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Modification of surface properties and enhancement of photocatalytic performance for g-C3N4 via plasma treatment
Accepted Manuscript Modification of surface properties and enhancement of photocatalytic performance for g-C3N4 via plasma treatment Zhiyong Mao, Jingjing Chen, Yanfang Yang, Lijian Bie, Bradley D. Fahlman, Dajian Wang PII:
S0008-6223(17)30806-0
DOI:
10.1016/j.carbon.2017.08.020
Reference:
CARBON 12282
To appear in:
Carbon
Received Date: 24 June 2017 Revised Date:
31 July 2017
Accepted Date: 11 August 2017
Please cite this article as: Z. Mao, J. Chen, Y. Yang, L. Bie, B.D. Fahlman, D. Wang, Modification of surface properties and enhancement of photocatalytic performance for g-C3N4 via plasma treatment, Carbon (2017), doi: 10.1016/j.carbon.2017.08.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Modification of Surface Properties and Enhancement of Photocatalytic Performance for g-C3N4 via Plasma Treatment Zhiyong Mao,a,∗, ⊥ Jingjing Chen,b,⊥ Yanfang Yang,a Lijian Bie,b Bradley D. Fahlman,c and Dajian Wangb,* Tianjin Key Laboratory for Photoelectric Materials and Devices, School of Materials Science and Engineering,
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a
Tianjin University of Technology, Tianjin 300384, China b
Key Laboratory of Display Materials and Photoelectric Devices, Tianjin University of Technology, Ministry of Education, Tianjin 300384, China
Department of Chemistry & Biochemistry and Science of Advanced Materials Program, Central Michigan
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c
University, Mount Pleasant, MI USA 48859
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Abstract:
In this work, a plasma treatment is employed to modify the surface properties of g-C3N4 photocatalyst to enhance its photocatalytic performance. A suite of characterization methods are used to investigate the influence of plasma treatment on the surface properties, such as
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morphology, hydrophilicity and chemical bonding states. The comparative photocatalytic performance of pristine g-C3N4 and the plasma-treated g-C3N4 (PT-g-C3N4) for the
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degradation of Rhodamine B (RhB) are demonstrated. The degradation efficiency under visible light irradiation for PT-g-C3N4 is >2.0 times as high as that of pristine g-C3N4. The
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remarkably enhanced photocatalytic performance for pollution degradation results from the optimization of the surface properties induced by plasma treatment. These findings may provide a promising and facile approach to design high-performance photocatalysts. Key words: Photocatalyst; Environmental pollutants; g-C3N4; Plasma treatment; Surface properties.
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Corresponding authors: Email: [email protected] (Prof. Zhiyong Mao), [email protected] (Prof. Dajian Wang) These authors contributed equally to this work and should be considered co-first authors.
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ACCEPTED MANUSCRIPT 1. Introduction In view of environmental pollution and energy crisis, photocatalysis for the degradation of organic pollutants, hydrogen generation from water splitting, CO2 photoreduction into
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hydrocarbon fuels, and even disinfection of bacteria, are fascinating challenging processes that employ the harvesting of solar energy by semiconductors. Graphitic carbon nitride (g-C3N4), a metal-free photocatalyst, has been vastly studied for these applications due to its
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merits in environmental benignity, good thermal and chemical stability, cost effectiveness,
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and visible-light response.[1-3] Nevertheless, pristine g-C3N4 still suffers drawbacks of low specific surface area, poor visible-light utilization, and fast recombination of photogenerated charge carriers. In this context, considerable and continuing efforts have been devoted to enhance the catalytic performance of g-C3N4 photocatalysts over the past decades.[4, 5]
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To date, several strategies have been proposed to address the above drawbacks of pristine g-C3N4. One strategy is optimizing the electron and band structures by doping heteroatoms to promote solar-light absorption in the visible-light region.[6,
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Another approach is the
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construction of heterojunctions through hybridization with other semiconductors to spatially
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separate and migrate the photogenerated electrons and holes; this results from the built-in electric field formed at the interface of the heterojunction compositions.[8,
9]
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photocatalytic system, photo-induced reactions take place at the surface of the catalyst. Surface properties, such as specific surface area, morphology, -OH content, defects, surface energy, and chemical bonding states of photocatalysts greatly influence its photocatalytic performance.[10-12] In view of this, nanostructural engineering is widely reported to develop nanostructural g-C3N4 with optimized surface properties, including nanosheets,[13]
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ACCEPTED MANUSCRIPT nanotubes,[14] quantum dots[15] and mesoporous arrays.[16] It is well recognized that g-C3N4 exhibits a unique layered feature analogous to graphite. Similar to the development of graphene from graphite, various exfoliation routes, such as ultrasonication-assisted liquid
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exfoliation,[17-19] acid exfoliation,[20, 21] and post-thermal oxidation etching exfoliation[22-26] were demonstrate to delaminate pristine g-C3N4 into a few layers g-C3N4 nanosheets with intriguing surface properties to advance the photocatalytic performances. For example, Xu et
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al. reported the exfoliation of pristine g-C3N4 in concentrated H2SO4 for the production of
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single layer graphene-like g-C3N4 structures with an atomic thickness of 0.4 nm, and lateral dimensions on the order of micrometers. The mono-layered g-C3N4 exhibited a surface area of 205.8 m2 g-1, which was nearly 50 times larger than pristine g-C3N4 (4.3 m2 g-1), noting vast active sites and decreasing charge carrier migration distance to the solid-liquid interface
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in the monolayer counterparts.[20] Liang et al. reported thermal etching exfoliation of pristine g-C3N4 under an NH3 atmosphere, which not only helps to exfoliate the pristine g-C3N4 into holey nanosheets with higher specific surface area, but also produces defects (carbon
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vacancies), resulting in 20 times higher photocatalytic activity for hydrogen production.[26]
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In addition to aforementioned nanostructural engineering, other physical techniques, such as UV irradiation, electron beam bombardment, and plasma treatment were reported to modify the surface properties of materials.[11] Non-thermal plasma consist of an abundance of excited atoms, ions, elections, and radicals, characterized by a high electron temperature (10,000-100,000 K) and low gas temperature (close to room temperature). These unique characteristics indicate that the chemically active species in plasma could easily react with the surface of materials at a low temperature, without destroying the fundamental structure of
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ACCEPTED MANUSCRIPT materials. As a result, plasma treatment has opened a new pathway in preparing high performance catalysts in terms of tailoring the surface properties. For example, Li et al. employed an O2-plasma treatment as an effective way to fabricate atomic-scale pores in the
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basal plane of 2D TaS2 catalysts, resulting in an increasing hydrogen evolution reaction.[27] Lee et al. reported an O2-plasma treatment on TiO2 photocatalysts and disclosed that the photocatalytic activity was significantly improved, owing to the increase in the hydrophilicity
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of TiO2, due to the formation of new surface hydroxyl groups on the surface induced by
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plasma treatment.[28] Similarly, Zhang et al. demonstrated the enhancement of photoactivity of TiO2 films for Rhodamine B (RhB) degradation induced by plasma treatment, which was proposed to create different locations of defects at the surface or interface. These generated defects act as functional electron traps in reducing the recombination of photogenerated
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electrons and holes.[12] Rahemiet et al. found that plasma treatment can form well-dispersed particles with uniform morphology without agglomeration, thereby promoting the interaction of the active component and support for Ni/Al2O3-CeO2 nanocatalysts.[29] However, to the
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best of our knowledge, there are no precedents concerning the effects of plasma treatment on
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the surface properties and photocatalytic performance for g-C3N4 photocatalyst. In this work, plasma treatment is performed for the first time to modify the surface properties of g-C3N4 photocatalysts. The influence of plasma treatment on the surface morphology, hydrophilicity, chemical bonding states, as well as the photocatalytic activity for the degradation of RhB are investigated in detail. A comprehensive comparison between pristine g-C3N4 and plasma-treated g-C3N4 (PT-g-C3N4) is also demonstrated to discuss the enhancement mechanism for the photocatalytic performance of PT-g-C3N4.
