Luminescence response and CL degradation of combustion synthesized spherical SiO2:Ce nanophosphor

Article abstract of DOI:10.1016/j.materresbull.2011.08.051

This study was aimed to systematically investigate the luminescence response of SiO₂:Ce³⁺ nanophosphors with different excitation sources. The powders were synthesized by using an urea assisted combustion method. SiO₂:Ce_1m% samples were also annealed at 1000 °C for 1 h in a charcoal environment to reduce incidental Ce ⁴⁺ to partial Ce³⁺ ions. High resolution transmission electron microscopy (HRTEM) images of the as synthesized and annealed powder samples confirmed that the particles were spherical and in the size range of 3-8 nm in diameter. X-ray diffraction (XRD) and electron dispersion spectroscopy (EDS) results showed that the SiO₂ was crystalline and pure. Diffused reflectance, photoluminescence (PL) and cathodoluminescence (CL) results of the SiO₂:Ce³⁺ samples were obtained and compared with each other. The CL degradation and the surface reactions on the surface of the SiO₂:Ce³⁺ were studied with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). A clear improvement in the chemical stability of the SiO₂:Ce³⁺ annealed at 1000 °C were obtained.

Full text of DOI:10.1016/j.materresbull.2011.08.051

Materials Research Bulletin 46 (2011) 2359–2366
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Luminescence response and CL degradation of combustion synthesized
spherical SiO :Ce nanophosphor
2
I.M. Nagpure *, Shreyas S. Pitale, K.G. Tshabalala, Vinay Kumar, O.M. Ntwaeaborwa,
J.J. Terblans, H.C. Swart *
Department of Physics, University of the Free State, P. O. Box 339, Bloemfontein ZA9300, South Africa
A R T I C L E I N F O
A B S T R A C T
3
+
Article history:
This study was aimed to systematically investigate the luminescence response of SiO :Ce nanopho-
2
Received 10 April 2011
sphors with different excitation sources. The powders were synthesized by using an urea assisted
Received in revised form 24 June 2011
Accepted 24 August 2011
Available online 1 September 2011
2
combustion method. SiO :Ce1m% samples were also annealed at 1000 8C for 1 h in a charcoal
4+
3+
environment to reduce incidental Ce to partial Ce ions. High resolution transmission electron
microscopy (HRTEM) images of the as synthesized and annealed powder samples confirmed that the
particles were spherical and in the size range of 3–8 nm in diameter. X-ray diffraction (XRD) and electron
Keyword:
dispersion spectroscopy (EDS) results showed that the SiO
reflectance, photoluminescence (PL) and cathodoluminescence (CL) results of the SiO
were obtained and compared with each other. The CL degradation and the surface reactions on the
2
was crystalline and pure. Diffused
A. Nanostructures
A. Optical materials
C. Photoelectron spectroscopy
C. X-ray diffraction
D. Luminescence
3
+
2
:Ce samples
3
+
2
surface of the SiO :Ce were studied with X-ray photoelectron spectroscopy (XPS) and Auger electron
3
+
spectroscopy (AES). A clear improvement in the chemical stability of the SiO
were obtained.
2
:Ce annealed at 1000 8C
D. Surface properties
ß 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
enough to achieve the required morphology. In the interest of
luminescence applications, Ce is one of the most extensively
employed activators in the optical industry. The Ce ion was
The fact that SiO
2
can exist in numerous crystalline and
3+
amorphous forms, and its status as one of the most prevalent
compounds on earth, has stimulated a long history of experimental
and theoretical investigations. In recent years, SiO
studied widely due to its various applications. Physical perfor-
mance of this matrix makes it suitable for various applications such
as surface coating, photo catalysts, magnetic materials and gas
observed in two different states, the luminescence trivalent Ce
state due to the allowed 5d-4f transition and the non-luminescent
4
+
2
has been
Ce state. The optical features of Ce in many crystalline and
amorphous SiO matrices were intensively investigated from both
2
experimental and theoretical points of view [7,8]. In fact, the strong
dependence of both the energy and the splitting of the 5d levels
upon the host matrix significantly affect the rare earth (RE) optical
response and works as a powerful probe of the host crystal field [9].
