Bhushan Gadgil

Chemical functionalization of multi-walled nanotubes (MWNTs) producing different oxygen-containing functionalities is very useful to enhance their dispersion in various solvents and ease further derivatization for several applications. Covalent attachment of carboxyl, carbonyl and hydroxyl groups on the surface of MWNTs have been extensively used in many applications. We present here the surface modification of MWNTs by different chemical routes to produce selectively the MWNT-COOH and MWNT-OH, and discuss the effect of functionality on futher derivatization via amination.

In this study, we report reductive electrosynthesis of a polyviologen derivative bearing amide linkage in its side chain, derived from a cyanopyridinium based monomer with amide functionality. The as grown film was characterized by cyclic voltammetry which displays a well-defined and reversible two step redox response characteristic of viologen. FTIR analysis show evidence of amide linkage and successful reduction of cyanopyridinium moieties to polyviologen. In situ multi ESR/UV-Vis-NIR spectroelec-trochemistry show a single line in ESR signal, thereby suggesting polarons as the only charge carrier involved during charging/discharging process. Furthermore, a characteristic UV–Vis absorption spectra confirms viologen formation. Finally, the polyviologen film is subjected to simultaneous voltammetric determination of heavy metal ions, Pb(II), Cu(II) and Hg(II). The amide linkage in the polyviologen derivative is supposedly complexing these divalent metal ions, enabling their sensitive and simultaneous determination with low detection limits.

We report a systematic study on the effect of graphene vs. reduced graphene oxide interfacial layers on
the electrochromic properties of most widely used electrochromes; i.e. methyl viologen, PEDOT, Prussian
blue and WO3. As a conclusive finding, an improved electrochromic switching kinetics is observed on
reduced graphene oxide in comparison to the pristine graphene sheets possibly due to the heterogeneous
electron transfer in graphene materials.

The importance of viologens in the field of electrochromic materials is well recognized due to their intensely colored radical cation formation. In this study, 1-[6-[(4-methyl-3 thienyl)oxy]hexyl]-4,40- bipyridium hexafluorophosphate (Th-V) was synthesized and electropolymerized in a solvent mixture comprising water and acetonitrile (v/v; 50 : 50) with 0.1 M lithium perchlorate (LiClO4) as an electrolyte salt, yielding a viologen bearing polythiophene (PTh-V) film on an electrode surface. The resulting polymer shows electrochemical activity from both the redox active viologen and the conjugated polythiophene moieties. The redox behavior of the polymer was studied by multi in situ
spectroelectrochemical technique by means of simultaneous recording of electron spin resonance and UV-Vis-near infrared (ESR/UV-Vis-NIR) spectra. The results indicate that only polaron charge carriers are created during both n- and p-doping of the PTh-V film. The polymer film shows enhanced electrochromic contrast due to the introduction of a pendant viologen group into the thiophene unit. The film switched reversibly between dark violet (at 0.6 V) and almost transparent (at 1.0 V) showing good optical contrast with a coloration efficiency of ca. 305 cm2 C1 at 610 nm. The switching transmittance kinetics demonstrate fast response times to attain a bleached state and excellent operational stability with repeatable voltage switching between colored/bleached states for 1000 cycles. The polythiophene backbone was found to strengthen the thermal stability of the conjugated PTh-V redox polymer. The excellent optical contrast with sharp color changes and high color efficiency combined with adequate thermal behavior suggests the potential of PTh-V in the electrochromic device (ECD) application.

Chemical functionalization of multi-walled carbon nanotubes (MWNTs) producing different oxygen-containing functionalities is very useful to enhance their dispersion in various solvents and ease further derivatization for several applications. Covalent aattachment of carboxyl, carbonyl and hydroxyl groups on the surface of MWNTs can be achieved by various chemical treatments. The COOH-functionalized MWNTs have been extensively used in many applications. We present here the surface modification of MWNTs by different chemical routes to produce selectively the MWNT-COOH and MWNT-OH, and discuss the effect of functionality on further derivatization via amination.

