Synthesis of four-angle star-like CoAl-MMO/BiVO4 p-n heterojunction and its application in photocatalytic desulfurization

Article abstract of DOI:10.1039/c7ra03012f

A four-angle star-like Co-Al mixed metal oxide (CoAl-MMO)/BiVO₄ heterojunction has been synthesized via a hydrothermal method and following sintering. The CoAl-MMO/BiVO₄ is derived from CoAl-LDHs/BiVO₄, in which CoAl-LDHs leads to a distribution of amorphous CoAl-MMO. The CoAl-MMO loading on BiVO₄ greatly enhances visible light absorption, improves charge separation by band offset charge transfer, and makes flat band potential more negative. The three effects together result in excellent photocatalytic activity. Under visible light irradiation, desulfurization efficiency of thiophene has achieved up to 98.58% on CoAl-MMO/BiVO₄ with molar ratio of 0.3:5.

Full text of DOI:10.1039/c7ra03012f

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Synthesis of four-angle star-like CoAl-MMO/BiVO4
p–n heterojunction and its application in
photocatalytic desulfurization
Cite this: RSC Adv., 2017, 7, 25455
a
a
Limin Yun,a Zhanxu Yang,
Zong-Bao Yu,
Tianfeng Cai,a Yue Li,b
*
Changyou Guo,c Chengyuan Qia and Tieqiang Rena
A four-angle star-like Co–Al mixed metal oxide (CoAl-MMO)/BiVO4 heterojunction has been synthesized
via a hydrothermal method and following sintering. The CoAl-MMO/BiVO4 is derived from CoAl-LDHs/
BiVO4, in which CoAl-LDHs leads to a distribution of amorphous CoAl-MMO. The CoAl-MMO loading on
BiVO4 greatly enhances visible light absorption, improves charge separation by band offset charge
transfer, and makes flat band potential more negative. The three effects together result in excellent
photocatalytic activity. Under visible light irradiation, desulfurization efficiency of thiophene has achieved
up to 98.58% on CoAl-MMO/BiVO4 with molar ratio of 0.3 : 5.
Received 13th March 2017
Accepted 25th April 2017
DOI: 10.1039/c7ra03012f
rsc.li/rsc-advances
band offset. Besides, one widely common strategy is to combine
BiVO4 with other noble metal or noble metal oxides,15,16 such as
Pt, Ag, RuO2. For example, Lin et al.17 reported a visible-light
Introduction
Organic sulfur from petroleum is a major contributor to envi-
ronmental pollution. Nowadays, the most widely employed
method to remove thiophenic compounds in fuel are hydro-
desulfurization (HDS)1 and adsorption desulfurization.2 Both of
the processes need hydrogen, relatively high-pressure condi-
tions and high energy consumption for deep desulfurization.
Therefore, the development of desulfurization process with low
energy consumption, mild operating conditions and environ-
mental friendliness has become the research focus.3
Photocatalytic desulfurization technology has attracted
much attention, because it can provide a cleaner and more
environmental friendly way to realize desulfurization. TiO2-
based photocatalyst has been widely studied and used in pho-
tocatalytic oxidative desulfurization.4 But since its band gap is
3.2 eV, it can only absorb UV light, which limits its application.5
Therefore, the development of photocatalysts with visible light
response has become a research hotspot. Bismuth vanadate
(BiVO4, n-type semiconductor), with a band gap of 2.4 eV, has
attracted much attention because it shows activation under
visible light irradiation.6–10 However, the photocatalytic activity
of pure BiVO4 is always low because of the rapid recombination
of carriers (electrons and holes). To overcome these drawbacks,
strategy of heterogeneous structure construction has been
developed,11–14 to spatially separate photogenerated carriers by
responsive photocatalyst BiVO4 co-loaded with Pt and RuO2
co-catalysts, which photocatalytically oxidized thiophene to SO3
and achieved over 99% of thiophene conversion. Gao et al.18
synthesized Ag–BiVO4 photocatalysts via hydrothemal method
and photocatalytic desulfurization efficiency under visible light
irradiation at pH ¼ 7 could be up to 95%. Although the desul-
furization efficiency of the above photocatalyst is high, the cost
of cocatalysts using noble metal or noble metal oxides is
expensive. It is necessary to develop low-cost cocatalysts to
combine with BiVO4 to achieve efficient desulfurization.
