RESUMO
ZnO is a widely studied gas sensor material and is used in many commercial sensor devices. However, selectivity towards any particular gas remains an issue due to lack of complete knowledge of the gas sensing mechanism of oxide surfaces. In this paper, we have studied the frequency dependent gas sensor response of ZnO nanoparticles of a diameter of nearly 30 nm. A small rise of synthesis temperature from 85 °C to 95 °C in the solvothermal process, shows coarsening by joining and thereby distinct loss of grain boundaries as seen from transmission electron micrographs. This leads to a substantial reduction in impedance, Z (GΩ to MΩ), and rises in resonance frequencyfres(from 1 to 10 Hz) at room temperature. From temperature dependent studies it is observed that the grain boundaries show a Correlated Barrier Hopping mechanism of transport and the hopping range in the grain boundary region is typically 1 nm with a hopping energy of 153 meV. On the other hand, within the grain, it shows a change of transport type from low temperature tunneling to beyond 300 °C as polaron hopping. The presence of disorder (defects) as the hopping sites. The temperature dependence offresagrees with different predicted oxygen chemisorbed species between 200 °C to 400 °C. As opposed to the traditional DC response, the AC response in the imaginary part of (Zâ³) shows gas specific resonance frequencies for each gas, such as NO2, ethanol, and H2. Among the two reducing gases, ethanol and hydrogen; the former shows good dependence on concentration in Zâ³ whereas the latter shows a good response infresas well as capacitance. Thus, the results of frequency dependent response allow us to investigate greater details of the gas sensing mechanism in ZnO, which may be exploited for selective gas sensing.
RESUMO
Molybdenum disulfide (MoS2) has emerged as a promising material for the development of efficient sensors. Here, we have exfoliated and decorated MoS2 flakes with the novel, single-phase multi-component silver-gold-copper-palladium-platinum (Ag-Au-Cu-Pd-Pt) alloy nanoparticles, popularly named High Entropy Alloy (HEA) nanoparticles, using facile and scalable low-temperature grinding, followed by the sonochemical method. It was found that the decoration of HEA nanoparticles imparts the surface-enhanced Raman scattering effect and reduction in the work function of the material from 4.9 to 4.75 eV as measured by UV photoelectron spectroscopy. This change in the work function resulted in a Schottky barrier between the gold contact and HEA decorated MoS2 flakes as a result of drastic changes in the surface chemical non-stoichiometry. The response to hydrogen gas was studied at temperatures in the range of 30 to 100 °C, and it showed an unusual p-type nature due to surface-adsorbed oxygen species. The nanoscale junction formed between HEA and MoS2 showed a ten-time increase in the response towards hydrogen gas at 80 °C. The experimental observations have been explained with DFT simulation showing more favourable hydrogen adsorption on HEA-decorated MoS2 resulting in an enhanced response.
