Our current research interests focus to carbon-based nanostructured materials toward the preparation of novel hybrid materials for diverse nanotechnological applications. In general, the carbon nanostructures include the spherical 0-D fullerenes, the elongated 1-D carbon nanotubes and carbon nanohorns and the flat 2-D grapheme. Special attention is given on their chemical functionalization, spectroscopic characterization and microscopy visualization as well as on the study of their electronic, photophysical and optical properties. Our research is interdiciplinary involving Chemistry, Physics and Materials Science while sometimes Biology and Medicine are also there. In particular, our group is engaged on the functionalization of carbon nanostructures with organic photo- and/or electro-active moieties giving rise to donor-acceptor hybrids suitable for mimicking photosynthesis, solar cells (photovoltaics), field-effect-transistors (FETs) and molecular electronics. Additionally, we are also interested on the modification of the carbon nanostructures with biologically active units (peptides, proteins, drugs, genes) for biotechnological applications mainly as drug and/or gene delivery systems.
Support
EURYI Award 2004, EU FP7-Capacities-NANOHOST project (GA 201729) and EU-Greek State/GSRT-PAVET 2005 programme.
Contact
Nikos Tagmatarchis
Theoretical and Physical Chemistry Institute,
National Hellenic Research Foundation,
48 Vassileos Constantinou Ave.,
Athens 11635, Greece
FULLERENES
In functionalized fullerenes, C60 retains most of its original novel properties, which synergistically associate with the intimate properties of the added functional moieties.
We have succeeded on the regioselective synthesis of an equatorial [60]fullerene bis-adduct via cycloaddition of in situ generated azomethine ylides upon thermal condensation of triphenylamine (TPA) bis-aldehyde and an a-amino acid. In such a way, we have achieved i) to incorporate the rigid TPA moiety as a tether to direct, with full regioselectivity, the introduction of the second fused pyrrolidine unit at the fullerene sphere and, ii) to prepare a new push-pull hybrid material consisting of the good electron acceptor [60]fullerene and electron donor TPA, as well as an efficient hole-transporter and electroluminescent material. Additionally, we have also succeeded on the formation of TPA-bridged bis-[60]fullerene.
AZAFULLERENES
Azafullerenes are generated upon substitution of a carbon atom by nitrogen from a fullerene cage. Due to valence difference between carbon and nitrogen, azafullerene is an open-shell and as such it rapidly dimerises to (C59N)2 or abstracts a hydrogen to form C59HN.
We have investigated the high-temperature transformations of hydroazafullerene C59HN and found evidence for a unique intermediate phase characterised by C59N-C59HN• bonding. In these structures the unpaired spin density is shifted away from the C59N• radical to a bonded C59HN cage, as seen from both Mulliken population analysis and EPR spectra. This structure appears to be remarkably stable even at temperatures as high as 500 K and even after losing its second hydrogen atom. Further high-temperature treatment is needed to gradually transform to the known bisazafullerene (C59N)2. We speculate that the present results could be relevant for the preparation of C59N•@SWNT peapod structures, recently proposed to be important building units in future spintronic devices.
The temperature dependence of the concentration of paramagnetic centres responsible for signal-1 (-) and C59HN• radical EPR signal (•) at high temperatures. Inset: experimental EPR spectrum taken at T = 580 K and corresponding fit with the signal-1 and C59N components.
We have investigated methods of insertion of azafullerenes in SWNTs at different temperatures and explore the effects of the conditions applied on the structure of azafullerene-based peapods,
C59N@SWNTs, as compared with the traditional gas-phase, high-temperature procedure. We have shown that C59N• inserted into carbon nanotubes at high temperature, from purified (C59N)2 in the gas phase, can undergo a variety of different transformations forming dimers, oligomers or existing in its monomeric form inside SWNTs due to the stabilization effect by nanotube side walls.
Representative HR-TEM micrographs of azafullerene peapods, prepared at 520 °C, illustrating (a) high filling rate and the presence of (b) monomers C59N stabilized in the interior of SWNTs, (c) simers (C59N)2, and (d) oligomers of azafullerene cages.
