Ultrathin silica-based nanosheets with a thickness of ∼1 nm are fabricated in a low-cost and high-yield system excluding alkali metal ions. Unlike the Na-zeolites with a long capillary and microporous structure, the ultrathin nanosheets exhibit highly effective surface and active sites. As a result, the 0.4 wt% Pt-loaded nanosheets exhibit superior catalytic activity in H₂O₂ decomposition with an O₂ evolution rate of 342.5 mL g⁻¹ min⁻¹ in a clean aqueous system.