# Synthesis of Nanostructured Nanoclay-Zirconia Multilayers:

Authors: Hao Chen; Guoping Zhang; Kathleen Richardson; Jian Luo
Journal: Journal of Nanomaterials
 (2008-02-20)

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## Abstract

This paper reports the first effort to synthesize a new class of inorganic nanostructured materials consisting of alternating ultrathin layers of nanoclays and oxide ceramics. A novel solution-based layer-by-layer (LBL) deposition technique was developed to prepare multilayers of hydrated Zr cations and nanoclays. This LBL deposition technique is devised by integrating an electrostatic-mediated dip coating method for making nanoclay-polymer multilayers with a successive ionic layer adsorption and reaction method for making ultrathin oxide films.  Nanostructured clay-zirconia multilayer composites formed through subsequent annealing.  Characterization by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction confirmed that these films are uniform and crack-free, consist of no detectable impurities, and possess nanoscale-layered structure. The incorporation of nanoclays facilitates the electrostatic-mediated assembling of multilayers, enhances the structural integrity, and provides a generic framework to construct functionally graded materials. Potential applications are envisaged.


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## Body

## 1. Introduction

The last decade has seen tremendous
developments in the “wet” colloid
chemical synthesis of nanosized and nanostructured materials, which was inspired by “biomineralization and
hierarchically organized self-assembly” [1]. This wet synthesis technique
utilizes alternating layer-by-layer (LBL) deposition process, which is enhanced by electrostatic assembling or
surface-mediated adsorption and reaction. It has several major advantages, such as relative ease of preparation, low cost, and high versatility. Through this technique, a variety of nanostructured materials with improved properties or enhanced
functionalities have been developed, including inorganic [2, 3] or hybrid organic-inorganic [4] films. In particular, inspired by the special mortar-brick nanostructure
of nacre and its superior mechanical properties, hybrid multilayers consisting of alternating organic polymer and inorganic
nanoclay layers have been made by an LBL deposition and assembly method [5, 6]. This method was modified from a more general
method for making polyelectrolyte multilayers where substrates are alternately
dipped into polycation and polyanion solutions to form polyelectrolyte thin
films [7].In parallel, a successive ionic
layer adsorption and reaction (SILAR) method [8–10]
was developed initially for the deposition of
sulfide
films. The SILAR method grows sulfide thin films by repeating deposition cycles
consisting of four steps: dipping in cationic precursor solutions for
adsorption of cations, rinsing to remove physisorbed cations, dipping into
anionic (S2−) solutions to form a monolayer or submonolayer of
sulfides, and rinsing again. This SILAR technique has recently been adapted to
deposit oxide (e.g., ZnO) films [9, 10]. Notably, this technique has
been used for making ultrathin (e.g., 1–10 nm) gate
dielectric oxide films [11, 12] and combined with a
hydrothermal annealing process for low-temperature synthesis of nanocrystalline
oxide films [13]. More recently, the
feasibility of a modified SILAR technique without rinsing steps to prepare thin
zirconia films has been reported [14] and the associated film
growth kinetics has been studied [15]. The latter approach can be
regarded as a surface hydrolysis and precipitation process assisted by
electrostatic interactions [14, 15].A key underlying mechanism in the wet chemical construction of
nanomaterials is the solution-based self-assembly aided by electrostatic
attraction force between oppositely charged components (e.g., ions, polyions).
The use of platy-shaped clay nanoplatelets with intrinsic, permanent negative surface charges in constructing nanostructured multilayers
is an innovation to extend the capabilities of the conventional ion/polyion
aided self-assembly techniques. Clay minerals are hydrous layered
aluminosilicates, and
montmorillonite (MMT) is a swelling clay mineral consisting of 2 : 1 layers (i.e., 2 tetrahedral and 1 octahedral sheets per layer, as shown in
Figure1(a)). These 2 : 1 layers can be separated in Na+ or Li+ rich
solutions forming exfoliated clay nanoplatelets of ~1 nm in thickness. The
permanent negative surface charges of nanoclays not only promote the
electrostatic self-assembly process and its efficiency, but also render a high lateral bond strength within the constructed multilayers
or their precursors [1]. In addition, the high aspect ratio of platy nanoclays provides
an ideal planar surface for self-assembly and growth of other nanomaterials. To
date, hybrid inorganic clay and
organic polymer (including biopolymer) nanostructured multilayers have been
developed [16], but development in making nanoclay-based inorganic nanocomposites or
multilayers has not been reported. Due to the inherent thermal and chemical
instability of organic polymers, hybrid organic and inorganic multilayers have limitations in some specialized
applications, such as in high-temperature
environments. In this study, we attempt
to develop a new class of inorganic oxide-clay nanostructured multilayers.(a) The structure of 2 : 1 clay nanoplatelets exfoliated from MMT. (b) A targeted multilayer structure to be synthesized
using a new LBL assembling technique via making use of the intrinsic negative
surface charges and unique platy shape of nanoclays (as shown in (a)). In
principle, the thickness and
chemical composition of each nanometer-thick oxide layer can be tuned
individually, offering versatility
in engineering these nanostructured films with a wide range of potential
applications to be explored in future studies.
(a)(b)The paper presents the preliminary findings of this research with a
particular focus on the feasibility of making hydrated inorganic oxide-clay
nanostructured multilayers using the newly developed solution-based LBL
synthesis technique. The ultimate goal of this study is to develop purely
inorganic, nanostructured nanoclay-oxide
multilayers, as shown in Figure1(b). Zirconia is
chosen as the model oxide because we have recently developed a modified SILAR
technique to make pure or doped zirconia films [14] and a relevant film growth
model [15]. Zirconia thin films
have a wide variety of applications, such as protective coatings, insulating or dielectric ceramic layers, and other functional ceramics.


