Abstract
The photocatalytic degradation of Rhodamine B dye was successfully carried out under UV irradiation over porous TiO2-SnO2 nanocomposites with various molar ratios of Ti/Sn (4–12) synthesized by hydrothermal method using polystyrene microspheres as template. The combination of TiO2 with SnO2 can obtain high quantum yield of TiO2, and then achieve the high photocatalytic activity. And its porous structure can provide large surface area, leading to more adsorption and fast transfer of dye pollutant. Structural and textural features of the samples were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and N2 sorption techniques. Both adsorption and UV irradiation contribute to decolorization of about 100% of Rhodamine B dye over the sample TiSn10 after 30 min of the photocatalytic reaction, while the decomposition of Rhodamine B dye is only 62% over pure titania (Degussa P25).
1. Introduction
The disposal of various toxic dyes from the textile industry has attracted extensive attention in the field of water pollution prevention and cure. Rhodamine B (RB) is one of the famous dyes and is widely used as a colorant in foodstuffs and textiles due to its high stability. It is harmful to human beings and animals and causes irritation of the skin, eyes, and respiratory tract. The removal methods of toxic dyes have been considered in recent studies in the literature. These include physical adsorption [1, 2], chemical degradation [3], biological degradation [4], photodegradation, or the synergic treatments of different methods. Recently, much attention has been paid to the photocatalytic degradation of dye pollutants using nanodispersed catalysts TiO2 [5], SnO2 [6], and so forth. It uses light energy to initiate chemical reactions in the presence of photocatalysts which are mostly semiconductor materials. Heterogeneous photocatalysis using semiconductors is an effective and rapid technique for the removal of dye pollutants from wastewater [7].
TiO2 is regarded as a promising semiconductor in degradation of various dye pollutants for its strong oxidizing power, nontoxicity, low cost, chemical stability, and high photocatalytic activity [8]. The degradation process involves the exposure of TiO2 to UV light which is associated with formation of positive hole and negative electrons in the valence and conduction bands that oxidize and reduce the dye pollutants. However, there are some issues that limit the photocatalytic activity of TiO2 materials, such as the low quantum yield, the wide band gap energy (3–3.2 eV), and the low mass transport rates. Therefore, the researchers conduct many works to solve these essential drawbacks. An effective strategy is to couple TiO2 with other semiconductors, transition elements, and noble metals which can improve the photodegradation activity owing to the role of dopants in improving the optical features of the samples by extending the absorption activity to visible region and preventing the recombination of the charge carriers [9–12]. And another useful means is to increase its surface area via synthesis of porous TiO2 monoliths, leading to more adsorption and fast transfer of dye pollutants. More attention has been paid to TO2 system in recent years by coupling TiO2 with SnO2 [13–17]. It is generally accepted that this new nanocomposite exhibits high photocatalytic reactivity compared with pure TiO2. It is well known that the band gaps of SnO2 and TiO2 are 3.6 and 3.2 eV, respectively. The combination of these two semiconductors leads to the accumulation of electron in the conduction band of SnO2 and the photogenerated holes in the valence band of the TiO2 particle, which improve the separation of photogenerated charges, shifting the photoexcitation of the sample toward visible light, and increase the oxidizing power of TiO2 [14–16].
In this work, we reported a novel hierarchical macroporous-mesoporous TiO2-SnO2 nanocomposite prepared by hydrothermal method using polystyrene microspheres as template. The textural and structural properties of the as-prepared photocatalysts were characterized by means of XRD, TEM, and N2 sorption. And the photocatalytic activities of the as-prepared catalysts were evaluated by the photocatalytic degradation of RB dye pollutant under UV light. The effect of the molar ratios of Ti/Sn on the crystal structure, morphology, and optical properties of the final products was investigated.
