Graphitic Carbon Nitride with Carbon Vacancies for Photocatalytic

Dec 28, 2018 - Xiaofei Liang, Guanlong Wang, Xiaoli Dong, Guowen Wang, Hongchao Ma, and Xiufang Zhang*. School of Light Industry and Chemical ...
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Graphitic Carbon Nitride with Carbon Vacancies for Photocatalytic Degradation of Bisphenol A Xiaofei Liang, Guanlong Wang, Xiaoli Dong, Guowen Wang, Hongchao Ma, and Xiufang Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02089 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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Graphitic Carbon Nitride with Carbon Vacancies for

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Photocatalytic Degradation of Bisphenol A

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Xiaofei Liang, Guanlong Wang, Xiaoli Dong, Guowen Wang, Hongchao Ma, Xiufang Zhang*

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School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China

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* Corresponding author: Xiufang Zhang; E-mail: [email protected]

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ABSTRACT

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Photocatalysis is intensely employed to remove refractory organic pollutants in water, but

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suffers from low efficiency due to rapid recombination of photogenerated electrons and holes. Here,

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carbon vacancies modified g-C3N4 (VC-C3N4) is prepared via a handy two-step calcination method

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and firstly applied in the photocatalytic removal of bisphenol A (BPA). Compared to pristine

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g-C3N4, the photocatalytic degradation activity of VC-C3N4 for BPA is largely enhanced, whose

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kinetic constant (k) of BPA degradation is 1.65 times as that of pristine g-C3N4. The enhanced

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photocatalytic performance of VC-C3N4 is ascribed to critical role of carbon vacancies: On the one

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hand, carbon vacancies serve as the reservoir of photogenerated electrons to inhibit the

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recombination of photogenerated holes and electrons. On the other hand, carbon vacancies as

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conversion centers transfer trapped photogenerated electrons to absorbed O2 for generation of

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abundant superoxide radical (•O2-), which takes a dominant effect in the photocatalytic degradation

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process.

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KEYWORDS: carbon vacancies, g-C3N4, bisphenol A, •O2- species, photocatalytic degradation.

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1 INTRODUCTION

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Recently, water pollution due to the unrestrained misuse of emerging organic materials (such

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as antibiotics, endocrine disrupters, and etc) has become one of important factors threating human

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health. Specially, bisphenol A (BPA) is one of the most widely used industrial organic materials in

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the world and long-time exposure to BPA could trigger abnormal release of human hormones and

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harm of liver and kidney function [1-4]. Due to the high toxicity, great stability and difficult

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degradation of these pollutants in aqueous system [5], people have been devoted to finding an

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efficient, fast, universally applicable method with no secondary pollution to eliminate or reduce

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these toxic and organic substances. Semiconductor photocatalytic technology has long been

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considered as a high-efficiency, low cost and green method to remove stubborn pollutants in the

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water or split water for hydrogen generation [6-13]. Oxidative active species produced by

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semiconductor, for instance, hydroxyl radicals (•OH), superoxide radicals (•O2-) and photogenerated

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holes (h+), can oxidize organic pollutants. Among kinds of semiconductor photocatalysts, graphitic

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carbon nitride (g-C3N4) is deemed as a potential catalyst for pollutant degradation under visible

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light [14-16] due to its low cost synthesis, high yield, appropriate band gap energy (Eg) (about 2.7

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eV ) and great chemical stability [17-22]. However, the performance of g-C3N4 is still restrained

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because of its rapid recombination of photogenerated electrons and holes [23]. Consequently,

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researchers have been trying to effectively inhibit the recombination of photogenerated holes and

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electrons of g-C3N4 by modifying their chemical structure via doping mental/non-mental elements

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(such as Fe, Ka, Ag, Be, N) [24-28], controlling morphology [29, 30], constructing heterojunction

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[31] or introducing vacancies [32]. Some excellent works addressed BPA degradation with

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modified g-C3N4. Oh et al. [33] fabricated Ag/S-doped g-C3N4 photocatalyst and its improved

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photocatalytic performance for BPA oxidation was determined. In his work, they also illustrated the

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oxidation pathways of BPA in the photocatalytic process. Qiu et al. [34] reported that the

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photocatalytic capacity of mesoporous g-C3N4 was improved under visible-light irradiation after the

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modification of Co3O4. What’s more, they also testified that both ·OH and ·O2- radical species take

