Metal Oxide and Oxynitride Semiconductors for Photoelectrochemical Water Splitting

Abstract

Photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes to convert solar energy directly into hydrogen is an elegant approach towards clean and renewable energy production. Although extensive research has been carried out since TiO2 was first used as a photocatalyst in 1972, achieving high efficiency, operational stability and low-cost remain the major obstacles for its practical implementation. In this dissertation, we investigated several strategies to address the challenges impeding the performance of TiO2. First, we improved the optical absorption and charge transfer properties of TiO2 through morphological modification by preparing 3D ordered macroporous TiO2 inverse opal (IO) films. The underlying interconnected TiO2 porous network improves light trapping efficiency and allows direct charge transfer pathways across the semiconductor/electrolyte interface (SEI). Photon absorption of the IO films is enhanced in the visible and near-infrared IR region due to slow photon effects and light scattering. In addition, greater surface area provides more active sites for photocatalysis. Next, we introduced controlled oxygen vacancies in TiO2 IO films to improve its photon absorption. TiO2 IO photoanodes were electrochemically reduced from 300 to 500 s, with the photoanode reduced for 400 s producing the highest current density. For these photoanodes, photon absorption in the visible region increased and the photoconversion efficiency in the UV region increasing by almost 3 folds. By introducing oxygen vacancies, we also increased photogenerated carrier lifetimes and improve the conductivity of TiO2. However, a longer reduction time generates excessive oxygen vacancies which act as carrier traps for recombination. Formation of a heterostructure by coupling different photocatalysts into a single photoelectrode can overcome the drawbacks of individual photocatalyst, allowing favourable properties from each photocatalyst to be combined. Tantalum oxynitride (TaOxNy) is an ideal candidate to form a heterostructure with TiO2 because it has band edges which straddles the redox potential of water and a tunable bandgap from 1.9 to 2.5 eV. Deposition of TaOxNy onto an affordable transparent substrate such as FTO-coated glass is desirable but FTO cannot withstand the high temperatures during nitridation. Here, we used plasma-enhanced atomic layer deposition to deposit TaOxNy directly onto the FTO-coated glass substates, and employ layered doping to control the oxygen and nitrogen stoichiometry by alternating the cycles of Ta-N and Ta-O. For 1 super-cycle of 50 Ta-N to 1 Ta-O ratio, we observed both photoanodic and photocathodic currents, known as photocurrent switching. Although, the exact mechanism for the photocurrent switching behaviour needs to be explored further; the ability to control this behaviour by changing the composition through layering rather than homogenous doping is intriguing, and a promising route to implement a single material PEC system to split water. Finally, TaOxNy was deposited onto TiO2 IO to form a type II heterojunction which improved the latter's overall water splitting performance. The deposition temperature of TaOxNy plays an important role in achieving uniformity of the heterojunction throughout the IO film and is crucial to its PEC performance. The photon absorption of TiO2 is extended into the visible spectrum, and an onset potential reduction and improved stability in alkaline electrolyte were achieved for TaOxNy. The appropriate band alignment between TiO2 and TaOxNy creates a large built-in electric field for minority carriers which inhibits electron-hole recombination, extends photogenerated carrier lifetimes and promotes charge transport. Reduction in photogenerated hole accumulation at the SEI and longer photogenerated carrier lifetimes suppress self-oxidation during photo-irradiation, thus improving the photoanode stability in alkaline electrolyte.