Auto-ignition based synthesis of Y2O3 for photo- and thermo-luminescent applications

Article abstract of DOI:10.1016/j.jallcom.2013.09.037

We present a simple route for synthesis of Y₂O₃ for both photoluminescent (PL) and thermoluminescent (TL) applications. We show that by simply switching the fuel from ethylene di-amine tetracetic acid (EDTA) to its disodium derivative (Na₂-EDTA), we obtain a better photoluminescent material. On the other hand, use of EDTA aids in formation of Y₂O₃ which is a better thermoluminescent material. In both cases pure cubic nano-Y₂O₃ is obtained. For both the material systems, structural characterization, photoluminescence, thermoluminescence, and absorbance spectra are reported and analyzed. Use of EDTA results in nano Y₂O₃ with crystallite size ～10 nm. Crystallinity improves, and crystallite size is larger (～30 nm) when Na₂-EDTA is used. TL response of Y₂O₃ nanophosphors prepared by both fuels is examined using UV radiation. Samples prepared with EDTA show well resolved glow curve at 140 C, while samples prepared with Na₂-EDTA shows a glow curve at 155 C. Effect of UV exposure time on TL characteristics is investigated. The TL kinetic parameters are also calculated using glow curve shape method. Results indicate that the TL behavior of both the samples follow a second order kinetic model.

Full text of DOI:10.1016/j.jallcom.2013.09.037

Accepted Manuscript
Auto-ignition based synthesis of Y O for photo- and thermo-luminescent ap‐
2 3
plications
R. Hari Krishna, B.M. Nagabhushana, H. Nagabhushana, R.P.S. Chakradhar, R.
Sivaramakrishna, C. Shivakumara, Tiju Thomas
PII:
S0925-8388(13)02176-2
DOI:
http://dx.doi.org/10.1016/j.jallcom.2013.09.037
JALCOM 29391
Reference:
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Please cite this article as: R. Hari Krishna, B.M. Nagabhushana, H. Nagabhushana, R.P.S. Chakradhar, R.
Sivaramakrishna, C. Shivakumara, T. Thomas, Auto-ignition based synthesis of Y O for photo- and thermo-
2 3
luminescent applications, (2013), doi: http://dx.doi.org/10.1016/j.jallcom.2013.09.037
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Auto-ignition based synthesis of Y2O3 for photo- and thermo-luminescent
applications
R. Hari Krishnaa,b*, B. M. Nagabhushanab* H. Nagabhushanac, R. P. S. Chakradhard
,
R. Sivaramakrishnae, C. Shivakumaraf, Tiju Thomasg
aDepartment of Chemistry, M. S. Ramaiah Institute of Technology, Bangalore 560 054, India
bVisvesvaraya Technological University, Belgaum 590 018, India
cCentre for Nanoscience Research (CNR), Tumkur University, Tumkur 572 103, India
dCSIR-National Aerospace Laboratories, Bangalore 560017 India
eDepartment of Chemistry, REVA Independent PU college, Bangalore 560 024, India
fSolid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
gMaterials Research Centre, Indian Institute of Science, Bangalore 560 012, India
Abstract
We present a simple route for synthesis of Y2O3 for both photoluminescent (PL) and
thermoluminescent (TL) applications. We show that by simply switching the fuel from ethylene di-
amine tetracetic acid (EDTA) to its disodium derivative (Na2-EDTA), we obtain a better
photoluminescent material. On the other hand, use of EDTA aids in formation of Y2O3 which is a
better thermoluminescent material. In both cases pure cubic nano-Y2O3 is obtained. For both the
material systems, structural characterization, photoluminescence, thermoluminescence, and
absorbance spectra are reported and analyzed. Use of EDTA results in nano Y2O3 with crystallite
size ~10 nm. Crystallinity improves, and crystallite size is larger (~30 nm) when
Na2-EDTA is used. TL response of Y2O3 nanophosphors prepared by both fuels is examined using
UV radiation. Samples prepared with EDTA show well resolved glow curve at 140 ºC, while
samples prepared with Na2-EDTA shows a glow curve at 155 ºC. Effect of UV exposure time on
TL characteristics is investigated. The TL kinetic parameters are also calculated using glow curve
shape method. Results indicate that the TL behavior of both the samples follow a second order
kinetic model.
Keywords: nanophosphor, Y2O3, solution combustion, photoluminescence, thermoluminescence
*Corresponding authors: E-mail: bmnshan@yahoo.com (B. M. Nagabhushana), Tel.: +91 9916030272
rhk.chem@gmail.com (R. Hari Krishna) Tel.: +91 9886434109
1

