VISIBLE LIGHT SENSITISED WO3−BASED PHOTOANODES FOR SOLAR ASSISTED WATER OXIDATION

The need for stable, storable, accessible and renewable forms of energy has been insistently growing in the last decades, along with the concern about the ongoing climate changes connected to the extensive use of fossil fuels. The efficient exploitation of solar energy, that strikes the Earth’s surface with a great amount of energy everyday, is a promising, though challenging, approach to the problem, with hydrogen produced from solar energy using photoelectrochemical water splitting representing an encouraging alternative. This PhD thesis aims at contributing to deepening the knowledge of the working principles and intrinsic characteristics of photoanode materials, responsible for driving the water oxidation reaction, the kinetic bottleneck of overall water splitting. Focus of the work is the study of photoactive metal oxide-based materials, such as tungsten trioxide (WO3), its ternary oxide derivative copper tungstate (CuWO4) and bismuth vanadate (BiVO4), which are among the most promising materials to be employed for these scopes. Most of the experimental work here presented has been conducted at the UniMi Photocatalysis Group of the Università degli Studi di Milano, Italy, under the supervision of prof. Elena Selli; a fruitful collaboration with the group of prof. Sixto Giménez at the INAM Institute of the Universitat Jaume I of Castellón de la Plana, Spain, has led to the results outlined in Chapter 7 and partially in Chapter 5. The present PhD thesis is organized as follows. Chapter 1 presents an introduction to the grounds and aims at the origin of the work. An outline on the global energy demand and consumptions is drawn, evidencing that fossil fuels still represent the most widely used energy source. A brief description of the new, emerging renewable energy sources is presented, with a more detailed view on the hydrogen fuel, in particular on that produced from solar assisted water splitting. Chapter 2 aims at giving the theoretical principles at the basis of photoelectrochemical water splitting. The main category of photocatalysts, i.e., metal oxide semiconductors, is presented along with their physico-chemical properties. The peculiar characteristics that make them suitable photoactive systems for driving the water oxidation reaction are highlighted. Finally, the working mechanism and the requirements of combined tandem PEC cells are illustrated. Chapter 3 is dedicated to both the experimental and theoretical methods, as well as the synthetic and characterization techniques, that have been used throughout my PhD for producing and interpreting the data presented in this thesis. A careful description of the employed experimental set ups, located in the laboratories of both prof. Selli and prof. Giménez, is also reported. Chapter 4 starts with the description of the experimental work carried out during my studies, which was firstly focused on copper tungstate, a ternary oxide deriving from the more investigated tungsten oxide, which is characterized by a relatively smaller band gap enabling its sensitisation towards the visible portion of the solar spectrum. For the first time, the effects on photoactivity induced by doping this material with nickel ions have been systematically investigated and demonstrated by means of multiple physical and photoelectrochemical methods. Chapter 5 deals with the study of the well-known WO3−BiVO4 heterojunction, a system combining two of the most promising photoanode materials. My work was focused on disclosing the effects on the overall performance of the composite material induced by selectively tuning the morphology of the tungsten oxide underlayer. WO3 and WO3−BiVO4 photoanodes characterized by either a planar or a nanostructured morphology have been synthetized and multiple experimental techniques have been applied to ascertain the intrinsic characteristics as well as the photoelectrochemical performances of the so obtained photoactive materials layers. Chapter 6 is focused instead on the role played by the thickness of either the WO3 and the BiVO4 layer and their reciprocal balance in tuning the overall efficiency of the WO3−BiVO4 composite systems. Multiple heterojunction samples displaying a planar morphology have thus been systematically prepared by varying both the under- and the overlayer thicknesses. The thorough screening of these photoelectrodes allowed an in-depth investigation of the mechanisms of photocurrent generation, and in particular on the detrimental charge recombination paths which can be activated when varying the thicknesses of the two oxides. Chapter 7 is dedicated to the characterization of the photoanodes presented in Chapter 6 by means of two advanced techniques: Electrochemical Impedance Spectroscopy (EIS) and Spectroelectrochemical (SEC) analyses. These techniques provide information on the charge storage and charge transfer mechanisms occurring in the WO3−BiVO4 heterojunction, in comparison with single-component samples. Additional insight was thus obtained into the role exerted by the metal oxides thickness on the overall behaviour of the composite photoanodes. Finally, Chapter 8 provides an overview of the results discussed in this thesis, as well as the future perspectives of each investigated research line.