Prabhakar Rai

Solvothermal synthesis of ZnO nanostructures and their morphology-dependent gas-sensing properties.

Single-crystalline ZnO nanostructures were synthesized by solvothermal method using methanol as solvent. The effect of counterions of zinc salts (nitrate, acetate, and chloride) on the morphology of ZnO nanostructures was investigated. ZnO nanorods (NRs) were formed for all kinds of zinc salts except zinc chloride, where nanoparticles (NPs) were formed. The length and width of ZnO NRs were 100-150 nm and 20-25 nm, respectively, whereas NPs were 20-25 nm in diameter. Replacing methanol to ethanol generated only NRs for all kinds of zinc salts and they were about 10 times larger than those in methanol. The effect of morphology on sensing property was investigated by comparing their response. ZnO NRs showed very high response as compared to ZnO NPs for NO2 and vice versa for CO, although the surface area of ZnO NPs (42.83 m(2)/g) was much higher than those of ZnO NRs (17.6 m(2)/g). The response of ZnO NRs was 30 times higher than those of NPs for NO2 gas, whereas 4 times lower for CO gas. The maximum response of as prepared ZnO NRs was 44.2 to 50 ppm of NO2 gas at 300 °C. A relationship between morphology and interelectrode gap was established. It was demonstrated that the number of grains present between interelectrode gaps has significantly affected the response.

Design of highly sensitive and selective Au@NiO yolk–shell nanoreactors for gas sensor applications

Au@NiO yolk-shell nanoparticles (NPs) were synthesized by simple solution route and applied for efficient gas sensor towards H₂S gas. Carbon encapsulated Au (Au@C core-shell) NPs were synthesized by glucose-assisted hydrothermal method, whereas Au@NiO yolk-shell NPs were synthesized by precipitation method using Au@C core-shell NPs as a template. Sub-micrometer Au@NiO yolk-shell NPs were formed having 50-70 nm Au NPs at the periphery of NiO shell (10-20 nm), which was composed of 6-12 nm primary NiO particles. Au@NiO yolk-shell NPs showed higher response for H2S compared to other interfering gases (ethanol, p-xylene, NH₃, CO and H₂). The maximum response was 108.92 for 5 ppm of H₂S gas at 300 °C, which was approximately 19 times higher than that for the interfering gases. The response of Au@NiO yolk-shell NPs to H₂S was approximately 4 times higher than that of bare NiO hollow nanospheres. Improved performance of Au@NiO yolk-shell NPs was attributed to hollow spaces that allowed the accessibility of Au NPs to gas molecules. It was suggested that adsorption of H₂S on Au NPs resulted in the formation of sulfide layer, which possibly lowered its work function, and therefore tuned the electron transfer from Au to NiO rather NiO to Au, which leaded to increase in resistance and therefore response.

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.

Role of Pd nanoparticles in gas sensing behaviour of Pd@In2O3 yolk–shell nanoreactors

Pd@In2O3 yolk–shell nanoparticles (NPs) were synthesized by a simple solution route using Pd@C core–shell NPs as template and applied for gas sensing. A glucose-assisted hydrothermal method was used for the synthesis of Pd@C core–shell NPs. Pd@In2O3 yolk–shell NPs were formed after calcination (450 °C for 3h) of Pd@C core–shell NPs containing indium precursor. In the Pd@In2O3 yolk–shell geometry, about 50–70 nm Pd NPs were present at the periphery of an In2O3 shell (10–20 nm thickness). The In2O3 shell was composed of ∼10 nm primary particles. The role of Pd NPs in gas sensing behavior of In2O3 has been investigated. The loading of In2O3 with Pd NPs improved the response for reducing gases, but reduced the response for oxidizing gases. The response of Pd@In2O3 yolk–shell NPs to ethanol was approximately 14 times higher than that of pure In2O3 hollow nanospheres at 350 °C. However, no response was recorded for NO2 for Pd@In2O3 as compared to In2O3 (resistance ratio Rs = 2.50) at 350 °C. The maximum response of Pd@In2O3 yolk–shell NPs to 5 ppm ethanol was 159.02 at 350 °C, which was approximately 2.5 times higher than those for other interfering gases (NO2, p-xylene, trimethylamine, HCHO, CO and H2). The effect of humidity on the gas sensing characteristics of Pd@In2O3 yolk–shell NPs suggested that the present sensor can be used to detect ppm-level ethanol even in highly humid atmosphere (80% RH). The improved gas sensing performance of Pd@In2O3 yolk–shell NPs was attributed to catalytic activity of Pd NPs as well as hollow spaces that allowed the accessibility of Pd NPs to gas molecules.

