Photoelectrochemical Degradation of Organic Compounds Coupled

Apr 26, 2017 - The potential bias of 1.64 VNHE did not induce any noticeable activity in EC condition. Similar trends were observed for the degradatio...
0 downloads 8 Views 3MB Size
Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Photoelectrochemical Degradation of Organic Compounds Coupled with Molecular Hydrogen Generation using Electrochromic TiO Nanotube Arrays 2

Min Seok Koo, Kangwoo Cho, Jeyong Yoon, and Wonyong Choi Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

Environmental Science & Technology

1

Photoelectrochemical Degradation of Organic Compounds

2

Coupled with Molecular Hydrogen Generation using

3

Electrochromic TiO2 Nanotube Arrays

4

Min Seok Koo1, Kangwoo Cho1, Jeyong Yoon2 and Wonyong Choi1* 1

5 6 7

Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea

2

School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742,

8

Korea

9

10

Submitted to

11

Environmental Science & Technology

12

(Revised)

13

2017

14

* To whom correspondence should be addressed (W. Choi)

15

E-mail: [email protected]

16

Phone: +82-54-279-2283

17

1

ACS Paragon Plus Environment

Environmental Science & Technology

18

Page 2 of 30

ABSTRACT

19

Vertically aligned TiO2 nanotube arrays (TNTs) were prepared by electrochemical

20

anodization, and then cathodically polarized with dark blue coloration for the dual-functional

21

photoelectrochemical water treatment of organic substrates degradation and accompanying

22

H2 generation. The resulting Blue-TNTs (inner diameter: ~40 nm; length: ~9 µm) showed

23

negligible shift in X-ray diffraction pattern compared with the intact TNTs, but the X-ray

24

photoelectron spectra indicated a partial reduction of Ti4+ to Ti3+ on the surface. The

25

electrochemical analyses of Blue-TNTs revealed a marked enhancement in donor density and

26

electrical conductivity by orders of magnitude. Degradations of test organic substrates on

27

Blue-TNTs were compared with the intact TNTs in electrochemical (EC), photocatalytic (PC),

28

and photoelectrochemical (PEC) conditions (potential bias: 1.64 VNHE; λ > 320 nm). The

29

degradation of 4-chlorophenol was greatly enhanced on Blue-TNTs particularly in PEC

30

condition, whereas the PC activities of the Blue- and intact TNTs were similar. The potential

31

bias of 1.64 VNHE did not induce any noticeable activity in EC condition. Similar trends were

32

observed for the degradation of humic acid and fulvic acid, where main working oxidants

33

were found to be the surface hydroxyl radical as confirmed by hydroxyl radical probe and

34

scavenger tests. H2 generation coupled with the organic degradation was observed only in

35

PEC condition, where the H2 generation rate with Blue-TNTs was more than doubled from

36

that of intact TNTs. Such superior PEC activity was not observed when a common TiO2

37

nanoparticle film was used as a photoanode. The enhanced electric conductivity of Blue-

38

TNTs coupled with a proper band bending in PEC configuration seemed to induce a highly

39

synergic enhancement.

40 41

Keywords: Titania nanotube, Photoelectrochemical water treatment, Advanced oxidation process,

42

Hydrogen production, Water-Energy nexus.

2

ACS Paragon Plus Environment

Page 3 of 30

Environmental Science & Technology

43

INTRODUCTION

44

The photochemistry and photoelectrochemistry of semiconductor metal oxides have been

45

intensely investigated over the past decades to meet the growing needs of controlling

46

recalcitrant pollutants by producing active oxidants (e.g., OH radicals), mostly under external

47

input of energy.1-5 In particular, TiO2 has been widely used for photocatalytic or

48

electrocatalytic degradation of organic compounds,5-7 mostly thanks to catalytic active sites

49

on TiO2 surface, a highly positive potential of the valence band edge, and an excellent

50

chemical stability in a large window of potential bias and pH. Low cost and low toxicity are

51

additional advantages of titanium oxides for environmental purification purposes.3 Moreover,

52

TiO2 has been a key material for storage and conversion of energy, exemplified by its use in

53

photocatalytic H2 generation and CO2 reduction as well as Li-ion battery. Among various

54

morphologies of TiO2 (such as nanorods,8 films,9 nanoparticles10 and nanotube arrays11)

55

investigated for environmental and energy applications, titanium nanotube arrays (TNTs)

56

may prove to be an attracting choice, owing to their unique and desirable properties such as

57

high specific surface area, open-channel structure that facilitates the mass transfer of

58

substrates, and reduced light-scattering loss.12-13 Nevertheless, the applications on TNTs for

59

photoelectrochemical (PEC) conversions are still limited by wide band gap, fast charge pair

60

recombination, and low electrical conductivity.

61

In order to enhance the activities of TiO2 either as a photocatalyst or an electrocatalyst,

62

doping of external elements14-15 has been tried to effectively improve the light absorption and

63

electrical conductivity. In addition, a partial reduction of Ti4+ to Ti3+, together with

64

intercalation of protons and/or formation of oxygen vacancies, could be a feasible self-doping

65

that can be done by hydrogenation, chemical and electrochemical reduction of TiO2.16-19 In

66

particular, a cathodization of TNTs proved to a simple and safe procedure for the self-doping

67

of TiO2, since the one-dimensional nanostructure of TNTs was suitable for the 3

ACS Paragon Plus Environment

Environmental Science & Technology

Page 4 of 30

68

electrochemical reduction treatment.20 The so-called electrochemical self-doping would

69

change the electronic state in the band gap which in-turn affects the optical properties of TiO2

70

(light absorption).21 In addition, an enhanced mobility of charge carriers associated with the

71

Ti3+ impurities makes the electrochromic TiO2 suitable for the application to water treatment

72

and energy conversion.22

73

The PEC approach in which an external potential bias coupled with light irradiation

74

effectively separate the charge pairs into a cathode and an anode provides an ideal method for

75

achieving various photochemical conversions including the degradation of aqueous

76

pollutants,2,23 via the generation of reactive oxygen species and the reduction of proton or

77

water to molecular hydrogen. Utilizing solar energy for water treatment with simultaneous

78

energy recovery (e.g., H2 generation) is an attractive technology. To this end, it is reasonable

79

to propose that the electrochromic TiO2 with an increased charge carrier density and

80

electrical conductivity would be desirable for its application to PEC water treatment and

81

energy conversion.