1. Introduction:
Rare earth oxides (REO) have been extensively studied in recent years due to their unique
electronic, optical, and chemical properties, which in turn enables their use in several engineering
applications [1, 2]. Among REO, Y2O3 has gained significant attention owing to its use as a
dielectric insulator in complementary metal oxide semiconductor (MOS) devices. Its wide energy
band gap (~5.5 eV) along with its relatively high dielectric constant, chemical stability, and
thermal stability (melting point ~ 2325 oC) makes it a robust engineering material [3]. Furthermore,
Y2O3 is an important material for optical applications because of its ability to host rare earth atoms
[4, 5]. In fact many of the best red phosphors used in fluorescent lamps and flat panel devices are
Y2O3 based systems [6-8].
A wide variety of processes are available for the synthesis of REO. Y2O3 powder has been
synthesized by various wet chemical routes, such as co-precipitation [9], hydrothermal [10], sol–gel
[11] and gas-phase condensation [12] and solution combustion [13, 14] route. With appropriate
processing parameters, these wet chemical processes result in nano-sized powders with narrow
particle size distribution. Among the synthesis processes explored so far, the solution combustion
method has the advantage of being rapid and low in cost. Another advantage of this method is that
the desired phase can be achieved with better control over stoichiometry with a relatively simple
experimental setup. In solution combustion method, the desired product is obtained through a self-
sustaining exothermic redox reaction, which takes place in a gel that is formed by dehydrating an
aqueous solution of a fuel (eg. polycarboxilic acid) and an oxidant (eg. metal nitrates). Powder
characteristics like crystallite size, surface area, extent and nature (hard or soft) of agglomeration
are primarily governed by enthalpy or flame temperature generated during combustion, which itself
is dependent on nature of the fuel, fuel-to-oxidant ratio, and amount of gaseous byproducts [15].
2

The choice of fuel significantly impacts the exothermicity and amount of gases that evolve during
the combustion process; this in turn has a strong influence on the properties of the product [16]. The
fuels employed so far in the combustion synthesis of Y2O3 are glycine [17], citric acid [18], urea
[19], oxalyldihydrazide [20], carbohydrazide [21] etc. Complexing ability is one of the primary
characteristics that a fuel should possess. Among the complexing agents, EDTA and Na2-EDTA are
known to be particularly efficient due the presence of amine groups in these molecules. Such
molecules can effectively complex metal ions of different sizes, and help in achieving chemical
homogeneity in the end products. However the high chelating abilities of EDTA and Na2-EDTA
has not been widely used as a fuel in solution combustion reactions.
In the present work, we prepare nano-Y2O3 powder by combustion technique using both
EDTA and Na2-EDTA. The combustion synthesized powers are characterized using powder x-ray
diffraction (PXRD), Fourier Transform Infrared (FTIR) spectroscopy, UV-Vis spectroscopy,
Brunauer-Emmett-Teller (BET) specific surface area analysis, scanning electron microscopy (SEM)
and transmission electron microscopy (TEM). We show that by simply switching the fuel from
EDTA to its disodium derivative (i.e. Na2-EDTA) we obtain a better photoluminescent material. On
the other hand, use of EDTA aids in formation of Y2O3 that is thermoluminescent.
1. Experimental
Chemicals
Analytic grade yttrium oxide (Y2O3: 99.99%, CDH Ltd.), nitric acid (HNO3: 99.99%, Merk Ltd.)
EDTA (C10H16N2O8: 99.99%, Merk Ltd.) and Na2-EDTA (C10H14N2O8Na2: 99.99%, Merk Ltd.) are
used as starting materials for the preparation of Y2O3 nanoparticles. All the chemicals are used
without further purification.
3

