In situ fabrication of alumina nanotube array and photoluminescence

Article abstract of DOI:10.1063/1.2337160

Aluminum foil was anodized in aged electrolyte under high voltage. The morphology observation shows that the alumina film has a three-layer structure from bottom to top and the middle layer shows large quantities of individual alumina nanotubes. Their formation mechanism is discussed in detail. Under ultraviolet excitation, the alumina film exhibits an emission centered at ～400 nm. Based on annealing behavior of the emission band and electron paramagnetic resonance result, the origin of the emission is considered to be due to optical transition in single ionized oxygen vacancy (F⁺ center) in the alumina. The experimental results can be expected to have favorable applications in optoelectronics and biotechnology.

Full text of DOI:10.1063/1.2337160

In situ fabrication of alumina nanotube array and photoluminescence
G. S. Huang, X. L. Wu, F. Kong, Y. C. Cheng, G. G. Siu, and Paul K. Chu
Citation: Applied Physics Letters 89, 073114 (2006); doi: 10.1063/1.2337160
View online: http://dx.doi.org/10.1063/1.2337160
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/7?ver=pdfcov
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APPLIED PHYSICS LETTERS 89, 073114 ͑2006͒
In situ fabrication of alumina nanotube array and photoluminescence
a͒
G. S. Huang, X. L. Wu, F. Kong, and Y. C. Cheng
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093,
People’s Republic of China and Department of Physics, Nanjing University, Nanjing 210093,
People’s Republic of China
G. G. Siu and Paul K. Chu
Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong,
People’s Republic of China
͑
Received 5 March 2006; accepted 28 June 2006; published online 17 August 2006͒
Aluminum foil was anodized in aged electrolyte under high voltage. The morphology observation
shows that the alumina film has a three-layer structure from bottom to top and the middle layer
shows large quantities of individual alumina nanotubes. Their formation mechanism is discussed in
detail. Under ultraviolet excitation, the alumina film exhibits an emission centered at ϳ400 nm.
Based on annealing behavior of the emission band and electron paramagnetic resonance result, the
origin of the emission is considered to be due to optical transition in single ionized oxygen vacancy
+
͑
F center͒ in the alumina. The experimental results can be expected to have favorable applications
in optoelectronics and biotechnology. © 2006 American Institute of Physics.
DOI: 10.1063/1.2337160͔
͓
In recent years, there has been a growth of attention in
synthesis of well-organized one-dimensional ͑1D͒ nanomate-
rials. Simple and convenient fabrication of 1D nanomaterials
using porous anodic alumina ͑PAA͒ membrane as a template
three groups, A, B, and C. They correspond to the preset
voltages of 60, 50, and 40 V, respectively, and a subscript
such as 190 in A190 is used to designate the initial current
2
density ͑mA/cm ͒. The surface morphologies were charac-
1
–5
has thus attracted more and more interest. Corresponding
terized using field emission scanning electron microscope
͑SEM, LEO 1530VP͒. PL and PL excitation ͑PLE͒ spectra
were taken on Fluoromax-2 fluorescence spectrometer
alumina nanotube ͑ANT͒, as a special nanostructure with
6
7
high flexibility and important dielectric properties, can be
considered as outer insulating protective layer and has been
proposed to synthesize with several methods, such as anod-
͑
Jobin-Yvon Company͒. Electron paramagnetic resonance
͑
EPR͒ spectra were obtained with a Bruker EMX-10/12
8
spectrometer. All the measurements were run at room
temperature.
ization of Si-based Al film, chemical etching in NaOH
solution, and ultrasonic treatment of PAA membrane.
9
10
Figure 1͑a͒ shows the SEM image of sample A190. A
three-layer-structure morphology can clearly be observed.
However, these methods have obvious disadvantages such as
short lengths of obtained nanotubes, fussy processes, inevi-
table introduction of impurity, and low productivity. These
shortcomings have limited more applications of the ANTs.
11
Recently, Chu et al. have reported fabrication of the PAA
membrane with ordered nanopore array under high voltage
in an aged electrolyte, which leads to the birth of an ex-
panded field of PAA-related materials. In this letter, we re-
port in situ fabrication of large quantities of ANTs without
any additional post-treatment. The convenient fabrication
method provides a good basis for seeking further applica-
tions of the ANTs. In addition, we found that the obtained
ANT array has an obvious blue photoluminescence ͑PL͒ un-
der ultraviolet excitation. The PL mechanism is discussed in
detail on the basis of its annealing behavior.
