Nano Metal Oxide/Graphene Composites For Study On Flexible Transparent Paper-based Gas Sensors

As a kind of electronic devices,flexible gas sensors have been widely used in wearable sensor as electronic skin and for implantable medical treatment.Campared with traditional ceramic and glass-based gas sensors,flexible devices are mostly prepared on polymer substrates by low-temperature?<200??fabrication process.Therefore,with the growth of research and application,a number of non-degradable plastic wastes are generated,which is unfriendly to the environment.In order to acheive the sustainable development of human society,scientists began to look for environmental friendly materials to replace existing plastics substrates to make"green"electronic devices.Therefore,transparent paper-based electronic devices with high potential to obtain mass and sustainable production of"green"devices has become a research hotspot in the research.How to fabricate a low cost environmental friendly gas sensor with good sensing performance at room temperature has been always concerned by the academia and the industry.In this thesis,based on the advantage of electrical and mechanical property of the two-dimensional graphene?graphene oxide,rGO?composite materials,by nanotechnology,through experiment composed with theoretical analysis,we had developed a variety of transparent paper-based gas sensors,which will expand the development and application of flexible electronics in the future.In this thesis,CNFs transparent paper for gas sensors is prepared in primary.The pulp was oxidated by TEMPO/NaBr/NaClO catalytic system,further treated by ultra-high pressure homogenization method to obtain nanocellulose?CNFs?.Nanocellulose?CNFs?and transparent paper were characterized by TGA and AFM.The results showed that CNFs transparent paper,with good property of 90%light transmittance,excellent mechanical properties,good thermal stability,nanometer surface roughness and easy printing,could be obtained by tuning the dosage of NaClO and homogenization times.With the method of screen printing,gas sensors are fabricated on different substrates?alumina plate,A4 paper,vegetable parchment and transparent paper?,and tested at room temperature.The result shown transparent paper-based gas sensor perform better sensing property than other substrate-based sensors,proved that to reduce the surface roughness help raise gas sensitive film stability,and is helpful for signal collection,to get better sensing performance.The most widely solid state chemical gas sensors are resistance sensors based on metal oxide semiconductors.In order to fabricate a printed paper-based sensor with high sensitivity and rapid response at room temperature,the problem of high operating temperature,low sensitivity,long response/recovery time and lack of flexibility of metal oxide should be solved primarily.One way to overcome this limitation is to combine metal oxides with carbon-based materials.Graphene,as a member of the carbon nanomaterials,not only has good electrical conductivity,high specific surface area,high electron mobility and low electrical noise,but also with its surface defects served as gas sensing active site,it will largely improve adsorption and reaction of the gas molecules for sensing test.With the property of good sensing selectivity of metal oxide,the preparation of composite not only can improve the performance of gas sensing,lower working temperature,higher response speed;will also meet the demand of flexibility for transparent paper sensor.In this thesis,two semiconductor metal oxide materials,TiO₂ and ZnO,were synthsised,which were reduced and compounded with GO to construct gas sensor on the prepared transparent paper base.?1?Anatase TiO₂ quantum dot clusters?QDs?were synthesized by hydrolysis at low temperature?80??and Ag decorated composites were further prepared by calcined at 550?.Mixture Slurry of TiO₂ QDs and GO were coated on paper substrate by screen printing method,and GO was in-situ reduced by certain amount of oxalic acid at 120?for 10 h to fabricate composites.The characterization results showed that TiO₂ QDs were spherical nanoparticles with size of 66 nm,which were formed by the agglomeration of 2-5 nm TiO₂ quantum dot.Due to this special property,it obtained a large specific surface area?315.74 m²/g?with uniform size.As we all known,particle materials cannot use as flexible materail,so to lay a foundation for the later preparation of paper-based devices,we use ceramic tube?curved?substrate to prepare gas sensors.Finally,XRD and Raman results of showed that GO was successfully reduced and TiO₂ QDs/rGO composite sensor was prepared by facile way.Gas sensing performance results showed that the gas sensor has a good sensing property towards ammonia,and the optimal addition amount is 3%?molar ratio?,which the response of Ag/TiO₂ QDs composites sensor is6 times higher than pure TiO₂ sensor.The detection range to ammonia gas was 10-100 ppm with the response value of 25.1-40.7,and a good linearity of sensing characteristics could be observed.Meanwhile,the response and recovery time of the sensor to 10 ppm acetone gas were as low as 183 s and 604 s,respectively.TiO₂ QDs/rGO composites transparent paper-based gas sensor has not achieved good gas sensing property,but it is significance for future experiments.?2?ZnO nanoroids were synthesized by solvothermal method,and two-dimensional ZnO/rGO composites were further prepared by hydrothermal method,in which the rGO addition amount was 1-15 wt%.Then,a room temperature gas sensor was fabricate on CNFs transparent paper by screen printing and its sensing performance toward ethanol was investigated.The characterization results showed that the length and width of ZnO nanorods were about 30-200 nm and 30 nm,and uniformly assembled on the surface of rGO.Gas sensing performance results showed that the optimal addition amount is 10 wt%,which the response of ZnO/rGO composites sensor is 10.5 times higher than pure ZnO sensor.The detection range to acetone gas was 0.2-9 ppm with the response value of 2.03-34.45,and a good linearity of sensing characteristics could be observed.Meanwhile,the response and recovery time of the sensor to 0.2 ppm acetone gas were as low as 18 s and 4 s,respectively.Finaly,it showed good sensing stability after bending as well.?3?ZnO quantum dots were synthesized by hydrolysis at low temperature?32??,and two-dimensional ZnO/rGO composites were further prepared by hydrothermal method,in which the rGO addition amount was 1-15 wt%.Then,a room temperature gas sensor was fabricate on CNFs transparent paper by screen printing and its sensing performance toward acetone was investigated.The characterization results showed that ZnO quantum dots were about 8 nm and uniformly assembled on the surface of rGO.Gas sensing performance results showed that the optimal addition amount is 10 wt%,which the response of ZnO/rGO composites sensor is 8.6times higher than pure ZnO sensor.The detection range to acetone gas was 1-16 ppm with the response value of 1.49-13.02,and a good linearity of sensing characteristics could be observed.Meanwhile,the response and recovery time of the sensor to 1 ppm acetone gas were as low as65 s and 33 s,respectively.Finaly,it showed good sensing stability after bending as well.Compared with the traditional gas sensor,transparent paper-based sensor has the characteristics of environmental friendly,flexibility and transparency,smart,miniaturization,low energy consumption at room temperature and mass production,which lays a solid foundation for the following scientific research and practical application.