# Biologically Inspired Synthesis Route to Three-Dimensionally Structured Inorganic Thin Films

Authors: Birgit Schwenzer; Daniel E. Morse
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Inorganic thin films (hydroxide, oxide, and phosphate materials) that are textured on a submicron scale have been prepared from aqueous metal salt solutions at room temperature using vapor-diffusion catalysis. This generic synthesis approach mimics the essential advantages of the catalytic and structure-directing mechanisms observed for the formation of silica skeletons of marine sponges. Chemical composition, crystallinity, and the three-dimensional morphology of films prepared by this method are extremely sensitive to changes in the synthesis conditions, such as concentrations, reaction times, and the presence and nature of substrate materials. Focusing on different materials systems, the reaction mechanism for the formation of these thin films and the influence of different reaction parameters on the product are explained.


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## Body

## 1. Introduction

Nanostructured
thin films as well as thin films with no surface texture are currently used for
many applications such as electro-optical devices [1], batteries [2], solar
cell technology [3], and gas sensors [4]. These applications require high
purity, defect-free materials. The use of metal organic chemical vapor
deposition (MOCVD), molecular-beam epitaxy (MBE), or liquid-phase epitaxy (LPE)
techniques represents the state-of-the-art fabrication processes  for such
high-quality semiconductor thin films.However, in recent years, lower-cost approaches to fabricate these films have emerged,
among them hydrothermal [5] and electrochemical [6] synthesis methods. In
addition, synthesis
techniques that mimic biomineralization processes have received much attention
because of the inherently benign and malleable conditions under which
biominerals are produced. An increasing number of research groups have been
studying biomimetic [7] or biologically inspired [8] pathways towards
preparation of semiconductor materials in the past few years using biological
principles of materials formation.Biomineralization,
including the formation of bones, teeth, shells, or the silica skeletons of
marine sponges, takes place in vivo under ambient conditions. These biomineralization processes often produce
highly ordered structures on the nanoscopic as well as the macroscopic scale,
and generally nano- or micrometer-sized defects within these structures do not
propagate, but are corrected on a short-range scale. The lessons learned from
these biomimetic systems follow a general model [9, 10]: the reaction
environment is anisotropically preorganized; this provides a framework from which
the material can grow in an anisotropic manner. Kinetic control is imposed
at the stage of nucleation, typically at an interface in the reaction
environment, by closely controlling the supply of molecular precursor
chemicals. In addition, crystal growth is vectorially regulated by a template,
as seen both in natural systems [11–13] and
artificial systems such as those based on virus protein coats [14], ferritin-like
cage proteins [14, 15], and others [16].Based
on previous research in our group on silicatein (forsilicaprotein) [13, 17, 18], an enzymatic biocatalyst discovered in the biologically fabricated needles
of silica made by a sponge (Tethya Aurantia), we used these biomimetic concepts for the development of a bioinspired
synthesis route to nanostructured inorganic materials.We observed that silicification inTethya Aurantia is controlled by
occluded protein filaments (silicateins) that serve as both templates and
catalysts for the deposition of opal-like SiO2 [17–19]. In vitro experiments with isolated
silicatein filaments yielded SiO2 nanoparticles similar to those
formed in vivo, when the proteins
were exposed to silicon tetraethoxide (TEOS) [18]. Recognizing silicatein to be
a specialized member of the superfamily of hydrolytic enzymes [17, 18], Zhou et
al. used site-directed mutagenesis to confirm the proposition that the
sidechains of two specific amino acids, serine and histidine, play an essential
role as partners in the catalytically active center of silicatein [19]. Based
on these findings, the following mechanism for the formation of SiO2 was proposed [18]: resulting from
the close proximity of the hydroxyl group of the serine, the hydrogen atom of
this functional group becomes a bridging hydrogen atom linking the oxygen atom
of the hydroxyl group and the nitrogen atom in the 3 positions of the imidazole ring on the
histidine. This partial withdrawal of the proton leads to an increased
nucleophilicity of the oxygen atom on the serine, facilitating a nucleophilic
addition onto the electron-deficient silicon center of TEOS. An EtO− group is cleaved from the TEOS precursor, reacting with the electron-deficient
bridging hydrogen to form EtOH, and the addition of a water molecule then
initiates  hydrolysis to yield silanol, with restoration of the enzyme’s catalytic
center with its –CH2–O⋯H⋯N– bond. Subsequently either further
hydrolysis takes place, or several silicon alkoxide molecules react via
condensation to form SiO2 as the final product [18].Mimicking
this hydrolysis/condensation mechanism with its –CH2–O⋯H⋯N– sequence as the reactive center for
catalysis, silica, silsesquioxanes, and metal oxide materials has been
prepared using block copolypeptides of the essential catalytic amino acids
[20] or small functionalized molecules [21] in an effort to remove the
biomolecule from the process. In a further translation step, self-assembled
monolayers (SAMs) of suitable hydroxyl- and imidazole-terminated molecules have
been patterned on gold-coated silicon wafers [22]. Similarly, the same
catalytic activity has been achieved by functionalizing gold nanoparticles with
the respective surface coating to mimic the catalytic center of silicatein
[23]. While these approaches were successful for the preparation of a variety
of metal oxide materials, such as TiO2 [24] and Ga2O3 [25], the use of an organic template to direct synthesis is not feasible for
device applications due to the contamination of the end product with the
remnants of the organic template.
Removing the organic elements from the process would alleviate this
source of contamination and can potentially yield high-purity materials.Translating
these biomimetic systems further to achieve carbon-free reaction conditions, we
recently reported a kinetically-controlled vapor-diffusion catalysis method as
a biologically-inspired synthesis approach to hierarchically-structured
inorganic thinfilm preparation [26–29]. This method
uses the principles of slow catalysis and vectorial control to achieve
anisotropic, kinetically-controlled reaction conditions. However, unlike the
previously mentioned approaches, the aqueous reaction solution and the gaseous
catalyst ensure an extremely low level of contamination for this general method
to fabricate metal oxide, hydroxide, and phosphate thin films. Other groups
[30, 31] have reported the same type of structured thin films or nucleation on
substrates using chemical bath deposition (CBD). In contrast to our work,
substrates are essential in those methods to nucleate the films, and in some
cases organic solvents are used as part of the precursor-containing solution.
We here summarize and cross-link previously reported results from our group and
others, and describe in more detail the mechanism for the formation of
substrate-free, unsupported films and the influence of different reaction
parameters on the resulting thin film products.


