Size Effects on Properties of NiO Nanoparticles ... - ACS Publications

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Size Effects on Properties of NiO Nanoparticles Grown in Alkalisalts W. J. Duan,† S. H. Lu,†,‡ Z. L. Wu,§ and Y. S. Wang*,† †

Department of Physics, Beijing Normal University, Beijing 100875, China Department of Security and Prevention, Chinese People’s Public Security University, Beijing 102614, China § Analytic and Testing Center, Beijing Normal University, Beijing 100875, China ‡

ABSTRACT: NiO nanoparticles with sizes of 3.5−12.4 nm were grown by thermal decomposing of nickel acetate at different temperatures in NaCl and Li2CO3 alkalisalts. The properties of the nanoparticles were characterized by X-ray diffraction spectrometer, transmission electron microscope, absorption spectrometer, micro-Raman microscope, and superconducting quantum interference device. The effects of the nanoparticle sizes on the crystal structure, exciton ground state energy, vibration modes, and magnetic properties were studied. Lattice parameter of NiO increases with a decrease in nanoparticle sizes. The band gap of NiO nanoparticles increases with a decrease in the nanoparticle size. LO modes of NiO nanoparticles shift red, and the intensity increases with a decrease in the nanoparticle sizes. Surface phonon modes are observed. Bifurcation temperature and blocking temperature of NiO nanoparticles shift to lower temperature with a decrease in nanoparticle sizes. Two peaks are present in all nanoparticles’ zero-field-cooled magnetization curves, and the saturation magnetization, remanet magnetization, and coercivity increase with a decrease in the nanoparticle sizes. The nanoparticles exhibit size-dependent anomalous magnetic properties that make the remagnetization curve surpass the initial magnetization curve in the M−H hysteresis curves taken at 5 K.

1. INTRODUCTION Nanomaterials with finite sizes usually exhibit a number of unique properties, which may strongly differ from those observed in bulk materials.1 Effects of size on crystal structure, vibration modes, and magnetic properties of NiO have attracted much attention due to its applications in catalysis, battery cathodes, antiferromagnetic layers, p-type transparent conducting films, and electrochromic films.2−4 Bulk nickel oxide (NiO) has a cubic structure above its N’eel temperature (TN, 523 K) and transforms from cubic to rhombohedral structure below TN.5 Anyhow, both cubic and rhombohedral structures of NiO nanoparticles with different sizes were reported by different groups.6−14 Recently, Aleksandar et al.15 observed a structural evolution of Ni0.9Zn0.1O nanoparticles from rhombohedral structure symmetry toward cubic symmetry when the nanoparticles were annealed. They also observed a deviation of the lattice parameters of the nanosized NiO from that of bulk NiO as a consequence of crystallite size reduction.16 It is obvious that the structure of nanoparticles is sensitive to their sizes. Raman vibrations of phonons and magnons reflect the existence of the finite size defects and nickel vacancies and help to identify the superexchange mechanism associated with the short-range magnetic interactions.17 One-phonon vibration and two-magnon vibration of 13 nm NiO nanoparticles12 and 32 nm nanowalls17 had been studied. Anyhow, there are seldom reports on vibration of NiO nanoparticles smaller than 10 nm. Otherwise, the effects of sizes and surface states on the magnetic properties of NiO nanoparticles have also attracted much attention in recent decades due to the observation of © 2012 American Chemical Society

anomalous magnetic properties such as magnetic enhancement, large coercive, spin glass freezing, and unusual hysteretic behavior.18−22 Several models such as two-sublattice model, multisublattice model,23 and core/shell model11,24−26 have been used to explain the anomalous properties of the nanoparticles. However, size effects and surface effects on the magnetic properties of NiO reported by different groups are inconsistent, even controversy. Recently, Aragon et al.11 observed the presence of two peaks at 160 K and around 7 K in the zero-field-cooled magnetization curve for 22 nm NiO nanoparticles. The blocking process around 160 K was attributed to the thermal relaxation of uncompensated spins in the particle cores and the peak at 7 K to the freezing of surface spin clusters. Ghosh et al.20 observed an increase in the surface spin freezing temperature from 10 to 15 K as the particle diameters increased from 3 to 7 nm. However, Winkler et al.6 observed the surface spin freezing temperature of 17 K in 3 nm NiO nanoparticles, which was much higher than that reported by Ghosh et al.20 They also observed a red shift of freezing temperature as the nanoparticles were dispersed in a polymer.6 Similar shift of surface spin freezing temperature in 5.8 nm NiO dispersed by polyvinyl-pyrrolidone was reported.21 Obviously, the dangling bonds at the surface affect the process of surface spin freezing. The results from different groups are inconsistent, and it is difficult to compare due to the differences Received: August 14, 2012 Revised: November 20, 2012 Published: November 20, 2012 26043