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crucial effect on BPA photodegradation capacity. Ag2O/g-C3N4 composites were also employed for

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the photocatalytic degradation of phenolic compounds under UV-/visible light. Ren et al. found that

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the highly dispersed Ag2O particles can promoted the photocatalytic performance of Ag2O/g-C3N4

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heterojunction photocatalyst, because it facilitates visible-light absorption of catalyst and efficient

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separation of photoinduced electron-holes [35]. It was investigated by Zhao and co-authors that

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adding persulfate (PS) in g-C3N4 can strengthen the photocatalytic ability of BPA under visible light

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[36]. Instead of SO4•-, •O2- and •OH were the major active radicals in the photocatalytic degradation

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process of BPA.

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Vacancy, which is a usual defect in catalysts, can play a crucial part in modifying electronic

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structure of photocatalytic material and thereby improve the photocatalytic degradation ability of

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the catalysts. Lately, there are a few studies about carbon vacancies defects in g-C3N4. For instance,

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Dong et al. prepared carbon vacancy regulated ultrathin VC-C3N4 nanosheets in photoreduction of

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NO to N2 [37], it was found that carbon vacancy of VC-C3N4 could accelerate the separation of

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photogenerated electron-holes. Li et al. proposed that carbon vacancy modulated g-C3N4 exhibited

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efficient hydrogen peroxide generation under visible light [38]. Carbon vacancies present unpaired

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electrons after the loss of carbon atoms, thereby it is easy for them to attract electrons from external

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substances, such as photogenerated electrons, organic pollutants with electron-rich aromatic

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structure or O2. The capture of photogenerated electrons by carbon defects could validly restrain the

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recombination of photoinduced electron-holes and the enhanced absorption towards organic

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pollutants is favorable for the photocatalytic degradation process. Moreover, carbon vacancies are

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likely to facilitate the reaction between photogenerated electrons and dissolved O2 for the

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generation of •O2- species, which may take significant effect in the photocatalytic degradation

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process. Inspired by it, it is assumed that maybe g-C3N4 with carbon vacancies modification could

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realize the enhanced the photocatalytic degradation of BPA. Nevertheless, there is no investigation

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about constructing carbon vacancies mediated g-C3N4 (VC-C3N4) for photocatalytic removal of

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organic pollutants such as BPA. Moreover, the intrinsic role of carbon vacancies in VC-C3N4 for

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photocatalytic removal of organic contaminants remains unknown.

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In our work, g-C3N4 catalyst with carbon vacancies was prepared via a handy two-step

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calcination method. The photocatalytic performance of VC-C3N4 for BPA removal was enhanced.

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The relevant mechanism of the photocatalytic oxidation of BPA, and especially the contribution of

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carbon vacancies to the enhanced photocatalytic ability were discussed in detail.

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2 EXPERIMENTAL SECTION

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2.1 Reagents

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All of the reagents were purchased from Tianjin Guangfu co., Itd and used without purification.

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The cellulose acetate membranes were purchased from Tianjin Jinteng co., Itd. Ar (≥99.999%) gas

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was purchased from Dalian Special Gases co., Itd.

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2.2 Samples Preparation

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The VC-C3N4 was prepared as follows: 10 g melamine was added into a semi-closed crucible

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(50 mL), and was heated at 550 ℃ for 2 h in muffle furnace. When it reached 25 ℃, the resulting

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yellow product was grinded to powder. Then, the as-prepared g-C3N4 sample was calcined again in

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tube furnace at 520 ℃ for another 2 h under pure Ar gas flow (120mL·min-1). The resulting bright

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yellow powder was denoted as VC-C3N4. Scheme 1 was the structural illustration of carbon

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vacancies modified g-C3N4 formation.

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Pristine g-C3N4 sample was prepared by the same method without Ar gas.

Scheme 1. Structural illustration of carbon vacancies modified g-C3N4 formation.

