Photoreduction properties of novel Z-scheme structured Sr0.8La0.2(Ti1?δ4+Tiδ3+)O3/Bi2MoO6composites for the removal of Cr(vi)

Article abstract of DOI:10.1039/d1ra01466h

Novel Z-scheme structured Sr_0.8La_0.2(Ti_1?δ⁴⁺Ti_δ³⁺)O₃/Bi₂MoO₆(LSTBM) composites were preparedviaa facile two-step solvothermal method. Several characterization techniques were employed to investigate the phases, microstructures, compositions, valence states, oxygen vacancies, surface oxygen absorption, energy band structures and lifetime of photoproduced carriers. It was found that the lifetime and transfer of the photoproduced carriers of LSTBM were better than those of Bi₂MoO₆(BMO) and Sr_0.8La_0.2(Ti_1?δ⁴⁺Ti_δ³⁺)O₃(LSTO). The LSTBM with a molar ratio of BMO/(LSTO + BMO) = 0.07 (denoted as LSTBM7) showed 1.9 and 3.1 times removal rates than those for BMO and LSTO, respectively. Importantly, the built-in electric field in the heterojunction of LSTBM and O_v-s, especially in O_v-s on the higher-Fermi-level side of the heterojunction, had co-played roles in prolonging the lifetime and improving the transfer of photogenerated carriers. The photoproduced e^?played a dominant role in reducing Cr(vi) to Cr(iii) and the produced Cr(iii) tends to form Cr(OH)₃and adsorb onto the surface of the photocatalyst to decrease the nucleation energy. The possible reduction route for Cr(vi) to Cr(iii) over LSTBM7 was figured out. This study implies that inducing O_v-s on the higher-Fermi-level side of the Z-scheme heterojunction is a more effective route for separating the photogenerated electrons and holes and improving the transfer of photogenerated carriers.

Full text of DOI:10.1039/d1ra01466h

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PAPER
Photoreduction properties of novel Z-scheme
4
+
3+
structured Sr La (Ti
Ti )O /Bi MoO
0
.8 0.2
1ꢀd
d
3
2
6
Cite this: RSC Adv., 2021, 11, 14007
composites for the removal of Cr(VI)†
Bin Liu, ‡ Zhili Fan,‡ Wangjian Zhai, Junfeng He, Shaofeng Wang,* Fuming Chen,
Yinzhen Wang, Wei Li, Xianhua Hou
and Qinyu He*
4+
3+
Novel Z-scheme structured Sr0.8La0.2(Ti1ꢀd Ti
d
3 2 6
)O /Bi MoO (LSTBM) composites were prepared via
a
facile two-step solvothermal method. Several characterization techniques were employed to
investigate the phases, microstructures, compositions, valence states, oxygen vacancies, surface oxygen
absorption, energy band structures and lifetime of photoproduced carriers. It was found that the lifetime
and transfer of the photoproduced carriers of LSTBM were better than those of Bi
2
MoO
(LSTO). The LSTBM with a molar ratio of BMO/(LSTO + BMO) ¼ 0.07 (denoted
as LSTBM7) showed 1.9 and 3.1 times removal rates than those for BMO and LSTO, respectively.
Importantly, the built-in electric field in the heterojunction of LSTBM and O -s, especially in O -s on the
6
(BMO) and
4+
3+
d
Sr0.8La0.2(Ti1ꢀd Ti
)O
3
v
v
higher-Fermi-level side of the heterojunction, had co-played roles in prolonging the lifetime and
ꢀ
improving the transfer of photogenerated carriers. The photoproduced e played a dominant role in
reducing Cr(VI) to Cr(III) and the produced Cr(III) tends to form Cr(OH)
the photocatalyst to decrease the nucleation energy. The possible reduction route for Cr(VI) to Cr(III) over
LSTBM7 was figured out. This study implies that inducing O -s on the higher-Fermi-level side of the Z-
3
and adsorb onto the surface of
Received 23rd February 2021
Accepted 29th March 2021
v
DOI: 10.1039/d1ra01466h
scheme heterojunction is a more effective route for separating the photogenerated electrons and holes
rsc.li/rsc-advances
and improving the transfer of photogenerated carriers.
improve the lifetime of photogenerated carriers. Another is to
enhance the transfer efficiency of the photogenerated carriers.
Introduction
With the development of industry, more and more waste water The routes to prolonging the lifetime of the photogenerated
containing heavy metal ions, such as hexavalent chromium carriers include establishing a built-in eld through hetero-
1
10
[
Cr(VI)], are being discharged into rivers and soil. Cr(VI) is toxic junction, introducing an electron-capturing level such as
11
2
and is a threat to human beings and animals. In order to solve oxygen vacancy (O
this problem, numerous methods have been explored, such as conductor (SM) possessing intrinsic local electric eld such as
adsorption, coagulation–occulation technique, membrane perovskites. Among the above-mentioned strategies, con-
separation, ion exchange, electrochemical method and pho- structing a heterojunction and introducing an electron-
tocatalysis. Due to the safety and simplicity of the process, capturing level in the band gap has attracted increasing atten-
) in the band gap, and employing semi-
v
3
4
12
5
6
7
8
photocatalytic technology has attracted increasing attention in tion due to its considerable effect on prolonging the lifetime of
9
11,13
removing Cr(VI). However, there are several shortcomings, photogenerated carriers.
The constructed heterojunctions
14
15
particularly low removal efficiency, that should be addressed.
include p–n heterojunction, type II heterojunction and Z-
To resolve the problem of low removal efficiency, many scheme heterojunction, among which Z-scheme hetero-
routes have been developed. Two approaches have dominantly junction is the most effective strategy to improve the lifetime of
16,21
attracted attention due to the high effectiveness. One is to photogenerated carriers.
accepted that introducing O
Meanwhile, it is unanimously
v
is also an effective way to separate
17–19
photogenerated electron–hole pairs by capturing electrons.
