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

Abstract

A TiO 2(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 TiO 2 surface by immersing the TiO 2 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 TiO 2 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 TiO 2 surface.