Abhishek Kumar Mishra

Synthesis, characterization and DFT studies of zinc-doped copper oxide nanocrystals for gas sensing applications

Due to their unique properties, p-type copper oxide nanostructures have demonstrated promising potential for various applications, especially for the detection of ethanol vapour and other volatile organic compounds (VOCs). In this work a simple and cost-effective synthesis from chemical solutions (SCS) at low temperatures (≤80 °C) and rapid thermal annealing (RTA) process were used to grow zinc-doped copper oxide (ZnxCu1−xOy) nanostructures. The structural, morphological, vibrational, chemical, electronic and sensorial characteristics of ZnxCu1−xOy nanocrystallite layers obtained by using such an efficient approach based on both, the SCS and RTA processes, have been studied. The investigations demonstrated the possibility to tune sensitivity from VOC to H2, as well as an improved response and high selectivity with respect to hydrogen gas for ZnxCu1−xOy nano-crystalline thin films with x = 0.03. Density functional theory calculations showed that the charge transfer together with changes in the Fermi level facilitate H2 gas sensing, which is further enhanced by Zn doping. Hydrogen gas sensing with a high response and selectivity using p-type hybrid semiconductor nanostructures has been reported. An improved stability in humid air was observed by exposure of doped samples to rapid thermal annealing process for the first time. The experimental and calculation results provide an alternative to sensitive and selective detection of ethanol and hydrogen gases, which would be of particular benefit in the area of public security, industrial and environmental applications.

Hybridization of Zinc Oxide Tetrapods for Selective Gas Sensing Applications

In this work, the exceptionally improved sensing capability of highly porous three-dimensional (3-D) hybrid ceramic networks toward reducing gases is demonstrated for the first time. The 3-D hybrid ceramic networks are based on doped metal oxides (MexOy and ZnxMe1-xOy, Me = Fe, Cu, Al) and alloyed zinc oxide tetrapods (ZnO-T) forming numerous junctions and heterojunctions. A change in morphology of the samples and formation of different complex microstructures is achieved by mixing the metallic (Fe, Cu, Al) microparticles with ZnO-T grown by the flame transport synthesis (FTS) in different weight ratios (ZnO-T:Me, e.g., 20:1) followed by subsequent thermal annealing in air. The gas sensing studies reveal the possibility to control and change/tune the selectivity of the materials, depending on the elemental content ratio and the type of added metal oxide in the 3-D ZnO-T hybrid networks. While pristine ZnO-T networks showed a good response to H2 gas, a change/tune in selectivity to ethanol vapor with a decrease in optimal operating temperature was observed in the networks hybridized with Fe-oxide and Cu-oxide. In the case of hybridization with ZnAl2O4, an improvement of H2 gas response (to ∼7.5) was reached at lower doping concentrations (20:1), whereas the increase in concentration of ZnAl2O4 (ZnO-T:Al, 10:1), the selectivity changes to methane CH4 gas (response is about 28). Selectivity tuning to different gases is attributed to the catalytic properties of the metal oxides after hybridization, while the gas sensitivity improvement is mainly associated with additional modulation of the electrical resistance by the built-in potential barriers between n-n and n-p heterojunctions, during adsorption and desorption of gaseous species. Density functional theory based calculations provided the mechanistic insights into the interactions between different hybrid networks and gas molecules to support the experimentally observed results. The studied networked materials and sensor structures performances would provide particular advantages in the field of fundamental research, applied physics studies, and industrial and ecological applications.

A density functional theory study of the adsorption behaviour of CO2 on Cu2O surfaces

Copper has many applications, particularly in electro-catalysis, where the oxidation state of the copper electrode plays a significant role in the selectivity towards products. Although copper-based materials have clear potential as catalysts in the reduction of CO2 and conversion to products, fundamental understanding of CO2 adsorption and activation on different copper oxide surfaces is still limited. We have used DFT+U methodology to study the surface reconstruction of the three most exposed (111), (110), and (001) surfaces of Cu2O with different possible terminations. Considering several adsorbate geometries, we have investigated CO2 adsorption on five different possible terminations and proposed eight different configurations in which CO2 binds with the surface. Similar to earlier findings, CO2 binds weakly with the most stable Cu2O(111):O surface showing no molecular activation, whereas a number of other surfaces, which can appear in the Cu2O particles morphology, show stronger binding as well as activation of the CO2 molecule. Different CO2 coverages were studied and a detailed structural and electronic charge analysis is presented. The activation of the CO2 molecule is characterized by structural transformations and charge transfer between the surface and the CO2 molecule, which is further confirmed by considerable red shifts in the vibrational frequencies.