PNNL scientist, Igor Lyubinetsky, to participate in International Conference to commemorate the invention and development of SPM (25 years of STM).
STM Study of Trimethyl Acetate Acid Adsorption and Photodecomposition on TiO2(110) Surface
I. Lyubinetsky, E. K. Vestergaard, Z. Yu, M. A. Henderson
Research on photochemical reactions on TiO2 continues to attract growing attention due to potential applications such as degradation of organic pollutants, hydrophilic coatings, and water splitting for hydrogen production. In this work, the trimethyl acetate acid (TMAA) adsorption and photoinduced decomposition at room temperature on a model rutile TiO2(110) surface have been examined in–situ with scanning tunneling microscopy (STM), combining the molecular beam dosing and ultraviolet (UV) irradiation on the STM stage. The TMAA has a relatively small functional group (carboxylate) and its adsorption structure has been reasonably well understood from previous studies . Still, the extent at which the oxygen vacancies, which normally exist on surface of reduced TiO2, affect the surface chemistry in this case has not been clear. It is well-known that many surface reactions on TiO2 are significantly influenced by the oxygen vacancies, with these intrinsic defects often being preferential adsorption cites. Furthermore, the oxygen vacancies may act as electron donors hence could have an effect on photoinduced charge transfer.
For the TMAA adsorption on TiO2, no direct evidences of oxygen vacancy effect have been found in this study at both low and saturation coverages. TMAA molecules adsorb dissociatively through deprotonation to form carboxylate group of trimethyl acetate (TMA), bridged over two Ti4+ sites of the same Ti row (along the <001> crystallographic direction). Despite each oxygen vacancy has two surface-exposed adjacent Ti sites, TMA typically does not fill or block oxygen vacancies. TMA species are distributed fairly randomly at low coverages, and no significant surface diffusion of TMA has been observed at room temperature. Coverage increase leads to formation of the highly ordered (1x2) arrays of TMA groups (saturation coverage is 0.5 ML). Observed lack of the TMA mobility and formation of the ordered structures probably indicates that incident TMAA adsorption occurs through a mobile precursor-mediated channel. After UV light irradiation, TMA groups disappear in STM images, with photodepletion taking place rather homogeneously across the surface. The results are discussed in terms of a charge transfer initiated by electron-hole pair formation in the TiO2.