82

Herein, we investigate an exemplified PEC system for simultaneous water treatment and

83

molecular hydrogen production, using electrochromic titania nanotube arrays (denoted as

84

Blue-TNTs) as a photoanode. A cathodic polarization of crystalline TNTs was found to

85

enhance the degradation of organic compounds and H2 production significantly whereas such

86

PEC activities could not be obtained with using intact TNTs or common TiO2 nanoparticle

87

film. This investigation demonstrates a good strategy for utilizing immobilized catalysts for

88

energy-recovering water purification process without the need of recovery and recirculation

89

of catalysts.

90 91

MATERIALS AND METHODS

92

Electrodes Preparation and Characterization.

TNTs were fabricated by a two-step 4

ACS Paragon Plus Environment

Page 5 of 30

Environmental Science & Technology

93

anodization procedure to obtain relatively stable and uniform structure.24-25 The anodization

94

process was performed in a single compartment cell with a Ti foil (Aldrich, 3 x 2 cm2, 0.127

95

mm thick, 99.7% purity) as a working electrode and a Pt wire as a counter electrode. Before

96

the anodization, Ti foils were cleaned with ethanol and deionized (DI) water by

97

ultrasonication, and then dried in air. The first anodization was performed at 60 V for 15 min

98

in ethylene glycol electrolyte containing 0.5 wt% NH4F (Sigma-Aldrich, 98% purity) and 3

99

wt% H2O. The resulting TNTs layer was removed by ultrasonication in a concentrated H2O2

100

solution (Junsei, 35% purity), then washed with DI water. The second anodization was done

101

at 60 V for 45 min in ethylene glycol electrolyte containing 0.2 wt% NH4F and 1 wt% H2O.

102

After washing with ethanol and DI water, the as-formed TNTs were dried in air and annealed

103

at 450 °C for 3 h (in air with a ramp rate of 2 °C min-1). In order to prepare Blue-TNTs, a

104

constant current (0.017 A/cm2) was applied to the TNTs electrode for 30 s in phosphate buffer

105

solution ([KH2PO4]0 = 0.1 M, pH = 7.2 with addition of NaOH) with using a Pt wire as a

106

counter electrode.26 The activities of the Blue-TNTs electrode were compared with those of

107

the TiO2 nanoparticulate film electrode as a control. For the preparation of TiO2

108

nanoparticulate film, commercial TiO2 (P25) of which particle size is about 20 - 30 nm was

109

used. P25 was coated on a glass substrate (2 x 2 cm) by a doctor-blade method.27 P25 powder

110

was thoroughly mixed with ethanol in a concentration of 0.15 g mL-1. The mixed paste was

111

cast on the substrate glass plate, dried under air, and then heated at 200 °C for 1 h to remove

112

residual ethanol in the immobilized photocatalyst electrode of which thickness was about ~10

113

µm. The TNTs and Blue-TNTs were characterized by diffuse reflectance spectroscopy (DRS,

114

Shimadzu UV-2401PC), X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with

115

monochromated Al-Kα radiation (1486.6 eV), X-ray diffraction (XRD, Max Science Co.,

116

M18XHF) using Cu-Kα radiation, and field emission scanning electron microscopy (FE-SEM,

117

JEOL, JSM-7401F). 5

ACS Paragon Plus Environment

Environmental Science & Technology

Page 6 of 30

118

Electroanalytical Measurements.

119

with a three-electrode configuration was employed in this study, which included a working

120

electrode (TNTs or Blue-TNTs, 2 x 2 cm2), a stainless steel (SS, 2 x 2 cm2) counter electrode,

121

and a Hg/Hg2SO4 reference electrode. The electrode module was connected to a computer-

122

controlled potentiostat (Gamry Instruments Reference 600). The distance between anode and

123

cathode was 5 mm and the supporting electrolyte for the following electroanalytical

124

measurements was 0.1 M Na2SO4 solution. Potentials need to be corrected for the

125

uncompensated resistance between the working electrode and the reference electrode. For a

126

planar electrode with a uniform current density, the uncompensated resistance (Ru) is given

127

by Ru = x/κA, where x is the distance from tip of working electrode to reference electrode

128

(cm), A is the area of the working electrode (cm2), and κ is the conductivity of solution.28 The

129

measured potentials (reported in this work) should be subtracted by iRu to give the corrected

130

potentials and our estimation yielded Ru = 9.6 Ω and iRu = 0.22 V. Linear sweep voltammetry

131

(LSV) data were collected in the potential range of -0.5 to +1.3 V (vs. Hg/Hg2SO4) at a scan

132

rate of 50 mV s-1. For electrochemical impedance spectroscopy (EIS) Nyquist plot, the

133

potential bias was set at open circuit voltage (OCV) with a frequency range of 1 MHz to 0.01

134

Hz and alternating current (AC) voltage of 100 mV rms. The Mott-Schottky measurements

135

were done at AC potential of 10 mV with a frequency of 100 Hz in the potential range of -0.5

136

to +0.2 V (vs. Hg/Hg2SO4). The LSV data and Nyquist plots were also collected under UV

137

irradiation, where the light source was a 300-W Xe arc lamp with a UV cutoff filter (λ > 320

138

nm).

139

Degradation of Organic Substrates Coupled with H2 Generation.

140

Sigma), humic acid and fulvic acid (HA, FA, Suwannee River) were chosen as test

141

compounds to compare their degradation rates in variable energy input conditions;

A single compartment cell (working volume: 80 mL)

4-chlorophenol (4-CP,

6

ACS Paragon Plus Environment

Page 7 of 30

Environmental Science & Technology

142

electrochemical (EC, potential bias only), photocatalytic (PC, irradiation only) and

143

photoelectrochemical (PEC, potential bias with irradiation). Coumarin (Sigma) and

144

terephthalic acid (TA, Sigma) were used as a trapping reagent of •OH radicals, and tert-butyl

145

alcohol (TBA, Sigma) and ethylenediaminetetraacetic acid (EDTA, Sigma) were used as •OH

146

and hole scavenger, respectively. The initial substrate concentration was 100 µM for 4-CP

147

and 1 ppm for FA and HA, while 0.1 M Na2SO4 was added as a supporting electrolyte (pH 6).

