Enhancement of photocatalytic property on ZnS/MoS2 composite under visible light irradiation

In this paper, the composite ZnS/MoS2 was obtained via two steps including solvothermal methods. The as-synthesized sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-Vis. diffuse reflectance spectra (DRS). The photocatalytic activity of the product was evaluated through photocatalytic degradation of Rhodamine B (Rh B) under UV-Vis. light irradiation; the electrical conductivity of ZnS/MoS2 composites was significantly improved compared to ZnS, MoS2, respectively. The results showed that the ZnS/MoS2 composite photocatalyst possesses better photocatalytic activity in degrading Rh B than the single ZnS or the single MoS2. The better photocatalytic properties may be due to the synergetic effect of two semiconductors, because of which electrons and holes were separated effectively. And its specific microstructure played an active role in evaluating photocatalytic performance.


Introduction
Environmental degradation is one of the challenging phenomenons due to the increasing rate of the industrialization in the world wide. Especially, organic dyes have been wildly used in the plastic, paper, textile, and other chemical industries in our lives [1][2][3]. And it can cause environmental pollution.
However, photocatalysis is one of the promising methods to remove the organic components from waste water. Semiconductor photocatlysts have attracted a subject of extensive interest due to their fundamental importance applications for solving energy and pollution problems. Recently, molybdenum disulfide (MoS 2 ), shows great potential for visible light irradiated photocatalysis applications. More and more researchers have been extensively investigated MoS 2 as a promising candidate for many materials, due to its excellent electrical carrier mobility, high chemical reactivity and optical properties [4][5][6][7]. For instance, Xiuquan Gu [8] et al. obtained MoS 2 /Ag 3 PO 4 hybrids by incorporating MoS 2 nanosheets during the preparation process of Ag 3 PO 4 microparticles. The paper pointed out that the photocatalytic activity of MoS 2 /Ag 3 PO 4 hybrid got improved owning to compositing with trace amounts MoS 2 . The group of Aiyong Zhang [9] prepared layer-controlled MoS 2 on three-dimensional Bi 2 S 3 micro-flower through sacrificial templating strategy. The paper showed that 3D flower-like hybrid nanostructures make MoS 2 / Bi 2 S 3 to display great photocatalysis under visible light irradiation.
Similarly, among the plenty of metal sulfide photocatalysts, especially, ZnS, have been paid more attention from the scientific research owing to their optical properties and unique catalytic functions. However, ZnS is responsive only in ultraviolet light region. To address this issue, many means have been used, such as nanomaterials morphology modification and semiconductor compositing. These methods are proved to be effective, and the best way is meeting the energy demands of ZnS with wide band-gap to maintain the catalytic activity [10][11]. Longxin Hu [12] et al. reported that SnO 2 /ZnS nanocomposite was prepared by one step hydrothermal method. It was found that the compound showed the great photocatalytic activity. The photocatalytic degradation of RhB (Rhoadmine B) reached 95% at given time. In Baibiao Huang's group [13], they prepared ZnS microspheres with Mn-doping by a facile solvothermal route. And it was found that Mn-doping ZnS exhibited better photocatalytic performance than pure ZnS.
Owing to there have been few studies related to ZnS/MoS 2 composite. In this paper, we successfully prepared semiconductor of MoS 2 decroated with ZnS as catalyst for efficient degradation of dye. Therefore, we pay attention to the ZnS nanoparticles coated MoS 2 nanosheets system connecting two chemically stable and inexpensive multifunctional semiconductor materials together. In this system, MoS 2 nanosheets work as narrow band gap p-type semiconductor and ZnS serves as wide band-gap n-type semiconductor. The experimental reagents are simple and easy to get and the reaction conditions are simple and controllable. Experiment results indicate that ZnS composite MoS 2 exhibit superior photocatalyst performance towards RhB.
In a typical procedure, 1.5 mmol Na 2 MoO 4 and 15 mmol C 3 H 7 NO 2 S were dissolved in 80 ml deionized water. Then, the solution was transferred into a Teflon-lined stainless-steel autoclave, which was sealed and heated at 220 for 24 h, and then cool down to room temperature. The as-prepared precipitates were washed by ethyl alcohol and distilled water three times respectively. Then the black powder was collected and dried in vacuum at 80 for 12 h. The black precipate was denoted as MoS 2 . 0.1g thioacetamide(CH 3 CSNH 2 ), 0.6758g zinc acetate (Zn(CH 3 COO) 2 ) and 0.245g Sodium Sulfide (Na 2 S) were dissolved in 20 ml deionized water. The mixture was full stirred for 2 h at room temperature. Then, the solution was transferred into a Teflon-lined stainless-steel autoclave, which was sealed and heated at 170 for 24 h, and then cool down to room temperature. Then, the precipitates were collected and washed by ethyl alcohol and distilled water three times respectively. Then obtained powder was dried in vacuum at 60 for 12 h. The final powder was called ZnS.
0.1 as-prepared MoS 2 , and 0.2g Sodium dodecyl sulfate(SDS), 1.3516g zinc acetate, 0.49g Sodium Sulfide were dissolved in 60 ml deionized water. Then, the solution was transferred into a Teflon-lined stainless-steel autoclave, which was sealed and heated at 170 for 2 h, and then cool down to room temperature. Then, the precipitates were collected and washed by ethyl alcohol and distilled water three times respectively. Then obtained powder was dried in vacuum at 60 for 12 h which denoted as ZnS/MoS 2 .
The as-prepared products were characterized by powder X-ray diffraction (XRD) in Bruker D8 GLIIUDFWRPHWHU XVLQJ &X .Į UDGLDWLRQ 7KH VXUIDFH morphology of the samples were observed by a HITACHI S3500 scanning electron microscopy (SEM). The light absorption properties were measured using UV-Vis. diffuse reflectance spectrophotometer U-4100 (DRS) with a wavelength range of 300~800 nm.
The photocatalytic activity of the samples was determined by degradation of Rhoadmine B (RhB) in an aqueous solution under visible light irradiation. The photo degradation experiments were carried out under UV-Vis. light irradiation whose source was a 300W Xe lamp. The distance between the xenon lamp and the sample was about 50 cm. In the each experiment, 20 mg photocatalyst was added into 20 mL RhB solution (C 0 =10 mg/L). Prior to irradiation, the suspension was sonicated for a few minutes and then magnetically stirred in dark for hours to obtain desorption-adsorption equilibrium. At given time intervals, 5 mL of the sample solution was collected and separated through centrifugation. The HITACHI U-4100 UV-Vis. spectrometer was used to determine the concentration of RhB solution during the photocatalytic degradation process.
The electrochemical properties of all the samples were studied using electrochemical workstation (PGZ402, VoltaLab, France) with a three-electrodecon figuration. The samples were then used to modify screen-printed carbon electrodes (SPCEs) to immobilize antibodies and enhance the electrochemical signals.1 l of 1mg/ml ZnS MoS 2 and ZnS/MoS 2 were spread on the working area of SPECs using a micropipette and the film was dried at room temperature, resepectively. All the cyclic voltammetry (CV) measurements were recorded in 0.1M phosphate buffer (PBS, pH7.4) containing 5mM K 3 [Fe(CN) 6 ] and K 4 [Fe(CN) 6 ] in the potential range of -1000 to +1000mV. A reproducible voltammogram was obtained under steady-state conditions after about five cycles. All the electrochemical impedance spectroscopy (EIS) measurements were carried out in 0.01M PBS containing 5mM K 3 [Fe(CN) 6 ] and K 4 [Fe(CN) 6 ] in a frequency range from 0.1 to 104 Hz at 220mV.

