Surface Amino Group Regulation and Structural Engineering of

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Functional Nanostructured Materials (including low-D carbon)

Surface Amino Group Regulation and Structural Engineering of Graphitic Carbon Nitride with Enhanced Photocatalytic Activity by Ultrafast Ammonia Plasma Immersion Modification Shifei Kang, Maofen He, Mengya Chen, Yanfei Liu, Yuting Wang, Yangang Wang, Mingdong Dong, Xijiang Chang, and Lifeng Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01068 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

Surface Amino Group Regulation and Structural Engineering of Graphitic Carbon Nitride with Enhanced Photocatalytic Activity by Ultrafast Ammonia Plasma Immersion Modification

Shifei Kang1,2, Maofen He1, Mengya Chen1, Yanfei Liu1, Yuting Wang2,3, Yangang Wang4, Mingdong Dong2, Xijiang Chang5 *, Lifeng Cui1 *

1. Department of Environmental Science and Engineering, University of Shanghai for Science and Technology, 200093, Shanghai, P.R. China 2. Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000, Aarhus C, Denmark 3. School of Environment and Civil Engineering, Dongguan University of Technology, 523808, Guangdong, P.R. China 4. College of Biological Chemical Science and Engineering, Jiaxing University, 314001, Jiaxing, P.R. China 5. School of Electrical & Electronic Engineering, Shanghai University of Engineering Science, 201620, Shanghai, P.R. China

*Corresponding authors. E-mail addresses: [email protected] (X. Chang), [email protected] (L. Cui). 1 / 21

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Abstract Surface amino group regulation and structural engineering of graphitic carbon nitride (g-CN)

for better catalytic activity has increasingly become a focus of academia and industry. In this work, the ammonia plasma produced by a microwave surface wave plasma generator was developed as a facile source to achieve fast, controllable surface modification and structural engineering of g-CN by ultrafast plasma treatment in minutes, thus enhancing its photocatalytic performance of g-CN. The morphology, surface hydrophilicity, optical absorption properties and states of C-N bonds, were investigated to determine the effect of plasma immersion modification on the g-CN catalyst. The structure and photoelectric features of the plasma modified samples were characterized by X-ray diffractometry, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The results indicate that the ammonia plasma treated gCN-NH3 exhibits an ultrathin nanosheet structure, an enriched amino groups and ideal molecular structure, a narrower band gap (2.35 eV) and extended light harvesting edges (560 nm), and enhanced electron transport ability. The remarkably enhanced photocatalytic activity demonstrated in the photoreduction and detoxification of hexavalent chromium (Cr

VI)

can be ascribed to the

optimization of the structural and photoelectric properties induced by the unique ammonia plasma treatment. The effective and ultrafast approach developed in this work is promising in the surface amino group regulation and the structural engineering of various functional materials.

Keywords: Graphitic carbon nitride, Photocatalytic activity, Surface modification, Ammonia plasma,

Structural engineering

1.

Introduction In the past few decades, the increasing consumption of fossil fuels has led the increasingly

serious problems of energy crisis and environmental pollution. Semiconductor photocatalysis, which is considered a green pathway, has been helpful to solve difficult problems, such as water splitting, elimination of organic pollutants, and CO2 photoreduction of carbon dioxide. As an environmentally applicable metal-free polymer semiconductor photocatalyst with a relatively good 2 / 21