Herein we report the photocatalytic oxidation of thiophene by
Co–Al mixed metal oxide (CoAl-MMO) loaded BiVO4. However,
investigation indicates that CoAl-MMO does not act as cocatalyst
of BiVO4, but gets three unprecedented effects. The CoAl-MMO
loading enhances visible light absorption, improves charge
separation by band offset charge transfer, and makes at band
potential more negative. Combination of these effects largely
enhances photocatalytic efficiency of thiophene oxidation. The
CoAl-MMO derives from a Co–Al-layered double hydroxide (CoAl-
LDH) precursor.19,20 Since LDH has a uniform distribution of
metal cations on the atomic level, sintering leads to CoAl-MMO
with a uniform distribution of cobalt and aluminum.21 It notes
that band gap of CoAl2O4 (ref. 22) is narrow while that of Al2O3 is
wide,23 but neither has been found from our sample by X-ray
diffraction (XRD). Besides XRD, samples were characterized by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM), ultraviolet-visible diffusive reectance spec-
troscopy (UV-vis DRS) and photoluminescence (PL) spectra. The
desulfurization activity has been explored under visible-light.
aCollege of Chemistry, Chemical Engineering and Environment Engineering, Liaoning
Shihua University, Fushun, Liaoning 113001, P. R. China. E-mail: zhanxuy@126.com
bSchool of Foreign Languages, Liaoning Shihua University, Fushun, Liaoning 113001,
P. R. China
cSINOPEC Fushun Research Institute of Petroleum and Petrochemicals, Fushun
113001, China
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atmospheric pressure and room temperature, using air and
H2O2 (30%) as oxidants. The model oil, with sulfur content of
200 ppm is prepared by dissolving thiophene into n-octane.
50 mg photocatalyst and 50 mL model oil were added to the
reactor. The suspension was stirred in dark for 30 min to obtain
an adsorption–desorption equilibrium between the CoAl/BiVO4
photocatalyst and model oil. Then, 0.128 mL H2O2 was added
and the suspension was irradiated by a 500 W Xe arc lamp (CHF-
XM35-500W), the airow velocity was of 5 mL minꢀ1. Desul-
furized oil was collected periodically and extracted with aceto-
nitrile. Sulfur content was determined by a TSN-5000 series
uorescence nitrogen/sulfur analyzer (Jiangfen Electroanalyt-
ical Instrument Co., Ltd., China).
Experiment
Chemicals
All the reagents were analytical grade and used without any
further purication.
Preparation of BiVO4 and CoAl-MMO/BiVO4 samples
In a typical synthesis, under stirring conditions, 0.17 mmol
P123 (analysis) and 5 mmol Bi(NO3)3$5H2O were dissolved in
a mixed solution of 5 mL HNO3 solution (3 mol Lꢀ1) and 20 mL
ethylene glycol solution to obtain solution A. 5 mmol NH4VO3
was dissolved in 20 mL deionized water with 70 ꢁC to obtain
solution B. Solution A and B at room temperature were stirred
for 30 min each, then solution A was added to B dropwise. Then
pH value was adjusted with NH3 H2O (14 wt%) and was stirred
for 60 min to obtain BiVO4 precursor. A certain amount of
Results and discussion
Co(NO3)2$6H2O and Al(NO3)3$9H2O was dissolved in deionized Fig. 1 shows XRD patterns of BiVO4 and CoAl-MMO/BiVO4
water (Co2+/Al3+ molar ratio ¼ 3 : 1), then a solution containing samples. The diffraction peaks at 2q of 18.6ꢁ, 28.9ꢁ, 30.5ꢁ, 34.4ꢁ,
1.0 mol Lꢀ1 NaOH and 0.30 mol Lꢀ1 Na2CO3 was added into it. 35.3ꢁ, 39.4ꢁ, 42.3ꢁ, 46.1ꢁ, 46.6ꢁ, 47.3ꢁ, 53.3ꢁ, 58.3ꢁ, and 59.9ꢁ can
Stirring for 60 min, Co–Al layered double hydroxide (CoAl- be observed, which index to monoclinic BiVO4 (JCPDS no. 14-
LDHs) precursor had been obtained. CoAl-LDHs precursor 0688) and correspond to the crystalline planes of (101), (013),
was added to BiVO4 precursor, pH value was adjusted to 10 with (112), (200), (020), (211), (105), (123), (204), (024), (301), (303)
NaOH (1.0 mol Lꢀ1) solution and stirred for 60 another min. and (224).24 It is clearly observed from the graph that as CoAl-
The mixed solution was transferred to a 100 mL Teon lined MMO loading ratio increases, the intensity of CoAl-MMO/
stainless steel autoclave and hydrothermal treatment was con- BiVO4 samples becomes weaker; and no other peaks can be
ꢁ
ducted at 180 C for 12 h. Aer that, the resulting CoAl-LDHs/ found in XRD patterns of CoAl-MMO/BiVO4 catalysts, which
BiVO4 catalysts were washed with deionized water and absolute may due to the amorphous characteristics of CoAl-MMO.