Synthesis:
◦
Yttrium oxide is dissolved in 1:1 nitric acid and heated in a sand bath at 80 C to get transparent,
highly viscous yttrium nitrate gel. The excess nitric acid is evaporated. EDTA is dissolved in
deionized water, and the solution is subsequently dropped into the yttrium nitrate solution.
Continuous stirring is done to ensure homogeneity of the solution. The petri dish containing the
homogeneous mixture of yttrium nitrate and EDTA is placed in a pre-heated muffle furnace which
◦
is maintained at 500 10 C. Initially the solution boils and undergoes dehydration, which is
followed by decomposition accompanied by evolution of large amounts of gases (oxides of carbon,
nitrogen). This is followed by spontaneous smoldering type combustion with enormous swelling of
the reaction mixture, which in turn produces foamy and voluminous Y2O3. Same procedure is
followed in the preparation of Y2O3 with disodium salt of EDTA as fuel. The theoretical equations
(F/O = 1.0) for the formation of Y2O3 nanoparticles using different fuels can be represented by the
following reactions:
EDTA: 8Y(NO3)3+3C10H16N2O8ꢀ
4Y2O3+30CO2+15N2+24H2O, ~17 moles of gases/mol of Y2O3
Na2-EDTA: 10Y(NO3)3+3C10H14N2O8Na2ꢀ
5Y2O3+6NaNO3+30CO2+15N2+21H2O, ~13 moles of gases/mol of Y2O3 .….(2)
…….(1)
Characterization
The phase purity and crystallinity of the nanophosphors are examined by powder X-ray
diffractometer (PANalytical X′Pert Pro) using CuKα (1.541Å) radiation with a nickel filter. The
surface morphology of the product is examined using an SEM (JEOL JSM 840A). A Quanta
Chrome Corporation NOVA 1000 gas sorption analyzer is used to find the surface area of the
4

powder samples using the BET approach. Transmission Electron Microscopy (TEM) analysis is
performed on a Hitachi H-8100 (accelerating voltage up to 200 kV, LaB6 filament) equipped with
EDS (Kevex Sigma TM Quasar, USA).
The FT-IR studies are performed on a Perkin Elmer Spectrometer (Spectrum 1000) with
KBr pellets. The UV-VIS absorption of the samples is recorded on SL 159 ELICO UV - VIS
Spectrophotometer. The photoluminescence (PL) measurements are performed on a Jobin Yvon
spectrofluorimeter (Fluorolog–3) equipped with a 450-W xenon lamp as an excitation source. This
instrument comes with a sample holder that ensures that the sample thickness is uniform
throughout. This in turn ensures that the sample area exposed to excitation source, and the distance
between the sample and the detector is kept the same for all the measurements. This is how we were
able to compare the PL intensities of different samples. TL measurements are carried out at room
temperature using Nucleonix TL reader, using UV- irradiation (250 nm wavelength) as excitation
with irradiation time in the range 20-140 min.
Fig 1a and b represents the XRD patterns of the products obtained using EDTA and
Na2-EDTA fuels respectively. The two products are labelled EDTA Y2O3 and Na2-EDTA Y2O3
henceforth. XRD patterns for both the samples show peaks corresponding to all the planes of
standard cubic phase Y2O3 (JCPDS 70-0134). Diffraction pattern for EDTA Y2O3 shows broad and
unresolved XRD lines, whereas XRD pattern for Na2-EDTA Y2O3 sample show crystalline and well
resolved peaks. This difference in crystallinity of samples with change in fuel may be due to the
difference in the number of moles of gases evolved during combustion.
The average crystallite size (D) of Y2O3 samples for different fuels are estimated from the full
width at half maximum (FWHM) of the (222) diffraction peak of the powders. This is done using
the Scherrer formula [22]. From the observed XRD patterns, it is evident that tensile lattice strain is
5

