# Synthesis and Characterization of Neodymium Oxide in  Silica Matrix by Solgel Protocol Method

Authors: Surender Duhan; Paveen Aghamkar; M. Singh
Journal: Research Letters in Physics
 (2008)

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## Abstract

Formation of nanocrystalline rare earth was prepared by solgel route, using tetra-ethoxysilane and Nd(NO3)3 as precursor materials and HCl as a catalyst. The prepared samples were submitted to thermal treatments at temperature 500∘C (5 hours) and 800∘C (10 hours). Structural changes were investigated by XRD, FTIR spectroscopy, and SEM. At 800∘C (10 hours) resulted in the formation of cubic Nd2O3 nanocrystallites with average size ∼ 20 nm.


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## Body

## 1. Introduction

There is growing
interest in nanostructured inorganic materials in large part because they often
exhibit properties distinct from those of the bulk, that can prove usefulness
in various applications. Recently, nanolanthanide oxides containing silica have
attracted a great deal of interest due to their macroscopical properties such
as high mechanical resistance, chemical stability, and heat resistance [1–5].
Specially for these applications, silica has been preferred as host matrix, due
to its higher softening temperatures, higher thermal shock resistance, and
lower index of refraction, over the other oxide glasses [6–12].There are many methods to synthesize
nanocomposites. Recently, methods, for example,
precipitation in high-boiling polyalcohol solutions, inverse
microemulsion, and hydrothermal solgel autocombustion, and so forth have been
used to synthesize neodymium oxides nanocrystallites in glass matrix. Many
researchers [10–12] have pointed out that the formation of rare-earth oxides
inside or at the surface of amorphous SiO2 matrix mainly depends on
the preparation method and calcination. In particular, Kępiński et al. [4]
synthesized and characterized thin film of Nd2O3 on the glass
slide and the stainless steel plate. For proper utilization of binary oxides
systems, specially nanocrystalline Ln2O3 (Ln, lanthanide)
containing SiO2, in scientific and technological applications,
requires a better understanding of the phase diagrams and interionic
interactions of the binary oxides. The phase evolution and interaction
mechanisms are deeply involved in the fundamental physics of rare-earth
ions/oxides and silica. Thus, in the present report, we have investigated
effect of the temperature as well as annealing time on the binary oxide and
found that the phase evolution of rare-earth oxides depends on thermal
treatment. The binary oxide was synthesized by the solgel protocol method. The
stem of present study is in the results of our earlier report [13], in which we
reported that the thermal annealing history plays crucial role in altering the
fundamental of size on Nd2O3-doped silica powder prepared
by the solgel protocol method. In that investigation [13], it was observed
that the nanostructures of the Nd2O3-doped silica powder can
be obtained annealing at temperature (1200°C) for (6 hours).
However, in the present paper we have shown that calcination at low temperature
(800°C) with prolonged annealing time (10 hours) mainly supports the
development of the cubic Nd2O3 nanocrystallites in case
of neodymium-containing silica. We found average size of the neodymium oxide
nanocrystallites in a silica matrix was ∼20 nm. The X-ray diffraction (XRD),
Fourier transformation infrared spectroscopy (FTIR), scanning electron
microscopy (SEM) data are obtained of heat treated samples.


## 2. Experimental

Using solgel technique, Nd-containing silica gel was prepared by refluxing high purity
reagents. Tetraethoxy silane (Aldrich 99.999), ethanol (Aldrich 99.9995), and
deionized water were mixed in the presence of hydrochloric acid as catalyst
(Aldrich 99.995). 8wt% neodymium oxide was introduced in the prehydrolyzed
solution in the form of nitrate under heating. The hygroscopic nature of the Nd
(NO)3 salt does not allow its exact weighing, thus the salt was
dissolved in deionized water and metal content was determined by standard
titration. The pH of the resultant solutions was 5. The solutions were filled
in a quartz (10×20×45 mm) and kept in a drying oven
(GFL-7105) at 100°C. It was observed that the gelation act after
approximately four days. Even after the gelation, the samples were still kept
inside the oven for 20 and 35 days for aging. The aging process allows further
shrinkage and stiffening of the gel. It was found that after 20 days, the
percentage of shrinkage of the samples was very low. To this end, it was observed
that the undoped samples were transparent and colorless, while the color of the
doped samples was glassy violet-purple due to the presence of neodymia. In
order to characterize the samples, complementary methods were used. X-ray
diffraction pattern of samples were carried out by a Philips X-ray
diffractometer PW/1710, with Ni filter, using monochromatized CuKα radiation of
wavelength 1.5418 A° at 50 KV and 40 mA. Scanning electron microscopy
(SEM) of the samples was done with JEOL-JSM-T330-A 35 CF microscope at an
accelerating voltage of 20 KV. Infrared spectra were collected from with a
Perkin Elmer 1600 (spectrophotometer) in 2000–500 cm-1 range.