148

The degradation of organic substances was conducted in a single compartment cell reactor

149

(working volume: 80 mL) with a three-electrode system, which included a working electrode

150

(TNTs or Blue-TNTs, 2 x 2 cm2), a counter electrode (SS, 2 x 2 cm2), and a reference

151

electrode (Hg/Hg2SO4). The solution in the reactor was air-saturated. The potential bias was

152

fixed at +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), and the UV irradiation (λ > 320 nm) source was

153

the 300-W Xe arc lamp. The incident light intensity was measured to be 1 W/cm2 (optical

154

power meter, Newport 1918-R). For the measurement of molecular hydrogen and oxygen gas

155

evolution, the reactor with 0.1 M Na2SO4 solution (with or without organic substrates) was

156

initially purged with Ar gas (Linde, 99.9995%) for 1 h to remove the dissolved oxygen.

157

During the reaction, gas samples were periodically withdrawn from the headspace (~40 mL)

158

with a 100 µL glass syringe (Hamilton 81030).

159

Analytical Methods. Analysis of 4-CP and the degradation intermediates of 4-CP was done

160

using a high performance liquid chromatograph (HPLC, Agilent 1100). Analysis of anionic

161

chlorine species (i.e. Cl-, ClO- and ClO3-) was done by an ion chromatograph (IC, DX-120).

162

HA and FA were analyzed by monitoring their fluorescence emission using a

163

spectrofluorometer (HORIBA fluoromax-4) under the excitation of 279 nm. H2 in gas

164

samples was analyzed by a gas chromatograph (GC, HP6890A) with a thermal conductivity

165

detector (TCD) and a 5 Å molecular sieve column. For the comparison of the TNTs and Blue7

ACS Paragon Plus Environment

Environmental Science & Technology

Page 8 of 30

166

TNTs electrode surface area, the adsorption of methylene blue (MB) dye was measured by

167

adsorbing MB on the electrode surface (in 0.5 M MB solution at pH 10 for 3 h) and the

168

subsequent desorption of the adsorbed dye at pH 2. The desorbed dye amount was quantified

169

spectrophotometrically using a UV/Visible spectrophotometer (Agilent 8453).

170 171

RESULTS AND DISCUSSION

172

Physicochemical Properties of the Photoanodes.

173

morphologies of intact TNTs (Figure S1 in Supporting Information (SI)) and Blue-TNTs

174

(Figure 1a and b) were comparable with each other in the SEM analysis. These nanotubes

175

were roughly characterized by an inner diameter of ~40 nm and an average length of ~9 µm.

176

The MB dye adsorption on each electrode was measured to be almost the same between

177

TNTs (9.0(±0.2) nmol cm-2) and Blue-TNTs (9.6(±0.3) nmol cm-2) electrode. Therefore, the

178

surface area difference between TNTs and Blue-TNTs can be ruled out as a main factor

179

influencing the electrode activity. Blue-TNTs and intact TNTs also exhibited almost identical

180

XRD patterns (Figure S2 in SI), primarily related with anatase form, whereas the cathodic

181

polarization process turned the color of intact TNTs to dark blue (Figure 1d (inset)). This

182

electrochromic phenomenon would arise from the surface defect generation associated with a

183

change in the oxidation state of Ti species during the cathodic polarization, which might be

184

accompanied with intercalations of monovalent cations (such as H+29). A formation of Ti3+

185

along with the intercalation of proton in the TiO2 lattice can be expressed by Eq. 1. 30-31

186

The horizontal and cross-sectional

> Ti + e + H → > Ti H

(1)

187

The XPS measurement (Figure 1c) confirmed the existence of Ti3+ on the surface of Blue-

188

TNTs. For the intact TNTs, typical maxima of Ti4+ were observed at binding energies of

189

465.1 (Ti 2p1/2) and 458.9 eV (Ti 2p3/2),32 while the Blue-TNTs peaks were shifted to lower 8

ACS Paragon Plus Environment

Page 9 of 30

Environmental Science & Technology

190

binding energies, owing to the partial contribution of Ti3+ state. The DRS shown in Figure 1d

191

indicated a red-shift of the spectrum and greater absorption of UV light for the Blue-TNTs,

192

which manifested the electrochromism. However, this enhanced light absorption of Blue-

193

TNTs was found to have an insignificant effect on the photocatalytic activity (vide infra).

194

The electrochemical analysis evaluated the charge transfer characteristics under dark and

195

light illumination conditions, which was shown as Nyquist and Mott-Schottky plots. In the

196

Nyquist plots (Figure 2a) obtained in the dark condition, the behavior of intact TNTs and

197

Blue-TNTs followed a transmission line model for TiO2 nanotube system,33 and both

198

electrodes exhibited very large charge transfer resistance. Under UV irradiation, on the other

199

hand, more distinct semicircular relation was observed in Blue-TNTs, owing to the enhanced

200

interfacial charge transfer. The charge transfer resistance of Blue-TNTs under irradiation was

201

observed to be far smaller than that of the intact TNTs, based on the low-frequency domain

202

intercepts of the arcs. This result indicates more facile electron migration through the

203

nanotube network in Blue-TNTs, which was also confirmed by Mott-Schottky plots (Figure

204

2b). Blue-TNTs exhibited a markedly flat slope, corroborating an improvement in electrical

205

conductivity. The charge carrier density (ND) was estimated through Mott-Schottky equation

206

expressed as Eq. 2: 34-35

207



= 



 

 E − E ! " −

#$ 

%

(2)

208

where Csc is the space charge capacitance (F cm-2); e is elementary charge (1.602 × 10-19 C); ε

209

is the relative dielectric constant of electrode material (48 for anatase TiO2; assumed to be

210

identical for Blue-TNTs)24, ε0 is the permittivity of vacuum (8.85 × 10-12 N-1 C2 m-2); ES is the

211

applied potential (V); EFB is the flat band potential (V); k is the Boltzmann’s constant (1.38 ×

212

10-23 J K-1), and T is the absolute temperature (K). Our estimation suggested the donor

213

density of TNTs and Blue-TNTs to be 9.92 × 1020 and 3.63 × 1023 cm-3, respectively. 9

ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 30

214

Consequently, the presence of Ti3+ as a defect in Blue-TNTs notably enhanced the electrical

215

conductivity and charge carrier density, which reconfirmed the previous report.26, 36-37 Our

216

results further showed that the corresponding improvement in interfacial charge transfer is

217

more pronounced in a PEC condition, rationalizing the application of Blue-TNTs as a

218

photoanode. More direct evidences are presented in the linear sweep voltammograms (Figure

219

2c) obtained with and without UV (λ > 320 nm) irradiation. Both electrodes showed

220

negligible dark current, whereas the photocurrent of Blue-TNTs was distinctly higher than

221

TNTs under irradiation. The photocurrent of TNTs was almost saturated at the anodic

222

potential bias of -0.2 V (vs. Hg/Hg2SO4), whereas that of Blue-TNTs monotonically

223

increased up to +1.3 V (vs. Hg/Hg2SO4). At +1.0 V (vs. Hg/Hg2SO4), the photocurrent

224

generated with Blue-TNTs (5.7 mA/cm2) was four times higher than that of the intact TNTs

225

(1.4 mA/cm2).

