Preparation of ZnS@In2S3 Core@shell Composite for Enhanced Photocatalytic Degradation of Gaseous o-Dichlorobenzene under Visible Light

In this study, novel ZnS@In2S3 core@shell hollow nanospheres were fabricated by a facile refluxing method for the first time, and the formation mechanism of hollow structure with interior architecture was discussed based on ion-exchange Ostwald ripening. As the photocatalytic material for degradation of gaseous o-Dichlorobenzene (o-DCB), the as-synthesized core@shell hollow nanospheres were found to show significantly enhanced catalytic performance for effective separation of photo-generated charges. Moreover, the mechanisms of enhanced activity were elucidated by band alignment and unique configuration. Such photocatalyst would meet the demands for the control of persistent organic pollutant (POPs) in the atmospheric environment.

Based on the above analysis, ZnS@In 2 S 3 core@shell hollow nanospheres have been proposed in this work as high-efficient hollow composite nanomaterials for photocatalytic applications. Herein, the ZnS@In 2 S 3 core@shell hollow nanospheres have been prepared by an anion-exchange reaction for the first time and then applied to photocatalytic degradation of gaseous o-Dichlorobenzene (o-DCB) under visible-light irradiation. o-DCB, as one of the typical persistent organic pollutants (POPs), has raised wide attention for their carcinogenicity, high toxicity and bioaccumulation in the environment 27,28 . It is obvious that the coupling of ZnS with In 2 S 3 is a promising strategy to enhance the photocatalytic activity for pollutant elimination. Furthermore, the enhancement mechanism of photocatalytic performance for the ZnS@In 2 S 3 core@shell hollow spheres has been discussed in detail.

Results and Discussion
Morphology and Formation Mechanism Analysis. The morphology and internal structure of the ZnS sample were investigated by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). As depicted in Fig. 1a, the ZnS spheres show no evident aggregation with the particle size of about 100 nm, and possess the coarse surface. In Fig. 1b, it is obvious that the surface of ZnS spheres is piled with small nanoparticles. Figure 1c demonstrates that the interior framework exhibits the solid structure, indicating that the morphology has been well obtained by the refluxing method. Moreover, a closer examination shows the coarse edge of the ZnS sample (Fig. 1d), which is consistent with the above SEM images.
After coupled with In 2 S 3 nanoparticles, the white powder ( Fig. 1a) was turned into faint yellow (Fig. 2a). The SEM images of ZnS@In 2 S 3 composite materials are depicted in Fig. 2a and b. The particle size and morphology of ZnS@In 2 S 3 are almost similar to those of ZnS, indicating that the uniform spheres have been fabricated and distributed on a large scale. Further investigations of interior structure have been performed through TEM and HRTEM. As shown in Fig. 2c and d, the fabricated ZnS@In 2 S 3 nanospheres possess the core-shell hollow structure. The specific details are shown in Fig. 2e, and the diameter of the outer shell is about 20 nm. In addition, a typical high-resolution TEM (HRTEM) image from the outer edge displays distinct lattice fringes with the spacing of 0.268 and 0.308 nm corresponding with the (400) and (222) planes of In 2 S 3 , respectively, as shown in Fig. 2f. Based on the details of SEM and TEM images, one of most possible formation mechanisms of ZnS@In 2 S 3 core@shell hollow nanospheres is illustrated in Fig. 3. It is known that the pyrrolidon ring of the capping agent PVP possesses the strong interaction with Zn 2+ , which could prevent the agglomeration of ZnS nanoparticles through the repulsive forces among the polyvinyl groups 10 . After added TAA, the S 2− ions are released from TAA in the aqueous solution when heated to 100 °C, and then react with Zn 2+ to form ZnS tiny nanocrystals. To minimize the surface energy, these nanocrystals tend to form into solid spheres by the self-assembled method 29 . As a result, the ZnS solid spheres composed of small grains exhibited greater coarseness. As the surface charge of ZnS materials was negative in aqueous solutions, the In 3+ ions would attach on the surface of ZnS spheres through the electrostatic attraction. Subsequently, the In 2 S 3 nanoparticles will be formed on the surface of ZnS crystals via interfacial diffusion 10 , which would form the core@shell ZnS@In 2 S 3 structure. Obviously, it is believed that the difference of solubility products (K sp ) of ZnS (1.6 × 10 −24 ) and In 2 S 3 (6.3 × 10 −36 ) is the main driving force for the formation of ZnS@In 2 S 3 composite 29,30 . At last, after adding TAA again, the S 2− concentration in the solution  environment is much higher than that in the composites such that the S 2− ions of ZnS are hardly to diffuse outside, and so the yolk@shell structure is formed. In the post-heating process, the yolk@shell structure would contract to form the core@shell hollow structure due to the instability.
