Synthesis and characterization of BiOCl–Cu2ZnSnS4 heterostructure with enhanced photocatalytic activity
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Cu2ZnSnS4 QDs (5–7 nm) were synthesized by chemical coprecipitation route. Using prepared Cu2ZnSnS4 QDs, a new type of BiOCl–Cu2ZnSnS4 heterostructures were prepared using bismuth nitrate as a precurssor and potassium chloride as a source of chlorine at 100 °C, 4 h. BiOCl–Cu2ZnSnS4 heterostructures were analyzed by XRD, TEM, UV–visible NIR, PL and surface area studies. The diffracted peaks of BiOCl–Cu2ZnSnS4 heterostructure show the presence of tetragonal BiOCl and did not show the intense peaks of Cu2ZnSnS4 QDs. TEM images of BiOCl–Cu2ZnSnS4 showed the deposition of Cu2ZnSnS4 QDs on the BiOCl microsphere surface. The surface area of BiOCl, Cu2ZnSnS4 and BiOCl–Cu2ZnSnS4-1 was of 1.91 m2/g, 3.06 m2/g and 13.39 m2/g, respectively. BiOCl–Cu2ZnSnS4-1 had a higher photodegradation rate for Congo red dye than BiOCl and Cu2ZnSnS4 QDs under sunlight, and that higher photoactivity was due to the heterostructure effect between BiOCl and Cu2ZnSnS4 QDs along with increased optical absorption in the visible region. Scavenger study endorses the involvement of superoxide and holes radicals in the degradation of Congo red.
KeywordsBiOCl–Cu2ZnSnS4 Heterostructure Nanocomposite Photodegradation Congo red Scavengers
Semiconductor photocatalysis is a useful, low-cost and environmentally benign technique for solving environmental pollution problems. The scientific community is focused on a very challenging and relevant research’s direction, for the synthesis of a better photocatalyst for hydrogen production, environmental pollution and many more. Bismuth oxychloride (BiOCl) is one of the good adsorbents and photocatalyst for the elimination of organic pollutants such as dyes, phenols, bacteria as it possesses a high chemical stability (Li et al. 2017). The good photocatalytic activity is attributed to layered structure consisting of alternate arrangement of [Bi2O2]2+ monolayer and dual Cl layers (Ao et al. 2016). BiOCl had better photodegradation performance than commercial TiO2 (P25) under UV light irradiation. However, due to its large band gap (3.17–3.54 eV) decreases BiOCl photoactivity (Liu et al. 2016). Thus, it is necessary to overcome these limitations and it can be performed by forming heterostructured composites through coupling of BiOCl with another semiconductor having narrow band gap. The designed BiOCl heterostructured composite will possess better visible light response and also facilitate the separation of photoinduced electron–hole pairs due to synergistic effect which eventually provides excellent photodegradation of contaminants.
Copper zinc tin sulfide (Cu2ZnSnS4) is a quaternary semiconducting compound of stannite structure. Cu2ZnSnS4 is an abundant, non-toxic component in the earth crust. The absorption coefficient of Cu2ZnSnS4 is 104 cm−1. Its band gap is about 1.4–1.7 eV (Malerba et al. 2014). Cu2ZnSnS4 has shown to be an excellent light energy-harvesting material for solar cells and good chemical stability in the photodegradation of organics (Arbouz et al. 2017; Hou et al. 2014; Barpuzary et al. 2015).
BiOCl-based constructed heterostructures revealed the enhanced photocatalytic activity, for example, BiOCl–La2Ti2O7 (Ao et al. 2016), Bi2S3–BiOCl (Xu et al. 2017), CdS–BiOCl (Pan et al. 2018), BiOCl–ZnFe2O4 (Sun et al. 2018), AgCl–BiOCl (Cheng et al. 2014), BiVO4–BiOCl (He et al. 2014), Co3O4–BiOCl (Tana et al. 2014), BiOCl–TiO2 (Li et al. 2016), Bi3O4Br/BiOCl (Gao et al. 2010), BiOCl/C3N4 (Huang et al. 2017), α-Fe2O3/BiOCl (Zheng et al. 2018), BiOCl–Ag3PO4 (Cao et al. 2013), BiOCl–Ag2CO3 (Fang et al. 2016), Ag-modified BiOCl (Yu et al. 2017), BiOCl–SnO2 (Sun et al. 2015), Bi2MoO6–BiOCl (Zuo et al. 2015), CdS/BiOCl (Lin et al. 2017), BiOCl–Ag8SnS6 (Shambharkar and Chowdhury 2018). This inspires us to fabricate a novel BiOCl–Cu2ZnSnS4 heterostructure with the hope of boosting visible light absorption capacity for better photocatalytic activity than their single component.