226

Photoelectrochemical Degradation of Organic Compounds.

227

Blue-TNTs were compared in terms of the degradation of test organic compounds (4-CP, FA

228

and HA) under variable energy input conditions (PEC, PC and EC). Figure 3a shows that the

229

degradation of 4-CP was negligible in EC condition for both TNTs. In PC condition, both

230

TNTs exhibited similar 4-CP degradation efficiency, ca. 25% within 2 hours. An intriguing

231

discrepancy was observed in PEC condition which achieved almost complete degradation of

232

4-CP with Blue-TNTs within 2 hours, whereas only 45% removal was observed with intact

233

TNTs. As a comparison to TNTs electrodes, a mesoporous TiO2 electrode that consisted of

234

nanoparticulate (P25) film of which thickness is similar to TNTs was prepared and tested

235

under the same conditions (PEC, PC, and EC) as shown in Figure 3b. The TiO2

236

nanoparticulate film electrode exhibited much lower activities than TNTs electrodes in PEC

237

condition. This clearly shows the superior nature of TNTs as a PEC electrode, which

The activities of intact and

10

ACS Paragon Plus Environment

Page 11 of 30

Environmental Science & Technology

238

distinguishes itself from the common nanoparticulate TiO2 film electrode. The TiO2

239

nanoparticulate film seems to be much less efficient than TNTs because the diffusion of

240

electrolytes and substrate molecules is much hindered within the mesopores of the film.

241

Figure 3c depicts the mass balance of chlorine species and H2 generation during the PEC

242

degradation of 4-CP with Blue-TNTs. The chloride ion, a degradation product of 4-CP, could

243

be sequentially oxidized to hypochlorite (ClO-), chlorite (ClO2-), and chlorate (ClO3-). The

244

concentration of ClO2- was below the detection limit owing to the short-lived characteristics

245

(i.e., facile oxidation to ClO3-).38 Therefore, the sum of Cl-, ClO- and ClO3- concentration

246

closely matched the reduction in 4-CP concentration which indicates the absence of

247

chlorinated organic byproducts. The H2 generation linearly increased with the PEC reaction

248

time. Despite the complete removal of 4-CP on Blue-TNTs in 2 h, the degree of

249

mineralization (TOC removal) was about 38% in 2 h while a complete mineralization was

250

achieved after 6 hours of PEC reaction (Figure 3d). At the same PEC condition, the TOC

251

removal efficiencies obtained with intact TNTs were 25% and 42.4% at 2 h and 6 h,

252

respectively. Although some intermediates (HQ, CC and BQ) were generated from the

253

degradation of 4-CP (see Figure S3), they were eventually mineralized on Blue-TNTs.

254

Analogous results were also obtained in the degradation of FA and HA using Blue-TNTs.

255

Figure 4 shows the time-dependent variations of FA and HA concentrations based on the

256

fluorescence emission intensity (at 451 nm for FA and at 464 nm for HA, Figure S4 and S5 in

257

SI). The removal of the humic substances was again negligible in EC condition, while

258

moderate degradation was observed in PC condition with the pseudo-first-order rate constant

259

of 0.48 and 0.46 h-1 for FA and HA, respectively. These degradation rate constants increased

260

dramatically (10.7 h-1 for FA and 4.6 h-1 for HA) in PEC condition.

11

ACS Paragon Plus Environment

Environmental Science & Technology

Page 12 of 30

261

PEC and PC degradations of organic compounds usually involve the surface hydroxyl

262

radicals (•OH) generated from water oxidation with holes (Eq. 3) and the direct transfer of

263

holes. h ! + H O or OH " → • OH

264

(3)

265

The results of 4-CP, FA, and HA degradation imply that the facilitated production of •OH as

266

a non-selective oxidant would primarily account for the highly enhanced PEC activities.39 In

267

order to specify the role of hydroxyl radicals, we used coumarin and terephthalic acid (TA) as

268

a selective probe reagent for •OH trapping. The reaction of coumarin and TA with •OH

269

produces 7-hydroxycoumarin (7-HC, Eq. 4) and 2-hydroxyterephthalic acid (2-HTA, Eq. 5),

270

respectively. The generation of the hydroxylated products was quantified by monitoring the

271

fluorescence emission (Figure S6 and S7).40

272

• OH

+ coumarin → 7-HC (emission monitored with λex = 332 nm)

(4)

273

• OH

+ TA → 2-HTA

(5)

(emission monitored with λex = 315 nm)

274

Figure 5 depicts the time profiles of 7-HC and 2-HTA production whose slope would reflect

275

a pseudo steady-state concentration of OH radicals on the electrode surface. Both TNTs

276

produced negligible amount of 7-HC and 2-HTA in EC condition, which implies that OH

277

radicals cannot be generated under the present potential bias condition. On the other hand, the

278

PC condition clearly generated the sign of 7-HC and 2-HTA, which is consistent with the

279

well-known behavior of TiO2 photocatalyst that generates OH radicals under UV

280

irradiation.3-5 In PEC condition, the productions of 7-HC and 2-HTA on Blue-TNTs were

281

almost doubled from that of intact TNTs. This observation is in good agreement with Figure

282

3 and 4, which showed markedly higher PEC activities of Blue-TNTs than intact TNTs. The

283

coumarin and TA tests suggest that the high PEC activities of Blue-TNTs for the degradation

284

of organic compounds should be ascribed mainly to the facile generation of OH radicals.

12

ACS Paragon Plus Environment

Page 13 of 30

Environmental Science & Technology

285

Another oxidant that might be involved in this system is sulfate radical that can be

286

generated from the oxidation of sulfate electrolyte. We additionally carried out 4-CP

287

degradation experiment in inert NaClO4 electrolyte as a control, which was compared with

288

that in Na2SO4 electrolyte to estimate the possible role of sulfate radicals (see Figure S8). The

289

degradation efficiency of 4-CP was moderately reduced when using NaClO4 as an inert

290

electrolyte, which implies that some sulfate radicals are generated from sulfate ions and

291

involved along with the hydroxyl radical in the degradation of organic compounds.