## 2. Experimental

### 2.1. Materials

Silicon wafers with thermally oxidized surface layers (Si/SiO2)
were purchased and cut into pieces of 1×2 cm2, which are used as substrates.
The Si/SiO2 wafers were ultrasonically cleaned in 0.1 M NaOH solution for 20 minutes, immersed into piranha solution (3 vol. of 95–98 wt.% H2SO4:1 vol. of 30 wt.% H2O2) for 20 minutes, rinsed with deionized water 4 times and dried in
air. A Na-MMT clay (Cloisite® Na+) was purchased from Southern Clay
Products (Gonzales, Tex, USA). The 0.03 wt.% clay suspension (pH =
7.24) was prepared by stirring clays with magnet at 1000 rpm speed in deionized
water for 20 minutes and then ultrasonicated for 20 minutes to achieve complete
dispersion and exfoliation. Other chemicals were purchased from Sigma
(Sigma-Aldrich, Saint Louis, MO, USA). Two different zirconium cationic precursor
solutions were used to examine their suitability for growing the zirconia nanolayers
on clay nanoplatelets, including 0.1 M zirconium chloride solution  (ZrCl4, pH  =  1.32) prepared from zirconium (IV)
oxychloride octahydrate and zirconium acetate solution  (ZrAc4, pH  =  4.14) prepared from zirconium (IV) acetate
hydroxide. Sodium poly(styrenesulfonate) (PSS) and poly(ethyleneimine) (PEI) were dissolved in
deionized water at concentrations of 1.5 mg/mL for the preparation of polymer-clay
multilayers.


### 2.2. Wet Chemical Synthesis and Annealing

Two steps were involved in the synthesis of oxide-clay multilayers: (1)
wet chemical construction of multilayer precursors (termed as-deposited films),
and (2) thermal dehydration and crystallization at elevated temperatures to
form the annealed films. This preliminary study particularly focused on
demonstrating the feasibility of the first step, which is more critical for
developing the targeted clay-oxide multilayers. Thus the dehydration and
thermal annealing conditions were preselected as a rule of thumb to be600∘C in
air for 2 hours under 1 atmospheric pressure, since most MMTs can maintain their crystal structure intact (i.e., no dehydroxylation) below temperatures of 600–650∘C.
However, the control parameters of thermal annealing or hydrothermal treatment
(i.e., the second step of synthesis)
will be further optimized in future studies.A computer-controlled programmable Nima dip coater (Nima Technology Ltd., Coventry, UK) was used to synthesize nanostructured
as-deposited multilayers. To verify the LBL deposition method and the functions
of the dip coater, nanoclay-polymer
films were prepared following the recipe in Lvov et al. [17]. The Si/SiO2 substrates were pretreated by dipping into PEI, PSS, and PEI solutions each for 20
minutes. The pretreated substrates were then dipped into a clay suspension  for 20
minutes, rinsed with water for 2 minutes, dipped into a PEI
solution for 20 minutes, and rinsed again with water for 2 minutes to
complete one deposition cycle. This four-step deposition cycle was repeated 8
times to make nanoclay-polymer multilayers, denoted as (PEI-PSS-PEI)-(MMT-PEI)8.
Both the dipping and withdrawing speeds were controlled at 20 mm per minute.To
prepare nanoclay-oxide multilayers, the precleaned silicon substrates were dipped into a zirconium cationic
precursor solution (ZrCl4 or ZrAc4) and the exfoliated
nanoclay suspension alternately without interdipping rinsing. The deposition
cycle was repeated 15 to 30 times to prepare the as-deposited multilayers. Both
the dipping and withdrawing speeds were controlled at 20 mm per minute. The
holding time was kept 45 seconds in air, Zr precursor
solution and clay suspension.
After completing a certain number (n) of deposition cycles, the substrates were then air-dried at
room temperature to form the as-deposited films or annealed isothermally to 600∘C for
2 hours to form the annealed films, denoted as (clay-oxide)n. A
constant heating and cooling rate of 5∘C per minute was used
during annealing.


### 2.3. Characterization

Both as-deposited (i.e., air-dried) and annealed films were characterized by a Hitachi S4800 field-emission
scanning electron microscope (FE-SEM) equipped with an energy dispersive X-ray
spectroscopy (EDXS) analyzer. SEM specimens were coated with Pt/Au for increased
conductivity. In addition, a Hitachi
S3500 variable-pressure SEM was employed to examine specimens without
conductive coating. Both in-plane and cross-sectional surfaces of the
multilayers were studied. Fresh cross-sectional surfaces were created by
carefully cleaving the Si wafers, while protecting  the films with
soft filter paper. Multiple
measurements on different points were conducted to obtain the film thickness on
cross-sectional surfaces under SEM. EDXS was also performed to analyze both
qualitatively and quantitatively the elemental composition of the multilayers.X-ray diffraction (XRD) was performed on both as-deposited and annealed
films in a Scintag diffractometer usingCu-Kα
radiation (λ=1.5418 Å) generated at 40 kV and
35 A at a scan speed of 1∘(2θ) per minute, step size of 0.01∘,
and a scan range of 1–60∘2θ. In addition, a bare Si/SiO2
substrate
was scanned by XRD to examine any
potential influence of the substrate on the observed diffraction patterns of
the ultrathin multilayers.


