Nitrogen Vacancy Structure Driven Photoeletrocatalytic Degradation of

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Nitrogen Vacancy Structure Driven Photoeletrocatalytic Degradation of 4-Chlorophenol Using Porous Graphitic Carbon Nitride Nanosheets Yang Hou, Jian Yang, Chaojun Lei, Bin Yang, Zhongjian Li, Yu Xie, Xingwang Zhang, Lecheng Lei, and Junhong Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00279 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Sustainable Chemistry & Engineering

Nitrogen Vacancy Structure Driven Photoeletrocatalytic Degradation of 4-Chlorophenol Using Porous Graphitic Carbon Nitride Nanosheets

Yang Hou,†,* Jian Yang,† Chaojun Lei,† Bin Yang,† Zhongjian Li,† Yu Xie,#,* Xingwang Zhang,† Lecheng Lei,† Junhong Chen‡,* †

Key Laboratory of Biomass Chemical Engineering of Ministry of Education,

College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang Province 310027, China ‡

Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, Wisconsin 53211, USA #

Department of Material Chemistry, Nanchang Hangkong University, 696 Fenghe South Ave, Nanchang, Jiangxi Province 330063, China

1

ACS Paragon Plus Environment

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Abstract Incorporating vacancies has been demonstrated as important approaches to alter the catalytic properties of photocatalysts. Herein, a novel porous graphitic carbon nitride (GCN) nanosheets with tunable nitrogen vacancies was synthesized through thermal treatment of bulk GCN under an H2 atmosphere. The resulting porous nanosheets possessed ～25 nm in thickness, several hundred nanometers in lateral size, and a high surface area of 114 m2 g−1. The systematic characterization results revealed that the H2 treatment induced the structure distortion of GCN with the creation of nitrogen vacancies. As a result, as-prepared nanosheets exhibited a considerably enhanced photoelectrocatalytic performance. After 180 min of simulated sunlight irradiation, only 19.5% of total organic carbon still remained while 4-chlorophenol was completely eliminated. This enhanced activity was mainly attributed to the increased specific surface area, improved light absorption, and effective separation and transfer of photogenerated charge carriers, which was confirmed by photoelectrochemical measurement results. Radicals trapping studies revealed that •OH radicals and holes were involved as the major oxygen active species for the degradation of 4-chlorophenol. Our findings offer new insights into designing and developing highly efficient photocatalysts for environmental purification.

Keywords: Graphitic carbon nitride; Nitrogen vacancies; Porous nanosheets; Photoeletrocatalysis; Oxygen active species.

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Introduction In the recent years, the elimination of 4-chlorophenol (4-CP) from paper making, petrochemical, plastic, and insecticidal industries wastewater have aroused much attentions due to their associated serious effects on human health and environment issues.1 Various methods have been explored to remove 4-CP from wastewater, including

chemical/electrochemical

oxidation,

biological

degradation,

and

photocatalysis.2-4 Among these methods, semiconductor-based photoelectrocatalysis has drawn enormous interests as one of the most promising technologies for solving energy and environmental issues.5, 6 Semiconducting TiO2 has been considered as the most promising photocatalyst for wastewater treatment due to its environmentally friendly,

photochemical stability, capable

of complete mineralization, and

nontoxicity.7 However, TiO2 can be only excited by the ultraviolet light due to its wide band gap (3.0-3.2 eV),8 thus making TiO2 an inefficient photocatalytic material for solar light utilization and limiting its practical applications to a large extent. Seeking earth-abundant and high-performance photocatalysts that can make full use of solar energy is of particular importance to achieve highly efficient photoelectrocatalytic environmental remediation.9, 10 A lot of new visible-light-driven photocatalysts have been developed in recent years, such as ZnFe2O4,10 Cu2O,9 Ag/AgBr,11 Ag3PO4,12 , etc. However, considering the side issues of sustainability and scalability, the semiconductor photocatalysts without any metal elements (e.g., carbon dots,13, 14 graphene quantum dots,15 and boron carbide16) are greatly desired. As a typical metal-free photocatalyst, graphitic carbon nitride (GCN) with a suitable band 3



gap has received considerable attention due to its advantageous features including high chemical and thermal stability, unique electronic and optical properties, and low cost.17, 18 However, pristine GCN suffers from unsatisfactory photocatalytic activities due to the insufficient utilization of solar light, fast recombination of photogenerated charge carriers, and low specific surface area, which greatly limit its practical applications. Therefore, it is necessary to enhance the photocatalytic activity of GCN for practical applications. Up to now, great efforts have been made by modifying GCN through chemical modifications and nanostructure designs. Chemical modifications can effectively adjust the band gap of GCN by doping it with non-metal atoms (e.g., P, S, I)19-21 or metal atoms (e.g., Fe, Co, Cu),22-24 and introduce nitrogen vacancies to increase solar light harvesting and improve the separation efficiency of photogenerated charge carriers. Constructing nanostructures, such as porous structures,25 nanospherical frameworks,26 and nanosheets,27 can increase the specific surface area for efficient photogenerated charge transfer across the interface, thus achieving an improved photocatalytic activity. In spite of the significant progress achieved, simultaneous band gap engineering and porous structure engineering in GCN nanosheets for achieving synergistically enhanced photocatalytic performance is still a major challenge.28 To the best of our knowledge, to date, little work has been reported concentrating on the design and synthesis of GCN nanosheets with tunable porous architectures and nitrogen vacancies to radically enhance the photoelectrochemical (PEC) and photocatalytic activity. Moreover, there is no study on the utilization of 4