Anodization of high-purity Al foil ͑99.99%͒ was con-
ducted in an aged 15 wt % sulfuric acid solution which un-
derwent a preanodization for 15 A h. This aging treatment of
the electrolyte can enhance breakdown potential of PAA
11
membrane prominently. During the anodization, the solu-
tion temperature was maintained at 5 °C. The anodization
was initially carried out at constant current. And the applied
voltage begins from zero to a preset value and then remains
unchanged for 1 h. The obtained products are classified as
FIG. 1. ͑a͒ SEM image of sample A190. A unique three-layer structure can
be clearly observed. The corresponding enlarged images of the three layers
are displayed in ͑b͒, ͑c͒, and ͑d͒, respectively.
a͒Author to whom correspondence should be addressed; electronic mail:
hkxlwu@nju.edu.cn
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003-6951/2006/89͑7͒/073114/3/$23.00 89, 073114-1 © 2006 American Institute of Physics
29.120.242.61 On: Sun, 30 Nov 2014 11:10:04
0
1

073114-2
Huang et al.
Appl. Phys. Lett. 89, 073114 ͑2006͒
FIG. 3. ͑a͒ PL spectra of sample B175, taken under excitation with the 310,
3
30, and 340 nm lines of a Xe lamp. ͑b͒ PLE spectra of sample B175, taken
by monitoring at the 390, 400, and 410 nm emission wavelengths.
chemical etching under high voltage/large current density is
a key factor in forming large quantity of ANTs. Here we
would like to mention that the temperature of the electrolyte
in the experiment by Chu et al. was set at a much lower
FIG. 2. ͑a͒ SEM image of sample C175. The inset shows an enlarged image.
b͒ SEM image of sample B175. ͑c͒ SEM image of sample C150
͑
.
11
The bottom layer ͑lower left corner͒ is alumina nanopore
array. Its enlarged image is shown in Fig. 1͑b͒. The nanopore
array shows a density-packed hexagonal structure. In our
value, which leads to a remarkably reduced chemical etch-
ing rate. This results in formation of bare ANTs. In addition,
we found that longer anodic time in our present work is also
an important factor for fabrication of a lot of ANTs. From
Fig. 2͑c͒ and the inset of Fig. 2͑a͒, we can see that some
nanopore walls are broken along the channels. This leads to
the formation of some slender alumina nanowires ͑see the
1
2
previous results, some protuberances were observed on the
boundaries of some ordered areas and considered to be due
1
3
to the suspension formed under internal stress. However,
the current morphology demonstrates concaves instead of
convexes on the boundaries ͓see the white arrows in
Fig. 1͑b͔͒. This feature indicates that the pore growth is rapid
and has not totally stopped under high voltage. The middle
layer can be seen to be individual ANT arrays ͓see the en-
larged image in Fig. 1͑c͔͒. Large quantities of ANTs with
inner and outer diameters of ϳ65 and ϳ110 nm, respec-
tively, have in situ been formed in this layer. The top layer
1
0
white arrow͒. This result indicates that the ANTs in these
samples are different from those in sample A190. According
6
,8,10
to previous literature,
we know that two different cleav-
age fashions of PAA membrane can lead to the formation of
two kinds of ANTs. One forms the ANTs with smooth outer
8
surface, as observed from sample A190. The other forms the
ANTs with resident tube walls by the sacrifice of six neigh-
bor nanopores, which is accompanied with the formation of
͓
the right side in Fig. 1͑a͔͒ and its enlarged view in Fig. 1͑d͒
1
0
contain bundles of collapsed ANTs. Some curly ANTs illu-
minate their high flexibility ͑see the white arrow͒. These
morphology features confirm that this simple method is con-
venient for bulk production of individual ANTs.
alumina nanowires. Our present work clearly shows the
existence of the two cleavage fashions in the anodizing pro-
cess of Al foil. So we may control the morphology of the
ANTs by adopting appropriate parameters.
