On the synthesis and optical absorption studies of nano-size magnesium oxide powder

Article abstract of DOI:10.1016/j.jpcs.2008.06.143

Magnesium oxide (MgO) nano-size powder is synthesized using magnesium nitrate hexahydrate and oxalic acid as precursors with ethanol as a solvent. The process involves gel formation, drying at 100 °C for 24 h to form magnesium oxalate dihydrate [α-MgC₂O₄ · 2H ₂O] and its decomposition at 500, 600, 800, and 1000 °C for 2 h to yield MgO powder (average crystallite size ～6.5-73.5 nm). The sol-gel products at various stages of synthesis are characterized for their thermal behaviour, phase, microstructure, optical absorption, and presence of hydroxyl and other groups like o = C = O, C = O, C-C, etc. MgO powder is shown to possess an f.c.c. (NaCl-type) structure with lattice parameter increasing with decrease in crystallite size (t_av); typical value being ～4.222(2) A? for t_av～6.5nm as against the bulk value of 4.211 A?. Infrared absorption has shown MgO to be highly reactive with water. Also, a variety of F- and M-defect centres found in MgO produce energy levels within the band gap (7.8 eV), which make it attractive for application in plasma displays for increasing secondary electron emission and reducing flickering effects. The possible application of the intermediate sol-gel products, viz., α-MgC₂O₄ · 2H₂O and anhydrous magnesium oxalate (MgC₂O₄) in understanding the plants and ESR dosimetry, respectively, has also been suggested.

Full text of DOI:10.1016/j.jpcs.2008.06.143

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
On the synthesis and optical absorption studies of nano-size
magnesium oxide powder
Ã
Ashok Kumar, Jitendra Kumar
Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnesium oxide (MgO) nano-size powder is synthesized using magnesium nitrate hexahydrate and
Received 9 January 2008
Received in revised form
5 June 2008
oxalic acid as precursors with ethanol as a solvent. The process involves gel formation, drying at 100 1C
for 24 h to form magnesium oxalate dihydrate [
a-MgC2O4 ꢀ 2H2O] and its decomposition at 500, 600,
800, and 1000 1C for 2 h to yield MgO powder (average crystallite size ꢁ6.5–73.5 nm). The sol–gel
products at various stages of synthesis are characterized for their thermal behaviour, phase,
microstructure, optical absorption, and presence of hydroxyl and other groups like OQCQO, CQO,
C–C, etc. MgO powder is shown to possess an f.c.c. (NaCl-type) structure with lattice parameter
Accepted 30 June 2008
Keywords:
A. Inorganic compounds
A. Oxides
˚
increasing with decrease in crystallite size (tav); typical value being ꢁ4.222(2) A for tavꢁ6.5 nm as
˚
against the bulk value of 4.211 A. Infrared absorption has shown MgO to be highly reactive with water.
B. Sol–gel growth
Also, a variety of F- and M-defect centres found in MgO produce energy levels within the band gap
(7.8 eV), which make it attractive for application in plasma displays for increasing secondary electron
emission and reducing flickering effects. The possible application of the intermediate sol–gel products,
C. Thermogravimetric analysis (TGA)
D. Optical properties
viz.,
a
-MgC2O4 ꢀ 2H2O and anhydrous magnesium oxalate (MgC2O4) in understanding the plants and ESR
dosimetry, respectively, has also been suggested.
& 2008 Elsevier Ltd. All rights reserved.
1. Introduction
at relatively low temperatures [17–22]. Factors such as temperature,
time, pH, catalytic agent for gel formation, and the environ-
mental conditions drastically affect the characteristics of the final
product [32–34]. MgO powders have been prepared taking
magnesium ethoxide as precursor with different hydrolysis
catalysts, namely, HCl, C2H4O2, C2H2O4, or NH4OH for evolution
in the temperature range of 150–900 1C and control of average
particle size (16–88 nm) [34]. Thermal decomposition of magne-
sium alcoholates (via alcoxy/hydroxy intermediates) in air at
450–560 1C can also produce MgO of distinct particle size
depending on the alcohol chain length [35]. The addition of
20 mol% diethanolamine (DEA) stabilizes the magnesium meth-
oxide and facilitates the formation of MgO at 400 1C in oxygen
[36]. Kim et al. [23] studied the effect of acetic acid addition to
magnesium methoxide on the stability of the precursor and the
crystallization behaviour of sol–gel-derived MgO nano-size
powder. Accordingly, the heat treatment of gel product with
addition of acetic acid is found to lower the crystallization
temperature significantly. Recently, Chowdhury and Kumar [37]
have synthesized MgO with high degree of crystallinity and
tubular morphology using magnesium acetate as a precursor. The
particular interest lies in producing high-quality MgO for
application in plasma display panels so as to provide high
secondary electron emission, reduced flickering, and extended
service life. To achieve this, introduction of various F- and M-
defect centres between valence band of oxygen (2p) and
The IIA–VIB compounds, viz., oxides, sulphides, selenides, and
tellurides of alkaline earth elements usually exhibit an f.c.c. (NaCl-
type) crystal structure. Among them magnesium oxide (MgO)
occupies a special position due to its interesting properties and
potential industrial applications in pharmaceuticals, semiconduc-
tors, liquid crystal/electroluminescence/plasma/fluorescent dis-
plays, ferroelectric and superconductor thin films as substrate,
reflecting and antireflecting coatings, additives to heavy fuel oils,
toxic waste remediation, catalysis, lithium ion batteries, etc.
