CHẾ tạo vật LIỆU nền Bi2WO6 có cấu TRÚC NANO và NGHIÊN cứu một số TÍNH CHẤT của CHÚNG tt tieng ang

PREFACE
Fujishima and Honda have investigated photocatalytic activity of TiO 2 in
1972. However, due to the fact that TiO2 have wide band gap semiconductor at
3.2 eV. It can only absorb ultraviolet light region. Currently, this is a major
obstacle preventing the application of TiO 2 in water streatment.
From these difficulties, seeking the kind of light – driven materials is
nescessary. Some materials which ability photocatalytic activity in visible light
have been studied such as MnWO4, BiVO4, Ag3PO4... In among light –driven
materials, Bi2WO6 material- the narrow band gap exhibited the highest
photocatalytic activity in visible light (2.7 eV). The effiency of photocatalist of
Bi2WO6 is stronger than that of TiO 2 in visible light region.
The Bi2WO6 nanopowders have sucessfully prepared using some
methods such as: Hydrothermal method, sol-gel method, coprecipitation
method, sonochemical method.... However, there are few published paper
using the microwave-assisted method. Moreover, the properties and
photocatalytic activities of Bi2WO6 depend on the preparation conditions of
each of method, therefore, controlling conditional preparation of microwave
assisted method to obtain good crystal of Bi2WO6 is necessary. One another
hand, to apply Bi2WO6 in water streatment, we investigate some modified
Bi2WO6 materials such as compound with another semiconductor, replacement
Bi, W, O by doping another elements.
From reason above, the Bi2WO6 material is objectives of my thesis with title
”The synthesis of the Bi2WO6 –based and the studies of its properties”.
Thesis Objectives: (1) to synthesize Bi2WO6 via a microwave assisted
method; study effect of preparation conditions on physical properties and
photocatalytic activities. (2) To study enhanced photocatalytic activities
Bi2WO6 material by composite with another semiconductor and doping; (3)
Combine some methods to obtain Bi2WO6 with high surface and modify
Bi2WO6 by doping some elements to enhance photocatalytic activities.
Study Objects and method approach: My thesis focus on studying physical
properties and photocatalytic activity of pure Bi2WO6 and modified Bi2WO6 in

term of experimental method. In my thesis, the mircowave assisted method was
used to synthesize Bi2WO6 with high photocatalytic activities. The Bi2WO6
materials were synthesize in division of solid state physics- electronics LAB,
Facculty of Physics, Hanoi National University of Education. The measurements
were carried out by modern equipments with high reliability at national research
centers, a few measurements were done in foreign laboratories.
Scientific Meaning and Practical Significance: The major of thesis
study experimental process to synthesis Bi2WO6 and modified Bi2WO6
materials via a microwave assisted method. Investigation of effect of
preparation conditions on physical properties and photocatalytic activity of
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Bi2WO6 via a microwave assisted method, These results will contribute to the
understanding of Bi2WO6 photocatalytic in terms of basics and applicationoriented research.
Thesis Contents: The content of the thesis include (i) general
introduction of Bi2WO6 materials; photocatalytic advantages and drawbacks of
Bi2WO6; methods to improve photocatalytic activities of Bi2WO6 and some
previous experimental and theoretical studies on Bi2WO6 and modified
Bi2WO6; (ii) study experiment process to synthesis Bi2WO6 material via a
microwave assisted method and the influence of preparation conditions on the
physical and photocatalytic properties of Bi2WO6; (iii) major results of the
influence of doping and composite on the photocatalytic activities of Bi2WO6
doping Gd and Bi2WO6/BiVO4 composites.(iv) new approach-microwave
assisted combining hydrothermal method to synthesis high photocatalytic
activity Bi2WO6 and N doped Bi2WO6, the mechanism of photocatalytic
activity enhancement in the nanocomposites was investigated in detail.
Thesis Layout: Thesis is presented in 136 pages with 73 Figures and 21
Tables, including the heading, 5 chapters, and conclusions; a list of
publications, and references. Structures of the thesis as follows:

Introduction: Introducing research situation and the necessary of the
thesis; the physical meaning, the content and the structure of the thesis.
Chapter 1: Overview of physical, chemical and photocatlytic properties
of Bi2WO6 in previous studies on understanding and improving
photocatalytic properties of Bi 2WO6-based material.
Chapter 2: Experimental methods and processes to synthesize materials,
basic principles of expermental measurements used to analyze crystal
structure and physical properties of materials;
Chapter 3: Presenting results of Bi2WO6 synthesized via a microwave
assisted method, the influence of preparation conditions on structure and
photocatalytic properties of Bi2WO6; the factors effect on the photocatalytic
process.
Chapter 4: Presenting the effect of doping of Gd and composite of BiVO4
on physical and photocatalytic properties of Bi2WO6, and the mechanism of
photocatalytic activity enhancement in the modified Bi2WO6 materials.
Chapter 5: Presenting the syntheisis of Bi2WO6 and N doped Bi2WO6 via
a two step microwave assisted – hydrothermal method. Influence of doping
N on the physical and photocatalytic properties of Bi2WO6. Role of electrons
– holes in photocatalytic reaction.
Conclusion: Presenting the major results of the thesis.
The research results of the thesis have been published in 5 scientific
works in which there are 4 articles in international journals, 1 articles in
national journals, some results are analysized to submits on journals.
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CHAPTER 1
INTRODUCTION TO Bi2WO6 MATERIAL
1.1. Introduction
1.1.1. Structural characterization of Bi2WO6

Bi2WO6 belongs to the Aurivillus typewith a general formlar of: Bi 2An1Bn O3n+3, with A being Ca, Sr, Ba, Pb, Bi, Na, K và B là Ti, Nb,Ta, Mo, W, Fe
for n=1 and B =W. It is well-known that Bi 2WO6possesses both the interesting
ferroelectric and ferromagnetic properties.
The crystal structure of Bi 2WO6belongs to orthorhombic lattice, space
group P21ab, with lattice parameters a= 5.456 Å, b=16.430 Å, c=5.438 Å;
α=90o, β=90o, γ=90o. Bi2WO6 can be considered as (Bi 2O2)2+layers that
interleaves with the perovskite-type (WO4)2-structure. Figure 1.1 shows the
illustrative model in which Bi, W, O atoms arranges in the unit cell of Bi 2WO6.

