Tuning the Photocatalytic Activity of Graphitic Carbon Nitride by

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Tuning the Photocatalytic Activity of Graphitic Carbon Nitride by Plasma-Based Surface Modification Xueqiang Ji, Xiaohong Yuan, Jiajie Wu, Lan Yu, Huiyun Guo, Hannian Wang, Haiquan Zhang, Dongli Yu, and Yuanchun Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06637 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 9, 2017

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ACS Applied Materials & Interfaces

Tuning the Photocatalytic Activity of Graphitic Carbon Nitride by Plasma-Based Surface Modification Xueqiang Ji, Xiaohong Yuan, Jiajie Wu, Lan Yu, Huiyun Guo, Hannian Wang, Haiquan Zhang,* Dongli Yu,* and Yuanchun Zhao* State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

KEYWORDS: graphitic carbon nitride, photocatalytic activity, surface modification, plasma treatment, photodegradation

ABSTRACT: In this study, we demonstrate that plasma treatment can be a facile and environment friendly approach to perform surface modification of graphitic carbon nitride (gCN), leading to a remarkable modulation on its photocatalytic activity. The bulk properties of gCN, including the particle size, structure, composition and electronic band structures, have no changes after being treated by oxygen or nitrogen plasma; however, its surface composition and specific

surface

area

exhibit

remarkable

differences

corresponding

to

an

oxygen

functionalization induced by the plasma post-treatment. The introduced oxygen functional

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groups play a key role to reduce the recombination rate of the photoexcited charge carries. As a consequence, oxygen-plasma treated sample shows a much superior photocatalytic activity, which is about 4.2 times higher than that of the pristine g-CN for the degradation of Rhodamine B (RhB) under visible light irradiation; while the activity of nitrogen-plasma treated sample exhibits a slight decrease. Furthermore, both the plasma-treated samples are found to possess impressive photocatalytic stabilities. Our results suggest that plasma treatment could be a conventional strategy to perform surface modification of g-CN in forms of both powders and thin films, which holds broad interests not only for developing g-CN-based high-performance photocatalysts, but also for constructing photoelectrochemical cells and photoelectronic devices with improved energy conversion efficiencies.

1. INTRODUCTION In recent years, graphitic carbon nitride (g-CN) has been attracted worldwide attention as a metal-free, multipurpose phtotocatalyst due to its high thermal/chemical stability, low cost and appealing electronic band structures.1−3 g-CN is a tri-s-triazine-based polymeric semiconductor, the incorporation of sp2-hybridized carbon and nitrogen establishes a specific π-conjugated system with a moderate band gap of ca. 2.7 eV, which can efficiently absorb and utilize light up to 460 nm to drive various photocatalytic reactions such as water splitting,4−6 CO2 reduction,7,8 degradation of organic pollutants9,10 and bacteria disinfection.11,12 However, the practical applications of g-CN are still hindered by the high recombination rate of the photogenerated charge carries and thus the relatively low photocatalytic activity. Therefore, great efforts have been made to further modify the chemical structure and/or surface properties of g-CN, such as designing specific nanostructures,13−15 controlling the morphologies,16-18 doping with


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heteroatoms19,20 and copolymerizing with other organic molecules,21,22 ect. As a result, the photocatalytic activity of g-CN has been properly improved. g-CN can be facilely synthesized via a direct thermal condensation of nitrogen-rich precursors.1,4,23 The promoted polycondensation of tri-s-triazine units produces a closely assembled layer structure that further bonded through the optimized van der Waals interactions, resulting in an extremely high chemical inertness similar to that of graphite.2,24 As a result, posttreatment strategies to modify the pristine g-CN generally require high temperature or severe acid environments. For example, highly disordered or even amorphous carbon nitride were obtained by a post-heating treatment of g-CN in argon atmosphere up to 620 °C, substantially suppressing the radiative electron-hole recombination and thus enhancing its photocatalytic activity.25,26 In addition, pristine g-CN can also be protonated by strong oxidative acids such as HCl, HNO3 and H2SO4,27-29 which is found to be effective to adjust the electronic band structures, provide higher surface area and better dispersion in aqueous solutions that available for sol-gel chemistry. Recently, activation of g-CN by a H2O2 hydrothermal procedure has been reported, and

the

obtained

oxygen-functionalized

g-CN

showed

an

improved

photocatalytic

performance.30,31 A facile and environmentally friendly modification strategy for g-CN, however, is still lacking. In the past several decades, plasma treatment has been developed to be a conventional technology in the area of materials processing. Nonthermal plasmas are of special interests for low-temperature surface treatment, because they still consist of highly active species for reaction with different surfaces.32-34 Polymeric materials are essentially suitable for plasma-based modification and functionalization, by which their surfaces can be modified without changing the bulk properties.35 Generally, oxygen-plasma treatment introduces carboxylic acid and


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hydroxyl groups to the polymer surface, and also leads to more pronounced changes in the surface microstructures due to its high oxidation nature; while nitrogen plasma exhibits lower etching effect and introduces amine and amide functional groups to the surface.32,36 Plasma treatment has also been used for functionalization of carbon-based nanomaterials such as carbon nanotubes37 and graphene,38 to modulate their surface defects and improve the electronic properties. Considering the polymeric nature of g-CN, it is expected that plasma treatment could also be effective for the surface modification of this tri-s-triazine-based system. However, to the best of our knowledge, research on modulation of the photocatalytic activity of g-CN by plasma post-treatment has not been reported yet. Herein, we report that plasma-based surface modification is effective and particularly suitable for modulating the photocatalytic activity of g-CN. We found that both oxygen- and nitrogenplasma treatments have no influence on the bulk properties of g-CN but remarkably change the corresponding surface properties including the surface composition and specific surface area. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra of the samples revealed a reduced recombination rate of the photogenerated electrons and holes after plasma treatment. Photocatalytic activities of the pristine and plasma-treated samples were evaluated by photodegradation of Rhodamine B (RhB), and the oxygen plasma-treated sample was found to possess a much superior activity than that of the pristine and nitrogen plasma-treated samples. The photocatalytic stability of the plasma-treated g-CN has also been investigated. 2. EXPERIMENTAL SECTION 2.1. Synthesis. Bulk g-CN was synthesized by using melamine as the starting material via a direct thermal condensation process.23,39 Melamine (9 g in powder) in an open quartz boat was heated to 550 °C in a nitrogen atmosphere with a ramp rate of 10 °C/min and held for 4 h. After


