Ultrathin WO₃ Nanosheets Converted from Metallic WS₂ Sheets by Spontaneous Formation and Deposition of PdO Nanoclusters for Visible Light-Driven C-C Coupling Reactions

It is not facile to obtain ultrathin two-dimensional (2D) WO₃ nanosheets through the exfoliation of their bulk counterpart in solution due to strong covalent interaction between interlayers. In addition, they require additional functionalization with cocatalysts to expand their applicability in photocatalytic organic reactions owing to their insufficient conduction band edge position. Here, we report a chemical approach for the simultaneous production and functionalization of ultrathin 2D WO₃ nanosheets through the direct conversion of metallic WS₂ nanosheets, accomplished by the spontaneous formation and deposition of PdO nanoclusters on the nanosheet surface in H₂O. When chemically exfoliated metallic WS₂ nanosheets were simply mixed with K₂PdCl₄ in H₂O under mild conditions (50 °C, 1 h), they were converted to semiconducting WO₃ nanosheets on which PdO nanoclusters of a uniform size (∼3 nm) were spontaneously formed, leading to the production of PdO-functionalized ultrathin WO₃ (PdO@WO₃) nanohybrids. The conversion yield of WO₃ nanosheets from metallic WS₂ nanosheets increased with increasing coverage of PdO nanoclusters on the nanosheet surface. In addition, the conversion of WO₃ nanosheets induced by PdO nanocluster formation was effective only in H₂O but not in organic solvents, such as N-methylpyrrolidone and acetonitrile. A mechanical study suggests that the chemisorption of hydrated Pd precursors on the chalcogens of metallic WS₂ nanosheets leads to their facile oxidation by water molecules, producing WO₃ nanosheets covered with PdO nanoclusters. The as-prepared PdO@WO₃ nanosheets exhibited excellent photocatalytic activity and recyclability in Suzuki cross-coupling reactions of various aryl halides under visible light irradiation.