In situ X-ray absorption spectroscopy/energy-dispersive X-ray diffraction studies on the hydrothermal formation of Bi2W1-xMo xO6 nanomaterials

Article abstract of DOI:10.1002/ejic.201101116

Bi₂WO₆ has attracted considerable interest as a visible-light-driven photocatalyst with a layered Aurivillius structure. The catalytic performance of bismuth tungstate is notably enhanced through the partial substitution of tungsten with molybdenum. Whereas hydrothermally obtained Bi₂W_1-xMo_xO₆ solid solutions maintain the Bi₂WO₆ structure, their morphologies vary with the molybdenum content. Lower Mo contents (x < 0.5) favor the formation of hierarchically nanostructured microspheres that consist of sheet-like building blocks. Their disintegration is observed for greater extents of W/Mo substitution, hand in hand with a decrease in the surface area. Raman spectra furthermore indicate changes in the local structure of the octahedral W/Mo moieties upon variation of the Mo content. As little is known about the growth kinetics and hydrothermal formation processes of nanostructured Bi₂W_1-xMo_xO₆ catalysts, in situ XAS investigations were performed to determine the onset of their hydrothermal formation from Bi(NO₃)₃·5H₂O, K ₂WO₄ andNa₂MoO₄. In situ energy-dispersive X-ray diffraction (EDXRD) experiments revealed a correlation between particle shape, Mo content and formation pathway of the Bi ₂W_1-xMo_xO₆ nanomaterials. The results are compared to related in situ studies on hierarchically structured W/Mo oxides. Hierarchical Bi₂W_1-xMo_xO ₆ nanostructures and nanosheets have been selectively obtained by adjusting the W/Mo ratio. The hydrothermal formation process of these solid solutions was monitored by complementary in situ X-ray absorption spectroscopy/energy-dispersive X-ray diffraction. Their different morphology types correlate with the observed reaction kinetics. Copyright

Full text of DOI:10.1002/ejic.201101116

FULL PAPER
DOI: 10.1002/ejic.201101116
In Situ X-ray Absorption Spectroscopy/Energy-Dispersive X-ray Diffraction
Studies on the Hydrothermal Formation of Bi W Mo O Nanomaterials
2
1–x
x
6
Ying Zhou,[a,b] Elena Antonova, Yuanhua Lin, Jan-Dierk Grunwaldt,
[c]
[d]
[e]
[
c]
[b]
Wolfgang Bensch, and Greta R. Patzke*
Keywords: Bismuth / Tungsten / Molybdenum / Hydrothermal synthesis / Nanostructures
Bi
2
WO
6
has attracted considerable interest as a visible-light-
the local structure of the octahedral W/Mo moieties upon
variation of the Mo content. As little is known about the
driven photocatalyst with a layered Aurivillius structure. The
catalytic performance of bismuth tungstate is notably en- growth kinetics and hydrothermal formation processes of
hanced through the partial substitution of tungsten with mo- nanostructured Bi catalysts, in situ XAS investi-
lybdenum. Whereas hydrothermally obtained Bi gations were performed to determine the onset of their hy-
solid solutions maintain the Bi WO structure, their morpho- drothermal formation from Bi(NO ·5H O, WO and
. In situ energy-dispersive X-ray diffraction (EDXRD)
experiments revealed a correlation between particle shape,
Mo content and formation pathway of the Bi
2
x 6
W1–xMo O
2
x 6
W1–xMo O
2
6
3
)
3
2
K
2
4
logies vary with the molybdenum content. Lower Mo con-
tents (x Ͻ 0.5) favor the formation of hierarchically nano-
structured microspheres that consist of sheet-like building
blocks. Their disintegration is observed for greater extents of
W/Mo substitution, hand in hand with a decrease in the sur-
face area. Raman spectra furthermore indicate changes in
2 4
Na MoO
2
x 6
W1–xMo O
nanomaterials. The results are compared to related in situ
studies on hierarchically structured W/Mo oxides.
Introduction
design. Although an increasing repertoire of combined in
situ techniques for monitoring growth processes in solution
or various catalytic reactions has been established, only a
Functional nanostructured oxides are essential building
blocks for tomorrow’s technological challenges, such as sus-
tainable energy sources and constant access to clean water
[5]
few of these studies have been devoted to the important
materials class of visible-light-driven oxide photocatalysts
[
1]
resources. Photocatalytic nano-oxides play a major role
in these developments,[ and their morphology is a crucial
parameter for catalytic efficiency. Therefore, tunable solu-
tion syntheses of oxide materials, such as hydro- and solvo-
[6]
as alternatives to TiO2. Bismuth oxide based materials are
2]
currently a focus of worldwide photocatalyst research with
[7]
special emphasis on BiVO4 and Aurivillius-type Bi WO6
2
[8]
with a layered structure. Over the past years, we have de-
[
3]
thermal techniques, attract increasing research interest.
veloped a new way to access hierarchically nanostructured
Despite constant progress in shaping nanoscale oxides for
tailor-made applications, only a small fraction of these sys-
tems are understood and accessible on a sound mechanistic
[9]
Bi WO spheres, and we have monitored their formation
2
6
pathway with in situ energy-dispersive X-ray diffraction
[10]
(EDXRD) techniques for the first time.
