Nanotech 2010

Incorporation of plasma modified multi-walled carbon nanotubes into TiO2 photoanode for dye-sensitized solar cells (DSSC)

Y.F. Chan
National Cheng Kung University, TW

Keywords: TiO2, MWNTs, DSSC, nanocomposites


Dye-sensitized solar cells (DSSC) are fabricated using photoanode comprises of nanocomposite material of TiO2 and multi-walled carbon nanotubes (MWNT) treated in situ by plasma and subsequent functionalization by grafting of maleic anhydride (MA). The carboxyl groups attached on the tube wall of the MWNTs grafted with MA (MWNTs-MA) enable the bonding with Ti when mixed into TiO2 matrix through the O-C=O opening and dehydration reactions among the groups. Fourier transform infrared (FT-IR) was used to characterize the functional groups on the surface of the MWNTs after the plasma modification. During the physical blending process, the functionalized MWNTs enable the nanotubes to disperse well among the TiO2 nanoparticles and become integral parts of the nanocomposites, as shown by the observations of scanning electronic microscopy (SEM) and transmission electron microscopy (TEM) images. The photoanodes of the DSSC were prepared using the mixture of MWNTs-MA and TiO2 (Degussa P25) with aid of organic surfactants (Triton X-100). Raman spectroscopy verifies the co-existence of the carbon nanotubes (CNTs) in TiO2 matrix. XRD analysis confirms the existence of anatase phase of TiO2 nanoparticles coated on the FTO glass, while suggesting there is no significant phase shift in spite of the MWNTs-MA addition. Moreover, MWNTs-MA/TiO2 nanocomposites presented obvious improvements in photocurrent and solar power conversion efficiency, whereby the CNT provides more efficient electron transfer through the film as well as the charge collection at the surface of counter electrode . At an optimum incorporation of 0.075wt.% MWNTs into the working electrode, the photo conversion efficiency of the DSSC was improved by more than 30%, under the Air Mass 1.5 illumination standard for an active area of 0.25cm2. On the contrary, with MWCNTs additives of over 0.2%, the conversion efficiency decreases gradually and yet comparable to the one of pure TiO2, indicating that the MWNTs-MA exhibited satisfactory dispersal throughout the mesoscopic TiO2 film without agglomeration that could apparently increase the optical absorbance and thus deteriorate the performance of the DSSC. The plasma treatment serves as a dry and nonpolluting method to modify the CNTs for promoting better dispersion in TiO2 matrix as opposed to the conventional chemical acid modifications that alter their unique characteristics by shortening and fracturing the tubes.
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