V14B-01 INVITED
Structure Determination of Nanocrystalline Materials
Nanocrystalline materials play an important role in pristine and contaminated geochemical systems. In particular, the reactive surfaces of nanosized ferric and aluminum hydroxides are effective scavengers and transporters of metals and metalloids under aqueous conditions. Establishing the atomic arrangements of these nanocrystalline phases is an essential step towards understanding their structure-property relationships and their roles in geochemical environments. Solving the structures of nanosized materials, often forming with finite particle sizes less than 10 nm, is extremely challenging and often not feasible using conventional structure determination methods. Nanocrystalline ferrihydrite is one example of a material often referred to as being X-ray amorphous because it exhibits poorly defined diffraction maxima in conventional powder patterns. Such features primarily result from the effects caused by disorder and extremely small particle sizes and inhibits quantitative structure determination. This can be avoided by evaluating the atomic arrangement in real-space using total scattering methods combined with pair distribution function (PDF) analysis. PDF analysis is proving to be an essential tool in the study of nanocrystalline materials because it enables the incorporation of both the Bragg and diffuse scattering components and therefore information regarding the short-, intermediate-, and long-range ordering is attainable. We recently used this method to investigate the atomic structures of a number of important nanosized transition metal phases, including ferrihydrite. The initial study of synthetically-derived ferrihydrite resulted in the development of a new structure model for this phase. We are now expanding this investigation by evaluating samples of ferrihydrite occurring in natural settings which are inherently more complex formation environments due to the presence of organics and other metal and metalloid species. The total X-ray scattering method and PDF analysis will be introduced and experimental high-energy scattering data collected at a synchrotron facility will be presented.
V14B-02 INVITED
Synthesis and structure determination of uranyl peroxide nanospheres in the presence of organic structure directing agents
Recently, actinyl peroxide nanoclusters containing 20, 24, 28, or 32 actinyl polyhedra have been synthesized and their structures identified with single crystal X-ray diffraction [1]. Most nanomaterials are composed of main group elements or transition metals, therefore, these actinyl nanospheres may display vastly different chemical and physical properties due to the presence of filled f-orbitals. A major goal of our research group is to create novel actinyl materials, focusing on nano- and mesoporous materials. The original nanosphere syntheses were limited to inorganic crystallization agents, such as monovalent cations. Over the last decade, the use of organic compounds and surfactants have received increased attention as structure-directing agents for the generation of novel inorganic materials. Using structure-directing organic amines we have successfully synthesized and determined the structures of uranyl nanospheres containing 40 and 50 uranyl polyhedra. The topology of the skeletal U-50 nanosphere is identical to the C50Cl10 fullerene [2]. The topographical relationship between the actinyl nanospheres and fullerene or fullerene-like material may provide additional insight into stable configurations for lower fullerenes. [1] Burns et al., Actinyl peroxide nanospheres. Angewandte Chemie, International Edition, 2005. 44(14): p. 2135. [2] Xie et al., Capturing the Labile Fullerene[50] as C50Cl10. Science, (2004) 305(5671): p. 699.
V14B-03
H2O Inner-Surface Interactions in Micro/Nanoporous Silicates: Thermodynamic Behavior and Low Energy Molecular Vibrations
Macroscopic thermodynamic and molecular-scale behavior related to silicate surface-fluid interactions in nature is complex and poorly understood. The study of confined H2O at inner surfaces in micro/ nanoporous silicates is helpful for understanding outer-surface interactions, because such phases offer simpler physicochemical systems for investigation. We are investigating the nature of H2O in various micro/nanoporous silicates. Low temperature calorimetric heat capacity (Cp) determinations have been made to determine thermodynamic behavior. Powder IR and polarized single-crystal Raman spectroscopy are used to investigate local features such as bonding and dynamics. In this report, Cp behavior and low energy external H2O modes related to van der Waals and/or hydrogen bonding at inner surfaces are emphasized. The first group of microporous silicates that includes cordierite, Mg2Al4Si5O18· xH2O, and beryl, Be2Al3Si6O18· xH2O, where x = 0 to 1, can hold single H2O molecules in small structural microcavities and exchange them with the environment with no change in volume. The Cp behavior of the confined H2O, which is characterized by weak van der Waals forces to the aluminosilicate framework, is roughly similar to that of steam at T > 100 