To enhance the transfer efficiency of the photogenerated
Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum
Materials, Guangdong Provincial Engineering Technology Research Center for
Quantum Precision Measurement, Guangdong Provincial Engineering Technology carriers, several strategies have been developed, such as
Research Center of Efficient Green Energy and Environmental Protection Materials,
hybridizing expensive metal nanoparticles, such as Au and Ag
School of Physics and Telecommunication Engineering, South China Normal
nanoparticles, with photocatalysts, and compositing good-
University, China. E-mail: gracylady@163.com
20
conductivity material such as g-C N with photocatalysts.
3
4
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra01466h
In this study, we designed a photocatalyst that was expected
to superpose the effects of O and Z-scheme heterojunction on
The authors contributed equally.
v
©
2021 The Author(s). Published by the Royal Society of Chemistry
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prolonging the lifetime of photogenerated carriers as well as
improving the transfer of photogenerated electrons. In general,
the matched energy band edge position between the two SM
Experimental
Synthesis of photocatalysts
needs to be considered for constructing Z-scheme hetero- The sample LSTO was fabricated using the following procedure.
junction. As depicted in Fig. S1,† the edge of the valence band 3.40 g titanium butoxide was dissolved in 30 mL ethylene glycol
(
VB) and conduction band (CB) of SM I are higher than those of solution, in which 1.69 g of Sr(NO ) and 0.87 g of La(NO ) $6H O
3 2 3 3 2
ꢁ
SM II. The CB of SM II is higher than the VB of SM I. Aer were dissolved and magnetically stirred at 38 C for 4 h. The ob-
contacting, electrons would transfer from SM I to SM II. Thus, tained solution was dried at 60 C for days and then transferred
ꢁ
a built-in electric eld with a direction from SM I to SM II will be into a 45 mL aqueous solution of NaOH (5 M). The mixture was put
formed. This built-in electric eld would drive photogenerated into a Teon-lined stainless-steel autoclave and sealed for reacting
ꢁ
electrons in the heterojunction to transfer from SM II to SM I at 180 C for 24 h. Aer cooling to room temperature, the sample
and drive photogenerated holes in the heterojunction to was washed with deionized water and ethanol until the pH was 7,
ꢁ
transfer from SM I to SM II. Eventually, the photogenerated then dried at 60 C for 12 h. The as-prepared powder is LSTO. For
electron–hole pairs are separated efficiently and the lifetimes of comparison, pure SrTiO was prepared using the above-described
3
photogenerated carriers are prolonged.
Recently, strontium titanate (SrTiO
skite, has received considerable attention because of its physi-
procedure without lanthanum source, and the as-prepared pure
3
), an ABO
3
-type perov- SrTiO was denoted as STO.
3
The LSTBM composites and BMO were synthesized through
cochemical stability, non-toxicity and interesting structure a solvothermal method. In the rst step, 0.05 g of Na MoO -
2
4
22–25
compatibility.
It was reported that doping with La in SrTiO3 $2H O and 0.2 g Bi(NO ) $5H O were dissolved in 15 mL
2 3 3 2
(LSTO) would transform it into an n-type degenerate semi- ethylene glycol and magnetically stirred for 85 min (denoted as
conductor, which can trap extra charge carriers (electrons) solution A). In the second step, 20 mL of absolute ethanol was
generated in the conduction band bottom of n-type semi- added to solution A under magnetic stirring for 10 min to
26
conductor, consequently resulting in inhibiting the recombi- obtain solution B. In the third step, a certain amount of LSTO
25
nation of photo-generated charge carriers. A previous study was added in 20 mL of absolute ethanol and sonicated for
showed that La-doping in SrTiO resulted in a higher concen- 30 min to obtain turbid liquid (denoted as mixture C). In the
3
27
tration of oxygen vacancy clustering. Furthermore, it has been fourth step, solution B and mixture C were mixed with the molar
reported that doping La at the Sr site of SrTiO could result in ratios of Mo/(Mo + Ti) of 5%, 7% and 10%, and stirred for
3
4
+
0
3+
1
the transformation of Ti (3d orbital) into Ti (3d orbital), 10 min to obtain a mixture (denoted as “mixture D”). Finally, the
for a charge balance. The pres- mixture D was poured into a Teon-lined stainless-steel auto-
ence of Ti would improve the transfer of photoinduced elec- clave and kept at 160 C for 6 h. The product was washed with
28
therefore the formation of O
v
3
+
ꢁ
29
ꢁ
trons. In addition, O -s can modulate both the coordination deionized water and ethanol and subsequently dried at 60 C for
v
structures and electronic states of adsorbates via the surface- 12 h. A series of LSTBM was obtained and denoted as LSTBM5,
dangling bonds and localized electrons, therefore improving the LSTBM7, and LSTBM10 for those with starting molar ratios of
30,31
3+
photocatalytic efficiency.
It has also been found that the Ti / Mo/(Mo + Ti) of 5%, 7% and 10%, respectively.
Ti (%) ratio, therefore the O concentration increases with the
In order to clarify the design effects of LSTBM on the pho-
increase in La concentration. On the other hand, it is well- tocatalytic properties, pure BMO was prepared through the
documented that a large amount of O -s will be generated at the identical hydrothermal procedure and parameters.
4+
v
2
8
v
heterostructure interface. Consequently, when the components of
the heterostructure had different crystal structures, it could
present excellent photocatalytic properties owning to the high
Characterization
v
separation rate of photo-generated carriers induced by O -s and the X-ray diffraction (XRD) was carried out to analyze the phases of
32
built-in electric eld. Based on the above discussion, we designed as-prepared samples using an X'Celerator detector (XRD, X'Pert
Z-scheme structured composites and chose LSTO to be one SM in Pro, Panalytical, Los Altos, CA) with Cu Ka radiation (l ¼ 1.5406
ꢀ
ꢁ
ꢁ
a Z-scheme structured composite, whereas another chosen SM A) in the 2q range of 20 –80 . The microstructures of the as-
with energy-structure-matching in this composite was Bi
2
MoO
6
prepared powders were investigated using scanning electron
(
BMO), which has Aurivillius structure, is non-toxic, low-cost, microscopy (SEM, ZEISS ULTRA 55). The elements in samples
33,34
chemically stable and visible-light responsible.
