Three-dimensional assemblies of zirconia nanocrystals via shape-preserving reactive conversion of diatom microshells

Article abstract of DOI:10.1111/j.1551-2916.2005.00734.x

The synthesis of three-dimensional (3-D) assemblies of zirconia nanocrystals via the shape-preserving reactive conversion of biologically reproducible, silica-based microtemplates (diatom microshells) is demonstrated for the first time. Silica diatom microshells were first converted into magnesia replicas via an oxidation-reduction displacement reaction with magnesium gas. The magnesia replicas were then converted into zirconia replicas via a metathetic displacement reaction with zirconium tetrachloride gas. By utilizing both types of shape-preserving displacement reactions, bioclastic silica microtemplates may be converted into functional nanocrystalline assemblies with a variety of complex, but well-controlled 3-D shapes and chemistries for catalytic/chemical, biological, electrical, thermal, mechanical, and other applications.

Full text of DOI:10.1111/j.1551-2916.2005.00734.x

J. Am. Ceram. Soc., 89 [2] 694–698 (2006)
DOI: 10.1111/j.1551-2916.2005.00734.x
r 2005 The American Ceramic Society
ournal
J
Three-Dimensional Assemblies of Zirconia Nanocrystals Via
Shape-Preserving Reactive Conversion of Diatom Microshells
,w
Samuel Shian,* Ye Cai, Michael R. Weatherspoon, Shawn M. Allan, and Kenneth H. Sandhage**
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
3
1–34
The synthesis of three-dimensional (3-D) assemblies of zirconia
nanocrystals via the shape-preserving reactive conversion of
biologically reproducible, silica-based microtemplates (diatom
microshells) is demonstrated for the first time. Silica diatom
microshells were first converted into magnesia replicas via an
oxidation–reduction displacement reaction with magnesium gas.
The magnesia replicas were then converted into zirconia replicas
via a metathetic displacement reaction with zirconium tetra-
chloride gas. By utilizing both types of shape-preserving dis-
placement reactions, bioclastic silica microtemplates may be
converted into functional nanocrystalline assemblies with a va-
riety of complex, but well-controlled 3-D shapes and chemistries
for catalytic/chemical, biological, electrical, thermal, mechani-
cal, and other applications.
of tens to hundreds of nanometers).
A given frustule mor-
phology is replicated with a high degree of fidelity upon diatom
reproduction. As a result, sustained culturing of a particular di-
atom species can yield enormous numbers of cells with similarly
shaped frustules (e.g., more than one trillion copies of a parent
3
4,35
cell could be generated after 40 reproduction cycles).
Such
massively parallel and genetically precise (species–specific) as-
sembly of complex 3-D microstructures under ambient (marine
or freshwater) conditions greatly exceeds the capabilities of cur-
rent synthetic (man-made) microfabrication processes.
The estimated 10 –10 extant diatom species provide a diverse
selection of frustule morphologies for use in various applica-
4
5
3
1,36,37
tions.
The range of potential applications would be sig-
nificantly enhanced if such frustules could be converted into
other compositions without a loss of the frustule shape. Recent
work has demonstrated that zirconia frustule replicas may be
generated by coating silica frustules with a thin, sol–gel-derived
film and then removing the underlying silica through selective
I. Introduction
3
8
dissolution. While such a sol–gel approach may be used to
produce frustule replicas with a variety of chemistries, alkoxide-
based precursors tend to be relatively expensive. The purpose of
this paper is to demonstrate that 3-D silica-based diatom
frustules may be converted into nanocrystalline zirconia repli-
cas through a series of shape-preserving gas/solid displacement
reactions with lower-cost reactants.
HE attractive chemical, electrical, thermal, optical, and me-
chanical properties of zirconia-based ceramics have led to
T
the use of these materials in a wide range of applications, in-
cluding gas sensors and gas separators, fuel cells, catalysts and
catalyst supports, thermal protection coatings, refractories, op-
tical filters, cutting tools and milling media, biomedical implants,
1
–11
and high-temperature electrodes and heating elements.
Microscale zirconia-based structures with intricate three-dimen-
sional (3-D) shapes and fine (submicrometer) features would be
very attractive for a number of these and other applications.
