Enhancing photocatalytic activity of Bi2MoO6 via surface co-doping with Ni2+ and Ti4+ ions

Article abstract of DOI:10.1016/j.jpcs.2019.01.014

Photocatalytic redox reactions occur on the surface of the catalyst, and therefore the surface properties of the photocatalyst are vital for efficient catalytic reactions. Bi₂MoO₆ surface co-doped with Ni²⁺ and Ti⁴⁺

Full text of DOI:10.1016/j.jpcs.2019.01.014

Accepted Manuscript
2+
Enhancing photocatalytic activity of Bi MoO via surface co-doping with Ni and
2
6
4
+
Ti ions
Jie Wang, Yangang Sun, Chongchong Wu, Zhe Cui, Pinhua Rao
PII:
S0022-3697(18)31955-3
https://doi.org/10.1016/j.jpcs.2019.01.014
PCS 8876
DOI:
Reference:
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 24 July 2018
Revised Date: 10 January 2019
Accepted Date: 12 January 2019
Please cite this article as: J. Wang, Y. Sun, C. Wu, Z. Cui, P. Rao, Enhancing photocatalytic activity of
2+
4+
Bi MoO via surface co-doping with Ni and Ti ions, Journal of Physics and Chemistry of Solids
2
6
(2019), doi: https://doi.org/10.1016/j.jpcs.2019.01.014.
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ACCEPTED MANUSCRIPT
Enhancing photocatalytic activity of Bi MoO6 via surface co-doping with
2
2+
4+
Ni and Ti ions
Jie Wang, Yangang Sun*, Chongchong Wu, Zhe Cui, Pinhua Rao*
College of Chemistry and Chemical Engineering, Shanghai University of Engineering
Science, Shanghai, 201620, China
*
Corresponding author. E-mail: syg021@sues.edu.cn. raopinhua@hotmail.com.
Abstract
Photocatalytic redox reactions occur on the surface of the catalyst, and
therefore the surface properties of the photocatalyst are vital for efficient
2+
4+
catalytic reactions. Bi MoO surface co-doped with Ni and Ti ions was
2
6
fabricated through an incipient-wetness impregnation technology and
2+
4+
calcination method. The influence of Ni and Ti co-doping with different
concentrations on the crystal structure, morphology, and optical properties of
Bi MoO were investigated by different instrumental methods, and the
2
6
characterization results confirm that the surface of Bi MoO was doped with
2
6
2+
4+
2+
4+
Ni and Ti ions. Under visible-light irradiation, the effects of Ni and Ti
surface co-doping on the photocatalytic activity of Bi MoO were evaluated by
2
6
2+
4+
the degradation of rhodamine B, with the results suggesting Ni and Ti
co-doping increases visible-light absorption by Bi MoO and promotes the
2
6
2
+
4+
4+
separation of photogenerated charge carriers. With optimum Ni and Ti
2+
co-doping, Bi MoO surface co-doped with 0.18% Ni and 0.192% Ti
2
6
exhibits the greatest photocatalytic activity and stability in the degradation of
rhodamine B.
Keywords
co-doping, surface, Bi MoO , photocatalyst
2
6

