Infrared spectroelectrochemical analysis of potential dependent changes in cobalt hexacyanoferrate and copper hexacyanoferrate films on gold electrodes

Abstract

Metal hexacyanometallates represent coordination network compounds with a large variety of structurally similar substances. It is well established that they can undergo multiple redox transitions accompanied by cation insertion/ejection. However, the exact assignment of structural changes has remained complicated partially due to the structural variability resulting from different preparation methods and the lack of in situ tools to follow the redox transition and structural changes simultaneously. In this communication we prepared cobalt hexacyanoferrate (CoHCF) and copper hexacyanoferrate (CuHCF) by electrochemical deposition and, additionally, CuHCF by layer-by-layer deposition. These hexacyanoferrate films have a suitable thickness for their spectroelectrochemical characterization by means of polarization modulation infrared reflection-absorption spectroscopy with electrochemical control. Potential-dependent structural changes were followed in both HCF films using the cyanide stretching mode ν(CN) whose detailed position reveals coordination and symmetry around the cyanide bridging ligand. CoHCF undergoes two redox reactions. The spectral analysis of the ν(CN) mode provides clear evidence that oxidation of N-coordinated Co(II) to Co(III) takes place in the first redox process, at E1°′=0.530V. The second redox process at E2°′=0.685V is assigned to a partial oxidation of the C-coordinated Fe(II)/Fe(III). In CuHCF films, one redox process involves predominantly Fe(II)/Fe(III) centers. However, in the electrochemically deposited film a small fraction of the Cu(II) centers is oxidized to Cu(III). The species containing Cu(III) ions are unstable and undergo electron transfer reactions leading to the oxidation of the Fe(II) to Fe(III). In addition, CuHCF shows ‘annealing’ in which some cyanide groups change the coordination to C-coordinated Cu ions and N-coordinated Fe ions.