Consortium
INCDFM,    National Institute of Materials Physics, Bucharest
ICF,              Institute of Physical Chemistry of the Romanian Academy
UPB,             University ”Polytehnica” Bucharest
INCDTIM    National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj

National Institute of Materials Physics, Bucharest
INCDFM   http://www.infim.ro
Project Director:  Dr. Ioan Baltog  <ibaltog@infim.ro>

Institute of Physical Chemistry of the Romanian Academy
ICF   http://www.icf.ro/
Responsible: Dr. F. Branzoi <fbranzoi@chimfiz.icf.ro>

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

Methods & Laboratory Technologies

Organic/inorganic hybrid materials are multifunctional systems with an important  applicative potential in the field of  super-capacitors, lithium rechargeable batteries, photovoltaic cells, solar cells, electromagnetic absorbing sensors and, last but not least, transistors. The project focused on the synthesis of new organic / inorganic hybrid materials based on carbon nanotubes (CNT) and ZnO semiconducting nano-particles whose utilization as active materials is possible in the applications field of sensors, optoelectronics and energy storage. Previous of utilization in various applications, a good knowledge of their physical and chemical properties is necessary, reason that explains the activities proposed in this project: Raman scattering, UV-VIS-NIR and FTIR spectroscopy, photoluminiscence (PL), and scanning electron microscopy (SEM) studies. The following problems were proposed to be solved: i) the determination of the dependence of the ZnO  photoluminescent and vibrational properties on the morphology and the dimensions of the particles intended to be used in the  CN/ZnO composite generation; ii) the functionalization by mechanico-chemical, chemical and electrochemical reactions of the carbon nanotubes with ZnO nano-crystals; iii) the revealing of the vibrational, photo-luminescent and electrochemical properties of the CN/ZnO hybrid material and iv) the use of these organic/inorganic hybrid materials in applications as chemical sensors, optoelectronics and energy storage. A special attention was given to the morphology of ZnO nanoparticles.

