Structure of duplex oxide layer in porous alumina studied by 27Al MAS and MQMAS NMR

Article abstract of DOI:10.1246/cl.2005.1286

A local structure analysis using ²⁷Al multiple-quantum magic-angle-spinning (MQMAS) and MAS NMR techniques has been performed on porous alumina fabricated by the anodic oxidation of aluminum. It is suggested that the main constituents of the inner and outer oxide layers composing porous alumina are [AlO₄ and AlO₅] and [AlO₆], respectively, and that both of the layers adopt the amorphous structure. Copyright

Full text of DOI:10.1246/cl.2005.1286

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Chemistry Letters Vol.34, No.9 (2005)
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Structure of Duplex Oxide Layer in Porous Alumina Studied by Al MAS and MQMAS NMR
ꢀ
Takahiro Iijima, Seiichi Kato, Ryuichi Ikeda, Shinobu Ohki, Giyuu Kido,
Masataka Tansho, and Tadashi Shimizu
National Institute for Materials Science, Tsukuba, 305-0003
(Received June 24, 2005; CL-050814)
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A local structure analysis using Al multiple-quantum mag-
ic-angle-spinning (MQMAS) and MAS NMR techniques has
been performed on porous alumina fabricated by the anodic
oxidation of aluminum. It is suggested that the main constituents
of the inner and outer oxide layers composing porous alumina
are [AlO4 and AlO5] and [AlO6], respectively, and that both
of the layers adopt the amorphous structure.
Figure 1. Scanning electron micrograph of porous alumina fabricated
by anodic oxidation using oxalic acid.
In recent years, porous alumina having uniform pores has
1
–4
attracted much interest in many fields. Since the size, depth,
and interval of the pores are highly controllable in the porous
alumina materials fabricated by an anodic oxidation method,
the application of them extends to accurate filters, catalyst sup-
ports, nanocomposites, photonic crystals, and so on. The struc-
ture of porous alumina has been proposed to be an assembly
of a hexagonal column with the pore in its center by Keller
1
and co-workers in 1953. Thompson and Wood have reported
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that porous alumina consists of duplex oxide layers; outer oxide
layer existing adjacent to the pore is composed of anion-incorpo-
rated alumina, whereas inner layer existing remote from it is of
pure alumina. The inhomogeneous distribution of the anionic
species concentrated in the intermediate part of the outer oxide
has been suggested in order to account for the nonuniform re-
3
fractive index observed. However, the structure of alumina in
each oxide layer has not been clarified yet, although such infor-
mation is essential when the material having nanofunction is de-
signed. This can be due to the amorphous structure of porous alu-
mina that is reported for the material synthesized by a sol–gel
method.5
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Figure 2. Al MAS NMR spectra of the powder sample of porous
alumina synthesized using (i) sulfuric, (ii) oxalic, and (iii) phosphoric
acids. (a) and (b) show the spectra for the samples without and with
ꢃ
drying, respectively. The 20 pulse of the width pw ¼ 0:9 ms with
the irradiation frequency ꢀ1 ¼ 31 kHz was used. The side view of po-
rous alumina consisting of the duplex oxide layers is schematically
shown on the right side of the spectra. The black, gray, and white areas
represent aluminum metal, inner oxide and outer oxide, respectively.
High-resolution solid-state NMR is a powerful tool for ana-
lyzing the local structure of both crystalline and noncrystalline
6
materials. For example, diffraction experiments are unable to
provide detailed information about the local structure for the
amorphous sample, while NMR remains sensitive to local bond-
and 0.3 M phosphoric acid at 130 V (C). Scanning electron
micrograph (SEM) was measured for the obtained membranes
of porous alumina using a JEOL JSM-6500 FEG-SEM. The
Al MAS and MQMAS NMR spectra were recorded at 21.9 T
on a JEOL ECA 930 spectrometer equipped with a JEOL
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ing parameters. In particular, Al (S ¼ 5=2) magic-angle spin-
ning (MAS) NMR that can distinguish the coordination environ-
ment around it has been ubiquitously used for the structural
characterization of the inorganic materials. Further detailed
structural information around 27Al nuclei is obtainable from a
two dimensional NMR method of multiple-quantum MAS
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4 mm MAS probe. The Al resonant frequency was 242.398
MHz. The MAS frequency of ꢀr ¼ 18 kHz was stabilized within
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7
ꢁ10 Hz by a JEOL MAS speed controller. The Al chemical
shift is referenced to 1.0 M AlCl3 solution at ꢂ0:1 ppm. Three
(
acquiring the high-resolution spectrum of half-integer quadru-
MQMAS) developed by Frydman and co-workers in 1995 for
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pulse sequence employing z-filter was used for acquiring the
triple-quantum (3Q) MAS NMR spectrum.
