Is C3N4 Chemically Stable toward Reactive Oxygen Species in

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Is C3N4 Chemically Stable towards Reactive Oxygen Species in Sunlight-Driven Water Treatment? Jiadong Xiao, Qingzhen Han, Yongbing Xie, Jin Yang, Qiaozhi Su, Yue Chen, and Hongbin Cao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04215 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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Environmental Science & Technology

1 2 3 4 5 6

Is C3N4 Chemically Stable towards Reactive Oxygen Species in Sunlight-Driven Water Treatment?

7 8

Jiadong Xiao,†,‡ Qingzhen Han,† Yongbing Xie,†,* Jin Yang,† Qiaozhi Su,† Yue

9

Chen,† and Hongbin Cao†,*

10 11 12

†

Beijing Engineering Research Center of Process Pollution Control, Division of

13

Environment Technology and Engineering, Institute of Process Engineering,

14

Chinese Academy of Sciences, Beijing 100190, China

15

‡

University of Chinese Academy of Sciences, Beijing 100049, China

16 17 18 19 20 21 22 23 24 25 26 27 28 1

ACS Paragon Plus Environment


29

ABSTRACT: Reactive oxygen species (ROS) are key oxidants for the degradation of organic

30

pollutants in sunlight-driven photocatalytic water treatment, but their interaction with the

31

photocatalyst is easily ignored and, hence, is comparatively poorly understood. Here we show

32

that graphitic carbon nitride (C3N4, a famous visible-light responsive photocatalyst) is

33

chemically stable towards ozone and superoxide radical, in contrast to which hydroxyl radical

34

(•OH) can tear the heptazine unit directly from C3N4 to form cyameluric acid and further

35

release nitrates into aqueous environment. The ratios of released nitrogen from a nanosheet-

36

structured C3N4 and bulk C3N4 that finally exists in the form of NO3− reach 9.5 mol% and 6.8

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mol% in initially ultrapure water, respectively, after 10 h treatment by solar photocatalytic

38

ozonation which can rapidly generate abundant •OH upon C3N4. On a positive note, in the

39

presence of organic pollutants which scramble against C3N4 for •OH, the C3N4 decomposition

40

has been completely or partially blocked and, therefore, the stability of C3N4 under practical

41

working conditions has been obviously preserved. This work supplements the missing

42

knowledge of the chemical instability of C3N4 towards •OH, and calls for attention on the

43

potential deactivation and secondary pollution of catalysts in •OH-involved water treatment

44

processes.

45

Keywords: carbon nitride, chemical instability, photocatalytic ozonation, hydroxyl radical,

46

water treatment

47 48 49 50 51 52 53 54 55 56 57 58

2


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 INTRODUCTION

60

Graphitic carbon nitride (C3N4) has aroused worldwide great concern since 2009,1 and

61

represents today a wide variety of applications mainly including degradation and

62

mineralization of organic pollutants,2,

63

splitting,4,

64

synthesis,9 fuel cell (oxygen reduction reaction (ORR))10 and bioimaging.11 C3N4 has

65

experimentally proven to be highly stable against acid, base and most organic solvents and

66

shown relatively stable activity during recycling and, therefore, it has been alleged to possess

67

high chemical stability in the above-mentioned applications.12,

68

because direct experimental evidence is missing. Although the investigations in the thermal

69

stability and recycling activity of C3N4 are numerous,12,

70

knowledge, no study that really focuses on its chemical stability in photo- or electrocatalytic

71

related applications.

5

air purification,6,

7

3

hydrogen and oxygen production from water

CO2 reduction into hydrocarbon fuels,8 selective organic

13

13

This sounds suspicious

there is, to the best of our

72

Reactive oxygen species (ROS) that mainly include hydroxyl radical (•OH) and superoxide

73

radical (•O2−) are often involved as a primary or side reaction product during C3N4 catalytic

74

reactions, such as water decontamination,2, 3 organic synthesis9 and ORR.10 •OH (E0 = 2.8 V

75

vs. normal hydrogen electrode (NHE)14) is capable to oxidize almost all the organics in water,

76

and has recently shown the potential to decompose graphene oxide and reduced graphene

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oxide.15, 16 In this case, it is rather doubtful whether C3N4 can withstand the onslaught of ROS

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during catalytic reactions, especially during photocatalytic water treatment which requires as

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many ROS as possible as the key oxidants for the degradation and mineralization of organic

80

pollutants.17 If the undesired chemical instability of C3N4 towards ROS is confirmed it would

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render a debate on the environmental benefit of using C3N4 for photocatalytic

82

decontamination and, moreover, special attention will be paid on the potential harm of

