Synthesis of Bi2MoO6/Bi2Ti2O7 Z-scheme heterojunction as efficient visible-light photocatalyst for the glycolic acid degradation

Article abstract of DOI:10.1166/jnn.2019.16779

A novel Bi₂MoO₆/Bi₂Ti₂O₇ heterojunction photocatalyst was ?rstly prepared by a simple solvothermal route. The as-synthesized photocatalysts were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultravioletvisible diffuse reflectance spectroscopy and photoluminescence spectra. The effect of the material ratio, solution pH, and the salinity on glycolic acid degradation were discussed. The Bi₂MoO₆/Bi₂Ti₂O₇ (2:1) nanocomposite exhibits highest photocatalytic performance and chemical stability. The glycolic acid degradation efficiency can reach 99.9% at pH 4 after 150 min irradiation under visible light. The enhanced photocatalytic activity can be ascribed to the heterojunction interface in the composite, which is beneficial to the efficient charge carriers transfer. The degradation conforms to the pseudo-first-orderkinetics. According to the oxidationspecies trappingexperiments, the O^- ₂, h⁺ and OH are found to play important roles in the photocatalytic degradation and O^- ₂ is the main active species. The possible photocatalytic mechanism was further discussed on account of valence band, conduction band positions and possible transition states.

Full text of DOI:10.1166/jnn.2019.16779

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Journal of
Nanoscience and Nanotechnology
Vol. 19, 7635–7644, 2019
Copyright © 2019 American Scientific Publishers
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Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction
as Efficient Visible-Light Photocatalyst for the Glycolic
Acid Degradation
Qianyun Rong1ꢀ†, Dapeng Zhang1ꢀ†, Ying Li1, Zhenxing Zha1, Xinxiang Geng1,
Shihai Cui1ꢀ2ꢀ∗, and Jing Yang1ꢀ∗
1Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biomedical Materials,
School of Chemistry and Materials Science, Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control,
Nanjing Normal University, Nanjing 210023, P. R. China
2Nanjing Lvshiyuan Environmental Protection Technology Co., LTD., Nanjing 210033, P. R. China
A novel Bi2MoO6/Bi2Ti2O7 heterojunction photocatalyst was firstly prepared by a simple solvother-
mal route. The as-synthesized photocatalysts were characterized by scanning electron microscopy,
transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet-
visible diffuse reflectance spectroscopy and photoluminescence spectra. The effect of the mate-
rial ratio, solution pH, and the salinity on glycolic acid degradation were discussed. The
Bi2MoO6/Bi2Ti2O7 (2:1) nanocomposite exhibits highest photocatalytic performance and chemical
IP: 5.62.159.44 On: Sun, 07 Jul 2019 02:09:21
stability. The glycolic acid degradation efficiency can reach 99.9% at pH 4 after 150 min irradiation
Copyright: American Scientific Publishers
under visible light. The enhanced photocatalytic activity can be ascribed to the heterojunction inter-
Delivered by Ingenta
face in the composite, which is beneficial to the efficient charge carriers transfer. The degradation
conforms to the pseudo-first-order kinetics. According to the oxidation species trapping experiments,
ꢀ
ꢀ
ꢀ
the O−2 , h+ and OH are found to play important roles in the photocatalytic degradation and O−2 is
the main active species. The possible photocatalytic mechanism was further discussed on account
of valence band, conduction band positions and possible transition states.
Keywords: Bi2MoO6/Bi2Ti2O7, Glycolic Acid, Heterojunction, Photocatalytic Degradation,
Mechanism.
1. INTRODUCTION
as adsorption,2ꢀ6 extraction7ꢀ8 and oxidation9ꢀ10 have been
reported before.
As one of the most important chemical intermedi-
ates, glycolic acid (GA) is widely applied to adhesives,
metal cleaning, polymers, dyeing, textiles and household
chemical products.1–3 It is reported that carboxylic acid
molecules take up the vast majority of organic compounds
in aerosols in the atmosphere. Approximately 3.6% of the
organic content of aerosols is made up of GA in a pol-
luted troposphere.4 As far as we know, wastewater from
the production of pesticide 2,4-dichlorphenoxyacetic acid
(2,4-D) often contains a large amount of by-product GA.5
High concentration of GA will pollute the environment.
