Layer-assembled 3D Bi2WO6 hierarchical architectures by Ti-doping for enhanced visible-light driven photocatalytic and photoelectrochemical performance

Article abstract of DOI:10.1016/j.jallcom.2019.03.321

Layer-assembled three-dimensional (3D) catalysis gain substantial attention and widespread application in the field of photocatalysis and solar energy conversion. The 3D hierarchical architecture possesses large surface area and plenty reactive sites which contribute highly in catalysis performance. In this work we report a tetrabutyl titanate (TBT) assisted simultaneous approach of layer-assembled single-unit-cell 3D Bi₂WO₆ hierarchical architecture by Ti-doping. The Ti-doping beside its morphological effect can also generate crystal defects in the crystal lattice of Bi₂WO₆, resulting in highly enhanced visible-light driven photocatalytic performance towards highly stable refractory pollutants and profound photoelectrochemical ability. Moreover, the crystal defects mediated by Ti-doping can generate abundant oxygen vacancies which remarkably improve the selective adsorption towards different charges pollutants. Furthermore, the Ti-doping beside its great influence on the electron dynamic and band structure can also introduce the redox couples (Ti³⁺/Ti⁴⁺), resulting in significance enhancement in reactive oxygen species during photocatalytic reaction. The present work provides a valid route for constructing simultaneous approach of layer-assembled 3D hierarchical architecture and Ti-substitution in the crystal structure of Bi₂WO₆ with great improvement in photocatalytic and photoelectrochemical performance.

Full text of DOI:10.1016/j.jallcom.2019.03.321

Accepted Manuscript
Layer-assembled 3D Bi WO hierarchical architectures by Ti-doping for enhanced
2
6
visible-light driven photocatalytic and photoelectrochemical performance
Muhammad Arif, Min Zhang, Jiacheng Yao, Hongfei Yin, Pengfei Li, Ijaz Hussain,
Xiaoheng Liu
PII:
S0925-8388(19)31159-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.03.321
JALCOM 50085
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 19 December 2018
Revised Date: 9 March 2019
Accepted Date: 23 March 2019
Please cite this article as: M. Arif, M. Zhang, J. Yao, H. Yin, P. Li, I. Hussain, X. Liu, Layer-assembled
3
D Bi WO hierarchical architectures by Ti-doping for enhanced visible-light driven photocatalytic
2 6
and photoelectrochemical performance, Journal of Alloys and Compounds (2019), doi: https://
doi.org/10.1016/j.jallcom.2019.03.321.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Layer-assembled 3D Bi
2
WO
6
hierarchical architectures by Ti-doping for enhanced visible-
light driven photocatalytic and photoelectrochemical performance
1
1
1
1
1
2
Muhammad Arif , Min Zhang , Jiacheng Yao , Hongfei Yin , Pengfei Li , Ijaz Hussain ,
1
*
Xiaoheng Liu ,
1
Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, School
of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094,
China
2
Jiangsu Key Laboratory for Chemical Pollution Control and Resources Reuse, School of
Environmental and Biological Engineering, Nanjing University of Science and Technology,
Nanjing 210094, China
Corresponding Author: Xiaoheng Liu
Email: xhliu@njust.edu.cn
Tel: +86-25-84315943
Address: Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education,
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing
2
10094, China

ACCEPTED MANUSCRIPT
ABSTRACT:
Layer-assembled three-dimensional (3D) catalysis gain substantial attention and widespread
application in the field of photocatalysis and solar energy conversion. The 3D hierarchical
architecture possesses large surface area and plenty reactive sites which contribute highly in
catalysis performance. In this work we report a tetrabutyl titanate (TBT) assisted simultaneous
approach of layer-assembled single-unit-cell 3D Bi WO hierarchical architecture by Ti-doping.
2
6
The Ti-doping beside its morphological effect can also generate crystal defects in the crystal
lattice of Bi WO , resulting in highly enhanced visible-light driven photocatalytic performance
2
6
towards highly stable refractory pollutants and profound photoelectrochemical ability. Moreover,
the crystal defects mediated by Ti-doping can generate abundant oxygen vacancies which
remarkably improve the selective adsorption towards different charges pollutants. Furthermore,
the Ti-doping beside its great influence on the electron dynamic and band structure can also
3
+
4+
introduce the redox couples (Ti /Ti ), resulting in significance enhancement in reactive oxygen
species during photocatalytic reaction. The present work provides a valid route for constructing
simultaneous approach of layer-assembled 3D hierarchical architecture and Ti-substitution in the
crystal structure of Bi WO with great improvement in photocatalytic and photoelectrochemical
2
6
performance.
Keywords:
Bi WO ,
Ti-doping,
3D
hierarchical
architecture,
photocatalysis,
2
6
photoelectrochemical

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1
. Introduction
Single and multilayers two-dimensional (2D) semiconductor photocatalysts such as graphitic
carbon nitride (g-C ), boron nitride (BN), graphene, dichalcogenides transition metal and
3
N
4
transition metal oxides, have got substantial attention in the field of catalysis [1-3], energy
storage [4] and electronics [5]. Importantly, they cast inordinate potential in the field of
photocatalysis and/or photoelectrochemistry, as the 2D ultrathin materials facilitate the
photogenerated charge carriers and efficient charge migration from the inside to the surface,
which consequently, suppress the recombination rate and hence improve the photocatalytic and
2
2
photoelectrochemical efficiency based on diffusion time formula of t = d /k D (where, k
represent the constant, D is the diffusion coefficient of photogenerated electron-hole pairs and d
is the particle size, respectively) [6].
Recently, layered Bi-based semiconductor photocatalysts such as BiOX (X= Cl, Br, I) [7-10],
Aurivillius phase Bi
Cl, Br) [15, 16] have gained significant attention and widespread application due to their
chemical and photo-stability, high photo-oxidation ability and abundant structural diversity [17-
2 6 2 5 2 6 4 8
WO , Bi SiO , Bi MoO [11-14] and Sillén-Aurivillius phase Bi NbO X (X
=
2
+
2
0]. A simplest Aurivillius-type Bi WO consist of alternating bismuth oxide layers (Bi O )
2 6 2 2
2
-
and octahedral (WO ) layers is a promising photocatalyst for organic pollutants removal, H
4
2
production and CO reduction. However, the photocatalytic application of Bi WO still limits
2
2
6
due to its fast recombination of photogenerated electron-hole pairs and slow charge carrier
migration during the photocatalytic reaction. To improve the photocatalysis performance of

