Effects of high-temperature annealing on structural and optical properties of highly ordered porous alumina membranes

Article abstract of DOI:10.1063/1.1815072

We report on the structural and optical properties of highly ordered porous alumina membranes (PAMs) under different annealing temperatures up to 1100 °C. Clear structural transition from amorphous to γ- and then to α-Al₂O₃ has been

Full text of DOI:10.1063/1.1815072

Effects of high-temperature annealing on structural and optical properties
of highly ordered porous alumina membranes
W. L. Xu, M. J. Zheng, S. Wu, and W. Z. Shen
Citation: Appl. Phys. Lett. 85, 4364 (2004); doi: 10.1063/1.1815072
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 19
8 NOVEMBER 2004
Effects of high-temperature annealing on structural and optical properties
of highly ordered porous alumina membranes
W. L. Xu, M. J. Zheng, S. Wu, and W. Z. Shena)
Laboratory of Condensed Matter Spectroscopy and Opto-Electronic Physics, Department of Physics,
Shanghai Jiao Tong University, 1954 Hua Shan Road, Shanghai 200030, People’s Republic of China
(Received 30 January 2004; accepted 8 September 2004)
We report on the structural and optical properties of highly ordered porous alumina membranes
(PAMs) under different annealing temperatures up to 1100 °C. Clear structural transition from
amorphous to ␥- and then to ␣-Al2O3 has been demonstrated in PAMs through x-ray diffraction and
micro-Raman scattering measurements. The crystallization of the PAMs under an annealing
temperature above 800 °C results in the appearance of two absorption structures in the transmission
spectra ranging from 1500 to 1700 cm−1 due to the formation of stable carboxylic impurities. We
have presented convincing evidence that the blue photoluminescence band in PAMs originates from
the coactions of the singly ionized oxygen vacancies (F+ centers) and the luminescent centers
transformed from oxalic impurities. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1815072]
The alumina membranes prepared through a typical elec-
trochemical procedure1 possess highly ordered nanopores
with controllable and homogeneous dimensions arranged in a
close-packed hexagonal pattern. The demonstrated
dimensions2 of interpore spacing ranging from
50 to 400 nm and pore size from 25 to 300 nm are suitable
for both quantum electronic effects and photonic crystals. In
recent years, owing to their unique regular nanoscale struc-
tures, porous alumina membranes (PAMs) have attracted
great interest and have been widely used as templates to
fabricate nanostructures with nanorod or nanowire arrays in
various materials.2–7 It should be noted that many fabrication
procedures, such as molecular-beam epitaxy and chemical
vapor deposition, are carried out under high temperatures.
Therefore, a detailed knowledge of self-organized PAMs
themselves annealed at different temperatures is of techno-
logical importance from the viewpoints of both the PAMs
and the synthesized nanomaterials.
The recent work on the self-properties of PAMs has been
focused on the photoluminescence (PL) properties of PAMs
annealed at a temperature below 800 °C. However, the ori-
gin of the observed blue emission band is still not clear. Du
et al.8 and Li et al.9 have attributed the blue emission band to
the singly ionized oxygen vacancies (F+ centers) and the
blueshift of the PL peak energy with the increase of anneal-
ing temperature to the gradual release of internal stress by
the volume expansion of aluminum during oxide formation
in PAMs. Nevertheless, based on the proposal of Yamamoto
et al.10 that the oxalic impurities can be transformed into
luminescent centers with a blue PL band around 470 nm,
Gao et al.11 suggested that the transformed oxalic impurities
(rather than the F+ centers) could be responsible for the blue
PL emission in PAMs. Though the PL spectra can be sample
dependent due to the amorphous nature of the PAMs, the two
different blue emission bands8,9,11 have been observed in the
PAMs prepared under the same synthetic conditions of 40 V
anodizing voltage and 0.3 M oxalic acid ͑C2H2O4͒.
It is well known that the self-properties of PAMs are
directly associated with their structure, which can be
changed by further increasing the annealing temperature. In
this letter, we report on the structural and optical properties
of PAMs (prepared the same as in Refs. 8, 9, and 11) an-
nealed up to a high temperature of 1100 °C. We have dem-
onstrated a clear structural transition from the amorphous to
␥- and then to ␣-Al2O3 phases in PAMs, which paves the
way for identifying the blue PL band to originate from the
coactions of the F+ centers and the luminescent centers trans-
formed from oxalic impurities.
