Photocatalytic water splitting reaction: The pathway from semiconductors to MOFs

In light of the ever-growing global energy demand, photocatalytic water splitting has emerged as a promising avenue for sustainable and persistent energy sources. However, the quest for an optimal photocatalyst suitable for industrial-scale applications remains a strenuous challenge. The journey to identify the optimal photocatalyst for the water splitting reaction has been extensive and remains ongoing. While the search started with the use of inorganic semiconductors based on metal oxides, such as TiO2, many new and promising materials, such as Metal-Organic Frameworks (MOFs), have started to attract the attention of the scientific community. However, in order to be able to improve the efficiency of any photocatalyst, it is important to first understand how the reaction is taking place, in other words, it results imperative to understand the reaction mechanism. The aim of the following review is to study and analyze different experimental techniques that can be used for the elucidation of the reaction mechanism covering both water splitting's half reactions: hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and overall water splitting (OWS). This work starts with the fundamentals of photocatalytic OWS under solar irradiation, followed by the systematical evaluation of distinct MOF-based photocatalysts, classifying them based on the specific metal ion in their composition which facilitates standardized comparisons. The mechanistic investigation of photocatalysts is then detailed, employing various spectroscopic techniques. While a higher focus has been given to the analysis of the mechanistic study on MOFs, other important photocatalysts counterparts are also explored, as they have helped to cement the bases in which new materials can be studied. Furthermore, by comparing results obtained for conventional photocatalysts (e.g., metal oxide semiconductors) with those obtained for newer materials like MOFs, we attempt to show the great amount of information that can be extracted for the elucidation of reaction mechanisms. This systematic approach aims to help better investigate the mechanistic study and designing the next generation of photocatalysts for HER, OER, and OWS.