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pole nuclei. In the present work, the local structure of porous
alumina fabricated by the anodic oxidation is investigated by
Figure 1 shows the top view of the SEM image of porous
alumina of the sample (B). The black spots in this picture repre-
sent the pore of porous alumina. The SEM images of two other
samples were similarly obtained. We estimated the diameters of
the pore of porous alumina (A), (B), and (C) as ca. 10, 50, and
150 nm, respectively.
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using Al MAS and MQMAS NMR spectra. We will discuss
the structure of both oxide layers by analyzing the obtained
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Al NMR spectra.
Three samples of porous alumina were prepared by the
anodic oxidation; 99.99% aluminum sheets were oxidized in
.3 M sulfuric acid at 22 V (A), 0.3 M oxalic acid at 40 V (B),
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Figure 2(a) shows the 27Al MAS NMR spectra of the powder
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.9 (2005)
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samples of porous alumina (A), (B), and (C). Each of these spec-
tra consists of three broad peaks positioned at 66, 38, and 9 ppm,
which can be assigned to four- (AlO4), five- (AlO5), and six-fold
free from the broadening due to the second-order quadrupole
coupling (SOQC). The asymmetric structure especially for the
peaks of Al in AlO4 and AlO5 in F2-projected spectrum is
caused by the SOQC, which becomes symmetric one in F1-pro-
jected spectrum. The NMR parameters of the isotropic chemical
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5
(
AlO6) oxygen-coordinated aluminum atoms, respectively. The
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spectra are normalized to the peak of Al in AlO5. The signifi-
cant fraction of AlO5 has been also detected for porous alumina
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ꢂ1
2
1=2
shift ꢁ and quadrupole product PQ ¼ e Qqh ð1 þ ꢂ =3Þ
,
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,9
2
ꢂ1
and related materials fabricated by the sol–gel method. The
pore size dependence of the spectrum can be clearly seen for
where e Qqh and ꢂ are the quadrupole coupling constant
and asymmetric parameter, respectively, can be estimated from
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the peak of Al in AlO6. The relative intensity of Al in AlO4
was 0:70 ꢁ 0:04 for the three samples, whereas that in AlO6 was
the peak position of the MQMAS spectrum. The values of (ꢁ,
PQ) for 27Al in AlO4, AlO5, and AlO6 are obtained as (72.2 ppm,
4.3 MHz), (41.0 ppm, 4.1 MHz), and (11.2 ppm, 3.2 MHz), re-
spectively. When the ꢁ and PQ values are distributed owing to
the amorphous structure, the peak of the MQMAS spectrum
elongates along the chemical shift (CS) and quadrupole-induced
0.99, 0.58, and 0.29 for the samples (A), (B), and (C), respective-
ly. The correlation between the pore size and the duplex oxide
3
structure has been already reported; increasing the pore diame-
ter accompanies the decrease of the thickness of the outer oxide
layer, leaving that of the inner oxide layer unchanged, as drawn
schematically on the right side of the spectra in Figure 2. There-
fore, we propose that the outer oxide layer is mainly composed
of AlO6, while the inner one is AlO4 and AlO5. The AlO6-rich
structure of the outer layer is reasonable, because the outer oxide
contacts the electrolysis solution during the synthesis and water
molecules can easily bond to the aluminum atoms. Figure 2b
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shift (QIS) axes, respectively. Since all of three Al peaks are
elongated along CS axis, both of the inner and outer oxide layers
are revealed to be amorphous. The distribution of the ꢁ value
may be caused by that of the Al–O–Al bond angle. On the other
hand, the distribution of the PQ value is different among the three
Al species. The peaks of Al in AlO4 and AlO5 are elongated
along F2 axis rather than QIS axis. The PQ value of Al is de-
termined by a tensor of the electric field gradient made by the
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1
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shows the Al MAS spectra for the dried samples. The relative
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intensity of Al in AlO6 for the sample (A) having the thickest
outer oxide layer decreased from 0.99 to 0.83 by the drying. This
indicates that some of the water molecules in the outer layer are
removed by the drying and that the coordination number of alu-
minum decreased from six to four or five.
atomic group surrounding Al. Since the inner oxide layer con-
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,3
sists of the pure alumina, the PQ value is mainly dominated by
the symmetry of the polyhedron of AlO4 and AlO5, and thus the
distribution of the distortion of them seems to be relatively
small. The peak for AlO6 elongated largely along QIS axis
shows that the electric field gradient is considerably distributed.
