Design of anodically oxidized Nb2O5 films as a diode-type H2 sensing material

H2 sensing properties of an anodically oxidized Nb2O5 film coupled with a noble metal electrode (M/Nb2O5, M: Au, Pt and Pd) have been investigated under various operating conditions. Among the sensors tested, Pd/Nb2O5(S), which was prepared by anodic oxidation in an aqueous solution of 0.5 M H2SO4 and coupled with a Pd electrode, showed the highest H2 response, and the logarithmic sensor current under a forward bias was proportional to the logarithmic H2 concentration in the whole range tested (10-8000 ppm). The current-voltage characteristics of the Pd/Nb2O5(S) sensor at 100 °C apparently showed a typical rectifying function of a metal-semiconductor junction, which was formed between the Pd electrode and the Nb2O5 thin film. Addition of water vapor in measurement atmosphere enhanced the magnitude of response to H2 in air. On the other hand, the magnitude of the CO response was relatively smaller than that to H2. It was confirmed that the sensor could also respond to H2 in dry air, even at 28 °C. In addition, the response to N2-diluted H2 was much larger than that to air-diluted H2. Based on AC impedance spectra of the Pd/Nb2O5(S) sensor, it is considered that variations in activation energy of two conductance components in air and 8000 ppm H2 balanced with air with forward bias voltages are well correlated with those in the I-V characteristics in air.