Journal of Nanomaterials

Volume 2013 (2013), Article ID 321459, 6 pages

http://dx.doi.org/10.1155/2013/321459

Research Article

Preparation, Characterization, and Photocatalytic Activity of TiO₂/ZnO Nanocomposites

Liqin Wang,1,2 Xiujun Fu,¹ Yang Han,¹ E. Chang,¹ Haitao Wu,¹ Haiying Wang,¹ Kuiying Li,² and Xiaowen Qi³

¹Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

²State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

³College of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China

Received 18 June 2013; Revised 6 September 2013; Accepted 9 September 2013

Academic Editor: Lavinia Balan

Copyright © 2013 Liqin Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Nanoparticles of the TiO₂/ZnO composite photocatalysts were prepared via sol-gel process. The crystalline structure, morphology, thermal stability, and pore structure properties of the composite photocatalysts were characterized by XRD, FE-SEM, TG-DTA, and N₂ physical adsorption measurements. The photocatalytic activity of the composite catalysts was evaluated by photocatalytic degradation reaction of methyl orange (MO) in aqueous solution. The best preparation parameters for the composite photocatalysts were obtained through systematical experiments. Furthermore, the photocatalytic degradation reaction of aqueous MO solution followed the first-order reaction kinetics; the relative equation can be described as , and the calculated correlation constant () is 0.9937 for the calibration curve.

1. Introduction

Photocatalytic technology has been increasingly demonstrating prominent superiority for the decomposition of organic compounds and pollutants coming from many industries. Among various semiconductor photocatalysts, TiO₂ has been proven to be the most important one due to series of merits such as good photocatalytic activity, good chemical and thermal stability for long term, nontoxicity, and low cost. Thus, it has been widely applied in environmental, optical, and electronic fields [1–7]. ZnO is another promising photocatalyst and suitable alternative to TiO₂ for the wider direct band gap as well as higher solar receive and utilization efficiency for organic pollutants photodegradation [8–15]. Recently, many studies for improving TiO₂ photocatalytic efficiency have become hot topics; one approach is to dope some kind of transition metals into TiO₂, forming doped photocatalyst, which would modify both physical and optical properties of TiO₂ [16], but the results are still unsatisfying. Another one is to couple other oxides in order to achieve higher photocatalytic efficiency, such as WO₃ [17], ZnO [18–20], SiO₂ [21, 22], SnO₂ [23], Fe₂O₃ [24], and MoO₃ [25], and these studies on this aspect are becoming more and more extensively.

In this paper, nanoparticles of TiO₂/ZnO composite catalysts were obtained via sol-gel process. The crystalline structure, morphology, thermal stability, and pore structure properties were characterized by means of XRD, FE-SEM, TG-DTA, and N₂ physical adsorption measurements. The photocatalytic activity of composite catalysts was investigated by photocatalytic degradation experiment of MO in aqueous solution.

2. Experimental

2.1. Materials

The starting materials, tetrabutyl titanate (TBOT), zinc acetate, ethanol absolute, and hydrochloric acid (36.5 wt.%) were purchased from Shanghai Chemical Reagent Company, and they were used to prepare TiO₂/ZnO composite catalysts. The above reagents were of analytic reagent grade, and they were used without further purification. Seven mL TBOT was mixed with 20 mL ethanol absolute and hydrochloric acid which varied from 0.10 mL, 0.15 mL, 0.20 mL, and 0.25 mL to 0.3 mL, respectively, forming solution A. Zinc acetate (for instance, 0.45 g, 0.68 g, 0.90 g, 1.13 g, 1.35 g, and 1.58 g) and 1.5 mL deionized water were mixed with 20 mL ethanol absolute to form solution B. The starting materials ratio was equal to ZnO/TiO₂ (molar ratio) varying from 0.10, 0.15, 0.20, 0.25, and 0.30 to 0.35, respectively. Then, solution B was slowly added into solution A under magnetic stirring for 0.5 h. The mixed sol was aged at room temperature until forming gel, and the mixed gel was dried and calcined. The calcination temperatures were 450°C, 480°C, 500°C, 550°C, and 600°C, respectively. Calcination time was changed among 1 h, 1.5 h, 2 h, 2.5 h, and 3 h when the calcination temperature has been determined. In this way, the nanoparticles of TiO₂/ZnO composite catalysts can be synthesized.

