Preparation of WO3-modified TiO2 thin film by peroxo sol–gel method and its photocatalytic activity in degradation of methylene blue

Article abstract of DOI:10.1007/s11164-020-04225-5

A series of WO₃-modified TiO₂ sols with various WO₃ contents were synthesized by peroxo sol–gel method using H₂O₂ as the agent. The as-synthesized WO₃-TiO₂ sols were light yellow in color. The maximum amount of WO₃ which could be incorporated into the TiO₂ sol was 4?wt.percent. The pH values of the as-prepared sols were near neutral. These sols were very stable even after 2?years in stock. The sols were used to coat on glass substrate. The XRD patterns and the SAED patterns indicated that the phase of TiO₂ was anatase. There were no characteristic XRD peaks of WO₃ in the WO₃-modified TiO₂ sol, possibly due to low WO₃ loading. The particles were rhombus shaped with the major axis and minor axis of 30–77?nm and 15–31?nm, respectively. WO₃ was incorporated into the surface structure of TiO₂ and modified the electronic state of Ti. The film coated onto glass substrate was very uniform, and the film thickness was ~ 160?nm. The XPS results exhibited that W existed as 5 + and 6 + oxidation states and Ti existed in Ti³⁺ and Ti⁴⁺. These four chemical states play a role of gathering the photogenerated holes and capturing the photogenerated electrons in the lower conduction band of WO₃, respectively. This can effectively enhance the separation rate of photogenerated electron–hole pairs and increase the photoactivity. WO₃/TiO₂ samples had high photocatalytic activity under UV light illumination to degrade methylene blue, because it had high OH and O^2? group contents.

Full text of DOI:10.1007/s11164-020-04225-5

Research on Chemical Intermediates (2020) 46:4627–4643
https://doi.org/10.1007/s11164-020-04225-5
Preparation of WO ‑modiꢀed TiO thin ꢀlm by peroxo
3
2
sol–gel method and its photocatalytic activity
in degradation of methylene blue
Jia‑Ying Wu · Yu‑Wen Chen1
1
Received: 14 July 2020 / Accepted: 30 July 2020 / Published online: 7 August 2020
©
Springer Nature B.V. 2020
Abstract
A series of WO -modiꢀed TiO sols with various WO contents were synthesized by
3
2
3
peroxo sol–gel method using H O as the agent. The as-synthesized WO -TiO sols
2
2
3
2
were light yellow in color. The maximum amount of WO which could be incorpo-
3
rated into the TiO sol was 4 wt.%. The pH values of the as-prepared sols were near
2
neutral. These sols were very stable even after 2 years in stock. The sols were used
to coat on glass substrate. The XRD patterns and the SAED patterns indicated that
the phase of TiO was anatase. There were no characteristic XRD peaks of WO in
2
3
the WO -modiꢀed TiO sol, possibly due to low WO loading. The particles were
3
2
3
rhombus shaped with the major axis and minor axis of 30–77 nm and 15–31 nm,
respectively. WO was incorporated into the surface structure of TiO and modi-
3
2
ꢀ
ed the electronic state of Ti. The ꢀlm coated onto glass substrate was very uni-
form, and the ꢀlm thickness was~160 nm. The XPS results exhibited that W existed
3
+
4+
as 5+and 6+oxidation states and Ti existed in Ti and Ti . These four chemi-
cal states play a role of gathering the photogenerated holes and capturing the pho-
togenerated electrons in the lower conduction band of WO , respectively. This can
3
eꢁectively enhance the separation rate of photogenerated electron–hole pairs and
increase the photoactivity. WO /TiO samples had high photocatalytic activity under
3
2
2
−
UV light illumination to degrade methylene blue, because it had high OH and O
group contents.
Keywords WO -doped TiO · Photocatalysis · Methylene blue destruction
3
2
*
Yu-Wen Chen
ywchen@cc.ncu.edu.tw
1
Department of Chemical and Materials Engineering, National Central University,
Jhong-Li 32001, Taiwan
Vol.:(012
1
3456789)
3

