UC Davis

With the increasing deterioration of the global climate, ushered in by fossil-fueled industrialization, a call to action is set forth to develop technologies that will transform our current manufacturing and energy landscapes into sustainable ones, where materials chemistry plays a central role. Among the biggest challenges of our time lies the realization of carbon neutral and negative emission technologies as ways to ameliorate the effects of global warming. From a materials’ perspective, this realization can be achieved by the design of efficient light absorbers that can harvest intermittent sunlight, through the development of higher capacity batteries wherein this collected energy can be stored, and by the conception of selective catalyst materials that can directly or indirectly utilize renewably-sourced energy inputs to enable the conversion of molecules, whose products can act as energy vectors or replace petroleum-derived commodities. Metal chalcogenides represent a material space with apt characteristics to tackle these problems owing to the wide range of properties that sulfides, selenides, and tellurides encompass.Chapter 1 provides a summary of the advantageous properties that metal chalcogenides present in the field of energy storage and conversion, with a focus on the fundamental principles that underpin them. Additionally, gaps in the current understanding of catalytic and sensing properties in these materials are introduced. Essential information related to important characterization techniques applied in this work is incorporated as well. Chapter 2 delves into the study of the structural and electronic changes induced by the intercalation of transition metals into the framework of Chevrel Phase chalcogenides (MxMo6T8, where M = transition metal, T = S, Se, Te) – a series of materials with promising applications as catalysts and battery electrodes, for which the connection between composition and function is still poorly understood. Using X-ray absorption spectroscopy (XAS) techniques our study indicates that significant changes to the local structure of the clusters occurs upon intercalation, as evidenced by the decrease in the Mo6 trigonal antiprism anisotropy in sulfides and selenides, and its increase in tellurides. Furthermore, the charge transfer that occurs upon intercalation is dominated by the chalcogen ions, with the molybdenum atoms remaining largely redox inactive. Chapter 3 comprehends a thorough investigation into the structural, electronic and photophysical properties of a series of lanthanoid oxysulfides of formula Ln10S14O (Ln = La, Ce, Pr, Nd, Sm). Reflectance spectroscopy shows that these materials have band gaps in the visible range of the electromagnetic spectrum, exhibiting n-type semiconductivity, with electrons as majority carriers, and striking long lifetimes, possibly indicating the presence of surface defects. Chapter 4 presents the first report of the high temperature reversible thermochromism of Ln10S14O. Color space analysis determines the temperature regions in which the most dramatic color transitions occur for each lanthanoid. In situ X-ray diffraction experiments provide a detailed look at the evolution of the structural parameters with temperature, and calorimetric measurements show the reversibility and susceptibility of these materials to their environments between 300 °C – 600 °C.