different for samples prepared using different fuels. To quantify this correlation, we followed a
method suggested by Williamson and Hall (W–H) [23]. The W-H method is applicable in cases
where both crystallite size effect and lattice deformation are simultaneously operative. The
combined effect of size effect and lattice deformation gives rise to the observed FWHM (β) in the
XRD patterns. β is the sum of β1 (grain size dependent broadening) and β2 (lattice distortion
dependent broadening). This relation assumes a negligibly small instrumental contribution
compared to the sample-dependent broadening. W–H equation may be expressed in the form
λ
β cosθ = ε
(
4sinθ
)
+
…………………..(3)
D
where β (FWHM in radian), ε is the strain developed and D is the grain size. The equation
represents a straight line between 4 sin θ (X-axis) and β cos θ (Y-axis), where 2θ is the Bragg angle
corresponding to XRD peaks. The slope of line gives the strain (ε) and intercept (λ/D) of this line
on the Y-axis gives grain size (D). Fig.2 shows W-H plot for Y2O3 nanopowders prepared with
different fuels. It is observed from W-H plot that a good linear fit is not observed. This is due to the
intrinsic lattice strains (Table 1) present in nanoparticles owing to their large surface to volume
ratio. This is consistent with the previous studies on similar nanoparticles [24, 25].
The crystallite sizes of the powders as determined from Scherrer’s formula and W-H
method are summarized in Table 1. Lowest crystallite size is observed for sample prepared using
EDTA fuel due to amorphous nature of the product. It is interesting to notice that crystallinity and
crystallite size increases for sample prepared using Na2-EDTA fuel. This is attributed to the
presence of sodium ions in Na2-EDTA sample, which could result in formation of molten NaNO3
flux. This in turn helps in homogeneous mixing of reactants and uniform distribution of reactant
ions that chelate with the fuel, which results in the uniform and homogeneous combustion reaction.
Molten NaNO3 formed in-situ is hence expected to result in particles with improved crystallization
6

and increased size. This is reasonable since Manjunatha et.al [26] recently studied the effect of NaF
flux on microstructure of CdSiO3. In their study they showed that the addition of NaF significantly
improves the crystallinity of the CdSiO3:Sm3+ phosphor.
From the X-ray line broadening data, we notice that the diffraction peaks become broader
for EDTA Y2O3. The observed increase in FWHM of XRD peaks in this sample is attributed to
reduced crystallinity or formation of smaller crystallites in these samples. This is likely due to the
fact that the solubility of EDTA is less compared to Na2-EDTA fuel; this results in less
homogeneous combustion with reduced flame temperature. Also the number of moles of gases
evolved is more for EDTA fuel than Na2-EDTA fuel. The reduced temperature coupled with
increased gas emission reduces the possibility of local sintering among the primary crystallites,
thereby reducing the crystallite growth.
Rietveld refinement is a method in which various parameters of the XRD pattern (FWHM of peaks,
asymmetry of peaks, peak shifts, etc.) can be used to estimate the crystal structure of the material
under study. In the present study the Rietveld method is applied mainly for the purpose of
evaluating unit cell parameters of the samples. Refinement is done on both EDTA Y2O3 and Na2-
EDTA Y2O3 (Fig. 3). Rietveld refinement is done using the FULLPROF program [27]. We utilize
the pseudo-Voigt function in order to fit parameters to the experimental data set. The parameters
used are: a scale factor, a zero shifting factor, four back ground factors, three cell parameters, five
shape and width of the peak factors, one global thermal factors and two asymmetric factors. The
final refinement analysis shows that the experimental and calculated PXRD patterns obtained by the
Rietveld refinement are in good agreement with each other, and all observed peaks have been
suitably indexed. For the Rietveld refinement reported, the statistical validity (Rbragg) obtained is
2.3, which is well within the permissible limit (~ 6). The packing diagram of EDTA Y2O3 obtained
7

using Rietveld refinement is shown in Fig. 4. The refined parameters such as occupancy and atomic
functional positions of the Y2O3 nanoparticles are summarized in Table 2. The fitting parameters
2
(Rp, Rwp and χ ) indicate good agreement between the refined and observed PXRD patterns for the
Y2O3 nanoparticles.
The phase formation and purity of the products are further confirmed by FTIR spectroscopy,
and results are shown in Fig. 5(a) and (b). The strong absorption peak ~ 552 cm-1 is ascribed to the
stretching vibration of the Y-O bond [28]. The absorption bands in the range 1400–1600 cm-1 are
2-
2-
ascribed to the bond stretching mode of CO3 . Presence of CO3 is due to the organic precursors
which are used as fuel. The absorption peak at 3445 cm-1 is due to O-H stretching vibration of H2O
[29]. This is typical of most of nanomaterials, since their high specific areas results in significant
adsorption of atmospheric water vapor. It is also worth noticing a band at ~ 2360 cm-1
-
corresponding to nitrate group (NO3 ). These residual nitrates come from the starting nitrate
precursors used. No noticeable difference is seen between the FTIR spectra of the samples prepared
with different fuels. Like the PXRD results discussed earlier, FT-IR studies further confirms the
formation of pure Y2O3 product with no other major impurities or secondary phases.
The scanning electron micrographs of EDTA Y2O3 and Na2-EDTA Y2O3 are shown in Fig. 6a
and 6c respectively. EDTA Y2O3 exhibits highly porous structure with loosely distributed fine
particles on the surface. This may be due to the reduced solubility of EDTA (when compared to
Na2-EDTA), and the reduced flame temperature that inhibits partial sintering of particles during
synthesis. Micrographs of the Na2-EDTA Y2O3 show porous and foamy large aggregates. This
sample has a disconnected structure, and has large voids. Na2-EDTA fuel has a much higher
solubility in water. When this fuel is used, NaNO3 is expected to act as an in-situ flux, which results
in highly homogeneous combustion. This results in higher flame temperature, which leads to partial
8

