Highly improved visible-light-induced photocatalytic performance over BiOI/Ag2CO3 heterojunctions

Article abstract of DOI:10.1039/c7ra10423e

A novel BiOI/Ag₂CO₃ composite was synthesized through the co-precipitation method and well characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, UV-vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. Its photocatalytic activity was evaluated for degrading various pollutants (rhodamine B, methylene blue, methyl orange) and phenol aqueous solution. Experimental results indicated that Ag₂CO₃ nanoparticles anchored on the BiOI sheet surface promoted the enhanced photocatalytic activity. Moreover, when the mass ratio of BiOI in BiOI/Ag₂CO₃ composites was 20%, the as-prepared BiOI/Ag₂CO₃ (20 wt%) composite exhibited the optimal photocatalytic activity for degrading rhodamine B; its rate constant was approximately 6.75 and 4.3 times higher than the separate BiOI and Ag₂CO₃. The enhanced photocatalytic performance could be attributed to the heterojunctions between BiOI and Ag₂CO₃. Furthermore, the possible mechanism for photodegradation over the BiOI/Ag₂CO₃ composite is proposed.

Full text of DOI:10.1039/c7ra10423e

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Highly improved visible-light-induced
photocatalytic performance over BiOI/Ag2CO3
Cite this: RSC Adv., 2018, 8, 537
heterojunctions†
a
b
Nan Wang,a Lei Shi,
Lizhu Yao,a Caiyun Lu,a Yan Shia and Jianmin Sun
*
*
A novel BiOI/Ag2CO3 composite was synthesized through the co-precipitation method and well
characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron
spectroscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray
spectroscopy, UV-vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. Its
photocatalytic activity was evaluated for degrading various pollutants (rhodamine B, methylene blue,
methyl orange) and phenol aqueous solution. Experimental results indicated that Ag2CO3 nanoparticles
anchored on the BiOI sheet surface promoted the enhanced photocatalytic activity. Moreover, when the
mass ratio of BiOI in BiOI/Ag2CO3 composites was 20%, the as-prepared BiOI/Ag2CO3 (20 wt%)
composite exhibited the optimal photocatalytic activity for degrading rhodamine B; its rate constant was
approximately 6.75 and 4.3 times higher than the separate BiOI and Ag2CO3. The enhanced
photocatalytic performance could be attributed to the heterojunctions between BiOI and Ag2CO3.
Furthermore, the possible mechanism for photodegradation over the BiOI/Ag2CO3 composite is proposed.
Received 20th September 2017
Accepted 8th December 2017
DOI: 10.1039/c7ra10423e
rsc.li/rsc-advances
photocorrosion, Ag2CO3 possesses weak activity aer recy-
cling.14,15 Hence, it is urgent to develop some method to improve
1. Introduction
the stability of Ag2CO3.
Semiconductor photocatalysis has received a great deal of
attention because it represents a promising alternative tech-
nology for efficient utilization of solar energy to solve energy
and environmental problems.1 Among all kinds of semi-
conductor photocatalysts, TiO2 has been the most widely
investigated due to its low cost, non-toxicity, strong oxidizing
activity and high chemical stability. Nevertheless, the large
band gap and the inefficient separation of photoinduced hole–
electron pairs result in its unsatisfactory photocatalytic perfor-
mance.2 Accordingly, to take full advantage of solar energy, it is
essential to exploit effective and sustainable photocatalysts with
visible-light-induced photocatalytic capability.
A group of efficient visible-light-driven photocatalysts, BiOX
(X ¼ Cl, Br, and I), has shown superb visible light activity for
energy conversion and environmental remediation, attributed
to particular physicochemical properties.16–18 Among the BiOX
catalysts, BiOI presents the lowest band gap (1.7–1.9 eV),
revealing excellent visible-light responsiveness and photo-
catalytic capacity.19 However, high recombination of photo-
generated electron–hole pairs restricts its photocatalytic
performance. Previous reports implied that heterojunctional
structure could promote the separation of photogenerated
electrons and holes.20–22 Hence, it is possible that the coordi-
nation of BiOI and Ag2CO3 might produce better photocatalytic
activity than pure BiOI or Ag2CO3. However, there are few works
focusing on the preparation and photocatalytic performance of
BiOI/Ag2CO3 composites.