2.3 Characterizations

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The crystalline properties of g-C3N4 and VC-C3N4 were tested by an X-ray diffractometer

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(XRD, Shimadzu LabX-6100) with Cu-Kα radiation (λ=1.5418 Å). The surface groups were

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determined by an IRAffinity-1 Fourier transform infrared (FT-IR). The micro-morphologies and

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element compositions were obtained with the field emission scanning electron microscope (SEM)

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(JSM‐7800F, JEOL). The UV-2450 UV-vis spectrophotometer (Shimadzu) was used to record the

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diffuse reflectance spectra (DRS) of catalysts. As for the photoluminescence spectra (PL) of

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catalysts, fluorescence spectrometer (LS-55) was used in this work. The electrochemical experiment

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and photoelectrochemical experiment were all performed by using a CHI-660D electrochemical

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workstation (Chenhua, China). X-ray photoelectron spectroscopies (XPS) were measured by the

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ESCALAB250. Electron Spin Resonance (ESR) spectra were recorded on a Brüker A200

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spectrometer (JES FA200) at 77K. Specific surface areas were calculated via the

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Brunauer-Emmett-Teller (BET) model. The concentrations of BPA were analyzed by Agilent 1260

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HPLC (G7115A) with a Discovery C18 (25 ℃). The operating conditions were as follows: C18

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reverse phase column, alcohol: water = 7:3, λmax = 278 nm, flow-rate = 0.8 mL·min-1. Cr (VI)

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concentration was obtained by the conventional diphenylcarbazide colorimetric method [30].

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2.4 Photocatalytic experiments

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All the photocatalytic experiments were conducted in a Pyrex photocatalytic reactor. A 350W

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Xenon lamp was used as the light source (equipped with a λ > 420 nm optical filter). 0.03 g

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photocatalyst was dispersed in 100 mL prepared BPA (10.0 mg·L-1) or Cr (VI) (15 mg·L-1 ) solution.

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The pH of Cr (VI) solution was adjusted to 3.0 by using diluted HCl. Before the irradiation, the

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mixture was mildly stirred in dark for 60 min to reach the adsorption-desorption equilibrium. At

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certain time intervals, 5 mL of samples was withdrawn and filtrated by 0.22 μm cellose acetate

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membrane filters.

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For the detection of active species in the photocatalytic process, disodium ethylenediamine

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tetra acetic acid (EDTA-Na2), benzoquinone (BQ) and tertiary butanol (TBA) solutions were added

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into BPA solution to trap holes (h+), hydroxyl radicals (•OH) and superoxide radicals (•O2-),

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respectively. The concentration of these radical scavengers was all 0.3 mmol·L-1.

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2.5 Photoelectrochemical Experiments

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The Mott-Schottky experiment and transient photocurrent experiment were all recorded in an

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electrochemical workstation with conventional three-electrode cell, in which a Pt wire serves as

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counter electrode, a saturated calomel electrode (SCE) as reference electrode, and a carbon rod as

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working electrode with 0.28 cm2 exposed area. In details, 5 mg of photocatalyst was added in 1.5

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mL of mixture which contained 0.5 mL ultrapure water and 1.0 mL alcohol. Then the obtained

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solution was ultrasonically dispersed for 30 min to get homogeneous dispersion. Finally, 5.0 μL

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above homogeneous solution was dropped on the top of carbon rod, which was then naturally dried.

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During the test process, three electrodes were all immersed in 0.1 M NaSO4 solution.

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3 RESULTS AND DISCUSSION

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3.1 Crystal Analysis

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Fig. 1 XRD patterns of g-C3N4 and VC-C3N4.

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The crystalline structure of Vc-C3N4 and g-C3N4 was measured by XRD. The spiky peak at

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2θ=27.3° corresponds to (002) (d=0.324 nm) diffraction plane, which represents the interlayer stack

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of graphene-like structure (Fig. 1). The other peak at 2θ=13.1° corresponds to (001) (d=0.682 nm)

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diffraction plane caused by the in-plane cellular structure [15]. The crystalline structure of VC-C3N4

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is alike to that of g-C3N4, explaining that calcination process under Ar atmosphere did not change

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its holistic crystal structure.

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3.2 Analysis of Carbon Vacancy Formation

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Fig. 2 FT-IR spectra of VC-C3N4 and g-C3N4.

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The surface groups on samples were determined by FT-IR spectra. The sharp peak of VC-C3N4

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at 808 cm-1, corresponding to the C-N stretching vibration of the characteristic 1-3-s-triazine cycles,

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is slightly weaker than that of g-C3N4 (Fig. 2). This could be ascribed to the appearance of carbon

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vacancies, which destroy the structural integrity of 1-3-s-triazine cycles. A series of small peaks at

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1200-1700 cm-1 are in concerned with the stretching of aromatic C-N. The broad peak at around

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3200 cm-1 in the spectra of g-C3N4 corresponds to the N-H stretching vibration modes. Clearly,

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there is no evident structure change of VC-C3N4 after high-temperature process compared with

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pristine g-C3N4.

