EPR/FMR STUDY OF nCoO/(1-n)ZnO
(n=0.4, 0.5, 0.6 AND 0.7)
NANOCOMPOSITES
1
1
1
2
2
N. Guskos , G. Zolnierkiewicz , J. Typek , D. Sibera , and U. Narkiewicz
1Institute of Physics, West Pomeranian University of Technology, Al.Piastow 48, 70-311 Szczecin, Poland.
2
Institute of Chemical and Environment Engineering, West Pomeranian University of Technology,
ul. Pulaskiego 10, 70-322 Szczecin, Poland
?Nanocomposites of the general formula nCoO/(1-n)ZnO (where the composition index n=0.4, 0.5, 0.6 and 0.7) were prepared by using a microwave hydrothermal synthesis at pressure 3.9 MPa applied
for 15 min. At first, a mixture of cobalt and zinc hydroxides was obtained by addition of 2 M solution of KOH to the 20% solution of a proper amount of Zn(NO )·6H O and Co(NO )·6H O in water and then treated in 3 2 3 2
a solvothermal microwave reactor. Next, the obtained materials were washed with deionized water to remove salt residues. Finally, the materials were dried at 100°C for 24 h.
?The morphology of samples was investigated using scanning electron microscope (SEM, Hitachi) followed by the phase composition of the samples determined by the X-ray diffraction (XRD, CoKá
radiation, X'Pert Philips). The specific surface area of the nanopowders was determined using the Brunauer–Emmett–Teller (BET) method with the equipment Gemini 2360 of Micromeritics. The helium pycnometer AccuPyc 1330 of Micromeritics was applied to determine the density of powders. Magnetic resonance study was carried out on a conventional magnetic resonance spectrometer Bruker E 500 with 100 kHz magnetic field modulation using an Oxford helium-flow cryostat.
Experimental
• Only three phases observed in XRD in all samples: ZnO, ZnCo O and 2 4
Co(OH) .2
• No isolated nanoparticles, strong agglomeration seen in SEM images. • Two main components in magnetic resonance spectrum: broad and
narrow.
• The resonance field of broad (symmetrical) line changes with temperature – it is in 0 - 3 kG range.
• The narrow component is asymmetrical and must be fitted with two lines. The resonance fields are in 1.5 - 1.9 kG range.
• The linewidth of the broad line generally increases with temperature decrease, but seems to have two local maxima, one in 60 - 90 K range (smaller) and the other in 5 - 15 K range (bigger).
• The integrated intensity of the broad line increases with temperature decrease, but has a local maximum at 65 K and another additional at 15 K but only for n=0.6 and 0.7 samples.
• The integrated intensity of the narrow line increases with temperature decrease for all samples and at low temperatures (T<50 K) the Curie-Weiss law is fulfilled with an effective ferromagnetic interaction decreasing with composition index increase.
Basic Facts
Fig. 1. XRD patterns for nCoO/(1-n)ZnO nanocomposites with n=0.5, 0.6, and 0.7. Peaks attributed to ZnO are marked as Z, peaks attributed to ZnCo O are marked as S. The not marked 2 4
peaks are attributed to Co(OH) .2
2+
Fig. 3. Magnetic resonance spectra of the nCoO/(1-n)ZnO nanocomposites: (a) n=0.4; (b) n=0.5; (c) n=0.6; (d) n=0.7. The insets in (a), (b), and (d) show EPR spectra of Co ions registered at low temperatures. Fig. 2. SEM images of nCoO/(1-n)ZnO nanocomposites with different composition index: (a) n=0.5; (b) n=0.6; (c) n=0.7.
Fig. 9. Temperature dependence of the reciprocal EPR integrated intensity for the
narrow line for all investigated samples.
Fig. 8. Temperature dependence of the integrated intensity for the narrow (top panel) and broad (bottom panel) line.