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Bi
2
WO
6
, many efforts have been made, such as heterojunction formation [21-23], ion doping
[24, 25] and deposition of noble metal [26, 27], etc., though the enhancement in photocatalysis
performance is still limited. Recently, cetyltrimethylammonium bromide (CTAB) assisted
monolayer Bi WO was hydrothermally synthesized and different dyes adsorption is investigated
2
6
[
28]. Liang, et al. reported the oleate assisted layered single-unit-cell Bi WO and demonstrated
2 6
their activity for photocatalytic CO reduction into methanol [29]. However, the monolayer and
2
2 6
multilayers Bi WO having dispersed nanosheets morphology have been studied in the above-
mentioned work, which suffers from limited surface area and/or inadequate visible light
absorption in comparison with three-dimensional (3D) nanostructured materials. Considering the
advantages of 3D nanostructure ultrathin nanosheets, fabrication of single layer 3D hierarchical
architectures Bi WO nanosheets may be of high importance. Moreover, metal ion doping is a
2
6
fundamental technique to create impurities in the crystal structure which could cause the crystal
defects and can arbitrate the properties the bulk semiconductors [30-34], consequently generate
oxygen vacancies which can affect light absorption, band structure and chemical state of
semiconductors photocatalysts, respectively [35, 36]. All these changes will also contribute in
the photocatalytic and photoelectrochemical performance of photocatalyst.
Inspired by the idea of crystal defects engineering, herein we developed tetrabutyl titanate
(
TBT)-assisted strategy to simultaneously fabricate layer-assembled single-unit-cell Bi
2 6
WO 3D
hierarchical architecture and generate crystal defects through Ti-doping for enhance
2
+
photocatalysis performance. The TB-chains can absorb on (Bi O ) layers and hence can favor
2
2
the formation of stacking layers structure. By varying the dopant contents, the well distorted
crystal with stacking layers structure can be engineered through ion substitution, charge
compensation and generation of oxygen vacancies. Furthermore, the layer-assembled 3D

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Bi
2
WO
6
hierarchical architecture proven to boost electron dynamic by trapping effects and can
also affect the band structure of Bi WO through introducing new energy levels, resulting in
2
6
enhancement in photocatalytic and photoelectrochemical performance. The photocatalytic
activity of as prepared photocatalyst was investigated for the adsorption, photodegradation of
various organic pollutant, which reveals that the Ti-doped layer-assembled 3D Bi WO shows
2
6
highly improved photocatalytic activity. In addition, the electrochemical and
photoelectrochemical characterizations was conducted to further explain the enhanced
2 6
photocatalysis performance. Particularly, in Ti-doped 3D Bi WO hierarchical architecture, the
3+
4+
redox couple Ti /Ti can favor the formation of reactive oxygen species under the
oxidizing/reducing condition, which can act as electron scavenger during the photocatalytic
reaction. So far in our knowledge, no article on TBT-assisted simultaneous layer-assembled 3D
Bi WO hierarchical architecture and Ti ion substitution photocatalyst has been reported.
2
6
2
. Experimental
2
. 1. Catalyst preparation
Chemicals: Bi(NO ) . 5H O, Na WO . 2H O (Sigma-Aldrich), tetrabutyl titanate (TBT),
3
2
2
2
4
2
acetylacetone were obtained from Sinopharm Chemical Reagent Co. Ltd. China. All the
chemical used in this work were pure analytical grade and used as received.
Bulk Bi WO and layers assembled Ti-doped hierarchical Bi WO with varying concentration
2
6
2
6
of Ti (1%, 5%, 10%, 20% and 30%) were prepared by single-step hydrothermal technique. In
particular, 1.98 mmol of Bi(NO ) .5H O and 1 mmol of Na WO .2H O were first dispersed in
3
2
2
2
4
2
deionized water and ultrasonicated for 1 h. After complete dispersion, the suspension was kept
on stirring and in mean time different amount of tetrabutyl titanate (TBT) in acetylacetone

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-1
solution (0.04 molL ) was dropwise added in the above suspension. To ensure the complete
dispersion of TBT, the solution was stirred for 2 h and then transferred into 100 mL Teflon lined
stainless-steel autoclave and maintained at 180 °C for 24 h. The obtained product was washed
several times with DI water and ethanol and collected by centrifugation. The product was then
dried in oven at 60 °C for 24 h. The pure Bi WO was prepared by the same method without the
2
6
addition of TBT solution. The obtained samples were denoted as BWO for bulk Bi WO and 1-
2
6
Ti-BWO, 5-Ti-BWO, 10-Ti-BWO, 20-Ti-BWO and 30-Ti-BWO for Ti-doped Bi WO
2
6
according to varying concentration of Ti-dopant.
2
. 2. Characterizations
The X-ray diffraction analysis was conducted on Bruker D8 advance X-ray diffractometer
with Cu Ka radiation (λ = 0.15418 nm) at 40 kV and 40 mA. Raman spectra of the sample were
analyzed by Raman spectrophotometer (Horiba Jobin-Yvon LabRam HR800) with laser
excitation at 320 nm. The scanning electron microscope (SEM, JOEL-JSM 6360 LV field
emission microscopy) coupled with energy dispersive X-ray spectroscopy (EDS) operated at 5.0
kV and transmission electron microscope (TEM/HRTEM, JOEL-2100) at 200 kV were
conducted to analyze the morphology and crystal structure of as prepared samples. UV-vis
diffuse reflectance spectra (DRS) were characterized by UV-vis spectrophotometer (Shimadzu
UV-2500) with an excitation range of 200-800 nm in BaSO as reflectance standard. The X-ray
4
photoelectron spectrometry (XPS) spectra were recorded at PerkinElmer PHI5300 with A1Kα
radiation (1= 8.4 Å) as an excitation source. Photoluminescence (PL) emission spectra of sample
were detected by FL3-TCSPS florescence spectrometer under 325 nm excitation.
2
. 3. Adsorption and photocatalytic degradation experiments

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The adsorption activity was conducted in dark for rhodamine B (RhB), methyl orange (MO),
-1
4
-nitrophenol (4-NP) and phenol (20 mgL ) as targeted organic pollutants. A 50 mg
photocatalyst was dispersed in the above contaminant solutions (50 mL) with constant stirring
and then 3 mL aliquot was collected with an interval of 10 min and centrifugated. UV-vis
spectrophotometer was used to determine the concentration based on their absorbance spectra.
After 30 min successive adsorption experiment, the photocatalytic activity of as prepared
photocatalyst was investigated under visible light irradiation (500 W, Xenon lamp, λ ≥ 420 nm).
At a certain interval of time, 3 mL of sample was collected, centrifugated and the concentration
was then determined by UV-vis spectrophotometer (Shimadzu UV2800 spectrophotometer). All
the photodegradation experiment was conducted at room temperature. The photodegradation
efficiency towards the pollutants was calculated by the following formula;
ꢁ
ꢃꢄꢅꢃꢂ
ꢃꢄ
degradationꢀꢁ%ꢂ =ꢀ
× 100
Total organic carbon (TOC) mineralization was measured with a vario TOC analyzer (Vario
TOC, German Elementar Company).
2
. 4. Analysis of photodegradation intermediates of acarbose
The photocatalytic degradation of acarbose was performed by the same procedure as
discussed above for the photocatalytic degradation of other organic pollutants. A 50 mg of
-1
photocatalyst was added in 50 mL (20 mgL ) acarbose solution. The solution was first stirred in
dark for 30 to attain adsorption-desorption equilibrium. Then under visible light irradiation (500
W, Xenon lamp, λ ≥ 420 nm), 3 mL suspension was sampled at given interval of time,
centrifugated and the concentration was determined by high-performance liquid
chromatography-mass spectrometry (HPLC-MS).

