Flower-like g-C3N4 assembly from holy nanosheets with nitrogen vacancies for efficient NO abatement

Applied Surface Science 492 (2019) 166–176

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Flower-like g-C3N4 assembly from holy nanosheets with nitrogen vacancies for efficient NO abatement

T

Youyu Duana,b, Xiaofang Lia, , Kangle Lvb, , Li Zhaoc, Yi Liua,d ⁎

⁎

a

College of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, PR China Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, College of Resources and Environmental Science, South-Central University for Nationalities, Wuhan 430074, PR China c Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, PR China d College of Chemistry and Molecular Sciences, Wuhan University, 430072, PR China b

ARTICLE INFO

ABSTRACT

Keywords: Carbon nitride NO Photocatalytic oxidation Visible Nitrogen vacancy

As graphitic carbon nitride (gCN) can only absorb visible light with wavelength < 450 nm, and the recombination of photo-generated carriers is quick, resulting in its moderate photoreactivity. Herein, flower-like gCN assembly from porous nanosheets with nitrogen vacancies was prepared by calcination of melamine-cyanuric acid (MCA) supramolecular aggregates at 500 °C. NO oxidation was used to evaluate the photoreactivity of the prepared gCN in a continuous-flow reactor using a visible LED lamp (λ ≥ 400 nm) as the light source. We systematically studied the effect of calcination time (2–14 h) on the structure, property and photocatalytic performance of the prepared flower-like gCN. Enhanced visible photoreactivity of flower-like gCN was observed with extension the calcination time, and the sample calcined for 10 h (F10 sample) exhibits the highest NO removal rate (59.7%), which is much higher than that of bulk gCN (45.8%, B10 sample) which was prepared by calcination of melamine at the same temperature for 10 h. The improved photocatalytic activity of flower-like gCN is ascribed to the condensed π–π layer stacking, breaking of intraplanar hydrogen bonds, enlarged BET surface area, and formation of nitrogen vacancies, which result in a broadened visible responsive range and improved separation of the photo-generated carriers.

1. Introduction In recent years, much attention has been paid to semiconductor photocatalysis because it provides a potential way to solve the problems related to energy crisis and environmental pollution [1–6]. Although TiO2 is the most studied semiconductor photocatalyst, it can only be excited under the irradiation of UV light, which merely contains 3–5% of the energy in solar spectrum [7–11]. Recently, graphitic carbon nitride (gCN) has aroused great interest because it is a metal-free semiconductor photocatalyst which exhibits visible-light-responsive property and good biocompatibility [12–14]. However, the photoreactivity of gCN is not high enough to satisfy the practical applications due to its small BET surface area (usually < 10 m2 g−1), poor crystallinity, easy recombination of photo-generated carriers and limited visible-light-responsive range [15–18]. Up to now, many strategies have been used to improve the photoreactivity of gCN such as doping gCN with metals [19] or nonmetals [20], modification of gCN with carbon materials [21,22], and coupling gCN with other semiconductors to form ⁎

heterojunction [23–26]. The layer stacking distance is one of the important roles to affect the separation and mobility of the carriers of gCN as it can determine the interlayer polarization. By condensation and calcination the mixture of urea and oxamide, the distance between interlayers of the obtained gCN reduces from 0.326 to 0.292 nm, which stimulates the dissociation of exciton and therefore results in an increased the hot charge carrier yield [27]. To reduce the surface defects which may become the recombination centers, deteriorating the photoreactivity, well crystalline gCN was fabricated by polymerization of dicyandiamide in a closed stainless autoclave [28]. It was found that the rate of the photocatalytic hydrogen production over gCN almost increases 10 times when compared with that of bulk gCN sample that was prepared in an open system. Recently, Dong et al. have developed a novel modification strategy to enhance the photocatalytic efficiency of gCN by adopting alkali intercalation [29]. It was found that intercalation of alkalis such as Rb and Cs results in the formation of an interlayer vertical channel for directional electron delivery, which therefore boosts the separation

Corresponding authors. E-mail addresses: [email protected] (X. Li), [email protected] (K. Lv).