were tested by energy-dispersive X-ray spectrometry (EDS). The
In this work, a facile two-step solvothermal method was detailed microstructures and crystallographic structures were
employed to prepare novel Z-scheme structured LSTO/BMO further observed using high-resolution transmission electron
composites (LSTBM). As expected, it exhibits excellent Cr(VI) microscopy (HRTEM, JEM-210HR, Japan). Brunauer–Emmett–
photoreduction efficiency, resulting from a longer lifetime of Teller (BET) specic surface area and pore size distribution were
photogenerated carriers and improved transfer. It is found that, measured on JW-BK200C. The electron spin resonance spec-
besides the Z-scheme structured heterojunction, the ionized O
v
v
trum (ESR) was utilized to conrm the presence of O by Bruker
is one of the dominant factors effectively separates the photo- EMXPLUS. X-ray photoelectron spectroscopy (XPS) investigation
generated electrons and holes. Furthermore, the presence of was carried out to identify the surface elemental composition
3
+
Ti in LSTO due to La doping leads to higher transfer ability.
and chemical states of samples using a Thermo Scientic K-
Alpha XPS system (Thermo Fisher Scientic, America). UV-vis
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diffuse reectance (UV-vis DRS) measurements were performed
on a UV-2600 spectrophotometer (Shimadzu, Japan) using
BaSO as a reference sample. Thermal gravimetric (TG) analysis
4
and derivative thermal gravimetric (DTG) analysis were per-
formed on a TGA5500 instrument at a heating rate of
ꢁ
ꢀ1
1
0 C min . Photoluminescence (PL) spectra of as-prepared
samples were recorded on a Hitachi F-4500 uorescence spectro-
photometer (Hitachi, Japan) to qualitatively characterize the
recombination rates of photogenerated electron–hole pairs. Mott–
Schottky curves (M–S) and electrochemical impedance spectroscopy
(EIS) data were collected using the CHI760E Electrochemical
Fig. 1 (a) XRD patterns of STO, LSTO, BMO, LSTBM5, LSTBM7 and
Workstation (Shanghai Chenhua Instrument Corp., China) at room LSTBM10; (b) the enlarged diffraction peak (110) of STO and LSTO.
temperature with an Ag/AgCl (0.5 M Na SO ) as the reference elec-
2
4
trode and a Pt wire as the counter electrode. Inductively coupled
plasma-atomic emission spectrometry (ICP-AES) measurements
were performed using the SPECTRO ARCOS instrument.
Results and discussion
Phases and microstructures
2 2
In order to examine the absorbed O , H temperature
The XRD patterns of the as-prepared samples are shown in
programming reduction analysis (H -TPR) was performed using
AutoChem1 II 2920. First, 120 mg catalyst was pretreated in
2
Fig. 1(a). The LSTO exhibits strong diffraction peaks at 2q ¼
ꢁ ꢁ ꢁ ꢁ ꢁ ꢁ ꢁ
2.71 , 32.33 , 39.83 , 46.37 , 57.71 , 67.77 and 77.01 , corre-
2
ꢀ
1
ꢁ
pure He ow (30–50 mL min ) for 1 h at 300 C for dry
sponding to the (100), (110), (111), (200), (211), (220) and (310)
faces of cubic SrTiO (PDF #35-0734), respectively. Aer the intro-
ꢁ
pretreatment. Aer cooling to 50 C, 10% H
2
/He gas with a ow
3
ꢀ1
3+
rate of 30–50 mL min was introduced for 1 h for saturation,
duction of La , diffraction peaks of LSTO were basically close to
those of STO. Fig. 1(b) shows the enlarged diffraction peak (110) of
ꢀ
1
and then He ow (50 mL min ) to purge for 1 h to remove the
3+
weak physically adsorbed gas on the surface. The temperature
LSTO, clearly indicating that aer introducing La , the peaks have
ꢁ
ꢀ1
ꢁ
of the furnace was increased at a rate of 10 C min to 700 C
and a TCD was used as the detector.
shied to lower angles, probably due to the substitution by a larger-
3+
2+
25
ionic-radius La (1.15 A) for a smaller-ionic-radius Sr (1.13 A˚ ).
˚
The peaks of LSTO are more diffusive than those of STO due to the
3
+
introduction of La . The above results indicate that La atoms were
successfully introduced into the crystal structure of SrTiO . BMO
Photocatalytic activity experiments
3
ꢁ
The photocatalytic properties of all catalysts were assessed by
testing the properties of photoreducing Cr(VI) in a quartz reactor
irradiating by a simulated solar light source generated by
a 350 W xenon lamp (Shanghai Lansheng Company, China). In
these experiments, 20 mg of the catalyst was plugged into
ꢁ
ꢁ
ꢁ
exhibits ve well-dened peaks at 2q ¼ 28.10 , 32.38 , 46.78 , 55.18
ꢁ
and 58.29 , which can be indexed to the (131), (200), (202), (331) and
(
2 6
262) planes, respectively, of crystalline Bi MoO (PDF #72-1524).
Aer the formation of composite catalysts, there was no obvious
ꢀ1
1
00 mL K Cr O solution (10 mg L ). Before irradiation, the
2 2 7
mixed solution was stirred in the dark for 30 min to attain the
adsorption–desorption equilibrium. During the photoreduction
process, a 1.5 mL-mixed solution was sampled at a given time
interval and centrifuged to remove the containing photocatalyst.
The concentration of Cr(VI) was tested using a UV-vis spectropho-
tometer at 540 nm based on the diphenyl carbazide colorimetric
35
method. The relationship between concentration (C) and absor-
bance (A) follows an empirical formula set up by our group as
2
2
shown in Fig. S2.† The correlation coefficient R was close to 1 (R
0.99996), suggesting a high tting degree of the regression line
¼
to the observed absorbance of Cr(VI). Therefore, the concentration
of Cr(VI) can be calculated from eqn (1).
C ¼ (A ꢀ 0.00197)/0.09639
(1)
where C is the concentration of Cr(VI), A the absorbance of Cr(VI).