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naturally cooling down to the room temperature, a yellow agglomerated sample was obtained with a yield of ~61 %. The agglomerate was carefully milled into fine powders in an agate mortar for further use, which is denoted as CN. 2.2. Plasma Treatment. A low-pressure plasma system (Femto SRS, Diener Electronic, Germany) was employed to perform the post-treatment of g-CN. The plasmas were created by a RF generator with a frequency of 13.56 MHz and an output power of 80 W. The discharge chamber was a stainless steel cylinder with an inner diameter of 100 mm and a length of 270 mm, and an aluminum plate with a length of 235 mm and a width of 75 mm was used as the standard electrode. With the aid of a sample sieve, g-CN powders were uniformly spread on the bottom of a glass culture vessel, which was placed below the standard electrode with a distance of ~40 mm. Oxygen and nitrogen gases of 99.999 % purity were respectively introduced into the discharge chamber to produce the required plasmas, and the pressure was maintained at 30 Pa. The pristine sample powders were exposed in oxygen or nitrogen plasma for 30 min. The treated samples are denoted as CN-O and CN-N, respectively. 2.3. Characterization. X-ray diffraction (XRD) patterns of the samples were collected by using a Rigaku D/max-2500/PC X-ray diffractometer (Cu Kα1 irradiation, λ = 1.540598 Å). Fourier transform infrared (FTIR) spectra were recorded on a Bruker Equinox 55 FTIR spectrometer at ambient conditions, and the sample powders were mixed with KBr at a concentration of ca. 1 wt.%. A Shimadzu UV-2550 spectrophotometer was employed to detect the UV–vis diffuse reflectance spectra of the samples by using BaSO4 as the reference. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Thermo Scientific ESCALAB 250Xi spectrometer with a monochromatic Al Kα X-ray source, and the binding energies were calibrated by the C 1s peak of surface adventitious carbon at 284.6 eV. Elemental


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analyses (EA) were performed by using on a C/H/N elemental analyzer (Vario MACRO cube EL). In order to examine the reproducibility of plasma treatment, three different batches of samples were measured by both XPS and EA, respectively. The Brunauer–Emmett–Teller (BET) surface area was calculated from the nitrogen adsorption–desorption isotherms measured at 77 K using a Quantachrome Autosorb-IQ-2-XR analyzer. Photoluminescence (PL) emission spectra were recorded at room temperature using a Perkin Elmer LS55 fluorescence spectrophotometer with an excitation at 325 nm. The microstructures of the samples were investigated by using a JEOL JEM-2010 transmission electron microscope (TEM) with an accelerating voltage of 200 kV. Electron paramagnetic resonance (EPR) measurements were carried out on a Bruker A300 spectrometer at room temperature. 2.4 Photoelectrochemical measurements. 5 mg of the sample powders were dispersed in 1 mL ethanol with 50 µL Nafion solution (5%) by sonication for 30 min, and then 5 µL of the obtained slurry was spread on a standard glassy carbon electrode with a diameter of 3 mm. The measurements were conducted on a CHI 650E electrochemical workstation using a conventional three-electrode cell with a platinum plate as the counter electrode and an Ag/AgCl (saturated KCl) electrode as the reference. The electrolyte was a 0.5 M Na2SO4 aqueous solution. Electrochemical impedance spectroscopy (EIS) was measured at a bias potential of 0.5 V in the frequency range from 105 to 0.1 Hz with an amplitude of 5 mV. A 300W Xe lamp (PLS-SXE300UV, Beijing Trusttech Co. Ltd.) attached with a long-pass cutoff filter (λ > 420 nm) was used as the light source for the photocurrent measurement at a bias voltage of 0.5 V. 2.5. Photocatalytic Activity Measurements. Photocatalytic activities of the pristine and plasma-treated samples were evaluated by photodegradation of RhB. Catalyst powders (5 mg) were dispersed in an RhB aqueous solution (10 mL, 1×10-5 M). The suspension was stirred in the


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dark for 60 min before exposure to the light. The light source was the same system used for the photocurrent measurements, and the reaction apparatus was continuously cooled by a circulating water system to eliminate the temperature effect. The suspension was withdrawn regularly at a time interval of 10 min to measure the UV-vis absorption spectra by using a Perkin Elmer Lambda 35 spectrophotometer. The degradation efficiencies of RHB were evaluated by monitoring the intensity evolution of the maximum absorption peak. 3. RESULTS AND DISCUSSION SEM observation reveals that the g-CN sample is composed of various particles with sizes ranged from several to 10 µm and most of them are of a flake-like morphology (Supporting Information, Figure S1), which is consistent with the previous reports.30,40 After being treated by oxygen or nitrogen plasma, the featured morphology and particle size distribution of the pristine sample have been unambiguously retained; meanwhile the changes of the sample weight before and after plasma treatment are not detectable. This can be explained by the high thermal/chemical stability of g-CN as well as the relatively low output power of the plasmas (80 W) used in this study. Figure 1 shows the XRD patterns of the pristine CN as well as the plasma-treated samples of CN-O and CN-N. All patterns exhibit two dominant diffraction peaks at ~27.5° and ~13.1°, corresponding to a typical g-CN layered structure.1,4 The former is attributed to the interplanar stacking of the tri-s-triazine-based layer system and assigned as the (002) diffraction peak with d = 0.322 nm; while the latter with a weaker intensity is related to the in-plane structural assembling motifs and can be indexed as the (210) peak with d = 0.676 nm.41 The obtained XRD patterns of the plasma-treated samples are almost identical to that of the pristine one, indicating


7


both oxygen- and nitrogen-plasma treatments have no influences on the bulk structure of g-CN powders.

Intensity (a.u.)

