Fabrication of ZnIn₂S₄–g-C₃N₄ sheet-on-sheet nanocomposites for efficient visible-light photocatalytic H₂-evolution and degradation of organic pollutants

Abstract

ZnIn₂S₄–g-C₃N₄ sheet-on-sheet nanocomposites with different g-C₃N₄ contents were synthesized by a facile hydrothermal method and characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), high-resolution transmission electron microcopy (HRTEM), N₂ adsorption–desorption, ultraviolet-visible diffuse reflection spectroscopy (DRS), photoluminescence (PL) spectroscopy and photoelectrochemical (PEC) experiments. The photocatalytic activities of these samples were evaluated by the photocatalytic H₂-production and degradation of organic pollutants (methyl orange and phenol) under visible-light illumination (λ > 420 nm). The results showed that the ZnIn₂S₄–g-C₃N₄ composite photocatalysts displayed higher photocatalytic activity than the pristine g-C₃N₄ and ZnIn₂S₄ both for H₂-evolution and degradation of pollutants. The optimal g-C₃N₄ content was determined to be 40 wt%, and the corresponding H₂-production rate was 953.5 μmol h⁻¹ g⁻¹, which was about 1.91 times higher than that of pure ZnIn₂S₄. The enhanced photocatalytic activity of ZnIn₂S₄–g-C₃N₄ composites should be attributed to the well-matched band structure and intimate contact interfaces between ZnIn₂S₄ and g-C₃N₄, which led to the effective transfer and separation of the photogenerated charge carriers. Moreover, the ZnIn₂S₄–g-C₃N₄ composites showed excellent stability during the photocatalytic reactions under visible light. A possible mechanism of the enhanced photocatalytic activity of ZnIn₂S₄–g-C₃N₄ composites was proposed and supported by the PL and PEC results.

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1. Introduction

Photocatalysis, as a green technology, is attracting considerable interests due to its great potential in solving global environmental and energy problems.^1–3 TiO₂ is the most widely studied photocatalyst due to its unique characteristics such as high photocatalytic activity, low cost, stability and nontoxicity. However, TiO₂ only works in the ultraviolet region due to its wide band gap (∼3.2 eV), which restricts its practical applications.^4,5 Therefore, exploring new photocatalytic materials with high photocatalytic activity under visible-light (∼43% of the solar spectrum) is indispensable in this field.

As a typical metal-free polymeric semiconductor, graphitic carbon nitride (g-C₃N₄) is a very promising material for solar energy utilization because of its high thermal and chemical stability, nontoxicity, easy to prepare and desirable band gap of 2.7 eV.^6–12 The potential applications of g-C₃N₄ in photocatalytic hydrogen evolution, CO₂ reduction, degradation of pollutants, and photocatalytic organic synthesis have been reported.^6,7,9–12 However, the photocatalytic efficiency of pure g-C₃N₄ is limited by the high recombination rate of photogenerated electron–hole pairs and low surface area.^6,9 To enhance its photocatalytic performance, a number of methods have been exploited, including preparing novel nanostructures (mesoporous structure^13,14 and nanosheets^7,11), doping with metal or nonmetal elements,^15–18 deposition of noble metal,¹⁹ protonating by strong acids²⁰ and construction of heterojunction composite.^21–23 Among them, designing heterojunction composite is regarded as one of the best methods to promote the separation of photogenerated electrons and holes, which can effectively improve the photocatalytic efficiency.

ZnIn₂S₄, as a typical member of the AB₂X₄ family of semiconductors, is a good candidate for a photocatalyst under the visible-light irradiation due to its suitable bandgap (2.34–2.48 eV).^24–26 ZnIn₂S₄ exhibits two distinct polymorphs based on cubic and hexagonal lattices. Previous studies revealed that both polymorphs of ZnIn₂S₄ are active for photocatalytic hydrogen generation and pollutants degradation under visible-light irradiations and show considerable chemical stability.^27–30 However, the photocatalytic efficiency over pure ZnIn₂S₄ is low due to the short lifetimes of the photo-generated electron–hole pairs. To enhance the photocatalytic performance of ZnIn₂S₄, numerous effects have been made. For example, by controlling morphologies of ZnIn₂S₄,^26,31–34 doping with metals,^35,36 and incorporation of multiwalled carbon nanotubes,³⁷ reduced graphene oxide^28,30 or other semiconductors (such as CdS,³⁸ MoS₂,^29,39 CdIn₂S₄ (ref. 40) and TiO₂ (ref. 41)), the photocatalytic performance of ZnIn₂S₄ have been enhanced to a certain degree. However, it should be noted that the photocatalytic activity of ZnIn₂S₄ still requires further improvement.

Similar to g-C₃N₄, hexagonal ZnIn₂S₄ has an intrinsic layered structure based on a stacking of packets of S–Zn–S–In–S–In–S layers.²⁹ It is expected that it is facile to grow layer ZnIn₂S₄ on g-C₃N₄ surface due to their analogous layered structures which can minimize the lattice mismatch. Besides this, the position of the conduction band (CB) of g-C₃N₄ (−1.12 eV vs. NHE⁴²) is more negative than that of ZnIn₂S₄ (−0.68 eV vs. NHE³⁹) and thus provides possibility for a directional transfer of the photo-generated electrons from g-C₃N₄ to ZnIn₂S₄. Moreover, due to the valence band (VB) of ZnIn₂S₄ (+1.62 eV vs. NHE³⁹) are more positive than that of g-C₃N₄ (+1.57 eV vs. NHE⁴²), the holes on VB of ZnIn₂S₄ will migrate to that of g-C₃N₄, achieving the separation of charge carriers in composites. Therefore, it can be supposed that combining ZnIn₂S₄ with g-C₃N₄ may result in an improved photocatalytic efficiency compared with individual g-C₃N₄ and ZnIn₂S₄. However, to the best of our knowledge, no work related to ZnIn₂S₄–g-C₃N₄ photocatalyst has been reported so far.

