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Studying the Solvothermal Formation of MoO3 Fibers by Complementary In Situ EXAFSEDXRD Techniques.

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Hydrothermal Synthesis
DOI: 10.1002/anie.200500514
Studying the Solvothermal Formation of MoO3
Fibers by Complementary In Situ EXAFS/
EDXRD Techniques**
Alexej Michailovski, Jan-Dierk Grunwaldt,
Alfons Baiker, Ragnar Kiebach, Wolfgang Bensch, and
Greta R. Patzke*
The reliable and straightforward fabrication of nanoparticles
is essential for developing future nanotechnology.[1?5] Nanoscale transition-metal oxides provide a wide spectrum of
important properties,[6, 7] and their functionalization and
alignment is greatly facilitated by anisotropic morphologies.[8]
Among the transition-metal oxides, MoO3 is the focus of
much attention owing to its numerous applications, for
example, in catalysis[9?11] or sensor technology.[12, 13] We have
recently developed a solvothermal procedure that provides a
convenient access to highly anisotropic nanoscale MoO3
fibers.[14, 15]
The solvothermal process[16] is famous for its unique
control facilities regarding the particle size of the material
produced.[17, 18] Its major drawback, however, is that the
predictive and rational preparation of a desired solid remains
a major preparative challenge?especially if the morphology
of the product is to be tuned as well. This problem is because
of the tremendous impact of the synthetic parameters on the
course of the reaction. Thus, the development of solvothermal
nanomaterials syntheses may require considerable ?trial and
[*] Dipl-Chem. A. Michailovski, Dr. G. R. Patzke
Laboratory of Inorganic Chemistry
ETH H/nggerberg
8093 Z4rich (Switzerland)
Fax: (+ 41) 1633-4692
Dr. J.-D. Grunwaldt, Prof. Dr. A. Baiker
Institute for Chemical and Bioengineering
ETH H/nggerberg, 8093 Z4rich (Switzerland)
Dipl.-Chem. R. Kiebach, Prof. Dr. W. Bensch
Institut f4r Anorganische Chemie
UniversitDt Kiel, 24098 Kiel (Germany)
[**] We thank Prof. Dr. R. Nesper (Laboratory of Inorganic Chemistry,
ETH Z4rich) for his steady interest and continuous support of this
work. We gratefully acknowledge HASYLAB (DESY, Hamburg) for
providing beamtime at beamline X1 and F3 for in situ EXAFS and
XRD experiments, respectively. The beamline staff, Julia Wienold
and M. Herrmann, as well as Matteo Caravati and Michael Ramin
(ICAB, ETH Z4rich) are gratefully acknowledged for their help
during the EXAFS measurements. This work was supported by the
ETH Zurich, by the Swiss National Science Foundation (MaNEP:
Materials with Novel Electronic Properties), and by the National
Research Program ?Supramolecular Functional Materials?.
EXAFS = extended X-ray absorption fine structure, EDXRD = energy
dispersive X-ray diffraction.
Supporting information for this article is available on the WWW
under or from the author.
Angew. Chem. Int. Ed. 2005, 44, 5643 ?5647
error? work. The elucidation of solvothermal reaction
mechanisms is therefore vital for devising more predictive
strategies so that the resulting materials can be optimized for
commercial purposes. As solvothermal reactions are usually
performed in thick-walled reaction containers, sophisticated
in situ techniques employing high-intensity synchrotron radiation are required for their direct monitoring.[19?22] Generally,
the development of high-pressure, in situ spectroscopic
approaches is a challenging topic of chemistry and materials
science.[23] Orientating kinetic studies may also be performed
by quenching hydrothermal reactions, but this information is
only valid if the recovered material is not affected by any
irreversible changes upon cooling and isolation. Among the
multitude of solvothermally generated materials, special
emphasis has been placed upon the in situ study of zeolites,[24?26] open-framework compounds,[27, 28] silicate minerals,[29, 30] and sulfide-based systems.[22, 31] For the transitionmetal oxides, the main focus has been on ternary compounds
of industrial interest (e. g. bismuth molybdates[32] or
BaTiO3[33]), whereas considerably fewer binary oxides (such
as ZrO2[34]) have been investigated by solvothermal in situ
methods. Very little mechanistic information is available on
their solvothermal transformation into nanomaterials.
