Rakesh K. Pandey

Ionic conductivity of Ni(II)-based metallo-supramolecular polymers: effects of ligand modification

Ni(II)-based metallo-supramolecular polymers (polyNiL1-4) were synthesized via complexation of Ni(II) ions with bis(1,10-phenanthroline) ligands bearing a different spacer and/or substituents. The ionic conductivity of the polymer films was greatly enhanced with increasing relative humidity. It was also found that the polymer without a hydrophobic spacer (polyNiL3) showed about 500 times as high ionic conductivity as the polymer with a spacer. The ionic conductivity of the polyNiL3 film reached 0.75 × 10−3 S cm−1 at 98%RH.

Platinum(II)-Based Metallo-Supramolecular Polymer with Controlled Unidirectional Dipoles for Tunable Rectification.

A platinum(II)-based, luminescent, metallo-supramolecular polymer (PolyPtL1) having an inherent dipole moment was synthesized via complexation of Pt(II) ions with an asymmetric ligand L1, containing terpyridyl and pyridyl moieties. The synthesized ligand and polymer were well characterized by various NMR techniques, optical spectroscopy, and cyclic voltammetry studies. The morphological study by atomic force microscopy revealed the individual and assembled polymer chains of 1-4 nm height. The polymer was specifically attached on Au-electrodes to produce two types of film (films 1 and 2) in which the polymer chains were aligned with their dipoles in opposite directions. The Au-surface bounded films were characterized by UV-vis, Raman spectroscopy, cyclic voltammetry, and atomic force microscopy study. The quantum mechanical calculation determined the average dipole moment for each monomer unit in PolyPtL1 to be about 5.8 D. The precise surface derivatization permitted effective tuning of the direction dipole moment, as well as the direction of rectification of the resulting polymer-attached molecular diodes. Film 1 was more conductive in positive bias region with an average rectification ratio (RR = I(+4 V)/I(-4 V)) ≈ 20, whereas film 2 was more conducting in negative bias with an average rectification ratio (RR = I(-4 V)/I(+4 V)) ≈ 18.

Real-time humidity-sensing properties of ionically conductive Ni(II)-based metallo-supramolecular polymers

Metallo-supramolecular polymers composed of Ni(II) ions and bis(2,9-dimethyl-1,10-phenanthroline) showed ionic conductivity, which enhanced by about four orders of magnitude with increasing relative humidity from 30 to 98% RH, in the film state. Four Ni(II)-based polymers with different counter anions were synthesized to investigate the ionic conduction mechanism. The highest ionic conductivity (5 × 10−2 S cm−1) at 98% RH was observed in the polymer with chloride ions as the counter anion. The Grotthuss-type proton transfer mechanism was indicated by the low activation energy (0.21 eV) for the ionic conduction. The polymer film prepared on an interdigitated electrode showed dynamic humidity-sensing properties based on the quick and large current-response to humidity change.

Effect of a three-dimensional hyperbranched structure on the ionic conduction of metallo-supramolecular polymers

Fe(II)-, Co(II)- and Ni(II)-based metallo-supramolecular polymers with three-dimensional (3D) hyperbranched structures (3D-polyFe, 3D-polyCo and 3D-polyNi) showed highly humidity-responsive ionic conductivity. The 3D polymers exhibited 10–100 times higher ionic conductivity than their 1D linear equivalents due to the improved ion conduction pathway in the amorphous 3D structure.

One-Dimensional Anhydrous Proton Conducting Channel Formation at High Temperature in a Pt(II)-Based Metallo-Supramolecular Polymer and Imidazole System

One dimensional (1D) Pt(II)-based metallo-supramolecular polymer with carboxylic acids (polyPtC) was synthesized using a new asymmetrical ditopic ligand with a pyridine moiety bearing two carboxylic acids. The carboxylic acids in the polymer successfully served as apohosts for imidazole loaded in the polymer interlayer scaffold to generate highly ordered 1D imidazole channels through the metallo-supramolecular polymer chains. The 1D structure of imidazole loaded polymer (polyPtC-Im) was analyzed in detail by thermogravimetric analysis, powder X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and ultraviolet-visible and photoluminescence spectroscopic measurements. PolyPtC-Im exhibited proton conductivity of 1.5 × 10-5 S cm-1 at 120 °C under completely anhydrous conditions, which is 6 orders of magnitude higher than that of the pristine metallo-supramolecular polymer.

