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PAPER Qingzhu Zhang et al. Catalytic mechanism of C–F bond cleavage: insights from QM/MM analysis of fluoroacetate dehalogenase
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DOI: 10.1039/C7CY01162H
In
situ
chemical
transformation
synthesis
of
2
Bi4Ti3O12/I-BiOCl 2D/2D heterojunction systems for water
3
pollution treatment and hydrogen production
Published on 31 July 2017. Downloaded by University of Waterloo on 07/08/2017 03:13:04.
4 Kun Qiana, Li Xiaa, Zhifeng Jiangb,c,*, Wei Weib, Linlin Chena, Jimin Xiea,*
5 6 7 8 9
a
School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang, 212013, PR
10
China
11
b
12
212013, PR China
13
c
14
Corresponding authors: Zhifeng Jiang & Jimin Xie; Tel.: +86-511-88791708;
15
Fax: +86-511-88791800;
16
E-mail:
[email protected],
[email protected].
Institute for Energy Research, Center of Analysis and Test, Jiangsu University, Zhenjiang,
School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong
17 18 19 20
ABSTRACT
21
Enhancing visible light response and inhibiting the recombination of
22
photogenerated charge carriers are vital for Bi4Ti3O12 nanosheets to
23
achieve high activity in the fields of hydrogen generation and water
24
pollutant treatment. Hence, in this work, Bi4Ti3O12/I-BiOCl 2D/2D
Catalysis Science & Technology Accepted Manuscript
1
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25
heterojunction systems have been constructed successfully for the first
26
time, via a modified in situ ion exchange approach at room temperature.
27
The intimate 2D/2D interface can provide sufficient contact surface to
28
enhance the charge transfer rate. And the doping of I− ions can
29
dramatically improve the visible light absorption of the composites,
30
ensuring the quantity of photogenerated electron-hole pairs. Moreover,
31
well-matched band structure can lead to the effective separation of
32
photogenerated charges. Optical and electrochemical measurements are
33
used to prove the above-mentioned points. Bi4Ti3O12/I-BiOCl exhibited
34
highly enhanced visible catalytic activity towards the hydrogen
35
production and organic pollutants degradation, due to the reduced
36
photogenerated carriers recombination rate and the dramatically enhanced
37
visible light absorption. The possible photocatalytic mechanism was
38
proposed based on the results of active species trapping experiment and
39
ESR analysis. This work may open up new prospect for constructing
40
other
41
distinguished visible light response and efficiently charge carries
42
separation rate.
2D/2D
bismuth-based
semiconductor
photocatalysts
43 44 45 46
Keywords: photocatalysis, Bi4Ti3O12, nanosheet, BiOCl, doping
with
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1. Introduction
48
As an eco-friendly and promising technology, semiconductor
49
photocatalysis has been extensively applied in hydrogen production [1-3],
50
carbon dioxide reduction [4-6] and visible light decomposion [7-9]. In
51
recent years, layered perovskites are of considerable interest in the field
52
of photocatalysis because of the outstanding catalytic activities [10-13].
53
One prominent part of them is Bi4Ti3O12 (BTO), which has a lower
54
optical band gap (Eg of 2.8 eV) compared with TiO2 (Eg of 3.2 eV)
55
[14,15]. It has been explored as a novel high-efficiency photocatalyst in
56
recent years, owing to its unique crystal structure of alternating (Bi2O2)2+
57
and perovskite (Bi2Ti3O10)2- layers, as well as the electronic structure
58
consisting of hybridized valence band at the Bi 6s and O 2p levels [16].
59
Meanwhile, it is generally accepted that the catalytic performance of
60
semiconductor is highly dependent on the exposed crystal facets, particle
61
size, and surface architecture [17]. Hence, various work has been focused
62
to synthesize novel Bi4Ti3O12 photocatalyst with different morphologies
63
and dimensionality, including nanofiber [18], nanoparticle [19], nanodot
64
[20], micro-cross sphere [21] and mesoporous network [22]. Among
65
various morphologies, two-dimensional (2D) Bi4Ti3O12 nanosheet is
66
particularly attractive for the unique properties of higher surface area,
67
shorter diffusion distance of photogenerated charge carriers, and more
68
photocatalytic active sites [23], which in turn enhance its photocatalytic
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47
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69
performance. Lots of effort have been spent on synthesizing 2D Bi4Ti3O12
70
nanosheet dominated with {001} facets. He and co-workers reported
71
size-controlled Bi4Ti3O12 nanosheets by a molten salt method [24]. The
72
as-prepared Bi4Ti3O12 nanosheet shows the better photocatalytic kinetics
73
in photodegradation of rhodamine B (RhB), which is about 8.65 times
74
faster than that of bulk one obtained by a traditional solid-state reaction
75
method. Hou et al. obtained single-crystalline Bi4Ti3O12 nanosheets via a
76
sol-gel hydrothermal process [25], whose photodegradation efficiency of
77
Rhodamine B was nearly 3 times than that of conventionally calcined
78
Bi4Ti3O12. However, the same as TiO2, perovskite nanosheets also have
79
two main drawbacks which greatly hinder its further practical application:
80
one is the relatively limited absorption of visible-light and the other is the
81
rapid recombination of photogenerated electron-hole pairs [26-28].
82
Therefore, enhancing visible light response and inhibiting the
83
recombination of photogenerated charge carriers become very prominent
84
strategies for Bi4Ti3O12 nanosheets to achieve higher activity in
85
photocatalytic application.
86
To address these problems, coupling Bi4Ti3O12 with other materials
87
such as TiO2 [29], CuFe2O4 [30], BiOI [31], g-C3N4 [32] and RGO [33] is
88
an effective method to enhance the photocatalytic performance of the
89
pure Bi4Ti3O12, because the heterostructure between semiconductors can
90
efficiently separate photogenerated electron and hole to reduce the
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91
recombination of charge carries [34]. Recently, emphasis has been given
92
on designing novel 2D nanosheet-based heterojunction systems
93
benefiting from 2D structural feature, via coupling 2D nanosheet
94
materials with other nanostructured semiconductor to form 0D/2D,
95
1D/2D, and 2D/2D heterostructures [35-38]. Particularly, owing to the
96
significant advantages of the efficient physical contact and the enhanced
97
charge transfer rate, 2D/2D heterostructures are more expected as
98
compared with the 0D/2D and 1D/2D heterostructures [39-41]. A number
99
of 2D/2D heterostructure systems have been demonstrated with enhanced
100
photocatalytic performance, but mostly limiting in coupling with g-C3N4
101
or RGO nanosheets [42-44]. Therefore, it is still a challenge to construct
102
novel 2D/2D heterojunction systems based on perovskite nanosheets.