Herein, we report the first comprehensive in situ study on
the growth of MoO3 fibers by complementary EDXRD
(energy dispersive X-ray diffraction) and XANES/EXAFS
(X-ray absorption near edge structure/extended X-ray
absorption fine structure) techniques. While XRD allows
the long-range order to be monitored, XANES/EXAFS
provides information on the short-range order.[35?37] This
information is indispensable for describing the solvothermal
crystallization of nanostructured matter, because amorphous
solids and small particles (typically below 50 C) cannot be
detected by Bragg diffraction. Moreover, the simultaneous
observation of both the solid and the liquid phase during the
reaction is vital for proposing a reaction pathway. For this
purpose, a special in situ-EXAFS cell was used (see Supporting Information).[38] The EXAFS technique is ideally suited to
monitor both crystalline and amorphous solid intermediates
as well as species in solution, whereas all crystalline components can be fully characterized in terms of the complementary EDXRD method.
The solvothermal access to MoO3 nanorods is quite
straightforward: yellow molybdic acid, MoO3�H2O, is quantitatively transformed into the fibrous product after a few
hours of autoclave treatment at 180?220 8C in water or acetic
acid.[14] The fibers often display high aspect ratios with lengths
above 10 mm and diameters between 100 and 150 nm.
Although the parameters (e.g. time, acid concentration, or
temperature) can be varied quite widely, the presence of
MoO3�H2O is a prerequisite for the formation of MoO3
fibers.[14] MoO3 and MoO3�H2O both have layered crystal
structures that are topotactically related by a stepwise
dehydration proceeding via b-MoO3稨2O.[39] Preliminary
experiments covering the formation of MoO3 fibers in various
media point to a different formation pathway under solvothermal conditions.[14]
The temperature interval for the onset of MoO3 rod
formation from MoO3�H2O in acetic acid was evaluated with
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ex situ quenching experiments between 80 8C and 180 8C
(Experimental Section). The reaction can be monitored
visually because the bright yellow starting material is
converted into a gray-blue product by a partial reduction of
MoO3 (the amount of product falls below the detection limit
of standard analytical techniques). At 80 8C, the yellow color
was maintained, whereas a sharp color change was observed
after 12 min of treatment at 100 8C. When the reaction
temperature was raised, this time interval further decreased
to 6 min (120 8C) and 2 min (180 8C). These observations are
supported by the corresponding scanning electron micrograph (SEM) images and powder diffraction patterns (Figure 1 I): at 80 8C, MoO3�H2O (JCPDS 39?363; P21/n, a =
10.618(5), b = 13.825(7), c = 10.482(5) C, b = 91.61(4)8)
undergoes no significant structural (Figure 1 I a, bottom) or
morphological (Figure 1 I a, top) changes. However, at 100 8C,
the dehydration to MoO3 is finished (Figure 1 I b, bottom)
and the formation of a fibrous material sets in (Figure 1 I b,
top). Between 100 8C and 180 8C both the fibrous morphology
(Figure 1 I c, d, top) and the crystallinity (compare the (061),
(002) and (112) reflections in Figure 1 I c,d, bottom) improve
further. The analogous quenching series in water proceeded
on a similar timescale and led to the quantitative formation of
MoO3 nanorods at 150 8C within 3?4 min. (Figure 1 II d, top/
bottom). The powder diffraction patterns of the flat, fibrous
products display intensities different from the literature data
(JCPDS 35?609; Pbnm, a = 3.963, b = 13.856, c = 3.697 C).
This deviation may be due to the presence of stacking faults in
the [001] growth direction[40] together with a preferred
orientation of the MoO3 rods on the flat sample holder.
The presence of stacking faults in the as-synthesized rods
grown after 20 min of ex situ treatment was confirmed by
TEM investigations and may account for the broadened
reflections occurring in the powder diffraction patterns
(Figure 1). Figure 2 demonstrates that separated rods with
Figure 2. Representative SEM images of MoO3 nanorods (scale
bar = 2 mm) after prolonged solvothermal treatment.
