Solar energy to hydrogen can be achieved by two pathways, i.e., the photocatalysis process with powder catalysts and the photoelectrochemistry process with photoelectrodes. Both ways base on light absorption by semiconductors. However, the fast photogenerated electron/hole recombination causes serious energy loss. Over decades, scientists have proposed many approaches to achieve higher solar conversion efficiency. One of those is the cocatalyst loading, which can promote the surface water splitting reaction. The second is band gap engineering. The wide band gap semiconductor was turned into a narrow band gap one, thus broadening the light absorption spectrum. The third approach is the combination of different semiconductors to form heterojunctions, which can both broaden the light absorption spectrum and facilitate the separation and transport of photogenerated carrier.<br/>Hetero-junctions can be classified into four categories, which are cocatalyst/semiconductor junction, metal/semiconductor junction, PN junction and NN junction. High density of surface states on the surface of semiconductors traps both electrons and holes, resulting in surface recombination. When an oxygen evolution reaction (OER) catalyst is deposited on the n-type semiconductor, the catalyst can capture the holes and slow down electron flow rate to the surface, thus suppressing surface charge recombination. For the metal/Semiconductor junction, when a metal with a lower Femi level and the semiconductor contact, the Fermi levels must match. Therefore, heterojunction is formed and potential gradient by band bending is developed. Then during light illumination, the electrons and holes are drived by the potential gradient and flow in opposite directions. Therefore, the separation and transfer of the electrons and holes are facilitated. For the PN junction, when an n-type semiconductor with Femi level close to the conduction band and a p-type one with Femi level close to the valence band are to contact, the Femi energy levels equilibrate, thus forming a large potential gradient in the junction. Similarly for an NN junction, when two kinds of n-type semiconductor contact, NN junction forms. With a slightly different of the band configurations, the NN junction can be classified into type I and type-II forms.

Design of photoelectrode for water splitting utilizing semiconductor hetero-junctions

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.