The main results obtained are:
-Electrochemical synthesis lead to ZnO particles with similar optical properties as of the ZnO powder furnished by the Aldrich-Sigma that has characteristic morphological structure of the hexagonal platelets;
 - At the temperature of reaction of about 65°C, as the anionic surfactant concentration grows (ex: sodium dodecyl sulfate – SDS) in the reaction mixture (Zn(NO₃)₂ and KOH), ZnO nanoparticles of  different morphological form ( ellipsoids and flacks (foils)) were obtained.
- At the temperature of  85°C, when the chemical synthesis is conducted in the ZnCl₂+KOH reaction mixture enriched with SDS surfactant one obtain one (1D) dimensional ZnO nanoparticles of rods type;
- An efficient characterisation  method of 3D or 1D dimensional ZnO nanoparticles is the Raman spectroscopy. The  main lines detected in the ZnO Raman spectra have maxima at about 330, 380, 440, 560 and 1000-1100 cm^-1 being associated with the vibration modes A₁ (E₂^high-E₂^low)_, A₁ (TO), E₂^high, A₁ (2LA) and A₁(TO+LO), respectivley. The relative intensities of the Raman lines situated in the 300-500 cm^-1 spectral range strongly depend on the  ZnO morphology.
- Additional information regarding the size of the ZnO nanoparticles can be obtained using UV-VIS spectroscopy. The shift towards higher energy of the fundamental absorption band, observed by a similar shift of the excitonic emission, is the mean signature of the particles size decreasing.  ZnO nanoparticles of about 40, 48 and 55 nm mean size show an excitonic emission at  about 361, 368 and 375 nm;
- ZnO samples prepared by chemical and electrochemical methods are characterized by different PL spectra. For the ZnO, electrochemically  prepared, the PL spectrum is characterized by an excitonic band situated at 384 nm and a wide band with a maximum at about 560 nm that is associated with lattice defects. The oxygen play an important role in the generation of an extra-excitonic emission as that at 560 nm, fact demonstrated by the variation of the luminescence intensity as function on pressure, i.e., on  oxygen content. Differently, the ZnO samples obtained electrochemical, are characterized by a PL emission band with a maximum at about 440 nm.
- SWNT interaction with chemical compounds like ZnCl₂, Zn(NO₃)₂ – used in ZnO chemical and electrochemical synthesis led to a partial decomposing of the metallic tube whose vibrational proprieties are restored through successive treatment with basic solutions type NH₄OH, NaOH, etc.
- the  mechanico-chemical interaction of SWNTs with anionic and cationic surfactants results in  both an ionic functionalization and  non-covalent functionalization of NTC with surfactant molecules. Dispersion of SWNT into SDS solution involves a breaking of nanotube bundles into isolated nanotubes. Raman spectra of bundled SWNTs are characterized by lines located in three spectral ranges as follows: a) 50-250 cm^-1, where are situated the Raman lines associated to radial breathing mode (RBM); b) 1100-1700 cm^-1, where one find two bands often reported as D band that indicates a disorder or defect state in nanotube architecture and a G band associated to the tangential vibration mode (TM). The later band manifest resonantly at l_exc = 676 nm when the metallic nanotube are excited. The G band is characterized by an asymmetric profile in its lower energy side; and c) 2250-3500 cm^-1 , spectral range that correspond to 2^nd order Raman spectrum, where most intense lines are those detected at twice the frequency of D and G Raman bands.
- Raman spectra of isolated nanotubes obtained by SDS functionalization show important changes in Raman lines intensities that constitute the complex band situated in 1400-1600 cm^-1 range associated with TM vibration modes. In order to foresee the adsorption mechanism of SDS molecules on CNs, the critical micelle concentration was determined by performing correlated studies of superficial tension and equivalent conductivity. 
- Some chemical synthesis methods of ZnO imply the use of certain reactants with high oxidative power, like H₂O₂ etc. Such a methods can not be use in the preparation of SWNTs/ZnO composite, because an irreversible destruction of the metallic nanotubes occurs in the presence of  H₂O₂, when part of the atomic oxygen is covalently bonded to CN and the other part is absorbed into the SWNT bundle as molecular oxygen.  In the presence of Zn powder, the non-hydrostatically compressed SWNTs transform into very small fragments that behave as  fullerene, which finally  form certain donor-acceptor complexes of the type M_xC₆₀.
- Both mechanico-chemical reaction in solid phase between  SWNTs and ZnO and electrochemical way  lead to a new composite material of the SWNTs/ZnO characterized  by: i) an intense PL emission band with maximum at about 3.26 eV associated with radiative excitonic recombining and a lower intense  band with maximum at about 2.2 eV due to the oxygen excess; ii) a Raman line with maximum at 440 cm^-1 assigned to E₂ mode of  ZnO,  an intense D band and a new Raman line in RBM range and the disappearance of the asymmetry  of the G band..
-  In presence of the NaOH solution with and without Na₂S addition, films based on ZnO nanoparticles as well as composites of the type CN/ZnO are obtained on the Zn electrode alone and modified with CNs.
- New aspects of chemical synthesis of ZnO nanoparticles in the presence of CN are revealed by complementary Raman scattering, FTIR spectroscopy and photoluminiscence studies. Thus, adding CN to the synthesis mixture of ZnO nanoparticles with morphological of the type whiskers one obtain two types of composite materials. Former are identical with those obtained in the absence of carbon nanoparticles and another  in which  whose  vibrational properties are quite different. In the latter case, new Raman lines situated at about 127, 148, 214-262, 368, 478, 717, 1050-1085 si 3200 cm^-1  are observed. Additionally, in this case is reported a up-shift of the  excitonic emission band  that indicates reducing size of the resulted ZnO particles.
- The forecasting of certain application in optoelectronic field for CN/ZnO composite is argued by photoluminescence studies in continuous and pulsed regime.
- Using Raman scattering and photoluminiscence studies, potential applications in gas sensors field are demonstrated to be possible both on ZnO particles as on CNs/ZnO composites. In this purpose is presented a dependency of the PL emission and excitation spectra of ZnO particles and CNs/ZnO composite from the surrounding oxygen ratio. Using Raman spectroscopy, it was demonstrated that CNs/ZnO composite can be used as acetic acid vapours sensor. An evaluation of these composites performances depending on the morphological structure of ZnO nanoparticles is also reported. In fact, using the ratio of Raman lines intensities with maximums at about 433 and 944 cm^-1 as evaluation parameter of the SWNTs/ZnO composite performances, it is demonstrated that, regarding the life time, the most performant material is based on ZnO nanoparticles with morphological structure type sheet.
- Using cyclic voltammetry in the two electrodes system, with the cathode based on ZnO nanoparticles or the CN/ZnO composite and the anode made from Li alkaline metal, high densities of current are recorded during the oxidation-reduction processes that occur in the potential range (0.5; 3.5) V vs. Li/Li⁺ and in the presence of 1M LiPF₆ electrolyte dissolved in the ethylene carbonate: dimethyl carbonate solvents mixture (the volume ratio of the two solvents EC: DMC is 1:1).
- For the composites based on CNs with 1, 2 and n walls (SWNTs, DWNTs and MWNTs) and ZnO nanoparticles with morphological of the type rod are forecasted applications in the rechargeable Lithium-ions batteries field. Discharge capacities of about 28 and 84 mA h g^-1 are determined after effecting of 20 charge-discharge cycles for batteries based on anodes having as active material ZnO nanoparticles and CN/ZnO composite, respectively.

