A high-performance gas sensor operating at room temperature is always favourable since it simplifies the device fabrication and lowers the operating power by eliminating a heater. Herein, we fabricated the ammonia (NH3) gas sensor by using Au nanoparticle-decorated TiO2nanosheets, which were synthesized via two distinct processes: (1) preparation of monolayer TiO2nanosheets through flux growth and a subsequent chemical exfoliation and (2) decoration of Au nanoparticles on the TiO2nanosheets via hydrothermal method. Based on the morphological, compositional, crystallographic, and surface characteristics of this low-dimensional nano-heterostructured material, its temperature- and concentration-dependent NH3gas-sensing properties were investigated. A high response of ~ 2.8 was obtained at room temperature under 20 ppm NH3gas concentration by decorating Au nanoparticles onto the surface of TiO2nanosheets, which generated oxygen defects and induced spillover effect as well.Supplementary InformationThe online version contains supplementary material available at 10.1186/s11671-023-03798-5.