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53
CHAPTER 4
WET-CHEMICAL SYNTHESIS AND
CHARACTERIZATION OF PURE AND RARE EARTH
IONS (Ce3+, Sm3+ AND Gd3+) DOPED Dy2O3
NANOPARTICLES
4.1 INTRODUCTION
Rare-earth (RE) oxides have been widely used in high-performance
luminescent devices and as magnetic, catalytic, and other functional materials
due to their unique electronic, optical and chemical characteristics arising
from their 4f electrons (Adachi and Imanaka 1998). Most of these advanced
functions strongly depend on the particle size and composition, which are
sensitive to the bonding states of RE atoms or ions. As these properties could
be enhanced by incorporating the RE trivalent cations to the RE oxide system
within the nanometer regime, highly functionalized materials can be obtained
as a result of both shape-specific and quantum confinement effects. They
could also act as electrically, magnetically, or optically functional host
materials (Sato et al 2009, Hosokawa et al 2008, Yin et al 2008).
In the RE oxide family, dysprosium oxide (Dy2O3) has peculiar
property to crystallize in C-rare-earth sesquioxide structure (cubic bixbyite
phase) below 1870ºC and exhibits monoclinic and/or hexagonal structure at
elevated temperatures. It is highly insoluble in water and thermally stable,
suitable for optical and laser devices (Kofstad 1972, Tanabe et al 1989).
54
Many efforts have been devoted to the synthesis and physico-chemical
properties of Dy2O3 nanostructures. The synthetic pathways investigated to
prepare doped RE oxides were the same as for pure RE oxides. In most cases,
the introduction of RE trivalent cations in the structure resulted in the
decrease in particle size. So it is necessary to have detailed study on the pure
and RE doped Dy2O3 nanoparticles, which could bring potential applications
owing to its unique properties. It is proven that the RE ions can be doped or
coupled with oxide semiconductors to improve their chemical, optical,
optoelectric and luminescent properties (Krämer et al 2006, Chang et al 2011,
Yan et al 2006).
Recently Salavati-Niasari et al (2010) have employed sonochemical
method for the synthesis and effective conversion of Dy2(CO3)3 nanoparticles,
Dy(OH)3 nanotubes to Dy2O3 nanoparticles. Xu et al (2003) reported the
preparation of Dy(OH)3 and Dy2O3 nanotubes by hydrothermal method.
Dysprosium hydroxide and oxide nanorods have been prepared directly from
bulk Dy2O3 crystals by hydrothermal process at 130ºC and 210ºC,
respectively, by Song et al (2008). All these methods are reported to be
cumbersome and time consuming experiments.
Many physical and chemical methods have been reported for the
synthesis of nanomaterials. Among them, the wet-chemical route has attracted
considerable attention due to its feasibility to synthesize nanomaterials. It has
emerged as the most flexible and promising technique as it is relatively
simple, reproducible, and economically feasible for large scale production.
The precipitates are dense, can be readily filtered and are of considerably high
purity. Furthermore, they decompose to the oxide form at relatively low
calcination temperature without change in the morphology.
55
Inspite of several application potentials, there are seldom
investigations on the preparation of RE ions (Ce3+, Sm3+ and Gd3+) doped
Dy2O3 nanoparticles and the influence of RE ions on the properties of Dy2O3
host lattices. In this chapter, wet-chemical synthesis method for pure and
RE ions doped Dy2O3 (Ce:Dy2O3, Sm:Dy2O3 and Gd:Dy2O3) nanoparticles is
reported, and their thermal, structural, morphological and optical properties
have been investigated. In addition, the formation mechanism of pure and
RE ions doped Dy2O3 nanoparticles were discussed in detail.
4.2 MATERIALS SYNTHESIS
4.2.1 Pure and RE Ions doped Dy2O3 Nanoparticles
To prepare pure Dy2O3 nanoparticles, aqueous dysprosium acetate
solution (0.1M) was prepared by dissolving dysprosium acetate tetra hydrate
(0.8g) in millipore water (Resistance ~18εΩ). For RE ions doped Dy2O3
nanoparticles, the dysprosium acetate and RE(NO)3.6H2O (RE = Ce, Sm and
Gd) were prepared with a molar ratio of RE/(RE+Dy)=0.1 by properly
dissolving into Millipore water. Hexamethylenetetramine (HMT) solution
(0.05M) was prepared by dissolving HMT (0.14 g) in 20 ml of Millipore
water and stirred homogeneously. After an hour of constant stirring, HMT
solution was added drop wise to the RE:dysprosium acetate solution, and the
molar ratio of RE:dysprosium acetate to HMT was adjusted to 1:2. After
constant stirring at room temperature, it was subjected to aging at 40ºC for
24 h to obtain sol-containing solution. Subsequently, the sol was dried
overnight at 70ºC to remove the solvent. The dried particles were collected,
washed with ethanol and water in order to remove the ionic impurities. The
sample was subjected to conventional thermal treatment up to 600°C for 2h.
