Effect of atomic layer deposition coatings on the surface structure of anodic aluminum oxide membranes

Article abstract of DOI:10.1021/jp0503415

Anodic aluminum oxide (AAO) membranes were characterized by UV Raman and FT-IR spectroscopies before and after coating the entire surface (including the interior pore walls) of the AAO membranes by atomic layer deposition (ALD). UV Raman reveals the presence of aluminum oxalate in bulk AAO, both before and after ALD coating with A1₂O₃, because of acid anion incorporation during the anodization process used to produce AAO membranes. The aluminum oxalate in AAO exhibits remarkable thermal stability, not totally decomposing in air until exposed to a temperature >900 ?°C. ALD was used to cover the surface of AAO with either Al₂O₃ or TiO ₂. Uncoated AAO have FT-IR spectra with two separate types of OH stretches that can be assigned to isolated OH groups and hydrogen-bonded surface OH groups, respectively. In contrast, AAO surfaces coated by ALD with Al₂O₃ display a single, broad band of hydrogen-bonded OH groups. AAO substrates coated with TiO₂ show a more complicated behavior. UV Raman results show that very thin TiO₂ coatings (1 nm) are not stable upon annealing to 500 ?°C. In contrast, thicker coatings can totally cover the contaminated alumina surface and are stable at temperatures in excess of 500 ?°C. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0503415

J. Phys. Chem. B 2005, 109, 14059-14063
14059
Effect of Atomic Layer Deposition Coatings on the Surface Structure of Anodic Aluminum
Oxide Membranes
†
‡
|,#
§
§
Guang Xiong, Jeffrey W. Elam, Hao Feng, Catherine Y. Han, Hsien-Hau Wang,
§
,†,§
§
|
|
Lennox E. Iton, Larry A. Curtiss,* Michael J. Pellin, Mayfair Kung, Harold Kung, and
Peter C. Stair†
,|
Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439, Energy Systems DiVision,
Argonne National Laboratory, Argonne, Illinois 60439, Materials Science DiVision, Argonne National
Laboratory, Argonne, Illinois 60439, Department of Chemistry and Center for Catalysis and Surface Science,
Northwestern UniVersity, EVanston, Illinois 60208, and Institute for EnVironmental Catalysis,
Northwestern UniVersity, EVanston, Illinois 60208
ReceiVed: January 19, 2005; In Final Form: March 29, 2005
Anodic aluminum oxide (AAO) membranes were characterized by UV Raman and FT-IR spectroscopies
before and after coating the entire surface (including the interior pore walls) of the AAO membranes by
atomic layer deposition (ALD). UV Raman reveals the presence of aluminum oxalate in bulk AAO, both
before and after ALD coating with Al O , because of acid anion incorporation during the anodization process
2 3
used to produce AAO membranes. The aluminum oxalate in AAO exhibits remarkable thermal stability, not
totally decomposing in air until exposed to a temperature >900 °C. ALD was used to cover the surface of
AAO with either Al O or TiO . Uncoated AAO have FT-IR spectra with two separate types of OH stretches
2 3 2
that can be assigned to isolated OH groups and hydrogen-bonded surface OH groups, respectively. In contrast,
AAO surfaces coated by ALD with Al display a single, broad band of hydrogen-bonded OH groups.
AAO substrates coated with TiO show a more complicated behavior. UV Raman results show that very thin
TiO coatings (1 nm) are not stable upon annealing to 500 °C. In contrast, thicker coatings can totally cover
the contaminated alumina surface and are stable at temperatures in excess of 500 °C.
2 3
O
2
2
1. Introduction
membranes and on the catalytic and separation properties of
the membranes. Therefore, it is of interest to characterize the
impurities and understand their behavior under conditions to
which the membrane might be exposed during use. It is also
useful to either eliminate or cover the contaminated surface by
annealing or by atomic layer deposition (ALD) encapsulation.