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such GCN nanosheets as a photoelectrode for photoelectrocatalytic wastewater treatment. In this work, we developed a facile and effective approach to synthesize novel porous GCN nanosheets with tunable nitrogen vacancies by thermal treatment of bulk GCN under an H2 atmosphere. The obtained porous GCN nanosheets with nitrogen vacancies had an average thickness of ～25 nm, a narrow band gap of 1.82 eV, and a high surface area of 114 m2 g−1. Benefiting from the enhanced light harvesting and increased separation and transfer of photogenerated charge carriers, the hydrogenated GCN

nanosheets

exhibited

significantly

improved

PEC

performance

and

photoelectrocatalytic activity for degradation of 4-CP compared with the bulk GCN under simulated sunlight irradiation. The role of oxygen active species and mechanisms for the enhancement of photoelectrocatalytic activity were studied in detail. Experimental section Synthesis of hydrogenated GCN (porous GCN nanosheets with N vacancies) The bulk GCN was prepared by direct thermal polymerization of dicyandiamide (5 g) in a tube furnace at 550 °C for 4 h with a heating rate of 5 oC min−1 under air atmosphere. To prepare the hydrogenated GCN, the obtained GCN powder was further heated at 550 and 600 oC for 2 h in an H2 atmosphere, respectively. The naturally cooled products were denoted as GCN-550-H2 and GCN-600-H2. Characterization The morphologies and corresponding elemental maps of hydrogenated GCN were 5



characterized by using field-emission scanning electron microscope (FESEM, Hitachi S-4800) and energy dispersive X-ray spectroscopy (EDS, Carl Zeiss NVision 40). Transmission electron microscopy (TEM) images, selected area electron diffraction (SAED) patterns, and high resolution TEM (HRTEM) images were taken on a Hitachi H 9000 NAR operated at 200 kV. X-ray diffraction (XRD) investigation was performed on a Scintag XDS 2000 X-ray powder diffractometer using Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) measurements were taken using an HP 5950A with Mg Kα radiation. All binding energies were calibrated with C 1s peak at 284.6 eV. Elemental analysis was carried out on a Vario MICRO elemental Analyzer (Elementar, Germany). Fourier Transform Infrared (FTIR) spectra were recorded on a Bruker Vector Fourier transform spectrophotometer. Raman spectra were obtained using a Renishaw 1000B spectrometer with a 633 nm laser excitation. UV-Vis absorption spectra were measured using a UV-Visible spectrometer (Ocean Optics). Photoluminescence (PL) spectra were obtained at room temperature by using a Spex Fluorolog-3 with an excitation wavelength of 325 nm. Electron spin resonance (ESR) spectra were recorded with a Bruker EC106 X-band spectrometer. Thermogravimetric analysis (TGA) was conducted on a TA SDT 2960 thermoanalyzer with a heating rate of 5 oC min−1 in Ar atmosphere. Nitrogen adsorption-desorption was measured with a Micromeritics ASAP 2020 surface area and porosimetry analyzer. The samples were degassed for 8 h at 200 oC before analysis. PEC measurement 6


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PEC performances were evaluated using a CHI 760 E electrochemical analyzer in a typical three-electrode configuration with a Ti electrode modified with hydrogenated GCN as photoelectrode, a Pt foil as counter electrode, and Ag/AgCl electrode as reference electrode. The exposed area of photoelectrodes was kept at 1.0 cm2 and the loading mass of samples on the Ti electrode was 0.5 mg cm−2. 0.01 M Na2SO4 aqueous purged with N2 was used as the electrolyte. The simulated sunlight source (100 mW cm–2) was supplied by a 200 W Xenon lamp coupled with an AM 1.5G filter (Newport). The electrochemical impedance spectroscopy (EIS) was carried out at a DC bias of 0.8 V with an AC voltage of 10 mV in the frequency range from 100 K to 0.01 Hz. Unless otherwise noted, all potential was relative to the reversible hydrogen electrode (RHE) potential, which was converted from the Ag/AgCl electrode using: ERHE = E(Ag/AgCl) + 0.6 V.29 Photoelectrocatalytic activity test To evaluate the catalytic activity of the hydrogenated GCN photoelectrode, the photoelectrocatalytic degradation of 10 mg L−1 4-CP in an aqueous solution was carried out (Figure S1). A 200 W Xenon lamp coupled with an AM 1.5G filter (Newport) was used to simulate the sunlight, and the average light intensity was 100 mW cm−2. Prior to irradiation, the suspensions were placed inside the quartz reactor with magnetic stirring for 60 min in darkness to establish adsorption-desorption equilibrium. At a given time interval, 1 mL aliquot was withdrawn to examine the degradation of 4-CP at 224 nm using a UV-Vis spectrophotometer. The mineralization of 4-CP was determined by the decrease in total organic carbon (TOC) value using a 7



