Unveiling the Role of Defects on Oxygen Activation and

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Remediation and Control Technologies

Unveiling the Role of Defects on Oxygen Activation and Photodegradation of Organic Pollutants Mengjiao Xu, Yao Chen, Jiangtao Qin, Yawei Feng, Wei Li, Wei Chen, Jian Zhu, Hexing Li, and Zhenfeng Bian Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03558 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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Environmental Science & Technology

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Unveiling the Role of Defects on Oxygen Activation and Photodegradation of Organic Pollutants

4

Mengjiao Xu1,† Yao Chen1,† Jiangtao Qin1, Yawei Feng1, Wei Li2, Wei Chen3, Jian

5

Zhu1, Hexing Li*,1 and Zhenfeng Bian*,1

6

1The

7

Laboratory of Rare Earth Functional Materials, Shanghai Normal University,

8

Shanghai 200234, PR China

9

2Department

1 2

Education Ministry Key Lab of Resource Chemistry and Shanghai Key

of Chemistry, Laboratory of Advanced Materials, Shanghai Key Lab of

10

Molecular Catalysis and Innovative Materials and iChEM, Fudan University,

11

Shanghai 200433, P. R. China

12

3Department

13

117543, Singapore

14

ABSTRACT: The status of defects of TiO2 are of fundamental importance in

15

determining its physicochemical properties. Here we report a simple chemical

16

deposition method for controllable synthesis of defective anatase TiO2 nanocrystals

17

under various calcination atmospheres. XPS and ESR analysis reveals that both the

18

oxygen vacancies (VO) and the trivalent titanium (Ti3+) defects exists in TiO2 after N2

of Chemistry, National University of Singapore, 3 Science Drive 3,

ACS Paragon Plus Environment


19

treatment (N-TiO2). Meanwhile, mainly VO defects can be obtained in TiO2 with air

20

calcination (A-TiO2). ESR spectra for reactive oxygen species determination, clearly

21

show that the visible light catalytic activity is mainly caused by the efficient

22

activation of oxygen molecules to •O2- species for A-TiO2, which play an important

23

role in hindering the accumulation of intermediates during p-chlorophenol (4-CP)

24

photodegradation process. However, the oxygen molecules cannot be activated for

25

N-TiO2 even with superior visible light absorption and thus the photogenerated

26

electron are reductant, which participated in the transformation of BQ to HQ via

27

electron shuttle mechanism. Moreover, A-TiO2 exhibits higher separation efficiency

28

of photogenerated carriers than that of N-TiO2, showing the critical role of VO with a

29

suitable concentration in transferring photogenerated charges.

30

Introduction

31

TiO2 is considered as an ideal photocatalyst in organic pollutants degradation,

32

photocatalytic hydrogen generation, dye-sensitized solar cells and lithium-ion

33

batteries because of its nontoxicity, chemical stability, and low cost.1-4 However, its

34

practical applications are still limited by fast recombination of photoinduced charges

35

and the poor absorbance of visible lights. To date, enormous strategies have been

36

expended to improve its quantum efficiency and light harvesting, such as ions

37

doping,5,6 surface sensitization,7,8 crystal structure9 and morphology control.10,11 Both

38

experimental results and theoretical prediction demonstrate that defect states can


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profoundly influence the physicochemical properties of TiO2, including the optical

40

behavior, dissociative adsorption performance, electronic structure and catalytic

41

efficiencies.12,13 Chen et al. reported that the black titanium dioxide prepared under

42

high pressure hydrogen atmosphere could remarkably enhance solar-driven

43

photocatalytic activity.14 Since then, various methods are being employed to fabricate

44

defected TiO2, such as chemical reduction approach (e.g., H2, NaBH4, Al, Carbon),

45

electrochemical reduction, high energy particle bombardment, vacuum thermal

46

treatment, and flame reduction.15 Furthermore, the distribution of defects in TiO2 is

47

another important hot research topic, which shows great influence on the

48

photocatalytic performance. For instance, Kong et al. reported that improvement of

49

the ratio between surface defects and bulk defects could greatly enhance the

50

separation efficiency of photogenerated carries and photocatalytic activity.16 Yu et al.