103
In order to realize the 2D/2D system mentioned above, Bi-based
104
oxychlorides (BiOCl) nanosheet was chosen as another semiconductor
105
because of its available plate-like structure and high photocorrosion
106
stability [45-47]. More importantly, similar literatures have reported the
107
successful synthesis of Bi2O3/BiOCl and Bi7F11O5/BiOCl composites by
108
the partial conversion [48,49]. Bi4Ti3O12 and BiOCl both belong to
109
bismuth-based semiconductor with similar sandwiched layers structure,
110
they may easily grow together through an ion exchange process [50].
111
However, the large band gap of pure BiOCl makes it only absorb the
112
ultraviolet light, limiting the solar-light photocatalytic activity of the
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113
composite photocatalyst. In other words, coupling Bi4Ti3O12 with pure
114
BiOCl can not overcome the defect of its limited visible light response.
115
Fortunately, it has been proved that the non-metal doping of I− ion
116
showed great potential for introducing visible light adsorbing
117
functionality to BiOCl, through raising the VB level [51]. In addition,
118
dopants can play a role as trapping centers to effectively separate the
119
photogenerated hole-electron pairs [52]. Coupling Bi4Ti3O12 nanosheet
120
with I-BiOCl nanosheet may be a good idea to over come the two main
121
drawbacks mentioned above. Therefor, it is necessary to provide an
122
special strategy to design novel Bi4Ti3O12/I-BiOCl 2D/2D heterojunction
123
systems with distinguished visible light response and efficiently charge
124
carries separation rate.
125
Based on the above considerations, in this work, we have
126
successfully
constructed
Bi4Ti3O12/I-BiOCl
2D/2D
heterojunction
127
systems for the first time, via a modified in situ ion exchange approach in
128
room temperature. By heating a stoichiometric composition of α-Bi2O3
129
and TiO2 in molten NaCl-KCl, Bi4Ti3O12 nanosheets were obtained and
130
served as raw materials. By further reacting with Cl− and I− in acidic
131
condition, I-BiOCl films grew on the surface of Bi4Ti3O12 nanosheets to
132
form a 2D/2D composite system. The as-obtained 2D/2D interface can
133
provide sufficient contact surface to enhance the charge transfer rate. And
134
the well-matched band between Bi4Ti3O12 and I-BiOCl can ensure the
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135
effectively separation of photogenerated charges. Meanwhile, significant
136
improvement of the visible-light adsorption ability can be observed both
137
in I-BiOCl and Bi4Ti3O12/I-BiOCl composites after the doping of I− ion.
138
Thus, the two main drawbacks of Bi4Ti3O12 nanosheets have been solved.
139
To test the actual photocatalytic activity of Bi4Ti3O12/I-BiOCl composites,
140
hydrogen generation and photodegradation experiments were performed.
141
All of the experimental results indicated that Bi4Ti3O12/I-BiOCl 2D/2D
142
composites exhibited the best photocatalytic efficiency, due to the
143
reduced photogenerated carriers recombination rate and the dramatically
144
enhanced visible light absorption. Also, the possible photocatalytic
145
mechanism was proposed based on the results of active species trapping
146
experiment and ESR analysis. It is anticipated that the repeatable and
147
imitable method provided here may open up an effective strategy to
148
design other 2D/2D bismuth-based semiconductor photocatalysts with
149
distinguished visible light response and efficiently charge carries
150
separation rate.
151
2. Experimental details
152
2.1. Preparation of the Bi4Ti3O12 nanosheet
153
Bi4Ti3O12 nanosheet was synthesized via a molten salt synthesis
154
(MSS) method. Firstly, according to the composition of Bi4Ti3O12,
155
powders of α-Bi2O3 (99.9 %, Sinopharm Chemical Reagent Co., Ltd.) and
156
P25 (Degussa) with appropriate stoichiometric amounts were mixed.
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157
Then, KCl and NaCl were taken as the cosolvents at a mole ratio of KCl :
158
NaCl : Bi4Ti3O12 = 50 : 50 : 1. Then the resulting mixture was headed to
159
800 oC at a heating rate of 5 oC min-1 for 2 hours in air after continuously
160
grinded for some time in the agate mortar. The obtained powder, which
161
named as BTO, was washed with deionized water and absolute ethyl
162
alcohol several times to wash off the inorganic salt and dried at 60 oC in
163
air.
164
2.2. Fabrication of the Bi4Ti3O12/I-BiOCl nanosheet heterojunctions
165
Bi4Ti3O12/I-BiOCl nanosheet heterojunctions were synthesized by a
166
simple in situ chemical transformation method in the room temperature.
167
Firstly, 1 mmol KI was dispersed in 100 ml 0.1 M HCl solution (the
168
molar ratio was 1 : 10). Secondly, 0.2 g as-prepared BTO was added into
169
the solution to form a homogeneous suspension. After being stirred for 12
170
h, the precipitates were then separated from the suspension by
171
centrifugation, followed by washing with deionized water for several
172
times and dried at 60 oC in air. The obtained sample was named as
173
BTO/I-BOC. For comparison, BTO/BOC were also synthesized by the
174
same method without adding KI. Pure BOC and I-BOC were prepared
175
according to the previous report [51]. According to ICP analysis, the the
176
actual molar ratios of BTO and BOC (or I-BOC) in BTO/BOC and
177
BTO/I-BOC are about 4.8 : 1 and 4.81 : 1 respectively, which are roughly
178
the same.