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Introduction
Bi MoO , which possesses alternate perovskite MoO
2+
4
2+
slabs and [Bi O ]
2
6
2
2
layers, has been reported as a promising solar-energy-conversion material for
water splitting and organic pollutant degradation under visible-light irradiation
[
1-3], which is attributed to its suitable band gap (2.5–2.8 eV) [4-6] and
distinctive structure, which facilitate the separation of charges because of
electrostatic interaction between layers.
The photocatalytic activities of Bi MoO materials are closely related to
2
6
their morphology, crystal microstructure, and composition. For example,
Bi MoO crystals with different morphologies [1], including nanosheets [7],
2
6
inverse opals, hollow microspheres [8], yolk-shell microspheres [9], nanofibers,
microtubes [10], ordered arrays, elm branch–like hierarchical structures [11],
and thin films [12], have been successfully fabricated by different preparation
methods, such as hydrothermal, solvothermal, molten salt, electrospinning,
microwave, and thermal evaporation deposition methods. The unique yolk-shell
structure is beneficial for charge separation and light absorption, and Bi MoO6
2
with a yolk-shell structure exhibits excellent photocatalytic performance and
recyclability [9]. At the same time, heterostructures of Bi MoO , such as
2
6
Bi MoO /Ag [13], Bi MoO /Bi [14], Bi MoO /Bi S [15], Bi MoO /BiOBr [16],
2
6
2
6
2
6
2
3
2
6
Bi MoO /TiO [17, 18], Bi MoO /NiTiO [19], and Bi MoO /Ag/CdS [20],
2
6
2
2
6
3
2
6
have been developed to enhance the photocatalytic activity and stability of
Bi MoO because the formation of well-defined heterojunctions effectively
2
6
promotes the separation and suppresses the recombination of photogenerated
charge carriers. Moreover, to improve the photocatalytic performance of
Bi MoO as a catalyst, doping with ions, including B, Cl, W, and lanthanide
2
6
ions [21, 22], was applied to influence the composition of Bi MoO by
2
6
changing and controlling the concentration of surface and internal defects [23].
However, it remains a challenge to realize practical applications of Bi MoO as
2
6
a catalyst because of the fast electron-hole recombination, slow carrier
migration, and poor surface chemical states. Hence it is necessary to explore

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ways to address these issues.
It is common knowledge that the three primary processes influencing the
photocatalytic performances of semiconductor photocatalysts are the generation,
separation, and surface diffusion of electron-hole pairs. Since the photocatalytic
redox reactions occur on the surface of the catalyst, and the surface properties
of the photocatalyst will greatly affect the three aforementioned processes, it
can be concluded that the surface properties are vital for efficient photocatalytic
n+
reactions. So far, metal ion (M ) doping on the surface of Bi MoO by the
2
6
incipient-wetness impregnation method has not been reported. We believe that
n+
n+
an M -doped layer is induced near the surface of Bi MoO by M diffusion
2
6
after annealing, which can change the generation, separation, and surface
diffusion of the charge carriers. Hence the surface tuning of the photocatalyst to
increase the activity of the photocatalyst and the utilization of solar energy
should be investigated.
2+
In this article, Bi MoO photocatalysts with the surface co-doped with Ni
2
6
4+
and Ti ions were fabricated through an incipient-wetness impregnation
4+
2+
technology and calcination method. The photocatalytic activity of the Ti /Ni
co-doped Bi MoO surface was investigated through the degradation of
2
6
rhodamine B (RhB) under visible-light irradiation. The effect of different
2
+
4+
doping amounts of Ni and Ti was also studied. Bi MoO surface co-doped
2
6
2+
4+
with 0.18% Ni
and 0.192% Ti
(Ni/Ti/BMO-3) showed excellent
visible-light-driven photocatalytic performance.
2
. Experimental
2
.1. Preparation of pure Bi MoO
2
6
All reagents were analytically pure and used without further purification. In a
typical solvothermal synthesis, first, 2 mmol of Bi(NO ) ·5H O (0.97 g) was
3
3
2
added to 30 mL of ethylene glycol and the resulting mixture was magnetically
stirred for 30 min Then 1 mmol of Na MoO ·2H O (0.242 g) was dissolved in
.
2
4
2
3
+
Bi solution and the resulting mixture was stirred for 20 min. Second, 10 mL of