In recent years, there has been increasing interest in the
synthesis and characterization of anodic aluminum oxidation
AAO) membranes arising from their applications as a scaffold
(
1
2
3
platform for nanofabrication, nanomask, information storage,
and catalysis.4 AAO is also used as a template structure for
,5
Modification of AAO membranes by ALD provides the
means of controlling the pore diameter as well as the pore wall
the fabrication of nanowires, nanotubes, nanocomposites, and
1
,6,7
so forth.
The most interesting feature of AAO membranes
10,11
composition.
ALD is a thin film growth technique that uses
is their highly aligned pores of uniform cylindrical shape and
size. The pore size, pore density, and AAO thickness can be
alternating, saturating reactions between gaseous precursors and
a substrate to deposit films in a monolayer-by-monolayer
4
controlled by the anodization conditions. For example, narrow
1
2
pore diameter distributions can be formed with diameters
ranging from 20 to 400 nm by changing the applied potential
and the electrolyte.
fashion. The controlled growth of thin films can be ac-
complished using an ABAB... binary reaction sequence. Fur-
thermore, a wide variety of materials including oxides, nitrides,
sulfides, and metals have been deposited using ALD.13
Potential uses of AAO membranes in separations require that
there be precise control of the pore diameter. Their use in
catalysis would be greatly expanded if the chemical pore wall
composition could be varied arbitrarily. The compositional
constraint comes because the porous oxide can be produced with
only a few materials (Al, Si) and also because impurities are
incorporated into the structure of AAO membranes during
synthesis.4 The presence of the incorporated impurities could
have a significant influence on the near-surface structure of the
Recently, it has been demonstrated that ALD can uniformly
coat each pore of AAO in a layer-by-layer fashion over the entire
10
pore length. Nanostructured membranes, fabricated by a
combination of AAO and ALD, have been used as the support
11
for the synthesis of oxidation catalysts. These membranes offer
unique catalytic environments including (1) precisely controlled
pore size, (2) tailored pore wall composition, and (3) controlled
flow of reagents in and out of the catalyst.
,8,9
In this paper, we report on a detailed structural study of AAO
membranes before and after ALD coating using ultraviolet (UV)
Raman spectroscopy and Fourier transform-infrared spectro-
scopy (FT-IR). The combination of these methods is used to
characterize the structure of AAO membranes before and after
ALD coating with both Al2O3 and TiO2. ALD coating is shown
*
Corresponding author.
†
Chemistry Division, Argonne National Laboratory.
Energy Systems Division, Argonne National Laboratory.
Materials Science Division, Argonne National Laboratory.
Department of Chemistry and Center for Catalysis and Surface Science,
‡
§
|
Northwestern University.
#
Institute for Environmental Catalysis, Northwestern University.
1
0.1021/jp0503415 CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/30/2005

14060 J. Phys. Chem. B, Vol. 109, No. 29, 2005
Xiong et al.
to encapsulate incorporated anions and to produce a more
uniform hydroxylated surface.
2. Experimental Section
Aluminum oxalate was purchased from Alfa Aesar and was
used without further purification.
A. Synthesis of AAO. AAO membranes were synthesized
by anodizing pure Al foil (0.5 mm, 99.999%, 100 × 100 mm,
Sigma-Aldrich). First, the Al foils were degreased in acetone
and were electrochemically polished in a polishing solution
(
perchloric acid:EtOH ) 1:9). Next, the foil was anodically
oxidized in a 0.3 M aqueous oxalic acid solution at 3 °C at a
potential of 40 V. The anodization was repeated twice. The first
anodization step took ∼15 h. Then, the AAO film was
chemically stripped by immersing the sample in a mixture of
Figure 1. UV Raman spectra of (a) amorphous Al
and (c) aluminum oxalate after annealing at 550 °C in Ar.
2
O
3
on Si, (b) AAO,
6% phosphoric and 1.8% chromic acids. The second anodization
was carried out under the same conditions as the first for an
additional 24 h. This two-step anodization process greatly
improves the ordering of the nanopore arrays. The remaining
unreacted Al foil was etched in 0.1 M CuCl2 37.5% HCl solution
to release the AAO membrane from the unreacted portion of
the Al foil. The barrier layer, a thin oxide film that formed the
interface between the Al foil and the bottoms of the nanopores,
was then removed by immersing the AAO membrane in 5%
H3PO4 at 30 °C for 70 min. This step opens the AAO membrane
pores providing parallel pores open at both the top and bottom
of the film. The resulting membranes are typically about 70-
µm thick.
in air. The AAO samples were 13 mm in diameter with a
thickness of 70 µm placed on top of a flat sample holder.