Figure1.1: Illustration of Bi2WO6 crystal structure
1.1.2. Optical property
Lie et al investigated the electronic structure and density of states of
Bi2WO6 using the density functional theory. It was found that Bi 2WO6 is a
direct-band gap semiductor with Eg~2.75 eV. Experimentally, Xu et al
reported the band gap of 2.8 eV from the UV-Vis spectrum, in good agreement
with that determined from the theoretical calculation.
1.2. Photocatalytic performance
Bi2WO6 shows a photodegredation behavior for various organic
compounds composed of highly stable carbon rings. In addtion, Bi 2WO6 also
exhibits photocatalytic characterization for antibodies, including ciprofloxacin,
tetracycline hydrochloride, norfloxacin, levofloxacin...
Based on those studies in the literature, Bi 2WO6 could be a promising
candidate for environmental applications.
1.3. Synthesis methods
Bi2WO6 has been successfully synthesized using a varitety of different
techniques, such as hydrothermal solvothermal, sol-gel, sonochemical methods.

3

1.4. The influence of experimental conditions on the physical properties
A number of reports indicated that experimental factors, e.g. pH in
solution, strongly affects on the phase formation of Bi 2WO6. A sequence of
chemical reaction to form Bi 2WO6 can be described in the following equations:
Na2WO4.2H2O + 2HNO 3
H2WO4 + 2NaNO3 + 2H2O
Bi(NO3)3 + H2O
BiONO3 + 2HNO3
BiONO3 + H2O
Bi 2O2(OH)NO3 + HNO3
Bi2O2(OH)NO3 + H2WO4
Bi2WO6 + HNO3 +H2O
However, when the pH is larger than 8, the reaction happens in an another
route,
Bi2O2(OH)NO3 + 2WO42- + 3OHBi14W2O27 + 7NO3- + 5H2O
These equations suggest that, the precursors Bi(NO3)3and Na 2WO4form
the Bi2WO6 or Bi14W2O27 which is dependent on the pH solution. The effect of
the pH solution on the morphology of Bi 2WO6was reported. The authors
showed that, the superstructure phase of Bi 2WO6 is unstable in the case of the
pH greater than 2.5 and completely vanished if pH=7.5. The morphology of
Bi2WO6 becomes rectangular sheets with width of 80 nm and length of 1-3 μm
in sizes.

Figure 1.10 Morphology of Bi2WO6 prepared at different values of pH: pH=1
(a), pH=4.5 (b), pH=7 (c).
The role of the morphology induced by the pH on the photocatalytic
performance on Bi 2WO6 is shown in Fig 1.11. It can be seen that Bi 2WO6 that
exists in the superstructure (low pH) has the higher photocatalytic performance
compared to that in the retangular sheets (pH=7.5).

Figure 1.11Dependence of photocatalytic efficiency on the morphology of
Bi2WO6.
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For pH=7.5, but with additional surfactants like Ethylene Glycol, Xu and
coworkers reported that the morphology transforms from the sheet-like shape to
nano-sizedspheres.
1.5. Enhance the photocatalytic performance
The photodegradation performace Bi 2WO6 depends on many factors,
including the electron-hole recombination, specific surface area, band gap. As a
result, in order to enhance its efficiency one can control these factors by
modifying the original material. Two common techniquesare dopands and
making a composite with other compounds.
1.5.1. Doping Bi2 WO6
The modification of Bi 2WO6 based on doping with various elements has
been widely used in the literature: Gd, Mo, Ce, Br, Ba, Lu, Eu, Y, F, N. The
reuslts found that the doping of Bi 2WO6 produces a better photocatalytic
performance in comparion with the pure Bi 2WO6 materials.
1.5.2. Composite Bi2WO6
In addition to the doping strategy, the fabrication of Bi 2WO6 in the
composite structure has attracted interest in the photocatalytic enhancement.
The advantage of such a method is that the composite materials possess both
the physical characterization of individual components. The investigations on
the Bi2WO6 composite can be divided into two parts: (i) Bi 2WO6 with the
semiconductor ZnWO 4, Co3O4, ZnO, BiVO4, Bi2O3, Bi2S 3, graphene oxide,
WO3, g-C 3N4, TiO 2, CeO2, Ag3PO4. (ii) Bi2WO6 with metal naoparticles with a
high conductivity such as: Ag, Au, Cu, Pt.
CHAPTER 2
EXPERIMENTAL TECHNIQUES AND SAMPLES – ANALYZED

TECHNIQUES
2.1 Synthesis Bi2WO6
The Bi2WO6 nanopowders were synthesized by microwave assisted
method, a schematic diagram of experimental process as follows:

Figure 2.7 The schematic diagram synthesis of Bi2WO6 nanostructure
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In typical synthesis: 2.5 mmol of sodium tungstate dihydrate
(Na2WO4.2H2O) and 5 mmol of bismuth nitrate (Bi(NO 3)3.5H2O) were first
dissolved in 100 ml distilled water with 30 min stirring at room temperature.
The obtained solution was heated by a Sharp- modified microwave oven with
power of 750 W for 20 min. After microwave processing, the solution was
cooled to room temperature. The suspended was separated by entrifugation,
washed with deionized water and acetone for several times, then dried in an
oven at 70 C for 24 h. To investigate the effect of experimental conditions on
the physical properties, (1) the samples were finally annealed in air for 5 h at
temperatures of 400, 500, 600, and 700 oC, respectively. (2) the pH of solution
before heating was controlled at pH=1, 3, 5, 7, 9. (3); the time of irradiation
microwave was changed 5, 10, 15, 20 minutes.
2.2. Synthesis of Bi2WO6/BiVO4 nanocomposites
Bi2WO6/BiVO 4
nanocomposites
were
synthesized using diagram as
shown Fig 4.1. In a typical
synthesis, 2.5 mmol of
Na2WO4.2H2O and 5 mmol
of Bi(NO3)3.5H2O were

dissolved in 100 ml distilled
water with stirring at room
temperature
to
obtain
Figure 2.8 The schematic diagram synthesis of
solution A; and 2.5 mmol of
Bi2WO6/BiVO4 nanocomposite
NH4VO3 and 2.5 mmol of
Bi(NO3)3.5H2O were dissolved in 100 ml distilled water with stirring at room
temperature to obtain solution B. Then solution A and B were mixed with
appropriate Bi 2WO6:BiVO 4 molar ratio. The mixed solution was heated by a
Sanyo microwave oven with 750W for 20 min. After microwave processing,
the solution was cooled to room temperature. The resulted precipitate was
separated by centrifugation, washed with deionized water and acetone for
several times then dried in an oven at 70 °C for 24 h.Photocatalytic activity of
the nanoparticles was evaluated by the decolorization of Rhodamine B (RhB)
under visiblelight-irradiation. In the experimental setup, a 300W Xe lamp was
employed as the light source and a 420 nm cut-off filter was used to provide
visible light irradiation. Samples with Bi2WO6:BiVO 4 ratio 100:0; 90:10; 80:20;
70:30; 60:40; 50:50, 0:100 were indexed Bi2WO6, M 90-10, M 80-20, M 70-30,
M 60-40, M 50-50, BiVO4, respectively.
2.3. Synthesis of Gd doped Bi2WO6
Experimental process of Bi 2WO6/BiVO 4 nanocomposites was shown in
Fig 2.9.
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Figure 2.9 schematic diagram synthesis of Bi2WO6 doping Gd via a microwave
assisted method.

