Efficient Photo Generated Charge Carrier Separation via Engineering Energy Band Bending in Semiconductor Heterojunctions

Abstract

Photoelectrochemical (PEC) overall water splitting (OWS) has garnered significant attention due to the increasing demand for solar-driven hydrogen as a substitute for fossil resources, posing a threat to the climate. To achieve successful commercialization of PEC OWS, the solar-to-hydrogen (STH) efficiency must exceed 10%. However, to date, no single material has met this goal. Various applications based on metal oxide materials, known for their PEC OWS capability and chemical stability in harsh electrolyte environments, have been explored. Strategies to enhance PEC performance, such as loading co-catalysts, doping, nanostructuring, and constructing heterostructures, have been developed, considering the mechanism of PEC OWS. This study investigates the impact of engineering energy band bending on PEC performance through the doping of a semiconductor heterojunction. Strontium titanate (STO) is chosen as the n-type material, while nickel oxide (NiO) serves as the p-type material in the heterojunction. Stoichiometric STO exhibits dielectric properties, but oxygen vacancies confer n-type characteristics. STO has a large band gap of 3.2 eV and is chemically stable in alkaline electrolytes. NiO, on the other hand, exhibits p-type characteristics with Ni2+ vacancies inducing Ni3+ states in neighboring Ni atoms. NiO has a large band gap of approximately 3.6 eV and catalytic activity for oxygen evolution reaction (OER), making it a well-known electrochemical (EC) material. Considering the electrical structure of the STO/NiO heterojunction, a type II band alignment is observed, where the conduction band (CB) and valence band (VB) energy levels of one material are higher than those of the other. Although type II band alignment allows for a rectifying effect allowing the transfer of electrons and holes in opposite directions, it does not guarantee the separation of photo-generated electron-hole pairs and the transfer of charge carriers without energy band bending at the heterojunction interface. Despite this, many studies on heterostructure PEC electrodes with type II band alignment assume band bending occuring at the interface without further investigation. To elucidate the relationship between PEC performance and the heterojunction, energy band bending is engineered through Nb and Li doping of STO and NiO, respectively. Nb doping in STO increases charge carrier density, while Li doping in NiO also increases charge carrier density and also creates active sites for OER on the NiO surface. Varying doping concentrations allows for the control of carrier density and energy band bending. Through quantitative calculations of the built-in potential at the interface, the degree of band bending at the heterojunction is assessed. The effects of band bending on photo-generated charge carriers are then derived through PEC measurements.