ethanol, dried at 80 ꢁC for 3 h. Molar ratio of CoAl-LDHs/BiVO4
The morphology of samples has been characterized by SEM
for 0 : 5 (BiVO4), 0.1 : 5, 0.3 : 5, 0.5 : 5 were controlled. Final and TEM (Fig. 2). The morphology of BiVO4 shows four angles
products were obtꢀai1ned by sintering in a muffle furnace at with star-like and the particle size is uniform. Aer loaded with
a ramp of 1 ꢁC min from 30 ꢁC to 400 ꢁC and maintaining for CoAl-MMO, the morphology becomes more regular. Low molar
4 h. The nal products were noted as CoAl-MMO/BiVO4.
loading ratio makes the surface of CoAl-MMO/BiVO4 particles
look smooth, as molar loading ratio reaches 0.3 : 5, the surface
become rough. And under magnied TEM many small particles
on the surface can be observed (Fig. 2(d)). As red dashed line
and red arrows point, CoAl-MMO nanoparticles are amorphous
and dispersed on BiVO4 surface. As shown in inset graph of
Fig. 2(b) and (c), the thickness of CoAl-MMO/BiVO4 with molar
ratio 0.1 : 5 and 0.3 : 5 are about 750 nm and 1 mm, respectively,
meaning that with the loading ratio increasing, more CoAl-
Characterizations
X-ray powder diffraction (XRD) measurements were performed on
a Bruker D8 Advance diffractometer operated at 40 kV and 40 mA
at the scanning range from 2 to 70 degree with Cu-Ka radiation (l
¼ 0.15406 nm). The particle morphologies of the products were
observed by scanning electron microscopy (SEM) (Hitachi SU 8010)
and by transmission electron microscopy (TEM) (JEM 2200FS). A
Cary 5000 UV-vis spectrometer (Agilent Technologies) was used to
obtain the reectance spectra of the samples over a range of 400–
800 nm. Electrochemical analysis was carried out with a standard
three-electrode system with a Pt plate as the counter electrode, Hg/
Hg2Cl2 (saturated with KCl) as a reference electrode, and ITO glass
coated with the sample was used as the working electrode. A 0.5 M
Na2SO4 solution was used as electrolyte. A 500 W Xe arc lamp
(CHF-XM35-500W) was utilized as light source. Transient short-
circuit photocurrent measurements and Mott–Schottky experi-
ments with amplitude of 50 mV and a frequency of 1000 Hz were
taken on a CHI660E workstation.
Photocatalytic oxidative desulfurization tests
The photocatalytic oxidative desulfurization tests were carried
Fig. 1 XRD patterns of samples (a) BiVO4 and CoAl-MMO/BiVO4 with
out in a quartz tube reactor with a water condenser at molar ratio (b) 0.1 : 5, (c) 0.3 : 5, (d) 0.5 : 5.
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Fig. 2 SEM images of samples (a) BiVO4 and CoAl-MMO/BiVO4 with
molar ratio (b) 0.1 : 5, (c) 0.3 : 5 and (d) TEM image of that with ratio of
0.3 : 5.
MMO formed on BiVO4 surface and the particles become
thicker. The particle size rises also, which may be due to CoAl-
LDH playing a role of template.
