Bluish-white emission from radical carbonyl impurities in amorphous Al 2O3 prepared via the Pechini-type sol-gel process

Article abstract of DOI:10.1021/ic700652v

Many efforts have been devoted to exploring novel luminescent materials that do not contain expensive or toxic elements, or do not need mercury vapor plasma as the excitation source. In this paper, amorphous Al₂O ₃ powder samples wer

Full text of DOI:10.1021/ic700652v

Inorg. Chem. 2008, 47, 49−55
Bluish-White Emission from Radical Carbonyl Impurities in Amorphous
Al O Prepared via the Pechini-Type Sol Gel Process
−
2
3
Cuikun Lin, Min Yu, Ziyong Cheng, Cuimiao Zhang, Qingguo Meng, and Jun Lin*
State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
Received April 5, 2007
Many efforts have been devoted to exploring novel luminescent materials that do not contain expensive or toxic
elements, or do not need mercury vapor plasma as the excitation source. In this paper, amorphous Al powder
samples were prepared via the Pechini-type sol gel process. The resulting samples were characterized by X-ray
diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy
FESEM), photoluminescence (PL) excitation and emission spectra, kinetic decay, and electron paramagnetic
resonance (EPR). The obtained amorphous Al powder samples annealed at 500 and 600 C exhibit bright
bluish-white emission centered at 430 and 407 nm, respectively. The luminescent mechanisms of the amorphous
Al powder samples can be ascribed to the carbon-related impurities such as radical carbonyl species. The
2 3
O
−
(
2
O
3
°
2 3
O
calculated band structure of the defective amorphous Al
the proposed luminescent mechanism.
2 3
O agrees well with the results of spectral analysis and
1
1
I. Introduction
cially multicomponent) metal oxide materials. This method
has been used for the synthesis of electric and magnetic
materials rather extensively, including ferroelectric and
capacitor materials, superconducting materials, and photo-
Luminescent materials play an important role in modern
society for information displays and lighting.1,2 Most of the
commercial lamp phosphors require excitation by short
ultraviolet (UV) light for operation, such as mercury vapor
plasma in fluorescent lighting products. In addition, the
activators in fluorescent lamp or cathodoluminescent display
1
1
catalytic materials. The improved material properties for
the Pechini-type sol-gel process with respect to other
methods (such as the solid-state reaction method and
amorphous citrate method) have been demonstrated for the
synthesis of superconductors and photocatalysts. In the past
five years, we have extended the application of the Pechini-
type sol-gel process to the systematic synthesis of various
kinds of optical materials, including luminescent powders
and thin films, core-shell structured phosphor, and pigment
materials.
Alumina is an important material for the ceramic industry.
Amorphous Al has found extensive application in
catalysis, coatings, microelectronics, and thin film devices.
3
1
-3
phosphors are often expensive or toxic elements. There-
fore, much effort has been devoted to exploring novel
luminescent materials which do not contain expensive or
11c
4
-9
toxic elements or do not need mercury vapor plasma as
the excitation source.2,10
The Pechini-type sol-gel process (also known as the
polymerizable-complex technique), is well-known and is
extensively used for the synthesis of homogeneous (espe-
1
2
2
O
3
13-18
*
To whom correspondence should be addressed. E-mail: jlin@ciac.jl.cn.
(
(
(
(
(
(
(
1) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer-
Verlag: Berlin, 1994.
(8) (a) Carlos, L. D.; S a´ Ferreira, R. A.; Pereira, R. N.; Assunc a˜ o, M.; de
Zea Bermudez, V. J. Phys. Chem. B 2004, 108, 14924. (b) Fu, L.; Sa´
Ferreira, R. A.; Silva, N. J. O.; Carlos, L. D.; de Zea Bermudez, V.;
Rocha, J. Chem. Mater. 2004, 16, 1507. (c) Carlos, L. D.; de Zea
Bermudez, V.; S a´ Ferreira, R. A.; Marques, L.; Assun c¸ a˜ o, M. Chem.
Mater. 1999, 11, 581.
2) Feldmann, C.; J u¨ stel, T.; Ronda, C. R.; Schmidt, P. J. AdV. Funct.
Mater. 2003, 13, 511.
3) Ropp, R. C. Luminescence and the Solid State; Elsevier: Amsterdam,
Netherlands, 1991; Vol. 12, p 283.
4) Green, W. H.; Le, K. P.; Grey, J.; Au, T. T.; Sailor, M. J. Science
1
997, 276, 1826.