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ACCEPTED MANUSCRIPT 2. Experimental 2.1 Sample preparation. Urea and Rhodamine B(RhB) were purchased from Aladdin Industrial Corporation (Shanghai, China), and were used without purification. Pristine g-C3N4
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was prepared by the high-temperature polycondensation method developed in our previous work.[30] Typically, urea was placed into a covered crucible, and heated at 550 °C for 6 h in static air in a muffle furnace, with a heating rate of 2.5 °C /min. Dielectric-barrier discharge
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atmosphere plasma was employed to modify the surface properties of g-C3N4. A schematic of
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the DBD plasma setup used in our experiment is shown in Figure 1. Two steel electrodes with a diameter of 50 mm were connected to high-voltage generator with a fixed gap distance of ca.15 mm. A quartz reactor chamber with a diameter of 90 mm and a wall thickness of 2.5 mm was placed between the two electrodes. The discharge gap between the inner surfaces of
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the quartz reactor is about 8 mm. A high-voltage generator (CTP-2000K, Corona Laboratory, Nanjing, China) supplying a voltage from 0 kV to 30 kV with a sinusoidal waveform at a
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frequency of about 22 kHz, was used to generate the plasma. The plasma was initiated at room temperature and atmospheric pressure air without flow. The pristine g-C3N4 powder
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was placed within the quartz reactor chamber and treated by the plasma for a certain time (2.5, 5.0, 10, 20 min) to obtain plasma treated g-C3N4, denoted as PT-g-C3N4.
Figure 1 Schematic presentation of the DBD atmosphere plasma generation setup used in the experiment. -5-
ACCEPTED MANUSCRIPT 2.2 Sample characterization. The crystallinity of the prepared g-C3N4 and PT-g-C3N4 samples was analyzed by an X-ray diffractometer (XRD) (Rigaku, RINT Ultima-III, Japan) using Cu-Kα radiation. The morphology and elemental analysis were measured by a scanning
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electron microscope (SEM) equipped with energy-dispersive spectroscopy (Hitachi, S-4800), and a transmission electron microscope (TEM, JEM-6700F, Japan). Specific surface area measurements were taken using the BET method (N2 absorption, Kangta AUTOSORB-1,
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America). Fourier transform infrared spectroscopy (FTIR, Bruker, WQE-410) and X-ray
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photoelectron spectroscopy (XPS, Thermal Fisher Scientific 250XI, America) were used to characterize the surface chemical functional groups and chemical bonding speciation, respectively. The surface hydrophilicity was measured by contact-angle (CA) measurements (Drop Shape Analyzer, DSA100). Ultraviolet-visible (UV-Vis) absorption spectra were
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measured using a UV-Visible spectrometer (TU-1901, China) with a 60-mm diameter integration sphere. The electron spin resonance (ESR) signals of radicals spin-trapped by spin-trap reagent DMPO (5,5‘-dimethyl-1-pirroline-N-oxide) were tested in methanol (for
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superoxide radical) and water (for hydroxyl radical), respectively. Ultraviolet photoelectron
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spectroscopy (UPS, Thermo scientific, ESCALAB 250Xi, America) was used to characterize the redox potential. Electrochemistry impedance spectroscopy (EIS) and photoelectric current (PC) response measurements were performed on an electrochemical workstation (CHI600E, China) based on a conventional three-electrode system with the as-prepared photocatalysts and PVDF (mass ratio 3:1) coated on FTO glass as a working electrode, platinum foil as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode, respectively. The electrode was immersed in 1 M Na2SO4 aqueous solution, the frequency
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ACCEPTED MANUSCRIPT range was from 0.01 Hz to100 kHz, and the amplitude of the applied sine wavepotential in each case was 5 mV for the EIS measurements. The photocurrent was measured at a bias voltage of 0.02 V under chopped illumination with 40 s light on/off cycles. Incident light was
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obtained from a 300 W xenon lamp (PLS-SXE 300C, Perfectlight, Beijing). 2.3 Photocatalytic RhB degradation testing. Rhodamine B (RhB) with high photostability and water solubility was used as a model pollutant. For testing, a photocatalyst (30 mg) was
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suspended in 100-mL of RhB(aq) (10 mg/L) within a Pyrex photocatalytic reactor with a
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circulating water system maintained at 7 °C by a flow of cooling water during the photocatalytic reaction. Prior to irradiation, the suspensions were magnetically stirred for 30 min in the dark to ensure that RhB could reach the absorption-desorption equilibrium on the photocatalyst surface. Then, suspensions were irradiated by a 300-W xenon lamp with an
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optical filter (≥420 nm). At certain time intervals, 4-mL aliquots were sampled and centrifuged to remove the photocatalyst particles. The filtrates were analyzed by recording variations of the absorption band maximum (554 nm) in the UV-Vis spectra of RhB using a
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UV-Vis spectrophotometer (TU-1901, China).
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3. Results and discussion
X-ray diffraction (XRD) patterns of pristine g-C3N4 and PT-g-C3N4 with duration of 5.0 min are displayed in Figure 2(a). As previously reported, g-C3N4 exhibits two diffraction peaks centered at 13.1º(100) and 27.3º (002), which are associated with the in-plane structural packing motif and the inter-planar stacking of conjugated aromatic rings, respectively.[31] Both samples show typical g-C3N4 peaks without any impurity phases, indicating that the plasma treatment did not affect the crystal structure of the parent g-C3N4. -7-
ACCEPTED MANUSCRIPT However, the peaks of PT-g-C3N4 have a reduced intensity. This mainly results from the decrease of crystallinity and the appearance of defects in PT-g-C3N4 induced by the plasma treatment. The surface morphology and microstructure of g-C3N4 and PT-g-C3N4 were
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investigated by SEM/TEM images and N2 adsorption-desorption measurements. Compared to the parent g-C3N4 sheets (Figure 2(b)), which consist of solid micron-sized agglomerates, PT-g-C3N4 (Figure 2(c)) features ultrathin nanosheets that appear as soft agglomerates with a
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thickness of a few nanometers (20-50 nm). EDX elemental analysis reveals that the C/N
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molar ratio and O content of PT-g-C3N4 (C/N = 0.75, O = 2.67 at %) are higher than that of g-C3N4 (C/N = 0.67, O = 2.53 at%), indicating the possible formation of nitrogen-vacancy defects in the PT-g-C3N4 framework. The transformation of terminal C-NH2 groups to C-O, C=O groups induced by plasma bombardment might be the possibility for this chemical
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composition change. TEM images also confirm the ultrathin nanosheets morphology for PT-g-C3N4, and clearly shows more curled edges than g-C3N4 (Figure. 2(d) and 2(e)). As shown in Figure 2(d), both g-C3N4 and PT-g-C3N4 exhibit a type-Ⅳ isotherm with an
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extremely high adsorption capacity at relatively high pressures (P/P0: 0.9-1), suggesting the
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presence of abundant mesopores and macropores. The Brunauer-Emmett-Teller specific surface area of PT-g-C3N4 is calculated to be 130.4 m2 g-1, which is 2.23 times higher than that of g-C3N4 (58.5 m2 g-1). In addition, the total pore volume of PT-g-C3N4 (0.43 cc g-1) is higher than that of g-C3N4 (0.33 cc g-1), as defined at P/P0 = 0.99. The changes in suface morphology and the enhancements of specific surface area and pore volume are mainly attributed to the etching effect that occurs during the plasma treatment. The plasma is rich in highly active and energetic species, which are responsible for the delamination of pristine
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ACCEPTED MANUSCRIPT g-C3N4 into ultrathin nanosheets. It is well known that the larger specific surface area and pore volume enable more active species and reactants to be absorbed on its surface. Thus, we can assume that the PT-g-C3N4 may display more efficient photocatalytic activity than
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g-C3N4.
