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.

Experimental procedure


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.

At the initial stage of reaction, Sn2+, Cu+, Zn2+ ions combine with EG to form Sn–EG, Cu–EG, Zn–EG. When thiourea was added into the reaction system, Sn2+, Cu+ and Zn2+ ions form complex with thiourea and yielded Sn–thiourea, Cu–thiourea and Zn–thiourea complexes. These complexes undergo thermal decomposition at 160 °C to produce EG-capped Cu2ZnSnS4 QDs. The possible reactions occurred during the preparation of Cu2ZnSnS4 QDs were as follows:

$$\begin{aligned} 2{\text{Cu}}^{ + } + n({\text{EG}}) & \to \left[ {2{\text{Cu}}({\text{EG}})_{n} } \right]^{ + } \\ {\text{Zn}}^{2 + } + n({\text{EG}}) & \to \left[ {{\text{Zn}}({\text{EG}})_{n} } \right]^{2 + } \\ {\text{Sn}}^{2 + } + n({\text{EG}}) & \to \left[ {{\text{Sn}}({\text{EG}})_{n} } \right]^{2 + } \\ 2{\text{Cu}}^{+} + n({\text{SCH}}_{2} {\text{H}}_{4} ) & \to \left[ {2{\text{Cu}}({\text{SCN}}_{2} {\text{H}}_{4} )_{n} } \right]^{2 + } \\ {\text{Zn}}^{2+} + n({\text{SCN}}_{2} {\text{H}}_{4} ) & \to \left[ {{\text{Zn}}({\text{SCH}}_{2} {\text{H}}_{4} )_{n} } \right]^{2 + } \\ {\text{Sn}}^{2 + } + n\left( {{\text{SCH}}_{2} {\text{H}}_{4} } \right) & \to \left[ {{\text{Sn}}\left( {{\text{SCN}}_{2} {\text{H}}_{4} } \right)_{n} } \right]^{2 + } \\ \left[ {2{\text{Cu}}\left( {{\text{SCN}}_{2} {\text{H}}_{4} } \right)_{n} } \right]^{2+} + \left[ {{\text{Zn}}\left( {{\text{SCN}}_{2} {\text{H}}_{4} } \right)_{n} } \right]^{2+} \left[ {{\text{Sn}}\left( {{\text{SCN}}_{2} {\text{H}}_{4} } \right)_{n} } \right]^{2+} & \mathop{\longrightarrow}\limits^{{160\;^{ \circ } {\text{C}},6{\text{h}}}}{\text{Cu}}_{2} {\text{ZnSnS}}_{4} \\ \end{aligned}$$

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).

Characterization techniques

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

XRD pattern of BiOCl, Cu2ZnSnS4 QDs and BiOCl–Cu2ZnSnS4 is shown in Fig. 1a–c. The diffracted peaks of the synthesized Cu2ZnSnS4 QDs were matched with JCPDS file 00-026-0575 (Kesterite, syn) with crystal system tetragonal. The crystallite size of Cu2ZnSnS4QDs was obtained from Debye Scherrer formula, D = 0.99λ/β cos θ, where β is full width at half maximum of the diffracted peak, λ is the X-ray wavelength and θ is the angle of diffraction and it was found to be 7 nm. BiOCl–Cu2ZnSnS4 heterostructure displayed the typical BiOCl signals (JCPDS file 01-085-0861). Characteristic peaks of Cu2ZnSnS4 QDs were not seen in the pattern of BiOCl–Cu2ZnSnS4 which may be due to low content of Cu2ZnSnS4 QDs in the composite (Fig. 1c). The surface morphology and the interaction of Cu2ZnSnS4 QDs with BiOCl were studied from TEM (Fig. 2a–f). From Fig. 2e–f, it was seen that Cu2ZnSnS4 nanoparticles (5–7 nm) were deposited on BiOCl microsphere surface to form interface linking between BiOCl and Cu2ZnSnS4 QDs. The EDX data (Fig. 3) of BiOCl–Cu2ZnSnS4 revealed the presence of elements O, S, Cl, Cu, Zn, Sn and Bi and were distributed homogenously in the composite.

