A zirconium oxide film self-assembled at the air-water interface

Article abstract of DOI:10.1016/j.physb.2004.11.013

A self-assembled zirconia-based film, produced at the air-water interface using sodium dodecyl sulphate (SDS) as the template, has been characterised by energy-dispersive X-ray reflectometry, X-ray diffraction, and X-ray fluorescence analysis. Long-range order due to the lamellar liquid crystalline arrangement of the surfactant micelles was significant enough to produce Bragg diffraction. X-ray fluorescence from the specimens in the electron microscope indicates that the principal component of the film contains zirconium, oxygen and sulphate, -suggesting that the film contains zirconium polyoxo ions and surfactant. Raman spectroscopy indicates the presence of a zirconium hydroxide.

Full text of DOI:10.1016/j.physb.2004.11.013

ARTICLE IN PRESS
Physica B 357 (2005) 27–33
www.elsevier.com/locate/physb
A zirconium oxide film self-assembled at the air–water
interface
Ã
M.J. Hendersona, A. Gibaudb, J.-F. Bardeaub, A.R. Renniec, J.W. Whitea,
aResearch School of Chemistry, Australian National University, Canberra, ACT 0200, Australia
b
´
´
Laboratoire de Physique de l’Etat Condense, UMR CNRS 6087, Universite du Maine, 72085, Le Mans, Cedex 09, France
cUppsala University, The Studsvik Neutron Research Laboratory, S-611 82 Nyko¨ping, Sweden
Abstract
A self-assembled zirconia-based film, produced at the air–water interface using sodium dodecyl sulphate (SDS) as the
template, has been characterised by energy-dispersive X-ray reflectometry, X-ray diffraction, and X-ray fluorescence
analysis. Long-range order due to the lamellar liquid crystalline arrangement of the surfactant micelles was significant
enough to produce Bragg diffraction. X-ray fluorescence from the specimens in the electron microscope indicates that
the principal component of the film contains zirconium, oxygen and sulphate, -suggesting that the film contains
zirconium polyoxo ions and surfactant. Raman spectroscopy indicates the presence of a zirconium hydroxide.
r 2004 Elsevier B.V. All rights reserved.
PACS: 61.10.Kw; 68.03.Hj; 81.16.Dn
Keywords: Zirconium oxide; Thin film; Air–water interface; Time-resolved X-ray reflectometry
1. Introduction
Our focus is the production of highly orientated
films at the air–water interface. This interface has
been used previously to prepare two-dimensional
arrays of inorganic compounds including calcium
carbonate, [12] titania [13,14] and zirconia
[11,13,15]. The utility of a surfactant assembly as
template for the production of inorganic materials
at the air–water interface is illustrated by hexade-
cyltrimethylammonium halide, C16TAX (X ¼ Cl,
Br), -templated silica [16–18] and more recently
sodium dodecyl sulphate SDS-templated titania
[19,20]. The titania-based film, prepared from an
acidic solution of a titanium alkoxide, TiðOBunÞ4;
Zirconia is an important functional material. It
is used in catalysis, e.g., for the isomerization of
alkenes [1], or in optics for smart thermo- [2] or
electro- [3] optical modulation. The ability to
produce porous zirconium oxide with a high
surface area via a template route either as a
powder [4–6] or as structurally uniform thin films
[7–11] is therefore of interest.
Ã
Corresponding author. Fax.: +61 2 6125 4903.
E-mail address: jww@rsc.anu.edu.au (J.W. White).
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2004.11.013