Physicochemical property characterization. The XRD pattern of the ZnS@In 2 S 3 core@shell spheres is shown in Fig. 4. For the ZnS sample, the characteristic peaks, appearing at 2θ: 28.6°, 47.6° and 56.5° would be assigned to the diffraction patterns of (111), (220) and (311) of ZnS cube phase given in JCPDS (65-0309) (space group: F-43m (216)), respectively, and no other peaks of impurities were detected. After coupled with In 2 S 3 to form the ZnS@In 2 S 3 structure, the composite exhibits extra diffraction peaks at 2θ: 51.6° and 56.0°, which correspond to the (112) and (201) plane reflections of the hexagonal phase In 2 S 3 given in JCPDS (33-0623) (space group: P-3m1(164)), respectively. This indicates that the core@shell composite structure was fabricated by coupling ZnS with In 2 S 3 . Moreover, in order to further confirm the surface components and chemical states of ZnS@In 2 S 3 structures, the composite was evaluated by XPS and the results are shown in Fig. 5. From the survey spectrum, it could be seen that the composite material contains Zn, In, S, O, and C elements, and the O element is from the adsorption of H 2 O on the surface of catalyst or residual substances after calcination 31 . In addition, the C element at the binding energy of 284.6 eV is from allogenic substances or referencing spectra 32 . In Fig. 5b, there are two characteristic peaks at 1045.3 eV and 1022.0 eV for Zn 2p 1/2 and Zn2p 3/2 , respectively, which are assigned to Zn 2+33 . Meanwhile, the bands at 425.5 eV and 445.1 eV are ascribed to the spectrum of In 3+34 . Due to its asymmetry of XPS spectrum of S 2p, two evident peaks located at 163.0 eV and 161.9 eV are deconvoluted in Fig. 5d, which are attributed to S 2− 17,34 .
The optical response properties of the catalytic materials obtained were investigated by UV-vis diffuse reflection spectra (DRS). From Fig. 6a, it is obvious that the pure ZnS shows no evident absorption peak in the region of visible light (400-550 nm), whereas the adsorption edge of the ZnS@In 2 S 3 composite significantly extend to the visible light region, suggesting that the composite exhibits the enhanced absorption ability under the visible-light irradiation. Furthermore, the band gap E g values are calculated by estimating the intercept of the tangent at the Tauc's plots 35 . As depicted in Fig. 6b, the band gap of the ZnS@In 2 S 3 material was calculated to be 2.61 eV, showing that the electrons could transfer under visible-light illumination. Regarding of the lower recombination rate of photo-generated charges with the addition of In 2 S 3 , the photocurrent and electrochemical impedance spectroscopy (EIS) of ZnS and ZnS@In 2 S 3 composite were tested under visible light in the electrochemical workstation. As shown in Figure S1a (see Supporting Information), the ZnS@In 2 S 3 has much higher transient photocurrent than the ZnS that did not respond to visible light, indicating that the ZnS@In 2 S 3 has higher efficiency of photo-induced charge separation. In addition, Figure S1b exhibits the EIS Nyquist plots of the ZnS and ZnS@In 2 S 3 electrodes under visible-light illumination in the Na 2 SO 4 electrolyte. Usually, a smaller radius of the arc on the EIS Nyquist plot represents the faster charge-transfer speed 36,37 . After coupling with In 2 S 3 , the impedance radius of the ZnS electrode reduced significantly, which implies that the ZnS@In 2 S 3 structure has good response to visible light, so as to effectively improve the separation of photon-generated charges and accelerate the photo-induced charge transfer.
