Sean W. Depner

The effects of monovalent and divalent cations on the stability of silver nanoparticles formed from direct reduction of silver ions by Suwannee River humic acid/natural organic matter

The formation and characterization of AgNPs (silver nanoparticles) formed from the reduction of Ag⁺ by SRNOM (Suwannee River natural organic matter) is reported. The images of SRNOM-formed AgNPs and the selected area electron diffraction (SAED) were captured by high resolution transmission electron microscopy (HRTEM). The colloidal and chemical stability of SRNOM- and SRHA (Suwannee River humic acid)-formed AgNPs in different ionic strength solutions of NaCl, KCl, CaCl₂ and MgCl₂ was investigated in an effort to evaluate the key fate and transport processes of these nanoparticles in natural aqueous environments. The aggregation state, stability and sedimentation rate of the AgNPs were monitored by Dynamic Light Scattering (DLS), zeta potential, and UV-vis measurements. The results indicate that both types of AgNPs are very unstable in high ionic strength solutions. Interestingly, the nanoparticles appeared more unstable in divalent cation solutions than in monovalent cation solutions at similar concentrations. Furthermore, the presence of SRNOM and SRHA contributed to the nanoparticle instability at high ionic strength in divalent metallic cation solutions, most likely due to intermolecular bridging with the organic matter. The results clearly suggest that changes in solution chemistry greatly affect nanoparticle long term stability and transport in natural aqueous environments.

Inside and Outside: X-ray Absorption Spectroscopy Mapping of Chemical Domains in Graphene Oxide

The oxidative chemistry of graphite has been investigated for over 150 years and has attracted renewed interest given the importance of exfoliated graphene oxide as a precursor to chemically derived graphene. However, the bond connectivities, steric orientations, and spatial distribution of functional groups remain to be unequivocally determined for this highly inhomogeneous nonstoichiometric material. Here, we demonstrate the application of principal component analysis to scanning transmission X-ray microscopy data for the construction of detailed real space chemical maps of graphene oxide. These chemical maps indicate very distinct functionalization motifs at the edges and interiors and, in conjunction with angle-resolved near-edge X-ray absorption fine structure spectroscopy, enable determination of the spatial location and orientations of functional groups. Chemical imaging of graphene oxide provides experimental validation of the modified Lerf-Klinowski structural model. Specifically, we note increased contributions from carboxylic acid moieties at edge sites with epoxide and hydroxyl species dominant within the interior domains.

Transport and deposition of Suwannee River Humic Acid/Natural Organic Matter formed silver nanoparticles on silica matrices: The influence of solution pH and ionic strength

The transport and deposition of silver nanoparticles (AgNPs) formed from Ag(+) reduction by Suwannee River Humic Acid (SRHA) and Suwannee River Natural Organic Matter (SRNOM) utilizing a silica matrix is reported. The morphology and stability of the AgNPs was analyzed by transmission electron microscopy (TEM), dynamic light scattering (DLS) and zeta potential measurements. The percentage conversion of the initial [Ag(+)] to [AgNPs] was determined from a combination of atomic absorption (AAS) and UV-Vis spectroscopy, and centrifugation techniques. The results indicate higher AgNP transport and consequently low deposition in the porous media at basic pH conditions and low ionic strength. However, at low acidic pH and high ionic strength, especially with the divalent metallic cations, the mobility of the AgNPs in the porous media was very low, most likely due to NP aggregation. Overall, the results suggest the potential for AgNP contamination of subsurface soils and groundwater aquifers is mostly dependent on their aggregation state, controlled by the soil water and sediment ionic strength and pH.

Ferroelastic Domain Organization and Precursor Control of Size in Solution-Grown Hafnium Dioxide Nanorods

We demonstrate that the degree of branching of the alkyl (R) chain in a Hf(OR)4 precursor allows for control over the length of HfO2 nanocrystals grown by homocondensation of the metal alkoxide with a metal halide. An extended nonhydrolytic sol-gel synthesis has been developed that enables the growth of high aspect ratio monoclinic HfO2 nanorods that grow along the [100] direction. The solution-grown elongated HfO2 nanorods show remarkable organization of twin domains separated by (100) coherent twin boundaries along the length of the nanowires in a morphology reminiscent of shape memory alloys. The sequence of finely structured twin domains each spanning only a few lattice planes originates from the Martensitic transformation of the nanorods from a tetragonal to a monoclinic structure upon cooling. Such ferroelastic domain organization is uncharacteristic of metal oxides and has not thus far been observed in bulk HfO2. The morphologies observed here suggest that, upon scaling to nanometer-sized dimensions, HfO2 might exhibit mechanical properties entirely distinctive from the bulk.

