This result indicates that the formation of the intact interface structure between the ZnS and CN is beneficial to the interfacial charge transfer. To further study the charge transfer properties, we conducted electrochemical impedance spectroscopy (EIS) measurements over the pristine g-C₃N₄, ZnS and 50% ZnS/g-C₃N₄ photocatalysts, and the result was shown in Fig. 9b. In the Nyquist plot, the radius of each arc is characteristic of the charge-transfer process at the corresponding electrode/electrolyte interface with a smaller radius corresponding with a lower charge-transfer resistance. Notable, the 50% ZnS/g-C₃N₄ exhibits the smallest charge transfer resistance among all tested electrodes under irradiation, suggesting that effective shuttling of charges between the electrode and the electrolyte, and faster interfacial charge transfer occurred at the nanocomposite interface owing to the formation of the heterojunctions between g-C₃N₄ and ZnS.

Fig. 9 (a) Photoluminescence (PL) spectra of the as-prepared samples at room temperature. Excitation wavelength λ_ex = 370 nm; (b) Nyquist impedance plots of pristine g-C₃N₄, ZnS and 50% ZnS/g-C₃N₄ photocatalysts under visible-light irradiation in 0.2 M Na₂SO₄ aqueous solution (pH 6.8).

4. Conclusions

In summary, this work shows a simple and feasible method to enhance the photocatalytic H₂-production activity of g-C₃N₄ by the combination of wide band gap semiconductor. ZnS microsphere/g-C₃N₄ composite photocatalyst were successfully prepared via a simple precipitation method. All the prepared ZnS microsphere/g-C₃N₄ nanocomposites exhibit higher photocatalytic activity for H₂ evolution from ethanol aqueous solution than ZnS or g-C₃N₄. The content of ZnS in the composite influences the photocatalytic performance, and the optimum proportion of ZnS is 50 wt%. Importantly, it is believed that the positive synergetic effect between ZnS microsphere and g-C₃N₄ can greatly suppress the charge recombination, thus markedly improving the photocatalytic H₂-production activity. On the basis of the results of this study, the ZnS/g-C₃N₄ nanocomposite sample is expected to be an effective visible-light H₂-production photocatalyst for practical applications.

Acknowledgements

This work was supported by the financial supports of National Nature Science Foundation of China (no. 21406091), Natural Science Foundation of Jiangsu Province (BK20140530), and College Natural Science Research Program of Jiangsu Province (13KJB610003).

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Footnote

† Electronic supplementary information (ESI) available: SEM and TEM images of the pure g-C₃N₄, ZnS and the samples with different weight ratios of ZnS to g-C₃N₄, XPS spectra of g-C₃N₄, ZnS, and 50% ZnS/g-C₃N₄ samples, and N₂ adsorption/desorption isotherms of g-C₃N₄, ZnS and 50% ZnS/g-C₃N₄. See DOI: 10.1039/c4ra11740a

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