Photocatalytic properties and optical absorption of ZnFe 2 O 4 doped TiO 2 filmsby J. X. Qiu, Z. H. Li, H. Zhang

Surface Engineering

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Photocatalytic properties and optical absorption of ZnFe2O4 doped TiO2 films

J. X. Qiu*, Z. H. Li and H. Zhang

TiO2 films doped with ZnFe2O4 were prepared by a sol–gel method. The crystalline structure, photocatalytic efficiency and optical absorption band were investigated using X-ray diffraction and Ultraviolet-Visible spectrometer. The doping of ZnFe2O4 improves the photocatalytic efficiency and broadens the optical absorption band of TiO2 films. The photocatalytic efficiency of TiO2 films increases from 12 to 32% with the presence of ZnFe2O4. The exciting wavelength of

TiO2 films shifts about 20 nm to the visible light regions when the molar ratio of ZnFe2O4 is 2%.

Keywords: TiO2 films, ZnFe2O4, Photocatalytic property, Optical absorption, Sol-gel

Introduction

As a semiconductor photocatalyst, titanium dioxide plays an important role in the environment field.

Because titanium dioxide can efficiently degrade organic pollutants, it is widely used in sewage treatment, air purification and self-cleaning glass.1 However, titanium dioxide has two disadvantages. Its optical absorption band is relatively narrow, and its excited electrons and cavities readily recombine again, which decreases the efficiency of degrading organic pollutants. Zinc ferrite is a synthesised semiconductor with a spinel crystal structure.2 Compared with TiO2, the energy gap of

ZnFe2O4 is smaller, and the optical absorption band is wider. Therefore, if ZnFe2O4 is doped in TiO2, maybe the photocatalytic property and the optical absorption band of TiO2 may well be improved. In this paper,

ZnFe2O4 doped TiO2 films were prepared on a glass surface, and its crystalline state, photocatalytic property and optical absorption band are discussed.

Experimental

ZnFe2O4 doped TiO2 films were prepared using a sol–gel method, according to the molar ratio of ZnFe2O4 for 1, 2 and 3%. Zn(CH3COO)2 and Fe(NO3)3 with a molar ratio of 1 : 2 having been added to 10 mL absolute alcohol. After being solved thoroughly, tetrabutyl titanate, diethanolamine and a little deionised water were added to the solution and stirred for 2 h. After standing for another two hours, the TiO2 sol was obtained. It was spin coated onto a glass plate followed by drying and calcination.

Methyl orange solution was chosen as the photodegradation pollutant. The solution concentration was 12 mg L21. A glass plate covered with TiO2 film was put into a silicon glass beaker, and 5 mL methyl orange solution was poured over it. The area of TiO2 film was 18618 mm. A xenon lamp illuminant was located above the beaker. The distance between the bottom of the lamp and the top surface of the liquid solution is 20 cm. The wavelength of xenon lamp ranges from 200 to 700 nm. The radiation time is 60 min. A Shimadzu

UV-240 Ultraviolet-Visible spectrometer was used to record the change of concentration of the methyl orange solution.

The crystalline phase of films was examined with Xray diffraction (XRD). It was performed on a D/max-3B

X-ray diffractometer. The absorption band of films was measured also using a Shimadzu UV-240 UltravioletVisible spectrometer.

Results and discussion

Crystalline structure of films

The XRD patterns of TiO2 films doped with ZnFe2O4 are shown in Fig. 1. The calcination temperature was 500uC.

The pattern (a) is composed of diffraction peaks for TiO2 with an anatase structure. The diffraction peak for

ZnFe2O4 does not appear, probably because its content is too small to display a diffraction peak. When the ratio of ZnFe2O4 is increased to 5%, its diffraction peaks appear in the pattern (b). The structure of ZnFe2O4 is spinel.

These XRD patterns show that ZnFe2O4 can successfully form when TiO2 is as a matrix. This is a precondition for the study, in which ZnFe2O4 has an effect on the photocatalytic property of an anatase TiO2 film.

Effect of ZnFe2O4 ratio on photocatalytic reactivity of TiO2 films

The change pattern of methyl orange degraded by TiO2 films is shown in Fig. 2. TiO2 films were doped with different ratios of ZnFe2O4. The photocatalytic property of TiO2 film is obviously affected by the doping of

ZnFe2O4. With the ratio of ZnFe2O4, the degrading efficiency increase from 12 to 32%. The doping of

ZnFe2O4 improves the photocatalytic property of TiO2 films. Maybe, with a smaller energy gap, ZnFe2O4 can delay the combination of excited electrons and cavities of

TiO2, which could improve its photocatalytic reactivity.

School of Environmental and Materials Engineering, Yantai University,

Yantai 264005, China *Corresponding author, email jianxun_qiu@yahoo.com.cn  2008 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 15 February 2008; accepted 21 February 2008 240 Surface Engineering 2008 VOL 24 NO 3 DOI 10.1179/174329408X293693

In Fig. 2, there is a strange phenomenon. When the ratio of ZnFe2O4 increases from 2 to 3%, the degrading efficiency of TiO2 changes slightly. Now this change can not be explained clearly. Perhaps, more defects of TiO2 result from the 3%ZnFe2O4. These defects can make the combination of excited electrons and cavities easy, so the photacatalytic reactivity is decreased. In the following experiments, the ratio of ZnFe2O4 is selected for 2%.

Effect of calcination temperature on photocatalytic reactivity of TiO2 films

TiO2 films doped with 2%ZnFe2O4 were calcined under 450, 500, 550 and 600uC respectively. The variation of the degradation efficiencies of TiO2 films is shown in Fig. 3.

With increasing temperature, the photocatalytic reactivity of TiO2 films increases first, and then decreases, with a maximum at 500uC. The calcination temperature is a critical factor for the photocatalytic reactivity of TiO2, because it influences the crystalline size and defect. When the calcination temperature is low, the crystalline particles do not grow in a integrated fashion, and many defects exist, which is not helpful for the separation of excited electrons and cavities.3 When the calcination temperature is too high, the crystalline particles can grow big, which can reduce the size effect of nanometre particles. Therefore it is also bad for the photocatalytic reactivity of TiO2. It is deduced from the pattern in Fig. 3, the optimum temperature for TiO2 films is 500uC.