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ethylene glycol was added to the aforementioned solution and the resulting mixture
was stirred for 120 min at room temperature. Third, the solution was transferred
into a 50 mL Teflon-lined stainless steel autoclave and heated at 160 � for 20 h.
After the reaction, the autoclave was cooled to room temperature in air. The
sample was collected by centrifugation, washed with ethanol and deionized
water several times, and then dried at 70 � for 10 h. Pure Bi MoO was
2
6
obtained.
2+
4+
2
.2. Preparation of Bi MoO with the surface co-doped with Ni and Ti
2 6
2+
4+
The catalysts with Ni and Ti doping on the surface of Bi MoO were
2
6
prepared by incipient-wetness impregnation using as-obtained Bi MoO as a
2
6
2+
4+
catalyst support. The Ni (or Ti ) monosupported catalysts were prepared
through impregnation of the aforementioned catalyst supports with Ni(NO3)2
(
or tetrabutyl titanate) aqueous solution, kept static at room temperature for 6 h
and then dried at 80 ° C overnight, and finally calcined at 400 °C for 2 h in air.
2+
4+
Ni
and Ti
co-supported catalysts were synthesized by the same
impregnation of the catalyst supports with the corresponding Ni(NO ) or
3
2
tetrabutyl titanate aqueous solution.
The catalysts obtained were denoted as Ni/BMO (where BMO refers to
2+
4+
Bi MoO , and the percentage of Ni is 0.06%), Ti/BMO (the percentage of Ti
2
6
is 0.064%), and Ni/Ti/BMO-
x
(where
x
= 1, 2, 3, 4, and 5, corresponding to
2+
4+
Ni /Ti loading percentages of 0.06%/0.064%, 0.12%/0.128%, 0.18%/0.192%,
2+
0
.24%/0.256%, and 0.30%/0.32%, respectively). The final content of Ni and
4+
Ti in Bi MoO was determined by inductively coupled plasma optical
2
6
emission spectrometry (Varian 715-ES, Varian, Australia). The results indicate
2+
4+
that the Ni and Ti contents in Ni/Ti/BMO-3 were 0.176% and 0.181%,
respectively.
2
.3. Characterization
The X-ray diffraction (XRD) data were obtained with a powder X-ray

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diffractometer (Rigaku D/max-2550 PC). X-ray photoelectron spectroscopy
(
ESCALab MKII spectrometer with an excitation source of Mg Kα radiation)
was used to determine the chemical binding states of the constituent elements.
Scanning electron microscopy (SEM; JEOL S-4800), transmission electron
microscopy (TEM; JEOL JEM-2100F) and high-resolution TEM (JEM-2100F)
images were obtained to investigate the morphology of the products. UV-vis
diffuse reflectance spectroscopy (DRS) was performed with a UV3600
spectrophotometer (SHIMADZU).
2
.4. Photocatalytic activity experiments
The catalytic activities of the samples were evaluated by our measuring the
degradation of RhB (maximum absorption peak at 554 nm) under visible light
at room temperature (25 °C) according to our previously reported method [24].
The experimental procedure was as follows: The photocatalyst sample (20 mg)
-1
was suspended in 50 mL of RhB solution (5.0 mg L ) to form a suspension.
Afterward, the suspension was stirred in the dark for 60 min to ensure an
adsorption-desorption equilibrium. Then the suspension under magnetic stirring
was placed approximately 10 cm below a xenon lamp (500 W, model
PLS-SXE300, Beijing PerfectLight ) and an optical filter glass was used to cut
out light of wavelength below 400 nm. About 3.5 mL of suspension was
sampled every 20 min and centrifuged. The solution of the clear upper layer
was measured with a UV-1901 spectrophotometer (Shanghai Aucy) to obtain
the absorption spectrum of RhB solution. After testing, the solution was
returned from the reactor top. Radical-trapping experiments were performed by
our adding 1 mM benzoquinone, 1 mM ammonium oxalate, 6 mM AgNO , and
3
-1
1
mM tert-butanol to the RhB (50 mL, 5 mg L ) solution before the
photocatalysis tests.
3
. Results and discussionThe crystalline structures of all as-obtained samples
were analyzed by powder XRD measurements. Fig. 1a shows the XRD patterns

