In(2)O(3) Nanotower Hydrogen Gas Sensors Based on Both Schottky Junction and Thermoelectronic Emission

Indium oxide (In(2)O(3)) tower-shaped nanostructure gas sensors have been fabricated on Cr comb-shaped interdigitating electrodes with relatively narrower interspace of 1.5 μm using thermal evaporation of the mixed powders of In(2)O(3) and active carbon. The Schottky contact between the In(2)O(3) nanotower and the Cr comb-shaped interdigitating electrode forms the Cr/In(2)O(3) nanotower Schottky diode, and the corresponding temperature-dependent I-V characteristics have been measured. The diode exhibits a low Schottky barrier height of 0.45 eV and ideality factor of 2.93 at room temperature. The In(2)O(3) nanotower gas sensors have excellent gas-sensing characteristics to hydrogen concentration ranging from 2 to 1000 ppm at operating temperature of 120–275 °C, such as high response (83 % at 240 °C to 1000 ppm H(2)), good selectivity (response to H(2), CH(4), C(2)H(2), and C(3)H(8)), and small deviation from the ideal value of power exponent β (0.48578 at 240 °C). The sensors show fine long-term stability during exposure to 1000 ppm H(2) under operating temperature of 240 °C in 30 days. Lots of oxygen vacancies and chemisorbed oxygen ions existing in the In(2)O(3) nanotowers according to the x-ray photoelectron spectroscopy (XPS) results, the change of Schottky barrier height in the Cr/In(2)O(3) Schottky junction, and the thermoelectronic emission due to the contact between two In(2)O(3) nanotowers mainly contribute for the H(2) sensing mechanism. The growth mechanism of the In(2)O(3) nanotowers can be described to be the Vapor-Solid (VS) process.