https://doi.org/10.1016/j.apsusc.2019.06.125 Received 20 January 2019; Received in revised form 4 June 2019; Accepted 12 June 2019 Available online 14 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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monochromatized Cu-Ka radiation with a scanning rate of 0.05°s−1. Fourier transform infrared spectroscopy (FTIR) was recorded on a spectrometer (NeXUS 470) using the KBr pellet technique. UV-vis diffuse reflectance spectroscopy was measured via a spectrophotometer (Shimadzu UV-2550) using BaSO4 as background. The BET surface area and pore size distribution curve were determined by nitrogen sorption apparatus (ASAP 2020, Micromeritic Instruments, USA). Prior to measurements, the photocatalysts were degassed at 150 °C. X-ray Multilab 2000 XPS system was used for photoelectron spectroscopy (XPS) measurement, and C1s peak at 284.4 eV of the surface adventitious carbon is used as reference. Electron paramagnetic resonance (EPR) was recorded by EPR spectrometer (JES-FA 200, JEOL), and specimens for electron spin-resonance (ESR) spectroscopy measurement was obtained in DMPO solution (methanol dispersion is used for DMPO-O2%− and aqueous dispersion is used for DMPO-%OH) under an LED lample irradiation (λ > 400 nm). F-700 fluorescence spectrophotometer (Hitachi, Japan) is used for photoluminescence (PL) spectrum measurement (λex = 380 nm). The transient-PL spectrum was monitored at FLSP-920 fluorescence spectrophotometer (λex = 330 nm).

of photo-generated carriers between layers. Exfoliation of bulk gCN to nanosheets is also an efficient way to improve the photoreactivity of gCN [15]. Till now, repeated oxidative calcination [15,30], sonicated exfoliation [31], solvent-assisted exfoliation [32] and even ball milling [33] have been used to prepare gCN nanosheets. However, these methods are energy- or time-consuming. Although gCN nanosheets possess high BET surface area, exfoliation usually provide an enlarged bandgap due to quantum size effect, which is not good for the utilization of visible light [34]. Creation of carbon or nitrogen vacancies in gCN is another way to improve its photoreactivity. By calcination of bulk gCN in argon gas flow, Li et al. successfully fabricated gCN with carbon vacancies [35]. It was found that carbon vacancies cannot only provide more active for oxygen adsorption, but also induce the electron delocalization. Cao et al. prepared gCN with nitrogen vacancies through calcination of melamine in nitrogen gas, and it was found that nitrogen vacancies facilitate the capture of photo-generated electrons, retarding the recombination of the carrier [36]. With the development of economy, our air is being polluted by exhausted gases from factories and automobile, which include SOx [37], NOx [38,39] and volatile organic compounds (VOCs) [40,41]. Semiconductor photocatalysis has been used for air purification such as oxidation of formaldehyde [42] and NO abatement [39,43,44]. Except gCN, some inorganic semiconductors such as TiO2 [45], bismuth-based photocatalysts such as BiOBr [46], Bi2WO6 [47], Bi2O2CO3 [48] and BiPO4 [49] have been used for photocatalytic NO oxidation. In this paper, flower-like gCN assembly from porous nanosheets with nitrogen vacancies (Nv) was prepared by in-situ transformation of flower-like melamine-cyanuric acid (MCA) supramolecular aggregates at high temperature in air, which was used for visible photocatalytic NO oxidation. The prepared flower-like gCN possesses the following important features: (1) porous structure benefitting the penetration of the NO gas; (2) large BET surface area for gas facilitating the adsorption and activation of NO; (3) broadened visible-light-responsive range and improved light-harvesting ability due to introduction of Nv and unique hollow structure [50]. Therefore, it is believed to be a good candidate photocatalyst for NO oxidation. Note that both %OH and %O2– are important reactive oxygen species (ROSs) that are responsible for the oxidization of NO [51].

2.3. Photoelectrochemical measurements Transient photocurrent, electrochemical impedance (EIS) spectroscopy and Mott-Schottky plots were performed on an electrochemical workstation (CHI760e, Shanghai, China) equipped with a standard there-electrode system, where a Pt wire, ITO/g-C3N4 electrode and Ag/ AgCl (in saturated KCl) electrode were used as the counter electrode, working electrode and reference electrode, respectively. 0.4 mol L−1 Na2SO4 aqueous solution was used as electrolyte solution. The ITO/gCN electrode was prepared by a drip coating method, and a 3 W LED lamp (λ = 420 ± 10 nm) was used as the light source (Shenzhen LAMPLIC, China). 2.4. Photocatalytic oxidation of NO A continuous-flow reactor (30 × 15 × 10 cm) was used for photocatalytic oxidation of NO, and a visible LED lamp (150 W) equipped with a cut-off filter (λ > 400 nm) was used as the light source. Firstly, 0.2 g of gCN photocatalyst was homogeneously deposited on the surface of a glass dish (diameter of 12.0 cm) by sonication and evaporation. Then the dish was moved into the reactor. After the adsorption-desorption equilibrium of NO over gCN photocatalyst was achieved, turn on the LED lamp to initiate the photocatalytic oxidation of NO. The concentration of NO was adjusted to 600 ppb in air before entering the reactor, and the flow rate was kept at 1.0 l min−1. The concentration of NO after photocatalytic oxidation was online detected by a NOx analyzer (Advanced Pollution Instrumentation, Model T200).

