Metal-Semiconductor Nanohybrids for Solar-to-Fuel Conversion: A Systematic Study on the Processes Limiting the Photocatalytic Yield

Abstract

Colloidal semiconductor-metal nanostructures can be used as photocatalysts for generating hydrogen from water. The use of nanostructures is especially promising due to their large surface-area-to-volume ratio, providing a large catalytically active surface area with small amounts of material. Additionally, due to their synthetically adjustable optoelectronic properties and composition, these nanostructures can serve as model systems for studying the efficiency-limiting parameters of the solar-to-fuel conversion processes. Literature agrees that the photocatalytic activity of the nanostructures critically relies on the separation of the photogenerated charge carriers, which can be altered by tuning the dimensionality and composition of the nanoparticles or by inducing charge scavenging and trapping processes. However, processes occurring at the nanoparticles’ surface were only partly considered in the present discussion, e.g. electron donating agent-specific surface interactions have not been addressed so far. To assess the impact of the various parameters limiting the photocatalytic yield, a systematic study was carried out, which correlates the charge carrier dynamics within 11-mercaptoundecanoic acid-capped, Pt-tipped CdSe/CdS dot-in-rods with their photocatalytic activity and colloidal stability. The charge carrier dynamics and photocatalytic activity of the nanoparticles were investigated in parameter studies by conducting transient absorption and time-resolved photoluminescence spectroscopy, as well as steady-state hydrogen production measurements. These measurements were combined with electron microscopy, dynamic light scattering, and static spectroscopy to examine the colloidal stability of the nanoparticles. Based on this large set of measurements, the study reveals that the photocatalytic activity is critically dependent on processes occurring at the nanoparticles’ surface. It was found that electron donating agents interact differently with the nanoparticles’ surface, and the two distinct groups of surface-active and diffusion-limited electron donating agents were identified. Besides these electron donating agent-specific surface interactions, the degree of surface passivation by ligands was observed to influence the nanoparticles’ photocatalytic activity. The largest photocatalytic activities were determined for nanoparticle agglomerates, forming upon ligand loss or induced by the presence of surface-active electron donating agents. These results indicate that the agglomeration of nanoparticles does not necessarily diminish the photocatalytic activity due to a loss of accessible surface area but instead may increase it by enabling charge transfer within the formed network. Combining these insights into the processes occurring at the nanoparticles’ surface with a detailed acquisition of the synthesis parameters for obtaining Pt-CdSe/CdS nanoparticles with functional components of distinct number and dimensionality, this study enables to produce tailored nanohybrids functioning as building blocks for assembling structured and highly efficient catalyst networks.