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44219-G10
Bridge-Mediated Interparticle Electron Transfer in Self-Assembled Hybrid Semiconductor Nanomaterials

David F. Watson, State University of New York at Buffalo

Research overview.  The objective of the research supported by this ACS PRF-G grant is to characterize the photoinduced interfacial electron transfer reactivity of coupled semiconductor nanomaterials.  During the first year of the project, we focused primarily on materials fabrication and characterization.  Specifically, we developed a method for tethering CdS and CdSe quantum dots (QDs), or approximately spherical particles with diameters of 2-6 nm, to the surfaces of nanocrystalline TiO₂ films through bifunctional organic linkers.  Our research on surface functionalization and materials assembly has led to the publication of three manuscripts in Langmuir, the ACS journal of colloids and surface chemistry.  During the second year of the project, we focused both on fundamental aspects of the surface chemistry of these systems and on using time-resolved spectroscopic techniques to characterize the excited-state interfacial electron transfer reactivity of the coupled semiconductor materials.

In our materials fabrication strategy, QDs are attached to the terminal thiol groups of TiO₂-adsorbed mercaptoalkanoic acids (MAAs).  We have measured higher surface loadings of QDs on TiO₂ surfaces functionalized with mixed monolayers of MAAs and unthiolated alkanoic acids than on pure MAA monolayers.  This unexpected finding prompted us to study mixed monolayers on TiO₂ surfaces in more detail.  We discovered that mixed monolayers of thiolated and nonthiolated surfactants undergo unprecedented time-dependent compositional changes after the establishment of saturation surface coverages.  The changes are caused by dimerization reactions between thiol groups, suggesting that intermolecular interactions play a key role in determining the compositions of mixed monolayer systems.

We have used time-resolved spectroscopic techniques (transient absorption and time-resolved emission) to characterize bridge-mediated electron transfer processes in tethered QD-MAA-TiO₂ assemblies.  The excited state of CdS QDs decays within tens to hundreds of nanoseconds.  However, interfacial electron transfer from photoexcited CdS to TiO₂ leads to a charge-separated state with a lifetime of several microseconds.  Notably, the electron transfer yield decreases dramatically with increasing linker length and interparticle separation.  Thus, the interfacial electron transfer reactivity can be tuned systematically by varying the nature of the molecular linkage between donor and acceptor nanoparticles.

Significance of research.  Our studies of mixed monolayers have immediate impact with regard to optimizing surfactant-mediated materials assembly processes.  In addition, and more generally, our findings illustrate that intermolecular interactions between surfactant functional groups can dramatically influence the composition, and therefore the reactivity and properties, of mixed monolayers.  Similar dynamic compositional changes may impact the assembly of a range of mixed monolayer systems for applications in sensing, molecular electronics, and materials assembly.  Our spectroscopic results suggest that molecularly-linked nanoparticles may serve as robust platforms for understanding and controlling the excited-state deactivation pathways and interfacial electron transfer reactivity of nanostructured materials.  Our results will serve as the basis for an ongoing systematic investigation of the influence of materials composition, morphology, and interconnectivity on the dynamics and efficiency of interfacial electron transfer processes.  Our findings may lead to the development and optimization of coupled semiconductor nanomaterials for a range of applications in photocatalysis and photovoltaics.

Impact of PRF funding.  PRF funds were used to acquire supplies for the fabrication and characterization of nanomaterials and surfaces and for spectroscopic experiments.  Funds were also used to support a postdoctoral researcher for six months and two graduate students over the summer months.  The postdoctoral researcher has since taken a position at the National Renewable Energy Laboratory (NREL) in Golden, CO.  The two graduate students are continuing their Ph.D. studies in our research group at the University at Buffalo.  Finally, funds were used to cover travel and registration expenses for the 2008 Gordon Research Conference on Electron Donor-Acceptor Interactions.  The PI contributed a poster and was selected by the conference organizers to give an oral presentation.  (A total of four posters were chosen.)  Speaking at the conference was an excellent opportunity to present our findings to key members of the electron-transfer community.  Over two years, the funding from the PRF was instrumental in supporting our research endeavors.  The preliminary results from this project provided the basis for a proposal which was funded by the National Science Foundation’s CAREER program.  Therefore, we enjoy continuing support for the research which was initiated under this grant.

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