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Fig. 3 SEM images and corresponding EDX results of (a-d) g-C3N4 and (e-h) Vc-C3N4.

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The SEM images and EDX analytic results of pristine g-C3N4 and VC-C3N4 are shown in the

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Fig. 3. Three random regions (A-C) are chosen to measure surface element composition of g-C3N4

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and Vc-C3N4. It is clear that three C/N radios of VC-C3N4 are all less than g-C3N4, and the average

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C/N value of VC-C3N4 and g-C3N4 is 0.43 and 0.55, respectively. This result verifies that the carbon

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vacancies of VC-C3N4 form owing to the loss of carbon atoms

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Fig. 4 ESR spectra of VC-C3N4 and g-C3N4.

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In addition to FT-IR and EDX analysis, ESR is also used to further confirm the generation of

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carbon vacancy. In Fig. 4, a characteristic ESR signal at g=2.00, which is caused by the unpaired

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electrons of aromatic rings [23], is in both g-C3N4 and VC-C3N4 samples. As shown in scheme 1, the

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loss of carbon atoms in g-C3N4 gives rise to the N atoms carrying unpaired electrons. Compared

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with the pristine g-C3N4 which only contains the C atoms with unpaired electrons at the edge, the

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carbon vacancy-modified g-C3N4 contains more atoms (unsaturated N atoms) carrying unpaired

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atoms. Distinctly, the VC-C3N4 possesses an exceedingly high ESR signal compared with g-C3N4,

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demonstrating the evident increase of unpaired electrons in VC-C3N4 (Fig. 4). The analysis of ESR

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spectra forcefully validates the existence of carbon vacancies in VC-C3N4, which is in accordance

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with previous FT-IR and EDX analysis.

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To sum up, the carbon vacancies are successfully introduced into g-C3N4. At high temperature,

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the carbon atoms maybe obtain energy from flowing Ar gas which carried enormous energy, which

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then flee from 1-3-s-triazine cycles and thus result in the electron-deficient carbon vacancies [39].

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3.3 Photocatalytic Capability

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Fig. 5(a) The degradation of BPA and (b) kinetic constants (k) with g-C3N4 and VC-C3N4 under visible light.

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The photocatalytic degradation ability of g-C3N4 and VC-C3N4 under visible light (λ > 420 nm)

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was assessed using BPA as a target pollutant (Fig. 5(a)). It was found that BPA degradation

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followed the pseudo-first-order kinetics ln(C0/Ct) = −kt (t is time and k is kinetic constant), and Fig.

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5(b) shows the corresponding kinetic constants (k) of BPA degradation by g-C3N4 and VC-C3N4. In

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the direct photolysis, the concentration of BPA was nearly unchanged under the visible light

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illumination, meaning that BPA cannot be degraded by direct photolysis (Fig. 5(a)). The BPA

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removal rate was only 10.3 % when came to adsorption-desorption equilibrium within 40 min. But

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the photocatalytic removal of BPA over VC-C3N4 reached 90.0% in 420 min illumination. The

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result is attributed to the great photocatalytic degradation ability of VC-C3N4. In comparison, the

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photocatalytic degradation efficiency of g-C3N4 was only 77.7% in the same duration, indicating

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that carbon vacancies facilitated the enhancement of the photocatalytic activity of VC-C3N4.

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Moreover, the k of VC-C3N4 (0.0056 min-1) was 1.65 times as that of g-C3N4 (0.0034 min-1).

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Fig. 6 Cycling runs of BPA degradation efficiency by VC-C3N4.

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The stability of catalysts is another vital property of the photocatalysts in the aspect of

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practical application, thereby, it is essential to evaluate the reusability of the VC-C3N4. The stability

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of the VC-C3N4 was measured by evaluating its photocatalytic performance in repeated use. After

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five cyclic experiments (Fig. 6), the photocatalytic ability of VC-C3N4 was almost unchanged,

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indicating good stability of VC-C3N4 in photocatalytic process.

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3.4 Proposed Mechanism Analysis

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Fig. 7(a) PL and (b) transient photocurrent respond of g-C3N4 andVC-C3N4.