## 2.1. Materials

Silicon wafers with thermally oxidized surface layers (Si/SiO2)
were purchased and cut into pieces of 1×2 cm2, which are used as substrates.
The Si/SiO2 wafers were ultrasonically cleaned in 0.1 M NaOH solution for 20 minutes, immersed into piranha solution (3 vol. of 95–98 wt.% H2SO4:1 vol. of 30 wt.% H2O2) for 20 minutes, rinsed with deionized water 4 times and dried in
air. A Na-MMT clay (Cloisite® Na+) was purchased from Southern Clay
Products (Gonzales, Tex, USA). The 0.03 wt.% clay suspension (pH =
7.24) was prepared by stirring clays with magnet at 1000 rpm speed in deionized
water for 20 minutes and then ultrasonicated for 20 minutes to achieve complete
dispersion and exfoliation. Other chemicals were purchased from Sigma
(Sigma-Aldrich, Saint Louis, MO, USA). Two different zirconium cationic precursor
solutions were used to examine their suitability for growing the zirconia nanolayers
on clay nanoplatelets, including 0.1 M zirconium chloride solution  (ZrCl4, pH  =  1.32) prepared from zirconium (IV)
oxychloride octahydrate and zirconium acetate solution  (ZrAc4, pH  =  4.14) prepared from zirconium (IV) acetate
hydroxide. Sodium poly(styrenesulfonate) (PSS) and poly(ethyleneimine) (PEI) were dissolved in
deionized water at concentrations of 1.5 mg/mL for the preparation of polymer-clay
multilayers.


## 2.2. Wet Chemical Synthesis and Annealing

Two steps were involved in the synthesis of oxide-clay multilayers: (1)
wet chemical construction of multilayer precursors (termed as-deposited films),
and (2) thermal dehydration and crystallization at elevated temperatures to
form the annealed films. This preliminary study particularly focused on
demonstrating the feasibility of the first step, which is more critical for
developing the targeted clay-oxide multilayers. Thus the dehydration and
thermal annealing conditions were preselected as a rule of thumb to be600∘C in
air for 2 hours under 1 atmospheric pressure, since most MMTs can maintain their crystal structure intact (i.e., no dehydroxylation) below temperatures of 600–650∘C.
However, the control parameters of thermal annealing or hydrothermal treatment
(i.e., the second step of synthesis)
will be further optimized in future studies.A computer-controlled programmable Nima dip coater (Nima Technology Ltd., Coventry, UK) was used to synthesize nanostructured
as-deposited multilayers. To verify the LBL deposition method and the functions
of the dip coater, nanoclay-polymer
films were prepared following the recipe in Lvov et al. [17]. The Si/SiO2 substrates were pretreated by dipping into PEI, PSS, and PEI solutions each for 20
minutes. The pretreated substrates were then dipped into a clay suspension  for 20
minutes, rinsed with water for 2 minutes, dipped into a PEI
solution for 20 minutes, and rinsed again with water for 2 minutes to
complete one deposition cycle. This four-step deposition cycle was repeated 8
times to make nanoclay-polymer multilayers, denoted as (PEI-PSS-PEI)-(MMT-PEI)8.
Both the dipping and withdrawing speeds were controlled at 20 mm per minute.To
prepare nanoclay-oxide multilayers, the precleaned silicon substrates were dipped into a zirconium cationic
precursor solution (ZrCl4 or ZrAc4) and the exfoliated
nanoclay suspension alternately without interdipping rinsing. The deposition
cycle was repeated 15 to 30 times to prepare the as-deposited multilayers. Both
the dipping and withdrawing speeds were controlled at 20 mm per minute. The
holding time was kept 45 seconds in air, Zr precursor
solution and clay suspension.
After completing a certain number (n) of deposition cycles, the substrates were then air-dried at
room temperature to form the as-deposited films or annealed isothermally to 600∘C for
2 hours to form the annealed films, denoted as (clay-oxide)n. A
constant heating and cooling rate of 5∘C per minute was used
during annealing.


## 2.3. Characterization

Both as-deposited (i.e., air-dried) and annealed films were characterized by a Hitachi S4800 field-emission
scanning electron microscope (FE-SEM) equipped with an energy dispersive X-ray
spectroscopy (EDXS) analyzer. SEM specimens were coated with Pt/Au for increased
conductivity. In addition, a Hitachi
S3500 variable-pressure SEM was employed to examine specimens without
conductive coating. Both in-plane and cross-sectional surfaces of the
multilayers were studied. Fresh cross-sectional surfaces were created by
carefully cleaving the Si wafers, while protecting  the films with
soft filter paper. Multiple
measurements on different points were conducted to obtain the film thickness on
cross-sectional surfaces under SEM. EDXS was also performed to analyze both
qualitatively and quantitatively the elemental composition of the multilayers.X-ray diffraction (XRD) was performed on both as-deposited and annealed
films in a Scintag diffractometer usingCu-Kα
radiation (λ=1.5418 Å) generated at 40 kV and
35 A at a scan speed of 1∘(2θ) per minute, step size of 0.01∘,
and a scan range of 1–60∘2θ. In addition, a bare Si/SiO2
substrate
was scanned by XRD to examine any
potential influence of the substrate on the observed diffraction patterns of
the ultrathin multilayers.


