In situ growth of CuS nanoparticles on g-C₃N₄ nanosheets for H₂ production and the degradation of organic pollutant under visible-light irradiation

Abstract

The solar-to-fuel conversion using a photocatalyst is an ideal method to solve the energy crisis and global warming. In this contribution, photocatalytic H₂ production and organic pollutant removal using g-C₃N₄/CuS composite was demonstrated. Well dispersed CuS nanoparticles (NPs) with a size of about 10 nm were successfully grown on the surface of g-C₃N₄ nanosheet via a facile hydrothermal method. The as-prepared g-C₃N₄/CuS nanocomposite at an optimized loading exhibited a much higher visible light photoactivity, giving up to 2.7 times and 1.5 times enhancements in comparison to pure g-C₃N₄ for photocatalytic H₂ production and methylene orange (MO) degradation, respectively. These enhanced photocatalytic activities are attributed to the interfacial transfer of photogenerated electrons and holes between g-C₃N₄ and CuS, which leads to effective charge separation on both parts. That is, under the visible light irradiation, electrons in the valence band (VB) of g-C₃N₄ can directly transfer to the CuS NPs, which can act as an electron sink and co-catalyst to promote the separation and transfer of photo-generated electrons, thus significantly improving the photocatalytic efficiency.

---

1. Introduction

Developing and using renewable energy sources is believed to be an effective approach to solve the environmental and energy challenges facing the world in the 21st century arising from the overuse of fossil fuels and the resulting serious environmental pollution problems.^1–3 Solar-light driven photocatalytic photocatalysis has been regarded as an ideal “green strategy” for environmental remediation and energy conversion.^4,5 Furthermore, the potential success of this strategy relies largely on the development of semiconductor materials which have some key requirements, including: the material should be able to absorb visible light to maximize use of the solar spectrum, and the electrons and holes will migrate to reactive sites instead of recombining with each other. The material should also be abundant, cheap, non-toxic, and stable in different reaction environments.^6–8

Polymeric graphitic carbon nitride (g-C₃N₄) is a paradigm photocatalyst due to its unique electric, optical, structural, and physiochemical properties.^9–11 State-of-the art progress has been achieved in the photocatalysis over g-C₃N₄ semiconductor materials since Wang and his co-workers first reported the photocatalytic H₂ evolution over g-C₃N₄ in 2009.¹² Unfortunately, a single g-C₃N₄ alone is very difficult to satisfy the practical application because of the ultrafast electron–hole recombination rate, low surface area of g-C₃N₄ (∼10 m² g⁻¹), and small surface active sites for sluggish uphill H₂-evolution reactions.¹³ In these circumstances, ever more endeavors are devoted to developing g-C₃N₄-based semiconductor materials through different thermodynamic (e.g., doping and sensitization) and kinetics (e.g., constructing heterojunctions, Z-scheme systems, fabricating micro/nano architectures, and loading proper cocatalysts) methods.^10,11,14 In recent years, researchers have carried out a lot of research in this area. For example, Cui et al. developed a facile and in-air chemical vapor deposition (CVD) method that produces onion-ring-like g-C₃N₄ microstructures.¹⁵ The optimal synthesized sample (RCN-350) shows a high H₂ evolution rate of 1900.0 μmol h⁻¹ g⁻¹ under visible light irradiation. The g-C₃N₄/V_o-ZnO hybrid photocatalyst with a g-C₃N₄ content of 1 wt% exhibited enhanced photocatalytic activity for degradation of organic contaminants than pure V_o-ZnO and g-C₃N₄ under visible light irradiation.¹⁶ Very recently, Xu et al. firstly reported that the photocatalytic activity of 1,1′-bis(4-carboxylatobenzyl)-4,4′-bipyridinium dichloride (denoted as CBV²⁺) coupled with g-C₃N₄ through hydrogen bonds.¹⁷ When 1 wt% CBV²⁺ is introduced, the hydrogen production rate of g-C₃N₄/CBV²⁺ dramatically increases up to 41.57 μmol h⁻¹, exceeding 85 times the rate over bare g-C₃N₄ (only 0.49 μmol h⁻¹).

