Exchange-Bias and Grain-Surface Relaxations in Nanostructured NiO

Article pubs.acs.org/JPCC

Exchange-Bias and Grain-Surface Relaxations in Nanostructured NiO/ Ni Induced by a Particle Size Reduction Aleksandar Kremenović,*,† Boštjan Jančar,‡ Mira Ristić,§ Milica Vučinić-Vasić,∥ Jelena Rogan,⊥ Aleksandar Pačevski,† and Bratislav Antić# †

Faculty of Mining and Geology, University of Belgrade, Đušina 7, 11000 Belgrade, Serbia Jožef Štefan Institute, Jamova 39, 1000 Ljubljana, Slovenia § Division of Materials Chemistry, Ruđer Bošković Institute, POB 180, HR-10002 Zagreb, Croatia ∥ Faculty of Technical Sciences, University of Novi Sad, Trg D. Obradovića 6, 21000 Novi Sad, Serbia ⊥ Faculty of Technology and Metallurgy, University of Belgrade, POB 494, 11000 Belgrade, Serbia # Institute of Nuclear Sciences ”Vinča”, University of Belgrade, POB 522, 11001 Belgrade, Serbia ‡

ABSTRACT: Transition-metal-oxide/transition-metal nanocomposites, such as NiO/Ni, FeO/Fe, and CoO/Co, have been the subject of much recent investigation (i) because of their potential applications and (ii) because they are good model systems for studies of some effects on the nanoscale. They are used, for example, as catalysts, fuel-cell electrodes, magnetic memories, etc. When a nanocomposite is composed of both ferromagnetic (FM) and antiferromagnetic (AFM) nanoparticles, interesting physical properties can occur, such as the phenomenon of exchange bias (EB). A Ni/NiO nanocomposite obtained by the thermal decomposition of nickel(II) acetate tetrahydrate, Ni(CH3COO)2·4H2O, at 300 °C is composed of NiO (62%) and Ni (38%) with crystallite sizes of 11 and 278 nm, respectively. We observed an increase in the crystallite size for NiO and decrease of crystallite size for Ni, a decrease in the microstrain for both and an increase in the NiO phase content with thermal annealing in air, while highenergy ball milling leads to a decrease of the crystallite size, an increase in the size of the agglomerates, and microstrain as well as reduction, NiO → Ni. The lattice parameters of the nanosized NiO and Ni show a deviation from the value for the bulk counterparts as a consequence of crystallite size reduction and the grain-surface relaxation effect. The exchange bias found in a milled sample with particles of 10 nm (NiO) and 11 nm (Ni) disappears for larger particles as a consequence of a coupling-area decrease between the antiferromagnetic and ferromagnetic particles. Due to reduction/ oxidation (NiO ↔ Ni) and size as well as surface-relaxation effects the saturation magnetization value increases/decreases with milling/annealing, respectively. Having in mind the effect of size on the exchange bias, coercivity, and magnetization values, it is possible, by annealing/milling, to tailor the composition and particle size and then control the exchange bias and improve the other magnetic properties of the Ni/NiO.

1. INTRODUCTION Nanocomposites have attracted much attention recently owing to the synergistic properties induced by the interactions between different nanometer-scale objects. Nanocomposites could show fascinating magnetic, magneto-optical, and semiconducting properties that can be modulated by the interfacial interactions between the different nanocomponents. This opens up a new opportunity to develop advanced, multifunctional nanomaterials for device concepts and applications.1 In particular, transition-metal-oxide/transition-metal nanocomposites (such as NiO/Ni, FeO/Fe, and CoO/Co) have recently been investigated because of (i) their potential applications and (ii) they are good model systems for studies of some effects on the nanoscale. They are used in catalysts, fuel-cell electrodes, magnetic memories, etc.2,3 For nanocomposite Ni/NiO, Lee et al.4 demonstrated the successful © 2012 American Chemical Society