## 3. Results and Discussion

### 3.1. Clay-Polymer Multilayers

SEM images of the planar and cross-sectional surfaces of the as-deposited
nanoclay-polymer multilayers [(PEI-PSS-PEI)-(MMT-PEI)8] are shown in
Figure 2. Some unevenly piled or protruded MMT platelets can be observed in the
planar surfaces, as shown in Figure 2(a). The cross-sectional surface, as shown in
Figure 2(b), exhibits nacre-like layered structure,
which is similar to that observed in the films developed by Tang et al. [18]. The average measured film thickness is 40±9 nm, of which 10 nm is from the precursor polymer film [17]. Thus the thickness of a
single (MMT-PEI)1  layer formed through a complete deposition cycle
is approximately ~3.75 nm, which is consistent with the previously reported
value of 3.3 nm per deposition cycle [17]. It was postulated that the
highly branched polycation PEI works as “electrostatic glue” [17] to assemble clay platelets. In
conclusion, polymer-nanoclay multilayers with similar nanostructure were
reproduced in our laboratory. This verified our equipment and procedures and
enabled further exploration of synthesizing the new nanoclay-oxide
multilayers.(a) Planar surface and (b) cross-sectional
surface SEM images of as-deposited nanoclay-polymer [(PEI-PSS-PEI)-(nanoclay-PEI)8]  films.
(a)(b)


### 3.2. Clay-Zirconia Multilayers

The SEM images of as-deposited films prepared by ZrCl4  and
ZrAc4  cationic precursor solutions are shown in
Figures 3 and  4, respectively. The as-deposited films were dried in air, thus either
nanoclays or hydrated Zr cations are not completely dehydrated, and water molecules
exist within the interface of clay nanoplatelets and Zr cations. Both films
appear to be reasonably uniform and crack-free. Furthermore, they both exhibit
observable layered structures in cross-sectional surfaces under SEM
(Figures 3(b)
and 4(b)).(a) Planar surface and (b) cross-sectional surface SEM images
of as-deposited nanoclay-zirconia multilayers made via ZrCl4  cationic solution.
(a)(b)(a) Planar surface and (b) cross-sectional surface SEM images
of as-deposited nanoclay-zirconia multilayers made by ZrAc4 cationic
solutions.
(a)(b)Evidently, the films made by ZrAc4  cationic precursor
solutions have significantly smoother surface and more regularly (parallel)
layered structure than those by ZrCl4  precursor solutions
(Figure 3 versus
Figure 4). The clay nanoplatelets are better oriented in the films prepared by
ZrAc4  cationic solutions, while peeled and protruded edges of clay
platelets are frequently observed in the ZrCl4  prepared films, as shown in Figure
3(a). For films prepared by ZrCl4  precursor solutions, the layered
structure is less close-packed with occasional, sparsely distributed open voids, as shown in
Figure 3(b). The nanoclay-oxide multilayers prepared using ZrAc4  precursor
solutions also appear to have lower surface roughness than those
polymer-nanoclay multilayers prepared in this and prior [17] studies. Based on these
results, ZrAc4  cationic precursor solutions are used exclusively to
make multilayers in further studies.SEM images of the planar and cross-sectional surfaces of annealed films
are shown in Figure5. The annealed films appear to be denser and more uniform than
the as-deposited ones. Some indications of the layered structure can be observed
in the cross-sectional surfaces under SEM, as shown in Figures 5(a)
and 5(b). It should be noted that the observed
cross-sectional surfaces were influenced by the actual (somewhat random)
cleaving process and hence the layered structure is not always clearly visible.
The surfaces of annealed films, as shown in Figure 5(c) are also smoother than those of
as-deposited films, as shown in Figure 4(a); yet, some clay nanoplatelets can still be
seen in Figure 5(c).(a),(b) cross-sectional surface, and (c) planar surface SEM
images of annealed nanoclay-zirconia films made by ZrAc4  precursor solutions with 25
deposition cycles.
(a)(b)(c)EDXS analysis of as-deposited and annealed films found the presence of Al, Si, and
Zr, without other metal impurities. A representative EDXS spectrum is shown in
Figure6. Quantitative elemental
analysis was also attempted with results shown in Table 1. We believe that the
excess amounts of Si and O measured by EDXS are from the Si/SiO2  substrates.
Since the films are thin (less than 100 nm) and the penetration depth of an
electron beam into a nonmetallic material is usually about 1 μm, the beam must be smeared. Although the quantitative compositional measurements
are probably not accurate due to the beam spreading effect, it can be concluded that these films consist of
oxides (or hydroxides) of Al, Si,
and Zr without significant impurities of other elements.Table 1
EDXS compositional analysis. Each
value is the average of three measurements.
The measured excessive amounts of Si and O are from Si/SiO2  substrates, an artifact due to the spread of electron beams.
ElementAtomic %As-depositedAnnealedO61.735.3Al3.61.2Si30.659.9Zr43.6Figure 6
A representative EDXS spectrum for an annealed film.XRD patterns of as-deposited and annealed multilayers are shown in Figure7, in which the d-spacings are
labeled for all peaks. As-deposited films appeared to be largely amorphous, but a series of low-angle peaks were observed. The
1.2 nm peak should result from hydrated montmorillonite, which usually exhibits the first peak (001) around 1.0–2.0 nm, depending
on the degree of hydration (e.g., relative humidity) and the types of interlayer cations [19]. The as-deposited films were
air-dried (i.e., the relative
humidity is unknown) and contain Zr4+ as interlayer cations.
Therefore, the montmorillonite in the air-dried films is likely to have a basal
spacing of 1.2 nm. The presence of this peak indicates that some multiple
layers of clay nanoplatelets are absorbed in dipping, which is not desired. The 5.7 nm peak represents the periodic thickness formed
by each deposition cycle for the multilayers without annealing. The origin of
the 2.3 nm peak is currently unclear and worth further investigation. It is
noteworthy that the 1.2 nm peak is the sharpest of all peak, as shown in
Figure 7(a).
Given the low crystallinity of clay minerals, all other broad peaks suggest that the as-deposited multilayers possess much
less ordered atomic structure or crystallinity, as expected. Nevertheless, a good
layered nanostructure is clearly present in the as-deposited film with a
periodic thickness of ~5.7 nm, as shown in Figure 7, indicating the success of the LBL deposition.XRD patterns of (a) as-deposited and
(b) annealed films formed by 20 deposition cycles and ZrAc4 cationic
solutions. The two XRD scans were carried out at the same conditions (scan rate
and slit size) and the patterns are shown in the same intensity scale. The
insets (which are shown in different intensity scales) are additional scans at
low angles with a smaller slit.
(a)(b)For annealed specimens
Figure7(b), only two peaks are observed. A
broad weak peak at 1.6 nm is probably caused by the (002) reflection of the
periodic thickness of 3.2 nm (see Section 3.3). The shape of the peak also indicates
that the periodicity of the layers is not perfect, which is probably caused by
the crystallization process during annealing. The low-angle scan did not find
the (001) reflection at 3.2 nm, and the reason is unclear at present. The
second peak at d = 0.3 nm matches ZrO2 (111) reflection, indicating the formation of nanocrystalline ZrO2. (It
is noted that a broad weak peak whose intensity varies with the specific
experimental conditions was sometimes observed around 30∘2θ, which is believed to be caused by
the Si/SiO2 substrates [14, 15]. This is also seen in the XRD
patterns of the as-deposited films, although a lower intensity was observed. Although
these two measurements were made in the same conditions, it is still uncertain
whether or not this peak at ~30∘⁢2θ is an artifact from the substrate). Assuming that this peak at ~30∘⁢2θ  is indeed from ZrO2 (111) reflection, the mean grain size of
the nanocrystalline ZrO2 was estimated to be ~9.7 nm based on the
peak width. Given that periodic thickness in the annealed film is approximately
3.2 nm (see Section 3.3), the ZrO2 crystallite size of 9.7 nm
indicates that the growth of nanocrystals may start to break the parallel
multilayer structure (by displacing or heaving up nanoclay platelets which
should act as barriers between two adjacent oxide layers), which is consistent
with the absence of or week low-angle reflections from the annealed multilayers.
Nonetheless, the existence of some layered structures is evident by SEM
observations on the cross-sectional surfaces, as shown in Figures 5(b)
and 5(c), but the
layered structure is probably less regular, presumably due to the growth of ZrO2 nanocrystals.It is generally expected that montmorillonites are chemically stable for heating
or annealing at600∘C
[20]. More annealing experiments
will be conducted to investigate how nanoclays react with Zr complexes during
high temperatures, which will help form high temperature endurable
nanoclay-oxide multilayers.It is worth emphasizing that the annealed films still remained layered
structure after cutting and no significant cracking or delaminating was
observed in Figure5, in spite of up to 50% linear shrinkage occurred during
annealing, as discussed below.


