Synthesis of Ag/AgBr/Bi4O5Br2 Plasmonic Heterojunction Photocatalysts: Elevated Visible-light Photocatalytic Performance and Z-scheme Mechanism

Article abstract of DOI:10.1002/cctc.201900529

The novel photocatalysts of Ag/AgBr/Bi₄O₅Br₂ layered microspheres (AAB-LMs) were firstly constructed via the in-situ deposition and visible-light photoreduction approach. Their compositions, structures, morphologies and optical properties were characterized by various techniques. The photocatalytic efficiencies of AAB-LMs-10 were 28 and 3.4 times higher than those of Bi₄O₅Br₂-LMs towards degrading acid orange II and aniline, respectively. The superior photocatalytic behavior of AAB-LMs benefited from efficient separation and transfer of photo-generated charges, which was mainly credited to the surface plasmon resonance effect of Ag nanoparticles and the synthetic interaction among Ag, AgBr and Bi₄O₅Br₂. O₂ ^? and h⁺ were characterized as the predominant photoactive species in this AAB-LMs-photocatalysis system. Moreover, due to the presence of radical species Br⁰, which was verified by the detection of bromoaniline and dibromoaniline during the photocatalytic degradation process of aniline, Z-scheme mechanism rather than the conventional heterojunction mode was more suitable to explain the enhanced photocatalytic performance of AAB-LMs.

Full text of DOI:10.1002/cctc.201900529

Accepted Article
Title: Synthesis of Ag/AgBr/Bi4O5Br2 Plasmonic Heterojunction
Photocatalysts: Elevated Visible-light Photocatalytic Performance
and Z-scheme Mechanism
Authors: Shanshan Ding, Mengshu Han, Shaogui Yang, and Cheng
Sun
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: ChemCatChem 10.1002/cctc.201900529
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A Journal of
www.chemcatchem.org

10.1002/cctc.201900529
ChemCatChem
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Synthesis of Ag/AgBr/Bi4O5Br2 Plasmonic Heterojunction
Photocatalysts: Elevated Visible-light Photocatalytic Performance
and Z-scheme Mechanism
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Shanshan Ding‡[a], Mengshu Han‡[a,b], Yuxuan Dai[a], Shaogui Yang*[c], Danjun Mao[a],
Huan He[c], Cheng Sun*[a]
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‡ These authors contribute equally to this article.
[a] S. S. Ding‡, M. S. Han‡, Y. X. Dai, D. J. Mao, Prof. Dr. C. Sun
State Key Laboratory of Pollution Control and Resource Reuse, School of the
Environment, Nanjing University, Nanjing 210046 (P. R. China)
[b] M. S. Han‡
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Key Laboratory of Information and Computing Science Guizhou Province, Guizhou
Normal University, Guiyang 550001 (P. R. China)
[c] Prof. Dr. S. G. Yang, Prof. Dr. H. He
School of the Environment, Nanjing Normal University, Nanjing 210046 (P. R. China)
* Corresponding Authors:
Prof. Dr. C. Sun, E-mail addresses: envidean@nju.edu.cn
Prof. Dr. S. G. Yang, E-mail addresses: yangdlut@126.com
20 ABSTRACT
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The novel photocatalysts of Ag/AgBr/Bi4O5Br2 layered microspheres (AAB-LMs)
were firstly constructed via the in-situ deposition and visible-light photoreduction
approach. Their compositions, structures, morphologies and optical properties were
characterized by various techniques. The photocatalytic efficiencies of AAB-LMs-10
were 28 and 3.4 times higher than those of Bi4O5Br2-LMs towards degrading acid
orange II and aniline, respectively. The superior photocatalytic behavior of AAB-LMs
benefited from efficient separation and transfer of photo-generated charges, which was
mainly credited to the surface plasmon resonance effect of Ag nanoparticles and the
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synthetic interaction among Ag, AgBr and Bi4O5Br2. •O2 and h+ were characterized as
the predominant photoactive species in this AAB-LMs-photocatalysis system.
Moreover, due to the presence of radical species Br0, which was verified by the
detection of bromoaniline and dibromoaniline during the photocatalytic degradation
process of aniline, Z-scheme mechanism rather than the conventional heterojunction
mode was more suitable to explain the enhanced photocatalytic performance of AAB-
LMs.
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Introduction
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The rapid progression in the world economy and urbanization as well as people's
unfailing demand for energy storage enormously influence on global environment.
What is more, there is a serious water pollution resulting from various assortment of
toxic and harmful contaminants discharged into aqueous environment for uncontrolled
anthropogenic activities.[1-3] Herein, seeking an effective, clean and low-cost method to
reclaim the polluted water including wastewater has become a hotspot.[4-6]
Semiconductor based photocatalysis, one of the most promising advanced oxidation
technologies, can effectively decompose some organic pollutants with structural
stability, high toxicity and anti-biodegradation.[7-9] Therefore, the exploration and
application of novel photocatalysts with high efficiency have been urgently needed.
During recent decades, visible-light-driven (VLD) photocatalysts have been a
prospective candidate for their efficient light absorption in the full spectrum of solar
energy.[10-12] Bismuth oxybromide (BiOBr) exhibits commendable visible-light
photocatalytic performance with highly anisotropic layered structure and suitable band
gap (Eg = 2.7 eV).[13-14] Despite these advantages, the too positive conduction band
minimum (CBM) of BiOBr and high recombination of photo-induced charges still limit
its practical application.[15] The result of density functional theory (DFT) indicates that
the conduction band (CB) of BiOBr is mainly made up of Bi 6p orbits,[16] thus some
researchers have suggested that the CB position can be adjusted by increasing the
content of bismuth in BiOBr to improve the hybridization of CB and decrease its band
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gap, which is in favor of charge transfer and visible-light absorption of Bi-O-Br
semiconductor photocatalysts such as the construction of Bi5O7Br,[17] Bi3O4Br[18] and
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Bi24O31Br10.[19] Among them, Di et al. reported that the photocatalytic efficiency of as-
prepared Bi4O5Br2 ultrathin nanosheets was 1.9 times higher than that of BiOBr
ultrathin nanosheets towards degrading ciprofloxacin.[20] Recently, our group have
successfully synthetized a novel Bi-rich Bi4O5Br2 with enhanced visible-light
absorption in an ionic liquid-in-water (IL/W) system, in which the ionic liquid could be
served as bromine source, dispersing agent and template synchronously. The
synthesized Bi4O5Br2 layered microspheres (Bi4O5Br2-LMs) performed excellent
photocatalytic activities for the degradation of o-phenylphenol and norfloxacin than
those of BiOBr nanosheets.[15] All these reports sufficiently demonstrate that Bi4O5Br2-
LMs has an outstanding photocatalytic performance in the removal of environmental
pollutants. However, since the single-component semiconductor photocatalysts have
the lower light utilization efficiency and higher recombination rate of photo-generated
electron-hole pairs than that of composite ones, which economically restrict their
further practical application.[21-23] Therefore, the construction of heterojunction
photocatalysts has rewarded extra consideration due to their unique optical and
electronic properties such as reinforced photo-absorption and photo-induced charge
transfer. Among them, Z-scheme heterojunction photocatalysts have been
incrementally studied, in which the enhanced photo-absorption efficiency, high charge-
separation and strong redox capacity can be realized synchronously through the
formation of Z-scheme heterostructure.[11, 24] For example, Zou et al. have prepared that
the ameliorated photocatalytic activity of g-C3N4/Au/C-TiO2 hollow spheres on H2
production could be attributed to the Z-scheme charge transfer mechanism.[25] Liu et al.
