Research on Chemical Intermediates

, Volume 39, Issue 9, pp 3981–3989

Effect of the TiO2 shell thickness on the photocatalytic activity with ZnO/TiO2 core/shell nanorod microspheres

Authors

    • Department of Educational ScienceHunan First Normal University
  • Jiansheng Tang
    • Department of Educational ScienceHunan First Normal University
  • Min Zheng
    • Department of Educational ScienceHunan First Normal University
  • Qi Lu
    • Department of Educational ScienceHunan First Normal University
  • Yao Chen
    • Department of Educational ScienceHunan First Normal University
  • Hongru Guan
    • Department of Educational ScienceHunan First Normal University
Article

DOI: 10.1007/s11164-012-0913-2

Cite this article as:
Mo, M., Tang, J., Zheng, M. et al. Res Chem Intermed (2013) 39: 3981. doi:10.1007/s11164-012-0913-2

Abstract

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.

Keywords

ZnO/TiO2 core/shellPhotocatalytic degradationPhenolThermolysis

Introduction

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. [1]. 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 [28]. The ZnO-based core/shell nanocomposites, such as ZnO/Al2O3 [9, 10], ZnO/TiO2 [11, 12], and ZnO/FeOx [13], has also been reported. As well-known photocatalysis materials, nano-TiO2 [14] and nano-ZnO [15] 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 [10] have prepared ZnO/TiO2 by an atomic layer deposition (ALD) for the dye-sensitized solar cells. Gao and coworkers [11] have synthesized the ZnO/TiO2 core/shell structure on the tetrapod-like ZnO with titania deposition by vapor hydrolysis method. Liao et al. [17] have fabricated the ZnO/TiO2 core/shell nanoparticles via a sol–gel process. Irannejad et al. [18] 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.

Experimental

Materials

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 [19]. 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.

Photocatalyst characterization

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

Figure 1 shows the SEM micrograph of the TiO2-coated ZnO nanorod microsphere and the EDS spectrum of TiO2-coated ZnO with a diameter of 180 nm. The EDS spectrum indicates that the ZnO nanorods coated with TiO2 are composed of Zn, Ti, and O elements at the Ti/Zn molar ratio of 1:1.02, which is higher than the prescribed ration of TiO2 to ZnO.
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Fig. 1

a SEM micrograph of TiO2-coated ZnO nanorod microsphere and b EDS spectrum taken on TiO2-coated ZnO

Figure 2 shows the TG–DTA curves of ZnO/TiO2 core/shell nanorod microspheres in air, indicating that a consecutive weight loss below 750 K is attributed to the removal of residual PEG and the organic components. The total weight loss of the ZnO/TiO2 nanocomposites is about 11 wt%. In fact, the shell deposited consists of TiO2 and residual organic components from tetrabutyl titanate.
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Fig. 2

TG–DTA curves of ZnO/TiO2 nanocomposites in air

The XRD pattern of ZnO nanorod microspheres is well indexed to the phase-pure wurtzite-type ZnO (Fig. 3a). With a shell of TiO2 deposited on ZnO nanorods, the XRD patterns of the ZnO/TiO2 nanocomposites are similar to the pure ZnO, implying that the deposited TiO2 is amorphous. After the ZnO/TiO2 core/shell nanocomposites are annealed at 873 K for 60 min, the core ZnO nanorods are removed by dispersing the ZnO/TiO2 nanocomposites in 5 wt% HAc solution for 60 min at room temperature. Figure 3b is the XRD pattern of TiO2 nanotubes. The powders obtained were confirmed to be anatase TiO2.
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Fig. 3

XRD patterns of a ZnO microspheres, b TiO2 nanotubes

The TEM image of ZnO microspheres (Fig. 4a) shows the length of ZnO nanorods is ~6 μm and the diameter is ~180 nm. The thickness of the TiO2 layer can be controlled by the amount of tetrabutyl titanate in the solution: the thickness is ~30 nm (Fig. 4b), ~70 nm (Fig. 4c), and ~180 nm (Fig. 4d), when the amount of tetrabutyl titanate is 50, 100, and 800 μL, respectively. Figure 4e is the TEM image of ZnO/TiO2 core/shell nanorods annealed at 873 K for 1 h. Figure 4f shows the TiO2 nanotubes after the core ZnO nanorods are removed by dispersing the ZnO/TiO2 nanocomposites in 5 wt% HAc solution. The nanotube wall is porous and consists of homogeneous overlapped nanoparticles. The unique porous structure means that the active molecules can diffuse inside the nanotube walls.
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Fig. 4