### 3.3. Film Growth Rate Through LBL Deposition

The thickness of as-deposited and annealed films versus
the number of deposition cycles prepared through ZrAc4 cationic
precursor solutions are shown in Figures 8(a)
and 8(b), respectively. As-deposited
multilayers are expected to exhibit low cohesion between layers, and part of
the multilayers might fall off when they were cut along with the substrate for
cross-sectional surface observations. This may be responsible for the measured
unusually small thickness values (i.e., the three open circles shown in
Figure 8(a)). If these data are excluded, the
measured film thickness for both as-deposited and annealed films increase
almost linearly with the number of deposition cycles. Through linear
regression, the average film growth rates of as-deposited and annealed multilayers
are ~6.8 nm and ~3.2 nm per cycle, respectively. In other words, annealing at
600∘C
for 2 hours caused a ~50% linear shrinkage of film thickness, which is nearly
the same as that previously observed for pure ZrO2 films [14, 15]. The shrinkage also indicates
dehydration and sintering have occurred during annealing. The periodic layer
spacing of the as-deposited multilayers was ~5.7 nm, as measured by XRD, as shown in
Figure 7, which should be more accurate than the value (~6.8 nm) obtained by SEM
measurement. However, the XRD and SEM measurements (5.7 nm versus 6.8 nm) are
rather consistent.Thickness versus the number of deposition cycles for
(a) as-deposited and (b) annealed films made by ZrAc4 precursor solutions. In
(a), it is
assumed that open circles represent measurement artifacts (as-deposited films
where the part of layered films peeled off during cleaving of the substrates). Error
bars represent the data range. Lines represent the best linear fits.
(a)(b)