5) Hayakawa, T.; Hiramitsu, A.; Nogami, M. Appl. Phys. Lett. 2003,
2, 2975.
6) (a) Brankova, T.; Bekiari, V.; Lianos, P. Chem. Mater. 2003, 15, 1855.
b) Bekiari, V.; Lianos, P. Langmuir 1998, 14, 3459.
7) Yold, B. E. J. Non-Cryst. Solids 1992, 147, 614.
(9) Lin, J.; Baerner, K. Mater. Lett. 2000, 46, 86.
(10) Wegh, R. T.; Donker, H.; Oskam, K. D.; Meijerink, A. Science 1999,
283, 663.
(11) (a) Pechini, M. P. U.S. Patent 3,330,697, 1967. (b) Kakihana, M.;
Yoshimura, M. Bull. Chem. Soc. Jpn. 1999, 72, 1427. (c) Kakihana,
M.; Domen, K. MRS Bull. 2000, 25 (9), 27.
8
(
1
0.1021/ic700652v CCC: $40.75
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 1, 2008 49
Published on Web 12/11/2007

Lin et al.
The photoluminescence properties in porous alumina mem-
brane have been paid much attention in the past decade.
II. Experimental Section
Preparation. Al powder samples were prepared via the
Pechini-type sol-gel method (SG).
NO ‚9H O (A. R., Beijing Beihua Chemicals Co., Ltd.) was
dissolved in an aqueous solution under vigorous stirring, and then
mixed with a 40 mL water-glycerol (v:v ) 1:7) solution containing
citric acid (A. R., Beijing Beihua Chemicals Co., Ltd.) as the
chelating agent for the metal ions. The molar ratio of the metal
ions to citric acid was 1:2. A 4.00 g sample of poly(ethylene glycol)
19-21
2 3
O
Zhang and coauthors reported on blue luminescence from
alumina nanoparticles suspended in toluene solution through
11,12
Typically, 1.88 g of Al-
(
3
)
3
2
21a
ultrasonic treatment of a porous anodic alumina membrane.
Mohanty etc. dispersed Eu nanoparticles in amorphous
Al
to enhance the photoluminescence.2 However, so far
2 3
O
1c
2 3
O
little attention has been paid to the emission properties in
this material prepared by the Pechini-type sol-gel process.
(PEG, 20000; A. R., Beijing Beihua Chemicals Co., Ltd.) was added
It has been reported that sol-gel derived SiO
2
-based
as a cross-linking agent. The solution was stirred for 1 h to form
a sol, and then the sample was immediately dried at 150 °C for 6
h. The obtained precursor material was preheated at 400 °C for 3
h, was fully ground, and was sintered at the desired temperature
(500-900 °C) in air for 3 h to produce the final sample. As a
comparison with the samples prepared via the Pechini-type sol-
materials, including SiO gels and organic/inorganic hybrid
2
silicones, show strong luminescence from the blue to red
spectral region. These materials are potentially used as a kind
of environmentally friendly luminescent material without
8
expensive or toxic metal elements as activators. Recently,
gel process, 3.00 g of Al(NO
°C in air for 3 h to obtain Al
sample. We denote the final samples as follows. AOSGx series (x
500, 600, 700, 800, and 900) denotes the Al samples prepared
3 3 2
) ‚9H O was annealed directly at 500
we found that Pechini-type sol-gel derived BPO
4
doped with
Ba emits an efficient bluish-white light. Nanocrystalline
tetragonal ZrO powders prepared by the Pechini-type sol-
gel method also show a strong bluish-white emission. The
luminescence mechanism of nanocrystalline ZrO can be
ascribed to the interstitial carbon defects (C ) in tetragonal
2+
12b
2 3
O (solid-state process, SS) powder
2
)
2 3
O
via the Pechini-type sol-gel process, where x is the annealing
temperature (°C). AOSS500 stands for the sample obtained by the
solid-state reaction, and here 500 denotes the annealing temperature
(500 °C).
2
i
1
2e
2
ZrO .
Characterization. The X-ray diffraction (XRD) of powder
samples was examined on a Rigaku-Dmax 2500 diffractometer
using Cu KR radiation (λ ) 0.15405 nm). Fourier transform infrared
Considering the above situations, it would be of great
interest and importance to check if such strong emission can
be observed in other oxide systems prepared by a similar
process. Furthermore, it would be possible to find some
useful luminescent materials via such an investigation.
