Introduction

Graphene-based semiconductor nanocomposites have been considered as one of the ultimate solutions for energy and environmental issues after the discovery of photocatalytic hydrogen production by Fujishima and Honda in 1972 (Fujishima and Honda 1972). Among different environmental issues, water pollution is one of the major problems throughout the whole world (Yek et al. 2020; Nasrollahzadeh et al. 2020). During several years, significant progresses have been built to solve the water pollution problems and numerous nanomaterials have been constructed to be effective for wastewater management by photocatalytic dye degradation (Akbari et al. 2020; Das et al. 2018; Liang et al. 2017). In contrast, explorations on using graphene-based ternary semiconductor nanocomposites for the photocatalytic reaction are very few. Recently, our group has reported (Tantubay et al. 2020) reduced graphene oxide-CuO/ZnO nanocomposite as an efficient catalyst highlighting an intimate contact that is necessary for the reduction of 4-nitroaniline to p-phenylenediamine.

Notably, several literature surveys show the tremendous photocatalytic applications in the case of ternary nanocomposites or graphene-based hybrid (e.g., CuO/ZnO, CuS/ZnS, CdS/ZnO, CdS/ZnS, ZnO/ZnS) (Prabhu et al. 2019; Feng et al. 2016; Ma et al. 2017; Tang et al. 2015; Lonkar et al. 2018) nanostructures. But there seems to be concurrence that enhancement of photocatalytic activity has been increased after combining the semiconductors with graphene which acts as an electron reservoir to accept the photo-generated electrons from the semiconductor resulting prolonged separation and lifetime between electron-hole pairs and this phenomenon is responsible for photo-activity enhancement. From the past decades, several metal sulfide semiconductor photocatalysts have been proven for wastewater treatment in the presence of different organic dyes or reagents (Wang et al. 2020; Mosleh et al. 2019) due to their visible light response. Among those photocatalysts, ZnS is a typical IIB–VIA group (Feng et al. 2016) semiconductor photocatalyst which exhibits a 90% quantum yield at 313 nm and it has a high conduction band position because it can only absorb ultraviolet light due to its wide band gap (~ 3.6 eV). However, to grow visible light-active ZnS-based photocatalysts, doping of different elements has been carried out and from the results; it is shown that doping of foreign elements with ZnS successfully enhances the absorbance of ZnS in the visible light region. Yu and his co-workers reported that (Zhang et al. 2011) doped ZnS shows significant enhanced photocatalytic activity under visible light irradiation after the surface moderation of ZnS nanomaterials by CuS.

In the recent work, CuS/ZnS-graphene (CZSG) nanocomposites have been synthesized by a simple green approach until now which is not reported as shown in Table 1. Green nanotechnology (Iravani and Varma 2020; Kanwar et al. 2019) is a term that creates outcomes by following an environmentally friendly path. Several processes have been developed to construct CuS/ZnS-graphene heterojunction such as solvothermal method, sulfuration process, and hydrothermal method (Wang et al. 2016; Shao et al. 2016; Harish et al. 2017) but all of them are chemical ways which have always some drawbacks. So, the green approach (Naghdi et al. 2018; Fahiminia et al. 2019; Nasrollahzadeh and Sajadi 2015, 2016; Nasrollahzadeh et al. 2014, 2015a, b) is the safest path to grow different shaped and dimensioned nanomaterials. It is noticed that the role of graphene in the nanocomposites is to perform as an organic dye-like photosensitizer (Zhang et al. 2012), and upon visible light irradiation, the photo-generated electrons can move from graphene to the conduction band of ZnS where ZnS is a wide band gap material. In this phenomenon, semiconductors do not participate in the photocatalytic process under visible light irradiation and this type of photocatalytic mechanism is almost distinct from the usual graphene-based semiconductor photocatalysts to date where graphene acts as an electron reservoir to arrest photo-generated electrons from the conduction band of the semiconductor (Ghosh and Sen 2017). Therefore, our research of such a unique photocatalytic reaction mechanism where graphene acts as a photosensitizer (Singh and Khare 2015) as well as with the increase of graphene content shows enhanced photocatalytic response and a decrease of the particle size of the CuS/ZnS NPs grown on the graphene surface.

