Is C3N4 Chemically Stable toward Reactive Oxygen Species in

Oct 24, 2017 - This work supplements the missing knowledge of the chemical instability of C3N4 toward •OH and calls for attention to the potential ...
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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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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

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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

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

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

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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

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

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

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

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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

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

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processes.

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

46

water treatment

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

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 INTRODUCTION

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

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represents today a wide variety of applications mainly including degradation and

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mineralization of organic pollutants,2,

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splitting,4,

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

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

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

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

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

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stability and recycling activity of C3N4 are numerous,12,

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

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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

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

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

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

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

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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

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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

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

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

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formed. 3

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

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

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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

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

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

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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

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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

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

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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

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

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

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Bisphenol

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

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

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

A

(BPA,

96.0%

pure),

sodium

4

valproate

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(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)

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

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and treatment.

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

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

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

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

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

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

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Material characterization. Solid-state

13

C nuclear magnetic resonance (NMR) spectra

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

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

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

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(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

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

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

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were comprehensively investigated in our previous work.18

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Photocatalytic oxidation and photocatalytic ozonation experiments. The photocatalytic

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

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

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

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(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

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

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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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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

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follows.

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

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

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

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

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

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

CNO - ×100.06 3

CNO - × 99.59 3

(1) (2)

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

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

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

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

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

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

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

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

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

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was obtained.

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

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

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

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

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The exchange and correlation energy was introduced via the generalized gradient

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

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

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

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

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

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

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

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described by the ultrasoft pseudopotential.

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

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

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

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

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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

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

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

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Esurf =

191

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

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

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two equivalent surfaces in the slab model.

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 RESULTS AND DISCUSSION

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

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

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

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H2O2-mediated

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

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

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

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

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

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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

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

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

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

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

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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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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)

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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

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

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

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

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

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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

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

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

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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

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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

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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.

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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

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

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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).

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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

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

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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

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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

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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

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

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(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

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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

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), the molecular

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Intensity (a.u.)

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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.

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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

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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

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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

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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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Table of Contents Art

Chemically stable

•O − 2

•OH

HO

Secondary pollutant

517

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