Engineered Half-Unit-Cell MoS 2 /ZnIn 2 S 4 Monolayer Photocatalysts and Adsorbed Hydroxyl Radicals-Assisted Activation of C α -H Bond for Efficient C β -O Bond Cleavage in Lignin to Aromatic Monomers.

Photocatalysis has high potential in the cleavage of C β -O bond in lignin into high-value aromatic monomers; however, the inefficient C α -H bond activation in lignin and a low hydrogen transfer efficiency on the photocatalyst's surfaces have limited its application in photocatalytic lignin conversion. This study indicates that the cleavage of the C β -O bond can be improved by the generation of the C α radical intermediate through C α -H bond activation, and the formation of desirable aromatic products can be significantly improved by the enhanced hydrogen transfer efficiency from photocatalyst surfaces to aromatic monomeric radicals. We elaborately designed the half-unit-cell MoS 2 /ZnIn 2 S 4 monolayer with a thickness of ∼1.7 nm to promote the hydrogen transfer efficiency on the photocatalyst surfaces. The ultrathin structure can shorten the diffusion distance of charge carriers from the interior to the surfaces and tight interface between MoS 2 and ZnIn 2 S 4 to facilitate the migration of photogenerated electrons from ZnIn 2 S 4 to MoS 2 , therefore improving the selectivity of desirable products. The adsorbed hydroxyl radical (*OH) on the surfaces of MoS 2 /ZnIn 2 S 4 from water oxidation can significantly reduce the bond dissociation energy (BDE) of C α -H bond in PP-ol from 2.38 to 1.87 eV, therefore improving the C α -H bond activation. The isotopic experiments of H 2 O/D 2 O indicate that the efficiency of *OH generation is an important step in C α -H bond activation for PP-ol conversion to aromatic monomers. In summary, PP-ol can completely convert to 86.6% phenol and 82.3% acetophenone after 1 h of visible light irradiation by using 3% MoS 2 /ZnIn 2 S 4 and the assistance of *OH, which shows the highest conversion rate compared to previous works.