The synthesis process is similar as that in our chapter 3. Different content
of Gd-doping was obtained by mixing Bi(NO 3)3·5H2O and Gd(NO3)3·6H2O
with atomic ratio of Gd:Bi = 0, 1.0, 2.5, 5.0, and 7.5%.
2.4. Synthesis of N doped Bi2 WO6
Experimental process to synthesis Bi2WO6 via a two step microwave
assisted – hydrothermal method as follows:
In the first step of microwave-assisted synthesis, 2.5 mmol Na 2WO4.
2H2O and 5 mmol Bi(NO3)3.5H2O were dissolved in 100ml distilled water with
continuous stirring for two hours, and the obtained solution was heated using a
75% power Sharp microwave oven for 20 min and then cooled to room
temperature. A range of N-doping content was obtained using molar ratios for
the reagents C4HN2O and Bi(NO 3)3.5H2O of 0%, 0.1%, 0.25%, 0.5%, and
0.75%. In the second step involving hydrothermal synthesis, the solution from
the first step was transferred into a Teflon-lined autoclave and filled to 80% of
the total volume. The autoclave was then sealed into a stainless steel tank and
kept at 180 C for 12 h. Following this, the reactor was left to cool to room
temperature naturally. The resulting precipitatewas separated by centrifugation,
washed several times with deionized water and ethanol, and then dried in an
oven at 70 oC for 24 h in air. The samples obtained using the above process are
denoted here as MH:N-x (x:0, 0.1, 0.25, 0.5, 0.75). H:N-0 and M:N-0 Bi2WO6
samples were also prepared using a one-step hydrothermal method and a onestep microwave-assisted method, respectively. The properties of M:N-0
Bi2WO6 nanoparticles have been investigated in chapter 3 and 4. In this chapter,
we present a study of MH:N-x and H:N-0 Bi2WO6 nanoparticles.
Photocatalytic activity of the nanoparticles was evaluated by the
decolorization of methylene-blue (MB) under visible-lightirradiation. In the
experimental setup, a 300 W Xe lamp was employed as the light source and a
420 nm cut-off filter was used to provide visible-light-irradiation. In every
experiment, 0.1 g of nanoparticles was added to 100 ml of MB solution (10 -5
mol/l). Before being irradiated, the suspension was magnetically stirred in the

dark for 3 h to ensure the establishment of and sorptionedesorption equilibrium
between the photocatalyst and MB. After a given irradiation time, the
7


suspension was centrifuged to remove the catalyst immediately, and UV-VIS
absorbance measurement was performed. The decolorization of MB was
monitored by the decrease of absorption peaks. For reusability test, the Bi 2WO6
nanoparticles were immersed in ethanol for 3.0 h and rinsed with deionized
water, and then dried at 370 K. After this, the cleaned Bi 2WO6 nanoparticles
were reused to test photocatalytic activity.
2.5 Samples-analyzed instrume nts and techniques
The crystallography of the obtained nanoparticles was analyzed using a
Bruker D5005 X-ray diffractometer (XRD). The UV–VIS diffuse reflectance
was performed using a Jasco V670 spectrophotometer. The morphology of the
nanoparticles was observed by scanning electron microscope (SEM, S4800Hitachi). High resolution transmission electron microscopy (HRTEM) images
were conducted on a JEOL 2010 electron microscope operated at 200 kV. The
surface areas of the samples were determined by using the Brunauer–Emmett–
Teller (BET) analysis of the nitrogen adsorption–desorption isotherm, which
were measured at 77 K using Autosorb, Quantachrome, USA. The
photoluminescence (PL) measurements were carried out with a
Spectrofluorometer of Horiba Jobin Yvon NanoLog using 390 nm excitation
from a 500W Xenon lamp.
CHAPTER 3
A STUDYING PROPERTIES, PHOTOCATALYTIC ACTIVITIES OF
Bi2WO6 SYNTHESIZED VIA A MICROWAVE ASSISTED METHOD
3.1. Effect of time irradiation and pH on the physical properties of Bi2WO6
The effect of microwave irradiation on the structure of Bi 2WO6 was
shown Fig 3.1. The results indicate that the good crystal Bi2WO6 was obtained
at the time of microwave irradiation of 20 minutes.

Figure 3.1 XRD patterns of Figure 3.2 XRD patterns of Bi WO
2
6
Bi2WO6 nanoparticles with microwave nanoparticles synthesized at pH=1, 3, 5,
irradiation time of 5, 10, 15, 20 minutes. 7, 9, 11.

8


Fig 3.2 presents XRD patterns of Bi2WO6 synthesized at pH = 1, 3, 5, 7,
9, 11 with microwave irradiation time of 20 minutes and annealing temperature
of 500 oC. The XRD results indicate that single crystal phase of Bi2WO6
obtained at low pH.
3.2 Effect of annealing temperature on the physical properties and
photocatalytic activity of Bi2WO6
Fig 3.3 shows the XRD patterns of Bi 2WO6 annealed temperature of 400,
500, 600, and 700 oC. The nanoparticles as-prepared sample would be in
amorphous state, thus yield no apparent diffraction peaks. With annealing
treatment above 500 oC, the crystalline quality of Bi2WO6 nanoparticles gets
better, and the impurity Bi 14W2O27 phase disappears. The diffraction peaks of
the nanoparticles obtained with annealing temperature above 500 oC can be
well-indexed to pure orthorhombic Bi2WO6 phase according to the JCPDS Card
(No.39-0256), as presented in Fig. 3.3. This indicates that good crystalline
quality Bi2WO6 nanoparticles can be synthesized by fast microwave-assisted
method with low temperature (500 oC) annealing treatment.

