Michael G. Weir

Dendrimer-encapsulated nanoparticles: New synthetic and characterization methods and catalytic applications

In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized using a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently reduced chemically to yield nearly size-monodisperse particles having diameters in the 1–2 nm range. Monometallic, bimetallic (alloy and core@shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica, but also as a stabilizer for the nanoparticle. In this perspective, we report on progress in the synthesis, characterization, and applications of these materials since our last review in 2005. Significant advances in the synthesis of core@shell DENs, characterization, and applications to homogeneous and heterogeneous catalysis (including electrocatalysis) are emphasized.

Structural Analysis of PdAu Dendrimer-Encapsulated Bimetallic Nanoparticles

PdAu dendrimer-encapsulated nanoparticles (DENs) were prepared via sequential reduction of the component metals. When Au is reduced onto 55-atom, preformed Pd DEN cores, analysis by UV-vis spectroscopy, electron microscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy leads to a model consistent with inversion of the two metals. That is, Au migrates into the core and Pd resides on the surface. However, when Pd is reduced onto a 55-atom Au core, the expected Au core-Pd shell structure results. In this latter case, the EXAFS analysis suggests partial oxidation of the relatively thick Pd shell. When the DENs are extracted from their protective dendrimer stabilizers by alkylthiols, the resulting monolayer-protected clusters retain their original Au core-Pd shell structures. The structural analysis is consistent with a study of nanoparticle-catalyzed conversion of resazurin to resorufin. The key conclusion from this work is that correlation of structure to catalytic function for very small, bimetallic nanoparticles requires detailed information about atomic configuration.

Synthesis, characterization, and electrocatalysis using Pt and Pd dendrimer-encapsulated nanoparticles prepared by galvanic exchange

In this report we present the synthesis and characterization of Pt and Pd dendrimer-encapsulated nanoparticles (DENs) using the method of galvanic exchange. Sixth-generation hydroxyl-terminated poly(amidoamine) dendrimers were used to prepare Cu DENs composed of 55 atoms. In the presence of either PtCl42− or PdCl42−, the less noble Cu DENs oxidize to Cu2+ leaving behind an equal-sized DEN of Pt or Pd, respectively. DENs prepared by direct reduction with BH4−, which is the common synthetic route, and those prepared by galvanic exchange have the same composition, structure, and properties as judged by UV-vis spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electrochemical methods. However, the galvanic exchange synthesis is much faster (3 h vs. 96 h), and the yield of reduced DENs is significantly higher (nearly 100% in the case of galvanic exchange).

In situ X-ray Absorption Analysis of ~ 1.8 nm Dendrimer- Encapsulated Pt Nanoparticles during Electrochemical CO Oxidation

We report an in situ X-ray absorption-fine structure (XAFS) spectroscopic analysis of ∼1.8 nm Pt dendrimer-encapsulated nanoparticles (DENs) during electrocatalytic oxidation of CO. The results indicate that Pt nanoparticles encapsulated within poly(amidoamine) (PAMAM) dendrimers and immobilized on a carbon electrode retain their electrocatalytic activity and are structurally stable for extended periods during CO oxidation. This is a significant finding, because nanoparticles in this size range are good experimental models for comparison to first-principles calculations if they remain stable.

Impact of annealing on the chemical structure and morphology of the thin-film CdTe/ZnO interface

To enable an understanding and optimization of the optoelectronic behavior of CdTe-ZnO nanocomposites, the morphological and chemical properties of annealed CdTe/ZnO interface structures were studied. For that purpose, CdTe layers of varying thickness (4–24 nm) were sputter-deposited on 100 nm-thick ZnO films on surface-oxidized Si(100) substrates. The morphological and chemical effects of annealing at 525 °C were investigated using X-ray Photoelectron Spectroscopy (XPS), X-ray-excited Auger electron spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, and atomic force microscopy. We find a decrease of the Cd and Te surface concentration after annealing, parallel to an increase in Zn and O signals. While the as-deposited film surfaces show small grains (100 nm diameter) of CdTe on the ZnO surface, annealing induces a significant growth of these grains and separation into islands (with diameters as large as 1 μm). The compositional change at the surface is more pronounced for Cd than for Te, as evidenced using component peak fitting of the Cd and Te 3d XPS peaks. The modified Auger parameters of Cd and Te are also calculated to further elucidate the local chemical environment before and after annealing. Together, these results suggest the formation of tellurium and cadmium oxide species at the CdTe/ZnO interface upon annealing, which can create a barrier for charge carrier transport, and might allow for a deliberate modification of interface properties with suitably chosen thermal treatment parameters.

Photoemission study of CdTe surfaces after low-energy ion treatments

We present a study of low-energy ion surface cleaning treatments and their impact on the surface electronic structure of an air-exposed CdTe thin film treated with CdCl2. In order to determine the electronic structure using surface-sensitive photoemission, surfaces need to be free of contaminants. This is achieved by subsequent low-energy ion treatment steps, carefully monitoring the chemical and electronic surface structure. We present data on the valence band maximum (VBM), and core-level binding energies, that suggest that neither preferential sputtering occurs nor metallic states are formed using our cleaning procedure. For a clean CdTe surface, the VBM is determined to be (0.8 ± 0.1) eV below the Fermi energy.