CN-N CN-O CN

10

20

30

40

50

60

70

2θ (degree)

Figure 1. XRD patterns of the pristine CN, oxygen-plasma treated CN-O and nitrogen-plasma treated CN-N.

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CN-N CN-O CN 3600

3000

2400

1800

1200

600

-1

Wavenumber (cm )

Figure 2. FTIR spectra of the pristine CN, oxygen-plasma treated CN-O and nitrogen-plasma treated CN-N. The measured FTIR spectra of the pristine and plasma-treated samples are shown in Figure 2, which further confirms that the effect of plasma treatment on the chemical structure of bulk g-


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CN powders is neglectable. The spectra exhibit typical IR absorptions for g-CN.23,42 The main absorption bands in range of 1150−1650 cm-1 are related to the characteristic C–N and C=N stretching vibrations from the skeletal tri-s-triazine units; while the sharp peak at ~808 cm-1 corresponds to breathing vibration mode of the tri-s-triazine system. Moreover, the broad brands ranged from 2900 to 3400 cm-1 can be assigned to the stretching vibrations of the uncondensed primary and secondary amino groups. The only notable difference among the spectra is that, the main absorption bands (as indicated in the rectangular area in Figure 2) of CN-O become slightly sharper, probably resulting from the selective removal of the disordered or amorphous components of the sample during oxygen plasma treatment,36 making the featured absorption peaks a little more pronounced. In general, however, the recorded three FTIR spectra are almost same and in good agreement with the XRD results.

(b)

C 1s

(c)

N 1s

CN-N

C3N

Cad

Intensity (a.u.)

CN-N

CN-O

O 1s

CN-O N3C

N2C

CN-N Intensity (a.u.)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CN-O Oad

OO-C

NN-H

CC-O CN

CN

CN

292 290 288 286 284 282 280

404 402 400 398 396 394

538 536 534 532 530 528 526

Binding energy (eV)

Binding energy (eV)

Binding energy (eV)

Figure 3. Deconvoluted (a) C 1s, (b) N 1s and (c) O 1s high-resolution XPS spectra of the pristine CN, oxygen-plasma treated CN-O and nitrogen-plasma treated CN-N.


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X-ray photoelectron spectroscopy was used to investigate the variations on the surface chemical states of g-CN before and after plasma treatment. The measured survey XPS spectra indicate all the samples only contain C, N and O elements (Supporting Information, Figure S2), and the deconvoluted high-resolution spectra are shown in Figure 3 (see Table S1 in Supporting Information for the details of data processing). As far as the pristine CN is concerned, the C 1s spectrum can be well fitted by three peaks centered at 289.0 eV for a trace amount of C–O bonds (CC–O), 287.9 eV for the three-coordinated carbon (C3N) in the tri-s-triazine ring and 284.6 eV originating from the adventitious graphitic carbon (Cad), respectively.20,30,43 The N 1s spectrum can also be deconvoluted into three peaks at 398.3 eV for C–N=C bonding state in the CN heterocycle (N2C), 399.7 eV for three-coordinated nitrogen (N3C) in the tri-s-triazine unit and 400.8 eV arising from the nitrogen in amino groups (NN–H). Moreover, its O 1s spectrum includes two deconvoluted peaks at 531.8 eV for the N–C–O groups (OO–C) and 533.2 eV for the adsorbed water and oxygen (Oad), respectively. To clarify the variations of each bonding state induced by the plasma treatment, we performed a detailed analysis on the deconvoluted spectra and the results are summarized in Table 1. As shown in Figure 3a, after plasma treatment the main C 1s peak (in the range of 286 to 290 eV) is slightly broaden toward the higher binding energy and thus presents a larger component of CC–O bonding state, revealing an newly induced oxygen surface functionalization; meanwhile, the sample surfaces are more active for graphitic carbon, hence the contribution of Cad also exhibits an increase (Table 1). For N 1s spectra shown in Figure 3b, both NN–H components of CN-O and CN-N show an obvious decrease, demonstrating that the amino groups outside the tri-s-triazine unit have been partially modified by the introduced functional groups such as carboxylic acid and hydroxyl for CN-O, or the free radicals for CN-N that will further react with the atmospheric oxygen to produce N–C–O


10

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Table 1. Comparison on the Relative Integrated Content of Each Bonding State Determined in the Deconvoluted C 1s, N 1s and O 1s XPS Spectra of Pristine CN, Plasma-Treated Samples CN-O and CN-N. C 1s

N 1s

O 1s

Cad (%) (284.6 eV)

C3N (%) (287.9 eV)

CC–O (%) (289.0 eV)

N2C (%) (398.3 eV)

N3C (%) (399.7 eV)

N N–H (%) (400.8 eV)

OO–C (%) (531.8 eV)

Oad (%) (533.2 eV)

CN

42.2

56.1

1.7

72.4

19.0

8.6

64.7

35.3

CN-O

47.7

49.1

3.2

71.9

22.0

6.1

72.8

27.2

CN-N

45.8

51.3

2.9

66.3

27.8

5.9

71.5

28.5

samples

functional groups.32 It should be noted that: i) the binding energy of the introduced N–C–O groups are essentially overlapped with the N3C state and difficult to be distinguished in the N 1s spectra of CN-O and CN-N,30,36 whereas leading to an corresponding increase (Table 1); ii) nitrogen plasma seems to be impossible to introduce amines or amides into this specific C–N–H system, but serves as an “inert” plasma similar to that of Ar and He to create active free radicals on the surface; iii) nitrogen plasma could be effective to remedy the existed nitrogen vacancies through an ion-implantation process32,38 to form new three-coordinated nitrogen state, further reducing the N2C intensity and increasing that of the N3C component for CN-N,44 as shown in Table 1. Last but not the least, as shown in Figure 3c, after plasma treatment the OO–C components shows an expected increase, which consequently suppresses the Oad contribution. To sum up, the XPS results reveal that the plasma treatment indeed implements a selective surface modification and introduces oxygen functional groups into the polymeric g-CN material.


