1 Introduction

Designing the metal oxide based photocatalyst for environmental pollution control has gained a lot of research interest worldwide [1,2,3]. Between all the various metal oxides, TiO2, ZnO, SnO2 based photocatalysts are some of the most studied systems [4,5,6]. ZnO is one of the best candidates which has great response as a photocatalyst. However, the commercial usage of ZnO is limited because low quantum efficiency and instability in wide pH range due to photo-corrosion. This limitation has led to research oriented towards improving the ZnO properties by forming heterostructure or doping with other materials [7,8,9]. In the case of heterostructures, core-shell structures have an advantage where the shell can be a physical barrier between the optically active core and the surrounding medium. It can also modify the charge, stability, functionality, reactivity, and dispersive ability of core material [10,11,12,13,14]. Core shell heterostructures also have better electrical, catalytic, optical, and magnetic properties in comparison to their bare counter parts [15,16,17]. Generally, the core-shell heterostructures have intermediate properties between the core and shell materials [18,19,20]. Khanchandani et al. have synthesized ZnO–CdS, ZnO–Ag2S and ZnO–In2S3 core-shell heterostructures. They have functionalized the ZnO surface by using the citric acid and then formed CdS or Ag2S or In2S3 shell over a ZnO core by chemical route. These core/shell nanostructures show superior photocatalytic behavior for the degradation of dyes under light illumination [21,22,23]. Subash et al. prepared the Ag2S–ZnO composite via a two-step process. They first precipitated the zinc oxalate and Ag2S and then calcined the mixture at 400 °C for 12 h to obtain Ag2S–ZnO composite. The photodegradation of Acid Black 1 was performed by using Ag2S–ZnO composite under solar light. The composite was more capable for mineralizing the Acid Black 1 in comparison to synthesized ZnO, bulk ZnO, TiO2–P25 and TiO2 (Merck) [24]. Shi et al. have synthesized the ZnO–PbS/GO photocatalyst for hydrogen evolution from water. The multiple exciton formation in ZnO–PbS/GO is responsible for the remarkable enhanced photoactivity of H2 evolution reaction [25]. The above researches show that the formation of core-shell heterostructures by incorporating the metal sulfide shell on to the ZnO core enhances its absorption in the visible range which further improves the photocatalytic reaction efficiency by increasing the stability of electron-hole pairs. Zinc sulphide (ZnS) is a wide band gap (3.7 eV) semiconductor material which is nontoxic and water insoluble [26, 27]. It can be used as a coating material for ZnO nanorods because it has shown its application in electroluminescent devices, sensors and lasers [28,29,30,31]. ZnO–ZnS based heterostructures have shown improved physicochemical properties in different applications [32,33,34,35,36]. Hence, a considerable efforts have been made to synthesize ZnO–ZnS based heterostructures [37,38,39,40,41,42]. Lin et al. have made an efficient ZnO–ZnS photocatalyst using template-assisted method. The photo-activity of ZnO–ZnS photocatalyst was analyzed by using degradation of rhodamine B. The surface coupling between the two semiconductors proposes their higher photon absorption efficiency and an enhanced exciton separation, which lead to a significant improvement in the photocatalytic efficacy in comparison with bare ZnO [43]. Lonkar et al. have prepared the ZnS–ZnO/graphene based nano-photocatalysts. The ZnO and ZnS nanoparticles are uniformly distributed over the graphene matrix in the synthesized nanohybrids. The synergic effect in ZnS–ZnO/graphene decreased the band gap in the composite system. They checked the photocatalytic efficiency of ZnS–ZnO/graphene nano–photocatalysts by a test photo-reaction with organic azo dyes and toxic phenol molecules from waste water which have harmful impact on the environment [44]. Based on the above literature survey, it appears that the heterojunction formation between the ZnO and ZnS will generate superior hybrid material having lower photoexcitation energy than the bare counter parts [32,33,34,35,36,37,38,39,40,41,42,43,44]. The reason behind this is ZnO–ZnS heterostructures induces a built-in electric field which improve the interfacial charge transfer and deliver remarkable photoactivity [32,33,34,35,36,37,38,39,40,41,42,43,44]. Theoretically, type II band alignment formed between ZnO and ZnS results in the better photocatalytic activity in the heterostructure. Therefore, ZnO–ZnS heterostructure represents a fascinating candidate for photocatalysis. Here, we have attempted to a simplistic one-pot bulk approach for synthesize of ZnO–ZnS core-shell heterostructures through solid state solvent free route. Earlier, the typical methods which are used for the synthesis of ZnO–ZnS heterostructures consist of solvothermal route, sol–gel process, microwave route, ion replacement etc. The large solvent usage and the complex synthetic procedure limited their usage for large scale productions. Here, we did the sulphurization of ZnO nanorods by utilizing the thiourea as sulphur source. The amount of ZnO and ZnS in the heterostructures can be controlled by varying the amount of thiourea used in the reaction. These fabricated heterostructures were thoroughly characterized using PXRD, TEM, EDX, UV-Vis spectroscopy, PL, EPR, EIS and XPS studies. Further, the as synthesized ZnO–ZnS heterostructures were utilized for removal of industrial waste water containing the organic pollutants (p-nitrophenol and methyl orange).

