Ammonia gas sensors based on undoped and Ca-doped ZnO nanoparticles.

Due to its large use in different industrial sectors, high toxicity, and corrosion, the demand for sensing techniques towards ammonia gas has become urgent. In this study we report on the sensing performances of a conductometric sensor for NH3 gas based on Ca-doped ZnO nanoparticles with different calcium concentrations (0, 1, and 3 at%) synthesized using the sol-gel process under supercritical dry conditions of ethanol. All samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier-transform infrared (FTIR) spectroscopy. Pure and Ca-doped ZnO are polycrystalline and well crystallized in the hexagonal wurtzite structure. TEM images revealed that pure ZnO is composed of spherical particles with dimensions in the nanometer range. Larger particles were observed after the incorporation of Ca ions. The average crystallite size, estimated by the Williamson-Hall method, was 43, 80, and 96 nm for pure, Ca-1 at% and Ca-3 at%, respectively. Furthermore, FTIR spectroscopy was used to prove the formation of ZnO and the incorporation of calcium ions in the Ca-doped ZnO samples. The gas sensing performances towards ammonia gas clearly ameliorated after the addition of Ca ions in the ZnO structure. The gas response to NH3, R0/Rg, of the 1% Ca-doped ZnO sensor reached a value of 33 for 4000 ppm of ammonia at T = 300 °C with good selectivity compared to other gases such as CO, CO2, and NO2. The response and recovery times were 5 s and 221 s, respectively. The reported good sensing performances indicate the potential application of Ca-doped ZnO as a sensor material for ammonia detection.

High performance acetone sensor based on γ-Fe2O3/Al-ZnO nanocomposites.

Ternary nanocomposites made of γ-iron oxide and aluminum-doped zinc oxide (γ-Fe2O3/Al-ZnO NCs), with different metal oxides ratio (0%-100%) were prepared through a solvothermal sol-gel process. The synthesized materials were characterized by x-ray diffraction, UV-vis spectroscopy, photoluminescence (PL), scanning electron microscope and BET analysis. Characterization results demonstrated that the ternary γ-Fe2O3/Al-ZnO NCs are mainly constituted by γ-Fe2O3 and Al-ZnO individual phases, while structural and physical properties like surface area, pore size, optical band gap, PL and electrical conductivity were deeply affected by the composition of nanocomposite. The synthesized γ-Fe2O3/Al-ZnO NCs were employed to prepare conductometric gas sensors, then their sensing performances toward acetone were also investigated. Results revealed enhanced sensing performance of nanocomposites than both pure γ-Fe2O3 and Al-ZnO phases. In particular, the γ-Fe2O3(33%)/Al-ZnO based gas sensor showed the best sensing properties, like a high response of R air/R gas = 29, a short response time of 3 s, in addition to an improved selectivity toward acetone versus ethanol at an operating temperature of 200 °C. Overall, ternary γ-Fe2O3/Al-ZnO NCs appear to be promising for the development of conductometric acetone sensors.