Biomolecule-assisted solvothermal synthesis of Cu₂SnS₃ flowers/RGO nanocomposites and their visible-light-driven photocatalytic activities

Abstract

Elimination of organic pollutants from wastewaters under visible-light irradiation is a venerable challenge in the fields of environmental and material science. In this work, we present a facile eco-friendly method to fabricate novel Cu₂SnS₃ flowers/RGO (Cu₂SnS₃/RGO) nanocomposites via a solvothermal method using L-cysteine as a sulphur source and linker. The crystal structure, morphology, purity, spectroscopic, and charge carrier separation properties of the synthesized samples were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission-scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), and X-ray energy-dispersive spectroscopy (EDS), Raman spectroscopy, UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and electrochemical impedance spectroscopy (EIS). The photocatalytic performance of the Cu₂SnS₃/RGO nanocomposites was evaluated by photocatalytic decolorization of eosin under visible-light irradiation. Results showed that the Cu₂SnS₃/RGO (3%) nanocomposite exhibits enhanced photocatalytic activity compared with the pure Cu₂SnS₃. The improved photocatalytic activity of the nanocomposite was mainly attributed to the formation of well-defined interface between Cu₂SnS₃ and graphene sheets, which greatly enhance the charge carrier separation efficiency in the Cu₂SnS₃ semiconductor.

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1. Introduction

In recent years, various aromatic compounds, dyes and pharmaceutical pollutants discharged from textile, agricultural and dyeing industries have had a negative impact on living things.¹ To solve these issues, great research works have been dedicated to develop new technologies to achieve degradation of these pollutants.² Among the applied strategies, a semiconductor-based photocatalytic process is a promising route to degrade the environmental pollutants.³ Applying narrow-band-gap semiconductors to degrade organic pollutants in air and water is an effective approach for solving environmental issues using the abundant sun light.⁴ Furthermore, photocatalytic processes have attracted significant interest, as a green technology, to solve the energy as well as environmental problems.⁵ However, conventionally used photocatalysts including ZnO and TiO₂ cannot be widely used in practical applications because of their poor photocatalytic activities under visible light.^6–8 Thus, it is highly required to provide novel, stable, low cost and efficient visible-light-driven photocatalysts. In order to harvest more visible-light irradiation, a great number of sulphur-based photocatalysts such as CdS,⁹ Bi₂S₃,¹⁰ CuInS₂,¹¹ Cu₂ZnSnS₄ (ref. 12) and Cu₂S¹³ have been reported. Unfortunately, these photocatalyst materials have been limited for commercial applications due to fast recombination rate, resulting in low photocatalytic activity. Recently, copper tin sulphide (Cu₂SnS₃) has largely investigated due to its interesting optical and electrical properties. The Cu₂SnS₃ system, as important ternary I–IV–VI group semiconductor with tunable band gap¹⁴ and outstanding optical thermo-mechanical properties¹⁵ has attracted significant attention owing to its broad applications in solar cells,¹⁶ thermoelectric,¹⁷ lithium ion batteries,¹⁸ and photocatalytic degradation of pollutants.¹⁹ Recently, Cu₂SnS₃ nanostructures with different morphologies were prepared by various techniques. Chen et al.²⁰ synthesized flower-like Cu₂SnS₃ nanostructures by solvothermal technique at 180 °C for 16 h. The average diameter of Cu₂SnS₃ flowers were about 1–1.5 mm. Liang et al.²¹ prepared Cu₂SnS₃ nanostructures using hazardous thiourea as sulfur source and applied to the absorbing layer of thin solar cell. Moreover, this Cu₂SnS₃ system is usually combined with advanced carbonaceous materials like activated carbon, CNT and graphene to improve the photocatalytic activity due to their large surface area and high electron mobility, which could greatly promote separation of the photogenerated electrons.²² Among these materials, graphene is a two-dimensional material made of carbon atoms in closely arranged hexagonal honeycomb like structure. It has some outstanding properties, such as large surface area, good thermal, and electrical conductivity²³ Existing literature show that coupling semiconductor catalysts with graphene can effectively improve their photocatalytic activity.²⁴ To date, several kinds of graphene-based composite materials containing metal phosphates, metal oxides and metal sulfides have been reported with the improved photocatalytic activity.²⁵ So, it is expected that the combination of Cu₂SnS₃ and graphene would lead to the formation of highly efficient catalyst for the degradation of organic pollutants. To best of our knowledge no reports on combining graphene with Cu₂SnS₃ for photocatalytic studies has been reported.