292

Superoxide anion radicals can be also generated on the SS counter electrode through the

293

reduction of dissolved O2 but their oxidation power is not strong enough to degrade

294

recalcitrant organic substrates.

295

To further investigate the role of •OH as an oxidant on Blue-TNTs photoanode, TBA and

296

EDTA were used as scavengers for free and surface-bound •OH and holes in the PEC

297

degradation of 4-CP (see Figure S9). 4-CP was fully degraded without scavenger reagents in

298

2 h PEC reaction, whereas, in the presence of either TBA or EDTA, the PEC degradation of

299

4-CP was significantly retarded, but not completely inhibited. On the other hand, the

300

degradation of 4-CP could be almost completely prohibited in the presence of both TBA and

301

EDTA. This indicates that both OH radicals and holes are responsible as main oxidants of 4-

302

CP in the PEC degradation using Blue-TNTs.

303

In dark condition (EC), the potential applied in this study (+1.64 VNHE) seemed to be

304

insufficient to overcome the kinetic barrier of •OH generation for both intact TNTs and Blue-

305

TNTs. We further carried out an additional EC test at a higher voltage (+2.75 VNHE) and

306

found that the EC system at 2.75 VNHE still exhibited a lower activity than the PEC system at

307

1.64 VNHE for the degradation of 4-CP (see Figure S10). That is, PEC requiring a lower

308

voltage is more efficient than EC requiring a higher voltage (by 1.11 V), which demonstrates

309

the merit of PEC system. Although a PEC system needs light irradiation which is an extra 13

ACS Paragon Plus Environment

Environmental Science & Technology

Page 14 of 30

310

energy input, the solar application should make PEC systems cost effective. On the other

311

hand, the PC activities of intact TNTs and Blue-TNTs exhibited little difference although

312

Blue-TNTs has higher light absorption in the UV region (see Fig. 1d). Possible explanations

313

include: (i) the Ti3+ as a surface defect site could also serve as a recombination center41 and

314

(ii) the enhanced electron mobility in Blue-TNTs would not effectively increase the

315

photocatalytic activity, since the sluggish interfacial electron transfer steps (e.g., reduction of

316

dissolved oxygen or molecular hydrogen evolution) are rate-limiting. The results in this study

317

clearly demonstrated that the positive effect of electrochemical self-doping in TNTs is

318

particularly outstanding in PEC condition. The band bending under an anodic potential bias

319

in synergy with an enhanced electrical conductivity of Blue-TNTs would facilitate the excited

320

electrons to migrate towards the cathode and effectively overcome the fast recombination

321

process.

322

Molecular Hydrogen Generation.

323

TNTs or Blue-TNTs in 0.1 M Na2SO4 solution with organic substrates (4-CP, HA, and FA) in

324

three conditions (EC, PC, and PEC). In PC and EC conditions, the hydrogen evolution was

325

absent with both TNTs and Blue-TNTs, corroborating that the interfacial electron transfer is

326

sluggish rate-determining step in PC condition. On the contrary, a distinct activity for H2

327

production was observed in PEC condition where the transfer of excited electrons in TNTs

328

and Blue-TNTs to a SS counter electrode could be facilitated under a potential bias for

329

generating H2 (Eq. 6). The hydrogen evolution amount did not depend on the kind of organic

330

substrates (4-CP, HA, and FA) in the PEC reactor.

331

Figure 6a compares the production of H2 with either

2e + 2H → H

(6)

332

The rate of hydrogen generation was observed to be far greater with Blue-TNT (72 µM cm-2

333

h-1) than intact TNTs (19 µM cm-2 h-1). Consequently, the superior activity of Blue-TNTs was 14

ACS Paragon Plus Environment

Page 15 of 30

Environmental Science & Technology

334

fully demonstrated in PEC condition, with respect to H2 evolution as well as organic

335

degradation, owing to more efficient migration of charge carriers. When the quantified

336

amounts of H2 were compared with the theoretical ones based on the measured current

337

(dashed lines in Figure 6), the average Faradaic efficiency of H2 evolution was close to unity

338

for both TNTs and Blue-TNTs.

339

On the other hand, the accompanying evolution of O2 in the PEC reactor was also

340

measured and compared with H2 evolution in Figure 6b. The production of O2 was lower than

341

that expected from the stoichiometric water splitting (1/2 of H2 production), which implies

342

that holes were consumed not only in water oxidation to O2 (2H2O + 4h+ → O2 + 4H+) but

343

also in hydroxyl radical generation (H2O + h+ → HO• + H+), sulfate radical generation (SO42−

344

+ h+ → SO4•−), and direct oxidation of organic substrate (4-CP + h+ → 4-CP•+). In addition, it

345

should be noted that the evolution of H2 and O2 was little affected by the presence and

346

absence of 4-CP, which indicates that the direct hole consumption by organic substrates is

347

negligible compared to other hole consuming reactions.

348

Stability.

349

dark ambient condition. However, the electrochromic reaction given by Eq. 1 may be

350

reversible under either potential bias or irradiation so that the backward reaction (Ti3+H+ →

351

Ti4+ + e- + H+) might deteriorate the PEC activity (see Figure S11). In order to check the

352

stability of self-doping in Blue-TNTs, the degradation of 4-CP was repeated up to five cycles

353

in the PEC condition (Figure 7). The pseudo-first order rate constants of 4-CP degradation

354

ranged from 1.8 h-1 to 1.4 h-1 for Blue-TNTs without a significant variation, while the intact

355

TNTs consistently showed much lower activity (0.40 ~ 0.25 h-1). In addition, the rate of H2

356

generation was also maintained during the repeated tests (72 ~ 66 µM cm-2 h-1) with Blue-

357

TNTs in PEC condition. The PEC activities of the electrochromic TNTs (Blue-TNTs) were

For virgin Blue-TNTs, the dark blue color was maintained for several weeks in a

15

ACS Paragon Plus Environment

Environmental Science & Technology

Page 16 of 30

358

maintained through multiple uses despite a shift in XPS Ti 2p band after PEC experiment

359

(Figure S11). This implies that the self-doping coupled with proton intercalation and/or

360

oxygen vacancy formation induced a permanent structural distortion to make Blue-TNTs

361

stable enough.42

362

Environmental Implications.

