Layered double hydroxides-based photocatalysts and visible-light driven photodegradation of organic pollutants: A review

Layered double hydroxides (LDHs) and their derivatives are a family of promising photocatalysts that have been widely used in photodegradation of organic pollutants. We review the most recent advances in visible-light driven photodegradation of organic pollutants using LDHs based materials with emphasis on the manipulation of their morphological, compositional, and electronic properties and the mechanistic understandings of the photocatalytic processes. Based on the characteristic structures of LDHs, i.e., stable layered structure, specific memory effect, switchable property of layered composites and high surface area, we overview the performance and mechanism of LDHs based catalysts for the photodegradation of common and persistent organic pollutants. First, LDHs-based photocatalysts were classified into five categories, LDHs-derived mixed metal oxides, supporting LDHs, intercalated LDHs, modified LDHs, and LDHs with unique structures (e.g., core-shell LDHs), and reviewed individually in terms of their synthetic methodologies, and structural, atomistic topological and electronic properties. Second, for mechanistic understandings of the photocatalytic processes, we summarize major factors that govern the performance of LDHs-based photocatalysts, including catalytically-relevant properties at the metal/LDHs heterojunctions, adsorption effect, acid-base pairs and the presence of vacancy sites. Third, depending on the photodegradation reactions, the targeting organic pollutants were classified into four types, azo dyes, phenols, persistent organic pollutants and other types of organic pollutants; LDHs-based photocatalysts with optimized performance for each type of molecule are summarized with mechanistic understandings. In addition, we review recent trend in the application of LDHs-based materials in new-emerging areas including CO2 reduction, hydrolysis to produce hydrogen, and photo-assisted organic synthesis with promising performances. Mechanistic details of the photocatalytic processes that lead to the different outcomes in terms of the efficiency, reaction routes and practically-relevant applications in energy harvesting and removal of organic pollutants are the primary focus of the present review. Outlook of major future directions in LDHs-based photocatalysis is outlined by the end.