Especially the incorporation of Ce as well as of other rare-earth ions
inside the glassy matrices, and its homogeneous distribution is not
straightforward and was the subject of several studies [9–12]. Kim
et al. [14] and Meng et al. [13] used a sol gel route for the
2
sensor materials [1,2]. SiO is used as a coating material due to its
favorable band alignment when forming a heterojunction with
other commercial cathode ray tube (CRT) phosphors [3]. Further-
more, an electron beam exhibits minimal losses when the
electrons penetrate the SiO
higher penetration depth in SiO
of light elements and has a low density. It is a really difficult task to
achieve spherical morphology with nano sized particles in the SiO
2
, and the electrons have therefore a
2
due to the fact that SiO , consists
2
3
+
3+
4+
incorporation of Eu and Tb , and Tb and their mutual energy
transfers into the silica matrices. Ntwaeaborwa et al. [6] also
followed a sol–gel technique in a different way for the incorpo-
2
glassy matrix. Spherical morphology is very important in terms of
application purposes. Several researchers used the melt method
3
+
3+
3+
ration of Ce , Tb and Eu ions and study their mutual energy
transfer into nano silica matrices. Nogami [15] and Ntwaeaborwa
[
4], precipitation [5] and sol–gel technique [6] as the prominent
routes to prepare the SiO matrix but these applied routes are not
2
et al. [16,17] provide a proof that rare-earth doped amorphous SiO
2
nanophosphor could be used in low voltage FEDs and other related
light emitting devices.
In the present case, investigations were done on the preparation
*
Corresponding authors. Tel.: +27 0 51 401 9749; fax: +27 0 51 401 3507.
E-mail addresses: indrajitnagpure@yahoo.co.in (I.M. Nagpure),
swarthc@ufs.ac.za (H.C. Swart).
2
of spherical nano sized undoped and Ce doped SiO matrix by using
the urea assisted combustion method. A charcoal environment is
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.08.051

2
360
I.M. Nagpure et al. / Materials Research Bulletin 46 (2011) 2359–2366
used during annealing to achieve the homogeneous distribution of
analyzer pass energy was maintained at 11.3 eV. Measurements
were performed using either a 1 eV/step and 45 min acquisition
times (binding energies ranging from 0 to 1400 eV) for survey
scans or a 0.1 eV/step and 20–30 min acquisition times for the high
resolution scans. The sample area analyzed was about 1 mm and
the pressure during acquisition was typically under 1 ꢂ 10ꢃ8 Torr.
3+
luminescence Ce ions in the SiO
such as diffused reflectance, photoluminescence (PL) and cath-
odoluminescence (CL) were studied in Ce doped SiO matrix. The
2
matrix. The optical properties
2
2
CL degradation was deeply studied in the as-prepared and
annealed samples. The oxidation state of the doped Ce ion and
elemental analysis before and after CL degradation were reported
using X-ray photoelectron spectroscopy (XPS) and Auger electron
spectroscopy (AES).
3. Results and discussion
3.1. Structural and morphology analysis
2
. Experimental
Fig. 1 shows the X-ray diffraction patterns of the as-prepared
Spherical nano sized SiO
prepared using the urea assisted combustion method. All the
chemicals used were of 99.99% purity. The starting materials were
2
(pure) and SiO
2
:Ce phosphors were
and annealed SiO2 matrix. The as-prepared and annealed SiO2
samples showed the formation of peaks corresponding to the
crystalline low quartz’s phase with a broad peak centred at (1 0 0),
and other prominent peaks at (0 1 1), which is characteristic
diffraction pattern of the low-quartz’s phase of a SiO2 glassy
matrix. The observed peak from the XRD data confirmed that the
sample which was annealed at 1000 8C for 1 h has an improved
crystallinity and all phases of the low quartz’s were clearly
observed (see also the inset of Fig. 1). The observed peaks were
assigned by comparing the XRD profile with the respective ICCD
data file No. 86-1565. Si and O were the only two major chemical
elements present as detected from the electron dispersive X-ray
spectrometer (EDS) data from the as-prepared and annealed
samples as shown in the inset of Fig. 2. Atomic and weight
percentages are consistent with expected results. The SEM and
silicic acid (SiO
2
ꢀnH
2
O), urea (NH
2
CONH
2
), ammonium nitrate
O. The ratio of the
(
NH NO ) and cerium nitrate Ce(NO
4
3
3
)
3
ꢀ7H
2
metal nitrates (oxidizers) and urea (fuel) was calculated using the
total oxidizing and reducing valencies of the components, which
served as the numerical coefficients to get an equivalent ratio of
unity. To avoid an incomplete combustion reaction, stoichiome-
trical equal amount of ammonium nitrate was added in the
reaction mixture and an excess heat was liberated during the
combustion process [18]. The chemical reaction taking place in the
combustion process can be described as follow:
SiO
2
ꢀnH
2
O þ 20NH
2
CONH
2
þ 20NH
" þ 151H
4
NO
3
! SiO
O "
2
2
HRTEM micrographs of as prepared and annealed SiO samples are
þ 80N " þ 29CO
2
2
2
shown in Fig. 2(a)–(d). Although some faceting exists, the SEM and
HRTEM micrographs show agglomerated SiO2 with individual
particles appearing circular implying spherical shapes. In addition,
bigger particles were encrusted with smaller particles, suggesting
that the agglomerated SiO2 powder was made of nano-sized
particles. The average particle sizes estimated from the HRTEM
images are ranging from 3 to 8 nm in diameter.