The removal rate of Cr(VI) at different reaction times can be
calculated from eqn (2),
Removal rate ¼ (C
0
ꢀ C
t
)/C
0
ꢂ 100
(2)
Fig. 2 Typical SEM image of (a) LSTO, (b) BMO, (c) and (d) LSTBM7 at
different magnifications; (e) EDS spectrum of LSTBM7 and (f) HRTEM
images of LSTBM7.
where C is the original concentration of Cr(VI), and C
Cr(VI) concentration at the illumination time t.
0
t
is the
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Table 1 BET surface area, pore volume and pore size of LSTO, BMO
and LSTBM7
Pore volume
Pore size
(nm)
2
ꢀ1
2
ꢀ1
Sample
BET area (m g
)
(m g
)
LSTO
BMO
LSTBM7
5.795
55.276
6.614
0.015
0.127
0.010
20.082
15.311
10.360
2
Fig. 3 (a) ESR spectra and (b) H -TPR profile of the BMO, LSTO and
LSTBM7.
shi of the diffraction peaks, indicating that BMO exists as a sepa-
rate phase rather than being incorporated into the LSTO lattice.
The morphologies and particle sizes of LSTO, BMO and
LSTBM7 were investigated using SEM. As displayed in Fig. 2(a),
the sample LSTO possesses an irregular spherical morphology
with a diameter of 100–160 nm, the surface of which has a small
island with a dimension of around 20 nm. From Fig. 2(b), it can
be found that BMO particles are basically spheres with the size
of 1.5–3 mm and assembled into nano-sheets. Additionally,
some BMO particles were merged into larger irregular particles
of the size of 6–9 mm. Fig. 2(c) is the SEM image of LSTBM7. It is
obviously seen from Fig. 2(c) that there are two particle types,
a larger one and a smaller one. The larger one possesses a size of
ꢃ9 mm and the smaller one possesses a size of ꢃ100 nm. By
comparing the sizes of BMO and LSTO in Fig. 2(a) and (b),
respectively, it can be concluded that the larger ones are prob-
ably the merged BMO and the smaller ones are LSTO. A high-
resolution image of LSTBM7 is shown in Fig. 2(d). It can be
seen that the larger particle in Fig. 2(c) possesses nanosheet-
structure, a characteristic of BMO, and the smaller particle
possesses a size of 100 nm and island surface, a characteristic of
LSTO. From Fig. 2(c) and (d), it can be concluded that LSTO
mainly lands on the surface of BMO, and some LSTO micro-
spheres are embedded in the layered structure of BMO to form
a 3D hierarchical structure. Moreover, such a hybrid structure
could increase the specic surface area and expose more active
sites to participate in photocatalytic reactions. The BET surface
area, pore volume and pore size of LSTO, BMO and LSTBM7 are
shown in Table 1. The sample LSTBM7 possesses a BET surface
that there is a peak at g ¼ 2.004, identied for electrons trapped
36
on O -s in the ESR spectra of the above-said samples, indi-
v
cating the presence of O -s in BMO, LSTO and LSTBM7. In
v
addition, we can see that the ESR intensity of LSTBM7 was
much higher than the sum of LSTO and BMO, suggesting that
LSTBM7 possesses much more O -s than that in LSTO and
v
BMO, probably due to lattice distortion and mismatch in the
30
heterojunction LSTO/BMO. Since the molar proportion of
BMO in LSTBM7 is only 7%, the above results show that a small
amount of heterojunction can produce considerable O -s, i.e.,
v
the heterojunction is a very effective tool to produce O -s.
v
H -TPR analysis was performed to investigate the reducibility of
2
2
the samples. The H -TPR diagrams of BMO, LSTO and LSTBM7 are
shown in Fig. 3(b). There is only one reduction region around
ꢁ
5
91 C in the H
2
-TPR prole of BMO, indicating that there is only
lattice oxygen in BMO. For LSTO, there is a wide reduction in the
ꢁ
37
range of 175–500 C (attributed to the surface lattice oxygen ) and
ꢁ
38
one reduction region around 600 C (attributed to lattice oxygen ).
LSTBM7 shows four distinctive reduction regions around 258 C,
98 C, 457 C and 591 C. The rst and second ones could be
ꢁ
ꢁ
ꢁ
ꢁ
3
ascribed to the absorbed O by O -s and surface lattice O, respectively,
v
ꢁ
whereas the reduction peak around 591 C could be attributed to
ꢁ
lattice oxygen. The peaks around 457 C could be attributed to the
oxygen absorbed by O
gesting that the absorbed oxygen by O
v
-s in the heterojunction of LSTO/BMO, sug-
-s has a higher binding energy
v
than that of the surface lattice oxygen. In the low-temperature region
ꢁ
around 150–300 C, LSTBM7 showed higher reduction temperature,
2
ꢀ1
2
ꢀ1
area of 6.614 m g , higher than that of LSTO (5.795 m g ).
Notably, from Fig. 2(c), it can be seen that the contact area
between LSTO and BMO is small. Fig. 2(e) is the EDS spectrum
of LSTBM7. It can be conrmed that LSTBM7 contains La, Sr, Ti,
Bi, Mo and O elements. Further information about LSTBM7
composites was obtained from HRTEM images as shown in
Fig. 2(f). We chose two contacted particles for HRTEM investiga-
indicating higher reducibility. At a higher reduction temperature
region, the peak of LSTBM7 moved to a higher temperature, possibly
3
+
3+
owing to the presence of Ti , since the presence of Ti ions in LSTO
3+
structure can lead to coupling between those Ti ions through O
ions (Ti –O –Ti ) or superexchange interaction.
3
+
2ꢀ
4+
28
tion, in which the smaller one was located on the surface of the Elemental valence state analysis
larger one. The smaller one possesses lattice spacing of 0.198 nm,
possibly corresponding to the crystal plane of (200) of LSTO and
The chemical compositions and chemical states of the as-
prepared samples were studied by XPS analysis. Fig. 4(a)
shows the XPS survey spectra of LSTO, BMO and LSTBM7.