In this study, ZnIn₂S₄–g-C₃N₄ sheet-on-sheet nanocomposites with different g-C₃N₄ contents were synthesized by a facile hydrothermal method. The as-prepared ZnIn₂S₄–g-C₃N₄ composites showed a significantly enhanced photocatalytic performance for hydrogen evolution and degradation of organic pollutants under visible light irradiation. A possible enhancement mechanism for the improved photocatalytic activity in the ZnIn₂S₄–g-C₃N₄ composites was also proposed.

2. Experimental

2.1 Materials preparation

All the chemicals were of analytical grade from the Sinopharm Chemical Reagent Co., Ltd.

Preparation of g-C₃N₄ nanosheets. The g-C₃N₄ nanosheets were prepared by a thermal oxidation etching method. Briefly, dicyandiamide was put into an alumina crucible with a cover. The crucible was then placed in a muffle furnace and heated to 550 °C with a heating rate of 2 °C min⁻¹, which has been held for 4 h at this temperature. When the temperature of alumina crucible was cooled down after reaction, the obtained bulk g-C₃N₄ products were grinded into powders. Next, 400 mg bulk g-C₃N₄ obtained as above was placed in an open ceramic container and heated to 500 °C with a ramp rate of 2 °C min⁻¹, and maintained at this temperature for 2 h. A light yellow powder of g-C₃N₄ nanosheets was finally obtained.

Preparation of ZnIn₂S₄–g-C₃N₄ composites. The ZnIn₂S₄–g-C₃N₄ composites were prepared by a hydrothermal method. In detail, 0.068 g ZnCl₂, 0.293 g InCl₃·4H₂O and 0.225 g thioacetamide (TAA) were dissolved in 40 mL deionized water by stirring at room temperature, and the pH of the solution was adjusted to 2.5 by hydrochloric acid. Meanwhile, an appropriate amount of as-prepared g-C₃N₄ nanosheets was dispersed in 30 mL deionized water by sonication for 30 min. Then, the obtained g-C₃N₄ solution was added to the above solution gradually under magnetic stirring. After being stirred for 3 h, the suspension was transferred to a 100 mL Teflon-lined autoclave and maintained at 80 °C for 6 h. The precipitate was collected by centrifugation, washed with deionized water and absolute ethanol for several times and dried at 60 °C under vacuum. The as-synthesized ZnIn₂S₄–g-C₃N₄ sample with 20 wt%, 30 wt%, 40 wt% and 50 wt% g-C₃N₄ was labeled as ZISCN20, ZISCN30, ZISCN40 and ZISCN50, respectively. For comparison, pure ZnIn₂S₄ was prepared using the same hydrothermal method without the addition of g-C₃N₄ nanosheets.

2.2 Characterization

The crystal structure of the samples was identified by an X-ray diffractometer (XRD, D-MAX-2550, λ = 0.15418 nm) using Cu Kα radiation. The morphologies and microstructures of the photocatalysts were evaluated by transmission electron microscopy (TEM FEI TECNAI G20) and high-resolution transmission electron microscopy (HRTEM FEI TECNAI G2F20). The surface elemental component and the chemical state of the sample were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). FT-IR spectrum was performed on a FT-IR spectrophotometer (AVATAR370) using KBr disks at room temperature. Ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) of the samples were obtained on an UV-vis spectrophotometer (Hitachi U-3010) using BaSO₄ as a reference. The photoluminescence (PL) spectra were measured using a fluorescence spectrophotometer (Hitachi RF-5301) at room temperature. The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were analyzed by nitrogen adsorption using a Micromeritics ASAP 2020 nitrogen adsorption apparatus. Thermogravimetric analysis (TGA) was performed on a Netzsch 409 PC thermal analyzer under nitrogen atmosphere with a heating rate of 10 °C min⁻¹.

2.3 Photocatalytic measurement

The photocatalytic hydrogen-production experiment was conducted in a closed gas circulation and evacuation system fitted with a top window Pyrex cell. A 300 W xenon lamp (PLS-SXE300C, Beijing Perfectlight Co. Ltd., China) coupled with a UV cut-off filter (λ > 420 nm) was used to provide the visible light. In each run, 0.05 g photocatalyst was dispersed in a 100 mL aqueous solution containing 10 mL trolamine, which served as a sacrificial agent. Before the irradiation with visible light, the suspension was degassed with N₂ for 1 h to drive away the O₂ in the system. The reaction cell was kept at room temperature with cooling water. The produced H₂ was detected using an online gas chromatography (GC7900, N₂ carrier, 5A molecular sieve column, TCD detector).