Obviously, the formation of the current ANTs is tightly
related to high anodic voltage and large current density used
during the anodization. When Al foil was anodized, massive
joule heat causes abrupt rise of local temperature of the PAA
membrane, which largely accelerates chemical etching of the
PL of the alumina membrane is interesting. We found
that although different samples have different morphologies,
all the samples show a similar blue PL band under ultraviolet
excitation. Figure 3͑a͒ displays the PL result from sample
B
175. A strong PL band appears at ϳ400 nm. Its peak posi-
+
membrane in such a strong acidic solution ͑H concentration
tion hardly changes with excitation wavelength. The corre-
sponding PLE spectra show a pinned peak at ϳ335 nm
͓Fig. 3͑b͔͒. The PL and PLE properties are completely dif-
ferent from those from ordinary PAA membranes formed in
of the aged electrolyte is about 1M with pH meter͒ and thus
results in a unique cleavage of the PAA membrane, i.e., the
formation of massive ANTs. The alumina nanopores in the
bottom layer are less affected by the strong chemical etching
and therefore their morphologies almost keep unchanged
1
2
sulfuric acid ͑not shown͒. Hence, we can rule out the pos-
2
4
−
sibility for the PL band to be due to SO group. To disclose
͓
Fig. 1͑b͔͒. To further clarify the formation mechanism of the
the origin of the PL band, sample B175 was annealed at
ANTs, the PAA membranes formed under different voltages
and current densities were characterized by SEM and the
obtained images are displayed in Fig. 2. One can see that the
surface-collapsed ANTs can also be observed from sample
B175 with a thin top layer ͓Fig. 2͑b͔͒, but compared with
those from sample A190 ͓Fig. 1͑d͔͒, the collapsed ANTs are
reduced in quantity and the underneath nanopores/tubes
become vaguely visible. For samples C175 and C150, the
top layer is even thinner ͓Fig. 2͑a͔͒ and almost disappears
400 °C in N for 0.5 h. Figure 4͑a͒ and its inset show the PL
2
and PLE spectra of the annealed sample, respectively. It can
be seen that the intensities of the ϳ400 nm PL and ϳ335 nm
PLE bands are dramatically decreased. This suggests that the
−
PL band cannot originate from optical transition in OH
−
group, because the intensity of an emission from OH group
should remarkably be increased in the sample annealed at
1
4
400 °C. Figure 4͑b͒ shows the PL spectra of the samples
formed under different voltages. We can see that although the
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͓Fig. 2͑c͔͒. These results indicate that joule heat produced by
two spectra have identical PL peak positions, the PL intensity
129.120.242.61 On: Sun, 30 Nov 2014 11:10:04

073114-3
Huang et al.
Appl. Phys. Lett. 89, 073114 ͑2006͒
1
2
duced in the alumina. This causes a significant increase of
the PL intensity, as shown in Fig. 4͑b͒. Here we should point
1
6,17
out that in previous work,
the intensity of the PL band
from oxygen vacancies increases in the annealed sample due
to the increased oxygen vacancies caused by Al oxidization
at alumina/aluminum interface. However, in our present ex-
periments, thick PAA membranes ͑Ͼ100 ␮m͒ makes the
light emission from the alumina/Al interface undetectable.
The collected PL signal in the experiments is mainly from
oxygen vacancies near the surface layer of a membrane.
Such PL obviously has intensity lower in the annealed
1
6
sample due to annihilation of some oxygen vacancies.
In conclusion, we have fabricated large quantities of in-
dividual ANTs using anodization of Al foil in an aged sulfu-
ric acid solution under high anodic voltage. The obtained
sample demonstrates a unique three-layer structure, with the
ANT array located in the middle layer. We have discussed
the formation mechanism of the ANTs. In addition, we have
also investigated the blue emission property of all the
samples under ultraviolet excitation. Based on annealing be-
havior of the blue emission band and our EPR result, we
have attributed the blue emission to optical transition in the
+
F centers in the alumina matrix.
This work was supported by grants ͑Nos. 10225416 and
BK2006715͒ from the National and Jiangsu Natural Science
Foundations as well as the LAPEM. Partial support was also
from the Major State Basic Research Project No.
G001CB3095 of China and City University of Hong Kong
Direct Allocation Grant No. 9360110.
1
FIG. 4. ͑a͒ Annealing behavior of the blue PL band, taken under excitation
with the 330 nm line of a Xe lamp. The inset shows annealing behavior of
the PLE band, taken by monitoring at 400 nm. ͑b͒ PL spectra of samples
B175 and C175. The inset shows the corresponding EPR spectra.
