Quantitative Analysis of Surface Reactive Species and Interfacial Charge Transfer Kinetics of Nanostructured Photoelectrodes with Advanced Electrochemical Methods for Solar Water Splitting

Abstract

Photoelectrochemical (PEC) water oxidation suffers from the sluggish kinetics with a high overpotential largely limiting the scale-up of solar water splitting. It is thus critical to explore efficient low-cost catalysts and deeply understand the interfacial reaction mechanism toward the PEC water oxidation. This dissertation presents the quantitative studies of interfacial charge transfer activities at the TiO_2 nanorods (NRs)-based materials under open-circuit and PEC water oxidation conditions. Firstly, TiO_2 NRs are incorporated with a Ti-based metal-organic framework (MOF), NH_2-MIL-125, in two strategies for developing MOF/TiO_2-derived hybrid catalytic "NanoCONT" materials toward electrochemical and PEC water oxidation. Improved electrocatalytic performance is obtained in MOF nanoparticles-incorporated system attributed to the abundant oxygen vacancies, well-dispersed active sites, large surface area, and highly conductive C and N matrix, whereas the strongly bonded MOF-derived thin film well retains the visible-light photoactivity by reducing the electrical conductivity and charge recombination. Secondly, the quantitative investigations of interfacial charge transfer activities are realized by a scanning electrochemical microscopy technique. The generation of reactive oxygen species in the electrochemical and PEC water oxidation, influenced by external potential, illumination time, and their spontaneous decay, are dynamically resolved with a surface interrogation mode. The interfacial photogenerated charge transfer kinetics at a microfabricated 25-μm TiO_2 NRs ultramicroelectrode is further quantified with the tip approach curves, where the charge carriers promote the interfacial IrCl_6^2-/IrCl_6^3- redox reaction in the presence of PEC water oxidation to a degree depending on the applied potential. A reducing capability of the illuminated TiO_2 NRs is also discovered under an open-circuit potential.Finally, the plasmon-induced charge transfer kinetics in an Au@TiO_2 NRs heterostructure is in situ quantified by measuring the transient built-up and decay of open-circuit photopotential. A wavelength-dependent maximum hot-electron accumulation rate (up to > 10 mV s^-1) can be observed at a few mV, followed by a > 1 min relaxation lifetime with a broad distribution owing to the deep charge trapping in TiO_2. Upon an in situ Ag photodeposition, hot-electron accumulation is slowed down, and relaxation is facilitated but with a broader lifetime distribution suggesting a more complex charge relaxation mechanism.