Photocatalytic degradation activity of the samples was estimated by monitoring the degradation of methyl orange (MO) and phenol in an aqueous solution under visible light irradiation. The reaction was performed in a photochemical reactor (BL-GHX-V, Shanghai Bilon Instruments Co., Ltd, China) equipped with a 500 W Xe lamp combined with a 420 nm cut-off filter as a light source. All experiments were conducted at room temperature in air. In a typical photocatalytic experiment, 0.02 g photocatalyst was added into 50 mL pollutant solutions (10 mg L⁻¹) in a reaction cell with a Pyrex jacket. Prior to irradiation, the suspension was magnetically stirred in the dark for 30 min to reach an adsorption–desorption equilibrium. Then, the suspension was exposed to visible light irradiation under magnetic stirring. At given time intervals, about 5 mL suspensions were collected and centrifuged (12 000 rpm, 6 min) to remove the photocatalyst particles. Then, the pollutant concentration of the obtained solution was analyzed by a UV-vis spectrophotometer (Agilent 8453) by checking the absorbance at 506 nm and 270 nm for MO and phenol, respectively. The total organic carbon (TOC) assays were carried out using a TOC analyzer (Multi N/C 2100). To probe the oxidizing species in the photocatalytic degradation, the disodium ethylenediamine tetraacetate (EDTA-2Na), isopropyl alcohol (IPA), or benzoquinone (BQ), was added in the solution. The concentrations of the quenching chemicals were 2 mM.

2.4 Photoelectrochemical measurements

Photocurrent measurements were performed on a CHI 660E electrochemical workstation (Chenhua Instrument, Shanghai, China) in a conventional three electrode configuration with a Pt foil as the counter electrode and a Ag–AgCl (saturated KCl) as the reference electrode. A 300 W Xe arc lamp severed as a light source. A 0.5 M Na₂SO₄ aqueous solution was used as the electrolyte. The working electrodes were prepared as follows: 10 mg of the prepared photocatalyst was ground with 20 μL of a poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDO/TPSS, 1.3–1.7%) aqueous solution and 100 μL of distilled water to make a slurry. The slurry was then spread on a 1.5 cm × 1.0 cm indium-tin oxide (ITO) glass substrate with an active area of about 0.3 cm² by the doctor-blade method, using adhesive tape as the space. The film was dried in air and annealed at 150 °C for 30 min in a flowing N₂ atmosphere. The photoresponses of the samples as light on and off were measured at 0.0 V. Electrochemical impedance spectroscopy (EIS) measurements were determined at an AC voltage magnitude of 5 mV with the frequency range of 10⁶ to 0.01 Hz with the initial potential (0 V) in 0.01 M Na₂SO₄.

3. Results and discussion

3.1 Structure and properties characterization

The contents of ZnIn₂S₄–g-C₃N₄ composites were determined by thermogravimetric analysis (TGA) under nitrogen atmosphere from room temperature to 800 °C with a heating rate of 10 °C min⁻¹. The decomposition of the g-C₃N₄ nanosheets start at 560 °C and is complete at ∼760 °C which is attributed to the burning of g-C₃N₄ as shown in Fig. 1. In all ZnIn₂S₄–g-C₃N₄ composites, this weight loss region can also be observed and the g-C₃N₄ content can be easily determined from the corresponding weight loss in the thermogram. As shown in Table 1, the real g-C₃N₄ contents are nearly consistent with the theoretical values.

Fig. 1 TGA curves of g-C₃N₄ nanosheets, ZnIn₂S₄ and ZnIn₂S₄–g-C₃N₄ composites.

Table 1 BET surface area and real g-C₃N₄ concentrations of the as-prepared samples

Samples	g-C3N4 content (wt%)	SBET (m^2 g^−1)
ZnIn2S4	0	66.9
ZISCN20	20.5	63.9
ZISCN30	27.4	42.3
ZISCN40	38.6	35.2
ZISCN50	52.9	29.8
g-C3N4 nanosheet	100	10.4

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Fig. 2 shows the XRD patterns of g-C₃N₄ nanosheets, ZnIn₂S₄ and ZnIn₂S₄–g-C₃N₄ composites with different g-C₃N₄ contents. The XRD pattern of the ZnIn₂S₄ can be assigned to a hexagonal phase of ZnIn₂S₄ (JCPDS no. 03-065-2023) and the characteristic peaks at 21.4°, 27.8° and 47.3° are attributed to (006), (102) and (110) crystal planes of ZnIn₂S₄.²⁹ Two distinct peaks around 13.1° and 27.7° are observed in the XRD pattern of g-C₃N₄ nanosheets, corresponding to the (100) and (002) diffraction planes, respectively.^6–12 For all ZnIn₂S₄–g-C₃N₄ composites, only broadened diffraction peaks for ZnIn₂S₄ have been observed due to the overlapping of the tow closely peaks for ZnIn₂S₄ (27.8°) and g-C₃N₄ (27.7°).

Fig. 2 XRD patterns of g-C₃N₄ nanosheets, ZnIn₂S₄ and ZnIn₂S₄–g-C₃N₄ composites.

The morphologies and microstructures of g-C₃N₄ nanosheets, ZnIn₂S₄ and ZISCN40 were characterized by TEM and HRTEM. It can be observed from Fig. 3a that pure g-C₃N₄ shows a two-dimensional nanosheet structure with a wrinkled surfaces. The pristine ZnIn₂S₄ sample (Fig. 3b) exhibits microsphere morphology with a diameter of about 1.5 μm. Further observation show that individual ZnIn₂S₄ microsphere are built by numerous nanosheets with a thickness of about 10 nm (Fig. 3c). After hybridization with g-C₃N₄, the ZnIn₂S₄ microspheres are unfolded and distributed uniformly as nanosheets on the surface of g-C₃N₄ nanosheets to form a 2D sheet-on-sheet structure (Fig. 3d). This phenomenon may be ascribed to the important effect of surface groups of g-C₃N₄ in the crystal growth and self-assembly process of ZnIn₂S₄ materials. The 2D sheet-on-sheet composite structure offers the maximum contact area between two components (ZnIn₂S₄ and g-C₃N₄ nanosheets), which will facilitate the transfer of photogenerated carriers and improve the photocatalytic performance. The HRTEM image of ZISCN40 (Fig. 3e) reveals that a lattice spacing of ZnIn₂S₄ crystallites is ∼0.324 nm, corresponding to the (102) plane of ZnIn₂S₄.³⁰ The EDS spectrum (Fig. 3f) of the composites clearly confirms the presence of Zn, In, S, C and N elements (other peaks originate from the substrate).

