Chemical, physical, and structural properties of oxide nanoparticles

Nanoparticles often have novel properties that can be exploited for the development of new devices or improved processing, making the controlled synthesis of nanoparticles vital to fields including chemistry, materials science and engineering, and the environmental sciences.  Nanoparticles are also common, natural components of the Earth’s surficial materials, are increasingly found as anthropogenic contaminants in the environment, and are often employed in water treatment systems and engineered systems to remediate pollutants in groundwater. Processes like dissolution, precipitation, growth, electron-transfer reactions, and phase transformations strongly influence the fate and transport of a wide range of natural and anthropogenic chemical species in aqueous systems, and many of these processes occur at the surfaces of or produce nanophase materials.  Furthermore, nanosized (i.e., smaller than tens of nanometers) particles often behave differently, both physically and chemically, than their larger counterparts. Thus, the synthesis of model materials for use in experiments aimed at elucidating the fundamental mechanisms of nanoparticle transformations, nucleation and growth as well as the physical and chemical properties of nanoparticles is critical to understanding their behavior in both natural and engineered systems.

Work in the Penn group focuses on the fundamental growth mechanisms of a wide range of environmentally and technologically important nanoparticles, with the goal of controlling particle size and shape, the distribution of elements throughout each particle, and defect concentration.  The well characterized materials are then used in experiments aimed at elucidating the link between the aforementioned physical and chemical properties and chemical reactivity and magnetism.  The group employs high-resolution transmission electron microscopy (HRTEM) to characterize solid-state changes resulting from reactions with both natural and anthropogenic chemicals so as to quantitatively assess reactivity, reactive surface area, and how reactivity evolves as reactions proceed. Finally, natural materials are used in parallel experiments so as to enable meaningful comparisons between the natural and model materials.

NANOPARTICLE GROWTH: We focus on a nonclassical crystal growth mechanism called oriented aggregation, which results in the formation of new secondary crystals that are composed of oriented primary units. Recent results have shown that this growth mechanism is strongly size dependent and can be exploited to control both size and shape of nanoparticles. We synthesize a wide range of materials, including iron oxides, zeolites, titanium dioxide, zinc oxide, and other transition metal oxides. We use these materials as model materials in order to improve our understanding of magnetism and the chemical behavior of nanoparticles in environmental systems - both natural and engineered.

A sampling of TEM images of iron oxide nanoparticles prepared in the Penn Labs.

NANOPARTICLE REACTIVITY: A second major area of research examines the properties of iron oxide nanoparticles, which are common at and near the earth's surface and often employed in engineered remediation systems.  Results from experiments using both synthetic and natural materials are particularly useful in understanding fate and transport of contaminants and in understanding the paleomagnetic record, which can serve as a sensitive record of climate change.  We examine reactivity by quantifying the relative rates of redox reactions as a function of varying nanoparticle properties (e.g., doping, size, shape, microstructure). Our results show that reactivity is strongly dependent on the presence of dopants (e.g., arsenic, which is a common dopant in natural iron oxide nanoparticles) and size.

We use a wide range of solid-state characterization techniques, including transmission electron microscopy, magnetic characterization, and small angle X-ray scattering, to characterize both natural and synthetic nanophase materials. 

OUTREACH:  MICROSCOPY CAMP - Atomic-structure imaging as a means for improving middle school students’ understanding of atoms (www.chem.umn.edu/microscopycamp)
With recent advances in the achievable resolution of electron microscopes, a direct method for demonstrating the atomic structure of solid crystals is available.  Microscopy Camp is designed to introduce and reinforce acceptable concepts of the atomic structure of solid crystals to middle school-age students. To date, twenty students have attended Microscopy Camp, which was held over two days at the University of Minnesota in August of 2005 and 2006.  The program was funded by Prof. Penn’s NSF CAREER award. Campers synthesized their own magnetite (Fe3O4) nanoparticles; characterized their particles using visual inspection, hand-held magnifiers, light microscopes, and hand-held permanent magnets; and participated in the characterization of their particles using scanning electron microscopy (SEM) and HRTEM (e.g., figure 5). For most students in this age group, this type of experience provides the first opportunity to directly observe the atomic structure of solid crystalline materials. Microscopy Camp 2007 (July 30 – August 3, 2007) was expanded, funded through the Department of Education, and serves middle school and high school science teachers.

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Specific Research Activities:
Exploring the chemical reactivity of nanoparticles:
     Crystallographic, compositional, morphologic, and microstructural dependence     
     Dissolution experiments: compare materials before and after dissolution, quantify products of dissolution.
     Contaminant degradation experiments: quantify degradation of contaminants like nitroaromatics, elucidate the link between surface structure and rates of reaction, quantify changes in surface structure as a function of reaction progress.
     Electrochemical reactions: quantify the degree of reaction as a function of particle size, morphology, microstructure, and doping.  
     Materials of interest include both synthetic nanomaterials and environmental nanoparticles.

     This type of work has broad relevance to areas like heterogeneous catalysis, photocatalysis, environmental remediation, biogeochemical cycling of anthropogenic and natural chemical species, mineral liquid crystals, magnetism, nanotechnology, and more.

Synthesis of materials
     Systematically changing pH, ionic strength, temperature, reaction rate, etc....
     Controlling "speed" of oxidation, nucleation, and growth.
     Adding "contaminants" and "dopants"
     Materials of interest: iron oxides, transition metal oxides, zeolites, sulfides, and more.
     One goal is to produce MODEL materials: In other words, materials that are similar to those found in the environment. We then use these materials in experiments aimed at elucidating the behavior of natural nanoparticles in surface and near-surface environments (e.g., groundwater system, river beds).

Characterization of naturally-occuring and synthesized nanoparticles
     X-ray Diffraction (XRD): identify bulk phases.
     Transmission Electron Microscopy (TEM): characterize size, morphology, composition, and microstructure (e.g., disorder and defects).
             Click here to see some images!
     Cryo-TEM: characterize aggregation state in situ. In this method, a suspension of nanoparticles is flash-frozen using liquid ethane, which results in fast solidification and prevents crystallization of ice. In this way, the aggregation state of the nanoparticles in suspension can be quantified and tracked as a function of reaction progress.
     Electron Diffraction (ED): identify phases at the nano-length scale.
     Atomic Force Microscopy (AFM): characterize building block organization within larger assemblies.
     Scanning Electron Microscopy (SEM) characterize size, morphology, and building block organization within larger assemblies.
     Small Angle X-ray Scattering (SAXS): characterize particle size and size distribution in situ.
     Magnetic characterization: Magnetism is uniquely sensitive to trace components, size, and size distribution. We collaborate with the folks at the Institute for Rock Magnetism!

Modeling aggregation and assembly morphologies
     Crystallographic constraints and surface charge considerations.
     Model disaggregation and aggregation.
     Predict particle size and size distribution.
     Synthesize materials and compare to model results.  

Dissolution as a probe for chemical reactivity: synthetic and naturally-occurringmaterials
      Using dissolution agents to probe surface reactivity and structure.
      Quantifying reactive surface area.
      Characterize materials using TEM: before and after.