Hierarchical
[
4]
basis. The lack of direct insight into hydrothermal and
related formation processes renders morphology control in
oxide synthesis an ongoing challenge that hinders informed
morphologies have been found to be favorable for other bis-
muth-containing photocatalysts as well, such as for
[11]
[12]
[13]
Bi O ,
bismuth titanate
or BiOCOOH,
but their
2
3
growth mechanisms have all been proposed on the basis of
ex situ data and quenched intermediates. Due to the lack
of in situ studies, the particle shaping of oxide materials
often remains an empirical question of trial and error.
Given that doping of binary oxides with additional metal
ions is a frequently used performance optimization strategy,
achieving morphology control in multi-component systems
[
[
a] State Key Laboratory of Oil and Gas Reservoir Geology and
Exploitation, Southwest Petroleum University,
Chengdu 610500, China
b] Institute of Inorganic Chemistry, University of Zurich,
8
057 Zurich, Switzerland
Fax: +41-44-63-56802,
E-mail: greta.patzke@aci.uzh.ch
[
[
c] Institute of Inorganic Chemistry, University of Kiel,
2
4098 Kiel, Germany
d] School of Materials Science and Engineering, Southwest Petro- has become a considerable challenge that needs to be re-
leum University,
solved.
Chengdu 610500, China
[
e] Institute for Chemical Technology and Polymer Chemistry,
Although Bi MoO and Bi WO have the same Aurivil-
2 6 2 6
Karlsruhe Institute of Technology (KIT),
lius structure in common, we have discovered notable differ-
ences in the humidity-sensing behaviors of both com-
pounds that are linked to their surface properties.[ Mo
7
6131 Karlsruhe, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101116.
14]
Eur. J. Inorg. Chem. 2012, 783–789
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
783

G. R. Patzke et al.
FULL PAPER
incorporation into nanostructured bismuth tungstate hosts lengths of the a and c axes increase for higher Mo contents,
has furthermore been identified as a new strategy to en- whereas the b-axis lengths significantly decrease (cf.
hance their photocatalytic activity by means of the emerg- Table 1). This agrees well with previous reports by Zhang
[15]
[15a]
ing Bi W Mo O phases. However, not only electronic et al.
and demonstrates that Bi WO and Bi MoO form
2
1–x
x
6
2 6 2 6
and structural effects play a role in photocatalyst design, a continuous Bi W Mo O solid solution from the facile
2
1–x
x
6
but crystallinity, specific surface area and morphology are one-step hydrothermal reaction of Bi(NO ) ·5H O, K WO
3
3
2
2
4
equally important parameters.[ As outlined above, they and Na MoO as readily available precursors.
16]
2
4
can only be efficiently steered through detailed knowledge
Typical SEM images of the Bi W Mo O samples are
2 1–x x 6
about the underlying materials formation processes. shown in Figure 1. Bi W0.75Mo0.25O bears a strong resem-
2
6
Whereas advanced in situ studies have been performed on blance to the morphology of hierarchical Bi WO micro-
2
6
the solution growth of Bi MoO catalysts[
4a,17,18]
and spheres that consist of individual nanosheet building
2
6
Bi WO has been investigated in our previous recent blocks. These hierarchical microspheres start to disintegrate
2
6
10]
[
work,
no such information is available for the growth upon higher Mo contents in the Bi W Mo O materials
2 1–x x 6
kinetics and processes of the catalytically more efficient
mixed Bi (W,Mo)O phases.
(
cf. Figure 1c), and this process continues until the in-
2
6
dividual nanosheet building blocks appear (cf. Figure 1d).
The present study now closes this gap, given that our
extensive studies on the formation mechanisms of molybde-
The morphology of Bi MoO6 is similar to that of
2
Bi W0.25Mo0.75O , but the overall particle sizes are larger.
