Sung Yong Kang

Selective Improvement of NO2 Gas Sensing Behavior in SnO2 Nanowires by Ion-Beam Irradiation

We irradiated SnO2 nanowires with He ions (45 MeV) with different ion fluences. Structure and morphology of the SnO2 nanowires did not undergo noticeable changes upon ion-beam irradiation. Chemical equilibrium in SnO2/gas systems was calculated from thermodynamic principles, which were used to study the sensing selectivity of the tested gases, demonstrating the selective sensitivity of the SnO2 surface to NO2 gas. Being different from other gases, including H2, ethanol, acetone, SO2, and NH3, the sensor response to NO2 gas significantly increases as the ion fluence increases, showing a maximum under an ion fluence of 1 × 10(16) ions/cm(2). Photoluminescence analysis shows that the relative intensity of the peak at 2.1 eV to the peak at 2.5 eV increases upon ion-beam irradiation, suggesting that structural defects and/or tin interstitials have been generated. X-ray photoelectron spectroscopy indicated that the ionic ratio of Sn(2+/)Sn(4+) increases by the ion-beam irradiation, supporting the formation of surface Sn interstitials. Using thermodynamic calculations, we explained the observed selective sensing behavior. A molecular level model was also established for the adsorption of NO2 on ion-irradiated SnO2 (110) surfaces. We propose that the adsorption of NO2-related species is considerably enhanced by the generation of surface defects that are comprised of Sn interstitials.

Microwave-Assisted Synthesis of Graphene–SnO2 Nanocomposites and Their Applications in Gas Sensors

We obtained extremely high and selective sensitivity to NO2 gas by fabricating graphene-SnO2 nanocomposites using a commercial microwave oven. Structural characterization revealed that the products corresponded to agglomerated structures of graphene and SnO2 particles, with small secondary SnOx (x ≤ 2) nanoparticles deposited on the surfaces. The overall oxygen atomic ratio was decreased with the appearance of an SnOx (x < 2) phase. By the microwave treatment of graphene-SnO2 nanocomposites, with the graphene promoting efficient transport of the microwave energy, evaporation and redeposition of SnOx nanoparticles were facilitated. The graphene-SnO2 nanocomposites exhibited a high sensor response of 24.7 for 1 ppm of NO2 gas, at an optimized temperature of 150 °C. The graphene-SnO2 nanocomposites were selectively sensitive to NO2 gas, in comparison with SO2, NH3, and ethanol gases. We suggest that the generation of SnOx nanoparticles and the SnOx phase in the matrix results in the formation of SnO2/SnO2 homojunctions, SnO2/SnOx (x < 2) heterojunctions, and SnO2/graphene heterojunctions, which are responsible for the excellent sensitivity of the graphene-SnO2 nanocomposites to NO2 gas. In addition, the generation of surface Sn interstitial defects is also partly responsible for the excellent NO2 sensing performance observed in this study.

Surprising synthesis of nanodiamond from single-walled carbon nanotubes by the spark plasma sintering process

Nanodiamond (ND) was successfully synthesized using single-walled carbon nanotubes (SWCNTs) as a pure solid carbon source by means of a spark plasma sintering process. Raman spectra and X-ray diffraction patterns revealed the generation of the cubic diamond phase by means of the SPS process. Lattice-resolved TEM images confirmed that diamond nanoparticles with a diameter of about ∼10 nm existed in the products. The NDs were generated mainly through the gas-phase nucleation of carbon atoms evaporated from the SWCNTs.