2. Experimental 2.1. Preparation The precursor, flower-like melamine-cyanuric acid (MCA) supramolecular aggregates, for the fabrication of flower-like gCN was prepared according to literature [52]. Briefly, 200 mL of DMSO solution containing 10.0 g of cyanuric acid was dropwise added into 400 mL of DMSO solution that has been dissolved 10.0 g of melamine under magnetic stirring. Ten minutes later, the resulted precipitates were collected by filtration. After washed with water and drying in an electronic oven at 80 °C for 10 h, 7.2 g of the MCA precursor was obtained. Flower-like gCN was synthesized by calcining 10.0 g of MCA precursor at 500 °C for certain time (2–14 h). The resulted flower-like gCN sample is denoted as Fx (see Table 1). F10 sample denotes the flowerlike gCN which was obtained by calcination of MCA precursor at 500 °C for 10 h, while B10 is the bulk gCN that was prepared by direct calcination of melamine (10.0 g) at 500 °C for 10 h.

3. Results and discussion 3.1. Morphology and phase structure of MCA Fig. 1a shows the digital image of MCA supramolecular aggregates. It can be seen that the MCA aggregates are condense white powders. After calcination of MCA at 500 °C for 10 h, loose yellow powders (F10 sample) were obtained (Fig. 1b), which are flower-like gCN assembly from nanosheets. When compared with flower-like gCN, the color of bulk gCN (B10) is much lighter (Fig. 1c), reflecting the improved lightharvesting ability of flower-like gCN. SEM images show that these MCA aggregates are relatively monodisperse, which exhibit flower-like structures in diameters of about 1–2 μm with the pedal thickness of ca. 20 nm (Fig. 2). Strong hydrogen bonds between melamine and cyanuric acid facilitate the self-assembly, resulting in the formation of stable MCA supermolecular aggregates (Fig. S1). The structure of MCA supramolecular aggregates is confirmed by XRD (Fig. S2) and FTIR spectrum (Fig. S3), corresponding to the

2.2. Characterization The morphology and microstructure of the photocatalyst were observed by a field emission scanning electron microscope (SEM) (Hitachi, Japan) and a transmission electron microscope (TEM) (G20, Tecnai, USA). The phase structure of the photocatalyst was characterized by powder X-ray diffraction (XRD) in a Bruker D8 Advance using 167

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Table 1 Physical property of the photocatalyst. Sample

Precursora

Calcination time (h)

SBET (cm2 g−1)

PV (cm3 g−1)

APS (nm)

Bandgap (eV)

Sub-bandgap (eV)

MCA F2 F4 F6 F8 F10 F12 F14 B10

– MCA MCA MCA MCA MCA MCA MCA Melamine

– 2 4 6 8 10 12 14 10

6.3 57.5 71.0 103.4 116.4 142.8 155.4 169.8 16.3

0.04 0.20 0.27 0.49 0.51 0.53 0.61 0.62 0.08

11.2 14.5 15.6 19.1 18.5 17.1 13.3 14.7 20.3

– 2.75 2.76 2.78 2.82 2.88 2.92 2.94 2.72

– 1.77 1.77 1.77 1.76 1.76 1.69 1.69 –

a

MCA means the melamine-cyanuric acid supramolecular aggregates.

