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

H-2 sensing properties of an anodically oxidized Nb2O5 film coupled with a noble metal electrode (M/Nb2O5, M: An, 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 H-2 response, and the logarithmic sensor current under a forward bias was proportional to the logarithmic H-2 concentration in the whole range tested (10-8000 ppm). The current-voltage characteristics of the Pd/Nb2O5(S) sensor at 100 degrees 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 H-2 in air. On the other hand, the magnitude of the CO response was relatively smaller than that to H-2. It was confirmed that the sensor could also respond to H-2 in dry air, even at 28 degrees C. In addition, the response to N-2-diluted H-2 was much larger than that to air-diluted H-2. 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 H-2 balanced with air with forward bias voltages are well correlated with those in the I-V characteristics in air. (c) 2005 Elsevier B.V. All rights reserved.