[1–16]. The novel and useful properties of MgO are further
enhanced when used as nano-size powder. Many synthesis routes
like sol–gel, hydrothermal, flame spray pyrolysis, laser vaporiza-
tion, chemical gas phase deposition, combustion aerosol synth-
esis, aqueous wet chemical, and surfactant methods provide
nano-size MgO [17–31]. However, the morphology and properties
of the resulting MgO differ and depend largely on the synthesis
route and processing conditions. The sol–gel process has assumed
a special significance for synthesis nowadays because it is simple,
cost effective, and capable of yielding unique properties (e.g., large
surface area-to-volume ratio, narrow particle size distribution, etc.)
Ã
Corresponding author. Tel.: +91512 2597107; fax: +91512 2597664.
E-mail address: jk@iitk.ac.in (J. Kumar).
0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2008.06.143

ARTICLE IN PRESS
A. Kumar, J. Kumar / Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
2765
conduction band of magnesium (3s and 3p) in MgO is essential.
F-and M-defect centres can be created by several methods, such as
electron-, neutron-, or -irradiation. An attempt has therefore
g
been made to synthesize nano-crystalline MgO with F- and
M-centres using an inorganic precursor, magnesium nitrate
hexahydrate [Mg(NO3)2 ꢀ 6H2O], and to study its formation,
morphology, infrared (IR) and ultraviolet–visible–near infrared
(UV–VIS–NIR) absorption characteristics.
2. Experimental
For synthesis of nano-size MgO, Mg(NO3)2 ꢀ 6H2O, and oxalic
acid [(COOH)2 ꢀ 2H2O] precursors in 1:1 molar ratio are first
dissolved separately in ethanol and stirred to obtain two clear
solutions. These are then mixed together to yield a thick white gel.
The gel product is digested for 12 h and dried subsequently at
100 1C for 24 h, ground, sieved through 240 mesh, calcined at 500,
600, 800, and 1000 1C for 2 h, and cooled at a rate of 10 1C/min.
Thermal analysis of the dried product is carried out with a
computer-controlled thermogravimetric analysis (TGA) set-up
developed in the laboratory to study its stability and formation
Fig. 1. Weight (W) vs. temperature (T) and (ꢂdW/dT) vs. T plots of sol–gel product
dried at 100 1C for 24 h; the temperature is raised at the rate of 4 1C/min in air. The
inset shows the differential thermal analysis (DTA) plot.
of MgO. While an X-ray powder diffractometer (Rich Seifert model
˚
ISO Debye flux 2002) with the CuK radiation (
l ¼ 1.5418 A) has
a
been used for the identification of the phase(s) and determining
the average crystallite size, electron microscopes (FEI Quanta
200HV SEM and FEI Tecnai G2 TEM) have been employed for
observing the microstructure and morphology. In addition, a
Fourier transform infrared (FTIR) spectrometer (BRUKER Vertex-70)
is used for the detection of hydroxyl (–OH) and other groups,
like OQCQO, CQO, C–C, etc., whereas a UV–vis–NIR spectro-
photometer (Varian model Carry 5000) is employed for optical
absorption measurements.
3. Results and discussion
3.1. Thermal analysis of sol–gel product
The thermal stability of the oven-dried precursor is monitored
through its weight by raising its temperature at the rate of 4 1C/min
in air up to 800 1C. Two main stages of weight loss are observed
(Fig. 1). In the first stage, a weight loss of ꢁ24% occurs in the
temperature range of 201–282 1C corresponding to an endother-
mic reaction as indicated by differential thermal analysis (DTA)
plot given in inset of Fig. 1. This weight loss can be attributed
to removal of crystallizing water of MgC2O4 ꢀ 2H2O, which is
ꢁ24.26%; close to the value mentioned above. The (ꢂdW/dT)
versus T plot shows a peak around 234 1C where the weight loss is
maximum. In the second stage, a weight loss of ꢁ47.5% takes place
in the temperature range of 416–564 1C. This reaction is also
endothermic in nature (inset of Fig. 1) and yields mainly MgO.
Needless to say, anhydrous magnesium oxalate (MgC2O4) in giving
off MgO should undergo a weight loss of ꢁ48.5% as against ꢁ47.5%
observed in the present case. Above 564 1C, the residues and other
volatile species come out with a weight loss of ꢁ1%. Considering
the above results, the decomposition temperature for the dried gel
powder is set in the range of 500–1000 1C for obtaining MgO in
the present study. The systematic steps involved in the synthesis
of pure MgO powder are shown in Scheme 1.
Scheme 1. The steps involved in sol–gel synthesis as per thermogravimetric
analysis.
having monoclinic structure with known lattice parameters
˚
˚
˚
a ¼ 12.680 A, b ¼ 5.391 A, c ¼ 9.977 A,
b ¼ 129.821, and Z ¼ 4
[JCPDS # 26-1223]. The decomposition of dried precursor powder
at 400 1C for 2 h causes complete removal of crystallizing water
and yields MgC2O4 (Fig. 3). Its XRD pattern corresponds to a
monoclinic phase and matches well with the lattice parameters
˚
˚
˚
a ¼ 9.413 A, b ¼ 6.229 A, c ¼ 5.724 A,
b ¼ 96.471, Z ¼ 4, and space
group C2/c [JCPDS # 26-1222]. On raising the decomposition
temperature further to 500 1C, a single f.c.c. phase of periclase
MgO emerges (Fig. 4a). This temperature lies in the range
416–564 1C, where decomposition of MgC2O4 takes place; the
maximum weight change occurring at 526 1C (Fig. 1). XRD
patterns recorded after decomposition of dried sol–gel precursor
at 500, 600, 800, and 1000 1C for 2 h each are shown in Fig. 4.
