Behavior of a Cu-Peptide complex under water oxidation conditions – Molecular electrocatalyst or precursor to nanostructured CuO films?

Abstract

The true electrocatalyst of water oxidation in a number of cases proved to be an in situ developed ‘CuO’ film instead of (or beside) the original Cu complex itself, since the breaking of metal-ligand interactions under the applied conditions often successfully compete with the homogeneous process. The decomposition of a catalyst candidate is obviously unwanted; on the other hand, inexpensive and controllable precursor complexes represent an exciting platform to fabricate nanostructured metal oxide coatings utilized in artificial photosynthesis. We investigated the Cu-triglycine complex equilibrium system with uniform {N,N,N,O}eq peptide binding mode throughout the pH range of ~7 to 10 in borate buffer. In the presence of the complex under the conditions of water oxidation electrocatalysis the development of a ‘CuO’ nanoparticle coating was observed on the indium tin oxide working electrode. This coating acted as a robust electrocatalyst of water oxidation. X-Ray photoelectron spectroscopy (XPS) indicated a mixed Cu2O/CuO/Cu(OH)2 surface composition. Fine coatings could be also fabricated on different electrodes including glassy carbon, boron-doped diamond and an n-type semiconducting α-Fe2O3 nanoarray, importantly, without ruining its morphology. We identified the interplay between the co-existent borate equilibrium species and the Cu-triglycine system as the key factor to define the dominant process and allow control over the deposition.