Sequestering High-Energy Electrons to Facilitate Photocatalytic Hydrogen Generation in CdSe/CdS Nanocrystals

T generation of H2 via photocatalytic water splitting has received considerable attention in recent years as efforts to optimize solar energy devices have accelerated. 3 Although visible photons are sufficiently energetic to split H2O and generate high-energy H2 fuel (requiring a 1.23 eV difference of its coupled half-reactions), materials that demonstrate this in any great yield are still not developed. This is due in part to significant overpotentials, rapid recombination of photogenerated charge carriers, poor absorption of the solar spectrum, and undesired photocorrosion reactions. The recent advent of semiconductor nanoparticle technology has provided researchers with the powerful flexibility to design novel materials to address these limitations. For example, band gaps can be tuned relative to their bulk state by quantum confining photogenerated excitons to nanometer dimensions; this increases the free energy (reduction potential) of charge carriers and can exceed the H2 reduction overpotentials necessary for photocatalysis. 5,9 By engineering stable materials that exhibit long-living excitons at elevated free energies, efficient water splitting photocatalysis should be achievable. Metal chalcogenides are promising materials for photocatalytic H2O reduction because their absorption spectra strongly overlap the visible region of the solar spectrum, which eliminates the need to design complicated multicomponent sensitizer/ catalyst systems. 14 While bulk CdSe is not catalytically active, the activity of CdSe nanoparticles has been theorized 17 and proven altogether with other semiconductor-based nanoparticles including ZnSe nanoribbons. Hodes and co-workers also demonstrated that nanostructured CdSe films performed as photoanodes in a cell employing a regenerative Na2SeSO3 electrolyte. 19 Recently, we demonstrated that colloidal two-dimensional CdSe nanoribbons (NR) also exhibit photocatalytic H2 evolution in the presence of hole-scavenging HS , which was attributed to an increase of the band gap in quantum-confinedCdSeNRs (2.7 eV) over bulk CdSe (1.74 eV). High-efficiency photocatalytic materials require the generation and preservation of high-energy electrons with suitable solvent access to initiate redox chemistry. This further requires the inhibition of competing mechanisms (i.e., electron/hole recombination, charge trapping, photocorrosion) that decrease catalytic activity. One approach of reducing the influence of these competing mechanisms is to design new multicomponent systems consisting of two (ormore) semiconductormaterials. Such systems introduce new possibilities of manipulating electron flow from one material to the other, depending on the energy level differences of the conduction bands (for electrons) and the valence bands (for holes). For example, excitation of a semiconductor material that is in electrical contact with a second semiconductor material with a lower-energy conduction band will result in electron transport into the second material. Furthermore, if the valence band of the second material were higher in energy, then hole transport to the first material would occur. For either of these cases, the composite material is referred to as “type II” and are of significant interest in photovoltaic and electrooptical developments.