(FT-IR) spectra were measured with a Perkin-Elmer 580B infrared
spectrophotometer with the KBr pellet technique. The morphology
and composition of the samples were inspected using a field
emission scanning electron microscope (FESEM; XL30, Philips)
equipped with an energy-dispersive X-ray spectrometer (EDS; JEOL
JXA-840). The excitation and emission spectra were taken on a
Hitachi F-4500 spectrofluorimeter equipped with a 150 W xenon
lamp as the excitation source. Luminescence lifetimes were
measured with a Lecroy Wave Runner 6100 digital oscilloscope
2 3
Accordingly, in this paper we prepared amorphous Al O
samples via the Pechini-type sol-gel process. It is interesting
to find that this material also shows an intense bluish-white
emission (λmax ) 407-430 nm) under a wide range of UV
light excitation (230-420 nm). Possible mechanisms have
been proposed to explain the observed luminescent phenom-
ena.
(
1 GHz) using 280 nm lasers (pulse width ) 4 ns) as the excitation
source (Continuum Sunlite OPO). Electron paramagnetic resonance
EPR) spectra were taken on the JESFE3AX electronic spin
(
(
12) (a) Yu, M.; Lin, J.; Fang, J. Chem. Mater. 2005, 17, 1783. (b) Lin, C.
K.; Luo, Y.; You, H.; Quan, Z.; Zhang, J.; Fang, J.; Lin, J. Chem.
Mater. 2006, 18, 458. (c) Yu, M.; Lin, J.; Wang, Z.; Fu, J.; Wang, S.;
Zhang, H. J.; Han, Y. C. Chem. Mater. 2002, 14, 2224. (d) Wang, H.;
Lin, C. K.; Liu, X. M.; Lin, J.; Yu, M. Appl. Phys. Lett. 2005, 87,
resonance spectrophotometer. All the measurements were performed
at room temperature. The band structures of the defective amor-
phous Al O were calculated using the CASTEP code (version 3.0,
2 3
Accelrys) based on the density functional theory (DFT).
181907. (e) Lin, C. K.; Zhang, C. M.; Lin, J. J. Phys. Chem. C 2007,
1
11, 3300.
III. Results and Discussion
(
13) (a) Costina, I.; Franchy, R. Appl. Phys. Lett. 2001, 78, 4139. (b) Hou,
Y. J.; Wang, Y. Q.; He, F.; Mi, W. L.; Li, Z. H.; Wu, W.; Min, E. Z.
Appl. Catal., A 2004, 259, 35. (c) Myung, S. T.; Izumi, K.; Komaba,
S.; Sun, Y. K.; Yashiro, H.; Kumagai, N. Chem. Mater. 2005, 17,
2 3
XRD and FT-IR. The XRD patterns of Al O samples
prepared by the Pechini-type sol-gel process and solid-state
reaction are shown in Figure S1: (a) AOSS500, (b)
AOSG500, (c) AOSG600, (d) AOSG700, (e) AOSG800, and
3
695.
14) (a) Stoloff, N. S.; Liu, C. T.; Deevi, S. C. Intermetallics 2000, 8, 1313.
b) Chang, H.; Choi, Y.; Kong, K.; Ryu, B. H. Chem. Phys. Lett. 2004,
91, 293.
(
(
3
(f) AOSG900, respectively (Supporting Information). Only
(
(
15) Jimen e´ z-Gonzales, A.; Schmeisser, D. Surf. Sci. 1991, 250, 59.
16) Ealet, B.; Elyakhloufi, M. H.; Gillet, E.; Ricci, M. Thin Solid Films
amorphous materials were produced at lower temperatures
(500-600 °C), as shown in Figure S1 (a-c).22 For the
samples annealed at 800 and 900 °C, diffraction peaks at
37.6, 45.8, and 67.6° are present, which belong to (311),
1
994, 250, 92.
17) Gautier, M.; Duraud, J. P.; Van, L. P.; Guittet, M. J. Surf. Sci. 1991,
50, 71.
(
(
(
(
(
2
18) Bianconi, A.; Bachrach, R. Z.; Hagstrom, S. H.; Flodstr o¨ m, S. A. Phys.
ReV. B 1979, 19, 2837.
19) Du, Y.; Cai, W. L.; Mo, C. M.; Chen, J.; Zhang, L. D.; Zhu, X. G.
Appl. Phys. Lett. 1999, 74, 2951.