The PAMs were fabricated through a typical two-step
anodizing electrochemical procedure1 with high-purity
(99.999%) aluminum foil as the anode in a 0.3 M C2H2O4
electrolyte. The first anodization lasted for 6 h under the
constant anodizing voltage of 40 V and electrolyte tempera-
ture of 16 °C. The specimens were then immersed in a mix-
ture of 6.0 wt % H3PO4 and 1.8 wt % H2CrO4 at 60 °C for
8 h to remove the alumina layers. After the second 8 h an-
odization under the same conditions as the first one, the
highly ordered porous alumina was formed. The rest of the
aluminum can be further removed in a saturated CuSO4 so-
lution. Figure 1 shows the atomic force microscope (AFM)
surface morphology of the as-prepared PAMs. It is clear that
the PAMs possess highly ordered nanopore arrays arranged
in a close-packed hexagonal pattern with the nanopore size
and interpore spacing of ϳ50 nm and 100 nm, respectively.
Figure 2 presents the x-ray diffraction (XRD) (by Bruker
D8 ADVANCE system) and micro-Raman [through Jobin
Yvon Lab Ram-INFINITY with a 514.5 nm Ar+ laser (spot
diameter ϳ1 ␮m) under backscattering geometry of
¯
Z͑X,−͒Z] spectra of the PAMs annealed at different tempera-
tures for 5 h. It should be noted that we have divided the
original set of samples into different portions, and each piece
of sample was heated at different temperatures. The XRD
results indicate that the as-prepared and low-temperature an-
nealed PAMs are amorphous and the PAMs are transformed
into ␥-Al2O3 phase under the annealing temperatures of
800–950 °C, since the two diffraction peaks at 2␪=45.92°
and 67.06° are well coincident with those of ␥-Al2O3.12 With
a)Author to whom correspondence should be addressed; electronic mail:
wzshen@sjtu.edu.cn
0003-6951/2004/85(19)/4364/3/$22.00
4364
© 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 85, No. 19, 8 November 2004
Xu et al.
4365
FIG. 3. Room-temperature infrared transmission spectra of the as-prepared
PAM and the PAMs annealed at different temperatures for 5 h. The curves
have been shifted for clarity.
FIG. 1. A top-view AFM photograph of the as-prepared PAM with highly
ordered nanopore arrays arranged in a close-packed hexagonal pattern.
nal oxalic impurities are decomposed into carboxylic groups,
with the remainder of oxalic impurities heated to produce
CO2. When the PAMs begin to transform into Al2O3 crystal-
line above 800 °C (Fig. 2), the periodic and ordered arrange-
ment of the atoms Al and O (or oxygen vacancies) contrib-
utes to the formation of stable carboxylic impurities, such as
aluminum–carboxylate complex in PAMs.10
Further evidence of the above argument can be observed
in the intensity variation of the sharp absorption structure at
2339 cm−1 in Fig. 3, which is associated with the trapped
CO2 in the lattice.17 The structure reaches its maximum after
being annealed at 700 °C, and then decreases with a further
increase of the annealing temperature. The 700 °C annealed
PAM is still in the amorphous phase, but with the largest
degree of the oxalic impurity decomposition into trapped
CO2 in the lattice. With the further increase of the annealing
temperature, the decrease in the intensity of the CO2 absorp-
tion is related to the formation of stable carboxylic impurities
and its gasification at such high temperatures.
the further increase of the annealing temperature to 1100 °C,
we can observe up to nine ␣-Al2O3 phase related diffraction
peaks.12 Accordingly, the micro-Raman spectrum also dis-
plays four ␣-Al2O3 Raman structures13 for the 1100 °C an-
nealed PAM. These results clearly demonstrate that the struc-
tural transition from the amorphous to ␥- and then to
␣-phases can be realized in PAMs through different high-
temperature annealing.
The infrared transmission/reflection measurements have
the ability to clearly reveal the transitions of lattice vibra-
tions and localized energies in defects, impurities, and etc.
Figure 3 shows the room-temperature infrared transmission
spectra of the PAMs annealed at different temperatures,
which were performed on a Nicolet Nexus 870 Fourier trans-
form infrared spectrometer. The broad absorption band of
2800–3700 cm−1 can be attributed to stretching vibrations of
H2O molecular and/or uOH group, which is not related
with the annealing temperature due to the air exposure of the
postannealing PAMs. Of particular interest is the appearance
of two absorption structures ranging from 1500 to 1700 cm−1
in the PAMs annealed above 800 °C, which are the charac-
teristics of CvO stretching vibrations in carboxylate ions
and carboxylic groups.14 It is well known that there are ox-
alic impurities in PAMs10,15,16 due to the competition be-
tween the water-splitting reaction ͑3/2H2O→3H+
+3/2O2−͒ and dissociation of acids ͑HC2O−4 →C2O24−+H+͒
to form conjugate base anions, which can replace O2− in the
oxide as substitution or contamination impurities within
some depth. During the annealing process, part of the origi-
Figure 4 presents the room-temperature PL emission
spectra of these PAMs annealed at different temperatures,
which were recorded by a Jobin Yvon ultraviolet–visible
monochromator under a 325 nm He–Cd laser excitation. It is
clear that these PAMs have a broad blue emission band in the
range of 350–550 nm. The as-prepared alumina shows the
PL peak position at 431 nm. With the increase of the anneal-
FIG. 4. Room-temperature PL spectra of the as-prepared PAM and the
PAMs annealed at different temperatures for 5 h under a 325 nm laser ex-
citation. The experimental spectrum of the as-prepared PAM has been de-
convoluted by two Gaussian functions (dotted curves) for the contribution of
the two emission bands, with a sum of these two shown as solid squares.
