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A B S T R A C T S FRIDAY, JULY 2 N A N O S E A 2 0 1 0 absorption and thus harvesting solar energy [1]. Investigations on the photo-electrochemical properties of single wall carbon nanotubes (SWCNTs) were carried out and the photon-to-photocurrent efficiency (IPCE) [2] reported remained low (about 0.15%) due to the rapid exciton annihilation [2]. For this reason a number of strategies were developed to enhance the photoconversion efficiency for SWCNTs. In this research field, of equal interest are multi-wall carbon nanotubes (MWCNTs), in which the presence of numerous concentric cylindrical walls provides additional pathways for the flow of photogenerated charge carriers. We have already demonstrated that MWCNTs can generate photocurrent in the visible and ultraviolet spectral range [3] and the maximum IPCE reported was approximately 7%, about 50 times higher than that of SWCNTs [2]. In addition, we also observed that a sizeable enhancement in the photocurrent generation can be obtained in MWCNTs decorated with Cu-metal clusters deposited by thermal evaporation [4]. 2 – Abstract In this work we show recent studies on the influence on the photocurrent response from Cu-MWCNTs as a function of the Cu clusters size and morphology. The Cu deposition produced crystalline nanoparticles localized on the outer walls of the CNTs. The mean dimension of the Cu nanoparticles depends on the quantity of deposited copper. The photocurrent response of the different sized hybrid samples (Cu-MWCNTS) was measured with a three-arm photo-electrochemical cell. It was found that a sizeable enhancement in the photocurrent over the visible and near ultraviolet energy range can be obtained. The IPCE increased for different sized-CuMWCNTs with respect to that of bare MWCNTs up to 0.5 nm Cu nominal thickness. The maximum in the IPCE obtained at 0.2nm Cu nominal thickness was 13% while that from bare MWCNTs was about 5.4%. Nevertheless, for Cu nominal depositions greater than 1nm the effect reversed and the IPCE from Cu-MWCNTs system started decreasing with respect to that of MWCNTs. The presence of a photocurrent signal is a clear-cut indication that MWCNTs act as photoconductive material. This property is essential for the conversion of light into electricity as upon illumination a fast charge separation and a slow charge recombination is required. In charge-separated species lifetimes of the order of microseconds have been evaluated in photocurrent generation in SWCNTs. Longer lifetimes are expected for MWCNTs than for SWCNTs, due to a enhanced delocalization and percolation of the electrons inside the concentric tubes which decelerate the recombination‟s decay dynamics. Copper is known to be a good electron donor upon photo excitation. Therefore, the photocurrent signal enhancement indicates that additional electrons coming from the Cu nano-particles are efficiently transferred to the numerous concentric cylindrical walls of carbon nanotubes which offer more pathways for the flow of photogenerated charge carriers. 3 – Conclusion The results presented in this study point out two major findings: (i) the ability of Cu nano-particles to significantly modify the electronic properties of the entire hybrid system facilitating the charge separation and subsequent collection at the electrode (ii) the ability to tune the photoelectrochemical response and photoconversion efficiency of MWCNTs via size control of Cu nanoparticles. The maximum powerconversion efficiency obtained with Cu-MWCNTs film highlights the usefulness of tubular morphology in facilitating charge transPort-Pin nanostructure-based solar cells. It was found that the photoactive Cu nano-particles greatly enhanced the intrinsic ability of MWCNTs to behave as an efficient low dimensional media for generating e-h carriers. The reported hybrid system seems to be as promising as that obtained from CNTs functionalization with organic molecules. 135
A B S T R A C T S FRIDAY, JULY 2 N A N O S E A 2 0 1 0 [1] Carbon nanotubes, Acc. Chem. Res. 35 (2002) 997. [2] S. Barazzouk, S. Hotchandani, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 108, 17015 (2004) [3] P. Castrucci, F. Tombolini, M. Scarselli, E. Speiser, S. Del Gobbo, W. Richter, M. De Crescenzi, M. Diociaiuti, E. Gatto, M. Venanzi, Appl. Phys. Lett. 89, 253107 (2006) [4] M. Scarselli, C. Scilletta, F. Tombolini, P. Castrucci, M. Diociaiuti, S. Casciardi, E. Gatto M. Venanzi, M. De Crescenzi, J. Phys. Chem C 113, 5860 (2009) 11H00-11H20 Carbon nanotube based composites: electrical and mechanical properties. BELLUCCI (INFN – LNF, via E. Fermi, 40, 00044 Frascati (RM) Italy). bellucci@lnf.infn.it We report on the results of a systematic study of the electrical properties of carbon nanotube-based polymeric composite materials. Our purpose is the production and characterization of a light, thin and mechanically strong new composite material able to cover electric circuits against external electromagnetic interference. Setting the resistivity properties of carbon nanotube-based composites against those containing micro-sized graphite particles as constituent we show the advantages of using carbon nanotubes. The change in the resistivity values for carbon nanotubes-based composites turns out to be significant, even for small changes in the added carbon nanotubes percentage [1,2,3]. Mechanical characterizations of the nanocomposites will be also discussed. The study of mechanical performances is carried out focusing on the influence on the mechanical properties of different parameters, such as [4,5]: • Aspect ratio (defining the matrix-CNTs interface extension); • CNTs chemical functionalization; • Synthesis method. The analysis carried out on the mechanical properties of nanocomposites shows interesting results: The existence of a “rheological percolation threshold” for some of the different CNTs, below which there seems to be a good dispersion, whereas above which, there seems to be a better dispersion of the agglomerates themselves. Accordingly, the ineffectiveness of the functionalization with organic polar groups, when the CNT dispersion does not reach a reasonable level. In spite of the industrial advantage of commercial CNTs (e.g. produced in bulk quantities, large area deposition), those obtained through arc discharge yield the highest strength improvement. The analysis carried out on the electrical properties of nanocomposites also shows interesting results : For all concentrations considered, CNTs show improved conducting properties, with respect to carbon black. At low CNT concentrations the lower defect density seems to prevail over the aspect ratio. As the CNT concentration is increased, the conductivity for SWNTs shows a sudden rise Þ the percolation threshold has been crossed. [1] S. Bellucci, et al. 2006 Nanotech 2006 Vol. 1, p. 129-133, ISBN 0-9767985-6-5 [2] S. Bellucci, et al. J Exp Nanosci, 2-3, (2007) [3] S. Bellucci, F. Micciulla, N. Pugno, The Nanomechanics in Italy, 2007: 197-210 ISBN: 978-81-308-0237-4, Ed.: N. Pugno, Research Signpost, Kerala, India, p. 197-210. [4] S. Bellucci, Carbon nanotubes composites for aerospace applications, submitted to Journal of Nanostructured Polymers and Nanocomposites [5] S. Bellucci, et al., Screening Electromagnetic Interference Effect using Nanocomposites, Macromolecular Symposia. vol. 263, pp. 21-29 ISSN: 1022 1360, 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 136
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