Direct Detection of Reactive Intermediates in Photocatalytic Water Splitting with Small Carbon Species on TiO2(100)

Direct detection of elusive intermediates is crucial to deciphering the mechanisms of photocatalytic reactions at semiconductor interfaces. Time-of-flight mass spectrometry under ultra-high-vacuum conditions in conjunction with ultraviolet laser irradiation is employed to investigate photocatalytic water (D2O) splitting and its coupling with methyl iodide (CH3I) fragments on TiO2(100). Direct detection of elusive intermediates, such as D, OD, and OOD species, unequivocally provides experimental evidence for the elementary steps of photocatalytic water splitting under ultra-high-vacuum conditions. In the presence of CH3I, additional pathways emerge, leading to methane (CH3D) and methanol (CH3OD) formation via reactions between photogenerated water and methyl iodide fragments. Temperature-dependent measurements indicate that photocatalytic water splitting proceeds most efficiently at room temperature, whereas density functional theory calculations elucidate the underlying mechanisms that govern surface reactions. These results reveal molecular-level pathways of light-driven reactivity and provide a framework for controlling selective photocatalytic transformations at semiconductor interfaces.