also have reported that the carbon cloth-supported MoS2/Ag2S/Ag3PO4 performed
enhanced photocatalytic efficiency in the degradation of rhodamine B via the direct Z-
scheme photocatalytic system.[26] These results manifest that Z-scheme mechanism is
in great significance to improve the photocatalytic activity of semiconductor in the fuel
production and water treatment. Nevertheless, constructing Z-scheme heterojunction
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photocatalysts based on Bi4O5Br2 to further improve the catalytic activity is yet to be
reported.
Lately, extensive concern has been focused on plasmonic photocatalysts, such as
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Ag/AgX (X = Cl, Br, I), for their high visible-light photocatalytic performance.[27-28]
Because the conduction electrons of Ag oscillate coherently with the oscillating electric
field after exciting by the light, the generated surface plasmon resonance (SPR) effect
can further expand the light response of AgX to the visible-light region.[28] Although
the instability and photo-sensitivity of AgX under light irradiation hinder their
commercialization, the construction of composite photocatalysts combined Ag/AgX
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with other semiconductors was proposed to overcome these weaknesses and boost their
[29]
photocatalytic behavior, such as Ag/AgI/BiOI,[27] Ag/AgBr/g-C3N4
and
Ag/AgCl/TiO2.[30] Specifically, most of the ternary semiconductor photocatalysts
anchored with Ag/AgX have complex preparation process, and need to be added other
chemical reagents as the halogen source during the synthesis process.[31-33] Therefore,
anchoring Ag/AgBr particles on the surface of Bi4O5Br2 can overcome the complexity
of preparation, because Bi4O5Br2 can serve as the template to offer bromine source for
the formation of AgBr, and Ag0 can also be generated simultaneously. The in-situ
growth of Ag/AgBr on Bi4O5Br2 via photoreduction can accomplish tight coupling
between Ag/AgBr on Bi4O5Br2, which is of great significance for the separation and
transfer of photo-induced charge. On the other hand, the Br- of AgBr can be oxidized
by holes to produce Br0, which could act as a reactive radical species to participate in
the degradation process of pollutants.[33] All of these inspire that synthesizing
Ag/AgBr/Bi4O5Br2 plasmonic heterojunction photocatalysts can efficaciously improve
the visible-light photocatalytic efficiency.
In this study, Ag/AgBr/Bi4O5Br2 layered microspheres (AAB-LMs) with Z-
scheme heterostructure were firstly fabricated via a facile in-situ deposition and
photoreduction approach. In the Bi4O5Br2 preparation, the ionic liquid 1-Octyl-3-
methylimidazolium bromide and the surfactant Triton X-100 were used as template and
stabilizer, respectively. Furthermore, the Bi4O5Br2-LMs could offer the bromine for the
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formation of AgBr, Ag+ could be reduced to Ag0 during the visible-light irradiation. The
obtained materials Ag/AgBr/Bi4O5Br2-LMs with different photoreduction time of 10
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min, 40 min, 90 min and 180 min were designated as AAB-LMs-10, AAB-LMs-40,
AAB-LMs-90 and AAB-LMs-180, respectively. The compositional, structural and
morphology characterization as well as optical properties of as-synthesized samples
were also analyzed. The photocatalytic proficiency of AAB-LMs was investigated via
the degradation of acid orange II (AO7), 4-tert-butylphenol (PTBP) and aniline under
visible-light. Moreover, the photocatalytic enhancement mechanism and structure-
activity relationships of AAB-LMs were discussed in detail. At last, the conjectural
degradation pathways of aniline in this AAB-LMs-photocatalysis system were also
proposed.
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13 Results and Discussion
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Compositional and Structural Information
XRD was utilized to characterize the crystalline structure of the samples, and the
resulting XRD patterns are shown in Figure 1. Clearly, the diffraction peaks of as-
prepared Bi4O5Br2-LMs were indexed to the monoclinic Bi4O5Br2 (JCPDS Card No.
37-0699).[20] The diffraction peaks of AAB-LMs which located at 31.0°, 44.3°,
64.5°and 73.3° belonged to (200), (220), (400) and (420) facets of AgBr (JCPDS Card
No. 06-0438), respectively,[28, 34] while the diffraction peaks at 44.3° and 64.4° were
attributed to the (200) and (220) facets of Ag0 (JCPDS Card No. 65-2871),
respectively.[30] It is noticeable that although the diffraction peaks intensity of samples
performed much sharper by extending the photoreduction time, the peak of Ag0 at 38.1°
was not be detected, which might be due to the low content, small size and
34]
homogeneous dispersion of Ag0.[28,
The mentioned XRD characterization
demonstrated that the crystallinities of the as-fabricated composites were getting better
gradually and Ag/AgBr had been successfully generated on the surface of Bi4O5Br2-
LMs. Moreover, no diffraction peak of impurities in crystallographic measurement was
found, indicating that no new crystalline phase was formed during the whole
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preparation process and the crystalline phase structure of Bi4O5Br2-LMs was not
changed after the introduction of Ag and AgBr.
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Figure 1. XRD patterns of as-prepared materials treated with different visible-light
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photoreduction time.
