Enhanced oxidation of nanoparticles through strain-mediated ionic transport

Pratt, Andrew; Lari, Leonardo; Hovorka, Ondřej; Shah, Amish B.; Woffinden, Charles; Tear, S.P.; Binns, C.; Kröger, Roland

doi:10.1038/nmat3785

Cited by 113 publications

(109 citation statements)

References 28 publications

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“…These observations are in agreement with what has been found also at the interface of Au-Fe 3 O 4 composites [48,49], Co/Co 3 O 4 nanooctahedra [25], and Fe oxidized cubic NP [50], suggesting that lattice mismatch correlates with changes in magnetic anisotropy. There is also evidence that lattice strain induced by acoustic waves applied to some magnetic structures [51,52] can influence the value of the anisotropy constant and its easy axis.…”

Section: Magnetic Characterizationsupporting

confidence: 92%

Probing core and shell contributions to exchange bias in Co/Co<inf>3</inf>O<inf>4</inf> nanoparticles of controlled size

De¹,

Iglesias

Majumdar

et al. 2017

2017 IEEE International Magnetics Conference (INTERMAG)

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Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenology. In this work, we have prepared core/shell structured nanoparticles with a total average diameter around ∼27 nm and a wide range of shell thicknesses through the controlled oxidation of Co nanoparticles well dispersed in an amorphous silica host. Structural characterizations give compelling evidence of the formation of Co 3 O 4 crystallite phase at the shells surrounding the Co core. Field cooled hysteresis loops display nonmonotonous dependence of the exchange bias H E and coercive H C fields, that become maximum for a sample with an intermediate shell thickness, at which lattice strain is also maximum for both phases. The EB effects persist up to temperatures above the ordering temperature of the oxide shell. Results of our atomistic Monte Carlo simulations of particles with the same size and composition as in experiments are in agreement with the experimental observations and have allowed us to identify a change in the contribution of the interfacial surface spins to the magnetization reversal, giving rise to the observed maximum in H E and H C .

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Section: Magnetic Characterizationsupporting

confidence: 92%

Probing core and shell contributions to exchange bias in Co/Co<inf>3</inf>O<inf>4</inf> nanoparticles of controlled size

De¹,

Iglesias

Majumdar

et al. 2017

2017 IEEE International Magnetics Conference (INTERMAG)

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“…3(g)-3(i). The data suggests that Ni nanoparticles possess the thinnest oxide shell with a thickness of about 1 nm, while Co and Fe nanoparticles develop an oxide shell of 2 to 3 nm in agreement with previous studies [60,66,67]. The actual particle sizes in UHV during the X-PEEM and RHEED investigations might therefore be smaller by about 1-2 nm when compared to the ex situ AFM results presented in Figs.…”

Section: B Structural Characterization By Means Of Rheedsupporting

confidence: 91%

“…Stacking fault energies in bcc Fe are comparably high; therefore, such defects are likely metastable in nanoparticles as for dislocations. In fact, stacking faults or twinning have been observed thus far only in bulklike systems under high mechanical stresses but not in bcc Fe nanoparticles [16,60,67,94]. Similarly, there is currently no evidence for dislocations or other defects in bcc Fe nanoparticles, including the present HR-STEM results.…”

Section: -9contrasting

confidence: 56%

Direct observation of enhanced magnetism in individual size- and shape-selected 3d transition metal nanoparticles

et al. 2017

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Magnetic nanoparticles are critical building blocks for future technologies ranging from nanomedicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly understood dispersion of magnetic properties is reported, even within monodisperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the magnetism of a large number of size-and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature, only superparamagnetic behavior is observed in our experiments for all Ni nanoparticles within the investigated sizes, which range from 8 to 20 nm. However, Fe and Co nanoparticles can exist in two distinct magnetic states at any size in this range: (i) a superparamagnetic state, as expected from the bulk and surface anisotropies known for the respective materials and as observed for Ni, and (ii) a state with unexpected stable magnetization at room temperature. This striking state is assigned to significant modifications of the magnetic properties arising from metastable lattice defects in the core of the nanoparticles, as concluded by calculations and atomic structural characterization. Also related with the structural defects, we find that the magnetic state of Fe and Co nanoparticles can be tuned by thermal treatment enabling one to tailor their magnetic properties for applications. This paper demonstrates the importance of complementary single particle investigations for a better understanding of nanoparticle magnetism and for full exploration of their potential for applications.

