Hydrogen and ethanol sensing performance of NiO nanowires

Summary In this contribution we present the growth of NiO thin nanowires (40 nm diameter) via hydrothermal method and subsequent annealing of the fabricated device. The smooth nanowires undergo a polycristallization during the annealing, which make them a series of nanograins. The nanogranular wires are then tested as hydrogen sensors in air at different temperatures (200 - 400°C) in a range of H2 concentrations (50 - 1000 ppm). Sensor response is higher than 100% at 200°C for 1000ppm H2 and decrease while increasing working temperature. Response and recovery times are long (around 700 seconds) at low temperature and dramatically decrease to 20-30 seconds while increasing it. The limit of detection is under the ppm range at every working temperature and is, as far as we know, the lowest LoD for such kind of sensors. Motivation and results Nanostructured metal oxides are nowadays attracting more and more interest due to their unique properties. Binary n-type semiconducting oxides (SnO2, In2O3 and ZnO among the others) have been broadly investigated as gas-sensing materials, whereas little has been done in the field of p-type semiconducting oxides for application in gas sensors. Nickel oxide (NiO), which is usually taken as model for p-type semiconduction, has a wide range of applications due to its good chemical stability as well as for its excellent optical and electrical properties. Until recently, the studies about NiO sensing properties were limited to thin films because of the intrinsic difficulty of growing NiO nanostructures [1,2]. In this investigation we studied and hydrothermal method to grow nickel oxide nanowires and optimized the process properly setting the growth parameters. The resulting wires, shown in Figure 1 with a higher magnification in the inset, have a diameter of 40-50 nm, a length around 1 micron and are smooth with a constant diameter. Once dropped between the metal electrodes to fabricate the sensing device, they are annealed at 500°C for 2 hours and this drastically change their morphology: the nanowires are then made of small nanoparticles connected to each other (see Figure 2). The polycrystalline structure is confirmed also by TEM studies (in the insets of Figure 2) and is very important for the sensing properties of the active material. While monocrystalline nanowires are indeed relying only on the depletion layer modulation mechanism, granular nanowires can also exploit the series of barriers at the grain boundaries, which are very sensitive to the gas concentration. The synergic presence of two sensing mechanism makes this type of nanostructured material even better for sensing applications. In fact the results of this preliminary experiments are very good, with intense response, complete recovery, fast responding and recovery to the H2 concentration peaks and good stability, as shown in Figure 3. Furthermore, extensive studies on the sensing mechanisms and performance of p-type metal oxides are very interesting also for the possibility of combining n-type and p-type materials obtaining junction-based sensors, which promise to be a next generation of semiconductor-based conductometric devices.