Abstract

The contents of the present thesis can be divided into three parts. The first three chapters introduce the general context in which this work was developed: the social impact, the motivations and the key concepts of our research field. In particular, in Chapter 1 we discuss about the energy issue, focusing on the problem of sustainability of the energy sources. Through an analysis of updated energy and population statistics, we come to the conclusion that solar energy is the most environmentally, economically and socially sustainable energy source. Then, in Chapter 2 we present the basics of photoelectrochemical water splitting, as a possible strategy of solar hydrogen production. This discussion is inserted in the more general topic of artificial photosynthesis toward solar fuel generation. In view of the experimental work presented in this thesis, we put attention on semiconductor-based photoelectrochemical water splitting and on heterogeneous catalysis with inorganic catalytic materials. More specifically, we propose physical vapor deposition techniques as synthetic methods suitable for the industrial production of thin films for photoelectrochemical applications. In Chapter 3 we introduce the fundamentals of physical vapor deposition techniques (namely, radiofrequency magnetron sputtering, electron-beam deposition and pulsed laser deposition). The second part highlights some fundamental mechanisms that are relevant in the pulsed laser ablation of metals. In particular, we review our recent results on the modeling of liquid nanodroplet formation in the nanosecond laser ablation of pure metals. Chapter 4 develops a simplified model of phase explosion, based on the theory of homogeneous boiling. Through a continuum approach, we describe the liquid nanoparticle formation in a metastable liquid metal, whose temperature is constant over time and space. The results of our computational simulations are presented here for a set of seven metals (Al, Fe, Co, Ni, Cu, Ag and Au), commonly used in pulsed laser deposition. Our modeling was further improved, taking into account a more realistic spatial and temporal dependence of the temperature. In Chapter 5 we design a simulation of the nanosecond laser ablation of aluminum, which considers phase explosion and vaporization mechanisms. A nanosecond Gaussian-shaped laser pulse was assumed and the spatial gradient of the temperature was calculated according to the heat conduction equation. In this way, space–time resolved homogeneous boiling was studied and the size distribution of the produced liquid nanodroplets is presented. After this long digression on the fundamentals of laser ablation mechanisms, we return to focus on the application of physical vapor deposition techniques to the synthesis of solid-state thin layers for photoelectrochemical water splitting. This third part is composed of three chapters, each one dealing with a different physical vapor deposition technique. Chapter 6 presents the synthesis and characterization of tin-doped hematite through radiofrequency magnetron sputtering. That study allowed us to shed some light on the effect of tin doping on the structural, optical and electrochemical properties of hematite. Indeed, tin-doped hematite was studied as a photoanodic material in some considerable experimental works, but the employed techniques made difficult to decouple the effect of the dopant from other structural and morphological features. Chapters 7 and 8 present the results of our work on the pulsed laser deposition and electron-beam deposition of water oxidation catalysts, respectively. In particular, Chapter 7 proposes the synthesis of a porous amorphous iron oxide catalyst employed to functionalize hematite photoanodes. The small-scale nanostructuring obtained through pulsed laser deposition allowed minimizing some issues such as the parasitic light absorption. In Chapter 8 we characterize pure and binary metal oxide thin films based on Fe, Co and Ni, deposited through electron-beam deposition. In our investigation of the electrocatalytic performance of these water oxidation catalysts, NiFe2Ox results as the most active material, in agreement with recent literature.