Thermally-induced phase changes in electrophoretically deposited titanate and niobate layered oxides
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Thin films of CsxTi(2−x/4)䊐x/4O4 (CsTiO), K4Nb6O17 (KNbO), and their proton-exchanged forms, i.e., HxTi(2−x/4)䊐x/4O4 (HTiO) and H4Nb6O17 (HNbO), were prepared using the electrophoretic deposition technique. The amine- and thiol-intercalated HTiO and HNbO films were prepared by exfoliation of powders in aqueous ethylamine and (mercaptoethyl)amine hydrochloride solutions, respectively. The heat-induced phase transformation of these films was investigated. Evidently, the CsTiO and thiol-intercalated HTiO films underwent phase transformation at relatively high temperatures due to the cations within the interlayer. CsTiO and HTiO films lost their layered structure and transformed, in turn, into the anatase and rutile phases with increasing temperature. However, the intercalated samples exhibited unidentified phases at in-between temperatures, eventually transforming to TiO2. The KNbO film transformed into a layered KNb3O8 structure, while the HNbO lost its layered structure completely to form Nb2O5. Thus, the phase change depended on the modification of the interlayers, and the heat treatment resulted in thin films with new crystal structures for the amine- and thiol-intercalated samples.
I. INTRODUCTION
The development of thin films with novel features is crucial for the advancement of thin-film applications in a diversity of fields. Production of thin films at the nanoscale by utilizing ion-exchange and modification capability and thermal stability imparts the films with advanced features, enabling their use as functional materials. To study the photocatalytic effect, layered-oxide thin films, such as K4Nb6O17,1,2 CsxTi(2−x/4)䊐x/4O4,3,4 and HxTi(2−x/4)䊐x/4O41,3– 6 were characterized. Cesium titanate and potassium niobate thin films were fabricated by spin coating,1,2,4– 6 a technique that overcomes some of the shortcomings of other techniques, such as higher cost, longer preparation time, and more complex experimental setup. Electrophoretic deposition (EPD) was recently reported to enable the fabrication of layered titanate oxide thin films3 with unique properties, in a comparatively more controllable and reliable fashion. The exfoliation and intercalation of layered oxide materials has been attracting great interest in recent years since these processes contribute unique properties to layered oxides. The exfoliation of layered titanates with
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J. Mater. Res., Vol. 17, No. 10, Oct 2002 Downloaded: 18 Mar 2015
organics such as amines has been reported.1,3–12 The exfoliation process provides a simple method for producing thin films with novel features and for modifying the interlayer. In addition, the intercalation of certain alkalimetal cations into the interlayer1,6 and pillaring of the interlayer with metal oxides11–14 were also studied with an eye toward enhancement of the thermal stability and photocatalytic properties of layered oxides. The structural change in layered oxides occurs at different temperatures depending on the par
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