dx.doi.org/10.1021/jp308073c | J. Phys. Chem. C 2012, 116, 26043−26051

The Journal of Physical Chemistry C

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in defects and the surface states of the studied nanoparticles. Recently, Tadic et al.22 embedded 4 nm NiO nanoparticles in silica matrix and observed an anomalous hysteresis behavior that the initial magnetization curve lay below the hysteresis loop at 2 and 5 K for a certain field range. Similar anomalous hysteresis behaviors were also observed in a granular film of αFe2O3 nanoparticles in amorphous alumina matrix27 and (Fe0.26Ni0.74)50B50 nanoparticles,28 which were attributed to a strong increase of the local surface anisotropy and interparticle interactions. Anyhow, α-Fe2O3 is not canonical antiferromagnetic, and (Fe0.26Ni0.74)50B50 is ferromagnetic, while NiO is antiferromagnetic. Furthermore, there are seldom reports on the evolution of the anomalous hysteresis behaviors with the nanostructure sizes. The origins of the anomalous hysteresis behaviors of NiO nanoparticles need to be investigated further. Otherwise, NiO is a wide band gap semiconductor with band gap of 3.6 eV29 and attracts much attention. There are seldom reports on variation of the band gap of NiO nanoparticles. In order to systematically explore the size effects on the properties of NiO nanoparticles, nanoparticles with different sizes were grown at different temperatures in alkalisalts. The crystal structure, absorption properties, vibration modes, and magnetic properties of NiO nanoparticles were analyzed. Size effects on those properties were investigated. Figure 1. XRD patterns of NiO nanoparticles grown at different temperatures along with the calculated patterns from the Rietveld refinements. The difference patterns are also shown below the observed patterns.

2. EXPERIMENTAL SECTION Mixtures of 0.5 g of Ni(CH3COO)2·4H2O, 0.5 g of Li2CO3, and 10 g of NaCl were grinded sufficiently for 2 h. Then, the mixtures were decomposed in air at 483, 523, 563, 603, and 643 K for 2 h, respectively. Subsequently, the obtained mixtures were washed thoroughly with deionized water several times, and NiO nanoparticles were prepared. The structure and size of the nanoparticles were characterized by X-ray diffraction (XRD) on a Shimadzu XRD-6000 with Cu Kα radiation. Continuous scan data were collected every 0.02° in the 2θ angular range of 10−80°. The structure and morphology of the nanoparticles were observed by a high-resolution transmission electron microscope (TecnaiG 220 S-TWIN). Absorption spectra were measured by dispersing NiO nanoparticles in deionized water on a UV1900 spectrometer. Raman spectra of the nanoparticles were acquired at room temperature from a Jobin−Yvon micro-Raman spectrometer using a 325 nm He− Cd laser as an excitation source. The laser beam was focused on a 1 μm spot, using a 40× objective. Magnetization measurements were performed on a superconducting quantum interference device (SQUID). The nanoparticles for measurement were put into a polymeric capsule. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were measured from 370 to 2 K in a field of 100 Oe. M−H hysteresis curves were measured at 5 K and a temperature just below the blocking temperature.

been reported in refs 6−14. The angles and relative intensity of the diffraction peaks from cubic and rhombohedral structures are very close. To determine the nanoparticle structure and investigate the lattice variation with the nanoparticle sizes, the crystal structure refinement was performed for nanoparticles using the Rietveld method with the GSAS program30 based on Fm3̅m and R3̅m models separately. The peak profiles were modeled using the Thompson−Cox−Hastings modified pseudo-Voigt (TCH-pV) function. The background was modeled by function of shifted Chebyshev polynomials. The fits based on R3̅m model are superior to that based on Fm3̅m model. The fits based on R3̅m model are shown in Figure 1. It can be seen that the theoretic fits are consistent with the experimental results. Diffraction peaks around 37.3°, 43.4°, 63.0°, 75.3°, and 79.3° can be indexed as diffraction from (111), (200), (220), (311), and (222) planes of NiO with rhombohedral structure, respectively. The sizes of the nanoparticles derived from the (111) plane diffraction using Scherrer’s formula are 3.5, 4.8, 5.6, 10.3, and 12.4 nm for nanoparticles grown at 483, 523, 563, 603, and 643 K, respectively. Nanoparticle sizes increase with an increase in the growth temperatures. The lattice parameters of the nanoparticles grown at different temperatures are extracted from the refined XRD data. Variation of lattice parameters with the nanoparticle sizes is shown in Figure 2. The lattice parameter increases with a decrease in nanoparticle sizes. This is similar to that observed in cubic NiO nanoparticles.20 The lattice parameters in ref 20 are also shown in Figure 2 for comparison. The lattice constant increases almost linearly with a decrease in the nanoparticle size. In our work, the lattice constant increases slowly as the nanoparticle size is larger, but it increases dramatically as the nanoparticle size is smaller. The reason causing the difference is unclear.