In this work, we successfully synthesized BiOI/Ag2CO3 het-
erojunction at room temperature. The obtained BiOI/Ag2CO3
composites were well characterized by XRD, FTIR, XPS, SEM,
TEM, ESX, UV-vis DRS and PL. Various pollutants, such as
rhodamine B (RhB), methylene blue (MB), methyl orange (MO)
and phenol, were selected as model molecules to investigate the
photocatalytic ability of the resultant BiOI/Ag2CO3 composite.
Experimental results indicated that as-prepared BiOI/Ag2CO3
composite exhibited clearly enhanced photocatalytic activity for
degrading various pollutants compared with pure Ag2CO3 and
BiOI, which could be attributed to the heterojunction between
Recently, Ag-based photocatalysts, including Ag2O,3
Ag3PO4,4,5 AgX (X ¼ Cl, Br, I),6–8 AgVO3,9 Ag2CrO4,10 and
Ag2WO4,11 have been regarded as promising photocatalysts.
Among them, Ag2CO3 has exhibited highly efficient photo-
catalytic property for decomposing methylene blue, rhodamine
B
and methyl orange dyes.12,13 However, due to self-
aCollege of Chemistry, Chemical Engineering and Environmental Engineering,
Liaoning Shihua University, Fushun 113001, China. E-mail: shilei_hit@qq.com; Tel:
+86-18340316558
bState Key Laboratory of Urban Water Resource and Environment, MIIT Key Laboratory
of Critical Materials Technology for New Energy Conversion and Storage, School of
Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin
150080, China. E-mail: sunjm@hit.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra10423e
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BiOI and Ag2CO3. In addition, the BiOI/Ag2CO3 composite exclude the UV light, and 0.1 M Na2SO4 was used as the
retained excellent photostability; the activity decreased slightly electrolyte.
aer ve recycles. Moreover, the possible mechanism for the
enhancement of the photocatalytic activity over the BiOI/
Ag2CO3 composite is also proposed.
2.3 Photocatalytic testing for pollutant degradation
The photocatalytic activity of the as-made BiOI/Ag2CO3
composites was evaluated in degrading RhB, MB, MO and
phenol under visible light. A 300 W Xe lamp with a 400 nm
cutoff lter was applied as the visible-light source. 50 mg
sample was dispersed into 50 mL of 5 mg Lꢁ1 pollutant and
stirred in the dark for 1 h to reach the adsorption–desorption
equilibrium. Then, 4 mL of solution was collected every given
time interval under irradiation, and the catalyst was removed by
centrifugation (12 000 rpm, 5 min). The concentration of RhB,
MB, and MO was detected via UV-vis spectrophotometer at the
wavelength of 552, 664 and 464 nm. The concentration of
phenol was detected through colorimetric method using
4-aminoantipyrine as the colour-developing agent at 510 nm.23
2. Experimental section
2.1 Synthesis of BiOI/Ag2CO3 photocatalysts
All the reagents were supplied from Sinopharm Chemical
Reagent Co. Ltd, China. Deionized water was used throughout
the study.
The preparation of BiOI was as follows: 1.212 g of Bi(NO3)3$
5H2O was added into 40 mL deionized H2O. Aer stirring for
1 h, 20 mL of H2O containing 0.415 g KI was dropped slowly into
the above mixed solution. The resultant solution was vigorously
stirred for 1 h at room temperature. Finally, the obtained
products were collected by ltration, then washeꢀd with water
and absolute alcohol thoroughly, and dried at 60 C.
3. Results and discussion
A series of BiOI/Ag2CO3 composites were produced through
the co-precipitation method. In a typical experimental process,
0.340 g AgNO3 was dissolved in 30 mL of deionized H2O, then
a certain amount of BiOI was added into the above solution.
