Development Of UV-enhanced Semiconducting Oxide Gas Sensors

Metal-oxide semiconductor gas sensor plays an important role in industrial andmining safety, gas detection in daily life and so on. However, gas sensor is usuallyoperated at high temperature for overcoming high reaction activation energy so thathas speedy response and recovery properties. Such high temperature not only can leadto ignite the flammable and explosive gases but also reduced the service life of sensor.In addition, heating system needs high power consumption and is disadvantage forintegration and miniaturization of sensors.In the recent years, some researchers have reported that illuminating sensors withultra-violet (UV) light can be used for detecting gas at room temperature needing noheating system. The research has just started in the field of UV-enhanced gas sensor,and the study is also very limited. Low-power UV light-emitting diode (LED) waschosen to be light source, and a series of novel UV-enhanced gas sensor has beendeveloped. The UV light stimulated gas sensing performance was studied on widebandgap semiconductor with good photoelectric performance such as SnO₂,ZnO,TiO₂,In₂O₃. By designing the device structure and morphology of sensing materials,the light absorption efficiency has been significantly improved. Light stimulationperformance was affected by rapid recombination of photo electron and hole. Hence,the guideline of our study is to improve photocarriers density and time. Meanwhile,mechanism of UV-activated gas sensing properties was methodical analyzed.Combined with heterojunction model and reaction kinetics equation, light andelectricity performance were deeply discussed.1. ZnO-SnO₂composite was prepared by solid phase mixture. Through controllingproportion of ZnO and SnO₂, a optimum material was obtained when the proportion of ZnO:SnO₂equaled to3:7. The gas sensor based on as-preparedmaterial had good gas sensing properties to ethanol with UV irradiation. Theoptimum operating temperature of sensor was reduced by the use of UV light.What else, the response was improved to be2.5times as without UV light. Theresponse and recovery time of UV-enhanced sensor were30s and150s,respectively. It confirmed the enhancement effect of UV irradiation on compositematerial.2. SnO₂-In₂O₃hierarchical microspheres were prepared by hydrothermal andsolvothermal method. The study on the proportion of In₂O₃showed that thecomposite with mole ratio of SnO₂and In₂O₃to be4:1(IT025) had the bestsensitivity and response/recovery speed. The sensors based on these compositesshowed outstanding sensing properties to O3at room temperature with UV light.The response to840ppb of O3reached10.12, response and recovery time were20s and55s, respectively, under UV illumination. The grow mechanism ofcomposite material was detailed analyzed. A gas sensing mechanism wassuggested and it found that gas sensing properties of the UV-enhanced dependedon the proportion of gas sensing materials and photo sensing materials in thecomposite.3. According to the previous study, the reasonable combination of gas sensing oxideand photo sensing oxide had great effect on the properties of composite. SnO₂hollow sphere loaded with TiO₂(HS SnO₂-TiO₂) was prepared, and it was used tofabricate high response sensor to ethanol with UV irradiation. HS SnO₂-TiO₂wasprepared by coating the TiO₂on the outer-and innersurfaces of HS SnO₂. Itrealized the fully contact between SnO₂and TiO₂, and improved the UV-enhancedgas sensing properties. Ultraviolet absorption and photoluminescence spectra wereintroduced to investigate the optical performance of materials. A sensingmechanism related to the SnO₂-TiO₂heterojunction as well as UV illuminationwas proposed.4. A simple ultrasound spray pyrolysis (USP) methods was used to first prepare SnO₂nanowires array. Through strict controlling the experiment parameter, it realized the controllable synthesis of SnO₂nanowires. The growth mechanism and crystalstructure of SnO₂nanowires in the aerosol condition was detailed analyzed. Thegas sensing properties of sensors based on SnO₂nanowires array wereinvestigated by heating and UV activation. The results showed that the highestresponse to10ppm NO2was6.9at150℃. Meanwhile, with UV illumination, thesimilar response was5.5at room temperature. Besides, it had speedier responsetime than heating, it just need50s to reach the maximum resistance.5. A UV enhanced planar semiconducting oxide based gas sensor attached withreflector was fabricated and evaluated. This sensor structure was beneficial tolight utilization efficiency. The planar type sensor with reflector had higher opticalpath length and UV light absorption so that improved the UV-enhanced gassensing properties. Although the sensor with reflector was operated at roomtemperature, the response to100ppm ethanol was as much as160under UVillumination, and the response and recovery time were50s and150s, respectively.Furthermore, the humidity effect on gas sensor was evaluated. Alternating currentcomplex impedance method and direct current polarization measurement revealedthat electron took the dominating part in conduction. The moisture resistantcapability of sensor was confirmed under UV light illumination.