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High-vacuum annealing reduction of Co/CoO nanoparticles
Abstract
Porous films of Co/CoO magnetic nanoparticles have been obtained by inert gas condensation and partially oxidized in situ in the deposition chamber. These nanoparticle films were subjected to thermal treatments in high vacuum and the chemical and structural changes monitored by x-ray diffraction, transmission electron microscopy, transport and magnetic measurements (with a focus on the exchange-bias phenomenon), which evidence that for vacuum annealing temperatures above 360 (°)C, most of the CoO phase is reduced to metallic Co without requiring the presence of an external reducing agent (e.g., H2) or a plasma. Additionally, there is a certain degree of particle coalescence resulting in the formation of greater nanoparticles as the annealing temperature increases. This yields a smaller proportion of CoO compared to metallic Co and a reduction of the Co/CoO interface density, pinpointed by a drastic decrease of the exchange-bias field. The crucial roles of the vacuum level and the surface-to-volume ratio are evidenced by magnetic measurements, highlighting the potential of magnetometry as a probe for the reduction/oxidation of composite nanostructures.
Supplementary resource (1)
... The effects of different parameters, such as interfacial roughness, particle size, magnetic and nonmagnetic defects, and the shape of nanoparticles in conventional core/shell structures, have been investigated [3]. The core/shell with a metal shell has rarely been studied [31][32][33][34][35]. Soares et al. synthesized the CoFe 2 O 4 /CoFe 2 by reducing CoFe 2 O 4 in the hydrogen to find out the critical shell thickness in which the core couples to the shell through the exchange-spring mechanism. ...
... Figure 10a indicates that the variation in coercivity with respect to the oxidation time is not uniform in the OMO-series and has a maximum for 40 min oxidation (90.9 kA/m) and a minimum for 20 and 60 min oxidation times (75.3 kA/m). The range of coercivity values reported in the literature on Co/Co-oxide nanoparticles varies from 16 to 800 kA/m [31,32,[56][57][58][59][60][61][62]. It was shown that the coercivity is highly influenced by the particle size, domain structure, shape of the particle, and the thickness of each layer [31,32,42,56,57,62,63]. ...
... The range of coercivity values reported in the literature on Co/Co-oxide nanoparticles varies from 16 to 800 kA/m [31,32,[56][57][58][59][60][61][62]. It was shown that the coercivity is highly influenced by the particle size, domain structure, shape of the particle, and the thickness of each layer [31,32,42,56,57,62,63]. Srikala et al. reported a range of coercivities from 16 to 56 kA/m for Co/CoO nanoparticles with different sizes, ranging from 14 to 44 nm and with spherical and cubic morphologies [57]. ...
In this study, we investigate the enhancement of exchange bias in core/shell/shell structures by synthesizing single inverted core/shell (Co-oxide/Co) and core/shell/shell (Co-oxide/Co/Co-oxide) nanostructures through a two-step reduction and oxidation method. We evaluate the magnetic properties of the structures and study the effect of shell thickness on the exchange bias by synthesizing various shell thicknesses of Co-oxide/Co/Co-oxide nanostructures. The extra exchange coupling formed at the shell-shell interface in the core/shell/shell structure leads to a remarkable increase in the coercivity and the strength of the exchange bias by three and four orders, respectively. The strongest exchange bias is achieved for the sample comprising the thinnest outer Co-oxide shell. Despite the general declining trend of the exchange bias with Co-oxide shell thickness, we also observe a nonmonotonic behavior in which the exchange bias oscillates slightly as the shell thickness increases. This phenomenon is ascribed to the dependence of the antiferromagnetic outer shell thickness variation at the expense of the simultaneous opposite variation in the ferromagnetic inner shell.
... 1d and S1b of ESI, a large orientated nanocrystal was observed in the TEM images after the annealing of S2. This result indicates a kinetic growth mechanism enhanced by the heating rate as reported in previous works 27,30,45,46 . Specifically, the growth of CoO nanoparticles is dependent on two mechanisms: a) Co diffusion into the remaining organic coating, into the lattice and intra lattice, and, principally, b) by coalescence and orientated attachment of CoO nanoparticles. ...
... In fact, the presence of low intensity peaks in the X-ray patterns as shown in Fig. 2c within the range of 2 θ ∼ 45 • -48 • corresponding to metallic hcp cobalt structure indicates to the presence of the metallic cobalt. Complementary, as previously reported, CoO phase may be reduced to metallic Co at high temperature in vacuum yielding Co/CoO interface in the nanoparticles 36,45 . Therefore, the presence of metallic Co in sample S2 after annealing is likely to be associated with site A. ...
We herein report a comprehensive investigation on the magnetic, structural, and electric properties of CoO nanoparticles with different sizes by local inspection through hyperfine interactions measured in a wide range of temperatures (10–670 K) by using radioactive 111\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{111}$$\end{document}In(111\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{111}$$\end{document}Cd) tracers with the perturbed angular correlations technique. Small cobalt oxide nanoparticles with the characteristic size of 6.5 nm have been prepared by the wet chemical route that turned out to be essential to incorporate radioactivity tracers during nucleation and growth of the particles. Nanocrystalline samples with 22.1 nm size were obtained by thermal treatments under low pressure of helium at 670 K. The hyperfine data were correlated with X-ray diffraction, ZFC–FC magnetic measurements, and transmission electron microscopy to describe the structure, magnetic properties, size, and shape of samples. An analysis of the temperature evolution of hyperfine parameters revealed that the structural distortion and the magnetic disorder in the core and on the surface layer play an important role in the magnetic behavior of CoO nanoparticles.
... As shown in Fig. 1d and Fig.S2b of ESI, a large orientated nanocrystal was observed by TEM images after annealing in sample S2. This result indicates a kinetic grown mechanism enhanced by the heating rate as reported in previous works 30,35,42,43 . Specifically, the growth of CoO nanoparticles is dependent on two mechanisms: a) Co diffusion into the remained organic coating, into the lattice and intra lattice, and, principally, b) by coalescence and orientated attachment of CoO nanoparticles. ...
... In fact, the presence of low intensity peaks in the X-ray patterns as shown in Fig. 2c within the range of 2θ ∼45 o -48 o corresponding to metallic hcp cobalt structure indicate the presence of metallic cobalt. Complementary, as previously reported, CoO phase may be reduced to metallic Co at high temperature in vacuum yielding Co/CoO interface in nanoparticles 42,59 . Therefore, the presence of metallic Co in sample S2 after annealing is likely to be associated with site A. ...
We herein report a comprehensive investigation on the magnetic, structural, and electric properties of CoO nanoparticles with different sizes by local inspection through hyperfine interactions measured in a wide range of temperatures (10-670 K) by using radioactive 111In( 111Cd) tracers with the perturbed angular correlations (PAC) technique. Small cobalt oxide nanoparticles with 6.5 nm have been prepared by the wet chemical route, which has shown essential to incorporate radioactivity tracers during nucleation and grown syntheses. Nanocrystalline samples with 22 nm size were obtained by thermal treatments under low pressure of helium at 670 K. The hyperfine data were correlated with X-ray diffraction (XRD), ZFC-FC magnetic measurements and transmission electron microscopy (TEM) to describe the structure, magnetic properties, size, and shape of samples. An analysis of the temperature evolution of hyperfine parameters revealed that the structural distortion and the magnetic disorder in the core and on the surface layer play an important role in the magnetic behavior of CoO nanoparticles.
