Why does bi2wo6visible-light photocatalyst always form as nanoplatelets?

Article abstract of DOI:10.1021/acs.inorgchem.0c01249

Bi2WO6 nanocrystals exhibit excellent photocatalytic properties in the visible range of the solar spectrum, and intense efforts are directed at designing effective synthesis processes with control of size, morphology, and hierarchical structure. All known hydrothermal syntheses produce either nanoplatelet morphology or hierarchical structures based on such primary entities. Here we investigate the nucleation and growth of Bi2WO6 nanocrystals under hydrothermal conditions using in situ X-ray total scattering (TS) and powder X-ray diffraction (PXRD) measurements. It is shown that the preferential growth of Bi2WO6 nanoplates is due to the presence of disordered layers of Bi2O22+ molecular complexes in the precursor solution with an approximate length of 13 ?. These layers interact with tetrahedral WO42- molecular units and eventually form the disordered cubic (Bi0.933W0.067)O1.6) crystalline phase. When enough tungsten units are intertwined between Bi2O22+ layers formation of Bi2WO6 pristine nanoplates takes place by necessary sideways addition of units in the ac plane. The experimentally observed formation mechanism suggests that the Bi/W atomic ratio must play a central role in the nucleation (assembly of initial crystal layers). Indeed, it is observed in separate continuous flow supercritical synthesis that for a stoichiometric (Bi/W = 2:1) precursor, a (Bi0.933W0.067)O1.6) impurity phase is always observed together with the main Bi2WO6 product. Excess tungsten is required in the precursor to form phase-pure Bi2WO6 material. Thus, the present study also reports a fast, scalable, and green method for production of this highly attractive photocatalyst.

Full text of DOI:10.1021/acs.inorgchem.0c01249

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Article
Why Does Bi2WO6 Visible-Light Photocatalyst Always Form as
Nanoplatelets?
Dipankar Saha, Espen D. Bøjesen, Aref Hasen Mamakhel, and Bo B. Iversen*
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ABSTRACT: Bi2WO6 nanocrystals exhibit excellent photocatalytic
properties in the visible range of the solar spectrum, and intense efforts
are directed at designing effective synthesis processes with control of
size, morphology, and hierarchical structure. All known hydrothermal
syntheses produce either nanoplatelet morphology or hierarchical
structures based on such primary entities. Here we investigate the
nucleation and growth of Bi2WO6 nanocrystals under hydrothermal
conditions using in situ X-ray total scattering (TS) and powder X-ray
diffraction (PXRD) measurements. It is shown that the preferential
growth of Bi2WO6 nanoplates is due to the presence of disordered
layers of Bi2O22+ molecular complexes in the precursor solution with an
approximate length of 13 Å. These layers interact with tetrahedral
2−
WO4 molecular units and eventually form the disordered cubic
2+
(Bi0.933W0.067)O1.6) crystalline phase. When enough tungsten units are intertwined between Bi2O2 layers formation of Bi2WO6
pristine nanoplates takes place by necessary sideways addition of units in the ac plane. The experimentally observed formation
mechanism suggests that the Bi/W atomic ratio must play a central role in the nucleation (assembly of initial crystal layers). Indeed,
it is observed in separate continuous flow supercritical synthesis that for a stoichiometric (Bi/W = 2:1) precursor, a
(Bi0.933W0.067)O1.6) impurity phase is always observed together with the main Bi2WO6 product. Excess tungsten is required in the
precursor to form phase-pure Bi2WO6 material. Thus, the present study also reports a fast, scalable, and green method for production
of this highly attractive photocatalyst.
he discovery of excellent visible light photocatalytic
properties of Bi2WO6 nanoplatelets in 2004 has spurred
are crucial for the photocatalytic activity. It has been observed
that ultrathin nanoplatelets and porous microstructures of
nanoplatelets could improve the surface area and adsorption
capability resulting in higher photocatalytic activity.16 There-
fore, exact control over the parameters governing the precise
nanostructure of the Bi2WO6 products is essential if the
optimal photocatalytic activity is to be achieved.