TOC analyzer (Shimadzu 5000A). Results and discussion Characterization of hydrogenated GCN nanosheets The morphology and microstructure of GCN and hydrogenated GCN were investigated by TEM. As shown in Figure 1a, the GCN displays a typical ultra-thin lamellar nanosheet structure. In contrast, the TEM images of hydrogenated GCN appear layered surface morphology with many pores (Figure 1b-1c), which are created by the chemical reaction between N atoms of GCN and active hydrogen atoms during the hydrogenation reaction.30 HRTEM image and SAED pattern demonstrate the partially crystalline feature of GCN-600-H2 (Figure 1d). FESEM image presented in Figure 1e reveals that the GCN-600-H2 has a ～25 nm in thickness and a lateral size up to several hundred nanometers. The elemental mapping images exhibit the homogeneous spatial distribution of C, N, and O elements in the GCN-600-H2 (Figure 1e-1h). The crystal structures of the GCN, GCN-550-H2, and GCN-600-H2 were investigated by XRD (Figure 2a). Both GCN-550-H2 and GCN-600-H2 show two peaks at 27.7° (CN graphitic-like layers) and 13.6° (in-planar repeated tri-s-triazine units) similar to those of bulk GCN,31 suggesting that the hydrogenated GCN nanosheets basically has the same crystal structure as their parent GCN. Compared with bulk GCN, the peaks of GCN-550-H2 and GCN-600-H2 clearly become weaker and positively shift towards a higher angle, indicating that heat treatment with H2 gas increased long-range disorder of the in-plane structural packing for graphitic structure and produced more 8


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defects due to the removal of N-containing species from the GCN skeleton.32 The microstructure of hydrogenated GCN was further revealed by FTIR (Figure 2b). The typical stretching modes of C–N heterocycles in the region of 1,200–1,600 cm−1 and the characteristic breathing mode of the triazine unit at 810 cm−1 are observed in GCN-550-H2 and GCN-600-H2.33 No obvious change in the feature bands is noticed between hydrogenated GCN and bulk GCN, suggesting that the molecular structure of original GCN backbone remains almost unchanged, in spite of the introduction of N-vacancies.34 Raman spectra and TGA analysis for hydrogenated GCN and GCN further confirm that no evident structural destruction occurs after the loss of N-atoms (Figure 2c-2d). In addition, TGA results show that the weight loss of bulk GCN is more pronounced than that of GCN-550-H2 and GCN-600-H2 (Figure 2d), suggesting the enhanced thermal stability of hydrogenated GCN, which may be attributed to the increase of amino groups (Table S1) by H2 heating treatment that facilitates the strong interaction between layers through the hydrogen bondings.35 The compositions and chemical states of hydrogenated GCN were investigated by XPS. The survey XPS spectra of GCN-600-H2 and GCN-550-H2 exhibit the presence of C, N, and O elements without any other detectable impurities (Figure S2). For high-resolution C 1s spectra of GCN-600-H2 (Figure 3a-3c), three peaks at 284.7, 287.0, and 293.0 eV correspond to characteristic graphite carbon (C=C bonds), sp2-hybridized carbon in the aromatic ring (N−C=N bonds), and C−O functional groups, respectively.36,

37

The peak-area ratio of C=C bonds to N−C=N bonds

increases from 0.06 in the GCN to 0.20 in the GCN-600-H2. The percentage increase 9



of graphite carbon content supports the assumption of nitrogen vacancies formation.38 In the high-resolution N 1s spectra (Figure 3d-3f), three peaks centered at 398.6, 399.3, and 400.6 eV, which are associated with sp2-hybridized nitrogen (C–N=C), sp3-hybridized nitrogen (N−(C)3), and amino groups (C−N−H) in the skeleton of GCN-600-H2, respectively.39 Quantitative analysis reveals that the C/N atomic ratios of GCN-600-H2 and GCN-550-H2 are determined to be 0.84 and 0.76, respectively, which are larger than that of the GCN (0.72). Elemental analysis also exhibits that the GCN-600-H2 has a larger C/N molar ratio (0.79) than that of GCN-550-H2 (0.72) and GCN (0.70), indicating the presence of nitrogen vacancies in the hydrogenated GCN (Table S1). With the increasing reaction temperature, the peak-area ratio of C–N=C to N−(C)3 decreases from 2.18 in the GCN to 1.75 in the GCN-600-H2, suggesting that the loss of N species mainly occurs at the C–N=C sites during the thermal treatment. Due to the reducing effect of H2 gas, partial N species of GCN are more easily reduced to NH3,30 resulting in the nitrogen loss, thus forming the nitrogen vacancies. The high-resolution O 1s spectrum of GCN-600-H2 shows the core level at 531.3 eV (Figure S3), which is ascribed to surface-adsorbed H2O. ESR measurement was employed to further confirm the presence of N-vacancies (Figure 4a). Both GCN-550-H2 and GCN-600-H2 possess one single Lorentzian line centered at g value of 2.003, which can be ascribed to unpaired electrons in sp2-carbon atoms of p-bonded aromatic rings due to the formation of carbon-based radicals.20 In contrast, no apparent ESR signal is detected from the bulk GCN. The carbon-based radicals ware generated as a result of chemical reaction between N 10