20) Huang, G. S.; Wu, X. L.; Mei, Y. F.; Shao, X. F.; Siu, G. G. J. Appl.
Phys. 2003, 93, 582.
21) (a) Zhang, W. J.; Wu, X. L.; Fan, J. Y.; Huang, G. S.; Qiu, T.; Chu,
P. K. J. Phys: Condens. Matter 2006, 18, 9937. (b) Chen, J. H.;
Huang, C. P.; Chao, C. G.; Chen, T. M. Appl. Phys. A: Mater. Sci.
Process. 2006, 84, 297. (c) Mohanty, P.; Ram, S. J. Mater. Chem.
(400), and (440) reflections of η-Al
2
O
3
(JCPDS Card No.
77-0396). No other phase was detected, indicating that phase
transformation from amorphous Al
take place around 800 °C.
2
O
3
2 3
to η-Al O begins to
(22) (a) Liu, M.; Li, H.; Xiao, L.; Yu, W.; Lu, Y.; Zhao, Z. J. Magn. Magn.
Mater. 2005, 294, 294. (b) Hung, P. K.; Vinh, L. T. J. Non-Cryst.
Solids 2006, 352, 5531.
2003, 13 (12), 3021-3025.
50 Inorganic Chemistry, Vol. 47, No. 1, 2008

2 3
Amorphous Al O Prepared Wia Pechini-Type Sol-Gel Process
Figure 1. FT-IR spectra of Al2O3 prepared by Pechini-type sol-gel process
SG) and solid-state reaction (SS). (a) AOSG500, (b) AOSG600, (c)
(
AOSG700, and (d) AOSS500.
2 3
The FT-IR spectra of Al O prepared by the Pechini-type
sol-gel process and solid-state reaction are shown in Figure
(a) AOSG500, (b) AOSG600, (c) AOSG700, and (d)
1
-
1
AOSS500, respectively. The band at 3457 cm is due to
the stretching mode of hydroxyl groups (from surface water
and adsorbed water), and the band at 1643 cm is due to
the bending mode of water molecules. The broad band at
-
1
-1
5,23
755 cm can be ascribed to the Al-O vibration of (AlO
4
).
-
1
The weak bands at 1435 and 1527 cm can be ascribed to
the stretching vibrations of carboxylate groups,1
2b,e,23
indicat-
ing that the carbon impurity has not been removed com-
pletely in the Pechini-type sol-gel derived samples due to
the fact that they have been annealed at relatively low
temperature (500 °C). The carbon impurity can also be
detected by energy-dispersive X-ray spectroscopy for the
AOSG500 (EDS; see next section).
Figure 2. (a) FESEM micrograph and (b) EDS of AOSG500 sample.
spectra cannot be detected (Figure 3, blue line). The
luminescence spectra of the AOSG800 and AOSG900
samples cannot be detected either and will not shown here.
For AOSG500, the sample shows a strong emission band
ranging from 350 to 600 nm, peaking from 407 to 450 nm
FESEM and EDS. Figure 2 shows the FESEM image
(
a) and the energy-dispersive X-ray spectrum (EDS) (b) of
the AOSG500 sample, respectively. From Figure 3a, we can
see that the Al (AOSG500) sample is porous, with pore
(
centered at 430 nm) (Figure 3b, black lines). The corre-
sponding excitation spectrum includes a broad band from
35 to 425 nm (Figure 3a, black line). The emission spectrum
2 3
O
sizes ranging from 50 to 150 nm in diameter. The EDS
examination confirms the presence of Al and O from the
sample, Si from the Si substrate, and Au from the coating
for SEM measurement. The detected carbon impurity (C,
also observed in the IR spectra) is from the sample prepared
by the Pechini-type sol-gel process, in which some organic
solvents and additives (glycerol and PEG) were employed.
Photoluminescence Properties. The AOSG500, AOSG600,
AOSG700, and AOSS500 samples exhibit brown, near-white,
white, and yellow color under sunlight, respectively. Under
UV-lighting excitation, the AOSS500 samples show no
luminescence, while the AOSG500, AOSG600, and AOSG700
samples exhibit bluish-white and weak blue luminescence,
respectively.
2
of AOSG600 sample exhibits luminescent properties similar
to those of AOSG500 except for the emission intensity. Its
emission spectrum also consists of a strong broad band
ranging from 350 to 600 nm with a maximum at 407 nm
(
4
Figure 3b, red line), whose integrated intensity is enhanced
times with respect to that of AOSG500. The AOSG700
sample also shows a similar emission with a maximum at
86 nm (Figure 3b, green line) except a decrease of the
3
intensity, indicating that the optimal annealing temperature
is 600 °C.
