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Taehoon Lim1 Alfredo Martinez-Morales1

1, University of California, Riverside, Riverside, California, United States

Hydrogen, as a next generation clean energy source, can be obtained from the electrolysis of water using a variety of catalysts such as Co3O4, Ni, and MoS2. Photoelectrochemical (PEC) water splitting is an electrolysis process that produces hydrogen and oxygen by the light-assisted catalysis of water. An excited electron and hole generated by the light absorption of a photocatalyst material are the catalyst for a redox reaction. ZnO can be used for photoelectrochemical water splitting devices due to its high photocatalytic activity. In its more occurring natural state ZnO has a hexagonal wurtzite crystal structure. This crystal structure prefers to grow toward the [001] direction (along c-axis). The characteristic of easily achieving 1-dimensional structure is one of the main reasons why ZnO has been intensively researched for low-dimensional applications. In 1-dimensional ZnO nanostructures, while the plane surface of (001) perpendicular to the c-axis is a polar surface terminated by Zn or O, the largest surfaces are non-polar ({100} and {110}). In PEC water splitting, surface polarity is critical because it greatly affects wettability at the interface.
In this work, an effective technique to control the reaction kinetics which extensively affects the product morphology is introduced. In low-temperature chemical vapor deposition, Zn vapor supply was controlled to obtain two different morphologies: hexagonal nanopyramid and hexagonal prism. The surfaces of pyramidal ZnO are semi-polar {112} family of planes while the prismatic ZnO is covered with polar (001) plane with non-polar side planes. The effect of surface polarity of synthesized ZnO was characterized by cyclic voltammetry. The morphology, crystal structure and defects, PEC performance, and operation mechanism are discussed.

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