3. RESULTS AND DISCUSSION 3.1. Nanoparticle Structure and Exciton Ground State Energy. XRD patterns of the nanoparticles grown at different temperatures are shown in Figure 1. Diffraction peaks are observed around 37.3°, 43.4°, 63.0°, 75.3°, and 79.3° in the nanoparticles grown at temperatures from 523 to 643 K. However, additional peaks at 19.1°, 33.05°, and 38.3° are observed in the nanoparticles grown at 483 K, which means that Ni(CH3COO)2·4H2O decomposed incompletely at 483 K. Both cubic and rhombohedral structured nanoparticles have 26044



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have good crystallization quality. Size distributions for nanoparticles grown at 483, 523, 563, and 643 K are shown in Figure 4 by counting more than three hundred particles from TEM

Figure 2. Variation of lattice parameters with the nanoparticle sizes in our work (○) and in ref 20 (■).

Figure 3a−d shows TEM images of NiO nanoparticles grown at different temperatures. The morphology of NiO nano-

Figure 4. Size distributions of NiO nanoparticles grown at different temperatures: (a) 483, (b) 523, (c) 563, and (d) 643 K. dav represents the average diameter, and σ represents the standard deviation of nanoparticle size.

images. The average diameter (dav) and standard deviation (σ) of NiO nanoparticles were calculated and are also shown in Figure 4. The average diameters of NiO nanoparticles grown at 483, 523, 563, 603, and 643 K are 3.5, 4.6, 5.5, 10.1 (not shown), and 12.4 nm, respectively, which are consistent with that estimated from the X-ray diffraction patterns. The size distributions are narrow for nanoparticles grown at 483, 523, 563, and 603 K, the standard deviations are 0.4, 0.6, 0.7, and 0.8 nm, respectively. However, the size distribution is broad for nanoparticles grown at 643 K; the standard deviation is 2.9 nm. Absorption spectra of NiO nanoparticles are shown in Figure 5. There is a strong absorption around 300−320 nm and a weak absorption around 387 nm. The absorption around 387 nm is more obvious in NiO nanoparticles with smaller sizes. It is ascribed to the band transition of Ni2+ from 3A2g to 3T1g(G).31 The electric dipole d−d transition is forbidden according to Laported’s rule, but because of spin−orbital interaction and phonon effects, it is weakly allowed.31−33 The strong absorption around 300−320 nm is attributed to NiO exciton transition. It can be seen that there is a continued red shift for the absorption edge with an increase in the nanoparticle sizes. Red shift of the nanoparticle absorption edge could be attributed to the quantum size effect. For a direct type of a semiconductor, the optical band gap Eg could be derived from the optical absorption spectra using Tauc’s relationship34

Figure 3. TEM images of NiO nanoparticles grown at different temperatures: (a) 483, (b) 523, (c) 563, and (d) 643 K. (e) HRTEM image of NiO nanoparticles grown at 643 K.

particles grown at 483 K is not very spherical, but the lattices of the nanoparticles are seen clearly, which indicates that the nanoparticles are single crystal. The nanoparticles grown at 523, 563, and 643 K are approximately spherical in shape. Figure 3e is a typical HRTEM image of NiO nanoparticles grown at 643 K; the lattice of the nanoparticle is seen clearly. The space between the adjacent lattice planes in the marked nanoparticle is about 2.09 Å, which is equal to the distance between two (100) planes of NiO in rhombohedral phase. The nanoparticles

αhυ = A(hv − Eg )1/2

(1)

where α is the absorption coefficient, hv is the photon energy, and A is a constant. The typical variation of (αhv)2 versus hv for NiO nanoparticle with a size of 10.1 ± 0.8 nm is shown in Figure 5b. By fitting the linear part of the plot and extrapolating the linear region on the energy axis, the optical band gap or the exciton ground state energy of NiO nanocrystals could be derived, and it is 3.62 eV for the NiO nanoparticle with a size of 26045