sintering of particles. This is the likely reason for the significant agglomeration observed in this
sample. Fig. 6b and d shows TEM images of Y2O3 powders obtained using EDTA and Na2-EDTA
respectively. TEM shows that nanopowders obtained with EDTA show porous frame with a
significant amount of the amorphous phase. The particles in this case are in the range ~10 nm. On
the other hand, Na2-EDTA Y2O3 exhibits highly agglomerated particles with sizes in the range ~30
nm. These results are consistent with SEM and XRD results.
Specific surface area (SSA) is one of the essential material parameters in nanopowders. The
surface area (m2/g) of powder samples is dependent on many parameters such as particle size,
shape, surface textures, size distribution. Porosity is also a crucial factor that determines the SSA.
Combustion-derived products usually exhibit good SSA since the release of heat (exothermicity)
during the combustion reaction is long enough for nucleus formation, but too short for grain growth
[30]. The surface area and pore size distribution of the samples prepared using EDTA and Na2-
EDTA is determined from the corresponding nitrogen adsorption and desorption isotherms, and are
presented in Fig. 7(a) and 8(b) respectively. The BET surface area of Y2O3 powders prepared with
EDTA and Na2-EDTA are 27.00 and 20.00 m2/g respectively (Table 3). The higher surface area for
sample prepared with EDTA is due to reduced flame temperature which results in reduced
crystallite size. The reason for this has been discussed above. Likewise decrease in surface area for
powder prepared using Na2-EDTA sample is attributed to more homogeneous combustion with
higher flame temperature which leads to partial sintering of particles.
Fig.7(c) and (d) shows the pore size distribution which are calculated using the Barrett-
Joyner-Halenda (BJH) model. For this calculation, N2 isotherms are used. The pore size distribution
of the both the samples show that pore diameter 10-160 Å with a maximum at about 22 Å. The
9

surface area, pore volume and pore diameter for the samples prepared with two different fuels are
summarized in Table 2.
UV–Vis spectra of Y2O3 nanoparticles prepared by different fuels are shown in Fig. 8A.
Two absorption peaks at ~254 nm (sharp), ~ 266 and 280 nm (broad) are recorded. The maximum
absorption (~254 nm) is due to transition between valence band and conduction band [31]. The
weak absorptions (~ 266 and 280 nm) observed in the UV–Vis region is attributed to transitions
involving extrinsic states such as surface traps/defect states/impurities [32]. It is well known that
smaller size particles have high surface to volume ratio. This results in increase in defect density.
This is also why nanomaterials often exhibit strong and broad absorption bands [33]
The optical band gap energy (Eg) is estimated using Wood and Tauc relation [34] given by,
(αhυ) α (hυ-Eg)1/n ……………….. (4)
where ‘a’ is the absorption coefficient, hν the photon energy, Eg the energy gap, and n = 2 for direct
allowed transition. For direct band gap materials, Eq. (4) can be rearranged and written in the form
(αhυ)2 α (hυ-Eg) ………………….. (5)
The energy gap is determined by plotting (αhυ)2 versus hυ and finding the intercept on the
hν axis, as shown in Fig.8B. The band gap in sample prepared with EDTA is less (Eg= 4.8 eV)
when compared to sample prepared with Na2-EDTA (Eg= 5.2 eV). The band gap of Na2-EDTA
Y2O3 are consistent with the reported band gap of Y2O3 [35]. The lower Eg of EDTA Y2O3 most
likely has to do with the higher structural disorder in its lattice. The optical band gap, Eg values
mainly depend on the preparation methods and processing conditions (heat treatment, reaction time,
reaction temperature, etc.). Sample prepared with EDTA fuel is slightly amorphous in nature. The
10

