Semiconductor Indium Oxide Gas-Sensing Materials

Formaldehyde is a kind of toxic volatile organic compound,which is widely existing indoors.It is mainly derived from the decoration of wood-based panel furniture,adhesives,paints,coatings,etc.Formaldehyde is listed as a class?of carcinogens,which is seriously harmful to human health.It is extremely important to research and develop a quick response,sensitive and economical formaldehyde detection method,because of the problems of formaldehyde pollution.Metal-oxide-semiconductor-based sensors have attained a large of research and application for their own advantages of portability,easy operation,and real-time monitoring.The key principle is to convert chemical signal into electrical signal through chemical reaction between the target gas and the surface chemisorbed oxygen of metal oxide semiconductor.The typical detection materials such as In₂O₃,WO₃,ZnO and SnO₂,hardly meet the needs of making sensors with high response and low detection limits.Some studies have shown that the gas sensing performance of semiconductors can be improved by doping element.However,for the reason of the complexity of the sensing mechanism and the influencing factors of surface response,the effect of doping on the gas sensing reaction is still unclear.It is also difficult to predict theoretically the effects of various chemical dopings on the gas sensing performances of semiconductors.We synthesized a series of metal cation-doped In₂O₃ semiconductor sensing materials,systematically studied the modification effects of doping on the energy level structure and surface properties of semiconductors,and further studied the effects of these modifications on gas sensing.Our result revealed that sensing performance of materials has a close relationship with its energy level structure and surface basicity,making Fermi levels become a suitable descriptor to screen rapidly high-efficiency sensing materials.Besides,we synthesized sensing materials with the high response,good selectivity and stability for the detection of formaldehyde.They have reached the ultra-low detection limit of ppb level,which meets the national indoor formaldehyde detection standards.The research of this paper includes the two parts:1.A series of heteroatom-doped indium oxide M-In₂O₃?M=Al,Ti,Zr,V,Cr,Mo,W,Sn,and Ga?semiconductor sensing materials were synthesized via an electrospinning method.In order to understand how doping alters energy level structure and its effect on the surface chemisorption of oxygen and subsequent sensing processes.The experimental results reveal that Al,Ga,and Zr substitutions lead to the elevation of Fermi level,whereas introduction of V,Cr,Mo,W,Sn,and Ti dopants lowers the Fermi level.However,only the former can improve the response to formaldehyde,indicating the Fermi level is an important factor affecting the gas sensing performance.The elevation of Fermi level increases energy level difference between oxide semiconductor and oxygen molecules and facilitates the surface absorption of oxygen species,resulting in superior formaldehyde sensing activity.Especially,we found that Al-doped In₂O₃(Al_0.15In_1.85O₃)exhibits remarkably enhanced sensing performances toward 100 ppm formaldehyde at low working temperature?150°C?with high response?R_a/R_g=60.3±4.9?,good selectivity,short response time?2 s?,and ultra-low limit of detection?60 ppb?.2.Rare earth metal oxides have unique electronic structures,high electron mobility and surface basicity.Rare earth doping is an effective way to modulate the electronic structure and surface basicity of semiconductors,thereby affecting the versatile functional properties.Based on this,we synthesized a series of rare earth-doped indium oxide sensing materials RE-In₂O₃?RE=Y,La,Nd,Ho and Tm?for formaldehyde detection.In order to further study the relationship between the gas sensing performances of semiconductors and their energy level structure and surface basicity.The experimental results show that the doping of rare-earth increase the Fermi level of In₂O₃ and increase the number of basic sites on the surface of In₂O₃,thereby enhancing the response to formaldehyde.Especially the Y-doped In₂O₃-based sensor?5Y-In₂O₃?gives the highest response to 100 ppm formaldehyde?R_a/R_g=91.7±7.9?,the response time is only 1 s,and the ultralow detection limit of 50 ppb.Experimental results indicate that the improved formaldehyde sensing performance should be attributed to the elevation of the Fermi level and the increase the number of surface basic sites.