Detecting and Preventing the Formation of Photosensitizer-Catalyst Colloids in Homogeneous Light-Driven Water Oxidation

Combining cationic ruthenium photosensitizers (RuPS) with anionic polyoxometalate water oxidation catalysts (POM-WOCs) is the standard approach for light-driven POM-based water oxidation. Here, we show that colloid formation by electrostatic aggregation of a molecular photosensitizer {e.g., [Ru(bpy)(3)](2+) (bpy = 2,2-bipyridine)} and a POM-WOC {e.g., [Co-4(H2O)(2)(-PW9O34)(2)](10-)} significantly affects catalytically relevant system parameters. A facile, quantitative procedure for colloid detection using syringe filtration and UV/Vis spectroscopy is presented, and we illustrate that photosensitizer-POM colloid formation is a general phenomenon under typical WOC conditions and is observed for a range of photosensitizers and POMs. It is further demonstrated that for some systems an increase in the ionic strength of the solution prevents colloid formation. Significant changes in the electronic interactions between RuPS and POM-WOC under colloidal and homogeneous conditions are reported, thus highlighting the need for fast and reliable colloid identification. In addition, the study raises awareness about colloid formation in homogeneous solar-energy conversion schemes driven by two or more ionic species.