All these patterns correspond to periclase phase (NaCl–type) of
MgO with the values of lattice parameter as listed in Table 1;
˚
3.2. Phase evaluation
standard value for bulk MgO being 4.211 A [JCPDS # 45-0946].
The diffraction peaks are somewhat broader in Fig. 4a but
become sharper with increase in the decomposition temperature
(Figs. 4b–d) due to resulting increase in the average crystallite size
(tav). Table 1 gives the values of average crystallite size as
The X-ray diffraction (XRD) pattern of a sol–gel precursor after
drying at 100 1C for 24 h is shown in Fig. 2. Its indexing suggests
the formation of magnesium oxalate dihydrate [
a-MgC2O4 ꢀ 2H2O]

ARTICLE IN PRESS
2766
A. Kumar, J. Kumar / Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
Fig. 4. X-ray diffraction patterns of dried sol–gel product after decomposition at
(a) 500, (b) 600, (c) 800, and (d) 1000 1C for 2 h each. These indicate formation of
f.c.c. (NaCl-type) phase of magnesium oxide.
Fig. 2. X-ray diffraction pattern of sol–gel product dried at 100 1C for 24 h;
showing emergence of monoclinic phase of
a-MgC2O4 ꢀ 2H2O.
Table 1
The lattice parameter and average crystallite size (tav) of MgO powder after
decomposition of dried sol–gel product at 500, 600, 800, and 1000 1C for 2 h each
Decomposition temperature
Lattice parameter
(nm)
Crystallite size tav
(1C)
(nm)
500
600
0.4222(2)
0.4221(2)
0.4213(2)
0.4212(2)
6.5
9.5
800
28.5
73.5
1000
Fig. 3. X-ray diffraction pattern of dried sol–gel product after decomposition at
400 1C for 2 h. It corresponds to monoclinic phase of anhydrous magnesium
oxalate (MgC2O4).
determined from Scherrer formula with full-width at half-
maximum (FWHM) of 200 and 220 diffraction peaks,
taking silicon as standard for accounting for the instrumental
broadening.
The lattice parameter of the MgO produced is found to
decrease with increase in tav (Fig. 5), as reported in other oxides,
namely, Fe2O3 and yttrium aluminum garnet (YAG) [38,39]. The
Fig. 5. Lattice parameter and surface area-to-volume ratio as a function of average
crystallite size in MgO powder.
surface area-to-volume ratio (
crystallites as spherical and using the relation
Z
; calculated by considering the
¼ 6/tav; tav being
surface of crystallites. Also, H2O being polar forms dipoles on
adsorption at the surface. Such features are enhanced with
increase in the surface area. The repulsion of dipoles (of polar
H2O and unpaired electrons) then produces tensile stresses on the
underlying layer, and, in turn, increases the inter-ionic separation.
The nitrogen adsorption method has been used to determine
the BET surface area and pore size distribution of a typical MgO
powder obtained by decomposition of oxalate at 600 1C for 2 h.
Z
the diameter) also varies in the same manner with tav (Fig. 5).
Thus, the increase in lattice parameter with decrease in tav is
essentially caused by the corresponding increase in the surface
area or underlying effects (e.g., surface stresses). In oxides, the
bonds are directional and there exist dangling bonds with
unpaired electrons because of reduced coordination on the outer

ARTICLE IN PRESS
A. Kumar, J. Kumar / Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
2767
Fig. 6 shows the pore size distribution curve derived from the
nitrogen adsorption isotherm (included in inset). The sample has a
BET-specific surface area of about 229.3 m2/g, pore volume of
0.84 cm3/g, and average pore diameter of 17 nm. The average
particle size as deduced from the BET-specific surface area is
7.3 nm, which matches well with the crystallite size (ꢁ9.5 nm)
obtained by XRD. The high values of BET surface area and pore
volume can improve characteristics of MgO powder appreciably to
make it suitable for catalytic activity (i.e., adsorption and
dissociation of molecules like, CH3OH, CH4, NO2, H2O, CO, and
CO2), toxic waste remediation, etc. [12–15].
3.3. Microstructure
Fig. 7 shows a few typical scanning electron micrographs (in
SE-mode) at various stages of the synthesis process. The gel after
drying at 80 1C for 24 h contains numerous rods of nearly the same
diameter (ꢁ175 nm) but of various lengths (Fig. 7a). On examining
the micrograph closely, rods appear to be made of particles
(Fig. 7a). Some spherical particles lying independently can also be
noticed. Drying the gel at 100 1C for 24 h exhibits
a-MgC2O4 ꢀ 2H2O
crystals of parallelopiped morphology with varying size (Fig. 7b).
