Abstract
Development of sensor materials based on metal oxide semiconductors (MOS) for selective gas sensors is challenging for the tasks of air quality monitoring, early fire detection, gas leaks search, breath analysis, etc. An extensive range of sensor materials has been elaborated, but no consistent guidelines can be found for choosing a material composition targeting the selective detection of specific gases. Fundamental relations between material composition and sensing behavior have not been unambiguously established. In the present review, we summarize our recent works on the research of active sites and gas sensing behavior of n-type semiconductor metal oxides with different composition (simple oxides ZnO, In2O3, SnO2, WO3; mixed-metal oxides BaSnO3, Bi2WO6), and functionalized by catalytic noble metals (Ru, Pd, Au). The materials were variously characterized. The composition, metal-oxygen bonding, microstructure, active sites, sensing behavior, and interaction routes with gases (CO, NH3, SO2, VOC, NO2) were examined. The key role of active sites in determining the selectivity of sensor materials is substantiated. It was shown that the metal-oxygen bond energy of the MOS correlates with the surface acidity and the concentration of surface oxygen species and oxygen vacancies, which control the adsorption and redox conversion of analyte gas molecules. The effects of cations in mixed-metal oxides on the sensitivity and selectivity of BaSnO3 and Bi2WO6 to SO2 and VOCs, respectively, are rationalized. The determining role of catalytic noble metals in oxidation of reducing analyte gases and the impact of acid sites of MOS to gas adsorption are demonstrated.
Highlights
The sensing mechanism related to chemisorption of oxidizing gases (Equation (5)) was experimentally proven by spectroscopy studies, e.g., NO2 adsorption on the metal oxide semiconductors (MOS) surfaces with the formation of NO2− was observed by in situ Raman measurements [41] and diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy [42]
In line with the increment of charge/radius ratio and electronegativity of the constituent cations, the metal-oxygen bond energy reported in literature and surface acidity probed by temperature programmed of ammonia (TPD) of ammonia increase in the order: In2 O3, ZnO, SnO2, TiO2, WO3
It was due to interplay of two opposite trends: enhancing oxygen vacancies formation and decreasing oxygen chemisorption with the increase of EM-O, as was shown by the concentrations of paramagnetic sites VO − and O2 − measured by electron paramagnetic resonance (EPR)
Summary
Nanomaterials based on metal oxide semiconductors (MOS) have been extensively investigated for use in resistive gas sensors. The fundamental approach is modification of the chemical composition of the sensor material It can be performed by functionalization of MOS surfaces [9,10,11,12], bulk doping of MOS [13,14], use of p-type MOS and mixed-metal oxides instead of conventionally employed n-type MOS (SnO2 , ZnO, WO3 , TiO2 , Ga2 O3 , In2 O3 , etc.) [15,16,17], or MOS-based composites with different additives, e.g., noble metals, metal oxides, graphene and carbon nanotubes, polymers [18,19,20,21]. That conditions the wide bandgap n-type semiconductor behavior These cations may be partially reduced through the formation of oxygen vacancies, and the concentration of point defects in these metal oxides falls in the 1016 –1019 cm−3 range, which is appropriate for the sensitivity of bulk conduction of localized charges at the surface [22]. Besides the predictable effect on electronic properties of the semiconductor, the additives influence the adsorption capacity and chemical reactivity of the material surface, and the latter has often been disregarded in the gas sensor studies
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