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of the eight synthesized samples. In the XRD pattern of pure Bi MoO , all
2
6
diffraction peaks match well with the standard orthorhombic Bi MoO structure
2
6
(
JCPDS card no. 21-0102). No obvious peaks of other impurities were detected,
indicating that high-quality orthorhombic phase Bi MoO was obtained. In the
2
6
2+
4+
XRD pattern of Bi MoO samples loaded with Ni and Ti , no new peak was
2
6
observed in the samples with different loadings. This is attributed to the
2+
4+
2+
4+
extremely low loading of Ni and Ti and high dispersion of Ni and Ti on
the Bi MoO surface. Careful observation of these XRD patterns (Fig. 1b)
2
6
2+
4+
indicates that the doping with Ni and Ti caused a decrease in the intensity of
n
+
the peak at 28.3° in M -doped Bi MoO compared with pure Bi MoO , which
2
6
2
6
2+
4+
suggests that Ni and Ti ions were successfully incorporated in the Bi MoO
2
6
lattice [25]. The XRD analysis results confirm that the surface of Bi MoO was
2
6
2+
4+
doped with Ni and Ti ions by a simple incipient-wetness impregnation
technology and calcination method.
X-ray photoelectron spectroscopy spectra of pure Bi MoO6 and
2
Ni/Ti/BMO-3 are shown in Fig. 2 and were used to determine the elemental
composition and chemical state. There are peaks at 159.2 and 164.5 eV for pure
Bi MoO and at 159.1 and 164.4 eV for Ni/Ti/BMO-3, corresponding to Bi 4f
7/2
2
6
and Bi 4f . In Fig. 2c, the binding energy peaks at 232.5 and 235.6 eV for pure
5/2
Bi MoO and at 232.4 and 235.6 eV for Ni/Ti/BMO-3 are assigned to Mo 3d
5/2
2
6
6+
and Mo 3d of Mo [26, 27]. The binding energy of O 1s is 530.1 eV for pure
3/2
Bi MoO and 530.0 eV for Ni/Ti/BMO-3. The binding energies of Ni 2p in
2
6
Ni/Ti/BMO-3 are 873.6 and 855.6 eV for Ni/Ti/BMO-3 [28, 29]. The binding
energy of Ti was observed to be 465.9 eV [30] in accordance with Ti 2p1/2 in
Ni/Ti/BMO-3. This confirmed that the surface of Bi MoO was successfully
2
6
2+
4+
doped with Ni and Ti ions.
The morphologies of pure Bi MoO , Ti/BMO, Ni/BMO, and Ni/Ti/BMO-
x
2
6
samples were analyzed by SEM. The images obtained are shown in Fig. 3.
From these images, we can see that the samples are all composed of flowerlike
microspheres with diameters of 1–3 µm. These microspheres are built from

ACCEPTED MANUSCRIPT
2+
4+
two-dimensional nanosheets. With increase of Ni and Ti doping/co-doping
n+
on the surface of Bi MoO , the M -doped Bi MoO microspheres display a
2
6
2
6
flowerlike shape similar to that of pure Bi MoO .
2
6
The TEM images in Fig. 4 further verify that the flowerlike microspheres of
Bi MoO and Ni/Ti/BMO-3 comprise two-dimensional nanosheets with a
2
6
thickness of 20–30 nm. Furthermore, the high-resolution TEM images in Fig.
b and d clearly reveal a set of fringes with lattice spacings of 0.235 and 0.316
nm, ascribed to the (221) and (131) planes of Bi MoO .
4
2
6
N adsorption-desorption measurements were performed (Fig. 5). The
2
2
-1
Brunauer-Emmett-Teller (BET) surface area of pure Bi MoO is 21.2 m g .
2
6
2
-1
Ni/Ti/BMO-3 exhibits a slight increase of the BET surface area (28.9 m g ).
Moreover, the pore size distributions calculated on the basis of
Barrett-Joyner-Halenda analysis reveal most pores are of size approximately 25
nm in both Bi MoO and Ni/Ti/BMO-3 (inset in Fig. 5). Ni/Ti/BMO-3 with
2
6
nanopores and a high BET surface area favors photocatalysis [31].
The optical absorption spectra of Bi MoO , Ni/BMO, Ti/BMO, and
2
6
Ni/Ti/BMO-x are shown in Fig. 6a. Bi MoO exhibits absorption peaks in both
2 6
the UV region and the visible region. The onset of the absorption edge for
Bi MoO is at approximately 470 nm (2.36 eV), which is in accordance with
2
6
2+
4+
the literature [31, 32]. It can be see that the introduction of Ni and Ti
broadens and increases the visible-light absorption intensity of Bi MoO . In
2
6
comparison with Bi MoO , Ni/BMO, and Ti/BMO, Ni/Ti/BMO-3 clearly
2
6
exhibits enhanced absorption in the visible range (~450–700 nm),
corresponding to a band gap of 2.23 eV (Fig. 6b), suggesting that Ni/Ti/BMO-3
might exhibit good photocatalytic behavior under visible light.
As indicated above, UV-visible absorption results for Ni/Ti/BMO-3 predict
its improved photocatalytic performance in visible light. The photocatalytic
performance was tested by the degradation of RhB. RhB displays a
characteristic absorption peak at a wavelength of around 554 nm and is a
popular probe molecule in heterogeneous catalytic reactions. For comparison,

