The first synthesis of Bi self-doped Bi2MoO6-Bi2Mo3O12 composites and their excellent photocatalytic performance for selective oxidation of aromatic alkanes under visible light irradiation

Article abstract of DOI:10.1039/c7cc02890c

We report the synthesis of Bi self-doped Bi₂MoO₆-Bi₂Mo₃O₁₂ composites with heterostructures by a one-step method using Bi_3.64Mo_0.36O_6.55 as precursor. Among them, Bi_2.1MoO₆-Bi_2.1Mo₃O₁₂ shows excellent selectivity as well as recyclability in the oxidation of aromatic alkanes to aldehydes under visible light irradiation.

Full text of DOI:10.1039/c7cc02890c

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First synthesis of Bi self-doped Bi2MoO6-Bi2Mo3O12 composite and its
excellent photocatalytic performance for selective oxidation of
aromatic alkanes under visible light irradiation
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Lu-Na Songa+, Lang Chena+, Jie Hea, Peng Chena, Huang-Kai Zenga, Chak-Tong Aub, Shuang-Feng
Yina*
www.rsc.org/
We report the synthesis of Bi self-doped Bi2MoO6-Bi2Mo3O12
Through
phase
transformation,
heterostructured
composites with heterostructures by a one-step method using photocatalysts containing the same elements but different in phase
5
Bi3.64Mo0.36O6.55 as precursor. Among them, Bi2.1MoO6-Bi2.1Mo3O12 such as Bi3.64Mo0.36O6.55/Bi2MoO6 and Bi7O9I3/Bi5O7I6 could be
shows excellent selectivity as well as recyclability for the oxidation fabricated. In the past years, there were reports on the
of aromatic alkanes into ketones under visible light irradiation.
transformation of cubic Bi3.64Mo0.36O6.55 to γ-Bi2MoO6.7 As
illustrated in Scheme 1, the structures of Bi3.64Mo0.36O6.55 are built
on a fluorite-type Bi-O network incorporated with MoO42-. As the
With suitable band location and superior spectral properties,
nontoxic bismuth molybdate (α-Bi2Mo3O12, β-Bi2Mo2O9, γ-Bi2MoO6
and cubic Bi3.64Mo0.36O6.55) has drawn extensive attention in recent
years,1 and many novel and simple methods were used to have
them synthesized.2 However, the application of the individual
bismuth molybdate as photocatalyst is limited due to rapid
electron-hole recombination and slow migration of charge carriers.
It was reported that γ-Bi2MoO6 nanoplates exhibited superior
performance in selective photocatalytic oxidation reaction.2b And a
mixture of α-Bi2Mo3O12 and γ-Bi2MoO6 showed positive synergism:
the oxygen species formed on Bi2MoO6 migrate onto the surface of
Bi2Mo3O12 to generate active sites.3 Generally, a composite often
has better photocatalytic activity than the corresponding mixture
because of better interparticle electron transfer (IPET) between the
semiconductors.4 In view of that, it is desirable to achieve a
composite of α-Bi2Mo3O12 and γ-Bi2MoO6 with a large number of
nanostructured heterojunctions. Nevertheless, to our knowledge,
there is no report on a composite of this kind. It is hence of
2-
time for hydrothermal treatment increases, [Bi2O2]2+ and MoO4
2-
moieties self-organize and MoO4 starts to transform into
octahedron MoO6. According to Kongmark et al.,7b with the
reorganization of [Bi2O2]2+ and MoO6, the growth of γ-Bi2MoO6 can
be facilitated. On the other hand, α-Bi2Mo3O12 with defective
scheelite structure is composed of Bi3+ and MoO42-, and the Mo
surrounding is the same as that in Bi3.64Mo0.36O6.55.8 On the basis of
these information, we envisage that the introduction of Bi3+ to the
2-
synthetic process would refrain the transformation of MoO4 to
2-
MoO6. Simultaneously, a part of MoO4 combines with the added
Bi3+ to form Bi2Mo3O12
, in-situ forming a composite of
Bi2MoO6-Bi2Mo3O12. In this study, we observed that this is indeed
the case.