Figure 3.3 XRD patterns of as-prepared Bi2WO6 and Bi2WO6 nanoparticles annealed
at, 400, 500, 600 and 700 oC for 5 h.

Fig 3.6 shows the SEM images of Bi 2WO6 with difference annealing
temperature. The increase of particle size with increasing annealing temperature
is clearly observed in the SEM images. The average particle size of Bi 2WO6
nanoparticles were found to be 30, 60, 80, and 400 nm for annealing
temperatures of 400, 500, 600, and 700 oC, respectively. For the Bi 2WO6
nanoparticles obtained with 700 oC annealing, the particle size obtained from
SEM is more than 10 times of the particle size estimated from XRD. We
suggest that this extreme difference would be mainly correlated with the
layered structure of Bi 2WO6 nanoparticles: the XRD results would mainly
indicate the thickness of layered nanoparticles, while SEM images would
mainly indicate the plane size of layered nanoparticles. The layered structure of
the Bi2WO6 nanoparticles is confirmed by HRTEM image, as shown in Fig.
3.6(e). This image was taken for the Bi 2WO6 nanoparticles obtained with
annealing temperature of 500 oC. Fig. 3.6(e) shows that the average space
9


between adjacent planes is 0.31 nm, which is assigned to the (131) planes of
orthorhombic layered structure of Bi 2WO6.

Figure 3.6 SEM images of the nanoparticles obtained with annealing
temperatures of 400 (a), 500 (b), 600 (c), 700 oC (d) and TEM image of Bi2WO6
nanoparticles obtained with annealing temperature of 500 OC (e).

Figure 3.7 Nitrogen absorption and
Figure 3.10 Uv-vis diffuse reflectance
desorption curve of Bi 2WO6 at
spectra of Bi 2WO6 with annealing
annealing temperature of 400 500, 600
temperatures of 400, 500, 600, and

o
and 700 C.
700 oC.
The surface areas of the Bi2WO6 nanoparticles were estimated by BET
experiments, the results are shown in Fig 3.7. With increasing annealing
temperature, the particle surface area deceases gradually. For the Bi2WO6
nanoparticles obtained with annealing temperatures of 600 and 700 oC, their
surface area is similar, although the SEM images indicated very different
particle size. This further suggests that the SEM images indicated only the
plane size of the layered nanoparticles. The thickness of the layered
nanoparticles would be similar for the Bi2WO6 nanoparticles obtained with
annealing temperatures of 600 and 700 oC, thus their surface areas are similar.
The above results show that with increasing annealing temperature, the
crystalline quality of Bi2WO6 nanoparticles gets better, which improves the
visible-light-absorption. This would result in more photo-generated electrons
and holes, thus be helpful to improve photocatalytic activity. However, with
increasing annealing temperature, the surface area of the Bi2WO6 nanoparticles
gets smaller. This would result in less active sites, thus decrease the
photocatalytic activity. Whether visible-light-absorption or surface-area plays
10


major role for improving photocatalytic activity would depend on the efficiency
of transportation of photogenerated charges to active sites. If surface area plays
a major role, this would indicate high efficiency of transportation of
photogenerated charges to active sites. Then, these nanoparticles would be
promising for achieving high photocatalytic activity. Fig. 3.10 shows the UVVIS diffuse reflectance spectra of the nanoparticles obtained with annealing
temperatures of 400, 500, 600, and 700 oC. According to the spectra, the
samples present photo-absorption properties from UV to visible light shorter
than 450 nm, which implied the possibility of good photocatalytic activity

under visible light-irradiation.The band gaps of the Bi2WO6 nanoparticles
obtained with annealing temperature of 500, 600 and 700 oC were estimated to
be 2.93, 2.89 and 2.83 eV, respectively. For the nanoparticles obtained with 400
o
C annealing, there are two phases of Bi2WO6 and Bi14W2O27. These two phases
would have different contribution for the absorption spectrum, thus the above
formula could be not simply applied to one spectrum for obtaining the band
gaps of two phases.

Figure 3.15 Cycling runs of the
Figure 3.13 Absorbance change of MB
photocatalytic degradation of MB
at 665 nm as a function of visible light
under visible light in the presence of
irradiation time in the presence of
Bi2WO6 nanoparticles annealed at 500
Bi2WO6 nanoparticles annealed at
o
C.
different temperatures.
To compare the photocatalytic activities of the nanoparticles annealed at
different temperatures. The absorbance time variations (A t/Ao) of the peaks at
668 nm for nanoparticles annealed at 400, 500, 600, and 700 oC are plotted in
Fig. 3.13 (At is time-dependent absorbance and Ao is initial absorbance). Fig.
3.13 shows that the nanoparticles obtained with annealing temperature of 400
o
C have the lowest photocatalytic activity. This would be correlated with the
significant impurity phase of Bi 14W2O27 nanoparticles. These nanoparticles
have similar visible-light absorption with Bi2WO6 nanoparticles, and they have
bigger surface area. Thus the low photocatalytic activity would indicate very

low efficiency of transportation of photo-generated charges to active sites in
Bi14W2O27 nanoparticles. For the Bi2WO6 nanoparticles obtained with annealing
temperature above 500 oC, the photocatalytic activity decreases gradually with
11


increasing annealing temperature. We have shown that the visible-light
absorption of the Bi2WO6 nanoparticles increases gradually with increasing
annealing temperature; while the surface area of the nanoparticles decreases
gradually with increasing annealing temperature. Thus, for the Bi2WO6
nanoparticles, the surface area plays more important role than visible-light
absorption for enhancing photocatalytic activity. This indicates efficient
transportation of photo-generated charges to active sites in the Bi2WO6
nanoparticles obtained with annealing at 500 oC.
Considering practical application, it is important and necessary to
investigate the reusability and stability of a photocatalyst. To confirm the
reusability and stability of the photocatalytic performance of the Bi2WO6
nanoparticles, circulating runs in the photocatalytic degradation of MB under
visible-light irradiation were checked. As shown in Fig. 3.15, comparing with
the first run, the photocatalytic activity losses ~5%, 6%, and 10% for second,
third, and fourth run, respectively.
CHAPTER 4
STUDY OF MODIFIED Bi2WO6 NANOPOWDERS VIA A
MICROWAVE ASSISTED METHOD
4.1. Results of synthesis and study properties of Bi2WO6/BiVO4
nanocomposites
Figure 4.3 presents the XRD patterns of
pure Bi2WO6 nanoparticles, Bi 2WO6/BiVO4
nanocomposites,
and

