Control of Oxidation State of Transition Metal Oxides by Designing Redox Reaction for Photoelectrochemical Energy Conversion

Abstract

Transition metals have various oxidation states because they have electrons in the d-orbital, which has small energy differences. Therefore, transition metals have the redox property that electrons can easily be lost and obtained. These metals can also bond with various anions, forming oxides, nitrides and sulfides with various compositions showing various material properties. Especially, transition metal oxides are widely used for photoelectrochemical catalyst because they show various band structures according to the oxidation states. The oxidation states in transition metal oxides is crucial for the selectivity and the activity of catalyst because they are related to the potential of photogenerated electrons, which is determined by the position of energy band. Among the photoelectrochemical energy conversion reactions, the CO2 reduction reaction is greatly affected by the electron potential of photogenerated electrons for its selectivity and efficiency. Because, in performing the reduction of CO2 in an aqueous condition, which is the requirement of mimicking natural photosynthesis process, the reduction of CO2 has to compete with the hydrogen evolution reaction. Making the CO2 reduction more dominant was difficult because the rate-determining step of CO2 reduction has a higher redox potential than that of hydrogen evolution reaction, which requires the precise control of active materials.<br/> The objective of this thesis is precisely controlling the oxidation state of transition metal oxide to selectively perform the photoelectrochemical CO2 reduction and designing the nanostructure to optimize the activity and stability of active materials. <br/> Firstly, 1-D nanostructured mono-phase Cu2O nanofiber photocathode for selective photoelectrochemical CO2 reduction was fabricated using electrospinning method and thermodynamically programmed calcination. The phase of Cu2O nanofiber should be precisely controlled to mono-phase without the impurity phase as CuO to selectively perform the photoelectrochemical CO2 reduction over hydrogen evolution reaction. Until now, the atmosphere during the calcination process was not precisely controlled either oxidative atmosphere by atmospheric pressure, or reductive atmosphere by Ar or H2 gas resulting mixing with CuO. The phase of copper oxide nanofibers was controlled by the nanoscale gas-solid reaction considering thermodynamics and kinetics. The driving force of the phase transformation between the different oxidation states of copper oxide is calculated by comparing the Gibbs free energy of each of the oxidation states. From the calculation, the kinetically processable window for the fabrication of Cu2O in which mono-phase Cu2O can be fabricated in a reasonable reaction time scale is discovered. Also, a hierarchical structure of Cu2O thin film underlayer and TiO2 passivation layer is developed to optimize the photoactivity and stability of Cu2O nanofiber electrode. By controlling the oxidation state of copper oxide nanofiber electrode and designing the appropriate nanostructure, a faradic efficiency of 93% for CO2 conversion to alcohol was achieved in an aqueous media.<br/> The second focus is to control oxidation state in nanostructured hematite (α-Fe2O3) and to develop hierarchically structured photoanode for effective oxygen evolution reaction (OER), which is the counter reaction of photoelectrochemical CO2 reduction. One of the most important challenges of hematite photoanode for water oxidation is the improvement of the electrical conductivity. To date, the conventional approach to overcome the drawback has been to identify heterogeneous dopants. We systematically controlled oxygen vacancy as a new intrinsic dopant source and investigated the interplay with external dopant, such as tin (Sn). Based on this understanding, we demonstrate that controlled generation of oxygen vacancies can activate the photoactivity of hematite significantly. Also we developed hierarchical structure that consists of undoped Fe2O3 underlayer, Ti-doped Fe2O3 nanorod, β-FeOOH overlayer to optimize the photoactivity of hematite. Undoped hematite underlayer negatively shifted the onset potential by 40 mV (0.84 V vs. RHE) and β-FeOOH overlayer enhanced the photocurrent density to 1.83 mA cm-2 at 1.4 V vs. RHE.<br/> This study provides useful information for understanding the methodology to precisely control the oxidation state of transition metal oxide and rational design of nanostructured electrodes, and their effects on activity and selectivity of photoelectrochemical reactions.