structural defects in this system are expected to introduce defect states between valence and
conduction band, which would reduce the observed optical band gap. On the other hand, Na2-EDTA
Y2O3 is highly crystalline and therefore the concentration of structural defects (oxygen vacancies,
distortion or strain in lattice) is minimal; this explains the large Eg (5.2 eV) observed.
Figure 9 (a and b) shows the photoluminescence spectrum of Y2O3 nanophosphors prepared using
EDTA and Na2-EDTA as fuels respectively. Upon 248 nm excitation, a series of emission bands
ranging from UV to green region are observed. The bands observed are centered at 380, 421, 485,
505, 515 and 527 nm. Since Y3+ itself is non-luminous, the observed luminescence from Y2O3
samples must be due to non-stoichiometry created by the oxygen deficiency in the system, which is
known to occur when Y2O3 forms in carbon rich ambient [36]. Carbon related impurities, caused
due to the use of carbon based fuels used in combustion synthesis, contribute to the luminescence of
Y2O3.
In the emission spectra, the broad emission peak at 380 nm in UV region is attributed to radiative
recombination of photo-generated hole with an electron occupying the oxygen vacancy [37].
Emission band centered at 421 nm is observed in both EDTA Y2O3 and Na2-EDTA Y2O3, and is
attributed to recombination of a delocalized electron close to the conduction band with a single
charged state of surface oxygen vacancy [38]. The emission band at 480 nm can be attributed to
self-trapped excitation luminescence [39]. Other weak emission peaks at ~505, 515 and 527 nm are
due to different kinds of oxygen vacancies. It is observed that intense and well resolved emission
peaks are recorded for Na2-EDTA Y2O3 when compared to EDTA Y2O3 fuel. This enhanced
photoluminescence response for Na2-EDTA sample is attributed to better crystallinty of the sample.
Fig. 10 and Fig. 11 shows TL glow curve characteristics of UV exposed (20-140 min) EDTA Y2O3
and Na2-EDTA Y2O3. A broad TL peak at 140 oC is recorded in EDTA Y2O3, whereas in the case of
11

Na2-EDTA Y2O3 a broad peak centered at 155 oC is recorded. Heating rate is kept constant (2.5 oC s-
1) in both cases. It is clear that the shape and position of TL peaks are almost the same in the dose
range studied, showing a dose independent character. However it is apparent the intensities of the
TL peaks changes with the increase in the UV exposure time. It is worth
noticing that the glow peak is well defined in case of EDTA Y2O3 when compared to
Na2-EDTA Y2O3.
Variation in TL intensity with different UV exposures is recorded and shown in Fig 12. The
peak heights are used for measuring the TL intensities. In EDTA Y2O3, TL intensity increases with
UV exposure up to 120 min, after which it decreases. This trend is generally due to competition
between radiative and non-radiative centers, or between different kinds of trapping centers.
However in the case of Na2-EDTA Y2O3, TL intensity increases with UV exposure. In the present
study UV exposure is used for creation of defects, and the observed TL glow peaks is mainly
attributed to surface defects, since UV radiation cannot penetrate deep into the material [40]. The
sublinear decrease in TL intensity with UV exposure can be explained on the basis of Horowitz’s
TIM (track interaction model) [41, 42]. At low exposure time the recombination of various
trapping/ luminescent centres (TCs/LCs) occurs entirely within the tracks. Electrons escaping the
tracks are intercepted by the non-radiative competitive centres (CC) in the intermediate region. The
TL signal, therefore, linearly increases with the irradiation and is simply proportional to the UV
dose (the exposure time). At higher dosage levels, the distance between neighbouring tracks
decreases and the electrons leaving the track reach the neighbouring tracks, resulting in the
increased recombinations, which results in greater TL intensity. In the present study, the TL
intensity increases with UV exposure time, since number of particles irradiated increases with
exposure time.
12