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visible light response, g-CN has been increasing widely uses as in environmental photocatalytic applications1-2. Unfortunately, the poor charge separation and transfer ability of graphitic carbon nitride, which was mainly due to incomplete polymerization defects, severely restricted its development and practical implementation3-5. Therefore, it is highly desirable to fundamentally overcome the abovementioned shortcomings and enhance the photocatalytic performance of g-CN photocatalysts. In view of the above mentioned drawbacks of raw g-CN, the conventional solution in improving the quality of g-CN is optimizing its electron and band structures by doping heteroatoms6 or developing semiconductor heterojunctions to benefit the migration of the photogenerated carriers7-8. Recently, surface group regulation and structural engineering of g-CN has increasingly become a promising strategy. In a typical photocatalytic system, photoinduced reactions occur at the surface of the catalyst. Therefore, the photocatalytic performance of photocatalysts can be significantly affected by its surface features such as the morphology, defects, hydrophilicity and specific surface area9-10. Meanwhile, raw g-CN materials obtained by most thermal polymerization approaches are far from high crystallinity, and are actually a mixture of ideal g-CN and incompletely reacted intermediates, which can result in undesirable structural defects and unwanted charge trapping sites11-12. Therefore, the main purpose of structural engineering of graphitic carbon nitride (g-CN) should not be limited to construction of the well-discussed nanostructure or mesopores and should include the removal of structural defects to optimize the crystal structure of g-CN13. Guo et al12. note that the conventional synthetic approach involves only simple heating cannot yield highquality g-CN and reported on the synthesis of high-quality g-CN with reduced structural defects through a confined microwave-assisted thermolysis method using melamine/cyanuric acid supramolecular aggregates as precursor. We previously reported11 an ultrafast "alternated cooling and heating" strategy in which highly crystalline few-layer g-CN were synthesized by judiciously combining flash freezing and microwave-assisted thermo exfoliation. In addition to the aforementioned structural engineering by the confined thermal polymerization method, many other physical techniques were introduced to optimize the surface and structural properties of materials, among which nonthermal plasma is the most promising14-16. Consisting of abundant excited electrons, atoms, ions, and radicals, nonthermal plasma with room3 / 21



temperature low gas temperature can display a remarkable high electron temperature (10,000100,000 K). The excited species in plasma are chemically highly active in reacting with the surface of materials at a low temperature to achieve surface group regulation and structural engineering of the materials. More importantly, the fundamental structure of the treated materials, especially the polymer materials such as g-CN, can survive during the modification process. Therefore, plasma modification has provided enlightenment for the preparation of high-quality catalysts in terms of surface and structural tailoring. For example, Kang et al. developed an Argon-diluted Nitrogen plasma method to prepare N-doped commercial P25 TiO2 with excellent photocatalytic activity in the photooxidation of organic molecules in water17. Mao et al. first reported that plasma method can be used in the surface properties modification of g-CN photocatalyst with improved photocatalytic activity18. Ji et al. found that the oxygen functional groups introduced through oxygen-plasma treatment play a key role in developing high-performance g-CN-based photocatalysts and photoelectronic devices15. However, as far as we know, there are few studies concerning material modification using ammonia plasma, which contains an abundance of NH3-related excited species. The unique ammonia plasma atmosphere is promising for the surface amino group regulation and structural engineering of g-CN. In this work, a simple, fast and scalable ammonia plasma immersion approach was developed for the first time to achieve the modification of the surface and structural features of g-CN photocatalysts. A self-built microwave surface wave plasma generator was used to ionize ammonia gas in a vacuum chamber and produce the unique ammonia plasma to achieve the controllable surface modification and structural engineering of g-CN powder within several minutes. The rational microwave discharge design can greatly reduce the consumption of ammonia gas, avoiding the corrosive influence of a high concentration of ammonia gas on the reaction device. The plasmatreated g-CN nanostructure showed strong visible light absorption and an ideal crystal/surface structure. The mechanism of improvement of the modified g-CN in electrochemical performance, visible light absorption behavior and active sites for photocatalysis were investigated by systematic physical chemical characterizations. Based on this work, a universal clean and efficient ammonia plasma surface activation treatment technology for nanomaterials is established.

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2.

Experimental Section

2.1 Material preparation Urea and potassium dichromate (Cr

VI)

were obtained from Aladdin Industrial Corporation

(Shanghai, China) and used without purification. Raw g-CN was prepared by simple thermal polymerization of urea, which was placed in a covered crucible and heated to 550 °C for 4 h in a muffle furnace and then natural cooled to room temperature, the heating rate is 5 °C min-1. The obtained yellow product was ground to make raw g-CN powder. 2.2 Plasma generator setup and material modification A diagram of the self-designed microwave plasma generator employing a microwave surface wave discharge strategy in a vacuum chamber is shown in Scheme 1. Compared with the plasmas produced in dielectric barrier discharge configurations or capacitive coupled plasma equipment, microwave plasmas provide high degree of ionization and high electron density that is promising in the modification of various kinds of functional materials18-21. The plasma-treated g-CN powder was developed by uniformly placing 1 g of g-CN powder on the sample stage in the center of the reaction chamber. The gas control system and pumping system worked together to adjust the working gas composition and pressure. A 1 kW microwave source (MUEGGE, MS1000B) was employed and plasma was excited in the processing chamber under a suitable gas condition after energy coupling in the surface-wave mode. Before plasma ignition, the processing chamber was pumped to a basis vacuum of approximately 10-2 Pa and filled with Ammonia gas (99.999%). The chamber was sustained by a mass flow controller at approximately 20 Pa. When the microwave was turned on and the stub-tuner was tuned, plasma could be excited in the chamber. We focused on the effect of ammonia plasma treatment on the samples for 1-15 min. The g-CN-NH3 refers to the g-CN sample treated by ammonia plasma for 5 mins as a representative sample. The influence of different ammonia plasma time duration is discussed in the photocatalytic performance and mechanism section. To distinguish the unique treatment effect of ammonia plasma with simple physical bombardment in common plasma treatment, Argon plasma modification in which only the simple physical bombardment effect was involved was also performed on the raw g-CN under the same equipment conditions for 30 min using Argon as gas carrier gas, and the resulting control sample was named g-CN-Ar. 5 / 21