Figure 2. (a) XRD patterns; (b-c) SEM images and EDX elemental analysis results; (d-e) TEM images; and (f) Nitrogen adsorption-desorption isotherms for g-C3N4 and PT-g-C3N4.
The influence of plasma treatment on the chemical functional groups and optical property were investigated by FTIR spectra and UV-Vis diffuse reflectance spectroscopy (DRS), respectively. From the FTIR spectra (Figure 3(a)), we find that both g-C3N4 and PT-g-C3N4
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ACCEPTED MANUSCRIPT show broad peaks assigned to -NH groups between 2900-3400 cm-1, characteristic stretching modes of CN heterocycles from 1200-1700 cm-1, and typical breathing modes of the triazine units at 810 cm-1, revealing their similar chemical structures. It is worth noting that the
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absorbance of a broad peak assigned to an -OH group between 3400-3700 cm-1 for PT-g-C3N4 is higher than that of g-C3N4. It is thought that the hydrophilic -OH functional group at the surface can be induced by plasma treatment. DRS (Figure 3(b)) demonstrate that
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g-C3N4 and PT-g-C3N4 displays the same absorption threshold at ~460 nm, suggesting
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plasma treatment did not change the optical band gap. The band gap energy can be estimated to be 2.86 eV via the Tauc plot calculated from the DRS data (Figure S1). It was well recognized that defects such as carbon or nitrogen vacancies show strong influence on the band gap and photocatalytic activity of g-C3N4 photocatalysts.[32] Tay and Niu et al. reported
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that nitrogen vacancies would narrow the band gap of g-C3N4 remarkably, and then resulting in the enhancement of photocatalytic performance.[32-34] From the DRS, we find that g-C3N4 and PT-g-C3N4 show an extremely same band gap without narrowing change, indicating the
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plasma treatment did not introduce nitrogen vacancies in PT-g-C3N4. This changeless band
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gap evaluated from DRS confirms the above discussed increase of C/N ratio is attributed to the transformation of terminal C-NH2 groups to C-O, C=O groups induced by plasma bombardment instead of the formation of nitrogen vacancies.
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Figure 3(a) FTIR spectra; (b) UV-Vis diffuse reflectance spectroscopy (DRS) for g-C3N4 and PT-g-C3N4.
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The influence of plasma treatment on the surface composition and chemical speciation
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were further investigated by X-ray photoelectron spectroscopy (XPS), as depicted in Figure 4. The XPS survey spectra (Figure 4(a)) of both g-C3N4 and PT-g-C3N4 contain three sharp peaks at ~286, ~396, and ~529 eV, which are assigned to C 1s, N 1s, and O 1s signals, respectively. Similar to the EDX elemental analysis results, quantitative analysis from XPS spectra reveals that the surface C/N atomic ratio and O content of PT-g-C3N4 (C/N = 1.35, O = 3.85 at%) is much larger than that of C3N4 (C/N = 1.14, O = 3.57 at%), suggesting there is a higher density of oxygen-containing species on the PT-g-C3N4 surface transferred from
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ACCEPTED MANUSCRIPT terminal amine groups (NHx) by plasma bombardment. Note that the high-resolution O 1s spectrum of g-C3N4 can be fitted by Gaussian curves with one peak centered at ~529.8 eV, which is described as absorbed water.[35] Besides the common response, PT-g-C3N4 shows a
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new generated peak at ~532.5 eV (Figure 4(b)) ascribed to -OH group, which is also confirmed by the FTIR spectra. The C 1s peak of g-C3N4 can be resolved into two peaks centered at ~282.5 eV and ~286.1 eV, which are attributed to C=C and N-C=N bonds,
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respectively. Compared to g-C3N4, the C 1s peak of PT-g-C3N4 produces a new peak at
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~288.1eV (Figure 4(c)), which may be due to a -COOH group.[36] The N 1s peak of both g-C3N4 and PT-g-C3N4 can be deconvoluted into four peaks centered at ~396.2, ~397.2,~399.1, and 402.1 eV (Figure 4(d)), corresponding to C=N-C, N-[C]3, C-NHx bonds and charging effects, respectively. It is found that the binding energies of O1s, C 1s and N 1s
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of PT-g-C3N4 are all higher than those of g-C3N4. This could be ascribed to the introduction of many oxygen-containing species, such as -OH and -COOH, on the surface of PT-g-C3N4 induced by plasma treatment. These -OH and -COOH groups will result in an increased
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hydrophilicity, which will improve the binding of polar molecules to the surface of
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PT-g-C3N4, leading to a higher photocatalytic activity.
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Figure 4 (a) XPS survey spectra; (b) high resolution O 1s; (c) high resolution C 1s; and (d) high resolution N 1s for g-C3N4 and PT-g-C3N4.
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In order to confirm the increase of hydrophilicity of PT-g-C3N4, contact angle measurements for pristine g-C3N4 and PT-g-C3N4 with duration of 5.0 min were performed by using deionized water and ethylene glycol as test liquids, as shown in Figure 5(a). After
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plasma treatment, the contact angle of water decreases from 74.1o to 51.8o, while that of
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ethylene glycol decreases from 34.6o to 29.1o. The decrease of contact angle indicates that plasma treatment can enhance the surface hydrophilicity of g-C3N4 through the introduction of hydrophilic -OH and -COOH groups. The surface energies were evaluated by using the contact angle data of the test liquids from the extended Fowkes equations as follows:[36]
(Eq-1)
(Eq-2)
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ACCEPTED MANUSCRIPT where σL and σS are the surface energies of the test liquids and the solid sample, θ is the contact angle, the superscripts D, P refer to the dispersive and polar components of the surface energy. The two components σSD and σSP can be calculated using Eq-1, and the total
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surface energy σS of the solid sample can be obtained from Eq-2. The calculated surface energy and its two components (dispersive and polar components) are depicted in Figure 5(b) for g-C3N4 and PT-g-C3N4, respectively. After plasma treatment, the surface energy of
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PT-g-C3N4 increases from 38.71 to 52.77 mN/m. This change can be attributed to the
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increase in the number of polar bonds resulted from the generation of hydrophilic functional groups on the surface of the PT-g-C3N4. This behaviour is in agreement with that found from
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FTIR and XPS analyses.
Figure 5(a) Photos of contact angle measurements using water and ethylene glycol as test liquids; and (b) calculated surface energies for g-C3N4 and PT-g-C3N4.
