Preparation, Characterization and Photocatalytic Activity of Cr-Doped Rutile TiO<sub>2</sub>(110) Single Crystal Thin Films

Abstract

The growth of Cr-doped rutile TiO2(110) homoepitaxial single crystal thin films using the pulsed laser deposition (PLD) method was investigated. Surface morphology and electronic structure were characterized using scanning tunneling microscopy/spectroscopy (STM/STS), and X-ray and ultraviolet photoemission spectroscopies (XPS/UPS). Optical absorption spectra were measured using ultraviolet- visible (UV-Vis) absorption spectroscopy. STM images revealed that the atomically flat TiO2(110)-(1×1) surface was maintained at a Cr doping concentration of 6% (atomic ratio), indicating that the Cr dopant had negligible effect on surface morphology. The Cr-doped rutile TiO2(110) film showed higher tunneling conductance than an undoped rutile single crystal. XPS and UPS spectra indicated that Cr atoms bound to lattice O, were present in +3 oxidation state and introduced an impurity state 0.4 eV above the valence band maximum. The UV-Vis absorption spectrum of the Cr-doped film showed an absorbance extending to ~650 nm in the visible range, which was consistent with UPS measurements. Using the Cr-doped TiO2 films, the dissociation of methanol molecules was only observed under irradiation with UV light (wavelength shorter than 430 nm). The dissociation reaction was not observed under irradiation with visible light (wavelength longer than 430 nm). Our results suggest that doping with Cr element alone may not be sufficient to promote the visible light photoactivity of rutile TiO2(110) surfaces.