Journal of Non-Crystalline Solids 325 (2003) 16–21

www.elsevier.com/locate/jnoncrysol

Thermal effects on ESR signal evolutionin nano and bulk CuO powder

A. Viano a,*, S.R. Mishra b, R. Lloyd c, J. Losby b, T. Gheyi c

a Department of Physics, Rhodes College, 2000 North Parkway, Memphis, TN 38112, USAb Department of Physics, The University of Memphis, Memphis, TN 38152, USA

c Department of Chemistry, The University of Memphis, Memphis, TN 38152, USA

Received 29 July 2002; received in revised form 26 May 2003

Abstract

In order to understand the effects of low dimension on the magnetic properties of CuO, a systematic electron spin

resonance study is carried out on CuO nano and bulk powders. Sol–gel produced CuO nanopowder was calcined at

temperatures from 200 to 1000 �C to produce nanoparticles of varying size. A broad electron spin resonance (ESR)

signal was obtained for the CuO nanoparticles, representative of antiferromagnetic ordering. This ordering persists even

at higher calcination temperatures. On the contrary, CuO bulk powder shows two separate ESR signals which merge

into one at higher temperatures. These data indicate that the antiferromagnetic ordering is preserved up to 800 �C in

CuO nanomaterial. In bulk CuO powder, the ESR signal breaks from antiferromagnetic ordering at lower tempera-

tures. This is a result of the high latent heat of bulk material, which leads to early decomposition of the powder. With

evidence from detailed thermal analysis, the observed differences in the ESR data for bulk and nano CuO as a function

of calcination temperature are explained on the basis of particle size difference.

� 2003 Elsevier B.V. All rights reserved.

PACS: 75.10; 75.50K; 61.46

1. Introduction

The magnetic properties of nanoparticles are of

great current interest, mainly because of their un-

ique properties and their diverse industrial appli-cations [1]. Most studies are concentrated on

understanding the magnetic properties of oxide

* Corresponding author. Tel.: +1-901 843 3912; fax: +1-901

843 3117.

E-mail address: viano@rhodes.edu (A. Viano).

0022-3093/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/S0022-3093(03)00317-X

nanoparticles such as ferritines; c-Fe2O3, NiO, etc.

CuO occupies a special place among the semi-

conductor 3d oxides because it has some unique

physical properties. Unlike NiO, CoO, FeO and

MnO, CuO has a low symmetry monoclinic crystalstructure and a magnetic susceptibility tempera-

ture behavior that is unusual for 3d antiferro-

magnets. In polycrystalline CuO the susceptibility

increases, rather than decreases, above the N�eeeltemperature, TN. Also, CuO has been investigated

extensively in relation to the high Tc oxide super-

conductors [2]. CuO has recently been studied in

ed.

A. Viano et al. / Journal of Non-Crystalline Solids 325 (2003) 16–21 17

nanostructured form, which is known to haveapplications in catalysis and optics [3]. Over and

above, the study of CuO by itself is essential since

many of its magnetic properties are governed by

the oxygen stoichiometry. The diverse results ob-

tained by different authors when measuring its

magnetic properties have been attributed to the

existence of intrinsic defects like cation or anion

vacancies [4,5], particle size effects [6], and un-compensated charge [7].

In the present investigation, an electron spin

resonance (ESR) technique is used to probe the

calcination temperature effect on magnetic order-

ing in nano and bulk CuO powder. The magnetic

measurement is coupled with thermal analysis to

understand the origin of ESR signals that may

arise from any phase transformation. Thermalanalysis also allows a check of the phase purity

and aids in an understanding of the relationship

between magnetic ordering and the oxide phases of

CuO. The importance of the present study lies in

the fact that sol–gel synthesis has evolved as a

standard technique for oxide nanoparticle fabri-

cation. In this process, particle size is often con-

trolled by varying the calcination temperature.Although calcination can increase the particle size,

its effects on other physical properties, such as

magnetic, electrical and thermal, need to be in-

vestigated. The influence of the calcination tem-

perature on these properties depends on the heat

distribution within the particle. Nanoparticles can

easily dissipate heat because of their high surface

area and low latent heat, while bulk particles haveunequal heat distribution and high latent heat.