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ACCEPTED MANUSCRIPT To investigate the photogenerated charge separation and transfer performance, electrochemical impedance spectroscopy (EIS) and photocurrents were measured. The experimental Nyquist impedance plots for g-C3N4 and PT-g-C3N4 without light illumination
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are shown in Figure 6(a). The arc radius on the EIS Nyquist plot can reflect the reaction rate on the surface of the electrode. A smaller arc radius corresponds to a more effective separation of photogenerated electron-hole pairs and a higher efficiency of charge migration
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across the electrode/electrolyte interface.[37] The arc radius for the PT-g-C3N4 electrode is
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smaller than that of the g-C3N4 electrode, suggesting that PT-g-C3N4 has a stronger electronic conductivity in the non-photoexcited state, and would be very beneficial for photoexcited charge separation. Figure 6(b) shows the transient photocurrent responses for g-C3N4 and PT-g-C3N4 under visible light irradiation in an on-and-off cycle mode. It is clearly seen than
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the photocurrent of PT-g-C3N4 is higher than that of g-C3N4, which suggests that the recombination of electrons and holes is greatly impeded. The EIS and photocurrent analyses are consistent with the results of PL results, indicating PT-g-C3N4 might exhibit better
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photocatalytic performance.
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Figure 6 (a) EIS; (b) transient photocurrents of g-C3N4 and PT-g-C3N4 in a 1 M Na2SO4 aqueous solution.
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The photocatalytic performance of g-C3N4 and PT-g-C3N4 with duration of 5.0 min were evaluated by studying the degradation of RhB under visible light irradiation (≥420), as
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shown in Figure 7. Prior to irradiation, the suspensions were magnetically stirred in dark for 30 min to obtain the absorption-desorption equilibrium between the photocatalyst and RhB.
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As depicted in Figure 7(a), the RhB aqueous solution under visible light irradiation is stable, indicating RhB self-degradation is almost negligible in the absence of a photocatalyst. With the presence of g-C3N4 and PT-g-C3N4 photocatalysts, the concentration of RhB decreases dramatically under same experimental conditions. After irradiation of 20 min, the degradation rate of RhB is ~ 45% and ~ 90% for g-C3N4 and PT-g-C3N4, respectively. Hence, the photocatalytic activity of PT-g-C3N4 is 2.0 times higher than that of g-C3N4 for the degradation of RhB. The influence of plasma treatment time (2.5, 5.0, 10, 20 min) on - 16 -
ACCEPTED MANUSCRIPT photocatalytic degradation performance for RhB were tested and depicted in Figure S2. It was found that the photocatalytic activity of g-C3N4 photocatalyst enhances obviously with the increasing plasma treatment time from 0 to 5.0 min. When the duration is longer than 5.0
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min, the enhancement extend of photocatalyti degradation performance is limited. The photocatalytic degradation kinetics was also studied. The degradation of RhB can be attributed to a pseudo-first-order reaction with a simplified Langmuir-Hinshelwood model
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when C0 was very small: -ln(C/C0) = kt, where k is the rate constant. The kinetic rate
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constants are obtained by plotting -ln(C/C0) with the visible light irradiation time (min.), which is shown in Figure 7(b). The kinetic rate constants are evaluated to be 0.102 min-1 for PT-g-C3N4 with duration of 5.0 min, which is 2.83 times higher than that of g-C3N4 (0.036 min-1). In addition, the absorbance spectra demonstrate the mineralization of RhB during the
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photodegradation process with the prescence of PT-g-C3N4 photocatalyst (Figure 7(c)). It is found that the characteristic absorption peak of RhB solution at 554 nm shifts to shorter wavelengths and the intensity decreases under visible light irradiation. These changes
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indicate the remove of an ethyl group from RhB. The absorption peak eventually disappears,
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which implies the removal of the benzene ring of RhB.[12] From Figure 7(d), one can see there is no obvious decrease of photodegradation after four cycles, confirming the high stability of the PT-g-C3N4 photocatalyst. The above results clearly demonstrate that the photocatalytic activity of g-C3N4 has been significantly improved by plasma treatment, which would optimize the surface properties of g-C3N4 in enlarging the specific surface area and increasing the hydrophilicity.
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Figure 7 (a) The photocatalytic degradation of RhB dye; (b) kinetic fit for the degradation of RhB for g-C3N4 and PT-g-C3N4; (c) changes in UV-visible absorption spectra of RhB solution with variable photodegradation reaction time for PT-g-C3N4; (d) recyclability of PT-g-C3N4 photocatalyst for the degradation of RhB.
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The role of reactive species including hole (h+), hydroxyl radical (.OH) and superoxide radical (.O2-) in the degradation of RhB for g-C3N4 photocatalytsts were indentified in our previous work by radicals trapping experiments,[30] using triethanolamine (TEOA),
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isopropanol (IPA), and vacuum conditions as h+, .OH and .O2- scavengers, respectively. To
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further confirme the role of .OH and .O2-, ESR spin-trap technique was employed to determine the active species in PT-g-C3N4. As shown in Figure 8(a) and 8(b), the characteristic peaks of DMPO-.O2- and DMPO-.OH adducts can be detected obviously under visible light for PT-g-C3N4, compared with the sample in dark. And the signal intensities of .OH and .O2- become stronger with the increase of light irradiation time. These results revealed that the .O2- and .OH active species could be generated in PT-g-C3N4, which might play important role in the photocatalytic degradation process.
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Figure 8 ESR spectra of (a) DMPO-.O2-; and (b) DMPO-. OH adducts in PT-g-C3N4 aqueous dispersion.
In order to illuminate the enhancement mechanism, band structures of g-C3N4 and
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PT-g-C3N4 were proposed by evaluating their conduction band and valence band levels with
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Ultraviolet photoelectron spectroscopy (UPS), as shown in Figure 9. The valence band energy (Ev) (equivalent to ionization potential) of g-C3N4 and PT-g-C3N4 is determined to be 6.92 and 7.12 eV by subtracting the width of the He I UPS spectra from the excitation energy (21.22 eV). The conduction band energy Ec is thus estimated at 4.06 and 4.26 eV for g-C3N4 and PT-g-C3N4 from formula Ev-Eg, respectively. The Eg value for g-C3N4 and PT-g-C3N4 was determined to be an equal value at 2.86 eV (Figure S1). The values of Ev and Ec in electron volts are converted to electrochemical energy potentials in volts according to the reference
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ACCEPTED MANUSCRIPT standard for which 0 versus RHE (reversible hydrogen electrode) equals -4.44 eV versus evac (vacuum level). According to these evaluated levels, the schematic energy band illustrations of g-C3N4 and PT-g-C3N4 with the redox potentials of the photocatalytic reaction were
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depicted in Figure 9(b). It is found that plasma treatment down-shifts the valence band of PT-g-C3N4 to a more positive potential than g-C3N4, then leading to the increase of hole oxidability and .OH formation for the photocatalytic degradation of RhB. Based on the above
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experimental results, the down-shift of valence band for PT-g-C3N4 is mainly attributed to the
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introduction of O-containing hydrophilic functional groups induced by plasma treatment.
Figure 9 (a) UPS spectra; and (b) the proposed band structure diagram for g-C3N4 and PT-g-C3N4.
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ACCEPTED MANUSCRIPT 4. Conclusions In summary, plasma treatment was employed to modify the surface properties of a g-C3N4 photocatalyst to enhance its photocatalytic performance. Experimental results showed that
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plasma treatment can exfoliate pristine g-C3N4 into ultrathin nanosheets with an increase in specific surface area. In addition, the surface energy of g-C3N4 was enhanced by plasma treatment through introducing hydrophilic functional groups, resulting in an increase of
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surface hydrophilicity. As a result, the degradation efficiency under visible light irradiation
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for plasma treatment g-C3N4 (PT-g-C3N4) is up to 2.0 times higher than pristine g-C3N4. The remarkably enhanced photocatalytic performance for pollution degradation is attributed to optimization of surface properties induced by plasma treatment. Acknowledgments
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We gratefully acknowledge the financial support by the National Natural Science Foundation of China (nos. 51102265 and 50872091) and “Foreign Experts” Thousand Talents Program
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(Tianjin, China).