pure
BiVO4
nanoparticles. The characteristic diffraction
peaks in Fig. 4.3a can be well-indexed to pure
orthorhombic Bi 2WO6 phase according to the
JCPDS No.39-0256. The characteristic
diffraction peaks in Fig. 4.3e can be wellindexed to pure monoclinic scheelite Figure 4.3 XRD patterns of pure
BiVO4 phase according to JCPDS No.75- Bi2WO6 (a), M 80-20 (b), M 70-30
1867. The
diffraction peaks
of (c), 50-50 (d) pure BiVO (e)
4
Bi2WO6/BiVO 4 (80–20) nanocomposites
in Fig. 4.3 have contributions from both orthorhombic Bi2WO6 and monoclinic
scheelite BiVO4 phases, and no impurity peaks were found. When BiVO 4
content increases from 20 to 50%, the intensities of diffraction peaks of BiVO 4
increases significantly, and the intensities of diffraction peaks of Bi2WO6
decreases significantly (Figs. 4.3b–d). The diffraction peak positions of
nanocomposites match precisely to values of pure nanoparticles, indicating
there would be no incorporation of W and V ions in the Bi 2WO6/BiVO 4
nanocomposites.
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Figure 4.8 Raman spectra of pure
Bi2WO6, BiVO4 and Bi2WO6/BiVO4
samples

Figure 4.5 HRTEM images and EDX
spectra of M 70-30 sample.

The HRTEM image of 70–30 Bi2WO6/BiVO 4 nanocomposites is shown
in Fig. 4.5. The lattice spacing of 0.31 nm corresponds to the d spacing between
adjacent (131) crystallographic planes of Bi 2WO6, while the fringes of 0.27 nm
match the (024) planes of m- BiVO4. The Raman spectra and TEM images
indicate that in the nanocomposites, the Bi 2WO6 and BiVO4 nanoparticles
would be in close contact, which could be helpful for separation of
photogenerated free carriers, thus improving photocatalytic activity.
Figure 4.7 presents the diffuse
reflection spectra of pure Bi 2WO6
nanoparticles,
Bi 2WO6/BiVO4
nanocomposites,
and
pure
BiVO4
nanoparticles.
The
pure
Bi2WO6
nanoparticles show photo-absorption from
UV to visible light shorter than 450 nm.
The
absorption
range
of
the
Bi2WO6/BiVO 4
nanocomposites
is

substantially extended toward visible light,
which is because the pure BiVO4 Figure 4.7 UV–Vis diffuse
nanoparticles have photo-absorption from reflectance spectra of pure
UV–VIS light up to 550 nm. The Bi2WO6,
BiVO4
and
significant increase of visible light Bi2WO6/BiVO4
absorption could be very helpful for
improving the photocatalytic activity. Figure 4.7 indicated that the absorption
of Bi2WO6/BiVO 4 nanocomposites is not a simple linear summation of the
absorption of pure Bi2WO6 and BiVO4 nanoparticles. This indicates that in the
nanocomposites, the Bi2WO6 and BiVO4 nanoparticles would be in close
contact and there would be charge transfer between Bi2WO6 and BiVO4
nanocrystallites, consistent with the SEM and TEM results
To compare the photocatalytic activity of the samples, the absorbance time
variation (At/Ao) of the 544 nm peak of RhB in the presence of pure Bi2WO6,
80–20 Bi2WO6/BiVO4, 70–30 Bi2WO6/BiVO4, 50–50 Bi2WO6/BiVO 4, and pure
13


BiVO4 nanoparticles are plotted in
Fig. 4.7 (At is timedependent
absorbance, and Ao is initial
absorbance).
In
order
to
quantitatively
analyze
RhB

degradation,
the
Langmuir–
Hinshelwood model was applied: ln(At/Ao) = kt, where k is the reaction
Figure 4.7 Absorbance change of 553
rate constant. The fitting results are
nm peak as a function of irradiation
presented in Fig. 4.7b. Figure 4.7b
time in the presence of pure BiVO 4,
indicated that the photocatalytic
Bi2WO6 and Bi2WO6/BiVO4
activity of pure Bi2WO6 nanoparticles
nanocomposite
is much higher than that of pure BiVO4 nanoparticles. The photocatalytic
activity of all Bi2WO6/BiVO 4 nanocomposites is higher than that of pure
Bi2WO6 nanoparticles. The photocatalytic activity of 70–30 and 50–50
Bi2WO6/BiVO 4 nanocomposites is higher than that of pure Bi2WO6
nanoparticles, while the photocatalytic activity of 80–20 Bi2WO6/BiVO4
nanocomposites is lower than that of pure Bi2WO6 nanoparticles. For the
Bi2WO6/BiVO 4 nanocomposites, as the content of BiVO4 increases, the
photocatalytic activity first shows a significant increase, then, slowly decreases;
and the 70–30 Bi2WO6/BiVO 4 nanocomposites have the highest photocatalytic
degradation efficiency. These phenomena indicate complex mechanism of
photocatalytic process in the Bi2WO6/BiVO4 nanocomposites.
The
recombination
rate
of
photogenerated electron-hole pairs of the
Bi2WO6/BiVO 4 nanocomposite samples was

investigated by PL emission experiment, and
the results are presented in Fig. 4.8. In the
figure, the PL intensities of all the samples
have been corrected taking into account the
different absorption of 390 nm excitation
light. PL emission is mainly resulted from
the recombination of free carriers. Figure 4.8 PL emission spectra
Therefore, PL experiment is a useful of pure Bi WO , BiVO and M
2
6
4
technique to survey the recombination rate 80-30;M 70-30 và M 50-50
of photogenerated electron–hole pairs in a samples nanocomposite.
semiconductor: in general, the lower the
corrected PL intensity, the lower the recombination rate of photogenerated
electron–hole pairs. As can be seen in Fig. 4.8, all the Bi 2WO6/BiVO4
nanocomposites have lower recombination rate of free carriers than that of pure
Bi2WO6 and BiVO 4 nanoparticles. This indicates that there would be charge
14


transfer between the Bi 2WO6 and BiVO4 nanocrystallites, and free carriers have
been separated in these two semiconductors, in agreement with the absorption
result in Fig. 4.8. Charge transfer could occur if nanoparticles are in close
contact, it is much easier than energy transfer process, which is strongly
depending on the separation between the centers of nanoparticles. Therefore,
our results support that in the nanocomposites, charge transfer would be happed
between the Bi 2WO6 and BiVO4 nanocrystallites.
4.2. Results of synthesis and study properties of Bi2WO6/BiVO4 nanocomposites
The XRD patterns of pure and Gddoped Bi2WO6 nanoparticles are presented

in
Fig. 4.9. As can be seen in Fig. 4.9, all
these diffraction peaks match well with
orthorhombic Bi 2WO6 (JCPDS Card No.
73-2020), and no peaks of impurity phases
can be observed. This indicates Gd-doping
did not modify the host crystalline
structure and also did not lead to the
generation of any new crystal phase. Figure 4.9 XRD patterns of
Since the ionic radius of Gd 3+ (0.094 nm) Bi2WO6 doping Gd (0, 1.0, 2.5,
is smaller than that of Bi 3+ (0.103 nm), 5.0, 7.5, and 10.0 %).
the lattice parameters of Bi 2WO6 nanoparticles would decrease after Gd 3+ ions
replacing Bi3+ ions.

