Ultrasound-assisted construction of a Z-scheme heterojunction with g-C3N4 nanosheets and flower-like Bi2WO6 microspheres and the photocatalytic activity in the coupling reaction between alcohols and amines under visible light irradiation

Article abstract of DOI:10.1016/j.mcat.2020.111122

Flower-like Bi₂WO₆ microspheres and bulk g-C₃N₄ were prepared by the hydrothermal method and high-temperature calcining method, respectively. A new method based on the combination of ultrasonic stripping and mechanical stirring was used to obtain uniform composite x-Bi₂WO₆/g-C₃N₄ (x is the Bi₂WO₆ mass ratio) photocatalysts with a Z-scheme heterojunction. In the reaction of benzyl alcohol and aniline to form imine, the optimal composite, 30percent-Bi₂WO₆/g-C₃N₄, showed a conversion rate of 87.6percent, which is much higher than that of pure Bi₂WO₆ and g-C₃N₄. Characterization by SEM and TEM showed that the ultrasonically stripped g-C₃N₄ significantly reduced the radial dimension compared to bulk g-C₃N₄. A uniformly dispersed photocatalytic material was formed, and it maintained a flower-like microsphere structure. Materials characterized by N₂ adsorption–desorption isotherms (BET) showed that the specific surface area of the g-C₃N₄ nanosheets increased by approximately ten times after ultrasonic stripping. The photostability of the composite was verified by cyclic experiments. Under visible light irradiation, the activation energy of the coupling reaction between benzyl alcohol and aniline was found to be decreased by 29.8 kJ mol^?1. A capture experiment was used to verify the active species in the photocatalytic system, with the reaction found to be mainly completed through the synergistic action of h⁺, e^? and ?O₂^?.

Full text of DOI:10.1016/j.mcat.2020.111122

Molecular Catalysis 494 (2020) 111122
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Ultrasound-assisted construction of a Z-scheme heterojunction with g-C N4
3
nanosheets and flower-like Bi WO microspheres and the photocatalytic
2
6
activity in the coupling reaction between alcohols and amines under visible
light irradiation
a,b
a
a
a
a
a,
b
Xiujuan Yu , Xueli Hao , Haiying Li , Zhiying Zhang , Yan Wang , Jingyi Li *, Zhibao Wang ,
Enbo Li
a
b
College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, People's Republic of China
Hebei Key Laboratory of Neuropharmacology, College of Pharmacy, Hebei North University, Zhangjiakou 075000, People's Republic of China
b
A R T I C L E I N F O
A B S T R A C T
6 3 4
WO microspheres and bulk g-C N were prepared by the hydrothermal method and high-tem-
Keywords:
Bi WO /g-C N
2 6 3 4
Z-scheme heterojunction
Coupling reaction
Visible light irradiation
Flower-like Bi
perature calcining method, respectively. A new method based on the combination of ultrasonic stripping and
mechanical stirring was used to obtain uniform composite x-Bi WO /g-C (x is the Bi WO mass ratio)
photocatalysts with a Z-scheme heterojunction. In the reaction of benzyl alcohol and aniline to form imine, the
optimal composite, 30%-Bi WO /g-C , showed a conversion rate of 87.6%, which is much higher than that of
pure Bi WO and g-C . Characterization by SEM and TEM showed that the ultrasonically stripped g-C
significantly reduced the radial dimension compared to bulk g-C
material was formed, and it maintained a flower-like microsphere structure. Materials characterized by N
adsorption–desorption isotherms (BET) showed that the specific surface area of the g-C
2
2
6
3
N
4
2
6
2
6
3 4
N
2
6
3
N
4
3
N
4
3
4
N . A uniformly dispersed photocatalytic
2
3 4
N nanosheets increased
by approximately ten times after ultrasonic stripping. The photostability of the composite was verified by cyclic
experiments. Under visible light irradiation, the activation energy of the coupling reaction between benzyl al-
−
1
cohol and aniline was found to be decreased by 29.8 kJ mol . A capture experiment was used to verify the
active species in the photocatalytic system, with the reaction found to be mainly completed through the sy-
+
−
−
nergistic action of h , e and •O
2
.
1
. Introduction
specific surface area [7,8]. At present, semiconductor photocatalysis
technology is mainly used in organic synthesis, photolysis of water to
produce oxygen and degradation of pollutants [9–14]. The mechanism
of semiconductor photocatalysis is mainly to absorb solar photons
greater than or equal to their forbidden band width (Eg) energy and is
stimulated to produce photogenerated electron and hole pairs with
redox capability, which can be captured and participate in photo-
catalytic redox reactions. Therefore, the performance of semiconductor
photocatalysts mainly depends on their light absorption capacity and
the separation rate between photogenerated electrons and holes
[15,16]. Based on the above basic factors, many methods have been
adopted to modify the catalyst to obtain highly efficient photocatalytic
materials, including the construction of heterojunctions [4,17], ion
doping [15,18], size effect and crystal structure effect [6,19]. Among
these methods, reducing the particle size of the material can facilitate
Imines are important intermediates in the synthesis of various
bioactive nitrogen heterocyclic compounds, fine chemicals, pharma-
ceuticals and agrochemicals [1–3]. One of the most commonly used
methods to form imines is the coupling of alcohols with amines. Alco-
hols are ideal starting materials because they are inexpensive, are easy
to obtain, and can theoretically produce only hydrogen or water as
byproducts, while the use of catalysts can control the selectivity of the
reaction. In recent decades, photocatalysis, as an efficient, green and
promising technology, has attracted extensive attention from re-
searchers [4–6]. In particular, some semiconductor-based photo-
catalysts with nanostructures have broad application prospects in the
fields of solar and chemical energy conversion and environmental
purification due to their good charge transport performance and large
⁎
Corresponding author.