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The determination of intermediate formation was investigated by high-performance liquid
chromatography-mass spectrometry (HPLC-MS) (Agilent 1260/6310, Ion Trap LC/MS)
equipped with a Poroshell 120 SB-C18 column (150 mm × 4.6 mm i.d., 2.7 ꢀm). Water (50%)
and methanol (50%) with 0.2 mL/min flow rate were used as mobile phase. The gradient started
with 20% B, increased to 30% in 3 min and then 80% n 4 min and finally returned to the initial
composition within 5.5 min and 4.5 min equilibration. A 10 ꢀL was injected and the temperature
of column was kept 30 °C. The MS was experimented by operating in positive ion mode using
ESI under the following condition: capillary, 27 nA; temperature of drying gas; nebulization
pressure, 30 psi; 300 °C; drying gas flow, 10 L/ min. MS was scanned in a mass range from m/z
5
0 to 1000.
2
. 5. Active reactive species analysis
To examine the active reactive species generated during the photodegradation towards RhB
over Ti-doped Bi WO , ethylenediaminetetraacetic acid disodium (EDTA-2Na), benzoquinone
2
6
-
+
(
BQ) and isopropanol (IPA) were used as superoxide radicals (ꢀO ) and hole (h ), hydroxyl
2
radicals (ꢀOH) scavengers, respectively [37, 38]. The experimental procedure is the same as
discussed above for the degradation of organic pollutant.
2
. 6. Photoelectrochemical measurements
The electrochemical and photoelectrochemical tests such as photocurrent, electrochemical
impedance spectroscopy (EIS), Mott-Schottky and cyclic voltammetry (CV) were recorded on
electrochemical workstation (CHI660A, CH instrument Co) at room temperature. All the tests
were performed in standard three electrode system in 0.1 M aqueous Na SO solution. The
2
4
platinum foil, Ag/AgCl and the sample were used as counter, reference and working electrodes.

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In the preparation of working electrode, the FTO glass was ultrasonically cleaned in acetone,
water and ethanol. The electrodes were prepared by dispersing 10 mg of photocatalyst in 1 mL
water-ethanol (Vwater:Vethanol = 1:1) and 10 µL Nafion solution and then dried in oven at 60 °C for
2
1
2 h. The area of electrodes covered with photocatalyst is about 1 × 1 cm . A 500 W Xenon lamp
with cut off filter of 420 nm was used as an excitation source.
3
. Results and discussion
3
.1. Morphological structure
The morphology of bulk and Ti-doped Bi WO was investigated by SEM and
2
6
TEM/HRTEM. As shown in Fig. 1, both the bulk Bi
2
6 2 6
WO and Ti-doped Bi WO present
hierarchical microstructure with several microns size well-organized nanoplates. Comparing the
morphological structure Bi WO with Ti-doped Bi WO , the bulk Bi WO nanoplates are
2
6
2
6
2
6
complanate, uneven and decayed with an average thickness of 24 nm (Fig. 1a, b). Whereas, the
Ti-doped Bi WO nanosheets are obviously thin and curly with well-organized hierarchical
2
6
architecture and the average thickness decreased to 13 nm after Ti-doping into the crystal lattice
of Bi WO (Fig. 1c, d). Furthermore, the EDS elemental spectra and mapping images confirm
2
6
the presence of Ti, Bi, W and O elements in the Ti-doped Bi WO in contrast to bulk Bi WO , as
2
6
2
6
shown in Fig. S1. The high and low magnification atomic force microscopic (AFM) images and
the corresponding height profiles (the inset) for bulk Bi WO and Ti-doped Bi WO are
presented in Fig. 1e, f and Fig. S2. It can be seen that the nanosheet thickness of bulk Bi WO is
2
6
2
6
2
6
1
0.6 nm, while the thickness of Ti-doped Bi WO is 1.8 nm (the inset of Fig. 1e, f). The
2 6
thickness of Ti-doped Bi WO nanosheets well accordance with the thickness of unit-cell of
2
6
Bi WO which is 1.64 nm along [001] direction. This result demonstrates that the layered
2
6

ACCEPTED MANUSCRIPT
2 6
assembled 3D Ti-doped Bi WO microstructures possesses well-defined single-unit-cell layer
synthesized by TBT-assisted hydrothermal process.
Fig. 2 present the TEM/HRTEM images of bulk Bi
2
6 2 6
WO and Ti-doped Bi WO samples.
The TEM and HRTEM images of bulk Bi WO (Fig. 2a, b) and Ti-doped Bi WO (Fig. 2c, d),
2
6
2
6
the formation of transparent and ultra-thin nanosheets can be observed and some single-layer
nanosheets can be directly observed in the HRTEM image of Ti-doped Bi WO (Fig. 2d). The
2
6
HRTEM images (Fig. 2b, d), reveals two kinds of lattice fringes with interplanar spacing of
.272 nm and 0.273 nm, which corresponds to the (020) and (200) planes of orthorhombic phase
Bi WO , respectively. It reveals that the Bi WO and Ti-doped Bi WO nanosheets were exposed
0
2
6
2
6
2
6
with (001) facet, which is consistent with the SAED pattern (the inset Fig. 2b, d). The SEM and
TEM images reveal that the Ti-doping could assist the formation of unique layered hierarchical
structures and the Ti-doping creates defects in the crystal structure of Bi WO which can
2
6
eventually causing the formation of redox couple and generation oxygen vacancies. Furthermore,
the defects caused by Ti-doping remarkably effect on the specific surface area of Bi WO , pore
2
6
volume and pore size distribution due the decrease in thickness of nanosheets. The BET specific
2
-1
surface area of bulk Bi WO is 29.6622 m g , while the S
of Ti-doped Bi WO is 59.3476
2 6
2
6
BET
2
-1
2 6
m g , which is almost double of the pure Bi WO nanosheets (Fig. S3). Moreover, the Ti-doping
could also favor the introduction of large number of mesopores and macropores (the inset Fig.
S3).
3
.2. Crystal structure and chemical state analysis
2 6 2 6
Fig. 3 display the XRD patterns of bulk Bi WO and a series of Ti-doped Bi WO (Ti-
BWO) samples with varying contents of Ti-dopant. All the diffraction peaks could be indexed to
the orthorhombic-phase Bi WO (JCPDS No. 079-2381) and no other peak can be seen,
2
6