Thus, the heat distribution during the fabrication

stage of nanoparticulate material can influence its

physical properties. This study investigates this

influence on the magnetic properties.

2. Experimental

CuO nanoparticles were synthesized using the

sol–gel technique described by Punnoose et al. [8].

This involved reacting aqueous solutions of copper

nitrate and sodium hydroxide at pH¼ 10 and at

room temperature. The resulting gel was washed

several times with distilled water until free of

nitrate ions. This gel was carefully filtered anddried in air at 70 �C. The bulk CuO powder was at

)22 mesh. Both CuO nano and bulk powder were

calcined in air at temperatures ranging from 200 to

1000 �C for approximately 10–12 h. The annealed

samples were cooled slowly to room temperature.

X-ray diffraction (XRD) patterns were obtained

with a diffractometer using CuKa radiation. The

Scherrer formula [9] was applied to determine theparticle size, taking instrumental broadening into

account. ESR data were obtained on a spectro-

meter interfaced to a PC for data acquisition and

analysis. The ESR was operated with a center field

of 2500 G, a modulation amplitude of 4.00, a time

constant of 0.25 s, a scan time equal to 4 min, and

a microwave power of 20 mW. Thermal analysis

was performed using a differential thermal ana-lyzer (DTA) with Pt/PtRd thermocouples and

alumina sample and reference cups. Al2O3 powder

was used as the reference material and scans were

carried out with a constant heating/cooling rate

of 10 �C/min. Transmission electron microscopy

(TEM) images were obtained at 80 kV.

3. Results

The results of structural analysis are shown in

Fig. 1. Fig. 1(a) shows a typical TEM micrograph

of CuO nanomaterial calcined at 200 �C. An ag-

glomeration of nanoscale particles is clearly visible,

showing a uniform distribution of particle sizes and

a homogeneous morphology. A typical XRD pat-tern for a wide range of 2h values for unannealed

CuO nanomaterial is shown in Fig. 1(b). Fig. 1(c)

shows the XRD patterns for the calcined samples

(for greater clarity, only the pertinent range in 2h isplotted). The particle sizes, as determined using the

Scherrer formula, are also listed next to their rep-

resentative spectra in Fig. 1(c).

Fig. 2 shows the evolution of the ESR signal ofthe CuO nanomaterial as a function of calcination

temperature, while Fig. 3 shows the ESR data for

bulk CuO powder. The line shapes of these ESR

spectra are typical of polycrystalline material.

Fig. 4 plots the ESR line widths of both bulk and

nano CuO powder as a function of the calcination

temperature.

Fig. 2. Overlay of ESR spectra of CuO nanopowder calcined at

different temperatures. The calcination temperature is written

just above each curve.

Fig. 1. (a) TEM image of CuO nanopowder calcined at 200 �C;(b) XRD pattern of CuO calcined at 70 �C and (c) temperature-

dependent XRD patterns of CuO nanopowder. The calcination

temperature and average particle size are written above each

spectra. All XRD spectra result from CuKa radiation.

Fig. 3. Overlay of ESR spectra of CuO bulk powder annealed

in air at different temperatures. The calcination temperature is

written just above each curve.

18 A. Viano et al. / Journal of Non-Crystalline Solids 325 (2003) 16–21

Thermal analysis data are shown in Fig. 5. The

heating curves for the CuO nanomaterial and un-

calcined bulk powder are seen in Fig. 5(a). In thesethermograms, the endothermic direction is a de-

creasing DT (down). Each curve is labeled with the

annealing temperature used for that sample. The

cooling curves, shown in Fig. 5(b), show a single

sharp exothermic peak. The sharp increase intemperature at the start of this transition (visible

as a linear increase on the high temperature side of

the peak) is due to a rapid heating of the material

at the start of this exothermic reaction. This very

rapid heating causes the entire sample to increase

slightly but rapidly in temperature, resulting in the

slant of the peak.

Fig. 4. Temperature dependence of ESR linewidth of: (r) CuO

nanoparticle and (j) CuO bulk powder.