... Hence, even for the Co-CoO system only, its value varies in a wide range from few tens to thousands Oersted. 3,[9][10][11][12][13][14][15][16] Therefore, both theoretical and experimental studies of mechanism and preparation of structured materials having a large stable EB effect are of high importance. 4 Various methods were applied to prepare samples for study of the EB effect. ...
... 8 The effect was studied frequently in thin films prepared by molecularbeam epitaxy, sputtering, gas condensation or chemical vapor deposition. [9][10][11]14,16 Many studies were done on core-shell nanoparticles prepared by chemical methods such as thermal decomposition, chemical reduction… 13,15 Ball milling method was also used because it is a simple technique and samples can be fabricated in a large scale. Ball milling was usually applied for mixtures of different constituents to produce nanocomposites. ...
Co-CoO composite powders were prepared by high-energy ball milling and subsequent annealing with the aim to probe an exchange bias (EB) effect. A microstructure consisting of flakes with a thickness of about 100 nm was revealed from scanning electron microscopy images. X-ray diffraction phase identification indicated that the optimal annealing temperature of as-milled Co for the formation of Co-CoO composite structure is 300C. Magnetic measurements showed that saturation magnetization, Ms, of annealed Co-CoO decreased as compared to that of as-milled Co. This implies that a fraction of the oxide phase was formed after heat treatment. Furthermore, the hysteresis loop measured at 5 K after cooling in a magnetic field of 50 kOe from 350 K showed a presence of the EB effect, which reached the value of 120 Oe. It is closely related to the formation of an antiferromagnetic (AFM) CoO phase, which interacts with the adjacent Co ferromagnetic (FM) phase. A Monte Carlo simulation was also performed to demonstrate the EB effect in FM–AFM structured materials. A better agreement between simulated and experimental hysteresis loops was obtained when averaged characteristics of the randomly-oriented individual powders were taken into account. Simulation results also showed that EB is largest when easy axes of FM and AFM phases are parallel to the magnetic field, and a critical fraction of AFM phase was suggested to be necessary for appearance of the EB effect.
... The sample was grown during 16 min with a deposition rate (measured at the beam axis position using a quartz crystal monitor) of 1.4 Å/s, thus leading to a film thickness greater (considering porosity) than 140 nm. The mean particle size (including the CoO shell) is about 6 nm (see Figure 1), similar to Co particles produced under the same synthesis conditions in previous studies [27]. The same sample was annealed in air at 100 • C for progressively longer times (up to a total cumulative time of t A = 1464 h) in order to monitor the change of magnetic properties resulting from the controlled variation of the Co/CoO volume ratio. ...
... The sample was grown during 16 min with a deposition rate (measured at the beam axis position using a quartz crystal monitor) of 1.4 Å /s, thus leading to a film thickness greater (considering porosity) than 140 nm. The mean particle size (including the CoO shell) is about 6 nm (see Figure 1), similar to Co particles produced under the same synthesis conditions in previous studies [27]. The same sample was annealed in air at 100 °C for progressively longer times (up to a total cumulative time of tA = 1464 h) in order to monitor the change of magnetic properties resulting from the controlled variation of the Co/CoO volume ratio. ...
Porous films of cobalt nanoparticles have been obtained by sputter gas aggregation and controllably oxidized by air annealing at 100 °C for progressively longer times (up to more than 1400 h). The magnetic properties of the samples were monitored during the process, with a focus on the exchange bias field. Air annealing proves to be a convenient way to control the Co/CoO ratio in the samples, allowing the optimization of the exchange bias field to a value above 6 kOe at 5 K. The occurrence of the maximum in the exchange bias field is understood in terms of the density of CoO uncompensated spins and their degree of pinning, with the former reducing and the latter increasing upon the growth of a progressively thicker CoO shell. Vertical shifts exhibited in the magnetization loops are found to correlate qualitatively with the peak in the exchange bias field, while an increase in vertical shift observed for longer oxidation times may be explained by a growing fraction of almost completely oxidized particles. The presence of a hummingbird-like form in magnetization loops can be understood in terms of a combination of hard (biased) and soft (unbiased) components; however, the precise origin of the soft phase is as yet unresolved.
... Most probably the surface oxygen was mainly in the form of O-H groups (O 1s observed around 531 eV [26]) which can desorb at lower temperatures. Another thing is that Co oxides could be also formed on the surface and then could decompose at elevated temperatures in UHV [27]. At higher temperatures >673 K the O concentration again increased but this is directly related to the decreasing amount of Co and exposition of bare TiO 2 (001) substrate (Ti concentration also increased above 573 K). ...
... Further annealing of Co/TiO 2 (001) at 573-673 K caused the disappearance of the CoO components. It means that CoO was reduced to metallic Co which is in line with the recent findings for CoO NPs [27] where the authors observed their reduction in UHV at temperatures as low as 633 K. The metallic component was observed also at higher temperatures and above 973 K it vanished almost completely even though the recorded spectra were additionally averaged from four subsequent measurements to improve signal-to-noise ratio. ...
Cobalt-and nickel-rich nanoparticles were produced by direct segregation from a single crystal rutile TiO2 in ultra-high vacuum conditions. It was shown that, similarly to the previous findings for Fe, also in the case of Co and Ni, the segregation of a noticeable amount of material is possible on the (001) surface. Segregation of these metals results in formation of nanoparticles and their distribution on the surface partially reflects the substrate symmetry in the case of Co and reveals some signs of selforganization for Ni. Cyclic appearance and disappearance of Co-and Ni-rich nanoparticles can be fully controlled by the temperature of annealing. The explanation of the mechanism of repeatable segregation is proposed basing on interaction of Co and Ni cations with Ti interstitials. These new findings can be especially important for catalysis and spintronic applications where the controlled nanoparticles size and concentration are crucial.
... It happens when ferromagnetic components (nickel or cobalt) is exchange coupled with an antiferromagnetic component (nickel or cobalt oxide) at the interface. This property is useful not only in spintronic devices but also for probing the properties at the ferromagnetic-antiferromagnetic interface [27]. Very recently, cobalt oxide has been reported to be partially reduced at high vacuum annealing conditions and showed significant reduction of exchange bias afterwards [27]. ...
... This property is useful not only in spintronic devices but also for probing the properties at the ferromagnetic-antiferromagnetic interface [27]. Very recently, cobalt oxide has been reported to be partially reduced at high vacuum annealing conditions and showed significant reduction of exchange bias afterwards [27]. ...
