Electrophoretic Deposition of Single-Walled Carbon Nanotubes Covalently Modified with Bulky Porphyrins on Nanostructured SnO2 Electrodes for Photoelectrochemical Devices

Single-walled carbon nanotubes (SWNTs) covalently modified with large porphyrin molecules have been prepared to construct photoelectrochemical devices with nanostructured SnO 2 electrodes on which the multiporphyrin-linked SWNTs are deposited electrophoretically. The film of the porphyrin-linked SWNTs on the nanostructured SnO 2 electrode exhibited an incident photon-to-photocurrent efficiency as high as 4.9% under an applied potential of 0.08 V vs SCE. The more uniform film and moderate photocurrent generation in the porphyrin-linked SWNT devices can be rationalized by the exfoliation abilities of the bulky porphyrins that yield large steric hindrance around the SWNTs. Direct electron injection from the excited states of the SWNTs to the conduction band of the SnO 2 electrode is responsible for the photocurrent generation. Despite the efficient quenching of the porphyrin-excited singlet state by the SWNTs in the porphyrin-linked SWNTs, the photocurrent action spectra revealed that the excitation of the porphyrin moieties makes no contribution to the photocurrent generation. The evolution of an exciplex between the porphyrin-excited singlet state and the SWNTs and the subsequent rapid decay to the ground state without generating the charge-separated state is proposed to explain the unusual photoelectrochemical behavior. The results obtained here will provide valuable information on the design of SWNT-based photoelectrochemical devices.