T
tremendous research activity.1 Numerous synthesis procedures
have been developed to synthesize samples exhibiting optimal
photocatalytic activity. Indeed, there is a plethora of references
discussing various synthesis approaches and their effect on the
performance of the products, and several excellent reviews
summarize these trends.2,3 Curiously, most synthesis pathways
produce either nanoplatelets of Bi2WO6 or hierarchical
structures based on such nanoplatelets. For example, square
nanoplates and hierarchical complex morphologies, namely,
flower-, tire-, and helixlike Bi2WO6, were successfully
synthesized by hydrothermal processes and the morphology
of these nanoparticles were controlled by varying hydrothermal
temperature and reaction time.4−10 Zhang et al. reports that
nanoparticles of Bi2WO6 could be obtained by calcining
amorphous complex precursor at a relatively low temperature
of above 450 °C.11 While low-temperature combustion
produced nanoflake Bi2WO6, nanosheets were obtained by
the microwave-assisted hydrothermal method.12,13 It has been
shown that the pH of the solution plays a role in controlling
whether the nanoplatelets remain suspended in the solution or
arrange into hierarchical structures.14,15 The morphology, size,
and any possible hierarchical assembly of these nanoplatelets
Bismuth tungstate (Bi2WO6) is not only an excellent
photocatalyst under visible light irradiation, but also like
many semiconductors holds a range of interesting physical
properties such ferroelectricity piezoelectricity, pyroelectricity,
and a nonlinear dielectric susceptibility.17−22 Composites of
bismuth tungstate have also been found with photocatalytic
activity.23−27 Bi2WO6 is an Aurivillius phase. The structure
consists of layers having the formula (Bin−1MnX3n+1)
2− (for n =
2+
1, M = W), intercalated between Bi2O2 layers. The WO6
Received: April 27, 2020
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octahedra share corners along the crystallographic a- and c-
reaction parameters obtained from in situ PXRD measurements
do not lead to high-quality products.40 This is because the
information on the crystallization is obtained after nucleation
has occurred since PXRD only provides access to the growth
mechanisms and kinetics of the crystalline parts of the
synthesis mixture by analysis of the Bragg diffraction signal.
However, the crucial nucleation step, which often determines
the fate of a given crystalline product, remains unexplored due
to the lack of techniques able to probe the solvated molecular
structure in solution under hydrothermal conditions. An
ensemble-averaged atomistic insight into the pristine crystal
nuclei can be obtained by measuring in situ X-ray total
scattering data on the reaction mixture and subsequently
calculating and modeling the atomic pair distribution function
(PDF).41,42 Total scattering (TS) studies enable extraction of
atomic-scale structural information from gases, liquids,
amorphous materials, nanocrystalline, and disordered materials
as well as crystalline structures. In several recent studies, it has
been shown that cluster structures in the precursor solution
play a crucial role not only in determining the crystal
polymorph of a particular material but also in directing the
microstructural evolution during nanoparticle formation and
growth.43−52 Here we probe the hydrothermal synthesis of
Bi2WO6 by in situ X-ray total scattering and PXRD
measurements to answer the basic questions of nucleation
and to understand the peculiar preference for nanoplatelet
formation in this important system. Furthermore, we use the in
situ information to develop an effective synthesis method to
obtain phase-pure Bi2WO6 in a continuous flow supercritical
reactor.
2+
axes, forming infinite WO6 layers with alternate Bi2O2 layers
along the b-axis (Figure 1).28 The tungsten polyhedra are
2+
connected to Bi2O2 layers through oxygen atoms shared
between polyhedra and layers.
Figure 1. Crystal structure of Bi2WO6 showing the arrangement of
2+
WO6 layers with alternate Bi2O2 layers.
Hydrothermal synthesis is one of the preferred methods for
obtaining Bi2WO6 nanocatalysts since it is robust, green, and
scalable, and it provides a high degree of control over the size
of the nanoplates. Previous efforts have been made to
understand the mechanism governing Bi2WO6 formation and
growth under hydrothermal conditions via both in situ and ex
situ powder X-ray diffraction (PXRD) studies.4,29,30 One major
result from these studies is the anisotropic growth of the
nanoplate crystallites which was attributed to a “crystal-
ripening” process.4 A particularly effective hydrothermal
synthesis method is to use one-step continuous flow
supercritical reactors,31,32 which can produce large amounts
of the product very quickly (reaction time in seconds).
