Hierarchically three-dimensional (3D) porous ZnO architectures were synthesized by a template-free, economical hydrothermal method combined with subsequent calcination. First, a precursor of hierarchical basic zinc carbonate (BZC) nanostructures self-assembled by sheet-like blocks was prepared. Then calcination of the precursor produced hierarchically 3D porous ZnO architectures composed of interconnected ZnO nanosheets with high porosity resulting from the thermal decomposition of the precursor. The products were characterized by X-ray diffraction, Fourier tranform infrared spectroscopy, thermogravimetric-differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller N 2 adsorption-desorption analyses. Control experiments with variations in solvent and reaction time respectively revealed that ethanol was responsible for the formation of the BZC precursor, and the self-assembly of BZC nanosheets into hierarchically 3D architectures was highly dependent on the reaction time. Gas sensing tests showed that these hierarchically porous ZnO architectures were highly promising for gas sensor applications, as the gas diffusion and mass transportation in sensing materials were significantly enhanced by their unique structures. Moreover, it is believed that this solution-based approach can be extended to fabricate other porous metal oxide materials with a unique morphology or shape.

Polypyrrole-coated SnO 2 hollow spheres hybrid materials have been synthesized through an in situ polymerization of pyrrole monomers in the presence of preprepared SnO 2 hollow spheres. The hybrids were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), and thermogravimetric analysis (TG). Experimental data showed certain synergetic interaction existed in the hybrids, probably resulting in the enhanced thermal stability of polypyrrole coatings. Gas sensing tests showed that the hybrids possessed very fast response and high sensitivity to ammonia gas at room temperature, implying its potential application for gas sensors.

A novel sensing hybrid-material of Au nanoparticles (Au NPs)-functionalized ZnO nanowires (Au-ZnO NWs) was successfully synthesized by a two-stage solution process. First, ZnO NWs were fabricated via a low-temperature one-pot hydrothermal method with SDSN introduced as a structure-directing agent. Afterward, the as-prepared ZnO NWs were used as supports to load Au NPs with small sizes via precipitating HAuCl4 aqueous solution with ammonia. The obtained samples were characterized by means of XRD, SEM, TEM and EDX. Both pristine and Au-ZnO NWs were practically applied as gas sensors to compare the effect of Au NPs on the sensing performances and the obtained results demonstrated that after functionalization by catalytic Au NPs, the hybrid sensor exhibited not only faster response and recovery speeds but also a higher response to benzene and toluene than the pristine ZnO sensor at 340 °C, especially showing high selectivity and long-term stability for low concentration toluene, which is rarely reported with this method, indicating its original sensor application in detecting benzene and toluene. To interpret the enhanced gas sensing mechanism, the strong spillover effect of the Au NPs and the increased Schottky barriers caused by the electronic interaction between Au NPs and ZnO NW support are believed to contribute to the improved sensor performance.

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