Thermodynamic and Atomic Vibrational Properties of Metal Nanoparticles: Size, Support, and Adsorbate Effects

The University Of Central Florida Board Of Trustees, Orlando FL

Investigators

Abstract

TECHNICAL SUMMARY: This project aims to gain insight into the size-dependent evolution of the melting and Debye temperatures, thermal expansion coefficient, sintering behavior and phonon density of states of supported metallic nanoparticles. As nanoparticle size decreases, it becomes progressively more difficult to produce samples with a well-defined size distribution. Small changes in size produce a large change in the fraction of atoms at free surfaces relative to those in contact with the support. Small nanoparticles are expected to be strongly affected by their local environment, such as the supporting materials and surface adsorbates. This study addresses these challenges through a combination of highly controlled sample synthesis to produce large volumes of well-defined ordered metal nanostructures through inverse micelle encapsulation and highly sensitive in situ and ex situ characterization techniques at shared user facilities (Brookhaven National Laboratory-BNL and Argonne National Laboratory) as well as in the PI's research lab. The material systems under investigation include Pt and Fe nanoparticles supported on strongly (Al2O3, SrTiO3) and weakly (SiO2) interacting substrates. The characterization techniques available for this project are: (i) atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) for structural characterization, (ii) X-ray photoelectron spectroscopy (XPS) for electronic and chemical characterization, and (iii) X-ray absorption spectroscopy (EXAFS and XANES) and nuclear resonant inelastic X-ray scattering (NRIXS) for structural, thermodynamic and lattice-vibrational characterization. The intellectual merit of this application relies on: (i) the improvement of our current understanding of the thermal properties of metal nanoparticles < 3 nm and (ii) how these characteristics change because of particle size-effects, nanoparticle-adsorbate and nanoparticle-support interactions. This study will contribute insight into the role of the support and its pre-treatment on nanoparticle sintering phenomena, temperature-dependent atomic order-disorder transitions and structural phase transitions involving soft phonon modes. NON-TECHNICAL SUMMARY: Anomalous thermodynamic properties, such as superheating, negative thermal expansion and ultra-low thermal conductivities, have been reported for nanostructured metals. The origins of these effects are heavily debated. This is due in part to the scientific challenge posed by the complexity of these systems and the need to consider the influence of a number of variables simultaneously, such as the nanoparticle geometry and environment. Through this project, insight into the evolution of important characteristics, such as the melting temperature, thermal expansion coefficient and sintering behavior, of nanoparticles with decreasing size will be sought. A deeper knowledge of the thermodynamic properties of nano-metallic systems may lead to future advances in nanotechnology and materials science, including improving the thermal stability and operation regime of nanoparticles, heat generation and distribution in plasmonic nano-antennae, thermoelectric applications, and nanostructured metal-organic composites for solar cell applications. This project will contribute to the training of PhD and undergraduate students in university and national laboratory settings and will provide the first research experience to two K-12 students. The PI will work with the local Science Center in the organization of an exhibit entitled "Art in Science, Science in Art," intended to draw the attention of the general public to state-of-the-art scientific research in the nanoscience area through visually appealing posters, including microscopy images from the proposed research.