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The elemental composition and chemical states of as-synthesized materials were
characterized by XPS. Figure 2a depicts the survey scan spectrum of AAB-LMs-10,
demonstrating that the composite mainly consists of Bi (157.8 eV，163.2 eV), O
(530.8eV), Br (67.3 eV) and Ag (366.5 eV，372.6 eV). The high-resolution spectra of
Bi 4f, O 1s, and Br 3d are shown in Figure 2b-d, respectively. Specifically, the binding
characteristic orbital energies of Bi 4f with peaks located at 157.7 eV and 163.1 eV
closely conformed to Bi 4f7/2 and Bi 4f5/2 of Bi ions with a valence of +3.[35] The broad
peaks at 528.6 eV and 531.0 eV were ascribed to O 1s of AAB-LMs-10, which was
mainly due to the lattice oxygen and the surface absorbed H2O or hydroxyl group (-OH)
of the sample.[21] The characteristic peaks of 67.3 eV and 68.6 eV corresponded to the
Br 3d5/2 and Br 3d3/2, indicating that Br ions presented a valence of -1. In order to further
verify the presence of Ag and AgBr, the high resolution XPS spectra of Ag 3d were
also conducted. As depicted in Figure S1, the characteristic peaks of Ag 3d were mainly
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detected at 367.0 eV and 372.5 eV, corresponding to Ag 3d5/2 and Ag 3d3/2, which could
be further divided into four peaks. 366.5 eV and 372.6 eV belonged to Ag+ 3d5/2 and
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Ag+ 3d3/2 respectively, whereas 366.8eV and 373.1 eV ascribed to Ag0 3d5/2 and Ag0
3d3/2 respectively. The above observation further validated the existence of Ag with
different valences of as-prepared photocatalysts.[22] Moreover, comparing the
characteristic peaks belonging to the Ag 3d5/2 and Ag 3d3/2 of the AAB-LMs composites
treated with different photoreduction time (Figure S1), it is easy to find that the shift of
these peaks to the left or right occurred, which may be due to the various amount of
Ag/AgBr loaded on the Bi4O5Br2-LMs or the different surrounding chemical
environment during the preparation process. In addition, the semi-quantitative analysis
of the molar ratio of Ag to AgBr in the AAB-LMs was also performed using the XPS
spectra of Ag 3d peak area (Table 1). It indicates that the content of Ag on the surface
of the composites decreased with the extension of the photoreduction time of the
Bi4O5Br2-LMs.
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（b）Bi 4f
（a）Survey
（c）O 1s
（d）Br 3d
528.6 eV
Br 3d5/2
531.0 eV
Br 3d3/2
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Figure 2. The XPS spectra of (a) survey, (b) Bi 4f, (c) O 1s and (d) Br 3d for as-
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synthesized AAB-LMs-10 composite.
Table 1 XPS Ag 3d peak area analysis of AAB-LMs with different photoreduction time.
Photocatalysts
AAB-LMs-10
Peak position (eV)
366.460
366.814
372.585
373.103
365.652
366.241
371.684
372.079
366.260
367.054
372.377
372.517
365.800
366.405
371.840
372.663
Peak area
1988.220
677.284
948.771
697.519
1826.777
673.654
1011.960
630.225
924.560
357.234
646.096
334.204
932.679
448.519
534.151
63.849
Peak attribution Ag/AgBr ratio
Ag+
Ag0
Ag+
Ag0
Ag+
Ag0
Ag+
Ag0
Ag+
Ag0
Ag+
Ag0
Ag+
Ag0
Ag+
Ag0
46.81%
45.93%
44.02%
34.93%
AAB-LMs-40
AAB-LMs-90
AAB-LMs-180
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Morphology and Textural Properties
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The morphologies of the Bi4O5Br2-LMs andAAB-LMs samples were observed by SEM.
As shown in Figure 3a-e, Bi4O5Br2-LMs with rough surfaces maintained its integrity
without much morphological change after the Ag/AgBr particles anchored. Since the
microsphere was composed of many sheets stacked on top of each other, some extra
pieces around the sphere could be found from the SEM images. Besides, many
nanoparticles could be obviously discovered on the outer surfaces of the Bi4O5Br2-LMs
owing to the formation of Ag/AgBr particles (20-80 nm). Some of the Ag/AgBr
nanoparticles were deeply embedded or attached on the surface of Bi4O5Br2-LMs,
confirming the combination of Bi4O5Br2-LMs with the Ag and AgBr. With respect to
the unembellished Bi4O5Br2-LMs, the particle sizes were mainly about 2.2 µm, whereas
the average size of AAB-LMs gradually decreased in order by prolonging
photoreduction time:AAB-LMs-10 (~1.1 µm) >AAB-LMs-40 (~0.9 µm) > AAB-LMs-
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90 (~0.75 µm) ≈ AAB-LMs-180 (~0.76 µm). Considering the bromine of AgBr mainly
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came from Bi4O5Br2-LMs and the exfoliation of the sheet structure of the Bi4O5Br2-
LMs occurred on the surface, the decrease of particle size of the overall AAB-LMs
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seemed reasonable, which was consistent with the increasing AgBr proportion of the
AAB-LMs mentioned in the XPS characterization. The SEM-EDS line scan and
elemental mapping spectra were also recorded to gain insight of the surface element
composition and distribution of the as-fabricated materials. Figure 3f depicts the SEM-
EDS spectrum of AAB-LMs-10, indicating that Bi, O, Br and Ag elements distributed
throughout the composite. Moreover, the SEM-EDS elemental mapping (Figure 4)
illustrates that all elements were uniformly distributed on the surface of the AAB-LMs-
10. All characterizations disclosed that the Ag/AgBr particles had been successfully
loaded during the photoreduction process.
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（a）
（b）
（c）
2 µm
2 µm
2 µm
（d）
（e）
（f）
2 µm
2 µm
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Figure 3. The SEM images of the (a) Bi4O5Br2-LMs, (b) AAB-LMs-10, (c) AAB-LMs-
40, (d) AAB-LMs-90 and (e) AAB-LMs-180 samples, and (f) SEM-EDS spectrum of
AAB-LMs-10 material.
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Figure 4. (a) SEM-EDS elemental mapping of AAB-LMs-10, (b) - (e) the EDS
elemental mapping of Bi, O, Br and Ag.
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TEM can be used to obtain more microstructural information about the as-prepared
material. In Figure 5a, the as-synthesized AAB-LMs-10 mainly presents the
microsphere structure with a certain degree of hollowness and layered structure, and
this distinctive morphology is extremely cooperative throughout the whole
photocatalytic process because of the continuous diffraction and reflection of the
absorbed light beam inside the material, thus further increasing the photon capture
rate.[15] Moreover, combined with the SEM characterization, convex granular structural
materials assembled on the surface of Bi4O5Br2-LMs might be the Ag/AgBr particles.
To prove the conjecture further, high-resolution transmission electron microscopy
(HRTEM) analysis was carried out, which is shown in Figure 5b. The interplanar
spacing of 0.333 nm and 0.261 nm corresponded to the crystallographic plane of AgBr
(111)[36] and Bi4O5Br2-LMs (220),[15] respectively. The lattice fringes with the spacing
of 0.236 nm referred to the cubic Ag (111) crystal plane,[28] which can be regarded as a
Z-scheme bridge to transfer the photo-induced electrons and holes. This
characterization adequately confirmed that metallic Ag and AgBr coexisted on the
surface of Bi4O5Br2-LMs and they tightly contacted each other, which could facilitate
the separation and transportation of photo-induced charges.
