Design of anodically oxidized Nb_2O_5 films as a diode-type H_2 sensing material

摘要

H_2 sensing properties of an anodically oxidized Nb_2O_5 film coupled with a noble metal electrode (M/Nb_2O_5, M: Au, Pt and Pd) have been investigated under various operating conditions. Among the sensors tested, Pd/Nb_2O_5(S), which was prepared by anodic oxidation in an aqueous solution of 0.5 M H_2SO_4 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/Nb_2O_5(S) sensor at 100℃ apparently showed a typical rectifying function of a metal-semiconductor junction, which was formed between the Pd electrode and the Nb_2O_5 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℃. 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/Nb_2O_5(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.