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atoms of GCN and active hydrogen atoms during the hydrogenation reaction, which leads to the partial removal of N-containing species from the GCN. Obviously, the GCN-600-H2 has the strongest intensity of ESR signal among the three samples, indicating the highest concentration of unpaired electrons (N-vacancies), which is beneficial to the photoelectrocatalytic process.28 Thus, both XPS and ESR results confirm the presence of N-vacancies in the hydrogenated GCN. UV−Vis diffuse reflectance spectra in Figure 4b show that the absorption edges of hydrogenated GCN are significantly red-shifted to the longer wavelengths compared to that of bulk GCN. The corresponding band gaps determined from Tauc plots are 2.54 and 1.82 eV for GCN-550-H2 and GCN-600-H2, respectively, smaller than that of GCN (2.78 eV),9 which results from the generation of N-vacancies.40 The light absorption of GCN from the UV into the visible region is enhanced after the hydrogen treatment, while the GCN-600-H2 exhibits the highest light absorption among three samples. The widened band gap of the hydrogenated GCN nanosheets can be further supported by the red-shifts of their photoluminescence emission peaks in Figure 4c. Moreover, the prepared GCN-600-H2 exhibits the lowest photoluminescence intensity, suggesting the lowest radiative recombination of photogenerated electron–hole pairs occurring in the GCN-600-H2, which may be attributed to the unique porous structure that allows for sufficient charge transport across the surface, thereby improving the charge separation.41 Time-resolved PL spectra further confirm the longer lifetime of the photogenerated charge carriers in the GCN-600-H2 compared with that of the GCN (Figure S4), signifying the increase of photogenerated charge involved in the 11



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photoelectrocatalytic reaction. N2 adsorption/desorption isotherms recorded on the hydrogenated GCN show typical type IV isotherms with H3-type hysteresis loop, indicative of the presence of porous structures (Figure 4d). The GCN-600-H2 possesses a high Brunauer–Emmett–Teller (BET) surface area of 114 m2 g−1 with a total pore volume of 0.62 cm3 g−1. This BET value is much higher than that of GCN-550-H2 (94 m2 g−1) and GCN (7 m2 g−1). The pore

size

distribution

curve

of

GCN-600-H2

calculated

by

the

Barrett–Joyner–Halenda (BJH) model confirms the presence of mesopores and macropores (Table S2). The high surface area and rich porosity result in more accessible active sites for efficient mass transport.42 Photoelectrochemical performance Figure 5a shows the transient photocurrent responses for GCN, GCN-550-H2, and GCN-600-H2 photoelectrodes with three on–off cycles of intermittent irradiation. All of the photoelectrodes exhibit instantaneous and reproducible photocurrent responses upon each irradiation. When the irradiation is interrupted, the photocurrent value rapidly decreases to zero, and the photocurrent is promptly returned to a constant value when the light is turned on. Under simulated sunlight irradiation (100 mW cm−2, AM 1.5G), the transient photocurrent densities of GCN-600-H2 (7.31 uA cm−2) and GCN-550-H2 (3.98 uA cm−2) are found to increase by 3.57 and 1.94 times than that of the GCN (2.05 uA cm−2) at 0.8 V, respectively. Although the photocurrent density of 7.31 uA cm−2 for GCN-600-H2 is still lower than that of some C3N4-based composite photoelectrodes (such as, C3N4/TiO243,

44

), there is considerable room for further

12


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improvement in the photoelectrochemical activity by constructing heterojunctions with other inorganic semiconductors.27, 45 This is beyond the scope of this study. The photocurrent enhancement indicates the more efficient separation of photoexcited electron–hole pairs for GCN-600-H2. When bias values of 1.0 or 1.2 V are applied to the GCN-600-H2, transient photocurrents also exhibit good switching behavior (Figure 5b). Moreover, no obvious change on the transient photocurrent of GCN-600-H2 photoelectrode is observed after 2,000 s of irradiation (Figure 5c). The EIS study reveals the smaller arc radius of Nyquist plots for GCN-600-H2 than those of GCN-550-H2 and GCN (Figure 5d), indicating a more efficient charge transfer.46 This result demonstrates that incorporating N-vacancies indeed can effectively improve the electronic conductivity of GCN to enhance the separation efficiency of photo-induced charge carriers. Photoelectrocatalytic activity The photoelectrocatalytic performance of GCN-600-H2, GCN-550-H2, and GCN was evaluated by degradation of 4-CP under simulated sunlight irradiation (100 mW cm−2, AM 1.5G), as shown in Figure 6a. The adsorption capacities of the three photocatalysts in dark are consistent with the order of their BET surface areas. Under simulated sunlight irradiation, the hydrogenated GCN clearly exhibits much higher photoelectrocatalytic performance than pure GCN. In particular, the GCN-600-H2 has the highest photoelectrocatalytic activity, and nearly 100% of the 4-CP is eliminated within 120 min irradiation; while only 73.8% and 41.1% of 4-CP is removed over GCN-550-H2 and GCN under the same condition, respectively. The excellent 13



photoelectrocatalytic performance of GCN-600-H2 can be attributed to the fact that it possesses the highest light absorption, lowest recombination of photogenerated electron–hole pairs, and largest specific surface areas among the three samples, which results from the highest concentration of N-vacancies. Moreover, no detectable degradation of 4-CP is measured during the electrocatalytic process (0.8 V, Figure 6b). Also, nearly no 4-CP is photodegraded in the absence of photocatalyst, which proves that the 4-CP itself is stable in both electrocatalytic and direct photolysis processes. The degradation efficiency of 4-CP over GCN-600-H2 in the photoelectrocatalytic process is much higher than the summation of the individual electrocatalytic and photocatalytic process, indicting the existence of cooperative interaction between photocatalytic and electrocatalytic process. The PEC degradation process of 4-CP was found to follow pseudo-first-order equation (Figure 6c). The GCN-600-H2 (0.017 min−1) exhibits the highest apparent degradation constant among all the samples, which is about 2.66 and 6.30 times higher than that of GCN-550-H2 (0.0064 min−1) and GCN (0.0027 min−1), respectively. The mineralization ratio of 4-CP over GCN-600-H2 is determined by TOC measurement (Figure 6d). After irradiation for 180 min, only 19.5% of TOC still remains while 4-CP is completely eliminated. The results indicate that most of the 4-CP is mineralized by GCN-600-H2 in the PEC process. Additionally, the GCN-600-H2 also exhibits a remarkable stability (Figure 6e), without any significant loss of photoelectrocatalytic activity even after four cycles of reuse. To further evaluate the photoelectrocatalytic performance of GCN-600-H2, the decomposition of phenol was 14