Aer stirring for 20 min, 10 mL of deionized water containing
0.106 g of Na2CO3 was dropped slowly into the suspension
solution. Aer continuous stirring for 1 h, the suspension was
ltered, washed with water and absolute alcohol, and dried at
3.1 The structure and property characterizations of BiOI/
Ag2CO3 composite
Fig. 1 gives the XRD patterns of resultant BiOI, Ag2CO3 and
BiOI/Ag2CO3 (20 wt%) composite. Sharp and high diffraction
peaks were observed in all samples, indicating that all the
resultant samples were well crystallized. For BiOI, the four main
diffraction peaks appeared at 29.8ꢀ, 31.9ꢀ, 45.6ꢀ and 55.4ꢀ,
which are good agreement with the (012), (110), (020) and (122)
diffraction planes of the tetragonal phase of BiOI (JCPDS no. 10-
0445).24 Ag2CO3 presented obvious diffraction peaks at 20.7ꢀ,
32.8ꢀ, 33.8ꢀ, 37.2ꢀ and 39.8ꢀ. These diffraction peaks could be
indexed to the crystallite planes of (110), (ꢁ101), (ꢁ130), (200)
and (031) of monoclinic Ag2CO3 (JCPDS no. 26-0339).25 Obvi-
ously, both BiOI and Ag2CO3 were found in the BiOI/Ag2CO3
(20 wt%) composite. No impurity peaks were observed, implying
the high purity of the composite.
ꢀ
35 C. According to the mass ratio of BiOI in the BiOI/Ag2CO3
composite, the as-made samples were denoted as BiOI/Ag2CO3
(10 wt%), BiOI/Ag2CO3 (20 wt%) and BiOI/Ag2CO3 (50 wt%). The
pure Ag2CO3 was synthesized in the same way without the
addition of BiOI.
2.2 Characterizations
The X-ray diffraction (XRD) patterns were obtained on a Bruker
D8 Advance X-ray powder diffractometer with Cu Ka radiation
(40 kV, 30 mA) for phase identication. Fourier transform
infrared spectra (FTIR) were recorded on Thermo Fisher
Scientic IS10. The X-ray photoelectron spectroscopy (XPS)
analysis was performed using the Thermo Fisher Scientic
Escalab 250. The scanning electron microscopy (SEM) images
were taken using a HITACHI SU8010 scanning electron micro-
scope. The transmission electron microscope (TEM) was carried
out via JEM-2100F with energy dispersive X-ray spectroscopy
(EDX). The UV-vis diffuse reectance spectra (DRS) were recor-
ded using an Agilent Cary 5000 UV-vis spectrometer. The pho-
toluminescence spectra (PL) were collected by PerkinElmer
LS55 spectrometer with the excitation wavelength of 220 nm.
Transient photocurrent response of the samples was detected
using an electrochemical system (CHI760, China) with three-
electrode quartz cells. The counter electrode was Pt wires, and
a saturated calomel electrode was used as the reference elec-
trode. The working electrode was the sample lms (BiOI,
Ag2CO3 and BiOI/Ag2CO3 (20 wt%) composite) coated on ITO
The FTIR spectra of pure BiOI, Ag2CO3 and BiOI/Ag2CO3
(20 wt%) composite are shown in Fig. 2. Clearly, Ag2CO3 and
BiOI/Ag2CO3 (20 wt%) composite presented similar stretching
Fig. 1 XRD patterns of various samples. (a) BiOI, (b) Ag2CO3 and (c)
glass. The light source was a 300 W Xe lamp with 400 nm lter to BiOI/Ag2CO3 (20 wt%) composite.
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similar to the SEM results. Ag2CO3 still exhibited ruleless
structure and was aggregated, and BiOI presented sheet struc-
ture with the size of 1–3 mm. From the TEM image of BiOI/
Ag2CO3 (20 wt%) (Fig. 4F), Ag2CO3 nanoparticles adhered to the
surface of the BiOI sheet, signicantly decreasing the size of
Ag2CO3 compared with pure Ag2CO3. HRTEM image (Fig. 4G)
shows that the Ag2CO3 nanoparticles tightly loaded on the BiOI
sheet surface. The lattice fringe spacings of 0.280 and 0.199 nm
respectively corresponded to the (110) and (020) planes of
tetragonal BiOI, and those at 0.264 and 0.226 nm are due to the
(ꢁ130) and (031) planes of the monoclinic Ag2CO3, respectively.
Obviously, this result indicates that the heterojunctions
between Ag2CO3 and BiOI have formed, and that there was
a clear interface contact between Ag2CO3 and BiOI, which might
contribute to facilitating the photogenerated electron transfer
between Ag2CO3 and BiOI, thus leading to the improved
photocatalytic property. Moreover, the EDX spectrum of the
BiOI/Ag2CO3 (20 wt%) composite is also shown in Fig. 4H. The
composite contains only C, O, Bi, Ag and I elements, except for
element of Cu from the supports. The above results agree well
with the XRD, FTIR and XPS results.
