book of abstracts - IM2NP

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.
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