2
6
[
19]
[20]
num and tungsten oxide nanomaterials and their com-
The crystallite sizes of the Bi W Mo O samples were de-
2
1–x
x
6
[
21]
bination into hierarchically structured (W/Mo) oxides
termined from the (131) reflection by using the Scherrer
equation as shown in Table 1. In line with the continuously
sharpened peak profile, the crystallite size decreases signifi-
cantly from about 38.2 nm for Bi MoO to approximately
have encouraged us to transfer this knowledge to the
exploration of morphology–mechanism relationships to
Bi W Mo O materials. Therefore, we present the first
2
1–x
x
6
2
6
complementary in situ X-ray absorption spectroscopy
XAS) and EDXRD studies on the hydrothermal incorpo-
1
0.4 nm for Bi WO . This trend is also reflected in the Bru-
2 6
(
nauer–Emmett–Teller (BET) surface areas (cf. Table 1). The
ration of Mo into hierarchical Bi WO6 architectures.
2 –1
2
surface area for Bi MoO is only 9.8 m g , and it increases
2 6
Whereas in situ XAS can both monitor the liquid and the
solid phase, in situ EDXRD techniques were used to cover
2 –1
to 18.4 m g for Bi WO . Moreover, the surface area of
Bi W Mo O6 decreases from 22.6 to 16.9 m g
2
6
2
–1
2
1–x
x
the entire Bi W Mo O (x = 0, 0.25, 0.50, 0.75 and 1.0)
2
1–x
x
6
with increasing Mo content. These results indicate that
Bi W Mo O samples have the same structural motif in
series. The results are correlated to our in situ investigations
on the formation of related hierarchical particle arrange-
ments in tungstate and (W/Mo) oxide based hydrothermal
2
1–x
x
6
common but can be divided into two morphological types:
hierarchical nanostructures (x = 0 and 0.25) and nanosheets
[
20,21]
systems.
(x = 0.50, 0.75 and 1.0).
Moreover, all solid products were collected after in situ
EDXRD experiments and were analyzed by ex situ XRD
and SEM. The XRD patterns of these samples agree well
with those of the phases grown under ex situ experimental
conditions. SEM images shown in Figure S2 in the Sup-
Results and Discussion
Hydrothermal Synthesis of Bi W Mo O Nanostructures
2
1–x
x
6
XRD patterns of Bi W Mo O products are shown in porting Information demonstrate that the products isolated
2
1–x
x
6
Figure S1 in the Supporting Information. All of the samples after the in situ EDXRD experiments already bear con-
exhibit a single Aurivillius phase. The Mo-free product (x siderable resemblance to the fully developed materials ob-
=
0) can be indexed to orthorhombic Bi WO with lattice tained after 24 h of hydrothermal reaction (cf. Figure 1).
2 6
parameters of a = 5.4314(6) Å, b = 16.418(1) Å and c =
To understand the local structural characteristics of the
5
.4505(5) Å (space group Pca2 , JCPDS no. 73-1126). For Bi W Mo O solid solutions, Raman spectroscopy was
1
2
1–x
x
6
x = 1.0, the γ-Bi MoO phase with lattice parameters a = employed, because it can detect structural variations in the
2
6
5
.4809(1) Å, b = 16.2239(5) Å and c = 5.4999(1) Å (space coordination polyhedra of a given material through differ-
[
22]
group Pca2 , JCPDS No. 77-1246) was obtained. The lat- ences in the observed modes. Figure 2 shows the Raman
1
tice parameters for mixed compounds with x = 0.25, 0.50 spectra recorded for the Bi W Mo O6 solid solution
2
1–x
x
and 0.75 fall within the range of the pristine phases: the series. Generally, the resonances of Bi WO and Bi MoO
2
6
2
6
2 x 6
Table 1. Unit-cell parameters and surface areas of Bi W1–xMo O products.
131 [nm][a]
SBET [m g ]
2 –1
Sample
Bi WO
a [Å]
b [Å]
c [Å]
D
2
6
5.4314(6)
5.4553(4)
5.4568(4)
5.4762(2)
5.4809(1)
16.418 (1)
16.367(1)
16.314(1)
16.2584(6)
16.2239(5)
5.4505(5)
5.4679(3)
5.4703(2)
5.4898(1)
5.4999(1)
10.4
11.6
17.9
23.4
38.2
18.4
22.6
18.8
16.9
9.8
Bi
Bi
Bi
2
2
2
W0.75Mo0.25
W0.50Mo0.50
W0.25Mo0.75
O
6
O
6
O
6
2 6
Bi MoO
[a] Crystallite size estimated from the (131) reflection with the Scherrer equation.
