| Transition metal oxide nanomaterials have been attracting much attention because of their unique physical and chemical properties as well as promising applications in micro/nanoscale devices.The controllable synthesis of the metal oxide nanostructures with different sizes,morphologies,chemical compositions,and structures have become a key progress in nanoscience and nanotechnology.Although the metal oxide nanostructures have been successfully grown by thermal oxidation method,which shows advantages of simplicity,efficient,low-cost,and large-scale preparation,the cracking problem caused by the low adhesion between the nanomaterial and substrate restricts their practical applications in functional nanodevices.Therefore,the synthesis strategies for the direct growth of oxide nanostructures on different substrates still needs to be developed.Furthermore,the functional properties of nanomaterials synthesized by thermal oxidation method also need further investigations.In this thesis work,the growth mechanisms and the approaches for enhancing the adhesion of CuO nanowires on the substrate were firstly investigated.By combining the thermal oxidation with sputtering deposition and thermal evaporation,CuO nanostructures and hetero-nanostructure were prepared on different substrates,including Si substrate,Ni foam,and C cloth.The physical and chemical properties,including field emission,gas sensing,nanoscale electrical,photoluminescence?PL?,and electrochemical properties,were investigated.The main researches and results of are as follows:Firstly,the conventional hotplate thermal oxidation technique was modified by introducing one or two thermal conductive plates to enhance the growth and the adhesion of CuO nanowires on the Cu foils.In detail,the Cu foils were heated on the hotplate directly,on one thermal conductive plate over the hotplate,or between two thermal conductive plates over the hotplate at 400 oC in ambient condition for 24 h.The results indicated that adding plates can supply a more stable oxidation environment.The sufficient activated oxygen,stable temperature,and proper temperature gradient caused by the two plates accelerated the oxidation of Cu and formation of CuO nanowires,resulting in the large length and diameter with moderate density.The grain-boundary diffusion and Kirkendall effect were proposed to explain the diffusion and aggregation of vacancies,the growth of nanowires,and the formation of cracks under different heating conditions.The modified thermal oxidation method not only enhanced the adhesion between the CuO nanowires and the substrate,but also improved the field emission properties of the obtained nanowires,which showed the lowest turn-on field of 5.31 V/?m and lowest threshold field of 9.8 V/?m.The excellent field emission properties and enhanced nanowire-substrate adhesion indicate that these nanowires have potentials for the application of the cathode of field emission displays.Secondly,an electric field was introduced into the modified thermal oxidation method to further control the thermal oxidation synthesis of CuO nanowires.The effects of electric field intensity and direction on the morphology,structure,and field emissions of the nanowires on both sides of the Cu foils were investigated.The results showed that the electric field improved the length and density of the nanowires with growing direction parallel to the electric field by accelerating the diffusion of Cu ions via the oxide layer and along the nanowires.Interestingly,the growth direction of nanowires was also changed by the field.The structure of the oxidized layer prepared with different electric fields was Cu foil Cu2O layer/CuO layer CuO nanowires.The growth of CuO with different morphologies on both sides of Cu foils can be successfully explained by the influences of electric field and temperature field on the ion diffusion and the different oxygen supplies on both sides.The related work provides a new approach to control the growth of metal oxides through adjusting temperature gradient and electric field.The employment of electric field makes it is possible to obtain CuO nanowires at a relatively low temperature,broadening the application ranges of thermal oxidation on different substrates.This work suggests that the CuO nanostructures can be further assembled onto Si substrate,Ni foam,and C cloth.Thirdly,to realize the application of CuO nanowires on nanodevices,single crystal CuO nanorods with good adhesion were fabricated on Si substrates via a facile two-step strategy of sputter depositing Cu films followed by thermal oxidation.The effects of radio frequency?RF?