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

Abstract

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.