Scott Calvin

Chemically prepared magnetic nanoparticles

Abstract Nanotechnology has spurred efforts to design and produce nanoscale components for incorporation into devices. Magnetic nanoparticles are an important class of functional materials, possessing unique magnetic properties due to their reduced size (below 100 nm) with potential for use in devices with reduced dimensions. Recent advances in processing by chemical synthesis and the characterisation of magnetic nanoparticles are the focus of this review. Emphasis has been placed on the various solution chemistry techniques used to synthesise particles, including: precipitation, borohydride reduction, hydrothermal, reverse micelles, polyol, sol–gel, thermolysis, photolysis, sonolysis, multisynthesis processing and electrochemical techniques. The challenges and methods for examining the structural, morphological, and magnetic properties of these materials are described.

MAGNETIC AND STRUCTURAL PROPERTIES OF NICKEL ZINC FERRITE NANOPARTICLES SYNTHESIZED AT ROOM TEMPERATURE

Nickel zinc ferrite nanoparticles (Ni0.20Zn0.44Fe2.36O4) have been produced at room temperature, without calcination, using a reverse micelle process. Particle size is approximately 7 nm as determined by x-ray powder diffraction and transmission electron microscopy. Saturation magnetization values are lower than anticipated, but are explained by elemental analysis, particle size, and cation occupancy within the spinel lattice. Extended x-ray absorption fine structure analysis suggests that a significant amount of Zn2+, which normally occupies tetrahedral sites, actually resides in octahedral coordination in a zinc-enriched outer layer of the particles. This “excess” of diamagnetic Zn can thus contribute to the overall decrease in magnetism. Further, this model can also be used to suggest a formation mechanism in which Zn2+ is incorporated at a later stage in the particle growth process.

Determination of crystallite size in a magnetic nanocomposite using extended x-ray absorption fine structure

An extended x-ray absorption fine structure was collected for a soft magnetic material comprising very fine nanoscale crystallites of nickel within coarser iron matrix grains. Using a simple spherical model and the spectra of bulk standards, the nickel crystallite size was estimated. Comparison with transmission electron microscopy images confirms that this technique yields a size weighted toward smaller crystallites, whereas Scherrer analysis yields sizes weighted toward larger crystallites. The iron crystallite size was also estimated by this technique in order to ascertain the effect of a nonspherical morphology. This technique shows promise for in situ analyses of materials containing nanoscale crystallites and as a complement to Scherrer analyses.

Comparison of extended x-ray absorption fine structure and Scherrer analysis of x-ray diffraction as methods for determining mean sizes of polydisperse nanoparticles

Curve fitting of extended x-ray absorption fine structure (EXAFS) spectra, transmission electron microscopy (TEM) imaging, and Scherrer analysis of x-ray diffraction (XRD) are compared as methods for determining the mean crystallite size in polydisperse samples of platinum nanoparticles. By applying the techniques to mixtures of pure samples, it is found that EXAFS correctly determines the relative mean sizes of these polydisperse samples, while XRD tends to be weighted more toward the largest crystallites in the sample. Results for TEM are not clear cut, due to polycrystallinity and aggregation, but are consistent with the other results.

Probing the electrode/electrolyte interface in the lithium excess layered oxide Li1.2Ni0.2Mn0.6O2

A detailed surface investigation of the lithium-excess nickel manganese layered oxide Li1.2Ni0.2Mn0.6O2 structure was carried out using X-ray photoelectron spectroscopy (XPS), total electron yield and transmission X-ray absorption spectroscopy (XAS), and electron energy loss spectroscopy (EELS) during the first two electrochemical cycles. All spectroscopy techniques consistently showed the presence of Mn(4+) in the pristine material and a surprising reduction of Mn at the voltage plateau during the first charge. The Mn reduction is accompanied by the oxygen loss revealed using EELS. Upon the first discharge, the Mn at the surface never fully recovers back to Mn(4+). The electrode/electrolyte interface of this compound consists of the reduced Mn at the crystalline defect-spinel inner layer and an oxidized Mn species simultaneously with the presence of a superoxide species in the amorphous outer layer. This proposed model signifies that oxygen vacancy formation and lithium removal result in electrolyte decomposition and superoxide formation, leading to Mn activation/dissolution and surface layer-spinel phase transformation. The results also indicate that the role of oxygen is complex and significant in contributing to the extra capacity of this class of high energy density cathode materials.