Table 1 Summary of photocatalytic performance of rGO/graphene-metal sulfide nanocomposites reported in recent papers till date
Full size table

Materials and methods

Materials

The materials consisted of ultrafine graphite powder (Aldrich), potassium permanganate (Merck), hydrogen peroxide (Merck, 30%), conc. sulfuric acid (Merck, 98% pure), hydrochloric acid (Merck, 30%), zinc sulfate (ZnSO4, 7H2O) (Merck), copper sulfate (CuSO4, 5H2O) (Merck), green tea leaves, thiourea (SC(NH2)2) (Merck), liquor ammonia (NH3), methylene blue (MB), doubled distilled water, ethanol (Merck), Mueller-Hinton agar (HiMedia), bacterial strains, culture conditions, supplements, and Gram-negative bacteria (Escherichia coli BL21 DE3 (E. coli), Aeromonas sp.) and Gram-positive bacteria (Bacillus subtilis (B. subtilis), Staphylococcus aureus (S. aureus), Cellulosimicrobium sp.).

Preparation of plant extract

Fresh leaves of green tea were collected from the local market of Burdwan town (W.B) and the resulting mixture was boiled and cooled down at room temperature, followed by severing into small parts of known weight of these plant leaves in 80 mL distilled water. Then, the obtained transparent filtrate was used as a reducing agent after passing through a Whatman 41 filter paper to remove the solid materials.

Synthesis of CuS/ZnS-graphene nanocomposite

Here, one of the precursors involved in the reaction is graphene oxide (GO) which was prepared from graphite powder followed by Hummers method (William et al. 1958). In order to develop CZSG nanocomposite, a certain amount of GO was dispersed in 150 mL of water through ultrasonic treatment for 20 min. Then, an aqueous solution of ZnSO4, 7H2O and CuSO4,5H2O was added at once to the above mentioned solution under stirring condition as shown in Scheme 1. The resulting reaction mixture was stirred vigorously for a few minutes, followed by adjusting the pH of the dispersion to 10 by adding liquor ammonia and then thiourea dropwise. After the addition of green tea extract, the mixture was agitated by heating at 80 °C for 2 h resulting in deep brown precipitate and transferred to a round bottom flask to get refluxed at 110 °C for 8 h. After completion of the reaction, the black product was collected through centrifugation and was washed using ethanol and doubled distilled water by several times and then dried under vacuum condition at 70 °C temperature. The same procedures were followed to prepare different compositions of the nanocomposites with and without GO and also graphene was prepared using the same protocol without the addition of metal salts. To synthesize different compositions, the weight ratios of the GO, the metal salts, and the sample names are given in Table 2.

Scheme 1
scheme 1

Synthesis diagram of CZSG nanocomposites

Full size image
Table 2 Weight ratios of ZnSO4, 7H2O, CuSO4, 5H2O, and GO used for the preparation of CZSG nanocomposites
Full size table

Characterization techniques

Powder X-ray diffraction (XRD) measurement was carried out using a powdered sample with X-ray powder diffraction patterns recorded in a Bruker (D8 Advance) diffractometer with Ni-filtered CuKα radiation (λ= 1.5414Å). For phase identification, recorded data was compared with standard JCPDS files (Joint Committee of Powder Diffraction System). To study the chemical bonding, Fourier transform infrared (FTIR) study was done by SHIMADZU IR Prestige-21 and the test specimens were prepared by the KBr disk method. The microstructure of the as-synthesized samples was studied by a transmission electron microscope (Model JEOL-1400) and UV-Vis absorption spectra were used to investigate the optical properties of the photocatalysts by recording the data on SHIMADZU UV-1800 spectrophotometer.

Photocatalytic studies

The evaluation of the photocatalytic activity of the as-prepared CZSG nanocomposites was performed at ambient temperature by examining the photo-assisted degradation of MB as the representative dye under visible light illumination. In each photocatalytic experimental procedure, the photocatalyst of a known dosage was added to the fixed amount of dye solution (10 ppm). Then, the mixed dispersion was magnetically stirred in the dark for 30 min before light irradiation at room temperature to ensure the adsorption-desorption equilibrium of MB. Then, visible light source was irradiated to initiate the photocatalytic reaction. The concentration of MB was monitored from time to time using a UV-visible spectrophotometer. The concentration of MB was detected from the decrease in intensity of the absorbance band at around 664 nm by a UV-Vis spectrophotometer. The decomposition efficiency of MB could be determined by the equation:

$$ \mathrm{D}\%=\frac{\left({A}_0-{A}_t\right)}{A_0}\times 100\% $$
(1)

where A0 refers to the absorption of MB after the adsorption equilibrium is achieved prior to the visible light illumination and At is the absorption of MB at time interval t under visible light illumination. Furthermore, photocatalytic kinetics was usually expressed by the following equation:

$$ \ln \frac{A_0}{A_t}= kt $$
(2)

where k is the apparent pseudo-first-order reaction rate constant and t is the irradiation time.

