Evidence for Vacancy Creation by Chromium Doping of Rutile Titanium Dioxide (110)

Chromium doped rutile TiO2(110) is studied using noncontact atomic force microscopy (NC-AFM) at room temperature. High-resolution NC-AFM images allow for investigating the effect of chromium doping on the surface structure at the atomic level. Besides added rows that are ascribed to Cr2O3 structures, we observe a considerable amount of surface hydroxyl defects, which are known to originate from oxygen vacancies. A careful analysis of the surface oxygen vacancy density is presented, providing strong experimental evidence for enhanced formation of surface oxygen vacancies upon chromium doping. We can explain these experimental results by a simple model based on maintaining charge neutrality while at the same time minimizing lattice stress. This simple model accounts for an increased oxygen vacancy density, in good agreement with our experimental findings.