Coated magnetite nanoparticles with a 6−8 nm average diameter were prepared. The surfactants used
to stabilize the nanoparticles and disperse them in organic solvents were oleic acid (OA), lauric acid ,
dodecyl phosphonate, hexadecyl phosphonate, and dihexadecyl phosphate. Transmission electron microscopy
analyses of the aggregation of the coated particles suggest that carboxylate surfactants provide the particles
with better isolation and dispersibility as compared with phosphonate surfactants. However, Fourier
transform infrared spectra of the phosphonate and phosphate coated particles suggest that these surfactants
cover the surface of the nanoparticles in islands of high packing density. The thermogravimetric and
differential scanning calorimetry measurements suggest that there is a quasi-bilayer of these surfactants
covering the surface of the nanoparticles, with varying amounts of surfactant in the outer layer and with
the second layer weakly bound to the primary layer through hydrophobic interactions between the alkyl
chains. The desorption temperatures of the alkyl phosphonates and phosphate are higher than those of
the carboxylate coated particles. The enthalpy of binding of the ligands suggests strong P−O−Fe bonding
on the surface. Nevertheless, regardless of binding strength, the OA coated particles are better dispersed
in organic solvents. Their higher hydrophobicity is likely due to different interactions among the oleyl
chains and/or a smaller tendency to form bilayer structures.
Molecular modification of dye-sensitized, mesoporous TiO2 electrodes changes their electronic properties. We show that the open-circuit voltage (V(oc)) of dye-sensitized solar cells varies linearly with the dipole moment of coadsorbed phosphonic, benzoic, and dicarboxylic acid derivatives. A similar dependence is observed for the short-circuit current density (I(sc)). Photovoltage spectroscopy measurements show a shift of the signal onset as a function of dipole moment. We explain the dipole dependence of the V(oc) in terms of a TiO2 conduction band shift with respect to the redox potential of the electrolyte, which is partially followed by the energy level of the dye. The I(sc) shift is explained by a dipole-dependent driving force for the electron current and a dipole-dependent recombination current.
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