### 3.4. Issues, Future Studies, and Potential Applications

As discussed above, the preliminary results are promising. However, the
combined XRD, SEM, and EDXS characterization indicates that the multilayers synthesized
in this study are not yet perfect, and there are some deviations from the
targeted nanostructure shown in Figure1(b).
First, the 1.2 nm peak in XRD patterns in  Figure 7(b) indicates that
nanoclays are not completely exfoliated into single 2 : 1 layers and multiple
layers of clay nanoplatelets are deposited within the films. This can be
further improved by fine tuning the clay suspension chemistry, exfoliation
method, and dipping process. Second, while as-deposited films exhibit fairly
good layered structure, the thermally annealed films appear to possess relatively
poor periodicity, as evident by both SEM and XRD. Annealing at 600∘C for
2 hours in air was the only dehydration process that has been tested so far
(since this study focused more on the feasibility of the LBL deposition). Further
optimization of the annealing procedure will be conducted to identify optimal conditions based on a trade-off between dehydration and the structure
integrity. Hydrothermal annealing [13] may be an alternative method
to crystallize the oxides while maintaining the stability of clays and
multilayered structure.Nonetheless, this study has
clearly demonstrated the basic feasibility of this new LBL technique for making
this new class of nanostructured nanoclay-oxide multilayers. Further research
should be conducted to finely tune the thickness and composition of each individual
oxide layers (i.e., oxides other than ZrO2) by changing the
deposition parameters and precursor solutions, which offers significant versatility for making nanostructured films with
diverse functionalities and applications.The clay-oxide multilayers are expected to have many potential
applications, which include but are not limited to coatings and free-standing
membranes (made by using sacrificial substrates) for MEMS/NEMS and sensors. This
is, in part, owing to the unique-layered nanostructure, as demonstrated by the
organic-inorganic hybrid counterparts—nanoclay-polymer multilayers. In particular, a
purely inorganic nanostructured composite consisting of zirconia and nanoclays
has high thermal resistance (e.g., stable up to800∘C) and high hardness and is
chemically inert. Thus, it can be used as protective and hard coatings. In
addition to the high hardness of nanocrystalline ZrO2, the presence
of platy nanoclay layer within the structure increases fracture toughness by
deflecting cracks, rendering even higher hardness of the multilayers. The high
lateral bond strength of nanoclays ensures the integrity of free-standing films
and membranes, and hence enhances the dynamic or vibrational performance. Potential
applications in cantilever sensing components can be envisioned. Finally, it is
expected that this purely inorganic nanostructured composite consisting of
zirconia and nanoclays exhibits much higher thermal and chemical stability than
their hybrid counterparts.


## 3.1. Clay-Polymer Multilayers

SEM images of the planar and cross-sectional surfaces of the as-deposited
nanoclay-polymer multilayers [(PEI-PSS-PEI)-(MMT-PEI)8] are shown in
Figure 2. Some unevenly piled or protruded MMT platelets can be observed in the
planar surfaces, as shown in Figure 2(a). The cross-sectional surface, as shown in
Figure 2(b), exhibits nacre-like layered structure,
which is similar to that observed in the films developed by Tang et al. [18]. The average measured film thickness is 40±9 nm, of which 10 nm is from the precursor polymer film [17]. Thus the thickness of a
single (MMT-PEI)1  layer formed through a complete deposition cycle
is approximately ~3.75 nm, which is consistent with the previously reported
value of 3.3 nm per deposition cycle [17]. It was postulated that the
highly branched polycation PEI works as “electrostatic glue” [17] to assemble clay platelets. In
conclusion, polymer-nanoclay multilayers with similar nanostructure were
reproduced in our laboratory. This verified our equipment and procedures and
enabled further exploration of synthesizing the new nanoclay-oxide
multilayers.(a) Planar surface and (b) cross-sectional
surface SEM images of as-deposited nanoclay-polymer [(PEI-PSS-PEI)-(nanoclay-PEI)8]  films.
(a)(b)


