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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.

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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

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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)

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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)

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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.

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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)

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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)

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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

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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

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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.