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Scheme 1. Simulation diagram of the self-designed microwave plasma generator.

2.3 Material Characterization The crystallinity of the prepared g-CN, g-CN-Ar and g-CN-NH3 samples was analyzed using an X-ray diffractometer (XRD) (Bruker Advanced D8 powder, Germany) using Cu-Kα radiation (λ = 1.5418 Å) at a scan step of 0.02°. The morphology analysis was conducted using a transmission electron microscope (TEM) (JEM-2100F) operated at an accelerating voltage of 200 kV. The hydrophilic water contact angle (WCA) on the sample base was measured using a KRUSS DSA100 drop shape analysis system; the sample base was placed on an aluminum stage and compressed under 20 MPa to form a compact smooth contact interface. Nitrogen adsorption-desorption analysis were performed using a Micromeritics Tristar 3000 surface area and pore size analyzer. The surface organic functional groups of g-CN samples were recorded by Fourier transform infrared spectroscopy (FTIR, Thermoelectric, Nicolet 66700). The X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) was employed to characterize the C-N chemical bonding state. The Cary 500 ultraviolet-visible (UV-vis) diffuse reflectance spectrophotometer (DRS) was run with BaSO4 as a reference. Electrochemical impedance spectra (EIS) were measured on a Chi660e electrochemical workstation from 0.1 Hz to 100 kHz at the circuit potential. The value of WCA, pore volume, specific surface area and band gap was obtained by averaging three original values. 2.4 Photocatalytic activity measurement Photocatalytic activity of the as-prepared g-CN, g-CN-Ar and g-CN-NH3 samples was monitored by the photocatalytic reduction and detoxification of hexavalent chromium (Cr 6 / 21


VI)

in

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aqueous suspension (40 mg/L, 50 mL) under visible-light irradiation. The photocatalytic reaction was performed in an open quartz tube with a diameter of 30 mm in the presence of 20 mg photocatalysts, the reaction temperature was kept at 25 ◦C by a recycle cooling water system. A 500W Xe arc lamp with a 420 nm cut-off optical filter was employed as the visible light source, and the average optical intensity on the Cr

VI

aqueous suspension was 20 mW/cm2. The Cr

VI

concentration was determined by measuring the absorbance at 540 nm using an Evolution Thermo 600 UV-vis spectrophotometer with diphenyl carbazide (DPC) as a chromogenic agent.

3.

Results and Discussion

3.1 Characteristics of catalysts

Fig. 1 TEM images of (a, b) g-CN; (c, d) of g-CN- NH3; insert is the corresponding hydrophilic water contact angle on g-CN and g-CN-NH3 surfaces, respectively

The morphology and microstructure of untreated g-CN and the ammonia plasma-treated g-CNNH3 samples were characterized by transmission electron microscopy (TEM). As shown in Fig. 1(a) and (b), the g-CN-NH3 displays a more transparent and smaller morphology of the carbon nitride in than the g-CN, which indicates the ultrathin layer structure of the g-CN-NH3. The ultrathin sheetlike structure verified by the TEM images may result in more exposed active edges for efficient photocatalytic reactions. The morphological change may occur because of the resolution of a large number of structural defects during ammonia plasma etching, which can serve as unwanted charge 7 / 21



trap sites in photocatalytic reactions. The thin and uniform layers may provide more catalytic active sites and improve photocatalytic activity22. The hydrophilic contact angle test of water on the untreated g-CN and g-CN-NH3 sample was measured on a compressed compact and smooth contact interface, and the significantly flattened drop shape on the g-CN-NH3 sample interface indicated the superior hydrophilicity of the ammonia plasma modified samples. The measured water contact angles on the untreated g-CN and g-CN-NH3 interfaces were 83.4±1.6° and 48.6±0.9°, respectively. The improvement of the hydrophilicity may be ascribed to the enrichment of amine groups stemming from the excited NH3 species in the ammonia plasma, and the good hydrophilicity plays an important role in facilitating the photocatalytic reaction in aqueous solutions.