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Based on the above study, it is speculated that the elevated photocatalytic capacity of VC-C3N4

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is owing to the high-efficiency separation of photoinduced electron-holes induced by carbon

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vacancies. To verify this hypothesis, we tested separation and migration efficiency of photoinduced

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charge carriers in semiconductors by PL and transient photocurrent. The PL peak of VC-C3N4 is

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weaker than pristine g-C3N4 (Fig. 7(a)), meaning that the recombination of photogenerated electrons

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and holes of VC-C3N4 is effectively limited. This phenomenon could be ascribed to valid capture of

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photogenerated electrons by carbon vacancies. Unexpectedly, the position of PL peak of VC-C3N4

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presents red shift from 450 nm to 468 nm. This phenomenon implies that the band gap of VC-C3N4

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may be changed. Moreover, the high-efficiency separation of photogenerated electron-holes can be

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further evidenced by transient photocurrent measurement. According to Fig. 7(b), the VC-C3N4

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exhibits a significantly larger photocurrent response than g-C3N4. The photocurrent density of

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VC-C3N4 reaches about 6 μA·cm-2, almost 3 times as that of g-C3N4 (about 2 μA·cm-2), doubtlessly

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confirming the high-efficiency separation of photogenerated electron-holes in VC-C3N4.

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Fig. 8(a) DRS, (b) the plot of (αhv)2 versus photo energy (hv), (c) Mott-Schottky plots and (d) the band position schematic of g-C3N4 and VC-C3N4.

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Optical absorption property is also an important factor in reflecting the photocatalytic ability of

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photocatalysts. Fig. 8(a) exhibits that the adsorption edge of VC-C3N4 exhibits an evident red shift

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(extended from 450 nm to 462 nm) compared to g-C3N4, the movement of adsorption edge is in

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good agreement with above PL results. Fig. 8(b) shows the corresponding Eg of two samples, which

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is calculated from the DRS spectra by the equation (1): (hν·α) =A(hν-Eg)n

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(1)

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where α is the absorption coefficient, hν is light frequency, and A is Planck’s constant.

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Moreover, n is 1/2 for direct band gap material. The Eg of VC-C3N4 is narrowed from 2.76 eV to

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2.65 eV, indicating that the introduction of carbon vacancies alters band gap. Narrow band gap

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gives rise to better visible-light response and then enhances photocatalytic degradation ability.

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Whether the conduction band (CB) position of VC-C3N4 is still high enough to reduce O2 (the

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potential of O2/superoxide radical (•O2-)) after the band gap was narrowed. The CB position of

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VC-C3N4 was determined by the Mott-Schottky measurement. The flat-band potentials (Vfb) of

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g-C3N4 and Vc-C3N4 are -1.10 V and -0.95 V (vs. SCE), respectively, corresponding to -0.86 V and

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-0.71 V (vs. NHE), respectively (Fig. 8c). As is well-known, the Vfb of n-type semiconductor is

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more positive by 0.2 V than its CB position [40]. Hence, the CB position of g-C3N4 and Vc-C3N4

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are -1.06 V and -0.91 V (vs. NHE), respectively, and the CB position of Vc-C3N4 is still higher than

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the position of O2/•O2- (-0.044 eV vs. NHE) [41]. Consequently, VC-C3N4 can theoretically generate

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•O2- in photocatalytic process. The EVB of g-C3N4 and Vc-C3N4 were calculated by the equation of

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Eg=EVB-ECB, therefore the EVB of g-C3N4 is 1.70 eV, and the EVB of Vc -C3N4 is 1.74 eV (vs. NHE)

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(Fig. 8d).

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The BET surface area is another vital factor, which can influence the photocatalytic

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performance of catalyst. By determination, the BET surface areas of g-C3N4 and VC-C3N4 were 30.1

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and 14.7 m2·g-1, respectively, indicating the calcination under Ar atmosphere decreased the specific

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surface area. This change would not contribute to the enhanced photocatalytic performance.

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Fig. 9(a) The degradation of BPA with different scavengers; (b) N2 and O2 blowing experiment; (c) The C/C0 for Cr (VI) photocatalytic degradation with g-C3N4 and VC-C3N4 and (d) the action mechanism of carbon vacancies under visible light irradiation.

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Obviously, BPA can be significantly photodegraded by VC-C3N4, and the oxidative species (h+,

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•O2- and •OH) generated in the photodegradation process are deduced to be possible oxidants.