Supercritical flow methods allow tuning of unique properties of
supercritical fluids, e.g., solvent strength, viscosity, diffusivity,
dielectric constant, ionic product, and surface tension.33,34 In
continuous flow reactors, the supercritical fluids provide very
rapid heating at the mixing point with the precursor solution
leading to high supersaturation conditions resulting in rapid
nucleation of nanoparticles.35−37 The particle size, size
distribution, crystallinity, and phase composition may be
controlled through simple variation of reaction parameters
such as temperature, pressure, precursor concentration, reactor
residence time, and choice of solvent.31,38,39
EXPERIMENTAL SECTION
■
All chemicals were purchased from commercial sources (Sigma-
Aldrich) and used as received. Bismuth citrate [O2CCH2C(OH)-
(CO2)CH2CO2]Bi (99.99%, CAS: 813−93−4) and sodium tungstate
dihydrate Na2WO4·2H2O (99%, CAS: 10213−10−2) were used as
precursors for the bismuth tungstate Bi2WO6 nanoparticle synthesis.
Bismuth citrate was dissolved in water and few drops of dilute
NH4OH were added to make a clear solution. Na2WO4·2H20 was
dissolved in water. The concentrations of bismuth citrate solution and
sodium tungstate solution were ∼2 and ∼1 M, respectively. Both
solutions were mixed to prepare the final precursor solution used, and
the pH of each was ∼13. An additional precursor mixture was made,
where the pH of the solution was adjusted to approximately 7
(measured by pH paper) by adding HNO3 for studying the effect of
pH on the reaction mechanism. The molar ratio of the final mixture
was Bi/W = 2:1 (denoted as F2 precursor solution).
These clear precursor solutions were used for the in situ
experiments, which were conducted in a custom-made capillary
reactor (fused silica capillary) pressurized to 250 bar and heated to
temperatures of 250 and 350 °C.53 The in situ total X-ray scattering
experiments were carried out at beamline ID11 at the European
Synchrotron Radiation Facility, Grenoble, France. The 14-bit dynamic
Frelon4M CCD detector (50 × 50 um2 pixel size) was placed off-
center, which provided a useful Qmax of 21.7 Å−1. The sample-to-
detector distance was ∼103 mm, and the monochromatic X-ray beam
had a wavelength of 0.18896 Å. Exposure time was set to 1 s, and with
a detector readout time of few milliseconds, this gives a time
resolution of ∼2.2 s. The flat-field and distortion-corrected data were
integrated using Fit2D.54 The integrated total scattering data were
converted into pair distribution function by the use of the PDFgetX3
program.55 Prior to the Fourier transformation, the data were
corrected for background scattering using measurements on the
deionized water in the same capillary at appropriate temperatures. A
Qmin and Qmax of 0.6 and 21.7 Å−1 were used for the Fourier
One of the major aims of the in situ studies is to use the
obtained information to develop effective methods for mass
production of nanomaterials for industrial applications. Even
though reactor configurations are different, the knowledge
gained from the in situ hydrothermal experiments possibly can
be applied to the continuous flow supercritical reactors as in
both cases the reaction involves rapid heating and supercritical
conditions.31 However, sometimes it has been found that the
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Figure 2. (a) Selected PDFs illustrating the reaction progress at 250 °C. (b) Fitted PDF for the precursor solution with both a Bi2O2 layer phase
and WO4 tetrahedra (Rw = 14.4%). Inset shows the low r-region. (c) Fitted PDF for the precursor solution with only the Bi2O2 layer phase (Rw =
20.1%). (d) Molecular clusters present in the precursor solution.
transformation. Structural models were refined against resulting PDFs
RESULT AND DISCUSSION
■
using PDFgui.56
In Situ X-ray Total Scattering. Figure 2a shows the
In situ PXRD experiments were performed at beamline I711,
MAXII, MAX-lab, Sweden. The wavelength used was 0.9941 Å. A
MAR165 CCD 2D detector was placed ∼90 mm behind the reactor.
evolution of the PDF with respect to time, where the bottom
curve represents the PDF obtained from the aqueous precursor
solution of bismuth citrate and sodium tungstate dihydrate.