4628
J.-Y. Wu, Y.-W. Chen
Introduction
TiO has been well known to be the good material for degradation of dyes and anti-
2
corrosion [1]. To enhance the photocatalytic activity of TiO , several strategies
2
have been developed [1–9]. Adding WO to TiO can promote the activity under
3
2
visible-light irradiation [9–14]. Ren et al. [15] have reported that TiCl and Na WO
4
2
4
were applied as precursors to prepare TiO /WO nanoparticles through microwave-
2
3
assisted hydrothermal process. This composite catalyst showed greater photoactiv-
ity than TiO in the degradation of Rhodamine B [15]. However, most of previous
2
studies used powder form or sol in acidic solution [15–26]. The powder WO -TiO
3
2
cannot be used to coat on some substrates. The acidic sol is diꢂcult to handle. There
are many papers revealing some information about preparation WO -modiꢀed TiO
3
2
and their application on the photocatalytic reaction [27–47]. The eꢁects of amount
of WO doping on the photocatalytic properties are not clear. Most of WO -TiO
2
3
3
sols reported in literature are acidic sols [43]. Some of them used conventional
sol–gel method with alkoxide as the starting material. Alkoxide is easy to hydro-
lyze and cannot expose to air. None of them used peroxo sol–gel method to pre-
pare WO -TiO sol. In previous studies [47–54], one of the authors has used peroxo
3
2
sol–gel method to prepare TiO -based sols. The sol was not acidic, and it has very
2
high photocatalytic activity. The advantages of using peroxo sol–gel method to pre-
pare TiO sols were (1) transparent, (2) neutral (~pH 7) sol, (3) easy to coat on the
2
substrate without using surfactant and strongly stick on the substrate, (4) formation
of nanocrystallite in sol and no annealing is needed after coating, (5) stable, and (6)
high photocatalytic activity.
In this study, WO was used to modify TiO since WO can play a role of
3
2
3
electron-accepting species. The objective of this study was to develop a method
to prepare WO -TiOs sol. The peroxo sol–gel method was used in this study. We
3
2
also tried to ꢀnd the optimum doping amount of WO and to investigate its appli-
3
cations on destruction of organic dyes.
Experimental
Synthesis of TiO sol
2
The peroxo sol–gel method was used as those reported in the literature [42–50].
5
.88 ml TiCl was slowly added into 300 ml 1 M HCl aqueous solution under con-
4
stant-speed magnetic stirring and kept at 0 °C ice bath for 30 min. 1 M NH OH
4
aqueous solution was then added dropwise to form white hydrated titanium oxide
gel, Ti(OH) . The pH value of the gel was around 8.5. After aging for 30 min,
4
Ti(OH) gel was ꢀltered and washed with distilled water several times to remove
4
the residual chloride ion.
In order to prepare 1 wt.% TiO sol, 10 g as-prepared gel was dispersed in
2
1
00 ml distilled water under magnetic stirring and ultrasonic vibration for 1 h
1
3

Preparation of WO ‑modiꢀed TiO thin ꢀlm by peroxo…
4629
3
2
to form well-dispersed milky solution. An aqueous hydrogen peroxide solution
(
30 wt.% H O ) was added dropwise into this milky solution under magnetic stir-
2 2
ring for 2 h to get uniformly titanium peroxide solution with orange color. The
molar ratio of TiO /H O was 1:6. This solution was then heated up to 95 °C and
2
2
2
kept at 95 °C for 12 h under magnetic stirring. The transparent TiO sol with light
2
yellow color was obtained.
Preparation of WO ‑modiꢀed TiO sols
3
2
Sodium tungstate (Na WO ) from Aldrich was used as the precursor of WO in this
2
4
3
study. The preparation method was the same as that of pure TiO sol except that
2
Na WO was added in the last heating step. The weight ratios of WO /TiO were
2
4
3
2
0
.5:100, 1:100, 2:100 and 4:100, respectively. The detailed preparation procedure
4
+
is shown in Fig. 1. TiCl was dissolved in water to form Ti ions. It was converted
4
to Ti(OH) gel after addition of NH OH. Ti(OH) was converted to TiO nanocrys-
4
4
4
2
tals and Na WO was converted to WO nanocrystals by adding H O and heating
2
4
3
2
2
at 90–95 °C for a few hours. The catalyst was denoted as WT(x), where W means
tungsten oxide, T means titanium oxide, and x stands for WO /TiO weight ratio
3
2
(
%).
Preparation of WO ‑modiꢀed TiO thin ꢀlms
3
2
TiO and WO -modiꢀed TiO ꢀlms were prepared by dip-coating method using
2
3
2
the as-prepared pure TiO and WO -modiꢀed TiO sols. The method is the same
2
3
2
as that reported in previous literature [48–53]. The substrate was soda-lime glass.
Fig. 1 The procedure of preparing WO -TiO sol
3
2
1
3

