In situ ion-exchange synthesis of SnS₂/g-C₃N₄ nanosheets heterojunction for enhancing photocatalytic activity

Abstract

In this paper, free standing graphitic carbon nitride (g-C₃N₄) nanosheets have been synthesized by mixed solvents (water/IPA = 2/1) liquid phase exfoliation, and a series of SnS₂/g-C₃N₄ heterojunctions with different contents of SnS₂ have been prepared via a simple ion-exchange process. Exfoliated g-C₃N₄ presents a two-dimension sheet-like structure with the thickness of 2.8 nm and small SnS₂ nanoparticles with diameter of 5–10 nm are well anchored on the surface of g-C₃N₄ nanosheets, which was proved by transmission electron microscopy (TEM) and atomic force microscope (AFM). The 4.0-SnS₂/g-C₃N₄ sample shows the highest photocurrent density of 13.66 μA cm⁻² at 0.8 V, which is about 1.5 time of the g-C₃N₄ nanosheets and 2 times of the bulk g-C₃N₄, respectively. Photocatalytic measurement also demonstrated that constructing heterojunction of SnS₂/g-C₃N₄ can improve the photocatalytic efficiency as compared to pure g-C₃N₄ and g-C₃N₄ nanosheets. The highly effective photoelectrochemical and photocatalytic activities of SnS₂/g-C₃N₄ heterojunctions are attributed to the efficient separation of photogenerated hole–electron pairs. This work may provide a novel concept for the rational design of high performance g-C₃N₄-based photocatalysts.

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1. Introduction

Semiconductor photocatalysis is regarded as an effective and green technique since it is able to decompose a variety of environmental contaminants under mild conditions.^1–3 Over the past several years, graphite carbon nitride (g-C₃N₄) has attracted considerable attention owing to its photocatalytic properties, as well as its promising applications in the fields of catalysis, photoelectron apparatus and chemical sensors. The high chemical stability and desirable band gap (2.7 eV) of g-C₃N₄ make it an exceptionally promising organic semiconductor photocatalyst for water splitting and degradation of pollutant.^4,5 Wang et al. first reported that g-C₃N₄ can catalyze water splitting and produce the hydrogen in visible light;⁵ Zou et al. confirmed that g-C₃N₄ can oxidize the organic pollution under visible light irradiation, thus achieve the goal of dispelling the pollutant.⁶

However, the photocatalytic activity of pure g-C₃N₄ is low owing to its poor light-absorbance and fast recombination of photo-generated electron–hole pairs.⁶ Recent investigations show that single or several atom thickness g-C₃N₄ nanosheets have improved photocatalytic property, strong electrochemical luminescence activity, good biocompatibility and high fluorescent quantum yield compared with their bulk counterparts.^7–9 Usually, g-C₃N₄ nanosheets are prepared by liquid phase separation method in water or organic solvent by ultrasonic exfoliating process.^7,10,11 Zhao et al. prepared single atomic layer of g-C₃N₄ nanosheet and adopt it for degradation rhodamine B,¹² as a result, they found that the photocatalytical activity of single layer graphitic g-C₃N₄ was about 3 and 10 times higher than that of few layer g-C₃N₄ and bulk g-C₃N₄.

Constructing heteroconjection is another way to improve the photocatalytic performance of C₃N₄. Among various carbon nitride crystal forms, g-C₃N₄ possesses the smallest band gap, so it is easy to form a heterojunction,^13,14 for instance ZnO/g-C₃N₄,¹⁵ Bi₂WO₆/g-C₃N₄,¹⁶ g-C₃N₄/TiO₂ (ref. 17) and g-C₃N₄/SrTiO₃.¹⁸ Katsumata et al. prepared the g-C₃N₄/Ag₃PO₄ heterojunction with different weight ratios of g-C₃N₄ by a facile in situ precipitation method, which showed highly effective photocatalytic activity under visible light irradiation. Hou et al. constructed 2D porous graphitic C₃N₄ nanosheets/nitrogen-doped graphene/layered MoS₂ ternary nanojunction.¹⁹ Chen et al. used ion exchange process to prepare the g-C₃N₄/NiS composite material and found that the sample containing 1.5% NiS exhibited the highly photocatalytic activity;²⁰ Young-Si Jun et al. adopt a new approach to assemble low-dimensional g-C₃N₄ into a three-dimensional structure, which significantly improved photocatalytic properties.²¹