All the reagents in the required amounts were dissolved in 5 ml
doubly distilled water by stirring vigorously for a 1/2 h and a
transparent homogeneous solution was obtained. This transparent
solution was transferred to a muffle furnace preheated to 550–
6
ꢁ
00 8C and a spherical nano sized SiO
2
:Ce powder was obtained in
5 min. The SiO :Ce powder samples were prepared with different
2
concentrations (1, 2 and 4 mol%) of Ce ions. The samples were then
annealed at 1000 8C for 1 h in a charcoal environment to reduce
incidental Ce to partial Ce ions.
3.2. Diffused reflectance, PL and CL analysis
4+
3+
The as-prepared and annealed powder was characterized by X-
ray powder diffraction (XRD) for their phase purity and crystallin-
The optical diffused reflectance spectrum of as-prepared and
2
2
2m%
annealed SiO and SiO :Ce powder sample was compared with
ity. XRD analysis was carried out using
˚
a
PAN-analytical
standard Ce(NO ) ꢀ6H O and CeO as shown in Fig. 3. The diffused
3
3
2
2
diffractometer with Cu-Ka
(
l
= 1.5418 A) X-rays at a scanning
reflectance spectrum of as-prepared and annealed SiO2 sample
shows weak absorption at 220, 257, 305 nm attributed to Si–O type
defect and stronger absorption at 420 nm which is due to the
step of 0.018; scanning time at each step-20 s, in the 2
u range from
1
08 to 608. The morphology and chemical composition analyses
were carried out using a Shimadzu’s SSX-550 scanning electron
microscope (SEM) and electron dispersive X-ray spectrometer
(EDS) respectively. High Resolution Transmission Electron Micros-
copy (HRTEM) images were taken with a Hitachi H-800 transmis-
sion electron microscope, using an accelerating voltage of 200 kV.
Perkin Elmer LS 950 lambda UV-Vis spectrophotometer was used
to study the diffused reflectance spectrum. The photolumines-
cence (PL) properties of the phosphors were measured using a
Shimadzu spectrophotometer (RFPC-5301) at room temperature
using a monochromatized Xenon flash lamp as an excited source.
The excitation and emission slit width were kept at 1.5 nm. The
Auger and Auger peak-to-peak height (APPH) data were collected
in a UHV chamber using a PHI Model 549 spectrometer. An Ocean
Optics PC2000 Spectrometer and OOI Base32 computer software
were used to record the cathodoluminescence (CL) data from the
powder phosphor. The Auger, APPH and CL data were recorded
simultaneously using the same electron beam accelerated at 2 kV
2
at a current density of 2 mA/cm . The electron beam irradiation
ꢃ6
was prolonged for 16 h at an O
2
pressure of 1 ꢂ 10 Torr. The XPS
measurements were carried out with a PHI 5000 versa probe
spectrometer using monochromatic Al Ka radiation
= 1253.6 eV). For higher resolution spectra, the hemispherical
Fig. 1. X-ray diffraction pattern of the SiO2 host lattice as cast as well as after
(
hn
annealing.