Figure 4.11 The lattice of Bi2WO6 doping
Gd (0, 1.0, 2.5, 5.0, 7.5 %)

Figure 4.14 Raman spectra of Gd doped
Bi2WO6.

Fig 4.11 shows the lattice parameters of orthorhrombic of Bi 2WO6, the
results indicate that with further increasing Gd-doping to 7.5%, the lattice
parameters do not show further decreasing. This may suggest that at higher Gd
concentration, only a partial of Gd 3+ ions could substitute the Bi 3+ ions in the
Bi2WO6 host lattice. Moreover, the peaks of 790 cm-1 also shift forward longer
wave number and with increasing Gd doping ro 5.7%, these peaks were not
shifted futher. The Raman results agree with that XRD patterns.

15

Table 4.1 initial Gd concentrations and Gd
concentration from EDXS analysis.

Initial Gd0% 1% 2.5% 5% 7.5%
doped
Gd
concentration 0 0.98 2.04 2.20 2.29
from EDXS
Figure 4.16 XPS analysis of
2.5% Gd-doped Bi2WO6
nanoparticles: (a) Bi 4f (b)
W 4f (c) O1s and (d) Gd 4d.

To analyze the Gd concentration in Gd- doped Bi2WO6 nanoparticles,
EDXS measurements were performed. Table 4.6 list the initial Gd-doped
Bi2WO6 nanoparticles and the Gd concentration results from EDXS analyses
for all Gd-doped samples. The EDXS analysis indicated that with initial Gddoping of 1.0, 2.5, 5.0, and 7.5%, the
obtained nanoparticles have the final Gd
concentration of about 0.98, 2.04, 2.20, and
2.29%, respectively. This is in good
agreement with the XRD results. The XRD
and EDXS results indicate that through fast
microwave assisted synthesis, only with
initial Gd-doping up to about 2.5%, Gd3+
ions can successfully substitute the Bi 3+ ions
in the Bi 2WO6 host lattice; while with further
Hình 4.15 UV–Vis diffuse
increasing Gd-doping, amorphous oxidation
reflectance spectra of pure,

state of Gd could be formed, which would
1.0, 2.5, 5.0 and 7.5% Gd
make the understanding of these samples
doped Bi2WO6 nanoparticles.
difficult. Figure 4 presents the chemical state
of 2.5% Gd-doped Bi2WO6 nanoparticles. The
high resolution XPS spectra of the three primary elements Bi 4f, W 4f, and O1s,
are shown in Fig. 4.16a–c, respectively. The results showed that the binding
energies of Bi 4f7/2, Bi 4f5/2, W 4f7/2, W 4f5/2, and O1s are 159.7, 165.2, 35.8,
37.9, and 530.7 eV, respectively. Figure 4.16 d is the XPS spectrum of Gd 4d.
The characteristic peaks of Gd 4d region are at 141.8 and 149.4 eV, which
would be correlated to Gd 4d5/2 and Gd 4d 3/2, respectively. These results
indicate that Gd modified Bi 2WO6 nanoparticles have been successfully
obtained, in good agreement with XRD results.

16


. Figure 4.15 presents the DRS spectra of pure, 1.0 and 2.5% Gd-doped Bi2WO6
nanoparitcles. As can be seen in Fig. 4.15, all the pure and Gd-doped Bi2WO6
nanoparitcles show similar photo-absorption property: having light absorption
from UV to visible light ~ 450 nm. The DRS spectra of these samples displayed
steep edges, inferring that the visible light
absorption should be caused by bandgap
transition but not by transition from impurity
level. With 1.0 and 2.5% Gd-doping, the
absorption edge has a weak redshift to
visible region, and the visible absorption
intensity has a weak increase. The red-shift
of absorption edge and increase of visible

absorption intensity would be helpful for the
enhancement
of
visible-light-driven
photocatalytic activity of Gd-doped
Figure 4.17 Absorbance change of
Bi2WO6 nanoparticles.
553 nm peak as a function of
irradiation time in the presence of
The time variations of absorbance of the pure and Gd -doped Bi WO
2
6
peak at 553 nm for pure, 1.0 and 2.5% Gd- nanoparticles
doped Bi2WO6 nanoparticles are plotted in
Fig. 8. It shows in Fig. 8 that the
photocatalytic
activity
of
Bi 2WO6
nanoparticles can be remarkably improved
with Gd-doping. After 120 min visible-lightirradiation, the RhB decolorization rate for
pure Bi2WO6 nanoparticles is only about
45%; with 2.5% Gd doping, it can be
increased to 100%. This shows that Gd
doping is very helpful to improve the
Figure 4.19 PL emission
photocatalytic
activity
of
Bi 2WO6

spectra of pure, 1.0 and 2.5%
nanoparticles. Gd3+ has high stability due to Gd-doped
Bi2WO6
the half-filled electronic configuration at nanoparticles
outermost electron shell. Thus, when Bi 3+
ions are substituted by more stable Gd3+ ions, photogenerated electrons can be
more easily transferred to the active sizes on the surface of nanoparticles.
Therefore, the recombination rate of electron–hole pairs would decrease and
photocatalytic activityof nanoparticles would increase with Gd-doping.
Figure 4.19 shows the PL emission spectra of pure, Gd-doped Bi2WO6
nanoparitcles. The emission peak around 470 nm can be assigned to the
intrinsic luminescence of Bi 2WO6, which would be correlated with the direct
electron–hole recombination of band transition from the hybrid orbit of Bi 6s and
O2p (VB) to the empty W5d orbit (CB) in the WO6 2− complex. As can be seen in
17


Fig. 4.19, the PL emission spectra of pure and Gd-doped Bi2WO6 nanoparticles
displayed the main PL peaks at similar positions but with significant different
intensities. Figure 4.19 showed that these samples displayed similar absorption
intensity. Therefore, the significant difference of PL intensity would be
correlated with very different recombination rate of electron–hole pairs in these
nanoparticles. This significant modification of recombination rate of electron–
hole pairs by Gd-doing would be of great importance for improving
photocatalytic activity of Bi 2WO6 nanoparticles.
CHAPTER 5
STUDY OF NITROGEN DOPING Bi2WO6 VIA A MICROWAVE
ASSISTED- HYDROTHERMAL METHOD
5.1. Effect of experimental method on physical and photocatalytic
properties of Bi2WO6.

