Surface density of states of TiO2(110) single crystal and adsorbed molecular observation by scanning tunneling microscopy and tunneling spectroscopy

Abstract

We present the evidence for the surface density of states (SDOS) of the hydroxylated and reduced rutile TiO2 (110) single crystal different from the bulk electronic property by tunneling spectroscopy using scanning tunneling microscopy technique (STM/TS). The surface band gap energy (SEg) was 1.6 eV as the tunneling-active free states. This experimentally obtained SEg (1.6 eV) was quite different from the bulk band gap energy (3.05 eV). The chemisorbed OH− on the Ti3+ –O vacancy state is located 1.35 eV below the EF . The observed tunneling current rectification originates in the surface electronic structure, band bending and surface coordination property. It became clear that the photo-oxidation of the chemisorbed OH− by photogenerated holes was energetically feasible through the study of SDOS using the STM/TS. Phenol molecules were adsorbed onto the hydroxylated TiO2 surface. The image of the phenol molecule was measured at a tip bias of +443 mV which is within the surface band gap and slightly larger than the vibration energy of phenol ring. The electron density of the phenol ring was apparent in the STM picture.