Abstract
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.
1. Introduction
Oxide semiconductors depended on their excellent performance in photocatalytic realm and has drawn scientific interests for ongoing research. Among various oxide semiconductor photocatalysts, TiO2 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 [1–4]. However, TiO2 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 TiO2 [5–8]. 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 TiO2 crystalline matrix significantly influences its photocatalytic properties. It has been reported that doping TiO2 with metal may extend its light absorption further into the visible region [9–15]. When TiO2 was doped by metal ion, new impurity levels could be introduced between the conduction and valence band of TiO2, and thus narrowing the band gap of TiO2 [16]. Xu et al. [17] synthesized the Bi-doped TiO2 by an electrospinning method, and the Bi-doped TiO2 exhibited higher activities than sole TiO2 in the degradation of rhodamine B. Coupled TiO2 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/TiO2 via microemulsion-mediated solvothermal method, and the ZnS/TiO2 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-TiO2 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-TiO2.
2. Experiment
2.1. Preparation of ZnS-Bi-TiO2
2.1.1. Preparation of ZnS
All chemicals were used as received without further purification. The equal molar quantities of ZnCl2 and Na2S·9H2O were put into the agate mortar and fully grinded, then the yellow solid was obtained. After the solid was washed, dried, and groud, the ZnS nanoparticle was gained.
2.1.2. Preparation of ZnS-Bi-TiO2
40 mL absolute ethyl alcohol and 0.2 mL HNO3 were added into 0.05 mol Tetra-n-butyl titanate (TnBT), then a certain amount of ZnS was put into the mixture, and then solution A was obtained. Solution B consisted of 20 mL absolute ethyl alcohol, 7 mL deionized water, 7.5 mL glacial acetic acid, and a constant amount of Bi(NO3)3·5H2O (Bi/Ti = 0.006). After solution A was ultrasonic treated for 15 min and stirred for 30 min, solution B was slowly dripped into solution A. The resulting colloid was aged at room temperature for 48 h; dried at 100°C for 7 h, and then calcined in tubular furnace at 400°C with protection of nitrogen for 5 h. Finally, the samples were obtained and named as ZnS-Bi-TiO2-, where denotes the molar ratio of ZnS to Ti.
2.2. Characterization
The composition of the samples was identified by X-ray photoelectron spectroscopy (XPS, ESCALAB250, Thermo VG) with Al Kα radiation. The microstructure of the samples was characterized by transmission electron microscopy (TEM, JEM-2000EX, JEOL). The crystallinity of the samples was identified by X-ray diffraction analysis (XRD, XRD-6100, SHIMADZU) using graphite-monochromatized Cu Kα radiation at 40 kV, 30 mA. Light absorption property was evaluated by UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS, CARY 100&300, VARIAN). The PL spectra of photocatalysts were measured using a fluorescence spectrophotometer (PE-LS55, USA) equipped with a xenon lamp at an excitation wavelength of 325 nm.
2.3. Photocatalytic Experiment
The photocatalytic activity of ZnS-Bi-TiO2 was evaluated by degradation of MO. The reaction was carried out in the multifunctional photochemical reaction instrument equipped with a 350 W xenon lamp. In each experiment, MO solution ( mg/L, 200 mL) was added into the instrument, and 0.4 g of ZnS-Bi-TiO2 was suspended in the solution. At every 20 min, a certain volume of suspension was sampled and centrifuged to remove the particles. The concentration of MO was determined by UV-Vis spectrophotometer at 464 nm.
The stability of ZnS-Bi3+-TiO2 was tested by repeating the same experiment for four times. Once each run of photodegradation experiment was finished, the used ZnS-Bi-TiO2 was centrifuged and washed with ethanol and deionized water for certain times, and then dried before reuse.
3. Results and Discussion
As shown in Figure 1, all diffraction peaks can be attributed to the TiO2 with anatase crystal structure. No peaks corresponding to bismuth oxide and ZnS phases were detected in the XRD patterns of the ZnS-Bi-TiO2-, which might be due to low amount of Bi3+ and ZnS. The mean crystal size of ZnS-Bi-TiO2- samples shown in Table 1 was calculated by the Scherrer equation (the peak of samples was taken into account for the Scherrer calculation). The crystal size of the Bi-TiO2 was smaller than that of TiO2, which suggested that doping TiO2 with Bi3+ could suppress the crystal growth of TiO2 during the annealing process [1]. In order to investigate the nanocrystal morphology and structure of the catalyst, the TEM observation for TiO2 and ZnS-Bi-TiO2-0.005 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 TiO2, and the size of TiO2 and ZnS-Bi-TiO2-0.005 particles was about 10 nm (which was similar to the average particle size obtained by XRD analysis).