This solution was heated in the capillary reactor and in situ
PDF data were collected while the reaction progressed. The
PDF obtained from the precursor solution represents solvated
molecular complexes, which have an average size of 13 Å as
there is no PDF signal observed beyond 13 Å in real space,
Figure 2b. The peak at 1.77 Å can be attributed to W−O bond
distances typically found for tungsten tetrahedrally coordinated
to oxygen, while the peak at 2.20 Å arises due to Bi−O bond
distances. Detailed modeling of the data revealed that the PDF
could best be described by a combined model (Rw = 14.4%)
The exposure time was 4 s, and with a detector readout time of 1 s,
this gave a time resolution of 5 s. The in situ PXRD data were
integrated using Fit2D.54 Structural information as a function of
reaction time was extracted from the as-integrated PXRD data by
sequential Rietveld refinement57 using FullProf.58 The sequentially
refined instrumental parameters were zero point displacement and
Chebychev polynomials (9) to fit the background. The sequentially
refined structural parameters for both phases [(Bi0.933W0.067)O1.6 and
Bi2WO6] were scale factor, unit cell parameters, and peak profile using
Thompson−Cox−Hastings−pseudo-Voigt (TCHpV) profile parame-
ters.58 Instrumental resolution was taken into account by refining
TCHpV profile parameters for the LaB6 standard and including them
in an instrumental resolution file (*.irf) implemented in the sequential
refinements.
In the continuous flow synthesis, the flow rate of the precursor
solution was 6 mL/min, while the flow rate of the preheated solvent
(water) was 12 mL/min. The temperature was 250, 300, 350, and 390
°C, and the pressure was 250 bar. Details of the flow reactor can be
found elsewhere.59 The collected product slurries were spun down by
centrifugation (6500 rpm), and the product was washed four times
with deionized water and once with ethanol. Subsequently, the
washed powders were dried overnight in a vacuum furnace set to 40
°C. The samples prepared by the flow method are named according
to the Bi/W molar ratio of the precursor solution with “F1” for Bi/W
= 1:1 and “F2” for Bi/W = 2:1. Ex situ PXRD data were measured for
the flow synthesis samples on a Rigaku SmartLab powder
diffractometer with Bragg−Brentano geometry. A Ge(111) single-
crystal monochromator was used to produce Cu Kα1 radiation.
TEM images were recorded on a TALOS F200A with a TWIN lens
system, X-FEG electron source, and a Ceta 16 M Camera operated at
200 keV using a Ceta 16M CMOS camera.
2+
2−
containing layers of Bi2O2 and additional WO4 tetrahedra
2+
2−
(Figure 2b,c, phase fractions of Bi2O2 74.5% and WO4
25.5%). The most intense peak at ca. 4 Å can be attributed to
2+
the Bi−Bi distances present in the Bi2O2 layers (Figure 2b).
2+
The average size of the Bi2O2 layers was determined to be
13.2(1) Å by the PDF refinement (Figure 2d). The existence
2+
2−
of Bi2O2 and WO4 units in the precursor solution has
previously been proposed, and moreover, it was hypothesized
that these molecular entities interact with each other to initiate
the nucleation event.14 Here we for the first time provide
2+
experimental evidence to support this hypothesis. The Bi2O2
layer presumably is positively charged, while the tungstate
tetrahedra are negatively charged. Therefore, it is a reasonable
expectation that they interact electrostatically with each other
and form the initial nuclei for eventual crystallization of
Bi2WO6. However, as demonstrated below, the formation
mechanism involves several steps.
PDF data were collected separately for a bismuth citrate
solution and a sodium tungstate solution. The two separate
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PDFs were added manually and compared with the PDF
obtained after mixing and injection of the solution into the
reactor tube (the cold precursor solution). Significant
differences (marked by arrows) were observed between them
(Figure 3). The W−O distance changes slightly from 1.76 to
initiation of heating, the PDF signal is only observed up to
distances of ∼20 Å in the real space. During step 1, there are
no significant changes in the PDF signal until after 1 min of the
reaction. This indicates two distinct feature of the nucleation
2+
process. First, it is clear that the Bi2O2 layers do not
disintegrate upon heating. Second, these layers do not initially
interconnect at the edge of the layer, but instead, they stack on
top of each other with WO42− in between the layers. While this
structural arrangement is occurring, no increase in the average
particle size is observed, i.e., no new peaks appear in the PDF,
indicating that rearrangement is limited to occur within the
layers themselves. In other words, new bond formation only
2−
occurs to connect the WO4 tetrahedra within each layer
resulting in corner-shared WO6 octahedra formation. At this
stage of the reaction presumably the clusters are very
disordered due to the continuously ongoing bond formation.