E-mail address: lijingyicn@163.com (J. Li).
https://doi.org/10.1016/j.mcat.2020.111122
Received 26 May 2020; Received in revised form 8 July 2020; Accepted 13 July 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
the transfer of carriers from the body to the surface, and increasing the
specific surface area is closely related to the active site [5,7]. In addi-
tion, since the photocatalytic activity of semiconductors is closely re-
lated to their structure, the construction of semiconductor heterojunc-
tions has proven to be an ideal method for improving photocatalytic
activity. Among them, it is considered to be a very effective strategy to
construct a Z-scheme structure with another semiconductor with an
appropriate energy band position. Some studies have shown that the
construction of a Z-scheme structure photocatalytic system and the
transfer and separation of electrons and holes through the interfacial
electric field formed between heterostructures can not only improve the
separation efficiency of electron and hole pairs but also maintain two
semiconductor materials with good redox ability [20–23].
Scientific Ltd. Company and used without further purification. All other
solvents and bases used in the experiments were supplied by local
commercial suppliers.
2.2. Preparation of the BWO/CN photocatalyst
2.2.1. Preparation of Bi
At room temperature, 5 mmol Na
solved in 40 mL deionized water by stirring to form solution A; 5 mmol
Bi(NO ·5H O was completely dissolved in 100 mL 2 M HNO by stir-
2 6
WO
2 4 2
WO ·2H O was completely dis-
3
)
0
2
3
ring to form solution B. Then, liquid B was slowly dropped into liquid A,
and the mixed solution was fully stirred for approximately 2 h with
NaOH to adjust the pH value of the mixture to 4–5 (2–3, 6–7, 8–9, or
10–11). The mixed solution was then transferred to a high-pressure
reaction kettle and heated at 160 °C (120 °C, 140 °C, 180 °C or 200 °C)
for 24 h. After heating, the sample was cooled naturally to room tem-
perature, and the precipitate was washed with deionized water and
anhydrous ethanol three times followed by drying at 70 °C for 12 h; the
As an excellent visible light photocatalyst, Bi
2 6
WO has attracted
2−
widespread attention because it is composed of a WO
4
perovskite
layers. With a band gap of
6
shows obvious photocatalytic performance under ul-
2
+
structure sandwiched between (Bi
.8 eV, Bi WO
2 2
O )
2
2
traviolet and visible light irradiation [17,24,25]. Studies show that
compared with ordinary nanomaterials, photocatalysts with a flower-
prepared Bi
2 6
WO powder catalyst was stored after sufficient grinding
like Bi
2
WO
6
structure have higher transfer rates of interface charges.
(Bi WO is abbreviated as BWO).
2
6
The main reason for these rates is that the flower-like structure uses
independently controllable mesopores or macropores as the transpor-
tation routes so that the reactant molecules reach the reaction site,
thereby improving the photocatalytic performance [26,27]. However,
the utilization rate of pure Bi WO is limited, and the recombination
2 6
rate of photogenerated electrons and holes is high, thus limiting the
photosensitive reaction of the catalyst [28,29]. Graphite-phase carbon
3 4
2.2.2. Preparation of g-C N
A certain amount of melamine was weighed into a crucible, which
was then placed into a muffle furnace. The temperature of the muffle
furnace was increased at a rate of 5 °C/min to 550 °C, and the calci-
nation was maintained at 550 °C for 4 h. After cooling to room tem-
3 4
perature, the prepared g-C N was ground to a powder, washed three
nitride (g-C
3
N
4
) is a polymer semiconductor material with a narrow
times with hot deionized water, washed once with anhydrous ethanol,
and then dried at 70 °C for 12 h; the obtained yellow powder was then
band gap of 2.7 eV. Due to its excellent chemical stability and unique
electronic structure, it is widely used as an inexpensive, stable, and
metal-free component polymer photocatalyst in the research field of
visible light photocatalysis [30–32]. Due to the suitable energy band
stored for later use. An appropriate amount of bulk g-C
solved in an aqueous solution with a volume of 50% ethanol content
and continuously sonicated for 4 h to obtain the stripped g-C
(stripped g-C is abbreviated as CN).
3 4
N was dis-
3 4
N
structure of g-C
3
N
4
, it has been selected as another component to
prepared by calcining
3 4
N
modify Bi WO to form a Z-type system. g-C N
2
6
3 4
melamine at a high temperature has a large particle size and a small
specific surface area. Therefore, it is necessary to reduce its volume and
increase its specific surface area [33,34]. Inspired by the above con-
clusions, a new method was adopted in this study: at room temperature,
2.2.3. Preparation of x-BWO/CN
3 4 2 6
The ultrasonically stripped g-C N and Bi WO were mixed ac-
cording to a certain mass ratio, dissolved in a certain volume of me-
thanol solution, sonicated for 1 h, and then continuously stirred for
12 h. The precipitate obtained by centrifuging the mixed solution was
washed several times with deionized water and anhydrous ethanol and
then dried at 70 °C for 12 h. After sufficient grinding, the BWO/CN
composite catalyst was obtained and recorded as x-BWO/CN, where x is
the mass fraction of BWO in the composite photocatalyst (15%, 30%,
45%, and 60%). Taking 15%-BWO/CN as an example, the composite
photocatalyst was prepared by mechanical stirring of 15 mg BWO and
85 mg CN.
a combination of ultrasonic stripping bulk g-C
stirring complex method was found to not only increased the specific
surface area of g-C nanosheets (CN) but also maintained the flower
globular structure of the composite material. Experiments found that a
Bi WO /g-C composite photocatalytic material (BWO/CN) com-
posed of flower-like microspheres Bi WO (BWO) and stripped g-C
CN) showed good photocatalytic activity, significantly higher than that
of the pure photocatalysts (Bi WO or g-C ).