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predicted by Kayanuma is also shown in Figure 5c. It can be seen that exciton ground state energy derived by extrapolating the absorption is much smaller than that predicted by Kayanuma mode. The Kayanuma mode usually overestimates the exciton ground state energy of the nanoparticles.38 Anyhow, the variation tendency of the exciton ground state energy of the nanoparticles with the sizes derived from absorption spectra is close to that predicted by Kayanuma mode when the nanoparticle size is smaller. Dependence of the exciton ground state energy on the size in the whole range is simulated as follows E = 3.613 + 2.59e−1.08r

(5)

The nanoparticle exciton ground state energy decreases exponentially with the increasing of the nanoparticle radius. 3.2. Size Effects on the Vibration of NiO Nanoparticles. Figure 6a shows the Raman spectra of NiO

Figure 5. (a) Absorption spectra of NiO nanoparticles with different sizes: (I) 3.5 ± 0.4, (II) 4.6 ± 0.6, (III) 5.5 ± 0.7, (IV) 10.1 ± 0.8, and (V) 12.4 ± 2.9 nm. (b) Variation of (αhv)2 versus hv for 10.1 ± 0.8 nm NiO nanoparticle. (c) Dependence of exciton ground state energy on nanoparticle sizes.

10.1 ± 0.8 nm. The exciton ground state energy of NiO nanoparticles with other sizes are also derived and shown in Figure 5c. It can be seen that the exciton ground state energy decreases with an increase in nanoparticle size. Dependence of the exciton ground state energy on the sizes of nanoparticles had been predicted by Kayanuma35 with the effective mass approximation. In strong confinement (where R < 2aB) ⎛ a ⎞2 E = Eg + π 2⎜ B ⎟ R y* − 0.248R y* ⎝R⎠

Figure 6. (a) Raman spectra of NiO nanoparticles with different sizes collected with a laser power of 0.38 mW. (b) Variation of 1LO frequency with the nanoparticle radius.

nanoparticles with different sizes. Measurements were performed with an excitation laser power of 0.38 mW. Vibration modes at 400−440, 550−580, 723, 900, 1080− 1140, and 1595 cm−1 are observed. The vibration at 1595 cm−1 is related to carbon phase resulted from the decomposition of nickel acetate.39 Vibration modes around 400−440, 550−580, 723, 900, and 1080−1140 cm−1 had also been reported in the literature and were attributed to 1TO, 1LO, 2TO, TO + LO, and 2LO modes of NiO separately.12,17,40,41 Two-magnon band (2M) at 1400−1500 cm−1 was reported in refs 12, 17, 41, and 42, but it is not observed in this work. 2M vibration is associated with Ni2+−O2−−Ni2+ superexchange interaction.43 Gandhi et al.17 observed a decrease of 2M intensity in NiO nanowalls compared with that of bulk NiO. Mironova-Ulmane et al.41 observed a dramatic decrease in 2M intensity of nanopowders prepared by evaporation of the coarse grained NiO powder, and the 2M vibration became undetectable for 100 nm crystallites at room temperature. They attributed the

(2)

where E is the exciton ground state energy of the nanocrystal, Eg is bulk band gap, R is the radius of the nanoparticle, aB is Bohr radius, and R*y is effective Rydberg energy. aB and R*y are calculated from the relationships ⎛ 1 1 ⎞ ⎟ aB = h2ε /e 2⎜ + mh* ⎠ ⎝ me* R y* =

2e 4π 2 ⎛ 1 1 ⎞ ⎟ ⎜ + 2 2 mh* ⎠ ε h ⎝ me*

(3)

(4)

where ε (11.9 for NiO) is the bulk dielectric constant, me* and m*h are, respectively, the effective mass of a conduction band electron and a valence band hole, which are 0.11 mo and 0.32 mo, respectively, for NiO.37 The dependence of the lowest exciton state energy on the radius of the nanoparticles 36

