Dependence of the Work Function of TiO2 (Rutile) on Crystal Faces, Studied by a Scanning Auger Microprobe

The work functions for atomic-level regulated (HF-treated and annealed) n-TiO2 (110) and (100) surfaces with a clear step and terrace structure, together with photoetched ones, were investigated with a scanning auger microprobe (SAM) using 0.05 wt % Nb-doped TiO2. Analysis of the threshold region of the secondary electron spectra has revealed that the work function of the atomic-level regulated TiO2 (100) surface (about 4.13 eV) is 0.07 eV lower than that of the (110) surface. The photoetching of the atomic-level regulated TiO2 (110) surface in 0.05 M H2SO4 decreased the work function by 0.07 eV, but that of the atomic-level regulated (100) surface did not cause any change. Both results were in harmony with previously reported results that the flat-band potential for the (110) surface is about 0.09 V more positive than that for the (100) surface and that the (100) face was selectively exposed by the photoetching, irrespective of original surfaces. The investigation of the work function at local areas of the atomically flat TiO2(110) surface after a half of it was photoetched showed that the work function continuously changed from the value for the (110) face to that of the (100) face as the photoetching proceeded. It was suggested that the SAM was a powerful method to investigate the work function of the solid surface with complex morphology.