51

observed that the TiO2 hydrogenated at the temperature over 873 K exhibited much

52

higher photocatalytic activity ascribed to the diffusion of defects from the bulk to the

53

surface.17 Zhang et al. demonstrated that the surface defects were more favorable for

54

photocatalytic hydrogen evolution than the bulk defects.18

55

Despite great progress has been made on defect engineering of TiO2, some of the

56

main issues are still under debate. One is that the role played by defects in producing

57

reactive species for photocatalysis is still not clearly understood. It was recognized

58

that capturing the photoinduced electrons by adsorbed molecular oxygen on the

59

surface was the rate-determining step for photo-oxidation processes, which was much



60

slower than that of capturing hole by the surface absorbed H2O or organic

61

pollutants.19,20 The interaction between oxygen and photocatalysts is so vital that

62

many strategies have been explored. Jing’s group developed a method of surface

63

modification with inorganic acids to promote O2 adsorption.19 Malwadkar et al.

64

declared that deposited Au nanoparticles on TiO2 was efficiently contributed to the

65

adsorption and activation of oxygen.21 Specially, defect sites are shown to be active

66

for the adsorption and activation of O2. It was demonstrated that O2 did not adsorb on

67

fully-oxidized, perfect TiO2 surface, while it adsorbed both in a molecular and an

68

irreversible dissociative manner on the defective surface with excess negative

69

charges.22,23

70

Additionally, only a few researches involved the mismatch between the superior

71

light absorption and photoactivity. Hu et al. demonstrated that the enhanced

72

photocatalytic hydrogen evolution from water splitting was mainly due to the incident

73

photon-to-electron conversion efficiency (IPCE) increased in the UV region instead of

74

the dramatic improvement in visible light absorbance for hydrogenated TiO2.24

75

Leshuk et al. revealed that the high-temperature hydrogenated TiO2 with strong

76

visible-light absorption exhibited much poor photocatalytic activity in comparison

77

with the untreated TiO2.25 They claimed that the decrease in photocatalytic activity

78

was due to the increase in the concentration of VO defects in the TiO2 lattice. In

79

contrast, Liu et al. found that the photocatalytic activity sharply decreased with


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increased content of VO defects though they exhibited enhanced visible light

81

absorption.26

82

Herein, we report a successful fabrication of defective anatase TiO2 with

83

controllable concentration of VO and Ti3+ defects using chemical deposition method.

84

Hydrazine hydrate was employed as both the depositing and the reducing agent.

85

Different calcination atmospheres were applied to control the defect states in TiO2.

86

Experimental evidence clearly demonstrate the presence of mainly oxygen vacancies

87

(VO) in the TiO2 after air-calcinations (A-TiO2). However, both VO and Ti3+ defects

88

co-existed in the TiO2 after N2-calcination (N-TiO2). It is found that the concentration

89

of defects has different impacts on the formation of free radical species and the

90

activity in visible light photocatalysis. The high photocatalytic performance of the

91

A-TiO2 is attributed to the fast photogenerated charge transfer and in particular the

92

efficient activation of oxygen molecules. N-TiO2 with a superior visible light

93

absorption cannot activate oxygen molecules because of a relatively high

94

concentration of defects, resulting in a dramatic decrease in visible light

95

photocatalytic performance.

96

Experiment

97

Chemicals: Titanium tetrafluoride (TiF4, 99%, J&K Scientific Ltd.), Hydrazine

98

hydrate (NH2NH2·H2O, 85%, sinopharm chemical reagent Co. Ltd.), used without

99

further purification. Deionized water was used in the whole experimental process.



100

Synthesis of defected TiO2 nanoparticles: The defected TiO2 nanoparticles were

101

prepared via a simple chemical precipitation method. In a typical procedure, 0.05 mol

102

TiF4 was resolved in 200 ml deionized water under magnetic stirring. Then, 1.6 ml

103

NH2NH2·H2O added into the solution at room temperature (stirring for 2 hours). The

104

white precursor powder was separated (washed for several times using water and

105

ethanol), and dried (353 K). Finally, the dried powder was calcined in air or N2 at 923

106

K (5 K/min, 2 hours) to obtain TiO2 with different defects, named as A-TiO2 and

107

N-TiO2, respectively.

