(Invited) Stabilizing Semiconductor-Solution Interfaces Via Chemically Stable but Electronically Defective Coatings

Abstract

Recently, a photoelectrode protection strategy, which uses an amorphous “leaky” TiO2 coating to form a complex semiconductor/TiO2/Ni structure, has shown to stabilize semiconductor-solution interfaces against photoanodic corrosion. Particularly, stabilizing semiconductor-aqueous solution interfaces for water oxidation is important because water oxidation is required for all solar fuel strategies, e.g. H2 production and CO2 reduction. We show that this TiO2protection strategy has stabilized a number of 1.1 – 1.9 eV band gap light absorbers including Si, group III-V, group II-VI semiconductors, their tandem structures and wire structures for up to 2000 hours continuous water oxidation in 1 M KOH(aq). Because of this, various otherwise unstable materials can be brought back to the table again for building photoelectrochemical devices. A water splitting reactor prototype of ~10% solar-to-hydrogen efficiency was also built. The n-doped Si/TiO2/Ni structure also operates as a high barrier height junction for solar energy conversion. To further understand the mechanism of “leaky” TiO2 protection and the role of Ni electrocatalyst overlayers, we used Si/TiO2/Ni structures as a model system to study solid-solution interfaces. TiO2 coatings of 3 – 143 nm in thickness introduce two interfaces: a TiO2/liquid interface and a TiO2/n-Si heterojunction. We have employed operando ambient-pressure X-Ray photoelectron spectroscopy (AP-XPS) to study TiO2/Ni/liquid and TiO2/liquid interfaces, and directly observed their ohmic and rectifying junction behavior, respectively. We also extensively investigated n-Si/TiO2 heterojunction interfaces by employing photoelectrochemical, solid-state electrical, and photoelectron spectroscopic techniques. The distinctive electrical behavior of n-Si/TiO2 heterojunctions will be compared with conventional semiconductor/metal and semiconductor/liquid junctions. Because electronic defect states exist in band gaps of “leaky” TiO2, their effects on the formation of n-Si/“leaky”TiO2 heterojunctions will be discussed. This interfacial study reveal several strategies for improving the performance of thin-layer protected photoelectrodes as well as for further expanding selection of “leaky” protective coatings.