ACCEPTED MANUSCRIPT
indicating the purity of the samples (Fig. 3a and b). However, a slight shift and gradual decrease
in peak intensity of (131) peak can be observed in the diffraction peaks for Ti-doped samples
after increasing the Ti contents (inset of Fig 3a and Fig. S4). This phenomenon could be
3+
attributed to synergic effect of nanosized and microstrain effect and the substitution of Bi ion
+
3
by Ti ions in the modified Ti-doped samples, which consist of ultra-thin nanosheets, revealing
the formation of single-unit-cell Bi WO . The results are also consistent with the SEM, TEM and
2
6
AFM results.
Furthermore, a careful examine of the (131) diffraction peak of bulk Bi WO and Ti-doped
2
6
2 6
Bi WO samples at 2θ = 28.39° and 2θ = 28.33° (inset of Fig. 3a and Fig. S4), signifies that the
Ti-doping could shift the diffraction peak towards lower 2θ value. The crystal structure of
2
+
2-
Bi WO is composed of alternative stacked (Bi O ) layers and (WO ) octahedral layers along
2
6
2
2
4
3
+
the b-axis. The introduction of Ti cation into crystal lattice of Bi WO will substitute the Bi
2
6
2
+
cation in the (Bi O ) layer due to similar charge. The crystal distortion of (131) diffraction
2
2
3
+
3+
2 6
peaks in Ti-doped Bi WO is due the dissimilar ionic radius of Ti and Bi [39]. According to
the Bragg’s law, the increase in the 2θ value, should be attributed to the decrease of lattice
parameters. Besides, the cell parameters of pure Bi WO and Ti-doped Bi WO determined from
2
6
2
6
XRD data, which are a = 5.347 Å, b = 16.430 Å and c = 5.458 Å for bulk Bi WO and a = 5.452
2
6
+3
Å, b = 16.427 Å and c = 6.436 Å for Ti-doped Bi WO . This result indicated that the Bi cation
2
6
+
3
is substituted by the Ti in Ti-doped Bi WO [39]. Furthermore, the substitution of host cation
2
6
3
+
3+
Bi with large ionic radius (103 pm) with a foreign cation Ti with smaller ionic radius (60.5
pm) causing lattice shrinkage [24], similar phenomenon has been observed in the Ce-doped
2 6 2 6
Bi MoO [40] and Gd-doped Bi WO [41].

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To further investigate the crystal structure distortion and generation of crystal defects in Ti-
doped Bi WO , the Raman spectra of bulk Bi WO and Ti-doped Bi WO was measured to
2
6
2
6
2
6
detect the substitution of element in Bi WO nanostructure and the results are shown in Fig. 4.
2
6
-1
-1
As shown in Fig. 4a and b, the peaks at 824.69 cm and 793.7 cm could be assigned to the
symmetric and asymmetric Ag modes of terminal O-W-O [42]. The tungstate chain asymmetric
-1
-1
bridging mode is associated with the band at 712 cm and the peaks at 306.2 cm could be
3
+
6-
assigned to the translational mode of simultaneous motions of Bi and WO
6
[24]. The
is changed in
6
. The magnified Raman spectra Fig. 4c and d (data is offsets for
-1
-1
vibration bands of O-W-O at 824.69 cm and 793.7 cm in the T-doped Bi
contrast with bulk Bi WO
clarity), it can be seen that the peak at 824.69 cm shifts towards higher, while the peak at 793.7
2 6
WO
2
-
1
-1
-1
cm and 712 cm changed slightly towards lower wavenumber, respectively. It should be due
3
+
3+
the substitution of Bi by Ti cation in the crystal lattice and the exposer of more atoms in
hierarchical single-unit-cell Ti-doped Bi WO , which consequently causing crystal defects. Fig
2
6
4
b, c and d (data is offsets for clarity), the peak broadening and the shifting of peaks towards
lower and higher wavenumber can be observed with an increasing Ti-dopant in the crystal lattice
of Bi WO , which is consistent with the XRD data.
The investigate the chemical state and change in each atom coordination environment
caused by crystal distortion in Ti-doped Bi WO , the X-ray photoelectron microscopy (XPS) was
performed for bulk Bi WO and Ti-doped Bi WO samples and the results are shown in Fig.5.
2
6
2
6
2
6
2
6
The survey XPS spectra (Fig. 5a) for Bi WO and Ti-doped B WO confirmed the detection of
2
6
2
6
Bi, W, O and Ti elements in samples. The binding energies peaks at 157. 5 eV and 162.9 eV
Fig. 5b) can be assigned to Bi 4f7/2 and Bi 4f5/2 in bulk Bi WO and Ti-doped Bi WO which
(
2
6
2
6
3
+
corresponds to Bi in the crystal structure [39]. In comparison to bulk Bi WO , the typical Bi 4f
2
6

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binding energies peaks shifted towards a higher binding energy after Ti-doping and the
origination of two less intense binding energies peaks at 155.5 eV and 160.9 eV, which can be
2
+
attributed to metallic Bi or Bi [22]. The shifting in the Bi 4f binding energies peaks should be
due to difference of electronegativity of bismuth and titanate elements, which demonstrate that
the coordination environment of Bi surrounding was influenced by Ti substitution in the crystal
lattice in Ti-doped B WO [43]. Fig. 5c display the XPS spectra of W 4f in bulk Bi WO and Ti-
2
6
2
6
doped Bi
2
WO
6
. The peaks at 34.4 eV and 36.4 eV can be correlated to W 4f 7/2 and W 4f5/2
,
6
+
which corresponds to W , respectively [22]. The W 4f binding energy peak in Ti-doped Bi
2
WO
6
6
+
5+
could be fitted to two spin-orbit doublets, which corresponds to the W and W oxidation sates
of tungstate element. The shifting of typical W 4f peaks towards higher binding energies and the
origination of two new binding energies peaks at 34.1 eV and 35.9 eV (Fig. 5c) in Ti-doped
6
+
5+
Bi WO should be due to the partial reduction of W to W after Ti-doping, consequently
2
6
causing the charge compensation [40]. In other words, the occurrence of crystal defects and
generation of oxygen vacancies formed on the surface of Ti-doped Bi WO [44]. The O 1s peaks
2
6
(
Fig. 5d) can be deconvoluted into three bands at 528.7 eV, 529.9 eV and 530.7 eV, which
corresponds to lattice oxygen, hydroxyl groups (O-H) and surface adsorbed H O [45]. The Ti-
doped Bi WO have higher (OH) amount in comparison to bulk Bi WO , confirming that the Ti-
2
2
6
2
6
doped Bi WO have abundant O atom on the surface, which is an evidence of crystal defect
2
6
formation and generation of more oxygen vacancies after Ti-doping. Moreover, the abundant OH
can makes the Ti-doped Bi WO more hydrophilic, which is beneficial for photocatalytic
2
6
reaction. In comparison of bulk Bi WO , the XPS spectra of Ti-doped Bi WO present an
2
6
2
6
obvious shoulder band of Bi 2p and Bi 4d peaks along the main peak as displayed in Fig.5 e. The
binding energies peaks at 457.1 eV and 464.9 eV is indicating the position of Ti 2p peaks in the

