Effect of substrate orientation on the crystal quality and surface roughness of Nb-doped TiO 2 epitaxial films on TiO 2
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Effect of substrate orientation on the crystal quality and surface roughness of Nb-doped TiO2 epitaxial films on TiO2 Y. Gao and S. A. Chambers Environmental Molecular Sciences Laboratory, Pacific Northwest Laboratory, P.O. Box 999, MS K2-12, Richland, Washington 99352 (Received 28 August, 1995, accepted 15 December 1995
We have grown Nb-doped TiO2 epitaxial films on (100) and (110)-oriented TiO2 rutile substrates by molecular beam epitaxy. Nb substitutionally incorporates at cation sites in the rutile lattice, forming Nbx Ti 12x O2 solid solutions. However, the crystal quality and surface roughness of the films depend strongly on the substrate orientation. Surface roughening and defect formation occur at lower values of x on (100) than on (110). This result is due to anisotropic changes in the metal-oxygen bond lengths within the rutile structure in going from TiO2 to NbO2 ; there are 1% and 12% changes in the metal atom to octahedron-base oxygen and metal atom to octahedron-vertex oxygen bond lengths, respectively. Every metal atom in the (100) growth surface has in-plane components of the 12% mismatch. However, only half of the metal atoms in the (110) growth plane have such components. Thus, there is substantially larger in-plane lattice mismatch when the growth surface is (100) compared to (110), resulting in surface roughening and formation of defects at a lower doping level for (100)-oriented Nbx Ti12x O2 epitaxial films.
I. INTRODUCTION
Titanium dioxide (TiO2 ) has been extensively investigated over the last two decades as a model catalyst for a large class of thermal and photoinduced chemical processes.1–3 While the surface structure and electronic and chemisorption properties of pure TiO2 rutile have been widely studied,4–11 substantial efforts have also been devoted to the influence of chemically inequivalent foreign atoms on the surface reactivity of TiO2 12–16 It has been shown that the addition of small amounts of WO3 , MoO3 , or Nb2 O5 to TiO2 significantly increases the photocatalytic activity and surface acidity of TiO2 .12–16 However, the mechanism(s) for enhanced photocatalytic activity are not at all understood since the surface structures of the oxides in these studies were unknown. Gaining insight into the cause(s) of this enhancement requires careful surface chemistry studies that employ high-quality, well-characterized single crystals or epitaxial films of doped TiO2 . However, such single crystals and/or epitaxial films with doping levels higher than 0.1 at. % have not been produced to the best of our knowledge. In this communication, we will report the successful growth of epitaxial Nb-doped TiO2 rutile films with doping levels up to 40 at. % by molecular beam epitaxy (MBE). We show that although epitaxial films of Nb-doped TiO2 rutile can be grown on both (100) and (110) TiO2 rutile substrates, the crystal quality and surface roughness of these films are largely dependent on the substrate orientation, particularly at high doping J. Mater. Re
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