application of NiO-coated Ni nanoparticles for the magnetic separation of Histidin-tagged proteins. When a nanocomposite is composed of both ferromagnetic (FM) and antiferromagnetic (AFM) nanoparticles an interesting physical property can occur, for example, the exchange bias (EB) phenomenon. Such an EB is explained in terms of the exchange interactions between the FM and the AFM phases at their interface. The main indication of the existence of an EB is the shift of the hysteresis loop, HE, along the field axis after field cooling from above the Néel temperature, TN, of the AFM (and below the Curie temperature, TC of the FM) in materials composed of FM−AFM interfaces.5 Received: July 13, 2011 Revised: January 11, 2012 Published: January 17, 2012 4356

dx.doi.org/10.1021/jp206658v | J. Phys. Chem. C 2012, 116, 4356−4364

The Journal of Physical Chemistry C

Article

The EB in Ni/NiO has already been reported, e.g.,6−8 Del Bianco et al.9 investigated EB in Ni/NiO composites with a Ni content varying between 4 and 69 wt %. They showed the EB dependence on the Ni content and (micro)structural characteristics of both phases.9 From the magnetic point of view, NiO changes the physical properties with the particle size: bulk NiO is antiferromagnetic with a Néel temperature TN = 523 K, while the nanosized counterpart is superparamagnetic or superantiferromagnetic.10 Nickel is ferromagnetic with a TC of 627 K.11 Ni/NiO nanocomposites were obtained by different methods, e.g., milling NiO under H2 atmosphere,2 by microwave irradiation of two different nickel organic salts (acetate and formiate),12 mechanical alloying of Ni and NiO,7 and by the sol−gel route.13 In this study we used a simple method to form a Ni/NiO nanocomposite by the thermal decomposition of nickel(II) acetate tetrahydrate, Ni(CH3COO)2·4H2O. The aims were (a) an integrated study of the thermal decomposition process of nickel(II) acetate tetrahydrate, (b) an investigation of the influence of temperature and milling on the phase composition, structure/ microstructure and magnetic properties, and (c) studies of the size effect on grain-surface relaxation and EB.