2 Materials and methods

2.1 Material used

For the preparation of ZnO, ZnS and ZnO–ZnS heterostructures, zinc acetate dihydrate (Sigma-Aldrich, purity 98%) and thiourea (Merck, purity 99%) were utilized. p-nitrophenol (PNP, Fluka, > 98%) and methyl orange (MO, Sigma-Aldrich, 98%) were used as representative chemicals for industrial waste effluent treatment. Benzoquinone (Sigma-Aldrich, purity 98%), Ammonium oxalate (Merck, purity 99%) and isopropanol (Merck, purity 99%) were used as trapping agents to identify the active species responsible for the photocatalytic activity of ZnO–ZnS heterostructures.

2.2 Synthesis procedure

ZnO nanorods were synthesized by solid state route. Briefly, zinc acetate was thermally decomposed in a muffle furnace for 12 h at 300 °C with a heating rate of 40 °C/h. Pure ZnS was synthesized by putting equimolar zinc acetate and thiourea in furnace under same condition. The ZnO–ZnS heterostructures were synthesized by taking different molar ratio of ZnO nanorods and thiourea in a mortar pestle and grounded to fine powder (Table 1). Then the resulting solid mixture was calcined in a muffle furnace under similar condition. The obtained product was washed with water and ethanol to remove unreacted thiourea.

Table 1 Parameters for the synthesis of ZnO–ZnS heterostructure formation
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2.3 Characterization

The characterization of phase formed, their purity and crystal structure of the products were carried out on a powder X-ray diffractometer (Bruker D8 Advance diffractometer) having Ni filtered Cu Kα radiation with wavelength 1.54060 Å in the range of 10°–80° with a scan rate 0.02°. The morphology studies of the samples were done on a transmission electron microscope (JEOL, JEM-2100) operated at 200 kV. The band gap of the samples was calculated using DRS studies on a UV-vis spectrophotometer (Shimadzu, UV-2600). The band gaps (Eg) were evaluated using following Tauc equation –

$$ \left( {\alpha h\nu } \right)^{n} = k\left( {h\nu - E_{g} } \right) $$
(1)

where h is Planck’s constant, ν is the frequency of photon, α is the absorption coefficient, k is a constant and n is 2 or ½ depending on direct transition or indirect transition respectively.

