The study of the growth process of metal nanoparticles is greatly simplified if reactants (i.e., metal ion and reducing agent) are physically separated from one another, by their location in two (immiscible) liquid phases. Nucleation and growth of the nanoparticles then takes place at the interface between these two liquid phases. This collaboration between the University of Illinois at Chicago and the University of Manchester in the United Kingdom uses state-of-the-art X-ray spectroscopy, surface scattering and electrochemistry techniques to investigate particle growth on the molecular level as it occurs at the liquid/liquid interface. Such localization at the interface allows for the use of X-ray absorption, which would not readily detect particles dispersed homogeneously throughout a solution volume, but can be applied in the interfacial case because the particles are highly concentrated at the interface. X-ray absorption spectroscopy probes the local geometric and electronic structure in non-crystalline systems, including determination of the chemical species and the chemical state of the atoms. In addition to this spectroscopic probe, the investigators use a structural probe, X-ray surface scattering, to study the in-plane and out-of-plane structure, including the shape, size, and organization of the particles, as well as the depletion of reactant species near the interface. These X-ray techniques are combined with electrochemical control of the interfacial reaction at the liquid/liquid interface, both to monitor the progress in particle growth as well as to investigate the influence of the applied potential in controlling particle production.
This project provides students with a research environment shaped by scientific expertise from the UK and USA. Students participate in joint group meetings (by video teleconferencing) and travel to laboratories in both countries to learn X-ray spectroscopy, surface scattering, and a broad range of electro-analytical chemistry techniques for the study of nanoparticles at interfaces. It is anticipated that a molecular-level understanding of metal nanoparticle nucleation and growth will allow for the production of nanoparticles with designed properties. This should influence the development of applications of nanoparticles in a number of areas, including the design of new materials for catalytic, opto-electronic, and coating applications.
Progress in nanotechnology relies upon the production of nanoparticles. During the past decade many recipes have been introduced for the synthesis of nanoparticles from the solution phase, including particles of different composition, shape, and architecture such as core and shell structures. In spite of this extensive work we lack a molecular level understanding of the nucleation and growth of nanoparticles that could lead to their rational, rather than empirical, design. The goal of this project was to determine a molecular level understanding of the nucleation and growth of nanoparticles. Our strategy was to study the electrochemically controlled formation of metal nanoparticles at liquid-liquid interfaces using X-ray techniques that allowed us to characterize the nanoparticles on the sub-nanometer molecular length scale. This project involved several stages. These included the development of a new, electrochemically-controlled route to nanoparticle production. Our collaborators at the University of Manchester accomplished this. Another stage consisted of the investigation of X-ray scattering techniques to characterize nanoparticles at the liquid-liquid interface. This was accomplished by studying both preformed and growing nanoparticles using synchrotron X-ray sources in several countries, including the Advanced Photon Source in the USA, the Diamond source in the UK and the PetraIII source in Germany. This led to a characterization of nanoparticles positioning, ordering, and transport at liquid-liquid interfaces. This capability has been used to characterize the nanoparticle growth process. This project provided for the development of human resources in an interdisciplinary area of relevance to nanotechnology. Post-doctorals and students were trained in the development of new techniques to grow and characterize nanoparticles. This Materials World Network collaboration leveraged expertise from both the USA and the UK in the development of these capabilities.
