Construction of Metal Organic Frameworks with Porphyrin on (110) Single Crystal Surfaces
Authors: Mitadera Mai , Tomizawa Eika , Kondo Yuri , Kondo Toshihiro 
Published: February 2020

Introduction

These days, the use of photovoltaics such as solar cells have been expanding since they are clean, cost-effective, and carbon-free. The most common solar cells are crystalline silicon solar cells, but the efficiency is relatively low as about 20 %. The silicon absorbs only infrared radiation even though about half of the solar radiation is constituted by visible-light. On the other hand, porphyrin based solar cells show strong absorption in the visible region, so they have been increasing attention recently. Among them, porphyrin based dye sensitized solar cells (DSSCs) composed of semiconductors including TiO2 are investigated enthusiastically. Since DSSCs are the combination of dye which absorbs visible-light and TiO2 which absorbs ultraviolet radiation, they are thought to be effective. Porphyrin has a planar structure which is composed of four pyrrole subunits and has orientation dependent absorption. It absorbs visible-light most effectively irradiated from normal to the ring plane. Thus, it is important to fix the porphyrin ring flatly to the solid substrate surface to maximize the energy conversion efficiency.
In order to control the orientation of molecules, metal organic frameworks (MOFs) which are highly crystalline porous coordination polymers can be utilized. MOFs are made from organic ligand and metal ion, and used as frameworks to introduce functional moiety such as porphyrin. MOFs can be fabricated by a layer-by-layer (LbL) fashion on solid substrate keeping the orientation and controlling the thickness and morphology without loss of bulk properties. As a linker between MOFs and substrates, self-assembled monolayers (SAMs) were employed to fix the MOFs on solid surface.
The aim of this study is to construct the three-dimensional MOFs with porphyrin on single crystal Au(110) and TiO2(110) surfaces. As linker SAMs, cysteamine (CA) and isonicotinic acid (IA) SAMs were constructed on Au(110) and TiO2(110), respectively. On both the CA SAM / Au(110) and IA SAM / TiO2(110), Cu - TCPP (tetra-carboxyphenyl porphyrin) - 4,4’ bipyridine (bpy) MOF was fabricated by the LbL fashion. Each preparation process was structurally investigated by scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS).

Experimental

After annealing and quenching, Au(110) was immersed in 1 mM CA ethanol solution at room temperature for 1 hour to construct CA SAM on Au(110). The substrate was then successively soaked in 0.2 mM Cu(OAc)2 ethanol solution, 0.1 mM TCPP ethanol solution, and 0.2 mM bpy ethanol solution for 10 minutes, keeping the temperature of 40°C (Cu-TCPP-bpy MOF / CA SAM / Au(110)). After every immersion, the substrate was sonicated with ethanol for 10 min. In the same way, annealed TiO2(110) was soaked in saturated IA ethanol solution at 40°C for 1 day to construct IA SAM on TiO2(110). Subsequently, the substrate was immersed in 0.1 mM Cu(OAc)2 ethanol solution for 3, 5, and 10 minutes for comparison, keeping the temperature 40° (Cu2+ / IA SAM / TiO2(110)). The substrate was sonicated with ethanol for 10 min after every immersion. At each step, the structure and elemental composition were estimated by STM and XPS.

Results and discussion

In the case of Cu-TCPP-bpy MOF / CA SAM / Au(110), STM and XPS measurements were conducted. STM image reveals that CA SAM was homogeneously fabricated on the Au(110) surface. It was also observed by STM that Cu2+ and TCPP layers were constructed on the CA SAM with the LbL fashion, and bpy orientates vertical to the MOF sheet with porphyrin. Additionally, XP spectra in S2p and N1s indicates that the CA SAM was existed on Au(110), and those in Cu2p and N1s show that Cu2+, TCPP, and bpy were coordinated after each immersion. On the other hand, for Cu2+ / IA SAM / TiO2(110), XPS was performed. From a peak in N1s region, the modification of IA SAM on TiO2(110) was confirmed. Furthermore, the XP spectrum in Cu2p region shows the deposition of Cu2+ on IA SAM after the immersion.