K. J. Klabunde

Digestive Ripening, Nanophase Segregation and Superlattice Formation in Gold Nanocrystal Colloids

A novel digestive ripening process is shown to narrow the particle size distribution from a highly polydisperse dodecanethiol ligated gold colloid. Unlike the Ostwald ripening process, the digestion occurs through transferring materials from large particles to small particles. Temperature-induced size segregation can further select the particle sizes. By using these two methods, highly ordered superlattices using nanocrystals as building blocks can be synthesized directly from a polydisperse colloid.

Preparation of manganese ferrite fine particles from aqueous solution

Fine manganese ferrite particles have been prepared by a coprecipitation method and subsequent digestion process (below 100°C). Manganous salts mixed with either ferric or ferrous salts were coprecipitated with sodium hydroxide. Particles produced from ferric salts were MnFe2O4 with relatively smaller sizes (5 to 25 nm) while ferrous salts created MnxFe3−xO4 (0.2 < × < 0.7) with bigger sizes up to 180 nm. In either case, the particle size appeared to be a unique function of the ratio of metal ion concentration to hydroxide ion concentration when the digestion conditions were fixed. For the system with ferric salts, the undigested samples were polycrystals with crystallite sizes of about 2 nm. Digestion, which could be described as an Ostwald ripening process, did not change the crystalline structure but increased both the crystallite size and the particle size. A basic solution was essential for an effective digestion process in this system. The system with ferrous salts, on the contrary, needed an acidic solution to create a single ferrite phase. Digestion changed both the crystalline structure and the particle size of the precipitated precursors. This process involved a dissolution and renucleation/growth mechanism. Cation and anion effects on the particle size and the evolution during digestion were also studied.

Exchange anisotropy in oxide passivated Co fine particles

Oxide passivated Co particles have been prepared by vapor deposition in the size range of 50–350 A. A strong exchange anisotropy was observed due to the core‐shell structure of the Co particles, where the core consists of metallic Co and the shell of Co oxides. The exchange anisotropy of the particles was studied using shifted hysteresis loops after the sample was field cooled (FC). The shift in the FC loop exhibited a maximum at a particle size of 80 A (shift=10.7 kOe) and is related to the amount of surface oxidation. The shift in the loops disappeared around 150 K in all the samples irrespective of their particle size, and this is attributed to the superparamagnetic behavior of the antiferromagnetic oxide shell.

Magnetic properties of nanophase cobalt particles synthesized in inversed micelles

Cobalt particles were prepared with the microemulsion method in the binary system of DDAB (didodecyldimethylammonium bromide)/toluene by reduction of CoCl2 with NaBH4. The average particle size of the as‐prepared samples could be varied from 1.8 to 4.4 nm by controlling the concentration of CoCl2 in the solution of DDAB in toluene. TEM studies showed that the particles were quite uniform and well isolated. The particle sizes determined from magnetic data were consistent with those measured by TEM. The coercivity of the particles at 10 K increased from 640 to 1250 Oe as particle size increased from 1.8 to 4.4 nm. The blocking temperature of the particles increased from 19 to 50 K for the same size range. The saturation magnetization σs at 2 K increased with decreasing particle size. The value of σs of the particles with average size of 1.8 nm was about 200 emu/g, which is 20% higher than the bulk value. This implies that the magnetic moment per atom is enhanced in the nanoparticle system.

Effect of oxide layer on the hysteresis behavior of fine Fe particles

The effects of surface oxidation on the structural and magnetic properties of fine Fe particles prepared by the evaporation technique have been studied using transmission electron microscopy, x‐ray photoelectron spectroscopy, superconducting quantum interference device magnetometry, and Mossbauer spectroscopy. By varying the argon pressure, particles were obtained with sizes in the range of 60–350 A. The hysteresis behavior was found to be strongly dependent on the variation in the amount of surface oxidation. The differences in the magnetic behavior due to variation in size and oxidation have been explained by considering a shell/core model for the particle morphology with the shell consisting of Fe oxides surrounding the α‐Fe core.