In the present study, we demonstrate a simple and eco-friendly route for the controllable synthesis of flower like Cu₂SnS₃ with hierarchical structures using L-cysteine as a sulphur source and excluded the hazardous sulphur sources such as thiourea and thioacetamide. The synthesized Cu₂SnS₃ was then incorporated with GO sheets to form Cu₂SnS₃/RGO nanocomposites. The as-prepared composites were characterized with various sophisticated analytical instruments. The photocatalytic activities of the prepared Cu₂SnS₃ microflowers and Cu₂SnS₃/RGO nanocomposites have been evaluated by degradation of eosin dye under visible-light irradiation.

2. Experimental

2.1. Materials

Copper nitrate trihydrate (Cu(NO₃)₃·3H₂O), stannous chloride (SnCl₂·2H₂O), ethylene glycol and L-cysteine were purchased from Himedia Pvt. Ltd. (Mumbai). Graphite powder (Extra pure mesh size 60) was obtained from Loba Chemie, sulphuric acid (H₂SO₄), potassium permanganate (KMnO₄), sodium nitrate (NaNO₃), and hydrogen peroxide (H₂O₂) were purchased from Merck. All the reagents were analytical grade and used without any further purification.

2.2. Synthesis of photocatalysts

Graphene oxide was prepared by the modified Hummer's method according to our previous reports.²⁶ The Cu₂SnS₃/RGO nanocomposites were prepared by one-step solvothermal approach. Typically, desired weight (2, 4, 8, 12, 16 mg) of graphene oxide was dispersed in 40 mL of ethylene glycol (pH of the solution was 6.8) through ultrasonication process for 30 min. Then, 2 mmol of Cu(NO₃)₃·3H₂O (0.75 g), 1 mmol of SnCl₂·2H₂O (0.40 g), and 3 mmol (0.70 g) of L-cysteine (pH of the ethylene glycol solution was declined to 6.4 after adding L-cysteine) were added into this solution under magnetic stirring for 2 h at room temperature. Finally, the solution was then transferred to 50 mL Teflon-lined stainless steel autoclave and heated in muffle furnace at 200 °C for 16 h and then was allowed to cool at room temperature. The black colour product was collected by centrifugation process and washed with water and acetone repeatedly for several times to remove the excess of solvents and unreacted L-cysteine. The resulted powder was dried at 60 °C for 6 h to obtain the black colour Cu₂SnS₃/RGO nanocomposite. The weight ratio of RGO to Cu₂SnS₃ were 0.5%, 1%, 2%, 3% and 4% and the obtained samples were labeled as Cu₂SnS₃/RGO (0.5%), Cu₂SnS₃/RGO (1%), Cu₂SnS₃/RGO (2%), Cu₂SnS₃/RGO (3%), and Cu₂SnS₃/RGO (4%) nanocomposites, respectively. The pure Cu₂SnS₃ was also prepared under the same condition without adding the graphene oxide.