FT-IR. The FT-IR measurements were performed using a
Nicolet 60 SX transmission FTIR spectrometer equipped with
a mercury cadmium telluride (MCT) detector and a quartz cell
connected to the gas manifold. The samples were calcined for
2
-3 h at 450 °C in an O2 flow and then were cooled to room
temperature. All spectra were recorded in He at room temper-
ature. Background measurement was performed using an empty
cell in a He flow at room temperature. The spectra were obtained
by subtracting the background from the raw data. Spectra were
B. Atomic Layer Deposition. The nanoporous AAO mem-
-
1
collected with a resolution of 4 cm (128 scans).
branes were coated by Al2O3 and TiO2 ALD using a viscous-
1
0,14
SEM. The scanning electron micrograph (SEM) images were
aquired using a Hitachi S4700 SEM with a field emission gun
flow ALD reactor operated in a quasi-static mode
at a
deposition temperature of 177 °C. The AAO substrates were
placed in the reactor with the long axes of the nanopores oriented
perpendicular to the carrier gas flow. Alternating exposures to
trimethyl aluminum (TMA, Aldrich) and water vapor from
(FEG) electron beam source.
3
. Results
1
0
deionized water were used for performing the Al2O3 ALD
while the TiO2 ALD utilized alternating titanium tetrachloride
The UV Raman spectra of AAO, amorphous Al O , and
2
3
aluminum oxalate are shown in Figure 1. Amorphous alumina
Figure 1a was used as a clean reference, since AAO is also
composed of amorphous alumina. The amorphous alumina was
synthesized by ALD as described in the Experimental Section.
The spectrum of AAO (Figure 1b) exhibits peaks at 757, 835,
(TiCl4, Aldrich) and deionized water exposures.
During each ALD cycle, the reactor was first evacuated using
a mechanical pump to a pressure below 0.02 Torr. Next, the
reactor was filled with the first ALD precursor to a pressure of
-
1
5
Torr for time t1. Following this exposure, the reactor was
943, 1183, 1385, and 1514 cm , respectively. As shown in
Figure 1a, there is no detectable signal from the amorphous
Al O sample, indicating that the peaks in the UV Raman
evacuated and then purged with ultrahigh purity nitrogen at a
mass flow rate of 200 sccm and a pressure of 1 Torr for time
t2. Next, the exposure to the second precursor was performed
at 5 Torr for time t1, followed by evacuation and nitrogen purge
for time t2. The exposure times and purge times were increased
during the course of the coating process to account for the slower
diffusion as the pores narrowed.10 The initial cycles used t1 )
2
3
spectrum from the AAO membrane (Figure 1b) are not from
the amorphous Al O , which forms the bulk of the AAO.
2
3
It has been reported that as much as 5% of the anodized
portion of the AAO membrane are acid anions incorporated into
4,8,9
the structure of AAO membrane during anodization.
These
1
)
0 s and t2 ) 30 s, while the final cycles used t1 ) 45 s and t2
studies demonstrated that two different alumina regions are
formed during the anodization process: a center framework of
relatively pure alumina and a pore wall region contaminated
by introduction of the acid anion. These two regions are clearly
evident in SEM micrographs of uncoated AAO membranes
(Figure 2a) because of their differing secondary electron yield.
An example of an ALD coated AAO membrane is shown in
Figure 2b. The uniformity over this region is clearly demon-
strated. Cross-sectional SEM studies (not shown) demonstrate
that the coating is uniform along the entire pore length.
To ascertain whether the Raman spectrum for our oxalic acid
derived AAO was a result of anion incorporation, a reference
compound, hydrated aluminum oxalate (Aldrich), was tested.
