Yeon-Tae Yu

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.

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 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.

Synthesis of Au/TiO2 Core–Shell Nanoparticles from Titanium Isopropoxide and Thermal Resistance Effect of TiO2 Shell

On the synthesis of Au/TiO2 core–shell structure nanoparticles, the effect of the concentration of Ti4+ on the morphology and optical property of Au/TiO2 core–shell nanoparticles was examined. A gold colloid was prepared by mixing HAuCl44H2O and C6H5Na32H2O. Titanium stock solution was prepared by mixing solutions of titanium(IV) isopropoxide (TTIP) and triethanolamine (TEOA). The concentration of the Ti4+ stock solution was adjusted to 0.01–0.3 mM, and then the gold colloid was added to the Ti4+ stock solution. Au/TiO2 core–shell structure nanoparticles could be prepared by the hydrolysis of the Ti4+ stock solution at 80 °C. The size of the as-prepared Au nanoparticles was 15 nm. The thickness of the TiO2 shell on the surface of gold particles was about 10 nm. The absorption peak of the Au/TiO2 core–shell nanoparticles shifted towards the red end of the spectrum by about 3 nm because of the formation of the TiO2 shell on the surface of the gold particles. The crystal structure of the TiO2 shell showed an anatase phase. The increase in the Au crystallite size of the Au/TiO2 nanoparticles with increasing heat treatment temperature is smaller than that in the pure Au nanoparticles. This may be due to the encapsulation of Au particles with the TiO2 shell that prevents the growth of the nanoparticle nucleation.

Spectroscopic investigation of S–Ag interaction in ω-mercaptoundecanoic acid capped silver nanoparticles

This paper deals with the synthesis of omega-mercaptoundecanoic acid (MUA) capped silver nanoparticles (NPs) with an average size of 15nm by citrate reduction technique and spectroscopic investigation of S-Ag interaction. We have studied the interaction of thiol with silver NPs in aqueous medium by employing UV-vis, Raman, FT-IR, and photoluminescence spectroscopy. The shifting of silver surface plasmon band in the UV-vis spectra shows the stabilization of the silver nanoparticles by MUA. The disappearance of S-H stretching in both the FT-IR and Raman spectra and the shifting of the NMR signals of the protons in close proximity to the metal center supported the existence of the S-Ag interaction in MUA capped silver NPs. The morphology of the thiol protected silver NPs was investigated by transmission electron microscopy (TEM) and was found to be distinct and spherical entities.

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 and characterization of Au/TiO2 core-shell structure nanoparticles

Au/TiO2 core-shell structure nanoparticles were synthesized by sol-gel process, and the morphology and crystallinity of TiO2 shell were investigated by TEM and UV-vis absorption spectrometer. Au/TiO2 core-shell structure nanoparticles could be prepared by the hydrolysis of TOAA (titanium oxide acethylacetonate) in gold sol ethanol solution with water. The thickness of TiO2 shell on the surface of gold particles was about 1 nm. To investigate the crystallinity of TiO, shell, UV light with 254 nm and radioactive ray of60Co were irradiated on the TiO2-coated gold sol ethanol solution. The surface plasmon band of gold nanoparticles appeared only when the radioactive ray was irradiated on the TiO2-coated gold sol ethanol solution. From these results, it was found that the TiO, shell was amorphous and the MUA (mercaptoundecanoic acid) layer on the Au particle for its dispersion in ethanol did not act as an obstacle to disturb the movement of electrons onto the surface of Au particles.

Quantum efficiency control of InGaN/GaN multi-quantum-well structures using Ag/SiO2 core-shell nanoparticles

Photoluminescence (PL) efficiency increase up to 2.8 times was observed for GaN/InGaN multi-quantum-well (MQW) structures as a result of deposition of a thin layer of about 40-nm-diameter Ag nanoparticles (NPs) surrounded by SiO2 shell. These Ag/SiO2 NPs were prepared by sol-gel method. The amount of PL intensity enhancement decreased with increasing the SiO2 shell thickness. PL intensity increase was accompanied by corresponding decrease of PL decay time and is ascribed to a strong coupling of MQW region to localized surface plasmons (LSPs) associated with Ag/SiO2 NPs.