Generally, the electronic feature of a semiconductor is a
critical factor influencing its photocatalytic activity. Self-doping can
introduce a balanced amount of defects which can not only tune
the band structure of photocatalyst but also promote charge-carrier
separation without introducing any foreign elements.9 In the
significance to develop
a method for the preparation of
Bi2Mo3O12-Bi2MoO6 composites.
E-mail address: sf_yin@hnu.edu.cn
Scheme
1
Illustration of Bi2MoO6-Bi2Mo3O12 heterojunction
formation.
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present study, we deliberately introduced an excess amount of Bi3+ that Bi3+ is evenly doped in the two bismuth molybdates, one can
for the tuning of valence band position. It was observed that the deduce that the amount of Bi3+ doping in Bi2MoO6 and Bi2Mo3O12 is
Bi2.1Mo3O12-Bi2.1MoO6 composite exhibits photocatalytic activity 6.4% and 4.6%, respectively. It is hence regarded that the
significantly higher than that of the individual bismuth molybdate composition of BMO-1.0 is roughly 58.6% Bi2.1MoO6 and 41.4%
components towards the partial oxidation of toluene into Bi2.1Mo3O12
.
benzaldehyde. Detailed information for the experimental process is
provided in the Electronic Supplementary Information, ESI†.
X-ray diffraction (XRD) with refinement was applied to
determine the structure of the as-prepared bismuth molybdate
(BMO) samples. As illustrated in Fig. 1a and Fig. S1 (ESI† ),
The morphology and structure of BMO-1.0 was investigated by
TEM analysis (Fig. 1b and c). The composites transferred from
Bi3.64Mo0.36O6.55 nanoparticles are of flake-like morphology (Fig. S3).
The corresponding HRTEM image of the nanocomposite reveals
lattice fringes with d-spacing of 0.264 and 0.308 nm, corresponding
Bi3.64Mo0.36O6.55 (JCPDS43-0446) was indeed synthesized following to the (220) planes of Bi2MoO6 and (104) planes of Bi2Mo3O12
the method reported by Lin et al.10 The Bi/Mo mole ratio of
,
respectively (Fig. 1c). The observed d-spacing values, however, are
slightly larger than those of Bi2MoO6 and Bi2Mo3O12 in perfect
crystallinity, and the enlargement is plausibly due to the
accommodation of extra Bi3+ in the lattice.12 The results confirm
that heterostructure Bi2.1Mo3O12-Bi2.1MoO6 composites can be
fabricated by this synthetic approach.
Bi3.64Mo0.36O6.55 was determined to be 9.81 by ICP analysis (Table S1,
ESI † ), which is close to the theoretical value. When cubic
Bi3.64Mo0.36O6.55 was hydrothermally treated at 160 °C for 24 h in
acidic environment as previously reported,7 it transformed to
phase-pure γ-Bi2MoO6 (JCPDS21-0102). However, with the addition
of Bi3+ to the precursor under acidic conditions, extra peaks
assignable to α-Bi2Mo3O12 (JCPDS23-1033) are clearly detected, and
there is a decrease of bismuth content in the catalyst as confirmed
by ICP analysis (Table S1, ESI†). The results demonstrate that
addition of Bi3+ in the hydrothermal synthesis has an effect on the
crystal phase of products. The Bi/Mo ratio of BMO-x increases with
the increase of Bi3+ addition which infers the doping of Bi3+ into the
composites. Meanwhile with increasing Bi3+ doping, the
characteristic (020) peaks of γ-Bi2MoO6 in the 2θ = 10–11◦ range
and the characteristic (211) peaks of α-Bi2Mo3O12 in the 2θ = 30–31◦
range slightly shift toward lower 2θ value (Fig. S1b and c, ESI†). The
XPS analysis was conducted to investigate the composition and
chemical states of the BMO-x samples. In the high-resolution Bi 4f
spectrum of BMO-1.0 (Fig. S4a, ESI†), Bi 4f5/2 and Bi 4f7/2 at binding
energy of 164.6 and 159.0 eV are in line with those of Bi3+ 13
The
.
