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The manner how nano building blocks assemble into hierarchical architectures exerts a tremendous influence on gas-sensing performance of the metal oxides. Here, we focus on tuning the 2D SnO 2 nanosheets into 3D hierarchical nanoflowers by manipulating the presence of NaOH, and investigate their gas-sensing functionalities. We find that the blooming SnO 2 nanoflowers assembled by ultrathin nanosheets (∼50 nm) are shrunk into semi-blooming state, and that the semi-blooming nanoflowers based sensor shows enhanced gas-sensing performance towards the ethanol, which is attributed mainly to the confined effect due to numerous nano or micro reaction rooms by keeping oxygen and ethanol molecules to complete gas-sensing reactions. While the semi-blooming nanoflowers turn into ordered mesoporous via thermally removing the periodically arranged polyvinyl pyrrolidone micelles, their gas-sensing performance is found to be improved dramatically, indicating that sufficient amount of gas diffusion is crucial to gas-sensing properties rather than the fast gas diffusion speed. As a final verification, we fabricate the sensors using the mesoporous semi-blooming SnO 2 nanoflowers and successfully monitor the existence of beer by a simple integrated device, making it a promising candidate in detecting drunk driving. [ABSTRACT FROM AUTHOR]

Quasi-one-dimensional (1D) heterostructures act as promising gas-sensing materials due to their high surface-to-volume ratio, large depletion area, and synergistic behaviors of the two components that contribute to dramatic change in resistance. To date, the noble metals and p-type metal oxides, a component of the heterostructure systems, have been extensively applied as good catalysts owing to their excellent selectivity and sensitivity to analyte gases. In particular, much effort has been devoted to the fabrication and processing of nanoscale quasi-1D metal-oxide-based heterostructural materials for a variety of practical applications especially as gas sensors. In this review, we first introduce the two-step preparation process of various quasi-1D nanostructures and then discuss how the primary parameters, e.g. the amount of activated adsorption sites, the gas diffusion ability, and the coverage the secondary materials, can have an impact on the gas-sensing performances. We finally review gas-sensing mechanism in terms of the species of heterojunctions, such as the Schottky-junction, p – n or n – p junction, and n – n or p – p junction. [ABSTRACT FROM AUTHOR]

NO 2 is an oxidizing gas, which is considered to be the most dangerous and most toxic gas that should be controlled. Because of the advantages of high sensitivity, selectivity, stability and fast response/recovery speed, the chemical resistance sensor is used as a NO 2 gas sensor. This article reviews the NO 2 gas sensor based on metal oxides. Firstly, the hazards of NO 2 and the major disadvantages of chemiresistive sensors based on metal oxides were discussed. Then, different sensitive materials of chemical resistance sensors were discussed, including metal oxides (MO), conductive polymers and carbon-based materials. Finally, the NO 2 gas sensors based on metal oxide (including ZnO, NiO, CuO, SnO 2 , WO 3) were discussed from the aspects of morphology, preparation method, modification, doping and composite. [ABSTRACT FROM AUTHOR]

Metal oxide semiconductor (MOS)-based core-shell nanostructures with the unique structural and synthetic superiority have been universally applied for the chemiresistive gas sensors. We summarize the recent work involving MOS@MOS and noble metal@MOS core-shell nanostructures and further relate the improved performance to the work function-induced electron transfer through the N-N, N-P or Schottky junction. Generally, the performance can be enhanced if electrons transfer from the shell to the core and the heterostructure shows the same N- or P- type behavior to the shell MOS. Besides, the shell thickness is regarded to play a significant role in the performance modulation including the N-to-P (or P-to-N) conversion. The utilization of noble metals can sometimes change energy barriers and reaction routes of gas sensing, especially the adoption of Pd in detecting H 2. Moreover, alloying which can prevent the oxidation of noble metals, and constructing more complicated multi-shell nanostructures are both discussed as the potential development directions in this field. [Display omitted] [ABSTRACT FROM AUTHOR]

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]