Historical reliance on non-renewable fossil fuels is a key contributor to environmental and humanitarian challenges facing the world today, underscoring a need for renewable and clean sources of energy. H₂ is a “future fuel” capable of releasing nearly three-times more energy than traditional petroleum-based fuels while producing only benign water as a byproduct. However, to make hydrogen generation clean and sustainable, it needs to be de-coupled from petroleum processing and tied to renewable energy sources like solar or wind. One path towards this goal is to use mature technologies like photovoltaics to drive water electrolysis; however, this path requires step-wise energy conversion from solar, to electrical, to chemical fuel, along with associated losses along the way. An alternative pathway is direct conversion of solar to chemical fuels via photocatalytic pathways. The major limitation for such light-driven water splitting reactions is the lack of efficient photocatalysts to oxidize water to O₂ and then transform the resultant protons and electrons into H₂. As this process requires simultaneous management of four electrons and four protons, systems that combine multiple photosensitizers to “feed” the active catalyst represent a promising design strategy. In this vein, the present work describes design and testing of a solar photocatalyst for the water reduction (hydrogen evolution reaction, HER) process exploiting CdS quantum dot (QD) porous assemblies (aerogels) as photosensitizers coupled with Ni₂P nanoparticles—established electrocatalysts for HER—to convert protons and photogenerated electrons to hydrogen.
The synthesis of CdS QDs aerogels incorporating Ni₂P nanoparticles will be reported, and the role of QD crystal structure (zinc blende vs. wurtzite), mass loading of Ni₂P nanoparticles, and annealing protocols on HER activity under visible-light irradiation in the presence of hole scavengers (sulfide/sulfite) will be described. These data will be correlated to photophysical analyses of the QD aerogels to provide mechanistic insight into exciton generation, charge separation, and carrier transport. These results will be discussed in terms of design of a Z-scheme architecture to enable overall visible light water splitting using efficient, earth-abundant, catalysts.

Kody Whisnant

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