83

catalyst deactivation and secondary pollution in the processes where ROS are likely to be

84

formed. 3



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Photocatalytic ozonation, the combination of photocatalysis with ozonation, was found to

86

be much more efficient than ozonation alone or photocatalytic oxidation, with much faster

87

and more complete mineralization of organic pollutants.3, 18 In our very recent work,18 we

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found that photocatalytic oxidation with C3N4 (Vis/O2/C3N4) generates abundant •O2− as the

89

dominating ROS. When a low dosage of ozone (2.1 mol% O3 in O2) was coupled into

90

Vis/O2/C3N4, 2-3 times more conduction band electrons (CB-e−) were trapped, and the ROS

91

generation pathway changed. Photocatalytic ozonation with C3N4 (Vis/O3/C3N4) generated

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almost exclusively •OH whose yield was 6-17 times as high as that in Vis/O2/C3N4 and, as a

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consequence, the pollutant mineralization rate increased by 41-84 folds.18 Although

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Vis/O3/C3N4 is much superior to Vis/O2/C3N4, the stability of C3N4 in this •OH-dominating

95

process is still unknown. Hence, in this work we comprehensively studied bulk C3N4 and a

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nanosheet-structured C3N4 (NS C3N4) in initially ultrapure water treated by photocatalytic

97

oxidation and photocatalytic ozonation, respectively, so as to distinguish their chemical

98

stabilities towards •O2− and •OH. Mechanistic insights into the interactions between C3N4 and

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its self-catalytically generated ROS were obtained. Moreover, the stability of C3N4 under

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practical working conditions (i.e., in the presence of organic pollutants) was also

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comparatively investigated.

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 EXPERIMENTAL SECTION

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Reagents. Melamine (99.5% pure), oxalic acid (OA, 99.5% pure), phenol (99.0% pure) and

104

thiophene (99.0% pure) were purchased from Sinopharm Chemical Reagent Co., Ltd., China.

105

Tert-butyl alcohol (TBA, 99.5% pure) was supplied by Xilong Scientific Co., Ltd., China.

106

Bisphenol

107

dihydroxyanthraquinone (quinizarin, 97.0% pure) were obtained from J&K Scientific Co.,

108

Ltd., China. Benzene (99.5% pure) was purchased from Beijing Modern Oriental Fine

109

Chemistry Co., Ltd., China. Oxygen gas (99.0% pure) was provided by Beijing Qianxi Gas

A

(BPA,

96.0%

pure),

sodium

4

valproate


(98.0%

pure)

and

1,4-

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Co., Ltd., China. Ultrapure water (Resistance: 18.2 MΩ; total organic carbon (TOC): 2 ppb)

111

was produced by a Direct 8 system (Merck Millipore, Germany) and used for all the synthesis

112

and treatment.

113

Material synthesis. The fabrication of bulk C3N4 and NS C3N4 has been reported

114

previously in our work.18 In brief, bulk C3N4 was synthesized by direct polycondensation of

115

melamine. NS C3N4 was prepared by a post-annealing of bulk C3N4 powder inside an open

116

alumina crucible at 550 °C for 3 h with a heating rate of 3 °C min−1. The chemical

117

compositions of bulk C3N4 and NS C3N4 are estimated to be C3.09H2.03N4.32 and C2.91H2.28N4.49,

118

respectively, as revealed by elementary analysis (Table S1).

119

Material characterization. Solid-state

13

C nuclear magnetic resonance (NMR) spectra

120

were recorded using an AVANCE III HD 500 MHz spectrometer (Bruker BioSpin, Germany).

121

The morphological variation of C3N4 during treatment was investigated by a JEM-2100F

122

field-emission transmission electron microscopy (FETEM, JEOL, Japan). X-ray diffraction

123

(XRD) data was obtained on an X’ PERT-PRO MPD instrument (Philips, Holland) using a

124

Cu Kα irradiation (λ= 0.15406 nm). The surface chemical composition variation of C3N4 was

125

characterized by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250Xi instrument

126

(Thermo Fisher Scientific, USA). The physicochemical properties of bulk C3N4 and NS C3N4

127

were comprehensively investigated in our previous work.18

128

Photocatalytic oxidation and photocatalytic ozonation experiments. The photocatalytic

129

ozonation experiments were carried out in a semi-batch reactor loaded with 40 mg C3N4 in

130

400 mL solution under light irradiation and O3 bubbling simultaneously (Figure S1). The

131

simulated sunlight (0.42 W cm−2 light intensity) was provided by an AM 1.5G solar simulator

132

(Aulight Co., Ltd., China) vertically placed above the quartz cap of the reactor. Gaseous

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ozone (100 mL min−1) was generated from pure oxygen using an ozone generator (Anseros

134

COM-AD-01, Germany) and continuously fed through a porous glass plate into the reactor.