In this paper, flower-like microsphere Bi
nosheets was prepared by a hydrothermal method with Bi(NO
and Na WO ·2H O as the precursors, and bulk g-C was prepared by
calcining melamine at a high temperature. Then, the combination of
ultrasonic stripping of bulk g-C and the mechanical stirring of a
3 4
N and a mechanical
3 4
N
2
6
3 4
N
2
6
3 4
N
(
2
6
3 4
N
2
WO
6
stacked by na-
·5H
3
)
3
2
O
2.3. Characterization methods
2
4
2
3 4
N
The crystal phases of the materials were analyzed by X-ray dif-
fraction (XRD, D/MAX-2500, Rigaku Industrial Corporation). An X-ray
diffractometer was used with Cu/Kα radiation (λ = 1.5405 nm) at
40 kV and 100 mA. The morphological characteristics of the materials
were observed by scanning electron microscopy (SEM, model S-4800,
Japan Electronic Hitachi Optical Laboratory Ltd. Co.). Transmission
electron microscopy (TEM) and high-resolution transmission electron
microscopy (HRTEM) images were obtained using an F20 S-double
electron microscope (Tecnai G2, FEI Ltd. Co.) at an accelerating voltage
of 200 kV. The UV–vis diffuse reflectance spectra of the materials were
measured using a UV–vis spectrophotometer based on a complete
spherical component of 100% barium sulfate (UV–vis DRS, U-3900,
Hitachi Limited Company). The X-ray photoelectron spectra (XPS,
ESCALAB 250Xi, Thermo Fisher Scientific Company) of the materials
were measured using a spectrometer equipped with an Al-Kα X-ray
source (hν = 1486.6 eV). The photoluminescence (PL, FLS920,
Edinburgh Instruments) spectra of the materials were measured by
3 4
N
compound yielded a series of BWO/CN composite photocatalyst mate-
rials with Z-scheme heterostructures with different loading mass ratios.
The effect of the composite material on the conversion of the coupling
reaction of alcohols and amines under different loading mass ratios was
studied in detail under visible light irradiation. On the basis of various
characterization analyses and quenching experiment results, the pos-
sible photocatalytic mechanism was also discussed.
2. Experimental
2.1. Materials
Bismuth nitrate pentahydrate, sodium tungstate dihydrate, mela-
mine, alcohols, amines, n-hexane, toluene, cyclohexane, n-heptane,
isopropanol, 1,4-dioxane, CH ONa, Cs CO were purchased from J&K
3
2
3
2

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
using a 450 W pulsed xenon lamp as the light source. The wavelength of
the excitation light was 325 nm, and the wavelength of the emission
spectrum ranged from 350 to 700 nm. The multipoint
Brunauer–Emmett–Teller (BET, Quantachrome Instruments) was used
to measure the specific surface area and pore size distribution of the
materials. The reaction products were quantitatively and qualitatively
analyzed by gas chromatography (GC-2030, Shimadzu) and gas chro-
matography and mass spectrometry (GC-MS, Trace DSQ II).
2.4. Photocatalytic experiment
The photocatalytic activity of the catalysts in the coupling reaction
between alcohols and amines was studied under visible light irradia-
tion. The activity assay was performed in a photochemical reaction box
(
Beijing Zhongjiao Jinyuan Technology Ltd. Co., China). The reactor
was equipped with an adjustable power-xenon light source
P ≤ 500 W), a circulating water cold trap and an ultraviolet light cut-
(
off filter (λ > 400 nm). In the experiment, 50 mg catalyst powder was
dispersed in a quartz tube containing 6 mL reaction solvent. Then, an
appropriate amount of alkali, alcohols, and amines were added into the
mixed solution and stirred continuously for 10 h under visible light ir-
radiation. The reaction temperature was 15 ± 3 °C.
Fig. 1. XRD patterns of the different catalysts.
purity.
3.2. Scanning electron microscopy (SEM) analysis
2.5. Recycling test
The morphology of the materials was observed by SEM character-
To test the stability of the 30%-BWO/CN catalyst, five cycle ex-
ization. Fig. 2 shows SEM images of the prepared photocatalytic ma-
terials. It can be seen from Fig. 2a and b that Bi WO has a flower-like
microsphere structure formed by stacking of ultrathin nanosheets.
Fig. 2c and d show the SEM images of bulk g-C and a nanometer-
sized CN sheet obtained after ultrasonic stripping of bulk g-C . By
periments were carried out. A certain number of parallel experiments
for activity determination were completed (n = 40), and the catalyst
was recovered and reused five times. The recovered catalyst was wa-
shed several times with deionized water and anhydrous ethanol and
then dried and reused. The results obtained from five cycles were
analyzed.
2
6
3 4
N
3 4
N
comparing Fig. 2c and d, the results show that the radial size of CN after
ultrasonic stripping is greatly reduced and that it has a thin nanosheet
structure. Fig. 2e shows an SEM image of 30%-BWO/CN. From Fig. 2e,
2
.6. Analyze the activation energy in the reaction
2 6
it can be seen that the structure of the flower-like Bi WO microspheres
is not changed, but the surface of the nanosheet is loaded with CN,
which is evenly distributed. In addition, not only does the combination
of ultrasonic assistance and mechanical stirring not change the flower-
like microsphere structure of Bi WO , but it can also be observed from
2 6
Fig. 2e that the size of the CN nanosheets is continuously decreased. At
the same time, the CN loading makes the gap between the petal
The experimental method for determining the activation energy is
the same as the experimental method used for measuring the photo-
catalytic activity. During the reaction, the temperature of the reactor
was adjusted by controlling the flow of cooling water. The reaction
times were 2 h, 4 h, 6 h, and 8 h, and the reaction temperature was 15 °C
(
25 °C, 35 °C, and 45 °C). The conversion rate of the reaction between
2 6
structure of Bi WO smaller. Therefore, it is speculated that such a
benzyl alcohol and aniline to imine in the reactor was measured. The
Arrhenius equation and the reaction kinetics data at different tem-
peratures were used to estimate the apparent activation energy.
structure is more conducive to the transfer of electrons on the surface of
the material according to the results of UV–vis DRS. Fig. 2f shows the
SEM image of 30%-BWO/CN after five cycles. Fig. 2f shows that the
2 6
structure of the flower-like Bi WO microspheres is changed slightly,
but the heterostructure of the composite material is found to remain
uniformly intact.