Figure 1. SEM images (top: I and II, scale bar = 1 mm) and XRD patterns (bottom: I and II) of the products resulting from mixtures of
MoO3�H2O and acetic acid (I) or water (II) at 80 8C (a), 100 8C (b), 120 8C (c) and 180 8C (acetic acid) or 150 8C (H2O) (d).
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 5643 ?5647
well-defined edges and tips are accessible after prolonged
solvothermal treatments (16?24 h at 180 8C). Even at 100 8C,
only few residual microcrystals of MoO3�H2O are still
present (see powder XRD pattern (Figure 1 II b, bottom)
and SEM images (Figure 1-II b, top)).
The temperature-dependent formation of MoO3 rods was
studied with in situ X-ray absorption spectroscopy. Our novel
autoclave cell allows the study of the liquid phase and the
solid/liquid interface. The cell can be positioned so that the Xray beam probes either the liquid phase or the solid/liquid
interface (see Supporting Information). In the present case,
the monitoring of the liquid phase is essential to distinguish a
strictly topotactic dehydration from alternative dissolution?
precipitation mechanisms. XANES spectra at the molybdenum K edge of the reaction of MoO3�H2O in water were
taken both at the bottom of the cell (mainly solid part,
Figure 3 a,b) and the middle of the cell (liquid part, Figure 3 c)
between 25 8C and 150 8C. Up to 50 8C no molybdenumcontaining species were observed in solution (Figure 3 c) and
the solid phase displayed the XANES spectrum of pure
MoO3�H2O. The first slight changes were observed at 90 8C,
and the reaction sets in around 99 8C (Figure 3 a,b). The
XANES and Fourier transformed EXAFS spectra (Figure 3 a,b) demonstrate the continuous transformation of
MoO3�H2O into MoO3 nanorods at 99 8C and do not indicate
the presence of any intermediates (for reference spectra see
the Supporting Information). Furthermore, the concentration
of water-soluble species increases significantly at around
95 8C and rises continuously up to 150 8C. Note that the
separation into individual MoO3 nanorods proceeds at these
elevated temperatures (cf. Figure 1-I/II d, top). The reaction
starts as soon as the first soluble Mo species are detected and
it accelerates at higher temperature so that the concentration
of Mo species in solution increases. Furthermore, the rod
formation is diffusion limited, as shown by the reaction being
slower when performed in glass wool (see Experimental
Section). All these observations strongly support a solutionbased mechanism rather than a topotactic transformation of
MoO3�H2O into MoO3.
In situ-EDXRD techniques were employed to study the
time-resolved formation of MoO3 rods from MoO3�H2O. A
typical result of the time-dependent evolution of the diffraction patterns at 120 8C in acetic acid is shown in Figure 4 a.
The Mo signals and the characteristic reflections of
MoO3�H2O (Figure 4 a, inset) are visible from the start.
After a temperature-dependent induction time (in this case:
8 min) during which the signals of the starting material vanish,
the first reflections of the product are observed and the
product starts to form without the occurrence of a crystalline
intermediate. All the reflections of the material grow
simultaneously. The reaction is complete after another 7?
8 min. The crystallization curve (a(t) vs. time) has a sigmoidal
shape (Figure 4 b) which is typical for the kinetics of many
solid-state reactions. Compared to other solid-state reactions,
the velocity of the MoO3 rod formation is very high.
Therefore, a further evaluation of the observed kinetic data
was not possible.
In summary, the formation of MoO3 rods is a rapid process
that is finished within minutes at temperatures well below
Angew. Chem. Int. Ed. 2005, 44, 5643 ?5647
Figure 3. a) Molybdenum K-edge X-ray absorption near edge spectra
during the hydrothermal reaction of MoO3�H2O to MoO3 fibers (solid
phase), inset: expansion of the 20.02?20.1 region, b) Fourier transforms of the EXAFS spectra, c) molybdenum K-edge XANES spectra of
the liquid phase during the reaction. The intensity of the absorption
step is referenced against the spectrum recorded at 150 8C. A and
j FT(ck2) j in arbitrary units.