135

The gaseous ozone concentration (45 mg l−1, i.e., 2.1 mol% O3 in O2) was determined by an 5



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ozone analyzer (Ozomat GM6000PRO, Anseros, Germany). Photocatalytic oxidation

137

experiments were carried out under the same condition for a fair comparison. Aqueous

138

samples were withdrawn at intervals and filtered through a 0.22 μm polytetrafluoroethylene

139

membrane (Tianjin Jinteng Instrument Factory, China) to remove solid C3N4.

140

Analytical methods. Aqueous total organic carbon (TOC) concentration was determined

141

with a TOC-VCPH analyzer (Shimadzu, Japan). The concentrations of NO2− and NO3− ions

142

were measured by a Dionex ICS-5000+ high pressure ion chromatography (HPIC). Dionex

143

IonPac AG11-HC (4 × 50 mm) and Dionex IonPac AG7 (4 × 50 mm) columns were used

144

with a 25 L sample loop. The eluent concentration was 32 mM KOH at a flow rate of

145

1.0 mL min−1, and the operation temperature was 30 °C. A Dionex AERS 500 anion

146

electrolytically regenerated suppressor was used in recycle mode. The detection limit of NO3−

147

and NO2− was 0.02 mg l−1. The determination of NH4+ ion was performed using a 761

148

Compact ion chromatography (IC) equipped with a Metrosep C 4-150/4.0 column (Metrohm,

149

Switzerland). A mixture of nitric acid (1.7 mM) and dipicolinic acid (0.7 mM) was used as the

150

mobile phase at 0.8 mL min−1. The detection limit of NH4+ was 0.02 mg l−1. The molar ratio

151

of released nitrogen from C3N4 that exists in the form of NO3− in water was determined as

152

follows.

153

Molar ratio of released N (%)= 62×100×4.49 ×100 (NS C3 N4 )

154

Molar ratio of released N (%)= 62×100×4.32 ×100 (bulk C3 N4 )

155

CNO3- denotes the released NO3− concentration (mg l−1). “62” and “100” indicate the molecular

156

weight (MW, g mol−1) of NO3− and 100 mg l−1 of C3N4 dosage, respectively. “100.06” and

157

“99.59” are the MW (g mol−1) of NS C3N4 and bulk C3N4, respectively, while “4.49” and

158

“4.32” indicate the nitrogen number in their respective chemical formulas (Table S1).

CNO - ×100.06 3

CNO - × 99.59 3

(1) (2)

159

The released aqueous products from C3N4 were investigated by a micrOTOF-Q

160

electrospray ionization tandem mass spectrometry (ESI MS/MS) (Bruker, Germany). The ESI 6


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161

interface was operated in the negative mode, and the parameters were set as follows: capillary

162

voltage at 3500 V; nebulizer gas (N2) pressure at 0.4 bar; dry heater temperature at 180 ◦C;

163

dry gas (N2) flow rate at 4.0 L min−1; collision energy at 5.0 V. Full-scan spectra were

164

obtained by m/z scanning from 50 to 1000. An external instrument calibration was performed

165

using sodium formate cluster by switching the sheath liquid to a solution containing 5 mM

166

sodium hydroxide in the sheath liquid of 0.2% formic acid in water/isopropanol 1: 1(v/v). An

167

exact calibration curve based on numerous cluster masses, each differing by 68 Da (NaCHO2),

168

was obtained.

169

Quantum chemical computation. A first-principles study based on the density functional

170

theory (DFT) was carried out to probe the effect of atomic layer number on the surface energy

171

of C3N4. All the calculations were performed by the planewave ultrasoft pseudopotential

172

method as implemented in the Cambridge Serial Total Energy Package (CASTEP) code. 19

173

The exchange and correlation energy was introduced via the generalized gradient

174

approximation (GGA) PW91 functional. A hybrid semi-empirical solution (OBS) was taken

175

to introduce the structure dispersion correction to C6R−6 in the DFT formalism. The cutoff

176

energy for the planewave basis set was 310 eV, and the k-points of the unit cell and surface

177

structure were 4 × 2 × 4 and 4 × 4 × 1, respectively. Furthermore, the total energy

178

convergence criteria for the self-consistent field (SCF) was 5.0×10−7 eV atom−1, and the

179

surface structure relaxation was carried out until all components of the residual forces were

180

lower than 0.01 eV Å−1. The interaction between valence electrons and atom cores was

181

described by the ultrasoft pseudopotential.