Recyclability study

To evaluate the photostability of the nanocatalyst, five cycles of dye degradation of MB have been carried out with the most efficient catalyst (CZSG4). After each cycle of photo-degradation test, the MB solution containing used photocatalyst in dispersion state was centrifuged, washed with water, and dried in vacuum, and thus, the used photocatalyst was separated from the MB solution to be used for the next cycle.

Antibacterial activity

The well diffusion method was used to determine the antibacterial activity of nanocomposites. The bacterial strains (mentioned in the Materials and methods section) were used for this study. Mueller-Hinton agar medium was made by adding 38 g of Mueller-Hinton agar in 1.0 L of Milli-Q water and sterilized by autoclaving for 20 min. One hundred-microliter log phase grown cells from each strain, E. coli, B. subtilis, S. aureus, Aeromonas sp., and Cellulosimicrobium sp., were spread separately on the solid surface of the plates. The specified volume of sample solutions was placed in the wells and the plates were incubated at 37 °C for 24 h in a bacterial incubator after making the wells of 3 mm in diameter using a sterile cork borer. The zone of inhibition surrounding the wells was noticed by the samples and each zone of inhibition was compared against standard antibiotics of ampicillin (10 μL) and tetracycline (30 μL).

Results and discussion

XRD analysis was applied to determine the crystalline structures of as-prepared CZSG composites. The XRD patterns of well-connected formation of CZSG composites with different GO concentrations are displayed in Fig. 1a. It is obvious that the XRD patterns of CZSG curves exhibit several diffraction peaks at 2θ value at 29.45, 31.79, 32.71, 48.11, 52.51, 58.50, and 59.16 corresponding to (102), (103), (006), (110), (108), (203), and (116) planes of the hexagonal phase of CuS (JCPDS no. 06-0464) (Guan et al. 2015; Shuaia et al. 2018) respectively. Besides, the diffraction peaks at 2θ at 28.53, 47.65, and 56.8 have corresponded to the (111), (220), and (311) planes of the cubic phase of ZnS (JCPDS no. 05-0566) (Abideen and Teng 2018; Huang et al. 2020). There is no peak of graphene oxide (GO) (001) in the composite samples which clearly indicates the successful reduction of GO to graphene.

Fig. 1
figure 1

a XRD graphs of CZSG1, CZSG2, and CZSG4 (pink color for ZnS planes and black color for CuS planes and b FTIR spectra of graphene and CZSG1

Full size image

The existence of different oxygen-containing functional groups on GO and the successful reduction of GO to graphene in the CZSG1 nanocomposite was confirmed by FTIR spectroscopy. As shown in Fig. 1b, the FTIR spectrum of GO exhibited several characteristic peaks. The bands at 3450 and 1602 cm−1 have belonged to the stretching and bending vibrations of the O–H group (Sennour et al. 2017; Khan et al. 2020). The band at 1726 cm−1 is attributed to C=O groups (Abida et al. 2020) and the stretching vibrations at 1231, 1149, and 1085 cm−1 are responsible for the C–O bond of epoxy groups (Guan et al. 2014; Liu et al. 2019). The decrease in intensity of these peaks and the appearance of a peak at 606 cm−1 (metal-S bond stretching) in the composite material confirm the formation of this nanocomposite (Cao et al. 2019; Dutta et al. 2017).

In Fig. 2a, GO exhibits an π–π* transition at 230 nm and a hump for n–π* transition at 305 nm, but after reduction, the π–π* transition shifts to 270 nm in graphene indicating that some groups are partially removed from the GO surface and the conjugation in the structure is restored (Shams et al. 2019). In CZS sample as shown in Fig. 2b, a hump around 300–380 nm has been found due to CuS/ZnS NPs (Corrado et al. 2010). In the case of CZSG nanocomposite samples, two peaks at around 267–271 nm and 368 nm are responsible due to the π-π* transition in graphene and the formation of CZS NPs on the graphene surface (Devi et al. 2015; Reddy et al. 2014).

