In Situ DRIFTS Study Of Gas-solid Interface Reaction On γ-Fe₂O₃-based Gas Sensors

The gas sensing property of matallic oxide semiconductor is the phenomenon thatthe resistance changes when it reacts with the gases （oxidizing or reducing） at a certaintemperature. In situ diffuse reflectance infrared Fourier transform spectroscopy （DRIFTS）technique is an important tool for the characterization of the surface reactivity of gassensing materials, as it allows the physicochemical processes taking place in an activesensing element to be followed in real time and under operating conditions. Therefore,matallic oxide semiconductor gas sensors were modified by doping noble metal Pd andmanufacturing heterojunction structure to improve their sensitivities to toxic gases likeformaldehyde and acetone in this paper. In addition, the adsorption and reactions offormaldehyde and acetone on nano γ-Fe₂O₃-based films were studied by DRIFTStechnique to understand the gas sensing mechanism of the sensors.（1） Nano γ-Fe₂O₃sensors were prepared by the screen printing technology. Theresults of XRD and FESEM indicated that the γ-Fe₂O₃films consisted of the original20³0nm spherical γ-Fe₂O₃nano-particles and they were in the cubic phase. Thesensitivities of the gas sensors were measured under steady-state conditions. Theadsorption and reactions of formaldehyde and acetone on nano γ-Fe₂O₃films werestudied by DRIFTS technique. The mechanism of formaldehyde and acetone sensing wasproposed according to the results of DRIFTS experiments, which indicated thatdioxymethylene, formate ions, polyoxymethylene and molecularly adsorbedformaldehyde surface species were formed during the interaction of formaldehyde withnano γ-Fe₂O₃films at different temperatures and acetates, methoxy groups, carbonatesand molecularly adsorbed acetone surface species were formed during the interaction ofacetone with nano γ-Fe₂O₃films at different temperatures. The mechanism of gas sensingproperties and gas-solid interface reactions was interpretated according to the results. （2） Pd-doped nano γ-Fe₂O₃sensors were printed on an Al2O3substrate by screenprinting-injecting hybrid technology. The sensitivities of the gas sensors were measuredunder steady-state conditions. The adsorption and reactions of formaldehyde and acetoneon Pd-doped nano γ-Fe₂O₃films were studied by DRIFTS technique. The results showedthat the Pd-doped γ-Fe₂O₃gas sensors could effectively improve the gas sensingproperties of formaldehyde. A possible mechanism by which the supported palladiumpromoted the activation of surface oxygen on the catalysts is proposed and a mechanismfor the reaction and decomposition of formaldehyde was proposed. Because electronswere released in the above reactions, the resistance of gas sensing materials decreasesand the detection of formaldehyde gas could be realized. The possible mechanism of thereaction process may promote the development of an improved gas sensor forformaldehyde.（3） Nano Nb₂O₅/γ-Fe₂O₃sensors were prepared by the screen printing technology.The sensitivities of the gas sensors were measured under steady-state conditions. Theadsorption and reactions of formaldehyde and acetone on Nb₂O₅/γ-Fe₂O₃films werestudied by DRIFTS technique. The results showed that the Nb₂O₅/γ-Fe₂O₃gas sensorscould effectively improve the gas sensing properties of formaldehyde and acetone. Onthe one hand, Nb₂O₅inhibited the grain growth, increasing the surface area of the sensinglayer and favoring the interaction with the targeted gases. On the other hand, theconductance of γ-Fe₂O₃was increased by addition of Nb₂O₅, which could greatlyfacilitate the transfer of electrons between gas diffusion and mass transport in sensormaterials.