Rama Kant

Poly(3,4-ethylenedioxythiophene)-Multiwalled Carbon Nanotube Composite Films: Structure-Directed Amplified Electrochromic Response and Improved Redox Activity

Composite thin films of poly(3,4-ethylenedioxythiophene) (PEDOT)-enwrapped functionalized multiwalled carbon nanotubes (MWCNTs) have been synthesized over multiple length scales by electropolymerization of the monomer without the use of any other supporting electrolyte. The functionalized MWCNTs are incorporated into the positively charged polymer deposit as counterions during oxidative electropolymerization. The morphology, electrochemistry, and electrochromism of the PEDOT-MWCNT films have been compared with those of control PEDOT films doped by triflate ions. Such a comparison enabled us to demonstrate the profound effect of MWCNTs as counterions, realized in terms of better electropolymerization rate, higher conductivity, faster color-bleach kinetics, higher charge storage capacity, and substantially amplified coloration efficiency (eta = 414 cm(2) C(-1), lambda(max) = 575 nm, E = -1.5 V) in comparison to the values of eta reported to date for PEDOT. The strong interaction between the polymer and MWCNTs, the interconnected nanotubular structures, and the porous framework of the film allow facile charge transport and larger ion uptake during redox switching. Electrochemical investigations on devices based on PEDOT-MWCNT and control PEDOT films established the practical utility of PEDOT-MWCNT films as they show lower charge-transfer resistance, higher diffusional capacitance, and a much smaller amplitude of impedance as compared to control PEDOT films.

Effect of surface roughness on diffusion-limited charge transfer

A theory is developed for diffusion-limited charge transfer on a non-fractally rough electrode. The perturbation expressions are obtained for concentration, current density and measured diffusion-limited current for arbitrary one- and two-dimensional surface profiles. The random surface model is employed for a rough electrode\electrolyte interface. In this model the gross geometrical property of an electrochemically active rough surface - the surface structure factor-is related to the average electrode current, current density and concentration. Under short and long time regimes, various morphological features of the rough electrodes, i.e. excess area (related to roughness slope), curvature, correlation length, etc. are related to the (average) current transients. A two-point Pade approximant is used to develop an all time average current expression in terms of partial morphological features of the rough surface. The inverse problem of predicting the surface structure factor from the observed transients is also described. Finally, the effect of surface roughness is studied for specific surface statistics, namely a Gaussian correlation function. It is shown how the surface roughness enhances the overall diffusion-limited charge transfer current.

Effect of surface roughness on interfacial reaction-diffusion admittance

We develop a theory for the electrical admittance of a rough interface under diffusion-limited and partial diffusion-limited charge transfer conditions. The large and small-frequency expansion brings out the relationships between the various morphological features of the rough electrode i.e. area, curvature and width, and the average admittance. The diffusion-limited reaction admittance is determined on an approximately self-affine corrugated random surface fractal. We obtain exact results for the low roughness and the asymptotic results (in three frequency regions) for the arbitrary and large roughness surfaces. These results show an anomalous frequency dependence for the mean admittance and the mean excess admittance for the large and small roughness surfaces, respectively. The intermediate frequency behavior of the reaction admittance for small roughness interfaces has the following form: 〈Y〉∼(iω)1/2+const.(iω)3/2−H, where H is Hursts exponent, for the large roughness interfaces it has same form as predicted earlier: 〈Y〉∼(iω)1−H/2. This non-universality and dependence of intermediate-frequency behavior on the strength of interfacial fractality has not been previously conceived. We also show the localization of the active zones in the presence of roughness. Finally, these results unravel the connection between the total reaction admittance and the crossover frequency to the roughness characteristics likes the fractal dimension, lower and upper fractal cutoff lengths, and the amplitude of the fluctuations of the fractal.