2.3. Characterization

X-ray diffraction (XRD) patterns of the synthesized samples were recorded using X-ray diffractometer (Xpert-Pro-PAN Analytical Instrument) at a scan speed of 3° min⁻¹. The phase structure and purity were ascertained using XRD analysis. The morphology of the synthesized photocatalysts was examined by field emission scanning electron microscopy (FE-SEM) with an SU660 Hitachi Japan. The distribution of particles was further examined using high-resolution transmission electron microscopy (HR-TEM) JEOL JEM2100 microscope. The X-ray photoelectron spectroscopy (XPS) spectra were recorded, using Ms Omicron Nanotechnology, GmbH, (Germany) XM 1000-AlKα monochromator at 1486 eV energy radiation. Raman spectra were recorded using Nanophoton confocal Raman micro spectrophotometer (visible light range 532 nm). UV-vis diffused reflectance spectra (UV-vis DRS) of the samples were obtained from a UV-2450 (Shimadzu) UV-vis spectrophotometer.

2.4. Photocatalytic degradation studies

Photocatalytic activity of the prepared samples was evaluated by degradation of eosin dye under visible-light irradiation. The light was provided by a tungsten-halogen lamp with 150 W and the distance between the solution and lamp was 15 cm. Typically, 0.1 g of photocatalyst was added to a 50 mL of eosin dye solution (7.5 mg L⁻¹). For all degradation studies neutral pH was maintained. Prior to the light irradiation, the suspension was stirred in dark condition for 20 min to establish adsorption–desorption equilibrium between eosin and the photocatalyst. The concentrations of eosin dye were determined at various time intervals using a Shimadzu UV-vis spectrophotometer. The photocurrent and EIS studies were performed in CHI660D instrument according to our previous reports.²⁶

3. Results and discussion

3.1. XRD studies

The phase structure and purity of as-synthesized Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) samples were characterized by XRD analysis. The XRD patterns of Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) composite are shown in Fig. 1. In the case of Cu₂SnS₃, the diffraction peaks at about 26.79°, 28.62°, 33.94°, 47.42°, 51.96°, 56.34°, and 77.02° are corresponding to the (−1−31), (1−31), (−206), (−2010), (303), (058), and (074) planes of triclinic (Mohite) phase of Cu₂SnS₃ (JCPDS no. 35-0684).²⁷

Fig. 1 XRD patterns of the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) nanocomposite.

As can be seen, no other impurities such as binary sulfides were detected in the XRD pattern, which confirms purity of the sample. The Cu₂SnS₃/RGO (3%) nanocomposite shows a XRD pattern similar to the pure Cu₂SnS₃, indicating that the introduction of RGO does not change the crystal phase of Cu₂SnS₃. Moreover, no obvious diffraction peaks of RGO are observed in the Cu₂SnS₃/RGO nanocomposite, which may be due to the low content and relatively low diffraction intensity of RGO in the nanocomposite.²⁸

3.2. Raman spectroscopy

The phases of GO and Cu₂SnS₃/RGO (3%) nanocomposite were further confirmed by Raman spectroscopy. The Raman spectra of GO and Cu₂SnS₃/RGO (3%) nanocomposite were represented in Fig. 2. The Raman spectrum of GO shows two peaks at 1346 and 1573 cm⁻¹ (D and G bands), which are attributed to the defects and E_2g phonon modes of sp² frameworks in graphite.²⁹ The Raman spectrum of Cu₂SnS₃/RGO (3%) nanocomposite shows slight shifts and the D and G bands appear at 1341 cm⁻¹ and 1568 cm⁻¹, respectively. Compared with the pure GO, the Cu₂SnS₃/RGO (3%) nanocomposite exhibits an increased I_D/I_G ratio (0.93 to 1.04), which is attributed to the decrease in the average size of sp² domains upon the reduction of GO and the removal of various oxygen functionalities in the GO matrix. This result confirms the reduction of GO to RGO during the solvothermal process.^27,30

Fig. 2 Raman spectra of the GO and Cu₂SnS₃/RGO (3%) nanocomposite.

3.3. Morphological studies

Fig. 3 shows the FE-SEM images of the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) samples.

Fig. 3 FE-SEM images for the (a and b) Cu₂SnS₃ and (c and d) Cu₂SnS₃/RGO (3%) nanocomposite.