Magnetic Ni nanoclusters were synthesized by electron beam deposition
utilizing CVD graphene as a scaffold. The subsequent clusters were subjected to
high vacuum (5-8 x10-7 torr) annealing between 300 and 600 0C. The chemical
stability, optical and morphological changes were characterized by X-ray
photoemission microscopy, Raman spectroscopy, atomic force microscopy and
magnetic measurement. Under ambient exposure, nickel nanoparticles was observed
to be oxidized quickly, forming antiferromagnetic nickel oxide. Here, we report
that the majority of the oxidized nickel is in non-stoichiometric form and can
be reduced under high vacuum at temperature as low as 300 0C. Importantly, the
resulting annealed clusters are relatively stable and no further oxidation was
detectable after three weeks of air exposure at room temperature.
... Three films of Co=CoO core or shell nanoparticles [4,[34][35][36] (5-7 nm) highly dispersed in an AFM NiO matrix (S series)-see Fig. 1(a)-or in a Nb matrix, for reference (R series), were grown by combining inert gas condensation (Co nanoparticles) and rf sputtering (NiO and Nb) [34][35][36][37][38][39][40]. The digits in the sample names refer to the cluster source power (W), which, together with the occasional use of a carrier gas (He), was varied to control the nanoparticle size [35]. ...
... Three films of Co=CoO core or shell nanoparticles [4,[34][35][36] (5-7 nm) highly dispersed in an AFM NiO matrix (S series)-see Fig. 1(a)-or in a Nb matrix, for reference (R series), were grown by combining inert gas condensation (Co nanoparticles) and rf sputtering (NiO and Nb) [34][35][36][37][38][39][40]. The digits in the sample names refer to the cluster source power (W), which, together with the occasional use of a carrier gas (He), was varied to control the nanoparticle size [35]. ...
Thermal activation tends to destroy the magnetic stability of small magnetic
nanoparticles, with crucial implications in ultra-high density recording among
other applications. Here we demonstrate that low blocking temperature
ferromagnetic (FM) Co nanoparticles (TB<70 K) become magnetically stable above
400 K when embedded in a high N\'eel temperature antiferromagnetic (AFM) NiO
matrix. The origin of this remarkable TB enhancement is due to a magnetic
proximity effect between a thin CoO shell (with low N\'eel temperature, TN; and
high anisotropy, KAFM) surrounding the Co nanoparticles and the NiO matrix
(with high TN but low KAFM). This proximity effect yields an effective AFM with
an apparent TN beyond that of bulk CoO, and an enhanced anisotropy compared to
NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations
via core-shell exchange-bias coupling, leading to the observed TB increase.
Mean-field calculations provide a semi-quantitative understanding of this
magnetic- proximity stabilization mechanism.
... Combine with the bulk-type SSLB over-lithiation behavior and the in operando TEM results, the over-lithiation study in in operando TEM suggests that LCO should be intrinsically not stable under low voltage bias, which leads to the decomposition of LCO into metallic Co, cobalt oxides, and Li 2 O. Cobalt oxides could further decompose into metallic Co and release oxygen due to the ultra-low pressure TEM environment and Joule heat from the electron beam of TEM. [102] However, the formation of cobalt oxide was not observed in the in operando TEM. This is similar to the report from in operando X-ray absorption near-edge spectroscopy and in operando transmission X-ray microscopy. ...
Li7La3Zr2O12 (LLZO)‐based all‐solid‐state Li batteries (SSLBs) are very attractive next‐generation energy storage devices owing to their potential for achieving enhanced safety and improved energy density. However, the rigid nature of the ceramics challenges the SSLB fabrication and the afterward interfacial stability during electrochemical cycling. Here, a promising LLZO‐based SSLB with a high areal capacity and stable cycle performance over 100 cycles is demonstrated. In operando transmission electron microscopy (TEM) is used for successfully demonstrating and investigating the delithiation/lithiation process and understanding the capacity degradation mechanism of the SSLB on an atomic scale. Other than the interfacial delamination between LLZO and LiCoO2 (LCO) owing to the stress evolvement during electrochemical cycling, oxygen deficiency of LCO not only causes microcrack formation in LCO but also partially decomposes LCO into metallic Co and is suggested to contribute to the capacity degradation based on the atomic‐scale insights. When discharging the SSLB to a voltage of ≈1.2 versus Li/Li+, severe capacity fading from the irreversible decomposition of LCO into metallic Co and Li2O is observed under in operando TEM. These observations reveal the capacity degradation mechanisms of the LLZO‐based SSLB, which provides important information for future LLZO‐based SSLB developments. Li7La3Zr2O12‐based all‐solid‐state Li battery is made to deliver a high areal capacity and stable cycle performance. In operando transmission electron microscopy is performed to observe the charge/discharge processes while three capacity degradation mechanisms, that is, interface delamination, microcrack formation, and LiCoO2 decomposition, are revealed.
... Unlike core, shell surfaces remain active and interact easily with the ambient. Property modulation of magnetic oxides in previous studies via surface treatment [15,16] (annealing/reduction/oxidation in reducing ambient) justified our choice of coating barium titanate with iron oxide. Previously, researchers synthesized Fe 2 O 3 -BaTiO 3 CSNP with Fe 2 O 3 as the core and BaTiO 3 as the shell component [17,18], which was useful but did not offer similar versatility. ...
Improvement of magnetic, electronic, optical, and catalytic properties in cutting-edge technologies including drug delivery, energy storage, magnetic transistor, and spintronics requires novel nanomaterials. This article discusses the unique, clean, and homogeneous physiochemical synthesis of BaTiO3/iron oxide core–shell nanoparticles with interfaces between ferroelectric and ferromagnetic materials. High-resolution transmission electron microscopy displayed the distinguished disparity between the core and shell of the synthesized nanoparticles. Elemental mapping and line scan confirmed the formation of the core–shell structure. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy detected the surface iron oxide phase as maghemite. Rietveld analysis of the X-ray diffraction data labeled the crystallinity and phase purity. This study provides a promising platform for the desirable property development of the futuristic multifunctional nanodevices.
... Similar results were obtained for Co particles electrodeposited onto twisted graphene [5]. Considering that H EB is inversely proportional to the size of ferromagnetic particles [23], the relatively small values of H EB , with respect to for example H EB = 6.5 kOe for Co-CoO particles at T = 10 K [24], are indicative of quite a large size of Co cores in the deposited Co-CoO particles. ...
... Different strategies can be found in literature to promote the coupling between the Co and the CoO phase in coatings, as in the case of multilayers [17][18][19][20] , nanocomposite coatings with a random distribution of Co and CoO phases 12,[21][22][23] or films fabricated by assembly of Co/CoO nanoparticles 24-25 14 . In these works the CoO phase is formed either by deposition in the presence of O 2 to generate Co/CoO nanocomposite coatings 21-23 or multilayers [19][20] ; by annealing treatments followed by exposure to oxygen 12,17,26 or simply by oxidation in contact with air 25 . The aim is to create a homogeneous distribution of CoO that generates an effective EB coupling with the Co phase. ...