182

The unit-cell structure of hexagonal heptazine-based C3N4 was set as Figure S2, containing

183

24 C atoms and 32 N atoms (C24N32).20 The C3N4 unit cell was further optimized, and the final

184

crystal parameters (a = 7.14 Å; c = 3.22 Å) is in accordance with the literature.1, 21 The surface

185

models with different atomic layers were established in the C3N4 (001) plane. The surface

186

energies of three- and six-layer models (1×1 supercell, Figure S3) were calculated to study the 7



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effect of C3N4 layer number on the relative stability of C3N4 slab. The thickness of vacuum

188

layer was set as 20 Å, and the two bottom layers were fixed. The surface energy Esurf (kJ m−2)

189

was calculated according to the equation as follow:11, 22

190

Esurf =

191

where Eslab (kJ) and Ebulk (kJ) denote the total energies of the slab model (the cell containing

192

N C3N4 units) and bulk crystal with a primitive cell, respectively. 2A (m2) is the total area of

193

two equivalent surfaces in the slab model.

194

 RESULTS AND DISCUSSION

195

ROS generation in Vis/O2/C3N4 and Vis/O3/C3N4. The ROS generation mechanism in

196

Vis/O2/C3N4 and Vis/O3/C3N4 has been explored in detail in our previous study,18 which is

197

also exhibited in Figure 1. O2 traps CB-e− to form •O2− which slowly converts into •OH via a

198

H2O2-mediated

199

Vis/O2/C3N4 (Figure 1a). According to the extent to which the DMPO-OH and DMPO-OOH

200

species contribute to the total EPR signal, the relative percentages of •O2− and •OH were

201

estimated to be 80% and 20%, respectively, verifying •O2− as the dominant ROS in

202

Vis/O2/C3N4.18 In contrast, electron capture by O3 generates •OH much faster through an •O3−-

203

mediated one-electron-reduction pathway (O3→•O3−→HO3•→•OH) (Figure 1b). O3 can

204

rapidly take the electron back from •O2− to form •O3− and, therefore, the slow H2O2-mediated

205 206 207

Eslab − NEbulk

(3)

2A

three-electron-reduction

pathway

(O2→•O2−→HO2•→H2O2→•OH)

in

•

OH generation route is blocked in the presence of O3 while the CB-e−-to-•OH conversion

efficiency is strongly enhanced via the •O3−-mediated pathway. The relative percentages of •

O2− and •OH were estimated to be 10% and 90%, respectively, confirming •OH as the

208

dominating ROS in Vis/O3/C3N4.18 As interpreted from the signal intensity of DMPO-OH and

209

DMPO-OOH, Vis/O3/C3N4 generates 6-18 times more •OH in comparison to Vis/O2/C3N4,

210

while the relative number of •O2− decreases notably due to the conversion of •O2− into •OH in

211

the presence of O3.18 Therefore, •O2− and •OH are the dominating ROS in Vis/O2/C3N4 and 8


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212


Vis/O3/C3N4, respectively. (a)

(b)

H2O2 H2O

HO2 O − (dominant) 2

OH

(minor)

O2

O2

O − 2  O3−

O2

CB-e−

O3

CB-e−

HO3

OH

O2

(dominant)

213 214

Figure 1. ROS formation pathways in (a) Vis/O2/C3N4 and (b) Vis/O3/C3N4.

215

Chemical stability assessment of C3N4 towards •O2− and •OH. In order to probe the

216

chemical stability of C3N4 exposed to •O2− and •OH, respectively, we performed a series of

217

sunlight/O2/C3N4 and sunlight/O3/C3N4 experiments in ultrapure water under simulated solar

218

irradiation. TOC, NH4+, NO2− and NO3− in solution were monitored during these two

219

processes so as to distinguish whether C3N4 decomposes or not. Since C3N4 is the only carbon

220

and nitrogen-containing compound, TOC and nitrogen species, if detected in aqueous

221

solution, only possibly derive from C3N4 decomposition. As shown in Figure S4, no TOC,

222

NH4+, NO2− and NO3− were formed under sunlight/O2/bulk C3N4 conditions and, similarly, no

223

formation of NH4+ and NO2− was observed in Vis/O2/NS C3N4. In contrast, we found a

224

gradual accumulation of TOC and NO3− in water under Vis/O2/NS C3N4 conditions but both

225

concentrations were less than 0.5 mg l−1 within 3 h. Similar results were obtained in

226

Vis/O3/C3N4 (Figure S5) but the TOC and NO3− formation in liquid phase which indicates the

227

decomposition of C3N4 was greatly promoted. The result presented above indicates obviously

228

that C3N4 is relatively chemically unstable in photocatalytic ozonation but almost stable in

229

photocatalytic oxidation, and that NS C3N4 is more vulnerable in comparison to bulk C3N4.