Among above mentioned various kinds of methods, loading proper cocatalysts is extensively considered one effective approach to improve the photocatalytic behavior of g-C₃N₄-based photocatalysts, which can simultaneously achieve the promoted charge separation, accelerated surface reaction kinetics, and suppressed surface back reactions. Up to now, many heterostructures such as g-C₃N₄/Ag, g-C₃N₄/AgCl,¹⁸ g-C₃N₄/Bi₂WO₆, g-C₃N₄/black phosphorus,¹⁹ have been reported to be active photocatalysts and were found to show excellent photocatalytic activity toward the decomposition of organic pollutants and H₂ generation from water splitting under visible-light illumination. Compared to the low natural abundance and high cost of noble metals, the earth-abundant metals and their compounds seem to be more promising for practical applications on a large scale. Copper sulfide (CuS), with a relatively narrow band gap, is a nontoxicity, good photosensitivity, and excellent physical and chemical stability, eco-friendly and economic suitable materials, which has gained immense interest.²⁰ Furthermore, the energy levels of CuS and g-C₃N₄ match and overlap each other to construct the heterojunction, thereby showing excellent visible-light photocatalytic activity.

In the present study, we report a visible light-driven photocatalytic H₂ production and organic pollutant removal based on g-C₃N₄/CuS material prepared by a simple in situ growth hydrothermal method. The as-prepared nanocomposite at an optimized loading exhibited a much higher visible light photoactivity, giving up to 2.7 times and 1.5 times enhancements in comparison to pure g-C₃N₄ for photocatalytic H₂ production and methylene orange (MO) degradation, respectively. Furthermore, the possible underlying mechanisms were proposed. This study may open a new avenue for developing high efficiency, non-toxic, and low-cost g-C₃N₄-based photocatalysts for the visible-light photocatalysis.

2. Experimental section

2.1 Materials

Urea, copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O), thioacetamide (TAA), methyl orange (MO), 1,4-benzoquinone (BQ), disodium triethylamine (TEA), and tert-butyl alcohol (t-BuOH) were purchased from Aladdin Reagent Co. Ltd. All chemicals were analytically pure and used without further purification. The absolute pure water, purified by a Millipore Ultrapure water system and having a resistivity of 18.2 MΩ cm at 25 °C, was used in the current investigation.

2.2 Synthesis of the g-C₃N₄/CuS nanocomposites

The synthesis procedures of g-C₃N₄/CuS nanocomposites are shown in Scheme 1. 30 g of urea was placed into an alumina crucible with a cover, and then the crucible was heated in a muffle furnace at 550 °C for 3 h with a heating rate of 1 °C min⁻¹. The yielded yellow powder was washed with nitric acid (0.1 mol l⁻¹) and distilled water to remove any residual alkaline species (e.g. ammonia) adsorbed on the surface of the product, and then the product was dried at 80 °C for 12 h. The g-C₃N₄/CuS nanocomposites were prepared by a hydrothermal method. Typically, 0.25 g of g-C₃N₄ was dispersed into 20 ml of deionized water under ultrasonication for 0.5 h, and then a certain amount of Cu(NO₃)₂·3H₂O and TAA were added slowly to the dispersion with the same molar of Cu²⁺ and S²⁻. After ultrasonication for another 0.5 h, the mixture was transferred into a 100 ml a Teflon bottle held in a stainless-steel autoclave, sealed, and maintained at 160 °C for 3 h. After naturally cooling down to room temperature, the solid product was collected and washed thoroughly by centrifugation with deionized water, and then dried at 60 °C for 12 h. According to this method, different weight ratios of g-C₃N₄/CuS samples with CuS contents of 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1.0 wt% were synthesized. Pure CuS was prepared similarly without introducing g-C₃N₄.

Scheme 1 Schematic illustration for the synthesis of the g-C₃N₄/CuS photocatalyst.

2.3 Characterization

The X-ray diffraction (XRD) patterns of the samples were recorded on a D8 Focus diffractometer (Bruker) with use of Cu Kα radiation (λ = 0.154 nm). The microstructure and composition of the photocatalysts were studied by a transmission electron microscope (TEM, JEOL 2100F, operated at 200 kV), combined with energy-dispersive X-ray (EDX) spectroscopy. The UV-vis diffuse reflectance spectra (DRS) were recorded on a UV-vis spectrometer (UV-2550, Shimadzu, Japan) equipped with an integrating sphere. The photoluminescence (PL) spectra were measured by using a fluorescence spectrophotometer (F4600, Hitachi, Japan) with the excitation wavelength of 300 nm.