## 2. Vapor-Diffusion Catalysis

A wide variety
of inorganic materials in thin film morphology have been prepared using
vapor-diffusion catalysis [26–29] or CBD [30, 32]. For growth on substrates, a heterogeneous nucleation mechanism has been
established. Figure 1 shows an illustration of the experimental setup for a
typical vapor-diffusion catalysis reaction.(a) Schematic illustration of the
vapor-diffusion catalysis method for unsupported film formation from aqueous
metal salt solutions; SEM images of the growth of Co5(OH)8(NO3)2·2H2O
from 0.1 M Co(NO3)2 solution showing (b) noncontinuous
network of partially fused Co5(OH)8(NO3)2·2H2O
nucleation islands after 15 minute reaction time, (c) extended network of further
fused Co5(OH)8(NO3)2·2H2O
nucleation islands after 30-minute reaction time; and (d) fully condensed
structured Co5(OH)8(NO3)2·2H2O
thin films after 60-minute reaction time.
(a)(b)(c)(d)In the vapor-diffusion catalysis method, a gaseous catalyst from an external source is
diffused into an aqueous metal salt solution (Figure1(a)). Adding the catalyst
as a gas at low concentrations by vapor-diffusion instead of adjusting the pH
homogeneously in solution leads to an anisotropic reaction environment that is
temporally and spatially controlled by the rate of diffusion. In the case
illustrated and described above, a pH gradient, decreasing from the
vapor-liquid interface towards the bottom of the reaction vessel, is
created.Rather than introducing the gaseous catalyst into the closed reaction environment by
connecting it to a gas cylinder, we rely on the in situ formation of a liquid/vapor solution equilibrium from a
suitable source to saturate the reaction chamber with gaseous catalyst. For example, an aqueous NH4OH solution is used to
yield an NH3 atmosphere (Figure 1(a)), which serves as the catalyst for the
reactions discussed here. Employing an environment that from the beginning is
completely saturated with the catalytic gas would significantly accelerate the
initial rates of hydrolysis, condensation and nucleation, and therefore would
not allow the maximum benefits of slow catalysis that we achieve through
vapor-diffusion catalysis as opposed to, for example, hydrothermal reactions.The slow diffusion and catalysis allow for very localized formation of nucleation
crystallites close to the meniscus of the reaction solution during the first
minutes of the synthesis. Measurements of pH changes at the vapor-liquid
interface have confirmed that the increase in pH, that is, [OH−]
concentration, can be correlated with the formation of nucleation crystallites
[27]. Figures 1(b)–1(d) show SEM images
of Co5(OH)8(NO3)2·2H2O films at various stages of formation. After 15 minutes, clusters that seem to
originate from discrete island nucleation can be observed. The shape of the
crystallites growing into the aqueous reaction solution (indicated by the arrow
in Figure 1(b)) is similar to the spindle-shaped hexagonal crystallites Yamabi
and Imai [31] observed for their CBD growth on glass, Si,
and other substrates.
Some fusion of these crystallites, originating from their nucleation centers
rather than the protruding crystalline spikes, can already be seen. Thirty
minutes after the start of the reaction, the formation of a continuous although
still porous plane connecting the individual crystallites has progressed
significantly (Figure 1(c)). This plane will eventually form the backplane of the
structured film, thickening to ∼1.5 μm [26] and incorporating the original
spindle-like crystallites into the base of the film. For the formation of
structured Co5(OH)8(NO3)2·2H2O thin films from a 0.1 M Co(NO3)2 solution, the fusion of
the backplane is complete after ∼60 minutes; simultaneously during this time,
plates with a thickness of ∼30–50 nm [26] formed
protruding orthogonally from the continuous backplane oriented into the aqueous
reaction solution (Figure 1(d)). The arrows in Figure 1(d) indicate the backplane
(white arrow) and the nanostructured side of the material (black arrow),
respectively.Hydrothermal methods, that rely on
the in situ formation of either one
of the precursor materials or the reaction catalyst, similarly achieve a
temporally controlled slow crystal formation [30, 33]. For example, thermal
decomposition of urea [30] or hexamethylenetetramine (HMT) [33] has been used