230

The decomposition of C3N4 could release organic products into aqueous environment, and

231

NO3− is most likely the terminal form of nitrogen-containing intermediates existing in liquid

232

phase. Therefore, NO3− were detected within 10 h of Vis/O2/C3N4 and Vis/O3/C3N4 treatments 9



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233

for a long-term stability assessment, and the molar ratios of released nitrogen were also

234

estimated by Eq. 1 and 2 (Figure 2). As shown in Figure 2a, 0.59 mol% and 0.14 mol% of

235

nitrogen in NS C3N4 and bulk C3N4 were finally released into aqueous environment under

236

Vis/O2/C3N4 conditions, respectively, while this ratio reached as high as 9.5 mol% and 6.8

237

mol% in Vis/O3/C3N4 (Figure 2b). Supposing a long-term operation of Vis/O2/C3N4, the

238

released N from NS C3N4 could reach up to 7 mol% within 5 day on the present trends.

239

Therefore, as for either solar photocatalytic oxidation or photocatalytic ozonation, the

240

chemical instability of C3N4 should be taken seriously.

black blue

0.8

1.5

0.6 1.0 0.4 0.5

0.2

0.0

Ratio of released N (mol%)

1.0

-

-1

[Released NO3 ] (mg l )

(a) 2.0

0.0 0

2

4

6

8

10

Time (h)

(b) 30

12 10

20

-

8 15 6 10

4

5

2

0

0 0

241

Ratio of released N (mol%)

black blue

-1

[Released NO3 ] (mg l )

25

14

2

4

6

8

10

Time (h)

242

Figure 2. The concentration of NO3− and ratio of released N in aqueous solution as a function

243

of time during (a) phtotocatalytic oxidation and (b) photocatalytic ozonation with NS C3N4

244

(■) and bulk C3N4 (●). Data reported are an average of three independent experiments.

10


Page 11 of 24


245

Almost no morphological change of C3N4 during photocatalytic oxidation can be observed

246

(not shown here), but the structural variations of C3N4 during photocatalytic ozonation are

247

obvious as seen from the TEM investigations in Figure 3. The initial NS C 3N4 exhibits a

248

large-scale sheet structure with wrinkles and curls (Figure 3a) while it starts to disintegrate

249

from the edge with generation of serried seaweed-like fragments after 3 h of photocatalytic

250

ozonation (Figure 3b). It is easier to find a large number of C3N4 debrises of different sizes

251

and shapes after 10 h (Figure 3c) though the sheet structure is still dominant (not shown here).

252

In comparison with NS C3N4, the morphological alteration of bulk C3N4 is minor (Figure 3d-

253

f). The fragments (Figure 3e and f) formed on the edge are almost in one order of magnitude

254

larger size (a few micrometers) than those of NS C3N4 (hundreds of nanometers, inset of

255

Figure 3c). The TEM results herein are in accordance with the accumulation of TOC and

256

NO3− in water (Figure S5 and Figure 2b), reconfirming strongly the decomposition of C3N4

257

under sunlight/O3/C3N4 conditions. (a)

(b)

(c)

(d)

(e)

(f)

258 259

Figure 3. FETEM images of NS C3N4 after photocatalytic ozonation treatment for (a) 0 h

260

(fresh material), (b) 3 h and (c) 10 h, and of bulk C3N4 with photocatalytic ozonation

261

treatment for (d) 0 h (fresh material), (e) 3 h and (f) 10 h.

262

As revealed by XPS (Figure S6a), a significant loss of nitrogen from 56.1 at.% to 49.1 at.%

263

upon NS C3N4 surface due to photocatalytic ozonation treatment further confirms the breakup 11



Page 12 of 24

264

of nitrogen-containing units from solid C3N4, which is in accordance with the accumulation of

265

NO3− in water (Figure 2). The increase of surface oxygen content is reasonable because ROS

266

generated in photocatalytic ozonation could pose an oxygen doping effect, which is similar to

267

the reported H2O2 hydrothermal procedure.23 The XRD peak at 13.1º (corresponding to in-plain

268

structural packing motif13) almost disappears after treatment (Figure S6b), which is well

269

consistent with the uniplanar fragmentation of NS C3N4 observed by TEM (Figure 3a-c).