2.2. Characterization

The morphology of TiO₂/ZnO composite catalysts was studied by the field emission scanning electron microscopy (FE-SEM, S4800, Japan), and the pipe pressure was 15 kV. The crystalline structure and crystal phases of the as-prepared composite catalysts were determined by the X-ray powder diffractometer (XRD, Rigaku, Japan) of D/MAX-rB, which was radiated by Cu Kα with the pipe pressure of 40 mV, the wave length () being 0.154056 nm, and the diffraction angle being in the range of 10°–80°. The N₂ adsorption/desorption isotherms, specific surface area, and pore size distribution plots were measured by automatic physical adsorption apparatus (NOVA4000e, Quantachrome, USA), and the thermogravimetric analysis and differential thermal analysis (TG-DTA) curves of the composite catalysts were carried out by CRY-2P and WRT-3P analyzers.

2.3. Photocatalytic Activity Test

0.1 g powders of TiO₂/ZnO nanocomposite catalysts were put into 100 mL MO aqueous solution; its concentration was 10 mg/L. A 250 W high-pressure mercury lamp was used as the light source. The absorbance of MO aqueous solution was analyzed by the 722s visible spectrophotometer at the wavelength of 465 nm, corresponding to maximum absorption wavelength of MO. The photocatalytic degradation rate of MO aqueous solution can be calculated by the formula

In the formula, represented the photocatalytic degradation rate of MO aqueous solution, was the absorbance of MO aqueous solution before the photocatalytic reaction, and was the absorbance of MO aqueous solution after being catalyzed by the TiO₂/ZnO nanocomposite powders at a moment within 5 hours.

3. Results and Discussion

3.1. Physical Properties of TiO₂/ZnO Composite Catalysts

3.1.1. XRD Pattern Analysis of TiO₂/ZnO Composite Catalysts

Figure 1 shows XRD pattern of the prepared TiO₂/ZnO nanocomposite samples. From the XRD pattern and corresponding characteristic 2θ values of diffraction peaks, it can be confirmed that TiO₂ particles in the samples are identified as anatase phase according to the standard card of PDF#21-1272, for the sharp diffraction peaks located at 2θ = 25.3°, 38.0°, 48.1°, 54.1°, 55.1°, 62.8°, 68.9°, and 75.2°, which are corresponding to the (101), (004), (200), (105), (211), (204), (116), and (215) planes, respectively. Meantime, several slight diffraction peaks located at 2θ = 31.7°, 34.4°, 36.3°, and 70.0° are also observed, they are corresponding to the (100), (002), (101), and (201) planes of hexagonal zincite phase ZnO particles according to the standard card of PDF#36-1451. Therefore, it can be suggested that the as-prepared TiO₂/ZnO composites samples are the combination of anatase TiO₂ particles and zincite ZnO particles. In addition, most diffraction peaks in the XRD pattern are sharp and symmetrical Gauss peaks, which further indicate that the particles of TiO₂ and ZnO in the composites samples have high crystallinity. The results are also identified with other research papers [26, 27].

Figure 1: XRD pattern of the TiO₂/ZnO composite catalysts.

3.1.2. FE-SEM Analysis of TiO₂/ZnO Composite Catalysts

Figure 2 has shown FE-SEM image of TiO₂/ZnO composite catalysts. A lot of TiO₂ and ZnO nanoparticles with a granular morphology can be seen clearly from Figure 2. According to the measurement, the particles size of the as-prepared TiO₂/ZnO samples is almost not more than 100 nm. It is also observed that TiO₂ particles are main components in the TiO₂/ZnO composites samples, and ZnO particles were widely dispersed on the surfaces of the obtained TiO2 bulks, which would be beneficial to improve the catalytic activity of the TiO₂/ZnO composite photocatalysts in essence. Furthermore, some amounts of TiO₂ and ZnO crystalline grains in the samples are aggregated to some extent for their nanoscale particles.