ACCEPTED MANUSCRIPT
2 6
crystal lattice of Ti-doped Bi WO [46]. The binding energies peaks at 457.1 eV and 464.9 eV
3
+
4+
can be ascribed to Ti 2p3/2 and Ti 2p , which corresponding to the Ti and Ti oxidation states,
1/2
respectively [47]. Comparing the binding energy peaks of Ti 2p in Ti-doped Bi WO with that
2
6
of bulk Bi WO , the Ti 2p band can be clearly seen in the Ti-doped Bi WO , which cannot be
2
6
2
6
3
+
4+
observed in the bulk Bi WO . The results confirmed that both the Ti and Ti cations coexist in
2
6
3
+
3+
Ti-doped Bi WO , which further strengthen that the Bi should be substituted by Ti in the
2
6
modified Ti-doped Bi
.3. Analysis of optical properties and band structure
The UV-vis light absorption properties of bulk Bi WO and a series of Ti-doped Bi WO
6
2 6
WO .
3
2
6
2
was investigated by UV-vis diffuse reflectance spectra (DRS) as shown in Fig. 6. It was found
that the Ti-doping influenced the visible light absorption of Bi WO . As exhibit in Fig. 6a and b,
in contrast to bulk Bi WO with an absorption edge of 454 nm, the Ti-doped Bi WO present
apparent blue-shift (about 18 nm) in absorption edge, this could be due to the substitution of Bi
2
6
2
6
2
6
3
+
3
+
by Ti cation and the nano-sizing (NS) effect [48]. Moreover, it can be seen that with increasing
Ti-dopant, a gradual blue-shift in the absorption edge of Ti-doped samples can be observed (Fig.
6
b). The optical band gap of bulk Bi WO and Ti-doped Bi WO was calculated by (αhυ = A(hv
2 6 2 6
n/2
-
E ) , where α, hυ, A and E are the optical absorption coefficient, photonic energy, the
g g
constant of proportionality and the band gap, respectively. The type of transition was defined by
the n value (n = 1 and 4 for direct and indirect absorption, respectively). According to the
2 6
previously reported research works, Bi WO is an indirect band gap semiconductor [24]. For
bulk Bi WO and Ti-doped Bi WO , the indirect band gap was found from the intercept of
2
6
2
6
1
/2
function of absorption plotted versus energy (the inset of Fig. 6a). Therefore, the band gap

ACCEPTED MANUSCRIPT
2 6 2 6 2 6
calculated for bulk Bi WO and Ti-doped Bi WO (10%-Ti-doped Bi WO ) was 2.71 eV and
2
.84 eV, respectively.
The band structure for bulk Bi
2 6 2 6
WO and Ti-doped Bi WO was investigated by Mott-
Schottky method as shown in Fig. 7 a, b. Based on the band gap of Bi WO (2.71 eV) and Ti-
2
6
doped Bi WO (2.84 eV), the CB level of Ti-doped Bi WO is supposed to be more negative in
2
6
2
6
comparison with pure Bi WO . The Mott-Schottky measurement was employed to confirm the
2
6
band edge position of Bi WO and Ti-doped Bi WO . The flat band potentials determined by
2
6
2
6
Mott-Schottky plot is -0.47 V and -0.54V (vs. Ag/AgCl, pH = 7) for Bi WO and Ti-doped
2
6
Bi
NHE, pH = 7). The positive slop of linear plot shows an n-type semiconductors characteristics of
bulk Bi WO and Ti-doped Bi WO , which have the CB level more negative by about 0.1 V than
the flat band potential [49, 50]. Therefore, the conduction band CB level of bulk Bi WO and Ti-
2 6
WO , respectively, which is equal to -0.23 V and -0.31 V (vs. normal hydrogen electrode
2
6
2
6
2
6
doped Bi WO are approximately estimated to be -0.33 eV and -0.41 eV, and their valance band
2
6
VB potential are calculated to be 2.28 eV and 2.33 eV, respectively. Hence, in contrast to bulk
Bi WO , the Ti-doped Bi WO possess more negative conduction band level and more positive
2
6
2
6
valance band level. Consequently, the Ti-doping could effectively improve the oxidation ability
of holes in VB of Bi WO , which allow to bring more influential photo-redox driving force.
2
6
3
2 6 2 6
.4. Photodegradation performance of Bi WO and Ti-doped Bi WO
The adsorption and photolysis performance of bulk Bi WO and Ti-doped Bi WO was
2
6
2
6
investigated for degradation of RhB, MO, phenol and 4-nitrophenol (4-NP) under visible light
irradiation (˃ 420 nm) and the results are shown in Fig. 8. To investigate the selective adsorption
efficiency, different charges bearing organic pollutants were taken as molecular probes. The pH
value determined in our experiments for dye solution is ∼ 7.0 for the selective adsorption and

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photodegradation experiments. At this pH value the pollutants can be ionized according to their
+
cationic and anionic composition. The charge containing for RhB, C H ClN O = C H N O
3
2
8
31
2
3
28 31
2
-
-
+
-
+
Cl , MO, C H N SO Na = C H1 N SO + Na , 4-NP, C H NO = C H NO and phenol as
14 14 3 3 14 4 3 3 6 5 3 6 4 3
neutral in the solution. Therefore, the RhB is positively charged, while the MO and 4-NP are
negatively charged, as shown in Fig. S5a. To study the catalyst-contaminants interaction in
solution, the point of zero charge (PZC) was measured for Ti-doped Bi WO , as shown in Fig.
2
6
2 6
S5b. The PZC determined for Ti-doped Bi WO photocatalyst is pHPZC = 3.2, which indicates
that the Ti-doped Bi WO is negatively charged in the reaction at pH of ∼ 7. The adsorption
2
6
efficiency curves for different organic pollutants are presented in Fig. S5c. It can be seen that the
positively charge RhB dye have high adsorption ability compare to negatively charge MO, 4-NP
and neutral phenol. This results further reveal that the Ti-doped in the crystal lattice of Bi WO
2
6
absorb the surface oxygen and work as electron trapping center to generate more super oxide
radical anions, consequently boosting the photocatalysis performance and selective adsorption
efficiency for positively charged cationic pollutants.
In the case of photodegradation, no photodegradation of RhB was observed in the absence of
photocatalyst, showing that RhB and MO are stable under visible light (Fig. S6). In comparison
to bulk Bi
2
WO
6
, the Ti-doped Bi
2
WO
6
presents excellent photocatalytic degradation
mineralization performance towards various organic pollutant such as RhB, MO, Phenol and 4-
nitrophenol. It is worth mentioning that the Ti-doped Bi WO (10-Ti-BWO) can degrade almost
2
6
9
9.9 % and 99.8% of RhB and MO and the mineralization rate reaches 91.3 % and 86.4 % within
a short interval of time (Fig. 8a, c, d and S7a). Similarly, phenol and 4-NP being very stable
refractory organic pollutants are photodegraded 68% and 42% over Ti-doped Bi WO with
2
6
2 6
mineralization rate of 55.4 % and 27.8 %, which is higher than the bulk Bi WO which is 22.2%

