Monday, October 17, 2011: 12:50 PM
200 I (Minneapolis Convention Center)
We have recently demonstrated continuous control over Pt nanoparticle size across the cluster-to-crystal transition region (ca. 0.5 – 2.7 nm) using a colloidal synthesis approach that involves autoreduction of a Sn-Pt complex in hydrochloric acid solution. This approach yields particle size distributions less than 10% and permits fine atomic structure contrast in this critical size range. Such uniform structures are needed to characterize the geometric and electronic configurations of nanomaterials used in catalysis and fields like nanoelectronics and optics. Despite this need, the underlying mechanisms leading to nanocrystal formation are poorly understood and often unexplored. Recent attempts at systematic studies to investigate size-controlled NP structures have had limited success especially with particle sizes less than about 1.5 nm leaving size effects occurring at < 1 nm unexplored. This size region is interesting because significant changes from bulk properties can occur here. We have previously demonstrated a 3-fold improvement in the electrocatalytic oxygen reduction activity versus control catalyst for 1.7 nm diameter particles that exhibit densely packed, yet non-crystallographic arrangements of atoms. In the present paper, we describe the kinetics of Sn-Pt autoreduction during colloidal synthesis and the formation of Pt metal using UV-Vis spectroscopy by determining and following spectroscopic features at several wavelengths. These features are used to monitor the generation of Pt-Sn complexes formed when Pt(IV) and Sn(II) are mixed. Broad features extending into the visible spectrum, ca 600 nm, characterize the appearance of the Pt metal. These broad features are induced by the photoexcitation and oscillations of the free electrons in the conduction band that occupy energy states near the Fermi level. Using this information, we propose a mechanism for the autoreduction synthesis scheme that yields precise particle size control. TEM, FFT, and EXAFS are used to characterize the particles observed following reaction completion. Understanding the mechanism of nanoparticle growth and stabilization for Pt will allow us to extend this technique to the other noble metals for both catalytic and non-catalytic applications.