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ascribed the Raman shift to the size-induced phonon confinement effect and surface relaxation. When the nanoparticle size is comparable to the exciton diameter of the semiconductor, the size confinement effect leads to a rapid decrease of LO frequency.47 Bohr radius of NiO derived from eq 4 is 7.7 nm. The lattice constant of NiO nanoparticles shown in Figure 2 expands dramatically as the nanoparticle size decreases from 5.5 nm to smaller. This also demonstrates that structure relaxation of the nanoparticle is more obvious as the nanoparticle size is much smaller than Bohr radius. The dramatic decrease of 1LO frequency in Figure 6b can be ascribed to size confinement and surface relaxation effects. Everall et al.49 observed a red shift of Raman frequency induced by the probe laser beam heating. Otherwise, laser heating would anneal out point defects such as vacancies in the nanoparticles. To avoid the probe beam heating effects on the Raman spectra, the laser power for collecting the spectra in Figure 6a was kept as low as possible to 0.38 mW. To see the heating effects, Raman spectra were collected first with an increase in probe laser power from 0.038 to 3.8 mW, then with a decreasing procedure of probe laser power at the same point of the sample. The spectra are shown in Figure 7. As the laser

decrease of 2M intensity to the decrease of antiferromagnetic spin correlations. However, they observed 2M vibration in 13 nm NiO nanoparticles prepared by precipitation method and annealed at 623 K.12 Recently, Cazzanelli et al.42 observed that 2M scattering decreased strongly upon dilution with magnesium ions due to the lowering of local symmetry at the Ni2+ sites. Obviously, both the decrease of spin correlation length resulted by small size and disorder induced by defects in nanoparticles would weaken the 2 M intensity of NiO nanoparticles. It is difficult to distinguish the size and defect effects in the particles in Figure 6. It can be seen from Figure 6 that the intensity of the band at 550−580 cm−1 increases and the intensity at 1080−1140 cm−1 decreases with a decrease in the nanoparticle sizes. This is similar to that observed in ref 17, but the bands are asymmetrical and the shapes vary with the particle sizes. The bands are fitted by Lorentz function. For NiO nanoparticles with a size of 12.4 ± 2.9 nm, the band at 550−580 cm−1 is composed of two peaks at 547 and 579 cm−1; the band at 1080−1140 cm−1 is also composed of two peaks at 1076 and 1137 cm−1. Surface vibration at 541 cm−1 in NiO was reported.44 The lowest frequency of the surface mode for a spherical nanoparticle can be calculated using the dielectric continuum model with the equation45 ωso1 = ωTO

ε0 + 2εM ε∞ + 2εM

(6)

where ωTO is the frequency of transverse optical mode, ε0 and ε∞ are the static and high-frequency dielectric constants of the nanoparticles, and εM is the host medium dielectric constant. ωTO = 405 cm−1, ε∞ = 5.7,44 ε0 = 11.9,36 and εM = 1 for air-host medium dielectric constants are used. A vibration at 547 cm−1 derived from Figure 6a for the 12.4 ± 2.9 nm NiO nanoparticle is very close to the calculated ωSO1 (544 cm−1). The vibrations at 547 and 1076 cm−1 can be ascribed to the first and secondordered surface vibration modes. Although 1LO scattering is not expected for perfect cubic and rhombohedral structured NiO,15 it was observed separately by different groups at 560− 571 cm−1.17,40,41,46 The appearance of 1LO is attributed to the disorder introduced by defects41 and scattering from the extended region other than the Brillouin zone center due to the damage of transmission symmetry in nanostructures.47 Vibrations at 579 and 1137 cm−1 in Figure 6 can be ascribed to vibrations of 1LO and 2LO modes. It also can be seen from Figure 6a that the relative intensity of the first- and secondorder SO modes respective to those of 1LO and 2LO modes increases with a decrease in nanoparticle sizes. Moreover, the relative intensity of 1LO to 2LO decreases with an increase in the nanoparticle sizes. The appearance of SO modes and enhancement of 1LO vibration mode in smaller nanoparticles suggest that the nanoparticles are imperfect. This is consistent with the diminishing of the 2M vibration. Except from the dependence of the vibration intensity on the nanoparticle sizes, the frequencies of the 1LO and 2LO modes have a red shift with a decrease in the nanoparticle sizes. To see the variation clearly, dependence of one-phonon LO frequency on the nanoparticle size is shown in Figure 6b. As the average nanoparticle radius decreases from 6.2 to 2.8 nm, the LO vibration frequency decreases slowly. With a further decrease in the nanoparticle size, the vibration frequency decreases dramatically. Red shift of Raman vibration with a decrease in the nanostructure sizes has been reported in many references and theoretically studied by Yang and Shi et al.47,48 Yang et al.