KBr. Precautions were taken to eliminate the influence of moisture from the air on the samples. Electron microprobe analyses were obtained using a JEOL JSM−6610LV scanning electron microscope (SEM) connected with a INCA 350 energy-dispersion X-ray (EDS) analysis unit. Acceleration voltages of 30 kV and 20 kV were used for the images and the analyses, respectively. FE SEM images were recorded using a thermal field-emission scanning electron microscope, JSM-7000F, Jeol Ltd., Japan. The FE SEM was coupled with an energy-dispersive X-ray analyzer, EDS/INCA 350, Oxford Instruments, England. The samples inspected with the FE SEM were not coated with a conductive layer. Transmission electron micrographs and selected-area diffraction patterns were collected with a Jeol JEM 2100 transmission electron microscope operating at 200 kV. The samples were prepared by dispersing the powders in acetone and dropping the suspension on a lacey carbon film supported on a 300-mesh copper grid. The hysteresis loops were measured at 5 K after being zerofield-cooled (ZFC) and field-cooled (FC) for the selected sample using an MPMS XL-5 SQUID magnetometer. For the collection of the X-ray powder-diffraction (XRPD) data a Philips PW1050 (phase identification) and Bruker D8 DISCOVER (Rietveld and size-strain refinement) automated X-ray powder diffractometers were used. The Philips PW1050 diffractometer was equipped with a Cu-tube, Ni filter and a Xefiled proportional counter. The generator was setup at 40 kV and 32 mA. The divergence and receiving slits were 1° and 0.1 mm, respectively. The scanning range was 30−110° in 2θ, with a step of 0.05° and a scanning time of 30 s per step. The Bruker D8 DISCOVER diffractometer was equipped with a Cu tube, Ge primary beam monochromator (yielded strictly pure Cu Kα1 radiation), LYNXEYE PSD detector (3° opening). The generator was setup at 40 kV and 40 mA. The scanning range was 10−110° in 2θ, with a step of 0.02° and a scanning time of 3 s per step. A microstructure determination from the XRPD data is one of the most frequently used techniques. The microstructural effects that are responsible for the profile shape of the diffraction peaks are the finite size of the crystals or domains and the microstrain. The crystallite size and microstrain values of phases NiO and Ni in the samples were determined with the standard procedure for extracting the crystallite size and microstrain parameters from the diffraction data based on the integral breadth of the line profiles.14 The microstructural analysis was performed by considering both the size and strain effects as isotropic, by using the Fullprof computer program15 The instrumental broadening was fully characterized through the instrumental resolution function that was obtained using a standard specimen of LaB6.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The nanocomposite Ni/NiO was obtained by the thermal decomposition of nickel acetate tetrahydrate, Ni(CH3COO)2·4H2O (Alfa Aesar). The starting compound was annealed in air at 300 °C for 5 min and fast cooled to room temperature. The so-obtained sample, denoted as S300, was further annealed at different temperatures up to 800 °C. The samples annealed at 550 (S550) and 800 °C (S800) were selected in order to analyze the effects of thermal treatment on the structure, microstructure, magnetic properties and phase content ratio (NiO:Ni). The as-prepared sample S300 was milled for 1 h using a planetary ball mill (Fritsch Pulverisette 5). A hardened-steel vial of 500 cm3 volume, filled with 40 hardened steel balls with a diameter of 13.4 mm, was used as the milling medium. The mass of the powder was 20 g and the balls-to-powder mass ratio was 20:1. The milling was done in air atmosphere without any additives. The angular velocity of the supporting disk and vial was 32.2 and 40.3 rad s−1, respectively. The resulting sample, S300_HEBM, was compared with unmilled S300. 2.2. Experimental Methods. The thermogravimetric (TGA) and differential thermal (DTA) analyses were performed simultaneously (30−800 °C range) on a SDT Q600 TGA/DSC instrument (TA Instruments). The heating rates were 20 °C min−1 and the sample mass was less than 10 mg. The furnace atmosphere consisted of air at a flow rate of 100 cm3 min−1. The particle (agglomerate) size distribution was determined using a Malvern Mastersizer 2000 Particle Size Analyzer, capable of analyzing particles between 0.01 and 2000 μm. A microprecision wet-dispersion unit, Hydroμp, was used. The measurement parameters were: pump speed = 2500 rpm; ultrasonic = on. FT-IR spectra were recorded at RT using a Perkin-Elmer spectrometer, model 2000. The FT-IR spectrometer was coupled to a personal computer loaded with the IRDM (IR Data Manager) program to process the recorded spectra. The samples were pressed into discs using spectroscopically pure

3. RESULTS AND DISCUSSION 3.1.1. Thermal Decomposition of Nickel(II) Acetate Tetrahydrate. TGA/DTA Analysis. The thermal decomposition of nickel(II) acetate tetrahydrate occurs in two steps: I, the dehydration of the Ni(CH3COO)2·4H2O and the partial hydrolysis of the acetate groups to the basic nickel acetate with general formula (1 − x)Ni(CH3COO)2·xNi(OH)2, and II, the further decomposition of the dehydrated intermediate generating solid products such as Ni, NiO or a mixture of Ni and NiO. The range of temperatures of such thermal events and the 4357

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resulting solid products may depend on the atmosphere, the heating rate and the origin of the powder.16,17 The TGA and DTA curves for Ni(CH3COO)2·4H2O are presented in Figure 1. The DTA curve shows an endothermic

Based on the TGA/DTA and XRPD results, the reactions occurring during the thermal decomposition of Ni(CH3COO)2·4H2O are as follows: Step I, partial hydrolysis (up to 294 °C) Ni(CH3COO)2 · 4H2O(s) = 0.82Ni(CH3COO)2 · 0.18Ni(OH)2 (s) + 3.64H2O(g) + 0.36CH3COOH(g)

and step II, oxidation (294−381 °C) 0.82Ni(CH3COO)2 · 0.18Ni(OH)2 (s) + xO2(g) = 0.28Ni(s) + 0.72NiO(s) + product(g)

where x depends on gaseous products, which can be very different.16 3.1.2. XRPD Analysis. The process of the thermal decomposition of nickel(II) acetate tetrahydrate was monitored by XRPD. The starting compound was annealed at different temperatures in air for 5 min and each resulting sample was analyzed using the XRPD technique. Figure 2 shows the Figure 1. TG and DTA curves for nickel(II) acetate tetrahydrate.