SnS₂ is a nontoxic and low-priced semiconductor material with a visible-light band gap of 2.2–2.35 eV. So, we choose SnS₂ to form heteroconjections with few layer g-C₃N₄ nanosheets for increasing photon-generated carrier transfer. In this paper, we have prepared free standing g-C₃N₄ nanosheets by liquid phase exfoliation, and successfully synthesized SnS₂/g-C₃N₄ nanosheet heterojunction via a simple ion-exchange process. Moreover, we explored the photoelectrochemical response and photocatalytic performance of SnS₂/g-C₃N₄ heterojunction, which exhibited the higher photocurrent and photocatalytic efficiency compared with pure g-C₃N₄ nanosheets. Additionally, a smaller charge transfer resistance was observed for the heterojunction under visible light irradiation.

2. Experimental section

2.1 Preparation of photocatalysts

All reagents were analytical grade and purchased from Shanghai Reagents Company (Shanghai, China). The bulk g-C₃N₄ was synthesized by polymerization of melamine molecules under high temperature. In brief, 3 g melamine was added in a crucible with a cover and heated at 500 °C for 2 h under air atmosphere with a ramp rate of about 10 °C min⁻¹, then heated at 520 °C for 2 h with a ramp rate of 5 °C min⁻¹. After cooling to room temperature, the obtained yellow product was the g-C₃N₄ powder.

The g-C₃N₄ nanosheets were obtained by liquid exfoliation of as-prepared bulk g-C₃N₄ in various solvents via sonication. Typically, 20 mg of bulk g-C₃N₄ was added into 20 mL flask with 15 mL different solvents such as isopropyl alcohol (IPA), water, ethanol, formamide, N-methyl-pyrrolidone (NMP), or the mixture solvents (water and IPA). Then, the flask was ultrasounded for 10 h. The initial formed suspension was centrifuged at about 3000 rpm to remove aggregates, and then, the supernatant was collected, the exfoliated g-C₃N₄ nanosheets were obtained, centrifuged and dried in vacuum for further characterization.

The SnS₂/g-C₃N₄ heterojunctions were prepared through an ion-exchange method at room temperature, as shown in Scheme 1. Firstly, 30, 60, 90 or 120 μL of 0.01 M SnCl₄ solution was dropped in the 10 mL g-C₃N₄ nanosheets dispersion (a concentration of 0.27 mg mL⁻¹). After stirring 20 min, 60, 120, 180 or 240 μL of 0.01 M Na₂S solution was added into the above mixed dispersion. The mixed solution was stirred for another 5 minutes, then the solution was centrifuged at 14 000 rpm for 10 min, and the precipitates were collected and washed with ethanol and deionized water 3 times, respectively. The products were dried for 12 h in a vacuum oven at 80 °C. The mass ratio of SnS₂ to g-C₃N₄ nanosheets are 2%, 4%, 6%, and 8% in the final samples, thus they are denoted as 2.0-SnS₂/g-C₃N₄, 4.0-SnS₂/g-C₃N₄, 6.0-SnS₂/g-C₃N₄ and 8.0-SnS₂/g-C₃N₄, respectively.

Scheme 1 Schematic representation of the deposition of SnS₂ nanoparticles on g-C₃N₄ sheets.

Bulk SnS₂ were synthesized by precipitation method. Briefly, 20 mL of 0.01 M SnCl₄ solution was dropped in 50 mL of 0.01 M Na₂S solution and the mixed solution was stirred for 30 minutes, then the precipitate was collected by centrifuged.

