18. Application of Layered Double Hydroxides (LDHs) in Photocatalysis

Layered double hydroxides (LDHs, [M2+1−xM3+x(OH)₂]x+(An−x/n)·mH₂O), also known as hydrotalcite-like compounds, are natural and/or synthetic clays consisting of highly ordered two-dimensional hydroxide sheets, where M²⁺, M³⁺, and A ⁿ⁻ are divalent and trivalent cations, and the interlayer anions of valence n. Recently, LDHs have attracted great attention in the field of photocatalysis because of their characteristic layer structures, remarkable adsorption properties, and large specific surface areas. Recent applications of LDHs to the photocatalytic reactions such as the degradation of organic compounds, the water splitting (H₂ and O₂ evolution in the presence of sacrificial reagents), and the conversion of CO₂ are reviewed in this chapter. Moreover, advances in synthesis techniques and characterization methods are also summarized. The variety of metal components in LDHs (M²⁺ and M³⁺) caused significant changes to the photocatalytic activities; in particular, the use of Ni−Al LDH enabled us to achieve the selective formation of CO in the photocatalytic conversion of CO₂ in an aqueous solution, whereas the reduction of proton (H+) to H₂ was suppressed.