TL characterization of the phosphor requires extraction of information regarding trapping
parameters such as activation energy (E) of traps, and order of kinetics (b) associated with the glow
peaks. Here E is a measure of energy required to eject an electron from the defect center to the
conduction band, while s is the rate of electron ejection. The order of kinetics b is a measure of the
probability that a free electron gets re-trapped. This re-trapping effect increases with density of
empty traps. For estimation of trapping parameters (s and E), we did deconvolution of the TL glow
curves using ORIGIN 8.1 software. This is shown in the Fig.13 (a and b) for EDTA Y2O3 and Na2-
EDTA Y2O3 respectively. The three curves obtained are needed to account for the two weak
shoulders, and one dominant maximum observed in the TL curve.
The following empirical formulae are used to estimate trapping parameters using the Chen’s
peak shape method [43]:
The activation energy (E)
2
⎛
⎜
⎜
⎞
⎟
⎟
kT
m
Eα = cα
− bα
(
2kTm
)
……… (6)
α
⎝
⎠
where α = τ, δ and ω with τ = Tm – T1, δ = T2 – Tm and ω = T2 – T1
(
)
Cτ = 1.51+ 3.0 μg − 0.42
(
)
b =1.58 + 4.2 μg − 0.42
τ
,
……… (7)
……… (8)
……… (9)
(
)
)
Cδ = 0.976 + 7.3 μg − 0.42
bδ = 0
,
,
(
Cω = 2.52 +10.2 μg − 0.42
bω = 1
13

Frequency factor (s)
Once E and b are known, frequency factor (s) can be evaluated
⎛
⎞
βE
E
−1
……… (10)
⎜
exp
⎜
⎟
s =
[
1+ (b −1)Δm
]
kT 2
kTm
⎟
⎝
⎠
m
2kTm
Where β is the linear heating rate, b is the order of kinetics and Δm =
E
Order of kinetics
To determine the order of kinetics (b), the form factor or symmetry factor is determined using the
following expression:
…..…… (11)
This involves calculation of T1 and T2. T1 and T2 are the temperatures corresponding to half of the
maximum intensities on either side of the glow peak maximum temperature (Tm). The nature of the
kinetics can be found by the form factor. Theoretically the value of geometrical form factor (μ ) is
g
close to 0.42 for first order kinetics, and value is 0.52 for second order kinetics.
The estimated kinetic parameters for EDTA Y2O3 and Na2-EDTA Y2O3 phosphor are given in
Table 4 and 5 respectively.
EDTA Y2O3 showed better TL response with good linearity. It is important to notice that using right
kind of fuel it is possible to maximize the TL efficiency, improve sensitivity, and ensure dose
linearity for a specific irradiation type. The traps responsible for TL observed in this case are due to
oxygen vacancies produced during the fabrication of nanophosphor and/or due to defects produced
by irradiation exposure [44]. The obtained experimental results shows that the TL characteristics of
a material (eg. efficiency, saturation, sensitivity and dose linearity) depends greatly on the nature of
the fuel used in the combustion synthesis.
14

Conclusions:
We show that a single experimental parameter, namely the fuel used in solution combustion
synthesis, can be changed to obtain Y2O3 that is either photoluminescent (PL) or thermoluminescent
(TL) active. We demonstrate that by simply switching the fuel from ethylene diamine tetracetic acid
(EDTA) to its disodium derivative (Na2-EDTA), we obtain a better photoluminescent material. On
the other hand, use of EDTA aids in formation of Y2O3 that is thermoluminescent. EDTA Y2O3 is
relatively disordered, has a crystallite size ~10 nm, and has a specific surface area of ~27 m2/g.
Crystallinity is better and crystallite size is larger (~30 nm) when Na2-EDTA is used. The TL
kinetic parameters are also calculated using glow curve shape method. Results indicate that the TL
glow curves of both samples follows second order kinetic model. The observed PL is due to oxygen
deficiencies in Y2O3, while the observed TL characteristics (eg. supra-linearity in the intensity
variation with dosage) is understood using Tract Interaction Model (TIM). This work shows that
appropriate choice of fuel alone can help to achieve significant improvement in the
photoluminescence or thermoluminescence response.
Acknowledgements
R.H.K. is grateful to the Management, Principal and HOD Chemistry of M. S. Ramaiah Institute of
Technology, Bangalore, for their constant support and encouragement.
15

