Elsevier

Thin Solid Films

Volume 516, Issue 23, 1 October 2008, Pages 8530-8536
Thin Solid Films

Characteristics of chromium-doped titanium oxide coatings synthesized by cathodic arc deposition

https://doi.org/10.1016/j.tsf.2008.05.014Get rights and content

Abstract

Cr-doped titanium oxide (Cr/TiO2) thin films were deposited on stainless steel and glass substrates by using the cathodic arc deposition technique. The characteristics of undoped and Cr-doped TiO2 thin films were compared by using several analytical approaches. The undoped TiO2 film as an as-deposited coating had an anatase structure, while the Cr/TiO2 film had an amorphous structure, as revealed by X-ray diffraction. After 3 h of heat treatment at 450 °C, the undoped and Cr-doped titanium oxide films became mixed anatase and rutile phases. The microstructure observed under a scanning electron microscope of the Cr-doped TiO2 film was more compact than that of the undoped film. UV–VIS spectroscopy indicated that chromium doping shifted the absorption edge of the titanium oxide film from the UV region to the visible region. Chromium doping extends the optical absorption range to the visible region and reduces the optical band gap of as-deposited films from 3.2 to 2.2 eV. The drop in the water contact angle from 28° to zero following visible light irradiation implies that the Cr/TiO2 thin film becomes a better photocatalytic material because of better hydrophilic property.

Introduction

Titanium dioxide (TiO2) has attracted much attention because it has industrial applications which exploit its favorable biocompatible, photocatalytic and hydrophilic properties [1], [2], [3], [4]. Since it is chemically inert and inexpensive characteristics, TiO2 has become one of the most frequently used photocatalysts. TiO2 has a suitable band gap (Eg = 3.2 eV) and is highly photoactive. Many works have revealed that the titanium oxide with an anatase structure is more photocatalytically active than that with a rutile phase and amorphous structures. Therefore, synthesis parameters were adjusted to favor the anatase structure. Additionally, for pure TiO2, only the ultraviolet part of the solar irradiation (about 5%) is active in the photoexcitation processes. The extensive applications of titanium dioxide based on visible light have motivated investigations of metal-doped TiO2 as photocatalysts [5], [6], [7], [8], [9]. Pure and metal-doped TiO2 films were deposited on various substrates using various techniques, including chemical vapor deposition (CVD), sol–gel deposition, sputtering deposition, vacuum arc deposition and plasma oxidation [10], [11], [12], [13], [14]. A comparison of these studies stated that the incorporation of metal into titanium oxide films can significantly extend the photocatalytic absorption of light in the visible region. However, the photocatalytic activity of the prepared metal-doped TiO2 photocatalysts depends strongly on the species and concentration of the dopant ion. Chromium (Cr) is commonly used as a dopant to enhance the photocatalytic activity of TiO2. Chromium doping by sputtering process increased the optical absorption range in the visible range and reduced the optical band gap from 3.27 to 2.91 eV [14].

Cathodic arc deposition (CAD) is an established method in industry. Common features of this technique are the high deposition rate and the strong adhesion of the films because of the high metal ion energy. CAD can be used to obtain TiO2 films with highly textured rutile layers on silicon (100) and (111) or amorphous phase on various substrates [12], [15]. The aim of this study is to investigate the characteristics of chromium-doped titanium oxide (Cr/TiO2) thin films that are synthesized by the CAD process to extend the photocatalytic activity of this material to the visible range.

Section snippets

Experimental procedures

Fig. 1 schematically depicts a typical CAD system with an opposed-cathode chamber which is employed to deposit coatings. Two cathodes are placed opposite each other in the chamber: one is a titanium disc and the other is a chromium disc: each has a diameter of 100 mm. The structure of the TiO2 (anatase) films reportedly depends on the substrate bias and the O2 pressure [15], [16]. Also, the structure of the TiO2 films deposited on unheated substrates was predominantly amorphous, but the anatase

Characteristics of films

Fig. 2 presents the XRD patterns of the pure and Cr-doped titanium oxide films on stainless steel substrates, as well as the heat-treated samples. The XRD spectra revealed that the as-deposited sample of the pure titanium oxide films was of the anatase type, and that the sample that was subjected to post heat treatment at 450 °C for 3 h was a mixture of anatase and rutile structures, as shown in Fig. 2(a) and (b), respectively. Unexpectedly, chromium doping was responsible for the amorphous

Conclusion

Cr-doped titanium oxide film was synthesized by cathodic arc deposition and characterized by XRD, UV–VIS absorption spectroscopy and water contact angle measurement. The XRD spectra indicated that the as-deposited pure titanium oxide film was of the anatase type, while the Cr-doped titanium oxide films were of the amorphous-type. Heat treatment at 450 °C for 3 h transformed the structure of Cr-doped titanium oxide film to rutile-type and the surface morphological image revealed many

Acknowledgements

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 94-2218-E-451-001. Ted Knoy is appreciated for his editorial assistance.

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