784
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Eur. J. Inorg. Chem. 2012, 783–789

2 x 6
Hydrothermal Formation of Bi W1–xMo O Nanomaterials
atoms of the (W/Mo)O octahedra are not affected by in-
6
15a]
[
creasing Mo content.
amount of Mo in Bi W Mo O does not only influence
It can be concluded that the
2
1–x
x
6
the final morphology (cf. Figure 1), but also the local struc-
tures of the obtained solid solutions. With increasing Mo
content up to x = 0.5, the apical (W/Mo)–O bond length
within the (W/Mo)O octahedra decreases.
6
In our previous work, we studied the hydrothermal
crystallization of Bi WO with in situ EDXRD monitoring
2
6
[
10]
techniques. We found the reaction kinetics of this hydro-
thermal system to be pH-dependent, accompanied by the
disintegration of hierarchical nanostructures into their
nanosheet building blocks. Therefore, it is to be expected
that the addition of Mo precursor to the reaction medium
influences the nucleation and growth of Bi WO6 par-
2
ticle architectures. This can be a reason why hierarchical
Bi W Mo O6 (x = 0.50, 0.75 and 1.0) materials with
2
1–x
x
Figure 1. Representative SEM images of (a) Bi
2 6
WO , (b)
higher Mo content have not yet been accessed. In
the following, in situ XAS and EDXRD techniques were
combined to elucidate the hydrothermal formation of
Bi W Mo O solid solutions.
Bi , (c) Bi and (d) Bi W0.25Mo0.75O .
2
W
0.75Mo0.25
O
6
2
W
0.50Mo0.50
O
6
2 6
in the range of 600–1000 cm–1 can be assigned to the
stretching of W–O and Mo–O bonds, and those below
2
1–x
x
6
–
1
4
00 cm
arise from bending, wagging and external
[
23]
modes.
More detailed investigations show that Bi WO6
2
In Situ XAS Investigation of Bi W Mo O Formation
2
1–x
x
6
and Bi W0.75Mo0.25O exhibit very similar Raman spectra.
2
6
–
1
The band at 303 cm can be assigned to translational
modes that involve simultaneous motions of Bi and (W/
Mo)O6.
In our previous work, we constructed a special in situ
cell that allowed us to monitor both the liquid phase and
3
+
[
24]
–1
The strong band at 790 cm , along with two
[25]
the solid–liquid interface during hydrothermal processes.
–
1
shoulder bands at 817 and 712 cm of Bi WO6 and
2
This provides essential insight into the formation of oxide
nanomaterials and thereby indispensable information for
their controlled large-scale production.
Bi W0.75Mo0.25O , corresponds to (W/Mo)–O stretching
2
6
modes of the distorted (W/Mo)O octahedra.
6
X-ray absorption near edge spectroscopy (XANES) spec-
tra at the Mo-K edge were recorded both at the middle (li-
quid part in Figure 3a) and at the bottom (mainly solid part
in Figure 3b) of the cell during the hydrothermal reaction.
Na MoO as the molybdenum source dissolved at a tem-
2
4
perature as low as 50 °C under acidic reaction conditions
Figure 3a). The decrease in Mo-containing species up to
0 °C furthermore indicates the formation of solid products
i.e., the reaction onset at relatively low temperatures (Fig-
(
8
[
ure 3b)]. As the Bi and W species showed strong X-ray ab-
sorption at realistic concentrations needed for the experi-
ments, high-quality in situ extended X-ray absorption fine
structure (EXAFS) data were extremely difficult to obtain.
In the following, we thus concentrated solely on the absorp-
tion edge with higher photon energy (Mo-K edge) rather
Figure 2. Raman spectra of as-prepared Bi
wavenumber range; (b) higher wavenumber range.
2 x 6
W1–xMo O : (a) lower
than on the lower-energy Bi L and W L edges. Figure 3b
3
3
shows a comparison of the in situ Mo-K edge XAS data
Interestingly, the resonance at 817 cm is shifted to a collected in the course of the reaction with the starting ma-
–
1
–
1
higher wavenumber at 842 cm with further increasing Mo terial Na MoO and the product Bi W0.75Mo0.25O as ref-
2
4
2
6
content in Bi W Mo O (x = 0.50, 0.75 and 1.0). It has erences. Firstly, it is obvious that Mo displays completely
2
1–x
x
6
been demonstrated in previous studies that lower fre- different coordination environments in precursor and prod-
quencies of the Raman stretching bands correspond to the uct. The Mo-K edge XAS of Na MoO exhibits very dis-
2
4
longer metal–oxyen bonds.[22] Therefore, Bi WO6 and tinctive pre-edge features close to 19995 eV, and the absorp-
2
Bi W0.75Mo0.25O display longer apical (W/Mo)–O bonds tion sharply rises at 20000 eV. The pre-edge feature is due
2
6
than Bi W Mo O (x = 0.50, 0.75 and 1.0). However, the to the forbidden transition from the 1s to the 4d level,
2
1–x
x
1
6
–
band at 712 cm observed for all samples displays no sig- which can only be observed due to a mixing of d and p
nificant shift, thus indicating that the equatorial oxygen orbitals in the final state (e.g., in the case of tetrahedral
Eur. J. Inorg. Chem. 2012, 783–789
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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785