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179
Scheme 1 shows the fabrication process of the BTO/I-BOC
180
heterojunctions. According to similar reports [47,50], the as-prepared
181
Bi4Ti3O12 nanosheets and HCl were served as substrate and transforming
182
reagent, respectively. When Bi4Ti3O12 nanosheets and HCl solution were
183
mixed, the outside of Bi4Ti3O12 nanosheets could be dissolved by the H+
184
(Eq. (1)). Then, an intermediate product BiO+ was generated because of
185
the hydrolysis of Bi3+ (Eq. (2)). Finally, BiO+, Cl− and I− gradually form
186
the second phase I-BiOCl on the surface of Bi4Ti3O12 nanosheets, due to
187
small solubility product constant (1.8×10-31 ) (Eq. (3)).
188
Bi4Ti3O12 + 24H+ → 4Bi3+ + 3Ti4+ + 12H2O
(1)
189
Bi3+ + H2O → BiO+ + 2H+
(2)
190
BiO+ + (1-x)Cl− + xI− → BiOCl1-xIx
(3)
191
The details of characterization, photoelectrochemical measurements,
192
photocatalytic hydrogen production and hazardous pollutant degradation
193
measurements are shown in Supporting Information.
194
3. Results and discussion
195
3.1. Structural and morphological analysis
196
The phase composition and crystal structure of the pure and
197
composite systems were examined by X-ray diffraction measurements
198
(XRD). Fig. 1A shows that pure BOC and I-BOC exhibit high intensity
199
peaks assigning to tetragonal phase BiOCl [54,55] (JCPDS card No.
200
82-0485). For BTO nanosheet, twelve main distinctive peaks at 2θ =
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201
16.07°, 21.64°, 23.31°, 30.06°, 32.85°, 37.04°, 38.35°, 39.88°, 47.32°,
202
47.80°, 51.53° and 57.23° can match well with perovskite Bi4Ti3O12
203
[24,25] (JCPDS card No. 72-1019). For BTO/BOC and BTO/I-BOC
204
composites, all the peaks can be assigned to Bi4Ti3O12 or BiOCl. Notably,
205
no characteristic peak of BiOI was detected in I-BOC and BTO/I-BOC.
206
Meanwhile, as shown in Fig. 1B, the (110) and (102) peaks are slightly
207
shifted to lower angle region for both I-BOC and BTO/I-BOC, while the
208
the (200) and (020) peaks of BTO/I-BOC show no obvious movement
209
compared with pure BTO and BTO/BOC composite, after doping with
210
little amount of I− ions. This tiny change could be another the important
211
evidence that I− ions with larger ionic radius were successfully
212
incorporated into the BiOCl lattice by replacing a portion of Cl− ions with
213
smaller ionic radius [51,56].
214
To
investigate
chemical
composition
of
the
as-prepared
215
photocatalyst, X-ray photoelectron spectroscopy (XPS) analysis was used
216
and the results are shown in Fig. 2. In Fig. 2A, the surface of BTO/I-BOC
217
sample is composed of C, Bi, Ti, O, Cl and I elements. The C 1s peak at
218
around 284.8 eV caused by carbon contained instrument was used for
219
calibration [57]. As shown in Fig. 2B, spectra of Bi 4f5/2 and Bi 4f7/2
220
located at 164.5 eV and 159.1 eV correspond to the trivalent bismuth
221
[58]. Fig. 2C shows the high-resolution XPS spectrum of O 1s, which is
222
also composed of two peaks at positions around 532.7 eV and 529.7 eV,
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223
after fitting processing. These two peaks can belong to surface-adsorbed
224
hydroxyl groups [59] and the crystal O of the sample [60], respectively.
225
As for Ti 2p element (Fig. 2D), two peaks at 466.1 eV and 457.8 eV can
226
be assigned to the Ti 2p1/2 and Ti 2p3/2 respectively, affirming that the
227
valence state of Ti was +4 in the BTO and BTO/I-BOC samples [61]. The
228
XPS signals in Fig. 2E are related to Cl 2p, the binding energies of Cl 2p
229
at 199.6 eV (Cl 2p1/2) and 198.1 eV (Cl 2p3/2) demonstrate the existence
230
state of Cl− in the samples [62]. Most importantly, characteristic peaks of
231
doped I element can be observed in Fig. 5F, where I 3d3/2 and I 3d5/2
232
peaks owned 630.8 eV and 619.3 eV binding energies. The observation of
233
the I 3d peaks prove that I exists as I− in BiOCl nanosheets of
234
BTO/I-BOC samples [51]. As a result, it is concluded that BTO/I-BOC
235
heterostrcure was successful constructed through in situ chemical
236
transformation process, along with the I− ion doping step. Notably,
237
compared with BTO and I-BOC, the peak positions and intensities of Bi,
238
Ti, O and Cl elements in BTO/I-BOC composites shows a slight shift.
239
This may prove the strong interactions at the interfaces between BTO and
240
I-BOC [40].
241
The surface morphologies of various samples were characterized by
242
SEM (Fig. 3). In Fig. 3A, inerratic rectangular BTO nanosheet could be
243
clearly observed with the average side length of 1.5 µm. The BOC and
244
I-BOC nanosheets synthesized by the precipitation method self-assemble
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245
to form microspheres with 1-2 µm in diameter (Fig. 3B and C). For
246
BTO/BOC sample, it can be observed that the BTO nanosheets exhibit
247
rectangular structure with more thickness, showing big difference from
248
the BOC nanosheets with circular shape (Fig. 3D). The well-defined
249
2D/2D structure has been obtained via the in situ chemical transformation
250
method. As shown in Fig. 3E, similar morphology can be obtained in
251
BTO/I-BOC, indicating that the adding of I− ions during the chemical
252
transformation process will not change the 2D/2D structure. Meanwhile,
253
elemental mappings were also performed to conform the formation of the
254
BTO/I-BOC heterostructures. As shown in Fig. 3E-J, the SEM image and
255
EDS elemental mappings from this area display evenly distributed points
256
of Bi, O, Ti, Cl and I elements. Obviously, the I-BiOCl nanosheet and the
257
EDS mapping of elements I are in the same region, further demonstrating
258
that I elements have been successfully doped into BiOCl nanosheet.
259
Highly homogeneous composition and structure affinity between BTO
260
nanosheets and I-BOC nanosheets have been obtained in this work [27].
