Development of Semicoducting Materials for Photoelectrochemical Cells

Photochemistry is a branch of chemistry that has been developed ever since the 19th century, which exploits light instead of heat as a source of energy to initiate some reactions in milder conditions. As a part of photochemistry, photocatalysis has also been discovered at the beginning of the 20th century, however it did not receive much attention up till the 70s, after the discovery of the versatility of titanium dioxide and with the stimulus of a difficult economic situation, that first posed the problem of a future scarcity of oil. From then on, an increasing attention was placed on the field, with the number of yearly published papers that grew exponentially. When a heterogeneous photocatalyst is paired with an electrochemical apparatus the practice of photo-electrocatalysis arises. Compared with simple photocatalysis, this approach offers the advantage of separating the reduction and oxidation sites preventing the issues of product crossover. The photo-electrocatalytic approach is quite versatile and is mainly applied in the fields of organic synthesis, pollutants degradation, carbon dioxide reduction and water splitting. In this work we investigated the potentials of a new kind of photocatalysts for hydrogen evolution in aqueous environment; these are some heterogeneous materials, composed by carbon nitride and metal halide perovskites and/or perovskite derivatives, the main characteristics of which are the possibility of including most of the elements of the periodic table and also many organic cations in their composition, and the ability of tuning their optical and electronical properties with the variation of their exact composition. In particular, the project was divided in three parts: during the first period, the materials employed for the hydrogen evolution catalysis were synthesised and characterized in their powder form starting from the pure components, continuing with the heterogeneous catalysts; during the second period, the building of the photo-electrodes was started, for the cathodic electrodes some of the previously characterized composite materials were employed, while for the anodic ones some bismuth vanadate-based materials, widely described in literature, were briefly optimized; during the third period, two of the individually characterized electrodes were selected and used together to close some complete photo-electrochemical cells, that were also employed to investigate some different kinds of electrolytes. Specifically, the crystal structures and optical properties of the materials were determined through X-Ray Diffraction (XRD) and UV-Vis absorption spectroscopy, the water stability and hydrogen photo-production abilities of the powdered cathodic photo-catalysts were probed through, respectively, the examination of samples that were dispersed in water for a prolonged time and photocatalytic tests on the powdered catalysts dispersed in an aqueous solution, other kinds of characterizations were performed only on some samples, when they were deemed necessary; in addition, in order to widen the characterization of these materials, the powders were used to also catalyse some organic reactions and pollutant degradations. On the other, for the photo-electrodes, the characterization mainly consisted in Open Circuit Voltage (OCV) readings, used to discover the potential naturally reached by the materials, Cyclic Voltammetry (CV), that showed the working potential of the materials, and Chronoamperometric (CRA) measures, that simulated the real working conditions. Lastly, for the complete cells, not only the working conditions were simulated through CRA measures, but the different electrolytes were also used in order to develop a basic understanding of what kind of environment could be more suitable for the materials.