5
,12–
1
8
II. Experimental Procedure
Ceramic microfabrication processes based on the buildup of
-D patterned layers (e.g., micro-stereolithography, ink jet print-
ing, co-firing of stacked ceramic tapes) have been used to gen-
2
Experiments aimed at the direct conversion of silica-based
diatom frustules into zirconium-bearing replicas were conduct-
ed by reacting the frustules with zirconium tetrafluoride vapor.
Cylindrical Aulacoseira diatom frustules (obtained in the form
erate 3-D structures with feature sizes on the order of 100 mm or
1
9–23
larger. However, the direct assembly of complex-shaped
-D zirconia microstructures with finer features in large quan-
3
tities at low cost remains a significant challenge.
of diatomaceous earth) and ZrF
Research Chemicals Inc., Catoosa, OK) were placed in two sep-
arate nickel crucibles. The molar ZrF :SiO ratio within the
4
powder (99% pure, Advanced
Numerous examples of 3-D assembly of mineralized struc-
Certain micro-organisms are
4
2
2
4–30
tures can be found in nature.
crucibles was varied from 0.36:1 to 3.6:1 in different experi-
ments. The crucibles were loaded inside a nickel alloy tube (5.1
cm diameter, 20.3 cm long, Inconel Alloy 600, CG Metals,
Duluth, GA) that was then welded shut within an argon atmos-
phere glovebox. The sealed ampoules were then heated at 51C/
min to a fixed temperature in the range of 2501–8001C and held
at this temperature for 2 h. After cooling to room temperature,
the sealed tubes were cut open and the reacted specimens were
removed.
adept at synthesizing oxide microstructures with complex shapes
and with fine (submicrometer) features. A spectacular range of
intricate 3-D structures are generated by aquatic micro-algae
3
0–34
known as diatoms (order Bacillariophyceae).
Diatoms are
single-celled eukaryotic micro-organisms that form rigid cell
3
1–34
walls (frustules) comprised of amorphous silica.
Each dia-
tom species assembles a frustule with a particular 3-D shape that
is decorated with specific patterns of fine features (e.g., pores,
ridges, protuberances with characteristic dimensions in the range
A series of displacement reactions was also examined for
converting silica-based frustules into zirconia replicas. An oxi-
dation–reduction reaction with magnesium gas was first used to
convert the silica frustules into magnesia-based replicas. A meta-
thetic reaction with zirconium tetrachloride gas was then used to
convert the magnesia replicas into zirconia. Magnesia-based
replicas of Aulacoseira frustules were generated by reaction
with Mg(g) at 9001C for 1.5 h in a manner similar to that
M. Firestone—contributing editor
Manuscript No. 20753. Received July 7, 2005; approved August 16, 2005.
Supported by the Air Force Office of Scientific Research (Hugh De Long, Joan Fuller,
Program Managers).
3
9,40
*Member, American Ceramic Society.
described elsewhere.
The MgO-based replicas were then
*
*Fellow, American Ceramic Society.
Author to whom correspondence should be addressed: e-mail: sandhage@mse.
w
exposed to a 0.49M NaOH solution at 601C for 3 h to selec-
tively dissolve residual silicon from the magnesia. MgO diatom
gatech.edu
6
94

February 2006
Communications of the American Ceramic Society
695
replicas and anhydrous ZrCl powder (99.9% pure, Advanced
4
Research Chemicals Inc.) were loaded into two separate nickel
crucibles. The molar ratio of ZrCl :MgO placed within the cru-
4
cibles was chosen to be 0.52:1, 0.86:1, or 1.2:1. The two crucibles
were then placed inside a nickel alloy tube (5.1 cm diameter, 20.3
cm long, Inconel Alloy 600, CG Metals) that was welded shut
within an argon atmosphere glovebox. The sealed ampoules
were then heated at 51C/min to 6501C and held at this temper-
ature for 2 h. After cooling to room temperature, the sealed
tubes were cut open and the reacted diatom frustules were with-
drawn. The reacted frustules were then immersed and stirred
(100 rpm) for 1 h in de-ionized water heated to 901C. The
frustules were then removed from the water using vacuum-
assisted filtration.
