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The Ni-SnO 2 /g-C 3 N 4 composite exhibits unique hierarchical structure, which presents the gas response of 142.5 toward 10 ppm acetone at 125 °C. [Display omitted] • Hierarchical Ni-doped SnO 2 /g-C 3 N 4 nanocomposites are successfully synthesized. • The highly ordered Ni-doped SnO 2 nanosheets in situ grew onto g-C 3 N 4 lamina. • Ni-doped SnO 2 /g-C 3 N 4 presented the gas response of 142.5 toward 10 ppm acetone at 125 °C. • Introduction of g-C 3 N 4 formed g-C 3 N 4 -SnO 2 heterojunction and resulted in sharp resistance change. • Doping of Ni produced more oxygen vacancies for SnO 2. The development of metal oxides semiconductor-based gas sensors for the accuracy detection of acetone in human exhaled breath is highly desirable for diabetes diagnosis. Herein, we use calcining and hydrothermal method to grow highly ordered Ni-doped SnO 2 nanosheets onto g-C 3 N 4 lamina. The prepared Ni-SnO 2 /g-C 3 N 4 composite presents high gas response of 142.5 (Resistance ratio, defined as S = R air /R gas) toward 10 ppm acetone at the optimized operating temperature of 125 °C, under the environment temperature of 25 °C, 80 %RH. Moreover, the Ni-SnO 2 /g-C 3 N 4 based senor also maintains fast response–recovery time, good stability and extraordinary humidity resistance for detecting acetone, including the theoretical detection limit (LOD) as low as 1.38 ppb. The greatly improved gas sensing properties of the Ni-SnO 2 /g-C 3 N 4 composite can be due to the highly ordered hierarchical structure, Ni dopant and SnO 2 /g-C 3 N 4 heterojunctions. We believe this novel Ni-SnO 2 /g-C 3 N 4 nanocomposite promises to be a potential gas sensing material for diabetes diagnosis. [ABSTRACT FROM AUTHOR]