The luminescence decay curves for AOSG500, AOSG600,
and AOSG700 are shown in Figure 4, parts a, b, and c,
respectively. These decay curves can be fitted to a single-
Figure 3 shows the excitation (a) and emission (b) spectra
of AOSG500, AOSG600, AOSG700, and AOSS500 samples,
respectively. For AOSS500, the excitation and emission
exponential function as I ) I exp(-t/τ) (τ is lifetime), from
which the lifetimes are determined to be 6.93, 5.82, and 5.64
ns for AOSG500, AOSG600, and AOSG700, respectively.24
0
(
23) Dean, J. A. Lange’s Handbook of Chemistry (Chinese version);
(24) Lin, C. K.; Wang, H.; Kong D. Y.; Yu, M.; Liu, X. M.; Wang, Z. L.;
Scientific Publisher of China: Beijing, 2003.
Lin, J. Eur. J. Inorg. Chem. 2006, 18, 3667.
Inorganic Chemistry, Vol. 47, No. 1, 2008 51

Lin et al.
Figure 3. (a) Excitation and (b) emission spectra for AOSG500 (black
line), AOSG600 (red line), AOSG700 (green line), and AOSS500 (blue
line). The inset of (b) is the luminescent photograph of AOSG500 and
AOSG600 under the excitation of a 365 nm UV lamp.
Note that the lifetime deceases a little with the increase of
the annealing temperature.
Possible Luminescence Mechanism. Because Al3 itself
+
is not able to show luminescence, the observed luminescence
from Al O (SG) must be related to some impurities and/or
2 3
defects in the host lattice, which can be confirmed by the
short lifetimes: 6.93, 5.82, and 5.64 ns for AOSG500,
AOSG600, and AOSG700, typical values for the lumines-
Figure 4. Decay curves for the luminescence of Pechini-type sol-gel
derived Al2O3: (a) AOSG500, (b) AOSG600, and (c) AOSG700.
25,26
cence caused by defects.
The amorphous Al
2 3
O is built
5
-
without a clear or exact identification of emission centers.
For room-temperature-obtained organic/inorganic hybrid
up by (AlO
atoms, like the SiO
SiO (including SiO
4
)
tetrahedra with corner sharing of oxygen
4
tetrahedral framework in amorphous
glass,4 SiO
,5,7
4,9
silicones containing -NH
proposed a mechanism based on NH /NH (or NH /N )
2
(or -NH) groups, Carlos et al.
2
2
2
gels, organic/inorganic
+
-
+
-
6
,8
27
3 2
hybrid silicones, silica nanotubes, etc.), all of which are
well-known to show luminescence from the blue to the red
spectral region. Moreover, molecular sieves such as MCM-
donor-acceptor pairs to explain the blue emission from these
8
materials. Du et al. ascribed the blue luminescence of
alumina membranes with ordered pore arrays to the singly
41 and MCM-48, open-framework phosphates and germi-
+
19
ionized oxygen vacancies (F centers). Hayakawa et al.
ascribed the white light emission in Al -SiO glasses to
nates, and porous zinc gallophosphates show excellent and
2
8-31
2
O
3
2
tunable luminescence properties in the visible region.
5
the radical carbonyl terminations on the surface of pores.
Another defect type associated with carbon impurities was
proposed as being the origin of the luminescence of sol-
gel derived silica gels based on tetraethyl orthosilicate
In all of the above materials, the origins of the luminescence
have been ascribed to defects and/ or impurities in the hosts
(
(
25) Fujimaki, M.; Ohki, Y.; Nishikawa, H. J. Appl. Phys. 1997, 81, 1042.
26) Pifferi, A.; Taroni, P.; Torricelli, A.; Valentini, G.; Mutti, P.; Ghislotti,
G.; Zanghieri, L. Appl. Phys. Lett. 1997, 70, 348.