In this paper Co/CoO thick layers (hundreds of nanometers) of different porosity and oxidation degree were prepared in a magnetron sputtering deposition process by tailoring the DC sputtering power, as well as the process gas and target composition. The control of the synthesis parameters allowed the nanostructuration of the films with a singular distribution of closed pores and a controlled amount of CoO. We observed an exchange bias field of 2.8 KOe for porous Co/CoO composites, similar to Co/CoO bilayers but for coatings thicker than 300 nm. Besides, it was observed that the coating presents bistable magnetic features when cooled under zero field conditions as a result of the unusual exchange coupling.
... Interestingly a shoulder peak at around 778 eV can be attributed to metallic cobalt on the surface of the CNTs/P900 sample likely due to a reduction of cobalt to lower oxidation states at the high temperature treatment in inert atmosphere [29][30][31] . XRD and HRTEM analysis of the lattice fringes in the catalyst nanoparticles (Fig. 1d and e, respectively) suggests both CoO and Co 3 O 4 phases present in the samples, in excellent agreement with electron diffraction measurements (Fig. 1f) 32,33 . The Raman spectrum of CoO x @CNTs/ P900 is vastly dominated by characteristic vibrations of crystalline Co 3 O 4 with clear peaks at 482 cm −1 (E g ), 519 and 621 cm −1 (F2 g ) and 690 cm −1 (A1 g ) (Supplementary Fig. S8b) [34][35][36] . ...
Herein we report a 3D heterostructure comprising a hierarchical macroporous carbon foam that incorporates mesoporous carbon nanotubes decorated with cobalt oxide nanoparticles as an unique and highly efficient electrode material for the oxygen evolution reaction (OER) in electrocatalytic water splitting. The best performing electrode material showed high stability after 10 h, at constant potential of 1.7 V vs. RHE (reversible hydrogen electrode) in a 0.1 M KOH solution and high electrocatalytic activity in OER with low overpotential (0.38 V vs RHE at 10 mA cm−2). The excellent electrocatalytic performance of the electrode is rationalized by the overall 3D macroporous structure and with the firmly integrated CNTs directly grown on the foam, resulting in a large specific surface area, good electrical conductivity, as well as an efficient electrolyte transport into the whole electrode matrix concurrent with an ability to quickly dispose oxygen bubbles into the electrolyte. The eminent properties of the three-dimensional structured carbon matrix, which can be synthesized through a simple, scalable and cost effective pyrolysis process show that it has potential to be implemented in large-scale water electrolysis systems.
... The roughness changes in the Ni 95 Cr 5 :Ag case, where there are altogether different grain sizes. The GIXRD pattern does not show any impurity antiferromagnetic phases such as Cr, CrO 2 , Cr 2 O 3 or NiO, which is further confirmed by exchange bias study (shown in supplementary information Fig. S3) at 3 K, a method described elsewhere 11,20 . ...
Cr-surface segregation is a main roadblock encumbering many magneto-biomedical applications of bimetallic M-Cr nanoalloys (where M = Fe, Co and Ni). To overcome this problem, we developed Ni95Cr5:Ag nanocomposite as a model system, consisting of non-interacting Ni95Cr5 nanoalloys (5 ± 1 nm) immersed in non-magnetic Ag matrix by controlled simultaneous co-sputtering of Ni95Cr5 and Ag. We employed Curie temperature (TC) as an indicator of phase purity check of these nanocomposites, which is estimated to be around the bulk Ni95Cr5 value of 320 K. This confirms prevention of Cr-segregation and also entails effective control of surface oxidation. Compared to Cr-segregated Ni95Cr5 nanoalloy films and nanoclusters, we did not observe any unwanted magnetic effects such as presence Cr-antiferromagnetic transition, large non-saturation, exchange bias behavior (if any) or uncompensated higher TC values. These nanocomposites films also lose their unique magnetic properties only at elevated temperatures beyond application requirements (≥800 K), either by showing Ni-type behavior or by a complete conversion into Ni/Cr-oxides in vacuum and air environment, respectively.
Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the l-series), the size is ≈100 nm, and in the other (the s-series, obtained using lower synthesis temperatures than the l-series), it is ≈10 nm. In the l-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the s-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic–antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores.
The development of low-cost electrocatalysts with high performance toward hydrogen evolution reaction (HER) is crucial for large-scale and clean hydrogen productions. Herein, we report the facile synthesis of the Co/CoO hybrids with oxygen vacancies on carbon cloth (CC) via the calcination treatment of Co3O4 precursor in reductive atmosphere (H2/Ar). Moreover, to avoid delamination and cracks during heat treatment and catalysts abscission during HER test, the precursor of Co(OH)2 was stabilized on CC by anion intercalation method, resulting in robust adhesion between film and substrate. The as-prepared Co/CoO hybrids rooted on CC substrate possessed good charge transfer properties, favorable kinetics, abundant active sites, and robust adhesion. A suitable reductive atmosphere was crucial for high catalytic activity. By optimizing, the sample obtained at 30% H2/Ar atmosphere (Co/CoO-30) possessed optimal balance between Co/CoO and oxygen vacancies, exhibiting optimum HER performance with the smallest overpotential of 158 mV at 10 mA cm-2 and Tafel slope of 68.1 mV dec-1 as well as good stability, operating at least 24 h.
In this contribution low temperature formation of Ruddlesden‐Popper (RP)‐type layered La2CoO4±δ perovskite was optimized via in situ X‐ray powder diffraction (XRPD). Starting from LaCoO3 a stoichiometric transformation to La2CoO4±δ and CoO can be achieved by controlled reduction with H2. The challenge of this reaction is the use of appropriate amounts of H2 in a defined temperature region. If the amount of H2 is too high, complete reduction of the perovskite occurs. If temperatures are not appropriate, intermediate phases seem to hinder the transformation to La2CoO4±δ or lead to a complete decomposition to simple oxides. Based on in situ XRPD experiments, temperature window and required amount of H2 for the transformation of LaCoO3 to La2CoO4±δ were determined. Systematic experiments reveal that 650 °C is the optimal temperature for the complete transformation of LaCoO3 into La2CoO4±δ and CoO/Co0. The information was then transferred to realize bulk synthesis of La2CoO4±δ at 650 °C in a tube furnace without extended heat treatments at elevated temperatures.
Cobalt (II) acetate is mixed with zinc methacrylate (ZnMAA) to form a photopatternable Co-doped zinc oxide precursor. By using deep-UV (DUV) interference lithography, Co-doped ZnMAA precursor can be patterned as negative tone resist and transformed into ferromagnetic Co:ZnO films after thermal treatment. Moreover, Co:ZnO patterns as small as 300 nm line-width can be easily obtained. To have an in-depth understanding to the effect of DUV-patterning process as well as thermal annealing on Co:ZnO films derived from Co-doped ZnMAA precursor, optical, magnetic, and electrical characterizations are performed on Co:ZnO films prepared in different conditions. For the Co:ZnO film prepared without DUV-patterning, large zero-field-cooling (ZFC)–field-cooling (FC) irreversibility appears in superconducting quantum interference device measurements after vacuum annealing, indicating that Co clusters have formed inside the film. On the other hand, no ZFC–FC bifurcation can be observed for the DUV-patterned Co:ZnO film after the vacuum annealing, suggesting that the uniformity of Co ion distribution inside ZnO lattice is improved by DUV-patterning.