## 3.2. Clay-Zirconia Multilayers

The SEM images of as-deposited films prepared by ZrCl4  and
ZrAc4  cationic precursor solutions are shown in
Figures 3 and  4, respectively. The as-deposited films were dried in air, thus either
nanoclays or hydrated Zr cations are not completely dehydrated, and water molecules
exist within the interface of clay nanoplatelets and Zr cations. Both films
appear to be reasonably uniform and crack-free. Furthermore, they both exhibit
observable layered structures in cross-sectional surfaces under SEM
(Figures 3(b)
and 4(b)).(a) Planar surface and (b) cross-sectional surface SEM images
of as-deposited nanoclay-zirconia multilayers made via ZrCl4  cationic solution.
(a)(b)(a) Planar surface and (b) cross-sectional surface SEM images
of as-deposited nanoclay-zirconia multilayers made by ZrAc4 cationic
solutions.
(a)(b)Evidently, the films made by ZrAc4  cationic precursor
solutions have significantly smoother surface and more regularly (parallel)
layered structure than those by ZrCl4  precursor solutions
(Figure 3 versus
Figure 4). The clay nanoplatelets are better oriented in the films prepared by
ZrAc4  cationic solutions, while peeled and protruded edges of clay
platelets are frequently observed in the ZrCl4  prepared films, as shown in Figure
3(a). For films prepared by ZrCl4  precursor solutions, the layered
structure is less close-packed with occasional, sparsely distributed open voids, as shown in
Figure 3(b). The nanoclay-oxide multilayers prepared using ZrAc4  precursor
solutions also appear to have lower surface roughness than those
polymer-nanoclay multilayers prepared in this and prior [17] studies. Based on these
results, ZrAc4  cationic precursor solutions are used exclusively to
make multilayers in further studies.SEM images of the planar and cross-sectional surfaces of annealed films
are shown in Figure5. The annealed films appear to be denser and more uniform than
the as-deposited ones. Some indications of the layered structure can be observed
in the cross-sectional surfaces under SEM, as shown in Figures 5(a)
and 5(b). It should be noted that the observed
cross-sectional surfaces were influenced by the actual (somewhat random)
cleaving process and hence the layered structure is not always clearly visible.
The surfaces of annealed films, as shown in Figure 5(c) are also smoother than those of
as-deposited films, as shown in Figure 4(a); yet, some clay nanoplatelets can still be
seen in Figure 5(c).(a),(b) cross-sectional surface, and (c) planar surface SEM
images of annealed nanoclay-zirconia films made by ZrAc4  precursor solutions with 25
deposition cycles.
(a)(b)(c)EDXS analysis of as-deposited and annealed films found the presence of Al, Si, and
Zr, without other metal impurities. A representative EDXS spectrum is shown in
Figure6. Quantitative elemental
analysis was also attempted with results shown in Table 1. We believe that the
excess amounts of Si and O measured by EDXS are from the Si/SiO2  substrates.
Since the films are thin (less than 100 nm) and the penetration depth of an
electron beam into a nonmetallic material is usually about 1 μm, the beam must be smeared. Although the quantitative compositional measurements
are probably not accurate due to the beam spreading effect, it can be concluded that these films consist of
oxides (or hydroxides) of Al, Si,
and Zr without significant impurities of other elements.Table 1
EDXS compositional analysis. Each
value is the average of three measurements.
The measured excessive amounts of Si and O are from Si/SiO2  substrates, an artifact due to the spread of electron beams.
ElementAtomic %As-depositedAnnealedO61.735.3Al3.61.2Si30.659.9Zr43.6Figure 6
A representative EDXS spectrum for an annealed film.XRD patterns of as-deposited and annealed multilayers are shown in Figure7, in which the d-spacings are
labeled for all peaks. As-deposited films appeared to be largely amorphous, but a series of low-angle peaks were observed. The
1.2 nm peak should result from hydrated montmorillonite, which usually exhibits the first peak (001) around 1.0–2.0 nm, depending
on the degree of hydration (e.g., relative humidity) and the types of interlayer cations [19]. The as-deposited films were
air-dried (i.e., the relative
humidity is unknown) and contain Zr4+ as interlayer cations.
Therefore, the montmorillonite in the air-dried films is likely to have a basal
spacing of 1.2 nm. The presence of this peak indicates that some multiple
layers of clay nanoplatelets are absorbed in dipping, which is not desired. The 5.7 nm peak represents the periodic thickness formed
by each deposition cycle for the multilayers without annealing. The origin of
the 2.3 nm peak is currently unclear and worth further investigation. It is
noteworthy that the 1.2 nm peak is the sharpest of all peak, as shown in
Figure 7(a).
Given the low crystallinity of clay minerals, all other broad peaks suggest that the as-deposited multilayers possess much
less ordered atomic structure or crystallinity, as expected. Nevertheless, a good
layered nanostructure is clearly present in the as-deposited film with a
periodic thickness of ~5.7 nm, as shown in Figure 7, indicating the success of the LBL deposition.XRD patterns of (a) as-deposited and
(b) annealed films formed by 20 deposition cycles and ZrAc4 cationic
solutions. The two XRD scans were carried out at the same conditions (scan rate
and slit size) and the patterns are shown in the same intensity scale. The
insets (which are shown in different intensity scales) are additional scans at
low angles with a smaller slit.
(a)(b)For annealed specimens
Figure7(b), only two peaks are observed. A
broad weak peak at 1.6 nm is probably caused by the (002) reflection of the
periodic thickness of 3.2 nm (see Section 3.3). The shape of the peak also indicates
that the periodicity of the layers is not perfect, which is probably caused by
the crystallization process during annealing. The low-angle scan did not find
the (001) reflection at 3.2 nm, and the reason is unclear at present. The
second peak at d = 0.3 nm matches ZrO2 (111) reflection, indicating the formation of nanocrystalline ZrO2. (It
is noted that a broad weak peak whose intensity varies with the specific
experimental conditions was sometimes observed around 30∘2θ, which is believed to be caused by
the Si/SiO2 substrates [14, 15]. This is also seen in the XRD
patterns of the as-deposited films, although a lower intensity was observed. Although
these two measurements were made in the same conditions, it is still uncertain
whether or not this peak at ~30∘⁢2θ is an artifact from the substrate). Assuming that this peak at ~30∘⁢2θ  is indeed from ZrO2 (111) reflection, the mean grain size of
the nanocrystalline ZrO2 was estimated to be ~9.7 nm based on the
peak width. Given that periodic thickness in the annealed film is approximately
3.2 nm (see Section 3.3), the ZrO2 crystallite size of 9.7 nm
indicates that the growth of nanocrystals may start to break the parallel
multilayer structure (by displacing or heaving up nanoclay platelets which
should act as barriers between two adjacent oxide layers), which is consistent
with the absence of or week low-angle reflections from the annealed multilayers.
Nonetheless, the existence of some layered structures is evident by SEM
observations on the cross-sectional surfaces, as shown in Figures 5(b)
and 5(c), but the
layered structure is probably less regular, presumably due to the growth of ZrO2 nanocrystals.It is generally expected that montmorillonites are chemically stable for heating
or annealing at600∘C
[20]. More annealing experiments
will be conducted to investigate how nanoclays react with Zr complexes during
high temperatures, which will help form high temperature endurable
nanoclay-oxide multilayers.It is worth emphasizing that the annealed films still remained layered
structure after cutting and no significant cracking or delaminating was
observed in Figure5, in spite of up to 50% linear shrinkage occurred during
annealing, as discussed below.


