Direct observation of atomic step edges on the rutile TiO₂(110)-(1 × 1) surface using atomic force microscopy

Clarifying the atomic configuration of step edges on a rutile TiO₂ surface is crucial for understanding its fundamental reactivity, and the direct observation of atomic step edges is still a challenge. AFM is a powerful tool for investigating surface structures with true atomic resolution, and it provides the opportunity to resolve the real structure of step edges with improved techniques. In this work, we successfully imaged the atomic configuration of 〈001〉 and 〈1-11〉 step edges on the surface of rutile TiO₂(110)-(1 × 1), and we present the direct observation of oxygen vacancies along the 〈1-11〉 step edges, indicating that one 〈1-11〉 step edge site corresponds to one oxygen vacancy using AFM. We also made use of the simultaneous AFM/STM measurements to explore the electronic structure of step edges, which enhanced the evidence of oxygen vacancies existing along the 〈1-11〉 step edges and further demonstrated that the 〈001〉 step edge was terminated by an O row. The effect of the reduced 〈1-11〉 step edges was explored by probing the O₂ adsorption and the nucleation behavior of gold clusters. It was found that oxygen vacancies along the 〈1-11〉 step edges could contribute to O₂ dissociative adsorption and there was no obvious difference compared with the oxygen vacancies on the flat terrace. The reduced step edge and terrace likewise acted as nucleation and growth sites for gold atoms/nanoparticles, in line with previous reports. The present study provides a complete characterization of the atomic configuration of the step edges on the TiO₂(110) surface and plays an important role in investigating the surface chemistry of metal oxides.