Arun K. Prasad

The role of SnO2 quantum dots in improved CH4 sensing at low temperature

The role of quantum dots (QDs) of SnO2 in detecting low concentrations of methane (CH4) at a relatively low temperature of ∼150 °C with high response (S ∼ 3.5%) and response time below 1 min is reported. A simple room temperature single step chemical process was adopted for the growth of SnO2 nanoparticles of a size around 2.4 nm. These nanoparticles were subsequently annealed at 800 °C to increase the grain size to 25 nm. The as-prepared SnO2 nanoparticles, being smaller than the corresponding Bohr radius (2.7 nm), showed a strong quantum confinement effect with a blue shift in the band gap energy from 3.6 eV for the bulk SnO2 to 4.37 eV for the QDs. These QDs exhibited a strong sensing response to CH4 in comparison to the annealed sample. A low activation energy of 90 meV, as estimated from the temperature dependent S plot for SnO2 QDs, was found to be the driving force for such unusual high sensitivity at a low operating temperature. X-ray diffraction, transmission electron microscopy, along with Raman spectroscopy measurements are used for the detailed structural studies. The critical role of the chemisorbed oxygen species present at different operating temperatures on the surface of the off-stoichiometric quantum sized SnO2 and bulk-like annealed samples are discussed in light of the adsorption kinetics.

Influence of in-plane and bridging oxygen vacancies of SnO2 nanostructures on CH4 sensing at low operating temperatures

Role of “O” defects in sensing pollutant with nanostructured SnO2 is not well understood, especially at low temperatures. SnO2 nanoparticles were grown by soft chemistry route followed by subsequent annealing treatment under specific conditions. Nanowires were grown by chemical vapor deposition technique. A systematic photoluminescence (PL) investigation of “O” defects in SnO2 nanostructures revealed a strong correlation between shallow donors created by the in-plane and the bridging “O” vacancies and gas sensing at low temperatures. These SnO2 nanostructures detected methane (CH4), a reducing and green house gas at a low temperature of 50 °C. Response of CH4 was found to be strongly dependent on surface defect in comparison to surface to volume ratio. Control over “O” vacancies during the synthesis of SnO2 nanomaterials, as supported by X-ray photoelectron spectroscopy and subsequent elucidation for low temperature sensing are demonstrated.

CNT-ZnO Nanocomposite Thin Films: O2 and NO2 Sensing

The CNT-ZnO nanocomposites were synthesized by addition of commercially available MWCNT during growth of ZnO nanoparticles employing a wet chemical route. These nanocomposites were then spin coated and characterized using X-ray diffraction, scanning electron microscopy, current-voltage characteristics and O2 (5-20%) / NO2 (2-20 ppm) gas sensing at 250°C operating temperature in N2 atmosphere (0.4±0.03 mbar). The addition of CNT in ZnO is found to increase the sensitivity for both O2 and NO2 gas sensing. The 0.1 wt % CNT addition in ZnO is observed to appreciably enhance the NO2 gas sensitivity while 1.0 wt % CNT addition in ZnO showed highest sensitivity for O2 gas detection.

The role of SnO 2 quantum dots in improved CH 4 sensing at low temperature

The role of quantum dots (QDs) of SnO2 in detecting low concentrations of methane (CH4) at a relatively low temperature of ∼150 °C with high response (S ∼ 3.5%) and response time below 1 min is reported. A simple room temperature single step chemical process was adopted for the growth of SnO2 nanoparticles of a size around 2.4 nm. These nanoparticles were subsequently annealed at 800 °C to increase the grain size to 25 nm. The as-prepared SnO2 nanoparticles, being smaller than the corresponding Bohr radius (2.7 nm), showed a strong quantum confinement effect with a blue shift in the band gap energy from 3.6 eV for the bulk SnO2 to 4.37 eV for the QDs. These QDs exhibited a strong sensing response to CH4 in comparison to the annealed sample. A low activation energy of 90 meV, as estimated from the temperature dependent S plot for SnO2 QDs, was found to be the driving force for such unusual high sensitivity at a low operating temperature. X-ray diffraction, transmission electron microscopy, along with Raman spectroscopy measurements are used for the detailed structural studies. The critical role of the chemisorbed oxygen species present at different operating temperatures on the surface of the off-stoichiometric quantum sized SnO2 and bulk-like annealed samples are discussed in light of the adsorption kinetics.

Operating temperature effect in WO 3 films for gas sensing

WO₃ film sputtered on Al₂O₃ was used to sense ppm concentrations of NO₂ and NH₃ gas. The optimum working temperature of the amorphous film was found to be 200°C for both the gases, in accordance with literature. However, thermo-gravimetric analysis of the films predicts a different mechanism from that in literature which speculates that there is loss of water molecules from WO₃ at 200°C and therefore better sensing response. Thermo-gravimetric analysis shows that there is an increase in weight percentage in air at 200°C, which we speculate to be due to optimum surface oxygen content which leads to better sensing response. Raman spectroscopy at 200°C supports our speculation by showing no structural change in the WO₃ compound and no shifting of the ~700 cm¹ peak which is indicative of water loss. Furthermore, there was a marked change in the heating-cooling hysteresis at 200°C, which could result from the optimum surface oxygen content changing the electron transport properties The 200°C may also be regarded as a new transition temperature in WO₃ although the transition is not structural (electron-phonon coupling) but is electronic in nature (electron-electron correlation) and this transition temperature could be linked to the optimum sensing temperature of WO₃.

