Core-shell Au@TiO2 nanoparticles with truncated wedge-shaped TiO2 morphology have been synthesized successfully by a simple and flexible hydrothermal route. Morphological evolution of TiO2 shells was investigated by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction technique. It has been revealed that the truncated wedge-shaped TiO2 shells experience an epitaxially segmented orientation growth. Also, the (101) crystal planes of TiO2 crystals grow preferentially on the surface of gold nanocrystals stabilizing the heterointerfaces, then faster [001] growth results in the "budding" process occurs, producing growth sites on the initial deposition TiO2 layers, where the TiO2 crystals grow up into truncated wedge-shaped morphologies. It is also found that morphological evolution of TiO2 shells is dependent on the produced F- ion concentration from hydrolyzed TiF4 precursors. The produced F- ions not only facilitate the formation of well-defined wedge-like TiO2 shells, but also contribute to the externally exposed truncated crystal {004} facets. As the representative photocatalyst, the catalytic activities of the resultant core-shell Au@TiO2 nanoparticles were investigated by photoinitiated oxidation degradation of gaseous acetaldehyde. It has been indicated that the nanostructured core-shell Au@TiO2 photocatalyst represents high photocatalytic activity when exposed to UV or visible light irradiation. The high phototocatalytic performance is also largely attributed to the preferentially grown TiO2 shell structures and metal (Au)-TiO2 heterointerfaces.

We successfully prepared Au@ZnO core-shell nanoparticles (CSNPs) by a facile low-temperature solution route and studied its gas-sensing properties. The obtained Au@ZnO CSNPs were carefully characterized by X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, and UV-visible spectroscopy. Mostly spherical-shaped Au@ZnO CSNPs were formed by 10-15 nm Au NPs in the center and by 40-45 nm smooth ZnO shell outside. After the heat-treatment process at 500 °C, the crystallinity of ZnO shell was increased without any significant change in morphology of Au@ZnO CSNPs. The gas-sensing test of Au@ZnO CSNPs was examined at 300 °C for various gases including H2 and compared with pure ZnO NPs. The sensor Au@ZnO CSNPs showed the high sensitivity and selectivity to H2 at 300 °C. The response values of Au@ZnO CSNPs and pure ZnO NPs sensors to 100 ppm of H2 at 300 °C were 103.9 and 12.7, respectively. The improved response of Au@ZnO CSNPs was related to the electronic sensitization of Au NPs due to Schottky barrier formation. The high selectivity of Au@ZnO CSNPs sensor toward H2 gas might be due to the chemical as well as catalytic effect of Au NPs.

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This feature article focuses on recent research progress in noble metal@metal oxides core@shell NPs for gas sensor applications.