Fig. 2 FTIR spectra of (a) BiOI, (b) Ag2CO3 and (c) BiOI/Ag2CO3
(20 wt%) composite.
vibrations and bending vibrations. The peaks at 1449, 1382, 883
The light harvest was investigated by UV-vis DRS (Fig. 5).
and 705 cmꢁ1 were assigned to the characteristic peaks of Obviously, the absorption band edge of Ag2CO3 was estimated
carbonate ion.26 Compared with bare Ag2CO3, a new peak near to be 574 nm, suggesting that Ag2CO3 exhibited good photo-
487 cmꢁ1 could be observed in the BiOI/Ag2CO3 (20 wt%) absorption from UV region to visible light. The absorption
composite. This peak was assigned to the absorption peak of Bi– band edge of BiOI was around 703 nm, presenting that BiOI had
O,24 which also further revealed that BiOI was successfully strong and broad absorption in the visible light region. Aer
composed with Ag2CO3.
BiOI was combined with Ag2CO3, the BiOI/Ag2CO3 (20 wt%)
The elemental composition and state of BiOI/Ag2CO3 composite showed enhanced photoharvest compared with pure
(20 wt%) composite were analyzed using XPS. In Fig. 3A, the Ag2CO3. Furthermore, the band gap energy of as-prepared
survey spectra demonstrate that the composite contains Ag, C, samples could be calculated through the equation Eg ¼ 1240/l
O, Bi and I elements, proving that the sample is composed of (where l is the absorption edge and Eg is the corresponding
Ag2CO3 and BiOI. Fig. 3B shows that the peaks of Ag 3d are band gap). Correspondingly, the band gaps of BiOI and Ag2CO3
observed at 367.8 and 373.8 eV, ascribed to Ag ions of Ag2CO3.27 were estimated to about 1.76 eV and 2.16 eV.
C 1s peak was deconvoluted into three peaks at 284.6, 286.5 and
Photoluminescence spectra emission originates from the
288.4 eV in Fig. 3C. The peak at 284.6 eV is attributed to the C–C recombination of photogenerated electron–hole pairs; thus, PL
and/or adventitious carbon; the peaks at 286.5 and 288.4 eV might reveal the transfer and recombination processes of the
correspond to the carbonate ion in the sample.28 The wide XPS photoinduced charge carrier.33 Generally, a lower PL intensity
of O 1s peak (Fig. 3D) could be divided into three peaks; the denotes a higher separation probability of photogenerated
peak at 530.7 eV corresponds to Bi–O,29 the peaks at 532.3 and charge carrier.34,35 Fig. 6 displays the PL spectra of BiOI and
533.3 eV are respectively attributed to the crystal lattice oxygen BiOI/Ag2CO3 (20 wt%) with the excitation wavelength of 220 nm.
of Ag2CO3 and surface absorbed oxygen species.30 The peaks at Pure BiOI exhibited higher emission intensity than BiOI/Ag2CO3
binding energies of 164.1 and 158.8 eV (Fig. 3E) belong to Bi (20 wt%) composite, indicating that the coordination of BiOI
4f5/2 and Bi 4f7/2 of Bi3+ ions.31 For I 3d in Fig. 3F, the two peaks and Ag2CO3 could promote the separation of photogenerated
located at 619.4 eV and 630.8 eV are associated with I 3d5/2 and I electrons and holes, and it is anticipated to contribute to the
3d3/2, respectively.32
improved photocatalytic activity.