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also studied. As shown in Figure 6f, phenol could not be readily degraded in both electrochemical and direct photolysis processes. The GCN-600-H2 is able to degrade more than 78% phenol after 180 min simulated sunlight irradiation. In comparison, the GCN and GCN-550-H2 degrade only less than 25% and 49% of phenol within 180 min. Role of radical scavenge In order to reveal the photoelectrocatalytic mechanism, the active species generated in the degradation process of 4-CP over GCN-600-H2 were inspected by using radical scavenging experiments. The tertiary butyl alcohol (t-BuOH), ethylenediamine tetraacetic acid disodium salt (EDTA-2Na), and p-benzoquinone (BQ) were chosen as scavengers for quenching •OH radicals, holes, and O2•− radicals, respectively.11, 47 As shown in Figure 7a, with the addition of t-BuOH, the degradation efficiency of 4-CP is significantly reduced to 80% after 180 min irradiation, which indicats that •OH radicals are major reactive species in the photoelectrocatalytic reaction system. In contrast, the presence of EDTA-2Na could inhibit the 33.4% of 4-CP degradation, suggesting the involvement of photogenerated holes either acting as the reactive species or the origination of •OH radicals in the reaction process. However, when the BQ is added, only a slight decrease of degradation efficiency of 4-CP is observed. To further

understand

photoelectrocatalytic

the

role

reaction

of

active

process,

oxygen

the

ESR

species

involved

technique

using

in

the

DMPO

(5,5-dimethyl-1-pyrroline-N-oxide) as trapping reagent was employed (Figure 7b). The characteristic peaks of DMPO−•OH are observed for GCN-600-H2 under light 15



irradiation, compared with that of the sample in dark, indicating that the •OH reactive species is really generated during the reaction. These results indicate that both •OH radicals and holes are dominant reactive species for 4-CP degradation over GCN-600-H2 in the photoelectrocatalytic system. Proposed photoelectrocatalytic mechanism Based on the above results, the mechanism of photoelectrocatalytic process over GCN-600-H2 is proposed as follows (Figure 8): Under simulated sunlight irradiation, the GCN-600-H2 could be easily excited. The photogenerated electrons of GCN-600-H2 are transited to the conduction band (CB) and holes remain in the valence band (VB). The introduction of N vacancies leads to the narrowed band gap of GCN, which contributes to the enhanced separation efficiency of photo-generated electron and hole pairs. The photogenerated electrons accumulated on the CB of GCN-600-H2 could pass through the interface between GCN-600-H2 and Ti to the external circuit under the external electric field, thus the photogenerated electron-hole pairs is effectively separated. The electrons arrived at the counter electrode are further transformed into O2− radicals, subsequently inducing the degradation of 4-CP. Meanwhile, the leftover photogenerated holes in the VB of GCN-600-H2 react with the surface-adsorbed 4-CP molecules or interact with H2O molecules or OH− to form •OH radicals,9 which is a strong oxidant for 4-CP mineralization. Conclusion In summary, we developed a facile synthetic approach for fabrication of porous GCN nanosheets with N vacancies by heating bulk GCN under an H2 atmosphere. The 16


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obtained GCN-600-H2 nanosheets exhibited a layered structure with an average thickness of ～25 nm and a high surface area of 114 m2 g−1. The presence of N-vacancies in the GCN-600-H2 was confirmed by XPS and ESR results. The introduction of N vacancies leaded to the narrowed band gap, enhanced separation efficiency of photogenerated charge carriers, and increased specific surface area of bulk GCN. The resulting GCN-600-H2 exhibited significantly enhanced PEC performance and photoelectrocatalytic activity toward degradation of 4-CP under simulated sunlight irradiation in contrast to the bulk GCN. After irradiation for 180 min, the degradation and mineralization percentages of 4-CP over GCN-600-H2 was up to 100% and 80.5%, respectively. The radicals trapping experiments revealed that the •OH radicals and photogenerated holes were considered as the main active species responsible for the degradation 4-CP over GCN-600-H2. This study provides valuable insights into design of highly efficient photocatalysts with many potential applications such as in water splitting, CO2 reduction, and solar energy conversion devices. Acknowledgements Y. Hou thanks the support of NSFC 51702284, Fundamental Research Funds for the Central Universities (112109*172210171) and the Startup Foundation for Hundred-Talent Program of Zhejiang University (112100-193820101/001/022). Supporting Information The supporting information is available free of charge on ACS Publication website. Notes The authors declare no competing financial interest. 17



Corresponding authors Yang Hou, E-mail: [email protected] Junhong Chen, E-mail: [email protected] Yu Xie, E-mail: [email protected]

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Hou, Y.; Li, X.; Zou, X.; Quan, X.; Chen, G. Photoeletrocatalytic Activity of a

