The correlation between surface defects and the behavior of hydrogen adsorption over ZnO under UV light irradiation†
Abstract
The H2 adsorption behaviors of a series of ZnO samples with different surface defects were characterized using a novel method of testing photo-assisted gas-sensitive response to H2 at room temperature under ultraviolet light irradiation. It was found that two types of H2 adsorption occur according to the electron transfer direction at the interface of the adsorbed H2 and the ZnO sample (type I: electrons transfer from ZnO to H2; type II: electrons transfer from H2 to ZnO). The adsorption type of oxygen vacancies (VOs) and zinc vacancies (VZns) can be changed by adjusting the Fermi level (EF) of ZnO. VOs were beneficial to type I H2 adsorption due to their higher EF, while VZns were beneficial to type II H2 adsorption due to their lower EF. The XRD, XPS and EPR results of the samples showed that VOs and VZns are preferentially formed at the {100} and {002} crystalline planes, respectively. Moreover, the in situ DRIFT results of H2 adsorption and the EPR results of hydroxyl radicals showed that surface hydroxyls and hydroxyl radicals play important roles in H2 adsorption, and two possible mechanisms of H2 adsorption were proposed on VOs and VZns over ZnO, respectively. Furthermore, the adsorbed H2 with different adsorption types induced by VOs and VZns exhibit different oxidation behaviors over ZnO under UV irradiation.