A microwave assisted solvothermal method is described for rapid preparation of nano-oxides. This method is based on exploiting differential dielectric constants to induce preferred heating and decomposition of the oxide precursors in the presence of suitable capping agents. This strategic approach has been used to prepare nanopowders of MgO, NiO, ZnO, Al2O3, Fe2O3 and ZrO2

Kumar, Sundeep

Mahesh, Krishnamurthy

Rao, K. J.

English

Publications of the IAS Fellows

Bull. Mater.  Sci., Vol.  28, No. 1, February  2005,  pp. 19–24. © Indian  Academy  of  Sciences. 
  19
A strategic approach for preparation of oxide nanomaterials 
K  J  RAO*,  KRISHNA MURTHY  MAHESH †  and  SUNDEEP  KUMAR  
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India 
 
MS received 30 November 2004 
Abstract. A microwave assisted solvothermal method is described for rapid preparation of nano-oxides. This 
method is based on exploiting differential dielectric constants to induce preferred heating and decomposition 
of the oxide precursors in the presence of suitable capping agents. This strategic approach has been used to 
prepare nanopowders of MgO, NiO, ZnO, Al2O3, Fe2O3 and ZrO2. 
Keywords. Microwave synthesis; nanomateri ls; oxide powders. 
1. Introduction 
 
Nanomaterials research has witnessed an exponential growth 
during the last decade (Edlstein and Cammarata 1998; 
Esaki 1999; Ding et al 2003; He t al 2003; Pang et al 
2003; Sankaranaray nan and Sreekumar 2003; Yin et al 
2004). In the nanometric range, metallic and small band 
gap semiconducting materials exhibit fascinating qua-
tum phenomena (Esaki 1999; Savchuk et al 2003). Large 
band gap materials such as oxides stabilize in their high 
temperature phases and exhibit enhanced surface pheno-
mena like catalysis and reduced reaction barriers for 
solid-state reactions (Agrafiotis et al 2000; Kaskel t al 
2004; Anderson et al 2004). Indeed, in the nanometric 
range, materials may be expected to b have quite diffe-
rently from both molecular and bulk states since the ratio 
of the number of surface atoms to the number of bulk 
atoms is quite high. As a consequence of this increased 
ratio the number of valence unsatiated atoms in the nano-
particle becomes significant. There is thus a curiosity to 
understand the behaviour of materials in the nano length 
scale and if possible to exploit the new properties exhi-
bited by materials purely as a consequence of the small-
ness of size. Nano oxide materials have found wide ranging 
applications particularly as catalysts and as starting mate-
rials for making advanced structural ceramics. During 
sintering and shaping of oxidic materials for practical
applications, use of nano sized particles as starting mate-
rials can be of great advantage because of the availability 
of large surface areas of the nanoparticles. It may be 
noted that surface areas of particles for a given quantity 
of the material scales as 1/<d>. Where <d> is the average 
diameter (linear dimension) of the particles. 
 Therefore, several methods have been develope  for 
the preparation of nanomaterials (Edelstein and Cammarata 
1998; Lee and Kim 2004; Pitisescu et al 2004). We have 
reported earlier (Rao et al 1998; Shen et al 2003; Lee and 
Choi 2004) from this laboratory that metal organics sub-
jected to microwave irradiation decompose and form oxides 
often i  the form of nanoparticles when subjected to micro-
wave irradiation. Nanometric products also result when a 
gaseous reactant is used in microwave synthesis as ob-
served in the preparation of aluminium nitride by carbother-
mal reduction cum nitridation method (Panneerselvam 
2002). Recently, a solvothermal microwave method has 
been described for the preparation of nanometric metal 
particles (Komarneni 2003). However, no general strategy 
has yet been described for the preparation of nanomaterials 
based on the use of microwaves. In this communication 
we report a novel microwave method for the preparation 
of oxide nanoparticles. The method is based on a strategy 
that is of very general applicability. Preparation of a few 
oxides is described which demonstrates the validity of 
the strategy. Of these oxides chosen, MgO, NiO, ZnO, 
Al2O3, Fe2O3 and ZrO2 have been pre ared in nanometric 
form by other methods and have been reported in the li-
terature. We suggest that the method described below can 
be extended for the preparation of nano materials other 
than oxides. 
2. Basis of the microwave strategy 
The approach is based on the following observations. 
When materials areir adiated with microwaves they absorb 
energy from the microwave field. The power abs rbed, P, 
is given by 
P = 2p f e0e¢ tan d  | E i |
 2. 
The frequency, f and the amplitude |E i | of the micro-
waves are instrumental parameters (controllable) while 
the dielectric constant, e ¢, and the loss tangent, tan d are 
 
*Author for correspondence (kjrao@sscu.iisc.ernet.in) 
†Under graduate Summer Research Fellow of the Indian Academy
of Sciences from IIT Madras. 
K  J  Rao,  Krishnamurthy  Mahesh  and  Sundeep  Kumar 
 
