Electrical characterization of hydrothermally synthesized manganese dioxide nanowires with regard to NO₂ adsorption/desorption thermodynamics

Single nanowires (NWs) are promising structures to improve sensitivity and selectivity of metal-oxide (MOX) gas sensors. Self-made MnO₂ nanowires were hydrothermally synthesized and electrically characterized in different ambient. The nanowires were approximately 4-10 µm long and about 100 nm in diameter. The nanowires were suspended in water and deposited on two parallel gold electrodes having separation distance of 4 µm. Single nanowires were aligned perpendicularly across the gold electrodes by dielectrophoresis (DEP) technique. The number of the NWs was determined by scanning electron microscopy. The conductivity was measured in synthetic air, nitrogen, and in NO₂ ambient. The tests consisted in measuring resistivity of the NWs in relation to temperature of the bottom-placed heater under the chip. The temperature went from room temperature up to 300°C. The resistivity changes were observed accounting for oxygen reduction on the NWs surface as the electrons were moving from the NWs to the oxygen. The resistivity was explored at a constant current arrangement test. Based on resistivity changes, electrical properties, such as activation energy and a type of semiconductor (p-type in case of MnO₂), were evaluated. Mott-Schottky analysis was applied to estimate acceptor concentration, as well as NWs permittivity. In future experiments, we plan to study other types of NWs (e.g., WO₃ NWs) and/or explore an effect of other gases on the NW’s electrical properties, e.g., ethanol or H₂.