Figure 7. Raman spectra for NiO nanoparticles with a size of 5.5 ± 0.7 nm collected under various excitation laser power, the measurements were in a sequence that the excitation laser power first increased from 0.038 to 3.8 mW (solid lines) and then decreased (dotted lines).

power is less than 1.9 mW, the line shape and vibration frequencies are unchangeable, and the heating effects can be ignored. When the exaction laser power exceeds 1.9 mW, the vibration peaks broaden and shift red slightly. After exposure to high power probe laser, the vibration modes shift blue, 2TO, TO + LO, and 2LO vibrations enhance even for the spectra collected with weaker laser power. 2M vibration is still not observed in nanoparticles after exposure to high power laser. Defects can induce red shift of phonon vibrations.47,50 Blue shift and intensity enhancement of 2LO mode after the nanoparticles are exposed to the high power laser suggest that there are defects in the as-grown nanoparticles, and exposing the nanoparticles to higher power laser would anneal out some defects. 3.3. Magnetic Properties of NiO Nanoparticles. Figure 8 shows the temperature dependence of the magnetization M(T) of NiO nanoparticles with different sizes under both zero-field-cooled (ZFC) and field-cooled (FC) conditions, from 370 to 2 K and applying a field of 100 Oe. The MFC and 26047



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pyrrolidone. Winkler et al.6 also observed a Tf of 17 K for 3 nm bare NiO powder and 15 K for capped nanoparticles. Tadic et al.22 observed a Tf of 6.5 K for 3 nm NiO nanoparticles embedded in silica matrix. Obviously, the surface states of the nanoparticles have significant effects on the freezing of the surface spins. The similar Tf for the nanoparticles with different sizes in Figure 8 may be ascribed to the similar surface states of the nanoparticles grown with the same method. Variation of MFC in Figure 8 could be divided into two regions. In the higher-temperature region, MFC value increases slowly with a decrease of the temperature. As the temperature is close to Tf, MFC value increases dramatically with a decrease in the temperature, and then, it tends to saturate below Tf. Winkler et al.6 observed a similar variation of MFC with temperature in 3 nm nanoparticles. NiO nanoparticles have been supposed to have an antiferromagnetically ordered core with uncompensated magnetic moment and magnetically disordered surface shells.6,22,26 In the high temperature region above the bifurcation temperature (Tirr) of the MZFC and MFC curves, all spins are unblocked, and NiO nanoparticles exhibit superparamagnetic (SPM) characteristics. With the decreasing of the temperature, thermal fluctuations slow down. When the temperature is close to TB, the uncompensated magnetic spins of antiferromagnetic cores block progressively, while the disordered surface spins are still free to fluctuate thermally. MFC would increase gradually with the decrease of temperature. As the temperature is lowered further, most of the uncompensated core moments have blocked and aligned along easy axes. Then, the magnetization behavior is mainly determined by the surface spins. Once the temperature is close to Tf, the surface spin fluctuations slow down, short-range magnetic correlations develop, and surface spins form spin clusters. The disorder and competition of magnetic interactions between the surface spin clusters in the shell region finally give rise to surface spins freezing.6 MFC would increase dramatically due to surface spin freezing. The obvious saturation behavior of MFC at a temperature blow Tf has been reported in γ-Fe2O3 nanoparticles52 and (Fe0.26Ni0.74)50B50 amorphous powders,28 which was attributed to the strong interparticle interactions. Winkler et al.6 observed a diminishing of saturation of MFC at low temperature when 3 nm nanoparticles were dispersed in polymer. NiO nanoparticles in this work are not capped. The nanoparticles may conglomerate during cleaning process after growth, which may result in the slight saturation of MFC at very low temperature. To determine the field dependence of the nanoparticle magnetism, M−H hysteresis curves were measured at 5 K for NiO nanoparticles with different sizes and are shown in Figure 9. The samples were cooled from room temperature to the measurement temperatures in zero fields. Then, the measurement was performed with the magnetic field first raising from zero field to 4.5 T (initial curve), subsequently decreasing to −4.5 T (demagnetization curve), then increasing to 4.5 T (remagnetization curve) again. It can be seen that a hysteretic behavior is observed in all samples. During the initial magnetization process, the magnetization increases with the increasing of magnetic field in lower-field region. As the magnetic field increases further, the magnetization increase slows down. Then, the magnetization increases linearly with magnetic field and does not saturate in high field. These indicate that the magnetization consists of two components: an easily magnetized component in low-field region and a nonsaturating component responsible for the almost linear

Figure 8. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves at a field of 100 Oe for NiO nanoparticles with different sizes.