20
material parameters (e0 is the permittivity of the free 
space). Therefore, in a given instrumental set up ( | D Ei
 |
and f held constants), P is determined by e ¢and tan d. e ¢  
is generally high for materials containing molecules or 
complex ions of high dipole moment and therefore, high 
molar polarization. Therefore, when a mixture of two ma-
terials, A and B of different dielectric constants, e A¢  and 
e B¢ , is exposed to microwave field, the material with 
higher values of e ¢ absorbs en rgy preferentially and gets 
heated rapidly compared to the other. For the microwave 
decomposition of a material in a mixture the differential 
absorption of microwave energy can be used to advan-
tage. A precursor material can be chosen such that it has 
a high value of e ¢and decomposes by preferential micro-
wave absorption to yield the desired oxide. Thus, if a metal 
organic or an inorganic complex salt, of reasonably high 
value of dielectric constant that yields the desired oxide 
upon decomposition is dissolved in a suitable liquid mdium 
of low dielectric constant and irradiated with microwaves, 
we can expect the formation of the metal oxide. The oi  
generally forms as a suspension in the chosen liquid. It 
would of course be better o choose a precursor, which 
has high values of both e ¢ and tan d, but tan d values are 
often not readily available and are also highly tempera-
ture dependent. This may result in a disadvantage occ-
sionally if tan d decreases with temperature (as in a post 
transition region) in the region where e ¢ increases. We 
have not investigated these aspects fully in this commu-
nication. The liquid chosen should be one, which does 
not decompose before its normal boiling temperatur . Also 
the resulting oxide should not be soluble in it. The insolu-
ble oxide product, however, should be prevented from 
growing into large particles. This is accomplished by the 
addition of a suitable capping agent to the initial solution. 
It is also necessary to control the microwave power so 
that the solvent does not boil violently. 
3. Experimental 
Six different oxides were prepared using the above stra-
tegy as described below. The starting materials used for 
their preparation are given in table 1 along with their di-
electric constant values. These dielectric constant value  
were determined by measuring e ¢ (w) over a wide fre-
quency range and extrapolating e ¢(w) to w = 0. Measure-
ments were done on HP4192A Impedance Anlys r. A 
special cell was designed to measure e ¢ (w) of liquid pre-
cursors. Generally a precursor concentration of 5 g/100ml 
of solvent was employed. 0×2 g (per 100 ml of solvent) of 
poly vinyl pyrrolidone (PVP) was added as a capping 
agent. A homogeneous solution so obtained was kept in a 
round bottom flask fitted with a water-cooled refluxing 
system. A domestic (LG Scientific MS283MC 800 W) 
microwave oven operating at 2×45 GHz and with a six-
stage variable power was slightly modified to accomm-
d te the round bottomed flask with the at ached refluxing 
system; the reflux column was made to project out verti-
cally from the top f the oven. Solutions were refluxed 
or 30 min in each case with the power level held at 20% 
(160 W). Sometimes it was necessary to lower or elevate 
the microwave power level so that the contents did not 
boil too violently. The contents were allowed to col, 
transferred to centrifugation tubes, centrifuged for 30 min 
and the fine deposits at the bottom of the tubes were re-
covered after decanting the supernatant liquid. The deposits 
were repeatedly washed with acetone and recentrifuged 
wherever necessary. The wet samples were then dried 
before further experiments. Decomposition f the metal 
organics and the act tes in table 1 leads to formation of 
the respective oxides. The organic products of decompo-
sition are ethers and esters, which escape or remain partly 
dissolved. It may be noted that the dielectric constant of 
the solvent in each case is significantly lower than that of 
the precursor (solute). 
 The dry samples were heated in a Thermolyn  furnace 
to rid the particles of the capping agent and to crystallize 
the amorphous products. The temperatures and the soaki g 
times are given in table 2. The X-ray diffraction (XRD) 
patterns of the samples were examined at several tem-
peratures on a Siemens Diffrac Puls D5005 diffractome-
ter. A typical evolution of the XRD patterns with heat 
treatment is shown in figure 1 in the case of Al2O3. The 
phase identification was done by comparing with pub-
lished data. In all the cases the as-obtained powders were 
amorphous and most of them exhibited crystalline pat-
terns when heated to 200–40 °C (table 2). The amorphous 
samples heated to 400°C or higher were free from residual 
org nics and the capping agent. This was confirmed us-
ing infrared spectra (obtained from the Perkin Elmer FT 
Table 1. Precursor and solvent materials used in the present study. 
      