108

Characterization: X-ray diffraction spectra (XRD, Rigaku D/MAX-2000, Cu Kα

109

source) were operated at 40 kV and 20 mA, scanned at a rate of 5° min-1, Raman

110

spectra (Jobin Yvon, SuperLabRam II), field emission scanning electron microscopy

111

(FESEM, HITACHI S4800, 50 kV), transmission electronic micrograph (TEM, JEOL

112

JEM-2010, 200 kV), X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI

113

5000C, Al Kα), UV-vis diffuse reflectance spectra (Hitachi UH4150), nitrogen

114

sorption (Micromeritics Tristar II 3020, at 77 K). Thermal gravimetric analyzer

115

(TGA, DTG-60H, and heating speed of 5 K/min) was used to analyze the impurities

116

of the precursor of TiO2. The photoluminescence (PL) spectra were obtained using a

117

FLS 980 fluorometer (Edinburgh Instruments Ltd.).

118

Photocatalytic activity test: Photocatalytic performance of the samples was tested by

119

the decomposition of 4-chlorophenol (4-CP). Typically, 10 mg catalyst was dispersed

120

in 50 ml of 4-CP aqueous solution (10 ppm), which was irradiated by a 400 W


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high-pressure Hg lamp. The wavelengths > 420 nm was cut by a glass filter (JB-420).

122

The solution was continuously stirred for 30 min in the dark before irradiation to

123

conform the adsorption-desorption equilibrium. The residual 4-CP was analyzed by a

124

UV-vis spectrophotometer (UV 7504/PC).

125

ESR Test: Electron spin resonance (ESR) spectra were recorded on a JEOL

126

JES-FA200 model spectrometer (9.05 GHz). Oxygen vacancy and Ti3+ defects were

127

detected in a 5 mm suprasil quartz tube at 77 K in vacuum. Samples were irradiated for

128

12 minutes before ·O2- and ·OH trapping tests (300 W Xenon lamp > 420 nm), which

129

were operated at room temperature in methanol and water respectively in a capillary

130

quartz tube, using 5,5-dimethyl-1-pyrroline-N-oxide (50 mM) as the spin-trapping

131

agent.

132

Analyses: The concentration of 4-CP was quantified by HPLC (Agilent, Zorbax

133

Eclipse XDB-C8 column (4.6 mm × 150 mm, 5 μm)), using an Infinity quaternary

134

pump (Agilent 1260) and a fluorescence detector (Agilent 1260). Methanol and water

135

(50:50, v/v) was used as the mobile phase (0.4 ml/min). The wavelength employed for

136

detecting 4-CP, BQ and HQ was 280 nm, 250 nm and 290 nm, respectively.

137

Photoelectrochemical Test: Photoelectrochemical measurements were used a

138

single-compartment quartz cell in a conventional three-electrode system (CHI 660E,

139

platinum sheet (10mm*20mm) counter electrode, a saturated calomel reference

140

electrode (SCE) and the TiO2 film working electrode). A TiO2 film electrode was



141

prepared on a FTO conducting glass substrate (sheet resistance ≤ 8 Ω). Using Scotch

142

tape as a spacer (2 cm*1 cm), colloidal TiO2 solution was deposited on the substrate.

143

The excess suspension was raked off with a glass rod. Then the film was dried and

144

heated at 573 K for 2 h in air (5 K/min). Na2SO4 aqueous solution (0.50 M) was used

145

as the electrolyte. A bias voltage of 0.5 V was applied to drive the migration of

146

photoinduced electrons. The irradiation source was 300 W xenon lamp with a cut-off

147

filter (λ > 420 nm) and positioned 8 cm above the reaction cell. The surface

148

photovoltage (SPV) spectra were measured on a home-made system consisting of

149

xenon lamp (Omni-5007, Zolix), amplifier (SR830-DSP) with a light chopper

150

(SR540) at the chopping frequency of 23 Hz.

