Controlable Synthesis Of ZnO Nanohierarchitectures And Their Electrochemical Properties

The unique and fascinating properties of nano-structured materials have triggered tremendousmotivation among scientists. Zinc oxide is a versatile semiconductor material, which has beenfound useful in many applications such as nanolasers, light-emitting diodes, piezoelectricnanogenerators, surface acoustic wave devices, transparent conducting materials and solarcells due to the unique optical and electrical properties. Meanwhile, ZnO nanostructures areone of the most promising materials for the fabrication of chemical and biosensors due tohaving exotic and versatile properties including biocompatibility, nontoxicity, chemical andphotochemical stability and so on. On the other hand, the properties of ZnO are closely relatedto its morphology, size and crystallinity. Therefore, developing a shape and size controlledZnO nanomaterials is indispensable for exploring the potential of ZnO as a source of smartand functional materials. Hierarchical micro-/nanostructures, assembled by various lowdimensional nanomaterials as building blocks have shown enhanced performs and haveattracted considerable attention in the past few decades because of their special morphology,size, and hierarchy. The synthesis and self-organization of micro-and nanoscale inorganicmaterials with special morphology, size, and hierarchy, and the systematic study ofstructure-property relationships are ones of the most central issues. In this work, ZnOnanomaterials with cotrolled structure have been successfully prepared by a simple one-stepsolution route using the trisodium citrate as a shape-directing agent. A possible controllableformation mechanism of the ZnO nanostructures was proposed. Furthermore,3D hierarchicalporous ZnO hollow microsphere has been successfully prepared and their applications inbiosensors have been studied. Finally, the sandwiched ZnO-graphene nanocomposites havebeen fabricated and their performances for supercapacitors were explored.The main works inthe thesisare given as follows:(1) A facile solution route to synthesize ZnO hierarchical architectures was carried outemploying trisodium citrate as structure-directing agent. FESEM, TEM, XRD and FT-IRspectroscope were used to investigate the structure character of the products. The influence ofthe citrate on ZnO microstructure was studied. It was found that the citrate concentrationplayed a key role in evolution of the morphology and crystal phase. On the basis ofexperimental results, a possible formation mechanism of the ZnO hierarchical nanostructureswas proposed.（2）Porous ZnO hollow microsphere has been successfully prepared by a simple one-stepsolution route using the trisodium citrate as a shape-directing agent. FESEM, FT-IR, TEM and XRD were used to characterize the samples. Serials of experiments show the reactiontime has a prominent impact on the phase and morphology transformation. At a point of thecontrolling growth time, porous ZnO hollow microspheres with average diameter of2-3μmand hole-opening diameter of~0.5μm were obtained which show a high surface area (117.36m2g1) and a large pore volume (0.50cm3g1). On the basis of experiment results, apossible growth mechanism for as-synthesized porous ZnO hollow microspheres was revealedto be a two-step process：The zine citrate complexion was preferentially formed, and then thedissolution of zine citrate complexion and the formation of ZnO crystal occurredsimultaneously, associated with the special role of the citrate anion on oriented growth.(3) A novel, biocompatible glucose senors based porous ZnO hollow microsphere wasdeveloped. The as-prepared three-dimensional (3D) nanohierarchically ZnO with controlledmorphology and dimensions was functionalized by Au nanoparticles (AuNPs) via in-situreduction of HAuCl4. The biosensor was fabricated by immobilizing the glucose oxidase(GOD) onto the Au-ZnO nanocomposit on the surface of glassy carbon electrode (GCE). TheDET beween GOD and electrode was obtained due to high surface area, good biocompatibleand fast electron transfer ability. The prepared biosensor exhibited high sensitivity,anti-fouling properties, lower detection limit and high stability for glucose sensing.（4）The biocompatible Au-ZnO nanocomposite was used to fabricate a sensitive sensor forthe detection of dopamine (DA). The prepared biosensor exhibited high sensitivity,, lowerdetection limit and wide line range for DA sensing due to high surface area ofnanohierarchically ZnO and high catalytic property of AuNPs.（5）Anovel label-free DNAhybridization biosensor was fabricated using3-dimensional (3D)ZnO superstructures as enhanced sensing platform, employing chitosan (CS) as film-formingmaterial. A highly sensitive electrochemical DNA sensor was constructed by homogenouslydistributing Au nanoparticles (AuNPs) on ZnO-CS matrix. The AuNPs/ZnO-CS filmexhibited good conductor for accelerating the electron transfer, which led to obvious signalamplification and low detection limit for electrochemical sensing.(6) The high-performance supercapacitor based ZnO-graphene nanocomposites wasdevelopped. The sandwiched ZnO-graphene nanocomposites have been fabricated via a facile,low-temperature in situ wet chemistry process. During this process, high dispersed ZnOnanoparticles are embedded in graphene nanosheets. Thus, intimate interfacial contactbetween ZnO nanoparticles and graphene nanosheets are achieved, which facilitateselectrochemical activity and enhance electrochemical properties due to fast electron transfer.The as-prepared ZnO-graphene nanocomposites exhibit a maximum specific capacitance of 786F g-1and excellent cycle life with capacity retention of about92%after500cycles. Thisfacile design and rational synthesis offers an effective strategy to enhance the electrochemicalperformance of supercapacitors and shows promising potential for large-scale application inenergy storage.