For observation of the morphology of as-prepared samples,
Photocurrent measurement is an efficient way to evaluate the
rstly, SEM images of pure Ag2CO3, BiOI and BiOI/Ag2CO3 separation efficiency of photocharges. Generally, higher
(20 wt%) composite are provided in Fig. 4. A large number of photocurrent intensity means better separation efficiency of
Ag2CO3 particles with irregularity were observed (Fig. 4A), and photocharges. As shown in Fig. 7, visible light irradiation could
the size of Ag2CO3 particles is between 0.2–1 mm. Pure BiOI is induce the BiOI, Ag2CO3 and BiOI/Ag2CO3 (20 wt%) composite
composed of abundant sheets with micron size (Fig. 4B). For as- to generate photocurrent signals. BiOI/Ag2CO3 (20 wt%)
prepared BiOI/Ag2CO3 (20 wt%) composite in Fig. 4C, Ag2CO3 composite possessed stronger photocurrent intensity than pure
nanoparticles with 20–200 nm are attached on the surface of BiOI and Ag2CO3, suggesting that the former exhibited better
BiOI (labeled by red circles). Further information about the separation efficiency for photogenerated electron–hole pairs,
microstructure of the resultant samples was supplied from the which was consistent with photoluminescence measurement
TEM and HRTEM images. Clearly, the obtained results were results.
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Fig. 3 XPS of BiOI/Ag2CO3 (20 wt%) composite. (A) Survey spectra, (B) Ag 3d, (C) C 1s, (D) O 1s, (E) Bi 4f and (F) I 3d.
RhB. The BiOI/Ag2CO3 (20 wt%) composite presented the
optimal photocatalytic activity, and almost 100% RhB was
degraded within 12 min under visible-light irradiation. The
signicantly enhanced photocatalytic activity might be attrib-
uted to the following facts. Firstly, the induced BiOI played the
role of photo-absorber to enlarge the photoharvest of BiOI/
Ag2CO3 composites, leading to the enhanced photocatalytic
degradation. Secondly, the heterojunction formed between BiOI
and Ag2CO3 contributed to the electron transfer, promoting the
separation of the photogenerated electrons and holes.
3.2 Photocatalytic activity and stability
The photocatalytic activity of pure BiOI, Ag2CO3 and BiOI/
Ag2CO3 composite for the degradation of RhB solution was
measured under visible light. Fig. 8A exhibits the concentration
changes of RhB. When bare BiOI and Ag2CO3 were used as
photocatalysts, about 44% and 60% of RhB decomposed in
12 min, respectively. The BiOI/Ag2CO3 composites exhibited
much higher photocatalytic activity under the same conditions;
comparatively, BiOI/Ag2CO3 (10 wt%) and BiOI/Ag2CO3 (50 wt%)
respectively possessed 95% and 97% degradation efficiency for
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Fig. 4 The SEM images of (A) pure Ag2CO3, (B) BiOI and (C) BiOI/Ag2CO3 (20 wt%) composite; the TEM images of (D) pure Ag2CO3, (E) BiOI, (F)
BiOI/Ag2CO3 (20 wt%) composite; (G) the HRTEM image and (H) EDX spectrum of BiOI/Ag2CO3 (20 wt%) composite.
Correspondingly, the RhB degradation rate constant was wherein C0 is the equilibrium concentration of pollutant, C is
investigated in Fig. 8B. In general, the photocatalytic degrada- the concentration of remaining dye solution at each irradiation
tion of RhB ts well with a pseudo-rst-order reaction within time, k is the rate constant, and t is reaction time. As shown in
the low-concentration region, and its kinetics could be calcu- Fig. 8C, the degradation rate constants (k) of RhB over BiOI/
lated as follows:
Ag2CO3 (10 wt%), BiOI/Ag2CO3 (20 wt%) and BiOI/Ag2CO3
(50 wt%) were 0.251, 0.327 and 0.287 minꢁ1, respectively. Also,
the BiOI/Ag2CO3 (20 wt%) composite exhibited the highest rate
ln(C0/C) ¼ kt
(1)
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Fig. 7 Transient photocurrent property of (a) BiOI, (b) Ag2CO3 and (c)
BiOI/Ag2CO3 (20 wt%) composite.
Fig. 5 UV-vis diffuse reflectance spectra of (a) BiOI, (b) Ag2CO3 and (c)
BiOI/Ag2CO3 (20 wt%) composite.
Aer 5 recycles, the used BiOI/Ag2CO3 composite maintained
80% of the original activity over the fresh catalyst, revealing the
excellent stability of BiOI/Ag2CO3 catalyst.
In addition, the leachability test was also performed. 50 mg
catalyst was added into the 50 mL distilled water, and aer
stirring for 2 h, the catalyst was removed by centrifugation. A
certain amount of RhB was added in the ltrate to obtain
5 mg Lꢁ1 RhB solution, which was irradiated by 300 W Xe lamp,
and 4 mL of the solution was collected every given time interval
and detected via UV-vis spectrophotometer. The result is shown
in Fig. S1.† Clearly, RhB was hardly degraded. This result
indicated that BiOI/Ag2CO3 composites were not leached.