MZFC curves split below a particular bifurcation temperature (Tirr). The MZFC curve presents two peaks: a broad peak centered at a temperature below the bifurcation temperature (Tirr) designated as TB, and another sharp peak in the lowtemperature region designated as Tf. These are similar to that reported in many references.6,9,11,19,20,51,52 Two peaks in the MZFC curve correspond to two blocking processes; the peak at high temperature (TB) is attributed to the thermal relaxation of uncompensated spins in the particle cores, and the peak at lower temperature to the freezing of surface spin clusters. It can be seen from Figure 8 that the peak (TB) of MZFC curve for 12.4 ± 2.9 nm NiO nanoparticles is rather broader than that of other nanoparticles with smaller sizes. TB of the MZFC curve is usually attributed to the average blocking temperature of the uncompensated antiferromagnetic core moment, while Tirr corresponds to the highest blocking temperature.53 The broadening of the peak (TB) of the MZFC curve for 12.4 ± 2.9 nm NiO nanoparticles indicates that the nanoparticles have a broader magnetic moment distribution, which originates from the broad size distribution of the nanoparticles (Figure 3). It also can be seen from Figure 8 that Tirr shifts to lower temperature from 300 to 46.4 K and that TB shifts progressively from 161 to 19.8 K with a decrease of NiO nanoparticle sizes from 12.4 to 3.5 nm. TB is proportional to the anisotropy energy barrier. For noninteracting particles, the anisotropy energy barrier is proportional to the volume of the particles.6 The blocking temperature TB should increase with the increase of nanoparticle size. Shift of Tirr and TB to lower temperature with a decrease in nanoparticle sizes in Figure 8 is ascribed to the decrease of nanoparticle sizes. Tf in Figure 8 are about 10.8, 10.5, 12.7, and 10.4 K for NiO nanoparticles with sizes of 3.5 ± 0.4, 4.6 ± 0.6, 5.5 ± 0.7, and 12.4 ± 2.9 nm, respectively, which is not very sensitive to the nanoparticle sizes. The origin of the sharp peak Tf of MZFC curve has been ascribed to the freezing of disordered surface spins of the nanoparticles.6,11,21 Meneses et al.21 obtained a Tf of 11 K for bare NiO nanoparticles with a size of 5.8 nm, 9 K for the nanoparticles dispersed in polyvinyl26048



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surface-to-volume ratio of particles increases with a decrease in the nanoparticle sizes. Therefore, the ratio of surface spins increases with a decrease in the particle sizes. The larger saturation magnetization, remnant magnetization and coercivity in the smaller nanoparticles at lower temperatures suggest that the magnetization of the nanoparticles is mainly from the surface spins. We attributed the anomalous behavior of the magnetization curves to the coupling between the core and shell. At the temperature below Tf, the surface spins are frozen into a spin-glass configuration, while the uncompensated spins in the cores align along the easy axis. In the initial magnetization process, when the magnetic field is lower, uncompensated spins in the cores rearrange along the magnetic field gradually, while the surface spins are still frozen in random directions. The magnetization increases with the increasing of magnetic field. With a further increase in the magnetic field, the surface anisotropy is overcome by the applied field, surface spins tend to align along the field direction. The magnetization increases with the increasing of magnetic field slowly in lower magnetic region. When the field is high-enough, most of the surface spins align along the field direction. Then, the interface spins couple with the core spins and are pinned by the cores. During the decrease of the applied field, some of the surface spins and uncompensated core spins align in the direction of the magnetic field when the field is lowered to zero. When the applied field is reversed, the interfacial spins rotate consistently with the core spins, the magnetization decreases quickly in the lower field region. During the remagnetization process, the interfacial spins rotate consistently with the core spins under lower field, which results in a quick increase in the magnetization. Then, the remagnetization curve would surpass the initial magnetization curve in some field. With the decreasing of nanoparticle size, the volume of core decreases and even vanishes, the pinning effect of interfacial spins decreases. So the anomalous magnetic property is more pronounced in nanoparticles with larger sizes than that in smaller particles. To further investigate the anomalous magnetization process of the NiO nanoparticles. The M−H hysteresis curves were collected just below the blocking temperature and are shown in Figure 10. The saturation magnetization, remanent magnetization, and coercivity are listed in Table 2. Compared with the hysteresis loops taken at 5 K, the hysteresis loops narrow down,

Figure 9. M−H hysteresis curves of NiO nanoparticles with different sizes at 5 K. The inset shows the magnification of the central part.