3
. Results and discussion
3.1. X-ray diffraction (XRD) analysis
The crystal structures of the photocatalytic materials were char-
3.3. Transmission electron microscopy (TEM) analysis
acterized by XRD. As shown in Fig. 1, pure stripped g-C
distinct peaks at 13.25° and 27.53°, corresponding to the (1 0 0) and
0 0 2) crystal planes, respectively. For Bi WO , intense diffraction
3 4
N exhibits two
Fig. 3 shows TEM and HRTEM images of the prepared photo-
catalytic material. Fig. 3a and b show the flower-like spherical structure
(
2
6
peaks at 2θ = 28.32°, 32.81°, 47.20°, 55.91°, and 58.48° are observed,
of Bi
nanosheet structures. This result is consistent with that obtained from
SEM. Fig. 3c shows a TEM image of stripped g-C (CN). In this figure,
2 6
WO , which is formed by an alternating superposition of thin
corresponding to the (1 3 1), (2 0 0), (2 0 2), (3 3 1), and (2 6 2) crystal
planes, respectively. All the diffraction peaks of Bi
good agreement with the orthogonal phase of Bi
126), and there is no impurity phase [35]. As shown in the XRD re-
sults, the main peak of g-C at 27.53° is overlapped with the main
peak of Bi WO at 28.32°. These results agree well with previous lit-
erature results [36]. In addition, the clear observation of g-C in the
BWO/CN composite structure suggests that g-C was successfully
combined with Bi WO ; the existence of g-C and Bi WO in the
2
WO
6
in Fig. 1 are in
3 4
N
2
WO
6
(JCPDS 73-
the mesoporous structure of CN can be observed. Mesopores are formed
due to gas release during thermal condensation [37]. Fig. 3d and e show
TEM images of the 30%-BWO/CN composite material, which confirm
the close contact between CN and BWO, indicating that a BWO/CN
heterostructure was successfully constructed. The presence of hetero-
structures can promote the transfer of photogenerated electron and hole
pairs, thereby improving the photocatalytic activity. Fig. 3f shows a
HRTEM image of 30%-BWO/CN, in which lattice spacings of 0.315 nm
and 0.272 nm can be observed, corresponding to the (1 3 1) and (2 0 0)
1
3 4
N
2
6
3 4
N
3 4
N
2
6
3
N
4
2
6
composite materials can be further confirmed by SEM, TEM, and XPS
analyses. The presence of additional phases was not detected in all
prepared materials, indicating that the synthesized catalysts have high
2 6
planes of Bi WO , respectively [38,39]. To further study the element
3

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
Fig. 2. SEM images of different catalysts: (a, b) Bi
2
WO
6
, (c) bulk g-C
N
3 4
, (d) stripped g-C
3
N
4
(CN), (e) 30%-BWO/CN, and (f) 30%-BWO/CN catalyst after cyclic
experiments.
Fig. 3. TEM images of the different catalysts: (a, b) Bi
0%-BWO/CN.
2
WO
6
, (c) stripped g-C
3
N
4
(CN), (d, e) 30%-BWO/CN, (f) HRTEM image of 30%-BWO/CN, and (g) EDX map of
3
4

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
Fig. 4. (a) UV–vis DRS patterns of different catalysts and (b) the relative band gap energy of different catalysts.
composition of 30%-BWO/CN, the EDX spectrum of 30%-BWO/CN was
measured. The results in Fig. 3g show the content of each element,
which was basically found to be consistent with the actual added value.
main peak at 398.3 eV corresponds to C=N–C, the main peak at
399.9 eV corresponds to the nitrogen atom of N–(C)
404.0 eV is attributed to oxidized N [32,43]. Fig. S1 shows a compar-
ison of the Bi 4f, W 4f, O 1s regions of 30%-BWO/CN and pure Bi WO
and a comparison of the C 1s, N 1s regions of 30%-BWO/CN and pure
stripped g-C . The results show that after the formation of the Z-
3
, and the peak at
2
6
,
3.4. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) analysis
3 4
N
scheme heterojunction, the chemical states of Bi, W, O, C and N all
show slight changes in Fig. S1.
Fig. 4a shows the UV–vis DRS spectra obtained for Bi
, and x-BWO/CN. As seen from Fig. 4a, both pure Bi
show an absorption in the visible region, indicating that visible
light can excite Bi WO and g-C . When Bi WO and g-C form
2
WO
6
, stripped
g-C
C N
3 4
3
N
4
2
WO
6
and g-
2
6
N
3 4
2
6
3
N
4
3.6. Photoluminescence (PL) spectra analysis
composite materials, the absorption band of x-BWO/CN shows a red-
shift of different degrees (the redshift of 30%-BWO/CN is relatively
obvious), and this redshift increases the absorption range of the com-
posite materials in the visible light region. Fig. 4b shows the band gap
Photoluminescence spectroscopy can be used to explain the transfer
and recombination of photogenerated electron and hole pairs in semi-
conductor photocatalysts. The lower the photoluminescence intensity
is, the lower the recombination rate of photoluminescent carriers under
energy spectrum (Eg) of Bi
2 6
WO and x-BWO/CN. The Eg values of these
samples can be calculated based on the UV–vis absorption spectra data
light irradiation [22]. Fig. 6 shows the PL spectra of stripped g-C
Bi WO and x-BWO/CN at an excitation wavelength of 325 nm. It can
be seen from Fig. 6 that the emission peak intensity of g-C is the
highest, much higher than that of Bi WO and x-BWO/CN, and that the
emission peak intensity of 30%-BWO/CN is the weakest. This outcome
indicates that the heterostructure formed by Bi WO and g-C at a
30% mass ratio has the best effect in inhibiting the recombination of
photogenerated electrons and holes. This result is in accordance with
the experimental results obtained of the performance of the catalysts.