G. R. Patzke et al.
FULL PAPER
symmetry). The intensity of that feature depends on the In Situ EDXRD Studies
amount of p character in the resulting state.[ In compari-
18]
The time-dependent crystallization of Bi W Mo O6
2
1–x
x
son, the pre-edge intensity of the Bi W0.75Mo0.25O product
2
6
nanomaterials was studied with in situ EDXRD, and the
typical evolution of the diffraction patterns of
Bi W0.75Mo0.25O is shown in Figure 4.
decreases sharply due to the transition from Mo in a tetra-
hedral environment to octahedrally coordinated Mo, even
in solution. Whereas previous in situ studies on the forma-
tion of Bi MoO had started from (NH ) Mo O ·4H O as
2
6
2
6
4 6
7
24
2
polyoxometalate precursor with octahedrally coordinated
[
18]
Mo centers, tetrahedral Mo coordination in the starting
material is equally suitable to bring forward mixed Auri-
villius-type compounds. As intermediate non-centrosym-
metric tetrahedral [MoO ] species have been observed prior
4
to the formation of Bi MoO from (NH ) Mo O ·4H O
2
6
4 6
7
24
2
[
17]
and Bi O ,
we here demonstrate that the isostructural
2
3
Bi W Mo O target compounds can also be accessed di-
2
1–x
x
6
rectly from a precursor material with tetrahedrally coordi-
nated molybdenum atoms. The pH influence is another key
parameter, because acidic media in combination with heat-
ing obviously promote the conversion of tetrahedrally to
octahedrally coordinated Mo.
Figure 4. Time-resolved in situ EDXRD patterns recorded during
the hydrothermal crystallization of Bi at 160 °C.
The indices of intense lines of the product phase as well as the Mo-
and W-K fluorescence lines are marked.
2
W0.75Mo0.25O
6
K
α
α
The W and Mo fluorescence lines were present from the
beginning of the reaction process, and several diffraction
peaks of orthorhombic Bi W0.75Mo0.25O appeared after a
2
6
very short induction time. Their intensity increased over
the course of the reaction, and no intermediates were de-
tected. In situ EDXRD monitoring of the formation of
Bi W Mo O (x = 0, 0.50, 0.75 and 1.0) closely resembles
2
1–x
x
6
the results for Bi W0.75Mo0.25O as shown in Figure 4, but
2
6
the individual induction times tind. and half-life times
t0.5 strongly depend on the Mo content among the
Bi W Mo O series (cf. Figure 5 and Table 2). The t
2
1–x
x
6
ind.
and t0.5 values for Bi WO are 8 and 16 min, respectively,
2
6
whereas the formation of Bi MoO6 sets in immediately:
2
within the time resolution of the experimental setup no in-
duction time could be observed, and t0.5 is only 6 min for
Bi MoO . The formation processes of Bi W Mo O (x =
2
6
2
1–x
x
6
0
.25, 0.50 and 0.75) display intermediate kinetics. In ad-
dition, different growth curves observed (cf. Figure 5) for
a) Bi W Mo O with x = 0, 0.25 and (b) Bi W Mo O6
(
2
1–x
x
6
2
1–x
x
with x = 0.50, 0.75 and 1.0 suggest that the crystallization
processes vary with the Mo content.
As the reaction kinetics of Bi W Mo O materials ap-
2
1–x
x
6
pear to depend strongly on the degree of W/Mo substitu-
tion, kinetic analyses were performed by fitting the experi-
mental data to a theoretical expression that related the ex-
Figure 3. (a) Mo-K edge XAS spectra of the hydrothermal conver- tent of reaction α (cf. Figure 5) to time. A more detailed
sion of Bi(NO ·5H O/K WO /Na MoO to Bi (li-
quid phase); (b) solid-phase Mo-K edge XAS spectra of the corre-
sponding hydrothermal Bi formation with reference
spectra (Mo-K edge XAS of the Na MoO precursor and of the
Bi product).
3
)
3
2
2
4
2
4
2 6
W0.75Mo0.25O
description of kinetic data evaluation can be found in the
[
26]
literature.