As a consequence of the lack of particle growth, these
prenucleation structures presumably act as nuclei for the
formation of the nanocrystal. Once the initial local bond
2+
2−
formation between layers of Bi2O2 and WO4 tetrahedra is
complete, the reaction progresses into step 2, where the growth
of the nanocrystals occurs. Since the molecular complexes exist
in layered form, it is easier to interconnect edgewise than
growing layers on top of each other. These platelet nuclei play
a crucial role in the intrinsic anisotropic growth since the
number of dangling bonds is much higher on the surface than
on the edge of the nuclei. According to Gibbs−Thomson
theory, the number of dangling bonds per atom over the entire
crystal determines the chemical potential of a surface.60−62
Therefore, it is safe to say that the platelet nuclei surface has a
higher chemical potential than the edge, and two-dimensional
growth is therefore preferred. Once a platelet has nucleated, it
uses other small platelets to form larger ones.60 Hence, this
process is more energy efficient since less bond formation is
required to join the layers. If the disordered nuclei formed in
Figure 3. Comparison of PDFs between manually added data
measured separately on bismuth citrate and sodium tungstate
solutions and the PDF collected after mixing to form the precursor
solution.
1.78 Å after mixing the solutions. Although there is no
significant change in the Bi−O distances, peaks corresponding
to Bi−Bi nearest neighbors at 3.76 Å get broader and next
nearest neighbor Bi−Bi peak position shifts toward higher r
values. This corroborates the fact that when bismuth citrate
and sodium tungstate solutions are mixed, the molecular
entities interact and form specific clusters. Probing these
changes in the PDF while mixing cold solution is a challenging
future project since it requires a new reactor setup.
step 1 were to stack on top of each other, then they would
Figure 4 shows the PDFs from the first 3 min of the reaction
after heating is commenced. The reaction can be divided into
two distinct steps. In the first step immediately after the
2−
need WO4
tetrahedra as connectors between them.
2−
However, this already occurred in step 2, and WO4
Figure 4. Evolution of the PDF with respect to time at a synthesis temperature of 250 °C. The structures present at the different reaction stages are
sketched.
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tetrahedra have been exhausted in the process. Furthermore, if
the stacking of the layers were to happen in step 2 instead of
sideways growth, then a steady appearance of new PDF peaks
would have been observed. Thus, the bond formation in
between layers of Bi2O2 of WO4 constitutes the initial
nuclei that subsequently interconnect along the edges of the
layers, resulting in the sudden appearance of additional peaks
in the PDF (Figure 4). We conclude that the crystallization in
the second step does not occur by stacking of nanoplates but
by connectivity in the ac planes.
(Bi/W = 2:1). The crystalline phase could be indexed in space
group Pca21 with cell parameters of a = 5.4687(14) Å, b =
16.503(3) Å, c = 5.4823(16) Å (see Figure S1). Figure 6 shows
the time-resolved PXRD patterns of Bi2WO6. From the time
evolution of the PXRD it is obvious that the reaction
progresses in two steps in agreement with the TS data.
Immediately after heating is initiated, the formation of the
disordered crystalline phase of cubic (Bi0.933W0.067)O1.6 is
observed. The (Bi0.933W0.067)O1.6 structure consists of inter-
2+
2−
2+
connected Bi2O2 layers with W partially occupying the Bi
The PDF of the heated reaction mixture after 20 min of
reaction time was fitted with the reported structure of
crystalline Bi2WO6 (Figure 5a). Interestingly, by applying a
positions. In step 2, peaks corresponding to the Bi2WO6 phase
start to appear. At increased temperature, the transformation of
(Bi0.933W0.067)O1.6 to Bi2WO6 is significantly faster. It has been
previously reported that the disordered cubic (Bi0.933W0.067)-
O
1.6 phase only is observed at higher pH (∼11) when bismuth
nitrate is used as the Bi source. The formation of the
2−
disordered phase is due to the increased solubility of WO4
tetrahedra in basic solution.29,63 In the present synthesis, we
use bismuth citrate and sodium tungstate dehydrate
precursors, which gives a high pH of about 13. However, as
seen in Figure 6b,d, the reaction is unchanged even when the
pH is adjusted to 7. In both cases, the disordered cubic phase
(Bi0.933W0.067)O1.6 is observed initially in the reaction steps.