peak at 113 °C and an exothermic peak at 380 °C. The endothermic DTA peak is due to the dehydration and the hydrolysis up to about 170 °C, with the weight loss found to be 33.0%. This process continues up to 294 °C in a subsequent, almost horizontal step with an additional 2.0% weight loss. The overall weight loss of 35.0% up to 294 °C is much larger than the theoretical value for four water molecules (about 29%). Therefore it should be ascribed to the hydrolysis of the acetate groups during the dehydration, resulting in the simultaneous evolution of acetic acid to the gas phase. According to this result, the formula of the obtained intermediary solid phase is 0.82Ni(CH3COO)2·0.18Ni(OH)2. This formula is very similar to the formula 0.86Ni(CH3COO)2·0.14Ni(OH)2 reported by De Jesus et al.16 The second decomposition step corresponds to the major decomposition of the dehydrated intermediate and to the exothermic DTA peak (Figure 1), which is most probably the combustion of organic residues in the air. The weight loss found (71.8%) up to 381 °C is consistent with the value expected for the formation of a mixture of Ni and NiO (about 72%). After these fragmentations at temperatures up to 800 °C, no thermal effects were observed in the DTA curve (Figure 1). The TGA and DTA curves show that the temperature of 381 °C (Figure 1) corresponds to a complete decomposition of nickel(II) acetate tetrahydrate with a residual mass of about 28%, which was due to the mixture of Ni and NiO. This mixture was also verified by the XRPD patterns (for details, see section 3.1.3). In conclusion, the partial hydrolysis of Ni(CH3COO)2·4H2O can be described by the general equation:

Figure 2. (a) Thermal degradation of nickel(II) acetate tetrahydrate monitored by XRPD (see text). Diffraction patterns for nickel(II) acetate tetrahydrate at ambient temperature (at the bottom) and after annealing (main panel) and after the formation of the nanostructured Ni/NiO (the inset). (b) Change in sample color after selected temperatures.

process of the thermal decomposition of nickel(II) acetate tetrahydrate and the formation of the Ni/NiO composite, as well as the process of the partial oxidation of nickel. The X-ray data were recorded for lowest temperatures until the complete decomposition of nickel acetate. Evidently, only slight change of intensity ration of main peaks in diffraction patterns of nickel acetate could be observed before structure collapse. It is possible that basic nickel acetate is formed as amorphous like phase. The inset of Figure 2 shows the process of nickel oxidation in air after the formation of Ni/NiO at 300 °C. The phase evolution of the Ni/NiO composite was studied by

Ni(CH3COO)2 · 4H2O(s) = (1 − x)Ni(CH3COO)2 ·x Ni(OH)2 (s) + (4 − 2x)H2O(g) + 2xCH3COOH(g) 4358

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Rietveld refinement using the Fullprof computer program15 Details of the refinement procedure are given in section 3.3. Table 1 shows the percentage of Ni and NiO in the Ni/NiO composite. At 300 °C there is 38(1) % Ni, but with an increase Table 1. Crystallite Size (εβ), Strain (e), and Percentage of Each Phase (wt %) for Ni/NiO NiO

Ni

sample

εβ [nm]

e·104

%

εβ [nm]

e·104

%

S300 S550 S800 S300_HEBM

11 67 166 10

27 12 8 43

62(1) 67(1) 94(2) 48(2)