2. Experimental Section
2.1. Materials
Titanium n-butoxide (TBOT, Ti(OC4H9)4), tin(IV) chloride, tetrahydrofuran (THF), and acetone were used without any further purification. All chemicals used in this study were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. or Tianjin Dengke Chemical Reagent Co., Ltd. Polystyrene (PS) microspheres were obtained by emulsion polymerization of styrene which had been discussed in our previous reports [18, 19]. In a typical procedure, 150 g deionized water was poured into a 300 mL jacket reactor, which was kept at 85°C until the end of the reaction. Then, 0.075 g sodium styrene sulfonate and 0.0633 g sodium hydrogen carbonate were dissolved in the deionized water. Under constant stirring, 17.50 mL styrene monomer was added to this solution under the nitrogen protection. After 1 h, 0.0833 g potassium persulfate was introduced into the solution. After 18 h polymerization, the monodispersed PS spheres with the diameter of 247 nm were obtained.
2.2. Preparation of TiO2-SnO2 Composites
The TiO2-SnO2 photocatalysts were prepared by hydrothermal method using polystyrene microspheres as template. In a typical procedure, 1.052 g SnCl4·5 and the calculated volume of TBOT with various molar ratios of Ti/Sn (4–12) were dissolved in 60 mL of anhydrous ethanol under stirring; then the calculated amount of PS microspheres (100 g PS microspheres per mole of solutes) was added to the prepared solution under magnetic stirring. After ultrasonic dispersion for 20 min, the distilled water was slowly dripped into the resulting suspension under vigorous stirring until the gel occurred; then distilled water for diluting the suspension continued to be added (ensure that the total volume of distilled water is 200 mL) and stirred for 30 min. Subsequently, the pH value of the suspension was regulated by adding aqueous ammonia and simultaneous stirring until pH = 8 and kept for 30 min. The solution and the resulting precipitates were placed inside a Teflon-lined stainless autoclave, and the autoclave was heated at 150°C for 5 h. Then the mixture was cooled down to room temperature and the precipitate was separated by filtration, washed several times with distilled water and anhydrous ethanol, and dried 10 h in air at 60°C. Finally, the obtained sample was placed in a glass soxhlet extractor and PS microspheres were extracted by the mixed solution of THF/acetone (1/1 of v/v) for four days, dried 6 h in air at 80°C, and ground to 60–80 mesh. The series of as-prepared samples were named as TiSn4, TiSn6, TiSn8, TiSn10, and TiSn12 corresponding to the samples with various molar ratios of Ti/Sn = 4, 6, 8, 10, and 12, respectively.
2.3. Characterization
X-ray diffraction (XRD) analysis was performed on a Bruker-AXS D8 Advance diffractometer, with Cu radiation at 40 kV and 25 mA in a scanning range of 10–80° (). The diffraction peaks of the crystalline phase were compared with those of standard compounds reported in the JCPDS Data File. N2 adsorption-desorption isotherms were collected at liquid nitrogen temperature using a Micromeritics ASAP 2020 adsorption apparatus. Before carrying out the measurement, each sample was degassed at 60°C for more than 6 h. The specific surface areas () of the samples were calculated following the multipoint BET (Brunauer-Emmett-Teller) procedure. The pore size distributions were determined from the adsorption branch of the isotherms using the BJH method. Transmission electron microscopy (TEM) analysis was performed on a JEOL JEM-2100 microscope, operating at 200 kV. The samples were dispersed in ethanol and treated with ultrasound for 5 min and then deposited on a copper grid coated with preformed holey carbon film.
2.4. Photocatalytic Activity Measurements
Rhodamine B aqueous solution under UV light was used as a model reaction to evaluate the photocatalytic activity of prepared samples. The light source was a 450 W high-pressure mercury lamp (center wavelength = 365 nm, Foshan Electrical And Lighting Co., Ltd., China) and the lamp was located 15 cm higher than the solution surrounded by a circulating water tube. A general procedure was carried out as follows. First, 100 mL aqueous Rhodamine B solution (20 mg/L) was placed in a 500 mL water-jacketed reactor, which was maintained at a temperature of 20°C. Then, 0.1 g catalyst was suspended in the solution. The suspension was stirred in dark for 30 min in order to reach the adsorption-desorption equilibrium. Finally, the suspension was irradiated under UV light. To monitor the photocatalytic process, 6 mL mixture solution was aspirated from the test tube with an interval of 10 min. The determination of Rhodamine B (RB) dye was done on a TU-1810 UV-vis spectrophotometer by measuring absorbance at of 553 nm. A calibration curve obtained at the RB dye concentration of 1–10 mg/L was used for determination of initial and final concentrations. The photocatalytic degradation rate was calculated by the following expression: where (mg/L) is the initial concentration of Rhodamine B solution which reached absorbency balance and (mg/L) is the concentration of the dye solution at the irradiation time ().