261
TEM and HRTEM images were used to further investigate the
262
microstructure of the products. Fig. 4A shows the representative TEM
263
image of the lamellar BTO nanosheet, which is correspond with the SEM
264
image. For BTO/BOC sample, BOC nanosheets are anchored on the
265
surface of BTO nanosheet to form a well-defined 2D/2D structure (Fig.
266
4B). As shown in Fig. 4C, similar morphology can be observed in
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267
BTO/I-BOC. HRTEM image of the BTO/I-BOC sample is shown in Fig.
268
4D. The lattice fringe spacing of 0.270 nm and 0.272 nm are for the (020)
269
and (200) crystal plane of BTO nanosheet, indicating that exposed facets
270
of BTO can be determined as (001) facets [24]. Meanwhile, other lattice
271
spacing are displayed and measured to be 0.274 nm and 0.157 nm, which
272
match well the separation of the I-BOC (110) and (212) planes [47].
273
Furthermore, STEM images are provided in Supporting Material (Fig.
274
S2). The above results may be other solid evidences that I-BOC
275
nanosheets successfully grew on the surface of BTO nanosheets.
276
3.2 Band structure analysis
277
Since the the light absorption properties played an crucial role in
278
photodegradation, UV-vis DRS was performed to investigate the optical
279
properties of as-prepared photocatalysts [63]. As shown in Fig. 5A, BOC
280
nanosheets only have photo-absorption at UV light region, with the
281
absorption edge near 360 nm. A little better than pure BOC, pure BTO
282
nanosheets exhibit an optical response at wavelengths around 450 nm,
283
indicating that the BTO sample can be excited by a narrow region of the
284
visible light [64]. However, when coupling BTO with pure BOC, the light
285
absorption range of BTO/BOC sample is situated between them and the
286
absorption edge occurs at around 420 nm. The limited absorption of
287
visible light may severely restrict its photocatalytic performance. It
288
should be noted that the fundamental absorption edge of I-BOC is greatly
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289
extended to the visible light region (450-600 nm) after I− doping. The
290
formed impurity levels by I− ions may account for this obvious red-shift
291
in the absorption threshold, indicating an decreased band-gap energies
292
[65-67]. As expected, compared with BTO/BOC, enhanced visible light
293
absorption can also be observed in BTO/I-BOC sample. The enhanced
294
capture of visible light may contribute to more photogenetated
295
electron-hole pairs and then lead to the favorably improved photocatalytic
296
activity [68].
297 298
299
Typically, the band gap of semiconductors can be calculated by Kubelka-Munk (KM) expression [69]: αhν = A(hν − Eg)n/2
(4)
300
where α, h, ν, Eg and A are the absorption coefficient, Planck’s constant,
301
light frequency, band gap and constant respectively. The value of the
302
exponent n denotes the nature of the sample transition. According to
303
previous reports, the n values of BOC and BTO are both 4 because they
304
are indirect band-gap materials [17,25]. The plots of (αhν)1/2 versus
305
energy (hν) is shown in Fig. 5B. The band gap energies of BTO, BOC
306
and I-BOC samples are calculated to be about 2.72 eV, 3.34 eV and 2.07
307
eV, respectively. These results are in good agreement with the reported
308
data [26,54]. Meanwhile, The VBM of BTO and the I− doping level of
309
I-BOC can be obtained by XPS valence spectra in Fig. 5C. According to
310
the formula of ECB = EVB − Eg, the corresponding CBM position can be
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311
calculated to be -1.56 eV for BTO and -0.23 eV for I-BOC. Jia et al.
312
reported their work about I-BiOCl/I-BiOBr composites. They measured
313
the CBM position of BiOCl and I-BiOCl by Mott-Schottky curves and
314
found the doping of I− ions could not change the the ECB values of
315
BiOCl [51,70]. Thus, the CBM position of BOC may be -0.23 eV, the
316
same as that of I-BOC. According to Eg of BOC, the VBM of BOC can be
317
calculated to be about 3.11 eV. Based on the above analysis, the
318
well-matched energy band structure between BTO and I-BOC can be
319
observed in Fig. 5D, which can play a crucial role in facilitating the
320
separation of electron-hole pairs.
321
3.3 PL analysis
322
PL measurements were applied to evaluate the separation efficiency
323
of photogenerated charge carriers in the as-obtained catalysts, since the
324
PL emission arises from the recombination of free carriers [71]. Fig. 6
325
shows the PL spectra of different samples in the wavelength range of
326
420-600 nm with a 320 nm excitation. In the spectrum of BOC, there are
327
four strong emission peaks at about 450 nm, 470 nm, 555 nm and 585
328
nm. The emission of BOC at 450 nm may derive from the self-trapped
329
excitons [72] and the distinct emission peaks at 470 nm and 555 nm may
330
belong to the emission of the band gap transition [73]. Meanwhile, the
331
emission at about 585 nm may originate from the radiative transitions
332
within the sub-states [74]. The peaks of BOC is similar to that of recent
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333
report [75]. BTO nanosheets also exhibit similar emission peaks, which
334
might be due to the intrinsic luminescence properties of Bi4Ti3O12, similar
335
to Bi2MoO6 [76]. Apparently, pure BTO and BOC show the highest
336
intensity of emission peak, indicating the highest recombination rate of
337
photogenerated electron-hole pairs [77]. For BTO/BOC and I-BOC
338
samples, obvious decreased PL intensity can be observed. Both the
339
formation of heterogeneous structure and the doping of I− ions can
340
efficiently facilitate the separation of photon generated carriers [51,78].
341
Predictably, when coupling BTO with I-BOC, the BTO/I-BOC sample
342
shows the lowest PL intensity and recombination rate, which benefits the
343
photocatalysis.
