Fabrication of nano-structured materials from aqueous solution by liquid phase deposition
Introduction
Titanium oxide is an important material because of its wide application such as in photocatalysis [1], [2], chemical sensors [3], [4], and phonic crystals [5], [6]. In recent years, a great deal of effort has been devoted to developing photocatalysts with high activities for possible application in detoxification of the environment. The improvement of these catalysts has been attempted by doping with metals or anions such as Fe, Cr, N, F, and so on [7], [8], [9], [10], [11]. On the other hand, Cao et al. [12] and Hattori et al. [13], [14] reported achieving high photocatalytic activity of SnO2/TiO2 multilayer films due to better electronic transfer properties. In this study, we report on the preparation and photocatalytic properties of TixSn1−xO2 films with gradient composition profiles by the liquid phase deposition process (LPD) method.
The LPD method was developed for the preparation of metal oxide and/or hydroxide thin films on various kinds of substrates with complex morphologies or a large surface area [15], [16], [17]. In this method, metal oxide thin films are formed on the substrate by means of ligand-exchange hydrolysis of a metal-fluoro complex and the F− consumption reaction with boric acid or aluminum metal. We have reported the successful preparation of TixSi1−xO2 oxide thin films with continuous gradient by the LPD method [18]. Further, TiO2 and SnO2 are known to dissolve into each other at all proportions, and homogeneous mixed oxides of TixSn1−xO2 have been grown for a wide range of compositions [19], [20], [21]. Therefore, it could be expected that fabrication of TixSn1−xO2 films with gradient composition profiles would be possible by the LPD method.
The microstructural and compositional characteristics of the deposited film were investigated by using a cross-sectional transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) microanalyzer. We also report on the photocatalytic activity of TixSn1−xO2 films with gradient composition profiles and discuss the photodegradation mechanism.
Section snippets
Materials
Tin (II) fluoride (SnF2; Nacalai Tesque, Inc.) was dissolved in distilled water. SnO2 · nH2O was precipitated by oxidation of fluoric solution upon addition of hydrogen peroxide (H2O2; Santoku Chemical, Inc.). After filtration, the precipitate was washed with distilled water repeatedly, dried at ambient temperature, then dissolved in hydrofluoric acid (HF 55%; Stella Chemifa Inc.) aqueous solution at a concentration of 1.5 mol dm−3; this solution was used as the parent solution. Ammonium
Cross-sectional TEM observations
Fig. 2(a), (b) show a typical cross-sectional TEM image of the TixSn1−xO2 film with gradient composition profile (flow rate: 1.0 ml min−1) and corresponding depth profile of Ti and Sn. The dark region seen on the left side of Fig. 2(a) is the Au wire used as the substrate. These images confirm that the film thickness was ca. 320 nm and that a continuous film was formed. No clear interface was observed in the deposited film because the composition was changed gradually on a nanometer scale. The
Conclusion
We prepared TixSn1−xO2 with gradient composition profiles by the LPD method and investigated its microstructure. The films were deposited from the solution in which the (NH4)2TiF6 solution had been added under a controlled flow rate to the SnO2 · HF parent solution at ambient temperature. TEM observations and nanoprobe EDX elemental analysis revealed that the composition and gradient profile of the deposited films in the direction of thickness could be controlled by adjusting the mixing
Acknowledgements
This study was supported by Grant-in-Aid for Scientific Research (A) (2) No. 15205026 from the Japan Society for the Promotion of Science.
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