Photo-Gated Intervalence Charge Transfer of Ethynylferrocene Functionalized Titanium Dioxide Nanoparticles

Abstract

Ethynylferrocene-functionalized titanium dioxide nanoparticles (TiO2-eFc) were synthesized, for the first time ever, by a modified two-phase hydrothermal method. Transmission electron microscopic measurements showed that the nanoparticles were rather uniform in size, with an average diameter of 4.0±0.5nm and well-defined lattice fringes that were consistent with those of anatase TiO2. 1H NMR, FTIR and XPS measurements confirmed the attachment of the ferrocenyl ligands onto the nanoparticle surface, most likely forming Ti−CC−Fc interfacial bonds. The resulting nanoparticles exhibited a bandgap of ca. 3.3eV, and two emission bands in photoluminescence measurements at 351 and 460nm, with the former due to the TiO2 cores whereas the latter from intraparticle charge delocalization between the nanoparticle-bound acetylene moieties under UV photoirradiation. In electrochemical measurements, only one pair of voltammetric peaks were observed in the dark, due to the redox reactions of the nanoparticle-bound ferrocenyl groups. However, when subject to photoirradiation with UV lights (254 and 365nm), two pairs of voltammetric peaks appeared, with a respective peak spacing of 174 and 198mV, suggesting intervalence charge transfer (IVCT) between the ferrocenyl moieties bound on the nanoparticle surface. This arose from photo-enhanced electrical conductivity of the TiO2 cores that served as part of the chemical linkage bridging the ferrocenyl moieties. Significantly, such photo-gated IVCT varied with the photoexcitation energy that dictated the intraparticle charge transfer pathways.