The curves have been shifted for clarity.
FIG. 2. (a) XRD and (b) micro-Raman spectra of the PAMs annealed at
different temperatures for 5 h. The curves have been shifted for clarity.
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4366
Appl. Phys. Lett., Vol. 85, No. 19, 8 November 2004
Xu et al.
ing temperature, the luminescence peak position blueshifts
below 700 °C. This is in agreement with the observations by
Du et al.8 and Li et al.,9 who have attributed the blue emis-
sion band to the singly ionized oxygen vacancies (F+ centers)
and the blueshift to the gradual release of internal stress by
the volume expansion of aluminum during oxide formation
in PAMs. However, a clear redshift of the blue emission band
can be observed with a further increase of the annealing tem-
perature above 800 °C, which cannot be explained definitely
by the above mechanism.
the further decomposition of carboxylic impurities. Conse-
quently, the luminescence due to the transformed carboxylic
impurities (around 470 nm) dominates the PL band. The
domination will be much clearer at higher annealing tem-
peratures, leading to the observation of redshift of the blue
emission band with the increase of annealing temperatures.
Of course, due to the high annealing temperature, the PL
intensity of the transformed carboxylic impurities is much
weaker than that of the F+ centers in the PAMs annealed
ഛ700 °C. The PL results in Fig. 4 clearly reveal the evolu-
tion of the luminescence characteristics with the PAM struc-
tures, demonstrating the effects of the high-temperature an-
nealing.
In summary, PAMs with highly ordered pore arrays were
fabricated through a typical two-step anodizing electro-
chemical procedure. We have demonstrated the clear struc-
tural transition in PAMs from the amorphous (for as-
prepared and low-temperature annealed ones) to ␥- (under
the annealing temperatures of 800–950 °C) and then to ␣-
(with an annealing temperature of 1100 °C) Al2O3. The
structure has played an important role in the optical proper-
ties of PAMs. The formation of stable carboxylic impurities
has been observed in the crystallized PAMs. The proposal
that the blue emission band in PAMs originates from the
coactions of the singly ionized oxygen vacancies (F+ centers)
and the luminescent centers transformed from oxalic impuri-
ties can well explain the observed annealing temperature de-
pendent luminescence peak position and intensity.
On the other hand, Yamamoto et al.10 have proposed, as
early as in 1981, that the oxalic impurities incorporated in
the PAMs can be transformed into luminescent centers by a
high electric-field setup inside the pores or by the Joule heat-
ing, showing a blue PL band around 470 nm. The recent
studies of Gao et al.11 further suggest that the transformed
oxalic impurities (rather than the F+ centers) could be re-
sponsible for the blue PL emission in PAMs through X-band
electron paramagnetic resonance (EPR) and infrared trans-
mission measurements. According to that origin, the lumi-
nescence intensity would be enhanced with the increase of
the annealing temperature. Nevertheless, our PL results in
Fig. 4 do not support this conclusion at high annealing tem-
peratures ͑500–1100 °C͒.
Based on our self-consistent results in Figs. 2–4, we sug-
gest that the observed blue PL band could be attributed to the
coactions of both the F+ centers and the luminescent centers
transformed from oxalic impurities. From the overall anneal-
ing temperature results in Fig. 4, we can actually observe
two PL bands, centered at ϳ400 nm and ϳ470 nm, corre-
sponding to the luminescence structures of F+ centers and
transformed carboxylic impurities, respectively. In fact, these
two emission bands can be clearly deconvoluted by two
Gaussian functions in PL spectrum of the as-prepared PAM
(dotted curves in Fig. 4) with the integrated emission contri-
bution of ϳ50% to 50%. With the annealing temperature
up to 500 °C, it has been confirmed that the formed oxygen
vacancies (F+ centers) due to the oxidation of the remaining
aluminum in PAMs under an oxygen-poor atmosphere are
more than the annihilation of F+ centers caused by the com-
bination with oxygen diffusing into the PAMs,9 while the
oxalic impurities in alumina membranes decrease because of
the decomposition. In this case, the F+ centers are predomi-
nant in the PL spectra, resulting in the increase of the PL
intensity around 400 nm and a blueshift of the PL peak po-
sition.
This work was supported by the Natural Science Foun-
dation of China under Contract No. 10125416, the Shanghai
Major Project of 03DJ14003, and the Shanghai Nanotechnol-
ogy Fundamental Research Project of 0352nm013.
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