Abstract
This chapter discusses the effects of TiO2 prepared by atomic layer deposition on various III–V p-type semiconductors for photoelectrochemical applications, mainly in the hydrogen evolution reaction (HER) and CO2 reduction reactions. In addition to providing protection against photocorrosion due to its chemical stability and high uniformity, the large valence band edge offset between n-type TiO2 and p-type III–V semiconductors acts as a hole-blocking barrier, thus decreasing the interfacial recombination rates for photogenerated electron–hole pairs. The conduction band edge of TiO2 is well aligned with that of III–V semiconductors, and the built-in potential further assists the electron extraction efficiency, leading to high photocurrent densities. Also, the highly catalytic surface Ti3+ defect states (i.e. oxygen vacancies) can lower the potential barrier of intermediate states and promote charge transfer. The photoelectrochemical (PEC) performance of TiO2-encapsulated III–V semiconductor photocathodes is further enhanced by depositing co-catalysts or plasmonic nanoparticles. In order to study further the stability of TiO2 under various applied potentials and pH values, the Pourbaix diagram of titanium is constructed based on thermodynamic equilibrium between the metal and the electrolyte. Combining the kinetic and structural studies, the Pourbaix diagram provides a powerful tool in understanding corrosion, passivation, and immunity under various electrochemical working conditions. Methods of characterizing the surface states of TiO2 passivation layers are discussed in the initial part of the chapter. Then the PEC performance and underlying enhancement mechanisms of the HER and CO2 reduction of three different TiO2-encapsulated III–V compound semiconductors (InP, GaAs, and GaP) are presented. Finally, the Pourbaix diagram of titanium is constructed and used to study the chemical stability of TiO2 passivation.
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