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Figure 5. (a) TEM and (b) HRTEM images of AAB-LMs-10.
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The BET surface area of the catalyst is also a non-negligible factor affecting its
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photocatalytic activity. Figure 6 displays the N2 adsorption-desorption isotherms and
BJH pore size distribution curves of the Bi4O5Br2-LMs and AAB-LMs-10. Both N2
adsorption-desorption isotherms exhibited the Ⅳ type isotherm, which was typical for
the mesoporous materials. The distinct hysteresis loops for both Bi4O5Br2-LMs and
AAB-LMs-10 in the relative pressure P/P0 range of 0.7 to 1.0 indicated that there was
the pore size distribution. The surface textural properties of as-prepared materials
obtained from BET characterization are listed in Table 2. It illustrates that all samples
were filled with many mesopores. Moreover, compared to that of the uncovered
Bi4O5Br2-LMs, a slight decrease in BET specific surface area of AAB-LMs with
prolonging the photoreduction time happened, which might owe to the increased
Ag/AgBr nanoparticles that entered the inner surface of Bi4O5Br2-LMs and occupied
their mesopores. This phenomenon was also accompanied by a reduction of the active
sites on the surface of AAB-LMs, leading to an attenuation in the catalytic performance
of the photocatalyst.
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Figure 6. N2 adsorption-desorption isotherms and inset BJH pore size distribution
curves of (a) the unembellished Bi4O5Br2-LMs and (b) the as-prepared AAB-LMs-10.
Table 2 BET areas, pore size and volume of the as-prepared materials.
Photocatalyst
Bi4O5Br2-LMs
AAB-LMs-10
AAB-LMs-40
AAB-LMs-90
AAB-LMs-180
Surface Area (m2g-1)
35.5395
Pore size (nm)
18.0511
16.992
Pore volume (cm3g-1)
0.132271
32.6872
0.134203
30.8193
9.2379
0.063651
28.943
17.7095
16.6258
0.120520
28.907
0.115338
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Optical Absorption Properties
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DRS has been regarded as a constructive characterization due to its sensitive
identification of the existence of metal ion coordinating in semiconductors, and the
DRS results could illustrate the spectral characteristics of the samples under light.[37]
After depositing Ag/AgBr nanoparticles, the color of the AAB-LMs composite changed
from the light yellow of the pristine Bi4O5Br2-LMs to brown, showing that the optical
properties of AAB-LMs had been changed. As revealed in Figure 7, the uncovered
Bi4O5Br2-LMs had strong absorption at 480 nm, which mainly derived from the
intrinsic band gap absorption. As for the AAB-LMs, there was a continuous absorption
in the wavelength range 200-800 nm and a noticeable bulge located at 550 nm, which
were the evidence of more significant enhancement in light absorption bands than pure
Bi4O5Br2-LMs. All of these oddities resulted from the synergistic effects among
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Bi4O5Br2-LMs, Ag and AgBr,[38] the SPR effect of Ag nanoparticles[39] and the
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involvement of isolated energy levels in the band gap of Bi4O5Br2-LMs,[40] which
demonstrated that the as-fabricated AAB-LMs possessed high effectiveness and wide
application as a VLD photocatalyst.[41] Since the photocatalytic behavior of
semiconductors can be greatly affected by their light absorption performance, the
extending of light absorption edges possibly worked out the boosted visible-light
photocatalytic behavior of AAB-LMs.
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Figure 7. UV-vis diffuse reflection spectra of the as-fabricated materials.
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It is well-known that the light absorption performance of the semiconductor is
strongly linked to its energy band structure for the intrinsic transition.[42] The smaller
the band gap energy (Eg), the higher the utilization rate of light. The Eg of as-prepared
sample is calculated by αhν = A (hν-Eg)n/2, where α, h, ν and A are the absorption
coefficient, Planck’s constant, light frequency and a constant, respectively. As for the
AgBr and Bi-O-Br, both of values of n are 4,[27] and their indirect band structures allow
the excited electrons of valence band (VB) transport to the conduction band (CB) of the
semiconductor through a certain k-space distance, which effectively suppresses the
combination of photo-induced electron-hole pairs and improves their photocatalytic
behavior.[13-14] The calculated Egs of as-prepared samples are displayed in Figure S2,
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which were 2.40 eV，2.33 eV，2.35eV，2.36 eV and 2.22 eV for Bi4O5Br2-LMs, AAB-
LMs-10, AAB-LMs-40, AAB-LMs-90 and AAB-LMs-180, respectively. Although
3
there were slight differences between the values of Egs of AAB-LMs, all of them were
lower than that of the pure Bi4O5Br2-LMs, demonstrating that the loading of Ag/AgBr
nanoparticles was conductive to generate more photo-induced electron-hole pairs under
visible-light irradiation, thus improving their photocatalytic activities.[35]
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Photoelectrochemical measurements mainly includes Electrochemical Impedance
Spectroscopy (EIS) and photocurrent analysis in this study. EIS can investigate the
transfer impedance of photo-generated charge and the relationship between charge
transfer and recombination. The radius of the arc of the EIS spectrum can indirectly
reflect the reaction rate happening on the surface of the electrode. The smaller diameter
of the semicircle assigned to charge transfer impedance, the lower recombination rates
of photo-induced charge, indicating that the semiconductor has a better photocatalytic
activity.[35] In Figure 8a, the measured diameter of AAB-LMs-10 was significantly
smaller than that of the uncovered Bi4O5Br2-LMs, affirming AAB-LMs-10 had a lower
charge transfer resistance and a more effective separation of photo-induced electron-
hole pairs. This was mainly attributed to the deposition of Ag/AgBr, which could
enhance the separation and transfer of electron-hole pairs of Bi4O5Br2-LMs. To gain
further insight into the improved photocatalytic behavior, the instantaneous
photocurrent responses of Bi4O5Br2-LMs and AAB-LMs-10 were studied. The intensity
of photocurrent could indirectly represent the charge transfer ability of the
semiconductor under light.[21] As depicted in Figure 8b, the as-prepared samples had a
fast and reproducible photocurrent response under light irradiation, and the maximum
photocurrent density of the sample after treated with photoreduction process (AAB-
LMs-10) was observably better than that of the pure Bi4O5Br2-LMs. It indicated that
AAB-LMs-10 had a lower recombination rate of the photo-generated electron-hole
pairs and an excellent photocatalytic property. Both photoelectrochemical
characterization illustrated that the loading of Ag/AgBr as well as the synthetic
interaction of Ag, AgBr and Bi4O5Br2-LMs had a positive effect to promote the
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separation and transfer of photo-generated charge.