International Projects  proposed/accepted

The results of this project were obtained in the frame of two bilateral scientific cooperations established between National Institute of Materials Physics (NIMP) Bucharest, Romania and Institut des Materiaux “Jean Rouxel” (IMN), Nantes, France.

Conferences & Workshops

1.   Photoluminescence and Raman studies concerning the modeling shape and size of ZnO    nanoparticles in the presence of carbon nanotubes
M. Baibarac, I. Baltog, T. Velula, C. Bucur, S. Lefrant
Sesiunea stiintifica de prezentare a rezultatelor obtinute in cadrul Programului Cercetare de Excelenta, 18-19 septembrie 2008, Iasi

2.   High quality double wall carbon nanotubes grown by a cold-wall radio frequency chemical vapor deposition process
R. Biris, D. Lupu, A. Gruneis, P. Ayala, M. H. Rummeli, T. Pichler, Z. Li, Y. Xu, I. Misan, E. Dervishi, A. S. Biris
235^th American Chemical Society National Meeting, 6-10 April, 2008, New Orleans, Louisiana, USA, Section D, poster 407

Published Papers

1.   Mechanico-chemical interaction of single-walled carbon nanotubes with ZnO evidenced   by photoluminescence and SERS spectroscopy
M. Baibarac, I. Baltog, S. Lefrant, J. Y. Mevellec, C. Godon,
      Journal of Optoelectronics and Advanced Materials 9, 1422, 2006
2.   Vibrational and photoluminescence properties of composites based on zinc oxide and single-walled carbon nanotubes
M. Baibarac, I. Baltog, S. Lefrant, J.Y. Mevellec, M. Husanu
      Physica E 40, 2556, 2007
3.   Photoluminescence properties of composites based on zinc oxide and single-walled carbon nanotubes
M. Baibarac, I. Baltog, M. Husnau, T. Velula, C. Bucur, L. Mihut, N. Preda
      Journal of Optoelectronics and Advanced Materials 10, 288, 2008
4.   Particular signature of isolated and bundled carbon nanotubes in their Raman spectra
      M. Husanu, M. Baibarac, I. Baltog,
      Romanian Reports on Physics 60 (3), pp. 691-699, 2008
5.   Synthesis of narrow diameter distribution carbon nanotubes on ZnO supported catalysts
D. Lupu, A. R. Biris, F. Watanabe, Z. Li, E. Dervishi, V. Saini, Y. Xu, A. S. Biris, M. Baibarac, I. Baltog
      Chemical Physics Letters 473, 299, 2009
6.   The effect of some organic surfactants on the corrosion of zinc in neutral and alkaline aqueous solutions
V. Branzoi, L. Pilan, F. Branzoi
      Revue Roumaine de Chimie 52, 587, 2007
7.   CO₂ enhanced carbon nanotube synthesis from pyrolysis of hydrocarbons
       Z. Li, Y. Xu, X. Ma, E. Dervishi, V. Saini, A. R. Biris, D. Lupu, A. S. Biris, 
      Chem. Commun., 3260, 2008

Resources