Fig. 2
figure 2

UV-Vis spectra of a GO and graphene b CZSG nanocomposites

Full size image

The optical band gap study has been investigated to ensure the band gap values in the nanocomposites. The band gap values for the nanocomposites have been explored from the optical absorption phenomenon which obeys Tauc’s formula (Zanatta 2019; Morales et al. 2019). The value of the band gap can be calculated using the fundamental absorption which corresponds to the electron jump from the valance band to the conduction band. The absorption coefficient (α) and the incident photon energy (hϑ) are connected with the Tauc’s formula given by the equation:

$$ {\left(\alpha h\vartheta \right)}^n=K\left( h\vartheta -{E}_{\mathrm{g}}\right) $$
(3)

where hϑ is the photon energy, α is the absorption coefficient, K is a constant, Eg is the band gap energy, and the value of n is 2 for direct allowed transitions while it is ½ for the indirect allowed transition. The variation of (αhν)2 vs. hν for the samples reveals that the exploration of the linear portion to the hν axis gives the energy gap of the direct band gap (Abdel-Kader and Mohamed 2020). From Fig. 3, it is shown that the values of the direct band gap for the CZS, CZSG1, CZSG2, and CZSG4 are 3.04, 2.93, 2.90, and 2.87 eV, respectively, which clearly indicate a band gap narrowing of the CZS sample after the coupling with graphene in CZSG nanocomposites. According to a literature study (Wang et al. 2015a, b; Lonkar et al. 2018), this result can be ascribed due to the hybridization of carbon materials in which doping of graphene into the CZS nanoparticles originates mid-gap energy levels resulting in a decrease in band gap energy.

Fig. 3
figure 3

UV-Vis absorption data fitted by Tauc’s formula for direct band gap of a CZS, b CZSG1, c CZSG2, and d CZSG4 nanocomposites

Full size image

With the increase of graphene content in the CZSG nanocomposites, the particle size decreases as shown in Fig. 4. The approximate sizes of the samples CZSG1, CZSG2, and CZSG4 are 175–215 nm, 125, and 9 nm respectively. With the increase of the graphene content in the nanocomposite samples, the NP size grown on the graphene surface follows the order CZSG1˃ CZSG2˃ CZSG4. This is because of the fact that there are many negative groups on GO which act as good nucleation sites for the NP growth (Stanek et al. 2016; Paranthaman et al. 2019). So, with the increase of the GO content, nucleation sites increase, and the probability of agglomeration decreases, as a result, the size of the NPs decreases (Khan et al. 2015; Estelléa et al. 2018).

Fig. 4
figure 4

TEM images of a CZSG1, c CZSG2, and e CZSG4 nanocomposites and b, d, and f are size distribution graphs of CZSG1, CZSG2, and CZSG4 nanocomposites

Full size image

Photocatalytic study

It was believed that the photocatalytic activity of a photocatalyst is enhanced depending on the (i) separation efficiency of the photo-excited electron-hole pairs and the (ii) amount of the reactive oxygen species (ROS) (Riente and Noël 2019). Under visible light irradiation, ZnS cannot respond due to a large band gap value (Wang et al. 2015a, b) but CuS is active in this condition (Adhikari et al. 2017). Therefore, a heterojunction is formed between ZnS and CuS to transfer the photo-generated electrons from CB of CuS to CB of ZnS leaving the holes in the VB of CuS (Shao et al. 2016) and the resulting suppression of recombination of electron-hole pairs occurs and thus the photocatalytic activity is increased (path (I)) as shown in Scheme 2. In Fig. 5a, the bare CZS sample shows the photocatalytic activity 63.66% within 100 min, but after the incorporation of graphene with the binary CZS nanocomposite, the enhanced photocatalytic activities become 79.71%, 87.48%, and 90.21% in the case of nanocomposite samples CZSG1, CZSG2, and CZSG4 respectively (Fig. 5b–d). The bare CZS exhibited less photocatalytic activity than the CZSG nanocomposites due to the aggregation of CZS NPs which results in larger sized NPs. Actually, graphene acts as an organic dye-like macromolecular ‘photosensitizer’ (Feng et al. 2014; Gálveza et al. 2019), where under visible light irradiation, the electrons on the highest occupied molecular orbital (HOMO) of graphene were at first excited to the lowest unoccupied molecular orbital (LUMO) of graphene and then the photo-excited electrons in graphene move to the CB of ZnS, resulting in the separation of electron-hole pairs. Thus, in CB of ZnS, the number of photo-excited electrons has been increased after the addition of graphene in CZSG nanocomposite (path (II)). For this reason, with the increase of graphene content, the photocatalytic activity gradually increases in the graphene-based nanocomposite samples than that of bare CZS nanocatalyst. The rate constants of the MB degradation for the samples are given in Table 3.