Synthesis of TiO2 hollow spheres by selective etching of Au@TiO2 core–shell nanoparticles for dye sensitized solar cell applications

Heterostructured Au@TiO2 core–shell nanoparticles (NPs) were synthesized by a microwave assisted hydrothermal method. A colloidal method was used to synthesize 40 ± 5 nm Au NPs, whereas a microwave-assisted hydrothermal method was used to deposit a TiO2 shell layer with 60 ± 10 nm shell thickness on Au NPs. The average size of TiO2 NPs was 17 ± 2 nm and size was increased with increasing reaction temperature without considerable change in shell thickness. The stability of Au@TiO2 core–shell NPs in iodide electrolyte solution was examined. It was found that the Au NPs are unstable in the iodide electrolyte and lost their surface plasmon resonance (SPR) characteristics. Hollow TiO2 NPs (150–200 nm in diameter) were produced by selective etching of as-prepared Au@TiO2 core–shell NPs in KCN solution. The final hollow TiO2 spheres were applied as a scattering layer on top of a nanocrystalline TiO2 film, serving as the photoanode of dye sensitized solar cells (DSCs). A high efficiency of 7.40% was achieved with TiO2 hollow spheres, compared with 5.21% for the electrode with commercial TiO2. It was also found that the efficiency increased with increasing crystallinity of TiO2 NPs. The increment in efficiency was related to efficient light scattering, electrolyte diffusing feasibility for better electron transport, and a high surface area for higher dye loading.

3D urchin-shaped Ni3(VO4)2 hollow nanospheres for high-performance asymmetric supercapacitor applications

3D urchin-shaped Ni3(VO4)2 hollow nanospheres were synthesized by a facile, template free, hydrothermal method. The size of the urchin-shaped Ni3(VO4)2 hollow nanospheres was ∼500 nm and they were composed of ∼10 nm thick sheet-like building block units. The morphological evolution was sensitive to alkaline media and ∼50 nm nanoparticles were formed when liquid ammonia was replaced by sodium hydroxide. The formation of [Ni(NH3)6]2+ complex ions (hexaamminenickel(II) ions) and subsequent slow release of nickel ions to the growing crystal seem to have resulted in the formation of hollow urchin-shaped nanostructures. The electrochemical supercapacitor properties of these two nanostructures were investigated and it was found that the urchin-shaped nanospheres exhibited better performance than the nanoparticles in all respects. The as-fabricated porous urchin-shaped Ni3(VO4)2 nanosphere electrode exhibited a specific capacity of 402.8 C g−1 at 1 A g−1 with enhanced rate capability and an excellent capacity retention of 88% after 1000 cycles. An asymmetric supercapacitor was fabricated using Ni3(VO4)2 nanospheres as the cathode and activated carbon (AC) as the anode and the electrochemical properties were studied at various scan rates in the potential range of 0.0–1.6 V. The as-fabricated asymmetric supercapacitor (Ni3(VO4)2//AC) achieved a high specific capacity (114 C g−1), energy density (25.3 W h kg−1) and power density (240 W kg−1). Moreover, this asymmetric supercapacitor displayed an excellent life cycle with 92% specific capacity retention after 1000 consecutive charge–discharge cycles. The impressive electrochemical performance of the Ni3(VO4)2 nanospheres, owing to their large surface area, pore volume and 3D structure, makes them a promising candidate for the future high energy storage systems.

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.

Facile synthesis of Cu2O microstructures and their morphology dependent electrochemical supercapacitor properties

In this study, Cu2O microcubes and microspheres were synthesized using facile hydrothermal methods by manipulating the synthesis parameters. The Cu2O microcubes (∼2 μm in diameter) were formed in presence of formic acid, whereas hierarchical Cu2O microspheres (∼5 μm in diameter) were formed in acetic acid. Transmission electron microscopy (TEM) confirmed the formation of single crystalline microcubes and polycrystalline microspheres. The possible growth mechanism suggested that microcubes were formed due to the cubic crystal structure of Cu2O and the formation kinetics, whereas microspheres were formed due to the orientational attachment of nuclei with similar aggregation velocities along every direction. The electrochemical properties of the Cu2O microcubes and microspheres were investigated to understand the role of the morphology on the supercapacitor properties. The Cu2O microcubes exhibited a higher specific capacitance, better rate capability and cycling stability as compared to microspheres, although the particle size and pore size were larger and surface area was lower. The specific capacitance of the Cu2O microcubes and microspheres were 660 and 516 F g−1, respectively, at a 1 A g−1 current density. The Cu2O microcubes showed 80% specific capacitance retention at a 5 A g−1 current density after 1000 cycles. The single crystalline nature and the presence of a smaller number of grain boundaries in the microcubes compared to the microspheres resulted in an increase in conductivity and an increase in capacitance. The results showed that the Cu2O microcubes can be a promising electrode material for high performance supercapacitors.