Facet Formation on Single Crystal TiO 2 Surfaces Studied by Atomic Force Microscopy
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FACET FORMATION ON SINGLE CRYSTAL TiO 2 SURFACES STUDIED BY ATOMIC FORCE MICROSCOPY M. D. ANTONIK, J. C. EDWARDS, AND R. J. LAD Laboratory for Surface Science & Technology, University of Maine, Orono, ME 04469. ABSTRACT Atomic force microscopy (AFM) and low energy electron diffraction (LEED) have been used to study the faceting behavior on (001) and (100) surfaces of a TiO 2 single crystal. On the TiO2 (001) surface, LEED patterns characteristic of (0111 facet planes develop after annealing below 900 IC in agreement with previous studies, but AFM shows a complex surface morphology consisting of a large distribution of facet sizes and orientations. After annealing at 1300 1C, facets do not form but rather a network of 5 - 30 nm high ridges develops over the entire surface. These ridges may be the result of surface defects produced by changes in bulk stoichiometry during annealing. On the TiO2 (100) surface, facets are also observed after annealing below 900 'C. However, these facets have extremely small height to width aspect ratios and are not discernible with LEED. INTRODUCTION Many surfaces exhibit thermal faceting in order to minimize their total surface free energy. The equilibrium surface morphology and facet orientations are determined by the shape of a Wulff plot, a polar plot of surface energy versus the surface orientation [I]. The Wulff plot is difficult to measure, however, and the thermodynamic driving force is often limited by the kinetics of mass transport. The structure and orientation of surface facets have traditionally been studied using diffraction methods, such as LEED [2,3], which yield average facet distributions. In this paper, we show how the atomic force microscope (AFM) provides information about the structure and size of thermal facets with nanometer scale resolution that complements the results from a LEED analysis. TiO 2 surfaces have received considerable attention, both as model oxide systems [4] and as photocatalysts [5], catalyst supports [6], and gas sensors [7]. Many fundamental studies on TiO 2 single crystal surfaces have been motivated by the fact that it is one oxide system for which stoichiometric surfaces can be reproduced in vacuum. The typical procedure to create a stoichiometric surface consists of Ar+ bombardment followed by annealing at 550 IC in lxl0-7 Torr 02 [8]. After this prescribed oxidation treatment on the (110) surface, sharp (lxl) LEED patterns are observed indicating that the (110) surface is very stable [4,8]; recent STM [9] and AFM [10] observations suggest that these surfaces still contain many defects. The (100) and (001) surfaces prepared in this manner, however, are unstable. The (100) surface exhibits (1x3), (lx5) and (1x7) reconstructed LEED patterns over the temperature range 600 - 1200 'C [11], and the (001) surface facets into 10111 planes below 923 IC and (114) planes at higher temperatures [3,11,12]. The nature of the structural rearrangement on these latter two surfaces is the subject of the present study. Recently, scanning tunneling microscopy
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