Fig. 2 (a) XRD patterns (b) FT-IR spectra (c) UV-vis diffuse reflectance spectra (d) Estimated band gaps (e) Nitrogen adsorption–desorption isotherms and (f) Barrett–Joyner–Halenda (BJH) pore-size distribution 8 / 21


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curves of g-CN, g-CN-Ar and g-CN-NH3.

The crystal structure of the synthesized g-CN, g-CN-Ar and g-CN-NH3 samples were characterized by mean of using the X-ray diffraction patterns. As shown in Fig. 2(a), the XRD patterns of all the three samples show two typical diffraction peaks at 2θ of 13.1° and 27.4°, respectively, corresponding to the characteristic (100) and (002) reflections of g-CN23-25. There are no impurity peaks can be found, verifying the fundamental C-N crystal structure survived in the ammonia plasma treatment. However, compared with the diffraction peaks of the control g-CN and g-CN-Ar, the diffraction peaks of g-CN-NH3 are reduced in intensity. This can mainly be ascribed to the formation of few-layer nanosheets in g-CN-Ar and g-CN-NH3 induced by the ammonia plasma etching effect, as reflected by the TEM results. The chemical functional groups on g-CN before and after the ammonia plasma modification was investigated by FTIR. The FTIR spectra shown in Fig. 2(b) exhibit typical absorption features for g-CN26. The sharp peak at 818 cm-1 can be attributed to the bending vibration of the tri-s-triazine units in g-CN, the IR groups located in the range of 845~1800 cm-1 are derived from the stretching vibrations of C–N and C=N in the CN heterocycles of the graphitic carbon nitride, and the broad peak centered at 3040~3400 cm-1 corresponds to the stretching vibrations of –NH2 and =NH amine groups. The FTIR peaks intensity of g-CN-Ar located at the tri-s-triazine units region and amine groups region decreased than that of the untreated g-CN because of the strong physical bombardment effect of Argon plasma can partly destroyed the intrinsic surface functional structure. Interestingly, FTIR spectra of g-CN-NH3 displayed slightly sharper peaks compared with bare gCN, indicating an optimized surface function groups structure can be created by ammonia plasma modification. These pronounced hydrophilic absorption peaks are in agreement with the hydrophilic contact angle results, likely due to the selective removal of unwanted disordered defects of the primary g-CN and the introduction of new amine groups during ammonia plasma treatment27. The optical absorption properties of g-CN, g-CN-Ar and g-CN-NH3 were investigated by UVvis DRS. As shown in Fig. 2(c), the absorption edge of the untreated g-CN is approximately 440 nm. After ammonia plasma modification, the absorption edge of g-CN-NH3 is redshifted to approximately 560 nm. The sample of g-CN-NH3 shows a remarkable redshift on the absorption edges, suggesting that the plasma treatment changed the optical band gap. The calculated band gap 9 / 21



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energy of g-CN-NH3 as shown in Table 1 and Fig. 2(d) is 2.35 eV, which is significantly narrower than that of g-CN (2.70 eV). The change in the light absorption edges and band gaps can be presumably ascribed to the formation of terminal amino groups or the production of ultrathin nanosheet morphology can be speculated28-29. This conjecture is consistent with the TEM and XRD data. The appropriate band edge and narrowed band gap of g-CN-NH3 satisfy the thermodynamic requirements for photocatalysis. Table 1 Structural and band characteristics of all samples

Sample

Pore volume (cm3/g)

SBET (m2/g)