The pure Cu₂SnS₃ is relatively uniform in shape and microstructure, which consists of large number of flower-like Cu₂SnS₃ nanocrystals, and the Cu₂SnS₃ flowers are constructed by aggregation of large-scale thicker nanosheets by Ostwald ripening mechanism Fig. 3(a and b). For the Cu₂SnS₃/RGO (3%) nanocomposite in Fig. 3(c and d), the near-transparent graphene sheets are fully exfoliated and decorated homogeneously with Cu₂SnS₃ flowers. The formation route to anchor Cu₂SnS₃ nanoparticles onto the exfoliated GO sheets may be proposed as the adsorption of copper and tin ions into the layered GO sheets, followed by the nucleation and growth of Cu₂SnS₃ crystals.³¹ Moreover, GO sheets can be reduced upon solvothermal reaction using ethylene glycol solvent, leading to formation of Cu₂SnS₃/RGO nanocomposite.³² The Cu₂SnS₃ flowers anchored on the surface of RGO sheets have a good dispersion behaviour and larger surface area, which can offer more active adsorption sites in photocatalytic pathway.³³ Therefore, it is expected that the nanocomposite could present an enhanced photocatalytic activity under visible light.

Control experimental work shows that the final morphologies of the Cu₂SnS₃ microflowers are strongly affected by the reaction condition as shown in Fig. 4. In the absence of ethylene glycol under the same experimental condition using ethanol as solvent, only irregular Cu₂SnS₃ nanoparticles was observed as the final product (Fig. 4a).

Fig. 4 FESEM images of the (a) Cu₂SnS₃ synthesis using ethanol–L-cysteine system (b) Cu₂SnS₃ synthesis using ethylene glycol–thiourea system.

Furthermore, using thiourea as a sulphur source keeping ethylene glycol as solvent hierarchical grown microflowers was appeared which is much similar to microflowers morphology obtained from L-cysteine as sulphur source (Fig. 4b). It seems that ethylene glycol plays an essential role in the self-assembly growth of the Cu₂SnS₃ microflowers. It is known that ethylene glycol serves as a surface-modifying reagent³⁴ in the microstructure formation. It was proposed that ethylene glycol might tends to lowers the surface energy of Cu₂SnS₃ nanocrystals by forming chemical bonds between the surface Cu and Sn atoms of Cu₂SnS₃ nanocrystals.³⁵

On the basis of the morphological studies, the plausible mechanism of the formation of Cu₂SnS₃ microflowers is proposed. L-Cysteine is a peptide molecule which consists of functional groups, such as –SH, –NH₂, and –COOH, which have a tendency to coordinate with cations to form high-affinity metal–ligand clusters. In the solvothermal process, firstly Cu²⁺ was reduced to Cu⁺ by L-cysteine and ethylene glycol, and then further complexed by L-cysteine to form stable Cu⁺–Cysteine complexes.²⁰ At elevated reaction temperature, the complexes decomposed to release Cu⁺. Because (0.17 V) is larger than (0.15 V),²¹ Sn²⁺ was slowly oxidized to Sn⁴⁺ by Cu²⁺ under ambient conditions. Furthermore, Cu²⁺ was reduced to Cu⁺ by Sn²⁺. During the experimental process, tin ions and L-cysteine homogeneously dispersed in the ethylene glycol and trace amounts water (from hydrated tin chloride SnCl₂·2H₂O). Thus, L-cysteine could react with trace amount of water to release H₂S slowly, then free Sn⁴⁺ reacts with H₂S to produce [Sn₂S₆]⁴⁻. Finally, Cu⁺ combines with [Sn₂S₆]⁴⁻ to form Cu₂SnS₃ nuclei. When the reaction time increases, the primary Cu₂SnS₃ nuclei tend to grow larger and preferentially accumulated with each other to form Cu₂SnS₃ lamellar nanostructures.²¹ Finally, the lamellar petals could easily aggregate to form flower-like Cu₂SnS₃ microflowers. Under solvothermal conditions, Cu⁺ and Sn⁴⁺ ions would be transferred to Cu₂SnS₃ and graphene oxide sheets would be simultaneously reduced to RGO by H₂S released from L-cysteine. The fresh Cu₂SnS₃ microflowers uniformly decorate on the surface of reduced graphene oxide, resulting in formation of the Cu₂SnS₃/RGO nanocomposite.³⁶