The gas phase synthesis and characterization of multifunctional core@shell, Au@TiOx nanoparticles is reported. The nanoparticles are produced in a one-step process by using a Multiple Ion Cluster Source under controlled environment that guaranties the purity of the nanoparticles. The growth of the Au cores (6 nm diameter) is stopped when these latter pass through the Ti plasma where they are covered by an ultra-thin (1 nm thick) and homogeneous titanium shell that is oxidized in-flight before the soft-landing of the nanoparticles. The Au cores are found to be highly crystalline with icosahedral (44%) and decahedral (66%) structures while the shell mainly composed by TiO2 (79%) is not ordered. The highly electrical insulating behaviour of the titanium oxide shell is evidenced by charging effect produced during x-ray photoemission spectroscopy.
In this article, Co-based metal organic frameworks (MOFs) with two shapes were used as pyrolysis precursor to synthesize multilayer core-shells composites loaded on rGO sheets. The core-shell structures were obtained by the formation of cores from metal ions and carbon shells from carbonization of ligands. Controllable oxidation of Co cores to CoOx shells generated multilayer core-shell structures anchored onto the surface of rGO sheets. The N-doped composites were obtained by adding poly vinyl pyrrolidone. The multilayer core-shells composites exhibited superior catalytic activity toward hydrogen generation compared to their single layer counterparts. Using the N-doped multilayer composites, high hydrogen generation specific rate of 5560 mL•min-1•gCo-1 was achieved at room temperature. The rGO sheets in composites improved their structure stability. These catalysts exhibited high stability after used five cycling. This synergistic strategy proposes simple, efficient and versatile blue-prints for the fabrication of rGO composites from MOFs-based precursors.
The physical principles underlying some current biomedical applications of magnetic nanoparticles are reviewed. Starting from well-known basic concepts, and drawing on examples from biology and biomedicine, the relevant physics of magnetic materials and their responses to applied magnetic fields are surveyed. The way these properties are controlled and used is illustrated with reference to (i) magnetic separation of labelled cells and other biological entities; (ii) therapeutic drug, gene and radionuclide delivery; (iii) radio frequency methods for the catabolism of tumours via hyperthermia; and (iv) contrast enhancement agents for magnetic resonance imaging applications. Future prospects are also discussed.
The fundamental question as to the relative importance of interparticle superexchange versus dipolar interaction between oxide magnetic particles in direct physical contact is addressed by examining the magnetic properties of a series of compacted samples comprising identical maghemite particles (8 nm in diameter) coated by nonmagnetic shells (oleic acid or silica) of varying thickness that control the distance between the magnetic cores and hence the packing density (particle volume fraction). A remarkably narrow maghemite particle size distribution is established by electron microscopy and small-angle X-ray scattering. The series includes a sample made up of bare particles in a random-close-packed configuration (therefore in direct contact) that exhibits ideal superspin-glass behavior with a relatively high freezing transition temperature. It is shown that interparticle superexchange interactions between the nanoparticles in this sample play a minor role compared to classical dipolar interactions in establishing the collective, superspin-glass state. This follows from the freezing temperature of the most concentrated samples in the series (those with 0 ≤ shell thickness < 3 nm), which are found to vary in direct proportionality with the volume fraction of the maghemite cores and therefore with the strength of dipolar interactions.
We describe here the use of high-temperature oxidation and reduction to produce highly crystalline nanoparticles of Ni and NiO. Starting with an amorphous Ni powder, we demonstrate that oxidation at 900 °C produces faceted NiO nanocrystals with sizes ranging from 20 to 60 nm. High-resolution transmission electron microscopy measurements indicate near-perfect atomic order, truncated by (200) surfaces. Magnetization measurements reveal that the Néel temperature of these NiO nanoparticles is 480 K, substantially reduced by finite-size effects from the bulk value of 523 K. The magnetization of these faceted NiO nanoparticles does not saturate in fields as large as 14 T while a loop offset is observed which increases from 1000 Oe at 300 K to its maximum value of 3500 Oe at 50 K. We have used high-temperature reduction to transform the faceted NiO nanoparticles into highly ordered Ni nanoparticles, with a Curie temperature of 720 K and blocking temperatures in excess of 350 K. Subsequent efforts to reoxidize these Ni nanoparticles into the core-shell morphology found that the Ni nanoparticles are much more resistant to oxidation than the original Ni powder, perhaps due to the relative crystalline perfection of the former. At 800 °C, an unusual surface roughening and subsequent instability was observed, where 50-nm-diameter NiO rods grow from the Ni surfaces. We have demonstrated that high-temperature oxidation and reduction in Ni and NiO are both reversible to some extent and are highly effective for creating the highly crystalline nanomaterials required for applications such as exchange-bias devices.
The origin of asymmetry in field cooled (FC) hysteresis loops exhibiting exchange bias (EB) is investigated by studying the static and dynamic magnetic properties of core-shell Co/CoO nanoparticles. Two distinct freezing temperatures coresponding to the core (Tf-cr ∼ 190 K) and the shell moments (Tf-sh ∼ 95 K) are obtained from the energy barrier distribution. The FC loops are symmetric in the temperature range Tf-sh ≤ T ≤ Tf-cr, however, asymmetry in hysteresis is observed immediately below Tf-sh. These intriguing features are also probed by radio frequency transverse susceptibility (TS) experiments. We show that the first anisotropy fields obtained from the demagnetization and return curves of field cooled TS measurement, shift along the negative field axis and strikingly resemble the temperature dependence of EB. Field cooled TS measurements reveal the effect of competing Zeeman and anisotropy energy above and below Tf-sh to account for the development of asymmetry. Our study indicates that asymmetry in FC hysteresis loops is intrinsic to core-shell nanoparticles and develops only below the freezing temperature of the shell due to enhanced magnetic anisotropy.
We have studied the magnetic properties of [Ag(tAg)∕Co(1.2 nm)]60 multilayers grown in an oxygen atmosphere. Partial oxidation of the Co layers results in the appearance of tAg dependent exchange-bias properties. A strong increase in the exchange-bias field HE together with a significant reduction in the coercivity HC are observed when tAg is decreased below tAg*=4 nm, whereas these two parameters adopt approximately constant values for tAg>tAg*. At tAg*, a transition from continuous to islandlike silver layers (on reducing tAg) is signaled by electrical resistivity and x-ray reflectivity results. From magnetic hysteresis loops recorded at room temperature and magnetization curves, it is concluded that this transition induces a granular morphology in the Co regions, which are previously (tAg>tAg*) plateletlike, and enhances the oxidized fraction of Co (fCoO). The increase (decrease) in HE (HC) with reducing tAg below tAg* is correlated to the increase in both the electrical resistivity and fCoO. From the latter correlation, we infer that the higher degree of oxidation in the granular Co layers is associated with effectively thicker antiferromagnetic (CoO) regions than in the tAg>tAg* (continuous multilayer) case—with correspondingly higher magnetic anisotropy energies—which may account for both the enhancement in HE and the reduction in HC. In addition, our study provides information on the surfactant effect of O2 in Ag sputter growth since the continuity thickness value (tAg*=4 nm) is found to be lower than those previously reported in nonreactive sputtering of Ag.