Fig. 1
figure 1

XRD pattern of a Cu2ZnSnS4 QDs, b BiOCl and c BiOCl–Cu2ZnSnS4-1

Fig. 2
figure 2

TEM images of BiOCl (a, b), Cu2ZnSnS4 QDs (c, d) and BiOCl–Cu2ZnSnS4-1(e, f)

Fig. 3
figure 3

EDX spectra of BiOCl–Cu2ZnSnS4-1

UV–Vis NIR absorption spectra of BiOCl, Cu2ZnSnS4QDs and BiOCl–Cu2ZnSnS4 are shown in Fig. 4. Cu2ZnSnS4 QDs absorbed UV–visible light up to near infrared region, while UV radiation was absorbed by BiOCl and BiOCl–Cu2ZnSnS4 composites exhibited a mixed absorption (Fig. 4b). The band gap energy (Eg) for BiOCl microspheres, Cu2ZnSnS4 QDs and BiOCl–Cu2ZnSnS4 was obtained from Tauc equation (Xiao et al. 2018), and the plots are shown in Fig. 5a–d. The obtained Eg value of BiOCl microspheres was closed to 3.46 eV (Zang et al. 2006). The Eg value was decreased on loading of Cu2ZnSnS4 QDs in composites (Table 1) which may be aroused from synergistic effect between Cu2ZnSnS4 QDs and BiOCl in lowering the band gap.

Fig. 4
figure 4

UV–visible light-NIR absorption spectrum of BiOCl microspheres, BiOCl–Cu2ZnSnS4-1, BiOCl–Cu2ZnSnS4-2 and Cu2ZnSnS4

Fig. 5
figure 5

Tauc plots of (αhv)1/2 versus photon energy (hv) of a BiOCl microspheres, b Cu2ZnSnS4 QDs, c BiOCl–Cu2ZnSnS4-1 and d BiOCl–Cu2ZnSnS4-2

Table 1 Rate constant, band gap energy and surface area of prepared BiOCl microspheres, Cu2ZnSnS4 QDs and BiOCl–Cu2ZnSnS4

To study the charge recombination over BiOCl–Cu2ZnSnS4 heterostructures, PL responses were taken and the plots were presented in Fig. 6. At excitation wavelength of 310 nm, BiOCl shows PL intensity with emission peaks at 313 nm, 360 nm and 625 nm. The emission intensity was decreased on incorporating Cu2ZnSnS4 QDs in the BiOCl–Cu2ZnSnS4-1. This was occurred due to the isolation of electrons and holes by coupling BiOCl with Cu2ZnSnS4. On further addition of Cu2ZnSnS4 in BiOCl–Cu2ZnSnS4-2, the PL intensity was increased because of the recombination of isolated electrons and holes. Thus, our findings have shown that BiOCl–Cu2ZnSnS4-1 heterostructure exhibited the best eh+ pair separation.

Fig. 6
figure 6

Photoluminescence spectra of BiOCl, BiOCl–Cu2ZnSnS4-1 and BiOCl–Cu2ZnSnS4-2

N2 adsorption–desorption isotherms of Cu2ZnSnS4 QDs, BiOCl and BiOCl–Cu2ZnSnS4 were of type III (Fig. 7), and the surface area of BiOCl–Cu2ZnSnS4 was increased after adding Cu2ZnSnS4 QDs in composite (Table 1). The pore volume of BiOCl–Cu2ZnSnS4 was of 0.076 cm3/g.

Fig. 7
figure 7

N2 adsorption–desorption isotherms of as-prepared BiOCl microspheres, Cu2ZnSnS4 QDs and BiOCl–Cu2ZnSnS4-1

Photocatalytic performance

After sunlight irradiation, absorbance spectra of Congo red dye solution in the presence of BiOCl microspheres, Cu2ZnSnS4 QDs and BiOCl–Cu2ZnSnS4 were measured (Fig. 8a–d). The Congo red molecules were degraded completely in the presence of BiOCl–Cu2ZnSnS4-1 photocatalyst, and the degradation efficiency was represented in Fig. 9a. BiOCl–Cu2ZnSnS4-1 showed higher photodegradation and degraded 94.9% of Congo red dye after 90 min of sunlight irradiation. The photodegradation rate constant of Congo red was higher for heterostructure than for bare BiOCl and Cu2ZnSnS4 QDs (Fig. 9c). The improved photocatalytic activity of BiOCl–Cu2ZnSnS4 was endorsed to the formation of heterostructure between BiOCl and Cu2ZnSnS4 QDs that improved separation and transport of photogenerated charge carriers. The degradation behavior of BiOCl–Cu2ZnSnS4 was compared with commercial P25 TiO2 and it was observed that the degradation efficiency of BiOCl–Cu2ZnSnS4 was lesser than that of P25 TiO2 towards Congo red (Shambharkar and Chowdhury 2018).

