Consortium
INCDFM,    National Institute of Materials Physics, Bucharest
UPB, University ”Polytehnica” Bucarest
NIRDIMT, National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj
ICF, INSTITUTE OF PHYSICAL CHEMISTRY OF THE ROMANIAN ACADEMY (Institutul de Chimie Fizica „Ilie Murgulescu” al Academiei Romane  - ICF)
ICPE-CA The national RESEARCH-DEVELOPMENT institute for electric engineering (Institutul National de Cercetare-Dezvoltare pentru Inginerie Electrica - ICPE-CA).

National Institute of Materials Physics, Bucharest
NCDFM   http://www.infim.ro
Project Director:  Dr.Mihaela Baibarac  <barac@infim.ro>

University ”Polytehnica” Bucarest
UPB   http://www.pub.ro/
Responsible: Prof. Dr. I. V. Branzoi <iv_branzoi@chim.upb.ro>

National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj
NIRDIMT http://www.itim-cj.ro
Responsable: Dr. D. Lupu <lupu@oc1.itim-cj.ro>

The national RESEARCH-DEVELOPMENT institute for electric engineering
ICPE http://www.icpe-ca.ro/
Responsible: Dr. Ana-Maria Bondar <abondar@icpe-ca.ro>

Results

The composite materials based on conducting polymers (CPs) and carbon nanotubes (CNs) form multifunctional systems, with a great applicative potential in the field of supercapacitors, lithium rechargeable batteries, photovoltaic cells and photodiodes, solar cells, sensors, electromagnetic absorbents and, last but not least, transistors. The carbon nanoparticles incorporation into polymeric matrix, leads to improvement of polymers mechanic and electric properties. The project was focused on the elaboration of synthesis methods of nanocomposites of the type poly N-vinyl carbazole / carbon nanotubes (PNVK/CNs) used as active materials into rechargeable lithium batteries and photovoltaic cells. Their characterization through Raman light scattering, infrared spectroscopy (IR), photoluminescence (PL), conduction and photoconduction was another objective of the project. The problems proposed to be solved in this project were: i) the functionalization of the carbon nanotubes with N-vinyl carbazole (NVK) and poly N-vinyl carbazole (PNVK), using the mechanico-chemical reactions, chemical and electrochemical polymerization; ii) the evidence of the vibrational, photoluminescence and electrochemical properties of the hybrid materials synthesized by above methods; iii) the testing of these hybrid materials in applications like rechargeable lithium batteries and photovoltaic cells. For this, the PNVK/NTC composites have been prepared by two ways: i) the chemical polymerization of the monomer in the presence of NTC and ii) the electrochemical synthesis of the polymer onto the CNs film deposited on the Pt electrode.
Using Raman spectroscopy and photoluminescence studies one demonstrated that the chemical polymerization of NVK in the presence of CNs results in a functionalization of SWNT with PNVK in un-doped state. The main experimental facts that plead for this conclusion were furnished by:
A) Raman spectroscopy when: i) at 676 nm excitation wavelength has been observed a significant decreasing in the intensity of Raman lines from the 125-225 cm-1 spectral range accompanied by an increasing in the intensity of the D band without any changes induced in the profile of the band associated to the tangential vibration mode (TM); ii) at 1064 nm excitation wavelength, an increase in intensity of the D band, and a change of the ratio between the intensities of the Raman lines peaked at 164 and 176 cm-1 is observed, which suggests that the relative ratio of isolated tubes decreases as a result of an additional roping induced into SWNTs bundle.
B) Photoluminescence (PL), where is observed that: i) PL spectrum of PNVK is characterized by a band with maximum at about 410 nm which is assigned to the lower energy excimer; ii) a decrease in the PL efficiency of PNVK takes place when SWNTs are added to the synthesis mixture, fact due to the interface electronic interaction that induces non-radiative recombination processes; iii) the appearance of a new PL band at about 500 nm whose intensity increases as a growth of the SWNT weight occurs into the synthesis mixture; this band is correlated with the covalent bonding of un-doped PNVK on the CNs wall.
The electrochemical polymerization of the monomer NVK on the CNs modified Pt electrode involves a covalent functionalization of the CN wall with doped PNVK, fact argued by cyclic voltammetry studies as follows: i) the cyclic voltammogram of PNVK recorded on Pt electrode immersed in the NVK+LiClO4+CH3CN solution is characterized by an oxidation maximum situated at about 1.4 V vs. Ag/Ag+ and a reduction maximum at about 1V vs. Ag/Ag+; as the number of cycles recorded on the CNs covered Pt electrode increases, a gradual shift of the cathodic maximum from 0.5 to -0.1 V vs. Ag/Ag+ is observed as an increase in the scans number takes place. Depending on the used carbon nanotube type (single walled carbon nanotube (SWNT) or multi wall carbon nanotubes (MWNT)) for the covering of the Pt electrode immersed in the NVK+LiClO4+CH3CN solution, the cyclic voltammogram shows one or two reduction peaks, respectively. These variations of cyclic voltammograms are correlated on the one hand with reaction mechanism, and on the other hand with the structure of the two types of CNs. The appearance of a supplementary reduction maximum in MWNT is argued through the existence of an additional reduction reaction induced by the defects on CNs which have a bamboo-like structure; ii) the NVK electrochemical polymerization process mechanism on the Pt electrode covered with a CNs film involves three stages with the chemical – electrochemical –chemical character. In the chemical stage is invoked an addition reaction of NVK molecules on the CNs film, followed in the electrochemical stage by the formation of a radical that induces the growth of macromolecular chain which end in the last stage, i.e. the chemical stage; iii) the electrochemical process is controlled by diffusion, fact argued through the linear dependence between cathodic maximum potential current and the sweep rate used to the NVK electrochemical polymerization on the CNs covered Pt electrode; iv) high current density are noticed when the NVK concentrations increase; this experimental fact suggests that the reaction rate is proportional with monomer concentration.
Maintaining a constant VK concentration and using different LiClO4 concentrations, higher and higher, large current responses are obtained. These high current densities may be correlated with the increase of the electrolyte conductivity. The modification of the position of cathodic maximum potential is assigned to the doping reaction of PNVK covalent functionalized SWNTs both with ClO4- ions and SWNT radical anions. Other results that plead for above conclusions were obtained by the Raman scattering, FTIR spectroscopy and photoluminescence (PL) studies. Thus, one observes that as the cyclic voltammograms number increases, from 50 to 100, in the Raman spectrum of CNs appear additional variations as follows: i) a sudden decrease in the intensity of the Raman band localized at 164 cm-1; ii) a gradual diminution of 178 cm-1 band intensity and iii) the appearance of new bands at about 1159, 1226, 1263, 1352, 1493 and 1622 cm-1, all belonging to PNVK Raman spectrum. To difference between a successively deposition of polymer layers on CNs and a functionalization process of SWNTs with PNVK, a post-treatment of the samples electrochemical obtained with aqueous 1M NH4OH solution was carried out. The interaction between the doped PNVK functionalized SWNT and NH4OH results in the passage of the polymer from a doping state into an un-doping one.
The Raman spectroscopy and attenuated total reflection infrared (ATR-IR) spectroscopy is used to argue the functionalization of SWNT with PNVK in un-doped state. Main conclusions resulting from the ATR-IR and PL spectroscopy studies regarding the proposed mechanism for electrochemical synthesis are: i) NVK chemical polymerization in the presence of CNs involves a head-to-tail addition of the monomer during the development of the macromolecular chain; ii) the steric hindrance effects induced to PNVK benzene ring are the result of the functionalization CNs with polymer and iii) the existence of defects into PNVK structure was evidenced through photoluminescence studies, where the appearance and the increase in intensity of the vibronic structure from 2.57 eV was explained on the base of the excited singlet states of the covalent functionalized SWNT wall with PNVK molecules.
Main applications demonstrated in this project are in the field of: i) rechargeable lithium batteries – when using the charge-discharge tests, high specific discharge capacities of about 45 and 115 mA h g-1 are reported for the composites of the type PNVK/SWNTs and PNVK/MWNTs, respectively, obtained on electrochemical way and ii) photovoltaic cells – where by the photoconduction measurements a percolation threshold is determined at a concentration of about 8% SWNTs in the PNVK/CNs composite weight.
Note: During of project development , at the end of each research stage has been presented a detailed research report.