4630
J.-Y. Wu, Y.-W. Chen
2
The total coating surface area of these substrates was 40 cm . Before coating, the
substrate was cleaned by washing solution, i.e., 1 M NH OH, and distilled water
4
several times. The glass substrate was dried in an oven at 50 °C. The glass substrate
was vertically soaked into the as-prepared sols for 2 min, and then the substrate was
pulled out with the constant speed of 60 cm/min for 6 times. The thickness of TiO
2
and WO -TiO ꢀlms was~130 nm. The ꢀlms were coated on both sides of the glass
3
2
substrate. After coating, the substrate was heated at 400 °C to promote the formation
of crystalline WO -TiO .
3
2
Characterization
X‑ray diꢁraction (XRD)
The crystalline structure of the as-prepared TiO and WO -TiO was performed
2
3
2
using Siemens D500 powder diꢁractometer using CuK radiation (λ=1.5405 Å) at
α1
a voltage and current of 40 kV and 40 mA, respectively. The scanning range of 2θ
was 20°–80° at a rate of 0.05°/min. The XRD sample was prepared by drying the
TiO or WO -TiO sol at 100 °C for 2 day to obtain the powder sample and the sam-
2
3
2
ple was calcined at 400 °C for 3 h.
Scanning electron microscopy (SEM)
The morphology and thickness of ꢀlms were observed by Hitachi-3000 with tung-
sten lamp at acceleration voltage of 10 kV and emission current of 81,000 nA. The
SEM sample was prepared by cutting the coated glass substrate into 0.5 cm×0.5 cm
pieces with diamond knife ꢀrst and then coated with platinum to increase its
conductivity.
Transmission electron microscopy (TEM) and high‑resolution electron microscopy
(HRTEM)
The morphologies of TiO and WO -TiO sols were observed by TEM (JEM-2000
2
3
2
EX II) operated at 160 kV or 200 kV and HRTEM (JEOL JEM-2100) operated at
00 kV. The TEM sample was prepared by dipping the carbon-coated copper grid
2
(
200 meshes) (Ted Pella) into the as-prepared sols for 3 times. The chemical compo-
sition of the sample was determined by energy-dispersive X-ray spectroscopy (EDS)
attached on HRTEM using silicon detector.
X‑ray photoelectron spectroscopy (XPS)
The XPS spectra were recorded with a Thermo VG Scientiꢀc Sigma Prob spec-
trometer. The XPS spectra were collected using Al K radiation at a voltage and
α
current of 20 kV and 30 mA, respectively. The base pressure in the analyzing
–
9
chamber was maintained in the order of 10 torr. The spectrometer was operated
at 23.5 eV pass energy, and the binding energy was corrected by contaminant
1
3