As the pore size and the corresponding specific surface area of ZnS@In 2 S 3 core@shell spheres play important roles in enhancing the photocatalytic performance, the N 2 adsorption/desorption isotherms were investigated. As shown in Fig. 6c, the typical IV isotherms with a H1 hysteresis loop were obtained. Besides, the pore size distribution was in the regions of mesoporous and micropore structures (shown in Fig. 6d). Moreover, the specific surface area of the ZnS@In 2 S 3 core@shell structure was about 105.4 m 2 g −1 . Compared to the mixed sulfides (66.2 m 2 g −1 ), the core-shell structure showed much larger surface, to some extent, which could promote the degradation efficiency.
The performance and mechanism of degradation gaseous o-DCB. Fig. 7a illustrates the photocatalytic performance for gaseous o-DCB degradation over all the samples obtained under visible light (λ > 400 nm). It was found that the substrate could hardly be decomposed as persistent organic pollutant without catalysts. When adding the visible-light responsive In 2 S 3 photocatalyst, the degradation rate could reach 30% after 8 h reaction time for lower charge recombination rate and lower quantum efficiency. Compared with the simple mixed ZnS-In 2 S 3 , the ZnS@In 2 S 3 core@shell hollow catalyst shows much higher degradation ratio (49%), suggesting the importance for having the hierarchical core-shell structure in photocatalytic reactions. Compared to other photocatalysts, the ZnS@In 2 S 3 material shows the superiority for degradation of gaseous o-DCB under the identical and parallel conditions 17,23 (Table S1, see Supporting Information). This is attributed to the following reasons: (1) The core@shell hollow structure possesses high specific surface area with the capability of modulating the index of refraction of light, which could provide more active sites and enhance the absorption ability; (2) The unique structure would increase the migration efficiency of photon-generated carriers, and then improve the quantum efficiency. In addition, the reaction kinetics over these catalysts were further investigated by Langmuir-Hinshelwood model 38 . As depicted in Figs 5h and 7b was selected as the targeted reaction time for some intermediates adsorbed on the surface of catalysts. It could be seen that all the degradation processes follow the pseudo-first-order fitting, and the kinetic constant over ZnS@In 2 S 3 core@shell material (0.0704 h −1 ) is 1.3 and 1.4 times higher than that over the mixed ZnS-In 2 S 3 (0.0538 h −1 ) and the pure In 2 S 3 (0.0492 h −1 ), respectively. The results indicate that the degradation efficiency of gaseous o-DCB could be significantly enhanced with using the ZnS@In 2 S 3 core@shell hollow structure. To explore the catalytic mechanism, the degradation experiments were conducted by in situ Fourier transform infrared (FTIR) spectroscopy and electron paramagnetic resonance (EPR) technique, which would be useful to investigate the reaction mechanism on the surface of catalysts 39 . As shown in Fig. 8a, some new peaks at 1684 and 1508 cm −1 were attributed to C=O stretching vibration of unsaturated aliphatic acid 40 , while those at 1558 and 1540 cm −1 were assigned to COO vibration of unsaturated aliphatic acid (formate and acetate) 41 . At last, the bands at 2361 and 2339 cm −1 (Fig. 8b) correspond to the C=O vibration of CO 2 , indicating that the gaseous o-DCB could be mineralized to CO 2 .
The possible degradation mechanism for gaseous o-DCB over ZnS@In 2 S 3 core@shell catalyst was shown in Fig. 8c. When the composite material is irradiated by the visible light, the electrons of the outermost In 2 S 3 shell would transfer from the VB to the CB for the narrow band-gap semiconductor, meanwhile, the In 2 S 3 catalyst can create the same number of holes (h + ) in the VB 42 . To these photo-induced electrons, the reduction reaction will occur with O 2 adsorbed on the surface of In 2 S 3 , and then the important product, O 2 ·-active species, is forming as more positive potential (E = −0.046 V) (see Supporting Information, Figure S2), which would further degrade the gaseous o-DCB to some products. On the other hand, the holes generated with strong oxidation ability will transfer to the VB of ZnS material with more positive potential (E = 1.30 V) 43 , and then react with the gaseous o-DCB to form other products 9 . In a certain extent, the characteristics of effective separation for photo-generated charges (e − and h + ) are beneficial to the enhancement of the photocatalytic performance of o-DCB degradation.