I.M. Nagpure et al. / Materials Research Bulletin 46 (2011) 2359–2366
2361
Fig. 2. SEM-EDS images of (a) as-prep. and SiO
2
(b) annealed SiO
2
(10008/1 h); HRTEM images of (c) as-prepared SiO
2
and (d) annealed SiO
2
(10008/1 h).
3
+
oxygen vacancy defects centres present in the SiO
depth of intensity peaks get reduced in the annealed sample,
suggesting that reduction in defect centres due to the annealing
2
host lattice. The
Ce ion with maximum at 320 nm was obtained. It is an allowed
1
1
transition of f to d configuration. The 320 and 257 nm absorption
are associated with the Ce band edge and this suggest that Ce
ions were present in both as-prepared and annealed sample. XPS
analysis was used to distinguish between the Ce and Ce redox
states. The detailed XPS analysis is discussed in Section 3.5. The
presence of Ce ions or non-homogeneous distribution of the Ce
and Ce ion into the SiO
Annealing in charcoal environment was used for the partial
conversion of non-luminescent Ce ions to luminescent Ce ions.
Fig. 4 shows the PL excitation and emission spectrum of as-
prepared and annealed SiO
3
+
3+
2
effect. In the as-prepared SiO :Ce2m% sample, weak absorption was
3
+
4+
observed at 247 nm and the broad absorption peak at 270–370 nm
is due to the doping with the Ce ion. The weaker band was observed
at 450–500 nm and could be due to inter-valence charge transfer
4
+
3+
3+
4+
4+
transition between Ce and Ce ion as shown in the inset of Fig. 3
2
glassy matrix could not be avoided [21].
4
+
[
4
19]. The absorption of Ce ion consist of two bands, one at 230–
50 nm and the other one at 450–500 nm and is most likely due to
4
+
3+
2
ꢃ
4+
the charge transfer between O and Ce [9,11,20]. The standard
3+
4+
Ce(NO
3
)
3
ꢀ6H
2
O and CeO
2
were used to compare Ce and Ce
2 2
and SiO :Ce samples. The PL data were
absorption to that of the synthesized samples. In the charcoal
annealed sample, Ce absorption is shifted towards a lower
recorded when the powder samples were excited using
a
4
+
wavelength, suggesting that there is a partial conversion of Ce
3+
ions to Ce ions. The well known broad band absorption peak of
2 2
Fig. 4. PL spectra of SiO and SiO :Ce phosphor obtained with a Xenon flash lamp as
Fig. 3. Optical diffused reflectance spectrum of SiO
2
2
and SiO :Ce as indicated.
source.

2
362
I.M. Nagpure et al. / Materials Research Bulletin 46 (2011) 2359–2366
4
+
monochromatized xenon lamp. The PL emission in the as-prepared
and annealed SiO sample at around 425 nm may be attributed to
the –O–O– type defect, an O – intrinsic defect and an irradiation
intrinsic defect and the emission at around 470 nm is related to the
triplet to ground state transition of neutral oxygen vacancy defects
(Ce ) states in SiO
2
:Ce. It can be inferred that an incident electron
4+
2
using in a high energy excitation CL process transformed the Ce
ion into a quasistable state of excited Ce ion, i.e. (Ce )* [24]. This
3
+
3+
2
9
1
4+
after undergoing 4d 4f transitions stabilizes back to Ce
1
0
0
(4d 4f ). The process may be summarized as follows:
[
22]. The PL emission for the annealed SiO
due to the defect stabilization and improvement in the crystallinity
after annealing. In the case of the SiO :Ce sample a prominent
excitation broad band is observed at around 340 nm due to the 4f-
2
was slightly suppressed
Ce4þð4d 4f Þ þ electronðe Þ ! quasi-permanentðCe3þÞꢄ
10
0
ꢃ
9
1
4þ
10
0
ꢃ
2
ð4d 4f Þ ! Ce ð4d 4f Þ þ hnradiativeð ! Þ þ electronðe Þ
(1)
3+
1
1
5
d transition of Ce ions [23]. It is an allowed transition of f to d
configuration. It is clear that the excitation spectrum is a
superposition of several bands. A weak excitation peak is also
observed at around 257 nm due to band to band host absorption.