Fig. 3 TEM images of (a) g-C₃N₄ nanosheets, (b and c) ZnIn₂S₄ and (d) ZISCN40, (e) HRTEM image and (f) EDS spectrum of ZISCN40.

Fig. 4 shows the FT-IR spectra of g-C₃N₄ nanosheets, ZnIn₂S₄ and the ZnIn₂S₄–g-C₃N₄ (40%) sample. In the FT-IR spectra of g-C₃N₄, the broad peak at 3250 cm⁻¹ can be attributed to stretching mode of the N–H bond.⁴³ And the peaks located at about 1240, 1320, 1410, 1460, 1575 and 1640 cm⁻¹ are ascribed to aromatic C–N stretching vibration modes and C=N stretching vibration modes.^44,45 The peak at 812 cm⁻¹ is originated from characteristic breathing vibration of triazine ring.⁴⁴ For ZnIn₂S₄, the broad band centered at 3430 cm⁻¹ and 1620 cm⁻¹ are contributed to the surface absorbed water molecules and hydroxyl groups.⁴⁶ In the case of the ZnIn₂S₄–g-C₃N₄ composite, the IR bands characteristic of g-C₃N₄ and ZnIn₂S₄ are also identified, confirming that the composite photocatalyst is composed of g-C₃N₄ and ZnIn₂S₄.

Fig. 4 FT-IR spectra of g-C₃N₄ nanosheets, ZnIn₂S₄ and ZISCN40.

The X-ray photoelectron spectra (XPS) were further employed to elucidate the surface elemental composition of g-C₃N₄, ZnIn₂S₄ and ZnIn₂S₄–g-C₃N₄ and to further study the interaction of ZnIn₂S₄ with the g-C₃N₄ support. The XPS survey spectrum of ZISCN40 (Fig. 5a) confirms that the composite is mainly composed of C, N, Zn, In and S. The C 1s spectrum of pure g-C₃N₄ (Fig. 5b) can be deconvoluted into three peaks at 284.7, 286.4 and 288.3 eV. The C peak at 288.3 eV is identified as sp²-bonded carbon (N–C=N), the C peak at 284.7 eV is corresponded to graphitic carbon which was usually observed on the XPS characterization for carbon nitrides.^44,45 The weak peak at 286.4 eV can be assigned to sp³-coordinated carbon bonds from the defects on g-C₃N₄ surfaces.^44,45 The N 1s spectrum of pure g-C₃N₄ (Fig. 5c) can be deconvoluted into three peaks at 398.1, 399.3 and 400.7 eV, respectively, corresponding to the nitrogen atoms in the aromatic rings (C=N–C), tertiary nitrogen (N–(C)₃) and C–N–H.^47,48 In comparison with pure g-C₃N₄, the negative shift of the C 1s peaks and positive shift of N 1s peaks are observed for the ZISCN40 sample, indicating the g-C₃N₄ structure has changed after interaction with ZnIn₂S₄ to form the heterojunction. The main binding energy peaks located at 1044.5 eV (Zn 2p_1/2), 1021.5 eV (Zn 2p_3/2), 452.4 eV (In 3d_3/2), 444.9 eV (In 3d_5/2), 161.5 eV (S 2p_3/2) and 162.7 eV (S 2p_1/2) can be observed from the high resolution spectra of the pristine ZnIn₂S₄ in Fig. 5d–f, respectively. The binding energy values are very close to the reported ones,^29,30,38,39 indicating that the valence states of Zn, In and S are +2, +3 and −2, respectively. After modification with g-C₃N₄, slight shifts toward lower binding energies can be observed from the high resolution spectra of ZISCN40 in Fig. 5d–f, implying that the binding energies of the core level electrons of those metal and sulfide ions are affected due to the possible chemical bonding actions among the composite components. The XPS results clearly indicate the presence of chemical bonds between g-C₃N₄ and ZnIn₂S₄, rather than a simple physical mixture.

Fig. 5 XPS spectra of g-C₃N₄ nanosheets, ZnIn₂S₄ and ZISCN40: (a) survey spectrum of ZISCN40, (b) C 1s, (c) N 1s, (d) Zn 2p, (e) In 3d, (f) S 2p.

The Brunauer–Emmett–Teller specific surface areas (S_BET) of pure g-C₃N₄ nanosheets, ZnIn₂S₄, and the as-prepared ZnIn₂S₄–g-C₃N₄ composites were measured by nitrogen adsorption and summarized in Table 1. As listed in Table 1, the BET specific surface area of the pristine ZnIn₂S₄ is about 66.9 m² g⁻¹. For the ZnIn₂S₄–g-C₃N₄ composites, the surface area decreases with the enhancement of g-C₃N₄ mass ratio because the surface area of g-C₃N₄ nanosheets is only 10.4 m² g⁻¹.

