Sanjit Manohar Majhi

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

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.

Facile Approach to Synthesize Au@ZnO Core–Shell Nanoparticles and Their Application for Highly Sensitive and Selective Gas Sensors

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.

Effect of Au nanorods on potential barrier modulation in morphologically controlled Au@Cu2O core-shell nanoreactors for gas sensor applications.

In this work, Au@Cu2O core-shell nanoparticles (NPs) were synthesized by simple solution route and applied for CO sensing applications. Au@Cu2O core-shell NPs were formed by the deposition of 30-60 nm Cu2O shell layer on Au nanorods (NRs) having 10-15 nm width and 40-60 nm length. The morphology of Au@Cu2O core-shell NPs was tuned from brick to spherical shape by tuning the pH of the solution. In the absence of Au NRs, cubelike Cu2O NPs having ∼200 nm diameters were formed. The sensor having Au@Cu2O core-shell layer exhibited higher CO sensitivity compared to bare Cu2O NPs layer. Tuning of morphology of Au@Cu2O core-shell NPs from brick to spherical shape significantly lowered the air resistance. Transition from p- to n-type response was observed for all devices below 150 °C. It was demonstrated that performance of sensor depends not only on the electronic sensitization of Au NRs but also on the morphology of the Au@Cu2O core-shell NPs.

Synthesis of plasmonic Ag@SnO2 core–shell nanoreactors for xylene detection

Ag@SnO2 core–shell nanoparticles (NPs) were prepared by a microwave-assisted hydrothermal method. The Ag NPs were synthesized by colloidal method and their size (10–24 nm) was controlled by the amount of reducing and stabilizing agents added. The size of Ag NPs was increased and subsequently their surface plasmon (SP) band was red-shifted with increasing reducing agent amount. A SnO2 NP shell was deposited on Ag NPs by microwave-assisted hydrothermal method. The size of Ag@SnO2 core–shell NPs was within 50 nm in diameter, which was composed of 15–18 nm Ag NPs and a 10–15 nm SnO2 shell. The SP band of Ag NPs was red-shifted with SnO2 shell formation. Ag@SnO2 core–shell NPs showed higher response to p-xylene as compared to other interfering gases (NO2, HCHO, CO and H2). The maximum response of Ag@SnO2 core–shell NPs to 5 ppm p-xylene was 16.17, whereas the maximum response of bare SnO2 was 10.79 to 5 ppm H2. The response of Ag@SnO2 core–shell NPs to 5 ppm p-xylene was approximately 7 times higher than that of bare SnO2 NPs. The improved gas sensing performance of Ag@SnO2 core–shell NPs was attributed to the electronic as well as catalytic activity of Ag NPs. It was proposed that the selective detection of p-xylene was attributed to the effective inwards diffusion of p-xylene through SnO2 shells and their subsequent dissociation into smaller and more active species by Ag NP catalysts on the inner part of the SnO2 shell.

Synthesis of Pd@ZnO Core-shell Nanoparticles with Different Size and Their Gas Sensing Properties.

Two different sizes of Pd@ZnO core-shell nanoparticles (NPs) have been prepared by using two different sizes of Pd NPs (15 and 50 nm) as metal cores and applied for acetaldehyde gas sensing. Transmission electron microscopy images revealed that the overall size of two sensing materials such as Pd15@ZnO and Pd50@ZnO core-shell NPs are 80-100 nm and 100-120 nm, respectively. Xray-diffraction pattern revealed that the oxidation of Pd metal core was started from 300C. The spherical shape and size are maintained after the Pd@ZnO core-shell NPs was calcined at 500C for 2 h. PdO15@ZnO core-shell NPs showed higher response to acetaldehyde. The maximum response of PdO15@ZnO core-shell NPs to 100 ppm of acetaldehyde at 350 C was 75, whereas the maximum response of PdO50@ZnO core-shell NPs to 100 ppm of acetaldehyde was 28 as compared to the pure ZnO NPs (Rs=18). The high response of PdO15@ZnO coreshell NPs than PdO50@ZnO core-shell NPs is due to the smaller size of PdO core, which has more catalytic activity than 50 nm sized PdO core.