Preparation of WO ‑modiꢀed TiO thin ꢀlm by peroxo…
4631
3
2
carbon (C = 284.6 eV) in order to facilitate the comparisons of the values
1
s
among the catalysts and the standard compounds. Peak ꢀtting was done by using
XPSPEAK 4.1 with Shirley background and 30:70 Lorentzian/Gaussian convolu-
tion product shapes.
Photocatalytic reaction test
For the photocatalytic reaction test, 50 ml 10 ppm methylene blue solution was
loaded into a quartz reactor. The glass substrate that was coated with TiO or
2
WO -TiO ꢀlm was then loaded into the reactor. The reactor was in the dark over-
3
2
night to reach saturation of adsorption of methylene blue (MB) in the catalyst
[
48–53]. The photocatalytic reaction under UV light was then started. The UV
light source was 20 W UVC lamp (wavelength = 254 nm). The concentration
of MB was determined by UV–visible spectrophotometer (Cary 300 Bio). The
detailed experimental procedure was reported in the previous literature [48–53].
Results and discussion
Characteristics of TiO and WO ‑TiO Sols
2
3
2
The as-prepared TiO and WO -TiO sols were synthesized with peroxo sol–gel
2
3
2
method. The TiO sol was light yellow transparent sol containing TiO nanoparti-
2
2
cles dispersed in water. The yellow color was due to the presence of small amount
of titanium peroxide [48]. As the amount of H O in preparation decreased, the
2
2
color became lighter. The pH values of TiO and WO -TiO sols are listed in
2
3
2
Table 1. The pH value of TiO sol was 8.7. With increase in the heating time,
2
the pH value of TiO sol increased, due to the decrease of the amount of H O
2
2
2
in heating process. Table 1 shows that adding WO to TiO sol decreased the pH
3
2
value slightly to be less basic, since the tungsten precursor was acidic. It was not
able to synthesize stable WO -TiO sol by peroxo sol–gel method if WO content
3
2
3
was greater than 4 wt.%.
Table 1 The pH values of the
Sample
TiO2
WT(0.5)
WT(1)
WT(2)
WT(4)
pH value
WO -TiO as-prepared sols
3
2
8.78
8.72
8.68
8.63
8.12
1
3

4632
J.-Y. Wu, Y.-W. Chen
=
Anatase
(B)
(A)
20
30
40
50
60
70
80
2θ(°)
Fig. 2 XRD patterns of a TiO nanoparticles obtained from the as-prepared TiO sol and calcined at 400
2
2
°
C for 30 min and b the as-prepared TiO nanoparticles without calcination
2
XRD
The as-prepared TiO and WO -TiO sols were heated at 100 °C for 2 days to get
2
3
2
powder samples. The XRD patterns are shown in Figs. 2. It shows the anatase
TiO diꢁraction peaks located at 2θ = 25.4102°, 37.9658°, 48.1227°, 62.7199°
2
and 75.2245° corresponding to the phase of (101), (004), (200), (204), and (215),
respectively [5–8, 10–15, 48–53]. The as-prepared TiO had low intensity peaks
2
for anatase, as shown in Fig. 2b, because the TiO crystallite size was very small.
2
Figure 3 shows the XRD patterns of WO -TiO samples. The diꢁraction peaks
3
2
of these patterns are consistent with the characteristic peaks of anatase TiO and
2
did not show any diꢁraction peak of WO , inferring WO crystal was too small to
3
3
detect. The characteristic peaks of WO ·H O are at 2θ = 25.7164°, 35.1076° and
3
2
5
2.7672°, respectively. Pan and Lee [16] reported that WO was highly dispersed
3
in the bulk phase of TiO particles and WO was not formed. Our results are in
2
3
accord.
SEM
The structure of TiO and WO -TiO ꢀlms was examined by SEM. The top-view
2
3
2
surface structure and cross-sectional view of the as-prepared thin ꢀlm are shown
in Fig. 4. Each sample had very uniform and smooth surface of ꢀlm on the top of
the substrate, indicating that dip coating is a good way to prepare the thin ꢀlm.
The particle size of pure TiO was smaller than those of WO -TiO samples.
2
3
2
1
3

Preparation of WO ‑modiꢀed TiO thin ꢀlm by peroxo…
4633
3
2
(G)
(F)
(E)
(D)
(C)
(B)
(A)
20
30
40
50
60
70
80
2
θ(°)
Fig. 3 XRD patterns of a TiO calcined by 400 °C, b WT(0.5), c WT(1), d WT(1.5), e WT(2), f WT(3),
2
g WT(4)
Fig. 4 Top view of the surface of a TiO ꢀlm, b WT(0.5) ꢀlm, c WT(1) ꢀlm, d WT(2) ꢀlm
2
1
3