The length of parallelopipeds lies between 680 and 3940 nm while
base dimensions are in the range 200–1500 nm. The parallelopi-
peds with square base and smaller area have higher population
density (Fig. 8a). The base grows asymmetrically (i.e., more in any
one direction) and thus takes a rectangular shape. However,
population density of parallelopipeds decreases progressively
with increase in the base area (Fig. 8a).
a-MgC2O4 ꢀ 2H2O is an
important biomineral found in thallus of Lecanora altra, rock–
lichen interface, and cactaceae plant species or is naturally formed
by interaction of living organism with mineral surroundings [40].
By adjusting the digestion and drying conditions of sol–gel
product, its morphologies may be varied (parallelopiped, sphere,
rod, etc.) and corresponding changes in characteristics can be
Fig. 6. The pore size distribution curve and the corresponding nitrogen isotherm
(inset) of MgO powder, obtained by decomposition of magnesium oxalate
dihydrate at 600 1C for 2 h in air.
utilized for a better understanding of the plants.
a-MgC2O4 ꢀ 2H2O
after decomposition at 400 1C for 2 h yields MgC2O4 crystals,
Fig. 7. Scanning electron micrographs of (a and b) sol–gel product after drying at 80 and 100 1C for 24 h each, respectively, showing morphology of
(c and d) MgC2O4 and MgO formed after sol–gel product dried at 100 1C for 24 h is further calcined at 400 and 600 1C for 2 h each, respectively, in air.
a-MgC2O4 ꢀ 2H2O, and

ARTICLE IN PRESS
2768
A. Kumar, J. Kumar / Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
Fig. 8. Population density of parallelopipeds as a function of base area for (a)
a-MgC2O4 ꢀ 2H2O and (b) MgC2O4.
emergence of small distorted rods (Fig. 7d) with diameter in the
range of 110–330 nm and an average diameter of ꢁ195 nm. Fig. 9
shows a typical transmission electron micrograph of MgO powder
obtained by decomposition of dried sol–gel product at 1000 1C for
2 h. Here, nano-size crystallites are closely seen as grouping
together and forming a rod-like morphology.
3.4. Optical absorption
3.4.1. IR absorption of sol–gel product
The FTIR spectrum of precursor gel after drying at 100 1C for
24 h is shown in Fig. 10. The broad absorption band around
3387 cmꢂ1 is composed of several peaks in the range
3100–3700 cmꢂ1. It corresponds to the –OH-group-stretching
vibrations. This observation provides evidence for the presence
of chemically bound H2O in the magnesium oxalate. The two
absorption bands around 2978 and 2851 cmꢂ1 are due to surface
–OH stretch arising from hydroxyl groups in dissociated state or
C–H stretch of organic residue [42]. An important absorption
peak at 1649 cmꢂ1 appears due to chelating with carbonyl group
(ua (CQO)) and formation of an inorganic ester (i.e., magnesium
oxalate, in this case). On careful examination, it is found that the
peak comprises of a doublet with wave numbers 1653 and
1630 cmꢂ1
, which represent symmetrical and asymmetrical
Fig. 9. Transmission electron micrograph of MgO powder obtained by decom-
position of dried sol–gel product at 1000 1C for 2 h.
vibrations of the carbonyl group, respectively. The deformational
mode of water usually seen around 1600 cmꢂ1 is, however,
masked by the above doublet. The unionized and uncoordinated
COO, characterized by a single stretching absorption band at
1750–1700 cmꢂ1, shifts towards the lower frequency and splits
into two bands (as observed above) when it combines with metal
ions as (C2O4)2ꢂ ion [43]. Thus, IR absorption confirms the
which also have parallelopiped morphology, but of relatively
reduced size (Figs. 7c and 8b). Their length varies from 550 to
3110 nm with base dimensions being in the range of 95–710 nm. It
can act as
concentration of
a
standard reference material for determining
-radiation-induced free radicals in electron spin
g
formation of
a-MgC2O4 ꢀ 2H2O (revealed by XRD in Section 3.2).
resonance (ESR) dosimetry because of its high sensitivity, stability,
and longer life time than alanine in use currently [41].
The addition of aliovalent ions in MgC2O4 may further improve
its sensitivity and application in a wide dynamic dose range.
A further increase in temperature (say, to 600 1C) leads to
decomposition of the precursor powder to MgO, exhibiting
The other sharp absorption peaks at 1371 and 1325 cmꢂ1 are
assigned to ns (C–O)+
d
(OCQO) modes. While the weak band at
1118 cmꢂ1 corresponds to C–O stretching, a peak at 688 cmꢂ1 is
attributed to bending mode of OQCQO and/or libration of
water. All the observed absorption bands with respective intensity
and possible origin/vibration modes are listed in Table 2.

ARTICLE IN PRESS
A. Kumar, J. Kumar / Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
2769
Fig. 11. Fourier transform infrared (FTIR) spectra of MgO powder obtained by
decomposition of magnesium oxalate dihydrate at (a) 500, (b) 600, (c) 800, and (d)
1000 1C for 2 h each in air.
Fig. 10. Fourier transform infrared (FTIR) spectrum of sol–gel product dried at
100 1C for 24 h in air.