ACCEPTED MANUSCRIPT
and 9% with mineralization rate of 13.2 % and 4.1 % (Fig. 8b, e, f and S7b). The absorbance
spectra of RhB and MO over bulk Bi WO and Ti-doped Bi WO are shown in Fig. 8c, d. The
2
6
2
6
RhB absorption peak at 552 nm gradually shifted towards lower wavelength and dramatically
decreases and almost disappeared within 40 min of irradiation, which shows the occurrence of
de-ethylation of RhB [51]. Similarly, in the absorbance spectra of 4-NP, new peak at 205 nm is
gradually increasing with decreasing the main absorbance for 4-NP at 318 nm after increasing
the irradiation time, which could be attributed to the formation of intermediates during the
photodegradation of 4-NP (Fig. 8f). All the Ti-doped samples present enhanced degradation
efficiency towards the degradation of RhB, MO, phenol and 4-NP. The order of degradation over
various Ti-doped Bi WO samples are 10-Ti-BWO ˃ 20-Ti-BWO ˃ 5-Ti-BWO ˃ 1-Ti-BWO ˃
2
6
Bi WO . Among these, the photocatalytic degradation performance of 10-Ti-BWO (10%-Ti-
2
6
Bi WO ) achieved the highest photolysis efficiency towards the photodegradation of various
2
6
organic pollutants, which indicates that the Ti-doping enhanced the photodegradation ability of
Bi WO under visible light irradiation. The results also show that increasing the Ti-dopant
2
6
contents in Bi
defect density generation in Ti-doped Bi
affect the photolysis performance of Ti-doped Bi WO .
2
WO
6
(20-Ti-BWO) could decrease the photodegradation ability because of higher
2
WO due to excess amount of Ti-dopant, consequently,
6
2
6
2 6
Fig. S8 present the photostability of Ti-doped Bi WO (10-Ti-BWO) towards the
photocatalytic degradation of RhB and MO. A slight decrease can be seen in the
photodegradation performance of 10-Ti-BWO, indicating the efficient photostability of Ti-doped
Bi WO . To further explain the enhanced photostability, cyclic XRD and SEM was investigated
2
6
for 10-Ti-BWO in contrast to pure 10-Ti-BWO and bulk Bi WO and the results are shown in
2
6
Fig. S9. It can be seen in the XRD pattern of 10-Ti-BWO (Fig. S9a), that no obvious distortion

ACCEPTED MANUSCRIPT
happened in the diffraction peaks after 4 cyclic degradation in contrast to pure 10-Ti-BWO and
bulk Bi WO (before degradation) which indicates the efficient photostability of Ti-doped
2
6
Bi WO . Similarly, the SEM images (Fig. S9b, c) of 10-Ti-BWO after 4-cylic degradation,
2
6
evidently justify that the Ti-doped Bi WO maintained the same layer assembled 3D
2
6
morphology, with a slight decay in the morphological structure.
3
.5. Possible intermediates of acarbose
The photocatalytic degradation of acarbose (ACB) over Ti-doped Bi WO (10-Ti-BWO)
2
6
was carried out and the intermediated was analyzed by HPLC-MS as shown in Fig. 9. Acarbose
produce the protonated form [M-H]+ at 646.25m/z. As the degradation time increases a lot of
intermediates formation occur which can be seen in Fig. 9 and the parent molecular ion peak of
acarbose become disappear which confirm the successful degradation (Fig. S10). According to
HPLC-MS chromatogram before and after irradiation of 15, 30 and 45 min (Fig. S8), some of
intermediate may form due to McLafferty rearrangement of molecules such as pent-1-ene at
7
0.18m/z and diethylamine at 73.10m/z according to literature study. Other intermediated were
identified as Hydrogen acetate at 60.10m/z, Phenylalanylleucine at 279.29m/z, Butanol at
4.24m/z, Heptanone at 114.23m/z, limonene at 138.22m/z and cardiomanol at 280.32m/z. The
complete absence of parental molecular ion peak identifies and confirm the complete degradation
of acarbose by Ti-doped Bi WO
.6. Photoluminescence PL properties
The photoluminescence PL spectra of semiconductor photocatalysts could effectively reflect
7
2
6
.
3
the photolysis efficiency, charge migration, trapping and charge transfer, which reveals the
photogenerated electron-hole pairs separation and recombination during photocatalytic reaction
[52]. Lower the PL emission intensity usually indicates the low recombination rate of

ACCEPTED MANUSCRIPT
2 6
photogenerated electron-hole pairs. The PL emission spectra for bulk Bi WO and Ti-doped
Bi WO was operated at 350 nm excitation wavelength and the results is shown in Fig. 10a. The
2
6
PL emission intensity for Bi WO and Ti-doped Bi WO is centered at 393 nm. The PL emission
2
6
2
6
intensity of Ti-doped Bi WO (10-Ti-BWO) is lower than bulk Bi WO , which indicates that Ti-
2
6
2
6
doping could effectively suppress the rate of recombination of photogenerated electron-hole
pairs. In the Ti-doped Bi WO , the Ti-doping cause crystal defect in the crystal structure, which
2
6
could effectively favor the generation of oxygen vacancies and redox couple. Therefore, plenty
of oxygen vacancies can be easily produced under the illumination of light. Moreover, the Ti-
2 6
cations on the surface of Bi WO could serves as oxygen trapping site which is beneficial for
interfacial charge transfer and redox couple formation, consequently, results in enhanced
photocatalytic performance, which is consistent with the degradation results.
3
.7. Enhanced photodegradation mechanism
The investigation of main reactive species in the photodegradation reaction is worth
important to elucidate the enhanced photocatalytic mechanism. The active reaction trapping
experiment involves the benzoquinone (BQ), ethylenediaminetetraacetic acid disodium (EDTA-
-
+
2
Na) and isopropanol (IPA) as superoxide (•O ), holes (h ) and hydroxyl radical (•OH)
2
scavengers, respectively [37, 38]. Fig. 10b present the active reaction species trapping
experiment for the photodegradation of RhB over Ti-doped Bi WO (10-Ti-BWO). It can be
seen that the degradation of RhB was slightly affected by the addition of IPA as a quencher for
OH, which indicates that the ꢀOH is a minor reactive species during the degradation process.
However, the photodegradation efficiency of Ti-doped Bi WO was effectively decreased by the
2
6
ꢀ
2
6
-
adding BQ and EDTA-2Na, respectively. This result indicates that the superoxide (•O ) and
2

