Enhanced Visible-Light-Driven Photocatalytic Disinfection

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Article

Enhanced Visible-Light-Driven Photocatalytic Disinfection Performance and Organic Pollutants Degradation Activity of Porous g-CN Nanosheets 3

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Jing Xu, Zhouping Wang, and Yongfa Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07657 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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ACS Applied Materials & Interfaces

Enhanced Visible-Light-Driven Photocatalytic Disinfection Performance and Organic Pollutants Degradation Activity of Porous g-C3N4 Nanosheets Jing Xu†,‡, Zhouping Wang*,†,‡, and Yongfa Zhu§ †

State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi 214122, P. R.

China ‡

School of Food Science and Technology, Jiangnan University, Wuxi 214122, P. R. China

§

Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China

ABSTRACT: Porous g-C3N4 nanosheets (PCNS) photocatalyst with a thickness of 2.0 nm, pore volume of 0.61 cm3 g-1 and surface area of 190.1 m2 g-1 was prepared by a simple two-step template-free approach without the addition of extra reagents. Compared with the bulk g-C3N4 (BCN), PCNS possesses more quantities of surface reactive sites, improved efficiency of charge transfer and accelerated separation of photogenerated electron-hole pairs. Accordingly, the visible-light-driven photocatalytic disinfection performance and organic pollutants degradation activity of PCNS are significantly enhanced. Escherichia coli (E. coli) cells can be killed completely by PCNS within 4 h, whereas only 77.1% of E. coli cells can be killed by BCN. The photodegradation rates of PCNS on methylene blue, Acid Red 27 and bisphenol A are almost 6.4, 4.0 and 1.9 times as fast as that of BCN, respectively. The photocurrent intensity of PCNS is about 3.7 times in comparison with that of BCN. Considering the easy preparation and excellent performance, PCNS could be a promising and competitive visible-light-driven photocatalyst in the field of environment remediation. KEYWORDS: photocatalysis, g-C3N4, mesoporosity, nanosheets, disinfection, 1

ACS Paragon Plus Environment


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pollutants degradation, visible light

INTRODUCTION Since the beginning of 21st century, water pollution has become a big problem

which exerts serious influence on ecological environment and human survival.1,2 Organic dyes,3,4 benzene based organics5,6 and pathogenic bacteria7,8 are the main pollutants of concern in wastewater due to their high toxicity and hardly degradable characteristic. Therefore, seeking efficient, green and safe purification technology and material which can effectively remove organic pollutants and pathogenic bacteria from wastewater is of great significance for environmental protection. Compared with the conventional methods, photocatalysis technology shows greater promise in environment remediation due to its low cost, no harmful by-products, and strong oxidative capability for contaminants under facile conditions.9-12 Graphitic carbon nitride (g-C3N4), a metal-free polymeric semiconductor, was first developed to be a visible-light-driven photocatalyst in 2009 by Wang and coworkers.13 Since then, g-C3N4 has shown great potential in plenty of application fields such as water splitting,13,14 CO2 reduction,15,16 contaminants degradation17,18, disinfection19,20 and sensitive detection21-24 due to its low price, high stability, and excellent capacity for solar utilization. However, the bulk g-C3N4 (BCN) usually exhibits a limited photocatalytic activity because of its small surface area and high possibility of charge carriers recombination.25 Researchers have made a considerable amount of effort to improve the photocatalytic activity of g-C3N4 in terms of doping noble metal,26-28 compositing with semiconductors or other carbon materials,29-33 2