Therefore, it is essential to develop a cost-effective and
safe technology for GA treatment. Some methods such
As an oxidation technology, photocatalysis is sup-
posed to be a very promising and effective method
for the photodegradation of organic pollutants. It has
many merits including strong oxidability, no secondary
pollution, wide applicability and simple operation.11–13
Among the various photocatalysts reported in litera-
ture, bismuth titanate has been considered to have
excellent photocatalytic activity and stability.14ꢀ15 In the
Bi–Ti–O system, there are lots of crystal structures includ-
ing sillenite (Bi12TiO20ꢁ, pyrochlore (Bi2Ti2O7ꢁ, mono-
clinic (Bi2Ti4O11ꢁ and Aurivillius-type layered perovskite
(Bi4Ti3O12ꢁ.16ꢀ17 Among these, Bi2Ti2O7 possesses a nar-
rower band gap (2.8–2.9 eV) than TiO2 (3.0–3.2 eV).18ꢀ19
However, fast recombination of photo-induced charge car-
riers restricts the energy-conversion efficiency of Bi2Ti2O7.
∗Authors to whom correspondence should be addressed.
†These two authors contributed equally to this work.
J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 12
1533-4880/2019/19/7635/010
doi:10.1166/jnn.2019.16779
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Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction as Efficient Visible-Light Photocatalyst
Rong et al.
To promote the separation of photoexcited electron–hole
pairs, a good method is to couple it with other materi-
als, such as TiO2,20 reduced graphene oxide,21 Bi4Ti3O12,22
transition metal ions Fe(III) and Mn(II).23ꢀ24
(EDS, Horiba, Japan) was employed with an EX250 spec-
troscope. X-ray powder diffraction (XRD, Rigaku, Japan)
were performed on a D/max-RC diffractometer with a
Cu Kꢂ radiation (ꢃ = 0ꢄ15406 nm). The surface compo-
sitions and chemical states of materials were identified
by X-ray photoelectron spectroscopy (XPS, ULVAC-
PHI, Japan) with a PHI 5000 Versa Probe system. The
ultraviolet-visible diffuse reflectance spectroscopy (UV-
Vis DRS, Varian, USA) was obtained by a Cary 5000
spectrophotometer. Photoluminescence (PL, Perkin-Elmer,
USA) spectra was conducted on an excitation wavelength
at 395 nm equipped with a Horiba Scientific FluoroMax-4
spectrofluorometer. Mott-Schottky plots were measured
by CHI-660E (chenhua, Shanghai, China) electrochemi-
cal workstation at the potential range of −0.5 V to 0.5 V
and the frequency of 1 KHz. The concentration of GA
was analyzed by a Waters 1525 high performance liquid
chromatography (HPLC, Waters, USA) with UV-vis 2489
detector.
Bi-based semiconductors, such as Bi2O3, Bi2MoO6,
Bi2WO6 and BiOCl, have attracted extensive attention due
to their unique structure and significant photocatalytic per-
formance. Wang et al. successfully synthesized novel plas-
monic Bi nanoparticles and BiOCl sheets co-decorated
ZnSn(OH)6 with various morphologies, which provided
original insights into Bi-based materials for effective
utilization of visible-light and wastewater treatment.25
Some modified Bi-based semiconductors such as mag-
netic Bi25FeO40/rGO catalyst and Ag/AgVO3/BiOCl com-
posite also exhibited superior photocatalytic activity.26ꢀ27
In especial, the Bi2MoO6 with the Aurivillius structure
contains a narrow energy bandgap (2.5–2.8 eV), which
are able to promote electron conductivity and improve
the photocatalytic activity.28–30 Recently, Bi2MoO6 based
heterojunction with Bi2WO6,31 TiO2,32 RGO,33 g-C3N4,34
MO (M = Cu, Co3/4 or Ni),35 Ag2O,36 Ag2MoO4,37 AgX
(X = Cl, Br or I)38–40 and so on have been reported before.
To the best of our knowledge, the photocatalytic perfor-
mance of Bi2MoO6/Bi2Ti2O7 nanocomposite has not been
investigated till date.
2.3. Synthesis of the Bi2MoO6/Bi2Ti2O7
Nanocomposite
2.3.1. Synthesis of Bi2Ti2O7
The Bi2Ti2O7 was prepared via a solvothermal method
with Bi(NO3ꢁ3 · 5H2Oꢀ and Ti(OC4H9ꢁ4 as precursors.41
In brief, 18 mL of ethanol and 7 mL of glycerol were
stirred until a homogeneous solution was obtained. The
In this work, a series of Bi2MoO6/Bi2Ti2O7 nanocom-
posites with different molar ratios have been prepared
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successfully through a facile solvothermal route. The
0.8537 g Bi(NO ꢁ3 · 5H2O was dissolved in the ethanol-
glycerol solution. Then 1.5 mL of Ti(OC4H9ꢁ4 and 5 mL
of diethyl ether were added to the solution above under
stirring, respectively. The mixture was transferred into a
Copyright: American Scientific Publish3ers
photocatalytic activity was estimated by GA degrada-
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tion under visible light irradiation. The results show that
Bi2MoO6/Bi2Ti2O7 heterojunction photocatalyst exhibits
superior photocatalytic performance. Furthermore, the pos-
sible mechanism for photodegradation process of the
Bi2MoO6/Bi2Ti2O7 nanocomposite has been investigated
in detail.
