Nanostructure Shape-Effects in ZnO heterogeneous photocatalysis

Selective oxidation of alcohols is an essential reaction for fine chemical production. Here, the photocatalytic oxidation of benzyl alcohol by zinc oxide (ZnO) nanocrystals was investigated to clarify the mechanism of selective oxidation with this process. Reactivity when in contact with three distinct ZnO nanocrystal shapes: nanocones, nanorods and nanoplates, was studied in order to compare crystal facet-specific effects in the reaction system. The same non-hydrothermal and non-hydrolytic aminolysis method was used to synthesise all three nanocrystal shapes. The ZnO catalysts were characterized using by a range of techniques to establish the key properties of the prominent ZnO crystal facets exposed to the reaction medium. The ZnO nanocrystals photocatalysed the benzyl alcohol oxidation reaction when irradiated by a 370 - 375 nm LED output and each ZnO crystal morphology exhibited different reaction kinetics for the oxidation reaction. ZnO nanocones displayed the highest benzyl alcohol conversion rate while nanorods gave the lowest. This established a facet-dependent kinetic activity for the benzyl alcohol reaction of (1011) > (0001) > (10 (1) over bar0). Experimental and density functional theory computation results confirm that the {10 (1) over bar1} facet is a surface that exposes undercoordinated O atoms to the reaction medium, which explains why the reactant benzyl alcohol adsorption on this facet is the highest. Light irradiation can excite valence band electrons to the conduction band, which are then captured by O-2 molecules to yield superoxide (O-2(center dot-)). In a non-aqueous solvent, the photogenerated holes oxidise benzyl alcohol to form a radical species, which reacts with O-2(center dot-) to yield benzaldehyde. This results in 100% product selectivity for benzaldehyde, rather than the carboxylic acid derivative. (c) 2021 Elsevier Inc. All rights reserved.

Automated summary

Selective oxidation of alcohols is crucial for fine chemical production, and the photocatalytic oxidation of benzyl alcohol using ZnO nanocrystals was investigated in this study. Different shapes of ZnO nanocrystals displayed varying kinetics for the benzyl alcohol oxidation reaction, with nanocones showing the highest conversion rate. The {10 (1) over bar1} facet of ZnO was found to expose undercoordinated O atoms, leading to the highest adsorption of benzyl alcohol and ultimately resulting in 100% selectivity for benzaldehyde as the product.