Geometrically isomeric Pt( ii )/Fe( ii )-based heterometallo-supramolecular polymers with organometallic ligands for electrochromism and the electrochemical switching of Raman scattering

Heterometallo-supramolecular polymers with Pt(II) and Fe(II) ions introduced alternately (cis-polyPtFe and trans-polyPtFe) in a precise way were prepared successfully by the 1:1 complexation of Fe(II) ions with cis- or trans-conformational organo-Pt(II) ligands. The conformational difference between cis- and trans-greatly changed the morphology, crystallinity, ionic conductivity, electrochromic properties, and redox-triggered fluorescence of the polymers. The cis-polyPtFe exhibited better crystallinity and low ionic conductivity, whereas trans-polyPtFe showed an amorphous nature with high ionic conductivity. Both the polymers exhibited reversible electrochromism between purple and yellow colors due to the redox of Fe(II)/(III) upon applying a potential of 0 V or +3 V. The trans-polyPtFe showed better electrochromic stability and response times compared to cis-polyPtFe. In addition, the trans-polyPtFe also showed an improved response in redox-triggered Raman scattering switching compared to cis-polyPtFe over a long time range.

Nano molar detection of Cd(II) ions by luminescent metallo-supramolecular polymer formation

A Cd(II)-based metallo-supramolecular polymer (polyCd) was formed via the 1 : 1 complexation of Cd(II) ions with bis(1,10-phenanthroline) bearing a fluorine moiety as a luminescent spacer. The stepwise complexation was observed in UV-vis spectral titration. Selective sensing of Cd(II) ions based on the luminescent polymer formation was successfully achieved with sensitivity down to 8 nM concentration.

Reversible and Continuously Tunable Control of Charge of Close Surfaces

Surfaces of almost all types of materials are often charged easily by contact electrification or deposition of ions; hence, surface charge is ubiquitous and has a vast range of influences in our lives and in industry. Since the 19th century, scientists have been measuring the charge of multiple materials collectively. The common expectation is that the total charge of multiple materials is equal to the sum of the charges of the individual materials. This study describes a previously unreported phenomenon in which the total charge of two insulating surfaces decreases when the surfaces are brought close to each other. The charge varies continuously and reversibly depending on the distance of separation between the surfaces. Experimental results derived from analyzing the movement of charge suggest that the changes are due to a rapid exchange of charge between the surfaces and their surrounding air. This change can be used to control the surface charge of the materials flexibly and reversibly.

Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge/Discharge Rates and Long-Lived Electrocatalysis

Nanostructured electrodes are at the forefront of advanced materials research, and have been studied extensively in the context of their potential applications in energy storage and conversion. Here, we report on the properties of core-shell (gold-polypyrrole) hybrid nanowires and their suitability as electrodes in electrochemical capacitors and as electrocatalysts. In general, the specific capacitance of electrochemical capacitors can be increased by faradaic reactions, but their charge transfer resistance impedes charge transport, decreasing the capacitance with increasing charge/discharge rate. The specific capacitance of the hybrid electrodes is enhanced due to the pseudocapacitance of the polypyrrole shells; moreover, the electrodes operate as an ideal capacitive element and maintain their specific capacitance even at fast charge/discharge rates of 4690 mA/cm3 and 10 V/s. These rates far exceed those of other types of pseudocapacitors, and are even superior to electric double layer-based supercapacitors. The mechanisms behind these fast charge/discharge rates are elucidated by electrochemical impedance spectroscopy, and are ascribed to the reduced internal resistance associated with the fast charge transport ability of the gold nanowire cores, low ionic resistance of the polypyrrole shells, and enhanced electron transport across the nanowires junctions. Furthermore, the hybrid electrodes show great catalytic activity for ethanol electro-oxidation, comparable to bare gold nanowires, and the surface activity of gold cores is not affected by the polypyrrole coating. The electrodes exhibit improved stability for electrocatalysis during potential cycling. This study demonstrates that the gold-polypyrrole hybrid electrodes can store and deliver charge at fast rates, and that the polypyrrole shells of the nanowires extend the catalytic lifetime of the gold cores.

An insight into ion-conduction phenomenon of gold nanocluster ligand based metallo-supramolecular polymers

A unique type of gold nanocluster ligand was prepared for metallo-supramolecular polymer synthesis. The nanocluster was capped with mercapto-phenyl-terpyridine and supramolecular polymers were prepared by complexation with Fe(II) salt. The obtained polymers exhibited a porous structure with high ionic conductivity. The ionic conductivity was found to be more than an order of magnitude higher than the polymer without gold nanoclusters.