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introducing heteroatoms,34,35 and controlling morphology36,37. Among these approaches, morphology controlling creates a promising way to optimize g-C3N4 since it can dramatically enlarge the surface area, increase the number of surface reactive sites, and improve the transfer and separation efficiency of photogenerated charge carriers. In the past few years, highly anisotropic g-C3N4 nanosheets (CNS) with a thickness of around 0.4-3 nm can be successfully prepared via the thermal etching,38 ultrasound exfoliation,39 liquid exfoliation40 and chemical exfoliation41 methods. Compared with BCN, CNS shows new physicochemical properties and more potential in the field of environmental purification. Meanwhile, porous g-C3N4 (PCN) fabricated usually by the soft templating method42,43 or hard templating method44,45 can result in well-developed porosity and superior photocatalytic activity. However, these templating methods involve costly functionalized templates and complex removal procedures, which are inconvenient and unfavorable for the practical application purposes.46 Considering the respective superiorities of CNS and PCN, it should be a feasible way to further develop the photocatalytic capability of g-C3N4 by combining both nanostructures together. Therefore, it is a challenging work to achieve the nanosheet structure and mesoporosity morphology of g-C3N4 simultaneously through a facile, simple and economical strategy. Herein, porous g-C3N4 nanosheets (PCNS) with the thickness of 2.0 nm, pore volume of 0.61 cm3 g-1 and surface area of 190.1 m2 g-1 was synthesized by a two-step template-free approach including the hydrothermal treatment and thermal etching 3



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process without the addition of extra reagents. The morphologies, structures and properties of PCNS were systematically characterized. The visible-light-driven photocatalytic disinfection performance and organic pollutants degradation activity of PCNS were carefully evaluated and found to be significantly enhanced in contrast with BCN. Moreover, the effect of the mesoporous nanosheet structure on enhanced photocatalytic activity in PCNS system was also elucidated. Considering the easy preparation and excellent performance, PCNS could be a promising and competitive visible-light-driven photocatalyst in the field of environment remediation.

EXPERIMENTAL SECTION

Synthesis of Porous g-C3N4 Nanosheet Photocatalyst. BCN was prepared by heating melamine (Sinopharm Chemical Reagent Co., Ltd., China) in a muffle furnace at 550 °C for 4 h with a heating rate of 2.5 °C min-1 in air atmosphere. PCNS was synthesized via a two-step template-free method including the hydrothermal treatment and thermal etching process. The synthetic strategy of PCNS is illustrated in Scheme 1. Step 1: 1.5 of BCN was ultrasonically dispersed in 135 mL of deionized water for 30 min. Then the dispersion was poured into a 150 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 12 h in an oven. The obtained suspension was centrifugalized, washed thoroughly with deionized water, and dried at 60 °C in air overnight. The pale yellow powder product was PCN. Step 2: the as-prepared PCN (0.7 g) was placed in an uncovered crucible and heated in a muffle furnace at 500 °C for 2 h with a heating rate of 5 °C min-1 in air. The final white powder product was PCNS. 4


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Scheme 1. Schematic Diagram of the Synthetic Strategy of PCNS

Characterization. Transmission electron microscopy (TEM) images of the samples were acquired using a JEOL JEM-2100 microscope. The Brunauer-Emmett-Teller (BET) surface area and pore volume were estimated on a TriStar II 3020 instrument (Micromeritics). X-ray diffraction (XRD) patterns were recorded by a Bruker D2-phaser X-ray diffractometer with a monochromatized Cu Kα radiation. Atomic Force Microscope (AFM) images were obtained on a Bruker Dimension ICON in phase mode. Fourier transformed infrared (FTIR) spectra were gained by a Thermo Fisher Nicolet iS10 spectrometer. Diffuse reflection spectra (DRS) of the powder samples were recorded using a UV-3600 plus spectrophotometer (Shimadzu). X-ray photoelectron spectroscopy (XPS) spectra were conducted on a Thermo Fisher ESCALAB 250Xi system and calibrated by C1s binding energy at 284.8 eV. Elemental analysis data were estimated by Elementar vario Micro cube. Photoluminescence (PL) spectra were investigated using a Hitachi F-7000 fluorescence spectrometer excited by incident light of 365 nm. Time-resolved fluorescence decay spectra were acquired on Edinburgh FLS920 spectrophotometer using a 375 nm nanosecond pulse laser as the excitation source. Electron spin resonance (ESR) analysis was conducted on a Bruker EMXplus spectrometer using 5,5-dimethyl-1-pirroline-N-oxide (DMPO) as the 5