2.2 Characterization

The crystal structures of the as-synthesized samples were characterized by PANalytical X'Pert PRO X-ray diffraction (XRD) with CuKα radiation (λ = 1.54 Å) in the range of 10 to 80°. Fourier transform infrared (FT-IR) spectra were obtained on a Fourier transform infrared spectroscopy (Nicolet iS10, Thermo Fisher). The microstructures of the as-prepared samples were characterized by the transmission electron microscopy (TEM, JEOL, JEM-2100F) and atomic force microscope (AFM, NT-MDT, Ntegra Prima SPM). The diffuse reflectance spectra (DRS) of the as-obtained samples were measured by a UV/VIS/NIR Spectrometer (PerkinElmer, Lambda 750) in the range of 200 to 1100 nm. BaSO₄ was used as the reflectance standard material. The X-ray Photoelectron Spectroscopy (XPS) were taken by ESCALAB 250Xi system (Thermo Electron Corporation, USA). The excitation source was AlKα radiation (photoelectron energy of 1253.6 eV). Survey scans and relevant core levels were recorded: Sn 3d, S 2p, C 1s and N 1s.

2.3 Photoelectrochemical and photocatalytic measurement

Photoelectrochemical measurements were performed on a conventional three-electrode cell under dark and simulated sunlight irradiation (350 W Xe lamp, a cutoff filter of 420 nm). The samples of the above were spin-dropped on the 1 cm × 1 cm indium tin oxide (ITO) glass at 600 rpm for 30 s, and then dried at 60 °C for 1 h for obtaining working electrodes with a similar film thickness. Pt wire worked as the counter electrode and Ag/AgCl electrode as the reference electrode and 0.2 M Na₂SO₄ solution was used as the electrolyte. For Nyquist plots measurements, the perturbation signal was 20 mV and the frequency ranged from 100 kHz to 10 mHz.

The photocatalytic activity of powder samples was evaluated by the degradation of methyl orange (MO) The optical system for the degradation reaction included a 350 W Xe lamp, a cutoff filter of 420 nm. In each experiment, 20 mg photocatalyst was added to 100 mL dye aqueous solution (10 mg L⁻¹). Before irradiation, the suspensions were magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption/desorption equilibrium between the photocatalyst and the dye. During the reaction process under visible light illumination, 3 mL of suspension was sampled at given time intervals and centrifuged to remove the photocatalyst particles. The concentration of MO was determined by monitoring the height of the maximum of the absorbance in ultraviolet-visible spectra.

To explore the photo-induced radical species in the photocatalytic degradation of MO, the reactive species trapping experiments were performed. The benzoquinone and IPA were utilized as the scavenger of the superoxide radical (·O₂⁻) and hydroxyl (·OH), respectively. For the h⁺ hole, disodium ethylenediaminetetraacetate (EDTA–2Na) was used as quencher. The trapping experiment taken was similar to the above photocatalytic activity test except that the different scavengers were added into the MO solution.

3. Results and discussion

Bulk g-C₃N₄ was suspended in various solvents such as IPA, NMP, water, ethanol, and formamide to create light yellow dispersions with the same concentration of 2 mg mL⁻¹, and then sonicated at room temperature for the gradual exfoliation of bulk g-C₃N₄ to nanosheets (as illustrated in Fig. S1†). Similar method is used to exfoliate various layered materials, such as boron nitride,²² MoS₂,^23,24 to single or few-layers nanosheets via the interaction of solvent molecules. Liquid exfoliation effect is significantly influenced by the surface energies of used solvents. Organic molecules could be similarly inserted into layers of the bulk material and generate interaction when surface energy matching. As a result, layer material was gradually exfoliated to single-or few-layer nanosheets. From Fig. S1,† it can be seen that g-C₃N₄ nanosheets can be well dispersed in water, isopropyl alcohol and NMP. Although g-C₃N₄ nanosheets easily disperse in water, it is not stable and tends to sedimentation. While, g-C₃N₄ nanosheets were very stably disperse in IPA (shown in Fig. S2†) just with a low yield after sonication. Meanwhile, NMP has the high boiling point, so it was difficult to remove. Thus, we choose the mixed solvents of water and isopropyl alcohol as a dispersant. From Fig. 1, it is observed that g-C₃N₄ nanosheets concentration in the solvent is gradually decreased with the reduction of the amount of water. The prepared g-C₃N₄ nanosheets have a relatively high concentration and a better stability when the volume ratio of water and IPA is 2 : 1 (as illustrated Fig. S2†). Therefore, we exfoliated bulk g-C₃N₄ for ensure the stability and dispersion using the mixed solvents of water and IPA with volume ratio of 2 : 1.

Fig. 1 Photographs of the dispersions of g-C₃N₄ nanosheets in water, IPA, and the mixture.