4634
J.-Y. Wu, Y.-W. Chen
Fig. 5 Cross-sectional image of WO -TiO ꢀlm
3
2
Adding WO to TiO created rough surface which could enhance the adsorption
3
2
of dye and increased surface area. This is beneꢀcial for the photocatalytic activity
as shown in the latter section. The typical cross-sectional view of WO -TiO ꢀlm
3
2
is shown in Fig. 5. The ꢀlm thickness of the WO -TiO ꢀlm was 163.4 nm. Since
3
2
WO -TiO formed anatase crystal in sol, no annealing is needed after coating on
3
2
glass. In addition, WO -TiO strongly sticks on the glass with 3H hardness.
3
2
TEM and HRTEM
The morphology of the as-prepared TiO and WO -TiO sols was analyzed by TEM
2
3
2
and HRTEM. HRTEM image in Fig. 6d shows that the TiO particles were rhombus
2
shaped with the major axis and minor axis of 30–77 nm and 15–31 nm, respectively.
The selected area electron diꢁraction (SAED) pattern conꢀrms that TiO nanopar-
2
ticles were anatase, as indicated by the plane of (101), (103) and (211) [6, 48–53].
The lattice space of 0.395 nm corresponding to the (101) anatase plane is observed
in Fig. 6c. Figure 7 shows the morphologies and particle sizes of the as-prepared
WO -TiO . Compared with pure TiO sol, the particle size of WO -TiO was a little
3
2
2
3
2
smaller than that of TiO . They also show rhombus shape.
2
In order to conꢀrm whether WO particle was dispersed in TiO sol or not, WO
3
2
3
sample was prepared. Figure 8c shows that the shape of WO was spherical-like and
3
its average particle size was~10 nm, which was smaller than that of TiO particle.
2
The WO -TiO sample was also examined by EDS as shown in Fig. 8b. None of the
3
2
particles were WO . One can conclude that WO was incorporated into the structure
3
3
of TiO and no WO particles were formed in this study, providing that the concen-
2
3
tration of WO was low. In conclusion, one has successfully developed a method to
3
prepare WO -TiO sol by peroxo sol–gel method.
3
2
1
3

Preparation of WO ‑modiꢀed TiO thin ꢀlm by peroxo…
4635
3
2
(A)
(B)
(D)
(C)
Fig. 6 a TEM image of TiO , b The SAED pattern of TiO , c the lattice space of TiO from HRTEM, d
2
2
2
HRTEM image of TiO
2
1
3

4636
J.-Y. Wu, Y.-W. Chen
Fig. 7 TEM image of a WT(0.5) sol, b WT(4) sol
XPS
XPS analysis was performed to investigate the electronic state of each element
4
+
of the sample. Based on the literature [45–50], the binding energy of Ti 2p
3
/2
4
+
and Ti 2p is centered at 458.6 eV and 464.4 eV, while the binding energy
1
/2
3
+
3+
of Ti 2p and Ti 2p is centered at 457.7 eV and 463.5 eV [51–53]. The
3
/2
1/2
4
+
spectra of Ti 2p region of all samples are shown in Fig. 9. The contents of Ti
3
+
4+
and Ti are listed in Table 3. All samples showed large amount of Ti and small
3
+
3+
amount of Ti . WO -TiO samples had higher concentrations of Ti than pure
3
2
TiO . WO was incorporated into the surface structure of TiO and modiꢀed the
2
3
2
4
+
electronic state of Ti. Ti 2p (Ti ) of the WT(4) was observed at lower binding
3
+
energy region than the Ti 2p (Ti ) of other 3 samples as shown in Fig. 9, pos-
sibly because this sample had higher content of WO than the other 3 samples.
3
1
3