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Table 2. Atomic C/N Ratio and O Composition Respectively Determined by EA and XPS measurements as well as the Specific Surface Area (SBET) of Pristine CN, Plasma-Treated Samples CN-O and CN-N. XPSa

EA C/N (atomic)

SDb

C/N (atomic)

SD

O (at. %)

SD

SBET (m2 g-1)

CN

0.67

0.0024

0.76

0.0126

4.85

0.0075

15.3

CN-O

0.68

0.0015

0.78

0.0135

7.48

0.0056

17.6

CN-N

0.67

0.0022

0.73

0.0154

6.34

0.0079

12.5

samples

a

Components of the adventitious graphitic carbon (Cad) and adsorbed water/oxygen (Oad) in the corresponding XPS spectra have been excluded to determine the atomic C/N ratio as well as the O composition. bStandard deviation (SD) is obtained from the EA and XPS data measured on three batches of the samples: SD = ∑ − /2/ , = ∑ /3, = 1, 2, 3 . Elemental analyses of the obtained samples provide convincing evidence that both oxygenand nitrogen-plasma treatments scarcely change the bulk composition of g-CN (Table 2), and the measured atomic C/N ratio of ~0.67 is well consistent with the previous reports by using melamine as the raw material.39,41 However, the C/N ratios determined by XPS are typically higher and associated with relatively larger experimental errors, probably due to the specific uncertainty of XPS measurements. Furthermore, the corresponding surface composition determined by XPS measurements highlights the modification effect of plasma treatment on the sample surfaces, As shown in Table 2, after oxygen-plasma treatment the corresponding atomic C/N ratio increases from 0.76 to 0.78, which could be explained by the preferential removal of amino groups from the sample surface. While the one for CN-N decreases to 0.73, probably due to the aforementioned nitrogen-plasma-induced remediation of N3C vacancies; meanwhile, the amino groups outside the tri-s-triazine unit will be partially modified to produce free radicals, which will further react with the atmospheric oxygen.32 As a result, the surface oxygen composition of both CN-O and CN-N exhibits a corresponding increase.


12

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Figure 4. TEM images showing the surface morphologies of (a) pristine CN, (b) oxygen-plasma treated CN-O and (c) nitrogen-plasma treated CN-N. The top-right insert in each image presents the corresponding selected area electron diffraction (SAED) pattern. The measured specific Brunauer−Emmett−Teller surface areas (SBET) of the samples are also listed in Table 2. The pristine CN possesses a SBET of 15.3 m2/g, which is typically higher than the previously reported values2,40 because in this study, it has been sufficiently milled into finer powders to facilitate a uniform plasma-based surface modification. The specific surface area of CN-O is found to be 17.6 m2/g, while for CN-N it decreases to 12.5 m2/g (see Figure S3 in Supporting

Information

for

the

corresponding

N2

adsorption–desorption

isotherms).

Subsequently, the surface morphology and microstructure of the samples has been carefully investigated by TEM coupled with selected area electron diffraction (SAED), which provides a convinced explanation on the variation of SBET induced by plasma treatment. It should be noted that all the sample powders were directly dispersed in ethanol and dropped onto the TEM grids, by which the original texture of the samples can be delicately retained. As shown in Figure 4a, the observed thin flake area of pristine CN is of a distinct layered texture, presenting the clue of mechanical exfoliation of the sample particles. The surface of the CN-O sheet area (Figure 4b), however, exhibits a uniformly flaky texture with a higher roughness arising from the effective surface etching by oxygen-plasma treatment, which leads to a higher SBET. Nevertheless, nitrogen


13


plasma is less effective for surface etching but could concurrently remedy the nitrogen vacancies in the sample surface through the ion-implantation process. Therefore, a rather smooth surface has been observed in the thin area of CN-N (Figure 4c), which thus produces a relatively lower SBET. Moreover, the corresponding SAED patterns for CN-O and CN-N indicate that the graphitic structure of pristine CN has been retained after the plasma treatment.

Absorbance (a.u.)

KM (a.u.)

(a) 2.71

2.4

2.72 2.7 3.0 3.3 Energy (eV)

3.6

CN-N CN-O CN 400

500

600

700

800

Wavelength (nm) (b)

CNN CNO CN

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.74 eV

20

15

10

5

0

-5

Binding energy (eV)

Figure 5. (a) UV-vis reflectance spectra and (b) XPS valence band spectra of the pristine CN, oxygen-plasma treated CN-O and nitrogen-plasma treated CN-N. The inset in (a) shows the corresponding Kubelka−Munk function plot vs the energy of the absorbed light for the bandgap determination.


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Figure 5a shows the UV-vis reflection spectra of the as-prepared samples. All the measured spectra show a typical UV-vis absorption edge around 450 nm for semiconducting g-CN, corresponding to the visible-light induced electronic transition from the valence band to the conduction band. The bandgap of pristine CN is found to be about 2.72 eV, which is well consistent with that of previous reports.4,24 After plasma treatment, both spectra of CN-O and CN-N show a very slight redshift corresponding to a bandgap of 2.71 eV, which could be attributed to the plasma-induced surface oxygen functionalization.30,31 The XPS valence band spectra of the samples are shown in Figure 5b, indicating the valence band of the three samples are all at 1.74 eV, which is well consistent with the previous works.25,44 Overall, the plasmabased surface modification does not notably change the electronic band structures of g-CN. Photoluminescence (PL) spectra of the as-prepared samples were measured to investigate the separation and recombination behaviors of the photoexcited charge carries. As shown in Figure 6a, all the samples present similar band-to-band PL characteristics with an emission peak appeared around 470 nm, further confirming their band structures basically keep unchanged before and after plasma treatment. However, the corresponding emission intensities reveal a significant difference. The PL emission of CN-O has been apparently suppressed, suggesting a much lower radiative recombination rate of the photogenerated electrons and holes. This can be explained by the efficient electron-trapping feature of the introduced oxygen functional groups,20,31 which could promote the separation of the electrons and holes migrated to the surface and thus inhibit their recombination. As discussed above, nitrogen-plasma treatment will also induce a less effective oxygen functionalization on the sample surface, therefore, the PL intensity of CN-N also exhibits a decrease but is less pronounced than that of CN-O. Electron paramagnetic resonance (EPR) was also employed to investigate the unpaired electrons in the


15


(a)

Intensity (a.u.)

