Effect of the TiO2 shell thickness on the photocatalytic activity with ZnO/TiO2 core/shell nanorod microspheres
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- Mo, M., Tang, J., Zheng, M. et al. Res Chem Intermed (2013) 39: 3981. doi:10.1007/s11164-012-0913-2
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TiO2 shell has been fabricated directly on the surface ZnO nanorod microspheres by thermal decomposition of tetrabutyl titanate in octadecane. The thickness of the coverage with TiO2 was controlled by the amount of tetrabutyl titanate added. The core/shell nanorods have anatase TiO2 shells after annealed at 873 K in air. This method enables us to tailor the thickness of TiO2 shell for desired photooxidation application in phenol degradation. ZnO nanorods showed a relatively low efficiency in the photooxidation reaction of phenol. After coating atanase TiO2, the photocatalytic activity of the ZnO/TiO2 core/shell nanocomposites was significantly enhanced in photocatalytic degradation of phenol. It was also found that the thickness of the TiO2 shell affected the catalytic efficiency of ZnO/TiO2 core/shell nanorod microspheres.
KeywordsZnO/TiO2 core/shellPhotocatalytic degradationPhenolThermolysis
ZnO, as a typical semiconductor of II–VI compounds with a wide and direct emission band gap, has been utilized for solar cells, transparent conducting films, piezoelectric nanogenerators, photocatalysts, waveguides, ultraviolet lasers, etc. . In recent years, well-defined ZnO nanostructures with various morphologies such as nanowires, nanobelts, nanohelices, nanotetrapods, nanotubes, and some complicated hierarchical nanostructures have been extensively investigated [2–8]. The ZnO-based core/shell nanocomposites, such as ZnO/Al2O3 [9, 10], ZnO/TiO2 [11, 12], and ZnO/FeOx , has also been reported. As well-known photocatalysis materials, nano-TiO2  and nano-ZnO  have received much attention with respect to the degradation of various environmental pollutants. The core/shell nanocomposites made by nano-ZnO coated with TiO2 are expected to be a novel material which had the merits of both of them. In fact, there are some works on core/shell structured ZnO/TiO2 for applications with photocatalysts [11, 12, 16]. There are also many methods to prepare the ZnO/TiO2 core/shell nanostructures. For example, Yang and coworkers  have prepared ZnO/TiO2 by an atomic layer deposition (ALD) for the dye-sensitized solar cells. Gao and coworkers  have synthesized the ZnO/TiO2 core/shell structure on the tetrapod-like ZnO with titania deposition by vapor hydrolysis method. Liao et al.  have fabricated the ZnO/TiO2 core/shell nanoparticles via a sol–gel process. Irannejad et al.  have reported that a thin layer of TiO2 was coated on ZnO nanorod arrays by chemical vapor deposition.
In this paper, an effective method combining chemical growth of ZnO nanorod microspheres with hydrothermal and deposition of TiO2 on the surface of ZnO nanorods through thermolysis of tetrabutyl titanate in octadecane was employed for the first time, by which ZnO/TiO2 core/shell nanorod microspheres with thickness-tunable anatase TiO2 shell coated uniformly on the ZnO nanorods were successfully fabricated. The ZnO/TiO2 core/shell nanorods could be used as a highly efficient and recyclable photocatalyst. Moreover, the photocatalytic activity of ZnO microspheres was significantly enhanced in the photooxidation of phenol by the coating of anatase TiO2, and the thickness of the TiO2 shell also has an effect on the photocatalytic performance. The developed method should be important for the control of TiO2 deposition in solution for core/shell nanomaterials fabrication, without use of the ALD technique.
All chemicals used were of analytical grade, without further purification. All aqueous solutions were prepared with distilled water.
ZnO nanorod microspheres preparation
In our experiments, the ZnO nanorod microspheres were fabricated in literature . An amount of 11.2 g of Zn(NO3)2·6H2O were dissolved in 45 mL of doubly deionized water, and 26.6 g of NaOH was added to the mixture solution. Then, 110 mL of anhydrous ethanol and 554 mL of deionized water added in turn to the above homogeneous solution. Finally, 45 mL of PEG 200 and 9 mL of 2 mol/L ammonia were mixed with the above solution. The solution experienced 10 min of supersonic (20 kHz) agitation in a pulverizer at a power of 200 W, and was then hydrothermally treated at 353 K for 17 h in a conical flask. The white precipitate was collected, and cleaned with water and absolute alcohol for the removal of the residual PEG, and dried at 313 K. Thus, the microsphere of ZnO nanorods is obtained.
ZnO/TiO2 core/shell preparation
An amount of 0.05 g of ZnO nanorod microspheres and the different amounts (50, 100, and 800 μL) of tetrabutyl titanate were mixed with 20 mL of octadecane, and then the mixture was magnetically stirred and refluxed at 593 K for an hour under a flow of nitrogen. After it was cooled to room temperature, cyclohexane and ethanol were added to the mixture. A primrose yellow deposition was observed and separated via centrifugation. The ZnO/TiO2 nanocomposites were annealed at 873 K in air for 1 h.
Powder XRD measurements were performed on a Philips X’Pert MPD Pro X-ray diffractometer, with graphite monochromatized high-intensity Cu Kα radiation at 40 kV and with 30 mA flux at a scanning rate of 0.066° s−1. Scanning electron microscopy (SEM) images were taken on a JSM-5900 instrument. The TEM images were collected on a JEOL TEM-200CX instrument at an acceleration voltage of 200 kV. TG profiles were recorded on a STA 449C-Thermal Star instrument to monitor the sample weight upon heating. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo ESCALAB 250 using Al Kα radiation (hν = 1,486.6 eV). The spectrometer was operated at 20 eV pass energy.
Photocatalytic activity determination
In the photocatalytic experiments, 0.1 g of photocatalysts were added into 250 mL of phenol solution (the initial concentration of phenol was 100 mg/L) and the reaction mixture was stirred in the dark for 1 h to ensure the adsorption/desorption equilibrium of phenol with the photocatalysts. Subsequently, solution was exposed to UV radiations from a high pressure Hg lamp at room temperature. The analytic samples exposed to the UV light for different time intervals were taken out from the reaction suspension and filtered off to remove the photocatalysts. The change in the phenol concentration was monitored by a Waters 515 high performance liquid chromatography.
Results and discussion
In summary, we have prepared thickness-tunable ZnO/TiO2 core/shell nanorods by thermolysis of tetrabutyl titanate in octadecane with ZnO nanorod microspheres as templates. By the coating of atanase TiO2 on the ZnO nanorods, the ZnO/TiO2 nanocomposites showed a significantly enhanced photocatalytic efficiency in the photooxidation reaction of phenol. The ZnO/TiO2 core/shell nanocomposites with ~30 nm shell thickness showed the highest efficiency in the photogradation of phenol compared to that with the thicker TiO2 shell. Moreover, these nancomposites showed a good recyclable capacity, and this method is expected to be able to fabricate a TiO2 coating on other materials.
This work was supported by the Research Foundation of Education Bureau of Hunan Province (11B027) and the Planned Science and Technology Project of Hunan Province (2011FJ3248, 2011FJ3125).