Enhanced ammonia sensing properties using Au decorated ZnO nanorods

Nanostructured Au-ZnO sensor systems have recently attracted attention for improving sensing behavior towards ethanol, acetone, hydrogen, CO, but rarely towards ammonia. In this report, an enhanced sensing response towards ammonia at near room temperatures using Au decorated ZnO nanorods (NR) is reported in comparison to pristine ZnO NR of hexagonal wurtzite structure. A facile two step synthesis method is adopted involving hydrothermal synthesis for the preparation of high aspect ratio ZnO NR of 40 nm diameter and 100 nm length followed by chemical growth methods for nanoparticle Au decoration of 5 nm average particle size. UV-Visible Diffuse Reflectance Spectroscopy confirms presence of Au nanoparticles along with ZnO NR with characteristic surface plasmon resonance and ZnO exciton peak. Gas sensing studies revealed that the Au-decorated ZnO NR can detect ammonia even at 50°C and shows highest response at 100°C than pristine ZnO NR which shows a response maximum at 300°C.

Native defect-assisted enhanced response to CH4 near room temperature by Al0.07Ga0.93N nanowires

Gas sensors at low operating temperature with high sensitivity require group III nitrides owing to their high chemical and thermal stabilities. For the first time, Al0.07Ga0.93N nanowires (NWs) have been utilized in CH4 sensing, and it has been demonstrated that they exhibit an improved response compared to GaN NWs at the low operating temperature of 50 °C. Al0.07Ga0.93N NWs have been synthesized via the ion beam mixing process using inert gas ion irradiation on the bilayer of Al/GaN NWs. The sensing mechanism is explained with the help of native defects present in the system. The number of shallow acceptors created by Ga vacancies (VGa) is found to be higher in Al0.07Ga0.93N NWs than in as-grown GaN NWs. The role of the O antisite defect (ON) for the formation of shallow VGa is inferred from photoluminescence spectroscopic analysis. These native defects strongly influence the gas sensing behaviour, which results in enhanced and low-temperature CH4 sensing.

Template-free synthesis of vanadium sesquioxide (V2O3) nanosheets and their room-temperature sensing performance

Until recently, it has been relatively hard to prepare vertically aligned vanadium sesquioxide (V2O3) nanosheets on glass substrates due to their extreme sensitivity to the atmosphere, doping, external pressure, and temperature. In this study, we present a simple one-step sputtering technique for the preparation of high-density vertically aligned V2O3 nanosheets on glass substrates without the use of any catalyst or template. To date, this structure has not been achieved using the V2O3 phase through sputtering. The definite oxidation state and phase of V2O3 with V3O5 inclusions were confirmed from the binding energies of the V2p3/2 and O1s peaks via X-ray photoelectron spectroscopy (XPS). The growth mechanism of nanosheets has been briefly explained. In this way, hierarchical V2O3 nanosheets interconnected with each other, which would allow easy electron transport between electrodes, favour remarkable sensing performance. The nanosheets exhibited trace-level ammonia (NH3) detection under ambient conditions for a wide concentration range of 10–500 ppm. Moreover, the selectivity and stability of the V2O3 nanosheets was studied. The present study may permit the design and fabrication of new electronic materials with multiple advantages based on V2O3 nanosheets.

The role of SnO2quantum dots in improved CH4sensing at low temperature

The role of quantum dots (QDs) of SnO2 in detecting low concentrations of methane (CH4) at a relatively low temperature of ∼150 °C with high response (S ∼ 3.5%) and response time below 1 min is reported. A simple room temperature single step chemical process was adopted for the growth of SnO2 nanoparticles of a size around 2.4 nm. These nanoparticles were subsequently annealed at 800 °C to increase the grain size to 25 nm. The as-prepared SnO2 nanoparticles, being smaller than the corresponding Bohr radius (2.7 nm), showed a strong quantum confinement effect with a blue shift in the band gap energy from 3.6 eV for the bulk SnO2 to 4.37 eV for the QDs. These QDs exhibited a strong sensing response to CH4 in comparison to the annealed sample. A low activation energy of 90 meV, as estimated from the temperature dependent S plot for SnO2 QDs, was found to be the driving force for such unusual high sensitivity at a low operating temperature. X-ray diffraction, transmission electron microscopy, along with Raman spectroscopy measurements are used for the detailed structural studies. The critical role of the chemisorbed oxygen species present at different operating temperatures on the surface of the off-stoichiometric quantum sized SnO2 and bulk-like annealed samples are discussed in light of the adsorption kinetics.