ꢁ
teflon-lined autoclave and heated at 110 C for 10 h. The
products were collected by centrifugation, washed with
ethanol and water alternatively, and dried at 70 ꢁC. Finally,
ꢁ
the powders obtained were calcined at 500 C for 3 h.
2. MATERIALS AND METHODS
2.3.2. Synthesis of Bi2MoO6/Bi2Ti2O7
2.1. Reagents
In a typical procedure, 0.64 mmol Bi(NO3ꢁ3 · 5H2O and
0.32 mmol Na2MoO4 · 2H2O were dissolved in 5 mL of
ethylene glycol under stirring, respectively. Then 20 mL
ethanol was added into the solution above, followed by
stirring for 15 min. The resulting transparent solution and
Bi2Ti2O7 (0.1 g)ꢁwere shifted into a teflon-lined autoclave
and heated 160 C for 24 h. Theꢁobtained materials were
filtered, washed and dried at 90 C for 2 h. For compar-
ison, Bi2MoO6/Bi2Ti2O7 composites with different mass
ratio (1:1, 2:1, 4:1) were synthesized and marked MT1,
MT2 and MT4, respectively. The Bi2MoO6 was prepared
by using the same experimental conditions in the absence
of Bi2Ti2O7.
Bismuth nitrate pentahydrate was purchased from Aladdin
(Shanghai, China). Sodium molybdate dihydrate, tetrabutyl
titanate, ethylene glycol, diethyl ether anhydrous, glyc-
erol and glycolic acid were obtained from Sinopharm
Chemical Reagent Company (Shanghai, China). Ethylene-
diaminetetraacetic acid (EDTA), p-benzoquinone (BQ)
and isopropanol (IPA) were purchased from Shanghai
Chemical Regent Company (Shanghai, China). Purified
water (Wahaha Group Ltd., Hangzhou, China) was used
throughout the experiments. All chemicals were used with-
out further purification.
2.2. Apparatus
2.4. Photocatalytic Degradation of GA
Transmission electron microscopy (TEM, Hitachi, Japan)
and scanning electron microscopy (SEM, JEOL, Japan)
were used to observe the morphology and microstructure
of nanomaterials. Energy dispersive X-ray spectrometry
The photocatalytic experiments were conducted in the pho-
tochemistry reaction instrument (XPA-7). In the experi-
ment, 60 mg material was added to 50 mL of GA solution
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Rong et al.
Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction as Efficient Visible-Light Photocatalyst
(30 mg/L) and stirred in the dark for 30 min to reach
an adsorption–desorption equilibrium. Then a 500 W Xe
lamp was used as the visible light source. The solutions
were collected and filtered by 0.45 ꢅm filter membranes
every 30 min of light irradiation. The concentration of the
GA was determined by HPLC with a ZORBAX ODS-
C18 chromatographic column (5 ꢅm, 150 mm ×4ꢄ6 mm)
Where c0 is the initial concentration of GA and c is the
concentration of GA with the irradiation time (mg/L).
3. RESULTS AND DISCUSSION
3.1. Characterization Analysis
3.1.1. Morphology Analysis
As shown in Figure 1, the morphology and microstruc-
ture of nanomaterials were characterized by SEM and
TEM. The Bi2MoO6 is composed of flower-like micro-
spheres with diameters of 1.0–1.5 ꢅm (Fig. 1(a)). These
microspheres are built from a large number of oval
nanosheets with about 20 nm in length and 10 nm in
width (Fig. 1(d)). In Figure 1(b), the Bi2Ti2O7 exhibits
ꢁ
at 25 C. The mobile phase was methanol/2‰ phospho-
ric acid aqueous solution (5:95, v/v) with a flow rate of
1.0 mL/min. The injection volume was 20 ꢅL and the UV
detection wavelength was 220 nm. The degradation per-
centage after experiment was calculated by Eq. (1):
Degradation percentage = ꢆꢇc0 −cꢁ/c0ꢈ×100%
(1)
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Figure 1. SEM images of Bi2MoO6 (a), Bi2Ti2O7 (b), MT2 (c); TEM images of Bi2MoO6 (d), Bi2Ti2O7 (e), MT2 (g, h); HRTEM image of MT2 (f);
EDS pattern of the MT2 nanocomposite (i); mapping images of Bi, Mo, Ti and O (j).
