Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

Controllable integration of noble metals (e.g., Au, Ag, Pt, and Pd) and metal oxides (e.g., TiO2, CeO2, and ZrO2) into single nanostructures has attracted immense research interest in heterogeneous catalysis, because they not only combine the properties of both noble metals and metal oxides, but also bring unique collective and synergetic functions in comparison with single-component materials. Among many strategies recently developed, one of the most efficient ways is to encapsulate and protect individual noble metal nanoparticles by a metal oxide shell of a certain thickness to generate the core–shell or yolk–shell structure, which exhibits enhanced catalytic performance compared with conventional supported catalysts. In this review article, we summarize the state-of-the art progress in synthesis and catalytic application of noble metal nanoparticle@metal oxide core/yolk–shell nanostructures. We hope that this review will help the readers to obtain better insight into the design and application of well-defined nanocomposites in both the energy and environmental fields.

Noble metal nanoparticle@metal oxide core/yolk-shell nanostructures as catalysts: recent progress and perspective.

With an ever-increasing number of applications in many advanced fields, gas sensors are becoming indispensable devices in our daily life. Among different types of gas sensors, conductometric metal oxide semiconductor (MOS) gas sensors are found to be the most appealing for advanced applications in the automotive, biomedical, environmental, and safety sectors because of the their high sensitivity, reduced size, and low cost. To improve their sensing characteristics, new metal oxide-based nanostructures have thus been proposed in recent years as sensing materials. In this review, we extensively review gas-sensing properties of core@ shell nanocomposites in which metals as the core and metal oxides as the shell structure, both of nanometer sizes, are assembled into a single metal@metal oxide core–shell. These nanostructures not only combine the properties of both noble metals and metal oxides, but also bring unique synergetic functions in comparison with single-component materials. Up-dated achievements in the synthesis and characterization of metal@metal oxide core–shell nanostructures as well as their use in MOS sensors are here reported with the main objective of providing an overview about their gas-sensing properties.

Metal-core@metal oxide-shell nanomaterials for gas-sensing applications: a review

As motivated by the driving force of the urgent demand for high-performance nitrogen dioxide (NO2) sensor, in this work, a novel structure of Au@WO3 core–shell nanospheres (CSNSs) was designed and successfully prepared for NO2 detection. The techniques of X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller measurement, and elemental mapping analysis were used for the characterizations of the obtained samples. The results demonstrated that Au nanoparticles with 25–50 nm in diameter were successfully encapsulated by WO3 shells with the thickness of 30–50 nm. The NO2 sensing performance of the Au@WO3 CSNSs as well as the pure WO3 nanospheres were systematically investigated. Compared with the pure WO3 nanospheres, Au@WO3 CNNSs exhibited overall enhanced NO2 sensing performances in terms of response, detection limit, and response/recovery times. At the optimal operating temperature of 100 °C, Au@WO3 CSNSs showed excellent NO2 selectivity and long-term stability. Notably, the excellent NO2 sensing performance of Au@WO3 CSNSs was slightly affected by humidity. The possible sensing mechanism of the enhanced NO2 sensing properties of the Au@WO3 CSNSs was discussed.

Design of Au@WO3 core−shell structured nanospheres for ppb-level NO2 sensing

Among the complex nanostructures, core@shell nanomaterials are gaining much attention, as the physical properties of the core and shell can be easily and separately tuned. Two materials in the form of core@shell nanostructures combine their individual properties and also bring unique properties in comparison with single-component materials. Recently, the formation of core@shell nanoparticles (NPs) having noble metals (Au, Ag, Pt and Pd) as a core and metal oxides semiconductors (TiO2, SnO2, and Cu2O) as a shell has attracted immense research interest in sensing, photo-catalysis, dye-sensitized solar cells and so on due to tailorability and functionality in the core and shell. Therefore, an overview of the advances in this exciting field of noble metals@metal oxides core@shell NPs has been presented in this feature article. It includes systematic synthesis approaches of noble metal@metal oxide core@shell NPs and their applications in the field of gas sensors, which is based on the literature and our own recent work. The synthesis of core@shell NPs with controllable sizes, compositions, morphologies, structures and functionalities has been presented considering the advantages and the demerits of the process. Applications of these core@shell NPs in the areas of gas sensing and their sensing mechanisms are discussed. The future prospects of such core@shell nanostructures for gas sensing applications are also highlighted.

Noble metal@metal oxide semiconductor core@shell nano-architectures as a new platform for gas sensor applications

A nano-composite core–shell Ag@SnO 2 material was synthesized by chemical reduction and characterized by temperature-programmed reduction (TPR), UV–visible reflective spectroscopy, high-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray spectroscopy (EDS). SnO 2 , Ag/SnO 2 and Ag@SnO 2 were used to sense ethanol (C 2 H 5 OH) in an ethanol gas detection system. The SnO 2 material under 200 ppm ethanol gas exhibited a sensor response ( S  =  R air / R ethanol  = (resistance of sensor signal of air)/(resistance of sensor signal of ethanol gas)), response time ( t 90 ) and recovery time ( t R90 ) were 1.54, 54 s and 85 s, respectively. Ag@SnO 2 materials had an improved sensor response of 2.24, with a shortened response time ( t 90 ) and recovery time ( t R90 ) of 34 s and 68 s, respectively. The bond strength between ethanol gas and the Ag@SnO 2 on which was adsorbed, determined by van der Waal's forces, and was calculated for a case in which more than 20 molecules were adsorbed. The mechanism of the sensing of ethanol gas, based on its adsorption, its reaction on the surface and its desorption is presented.

Da-Jun Lin

Papers

Ag@SnO2 core–shell material for use in fast-response ethanol sensor at room operating temperature