363

photocatalysts for water treatment application would be a requirement of additional

364

separation step for reuse of catalysts as well as effluent clarification. Therefore, much

365

research efforts have been given to the development of immobilized photocatalysts, which

366

were prepared by template synthesis, sol-gel binding and direct growth on supports.43-44 On

367

the other hand, the activities of the immobilized photocatalysts would be hampered by a loss

368

in effective surface area and the diffusion limitations across the immobilized catalyst layers.

369

To this end, it has been proposed that the self-organized TNTs directly grown on Ti substrate

370

by anodic oxidation have an open channel structure which is suitable for efficient diffusion of

371

reactants and dioxygen molecules.12 More importantly, this study clearly demonstrated that a

372

PEC configuration could boost the photoactivity of the prepared electrochromic TNTs for

373

organic compounds degradation and simultaneous H2 generation. The cathodic polarization of

374

TNTs highly enhanced the electrical conductivity to induce much higher photocurrents under

375

a potential bias. At the same time, the photocurrent can be utilized for H2 production on an

376

inexpensive counter electrode. In a practical perspective, various renewable energy sources

377

such as solar cells can provide the relatively low potential bias for the PEC condition. Such

378

combination of a PEC reactor and solar cells can provide an ideal solution for water treatment

379

and energy recovery in remote areas that are not connected to a power grid system. The

380

electrochemical self-doping of TNTs is a simple and versatile method to prepare an

381

immobilized photoanode which shows high activity and durability in PEC condition. The

One of the major limitations of heterogeneous

16

ACS Paragon Plus Environment

Page 17 of 30

Environmental Science & Technology

382

properties of Blue-TNT might be further tuned by the adjustment of cathodization conditions

383

for more desirable band bending and visible light adsorption, which requires further

384

investigation.

385

386

ACKNOWLEDGMENT.

387

This work was supported by the Global Research Laboratory (GRL) Program (NRF-

388

2014K1A1A2041044) and KCAP (Sogang Univ.) (No. 2009-0093880) funded by the Korea

389

government (MSIP) through National Research Foundation of Korea (NRF) and, in part, by

390

KIST-UNIST partnership program (2V05120/1.160097.01).

391 392

Supporting Information Available.

393

SEM image of intact TNTs (Figure S1); XRD patterns of TNTs and Blue-TNTs (Figure S2);

394

HPLC analysis of 4-CP degradation intermediates (Figure S3); Fluorescence emission spectra

395

of FA (Figure S4) and HA (Figure S5); Fluorescence emission spectra of 7-HC (Figure S6)

396

and 2-HTA (Figure S7) generated on TNTs and Blue-TNTs; Effects of electrolytes on the

397

PEC degradation of 4-CP (Figure S8); Effects of TBA and EDTA on PEC degradation of 4-

398

CP (Figure S9); Comparison of PEC and EC degradation of 4-CP with different bias

399

potentials (Figure S10); XPS Ti 2p band of Blue-TNTs before and after PEC experiment

400

(Figure S11).

401 402

References

403

(1) Ochiai, T.; Fujishima, A. Photoelectrochemical properties of TiO2 photocatalyst and its

404

applications for environmental purification. J. Photochem. Photobiol., C 2012, 13, 247-262.

405

(2) Zhang, H.; Chen, G.; Bahnemann, D. W. Photoelectrocatalytic materials for 17

ACS Paragon Plus Environment

Environmental Science & Technology

Page 18 of 30

406

environmental applications. J. Mater. Chem. 2009, 19, 5089−5121.

407

(3) Park, H.; Park, Y.; Kim, W.; Choi, W. Surface modification of TiO2 photocatalyst for

408

environmental applications. J. Photochem. Photobiol. C 2013, 15, 1-20.

409

(4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications

410

of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69−96.

411

(5) Park, H.; Kim, H. I.; Moon, G. H.; Choi, W. Photoinduced charge transfer processes in

412

solar photocatalysis based on modified TiO2. Energy Environ. Sci. 2016, 9, 411−433.

413

(6) Cho, K.; Qu, Y.; Kwon, D.; Zhang, H.; Cid, C. A.; Aryanfar, A.; Hoffmann, M. R. Effects

414

of anodic potential and chloride ion on overall reactivity in electrochemical reactors designed

415

for solarpowered wastewater treatment. Environ. Sci. Technol. 2014, 48, 2377−2384.

416

(7) Cho, K.; Hoffmann, M. R. Urea degradation by electrochemically generated reactive

417

chlorine species: Products and reaction pathways. Environ. Sci. Technol. 2014, 48,

418

11504−11511.

419

(8) Wu, J. M. Photodegradation of Rhodamine B in water assisted by titania nanorod thin

420

films subjected to various thermal treatments. Environ. Sci. Technol. 2007, 41, 1723−1728.

421

(9) Miao, H.; Hu, X.; Fan, J.; Li, C.; Sun, Q.; Hao, Y.; Zhang, G.; Bai, J.; Hou, X.

422

Hydrothermal synthesis of TiO2 nanostructure films and their photoelectrochemical

423

properties. Appl. Surf. Sci. 2015, 358, 418-424.

424

(10) Long, M.; Brame, J. A.; Qin, F.; Bao, J.; Li, Q.; Alvarez, P. J. Phosphate changes effect

425

of humid acids on TiO2 photocatalysis: from inhibition to mitigation of electron-hole

426

recombination. Environ. Sci. Technol. 2017, 51, 514–521.

427

(11) Hu, L.; Fong, C.-C.; Zhang, X.; Chan, L. L.; Lam, P. K. S.; Chu, P. K.; Wong, K.-Y.;

428

Yang, M. Au nanoparticles decorated TiO2 nanotube arrays as a recyclable sensor for

429

photoenhanced electrochemical detection of bisphenol A. Environ. Sci. Technol. 2016, 50,

430

4430−4438. 18

ACS Paragon Plus Environment

Page 19 of 30

Environmental Science & Technology

431

(12) Weon, S.; Choi, W. TiO2 nanotubes with open channels as deactivation-resistant

432

photocatalyst for the degradation of volatile organic compounds. Environ. Sci. Technol. 2016,

433

50, 2556−2563.

434

(13) Weon, S.; Choi, J.; Park, T.; Choi, W. Freestanding doubly open-ended TiO2 nanotubes

435

for efficient photocatalytic degradation of volatile organic compounds. Appl. Catal. B 2017,

436

205, 386-392.

437

(14) Choi, W.; Termin, A.; Hoffmann, M. R. The role of metal ion dopants in quantum-sized

438

TiO2: correlation between photoreactivity and charge carrier recombination dynamics. J. Phys.