Fig. 4(b), (c) and (d) shows the high-resolution XPS spectra of Sr
slightly larger than that of SrTiO (PDF #35-0734). The larger one
3
possesses a lattice spacing of 0.312 nm, consistent with the (131)
crystal plane of BMO. The above-mentioned results imply that the
heterostructure of LSTO/BMO is in LSTBM7.
3
d, Ti 2p and La 3d in LSTO and LSTBM7, respectively. Fig. 4(e)
and (f) exhibit the high-resolution XPS spectra of Bi 4f and Mo
d in BMO and LSTBM7, respectively. Fig. 4(g) shows high-
3
2
ESR and H -TPR analysis
resolution XPS spectra of O 1s in LSTO, BMO and LSTBM7.
To conrm the existence of O
v
, ESR spectra of BMO, LSTO and
From Fig. 4(a), it is easy to nd that the expected coexistence
LSTBM7 were recorded and are shown in Fig. 3(a). It can be seen of Sr, Ti, La, Bi, Mo, O, and C elements (C came from the
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XPS peaks of La 3d5/2 and La 3d3/2 of LSTO in Fig. 4(d) are
43
located around 833.56 eV (ref. 42) and 850.72 eV, respectively.
The shake-up satellite peaks of La 3d and La 3d3/2 are located
5
/2
44
at 838.11 eV and 854.90 eV. Apparently, LSTBM7 possesses
a higher binding energy for La 3d5/2, La 3d3/2, shake-up satellite
peaks of La 3d5/2 and La 3d3/2 as 833.92 eV, 851.02 eV, 838.39 eV
and 855.04 eV, respectively.
Fig. 4(e) shows two strong peaks for Bi 4f5/2 and Bi 4f7/2
located at 164.42 eV and 159.11 eV, respectively, that belong to
45
BMO. It is noteworthy that peaks of Bi 4f5/2 and Bi 4f7/2 in
LSTBM7 composites exhibit a slight shi towards the lower
binding energy compared to those in BMO.
In Fig. 4(f), two peaks at 232.36 eV and 235.52 eV are
observed in the XPS spectrum of BMO. The two peaks are
45
ascribed to Mo 3d5/2 and Mo 3d3/2
, respectively. The peaks of
Mo 3d5/2 and Mo 3d3/2 of LSTBM7 also shi to lower values as
231.98 eV and 235.23 eV, respectively.
Fig. 4(g), (h) and (i) show high-resolution XPS spectra of O 1s
in BMO, LSTO and LSTBM7, respectively. Three peaks can be
tted for XPS spectra of O 1s of LSTO at 529.23 eV, 530.75 eV and
33.29 eV. The peak at 529.23 eV could be attributed to the
lattice oxygen of SrTiO , while the middle peak at 530.75 eV
could be attributed to O
usually assigned to the loosely bound oxygen on the LSTO

5
3
46
v
in LSTO. The peak at 533.29 eV is
47
surface. BMO exhibits two peaks at 529.80 eV and 530.49 eV,
which are ascribed to the Bi–O and Mo–O bonds, respectively, of
45
Bi MoO . Six peaks can be tted at 529.73 eV, 529.38 eV,
2
6
5
5
5
30.17 eV, 530.97 eV, 531.90 eV and 533.29 eV. The peak at
29.38 eV can be assigned to the lattice oxygen of LSTO, the one at
30.97 eV can be attributed to O of LSTO, and the one at 533.29 eV
v
Fig. 4 (a) XPS survey spectrum of LSTO, BMO and LSTBM7; (b)–(d)
high-resolution XPS spectra of Sr 3d, Ti 2p and La 3d in LSTO and
LSTBM7, respectively; (e) and (f) high-resolution XPS spectra of Bi 4f
and Mo 3d in BMO and LSTBM7, respectively; (g)–(i) high-resolution
XPS spectra of O 1s in BMO, LSTO and LSTBM7.
is assigned to the loosely bound oxygen on the LSTO surface. The
peaks at 529.73 eV and 530.17 eV can be assigned to Bi–O and Mo–
O bonds, respectively, of Bi MoO . It is worth noting that a new
2 6
peak appears at 531.90 eV in LSTMB7. This peak has also been
reported in ref. 48, which was assigned to surface hydroxyl groups.
In our sample LSTBM7, there are a few surface hydroxyl groups. On
reference sample) in LSTBM7, which is consistent with the
results obtained from EDS investigations.
the other hand, the oxygen absorbed by O
energy is found in the heterojunction based on the above
conclusion from H -TPR data. We would like to attribute the peak
at 531.90 eV to O on the LSTO side of the heterojunction LSTO/
v
-s with higher binding
Fig. 4(b) shows that the Sr 3d spectra of LSTO consist of two
individual peaks at 132.78 eV and 134.55 eV, which can be
2
25
attributed to Sr 3d5/2 and Sr 3d3/2 of SrTiO
3
,
respectively.
v
Whereas in LSTBM7, both peaks of Sr 3d5/2 and Sr 3d3/2 shi to
higher values of 133.01 eV and 134.83 eV, respectively.
Ti 2p spectra in Fig. 4(c) reveals that LSTO exhibits two
BMO. More reasons would be given below.
The above-mentioned XPS results reveal that the binding
energies of Sr 3d, La 3d, Ti 2p in LSTBM7 are accordingly higher
than those in LSTO, while the binding energies of Bi 4f and Mo
3d in the LSTBM7 composite are accordingly lower than those
in BMO. Previous works revealed the internal eld in hetero-
junction could result in a shi of binding energy.
4
0
4+
peaks at 458.08 eV (ref. 39) and 463.92 eV, attributed to Ti
4
+
2
p3/2 and Ti 2p1/2, respectively, in SrTiO , while there are
3
4
+
higher binding energies of 458.36 eV and 464.33 eV for Ti
4
+
2
p
3/2 and Ti 2p1/2 in the LSTBM7, respectively. Furthermore,
49,50
From the
the peaks located at 457.30 eV and 462.70 eV of LSTO corre-
above-mentioned results, it seems that the produced internal
eld in the heterojunction LSTO/BMO in LSTBM7 led to a rise in
3
+
3+
41
spond to Ti 2p3/2 and Ti 2p1/2
,
v
respectively. So, O -s may

have been introduced in the LSTO by mixed-valence of tita-
binding energies on the LSTO side and reduction in binding
energies on the BMO side, suggesting that the internal eld
direction is from LSTO to BMO.