Preparation of WO ‑modiꢀed TiO thin ꢀlm by peroxo…
4637
3
2
Fig. 8 a TEM image of WO , b EDS analysis of WO particle
3
3
Some WO had big particles and was not incorporated into surface structure of
3
TiO . Further research is needed to address this diꢁerence.
2
Figure 10 shows the O 1s spectra of various samples. The spectra were decon-
voluted to ꢀve peaks, i.e., 532.1 eV corresponding to oxygen in TiO surface
2
−
adsorption of OH , 529.9 eV corresponding to oxygen in TiO lattice, 529.2 eV
2
corresponding to oxygen in Ti–O–Ti bond, 530.2 eV corresponding to oxygen in
TiO surface adsorption of H O, 530.6 eV corresponding to oxygen in WO lat-
2
2
3
2
−
−
−
tice. The percentages of O and OH are listed in Table 3. The presence of OH
2
−
and O species on the samples infers that they can become captives of photogen-
erated electron–hole pairs directly or indirectly to suppress the recombination of
the photogenerated electron–hole pairs [46–53]. WT(4) sample had a lower con-
−
2−
tents of OH and O species, compared to other samples.
1
3

4638
J.-Y. Wu, Y.-W. Chen
Ti 2p
3/2
Ti 2p1/2
4+
Ti
3
+
(D)
4+
Ti
Ti
3+
Ti
(C)
(
(
B)
A)
4
70
468
466
464
462
460
458
456
454
452
Binding energy (eV)
Fig. 9 Ti 2p XPS spectra of a WT(0.5) ꢀlm, b WT(1) ꢀlm, c WT(2) ꢀlm and d WT(4) ꢀlm
O 1s
TiO2
WO
-
3
(D)
Ti-O-Ti
OH
(C)
2-
(B)
O
(
A)
A)
(
536 535 534 533 532 531 530 529 528 527 526 525
Binding energy (eV)
Fig. 10 O 1s XPS spectra of a WT(0.5) ꢀlm, b WT(1) ꢀlm, c WT(2) ꢀlm and d WT(4) ꢀlm
The XPS spectra of W 4f are shown in Fig. 11. It shows four peaks, because of
W 4f and W 4f transition with diꢁerent oxidation states of W atoms. The main
7
/2
5/2
peaks, having a W 4f at 37.8 eV and a W 4f at 35.7 eV, are attributed to the W
5
/2
7/2
atoms in 6+oxidation state. The other oxidation state in W atoms is 5+oxidation
state with a W 4f at 34.6 eV and W 4f at 36.7 eV peaks [10–12]. The binding
5
/2
7/2
energy of W 4f in WT(4) sample had lower binding energy than the other 3 samples
1
3

Preparation of WO ‑modiꢀed TiO thin ꢀlm by peroxo…
4639
3
2
Fig. 11 W 4f XPS spectra of a WT(0.5), b WT(1), c WT(2) and d WT(4)
as shown in Fig. 11. The results are in accordance with the results of Ti 2p spectra.
This is possibly because this sample had higher content of WO than the other 3
3
samples. Some WO had big particles and was not incorporated into surface struc-
3
ture of TiO . Further research is needed to address this diꢁerence (Table 2).
2
Photocatalytic destruction of methylene blue
The photocatalytic activities of TiO and WO -TiO thin ꢀlms were tested by the
2
3
2
photocatalytic destruction of MB. The saturation adsorption of MB in various
samples is shown in Table 4. The results show that adding WO in TiO increased
3
2
adsorption capacity of MB, because it had rougher surface than the undoped one.
The photocatalytic activities of the as-prepared TiO and WO -TiO ꢀlms are
2
3
2
shown in Figs. 12. The highest photocatalytic activities of ꢀlm were WT(4) and
−
1
WT(4) with the rate constant of 0.5138 h , as obtained by plotting ln(C /C) against
0
irradiation time corresponding to ꢀrst-order reaction. The rate constants are listed
in Table 5. It indicates that doping suitable amount of WO in TiO could increase
3
2
the photocatalytic activity. The optimal weight ratio of WO /TiO was 2/100. The
3
2
4
+
3+
6+
5+
WO -modiꢀed TiO samples existed in Ti , Ti , W and W chemical states.
3
2
3
+
5+
Ti and W can gather the photogenerated holes from conduction band of TiO ;
2
4
+
6+
however, Ti and W can capture the photogenerated electrons gathered in the
lower conduction band of WO . This can eꢁectively enhance the separation of pho-
3
togenerated electron–hole pairs and increase the photocatalytic activity. It was not
able to synthesize stable WO -TiO sol by peroxo sol–gel method if WO content
3
2
3
was greater than 4 wt.%.
1
3