Table 3
Table 2
FTIR spectroscopic data of MgO obtained by decomposition of magnesium oxalate
dihydrate at 500, 600, 800, and 1000 1C for 2 h each
FTIR spectroscopic data of magnesium oxalate dihydrate obtained by sol–gel
process after drying the sol–gel product at 100 1C for 24 h [37,42,43]
Temperature (1C)
–OH stretching
(cmꢂ1
FWHM
(cmꢂ1
)
Relative amount
of water
Peak
Intensity
Vibrational mode(s)
)
position
(cmꢂ1
)
500
600
3432.2
3432.1
3429.0
3419.4
840
800
732
500
100
71
3386.6
2977.8
2850.5
2314.0
1909.3
Strong
–OH stretching
800
69
12
Weak
–OH stretching (in phase)/C–H stretching
–OH stretching (out phase)
CQO asymmetric stretch
Bending and asymmetric stretching v2 modes
polarized in the H–H direction of the H2O
molecule
1000
Very weak
Medium
Weak
distribution of hydroxyl groups is narrowing with increase of the
crystallite size. But, the peak itself undergoes a minor red shift of
ꢁ1 cmꢂ1, suggesting weak binding of the hydroxyl groups at Mg2+
sites in bigger crystallites. The absorption band in the wave
number range 3660–3000 cmꢂ1 is quite broad (FWHM being
500–840 cmꢂ1, Table 3). This is assigned to –OH-stretching mode
of adsorbed H2O on the surface of MgO crystallites. The broadness
in the spectra confirms a high degree of hydrogen bonding of
water molecules among themselves and with the crystallite
surface. Also, the absorption peak undergoes a red shift from
1649.4
Strong
na (CQO)/H–O–H stretch and bending
association
1371.0
1325.0
1170.0
1118.0
831.2
Strong
ns (C–O)+
ns (C–O)+
ns (C–O)+
d
d
d
(OCQO)
(OCQO)
(OCQO)
Strong
Very weak
Very weak
Strong
C–O/C–C stretching
(O–CQO)+ (Mg–O)
Water libration, OQCQO bending
(Mg–O)+ (C–C)
d
n
688.5
499.5
Strong
Strong
n
n
3432 to 3419 cmꢂ1 and becomes weaker with increase in tav
,
3.4.2. IR absorption of MgO powder
indicating progressive reduction in the degree of binding of H2O
molecules at the crystallite surface. Table 3 summarizes the
frequency, FWHM, and intensity values of –OH-stretching vibra-
tion modes.
Fig. 11 shows the FTIR transmittance spectra in the wave
number range of 4000–3000 cmꢂ1 for MgO powders produced by
decomposition of
a-MgC2O4 ꢀ 2H2O (derived from sol–gel process)
at 500, 600, 800, and 1000 1C each for 2 h. These contain
signatures of adsorption and chemisorption of water. The sharp
peak centred around 3700 cmꢂ1 (Figs. 11a–c) indicates the
presence of hydroxyl group at the low-coordination sites or
defects [44]. Since the population of low-coordination sites
decreases with increase in crystallite size, the effect is expected
to be less pronounced at relatively larger tav. In the present case,
tav increases with increase in the decomposition temperature. So,
the absorption peak centred around 3700 cmꢂ1 is weakened
gradually with increase in the decomposition temperature and
disappears completely in MgO powders formed by decomposition
at higher temperature ꢁ1000 1C. Further, the FWHM of this
absorption peak decreases and the values are 26, 22, and 12 cmꢂ1
for MgO powders obtained by decomposition of sol–gel product at
500, 600, and 800 1C, respectively. This means that the energy
3.4.3. UV–VIS–NIR absorption of MgO powder
Fig. 12 shows absorption spectra of MgO powder obtained by
decomposition of the dried sol–gel product in air at 500, 600, 800,
and 1000 1C for 2 h each in the wavelength range of 190–1450 nm.
While the nature of the spectrum remains essentially the same,
the degree of absorption varies somewhat with the crystallite size.
A prominent absorption peak observed at ꢁ6.2 eV in all MgO
samples except that obtained by decomposition of sol–gel product
at 1000 1C for 2 h (Fig. 12) is due to interband transition associated
with surface ions of five-fold coordination; notice that the energy
value is smaller than the band gap (7.8 eV) of bulk MgO. Other
absorption bands can be understood in terms of oxygen vacancy/
defect centres created both in the bulk and on the surface of MgO

ARTICLE IN PRESS
2770
A. Kumar, J. Kumar / Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
Fig. 12. UV–VIS–NIR spectra of MgO powder obtained by decomposition of magnesium oxalate dihydrate at (1) 500, (2) 600, (3) 800, and (4) 1000 1C for 2 h each in the
wavelength range of (a) 190–230, (b) 230–270, (c) 275–325, (d) 350–550, (e) 800–1000, (f) 1050–1250, (g) 1260–1350, and (h) 1350–1450 nm.
near the valence band. In F+ and F+s centres the singly occupied
2
ground state is A1g(1s)1. Further, if both the electrons are
removed or promoted to the conduction band, a doubly charged
vacancy, F2+ (or Fs2+
) is formed and the associated energy
level moves still closer to the valence band. A group of two same
defect centres (F, F+, or F2+) in close proximity gives rise to
M-centres (M, M+, or M2+).
Scheme 2. Decomposition of magnesium oxalate; open square stands for oxygen
vacancy.
