Electrochemical Activity and Patterning of Functionalized Carbon Nanotubes

Open Access
Author:
Gross, Matthew L
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 04, 2010
Committee Members:
  • Michael Anthony Hickner, Thesis Advisor
Keywords:
  • Functionalization
  • Carbon Nanotubes
  • Materials Science
  • Conductivity
  • Patterning
Abstract:
Carbon nanotubes (CNTs) have been shown to be useful in applications ranging from sensing cancer cells to the production of electricity. However, to be useful in many applications, carbon nanotubes must undergo post-synthesis modification. These modifications take carbon nanotubes from their virgin state with primarily sp2 hyridized carbon and transform them into functional niche molecules and materials that can be further processed for specific uses. Modifications can include chemical functionalization, non-covalent asociations, or decoration by metal nanoparticles, among other methods. With many potential modification routes, it is important to understand the implications of these changes on the bulk properties of CNTs. When functionalizing CNTs, reactions are dependant on CNT size, chirality, and dispersion, thus, yielding a distribution of CNTs in bulk samples. It is known that covalent functionalization of singularly isolated CNTs changes their intrinsic behavior from a metallic state to semi-conducting state, but the effect that this change has on the conductivity and electrochemical performance of macroscopic samples has yet to be fully explored. In this thesis, the electrical and electrochemical properties of covalently functionalized CNTs were explored, and the ability to control and create defined patterns of surfactant-stabilized CNTs was demonstrated. Additonally, nanoparticle functionalized CNTs were used as electrodes to create a planar fuel cell. CNT-based electrodes were created using a mixed cellulose ester (MCE) membrane filtering process. Electrodes made included unmodified raw tubes, covalently functionalized tubes, and tubes with metal nanoparticles. These electrodes underwent electrochemical characterization in an aqueous environment. The results show that the electrochemical kinetics of diazonium functionalized CNTs are dramatically affected as compared to their pure state. An anodic/cathodic peak separation of 0.46 V for purified CNTs increased to 1.13 V upon functionalization, indicating decreased electron transfer in the functionalized nanotubes as compared to the unfunctionalized sample. This decrease in electrochemical electron-transfer kinetics can be attributed to the steric hindrance between the Fe(CN)6 3-/4- complex and the surface phenyl moieties on the functionalized CNTs. The diazonium functionalization reaction used in this work has been shown to attach a moiety to every 1 in 9 carbons, thus providing sufficient sidewall coverage which hindered access to the CNT surface. Electrical characterization of bulk CNT films shows that there was also a change in the conductivity of the samples upon functionalization. Electrical measurements on nanotube mats were performed using a 4-point probe measurement. The electrical conductivity of functionalized samples decreased from 545 S cm-1 for unmodified CNTs to 134 S cm-1 and 77 S cm-1 for BrCNTs and NO2CNTs, respectively. Micron-scale conductive planar arrays were fabricated by assembly of carbon nanotube (CNT)/polydiallyl dimethyl ammonium chloride (PDADMAC) thin films on interdigitated patterns using a layer-by-layer (LbL) deposition process from aqueous solutions. The LbL hybrid film was assembled on a quaternized surface pattern of 3-aminopropyltrimethoxysilane (APTMS), which was defined on a Si wafer substrate using microcontact printing or drop coating. Deposition of the CNT/polymer bilayers was shown to be limited to the quaternized APTMS regions as confirmed by Raman microspectroscopic mapping, optical and atomic force microscopy, and electrical probe measurements. Patterns of three, five, and eight bilayers had conductivities ranging from 30 to 130 S cm-1 for the hybrid structures, depending on number of bilayers and measurement technique, which agrees with other reported values for CNT/polymer thin film composites. CNTs functionalized with Pt and Au nanoparticles were tested for their electrocatalytic behavior in oxygen reduction and methanol oxidation. Nanoparticle functionalization was confirmed using TEM and XRD, which showed an average metal particle size of 5 nm to 25 nm on the tubes. The samples were found to have conductivities comparable to that measured for unmodified CNTs of ~550 S cm-1. On a standard rotating disk glassy carbon electrode, the PtCNTs were active for MeOH oxidation (AuCNT showed little or no MOR activity) while the AuCNTs were as active as the PtCNTs for ORR in 0.5 M KOH. Two-dimensional electrodes (2DE) of the Pt/Au CNTs were created on glass substrates using a MCE membrane cut and paste technique. The 2DE’s were shown to be an effective geometry for creating a membraneless, passive, mixed-fuel, two-dimensional fuel cell. A stable open circuit voltage of 0.31 V was recorded with a maximum power produced of 0.05 mW cm-1. This data confirms that the planar electrodes were in electrochemical communication with one another and this strategy may be used to build planar electrochemical devices. This thesis demonstrates the utility of functionalization and resulting properties of CNTs as used in electrodes. Functionalization can be used to control bandgaps, electrical conductivity, modulate electrochemical electron-transfer, improve catalytic activity, and also to control the deposition of CNTs for patterning. CNTs functionalized with electrocatalytically active metals, such as Pt and Au, are shown to be useful in creating a two-dimensional fuel cell.