Results and discussion
The XRD patterns of the samples are presented in Figure.   The morphologies of the pure MoS 2 , ZnS and ZnS/MoS 2 composite are observed by SEM. Figure.2a exhibits a representative SEM image of pure MoS 2 . The highly monodispersed rough ball-like spheres are obtained which is composed of thin plates as shown in Figure.  The absorption property is one of the most important properties to characterize the optical property of semiconductor. Figure.  The photocatalytic activities of as-prepared samples were evaluated by degrading Rhodamine B (Rh B) under UV-Vis. light irradiation. The photodegradation of the RhB as a function of the irradiation time over ZnS/MoS 2 composite is presented in Figure 4. We can see there is no significant change in the concentration curve with time when the photocatalysis is absent, which means Rh B is very stable without photocatalysis only under UV-Vis. light. From the curves of the concentration changes of RhB dye, it can be seen that the photocatalytic degradation rate of all as-prepared samples are increased with the increasing of the reaction time. In 80 minutes, nearly 70% RhB is degraded by ZnS/MoS 2 . whereas in 80 minutes, the MoS 2 and ZnS degraded the RhB concentration by 64%, and 82%, respectively. It could be seen that the ZnS/MoS 2 composite exhibits the best photocatalytic activity among the samples.  In order to further confirm the enhanced conductivity of ZnS/MoS 2 CV curves of ZnS/MoS 2 , ZnS, MoS 2 were measured in 0.1M phosphate buffer (pH 7.2) containing 5mM K 3 [Fe(CN) 6 ] and K 4 [Fe(CN) 6 ] as electrolyte. In Figure.5 a pair of symmertric and well-separated redox peaks was observed, indicating that the measured electroconductivity was based on the redox mechanism [14][15][16][17]. It was also noted that ZnS/MoS 2 had another cathodic peak at -700 mV may due to the redox of MoS 2 . ZnS/MoS 2 exhibited higher current density than single ZnS and MoS 2 due to the accelerated electron transfer by MoS 2 . This observation indicates an irreversible electrochemical reduction of MoS 2 when CV is conducted in the inorganic electrolyte at the potential range from 1000 to -1000 mV. It means that the MoS 2 was successfully reduced. It is obvious that ZnS/MoS 2 composite nanostructures possess more powerful photocatalytic capacity for the degradation of Rh B. As a consequence, a possible mechanism is proposed. According to some papers [18][19][20][21], we deduce that MoS 2 and ZnS have matching band potential.
Under irradiation, the photocatalyst could generate electrons and holes. The conduction band and valence band edge positions of MoS 2 are associated with those of ZnS, so both electrons and holes are transferred to MoS 2 . Holes transfer to MoS 2 nanoflakes, which increases charge separation. Thus, MoS 2 acts as an oxidation site. In another words, the band potentials of both MoS 2 and ZnS semiconductors are conductive to prevent the recombination of electrons and holes in the ZnS/MoS 2 composite. In general, the ZnS/MoS 2 composite with special nanostructure and interface increases the visible light absorption and restrains the recombination of electron-hole pairs.

Conclusion
In summary, ZnS/MoS 2 composite photocatalyst was got by two steps which in order were solvothermal methods. The microsphere of the composite was composed of hundreds of MoS 2 thin nanoplates and some ZnS plates were together with the sphere about a diameter of 1ȝP from the view of morphology. From the photodegradation of the Rh B organic dye we found that the ZnS/MoS 2 composited photocatalyst displayed better photocatalytic activity than both single MoS 2 and single ZnS. As a proof of concept, the ZnS/MoS 2 with good conductivity, superior electron transfer rate and high electrochemical activity is successfully used to detect photocatalytic property.