International Projects  proposed/accepted

1.National Institute of Materials Physics (NIMP) Bucharest, Romania and Institut des Materiaux “Jean Rouxel” (IMN), Nantes, France

2.National Institute of Materials Physics (NIMP) Bucharest, Romania and Materials Science Institute , Department of Crystallography and Solid State Chemistry, Barcelona, Spain

Conferences & Workshops

1. Electropolymerization of N-ethyl carbazole on single-walled carbon nanotubes – cyclic voltammetry, Raman and FTIR studies
M. Baibarac, I. Baltog, L. Mihut, J. Y. Mevellec, S. Lefrant
ChemonTubes, 6-9 April 2008, Zaragoza, Spain

2. Poly(N-vinyl carbazole) and carbon nanotubes based composites and their application to rechargeable lithium batteries
M. Baibarac, I. Baltog, L. Mihut, C. Bucure, P. Gomez-Romero
Sesiunea stiintifica dedicate Programului de Cercetare de Excelenta, Modulul I, Ariile tematice 8, 10 si 11, 18-19 Septembrie 2008, Iasi

Published Papers

1. Nanocomposite materials based on conducting polymers and carbon nanotubes. From fancy materials to applications,
M. Baibarac, P. Gomez Romero,
Journal of Nanoscience and Nanotechnology 6, 289, 2006

2. Electrochemically functionalized carbon nanotubes and their application to rechargeable lithium batteries,
M. Baibarac, M. Lira Cantu, J. Oro Sol, P. Gomez Romero,
Small 8-9, 1075, 2006

3. Electrosynthesis of the poly(N-vinyl carbazole)/carbon nanotubes composite for applications in the supercapacitors field,
M. Baibarac, P. Gomez-Romero, M. Lira-Cantu, N. Casan Pastor, N. Menstrel, S.Lefrant, European Polymer Journal, 42, 2302, 2006
4. Poly(N-vinyl carbazole) and carbon nanotubes based composites and their application to rechargeable lithium batteries,
M. Baibarac, M. Lira Cantu, J. Oro Sol, I. Baltog, N. Casan Pastor, P. Gomez Romero, Composite Science and Technology, 67, 2556, 2007

5. Spectroscopic evidence for the bulk polymerization of N-vinyl carbazole in the presence of single-walled carbon nanotubes,
M. Baibarac, I. Baltog, S. Lefrant, P. Gomez Romero,
Polymer, 48, 5279, 2007

6. Electropolymerization of N-ethyl carbazole on single-walled carbon nanotubes – cyclic voltammetry, Raman and FTIR studies
M. Baibarac, I. Baltog, L. Mihut, J. Y. Mevellec, S. Lefrant
Journal of Nanoscience and Nanotechnology, 2009 proof.

Resources