3 4
N ,
n/2, where
by using the equation αhν = A(hν −
α, h, ν, A and Eg represent
2
6
the absorption coefficient, Planck's constant, frequency, a constant and
the optical band gap energy, respectively. In this equation, n depends
on the characteristics of the transition in a semiconductor, i.e., direct
3 4
N
2
6
transition (n = 1) or indirect transition (n = 4). Both Bi
belong to indirect transitions, and n is 4 [40]. Thus, the bandgaps
WO and 30%-BWO/CN were calculated to be 2.75 eV and
.31 eV, respectively. The composite material with the Z-scheme het-
2
WO
6
and g-
2
6
3 4
N
C
N
3 4
of Bi
2
6
2
erostructure narrowed the band gap, thereby increasing the sample
response to visible light.
3.7. N
2
adsorption–desorption isotherm (BET) analysis
3.5. X-ray photoelectron spectroscopy (XPS) analysis
It is well known that the particle size and specific surface area of a
photocatalyst has a considerable influence on the photocatalytic ac-
tivity. To determine the reason for the observed improvement in pho-
tocatalytic performance, the surface area was first studied by mea-
To further analyze and identify the surface chemical composition
and binding energy of the 30%-BWO/CN composites, XPS analysis was
carried out, as shown in Fig. 5. Fig. 5a shows a typical full scanning
spectrum of the 30%-BWO/CN sample. Fig. 5b–f show the XPS analysis
results of Bi 4f, W 4f, O 1s, C 1s and N 1s. The result in Fig. 5b shows
that the main peaks at 158.5 eV and 163.8 eV correspond to the binding
energies of Bi 4f7/2 and Bi 4f5/2, respectively. Fig. 5c shows that the
2 3 4
surement of N adsorption–desorption isotherms of stripped g-C N ,
Bi
the N
2
WO
6
and 30%-BWO/CN, as shown in Fig. 7a. The results show that
adsorption and desorption curves of the tested materials are
2
similar and that all are classified as type IV [33]. Fig. 7b shows the pore
size distribution curves corresponding to the N adsorption–desorption
isotherms. The results show that the pore sizes of the stripped g-C
Bi WO , and 30%-BWO/CN nanomaterials range from 370 to 470 nm.
The specific surface areas of the bulk g-C , stripped g-C (CN),
Bi WO and 30%-BWO/CN composites are provided in Fig. 7c. The
specific surface areas of the bulk g-C , stripped g-C (CN), Bi WO
2
main peaks at 34.6 eV and 36.8 eV correspond to W 4f7/2 and W 4f5/2
,
3 4
N ,
respectively. Fig. 5d shows the high-resolution XPS spectrum of O 1s,
which can be divided into three peaks at 529.5 eV, 531.8 eV, and
2
6
3
N
4
3 4
N
5
32.9 eV, with the main peak at 529.5 eV corresponding to the lattice
2
6
2
+
oxygen of Bi
2
O
2
, the main peak at 531.8 eV corresponding to the
3
N
4
3
N
4
2
6
,
2−
2
−1
2
−1
2 −1
acidic oxygen of WO
hydroxyl groups on the sample surface or H
4
and the peak at 532.9 eV corresponding to
O adsorbed onto the sur-
and 30%-BWO/CN are 7.05 m g , 67.92 m g , 61.1 m g , and
2
−1
2
70.83 m g , respectively.
face [41]. Fig. 5e shows the high-resolution C 1s XPS spectrum of 30%-
BWO/CN; the peak at 284.6 eV is the binding energy of the C–C bond,
and that at 287.9 eV corresponds to the binding energy of N–C=N in g-
3 4
The result shows that the specific surface area of g-C N obtained by
ultrasonic stripping of bulk g-C N is increased by approximately ten
3 4
times. The increase in specific surface area leads to an increases in the
number of reactive sites and adsorption capacity of the catalysts. The
specific surface area of the Z-scheme 30%-BWO/CN heterogeneous
C
N
3 4
[42]. Fig. 5f shows a high-resolution N 1s XPS spectrum, which
can be divided into three peaks at 398.3 eV, 399.9 eV and 404.0 eV; the
5

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
Fig. 5. XPS spectra: (a) full spectrum of 30%-BWO/CN, (b) Bi 4f, (c) W 4f, (d) O 1s, (e) C 1s, and (f) N 1s.
composite increases slightly relative to that of pure Bi
stripped g-C (CN). Therefore, the specific surface area is not the only
factor responsible for the enhancement of photocatalytic activity of the
0%-BWO/CN composite material, but it is also strongly related to the
Z-scheme heterostructure of the material. The Z-scheme heterostructure
improves the separation rate of photogenerated electrons and holes and
is a key factor for the increase in photocatalytic activity.
2 6
WO and
3 4
N
3
In this paper, the activity testing experiments of all photocatalysts
including all light reactions and corresponding dark reactions) were
(
carried out in a photochemical reaction box. The coupling reaction
between benzyl alcohol and aniline occurred continuously for 10 h, and
GC–MS and GC were used to conduct qualitative analysis experiments
and quantitative analysis of the conversion rate and selectivity of the
reaction. All the experiments were performed twice, and the average
values were calculated. The results obtained from the GC–MS
3.8. Activity test of the catalyst
The coupling reaction between benzyl alcohol and aniline is shown
in the following formula:
6

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
Table 1
Effect of different solvents on the coupling reaction of benzyl alcohol and
aniline.