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S0008-6223(17)30806-0
DOI:
10.1016/j.carbon.2017.08.020
Reference:
CARBON 12282
To appear in:
Carbon
Received Date: 24 June 2017 Revised Date:
31 July 2017
Accepted Date: 11 August 2017
Please cite this article as: Z. Mao, J. Chen, Y. Yang, L. Bie, B.D. Fahlman, D. Wang, Modification of surface properties and enhancement of photocatalytic performance for g-C3N4 via plasma treatment, Carbon (2017), doi: 10.1016/j.carbon.2017.08.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Modification of Surface Properties and Enhancement of Photocatalytic Performance for g-C3N4 via Plasma Treatment Zhiyong Mao,a,∗, ⊥ Jingjing Chen,b,⊥ Yanfang Yang,a Lijian Bie,b Bradley D. Fahlman,c and Dajian Wangb,* Tianjin Key Laboratory for Photoelectric Materials and Devices, School of Materials Science and Engineering,
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a
Tianjin University of Technology, Tianjin 300384, China b
Key Laboratory of Display Materials and Photoelectric Devices, Tianjin University of Technology, Ministry of Education, Tianjin 300384, China
Department of Chemistry & Biochemistry and Science of Advanced Materials Program, Central Michigan
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c
University, Mount Pleasant, MI USA 48859
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Abstract:
In this work, a plasma treatment is employed to modify the surface properties of g-C3N4 photocatalyst to enhance its photocatalytic performance. A suite of characterization methods are used to investigate the influence of plasma treatment on the surface properties, such as
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morphology, hydrophilicity and chemical bonding states. The comparative photocatalytic performance of pristine g-C3N4 and the plasma-treated g-C3N4 (PT-g-C3N4) for the
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degradation of Rhodamine B (RhB) are demonstrated. The degradation efficiency under visible light irradiation for PT-g-C3N4 is >2.0 times as high as that of pristine g-C3N4. The
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remarkably enhanced photocatalytic performance for pollution degradation results from the optimization of the surface properties induced by plasma treatment. These findings may provide a promising and facile approach to design high-performance photocatalysts. Key words: Photocatalyst; Environmental pollutants; g-C3N4; Plasma treatment; Surface properties.
∗ ⊥
Corresponding authors: Email: [email protected] (Prof. Zhiyong Mao), [email protected] (Prof. Dajian Wang) These authors contributed equally to this work and should be considered co-first authors.
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ACCEPTED MANUSCRIPT 1. Introduction In view of environmental pollution and energy crisis, photocatalysis for the degradation of organic pollutants, hydrogen generation from water splitting, CO2 photoreduction into
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hydrocarbon fuels, and even disinfection of bacteria, are fascinating challenging processes that employ the harvesting of solar energy by semiconductors. Graphitic carbon nitride (g-C3N4), a metal-free photocatalyst, has been vastly studied for these applications due to its
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merits in environmental benignity, good thermal and chemical stability, cost effectiveness,
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and visible-light response.[1-3] Nevertheless, pristine g-C3N4 still suffers drawbacks of low specific surface area, poor visible-light utilization, and fast recombination of photogenerated charge carriers. In this context, considerable and continuing efforts have been devoted to enhance the catalytic performance of g-C3N4 photocatalysts over the past decades.[4, 5]
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To date, several strategies have been proposed to address the above drawbacks of pristine g-C3N4. One strategy is optimizing the electron and band structures by doping heteroatoms to promote solar-light absorption in the visible-light region.[6,
7]
Another approach is the
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construction of heterojunctions through hybridization with other semiconductors to spatially
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separate and migrate the photogenerated electrons and holes; this results from the built-in electric field formed at the interface of the heterojunction compositions.[8,
9]
In a
photocatalytic system, photo-induced reactions take place at the surface of the catalyst. Surface properties, such as specific surface area, morphology, -OH content, defects, surface energy, and chemical bonding states of photocatalysts greatly influence its photocatalytic performance.[10-12] In view of this, nanostructural engineering is widely reported to develop nanostructural g-C3N4 with optimized surface properties, including nanosheets,[13]
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ACCEPTED MANUSCRIPT nanotubes,[14] quantum dots[15] and mesoporous arrays.[16] It is well recognized that g-C3N4 exhibits a unique layered feature analogous to graphite. Similar to the development of graphene from graphite, various exfoliation routes, such as ultrasonication-assisted liquid
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exfoliation,[17-19] acid exfoliation,[20, 21] and post-thermal oxidation etching exfoliation[22-26] were demonstrate to delaminate pristine g-C3N4 into a few layers g-C3N4 nanosheets with intriguing surface properties to advance the photocatalytic performances. For example, Xu et
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al. reported the exfoliation of pristine g-C3N4 in concentrated H2SO4 for the production of
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single layer graphene-like g-C3N4 structures with an atomic thickness of 0.4 nm, and lateral dimensions on the order of micrometers. The mono-layered g-C3N4 exhibited a surface area of 205.8 m2 g-1, which was nearly 50 times larger than pristine g-C3N4 (4.3 m2 g-1), noting vast active sites and decreasing charge carrier migration distance to the solid-liquid interface
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in the monolayer counterparts.[20] Liang et al. reported thermal etching exfoliation of pristine g-C3N4 under an NH3 atmosphere, which not only helps to exfoliate the pristine g-C3N4 into holey nanosheets with higher specific surface area, but also produces defects (carbon
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vacancies), resulting in 20 times higher photocatalytic activity for hydrogen production.[26]
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In addition to aforementioned nanostructural engineering, other physical techniques, such as UV irradiation, electron beam bombardment, and plasma treatment were reported to modify the surface properties of materials.[11] Non-thermal plasma consist of an abundance of excited atoms, ions, elections, and radicals, characterized by a high electron temperature (10,000-100,000 K) and low gas temperature (close to room temperature). These unique characteristics indicate that the chemically active species in plasma could easily react with the surface of materials at a low temperature, without destroying the fundamental structure of
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ACCEPTED MANUSCRIPT materials. As a result, plasma treatment has opened a new pathway in preparing high performance catalysts in terms of tailoring the surface properties. For example, Li et al. employed an O2-plasma treatment as an effective way to fabricate atomic-scale pores in the
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basal plane of 2D TaS2 catalysts, resulting in an increasing hydrogen evolution reaction.[27] Lee et al. reported an O2-plasma treatment on TiO2 photocatalysts and disclosed that the photocatalytic activity was significantly improved, owing to the increase in the hydrophilicity
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of TiO2, due to the formation of new surface hydroxyl groups on the surface induced by
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plasma treatment.[28] Similarly, Zhang et al. demonstrated the enhancement of photoactivity of TiO2 films for Rhodamine B (RhB) degradation induced by plasma treatment, which was proposed to create different locations of defects at the surface or interface. These generated defects act as functional electron traps in reducing the recombination of photogenerated
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electrons and holes.[12] Rahemiet et al. found that plasma treatment can form well-dispersed particles with uniform morphology without agglomeration, thereby promoting the interaction of the active component and support for Ni/Al2O3-CeO2 nanocatalysts.[29] However, to the
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best of our knowledge, there are no precedents concerning the effects of plasma treatment on
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the surface properties and photocatalytic performance for g-C3N4 photocatalyst. In this work, plasma treatment is performed for the first time to modify the surface properties of g-C3N4 photocatalysts. The influence of plasma treatment on the surface morphology, hydrophilicity, chemical bonding states, as well as the photocatalytic activity for the degradation of RhB are investigated in detail. A comprehensive comparison between pristine g-C3N4 and plasma-treated g-C3N4 (PT-g-C3N4) is also demonstrated to discuss the enhancement mechanism for the photocatalytic performance of PT-g-C3N4.