Diffusion to rough interfaces: finite charge transfer rates

We develop a perturbation formalism for diffusion accompanying the finite charge transfer rates on an arbitrary rough electrode. Second-order perturbation expressions are obtained for the concentration, current density and measured current transients for an arbitrary surface profile electrode. Various results obtained by earlier workers in the area of complex interfacial geometry are generalized. The results reported earlier for diffusion-limited (Nernstian) charge transfer at an arbitrary roughness about plane, sinusoidal, curvature expansion and realistic random surface models are generalized for the case where the effects of finite rates of charge transfer are incorporated. The problems of the screened electrostatic potential of the electric double layer for the one-dimensional arbitrary roughness are generalized for a two-dimensional arbitrary surface. In the random surface model (the gross geometrical property of an electrochemically active rough surface) the surface structure factor is related to the average electrochemical current, the current density and the concentration. Under the short- and long-time regimes, various morphological features of the rough electrode, i.e. roughness factor (related to excess area and slope due to roughness), curvature, correlation length etc. are related to the (average) current transients. The expressions for inner and outer cross-over times are obtained. Finally, the effect of surface roughness is studied for specific surface statistics, namely a Gaussian correlation function. It is shown how roughness and finite charge transfer rates affect the overall quasi-reversible charge transfer current.

Structure–conductivity correlation in ferric chloride-doped poly(3-hexylthiophene)

Poly(3-hexylthiophene) (P3HT) matrix has been chemically doped (redox doping) by ferric chloride (FeCl3) with different molar concentrations to get P3HT–FeCl3 charge-transfer complexes. The effect of redox doping on photo-physical, structural, and morphological properties and dc electrical conductivity of P3HT matrices has been examined. The dc conductivity has been measured on films of pristine P3HT and P3HT–FeCl3 charge-transfer complexes in the temperature range 6–300 K. Analysis of dc conductivity data reveals that in the temperature range 40–300 K, the dc conductivity is predominantly governed by Motts 3-dimensional variable range hopping (3D-VRH); however, below 40 K tunnelling seems to dominate. A slight deviation from 3D-VRH to 1D-VRH is observed with an increase in doping level or precisely with an increase in the extent of P3HT–FeCl3 charge-transfer complexes. We attribute this deviation to the induced expansion in crystallographic lattices as revealed by x-ray diffraction data and formation of discrete conducting domains as observed by atomic force microscope imaging.

Hydrodynamic effects on the extension of stars and dendrimers in external fields

We assess the influence of hydrodynamic interactions on the extension of branched polymers subjected to external forces. We envisage that the macromolecules move under external applied fields, exemplified by mechanical or electrical micromanipulations. We focus our attention on the difference of the results obtained using the Rouse and the Zimm models, both of which represent an extreme situation, so that the realistic behavior is encompassed by the two models. We focus on the mean displacement of a specified monomer, on the shape which the macromolecule attains and on the structural average of the displacements involved. We discuss how these dynamic properties depend on the underlying topology, such as the number of branches and their length for the stars and on the number of generations for the dendrimers. Interestingly, although there exist quantitative differences between the results of the Rouse and of the Zimm model, in both models there appear typical dynamical features which depend on the topology only. This stresses the role of the structure on the dynamics and offers the possibility of tracking it under realistic, experimental conditions.

Electrochemical impedance based chiral analysis of anti-ascorbutic drug: l-Ascorbic acid and d-ascorbic acid using C-dots decorated conductive polymer nano-composite electrode

Clinical manifestations owing to l-ascorbic acid for scurvy as comparison to d-ascorbic acid and challenges of chiral purity are overcome by using chiral selective conductive polymer nanocomposite which mimics antibodies and enzymes. A novel chiral selective imprinted polyaniline-ferrocene-sulfonic acid film has been electrochemically fabricated on C-dots modified pencil graphite electrode. The performance of the obtained l-ascorbic acid or d-ascorbic acid chiral selective sensor was investigated by electrochemical impedance spectroscopy, cyclic and differential pulse voltammetry. The surface characteristics of the C-dots, chiral sensor before and after the de-doping of chiral d- and l-ascorbic acid were characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction spectroscopy. Excellent recognition results were obtained by difference in electron transfer resistance. The proposed chiral sensor is capable of measuring d-ascorbic acid or l-ascorbic acid in aqueous as well as in real and commercial samples within the range of 0.020-0.187 nM and 0.003-0.232 nM with detection limit of 0.00073 nM and 0.00016 nM, respectively. The proposed method has also been examined for the chiral selective recognition of ascorbic acid isomers (d- and l-) quantitatively, in complicated matrices of real samples.

