Electrochemical behavior of [Cu3(BTC)2] metal–organic framework: The effect of the method of synthesis

For the construction of the metal–organic framework [Cu3(BTC)2] (commonly known as MOF-199 or HKUST-1), three different synthesis strategies were employed: room temperature stirring (RT), solvothermal (ST) and ultrasonic assisted (US); which afforded the desired material in moderate yields. Several characterization techniques (XRD, FT–IR, SEM and TGA) showed that HKUST-1 was obtained in every method. However, there were found interesting differences in functionalization depending on the synthesis conditions which produced the same [Cu3(BTC)2] host net but with different coordinative and bulk molecular guests. By XRD pattern analysis, it was possible to identify characteristic peaks of Cu2O in the ST obtained material as secondary phase, showing that this methodology is quite harsh giving place to this product, although the [Cu3(BTC)2] complex was obtained in good yield. Regardless the energy source applied, in all three cases the desired [Cu3(BTC)2] MOF was obtained. In order to determine if the method of synthesis impacts directly on the electrochemical behavior of de material, applied cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were investigated. The electrochemical results have shown that the resistance to the faradic process increases in the following order: Cu–MOFUS, Cu–MOFRT and Cu–MOFST. This behavior can be associated to the presence of Cu2O in Cu–MOFST. However, the synthesized MOF at room temperature showed a major content of copper, which could be electrochemically transformed, as well as purity, in comparison with those obtained by solvothermal and ultrasonic assisted methods.

Graphical abstractCyclic voltammetry and structure of Cu–MOF compared with glassy carbon (UGC) in 0.1 M LiCl, prepared by tree different methods.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Differences in functionalization of [Cu3(BTC)2] depending on synthetic conditions. ► Synthesis method impacts directly on the electrochemical behavior of [Cu3(BTC)2]. ► The electrochemical behavior of MOF can be associated to presence of Cu2O. ► The reduction of copper ions into the structure occurs through two monoelectron steps.