4640
J.-Y. Wu, Y.-W. Chen
1
3

Preparation of WO ‑modiꢀed TiO thin ꢀlm by peroxo…
4641
3
2
Table 3 Concentrations of each chemical state of Ti, O and W by XPS analysis
Sample
W
Ti
O
W6+
W5+
Ti4+
Ti3+
OH−
TiO2
Ti–O–Ti
O2−
WO3
TiO2
–
–
7.58
92.42
70.97
29.89
44.76
38.65
31.76
27.72
16.05
35.69
24.34
–
29.80
15.78
22.66
16.81
24.99
38.44
0
25.27
6.44
2.66
–
WT(0.5)
WT(1)
WT(2)
WT(4)
70.01
52.12
56.72
51.25
29.99
47.88
43.28
48.75
29.02
70.12
55.25
61.36
28.02
23.80
18.51
26.44
28.48
12.23
22.56
21.57
Table 4 The saturation
Sample
MB concentration
adsorbed by sample
adsorption of methylene blue
solution in the dark
(
%)
TiO2
8.840
9.579
16.191
12.686
13.926
WT(0.5)
WT(1)
WT(2)
WT(4)
1
0
8
6
4
2
0
-1
0
1
2
3
4
5
6
7
8
9
10
time (h)
Fig. 12 Photocatalytic degradation of methylene blue aqueous solution. Black circle TiO ; black triangle
2
WT(0.5); black square WT(1); black diamond WT(2); black star WT(4)
1
3

4642
J.-Y. Wu, Y.-W. Chen
Table 5 Rate constant of
Rate constant (h−1
Catalyst
)
reaction of MB destruction from
pseudo-ꢀrst-order kinetic
TiO2
0.3728
0.3823
0.4108
0.4273
0.5138
WT(0.5)
WT(1)
WT(2)
WT(4)
Conclusion
The results of this study are summarized as follows:
1
. A series of WO -modiꢀed TiO sols with various WO contents were synthesized
3 2 3
by peroxo sol–gel method. The WO -TiO sols were light yellow in color. The pH
3
2
values of the as-prepared sols were neutral to weak basic. These sols were very
stable even after 2 years in stock. The sols can be used to coat on glass substrate.
2
. The XRD patterns and the SAED patterns indicated that the phase of TiO was
2
anatase. There were no WO peaks, and particles appeared in the WO -modiꢀed
3
3
TiO sol. The particles were rhombus shaped with the major axis and minor axis
2
of 30–77 nm and 15–31 nm, respectively. WO was incorporated into the surface
3
structure of TiO and modiꢀed the electronic state of Ti.
2
3
4
. The SEM results showed that the ꢀlm coated onto glass substrate was very uni-
form and the ꢀlm thickness was~160 nm.
. The XPS results exhibited that W existed as 5+and 6+oxidation states and Ti
3
+
4+
existed in Ti and Ti . These four chemical states play a role of gathering the
photogenerated holes and capturing the photogenerated electrons in the lower
conduction band of WO , respectively. This can eꢁectively enhance the separation
3
rate of photogenerated electron–hole pairs and increase the photoactivity.
5
. The optimal doping ratio of TiO /WO was 100:4. It had high photoactivity under
2
3
2
−
UV light illumination to degrade organic dye, because it had high OH and O
group contents.
Acknowledgements This research was supported by Ministry of Science and Technology, Taiwan.
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