Influence of hydroxyl groups on the adsorption of HCHO on TiO2-B(100) surface by first-principles study

The adsorption of formaldehyde (HCHO) on both clean and hydroxylated TiO2-B(100) surfaces with terminal and bridging hydroxyl groups is systematically investigated by using first principles density functional theory calculations. The discussion is mainly focused on the two different chemical adsorption configurations of HCHO in periodicity (2 × 2), in which the C atom of HCHO is bonded with two coordinated O atoms on a step (Structure I) or on a terrace (Structure II). The study indicates that bridging hydroxyl groups on most of the adsorption sites near to HCHO will weaken the adsorption of HCHO, while terminal hydroxyl groups on most of adsorption sites will facilitate it. The investigation of the effects of hydroxyl groups and H2O molecule on HCHO in different periodicities shows that the terminal hydroxyl groups or H2O molecules have significantly facilitated the adsorption of H2O at larger periodicities, while bridging hydroxyl groups do not have this trend. The analysis of the adsorption mechanisms of HCHO molecules on both clean and hydroxylated surfaces indicate that the terminal hydroxyl groups can extract electrons from the surface and facilitate adsorption of HCHO due to the adsorption energy being higher than that on the clean surface, while the bridging hydroxyl groups donate electrons to the surface and weaken the adsorption. In all chemical adsorption configurations, HCHO acts as an electron acceptor. Interestingly, though the adsorptions are weaker, HCHO in Structure II gains more electrons on both the clean and hydroxylated surfaces than in Structure I. This unique mechanism provides a novel angle to understand the interaction of HCHO with the hydroxylated TiO2 surface.