278 254 185 11

10 6 5 47

38(1) 33(1) 6(2) 52(2)

of the temperature due to the oxidation of nickel, the NiO content increases up to 94(2) % at 800 °C. We made an interesting observation in the milled sample previously prepared by the decomposition of Ni(CH3COO)2·4H2O at 300 °C, i.e., the phase content ratio NiO:Ni was found to be 48(2):52(2). It is worth mentioning that the inverse process, the reduction of NiO into Ni with the formation of the Ni/NiO composite, was achieved in different ways, e.g., by mechanical milling under a H2 atmosphere.2,12 The initial microstructure of the NiO powder is shown to play an important role in the reduction rate and in the final microstructure of the nanocomposites. Already-published results indicate that the mechanically induced defects have a strong influence on the kinetics of the reduction process.12 Furthermore, it should be noted that the color of the sample was dark-green after the thermal treatment, but before the thermal treatment the sample color was light-green (see Figure 2b). 3.1.3. FTIR Analysis. To better understand the possibilities of the obtained nanosize composites by high-energy ball milling, the compound obtained after the exposure of the starting material at 240 °C (NiAc-240) was milled for 1 h. The influence of the milling was studied with the aid of the FTIR spectra. The FTIR spectrum of the milled NiAc-240 (mNiAc-240) is similar to the spectra of the starting NiAc-240 sample and no significant difference could be obtained (Figure 3). As we expected, the corresponding bands of the starting materials in relation to the milled ones are always stronger and better defined. The only apparent difference is the existence of a strong band at 3478 cm−1 for NiAc-240. This band arose from the ν(O−H) stretching vibrations and confirmed the intermediate basic nickel acetate up to 240 °C due to the dehydration and the hydrolysis of acetate groups in the first step of the thermal decomposition. The formation of basic nickel acetate up to 170 °C was also confirmed by the TG analysis. The presence of H 2O molecules caused the appearance of a strong and broad ν(O−H) stretching band centered at about 3500−3000 and 3300−3000 cm−1 in the FTIR spectra of mNiAc-240 and NiAc-240, respectively. The shape and position of these bands indicate that both samples did not dehydrate totally, but the hydrolysis had started only in the case of NiAc-240. The reason for such behavior is the fact that milling inhibits the hydrolysis. Additionally, two very strong and broad bands in the 1600− 1360 cm−1 region are known as asymmetric, νas(COO), and symmetric, νs(COO), stretching vibrations of the acetato groups.18 The comparison of their difference, Δν, with the value for a “purely ionic” salt, NaCH3COO, (Δνi = 164 cm−1)

Figure 3. FTIR spectra for nickel(II) acetate tetrahydrate after annealing at 240 °C NiAc-240, and after its milling, mNiAc-240.

indicates the nature of the carboxylate coordination.19 It is evident that the Δν values for mNiAc-240 and NiAc-240 are smaller than Δνi: 149 cm−1 [νas(COO) = 1569 cm−1 and νs(COO) = 1420 cm−1] and 131 cm−1 [νas(COO) = 1551 cm−1 and νs(COO) = 1420 cm−1], respectively. Such behavior is in accordance with the presence of chelating COO groups in the mNiAc-240 and NiAc-240. The very similar intensity and the shape of the COO bands also support this conclusion. Characteristic C−H deformational vibrations in the range 750−700 cm−1 were observed for both samples. 3.2. Morphology, Microstructure, and Agglomeration. We investigated the samples with SEM and FE SEM at different magnifications. The characteristic FE SEM images of the investigated samples S300 (prepared at 300 °C) and milled S300_HEBM (after high-energy ball milling) are presented in Figure 4a,b. At low magnification the FE SEM showed particles in the form of aggregates of irregular shape in the micrometer size range (not shown). The size of these aggregates increases with the temperature of the thermal treatment. At higher magnification it can be seen that they are built from much smaller particles. The size of these smaller particles varies from 15 to 20 nm. Consequently, the bigger particles are in fact secondary particles, built up from smaller, pseudospherical primary particles. A smaller number of larger secondary particles, up to 500 nm, are also visible. If we compare the FE SEM images of the samples S300 and S300_HEBM it seems that the particles are approximately the same in size, but the aggregation is more pronounced in the milling process. As the XRPD analysis showed, the thermal decomposition products contain two kinds of particles, NiO and Ni, which are of significantly different sizes (for details, see section 3.3). It should be noted that the FE SEM cannot distinguish these two kinds of particles with any certainty, i.e., it is not possible to distinguish between the secondary NiO particles and the larger primary particles of metallic nickel. The elemental composition of the samples was additionally determined by EDS analyses at different positions on the samples. However, the EDS analyses obviously show the increase in the NiO content as the temperature of the thermal treatment increases from 300 to 800 °C, which can be related to the XRPD results. 4359