The crystal structure of the obtained samples was investigated by X-ray diffraction (XRD). As seen clearly from Fig. 2, the pure g-C₃N₄ nanosheets and SnS₂/g-C₃N₄ heterojunction have two distinct diffraction peaks at 27.5° and 13.2°, which can be indexed for graphite-like materials as the (002) and (100) diffraction plane. The (002) peak at 27.5° and the (100) peak at 13.2° corresponding to distance of 0.324 and 0.670 nm reflects the characteristic inter-planar stacking peak of conjugated aromatic systems and in-plane structural packing motif, respectively.²⁵ In addition, the characteristic diffraction peaks of SnS₂ appear at 51.8°. Seen from the illustration of Fig. 2, the diffraction peak at 51.8° gradually increase with the increasing amount of SnS₂ for SnS₂/g-C₃N₄ heterojunctions.

Fig. 2 XRD patterns of pure g-C₃N₄, pure SnS₂ and SnS₂/g-C₃N₄.

The atomic structure of bulk g-C₃N₄ and g-C₃N₄ nanosheets were characterized by FT-IR spectroscopy. As shown in Fig. 3, the absorption band of 3000–3600 cm⁻¹ is due to NH stretching vibration mode, and absorption band of 1200–1700 cm⁻¹ is ascribed to the stretching vibration mode of CN hetero-ring.²⁶ Note that, strong absorption peak at 809 cm⁻¹ is the bending vibration peak of the s-triazine ring.²⁷ In the case of SnS₂ nanosheets, a broadband peaked at 545 cm⁻¹ is originated to the vibration of Sn–S bands.²⁸ However, this feature band is difficult to distinguish on the curve of SnS₂/g-C₃N₄ heterojunction nanosheets due to the low content of SnS₂ nanoparticle.

Fig. 3 FT-IR spectra of bulk g-C₃N₄ and g-C₃N₄ nanosheets.

The chemical compositions of the as-prepared photocatalysts are characterized by XPS spectra, as shown in Fig. 4. The appearance of elements Sn, S, C, N in survey XPS spectrum of 4.0-SnS₂/g-C₃N₄ hybrid confirms the presence of both components C₃N₄ and SnS₂, as shown in Fig. 4a. The high-resolution spectrum of C 1s (Fig. 4b) can be deconvoluted into three peaks at 284.6 eV, 285.8 eV and 288.1 eV, corresponding to sp²-bonded aromatic structure (N–C=N), graphitic carbon (C–C), and residual C–O, respectively.^10,29 As to the N 1s spectrum of g-C₃N₄ nanosheets (Fig. 4c), N 1s peak at 398.5 eV corresponds to sp² hybridized aromatic N bonded to carbon atoms (C=N–C). The peak at 399.56 eV is assigned to the tertiary N bonded to carbon atoms in the form of N–(C)₃ or H–N–(C)₂. The weaker peak with a high binding energy at 400.9 eV is attributed to quaternary N bonded to three carbon atoms in the aromatic cycles and the peak centered at 404.3 eV correspondes to the π excitations.^30,31 The evolution of Sn 3d with two strong peaks is observed at 486.5 and 494.9 eV (Fig. 4d), which were attributed to Sn 3d_5/2 and Sn 3d_3/2, respectively.³² However, the peak relating to the C=N–C bonds of g-C₃N₄ nanosheets is shifted slightly to the higher binding energy side after loading SnS₂ and the S 2p peaks of SnS₂, reflecting to S–Sn bond,³² are also moved to the low binding energy side (Fig. 4e), when coupling with g-C₃N₄ nanosheets. It is indicated that there might be some interactions between the “nitrogen pots” of g-C₃N₄ with the Sn species of SnS₂. The observed binding energies of Sn 3d and S 2p spectra are in good accordance with Sn⁴⁺ and S²⁻ of SnS₂.

Fig. 4 XPS survey spectrum (a), high resolution of C 1s (b), N 1s (c), Sn 3d (d), S 2p (e) spectra of as-prepared photocatalysts.