Correspondingly, Fig. 10 provides the XRD patterns of BiOI/
Ag2CO3 (20 wt%) composite before and aer degradation of
RhB. Aer three catalytic runs, the Ag0 peak intensity at 38.1ꢀ
appeared and intensied with the recycle runs. However, the
other peaks did not change compared with the fresh catalyst.
The existence of metal Ag in the spent BiOI/Ag2CO3 composite
suggested that Ag2CO3 gradually changed into Ag0 during the
photodegradation process. The change in chemical state of Ag
species was possibly caused due to the combination of photo-
generated electrons with Ag+, which made the amounts of Ag
gradually increase in photocatalytic reactions.
Fig. 6 Photoluminescence spectra of (a) BiOI and (b) BiOI/Ag2CO3
(20 wt%).
constant, which was about 6.75 times larger than that of pure
BiOI (0.0484 minꢁ1) and 4.3 time larger than that of bare
Ag2CO3 (0.0748 minꢁ1).
3.3 Photocatalytic mechanism detection
To determine the universality of the BiOI/Ag2CO3 catalyst for
photodegrading other pollutants, the photocatalytic degrada- As we know, some active species, such as holes (h+), electrons
tion of MB, MO and phenol solution over pure BiOI, Ag2CO3 and (eꢁ), superoxide radicals (cO2ꢁ) and hydroxyl radicals (cOH), are
BiOI/Ag2CO3 (20 wt%) composite were also detected. As shown generated under light irradiation in the photodegradation
in Fig. 7D–F, BiOI/Ag2CO3 (20 wt%) composite still showed process. Therefore, in order to measure the dominating reactive
remarkably improved photocatalytic activity compared with species during photodegradation of RhB over BiOI/Ag2CO3
separate BiOI and Ag2CO3. The degradation conversation of MB composite and investigate the mechanism of the photocatalytic
was 95.5% within 12 min; 96% of MO was converted within reaction, reactive species trapping experiments were performed.
20 min; and for phenol, 90.1% was degraded aer 40 min Ethylene diamine tetraacetic acid (EDTA), tert-butyl alcohol
irradiation, exhibiting the composite's universal application for (t-BuOH) and p-benzoquinone (p-BQ) were used to quench h+,
ꢁ
photodegrading cationic and anionic dyes together with phenol cOH and cO2 , respectively.36–41 As shown in Fig. 11, the intro-
pollutant.
duction of p-BQ and EDTA caused signicant reduction of
ꢁ
The stability of a catalyst is also a signicant factor for its activity, implying that both cO2 and h+ were the main active
practical application. Hence, recycling experiments for the species. However, only a slight suppression of activity was found
photocatalytic degradation of RhB over BiOI/Ag2CO3 (20 wt%) with t-BuOH adding, indicating that cOH radical gave a rela-
composite were performed, and results are presented in Fig. 9. tively small contribution to the photocatalytic reaction.
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Fig. 8 (A) The photodegradation curves of RhB; (B) first-order kinetic plots for the photodegradation of RhB and (C) the corresponding rate
constants; (D) the photodegradation curves of MB; (E) the photodegradation curves of MO and (F) phenol. (a) Ag2CO3, (b) BiOI, (c) BiOI/Ag2CO3
(10 wt%), (d) BiOI/Ag2CO3 (20 wt%) and (e) BiOI/Ag2CO3 (50 wt%).
In addition, the valence band (VB) and conduction band (CB) the energy of free electrons on the hydrogen scale at about
potentials of BiOI and Ag2CO3 were calculated based on the 4.5 eV, and Eg is the band gap energy of the semiconductor (for
following empirical equations.42,43
BiOI, Eg ¼ 1.76 eV; for Ag2CO3, Eg ¼ 2.16 eV). Thus, the EVB and
ECB of BiOI were calculated to be +2.32 and +0.56 eV, and the EVB
and ECB of Ag2CO3 were at +2.6 and +0.44 eV.