variation in high-field region. Similar variation of M with H have been reported in NiO nanoparticles.6,9,21,22 The easily magnetized component is associated with the uncompensated spins in the cores, and the nonsaturating component is related with the surface spins.6 It also can be seen from Figure 9 that the coercive field decreases quickly in the lower-field region during the decrease of the magnetic field. Moreover, the remagnetization curve increases more quickly and surpasses the initial magnetization curve, which is an anomalous behavior. Tadic et al.22 observed similar anomalous behaviors in NiO nanoparticles embedded in the silica glass. They attributed them to the coupling effects between the core and shell. Similar anomalous behaviors were also observed in a granular film of αFe2O3 nanoparticles in amorphous alumina matrix27 and (Fe0.26Ni0.74)50B50 amorphous nanoparticles,28 which were ascribed to a strong increase of the local surface anisotropy and interparticle interactions at lower temperature. The origin of the anomalous behavior in the magnetization process is still controversy. In order to see the variation of the anomalous behavior with the nanoparticle sizes clearly, the enlarged part of the corresponding M−H hysteresis curve is inserted in Figure 9. It can be seen that the anomalous behavior is more pronounced for NiO nanoparticles with larger sizes, and the cross-point of the remagnetization and initial magnetization curves shifts to higher magnetic field with a decrease of nanoparticle sizes. The remnant magnetization (Mr) and coercivity (Hc) for NiO nanoparticles with different sizes are given in Table 1. The Table 1. Magnetic Parameters Obtained from the Hysteresis Loops Taken at 5 K sample size (nm)

temp (K)

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

3.5 ± 0.4 4.6 ± 0.6 5.5 ± 0.7 12.4 ± 2.9

5 5 5 5

12.07 ± 0.54 5.60 ± 0.15 5.13 ± 0.10 2.00 ± 0.07

0.91 0.41 0.45 0.18

± ± ± ±

433 ± 3 1462 ± 6 393 ± 3 332 ± 3

0.01 0.01 0.01 0.01

saturation magnetization (Ms) was obtained by extrapolating 1/ H to infinite field and is also shown in Table 1. The saturation magnetization, remnant magnetization, and the coercivity of NiO samples increase with a decrease in the nanoparticle sizes, and remnant magnetization and coercivity are extraordinary larger for NiO nanoparticles with a size of 4.6 ± 0.6 nm. The

Figure 10. M−H hysteresis curves measured at a temperature just below the blocking temperature for NiO nanoparticles with different sizes. 26049



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Table 2. Magnetic Parameters Obtained from the Hysteresis Loops Taken Just below the Blocking Temperature sample size (nm)

temp (K)

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

3.5 ± 0.4 4.6 ± 0.6 5.5 ± 0.7 12.4 ± 2.9

19 50 43 150

11.51 ± 0.59 4.83 ± 0.11 4.50 ± 0.11 1.62 ± 0.05

0.19 0.07 0.07 0.03

± ± ± ±

168 ± 2 131 ± 2 91 ± 1 111 ± 2

0.02 0.01 0.01 0.005

and the saturation magnetization, remanent magnetization, and the coercivity all decrease. The remagnetization process becomes normal. This is similar to that reported by Tadic et al.22 and Zysler et al.,28 which also demonstrates that the anomalous magnetization process is related to the freezing of the surface spins.

4. CONCLUSIONS NiO nanoparticles with sizes of 3.5−12.4 nm distort to rhombohedral structure at room temperature. The lattice parameter of the nanoparticles increases with a decrease of nanoparticle sizes. The band gap of NiO nanoparticles increases with a decrease in the nanoparticle size due to the size confinement of electrons. Decreasing of the spin correlation length and disorders induced by defects result in the diminishing of two magnon vibration in nanoparticles. Disorders introduced by defects and the damage of transmission symmetry in nanoparticles would induce the appearance of 1LO vibration. Size confinement and surface relaxation result in a red shift of LO vibration with a decrease of the nanoparticle sizes. Exposure of the nanoparticles to a high power probe laser beam would anneal out some defects and introduce a blue shift of 1LO vibration. Two blocking processes, respectively, corresponding to the blocking of the uncompensated magnetic spins in nanoparticle cores at higher temperature and freezing of disordered surface spins in shells at lower temperature are observed. Bifurcation temperature and blocking temperature of NiO nanoparticles shift to lower temperature, and the saturation magnetization, remanet magnetization, and coercivity increase obviously with a decrease of nanoparticle sizes. The coupling between the surface and core spins would result that the remagnetization curve surpasses the initial magnetization curve as the temperature is lower than the surface spin freezing temperature.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-10-58806921. Fax: 86-10-58800141. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (Projects 10974917 and 10574011) and the Fundamental Research Funds for the Central Universities.

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REFERENCES

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Size Effects on Properties of NiO Nanoparticles ... - ACS Publications

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