.50, 0.75 and 1) was too fast to permit detailed kinetic
evaluations, so Sharp–Hancock plots based on the (260) re-
The speed of Bi W Mo O formation (x =
2 1–x x 6
2
W0.75Mo0.25O
6
0
2
4
2
W
0.75Mo0.25
O
6
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Eur. J. Inorg. Chem. 2012, 783–789

2 x 6
Hydrothermal Formation of Bi W1–xMo O Nanomaterials
whether the particles are formed by self-assembly or other
routes, such as growth from a polycrystalline core.
Comparison with Hierarchical W/Mo Oxide Growth
Mechanisms
As outlined in the Introduction, the majority of the
growth mechanisms for the self-organization of oxide nano-
particles into hierarchical arrangements have been proposed
on the basis of ex situ observations. Only few examples of
the systematic in situ monitoring of oxide formation pro-
cesses in hydrothermal systems that contain two or more
cations have been reported. One of the few comprehensive
studies in this field is our combined in situ EDXRD/XAS
investigation on the influence of molybdenum on the
growth process of hexagonal W/Mo oxide nanoarchitec-
Figure 5. Extent of reaction α versus time for the growth of the
Bi series at 160 °C.
2
x 6
W1–xMo O
[
21]
tures
in comparison with pristine hexagonal tung-
In close analogy to the stepwise W/Mo substitu-
[
20]
states.
tion strategy towards the present Bi W Mo O solid solu-
tions, we first established hydrothermal access to hierarchi-
cally nanostructured hexagonal alkali tungstates that were
Table 2. Kinetic data for the formation of Bi
solutions.
2
W
1–xMo
x
O
6
solid
m[a]
2
1–x
x
6
Samples
Bi WO
t
ind. [min]
t0.5 [min]
+
+ [20]
obtained in the presence of Rb and Cs .
EDXRD studies showed that they were formed by means
of a nucleation-controlled mechanism with t values of
In situ
2
6
8
4
2
2
0
16
14
8
8
6
0.52
0.63
Bi
Bi
Bi
2
2
2
W
W
W
0.75Mo0.25
0.50Mo0.50
0.25Mo0.75
O
O
O
6
6
6
[
[
[
b]
b]
b]
ind.
+
+
11 min for Rb and 9 min for Cs . This scenario changed
upon addition of Mo to the hydrothermal system: hexago-
Bi
2
MoO
6
[
a] Avrami exponent. [b] The reaction was too fast to generate suf- nal Rb–W/Mo tungstates and Cs–W/Mo tungstates with
ficient data points for analysis.
more pronounced and better organized spherical growth
patterns emerged from a diffusion-controlled mechanism
after shorter induction times (Rb: tind. = 2 min; Cs: t
=
ind.
flection were only obtained for Bi WO6 and 6 min) and drastically reduced t0.5 values.[ The introduc-
21]
2
Bi W0.75Mo0.25O , respectively (Figure S3 in the Support- tion of Mo into the hexagonal tungstates did not alter their
2
6
ing Information). The linear shape of both graphs in the channel-structure motif and promoted their transformation
.2 Ͻ α Ͻ 0.85 range points to a common crystallization into nanoscale hierarchical morphologies in comparison
mechanism of both Bi WO and the Bi W0.75Mo0.25O solid with the hydrothermal formation of hexagonal tungstates in
0
2
6
2
6
solution with respective m values of 0.52 and 0.63 (cf. the absence of Mo. Furthermore, the accelerated formation
Table 2). kinetics of the alkali W/Mo oxides with respect to the corre-
Given that m values around 1.0 were reported for the sponding alkali tungstates can be explained by our observa-
[18]
hydrothermal formation of Bi MoO nanoplates, the for- tion that pristine MoO nanorods are formed very rapidly
2
6
3
mation of Bi W0.75Mo0.25O already exhibits a clear trend under hydrothermal conditions (i.e., on a minute scale that
2
6
[
19]
towards the upper limit of m ranges for diffusion-controlled does not permit further kinetic evaluations). The above-
growth that were determined to be 0.54–0.62 in previous described Bi WO /Bi (W,Mo)O /Bi MoO series shows the
2
6
2
6
2
6
[
27]
works.
