Formation and properties of the nanocluster structure of iron oxides

Abstract

Nanostructures based on iron oxide clusters 1-300 nm in size were synthesized and studied. Thermodynamic models of nanocluster nucleation resulting in the formation of both primary nanoclusters and nanocluster aggregates with the sizes up to 70-80 nm were consid ered. Models of heat capacity of the nanoclusters were examined, and the twofold increase in the heat capacity of the iron oxide clusters 2-3 nm in size compared to that of the bulk iron oxide samples was found. The size of the primary nanoclusters and the intercluster interaction make it possible to vary the magnetic properties of the nanostructures in a wide range from paramagnetic to magnetically ordered α Fe 2 O 3 -γ Fe 2 O 3 nanostructures with the first order magnetic phase transitions, magnetic twinning, and a strong magnetic field (10 Oe) effect on the magnetization increase at low temperatures.Key words: nanoclusters and nanostructures, nanocluster design, heat capacity, magnetic phase transitions, magnetization, Mössbauer spectroscopy, atomic force microscopy.Nucleation and formation of nanoclusters from the gas phase, solutions, and melts are known and well stud ied processes characterized by critical nucleation sizes. 1,2 Processes of cluster nucleation in the solid phase are much more complicated and less controllable. In this case, topochemical reactions leading to the appearance of a new phase can be used. For homogeneous nucleation, the minimum cluster size is determined by the critical nuclea tion radius, and the maximum radius is determined by diffusion restraint of mobility of components of the new phase formed due to the topochemical reaction. 3 In the case of matrix or template restraint, the minimum and maximum cluster sizes are determined by the nanoreactor sizes. 4 The further growth of clusters, for instance, with the temperature increase, can result in cluster agglomera tion; in this case, the initial weak intercluster interaction characteristic of the minimum critical cluster sizes is re placed by the strong intercluster interaction. 5 Solid phase nucleation also occurs when nanocrystallites are formed from the amorphous phase (metals and alloys, amorphous polymers) upon detonation processes, explosions, etc. 6The formation of nanoclusters and a solid phase nanostructure is very complicated; however, these are the processes that determine the final properties of such nanomaterials as metal alloys, ceramics, glasses, and or ganic polymers. Therefore, it is significant to develop models of nucleation interpreting the change or control of the properties of nanostructures. The model of homo geneous nucleation and agglomeration of iron oxide nanoclusters upon the thermal decomposition of iron oxalate was proposed. 3,5 It was shown that both weakly interacting, virtually separated with the media iron oxide nanoclusters and iron oxide nanoclusters of the initial agglomeration stage are stabilized under certain condi tions. This reaction makes it possible to obtain a wide range of iron oxide nanoclusters with the size from one to severa...

Photodissociation of Iron Oxide Cluster Cations

“…Nanoparticle oxides have applications in areas such as magnetism, catalysis, and medicine. 4,[9][10][11][12][13][14][15][16][17][18][19][20][21][22] Iron oxide is particularly interesting because it consists of four major bulk phases, all of which exhibit unique properties. [1][2][3] FeO, wüstite, has a rock salttype structure with defects, generally referred to as Fe 1-x O.…”

“…[1][2][3] These individual phases have been the subjects of many experiments involving films 12 and nanoparticles. 13,14,17,19,21 Gas phase experiments have also been performed to investigate properties such as bonding, reactivity, and structure of small cluster oxides. 23-56 Theory has provided a further understanding of the structures and stabilities of these systems.…”

Colloidal catalysts based on iron(III) oxides. 1. Decomposition of hydrogen peroxide

“…The fluctuation rate dramatically decreases with increasing particle size, and the mag netic moment becomes fixed relative to a particle. By comparing the results of this study with the data reported in [26,27], one may assume that the varia tions in the catalyst activity are associated with changes in both sizes and, probably, magnetic proper ties of the particles.…”

“…The Mössbauer spectra and magnetic properties were studied in [26,27] for Fe 2 O 3 particles obtained using the same method as in [25], i.e., the thermal decomposition of iron oxalate at different tempera tures, which was accompanied by changes in both the sizes and magnetic properties of the particles. Based on the performed measurements, the authors of [27] concluded that the first order magnetic phase transi tion occurs upon increasing particle size; i.e., super paramagnetic particles are transformed into magneti cally ordered ones.…”

“…Based on the performed measurements, the authors of [27] concluded that the first order magnetic phase transi tion occurs upon increasing particle size; i.e., super paramagnetic particles are transformed into magneti cally ordered ones. The Mössbauer spectrum corre sponding to the superparamagnetic particles, in which the magnetic moment vector undergoes rapid thermal fluctuations, has the form of a doublet similar to that shown in Fig.…”