2.4 Photocatalytic hydrogen evolution

The photocatalytic hydrogen production reactions were performed in Perfect Light Labsolar-6A automatic online photocatalytic analysis system (Labsolar-6A, Beijing Perfect light Co., Ltd.). In a typical experiment, 50 mg photocatalyst was well dispersed in a 100 ml aqueous solution containing 10 ml of triethanolamine as the sacrificial reagent. The reactant solution and the system were thoroughly evacuated several times to insure the complete removal of the air. The photocatalytic reaction was triggered by the visible-light irradiation offered by a 300 W Xe lamp (PLS-SXE 300 W xenon lamp Beijing Perfect light Co., Ltd.) with a 420 nm cut-off filter (Fig. S1†) under continuous stirring. The temperature of the reaction solution was maintained at room temperature by a water-cooling system. The amount of evolved H₂ was analyzed by an online gas chromatograph (Shimadzu GC-2014) equipped with a thermal conductivity detector and high-purity Ar carrier gas was used to analyze reaction-evolved gases.

2.5 Photodegradation of MO and the detection of active species

The photodegradation performances of the as-prepared g-C₃N₄/CuS photocatalysts were investigated by photodegrading MO dye in aqueous solution. 10 mg of the as-prepared photocatalysts were added into 20 ml of MO (10 mg l⁻¹) solution in a 50 ml quartz reactor with circulating cooling water to keep the reaction temperature constant. Before illumination, the mixed suspension was magnetically stirred in the dark for 30 min to obtain the adsorption–desorption equilibrium. A 300 W xenon lamp filtered by a UV cut-off filter (λ > 420 nm) was used as the visible light source. At certain time intervals, 0.5 ml of the reaction solution was taken out and centrifuged to remove the catalyst, then analyzed on UV-vis spectrometer to detect the residual concentration of MO in the solution. In addition, in order to detect the generated active species in the photocatalysis, 1,4-benzoquinone (BQ) (1 mM), disodium ethylenediaminetetraacetate (Na₂EDTA) (1 mM), and tert-butyl alcohol (t-BuOH) (1 mM) were used as superoxide radical (·O₂⁻), hole, and hydroxyl radical (·OH) scavengers, respectively, with all other conditions being the same.

2.6 Electrochemical analysis

The photoelectrochemical (PEC) measurements were performed with an electrochemical workstation (CHI 660E, CH Instruments) in a standard three electrode cell, using a Pt wire and a Ag/AgCl electrode (3 M KCl) as the counter and reference electrode, respectively. The working electrode was prepared on fluorine-doped tin oxide (FTO) glass with its boundary being protected by Scotch tape. Five milligrams of as-synthesized powder was dispersed into 1 ml of dimethylformamide under sonication for 30 min to get a colloidal dispersion. The dispersion was drop-casted onto the FTO glass. After natural air drying, the uncoated part of the FTO glass was isolated with epoxy resin glue. The 0.2 M of Na₂SO₄ (pH = 6.8) aqueous solution prepurged with nitrogen for 30 min was used as an electrolyte. A solar simulator was utilized as a light source for the measurements. Nyquist plots were recorded over the frequency range of 100 mHz to 100 kHz at a bias of 0.2 V.

3. Results and discussion

3.1 Characterization

The crystallinity and phase purity of the as-prepared samples were characterized by XRD (Fig. 1). For the pure CuS, it can be seen that all the peaks of the sample can be readily indexed to a pure hexagonal phase CuS according to the JCPDS card no. 06-0464 (a = b = 3.792 Å and c = 16.344 Å). As for the pure g-C₃N₄ sample, it shows diffraction peaks at 13.1° and 27.4°, which can be indexed as the (100) crystal plane of tri-s-triazine units and the (002) diffraction for interlayer stacking of aromatic systems of graphitic materials, respectively. For the g-C₃N₄/CuS samples, all the samples show quite similar profiles, that is, the XRD patterns show g-C₃N₄ and CuS phases. With an increasing amount of CuS from 0.25 wt% to 1.0 wt%, the diffraction peaks of CuS are intensified gradually, whereas the intensities of the peaks of g-C₃N₄ are weakened. These results indicated that CuS NPs were successfully loaded on the g-C₃N₄ by the present synthetic route.