The UV-vis diffuse reflectance spectra of the ZnIn₂S₄–g-C₃N₄ composites with different weight ratios of g-C₃N₄, as well as g-C₃N₄ nanosheets and ZnIn₂S₄ are compiled in Fig. 6a. Pure g-C₃N₄ sample shows absorption wavelengths from the UV to the visible range up to 460 nm. For ZnIn₂S₄, the absorption edge is about 600 nm, exhibiting the broad absorption region of visible-light. The ZnIn₂S₄–g-C₃N₄ composites have absorption edges to longer wavelengths in comparison with the pure g-C₃N₄. The observations are attributed to the interaction between g-C₃N₄ and ZnIn₂S₄ in the composites.^48,49 The interaction probably plays a significant role in improving the separation of the photogenerated electron–hole pairs to enhance the photocatalytic activity. The band gap energy of a semiconductor can be calculated by the following equation: αhν = A(hν − E_g)^n/2, in which α, h, ν, E_g, and A are the absorption coefficient, Planck constant, the light frequency, the band gap, and a constant, respectively. The value of n depends on the type of optical transition of the semiconductor (n = 1 for direct transition and n = 4 for indirect transition). For g-C₃N₄ and ZnIn₂S₄, the n value of 1 is used.^50,51 Therefore, as can be seen from Fig. 6b, the corresponding band gap values of ZnIn₂S₄, ZISCN20, ZISCN30, ZISCN40, ZISCN50 and g-C₃N₄ nanosheets are found to be 2.32, 2.34, 2.35, 2.38, 2.40 and 2.70 eV, respectively. The valence band edge potential and the conduction band edge potential of a semiconductor material can be determined by using the following equation: E_VB = X − E^e + 0.5E_g, where E_VB represents valence band edge potential, X is the electronegativity of the semiconductor estimated by the geometric mean of the electronegativity of the constituent atoms. E^e is the energy of free electrons on the hydrogen scale (∼4.5 eV), E_g is the band gap energy of the semiconductor. The CB edge potential E_CB can be determined by E_CB = E_VB − E_g. The X value of pure ZnIn₂S₄ and g-C₃N₄ is about 4.86 and 4.63 eV. The E_VB of bare ZnIn₂S₄ and g-C₃N₄ nanosheets can be assigned to be +1.52 and +1.48 eV, respectively, and the corresponding E_CB of ZnIn₂S₄ and g-C₃N₄ can be estimated to be −0.80 and −1.22 eV, respectively.

Fig. 6 (a) UV-vis spectra and (b) band gap energies of as-synthesized samples.

3.2 Photocatalytic activity

Photocatalytic hydrogen evolution activities of as-prepared samples were evaluated under visible light (λ > 420 nm) irradiation using trolamine as the sacrificial reagent to quench photoinduced holes. As shown in Fig. 7, the photocatalytic H₂-production rate is negligible when g-C₃N₄ alone is used as photocatalyst due to fast recombination of photocatalytic electron–hole pairs, indicating that pure g-C₃N₄ nanosheets is not active for photocatalytic hydrogen production. Pristine ZnIn₂S₄ without g-C₃N₄ shows a low photocatalytic activity with the H₂ evolution rate of 500.0 μmol h⁻¹ g⁻¹. After introduction of g-C₃N₄, the H₂-production activity of ZnIn₂S₄ is significant enhanced. The rate of H₂ evolution over ZnIn₂S₄–g-C₃N₄ composites increases with increasing g-C₃N₄ content, achieving a maximum of 953.5 μmol h⁻¹ g⁻¹ at the g-C₃N₄ content of 40 wt%, which is about 1.91 times higher than that of pure ZnIn₂S₄. A further increase in the content of g-C₃N₄ leads to a reduction in the photocatalytic activity. This decrease is probably due to the fact that excessive amount of g-C₃N₄ may cover the active sites on the surface of ZnIn₂S₄.^45,52 Therefore, an appropriate g-C₃N₄ loading amount is crucial to achieve the optimized photocatalytic activity of ZnIn₂S₄ photocatalyst.

Fig. 7 (a) Plots of photocatalytic H₂ evolution amount vs. irradiation time and (b) comparison of H₂ evolution rate for g-C₃N₄ nanosheets, ZnIn₂S₄ and ZnIn₂S₄–g-C₃N₄ composites under visible-light irradiation.

The photocatalytic degradation performances of as-prepared samples were also evaluated by using MO and phenol as the model pollutant under visible light irradiation (Fig. 8). The blank experiments (without any photocatalyst) show that direct photolysis of MO and phenol under visible light can be neglected. Fig. 8a shows that the pure g-C₃N₄ nanosheets has a poor activity, in which only 5.7% of MO is decomposed after irradiation for 120 min. Moreover, only 72.6% of MO can be removed by ZnIn₂S₄ for the same irradiation time. After coupling with g-C₃N₄, the photodegradation activity of ZnIn₂S₄ is remarkably enhanced. It can be observed that the optimum g-C₃N₄ mass ratio is 40 wt%. The degradation ratio of MO is up to 95.3% in the presence of ZISCN40 after 120 min irradiation. Fig. 8b displays the photodegradation activity of phenol over the as-prepared samples. Both pure g-C₃N₄ and ZnIn₂S₄ samples show low activities, in which only 21.4% and 52.5% of phenol are degraded after reacting for 240 min, respectively. In contrast, all the ZnIn₂S₄–g-C₃N₄ samples display much improved activities. The trend of activity change of these ZnIn₂S₄–g-C₃N₄ samples for the photodegradation of phenol is the same as that of MO degradation. The degradation ratio of phenol on the optimal photocatalyst (ZISCN40) reaches to 72.3% after 240 min irradiation. In addition, after 120 min or 240 min irradiation the mineralization rate of MO and phenol over ZISCN40 reaches to 70.0% and 58.0%, respectively (Fig. 9).

