Restraining SnO2 gas sensor response degradation through heterovalent doping

Abstract

Degradation of SnO₂ gas sensor response during long-term operation is a major obstacle, which hinders the penetration of metal oxide gas sensor technology in new application fields. The concept has been proposed, that heterovalent doping with transition metal cations may counteract the effects of diffusion related evolution of nanocrystalline material morphology and annealing of quenched defects, which affect electrical properties of material during long term operation. A series of Nb- and/or Cr-doped materials was synthesized via flame spray pyrolysis technique. The characterization was made using XRD, BET, TEM, XPS and EPR methods. Electrical measurements were done in the range of 100–400 °C working temperature range in the DC mode with the use of MEMS-microhotplates. Gas sensor experiments were made in the flow of air with controlled humidity and trace impurity gases (CO, CH₄, NH₃, H₂S, methanol, acetaldehyde, acetone, benzene, formaldehyde) concentration. The observed long-term electrical effects of gas sensor response degradation and baseline drift were corresponded with the ex-situ characterization of the materials via XRD, BET and EPR methods. The processes of grain agglomeration and intergrain neck growth are coinciding with the annealing of oxygen vacancies. The latter leads to the increase of the oxidation state of cations in the node positions of rutile structure and decrease in the charge carrier concentration. The introduction of n-type donor Nb(V) defect alongside with Cr(III) doping leads to two-fold decrease in sensor response drop over operational time compared to pure SnO₂ due to effect of native free electrons substitution by dopant-generated donor ones.