A weak and broad absorption band around 5.02 eV in powder
produced at the decomposition temperature of 500 1C corre-
sponds to 1A1g-1T1u transition of F-centres, as observed in a MgO
single crystal after reduction at ꢁ1600–1800 1C under magnesium
vapour (pressure ꢁ4000 Torr) and predicted by the second-order
perturbation theory [45,46]. The characteristic peaks at 2.85, 1.31,
and 0.96 eV disappear completely in samples formed at the
decomposition temperature of 800 1C and above (i.e., having
larger tav and hence reduced surface area). Therefore, the above
peaks possibly originate from species located at/or near the
surface in smaller crystallites only. Also, the theoretical calcula-
tions show splitting of Ms centres into bonding and anti-bonding
singlet states, lying 2.18 and 0.82 eV (instead of 1.22 eV in the case
during decomposition of magnesium oxalate itself. The process
can be described by Scheme 2.
If the oxygen vacancy retains its two electrons, the defect is
called an F-centre (i.e., F for bulk and Fs for surface). This produces
an energy level in middle of the valence band (2p oxygen) and
conduction band (3s and 3p magnesium) edges, which is
symmetric (a1g) in both Oh and C4 configurations, and can be
classified as a 1s-type orbital. The ground state is therefore
1A1g(1s)2 for both bulk and surface defects. When one of the two
electrons of F-centre is promoted to the conduction band, the
defect F+ (or F+s) is created and its energy level moves downwards,

ARTICLE IN PRESS
A. Kumar, J. Kumar / Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
2771
of Fs centre) below the conduction band [47]. As a consequence,
the observed peaks at 1.31 and 0.96 eV are assigned to 1A1-1A1
and 1A1-1B1 transitions, respectively, of Ms centres. Time-
dependent density functional theory predicts the peaks for the
above transitions at somewhat higher energies, viz., 1.48 and
1.19 eV, respectively [45]. A shoulder at 2.85 eV occurs due to
epitaxial thin films on (0 01) MgO for room temperature high-frequency
tunable microwave elements, Appl. Phys. Lett. 87 (2005) 142905–142907.
[7] J.M. Phillips, Substrate selection for high-temperature superconducting thin
films, J. Appl. Phys. 79 (1996) 1829–1848.
[8] J.F. Hamet, B. Mercey, M. Hervieu, B. Raveau, a-Axis oriented YBa2Cu3O7
superconducting thin film grown on MgO substrate, Physica C 193 (1992)
465–470.
[9] J.F. Hamet, B. Mercey, M. Hervieu, G. Poullain, B. Raveau, a-Axis oriented
superconductive YBCO thin films: growth mechanism on MgO substrate,
Physica C 198 (1992) 293–302.
1A0(gs)-1A0 (1s-2px,y
) transition from a four-coordinated
F-centre, as observed in epitaxially grown thin film of MgO on
Ag(10 0) substrates [46,47]. The splitting of 2p-like levels at the
surface gives rise to two singlet-to-singlet, 1A1g-1A1g (1s-2pz)
and 1A1g-1E (1s-2px or 2py) transitions and are associated with
absorption peaks at 3.2 and 4.2 eV, respectively. The absorption
¨
[10] O. Duyar, H.Z. Durusoy, Design and preparation of antireflection and reflection
optical coatings, Turk. J. Phys. 28 (2004) 139–144.
[11] P. Davini, R. Tartarelli, Emissions from the combustion of heavy oils: effect of
granulometry and surface area of MgO additive, Fuel 64 (1984) 380–383.
[12] P. Jeevanandam, K.J. Klabunde, A study on adsorption of surfactant molecules
on magnesium oxide nanocrystals prepared by an aerogel route, Langmuir 18
(2002) 5309–5313.
1
peak at 1.5 eV is related to A0(gs)-1E (1s-2px,y) transition of
three-coordinated F-centres. The absorption band found at 1.1 eV
is attributed to spin forbidden 1A1g-3A1g (1s-2pz) transitions of
Fs centres. Further, a broad intense peak at 0.9 eV perhaps arises
due to bulk M-centres. The observed peaks at 4.2, 3.2, 1.5, 1.1, and
0.9 eV have also been predicted by multi-reference (MR) and
difference dedicated (DD) configuration interactions but with
energies higher by ꢁ1 eV. Also, MgO thin films exhibit peaks at 3.4
and 1.0 eV in their low energy loss spectra [48,49]. Many
absorption peaks, instead of a few observed earlier [50], appear
possibly due to different work functions of crystallites because of
(i) prevailing size distribution and (ii) variation in the density of
H2O dipoles.
[13] J.V. Stark, K.J. Klabunde, Nanoscale metal oxide particles/clusters as chemical
reagents. Adsorption of hydrogen halides, nitric oxide, and sulfur trioxide on
magnesium oxide nanocrystals and compared with microcrystals, Chem.
Mater. 8 (1996) 1913–1918.
[14] R. Kakkar, P.N. Kapoor, K.J. Klabunde, Theoretical study of the adsorption
of formaldehyde on magnesium oxide nanosurfaces: size effects and the
role of low-coordinated and defect sites, J. Phys. Chem.
18140–18148.
B 108 (2004)
[15] L. Qingwen, Y. Hao, C. Yan, Z. Jin, L. Zhongfan, A scalable CVD synthesis of
high-purity single-walled carbon nanotubes with porous MgO as support
material, J. Mater. Chem. 12 (2002) 1179–1183.
[16] Y. Gu, D. Chen, X. Jiao, F. Liu, LiCoO2–MgO coaxial fibers: co-electrospun
fabrication, characterization and electrochemical properties, J. Mater. Chem.