20 30 40 50 60 70 Z Z Z Z Z Z Z Z S S S S S S S n=0.7 n=0.6 In te n si ty [A rb . u n its ] 2Q [deg] n=0.5 S Z 0 2 4 6 8 -4 -2 0 2 4 0.0 0.5 1.0 1.5 2.0 2.5 -4 0 4 8 12 16 a) d c " /d H [A rb . u n it s ] Magnetic field H [kG] 60 K 70 K 100 K n = 0.4 17 K 13 K 7 K 4 K 0 1 2 3 4 5 6 7 8 -6 -5 -4 -3 -2 -1 0 0 1 2 3 -10 0 10 20 30 40 b) d c " /d H [A rb . u n it s ] Magnetic field H [kG] 90 K 60 K 50 K n = 0.5 4 K 7 K 13 K 17 K 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -6 -3 0 3 6 9 12 15 c) d c " /d H [A rb . u n it s ] Magnetic field H [kG] 4 K 7 K 13 K 17 K n=0.6 0 2 4 6 8 -10 -8 -6 -4 -2 0 0.0 0.5 1.0 1.5 2.0 2.5 -4 -2 0 2 4 6 8 10 d) d c " /d H [A rb . u n it s ] Magnetic field H [kG] 70 K 55 K 45 K 35 K n = 0.7 4 K 7 K 13 K 17 K 0 1 2 3 4 5 6 7 8 -7 -6 -5 -4 -3 -2 -1 0 1 d c ''/ d H [A rb . u n it s ] Magnetic field [kG] n=0.5 T=50 K 20 30 40 50 0 1 2 3 4 R e s o n a n c e fi e ld Hr [k G ] Temperature [K] 0.4 0.5 0.6 0.7 0 20 40 60 80 1.50 1.55 1.60 1.65 1.70 1.75 1.80 R e s o n a n c e fi e ld Hr [k G ] Temperature [K] 0.4 0.5 0.6 0.7 0 10 20 30 40 50 60 70 80 90 5 10 15 20 25 30 L in e w id th D Hp p [k G ] Temperature [K] n=0.4 n=0.5 n=0.6 n=0.7 0 2 4 6 8 0 10 20 30 40 50 60 70 80 90 100 110 0 1 2 3 4 n=0.4 n=0.5 n=0.6 n=0.7 Narrow line In te g ra te d in te n s it y [A rb . u n it s ]
Temperature [K]
n=0.4 n=0.5 n=0.6 n=0.7 Broad line 0 5 10 15 20 25 30 35 40 45 50 0 2 4 6 8 10 R e c ip ro c a l E P R in te g ra te d in te n s it y [A rb . u n it s ] Temperature [K] 0.4 0.5 0.6 0.7According to the results of the XRD analysis, the XRD spectra reveal a presence of ZnO, Co(OH) and ZnCo O phases (Fig. 1). Spinel phase ZnCo O content increases with increasing CoO content in the 2 2 4 2 4
samples, while the ZnO content decreases simultaneously. The mean crystallite size of the detected phases was determined using Scherrer's formula. In particular, the mean crystallite size of ZnCo O varied 2 4
from 8 to 12 nm. SEM images of nCoO/(1-n)ZnO nanocomposites with different composition index n=0.5, 0.6, and 0.7 are shown in Fig. 2. The investigations by SEM allowed to distinguish three different types
3 2
of morphology: small spheroidal forms and large plates or rods (Fig. 2). The values of helium density of the investigated samples were in 4.6-4.7 g/cm range, the specific surface area in 19-21 m /g range. The obtained results shows that the helium density and specific surface area of the samples is at a similar level in all studied samples. The low density of samples may be due to the presence of cobalt hydroxide, the presence of this phase was confirmed by XRD analysis.
Results
Fig. 4. Magnetic resonance spectrum of n=0.5 sample at T=50 K (points) and the fitted
spectrum (solid line).
Fig. 5. Temperature dependence of the resonance field of the broad line.
Fig.6. Temperature dependence of the resonance field of the main component of narrow line.
Fig. 7. Temperature dependence of the linewidth of the broad line.
a)
b)
c)
• The broad line is a ferromagnetic resonance due
t o ( m o s t l y ) Z n C o O a g g l o m e r a t e d 2 4
nanoparticles. Due to agglomeration no superparamagnetic resonance is observed. For higher concentration of CoO in initial material (for n=0.6 and 0.7) another ferromagnetic components participating in formation of broad
line are possible (nanoparticles Co O , Co O , 3 4 2 3
CoO , Co etc.) what is suggested by two maxima 2
in temperature dependence of integrated intensity and linewidth.
• The narrow EPR line is produced by relatively
2+
isolated Co ions, likely to exist in ZnO, ZnCo O 2 4
and (mostly) in Co(OH) phases. The effective 2
2+
interaction of Co ions is ferromagnetic, but is weakened by additional antiferromagnetic interaction in more cobalt containing samples.