TEM images of a ZnO nanorod microsphere; bd the thickness of the TiO2 is about ~30 nm (b), ~70 nm (c), and ~180 nm (d); e ZnO/TiO2 core/shell nanorod microsphere annealed at 873 K for 60 min; f TiO2 nanotubes

XPS in Fig. 5 clearly show the Zn 2p, O 1s, and C 1s peaks for both ZnO nanorods and ZnO/TiO2 core/shell nanocomposites when all samples were annealed at 873 K for 60 min. In Fig. 5a, the XPS peaks with binding energies of 284.98, 1,021.40, and 530.13 eV correspond to Zn 2p3/2, O 1s, and C 1s, respectively. For TiO2-coated ZnO nanorods, an additional peak with a binding energy of 458.50 eV corresponding to Ti 2p3/2 was present (Fig. 5a). These XPS spectra serve as evidence for the formation of TiO2 shell on the ZnO nanorods. Figure 5b shows the narrow scan of Ti 2p which confirm the presence of peaks with binding energies of 458.45 and 464.20 eV corresponding to Ti 2p3/2 and Ti 2p1/2, respectively. Figure 5c shows the O 1s XPS spectra for the two types of samples. The peak position of the ZnO/TiO2 core/shell nanocomposites was shifted to a high binding energy due to the O 1s band position at higher binding in TiO2 compared with that in ZnO. This also confirms the formation of TiO2 shell coated on ZnO [20].
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Fig. 5

Wide scan survey XPS spectra of a ZnO nanorods and ZnO/TiO2 core/shell nanorod microspheres, b narrow scan of Ti 2p, and c XPS spectrum in the O 1s region for ZnO nanorods and ZnO/TiO2 core/shell nanorod microspheres

We evaluated the photocatalytic activity of ZnO/TiO2 core/shell nanorods in the photocatalytic degradation of phenol when all samples were annealed for 60 min at 873 K. Figure 6a shows the concentration changes in phenol over ZnO, ZnO/TiO2 core/shell nanocomposites with ~30 nm shell, and P-25 TiO2 photocatalyst. Much lower activity of ZnO nanorod microspheres is observed compared to Degussa P-25 TiO2 photocatalysts. However, the photocatalytic activity of ZnO/TiO2 core/shell nanocomposites is improved due to the ZnO-coated TiO2. It is worth noting that the photocatalytic degradation rate of phenol by anatase TiO2-coated ZnO nanorods is approaching that of Degussa P-25 TiO2 photocatalysts. The phenol molecules can diffuse into the ZnO cores through the porous TiO2 shell. The photo-degrading capability of ZnO/TiO2 core/shell nanocomposites is stronger than that of ZnO nanorods because of the synergetic effect of the coupling of ZnO and TiO2 metal oxides. Figure 6b compares the photocatalytic activity of ZnO/TiO2 core/shell nanocomposites with different thicknesses of TiO2 shell. It was found that the ZnO/TiO2 core/shell nanocomposites with ~30 nm thick shell has the highest efficiency, but the thicker shell with ~180 nm results in poorer efficiency because the phenol molecules are more difficult to diffuse into the ZnO cores. Moreover, in order to evaluate the activity stability of the ZnO/TiO2 core/shell catalyst, a reuse experiment was carried out. The catalytic activity stayed constant and no obvious deactivation was observed after being recycled five times. Additionally, the ZnO/TiO2 core/shell nanocomposites show an easily recyclable capacity via centrifugation.
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Fig. 6

The concentration changes in phenol over a different photocatalysts: ZnO nanorod microspheres, the ZnO/TiO2 core/shell nanocomposites, and Degussa P-25, b different TiO2 shell thicknesses: ~30, ~70, and ~180 nm

Conclusions

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.

Acknowledgments

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).

Copyright information

© Springer Science+Business Media Dordrecht 2012