56
4.3 RESULTS AND DISCUSSION
4.3.1 Thermal Property Studies
Thermal behaviour of pure and RE ions doped Dy2O3 powders was
investigated by TG and the results are shown in Figure 4.1. The
decomposition of both the precursors proceeds through three distinct weight
loss steps. The first weight loss step (~ 17%) between 35 and 110ºC is related
to the loss of moisture and trapped solvents (water and carbon dioxide). HMT
hydrolyzes above 150°C to form NH3 and CO32- along with OH- ions.
Initially, Dy3+ cation combines with OH- to form Dy(OH)2+
polyatomic group. At around 380°C, the bonding occurs between CO32- and
the positively-charged group Dy(OH)2+, which yields the solid DyCO3OH at
supersaturation. The second drastic weight loss step (~34%) is attributed to
the decomposition of precursor from DyCO3OH to the formation of Dy2O3.
Further increase in temperature beyond 400ºC triggers the decomposition of
the anhydrous salt, contributing to the third minor weight loss. The third step
accounts for weight loss due to the decomposition of residual acetate. Above
600ºC, the mass does not show any pronounced change. Hence by analysing
the TG curves, it is evident that the RE ions doping has appreciable effect on
the thermal decomposition process. Even though, the RE ions doped precursor
show similar TG curves, it requires a higher temperature for decomposition
than the pure one. Based on the above observations, the calcination
temperature for pure and RE ions doped Dy2O3 powders is chosen to be at
600ºC.
57
Figure 4.1 TG curves of the pure and RE:Dy2O3 nanoparticles
4.3.2 X-ray Diffraction Analysis
Powder XRD patterns for the pure Dy2O3 nanoparticles calcined at
different temperatures are compared in Figure 4.2. The XRD pattern
(Figure 4.2 (a)) of the as-prepared Dy2O3 nanoparticles shows amorphous
nature. The calcined samples of 250ºC and 400ºC show (Figure 4.2 (b) and
(c)) some amorphous humps around 30 and 45° of 2θ, which is the indication
of Dy2O3 compound formation. The XRD pattern of the Dy2O3 sample
calcined at 600ºC (Figure 4.2 (d)) confirms the Dy2O3 with cubic bixbyite
phase (JCPDS # 86-1327), with lattice constant a of cubic unit cell:
10.671(2) Å.
58
Figure 4.2 XRD patterns of (a) as-prepared, (b) 250ºC, (c) 400ºC,
(d) 600ºC calcined Dy2O3 nanoparticles
Figure 4.3 shows typical XRD spectra of pure and RE:Dy2O3
nanoparticles calcined at 600ºC in air. The pronounced broad diffraction
peaks indicate the amorphous nature of the samples. All reflections of pure
and RE:Dy2O3 samples are assigned to cubic bixbyite phase of Dy2O3 and are
indexed on the basis of JCPDS card No. 86-1327. There is a considerable
peak shift towards lower 2θ angles observed for four main Bragg reflections
in RE:Dy2O3 in comparison with pure Dy2O3. There are no peaks
corresponding to cerium, samarium, gadolinium or its oxide CeO2, Ce2O3,
Sm2O3 or Gd2O3 suggesting that the RE element may be doped into Dy2O3.
The peak shift also justifies the incorporation of the RE dopant into the host
Dy2O3 lattice along with the change in lattice parameter.
59
Figure 4.3 XRD patterns of (a) pure Dy2O3, (b) Ce:Dy2O3,
(c) Sm:Dy2O3 and (d) Gd:Dy2O3 nanoparticles calcined at
600ºC in air
Table 4.1 lists the comparison of lattice constant, particle size and
lattice strain between pure and RE:Dy2O3 nanoparticles. The lattice constant
of pure, Ce:Dy2O3, Sm:Dy2O3 and Gd:Dy2O3 nanoparticles are found to be
10.6712(0), 10.6725(3), 10.6722(7) and 10.6719(5) Å respectively. The
calculated lattice constant and observed peak shift are reflected in almost all
lattice planes of the XRD patterns of Ce:Dy2O3, Sm:Dy2O3 and Gd:Dy2O3
with respect to pure Dy2O3 nanoparticles. It is clear that the calculated lattice
parameters are almost close to the JCPDS value of bulk Dy2O3. The small
variation in the lattice parameter occurs due to RE ions incorporation and
slight mismatch between Dy and RE ions. It indicates that RE ions are
systematically substituted without changing the crystal structure.
