Researches On The Differences Of Electrochemical Behaviors Of Carbon Nanotubes With Different Orientations And Layers

Carbon nanotubes (CNTs) has been widely studied and applied in various areasof the scientific community since it was discovered. As a kind of nano materials,CNTs have excellent heat transfer properties, electrical properties, mechanicalproperties, optical properties and hydrogen storage properties, etc. Thesecharacteristics have attracted numerous researching enthusiasms. Researchers haveapplied the feature of nanotubes such as wide potential window, large specificsurface area and excellent conductivity into the field of electrochemistry andproduced a lot of electrochemical sensors, super capacitors and so on. In recentyears, with the proposing of aligned carbon nanotubes array (ACNTA) electrode,anisotropy of CNTs as a quasi-one-dimensional structure material is gradually beingconcerned. And the relationship between the arrangement and conductivity of CNTsis also worth further exploration. As the best model to study this issue, themanufacturing of ACNTA electrode is especially important. Langmuir-Blodgett (LB)film is a kind of molecular film assembly technology with a lot of advantages suchas thin, uniform, thickness controllable and molecular layer, highly anisotropic,without damage to the substrate etc. And LB has become a common means ofelectrode surface modification. LB film modified electrodes have the characteristicsof close and orderly arrangement, large activity center density, sensitiveelectrochemical response, and fast transmission of electron or substance. Therefore,in this thesis, we did the following researches focused on film-forming propertiesand film electrochemical properties of CNTs based on LB technology:(1) The carboxylated single-walled carbon nanotubes (SWCNTs) were used tomodify glassy carbon electrode (GCE) surface in two different ways: one was theelectrode modified by LB film technology to get the SWCNTs-LB/GCE, and theother was the direct dropped SWCNTs on GCE surface. We not only studied theisothermal surface pressure-surface area curve (π-A curve) of thecarboxyl-functionalized SWCNTs in LB film formation process, but also studied the electronic conduction velocity of SWCNTs on the mentioned two electrodes usingcyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) andchronocoulometry (CC) methods. On the surface of SWCNTs/GCE electrode,SWCNTs were arranged randomly and disorderly, which means some of SWCNTsare perpendicular to the electrode surface, while some were parallel with theelectrode surface or even more at different angles with the electrode surface. On thesurface of SWCNTs-LB/GCE electrode, SWCNTs were completely parallel to theelectrode surface just as a carpet tiled on it. The results showed that the electrontransfer rate on the SWCNTs/GCE surface is much faster than on theSWCNTs-LB/GCE surface which means that the longitudinal conduction parallel tothe tube is much prior. It can be speculated if SWCNTs could be arrangedcompletely perpendicular to the electrode surfaces as an electrochemical biosensor,the rate of electron conduction will greatly increase.(2) The carboxylated SWCNTs and carboxylated muti-walled carbon nanotubes(MWCNTs) were modified on GCE respectively based on LB film technology. Bystudying the π-A curves of MWCNTs, we compared it with SWCNTs in the filmformation process. Electrochemical properties of different carbon nanotube modifiedelectrodes were researched by CV, EIS and CC methods respectively. As a result, wefurther proofed that since the electron-conductive carbon nanotubes were moreinclined to conduct in the longitudinal direction of the tube. Because of tiledMWCNTs having more layers on the electrode surface, the mechanism of theelectron transmission limited the excellent electrical conductivity and blocked theelectrons from transferring. Compared to MWCNTs, SWCNTs had thecharacteristics of relatively high peak current, smaller semi-circle in EIS, largereffective electrode area and larger capacitance. Although the cost of SWCNTs isexpensive, we can predict that SWCNTs have even greater potential in the field ofbio-capacitance sensor.(3) Multi-SWCNTs-LB films and multi-MWCNTs-LB films were assembledby LB filmwith controllable thinkness, and then transferred onto the GCE surface tobuild two kinds of dense multilayer CNTs-LB film modified electrode. The responseof electrical activity and capacitance values were also studied and compared through CV, EIS and CC methods. The results indicated that no matter on SWCNTs-LB filmmodified electrode or MWCNTs-LB film modified electrode, the electrical activitywas obviously improved with the increasing layers of CNTs-LB film: the peakcurrent of Fe2+/Fe3+increased; the electrode resistance value in the contacting layerbetween the electrode surface and the solution decreased; the effective area of theelectrodes increased; capacitor charging increased. All these indicated that afterincreasing the amount of modified CNTs on electrode surface, their excellentelectrochemical performance has been further improved. Meanwhile in multilayers,the capacitance value of SWCNTs was higher than that of MWCNTs. Therefore,based on the characteristics such as thin, uniform, controllable molecular layer, andthe thickness, the height of the anisotropic of LB film, we have prepared a newelectrochemical sensor with a high capacitance value of the charge and dischargecapacity and even further applications.