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22. Le, S.; Jiang, T.; Zhao, Q.; Liu, X.; Li, Y.; Fang, B.; Gong, M. Cu-doped Mesoporous

Graphitic

Carbon

Nitride

for

Enhanced

Visible-light

Driven

Photocatalysis. RSC Adv. 2016, 6, 38811-38819. 23. Ma, J.; Yang, Q.; Wen, Y.; Liu, W. Fe-g-C3N4/graphitized Mesoporous Carbon Composite as An Effective Fenton-like Catalyst in A Wide pH Range. Appl. Catal., B 2017, 201, 232-240. 24. Liu, Q.; Zhang, J. Graphene Supported Co-g-C3N4 as a Novel Metal–macrocyclic Electrocatalyst for the Oxygen Reduction Reaction in Fuel Cells. Langmuir 2013, 29, 3821-3828. 25. Mane, G. P.; Talapaneni, S. N.; Lakhi, K. S.; Ilbeygi, H.; Ravon, U.; Al-Bahily, K.; Mori, T.; Park, D. H.; Vinu, A. Highly Ordered Nitrogen-Rich Mesoporous Carbon Nitrides and Their Superior Performance for Sensing and Photocatalytic Hydrogen Generation. Angew. Chem. Int. Ed. 2017, 56, 8481-8485. 26. Huang, J.; Antonietti, M.; Liu, J. Bio-inspired Carbon Nitride Mesoporous Spheres for Artificial Photosynthesis: Photocatalytic Cofactor Regeneration for Sustainable Enzymatic Synthesis. J. Mater. Chem., A 2014, 2, 7686-7693. 27. Hou, Y.; Wen, Z.; Cui, S.; Guo, X.; Chen, J. Constructing 2D Porous Graphitic C3N4 Nanosheets/Nitrogen-Doped Graphene/Layered MoS2 Ternary Nanojunction with Enhanced Photoelectrochemical Activity. Adv. Mater. 2013, 25, 6291-6297. 28. Ho, W.; Zhang, Z.; Xu, M.; Zhang, X.; Wang, X.; Yu, H. Enhanced Visible-light-driven Photocatalytic Removal of NO: Effect on Layer Distortion on g-C3N4 by H2 heating. Appl. Catal., B 2015, 179, 106-112. 21



29. Hou, Y.; Zuo, F.; Dagg, A.; Feng, P. A Three-Dimensional Branched Cobalt-Doped α-Fe2O3 Nanorod/MgFe2O4 Heterojunction Array as A Flexible Photoanode for Efficient Photoelectrochemical Water Oxidation. Angew. Chem. Int. Ed. 2013, 52, 1248-1252. 30. Wang, X. L.; Fang, W. Q.; Yao, Y.; Liu, P.; Wang, Y.; Zhang, H.; Zhao, H.; Yang, H. G. Switching the Photocatalytic Activity of g-C3N4 by Homogenous Surface Chemical Modification with Nitrogen Residues and Vacancies. RSC Adv. 2015, 5, 21430-21433. 31. Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Phosphorus-doped Graphitic Carbon Nitrides Grown In Situ on Carbon-Fiber Paper: Flexible and Reversible Oxygen Electrodes. Angew. Chem. Int. Ed. 2015, 54, 4646-4650. 32. Li, S.; Dong, G.; Hailili, R.; Yang, L.; Li, Y.; Wang, F.; Zeng, Y.; Wang, C. Effective Photocatalytic H2O2 Production under Visible Light Irradiation at g-C3N4 Modulated by Carbon Vacancies. Appl. Catal., B 2016, 190, 26-35. 33. 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. 2016, 28, 6471-6477. 34. 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. 2015, 27, 4572-4577. 35. Ho, W.; Zhang, Z.; Xu, M.; Zhang, X.; Wang, X.; Huang, Y. Enhanced Visible-light-driven Photocatalytic Removal of NO: Effect on Layer Distortion on 22


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g-C3N4 by H2 hHeating. Appl. Catal., B 2015, 179, 106-112. 36. Li, X.; Hartley, G.; Ward, A. J.; Young, P. A.; Masters, A. F.; Maschmeyer, T. Hydrogenated Defects in Graphitic Carbon Nitride Nanosheets for Improved Photocatalytic Hydrogen Evolution. J. Phys. Chem., C 2015, 119, 14938-14946. 37. Li, X.; Feng, Y.; Li, M.; Li, W.; Wei, H.; Song, D. Smart Hybrids of Zn2GeO4 Nanoparticles and Ultrathin g-C3N4 Layers: Synergistic Lithium Storage and Excellent Electrochemical Performance. Adv. Funct. Mater. 2015, 25, 6858-6866. 38. Fang, J.; Fan, H.; Li, M.; Long, C. Nitrogen Self-doped Graphitic Carbon Nitride as Efficient Visible Light Photocatalyst for Hydrogen Evolution. J. Mater. Chem., A 2015, 3, 13819-13826. 39. 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. 40. Niu, P.; Liu, G.; Cheng, H. M. Nitrogen Vacancy-promoted Photocatalytic Activity of Graphitic Carbon Nitride. J. Phys. Chem., C 2012, 116, 11013-11018. 41. Tong, Z.; Yang, D.; Li, Z.; Nan, Y.; Ding, F.; Shen, Y.; Jiang, Z. Thylakoid-inspired Multishell g-C3N4 Nanocapsules with Enhanced Visible-light Harvesting and Electron Transfer Properties for High-Efficiency Photocatalysis. ACS Nano 2017, 11, 1103-1112. 42. Hou, Y.; Li, X.; Zhao, Q.; Chen, G. ZnFe2O4 Multi-porous Microbricks/graphene Hybrid Photocatalyst: Facile Synthesis, Improved Activity and Photocatalytic 23