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ACCEPTED MANUSCRIPT 2. Experimental 2.1 Sample preparation. Urea and Rhodamine B(RhB) were purchased from Aladdin Industrial Corporation (Shanghai, China), and were used without purification. Pristine g-C3N4
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was prepared by the high-temperature polycondensation method developed in our previous work.[30] Typically, urea was placed into a covered crucible, and heated at 550 °C for 6 h in static air in a muffle furnace, with a heating rate of 2.5 °C /min. Dielectric-barrier discharge
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atmosphere plasma was employed to modify the surface properties of g-C3N4. A schematic of
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the DBD plasma setup used in our experiment is shown in Figure 1. Two steel electrodes with a diameter of 50 mm were connected to high-voltage generator with a fixed gap distance of ca.15 mm. A quartz reactor chamber with a diameter of 90 mm and a wall thickness of 2.5 mm was placed between the two electrodes. The discharge gap between the inner surfaces of
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the quartz reactor is about 8 mm. A high-voltage generator (CTP-2000K, Corona Laboratory, Nanjing, China) supplying a voltage from 0 kV to 30 kV with a sinusoidal waveform at a
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frequency of about 22 kHz, was used to generate the plasma. The plasma was initiated at room temperature and atmospheric pressure air without flow. The pristine g-C3N4 powder
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was placed within the quartz reactor chamber and treated by the plasma for a certain time (2.5, 5.0, 10, 20 min) to obtain plasma treated g-C3N4, denoted as PT-g-C3N4.
Figure 1 Schematic presentation of the DBD atmosphere plasma generation setup used in the experiment. -5-
ACCEPTED MANUSCRIPT 2.2 Sample characterization. The crystallinity of the prepared g-C3N4 and PT-g-C3N4 samples was analyzed by an X-ray diffractometer (XRD) (Rigaku, RINT Ultima-III, Japan) using Cu-Kα radiation. The morphology and elemental analysis were measured by a scanning
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electron microscope (SEM) equipped with energy-dispersive spectroscopy (Hitachi, S-4800), and a transmission electron microscope (TEM, JEM-6700F, Japan). Specific surface area measurements were taken using the BET method (N2 absorption, Kangta AUTOSORB-1,
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America). Fourier transform infrared spectroscopy (FTIR, Bruker, WQE-410) and X-ray
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photoelectron spectroscopy (XPS, Thermal Fisher Scientific 250XI, America) were used to characterize the surface chemical functional groups and chemical bonding speciation, respectively. The surface hydrophilicity was measured by contact-angle (CA) measurements (Drop Shape Analyzer, DSA100). Ultraviolet-visible (UV-Vis) absorption spectra were
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measured using a UV-Visible spectrometer (TU-1901, China) with a 60-mm diameter integration sphere. The electron spin resonance (ESR) signals of radicals spin-trapped by spin-trap reagent DMPO (5,5‘-dimethyl-1-pirroline-N-oxide) were tested in methanol (for
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superoxide radical) and water (for hydroxyl radical), respectively. Ultraviolet photoelectron
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spectroscopy (UPS, Thermo scientific, ESCALAB 250Xi, America) was used to characterize the redox potential. Electrochemistry impedance spectroscopy (EIS) and photoelectric current (PC) response measurements were performed on an electrochemical workstation (CHI600E, China) based on a conventional three-electrode system with the as-prepared photocatalysts and PVDF (mass ratio 3:1) coated on FTO glass as a working electrode, platinum foil as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode, respectively. The electrode was immersed in 1 M Na2SO4 aqueous solution, the frequency
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obtained from a 300 W xenon lamp (PLS-SXE 300C, Perfectlight, Beijing). 2.3 Photocatalytic RhB degradation testing. Rhodamine B (RhB) with high photostability and water solubility was used as a model pollutant. For testing, a photocatalyst (30 mg) was
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suspended in 100-mL of RhB(aq) (10 mg/L) within a Pyrex photocatalytic reactor with a
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circulating water system maintained at 7 °C by a flow of cooling water during the photocatalytic reaction. Prior to irradiation, the suspensions were magnetically stirred for 30 min in the dark to ensure that RhB could reach the absorption-desorption equilibrium on the photocatalyst surface. Then, suspensions were irradiated by a 300-W xenon lamp with an
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optical filter (≥420 nm). At certain time intervals, 4-mL aliquots were sampled and centrifuged to remove the photocatalyst particles. The filtrates were analyzed by recording variations of the absorption band maximum (554 nm) in the UV-Vis spectra of RhB using a
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UV-Vis spectrophotometer (TU-1901, China).
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3. Results and discussion
X-ray diffraction (XRD) patterns of pristine g-C3N4 and PT-g-C3N4 with duration of 5.0 min are displayed in Figure 2(a). As previously reported, g-C3N4 exhibits two diffraction peaks centered at 13.1º(100) and 27.3º (002), which are associated with the in-plane structural packing motif and the inter-planar stacking of conjugated aromatic rings, respectively.[31] Both samples show typical g-C3N4 peaks without any impurity phases, indicating that the plasma treatment did not affect the crystal structure of the parent g-C3N4. -7-
ACCEPTED MANUSCRIPT However, the peaks of PT-g-C3N4 have a reduced intensity. This mainly results from the decrease of crystallinity and the appearance of defects in PT-g-C3N4 induced by the plasma treatment. The surface morphology and microstructure of g-C3N4 and PT-g-C3N4 were
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investigated by SEM/TEM images and N2 adsorption-desorption measurements. Compared to the parent g-C3N4 sheets (Figure 2(b)), which consist of solid micron-sized agglomerates, PT-g-C3N4 (Figure 2(c)) features ultrathin nanosheets that appear as soft agglomerates with a
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thickness of a few nanometers (20-50 nm). EDX elemental analysis reveals that the C/N
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molar ratio and O content of PT-g-C3N4 (C/N = 0.75, O = 2.67 at %) are higher than that of g-C3N4 (C/N = 0.67, O = 2.53 at%), indicating the possible formation of nitrogen-vacancy defects in the PT-g-C3N4 framework. The transformation of terminal C-NH2 groups to C-O, C=O groups induced by plasma bombardment might be the possibility for this chemical
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composition change. TEM images also confirm the ultrathin nanosheets morphology for PT-g-C3N4, and clearly shows more curled edges than g-C3N4 (Figure. 2(d) and 2(e)). As shown in Figure 2(d), both g-C3N4 and PT-g-C3N4 exhibit a type-Ⅳ isotherm with an
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extremely high adsorption capacity at relatively high pressures (P/P0: 0.9-1), suggesting the
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presence of abundant mesopores and macropores. The Brunauer-Emmett-Teller specific surface area of PT-g-C3N4 is calculated to be 130.4 m2 g-1, which is 2.23 times higher than that of g-C3N4 (58.5 m2 g-1). In addition, the total pore volume of PT-g-C3N4 (0.43 cc g-1) is higher than that of g-C3N4 (0.33 cc g-1), as defined at P/P0 = 0.99. The changes in suface morphology and the enhancements of specific surface area and pore volume are mainly attributed to the etching effect that occurs during the plasma treatment. The plasma is rich in highly active and energetic species, which are responsible for the delamination of pristine
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ACCEPTED MANUSCRIPT g-C3N4 into ultrathin nanosheets. It is well known that the larger specific surface area and pore volume enable more active species and reactants to be absorbed on its surface. Thus, we can assume that the PT-g-C3N4 may display more efficient photocatalytic activity than
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g-C3N4.