The morphologies and microstructures of the Cu₂SnS₃ flowers and Cu₂SnS₃/RGO (3%) nanocomposite were further characterized by TEM and SAED analysis.

Fig. 5a shows representative TEM images of the pure GO sheets, revealing a stacked and rippled structure with high transparency. Fig. 5b–e shows representative TEM images of the Cu₂SnS₃/RGO (3%) nanocomposite. It can be observed that the sample consist of flower-like Cu₂SnS₃ was uniformly anchored on the RGO sheet, which is in accordance with the FE-SEM results. The HRTEM analysis further supports to assert of crystallinity for the Cu₂SnS₃. Fig. 5e shows a HRTEM image at the edge of individual flowerlike Cu₂SnS₃. From the SAED pattern Fig. 5f the typical diffraction circles of the Cu₂SnS₃ sample shows obvious polycrystalline nature. The periodic fringe spacing of 0.311 nm corresponds to the interplanar spacing between the (1−31) planes of Cu₂SnS₃, which is in good agreement with the XRD results.

Fig. 5 TEM images for (a) RGO (b–e) Cu₂SnS₃/RGO (3%) nanocomposite, and (f) SAED patterns of Cu₂SnS₃.

3.4. EDX mapping analysis

In order to confirm the elemental composition and distribution of the Cu₂SnS₃ particles on the surface of RGO, EDAX mapping studies were performed.³⁷

The elemental mapping images show the clear profiles only for Cu, Sn, C, O, and S elements exist in the Cu₂SnS₃/RGO composite (Fig. 6). A large quantity of Cu, Sn, and S elements uniformly distributed on the RGO surface has been noted.

Fig. 6 (a) FESEM image with elements distribution of the Cu₂SnS₃/RGO (3%) nanocomposite (b) EDAX spectrum of the Cu₂SnS₃/RGO (3%) nanocomposite and the corresponding elemental maps for (c) Cu, (d) Sn, (e) C, (f) O, and (g) S.

3.5. XPS studies

To analyse the chemical composition of the as-synthesized Cu₂SnS₃/RGO (3%) nanocomposite and to identify the chemical states of Cu, Sn, S, and C elements in the nanocomposite, XPS analysis was performed. The XPS survey spectrum of Cu₂SnS₃/RGO (3%) nanocomposite is shown in Fig. 7a. In the survey spectrum, the peaks emanating from Cu2p, Sn3d, S2p, C1s, and O1s were obviously observed at their standard values of binding energies, which confirm the existence of these elements in the synthesized nanocomposite.

Fig. 7 XPS spectra of the Cu₂SnS₃/RGO (3%) nanocomposites: survey spectrum (a), Cu2p spectra (b), Sn3d spectra, (c) S2p spectra, (d) and C1s spectra (e).