ACCEPTED MANUSCRIPT
+
holes (h ) are the main reactive species during the photocatalytic degradation of RhB over Ti-
doped Bi WO under visible light irradiation.
2
6
According to Mott-Schottky plot, the CB and VB band position calculated for Ti-doped
Bi WO is -0.46 eV and 2.38 eV, respectively. Based on the catalyst characterization and the
2
6
active species trapping experiment, a possible photocatalytic degradation mechanism for Ti-
doped Bi WO was proposed as shown in Fig. 11. The enhanced photocatalytic activity of Ti-
2
6
doped Bi WO can be attributed to the Ti-doping, increase BET specific surface area and the
2
6
formation of layer-assembled single-unit-cell hierarchical architecture of Ti-doped Bi WO .
2
6
According to DRS results (Fig. 6a, b), the introduction of Ti-dopant into the crystal structure
of Bi WO could results in red-shift in the optical absorption band, which indicates the formation
2
6
2 6
of new energy level (Ti-doped energy level) on the top of valance band of Bi WO [53].
Similarly, the electrons can be easily exciting from Ti-doped energy level to reach the
conduction band of Bi WO in Ti-doped Bi WO . Therefore, it can boost the photocatalytic
2
6
2
6
performance of Ti-doped Bi WO compare to bulk Bi WO . Moreover, the ion-doping could also
2
6
2
6
generate crystal defects which is benefit for the formation of redox couple, which play an
3
+
important role in the production of reactive oxygen species. The Ti-dopant substitute the Bi in
3
+
4+
the crystal structure of Bi WO . The redox couples Ti /Ti serves as electrons trapping center,
2
6
-
2
which could generate superoxide radical anions (ꢀO ) by the interaction with absorbed
molecular oxygen on the surface of photocatalyst and photogenerated electrons according to the
following steps:
+
-
Ti-Bi
2
WO
6
+ visible-light → Ti-Bi
2
WO
6
(h + e )
(1)
(2)
-
-
O
2
+ e → ꢀO
2

ACCEPTED MANUSCRIPT
3
+
4+
Similarly, the redox couples (Ti /Ti ) act as electron scavenger to generate superoxide radical
anions as following:
4
+
-
3+
Ti +e → Ti
(3)
(4)
(5)
3
+
-
4+
Ti + O
2
2
→ ꢀO + Ti
-
ꢀ
O
2
+ pollutants → degradation products
It is proposed from the active species trapping experiment that the hydroxyl radical ꢀOH is a
minor oxidative species during the photocatalytic degradation process. The photogenerated holes
can directly react with the pollutant to yield the degradation products according to the following
steps:
+
+
H
2
O + h → ꢀOH + H
(6)
(7)
(8)
+
h + pollutants → degradation products
ꢀ
OH + pollutants → degradation products
4
+
Under visible-light irradiation, the photoexcited electrons could be captured by the Ti , and
4
+
3+
3+
4+
the Ti can be further reduced to Ti cation. Similarly, the Ti could be oxidize back to Ti by
the oxygen molecule absorbed on the surface of photocatalyst in the reaction system. In the
3
+
meantime, the molecular O will react with Ti and electron to produce superoxide radical anion
2
-
3+
4+
(
ꢀO ). Therefore, the redox couples Ti /Ti not only facilitate the generation of superoxide
2
radicals but also efficiently sperate the photogenerated electron-hole pairs during the
+
photocatalytic degradation process. Moreover, the photogenerated h will either react with water
molecule to produce hydroxyl radical or it can directly react with the pollutants. However,
2 6
increasing the Ti-dopant than optimal level (10%-Ti-Bi WO ) could cause higher crystal defects

ACCEPTED MANUSCRIPT
3
+
4+
which can produce excessive existence of electron trapping such as redox couples (Ti /Ti ) and
oxygen vacancies. Hence providing a recombination center for the photogenerated electron-hole
pair, resulting in lower photocatalytic performance of 20-Ti-BWO. Therefore, 10% Ti-dopant is
the optimal concentration of Ti-doping and 10%-Ti-doped Bi WO possess the highest
2
6
photocatalytic degradation efficiency towards organic pollutants and efficient charge separation
during the degradation reaction.
3
.8. photogenerated charge separation and transfer analysis
The improved photocatalytic performance of bulk Bi WO and Ti-doped Bi WO was
2
6
2
6
investigated by a series of photoelectrochemical measurements and the results are shown in Fig.
2. The photocurrent response of a photocatalyst can designate the interfacial charge generation
and charge separation dynamics of photoelectrode [54]. The transient photocurrents responses
for bulk Bi WO and Ti-doped B WO electrodes was measured under visible-light irradiation (λ
420 nm) at 0.0 eV as shown in Fig. 12a, b. The fast generation of charge of both the electrodes
1
2
6
2
6
˃
under illumination indicates that the they are photo-sensitive. It is worth important to note that
the initial current density of bulk Bi WO and Ti-doped Bi WO photoelectrodes can reach 7.5 A
2
6
2
6
and 8.7 A, as light switched-on and then start to decay. The steady transient photocurrent density
of Ti-doped Bi WO electrode is about 5.6 A, which is much higher than bulk Bi WO with only
2
6
2
6
1
.3 A and higher current decay of the initial current. To further explain the excellent charge
generation and photostability of transient photocurrent responses of Ti-doped Bi WO samples in
contrast to bulk Bi WO , the experiment was repeated with same electrodes with a higher
2
6
2
6
interval of time (Fig. 12b). It can be seen that the transient photocurrent response of Ti-doped
Bi WO even more stable and increasing with increasing the irradiation time. Among the various
2
6
Ti-doped samples, the photocurrent response of 10-Ti-Bi WO is more stable and gradually
2
6