Fig 5.1 shows XRD patterrns of Bi2WO6 samples synthesisized by
microwave assisted method, hydrothermal method and microwave assisted –
hydrothermal method.

Figure 5.1 XRD patterrns of
Bi2WO6 samples synthesisized by
microwave
assisted
method,
hydrothermal
method
and
microwave
assisted
–
hydrothermal method.

Figure 5.2 SEM images of Bi2WO6 samples
synthesisized by microwave assisted method,
hydrothermal method and microwave assisted –
hydrothermal method and specific surface
area.

The XRD results indicate that the samples have good crystal structure.
Fig 5.2 show the SEM images and specific surface area of samples. It can be
seen that mophorlogy of Bi2WO6 synthesized by microwave assisted method is
nanoparticles, the Bi2WO6 synthesized by hydrothermal method have with
border not clearly while superstructure flower of Bi2WO6 synthesized by
microwave assisted-hydrothermal method consist of small nanoparticles. The
results of specific surface area of Bi2WO6 synthesized by microwave assisted

method is higher than that of Bi2WO6 synthesized by other method.Specific
surface area plays importance roles in photocatalytic process. Therefor, Bi2WO6
18


synthesized by Microwave assisted – hydrothermal method can be improved
photocatalytic activity.
5.2. Effect of N doping on the physical and photocatalytic properties of
Bi2WO6
Fig. 5.4 shows the XRD patterns of H:N-0, MH:N-0, MH:N-0.1, MH:N0.25, MH:N-0.5, and MH:N-0.75 nanoparticles. Five diffraction peaks at 28.3,
32.7, 47.0, 55.9, and 58.6 can be clearly observed in all the samples. These
peaks match very well with the orthor hombic phase of Bi2WO6 (JCPDS 390256). Thus, N-doping does not change the phase structure of the crystalline
Bi2WO6 host. The inset of Fig. 5.5 shows that the diffraction peak position
undergoes a very weak shift to a lower angle with increasing N doping. The
change in lattice parameters from N-doping can be attributed to nitrogen
replacing the common oxygen in the crystal lattice of Bi2WO6. The XPS results
below also indicate that oxygen sites are replaced by nitrogen ions. The ion
radius of nitrogen is slightly smaller than that of oxygen; thus, when some of
the oxygen sites are replaced by nitrogen ions, the lattice parameters slightly
increase. Moreover, all XRD patterns in Fig. 5.5 show sharp and high intensity
peaks, indicating the high crystalline quality of the samples.

Figure 5.4 XRD patterns of samples
H:N-0, MH:N-0, MH:N-0.1, MH:N0.25, MH:N-0.5, MH:N-0.75.

Figure 5.5 XPS spectra of sample
MH:N-0.5.

Fig. 5.5 presents the XPS spectra of the MH:N-0.5 nanoparticles. The
XPS spectra show that the binding energies of Bi 4f7/2, Bi 4f5/2, W 4f7/2, W 4f5/2,

Ov, and O1s are 164.2, 158.9, 38.0, 35.8, 532.0 and 530.0 eV, respectively.
These values are consistent with previously reported results. Fig. 5.6d shows
the XPS spectrum of the doping element N1s. The N1s peak was found only at
398.9 eV; no other peaks appeared. The binding energy at 398.9 eV can be
attributed to nitrogen replacing the common oxygen in the crystal lattice of
Bi2WO6 to form the O-Bi-N-W-O bond. This binding energy cannot represent
O-Bi-N or O-W-N alone, since both the binding energies of Bi 4f and W 4f
have changed to some extent, as shown in Fig. 5.6 (a) and (b); the common
oxygen ion in the O-Bi-O-W-O bond is therefore likely to have been replaced
by a nitrogen ion. The XPS results indicate that N-doped Bi2WO6 was
19


successfully prepared using the two-step microwave-assisted and hydrothermal
method, and show good agreement with the XRD results.

Figure 5.6 SEM images of samples MH:N-0
(a), MH:N-0.1(b), MH:N-0.25(c), MH:N0.5(d), MH:N-0.75(e),MH:N-0.25 with scale
bar 5μm (f).

Figure 5.7 UV-VIS spectra of
samples MH:N-0, MH:N-0.1,
MH:N-0.25, MH:N-0.5, MH:N0.75.

Fig. 5.7 shows the DRS spectra of the H:N-0, MH:N-0, MH:N-0.1,
MH:N-0.25, MH:N-0.5 and MH:N-0.75 nanoparticles. These DRS spectra
indicate that all of the samples have steep edges of visible light absorption at
about 450 nm. This absorption originates from the bandgap transition, rather
than the impurity level. The two-step method slightly increases absorption
intensity and causes a weak redshift of the absorption edge. With N-doping, the

intensity of absorption undergoes a further weak increase, and the edge of
absorption has a further weak redshift. N-doping is therefore not very helpful
for improving the visible light absorption of Bi2WO6 nanoparticles, and mainly
helps to decrease therecombination rate of electron-hole pairs.

Figure 5.8 Absorbance change of Figure 5.9 First-order kinetics plot for
553 nm peak as a function of RhB degradation of samples H:N-0,
irradiation time in the presence of
MH:N-0, MH:N-0.1, MH:N-0.25,
H:N-0,
MH:N-0,
MH:N-0.1,
MH:N-0.5, MH:N-0.75.
MH:N-0.25, MH:N-0.5, MH:N0.75.