Fig. 7b–e depicts the high resolution elemental scans of Cu2p, Sn3d, S2p, and C1s. The Cu2p peaks at 931.35 eV and 951.42 eV are ascribed to the Cu2p_2/3 and Cu2p_1/2 spins, which confirms the oxidation state of Cu is +1 respectively (Fig. 7b).³⁸ Moreover, the Sn3d spectrum contains a doublet with binding energies at 487.3 eV and 495.8 eV, which can be assigned as Sn3d_5/2 and Sn3d_3/2 spins which confirms the oxidation state of Sn is +4 (Fig. 7c).³⁹ Fig. 7d shows the elemental scan of the core-level binding energy of S2p in the energy range of 158–175 eV. The doublet with binding energies of 161.8 eV and 163.1 eV can be attributed to S2p_3/2 and 2p_1/2 spins, respectively. Since this spectrum matches well with that of the S²⁻ ions of Cu₂SnS₃, the XPS results confirm that sulphur exists only in the form of sulfide in the Cu₂SnS₃/RGO (3%) nanocomposite.⁴⁰ The C1s spectrum shows four peaks at binding energy of 284.47, 285.2, 286.4, and 287.8 eV after fitting, which correspond to the C–C or C=C bonds, C–OH group, C–O–C and C=O bonds due to the presence of residual oxygen-containing groups from the solvothermal process (Fig. 7e).⁴¹

3.6. UV-vis DRS studies

Fig. 8 shows the UV-vis diffuse reflectance spectra (DRS) of the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) nanocomposite, where all of the samples exhibit strong visible-light absorption ability. As can be seen, intensity of the absorption for Cu₂SnS₃ remarkably increased after the loading of RGO. This increase is likely originated from the hybridization and strong electronic coupling between the Cu₂SnS₃ and RGO matrix. Hence, it was revealed that the Cu₂SnS₃/RGO (3%) nanocomposite has a stronger visible-light absorption ability than the pure Cu₂SnS₃ which could facilitate the photocatalytic performance to some extent.^42,43

Fig. 8 UV-vis DRS of the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) nanocomposite.

It is well known that, the photocatalytic activity of photocatalysts mainly depends on its band structure. The conduction band (CB) and valence band (VB) edge potentials of Cu₂SnS₃ was calculated by eqn (1) and (2)⁴⁴

E_CB = X − E^e − 0.5E_g (1)

E_VB = E_CB + E_g (2)

where X is the absolute electronegativity, E^e is the energy of free electrons on the hydrogen scale (4.5 eV), and E_g is the band gap of the semiconductor. From the above equation the E_VB and E_CB of Cu₂SnS₃ is calculated as 1.12 and −0.28 eV, respectively.

3.7. Photocurrent studies

The photocurrent measurements were recorded to elucidate the internal charge transfer dynamics in the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) samples. Fig. 9 shows the transient photocurrent responses of the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) nanocomposite measured under the visible-light irradiation. Prompt photocurrent study is obtained during “on” and “off” cycles of visible-light illumination. Compared with the pure Cu₂SnS₃, the Cu₂SnS₃/RGO (3%) nanocomposite exhibits higher photocurrent values, suggesting the surface recombination of photo-generated charge carriers have been effectively prevented by RGO incorporation. The higher photocurrent of the Cu₂SnS₃/RGO (3%) nanocomposite is originated from an enhanced photo induced electron–hole separation during visible-light illumination.⁴⁵

Fig. 9 Photocurrent generations in the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) nanocomposite.

3.8. Electrochemical impedance spectroscopy (EIS) analysis

The separation efficiency of electron–hole pairs is a crucial factor, determining photocatalytic activity. Hence, the interfacial charge separation efficiency was investigated by EIS spectroscopy and the results are displayed in Fig. 10. In this figure, one can see that the EIS Nyquist plots for the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) nanocomposite. The semicircle arc radius in the EIS spectrum of Cu₂SnS₃/RGO (3%) nanocomposite is much smaller than that of the Cu₂SnS₃, indicating that the Cu₂SnS₃/RGO (3%) nanocomposite has lower resistance, good conducting nature and better charge transport nature than the pure Cu₂SnS₃.⁴⁶

Fig. 10 Electrochemical impedance spectra for the Cu₂SnS₃ and Cu₂SnS₃/RGO (3%) nanocomposite.

