# Preparation and Application of ZIF-8 Thin Layers
Figure 1: Different contact angle cases for rough homogeneous surfaces: Wenzel state and Cassie-Baxter state. The electrochemical synthesis of an MOF powder was first developed and pioneered by the chemical company BASF [28]. In this synthesis, a metal electrode is used as metal source for the electrochemical oxidization of ions. The formed metal ions build the corresponding organometallic network with the dissolved linker. The rapid synthesis creates a good route for the preparation of powdered materials. However, a thin layer deposition of MOFs using an electrochemical route has been rarely explored so far. For example, some attempts were undertaken to achieve electrochemical layer deposition for MOFs [29], such as Cu-BTC framework HKUST-1 [30] and ZIF-8 [31]. Especially by using electrochemical techniques, formation of a thin, homogeneous and porous layer of MOFs on a surface may represent a very promising approach to fabricate stable, hierarchically superstructured surfaces of a complex shape for liquid infusion. There are two common ways of electrochemical MOF synthesis. One represents the cathodic deposition [32], where the electrolyte contains a metal salt, the organic linker as well as a pro-base. The reduction of the pro-base generates a base, thus increasing the pH near the cathode. Then, the deprotonation of the organic linker is induced, which further coordinates the metal ions, and MOF precipitation occurs. Although this method is independent of the substrate used, it is not considered further in this work, since the method allows only a minor control of crystals formation during synthesis. The anodic electrochemical route was introduced by [PERSON] and co-workers [28]. The metal ions of the electrode are released into the electrolyte when an appropriate voltage is applied. Then, they become coordinated with the organic linker form the electrolyte and precipitate on the electrode, forming a thin MOF layer. The major advantage of this method, apart from high mass transfer and application in flow reactors, is the high influence on the resulting MOF properties by changing synthesis conditions. The use of an MOF as host material for LIPS was first introduced in 2018, via electrodeposition of Cu-BTC HKUST-1 [33]. In this work, an electrochemically deposited layer of HKUST-1 was used as supportive host. This MOF-layer was then infused with a perfluorated oil resulting in high contact angles for polar, non-polar and low-surface-tension liquids (114\({}^{\circ}\) for water, 95 \({}^{\circ}\) for diiodomethane and 61\({}^{\circ}\) for acetone). Since HKUST-1 shows larger contact angles and is probably more sensitive to water due to its components, we chose ZIF-8 for our studies. Besides the resistance of penetration and flooding, omniphobic lubricant-infused surfaces show a small contact angle hysteresis. This can help to achieve dropwise condensation of low surface tension fluids, even if the contact angle is lower than 90\({}^{\circ}\)[34]. Figure 2: Crystal Structure of ZIF-8 (space group I-43 m, a = 16.8815(5) Å, V = 4811.0(4) Å, Z = 12) [27]. Since we want to test the applicability of these coatings for the further development of heat exchangers in future studies, copper is used as the substrate material for all coatings, because it has a particularly high thermal conductivity and is a typical material for heat exchangers. In this work, we report wetting properties of omniphobic lubricant-infused surfaces, created by the deposited host material. We compare electrochemically deposited ZIF-8 layers with two different types of ZIF-8/PVDF composite layers and discuss advantages of each preparation.