The evaluation of photocatalytic activity of ZnO–ZnS heterostructures for p-nitrophenol and methyl orange dye has been done under a 300 W Xenon lamp (Newport) with 390 nm filter. The absorbance spectra were measured using an UV-Vis spectrophotometer (Shimadzu 2600, Japan). The absorbance spectra as measured after every 10 min under light illumination. The details of photocatalytic procedure are given in supplementary information. The photocatalytic measurements for degradation of MO dye using the ZnO–ZnS heterostructures were also performed under the standard conditions by using commercial AM 1.5G solar simulators. Electrochemical studies for measuring the EIS and transient photoresponse of the heterostructure performed in a three-electrode PGSTAT-30 (Autolab) electrochemical work station. EPR studies were performed on Bruker EMX Plus instrument. X-ray photoelectron spectroscopy (XPS) studies were carried out with an ESCALAB 250Xi (Thermo Scientific) operating with an Al Kα source (1486.6 eV) under an ultrahigh vacuum of 3.5 × 10−7 Pa.

3 Results and discussion

In the present study, an attempt has been made to design an efficient photocatalyst for industrial pollutant degradation. The work has been done in two stages and in the first stage, ZnO–ZnS heterostructures were synthesized via solid state (solvent free) route by changing the molar ratio of ZnO and thiourea. Second stage deals with the photocatalytic studies of the synthesized heterostructures for removal of industrial pollutants (here methyl orange and p-nitrophenol) and their detailed mechanistic studies. The phase and crystal structure of all synthesized samples was determined by PXRD. Figure S1a shows the PXRD pattern of the product obtained after the thermal decomposition of zinc acetate at 300 °C for 12 h. After the thermal treatment, the observed PXRD pattern of Z1 indicates the formation of hexagonal ZnO having space group P63mc (JCPDS-01-071-6424). Similarly, the product obtained from the thermal decomposition of mixture of equimolar ratio of zinc acetate and thiourea (Z2) has been analyzed by PXRD (Fig. S1b). All the reflection planes could be indexed using the hexagonal unit cell of ZnS to the P3m1 space group (JCPDS-01-072-9267). As discussed earlier, here we aim to synthesis ZnO–ZnS heterostructure and from above phase analysis; we found that utilization of zinc acetate leads to the formation of either ZnO or ZnS. Hence, to make the heterostructures, we have first attempted to synthesize the ZnO nanorods by calcining the zinc acetate at slow heating rate. The as obtained ZnO was further sulphurized using thiourea. For this purpose, ZnO was ground with thiourea in different molar ratio and heated at 300 °C for 12 h (Table 1 and experimental section). PXRD pattern of Z3, Z4 and Z5 are given in Fig. 1. All the diffraction peaks in PXRD pattern was well matched with ZnO (JCPDS no.01-071-6424) and ZnS (JCPDS no. 01-072-9267). As we increase the quantity of thiourea, the percentage of ZnS increased (Fig. 1b–d). Additionally, there are no extra peaks in PXRD pattern which confirms the formation of ZnO–ZnS heterostructures. From the powder x-ray diffraction study, it appears that when ZnO were thermally treated with thiourea, there is partial conversion of ZnO into ZnS by interaction of zinc oxide surface with thiourea at 300 °C [45]. The size and morphological characteristics of synthesized samples were analyzed under transmission electron microscopy (TEM). Figure 2 displays the TEM micrographs of the as-synthesized Z1, Z2, Z3, Z4 and Z5. In the case of Z1 (bare ZnO), the nanorods of size 25 nm × 390 nm with an aspect ratio of 14 were observed. As discussed earlier, zinc acetate and thiourea were heated together and ZnS was formed. TEM micrograph indicates the formation of 10–15 nm particles. To make the heterostructures, ZnO nanorods were heated with thiourea. These heterostructures have core-shell type of structure with inner core of ZnO and ZnS shell as shown in Fig. 2c,e. The TEM micrographs of the ZnO–ZnS heterostructures show the formation of nanorods of size 65 nm × 375 nm for Z3 with shell thickness 15 nm, 75 nm × 335 nm with shell thickness 20 nm, 90 nm × 300 nm for Z5 with shell thickness 