(Fig. 4b). The porous structures may be caused by the emission of the gases such as NH3 in the polymerization of MCA. Phase transformation (crystallization) usually begins from the surface of the microsphere. With increase the reaction time, hollow interials are produced due to redistribution of the mass from core to shell (Ostwald Rippening) [53,54]. When compared with that of flower-like gCN, the density of bulk gCN is much larger (Fig. 1b and c), which is mainly due to its dense structure (Fig. S4). According to AFM image (Fig. 5), the thickness of a petal from F10 sample is about 2–3 nm, further confirming its sheet-like structure. The thickness of a gCN nanoparticle from bulk gCN (B10) was measured to be 10–20 nm (Fig. S5), about 5–10 times thicker than that of gCN nanosheet from F10. The hollow interials and porous structures of flower-like gCN not only facilitate the diffusion of the gas, but also result in multi-reflection of the light within the cave, which make it a good candidate photocatalyst for air purification. Fig. 6A illustrates the effect of calcination time on the XRD patterns of the obtained flower-like gCN. It can be seen that all samples have two peaks at around 27° and 13°, the typical XRD peaks of carbon nitride (gCN). The former intense peak is identified as the (002) peak of gCN originated from the interplanar stacking of aromatic systems, while the latter weak peak is attributed to the in-plane structural packing motif. When compared with these of bulk gCN sample (B10), both the (002) and (100) peak intensities of the flower-like gCN (F10) are much weaker (Fig. 6B), which is possibly due to the fact that flower-like gCN is assembled from nanosheets. It can also be seen from Fig. 6B that the diffraction angle of the (002) peak for F10 sample is 27.60°, larger than that of B10 sample (27.36°). That was to say, the polymerization of MCA aggregates results in a decrease in the interlayer stacking distance from 0.326 nm (B10) to 0.323 nm (F10), reflecting the stronger van der Waals attraction between the neighboring heptazine layers of flowerlike gCN. The potential barrier between the layers were estimated to be

Fig. 1. Digital images of the powders for MCA aggregates (a), flower-like gCN (b) and bulk gCN (c), respectively.

results in literatures [52]. 3.2. Transformation of MCA aggregates to gCN After calcination of MCA at 500 °C in air, flower-like gCN was obtained (SEM in Fig. 3), indicating that the transformation is a selftemplated in-situ transformation process, although the petals become much thinner. Carefully view shows that the flower-like gCN has hollow interials (Fig. 4a), and the building-blocks are porous nanosheets

Fig. 2. SEM images of MCA aggregates at low (a) and high (b) magnification. 168

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Fig. 3. SEM images the obtained flower-like F2 (a) and F10 (b) gCN samples which were prepared by calcination of MCA for 2 h and 10 h, respectively.

33.2 eV, which makes the electron transfer between layers very difficult [55]. Reducing the distance of π–π* layer stacking between heptazine layers can improve the transport of lateral charge, beneficial to improve the photoreactivity. On the contrary, the diffraction angle of the (100) peak for F10 sample decreases from 12.88° to 12.72°, indicating an extended interplanar packing distance of the flower-like gCN (0.696 nm) to that of bulk gCN (0.687 nm). Extending the interplanar packing distance can break the hydrogen bonds originate from the neighboring melon strands, facilitating the electron transfer within layers [27]. The structures of flower-like gCN and bulk gCN were further confirmed by FTIR spectra (Fig. 7). The absorption peaks in the range of 3000–3300 cm−1 originate from the stretching vibration of NeH, while the absorption in the range of 1200–1600 cm−1 is attributed to the typical stretching vibration of CeN heterocycles, and the absorption peak at 810 cm−1 is attributed to the breathing mode of triazine units of gCN [56].

prepared gCN samples. It can be seen that flower-like gCN photocatalysts exhibit improved light-harvesting ability compared to bulk gCN both in UV and visible light regions. Fig. 8B shows the UV–vis absorption spectra for flower-like gCN (F10) and bulk gCN (B10) photocatalysts. From it we can see that the band absorption edge of flowerlike gCN, which is originated from the π–π* electron transition upon irradiation, is 431 nm (bandgap of 2.88 eV). While the band adsorption edge of B10 is 456 nm, corresponding to a reduced bandgap of 2.72 eV. The quantum confinement effect of the building blocks of nanosheets (Fig. 3) is responsible for the blue-shifted adsorption edge or enlarged bandgap of flower-like gCN when compared with bulk gCN. In addition, obviously improved visible light absorption of flower-like gCN is also observed. This is due to the introduction of nitrogen vacancy (Nv), which will be shown below. Surface area also plays very important role on the photoreactivity as larger specific surface area means more active sites can participate in the photocatalytic reaction [57]. Fig. 9 compares the nitrogen adsorption isotherms of MCA, flow-like gCN (F10) and bulk gCN (B10) photocatalysts. We can see that the adsorption curve of flower-like gCN shifts upward, indicating its larger surface area. The BET surface area (SBET) of F10 is as high as 142.8 m2 g−1, which is 8.8 and 22.7 times larger than that of bulk gCN (16.3 m2 g−1) and precursor MCA (only 6.3 m2 g−1), respectively. The isotherm of flower-like gCN photocatalyst is type IV with a hysteresis loop at relative high pressure range of 0.9–1.0. This H3 type mesopores is consistent with its flower-like morphology (Fig. 3) [15,57]. Inset of Fig. 9 shows the distribution curves of pore size for

