Hydrogen sensing enhancement of zinc oxide nanorods via voltage biasing

The capability of zinc oxide (ZnO) as a hydrogen sensing element has been pushed to its limits. Different methods have been explored to extend its sensing capability. In this paper, we report a novel approach which significantly improves the hydrogen sensing capability of zinc oxide by applying a bias voltage to ZnO nanorods as the sensing elements. Zinc oxide in the form of aligned nanorods was first synthesized on an Au-coated Si(111) substrate using a facile method via the galvanic-assisted chemical process. The sensing performance of the zinc oxide nanorods was investigated in response to the applied biasing voltage. It was found that the sensitivity, response time and detection limit of the ZnO sensing elements were dramatically improved with increasing bias voltage. A 100% increment in sensing response was achieved for the detection of 2000 ppm hydrogen gas when the bias voltage was increased from -2 to -6 V with 70% reduction in response and recovery times. This remarkable sensing performance is attributed to the reaction of hydrogen with chemisorbed oxygen ions on the surface of the ZnO nanorods that served as the electron donors to increase the sensor conductance. Higher reverse bias voltages sweep the electrons faster across the electrodes. This shortened the response time and, at the same time, depleted the electrons in the sensor elements and weakens oxygen adsorption. The oxygen ions could then be readily removed by hydrogen, leading to a higher sensitivity of the sensors. This, therefore, envisages a way for high-speed hydrogen gas sensing with high detection sensitivities.