We suppose that this is possible only under electron beam
excitation. Under xenon flash lamp PL emission in the as-prepared
2
SiO :Ce sample is very small compared to the annealed sample.
4
+
3+
PL emission spectrums of the as-prepared and annealed SiO
are similar in nature. The PL emission intensity of the as-prepared
SiO :Ce sample that extends from 390 to 525 nm is much lower
compared to the annealed sample. The weak PL emission intensity
2
:Ce
The conversion of Ce state into quasi-stable (Ce )* results in an
increase of the CL intensity.
2
The shift in the emission spectra of the SiO :Ce sample was
2
observed under two different excitation sources. The mechanisms
of incident energy from these sources to excite the materials are
different. It is observed that, when using a electron beam as an
3+
in the as-prepared sample may be due to the lack of Ce ions. The
annealed sample in the charcoal environment shows enhancement
in the PL intensity due to the improved crystallinity and possibly to
2
excitation source the CL intensity in the SiO :Ce sample shifted
4+
3+
the partial conversion of Ce ions to Ce ions. The observed PL
towards higher wavelengths compared to a xenon flash lamp. In
the case of the xenon flash lamp, the 338 nm excitation wavelength
emission peaks at 412 and 444 nm are due to the d ! f
0
1
1 2
3+
)] transitions of the Ce ion from the 5d excited
3+
[
4f 5d ! 4f ( F
J
was sufficient to excite the 4f-5d transition band of the Ce ions.
state to the 2F ground state. Normally this exhibits a broad
emission peak being regarded as the electric dipole allowed
The total luminescence output is from the Ce ions, divided in two
2
sub-bands at 412 and 444 nm. The obtained CL emission in SiO Ce
3+
emission. Since the ground state of the Ce ion appeared as a
doublet emission band at 412 and 444 nm due to spin–orbit
splitting of the 2F5/2 and 2F7/2 state (i.e. the peak at 444 nm is due
is shifted towards a higher wavelength (red shifted) compared to
the PL emission. The red shift in the CL emission is due to the effect
of surface Si–O bonds on the band structure of the SiO
2
2
2
to D(5d) ! F7/2(4f)) and the shoulder at 412 nm is due to the
nanocrystals, which suggests that for nanoparticles with diameters
of less than ꢁ3–8 nm, the expected red shift (ꢅ20–30 nm) is
determined by the optical transitions between the levels of the Si–
O bonds stimulated by the incident cathode rays at the crystallites
surface [25]. The corresponding surface bond traps an exciton and
produces a localized state that lies within the band gap. The role of
the oxygen on the surface in the Si–O bond significantly influence
2
2
transition from the D(5d) ! F5/2(4f). The PL intensity initially
increased with an increase in concentration from 1 to 2 mol% of
3+
Ce
and it decreased slightly when the concentration was
increased to 4 mol%.
The CL emission spectrum that was measured is shown in Fig. 5.
The CL broad band emission spectrum of the as-prepared and
annealed SiO
emission spectrum of undoped SiO
behavior as observed in the PL emission spectrum. The as-prepared
and annealed SiO :Ce samples consist of comparable CL intensities
with a characteristic doublet emission of 2F5/2 (460 nm) and 2F7/2
2
sample were observed at around 380–550 nm. CL
3
the Si-Sp network thus reducing the rate at which the band gap
samples showing similar
widens with the decrease of the diameter of the crystallites and CL
should be red shifted compared to the PL spectrum [26,27]. To
understand the definite mechanism, further deep investigation is
needed.
2
2
3
+
(
487 nm) state of the Ce ion. The CL emission of the as-prepared
SiO :Ce was much more intense under electron beam excitation
than PL emission. As inferred from diffused reflectance and XPS
3.3. CL degradation, AES, APPH and AES analysis
2
3+
studies, Ce ions were found in both trivalent (Ce ) and tetravalent
Fig. 6 shows the comparative CL degradation spectra in
2
logarithmic scale of the as-prepared and annealed SiO :Ce2m%
Fig. 5. CL emission spectrum of SiO
2
2
and SiO :Ce phosphor under electron beam
2
Fig. 6. Comparative normalized CL degradation spectrum of SiO :Ce2m% phosphor at
excitation.
