To develop a highly sensitive carbon monoxide (CO) sensor with a wide range of humidity resistance, we focused on the Pd loading method on SnO2 nanoparticles and the thickness of the sensing layer. The Pd nanoparticles were loaded on the SnO2 surface using the surface immobilization method (SI-Pd/SnO2) and the colloidal protection method (CP-Pd/SnO2). The XPS analysis indicated that the Pd nanoparticles were a composite of PdO and Pd, regardless of the loading method. According to the evaluation of the electrical properties at 350 °C, the CO response in a humid atmosphere and the resistance toward humidity change using CP-Pd/SnO2 were higher than those using SI-Pd/SnO2, even though the Pd loading amount of SI-Pd/SnO2 was slightly larger than that of CP-Pd/SnO2. In addition, Pd/SnO2 prepared via the CP method with a thinner sensing layer showed a higher sensor response and greater stability to humidity changes at 300 °C, even though the humidity change influenced the CO response at 250 and 350 °C. Thus, the overall design of the surface Pd, including size, dispersity, and oxidation state, and the sensor fabrication, that is, the thickness of the sensing layer, offer a high-performance semiconductor-type CO gas sensor with a wide range of humidity resistance.
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