Engineered Half-Unit-Cell\nMoS<sub>2</sub>/ZnIn<sub>2</sub>S<sub>4</sub> Monolayer Photocatalysts\nand Adsorbed Hydroxyl\nRadicals-Assisted Activation of C<sub>α</sub>–H Bond\nfor Efficient C<sub>

Photocatalysis has high potential in the cleavage of\nC<sub>β</sub>–O bond in lignin into high-value aromatic\nmonomers; however,\nthe inefficient C<sub>α</sub>–H bond activation in lignin\nand a low hydrogen transfer efficiency on the photocatalyst’s\nsurfaces have limited its application in photocatalytic lignin conversion.\nThis study indicates that the cleavage of the C<sub>β</sub>–O\nbond can be improved by the generation of the C<sub>α</sub> radical\nintermediate through C<sub>α</sub>–H bond activation,\nand the formation of desirable aromatic products can be significantly\nimproved by the enhanced hydrogen transfer efficiency from photocatalyst\nsurfaces to aromatic monomeric radicals. We elaborately designed the\nhalf-unit-cell MoS<sub>2</sub>/ZnIn<sub>2</sub>S<sub>4</sub> monolayer\nwith a thickness of ∼1.7 nm to promote the hydrogen transfer\nefficiency on the photocatalyst surfaces. The ultrathin structure\ncan shorten the diffusion distance of charge carriers from the interior\nto the surfaces and tight interface between MoS<sub>2</sub> and ZnIn<sub>2</sub>S<sub>4</sub> to facilitate the migration of photogenerated\nelectrons from ZnIn<sub>2</sub>S<sub>4</sub> to MoS<sub>2</sub>, therefore\nimproving the selectivity of desirable products. The adsorbed hydroxyl\nradical (*OH) on the surfaces of MoS<sub>2</sub>/ZnIn<sub>2</sub>S<sub>4</sub> from water oxidation can significantly reduce the bond dissociation\nenergy (BDE) of C<sub>α</sub>–H bond in PP-ol from 2.38\nto 1.87 eV, therefore improving the C<sub>α</sub>–H bond\nactivation. The isotopic experiments of H<sub>2</sub>O/D<sub>2</sub>O indicate that the efficiency of *OH generation is an important\nstep in C<sub>α</sub>–H bond activation for PP-ol conversion\nto aromatic monomers. In summary, PP-ol can completely convert to\n86.6% phenol and 82.3% acetophenone after 1 h of visible light irradiation\nby using 3% MoS<sub>2</sub>/ZnIn<sub>2</sub>S<sub>4</sub> and the\nassistance of *OH, which shows the highest conversion rate compared\nto previous works.