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Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction as Efficient Visible-Light Photocatalyst
Rong et al.
smooth and regular spherical morphology with diameter
ranging from 0.5–1.5 ꢅm, which is consistent with the
TEM image (Fig. 1(e)). As for the MT2 nanocompos-
ite (Figs. 1(c, g and h)), it can be seen that Bi2Ti2O7
acts as a support and the nanosheets of Bi2MoO6 are
well deposited on the surface of Bi2Ti2O7. The HRTEM
image (Fig. 1(f)) clearly shows that the lattice spacing
(0.315 nm) corresponds to the (131) plane of Bi2MoO6
and the lattice distance (0.298 nm) can be assigned to the
(444) plane of Bi2Ti2O7. The results qualitatively agree
with the XRD pattern as well. Meanwhile, Bi, Mo, Ti,
and O elements can be detected from the EDS pattern of
MT2 (Fig. 1(i)), which implies the co-existence of both
Bi2MoO6 and Bi2Ti2O7. The EDS mappings (Fig. 1(j)) fur-
ther confirms the presence and distribution of correspond-
ing elements accurately.
spectra (Fig. 3(a)) demonstrated that Bi, Mo, O, and Ti
elements exist on the surface of MT2. The carbon peak at
a binding energy of about 284.12 eV is derived from both
the carbon tape used for fixing material and CO2 in the
air adsorbed on the material surface.44 In the Bi4f spec-
trum (Fig. 3(b)), two peaks at binding energy of 159.01 eV
and 164.34 eV can be assigned to Bi4f7/2 and Bi4f5/2 of
Bi3+, respectively.36 Two peaks located at 232.30 eV and
235.45 eV are observed in the Mo3d spectrum (Fig. 3(c)),
which are ascribed to Mo3d5/2 and Mo3d3/2 of Mo6+ sig-
nals, respectively.45 Figure 3(d) shows that the O1s spectra
of Bi2MoO6 at 529.52, 530.74 and 531.09 eV correspond
to the O atoms in Bi–O, Mo–O, and surface hydroxyl
groups (–OH), respectively.46 Peaks located at 529.61,
530.04 and 530.90 eV of Bi2Ti2O7 may related to oxy-
gen in forms of Bi–O, Ti–O, and –OH.17 For MT2, the
O1s spectrum is deconvolved into four characteristic peaks
of 529.65, 530.22, 530.84 and 531.50 eV, corresponding
to Bi–O, Mo–O, Ti–O, and –OH, respectively. The O1s
peaks shifting to higher binding energy can be ascribed
to the interaction between Bi2Ti2O7 and Bi2MoO6. In
Figure 3(e), the peaks of Ti2p1/2 and Bi4d3/2 are par-
tially overlapped, leading to a broad crest at 465.86 eV.
The two peaks at 458.42 eV and 463.01 eV are due to
the Ti2p3/2 and Ti2p1/2 binding energies,13ꢀ47 respectively.
These peaks in MT2 are also shifted up by 0.3 eV. In
summary, the XPS analysis confirms the successful com-
3.1.2. XRD Analysis
The crystal structures of Bi2MoO6, Bi2Ti2O7 and
MT2 were investigated by XRD. As demonstrated in
Figure 2(a), Bi2MoO6 shows diffraction peaks of (131),
(200), (151), (062), (331) and (262) planes at 2ꢉ equal
to 28.3ꢁ, 32.9ꢁ, 35.9ꢁ, 47.1ꢁ, 55.9ꢁ and 58.5ꢁ, which can
be indexed to orthorhombic phase of Bi2MoO6 (JCPDS
No. 76-2388).42 The characteristic peaks of the diffrac-
tion patterns for Bi2Ti2O7 were well matched to the stan-
dard values of Bi2Ti2O7 (JCPDS No. 32-0118). The strong
diffraction peak located at 29.9ꢁ can be indexed to the
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position of MT2 nanomaterial.
Copyright: American Scientific Publishers
(444) plane.43 In the case of the MT2 nanocomposite,
the diffraction patterns of Bi2MoO6 and Bi2Ti2O7 are all
detected. It shows that the MT2 nanocomposite was pre-
pared successfully.
