Characterization and Photoelectrochemical Properties of Nanostructured Thin Film Composed of Carbon Nanohorns Covalently Functionalized with Porphyrins Georgia Pagona, † Atula S. D. Sandanayaka, ‡ Taku Hasobe,* ,‡,§ Georgios Charalambidis, | Athanassios G. Coutsolelos, | Masako Yudasaka, # Sumio Iijima, # and Nikos Tagmatarchis* ,† Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou AVe., Athens 116 35, Hellas, School of Materials Science, Japan AdVanced Institute of Science and Technology (JAIST), Nomi, Ishikawa 923 1292, Japan, PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, Chemistry Department, Laboratory of Bioinorganic Chemistry, UniVersity of Crete, PO Box 2208, Heraklion 710 03, Crete, Hellas, and Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan ReceiVed: June 18, 2008; ReVised Manuscript ReceiVed: August 3, 2008 Carbon nanohorns (CNHs) covalently functionalized at the conical tips with porphyrin (H 2 P) moieties were used to construct photoelectrochemical solar cells. Electrophoretic deposition was applied to fabricate films of the modified CNHs onto optically transparent electrodes (OTE) while nanostructured SnO 2 films were cast onto the OTE (OTE/SnO 2 ). The CNH-H 2 P film on the nanostructured SnO 2 electrode exhibited an incident photon to current conversion efficiency (IPCE) of 5.8% at an applied bias of +0.2 V vs SCE in a standard three-compartment electrochemical cell. The measured IPCE was found greater than the one observed for the sum of the single components, namely CNHs and H 2 P films onto the SnO 2 electrode. Fluorescence lifetime measurements revealed that photoinduced electron transfer from the singlet excited state of the porphyrin to the nanohorns takes place, while direct electron injection from the reduced nanohorns to the conduction band of the SnO 2 electrode occurs. These processes are responsible for the photocurrent generation. Introduction Nanometer-sized carbon-based materials such as fullerenes and nanotubes possess enormous potential as integrative com- ponents of energy conversion devices because of their unique robustness as well as their novel optical and electrical properties. 1,2 Particularly, the great electron-accepting properties of fullerenes together with the small reorganization energy of electron transfer, 3 leading to a high acceleration of photoinduced charge separation (as well as deceleration of charge recombination), have paved the way toward the construction of some fullerene- based photoelectrochemical devices and solar cells. 4-10 More- over, in the nanowire-like structure of carbon nanotubes efficient electron and hole transport on electrodes occurs, thus allowing their incorporation into integrative components on photovoltaic and photoelectrochemical devices. 11-16 Recently the photoelec- trochemical properties of single-walled carbon nanotubes (SWCNT) as well as of cup-stacked carbon nanotube films cast on conducting glass electrodes have also been reported. 17-19 Carbon nanohorns (CNH), 20 a nanostructured graphene-based material within the family of SWCNT, are receiving great attention especially because of their unique morphology and unusually high purity. The CNH has a tubular form with a diameter of 2-5 nm and a length of 40-50 nm with a conical end. The CNH form a secondary aggregate having dahlia flower- like spherical morphology with an aggregate diameter of 80-100 nm. The absence of transition metal catalyst during production (i.e., CO 2 laser ablation of a graphite pole, at room temperature under noble gas conditions), greatly differentiates them from conventional carbon nanotubes. In common with carbon nanotubes, CNHs are insoluble in any solvent. However, chemical functionalization of CNHs, resulting in soluble hybrid materials, has been investigated both theoretically 21 and experimentally. 22-25 In this framework, photophysical assays with CNH-based donor-acceptor hybrid materials have been reported. Importantly, these tests provide initial insight into potential applications of CNH in solar energy conversion. Model systems that have been prepared and tested include CNH-based conjugates and hybrids that bear a variety of photosensitizers/ electron donors: pyrene, 26 porphyrin, 27-30 ferrocene, 31 and tetrathiafulvalene. 32 These functional building blocks are either covalently attached or interact supramolecularly with the CNH skeleton. Recently, the covalent attachment of R-5-(2-aminophenyl)- R-15-(2-nitrophenyl)-10,20-bis(2,4,6-trimethylphenyl)porphy- rin (H 2 P) to CNH via an amide bond was accomplished. 29 Fluorescence quenching and nanosecond transient absorption spectroscopy results indicated that the photoexcited H 2 P moieties serve as electron donors, while CNH serve as electron acceptors, resulting in the formation of a charge-separated state CNH •- -H 2 P •+ . Having these results in mind, we investigated the photoelectrochemical behavior of CNH-H 2 P fabricated onto an optically transparent electrode (OTE) while nanostructured SnO 2 electrodes were cast onto the OTE. Such light energy conversion systems composed of CNH have yet to be fabricated so far. Herein, the details of the morphology and photoelectro- chemical behavior of CNH-H 2 P films (OTE/SnO 2 /CNH-H 2 P) are presented and compared with pristine CNH films (OTE/ SnO 2 /CNH). In this context, CNH-H 2 P is employed as a * Corresponding authors. (N.T.) Fax: + 30 210 7273794; tel: + 30 210 7273835; e-mail: tagmatar@eie.gr. (T.H.) E-mail: t-hasobe@jaist.ac.jp. † National Hellenic Research Foundation. ‡ Japan Advanced Institute of Science and Technology (JAIST). § PRESTO. | University of Crete. # NEC Corporation. J. Phys. Chem. C 2008, 112, 15735–15741 15735 10.1021/jp805352y CCC: $40.75  2008 American Chemical Society Published on Web 09/13/2008