No. Solvent
Polar index Light irradiation
In dark
Conv. (%) Sel. (%) Conv. (%) Sel. (%)
1
2
3
4
5
6
7
8
Petroleum ether 0.01
34.2
45.1
50.4
47.3
18.4
11.0
10.7
8.1
> 99
> 99
> 99
> 99
92.7
88.4
85.3
86.2
11.9
12.2
12.0
11.6
4.1
0
> 99
> 99
> 99
> 99
90.1
> 0
n-Hexane
Cyclohexane
n-Heptane
Toluene
0.06
0.1
0.2
2.4
4.3
4.8
6.4
Isopropanol
1,4-Dioxane
DMF
0
0
> 0
> 0
show that when cyclohexane was used as the reaction solvent, the
conversion rate of the reaction reached 50.4%, and the selectivity of the
product was greater than 99%. In addition, the data in Table 1 also
show that the conversion rate and selectivity under visible light irra-
diation are much higher than that in the corresponding dark reaction.
Table 2 compares the effects of different bases and their dosages on
the coupling reaction between benzyl alcohol and aniline. The reaction
was carried out under a light source intensity of 2.5 × 10 W/cm .
The reaction substrate included 50 mg 15%-BWO/CN catalyst, 1 mmol
benzyl alcohol, 1 mmol aniline, 6 mL cyclohexane and a certain amount
of base.
The experiment explored the effect of seven different bases and their
dosages on the coupling reaction between benzyl alcohol and aniline.
The data in Table 2 show that the use of a base has a very important
role in this reaction, greatly improving the conversion rate of the re-
action. The data in Table 2 show that the conversion rate of the reaction
Fig. 6. PL spectra of the different catalysts.
experiment are shown in the Supporting Information.
Table 1 compares the effects of eight different polar solvents on the
coupling reaction between benzyl alcohol and aniline. The reaction was
carried out under a light source intensity of 2.5 × 10 W/cm . The
reaction substrate included 50 mg 15%-BWO/CN catalyst, 1.0 mmol
NaOH, 1 mmol benzyl alcohol, 1 mmol aniline, and 6 mL solvent.
As shown in the data in Table 1, with an increase in solvent polarity,
the conversion rate of the coupling reaction between benzyl alcohol and
aniline first increased and then decreased, and when the polarity of the
solvent was more than 4.3, the selectivity dropped below 90%. The data
−
2
2
−2
2
2
Fig. 7. (a) N adsorption–desorption isotherms of the different catalysts, (b) the corresponding pore size distribution of the different catalysts, and (c) specific surface
area of the different catalysts.
7

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
Table 2
Table 4
Effects of different bases and their dosages on the coupling reaction of benzyl
Effect of reactant amount on the coupling reaction of benzyl alcohol and ani-
alcohol and aniline.
line.
No.
Base
Light irradiation
In dark
No. Benzyl alcohol
mmol)
Aniline
(mmol)
Light irradiation
In dark
(
Conv. (%)
Sel. (%)
Conv. (%)
Sel. (%)
Conv. (%) Sel. (%) Conv. (%) Sel. (%)
1
2
3
4
5
6
7
8
9
1.0 mmol KOH
1.0 mmol NaOH
1.0 mmol LiOH
51.5
50.4
25.1
44.2
46.7
30.9
71.8
73.7
67.1
53.2
38.5
4.2
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
10.4
12.0
6.1
8.1
7.2
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 0
1
2
3
4
5
3.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
3.0
83.4
87.6
81.3
72.9
69.3
> 99
> 99
> 99
> 99
> 99
12.3
12.2
11.9
12.0
11.8
> 99
> 99
> 99
> 99
> 99
1.0 mmol K
1.0 mmol Na
1.0 mmol CH
2
CO
3
2
CO
3
3
ONa
6.4
1.4 mmol Cs
1.2 mmol Cs
1.0 mmol Cs
0.8 mmol Cs
0.6 mmol Cs
None
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
12.3
12.1
11.7
11.1
3.6
reactants was 2–4 mmol, the molar ratio of benzyl alcohol and aniline
was 3:1, 2:1, 1:1, 1:2, or 1:3. As shown in Table 4, when the molar ratio
of benzyl alcohol and aniline was 2:1, the conversion rate of the reac-
tion was 87.6%, and the selectivity of the product was greater than
10
11
12
0
99%. This result may have occurred because part of the benzyl alcohol
was first oxidized into the intermediate product benzaldehyde during
the coupling reaction. Then, the aldehyde group in benzaldehyde could
undergo an affinity substitution reaction with the amine group in ani-
line. Therefore, the coupling reaction will require a higher proportion
of benzyl alcohol, but the excess benzyl alcohol will occupy the active
site of the 30%-BWO/CN photocatalyst, which is not conducive to the
progress of the amination reaction. In summary, the best reaction ac-
tivity was observed for a total reactant amount of 3 mmol and benzyl
alcohol to aniline molar ratio of 2:1.
2 3
was the best when Cs CO was used and that the amount of added base
can also affect the conversion rate of the reaction. The results show that
as the amount of base was increased, the conversion rate first increased
and then decreased. When 1.2 mmol Cs
rate reached 73.7%, which may be due to the better solubility of
Cs CO in the organic solution. The addition of excess base will occupy
2 3
CO was added, the conversion
2
3
the active site of the catalyst itself, thus affecting the reaction effect.
Table 3 compares the effects of different catalysts on the coupling
reaction between benzyl alcohol and aniline. The reaction was carried
To study the effect of the 30%-BWO/CN Z-scheme heterostructure
photocatalyst on the coupling reaction of different alcohol derivatives
and different amine derivatives, eight different alcohols and eight dif-
ferent amines were selected as research objects. The reaction was car-
−2
2
out under a light source intensity of 2.5 × 10 W/cm . The reaction
substrate included 50 mg catalysts, 1 mmol benzyl alcohol, 1 mmol
aniline, 6 mL cyclohexane, and 1.2 mmol Cs
The experiments compared the photocatalytic performance of pure
(including bulk g-C and stripped g-C (CN)), Bi WO , and
2 3
CO .
