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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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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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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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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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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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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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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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Catalysis Science & Technology Accepted Manuscript

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DOI: 10.1039/C7CY01162H

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DOI: 10.1039/C7CY01162H

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DOI: 10.1039/C7CY01162H

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715 716 717 718 719 720

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)

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400

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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

Catalysis Science & Technology

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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

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·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