Generalization of the Gouy-Chapman-Stern model of an electric double layer for a morphologically complex electrode: deterministic and stochastic morphologies.

We generalize the linearized Gouy-Chapman-Stern theory of an electric double layer for morphologically complex and disordered electrodes. An equation for capacitance is obtained using a linear Gouy-Chapman or Debye-Hückel equation for the potential near the complex-geometry electrode-electrolyte interface. The effect of the surface morphology of an electrode on an electric double layer is obtained using multiple scattering formalism in surface curvature. The result for capacitance is expressed in terms of the ratio of Gouy screening length to the local principal radii of curvatures of the surface. We also include a contribution of a compact layer, which is significant in the overall prediction of capacitance. Our general results are analyzed in detail for two special morphologies of electrodes, i.e., a nanoporous membrane and a forest of nanopillars. Variations of local shapes and global size variations due to residual randomness in morphology are accounted for as curvature fluctuations over a reference shape element. In particular, the theory shows that the presence of geometrical fluctuations in porous systems causes an enhanced dependence of capacitance on mean pore sizes and suppresses the magnitude of capacitance. This theory is further extended to include contributions to capacitance from adsorption of ions and electrode material due to electronic screening. Our predictions are in reasonable agreement with recent experimental measurements on supercapacitive microporous and mesoporous systems.

Post-polymerization functionalization of poly(3,4-ethylenedioxythiophene) films by 1-fluoro-2-nitro-4-azidobenzene: electrochromism and redox behavior

Poly(3,4-ethylenedioxythiophene) (PEDOT) films electropolymerized from an aqueous micellar solution encompassing the monomer (EDOT) and dopant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), have been functionalized after polymerizationvia a photochemical nitrene reaction using the reactive 1-fluoro-2-nitro-4-azidobenzene (FNAB), typically used for biochemical applications. Prior to polymer functionalization, the reaction of the monomer with FNAB was followed by Fourier transform infrared, absorption and light emission spectral studies. The functionalization of the polymer has been confirmed by characteristic FTIR absorptions and X-ray photoelectron spectroscopic signals due to –NH, C–F and –NO2groups of FNAB. The unusual surface morphology of the functionalized PEDOT–AOT film with micron sized outgrowths with a partially crystalline structure is responsible for its superior coloration efficiency (340 cm2C−1 at 550 nm), higher charge storage capacity (0.7 mC cm−2 at 5 mV s−1), better electrochemical cycling stability (high optical contrast is retained even after 3000 clear to dark and dark to clear cycles) and faster switching kinetics as compared to the electrochemical response of the pristine PEDOT–AOT film. These results demonstrate the unparalleled potential this functionalizing methodology has for creating electroactive polymer films with unusual morphologies and electrochemical properties for electrochromic applications.

Chronopotentiometry with power-law perturbation functions at an expanding plane electrode with and without a preceding blank period for systems with a coupled first-order homogeneous chemical reaction

The systems formalism is used to obtain the interfacial concentration transients for power-law current input at an expanding plane electrode. The explicit results for the concentration transients obtained here pertain to arbitrary homogeneous reaction schemes coupled to the oxidant and reductant of a single charge-transfer step and the power-law form without and with a preceding blank period (for two types of power-law current profile, say, (i) I(t) = I0(t−t0)q for t greater-or-equal, slanted t0, I(t) = 0 for t < t0; and (ii) I(t) = I0tq for t greater-or-equal, slanted t0, I(t) = 0 for t < t0). Finally the potential transients are obtained using Pade approximants. The results of Galvez et al. (for E, CE, EC, aC) (J. Electroanal. Chem., 132 (1982) 15; 146 (1983) 221, 233, 243), Molina et al. (for E) (J. Electroanal. Chem., 227 (1987) 1 and Kies (for E) (J. Electroanal. Chem., 45 (1973) 71) are obtained as special cases.