Research On The Preparation Of Graphene Oxide/Zinc Oxide Nanocomposites And Their Photoelectric Detecting Properties

Nanostructured Zinc Oxide（ZnO）has been an ideal semiconductor material for UV photodetecter because of its wide band-gap,high excitation binding energy,high breakdown voltage and saturated drift rate,large specific surface area,strong anti-radiation,abundant and low-cost raw materials.However,ZnO nanostructures tend to aggregate together due to their high surface activity,resulting in higher recombination rate of photogenerated electron-hole pairs and the deterioration of the photoelectric performance of the devices.Fortunately,it is an effective approach to facilitate the separation of the electron-hole pairs by compounding with graphene materials.Graphene oxide（GO）,act as a derivative of the graphene,not only possesses the similar excellent properties with graphene,but also is easier to composite with many organic/inorganic materials than graphene due to the oxygen-containing functional group on its surface and edge.However,the preparation of GO/ZnO nanocomposites and reasonable device stuctures are still the critical issues of GO/ZnO UV photodetector.Therefore,taking GO/nanostructured ZnO as the research object,solvothermal method was employed to prepare GO/ZnO nanocomposites.To achieve excellent performance for GO/ZnO nanocomposites UV photodetector,taking advantage of the adjustable physical and chemical performance of GO/ZnO nanocomposites,the means of low-temperature annealing,atmosphere annealing and growing ZnO nanorods arrays were used to regulate the crystallization characteristics,the defects state and morphology of GO/ZnO nanocomposites.Meanwhile,the relationship between UV photoresponse properties and microstructure charateristics of GO/ZnO nanocomposites were studied.Besides,high spectrum selective UV photodetectors with multilayer structures were designed and constructed.ZnO nanocrystals with the grain size of 6⁹ nm were synthesized by solvothermal method.Due to the poor crystal quality and the reaction residual layer on the surface of ZnO nanocrystals which could hinder the nanocrystals coupling and carriers transmission would worsen the photoelectrical properties,thermal annealing in vacuum oven was applied to regulate the crystallization and improve the optical properties of ZnO nanocrystals by adjusting the annealing temperature.Then,ZnO nanocrystals films UV photodetectors were fabricated by spin-coating at the room temperature.When the annealing temperature was 150℃,the optimal performance of the detector was obtained,and the responsivity of the devices was 52.84 mA/W,corresponding normalized detectivity was 8.55×10¹⁰ Jones.Based on the synthesis of ZnO nanocrystals,GO was introduced into the system to prepare GO/ZnO nanocomposites with ZnO nanocrystals uniformly dispersed on the surface of the GO.Low-temperature annealing was firstly adopted to“one-step”achieve the reduction of the GO and the improvement of crystallinity of ZnO nanocrystals.The controlling of the content of oxygen-containing functional group on GO was realized by adjusting the annealing temperature.Due to low-resistance channel between reduced GO and ZnO nanocrystals,which promoted the separation and transimssion of carrier,the photocurrent of GO/ZnO nanocomposites was significantly improved.After the low-temperature annealing,the responsivity of the devices of 131.36 mA/W was obtained,corresponding normalized detectivity was 8.22×10¹⁰ Jones.According to the fact that photoelectrical properties ofⅡ-Ⅵsemiconductors are easy to be affected by the defects in lattice,the defect status in ZnO nanocrystals and GO/ZnO nanocomposites were adjusted by changing the annealing atmosphere（air and vacuum conditions）,and the influence of defect status on the photoelectrical performance of the materials was studied.The samples annealed in vacuum possessed a higher concentration of oxygen vacancy defects due to the oxygen deficient condition,which was in favor of the improvement of conductivity of the films.Besides,the reduction of GO in the annealing process could also promote the photoresponse performance of GO/ZnO nanocomposites.Under the combined action of the oxygen vacancy and the reduced GO,the responsivity of GO/ZnO nanocomposites annealed in vacuum was improved to 204.7 mA/W,and the normalized detectivity was promoted to 6.01×10¹¹ Jones.In addition,the XPS analysis of ZnO nanocrystals and GO/ZnO nanocomposites showed that the oxygen vacancy in ZnO lattice could be“healed”by the oxygen in GO,resulting in the decrease of oxygen vacancy concentration in GO/ZnO nanocomoposites.In order to further promote the photoresponse performance of the detectors,taking advantage of the higher light absorption for one-dimensional ZnO nano arrays than zero-dimensional ZnO nanocrystals,GO/ZnO nanorods arrays were prepared by using GO/ZnO nanocomposites as the seed layers,and the growth properties and photoresponse performance of GO/ZnO nanorods arrays were controlled by adjusting the content of GO and the growth time of hydrothermal process.Because of the increased ZnO nucleation and growing sites supplied by the oxygen-containing functional group on GO,more dense ZnO nanorods array was obtained;what’s more,ZnO nanorods were likely to like-epitaxial growth on the surface of GO due to the similar lattice structures between GO and ZnO,resulting in better well-oriented nanorods arrays than pure ZnO nanorods arrays.Dense and well-oriented nano arrays effectively enhanced the absorption of incident light,improving the photoelectric conversion ability of the detectors.When the content of GO and growth time were proper,the responsivity and normalized detectivity were up to 1005.81 mA/W and 1.604×10¹² Jones respectively.In order to improve the spectrum selectivity of ZnO-based UV photodetctors,taking advantage of the tunable bandgap of ZnO nanocrystals,multilayer structures（filter/blocking/response）were designed to fabricate spectrum selectivity response UV photodetectors.When the response material was GO-ZnO nanorods arrays,spectrum selectivity photodetector with a“response window”from 350 nm to 390 nm（corresponding half high width was 21 nm）was obtained.The device showed a optimal response of 459mA/W and a normalized detectivity of 5.9×10¹¹ Jones under 370 nm incident light.When the response material was rGO-ZnO nanocomposites,spectrum selectivity photodetector with a“response window”from 350 nm to 375 nm（corresponding half high width was 14nm）was obtained.The device showed a optimal response of 64.8 mA/W and a normalized detectivity of 2.3×10¹¹ Jones under 365 nm incident light.