344
3.4. Photoelectrochemical measurements
345
Photocurrents were measured for as-prepared samples to investigate
346
the recombination of photogenerated electrons-holes in another way as
347
the photocurrent response results from the segregation of free charge
348
carriers [79]. Generally, the higher photocurrent density demonstrates
349
more efficient charge separation and transportation. As shown in Fig. 7A,
350
all of the samples show reproducible response swiftly to on/off visible
351
light irradiation cycles. Compared with pure BOC, I-BOC sample shows
352
the higher photocurrent density. The improved visible light absorption
353
can increase the quantity of photogenerated electron-hole pairs and the
354
doping of I− ions can play a role as trapping centers to effectively separate
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355
them [52,80]. As expected, the highest photocurrent intensity can be
356
observed in BTO/I-BOC sample, which is about 2.2 and 3 times as high
357
as that of BTO and I-BOC. Meanwhile, the photo-generated carrier
358
separation efficiency of BTO/I-BOC is more superior than that of
359
BTO/BOC, demonstrating the advantage of I− ions doping and better
360
photocatalytic performance of it. Specific explanation can be observed in
361
Fig. S3. In order to further conform this result, electrochemical
362
impedance spectra (EIS) measurements was performed and the Nyquist
363
plots are shown in Fig. 7B. It is well accepted that the smaller the arc
364
radius indicates the higher electron-hole pairs separation efficiency [81].
365
As expected, BTO/I-BOC sample exhibits the smallest arc radius due to
366
the lowest charge transfer resistance (Rct). The charge transfer resistance
367
values of the samples follow the order of BTO > BOC > I-BOC >
368
BTO/BOC > BTO/I-BOC, which is consistent with the PL and
369
photocurrents measurement. Thus, an excellent photocatalyst with
370
dramatic response of visible light and efficient separation of
371
photogenerated electron-hole pairs have been successfully constructed.
372
The photocatalytic performance of BTO/I-BOC is worth to be desired.
373
4. Photocatalytic activity measurements
374
4.1 photocatalytic performance of H2 production
375
H2 production experiment was first performed to evaluate the
376
accurate photoactivity of as-prepared samples, by adding 50 mg
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377
photocatalyst into 80 mL 25 vol % methanol solution. To ensure that H2
378
was photogenerated from methanol solution by catalysts, reference
379
experiments were carried without either photocatalyst or irradiation and
380
no appreciable amount of H2 was detected. That is to say, all the collected
381
H2 is the result of the photocatalytic reaction on photocatalysts. Fig. 8A
382
shows the H2 evolution activities of different catalysts. It can be seen
383
clearly that pure BOC makes no contribution to hydrogen evolution, due
384
to the only photo-absorption at UV light region. Similarly, the H2
385
evolution rate (HER) of pure BTO is very low, which is only 21.2 µmol
386
h-1 g-1 [26]. The rapid recombination rate of photogenerated electron-hole
387
pairs may account for this result [82]. Compared with the limited HER
388
performance of pristine BOC and BTO, BTO/BOC exhibits enhanced
389
photocatalytic activities, ascribed to the advantage of the heterojunction
390
structure. Notably, greatly increased HER value can observed in
391
BTO/I-BOC. The HER is determined as 91.7 µmol h-1 g-1, 3.2 and 18
392
times higher than that of BTO/BOC (28.7 µmol h-1 g-1) and I-BOC (5.1
393
µmol h-1 g-1), due to the further improved photic and electronic property,
394
resulted from I− ions doping. In addition, the HER of BTO/I-BOC is
395
higher than that of many similar BiOCl-based or perovskite
396
photocatalysts. Comparison of the results is shown in the Table S1.
397
Moreover, the H2 production cycle experiments were also carried to to
398
investigate the stability of as-prepared photocatalysts. No apparent
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399
deactivation of HER performances can be observed during the three times
400
cycles,
401
photocatalysts during H2 production (Fig. 8B).
402
4.2 photodegradation of dye and antibiotic
indicating
the
excellent
reusability of
the
as-prepared
403
The photocatalytic performance of single and composite samples
404
were further evaluated in degradation of RhB and MB. Prior to the light
405
irradiation, adsorption-desorption behavior of the catalyst was carried,
406
avoiding the error of the adsorption process during photodegradation.
407
According to the results of photocatalysts without light irradiation, the
408
adsorption-desorption balance has been achieved between the catalyst and
409
pollutant solution after 1 h adsorption. Because there is almost no change
410
of the concentration of pollutants, when compared with the result of 30
411
min adsorption. In Fig. 9A and C, photocatalytic activities towards the
412
degradation of RhB and MB over different photocatalysts have been
413
displayed. Without adding catalysts, the concentration of RhB/MB
414
approximately remains unchanged over time, excluding the possibility of
415
the self-photolysis process. Apparently, the degradation ratios of RhB
416
only reach 29 % and 44 % in 90 min respectively, using pristine BOC and
417
BTO as catalysts. Similarly, BOC and BTO can only degrade 38 %/41 %
418
of MB after 180 min of visible light irradiation because of limited visible
419
light response and fast combination of electron-hole pairs. When coupling
420
BTO with BOC, an passable enhanced performance of RhB/MB
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421
degradation can be observed in BTO/BOC. However, based on the optical
422
and electrochemical analysis, it can be speculated that though the
423
outstanding physical contact and well-matched band structure ensure the
424
transfer and separation of photogenerated electron-hole pairs, the limited
425
visible light response cannot generate abundant charge carriers (Fig. S2).
426
After solving this problem by I− ions doping, more than 90 % of RhB and
427
MB have been degraded by BTO/I-BOC. Additional, the corresponding
428
RhB/MB degradation kinetic curves over the as-prepared catalysts are
429
shown in Fig. 9B and D. The pseudo-first-order rate constants of
430
RhB/MB over BTO, BOC, I-BOC, BTO/BOC and BTO/I-BOC are
431
0.0035/0.0028,
432
0.0251/0.0130 min-1, respectively. As expected, BTO/I-BOC sample with
433
the best photocatalytic activity shows the highest reaction rate constant
434
values, which are nearly 7.1/5.4 and 4.7/3.8 times than that of pure BOC
435
and BTO.