A series of Co(x)CoO(1−x) thin films have been synthesized by sputtering Co in an oxygen atmosphere using different rf sputtering powers as a means to vary the concentration x. The highest exchange-bias field He (2.7 kOe at T=90 K) was measured in a sample with an estimated metallic Co fraction of x=0.53. In this sample, and in those with lower Co concentrations, the temperature dependence of the magnetization shows a pronounced dip, both in zero-field- and field-cooled curves, at temperatures close to the Néel temperature of bulk CoO. This feature, which disappears with moderate annealing, is associated to the destabilization of small isolated Co nanoparticles when the exchange coupling with the CoO matrix vanishes above its Néel temperature. A simple structural model is proposed to account for the presented magnetization. X-ray diffraction and
electrical resistivity results are also presented, which support the studies of magnetic properties.
A series of nanogranular Co/Nb samples has been prepared using an unfiltered beam of Co nanoparticles preformed by inert gas condensation. The preparation technique is shown to be a simple and effective method for fabricating, in a single deposition step, a sample series across which both particle size and concentration vary. We estimate the presence of weak interparticle (dipole-dipole) interactions ranging from 7 to 19% in strength (normalized to the median anisotropy energy barrier) across the present series. With the aim of elucidating the effect of such interactions on the blocking behavior of such nanogranular material, we have studied the field and temperature dependence of the magnetization in the films. For each sample, the temperature of the maximum in the zero-field-cooled magnetization curve (TMAX) is found to lie between the values of blocking (TB) and freezing (TF) temperature estimated from the experimentally determined particle size and concentration; i.e., TB<TMAX<TF. Furthermore, the deviation of TMAX with respect to TB correlates with the estimated strength of the interparticle interaction. These results support the Dormann-Bessais-Fiorani model, which predicts an enhancement of the effective particle anisotropy barrier in the weak-interaction regime. Our study also provides information on (i) the oxidation of nanoparticles in granular systems and (ii) the size-dependent divergence of nanoparticles ejected from a cluster source.
Co-based nanostructures ranging from core/shell to hollow nanoparticles were prepared by varying the reaction time and the chemical environment during the thermal decomposition of Co(2)(CO)(8). Both structural characterization and kinetic model simulation illustrate that the diffusivities of cobalt and oxygen determine the growth ratio and the final morphology of the nanoparticles. Exchange coupling between Co and Co-oxide in core/shell nanoparticles induced a shift of field-cooled hysteresis loops that is proportional to the shell thickness, as verified by numerical studies. The increasing nanocomplexity, when passing from core/shell to hollow particles, also leads to the appearance of hysteresis above 300 K due to an enhancement of the surface anisotropy resulting from the additional spin-disordered surfaces.
Gas phase nanoparticle production, manipulation and deposition is of primary importance for the synthesis of nanostructured materials and for the development of industrial processes based on nanotechnology. In this review we present and discuss this approach, introducing cluster sources, nanoparticle formation and growth mechanisms and the use of aerodynamic focusing methods that are coupled with supersonic expansions to obtain high intensity cluster beams with a control on nanoparticle mass and spatial distribution. The implication of this technique for the synthesis of nanostructured materials is also presented and applications are highlighted.
The technique of gas-phase aggregation has been used to prepare partially oxidized Co nanoparticles films by allowing a controlled
flow of oxygen gas into the aggregation zone. This method differs from those previously reported, that is, the passivation
of a beam of preformed particles in a secondary chamber and the conventional (low Ar pressure) reactive sputtering of Co to
produce Co–CoO composite films. Transmission electron microscopy shows that the mean size of the particles is about 6nm.
For sufficiently high oxygen pressures, the nanoparticles films become super-paramagnetic at room temperature. X-ray diffraction
patterns display reflections corresponding to fcc Co and fcc CoO phases, with an increasing dominance of the latter upon increasing
the oxygen pressure in the aggregation zone, which is consistent with the observed reduction in saturation magnetization.
The cluster films assembled with particles grown under oxygen in the condensation zone exhibit exchange-bias fields (about
8kOe at 20K) systematically higher than those measured for Co–CoO core-shell nanoparticles prepared by oxidizing preformed
particles in the deposition chamber, which we attribute, in the light of results from annealing experiments, to a higher ferromagnetic–antiferromagnetic
(Co–CoO) interface density.
The superparamagnetic blocking temperature of a series of granular Co <sub>22</sub> Ag <sub>78</sub> alloys grown by reactive sputtering under oxygen atmosphere exhibits a minimum when plotted as a function of the oxygen pressure. The magnetic stabilization observed above this minimum is found to arise from the exchange-bias between the Co core and a CoO shell, which appears to be spin disordered. The initial decrease in the blocking temperature at low oxygen pressures is mainly attributed to the inhibition of RKKY-like interactions by the formation of an insulating oxide layer, too thin to give rise to exchange bias, around the ferromagnetic cores.
Rechargeable solid-state batteries have long been considered an attractive power source for a wide variety of applications, and in particular, lithium-ion batteries are emerging as the technology of choice for portable electronics. One of the main challenges in the design of these batteries is to ensure that the electrodes maintain their integrity over many discharge-recharge cycles. Although promising electrode systems have recently been proposed, their lifespans are limited by Li-alloying agglomeration or the growth of passivation layers, which prevent the fully reversible insertion of Li ions into the negative electrodes. Here we report that electrodes made of nanoparticles of transition-metal oxides (MO, where M is Co, Ni, Cu or Fe) demonstrate electrochemical capacities of 700 mA h g(-1), with 100% capacity retention for up to 100 cycles and high recharging rates. The mechanism of Li reactivity differs from the classical Li insertion/deinsertion or Li-alloying processes, and involves the formation and decomposition of Li2O, accompanying the reduction and oxidation of metal nanoparticles (in the range 1-5 nanometres) respectively. We expect that the use of transition-metal nanoparticles to enhance surface electrochemical reactivity will lead to further improvements in the performance of lithium-ion batteries.
A method of constructing <30-nanometer structures in close proximity with precise spacings is presented that uses the step-by-step
application of organic molecules and metal ions as size-controlled resists on predetermined patterns, such as those formed
by electron-beam lithography. The organic molecules serve as a ruler for scaling down a larger “parent” structure. After metal
deposition and lift-off of the organic multilayer resist, an isolated smaller structure remains on the surface. This approach
is used to form thin parallel wires (15 to 70 nanometers in width and 1 micrometer long) of controlled thickness and spacing.
The structures obtained were imaged with field emission scanning electron microscopy. A variety of nanostructures could be
scaled down, including structures with hollow patterns.