Conclusions
In summary, the ZnS@In 2 S 3 core@shell hollow composite was fabricated by the ion-exchange reaction between ZnS and In 3+ ions in solution, and the formation mechanism is ascribed to the difference of solubility products (K sp ) of ZnS and In 2 S 3 . The ZnS@In 2 S 3 core@shell material exhibited the degradation rate of gaseous o-DCB of ~49% after 8 h reaction time under visible-light irradiation (λ > 400 nm), which is mainly attributed to the effective separation for photo-generated charges (e − and h + ). The ion-exchange method provides a new insight into the development of hollow photocatalysts with interior architecture.

Methods and Materials
Synthesis of Pure Zinc Sulfide (ZnS). The synthesis process is similar to the previous reports 10 . In the typical synthesis, 0.734 g of Zn(AC) 2 and 2 g of surfactant PVP were dissolved into 400 ml of ultrapure water and stirred for about 30 min to form a clear solution A. Meanwhile, 0.3 g of CH 3 CSNH 2 (TAA) was also added into  Materials Characterization. The X-ray diffraction (XRD) patterns were performed by an X-Ray diffractometer with Cu Kα radiation (Rigaku Corporation D/max-2400, Japan). The surface morphology of the samples was characterized by using a field emission scanning electron microscope (FESEM, Hitachi SU8010, Japan). The transmission electron microscopy (TEM) images were captured by using a FEI Tecnai G20 (USA). In addition, the chemical states and surface elements compositions were conducted by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI). The UV-Visible diffuse reflection spectra (DRS) were recorded by using an absorption spectrophotometer (JASCO, UV-550) from 200 to 800 nm. The specific surface area and pore distribution were investigated by nitrogen adsorption/desorption isotherms using an automated gas sorption analyzer (Autosorb-IQ, USA). Photoelectrochemical Measurements. The working electrode was prepared as follows: 0.01 g photocatalyst was mixed with ethanol under ultra-sonication for about 5 minutes, and the dissolved solution was then coated onto a piece of an indium-tin oxide glass (2 × 4 cm 2 ) dropwise. The as-prepared electrode was dried and treated at 200 °C for about 1 hour in the N 2 atmosphere. All electrodes had similar film thicknesses. Photocurrents and electrochemical impedance spectra (EIS) were measured by the electrochemical workstation (CHI760c) in a standard three electrode system using the prepared sample film as the working electrode, Ag/AgCl as the reference electrode, and the Pt flake as the counter electrode. Finally, a 500 W Xenon lamp with a UV-cut off filter (λ > 400 nm) was served as a visible-light source to irradiate the working electrodes, and 0.5 M Na 2 SO 4 solution was used as the electrolyte.
The Evaluation of Photocatalytic Degradation. To evaluate the photocatalytic performance of the as-prepared catalysts, the degradation efficiency of gaseous o-DCB was conducted in a home-built quartz reaction cell with the volume of about 130 mL 31 . First, the catalyst (0.02 g) pressed into the circular pieces was fixed to the holder and then the micro-reactor was sealed up immediately. Then, the gaseous pollutant was introduced into the reaction cell by injecting liquid o-DCB (5 µL) with a micro syringe. After an hour in the dark, the o-DCB was completely evaporated to gaseous pollution that could reach the adsorption equilibrium on the surface of catalysts. At that moment, the concentration of o-DCB was designated as the initial value, and the light (A 500 W Xenon lamp) equipped with a UV-cut off filter (λ > 400 nm) was turned on. Finally, in situ IR spectra were recorded by FTIR (Bruker VERTEX 70) in the region of 2400 and 1700 cm −1 , and the concentrations of gaseous o-DCB were calculated in the reaction process according to the previous report 17 .