Figure 2. (a) XRD patterns; (b-c) SEM images and EDX elemental analysis results; (d-e) TEM images; and (f) Nitrogen adsorption-desorption isotherms for g-C3N4 and PT-g-C3N4.
The influence of plasma treatment on the chemical functional groups and optical property were investigated by FTIR spectra and UV-Vis diffuse reflectance spectroscopy (DRS), respectively. From the FTIR spectra (Figure 3(a)), we find that both g-C3N4 and PT-g-C3N4
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absorbance of a broad peak assigned to an -OH group between 3400-3700 cm-1 for PT-g-C3N4 is higher than that of g-C3N4. It is thought that the hydrophilic -OH functional group at the surface can be induced by plasma treatment. DRS (Figure 3(b)) demonstrate that
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g-C3N4 and PT-g-C3N4 displays the same absorption threshold at ~460 nm, suggesting
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plasma treatment did not change the optical band gap. The band gap energy can be estimated to be 2.86 eV via the Tauc plot calculated from the DRS data (Figure S1). It was well recognized that defects such as carbon or nitrogen vacancies show strong influence on the band gap and photocatalytic activity of g-C3N4 photocatalysts.[32] Tay and Niu et al. reported
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that nitrogen vacancies would narrow the band gap of g-C3N4 remarkably, and then resulting in the enhancement of photocatalytic performance.[32-34] From the DRS, we find that g-C3N4 and PT-g-C3N4 show an extremely same band gap without narrowing change, indicating the
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plasma treatment did not introduce nitrogen vacancies in PT-g-C3N4. This changeless band
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gap evaluated from DRS confirms the above discussed increase of C/N ratio is attributed to the transformation of terminal C-NH2 groups to C-O, C=O groups induced by plasma bombardment instead of the formation of nitrogen vacancies.
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Figure 3(a) FTIR spectra; (b) UV-Vis diffuse reflectance spectroscopy (DRS) for g-C3N4 and PT-g-C3N4.
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The influence of plasma treatment on the surface composition and chemical speciation
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were further investigated by X-ray photoelectron spectroscopy (XPS), as depicted in Figure 4. The XPS survey spectra (Figure 4(a)) of both g-C3N4 and PT-g-C3N4 contain three sharp peaks at ~286, ~396, and ~529 eV, which are assigned to C 1s, N 1s, and O 1s signals, respectively. Similar to the EDX elemental analysis results, quantitative analysis from XPS spectra reveals that the surface C/N atomic ratio and O content of PT-g-C3N4 (C/N = 1.35, O = 3.85 at%) is much larger than that of C3N4 (C/N = 1.14, O = 3.57 at%), suggesting there is a higher density of oxygen-containing species on the PT-g-C3N4 surface transferred from
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ACCEPTED MANUSCRIPT terminal amine groups (NHx) by plasma bombardment. Note that the high-resolution O 1s spectrum of g-C3N4 can be fitted by Gaussian curves with one peak centered at ~529.8 eV, which is described as absorbed water.[35] Besides the common response, PT-g-C3N4 shows a
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new generated peak at ~532.5 eV (Figure 4(b)) ascribed to -OH group, which is also confirmed by the FTIR spectra. The C 1s peak of g-C3N4 can be resolved into two peaks centered at ~282.5 eV and ~286.1 eV, which are attributed to C=C and N-C=N bonds,
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respectively. Compared to g-C3N4, the C 1s peak of PT-g-C3N4 produces a new peak at
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~288.1eV (Figure 4(c)), which may be due to a -COOH group.[36] The N 1s peak of both g-C3N4 and PT-g-C3N4 can be deconvoluted into four peaks centered at ~396.2, ~397.2,~399.1, and 402.1 eV (Figure 4(d)), corresponding to C=N-C, N-[C]3, C-NHx bonds and charging effects, respectively. It is found that the binding energies of O1s, C 1s and N 1s
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of PT-g-C3N4 are all higher than those of g-C3N4. This could be ascribed to the introduction of many oxygen-containing species, such as -OH and -COOH, on the surface of PT-g-C3N4 induced by plasma treatment. These -OH and -COOH groups will result in an increased
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hydrophilicity, which will improve the binding of polar molecules to the surface of
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PT-g-C3N4, leading to a higher photocatalytic activity.
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Figure 4 (a) XPS survey spectra; (b) high resolution O 1s; (c) high resolution C 1s; and (d) high resolution N 1s for g-C3N4 and PT-g-C3N4.
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In order to confirm the increase of hydrophilicity of PT-g-C3N4, contact angle measurements for pristine g-C3N4 and PT-g-C3N4 with duration of 5.0 min were performed by using deionized water and ethylene glycol as test liquids, as shown in Figure 5(a). After
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plasma treatment, the contact angle of water decreases from 74.1o to 51.8o, while that of
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ethylene glycol decreases from 34.6o to 29.1o. The decrease of contact angle indicates that plasma treatment can enhance the surface hydrophilicity of g-C3N4 through the introduction of hydrophilic -OH and -COOH groups. The surface energies were evaluated by using the contact angle data of the test liquids from the extended Fowkes equations as follows:[36]
(Eq-1)
(Eq-2)
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ACCEPTED MANUSCRIPT where σL and σS are the surface energies of the test liquids and the solid sample, θ is the contact angle, the superscripts D, P refer to the dispersive and polar components of the surface energy. The two components σSD and σSP can be calculated using Eq-1, and the total
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surface energy σS of the solid sample can be obtained from Eq-2. The calculated surface energy and its two components (dispersive and polar components) are depicted in Figure 5(b) for g-C3N4 and PT-g-C3N4, respectively. After plasma treatment, the surface energy of
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PT-g-C3N4 increases from 38.71 to 52.77 mN/m. This change can be attributed to the
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increase in the number of polar bonds resulted from the generation of hydrophilic functional groups on the surface of the PT-g-C3N4. This behaviour is in agreement with that found from
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FTIR and XPS analyses.
Figure 5(a) Photos of contact angle measurements using water and ethylene glycol as test liquids; and (b) calculated surface energies for g-C3N4 and PT-g-C3N4.
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ACCEPTED MANUSCRIPT To investigate the photogenerated charge separation and transfer performance, electrochemical impedance spectroscopy (EIS) and photocurrents were measured. The experimental Nyquist impedance plots for g-C3N4 and PT-g-C3N4 without light illumination
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are shown in Figure 6(a). The arc radius on the EIS Nyquist plot can reflect the reaction rate on the surface of the electrode. A smaller arc radius corresponds to a more effective separation of photogenerated electron-hole pairs and a higher efficiency of charge migration
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across the electrode/electrolyte interface.[37] The arc radius for the PT-g-C3N4 electrode is
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smaller than that of the g-C3N4 electrode, suggesting that PT-g-C3N4 has a stronger electronic conductivity in the non-photoexcited state, and would be very beneficial for photoexcited charge separation. Figure 6(b) shows the transient photocurrent responses for g-C3N4 and PT-g-C3N4 under visible light irradiation in an on-and-off cycle mode. It is clearly seen than
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the photocurrent of PT-g-C3N4 is higher than that of g-C3N4, which suggests that the recombination of electrons and holes is greatly impeded. The EIS and photocurrent analyses are consistent with the results of PL results, indicating PT-g-C3N4 might exhibit better
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photocatalytic performance.
