STM imaging of a model surface of Ru(4,4′-dicarboxy-2,2′-bipyridine)2(NCS)2 dye-sensitized TiO2 photoelectrodes

A TiO2(1 1 0)-(1 × 1) surface was prepared in an ultra-high vacuum, transported in laboratory air, and observed with a scanning tunneling microscope (STM) operated in a vacuum of 10−4 Pa. Empty state images showed atomically flat terraces separated by single-height steps, on which 5-fold-coordinated surface Ti atoms were observed as spots arranged in a rectangular lattice. The Ru(4,4′-dicarboxy-2,2′-bipyridine)2(NCS)2 (N3) dye was adsorbed on the TiO2 surface by immersing the TiO2 wafer into an acetonitrile solution of the dye. In the empty state images, individual N3 molecules were observed as oval particles protruding by 0.6 nm from the TiO2 surface. The oval shape elongated to the [1 1 ¯ 0] directions was attributed to electron tunneling from tip to unoccupied states localized at the two carboxyl groups bound to the TiO2 surface.