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radical scavenger. Antimicrobial Experiments. Gram-negative E. coli was selected as the model bacteria. The bacteria were incubated in 100 mL of Luria Bertani (LB) nutrient solution and shaken at 37 °C for 4 h. After centrifugation, the bacteria cell pellet was washed twice with sterilized saline (0.9% NaCl) solution and then resuspended in sterilized saline solution. The photocatalytic disfection of E. coli cells was irradiated by a 500 W xenon lamp with a 420 nm cutoff filter in a photochemical reactor (XPA-7). The bacteria cells density and photocatalyst concentration were about 5×106 cfu mL-1 and 0.4 mg mL-1, respectively. After the visible light irradiation, aliquots of suspension was sampled at the certain time intervals and serially diluted with sterilized saline solution. 100 µL of the diluted saline solution was spread on the LB solid culture medium and then incubated at 37 °C for 12 h. Colony counting method has been used to estimate the viable E. coli cells density (in cfu). By comparison, the light control experiment was carried out without the photocatalyst under visible light irradiation, and the dark control experiment was conducted with as-prepared PCNS photocatalyst in the dark. Every antimicrobial experiment was carried out in triplicate. In order to ensure the sterility, all the glass wares were heated at 121 °C for 20 min in an autoclave. Photocatalytic Degradation Experiments. Visible-light-driven photocatalytic activities of the samples were also evaluated via the degradation of cationic dye (methylene blue, MB), anionic azo pigment (Acid Red 27, AR27) and colorless benzene based endocrine-disrupting chemical (bisphenol A, 6


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BPA) in solution in a photochemical reactor (XPA-7). Visible light source was also equiped with a 500 W xenon lamp and a 420 nm cutoff filter. The concentrations of MB, Acid Red 27 and BPA were 1.2× 10-5 M, 1.3× 10-5 M and 10 ppm, respectively. 10 mg of photocatalyst was ultrasonically dispersed in 50 ml of probe solution for 15 min. Then the dispersion was stirred for 1 h without irradiation to reach the adsorption-desorption equilibrium. After the irradiation, a suspension (2 mL) was extracted at certain time intervals and centrifugalized to remove the photocatalyst. The concentrations and intermediates of target pollutants were analyzed by Shimadzu UV-1800 UV-vis spectrophotometer and Waters HPLC system with a C18 reversed phase column. Photoelectrochemical Measurements. The photoelectrochemical properties of the samples were studied using a Chenhua CHI 660B electrochemical workstation with a three-electrode cell system which consists of a saturated calomel electrode (SCE) reference electrode, a Pt wire counter electrode and a self-made working electrode. The working electrode was produced by dip-coating photocatalyst slurry on an indium-tin oxide (ITO) glass and heating at 200 °C for 5 h. Na2SO4 solution (0.1 M) was chosen as the electrolyte. Visible light source was equiped with a 300 W xenon lamp (CEL-HXF300, Ceaulight) and a 400 nm cutoff filter. The photoelectric response measurement was carried out at 0.0 V bias. Electrochemical impedance spectroscopy (EIS) spectra were recorded in the range from 0.05 Hz to105 Hz at an AC voltage of 5 mV.

RESULTS AND DISCUSSION 7



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Photocatalytic Disinfection Performance.

E. coli, one kind of common bacteria, was chosen as the probe microbe to evaluate the photocatalytic disinfection performance. The corresponding inactivation efficiencies of the bacteria are summarized in Figure 1a. We first conducted the light control experiment without the photocatalyst. The results indicate that visible light irradiation almost lead to no photolysis of E. coli cells. The dark control experiment using PCNS alone reveals that the adsorption between PCNS and E. coli cells achieves equilibrium within 1 h, and about 85.5% of bacteria remain after 4 h, which demonstrates that PCNS has no toxic effect on E. coli cells without the irradiation. However, under the visible light irradiation, almost 100.0% of E. coli can be killed by PCNS within 4 h (Figure 1b-1d), whereas only 77.1% and 89.6% of E. coli can be killed over BCN and PCN under the same condition, respectively. Additionally, PCNS also exhibits greatly enhanced bactericidal rate in contrast with BCN and PCN. To further confirm the destruction of bacteria, the morphologies of E. coli cells before and after the photocatalytic disinfection were investigated by TEM measurement. Before the photocatalytic disinfection, E. coli cells exhibit a rod-shaped morphology with well-preserved membranes, flagellums and capsules (Figure 1e). After 4 h irradiation, E. coli cells are found to be tightly bound with PCNS which can accelerate the disinfection process. Some E. coli cells are absorbed on the surface of PCNS, the flagellums and capsules of these cells have been eliminated, and the outer membranes are partly damaged which can cause the leakage of intracellular components (Figure 1f).20 Meanwhile, some E. coli cells are embedded in PCNS, and 8