−2
2
ried out under a light source intensity of 2.5 × 10 W/cm . The re-
action substrate included 50 mg 30%-BWO/CN, 6 mL cyclohexane,
g-C
3
N
4
3
N
4
3
N
4
2
6
x-BWO/CN in the coupling reaction of benzyl alcohol and aniline. The
results in Table 3 show that the catalytic performance of CN obtained
after ultrasonic stripping is higher than that of bulk g-C N , which may
3 4
2 3
1.2 mmol Cs CO , 2.0 mmol alcohols, and 1.0 mmol amines. The results
are shown in Table 5.
From the results shown in groups 4 and 5 in Table 5, the conversion
rate of the reaction was found to be ideal when an electron-with-
drawing group was attached to the counterpoint of the alcohol re-
actants; the reaction conversion rate was higher than that obtained
when benzyl alcohol was used as the reactant. Conversely, the con-
version rate was found to be lower when an electron-donating group
was attached to the counterpoint of the alcohol reactants. This may
occur because the presence of an electron-withdrawing group promotes
the shedding of H on the alcoholic hydroxyl group, which is more
conducive to the formation of the intermediate product benzaldehyde.
As seen from the data in groups 2 and 6, the coupling reaction be-
tween the secondary alcohol and aniline is not favorable in this reaction
system. This is because the steric hindrance of the secondary alcohol is
relatively large, and the alcohol is easily oxidized to a ketone, which is
more stable than an aldehyde and not conducive to the amination re-
action. From the data in groups 3, 7, 8, 9, and 16, it can be seen that in
this reaction system, fatty alcohols (or fatty amines) and aromatic al-
cohols (or aromatic amines) with more branched C chains are not
conducive to the progress of the amination reaction. A possible reason
for this outcome is that such alcohols do not easily form intermediate
aldehydes, and that the nucleophilic reaction with amines is not strong,
so the conversion rate of the reactions is very low.
be due to the increase in the specific surface area of CN after stripping
and the increase in the number of reactive sites. The conversion rate of
x-BWO/CN with a Z-scheme heterostructure as a photocatalyst is much
higher than that of pure g-C
photocatalyst was added, the conversion effect was the best, reaching
1.3%. The conversion rate of the coupling reaction between benzyl
3 4 2 6
N and Bi WO . When 30%-BWO/CN
8
alcohol and aniline can be greatly improved by constructing a Z-scheme
heterostructure of x-BWO/CN with an appropriate mass ratio.
Table 4 compares the effects of the reactant amount on the coupling
reaction between benzyl alcohol and aniline. The reaction was carried
−2
2
out under a light source intensity of 2.5 × 10 W/cm . The reaction
substrate included 50 mg 30%-BWO/CN, 6 mL cyclohexane, and
1.2 mmol Cs
2 3
CO , and benzyl alcohol and aniline with different molar
ratios.
The influence of the amount of reactants on the coupling reaction of
benzyl alcohol and aniline was explored. When the total amount of
Table 3
Effect of different catalysts on the coupling reaction of benzyl alcohol and
aniline.
No.
Catalyst
Light irradiation
In dark
From the data in groups 11, 12, and 13, the conversion rate of the
reaction was found to be ideal when an electron-donating group was
attached to the para position of the amine reactant and was found to be
higher than that obtained when aniline was used as the reactant. In
contrast, the conversion rate was found to be lower when the electron-
withdrawing group was attached to the para position of the reactant.
This may occur because the electron-donating group increases the
density of the amido electron cloud of aniline, which is beneficial to the
nucleophilic reaction. According to the data for groups 14 and 15, the
Conv. (%)
Sel. (%)
Conv. (%)
Sel. (%)
1
2
3
4
5
6
7
Bulk g-C
3
N
4
11.3
32.4
38.6
73.7
81.3
70.9
62.6
> 99
> 99
> 99
> 99
> 99
> 99
> 99
2.2
6.9
7.2
12.1
11.9
11.7
11.3
> 99
> 99
> 99
> 99
> 99
> 99
> 99
Stripped g-C
Bi WO
N
3 4
2
6
15%-BWO/CN
30%-BWO/CN
45%-BWO/CN
60%-BWO/CN
8

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
Table 5
The coupling reaction between different alcohols and different amines.
No.
Alcohols
Amines
Light irradiation
Conv. (%)
87.6
In dark
Sel. (%)
> 99
Conv. (%)
12.2
Sel. (%)
> 99
1
2
Aniline
Aniline
10.4
84.2
0
> 0
3
4
Aniline
Aniline
26.7
39.6
89.3
92.8
5.2
8.3
90.6
94.1
5
Aniline
Aniline
93.1
> 99
14.4
> 99
6
7
13.3
30.1
91.5
98.6
0
> 0
Aniline
Aniline
5.4
93.6
8
9
13.8
47.9
94.6
0
> 0
Benzyl alcohol
Benzyl alcohol
> 99
5.7
> 99
1
1
1
0
1
2
52.1
91.0
89.9
> 99
> 99
> 99
9.0
> 99
> 99
> 99
Benzyl alcohol
Benzyl alcohol
12.9
14.8
1
1
3
4
Benzyl alcohol
Benzyl alcohol
78.4
89.7
> 99
> 99
12.4
16.0
> 99
> 99
1
1
5
6
Benzyl alcohol
Benzyl alcohol
3.1
85.2
88.3
0
0
> 0
> 0
16.7
3.9. Effect of light intensities and wavelengths on the coupling reaction of
benzyl alcohol and aniline
Fig. S2 shows the effect of five different light intensities on the
coupling reaction of benzyl alcohol and aniline. As seen from the figure,
the light intensity has a great influence on the conversion rate of the
reaction. When the light intensity increases, the conversion rate gra-
dually increases. When the light intensity reaches approximately
−
2
−2
2
.5 × 10 W cm , the conversion rate basically reaches an optimal
value. To protect the life of the xenon lamp and save energy, the final
−2
−2
light intensity was set to be 2.5 × 10 W cm
.