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consist of many crystallites or particles, and thus are many orders of magnitude larger than the monocrystalline and polycrystalline particles. One of the PSD parameters, named d(0.5)n.b., indicates that 50% the particles (agglomerates) measured on the number basis were smaller than, or equal to, the size stated. For sample S300 this value was ∼280 nm, approximately the same as the average crystallite size of the Ni obtained from the XRPD data (the average crystallite size is given in Table 1). As can be seen from Figure 4c, the particlesize distribution curve for S300 was monodispersed, with the most common value of the frequency number distribution curve (mode) being 252 nm. An increase in the degree of agglomeration caused by the milling is evident: the distribution curve for the milled sample S300_HEBM was shifted toward higher values and the fraction of agglomerates greater than 1 μm increased from 2.2% (for S300) to 17.1% for S300_HEBM. As a result of this agglomeration, the formation of three size fraction is evident. The mode values of these fractions were 279 nm, 365 and 489 nm, in increasing order. The increase in the degree of agglomeration caused by the milling was evaluated quantitatively using the ratio between the number-weighed means: D[1,0]S300_HEBM/D[1,0]S300 = 2.08. The significant agglomeration in all the samples (especially the milled samples) was confirmed by the bright-field TEM (transmission electron microscopy) images (Figure 5a,b). The selected-area electron diffraction (SAED) pattern collected from agglomerate (Figure 5a) revealed the presence of both Ni and NiO nanoparticles. The experimental and simulated patterns of the Ni and NiO, calculated using the data obtained from ICSD #162279 and #87108, respectively, are shown in Figure 5c. Apart from the composite agglomerates containing both Ni and NiO, separate agglomerates containing either mostly Ni or mostly NiO were found. Furthermore, carbon traces in the form of graphite sheets (Figure 6) were observed among the particles within the agglomerate, which we suggest were formed during the decomposition of the nickel(II) acetate tetrahydrate. 3.3. Structure and Microstructure Analysis: Rietveld and XRPD Line-Broadening Analysis. The collected XRPD data were used to refine the crystal structure and microstructure of the samples using the Fullprof program.15 The diffraction patterns of the samples produced after the thermal treatment at 300 °C, 550 °C and 800 °C as well as the milled sample showed the presence of the NiO and Ni phases only (Figure 2). A two-phases structural model, NiO S.G. Fm3m,20 and Ni S.G. Fm3m,21 was used for the crystal-structure refinement. The Rietveld refinement results are shown in Figure 7 for the S300 and S300_HEBM. The microstructure analysis results are given in Table 1. It is clear that the crystallite size effect is the main source of the peak broadening. However, a non-negligible microstrain is present. Interestingly, it was found that the decomposition temperature did not influence the crystallite size of both phases to the same extent. The difference between the diffraction peaks’ breadth for the NiO and Ni is clearly evident for the S300 sample’s diffraction pattern (Figure 7a). The values of the half width at half maxima (HWHM) are greater for the NiO than for the Ni. The diffraction peak’s HWHM difference is a consequence of the different microstructure of the NiO and Ni. The lower decomposition temperatures resulted in the largest difference between the crystallite sizes of the NiO and Ni: the ratio between the crystallite sizes of the phases (εβNi/εβNiO) decreased from approximately 25 to approximately 1 with an

Figure 4. FE SEM images of S300 (a), S300_HEBM (b), and aggregate distribution (c) (see text).

The particle size analyses, performed with the Mastersizer 2000, gave the particle volume percentage in 100 discrete size ranges between 0.02 and 2000 μm. However, any comparison between different particle-sizing techniques is difficult, and sometimes meaningless. The electron microscopy results could, fortunately, be compared with a number based on the results obtained from the laser diffraction. For this reason the results were, after the measurement, recalculated as the particle number percentage. The number-based results of the particle size distribution (PSD) analysis are shown in Figure 4c. This number-based PSD showed that the number fraction of particles (agglomerates) smaller than 0.2 μm is negligible (