Fig. 8 Photocatalytic activities of g-C₃N₄ nanosheets, ZnIn₂S₄ and ZnIn₂S₄–g-C₃N₄ composites on the degradation of the target organic pollutions under visible-light irradiation: (a) MO, (b) phenol.

Fig. 9 TOC removal during (a) MO and (b) phenol degradation over ZISCN40 under visible light illumination.

To understand the photocatalytic mechanism, the main active oxidants in the photocatalytic degradation process were identified. It is well-known that the oxidants generated in the photocatalytic process can be measured through trapping by IPA (a scavenger of hydroxyl radicals), EDTA-2Na (a scavenger of holes) or BQ (a scavenger of ˙O₂⁻). It can be clearly seen from Fig. 10 that the addition of EDTA-2Na greatly reduces the photodegradation rate of MO and phenol in the ZISCN40 suspension, whereas the addition of BQ has little effect on the photodegradation rate of MO and phenol. In addition, the addition of IPA as hydroxyl radicals' scavenger causes a minor change in the photocatalytic degradation of the above pollutants. It indicates that both the holes and hydroxyl radicals are the active species in the photocatalytic degradation of MO and phenol, while ˙O₂⁻ can be negligible in the reaction.

Fig. 10 Photocatalytic degradation of MO (a) and phenol (b) with the addition of hole and radical scavenger.

To reveal the stability of as-synthesized photocatalysts, the cycling hydrogen evolution and MO degradation experiments were performed over ZISCN40 under the same conditions. As shown in Fig. 11, there is no significant decrease in H₂-production rate and photodegradation rate after four consecutive cycles, indicating the good stability of the ZnIn₂S₄–g-C₃N₄ composite during photocatalytic reactions.

Fig. 11 Cycling runs for (a) the photocatalytic hydrogen evolution and (b) MO degradation in the presence of ZISCN40 under visible light illumination.

3.3 Mechanism for the enhanced photocatalytic activity of ZnIn₂S₄–g-C₃N₄ composites

It has been widely accepted that the separation efficiency of electrons and holes played a crucial role in the photocatalytic reaction. Photoluminescence (PL) analysis was applied to reveal the efficiency of charge carrier trapping and separation of the photoinduced electrons and holes in semiconductors. Fig. 12a shows the PL spectra of pure g-C₃N₄ nanosheets, ZnIn₂S₄ and ZISCN40 sample with an excitation wavelength of 385 nm. It can be seen that the pure g-C₃N₄ has a strong emission peak at around 460 nm, which corresponds to band gap recombination of electron–hole pairs. The PL intensity of bare ZnIn₂S₄ is weak probably because the amount of photogenerated electron–hole pairs is less under the same irradiation conditions.^53,54 Similar results were also observed on the BiOBr/g-C₃N₄,⁵⁴ Ag₃PO₄/g-C₃N₄,⁵⁵ Fe₂O₃/g-C₃N₄,⁵⁶ ZnFe₂O₄/g-C₃N₄,⁵⁷ ZnS/g-C₃N₄,⁵¹ BiVO₄/g-C₃N₄ (ref. 58) and MnFe₂O₄/g-C₃N₄ (ref. 59) heterojunction systems by other groups. The ZnIn₂S₄–g-C₃N₄ composite has a lower peak than that of g-C₃N₄, which indicates that the composite has a lower electrons and holes recombination rate. Fig. 12b shows the photocurrent transient response for the electrodes of bare g-C₃N₄ nanosheets, ZnIn₂S₄ and ZISCN40 under visible light irradiation. As it can be seen, with the light switched-on and -off cycles, the ZISCN40 sample exhibits the highest photocurrent transient response under visible light irradiation, which is greatly larger than that for bare g-C₃N₄ and ZnIn₂S₄. It suggests the remarkably enhanced carrier separation ratio over ZISCN40. To further confirm the above results, electrochemical impedance spectroscopy (EIS), a useful measurement to characterize charge carrier transportation, was also performed. As can be seen from Fig. 12c, the diameter of the Nyquist semicircle for the ZISCN40 sample is smaller than that of ZnIn₂S₄ and g-C₃N₄, which indicates that the hybrid composite has a lower resistance than that of ZnIn₂S₄ and g-C₃N₄. The PL and photoelectrochemical results confirmed the superior charge transfer and recombination inhibition in the ZnIn₂S₄–g-C₃N₄ composite photocatalyst, which is responsible for the enhanced photocatalytic activity.

Fig. 12 (a) PL spectra, (b) transient photocurrent responses and (c) electrochemical impedance spectroscopy of g-C₃N₄ nanosheets, ZnIn₂S₄ and ZISCN40 under visible light irradiation.

The high separation efficiency of photo-generated carriers should be ascribed to the suitable band potentials of g-C₃N₄ nanosheets and ZnIn₂S₄. The possible mechanism of separation and transportation of electron–hole pairs at the interface of ZnIn₂S₄–g-C₃N₄ composite under visible light is proposed and illustrated in Scheme 1. Under the visible light illumination, both g-C₃N₄ and ZnIn₂S₄ can be excited and produce photogenerated electron–hole pairs. Since the CB edge potential of g-C₃N₄ nanosheets (∼−1.22 eV vs. NHE) is more negative than that of ZnIn₂S₄ (∼−0.80 eV vs. NHE), the photoinduced electrons on the CB of g-C₃N₄ can directly transfer to the CB of ZnIn₂S₄. While the corresponding VB edge potential of ZnIn₂S₄ (∼+1.52 eV vs. NHE) is more positive than that of g-C₃N₄ nanosheets (∼+1.48 eV vs. NHE), the photogenerated holes on the VB of ZnIn₂S₄ can immigrate to the VB of g-C₃N₄. Thus, the photogenerated electrons and holes move in opposite directions, which can effectively reduce the recombination probability and enhance the charge separation efficiency, leading to a remarkable enhancement of the photocatalytic activity.