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the shapes of these cells have become deformed which can lead to the inactivation of the bacteria (Figure 1g).8

Figure 1. (a) Visible-light-driven photocatalytic disinfection performance against E. coli over BCN, PCN and PCNS. Images of E. coli colonies on solid culture medium (b) before irradiation, (c) after disinfection for 2 h, (d) for 4 h using PCNS. TEM images of E. coli cells (e) before irradiation, (f, g) after disinfection for 4 h using PCNS.

Photocatalytic Degradation Activity. The superiority of PCNS for photocatalysis application was further confirmed by the contaminants degradation experiments. MB, a common hazardous cationic dye, was chosen as the main probe pollutant to evaluate the photodegradation activity. The apparent rate constant k of the photocatalytic reaction can be calculated by fitting the pseudo-first-order equation.46 Under visible light irradiation, the apparent k of PCNS is 0.551 h-1, which is almost 6.4 and 1.6 times in comparison with that of BCN (0.0862 h-1) and PCN (0.338 h-1), respectively (Figure 2a). The HPLC results of MB solutions at different visible-light irradiation time of photodegradation by PCNS are shown in Figure 2b. The prominent peak of MB at 3.8 min is observed to diminish gradually with the increased irradiation time. The peaks 9



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at lower retention times are attributed to the intermediates formed during the MB photodegradation process, which can be interpreted as the elimination of one or more –CH3 groups from MB molecule.46 Additionally, the intensities of these intermediate peaks increase firstly and then decrease along with the irradiation time, indicating the degradation ability of PCNS for all the intermediates. Finally, it can be found that MB molecules have been completely degraded to CO2 and H2O after 5 h irradiation. As shown in Figure S1, the adsorptivities of these three photocatalysts were tested by the adsorption of MB. After equilibrium achieved within 30 min, 15.2%, 18.1%, and 44.7% of MB have been adsorbed from the solution by BCN, PCN and PCNS respectively. The enhanced adsorptivity of PCNS should be ascribed to the enlarged BET surface area and the formation of mesoporous nanosheet structure. As the adsorptivity of PCNS is much higher than that of BCN, it should be a crucial factor for the photocatalytic reaction. The photocatalytic performance of PCNS was also investigated via the degradation of anionic azo pigment (AR27) and endocrine-disrupting chemical (BPA) under visible light irradiation (Figure 2c). It can be found that the apparent k of PCNS on AR27 and BPA are almost 4.0 and 1.9 times as fast as that of BCN, respectively. Therefore, PCNS shows potential in a wide application of contaminants degradation.

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Figure 2. (a) Comparison of visible-light-driven photodegradation rate of MB over BCN, PCN and PCNS. (b) HPLC results of MB solutions at different visible-light irradiation time of photodegradation by PCNS. (c) Apparent rate constants for the visible-light-driven photodegradation of AR27 and BPA over BCN and PCNS.

Characterization of Porous g-C3N4 Nanosheets Photocatalyst. The morphologies of as-prepared photocatalysts were studied by TEM and AFM measurements. The typical TEM images of BCN display large particles shape with bulk morphology (Figure S3a and S3b), and PCN possesses a loose structure with 10-20 nm of nanopores (Figure S3c and S3d). PCNS exhibits an anisotropic 2D sheet-like mesoporous morphology (Figure 3a) whose edge region illustrates the transparent feature of nanosheets (Figure 3b). As shown in Figure 3c, the typical AFM image of PCNS shows the lateral size of these porous nanosheets is about tens of nanometers. The cross-section analysis of PCNS reveals the average thickness is around 2.0 nm (Figure 3d), which corresponds to about 6-7 C–N layers.41 Similar AFM results had also been reported in the literature.38 By contrast, the average thickness of PCN is more than 4 nm which is roughly twice of that of PCNS, indicating the effectiveness of the thermal etching process (Figure S4).

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Figure 3. TEM images of (a) PCNS, (b) the edge region of PCNS. (c) Typical AFM image, (d) cross-section analysis of PCNS.