The photocatalytic activities of the nanoparticles were investigated using
the visible light irradiation of RhB in an aqueous solution. Fig. 5.8 shows the
temporal evolution of UV-VIS absorption of an RhB solution with MH:N-0.5
nanoparticles. With increasing irradiation time, the intensity of all absorption
20


peaks of RhB decreases rapidly, indicating very good photocatalytic activity for
the MH:N-0.5 nanoparticles. In addition to the rapid decrease in intensity, the
absorption peak at 545 nm is blue-shifted and broadened, correlating with the
N-demethylation and deethylation processes. It can be seen that the MH:N-0
nanoparticles shows much higher photocatalytic activity than the H:N-0
nanoparticles. This is well explained by the much larger surface area of MH:N0 nanoparticles compared to the H:N-0 nanoparticles. Interestingly, all N-doped
Bi2WO6 nanoparticles show much better photocatalytic activity than pure
Bi2WO6 nanoparticles. This enhancement of photocatalytic activity with Ndoping mainly correlates with the decrease in the recombination rate of

photogenerated electron-hole pairs. With increasing N-doping, the
photocatalytic activity first significantly increases, then quickly decreases with
further increases in Ndoping, and the highest photocatalytic activity is obtained
for the MH:N-0.5 nanoparticles. This agrees well with the PL results in Fig.
5.10. With increasing N-doping, the recombination rate of electron-hole pairs
first decreases, and then quickly increases with further increases in N-doping,
and lowest recombination rate is obtained for the MH:N-0.5 nanoparticles.
After being irradiated for 30 min, about 17%, 23%, 51%, 59%, 81%, and 45%
of the RhB is degraded by the H:N-0, MH:N-0, MH:N-0.1, MH:N-0.25, MH:N0.5 and MH:N-0.75 nanoparticles, respectively.This suggests that for these
samples, both the surface area and the recombination rate of the electron-hole
pairs are important in improving the rate of decolorization of RhB under visible
light irradiation, and that the recombination rate is more significant. For the
nanoparticles prepared using the two-step microwave-assisted and
hydrothermal method, the surface area is significantly improved over the
hydrothermal method, although if the recombination rate is relatively high, the
ability to quickly decolorize RhB is still limited. This indicates that the surface
area of the N-doped Bi2WO6 nanoparticles is still relatively small. Thus, a more
significant improvement in surface area would be helpful in the further
enhancement of photocatalytic activity. Graphene has very large surface area
and superb optical properties, and we are currently applying graphene in
preparing N-doped Bi2WO6 nanoparticles with a larger surface area and superb
optical properties, to achieve a better enhancement of photocatalytic activity.
Fig. 5.10 shows the PL spectra of MH:N-0, MH:N-0.1, MH:N-0.25,
MH:N-0.5, and MH:N-0.75 nanoparticles. The emission peak at ~544 nm
originates from the transition of the Bi 6s and O2p hybrid orbit (VB) to the W
5d orbit (CB) in the WO 62- complex. It was found that the emission intensities
of all N-doped Bi2WO6 nanoparticles were lower than those of pure Bi2WO6
nanoparticles, suggesting a slower recombination rate of excited electron-hole
pairs in the N-doped Bi2WO6 nanoparticles. Interestingly, the MH:N-0.5
nanoparticles show the lowest emission intensity of all the N-doped Bi2WO6

21


nanoparticles, and are thus the most promising in terms of achieving high
photocatalytic activity.

Figure 5.10 PL spectra of samples
MH:N-0, MH:N-0.1, MH:N-0.25,
MH:N-0.5, MH:N-0.75.

Figure 5.11 Degradation
efficiency of RhB by MH:N-0.5
nanoparticles alone and with the
addition of IPA and KI.

In the investigation of the photocatalytic mechanism in MH:N-0.5
nanoparticles, trapping experiments were performed to determine the main
active species in the photocatalytic process. The results are presented in Fig.
5.11. Increasing the addition of isopropyl alcohol as a scavenger of
photogenerated electrons causes the photocatalytic activity to be very weakly
affected, indicating that photogenerated electrons do not contribute a great deal
to the degradation of RhB. Upon addition of potassium iodide as a scavenger of
photogenerated holes, the photocatalytic activity decreased significantly, and
with increasing concentration of potassium iodide, the photocatalytic activity is
almost quenched. This indicates that photogenerated holes are the dominant
active species in the degradation of RhB.

22

1.

2.

3.

4.

5.

CONCLUSION
The Bi2WO6 was successfully synthesized by microwave assisted
method. The good crystal Bi2WO6 was obtained at pH=1, microwave
irradiation time of 20 mintes and annealing temperature of 500 oC.Our
results indicated that surface area of the nanoparticles plays major role for
increasing photocatalytic activity: the photocatalytic activities increases
about linearly with increasing particle surface area; but weakly affected
by crystal quality. This suggested high efficiency of transportation of
photo-generated electrons and holes to the active sites before
recombination.
The Bi2WO6 modified with Gd doping and composite with BiVO4 was
also synthesisized via a microwave assisted method. The Gd-doped
Bi2WO6 nanoparticles exhibit significantly higher visible-light-driven
photocatalytic activity than pure Bi2WO6 nanoparticles, and with about
2.5% Gd doping.The enhancement of photocatalytic performance of Gd
doped Bi2WO6 nanoparticle would be mainly correlated with the decrease
of recombination rate of hotogenerated electron–hole pairs with Gddoping.
The Bi2WO6 composite with BiVO 4 was also synthesisized via a
microwave assisted method. The Bi2WO6/BiVO 4 nanocomposite could
enhance the photocatalytic activity of Bi 2WO6 nanoparticles, and the 70–

30 Bi2WO6/BiVO 4 nanocomposites exhibited highest photocatalytic
degradation efficiency of RhB under visible. Mechanism of
photocatalytic activity of Bi 2WO6/BiVO4 suggested that particle surface
area plays the most important role for improving photocatalytic activity,
and the recombination rate of photogenerated electron-hole pairs plays
more important role than the amount of light absorbed by nanoparticles
for improving photocatalytic activity
A two-step microwave-assisted and hydrothermal method is presented
here for the synthesis of Bi 2WO6 nanoparticles. The results show that this
two-step method can significantly enlarge the surface area of
nanoparticles compared with a conventional hydrothermal approach. Ndoped Bi2WO6 was also synthesized by microwave assisted –
hydrothermal method. Nitrogen not only replace Oxygen in host Bi2WO6.
N doped Bi2WO6 can significantly decrease the recombination rate of
photogenerated electron-hole pairs, and the lowest recombination rate can
be achieved by N-doping with an atomic ratio of N:Bi = 0.5. It is also
shown that both the surface area and recombination rate of
photogenerated electron-hole pairs are important in improving the
photocatalytic activity of Bi2WO6 nanoparticles. Furthermore, trapping
experiments indicated that the photogenerated holes are the dominant
active species in the photocatalytic process.
23