# Preparation and Application of ZIF-8 Thin Layers
## 3 Results  
### Electrochemical Thin Layer Deposition and Effect of Synthesis Conditions  
Electrochemical synthesis of MOF materials represents a method to create a thin layer coating onto complex shaped surfaces in a fast and scalable way.  
We applied the anodic method to create a thin layer coating of ZIF-8, in which zinc ions are electrochemically dissolved in the electrolyte, containing 2-methylimidazole as the linker. The two educts then form the resulting ZIF-8 network that is deposited on the substrate, because the concentration of zinc ions is significantly higher close to the surface of the working electrode. Since ZIF-8 must be deposited on the Cu substrate, the first step included deposition of metallic Zn on the Cu substrate, followed by the formation of ZIF-8. To examine the effect of different synthesis parameters for the anodic electrodeposition of ZIF-8, we changed individual parameters, based on a standardized synthesis process reported in Section 2.1. This standard synthesis is based on previous publications [31] and subsequent optimization for our application. The following parameters were gradually changed: linker concentration, potential, synthesis time and electrolyte composition. The resulting coatings were pre-evaluated for a homogeneous, stable and flat coating and then characterized by XRD and SEM for chemical purity and particle shape, as well as detailed information about microscopic coating quality. The electrochemical setup for the synthesis with a Cu plate as the working electrode is shown in Figure S3.  
The deposited ZIF-8 particles of the standardized procedure were scratched off from the surface, and the resulting powder was measured with X-ray diffraction. The measured pattern fits well with the corresponding reflections of the ZIF-8 reference [35], as shown Figure 3. However, an additional reflection was observed at around 8.5\({}^{\circ}\), corresponding to triclinic Cu\({}_{2.5}\)SO\({}_{4}\)(OH)\({}_{3}\)*2H\({}_{2}\)O [36]. Most probably, during anodic deposition of ZIF-8, some bare Cu surface was created upon Zn dissolving, and reacted with electrolyte species. Note that the background of the measured diffractogram is quite high, especially in the starting region, indicating the presence of some amorphous components in the sample.  
A selection of SEM images for specific conditions is shown in Figure 4. The Pictures 4B, 4E and 4H represent the standard synthesis conditions.  
Figure 3: X-ray diffraction pattern of the deposited material (red) and corresponding ZIF-8 reference [35] (black), Cu–K\(\alpha\)1 radiation, \(\lambda=1.54056\) Å. The signal at 8.5\({}^{\circ}\), marked with an asterisk, does not belong to the Bragg reflections of ZIF-8 material and corresponds to Cu\({}_{2.5}\)SO\({}_{4}\)(OH)\({}_{3}\)*2H\({}_{2}\)O [36], which was formed as an admixture during anodic deposition of ZIF-8.  
Figure 4: SEM images of ZIF-8 coating on a Cu substrate showing the impact of different synthesis conditions. (**A**–**C**) correspond to the varied linker concentration (with (**A**) = 0.5 M; (**B**) = 1.0 M; (**C**) = 2.0 M); (**D**–**F**) show the results of changing electrochemical potentials during the reaction (with (**D**) = \(-1.0\) V; (**E**) = \(-1.1\) V; (**F**) = \(-1.4\) V); (**G**–**I**) demonstrate the effect of the reaction time (with (**G**) = 5 min; (**H**) = 30 min; (**I**) = 60 min), (**K**–**M**) represent the result of different electrolyte compositions ((**K**) = 25% EtOH, (**L**) = 50% EtOH, (**M**) = 75% EtOH).