However, under milder conditions (i.e., lower temperature), bisazafullerene (C59N)2 can be inserted into nanotubes in its pristine dimeric form, which potentially can be used to generate free radical
C59N• species, isolated from the external environment, for subsequent chemical reactions in a confined space inside SWNTs.
Representative HR-TEM micrograph of azafullerene peapods prepared in supercritical-CO2, at 50 °C. The double arrows indicate positions of dimer (C59N)2 molecules inside the SWNTs, while some monomeric C59N are also present.
CARBON NANOHORNS
Carbon nanohorns (CNHs) are mass produced in high yields via CO2 laser ablation of graphite in the absence of any metallic catalyst and hold strong promise for diverse applications ranging from materials science technology to biomedicine. CNHs are morphologically different from nanotubes as they i) possess one conical-shaped tip, ii) are much shorter in length, and, iii) aggregate in spherical superstructures. We have succeded the chemical modification of CNHs via covalent and non-covalent diverse methodologies.
Covalent Functionalization
1,3-dipolar cycloaddition of azomethine ylides
Donor-acceptor CNHs-based hybrids for energy conversin systems and solar cells
New metallo-nanostructured materials of CNHs have been prepared by the coordination of copper(II)-2,2':6',2"-terpyridine (CuIItpy) with oxidized carbon nanohorns (CNHs-COOH). Photoexcitation of CuIItpy resulted in the one-electron reduction of CNHs with a simultaneous one-electron oxidation of the CuIItpy unit (CNHs• --COO-(CuIItpy)• +) as revealed by transient absorption measurements.
Novel CNHs-H2P hybrid has been accomplished. Photoexcitation of H2P results in the reduction of the CNHs and the simultaneous oxidation of the porphyrin unit. The formation of a charge-separated state, CNHs• --H2P• +, has been corroborated and the charge-separated CNHs• --H2P•+ states have been identified.
Photoelectrochemical cells
Carbon nanohorns (CNHs) covalently functionalized at the conical tips with porphyrin (H2P) moieties were used to construct photoelectrochemical cells. Electrophoretic deposition was applied to fabricate films of the modified CNHs onto optically transparent electrodes (OTE), while nanostructured SnO2 films were cast onto the OTE (OTE/SnO2). The CNH-H2P film on the nanostructured SnO2 electrode exhibited an incident photon-to-photocurrent efficiency (IPCE) of 5.8% at an applied bias of +0.2 V vs SCE in standard three-compartment electrochemical cell. Fluorescence lifetime measurements revealed that photoinduced electron transfer from the singlet excited state of the porphyrin to the nanohorns takes place, while, direct electron injection from the reduced nanohorns to the conduction band of the SnO2 electrode occurs. These processes are responsible for the photocurrent generation.
Carbon nanotubes non-covalently modified with a block copolymer were used for the formation of CdS semiconductor nanohybrids. The hybrid material was deposited onto ITO electrodes and the film formed exhibited an incident photon-to-photocurrent-efficiency (IPCE) of 7%.
CARBON NANOTUBES
We have demonstrated the orientation of SWNTs using very weak magnetic fields, which is a significant breakthrough as it shows that physical manipulation and therefore control of certain intrinsic physical properties of nanotubes is possible using a simple electromagnet. The use of modified nanotubes to achieve solubility to exploit the true aspect ratio and avoid bundling but instead allow rope formation is clearly seen. Furthermore, linear dichroism with high-level orientation in the magnetic field has allowed the characterization of both the electronic structure of carbon nanotubes and, more importantly, the interaction of small molecules with the SWNTs.
We have prepared soluble nanoPd-CNTs hybrids by reduction of palladium acetate and in situ stabilization and deposition of Pd nanoparticles onto SDS solubilized CNTs. The nanoPd-CNTs material has shown very good catalytic activity toward hydrogenation of olefinic compounds and C-C Suzuki and Stille coupling reactions, which is rationalized in terms of high surface area of nanoPd coated onto the surface of CNTs.