0.0046/0.0034,
0.0067/0.0043,
0.0084/0.0056
and
436
As the most commonly used antibiotic drugs in treating bacterial
437
infections, untreated ciprofloxacin (CIP) and tetracycline hydrochloride
438
(TC) in natural water may strengthen the drug resistance of bacteria,
439
which will pose a serious threat to human health. Thus, photodegradation
440
experiments of these pollutants was performed and the results are shown
441
in Fig. 9E. Details of the degradation curves are shown in the Supporting
442
Information (Fig. S4). Similarly, after 120 min visible light irradiation,
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443
enhanced photocatalytic activity can be observed in BTO/BOC and
444
I-BOC samples. And BTO/I-BOC exhibits the highest photoactivity as
445
expected.
446
To gain deeper insight into the RhB/MB/CIP/TC degradation, total
447
organic carbon (TOC) measurement was performed to study the
448
mineralization rate of these four pollutants, using BTO/I-BOC as the
449
representative photocatalyst. After 180 min of visible light irradiation,
450
95.1%/93.2%/88.4%/80.7%
451
RhB/MB/CIP/TC, respectively (Fig. 9F). Most of the pollutants have
452
been mineralized after photodegradation, further demonstrating the
453
dramatic photocatalytic performance of BTO/I-BOC.
of
TOC
were
eliminated
for
454
For the long-term use in practical application, the reusability of the
455
photocatalysts were investigated by cycle experiments of BTO/I-BOC
456
sample. It can be seen clearly in Fig. 10A that the photocatalytic activity
457
of BTO/I-BOC shows no apparent deactivation. Furthermore, the XRD
458
result of the BTO/I-BOC sample before and after four photocatalytic tests
459
is shown in Fig. 10B. It can be found that there is no observable change
460
of the crystalline structure and phase of the nanocomposites, further
461
demonstrating the sufficient stability of the catalysts during the
462
photodegradation process.
463
4.3 Species trapping experiments and ESR analysis
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464
In order to study the active species generated in the photo-catalytic
465
system, species trapping experiments were first performed. As shown in
466
Fig. 11, the RhB degradation efficiency decreases slightly, when 1 mmol
467
of t-BuOH (hydroxyl radical scavenger) was added, implying that •OH is
468
not the main active species. On the contrary, the degradation rate of RhB
469
is depressed obviously when 1 mmol of AO (hole scavenger) was added
470
into the reaction system, suggesting that holes (h+) are of great important
471
towards the degradation of RhB. Especially, dramatic reduction of the
472
RhB degradation rate can be observed when choosing BQ as the
473
superoxide radical (•O2-) scavenger, indicating that the •O2- plays a
474
crucial role towards the RhB oxidation process. Furthermore, the
475
controlled experiment in a N2 atmosphere was conducted to conform this
476
points. The removal efficiency of RhB was obviously inhibited. This
477
result further demonstrates the crucial role of •O2- during the
478
photodegradation of RhB. In summary, •O2- and h+ may be the major
479
reactive species during the photodegradation of RhB.
480
Additionally, to further notarize the main active species during
481
photodegradation, ESR spin-trap technique was applied and the
482
BTO/I-BOC samples were chosen as the representative photocatalysts. It
483
is apparent that four characteristic peaks of the DMPO-•O2- appeared
484
when the light is on. When the light is off, no ESR signal can be observed
485
(Fig. 12A), implying the crucial part •O2- played in the photodegradation
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486
process in this system. However, for •OH, no characteristic signals of the
487
DMPO-•OH can be detected whatever the light is on or off (Fig. 12B).
488
This result is corresponding with the active species trapping experiment.
489
4.4 Possible photocatalytic mechanism
490
Additionally, a possible mechanism over the BTO/I-BOC sample is
491
proposed in Scheme 2. According to band structure analysis above, the
492
CB and VB edge potentials of BTO vs. the normal hydrogen electrode
493
(NHE) are estimated at -1.56 and +1.16 eV [19], while the conduction
494
and valence band edge potentials of BOC vs. the normal hydrogen
495
electrode (NHE) are about -0.23 and +3.11 eV [79], respectively. Notably,
496
compared with the no response to visible light of pure BOC, I-BOC can
497
also be stimulated under visible light irradiation due to the I− ions doping.
498
The electrons can be excited from the I doped level (1.84 eV) to the
499
corresponding CB. Because the CB edge potential of BTO (-1.56 eV) is
500
more negative than that of I-BOC (-0.23 eV), the excited electron in the
501
CB of BTO tends to transfer to the CB of I-BOC nanosheet. At the same
502
time, a great deal of h+ on the I doped level of I-BOC shifted to that of
503
BTO. Thus, high-efficiency charge separation has been achieved. Since
504
the conduction edge potential of I-BOC (-0.23 eV) is more negative than
505
that of O2/•O2- (-0.046 eV) and H+/H2, the separated electron on the
506
surface of the I-BOC can convert the adsorbed O2 to •O2- radicals (or
507
directly reduce water to H2), which is the main participant in
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508
photodegration as we demonstrated in ESR test (Fig. 11B). At the same
509
time, organic pollutants can be oxidized directly by the holes in the VB of
510
BTO. As a result, the BTO/I-BOC system shows much better
511
photocatalysis activity compared with pure BTO and I-BOC.
512
Besides the well-matched band discussed above, it is believed that
513
there are various facts contributing to the dramatic photocatalytic
514
performance. The advantages of the elaborately designed photocatalyst
515
are listed below:
516
(1) The well-defined 2D/2D interface. By a simple in situ chemical
517
transformation method, I-BOC nanosheets successfully growed on the
518
surface of BTO nanosheets. It is well accepted that the intimate 2D/2D
519
interface would be helpful in achieving rapid interfacial charge
520
transferring and separation [40], leading to the dramatic photocatalytic
521
performance.
522
(2) The doping of I− ions. Based on the optical and electrochemical
523
measurement, the doping of I− ions can dramatically improve the visible
524
light
525
photogenerated electron-hole pairs [52,80].