Such a distribution is caused by the random distortion of AlO6
and/or the anions incorporated inhomogeneously in the outer
oxide layer. Finally, we note that the line width of F1-projected
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Figure 3 shows the Al 3QMAS spectrum of the powder
sample of porous alumina (B). The 3QMAS spectra for two other
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7
samples were almost the same except for the peak height of Al
in AlO6. F1 and F2 axes represent the isotropic and MAS axes of
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Al, respectively. The projection of the MQMAS spectrum onto
the F2 axis corresponds to the 27Al MAS NMR spectrum, while
spectrum for Al in AlO6 is smaller than that in AlO4 and AlO5,
which is caused by the degree of the distribution of the ꢁ value
27
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that onto the F1 axis to the Al high-resolution spectrum
for each Al species.
In summary, we have measured the 27Al MAS and MQMAS
spectra of porous alumina in order to clarify the local structure of
it. Such structure has not been analyzed so far by any methods
other than NMR, which can be due to the amorphous structure.
It is suggested that the oxides in the inner and outer layers of
porous alumina are mainly composed of [AlO4 and AlO5] and
[AlO6], respectively, and that the structures of both layers are
amorphous.
References
1
F. Keller, M. S. Hunter, and D. L. Robinson, J. Electrochem. Soc., 100, 411
1953).
(
2
3
G. E. Thompson and G. C. Wood, Nature, 290, 230 (1981).
J. Choi, Y. Luo, R. B. Wehrspohn, R. Hillebrand, J. Schilling, and U.
Gosele, J. Appl. Phys., 94, 4757 (2003).
4
5
H. Masuda and K. Fukuda, Science, 268, 1466 (1995).
L. Wilcox, G. Burnside, B. Kiranga, R. Shekhawat, M. K. Mazumder, R. M.
Hawk, D. A. Lindguist, and S. D. Burton, Chem. Mater., 15, 51 (2003).
K. Schmidt-Rohr and H. W. Spiess, ‘‘Multidimensional Solid-State NMR
and Polymers,’’ Academic Press, London (1994).
L. Frydman and J. S. Harwood, J. Am. Chem. Soc., 117, 5367 (1995); A. L.
Medek, J. S. Harwood, and L. Frydman, J. Am. Chem. Soc., 117, 12779
6
7
2
7
Figure 3. Al 3QMAS NMR spectrum of the powder sample of
porous alumina synthesized using oxalic acid and projections to F1
ꢀ
(
isotropic) and F2 (MAS) dimensions. The peaks marked by show
(
1995).
J.-P. Amoureux, C. Fernandez, and S. Steuernagel, J. Magn. Reson., Ser. A,
23, 116 (1996).
the spinning side bands of the main peak. CS and QIS represent the
chemical shift and quadrupole-induced shift axes, respectively. The
lines parallel to F2 and QIS axes are also shown for each peak. The
pulse parameters were (ꢀ1, pw) of (150 kHz, 2.8 ms) for the first pulse
exciting triple-quantum coherence, (150 kHz, 1.0 ms) for the second
pulse converting to zero-quantum coherence, and (14 kHz, 15 ms) for
the final pulse generating single-quantum coherence.
8
1
9
1
D. Coster and J. J. Fripiat, Chem. Mater., 5, 1204 (1993).
J.-P. Amoureux and C. Fernandez, Solid State Nucl. Magn. Reson., 10, 211
(1998); A. Goldbourt and P. K. Madhu, Monatsh. Chem., 133, 1497 (2002).
0
11 D. Muller, E. Jahn, G. Ludwig, and U. Haubenreisser, Chem. Phys. Lett.,
109, 332 (1984).
Published on the web (Advance View) August 20, 2005; DOI 10.1246/cl.2005.1286