CN CN-N CN-O

400 (b)

450 500 550 Wavelength (nm)

600

CN-O CN-N CN

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Dark

Light

3450 3525 3600 3450 3525 3600 Magnetic field(Gaussian)

Figure 6. (a) Fluorescence emission spectra of the pristine CN, oxygen-plasma treated CN-O and nitrogen-plasma treated CN-N. The excitation wavelength for fluorescence emission spectra was 325 nm. (b) Electron paramagnetic resonance (EPR) spectra of the three samples in the dark and visible light irradiation (λ > 420 nm), respectively. samples with or without visible light illumination, respectively. As shown in Figure 6b, one single Lorentzian line centered g = 2.0032 has been detected for each sample, corresponding to the generated conduction electrons in the localized π system of g-CN.14 Enhanced EPR signals were observed for the samples treated by both nitrogen and oxygen plasmas, which is consistent with the PL results.


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(a)

360

-Z'' (kΩ)

270

CN CN-N CN-O

180 90 0 0

30

60

90

Z' (kΩ) (b)

CN-O CN-N CN

0.25

-2

Current density (µA/cm )

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0.20

off

0.15 0.10 0.05 0

on 200

400 Time (s)

600

Figure 7. (a) Nyquist plots of electrochemical impedance spectroscopy (EIS) of the pristine CN, oxygen-plasma treated CN-O and nitrogen-plasma treated CN-N. (b) Photocurrent response of the as-prepared samples measured in the dark or under visible light irradiation (λ > 420 nm). Figure 7a shows the electrochemical impedance spectra of the as-prepared samples. The measured semicircular Nyquist plots for the pristine CN as well as the treated CN-N and CN-O present decreasing diameters, indicating the plasma-treated samples possess lower resistances for charge transport compared with the pristine one.18,21 The photocurrent response has further been measured to investigate the photoinduced charge transfer process of the samples. As shown in Figure 7b, the plasma-treated samples indeed generate larger photocurrents under visible light


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irradiation, illustrating the lower recombination rates and faster transport process of the photoexcited charge carries. 1.0 No catalyst

0.8 0.6 C/C0

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0.4

CN-N

0.2 CN CN-O

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20

40 Time (min)

60

80

Figure 8. Photocatalytic activities of the pristine CN, oxygen-plasma treated CN-O and nitrogenplasma treated CN-N evaluated by the degradation of RhB under visible light irradiation (λ > 420 nm). The photocatalytic activities of the samples were evaluated by monitoring the degradation of RhB under visible light irradiation (λ > 420 nm), as shown in Figure 8. As expected, CN-O shows the best photocatalytic performance, and the RhB can be completely decomposed within 30 min exposed under visible light irradiation, while around 51 % RhB has been degraded by the pristine CN under the identical condition. However, nitrogen-plasma treatment is found to reduce the photocatalytic activity of CN-N, after 30 min of visible light irradiation only about 32 % RhB has been decomposed; even prolonging the irradiation time to one hour, the degradation is found to be just proceeded over 60 %. The corresponding time-dependent UV-vis absorption spectra for RhB photodegradation are displayed in Figure S4 (Supporting Information). The degradation constant (~0.072 min-1) of RhB with CN-O is estimated to be about 4.2 and 6.5 times larger than


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that with the pristine CN (~0.017 min-1) and CN-N (~0.011 min-1) catalysts, respectively. It has been reported that a H2O2 hydrothermal treatment can also facilitate oxygen functionalization of g-CN.30,31 Under similar experimental conditions (with visible light irradiation in air), the degradation efficiencies of the treated samples were found to be about 6 times for RhB and 4 times for methyl blue (MB) higher than that of the pristine one, respectively. Note that the H2O2 treatment has obviously changed the bulk properties of g-CN such as the structure and the optical band gap, which is very different from the plasma-based surface modification used in this study. Therefore, our work further reveals the importance of surface states to affect the pohotcatalytic activity of g-CN. A control experiment has been performed to reveal the difference of RhB solution before and after degradation by CN-O under visible light irradiation for 60 min, illustrating that the RhB has been fully degraded (Supporting Information, Figure S5). We have further investigated the plasma-treatment time effect on the photocatalytic activity of CN-O. As shown in Figure S6, the degradation efficiency of RhB exhibits a gradual increase with the plasma-treatment time and becomes stable after the catalyst has been treated for 30 min. Moreover, the as-prepared samples have been employed to degrade 4-chlorophenol, and the results also indicate that the CN-O possesses the best photocatalytic activity (Supporting Information, Figure S7). It is well known that the activity of a photocatalyst can be affected by its particle size, specific surface area, electronic band structure as well as the surface chemical structures.44 The featured nonthermal plasma treatment used in this study induces a significant surface modification and functionalization; meanwhile the bulk properties of pristine CN, such as the particle size distribution, structure, composition and band structure, are basically kept unchanged. This allows us to perform a convinced analysis to reveal the origin of the remarkable modulation on their


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photocatalytic activities induced by plasma post-treatment. It is suggested that the activities of the plasma-treated samples are affected by not only the variation on the specific surface area, but also the newly introduced oxygen functional groups. After photoexcitation, a certain fraction of the generated electrons and holes will migrate to the surface of the catalyst, whereas they might not have the chance to react with the RhB molecules but still conduct a recombination. It has been reported that the introduced oxygen functional groups can efficiently trap the electrons migrated to the surface and promote the separation of the excitons, which leads to a lower recombination rate of the photogenerated electrons and holes (Figure 6).20,31 The separated electrons further induce a multistep reduction of the molecular oxygen in the aqueous solution and generates •OH radicals, which, together with the separated holes, drives the degradation of RhB.9,19 As for CN-O catalyst, the significant oxygen surface functionalization can efficiently improve its photocatalytic performance and the corresponding larger SBET also plays a positive role. As consequence, CN-O catalyst exhibits a much superior photocatalytic activity. On the other hand, nitrogen-plasma treatment is found to be less effective to perform surface oxygen functionalization while leads to a decreased SBET, this counteraction finally results in a relative lower photocatalytic activity of CN-N.