Fig. 3(A) shows UV-vis diffuse reectance spectra of BiVO4
and CoAl-MMO/BiVO4. BiVO4 shows an absorption region
between 200–530 nm, which covers both UV and partial visible
light region. Interestingly, aer loaded with CoAl-MMO the
absorption region extends to 700 nm, which is throughout the
UV to visible light region. For crystalline semiconductor, the
optical absorption near the band edge follows the formula:25
ahn ¼ A(hn ꢀ Eg)n/2
a, n, Eg and A refer to coefficient, light frequency, band gap
and a constant (A ¼ 1), respectively. n depends on the charac-
teristics of the transition in a semiconductor, for direct transi-
tion, n ¼ 1, for indirect transition n ¼ 4. For BiVO4, n ¼ 1.26 The
plots of (ahn)2 versus photon energy (hn) is shown in the inset
graph of Fig. 3(A), and the band gap of BiVO4 and CoAl-MMO/
BiVO4 can be obtained by extroplating the curve to a ¼ 0. As
a result, the band gaps CoAl-MMO/BiVO4 with loading ratio of
0 : 5, 0.1 : 5, 0.3 : 5 and 0.5 : 5 obtained are 2.4 eV, 2.35 eV,
2.06 eV and 2.08 eV, respectively. Compared to BiVO4, CoAl-
MMO/BiVO4 samples exhibit stronger absorption in the visible
light range and narrower band gap. BiVO4 is a n-type semi-
conductor27 while CoAl2O4 p-type,28 (although not detected by
XRD but may exist as amorphous), a p–n heterojunction should
have been formed at the interface of CoAl2O4 and BiVO4 parti-
cles, which extends the absorption range and makes the band
gap narrower. For sample with molar loading ratio of 0.3 : 5,
these small particles on the surface leads to expanding of
contact area and stable heterojunction structure. However, for
sample 0.5 : 5, the absorption capacity become lower and the
Fig. 3 (A) UV-vis diffuse reflectance spectra of BiVO4 and CoAl-MMO/
BiVO4 samples, the inset shows plots of (ahn)2 vs. photon energy (hn),
(B) Photoluminescence (PL) spectra of BiVO4 and CoAl-MMO/BiVO4
samples, (C) transient photocurrent response for BiVO4 and CoAl-
MMO/BiVO4 samples under an applied potential of 0.5 V vs. SCE with
several on–off cycles of intermittent visible-light irradiation.
band gap slightly increases. It may be due to that some growing
amount of CoAl-MMO particles are not well-loaded on the
surface of BiVO4, but just scatter around, which makes the
heterojunction structure unstable and restrain the interface
interaction between CoAl-MMO and BiVO4.
Photoluminescence (PL) spectra of semiconductor materials
derive from recombination of photo-induced charge carriers.
Higher PL intensity means higher recombination rate of
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carriers (electrons and holes) and photocatalytic activity
becomes correspondingly lower.29–31 Fig. 3(B) shows PL spectra
of BiVO4 and CoAl-MMO/BiVO4 samples with excitation wave-
length of 320 nm. The PL emission wavelength of all samples is
centered at 423 nm, different from 590 nm reported by R. Tang
et al. on BiVO4 nanosheets and its complex with graphene.8
BiVO4 alone has the highest PL intensity, and the PL intensity
decreases by different extent with CoAl-MMO loading amount,
the least one obtained on sample ratio of 0.3 : 5. It's caused by
the electron transfer from CoAl2O4 conduction band (CB) to
BiVO4 CB, and meanwhile holes transfer from BiVO4 valence
band (VB) to CoAl2O4 VB under the potential of band energy
difference, namely, band offset.28,32 The above migration of
photogenerated carriers makes electrons and holes spatial
separation, and tremendous reduces their recombination
probability. Accordingly, photogenerated carriers have longer
life time to take part in photocatalytic reactions, and the pho-
tocatalytic activity of CoAl-MMO/BiVO4 would be improved
upon BiVO4. As a photocatalytic activity test, transient photo-
current responses of the composite electrodes with on–off
cycles of intermittent visible-light irradiation are studied, as
shown in Fig. 3(C). The anodic photocurrent in Na2SO4 solution
represents photocatalytic water oxidation efficiency of gener-
ated holes on surface of sample electrode. Among the catalysts,
BiVO4 shows the lowest photocurrent, meaning that the least
amount of holes participates in water oxidation, caused by the
worst separation efficiency between carriers.33 With the molar
ratio being 0.3 : 5, CoAl-MMO/BiVO4 generates the highest
photocurrent density.
Fig. 4 (A) Mott–Schottky (MS) plots of BiVO4 and CoAl-MMO/BiVO4.
(B) Photocatalytic desulfurization of thiophene on BiVO4 and BiVO4
loaded with various ratios of CoAl-MMO (conditions: initial thiophene
content: 200 ppm; airflow velocity: 5 mL minꢀ1; H2O2: 0.128 mL; the
concentration of photocatalyst: 1 g Lꢀ1; reaction time: 6 h).