Fig. S3 shows the effect of six different wavelengths on the coupling
reaction of benzyl alcohol and aniline. The results show that in the
visible light range, the conversion of the reaction decreases as the
wavelength range narrows. When the wavelength range narrows to
510–800 nm, the conversion rate decreases to approximately half of the
optimal value, which may be related to the absorption range of the
composite photocatalyst in UV–vis DRS. Between 400 and 500 nm, the
Fig. 8. Recyclability tests of the coupling reaction of benzyl alcohol and aniline.
3
0%-BWO/CN composite shows an obvious visible light absorption,
conversion rate of the secondary aromatic amine as reactant was higher
in this reaction system, while the conversion rate was very poor when a
secondary fatty amine was used as the reactant. In addition, the data for
group 10 indicate that the conversion rate of aniline with intersite
substituents is not ideal, which may be due to its high steric hindrance.
which promotes the photocatalytic reaction.
3.10. Cycle capability test of the catalyst
To explore the stability of the 30%-BWO/CN composite, five cycle
determinations were carried out. The results are shown in Fig. 8. The
9

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
Fig. 10. Scheme of the reduced activation energy due to light irradiation.
Fig. 9. Capturing experiment of the active species of the catalyst.
2 6
imine. Since the CN nanosheets are embedded into the Bi WO mi-
crospheres (Fig. 2e), most of the heterojunction interfaces of 30%-
BWO/CN are shown in Scheme 1. As observed, photogenerated elec-
conversion rate of the fifth cycle was reduced by 8.4% compared with
that of the first experiment, in which the conversion rate of the first
three cycles shows a slight changes, and the conversion rate of the fifth
cycle shows a decreasing trend. The data show that the prepared 30%-
BWO/CN Z-scheme composite has good photostability and thermal
stability.
trons are injected from the CB of g-C , and the
3
N
4
to the CB of Bi
2
WO
6
holes in the VB of Bi WO are injected into the VB of g-C N . In sum-
2
6
3 4
mary, this mechanism is consistent with the results obtained from the
trapping experiment (Fig. 9).
3.12. Analysis of the activation energy of the reaction
3.11. Effect of reactive groups on the mechanism
B
Fig. S4a and b show the relationship for ln(1 − X ) at different
temperatures and reaction times t under light and dark conditions.
According to the Arrhenius equation, the activation energies of the
photoluminescence (Eap) and thermally driven (Eat) reactions of benzyl
alcohol and aniline under visible light and dark conditions were cal-
To explore the active species in the reaction, a capturing experiment
of the active species of the 30%-BWO/CN photocatalyst was carried
out: 2 mmol benzyl alcohol and 1 mmol aniline were used as the re-
actant, and 5 mmol carbon tetrachloride (CCl
4
), ethylenediamine tet-
−1
−1
culated to be 30.7 kJ mol
and 60.5 kJ mol , respectively. These
raacetic acid (EDTA), tert-butanol (t-BuOH), and 1,4-benzoquinone
−
+
−
results show that the 30%-BWO/CN catalyst under light irradiation
(
2
BQ) were added as quenchers for e , h , •OH, and •O , respectively.
−1
conditions can decrease the activation energy by 29.8 kJ mol
(ΔE = Eat − Eap, Fig. 10)[47].
As shown in Fig. 9, the results indicate that the effect of reactive groups
+
−
a
on the reaction was mainly controlled by the synergy between h , e
−
and •O
2
. The proposed mechanism of the Z-scheme heterostructure
3
0%-BWO/CN in the photocatalytic synthesis of imines is shown in
Scheme 1.
Electrons are transferred from the CB of g-C
4. Conclusions
3
N
4
to O
2
, forming su-
A 30%-BWO/CN photocatalyst with a Z-scheme heterostructure was
successfully constructed. The performance of this composite was ef-
fectively enhanced in the photocatalytic generation of imines and
showed good stability. The construction of a Z-scheme heterostructure
improved the separation efficiency of electron and hole pairs. At the
same time, ultrasonic stripping and mechanical stirring of the compo-
site were carried out at room temperature, which did not change the
−
peroxide radicals (•O
deprived hydrogen by the •O
benzaldehyde) and H are produced [44,45]. The other route in-
volves benzyl alcohol producing benzaldehyde and water directly by
[46]. Finally, imine is rapidly produced by aniline and benzal-
2
), while benzyl alcohol is oxidized by holes and
−
2
radical. Then, the vital intermediate
(
2 2
O
2 2
H O
−
+
−
dehyde. Therefore, •O
2
, h and e participates in the synthesis of the
Scheme 1. Possible mechanism of the synthesis of imine over 30%-BWO/CN.
10

X. Yu, et al.
Molecular Catalysis 494 (2020) 111122
crystal type and structure of the semiconductor but did lead to the
formation of a very uniform composite material, with the composite
photocatalyst found to still maintain a flower-like microspheres struc-
ture. In addition, the ultrasonic stripping of bulk g-C
proved the specific surface area of g-C prepared by the traditional
method and played an active role in the construction of composite
materials as excellent photocatalysts. This work provides a reference for
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and 2020MS02012), the 2013 Annual Grassland Talents Project of
Inner Mongolia Autonomous Region, the 2013 Annual Inner Mongolia
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Appendix A. Supplementary data
[
[
[
[
[
[
[
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Supplementary data associated with this article can be found, in the
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