4
+
3+
nium (Ti /Ti ) to balance the charge of the compound. The
3
+
amounts of Ti and O in La-doped SrTiO were calculated
v
3
from the XPS peak area, and the results are illustrated in
v
It is well known that O possesses a positive charge. There-
3
+
Fig. S3 and Table S1.† We can conclude that Ti
amounts in La-doped SrTiO increased with the increase of La
concentration. The results of Ti 2p spectra agree with those
obtained from H -TPR analysis.
and Ov
fore, O would tend to combine long-distance electrons. Aer
v
3
the heterojunction LSTO/BMO is formed, a built-in electric eld
would form in the junction, in case of which on the LSTO side,
2
some O
closer to the interface LSTO/BMO would lose the
v
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combined electrons and some O
face would not lose the combined electrons. Consequently, the
binding energy of O 1s would possess reducing value according to
v
located away from the inter-
ENHE ¼ Evs. Ag/AgCl + 0.197
(4)
(5)
g
Evb ¼ E + Ecb
the order, O of LSTO in the junction and closer to the interface, the
v
Consequently, the evaluated Evb of LSTO and BMO are 1.75 V
O
v
of LSTO in the junction and located away from the interface, the
other O of LSTO and BMO outside the junction LSTO/BMO. Thus,
in the XPS spectrum of LSTBM7, the peak located at 530.97 eV could
be attributed to O which did not lose combined electrons, while the
peak located at 531.90 eV could be ascribed to O which lost
and 2.49 V, respectively. The E -s and band edge positions of
g
v
LSTO and BMO are summarized in Table 2.
v
v
Photoreduction ability and stability of photocatalysts
combined electrons. The mechanism is shown in Fig. S4.† It can be
The photocatalytic activity of the as-fabricated samples was
evaluated by the photoreduction of simulated wastewater con-
taining Cr(VI). Fig. 5(a) and (b) are the dark adsorption–time
curve and photoreduction rate–time curves of aqueous-phase
Cr(VI) over the as-prepared samples. As shown in Fig. 5(a), it
can be found that all samples had reached the dark adsorption–
desorption equilibrium within 30 min. It can be seen from
Fig. 5(b) that LSTBM7 has the highest photoreduction rate of
expected that if the O was exposed to built-in eld produced by low-
v
energy electrons, the O is more likely to combine photogenerated
v
electrons, therefore resulting in a longer lifetime for photo-
generated carriers.
Band structure
The UV-vis DRS is commonly used to investigate the light
absorption and energy band features of SM. As shown in
Fig. S5(a),† LSTO presents a similar light response to the re-
99.7% within 180 min, much higher than that of pure BMO and
LSTO. The samples, LSTBM5 and LSTBM10 also display
a higher removal rate than that of pure BMO and LSTO.
Meanwhile, all kinetic curves are linear and the photoreduction
of Cr(VI) follows the pseudo-rst-order kinetic model:
25
ported results. BMO exhibits absorption in the visible light
range with an absorption edge approaching 437 nm. It can be
found that composites LSTBM5, LSTBM7 and LSTBM10 possess
stronger absorption within the visible light range than that of
LSTO but lower than that of BMO.
ꢀln(C
t
/C
0
) ¼ kt
(6)
Based on the absorption spectra, the band gap (E
g
) of SM can where, t is the irradiation time, k the kinetic rate constant, C0
be calculated from eqn (3).
and C the concentrations of Cr(VI) at the initial irradiation time
t
and at irradiation time t. As shown in Fig. 5(c), LSTBM7 displays
n/2
ahn ¼ A(hn ꢀ Eg)
(3)
the highest k value, almost 1.9 and 3.1 times those of BMO and
LSTO, respectively. These results reveal that the construction of
the LSTO/BMO heterostructure has greatly improved photo-
catalytic performance. The structural stability of LSTBM7 was
evaluated using ICP-AES during the photocatalytic cycle test. As
shown in Table S2,† aer three photocatalytic reactions, few
where a is the absorption coefficient, h is the Planck's constant,
n is the light frequency, A is the absorption coefficient, and n
represents 1 or 4 for direct or indirect band gap SMs. The
intersection of the tangent with the x-axis of Tauc plots
Fig. S5(b) and (c)†) can give a good approximation of E
51
(
(
g
values
ꢀ1
ions leaching from the catalyst except Sr (0.44788 mg L ),
Table 2) of the samples.
ꢀ1
which is far lower than the safe concentration (5 mg L ), as
According to ref. 8 and 33, SrTiO and Bi MoO are indirect
ꢀ1
3
2
6
well as except for Mo (0.44953 mg L ), which is larger than the
and direct SMs, respectively. Based on this knowledge, the
estimated-E -s of LSTO and BMO are 3.14 eV and 2.84 eV,
respectively.
ꢀ1
safe concentration (0.07 mg L ), even though Mo is a necessary
g
Mott–Schottky plots were used to determine the SM type, as
well as the band edge positions of LSTO and BMO. Since the
slopes of the tting curves are positive, as shown in Fig. S6,†
both BMO and LSTO are typical n-type SMs. For an n-type SM,
the at band potential (E ) is located near the conduction band

minimum (Ecb) and is readily obtained from the horizontal
2
52
intercepts as 1/C equals zero. According to eqn (4),
Ecb of
LSTO and BMO are estimated to be ꢀ1.39 V and ꢀ0.35 V vs.
NHE, respectively. The valence band maximum (Evb) of LSTO
49
and BMO are calculated using eqn (5).
g
Table 2 The E -s and band edge positions of LSTO and BMO
Sample
E
g
(eV) cb (V) Evb (V)
E
Fig. 5 (a) Adsorption experiments of the samples in dark; (b) normal
photocatalytic reduction of Cr(VI) and (c) pseudo-first-order kinetic
plots for the photocatalytic reduction of Cr(VI).