III. Results and Discussion
Initial experiments were conducted to directly convert cylindri-
cal Aulacoseira diatom frustules, of the type shown in Fig. 1(a),
into zirconium oxyfluoride replicas via a metathetic reaction
4
with ZrF gas (in a manner analogous to the conversion of di-
4
atom frustules into titanium oxyfluoride ). For reaction tem-
1
peratures at or below 3501C, no reaction was detected within 2
h. At higher temperatures, and for all starting molar ZrF :SiO
4 2
ratios examined, the silica frustules were found to disintegrate.
A partially disintegrated frustule is shown in Fig. 1(b). Rela-
tively large, faceted crystallites of the type shown in Fig. 1(c)
were observed to form away from the surfaces of these disinte-
grating frustules. Energy-dispersive X-ray (EDX) analyses
Fig. 1. Secondary electron images of: (a) a starting, cylinder-shaped
Aulacoseira diatom frustule; (b) a partially disintegrated frustule after
exposure to ZrF (g) for 2 h at 6001C (molar ZrF :SiO reactant ra-
tio 5 0.36:1); and (c) faceted Zr–O–F-bearing crystals formed near the
partially disintegrated frustule shown in (b). (d) Energy-dispersive X-ray
analysis of a crystal obtained at the position X in (c).
(
and O, which was consistent with a zirconium oxyfluoride com-
Fig. 1(d)) indicated that these crystallites contained Zr, F,
4
4
2
42
pound. The frustule disintegration, and concurrent formation
of Zr–O–F-bearing crystals away from the frustule surfaces,
suggested that the silica had reacted to form an oxygen-bearing
vapor species (e.g., Si OF
2
from the frustule surface and then reacted further with ZrF
6
(g), or SiOF
2
(g)) that migrated away
(g)
4
vapor required for reaction (2) to proceed spontaneously at
to form the Zr–O–F-bearing crystals. A similar type of reactive
frustule disintegration, and faceted Ti–O–F crystal formation
away from the frustules, was observed upon exposure of diatom
6
501C (assuming unit activities for pure condensed phase reac-
ꢀ2
tants and products) is only 4.3 ꢁ 10 Torr, which is much
smaller than the pressure of ZrCl
densed ZrCl at this temperature (the sublimation temperature
of ZrCl is only 3361C).
4
vapor generated from con-
silica frustules to TiF
4
(g) at temperatures in the range of 5001–
41
4
7
001C and at TiF :SiO molar ratios in excess of 4.9:1.
43,44
4
2
Upon cooling to room temperature,
product of this reaction may be selectively removed
from the ZrO product by dissolution in water. Indeed, zirco-
4
Although the exact nature of such reactive frustule evaporation
in the presence of ZrF (g) was not evaluated, the occurrence of
the MgCl
2
4
2
such disintegration under all of the reaction conditions exam-
ined indicated that a different reaction path was needed in order
to generate zirconia frustule replicas.
nium chloride was preferred over zirconium fluoride as a gase-
ous reactant because the solubility of magnesium chloride in
water is much higher than for magnesium fluoride.
45
An alternative approach was adopted that avoided the direct
exposure of silica frustules to a zirconium halide reactant. A se-
ries of displacement reactions was used to convert the silica
frustules first into magnesia-based replicas (via reaction with
magnesium gas) and then into zirconia replicas (via reaction of
the magnesia with zirconium tetrachloride gas). Prior work has
shown that diatom frustules can be converted into magnesia-
based replicas via the following net oxidation–reduction dis-
A secondary electron image of a magnesia replica of an
Aulacoseira frustule is shown in Fig. 2(a). The shape of the
starting SiO -based frustule (Fig. 1(a)) was preserved after con-
2
version into MgO. Secondary electron images obtained after re-
action of the magnesia with zirconium chloride gas at 6501C for
2
h are shown in Figs. 2(b) and (c). EDX analysis obtained at the
position X in Fig. 2(b) is shown in Fig. 2(d). The reacted
frustules were embedded in a matrix phase that was Mg and
Cl rich, which was consistent with a solidified MgCl -rich liquid
3
9,40
placement reaction
SiO2 þ 2MgðgÞ ! 2MgO þ fSig
where {Si} refers to silicon dissolved within a Mg–Si liquid
:
2
(
note: while the melting point of MgCl is 7141C, a eutectic liq-
2
46
(1)
2 4
uid can form in the MgCl –ZrCl system at only 4261C ). The
Mg–Cl-rich matrix phase could be selectively removed from the
reacted frustules by dissolution in water. Secondary electron
images of isolated frustules exposed after this dissolution treat-
ment are shown in Figs. 3(a)–(c). EDX analysis of such an iso-
lated frustule is shown in Fig. 3(d). The absence of a magnesium
peak (at 1.25 keV) in the EDX pattern indicated that the
frustules had been fully converted into zirconia within 2 h at
(
excess magnesium vapor) that tended to migrate away from the
generated by the continued reaction of elemental silicon with
3
9,40
MgO-based frustule replica.