TEM image of the g-C₃N₄ nanosheets (Fig. 5c) clearly presents a two-dimension sheet-like structure with relatively smooth surface. For revealing microstructures and the thickness, the diluted dispersion of the products obtained after ultrasonication treatment of bulk g-C₃N₄ was further investigated by AFM analyses. The AFM image (Fig. 5a) shows the exfoliated products are free-standing nanosheets with a large lateral size of 300–400 nm. Fig. 5b presents the thickness of the products is about 2.8 nm, suggesting 8–9 atomic monolayers.^33,34 In addition, the exfoliated sample can be formed to stable colloidal dispersion over a period of two weeks and exhibits a typical Tyndall effect. The TEM image of SnS₂/g-C₃N₄ hybrid is displayed in Fig. 5d, which shows that some small SnS₂ nanoparticles with diameter of 5–10 nm are well anchored on the surface of g-C₃N₄ nanosheets. These results indicated that SnS₂ has combined with ultrathin g-C₃N₄ nanosheets to form effective SnS₂/g-C₃N₄ heterojunction.

Fig. 5 AFM image (a) and the corresponding height analyses of g-C₃N₄ nanosheets (b), the TEM image of the g-C₃N₄ nanosheets (c) and the SnS₂/g-C₃N₄ hybrid (d).

The UV-vis diffuse reflection spectra of the SnS₂/g-C₃N₄ hybrids with different mass percentage of SnS₂, together with those of pure g-C₃N₄ nanosheets and pure SnS₂, were investigated and shown in Fig. 6a. The pure g-C₃N₄ nanosheets show an absorption edge at 450 nm, corresponding to band gap of ∼2.65 eV presented in Fig. 6b, and it is well be in consistent with the value of previously reported literature.^35,36 The pure SnS₂ displays light absorption over nearly entire visible light range. After introducing SnS₂, the SnS₂/g-C₃N₄ samples show the similar absorption edge and a broader absorption in the visible region as comparison to pure g-C₃N₄ nanosheets. Moreover, the absorption intensity of SnS₂/g-C₃N₄ samples in the visible region is gradually strengthened with increasing SnS₂ contents.

Fig. 6 (a) UV-vis diffuse reflection spectra of pure g-C₃N₄ nanosheets, pure SnS₂ and SnS₂/g-C₃N₄ heterojunctions; (b) band gap energies of SnS₂ and g-C₃N₄ nanosheets.

The band gap energy of the semiconductors can be calculated by the following equation:^37–39

αhv = A(hv − E_g)^n/2 (1)

by applying the above equation, the band gaps of g-C₃N₄ and SnS₂ are 2.65 and 2.23 eV, respectively. It accords with the previous literatures very much.^{35,36,40–44}

The band edge positions of CB and VB for semiconductor can be determined by the following equations:^44–46

E_VB = X − E_e + 0.5E_g (2)

E_CB = E_VB − E_g (3)

where E_VB is the VB edge potential, X is the electronegativity of the semiconductor, E_e is the energy of free electrons (4.5 eV on the hydrogen scale), and E_g is the band gap energy of the semiconductor calculated from the DR-UV-vis spectra. The conduction band edge potential (E_CB) was acquired using the eqn (3). After calculation, it is known that the E_CB of g-C₃N₄ and SnS₂ are −1.09 and −0.14 eV, respectively. Then, the E_VB of g-C₃N₄ and SnS₂ are 1.56 and 2.09 eV, respectively. These results are very close to the reported values.^39,41,44,47

To investigate photoelectrochemical performance of SnS₂/g-C₃N₄ heterojunction, typical current–voltage measurements of various samples were performed under light switch (Fig. 7). The 4.0-SnS₂/g-C₃N₄ heterojunction shows the highest photocurrent density of 13.66 μA cm⁻² at 0.8 V, which is about 1.5 and 2 times of g-C₃N₄ nanosheets (9.02 μA cm⁻²) and bulk g-C₃N₄ (6.82 μA cm⁻²), respectively.

Fig. 7 Current–potential plots for bulk g-C₃N₄, pure g-C₃N₄ nanosheets and SnS₂/g-C₃N₄ hybrids under light switch.