to liberate a basic catalyst. However, this approach does not make use of
spatial control of nucleation. The reaction environment in hydrothermal
synthesis is isotropic, with homogeneous nucleation in solution competing with
heterogeneous nucleation on the supplied substrate.The density, size, and shape of the structures protruding from the continuous
backplane of inorganic films prepared using vapor-diffusion catalysis can be
modified without the use of organic surfactants or growth inhibitors. We have
shown this control to depend on the type and concentration of the metal salt
precursor and gaseous catalyst and on the reaction time [26, 27].As can be seen
in Figure2, the morphology and shape of the nanostructured features and their
3-dimensionality vary depending on the chemical composition of the final
products. Both materials (ZnO, Figure 2(a); Zn5(OH)8(NO3)2⋅2H2O,
Figure 2(b)) were prepared by vapor-diffusion catalysis, diffusing ammonia into an
aqueous Zn(NO3)2 solution under differing conditions of
precursor concentration and reaction time [26, 27].(a) High-resolution SEM image of
the nanostructures protruding from a ZnO thin film prepared by
low-concentration vapor-diffusion from a 0.05 M Zn(NO3)2 solution. Reprinted by permission
from Journal of the American Chemical
Society, vol. 128, no. 31, pp. 10276–10280, 2006©American Chemical
Society. (b) SEM image of the nanostructures protruding
from a Zn5(OH)8(NO3)2⋅2H2O thin film prepared by vapor-diffusion from a
0.1 M Zn(NO3)2 solution.
(a)(b)The
nanostructured ZnO film seen in Figure2(a) was crystallized by vapor-diffusion
catalysis from a 0.05 M Zn(NO3)2 solution, whereas the
Zn(NO3)2 concentration used to obtain the structured Zn5(OH)8(NO3)2⋅2H2O
film (Figure 2(b)) was twice as high. With the slower nucleation rate, hexagonal
ZnO is formed [27]. In contrast, the rate of initial nucleation is faster when
higher concentrations of metal salt precursor solution and gaseous catalyst are
used, leading to the incorporation of anionic counterions from the precursor
material. To accommodate the incorporation of bulky NO3−
ions, the films that are formed from the higher concentration of Zn(NO3)2 precursor do not crystallize in a close-packed hexagonal crystal structure,
instead a layered hydrotalcite-like structure is formed [26, 34]. This
interesting structure consists of Zn2+ containing layers, that carry
a net positive charge from the incorporation of tetrahedrally coordinated Zn2+ ions into the crystal structure of otherwise octahedrally coordinated ions
[34]. The layers are held together by the intercalation of NO3−
counterions, resulting in a Zn2+ : NO3−
ratio of 5:2. The
overall crystal structure of this layered compound is monoclinic [35]. We
believe that the difference in nanostructured morphologies of the two materials
is a direct consequence of the difference in their crystal structures. Further
experiments to confirm this hypothesis are in progress.To
study the effect on crystallinity and morphology of films prepared using
vapor-diffusion catalysis when substrates are introduced into the reaction
system, different substrates were used as templates for the growth of thin
films from aqueous Zn(NO3)2 solutions [28]. Unlike the
results of CBD [32], again the reaction solution only contained the metal salt
precursor and the catalyst was added by diffusion as a gaseous medium. The
reaction solution was not stirred during the synthesis to not disturb the
anisotropic reaction environment.Two examples of
structured thin films grown from the same 0.1 M Zn(NO3)2 solution can be seen in Figure 3. The observed difference in morphology in this
case is not due to different reaction conditions but to nucleation sites
provided by different substrates that offer a more or less suitably
lattice-matched base for ZnO [28]. The film in Figure 3(a) has been grown on an
amorphous glass substrate and consists of amorphous Zn5(OH)8(NO3)2·2H2O.
The 3-dimensional structure of the film with its individual plates protruding
from the backplane attached to the nucleation surface closely resembles that of
the unsupported Zn5(OH)8(NO3)2·2H2O
films (Figure 2(b)). When polished (0001) oriented epitaxially grown ZnO was used
as substrate, the difference of this substrate in crystallinity,
crystallographic lattice matching with respect to Zn5(OH)8(NO3)2·2H2O