Fig. 1 XRD patterns of CuS, g-C₃N₄, g-C₃N₄/x wt% CuS (x = 0.25, 0.5, 0.75, and 1.0).

The further morphology and microstructure of the as-prepared samples were analyzed by transmission electron microscopy (TEM) technique in Fig. 2. The typical TEM image (Fig. 2A) of g-C₃N₄ shows 2D sheet-like nanostructures with transparent thin layers resembling the graphene nanosheets. For CuS sample (Fig. 2B), irregularly aggregation can be observed. However, surprisingly, by the hydrothermal method to prepare g-C₃N₄/CuS nanocomposite, the TEM result (Fig. 2C) clearly confirmed the successful loading of CuS NPs onto the g-C₃N₄ nanosheet surface, and no free CuS NPs were present in the suspension. A higher magnification TEM image (Fig. 2D) clearly demonstrate that the average size of CuS NPs is around 11.0 ± 4.4 nm (Fig. 2E). The HRTEM image (inset in Fig. 2D) shows that the lattice fringes with d-spacing of 0.31 nm can be assigned to the (102) crystal plane of hexagonal CuS phase. These results indicate that the g-C₃N₄ nanosheets significantly influence the growth of CuS NPs and effectively restrain their aggregations. It could be attributed to two possible reasons. First, the g-C₃N₄ nanosheet may act as a two-dimensional “mat” that interacts with CuS NPs through physisorption to hinder their aggregation. Second, the oxygen-containing defects and the amino groups on g-C₃N₄ surface could serve as anchor sites to immobilize Cu²⁺ ions as well as CuS NPs on g-C₃N₄ nanosheet, which prevents the aggregation of CuS NPs from surface diffusion on g-C₃N₄ nanosheet.²¹ The energy dispersive X-ray (EDX) results explicitly illustrated the existence of C, N, Cu and S elements in the region (Fig. 2F).

Fig. 2 TEM images of (A) pure g-C₃N₄, (B) pure CuS, (C) g-C₃N₄/CuS. (D) The EDX of g-C₃N₄/CuS sample. The inset in (C) is the HR-TEM image of the g-C₃N₄/CuS sample. (E) The size distribution profile of the CuS NPs. (F) The EDX of g-C₃N₄/CuS sample.

The UV-vis spectra were used to study the optical character of the as-obtained samples. As shown in Fig. 3A, the pure g-C₃N₄ displays intense absorption bands with absorption edges at around 450 nm, suggesting a band gap of about 2.70 eV. When the CuS NPs were loaded on the g-C₃N₄ nanosheet, intensities of the absorption in the visible light region (λ > 450 nm) are gradually enhanced with increasing the CuS content. This enhancement is ascribed that pure CuS has an obvious wide absorption range from 300 nm to 800 nm. Thus, the presence of CuS NPs in the composites can reduce reflection of light and enhance the absorption. What is more, the g-C₃N₄/CuS composites show almost the same absorption edge in comparison to the pure g-C₃N₄, indicating CuS is not incorporated into the lattice of g-C₃N₄ and only are deposited on it's surface. We also estimated the band gap energies (E_g) of g-C₃N₄ CuS by using the Kubelka–Munk transformation:²²

αhv = A(hv − E_g)^n/2

where α, h, v, and A are the absorption coefficient, Planck constant, light frequency, and a constant. The n depends on the characteristics of the transition in a semiconductor. According to previous report, the values of g-C₃N₄ and CuS are 4 and 1, respectively.^23,24 Thus, as displayed in Fig. 3B and C, the value of band gap for g-C₃N₄ and CuS are determined to be 2.7 eV and 1.9 eV, respectively. In addition, the potentials of valence band (E_VB) and conduction band (E_CB) of a semiconductor material could be calculated via the following empirical equations:

E_VB = X_{semiconductor} − E^e + 0.5E_g

E_CB = E_VB − E_g

where X_{semiconductor} is the electronegativity of the semiconductor, E^e is the energy of free electrons vs. hydrogen (about 4.5 eV/NHE). The X_{semiconductor} values of g-C₃N₄ and CuS are 4.64 eV and 5.27 eV, respectively. So the E_VB and E_CB of g-C₃N₄ and CuS are 1.49 eV/NHE, −1.21 eV/NHE and 1.72 eV/NHE, −0.18 eV/NHE, respectively.