Cobalt(II,III) oxide (Co3O4), synthesized by gel-combustion method, thermally treated in air at temperatures 400, 600 and 800 °C, was subjected to the reduction to metallic state in hydrogen atmosphere. The reduction was carried out at several constant heating rates and controlled by thermogravimetry. The mass vs. temperature curves were analyzed from the kinetic aspect. The two-step reduction was confirmed. Both the hydrogen partial pressure and the sample history influenced the reduction kinetics. The activation energies of both reduction steps were determined by means of Friedman isoconversional method and Kissinger methods, and the values obtained by Kissinger method were used to fit the experimental data by means of expanded Prout–Tompkins reaction model.
The catalytic oxidation of CO on various CoO catalysts was studied using a static technique at temperatures of 110 and 170 °C and a pressure of 2 Torr (). The CoO samples were prepared by the thermal decomposition of basic cobalt carbonate under a reduced pressure of 10-6 Torr at temperatures varying between 225 and 350 °C. The catalytic activity of the various samples in the oxidation of CO at 110 °C was found to be dependent on the specific surface area, the pore size and the degree of deviation from stoichiometric composition. In contrast, these properties had no influence on catalyst activity when the catalytic reaction was carried out at 170 °C. These results are attributed to the formation of the Co3O4 phase by the interaction between the oxygen of the reaction mixture and the CoO. The newly formed phase catalyses the oxidation of CO with much more efficiency than CoO does. This enhanced activity overlaps the role played by the surface characteristics of the solid. The results obtained for the oxidation of CO on CoO catalysts at 110 °C are in agreement with the electronic theory of catalysis.
Films of cobalt oxide were prepared by r.f. reactive sputtering of a cobalt target in Ar-O2 mixtures, and their optical properties were determined by absorption spectroscopy and scanning ellipsometry. The films were further characterized by electron microscopy and X-ray diffraction. The films were found to consist of Co3O4 when the oxygen flow rate exceeded a critical value, in agreement with previous studies. The optical properties of this oxide are due to intense ligand field absorption by Co2+ and Co3+ ions in a spinel structure. The optical properties of cobalt films oxidized near 500 °C were very similar to those of the sputtered films. Reactive co-sputtering of nickel and cobalt blackened the films for low Ni:Co ratios as a result of charge transfer absorption. Smooth films of Co3O4 did not function well as selective thermal absorbers of solar radiation.
We present results of the studies of structural and chemical transformations in Co/CoO nanoparticles prepared by inert gas condensation. The effect of the morphology and agglomeration on the phase transformation reaction path in self-oxidation and in controlled reduction processes are discussed in detail. As-prepared samples show self-oxidation related to the non-core/shell morphology of the particles. Annealing of particles at 250°C in reducing atmosphere leads to the oxidation of the particles showing coexistence of CoO and Co3O4 structures. This is explained by the diffusion of oxygen from the amorphous oxide surface to the bulk of the nanoparticles. Upon increasing the reaction temperature beyond 250°C, reductive transformation of the samples occurs systematically, from CoO/Co3O4 to CoO to Co (HCP+FCC) and eventually to Co (FCC). We have presented X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and magnetic data to track the structural and chemical transformation paths. We found strong correlation between structural and magnetic properties. Thermodynamic stability as a function of reaction temperature on the phase/chemical transformation is also discussed.
Polycrystalline Co3O4 films were prepared by normal pressure chemical vapour deposition, and the d.c. electrical conduction was investigated at temperatures from 170 to 400 K. A minimum resistivity of 14.2 Ω cm was obtained at a film forming temperature of 773 K. The Seebeck coefficient of films indicated the films to be p-type semiconductors. For the temperature range 220–400 K conduction of the films was confirmed to be due to polaronic hopping of holes. For temperatures from 170–220 K. the conduction was attributed to variable-range hoping of holes. Optical absorption edge analysis gave band gap energies of 1.50–1.52 eV corresponding to the edge of the charge transfer band (Co3+-Co2+), which was responsible to variable-range hoping, and 1.88–1.95 eV relative to the O2−-Co2+ charge transfer band.
The uncompensated spins on the surfaces of antiferromagnetic CoO films exhibit a thermoremanent magnetization after field cooling from TN that has the same temperature dependence as the exchange field of Ni 81Fe 19/CoO bilayers after field cooling. This suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on interfacial grain size.
The phenomenology of exchange bias and related effects in nanostructures is reviewed. The types of systems discussed include: lithographically fabricated ferromagnetic (FM)—antiferromagnetic (AFM) nanostructures, chemically surface modified FM particles, FM particles embedded in an AFM matrix, controlled core–shell particles, nanoparticles with surface effects and coupled AFM–AFM systems. The main applications of exchange biased nanostructures are summarized. Finally, the implications of the nanometer dimensions on some of the existing exchange bias theories are briefly discussed.
The electrolytic deposition of Co3O4 thin films on stainless steel was conducted in Co(NO3)2 aqueous solution for anodes in lithium-ion thin film batteries. Three major electrochemical reactions during the deposition were discussed. The coated specimens and the coating films carried out at −1.0V (saturated KCl Ag/AgCl) were subjected to annealing treatments and further characterized by XRD, TGA/DTA, FE-SEM, Raman spectroscopy, cyclic voltammetry (CV) and discharge/charge cyclic tests. The as-coated film was β-Co(OH)2, condensed into CoO and subsequently oxidized into nano-sized Co3O4 particles. The nano-sized Co3O4, CoO, Li2O and Co particles revealed their own characteristics different from micro-sized ones, such as more interfacial effects on chemical bonding and crystallinity. The initial maximum capacity of Co3O4 coated specimen was 1930mAhg−1 which much more than its theoretical value 890mAhg−1, since the nano-sized particles offered more interfacial bondings for extra sites of Li+ insertion. However, a large ratio of them was trapped, resulting in a great part of irreversible capacity during the first charging. Still, it revealed a capacity 500mAhg−1 after 50 discharged-charged cycles.
Ordered Co(3)O(4) with high surface area (until 173 m(2)/g) has been successfully obtained through a nanocasting route using mesoporous KIT-6 silica as a hard template and tested in the deep oxidation of a series of representative volatile organic compounds (VOCs): propane as a model of short chain alkane and toluene as a model of monoaromatic hydrocarbon. It has been demonstrated that the catalytic activity for VOC deep oxidation is very elevated and its catalytic stability at moderate temperatures very good. However, the role of the ordered structure in the catalytic performance does not seem to be beneficial. The enhanced catalytic activity has been explained in terms of both the high surface area and the presence of oxygen vacancies. In the present work we show for the first time the catalytic behaviour of a nanocasted cobalt oxide for the elimination of different VOCs.
