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Article
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?
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Jiadong Xiao,†,‡ Qingzhen Han,† Yongbing Xie,†,* Jin Yang,† Qiaozhi Su,† Yue
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Chen,† and Hongbin Cao†,*
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†
Beijing Engineering Research Center of Process Pollution Control, Division of
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Environment Technology and Engineering, Institute of Process Engineering,
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Chinese Academy of Sciences, Beijing 100190, China
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‡
University of Chinese Academy of Sciences, Beijing 100049, China
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ABSTRACT: Reactive oxygen species (ROS) are key oxidants for the degradation of organic
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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
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(•OH) can tear the heptazine unit directly from C3N4 to form cyameluric acid and further
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release nitrates into aqueous environment. The ratios of released nitrogen from a nanosheet-
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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
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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,
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water treatment
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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.
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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.
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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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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
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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
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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
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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,
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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
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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
174
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
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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 =
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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
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206
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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
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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)
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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
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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
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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
247
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
251
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
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.
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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
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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
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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
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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
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
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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
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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.
333
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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
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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
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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
*ybxie@ipe.ac.cn
397
*hbcao@ipe.ac.cn
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
 REFRENCES
406
(1) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.;
407
Antonietti, M., A metal-free polymeric photocatalyst for hydrogen production from water
408
under visible light. Nature Mater. 2009, 8 (1), 76-80.
409
(2) Zheng, Q.; Durkin, D. P.; Elenewski, J. E.; Sun, Y.; Banek, N. A; Hua, L.; Chen, H.;
410
Wagner, M. J.; Zhang, W.; Shuai, D. Visible-light-responsive graphitic carbon nitride:
411
Rational design and photocatalytic applications for water treatment. Environ. Sci. Technol.
412
2016, 50 (23), 12938-12948.
19
ACS Paragon Plus Environment
13
C NMR
Environmental Science & Technology
Page 20 of 24
413
(3) Xiao, J. D.; Xie, Y. B.; Nawaz, F.; Wang, Y. X.; Du, P. H.; Cao, H. B., Dramatic coupling
414
of visible light with ozone on honeycomb-like porous g-C3N4 towards superior oxidation of
415
water pollutants. Appl. Catal. B: Environ. 2016, 183, 417-425.
416
(4) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.;
417
Kang, Z., Metal-free efficient photocatalyst for stable visible water splitting via a two-electron
418
pathway. Science 2015, 347 (6225), 970-974.
419
(5) Zhang, X.; Peng, T.; Yu, L.; Li, R.; Li, Q.; Li, Z., Visible/near-infrared-light-induced H2
420
production over g-C3N4 co-sensitized by organic dye and zinc phthalocyanine derivative. ACS
421
Catal. 2014, 5 (2), 504-510.
422
(6) Dong, F.; Wang, Z.; Li, Y.; Ho, W.-K.; Lee, S. C., Immobilization of polymeric g-C3N4 on
423
structured ceramic foam for efficent visible light photocatalytic air purification with real
424
indoor illumination. Environ. Sci. Technol. 2014, 48 (17), 10345-10353.
425
(7) Cui, W.; Li, J.; Dong, F.; Sun, Y.; Jiang, G.; Cen, W.; Lee, S. C.; Wu, Z., Highly efficient
426
performance and conversion pathway of photocatalytic NO oxidation on SrO-
427
clusters@amorphous carbon nitride. Environ. Sci. Technol. 2017, 51 (18), 10682-10690.
428
(8) He, Y.; Zhang, L.; Teng, B.; Fan, M., New application of Z-scheme Ag3PO4/g-C3N4
429
composite in converting CO2 to fuel. Environ. Sci. Technol. 2014, 49 (1), 649-656.
430
(9) Chen, X.; Zhang, J.; Fu, X.; Antonietti, M.; Wang, X., Fe-g-C3N4-catalyzed oxidation of
431
benzene to phenol using hydrogen peroxide and visible light. J. Am. Chem. Soc. 2009, 131
432
(33), 11658-11659.
433
(10) Zheng, Y.; Jiao, Y.; Chen, J.; Liu, J.; Liang, J.; Du, A.; Zhang, W.; Zhu, Z.; Smith, S. C.;
434
Jaroniec, M.; Lu, G. Q.; Qiao, S. Z. Nanoporous graphitic-C3N4@carbon metal-free
435
electrocatalysts for highly efficient oxygen reduction. J. Am. Chem. Soc. 2011, 133 (50),
436
20116-20119.
20
ACS Paragon Plus Environment
Page 21 of 24
Environmental Science & Technology
437
(11) Zhang, X.; Xie, X.; Wang, H.; Zhang, J.; Pan, B.; Xie, Y., Enhanced photoresponsive
438
ultrathin graphitic-phase C3N4 nanosheets for bioimaging. J. Am. Chem. Soc. 2012, 135 (1),
439
18-21.
440
(12) Wang, Y.; Wang, X.; Antonietti, M., Polymeric graphitic carbon nitride as a
441
heterogeneous organocatalyst: From photochemistry to multipurpose catalysis to sustainable
442
chemistry. Angew. Chem. Int. Ed. 2012, 51 (1), 68-89.
443
(13) Cao, S.; Low, J.; Yu, J.; Jaroniec, M., Polymeric photocatalysts based on graphitic
444
carbon nitride. Adv. Mater. 2015, 27 (13), 2150-2176.
445
(14) Martínez-Huitle, C. A.; Andrade, L. S., Electrocatalysis in wastewater treatment: Recent
446
mechanism advances. Quim. Nova 2011, 34 (5), 850-858.
