Titanium dioxide (TiO₂) is an important material in a number of energy-related applications such as photocatalytic water splitting, and the conversion of solar energy to electricity. Most of these applications rely on nano-crystalline TiO₂ samples that consist principally of two polymorphs: anatase and rutile. In some commercial TiO₂ samples, anatase nano-crystals account for up to 75% of the product, and anatase is generally regarded as having a higher surface reactivity than rutile.

Despite the pivotal role of anatase as reactive component in TiO₂ samples used as the active phase in commercial catalysts, there is still a relatively scarce amount of experimental studies on anatase surfaces in comparison with rutile. More research is required to better understand the surface properties that define anatase as such a good photocatalyst. In particular, the real space characterization of anatase substrates at the atomic scale is essential to elucidate the basic principles that govern the photocatalytic and photovoltaic applications of this TiO₂ polymorph.

In this contribution, we combine atomic force microscopy (AFM) and scanning tunneling microscopy (STM), supported by first-principles calculations, for the simultaneous imaging and unambiguous identification of atomic species at the (101) anatase surface [1]. By using single water molecules as atomic markers, we demonstrate that AFM images the topmost oxygen atoms of the surface, while the main contribution to the STM signal comes from the titanium atoms at the third atomic layer. We show that the STM signal is sensitive to sub-surface defects, and that simultaneous AFM-STM allows the acquisition of STM images with atomic resolution within the surface band gap, where standard STM imaging is challenging.

Based on key distinguishing features extracted from experiments and first principles calculations, we identify candidates for the most common surface defects of the (101) anatase surface and characterize the surface local chemical reactivity with the AFM probe at atomic scale.

The findings reported here provide the foundation for future work on anatase, and pave the way for the study of more complex anatase systems related to water splitting and organic photovoltaics, including the adsorption geometries and binding sites of photoactive molecules as well as metal dopants to enhance hydrogen production.

[1] O. Stetsovych et al. Nature Communications 6, 7265 (2015)