Room-temperature hydrogen gas sensor composed of palladium thin film deposited on NiCo2O4 nanoneedle forest

This paper reports the performance and fabrication method of a palladium (Pd) gas sensor with a NiCo2O4 nanoneedle forest (NNF) that is very sensitive to hydrogen (H2). The Pd surface area that could react with H2 was maximized by sputtering Pd on NiCo2O4 NNF, which resulted in excellent sensitivity and response speed. Also, the temperature sensor and heater are integrated by using microelectromechanical system (MEMS) processes, where the sensitivity, as well as the response and recovery speeds, can be adjusted by controlling the operation temperature. The proposed sensor showed relative resistance changes of at RT and at 300 °C when saturated with a H2 concentration of 300 ppm. The response times were as fast as 117 and 102 s at RT for the rising and recovering cycles, respectively. However, at 300 °C, the response times in the rising and recovery cycles were 26 and 23 s, respectively, which were faster than those at RT. The sensor response at RT was linear throughout the measurement range, with a sensitivity of and a detection limit of 28 ppb. Therefore, the proposed sensor can be used as a low-power, high-sensitivity sensor at RT or as a sensor with a fast response time at a high operating temperature using an integrated heater.