K up to moderate T's. Cp is greater than that for ice at T < 100 K. Raman, IR and inelastic neutron scattering measurements show that the H2O molecule is quasi free in both phases. In cordierite, low energy T(H2O) modes occur at ~80 and ~48 cm-1 (Winkler and Hennion, 1994), and possibly at ~31 cm-1 . For beryl, for an external mode T(H2O) at ~ 10 cm-1 is present. The second type of microporous silicate, namely zeolites (those studied are bikitaite Li2[Al2Si4O12]·2H2O, natrolite - Na16[Al16Si24O80]·16H2O, scolecite - Ca8[Al16Si24O80]·24H2O, gmelinite - (Na2,Ca)[Al2Si4O12]·6H2O) are strongly hydrophilic and their intrachannel H2O molecules are hydrogen bonded. Zeolites show measurable changes in volume with loss or gain of H2O. The Cp behavior of H2O in natrolite is similar to that for ice at T < 100 K, but its Cp increases roughly linearly with increasing T and is greater than the Cp of ice at T > 100 K and also for H2O in cordierite at T > 250 K. At 298 K, T(H2O) modes between 45 and 180 cm-1 occur in natrolite and scolecite. Gmelinite shows similar Cp behavior at T < 100 K but anomalously high Cp behavior above 170 K. Conclusions from our investigations are: 1) At T < 200 K, Cp behavior of confined H2O is controlled by low-energy external T(H2O) modes. 2) T(H2O) mode energies reflect weak van-der-Waals to moderately strong hydrogen-bond forces at inner-surfaces. 3) The marked increase in Cp in some zeolites with increasing T indicates a change in hydrogen-bonding behavior from more "ice-like" to increasing "liquid- water-like". It is possible that this "transition" can account for the Cp behavior observed at T > 170 K in gmelinite. 4) Cp and entropy values for confined H2O in silicates cannot be considered similar, as is done in crystal-chemical-based schemes used for calculating thermodynamic properties of H2O-bearing silicates.
V14B-04
Rates of Ligand Exchange at Fe(III) Sites in a Nanometer-Size Aqueous Ion from 17O- NMR
Some of the pressing questions in geochemistry concern the dynamics and reactivities of individual functional groups at mineral surfaces, meaning their Broensted acid-base properties, the rates of ligand substitution and the rates of electron exchange. The field relies heavily on computer simulation because so few experimental models exist with well-constrained aqueous surface structures. We take a different approach. We use nanometer-size aqueous oxide clusters with well-known structures to isolate the key functional groups for spectroscopic study. Here we discuss a particularly relevant example and report rates of exchange between waters bound to isolated Fe(III) atoms at the surface of a 2.5-nm size nanoacid and bulk solution. The molecule [Mo72Fe30] is part of a class of nanometer-size ions called Keplerates. We apply the 17O-NMR line-broadening method and show that the rates are not dissimilar to Fe(III) monomer ions. A correlation suggests itself between Fe-OH2 bond lengths and rates over several orders of magnitude. We can estimate rates of ligand substitution at mineral surfaces if the correlation can be expanded and if bond lengths can be estimated for surface functional groups. Such a result was recently established for Al(III)-hydroxide minerals (Wang et al. 2007, Inorg. Chem. 46, 2962-2964).
V14B-05 INVITED
Kinetic competition during chemical and photochemical reactions at iron oxide nanoparticle surfaces
The adsorption of aqueous reductants to the surface of ferric iron bearing minerals and the subsequent creation and dissolution of ferrous iron are crucial steps in the geochemical cycling of this transition metal. While conceptual models of reductive dissolution are well established, the details of the interfacial electron transfer step are not well understood at the molecular scale because of the difficulties in probing chemical reactions at the length and time scales of elementary atomic and electronic processes. The goal of our research is to capture, using ultrafast x-ray spectroscopic techniques, the evolution of the chemistry and coordination geometry of surface metal atoms in iron oxide nanoparticles following electron transfer. Although ultrafast x-ray methods are undergoing a period of rapid development, there are numerous technical challenges for studying interfacial reactions of colloidal particles. In particular, the reaction must be initiated using a pulse of laser light in order to synchronize every site; and competing chemical reactions that lead to sample degradation must be minimized. However, addressing these challenges provides broadly relevant insights into kinetic controls on the reactivity of nanoparticles, which will be summarized in this talk. We combined studies of the chemical and photoreductive dissolution of iron oxide nanoparticles in order to indirectly probe the relative rates of competing processes. Of particular importance is the relative rate at which photogenerated structural ferrous iron can reduce surface bound species before detaching without reaction to produce aqueous Fe(II). We find that many elementary processes that occur during (photo)redox reactions of nanoparticles can exhibit a dependence on particle size.