Band gap (eV)

g-CN

0.6146 ± 0.018

103.26 ± 2.81

2.70 ± 0.01

g-CN-Ar

0.5105 ± 0.020

80.92 ± 3.21

2.65 ± 0.01

g-CN- NH3

0.5584 ± 0.012

86.41 ± 2.06

2.35 ± 0.02

The nitrogen adsorption–desorption isotherms of the g-CN, g-CN-Ar and g-CN-NH3 samples as shown in Fig. 2(e) display a similar typical type-IV isotherm and H3 hysteresis loop according to the IUPAC classification30. This result suggests that g-CN-NH3 exists as a mesoporous structure. The specific surface area (SBET) of all obtained samples was calculated by Brunauer-Emmett-Teller (BET) method and the corresponding pore size distribution curves (Fig. 2(f)) was determined by the Barrett-Joyner-Halenda (BJH) method. The SBET and BJH pore volumes of the products are listed in Table 1. The N2 adsorption-desorption measurements show no obvious differences in the specific surface area of g-CN and g-CN-NH3, indicating that the pore structure is not the main factor controlling the activity of g-CN in this work. After the ammonia plasma modification, the remaining SBET of g-CN-NH3 (86.41 m2 g-1) is still sufficiently large to support fast photocatalytic reactions.

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Fig. 3 (a) XPS survey spectrum and (b) high-resolution C 1s peaks and (c) N 1s Peaks of g-CN, g-CN-Ar and g-CN-NH3.

To further understand the composition and surface chemical features of CN related species in the g-CN sample before and after the ammonia plasma immersion modification, XPS determination of the samples was carried out. The measured survey XPS spectra of g-CN, g-CN-Ar and g-CNNH3 are shown in Fig. 3(a) suggest the samples mainly contain C, N and O. For the untreated gCN, the C 1s spectra illustrates the presence of graphitic carbon at 284.47 eV and C-N3 at 288.04 eV, revealing the formation of characteristic g-CN structure31-32. The N 1s spectra can be deconvoluted into three typical g-CN structural units peaks corresponding to the sp2-hybridized nitrogen in C-N=C at 398.31 eV, tertiary nitrogen in N-(C)3 groups at 398.92 eV and terminal amino-groups (-NH2) at 400.32 eV, respectively30. To better compare and understand the bonding state variations of each CN peak caused by the ammonia plasma immersion modification, a detailed analysis of the deconvoluted spectra was performed. As shown in Fig. 3(b), the main C 1s peak is almost unchanged after the ammonia plasma modification, but the surfaces of the g-CN-NH3 sample are more active for the three-coordinated carbon (C-N3), hence the contribution of C-N3 exhibits an increase, revealing that ammonia plasma treatment can introduce amino groups. For the N 1s spectra shown in Fig. 3(c), the -NH2(or =NH) components of g-CN-NH3 show an obvious increase (from 7.32% to 23.67%), demonstrating that the ammonia plasma treatment can introduce amino groups, 11 / 21



which significantly improved the hydrophilicity of g-CN-NH3 and benefitted photocatalytic performance. The C-N=C components of g-CN-NH3 show an obvious decrease (from 55.81% to 32.82%) and N-C3 components of g-CN-NH3 show an obvious increase (from 36.87% to 43.52), revealing that the plasma treatment can etch the intermediate products in the process of carbon nitride synthesis. Therefore, ammonia plasma could effectively optimize the surface amino group and reduce undesired structural defects. In sum, the XPS results reveal that effective selective polymeric structural modification and strong amino functional group regulation of the original gCN material were achieved.

Scheme 2. Structural optimization mechanism of the unique ammonia plasma modification by efficient etching of unpolymerized intermediate melamine monomers.

The possible optimization mechanism of the structural modification is shown in Scheme 2. Sp2-hybridized nitrogen in triazine rings (C-N=C) structure were observed in both the graphitic carbon nitride (g-CN) and unpolymerized intermediate melamine monomers whereas the tertiary nitrogen (N-(C)3) structure were only observed in highly polymerized graphitic carbon nitride (gCN). The structure of tertiary nitrogen N-(C)3 groups increased obviously in the sample treated with ammonia plasma. The ratio of the sp2 -hybridized nitrogen N in the C-N=C rings to tertiary N-(C)3 groups and amino groups in g-CN-NH3 become closer to that of the ideal g-CN model as reported15, indicating the incompletely reacted intermediates in bulk g-CN were efficiently etched by the unique ammonia plasma treatment. Therefore, good photoelectric and photocatalytic activity of the g-CNNH3 sample can be expected. 12 / 21


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Fig. 4 (a) and (b) Photocatalytic curves for Cr VI removal over different photocatalysts under visible light irradiation; (c) corresponding kinetic curves over different photocatalysts; (d) EIS spectra of different samples.

3.2 photocatalytic performance and mechanism As a typical highly toxic heavy metal substance which have been widely used in electroplating and tanning industry, the hexavalent chromium (Cr VI) entering natural waters cause serious harm to humans and ecological environment33. Since the biological toxicity of Cr III only accounts for less than 1% of Cr VI, photoreduction of Cr VI to Cr III is an effective method to achieve the detoxification of Cr

VI.