LSTO
BMO
3.14
2.84
ꢀ1.39
ꢀ0.35
1.75
2.49
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element for the human body. It can be concluded that the dark absorption for 30 min and photoreduction for 180 min)
structure is basically stable, and no toxic material but the was nished, the color of LSTBM7 and the simulated Cr(VI)
treated wastewater should be diluted before discharge.
wastewater changed into grey-green and nearly colorless,
respectively. These changes meant a decrease of Cr(VI) in the
testing solution and something deposited on the surface of
LSTBM7 particles aer being used (the word “used” means
The ability of separation and transfer of photogenerated
carriers
“over which the Cr(VI) was photoreduced”). The XPS spectra
PL spectroscopy was utilized in this work to investigate the
recombination of photo-generated electron–hole pairs in SMs.
In general, a lower PL intensity indicates a lower recombination
rate of the photo-generated electron–hole pairs. Fig. 6(a) shows
the PL spectra of as-prepared samples under the excitation of
shown in Fig. S7(b) and (c)† indicate that only Cr(III) species
40
(
576.96 eV and 586.90 eV) exist on the surface of the used
58,59
9
LSTBM7 photocatalyst.
Since Cr(OH) is a grey-green color,
3
it can be concluded that it is Cr(OH) is deposited on the surface
3
of LSTBM7 aer the photoreduction of Cr(VI) solution. The
325 nm-wavelength light. It was found that the PL intensity
ꢁ
chemical reaction of the used LSTBM7 around 472 C, exhibited
around 370 nm is caused by the recombination of photo-
in TG and DTG curves (Fig. S7(d)†), further conrms the exis-
53
produced electron–hole pairs in LSTO. Peaks around 400–
tence of Cr(OH)
the temperature reaches 400, Cr(OH)
and completely transforms into Cr
3
on the surface of the used LSTBM7 since when
53–55
5
00 nm could be ascribed to O
v
in materials.
Obviously, the
3
gradually lose the OH-
PL intensity of LSTBM5, LSTBM7 and LSTBM10 was lower than
that in LSTO, suggesting that the hybridization of LSTO with
BMO can effectively impede the recombination of the photo-
generated carriers.
ꢁ
60,61
2
O
3
at 500 C.
In short, all
of the above-mentioned investigations showed the deposition of
Cr(OH) on the surface of the used LSTBM7. But there are no
3
XRD peaks seen in the used LSTBM7 (Fig. S7(e)†). This result
means the deposited Cr(OH) is amorphous.
3
In order to further gure out the possible mechanism for
photoreduction from Cr(VI) to Cr(III), the active species of LSTBM7
during photoreduction were investigated by performing the active
species trapping experiments. Different scavengers, including
The electrochemical impedance spectroscopy (EIS)
measurements can be utilized to investigate the ability of elec-
tron transfer in the as-prepared samples since a small arc radius
means good electronic conductivity and efficient electron
56
separation. As shown in Fig. 6(b), LSTBM5, LSTBM7 and
LSTBM10 show smaller Nyquist radii than those in BMO and
LSTO, suggesting that hybridization of LSTO and BMO would
favor the charge transfer. The possible reason will be given in
the mechanism section. The possible reason accounting for the
better transfer ability of LSTBM7 could be ascribed to the
higher-binding-energy O aer more ionized-O was freed by the
carbon tetrachloride (CCl ), ethylenediaminetetraacetic acid diso-
4
dium (EDTA-2Na), isopropanol (IPA) and nitrogen (N ) purging were
2
ꢀ
ꢀ
employed for quenching the photogenerated electrons (e ), holes
+
(
h ), hydroxyl radicals ($OH) and superoxide radicals ($O
2
) gener-
Generally, the inhibi-
tion of photocatalytic activity aer adding the scavenger mean the
6
2,63
ated during the photoreduction process.
v
v
57
built-in eld in the heterojunction.
3
5,62,64,65
existence of corresponding active species.
Fig. 7 displays the
results of active species trapping experiments. Obviously, there are
three active species, namely, – e , $O and $OH – during photo-
Possible reaction mechanism and route from Cr(VI) to Cr(III)
ꢀ
ꢀ
2
In order to examine the reaction route to reducing Cr(VI) to reduction. The three active species play a rising role in photore-
ꢀ
< e . The addition of
2
ꢀ
Cr(III), a series of investigations were performed to compare the duction according to the order, $OH < $O
compositions and phases between the fresh LSTBM7 and used EDTA–2Na shows an abnormal phenomenon that the photocatalytic
LSTBM7. There are color changes (Fig. S7(a)†) that occurred activity was reinforced by introducing EDTA–2Na. This reinforce-
+
during light irradiation in LSTBM7, as well as a color change in ment was attributed to the reaction between EDTA–2Na and h ,
testing solution (simulated Cr(VI) wastewater at the beginning of a reaction that could reduce the recombination of photogenerated
photoreduction), accompanying a pH value change from 7.03 to electron–hole pairs, allowing more electrons, and therefore, more
ꢀ
7
.81 (Fig. S7(a)†). Before plugging into the simulated Cr(VI) $O
2
-s to participate in the reduction reaction, resulting in the rise
62,66
wastewater, the color of LSTBM7 and simulated Cr(VI) waste- in photocatalytic activity.
water were light yellow color and yellow (heavier than that of
LSTBM7), respectively. However, aer the process (including
LSTBM7 plugging into the simulated Cr(VI) wastewater, then
Fig. 7 Effects of various scavengers on the photoreduction rate of
Fig. 6 (a) PL spectra and (b) Nyquist plots of as-prepared samples.
Cr(VI) of LSTBM7.