The origin of the formation of this disordered phase is directly
seen in in situ TS data. At the beginning of the reaction, there
2+
2−
are Bi2O2 layers with WO4 tetrahedra in between, and the
amount of available WO4 is not sufficient to produce the
2−
fully ordered Bi2WO6 phase with a corner-shared WO6 unit.
Therefore, the average structure, which is probed by PXRD, is
the disordered phase (Bi0.933W0.067)O1.6. As the reaction
2−
progresses, more WO4 fills the layers, and once there is a
2−
sufficient number of WO4 tetrahedra to hold the layers
together, the formation Bi2WO6 begins.
The PXRD data in Figure 7a represent the reaction progress
with respect to time. They demonstrate that preferential
growth of the nanoparticles occurs along the ac plane since the
profile widths of the (200) and (002) reflections get
significantly sharper compared with the remaining reflections.
Figure 7b,c shows the variation of cell parameters and particle
size, respectively, extracted from sequential Rietveld refine-
ment. Initially, only the disordered cubic phase was observed
and the cell parameter is represented in Figure 7b with open
diamonds. After 1.5 min of reaction, this phase starts to
transform to Bi2WO6. The cell parameters are larger when the
nanoparticles are small, and they reach a smaller stable value
during the growth of the nanoparticles. The growth kinetics
were monitored for Bi2WO6 nanoparticle with Avrami−
Erofe’ev equation, which is a model to simulate nucleation-
growth crystallization.64−66
Figure 5. (a) PDF fitted with Bi2WO6 structure including unreacted
precursor complexes (Rw = 21.1%). (b) PDF fitted with Bi2WO6
structure without unreacted precursor complexes (Rw = 29.8%)
showing a poorer fit.
solely Bi2WO6 structural model, no satisfactory fit of the data
could be obtained (Figure 5b). Only by including unreacted
precursor molecules, it was possible to obtain a good fit to the
total PDF data. This indicates that under the applied reaction
conditions some amount of unreacted precursor complexes
remains present even after 20 min. Furthermore, it is observed
that the intensities of the PDF peaks at higher r-values (20−40
Å) do not match the calculated model (Figure 5). This is
attributed to the anisotropic size of the crystallites (nano-
platelets) not accounted for in the model. Therefore, the
average size of the nanoparticles obtained by fitting PDF is
expected to be smaller than observed by PXRD.
(−[k(t−tind)]n)
α = 1 − e
The extent of the reaction, α, can be evaluated from the
normalized particle size with respect to time. Here k denotes
the rate constant, and the exponent n is often used to
distinguish between different reaction mechanisms. tind is the
induction time defined as the time interval until the first
detectable particle size is observed. Growth kinetics of both the
platelet diameter and the thickness were evaluated by fitting
the above equation to the nanoplatelet growth (Figure 8).
The analysis reveals that the growth rate of the platelet
diameter (k = 0.86(1) min−1) is more than three time faster
than the thickness growth ((k = 0.25(1) min−1)). This is in
accordance with the finding from the TS studies. The exponent
Separate in situ PXRD measurements were carried out under
the same reaction conditions as used for the in situ total
scattering experiment. Pristine Bi2WO6 nanocrystals were
obtained after 20 min of reaction using the F2 precursor
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Figure 6. In situ PXRD data using precursor (Bi/W = 2:1) at different temperatures and pH: (a) pH 13 and T = 250 °C, (b) pH 7 and T = 250 °C,
(c) pH 13 and T = 300 °C, and (d) pH 7 and T = 300 °C. For clarity, only 5 min of the reaction is shown in this figure.
Figure 7. (a) Selected PXRD patterns illustrating the reaction progress at 250 °C. Variation of cell parameters (b) and particle size (c) with respect
to time of in situ PXRD at temperature 250 °C.