1 ꢂ 10ꢃ6 Torr O
2
atmosphere.

I.M. Nagpure et al. / Materials Research Bulletin 46 (2011) 2359–2366
2363
ꢃ6
samples during degradation at 1 ꢂ 10 Torr O
intensity of the emission peaks at 460 and 487 nm was reduced to
0% of the initial intensity after 7 h of prolonged electron
2
pressure. The CL
sample with respect to the as-prepared sample. This may be due to
the improvement of the crystallinity of the annealed sample.
5
Based on vaporization and thermochemical stability of SiO
decomposition of SiO
according to the following equation [29]:
2
the
bombardment of the as-prepared sample and after 16 h of
bombardment of the annealed sample under continuous electron
dose as shown in Fig. 5(4) and (5). Fig. 7(a) and (b) shows the
2
can result in the formation of new species
SiO
2
! SiOx ðx < 2Þ þ O "
ð Hf298 KÞ ¼ þ181 kJ=mol
D
(2)
normalized CL intensity and APPH as a function of electron dose at
ꢃ6
1
ꢂ 10 Torr in the O
2
atmosphere for the as-prepared and
Fig. 8 shows the AES spectrum of the as-prepared and annealed
sample before and after degradation with characteristic Auger
peaks of Si (82 eV), C (272 eV) and O (510 eV). The AES peaks show
that the C is present on the as-prepared and annealed sample
before degradation. After prolonged electron beam bombardment
the C left the surface completely. The escape of gaseous CO
be responsible for the decrease of the oxygen concentration. The Si
concentrations increase initially and then stabilize whereas the O
concentration is found to decrease progressively as shown in Fig. 7.
It is also clear that there is a change in the Si peak shape, specially
on the low energy side of the peak, which suggests a change in the
chemical environment on the surface. Due to the electron beam
irradiation local heating of the specimen is also possible, so that O
and presumably CO escape out of the SiO matrix. As a result, the
annealed sample as function of the electron dose. The decrease of
CL intensity in Fig. 7 suggested that, as far as an increase in electron
dose is concerned, there is a direct correlation between the
degradation of the CL intensity and the desorption of O
bombardment of the SiO :Ce2m% phosphors resulted in the relative
slightly increase of the Si peak intensity with a direct decrease in
the O peak intensity. The C present on the surface of the phosphor
2
. Electron
2
2
could
2
2
and into the pores of the silica decreases continuously with an
increase in the electron dose. It is clear that electron stimulated
surface chemical reactions (ESSCR) occurred during the electron
bombardment. The ESSCR mechanism [28] is use to explained
desorption of O
silicon according to this mechanism. O
reaction with other O species resulting from electron beam
induced dissociation of vacuum ambient gases such as H O, O , and
CO or as oxygen will now desorbs from the surface as ions [16,17].
2
. Si–O bonds are broken into free oxygen and
2
2
molecule following a
2
2
chemical composition on the phosphor surface becomes O-
deficient as predicted by Eq. (2). This may lead to a change of
chemical composition from SiO
completely. The rate at which the CL intensity decreases depends
on the O removal, which governs the stability of the matrix.
2
2
2
2 x
to SiO (x < 2) partially if not
Simultaneous with the process of desorption, it is possible that the
electron beam dissociated oxygen and silicon recombined to form
2
SiOx (x < 2) on the surface [29]. The SiO
cause of the CL intensity degradation of the SiO
rate of O decrease and CL degradation is slower for the annealed
x
layer could be the major
2
:Ce phosphor. The
3.4. XPS analysis of degraded and undegraded sample
An XPS study was carried out on the as-prepared and annealed
SiO :Ce sample before and after electron beam exposure (140–
60 C/cm ). The spectra are shown in Fig. 9(a)–(c). The SiO
matrix consist of a Si–O complex network where the doped Ce is
2
2
3
2
:Ce
2
attached to oxygen atoms. Fig. 9(a) shows the high resolution
peaks of the Si 2p and Fig. 9(b) for O 1s before and after electron
beam exposure of the as prepared and annealed samples. The
TM
peaks were deconvoluted using the MultiPak
Version 9 [30]
software. In the as-prepared sample the Si 2p peak before electron
beam exposure deconvoluted into three peaks with maxima at
1
1
03.7, 104.9 and 100.8 eV in as-prepared sample. The peak at
03.7 eV which can be ascribed to the Si–O bonding in pure quartz
silica and 104.9 eV peak corresponds to chemisorbed species
formed on the SiO surface. SiO is more sensitive to the
2
2
Fig. 7. Normalized CL and APPH spectra of SiO
2
:Ce2m% phosphor at 1 ꢂ 10ꢃ6 Torr O
2
Fig. 8. AES spectra of SiO
2
:Ce phosphor before and after degradation at
ꢃ6
atmosphere.
