| Summary: | To develop a highly sensitive carbon monoxide (CO) sensor with a wide range of humidity resistance, we focused on the Pd loading method on SnO<sub>2</sub> nanoparticles and the thickness of the sensing layer. The Pd nanoparticles were loaded on the SnO<sub>2</sub> surface using the surface immobilization method (SI-Pd/SnO<sub>2</sub>) and the colloidal protection method (CP-Pd/SnO<sub>2</sub>). 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/SnO<sub>2</sub> were higher than those using SI-Pd/SnO<sub>2</sub>, even though the Pd loading amount of SI-Pd/SnO<sub>2</sub> was slightly larger than that of CP-Pd/SnO<sub>2</sub>. In addition, Pd/SnO<sub>2</sub> 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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