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3.1.4. DRS Analysis
The UV-vis diffuse reflectance spectra of Bi2MoO6,
Bi2Ti2O7, and MT2 nanocomposite is shown in
Figure 4(a). It indicates that the absorption edges of
Bi2MoO6 and Bi2Ti2O7 are about 520 nm and 450 nm,
respectively, consistent with Refs. [48, 49]. The absorption
strength and boundary of MT2 photocatalyst are located
between Bi2MoO6 and Bi2Ti2O7, which can be related to
the heterojunction effect in MT2 nanocomposite.50 The
band gaps (Egꢁ of the semiconductors are obtained by
Eq. (2):35
3.1.3. XPS Analysis
The surface compositions and chemical states of the pho-
tocatalysts were further investigated by XPS. The survey
ahv = Aꢇhv −Egꢁn/2
(2)
Where h, ꢂ, v, A and n are Planck constant, the absorption
coefficient, the light frequency, the proportionality con-
stant, and the parameter related to the property of the
semiconductor transition, respectively. Direct and indirect
band-gap transition can be expressed as n = 1 or n = 4.37
For the Bi2MoO6 and Bi2Ti2O7 material, n is equal to
1.49ꢀ51 Figure 4(b) shows that the Eg of the Bi2Ti2O7 and
Bi2MoO6 are 2.90 and 2.60 eV, respectively, which are
close to the reported values.16ꢀ52
a Bi2MoO6
b Bi2Ti2O7
c MT2
10
20
30
40
50
60
70
80
90
3.1.5. PL Analysis
2 Theta/degree
The PL spectra was applied to study the transfer and
separation efficiency of photogenerated charge carriers.
As we know, the higher PL intensity indicates a higher
Figure 2. XRD patterns of Bi2MoO6 (a), Bi2Ti2O7 (b) and MT2
nanocomposite (c).
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Rong et al.
Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction as Efficient Visible-Light Photocatalyst
158.98
b
Bi4f
a
164.32
a Bi2MoO6
Bi2MoO6
158.97
159.01
164.27
164.34
b Bi2Ti2O7
Bi2Ti2O7
MT2
c MT2
800
1000
600
400
200
0
168
166
164
162
160
158
156
Binding energy/eV
Binding energy/eV
232.25
235.40
c
Mo3d
O1s
d
529.62
529.61
530.74
531.09
Bi2MoO6
Bi2MoO6
530.04
530.90
232.30
235.45
Bi2Ti2O7
530.22
530.84
531.50
MT2
529.65
MT2
240
238
236
234
232
230
228
538
536
534
532
530
528
526
524
Binding energy/eV
e
465.86
Ti2p
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Copyright: American Scientific Publishers
Bi2Ti2O7
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463.01
458.42
466.16
MT2
458.71
463.30
472 470 468 466 464 462 460 458 456
Binding energy/eV
Figure 3. XPS survey spectra of Bi2MoO6, Bi2Ti2O7 and MT2 (a); high-resolution XPS spectra of the Bi4f (b), Mo3d (c), O1s (d) and Ti2p (e) of
Bi2MoO6, Bi2Ti2O7 and MT2.
recombination of photo-generated electron–hole pairs.53
The PL spectra of Bi2Ti2O7, Bi2MoO6 and MT2 with an
excitation wavelength at 395 nm were shown in Figure 5.
MT2 shows the lower the fluorescence emission intensity
than those of Bi2MoO6 or Bi2Ti2O7. The result of PL
spectra manifests that the composition of Bi2Ti2O7 and
Bi2MoO6 is beneficial to the separation of photoinduced
electron–hole pairs.
was low when Bi2MoO6 or Bi2Ti2O7 was used individually
as photocatalysts. It may be related to the high recombi-
nation of the photoinduced charge carriers. By compari-
son, the MT nanocomposites show a higher photocatalytic
activity for GA degradation. According to the experiments,
the mass ratio of Bi2MoO6 and Bi2Ti2O7 in the composites
plays a dominant role in photocatalytic efficiency. With
the increase of the mass ratio of Bi2MoO6, the degradation
efficiency first increases and then decreases. MT2 nanoma-
terial shows the highest degradation rate. The removal rate
of GA can reach close to 99.9% after 150 min irradiation
under visible light. It may be ascribed to the formation of
more heterojunctions in MT2 nanocomposite and the better
separation of photo-generated electron–hole pairs.50 In the
following experiments, MT2 nanocomposite is adopted.
3.2. Photocatalytic Degradation of GA
3.2.1. Effect of the Component Ratio of
MT Nanocomposites
The photocatalytic activities of the materials under visi-
ble light irradiation were evaluated by GA, as shown in
Figure 6. The photocatalytic efficiency of GA degradation
J. Nanosci. Nanotechnol. 19, 7635–7644, 2019
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Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction as Efficient Visible-Light Photocatalyst
Rong et al.
a
b
Bi2Ti2O7
Bi2MoO6
MT2
Bi2MoO6
Bi2Ti2O7
3.2
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.3
300
400
500
600
700
800
wavelength/nm
hv/eV
Figure 4. UV-vis absorbance spectra (a), and energy band gaps of Bi2MoO6, Bi2Ti2O7, and MT2 nanocomposite (b).