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Figure 8. (a) EIS spectra and (b) photocurrent response of the pure Bi4O5Br2-LMs and
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AAB-LMs-10.
6
Photocatalytic Performance
7
The potential photocatalytic activity of the as-fabricated materials should be reflected
from the degradation performance of refractory organics pollutants, which is significant
for the environmental restoration and protection. In this study, it was mainly evaluated
by degradation of AO7, PTBP and aniline under visible-light irradiation. AO7, an
anionic azo dye, has been widely used in the textile industry but hardly biodegraded.[43]
Besides, Ember et al. reported that AO7 did not deactivate or form precipitates over a
long reaction time (several days) even in bicarbonate buffer solutions containing
oxidants with high concentrations.[44] In this study, the concentration of AO7 was
indirectly detected by a UV-vis. spectrophotometer at 484 nm. The visible-light
photocatalytic degradation efficiency (η) was reckoned with η = (1 - Ct / C0) × 100%,
in which Ct and C0 are the equilibrium concentration of target pollutant at illumination
time t and initial time, respectively. As a comparison, the photodegradation of AO7
without any catalyst (Figure 9a) was performed, and the corresponding results declared
that AO7 itself rarely degraded with visible-light illumination alone. With respect to the
degradation rates of AO7 with adding diverse as-synthesized photocatalysts, the
photocatalytic efficiencies of the as-fabricated AAB-LMs were dramatically higher
than that of the pristine Bi4O5Br2-LMs, in which AAB-LMs-10 performed the highest
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photocatalytic performance with the degradation efficiency of about 95 % in 60 min.
Compared with the reported CdS/TiO2 photocatalyst, which only degraded 40 % AO7
3
within 35 min under the visible-light illumination,[45] the AAB-LMs had an eminent
visible-light photocatalytic activity in the removal of AO7. Interestingly, the catalytic
behavior reduced with the increment of loading amount of Ag/AgBr, manifesting that
although the surface silver nanoparticles (Ag NPs) promoted the separation and
migration of photo-induced charge, the presence of excessive AgBr might cover the
active sites on the surface of Bi4O5Br2-LMs and the reduction of surface area of the
sample could also inhibit the light absorption. Herein the degradation rate of AO7 with
adding AAB-LMs-40, AAB-LMs-90 and AAB-LMs-180 as the photocatalysts
decreased gradually. In order to determine the optimal photoreduction time for the
preparation of AAB-LMs composites from the practical application view,
photodegrading PTBP over different as-synthesized photocatalysts was also
comparatively performed under visible-light irradiation (Figure S3). Surprisingly, the
visible-light catalytic activity of AAB-LMs to the degradation of PTBP was opposite to
that of AO7, which AAB-LMs-180 showed the best catalytic activity by the little
margin among the series of AAB-LMs. Although AAB-LMs had completely different
catalytic efficiency on different pollutants, the small disparity did not affect the fact that
AAB-LMs had splendid visible-light catalytic activity compared with BiOBr, Ag/AgBr,
Bi4O5Br2-plate and Bi4O5Br2-LMs. Therefore, combining the structural and
morphological characterizations and photocatalytic activities of AAB-LMs, 10 min was
considered as the optimal photoreduction time during the preparation process of AAB-
LMs, which not only saved energy consumed during the whole experimental process
but also reduced the whole preparation time of the composite photocatalyst. In addition,
the photodegradation process of AO7 also abided by the pseudo-first-order reaction and
the first-order rate constant (kapp) could be obtained by fitting the equation (–In (Ct / C0)
= kappt, where Ct, C0 and t represent the equilibrium concentration of the reaction
solution at instantaneous and initial time, and the visible-light irradiation time,
respectively). While Figure 9c plots the kinetic curves for AO7 photodegradation over
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as-fabricated catalysts, Figure 9e displays the values of kapp for different photocatalytic
degradation systems. Specifically, AAB-LMs-10 (0.04298 min-1) had the highest value
3
of kapp, which was nearly 28 times larger than that of the unembellished Bi4O5Br2-LMs
(0.0015 min-1). The above facts disclosed that Bi4O5Br2-LMs coupling with Ag/AgBr
could efficiently facilitate the separation of photo-generated electron-hole pairs, thereby
enhancing the photocatalytic efficiency of the composite AAB-LMs.
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Aniline is one of the most vital intermediates and raw materials in the manufacture
of dyes, pesticides, pharmaceutical sulfa medicines and anti-explosive agent of
gasoline.[46-47] Moreover, as one of the 129 priority contaminants (U.S Environmental
Protection Agency, U.S.EPA), aniline can even destroy the ecological balance of the
aquatic system and endanger the growth and survival of aquatic organisms after being
discharged into the environment.[48-49] Therefore, it is imperative to explore an efficient
and green way to remove aniline from the environment. With respect to the
photocatalytic degradation of aniline, the catalytic efficiencies of the as-fabricated
BiOBr, Ag/AgBr, Bi4O5Br2-plate, Bi4O5Br2-LMs and AAB-LMs were carried out in
this study. As depicted in Figure 9b, the photocatalytic degradation capability of
Bi4O5Br2-LMs (27.7 %) was better than that of BiOBr (18.2 %) and Bi4O5Br2-plate
(22.4 %), which revealed that the Bi-rich Bi4O5Br2-LMs had more salient visible-light
catalytic activity, and might mainly attribute to the 3D-monodisperse layered
microsphere structures and the presence of ionic liquid acted as the dispersing agent as
well as TX-100 served as stabilizer.[15] Interestingly, the degradation rate of aniline with
adding Ag/AgBr (43.2 %) as the photocatalyst was higher than that of as-prepared
single-phase photocatalysts but lower than that of the AAB-LMs-10 composite (about
70 %), demonstrating that the deposition of Ag/AgBr had a positive influence to boost
the photocatalytic efficiency of the composite. Moreover, the photodegradation of
aniline with adding different as-fabricated catalysts was also investigated by fitting the
obtained data in –In (Ct / C0) = kappt. The kinetic curves and the values of kapp for
degrading aniline are displayed in Figure 9d and Figure 9f, respectively. The data
showed that the photocatalytic degradation process of aniline corresponded to the
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pseudo-first-order kinetics and the value of kapp obtained from AAB-LMs was much
higher than those from other as-prepared catalysts, which further ensured that the AAB-
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LMs possessed higher photocatalytic activity.