The reference compound was dried at 550 °C in Ar and was
subsequently measured using UV Raman at room temperature
60 s.
Al2O3 ALD was also used to coat a Si(111) wafer to serve
as a Raman standard for amorphous alumina. This deposition
was performed using the ALD reactor in a conventional flowing
mode for 1000 TMA/H2O cycles with the timing sequence
1
-5-1-5 with a resulting thickness of about 130 nm.
C. Characterization of AAO membranes. UV Raman
Spectroscopy. The UV Raman spectra were measured using a
UV Raman instrument built at Northwestern University. Details
15,16
of the UV Raman instrument have been provided elsewhere.
The Raman spectra were excited at 244 nm generated by an
intracavity, frequency-doubled argon ion laser (Lexel 95 SHG).
The laser power at the sample is 4 mW, and a typical spectrum
collection time is 20 min. The UV Raman spectra were recorded

Anodic Aluminum Oxide Membranes
J. Phys. Chem. B, Vol. 109, No. 29, 2005 14061
Figure 2. SEM top view micrographs of (a) an uncoated AAO and
(b) a 15-nm alumina-coated (ALD) membrane showing 10-nm pores
remaining. The uncoated SEM membrane shows the different secondary
electron yields of the AAO membrane. The bright regions are from
what is suggested to be relatively pure alumina, while the gray portion
has incorporated oxalate anions. The dark portions are the pores
themselves.
Figure 4. UV Raman spectra of (a) an AAO membrane, (b) a 15-nm
ALD coating of Al
O
2 3
on an AAO membrane, and (c) a 15-nm ALD
coating of Al
air.
2 3
O on an AAO membrane after annealing at 500 °C in
When using these membranes for catalysis, thermal decom-
position of the oxalate may not be desirable for at least two
reasons. First, the AAO structure may be significantly changed
at the high temperatures and, second, the carbon contaminants
may still be present in the AAO structure, just in a different
form (possibly carbonate). Either of these scenarios could change
the catalytic properties of these structures. Therefore, an
alternative method that involves covering the contaminated
surface using ALD was attempted. In this study, a 15-nm Al2O3
film was deposited on the surface of an AAO membrane. This
coating reduces the pore diameter from 40 to 10 nm. Figure 4
displays the UV Raman spectra of the AAO membrane before
and after the 15-nm Al2O3 coating. The UV Raman spectra of
the coated and uncoated samples are nearly identical. This is
not surprising since (1) the excitation laser and the Raman
scattered signals are not absorbed by the 15-nm alumina film
Figure 3. UV Raman spectra of an AAO membrane at room
temperature after a heat treatment at room temperature (R.T.), 500 °C,
(
alumina does not absorb UV light at 244 nm) and (2) ALD
alumina coatings do not display any significant peaks (Figure
a).
To obtain direct evidence of the ALD layer from UV Raman
8
00 °C, and 900 °C in air.
1
under ambient conditions (Figure 1c). The Raman spectra of
the dehydrated aluminum oxalate and that of AAO membrane
measurements, TiO2 was chosen as a coating material. Because
TiO2 strongly absorbs UV light, the UV laser beam will not
penetrate the TiO2 film. Figure 5 shows the UV Raman spectra
of the AAO membrane coated by a 1-nm TiO2 film. The
intensities of aluminum oxalate UV Raman peaks are greatly
reduced. (Figure 5b) This indicates that the UV Raman spectrum
samples only the near surface region of the TiO2 coated sample.
After annealing at 500 °C, the peaks of aluminum oxalate return.
This result suggests that TiO2 dissolves into the bulk of the
alumina or forms clusters on the surface during annealing,
exposing the underlying aluminum oxalate.
(Figure 1b) are very similar, suggesting that the peaks from the
AAO membrane can be assigned to aluminum oxalate trapped
in the AAO alumina structure.