151

XAFS Study: Ti K-edge X-ray absorption near edge spectra (XANES) were collected

152

at the BL 14W1 of Shanghai Synchrotron Radiation Facility using a double Si

153

(111)-crystal monochromator at transmission mode. Energy of the electron beam in

154

storage ring was 3.5 GeV. Demeter was used for data analyse.27 Theoretical phase and

155

amplitude functions were obtained from the program FEFF 9.0.28 The k3-weighted

156

FT-EXAFS from 3 to 12 Å−1and an R window of 1-4 Å was applied for date fitting.

157

Results and discussion

158

XRD patterns in Figure 1a display several strong diffraction peaks with similar

159

intensity characteristic of anatase phase (JCPDS, No. 21-1272). Based on Scherrer

160

equation, the average crystallite size of A-TiO2 and N-TiO2 is estimated as both about


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41 nm. The absorption at 146 cm-1 in Raman spectra (Figure 1b) also indicates the

162

anatase crystalline structure. No significant difference in either XRD or Raman

163

spectra has been observed, suggesting the similar crystal phase, crystallinity, and

164

particle

165

atmosphere-calcinations. TEM micrographs (Figure S1) further reveal that A-TiO2

166

and N-TiO2 both show the similar morphology and the grain size ranging from 34 to

167

44 nm. The insert selected area electron diffractions (SAED) in Figure 1c-d

168

demonstrate the polycrystal characteristics of both A-TiO2 and N-TiO2.

of

the

A-TiO2

(a)

and

(b)

Intensity (a.u.)

A-TiO2 N-TiO2

JCPDS 21-1272 10

20

30

40

50

60

70

N-TiO2

Intensity (a.u.)

size

80

(c)

Eg

under

different

A-TiO2 N-TiO2

B1g A1g+B1g E g

Eg 200

2 Theta

obtained

400

600

-1

Wavelength (cm )

800

(d)

169 170

Figure 1. XRD patterns (a) and Raman spectra (b) of A-TiO2 and N-TiO2 sample,

171

respectively. TEM micrographs of A-TiO2 (c) and N-TiO2 (d) (The insert is SAED).



172

The N2 adsorption-desorption isotherms (Figure S2) demonstrate that both

173

A-TiO2 and N-TiO2 are present as Langmuir isotherm structure. The specific surface

174

area (SBET) and pore volume (VP) of the A-TiO2 are determined to be 40.6 m2·g-1 and

175

0.225 cm3·g-1, similar to those of N-TiO2 (SBET = 39.6 m2 g-1 and VP = 0.223 cm3 g-1).

176

Thermogravimetric measurements (TG, Figure S3) indicate that the precursor can be

177

totally converted into pure TiO2 under either air or N2 calcinations at 923 K. No

178

significant N and F were detected by XPS spectra (Figure S4), further confirming the

179

complete removal of impurities on the TiO2 surface.

180

As shown in Figure 2a, the XPS spectrum of A-TiO2 displayed two typical Ti4+

181

peaks with binding energies around 464.3 eV and 458.5 eV in Ti 2p1/2 and Ti 2p3/2

182

levels.29 The N-TiO2 exhibits slightly negative shift (0.1 eV) in comparison with

183

A-TiO2, indicating the presence of Ti3+ species. The fine scan spectra of O 1s are

184

asymmetric both in A-TiO2 and N-TiO2 (Figure 2b). The main peak of O 1s located at

185

529.8 eV corresponds to lattice oxygen of TiO2.30 While, a shoulder peak at a higher

186

binding energy of 531.8 eV can be ascribed to the VO defect.31 According to the peak

187

area ratio of VO, we estimate the content of VO as 8.9% in A-TiO2 and 14.8% in

188

N-TiO2, respectively. Clearly, the concentration of VO defects in N-TiO2 is much

189

higher than that in A-TiO2.


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(a)

191

464

462

460

A-TiO2

Intensity (a.u.)

