UC Berkeley

Consecutive eight study phases under the successive presence and absence of UV irradiation, water vapor, and oxygen were conducted to characterize surface changes in the photocatalytic TiO₂ powder using near-ambient-pressure X-ray photoelectron spectroscopy (XPS). Both Ti 2p and O 1s spectra show hysteresis through the experimental course. Under all the study environments, the bridging hydroxyl (OH_br) and terminal hydroxyl (OH_t) are identified at 1.1-1.3 eV and 2.1-2.3 eV above lattice oxygen, respectively. This enables novel and complementary approach to characterize reactivity of TiO₂ powder. The dynamic behavior of surface-bound water molecules under each study environment is identified, while maintaining a constant distance of 1.3 eV from the position of water vapor. In the dark, the continual supply of both water vapor and oxygen is the key factor retaining the activated state of the TiO₂ powder for a time period. Two new surface peaks at 1.7-1.8 and 4.0-4.2 eV above lattice oxygen are designated as peroxides (OOH/H₂O₂) and H₂O₂ dissolved in water, respectively. The persistent peroxides on the powder further explain previously observed prolonged oxidation capability of TiO₂ powder without light irradiation.