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the photocatalytic activities of Bi MoO , Ni/BMO, Ti/BMO, and Ni/Ti/BMO-
x
2
6
(x = 1, 2, 4, and 5) were also evaluated under the same conditions. Fig. 7a
shows the temporal evolution of the absorption spectrum of RhB solution with
Ni/Ti/BMO-3 as the catalyst after exposure to visible light. It can be seen that
the main absorption peak at 554 nm decreases rapidly with increase of the
exposure time and completely disappears after about 60 min, confirming that
the RhB solution can be efficiently photodegraded by Ni/Ti/BMO-3. The
photographs of the color change of RhB solution clearly reveal that RhB was
successfully degraded within 60 min, as shown in Fig. 7b. Fig. 7c shows the
photodegradation curves for RhB over different catalysts. From the blank
experiment, we confirm that no RhB is degraded without the catalysts after 60
min of irradiation. From the adsorption equilibrium curves (dark), it can be seen
that the different catalysts show very little adsorption ability for RhB after
treatment for 60 min. When P25 and pure Bi MoO are used as the
2
6
photocatalyst, only 39.1% and 64.3% of RhB is degraded, respectively. All the
surface-doped catalysts, such as Ni/BMO, Ti/BMO, and Ni/Ti/BMO- = 1, 2,
, 4, and 5) display higher activities than pure Bi MoO , and the degradation
x (x
3
2
6
rate of RhB solution is 87.1%, 92.2%, 88.4%, 90.4%, 98.8%, 76.1%, and
8.7%, respectively. Among them, Ni/Ti/BMO-3 displays the highest
8
+
+
photocatalytic activity, suggesting the optimal weight ratio of Ni2 , Ti4 , and
Bi MoO is 0.075:0.075:1. In comparison with other Bi MoO morphologies
2
6
2
6
for degradation of RhB [21, 23, 32], Ni/Ti/BMO-3 exhibited enhanced
photocatalytic activity. Moreover, the photocatalytic degradation kinetics of
RhB was investigated. The photocatalytic degradation data were fit to a
pseudo-first-order model [33], -ln(C/C0) = kt, where
reaction rate constant and is the light irradiation time. Fig. 7d shows the
kinetic curves for RhB under visible-light irradiation. The reaction rate constant
is equal to the corresponding slope of the simulation curve. The values for
RhB with P25, Bi MoO , Ni/BMO, Ti/BMO, and Ni/Ti/BMO- = 1, 2, 3, 4,
and 5) are 0.0075 ± 0.0002 min , 0.00147 ± 0.00046 min , 0.00320 ± 0.00024
k is the photocatalytic
t
k
k
x (x
2
6
-1
-1