3.3. UV–visible light absorption spectrum and nitrogen sorption The efficiency of photocatalysis mainly depends on the following three processes: (1) separation of photo-generated carriers after absorption of the light, (2) migration of the photo-generated electrons and holes to the surface of the photocatalyst, and (3) capture of the carriers by surface adsorption substrates to initiate photoreaction. Therefore, light-harvesting ability plays an important role on the photoreactivity of the photocatalyst. Fig. 8A shows the UV–vis absorption spectra of the

Fig. 4. TEM images of flower-like gCN (F10 sample). 169

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Fig. 5. AFM image of a gCN nanosheet detached from F10 sample and the corresponding height curves determined along different lines.

Fig. 6. XRD patterns of flower-like gCN samples (A) and the comparison of the XRD spectra between B10 and F10 samples.

flower-like gCN and bulk gCN. It can be clearly seen that flower-like gCN has much larger pore volume (0.53 cm3 g−1), exceeding that of bulk gCN sample (0.08 cm3 g−1) by a factor of 6.6. Table 1 lists SBET and pore structures of all the prepared gCN photocatalysts. It can be observed that both SBET and pore volume (PV) of the photocatalyst increase with extending the calcination time of MCA precursor, and all the flower-like gCN sample possesses much larger SBET and PV than bulk gCN (B10), which benefit the adsorption and diffusion of the gas, enhancing the photoreactivity [50]. 3.4. Photocatalytic NO oxidation As a typical pollutant in air, NO is mainly produced from the incineration of fossil fuels and the emission of the vehicle exhaust, which can cause acid rain even photochemical smog, incurring irritation in respiratory system [58,59]. Therefore, removal of NO from the air is of great importance. Then NO oxidation was used to evaluate the photoreactivity of the gCN photocatalyst. From Fig. 10A, it can be seen that the NO is very stable even under the irradiation of visible LED lamp. However, obvious NO decay was observed in illuminated gCN. With increase in the calcination time from 2 h (F2 sample) to 10 h (F10

Fig. 7. FTIR spectras of gCN samples.

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Fig. 8. UV–vis diffuse reflectance spectra of gCN samples (A), and the comparison of the diffuse reflectance spectra between B10 and F10 samples (B).

only 45.8%. To evaluate the photo-stability of the gCN [34,60], recycling use of flower-like gCN (F10) for NO oxidation was also performed (Fig. 10B). It was found that NO removal rate reduced from 59.7% to 50.6% after reuse for 5 times, which infers that flower-like gCN is relatively stable. The decrease in photoreactivity of flower-like gCN is possibly due to the lost of the photocatalyst during recycle. 3.5. XPS and EPR analysis To account for the high photoreactivity of flower-like gCN, we also measured the XPS and EPR spectra. Fig. 11A compares the XPS spectra of flower-like gCN and bulk gCN. Both photocatalysts contain C, N and O elements with binding energies of 288 eV (C 1s), 399 eV (N 1s) and 533 eV (O 1s), respectively [28,61]. Fig. 11B is the high resolution XPS spectra in N 1s region for flowerlike gCN and bulk gCN. It can be seen that both samples contain three kinds of N element, namely, two-coordinated NeC2 (397.7 eV), threecoordinated NeC3 (398.7 eV), and NeH (400.2 eV), respectively [13,62]. According to the XPS characterization results, F10 sample contains a C:N molar ratio of 0.75, which is larger than B10 gCN sample (C:N molar ratio of 0.70). The higher C/N molar ratio of F10 than that of B10 indicates the introduction of Nv into the flower-like gCN photocatalyst. The peak-area ratio of NeC3 for F10 sample is 33.9%, much lower than that for B10 (37.2%) sample, indicating that Nv mainly locates at the NeC3 lattice sites (Fig. 11C). Fig. 11D compares the high resolution XPS spectra of C 1s between bulk gCN (B10) and flower-like gCN (F10). It can be seen that each