Intensity (a.u.)

N-TiO2

466

190

(b)

A-TiO2

458

Binding energy (eV)

456

N-TiO2

536

534

532

530

528

526

Binding Energy (eV)

Figure 2. XPS spectra of (a) Ti 2p and (b) O 1s of A-TiO2 and N-TiO2.

192

193

The existence of defects in A-TiO2 and N-TiO2 is also confirmed by ESR

194

spectroscopy. As shown in Figure 3, an intense sharp resonance signal appears at g =

195

2.003, which is assigned to the shallow donor states due to VO defects in TiO2.32-36

196

Besides, the resonance signals at g = 1.983 and 2.023 were ascribed to Ti3+ defects in

197

TiO2.37-41 There is only one ESR signal at g = 2.003 observed in A-TiO2, suggesting

198

the absence of Ti3+ defects. Therefore, from the above results, the N-TiO2 contains

199

both Ti3+ and VO defects, while A-TiO2 contains only VO defects with a much lower

200

concentration (see scheme S1).



3+

VO

3+

Ti

Intensity (a.u.)

Ti

Page 12 of 32

N-TiO2

A-TiO2 1.94 1.96 1.98 2.00 2.02 2.04 2.06

201 202

g-value

Figure 3. ESR spectra of samples at 77 K in vacuum under visible light irradiation.

203

Since VO and Ti3+ defects are significant for the optical property, optical

204

absorption spectroscopy of either A-TiO2 or N-TiO2 has been detected. The UV-Vis

205

absorption and diffuse reflectance spectra (Figure S5) reveal that there is very little

206

difference in UV region absorption corresponding to the intrinsic band gap

207

absorption. However, spectra in the visible region are quite different. N-TiO2 exhibits

208

significantly enhanced absorption (Figure S5a) and decreased diffuse reflectance

209

(Figure S5b) compared with A-TiO2. It is widely accepted that the color center can be

210

induced by VO, leading to the enhanced visible light absorption.12 The Ti3+ defects

211

form a shallow donor level below the conduction band (CB), which can also increase

212

the visible light absorption.42 Based on the above results, we can deduce that the

213

higher concentration of defects in TiO2 results in more visible light absorbance.


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Abs.

0.6 0.4

220

(c) 8

240 260 280 Wavelength / nm

6

300

4

4

2 0

0 2 3 Time / h

4

5

0.0 200

320

2

1

4-CP HQ

0.2

4-CP BQ HQ

0

BQ

(d)

220

240 260 280 Wavelength / nm

300

320

8

4 4-CP BQ HQ

6 4

2

2 0

Intermediate / ppm

0.0 200

0h 2h 4h 6h

0.6 0.4

4-CP HQ

BQ

0.2

4-CP

0.8

Intermediate / ppm

Abs.

0.8

4-CP / ppm

(b)1.0

0h 2h 4h 6h

4-CP

4-CP / ppm

(a) 1.0

0 0

1

2 3 Time / h

4

5

214 215

Figure 4 UV-vis absorbance spectra changes during 4-CP photodecomposition by

216

A-TiO2 (a) and N-TiO2 (b). Photodegradation of 4-CP and the formation of BQ and

217

HQ as a function of time for A-TiO2 (c) and N-TiO2 (d).

218

219

Visible-light-driven photocatalytic performance was carried out (Figure 4 and

220

Figure S6). All spectra show characteristic absorption peaks at 228 nm (4-CP) in

221

Figure 4. For A-TiO2, the absorption intensity of 4-CP drops monotonically under

222

visible-light irradiation. The increased absorption intensity at 289 nm in the first 2

223

hours during photocatalytic 4-CP degradation is due to the generation of

224

hydroquinone (HQ).43,

225

degradation rapidly suggested by the decreased absorption intensity of HQ. By

44

Then the formed organic intermediates undergo a further



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226

contrast, benzoquinone (BQ) and higher concentrations of HQ are generated for

227

N-TiO2, and the number of intermediates reaches the maximum value after irradiation

228

for 4 hours, then gradually decreased. 4-CP undergoes a more complex process of

229

destruction at the initial stage, almost no change within the next 2 hours, and slower

230

destruction in the final 2 hours. Quantification of 4-CP, BQ and HQ made

231

chromatographically as shown in Figure 4 c-d.