Lattice constants of Ce:Dy2O3, Sm:Dy2O3 and Gd:Dy2O3 are
slightly larger than those of pure Dy2O3, which is attributed to the larger ionic
radius of Ce3+ (0.115 nm), Sm3+ (0.110 nm) and Gd3+ (0.108 nm) than that of
60
Dy3+ (0.105 nm) (Sakabe et al 2002). If the Dy3+ ion is replaced with Ce4+ ion,
there is a decrease in the lattice constant with respect to standard JCPDS
value of bulk Dy2O3 and is ascribed due to the smaller ionic radius of
Ce4+ (0.101 nm) than the Dy3+ ion. Hence the lower degree shift in diffraction
peaks and the increase in lattice constant confirm that Ce3+ ion is substituted
in Dy2O3 lattice. Samarium and gadolinium has only 3+ oxidation state.
Table 4.1 Lattice constant, particle size and strain calculation of pure
and RE:Dy2O3 nanoparticles
Sample h k l
d spacing Lattice
constant
‘a’
(Å)
Particle
Size
(nm)
Strain (calc) (exp)
Pure Dy2O3
2 2 2 3.0809(0) 3.0961(1)
10.6712(0) 14 4.2 x 10-3 4 0 0 2.6681(3) 2.6714(0)
4 4 0 1.8866(6) 1.8926(4)
6 2 2 1.6089(4) 1.6135(9)
Ce:Dy2O3
2 2 2 3.0805(2) 3.0900(8)
10.6725(3) 10 8.1 x 10-3 4 0 0 2.6678(1) 2.6669(5)
4 4 0 1.8864(2) 1.8905(1)
6 2 2 1.6087(5) 1.6121(0)
Sm:Dy2O3
2 2 2 3.0801(6) 3.1361(7)
10.6722(7) 11 6.7 x 10-3 4 0 0 2.6675(0) 2.7112(0)
4 4 0 1.8862(1) 1.9084(9)
6 2 2 1.6085(6) 1.6232(3)
Gd:Dy2O3
2 2 2 3.0801(6) 3.1123(8)
10.6719(5) 12 5.1 x 10-3 4 0 0 2.6675(0) 2.6909(4)
4 4 0 1.8862(1) 1.8969(4)
6 2 2 1.6085(6) 1.6174(5)
61
4.3.3 Lattice Strain Analysis
Williamson-Hall plot of pure and RE:Dy2O3 nanoparticles calcined
at 600˚C are shown in Figure 4.4. The broadening effect of XRD peaks
reflects the nanocrystalline nature of the resulting pure and RE:Dy2O3
samples. Since the effective XRD peak broadening can be caused by lattice
strain and small crystallite size, these two effects have to be distinguished
using W-H plot. The dependence is linear, with the slope determining the
lattice strain = 4.204x10-3, 8.7 x10-3, 6.7 x10-3 and 5.1 x10-3 for pure Dy2O3,
Ce:Dy2O3, Sm:Dy2O3 and Gd:Dy2O3 nanoparticles, respectively. It can be
clearly seen in the Figure 4.4 that the slope of the linear fit for RE:Dy2O3 are
much higher than that in pure Dy2O3, indicating the presence of strain in the
doped nanoparticles. The shift in the XRD patterns is also reflected in the
W-H plot, which is attributed due to the highly strained and distorted
environment around the RE3+ ions in the Dy2O3 lattice (Bueno-Ferrer et al
2010).
The crystallite size of pure Dy2O3, Ce:Dy2O3, Sm:Dy2O3 and
Gd:Dy2O3 nanoparticles estimated from the intercept are 14 nm, 10 nm, 11 nm
and 12 nm respectively. Because of RE3+ doping, the diffraction peaks of
RE:Dy2O3 become broad with reduced intensity implying that the RE ions
doping results in decreased nanocrystallite size. This may be ascribed to the
segregation of dopant cations at the grain boundary preventing the growth of
the particles (Ikuma et al 2003).