270

However, the XRD peak intensity increment at 27.4º (corresponding to interlayer stacking of

271

aromatic segments13) is a trick because the natural drying of wet used NS C3N4 during

272

recycling could undesirably promote the aggregation and re-stacking of exfoliated sheets.24

273

Mechanistic insights into the chemical instability of C3N4 towards •OH. As shown in

274

Figure 4, negligible NO3− was formed in initially ultrapure water during ozonation of NS

275

C3N4, indicating clearly that O3 is unable to decompose C3N4. The remarkable chemical

276

instability of C3N4 in photocatalytic ozonation is most likely due to high yield of •OH, which

277

has been proven by adding tert-butyl alcohol (TBA) to the system, which is a typical •OH

278

scavenger25 and inhibited the release of NO3− significantly (Figure 4). Almost no

279

morphological variation of C3N4 and aqueous NO3− formation (Figure 2a) have been observed

280

during photocatalytic oxidation which involves •O2− as a dominating oxidant (Figure 1a),

281

suggesting that C3N4 possesses high chemical stability towards •O2−. This is easy to

282

understand because O3 cannot decompose C3N4, neither should •O2−, a less reactive oxidant

283

(E0 (O3/•O3−) = 1.03 V and E0 (O2/•O2−) = −0.18 V vs. NHE),26 destroy C3N4. The very slight

284

decomposition of NS C3N4 under Vis/O2/C3N4 conditions as seen from the slow release of

285

NO3− (Figure 2a) is also due to •OH (a minor ROS, Figure 1a) because TBA could completely

286

block the NO3− formation in Vis/O2/C3N4 (inset of Figure 4). Therefore, it is evident that C3N4

287

is chemically stable with O3 and •O2−, but rather unstable in the presence of •OH. It is due to

288

the usually quite low •OH yield in photocatalytic oxidation that shield the truth from being

289

disclosed, while in photocatalytic ozonation where •OH is largely generated, the chemical 12


Page 13 of 24


290

instability of C3N4 becomes easily found. As seen from the less aqueous NO3− formation

291

(Figure 2) and minor morphological change (Figure 3), bulk C3N4 exhibits higher chemical

292

stability than NS C3N4 in photocatalytic ozonation. This is mainly due to two reasons: (i) the

293

•

OH yield over bulk C3N4 is lower because of its smaller surface area and less negative CB

294

edge potential in comparison with NS C3N4;18 (ii) the stacking of more polymeric C3N4 layers

295

in bulk C3N4 assists to preserve its structural stability, which can be seen from the quantum

296

chemical computation result that C3N4 with more stacking layers exhibits lower surface

297

energy (Table S2).

[Released NO3-•] (mg l-•1)

15

Ozonation Photocatalytic oxidation Photocatalytic oxidation/TBA Photocatalytic ozonation Photocatalytic ozonation/TBA

10

0.4 0.2

5

0.0

0

60 120 180

0 0

298

60

120

180

Time (min)

299

Figure 4. Detection of NO3− in initially ultrapure water released from NS C3N4 during

300

treatment by ozonation (O3/NS C3N4), photocatalytic oxidation (sunlight/O2/NS C3N4) and

301

photocatalytic ozonation (sunlight/O3/NS C3N4) with and without TBA.

302

Furthermore, we aim to find out what products have been released from solid C 3N4 that

303

constitute TOC in solution using ESI MS/MS. As shown in Figure 5a-d, three significant

304

peaks at m/z 220.0248, 178.0092 and 152.0282 were detected in initially ultrapure water

305

samples after photocatalytic ozonation with NS C3N4 or bulk C3N4, which possibly represents

306

three main possible intermediates dissociated from solid C3N4. However, when a 5.0 V of

307

collision energy was applied on the precursor ions at m/z 178.0092 and 152.0282, no MS/MS

308

signals were present (Figure 5f and g). In contrast, collision on the ion at m/z 220.0248 with 13



Page 14 of 24

309

the same energy brought two scraps at m/z 178.0009 and 152.0217 (Figure 5e). This indicates

310

only one pristine ion at m/z 220.0248 as the main degradation by-product from C3N4.

311

Theoretical isotope matching using Compass Isotope Pattern software (Bruker Daltonik)

312

defines [M − H]− as C6H2N7O3− (Figure S7). Given this molecular formula is quite close to

313

tri-s-triazine (heptazine, a common building unit of C3N4 network12,

314

structure of NS C3N4 and bulk C3N4 was further investigated by solid-state 13C NMR (Figure

315

S8a). The NMR signals at approximately 155.8 and 164.3 ppm confirms a poly(tri-s-triazine)

316

structure of the final carbon nitride (Figure S8b),27, 28 which is also in accordance with the

317

FTIR result reported previously.18 It is herein concluded that cyameluric acid (C6H3N7O3), an

318

oxidized compound of tri-s-triazine (heptazine), is the main product disassociated from C3N4

319

owing to the •OH cleavage effect. Scheme 1 illustrates the decomposition pathway of C3N4

320

during photocatalytic ozonation. Ozone captures CB-e− to generate abundant •OH, which tears

321

the heptazine unit from solid C3N4 to form cyameluric acid into aqueous environment.

