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40548-B10
Self-Assembled Films of Semiconductor and Metal Nanoparticles in Polyelectrolytes: Assembly Dynamics and Photo-Induced Charge Transfer and Transport Processes

Lara Halaoui, The American University of Beirut

Electrocatalytic and photo-induced charge transfer processes are investigated at metal and semiconductor nanoparticles assembled in polyelectrolyte. These studies have implications for solar cells, fuel cells, fuel generation, and catalysis. In this 3-year work we have made significant progress in furthering our understanding of charge transfer at nanosize metals and semiconductors. Several undergraduates (4 males, 4 females) have worked on this project, 3 of them are authors or co-authors of published work. We summarize here the scientific progress and describe on-going work.

Polyacrylate (PAC)-modified Pt nanoparticles (nano-Pt) and CdS quantum dots (Q-CdS) were synthesized and assembled on oppositely charged poly(diallyldimethylammonium chloride), PDDA, by electrostatic and hydrophobic interactions as films of PAC-nano-Pt/PDDA, PAC-Q-CdS/PDDA, or heterostructures of nano-Pt and Q-CdS in PDDA on different surfaces for electrochemical, spectroscopic, and imaging characterization. The assembly permitted control of nanoparticle surface density in one layer or in multilayers. It also allows varying interlayer separation between the metal nanoparticles and the Q-dots by intercalation of polyelectrolyte multilayers (PDDA/poly(styrenesulfonate), PSS).

Atomic hydrogen underpotential deposition (H_upd) was reported at crystallite sites of PAC-nano-Pt (=2.5 ± 0.6 nm) in PDDA, and charge transfer and transport in the assemblies was discussed (J. Phys. Chem. B 2005). Resolving H_upd states showed that the surface modifier does not irreversibly block adsorption sites, and provided surface characterization of the Pt nanoparticles. We also showed (Electrochem. Solid State lett. 2006) that electrocatalytic activity was retained at the nano-Pt/PDDA assemblies for some significant reactions (hydrogen evolution, hydrogen oxidation, and oxygen reduction). Nanostructured Pt electrodes were then assembled with varying nanoparticle density, controlled laterally by varying the adsorption time, or vertically with the number of layers. The electrochemical behavior towards oxygen reduction and related charge transfer processes was studied, and showed dependence on surface coverage and multilayer architecture (J. Phys. Chem. C 2007). In a related study emerging from the nano-Pt catalytic activity towards hydrogen peroxide oxidation, and in a similar trend, a highest intrinsic sensitivity towards hydrogen peroxide was measured at the lowest nano-Pt coverage in nano-Pt/PDDA arrays (has been prepared for submission).

Another aspect followed directly from electrochemical studies at nano-Pt/PDDA films, and was also funded by the University Research Board at AUB. Nanostructured electrodes of PAC-nano-Pt can now be assembled with control of surface density, and since PAC-Pt nanoparticles have been reportedly prepared in different shapes (in the literature by El. Sayed et al.), this procedure promises to control independently particle size, shape, and interparticle separation to study the effect of these factors on electrocatalysis. We have been working on reproducing and investigating factors affecting growth of facetted Pt nanoparticles from Pt(IV) salt reduced with hydrogen and working out the assembly. We use hydrodynamic electrochemical techniques to explore the effect of interparticle separation (and then size and shape) on the oxygen reduction reaction at nano-Pt/PDDA. Understanding the Pt crystallite size effect on this reaction is significant both fundamentally and for its implications in fuel cells.

We continued exploring the photoelectrochemical (PEC) characteristics of Q-CdS assembled in PDDA to gain a better understanding of the working mechanism prior to dissemination of results. Photoelectrochemistry with different liquid interfaces (i.e., sodium sulfide, ascorbic acid, tartaric acid) showed significant differences between the PEC properties of the Q-dot films and bulk CdS. For instance, while electrodeposited bulk CdS (n-type) is a photoanode in the presence of hole scavengers, the response of Q-CdS/PDDA films leads to either photo-anodic or photo-cathodic current, depending on the applied potential, the redox species, and pH. The response in one case was even predominantly photo-cathodic. Competing photo-induced oxidation or reduction, and charge transport in the Q-dot film will depend on the relative energetics of the Q-semiconductor in relation to that of solution and the underlying electrode potential, thus leading to this behavior. This work is being prepared for publication (draft).

Pt nanoparticles and Q-CdS were co-assembled in different layers in PDDA, and photoelectrochemistry at the resulting heterostructures was studied. The goal was to explore the possibility of driving reactions that are sluggish at the Q-dots (for example hydrogen evolution) by incorporating a co-catalyst. Several architectures are being explored: (PDDA/Q-CdS)₄, (PDDA/Q-CdS)₄/PDDA/nano-Pt, (PDDA/Q-CdS)₂/PDDA/nano-Pt/(PDDA/Q-CdS)₂/PDDA/nano-Pt, and (PDDA/Q-CdS)₄/ (PDDA/PSS)_n/PDDA/nano-Pt (subscript indicates the number of bilayers). This assembly would allow some control of interlayer separation between the Q-dots and nano-Pt, possibly affecting charge separation. We are conducting generation-collection experiments using a 4-electrode PEC cell with a rotating Pt ring-GC disk electrode, having a removable disk for assembly. Our preliminary results show a lower photocurrent at Q-CdS films with co-assembled nano-Pt, indicating photogenerated charge transfer to the metal (indicated earlier through Q-CdS photoluminescence quenching by nano-Pt). The lower photocurrent was accompanied by an increase in the collection ring current indicating further charge transfer from the metal nanoparticle to solution species. We continue these experiments in the presence of different redox couples to understand charge transfer at the heteronanostructures.

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