(
TEOS) and tetramethoxysilane (TMOS) incorporating a
4
27) Jang, J.; Yoon, H. AdV. Mater. 2004, 16, 799.
variety of carboxylic acids. Under adequate heat treatments
at least above 247 °C) a carbon impurity is created in the
O-Si-O- network forming -O-C-O- and/or -Si-
(
-
(
29) Lee, Y. C.; Liu, Y. L.; Shen, J. L.; Hsu, I. J.; Cheng, P. W.; Cheng,
C. F.; Ko, C.-H. J. Non-Cryst. Solids 2004, 341, 16.
C- bonds. The carbon-related defects are luminescent active
(
30) Feng, P. Chem. Commun. 2001, 1668.
only after a heat treatment because these xerogels are not
(31) Liao, Y. C.; Lin, C. H.; Wang, S. L. J. Am. Chem. Soc. 2005, 127,
8
9986.
luminescent prior to this procedure.
52 Inorganic Chemistry, Vol. 47, No. 1, 2008

Amorphous Al
2 3
O Prepared Wia Pechini-Type Sol-Gel Process
It is well-known that different preparation processes can
12e
produce different kinds of defects. For the Pechini-type
sol-gel derived Al , we assume the luminescence band
2 3
O
at 380-470 nm (which is the main contributor to the whole
emission, diminished when the sample is annealed above 700
°
C) must be related to some defects from the carbon
3+
impurities, since Al itself cannot show luminescence.
Considering the similar structure among Al , SiO , and
O
2 3
2
Al
2
O
3
-SiO
2
, the most probable luminescent center is the
•
radical carbonyl defects such as tAlsOsC dO on the pore
surface. We have the following evidence to support this
conclusion. First, the profile of the emission spectrum
(
emission range 350-600 nm; λmax ) 386-430 nm) and
the photoluminescence (PL) lifetimes (6.93, 5.82, and 5.64
ns) of our Pechini-type sol-gel derived Al are similar
to those of C-SiO gels (emission range 350-700 nm; λmax
Figure 5. EPR curves for (a) AOSG500, (b) AOSG600, (c) AOSG700,
d) AOSG800, (e) AOSG900, and (f) AOSS500.
2 3
O
(
2
varies between 405 and 550 nm depending on the preparation
conditions; PL lifetimes <10 ns). Second, carbon element
has been detected by FT-IR spectra (Figure 1) and EDS in
centers. These defect centers include an electron being
localized in a 2p orbital of the single-bonded carbon and
the single-bonded oxygen. This would give rise to photo-
luminescence through a strong electron-photon coupling.3
With raising the annealing temperature, the peak position
of the PL band exhibits a blue shift. This can be explained
as follows. With the increase of the temperature, the carbon
defects decrease. Since the energy levels of carbon defects
are located in the band gap of the alumina, the decrease of
the carbon defects will induce the spacing of the energy
4
2-34
our Pechini-type sol-gel derived Al
to check the effects of carbon impurity, we prepared Al
2 3
O (Figure 3b). In order
2 3
O
through the solid-state reaction (SS) process (without using
any organic solvents and additives, thus no carbon impurity
involved). This SS-derived Al O without carbon impurity
2 3
did not show any luminescence under the same excitation
condition (Figure 3b, blue line). From the above results, we
can safely conclude that the luminescence peak at 430 nm
1
9
levels of defects to become wide. It will lead the PL band,
which originated from electronic transitions between the
energy levels of the carbon defects, to move toward the short
wavelength (blue shift).
in the Pechini-type sol-gel derived Al
carbon impurities in the host lattice. In the preparation of
Al via the Pechini-type sol-gel process, glycerol and
2 3
O arises from the
2
O
3
PEG were employed as additives that contain large amounts
of carbon. In the subsequent annealing process, most of the
additives decomposed into H O and CO and escaped from
2 2
the system, but minor amounts of them may decompose to
create a carbon interstitial defect, which is assumed to be
the luminescent species in the lattice (like -O-Al-C-O-
Al-O-). The bond energies of the Al-O, C-O, and C-C
To prove the proposed assumption, EPR spectra for
AOSG500, AOSG600, AOSG700, AOSG800, AOSG900,
and AOSS500 samples were measured. Here we take the
Al O samples prepared by the solid-state reaction as the
2
3
standard sample. The EPR signal is very weak for AOSS500
(Figure 5d), while AOSG500 exhibits a strong and sharp
EPR band at g ) 2.02 (Figure 5a). It can be seen clearly
that with the increase of the annealing temperature, the EPR
signal deceases. Moreover, the sharp EPR band cannot be
seen in the EPR spectra for AOSG800 and AOSG900
samples, agreeing with the results of the emission spectra.