439

Chem. 1994, 98, 13669-13679.

440

(15) Lim, J.; Murugan, P.; Lakshminarasimhan, N.; Kim, J. Y.; Lee, J. S.; Lee, S. H.; Choi, W.

441

Synergic photocatalytic effects of nitrogen and niobium co-doping in TiO2 for the redox

442

conversion of aquatic pollutants under visible light. J. Catal. 2014, 310, 91-99.

443

(16) Lu, X.; Wang, G.; Zhai, T.; Yu, M.; Gan, J.; Tong, Y.; Li, Y. Hydrogenated TiO2 nanotube

444

arrays for supercapacitors. Nano lett. 2012, 12, 1690-1696.

445

(17) Wu, H.; Li, D.D.; Zhu, X.F.; Yang, C.Y.; Liu, D.F.; Chen, X.Y.; Song, Y.; Lu, L.F. High-

446

performance and renewable supercapacitors based on TiO2 nanotube array electrodes treated

447

by an electrochemical doping approach. Electrochim. Acta 2014, 116, 129−136.

448

(18) Singh, A. P.; Kodan, N.; Mehta, B. R. Enhancing the photoelectrochemical properties of

449

titanium dioxide by thermal treatment in oxygen deficient environment. Appl. Surf. Sci. 2016,

450

372, 63-69.

451

(19) Kim, C.; Kim, S.; Lee, J.; Kim, J.; Yoon, J. Capacitive and oxidant generating properties

452

of black-colored TiO2 nanotube array fabricated by electrochemical self-doping. ACS Appl.

453

Mater. Interfaces 2015, 7 (14), 7486−7491.

454

(20) Tokudome, H.; Miyauchi, M. Electrochromism of titanate- based nanotubes. Angew.

455

Chem., Int. Ed. 2005, 44, 1974-1977. 19

ACS Paragon Plus Environment

Environmental Science & Technology

Page 20 of 30

456

(21) Granqvist, C. G.; Lansåker, P. C.; Mlyuka, N. R.; Niklasson, G. A.; Avendano, E.

457

Progress in chromogenics: new results for electrochromic and thermochromic materials and

458

devices. Sol. Energy Mater. Sol. Cells 2009, 93, 2032-2039.

459

(22) Yang, Y.; Hoffmann, M. R. Synthesis and stabilization of Blue-Black TiO2 nanotube

460

arrays for electrochemical oxidant generation and wastewater treatment. Environ. Sci.

461

Technol. 2016, 50, 11888-11894.

462

(23) Zhao, X.; Guo, L.; Zhang, B.; Liu, H.; Qu, J. Photoelectrocatalytic oxidation of CuII-

463

EDTA at the TiO2 electrode and simultaneous recovery of CuII by electrodeposition. Environ.

464

Sci. Technol. 2013, 47, 4480−4488.

465

(24) Zhang, Z.; Wang, P. Optimization of photoelectrochemical water splitting performance

466

on hierarchical TiO2 nanotube arrays. Energy Environ. Sci. 2012, 5, 6506-6512.

467

(25) Kim, H. I.; Monllor-Satoca, D.; Kim, W.; Choi, W. N-doped TiO2 nanotubes coated with

468

a thin TaOxNy layer for photoelectrochemical water splitting: dual bulk and surface

469

modification of photoanodes. Energy Environ. Sci. 2015, 8, 247-257.

470

(26) Kim, C.; Kim, S.; Choi, J.; Lee, J.; Kang, J. S.; Sung, Y. E.; Choi, W.; Yoon, J. Blue

471

TiO2 nanotube array as an oxidant generating novel anode material fabricated by simple

472

cathodic polarization. Electrochim. Acta 2014, 141, 113−119.

473

(27) Kim, H.; Choi, W. Effect of surface fluorination of TiO2 on photocatalytic oxidation of

474

gaseous acetaldehyde. Appl. Catal., B 2007, 69, 127−140.

475

(28) Bard, A. J.; Faulkner, L. R. Fundamentals and Applications. In Electrochemical Methods,

476

2nd ed.; Wiley: New York, 2001.

477

(29) Wang, C. M.; Lin, S. Y. Electrochromic properties of sputtered TiO2 thin films. J. Solid-

478

State Electrochem. 2006, 10, 255-259.

479

(30) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Highly hydrophilic surfaces of

480

cathodically polarized amorphous TiO2 electrodes. J. Electrochem. Soc. 2001, 148, E39520

ACS Paragon Plus Environment

Page 21 of 30

Environmental Science & Technology

481

E398.

482

(31) Lyon, L. A.; Hupp, J. T. Energetics of the nanocrystalline titanium dioxide/aqueous

483

solution interface: approximate conduction band edge variations between H0 = -10 and H- = +

484

26. J. Phys. Chem. B 1999, 103, 4623-4628.

485

(32) Erdem, B.; Hunsicker, R. A.; Simmons, G. W.; Sudol, E. D.; Dimonie, V. L.; El-Aasser,

486

M. S. XPS and FTIR surface characterization of TiO2 particles used in polymer

487

encapsulation. Langmuir 2001, 17, 2664-2669.

488

(33) Zhou, He; Yanrong Zhang. Electrochemically self-doped TiO2 nanotube arrays for

489

supercapacitors. J. Phys. Chem. C 2014, 118, 5626-5636.

490

(34) Bard, A. J.; Faulkner, L. R. Electrochemical methods: fundamentals and applications.

491

Wiley and Sons: New York, 1980

492

(35) Yun, H. J.; Lee, H.; Joo, J. B.; Kim, W.; Yi, J. Influence of aspect ratio of TiO2 nanorods

493

on the photocatalytic decomposition of formic acid. J. Phys. Chem. C 2009, 113, 3050-3055.

494

(36) Kim, C.; Kim, S.; Hong, S. P.; Lee, J.; Yoon, J. Effect of doping level of colored TiO2

495

nanotube

496

properties. Phys. Chem. Chem. Phys., 2016, 18 (21), 14370-14375.

497

(37) Kim, C.; Lee, S.; Kim, S.; Yoon, J. Effect of annealing temperature on the capacitive and

498

oxidant-generating

499

array. Electrochim. Acta, 2016, 222, 1578-1584.

500

(38) Jung, Y. J.; Baek, K. W.; Oh, B. S.; Kang, J. W. An investigation of the formation of

501

chlorate and perchlorate during electrolysis using Pt/Ti electrodes: The effects of pH and

502

reactive oxygen species and the results of kinetic studies. Water Res. 2010, 44, 5345-5355.