ACCEPTED MANUSCRIPT
-1
-1
-1
-1
min , 0.0385 ± 0.0020 min , 0.0308 ± 0.0035 min , 0.0332 ± 0.0030 min ,
-
1
-1
-1
0
.0584 ± 0.0011 min , 0.0246 ± 0.0025 min , and 0.0316 ± 0.0031 min ,
respectively. Notably, is the highest for Ni/Ti/BMO-3 among the catalysts,
and is 4.0, 1.5, and 2.4 times higher than for Bi MoO , Ti/BMO, and
k
2
6
Ni/Ti/BMO-4, respectively.
The catalyst stability is also an important factor for practical application,
and a five-cycle photocatalytic experiment with Ni/Ti/BMO-3 was conducted.
Fig. 8a illustrates the variation of the RhB relative concentration
C/C0 with
irradiation time in the five cycles. In the first cycle the degradation rate of RhB
was 98.3%, and in the second cycle was 98.4%. After the procedure had been
performed five times, the degradation rate was 95.5%. Moreover, the used
Ni/Ti/BMO-3 (Fig. 8b) exhibits SEM features similar to those of fresh
Ni/Ti/BMO-3 (Fig. 3f). These results reveal that the photocatalytic activity of
Ni/Ti/BMO-3 can remain stable and efficient during organic dye degradation.
To better understand the photocatalytic mechanism of Ni/Ti/BMO-3, the
dominant active species in the photocatalytic process were investigated (Fig. 9).
The scavengers benzoquinone, ammonium oxalate, AgNO , and tert-butanol
3
•-
2
are usually used to capture superoxide (O ), photogenerated holes,
photogenerated electrons, and hydroxyl radicals (•OH), respectively [34, 35].
As shown in Fig. 9a, RhB degradation was greatly inhibited when
benzoquinone was added: the RhB degradation efficiency decreased from 98.8%
to 0.031% after 60 min of reaction, and the degradation rate decreased from
-1
•-
0
.0584 to 0.00049 min (Fig. 9d), indicating that superoxide (O ) is the
2
dominant active species. The RhB degradation efficiency decreased from 98.8%
to 76.6% when ammonium oxalate was used as the scavenger, indicating that
photogenerated holes were the active species. However, the photocatalytic
efficiency of RhB was not affected by the addition of tert-butanol (Fig. 9b) and
silver nitrate (Fig. 9c), suggesting that photogenerated electrons and •OH are
•-
not the dominant active species. Therefore it can be concluded that O2 and
photogenerated holes are the dominant active species.

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On the basis of the results of the characterization and photodegradation
tests, three reasons are proposed to account for the enhanced photocatalytic
2+
4+
activity induced by Ni and Ti surface doping. The first is the increase in
2+
optical absorption. The UV-visible spectra confirmed that doping with Ni and
4+
Ti could enhance visible-light absorption by Bi MoO and decrease the band
2
6
gap of 2.23 eV. The second reason is that Ni/Ti/BiMO-3 exhibited an increase
2+
4+
of surface area compared with Bi MoO due to Ni and Ti surface doping.
2
6
2+
4+
The third reason is that Ni and Ti on the surface of Bi MoO can capture the
2
6
2+
4+
photogenerated electrons, and Ni and Ti ions as electron scavenger agents
react with the photogenerated electrons and restrain the recombination of the
2+
electron-hole pairs during the photocatalytic reaction [25,36]. Then the Ni and
4+
Ti ions transfer the electrons to O adsorbed on the surface of Ni/Ti/BMO-
x
to
2
•-
2
form superoxide radical (O ). At the same time, extra holes are produced.
•-
2
Therefore it is well recognized that O radicals plays an important role, and
holes play a secondary role in the photodegradation process. The photocatalytic
mechanism is consistent with the results of the trapping experiment, which
•-
2
prove that O and photogenerated holes are the dominant active species.
4
. Conclusions
2+
4+
Bi MoO photocatalysts surface co-doped with Ni and Ti ions were
2
6
successfully fabricated through an incipient-wetness impregnation technology
2+
4+
and calcination method. The influence of Ni and Ti concentration on the
morphology and optical properties of Bi MoO catalysts was investigated. The
2
6
photocatalytic activity experiment on the degradation of RhB under
visible-light irradiation indicated that the optimum Ni/Ti/BiMO-3 resulted in
about 4.0, 1.5, and 2.4 times increase in the degradation rate with respect to
pure Bi MoO , Ti/BMO, and Ni/Ti/BMO-4, respectively. At the same time,
2
6
2+
4+
Ni and Ti co-doping can obviously increase visible-light absorption by
Bi MoO and promote the separation of photogenerated electrons and holes.
2
6
2+
4+
This study on Bi MoO surface co-doped with Ni and Ti is not only
2
6