3. Results and Discussion
3.1. Characterization of Nanoparticles
3.1.1. X-Ray Diffraction
Figure 1 shows the typical XRD patterns of the as-prepared samples TiSn4, TiSn6, TiSn8, TiSn10, and TiSn12. When the molar ratios of Ti/Sn > 4, the diffraction peaks at , 37.8, 48, 53.9, and 62.6° reveal the existence of predominant anatase phase (JCPDS number 21-1272). And, the relatively weak peaks detected at , 35.5, and 53.5° are referred to as rutile phase (JCPDS number 21-1276). Simultaneously, along with the molar ratios of Ti/Sn decrease (which means the increase of SnO2 content), the peaks intensity of anatase phase decreased and that of rutile phase was enhanced. When the molar ratios of Ti/Sn = 4, the main phase of TiO2 converts to rutile phase. The diffraction peaks showed at , 34.8, and 52.8° are referred to as cassiterite phase (JCPDS number 41-1445), and they are enhanced with the decrease of the molar ratios of Ti/Sn. On examining the figure, one can notice that the SnO2 content impacts the crystalline phase of TiO2 significantly. The excessive SnO2 can make the main phase of TiO2 convert from anatase to rutile phase. In addition, the sharp peaks in Figure 1 indicate the well crystallization of all the samples via the hydrothermal action.
3.1.2. TEM Analysis of TiO2-SnO2 Nanoparticles
Figure 2 shows the low resolution TEM (LRTEM) images of the prepared TiSn10 nanoparticles from different areas. The images clearly demonstrate that the TiSn10 sample has a disordered spherical macroporous structure, formed by the agglomeration of the uniform nanoparticles on the surface of polystyrene microspheres and the extraction of polystyrene microspheres by the mixed solution of THF/acetone. The average pore size of spherical macropores is about 580 nm, and it is obviously larger than the average particle size 247 nm of polystyrene microspheres. This may be owing to the fact that the temperature of hydrothermal reaction (150°C) is higher than the glass transition temperature of polystyrene microspheres (120°C), which leads to the flow and fusion of liquefied polystyrene microspheres. Furthermore, it also can be seen that the spherical macroporous structure has been slightly damaged due to the hydrothermal action and grind during the preparation process.
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The TEM images of the walls of spherical macropores recorded at different magnification are displayed in Figure 3. It clearly demonstrates that the walls of spherical macropore have a disordered mesoporous structure, which is formed by the agglomeration of the uniform nanoparticles. The accessible mesopores are connected randomly, lacking discernible long-range order in the pore arrangement among the small particles. And the nanoparticles in the sample are of regular morphology with the size around 10 nm (Figure 3(b)). In conclusion, the as-prepared TiSn10 sample has a hierarchical macroporous-mesoporous structure formed by nanoparticles.
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3.1.3. -Sorption Analysis
Figure 4 depicts N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of the prepared samples TiSn4, TiSn6, TiSn8, TiSn10, and TiSn12. The textural properties of the samples are listed in Table 1. The surface area of TiSn4 and TiSn10 is 71 m2/g and 73 m2/g, respectively, while the total pore volume of them is 0.28 cm3/g and 0.23 cm3/g, respectively. The highest surface area and pore volume (162 m2/g and 0.51 cm3/g) are obtained by the TiSn8 sample. From Figure 4(a), it can be seen that the isotherms of all the prepared samples are of classical type IV, characteristic of mesoporous materials according to the IUPAC, and the adsorption isotherms of the samples TiSn4, TiSn6, TiSn8, and TiSn10 exhibit a large increase at the above 0.8, indicating the presence of the macroporous structure (consistent with the TEM analysis results in Figures 2 and 3). In comparison to the well-known nonporous TiO2 material (P25), the hierarchical macroporous-mesoporous structure of the prepared samples is believed to facilitate the absorption of RB dye (Table 2) and the transport of photodegradation product molecules. The pore size distribution curves of the prepared samples, which are determined by the BJH method from the adsorption branch of the isotherm, exhibit one single narrow peak centered at 7.7–13.9 nm (Figure 4(b)), indicating the good homogeneity of the pores. Besides, one can observe a progressive increase in average pore size upon decreasing the molar ratios of Ti/Sn from 12 to 4 (namely, increasing the SnO2 content from 7.7 to 20.0%).