A-TiO2

Intensity (a.u.)

N-TiO2

319.4

232

319.6

319.8

G (mT)

320.0

(b)

A-TiO2 N-TiO2

Intensity (a.u.)

(a)

318

320

322

324

326

328

G (mT)

233

Figure 5. ESR test for reactive oxygen species (ROS) under visible light (a) ·OH

234

species and (b) ·O2- species).

235

To obtain further insight into the roles played by different defect stages, reactive

236

oxygen species (ROS) involved under visible light irradiation were determined by

237

ESR. As shown in Figure 5, ·OH species are detected for both A-TiO2 and N-TiO2.

238

However, only ·O2- species are determined for A-TiO2. It has been demonstrated that

239

a low concentration of defects creates a miniband below the conduction band edge,

240

while adequately high concentration leads to the band width increases and induces a

241

continuous vacancy band of electronic states to overlap with the conduction band.45-47


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242

Namely, a strong tail of conduction band can be formed for N-TiO2 and the generated

243

defect level is deeper compared to A-TiO2 because of a higher density of defects for

244

N-TiO2.15, 18 When the sample is irradiated by visible light, electrons can be excited

245

into the acceptor below the conduction band minimum (CBM). It is known that the

246

lower energy level, the weaker of the reduction ability of located electrons. As a

247

result, the photoexcited electrons cannot reduce O2 to ·O2-, thus there is no ·O2-

248

generation by N-TiO2. From the derivation of the Mott–Schottky equation (Figure

249

S7), the flat-band potential was determined to be -0.74 V and -0.66 V (versus SCE)

250

for A-TiO2 and N-TiO2, respectively. The generated defect level in A-TiO2 is more

251

positive than N-TiO2, which indicates that A-TiO2 has a stronger reduction ability

252

than N-TiO2. The reduction ability using MV2+ to estimate their energy level have

253

also been tested as shown in (Figure S8). Much more MV2+ can be reduced to MV+.

254

for A-TiO2, which indicates that photogenerated electrons for A-TiO2 have a much

255

stronger reduction ability compared with N-TiO2. Based on the equilibrium potential

256

of MV2+/ MV+. (-0.69 V vs. SCE) and the above results, we estimated the energy level

257

for A-TiO2 is above -0.69V and below -0.69V for N-TiO2. It is consistent with the

258

result of flat band potential test.

259

Thus, the different photocatalytic 4-CP degradation process can be explicated as

260

illustrated in Scheme 1. Attacked by ·OH radicals in the para position and a following

261

dichlorination, HQ is formed.43,

262

mechanism existence between HQ and BQ.43, 50 Namely, HQ can be oxidized to BQ

48-49

It’s reported that a fast electron shuttle



Page 16 of 32

263

by oxygen molecule (eq. 1), whereas BQ can be rapidly reduced to HQ by electrons

264

(eq. 2), leading to the consuming of oxygen molecules and photoinduced electrons in

265

an ineffective manner and consequently inhibits the mineralization process for

266

N-TiO2.50

267

HQ + 1/2 O2 → BQ + H2O

(eq. 1)

268

BQ +2e- + 2H+ → HQ + H2O

(eq. 2)

269

As for A-TiO2, ·O2- originates from the efficient activation of O2 molecule. BQ,

270

a common scavenger for ·O2- in photocatalysis, can be decomposed effectively by

271

·O2-, thus leading to the fast photocatalytic degradation of 4-CP by impeding the

272

accumulation of BQ and HQ.44 Therefore, the higher performance for A-TiO2 is not

273

only ascribed to the fast charges transfer, more importantly, it can activate oxygen

274

molecular effectively. 4-CP degradation of A-TiO2 under N2 atmosphere have been

275

performed to suppress the formation of superoxide anions (·O2-) (Figure S9). An

276

inhibited performance for A-TiO2 under N2 atmosphere suggests ·O2- are indeed one