## 3.3. Film Growth Rate Through LBL Deposition

The thickness of as-deposited and annealed films versus
the number of deposition cycles prepared through ZrAc4 cationic
precursor solutions are shown in Figures 8(a)
and 8(b), respectively. As-deposited
multilayers are expected to exhibit low cohesion between layers, and part of
the multilayers might fall off when they were cut along with the substrate for
cross-sectional surface observations. This may be responsible for the measured
unusually small thickness values (i.e., the three open circles shown in
Figure 8(a)). If these data are excluded, the
measured film thickness for both as-deposited and annealed films increase
almost linearly with the number of deposition cycles. Through linear
regression, the average film growth rates of as-deposited and annealed multilayers
are ~6.8 nm and ~3.2 nm per cycle, respectively. In other words, annealing at
600∘C
for 2 hours caused a ~50% linear shrinkage of film thickness, which is nearly
the same as that previously observed for pure ZrO2 films [14, 15]. The shrinkage also indicates
dehydration and sintering have occurred during annealing. The periodic layer
spacing of the as-deposited multilayers was ~5.7 nm, as measured by XRD, as shown in
Figure 7, which should be more accurate than the value (~6.8 nm) obtained by SEM
measurement. However, the XRD and SEM measurements (5.7 nm versus 6.8 nm) are
rather consistent.Thickness versus the number of deposition cycles for
(a) as-deposited and (b) annealed films made by ZrAc4 precursor solutions. In
(a), it is
assumed that open circles represent measurement artifacts (as-deposited films
where the part of layered films peeled off during cleaving of the substrates). Error
bars represent the data range. Lines represent the best linear fits.
(a)(b)


## 3.4. Issues, Future Studies, and Potential Applications

As discussed above, the preliminary results are promising. However, the
combined XRD, SEM, and EDXS characterization indicates that the multilayers synthesized
in this study are not yet perfect, and there are some deviations from the
targeted nanostructure shown in Figure1(b).
First, the 1.2 nm peak in XRD patterns in  Figure 7(b) indicates that
nanoclays are not completely exfoliated into single 2 : 1 layers and multiple
layers of clay nanoplatelets are deposited within the films. This can be
further improved by fine tuning the clay suspension chemistry, exfoliation
method, and dipping process. Second, while as-deposited films exhibit fairly
good layered structure, the thermally annealed films appear to possess relatively
poor periodicity, as evident by both SEM and XRD. Annealing at 600∘C for
2 hours in air was the only dehydration process that has been tested so far
(since this study focused more on the feasibility of the LBL deposition). Further
optimization of the annealing procedure will be conducted to identify optimal conditions based on a trade-off between dehydration and the structure
integrity. Hydrothermal annealing [13] may be an alternative method
to crystallize the oxides while maintaining the stability of clays and
multilayered structure.Nonetheless, this study has
clearly demonstrated the basic feasibility of this new LBL technique for making
this new class of nanostructured nanoclay-oxide multilayers. Further research
should be conducted to finely tune the thickness and composition of each individual
oxide layers (i.e., oxides other than ZrO2) by changing the
deposition parameters and precursor solutions, which offers significant versatility for making nanostructured films with
diverse functionalities and applications.The clay-oxide multilayers are expected to have many potential
applications, which include but are not limited to coatings and free-standing
membranes (made by using sacrificial substrates) for MEMS/NEMS and sensors. This
is, in part, owing to the unique-layered nanostructure, as demonstrated by the
organic-inorganic hybrid counterparts—nanoclay-polymer multilayers. In particular, a
purely inorganic nanostructured composite consisting of zirconia and nanoclays
has high thermal resistance (e.g., stable up to800∘C) and high hardness and is
chemically inert. Thus, it can be used as protective and hard coatings. In
addition to the high hardness of nanocrystalline ZrO2, the presence
of platy nanoclay layer within the structure increases fracture toughness by
deflecting cracks, rendering even higher hardness of the multilayers. The high
lateral bond strength of nanoclays ensures the integrity of free-standing films
and membranes, and hence enhances the dynamic or vibrational performance. Potential
applications in cantilever sensing components can be envisioned. Finally, it is
expected that this purely inorganic nanostructured composite consisting of
zirconia and nanoclays exhibits much higher thermal and chemical stability than
their hybrid counterparts.


## 4. Conclusions

A novel layer-by-layer deposition technique has been developed for
manufacturing nanoclay-zirconia multilayers by sequentially dipping a substrate
in an exfoliated nanoclay suspension and a zirconia cationic precursor solution
followed by annealing at600∘C for 2 hours.
Nanoscale layer-by-layer growth is achievable, as evidenced by a linear
film growth rate per deposition cycle measured on the cross-sectional surfaces
under SEM and by X-ray diffraction. It is
found that ZrAc4  is a better cationic precursor solution for growing
more uniformly layered films. The growth rate of as-deposited films is ~5.7 nm
per cycle; the growth rate can be further tuned by changing the deposition
parameters and dipping settings. Denser dehydrated films form after annealing
at 600∘C
for 2 hours in air and the film thickness shrinks by ~50%; yet, the annealed
films remain uniform and crack-free, and still possess nanoscale layered structure (although less clearly observable
under SEM). In summary, the
feasibility of the new LBL deposition technique has been clearly demonstrated, but the postdeposition dehydration and annealing processing
should be further optimized.The success of this study enables further endeavors to make a variety of
novel oxide-based nanostructured films. This LBL deposition technique can be
conveniently adapted for making periodic multilayers or functionally graded
oxide films via changing the composition or concentration of oxide cationic
precursor solutions periodically or continuously. Nanoclays with permanent
surface charges are used to further assist electrostatic assembling, serve as
diffusion barriers for functionally graded films, and incorporate other
functional materials. It is expected that such purely inorganic nanostructured
thin films exhibit unique mechanical properties, permeability, and corrosion
resistance. Further research to optimize the synthesis methods (particularly
the postdeposition dehydration process) and investigate their mechanical and
physical properties is currently in progress.



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*Source: 749508-2008-03-30.xml*