180 8C. All the experimental results point to a quick
dissolution?precipitation mechanism without a crystalline
intermediate phase. Thus, the high anisotropy of the MoO3
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Experimental Section
Figure 4. a) Time-resolved, background-corrected powder patterns
recorded at 120 8C. The indices of the starting material (*), the product, and the Mo fluorescences are marked (the inset shows a magnification of the first spectrum), b) time dependence of the extent of reaction a for reflection (021) at 120 8C. The crystallization curve has a sigmoidal fit.
For the quenching experiments, MoO3�H2O (180 mg, 1 mmol),
glacial acetic acid (0.5 mL), and water (1.5 mL) were added to a
glass ampoule (volume 23 mL) that was sealed under atmospheric
pressure. The ampoules were heated for 20 min at 80, 100, 120, or
180 8C and subsequently quenched at 8 8C in a NaCl/ice mixture.
The products were centrifuged several times in water, ethanol, and
acetone and then subjected to scanning electron microscopy (SEM,
LEO 1530 (FEG) microscope with 1 kV electrons) and to X-ray
powder analysis (STOE STADI-P2 diffractometer, flat sample
holders, CuKa radiation).
The in situ EXAFS and XANES experiments were performed at
the beamline X1 at HASYLAB (DESY, Hamburg, Germany).
MoO3�H2O (50 mg) was embedded between glass wool plugs to fix
the solid phase and were loaded together with water (3 mL) into a
newly designed spectroscopic cell (see Supporting Information). The
EXAFS scans around the molybdenum K edge were recorded
between 19 850 and 21 200 eV under stationary conditions (liquid
solution and solid phase) and were calibrated with a Mo reference
foil. Faster spectra during the change of the reaction conditions were
recorded in the QEXAFS mode between 19 950 and 20 450 eV (110 s/
spectrum). The following procedure was applied: Heating to 150 8C in
1.5 8C min1 steps from RT to 50, 94, 120, and 150 8C. The raw data
were energy-calibrated with the according reference foil, background
corrected and normalized (WINXAS 3.0 software[43]). Fourier transformation for EXAFS data during dynamic changes was applied to
the k3-weighted functions in the interval k = 3.5?9.3 C1.
MoO3 nanorods were obtained during the in situ EDXRD
experiments (HASYLAB beamline F3, Hamburg, Germany) from
(230.0 mg, 1.27 mmol) MoO3�H2O in a solution of glacial acetic acid
(0.4 mL) and H2O (1.1 mL). The reaction was carried out under
autogeneous pressure and isothermal conditions in sealable glass
tubes as inserts for autoclaves (see Supporting Information).[22, 31] The
reaction vessel was transferred into the autoclave, and the reaction
time was started after a delay time of approximately 1 min. The
reaction temperatures were then reached after about 30 s, and X-ray
powder patterns were recorded with acquisition times of 120 s. The
spectra were evaluated with EDXPow.[44] The reflection profiles were
modeled with a Gaussian peak shape, and the intensities of the
product reflections were normalized against the peak intensities of
the MoKa resonance.
Received: February 10, 2005
Revised: May 17, 2005
Published online: August 1, 2005
fibers presumably arises from the higher growth velocity of
the [001] crystal direction in the orthorhombic a-MoO3 lattice
cell (Pbnm, a = 3.9630, b = 13.856 , c = 3.6966 C).[41, 42] Our
approach provides a versatile strategy towards understanding
solvothermal reactions. After a series of orientating ex situ
quenching experiments, the temperature-dependent solid?
liquid equilibria of a given reaction can be monitored with a
newly developed autoclave suitable for in situ-EXAFS spectroscopy. A kinetic study based on in situ-EDXRD provides
the essential complementary information. In this way, the
solvothermal mechanisms and the role of the precursor
material are analyzed from different perspectives. Such
insights are essential for optimizing syntheses for industrial
scale-up processes. In conclusion, the presented approach is a
step towards the rational solvothermal design of nanomaterials and of syntheses proceeding by amorphous and/or liquid
phases in general.
Keywords: EXAFS spectroscopy � hydrothermal synthesis �
in situ spectroscopy � molybdenum oxide � nanostructures
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