We report a facile one-pot synthesis of Au/Au2S multicomponent nanoparticles supported on viologen (V) as a hybrid nanocomposite. Sodium dithionite was used as the reducing agent for both Au and V precursors and SDS as the stabilizing agent for Au nanoparticles in aqueous reaction medium. Spectroscopy, microscopy and electrochemical measurements confirm the successful composite formation. The as prepared flower-like composite showed excellent catalytic properties towards the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The catalytic activity factor of the Au/Au2S–V hybrid was highest in comparison to those reported for polymer supported Au nanoparticle catalysts. The enhanced catalytic performance can be attributed to the strong adsorption of 4-NP molecules onto the cationic V matrix and Au2S nanoparticle surface along with effective donor–acceptor interactions between Au–Au2S nanoparticles and V facilitating the electron transfer at the hybrid interface.

Polyviologen (PV)-reduced graphene oxide (rGO) nanocomposite films were fabricated by simple, one-step reductive electropolymerization of cyanopyridinium based precursor monomer (CNP) in an aqueous dispersion of graphene oxide (GO). Since the polymer formation and reduction of graphene oxide occurs within the same potential window, electrocodeposition method was preferred for obtaining nanostructured PV-rGO films. Cyclic voltammetry experiments of PV-rGO displayed two well resolved, reversible one-electron redox processes typical of viologen. Being a redox polymer, incorporation of rGO further enhances the electroactivity of the PV in the composite films. Vibrational spectral analysis with surface characterization revealed structural changes after composite formation along with subsequent reduction of GO within the polymer matrix.  The PV-rGO nanostructured film exhibits a high-contrast electrochromism with low driving voltage induced striking color changes from transparent (0 V) to purple (-0.6 V), high coloration efficiency, fast response times and better cycling stability compared to a pristine PV film. This performance can be attributed to the high stability of the electrochrome in the composite assembly induced by electrostatically driven non-covalent interactions between redox PV2+ and negatively charged rGO, improved electrical conductivity and enlarged surface area accessed through reinforced nanostructured graphene sheets for tethering PV molecules.

Electrochemical synthesis of a new copolymer film based on azulene and 3-thiophene acetic acid was
accomplished by using tetrabutylammonium hexafluorophosphate and acetonitrile as electrolyte. Both
homopolymers and the copolymer were characterized by cyclic voltammetry, electrochemical impedance
spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and in situ UV–Vis
spectroscopy. The effect of monomer concentration on electropolymerization process was studied by the
cyclic voltammetry technique. Electrochemical impedance spectroscopy is used to study the characteristics
of the film structures and kinetics, as well as the mechanisms for charge transfer and ion transport at
polymer film/electrolyte interface. Fourier transform infrared spectroscopy results confirm the presence
of both monomers in the resulting copolymer. Spectroelectrochemical measurements show the changes
in the colour of the polymeric films with applied potentials. The synthesized copolymer film was used in
voltammetric sensor development for H2O2 determination. A low detection limit of 10 nM H2O2 has been
obtained.

It is well-known that reductive electropolymerization
of cyanopyridinium moieties yields to viologenic materials. In
this work, a monomer with two electropolymerizable
cyanopyridinium groups separated by a six carbon spacer
(CNP) has been synthesized. Its electropolymerization in aqueous
electrolyte results in a linear polyviologen (PV) derivative, a
purple-colored film deposited on the electrode surface. Cyclic
voltammetry (CV) of PV films displays two well-resolved oneelectron
redox processes at c.a. −0.5 and −1.0 V vs. Ag/AgCl.
Fourier transform infrared (FTIR) spectral analysis
shows successful polymerization of PV from the CNP
monomer. In situ electron spin resonance (ESR)/UV-vis-
NIR spectroelectrochemistry was used in order to simultaneously
determine the polycation radical as well as the
magnetic and optical response of the redox PV system.
The single-line ESR spectrum observed at the first reduction
peak of PV film was assigned to the formation
of stable viologen cation radical species within the
polymer matrix, exhibiting the characteristic UV-vis-
NIR viologen cation radical absorption bands. The
electrosynthesized linear PV system represents a promising
stable redox active n-type material for organic
rechargeable devices.