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Figure 6 (a) EIS; (b) transient photocurrents of g-C3N4 and PT-g-C3N4 in a 1 M Na2SO4 aqueous solution.
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The photocatalytic performance of g-C3N4 and PT-g-C3N4 with duration of 5.0 min were evaluated by studying the degradation of RhB under visible light irradiation (≥420), as
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shown in Figure 7. Prior to irradiation, the suspensions were magnetically stirred in dark for 30 min to obtain the absorption-desorption equilibrium between the photocatalyst and RhB.
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As depicted in Figure 7(a), the RhB aqueous solution under visible light irradiation is stable, indicating RhB self-degradation is almost negligible in the absence of a photocatalyst. With the presence of g-C3N4 and PT-g-C3N4 photocatalysts, the concentration of RhB decreases dramatically under same experimental conditions. After irradiation of 20 min, the degradation rate of RhB is ~ 45% and ~ 90% for g-C3N4 and PT-g-C3N4, respectively. Hence, the photocatalytic activity of PT-g-C3N4 is 2.0 times higher than that of g-C3N4 for the degradation of RhB. The influence of plasma treatment time (2.5, 5.0, 10, 20 min) on - 16 -
ACCEPTED MANUSCRIPT photocatalytic degradation performance for RhB were tested and depicted in Figure S2. It was found that the photocatalytic activity of g-C3N4 photocatalyst enhances obviously with the increasing plasma treatment time from 0 to 5.0 min. When the duration is longer than 5.0
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min, the enhancement extend of photocatalyti degradation performance is limited. The photocatalytic degradation kinetics was also studied. The degradation of RhB can be attributed to a pseudo-first-order reaction with a simplified Langmuir-Hinshelwood model
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when C0 was very small: -ln(C/C0) = kt, where k is the rate constant. The kinetic rate
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constants are obtained by plotting -ln(C/C0) with the visible light irradiation time (min.), which is shown in Figure 7(b). The kinetic rate constants are evaluated to be 0.102 min-1 for PT-g-C3N4 with duration of 5.0 min, which is 2.83 times higher than that of g-C3N4 (0.036 min-1). In addition, the absorbance spectra demonstrate the mineralization of RhB during the
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photodegradation process with the prescence of PT-g-C3N4 photocatalyst (Figure 7(c)). It is found that the characteristic absorption peak of RhB solution at 554 nm shifts to shorter wavelengths and the intensity decreases under visible light irradiation. These changes
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indicate the remove of an ethyl group from RhB. The absorption peak eventually disappears,
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which implies the removal of the benzene ring of RhB.[12] From Figure 7(d), one can see there is no obvious decrease of photodegradation after four cycles, confirming the high stability of the PT-g-C3N4 photocatalyst. The above results clearly demonstrate that the photocatalytic activity of g-C3N4 has been significantly improved by plasma treatment, which would optimize the surface properties of g-C3N4 in enlarging the specific surface area and increasing the hydrophilicity.
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Figure 7 (a) The photocatalytic degradation of RhB dye; (b) kinetic fit for the degradation of RhB for g-C3N4 and PT-g-C3N4; (c) changes in UV-visible absorption spectra of RhB solution with variable photodegradation reaction time for PT-g-C3N4; (d) recyclability of PT-g-C3N4 photocatalyst for the degradation of RhB.
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The role of reactive species including hole (h+), hydroxyl radical (.OH) and superoxide radical (.O2-) in the degradation of RhB for g-C3N4 photocatalytsts were indentified in our previous work by radicals trapping experiments,[30] using triethanolamine (TEOA),
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isopropanol (IPA), and vacuum conditions as h+, .OH and .O2- scavengers, respectively. To
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further confirme the role of .OH and .O2-, ESR spin-trap technique was employed to determine the active species in PT-g-C3N4. As shown in Figure 8(a) and 8(b), the characteristic peaks of DMPO-.O2- and DMPO-.OH adducts can be detected obviously under visible light for PT-g-C3N4, compared with the sample in dark. And the signal intensities of .OH and .O2- become stronger with the increase of light irradiation time. These results revealed that the .O2- and .OH active species could be generated in PT-g-C3N4, which might play important role in the photocatalytic degradation process.
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Figure 8 ESR spectra of (a) DMPO-.O2-; and (b) DMPO-. OH adducts in PT-g-C3N4 aqueous dispersion.
In order to illuminate the enhancement mechanism, band structures of g-C3N4 and
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PT-g-C3N4 were proposed by evaluating their conduction band and valence band levels with
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Ultraviolet photoelectron spectroscopy (UPS), as shown in Figure 9. The valence band energy (Ev) (equivalent to ionization potential) of g-C3N4 and PT-g-C3N4 is determined to be 6.92 and 7.12 eV by subtracting the width of the He I UPS spectra from the excitation energy (21.22 eV). The conduction band energy Ec is thus estimated at 4.06 and 4.26 eV for g-C3N4 and PT-g-C3N4 from formula Ev-Eg, respectively. The Eg value for g-C3N4 and PT-g-C3N4 was determined to be an equal value at 2.86 eV (Figure S1). The values of Ev and Ec in electron volts are converted to electrochemical energy potentials in volts according to the reference
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ACCEPTED MANUSCRIPT standard for which 0 versus RHE (reversible hydrogen electrode) equals -4.44 eV versus evac (vacuum level). According to these evaluated levels, the schematic energy band illustrations of g-C3N4 and PT-g-C3N4 with the redox potentials of the photocatalytic reaction were
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depicted in Figure 9(b). It is found that plasma treatment down-shifts the valence band of PT-g-C3N4 to a more positive potential than g-C3N4, then leading to the increase of hole oxidability and .OH formation for the photocatalytic degradation of RhB. Based on the above
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experimental results, the down-shift of valence band for PT-g-C3N4 is mainly attributed to the
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introduction of O-containing hydrophilic functional groups induced by plasma treatment.
Figure 9 (a) UPS spectra; and (b) the proposed band structure diagram for g-C3N4 and PT-g-C3N4.
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ACCEPTED MANUSCRIPT 4. Conclusions In summary, plasma treatment was employed to modify the surface properties of a g-C3N4 photocatalyst to enhance its photocatalytic performance. Experimental results showed that
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plasma treatment can exfoliate pristine g-C3N4 into ultrathin nanosheets with an increase in specific surface area. In addition, the surface energy of g-C3N4 was enhanced by plasma treatment through introducing hydrophilic functional groups, resulting in an increase of
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surface hydrophilicity. As a result, the degradation efficiency under visible light irradiation
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for plasma treatment g-C3N4 (PT-g-C3N4) is up to 2.0 times higher than pristine g-C3N4. The remarkably enhanced photocatalytic performance for pollution degradation is attributed to optimization of surface properties induced by plasma treatment. Acknowledgments
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We gratefully acknowledge the financial support by the National Natural Science Foundation of China (nos. 51102265 and 50872091) and “Foreign Experts” Thousand Talents Program
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(Tianjin, China).
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