(n) of the diameter and thickness of nanoplate growth were
found to be 1.36 and 1.63, respectively, indicating nucleation
controlled growth kinetics of Bi2WO6.29,67 Earlier growth
kinetics analysis of Bi2WO6 reported n values in the range
between 0.54 and 0.77 using the same kinetic model as the
present study. It was suggested that the growth is diffusion-
controlled.27 However, in the report of Zhou et al. the intensity
of the Bragg diffraction was used to evaluate the extent of the
reaction; furthermore, no preferential particle growth was
considered. The present study not only shows the preferential
growth of nanoplatelets but also establishes the anisotropic
growth rates of the nanoparticle.
Continuous Flow Hydrothermal Synthesis of Bi2WO6.
The knowledge gained from the in situ experiments was used
to develop conditions for synthesis of phase-pure Bi2WO6 in a
continuous flow supercritical reactor.39 Flow syntheses were
carried out with a stoichiometric mixture of Bi and W
precursor (2:1, sample F2) at different temperatures 250, 300,
350, and 390 °C. The product obtained was always found to
consist of mixed phases of Bi2WO6 and disordered
(Bi0.933W0.067)O1.6 with phase fractions of 48.5:51.5%,
53.39:46.61%, 63.68:36.32%, and 68.79:31.21% at 250, 300,
350, and 390 °C, respectively (see the Supporting Information
sections S2 and S3). This is in contrast to the in situ TS and
PXRD results where pure phase Bi2WO6 was obtained at the
F
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Figure 8. Fitted curve with growth of Bi2WO6 nanoplate diameter and
thickness.
end of the reaction when using stoichiometric precursor.
Although the amount of impurity decreased with increasing
synthesis temperature, completely phase-pure Bi2WO6 could
not be obtained with the F2 precursor. TEM images clearly
illustrate the presence of two different types of particles in all
the F2 samples (Figure 9). Note that the TEM image of the
F2@250 °C sample only has few platelets indicating a lower
percentage of Bi2WO6 than that obtained from Rietveld
refinement on representative samples. The explanation of the
formation of the impurity phase may be gathered from the in
situ PXRD and TS analysis. The in situ PXRD data shows that
at 250 °C the transformation from (Bi0.933W0.067)O1.6 to
Bi2WO6 is very slow. Therefore, it might be concluded that
(Bi0.933W0.067)O1.6 did not have enough time to completely
transform to Bi2WO6 as the flow synthesis is very fast. At high
temperature, the transformation is faster, but phase-pure
Bi2WO6 still could not be obtained in flow synthesis with
the F2 precursor. The in situ TS analysis shows that the bond
2+
2−
formation between Bi2O2 and WO4 requires significant
2−
incubation time as more and more WO4 molecules have to
arrange themselves in between the Bi2O2 layers to initiate
2+
Figure 9. TEM images for all flow synthesized samples. F2 denotes
the molar ratio of Bi and W was 2:1, while F1 represents a molar ratio
of 1:1.
bond formation between the layers. This step is crucial for the
formation of a completely pure phase Bi2WO6 sample. If more
2−
WO4 tetrahedra are present in the solution, then the
2+
2−
the first step, precursor molecular complexes of disordered
interaction between Bi2O2 layers and WO4 tetrahedra
should be better facilitated. This can be achieved by increasing
the molar ratio of Bi to W to 1:1 (precursor F1).
2−
Bi2O22+(length ∼ 13 Å) layer interact with WO4 tetrahedra
2−
leading to the stacking of WO4 tetrahedra in between the
2+
Bi2O2 layers. This interaction leads to the formation of the
Indeed, when the molar ratio was changed to 1:1 (F1), the
impurity phase fraction of disordered (Bi0.933W0.067)O1.6 at 250
°C decreases to 3−4% (see Figure S4). This change is also
clear in the corresponding TEM images (Figure 9), where the
Bi2WO6 platelets now are present along with a minor
(Bi0.933W0.067)O1.6 phase. Completely phase-pure Bi2WO6 is
obtained at higher temperatures of 300, 350, and 390 °C with
the F1 precursor (see Figure S5). The TEM images also
confirm the formation of phase pure samples as no other
particles are seen except the Bi2WO6 platelets.
