Effect of Pd doping on the microstructure and gas-sensing performance of nanoporous SnOx thin films

Pristine and Pd-doped nanoporous SnO_x thin films were fabricated via a sol-gel route. The Pd-doped film exhibited enhanced H₂ gas-sensing performance, in terms of higher sensitivity and shorter response time. Structural characterization was performed to investigate the effect of Pd doping on the microstructure evolution of the films. The grain and pore size of Pd-doped film, as measured using transmission electron microscopy and grazing-incidence small-angle X-ray scattering (GISAXS), are both smaller than those of undoped film. In particular, the pore size evolution of the films during annealing was quantitatively monitored in situ using synchrotron-based GISAXS. Knudsen gas diffusion and depletion layer models were employed to evaluate the microstructure influence on the gas sensitivity semi-quantitatively. The results suggest that the microstructure of the Pd-doped film is critical for improving the gas sensitivity but cannot account for the total sensitivity enhancement, implying other mechanisms could play a more important role.