/direct current?DC?sputtering,Cr buffer layer,and step deposition on the morphology and adhesion of oxidized CuO were analyzed.The optimal oxidized sample showed a structure of Si wafer/thin oxide layer with dispersed small voids/thick CuO layer/CuO nanorods.The in-situ current-voltage?I-V?curves of the CuO film and CuO nanorod on Si substrate were directly characterized by the conductive atomic force microscopy?C-AFM?.The CuO film showed an almost symmetric and incomplete linear characteristic,while the CuO nanorod showed a p-type based nonlinear and asymmetric characteristic.The good rectifying characteristic of the upright standing CuO nanorod was attributed to the high specific surface area caused surface states and the metal-semiconductor contacts with different morphologies on the top and bottom surfaces.The room temperature PL spectra of CuO nanorods excited with an ultraviolet light showed a main and wide luminescence band from dark violet to bright blue in the range from 390 nm to470 nm and some weak emission peaks in visible light range.Compared with the bulk CuO,the blue shift of about 7 nm is attributed the enhancement of the quantum confinement effect caused by the decreased dimension and size of nanomaterials.Fourthly,the core-shell structured CuO/ZnO nanorods were directly fabricated on Si substrate via sputtering deposition of ZnO on the CuO nanorods with good adhesion.The nanorod was consisted of a single-crystalline p-type CuO core covered with the nanocrystalline n-type ZnO shell,which was confirmed by the composition and structure characterizations.The in-situ I-V characterizations by C-AFM of the core-shell structured CuO/ZnO nanorods revealed a novel p-type based self-rectifying resistive switching behavior,providing a solution to suppress the sneak current issue in a crossbar structure for nonvolatile memory devices.The special switching characteristic can be explained by the asymmetric back-to-back structure of C-AFM tip/ZnO and ZnO/CuO under different voltage directions due to the special p-n core-shell structure of nanorod.In addition,a red-shift of about 20 nm and increased intensities of PL peaks for the CuO/ZnO core-shell structures were attributed to the deposited shell and special heterostructure.The above results demonstrated that the electrical and optical properties of CuO nanorods can be successfully tuned by adding a ZnO shell.Fifthly,the CuO nanorods and CuO nanorods-based hetero-nanostructures,which can be directly used as a binder-free anode for lithium-ion batteries?LIBs?,were successfully synthesized on Ni foam by combining magnetron sputtering,thermal oxidation,and thermal evaporation methods.The thermal oxidation of Cu/C multilayer film and thermal evaporation Mo were proposed to improve the electrochemical performance of CuO nanorods on Ni foam.The results showed that the CuO nanorods anode obtained at 400°C showed a higher capacity than that obtained at 330°C.The better electrochemical properties by thermal oxidation of Cu/C multilayers can be attributed to the moderate length,diameter and density of the nanorods.Moreover,a special morphology in which a wide range of nanoflakes were formed in the middle of slender nanorods were formed after thermal evaporation of Mo on the CuO nanorods obtained at 330 oC on Ni foam.Differently,a dendritic structure with CuO nanorods as tree trunks and copper molybdenum oxide as the branches were formed by thermal evaporation of Mo on the CuO nanorods obtained by thermal oxidation at 400 oC on Si substrate and Ni foam.The electrochemical performances of these two heterostructures were successfully improved,and the obtained highest initial discharge areal specific capacity and reversible capacity after50 and 500 cycles on Ni foam were 4.80,1.38,and 0.88 mAh cm-2,respectively.This structure can be also used in other oxides to improve their electrochemical performance.Finally,different CuO nanostructures were successfully prepared on C cloth through combining the sputtering deposition and electric field assisted thermal oxidation at 330-400 oC.Although the obtained CuO nanorods anode at 400°C on Ni foam showed better electrochemical properties,the CuO nanocone structure obtained at 400 oC on C cloth showed worse electrochemical and PL properties due to low heat-resistance of C cloth and the lack of nanorods.A hierarchical network composite composed of C cloth/Cu2O layer/CuO layer/C-doped CuO nanorods were successfully fabricated by heating the Cu film on C cloth at a lower temperature of 330°C under an applied electric field.The as-synthesized composite can be directly used as the binder-free flexible anode for LIBs,delivering a higher specific capacity than Ni foam and excellent cycling stability,including high initial discharge specific capacity of 5.15 mAh cm-2(1300 mAh g-1)and high stable-reversible capacity of 2.38 mAh cm-2(646mAh g-1)after 150 cycles.The superior performance is attributed to the advantages of high specific surface area,defects,short diffusion path,stable structure and excellent conductivity.The flexibility of C cloth exhibits potential applications in the flexible LIBs.In addition,a novel green PL emission band ascribed to the singly ionized oxygen vacancies and the blue-shift of PL peaks were detected from the C-doped CuO nanorods on C cloth.Overall,in this work,based on the synthesis,structural design and performance exploration of the CuO-based one-dimensional nanomaterials,the adhesion of nanowires/nanorods on the Cu foils,Si substrates,and other substrates was enhanced.The heterostructured nanomaterials,such as core/shell structure,were also developed and prepared,which showed novel nanoscale electrical properties and excellent electrochemical performance as anode of LIBs.The present results have shown significance in development of functional nanodevices based on thermal oxidized oxide nanomaterials. |
PDF Full Text Request