Preparation And Properties Of Room Temperature Heterojunction Film Carbon Monoxide Sensor

The environmental pollution of carbon monoxide has become a global problem,in which artificially made carbon monoxide is the main source.On the one hand,carbon monoxide is the main component of the exhaust gas produced by more and more cars.On the other hand,the incomplete burning of fossil fuels on a large scale has caused carbon monoxide to rise rapidly worldwide.Carbon monoxide is a serious threat to human life and health.It is colorless and odorless,it is difficult to be perceived by the human body,and it can cause discomfort after inhalation.In many detecting processes,hydrogen with the same reducibility will produce similar sensing signals to that from carbon monoxide,interfering the detection results.Therefore,it is particularly important to develop highly sensitive and selective carbon monoxide gas sensors.Currently,metal oxide semiconductors are widely used as carbon monoxide gas sensor materials,such as n-type semiconductors tin dioxide（SnO₂）,zinc oxide（ZnO）,titanium dioxide（TiO₂）,indium oxide（In₂O₃）,etc.,and p-type semiconductors such as cobalt tetroxide（Co₃O₄）and copper oxide（CuO）.TiO₂ has become one of the most widely used materials due to its easy synthesis,good physical and chemical stability and biocompatibility.CuO is a typical p-type gas-sensitive material with good detection ability of carbon monoxide.Construction of p-n heterojunction,doping and noble metal modification are effective ways to improve the response and selectivity of a sensor.In this paper,TiO₂ and CuO thin films were assembled into a p-n heterojunction,the structure and interface properties of the heterojunction are regulated,so as to design and prepare a room temperature working carbon monoxide gas sensor with high sensitivity and selectivity.Specific research contents are as follows:1.Preparation of CuO-based heterojunction films and their CO gas sensing properties at room temperature.The p-type CuO and n-type SnO₂ and TiO₂ are assembled to form p-n junction CO sensors.SnO₂ nanorods and TiO₂ nanorods were synthesized by hydrothermal method,and SnO₂/CuO and TiO₂/CuO heterojunction films were obtained by in-situ growth of CuO nanorods by water bath method on top of the SnO₂ and TiO₂ films.For the SnO₂/CuO samples,ethanol content and tin source content in the precursor solution were controlled to adjust the morphology of SnO₂ structure,so as to prepare suitable morphology for the better growth of CuO on its surface.The heterojunction interface was adjusted to improve the sensitivity of the sensor to CO gas at room temperature.The SnO₂/CuO heterojunction samples all exhibit a typical p-type response to carbon monoxide,with the best sensors having a response value of 2.7 when exposed to very low concentrations of 1 ppm and 6.3 when exposed to 200 ppm.For the TiO₂/CuO samples,the copper source content in the precursor solution was changed to adjust the morphology and thickness of the TiO₂/CuO bilayer films during the preparation of CuO layer.The gas sensitivity of TiO₂/CuO heterojunction devices to carbon monoxide was tested at room temperature（25±1℃）.The best performance sample CT2 has a response value of 10.8 at 200 ppm CO,which is a great improvement compared with the single layer TiO₂ or CuO sample.TiO₂/CuO samples have better gas-sensitive response than the SnO₂/CuO samples,mainly because TiO₂/CuO sensor has a better contacted interface than SnO₂/CuO,the TiO₂ nanorods are more conducive to charge transfer and more porous than SnO₂ nanorods.In addition,the TiO₂/CuO sensor shows excellent selectivity and humidity resistance,especially the obvious inhibition behavior for hydrogen.The calculated I_F（CO,H₂）value of CT2 reaches the maximum value of 8.9,which can effectively solve the cross-sensitive behavior existing in the test process.The important role of oxygen vacancy on the surface of CuO layer on the improvement of CO performance was analyzed.The adsorption mechanism of the two gases,the diffusion length and the space charge region formed at the interface of the two materials were compared to analyze the reasons for the different response behaviors to CO and H₂.2.Improvement of CO gas sensing performance of Nb-doped TiO₂/CuO heterojunction films:As the TiO₂/CuO heterostructure thin films have long response time to CO,Nb was adopted to dope the TiO₂ layer to improve the CO sensing performance of the TiO₂/CuO heterostructure.Nb precursor was introduced into the solution of hydrothermal prepared TiO₂thin film in the first step,to regulate the crystal phase and micro-morphology of the thin film.The successful doping of Nb prevented the TiO₂ nanorods from growing vertically on the substrate,while increased defects and generated more oxygen vacancies.Nb⁵⁺replaces Ti⁴⁺,bringing in more electrons.CuO was grown with a secondary step water bath on the surface of the Nb doped TiO₂ films to form a Nb-TiO₂/CuO（NCT）heterostructure.The change of TiO₂ layer morphology after doping affects the growth of CuO on the surface and the change of CuO crystal plane orientation.The response time of the NCT samples is shorter than that of the CT samples.Among the prepared samples,the NCT3 sample has the shortest response time.When the sample is exposed to 200 ppm CO,the response time of the CT sample is 300～400 s,while the response time of the NCT sample is reduced to 40-80 s,achieving a significant reduction in response time.This is attributed to the effect of a large number of electron and space charge regions brought by Nb doping and the change of potential barriers.The doping of Nb produces more oxygen vacancies and lattice oxygen in the sample,which provides a large number of electrons on the material surface and effectively promotes the adsorption of gas,rapid electron capture and chemical reactions on the material surface.It helps to form more O^2-on the material surface and also promotes the interaction with CO gas,so it shortens the response time.In this paper,CuO-based heterojunction thin film gas sensor devices are designed for carbon monoxide gas sensing at room temperature.The TiO₂/CuO heterojunction thin film samples have higher response than SnO₂/CuO heterojunction thin film samples.In addition,the Nb-TiO₂/CuO heterojunction thin film sample was prepared by doping to solve the problem of slow gas response and improve the gas-sensitive performance of carbon monoxide sensor at room temperature.