62
Figure 4.4 Williamson-Hall plot of (a) pure Dy2O3, (b) Ce:Dy2O3,
(c) Sm:Dy2O3 and (d) Gd:Dy2O3 nanoparticles calcined
at 600 ºC in air
4.3.4 Transmission Electron Microscopy
TEM micrograph of Dy2O3 nanoparticles calcined at 600°C is
shown in Figure 4.5. As seen in Figure 4.5 (a) and (b), the sample is
composed of the agglomerated solid particles of about 14 nm in size, which is
consistent with the particle size determined from W-H plot. The SAED
pattern (Figure 4.5 (c)) shows ring structure, indicating the amorphous nature.
(c) (d)
63
Measured interplanar spacings (dhkl) from SAED pattern can be indexed to the
cubic phase of Dy2O3, which is also in good agreement with XRD results.
Figure 4.5 (a) and (b) TEM micrograph (c) SAED pattern of Dy2O3
nanoparticles calcined at 600oC
TEM micrograph in Figure 4.6 shows well-formed nanocrystallites
of Ce:Dy2O3, Sm:Dy2O3 and Gd:Dy2O3 with anomalistic sphericity in shape. It
clearly shows that the particles are in nanoscale regime within a diameter of
10 nm, 11 nm and 12 nm respectively for Ce:Dy2O3, Sm:Dy2O3 and
Gd:Dy2O3, which is consistent with the results of particle size determined
from W-H plot.
64
Figure 4.6 TEM micrograph of (a) and (b) Ce:Dy2O3, (c) and (d)
Sm:Dy2O3 and (e) and (f) Gd:Dy2O3 nanoparticles calcined
at 600ºC in air
65
4.3.5 Elemental Analysis
No impurities except dysprosium and oxygen elements were
detected for the EDS spectrum of pure Dy2O3 nanoparticles in Figure 4.7 (a).
EDS spectrum of RE:Dy2O3 nanoparticles in Figure 4.7 (b), (c) and (d) shows
the presence of major elements namely dysprosium, oxygen, cerium (for
Ce:Dy2O3), samarium (for Sm:Dy2O3) and gadolinium (for Gd:Dy2O3). The
elemental composition of cerium, samarium and gadolinium is 8.56%, 8.41%
and 8.74% respectively, which also confirms that majority of Ce3+, Sm3+ and
Gd3+ ions are doped with Dy2O3 system.
Figure 4.7 EDS analysis of (a) pure Dy2O3, (b) Ce:Dy2O3, (c) Sm:Dy2O3
and (d) Gd:Dy2O3 nanoparticles calcined at 600ºC in air
(a) (b)
(c) (d)
66
4.3.6 Fourier Transform Infrared Spectroscopy
FT-IR transmission spectra of as-prepared and calcined samples at
different temperatures of pure Dy2O3 nanoparticles along with RE ions doped
Dy2O3 nanoparticles calcined at 600°C in air are shown in
Figures 4.8 and 4.9. By comparing both the figures, The broad absorption
band located around 3400 cm-1 corresponds to the O–H stretching vibration of
residual water and hydroxyl groups, while the absorption band at 1630 cm-1 is
due to the ‘‘scissor’’ bending mode of associated water (Xu et al 2008).
The peaks in the region 2900-2800 cm-1 correspond to the stretching and
bending modes of the hydrocarbon chain of residual surfactant in the sample.
The absorption bands at 1500 cm-1 are attributed to the C=O bond of
carbonate ions which is formed during the hydrolysis of HMT.
There is some notable splitting of band, which could be due to the
location of the carbonate ions at the non-equivalent site of the crystals (Happy
et al 2007). It is also observed from the spectra that the surfaces are covered
by several layers of carbonate-like species especially the bidentate carbonates
(Han et al 2000), which are characterized by the absorption bands at 1520,
1350, 1053, and 848 cm-1 respectively. From the Figure 4.8 (b) and (c),
calcination of the samples at 250ºC and 400ºC did not result much change in
the vibration modes, but the elimination of C=O vibrations of carbonate ions
and the reduction of intensity of the hydroxyl group are observed.
These carbonate species are coordinated on the sample surfaces by
unsaturated chemical bonding, which has some impact on the thermal
behaviour and surface structural characteristics. Therefore, further calcination
at higher temperature or longer duration is required to eliminate those traces.