503

(39) Garcia-Seguraa, S.; Dostab, S.; Guilemany, J. M.; Brillas, E. Solar photoelectrocatalytic

504

degradation of acid orange 7 azo dye using a highly stable TiO2 photoanode synthesized by

505

atmospheric plasma spray. Appl. Catal., B 2013, 132−133, 142−150.

arrays

fabricated

properties

by

of

electrochemical

an

self-doping

electrochemically

on

reduced

electrochemical

TiO2

nanotube

21

ACS Paragon Plus Environment

Environmental Science & Technology

Page 22 of 30

506

(40) Ishibashi, K.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Detection of active oxidative

507

species in TiO2 photocatalysis using the fluorescence technique. Electrochem. Commun. 2000,

508

2, 207–210.

509

(41) Weidmann, J.; Dittrich, T.; Konstantinova, E.; Lauermann, I.; Uhlendorf, I.; Koch, F.

510

Influence of oxygen and water related surface defects on the dye sensitized TiO2 solar cell.

511

Sol. Energy Mater. Sol. Cells 1999, 56, 153-165.

512

(42) Zheng, Q.; Lee, H. J.; Lee, J.; Choi, W.; Park, N. B.; Lee, C. Electrochromic titania

513

nanotube arrays for the enhanced photocatalytic degradation of phenol and pharmaceutical

514

compounds. Chem. Eng. J. 2014, 249, 285−292.

515

(43) Pan, X.; Zhao, Y.; Liu, S.; Korzeniewski, C. L.; Wang, S.; Fan, Z. Comparing graphene-

516

TiO2 nanowire and graphene-TiO2 nanoparticle composite photocatalysts. ACS Appl. Mater.

517

Interfaces 2012, 4, 3944−3950.

518

(44) Guo, W.; Zhang, F.; Lin, C.; Wang, Z. L. Direct growth of TiO2 nanosheet arrays on

519

carbon fibers for highly efficient photocatalytic degradation of methyl orange. Adv. Mater.

520

2012, 24, 4761−4764.

521

22

ACS Paragon Plus Environment

Page 23 of 30

Environmental Science & Technology

522 523 524 525 526 527

Figure 1. (a) The horizontal and (b) cross-sectional images of Blue-TNTs, together with (c) the difference XPS spectra of Ti 2p band between Blue-TNTs and intact TNTs. (d) DRS spectra of intact TNTs and Blue-TNTs. The inset of (d) shows the variation of apparent color (electrochromism) during the cathodic polarization of intact TNTs.

23

ACS Paragon Plus Environment

Environmental Science & Technology

528 529 530 531 532 533 534 535 536

Figure 2. (a) Nyquist plots of intact TNTs and Blue-TNTs with and without light irradiation. Nyquist plots were obtained in the range from 1 MHz to 0.01 Hz at OCV under UV irradiation. The inset figures show the magnification of the regions of interest. (b) MottSchottky plots in electrochemical impedance spectroscopy. The Mott-Schottky measurements were done at AC potential of 10 mV with a frequency of 100 Hz in the potential range of -0.5 to +0.2 V (vs. Hg/Hg2SO4). (c) Linear sweep voltammograms of intact TNTs and Blue-TNTs with or without light irradiation (λ > 320 nm). 24

ACS Paragon Plus Environment

Page 24 of 30

Page 25 of 30

Environmental Science & Technology

537 538 539 540 541 542 543 544

Figure 3. (a) Time profiles of 4-CP degradation with intact and Blue-TNTs in PEC, PC and EC conditions. (b) Time profiles of 4-CP degradation with TiO2 nanoparticulate (P25) film electrode (with a thickness similar to TNTs) in PEC, PC, and EC condition. (c) Mass balance of chlorine species concentrations and H2 generation during the time course of 4-CP degradation with Blue-TNTs in PEC condition. (d) Time profiles of TOC removal with intact and Blue-TNTs. ([4-CP]0 = 100 µM, pH 6, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm).

25

ACS Paragon Plus Environment

Environmental Science & Technology

Page 26 of 30

545 546 547 548 549 550

Figure 4. (a) Time-dependent variation of the fluorescence emission intensity (λex = 279 nm) of fulvic acid (FA: emission at λ = 451 nm) and (b) humic acid (HA: emission at λ = 464 nm) in PEC, PC and EC conditions. ([FA]0 and [HA]0 = 1 ppm, pH 6, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm).

26

ACS Paragon Plus Environment

Page 27 of 30

Environmental Science & Technology

551 552 553 554 555 556 557

Figure 5. Time profiles of fluorescence emission intensity that indicates the generation of (a) 7-HC (emission at λ = 456 nm) and (b) 2-HTA (emission at λ = 425 nm) as an indicator of •OH generation on intact TNTs and Blue-TNTs in PEC, PC and EC conditions. Excitation wavelength was 332 nm (coumarin) and 315 nm (TA). [Coumarin]0 = 1 mM, [TA]0 = 500 µM in 2 mM NaOH, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm.

27

ACS Paragon Plus Environment

Environmental Science & Technology

Page 28 of 30

558 559 560 561 562 563 564 565 566

Figure 6. (a) Production of H2 in PEC, PC and EC conditions with intact TNTs and BlueTNTs. The dashed lines represent the theoretical amount of H2 evolution based on the measured current with assuming Faradaic efficiency of unity. The reactions were conducted in 0.1 M Na2SO4 electrolyte. (b) Production of H2 and O2 with and without 4-CP in PEC condition with Blue-TNTs. The dashed line represents the stoichiometric amount of O2 that is expected from the stoichiometric water splitting (1/2 of H2 production). ([4-CP]0 = 100 µM, pH 6, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm).

28

ACS Paragon Plus Environment

Page 29 of 30

Environmental Science & Technology

567 568 569 570 571 572

Figure 7. The PEC degradation of 4-CP on intact TNTs and Blue-TNTs and the concurrent H2 generation on Blue-TNTs for five repeated runs. For the H2 generation, the reactions were conducted in 0.1 M Na2SO4 electrolyte (with 4-CP). ([4-CP]0 = 100 µM, +1.0 V vs. Hg/Hg2SO4 (+1.64 VNHE), λ > 320 nm).

29

ACS Paragon Plus Environment

Environmental Science & Technology

573

Page 30 of 30

Table of Content (TOC)

574

30

ACS Paragon Plus Environment