Group Research Activities
B. Monolayer Protected Clusters
C. High aspect ratio metallic and
semiconducting/functional oxide nanostructures
D. Chemical and Electrochemical Gas Sensors
F. Materials for electrochemical power
sources
E. Micro Electro Mechanical Systems (MEMS)
G. Polymer Electrolyte Membrane Fuel
Cells
(A) Self-assembled
monolayer
The work in this area of ultrathin
organic films is aimed at a fundamental understanding of the monolayer packing,
pinhole distribution, and electron transfer behavior of gold and silver
electrodes when a long chain thiol forms a monomolecular barrier on the
surface.
During
the last few years we have studied the self-assembled monolayer formation
tendency of several organic disulfides in order to correlate their structure,
chemistry and molecular interactions. The effect of geometric constraints on
the kinetics and efficiency of SAM formation has been evaluated by taking
related systems such as naphthalene disulphide, diphenyl disulphide and
diphenyl diselenide With structures inspired by those found in nature, we have
also illustrated the importance of self-assembled monolayers in controlling
nucleation and growth during the biomemetic electrochemical synthesis of
crystalline zirconia at room temperature.
Recently,
we have demonstrated the use of diphenyl disulphide monolayer covered Cu, Au
substrate as a cathode material for rechargeable Li batteries for the first
time.
This has potential advantages, including
high-surface area, facile fabrication and scalability, conducive for creating
batteries with high power-weight ratios. Recent studies are focused on the
applications of such monolayers on Si in micro-electromechanical systems
(MEMS).
(B) Monolayer protected Nanoclusters and their application
Ordered
arrays of semiconducting and metallic quantum dots have received considerable
attention in recent times due to the interest in harnessing their attractive
properties for a large number of potential applications in molecular
electronics In contrast to the known methods of nanocluster organizations using
covalent and electrostatic interactions of the tunable surrounding monolayers,
we were the first to demonstrate that even hydrophobic interactions could be
used to organize Au nanoclusters with controllable cluster-cluster spacing.
This method also has been used to anchor Au nanoclusters on single walled
carbon nanotube surface.
Interestingly,
the electronic and optical properties of these films show that the Au colloids
maintain their individual characteristics with out fusion to larger units and
the current voltage behaviour show significant non-linearity. The recent
finding about the insulating to metallic transition at low temperatures using
gold, silver and copper nanoclusters has potential implications in tuning
single electron behaviour.
Recently, we have illustrated for the first time that even
larger sized particles are accessible for the single electron charging despite
their higher capacitance value by means of several electrochemical techniques,
where the presence of different charge steps have been isolated. This type of
solution single electron charging phenomena is known as quantized double layer
charging behavior, which we are currently systematically investigated for
various larger sized particles.
(C) High aspect ratio metallic and
semiconducting/functional oxide nanostructures
Recently
we have demonstrated the role of solvent on controlling the aspect ratio of
silver nanostructures during their growth, using a single-step preparation of
different aspect ratio silver nanostructures (R, 1-100) in aqueous acetonitrile
and 4-Aminothiophenol (ATP).
(D) Chemical and Electrochemical Gas Sensors
Surface
modification of semiconducting oxides like tin oxide has been found to be
useful in tuning the selectivity of hydrocarbon adsorption. The preparation of
surface ruthenated tin oxide was carried out by the covalent linking of noble
metals such as Ru, Pd, Pt etc., which was found to create surface states in the
mid-gap region thereby giving rise to interesting control over selectivity.
This
method has given enhanced sensitivity with controlled selectivity, towards
reducing gases such as butane. The conductivity changes involved in the sensing
mechanism of surface ruthenated tin oxide towards butane were explained using a
scheme based on surface acetate formation. These and other studies have
strongly suggested that tin oxide surface can be tailored to improve gas
sensing properties leading to the fabrication of more efficient sensing
elements.
Recently
focusing on the synthesis of shape selective Ru doped SnO2 nanostructures
such as wires, tube, bi-pyramid, cube etc. and systematically investing the
role of various shapes on several applications including gas sensors,
electrical and optical device formation.
(E) Materials for Electrochemical Power Sources
Several hybrid materials such as carbon fibers and
nano-composites for rechargeable lithium batteries and ultracapacitors have
been prepared and characterized. One of
the key efforts was to improve weight reduction, flexibility and robustness to
these electrode materials while enabling their potential advantages such as
large power density and energy density.
