Cobalt salophen complexes for light-driven water oxidation

Abstract

Earth-abundant molecular complexes have been found to be excellent catalysts for the light-driven water oxidation reaction. Here, we demonstrate the photochemical water oxidation catalysed by three cobalt salophen complexes with different axial ligands in the presence of [Ru(bpy)₃]²⁺ as a photosensitizer and Na₂S₂O₈ as an electron acceptor in phosphate buffer of pH 9 and 7. The electrochemical investigation of the complexes in pH 9 including cyclic voltammetry (CV) and linear sweep voltammetry (LSV) verify the deposition of catalytic films on the surface of the working electrode. The deposited film characterization using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) confirms that the complexes decomposed to form nanoparticles. Electrospray ionization mass spectrometry (ESI-MS) studies along with dynamic light-scattering (DLS) measurements of the catalyst solution during the course of photochemical water oxidation suggest the catalyst decomposition and the formation of nanoparticles. The XPS measurements of the produced nanoparticles suggest that the surface of the particles is composed of Co(II) and OH species. In contrast, multiple experiments argue that light-driven water oxidation using the same complexes in pH 7 is homogeneous. We thus conclude that cobalt salophen complexes act as precatalysts that decompose under basic conditions to form cobalt hydroxide nanoparticles which act as a real catalyst for the light-driven water oxidation reaction, whereas the same complexes act as a homogeneous catalyst in the photochemical water oxidation reaction under neutral conditions.