Nano material Precursor material Solvent       
MgO Magnesium ethylate (49) Ethyl acetate (6) 
ZnO Zinc acetate tetrahydrate (41) Ethanol (24×3) 
NiO Nickel acetate tetrahydrate (37) Ethanol (24×3) 
Al2O3 Aluminium-tri-sec butoxide (89) Ethyl acetate (6) 
Fe2O3 Ferric acetate (63) Ethanol (24×3) 
ZrO2 Zirconium (IV) isoprooxide (162) Ethyl acetate (6)       
A  strategic  approach for preparation  of oxide  nanomaterials 
 
21
Spectrum 1000) as shown in figure 2 for the typical case 
of zirconia in which the absorption in the region of 
~ 1500 cm–1 due to propyl oxy groups disappeared. Parti-
cle sizes were determined using Scherrer formula, 
D = kl /b cosq, 
where D is the mean diameter of the grains, k (= 0×96) is 
size factor, b the full width at half maximum, l the wave 
length of the X-rays used and 2q the angle at which the 
 
 
Figure 2. Infrared spectroscopy of the zirconia samples heated
to various temperatures. 
 
Table 2. Soaking time and sizes of the oxide particles*.     
Temperature (°C) 
 
 
 
Material 200 300 400 600 700 800 1000 1200                   
MgO  6 h 
a 
_ 
 
 6 h 
 9 Å 
 6 h 
14 Å 
_  6 h 
27 Å 
 1 h 
29 Å 
 6 h 
37 Å 
 
ZnO  6 h 
16 Å 
 6 h 
17 Å 
_ _  6 h 
34 Å 
_  6 h 
41 Å 
 
_ 
NiO  6 h 
a 
 6 h 
14 Å 
 
_ _ _ _ _ _ 
Al2O3  6 h 
a 
_  6 h 
a 
 6 h 
a 
_  6 h 
 7 Å 
 1 h 
 9 Å 
 2 h 
32 Å 
 
Fe2O3  6 h 
a 
_  6 h 
18 Å 
 6 h 
32 Å 
 
_ _ _ _ 
ZrO2  6 h 
a 
_  6 h 
14 Å 
 6 h 
16 Å 
_  6 h 
16 Å 
 1 h 
28 Å 
 2 h 
30 Å                   
*Particle sizes are not given when the phases are amorphous (indicated as ‘a’). 
 
 
 
Figure 1. Evolution of the X-ray diffraction pattern with heat 
treatment at elevated temperatures for the case of Al2O3. 
 
K  J  Rao,  Krishnamurthy  Mahesh  and  Sundeep  Kumar 
 
22
maximum intensity was observed. Wherever possible two 
or three diffraction peaks were chosen and consistency in 
the particle sizes obtained from using their widths was 
confirmed. The spread w s always within ± 2×5%. 
4. Results and discussion 
Particle sizes determined in this manner are given in ta-
ble 2 for samples heated to various temperatures. Particle 
sizes were also determined using light scattering meth-
ods, by dispersing the samples in ethyl acetate. Particle 
size distributions for all the samples are given in figure 3 
and the thermal histories are indicated in the figure itself. 
The agreement between particle sizes obtained from 
XRD and the sizes indicated by the highest frequency 
peaks in distribution from the light scattering experiment 
appear to be quite satisfactory. The particle size distribu-
tions are significantly narrow in the case of ZrO2 and 
Fe2O3. Particles of Al2O3, ZrO2, and MgO were also exa-
mined using transmission electron microscopy (TEM) 
and the TEM micrographs are shown together in figure 4. 
Although the particles agglomerate, they are distinguish-
able and the sizes can be measured. Particle sizes ob-
served in microscopy may be seen to be consistent with 
the sizes obtained from XRD (table 2) and light scatter-
ing measurements. 
 
Figure 3. The particle size distributions obtained from the dynamic light scattering experiment. 
 
A  strategic  approach for preparation  of oxide  nanomaterials 
 
23
 Therefore, in the present microwave approach, nano 
particles are prepared in a novel and efficient manner as 
the approach is based on a sound strategy. The primary 
requirement is that the oxide precursor has to have a di-
electric constant, significantly higher than the solvent. 
Ethanol is a good solvent in which most of the metal or-
ganics are soluble in the range of 5% by volume. Pre-
sence of a suitable capping agent ensures the formation of 
nanoparticles, which can be later, decomposed. In cases 
like ZrO2 and Al2O3, we found formation of nanoparticles 
even without the use of capping agent. Post preparative 
heat treatment generally leads to increased nanoparticle 
sizes. Thus, heat treatment also provides control on the 
particle sizes of the resulting oxide nanomaterials. The 
method can be used to prepare other classes of compounds 
like nitrides and chalcogenides using suitable complexes 
dissolved in amines and mercaptans, respectively. 
5. Conclusions 
A new strategy of microwave assisted solvothermal 
method has been applied for the preparation of nano-
oxides, MgO, NiO, ZnO, Al2O3, Fe2O3 and ZrO2. Based 
on this new strategy, other nano materials can also be pre-
pared using suitable starting metal organics and capping 
agents. 
Acknowledgement 
The authors acknowledge the financial support provided 
by the Indian Academy of Sciences, Bangalore. 
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K  J  Rao,  Krishnamurthy  Mahesh  and  Sundeep  Kumar 
 
24
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