The corresponding decrease in the intensity of the carbonate ion bond results
in increase in the absorption band of cubic phase of Dy2O3, which appears at
563 cm-1. No additional absorption peaks are observed in Figure 4.9 (b), (c)
67
and (d) with RE ions doping, indicating its homogeneous dispersion in the
parent material.
Figure 4.8 FTIR spectra of (a) as-prepared, (b) 250ºC, (c) 400ºC,
(d) 600ºC calcined Dy2O3 nanoparticles
Figure 4.9 FTIR spectra of (a) pure Dy2O3, (b) Ce:Dy2O3, (c) Sm:Dy2O3
and (d) Gd:Dy2O3 nanoparticles calcined at 600ºC in air
68
4.3.7 Formation Mechanism of Pure and RE:Dy2O3 Nanoparticles
Based on the above results, a possible reaction mechanism is
presented. For pure and RE ions doped system, the morphology, particle size
and the physicochemical nature of pure and RE:Dy2O3 can be easily
controlled by using hydroxycarbonates as decomposition precursors. It is
observed that above 150ºC, HMT hydrolyze to form NH3 and CO32−,
the carbonate and hydroxide ion react with Dy3+ or RE3+ to form
RE:DyCO3OH. The trivalent Dy3+ or RE3+ have a strong affinity with OH−.
The cation thus combine with OH−, forming the RE:DyCO3OH polyatomic
group. At elevated temperatures, CO32− bond with the positively charged
groups to yield the solid RE:DyCO3OH at supersaturation.
3 3 2
3 3/ :Dy RE CO OH RE DyCO OH (4.1)
The phase transformation of RE:DyCO3OH into RE:Dy2O3 after
calcination can be elucidated by the following equation:
22 3 2 22 : 3 : 2ORE DyCO OH RE Dy O CO H O (4.2)
The HMT not only acts as a mineralizer but also as a surfactant in
the wet-chemical process. HMT hydrolyzes to form NH3, subsequently
hydrolyzes to form OH− ions, which are also responsible for the formation of
tiny particles of pure and RE:Dy2O3 (Han et al 2000).
4.3.8 UV-Visible Spectroscopy
From the optical absorption spectra in Figure 4.10, a well-defined
sharp and strong absorbance peaks located in the UV region is observed for
the as-prepared and calcined Dy2O3 nanoparticles. It is worth noting that the
69
spectra show a strong blue-shift as compared with bulk material, indicating
the narrow and uniform particle size distribution obtained via this synthesize
route. In the Figure 4.10, inset plot of (αhν)2 versus energy gap in eV, reveals
that the bandgap decreases from 4.79 to 4.26 eV as the calcination
temperature increases, which is attributed to the growth of Dy2O3
nanoparticles.
Figure 4.10 UV-Vis absorbance spectra and inset plot of (αhν)2 versus
eV of (a) as-prepared, (b) 250ºC, (c) 400ºC and (d) 600ºC
calcined Dy2O3 nanoparticles
UV-Vis absorption spectra of pure and RE:Dy2O3 samples calcined
at 600ºC are plotted in Figure 4.11. The spectra show that the pure and
RE:Dy2O3 particles have no absorption in the visible region (>400 nm). It is
also observed that the RE3+ incorporation to Dy2O3 induces a considerable red
shift in the electronic absorption with respect to the pure sample. The
variation in the spectrum of RE:Dy2O3 with respect to pure Dy2O3 is due to
the presence of a dispersed RE3+ component, in the Dy2O3 support (Xiao et al
2006). Estimated band gap energy is 4.26, 4.01, 4.05 and 4.10 eV for pure
70
Dy2O3, Ce:Dy2O3, Sm:Dy2O3 and Gd:Dy2O3 respectively. The band gap of
Dy2O3 nanoparticles is reduced by RE3+ doping and the band gap narrowing is
primarily attributed to the substitution of RE3+ ions which introduces electron
states into the band gap of Dy2O3 to form the new lowest unoccupied
molecular orbital.
Figure 4.11 UV-Vis absorbance spectra and inset plot of (αhν)2 versus
energy gap of pure Dy2O3, Ce:Dy2O3, Sm:Dy2O3 and
Gd:Dy2O3 nanoparticles calcined at 600 ºC in air
4.3.9 Photoluminescence Studies
The fluorescence of RE compounds originates from electron
transitions within the 4f shell, which is peculiar for the lanthanides (Lu et al
2009). The luminescence of Dy3+ has attracted much attention because of its
white light emission. Compared with the absorption and emission spectra of
pure Dy2O3, a blue-shift phenomenon has been observed, which could be
71
attributed to quantum confinement on the nanoparticles leading to the
existence of a large number of defects. This may explain the blue-shift in the
absorption edges of the samples in the UV-Vis and PL spectra
(Salavati-Niasari et al 2010).