Fig. 3 (A) UV-vis diffuse reflectance spectra of CuS, g-C₃N₄, g-C₃N₄/x wt% CuS (x = 0.25, 0.5, and 1.0). The plot of (αhv)² versus energy (hv) for the band gap energy of (B) g-C₃N₄ and (C) CuS.

3.2 Photocatalytic performance

The photocatalytic activity of g-C₃N₄/CuS composites was first evaluated by hydrogen production reaction under visible light (λ > 420 nm), and the influence of photocatalysts containing different mass ratios of CuS on the H₂ evolution is presented in Fig. 4. Fig. 4A shows the typical H₂ evolution kinetics of different mass ratios of CuS under visible light irradiation. It should be noted that linearly increasing amounts of H₂ evolved in all samples could be easily observed over the entire time range of light irradiation, confirming the relatively excellent photostabilities for all samples. There is a phenomenon is observed that no H₂ can be detected when CuS alone is used as the photocatalyst, suggesting that pure CuS is not active for photocatalytic H₂ production. However, CuS has a significant influence on the photocatalytic H₂ production activity in the g-C₃N₄/CuS composites. The photocatalytic activities are gradually enhanced by increasing the CuS content, and it reaches a maximum value of 11.80 μmol h⁻¹ over g-C₃N₄/0.50 wt% CuS sample, which is about 2.7 times higher than pure g-C₃N₄ (4.37 μmol h⁻¹) (Fig. 4B). When CuS content is increased beyond 0.50 wt%, a decrease appeared in the photocatalytic H₂ evolution results. Especially for g-C₃N₄/1.0 wt% CuS sample, the activity of H₂ production decreases obviously, even slower than g-C₃N₄. The experiment on cyclic performance for H₂ production was run under visible-light illumination (λ > 420 nm) for 5 consecutive cycles; no obvious decay of H₂ production was observed, suggesting their quite good stability during water splitting (Fig. 4C).

Fig. 4 (A) The photodegradation activities of MO as a function of time over different photocatalysts under visible light irradiation (>420 nm), (B) the value of the rate constant k of the photodegradation of MO in the presence of as-prepared photocatalysts, (C) cycling runs of g-C₃N₄/0.5 wt% CuS photocatalyst in the photodegradation of MO under visible light irradiation (>420 nm), and (D) photodegradation of MO in the presence of three types of scavengers and g-C₃N₄/0.5 wt% CuS photocatalyst under visible light irradiation (>420 nm).

The photocatalytic activity of the g-C₃N₄/CuS composites was further assessed by the photodegradation of MO at room temperature under visible light (λ > 420 nm) irradiation (Fig. 5). MO was used as a target pollutant to evaluate the photocatalytic activity of the as-prepared samples because the photodegradation of MO is negligible under visible light, confirming that the photocatalytic activity indeed originates from the photocatalyst. Prior to irradiation, the suspensions were magnetically stirred in dark for 30 min to obtain the absorption–desorption equilibrium between the photocatalysts and MO. The variation in the absorption intensity of MO solution over CuS, C₃N₄, and g-C₃N₄/x wt% CuS (x = 0.25 wt%, 0.50 wt%, 0.75 wt%, and 1.0 wt%) at different irradiation times is recorded in Fig. S2 in ESI.† As shown in Fig. 5A, 0% and 40.5% MO are photodegraded by CuS and pure g-C₃N₄, respectively. While the g-C₃N₄/CuS composites show obviously enhance photocatalytic activities than pure g-C₃N₄ or CuS, especially the g-C₃N₄/0.50 wt% CuS shows the highest photodegradation ratio after 60 min, about 53.9% MO was decomposed. Furthermore, as the CuS content in the composites increases, the photocatalytic activity of the composite starts to decrease. The kinetic rate constants of the samples were obtained by plotting ln(C₀/C) with the visible light irradiation time (minute), which were fitted with the pseudo-first-order model (Fig. 5B). The apparent rate constants calculated were 0.00889 min⁻¹, 0.01301 min⁻¹, 0.01355 min⁻¹, 0.01014 min⁻¹, 0.00860 min⁻¹ and 0 min⁻¹ for g-C₃N₄, g-C₃N₄/0.25 wt% CuS, g-C₃N₄/0.50 wt% CuS, g-C₃N₄/0.75 wt% CuS, g-C₃N₄/1.0 wt% CuS and CuS, respectively. The highest rate constant (0.01355 min⁻¹) is achieved by the g-C₃N₄/0.50 wt% CuS, which is abut 1.5 times higher than the g-C₃N₄ (0.00889 min⁻¹). Meanwhile, CuS doesn't show any visible light activity in the photocatalytic degradation of MO. To be served as a good photocatalyst, except for the enhanced visible light activity, the reusability and stability are also extremely important. The photocatalytic degradation of MO under visible light was carried out for 5 cycles in the presence of g-C₃N₄/0.50 wt% CuS photocatalyst (Fig. 5C). After 5 consecutive cycles, the g-C₃N₄/0.50 wt% CuS photocatalyst still shows very high photocatalytic degradation rate of MO. This result indicates that the as-prepared g-C₃N₄/CuS photocatalyst possesses excellent reusability and photostability.