In the present study, chemical coprecipitation method was employed for synthesizing Cu2ZnSnS4QDs (5–7 nm). The prepared Cu2ZnSnS4 QDs were used for constructing BiOCl–Cu2ZnSnS4 heterostructure through deposition of Cu2ZnSnS4 particles on the surface of BiOCl with the help of coprecipitation method. BiOCl–Cu2ZnSnS4 heterostructure can extend the response of visible light and separate the photogenerated excitons very effectively and also increases the amount of reactive oxidative species under sunlight. As result, BiOCl–Cu2ZnSnS4 heterostructure had a better photocatalytic performance than bare BiOCl and Cu2ZnSnS4 QDs. Till date, no study has been conducted on the synthesis of BiOCl–Cu2ZnSnS4 heterostructured nanocomposites and evaluated its photocatalytic performance in the degradation of Congo red.
Cu(NO3)2·3H2O, ethylene glycol, Bi(NO3)3.5H2O were procured from Fisher Scientific Ltd. ZnCl2, SnCl2·2H2O, thiourea, potassium chloride were purchased from Merck. Congo red dye was procured from SRL. Distilled water was used to carry out the experiments.
Production of Cu2ZnSnS4 QDs, BiOCl and BiOCl–Cu2ZnSnS4 heterostructures
Cu2ZnSnS4 QDs were prepared by chemical coprecipitation (Shambharkar and Chowdhury 2018; Shambharkar and Chowdhury 2016). 0.1 M Cu(NO3)3, (0.05 M) ZnCl2, (0.05 M) SnCl2 solutions were taken in a round-bottomed flask, and 50 ml of ethylene glycol (EG) and 10 g of thiourea were added into it and then heated it at 160 °C, 6 h. The black precipitate of Cu2ZnSnS4 QDs was acquired and washed with methanol to remove EG and dried.
BiOCl–Cu2ZnSnS4-1 heterostructured nanocomposite was obtained from coprecipitation method. 100 mg of Cu2ZnSnS4 QDs was mixed with bismuth nitrate solution (0.01 M), 20 ml of EG, and 1 g of KCl. The reaction mixture was stirred and heated at 100 °C, 4 h. Gray color precipitate of BiOCl–Cu2ZnSnS4 was formed. Subsequently, the final product was washed to remove EG with methanol and dried at 80 °C. Similar procedure was followed for preparing BiOCl–Cu2ZnSnS4-2 using 150 mg of Cu2ZnSnS4 QDs. BiOCl microspheres were obtained from our published procedure (Shambharkar and Chowdhury 2018).
Crystallinity and purity of Cu2ZnSnS4 QDs, BiOCl and BiOCl–Cu2ZnSnS4 were determined by conducting X-ray diffraction (XRD) on Bruker AXS D8 ADVANCE X-ray diffractometer. Transmission electron microscope was performed on JEOL Model JEM-2100 to assess the morphology of samples. Band gap energy was obtained from UV–visible diffuse reflectance spectra (Varian, Cary 5000 UV–Vis NIR spectrophotometer). Surface area of the samples was determined from N2 adsorption–desorption isotherms (Micromeritics ASAP 2020), and photoluminescence (PL) spectra were recorded by JY Fluorolog-3-11 fluorescence spectrofluorimeter with Xenon Lamp (450 W) as excitation source with excitation wavelength 310 nm.
Photocatalytic activity of BiOCl, Cu2ZnSnS4 QDs and BiOCl–Cu2ZnSnS4 was evaluated by conducting degradation studies using Congo red dye (20 mg/l) under direct sunlight irradiation. BiOCl–Cu2ZnSnS4 powder (100 mg) was added in 80 ml of Congo red solution and agitated in the dark to ensure adsorption–desorption equilibrium before light irradiation. A solution of 5 ml from the mixture was taken out after irradiation time was over and centrifuged. Absorbance the dye was measured at λmax = 497 nm on Cary 100 Bio UV–Vis spectrophotometer. The similar technique was accepted for BiOCl and Cu2ZnSnS4 nanoparticles, and the photodegradation performance of samples was studied.
Results and discussion
XRD, TEM, UV–visible NIR, PL and BET studies
Rate constant, band gap energy and surface area of prepared BiOCl microspheres, Cu2ZnSnS4 QDs and BiOCl–Cu2ZnSnS4
Indirect band gap (eV)
Surface area (m2/g)
Rate (k) (min−1)
Facile coprecipitation method was employed for constructing a new BiOCl–Cu2ZnSnS4 heterostructure. BiOCl–Cu2ZnSnS4-1 heterostructured composite showed superior photocatalytic activity in the degradation of Congo red than single phase BiOCl and Cu2ZnSnS4 QDs in the presence of sunlight. The degradation of Congo red was achieved 94.9% in 90 min of sunlight irradiation. The photoactivity of BiOCl–Cu2ZnSnS4-1 was due to the generation of heterostructure between BiOCl and Cu2ZnSnS4, which separates the photoexcited electron–hole pairs in the interface of BiOCl and Cu2ZnSnS4 on absorption of visible light radiation. The recombination of e− and h+ in BiOCl was removed by constructing BiOCl–Cu2ZnSnS4 heterostructure. The created BiOCl–Cu2ZnSnS4 heterostructure can be used as a promising photocatalyst for environmental cleanup.
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