ACCEPTED MANUSCRIPT
scientifically interesting and technologically promising for Bi MoO materials
2
6
but is also useful for fabrication of other Bi-based materials.
Acknowledgments
This work was financially supported by the Foundation of Innovation Program of
Shanghai Municipal Education Commission (grant no. 14ZZ160), the Open Fund of
the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
Donghua University (grant no. LK1209), and Shanghai University of Engineering
Science (grant nos. 2012gp13 and E1-0501-15-0105).
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Fig. 1. X-ray diffraction patterns of different catalysts. BMO, Bi MoO ;
2
6
2+
Ni/BMO, Bi MoO surface doped with 0.06% Ni ; Ti/BMO, Bi MoO surface
2
6
2
6
4+
doped with 0.064% Ti ; Ni/Ti/BMO-1, Bi MoO surface co-doped with 0.06%
2
6
2+
4+
Ni and 0.064% Ti ; Ni/Ti/BMO-2, Bi MoO surface co-doped with 0.12%
2
6
2+
4+
Ni and 0.128% Ti ; Ni/Ti/BMO-3, Bi MoO surface co-doped with 0.18%
2
6
2+
4+
Ni and 0.192% Ti ; Ni/Ti/BMO-4, Bi MoO surface co-doped with 0.24%
2
6
2+
4+
Ni and 0.256% Ti ; Ni/Ti/BMO-5, Bi MoO surface co-doped with 0.30%
2
6
2+
4+
Ni and 0.32% Ti .
Fig. 2. (a) X-ray photoelectron spectroscopy (XPS) survey spectrum of
2+
4+
Bi MoO surface co-doped with 0.18% Ni and 0.192% Ti (Ni/Ti/BMO-3).
2
6
XPS spectra of Bi (b), Mo (c), and O (d) in Bi MoO and Ni/Ti/BMO-3. XPS
2
6
spectra of Ni (e) and Ti (f) in Ni/Ti/BMO-3.
Fig. 3. Scanning electron microscopy images of pure Bi MoO (a), Bi MoO
2
6
2
6
4+
2+
surface doped with 0.064% Ti (b), Bi MoO surface doped with 0.06% Ni
2
6
2+
4
(
c), Bi MoO surface co-doped with 0.06% Ni and 0.064% Ti (d), Bi MoO
2 6 2 6