Fig. 9. Comparison of the nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves (inset) of MCA, bulk gCN (B10) and flower-like gCN (F10).

sample), the NO removal rate over flower-like gCN steady increases from 32.2% (F2 sample) to 59.7% (F10 sample) at illumination time of 30 min. Further increase in the calcination time, the photoreactivity of the prepared flower-like gCN begins to decrease, which is possibly due to the collapse of the hierarchical structure. Under other identical conditions, the removal rate of NO over illuminated bulk gCN (B10) is

Fig. 10. Photocatalytic NO oxidation curves in the presence of different gCN photocatalysts under visible light irradiation (A) and the stability test of F10 for photocatalytic NO oxidation. 171

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Fig. 11. Comparison of the XPS survey spectra (A) and deconvoluted high-resolution N 1s (B) and C 1s (D) XPS spectra between bulk gCN (B10) and flower-like gCN (F10). The schematic atomic model of incomplete g-C3N4 constructed from melon units with three kinds of nitrogen (C) are labeled as NeC2, NeC3 and NeH, respectively.

framework, we can observe a Lorentzian line with a g value of about 2 [63]. This is because the presence of Nv is accompanied with the increase of the number of unpaired electrons [36]. From Fig. 12, we can see that flower-like gCN sample exhibits much stronger EPR signal at g = 1.9993 compared to bulk gCN, indicating the higher concentration of Nv in flower-like gCN photocatalyst. As the intensity of EPR signal is positively related to the concentration of Nv in the g-C3N4 framework, the much stronger EPR signal in flower-like gCN than in bulk gCN indicates that F10 sample possesses high concentrated Nv. It was reported that the presence of Nv in gCN photocatalyst can facilitate the trap of photo-generated electrons, enhancing the photocatalytic activity [64]. Therefore, we can predict the high photoreactivity of flower-like gCN sample. 3.6. (Photo)electrochemical measurements and photoluminescence analysis Not only the excitation, but also the separation and recombination of photo-generated carriers can affect the photocatalytic efficiency. Therefore, we compared the photocurrents, electrochemical impedance spectroscopy (EIS) spectra, steady and transient photoluminescence spectra between flower-like gCN and bulk gCN samples. From Fig. 13A, it can be seen that the photocurrent of flower-like gCN can be as high as 37.4 nAcm−2, which is 3.5 times larger than that of bulk gCN (10.8 nAcm−2), reflecting the higher efficient separation of photogenerated carriers over illuminated flower-like gCN. Careful view shows that the shapes of the photocurrents for flower-like gCN and bulk gCN are totally different. The photocurrent of bulk gCN increases smoothly before levelling off upon illumination (Zone 1 in Fig. 13A), indicating that almost all the holes are injected to the electrolyte.

Fig. 12. EPR spectra for bulk gCN (B10) and flower-like gCN (F10).

spectrum can be fitted with three peaks corresponding to the sp2 CeC (284.6 eV), CeNH (285.9 eV) and CeN3 (287.6 eV) species, respectively. Further observation shows that the peak intensity of CeNH species is weakened in F10 when compared with in B10 sample, further confirming the formation of Nv in flower-like gCN sample. The formation of Nv in flower-like gCN is also confirmed by EPR spectra (Fig. 12). Generally, if there is no Nv in perfect gCN framework, the ESR signal will be very weak. However, if there is Nv in gCN 172

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Fig. 13. Comparison of the photocurrents (A), electrochemical impedance spectroscopy (B), photoluminescence spectra (C) and transient photoluminescence spectra (D) between bulk gCN (B10) and flower-like gCN (F10).

Fig. 14. Mott-Schottky plots of bulk gCN (B10) and flower-like gCN (F10) measured in Na2SO4 solution (0.4 mol L−1).

Fig. 15. Comparison of the calculated DOS between pristine gCN (a) and NvgCN (b) photocatalysts.

