New Mechanism Of Pd-SnO₂Based CO Gas Sensor Studied By In-situ Dynamic XPS

SnO₂-based gas sensor, one of the most widely used gas sensors, has theadvantage of having a small size and being lightweight and simple fabricationprocedures. Thus, they are widely utilized for detecting toxic gases orcontamination gases in the atmosphere, such as CO, NOX, SO₂, and CO₂, aswell as combustible gases, like H₂, CH₄, and C₂H₅OH. In the past decades,researchers had significantly improved its gas sensitivity, selectivity, responseand recovery speed by reducing particle size, adding new dopant, improvingfabrication procedures and so on. But, the functional mechanism of them isstill a controversial topic without a complete theory system.X-ray photoelectron spectroscopy （XPS）, as surface sensitive chemicalanalysis equipment, is well suited for the study of surface-controlled gassensing materials like SnO₂. But the high vacuum environment of XPS is inconflict with the high concentration of reaction gas of SnO₂sensors. To solvethis problem, we developed a flow-controllable retractable gas doser. Theangle between the gas outlet and the sensor surface is set at30°; Here, thegas outlet could be very close to the surface of the gas sensor. Then on thegas sensor surface there could be outlet gas with a high enough concentrationfor detection through surface reactions. At the same time, the entireanalytical system could be kept at a higher vacuum still.First, the40nm SnO₂nano-powders of rutile structure have beensynthesized by hydrothermal treatment of SnS2and H₂O₂. Then, we preparedthe flat and tube samples of Pd-SnO₂gas sensors, and selected typically CO asreaction gas. At last, we studied the samples by in-situ dynamic XPS, XRD,SEM and sensor characteristics test, and got a series of very valuable results: （1） There are two kinds of lattice oxygen in the SnO₂of rutile structure:the surface lattice oxygen （OI,530.5eV） and the bulk lattice oxygen （O^II,431.9eV）.（2） At the working conditions of a Pd-doped SnO₂-based CO gas sensor, Pdatoms were used to form a transient compound as Pd（CO）₄. In turn, thistransient compound catches up a surface lattice oxygen atom from SnO₂toform a new compound [Pd（CO）₄]O₄, and then oxygen vacancies were left onthe surface of SnO₂. Thus, the resistance of the gas sensor is decreasedrapidly. Next, the new compound would be decomposed into CO₂and Pd. Atthe same time, atmospheric oxygen would be attracted by those oxygenvacancies to become a surface lattice oxygen of SnO₂. In this way, a catalyticcircle would then be completed. Thus, in the reaction process of CO+O=CO₂,the essential points of our new sensing mechanism are Pd is the transportcarrier for CO, whereas the surface oxygen of SnO₂is the transport carrier foroxygen. The chemical equations of whole process can be expressed as:Pd+4COâ†'Pd（CO）₄Pd（CO）₄+4SnO₂â†'[Pd（CO）₄]O4+4SnO+4Vo[Pd（CO）₄]O4â†'Pd+CO₂↑（3） Comparing the peak intensities of the photoelectron spectroscopicspectrum displayed by in-situ dynamic data collection to that of offlinein-situ data collection, the former is about30%stronger. This is related to thetransient weak magnetization phenomenon happening in the gas-sensingprocess. Because of this focused magnetization effect, scattered emittedphotoelectrons off the normal collection angle as designed for the detectionsystem would then be collected also. Thus, the peak intensities from in-situdynamic photoelectron are enhanced. This indicates why the in-situ dynamic photoelectron detection techniquehas special advantages in the study of functional mechanisms for both thegas-sensing and the related catalytic processes.