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Mechanism. Appl. Catal., B 2013, 142–143, 80-88. 43. Su, J.; Geng, P.; Li, X.; Chen, G. Graphene-linked Ggraphitic Carbon Nitride/TiO2 Nanowire Arrays Heterojunction for Efficient Solar-driven Water Splitting. J. Appl. Electrochem. 2016, 46, 807-817. 44. Su, J.; Zhu, L.; Chen, G. Ultrasmall Graphitic Carbon Nitride Quantum Dots Decorated

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Highly

Efficient

Photoelectrochemical Activity. Appl. Catal., B 2016, 186, 127-135. 45. Hou, Y.; Wen, Z.; Cui, S.; Feng, X.; Chen, J. Strongly Coupled Ternary Hybrid Aerogels

of

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Porous

Graphitic-C3N4

Nanosheets/N-Doped

Graphene/NiFe-Layered Double Hydroxide for Solar-Driven Photoelectrochemical Water Oxidation. Nano Lett. 2016, 16, 2268-2277. 46. Zhou, W.; Li, W.; Wang, J. Q.; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K.; Wang, L.; Fu, H.; Zhao, D. Ordered Mesoporous Black TiO2 as Highly Efficient Hydrogen Evolution Photocatalyst. J. Am. Chem. Soc. 2014, 136, 9280-9283. 47. Ding, Y.; Zhang, G.; Wang, X.; Zhu, L.; Tang, H. Chemical and Photocatalytic Oxidative Degradation of Carbamazepine by using Metastable Bi3+ Self-doped NaBiO3 Nanosheets as A Bifunctional Material. Appl. Catal., B 2017, 202, 528-538.

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Page 25 of 33 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


(a)

GCN

(b)

50 nm

50 nm (c)

GCN-600-H2 (d)

50 nm

5 nm

GCN-600-H2

2 1/nm

(f)

(e)

300 nm

C

(g)

N

GCN-550-H2

300 nm

(h)

300 nm

O

300 nm

Figure 1. TEM and HRTEM images of GCN (a), GCN-550-H2 (b), and GCN-600-H2 25



(c-d). FESEM image (e) and corresponding elemental mappings (f-h) of GCN-600-H2. Inset in (d) is the corresponding SAED pattern of GCN-600-H2.

GCN GCN-550-H2

(a) 24

28

(b)

Intensity (a.u.)

Intensity (a.u.)

GCN-600-H2

32

GCN GCN-550-H2 GCN-600-H2

10

20

30

40

50

4000

2θ (degree)

3000

2000

-1

1000

Wavenumber (cm ) 100

(d)

GCN-600-H2

Weight loss (%)

GCN GCN-550-H2

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

Page 26 of 33

75

50 GCN-600-H2 GCN-550-H2

25

GCN

(c) 0 500

1000

1500

2000

-1

2500

3000

0

200

400

o

600

800

Temperature ( C)

Raman shift (cm )

Figure 2. (a) XRD patterns, (b) FTIR spectra, (c) Raman spectra, and (d) TGA curves of GCN, GCN-550-H2, and GCN-600-H2. Inset in (a) is the enlarged image for specific refraction degrees.

26


Page 27 of 33

287.2 eV

(a)

(d)

C 1s

Intensity (a.u.)

Intensity (a.u.)

284.3 eV

293.0 eV

296

292

288

284

280

405

(b)

287.2 eV

N 1s

Intensity (a.u.)

Intensity (a.u.)

396

GCN-550-H2

284.3 eV

293.0 eV

292

399

(e)

C 1s

GCN-550-H2

296

402

Binding energy (eV)

Binding energy (eV)

288

284

280

405

(c)

287.0 eV

402

399

396

Binding energy (eV)

Binding energy (eV)

296

N 1s

GCN

GCN

(f)

C 1s

N 1s GCN-600-H2

GCN-600-H2

Intensity (a.u.)

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


284.7 eV 293.0 eV

292

288

284

280

405

402

399

396

Binding energy (eV)

Binding energy (eV)

Figure 3. High-resolution C 1s and N 1s XPS spectra of GCN (a, d), GCN-550-H2 (b, e), and GCN-600-H2 (c, f).

27



(a)

GCN-600-H2

GCN-600-H2

GCN-550-H2

Absorbance (a.u.)

Intensity (a.u.)

GCN-550-H2 GCN

GCN

(b)

EPR

3350

3400

3450

300

450

400

GCN-600-H2

450

500

550

750

400

-1

GCN GCN-550-H2

GCN-600-H2

3

Volume adsorbed (cm g )

(c)

600

Wavelength (nm)

Magnetic field (G)

GCN-550-H2

300

GCN

200

100

0

Wavelength (nm)

100

200

300

400

500

Pore size (nm)

0 0.0

600

(d) dV/dD

3300

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

Page 28 of 33

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Figure 4. (a) ESR spectra, (b) UV−Vis diffuse reflectance spectra, (c) PL spectra, and (d) Nitrogen adsorption–desorption isotherm curves of GCN, GCN-550-H2, and GCN-600-H2. Inset: the pore size distribution curves calculated from the desorption branch of nitrogen isotherms of GCN, GCN-550-H2, and GCN-600-H2.