Magnetic properties of microemulsion synthesized cobalt fine particles

Two microemulsion systems, the ternary system H2O/AOT/isooctane and the binary system DDAB/toluene, were used to prepare metallic cobalt particles by borohydride reduction of a cobalt salt. The particles prepared in the AOT system were extremely small and superparamagnetic with magnetic moment per particle of 11.5μB, a blocking temperature of 2 K, and σs=146 emu/g at 1.7 K. The magnetic moment and σs combined imply a particle size of about 5.4 A. However, the particles made in the DDAB system were relatively large, ∼10 nm. They were ferromagnetic at room temperature but with no coercivity.

COMPLEX FORMATION IN SOLUTIONS FOR CHEMICAL SYNTHESIS OF NANOSCALED PARTICLES PREPARED BY BOROHYDRIDE REDUCTION PROCESS

Abstract Complex forming agents are often used in chemical synthesis of nanoscale particles by means of a borohydride reduction process. Their influence on the rate of nucleation and a growth of particles without oxide/hydroxides shell are estimated earlier. In this paper experimental data are presented about the synthesis of Fe-Nd-B-H particles from chloride water solutions by reduction with sodium borohydride, as well as synthesis of Fe-Nd-Si-B-H particles, containing silicon, by using hexafluorosilicate of iron(II). According to the estimated influence of the aqua-complex structures on the synthesis of amorphous or nanocrystalline particles, synthesis of initial complex salts — precursors trans- and cis-[bis(ethylenediamine) dichloro cobalt(III)] chloride is carried out. The influence of the metal complex used in water solutions by a reduction with sodium borohydride on the composition, the structure and specific surface area of prepared nanoscale particles is investigated.

Synthesis and characterization of stable colloidal De/sub 3/O/sub 4/ particles in water-in-oil microemulsions

The authors have prepared nanometer-size magnetite particles in Aerosol OT (AOT)/water/iso-octane water-in-oil (w/o) microemulsions. The sizes of the particles in the colloids were inferred from dynamic light scattering (DLS) and were on the order of one nanometer. Those in dried samples were measured using transmission electron microscopy (TEM) and were 5-20 nm. This implies that aggregation occurs during drying. X-ray diffraction showed that the particles phase was consistent with magnetite. Magnetic properties were studied using a superconducting quantum interference device (SQUID) magnetometer. The colloids showed a superparamagnetic behavior and the dried samples showed a ferromagnetic behavior, supporting the size measurements using DLS and TEM, respectively. >

Magnetic and structural properties of ultrafine Co-B particles

Abstract Fine particles of cobalt-boron (Co-B) were produced by chemical reduction of aqueous solutions of cobalt chloride with sodium borohydride. The chemical composition, particle size, magnetic properties, and crystallization behavior have been studied as a function of NaBH4 concentration. The particle size was in the range of 200–1000 A. Chemical analysis showed the B content to be in the range 32–40 at%. The initially reduced powder was amorphous having very low magnetization and negligible coercivity. The particles were gradually crystallized by varying the annealing temperature. The saturation magnetization Ms for the annealed samples was in the range of 70–135 emu/g and the maximum coercivity Hc obtained was 2000 Oe. X-ray diffraction results indicated that the reason for the drastic change in magnetic properties is the formation of crystalline phases upon heating which included mostly Co3B together with Co2B and Co.

Magnetic hysteresis and Mössbauer studies in ultrafine iron particles

The magnetic hysteresis behavior of ultrafine Fe particles has been examined by SQUID magnetometry and Mossbauer spectroscopy. Ultrafine Fe particles have been prepared by vapor deposition under argon atmosphere in pressures from 1 to 6 Torr and passivated by exposure to oxygen at low pressure. Particles with size below 10 nm showed a small coercivity (∼100 Oe) at room temperature, which increased drastically upon cooling to cryogenic temperatures (1000–1500 Oe). The larger particles had the highest coercivity at room temperature (∼1000 Oe) which increased by 100% at 10 K. Mossbauer studies revealed that the Fe particles are surrounded by an Fe oxide layer composed of ultrafine Fe3O4 or γ‐Fe2O3 grains. At room temperature the oxide component gives a very broad absorption superimposed on an α‐Fe spectrum. The enhanced coercivity at room temperature may be attributed to anisotropy induced by exchange interaction between the ferromagnetic core and the ferri‐ or antiferromagnetic oxide layer.