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Hence, the quenching experiment was conducted to identify which oxidation species contribute in

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photodegradation process of VC-C3N4 (Fig. 9(a)). EDTA-Na2, BQ, and TBA were used to quench h+,

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•O -, 2

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BPA was not obviously affected when EDTA-Na2 was added, meaning that h+ specie generated tiny

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effect in BPA photocatalytic process. After the addition of TBA and BQ, the photocatalytic

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degradation efficiency of BPA obviously decreased from 92.0% to 48.0% and 21.3 %, respectively,

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which indicated that •O2- was the leading active radical and •OH was secondary important active

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radical in the photodegradation process. For further testify the role of •O2-, N2 and O2 blowing

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experiment were carried. Blowing N2 was to eliminate the dissolved O2 which was the source of

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•O2- species in solution, and blowing O2 was to increase the concentration of dissolved O2. From the

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Fig. 9(b), the photocatalytic removal of BPA was tremendously decreased after N2 blowing, but it

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was raised after O2 blowing. The two results consistently certify that •O2- species were the dominant

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oxidation species during photocatalytic process and were generated from the dissolved O2. To

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further explore the role of carbon vacancies, Cr (VI) was selected to reveal the photoreduction

and •OH active species, respectively. Obviously, the photocatalytic degradation efficiency of

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capacity of VC-C3N4 under visible light irradiation. Interestingly, the photoreduction ability of

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VC-C3N4 for Cr (VI) was inferior to g-C3N4 (Fig. 9(c)). This phenomenon signifies that compared

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with g-C3N4, less photogenerated electrons of VC-C3N4 are used in reduction of Cr (VI). Generally,

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photogenerated electrons of semiconductor have two main depletion ways: some photogenerated

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electrons directly participate in the reduction reaction with pollutants; some others react with O2 for

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the generation of •O2-. Since less photogenerated electrons of VC-C3N4 were consumed in reduction

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process, it is more likely that the photogenerated electrons react with O2 produce •O2-, which is

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different from that of g-C3N4. Consequently, carbon vacancies serve as the conversion center

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beneficial to transfer photogenerated electrons to O2 and produce abundant •O2-, leading to the

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improved photocatalytic ability of VC-C3N4.

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Based on the previous characterization and discussion results, a possible mechanism for the

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enhanced photocatalytic ability of VC-C3N4 was presented and demonstrated in Fig. 9(d). After the

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escape of carbon atoms from g-C3N4, the unpaired electron around the carbon vacancies makes

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them exceedingly electrophilic. Hence, the negatively charged molecules such as the BPA with

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electron-rich aromatic structure and the dissolved O2 are prior to adsorb on these carbon vacancies.

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Once illuminated by visible light, VC-C3N4 is immediately excited, and then many photogenerated

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holes and electrons are generated. Electron-deficient carbon vacancies of VC-C3N4 can promptly

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capture these electrons, and thus restrain its recombination with photogenerated holes. Then these

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photogenerated carriers migrate to the surface of VC-C3N4 and participate in the reaction. Therefore,

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the improved photocatalytic degradation ability of VC-C3N4 is partly ascribed to efficient separation

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of photogenerated electron-holes. More importantly, the carbon vacancies, serving as the

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conversion center, transfer the most of captured photogenerated electrons to the absorbed O2 for the

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formation of •O2-, which is the dominant oxidative specie for degradation of BPA.

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CONCLUSION

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To sum up, the carbon vacancies modified g-C3N4 with enhanced photocatalytic capacity was

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fabricated via a handy calcination method. The kinetic constant of BPA degradation with VC-C3N4

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is nearly 1.65 times as that of g-C3N4. After the carbon vacancies introduction, the recombination

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rate of photogenerated electron-holes is largely decreased. Moreover, carbon vacancies, serving as

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conversion centers, transfer most trapped photoinduced electrons to absorbed O2 for the generation

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of •O2-. This work provides an attractive avenue for designing and constructing novel photocatalyst

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towards efficient removal of refractory organic pollutants.

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AUTHOR INFORMATION

316

Corresponding author

317

E-mail: [email protected]

318

Notes

319 320 321

There is no competing financial interest.

Acknowledgments We thank the financial supported from the Natural Science Foundation of China (21577008,

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21878031), and Fundamental Research Funds for Central Universities (2016J004).

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