277

of the active oxidative species in the degradation process. When BQ selected as a

278

parent compound (Figure S10), the concentration of BQ gradually decreased under

279

visible-light irradiation, and the concentration of HQ reached the maximum value

280

after irradiation for 1 hours for A-TiO2. While as for N-TiO2, the concentration of BQ

281

decreased apparently within the first 1 hour, and almost no change within the next 5

282

hours. HQ reached the maximum value after irradiation for 2 hours, then gradually


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283

decreased. The different photocatalytic BQ degradation process is due to the electron

284

shuttle mechanism existence between HQ and BQ.

285 286

Scheme 1. The proposed mechanism of photocatalytic process under visible light

287

irradiation for A-TiO2 and N-TiO2.

288

To make clear why the photoactivity does not match their visible light

289

absorption, transfer efficiency of photogenerated charges is investigated by

290

determining the photocurrent by visible light irradiation (Figure S11a). A-TiO2

291

exhibits higher photocurrent than N-TiO2 at steady state, implying the efficient

292

photocharge migration which could diminish the recombination of photo carries. This

293

could be further confirmed by examining the photo-induced current profile (Figure

294

S11 a). It could be seen that the instant photocurrent of N-TiO2 was much higher than

295

that of A-TiO2, which is consistent with its stronger absorption for visible lights to

296

generate photocharges.51 The photocurrent decreases rapidly due to the easy



297

photoelectron-hole recombination. And thus, the distinct spike-like current signals

298

generate. The charge transfer property is also characterized by SPV spectroscopy

299

(Figure S11 b). Both A-TiO2 and N-TiO2 exhibit obvious response under 400-600 nm

300

visible light irradiation, which is due to the band to sub-band charge transition of

301

TiO2.52 Moreover, the SPV response of A-TiO2 is stronger than that of N-TiO2, which

302

can also confirm the high separation efficiency of photogenerated charge carriers.

303

PL spectra are used to understand the correlation of the recombination behavior

304

to various defect stages.53 The emission spectra of A-TiO2 and N-TiO2 were obtained

305

by 345 nm laser at 298 K (as shown in Figure S12), which exhibit similar emissions

306

peaks. The strong peak located at 398 nm (3.18 eV) is caused by the emission of

307

bandgap transition, corresponding to the bandgap energy of anatase.54 Meanwhile,

308

there are three main emission peaks located in the range from 450 nm to 500 nm,

309

which are mainly resulted from defects. In addition, the peaks at 450 nm (2.75 eV)

310

and 468 nm (2.65 eV) are ascribed to free band-edge excitons. The last peak located

311

at 485 nm (2.55 eV) is due to bound excitations.53 The N-TiO2 displays higher PL

312

peak than A-TiO2, indicating a high recombination rate of photocarriers,55 which, in

313

agreement with the experimental results.


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Page 19 of 32


C A2

A3

A-TiO2

B

N-TiO2

A1

4960

314

Ti K-edge XANES

D

4980

5000

5020

Energy (eV)

(b)

Ti-O Ti-Ti

FT Magnitude

Intensity (a.u.)

(a)

5040

A-TiO2

N-TiO2 0

2

4

6

8

R+D (Å)

315

Figure 6. (a) Normalized X-ray absorption near-edge spectra and (b) Fourier

316

Transform and corresponding fits at the Ti K-edge of samples.