and ZnO, and the polycrystalline versus epitaxially grown surface morphology
evidently suppressed the nucleation of Zn5(OH)8(NO3)2·2H2O
in favor of the formation of a 3-dimensional network of highly crystalline ZnO
[28]. These results indicate that substrates of different chemical composition
and degrees of crystallinity can be used to induce the preferential growth of
specific materials and govern the morphology of the resulting thin films.
Furthermore, the integrability of our vapor-diffusion synthesis method into
existing manufacturing processes such as MOCVD or MBE has been demonstrated,
and these results confirm its potential use as a low-cost alternative to these
fabrication methods.SEM images of thin films grown on
different substrates from an aqueous 0.1 M Zn(NO3)2 solution by vapor-diffusion catalysis over the course of 6 hours: (a) Zn5(OH)8(NO3)2·2H2O
on amorphous glass; (b) ZnO on the (0001) face of epitaxially grown ZnO.
(a)(b)Another way to modify the chemical
composition of structured thin films prepared by vapor-diffusion catalysis is
to supply different counterions to be incorporated into the crystal structure.
This method allows subtle changes to be made in the crystal structure, such as
replacing the anions that hold together the positively charged metal ion sheets
in hydrotalcite-like structures, and to thereby change the lattice spacing along
thec-direction within the
polycrystalline material. For example, Co5(OH)8(NO3)2·2H2O,
Co5(OH)8SO4·2H2O, and Co5(OH)8Cl2·3H2O
have been prepared from their respective metal nitrate, sulfate, and chloride
salts, with diffraction analyses confirming changes in the lattice spacing
proportional to the counterion diameter [26]. While the rates of formation of
the hierarchically structured films of these cobalt-containing
hydrotalcite-like materials vary slightly, their overall morphologies are very
similar. This is another indication that similar crystal structures favor the
formation of similar types of nanostructures that grow from the continuous
backplane of the thin film material.The chemical composition of thin
films can be changed more significantly, when additional anions are added to
the reaction solution. If the incorporation of these anions is more favorable
than the formation of hydroxide or oxide materials, different types of
inorganic materials can be formed using the vapor-diffusion catalysis method.
As one example, we have identified synthesis conditions that favor the
formation of metal phosphate thin films under very similar conditions than have
been used to prepare the metal oxide or hydroxide films [26, 29]. Thus, we
found that the addition of (NH4)2HPO4 to
aqueous chromium or manganese salt solutions leads to the formation of CrPO4·6H2O [29] and Mn3(PO4)2·7H2O [26],
respectively. These films also display the same general morphology as described
above, with submicron-scale structures protruding from the side of the
continuous film that had been facing the aqueous solution during the 18–24 hour reaction
time.


## 3. Conclusion

A generic
low-temperature method, for the preparation of structured inorganic metal
oxide, hydroxide and phosphate films, has been introduced. The underlying
biomimetic principles of materials formation have been reviewed, and their
importance has been discussed. Because the chemical composition, crystallinity
and morphology of the films, is very susceptible to changes in the reaction
environment, this opens many possibilities to tailor the properties of films
prepared by this environmentally benign vapor-diffusion catalysis. The method
allows the synthesis of freestanding, unsupported films as well as film growth
on substrates. We have proposed a nucleation sequence for the formation of
structured films grown by this kinetically-controlled synthesis approach, and
discussed previously reported results, cross-linking the findings and
discussing different possibilities for modification of selected aspects of thin
film growth.In summary, we have reviewed a
generic synthesis approach for low-cost, low-temperature fabrication of
structured inorganic metal oxide, hydroxide and phosphate films. The
integrability of the vapor-diffusion catalysis method into existing
manufacturing processes such as MOCVD or MBE has been demonstrated.



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