Synthesis, Growth Kinetics And Applications Of Metal Oxide Nanostructured Films/Arrays On Metallic Substrates

Metal oxide nanostructured films/arrays have potential applications in micro-electronic devices, energy conversion and storage(photovoltaic cells,lithium batteries,supercapacitors and hydrogen storage,etc.),chemical/biosensors,light-emitting displays,drug delivery/separation,optic storage and so on.The efficient control of the orientation,morphology,surface area,porosity and material size of the films/arrays is the prerequisite for practical applications.However,the mature methods nowadays for fabricating high-quality nanostructured films/arrays require high temperature,the use of seeds or multi-steps.Moreover,limited by the fabrication conditions and methods,most films/arrays were previously attained only on a series of small-area semiconducting/insulating substrates or on ITO. This did not benefit their applications in specific optoelectronic,energy and sensing-related devices. The current thesis aims to develop simple and general methods to realize the direct growth of several metal oxide nanostructures on large-area metallic substrates,ensuring the robust mechanical and electrical contact.More importantly,the direct use of some typical metal oxide(ZnO,SnO₂ and Fe₂O₃)-based nanostructured films/arrays as electrodes in field emission,electrochemical battery devices and biosensors will be discussed in detail(without the tedious electrode fabrication process).The main contents include the following:1.ZnO nanostructures with various morphologies(nanoneedle,nanowire,pencil-like nanorod and nanosheet) are fabricated on different kinds of flexible conducting metal substrates(including Fe-Co-Ni alloy,Ni,Cr,Ti,etc.) by a simple solution method(60℃＜T＜100℃).The growth kinetics is studied.The proposed inexpensive method is highly reproducible and suitable for large-scale fabrication(-10×10 cm² or even larger).We further study the photoluminescence and field emission (FE) property of ZnO arrays.It is found that ZnO nanoneedle array exhibits excellent FE performance and has potential application in flexible FE displays.2.Synthesis of SnO₂ nanorod arrays on Ni,Ti and alloy substrates by a simple hydrothermal method at 200℃and the efficient control of the array parameters such as the diameter/length of nanorods and the array density.The growth mechanism is also studied.3.Fabrication ofα-Fe₂O₃ top-closed porous nanotube array by room-temperature immersing ZnO needle-like nanorod array into ion salt solution followed by annealing in air.Interestingly,ZnO template can be in-situ removed during the immersion and the surface structure ofα-Fe₂O₃ nanotube can be effectively controlled by varying the ion salt concentration.A"sacrificed template-accelerated hydrolysis"mechanism is proposed.More importantly,α-Fe₂O₃ nanotube array can be transferred onto other flexible metal substrates by using ZnO array growing on these substrates.ZnO nanostructures disordered or with low orientation to the substrates can also be utilized to fabricateα-Fe₂O₃ nanotubes.In addition,annealing the immersed product in H2 leads to the formation of Fe₃O₄ top-closed porous nanotube array.4.We study for the first time the direct use of ZnO nanorod array as the anode for the lithium ion batteries(LIBs).The ZnO array exhibits higher reversible capacity and cycling life as compared to traditional ZnO films.Further,coating a thin layer of carbon onto the surface of ZnO nanorods will improve the electrochemical performance of ZnO arrays,especially when cycled at higher discharge and charge rates.The good conductivity and electroactivity of carbon play an important role.5.Development of a novel method to fabricate Zn-Al LDH nanosheet film directly from Al substrate and discuss of formation mechanism.Synthesis of morphology-controlled Zn-Al LDH ordered nanosheets on Zn-coated Fe-Co-Ni alloy by using a room-temperature "two metal substrates co-immersion" method.The route can be extended to LDH structures on other kinds of "bivalent metal"-coated substrates,such as Cu-Al LDH on Cu-coated alloy substrate.Calcination of Zn-Al LDH film under an argon atmosphere leads to the formation of porous ZnO/ZnAl₂O₄ nanosheet film. Each nanosheet consists of ZnO and ZnAl₂O₄ nanoparticles,and spinel ZnAl2O4 distribute uniformly in the ZnO nanoparticle matrix.We find that inactive spinel ZnAl₂O₄ can act as a buffer to relieve the stress caused by the volume change during charge-discharge cycling,leading to improved electrochemical performance of ZnO anode.This work represents a successful example for the development of promising anode materials for Li-ion batteries.6.We study the electrochemical performance of SnO₂ nanorod array and C-SnO₂ nanorod array as the anodes for LIBs.The effect of structural parameters of the array on the battery performance is systematically discussed.An array consisting of SnO₂ nanorods of average 60 ran in diameter and 670 nm in length delivers a reversible capacity of as high as 580 mAh/g after 100 cycles(at 0.1C) and shows excellent rate capability(2C-5C range).7.The LIB application ofα-Fe₂O₃(C/α-Fe₂O₃) top-closed porous nanotube arrays has been investigated.Based on the uniform distribution of carbon in the nanotubes and the hollow tubular configuration,we expatiate how to improve the electrochemical performance of LIB anodes by creating novel nanostructures of active materials.8.The first use of C-ZnO nanorod array on Ti as the working electrode for the immobilization of enzymes has been demonstrated.We construct the first metal oxide nanostructure array electrode-based direct electrochemistry biosensor.This biosensor is used to detect the concentration of glucose,showing excellent performance. 9.The K_0.33WO₃ nanosheet film is synthesized by direct heating W foil in air that is pre-immersed in the KOH aqueous solution.The obtained film exhibits high conductivity(five orders of magnitude higher than ZnO),good hydrophilicity and biocompatibity.It is further employed to act as the working electrode of glucose electrochemical biosensor.After modified with glucose oxidase, the K_0.33WO₃ nanosheet film electrode shows very high sensitivity to glucose,low detection limit and good selectivity.In addition,we discuss briefly the synthesis of ternary oxides such as sheelite CaMoO₄,SrMoO₄ and BaMoO₄ multilayered nanosheet films on Al(Ti) substrates.This opens up the possibility of fabricating multicomponent oxide nanostructure arrays/films directly on metallic substrates.In summary,this thesis develops several simple solution-based methods to fabricate a variety of functional metal oxide nanostructures on inert metallic substrates.The direct fabrication of these nanostructure arrays/films on conducting metal substrates ensures the robust adhesion between materials and substrates,making it possible to directly integrate them into various nanodevices.We particularly exploit the applications of nanostructure films/arrays in some forefront areas such as LIBs and electrochemical biosensors.