317

Ti K-edge XANES chiefly provides a signal of the bulk phase and is sensitive to

318

the environment of the surrounding atoms.56. The intensity of the pre-edge transitions

319

increases as the environment is distorted.45 As shown in Figure 6 a, the pre-peak A1 is

320

the result from a t2g bandlike state. While A2 and A3 pre-peaks are due to eg bandlike

321

states.57 The Ti 4s and/or O 2p orbitals are hybridized with Ti 4p characteristics, and

322

is used to transition B. C Pre-peaks, corresponding to transitions of core electron to O

323

2p states hybridized with Ti 4p states and Pre-peaks D is result from higher lying p

324

AOs.57 The post edge region from 4.98 to 5.02 keV involves many poorly resolved

325

3s-np dipole-allowed transitions.58 The slightly shift of the absorption edge for sample

326

A-TiO2 implies the presence of VO in the bulk. Besides, an increased intensity of A3

327

peak verifies the existence of TiO6 octahedron distorted structures. Figure 6 b shows

328

the radial structure functions (RSFs) of the samples. It is seen that both samples

329

exhibit three peaks at approximately 1.56, 2.50, 3.46 Å. The first peak (shell) arises



330

from the Ti-O bonding with a distance of 1.56 Å, the second peak (shell) arises from

331

the Ti-Ti bonding with a distance of 2.50 Å and the third peak (shell) arises from a

332

mixture of Ti-Ti and Ti-O bonding with a distance of 3.46 Å, which is consistent with

333

the previous results.56, 59 Table S1 shows the best fitting results, which indicates that

334

the surface and bulk of both samples are distorted, and Ti atom coordinated with

335

fewer O atoms in each central. The coordination number of Ti-1Ti and Ti-O for the

336

sample A-TiO2 is lower than that of N-TiO2 (3.3±0.6 vs. 3.9±0.6 for Ti-1Ti and

337

3.1±0.8 vs. 4.6±0.5 for Ti-O). Low coordinated Ti will be more favorable sites for

338

reactants in photocatalytic reactions.12

339

Defective anatase TiO2 nanocrystals is successfully synthesized. XPS and ESR

340

spectra indicate that oxygen vacancies (VO) and trivalent titanium (Ti3+) defects exist

341

simultaneously in the N2-calcinated sample, while mainly VO defects are present in

342

the air-calcinated sample. Investigations on photocurrent response, SPV and PL reveal

343

that A-TiO2 has a higher separation efficiency of photogenerated carriers. Moreover,

344

A-TiO2 with a low coordinated Ti can effectively activate oxygen molecular, leading

345

to the superior photocatalytic activity for deterioration of organics under visible light

346

irradiation. It is observed that various intermediates can be formed and accumulate in

347

the reaction system before their complete mineralization, which may be more toxic

348

than their parent. Therefore, a better insight into the formation of these intermediates

349

are of fundamental importance for the practical application of photocatalysis. Since

350

4-CP and VOCs are fundamental and toxic organic compounds, the results are useful


Page 20 of 32

Page 21 of 32


351

for deepening the understanding of photodegradation mechanism of organic materials

352

and the impact of defects on photocatalytic performance. Besides, it is also instructive

353

for the design and development of highly efficient visible light response

354

photocatalysts.

355

ASSOCIATED CONTENT

356

Supporting Information

357

The Supporting Information is available free of charge on the ACS Publications

358

website. TEM images, N2 adsorption-desorption isotherms, Thermal analysis profiles

359

of precursor, XPS spectra, UV-Vis absorbance spectra, optical reflectance spectra,

360

Proposed sample lattice structure, Photocatalytic performance, photocurrent spectra,

361

Photoluminescence spectra, SPV spectra of A-TiO2 and N-TiO2 and their structural

362

parameters.

363

AUTHOR INFORMATION

364

Corresponding Author

365

*[email protected], [email protected]

366

Author Contributions

367

†These two authors made equal contributions to this work.

368

ACKNOWLEDGMENTS



Page 22 of 32

369

This work is supported by NSF of China (21761142011, 21522703, 21876114,

370

21603036, 21377088), Shanghai Government (13SG44, 15520711300), Shanghai

371

Rising-Star Program, and Singapore National Research Foundation under the grant of

372

NRF2017NRF-NSFC001-007. WL thanks the support from the Major State Basic

373

Research Development Program (2016YFA0204000). Research is also supported by

374

The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai

375

Institutions

376

(PCSIRT_IRT_16R49), International Joint Laboratory on Resource Chemistry of

377

China (IJLRC).

378

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557 558 559

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Unveiling the Role of Defects on Oxygen Activation and

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