disordered, but crystalline, phase of (Bi0.933W0.067)O1.6, as
confirmed by in situ PXRD. As the reaction progresses, the
2−
disordered phase stacks more WO4 tetrahedra between the
layers of Bi2O22+. In the second step, the disordered phase gets
converted to Bi2WO6 nanoplatelets by “sideways” attachment/
growth along the ac plane. Kinetic analysis of the growth
reveals that the “sideways” ac plane growth is three times faster
than that along the “stacking” b direction explaining the
universal observation of nanoplatelet morphology. The atom-
istic information obtained from in situ measurement was used
to obtain phase-pure Bi2WO6 in continuous flow hydrothermal
synthesis. Since the in situ data reveal that the crucial step in
formation of Bi2WO6, is the presence of WO42− tetrahedra, the
molar ratio of tungsten must be increased relative to the
stoichiometric ratio of Bi/W = 2:1. The in situ data also clearly
show that the two-step reaction progresses much faster at high
CONCLUSIONS
■
The present in situ PXRD and PDF study demonstrates that
the initial presence of layers of Bi2O2 is responsible for the
2+
preferential growth of nanoplatelets of Bi2WO6 in hydro-
thermal synthesis. The crystallization occurs in two steps. In
G
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temperatures. Indeed, by using the surplus of W in the
precursor and perform synthesis at high reaction temperatures,
it is possible to obtain phase-pure Bi2WO6 nanoplatelets in a
scalable and sustainable continuous-flow supercritical process.
The present study provides a rationale for the observed
Bi2WO6 nanoplatelet morphology reported in the large body of
papers on the subject, and it reiterates the importance of
having atomic-scale information about the prenucleation
clusters if we are to properly understand and control
nucleation and growth phenomena.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01249.
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Total scattering data treatment and fitting protocol for
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(PDF)
AUTHOR INFORMATION
■
Corresponding Author
Bo B. Iversen − Center for Materials Crystallography,
Department of Chemistry and iNANO, Aarhus University,
8000 Aarhus, Denmark; orcid.org/0000-0002-4632-1024;
Email: bo@chem.au.dk
Authors
Dipankar Saha − Center for Materials Crystallography,
Department of Chemistry and iNANO, Aarhus University,
8000 Aarhus, Denmark
Espen D. Bøjesen − Center for Materials Crystallography,
Department of Chemistry and iNANO, Aarhus University,
8000 Aarhus, Denmark; orcid.org/0000-0002-9352-9514
Aref Hasen Mamakhel − Center for Materials Crystallography,
Department of Chemistry and iNANO, Aarhus University,
8000 Aarhus, Denmark
(15) Zhou, Y.; Vuille, K.; Heel, A.; Patzke, G. R. Studies on
Nanostructured Bi2WO6: Convenient Hydrothermal and TiO2-
Coating Pathways. Z. Anorg. Allg. Chem. 2009, 635 (12), 1848−1855.
(16) Li, W.; Zhu, Y.; Yu, J.; Jaroniec, M.; Jiang, C. Chapter 12 -
Synthesis and performance enhancement for Bi2WO6 photocatalysts.
Interface Sci. Technol. 2020, 31, 379−413.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01249
́
́
(17) Jimenez, R.; Castro, A.; Jimenez, B. Evidence of ferroelastic−
ferroelastic phase transition in BiMoxW1-xO6 compounds. Appl. Phys.
Lett. 2003, 83 (16), 3350−3352.
Notes
The authors declare no competing financial interest.
́
́
́
́
(18) Castro, A.; Begue, P.; Jimenez, B.; Ricote, J.; Jimenez, R.; Galy,
J. New Bi2Mo1-xWxO6 Solid Solution: Mechanosynthesis, Structural
Study, and Ferroelectric Properties of the x = 0.75 Member. Chem.
Mater. 2003, 15 (17), 3395−3401.
(19) Noguchi, Y.; Satoh, R.; Miyayama, M.; Kudo, T. New
Intergrowth Bi2WO6‑Bi3TaTiO9 Ferroelectrics. Nippon Seramikkusu
Kyokai Gakujutsu Ronbunshi 2001, 109 (1265), 29−32.
ACKNOWLEDGMENTS
■
The work was supported by the Villum Foundation, and the
Danish National Research Foundation (DNRF93). We thank
ESRF, France, and MAXLabII, Sweden, for beamtime, and we
gratefully acknowledge beamline scientist Dr. Andrea Bernas-
coni for help during data collection at beamline ID11 at ESRF.
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