Magnesia replicas generated by
reaction (1) may then be exposed to zirconium tetrachloride gas
to form zirconia by the following metathetic displacement reac-
tion:
6501C. The cylindrical frustule shape, and the protruding rim
and open hole at the end of the frustule, were preserved after
conversion into zirconia.
X-ray diffraction (XRD) analyses obtained from the zirconia-
converted frustules are shown in Fig. 4. Diffraction peaks for the
monoclinic and tetragonal zirconia polymorphs were detected,
whereas diffraction peaks for magnesium oxide were absent
2
MgO þ ZrCl4ðgÞ ! ZrO2 þ 2MgCl2
(2)
This net reaction is thermodynamically favored. The standard
Gibbs-free energy change per mole of reaction (2) at 6501C is
75.0 kJ/mol.
4
3,44
ꢀ
The equilibrium partial pressure of ZrCl
4

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Communications of the American Ceramic Society
Vol. 89, No. 2
Fig. 3. (a, b) Secondary electron (SE) images of nanocrystalline ZrO
replicas of Aulacoseira diatom frustules generated by the reaction of
MgO frustule replicas with ZrCl (g) at 6501C for 2 h (molar ZrCl :MgO
2
4
4
reactant ratio 5 0.52:1) and then immersion in de-ionized water at 901C
for 1 h. (c) Higher magnification SE image of the region indicated by the
rectangle shown in (b). (d) Energy-dispersive X-ray analysis of a nano-
Fig. 2. Secondary electron images of: (a) a MgO-based frustule replica
after exposure to Mg(g) at 9001C for 1.5 h and then immersion in a
0
.49M NaOH solution at 601C for 3 h; (b), (c) the products of reaction
between MgO diatom replicas and ZrCl (g) at 6501C for 2 h (molar
ZrCl :MgO reactant ratio5 0.86:1); and (d) energy-dispersive X-ray
analysis obtained at the position X in (b).
crystalline ZrO replica.
2
4
4
reactant ratio of 1.2:1 (Fig. 5(c)), although a higher concentra-
tion of the larger crystals was detected. The enhanced formation
of relatively large zirconia crystallites, and of monoclinic
(
1
Powder Diffraction File, Card No. 45–946 for MgO, No. 37–
484 for ZrO (monoclinic), No. 50–1089 for ZrO (tetragonal),
International Center on Diffraction Data, Newtown Square,
PA). As the molar ZrCl :MgO reactant ratio was increased from
.52:1 (near the stoichiometry of reaction (2)) to 1.2:1 (a signif-
2
2
zirconia, at 6501C with an increase in the amount of the ZrCl
4
reactant was consistent with prior reports on the effect of chlo-
rine on the tetragonal-to-monoclinic zirconia transformation at
4
4
8,49
48
Karapetrova et al. reported
0
temperatures below 10001C.
icant excess of ZrCl ), the relative amount of monoclinic zir-
4
conia generated upon reaction at 6501C for 2 h increased at the
expense of tetragonal zirconia.
that zirconia specimens with relatively high initial chlorine con-
tents (up to several percent, synthesized from oxychloride pre-
cursors) exhibited rapid grain growth between 6001 and 8001C,
and transformed more rapidly to monoclinic zirconia over this
temperature range, than specimens containing little initial chlo-
rine (on the order of 200 ppm, synthesized from nitrate precur-
Transmission electron micrographs of cross-sections of
zirconia-converted frustules generated with varied molar
ZrCl :MgO reactant ratios are shown in Figs. 5(a)–(c). Corre-
4
4
8
49
sponding selected area electron diffraction (SAED) patterns ob-
tained from these cross-sections are presented in Figs. 5(d)–(f).