Partitioning behavior and stabilization of hydrophobically coated HfO2, ZrO2 and HfxZr1−xO2 nanoparticles with natural organic matter reveal differences dependent on crystal structure

The interactions of engineered nanomaterials with natural organic matter (NOM) exert a profound influence on the mobilities of the former in the environment. However, the influence of specific nanomaterial structural characteristics on the partitioning and colloidal stabilization of engineered nanomaterials in various ecological compartments remains underexplored. Herein, we present a systematic study of the interactions of humic acid (HA, as a model for NOM) with monodisperse, well-characterized, ligand-passivated HfO(2), ZrO(2), and solid-solution Hf(x)Zr(1-x)O(2) nanoparticles (NPs). We note that mixing with HA induces the almost complete phase transfer of hydrophobically coated monoclinic metal oxide (MO) NPs from hexane to water. Furthermore, HA is seen to impart appreciable colloidal stabilization to the NPs in the aqueous phase. In contrast, phase transfer and aqueous-phase colloidal stabilization has not been observed for tetragonal MO-NPs. A mechanistic model for the phase transfer and aqueous dispersal of MO-NPs is proposed on the basis of evidence from transmission electron microscopy, ζ-potential measurements, dynamic light scattering, Raman and infrared spectroscopies, elemental analysis, and systematic experiments on a closely related set of MO-NPs with varying composition and crystal structure. The data indicate the synergistic role of over-coating (micellar), ligand substitution (coordinative), and electrostatic processes wherein HA acts both as an amphiphilic molecule and a charged chelating ligand. The strong observed preference for the phase transfer of monoclinic instead of tetragonal NPs indicates the importance of the preferential binding of HA to specific crystallographic facets and suggests the possibility of being able to design NPs to minimize their mobilities in the aquatic environment.

Precursor control of crystal structure and stoichiometry in twin metal oxide nanocrystals

Nanocrystals of solid-solution twin metal oxides HfxZr1−xO2 have been prepared by the non-hydrolytic sol–gel condensation of a metal alkoxide with a metal halide. We demonstrate that the alkoxide group can be used to tune the stoichiometry and crystal structure of the obtained nanocrystals. Increased branching of the alkyl group in the metal alkoxide results in the formation of monoclinic rather than tetragonal nanocrystals, whereas longer alkyl chains favor the increased incorporation of Zr in the HfxZr1−xO2nanocrystals regardless of whether a hafnium or zirconium alkoxide is used as the precursor. Remarkably, the monoclinic phase has been stabilized for a Zr-rich species, pointing to the significant role of the alkoxide precursors in influencing the growth and phase transformation of nanocrystalline materials in hot colloidal synthesis. Considerable control can thus be established over the phase diagram of nanocrystalline Hf/Zr oxides by appropriate choice of precursors.

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.

Microwave-induced nucleation of conducting graphitic domains on silicon carbide surfaces

Microwave irradiation of the C-rich (0001¯) surface of 6H-SiC is seen to rapidly induce the nucleation of conductive nanoscopic graphitic grains. Discrete graphitic islands are observed and Raman spectroscopy suggests turbostratic stacking with minimal electronic coupling between adjacent graphene layers. Ensemble Raman and near-edge x-ray absorption fine structure (NEXAFS) spectroscopies are used in conjunction with spatially resolved atomic force microscopy, scanning Kelvin probe microscopy (SKPM), and colocalized Raman imaging to characterize the topography and electronic structure of the obtained graphitic domains and to develop a mechanistic description of the nucleation process. SKPM provides a direct spatially resolved means to differentiate conductive graphitic grains from the wide-bandgap SiC semiconductor. NEXAFS spectroscopy allows for evaluation of the planar alignment of the graphitic nuclei. The microwave processing method demonstrated here provides a facile route to patterning conductive do...

Direct Observation of Hafnia Structural Phase Transformations

1. Department of Chemistry, University of Kentucky, Lexington, Kentucky, USA 2. Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY, USA 3. Department of Chemistry, Texas A&M University, College Station, TX, USA 4. Department of Materials Science and Engineering, Texas A&M University, College Station, TX, USA 5. Materials Science and Technology Division, Oak Ridge National Lab, Oak Ridge, TN, USA

Ferroelastic Domain Organization in Solution-Grown HfO2 Nanorods

Hafnia (HfO2) is a technologically important hard material with a relatively high dielectric constant and wide band gap [1]. However, the basic properties of HfO2 have been little studied in the literature due to its close resemblance to zirconia in chemical properties. In the bulk, the monoclinic (space group: P21/c) phase represents the thermodynamic minimum; phase transformation to a distorted fluorite tetragonal (space group: P42/nmc) phase is induced upon heating to 1720 o C. [2] The symmetry-lowering tetragonal to monoclinic phase transition in HfO2 closely parallels the better studied phase transition in ZrO2 and is believed to be Martensitic and athermal in nature. The transformation is thought to proceed through a diffusionless process wherein bond distances and angles are altered without disrupting the atomic connectivity within the lattice with preservation of a mirror plane symmetry element. Similar to ZrO2, this tetragonal to monoclinic phase transformation in HfO2 is strongly anisotropic being most pronounced along the aand c-axes and negligible along the crystallographic b-axis. This anisotropic expansion induces a substantial shear strain within the materials that can be variously accommodated. One approach by which this transformation strain can be relieved in this symmetry-lowering transition is by formation of periodic sequences of twin variants. The system seeks to establish a balance between compensating for macroscopic strain and the inevitable energetic penalty for establishing a new interface (the twin boundary). In other words, by forming a sequence of periodic domains separated by a coherent twin boundary, the structural integrity of the material can be retained and the macroscopic strain can be relaxed without extensive deformation or crack propagation. Evidence for formation of twinned domains has been reported in HfO2 polycrystalline samples [3] thin films [4] and nanorods [5]. However, periodic organization of twinned domains along 1D nanorods has not thus far been evidenced in either thin films or bulk ceramic grains. In this study, we demonstrate the organization of ferroelastic domains that create a nanoscopic stripe pattern within colloidal HfO2 nanorods.