USE OF MULTIPLE-EDGE REFINEMENT OF EXTENDED X-RAY ABSORPTION FINE STRUCTURE TO DETERMINE SITE OCCUPANCY IN MIXED FERRITE NANOPARTICLES

The site occupancy of manganese zinc ferrite (MZFO) nanoparticles is determined by a multiple-edge refinement of the extended x-ray absorption fine structure of the manganese, zinc, and iron absorption edges. The MZFO nanoparticles are generated by a reverse micellar synthetic route and compared to a ceramic standard. The simultaneous fitting of multiple absorption edges to a constrained model is found to yield site occupancies accurate to within eight percentage points.

Preparation and characterization of MnZn-ferrite nanoparticles using reverse micelles

Research on manganese zinc ferrites (MZFO) has undergone a renewal in recent years as advances in synthetic techniques promise smaller grain sizes and corresponding changes in material properties. Current techniques for nanoscale synthesis of ferrites, however, produce a broad distribution of particle sizes, thus limiting the density of compacted materials, and consequently altering coercivity [C. Rath et al., J. Appl. Phys. 91, 2211 (2002)]. To minimize porosity, bulk materials need to be pressed from uniform particles. Wet chemical synthesis performed in reverse micelles, in which pools of water are encased by surfactant molecules in an excess volume of oil, provides the greatest control over size and morphology. During synthesis, surfactant molecules keep particles separated and restrict particle growth. This affords greater control over the size and shape of the particles grown in the micelles and commonly results in highly uniform morphologies [J. P. Chen et al., J. Appl. Phys. 76, 6316 (1994); C. Li...

Extended X-ray absorption fine structure analysis of cation distribution in MnFe/sub 2/O/sub 4/ single crystal films and artificial ferrite structures

The cation distribution in MnFe/sub 2/O/sub 4/ single crystal films and artificial ferrite structures is determined by a multiple-edge analysis of the extended X-ray absorption fine structure (EXAFS) of the manganese and iron absorption edges. Compared with conventional manganese ferrite films processed by pulsed laser ablation (PLD), artificial ferrites that are constructed by a unique layer-by-layer PLD deposition were found to have inversion parameters as high as 58%. Magnetic properties of these ferrite films are compared as a function of cation inversion.

Cation occupancy determination in manganese zinc ferrites using Fourier transform infrared spectroscopy

The magnetic and electric properties of ferrites are influenced by the cation distribution within the crystalline spinel lattice. Methods such as extended x-ray-absorption fine structure (EXAFS) have been used to determine cation occupancies within the crystalline structure of materials such as manganese zinc ferrite (MZFO); however, it is not practical to be used for daily analysis. Fourier transform infrared (FTIR) spectroscopy is another technique which has the potential to determine cation occupancy while offering speed and convenience. In the literature it has been demonstrated that in ferrite systems FTIR data can be correlated to cation percentages when comparing tetrahedral (Td) and octahedral (Oh) sites. FTIR spectra were collected on a series of MZFO nanoparticles in the range from 200 to 600cm−1 and two absorbance peaks were observed. The first absorption region shifted with changing sample composition as calculated from transmission EXAFS experiments and elemental analysis. The data was normal...

Automated system for x-ray absorption spectroscopy of nanoparticle nucleation and growth

X-ray absorption spectroscopy (XAS) is a useful tool for studying nanoparticle synthesis and growth. Described here is a system for automating synthesis and data collection, allowing time-resolved XAS measurements at a synchrotron to be accurately combined with measurements made under identical conditions elsewhere, and promising the ability to use XAS with experiments in combinatorial chemistry. The primary components of this system are a commercial parallel processor and a custom flow cell. The system has been used to collect data on the synthesis of iron oxides from iron(II) acetylacetonate.