By analyzing the photocatalytic reduction Cr

VI

performance of samples, we can better

understand the photocatalytic potential of ammonia plasma-treated g-CN samples. As shown in Fig. 4(a), we evaluated the photocatalytic performance of ammonia plasmasprayed g-CN for different times (1, 3, 5, 7, and 15 min) by studying the visible-light irradiation removal of Cr VI under. No obvious photoreduction was detected in the blank experiment, verifying the stability of Cr VI in this test condition without the presence of photocatalysts. The photocatalytic activity of the g-CN-NH3 photocatalysts were basically similar, showing a slight increase from 1 to 5 min followed by a slight decrease. Notably, the g-CN-NH3 (1 min) sample treated for only 1 min also showed greatly improved activity. To protect instrument pipelines from corrosion and realize commercial-scale production, the 1 min ammonia plasma treatment time was reasonable. Fig. 4(b) 13 / 21



shows the Cr

VI

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photoreduction efficiencies of g-CN, g-CN-Ar and g-CN-NH3 under visible-light

irradiation. The 90 min photocatalytic Cr VI photo detoxification efficiency over bare g-CN accounts only 49.83%. Remarkably, g-CN-NH3 was much more efficient in photocatalytic reduction and removed nearly 97.3% of Cr VI after 90 min of visible light irradiation. The kinetic curves of Cr VI removal are shown in Fig. 4(c). The pseudo-first order reaction kinetic (K) of Cr VI removal over gCN-NH3 (40.7±5.6×10-3) was much higher than that of g-CN-Ar (14.6±1.4×10-3), g-CN (7.3±0.9×10-3) and the blank (0. 5±0. 0×10-3). The results show that the photocatalytic activity of unique immersed ammonia plasma modified g-CN-NH3 is 5.58 times higher than that of untreated g-CN. These results indicate g-CN-NH3 should be a superior high-quality photocatalyst compared with raw g-CN. To further determine the improved mechanism of ammonia plasma-treated g-CN, the electrochemical impedance spectra were measured. Fig. 4(d) shows the electrochemical impedance spectra (EIS) of g-CN and g-CN-NH3 electrodes. The recorded Nyquist plots for the pure g-CN and the ammonia-plasma-treated g-CN-NH3 present decreasing semicircular diameters in the highfrequency region and a diminishing slope in the low-frequency region, indicating that the g-CNNH3 has lower resistances for charge transport than the untreated g-CN sample, especially on the sample/electrolyte interface. The improved photoelectrochemical properties can be attributed to the etching out of the structural defects in the intermediate, which can serve as charge trap sites in photocatalytic reactions, thus greatly improving photogenerated charge mobility. The rapid and lowcost material activation of plasma treatment make this plasma treatment technology promising in industrial applications to produce high-quality catalysts.

4.

Conclusions An efficient and scalable ammonia plasma immersion treatment method was developed and

used to prepare high-performance g-CN photocatalysts with great electron transport ability. The photocatalytic abilities of untreated g-CN and the ammonia plasma-treated g-CN (g-CN-NH3) in the photoreduction and detoxification of hexavalent chromium (Cr

VI)

were demonstrated under

visible light irradiation. The 90 min Cr VI removal efficiency with the best g-CN-NH3 (5 min) was 97.30%, which is superior to that of the untreated g-CN (49.82%). The g-CN-NH3 (1 min) sample also showed roughly equivalent activity, which means the ultrafast ammonia plasma treatment 14 / 21


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method is feasible and promising. The significantly improved photoactivity of the g-CN-NH3 sample can be ascribed to the remarkable bordered visible light absorption region and fast charge separation, which was introduced by the unique immersed ammonia plasma treatment for the regulation of surface amino groups and optimization of the crystal structure of graphitic carbon nitride. This work provides insights for the industrial application of plasma technology in material modification.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51671136, 11705115 and 51502172), the International Technological Collaboration Project of Shanghai (Grant No. 17520710300), the Program for Professor of Special Appointment (Young Eastern Scholar) at Shanghai Institutions of Higher Learning (No. QD2016036), Danish National Research Foundation AUFF-NOVA project (Grant No. AUFFE-2015-FLS-9-18) and EU H2020 RISE 2016-MNR4S Cell project.

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