Layer-By-Layer Construction of Three-Dimensional MOF [Cu2(bdc)2dabco]n on Au Surface
Authors: Hase Mariko , Chun Wang-Jae , Kondo Toshihiro 
Published: February 2020

Introduction.

Metal-Organic frameworks (MOFs), which have a high surface area, flexibility, and designability, are highly porous materials formed from a combination with metal ion and organic ligand. Therefore, MOFs are widely studied and expected to apply them to gas storages, separations, and catalysts [1]. Moreover, MOFs are expected as not only bulk materials but also composite materials, such as electronic and photo devices, by constructing MOF thin films on various solid substrates.
Layer-by-layer (LbL) fashion can control a growth orientation, thickness, and morphology without losing bulk phase properties on oxide substrates, semiconductors, and self-assembled monolayers (SAMs) on metals [1-3]. However, their structures were not completely matched to those of their bulk. For understanding the growth mechanism of MOFs films to construct the same structures of their bulk, in this study, we have examined the structure of [Cu2(bdc)2dabco]n [4]; (bdc = 1,4-benzdicarboxylate, dabco = 1,4-diaza bicyclo[2.2.2]octane) thin films on a single-crystal Au(100) surface, which was modified by 4-mercaptobenzoic acid (4-MBA) SAMs. In order to investigate the preparation process of this MOF thin films, polarization modulation – fourier transform infrared reflectance absorption spectroscopy (PM-FTIR-RAS), x-ray photoelectron spectroscopy (XPS), and x-ray absorption fine structure (XAFS) [5], were performed.

Experimentals.

The Au(100) substrates were pre-treated via annealing/quenching to clean those surface. For [Cu2(bdc)2dabco]n thin films on Au(100) ([Cu2(bdc)2dabco]n / 4-MBA / Au(100) ), pre-treated Au(100) was initially immersed in an ethanol solution containing 1 mM 4-MBA for 10 min. The substrate was then alternatively immersed in the ethanol solutions containing 1 mM Cu(CH3COO)2 for 30 min, 0.1 mM bdc for 1 h, and 0.1 mM dabco for 1 h at room temperature to fabricate thin films.
After every immersion, the sample substrates were thoroughly rinsed with ethanol and dried in a stream of N2 gas. The structures of the samples at each step were investigated by PM-FTIR-RAS, XPS and XAFS.

Results and Discussion.

The XP spectra in the Cu2p 2/3 region showed the construction of [Cu2(bdc)2dabco]n on the Au(100) substrate modified by the 4-MBA SAM successively by LbL fashion immersed in each solution. The PM-FTIR-RA spectra showed more details for initial formation process of [Cu2(bdc)2dabco]n thin films on Au(100) modified by the 4-MBA SAM. The peak of C=O stretching vibration of terminal carboxyl group of the 4-MBA SAM for 4-MBA SAM / Au(100) was observed at 1724 cm-1, indicating the presence of a monomer hydrogen bonding of a couple of the carboxyl group. After immersion of 4-MBA SAM / Au(100) into 1 mM Cu(CH3COO)2, this peak observed at 1724 cm-1 for [Cu(CH3COO)2] / 4-MBA SAM / Au(100) was disappeared. Instead of this peak, another peak was observed at 1763 cm-1, indicating the disappearance of a hydrogen bond. Therefore, carboxyl-terminals for the 4-MBA SAM were partially coordinated to Cu(CH3COO)2. After immersion into 0.1 mM bdc, in addition of the coordination of the bdc molecules as a bridging legand, the peak due to C=O stretching vibration for [Cu(bdc)2]n / 4-MBA SAM / Au(100) was observed at 1698 cm-1. This result shows the presence of a dimer hydrogen bonding. After immersion into 0.1 mM dabco, the peak intensity of COO stretching vibration of a carboxylate group for [Cu2(bdc)2dabco]n / 4-MBA SAM / Au(100) decreased, as compared to that for [Cu(bdc)2]n / 4-MBA SAM / Au(100), suggesting that the direction of COO stretching vibration became parallel to the substrate surface.

References.

[1] D. Zacher et al., Chem. Soc. Rev., 38, 1418-1429 (2009).
[2] O. Shekhah et al., J. Am. Chem. Soc., 129, 15118-15119 (2007).
[3] H. Gliemann et al., Materials Today, 15, 110-116 (2012).
[4] K. Seki et al., Chem Lett., 30, 332 (2001).
[5] W.-J. Chun et al., J. Phys. Chem. B, 102, 9006-9014 (1998).