Morphology and size effect of Pd nanocrystals on formaldehyde and hydrogen sensing performance of SnO2 based gas sensor

In order to investigate the morphology and size effect of Pd nanocrystals on gas sensing performance of SnO2, Pd nanocrystals with different sizes (3, 20, 50 nm) and morphologies were synthesized and a series of Pd-SnO2 composites were prepared by decorating Pd nanoparticles (PdNP, ~3 nm), Pd nanocube (PdNC1, ~20 nm), Pd nanocube (PdNC2, ~50 nm), Pd rhombic dodecahedron (PdRD, ~50 nm), Pd octahedron (PdOC, ~50 nm) on the surface of SnO2 nanosheets. The test results showed that all Pd-SnO2 sensors exhibited dual selectivity to H2 at 200 °C and HCHO at 260 °C. In all Pd-SnO2 composites with different sizes of Pd, 2%PdNP-SnO2 sensor exhibited the best sensing performance. In all Pd-SnO2 composites with different shapes of Pd but similar size, PdNC1-SnO2 and PdOC-SnO2 sensors exhibited better sensing performance to HCHO and H2, respectively. Pd-SnO2 composites showed Pd shape and size-dependent sensing performance, and the size effect is more significant than morphology effect. Size effect is ascribed to the smaller size with more active sites and PdO species, and shape effect is attributed to the different adsorption energies of different facets to HCHO, which is proved by DFT calculation. In addition, the improved performance of SnO2 can be ascribed to two aspects: one is the suitable catalytic activity of Pd to HCHO and H2 oxidation; the other is that the transformation of Pd⇋PdO in air and HCHO or H2 leads to a large resistance change of SnO2.