To gain deeper insights into the charge transport behavior in the system, we conducted electrochemical impedance spectroscopy (EIS) measurements (Fig. 8). In the Nyquist diagram, the radius of each arc is associated with the charge-transfer process at the corresponding electrode/electrolyte interface, and a smaller radius corresponds to a lower charge-transfer resistance. In Fig. 8a, the impedance of the g-C₃N₄ nanosheets is significantly lower than the bulk material, and when the light source turned on, the impedance is further reduced, which also confirms that the g-C₃N₄ nanosheets has better photoelectric properties. Obviously, the 4.0-SnS₂/g-C₃N₄ heterojunction exhibits minimal charge transfer resistance under simulated sunlight irradiation, indicating that effective shuttling of charges between the electrode and the electrolyte. Faster interfacial charges transfer occurred at composite interface, thereby generating the increasing photocurrent. So, SnS₂ may serve as electron capture centres, speeding up charges transfer.

Fig. 8 (a) EIS Nyquist plots of the bulk g-C₃N₄, pure g-C₃N₄ nanosheets at a bias of 0.5 V under dark and simulated sunlight irradiation. (b) EIS Nyquist plots of pure g-C₃N₄ nanosheets, mechanical mixture of SnS₂/g-C₃N₄ and 4.0-SnS₂/g-C₃N₄ at a bias of 0.5 V under simulated sunlight irradiation.

Fig. 9a shows a comparison of the visible light photocatalytic activity of bulk g-C₃N₄, SnS₂ nanoparticles, g-C₃N₄ nanosheets and SnS₂/g-C₃N₄ composites. Prior to irradiation, the MO solution containing the catalyst was kept in the dark for 30 min to reach an adsorption/desorption equilibrium. As we can see, with absence of any photocatalyst the decomposition of MO hardly occurred when subjected to visible light irradiation. It is also suggested that the SnS₂ nanoparticles content has a significant effect on the photocatalytic activity of g-C₃N₄ nanosheets. In the case of bulk C₃N₄, approximately 82.6% of MO was decomposed after visible light irradiation for 40 min. When it comes to the pure g-C₃N₄ nanosheets, the degradation rate of MO is 90.7% and higher than that of SnS₂ nanoparticles which only display only 87.4% after irradiation for 40 min. However, the photoactivities of g-C₃N₄ nanosheets are obviously enhanced with the presence of SnS₂ nanoparticles on its surface. The photoactivities of SnS₂/g-C₃N₄ heterojunctions are higher than that of g-C₃N₄ nanosheets and SnS₂ nanoparticles. Especially, 4.0-SnS₂/g-C₃N₄, can decompose 95.3% of MO after only 25 min for visible light irradiation. The average decomposition rate of the MO dye over bulk g-C₃N₄, SnS₂ nanoparticles, g-C₃N₄ nanosheets and 4.0-SnS₂/g-C₃N₄ heterojunction is estimated to be about 0.0135 mg min⁻¹, 0.0166 mg min⁻¹, 0.0200 mg min⁻¹ and 0.0324 mg min⁻¹. It reveals that constructing heterojunction of SnS₂/g-C₃N₄ could improve the photocatalytic efficiency of g-C₃N₄ nanosheets.

Fig. 9 (a) Photocatalytic degradation of MO with various powder samples, (b) the cycling runs of 4.0-SnS₂/g-C₃N₄ heterojunction for the photodegradation of MO under visible light.

The reusability and stability of SnS₂/g-C₃N₄ heterojunction was evaluated by recycling experiments. By utilizing the same photocatalyst and a fresh MO solution, five consecutive cyclic photodegradation experiments were carried out for the 4.0-SnS₂/g-C₃N₄ heterojunction. As shown in Fig. 9b, MO was decomposed and decreased in every cycling run. The percentage of degradation of MO after the first cycle was 94.9% by 25 min visible light irradiation; while it was 89.9% at the fifth cycle with the same time irradiation. Only a little decrease in catalytic activity was observed after the fifth-cycle decomposition process. So SnS₂/g-C₃N₄ nanosheets heterojunction could be regarded as a stable and durable photocatalyst.