The temperature dependence of the UV Raman spectra of an
AAO membrane is displayed in Figure 3. For each spectrum,
the AAO membrane was calcined at the indicated temperature
in flowing air for 5 h and then was cooled to room temperature
before the spectrum was recorded. The spectrum of the AAO
membrane remained essentially unchanged up to 800 °C and
then changed significantly when annealed to 900 °C. This
suggests that the trapped aluminum oxalate remained stable up
to 800 °C before decomposing at 900 °C. The spectrum at 900
Figure 6 exhibits the UV Raman spectra of an AAO
membrane coated with a two-layer ALD film comprised of a
1-nm Al2O3 film and a 14-nm TiO2 film. The 1-nm Al2O3 film
allows coating of the TiO2 film on a compositionally uniform
-
1
°
C presents a broad peak ranging from 500 to 1000 cm
-
1
together with peaks at 1084, 1140, 1277, and 1382 cm . To
-
1
assign these peaks, reference spectra of R-, γ-, and δ-Al2O3
Al2O3 film. The sharp peak at 1550 cm is from O2 in the air.
No peak associated with aluminum oxalate is detectable at either
room temperature (R.T.) or 500 °C. This result indicates that
the thicker coating can totally cover the contaminated alumina
surface. Furthermore, the outer surface of the composite
membrane remains intact as a complete layer of TiO2 after
annealing to 500 °C. This does not preclude the possibility of
some subsurface interfacial mixing of the TiO2 and Al2O3 having
occurred.
-
1
were acquired. In the range of 600-2000 cm , peaks at 740
-
1
and 830 cm were observed for δ-Al2O3, and a peak at 740
-1
cm was found for R-Al2O3. Obviously, it is difficult to confirm
the presence of crystalline Al2O3 from the UV Raman spectra
since a broad band masks every peak between 500 and 1000
-
1
cm . However, evidence from unpublished in situ X-ray
diffraction measurements at 900 °C establish that γ-Al2O3 is
gradually formed.

14062 J. Phys. Chem. B, Vol. 109, No. 29, 2005
Xiong et al.
Figure 5. UV Raman spectra of (a) an AAO membrane, (b) a 1-nm
ALD coating of TiO on an AAO membrane, and (c) a 1-nm ALD
coating of TiO on an AAO membrane after annealing at 500 °C.
2
2
Figure 7. FT-IR spectra of (a) a 15-nm ALD coating of Al O on an
2
3
AAO membrane, (b) an uncoated AAO membrane, and (c) an Anopore
porous membrane (Whatman). All spectra are taken after annealing at
4
2
50 °C in O .
Figure 6. UV Raman spectra of (a) a 14-nm ALD coating of TiO
a 1-nm ALD coating of Al on an AAO membrane (b) sample in (a)
after annealing at 500 °C in air.
2
on
2 3
O
The FT-IR spectra of AAO membranes dehydrated at 450
C for 2 h in O2 followed by cooling to R.T. are shown in Figure
Figure 8. UV Raman spectra of aluminum oxalate powder after
annealing in dry air at different temperatures. The bottom spectrum is
of an AAO membrane calcined to 550 °C shown for comparison.
°
7. A commercial AAO membrane (Whatman) was measured
under the same conditions (Figure 7c). The spectra of our AAO
membranes and the commercial (Whatman) membranes exhibit
aluminum oxalate at room temperature exhibits peaks at 950,
-
1
FT-IR bands at 3556, 3677, 3746, and 3793 cm . Following
-
1
1
135, 1375, and 1610 cm . After calcining to 200 °C, a new
17-19
the literature assignments,
3
the peaks at 3677, 3746, and
793 cm are assigned to three types of isolated hydroxyl
groups, which are bound to three, two, and one aluminum atoms,
-
1
-1
peak at 860 cm appears, the peak at 950 cm intensifies,
-
1
-
1
-1
and the peak at 1135 cm shifts to 1181 cm . After annealing
-
1
at 550 °C, peaks at 775 and 1027 cm appear. The spectrum
at 550 °C is similar to that of the AAO membrane. After
annealing at 800 and 900 °C, all the sharp peaks disappear
indicating decomposition of the oxalate.