On the basis of an Avrami exponent m close to same kinetic trend for the tind. values: whereas bismuth mo-
0.5, we had assigned a diffusion-controlled reaction mecha- lybdate formation proceeds too fast to assign a mechanistic
nism to the hydrothermal formation of Bi WO6 nano- model, tungsten-rich phases grow more slowly and under
2
structures, because m should adopt values of 1 or above for diffusion control. Therefore, the morphological difference
other reaction mechanisms.[
10]
As
a
consequence, between platelet-shaped Bi MoO and hierarchical Bi WO
2 6 2 6
Bi W0.75Mo0.25O is formed by means of an intermediate and Bi (W,Mo)O is linked to the change of hydrothermal
2
6
2
6
reaction mechanism on the diffusion-controlled side, which reaction kinetics (i.e., to the crystal growth mechanism).
can be explained in terms of the partial substitution of W Given that hierarchical nanostructuring among W/Mo hex-
by Mo that speeds up the reaction kinetics. An increase of agonal tungstates is connected to diffusion-controlled
the reaction exponent m from 0.5 to 1 goes hand in hand growth as well, there are strong indications that hierarchical
with the disintegration of Bi WO spheres into their plate- nanostructures are preferably formed in a “kinetic window”
2
6
let-shaped building blocks. Therefore, the fast formation ki- for the organized diffusion of the components to the grow-
netics of Bi MoO may be the reason that the organization ing crystals. Interestingly, the combination of a slow-grow-
2
6
of nanoplates into hierarchical nanostructures has not been ing W-oxide with a fast-growing Mo-oxide component gen-
observed. Further investigations are required to elucidate erated optimum conditions for the hierarchical self-organi-
Eur. J. Inorg. Chem. 2012, 783–789
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
787

G. R. Patzke et al.
FULL PAPER
zation process among two structurally different systems, and Na
2
MoO
4
(0.1–x mmol) were placed into acetic acid (10 mL,
2
0 vol.-%) at room temperature and stirred magnetically for 20 min
namely, Aurivillius-type Bi MO compounds and hexago-
2
6
to disperse all reagents homogeneously. The resulting precursor
suspension was transferred into a Teflon-lined stainless steel auto-
clave with a capacity of 23 mL, maintained at 160 °C for 24 h and
subsequently cooled to room temperature. The precipitate was col-
lected after filtration, washed with distilled water and dried in air.
nal tungsten bronzes. Consequently, not only crystallo-
graphic features but also crystallization kinetics are a key
steering parameter for hydrothermally obtained particle
shapes. This renders kinetic monitoring a promising strat-
egy to achieve morphology control over a wider product
range, thereby covering a variety of structural motifs.
Characterization: PXRD was conducted with a STOE STADI P
diffractometer in transmission mode (flat sample holders, Ge mon-
ochromator, Cu-Kα1 radiation) operated at 40 kV and 40 mA. The
unit cells were refined with the ReflexPlus Package of Materials
Studio (Accelrys Inc.). For SEM, which was performed with a Zeiss
SUPRA 50VP microscope, samples were dispersed in ethanol and
Conclusion
A
series
of
nanostructured
Aurivillius-type
Bi W Mo O solid solutions was obtained from the one- subsequently deposited on a silicon wafer. The specimen was inves-
2
1–x
x
6
step hydrothermal reaction of Bi(NO ) ·5H O, K WO and tigated without conductive coating at a rather low voltage (2 kV)
3
3
2
2
4
Na MoO . The extent of hierarchical organization de- to minimize charging effects. BET surface area measurements were
2
4
2
performed with a Quadrasorb SI in N -adsorption mode. The sam-
ples were degassed under vacuum at 150 °C for 5 h. The BET spe-
cific surface area was determined by a multipoint BET method
using the adsorption data in the equilibrium relative pressure (p/p )
0
range of 0.05–0.30.
creases with the Mo content among the series. Whereas
Bi WO and Bi W0.75Mo0.25O display hierarchical self-or-
2
6
2
6
ganization of individual nanoplatelets into microspheres,
W/Mo ratios of 0.5 and above induce the segregation of
the individual building blocks into sheet-like morphologies.
Raman spectra of the Bi W Mo O series show that the
In Situ XAS: In situ XAS experiments were performed at the su-
perXAS beamline at the Swiss Light Source (SLS, Villigen, Switzer-
2
1–x
x
6
Mo content also influences the local structure of the (W/
land). Bi(NO
6 mg) were embedded between two glass wool plugs to fix the solid
phase, and they were loaded together with acetic acid (3 mL,
3 3 2 2 4 2 4
) ·5H O (30 mg), Na MoO (6 mg) and K WO
Mo)O octahedra through a decrease in their apical bond
6
(
length. In situ XAS investigations revealed that the forma-
tion of the Bi W Mo O compounds sets in at as low as
2
1–x
x
6
[25]
2
0 vol.-%) into a homemade spectroscopic cell.