V14B-06
The Structure of Water around Hematite Nanoparticles
The interactions between water and mineral surfaces are crucial for stabilizing surface structures [1] and mediating interfacial adsorption reactions of aqueous ions [2]. Nanoscale minerals, such as the iron oxides, are extremely common natural products of biomineralization and chemical weathering reactions. Although frequently a minority fraction, mineral nanoparticles can have a profound impact on their environment, having high surface areas and hence high reactivity and total energy relative to macroscopic minerals. Therefore an understanding of the interfacial region between water and nanoparticles is crucial in determining its transport and reactivity. Computer simulations has provided a useful tool in understanding the structure of water on mineral surfaces on the atomistic level [3]. Previous studies has focused on the structure of water on 2-dimentional flat surfaces [2, 3], however, we use molecular dynamics simulations to describe the structure of water around 3-dimensional hematite nanoparticles. We will show that the layering of water is very ordered and begin to evaluate the orientation of water molecules at different positions around the surface. We will describe how the change in surface charge can affect, not only the order and orientation of water, but also the transport of aqueous ions to the surface. In the final part we evaluate how the structure of water can influence aggregation of two or more hematite nanoparticles and suggest a mechanism to oriented aggregation growth. [1] H. Z. Zhang, B. Gilbert, F. Huang, and J. F. Banfield, Nature 424, 1025 (2003). [2] S. Kerisit, and S. C. Parker, J. Am. Chem. Soc. 126, 10152 (2004). [3] D. Spagnoli, D. J. Cooke, S. Kerisit, and S. C. Parker, J. Mater. Chem. 16, 1997 (2006).
V14B-07
Spectroscopic Investigations of Metal Speciation on Nanoscale Iron Oxyhydroxide Aggregates
Iron oxyhydroxide nanomaterials play an important role in the mobility of aqueous metal species through both sorption and desorption processes. The natural aggregation of such nanophases in aqueous environmental systems can lead to changes in their structure, available surface area, and reactivity and may additionally modify the mechanisms by which metal ions are adsorbed and retained. In this study, spectroscopic techniques were used to investigate the speciation of metals onto and within nanoscale iron oxyhydroxides exposed to various geochemical conditions which induce nanoparticle aggregation and growth. Suspensions of ~5 nm diameter iron oxyhydroxide nanoparticles were synthesized and aggregated through variations in pH (7-10), ionic strength (0.001-1 M), and temperature (25,50,75°C)/time (up to 96 hours) in the presence of 0.5 mM Cu(II) or Zn(II). A second set of experiments introduced the metals after aggregation had been induced (pH and ionic strength-based aggregation only). One aliquot of each suspension was then analyzed with atomic absorption spectroscopy to determine the initial uptake, while a second aliquot was subjected to a desorption step in which the pH was lowered to values below the macroscopic absorption edges of the relevant metals prior to AA analysis. X-ray absorption spectroscopy (XAS) analysis of selected samples was then conducted to assess the speciation of the metals associated with the solid phase following both the adsorption and desorption steps and determine the conditions that yielded the most strongly bound, incorporated metal species. Results indicate that the desorption step removes weakly-bound metal species but retains metals that appear to be more structurally incorporated into/onto the nanoparticle aggregates. Samples exposed to metals before aggregation were also found to incorporate metals more thoroughly than those that were aggregated before being exposed to metals. These findings have implications for the long-term removal of hazardous metals from the aqueous phase and the development of possible remediation strategies targeting contaminated environments.
V14B-08
X-ray Spectroscopic Tools for Structural Determination of Cr(VI)-Treated Iron Nanoparticles
Chromate (Cr(VI)) is a highly soluble groundwater contaminant of environmental concern. We are developing and investigating zero-valent iron nanoparticles (nano-Fe) for possible use in remediation of Cr(VI). In this paper we focus on the application of X-ray techniques to determine the electronic environment and solid phase chemistry of Cr and Fe in Cr(VI)-and Cr(III)-treated nano-Fe. Results from X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) confirmed that Cr(VI) was quantitatively reduced to Cr(III) by nano- Fe. Parallel experiments with 100-mesh Fe filings as a model system show similar Cr(VI) reduction properties but with a somewhat different Fe(III) corrosion product. Detailed analysis of XPS O 1s line spectra revealed that both Cr(III)- and Cr(VI)-treated nano-Fe yielded a predominantly Cr(III) hydroxide product. The Cr local atomic structure in Cr(III)- and Cr(VI)-treated nano-Fe was determined using extended x-ray absorption fine structure spectroscopy (EXAFS) and revealed octahedral Cr(III) with Cr-O interatomic distances between 1.97-1.98 angstroms for both Cr(III) and Cr(VI) treatments. A pronounced second Cr-Cr (and/or Cr-Fe) interatomic shell at 3.01 angstroms was also detected. Our results suggest that the reaction product of Cr(VI)-treated nano-Fe is an insoluble, poorly ordered Cr(III) precipitate and/or a mixed-phase Cr(III)/Fe(III) hydroxide.