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0.12

0.08

0.10

CN-O CN CN-N

0.08 0.06 0.04 0.02 0.00

0 1

3 5 Time (day)

7

(b)

-1

(a)

Degrad. constant (min )

-1

Degrad. constant (min )

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0.06

0.04

0.02

0.00 CN-N

CN

CN-O

Figure 9. Photocatalytic stability of the as-prepared samples. (a) The degradation constant determined from the corresponding RhB photodegradation for each g-CN sample continually measured over 7 days. (b) Statistical data based on the obtained degradation constants for each sample. Ageing effect is very common in the plasma-modified polymer surfaces, because the newly introduced functional groups are usually unstable with time and the treated surface is inclined to recover to its untreated state.36 Therefore, it is crucial to test the photocatalytic stabilities of the plasma-treated polymeric g-CN samples. We have continually measured the RhB degradation performance of the as-prepared catalysts under identical conditions for one week using the same batch of samples. As shown in Figure 9, both the CN-O and CN-N catalysts, as well as the pristine CN, show impressive photocatalytic stabilities (see the raw experimental data in Supporting Information, Figure S8). It has been reported that the mesoporous TiO2 films treated with N2/Ar plasma exhibited an enhanced photocatalytic performance, which was attributed to the induced nitrogen doping as well as the promoted oxygen vacancies.45-47 Therefore, the plasma-induced enhancement on the photocatalytic activity of inorganic catalysts is of a completely different mechanism in comparison to that of the polymeric g-CN reported in this


21


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study. Furthermore, the unexpected photocatalytic stability of the plasma-treated sample highlights the novelty of this post-treatment strategy, which apparently arises from the unique thermal/chemical stability of g-CN. From this point of view, oxygen-plasma surface modification is of particular advantages for efficiently enhancing the photocatalytic performance of g-CN. 4. CONCLUSIONS In summary, we have presented a facile and environmental friendly strategy to modulate the photocatalytic activity of polymeric g-CN by plasma-based surface modification. It is found that both oxygen- and nitrogen-plasma treatments have no influence on the bulk properties of g-CN but remarkably change the corresponding surface properties. Oxygen plasma treatment not only produces a higher surface area but also efficiently reduces the recombination probability of the photogenerated electron and holes migrated to the surfaces. As a consequence, the treated sample CN-O shows an apparently enhanced photocatalytic activity with an RhB degradation constant ~4.2 times higher than that of the pristine one. On the other hand, nitrogen plasma treatment reduces the surface area of the sample and leads to a suppressed photocatalytic activity. Furthermore, both the plasma-treated samples exhibit an impressive stability on their photocatalytic performances thanks to the unique high thermal/chemical stability of g-CN. Therefore, plasma-based surface modification could be a conventional and particularly suitable strategy for surface modification of g-CN system including powders and thin films, which would be of broad interests for developing g-CN-based high-performance photocatalysts as well as photoelectrochemical cells and photoelectronic devices.48,49


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ******. SEM image showing the particle size of g-CN, XPS survey spectra, elemental analysis results, N2 adsorption–desorption isotherms, time-dependent UV-vis absorption spectra for RhB photodegradation, and RhB degradation curves showing the photocatalytic stability (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H.Q.Z.). *E-mail: [email protected] (D.L.Y.). *E-mail: [email protected] (Y.C.Z.). Author Contributions Y.C.Z conceived the project. Y.C.Z, D.L.Y and H.Q.Z. discussed and designed the experiments. X.Q.J., X.H.Y and J.J.W. prepared the samples and carried out the characterization; X.Q.J. and H.N.W. performed the plasma treatment; X.Q.J, L.Y. and H.Y.G. conducted the photodegradation of RhB. All authors discussed the results. The manuscript was written mainly by Y.C.Z. and X.Q.J., and all authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This research was financially supported by the “100 Talents Project” of Hebei Province (Grant No. E2015100009), National Natural Science Foundation of China (Grant Nos. 51572235, 51332005, 51121061 and 91022029) and Natural Science Foundation of Hebei Province (Grant No. E2015203285). REFERENCES (1) Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; Muller, J.-O.; Schlogl, R.; Carlsson, J. M. Graphitic Carbon Nitride Materials: Variation of Structure and Morphology and Their Use as Metal-Free Catalysts. J. Mater. Chem. 2008, 18, 4893–4908. (2) Cao, S.; Low, J.; Yu, J.; Jaroniec, M. Polymeric Photocatalysts Based on Graphitic Carbon Nitride. Adv. Mater. (Weinheim, Ger.) 2015, 27, 2150−2176. (3) Ong, W.-J.; Tan, L.-L.; Ng, Y. H.; Yong, S.-T.; Chai, S.-P. Graphitic Carbon Nitride (gC3N4)‑Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Chem. Rev. 2016, 116, 7159−7329. (4) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76−80. (5) Liu, J.; Liu, Y.; Liu ,N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a TwoElectron Pathway. Science 2015, 347, 970–974. (6) Zhang, G.; Lan, Z.; Lin, L.; Lin, S.; Wang, X. Overall Water Splitting by Pt/g-C3N4 Photocatalysts without Using Sacrificial Agents. Chem. Sci. 2016, 7, 3062–3066.


