Surface Reactivity in Low Temperature Deposited Amorphous/Crystalline SnO2 Thin Films: Chemisorbed Oxygen Activity and CO Oxidation Pathways Revealed by In Situ XPS and Mass Spectrometry (2510.16512v1)

Abstract: This study investigates two critical aspects of the gas sensing mechanism in metal oxide sensors: (1) the conditions that maximize chemisorbed oxygen concentration as a function of temperature and oxygen partial pressure, and (2) which surface oxygen species (chemisorbed or lattice-bound) are primarily responsible for interaction with carbon monoxide (CO). SnO2 thin films, deposited at temperatures as low as 60 C and exhibiting mixed amorphous-crystalline phases with open, tortuous porosity, were evaluated for CO sensing at 200 C. Comprehensive characterization using EIS, MS, XPS, TEM, and sensor tests revealed a strong correlation between high sensing performance and the structural/electronic features of the defect rich low-temperature-deposited SnO2. Electrochemical impedance spectroscopy (EIS) was employed to identify the optimal sensing temperature. Mass spectroscopy (MS) used to analyze the exhaust gases after sensing reactions. The films exhibited oxygen under-stoichiometry and high concentrations of chemisorbed oxygen species. In situ XPS under 1 mbar (10000 ppm) O2 and CO exposures showed that chemisorbed oxygen, not lattice oxygen, was actively involved in CO oxidation, as further confirmed by CO2 detection via Mass spectroscopy (MS). Quantitative analysis revealed dynamic surface chemical status alternations, emphasizing the pivotal role of chemisorbed oxygen in the sensing mechanism at 200 C.