Fig. 8
figure 8

UV–visible absorbance spectra of Congo red before and after solar light irradiation in the presence of a BiOCl microspheres, b Cu2ZnSnS4 QDs, c BiOCl–Cu2ZnSnS4-1 and d BiOCl–Cu2ZnSnS4-2

Fig. 9
figure 9

a Percentage efficiency of photodegradation of Congo red with time. b Effect of different scavengers. c Plot of ln(C0/C) versus irradiation time. d Recyclability

The band edge positions of BiOCl and Cu2ZnSnS4 (Zheng et al. 2018; Cao et al. 2013) were determined (Table 1). The χ value for Cu2ZnSnS4 QDs was calculated to be 5.29, and χ value for BiOCl was calculated to be 6.36 (Sun et al. 2015). The CB of Cu2ZnSnS4 QDs was higher than CB of BiOCl. Therefore, the photoinduced electrons from the conduction band of Cu2ZnSnS4 QDs were inserted to the conduction band of BiOCl. Meanwhile, the photogenerated holes on the valence band of BiOCl were moved to valence band of Cu2ZnSnS4. This process resulted in heterojunction at the interface of the two compounds (Fig. 10). The holes react with H2O and generate ·OH radicals. The electrons combine with O2 to yield active superoxide radical anion of oxygen and decomposed the Congo red dye on the catalyst surface. The degradation experiment was repeated using the BiOCl–Cu2ZnSnS4. BiOCl–Cu2ZnSnS4 heterostructure had retained its stability up to four cycles.

Fig. 10
figure 10

Scheme of electron transfer in BiOCl–Cu2ZnSnS4 heterostructure

Various scavengers like isopropanol (Pr) for OH radicals, ammonium oxalate (AO) for h+, 1,4-benzoquinone (BQ) for O ∙−2 radicals were mixed with Congo red solution containing BiOCl–Cu2ZnSnS4 compound, and the photocatalytic activity was executed. From Fig. 9b, it was observed that AO and BQ were inhibited the photodegradation reaction. Therefore, the possible reactions happened over BiOCl–Cu2ZnSnS4 heterostructure in the degradation of the dye were given below.

$$\begin{aligned} {\text{BiOCl}}{-}{\text{Cu}}_{ 2} {\text{ZnSnS}}_{4} + hv & \to {\text{BiOCl}}{-}{\text{Cu}}_{ 2} {\text{ZnSnS}}_{4} \left( {h_{\text{VB}}^{ + } + e_{\text{CB}}^{ - } } \right) \\ \left( {h_{\text{VB}}^{ + } + e_{\text{CB}}^{ - } } \right) & \to {\text{Energy}} \\ e_{\text{CB}}^{ - } + {\text{O}}_{2} & \to {\text{O}}_{2}^{ \cdot - } \\ h_{\text{VB}}^{ + } + 2{\text{H}}_{2} {\text{O}} & \to {}^{ \cdot } {\text{OH}} + {\text{H}}_{ 3} {\text{O}}^{ + } \\ {\text{O}}_{2}^{ \cdot - } + {\text{H}}^{ + } & \to {}^{ \cdot } {\text{OOH}} \\^{ \cdot } {\text{OOH}} + {}^{ \cdot } {\text{OOH}} & \to {\text{H}}_{2} {\text{O}}_{2} + {\text{O}}_{2} \\ {\text{O}}_{2}^{ \cdot - } + {\text{Congo red}} & \to {\text{Intermediates}} \to {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}} \\^{ \cdot } {\text{OOH}} + {\text{Congo red}} & \to {\text{Degradation products}} \\^{ \cdot } {\text{OH}} + {\text{Congo red}} & \to {\text{Degradation products}} \\ \end{aligned}$$


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.