Suitable Location to Control Electron Transfer and Gap-mode Plasmon Interactions: Photocurrent Generation from Silver Nanoparticle–Porphyrin Composite Layers

Abstract

Two types of silver nanoparticle (Ag NP)dye nanocomposite layers were prepared on an indium-tin oxide (ITO) transparent electrode, where porphyrins were fixed on the Ag NPs by chemical bonding through their carboxylate moieties or by hydrophobic interactions. Stable cathodic photocurrents were generated from the nanocomposite layer structures and were specifically enhanced by the overlap of the Q-band excitation of the porphyrins between 500 and 600 nm with the gap-mode plasmon band of the Ag NPs. The photocurrent efficiency of the chemical bonding system was significantly higher than that from the hydrophobic interaction system.Creation of effective electron-transfer systems is one of the important challenges in the development of solar cells, electroluminescent displays, and photocatalytic devices. 15 Recently, a combination of metal nanoparticles (NPs) and dyes has been used for the construction of light-harvesting systems because of their mutual photocurrent enhancement behavior.68 Colored Ag NPs have received more attention as promising photon-harvesting materials because of their intense absorption of visible light and their plasmonic interaction due to the near-field effect (localized surface plasmon resonance (LSPR)).914 Hupp et al. have reported on the distance dependence of the LSPR effect using vapor-deposited bare Ag NPs coated with TiO 2 .15 The LSPR effect weakened with the increase in the TiO 2 thickness. We have reported on the simple, high-yield, low-cost, and largescale synthesis of organic solvent-dispersible Ag NPs protected by long-chain aliphatic molecules and their application in printed electronics. 16,17 By simple solution techniques with a dispersion solution of the Ag NPs, we successfully fabricated an effective and stable photocurrent generation system consisting of the Ag NPs and porphyrins on an indium-tin oxide (ITO) electrode. 8 In this system, specifically enhanced photocurrents could be stably generated in the visible region, because of the porphyrin Q-band absorption between 500 and 600 nm. In our efficiently enhanced photocurrent system, we speculated that (i) 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin (1) was chemically bonded onto the Ag NPs via a ³-conjugated rigid anchor carboxyphenyl group, (ii) the porphyrins were located with high probability in the narrow spaces (gaps) surrounded by the Ag NPs responsible for the so-called gap-mode plasmon resonance at around 470 nm, and (iii) electron transfer occurred from the Ag NPs to the porphyrins through mutual chemical bonding (Figure 1a).In order to identify the suitable locations between the Ag NPs protected by long-chain aliphatic molecules and common dyes such as porphyrins and ruthenium complexes controlling both their mutual electron transfer and the gap-mode plasmon interactions, we investigated and compared the photocurrent generation behavior of Ag NPporphyrin nanocomposite layers using 1 (Figure 1a) and 5,10,15,20-tetraphenylporphyrin (2) without any anchor moieties (Figure 1b). We adopted highly monodisperse A...