Ultrathin two-dimensional (2D) materials have attracted considerable attention for their unique physicochemical properties and promising applications;however,preparation of freestanding ultrathin 2D noble metal remains a significant challenge.Here,for the first time,we report use of a wet-chemical method to synthesize partially hydroxylated ultrathin Ir nanosheets (Ir-NSs) of only five to six atomic layers’ thickness.Detailed analysis indicates that the growth confinement effect of carbon monoxide and the partially hydroxylated surface play a critical role in formation of the ultrathin structure.The ultrathin Ir-NSs exhibit excellent performance for both the hydrogen evolution reaction and oxygen evolution reaction in a wide pH range,outperforming the state-of-the-art Pt/C and IrO2,respectively.Density-functional theory calculations reveal that the partial hydroxylation not only enhances the surface electron transfer between Ir-sites and intermediate O-species,but also guarantees efficient initial activation of bond cleavage of H-O-H for first-step H2O splitting.This,ultimately,breaks through barriers to full water splitting,with efficient electron transfer essentially maintained.