## 3. Results and Discussion

### 3.1. XRD

Figure1 shows the
XRD pattern of neodymium oxide doped silica powder calcined in air at different
temperatures (500–800°C)
for different hours. The powdered sample calcined at 500°C (5 hours)
shows no particular reflection peak, which infers that the powder is still
amorphous. Thus, we may say that the annealing at much below the melting
temperature of the binary oxides even for five hours did not play any effective
role in altering the amorphous phase of the Nd2O3–SiO2.
When the calcination temperature was increased up to 800°C and
clamped for the 10 hours, a significant change in the pattern of reflections
can be clearly seen. The two major reflections appeared at angle 2θ∼21.9° and 27.8°. The broad peak
centered about 2θ∼21.9 may be assigned (101) reflection of cristobalite
structure [JCPDS file no. 39-1425]. The crystoballite phase indicates
persistence of water molecules in the sample. However, the sharp peak may be attributed
to Miller indices (222) reflection of cubic Nd2O3 phase
[JCPDS file no. 21-0579]. It is expected that the heat treatment of the sample at
800°C temperature for 10 hours reduces the number of pores and their
connectivity and thus significantly alter the amorphous phase. Here, it is
worth pointing that in the previous investigation [4] such major reflections were
not observed in annealed (1000°C) high Nd2O3 loaded sample. However, when the sample was annealed in vacuum at 850°,
a weak reflection was appeared [4]. The narrow diffraction pattern around 27.8° was employed to estimate the mean crystallite size from Scherrer formula and
found ∼20 nm. These results suggest that crystallite size increases during
sintering for longer annealing time because of the coalescence of
nanoparticles. Above results suggests that the heat treatment at low
temperature (500–800°C)
with prolonged annealing time increases the crystallinity as well as size of
nanocomposites.Figure 1
XRD pattern of the Nd2O3-doped silica powder sample
annealed at “a”: 500°C (5 hours) and Sample
“b”: 800°C (10 hours).


### 3.2. FTIR

Figure2 shows
FTIR transmittance spectra (range of 2000–500 cm-1)
of the heat treated doped samples. At temperature 800°C, many
discrete bands appeared 650, 800, 970, and 1040 cm-1 which may be
assigned to Si–O–Si symmetric bond stretching vibration or vibration mode of
ring structure of SiO2 tetrahedra, stretching mode Si–OH typical of
the gel structure, TO mode of the Si–O–Si asymmetric bond stretching vibration
and bending modes of water adsorbed at the silica surface, respectively. In low
frequency region of the FTIR spectra, the strong band centered about 650 cm-1 may be assigned to Nd–OH bond. The heat treatment of the sample at high
temperature prolonged sintering transforms Nd–OH into cubic Nd2O3 phase [14].Figure 2
FTIR spectra of Nd2O3-doped silica at different
temperatures: Sample “a”: 500°C (5 hours) (glass)
and Sample “b”: 800°C (10 hours).Interestingly, the TO mode the Si–O–Si
slightly shifted toward a higher wave number as the  calcinations temperature of the sample was
increased up to 800°C calcined for 10 hours. Calcinations at high
temperature with prolonged plateau-sintering, the band centered at 1640 cm-1 disappeared. The disappearance of this band
allowed the binary oxide to act almost transparent material in spatial frequency
ranges from 2000 to 1500 cm-1. Results of FTIR complement and
support the XRD data.


### 3.3. SEM

Figure3 shows different types of morphologies of neodymium oxide as viewed under
scanning electron microscope. Micrograph “a” (calcined at 500°C (5 hours))
shows the morphology of the amorphous Nd-containing silica. As expected, micrograph
“b” (calcined 800°C (10 hours)) clearly shows that prolonged
sintering significantly alter the shape and crystalinity of the neodymium oxide
doped silica. The shape of crystallites appears to be nearly spherical. SEM
data support the XRD data of that condition.SEM photograph of Nd2O3: SiO2. Sample “a”:
500°C (5 hours) and Sample “b”: 800°C (10 hours).
(a)(b)