new peaks at 160.2 and 165.9 eV can be assigned to Bi(3+n)+ (n=1 or
2) of nonstoichiometric bismuth molybdate.14 Increasing the extent
of Bi3+ addition, the Bi 4f peaks downward shifts slightly. It is
understandable because with the generation of molybdenum
vacancies, elementary bismuth is surrounded by more oxygen
species that are higher in electron density than molybdenum
species.4a Moreover, the O 1s peaks at 529.9 and 531.8 eV can be
phenomena can be attributed to the formation of molybdenum ascribed to O–Mo and O–Bi entities of Bi2MoO6,11 while peaks at
vacancies.11 Despite the composition of BMO-1.0 cannot be 530.4 and 532.6 eV can be assigned to O–Mo and O–Bi bonds of
accurately obtained, a semi-quantitative analysis is conducted by Bi2Mo3O12, respectively.15 The slight shifts in peak position indicate
that the formation of heterojunctions influences the environment
of chemical bonding in the crystals. Moreover, the percentage of O–
Bi entities increases for BMO-2.5, suggesting that there is
strengthening of O–Bi bond as a result of the self-doping process
(Fig. S4b, ESI†).
means of XRD (Fig. S2, Table S2, ESI†). The composition of BMO-1.0
can be regarded as roughly 58.6% Bi2MoO6 and 41.4% Bi2Mo3O12
before self-doping. The Bi/Mo ratio increases from 1.45 of undoped
composite to 1.63 of BMO-1.0, implying that about 11.0% of
bismuth is doped in the crystal lattice. Then with the assumption
In order to understand the change of band structure after
self-doping, we determined the position of valence band maximum
(VBM). As shown in Fig. 1d, the VBM value of Bi2MoO6, Bi2.1MoO6,
Bi2Mo3O12 and Bi2.1Mo3O12 is 2.1, 2.2, 2.4 and 2.6 eV, respectively,
suggesting that the presence of molybdenum vacancies causes
decrease of Bi2MoO6 and Bi2Mo3O12 VBM values. The corresponding
band gap energy is 2.40, 2.50, 2.49 and 2.67 eV respectively (Fig.
S5a, ESI†). According to the empirical formula Eg = EVB – ECB, the
conduction band minimum (CBM) of Bi2MoO6, Bi2.1MoO6, Bi2Mo3O12
and Bi2.1Mo3O12 is -0.3, -0.3, -0.09 and -0.07 eV, respectively,
demonstrating that the molybdenum vacancies almost does not
result in the occurrence of distortion in the CBM. More importantly,
at the heterojunctions photogenerated electrons can easily transfer
from the CB of Bi2.1MoO6 to that of Bi2.1Mo3O12, whereas the
photogenerated holes in the VB of Bi2.1Mo3O12 flow into the VB of
Bi2.1MoO6, thus forming a long-lived electron-transfer state, which
is beneficial for the enhancement of overall photocatalytic activity
and in line with the following PL analysis and photoelectrochemical
Bi2MoO6 (JCPDS21-0102)
:
(d)
(a)
Bi2Mo3O12
Bi2.1Mo3O12
Bi2MoO6
*
*
@@:Bi2Mo3O12 (JCPDS23-1033)
@
Bi2.1MoO6
@
*
@
*
@
@@
@
@
@
@
*
@
**
*
@
@
*
*
@
@
@
@
-3 -2 -1
0
1
2
3
4
5
6
7
8
9
10
10
20
30
40
50
60
70
Binding energy (eV)
2-Theta (degree)
Fig. 1 (a) XRD pattern, (b, c) TEM and HRTEM images of BMO-1.0, (d)
valence band XPS spectra of the synthesized samples.
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data. It is observed that the BMO-x samples show increased and the increase of vacancy density even in relatively small amount
could enhance photocatalytic activity. However, vacancies in excess
can result in the formation of centers for the recombination of
charge carriers.20 In addition, the effect of substrate concentration
on the formation rate of benzaldehyde was investigated (Fig. S6, ESI
†). With the rise of substrate concentration, the formation rate of
benzaldehyde shows a trend of ascending followed by leveling off.