447
(15) Radich, J. G.; Krenselewski, A. L.; Zhu, J.; Kamat, P. V., Is graphene a stable platform
448
for photocatalysis? Mineralization of reduced graphene oxide with UV-irradiated TiO2
449
nanoparticles. Chem. Mater. 2014, 26 (15), 4662-4668.
450
(16) Wang, Z.; Sun, L.; Lou, X.; Yang, F.; Feng, M.; Liu, J., Chemical instability of graphene
451
oxide following exposure to highly reactive radicals in advanced oxidation processes. J.
452
Colloid. Interf. Sci. 2017, 507, 51-58.
453
(17) Augugliaro, V.; Litter, M.; Palmisano, L.; Soria, J., The combination of heterogeneous
454
photocatalysis with chemical and physical operations: A tool for improving the photoprocess
455
performance. J. Photoch. Photobio. C 2006, 7 (4), 127-144.
456
(18) Xiao, J.; Rabeah, J.; Yang, J.; Xie, Y.; Cao, H.; Brückner, A., Fast electron transfer and
457
•
OH formation: Key features for high activity in visible-light-driven ozonation with C3N4
458
catalysts. ACS Catal. 2017, 7 (9), 6198-6206.
459
(19) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. I. J.; Refson, K.;
460
Payne, M. C. First Principles Methods Using CASTEP. Z. Kristallogr. 2005, 220 (5/6), 567-
461
570.
21
ACS Paragon Plus Environment
Environmental Science & Technology
Page 22 of 24
462
(20) Ma, X.; Lv, Y.; Xu, J.; Liu, Y.; Zhang, R.; Zhu, Y., A strategy of enhancing the
463
photoactivity of g-C3N4 via doping of nonmetal elements: a first-principles study. J. Phys.
464
Chem. C 2012, 116 (44), 23485-23493.
465
(21) Zheng, Y.; Liu, J.; Liang, J.; Jaroniec, M.; Qiao, S. Z., Graphitic carbon nitride materials:
466
controllable synthesis and applications in fuel cells and photocatalysis. Energy Environ. Sci.
467
2012, 5 (5), 6717-6731.
468
(22) Skorodumova, N.; Baudin, M.; Hermansson, K., Surface properties of CeO2 from first
469
principles. Phys. Rev. B 2004, 69 (7), 075401.
470
(23) Li, J.; Shen, B.; Hong, Z.; Lin, B.; Gao, B.; Chen, Y., A facile approach to synthesize
471
novel oxygen-doped g-C3N4 with superior visible-light photoreactivity. Chem. Commun. 2012,
472
48 (98), 12017-12019.
473
(24) Si, Y.; Samulski, E. T., Exfoliated graphene separated by platinum nanoparticles. Chem.
474
Mater. 2008, 20 (21), 6792-6797.
475
(25) Khodja, A. A.; Sehili, T.; Pilichowski, J.-F.; Boule, P., Photocatalytic degradation of 2-
476
phenylphenol on TiO2 and ZnO in aqueous suspensions. J. Photoch. Photobio. A 2001, 141
477
(2), 231-239.
478
(26) Koppenol, W. H.; Stanbury, D. M.; Bounds, P. L., Electrode potentials of partially
479
reduced oxygen species, from dioxygen to water. Free Radical Bio. Med. 2010, 49 (3), 317-
480
322.
481
(27) Seyfarth, L.; Seyfarth, J.; Lotsch, B. V.; Schnick, W.; Senker, J., Tackling the stacking
482
disorder of melon-structure elucidation in a semicrystalline material. Phys. Chem. Chem. Phys.
483
2010, 12 (9), 2227-2237.
484
(28) Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.; Wang, X., Bioinspired hollow
485
semiconductor nanospheres as photosynthetic nanoparticles. Nature Commun. 2012, 3, 1139.
486
(29) Faria, P.C.C.; Órfão, J.J.M.; Pereira, M.F.R., Activated carbon catalytic ozonation of
487
oxamic and oxalic acids. Appl. Catal. B: Environ. 2008, 79 (3), 237-243.
22
ACS Paragon Plus Environment
Page 23 of 24
Environmental Science & Technology
488
(30) Bigda, R. J., Consider Fentons chemistry for wastewater treatment. Chem. Eng. Prog.
489
1995, 91 (12), 62-66.
490
(31) Kanagawa, T.; Kelly, D. P., Degradation of substituted thiophenes by bacteria isolated
491
from activated sludge. Microb. Ecol. 1987, 13 (1), 47-57.
492
(32) Benotti, M. J.; Trenholm, R. A.; Vanderford, B. J.; Holady, J. C.; Stanford, B. D.; Snyder,
493
S. A., Pharmaceuticals and endocrine disrupting compounds in US drinking water. Environ.
494
Sci. Technol. 2008, 43 (3), 597-603.
495
(33) Funai, D. H.; Didier, F.; Giménez, J.; Esplugas, S.; Marco, P.; Machulek, A., Photo-
496
Fenton treatment of valproate under UVC, UVA and simulated solar radiation. J. Hazard.
497
Mater. 2017, 323, 537-549.
498
(34) Quinti, L.; Allen, N. S.; Edge, M.; Murphy, B. P.; Perotti, A., A study of the luminescent
499
complexes formed by the dye 1, 4-dihydroxyanthraquinone (quinizarin) and Ga (III) and In
500
(III). J. Photoch. Photobio. A 2003, 155 (1), 93-106.
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
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Table of Contents Art
Chemically stable
•O −
2
•OH
HO
Secondary pollutant
517
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