around the Mo-K edge were recorded between 19850 and 20240 eV
rahedra in the Na MoO precursor into the octahedral under stationary conditions (liquid solution and solid phase) and
XAS scans
8
0 °C, and they illustrate the conversion of the [MoO ] tet-
4
2
4
moieties of the mixed oxide products, even in solution. were calibrated with an Mo reference foil. The following procedure
–
1
Their crystal-growth kinetics were furthermore studied with was applied: heating to 160 °C at a rate of 1.5 °Cmin from room
temp. to 50, 80, 120 and 160 °C. Background subtraction and nor-
in situ EDXRD monitoring experiments. Interestingly, the
malization were carried out by fitting a linear polynomial to the
different product-morphology types correlate with the ob-
pre-edge region and a cubic polynomial to the post-edge region of
served reaction kinetics. Whereas the Mo-rich nanosheet
the absorption spectra using WinXAS 3.20.[
28]
structures among the Bi W Mo O series emerge from a
2
1–x
x
6
rapid growth process that does not permit further kinetic In Situ EDXRD Measurements: In situ EDXRD experiments were
performed at HASYLAB (Hamburg) bending magnet beamline
evaluations, the self-organization of nanoparticles into hier-
F3, which provides an energy range from 13.31 to 62.54 keV with
archical Bi WO and Bi W0.75Mo0.25O microspheres pro-
2
6
2
6
a maximum intensity of about 26 keV. The d-spacing range is given
by d = 6.199/(E·sinθ). With a detector angle of approximately 4.9°,
the observable d-spacing range is 1.16–5.46 Å. The energy resolu-
ceeds by means of slower and diffusion-controlled growth.
The results are in line with our previously observed kinetic
and mechanistic trends for the hydrothermal formation of
hierarchically nanostructured hexagonal W/Mo oxides: in
analogy to the present study, the introduction of Mo into
–2
tion Δd/d is about 10 above 26 keV. The beam was collimated to
0.2 mm to give the best results. For the in situ experiments, auto-
claves with glass liners of an internal diameter of 10 mm and a
this hydrothermal W/Mo system accelerates the growth ki- volume of 10 mL were used. A typical experiment was performed
netics and changes the formation mechanism of the prod- with Bi(NO ·5H O (97 mg, 0.2 mmol), K WO (x mmol; x = 0,
ucts as well. As a consequence, diffusion-controlled path- 0.025, 0.050, 0.075 and 0.1), Na MoO (0.1 – x mmol) and acetic
3
)
3
2
2
4
2
4
acid (2 mL, 20 vol.-%) as reactants. The in situ reactions were run
at 160 °C. Data collection was started immediately after the intro-
duction of the autoclave in the oven. The acquisition time for the
in situ EDXRD data measured for the given reactions was 120 s.
After the in situ experiments, the solid products were collected by
filtration, washed with distilled water and ethanol and dried in air
for further characterization. Note that due to sedimentation effects
of the (B/W)-containing products during the reaction, the in situ
ways appear to generally favor the formation of hierarchical
nanoarchitectures that offer superior materials properties
(e.g., sensor development and photocatalysis). The present
study demonstrates how optimal morphologies of a multi-
component material can be obtained through systematic in
situ monitoring of the formation of solids under hydrother-
mal conditions and exploiting the accompanying mechan-
istic changes. Further investigations on bismuth-containing experiments had to be performed under rapid stirring.
sensor and catalyst materials are under way.
Supporting Information (see footnote on the first page of this arti-
cle): XRD patterns of Bi
SEM images of Bi
EDXRD experiments (Figure S2); Sharp–Hancock plots derived
from the kinetic data of Bi WO and Bi recorded
at 160 °C over a data range of 0.2 Ͻ α Ͻ 0.85 (Figure S3).
2
W
1–xMo
1–xMo
x
O
6
solid solutions (Figure S1);
2
W
x 6
O solid solutions after in situ
Experimental Section
Synthesis: In
a
typical procedure, Bi(NO
3
)
3
·5H
2
O
(97 mg,
2
6
2 6
W0.75Mo0.25O
0
.2 mmol), K
2
WO
4
(x mmol; x = 0, 0.025, 0.050, 0.075 and 0.1)
788
www.eurjic.org
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Eur. J. Inorg. Chem. 2012, 783–789

2 x 6
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We thank HASYLAB (DESY, Hamburg) for beamtime at beamline
F3 for in situ EDXRD experiments and the Swiss Light Source
(
SLS, Villigen, Switzerland) for beamtime at the super XAS beam-
line for in situ XAS experiments. Beatrix Seidlhofer and Jing Wang
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Received: October 13, 2011
Published Online: January 3, 2012
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Eur. J. Inorg. Chem. 2012, 783–789
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
789