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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]