Surface photosensitization of ZnO by ZnS to enhance the photodegradation efficiency for organic pollutants

It is challenging to develop a material which has low cost, high activity, good stability and recyclability under light exposure. Apart from these properties, the photocatalyst should also have good visible region absorbance and low electron-hole pair recombination rate. Keeping all this in view, we have designed a simple scalable synthesis of ZnO–ZnS heterostructures for the photocatalytic treatment of industrial waste (p-nitrophenol and methyl orange). The ZnO–ZnS heterostructures are synthesized via a solvent-free route by thermal annealing of solid-state mixture of ZnO and thiourea (a sulphur source) which results in ZnO–ZnS core shell kind of heterostructure formation. The interface formation between the ZnO–ZnS heterostructure favored the band-gap reduction in comparison to the bare ZnO and ZnS nanoparticles. Further, these ZnO–ZnS heterostructures were utilized as a photocatalyst for the degradation of toxic phenolic molecules (p-nitrophenol) and harmful organic dyes (methyl orange) present in the water under the light exposure (> 390 nm).


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, TiO 2 , ZnO, SnO 2 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] then formed CdS or Ag 2 S or In 2 S 3 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 Ag 2 S-ZnO composite via a two-step process. They first precipitated the zinc oxalate and Ag 2 S and then calcined the mixture at 400 °C for 12 h to obtain Ag 2 S-ZnO composite. The photodegradation of Acid Black 1 was performed by using Ag 2 S-ZnO composite under solar light. The composite was more capable for mineralizing the Acid Black 1 in comparison to synthesized ZnO, bulk ZnO, TiO 2 -P25 and TiO 2 (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 H 2 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).

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.

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 (E g ) were evaluated using following Tauc equationwhere 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.

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 P6 3 mc (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  [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 Zn2p 3/2 and Zn2p 1/2 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 S2p 3/2 and S2p 1/2 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 NaBH 4 with addition of the catalyst (1 mg/mL) are shown in Fig. 5. The pure PNP shows a λ max at 317 nm in aqueous medium and when the NaBH 4 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  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 (2021) 3:689 | https://doi.org/10.1007/s42452-021-04643-z Research Article 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 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 photoexcited electrons and holes will combine with surface adsorbed H 2 O and O 2 and generate superoxide radical anions ( • O 2 − ) and hydroxyl radicals (OH • ). These in-situ generated four active species (e − , h + , OH • and • O 2 − ) 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, ) 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 electronhole 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   [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.

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.
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