24

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Kuriki, R.; Sekizawa, K.; Ishitani, O.; Maeda, K. Visible-Light-Driven CO2 Reduction with Carbon Nitride: Enhancing the Activity of Ruthenium Catalysts. Angew. Chem., Int. Ed. 2015, 54, 2406−2409. (8) Kuriki, R.; Ishitani, O.; Maeda, K. Unique Solvent Effects on Visible-Light CO2 Reduction over Ruthenium(II)-Complex/Carbon Nitride Hybrid Photocatalysts. ACS Appl. Mater. Interfaces 2016, 8, 6011−6018. (9) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation of Rhodamine B and Methyl Orange over Boron-Doped g-C3N4 under Visible Light Irradiation. Langmuir 2010, 26, 3894−3901. (10) Liu, J.; Xie, S.; Geng, Z.; Huang, K.; Fan, L.; Zhou, W.; Qiu, L.; Gao, D.; Ji, L.; Duan, L.; Lu, L.; Li, W.; Bai, S.; Liu, Z.; Chen, W.; Feng, S.; Zhang, Y. Carbon Nitride Supramolecular Hybrid Material Enabled High-Efficiency Photocatalytic Water Treatments. Nano Lett. 2016, 16, 6568−6575. (11) Huang, J.; Ho, W.; Wang, X. Metal-Free Disinfection Effects Induced by Graphitic Carbon Nitride Polymers under Visible Light Illumination. Chem. Commun. 2014, 50, 4338−4340. (12) Muñoz-Batista, M. J.; Fontelles-Carceller, O.; Ferrer, M.; Fernández-García, M.; Kubacka, A. Disinfection Capability of Ag/g-C3N4 Composite Photocatalysts under UV and Visible Light Illumination. Appl. Catal., B 2016, 183, 86−95. (13) Jin, X.; Balasubramanian, V. V.; Selvan, S. T.; Sawant, D. P.; Chari, M. A.; Lu, G. Q.; Vinu, A. Highly Ordered Mesoporous Carbon Nitride Nanoparticles with High Nitrogen Content: A Metal-Free Basic Catalyst. Angew. Chem., Int. Ed. 2009, 48, 7884−7887. (14) Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.; Wang, X. Bioinspired Hollow Semiconductor Nanospheres as Photosynthetic Nanoparticles. Nat. Commun. 2012, 1139.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

(15) Liu, J.; Huang, J.; Zhou, H.; Antonietti, M. Uniform Graphitic Carbon Nitride Nanorod for Efficient Photocatalytic Hydrogen Evolution and Sustained Photoenzymatic Catalysis. ACS Appl. Mater. Interfaces 2014, 6, 8434−8440. (16) Zhao, Y.; Liu, Z.; Chu, W.; Song, L.; Zhang, Z.; Yu, D.; Tian, Y.; Xie, S.; Sun, L. LargeScale Synthesis of Nitrogen-Rich Carbon Nitride Microfibers by Using Graphitic Carbon Nitride as Precursor. Adv. Mater. (Weinheim, Ger.) 2008, 20, 1777– 1781. (17) Zhao, Z.; Dai, Y.; Lin, J.; Wang, G. Highly-Ordered Mesoporous Carbon Nitride with Ultrahigh Surface Area and Pore Volume as a Superior Dehydrogenation Catalyst. Chem. Mater. 2014, 26, 3151−3161. (18) Liang, Q.; Li, Z.; Yu, X.; Huang, Z.-H.; Kang, F.; Yang, Q.-H. Macroscopic 3D Porous Graphitic Carbon Nitride Monolith for Enhanced Photocatalytic Hydrogen Evolution. Adv. Mater. (Weinheim, Ger.) 2015, 27, 4634−4639. (19) Zhang, J.; Zhang, M.; Zhang, G.; Wang, X. Synthesis of Carbon Nitride Semiconductors in Sulfur Flux for Water Photoredox Catalysis. ACS Catal. 2012, 2, 940−948. (20) Huang, Z.-F.; Song, J.; Pan, L.; Wang, Z.; Zhang, X.; Zou, J.-J.; Mi, W.; Zhang, X.; Wang, L. Carbon Nitride with Simultaneous Porous Network and O-Doping for Efficient SolarEnergy-Driven Hydrogen Evolution. Nano Energy 2015, 12, 646−656. (21) J. D.; Fu, X.; Antonietti, M.; Wang, X. Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization. Angew. Chem., Int. Ed. 2010, 49, 441−444. (22) Ho, W.; Zhang, Z.; Lin, W.; Huang, S.; Zhang, X.; Wang, X.; Huang, Y. Copolymerization with 2,4,6-Triaminopyrimidine for the Rolling-up the Layer Structure, Tunable Electronic Properties, and Photocatalysis of g-C3N4. ACS Appl. Mater. Interfaces 2015, 7, 5497−5505.


26

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Zhao, Y.; Yu, D.; Zhou, H.; Tian, Y.; Yanagisawa, O. Turbostratic Carbon Nitride Prepared by Pyrolysis of Melamine. J. Mater. Sci. 2005, 40, 2645−2647. (24) Wang, Y.; Wang, X.; Antonietti, M. Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angew. Chem., Int. Ed. 2012, 51, 68−89. (25) Kang, Y.; Yang, Y.; Yin, L.-C.; Kang, X.; Liu, G.; Cheng, H.-M. An Amorphous Carbon Nitride Photocatalyst with Greatly Extended Visible-Light-Responsive Range for Photocatalytic Hydrogen Generation. Adv. Mater. (Weinheim, Ger.) 2015, 27, 4572−4577. (26) Kang, Y.; Yang, Y.; Yin, L.-C.; Kang, X.; Wang, L.; Liu, G.; Cheng, H.-M. Selective Breaking of Hydrogen Bonds of Layered Carbon Nitride for Visible Light Photocatalysis. Adv. Mater. (Weinheim, Ger.) 2016, 28, 6471–6477. (27) Zhang, Y.; Thomas, A.; Antonietti, M.; Wang, X. Activation of Carbon Nitride Solids by Protonation: Morphology Changes, Enhanced Ionic Conductivity, and Photoconduction Experiments. J. Am. Chem. Soc. 2009, 131, 50−51. (28) Zhang, J.; Zhang, M.; Lin, L.; Wang, X. Sol Processing of Conjugated Carbon Nitride Powders for Thin-Film Fabrication. Angew. Chem., Int. Ed. 2015, 54, 6297−6301. (29) Zhou, Z.; Wang, J.; Yu, J.; Shen, Y.; Li, Y.; Liu, A.; Liu, S.; Zhang, Y. Dissolution and Liquid Crystals Phase of 2D Polymeric Carbon Nitride. J. Am. Chem. Soc. 2015, 137, 2179−2182. (30) Li, J.; Shen, B.; Hong, Z.; Lin, B.; Gao, B.; Chen, Y. A Facile Approach to Synthesize Novel Oxygen-Doped g-C3N4 with Superior Visible-Light Photoreactivity. Chem. Commun. 2012, 48, 12017–12019.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