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18

Figure captions
Fig.1. X-ray diffraction patterns of Y2O3 samples synthesized using (a) EDTA and (b) Na2-EDTA
as fuels. (samples are labeled as EDTA Y2O3 and Na2-EDTA Y2O3 respectively)
Fig.2. Williamson−Hall plots of (a) EDTA Y2O3 (b) Na2-EDTA Y2O3. The scatter around the linear
fit indicates the presence of lattice strains in these nanoparticles.
Fig.3. Reitveld refinement of cubic Y2O3 nanoparticles prepared by EDTA and Na2-EDTA. All the
observed peaks are fitted (with statistical validity RBragg= 2.3) and indexed using JCPDS 70-0134.
Fig.4. Packing diagram of cubic Na2-EDTA Y2O3.
Fig.5. FTIR spectra of (a) EDTA Y2O3 and (b) Na2-EDTA Y2O3.
Fig.6. Scanning electron micrographs (SEM) and transmission electron micrographs (TEM) of
EDTA Y2O3 (a, b) and Na2-EDTA Y2O3 (c, d).
Fig.7. Nitrogen adsorption/desorption isotherms of (a) EDTA Y2O3, (b) Na2-EDTA Y2O3. The pore
size distribution of (c) EDTA Y2O3 and (d) Na2-EDTA Y2O3 particles is obtained through the
Barrett-Joyner-Halenda method.
Fig.8. (A) UV-Vis absorption spectra, and (B) optical energy band gap of (a) EDTA Y2O3, (b) Na2-
EDTA Y2O3.
Fig.9. Photoluminescence (PL) spectra of (a) EDTA Y2O3, (b) Na2-EDTA Y2O3. Oxygen deficiency
in Y2O3, which occurs due to the carbon rich ambient occurring during the synthesis, plays a crucial
role in determining the observed PL.
Fig.10. Thermoluminescence (TL) glow curves of UV-irradiated (20–140min) EDTA Y2O3. This
sample shows TL response with good dose linearity.
Fig.11. Thermoluminescence glow curves of UV-irradiated (20–140min) Y2O3 nanoparticles
prepared by Na2-EDTA.
Fig.12. Variation of Thermoluminescence glow peak intensity with UV-irradiated (20–140min)
(a) EDTA Y2O3, (b) Na2-EDTA Y2O3. EDTA Y2O3 shows TL response with better dose linearity.
Fig.13. Glow curve deconvolution of (a) EDTA Y2O3 exposed to 100 min of UV radiation, (b) Na2-
EDTA Y2O3 exposed to 80 min of UV radiation.
19

Table captions
Table 1. Estimated crystallite size and strain of Y2O3 nanoparticles prepared using EDTA and Na2-
EDTA fuels.
Table 2. Rietveld refined structural parameters of cubic Y2O3 nanoparticles prepared using EDTA
and Na2-EDTA as fuels.
Table 3. Physical parameters extracted of Y2O3 nanoparticles prepared using different fuels. The
parameters are obtained using Brunauer-Emmett-Teller analysis.
Table 4. Estimated trap parameters of irradiated Y2O3 nanoparticles which are prepared using
EDTA fuel. UV dose is varied by changing the irradiation time.
Table 5. Estimated trap parameters of irradiated Y2O3 nanoparticles which are prepared using Na2-
EDTA fuel. UV dose is varied by changing the irradiation time.
20

(a)
(b)
10
20
30
40
50
60
70
80
2θ (degree)
Fig. 1
21

a
b
0.75 1.00 1.25 1.50 1.75
4 Sinθ
Fig. 2
22

Fig. 3
23

Fig. 4
24

(a)
(b)
500 1000 1500 2000 2500 3000 3500 4000
Wavenumber (cm-1)
Fig. 5
25

Fig. 6
26

140
120
100
80
160
140
120
100
80
(a)
(b)
60
60
40
40
20
20
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Relative Pressure (P/P0)
(c)
0.25
0.20
0.15
0.10
0.05
0.00
(d)
2.5
2.0
1.5
1.0
0.5
0.0
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
Pore Diameter (A)
Pore Diameter (A)
Fig. 7
27

A
(b)
(a)
250
300
350
400
450
500
550
Wavelength (nm)
1.6x1011
B
1.2x1011
8.0x1010
4.0x1010
0.0
(b)
(a)
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
E (eV)
g
Fig. 8
28

(b)
(a)
470 480 490 500 510 520 530
Wavelength (nm)
350
400
450
500
550
Wavelength (nm)
Fig. 9
29