3.2.2. Effect of pH
was greatly reduced to 42.0% after 150 min. It is attributed
to the instability of catalyst resulting in the reduction of
the activity of the photocatalyst under strong acid con-
dition. The degradation rate of GA also decreased when
the solution pH was increased from 4 to 10. On the one
hand, GA (pKa = 3.83) is relatively unsteady under neu-
tral or alkaline condition.54 On the other hand, the weak
acidic environment is beneficial to the adsorption of GA
molecules on MT2.55 Therefore, the following experiments
were carried out in pH 4.
To study the effect of pH on GA degradation, a series of
experiments in the pH range 2–10 were conducted, and
the results are shown in Figure 7. Obviously, the opti-
mum photocatalytic efficiency was observed to be 99.9%
at pH 4. When the pH was 2, the degradation percentage
a Bi2MoO6
b Bi2Ti2O7
c MT2
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a
3.2.3. Effect of Salinity
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In order to explore the effect of salinity on GA degrada-
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tion, different amounts of NaCl (0.05%, 0.10%, 0.15%,
0.20%) were added into GA solutions, respectively. As
exhibited in Figure 8(a), the degradation rate of GA
decreases from 99.9% to 33.5% with the increase of the
NaCl. There are two reasons accounting for its inhibitory
effect on GA degradation. First, Cl− can displace the
bound OH− on the MT2 surface resulting in the decrease
b
c
360
380
400
420
440
460
480
500
520
Wavelength/nm
ꢀ
of OH−, which prevents the generation of OH. Second,
the adsorption of Na+ on the surface of the photocatalyst
Figure 5. PL spectra of Bi2Ti2O7, Bi2MoO6 and MT2 nanocomposite.
1.0
0.8
0.6
1.0
0.8
0.6
Bi2MoO6
0.4
0.4
0.2
0.0
pH=2
pH=4
pH=6
pH=8
pH=10
Bi2Ti2O7
MT1
0.2
MT2
MT4
0.0
–30
0
30
60
90
120
150
–30
0
30
60
90
120
150
Time/min
Time/min
Figure 6. Effect of the component ratio of MT nanocomposites on the
degradation of GA (c0 = 30 mg/L, pH = 4).
Figure 7. The degradation effect of GA by MT2 nanocomposite at dif-
ferent pH (c0 = 30 mg/L).
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Rong et al.
Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction as Efficient Visible-Light Photocatalyst
3.0
a
0.00% NaCI K=0.01354 min–1R2=0.99450
1.0
0.8
0.6
0.4
0.2
0.0
b
0.05% NaCI K=0.00458 min–1R2=0.98391
2.5
0.10% NaCI K=0.00415 min–1 R2=0.98941
0.15% NaCI K=0.00392 min–1 R2=0.99466
2.0
0.20% NaCI K=0.00266 min–1 R2=0.99651
1.5
0.00% NaCI
0.05% NaCI
0.10% NaCI
0.15% NaCI
0.20% NaCI
1.0
0.5
0.0
–30
0
30
60
90
120
150
0
30
60
90
120
150
Time/min
Time/min
Figure 8. Effect of salinity on GA degradation (c0 = 30 mg/L, pH = 4) (a), the kinetics study (b).
prohibits the GA molecules from moving onto the active
sites (site-blocking effect) and reduces the absorbance
of visible light by the MT2 nanocomposite.56ꢀ57 Kinet-
ics research has been shown in Figure 8(b). The degra-
dation conforms to the pseudo-first-order kinetics because
the lnꢇc0/cꢁ is linearly dependent on the reaction time
(Eq. (3)).
cycle experiments. It can be observed that the photocat-
alytic performance of MT2 shows no apparent deactivation
(Fig. 9(a)). The XRD result of the MT2 nanocomposite
before and after four photocatalytic experiments is demon-
strated in Figure 9(b). It can be found that there is no
obvious change of the crystalline structure, further demon-
strating the sufficient stability of the catalyst during the
photodegradation process.