As revealed in Fig. S7a (ESI†), there is no significant loss of
absorption in the visible light region with the increase of“x”value
that can be attributed to Bi self-doping,16 and the corresponding
band gap values of BMO-x are determined to be 2.49, 2.41, 2.36,
2.27, 2.22 (Fig. S5b, ESI†). Combining the results of XRD, XPS, TEM
analysis and Bi/Mo measurement by ICP, it is inferred that Bi can be
successfully doped in Bi2MoO6-Bi2Mo3O12 under mild conditions.
In view of the special structure and optical properties of
self-doped Bi2Mo3O12-Bi2MoO6, we used the selective oxidation of photoactivity across a test of 5 runs, indicating the high stability of
aromatic alkanes as a reaction model to examine its photocatalytic the BMO-1.0 photocatalyst. The results are further confirmed by
performance. The selective activation and transformation of sp3 C– careful characterization of the catalysts before and after the
H bonds in alkanes to produce value-added products is of great photocatalytic process using XPS and XRD techniques. There is no
importance.17 Compared to conventional thermocatalytic
change of defect population (Fig. S7b, ESI†) and phase structure (Fig.
approaches, semiconductor-based heterogeneous photocatalysis
features the unique advantages of operation at room temperature
and using solar light as driving source.18 Previously, we reported a
green process for alkane selective oxidation using flower-like
Bi2WO6 as active photocatalyst,19 but its poor stability restricts its
application. Therefore, developing a more efficient and stable
catalyst is worth studying.
S7c, ESI†) of the photocatalyst during the reaction.
It was found that the selective oxidation of toluene derivatives
over BMO-1.0 is also effective (Scheme S1, ESI†). Compared with
the oxidation of xylene, the transformation of chlorotoluene is less
significant, and the phenomenon can be associated with the
presence of electron withdrawing group (–Cl) that hinders the
oxidation of the methyl group, resulting in the methyl group of
chlorotoluene less active than that of xylene. For the same reason,
the selectivity to chlorobenzaldehyde is nearly 100%, whereas in
the case of xylene, the generation of corresponding alcohols as
by-products is observed. In all cases of using BMO-x, there is no
detection of carbon dioxide, indicating the absence of over
oxidation.
Herein we tested the photocatalytic activity of BMO-x to
establish how the extent of self-doping can exert an influence on
the photocatalytic activity of Bi2Mo3O12-Bi2MoO6 (Table 1). Results
of entries 1–3 indicate the oxidation is a photocatalytic process and
O2 is indispensable. It can be found that all five samples exhibit
fairly high selective oxidation efficiency (entries 6–10), much better
than those of their components. Among the BMO-x catalysts,
BMO-1.0 shows the highest formation rate (508 μmol·g-1·h-1) as well
as toluene conversion (15.2%), a performance even better than that
of flower-like Bi2WO6 under the same reaction conditions. It is
noted that the conversion first increases with the rise of Bi
self-doping amount and reaches a maximum at x = 1.0 and then
decreases with further rise of Bi3+ doping. That is because the
lowering of VBM value would result in a change of oxidation ability,
In order to understand the reason why BMO-1.0 shows the
enhanced photocatalytic activity, we performed experiments of
selective oxidation of toluene over various BMO catalysts (Fig. S8,
ESI†). The high photocatalytic activity of Bi2.1MoO6 can be ascribed
to the change of valence band position as well as to the fast
charge-carrier separation caused by molybdenum vacancies. The
fact that even much higher photocatalytic activity is observed over
BMO-1.0 (benzaldehyde formation rate
=
508.0 ꢀmol·gꢁ1·h-1)
suggests that the excellent activity could be related to its special
heterojunction structure as well as to the molybdenum vacancies
which are indispensable for the activation of methyl C–H bond of
toluene. In other words, together the two factors work to facilitate
the separation/transfer of charge carriers and lengthen the lifetime
of photogenerated electron-hole pairs of individual bismuth
molybdates. This inference has been verified by comparative
analysis of the photoelectrochemical and photoluminescence (PL)
results collected over BMO-x (Fig. S9, ESI†). It was found that the
photocurrent generated in BMO-1.0 is significantly higher than that
in other BMO-x samples, suggesting that photogenerated electrons
are efficiently produced in BMO-1.0 under visible light irradiation
(Fig. S9a, ESI†). On the other hand, the smallest arc radius of the
electrochemical impedance spectroscopic (EIS) plot of BMO-1.0
demonstrates the better charge-transfer ability of BMO-1.0 among
the BMO-x samples (Fig. S9b, ESI†).21 As shown in Fig. S9c, BMO-1.0
exhibits lower photoluminescence intensity, implying the
recombination of electron-hole pairs in BMO-1.0 is effectively
suppressed.22 With x >1.0, the photoluminescence intensity of
Table 1 Selective oxidation of toluene into benzaldehyde over the
as-prepared samplesa.