Moreover, the electron lifetime τ_n can be calculated by the formula that τ_n = 1/(2πf),⁴⁷ where f denotes the frequency of superimposed AC voltage. For the Cu₂SnS₃ sample, τ_n is calculated as 17 ms, but the Cu₂SnS₃/RGO (3%) nanocomposite exhibits τ_n = 30 ms. Hence, it is evident that τ is larger in the Cu₂SnS₃/RGO (3%) nanocomposite, which confirms the slower recombination process in this electrode material. This is obviously beneficial for higher photocatalytic activity of the Cu₂SnS₃/RGO (3%) nanocomposite.

3.9. Photocatalytic measurements

Photocatalytic activity of the Cu₂SnS₃ and Cu₂SnS₃/RGO nanocomposites were evaluated by degradation of eosin aqueous solution. Prior to the irradiation, the mixed solution of eosin and the photocatalyst was stirred in the dark for 20 min to establish the equilibrium adsorption state. Fig. S1† displays the concentration changes of eosin aqueous solution in the presence of the Cu₂SnS₃/RGO (3%) nanocomposite under the visible-light irradiation. During the photocatalytic degradation process, the main absorption peak of eosin at about 519 nm decreased gradually upon increasing the irradiation time, indicating the decolorization of this dye in the solution. After 140 min of the light irradiation the major absorption peak of eosin dye solution has almost disappeared. The absorption in the UV region indicates that stable intermediates have not formed during the decolorization process of eosin solution. Fig. 11 shows the comparison between photocatalytic activity of the Cu₂SnS₃ and Cu₂SnS₃/RGO nanocomposites under visible-light irradiation. The photodegradation of eosin is almost negligible without any photocatalyst. It should be noted that the Cu₂SnS₃/RGO nanocomposites exhibits the increased adsorption property than the pure Cu₂SnS₃. The higher adsorption capacity of eosin dye on the Cu₂SnS₃/RGO nanocomposites can be beneficial for the improvement of photocatalytic degradation. Compared with the Cu₂SnS₃, photocatalytic activity of the Cu₂SnS₃/RGO nanocomposites was improved significantly. Moreover, photocatalytic activity of the nanocomposites enhances with increasing weight percentage of RGO from 0.5 wt% to 3 wt%. However, further increase of RGO content results a decrease in the photocatalytic activities.

Fig. 11 Photodegradation curves of eosin over the Cu₂SnS₃ and Cu₂SnS₃/RGO nanocomposites.

In Fig. 11 it can be seen that when RGO dosage was 4 wt%, an obvious decrease on the degradation activity was observed, which is probably due to the fact that higher amount of RGO could shield surface of the catalyst and also rapidly decreasing intensity of the light through the depth of the reaction medium, which is called as “shielding effect” of graphene.⁴⁸ The higher degradation efficiency (92%) of the Cu₂SnS₃/RGO (3%) nanocomposite indicates that loading of graphene is beneficial for increasing the photocatalytic activity. These results indicate that the suitable amount of graphene incorporation plays a major role for degradation of eosin dye molecule.⁴⁹

In order to realize the importance of L-cysteine in the photocatalyst activity Cu₂SnS₃/RGO (3%) nanocomposite, this sample prepared by L-cysteine and thiourea as sulphur sources and results are illustrated in Fig. 12 As can be seen, photocatalytic activity of the Cu₂SnS₃/RGO (3%) nanocomposite prepared using L-cysteine is higher than the sample prepared using thiourea as sulphur source.

Fig. 12 Comparison of photocatalytic activity for the Cu₂SnS₃/RGO (3%) nanocomposite prepared using L-cysteine and thiourea.

To test the stability of Cu₂SnS₃/RGO (3%) nano composite, the photocatalyst was reused three times, and the results are presented in Fig. 13a. It is evident that there is a slight deactivation for the nanocomposite during the degradation reaction under visible-light irradiation (92% to 84%).

Fig. 13 (a) Stability test of the Cu₂SnS₃/RGO (3%) nanocomposite towards eosin degradation under visible-light irradiation (b) XRD pattern of recycled nanocomposite after three cycles.