For example, electrochemical lithium insertion into a
conducting polymer based nanocomposite of vanadium pentoxide gives about 300
mAh/g discharge capacity as a high energy density cathode for lithium battery
Recently we are focusing on the synthesis and application
of functionalized Carbon nanotubes based materials. This includes organic,
polymer and inorganic materials such RuO2, Au, Ag and Pt nanoparticle
etc. Activated carbon have also been prepared for couple of years from coconut
shell and its various functionalization including RuO2, Pt, Pd
crystallites to use them as effectively for supercapacitor, sensor and fuel
cell catalyst preparation. Our recent shows that the carbon nanotube based
materials are particularly interesting for energy storage device application.
We are currently focusing on fuel cell and hydrogen energy applications of
these materials.
(F) Micro Electro Mechanical Systems
The adsorption kinetics and
thermodynamic stability of OTS monolayer formation on Si (100).
Ultra-thin organic films like self-assembled monolayers
(SAMs) and Langmuir–Blodgett (LB) films are extensively studied due to their
fundamental importance in surface modification and also for their diverse
potential applications in nanotechnology. The presence of a simple
monomolecular film on a metallic or semiconducting surface can cause dramatic
changes in its surface properties and these SAMs are particularly important due
to their ability to control wetting, adhesion, lubrication and corrosion on
surfaces and interfaces. For example, the presence of SAM on Si (100) surface
can tackle the stiction problem of micro-electromechanical systems (MEMS) by
providing a suitable low energy surface coating . Usually alkyltrichlorosilane
based self-assembled monolayers are used for reducing stiction in silicon
micromachines and suitable candidates include octadecyltrichlorosilane (OTS),
perflorodecyltrichorosilane (FDTS) etc. We study the adsorption kinetics and
thermodynamic stability of OTS monolayer formation on Si (100) by using time
dependent contact angle measurements in accordance with Langmuir adsorption
isotherm.
Figure 1. Adsorption
kinetics of OTS monolayer on Si (100): a) Contact angle variation and b)
fractional coverage as a function of dipping time for various OTS
concentrations.
Figure 1 shows the variation of contact angle (with a
sessile water drop) as a function of dipping time for different OTS
concentrations. A clean Si surface has a contact angle less than 15°, which indicates its
hydrophilic nature. The SAM functionalized substrate shows increased contact
angle only in the initial stages of
immersion and later reach a steady state contact angle 1090 corresponding
to the full surface coverage. The contact angle data was normalized with
respect to the steady state value to calculate the fractional coverage, as
shown in the inset of Figure 1.
Fig.
2. AFM images of OTS monolayer on Si(100) , A) partially
grown OTS monolayer after 2.5 s soaking time and its corresponding IR spectra,
surface coverage is about 50 % b) after 5 s, 70% surface coverage c) 10 s, 90 %
surface coverage d) fully covered OTS monolayer after 60 s and its
corresponding IR spectra.
Figure 2 shows AFM images (968 x 986 nm2) and
corresponding IR spectrum of partially and fully covered OTS-monolayers by
exposing freshly cleaned Si substrate at various time intervals in OTS
solution. Image ‘3A’ is taken after 2.5 s immersion, corresponding to a contact
angle of 57°, (calculated fractional
coverage was ~50 %), the monolayer is not
fully covered and the OTS islands are randomly distributed on the surface. The
growth may be due to the accumulation of clusters of OTS molecules on the substrate. The average height of the OTS film is found
to be ~0.55 nm as the molecules are
randomly arranged lying flat on the surface. The inset of figure 3a shows the
corresponding IR spectrum of the partial monolayer. The observed methylene peaks (for sample 1) due to C-H symmetric
and antisymmetric stretching at 2852 and 2924 cm-1 respectively
indicate that the monolayer is not densely packed and oriented. The
intensity growth and peak positions of the C-H stretching vibrational modes
(–CH2 ) of the monolayer film provide information on the monolayer
formation rates and structural changes during in the course of the growth.
The images ‘3B’ and
‘3C’ were taken after 5 and 10 s soaking of the samples respectively in OTS
solution followed by extensive rising and drying. The contact angle for former
sample is 85°, corresponding to a
fractional coverage of ~ 76 % while that for
latter is 100° showing ~90 % coverage with an
average height of the OTS monolayers ~1.31 and 1.83 nm respectively. This indicates that
the OTS monolayer still possesses disordered alkyl chains if it follows a
uniform growth mechanism. The two major peaks of methylene vibrations assigned
to asymmetric and symmetric modes show red shift with increasing surface
coverage perhaps, due to conformational changes. It also suggests that the film
growth follows essentially a Langmuir model of irreversible adsorption.
(G) Polymer Electrolyte Membrane Fuel
Cells
The work is divided into following three parts.
- Synthesis,
characterization and electrochemical properties of polytriazole and novel
polyimide (PI) polymer electrolytes.