A. Viano et al. / Journal of Non-Crystalline Solids 325 (2003) 16–21 19

4. Discussion

The structural data give a clear indication of the

particle size dependence upon calcination temper-

ature. While the TEM images show a uniform size

for particles calcined at any one temperature, the

XRD data show an increase in size for increasing

calcination temperatures. All of the peaks in the

XRD spectra are identified as reflections from a

pure CuO phase, with the ()1 1 1) and (1 1 1) beingthe most prominent peaks. Clearly, these XRD

line widths decrease with increasing calcination

temperature (Fig. 1(c)), representing the growth of

the CuO particles. At 200 �C, the particles have an

Fig. 5. DTA (a) heating and (b) cooling curves for CuO sol–gel nanop

with its calcination temperature. A decreasing DT corresponds to an

average size of 15 nm, and this average size in-creases threefold, to 46 nm, for the material pre-

pared at 800 �C. The line width, and therefore

particle size, shows no further changes after 800

�C, and so those XRD spectra are not included.

ESR data for both the nanomaterial and bulk

CuO have the appearance of a broad signal, which

is ascribed to weak antiferromagnetic ordering in

CuO. Magnetic susceptibility measurements indi-cate that CuO has a N�eeel temperature of 453 K

[10] and therefore no ESR signal below this tem-

perature is expected. The presence of the broad

ESR signal has been a matter of interest for many

years. As discussed by Muraleedharan et al. [5],

CuO has three magnetic phases: (1) the antiferro-

magnetic phase below N�eeel temperature, (2) a fluid

like phase between the N�eeel temperature and 630K, and (3) a paramagnetic phase above 630 K.

Above the N�eeel temperature, paramagnetic or-

dering is observed in CuO.

For the nanomaterial, this broad signal, indi-

cating weak antiferromagnetic ordering, persists at

high calcination temperatures (Fig. 2), up to 800

�C. The bulk CuO data, however, show two sig-

nals; one broad and one sharp, as determined bycomputer simulation (not shown). These signals

gradually intensify and come closer together, fi-

nally merging at 800 �C (Fig. 3), and intensify

upon further heating up to 1000 �C. This is in

contrast to the nanomaterial, where the broad

ESR signal, indicating weak antiferromagnetic

articles calcined at different temperatures. Each curve is labeled

endothermic reaction.

20 A. Viano et al. / Journal of Non-Crystalline Solids 325 (2003) 16–21

ordering, is preserved up to 800 �C. Thus, theweak antiferromagnetic ordering persists to a

higher temperature for the nanomaterial than for

the bulk CuO. Since nanoparticles are very small

in size and therefore have high surface area and

many defects, the Cu2þ near the defects likely

contribute an antiferromagnetic interaction with

neighboring atoms, but of varying strength

throughout the material. This could result in localcanted structures, an effect which is well known in

disordered spinel ferrites [11]. Crystal field calcu-

lations performed by Grebinnik et al. [12], which

show that the shape of observed ESR spectra re-

sults from the exchange interactions in CuO, sup-

port this inference.

The ESR signal for bulk CuO powder also

shifts towards a higher magnetic field value uponannealing. A breaking of the antiferromagnetic

ordering is seen at higher temperatures with the

appearance of a singlet shifted to a higher field

value. It is re-emphasized here that the ESR

measurements have been performed after cooling

the samples to room temperature. Also, these

calcination temperatures are not high enough to

initiate any phase transition, e.g. CuO to Cu2O, aswill be discussed below. Thus, the observed

breakage of antiferromagnetic ordering in particles

annealed at temperatures above 800 �C can be

attributed to the weakening of the exchange in-

teraction between nonequivalent copper–oxygen

complexes, mainly resulting from oxygen defi-

ciency. The broad ESR signal clearly indicates the

presence of a large number of defects in CuOnanoparticles. In bulk CuO powder, the sharpen-

ing of the ESR signal indicates a complete de-

struction of antiferromagnetic ordering [13]. Even

though the N�eeel temperature of CuO is 230 K, the

actual magnetic transition from the antiferromag-

netic state to paramagnetic state takes place only

at higher temperatures. The presence of broad

lines thus observed is typical of the fluid phase.The appearance of the narrow line at the high