322

Cyameluric acid is continuously generated and degraded simultaneously by •OH, which may

323

be responsible for the fluctuation of TOC concentration in solution as indicated by the large

324

error bar in Figure S5a. The further oxidation of cyameluric acid will give CO2, H2O and

325

NO3− (the only terminal form of nitrogen in solution).

326 327 328

14


13

), the molecular


Intensity (a.u.)

Page 15 of 24

1000 800 600 400 200 0

a

Intensity (a.u.)

3000 2500 2000 1500 1000 500 0 10000 8000 6000 4000 2000 0

Intensity (a.u.)

400

500 m/z

600

700

800

900

1000

-MS, 0.2-0.3min #(13-17)

b

500 m/z c

600

500 m/z d

600

500 m/z

600

700

800

900

1000

-MS, 0.2-0.3min #(13-17)

178.0081 238.0366

200

300

400

220.0248

700

800

900

1000

-MS, 0.2-0.3min #(13-17)

152.0282 178.0092

100

Intensity (a.u.)

300

152.0278 220.0249

100

Intensity (a.u.)

200

1000 800 178.0080 220.0245 600 152.0259 225.9372 400 238.0366 200 0 100 200 300 400

Intensity (a.u.)

Intensity (a.u.)

100

-MS, 0.2-0.3min #(11-13)

2000 1600 1200 800 400 0 50

200

300

400

700

800

900

1000

-MS2(220.0000), 0.2-0.3min #(13-17)

e 220.0257 152.0217 178.0009 100

150

200

250

300

350

400

m/z

600

-MS2(178.0000), 0.2-0.3min #(14-18)

f 400 200 0 50

178.0063

100

150

200

250

300

350

400

m/z

200

-MS2(152.0000), 0.2-0.4mini #(13-22)

g

150 100 152.0282

50 0 50

100

150

200

250

300

350

400

m/z

329 330

Figure 5. ESI MS spectra of the initially ultrapure water samples after photocatalytic

331

ozonation with NS C3N4 for (a) 0 min, (b) 3 h and (c) 10 h, and (d) with bulk C3N4 for 10 h.

332

ESI MS/MS spectra from precursor ion at m/z (e) 220.0248, (f) 178.0092 and (g) 152.0282.

333 15



334

Page 16 of 24

Scheme 1. C3N4 decomposition pathway in photocatalytic ozonation

335 336

Competition between pollutants and C3N4 for •OH. During C3N4 photocatalyttic

337

ozonation of wastewater, organic pollutants are capable to competitively react with •OH in

338

place of C3N4. Therefore, it could be interesting to know the stability of C3N4 in the presence

339

of water pollutants. Here we select a wide variety of typical hazardous water pollutants with

340

different sizes and functionalities including (i) OA, a common refractory intermediate during

341

organics degradation by advanced oxidation processes;29 (ii) benzene and phenol, toxic

342

chemicals present in diverse industrial wastewaters;30 (iii) thiophene, a sulfur heterocyclic

343

component of coals and oils and known hypertoxic;31 (iv) BPA, an endocrine disrupting

344

chemical widely used in plastic and paper industries;32 (v) valproate, an essential antiepileptic

345

drug whose exposure would cause great harm to fishes;33 and (vi) quinizarin, a multi-role

346

chemical as a dye, photoinitiator, fungicide and pesticide.34 Noting that all the model

347

compounds contain no nitrogen, NO3−, if detected in aqueous solution, only possibly derives

348

from C3N4 decomposition. As shown in Figure S9-15, TOC and NO3− in the polluted water

349

during treatment by sunlight/O3/NS C3N4 were monitored so as to simultaneously characterize 16


Page 17 of 24


350

the activity and stability of NS C3N4 under working conditions. A summary of the results is

351

presented in Table 1. The decomposition of NS C3N4 has been completely or partly inhibited

352

in the presence of pollutants since no or less NO3− was detected in the polluted water during

353

treatment (Table 1, Figure S9-15). In the presence of micro-molecular pollutants including

354

OA, benzene, phenol and thiophene, NO3− was formed immediately after the TOC had

355

dropped approximately to zero (Figure S9a and S10-12), indicating that •OH reacts with the