Consistent with XRD analyses, the SAED analyses indicated an
increase in the amount of monoclinic zirconia relative to te-
tragonal zirconia with an increase in the starting molar
sors). Pasquevich et al. found that the growth rate of isolated
crystallites, and the tetragonal-to-monoclinic transformation,
ZrCl
also observed to change with an increase in this reactant ratio.
The zirconia crystallites formed with a ZrCl :MgO molar ratio
4
:MgO reactant ratio. The crystallite size distribution was
4
of 0.52:1 (Fig. 5(a)) were relatively small (B40 nm) and uniform
in size. However, for a ZrCl :MgO molar ratio of 0.86:1
4
(
Fig. 5(b)), the zirconia crystallites exhibited a bimodal size dis-
tribution; that is, a population of considerably larger crystallites
several hundreds of nanometers in size) was detected along with
(
the finer crystals. This bimodal crystallite size distribution was
consistent with XRD analyses of the average tetragonal and
monoclinic zirconia grain sizes. The widths of major, non-over-
lapped diffraction peaks for monoclinic zirconia (the (ꢀ111) and
(
111) peaks near 28.21 and 31.51, respectively, in Fig. 4) and te-
tragonal zirconia (the (011) peak near 30.31 in Fig. 4) were
measured at half of the maximum intensity values and these
4
7
values were inserted into the Scherrer equation to calculate the
average crystallite sizes for the tetragonal and monoclinic zir-
Fig. 4. X-ray diffraction (XRD) analyses of the ZrO
generated by the reaction of MgO-based replicas with ZrCl
for 2 h and then immersion in de-ionized water at 901C for 1 h. The three
XRD patterns are associated with different molar ZrCl :MgO reactant
ratios.
2
frustule replicas
(g) at 6501C
4
conia phases. For specimens with a ZrCl :MgO reactant ratio of
4
0.86:1, the average crystallite sizes for tetragonal and monoclinic
zirconia were 48 and 380 nm, respectively. A similar bimodal
crystallite size distribution was detected for a molar ZrCl :MgO
4
4

February 2006
Communications of the American Ceramic Society
697
ceramic compounds, with well-controlled 3-D morphologies
for a variety of catalytic/chemical, biological, electrical, ther-
mal, mechanical, or other applications.
IV. Conclusions
The conversion of silica-based diatom frustules into nanocrys-
talline zirconia replicas, via the use of gas/solid displacement
reactions, has been demonstrated for the first time. Initial ex-
periments involving the reaction of the silica frustules with zir-
conium tetrafluoride gas resulted in disintegration of the
microshells by a reactive evaporation process. Such reactive dis-
integration was avoided by converting the silica frustules into
magnesia replicas prior to reaction with a zirconium halide gas
(
thereby avoiding the direct exposure of the silica frustules to a
zirconium halide gas reactant). Magnesia-based replicas were
generated via an oxidation–reduction displacement reaction be-
tween cylinder-shaped Aulacoseira diatom frustules and magne-
sium gas at 9001C. A metathetic displacement reaction between
the magnesia replicas and zirconium tetrachloride gas at 6501C
then yielded zirconia replicas embedded in a magnesium chlo-
ride-rich matrix. Selective dissolution of the magnesium chloride
in water yielded freestanding, nanocrystalline assemblies of zir-
conia that retained the starting cylindrical Aulacoseira frustule
shape. This serial displacement reaction process may be used to
generate microscale zirconia assemblies with a wide variety of
other controlled 3-D shapes available among the tens of thou-
sands of species of extant diatoms and other silica-forming mi-
croorganisms. By utilizing self-replicating microshells as
templates, large numbers of nano-crystalline zirconia micro-
assemblies with similar, specific 3-D shapes may be produced
for use as controlled-shape powders, reinforcement phases, or
microdevice components for a host of applications.
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