H. Masuda and K. Fukuda, Science 268, 1466 ͑1995͒.
2
Z. L. Yang, Z. W. Niu, X. Y. Cao, Z. Z. Yang, Y. F. Lu, Z. B. Hu, and C.
C. Han, Angew. Chem. Int. Ed. 42, 4201 ͑2003͒.
G. S. Wu, L. D. Zhang, B. C. Cheng, T. Xie, and X. Y. Yuan, J. Am.
3
Chem. Soc. 126, 5976 ͑2004͒.
4
K. K. Kim and J. I. Jin, Nano Lett. 1, 631 ͑2001͒.
is stronger in the sample formed under higher anodic volt-
age. This implies that the sample formed under high voltage
has high concentration of luminescent centers. To reveal the
nature of the luminescent center, we measured the EPR spec-
tra of the corresponding samples with the same masses and
present the obtained results in the inset of Fig. 4͑b͒. Obvi-
ously, the EPR signal intensity tracks with the PL intensity.
This provides a good argument that the luminescent center
for the ϳ400 nm PL band is paramagnetic. From the ob-
tained EPR signals, we can calculate the Landé g value in
Zeeman interaction term to be 2.0071. In crystalline Al O , it
5
M. Steinhart, S. Zimmermann, P. Göring, A. K. Schaper, U. Gösele, C.
Weder, and J. H. Wendorff, Nano Lett. 5, 429 ͑2005͒.
6
Y. F. Mei, X. L. Wu, X. F. Shao, G. G. Siu, and X. M. Bao, Europhys. Lett.
62, 595 ͑2003͒.
7
H. J. de Wit, C. Wijenberg, and C. Crevecoeur, J. Electrochem. Soc. 123,
1
479 ͑1976͒.
8
Y. F. Mei, X. L. Wu, X. F. Shao, G. S. Huang, and G. G. Siu, Phys. Lett.
A 309, 109 ͑2003͒.
Z. L. Xiao, C. Y. Han, U. Welp, H. H. Wang, W. K. Kwok, G. A. Willing,
9
J. M. Hiller, R. E. Cook, D. J. Miller, and G. W. Crabtree, Nano Lett. 2,
1
293 ͑2002͒.
1
0
X. F. Shao, X. L. Wu, G. S. Huang, T. Qiu, M. Jiang, and J. M. Hong,
Appl. Phys. A: Mater. Sci. Process. 81, 621 ͑2005͒.
2
3
+
has experimentally been proven that the F center ͑single
11
S. Z. Chu, K. Wada, S. Inoue, M. Isogai, and A. Yasumori, Adv. Mater.
͑Weinheim, Ger.͒ 17, 2115 ͑2005͒.
G. S. Huang, X. L. Wu, Y. Xie, X. F. Shao, and S. H. Wang, J. Appl. Phys.
ionized oxygen vacancy͒ can emit a PL peak at about
1
5
16
12
4
13 nm. Du et al. have also reported a similar EPR signal
94, 2407 ͑2003͒.
with a Landé g value of 2.0085 and considered it to be from
1
3
+
G. D. Sulka, S. Stroobants, V. V. Moshchalkov, G. Borghs, and J.-P. Celis,
J. Electrochem. Soc. 151, B260 ͑2004͒.
14
the F center in the PAA membrane. Hence, it is reasonable
+
to attribute the PL band to optical transition in the F center
H. Tamura, M. Rückschloss, T. Wirschem, and S. Vepřek, Appl. Phys.
Lett. 65, 1537 ͑1994͒.
W. Chen, H. G. Tang, C. S. Shi, J. Dang, J. Y. Shi, Y. X. Zhou, S. D. Xia,
Y. X. Wang, and S. T. Yin, Appl. Phys. Lett. 67, 317 ͑1995͒.
Y. Du, W. L. Cai, C. M. Mo, J. Chen, L. D. Zhang, and X. G. Zhu, Appl.
Phys. Lett. 74, 2951 ͑1999͒.
located in alumina matrix. The concentration of oxygen va-
1
1
1
5
6
7
−
cancy in the alumina is inversely proportional to that of OH
1
2
in the electrolyte. When anodization is conducted under a
large current density, the consumption of OH increases. As
a result, the OH concentration is lowered in electrolyte near
−
−
Y. Li, G. H. Li, G. W. Meng, L. D. Zhang, and F. Phillipp, J. Phys.:
Condens. Matter 13, 2691 ͑2001͒.
the anode and large amount of oxygen vacancies are pro-
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