Methanol Photo-Oxidation on Rutile TiO₂ Nanowires: Probing Reaction Pathways on Complex Materials

Abstract

Understanding photochemical processes on nanomaterials is key to developing effective photocatalysts. Herein, methanol oxidation and reduction is used to probe the thermal and photochemical properties of rutile titania nanowires grown using a hydrothermal method. The presence of oxygen vacancy defects leads to methoxy formation and subsequent disproportionation to formaldehyde and methanol at 700 K. Methane and dimethyl ether are also produced in minor quantities. Oxygen adatoms enhance the formation of methoxy, which led to an increase in the disproportionation products and dimethyl ether at high temperature and a decreased amount of methane. The thermal reactivity of the nanowires parallels that of rutile TiO2(110) single crystals. Photo-oxidation of methoxy using UV light produced formaldehyde and methyl formate. These product yields were enhanced on nanowires with oxygen adatoms, but a majority of methoxy (∼70%) is not photoactive. In contrast, all methoxy is photo-oxidized on rutile TiO2(110) when O-adatoms are present. This difference indicates that holes created in the nanowires during UV excitation do not migrate to most of the methoxy—a required step for methoxy photo-oxidation. This lack of activity could be due to either trapping of holes in the material or different binding of the inactive methoxy. These studies demonstrate that while charge carriers can be efficiently created in nanowires differences in chemical properties can suppress photo-oxidation.