Ultrathin 2D silver sulphate nanosheets for visible-light-driven NO2 sensing at room temperature

Two-dimensional (2D) semiconductors, derived from the layered bulk crystals, have been attracting considerable interest due to their atomically thin structures that lead to unique electronic, optical, and surface properties. However, many important semiconductors, such as noble metal chalcogenides and their derivatives, are difficult to directly exfoliate down to the 2D ultrathin morphology given their non-layered crystal structures. Herein, we realise 2D layered-like silver sulphate (Ag2SO4) nanosheets from non-layered Ag2S via a two-step combined annealing-exfoliation process in which annealing in air converts bulk Ag2S into Ag2SO4 that is exfoliated by the strong localized mechanical force in the solvent phase. Ultrathin 2D Ag2SO4 nanosheets exhibit a strong absorption over the visible region with a direct bandgap of 1.87 eV and a prolonged exciton lifetime of 2.06 ns, which are favourable for optoelectronic gas sensing. The sensor demonstrates a room-temperature reversible response to NO2 gas at the parts per billion (ppb) level upon blue-light illumination, with a response factor of 8.39% for 160 ppb NO2 and a limit of detection (LOD) of 0.357 ppb. Such an approach can possibly be applied to a wide range of non-layered oxysulphide materials to provide a promising avenue for developing future optoelectronic gas sensors.