Lithium adsorption on TiO2: studies with electron spectroscopies (MIES and UPS)

The adsorption of lithium atoms on rutile TiO2(110) single crystals was studied with metastable-induced electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS(Hei)) between 130 K and room temperature. Some auxiliary measurements on W(110) required for data interpretation are also reported. At 130 K ionic adsorption at titania prevails up to 0.3 monolayer equivalents (MLE) as judged from the weak Li(2s) emission in MIES for these exposures. The reduction of the Ti4+ cation is manifested by the growth of an occupied bandgap state in UPS: the alkali s-electron is transferred to a near-surface cation, thereby reducing it to Ti3+ 3d. The transfer of the s-electron is responsible for the observed work function decrease up to similar to0.5 MLE coverage. From the analysis of the UPS Ti3+ 3d signal, as well as from the Li(2s) emission, it is concluded that the degree of ionicity of the adsorbed Li decreases from 100% at 0.3 MLE to 40% at 0.7 MLE. Above 0.5 MLE the MIES spectra are dominated by an Li(2s)-induced peak indicating the presence of Li with an at least partially filled 2s orbital. At temperatures above 160 K this peak is almost absent. Excluding Li desorption at these temperatures, we suggest that Li moves into or below the rutile TiO2(110) surface above 160 K. Lithium insertion into the surface and intercalation are discussed. Copyright (C) 2004 John Wiley Sons, Ltd.