Surface structure of anataseTiO2(001):Reconstruction, atomic steps, and domains

Abstract

The surface structure of anatase ${\mathrm{TiO}}_{2}(001)$ was investigated using scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), reflection high-energy electron diffraction (RHEED), and low-energy electron diffraction (LEED). A two-domain $(1\ifmmode\times\else\texttimes\fi{}4)/(4\ifmmode\times\else\texttimes\fi{}1)$ reconstruction, similar to those previously reported in LEED and ion scattering studies, was observed by STM and RHEED. This reconstruction was found to be stable not only from room temperature to 850 \ifmmode^\circ\else\textdegree\fi{}C in ultrahigh vacuum and oxygen-rich environments, but also during the anatase film growth. High-resolution STM images obtained at positive sample biases revealed two types of atomic row within each surface unit cell, indicating different Ti-derived states at the surface. At the same time, XPS of the reconstructed surfaces showed no evidence of ${\mathrm{Ti}}^{3+}.$ Based on the STM, XPS, RHEED, and LEED results, an ``added''-and-``missing''-row model is proposed to account for the $(1\ifmmode\times\else\texttimes\fi{}4)$ reconstruction. Atomic steps and their relationship to the population of $(1\ifmmode\times\else\texttimes\fi{}4)$ and $(4\ifmmode\times\else\texttimes\fi{}1)$ domains were also investigated. The results showed that for vicinal surfaces the domain population depended strongly on the overall surface step orientation. While populations of the $(1\ifmmode\times\else\texttimes\fi{}4)$ and the $(4\ifmmode\times\else\texttimes\fi{}1)$ domains were nearly equal on flat (001) surfaces, they became significantly lopsided on a surface with its normal 2\ifmmode^\circ\else\textdegree\fi{} away from the (001) direction, demonstrating a strong correlation between surface steps and domain population on vicinal surfaces.