The influence of the absolute surface area on the NO2 and H2 gas responses of ZnO nanorods prepared by hydrothermal growth

In this paper we report the influence of the absolute surface area (ASA) and aspect ratio (L/D) of hydrothermally grown ZnO nanorods (NRs) on the NO2 and H2 gases relative responses (RRs) in the 20 ppb − 1 ppm and 50 ppm-1000 ppm concentrations range. We used films of NRs characterized by different geometrical features, such as diameter, length, L/D and ASA, obtained using different growth times, but all showing a well-defined hexagonal shape and an alignment perpendicular to the substrate. Values of L/D up to 129 and NRs lengths up to 14 μm have been obtained; one of the highest reported for quasi-one-dimensional ZnO nanostructures obtained through solution-based one-pot syntheses. Electrical tests at 150 °C (NO2) and 200 °C (H2) carried out using films of NRs presenting different L/D evidenced a direct relationship between the increase of the ASA and L/D and gas RRs. The influence of geometrical and chemical characteristics, such as diameter, size and surface-defects concentration is also discussed in light of the existing literature. Cross sensitivity, investigated measuring the H2 (target) response in the presence of NO2 (interfering) gases as well as dynamic and cumulative NO2 gas reproducibility test are also presented. Beside the considerable interference played by NO2 to the H2 response, our long NRs demonstrate an excellent reproducibility of the electrical signal and fast recovery of the base line, evidencing the absence of irreversible chemisorption or sample-surface “history-related” phenomena.