1 ꢂ 10 Torr O
2
atmosphere.

2
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I.M. Nagpure et al. / Materials Research Bulletin 46 (2011) 2359–2366
2
Fig. 9. (a)–(c) XPS spectrum from SiO :Ce phosphor before and after electron exposure.
environmental hydroxide/oxygen. They readily adsorb on the
surface and gives rise to the unknown chemical species [31] in the
as-prepared and the annealed samples. The oxygen O 1s peak
before electron beam exposure (Fig. 9(b)) is deconvoluted into
three peaks at 534.1, 532.7 and 530.2 eV in as prepared sample.
The peak at 530.2 eV is arising from oxygen sites of the chemically
bonded Ce–O system, the 532.7 eV peak is possibly arising from the
while the peak at 532.7 eV is arising from the oxygen in the SiO
2
lattice. The 534.1 eV peak arising from the chemisorbed species
that forms an oxygen/hydroxide layer on the surface during
atmospherically exposure. The O 1s peak at 531.5 arise from
oxygen deficient species such as the formation of SiO
x (x < 2)
corresponds to the 102.3 eV Si 2p peak. The peak of O 1s at 531.5 eV
confirms that the SiOx (x < 2) layer formed on the surface as a result
of the electron beam stimulated surface chemical reactions.
It is concluded that after electron beam exposure the Si and O
2
oxygen present in the SiO lattice as Si–O bonding from quartz
structure. The 534.1 BE arising from the chemisorbed species that
forms oxygen/hydroxide. The Si 2p peak of the annealed sample of
the Si–O bonding energy slightly shifted from 103.7 to 103.3 eV
and in O 1s peak shifted from 532.7 to 532.5 eV due to stable quartz
silica obtained after annealing. The Si 2p peak at 104.9 eV in as-
prepared sample and 104.8 eV in annealed sample due to the
chemisorbed species. The peak at 100.8 eV in as-prepared and
2
atom is present in the SiO matrix and a SiOx (x < 2) layer formed on
the surface. The presence of binding energy peak at 104.9 eV of Si
2p and 534.1 eV of O 1s in as-prepared and annealed sample before
and after electron exposure suggests the formation of chemisorbed
species having no relation with the electron exposure and
degradation. The shift in the BE peak position of Si 2p and O 1s
in annealed sample compared to the as-prepared sample before
and after electron beam exposure suggest the improved crystal-
linity in annealed sample silica, which responsible to decrease the
rate degradation compared to as-prepared sample. The deconvo-
lution XPS results from Si 2p and O 1s before and after electron
beam exposure therefore confirms the presence of an oxygen
1
00.9 eV in annealed sample corresponds to the Si–C bonding,
which indicated the presence of C is still remaining in the SiO
2
surface during synthesis and after annealing.
2
After an electron dose of 140–360 C/cm , the Si 2p peak as
shown in Fig. 9(a) is deconvoluted into three peaks positioned at
1
02.3, 103.7 and 104.9 eV. The first peak at 102.3 eV of the Si 2p
spectra arise from oxygen deficient species such as the formation
of SiOx (x < 2). The SiOx (x < 2) layer formed on the surface as a result
of the electron beam stimulated surface chemical reactions. The
deficient SiO
Oxidation state and chemical bonding in SiO
and after degradation is summarized in Table 1.
x
(x < 2) layer on the surface of the SiO
2
phosphor.
2
:Ce sample before
1
03.7 binding energy peak in the Si 2p spectrum arises from Si–O
3
+
4+
bonding sites of SiO and 104.9 peak due to chemisorbed species.