lnꢇc0/cꢁ = kt
Where c0 and c have the same meaning with those
in Eq. (1); k is the reaction rate constant (min−1
(3)
3.3. Possible Photocatalytic Mechanism
In order to investigate the photocatalytic mechanism of GA
by the MT2 nanocomposite, the main oxidative species
produced in the photocatalytic process were detected
ꢁ
IP: 5.62.159.44 On: Sun, 07 Jul 2019 02:09:21
and t is the irradiation time (min). In Figure 8(b),
the k of GA degradation with 0.00%, 0.05%, 0.10%,
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0.15% and 0.20% NaCl are 0.01354, 0.00458, 0.00415,
0.00392 and 0.00266 min−1, respectively. The MT2
nanocomposite without NaCl has the highest reac-
tion rate constant. Hence, the existence of NaCl
in the photocatalytic reaction is unfavorable for GA
degradation.
by the oxidative species capture experiments. 1 mmol/L
BQ, EDTA and IPA were used as trapping agents of
superoxide radical (ꢀO−2 ꢁ, holes (h+ꢁ and hydroxyl rad-
icals (ꢀOH), respectively. The results are exhibited in
Figure 10. It can be noted that three active species
play a certain suppression influence on GA degradation.
The order of the rate coꢀnstant is expressed as BQ <
3.2.4. Reusability of the Photocatalyst
To evaluate the stability of the MT2 nanocomposite,
the reusability of the photocatalyst was investigated by
ꢀ
EDTA < IPA. Therefore, O−2 , h+ and OH are all the
primary active species participated in the photocatalytic
1.0
b
a
1st
2nd
4th
3rd
0.8
0.6
0.4
0.2
0.0
before cycle
after 4 cycles
10
20
30
40
50
60
70
80
90
0
100
200
300
400
500
600
700
2 Theta/degree
Time/min
Figure 9. Cycling runs of MT2 for GA degradation (a), XRD patterns of the MT2 nanocomposite before and after 4 recycling runs (b).
J. Nanosci. Nanotechnol. 19, 7635–7644, 2019
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Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction as Efficient Visible-Light Photocatalyst
Rong et al.
120
the VB to corresponding CB under visible light irradiation.
Two possible photocatalytic mechanisms for the GA
degradation are illustrated in Figure 11. Figure 11(b)
describes one possible separation process of photoinduced
carriers. The CB and VB potential of Bi2Ti2O7 are more
negative than those of Bi2MoO6. The e− in Bi2Ti2O7 can
shift freely to Bi2MoO6 while the h+ can move from
Bi2MoO6 to Bi2Ti2O7.59 The CB potential of Bi2MoO6 is
positive than O2/ꢀO2− (O2/ꢀO2− = −0ꢄ046 eV vs. NHE),60
suggesting that the e− accumulated in CB of Bi2MoO6
cannot combine with O2 and form ꢀO2−. The mecha-
nism speculation is incompatible with the trapping exper-
100
80
60
40
20
0
ꢀ
iments, which reveals that O−2 plays a leading role in
No scavenger
IPA
EDTA
BQ
the photocatalytic reaction. Therefore, the separation pro-
cess of the photoexcited electron–hole pairs doesn’t follow
Figure 11(b). Figure 11(c) describes the second possible
process, which is generally regarded as Z-scheme het-
erojunction mechanism. The e− in CB of Bi2MoO6 can
be consumed fast by reacting with the h+ in the VB of
Bi2Ti2O7.61 The e− in the CB of Bi2Ti2O2 have enough
Trapping agent
Figure 10. The degradation rate constants of GA with the MT2 in the
presence of various trapping agents.
reaction. The impact of active species follows the order of
ꢀO−2 > h+ >ꢀOH.
ꢀ
reduction ability to reduce the O2 in air to generate O2−
The possible photocatalytic mechanism was further dis-
cussed according to conduction band (CB) and valence
band (VB) positions of the semiconductor heterojunc-
tion. The CB and VB potential of Bi2MoO6 and
Bi2Ti2O7 were calculated by the Mulliken electronegativ-
ity theory (Eqs. (4)–(5)) or Mott-Schottky measurements
(Fig. 11(a)):
because the ECB of Bi2Ti2O7 is more negative than that
of O2/ꢀO−2 (O2/ꢀO2− = −0ꢄ046 eV vs. NHE). Afterwards,
ꢀO−2 can directly oxidize GA or react with H+ to produce
H2O2 (O2/H2O2 = +0.682 eV vs. NHE).62 Thereafter, the
ꢀ
H2O2 can reꢀact with the e− to form OH. Because the
potential of OH/H2O (ꢀOH/H2O= +2ꢄ72 eV vs. NHE)
+
is mo+re negative than the potential of h on Bi2MoO6,63
IP: 5.62.159.44 On: Sun, 07 Jul 2019 02:09:21
EVB = X −E +0ꢄ5E
(4)
Copyright: American Scientific Publishers
c
g
the h can directly oxidize the target pollutants or react
Delivered by Ingenta
with H2O to produce ꢀOH. So the separation process of the
ECB = EVB −Eg
(5)
photoinduced charge carriers follows Figure 11(c). On the
basis of the analysis above, the possible transition states
and reactions are presented for GA photodegradation as
follows:
Where X is the absolute electronegativity of the semicon-
ductor, Ec is the energy of free electrons on the hydrogen
scale (about 4.50 eV). The X values for Bi2MoO6 and
Bi2Ti2O7 are 6.13 and 5.18 eV.58 According to the equa-
tions above, the VB and CB positions for Bi2MoO6 are
calculated to be +2.93 and +0.33 eV versus NHE, respec-
tively. The VB and CB positions for Bi2Ti2O7 are +2.13
and −0.77 eV versus NHE, respectively. The results are
close to the analysis of Mott-Schottky measurements.