- Synthesis,
characterization and electrochemical properties of polybenzimidazole (PBI)
polymer electrolytes.
|
Nafion-117 |
Temp (0C) |
Ru(W) |
RCT(W) |
R (W) |
C (F) |
|
28 |
1.718 |
0.77623 |
0.7733 |
3.914*10-7 |
|
|
40 |
1.7623 |
1.2544 |
1.2542 |
2.688*10-7 |
|
|
50 |
2.0398 |
1.7968 |
1.7968 |
1.975*10-7 |
|
|
60 |
5.5981 |
8.735 |
8.651 |
5.044*10-8 |
- Studies in planar
fuel cell based on surface modified polymer electrolytes.
Achievements of Fuel Cell Work
[1] New monomers namely 4’-sulfonic 2,4-diaminodophenyl ether
(SDADPE), 4-(4’,3-pentadecylphenoxy) benzene-1,3 diamine sulfonic acid (PBDSA)
and 2,4 diamino 4’carbonyl diphenyl ether were successfully synnthesised.
[2] Based on these monomers a large number of
new polyimides and copolyimdes were synthesised and characterised with a view
to synthesise polyimide polymer electrolyte for fuel cell application.
[3] New polyimides containing sulfonic acid group were synthesised
and evaluated as polymer electrolyte for fuel cell.
[4] Based on these polymers two patents were filed and two papers
are being communicated for publications.
[5] Based on the surface modified polymers as polymer electrolytes,
a new concept on planar fuel cell was studied and has been demonstrated that
the concept is practical and has a potency in planar fuel cell applications
where fuel cell in miniature form is desired.
Patents:
(a)Proton Transport At
Polymer Surfaces, US patent filed,
(Pat. No.- NF-289/2001) S. P. Vernekar, A. S.
Patil, K. I. Khalil, K. Vijayamohanan,
I. S. Mulla, T. M.
Maddanimath
(b)An Improved Humidity Sensing Instrument, US patent filed
(Pat. No- NF-280/2001) K.
Vijayamohanan, T. M. Maddanimath, I. S. Mulla, S. P.
Vernekar, A. S. Patil
(c ) An Improved Polymer Electrolyte Membrane for Planar Fuel
Cell, US patent
applied S. P.
Vernekar, S. S. Kothawade, M. P. Kulkarni., R. A. Potrekar, K.
Vijayamohanan, I. S. Mulla, N. S. Ramgir,
Papers:
(A) Comparative Performance of Surface Modified Carbon as
anode for Li-ion rechargeable cells,
Trupti Maddanimath, I. S. Mulla, K.
Vijayamohanan, K. I. Shaikh, A.S. Patil, S. P. Vernekar, 2nd
International Conference on Electrochemical Power Systemsheld at Chennai on 9th
and 10th November, 2000.
(B) Humidity sensing properties of surface modified
polyethylene and polypropylene, Trupti Maddaniamath et al.,
Sensors and Actuators B, 81(2-3)2002,
141-151
(D) Temperature Dependent Impedance Studies of Nafion-117 Membrane
for PEMFC Applications, N. S. Ramgir,
S. S. Kothawade, M. P. Kulkarni, S. P.
Vernekar, I. S. Mulla, K.
Vijayamohanan. Proceedings of National Seminar on “Fuel Cell – Materials,
Systems and Accessories”. September 25 and 26, 2003 organized by Naval
Materials Research Laboratory, Ambernath.
(E) Comparison of Surface
Functionalized High Density Polyethylene, Block Copolymers of Polystyrene-block-poly
(ethylene-ran-butylene)-block-Polystyrene
and PVDF Polymer Membranes for Planar Fuel Cell Applications, S.S. Kothawade, M. P. Kulkarni, N. S. Ramgir S. P. Vernekar, K.
Vijayamohanan in Proceedings of National Seminar “ Fuel to
Fuel Cells” 4-5 December 2003 organized by Indian Institute of Chemical Technology,
Hyderabad.
(F) Synthesis and Characterization of Polyimides Containing Pendant Phenoxy Group
Sandeep S. Kothawade,
M. P. Kulkarni, Khalil Shaikh, A. S. Patil, S. P. Vernekar, T.
Madanimatth ,
K. Vijaymohanan to be
communicated .
(G) Synthesis and Characterization of
Polyimides and Copolyimides Having Pendant
Benzoic Acid Moiety ,
M. P. Kulkarni, S. S. Kothawade, Girish
Arabale, Deepali
Wagh, K. Vijayamohanan
and S. P. Vernekar to be communicated .
Contact: Dr.
K. Vijayamohanan
Head,
Materials Electrochemistry Group,
Physical and
Materials Chemistry Division
National
Chemical Laboratory,
Pune –
411008, Maharashtra, India
E-mail: vk.pillai@ncl.res.in
Tel: +
91-020-25893300 Ext 2276
Fax: +
91-020-25893044
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