The results of BET specific surface areas and pore volumes of as-prepared photocatalysts are displayed in Figure 4a. As expected, PCNS exhibits large surface area of 190.1 m2 g-1, which is about 19.8 and 6.7 times as high as that of BCN (9.6 m2 g-1) and PCN (28.3 m2 g-1), respectively. The pore volume of PCNS is also enlarged to 0.61 cm3 g-1 which is 12.0 and 5.1 times higher than that of BCN (0.047 cm3 g-1) and PCN (0.10 cm3 g-1), respectively. Compared with BCN and PCN, the pore size distribution curve of PCNS is more apparent with a broad peak ranged from 5 to 25 12


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nm (Figure 4b), which indicates that there are larger amount of mesopores existed in PCNS. Therefore, the hydrothermal treatment can make some unstable portion of BCN hydrolyze into ammonia and carbonate ion,47 leading to the formation of mesoporosity imparted into g-C3N4 frameworks. Additionally, the thermal etching treatment also can generate gases, which exfoliates PCN into small nanosheets with more apparent mesoporous structure.38 These results are consistent with the observed morphological changes. The structure properties of the photocatalysts were investigated by XRD measurement. As shown in Figure 4c, the strong 002 diffraction peak at 27.4° is assigned to the interlayer stacking of conjugated aromatic C-N heterocycles, and the 100 diffraction peak at 13.0° indicates the interplanar repeat of tri-s-triazine units.46 For PCN, the 002 and 100 diffraction peaks are both found to be stronger and sharper than that of BCN, and a new peak appears at 10.6°. This phenomenon is related to the hydrolysis and oxidation reactions occurred between g-C3N4 and H2O molecules under the hydrothermal condition, resulting in the partial dissociation of tri-s-triazine units and the formation of oxygen-containing groups.47 For PCNS, the 002 and 100 diffraction peaks are found to be broadened and weakened after the exfoliation treatment, which could be attributed to the successful delamination of PCN during the thermal etching process.41 FTIR spectra were obtained to investigate the chemical structure differences among BCN, PCN and PCNS (Figure S5). Besides the typical stretching vibrations of aromatic C-N heterocycles between 1200 and 1650 cm-1, the bending vibrations of triazine rings at 806 cm-1, and the O–H and N-H bonds between 13



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2900 and 3500 cm-1, a small signal emerge at 1084 cm-1 for PCN and PCNS, indicating the existence of carboxyl groups and the partially oxidation after the hydrothermal process.47 The optical properties were investigated by DRS measurement. In the DRS spectra (Figure 4d), it can be found that all the three samples possess visible light absorbance. BCN and PCN show similar absorption edge at around 454 nm. However, because of the existence of electron withdrawing groups, the absorbance value of PCN exhibits a slight blue shift.47 In contrast with BCN, the absorption edge of PCNS is blue shifted to 439 nm as a result of the quantum confinement effect.48 Correspondingly, the band gap of PCNS increases to 2.82 eV, leading to a strengthened photoredox ability.48

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Figure 4. (a) BET surface area and pore volume, (b) pore size distribution, (c) XRD patterns, (d) DRS spectra of BCN, PCN and PCNS.

XPS measurement is a useful tool to investigate the chemical states and compositions of BCN, PCN and PCNS. In the XPS survey spectra (Figure 5a), C and N are the main elements contained in all the three samples. As shown in Figure 5b, there are two peaks at 284.8 and 288.2 eV in the C1s spectra, which could be assigned to the sp2 C–C bonds in adventitious carbon species and the sp2-hybridized C (N–C=N) in the triazine rings, respectively.46 For PCN, a new peak appears at 289.5 eV, which attributes to the general C–O bonds.47 The N1s spectra signal (Figure 5c) can be deconvoluted into three peaks at 398.8, 400.1 and 401.2 eV, respectively 15



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corresponding to the sp2-bonded N (C–N=C) in the triazine rings, the tertiary N involved in N–(C)3 and the N atoms bonded with H.41 For PCN, the intensity of the peak located at 398.8 eV decreases, while that at 400.1 eV increases in contrast with BCN. This phenomenon should be ascribed to the existence of N–O species.47 The O1s spectra (Figure 5d) show a broad peak at 532.0 eV, which relates to the O atoms in N–C–O. The O1s peak of PCN has greatly strengthened, indicating the formation of the oxygen-containing groups during the hydrothermal treatment.47 Compared with BCN, there is no apparent change in the C1s, N1s and O1s spectra of PCNS, suggesting that PCNS still remains the primary chemical state and composition of BCN. In other words, a large portion of the oxygen-containing groups existed in PCN could be removed after the thermal etching process.

