Photon Factory Activity Report 2014 #32 (2015) B

BL-8B/2011G511, 2013G523

Particle size and distortion effects on NiO nanoparticles embedded in
one-dimensional pores of mesoporous silica
Takayuki Tajiri1,*, Seiya Saisho2, Masaki Mito2, Hiroyuki Deguchi2, Atsushi Kohno1
1Faculty of Science, Fukuoka University, Fukuoka 814-0180, Japan
2
Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan

3 Results and Discussion
Figure 1 shows the powder XRD patterns for the NiOnanoparticles with particle size of 2.6, 4.3, 8.6, and 17.2nm in SBA-15 at room temperature and simulated pattern
for bulk crystal with rhombohedral symmetry. In
previously reported results, the structure of different sized
NiO nanoparticles at room temperature has been reported
for the so-called rock-salt structure with cubic[1] andrhombohedra[2] symmetries. The obtained XRD patternsof all NiO nanoparticles in SBA-15 exhibited some broad
Bragg peaks, which could be attributed to therhombohedral distorted rock-salt structure. The average

Intensity [arb. unit]

2.6 nm

4.3 nm
8.6 nm17.2 nm
simulation
1.0

1.5

2.0
d [Å]

2.5

3.0

Fig. 1: Powder XRD patterns for NiO nanoparticles withparticle size of 2.6, 4.3, 8.6, and 17.2 nm in SBA-15 and asimulated pattern with a rhombohedral distorted rock-salt
structure.

a [Å]

3.02

(a)

3.00
2.98
2.96
2.94
7.8

c [Å]

2 Experiment
The NiO nanoparticles were synthesized in onedimensional pores with pore size of about 7 nm of
mesoporous silica SBA-15. Powder X-ray diffraction(XRD) measurements for the NiO nanoparticles in SBA15 were performed at room temperature. The energy of
the incident X-rays was 18 keV, and their wavelength wascalibrated using the CeO2 powder XRD pattern. Themagnetic properties of the NiO nanoparticles wereinvestigated using a commercial SQUID magnetometer.

particle size of the NiO nanoparticles was evaluated byusing Scherrer’s equation. The average particle size for
the NiO nanoparticles in the SBA-15 pores ranged from
2.6 to 22 nm. The NiO nanoparticles with d > 7 nm
should have spheroidal or rodlike shapes in the onedimensional SBA-15 pores with diameters ofapproximately 7 nm, as observed by TEM.[3](b)

7.6
7.4
7.2
2.60

c/a

1 Introduction
Crystal structure and physical properties fornanoparticles differ from those for the bulk crystal owing
to the energy state and translation symmetry changes atthe surface of a nanoparticle. In terms of the crystal
structure, the surface states of nanoparticles are likely
changed by the development of the lattice strain owing to
the existence of edges and defects at the surface of aparticle as well as the difference of the crystallographic
structure from that of the bulk crystal. For
antiferromagnetic nanoparticles, the surface state ofnanoparticles greatly affects the magnetic properties
because uncompensated spins of sublattices appear on thesurface of the particle and critically determine themagnetic properties of antiferromagnetic nanoparticles.
We investigated the correlation between crystal structure
and magnetic properties of nanoparticles of an
antiferromagnetic compound NiO. The crystal structure
and magnetic properties of NiO nanoparticles are
sensitive to the particle size and have exhibited uniquesize dependences. To the best of our knowledge, there
have been no reports of a correlation between crystal
structure and magnetic properties based on detailed
systematic experimental studies for NiO nanoparticles
with particle size of 2-20 nm.

(c)

2.55
2.50
2.45
234

567 8 910particle size [nm]2Fig. 2: (a, b) Particle size dependence of lattice constants,
a and c, for the NiO nanoparticles in SBA-15. Horizontal
dashed lines show the lattice constants for the bulk crystal.
(c) Particle size dependence of lattice constant ratio c/a.
Horizontal dashed line is equivalent to a value of c/a for acrystallographic structure with cubic symmetry.

Photon Factory Activity Report 2014 #32 (2015) B

The lattice constants for the NiO nanoparticle in SBA15 estimated by Rietveld analysis. Figure 2 shows thesize dependence of the lattice constants for thenanoparticles. The a and c values for the nanoparticles
increased with increasing particle size and exhibited
maxima around 3 nm. The lattice constant ratio c/a of √6
corresponds to the cubic symmetry. As the particle size
increased, the value of c/a increased rapidly and exhibiteda maximum at d ≈ 3 nm. Above d ≈ 3 nm, c/a decreased
monotonically down to about √6 at d = 10.6 nm. These
results indicate that the rhombohedral distortion of therock-salt structure of the NiO nanoparticles becomes
more pronounced, and its magnitude increases withdecreasing particle size for particles sizes below
approximately 10 nm.
The size dependences of the magnetic properties, suchas coercive field, shell moment, blocking temperature,and magnetic anisotropy energy, exhibited qualitativelysimilar behaviors, which were characterized by themaxima for particle sizes between 3 and 11 nm and nearly
constant values for particle sizes above ≈11 nm.[3] The
results of crystal structural analysis showed that thelattice constants and the rhombohedral distortion of therock-salt structure exhibited maxima at d ≈ 3 nm andattained nearly constant values for particle sizes above ≈9nm. The present results indicate that the magnetic
properties were strongly correlated with rhombohedral
distortion of the rock-salt structure. The variation in c/a
led to the change in magnetic exchange interactions in
NiO nanoparticles. The present results suggest that, forNiO nanoparticles, the rhombohedral distortion of therock-salt structure strongly influences the local magneticordering in the NiO nanoparticles. Therefore, thissuggests that the changes in the rhombohedral distortion
produced the changes in the magnetic interaction andmagnetic anisotropy constant of NiO nanoparticles. Thus,
the distortion of crystal structure and change in sublatticespin order led to the observed unique size dependence ofmagnetic properties. The present experimental resultssuggest that the crystallographic structural distortion
originating from the strong correlation between the spinand charge in NiO nanoparticles can be controlled bychanging the particle size. In addition, the distortion ofparticle shape of the NiO nanoparticle affects magneticproperties.
References[1] E. Winkler et al., Nanotechnology 19, 185702 (2008).
[2] M. Feygenson et al., Phys. Rev. B 81, 014420 (2010).
[3] T. Tajiri et al., J. Phys. Chem. C 119, 1194 (2014).
* tajiri@fukuoka-u.ac.jp