23 nm. In Fig. 2, TEM micrographs of the ZnO–ZnS heterostructure shows that ZnS nanoparticles are attached on to the surface of ZnO nanorods uniformly which enables effective interfacial electron transfer, thus, an increased photocatalytic activity is expected in heterostructures. The results of line EDX mapping (Fig. 3) show that there is co-existence of Zn, O and S elements in ZnO–ZnS core/shell heterostructures. Fig 3 demonstrates the results of TEM line-EDX mapping acquired by positioning the electron beam across the ZnO–ZnS nanorods. Positioning the electron beam across the ZnO–ZnS heterostructure resulted in significantly intense homogeneous signals of Zn and S in the shell and weaker signals of O beam across the ZnO–ZnS nanorods respectively. This analysis confirms the composition of ZnO–ZnS heterostructures have ZnO as core and ZnS as the shell material. Thus, the EDX analysis, gives the clear evidence of the formation of a core/shell heterostructure [46]. The insights of surface properties and components in the synthesized samples were obtained by XPS studies. The plots for all the samples are shown in Fig. 4 and S2. The surface O 1 s peak contained contributions from different chemical species in relative to the processing conditions. The Zn2p3/2 and Zn2p1/2 peaks at ~ 1020 and ~ 1043 eV, respectively, are shown in Fig. 4d-f. Besides the presence of lattice oxygen located at 530.0 eV which is an indication of ZnO formation, a second band centered at ~ 532 eV is assigned to the chemisorbed oxygen caused by surface hydroxyl groups. In the S2p XPS spectra (Fig. 4g–i), the S2p3/2 and S2p1/2 peaks centered at ~ 161 and ~ 163 eV, respectively, are related to Zn−S bonding, indicating that ZnS has been successfully synthesized. Compared with the pure ZnO, O 1 s peak at around 531.9 eV was detected and shifted after the deposition of ZnS nanoparticles, and the peak area at 530.0 eV of lattice oxygen decreased sharply (Fig. 4 and S2), which shows that almost no ZnO crystal exists in the ZnS shell layer [47,48,49]. The optical band gap determined for ZnO, ZnS and ZnO–ZnS heterostructures by using UV/vis diffuse reflectance spectroscopy (DRS) and the values calculated for Z1, Z2, Z3, Z4 and Z5 were found to be 3.3, 3.6, 3.1, 3.1 and 3.1 eV, respectively (Fig. S3). This shift in the absorption energy can be ascribed to the sensitization of ZnS. When the ZnO nanorods are covered with the ZnS shell, there is Type II kind of heterojunction formation. In a Type II heterostructure, the bandgap effectively reduced in comparison to the core and shell material. Decrease in bandgap will make the heterostructure to absorb the lower energy light to generate the charge carriers and also favors the fast charge transport across the heterojunction. This observation shows that ZnO–ZnS heterostructures can show better photocatalytic activity. BET adsorption–desorption isotherms of ZnO, ZnS and ZnS–ZnO heterostructures are shown in Fig. S3a. BET surface area of the ZnS–ZnO heterostructures are larger than those of bare ZnO, which indicates that the resulting heterostructures can provide a larger interface to enhance the contact between the dye and the active catalyst and facilitate photocatalytic reaction. The BJH pore size distribution curves of all the samples are shown in Fig. S3b. The mesoporosity in the resulting heterostructures was further confirmed from the pore size distribution peaks that are centered on a 2 nm and a broad peak around 6–8 nm. The measured BET surface areas and average pore radius for ZnO, ZnS and ZnS–ZnO heterostructures are presented in Table S1. After heterojunction formation, the surface areas tend to increase due to surface decoration of ZnO nanorods with ZnS nanoparticles, this further evidence the in-situ formation of nanostructured heterostructures [50]. Photocatalytic behavior of the synthesized ZnO, ZnS and ZnO–ZnS heterostructures is evaluated in comparison to bare ZnO and ZnS through photo-degradation studies of p-nitrophenol (PNP) and Methyl Orange (MO) dye under light exposure (> 390 nm). Before illuminating the light over catalyst containing PNP or MO solution, the solution is kept in dark to acquire the adsorption–desorption equilibrium. The