H 2 gas sensor performance of NiO at high temperatures in gas mixtures

Abstract

Gas sensors operating at elevated temperatures ( T > 500 °C) near combustion engines and power plants are needed to improve combustion control and to monitor and control emission. We investigated the effects of film thickness and operating temperature on the gas sensitivity of NiO films. 50 nm films had the best sensor characteristics and were further characterized for repeatability and selectivity. We present data on the structure, surface morphology, and chemical composition of 30–130 nm thick NiO films using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. This report includes gas sensor responses (GSRs) at operating temperatures from 300 to 650 °C for H 2 test gas concentrations ranging from 500 to 10,000 ppm in a synthetic air carrier gas. After annealing, the 50 nm thick films showed the highest surface roughness of 14.6 nm and the highest GSR at all operating temperatures tested. Surface roughness and area appear to be correlated to the gas sensor response of NiO thin films. An average GSR of 55 for 5000 ppm H 2 was found at 600 °C with a standard deviation of ±6.23. Repeated measurements for gas sensitivity were collected at 600 °C for 8 h where NiO was exposed to H 2 11 times at concentrations varying from 500 to 10,000 ppm and each exposure lasted for 20 min, yielding GSRs that follow a power law behavior. Cross sensitivity of NiO for 1100 ppm CO 2, 150 ppm NH 3 and 50 ppm NO x was investigated for individual gases and mixtures, all using synthetic air carrier gas. Selectivity was observed to decrease as the operating temperature increased.