Changhyun Jin

Hydrogen gas sensing of Co3O4-Decorated WO3 nanowires

Co3O4 nanoparticle-decorated WO3 nanowires were synthesized by the thermal oxidation of powders followed by a solvothermal process for Co3O4 decoration. The Co3O4 nanoparticle-decorated WO3 nanowire sensor exhibited a stronger and faster electrical response to H2 gas at 300 °C than the pristine WO3 nanowire counterpart. The former showed faster response and recovery than the latter. The pristine and Co3O4-decorated WO3 nanowire sensors showed the strongest response to H2 gas at 225 and 200 °C, respectively. The Co3O4-decorated WO3 nanowire sensor showed selectivity for H2 gas over other reducing gases. The enhanced sensing performance of the Co3O4-decorated WO3 nanowire sensor was explained by a combination of mechanisms: modulation of the depletion layer width forming at the Co3O4-WO3 interface, modulation of the potential barrier height forming at the interface, high catalytic activity of Co3O4 for the oxidation of H2, active adsorption of oxygen by the Co3O4 nanoparticle surface, and creation of more active adsorption sites by Co3O4 nanoparticles.

Temperature-dependent selectivity of bead-like TeO2 nanostructured gas sensors

Bead-like p-TeO2 nanowires (NWs) were obtained directly from Te powder by thermal evaporation along with assisting 40 sccm of O2 gas. Scanning and transmission electron microscopy showed that the two different formation origins for the two types of bead-like TeO2 NWs take place at the vicinity of the surface and are characterized by (1) the presence of locally higher concentration and saturation of O2, and (2) the surface state (terraces, ledges, and kinks (TLK)) of Te reacting with adsorbed oxygen atoms. In the present work, the gas-sensing performances of bead-like p-TeO2 NW gas sensors fabricated using a facile and low-temperature route has been investigated for the detection of nitric dioxide (NO2), ethanol (C2H5OH), and hydrogen sulfide (H2S) gases contained in human breath. Specifically, this work systematically investigates the gas response of bead-like p-TeO2 NW sensors in dependence of temperature in the range from 200 °C to 400 °C. The sensing capabilities of bead-like p-TeO2 NWs are investigated with respect to C2H5OH, NO2, and H2S without any artificial handling, such as surface modification, n- and/or p-type doping, and heterostructure formation. The selectivity of bead-like p-TeO2 NWs by adjusting the operating temperature is dependent on the target gas, which is attributed to an interaction between p-TeO2 NWs and gas molecules at a specific operating temperature. In particular, the bead-like p-TeO2 NWs show quite superior sensitivity and selectivity to C2H5OH, NO2, and H2S at 250, 350, and 400 °C, respectively. Thus, bead-like TeO2 NWs are promising for the detections of C2H5OH, NO2, and H2S with high selectivity at the ppm level and may contribute to the realization of more selective NW sensors.

Design and synthesis of amorphous SiOx structures generated by Sn quantum dots: growth mechanism and luminescent origin.

SiOx structures with different diameters of a few hundreds of nanometers and/or a few micrometers are prepared using applied thermal evaporation. Subsequently, Sn quantum dot-based SiOx architectures are synthesized via the continuous steps of the carbothermal reduction of SnO2, substitution of Sn(4+) for In(3+), thermal oxidation of Si, Sn sublimation, interfacial reaction, and diffusion reaction consistent with corresponding phase equilibriums. Several crystalline and spherical-shaped Sn quantum dots with diameters between 2 and 7 nm are observed in the amorphous SiOx structures. The morphological evolution, including hollow Sn (or SnOx) sphere and wire-like, worm-like, tube-like, and flower-like SiOx, occurs stepwise on the Si substrate upon increasing the given process energies. The optical characteristics based on confocal measurements reveal the as-synthesized SiOx structures, irrespective of whether crystallinity is formed, which all have visible-range emissions originating from the numerous different-sized and -shaped Sn quantum dots permeating into the SiOx matrix. In addition, photoluminescence emissions ranging between ultraviolet and red regions are in agreement with confocal measurements. The origins of the morphology- and luminescence-controlled amorphous SiOx with Sn quantum dots are also discussed.