Scheme 2
scheme 2

a, b Pictorial and schematic representation of photocatalytic mechanism respectively

Full size image
Fig. 5
figure 5

Absorption spectra of MB solution in the presence of a CZS, b CZSG1, c CZSG2 and d CZSG4 respectively under visible light irradiation. e, f Photocatalytic degradation of MB and rate kinetics in the presence of different CZSG photocatalysts under visible light irradiation

Full size image
Table 3 Comparison of the degradation efficiency, the rate constant, and relative correlation coefficient of different as-prepared nanocomposite samples
Full size table

Recyclability tests

Since CZSG4 nanocomposite shows excellent photo-degradation efficiency, the photocatalytic stability was investigated using this composite by recycling tests (Rajendran et al. 2016). The results of the recyclability test are shown in Fig. 6a, which shows a slight decrease in its catalytic performance after each cycle. This photocatalyst exhibits excellent photostability even after five cycles which further confirms the excellent stability of the nanocomposite under visible light irradiation.

Fig. 6
figure 6

a Recyclability tests of CZSG4 nanocatalyst for degradation of MB dye. b Bar diagram showing zone of inhibition (in mm) introduced by CZS (P), CZSG1 (Q), CZSG2 (R), and CZSG4 (S) against various microorganisms

Full size image

Antibacterial activity

In the present study, the antibacterial activity of the synthesized samples was tested by a well diffusion method using bacterial strain E. coli BL21 DE3 Gram-negative, S. aureus Gram-positive and B. subtilis Gram-positive, Aeromonas sp. Gram-negative, and Cellulosimicrobium sp. Gram-positive. The CZS (P), CZSG1 (Q), CZSG2 (R), and CZSG 4 (S) were promising in exhibiting their ability to destroy B. subtilis, a Gram-positive bacterium. The zone of inhibition was measured for P, Q, R, and S at 50 mg mL−1 concentration, shown in Table 5. The antimicrobial activities of P, Q, R, and S were compared against two standard antibiotics (Fig. 7 and Table 4) ampicillin and tetracycline. As shown in Fig. 6b, it was clear from the study that P, Q, R, and S nanoparticles have good antimicrobial activities on bacterial strains (Table 5).

Fig. 7
figure 7

Antibacterial activity of CZS, CZSG1, CZSG2, and CZSG4 (P, Q, R, and S in the picture respectively) nanocomposites against a E.coli, b B. subtilis, c S. aureus, d Aeromonas sp., and e Cellulosimicrobium sp. respectively

Full size image
Table 4 Antibacterial activity of standard antibiotics against bacterial strains
Full size table
Table 5 Antibacterial activities of P, Q, R, and S NPs
Full size table

± Standard deviation

Conclusions

In summary, CuS/ZnS-graphene nanocomposites with different graphene content have been fabricated via a simple greener approach and different characterization techniques confirm the synthesis of these nanocomposites. Microscopic study reveals that with the increase of the amount of graphene in the nanocomposites, the particle size of the CZS nanoparticles decreases due to a decrease in aggregation in high surface area graphene sheet and band gap energy also diminishes. The CZSG nanocomposites can act as visible light photocatalysts towards organic dye degradation. It has been exemplified that the visible light–driven nanocomposites/photocatalysts show remarkably efficient dye degradation capability due to the role of graphene as a photosensitizer. Due to having promising effects in various strains of microorganisms, the nanocomposites can act as potential antibacterial agents. These findings could assist to fabricate new graphene-based photocatalysts with better photocatalytic performance and stability in visible light irradiation.