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ꢀ
+
$
O2 + H / $HO
(7)
(8)
2
+
$
HO
2
+ $O + H / H
2ꢀ
2
O
2
+ O
2
+
H
2
O
2
+ Cr(III) + H / Cr(VI) + H
2
O + $HO
(9)
Meanwhile, because the energy difference (0.5 V) between
ꢀ
the VB of BMO and the potential of OH /$OH is smaller than
that (1.06 V) between the CB of LSTO and the potential of O
2
/
ꢀ
$
O
2
, the production of $OH is smaller. That is why during
photoreduction of Cr(VI) over LSTBM7, $OH play less role than
ꢀ
$
O
2
.
Fig. 8 Proposed mechanism for the separation of photogenerated
All in all, the above discussion conrmed the Z-scheme
charge carriers in LSTBM7.
mechanism in LSTBM7. The Z-scheme structured hetero-
junction and O in the junction have enhanced the lifetime of
v
photogenerated carriers and improved the transferability of
photoproduced carriers. On the basis of experimental results
and the discussion above, the process for Cr(VI) photoreduction
of LSTBM7 can be described as follows:
Based on the aforementioned results and analysis, a possible
energy band structure and reaction mechanism are put forward
in Fig. 8. Since Ecb and Evb of LSTO and BMO are (ꢀ1.39 V, 1.75
V), (ꢀ0.35 V, 2.49 V), respectively. LSTO and BMO could form
a Z-scheme heterojunction in LSTBM7. In the case of this Z-
scheme mechanism, the following inference can be obtained.
When BMO and LSTO contact closely, the electrons will move
from LSTO to BMO and form a built-in eld in the interface with
the direction of LSTO to BMO. The built-in eld direction has been
conrmed from the above-discussed XPS analysis. Under irradia-
tion, as shown in Fig. 8, the electrons in the VB of LSTO and BMO
would be excited and jump to their CB. Due to the existence of
a built-in electric eld, the photogenerated electrons of BMO
would move to LSTO and tend to recombine with photogenerated
holes in VB of LSTO, leaving behind photogenerated electrons in
the CB of LSTO and photogenerated holes in the VB of BMO,
resulting in a longer lifetime of photogenerated carriers. This
inference agrees well with the PL results.
A comparison of high-solution XPS between the samples
aer 20 min of photoreduction and 180 min of photoreduction
can give detailed evidence of the photoreduction route from
Cr(VI) to Cr(III) in Fig. S8.† It indicates that aer 20 min of
photoreduction, there appeared Cr(VI) on the surface of LSTBM7
particles, while at the end of photoreduction (180 min), only
Cr(III) existed on the surface of LSTBM7, suggesting that Cr(VI) in
the simulated wastewater was adsorbed on the surface of
LSTBM7 rst, then photo-reduced into Cr(III). In addition,
during the photoreduction, the produced Cr(III) tends to form
ꢀ
+
LSTBM7 + hn / e + h
(10)
(11)
(12)
(13)
(14)
42ꢀ
ꢀ
ꢀ
+ 5OH
CrO
+ 4H
2
O + 3e / Cr(OH)
3
ꢀ
2ꢀ
O
2
+ e / $O
2
ꢀ
ꢀ
ꢀ
CrO4 + 4H O + 3$O / Cr(OH) + 3O + 5OH
2
2
3
2
+
+
2
H O + h / $OH + H
Conclusion
4
+
3+
d
In summary, a novel heterogeneous Sr0.8La0.2(Ti1ꢀd Ti
Bi MoO composites were prepared via a facile two-step sol-
vothermal method. The energy band structure analyses showed
3
)O /
2
6
4
+
3+
that the heterojunction Sr0.8La0.2(Ti1ꢀd Ti
forms Z-scheme. The Z-scheme
Sr0.8La0.2(Ti Tid )O /Bi MoO photocatalyst showed excel-
d
)O
3 2 6
/Bi MoO
a
structured
4
+
3+
1ꢀd
3
2
6
lent photocatalytic reduction performance for Cr(VI) removal.
The photocatalytic reduction of the sample LSTBM7 is 1.9, 3.1
and 10.6 times that of BMO, LSTO and STO, respectively. The
dominant factors which led to improved photocatalytic effi-
ciency are the prolonged lifetime of photoproduced carriers and
improved transfer of carriers. The prolonged lifetime possibly
originated from the effective separation of photogenerated
electrons and holes by the build-in electric eld in the hetero-
3
Cr(OH) in case of pH 7.81 and adsorb onto the surface of the
photocatalyst to decrease the nucleation energy.
Based on all of the above results and analyses, a possible
reaction mechanism for Cr(VI) to Cr(III) over LSTBM7 composites is
proposed as follows. Actually, during the photoreduction, there
was a competition between the reaction of photogenerated elec-
4
+
3+
junction of Sr0.8La0.2(Ti
Tid )O /Bi MoO , as well as O
3 2 6 v
1ꢀd
which lost combined electrons in the junction. The improved
transfer of photogenerated carriers could be attributed to the
6
+
3+
9
trons with Cr(VI) (Cr /Cr , ꢀ0.13 V) and the reaction of photo-
increased O
v
which lose captured electrons in the built-in eld.
ꢀ
ꢀ
generated electrons with O
results of active species experiments, it can be seen that the pho-
2
2
(O /$O
2
, ꢀ0.33 V). But from the
It was also found that e plays a dominant role in reducing
Cr(VI) to Cr(III). Meanwhile, it was found that during the
togenerated electrons tend to directly reduce Cr(VI) to Cr(III) rather
3
photoreduction, the produced Cr(III) tends to form Cr(OH) in
the case of pH 7.81 and adsorb onto the surface of the photo-
catalyst to decrease the nucleation energy.
ꢀ
than form $O
2
. As for the phenomenon that $OH played less role
in photoreduction, the previous research can explain it. According
62,67
to previous research,
$OH can be attributed to the equilibrium
v
This work implies that inducing O -s on the higher-Fermi-
transmitting to Cr(VI) as shown in eqn (7)–(9),
level side of the Z-scheme heterojunction is an effective route
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to separating the photogenerated electrons and holes and 18 D. Chen, S. Wu, J. Fang, S. Lu, G. Zhou, W. Feng, F. Yang,
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Conflicts of interest
1
There are no conicts to declare.
Acknowledgements
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