Raman Santhanam

In situ cyclic voltammetry-surface-enhanced Raman spectroscopy: Studies on the doping-undoping of polypyrrole film

The redox behavior of polypyrrole (PPy) film in 0.2 M aqueous KNO3 solution was investigated by cyclic voltammetry (CV) in situ surface-enhanced Raman spectroscopy (SERS). The results show that the Raman intensity of CC backbone stretching decreases and increases regularly with the anodic and cathodic scans, respectively, owing to the change of surface roughness and the possibility of state transition. The peak position of CH in-plane deformation remains unchanged both in anodic and cathodic scans. Furthermore, the peak positions of NH in-plane deformation and ring deformation shift to lower and higher wavenumbers with anodic and cathodic scans, respectively. Also the Raman peak of the NO3− dopant ions appears significant when PPy is in a more oxidized state.

Effect of Al-substitution on the stability of LiMn2O4 spinel, synthesized by citric acid sol–gel method

Abstract Spinel LiMn2O4 and LiAlxMn2−xO4 (x=0.05, 0.15) are synthesized by a sol–gel method using citric acid as a chelating agent. The effect of calcination temperature on the purity of the Al-substituted spinel is examined by X-ray diffraction measurements which suggests that pure material is obtained by calcination at 800°C. Cyclic voltammetry is employed to characterize the reactions of lithium insertion into and extraction from the spinel materials. Surface morphology changes are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that Al-substitution enhances sintering of the spinel LiMn2O4. Charge–discharge cycling studies show that Al-substitution improves substantially the capacity retention of the spinel LiMn2O4.

Effect of various synthetic parameters on purity of LiMn2O4 spinel synthesized by a sol–gel method at low temperature

Abstract Spinel lithium manganese oxide, LiMn 2 O 4 , prepared by the sol–gel method using citric acid as a chelating agent under different (i) pH conditions, (ii) molar ratio of citric acid to total metal ion, (iii) amount of water, (iv) calcination temperature, and (vi) starting materials. The effects of various synthetic parameters on the purity of this oxide are analysed by means of X-ray diffraction measurements. The results show that pure LiMn 2 O 4 can be prepared from nitrate salts as starting materials at a low temperature of 600°C. The optimum pH and molar ratio of chelating agent to total metal ions are 6.0 and 1.0, respectively.

Nucleation and Growth Mechanism of Electropolymerization of Polypyrrole on Gold/Highly Oriented Pyrolytic Graphite Electrode

Nucleation and growth mechanism of polypyrrole films was investigated on vapor deposited gold/highly oriented pyrolytic graphite by current-time transient (i-t) measurements and atomic force microscopy. It was found that the nucleation and growth was an instantaneous two-dimensional process before nuclei overlapping and progressive three-dimensional after nuclei overlapping. The polypyrrole structure was observed to be zigzag or screw-type and particle-like for thin and thick films, respectively.

Nucleation and growth mechanism of electroformation of polypyrrole on a heat-treated gold/highly oriented pyrolytic graphite

Abstract Nucleation and growth mechanism of polypyrrole (PPy) films was investigated on vapor deposited gold/highly oriented pyrolytic graphite (Au/HOPG) after heat treatment by means of current–time transient ( i – t ) measurements and tapping mode atomic force microscopy. It was found that the nucleation and growth was a progressive 3-D before and after nuclei overlapping. However, the mechanism was 2-D instantaneous before nuclei overlapping and 3-D progressive after nuclei overlapping on a Au/HOPG substrate without heat treatment. The change in mechanism was due to lower surface defects on a heat-treated Au/HOPG substrate than on a bare Au/HOPG substrate. Images of PPy on a heat-treated Au/HOPG contained zig-zag and nodular structures.