2
3.5. Oxidation state of Ce and Ce (XPS)
The oxygen O 1s peak after electron beam exposure (Fig. 9(b)) is
deconvoluted into four peaks at 534.1, 532.7, 531.5 and 530.1 eV.
The peak 530.1 eV corresponds to the Ce–O bonding in the matrix,
3
+
4+
XPS was further used to distinguish between the Ce and Ce
redox states as shown in Fig. 9(c). The most intense core level XPS

I.M. Nagpure et al. / Materials Research Bulletin 46 (2011) 2359–2366
2365
Fig. 9. (Continued).
1
4
5
peaks of Ce (3d) at high binding energies between 870 and 930 eV
were used for the confirmation. The spectra are roughly separated in
A (887 eV), A (905.2 eV), A (908 eV) and a satellite peak as
6
4+
A (917 eV) confirm the presence of the Ce ions in the matrix
[32]. For the annealed SiO :Ce sample the large peak marked as
a 3d5/2 (M
5
) and a 3d3/2 (M
4
4+
) multiplet, due to the large 3d spin-orbit
2
3+
1
2
0
splitting. The Ce and Ce spectra are quite distinct, but they will
overlap when the two species are present together. The changes in
the peak area and peak position were used to distinguish between
B (885.2 eV), B (900.2 eV) and small peak marked as B (882 eV),
3
3+
B (906 eV) confirm the presence of the Ce ions in the matrix [33].
4
The presence of a satellite peak in the annealed sample at B (917 eV)
confirms that Ce ions are still present after annealing in the
charcoal environment. It is concluded that from the trend in the XPS
spectra the annealing process in the charcoal environment was
useful for the partial conversion of Ce ions to Ce ions.
3+
4+
4+
the Ce and Ce ions. The XPS peak positions of the as-prepared
phosphor and annealed sample are shown as solid lines and dotted
lines, respectively. Intheas-preparedSiO
marked as A (883 eV), A (901 eV) and small peaks marked as
2
:Ce samplethelargepeaks
0
3
4+
3+

2
366
I.M. Nagpure et al. / Materials Research Bulletin 46 (2011) 2359–2366
Table 1
Summary of chemical bonding and XPS peak position before and after electron exposure.
Oxidation state
Before electron exp.
BE (eV)
After electron exp
BE (eV)
Chem. bond.
Chem. bond
Si 2p
As prep.
100.8
Si–C
–
–
1
–
1
03.7
Si–0
103.7
102.3
104.9
–
Si–O
–
Si–O (SiO2 (2 < x))
04.9
Chemisorbed sp.
Chemisorbed sp.
Ann.
100.9
Si–C
–
1
–
1
03.3
Si–O
103.3
102.1
104.8
Si–O
–
Si–O (SiO2 (2 < x))
04.8
Chemisorbed sp.
Chemisorbed sp.
O ls
As prep.
530.2
Ce–O
530.1
532.7
531.5
534.1
–
Ce–O
Si–O
5
–
5
32.7
Si–O
–
Si–O (SiO2 (2 < x))
34.1
Chemisorbed sp.
Chemisorbed sp.
Ann.
530.1
Ce–O
–
5
–
5
32.5
Si–O
532.5
531.2
534.0
Si–O
–
Si–O(SiO2 (2 < x)
)
34.2
Chemisorbed sp.
Chemisorbed sp.
Ce 3d
As prep.
883, 887,
Ce–O
Ce–O
9
01, 905.2
08, 917
9
Ann.
882, 885.2,
9
00.2, 906,
17.2
9
[
[
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. Conclusion
SiO
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combustion method and latter annealed at 10008C for 1 h. It is
concluded that the as-prepared and annealed powder showed
dense partly crystalline spherical morphology with 3–8 nm sized
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[
[
[
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[
[
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2
2
spin–orbit splitting of F5/2 and F7/2 state. Annealed sample shows
better CL degradation than as-prepared sample due to improve in
the morphology of the sample and formation of SiO2 (x < 2) layer on
the surface major cause of degradation in CL intensity were
confirmed by XPS analysis.
[
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Authors are thankful to Prof. J. R. Botha for providing the PL
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thankful to the University of the Free State (UFS) and South African
National Research Foundation (NRF) for the financial support.
Centre for Microscopy at UFS for SEM/TEM/EDS are also gratefully
acknowledged.
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