In-situ Evidence of the Redox-State Dependence of Photoluminescence in Graphene Quantum Dots

Changes in the optical properties of graphene quantum dots (GQD) during electrochemical reduction and oxidation were investigated by photoluminescence (PL) spectroelectrochemistry, which provided direct in situ evidence of the dependence of GQD luminescence on their redox state. We demonstrated that GQD PL intensity was enhanced upon reduction (quantum yield increased from 0.44 to 0.55) and substantially bleached during oxidation (quantum yield ∼0.12). Moreover, PL emission blue/red-shifted upon GQD reduction/oxidation, rendering information about electronic transitions involved in the redox processes, namely, the π → π* and the <i>n</i> → π* transitions between energy levels of the aromatic sp² domains and the functional groups, respectively. PL intensity changes during GQD reduction/oxidation resulted from a variation in structural changes in GQD as a result of charge injection, as corroborated by in situ Raman spectroelectrochemistry

Adsorption Behavior of Perfluorinated Sulfonic Acid Ionomer on Highly Graphitized Carbon Nanofibers and Their Thermal Stabilities

A systematic adsorption study of perfluorinated sulfonic acid Nafion ionomer on ribbon-type highly graphitized carbon nanofibers (CNFs) was carried out using fluorine-19 nuclear magnetic resonance spectroscopy. On the basis of the values obtained for the equilibrium constant (<i>K</i>_eq, derived from Langmuir isotherm), the ionomer has varying affinities for CNFs (<i>K</i>_eqbetween 5 and 22) as compared to Vulcan (<i>K</i>_eq = 18), depending on surface treatments. However, the interactions are most likely governed by different adsorption mechanisms depending on hydrophilicity/hydrophobicity of the adsorbent carbon. The ionomer is probably adsorbed via the polar sulfonic group on hydrophilic Vulcan, whereas it is adsorbed primarily via hydrophobic −CF₂– backbone on the highly hydrophobic pristine CNFs. Ionomer adsorption behavior is gradually altered from apolar to polar group adsorption for the acid-modified CNFs of decreasing hydrophobicity. This is indicated by the initial decrease and then increase in the value of <i>K</i>_eqwith the increasing strength of the acid treatment. The corresponding carbon–ionomer composite also showed varying thermal stability depending on Nafion orientation. The specific maximum surface coverage (Γ_Smax) of the CNFs is 1 order of magnitude higher than that of Vulcan. The large discrepancy is due to the fact that the ionomers are inaccessible to the internal surface area of Vulcan with high microporosity

Ultrafast and High-Contrast Electrochromism on Bendable Transparent Carbon Nanotube Electrodes

Poly(3,4-ethylenedioxythiophene) and tungsten oxide films with enhanced electrochromism were prepared by templated electrodeposition on flexible transparent carbon nanotube electrodes. The nanostructured porous architectures obtained by coating the nanotube templates exhibited high contrast with faster switching response, higher coloration efficiencies, and better long-term redox switching stability and resistance to flexure than their homologues on ITO-coated plastic electrodes