Figure 4.12 PL spectra of Dy2O3 nanoparticles taken with the excitation
wavelength of 350 nm. (a) as-prepared, and (b) calcined at
600ºC
The PL spectra (Figure 4.12) for as-prepared and calcined samples
show emissions at 486 nm (blue), 575 nm (yellow) and a small peak at 666
nm (red). These three different emission bands originate from one origin
because of the same excitation wavelength. The transitions involved in blue,
yellow and red bands of Dy3+ ion are well known and identified as 4F9/2 → 6H15/2,
6H13/2, 6H11/2 transitions, respectively. The energy levels of
Dy3+ ion and emission transitions are presented in Figure 4.13 (Reddy et al
2011). It is known that Dy3+ emission around 486 nm (4F9/2 → 6H15/2) is of
magnetic dipole origin and 575 nm (4F9/2 → 6H13/2) is of electric dipole origin.
72
4F9/2 → 6H13/2 is predominant only when Dy3+ ions are located at
low-symmetry sites with no inversion centers (Borja-Urby et al 2011).
The low-symmetry location of Dy3+ results in the predominate
emission of 4F9/2 → 6H13/2 transition. Since the emission at 575 nm is
predominant, it suggests that there is a very little deviation from inversion
symmetry in this matrix. For 600˚C calcined sample the emission peaks can
be seen almost same as for the as prepared sample, and there are only a very
small difference between them which is attributed to the increased particle
size (Sujana et al 2008). By comparing the PL intensity of as-prepared and the
calcined samples, it is worth noting that the as-prepared nanoparticles might
have higher activity than relatively bigger particles. These experimental
results imply that there exists a relationship between the product size and its
optical properties.
Figure 4.13 The energy levels of Dy3+ ion and emission transitions
73
The PL spectra of pure and RE:Dy2O3 nanoparticles calcined at
600ºC, under 350 nm light excitation are shown in Figure 4.14. It is observed
that the pure and RE:Dy2O3 nanoparticles exhibit obvious PL signals with
similar curve shape, demonstrating that RE3+ dopant does not give rise to new
PL emission. It is also observed that the bands of RE:Dy2O3 samples are
shifted and become less intense compared to pure sample. This is attributed to
the small crystallite size of RE:Dy2O3, which is well supported by XRD and
TEM studies. The major difference in the intensities of pure and RE:Dy2O3
samples may be attributed to surface specific defects. These surface defects
induced by trivalent doping do not play a major role in the photoluminescence
behavior of these samples (Han et al 2009). Usually the fluorescence emission
of doping ions has higher photostability than the defect related luminescence
of semiconductive nanomaterials, because the defects are greatly affected by
synthesis conditions and environments (Palard et al 2010).
Figure 4.14 PL spectra of (a) pure Dy2O3, (b) Ce:Dy2O3, (c) Sm:Dy2O3
and (d) Gd:Dy2O3 nanoparticles calcined at 600ºC in air
74
4.4 CONCLUSION
Pure and RE ions doped Dy2O3 nanoparticles have been
synthesized by wet-chemical synthesis route. The TG studies reveal the
decomposition of DyCO3OH to form Dy2O3 and the calcination temperature
for pure and RE:Dy2O3 powders was chosen to be 600ºC. The XRD showed
the formation of pure and RE:Dy2O3 nanoparticles with the cubic bixbyite
structure. The lower degree shift in RE:Dy2O3 diffracted peaks and the
increase in lattice constant justifies that the Ce3+, Sm3+ and Gd3+ ions are
substituted in Dy2O3 lattice. TEM micrograph showed the size of pure Dy2O3,
Ce:Dy2O3, Sm:Dy2O3 and Gd:Dy2O3 nanoparticles to be 14, 10, 11 and
12 nm. The FTIR results clearly showed that the surface of pure and
RE:Dy2O3 nanoparticles was chemically bonded with the surface modifier.
RE:Dy2O3 nanoparticles showed considerable red-shift and enhanced optical
absorption in UV region with respect to pure sample and the direct bandgap
was determined to be 4.26 and 4.01 eV respectively. The PL results
confirmed that the samples possess strong visible emission and the difference
in intensity of RE:Dy2O3 sample may be due to the surface specific defects.