Silicon nanocrystal hybrid photocatalysts as models to understand solar fuels producing assemblies

Direct coupling of light-harvesting semiconductors and molecular catalysts is an attractive approach in designing new systems for artificial photosynthesis. Understanding the energetic requirements to enable photogenerated charge transfer from the semiconductor to the molecular catalyst is an essential design component of photoelectrochemical schemes. Here, we explore a model system to study these requirements by tethering a rhenium carbonyl coordination complex to a dodecyl-terminated silicon nanocrystal using a two-step surface functionalization process. Diffuse reflectance infrared Fourier transform spectroscopy and cyclic voltammetry characterized successful surface attachment and redox properties of the hybrid structure. UV-visible spectroelectrochemical measurements confirmed the formation of a known Re(0) reduction product in thin films of the hybrid assembly under cathodic potentials. During reduction, only spectroscopic features associated with the monomer were observed, not the Re-Re dimer, suggesting that the hybrid structure prevents a common deactivation pathway for Lehn-type CO2 reduction catalysts. Initial photocatalytic data showed that the NC-catalyst hybrid structure retained CO2 reduction activity to CO. Finally, transient absorption spectroscopy was used to examine the ultrafast dynamics of the system and shed light on the energetic alignment between the silicon semiconductor and the rhenium carbonyl complex. A rhenium metal complex tethered to a silicon quantum dot allows exploration of the effects of catalyst anchoring and Re-Si energetic alignment on photocatalytic CO2 reduction activity.