Photo-Electrochemical Characteristics of Two-Dimensional Porphyrin Molecular Layers on Semiconductor Surfaces
Authors: Kondo Yuri , Shokai Ayumi , Hase Mariko , Kondo Toshihiro 
Published: February 2020

Metallo-porphyrins have planar structure and effectively absorb visible-light normal to the ring plane. Thus, they can be applied to artificial photosynthetic systems and photovoltaic cells when the porphyrin ring was adsorbed parallel to the solid surface [1].
Si is one of the most basic and important semiconductor materials and used to the crystal Si-based solar cell. Si mainly absorbs infrared-light and cannot absorb visible-light. On the other hand, TiO2 is a widely used semiconductor material and strongly absorbs ultraviolet-light. Because most of the sunlight is in a visible-light region, combination of Si or TiO2 with metallo-porphyrin should be required in order to construct the solar-active devices. However, the interaction between the Si and TiO2 substrate surfaces and metallo-porphyrin is very weak. Thus, we have to construct the self-assembled monolayers (SAMs), with a binding group such a carboxylate group to metallo-porphyrin, on the Si and TiO2 surface as a first and then the metallo-porphyrin layers were constructed on them.
In this study, we constructed 5,10,15,20- tetraphenyl-porphyrin zinc (ZnTPP) in a planar array on the Si(111) and TiO2(110) single-crystal substrates through the carboxy-terminated SAM and investigated photo-electrochemical characteristics for the visible-light active photodevices.
Ester-terminated SAMs of methyl acrylate (MA) and ethyl undecylenate (EU) SAMs were self-assembled on the hydrogen terminated Si(111) substrate by heating. After the SAM preparation on the Si surface, the terminated ester groups of the SAMs were hydrolyzed by the immersion into a hot HCl solution [2,3]. Carboxylate-terminated TiO2 surface was prepared just by dipping the pre-cleaned TiO2(110) substrate into an ethanol solution containing saturated 1,4-benzenedicarboxylic acid (BDC) for overnight. These carboxylate-terminated Si(111) and TiO2(110) substrates were dipped into the ethanol solution containing ZnTPP. In each step, structure, orientation, and surface coverage of the molecular layers were estimated by attenuated total reflection Fourier transformed infrared spectroscopy (ATR-FTIRS) and x-ray photoelectron spectroscopy (XPS), in order to determine the optimum condition. Finally, the photo-electrochemical characteristics of the constructed porphyrin molecular layers on Si(111) and TiO2(110) were investigated in the electrolyte solution containing electron donor and/or acceptor with and without Xe lamp irradiation.
From the results of ATR-FTIRS and XPS [4], it was found that the longer the dipping time of each SAM-modified Si(111) and TiO2(110) substrates, the more the absorbed amount of ZnTPP. On the both surfaces, potential-dependent anodic photocurrent was observed by visible-light irradiation. Excited energy dependence is now under investigation.

References
[1] D. Dolphin, “The Porphyrins”, Academic Press, New York (1978).
[2] H. Asanuma, E. M. Bishop, and H.-Z. Yu, Electrochim. Acta, 52 (2007) 2913.
[3] K. Rück-Braun, M. Å. Petersen, F. Michalik, A. Hebert, D. Przyrembel, C. Weber, S. A. Ahmed, S. Kowarik, and M. Weinelt, Langmuir, 29 (2013) 11758.
[4] D. H. Karweik and N. Winograd, Inorg. Chem., 15 (1976) 2336.