However, no steady Faradaic current, but an obvious anodic photocurrent spike is observed on illuminated flower-like gCN electrode (Zone 2 in Fig. 13A), which reflects that photo-generated holes are accumulated at the electrode/electrolyte interface instead of injection to the electrolyte due to the low potential [65]. The smaller arc radius on the EIS Nyquist plot of flower-like gCN than that of bulk gCN also indicates the effective separation of the carrier and a faster interfacial charge migration of flower-like gCN

(Fig. 13B). The recombination of the photo-generated carriers is also effectively retarded in flower-like gCN judging from its lower PL intensity (Fig. 13C) and longer average life time (Fig. 13D). 3.7. Band structures and mechanism To compare the band structure of flower-like gCN with that of bulk gCN photocatalyst, Mott-Schottky analysis was employed to determine 173

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resulting in an broadened visible-light-responsive region (Fig. 8B). The calculated DOS results also confirm the negatively-shifted CB potential of flower-like gCN with Nv (Fig. 15). The bandgap of flower-like gCN is 2.88 eV, which is 0.16 eV larger than that of bulk gCN (2.72 eV), which is originated from the π–π* electron transition in the conjugated aromatic ring system. However, the Nv-induced electron transition in flower-like gCN results in the formation of a smaller sub-bandgap (1.76 eV), benefiting the visible photoreactivity of gCN photocatalyst (Scheme 1). Both hydroxyl radicals (%OH) and super oxygen radicals (O2%–) are important ROSs for photocatalytic NO oxidation [59,66]. From the radicals-trapping experiments (Fig. 16), we can see that both the ESR signals of the trapped hydroxyl radicals (%OH) and super oxygen radicals (O2%−) over illuminated flower-like gCN are much stronger than over bulk gCN. In addition, according to TPR results, the amount of NO adsorbed on the surface of flower-like gCN is 1.58 times larger than on that of bulk gCN (Fig. S6). More active sites for the adsorption of NO should also facilitate its oxidation. Therefore, we can understand the enhanced photoreactivity of flower-like gCN. Scheme 1. Determined band structures of bulk gCN (B10) and flower-like gCN (F10).

4. Conclusions Flower-like gCN assembly from porous nanosheets was successfully fabricated by direct calcination of melamine-cyanuric acid (MCA) supramolecular aggregates. The flower-like gCN exhibits improved photocatalytic activity towards NO oxidation under visible light irradiation when compared with bulk gCN. The higher photoreactivity of flowerlike gCN than of bulk gCN is attributed to the synergistic effects of compacted layered-stacking, breaking of hydrogen bonds that exist in the intralayer framework, enlarged BET surface area, introduction of nitrogen vacancy and negatively shifted CB potentials, resulting in the improved visible-light harvesting ability and efficient separation of photo-generated electron-hole pairs. In addition, the unique hollow structure of flower-like gCN also favors for the multi-reflection of incident light and gas diffusion, benefiting the photocatalytic NO oxidation.

the flat band potentials (Efb), which is near the conduction band (CB) position of the photocatalyst [34]. From the Mott-Schottky plots (Fig. 14), the CB potentials of flower-like gCN and bulk gCN were determined to be −0.93 and −0.76 V vs Ag/AgCl, or −0.71 and −0.54 V vs NHE, respectively. By combination the band absorption edge of UV–visible diffuse reflectance spectrum (Fig. 8B), the CB potentials of F10 and B10 were calculated to be +2.18 V and + 2.17 V vs. NHE, respectively. That was to say, flower-like gCN and bulk gCN have similar VB potentials, while the CB position of flower-like gCN is more negatively shifted compared to bulk gCN. The negatively shifted CB potential favors for the production of %O2−, an important ROSs that are responsible for the oxidation of NO [51]. To account for the increased absorption in visible region of the flower-like gCN to that of bulk gCN (Fig. 8B), DFT was used to calculate the band structure. It was found that the calculated bandgap of pristine gCN is 0.72 eV, which is smaller than that of gCN with Nv (0.99 eV). This confirms the enlarged bandgap of flower-like gCN with Nv (F14). In addition, the introduction of Nv also results in the formation of defect states. The Nv-induced sub-band excitation from defect states to the CB of gCN therefore significantly changes the optical property of gCN,

Acknowledgements This work was supported by the National Natural Science Foundation of China (51672312, 21571192 & 21373275) and the Fundamental Research Funds for the Central Universities, SouthCentral University for Nationalities (CZT19006).

Fig. 16. ESR signals of the DMPO-%OH (A) and DMPO-%O2– (B) adducts formed in the suspensions of B10 and F10 photocatalysts.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.06.125.

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