This indicates that there indeed exist paramagnetic defects
in Pechini-type sol-gel derived Al O . Since the EPR signal
2
3,32
bonds are ∼512, ∼358, and ∼368 kJ/mol, respectively.
Note that the bond energy of Al-O is much larger than those
of the C-O and C-C bonds and the bond energies lead to
a significant dependence of the order of bond cleavage on
the chemical and thermal environment.1 Therefore, the
initial cleavage took place in the C-O bonds, which modified
the intermolecular chemical environment and removed a
portion of the network oxygen along with carbon. For
example
5a
2
3
3
+
cannot be caused by Al (no single electron in these ions),
it must arise from some carbon-related defects, such as
5
,12e
radical carbonyl defects.
In an ideal amorphous Al
2 3
O , only Al-O bonds exist and
•
Al-O-C-R f Al O + R xO
(1)
2
3-x
it will not show any luminescence. The presence of structural
defects will introduce electronic states into the band gap,
resulting in luminescence of this material. As a summary,
•
•
tAlsOsCVsOsAlt f tAlsOsC + OsAl (2)
where x is the net oxygen vacancies created by the annealing
process, and the vertical arrow shows the bond cleavage site.
The above bond cleavages occur at a temperature where
healing of the fragmented bonds and normalization of the
coordination state are thermodynamically prohibited. Charge
unbalances created at these sites must be rectified by
localization of electrons and electron holes. The residual
fragmented bonds are apparently the precursors for various
(
32) (a) Yoldas, B. E. J. Mater. Res. 1990, 5, 1157. (b) Yold, B. E. J.
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(
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341, 16.
(
Inorganic Chemistry, Vol. 47, No. 1, 2008 53

Lin et al.
Figure 6. Energy band diagram showing the possible defects and emission
process in Pechini-type sol-gel derived Al2O3.
the whole emission process in our Pechini-type sol-gel
derived Al
In order to further confirm the proposed mechanism, the
electronic structures of the amorphous Al and defective
amorphous Al were calculated. The atomic structure of
amorphous alumina was built using molecular dynamics
MD) according to refs 35 and 36. It has been reported that
the coordination number of Al atom in amorphous Al O
3
2 3
O is schematically shown in Figure 6.
2
O
3
2 3
O
Figure 7. (a) Amorphous structure of Al2O3 (produced by simulated
quenching); (b) model structure of defective amorphous Al2O3.
(
2
22
to many materials, such as amorphous ice, amorphous silicon,
has a value between 4.1 and 4.8. Therefore, MD was carried
out for a microcanonical ensemble (NVE) of N ) 160 atoms
taken in a (2 × 2 × 1) supercell of orthorhombic primitive
unit cells (a ) 0.484 nm, b ) 0.833 nm, c ) 0.895 nm) of
4
3-48
amorphous indium phosphide, and so forth.
In this
calculation, the structural model of defective amorphous
alumina was established containing 161 atoms as a super-
lattice with constant volume and under periodic boundary
conditions (Figure 7b). The calculated band structures of
amorphous alumina and defective amorphous alumina are
given in parts a and b, respectively, of Figure 8. It can be
seen that the calculated band gap for amorphous alumina is
larger than that of defective amorphous alumina, confirming
the trend of the model proposed in the luminescent mech-
anisms. The energy levels of defective amorphous alumina
are located close to the Fermi level (determined by the
calculated partial density of state (DOS) plots, Figure 9) and
are ascribed to the energy levels of C(2p). From Figure 9, it
can be seen clearly that these levels (∼ -0.04 eV, ∼1.9 eV)
are between the Al(3s) levels in the conduction band (∼4.2
eV) and O(2p) levels in the valence band (∼ -5.8 eV). This
indicates that the wavelength of the white emission arises
from the carbon-induced defects, agreeing well with the
results of spectra analysis and the proposed luminescent
mechanism. It should be noted that the calculated band gaps
in semiconductors may be underestimated using this first-
principles energy band calculation, probably due to many
factors such as the action of the crystal field, the structure
model, the defect density, the limits of method itself, and so
3
7
2 3
κ-Al O . All the MD presented here was performed using
3
8
GULP code. The three-body and two-body potentials
suggested by Blonski and Garofalini were employed in the
calculations, which have previously been shown to provide
good descriptions of the silica-alumina interface and of silica
3
9,40
glasses.
at the temperature of 4000 K, which is much higher than
the melting temperature of Al . The liquid phase was
equilibrated during 4000 time steps, where a time step was
taken as ∆t ) 0.5 fs. Amorphous Al was then obtained
by simulated quenching of the liquid alumina at 300 K. The
First, the liquid phase of alumina was simulated
2 3
O
2 3
O
1
3
cooling rate of this simulated quenching was 5 × 10
K
-1
s , and the total simulating time for quenching was 74 ps.