ACCEPTED MANUSCRIPT
2 6
increasing with increasing irradiation time. This result indicates that the Ti-doped Bi WO have
fast charge generation and low charge recombination rate as compare to bulk Bi WO .
2
6
Moreover, the transient photocurrent density of Ti-doped Bi WO are more stable with several
2
6
times light on/off cycles indicating an efficient separation of photogenerated charges on the
surface of Ti-doped Bi WO .
2
6
The electrochemical impedance spectroscopy (EIS) is important technique to investigate the
interfacial charge dynamic of photogenerated electron-hole pairs of photocatalyst. The radius of
arc in EIS spectra indicates the interfacial layer resistant on the surface of photocatalyst. The
smaller the arc radius, the higher the charge transfer efficiency and vice-versa [55, 56]. To
further investigate the enhanced charge transfer and separation dynamic of as prepared Ti-doped
2 6
Bi WO photocatalyst, the Nyquist plots was measured under visible-light (λ ˃ 420 nm) and in
dark, as shown in Fig.12c. It was found that the arc radius for Ti-doped Bi WO electrode both in
2
6
the case of illumination and in dark are smaller than the bulk Bi WO . This implies that the Ti-
2
6
doped Bi WO possess higher ability of photogenerated charge transfer and charge separation,
2
6
consequently boosting the photocatalytic performance.
To investigate the efficient photocurrent density of Ti-doped Bi WO , the linear sweep
2
6
voltammetry (LSV) measurement at a scan rate of 50 mV/s under visible-light (λ ˃ 420 nm) and
in dark was measured and the photocurrent density-voltage curve (J-V) are shown in Fig. 12 d.
The photocurrent densities of bulk Bi
increasing potential. It can be seen that the Ti-doped Bi
photocurrent density in dark and under visible-light in comparison to bulk Bi WO . This result
2
WO
6
and Ti-doped Bi
2
6
WO electrodes increases with
2
WO
6
electrodes present a higher
2
6
infers to the efficient photogenerated charge separation ability of Ti-doped Bi WO . It is
2
6
important to note that the J-V curve can specify the density of photogenerated charge carriers in

ACCEPTED MANUSCRIPT
photocatalyst and in a voltammograms the onset potential of photocurrent can determine the
quasi Fermi-level of majority carries [57, 58]. The relationship among quasi Fermi-level and the
carrier density of bulk Bi WO and Ti-doped Bi WO can be explained by the following Nernst
2
6
2
6
equation [59, 60].
E -E = kTln(N -N )/e
(9)
f1 f2
f1
f2
Where E and E represent the quasi Fermi-level of sample 1 and sample 2, k is the
f1
f2
Boltzmann’s constant, T and e are the temperature and elementary charge and N , N are the
f1
f2
carrier density. The inset of Fig. 12d, it is shown the potentials under visible-light irradiation and
in dark is -0.039 V, -0.061 V and -0.006 V, -0.044 V for bulk Bi WO and Ti-doped Bi WO
2
6
2
6
electrodes, respectively. The difference in the potential (V) values under visible-light and in dark
between the quasi Fermi-levels of bulk Bi WO and Ti-doped Bi WO represents the higher
carrier density of Ti-doped Bi WO photoelectrode. Hence, this result concludes that the Ti-
doped Bi WO produces a higher carrier density in contrast to bulk Bi WO , which is benefit for
2
6
2
6
2
6
2
6
2
6
enhanced photocatalytic and photoelectrochemical performance of Ti-doped Bi WO
2
6
photocatalyst.
To further study the enhanced electrochemical performance of Ti-doped Bi WO in dark and
2
6
under visible-light irradiation, the cyclic voltammetry (CV) profile was investigated as shown in
Fig. 13. Fig. 13a present the comparison of dark and under illumination CV curves for Ti-doped
Bi WO , measured at a scan rate of 50 mV/s and at a potential between 1.0V to 0.0V,
2
6
respectively. A reduction peak at 0.22 V and the corresponding oxidation peak at 0.26 V can be
seen in the CV curve for Ti-doped Bi WO under illumination. Similarly, a reduction peak at
.24 V and the corresponding oxidation peak at 0.38 V can be observed in the CV curve of Ti-
doped Bi WO in dark. The cathodic and anodic peaks the in the CV plot of Ti-doped Bi WO
2
6
0
2
6
2
6

ACCEPTED MANUSCRIPT
3
+
4+
can be attributed to the generation of redox couple (Ti /Ti ) on the surface of photocatalyst
[61]. The minor variation observed of anodic and cathodic peaks potentials in the CV curves in
dark and under light irradiation could be due to the difference in experimental parameters [62].
However, the cathodic and anodic peaks potentials of bulk Bi WO are broaden and featureless
2
6
and no oxidation and reduction peaks can be observed (Fig. 13c). The decrease current density of
bulk Bi WO could be due to the limited electronic conduction on the surface in dark and under
2
6
illumination. Moreover, the electrochemical cyclic voltammetry profile of Ti-doped Bi
2 6
WO and
bulk Bi WO were performed for 20 segments at the same scan rate and same potential in dark
2
6
and under illumination, as shown in Fig. 13b, d. A reduction and the corresponding oxidation
peaks can be observed in the CV profile of Ti-doped Bi WO with a slight decrease in the
2
6
current density under visible-light irradiation (Fig. 13b). However, no cathodic and anodic peak
are observed in the CV plot for bulk Bi WO and a large decrease in the current density can be
2
6
seen under illumination of light (Fig. 13d). Hence, the remarkable improvement in the current
density of Ti-doped Bi WO can be attributed to the Ti-doping which could favor the generation
2
6
3+
4+
2 6
of redox pairs (Ti /Ti ). The CV profile of Ti-doped Bi WO reveals more efficient separation
of photogenerated electron-hole pairs under visible-light irradiation, consequently, enhance the
photocatalytic activity.
4
. Conclusions
In summary, tetrabutyl titanate assisted layer-assembled single-unit-cell Bi
2
6
WO hierarchical
architecture have been fabricated by Ti-doping. Experimental results proven that the Ti-doping
mediate crystal defects in the crystal lattice of Bi WO which resulting in the generation of
2
6
oxygen vacancies, and thus show excellent selective adsorption towards positive charge
pollutants. By changing the Ti-dopant content, the process of ion substitution, charge

ACCEPTED MANUSCRIPT
compensation and crystal structure distortion was demonstrated. The XPS analysis revealed the
3
+
4+
existence of redox couples Ti /Ti present in Ti-doped Bi WO . The photodegradation
2
6
experiment proven that the Ti-doping in the crystal structure of Bi WO and the formation of 3D
2
6
hierarchical nanostructures greatly enhanced the photodegradation of organic pollutants in
contrast to bulk Bi WO . The electrochemical study revealed that the high current density and
2
6
3
+
4+
efficient charge separation ability in the Ti-doped Bi WO is attributed to redox couple Ti /Ti
2
6
3+
4+
generated upon visible-light irradiation. Furthermore, the generation of redox couples Ti /Ti in
Ti-doped Bi WO could promote the reactive oxygen species, which boost the photocatalytic
degradation and photoelectrochemical performance of Ti-doped Bi WO . This work may provide
2
6
2
6
general strategy for single-step simultaneous approach to fabricate layer-assembled hierarchical
nanostructures and ion-substitution in semiconductors catalyst for photocatalytic and
photoelectrochemical applications.
Acknowledgments
The corresponding author wish to acknowledge the financial support by the National Natural
Science Foundation of China (grant no. 51572126, 51872141).
Appendix A.
Supplementary data

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