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2+
4+
surface co-doped with 0.12% Ni and 0.128% Ti (e), Bi MoO surface
2
6
2+
4+
co-doped with 0.18% Ni and 0.192% Ti (f), Bi MoO surface co-doped with
2
6
2+
4+
0
.24% Ni and 0.256% Ti (g), and Bi MoO surface co-doped with 0.30%
2 6
2+
4+
Ni and 0.32% Ti (h).
Fig. 4. Transmission electron microscopy images of Bi MoO (a, b) and Ni
2
6
2+
4+
Bi MoO surface co-doped with 0.18% Ni and 0.192% Ti (c, d).
2
6
Fig. 5. N adsorption-desorption isotherms of Bi MoO (BMO) and Bi MoO
6
2
2
6
2
2+
4+
surface co-doped with 0.18% Ni and 0.192% Ti (Ni/Ti/BMO-3). The inset
shows the pore size distribution.
Fig. 6. (a) UV-visible spectra and (b) the band gaps of different catalysts. BMO,
2+
Bi MoO ; Ni/BMO, Bi MoO surface doped with 0.06% Ni ; Ti/BMO,
2
6
2
6
4+
Bi MoO surface doped with 0.064% Ti ; Ni/Ti/BMO-1, Bi MoO surface
2
6
2
6
2+
4+
co-doped with 0.06% Ni and 0.064% Ti ; Ni/Ti/BMO-2, Bi MoO surface
2
6
2+
4+
co-doped with 0.12% Ni and 0.128% Ti ; Ni/Ti/BMO-3, Bi MoO surface
2
6
2+
4+
co-doped with 0.18% Ni and 0.192% Ti ; Ni/Ti/BMO-4, Bi MoO surface
2
6
2+
4+
co-doped with 0.24% Ni and 0.256% Ti ; Ni/Ti/BMO-5, Bi MoO surface
2
6
2+
4+
co-doped with 0.30% Ni and 0.32% Ti .
Fig. 7. (a) UV-visible spectra of rhodamine B (RhB) solution with time. (b)
Photographs of the color change of RhB solution in the presence of Bi MoO6
2
2+
4+
surface co-doped with 0.18% Ni and 0.192% Ti
visible-light irradiation. (c, d) Photodegradation curves and degradation rate constants
d, inset) for RhB over different catalysts BMO, Bi MoO ; Ni/BMO, Bi MoO
6
(Ni/Ti/BMO-3) under
(
.
2
6
2
2+
surface doped with 0.06% Ni ; Ti/BMO, Bi MoO surface doped with 0.064%
2
6
4+
2+
Ti ; Ni/Ti/BMO-1, Bi MoO surface co-doped with 0.06% Ni and 0.064%
2
6
4+
2+
Ti ; Ni/Ti/BMO-2, Bi MoO surface co-doped with 0.12% Ni and 0.128%
2
6
4+
2+
Ti ; Ni/Ti/BMO-4, Bi MoO surface co-doped with 0.24% Ni and 0.256%
2
6
4+
2+
Ti ; Ni/Ti/BMO-5, Bi MoO surface co-doped with 0.30% Ni and 0.32%
2
6
4+.
Ti
Fig. 8. (a) Five cycles in the photocatalytic degradation of rhodamine B by
Ni/Ti/BMO-3. (b) Scanning electron microscopy image of the used Bi MoO6
2

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2+
4+
surface co-doped with 0.18% Ni and 0.192% Ti
.
Fig. 9. UV-visible spectra of rhodamine (RhB) solution with time of the RhB
2+
4+
solution over Bi MoO surface co-doped with 0.18% Ni and 0.192% Ti
2
6
(
(
Ni/Ti/BMO-3) in the presence of (a) benzoquinone (BQ) and (b) tert-butanol
TBA). (c, d) Photodegradation curves and degradation rate constants (d, inset) for
RhB over Ni/Ti/BMO-3 in the presence of different scavengers. AO, ammonium
oxalate.
Graphical abstract
2+
4+
Bi MoO surface co-doped with Ni and Ti ions was fabricated through an
2
6
incipient-wetness impregnation technology and calcination method. Surface
2+
4+
co-doping of Ni and Ti obviously increased visible-light absorption by
Bi MoO and promoted the separation of photogenerated charge carriers.
2
6
2
+
4+
Bi MoO surface co-doped with 0.18% Ni and 0.192% Ti exhibits the
2
6
greatest photocatalytic activity and stability in the degradation of rhodamine B.

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Highlights
•
•
Ni/Ti/Bi MoO was fabricated through incipient-wetness impregnation.
2 6
2+
4+
Surface co-doping with Ni and Ti increased the photocatalytic
activity of Bi MoO .
2
6
2+
4+
•
The amount of Ni and Ti surface co-doping affects the catalytic
activity of Bi MoO .
2
6