(31) Dong, G.; Ai Z.; Zhang, L. Efficient Anoxic Pollutant Removal with Oxygen Functionalized Graphitic Carbon Nitride under Visible Light. RSC Adv. 2014, 4, 5553–5560. (32) Desmet, T.; Morent, R.; De Geyter, N.; Leys, C.; Schacht, E.; Dubruel, P. Nonthermal Plasma Technology as a Versatile Strategy for Polymeric Biomaterials Surface Modification: A Review. Biomacromolecules 2009, 10, 2351– 2378. (33) Liu, F.; Leung, Y. H.; Djurišić, A. B.; Ng, A. M. C.; Chan, W. K.; Ng, K. L.; Wong, K. S.; Liao, C.; Shih, K.; Surya, C. Effect of Plasma Treatment on Native Defects and Photocatalytic Activities of Zinc Oxide Tetrapods. J. Phys. Chem. C 2014, 118, 22760−22767. (34) Sheng, J.; Park, J.; Choi, D.-W.; Lim, J.; Park, J.-S. A Study on the Electrical Properties of Atomic Layer Deposition Grown InOx on Flexible Substrates with Respect to N2O Plasma Treatment and the Associated Thin-Film Transistor Behavior under Repetitive Mechanical Stress. ACS Appl. Mater. Interfaces 2016, 8, 31136−31143. (35) Yoshida, S.; Hagiwara, K.; Hasebe, T.; Hotta, A. Surface Modification of Polymers by Plasma Treatments for the Enhancement of Biocompatibility and Controlled Drug Release. Surf. Coat. Technol. 2013, 233, 99–107. (36) Vesel, A.; Junkar, I.; Cvelbar, U.; Kovac, J. Mozetic, M. Surface Modification of Polyester by Oxygen- and Nitrogen-Plasma Treatment. Surf. Interface Anal. 2008, 40, 1444– 1453. (37) Chen, C.; Liang, B.; Ogino, A.; Wang, X.; Nagatsu, M. Oxygen Functionalization of Multiwall Carbon Nanotubes by Microwave-Excited Surface-Wave Plasma Treatment. J. Phys. Chem. C 2009, 113, 7659–7665. (38) Dey, A.; Chroneos, A.; Braithwaite, N. St. J.; Gandhiraman, R. P., Krishnamurthy, S. Plasma Engineering of Graphene. Appl. Phys. Rev. 2016, 3, 021301.


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Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Chen, Y.; Wang, B.; Lin, S.; Zhang, Y.; Wang, X. Activation of n → π* Transitions in Two-Dimensional Conjugated Polymers for Visible Light Photocatalysis. J. Phys. Chem. C 2014, 118, 29981−29989. (40) Lou, S.; Zhou, Z.; Shen, Y.; Zhan, Z.; Wang, J.; Liu, S.; Zhan, Y. Comparison Study of the Photoelectrochemical Activity of Carbon Nitride with Different Photoelectrode Configurations. ACS Appl. Mater. Interfaces 2016, 8, 22287−22294. (41) Fina, F.; Callear, S. K.; Carins, G. M.; Irvine, J. T. S. Structural Investigation of Graphitic Carbon Nitride via XRD and Neutron Diffraction. Chem. Mater. 2015, 27, 2612−2618. (42) Lotsch, B. V.; Doblinger, M.; Sehnert, J.; Seyfarth, L.; Senker, J.; Oeckler, O.; Schnick, W. Unmasking Melon by a Complementary Approach Employing Electron Diffraction, Solidstate NMR Spectroscopy, and Theoretical Calculations-Structural Characterization of a Carbon Nitride Polymer. Chem. - Eur. J. 2007, 13, 4969−4980. (43) Martin, D. J.; Qiu, K.; Shevlin, S. A.; Handoko, A. D.; Chen, X.; Guo, Z.; Tang, J. Highly Efficient Photocatalytic H2 Evolution from Water Using Visible Light and Structure-Controlled Graphitic Carbon Nitride. Angew. Chem., Int. Ed. 2014, 53, 9240−9245. (44) Niu, P.; Liu, G.; Cheng, H.-M. Nitrogen Vacancy-Promoted Photocatalytic Activity of Graphitic Carbon Nitride. J. Phys. Chem. C 2012, 116, 11013−11018. (45) Huang, C.-M.; Chen, L.-C.; Cheng, K.-W.; Pan G.-T. Effect of Nitrogen-Plasma Surface Treatment to the Enhancement of TiO2 Photocatalytic Activity under Visible Light Irradiation. J. Mol. Catal. A: Chem. 2007, 261, 218−224. (46) Islam, S. Z.; Reed, A.; Kim, D. Y.; Rankin, S. E. N2/Ar Plasma Induced Doping of Ordered Mesoporous TiO2 Thin Films for Visible Light Active Photocatalysis. Microporous Mesoporous Mater. 2016, 220, 120−128.


29


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Page 30 of 30

(47) Islam, S. Z.; Reed, A.; Wanninayake, N.; Kim, D. Y.; Rankin, S. E. Remarkable Enhancement of Photocatalytic Water Oxidation in N2/Ar Plasma Treated, Mesoporous TiO2 Films. J. Phys. Chem. C 2016, 120, 14069−14081. (48) Bian, J.; Li, Q.; Huang, C.; Li, J.; Guo, Y.; Zaw, M.; Zhang, R.-Q. Thermal Vapor Condensation of Uniform Graphitic Carbon Nitride Films with Remarkable Photocurrent Density for Photoelectrochemical Applications. Nano Energy 2015, 15, 353−361. (49) Xu, J.; Shalom, M.; Piersimoni, F.; Antonietti, M.; Neher, D.; Brenner, T. J. K. ColorTunable Photoluminescence and NIR Electroluminescence in Carbon Nitride Thin Films and Light-Emitting Diodes. Adv. Opt. Mater. 2015, 3, 913−917.

SYNOPSIS TOC


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Tuning the Photocatalytic Activity of Graphitic Carbon Nitride by

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