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In general, the photocatalytic efficiency of as-prepared AAB-LMs for degrading
AO7 was markedly stronger than that of aniline under visible-light irradiation, which
was mainly because the π-conjugated molecular structure of aniline gave it stable and
inert electronic structure.[38] As for the commendable photocatalytic degradation of
AO7 over AAB-LMs, the self-photosensitization mechanism of dye molecules
interacted with AAB-LMs photocatalyst should not be ignored.[50] The AO7 molecules
adsorbed on the AAB-LMs could absorb visible-light to generate a singlet and/or a
triplet state of AO7, in which the former was more reactive than the latter. After that,
the electrons generated from excited state AO7 molecules could transport to the surface
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of AAB-LMs and react with adsorbed oxygen to form active species (•O2 ), which was
beneficial to improve the photodegradation efficiency of AO7.[41, 51] Therefore,
combined with the above photocatalytic degradation performance of AO7, PTBP and
aniline, the synergetic effect among Ag, AgBr and Bi4O5Br2-LMs was conductive to
boost the visible-light catalytic activity of the composite. All in all, the as-prepared
AAB-LMs composite photocatalyst had prominent photocatalytic performance and
bright application prospect in the removal of environmental pollutants.
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Figure 9. Photocatalytic degradation, pseudo-first-order kinetics curves and kapp of (a,
c, e) AO7 (10 mg/L) and (b, d, f) aniline (16 mg/L) with adding different as-prepared
photocatalysts under visible-light irradiation.
It is well-known that the recyclability and stability of catalyst are also imperative
for its long-time existence and repeated use in terms of practical applications.[1] To
verify the recyclability of as-synthesized AAB-LMs, repeatedly photocatalytic
degradation experiments of AO7 under the same reaction condition were designed and
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carried out. In Figure 10a, the photocatalytic proficiency of AAB-LMs-10 for degrading
AO7 still maintained high even after four consecutive cycles. Furthermore, the stability
3
of AAB-LMs was also characterized by XRD, XPS and SEM measurements. As
depicted in Figure 10b, the crystalline structure of AAB-LMs-10 used before and after
four photocatalytic degradation circles had hardly changed, and their position of the
peaks still corresponded well to the standard characteristic peaks of Bi4O5Br2 (JCPDS
No. 37-0699), Ag (JCPDS No.65-2871) and AgBr (JCPDS No.06-0438). High-
resolution XPS spectra of the used AAB-LMs (Figure S4a-e) demonstrated that
although these corresponding characteristic peaks shifted slightly to the left, which may
be ascribed to the change of the chemical environment, its composition and valence
states of elements had not changed. Moreover, the morphology of AAB-LMs (Figure
S4f) after the photodegradation still kept the complete spherical structure with some
nanoparticles embedded or attached on their surface. These described consequences
indicated that the as-prepared AAB-LMs-10 behaved favorable reusability and
structural stability and had a potentially practical application.
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Figure 10. (a) Recycling runs of as-prepared AAB-LMs-10 for AO7 (10 mg/L)
degradation in the photocatalytic reaction process, (b) XRD patterns of as-prepared
AAB-LMs-10 before and after photocatalytic degradation reaction under visible-light
irradiation.
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Photocatalytic Mechanism Insight
For better understanding the potential photocatalytic mechanism of the as-prepared
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AAB-LMs, the photocatalytic degradation experiments of aniline over AAB-LMs-10
with adding diverse radical scavengers were conducted. Specifically, isopropanol (IPA),
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4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPOL), potassium dichromate
(K2Cr2O7), and ammonium oxalate (AO) were selected to serve as scavengers for
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hydroxyl radical (•OH), superoxide radical (•O2 ), electrons (e-) and hole (h+),
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respectively.[52-53] As shown in Figure S5, the photocatalytic efficiency of AAB-LMs-
-
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10 dropped significantly with the presence of TEMPOL and AO, indicating that •O2
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and h+ were the primary active species in this photocatalysis system. Since photo-
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induced electrons (e-) can be captured by the surface adsorbed O2 of catalysts to
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generate •O2 , the addition of K2Cr2O7 could also suppress the photocatalytic
proficiency of AAB-LMs-10, thereby inhibiting the degradation of aniline. However,
the degradation rate of aniline was not affected by the addition of IPA, indicating that
•OH played a minor role in this photocatalytic degradation process.
The dominant radical species of the AAB-LMs-photocatalysis system was further
verified by EPR (Figure 11). No distinct characteristic peak was found in the dark,
indicating that the light irradiation was indispensable for the generation of active
-
species. Besides, the characteristic peaks of DMPO-•O2 were observed in both the
AAB-LMs-10 and Bi4O5Br2-LMs methanol dispersion, while no obvious characteristic
peak belonging to DMPO-•OH was observed in either EPR spectra of AAB-LMs-10 or
that of Bi4O5Br2-LMs in aqueous dispersion under visible-light illumination. Therefore,
-
•O2 but not •OH was the predominant radical species in the AAB-LMs-photocatalytic
degradation system, which was in well agreement with the conclusion drawn from the
quenching experiments.
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Figure 11. DMPO spin-trapping EPR spectra of the uncovered Bi4O5Br2-LMs and as-
−
synthesized AAB-LMs-10 in (a) methanol dispersion for DMPO - •O2 and (b) aqueous
dispersion for DMPO - •OH in dark or under visible-light irradiation.
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The degradation pathway of aniline in this AAB-LMs-photocatalysis system was
deduced based on the theoretical calculation and the gas chromatography-mass
spectrometer (GC-MS) data of degradation products. The detailed analytical methods
of GC-MS and frontier electron densities (FEDs) analysis of aniline can be found in the
Supplementary Material. Table S1 lists the formulas of identified relevant degradation
intermediates of aniline as well as their retention times and chemical structures, and
Figure S7 displays the resulting GC-MS chromatograms detected with different
degradation time of the aniline in the AAB-LMs-photocatalysis system. In the figure,
the relative abundance of characteristic peak belonging to aniline (retention time (RT)
= 5.47 min) was sharply reduced during the photocatalytic degradation process,
demonstrating that aniline was degraded in the AAB-LMs-photocatalysis system. The
main photocatalytic degradation routes are as follows (Figure 12). (1) Aniline was
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attacked by the active species (•O2 and h+) to produce phenol, which was conductive
to increase the absorbance, and then further to convert into hydroquinone (G); (2) The
−
excited imino radicals could be attacked by •O2 and h+ to form nitrophenols (B), and
then generated hydroquinone (G); (3) Due to the presence of AgBr in the AAB-LMs-
photocatalysis system, the strong oxidizing agent Br0 might be generated during the
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photocatalytic process. Therefore, aniline underwent the ortho, meta and para
substitution reactions with Br0 to form bromoaniline (C) and dibromoaniline (D), which
3
was also an important phenomenon to illustrate the generation and participation of Br0.