(a)
(b)
Investigation of the surface chemical compositions and their electronic states of the samples were carried out by XPS. The survey spectra of TiO2, Bi-TiO2 and ZnS-Bi-TiO2-0.005 samples are shown in Figure 3(a). As shown in Figure 3(a), a detectable Bi4f peak could be observed in the Bi-TiO2 and ZnS-Bi-TiO2-0.005 samples compared to that of pure TiO2, which confirmed the successful incorporation of Bi atoms into the composites. Furthermore, the detectable high-resolution peak of Zn2p was observed for ZnS-Bi-TiO2-0.005 sample in Figure 3(b). The Zn2p3/2 and 2p1/2 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-TiO2-0.005 composite. Although the above XPS results confirmed the existence of Zn element in the ZnS-Bi-TiO2-0.005 sample, but it is difficult to identify the S2p peak in ZnS-Bi-TiO2-0.005 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-TiO2 and ZnS-Bi-TiO2-0.005 shows a positive shift of approximately 0.2 eV as compared to those of pure TiO2 (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.
(a)
(b)
(c)
(d)
The high-resolution XPS spectrum of Bi in the 4f region for Bi-TiO2 and ZnS-Bi-TiO2-0.005 samples is displayed in Figure 3(d). There are four peaks centered at 164.23, 162.45, 159.01, and 157.06 eV in the 4f region of Bi. The peaks of 162.45 and 157.06 eV could be attributed to the Bi3+ in Bi2O3, which is in good agreement with other studies [20]. The 164.23 and 159.01 eV peaks at high binding energy could be assigned to the Bi4+ doped into the TiO2 lattice (the doped Bi is oxidized to Bi4+ due to strong interaction with TiO2) [21–23]. The presence of Bi4+/Bi3+ species in the catalyst is favorable to trap electrons, and improves the separation of the electron-hole pairs in the photocatalytic process.
Figure 4(a) shows the UV-Vis absorption spectra of the pure TiO2, Bi-TiO2, and ZnS-Bi-TiO2 with various ZnS content. Compared with the pure TiO2, the Bi-TiO2 samples showed remarkable absorption in the visible light region and red shift of absorption edge. This wide visible light response of Bi-doped TiO2 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 TiO2 (see Figure 4(b)), which may be ascribed to the introduction of new impurity levels between the conduction and valence band of TiO2 by the doping of Bi. After introducing ZnS, the absorption in visible light region is increased and the absorption edge of TiO2 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-TiO2 samples implies that the ZnS-Bi-TiO2 composites have better photocatalytic activities than those of pure TiO2 and Bi-TiO2 under visible light irradiation.
(a)
(b)
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 TiO2, Bi-TiO2, and ZnS-Bi-TiO2-0.005. As shown in Figure 5, the PL intensity decreased with successive introducing of Bi and ZnS. The PL spectra indicate that the recombination rate of photogenerated charge carriers is lower on the surface of the Bi-TiO2 and ZnS-Bi-TiO2-0.005 samples than pure TiO2. The lower PL spectra intensity for composites may be contributed to the doping of Bi and ZnS/TiO2 heterostructures in the composites. The Bi impurity energy level as a separation center can avoid volume recombination of photoinduced cariers. Moreover, the ZnS/TiO2 heterostructures improved the interfacial charge transfer rate and inhibited the recombination of photoinduced electron-hole pairs. The slower recombination of the photogenerated charges is advantageous for photocatalysis.
The photocatalytic activity of ZnS-Bi-TiO2 was evaluated by photocatalytic degradation of MO under simulated sunlight illumination. As shown in Figure 6(a), the Bi-TiO2 and ZnS-Bi-TiO2- samples exhibited higher photocatalytic activity compared with that of the TiO2. It was found that the photocatalytic elimination of MO with TiO2, Bi-TiO2, and ZnS-Bi-TiO2 followed the pseudo-first-order kinetics by formula (1). Consider
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(b)
As shown in Figure 6(b), the value ( is kinetic constant) of the Bi-TiO2 and ZnS-Bi-TiO2- samples was larger than that of TiO2. The enhanced photocatalytic performance of the Bi-TiO2 and ZnS-Bi-TiO2- samples could be attributed to that the Bi doping and heterostructure of ZnS/TiO2 could enhance light absorption and separation efficiency of photo-induced electron-hole pairs. It is noteworthy that the ZnS-Bi-TiO2-0.005 showed the best MO degradation rate, and value of ZnS-Bi-TiO2-0.005 (0.0141 min−1) was 4.9 times as great as that of TiO2 (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-TiO2-0.005, 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.
4. Conclusion
ZnS-Bi-TiO2 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 TiO2, the remarkable enhancement of photocatalytic capability was achieved, which was attributed to the doped Bi3+ and coupled ZnS that improved the ability to visible light absorption by TiO2. 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-TiO2, it is believed that ZnS-Bi-TiO2 photocatalyst may have potential application in the field of water pollution control.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The research was supported by Program for Science and Technology, Public-Benefit Foundation of Liaoning of China (2011001002) and Program for Liaoning Excellent Talents in University (LJQ2011050).