Prepareation And Sensing Properties Of ZnO Nanowire-Graphene Composites

Graphene has been considered as a promising candidate for electrically based gas sensors and widely used in the fields of photoluminescence,solar cells,and field-effect transistors because of the excitation binding energy,wide band gap,and property of easy fabrication.Its high quality of its crystal lattice leads to a quite low electronic noise and the two-dimensional structure provides great sensing area per unit volume.In this thesis,ZnO nanowire-graphene（ZnO NW-GR）composites were successfully prepared in different conditions and used to detect ammonia（NH₃）at room temperature for the first time.Besides,we built a portable gas sensing system based on the hybrid composites.In details,we obtained the achievements as follows.（1）ZnO nanowires were fabricated by a modified carbothermal vapor method,graphene oxide sheets were synthesized by a modified Hummers method.The composites were synthesized through hybridization of ZnO nanowires and graphene oxide sheets,which had been prepared in large scale through mechanical mixing and low-temperature thermal reduction processes.A series of characterization were taken to characterize the hybrid composites.（2）The NH₃ gas sensors based on the composites were fabricated and used to detect ammonia at room temperature.The devices exhibited an excellent response,which was much better than those of individual ZnO nanowires and graphene based sensors.When the ZnO nanowires content was 25 wt%,the hybrids exhibited a 19.2%resistance change to 50 ppm NH₃ at room temperature.Moreover,the sensor shows a fast response,full recovery,excellent long-term stability and selectivity toward NH₃.（3）A ZnO NW-GR based portable gas sensing system working at room temperature has been demonstrated for the first time.The system is mainly consist of six parts,including ZnO NW-GR based gas sensor,the power supply,signal modulating circuits,A/D conversion circuits,micro-programmed control unit（MCU）and upper computer,which enable the real-time detection of NH₃ at room temperature.Analog circuits and digital circuits were separated to use different power supplies.The signal modulating circuits converted the analog voltage signal into digital records.MCU was in charge of operating working procedures of the A/D converter,dealing with the converted data and communicating with the hosting computer using a pair of bluetooth.The portable gas sensing system can detect low-concentration NH₃ with higher sensitivity.