Fig. 5 (A) Photocatalytic H₂ production of the as-prepared photocatalysts under visible light irradiation (λ ≥ 420 nm). (B) The rate of H₂ generation over the as-prepared photocatalysts. (C) Stability study of g-C₃N₄/0.5 wt% CuS for H₂ evolution under visible light irradiation (λ ≥ 420 nm).

In order to detect the main active species in the photocatalytic process, such as photogenerated holes, superoxide radicals (·O₂⁻) and hydroxyl radicals (·OH), the trapping experiments in the presence of various scavengers were operated. As shown in Fig. 4D, under visible light irradiation of the as-prepared g-C₃N₄/0.50 wt% CuS photocatalyst, the photodegradation rate of MO was slightly suppressed by the addition of ·OH radical scavenger (t-BuOH, 1 mM), which reveals that ·OH radicals are not the main active species for the photodegradation of MO in current photocatalytic systems. However, in the presence of the ·O₂⁻ radical scavenger (BQ, 1 mM) and the hole scavenger (Na₂EDTA, 1 mM), the photodegradation rate of MO was decelerated significantly, with the photocatalytic degradation rate being reduced. It means that the ·O₂⁻ and holes play the major roles in the photodegradation of MO over the as-prepared g-C₃N₄/0.50 wt% CuS photocatalyst under visible light irradiation.

Photoelectrochemical (PEC) measurements, including electrochemical impedance spectroscopy (EIS) and transient photocurrent responses, of g-C₃N₄ and g-C₃N₄/0.5 wt% CuS samples are shown in Fig. 6. The high frequency region of Nyquist plots, providing useful information on charge transfer resistance, are shown in Fig. 6A. The arc radius on the EIS Nyquist plots of the g-C₃N₄/0.5 wt% CuS is smaller than that of g-C₃N₄, indicating that the loading of CuS NPs can reduce the charge transfer resistance and thus accelerate the interfacial charge transfer. Furthermore, the transient photocurrent measurement was carried out during light on–off cycles (Fig. 6B) to assess the charge carrier generation and transfer performance in the photoreaction system. The saturation photocurrent densities remained constant with light on, and immediately dropped to nearly zero once the light was switched off. The photocurrent density over g-C₃N₄/0.5 wt% CuS (∼1.05 μA cm⁻²) is about 3.2 times higher than that of the g-C₃N₄ (∼0.33 μA cm⁻²). The increased photocurrent confirms that the loaded CuS NPs could facilitate the separation and prolong the lifetime of the photoinduced charge carriers, which is responsible for the enhanced photocatalytic activities in water splitting and MO degradation. Moreover, the almost unchanged photocurrent response during repeated light on–off cycles is another evidence of the excellent stability of the as-prepared g-C₃N₄/CuS photocatalysts.

Fig. 6 (A) EIS Nyquist plots of g-C₃N₄ and g-C₃N₄/0.5 wt% CuS. (B) Transient photocurrent density response of g-C₃N₄ and g-C₃N₄/0.5 wt% CuS during on/off cycles under 0.2 V bias vs. Ag/AgCl in a 0.2 M Na₂SO₄ electrolyte solution under solar simulator irradiation.

The PL spectrum is widely employed to evaluate the separation efficiency of charge carriers in photocatalysts. The PL spectra of g-C₃N₄ and g-C₃N₄/CuS samples are shown in Fig. 7. After 300 nm excitation, g-C₃N₄ and g-C₃N₄/CuS samples exhibit broad emission peaks in the range of 420–600 nm, representing that the photo-excited electrons recombine with holes. However, the PL intensity decreases after the introduction of CuS NPs, which indicates that the charge carries recombination in g-C₃N₄/0.5 wt% CuS was largely suppressed compared with g-C₃N₄.