526
4. Conclusions
absorption
of
BTO/I-BOC,
increasing
the
quantity
of
527
In summary, novel BTO/I-BOC 2D/2D heterojunction systems have
528
been synthesized for the first time via a modified in situ ion exchange
529
approach at room temperature. The intimate 2D/2D interface can provide
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530
sufficient contact surface to enhance the charge transfer rate. And the
531
doping of I− ions can dramatically improve the visible light absorption of
532
the composites, ensuring the quantity of photogenerated electron-hole
533
pairs. Moreover, well-matched band structure can lead to effective
534
separation of photogenerated charges. Thus, BTO/I-BOC exhibited highly
535
enhanced visible photocatalytic activity towards the hydrogen production
536
and organic pollutants degradation. According to experimental results, the
537
possible photocatalytic mechanism has been proposed. This work may
538
provide
539
bismuth-based semiconductor photocatalysts with distinguished visible
540
light response and efficiently charge carries separation rate.
a
promising
platform
for
constructing
other
2D/2D
541 542 543
Conflict of interest There are no conflicts to declare.
544 545
Acknowledgments
546
The authors gratefully acknowledged the National Natural Science
547
Foundation (21506079, 21676129, 21607063), China Postdoctoral
548
Science Foundation (No. 2016M590421) and the Science & Technology
549
Foundation of Zhenjiang (GY2014028 and GY2016021).
550 551
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DOI: 10.1039/C7CY01162H
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DOI: 10.1039/C7CY01162H
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Catalysis Science & Technology Accepted Manuscript
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DOI: 10.1039/C7CY01162H
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DOI: 10.1039/C7CY01162H
A
40
(137)
50
I-BOC BTO/I-BOC (020)
BTO/BOC
BTO
BTO
60
31.5
70
32.0
32.5
2 Theta (degree)
33.0
33.5
34.0
34.5
2 Theta (degree)
721 722
Fig. 1. XRD patterns of the as-prepared BTO, BTO/BOC, BOC, I-BOC
723
and BTO/I-BOC samples. A
B
C
Bi 4f
I 3d
O 1s
Bi 4f
Ti 2p C 1s
O 1s
BTO
Cl 2p Bi 4f
Ti 2p C 1s
O 1s
O 1s surface O-H
I-BOC
BTO/I-BOC
I-BOC
BTO/I-BOC
BTO BTO
800
600
400
200
0
Binding energy (eV)
168
166
E
D
164
Ti 2p3/2
Cl 2p1/2
Intensity (a.u.)
BTO
BTO/I-BOC
468
464
460
Binding energy (eV)
456
160
Cl 2p3/2
Ti 2p
Ti 2p1/2
472
162
158
156
Binding energy (eV)
I-BOC
BTO/I-BOC
536
534
532
530
528
526
Binding energy (eV)
F Cl 2p
I 3d
I 3d5/2 I 3d3/2
203 202 201 200 199 198 197 196 195
Binding energy (eV)
538
Intensity (a.u.)
1000
724
Intensity (a.u.)
Intensity (a.u.)
BTO/I-BOC
C 1s Cl 2p
Intensity (a.u.)
I 3d O 1s
I-BOC
Bi 4f
Bi 4f7/2
Bi 4f5/2
I-BOC
BTO/I-BOC
636
632
628
624
620
616
612
Binding energy (eV)
725
Fig. 2. XPS spectra of the as-prepared BTO, I-BOC and BTO/I-BOC
726
samples: (A) survey spectrum, (B) Bi 4f, (C) O 1s, (D) Ti 2p, (E) Cl 2p
727
and (E) I 3d.
728 729
Catalysis Science & Technology Accepted Manuscript
30
(220) (1115) (0214)
(026) (0014) (028)
(200)
(006)
(008) (111)
(117)
BTO/BOC
20
(102)
(110)
Intensity (a.u.)
BTO/I-BOC
BOC
(200)
(211) (104) (212)
(004)
(112) (200)
(102)
(110)
(003)
Intensity (a.u.)
BOC I-BOC
10
Intensity (a.u.)
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(001)
(002) (101)
B
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A
Published on 31 July 2017. Downloaded by University of Waterloo on 07/08/2017 03:13:04.
1 µm
B
C
1 µm
1 µm
E
F I-BOC
D
Bi 2.0 µm
BTO
Bi M
G
BOC 1 µm
Ti 2.0 µm
H
I
Ti K
J
1 µm
Cl
730
I
Cl K
2.0 µm
O 2.0 µm
O K
731
Fig. 3. SEM images of (A) BTO, (B) BOC, (C) I-BOC, (D) BTO/BOC,
732
(E) BTO/I-BOC; (F-J) elemental mappings of BTO/I-BOC.
A
B BTO BOC 500 nm
500 nm
D
C
d(212)=0.157 nm d(110)=0.274 nm
BTO I-BOC d(020)=0.270 nm 90° °
500 nm
I-BOC
5 nm
d(200)=0.272 nm
BTO
733 734
Fig. 4. TEM images of (A) BTO, (B and C) BTO/I-BOC; (D) HRTEM
735
image of BTO/I-BOC.
Catalysis Science & Technology Accepted Manuscript
DOI: 10.1039/C7CY01162H
Catalysis Science & Technology
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DOI: 10.1039/C7CY01162H
B 0.8
2.0
(αhν)1/2 (eV)1/2
Intensity (a.u.)
2.5
I-BOC BTO/I-BOC BTO BTO/BOC BOC
0.6 0.4 0.2
1.5 I-BOC BTO/I-BOC BTO BTO/BOC BOC
1.0 0.5
0.0 200
300
400
500
600
700
800
Wavelength (nm)
C
0.0 2.0
2.5
V/NHE
1.16 eV
5
-1 0
2.72 eV
1 3
4
3
2
1
0
Binding energy (eV)
736
-1
-2
4.0
4.5
5.0
-2 CB
2 1.84 eV
3.5
hν (eV) hν (eV)
-2
BTO
6
3.0
D I-BOC
3.34 eV
2.72 eV
2.07 eV
Intensity (a.u.)
Published on 31 July 2017. Downloaded by University of Waterloo on 07/08/2017 03:13:04.
1.0
BTO
VB
-1 0
CB
1
2.07 eV I-doping
2 3.34 eV level VB 3
I-BOC
737
Fig. 5. (A) UV-vis diffuse reflectance spectra and (B) plot of (αhν)1/2 for
738
the band gap energy of as-prepared samples; (C) normalized XPS valence
739
band spectra and (D) scheme for the energy band structure of BTO and
740
I-BOC.
