To improve the sensor response to low concentrations of methane (CH4) at low operating temperatures in humid atmospheres, we prepared Pd-loaded SnO2 (Pd-SnO2) nanoparticles via two different Pd-loading processes: (i) a general impregnation method and (ii) a new loading method using poly(N-vinyl-2-pyrrolidone) (PVP) as a protective agent for Pd receptor particles. According to the measured electric resistances, the Pd particles limited the hydroxyl-poisoning of the SnO2 particle surface. Because Pd is oxidized to PdO, a p–n junction is formed at the interface between PdO and SnO2, and such interface gives the enlargement of the electron depletion layer. Therefore, Pd further improved the resistance against hydroxyl poisoning of the SnO2 surface in humid air. In addition, although the sensor based on neat SnO2 did not respond to low-concentration CH4 at 200–400 °C, both the sensors based on the Pd-loaded SnO2 samples exhibited high sensor response to 200 ppm CH4 in a humid atmosphere. The Pd-SnO2 obtained by the new loading method exhibited a higher response to CH4 at lower concentrations in the lower operating temperature range (200–250 °C). This improvement in the sensor response is probably due to the catalytic activity of the larger Pd nanoparticles. According to high-resolution transmission electron microscopy–energy-dispersive X-ray spectroscopy images, the new loading method successfully provided Pd-loaded SnO2 nanoparticles with Pd nanoparticles dispersed uniformly on the SnO2 particle surface. The average particle size of Pd nanoparticles loaded on the surface of SnO2 by the new loading method was slightly larger than that of the Pd nanoparticles loaded by the impregnation method. As the Pd particle size increases, it is thought that crystalline PdO particles are formed more easily, thereby improving the combustion activity of CH4 under humid conditions. These results are of great significance for further decreasing the energy consumption of the CH4 sensor and increasing its sensor response in humid atmospheres.
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