UV-vis absorption spectra for the reduction of PNP in aqueous medium using NaBH4 with addition of the catalyst (1 mg/mL) are shown in Fig. 5. The pure PNP shows a λmax at 317 nm in aqueous medium and when the NaBH4 is added to it, a red shift occurs from 317 to 403 nm due to the formation of p-nitrophenolate ions. The yellow color of the PNP solution thus disappeared as the photocatalytic reaction proceeds. In the case of bare Z1 or Z2, there was insignificant photodegradation of PNP (Fig. 5a, b), whereas, in the case of Z3, Z4 and Z5 heterostructures, there is an increased photodegradation of PNP under light illumination (Fig. 5c, d). In case of Z3, Z4 and Z5, there is appearance of a new peak ~ 297 nm which corresponds to formation of p-aminophenol [51]. Figure 5f shows that in the absence of any photocatalyst, there is no degradation of PNP under light irradiation which indicates the photocatalytic nature of ZnO–ZnS heterostructures. The comparison and the kinetics of photodegradation of PNP in the presence of heterostructures are shown in Fig. 6 and Table 2. The reaction rates of Z1, Z2, Z3, Z4 and Z5 were 0.012, 0.002, 0.037, 0.030 and 0.024 min−1 respectively. The Z3 sample is showing the best results for PNP photodegradation with an efficiency of ~ 90%. Further, the photodegradation of MO dye has been done and mechanistic studies have been explored. Fig 7 shows a typical UV-vis spectrum of MO by Z1, Z2, Z3, Z4 and Z5 for 60 min. Methyl orange dye has an azo group which acts as chromophore which gives its characteristic absorption maximum at 464 nm. The illumination of light over MO in the presence of photocatalyst causes the breakage of this azo bond hence the intensity is decreased at 464 nm with time. Fig 8 depicts the comparison of photo-catalytic activities of ZnO–ZnS heterostructures with respect to bare ZnO and ZnS under light exposure. The efficiency of bare ZnO or ZnS is comparatively lower than those of heterostructures (Fig. 8a). Z3 is showing the maximum efficiency ~ 97% for MO degradation. The rate kinetics for the photodegradation of MO by all the samples follows the pseudo first order reaction (Fig. 8b and Table 3). The reaction rates were for pure Z1, Z2, Z3, Z4 and Z5 are 0.006, 0.001, 0.049, 0.039 and 0.014 min−1, respectively. Fig 8c shows the reusability of Z3 for photocatalytic degradation of MO up to three cycles. The decrease in the photocatalytic efficiency of Z3 after the 3rd cycle is from 97 to 89%. Table 4 shows the comparison of the prepared ZnO–ZnS photocatalysts with the literature. The photocatalytic measurements for degradation of MO dye using the ZnO–ZnS heterostructures were also performed under the standard conditions by using commercial AM 1.5G solar simulators (Fig. S5). The results obtained using commercial AM 1.5G solar simulators were corroborated with the results obtained from Xenon lamp experiments (Fig. S6 and Table S2). The mechanism of photocatalysis in ZnO–ZnS heterostructures can be explained on the basis of band structures of the samples (Fig. 9). The type II kind of photocatalytic heterostructure is formed and the formation of ZnS shell has resulted in a red shift in the absorption spectra of ZnO core. This results an effective smaller band gap when compared to core and shell materials. Under light exposure, the valence band (VB) electrons get excited to conduction band (CB) in both ZnO and ZnS. The excited electrons move from CB (ZnS) to CB (ZnO) and generated holes from VB (ZnO) to VB (ZnS). This transfer process increases the separation between the electrons and holes which slowdown the recombination of electron-hole pair in the heterostructure. The reactive oxygen species are generated due to the reaction of photo-generated electron and holes in the semiconductor with surface adsorbed water molecules. The electrons in CB (ZnO) will combine with adsorbed O2 and produce •O2− species. The holes accumulated in VB (ZnS) react with adsorbed H2O molecules and generate OH• species in the solution. These OH• and •O2− species are very oxidative and reductive respectively and consequently convert the MO molecules into smaller degraded molecules, H2O and CO2. The reactions in the process involved are given below [2] -