Fig. 7 Room temperature PL spectra of g-C₃N₄, g-C₃N₄/0.5 wt% CuS under the excitation wavelength of 350 nm.

3.3 Possible photocatalytic mechanism

Based on the above results, the superior photocatalytic performance of the g-C₃N₄/CuS composites can be assigned to the interfacial transfer of photogenerated electrons and holes between g-C₃N₄ and CuS, which leads to effective charge separation on both parts. We illustrate a possible visible light photocatalytic mechanism of H₂ production and MO degradation in Fig. 8. For pure g-C₃N₄ and CuS, the photogenerated electron–hole pairs in both photocatalysts tend to recombine and only a small fraction of charge carriers could participate in the photocatalytic reaction because both pure photocatalysts showed less photocatalytic activities compared with composite materials. After heterojunction formation, g-C₃N₄ and CuS were excited simultaneously when the g-C₃N₄/CuS composites were irradiated with visible light, leading to generation of electron–hole pairs in the conduction band (CB) and valence band (VB), respectively. Since the CB of g-C₃N₄ (−1.21 eV vs. NHE) is more negative than that of CuS (−0.18 eV vs. NHE),^22,25 the photoinduced electrons on the CB of g-C₃N₄ would easily transfer to the CB of CuS, and the photoinduced hole on the VB of CuS would easily transfer to the VB of g-C₃N₄, which both would restrain the photoinduced electron–hole pairs recombination and also prolong the reaction time, thereby improving the efficiency of H₂ production from water and photodecomposition of MO. As a result, these electrons have longer time to reduce H⁺ to H₂ on the CuS surface. In the meantime, the holes on the VB of the g-C₃N₄ reacted with the sacrificial agent (triethanolamine) (Fig. 8A). In the case of photocatalytic degradation of MO (Fig. 8B), the electrons on the CB of g-C₃N₄ could react with O₂ to form ·O₂⁻ radicals and the holes on the VB of g-C₃N₄ are considered as the active species. While the electrons on the CB of CuS is high enough to react with O₂ to generate H₂O₂ for the reduction potential of O₂/H₂O₂ is 0.695 eV/NHE.²² This possible photocatalytic degradation mechanism of g-C₃N₄/CuS composite was also proved by using the active species trapping experiment before.

Fig. 8 Schematic diagram illustrating (A) the photocatalytic degradation of MO and (B) the photocatalytic H₂ production mechanisms over g-C₃N₄/CuS material under visible light irradiation.

4. Conclusion

In summary, highly active g-C₃N₄/CuS composites have been successfully prepared via the in situ hydrothermal method. The resulting composites photocatalysts exhibits significantly enhanced photocatalytic activities for H₂ production and MO degradation under visible-light irradiation in comparison with the pure g-C₃N₄ and CuS. The g-C₃N₄/0.50 wt% CuS composite shows the best photocatalytic H₂ production rate of 11.80 μmol h⁻¹, which is about 2.7 times higher than that of pure g-C₃N₄. In addition, the g-C₃N₄/0.50 wt% CuS composite presents the superior degradation efficiency of MO and reuse ability. This work not only introduces a simple strategy to in situ grow metal sulfide NPs on carbon nitrides surfaces, but also provides useful insights into the development of low-cost CuS as a substitute for noble materials in the photocatalysis via a facile method based on g-C₃N₄.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC 51402198, 21671139, 21201123), the National Key R&D Program of China (2018YFF01011400), the Natural Science Foundation of Liaoning Province (20170540715), the Fundamental Research of Educational Bureau of Liaoning Province (LQ2017005), the Liaoning Excellent Talents in University (LR2016016), Science and Technology Innovation Program for Middle-aged and Young Scientist of Shenyang (RC180066, RC180086), the Open Funds of the State Key Laboratory of Rare Earth Resource Utilization (RERU2018015, RERU2019009).

