Doped-vanadium oxides as sensing materials for high temperature operative selective ammonia gas sensors

Resistive electrochemical sensors based on vanadium oxides equipped with a pair of interdigital Au electrodes can detect NH₃ gas selectively at high temperature (500 °C). NH₃ addition in a base gas increased the relative conductance (σ/σ₀). Addition of less electronegative cation (Ce, Zr, Mg) to V₂O₅ increased the response and recovery rates, while electronegative cation (Al, Fe, Ni) increased sensor response magnitude. Among the samples tested, Al and Ce co-doped sample (VAlCe) was the most suitable sensor. The VAlCe sensor responded rapidly and linearly to change in concentration of NH₃ in the oxygen rich gas mixture and showed high selectivity in the presence of coexisting gases (NO, CO, H₂). The presence of water vapor did not markedly decrease the response magnitude but increased the response rate; the 90% response and 50% recovery times were less than 15 s. Based on the in situ UV–vis results, a possible sensing mechanism is proposed; adsorbed NH₃ causes reduction of V⁵⁺ to V⁴⁺, which results in the conductivity increase. Role of surface acidity on the selective detection of NH₃ as a basic molecule is also discussed.