It is well known that the ·O₂⁻, ·OH and h⁺ are the major reactive species for the photocatalytic decomposition. To uncover the possible photocatalytic mechanism of 4.0-SnS₂/g-C₃N₄ nanosheets heterojunction, the radical species trapping experiments were carried out to explore the main reactive species in the process of photocatalytic degradation. Herein, three chemicals including benzoquinone (BQ), disodium ethylenediaminetetraacetate (EDTA–2Na) and isopropyl alcohol (IPA) were utilized as the scavenger of ·O₂⁻, h⁺ holes and ·OH, respectively. As listed in Fig. 10a, the degradation efficiency of MO is 95% with absence of scavenger. When the IPA was added into the reaction solution, the degradation efficiency of MO is down to 73.5%. And adding the EDTA–2Na into the reaction solution, the degradation efficiency of MO is decreased to 43%. The degradation efficiency of MO is even whittled down into 28% with the presence of BQ. The phenomenon suggested that the photocatalytic activity of 4.0-SnS₂/g-C₃N₄ is extremely suppressed by BQ and EDTA–2Na, while slightly inhibited by IPA. It also can be concluded that ·O₂⁻, ·OH and h⁺ are the major reactive species for the 4.0-SnS₂/g-C₃N₄ nanosheets photocatalyst. The influencing degree is belonged to ·O₂⁻ > h⁺ > ·OH.

Fig. 10 (a) Effects of various scavengers on the photocatalytic efficiency of 4.0-SnS₂/g-C₃N₄ heterojunction, (b) schematic model of the photocatalytic mechanism for SnS₂/g-C₃N₄ heterojunction.

On the basis of radical species trapping experiments and the aforementioned band gap energy analysis, the possible photocatalytic mechanism is speculated. As illustrated in Fig. 10b, the g-C₃N₄ nanosheets and SnS₂ semiconductor possess overlapping band potentials, indicating that the band potentials of g-C₃N₄ nanosheets and SnS₂ nanoparticles can be well matched to form an effective heterojunction. Under visible light irradiation, both components are readily excited to generate photo-induced electrons–holes pairs. And the separation process of photo-induced electrons–holes can be vividly described in Fig. 10b. When the SnS₂/g-C₃N₄ nanosheets heterojunctions absorb the visible light and then are excited, the quick and strong combination is occurred between the photo-induced electrons in the conduction band (CB) of SnS₂ and photo-induced holes in the valence band (VB) of g-C₃N₄ nanosheets. Meanwhile, the photo-induced electrons in the CB of g-C₃N₄ nanosheets, which have more negative potential than the standard redox potential of E°(O₂/·O₂⁻) = −0.33 eV (vs. NHE), can react with the O₂ to produce ·O₂⁻. And then the generated holes in the VB of SnS₂ can directly decompose the MO molecules. Moreover, the generated holes in the VB of SnS₂, which have more positive potential than the standard redox potential of E°(·OH/OH⁻) = 1.99 eV (vs. NHE),^48–52 can oxidize OH⁻ to yield ·OH radicals. Therefore, a typical Z-scheme separation mechanism of photo-induced electrons–holes pairs is proposed and favourable for the generation of ·O₂⁻ and ·OH reactive species. As a result, efficient separation of photo-generated hole–electron pairs is achieved by the distribution of electrons and holes on different semiconductors, thus enhance the corresponding photocatalytic performance.

4. Conclusions

In summary, we have prepared free standing g-C₃N₄ nanosheets by mixed solvents exfoliation and subsequently obtained SnS₂/g-C₃N₄ heterojunction via a simple ion-exchange process. It was observed that small SnS₂ nanoparticles with diameter of 5–10 nm are well anchored on the surface of g-C₃N₄ nanosheets. Photoelectrochemical and photocatalytic measurement proved that constructing heterojunction of SnS₂/g-C₃N₄ could significantly improve the photoelectrochemical performance and photocatalytic efficiency as compared to pure g-C₃N₄ and g-C₃N₄ nanosheets. The highly effective photoelectron–chemical activities of g-C₃N₄/SnS₂ are attributed to the efficient separation of photo-generated hole–electron pairs on different semiconductors. Our findings pave the way to build the SnS₂/g-C₃N₄ heterojunction materials for photoelectrochemical application.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51202040, 21363006, 21503051), Guangxi Natural Science Foundation (2013GXNSFBA019042, 2012GXNSFBA053032), Scientific Research Projects of Guangxi Education (YB2014161) and Guilin Scientific Research and Technology Development Projects (KY2015ZL109).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21506d

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