Scheme 1 Mechanism for the enhanced photocatalytic activity of ZnIn₂S₄–g-C₃N₄ composites.

4. Conclusions

Novel ZnIn₂S₄–g-C₃N₄ sheet-on-sheet nanocomposites were prepared by a facile hydrothermal method. Under visible light irradiation, the obtained ZnIn₂S₄–g-C₃N₄ composites exhibited a significantly enhanced photocatalytic activity for hydrogen evolution and degradation of pollutants than pure ZnIn₂S₄ and g-C₃N₄. The optimal content of g-C₃N₄ was found to be 40 wt%. The enhanced photocatalytic performance of the composites was ascribed to the efficient charge separation and transfer on the interface between ZnIn₂S₄ and g-C₃N₄. More importantly, the as-prepared nanocomposites possessed high reusability. Therefore, they can be used as a promising photocatalyst for practical applications in environmental purification and clean hydrogen energy production from water splitting.

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (11472164) and Innovative Research Team (IRT13078) for financial support. The authors also thank Lab for Microstructure, Instrumental Analysis and Research Center, Shanghai University, for materials characterizations.

Notes and references

1. H. Tong, S. X. Ouyang, Y. P. Bi, N. Umezawa, M. Oshikiri and J. H. Ye, Adv. Mater., 2012, 24, 229–251.
2. M. R. Hoffmann, S. T. Martin, W. Y. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96.
3. X. B. Chen, S. H. Shen, L. J. Guo and A. S. Mao, Chem. Rev., 2010, 110, 6503–6570.
4. H. Y. Li, H. W. Yu, L. Sun, J. L. Zhai and X. R. Han, Nanoscale, 2015, 7, 1610–1615.
5. Z. D. Meng, L. Zhu, J. G. Choi, M. L. Chen and W. C. Oh, J. Mater. Chem., 2011, 21, 7596–7603.
6. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
7. S. B. Yang, Y. J. Gong, J. S. Zhang, L. Zhan, L. L. Ma, Z. Y. Fang, R. Vajtai, X. C. Wang and P. M. Ajayan, Adv. Mater., 2013, 25, 2452–2456.
8. W. J. Wang, J. C. Yu, D. H. Xia, P. K. Wong and Y. C. Li, Environ. Sci. Technol., 2013, 47, 8724–8732.
9. S. W. Cao and J. G. Yu, J. Phys. Chem. Lett., 2014, 5, 2101–2107.
10. J. Mao, T. Y. Peng, X. H. Zhang, K. Li, L. Q. Ye and L. Zan, Catal. Sci. Technol., 2013, 3, 1253–1260.
11. H. X. Zhao, H. T. Yu, X. Quan, S. Chen, H. M. Zhao and H. Wang, RSC Adv., 2014, 4, 624–628.
12. F. Z. Su, S. C. Mathew, G. Lipner, X. Z. Fu, M. Antonietti, S. Blechert and X. C. Wang, J. Am. Chem. Soc., 2010, 132, 16299–16301.
13. J. S. Zhang, F. S. Guo and X. C. Wang, Adv. Funct. Mater., 2013, 23, 3008–3014.
14. J. Xu, Y. J. Wang and Y. F. Zhu, Langmuir, 2013, 29, 10566–10572.
15. X. F. Chen, J. S. Zhang, X. Z. Fu, M. Antonietti and X. C. Wang, J. Am. Chem. Soc., 2009, 131, 11658–11659.
16. X. C. Wang, X. F. Chen, A. Thomas, X. Z. Fu and M. Antonietti, Adv. Mater., 2009, 21, 1609–1612.
17. Y. Wang, J. S. Zhang, X. C. Wang, M. Antonietti and H. R. Li, Angew. Chem., Int. Ed., 2010, 49, 3356–3359.
18. G. Liu, P. Niu, C. H. Sun, S. C. Smith, Z. G. Chen, G. Q. Lu and H. M. Cheng, J. Am. Chem. Soc., 2010, 132, 11642–11648.
19. L. Ge, C. C. Han, J. Liu and Y. F. Li, Appl. Catal., A, 2011, 409–410, 215–222.
20. Y. J. Zhang, A. Thomas, M. Antonietti and X. C. Wang, J. Am. Chem. Soc., 2009, 131, 50–51.
21. Y. Hou, Z. H. Wen, S. M. Cui, X. R. Guo and J. H. Chen, Adv. Mater., 2013, 25, 6291–6297.
22. Q. Li, N. Zhang, Y. Yang, G. Z. Wang and D. H. L. Ng, Langmuir, 2014, 30, 8965–8972.
23. K. Katsumata, R. Motoyoshi, N. Matsushita and K. Okada, J. Hazard. Mater., 2013, 260, 475–482.
24. J. Shen, J. T. Zai, Y. P. Yuan and X. F. Qian, Int. J. Hydrogen Energy, 2012, 37, 16986–16993.
25. L. Shang, C. Zhou, T. Bian, H. J. Yu, L. Z. Wu, C. H. Tung and T. R. Zhang, J. Mater. Chem. A, 2013, 1, 4552–4558.