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Figure 5. (a) XPS survey spectra, (b) C1s XPS spectra, (c) N1s XPS spectra, (d) O1s XPS spectra of BCN, PCN and PCNS.

Fluorescence spectroscopy is a powerful tool to study the photophysical properties of charge carriers in semiconductors. The PL spectra of the BCN, PCN and PCNS excited at 365 nm are presented in Figure 6a. The emission peaks of BCN and PCN locate at around 465 nm and 462 nm. By contrast, the emission peak of PCNS is blue shifted to 456 nm, which coincides with the result of DRS measurement. Compared with BCN and PCN, the PL emission peak intensity of PCNS has increased which could be attributed to its mesoporous nanosheet structure and the reduced number of structural defects.14,38 Time-resolved fluorescence decay spectra of BCN, PCN, and 17



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PCNS are shown in Figure 6b. The fluorescent intensities of all the three photocatalysts decay exponentially. Compared with BCN and PCN, PCNS exhibits slower decay kinetics. Two radiative lifetimes with different relative percentages can be calculated by fitting the biexponential equation. As shown in Table 1, the short lifetime of PCNS is 2.02 ns, which is longer than that of BCN (1.48 ns) and PCN (1.79 ns). Moreover, the long lifetime of charge carriers increases from 5.67 ns for BCN and 6.22 ns for PCN to 7.48 ns for PCNS, and the corresponding relative percentage of long lifetime rise simultaneously. For PCNS, the radiative lifetimes of both charge carriers have been effectively prolonged, resulting in the enhanced probability of their participation in photocatalytic process.38 Moreover, the prolonged lifetimes should be related to the improved charge transfer as the result of the mesoporous nanosheet structure of PCNS, which is advantageous for accelerating the separation of photogenerated electron-hole pairs.40

Figure 6. (a) PL spectra of BCN, PCN and PCNS under photoexcitation at 365 nm. (b) The nanosecond-level time-resolved fluorescence decay spectra of BCN, PCN and PCNS monitored under a 375 nm laser excitation. 18


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Table 1. The fluorescence lifetimes and the corresponding relative percentages of charge carriers for BCN, PCN and PCNS samples Sample

τ1 (ns)-Rel %

τ2 (ns)-Rel %

BCN

1.48-50.96

5.67-49.04

PCN

1.79-48.47

6.22-51.53

PCNS

2.02-41.17

7.48-58.83

Mechanism of Photocatalytic Activity Enhancement. The separation of photogenerated charge carrier is the rate-determining step in photocatalytic reaction. The photoelectric response and EIS measurements were conducted to investigate the separation and transfer efficiency of charge carrier in PCNS. It can be found in Figure 7a that all the three electrodes have generated quick and constant photocurrent responses as the light is turned on or off. Under visible light irradiation, the photocurrent intensity of PCNS is about 3.7 and 1.5 times in comparison with that of BCN and PCN, respectively, which indicates that the separation efficiency of photogenerated electron-hole pairs is significantly improved as a result of the mesoporous nanosheet structure.41 As shown in Figure 7b, the arc radiuses of PCNS electrode are smaller than that of BCN and PCN electrodes under dark and visible light conditions. As a smaller arc radius of EIS Nyquist plot could be indicative of a faster surface reaction rate of the working electrode, PCNS possesses more effective charge transfer and lower possibility of charge recombination.46 To confirm the mechanism further, trapping experiments were carried on via the photodegradation of MB by PCNS under visible light irradiation. It can be found in Figure 7c that the photodegradation activity of PCNS reduces slightly after adding the hydroxyl radical scavenger (t-BuOH) but decreases significantly in the presence of 19