LiMn2 O 4 Core Surrounded by LiCo x Mn2 − x O 4 Shell Material for Rechargeable Lithium Batteries Synthesis and Characterization

We demonstrate a novel method for the synthesis of LiMn 2 O 4 core surrounded by LiCo 3 Mn 2-x O 4 shell material for rechargeable lithium batteries. Surface and composition analysis by electron spectroscopy for chemical analysis and inductively coupled plasma method, indicated that cobalt mostly distributed on the surface of the LiMn 2 O 4 particles. X-ray diffraction studies revealed that cobalt was doped into the lattice of the LiMn 2 O 4 particles. X-ray absorption fine structure analysis further demonstrated that cobalt was doped mainly on the surface of the LiMn 2 O 4 particles, LiCo x Mn 2-x O 4 solid solution thus formed on the surface of the LiMn 2 O 4 particles greatly enhanced the initial capacity, capacity retention, and rate capability of the spinel LiMn 2 O 4 . The enhanced initial capacity of the LiMn 2 O 4 core/LiCo x Mn 2-x O 4 shell material may be due to the involvement of Co in the charging/discharging behavior and the improved capacity retention and the rate capability were most probably due to less cation disordering during charge/discharge cycling experiments at low as well as at high C-rates.

Structure stabilization of LiMn2O4 cathode material by bimetal dopants

The structural changes of spinel Li 1.02 Mn 2 O 4 and Li 1 0.2 Co 0.1 Ni 0.04 Mn 1 85O 4 cathode materials have been studied by synchrotron powder X-ray diffraction and differential scanning calorimetry (DSC) measurements. The results show that spinel Li 1.02 Mn 2 O 4 undergoes a phase transition from cubic (Fd3m) to orthorhombic symmetry (Fddd) at T = 285 K. However, substitution of a small amount of Co 3+ and Ni 3+ ions suppresses phase transition and the cubic phase is maintained at low temperature due to a decrease in the concentration of Jahn-Teller active Mn 3+ ions.

Local structure transformation of nano-sized Al-doped LiMn2O4 sintered at different temperatures

LiMn2O4 and LiAl0.15Mn1.85O4 were synthesized via the sol–gel process using citric acid as the chelating agent, followed by sintering at various temperatures. The electronic and atomic structures of LiMn2O4 and LiAl0.15Mn1.85O4 powders were probed by means of Mn K-edge X-ray absorption spectroscopy (XAS). Al-doping was found to promote the sintering of spinel LiMn2O4 so that the degree of structural disorder around Mn atoms in LiAl0.15Mn1.85O4 becomes lower than that of LiMn2O4, leading to an excellent capacity retention of this cathode material for lithium battery in charge–discharge cycle. # 2003 Published by Elsevier Science B.V.

Nucleation and Growth Mechanism of Electropolymerization of Aniline on Highly Oriented Pyrolytic Graphite at a Low Potential

In this work, the nucleation and growth mechanism for the electropolymerization of aniline was investigated at higher potentials on highly oriented pyrolytic graphite by potentiostatic current-time transient and atomic force microscopic (AFM) measurements. The electrochemical data fitted to the theoretical curves for the nucleation and growth suggest that electropolymerization of aniline follows the three-dimensional growth and progressive nucleation mechanism. These results were also compared with the results obtained at lower potentials. The results obtained from transient analysis, at higher potentials, were in good agreement with the results of AFM images.

Comparison of fluoride intercalation/de-intercalation processes on graphite electrodes in aqueous and aqueous methanolic HF media

The solvent can play a major role in the intercalation/de-intercalation process and the stability of graphite substrates towards this process. This fact is established in the present work that involves fluoride intercalation/de-intercatlation on graphite electrodes in aqueous and aqueous methanolic HF solutions where the HF concentration is varied between 1.0 and 18.0 M. In addition to cyclic voltammetry and potentiostatic polarization, open-circuit potential decay measurements, scanning electron microscopy and X-ray diffraction measurements have been employed. In general, addition of methanol and increasing concentration of HF raise the overall intercalation/de-intercalation efficiency. Methanol is adsorbed preferentially on the graphite lattice and, hence, suppresses both oxygen evolution and the formation of passive graphite oxides. In 15.0 M HF, the optimum methanol concentration is 5 vol.%. This suggests that, in addition to the adsorption effect, there is some weakening of the structured water molecules that facilitates the solvated fluoride ions for efficient intercalation.