field side in the bulk CuO data is attributed to

unassociated CuO molecules. As the annealing

temperature increases, the number of unassociated

CuO molecules grows at the expense of the mag-

netically coupled ones and hence the observed in-

tensification of the high field signal. As discussed

above, the weak antiferromagnetic ordering per-sists to a higher temperature in the nanomaterial

than in the bulk. These differences likely occur

because of the high latent heat of particles in the

CuO bulk powder. Thus, although the calcination

temperature may be lower, because of the high

latent heat the bulk powder is heated sufficiently to

initiate CuO dissociation. Furthermore, the ESR

signals for bulk powder are comparatively nar-rower, indicating a relatively low defect concen-

tration in bulk powder. The ESR line widths (Fig.

4) for the nanomaterial remain almost constant up

to 800 �C. The decrease in linewidth for the bulk

powder results from the continuous growth of a

singlet component in the ESR signal, which arises

from the increase in unassociated CuO molecules,

as explained above.The heating curves obtained by thermal analy-

sis show two transitions. The one occurring below

1050 �C corresponds to the CuO to Cu2O transi-

tion [3], and the onset of the transition is taken as

the first deviation from the baseline. The higher

temperature transition is the melting transition.

The high temperature of the CuO to Cu2O tran-

sition is evidence that our calcination temperaturesare not high enough to induce a structural phase

transition. Two observations from this graph re-

late to the nanoparticle size distribution. First, the

onset of the CuO to Cu2O transition shows a

steady increase with annealing temperature. The

melting transition also shows this general trend

towards a higher transition temperature. These

shifts in transition temperatures result from theincrease in particle size with increased calcination

temperature. The supplied heat distributes readily

in the smaller particles but is not rapidly distrib-

uted in larger ones, leading to a delay in the phase

transition for material calcined at higher temper-

atures. Second, the CuO to Cu2O transition shows

a general broadening as the annealing temperature

increases, or as the particle size increases. This isan indication of a variation of particle sizes within

the sample. A broader peak suggests a large size

distribution, with the small particles undergoing

the transition early and larger ones transforming

later.

The exothermic peaks seen in the cooling curves

indicate a rapid phase transition from Cu2O to the

A. Viano et al. / Journal of Non-Crystalline Solids 325 (2003) 16–21 21

stable CuO phase. This phase transition again isparticle size dependent. CuO nanoparticles an-

nealed at lower temperature regain the CuO phase

at a lower temperature compared to those annealed

at higher temperatures. The width of the exother-

mic peak decreases with particle size, again an

indication of rapid heat dissipation from nano-

particles. DTA results thus clearly indicate the

influence of particle size on the heat distribution.

5. Conclusion

This comparative study of bulk and nano CuO

powder highlights the importance of particle size

on the magnetic properties of nanoparticles. The

presence of a broad ESR signal in both bulk andnanopowder arises from a weakening of anti-

ferromagnetic ordering in CuO. This is also evi-

denced by the shift of this signal towards the

higher field side. The difference in the linewidths of

the ESR signals between bulk and nanopowder

indicates a high defect concentration in nano-

powder, which must arise from the comparatively

high surface area of nanoparticles. The calcinationdoes increase the particle size of CuO nanoparti-

cles prepared via a sol–gel route, but does not

destroy the antiferromagnetic ordering until it is

above 800 �C. The observed higher stability of

antiferromagnetic ordering for the nanomaterial

arises from the rapid heat dissipation capability of

the high surface area nanoparticles. On the con-

trary, antiferromagnetic ordering breaks down at amuch lower temperature, around 400 �C, for the

bulk material. The uneven heat distribution and

high latent heat of bulk powder are the reasons for

this difference. Thus the unique characteristic of

nano CuO, such as a high surface to volume

fraction, is responsible for the preservation of

antiferromagnetic ordering. The unusual magnetic

behavior of nanomaterial is intimately coupled to

its unique physical properties. Further studies willinclude investigations of the magnetic moment as a

function of particle size.

Acknowledgements

The authors would like to thank Ms Sharon

Frase at the University of Memphis� IntegratedMicroscopy Center for assistance acquiring the

TEM images. This work was funded in part by a

grant from Rhodes College.

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