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“…Even though the study of the surface of such materials is in its infancy there are hints that novel effects occur. For example, Pratt et al [614] recently showed that strain at the surface of iron oxide nanocubes leads to increased diffusion of cations, potentially explaining enhanced oxidation rates that have been observed for Fe nanoparticles. It would be fascinating to see if cuboid Fe 3 O 4 nanoparticles exposing (001) facets exhibit the SCV reconstruction, and whether similar Fe rich terminations appear in reducing conditions.…”

Section: Realistic Materialsmentioning

confidence: 99%

Iron oxide surfaces

Parkinson

2016

Surface Science Reports

487

463

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The current status of knowledge regarding the surfaces of the iron oxides, magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), hematite (α-Fe 2 O 3 ), and wüstite (Fe 1-x O) is reviewed. The paper starts with a summary of applications where iron oxide surfaces play a major role, including corrosion, catalysis, spintronics, magnetic nanoparticles (MNPs), biomedicine, photoelectrochemical water splitting and groundwater remediation. The bulk structure and properties are then briefly presented; each compound is based on a close-packed anion lattice, with a different distribution and oxidation state of the Fe cations in interstitial sites. The bulk defect chemistry is dominated by cation vacancies and interstitials (not oxygen vacancies) and this provides the context to understand iron oxide surfaces, which represent the front line in reduction and oxidation processes. The atomic-scale structure of the low-index surfaces of iron oxides is the major focus of this review. Fe 3 O 4 is the most studied iron oxide in surface science, primarily because its stability range corresponds nicely to the ultra-high vacuum environment, and because it is an electrical conductor, which makes it straightforward to study with the most commonly used surface science methods such as photoemission spectroscopies (XPS, UPS) and scanning tunneling microscopy (STM). The impact of the surfaces on the measurement of bulk properties such as magnetism, the Verwey transition and the (predicted) half-metallicity is discussed.The best understood iron oxide surface at present is probably Fe 3 O 4 (100); the structure is known with a high degree of precision and the major defects and properties are well characterised. A major factor in this is that a termination at the Fe oct -O plane can be reproducibly prepared by a variety of methods, as long as the surface is annealed in 10 Such straightforward preparation of a monophase termination is generally not the case for other iron oxide surfaces. All available evidence suggests the oft-studied (√2×√2)R45° reconstruction results from a rearrangement of the cation lattice in the outermost unit cell in which two octahedral cations are replaced by one tetrahedral interstitial, a motif conceptually similar to well-known Koch-Cohen defects in Fe (0001) is the most studied hematite surface, but 2 difficulties preparing stoichiometric surfaces under UHV conditions have hampered a definitive determination of the structure. There is evidence for at least three terminations: a bulk-like termination at the oxygen plane, a termination with half of the cation layer, and a termination with ferryl groups. When the surface is reduced the so-called "bi-phase" structure is formed, which eventually transforms to a Fe 3 O 4 (111)-like termination. The structure of the bi-phase surface is controversial; a largely accepted model of coexisting Fe 1-x O and α-Fe 2 O 3 (0001) islands was recently challenged and a new structure based on a thin film of Fe 3 O 4 (111) on α-Fe 2 O 3 (0001) was proposed. The merits of the compet...

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Enhanced oxidation of nanoparticles through strain-mediated ionic transport

Cited by 113 publications

References 28 publications

Probing core and shell contributions to exchange bias in Co/Co<inf>3</inf>O<inf>4</inf> nanoparticles of controlled size

Probing core and shell contributions to exchange bias in Co/Co<inf>3</inf>O<inf>4</inf> nanoparticles of controlled size

Direct observation of enhanced magnetism in individual size- and shape-selected 3d transition metal nanoparticles

Iron oxide surfaces

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