Formation rate Conv.
Sel.
(%)
Entry
Catalyst
(μmol·g-1·h-1)
(%)
-
1
2b
3c
4
none
BMO-1.0 (N2)
BMO-1.0 (dark)
Bi2.1MoO6
-
-
-
-
-
-
-
-
185.4
30.0
383.3
508.0
348.4
5.5
0.9
11.5
15.2
10.4
97.0
98.6
97.8
98.5
98.3
5
Bi2.1Mo3O12
BMO-0.5
6
7
BMO-1.0
8
BMO-1.5
9
BMO-2.0
BMO-2.5
363.7
103.3
330.7
7.1
3.0
9.9
97.3
97.7
96.2
10
11
Flower-like Bi2WO6
a
Photocatalyst (100 mg), substrate (1 mmol), benzotrifluoride (3
mL), room temperature, O2 flow rate (3 mL·min-1), visible-light
irradiation (λ≥400 nm, 3 h); replacing O2 with N2; without
b
c
irradiation.
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Notes and references
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Scheme
2 Schematic of the plausible mechanism for the
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photocatalytic selective oxidation of toluene to benzaldehyde over
BMO-1.0 under visible-light irradiation
BMO-x increases, indicating the formation of centers suitable for
the recombination of charge carriers. In addition, the BET surface
area of BMO-x samples is much larger than that of individual
bismuth molybdate (Table S3, ESI†). On the basis of the above
analysis, we proposed that the heterojunction, vacancies as well as
high specific surface area of BMO-1.0 play a critical role in the
enhancement of photocatalytic performance.
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–
+
– 23
•
(O2 ), holes (h ), and electrons (e ), as shown in Fig. S10, ESI†.
Based on the discussions in ESI † , reaction mechanism is
a
tentatively proposed (Scheme 2). Under visible light irradiation,
there is the generation of electron-hole pairs, and the toluene
adsorbed on the surface of BMO-1.0 is oxidized to cationic radicals
by the positive holes. The photogenerated electrons from the CB of
Bi2.1MoO6 migrate to the CB of Bi2.1Mo3O12, thus forming a
long-lived electron-transfer state. Meanwhile, the electrons react
with adsorbed O2 to give activated oxygen species (e.g., O2 –). The
•
activated oxygen species then selectively oxidize the cationic
radicals, finally leading to the formation of benzaldehyde.
In summary, for the first time a simple protocol was developed
for the synthesis of a Bi2MoO6-Bi2Mo3O12 heterostructure by adding
Bi3+ to control the phase transformation of Bi3.64Mo0.36O6.55. And the
band structure of bismuth molybdates can be tuned according to
the extent of Bi self-doping. The synthesized Bi2.1MoO6-Bi2.1Mo3O12
composite is apt for the oxidation of saturated sp3 C–H bonds using
dioxygen as oxidant, showing excellent visible-light-driven
photocatalytic activity, stability, as well as selectivity. The good
performance is attributed to the formation of heterostructure and
molybdenum vacancies generated in the self-doping process. This
method of using self-doped Bi2MoO6-Bi2Mo3O12 endowed with
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This project was financially supported by the NSFC (Grants
21401054, 21476065 and 21671062), Hunan Provincial Natural
Science Foundation (Grant 2015JJ3033), and the Hunan Provincial
Science and Technology Project (2015JC3051). C. T. Au thanks the
HNU for an adjunct professorship.
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ChemComm
View Article Online
DOI: 10.1039/C7CC02890C
Bi self-doped Bi2MoO6-Bi2Mo3O12 composites were first synthesized by a one-step method and
they showed excellent photocatalytic performance for selective oxidation of aromatic alkanes
under visible light irradiation.