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M.J. Henderson et al. / Physica B 357 (2005) 27–33
and SDS, shows Bragg diffraction consistent with
lamellar mesostructure in which the film
poured gently over the zirconium-containing solu-
tion causing some precipitation. The container was
sealed and left undisturbed until the mixture began
to clarify 2 h at room temperature. The solution
was then heated to 80 ꢀC for 1 h. After allowing
any precipitate to settle, the material at the
interface was retrieved by gently adding water to
raise the film high enough to be transferred to a
glass substrate. A film examined ex situ was the
insoluble part of the film remaining after it had
been bathed in water and then allowed to dry in
air. The rinsed film is assumed to contain only
surfactant that is anchored firmly to the inorganic
componentof ht e film.
a
components, titania and dodecyl sulphate exhibit
a regular 35 A arrangement. In this paper, we
present our work on the preparation and char-
acterisation of the analogous zirconia-based film.
Mesoporous zirconias have been prepared as
powders from zirconium alkoxides and aliphatic
anionic surfactants containing a sulphate head-
group, including sodium hexadecyl sulphonate [21]
and SDS [21,22]. When bitailed sodium-7-ethyl-2-
methyl-undecyl-sulphonate was used as the tem-
plate [23,24], a microporous solid was obtained.
To our knowledge, our work here offers the first
reportof an ordered zirconia film self-assembled at
the air–water interface using a surfactant assembly
as the template.
2.3. Energy-dispersive reflectometry
The energy-dispersive experimentwas per-
formed at25 ꢀC in the Research School of
Chemistry at the Australian National University,
using the instrument described elsewhere [19,20] in
a sealed temperature-controlled Teflon trough
(140 Â 40 Â 2 mm). The data were recorded as X-
ray reflectivity, I, as a function of scalar momen-
tum transfer,
2. Experimental procedure
2.1. Chemicals
Milli-Qs water was used for all film syntheses.
Zirconium (IV) n-butoxide (Aldrich, 80% solution
in butanol), hydrochloric acid (36–39% w/w) and
alizarin (BDH) were used as received. SDS (Lan-
caster 99%) was recrystallised from hot water:-
ethanol (1:9).
4p
qz ¼
sin y;
(1)
l
where l is the X-ray wavelength and y the specular
reflectance angle (0:6 ꢀÞ: The incidentangle was
evaluated experimentally by recording the specular
Bragg reflections from a highly ordered mesos-
tructured silicate film templated at the air–water
interface by hexadecyltrimethylammonium chlor-
ide, C16TAC [17]. The trough was hermetically
sealed to maintain a constant vapour pressure
2.2. Synthesis
A film was produced using the following
millimolar ratios of H2O:HCl:ZrðOBunÞ4:SDS,
100:0.6–1.2:0.15:0.05–0.08, at times between 1 h
ꢀ
at80 C or 2 weeks at room temperature. Where a
room temperature synthesis was performed, e.g.,
for the film used for GISAX analysis, film
formation was facilitated by the addition of water
about 1 week into the experiment.
above
the
interface.
Maestro
software
(Maestros-32; EG & Gs) was used to record
the energy-dispersive reflectivity profiles at 5 min
intervals.
A typical preparation is as follows. Zirconium
(IV) butoxide (0.28 g) was added to hydrochloric
acid (36–39% w/w, 0.48 g) and the mixture stirred
for 10 min. The thick white precipitate was treated
with water (0.45 g) until the precipitate dissolved
to give a clear and colourless solution. This
solution was then transferred to a Nalgene bottle.
A solution of SDS (0.09 g) in water (6.9 g) was
2.4. Scanning electron microscopy (SEM) and X-
ray fluorescence spectroscopy
Measurements were performed using a Jeol 6400
microscope. The sample was gold-coated prior
to analysis to prevent charging. The result of the

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M.J. Henderson et al. / Physica B 357 (2005) 27–33
29
solution on top of these drops to bring the level
just over the lip of the trough. The trough lid was
then secured and the energy-dispersive reflectivity
profile was recorded at5 min intervals over an 86 h
period. The data were recorded as reflectivity
intensity I, as a function of scalar momentum
transfer qz:
The firstspecrtum recorded displayed a broad
Bragg peak between qz ¼ 0:18 and 0.19 AÀ1: The
intensity of the peak gradually lessened until, after
about 2 h, only a fringe characteristic of surfactant
Zr
5000
4000
3000
2000
1000
0
CO
S
Si
Au
0
1
2
3
4
5
6
7
8
9
10
Energy / keV
was noticeable.
A
sharp Bragg peak at
Fig. 1. X-ray fluorescence analysis of a zirconia-based film
obtained at the air–water interface from an acidic solution of
ZrðOBunÞ4 and SDS.
qz ¼ 0:19 AÀ1 appeared 2.75 h into the experiment
and continued to develop until at 36 h a fall in the
count rate was noted. The period following the
drop in the count rate was characterised by a shift
in the Bragg peak position from qz ¼ 0:19 to
qz ¼ 0:18 AÀ1; corresponding to a change in the d
spacing from 33 to 35 A. The I vs. qz vs. t profiles
for this period where the Bragg peak shifted are
shown in Fig. 2. Beyond this time, the Bragg peak
at qz ¼ 0:18 AÀ1 was established and the intensity
increased dramatically until the experiment was
terminated. We attribute this behaviour to the
incorporation of a cosurfactant, dodecanol, pro-
duced from the hydrolysis of SDS [25] into the film
as reported for a titania-based film self-assembling
at the air–water interface [19].
X-ray fluorescence spectroscopic analysis is pre-
sented in Fig. 1.
2.5. X-ray diffraction (XRD)
Diffraction patterns were measured with a
Philips X’Pert Materials Research Diffractometer
equipped with a humidity-controlled cell, using
a
monochromatic CuK radiation operated at 40 kV
and 30 mA.
2.6. Grazing incidence small-angle X-ray scattering
(GISAXS)
A chemical spottestfor zirconium [26] using an
alcoholic solution of alizarin dye on a film that was
The pattern was obtained on the X22B beamline
at the National Synchrotron Light Source, Broo-
khaven National Laboratory, US.
2.7. Raman spectroscopy
Raman spectra were measured using a Horiba
T64000 instrument calibrated using the spectrum
obtained from diamond (1332 cmÀ1).
3. Results and discussion
To perform the time-resolved experiment at the
air–water interface, an aqueous acidic zirconia
solution was placed in evenly spaced aliquots
across the area of the trough. Fast mixing was
achieved by pouring the aqueous surfactant
Fig. 2. Energy-dispersive X-ray I versus qz versus t profiles of a
zirconia-based film self-assembling at the air–water interface
showing a change in the Bragg peak position. Angle of
incidence, y ¼ 0:6ꢀ:

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M.J. Henderson et al. / Physica B 357 (2005) 27–33
removed from the interface and then rinsed and
dried was positive, suggesting that zirconia within
the film was accessible for chemical post-treat-
ment. Post-functionalisation of mesoporous zirco-
nia-based films using organic ligands has been
reported previously [10]. SEM images of the
synthesised zirconia film (not shown) show that
the film surface was smooth. An X-ray fluores-
cence analysis of the zirconia-based film, Fig. 1,
indicated that carbon, oxygen, sulphur and zirco-
nium were present.
10-1
10-2
10-3
10-4
10-5
The XRD pattern, Fig. 3(a), shows a pattern
consistent with a structure having a lamellar
alternation of surfactant and oxide layers. The
reflections could not be indexed to other unit cells,
e.g., that for hexagonal mesostructuring as de-
monstrated for surfactant templated zirconia, [6]
alumina [27] and titania [28] as the (1 0 0), (1 1 0)
and (2 0 0) diffraction peaks of this unit cell were
absent. Similarly, a cubic unit cell was ruled out
[28]. The lamellar ordering of the zirconia film was
confirmed by a GISAX measurement, Fig. 4. The
d spacing, 36 A, is in agreement with that reported
for a titania-based film produced at the air–water
interface [19,20] and that of a silica-based film
obtained by a dip-coating process from a sol
containing tetraethoxysilane and SDS [29]. We
note here that a small-angle X-ray scattering
pattern (not shown) displayed (0 0 1) and (0 0 2)
diffraction peaks of the lamellar phase. An
estimate of the extent of the regular layer lattice
was obtained from the broadening of the first
diffraction peak using the treatment of Warren
[30]. If y is the Bragg angle and b the full width of
the peak at half maximum intensity in radians, the
coherence length Lc is
0.2
0.4
0.6
qz / Å-1
0.8
1.0
(a)
0.8
0.6
0.4
0.2
0.0
10 % RH
35 %RH
90 %RH
37.0
36.5
36.0
35.5
20 40 60 80
Relative Humidity %
0.10
0.15
0.20
0.25
0.30
0.35
qz / Å-1
(b)
Fig. 3. (a) X-ray diffraction pattern of a zirconia-based film
obtained at the air–water interface. (b) X-ray diffraction
patterns of a zirconia-based film at relative humidity 10%
(solid line), 35% (solid line, circle) and 90% (dotline). Inset
shows a plotof hte response of film
humidity. The line is a guide to the eye.
d spacing to relative
The change in the first two orders of diffraction
as a resultof swelling hte film atincreasing
humidity is shown in Fig. 3(b). The data corre-
spond to relative humidities 10% (solid line), 35%
(solid line, circle) and 90% (dotline). The insetof
Fig. 3(b) shows the change in d spacing evaluated
from the (0 0 1) diffraction peak plotted as a
function of relative humidity. The film was first
measured in 50% relative humidity. The film was
then saturated in 95% relative humidity before
reducing the humidity to about 10%. Thereafter,
the humidity was raised to 35, 50, 60, 70 and
0:89l
b cos y
Lc ¼
;
(2)
where l is the incident wavelength. The coherence
length obtained was 2017 A, more than twice the
length obtained for a mesostructured silica-SDS
film [29]. The coherence length is significantly
smaller than the thickness of the film as revealed
by SEM cross-section. This result suggests that the
value is a lower limitof ht e degree of long-range
ordering of the ZrO2–SDS layers.

