Fabrication of ZnS-Bi-TiO2 Composites and Investigation of Their Sunlight Photocatalytic Performance

The ZnS-Bi-TiO2 composites were prepared by the sol-gel method and were characterized by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-visible diffuse reflectance spectroscopy (UV-Vis DRS). It is found that the doped Bi as Bi4+/Bi3+ species existed in composites, and the introducing of ZnS enhanced further the light absorption ability of TiO2 in visible region and reduced the recombination of photogenerated electrons and holes. As compared to pure TiO2, the ZnS-Bi-TiO2 exhibited enhanced photodegradation efficiency under xenon lamp irradiation, and the kinetic constant of methyl orange removal with ZnS-Bi-Ti-0.005 (0.0141 min−1) was 3.9 times greater than that of pure TiO2 (0.0029 min−1), which could be attributed to the existence of Bi4+/Bi3+ species, the ZnS/TiO2 heterostructure.


Introduction
Oxide semiconductors depended on their excellent performance in photocatalytic realm and has drawn scientific interests for ongoing research. Among various oxide semiconductor photocatalysts, TiO 2 has been widely considered as the most promising photocatalyst owing to its high photocatalytic activity, good chemical and biological stability, nontoxicity, low cost, and so forth [1][2][3][4]. However, TiO 2 could only be excited under UV irradiation due to its large energy band gap of 3.2 eV, which implied that only about 5% of solar energy can be utilized by TiO 2 [5][6][7][8]. In recent years, great interests have been focused on the development of photocatalysts with visible light response.
The presence of metal ion dopants in the TiO 2 crystalline matrix significantly influences its photocatalytic properties. It has been reported that doping TiO 2 with metal may extend its light absorption further into the visible region [9][10][11][12][13][14][15]. When TiO 2 was doped by metal ion, new impurity levels could be introduced between the conduction and valence band of TiO 2 , and thus narrowing the band gap of TiO 2 [16].
Xu et al. [17] synthesized the Bi-doped TiO 2 by an electrospinning method, and the Bi-doped TiO 2 exhibited higher activities than sole TiO 2 in the degradation of rhodamine B. Coupled TiO 2 with other semiconductors also has been confirmed as an effective approach to enhance the visible light response. When both semiconductors are illuminated, electrons accumulate at the low-lying conduction band of one semiconductor while holes accumulate at the valence band of the other compound. These processes of charge separation are very fast and the efficiency of reduction or oxidation of the adsorbed organics remarkably increases. Yu et al. [18] successfully synthesized the ZnS/TiO 2 via microemulsionmediated solvothermal method, and the ZnS/TiO 2 exhibited enhanced visible light photocatalytic activity for the aqueous parathion-methyl degradation.
In this study, we integrated the above two methods and successfully synthesized the ZnS-Bi-TiO 2 photocatalyst by sol-gel method. Methyl orange (MO), which is a common pollutant in the industry effluents, was chosen to test the photocatalytic ability of ZnS-Bi-TiO 2 .

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The Scientific World Journal    The stability of ZnS-Bi 3+ -TiO 2 was tested by repeating the same experiment for four times. Once each run of photodegradation experiment was finished, the used ZnS-Bi-TiO 2 was centrifuged and washed with ethanol and deionized water for certain times, and then dried before reuse.