$$ ZnO - ZnS \, + \, hv \to ZnO - ZnS \, \left( {h^{ + } + \, e^{ - } } \right) $$
(2)
$$ H_{2} O + \, h^{ + } \to OH^{\bullet} $$
(3)
$$ O_{2}^{ - } + \, e^{ - } \to {^{\bullet}}O_{2}^{ - } $$
(4)
$${}^{\bullet}O_{2} - /h + /OH^{\bullet} + MO \to CO_{2} + H_{2} O $$
(5)
Fig. 1
figure 1

Powder X-ray diffraction patterns of a standard peaks for ZnO and ZnS, b Z3,c Z4 and d Z5

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Fig. 2
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TEM micrographs for the a Z1, b Z2, c Z3, d Z4 and e Z5

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Fig. 3
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STEM micrographs and their corresponding line mapping a, d Z3, b, e Z4 and c, f Z5

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Fig. 4
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a-c Fully scanned XPS spectra, high-resolution XPS spectrum of d-f Zn 2p, g-i O 1 s and j-l S 2p of Z3, Z4 and Z5 heterostructures respectively

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Fig. 5
figure 5

Photo-degradation study of p-nitrophenol under light illumination containing photocatalyst a Z1, b Z2, c Z3, d Z4, e Z5 and f no catalyst

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Fig. 6
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a Comparison and b kinetics of Photo-degradation of p–nitrophenol under light illumination containing different photocatalyst

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Table 2 Comparison of photodegradation efficiency of the synthesized photocatalysts for p-nitrophenol
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Fig. 7
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Photo-degradation study of methyl orange dye under light illumination containing photocatalyst a Z1, b Z2, c Z3, d Z4 and e Z5

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Fig. 8
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a Comparison, b kinetics of Photo-degradation of methyl orange under light illumination containing different photocatalyst and c recyclability test of Z3 for the degradation of MO up to 3 cycles

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Table 3 Comparison of photodegradation efficiency of the synthesized photocatalysts for methyl orange
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Table 4 Comparison of ZnO–ZnS core shell heterostructures with literature
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Fig. 9
figure 9