356

pollutant and its degradation intermediates in preference to NS C3N4. This is further verified

357

by the fact that no NO3− was formed in solution with excess OA (25 mM) that cannot be

358

completely mineralized within 3 h (Figure S9b). In contrast, in the case of pollutants with

359

larger molecular size (BPA, valproate and quinizarin), NO3− was gradually formed almost

360

from the beginning of the reactions (Figure S13-15), indicating that these pollutants and NS

361

C3N4 are almost simultaneously decomposed by •OH. BPA, valproate and quinizarin were

362

much harder to be mineralized as seen from the slow TOC removal (Table 1), which closes

363

the gap in degradation difficulty between the pollutant and NS C3N4 and, therefore, the

364

possibility for NS C3N4 to be attacked by •OH has been somewhat raised.

365

Table 1. Competitive mineralization of model pollutant versus NS C3N4 in photocatalytic

366

ozonation Priority

TOC removal rate (%)

Released NO3− in the polluted/ultrapure water (mg l−1)

OA

+

96.6 (0.25 h)

~ 0/2.1 (0.25 h)

Benzene

+

96.0 (1.75 h)

~ 0/7.6 (1.75 h)

Phenol

+

96.3 (1.75 h)

~ 0/7.6 (1.75 h)

Thiophene

+

95.3 (1.25 h)

~ 0/6.0 (1.25 h)

BPA

=

93.8 (2 h)

6.7/8.5 (2 h)

Pollutant

Structure

a

17



Page 18 of 24

Valproate

=

37.7 (3 h)

4.9/13.2 (3 h)

Quinizarin

=

58.0 (3 h)

12.0/13.2 (3 h)

Priority denotes the decomposition precedence of each model pollutant (1 mM except 50 mg l−1 for BPA)

367

a

368

versus NS C3N4 (100 mg l−1 ≈ 1 mM) in photocatalytic ozonation. “+” indicates that C3N4 starts to

369

decompose only after the pollutant has been almost completely mineralized. “=” indicates that C3N4 and the

370

pollutant are almost simultaneously decomposed. Note that the mineralization of organic pollutants is

371

mainly due to •OH rather than O3 alone.3, 18

372

In summary, we have, for the first time, studied the chemical stability of C3N4 under

373

exposure of ROS during photocatalytic water treatment. •OH can directly tear the heptazine

374

unit from C3N4 photocatalyst further to produce secondary pollutants into aqueous

375

environment, while C3N4 is chemically stable towards •O2− and O3. On a positive note, in the

376

presence of organic pollutants the decomposition of C3N4 can be completely or partly

377

inhibited due to their competition for •OH and, thus, the high activity and operation stability

378

of C3N4 have been obviously preserved. This work strongly calls for attention on the chemical

379

instability of C3N4-based materials in •OH-involved applications (e.g., water treatment,

380

organic synthesis and ORR), which may bring detrimental effects, such as deactivation and

381

secondary pollution that are ignored. This is a preliminary result and future studies will

382

attempt to optimize the balance between high efficiency and chemical instability of C 3N4 in

383

solar photocatalytic ozonation for wastewater treatment.

384

 ASSOCIATED CONTENT

385

Supporting Information

386

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

387

(Table S1-S2 and Figure S1-S15). Elementary analysis of C3N4 materials; experimental set-

388

up; quantum chemical computation results; detection of TOC, NH4+, NO2− and NO3− in 18


Page 19 of 24


389

initially ultrapure water during C3N4 photocatalytic oxidation and photocatalytic ozonation

390

within 3h; XPS and XRD characterization of NS C3N4 before and after photocatalytic

391

ozonation; theoretical isotope matching of MS peak at m/z 220.0048; solid-state

392

spectra of C3N4 materials; detection of TOC and NO3− in various polluted waters treated by

393

sunlight/O3/NS C3N4.

394

 AUTHOR INFORMATION

395

Corresponding Author

396

*[email protected]

397

*[email protected]

398

Notes

399

The authors declare no competing financial interests.

400

 ACKNOWLEDGEMENTS

401

The authors greatly appreciate the financial support from Natural Science Foundation of

402

Beijing Municipality (No. 8172043) and the National Science Fund for Distinguished Young

403

Scholars of China (No. 51425405). In addition, we specially thank Prof. Baohua Xu and Ling

404

Wang for the helpful discussion.

405

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516

Table of Contents Art

Chemically stable

•O − 2

•OH

HO

Secondary pollutant

517

24


Page 24 of 24

Is C3N4 Chemically Stable toward Reactive Oxygen Species in

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