Fig. 13b shows the XRD pattern of Cu₂SnS₃/RGO (3%) nanocomposite after recycling for three times. As can be seen, position and intensity of the peaks are similar to the fresh sample, confirming high stability of Cu₂SnS₃/RGO (3%) composite during the degradation reaction. Therefore, this Cu₂SnS₃/RGO (3%) nanocomposite can serve as an attractive and enduring photocatalyst for organic pollutant degradation.

During the photocatalytic degradation process, holes (h⁺), hydroxyl radicals (˙OH), and superoxide ion radicals (˙O₂⁻) are generally involved as the main active species to degrade contaminants. So the reactive species trapping experiments on the photocatalytic degradation of eosin dye was conducted to understand the photocatalytic mechanism over the Cu₂SnS₃/RGO (3%) nanocomposite. For this purpose, different scavengers were used to consume the corresponding active species. Generally, benzoquinone (BQ), triethanolamine (TEOA), isopropanol (IPA) are used for trapping ˙O₂⁻, h⁺, ˙OH, respectively. Hence, according to the changes on the photocatalytic reaction process, the effects of different reactive species in the photocatalytic process could be explained. The experimental results (Fig. 14) showed that when BQ was added into the photocatalytic system, the photocatalytic degradation efficiency experienced a fast deactivation, implying ˙O₂⁻ is the important active species (37% of degradation), implying that ˙O₂⁻ is important active species.^50,51

Fig. 14 Effect of different scavengers for eosin degradation over the Cu₂SnS₃/RGO (3%) nanocomposite.

Besides, the addition of TEOA into the photocatalytic system decreased the photocatalytic degradation of eosin from 92% to 49% in 140 min, suggesting h⁺ also plays a predominant role in the photocatalytic process. However, when the introduction of IPA into the photocatalytic process, only a slight decreased, suggesting that ˙OH has moderate role on the photocatalytic process.

Based on photocurrent and EIS studies, the possible photocatalytic mechanism was proposed and it was shown in Fig. 15. Herein, graphene sheets facilitate charge transfer efficiency and suppress the recombination rate of photogenerated electron–hole pairs produced by Cu₂SnS₃ particles.⁵² It is noted that efficient charge separation as well as charge transfer behaviour play crucial role in the improvement of photocatalytic activity of the Cu₂SnS₃/RGO nanocomposite. Graphene has been shown to be an effective electron transporter and acceptor in various semiconductor systems.⁵³ In the Cu₂SnS₃/RGO nanocomposites, graphene could act as the electron acceptor and Cu₂SnS₃ as the electron donor. In addition, high adsorption ability of eosin over the catalyst surface could help to enhance the photocatalytic activity.⁵⁴

Fig. 15 Schematic representation of possible photocatalytic degradation mechanism over the Cu₂SnS₃/RGO nanocomposite under visible-light irradiation.

4. Conclusion

In summary, the Cu₂SnS₃/RGO nanocomposites, as visible-light-driven photocatalysts, were synthesized by a simple solvothermal route using L-cysteine, as sulphur source. The XRD and Raman studies confirmed that RGO was successfully incorporated in Cu₂SnS₃ matrix. UV-vis DRS results showed that the Cu₂SnS₃/RGO nanocomposite have stronger visible-light absorption ability than the pure Cu₂SnS₃. From the EIS spectroscopy, it is found that the Cu₂SnS₃/RGO nanocomposite has lower resistance and good conducting nature than the pure Cu₂SnS₃. Moreover, all of the as-synthesized Cu₂SnS₃/RGO nanocomposites exhibited improved photocatalytic performance than the pure Cu₂SnS₃. The improved photocatalytic activity of the composite should be ascribed to efficient separation and transfer of charge carriers, provided by RGO incorporation. The present study could provide new insights into the preparation of novel ternary semiconductor photocatalysts.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12068g

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