Results and Discussion
As shown in Figure 1, all diffraction peaks can be attributed to the TiO 2 with anatase crystal structure. No peaks corresponding to bismuth oxide and ZnS phases were detected in the XRD patterns of the ZnS-Bi-TiO 2 -, which might be due to low amount of Bi 3+ and ZnS. The mean crystal size of ZnS-Bi-TiO 2 -samples shown in Table 1 was calculated by the Scherrer equation (the {101} peak of samples was taken into account for the Scherrer calculation). The crystal size of the Bi-TiO 2 was smaller than that of TiO 2 , which suggested that doping TiO 2 with Bi 3+ could suppress the crystal growth of TiO 2 during the annealing process [1]. In order to investigate the nanocrystal morphology and structure of the catalyst, the TEM observation for TiO 2 and ZnS-Bi-TiO 2 -0.005 samples was carried out. It can be seen from Figures 2(a) and 2(b) that the comodification of Bi and ZnS did not change the morphology of TiO 2 , and the size of TiO 2 and ZnS-Bi-TiO 2 -0.005 particles was about 10 nm (which was similar to the average particle size obtained by XRD analysis).
Investigation of the surface chemical compositions and their electronic states of the samples were carried out by XPS. The survey spectra of TiO 2 , Bi-TiO 2 and ZnS-Bi-TiO 2 -0.005 samples are shown in Figure 3(a). As shown in Figure 3 atoms into the composites. Furthermore, the detectable highresolution peak of Zn2p was observed for ZnS-Bi-TiO 2 -0.005 sample in Figure 3(b). The Zn2p 3/2 and 2p 1/2 core levels centered at 1021.89 and 1044.9 eV, which was completely matched with the binding energy of Zn2p in ZnS reported by Brion [19]. It suggested that the ZnS was present mainly as separate phases in ZnS-Bi-TiO 2 -0.005 composite. Although the above XPS results confirmed the existence of Zn element in the ZnS-Bi-TiO 2 -0.005 sample, but it is difficult to identify the S2p peak in ZnS-Bi-TiO 2 -0.005 sample because the binding energies of S2p and Bi4f are very close and due to the low content and sensitivity factor of S element. Nevertheless, the peak of Ti2p centered at 458.7 eV for Bi-TiO 2 and ZnS-Bi-TiO 2 -0.005 shows a positive shift of approximately 0.2 eV as compared to those of pure TiO 2 (see Figure 3(c)). This might be due to the fact that Ti atom was probably substituted by Bi atom and the chemical environmental surrounding of Ti may be Ti-O-Bi. The high-resolution XPS spectrum of Bi in the 4f region for Bi-TiO 2 and ZnS-Bi-TiO 2 -0.005 samples is displayed in Figure 3 [20]. The 164.23 and 159.01 eV peaks at high binding energy could be assigned to the Bi 4+ doped into the TiO 2 lattice (the doped Bi is oxidized to Bi 4+ due to strong interaction with TiO 2 ) [21][22][23]. The presence of Bi 4+ /Bi 3+ species in the catalyst is favorable to trap electrons, and improves the separation of the electronhole pairs in the photocatalytic process. Figure 4(a) shows the UV-Vis absorption spectra of the pure TiO 2 , Bi-TiO 2 , and ZnS-Bi-TiO 2 with various ZnS content. Compared with the pure TiO 2 , the Bi-TiO 2 samples showed remarkable absorption in the visible light region and red shift of absorption edge. This wide visible light response of Bi-doped TiO 2 could be attributed to the formation of surface defect centers, which are associated with existence of oxygen vacancies created by the doping process [24,25]. The red shift of absorption edge corresponded to the band gap narrowing of TiO 2 (see Figure 4(b)), which may be ascribed to the introduction of new impurity levels between the conduction and valence band of TiO 2 by the doping of Bi. After introducing ZnS, the absorption in visible light region is increased and the absorption edge of TiO 2 was further shifted to the visible light region with the increase of ZnS content. This may be attributed to the sulphur from ZnS is able to dope into the surfaces TiO2 particles [26] and decreases the value of band-gap energy. The enhancement of optical absorption intensity in the visible region for ZnS-Bi-TiO 2 samples implies that the ZnS-Bi-TiO 2 composites have better photocatalytic activities than those of pure TiO 2 and Bi-TiO 2 under visible light irradiation.
PL spectra can provide information about the features of excited states and related defects based on the electronic structure and optical characteristics [27]. Figure 5 shows room temperature photoluminescence spectra for pure TiO 2 , Bi-TiO 2 , and ZnS-Bi-TiO 2 -0.005. As shown in Figure 5 recombination of the photogenerated charges is advantageous for photocatalysis. The photocatalytic activity of ZnS-Bi-TiO 2 was evaluated by photocatalytic degradation of MO under simulated sunlight illumination. As shown in Figure 6(a), the Bi-TiO 2 and ZnS-Bi-TiO 2 -samples exhibited higher photocatalytic activity compared with that of the TiO 2 . It was found that the photocatalytic elimination of MO with TiO 2 , Bi-TiO 2 , and ZnS-Bi-TiO 2 followed the pseudo-first-order kinetics by formula (1). Consider As shown in Figure 6(b), the value ( is kinetic constant) of the Bi-TiO 2 and ZnS-Bi-TiO 2 -samples was larger than that of TiO 2 . The enhanced photocatalytic performance of the Bi-TiO 2 and ZnS-Bi-TiO 2 -samples could be attributed to that the Bi doping and heterostructure of ZnS/TiO 2 could enhance light absorption and separation efficiency of photoinduced electron-hole pairs. It is noteworthy that the ZnS-Bi-TiO 2 -0.005 showed the best MO degradation rate, and value of ZnS-Bi-TiO 2 -0.005 (0.0141 min −1 ) was 4.9 times as great as that of TiO 2 (0.0029 min −1 ). However, too much loading of ZnS shows adverse effects because some ZnS sites may act as charge recombination centers. Stability is one of the most important performances of photocatalysts, which could make photocatalysts to be reused. In order to evaluate the stability of photocatalysts, ZnS-Bi-TiO 2 -0.005, as a representative sample, was chosen for four recycles of photodegradation of MO. As presented in Figure 7, the photocatalytic activity exhibited no significant decrease after four recycles. Clearly, the photocatalytic activity of the ZnS-Bi-Ti-0.005 was quite stable.

Conclusion
ZnS-Bi-TiO 2 photocatalyst with visible light response was fabricated by a facile sol-gel method and its photocatalytic performance was tested by MO degradation under xenon lamp irradiation. Compared to TiO 2 , the remarkable enhancement of photocatalytic capability was achieved, which was attributed to the doped Bi 3+ and coupled ZnS that improved the ability to visible light absorption by TiO 2 . Furthermore, no significant decrease of activity was observed after four cycles for photodegradation of MO. Considering the photocatalytic ability under sunlight and the stability of ZnS-Bi-TiO 2 , it is believed that ZnS-Bi-TiO 2 photocatalyst may have potential application in the field of water pollution control.