-
1
respectively. The broad band at 3556 cm is associated with
hydrogen-bonded hydroxyl groups. Generally, the hydroxyl
group peaks are broad, but the bands assigned to isolated
hydroxyl groups in Figure 7b and c are relatively sharp,
indicating that the AAO surface is very uniform. After coating
by ALD, all peaks of the isolated OH groups disappear and
only that of the hydrogen-bonded OH group was observed. This
suggests that all hydroxyl groups are in environments where
the hydroxyl density is high enough for hydrogen bonding and
that the number of more isolated hydroxyl group sites has
substantially decreased. The lack of isolated hydroxyl group
sites indicates that the ALD alumina coatings are free of defects
which lead to missing hydroxyls.
4
. Discussion
The UV Raman results confirm that acid anions are incor-
porated into the structure of AAO membranes. The spectrum
of the trapped anions in AAO is very similar to that of aluminum
oxalate after annealing at 550 °C. The decomposition temper-
ature of the aluminum oxalate in AAO is 900 °C (see Figure
9
3). Xu et al. demonstrated by thermogravimetric analysis that
a weight loss in AAO membranes (produced in oxalic acid)
takes place at 890-930 °C, which is the same as the decom-
position temperature observed in this study. From the compari-
The temperature dependence of the UV Raman spectra of
aluminum oxalate powder is shown in Figure 8. The hydrated

Anodic Aluminum Oxide Membranes
J. Phys. Chem. B, Vol. 109, No. 29, 2005 14063
son between Figure 3 and Figure 8, it is clearly seen that the
decomposition temperature of aluminum oxalate in AAO is
higher than that of aluminum oxalate powder, indicating that
the aluminum oxalate in AAO is somewhat more stable. This
is probably because most of the aluminum oxalate in the bulk
of AAO cannot directly contact O2 in air.
ALD is an alternative method to obtain a compositionally
uniform pore wall surface. A 15-nm titania layer was found to
totally cover the contaminated alumina surface. Coating of AAO
membranes by ALD not only forms a compositionally uniform
surface but also modifies the types of hydroxyl groups present
on the surface.
Since annealing does not decompose contaminate anions until
high temperature, ALD is an alternative method to obtain a
compositionally uniform pore wall surface. The UV Raman
results show that a 1-nm TiO2 layer can cover the contaminated
surface but that this film is not stable upon annealing to 500
Acknowledgment. This work was supported by the U.S.
Department of Energy Basic Energy Sciences under Contract
No. W-31-109-ENG-38. The SEM analysis was performed in
the Electron Microscopy Center, Materials Science Division,
Argonne National Laboratory, Argonne, IL.
°
C. Ti atoms may dissolve into bulk alumina or aggregate to
form TiO2 nanoparticles on the surface. In contrast, a 15-nm
titania layer is quite stable even upon calcination to 500 °C
continuing to cover the pore walls.
References and Notes
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hydroxyl groups present on the surface. Hydroxyl groups play
an important role in surface chemistry and catalysis. For
example, hydroxyl groups can be acidic or basic, which is
reflected in their vibrational frequencies, and can be the active
sites in catalytic reactions. Moreover, the preparation of catalysts
using impregnation or grafting methods is carried out through
the interaction of precursor chemicals with hydroxyl groups.
Following Al2O3 coating of AAO by ALD, the isolated hydroxyl
groups disappear and only hydrogen-bonded hydroxyl groups
are observed. This suggests that ALD coating of AAO by Al2O3
produces a more uniform surface layer.
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Anodic aluminum oxide (AAO) membranes were character-
ized by UV Raman and FT-IR spectroscopies before and after
coating the entire surface (including the interior pore walls) of
the AAO membranes by atomic layer deposition (ALD). The
UV Raman spectrum of AAO membrane gives direct evidence
that dehydrated aluminum oxalate is incorporated into the
structure of AAO because of acid anion incorporation during
the anodization process. The elimination of the contaminant
anions can be accomplished only by annealing to high temper-
ature. The decomposition of the trapped aluminum oxalate starts
at 550 °C but is not completed until 900 °C. Since annealing
does not remove contaminate anions until high temperature,
2
(
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(
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