Subsequently, these intermediates were converted to hydroquinone (G) with the attack
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of •O2 and h+. After that, the resulting hydroquinone could be attacked to form
6
benzoquinone (H), which was accompanied by the aromatic ring-opening reaction to
produce a series of low-toxic small molecule organic compounds (I, J, K and L). These
degradation routes and the presence of Br0 were verified by the gradually decreased
characteristic peaks of bromoaniline (RT=11.26 min) and dibromoaniline (RT=19.09
min) as well as the enhanced peaks of hydroquinone (RT=18.50 min) and benzoquinone
(RT=23.11 min) with the extended photocatalytic degradation time, which were also
depicted in Figure S7; (4) The intermediates (E) and (F) were mainly produced by the
combination of aniline radicals excited by the active species and aniline or phenol
radicals, which could be in turn cleaved into some small molecular organics (I, J, K and
L). In conclusion, the generation and disappearance of bromoaniline and
dibromoaniline during the photocatalytic degradation process were mainly attributed to
the fact that aniline was attacked by the active species Br0 generated from the AgBr in
the AAB-LMs. Besides, considering the deamination efficiency of aniline was faster
than the aromatic ring-opening rate in the first 30 min,[54] the aromatic ring-NH2 bond
of aniline was broken preferentially, followed by a ring-opening reaction. Since the
benzene ring could not be directly oxidized to small molecular organics for its strong
stability, it was converted to a benzoquinone. Moreover, the presence of phenolic
compounds was mainly because the lone pair of electrons on the nitrogen atom together
with the benzene ring formed a delocalized large π bond (P-π conjugate) after the
hydrogen atom on the benzene ring was replaced by -NH2.
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Figure 12. Possible photocatalytic degradation pathways of aniline on the as-prepared
AAB-LMs under visible-light illumination.
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In order to analyze the elevated photocatalytic capability of AAB-LMs from the
theoretical viewpoint, the relationships between the atomic structure and electronic
structures of Bi4O5Br2 and AgBr were analyzed by using Density Function Theory
(DFT). The corresponding data are depicted in Figure 13. For Bi4O5Br2, the CB was
primarily dominated by Bi 6p orbit, while the VB was basically composed of Bi 6s, O
2p, Br 4p, Br 4s and O 2s orbits. Owing to the formed large hybrid electron density of
O atoms with vicinal Bi and Br atoms enhanced the electrostatic attraction between
nuclear and electrons, all of atoms in the VB were hybrid together very well, so it was
hard for electrons in the VB to be excited. On the other hand, since the low hybridization
of Bi atoms with close Br atoms in the CB was in favor of the decrease of electrostatic
attraction between nuclear and electrons, electrons could readily transfer to the surface
of Bi4O5Br2, which availably facilitated the separation of electron-hole pairs. Compared
with Bi4O5Br2, the CB of AgBr was mainly made up of Br 4s and Ag 5s orbits, while
the VB consisted of Br 4p and Ag 4d orbits. Considering the lower hybrid electron
density of Ag atoms with nearby Br atoms in the VB, the electrons of VB could be
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effortlessly excited. Nevertheless, because Ag atoms with nearby Br atoms had the
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higher hybridization in the CB of AgBr, the migration of electrons of CB mostly
occurred inside of the AgBr, resulting in a higher recombination of electron-hole pairs.
3
Therefore, the hybrid electron density in the VB of Bi4O5Br2 would decline after the
generation of the new composite system (AAB-LMs), which was conductive to the
migration of electrons in the VB to the CB of Bi4O5Br2 and the VB of Bi4O5Br2 to the
Ag. Moreover, since Ag might perform as the electron collector for its SPR effect and
a Z-scheme bridge, its presence could also reduce the hybridization in the CB of AgBr,
which made for the transfer of electrons from the CB of AgBr to the Ag and improved
the separation of electron-hole pairs. In summary, the successful preparation of AAB-
LMs could efficaciously suppress the recombination of electron-hole pairs, thereby
enhancing the photocatalytic performance.
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The conjectural mechanism for the robust stability and photocatalytic property of
as-prepared AAB-LMs could be concluded based on the above-mentioned
characterizations and theoretical calculation. In detail, visible-light driven (VLD)
photocatalysts AgBr[27] and Bi4O5Br2-LMs[20] could be effectively excited by the
visible-light to produce electron-hole pairs. As described in Figure 13e, AgBr adhered
to the Bi4O5Br2-LMs by a typical heterojunction mode, so that their matching band
potentials, in which the conduction band edge (ECB) of Bi4O5Br2-LMs (ECB = -1.21
eV)[15] was more negative than that of AgBr (ECB = -0.45 eV),[55] were favorable to the
transportation of photo-excited charges between them. Under visible-light irradiation,
the excited electrons could migrate away from the VB to CB of Bi4O5Br2-LMs and
immediately be trapped by the CB of AgBr, while the photo-generated holes on the VB
of AgBr would transfer to that of Bi4O5Br2-LMs. Therefore, the effective separation of
photo-induced electron-hole pairs of Bi4O5Br2-LMs could be achieved. However, the
Ag+ of AgBr would be reduced to Ag via the electrons gathered on the CB of AgBr, and
the resulting photo-corrosion of AgBr would cause the unsteadiness of the AAB-LMs,
which was contrary to the results of our recyclability and stability experiments results
and even the DFT analysis. Furthermore, since the h+ in the VB of AgBr could react
with Br- to produce Br0, which might be reduced to Br- again during the oxidation of
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organic pollutants, the faster transportation of h+ from the CB of AgBr to that of
Bi4O5Br2-LMs would have a negative impact on the production of Br0 and the stability
3
ofAAB-LMs to some extent. Therefore, the conventional heterojunction mode between
AgBr and Bi4O5Br2-LMs was not suitable to illustrate the enhanced photoactivity and
photostability of as-synthesized AAB-LMs.