17 (2007) 1769–1776.
[17] J. Jiu, K-i. Kurumada, M. Tanigaki, M. Adachi, S. Yoshikawa, Preparation of
nanoporous MgO using gel as structure-direct template, Mater. Lett. 58
(2003) 44–47.
[18] A.V. Chadwick, I.J.F. Poplett, D.T.S. Maitland, M.E. Smith, Oxygen speciation
in nanophase MgO from solid-state 17O NMR, Chem. Mater. 10 (1998)
864–870.
4. Conclusions
[19] X. Bokhimi, A. Morales, M. Portilla, A. Garcıa-Ruiz, Hydroxides as precursors
of nanocrystalline oxides, Nanostruct. Mater. 12 (1999) 589–592.
[20] A. Subramania, G.V. Kumar, A.R. Sathiya Priya, T. Vasudevan, Polyol-mediated
thermolysis process for the synthesis of MgO nanoparticles and nanowires,
Nanotechnology 18 (2007) 225601, (1–5).
MgO powder (average crystallite size ꢁ6.5–73.5 nm, NaCl-type
˚
structure with lattice parameter a ¼ 4.222–4.212 A) can be
prepared by a simple and cost-effective sol–gel process using
´
´
´
[21] T. Lopez, R. Gomez, J. Navarrete, E. Lopez-Salinas, Evidence for Lewis and
Brønsted acid sites on MgO obtained by sol–gel, J. Sol–Gel Sci. Technol. 13
(1998) 1043–1047.
Mg(NO3)2 ꢀ 6H2O and (COOH)2 ꢀ 2H2O as precursors with ethanol
as a solvent. The sol–gel product is
a-MgC2O4 ꢀ 2H2O (formed
naturally in rocks, plants, etc.) of parallelopiped morphology,
which on decomposition at 500 1C or above yields pure MgO.
Observations indicate that the synthesized MgO is quite suitable
for adsorption and dissociation of polar molecules, toxic waste
remediation, etc. Further, it contains F- and M-defect centres,
which are responsible for creating energy levels within the band
gap (7.8 eV) of MgO. Such a characteristic is vital for enhancement
of secondary electron emission efficiency, reduction of flickering,
etc., and therefore, for its application in plasma display panels.
[22] S. Utamapanya, K.J. Klabunde, J.R. Schlup, Nanoscale metal oxide particles/
clusters as chemical reagents. Synthesis and properties of ultrahigh surface
area magnesium hydroxide and magnesium oxide, Chem. Mater. 3 (1991)
175–181.
[23] J.Y. Kim, H.S. Jung, K.S. Hong, Effects of acetic acid on the crystallization
temperature of sol–gel-derived MgO nano-powders and thin films, J. Am.
Ceram. Soc. 88 (2005) 784–787.
´
´
[24] J.A. Wang, O. Novaro, X. Bokhimi, T. Lopez, R. Gomez, J. Navarrete, M.E. Llanos,
´
E. Lopez-Salinas, Characterizations of the thermal decomposition of brucite
prepared by sol–gel technique for synthesis of nanocrystalline MgO, Mater.
Lett. 35 (1998) 317–323.
[25] J.A. Wang, X. Bokhimi, O. Novaro, T. Lo´pez, R. Go´mez, Effects of the surface
structure and experimental parameters on the isopropanol decomposition
catalyzed with sol–gel MgO, J. Mol. Catal. A 145 (1999) 291–300.
[26] Y. Ding, G. Zhang, H. Wu, B. Hai, L. Wang, Y. Qian, Nanoscale magnesium
hydroxide and magnesium oxide powders: control over size, shape, and
structure via hydrothermal synthesis, Chem. Mater. 13 (2001) 435–440.
[27] M.S. El-Shall, W. Slack, W. Vann, D. Kane, D. Hanley, Synthesis of nanoscale
metal oxide particles using laser vaporization/condensation in a diffusion
cloud chamber, J. Phys. Chem. 98 (1998) 3067–3070.
Acknowledgement
One of the authors (A.K.) is thankful to the Council of Scientific
and Industrial Research (CSIR), New Delhi for providing a senior
research fellowship.
[28] J.S. Matthews, O. Just, B. Obi-Johnson, W.S. Rees Jr., CVD of MgO from a
Mg(b-ketoiminate)2: preparation, characterization, and utilization of an
intramolecularly stabilized, highly volatile, thermally robust precursor, Chem.
Vap. Deposition 6 (2000) 129–132.
References
[29] J.J. Helble, Combustion aerosol synthesis of nanoscale ceramic powders,
J. Aerosol Sci. 29 (1998) 721–736.
[1] S. Shen, P.S. Chow, F. Chen, R.B. Tan, Submicron particles of SBA-15 modified
with MgO as carriers for controlled drug delivery, Chem. Pharm. Bull. 55
(2007) 985–991.
[2] Z. Zhou, S. Xie, D. Wan, D. Liu, Y. Gao, X. Yan, H. Yuan, J. Wang, L. Song, L. Liu,
W. Zhou, Y. Wang, H. Chen, J. Li, Multidimensional magnesium oxide
nanostructures with cone-shaped branching, Solid State Commun. 131
(2004) 485–488.