Figure 2: FE-SEM image of the TiO₂/ZnO composite catalysts.

3.1.3. N₂ Physical Adsorption Analysis of TiO₂/ZnO Composite Catalysts

Figure 3 gives the N₂ adsorption/desorption isotherms, pore diameter, and pore size distribution plots of the TiO₂/ZnO composite catalysts. The adsorption/desorption isotherms are corresponding to the typical type-IV isotherms (IUPAC, 1985) which indicate the mesoporous nature of TiO₂/ZnO composite particles. In addition, the desorption hysteresis loop appears when the relative pressure () is in the range of 0.42 to 0.83. The hysteresis type of the isotherms can be classified as H1 which is related to the capillary condensation associated with the mesoporous channels of TiO₂/ZnO composites, and this type of hysteresis loop is normally attributed to the cylindrical pore geometry, and high degree of pore uniformity and connectivity in the composite catalysts [28].

Figure 3: N₂ adsorption/desorption isotherms, pore diameter, and pore distribution plots of the TiO₂/ZnO composite catalysts.

As shown in Figure 3, it also can be seen that the pore diameter is 6.602 nm, the biggest pore volume is 0.0361 cm³/g, and the average pore volume is 0.0119 cm³/g. In addition, the average specific surface area is 76.258 m²/g according to BET calculated results, which are bound up with the morphology and the size of lots of the nanoparticles in the TiO₂/ZnO composite catalysts [29]. These results show that there are rich and uniform pores as well as big specific surface areas in the obtained TiO₂/ZnO composite catalysts.

3.1.4. TG-DTA Curves Analysis of TiO₂/ZnO Composite Catalysts

In order to verify the thermal stability of the TiO₂/ZnO composite catalysts, the TG-DTA analysis was conducted using the dried composites gel. As shown in Figure 4, there is an endothermic peak at 80°C in the DTA curve, and the corresponding mass loss is about 20% in the TG curve, which has inferred that an amount of the absorbed water in the composites samples has been evaporated. When the temperature is increased to 275°C, an obvious exothermic peak can be observed in the DTA curve, and there is a corresponding mass loss in the TG curve, which can be assigned to the dehydroxylation of precursor powders and the formation of some brookite phase TiO₂ particles. There is another endothermic peak appearing at 525°C in the DTA curve, which is attributed to the dehydration of bound water. Then, there is a sharp and strong exothermic peak at 620°C in the DTA curve, which can be attributed to the polymorphic transformation of TiO₂, which is from anatase phase to rutile phase, and this is a steady and slow process. After 650°C, there is no peak in the DTA curve, and the mass of composites samples shows a little change in the TG curve. The total mass loss of the samples is about 40%. The results have shown that the obtained TiO₂/ZnO composite catalysts have good thermal stability.

Figure 4: TG-DTA curves of the TiO₂/ZnO composite catalysts.

3.2. Photocatalytic Performance Testing of TiO₂/ZnO Composite Catalysts

3.2.1. Evaluation of Preparation Parameters of TiO₂/ZnO Composite Catalysts

As shown in Figure 5, among the six kinds of the starting materials ratio, when ZnO/TiO₂ (molar ratio) is 0.25, the decolorization rate of MO aqueous solution is the highest; the maximum value is up to 72.34%. The effect of hydrochloric acid dosage on the photocatalytic activity of TiO₂/ZnO composite samples has been shown in Figure 6, when hydrochloric acid dosage is 0.15 mL, the decolorization rate of MO aqueous solution can be up to the maximum value 71.43%. Through investigating the calcination temperature of TiO₂/ZnO composite catalysts as shown in Figure 7, the most proper calcination temperature is 500°C. Furthermore, the calcination time has been inspected. It can be seen from Figure 8 that when the calcination time is 2 h, the best photocatalytic activity of TiO₂/ZnO composite catalysts has been obtained; at that time, the decolorization rate of MO aqueous solution has reached the highest value 74.03%. From the above, proper preparation conditions of TiO₂/ZnO composite catalysts are as follows: ZnO/TiO₂ (molar ratio), 0.25; hydrochloric acid dosage, 0.15 mL; calcination temperature, 500°C; and calcination time, 2 h.