Thus, a possible mechanism for the enhanced photocatalytic
activity of BiOI/Ag2CO3 composite is proposed. Apparently, the
matched band edges of BiOI (EVB ¼ 2.32 eV, ECB ¼ 0.56 eV) and
Ag2CO3 (EVB ¼ 2.6 eV, ECB ¼ 0.44 eV) could not be constructed so
that it was difficult promote the separation of the photoinduced
carriers. Moreover, the experiments of active species indicated
EVB ¼ X ꢁ Ee + 0.5Eg
ECB ¼ EVB ꢁ Eg
(2)
(3)
where EVB and ECB are the valence and conduction band edge
potentials. X is the electronegativity of the semiconductor, and
the X values for BiOI and Ag2CO3 are 5.94 and 6.02 eV;44,45 Ee is
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Fig. 11 Effects of the addition of different scavengers on the degra-
dation of RhB over BiOI/Ag2CO3 (20 wt%) composite: (a) no scavenger,
(b) adding 1 mM EDTA, (c) adding 1 mM p-BQ and (d) adding 5 mM
t-BuOH.
Fig. 9 Recycling experiments for photodegrading RhB over BiOI/
Ag2CO3 (20 wt%) composite.
ꢁ
that cO2 radical played the main role in the process of
degꢁrading pollutants, but the standard redox potential of O2/
cO2 (+0.13 eV vs. SHE)46 was more positive than that of BiOI or
Ag2CO3; theoretically, the electrons on the CB of BiOI or Ag2CO3
could not reduce O2 to cO2ꢁ. However, under visible light irra-
diation with energy less than 3.10 eV (l > 400 nm) in the BiOI/
Ag2CO3 system, similar to the previous reports,47–50 the VB edge
of BiOI and Ag2CO3 could be boosted to a higher potential edge,
respectively reaching ꢁ0.78 and ꢁ0.50 eV. Therefore, the
reformed and complementary band edges for the BiOI/Ag2CO3
composite are illustrated in Fig. 12. Under visible-light irradi-
ation, both BiOI and Ag2CO3 were excited to generate electrons
(eꢁ) and holes (h+). The eꢁ on the CB of BiOI could inject into
the CB of Ag2CO3, and Ag2CO3 is partly reduced to form Ag
nanoparticles by eꢁ; then, a part of eꢁ could easily be captured
by metal Ag through the Schottky battier. Finally, eꢁ on the
reformed CB potential could reduce the adsorbed O2 to yield
cO2ꢁ radical, which combined with H2O to further transform tꢁo
the active cOH species.51 This result might explain why the cO2
radical was the main active species. The photogenerated h+ on
the VB of Ag2CO3 could transfer to the VB of BiOI and become
directly involved in the degradation of pollutants because the
VB levels of BiOI and Ag2CO3 are less positive than the standard
redox potential of cOH/H2O (2.68 eV vs. SHE).52 This might
explain the reason that cOH radical exhibited small effect on the
photodegradation of RhB. Finally, the obtained main active
species h+ and cO2ꢁ reacted with the pollutants to generate the
degradation products. As a result, in the photocatalytic process,
the recombination rates of the photoinduced electron–hole
pairs were restricted to reach efficient charge separation, which
subsequently promoted the photocatalytic efficiency.
According to the previous literature,53–55 the intermediate
and nal products of RhB degradation are shown in Fig. 13. The
photodegradation of RhB has four main processes, including
N-deethylation, chromophore cleavage, opening-ring and
mineralization. During the photodegradation process, various
active intermediate species, such as cO2ꢁ, cOH, h+ and eꢁ, are
generated, which are involved in destroying RhB structure.
Fig. 10 The XRD patterns of BiOI/Ag2CO3 (20 wt%) composite before Fig. 12 Proposed degradation mechanism of pollutants over the BiOI/
and after reuse for photodegrading RhB.
Ag2CO3 composite.
544 | RSC Adv., 2018, 8, 537–546
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Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
We sincerely acknowledge the nancial support from National
Natural Science Foundation of China (21673060), Fund for
Research and Development of Science and Technology in
Shenzhen (No. JCYJ20160427184531017), and State Key Lab of
Urban Water Resource and Environment of Harbin Institute of
Technology (HIT2017DX10), talent scientic research fund
of LSHU (No. 2016XJJ-080) and basic research projects of
Liaoning Provincial Education Department (L2017LQN004).
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