[3] F. Haraguchi, K-i. Inoue, N. Toshima, S. Kobayashi, K. Takatoh, Reduction of the
threshold voltages of nematic liquid crystal electrooptical devices by doping
inorganic nanoparticles, Jpn. J. Appl. Phys. 46 (2007) L796–L797.
[4] J.P. Boeuf, Plasma display panels: physics, recent developments and key
issues, J. Phys. D 36 (2003) R53–R79.
[30] A. Bhargava, J.A. Alarco, I.D.R. Mackinnon, D. Page, A. Ilyushechkin, Synthesis
and characterisation of nanoscale magnesium oxide powders and their
application in thick films of Bi2Sr2CaCu2O8, Mater. Lett. 34 (1998) 133–142.
[31] G. Kordas, Sol–gel preparation of MgO fibers, J. Mater. Chem. 10 (2000)
1157–1160.
[32] S.H. Liang, I.D. Gay, Adsorption and thermal decomposition of methanol on
magnesium oxide.13C NMR studies, Langmuir 1 (1985) 593–599.
[33] K.J. Klabunde, J. Stark, O. Koper, C. Mohs, D.G. Park, S. Decker, Y. Jiang,
I. Lagadic, D. Zhang, Nanocrystals as stoichiometric reagents with unique
surface chemistry, J. Phys. Chem. 100 (1996) 12142–12153.
[34] X. Bokhimi, A. Morales, T. Lopez, R. Gomez, Crystalline structure of MgO
prepared by the sol–gel technique with different hydrolysis catalysts, J. Solid
State Chem. 115 (1995) 411–415.
[5] J. Zhang, F. Pan, W. Hao, T. Wang, Effect of MgO doping on the luminescent
properties of ZnO, Mater. Sci. Eng. B 129 (2006) 93–95.
[6] S.W. Liu, J. Weaver, Z. Yuan, W. Donner, C.L. Chen, J.C. Jiang, E.I. Meletis,
W. Chang, S.W. Kirchoefer, J. Horwitz, A. Bhalla, Ferroelectric (Pb, Sr)TiO3
[35] H. Thoms, M. Epple, H. Viebrock, A. Reller, Magnesium alcoholates as
precursors for magnesia, J. Mater. Chem. 5 (1995) 589–594.

ARTICLE IN PRESS
2772
A. Kumar, J. Kumar / Journal of Physics and Chemistry of Solids 69 (2008) 2764–2772
[36] H.S. Jung, J.-K. Lee, J.-Y. Kim, K.S. Hong, Crystallization behaviors of nanosized MgO
particles from magnesium alkoxides, J. Colloid Interface Sci. 259 (2003) 127–132.
[37] A. Chowdhury, J. Kumar, Structural, thermal, dielectric studies on sol–gel derived
MgO from non-alkoxide route, Mater. Sci. Technol. 22 (2006) 1249–1254.
[38] P. Vaqueiro, M. Arturo Lo´pez-quintela, Synthesis of yttrium aluminium garnet
by the citrate gel process, J. Mater. Chem. 8 (1998) 161–163.
[39] P. Ayyub, M. Multani, M. Barma, V.R. Palkar, R. Vijayaraghavan, Size-induced
structural phase transitions and hyperfine properties of microcrystalline
Fe2O3, J. Phys. C: Solid State Phys. 21 (1988) 2229–2245.
[40] P.V. Monje, E.J. Baran, Evidence of formation of glushinskite as a biomineral in
a Cactaceae species, Phytochemistry 66 (2005) 611–614.
[41] G.M. Hassan, U. Ulusoy, M. Ikeya, Radical formation in lithium and
magnesium oxalate, Jpn. J. Appl. Phys. 39 (2000) 6236–6242.
[44] M. Foster, M. Furse, D. Passno, An FTIR study of water thin films on
magnesium oxide, Surf. Sci. 502–503 (2002) 102–108.
´
[45] D. Domınguez-Ariza, C. Sousa, F. Illas, D. Ricci, G. Pacchioni, Ground- and
excited-state properties of M-center oxygen vacancy aggregates in the bulk
and surface of MgO, Phys. Rev. B 68 (2003) 054101–054109.
[46] L.A. Kappers, R.L. Kroes, E.B. Hensley, F+ and F centers in magnesium oxide,
Phys. Rev. B 1 (1970) 4151–4157.
[47] A. Gibson, R. Haydock, J.P. LaFemina, Stability of vacancy defects in MgO: the
role of charge neutrality, Phys. Rev. B 50 (1994) 2582–2592.
[48] J. Kramer, W. Ernst, C. Tegenkamp, H. Pfnu¨r, Mechanism and kinetics of
color center formation on epitaxial thin films of MgO, Surf. Sci. 517 (2002)
87–97.
[49] C. Sousa, G. Pacchioni, F. Illas, Ab initio study of the optical transitions of F
centers at low-coordinated sites of the MgO surface, Surf. Sci. 429 (1999)
217–228.
[50] M.-C. Wu, C.M. Truong, D.W. Goodman, Electron-energy-loss-spectroscopy
studies of thermally generated defects in pure and lithium-doped MgO(10 0)
films on Mo(10 0), Phys. Rev. B 46 (1992) 12688–12694.
[42] L.D. Site, A. Alavi, R.M. Lynden-Bell, The structure and spectroscopy of
monolayers of water on MgO: an ab initio study, J. Chem. Phys. 113 (2000)
3344–3350.
[43] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds,
Wiley, London, 1963, p. 206.