References

1. X. Chen, L. Liu, P. Y. Yu and S. S. Mao, Science, 2011, 331, 746.
2. Z. Xu, M. Quintanilla, F. Vetrone, A. O. Govorov, M. Chaker and D. Ma, Adv. Funct. Mater., 2015, 25, 2950–2960.
3. Z. Xu, Y. Liu, F. Ren, F. Yang and D. Ma, Coord. Chem. Rev., 2016, 320–321, 153–180.
4. A. Fujishima and K. Honda, Nature, 1972, 238, 238–245.
5. H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu and X. Wang, Chem. Soc. Rev., 2015, 45, 5234–5244.
6. M. Nolan, A. Iwaszuk, A. K. Lucid, J. J. Carey and M. Fronzi, Adv. Mater., 2016, 28, 5425–5446.
7. Q. Zhang, X. Jin, Z. Xu, J. Zhang, U. F. Rendón, L. Razzari, M. Chaker and D. Ma, J. Phys. Chem. Lett., 2018, 9, 5317–5326.
8. Y. Gao, C. Shi, J. Feng, G. Zhao, H. Yu, Y. Bi, F. Ding, Y. Sun and Z. Xu, RSC Adv., 2017, 7, 54555–54561.
9. Z. Xu, M. G. Kibria, B. AlOtaibi, P. N. Duchesne, L. V. Besteiro, Y. Gao, Q. Zhang, Z. Mi, P. Zhang, A. O. Govorov, L. Mai, M. Chaker and D. Ma, Appl. Catal., B, 2018, 221, 77–85.
10. Y. Gao, J. Lin, Q. Zhang, H. Yu, F. Ding, B. Xu, Y. Sun and Z. Xu, Appl. Catal., B, 2018, 224, 586–593.
11. Q. Zhang, J. Deng, Z. Xu, M. Chaker and D. Ma, ACS Catal., 2017, 7, 6225–6234.
12. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
13. J. Wen, J. Xie, H. Zhang, A. Zhang, Y. Liu, X. Chen and X. Li, ACS Appl. Mater. Interfaces, 2017, 9, 14031–14042.
14. Y. Wang, Q. Xia, X. Bai, Z. Ge, Q. Yang, C. Yin, S. Kang, M. Dong and X. Li, Appl. Catal., B, 2018, 239, 196–203.
15. L. Cui, J. Song, A. F. McGuire, S. Kang, X. Fang, J. Wang, C. Yin, X. Li, Y. Wang and B. Cui, ACS Nano, 2018, 12, 5551–5558.
16. Y. Liu, R. Wang, Z. Yang, H. Du, Y. Jiang, C. Shen, K. Liang and A. Xu, Chin. J. Catal., 2015, 36, 2135–2144.
17. Y.-N. Liu, C.-C. Shen, N. Jiang, Z.-W. Zhao, X. Zhou, S.-J. Zhao and A.-W. Xu, ACS Catal., 2017, 7, 8228–8234.
18. J.-X. Li, C. Ye, X.-B. Li, Z.-J. Li, X.-W. Gao, B. Chen, C.-H. Tung and L.-Z. Wu, Adv. Mater., 2017, 29, 1606009.
19. M. Zhu, S. Kim, L. Mao, M. Fujitsuka, J. Zhang, X. Wang and T. Majima, J. Am. Chem. Soc., 2017, 139, 13234–13242.
20. A. Manzi, T. Simon, C. Sonnleitner, M. Doblinger, R. Wyrwich, O. Stern, J. K. Stolarczyk and J. Feldmann, J. Am. Chem. Soc., 2015, 137, 14007–14010.
21. S.-W. Cao, Y.-P. Yuan, J. Fang, M. M. Shahjamali, F. Y. C. Boey, J. Barber, S. C. Joachim Loo and C. Xue, Int. J. Hydrogen Energy, 2013, 38, 1258–1266.
22. X. Chen, H. Li, Y. Wu, H. Wu, L. Wu, P. Tan, J. Pan and X. Xiong, J. Colloid Interface Sci., 2016, 476, 132–143.
23. W. Zhao, Z. Wei, H. He, J. Xu, J. Li, S. Yang and C. Sun, Appl. Catal., A, 2015, 501, 74–82.
24. L. Chen, J. He, Q. Yuan, Y.-W. Zhang, F. Wang, C.-T. Au and S.-F. Yin, RSC Adv., 2015, 5, 33747–33754.
25. Q. Liang, J. Jin, C. Liu, S. Xu, C. Yao and Z. Li, Inorg. Chem. Front., 2018, 5, 335–343.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra03532j

---