Schematics for the electron and hole transfer in the ZnO–ZnS heterostructures

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Therefore, the heterostructure shows higher photodegradation efficiency than that of pure ZnO and ZnS [35, 36]. The light illumination over the catalyst will generate electrons (e−) and holes (h+). Further, these photo-excited electrons and holes will combine with surface adsorbed H2O and O2 and generate superoxide radical anions (•O2−) and hydroxyl radicals (OH•). These in-situ generated four active species (e−, h+, OH• and •O2−) are responsible for the oxidation of organic dye pollutants. For identification of the principal active species during the degradation of MO with ZnO–ZnS as photocatalyst, different trapping agents were added to reaction solution i.e. ammonium oxalate for holes (h+), isopropanol for hydroxyl radicals (OH•) and benzoquinone for superoxide radical anions (•O2−) before the addition of photocatalyst (Fig. 10). In the absence of any added trapping agent, the photocatalyst degraded MO up to ~ 97.1% in 60 min. When the benzoquinone is added to the dye solution, the maximum quenching was observed (~ 28.7%) in photocatalysis. In case of addition of isopropanol (~ 84.3%) and ammonium oxalate (~ 95.2%), there is not much reduction in photocatalysis rate was observed. From these results, it appears that superoxide radical anions generated in solution are mainly responsible for the degradation of the dyes [52]. Photoluminescence (PL) technique is used for monitoring the recombination of electron hole pair within a semiconductor. It is known that when the rate of recombination of electron- hole is slow down, then, there is a reduction in PL peak intensity. The PL spectra of ZnO–ZnS heterostructures in comparison to bare ZnO excited at a wavelength of 350 nm are shown in Fig. 11. The emission peak for bare ZnO nanorods is obtained at ~ 390 nm. This UV emission arises from the recombination of the free electron-hole pairs via collision of exciton-exciton within the semiconductor [53]. The delay in the recombination of electron and hole due to their separation at the heterojunction of ZnO–ZnS heterostructures result the suppression in the emission signal [54]. PL also shows that Z3 has lowest intensity which indicates that it has slower recombination rate for electron and holes therefore, they can be available for photocatalytic reaction, and hence, it is showing highest catalytic activity. In order to compare the dark and light illuminated behavior of heterostructures with its bare counterparts, transient current response was measured. The typical photocurrent density (J) vs. time plot is shown in Fig. 12a. There was negligible photo-response observed in the bare ZnS whereas photocurrent was degrading with time in case of ZnO due to fast recombination rate of excitons. In the case of ZnO–ZnS heterostructures, there was a significant enhancement in the current on light illumination which again drops gradually after the removal of light source. The EIS studies were carried out to acquire the insights of the enhanced photocatalytic properties in case of Z3 heterostructure. The EIS studies were done in the absence and presence of light. The Nyquist plot of (Fig. 12b) showed a smaller arc radius in the light illumination than dark conditions. This confirmed that there is a low transfer barrier of electron and faster interfacial charge transfer occurs on heterostructure interface, hence, more efficient charge separation happens when it is illuminated under light compared to dark conditions. This observation is in closed agreement with earlier reports wherein SnO2/SnS2 heterostructure showed higher current density when light is switched on and it drops again when the light is off [55]. The micro-structural characterization of ZnO and its heterostructures have been also done by performing EPR spectroscopy (Fig. S7). This technique is a useful tool to monitor the presence of radical species and the behavior of the present native defects, such as oxygen and/or zinc vacancies. EPR spectra of the bare ZnO showed two signals at g = 1.9580 and g = 2.0036. The first peak is usually attributed to the shallow donor caused by the surface oxygen vacancies and interstitial Zn atoms. At the same time, the signal at g = 2.0024 is attributed to the presence of Zn vacancies. These results evidence the presence of a high concentration of defects in the ZnO nanorods. Also, a sharp decrease in the intensity of peak at g = 1.9580 has been observed for the ZnO–ZnS heterostructures with respect to the bare ZnO. This clearly indicates that the relative lower concentration of oxygen vacancies in heterostructures, confirming that Sulphur is occupying the oxygen vacancies [56]. The synthesis of ZnO–ZnS core shell heterostructures with optimized composition can be utilized as an effective photocatalyst for the removal of toxic pollutant from the industrial aqueous waste.

Fig. 10
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The scavenger studies for the detection of active species for the photodegradation of methyl orange in the presence of Z3

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Fig. 11
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Photoluminescence studies of ZnO–ZnS heterostructures in comparison to ZnO

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Fig. 12
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a Transient photoresponse of ZnO–ZnS heterostructures and b EIS studies for Z3 in presence and absence of light

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4 Conclusions

ZnO–ZnS heterostructures were successfully synthesized by solid state solvent free route. Sulphurization of ZnO nanorods has been done by utilizing the thiourea as sulphur source. The ratio of ZnO and ZnS can be controlled by varying the amount of sulphur source. Further, the as synthesized ZnO–ZnS heterostructure were utilized for removal of industrial waste (p-nitrophenol and methyl orange) from contaminated water. The Z3 has shown the maximum efficiency for PNP (~ 90%) and MO (~ 97.1%) under the light exposure. It is observed that Z3 consists of optimum amount of ZnO and ZnS in the ZnO–ZnS core–shell heterostructure. The heterostructure formation led to the effective separation of the electron and hole which in result decrease the recombination time, hence, these electrons and holes are available for photocatalytic activity. To confirm this, PL studies of ZnO–ZnS heterostructures has been done. We observed that Z3 shows maximum decrease in the PL intensity in comparison to bare ZnO. The details of photocatalytic activity have been explored by trapping agent studies. Thus, our studies show that ZnO–ZnS are the potential material with non-toxic nature which can work as an efficient photocatalyst for the waste water treatment to address the environmental pollution issues.