The obtained atomic structure of amorphous alumina is
shown in Figure 7a.
The electronic structure of amorphous alumina and defec-
tive amorphous alumina using CASTEP code based on the
density functional theory (DFT), and exchange and correla-
tion have been treated by the generalized gradient ap-
proximation (GGA) within the scheme due to Perdew-
Burke-Ernzerhof (PBE).4
1,42
This method has been applied
(
35) (a) Chang, H.; Choi, Y.; Kong, K.; Ryu, B. H. Chem. Phys. Lett. 2004,
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Zheng, W. T.; Zhang, K.; Wen, Q. B.; Zhu, J. Q.; Meng, S. H.; He,
X. D.; Han, J. C. Carbon 2006, 44, 962. (c) Zheng, B.; Zheng, W. T.;
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S. H.; He, X. D.; Han, J. C. Carbon 2005, 43, 1976.
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Wagastsuma, K.; Oku, M.; Shoji, F. Phys. ReV. B 1999, 59, 11125.
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(
36) (a) San Miguel, M. A.; Sanz, J. F.; Alvarez, L. J.; Odriozola, J. A.
Phys. ReV. B 1998, 58, 2369. (b) Hemmati, M.; Wilson, M.; Madden,
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2717.
54 Inorganic Chemistry, Vol. 47, No. 1, 2008

2 3
Amorphous Al O Prepared Wia Pechini-Type Sol-Gel Process
Figure 8. Calculated band structures according to the model of Figure 7: (a) amorphous Al2O3 and (b) defective amorphous Al2O3.
IV. Conclusions
Amorphous Al
prepared via the Pechini-type sol-gel process. The Pechini-
type sol-gel derived amorphous Al annealed at 500 and
00 °C exhibits white emission centered at 430 nm and bright
2 3
O powder samples have been successfully
2
O
3
6
bluish-white emission centered at 407 nm, respectively. The
results of FT-IR and EDS characterizations indicates that
the carbon impurity has not been removed completely in the
Pechini-type sol-gel derived samples due to the relatively
low annealing temperatures. The EPR spectra confirm that
there exist paramagnetic defects in amorphous Al
the results it can be concluded that the luminescence of the
amorphous Al powders is from the carbon-related defects
such as radical carbonyl defects. The DFT calculation of the
defective amorphous Al further proves that the bluish-
white emission arises from the carbon-related impurities
(defects), agreeing well with the results of spectral analysis
and the proposed luminescent mechanisms.
2 3
O . From
2 3
O
Figure 9. Calculated DOS of defective amorphous Al2O3.
forth. 49 These factors are expected to give some contributions
to the change of the calculated energy level, but were not
included due to the difficulty of doing so correctly. Never-
theless, the results of the calculation can well explain the
luminescence phenomenon and the proposed mechanism.
Here, we can only say that the Pechini-type sol-gel derived
2 3
O
Acknowledgment. This project is financially supported
by the foundation of “Bairen Jihua” of the Chinese Academy
of Sciences, the National Natural Science Foundation of
China (50572103 and 20431030), and the MOST of China
2 3
amorphous Al O samples show the mentioned luminescence
properties and the emission intensities can be controlled by
the annealing temperature. It is expected that the lumines-
cence properties (emission color and intensity) can be further
tuned by controlling the concentration of PEG, adding some
(Nos. 2003CB314707 and 2007CB935502).
2 3
Supporting Information Available: XRD patterns of Al O
+
+
+
prepared by Pechini-type sol-gel process (SG) and solid-state
reaction (SS) of (a) AOSS500, (b) AOSG500, (c) AOSG600, (d)
AOSG700, (e) AOSG800, and (f) AOSG900. (Figure S1). This
material is available free of charge via the Internet at http://
pubs.acs.org.
nontoxic inorganic ions (Li , Na , K , etc.), and so on. This
will be done in other systems (such as BPO
in the near future.
4
) and reported
(49) Fu, Z. L.; Zhou, S. H.; Zhang, S. Y. J. Solid State Chem. 2005, 178,
230.
IC700652V
Inorganic Chemistry, Vol. 47, No. 1, 2008 55