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3.2. Photocatalytic Activity Studies
Figure 5 displays the photocatalytic degradation rate and the pseudo-first-order kinetics of the prepared samples and pure TiO2 (P25, the surface area of 50 m2/g). The absorption (in dark) and photodegradation rates (under UV light) of RB dye by different photocatalysts within 30 min are presented in Table 2. From Figure 5(a), it is seen that all the prepared TiO2-SnO2 composites possess high degradation rate compared with pure titanium oxide P25, indicating the effectiveness of SnO2 doping and hierarchical macroporous-mesoporous structure. And, for these prepared samples, the relative order of photodegradation rate of RB dye solutions is TiSn10 > TiSn4 > TiSn6 TiSn12 > TiSn8. Although the samples TiSn6 and TiSn8 possess high surface area and pore volume (Table 1), they exhibit lower degradation effect. The lower activity of these two samples may be related to the smaller average pore size, which leads to the less absorption of RB dye compared with the samples TiSn10 and TiSn4 (Table 2). Furthermore, the photocatalytic activity of the sample TiSn10 is obviously higher than that of TiSn4. This may be owing to the lack of anatase phase in TiSn4, which is photocatalytic active phase that considered a predominant factor in influencing the photodegradation process. However, the sample TiSn4 with predominant rutile phase exhibits higher degradation rate than the sample TiSn6 with main anatase phase. This is possibly due to the increase of SnO2 content in TiSn4, which leads to more Sn doping, forming smaller energy gap among the band gap of TiO2, producing more electron-hole pairs, and thus improving the photocatalytic activity [13]. Figure 5(b) depicts the approximate linear relationship of versus irradiation time for the different samples, which indicates the photodegradation process of RB dye can be considered as a pseudo-first-order kinetics reaction. And the apparent rate constants were calculated to be 0.03385, 0.10032, 0.09149, 0.0764, 0.13363, and 0.09023 min−1 for pure TiO2 (P25), TiSn4, TiSn6, TiSn8, TiSn10, and TiSn12 photocatalysts, respectively.
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Figure 6 shows the absorption spectra of RB dye solution over the sample TiSn10. From Figure 6, it can be seen that the RB dye solution exhibits an obvious absorption peak at 553 nm at zero time, and the absorbance of RB dye solution at 553 nm is basically reduced to zero within 30 min, indicating the effective photodegradation of RB dye under the catalysis of TiSn10. On the other hand, the absorption peak gradually shifts to left with prolonged irradiation time, then stays around at 6 nm (the absorption peak of one intermediate) at 40 min, and finally disappears within 70 min. This demonstrates that the RB dye in solution is degraded completely within 30 min, and the whole RB dye along with its intermediate products is fully degraded within 70 min.
4. Conclusions
The photocatalytic degradation of Rhodamine B dye was successfully carried out under UV irradiation over porous TiO2-SnO2 nanocomposites prepared by hydrothermal method by using polystyrene microspheres as template. The photocatalytic activities exhibit an order of TiSn10 > TiSn4 > TiSn6 TiSn12 > TiSn8 > pure TiO2 (P25). The observed high photocatalytic activity is comprehensively affected by the molar ratios of Ti/Sn, hierarchical macroporous-mesoporous structure, and high surface area.
Conflict of Interests
The authors have no conflict of interests in relation to the instrumental companies mentioned in this paper directly or indirectly.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51172065) and State Key Laboratory Cultivation Base for Gas Geology and Gas Control (WS2013B03).