Colloidal magnetic nanoparticles (NPs) have been applied in magnetic separations, in medicine and in biochemistry. They are also potentially applicable in magnetic recording media. In this paper, we report a systematic investigation of the magnetic properties of colloidal Co NPs after three extents of oxidation. The native sample has a thin (1.0 nm) CoO shell and exhibits no exchange biasing. The purposefully partially oxidized sample has a thicker CoO shell (3.2 nm), and is exchange biased. The sample fully oxidized to CoO loses exchange biasing. We observe three distinct magnetic properties that result from the finite-thickness antiferromagnet shell exchange coupled to a finite-size ferromagnet core, and from crystal and stoichiometric defects: (1) an enhancement of the thermal stability of the orientation of the magnetic moment due to exchange biasing in the partially oxidized sample, (2) a low-temperature paramagnetic response in the partially and fully oxidized samples due to defects in the CoO shell, and (3) an asymmetry in the field-dependent magnetization for the partially oxidized sample at low temperature due to small clusters of Co in a diffusion layer around the Co core. We propose a simple model to interpret these effects.
The kinetics of oxidation of cobalt is a complex problem since the metal forms two or more oxides and the metal itself has a phase transition from hexagonal to face‐centered cubic at a temperature between 300° and 500°C.
The vacuum microbalance method is used to study the effect of time, temperature, pressure, pretreatment, etc., on the rate of the reaction at temperatures below 700°C. The hexagonal (cold‐worked) form of cobalt was found to oxidize more rapidly than the cubic (annealed) form. The rate of reaction was found to increase abruptly between 600° and 700°C. The rate data are interpreted in terms of the transition state theory and the energy and entropy of activation for the reaction calculated.
Valensi has discussed the oxidation mechanism above 700°C in terms of diffusion of oxygen through the oxide lattice. Electron diffraction studies on the solid phase reaction of to give indicate that the cobalt diffuses through the oxide at temperatures as low as 450°C. An alternate mechanism to that proposed by Valensi is suggested to account for the reaction based on the diffusion of cations and electrons.
Amorphous and crystalline Al2O3 nanoparticles were synthesized by a sol–gel method with postannealing at different temperatures. Magnetism measurements have indicated that all Al2O3 nanoparticles exhibit intrinsic room temperature ferromagnetism, and the saturation magnetism of the samples increases after vacuum annealing, whereas bulk Al2O3 presents paramagnetism. Electron spin resonance and fitting results of O 1s X-ray photoelectron spectroscopy reveal that the origin of the ferromagnetism in Al2O3 nanoparticles could be attributed to the singly charged oxygen vacancies (F+ centers). The variation of the relative area of oxygen vacancies and the number of free electrons is consistent with the change of saturation magnetization for the samples. Combined with these results, a direct correlation of ferromagnetism with F+ centers exchange mechanism is established.
Single-crystalline CoO nanospheres with the size distribution between 40 and 250 nm were prepared by a solvothermal method. Magnetic measurements indicate that the vacuum-annealed samples show room temperature ferromagnetism except for the CoO nanospheres of 250 nm, which still exhibit paramagnetism after being postannealed in vacuum atmosphere (10−3 Pa) at 250 °C as others. The saturation magnetization of all postannealed samples monotonically increases with the decrease of nanosphere diameter. No other impurity phases are observed for the postannealed samples, indicating that the revealed ferromagnetism is an intrinsic property. The fitted XPS results of O 1s spectra indicate that the variations of oxygen vacancies concentration are consistent with the variations of saturation magnetization for the vacuum-annealed samples, suggesting that the formed oxygen vacancies at the surface of the CoO nanospheres during the vacuum-annealing process account for the observed ferromagnetism.
Oxidation of cobalt nanoparticles supported on montmorillonite was studied using transmission electron microscopy and temperature programmed reduction of the oxide particles. It has been shown that hollow shells of cobalt oxide were formed in this process, consisting of the mixture of CoO and Co3O4 oxides. A linear relationship between the thickness of shells and the initial particles size was established which suggests that both diffusion of cobalt ions through the oxide layer and diffusion of the oxygen into cobalt occur, but particles sintering is negligible compared with the reverse diffusion of cobalt. The lower critical diameter of cobalt particles was estimated, above which formation of hollow shells occurred. The hollow shells formation is suggested to be the reason for cobalt redispersion upon oxidation−reduction cycling of supported heterogeneous catalysts.
The interaction of nitrogen, oxygen, and hydrogen plasmas with spin-coated arrays of colloidal cobalt–platinum particles was investigated with a large variety of microscopic and spectroscopic techniques. It could be demonstrated that the organic ligands of the nanoparticles can be completely removed. Yet, due to the short (∼1.6 nm) interparticle distances within the layers, strong degradation and sintering effects are observed after hydrogen and nitrogen plasma treatments. In the case of oxygen plasma, the shape and size of the individual particles are unaffected and can be preserved, even if a short hydrogen plasma is subsequently applied to reduce the particles back to their metallic state. Nevertheless, the mesoscopic order of the particle arrays is slightly decreased as observed by the breakup of larger ordered areas into smaller domains forming island–trench structures. Probing the surface chemistry of the particles with temperature programmed desorption, a rather complex surface chemistry is found to result from the plasma treatments. The first TPD spectrum after the cleaning process with oxygen and subsequent hydrogen plasmas reveals that the particles are loaded with adsorbed and implanted hydrogen. After removal of this hydrogen, subsequent TPD spectra using CO as a probe molecule, show broad signals between 190 and 360 K pointing to nonmetallic surface properties. While the platinum was found to be completely reduced, XPS measurements reveal a remaining fraction of oxidic cobalt species which are enriched at the surface. Thus, although the structure of the close-packed Co–Pt nanoparticle arrays can be qualitatively preserved during plasma-based ligand removal, the treatment leads to a complex materials system the chemical properties of which are influenced by the particle components, the substrate, and the plasma media.
The magnetic properties of nanoparticles can be subject to strong variations as the chemical composition of the particle surface is modified. To study this interrelation of surface chemistry and magnestism, self-assembled layers of colloidal 9.5 nm Co/CoO core/shell nanoparticles were exposed to mild reactive hydrogen and oxygen plasmas. The consecutive oxygen/hydrogen plasma treatment transforms the particle layer into an array of metallic nanomagnets with complete reduction of the oxide and removal of the organic surfactants. The original arrangement of the particle array and the number of Co atoms per particle remains unchanged within the experimental error, and thus this is a possible route for the fabrication of ultrahigh-density magnetic bit structures from colloidal dispersions. The magnetic properties can be tuned by controlling the thickness of the surface oxide layer, which magnetically hardens the particles, as evidenced by element-specific magnetic hysteresis loops.
The stacking fault energy has been measured as a function of temperature between 20°C and 710°C in hexagonal and cubic Co, hexagonal 85% Co 15% Ni and cubic 67% Co 33% Ni. The so-called node method has been used for the calculation of the stacking fault energy. Possible sources of errors in the node method are discussed. It has been found that the stacking fault energy decreases with increasing temperature in the hexagonal materials and increases in the cubic materials and that the stacking fault energy is not zero at the temperature of the h.c.p. ai f.c.c. transformation but between 10 and 20 erg/cm2 in both the cubic and the hexagonal phase. The similarity between ε-phase formation in Cr-Ni-Fe alloys and the h.c.p. ai f.c.c. transformation in Co-Ni is pointed out.
Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called 'superparamagnetic limit' in recording media. Here we show that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems can provide an extra source of anisotropy, leading to magnetization stability. We demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.
This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.
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