To investigate the electronic effect, Mott–Schottky (MS)
measurement34,35 has been performed on BiVO4 and CoAl-
MMO/BiVO4 series, as shown in Fig. 4(A). For CoAl-MMO/BiVO4
with molar loading ratio of 0 : 5, 0.1 : 5, 0.3 : 5 and 0.5 : 5, the
V are ꢀ0.54 V, ꢀ0.61 V, ꢀ0.72 V and ꢀ0.64 V vs. SCE (equiv-
alent to ꢀ0.06 V, ꢀ0.13 V, ꢀ0.24 V and ꢀ0.16 V vs. NHE at PH ¼
0), respectively. It is well known that CB potential (ECB) of a n-
type semiconductor is 0–0.2 V more negative than V and is
dependent on carrier concentration and electron effective mass.
It can be seen that ECB of the samples are more negative than
CoAl-MMO loading amount, and 0.3 : 5 is the best in each
aspect. Thus, based on the three merits, highly efficient thio-
phene desulfurization has been realized on CoAl-MMO/BiVO4.
Schematic description of the mechanism for thiophene desul-
furization on CoAl-MMO/BiVO4 has been shown in Scheme 1.
Along with CoAl-MMO loading amount modifying, band gap
ꢀ
the standard redox potential of O2/cO2 (0.28 V vs. NHE). It
indicates the photogenerated electrons could react readily with
ꢀ
ꢀ
adsorbed O2 to produce cO2 (ref. 36), and cO2 is active free
radical in desulfurization reactions. Photocatalytic desulfur-
ization activity has been investigated via thiophene decompo-
sition, the result shown in Fig. 4(B). The most negative V of
ꢀ0.24 V corresponds to the highest desulfurization efficiency, is
obtained with CoAl-MMO/BiVO4 ratio of 0.3 : 5.
In summary, there are three signicant merits of CoAl-MMO
loading on BiVO4 via sintering mixture of CoAl-LDH and BiVO4.
Firstly, it exhibits red shi of the whole band edge upon CoAl-
MMO loading. That widens and enhances visible light absorp-
tion and utilization. Secondly, nanosized n–p heterojunction
has been formed between CoAl-MMO particles and BiVO4. It
endows carriers' spatial separation, which has been proved by
PL intensity decrease. Thirdly, more negative V has been ob-
tained (dashed lines in Scheme 1), which benets formation of
Scheme 1 Schematic description of the mechanism for the photo-
cO2ꢀ. Interestingly, the above three effects changes along with catalytic oxidation of thiophene on CoAl-MMO/BiVO4.
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energy of CoAl-MMO/BiVO4 becomes smaller and more visible
light has been absorbed to excite carriers. 3 colored lines above
EVB in Scheme 1 indicate that EVB moves upward, corresponding
to band gap shortening, caused by changing loading amount of
CoAl-MMO. Due to the band offset of heterojunction, photo-
generated holes and electrons transfer to CoAl-MMO and BiVO4
respectively (black curved arrows in Scheme 1). Electron on
BiVO4 reacts with absorbed O2 to cO2ꢀ. On the other hand, hole
on CoAl-MMO reacts with OHꢀ forming cOH, meanwhile some
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Conclusions
Four-angle star-like CoAl-MMO/BiVO4 photocatalysts has been
synthesized by a hydrothermal method and later sintering.
CoAl-MMO loading on BiVO4, dispersed as amorphous particles
on its surface, brings advantages to photocatalytic performance
from three aspects, such as visible light enhancement, hetero-
junctions for carrier spatial separation, and making V more
negative as well. Therefore, photocatalytic desulfurization effi-
ciency by CoAl-MMO/BiVO4 has been improved largely,
compared to BiVO4 under visible light irradiation, from under
75% to over 97%. Aer optimizing the loading amount, 98.58%
conversion of thiophene has been achieved with molar loading
ratio of 0.3 : 5. This work has demonstrated that CoAl-MMO is
not only cost-effective, but also plays a signicant role in
enhancing the photocatalytic activity of BiVO4. It points that
low-cost and effective mixed metal oxide loading on other
photocatalysts may be a promising choice in photocatalytic
desulfurization.
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Acknowledgements
This study was supported by the National Natural Science
Foundation of China (21401093), Program for Liaoning
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