Figure 5: The effect of ZnO/TiO₂ (molar ratio) on the photocatalytic activity of the TiO₂/ZnO composite catalysts.

Figure 6: The effect of hydrochloric acid dosage on the photocatalytic activity of the TiO₂/ZnO composite catalysts.

Figure 7: The effect of calcination temperature on the photocatalytic activity of the TiO₂/ZnO composite catalysts.

Figure 8: The effect of calcination time on the photocatalytic activity of the TiO₂/ZnO composite catalysts.

3.2.2. Photocatalytic Activity of TiO₂/ZnO Composite Catalysts

The photocatalytic activity of TiO₂/ZnO nanocomposites prepared under the best conditions has been shown in Figure 9. When MO aqueous solution alone has been irradiated by high-pressure mercury lamp, its decolorization rate is only 2.22% (Figure 9, MO alone under the light). In the dark condition, TiO₂/ZnO composites have reached the saturation adsorption amount after 0.5 h, the decolorization rate of MO aqueous solution is 4.9% (Figure 9, Adsorption in the dark). Adding TiO₂/ZnO composite catalysts under the same light source, along with the reaction time extending, the photocatalytic decolorization rate of MO aqueous solution increased gradually. When the reaction time is 5 h, the decolorization rate of the MO aqueous solution is up to 93.30% (Figure 9, Photocatalytic reaction), and the increasing tendency of MO decolorization rate turns slowly after 4 h. It can be inferred that the MO aqueous solution would be nearly degraded completely when catalyzed by TiO₂/ZnO composite catalysts in proper time.

Figure 9: The effect of reaction time on MO aqueous solution degradation catalyzed by the TiO₂/ZnO composite catalysts.

3.2.3. Reaction Kinetics of MO Aqueous Solution Catalyzed by TiO₂/ZnO Composite Catalysts

As shown in Figure 10, photocatalytic reaction kinetics of MO aqueous solution has been studied, and the good linearity between the MO aqueous solution concentration and the reaction time can be observed. According to calculated results, the reaction kinetics equation can be described as , the reaction rate constant () is equal to 0.5689, and the calculated correlation constant () is 0.9937 for the calibration curve. The above results indicate that the photocatalytic reaction of MO aqueous solution follows the first-order reaction kinetics, which is consistent with the research results of Gao et al. [30].

Figure 10: The reaction kinetics of MO aqueous solution catalyzed by the TiO₂/ZnO composite catalysts.

4. Conclusions

In this study, TiO₂/ZnO composite catalysts were successfully prepared via sol-gel process. According to the above characterization and experiment results, TiO₂/ZnO composite catalysts have a granular morphology, and the particles size is almost not more than 100 nm. The composite catalysts with high crystallinity are the combination of anatase TiO₂ and zincite ZnO particles. The adsorption/desorption isotherms are the typical type-IV isotherm with H1 desorption hysteresis loop in desorption branch curve, which indicates the mesoporous nature of TiO₂/ZnO composite catalysts. The pore diameter is 6.602 nm, the biggest pore volume is 0.0361 cm³/g, the average pore volume is 0.0119 cm³/g, and the average specific surface area is 76.258 m²/g. In addition, the as-prepared TiO₂/ZnO composite catalysts have good thermal stability. Meantime, the better preparation conditions for the TiO₂/ZnO composite catalysts have been obtained, which are as follows: ZnO/TiO₂ (molar ratio), 0.25; hydrochloric acid dosage, 0.15 mL; calcination temperature, 500°C and; calcination time, 2 h. The decolorization rate of the MO aqueous solution is up to 93.30% after 5 h, and the experimental result is better than the research ones of Tian et al. [31] and Zhang and Song [32]. The reaction kinetics equation can be described as , which follows the first-order reaction kinetics. From the above results, it is reasonable to believe that TiO₂/ZnO composite catalysts will be applied more and more in environmental protection field and other catalytic fields.

Acknowledgments

This work was financially supported from the Natural Science Foundation of Hebei Province (no. E2013203296) and the Science & Technology Pillar Program of Hebei Province (no. 12276716D).

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