Spectroelectrochemical studies of oxidation reactions on hematite photoelectrodes

Abstract

In this thesis, the key factors governing the solar fuels catalytic process, i.e. photoelectrochemical water splitting, are studied by means of different spectroelectrochemical techniques. Photo-induced absorption spectroscopy together with transient photocurrent measurements are employed to study the catalytic function of different metal oxide photoanodes, focusing particularly on hematite, by the construction of rate law analyses. An introduction to the field is given in Chapter 1 followed by a description of the methods used herein in Chapter 2. Chapter 3, the first results chapter, focuses on the kinetics of water oxidation on hematite photoanodes at conditions of suppressed surface recombination. This chapter provides evidence of 2 different water oxidation mechanisms, with different activation energies, upon increasing the irradiation intensity. These mechanisms show faster kinetics upon surface deprotonation of the photoanode and a secondary kinetic isotope effect. In this chapter, a detailed water oxidation mechanism at a molecular level for hematite photoanodes is proposed and compared with biological systems such photosystem II. Further comparisons with other metal oxides such titania, bismuth vanadate and tungsten oxide indicate similar mechanisms whose kinetics are sensitive to the valence band edge of the materials. Chapter 4 explores the use of methanol oxidation on hematite, as a model organic substrate reaction, to improve the hydrogen production rate. This is proposed to be done concomitant with either, the degradation of organic pollutants or the synthesis of high value-added products when these reactions are selective. This chapter shows the selective oxidation of methanol to formaldehyde with kinetics faster than those for water oxidation and sensitive to the valence band edge when the reaction was performed on titania. The data collected allow the proposal of a detailed mechanism of reaction for the selective oxidation of methanol to formaldehyde. Finally, Chapter 5 focuses on the decay kinetics of the photogenerated holes in state-of-the-art thin, flat hematite films. This chapter discusses the recombination, accumulation and reaction dynamics of photogenerated holes upon laser excitation. Upon hole accumulation at the photoanode surface, this chapter provides evidence of a kinetic competition between surface recombination and trapping and hole extraction by the electrolyte.