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M.J. Henderson et al. / Physica B 357 (2005) 27–33
31
phase. All of the characterisation experiments of
the zirconia-based film here, including XRD and
GISAXS, were performed atambienttemperature
below 30 ꢀC: As these experiments indicated a
lamellar ordering of zirconium oxide and dodecyl
sulphate, there was no evidence to suggest that a
phase transformation caused by heat has contrib-
uted measurably to the diffraction signals.
Increased humidity decreased the intensity
greatly for both orders without significant change
in their width. This is consistent with a reduction
in contrast between the zirconium oxide and
surfactant layer. An interesting possibility is the
incorporation of the added water into the zirco-
nium layer. Inspection of the estimated X-ray
Fig. 4. GISAX pattern of a zirconia-based film obtained at the
air–water interface from an acidic solution of ZrðOBunÞ4 and
SDS.
scattering
(3:88 Â 10À5
length
densities
for
Zr(OH)4
and
finally 80%. This large swelling is an interesting
pointer to the structure of the zirconium oxide
layer since hydration of the SDS headgroups alone
might be expected to produce a contraction of the
layer spacing (due to a change in the chain tilt
angle) as itdoes in SDS crystalline phases [31]. The
change in the tilt angle from 15ꢀ (with respect to
the long axis of the anion and the unit cell) in
anhydrous SDS to 45ꢀ for the monohydrate
represents a 5 A change in the lamellar thickness.
However, the effect of increasing the hydration
state of the crystalline phases of SDS is to decrease
the lamellar thickness, an effect attributed to
electrostatic interactions between the headgroups
that in turn affect the alignment of the tails. In
contrast, the lamellar thickness of the mesostruc-
tured zirconia-based film presented in this study
increases with increasing humidity. We conclude
that although a change in tilt angle might have
occurred during hydration, the layer lattice change
is dominated by the swelling.
Although the effect of temperature of the film
mesostructure was outside the scope of this study,
heat is also known to induce phase transforma-
tions of mesostructured metal oxides [32]. This
behaviour is illustrated by the hexagonal to
lamellar transformation of aluminophosphate,
templated in alcoholic solution by dodecyl phos-
phate, between the rather low temperature range
of 35–43 ꢀC [33]. We note that the preparation of
the aluminophosphate in aqueous media between
20 and 120 ꢀ C yielded exclusively the lamellar
A
À2), SDS (0:93 Â 10À5AÀ2
)
water (0:94 Â 10À5
A
À2) supports this hypothesis.
For the estimation, the stoichiometry of the metal
oxide was considered to be that of the uncon-
densed hydroxide Zr(OH)4 with a density of
4:8 g cmÀ3; a density reported for an amorphous
hydrous zirconium oxide gel [34]. The contrast is
only reduced significantly when water is added to
the zirconia layer. The (0 0 1) intensity change to
full hydration was a decrease by a factor of three
so the contrast change was a factor of
p
3:
Assuming that all the absorbed water enters the
Zr(OH)4 layer (considered an hydrophilic oligo-
mer), this contrast change allows the mole
percentage water in that layer to be evaluated as
about25%. The presence of a zirconium hydro-
xide within the film is supported by the Raman
spectrum of the film material shown in Fig. 5.
Though the Raman bands observed below
600 cmÀ1 are attributed to the vibration modes of
Zr–O–Zr bonds, the lines do not correspond to
those of either the cubic, monoclinic or tetragonal
phases of zirconia [35]. However, the profile of the
spectrum between 100 and 600 cmÀ1 does show
some similarity with that obtained from a zirco-
nium hydroxide gel precipitated from aqueous
zirconyl chloride solution, pH 4 [36]. The absence
of long-range order within the zirconium hydro-
xide can be reconciled with the order that gives rise
to the diffraction peaks as the latter is not due to
atomic ordering but to the lamellar liquid crystal-
line arrangement of the surfactant micelles.

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M.J. Henderson et al. / Physica B 357 (2005) 27–33
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Financial supportfrom an Innovaiton Access
Programme (CG050120) administered by the
Department of Education Science and Training
is gratefully acknowledged. Messrs D. King and T.
Dowling are thanked for their assistance with the
energy-dispersive instrumentation. Dr. B. Ocko
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