The Key Role of Active Sites in the Development of Selective Metal Oxide Sensor Materials

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.

Ga2O3(Sn) Oxides for High-Temperature Gas Sensors

“…In the presence of carbon monoxide, the electrical resistance of all samples decreases (Figure 11). This is consistent with the fact that CO reacts with oxygen chemisorbed on the surface of gallium oxide in accordance with the following reaction [3,26]:…”

“…In the presence of ammonia, the electrical resistance of all samples also decreases: NH3 reacts with oxygen chemisorbed on the surface of semiconductor material in accordance with the following reaction [3,26]:…”

“…For samples containing 0.7 < x < 13 at.% Sn, the gradual increase in sensor signal with tin content is observed that can be explained by formation of more conductive (as compared to αand β-Ga 2 O 3 ) SnO 2 phase segregations. The n-n heterojunction between the Ga 2 O 3 (bandgap E g = 4.9 eV [2], electron affinity χ = 4.0 eV [27]) and SnO 2 (E g = 3.6 eV [26], electron affinity χ = 4.9 eV [28]) can also affect gas sensors properties of Ga 2 O 3 (Sn) materials. However, the possibility of mutual doping (formation of Sn-doped Ga 2 O 3 , n-type doping, and Ga-doped SnO 2 , p-type doping) complicates the interpretation of this effect.…”

Improving Gas-Sensing Performance Based on MOS Nanomaterials: A Review

“…Therefore, we should research new preparation processes to solve these problems. Besides, the external additives influence the adsorption capacity and chemical reactivity of the host-sensitive material’s surface [ 118 ], which has often been disregarded in gas sensor studies. So, when we select suitable materials to make the C-S nanostructure, we should pay attention to the interaction of the C-S interface in the context of density functional theory (DFT), molecular dynamics, or other theories.…”

Obituary for Prof. Dr. Alexander Gaskov

“…Later on, the focus of research in the laboratory was placed on active centers located on the surface of nanocrystalline semiconductor oxides [ 2 , 3 , 4 ]. This led to the development of new in situ and operando techniques for spectral studies of the reactivity of sensor materials in the interaction with gas phase.…”