Exploring the Environmental Photochemistry on the TiO₂(110) Surface in Situ by Near Ambient Pressure X-ray Photoelectron Spectroscopy

Heterogeneous reactions of nitrogen oxides on metal oxide surfaces have been suggested to play a significant role in environmental chemistry, physics, and engineering. Many of the metal oxide compounds found among atmospheric mineral dust particles are inherently semiconducting substrates. Due to their low band gap, they are effective photoactive materials in the environmentally relevant ultraviolet (UVA) range of solar radiation. Here, we have studied nitrogen oxide species evolution and photochemistry on TiO 2 (110) surfaces in the context of atmospheric chemistry by means of near ambient pressure X-ray photoelectron spectroscopy (AP-XPS) coupled with a 375 nm UV-laser module. In the presence of molecular O 2 only, changes in TiO 2 surface potential under UV irradiation were observed, attributed to band flattening. Under humid conditions, a significant increase in the BE range attributed to surface hydroxyl groups was observed, which may be the basis for the light-induced superhydrophilicity observed elsewhere with titania-based nanomaterials. The formation of surface nitrite and nitrate was observed after exposure to NO 2 in the dark. Core-level metal cation, O, and N XPS spectra were measured at elevated pressures of O 2, NO 2, and H 2 O. By selective UV irradiation of only the XPS measurement spot on the sample, we obtained differential information on the surface chemical state on the UV-irradiated compared to dark reference spots. Upon UV irradiation, increased oxidation of NO 2 was observed, while in turn a substantial increase of a reduced nitrate species possibly from electron transfer to nitrate and of a further reduced nitrogen species was observed during exposure to UV-radiation. The effect of surface hydroxylation and the involvement of carbon-containing surface compounds in the formation of nitrogenated organic species are emphasized.