28


0.8 V

ON

(a)

-2

Photocurrent density (µA cm )

-2

10 GCN-600-H2 GCN-550-H2

OFF

GCN

5

0 0

100

200

GCN-600-H2

(b)

1.2 V

1.0 V

0.8 V

300

0

100

Time (s) 9

(c)

6

GCN-600-H2

3

1.0x10

5

8.0x10

4

6.0x10

4

4.0x10

4

2.0x10

4

200

300

Time (s) (d)

0.8 V

-Z'' (ohm)

-2


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 29 of 33

GCN GCN-550-H2

Dark

GCN-600-H2

0.0

0 500

1000

1500

2000

0

Time (s)

10000

20000

30000

40000

Z' (ohm)

Figure 5. (a) Transient photocurrent responses of GCN, GCN-550-H2, and GCN-600-H2 in 0.01 M Na2SO4 under simulated sunlight irradiation (100 mW cm–2) at 0.8 V. (b) Transient photocurrent responses of GCN-600-H2 under simulated sunlight irradiation at 0.8, 1.0, and 1.2 V, respectively. (c) Transient photocurrent response of GCN-600-H2 under simulated sunlight irradiation at 0.8 V. (d) EIS Nyquist plots of GCN, GCN-550-H2, and GCN-600-H2 in 0.01 M Na2SO4 under dark.

29



1.0

1.0

0.8

(a)

0.4

(b)

0.6

Ct/C0

0.6

Ct/C-60

0.8

Dark

0.4

Direct photolysis GCN GCN-550-H2

0.2

Electrocatalytic process Direct photolysis Photocatalysis Photoelectrocatalysis

0.2

GCN-600-H2

0.0 -60

0.0 0

60

120

180

0

60

Time (min)

Removal efficiency (%)

-1

k = 0.0027 min

Ln (Ct/C0)

(c)

-1

k = 0.0064 min

-1.2

-1.8

Direct photolysis GCN GCN-550-H2

-2.4

GCN-600-H2

0

180

100

-1

k = 0.003 min

-0.6

120

Time (min)

0.0

-1

k = 0.017 min

4-CP TOC

75

(d)

50

25

0

60

120

180

0

60

Time (min)

120

180

Time (min) 1.0

1.0 4

3

2

1

(e)

0.8

(f)

0.8

Phenol 0.6

Ct/C0

Ct/C0

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 30 of 33

0.4

0.6 0.4

0.2

0.2

0.0

0.0 0

3

6

9

12

Electrocatalytic process Direct photolysis GCN GCN-550-H2 GCN-600-H2

0

60

120

180

Time (min)

Time (h)

Figure 6. (a) Degradation of 4-CP by direct photolysis and photoelectrocatalysis with GCN, GCN-550-H2, and GCN-600-H2 under simulated sunlight irradiation (100 mW cm–2, 0.8 V). (b) Process of photocatalytic degradation of 4-CP over GCN-600-H2. (c) Corresponding kinetic linear fitting for the concentration changes of 4-CP from (a). (d) Photoelectrocatalytic degradation of 4-CP and corresponding TOC removal over GCN-600-H2 under simulated sunlight irradiation. (e) Cycling runs for the photoelectrocatalytic degradation of 4-CP over GCN-600-H2 under simulated sunlight irradiation. (f) Degradation of phenol by electrocatalytic process, direct photolysis, 30


Page 31 of 33

and photoelectrocatalysis with GCN, GCN-550-H2, and GCN-600-H2 under simulated sunlight irradiation (100 mW cm–2, 0.8 V).

1.0 GCN-600-H2

Hydroxyl radical

0.8 Light on

Ct/C0

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


0.6

(a)

(b)

0.4 Dark

BuOH EDTA-2Na BQ No scavenger

0.2 0.0 0

60

120

180

Time (min)

3440

3460

3480

3500

3520

Magnetic field (G)

Figure 7. (a) Plots of photogenerated carriers trapping in the system of photoelectrocatalytic degradation of 4-CP over GCN-600-H2, GCN-600-H2/1 mM BQ, GCN-600-H2/1 mM EDTA-2Na, and GCN-600-H2/1 mM tBuOH under simulated sunlight irradiation (100 mW cm–2, 0.8 V). (b) DMPO spin-trapping ESR spectra recorded with GCN-600-H2 in aqueous dispersion in dark and under simulated sunlight irradiation.

31



Page 32 of 33

e Simulated sunlight Ec

e- (electron) + O2 = O2 O2 + 4-CP = CO2 + H2O ● --

● --

Ev

ee-

h (hole) + H2O/OH- = ●OH ● OH + 4-CP = CO2 + H2O h (hole) + 4-CP = CO2 + H2O

Hydrogenated GCN

e-

Electrolyte

ee-

Pt foil

Figure 8. Schematic diagram of the routes of photogenerated charge carrier transfer and 4-CP degradation.

32


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Table of Content

N Vacancy

3300

3350

3400

3450

Magnetic field (G)

The porous graphitic carbon nitride with tunable N vacancy demonstrated high efficiency for photoelectrocatalytic degradation of 4-chlorophenol.

33


Nitrogen Vacancy Structure Driven Photoeletrocatalytic Degradation of

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