An Investigation To CuO Nanowires Based Ionization Gas Sensors

The environment is full of gaseous pollutants with a growing volume,which are crucial to both the environment and human health.Therefore,various gas sensors are invented and extensively applied in both the industry and daily life,such as defense,aerospace,manufacturing,medical care,and transportation.In these gas sensors,ionization gas sensors have been widely utilized because of their remarkable characteristics,including high sensitivity and stability but short response or recovery time.This type of sensor is based on the ionization of the target gas,where a specific current-voltage feature can be generated as the“fingerprint”of a certain gas component.However,in the“Internet of Things”era,the gas sensors emphasize the portability and low power consumption.However the high operation voltage,the most suffering disadvantage of ionization gas sensors,hinders their practical application.Hence in this work,a series of enhanced ionization gas sensors based on CuO nanowires are studied,in order to decrease the operation voltage by optimizing the material system of the cathode.The items of the research in this paper are mainly listed as follows:1)CuO nanowire-based ionization gas sensor with crystalline-to-amorphous phase transformation.There is a remarkable reduction of operation voltage（110 V to20 V）of an ionization gas sensor using the CuO nanowires,which is caused by the crystalline-to-amorphous phase transformation during an initial gas discharge.According to theoretical calculation and analysis,the crystallography change is considered as the result of ion bombardment and heating effect during the gas discharge,which brings about the formation of a large number of nanocrystallites and surface states to enhance the following gaseous ionization procedures.Subsequently,the gas sensing properties of this sensor are examined by detecting the volatile organic compounds with various types or concentrations diluted in nitrogen or air.The gas test results confirm that there is a low detection limit of the sensor down to ppm level.Finally,a sensing mechanism based on charge redistribution by electron-gas collision related to the specific ionization energy is proposed.2)Ionization gas sensor based on the aluminum coated CuO nanowires.CuO nanowires with an aluminum coating layer are assembled in an ionization gas sensor as the cathode.The surface of aluminum is then oxidized naturally,which brings about the formation of CuO/Al/Al₂O₃ hybrid cathode material system.This sensor is sensitive to humidity and further enables the discharge of volatile organic compounds at ppm level at an ultralow operation voltage（～25.4 V to air）.It also can differentiate isopropanol with a low concentration limit around 5 ppm.The gas sensing mechanism can also be summarized as the variation of voltages that are required to ionize a specific gas component.At last,according to the comparative experiments,the synergetic enhancement of gas ionization is attributed to low work function amorphous Al₂O₃ with abundant surface states and Al interlayer which improves the transmission of internal electrons,in addition to the high aspect ratio of CuO nanowire substrate.3)An ionization gas sensor on the basis of CuO nanowires with titanium oxide coating.The titanium oxide is directly decorated on CuO nanowires by atomic layer deposition technique to form CuO/TiO₂ heterogeneous cathode material system as an ionization gas sensor.The detection range of this device as a humidity sensor is 10RH%to 80 RH%.It is further able to detect the volatile organic compounds,e.g.isopropanol with the concentration at a low limit down to 5 ppm.Finally,based on energy band analysis,low work function of amorphous TiO₂ and abundant surface states are considered as the root cause of operation voltage reduction.