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superoxide radical scavenger (N2) and hole scavenger (formic acid). The above results indicate that the photogenerated holes and superoxide radicals are the dominant reactive oxygen species of PCNS.49 The ESR analysis was also performed to detect radicals in the photocatalytic process. DMPO as a radical scavenger can give distinct reflection signals of hydroxyl radical (•OH) or superoxide radical (•O2−) owing to the formation of DMPO−•OH or DMPO−•O2− adducts. Since •O2− is very unstable in water, the generation of •O2− is measured in dimethyl sulfoxide (DMSO).50 It can be found in Figure S6 that there is no signal of DMPO−•OH adducts in H2O with/without visible light irradiation, implying that no •OH has been generated in PCNS suspension. Oppositely, noticeable signals of DMPO−•O2− adducts in DMSO can be observed after irradiation for PCNS (Figure 7d), suggesting that photogenerated electrons (e−) can efficiently reduce the adsorbed O2 into •O2−. In addition, the •O2− signals of PCNS are stronger than that of BCN and PCN, indicating that the amount of superoxide radicals generated in PCNS system is much more compared with BCN and PCN.

20


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Figure 7. (a) Photocurrent responses of BCN, PCN and PCNS under visible light irradiation. (b) EIS Nyquist plots of BCN, PCN and PCNS under dark and visible light conditions. (c) Visible-light-driven photodegradation activities of PCNS against MB with the addition of different scavengers. (d) ESR spectra of BCN, PCN and PCNS in DMSO solvent when visible light is turned on and off (DMPO as radical trapper).

On the basis of the above results, a plausible mechanism for the charge separation and photocatalytic process in PCNS system is proposed and shown in Scheme 2. Compared with BCN, the greatly improved visible-light-driven photocatalytic performance of PCNS in both disinfection and contaminants degradation could be ascribed to several reasons: (1) the enlarged surface area of PCNS can produce more surface reactive sites for the photocatalytic reaction; (2) the improved adsorptivity of 21



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PCNS for the probe pollutants can accelerate the rate of photocatalytic reaction; (3) the expanded band gap of PCNS can result in a strengthened photoredox ability; (4) the increased amounts of superoxide radicals and holes dominating the photocatalytic reaction can be generated from the PCNS system under visible light irradiation; (5) the decreased thickness and well-developed porosity of PCNS can make the transportation and separation of photogenerated charge carriers more effective. 46 Scheme 2. Schematic Diagram for the Mechanism of Charge Separation and Photocatalytic Process over PCNS Photocatalyst.

CONCLUSIONS In summary, porous g-C 3 N 4 nanosheets (PCNS) photocatalyst was prepared

through a two-step template-free approach including the hydrothermal treatment and thermal etching process without the addition of extra reagents. Compared with the bulk g-C 3 N 4 , PCNS possesses larger BET surface area, more quantities of surface reactive sites and improved efficiency of charge 22


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transfer and

seperation.

Consequently,

the

photocatalytic

disinfection

performance, the organic pollutants degradation activity and photocurrent generation capability of PCNS under visible light irradiation are significantly enhanced. This research may provide a simple and efficient strategy for achieving the nanosheet and mesoporous structure of g-C 3 N 4 simultaneously, and also extend the application prospect of g-C 3 N 4 photocatalyst in the field of solar energy conversion environmental purification.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. The adsorptivities of BCN, PCN and PCNS for MB in the dark, stability test on PCNS for the photodegradation of MB under visible light irradiation, the adsorptivity changes of PCNS during the repeated experiments, TEM images of BCN and PCN, AFM analysis of PCN, FTIR spectra of BCN, PCN and PCNS, ESR spectra of BCN, PCN and PCNS in H2O solvent when visible light is turned on and off, elemental analysis results of BCN, PCN and PCNS.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected].

ORCID Zhouping Wang: 0000-0002-3868-8125 23



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Notes The authors declare no competing financial interest.

ACKNOWLEDMENTS This work was supported by the National S&T Support Program of China (2015BAD17B02),

National

Natural

Science

Foundation

of

China

(21375049), China Postdoctoral Science Foundation (2016M601718), JUSRP51714B and Collaborative Innovation Center of Food Safety and Quality Control in Jiangsu Province.

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Enhanced Visible-Light-Driven Photocatalytic Disinfection

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