741
Catalysis Science & Technology Accepted Manuscript
A
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I-BOC BTO/BOC BTO/I-BOC
480
520
560
600
Wavelength (nm)
742 743
Fig. 6. PL spectra of as-prepared BTO, BTO/BOC, BOC, I-BOC and
744
BTO/I-BOC samples.
A
B 1.6 1.4
2500
-Z '' (O hm )
1.2 1.0 0.8 0.6 0.4
BTO BOC I-BOC BTO/BOC BTO/I-BOC
2000 1500 1000 500
0.2
0
0.0 40
745
3000
BTO/I-BOC BTO/BOC I-BOC BOC BTO
60
80
100 120 140 160 180
Irradiation (sec)
0
1000
2000
3000
4000
5000
6000
Z' (Ohm)
746
Fig. 7. (A) Time-based photocurrent response and (B) Nyquist plots of
747
as-prepared BTO, BTO/BOC, BOC, I-BOC and BTO/I-BOC samples.
748 749 750 751
Catalysis Science & Technology Accepted Manuscript
Intensity (a.u.)
BTO BOC
440
P hotocurrent (µA cm -2 )
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DOI: 10.1039/C7CY01162H
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DOI: 10.1039/C7CY01162H
A
B
BTO/BOC BTO
200
I-BOC BOC
80 60 40
2
100
1 st 2 nd 3 rd
20 0
0 0
752
1
2
3
4
I-BOC
BTO
BTO/I-BOC
BTO/BOC
Irradiation time (h)
753
Fig. 8. (A) Photocatalytic hydrogen evolution activity and (B) reusability
754
of as-prepared catalysts.
755 756 757 758 759 760 761 762 763 764 765 766 767 768
Catalysis Science & Technology Accepted Manuscript
100 BTO/I-BOC
300
r H (µmol h -1 g-1)
H 2 generation (µmol/g)
Published on 31 July 2017. Downloaded by University of Waterloo on 07/08/2017 03:13:04.
400
Page 39 of 42
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DOI: 10.1039/C7CY01162H
B
6
1.0 BTO/I-BOC BTO/BOC I-BOC
4
ln(C 0/C)
Published on 31 July 2017. Downloaded by University of Waterloo on 07/08/2017 03:13:04.
C/C 0
0.8 0.6 0.4 Blank BOC BTO I-BOC BTO/BOC BTO/I-BOC
0.2 0.0 0
30
60
BTO BOC Blank
2
0
90
120
150
0
180
30
60
Irradiation Time (min)
C
D 0.8
ln(C 0/C)
C/C 0
0.6 Blank BOC BTO I-BOC BTO/BOC BTO/I-BOC
0.0 0
30
60
180
150
180
1
0 90
120
150
180
Irradiation time (min)
E
150
BTO/I-BOC BTO/BOC I-BOC BTO BOC Blank
2
0.2
120
3
1.0
0.4
90
Time (min)
0
30
60
90
120
Time (min)
F 1.0
CIP TC
80
TOC/TOC 0
Degradation ratio (% )
100
60 40 20
RhB MB TC CIP
0.8 0.6 0.4 0.2 0.0
0 BT
O
BO
C I- B
769
OC O BT
/B
OC BT
IO/
BO
C
0
30
60
90
120
150
180
Time (min)
770
Fig. 9. Photocatalytic activity of the as-prepared samples for degradation
771
of (A and B) RhB, (C and D) MB and (E) CIP/TC; (F) mineralization rate
772
of RhB/MB/CIP/TC.
773 774
Catalysis Science & Technology Accepted Manuscript
A
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DOI: 10.1039/C7CY01162H
0 20
1 st
2 nd
3 rd
4 th
40 60 80
Before cycle
After 4 cycles
100 0
90
10
180 270 360 450 540 630 720
20
Time (min)
775
30
40
50
60
2 Theta (degree)
776
Fig. 10. (A) The cycling runs for the degradation of RhB in the presence
777
of the BTO/I-BOC sample under visible light irradiation, (B) XRD
778
patterns of the BTO/I-BOC sample before and after 4 recycling runs.
1.0
C/C 0
0.8 0.6 0.4 BQ N2 AO t-BuOH no scavanger
0.2 0.0 0 779
30
60
90
120
150
180
Time (min)
780
Fig. 11. Active species trapping experiment for the photocatalytic
781
degradation of RhB over the BTO/I-BOC sample.
782 783 784
70
Catalysis Science & Technology Accepted Manuscript
B Intensity (a.u.)
Degradation ratio (%)
Published on 31 July 2017. Downloaded by University of Waterloo on 07/08/2017 03:13:04.
A
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DOI: 10.1039/C7CY01162H
B
Light on
♣
♣
♣
♣
Dark
3480 3490 3500 3510 3520 3530 3540
DMPO-hydroxyl radical
Intensity (a.u.)
Intensity (a.u.)
Published on 31 July 2017. Downloaded by University of Waterloo on 07/08/2017 03:13:04.
DMPO-superoxide radical
Light on
Dark
3480
3500
3510
3520
3530
Magnetic Field (G)
Magnetic Field (G)
785
3490
786
Fig. 12. DMPO spin-trapping ESR spectra for the BTO/I-BOC sample
787
(A) in methanol dispersion for DMPO-•O2- and (B) in aqueous dispersion
788
for DMPO-•OH. Bi3+ H+ BTO
BTO
Bi3+
Eq. (1) Bi3+, H2O
Cl− I− BiO+ I-BOC BTO
Eq. (3)
789
I-BOC
BiO+
BiO+ BTO Eq. (2)
790
Scheme 1. The schematic of fabrication process for 2D/2D BTO/I-BOC
791
sample.
792 793
Catalysis Science & Technology Accepted Manuscript
A
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H+ CB
H2 CB
Published on 31 July 2017. Downloaded by University of Waterloo on 07/08/2017 03:13:04.
·O2O2 VB
BTO
I- doping level VB
I-BOC 794 795
Scheme 2. The separation and transfer of photogenerated charges in
796
BTO/I-BOC photocatalysts combined with the possible reaction
797
mechanism.
798
Catalysis Science & Technology Accepted Manuscript
DOI: 10.1039/C7CY01162H