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Furthermore, a novel Z-scheme mechanism via Ag NPs as a bridge was more
appropriate to explain the separation and transfer of photo-induced charges in the AAB-
LMs-photocatalysis system (Figure 13f). When AAB-LMs was excited by visible-light,
the photo-induced electrons of AgBr could more likely transfer to the Ag NPs through
the Schottky barrier and react with h+ migrated from the VB of Bi4O5Br2-LMs, which
further facilitated the efficacious separation of photo-induced charges of AgBr and
Bi4O5Br2-LMs. Moreover, the potential barrier witnessed by the gathered electrons at
the interface of the junction could not only make electrons back into the Bi4O5Br2-LMs,
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−
which promoted the formation of •O2 , but also reflect holes back into AgBr, so
promising the production of Br0.[24, 56] On the other hand, the migration of electrons on
the CB of the AgBr could also effectively avoid the reduction of Ag+ and guarantee the
−
stability of AAB-LMs. Considering the reduction potential of O2/•O2 was -0.33 eV, the
photo-induced electrons of Bi4O5Br2-LMs could reduce the surface adsorbed oxygen to
−
produce active species •O2 during the photocatalytic process. On the other hand,
compared with the reduction potentials of •OH/H2O (2.27 eV) and •OH/OH- (2.38
eV), the h+ in the VB of AgBr did not have enough oxidation ability to generate •OH
as the active species for this AAB-LMs-photocatalysis system.[21] Concomitantly, the
Br- of AgBr could be oxidized to Br0 by h+. As the reactive radical species, Br0 would
be reduced to Br- again by the oxidation of organic pollutants, which was verified by
the photocatalytic degradation process of aniline over AAB-LMs mentioned before.
Meanwhile, free Br- recombined with Ag+ to form AgBr, thus guaranteeing the stability
of the whole material.[57] Herein, the organic pollutant molecules could be efficiently
−
decomposed by the oxidation of •O2 , Br0 and h+ in the AAB-LMs-photocatalysis
system under visible-light irradiation.
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Figure 13. Schematic representation of crystal structure, calculated band structures and
density of states (DOS) of (a, b) Bi4O5Br2 and (c, d) AgBr; Schematic diagram of the
separation and transportation of photo-generated charges over the as-prepared AAB-
LMs composites (e) the typical heterojunction and (f) the novel Z-scheme
heterojunction combined with the proposed photocatalytic degradation mechanism
under visible-light irradiation.
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Conclusions
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In summary, visible-light-driven plasmonic heterojunction photocatalysts
Ag/AgBr/Bi4O5Br2-LMs performed a significant potential for the catalytic degradation
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10.1002/cctc.201900529
ChemCatChem
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of refractory organic contaminants. After the decoration of Ag/AgBr, AAB-LMs
exhibited a remarkable improvement of photocatalytic efficiency than the pristine
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Bi4O5Br2-LMs and Ag/AgBr for the degradation of AO7, PTBP and aniline under
visible-light. The boosted visible-light absorption and appreciable photocatalytic
performance of AAB-LMs were mainly ascribed to the surface plasmon resonance
effect of Ag nanoparticles and the synthetic interaction between Bi4O5Br2-LMs, Ag and
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AgBr. •O2 and h+ were testified as the predominant radical species in this AAB-LMs-
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photocatalysis system. Moreover, the possible photodegradation routes of aniline in this
AAB-LMs-photocatalysis system were also proposed on the basis of theoretical
analysis and GC-MS measurement. Since the active species Br0 taking part in the
photocatalytic degradation process, the Z-scheme mechanism rather than the
conventional heterojunction mode of as-prepared AAB-LMs was more convincing to
illustrate the elevated photocatalytic performance. Therefore, the in-situ deposition and
photoreduction strategy to construct the novel AAB-LMs may point out some new hints
on designing novel Z-scheme plasmonic heterojunction photocatalysts for wide
application in environmental remediation and solar-to-fuel conversion.
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18 Experimental Section
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Experimental Materials
Ionic liquid 1-Octyl-3-methylimidazolium bromide ([Omim]Br, 99%) was obtained
from Shanghai Chengjie Chemical Co., Ltd. Triton X-100 (TX-100, p-Octyl
polyethylene glycol phenyl ether), silver nitrate (AgNO3), ethanol and
sodium hydroxide (NaOH) were bought from Sinopharm Chemical Reagent Co., Ltd.
Bismuth pentahydrate (Bi(NO3)3•5H2O) and nitric acid were obtained from Nanjing
Chemical Reagent Co., Ltd. Acid orange II (AO7), 4-tert-butylphenol (PTBP) and
aniline were purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd, Aladdin
Reagent Co. Ltd and Beijing Bellingway Technology Co., Ltd, respectively. Milli-Q
deionized water was used during this experiment.
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10.1002/cctc.201900529
ChemCatChem
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Preparation of Bi4O5Br2-LMs
Bi-rich bismuth oxybromide layered microspheres (Bi4O5Br2-LMs) were synthesized
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via an ionic liquid-in-water (IL/W) microemulsion-mediated approach. First of all, in
order to obtain the precursor, dissolving 0.56 g [Omim]Br and 0.2 g TX-100 into 25 mL
deionized water to get the solution A, while the solution B was prepared by dissolving
0.97 g of Bi(NO3)3•5H2O into 5 mL of nitric acid (1M) to avoid hydrolyzation of Bi3+
ions. Afterwards, the yellow homogeneous solution could be clearly observed when
solution B was added drop by drop into the freshly ionic liquid microemulsion system
(solution A) with continuously energetic stirring at room temperature. And then,
adjusting the pH of the mixed system with NaOH (2M) to 10.5, which should be
continuously stirred for another 1h later. Afterwards, the reaction liquid was transferred
into a 50 mLTeflon-lined autoclave and reacted at 160 ℃ for 12 h. Finally, the obtained
yellow precipitate needed to be washed three times each with deionized water and
ethanol and dried at 60 ℃ for 24 h. To compare the photocatalytic efficiency of the as-
prepared materials, the same preparation method was used to synthesize BiOBr and
Bi4O5Br2-plate. The former was prepared under the condition that the reaction system
was at pH of 3, and the latter was synthesized in the absence of TX-100.
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Preparation of Ag/AgBr/Bi4O5Br2-LMs
Silver/silver bromide supported Bi-rich bismuth oxybromide layered
microspheres (Ag/AgBr/Bi4O5Br2-LMs, AAB-LMs) were constructed via a facile in-
situ deposition and visible-light photoreduction method. The details are as follows: 0.1g
of as-synthesized Bi4O5Br2-LMs was added into 48 mL deionized water, and then
dissolved 2 mLAgNO3 (0.1M) with ultrasonicating for 5 min to obtain homogeneously
dispersed solution. The photoreduction reaction was performed on a photochemical
reaction instrument (XPA-7, Nanjing, China). After stirring for 30 min under dark, the
suspension was irradiated by 500 W Xenon lamp with 400 nm cut-off filter for 10 min,
40 min, 90 min and 180 min, respectively. Subsequently, the obtained composites were
washed and dried at 60 ℃ for 24 h. The detailed preparation process of samples is
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