Optical and Electrochemical Effects of Semiconductor Nanoparticle Clustering on Photocatalytic Water Splitting Half-Reactions

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To meet increasing demands for carbon-reduction, renewable and low-emission fuel alternatives have been proposed as a replacement for carbon-intense natural gas and coal. Specifically, hydrogen and oxygen evolution have been demonstrated at the lab-scale through a room temperature water-splitting process using only water, sunlight, and suspended semiconductor nanoparticles as inputs. Recent advancements in improving the efficiency of this water-splitting process focus on methods to boost charge separation, chemical selectivity, and optical band-engineering. However, there is a growing need for demonstrations at scale that also consider nano-particle interactions as an ensemble. Additionally, Z-scheme reactor designs have motivated the study of oxidation and reduction half-reactions using charge-carrying ions as an intermediary. This work studies how photocatalytic rates of half-reactions driven by semiconductor nano-particle suspension are affected by the inevitable presence of aggregation and agglomeration. The effects of solution pH, Fe(III) concentration, input light intensity, reaction temperature, stirring and sonication, nanoparticle concentration and size, and material quality are experimentally characterized by the desired ion evolution reaction rates and external quantum yield. Additionally, the effects of these variables on the zeta potential, optical scattering, particle aggregation, and mass transport are also experimentally measured to probe ensemble behavior and nano-particle interactions. Results indicate the importance of balancing ion concentration and pH to reduce aggregation via enhanced zeta potential nanoparticle repulsion, while also avoiding competitive absorption from suspended intermediary ions and their respective pH-dependent hydrolysis states. Reaction modeling further motivates investigation into charge-transfer limitations and enhancements due to aggregation states that cannot be explained by optical effects alone.