## 3.1. XRD

Figure1 shows the
XRD pattern of neodymium oxide doped silica powder calcined in air at different
temperatures (500–800°C)
for different hours. The powdered sample calcined at 500°C (5 hours)
shows no particular reflection peak, which infers that the powder is still
amorphous. Thus, we may say that the annealing at much below the melting
temperature of the binary oxides even for five hours did not play any effective
role in altering the amorphous phase of the Nd2O3–SiO2.
When the calcination temperature was increased up to 800°C and
clamped for the 10 hours, a significant change in the pattern of reflections
can be clearly seen. The two major reflections appeared at angle 2θ∼21.9° and 27.8°. The broad peak
centered about 2θ∼21.9 may be assigned (101) reflection of cristobalite
structure [JCPDS file no. 39-1425]. The crystoballite phase indicates
persistence of water molecules in the sample. However, the sharp peak may be attributed
to Miller indices (222) reflection of cubic Nd2O3 phase
[JCPDS file no. 21-0579]. It is expected that the heat treatment of the sample at
800°C temperature for 10 hours reduces the number of pores and their
connectivity and thus significantly alter the amorphous phase. Here, it is
worth pointing that in the previous investigation [4] such major reflections were
not observed in annealed (1000°C) high Nd2O3 loaded sample. However, when the sample was annealed in vacuum at 850°,
a weak reflection was appeared [4]. The narrow diffraction pattern around 27.8° was employed to estimate the mean crystallite size from Scherrer formula and
found ∼20 nm. These results suggest that crystallite size increases during
sintering for longer annealing time because of the coalescence of
nanoparticles. Above results suggests that the heat treatment at low
temperature (500–800°C)
with prolonged annealing time increases the crystallinity as well as size of
nanocomposites.Figure 1
XRD pattern of the Nd2O3-doped silica powder sample
annealed at “a”: 500°C (5 hours) and Sample
“b”: 800°C (10 hours).


## 3.2. FTIR

Figure2 shows
FTIR transmittance spectra (range of 2000–500 cm-1)
of the heat treated doped samples. At temperature 800°C, many
discrete bands appeared 650, 800, 970, and 1040 cm-1 which may be
assigned to Si–O–Si symmetric bond stretching vibration or vibration mode of
ring structure of SiO2 tetrahedra, stretching mode Si–OH typical of
the gel structure, TO mode of the Si–O–Si asymmetric bond stretching vibration
and bending modes of water adsorbed at the silica surface, respectively. In low
frequency region of the FTIR spectra, the strong band centered about 650 cm-1 may be assigned to Nd–OH bond. The heat treatment of the sample at high
temperature prolonged sintering transforms Nd–OH into cubic Nd2O3 phase [14].Figure 2
FTIR spectra of Nd2O3-doped silica at different
temperatures: Sample “a”: 500°C (5 hours) (glass)
and Sample “b”: 800°C (10 hours).Interestingly, the TO mode the Si–O–Si
slightly shifted toward a higher wave number as the  calcinations temperature of the sample was
increased up to 800°C calcined for 10 hours. Calcinations at high
temperature with prolonged plateau-sintering, the band centered at 1640 cm-1 disappeared. The disappearance of this band
allowed the binary oxide to act almost transparent material in spatial frequency
ranges from 2000 to 1500 cm-1. Results of FTIR complement and
support the XRD data.


## 3.3. SEM

Figure3 shows different types of morphologies of neodymium oxide as viewed under
scanning electron microscope. Micrograph “a” (calcined at 500°C (5 hours))
shows the morphology of the amorphous Nd-containing silica. As expected, micrograph
“b” (calcined 800°C (10 hours)) clearly shows that prolonged
sintering significantly alter the shape and crystalinity of the neodymium oxide
doped silica. The shape of crystallites appears to be nearly spherical. SEM
data support the XRD data of that condition.SEM photograph of Nd2O3: SiO2. Sample “a”:
500°C (5 hours) and Sample “b”: 800°C (10 hours).
(a)(b)


## 4. Conclusions

Upon heat treatment of xerogel, nanostructure
cubic neodymium oxide in SiO2 matrix
was successfully prepared. The phase evolution, absorption spectra, and
morphology of the Nd-containing silica have been studied with the objective to
better understand the effect of thermal annealing. Calcinations of the
Nd-containing silica at 800°C for 10 hours mainly support the
formation of cubic neodymium oxidenanocrystallites in silica matrix
because of coalescences of individual nanoparticles.



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*Source: 237023-2008-08-03.xml*