26. B. Chai, T. Y. Peng, P. Zeng, X. H. Zhang and X. J. Liu, J. Phys. Chem. C, 2011, 115, 6149–6155.
27. Z. B. Lei, W. S. You, M. Y. Liu, G. H. Zhou, T. Takata, M. Hara, K. Domen and C. Li, Chem. Commun., 2003, 17, 2142–2143.
28. Y. J. Chen, H. Ge, L. Wei, Z. H. Li, R. S. Yuan, P. Liu and X. Z. Fu, Catal. Sci. Technol., 2013, 3, 1712–1717.
29. L. Wei, Y. J. Chen, Y. P. Lin, H. S. Wu, R. S. Yuan and Z. H. Li, Appl. Catal., B, 2014, 144, 521–527.
30. L. Ye, J. L. Fu, Z. Xu, R. S. Yuan and Z. H. Li, ACS Appl. Mater. Interfaces, 2014, 6, 3483–3490.
31. X. L. Gou, F. Y. Cheng, Y. H. Shi, L. Zhang, S. J. Peng, J. Chen and P. W. Shen, J. Am. Chem. Soc., 2006, 128, 7222–7229.
32. N. S. Chaudhari, A. P. Bhirud, R. S. Sonawane, L. K. Nikam, S. S. Warule, V. H. Rane and B. B. Kale, Green Chem., 2011, 13, 2500–2506.
33. F. Fang, L. Chen, Y. B. Chen and L. M. Wu, J. Phys. Chem. C, 2010, 114, 2393–2397.
34. X. F. Bai and J. S. Li, Mater. Res. Bull., 2011, 46, 1028–1034.
35. S. H. Shen, J. Chen, X. X. Wang, L. Zhao and L. J. Guo, J. Power Sources, 2011, 196, 10112–10119.
36. F. Li, G. P. Chen, J. H. Luo, Q. L. Huang, Y. H. Luo, Q. B. Meng and D. M. Li, Catal. Sci. Technol., 2013, 3, 1993–1999.
37. B. Chai, T. Y. Peng, P. Zeng and X. H. Zhang, Dalton Trans., 2012, 41, 1179–1186.
38. J. G. Hou, C. Yang, H. J. Cheng, Z. Wang, S. Q. Jiao and H. M. Zhu, Phys. Chem. Chem. Phys., 2013, 15, 15660–15668.
39. G. H. Tian, Y. J. Chen, Z. Y. Ren, C. G. Tian, K. Pan, W. Zhou, J. Q. Wang and H. G. Fu, Chem.–Asian J., 2014, 9, 1291–1297.
40. Y. G. Yu, G. Chen, G. Wang and Z. S. Lv, Int. J. Hydrogen Energy, 2013, 38, 1278–1285.
41. S. Yang, L. Li, W. H. Yuan and Z. L. Xia, Dalton Trans., 2015, 44, 6374–6383.
42. H. Katsumata, Y. Tachi, T. Suzukib and S. Kaneco, RSC Adv., 2014, 4, 21405–21409.
43. Y. L. Chen, J. H. Li, Z. H. Hong, B. Shen, B. Z. Lin and B. F. Gao, Phys. Chem. Chem. Phys., 2014, 16, 8106–8113.
44. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2009, 25, 10397–10401.
45. Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Phys. Chem. C, 2011, 115, 7355–7363.
46. H. F. Li, H. T. Yu, S. Chen, H. M. Zhao, Y. B. Zhang and X. Quan, Dalton Trans., 2014, 43, 2888–2894.
47. L. Q. Ye, J. Y. Liu, Z. Jiang, T. Y. Peng and L. Zan, Appl. Catal., B, 2013, 142–143, 1–7.
48. L. Ge, F. Zuo, J. K. Liu, Q. Ma, C. Wang, D. Z. Sun, L. Bartels and P. Y. Feng, J. Phys. Chem. C, 2012, 116, 13708–13714.
49. X. S. Zhou, Z. H. Luo, P. F. Tao, B. Jin, Z. J. Wu and Y. S. Huang, Mater. Chem. Phys., 2014, 143, 1462–1468.
50. J. S. Chen, F. Xin, X. H. Yin, T. Y. Xiang and Y. W. Wang, RSC Adv., 2015, 5, 3833–3839.
51. Y. Q. Shi, S. H. Jiang, K. Q. Zhou, B. B. Wang, B. Wang, Z. Gui, Y. Hu and R. K. K. Yuen, RSC Adv., 2014, 4, 2609–2613.
52. J. G. Yu, S. H. Wang, B. Cheng, Z. Lin and F. Huang, Catal. Sci. Technol., 2013, 3, 1782–1789.
53. Q. Li, H. Meng, J. G. Yu, W. Xiao, Y. Q. Zheng and J. Wang, Chem.–Eur. J., 2014, 20, 1176–1185.
54. Y. J. Sun, W. D. Zhang, T. Xiong, Z. W. Zhao, F. Dong, R. Q. Wang and W. K. Ho, J. Colloid Interface Sci., 2014, 418, 317–323.
55. H. Katsumata, T. Sakai, T. Suzuki and S. Kaneco, Ind. Eng. Chem. Res., 2014, 53, 8018–8025.
56. Y. Liu, Y. X. Yu and W. D. Zhang, Int. J. Hydrogen Energy, 2014, 39, 9105–9113.
57. J. Chen, S. H. Shen, P. H. Guo, P. Wu and L. J. Guo, J. Mater. Chem. A, 2014, 2, 4605–4612.
58. N. Tian, H. W. Huang, Y. He, Y. X. Guo, T. R. Zhang and Y. H. Zhang, Dalton Trans., 2015, 44, 4297–4307.
59. K. Vignesh, A. Suganthi, B. K. Min and M. Kang, J. Mol. Catal. A: Chem., 2014, 395, 373–383.

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