Due to the narrow band gaps, both the photo-generated
electrons of Bi2MoO6 and Bi2Ti2O7 can be excited from
MT2 +hv → e− +h+
O2 +e− → ꢀO−2
(6)
(7)
ꢀO−2 +H+ → ꢀHO2
(8)
ꢀHO2 +H+ → H2O2
H2O2 +e− → ꢀOH+OH+
(9)
(10)
3.5×108
a
3.0×108
2.5×108
2.0×108
Bi2MoO6
Bi2Ti2O7
1.5×108
1.0×108
5.0×108
0
–0.6 –0.4 –0.2 0.0
0.2
0.4
0.6
E (V/NHE)
Figure 11. Mott-Schottky plots for Bi2MoO6 and Bi2Ti2O7 (a); the schematic diagram of possible photodegradation mechanisms of the MT2 hetero-
junction (b, c).
7642
J. Nanosci. Nanotechnol. 19, 7635–7644, 2019

Rong et al.
Synthesis of Bi2MoO6/Bi2Ti2O7 Z-Scheme Heterojunction as Efficient Visible-Light Photocatalyst
h+ +H2O → ꢀOH
(11)
(12)
14. G. Zaccariello, M. Back, M. Zanello, P. Canton, E. Cattaruzza,
P. Riello, A. Alimonti, and A. Benedetti, ACS Appl. Mater. Inter.
9, 1913 (2017).
ꢀO−2 /h+/ꢀOH+GA → CO2 +H2O
15. B. Allured, S. Delacruz, T. Darling, M. N. Huda, and V. R.
Subramanian, Appl. Catal. B Environ. 144, 261 (2014).
16. G. F. Samu, Á. Veres, B. Endro˝di, E. Varga, K. Rajeshwar, and
C. Janáky, Appl. Catal. B Environ. 208, 148 (2017).
17. H. F. Shi, H. Q. Tan, W. B. Zhu, Z. C. Sun, Y. J. Ma, and E. B.
Wang, J. Mater. Chem. A 3, 6586 (2015).
18. H. Liu, Y. J. Chen, G. H. Tian, Z. Y. Ren, C. G. Tian, and H. G. Fu,
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4. CONCLUSION
In summary, a facile route for the preparation of
Bi2MoO6/Bi2Ti2O7 material was introduced. The physico–
chemical characterizations were conducted by SEM, TEM,
XRD, XPS, UV-vis DRS, and PL analysis. MT2 nanocom-
posite exhibited nearly 99.9% photocatalytic activity for
GA degradation within 150 min. The enhanced photo-
catalytic performance of MT2 can be attributed to the
formation of heterojunction interface between Bi2MoO6
and Bi2Ti2O7, which facilitated the transfer and separa-
tion of photogenerated electron–hole pairꢀs. The mecha-
ꢀ
nism research indicates the O−2 , h+ and OH are active
species and O−2 displays the main role. Moreover, the
ꢀ
photocatalytic mechanism of Z-scheme heterojunction
was proposed. The study shows the Bi2MoO6/Bi2Ti2O7
nanocomposite has a bright application prospect in the
photocatalysis of wastewater treatment.
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Acknowledgments: This work was supported by
National Natural Science Foundation of China (21177061),
Natural Science Foundation of Jiangsu Province
(BK20150968) and Technology-Based Pioneering Enter-
prise Incubation Project of Jiangsu Province (BC2016003).
29. D. Jie, Z. Q. Yang, C. He, X. W. Tong, Y. Li, X. J. Niu, and H. G.
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J. Nanosci. Nanotechnol. 19, 7635–7644, 2019