Liquid-Phase Molecular-Ionic Layer Deposition of Surface-Supported Metal-Organic Frameworks of Cu3(BTC)2 HKUST-1 for Electrochemical Sensors
Authors: Turkevych Ivan , Shirakawa Naoki , Fukuda Nobuko 
Published: December 2020

Metal-organic frameworks (MOFs) are traditionally developed as effective gas separation/storage materials, however, recently they are receiving increasing attention as multifunctional layers in electronic devices [1]. MOFs can possess unique combinations of properties derived from complex interaction between inorganic and organic constituents that can provide better functionality in comparison to conventional materials. Recently, applications of electrochemically active MOFs for small molecules sensing, such as glucose [2], ethanol [3], NADH [4] and H2O2 [2, 5] have been demonstrated. However, MOFs are still rarely used in electrochemical sensors, because fabrication of thin films of MOFs and their integration into electronic devices is challenging due to incompatibility of MOFs with conventional physical or chemical vacuum deposition methods, while conventional solvent-assisted methods, such as spin-coating and die-coating, or solvothermal methods are difficult to realize in controllable manner due to extremely low solubility of MOFs. From the other hand, the low solubility of MOFs becomes an advantage in the case of liquid-phase molecular ionic layer deposition (LP-MILD) method, which belongs to the family of layer-by-layer deposition methods similar to the atomic layer deposition (ALD). In the LP-MILD method the growing film is sequentially exposed to solutions of reactive precursors mediated by washing steps that remove excess of precursors and reaction byproducts, and effectively confine the reaction space to only one surface monolayer of the growing film. As a result, the film grows in steps of a half monolayer per process cycle, which allows easy control over film thickness and composition, including heteroepitaxy and/or doping. The LP-MILD method can be technically realized in different flavors, where sequential substrate exposures to precursor solutions and washing steps are facilitated by dip-coating [6], spray-coating [7] or spin-coating [8].
In this work we demonstrate that spin-coting facilitated LP-MILD can be effectively used to fabricate layers of surface supported metal organic framework (SURMOF) of Cu2(BTC)3, also known as HKUST-1, on fluorine-doped tin oxide (FTO) transparent conductive layers. Glass/FTO substrates cleaned by consecutive sonication in detergent, water, acetone and IPA and then treated in oxygen plasma were placed on a vacuum chuck of spin-coater and sequentially exposed to (1) solution of copper (II) acetate in ethanol (2 mM, 1000 rpm, 10 s), (2) ethanol (500 rpm, 20 s), (3) 1,3,5-benzenetricarboxylic acid (BTC) in methanol (5 mM, 1000 rpm, 10 s) and (4) methanol (500 rpm, 20 s). The precursor solutions and the substrate were kept at the same temperature during the LP-MILD process. We have found that the growth per cycle (GPC) for the deposition at 25 oC is larger than should be in the expected half-layer GPC mechanism, but nevertheless smaller than the full-layer GPC. This is already better result than in the previously reported works [7, 8], where the growth rate significantly exceeds even the full-layer GPC, which can be explained by entrapment of precursors in the cages of the growing SURMOF due to insufficient rinsing. We have found that the deposition at 40 oC approaches the ideal half-layer GPC mechanism, which indicates effective rinsing at elevated temperatures.
In this presentation we are going to compare various fabrication methods of HKUST-1 SURMOFs and demonstrate advantages of our LP-MILD method supported by morphological, structural and electrochemical characterizations of our HKUST-1 SURMOF layers as active layers for electrochemical sensors.

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