Graphitic carbon nitride synthesized at different temperatures for enhanced visible-light photodegradation of 2-naphthol

Accepted Manuscript Full Length Article Graphitic carbon nitride synthesized at different temperatures for enhanced visible-light photodegradation of 2-naphthol Yunlong Lan, Zesheng Li, Dehao Li, Guangxu Yan, Zhenxing Yang, Shaohui Guo PII: DOI: Reference:

S0169-4332(18)32920-9 https://doi.org/10.1016/j.apsusc.2018.10.152 APSUSC 40720

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

25 June 2018 12 September 2018 16 October 2018

Please cite this article as: Y. Lan, Z. Li, D. Li, G. Yan, Z. Yang, S. Guo, Graphitic carbon nitride synthesized at different temperatures for enhanced visible-light photodegradation of 2-naphthol, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.10.152

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Graphitic carbon nitride synthesized at different temperatures for enhanced visible-light photodegradation of 2-naphthol Yunlong Lan a, Zesheng Li b, Dehao Li b, Guangxu Yan a, Zhenxing Yang a, Shaohui Guo a,* a

State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Oil and Gas Pollution Control, China University of Petroleum, Beijing 102249, China b

Technology Research Center for Petrochemical Resource Cleaner Utilization of

Guangdong Province, Guangdong University of Petrochemical Technology, Maoming 525000, China.

* Corresponding author. Tel: 86-010-89739001 email: [email protected] (S. Guo)

Abstract Graphitic carbon nitride (g-C3N4) was synthesized by one-step calcination at different temperatures for enhanced photodegradation of 2-naphthol under visible light (λ ≥ 420 nm). The g-C3N4 photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR) and ultraviolet-visible diffuse reflectance spectra (UV-vis DRS). The morphology, band structure and optical property of g-C3N4 could be controlled by the calcination temperature. The formation of genuine g-C3N4 actually began when the temperature was higher than 500 ℃. Moreover, compared to the primary stage (450 °C) and the intermediate stage (500 °C and 550 °C), the g-C3N4 synthesized at the advanced stage (600 °C and 650 °C) had not only the strongest absorption of visible light but also the narrowest band gap, enabling controllable adjustment of the band structure of catalysts. Furthermore, the photodegradation efficiency of 2-naphthol was improved as the calcination temperature of g-C3N4 rising up. Meanwhile, g-C3N4 synthesized at 600 °C performed the best with a rate constant of 1.949 h-1 for the first-order kinetic model, which was 2.67 times that of the catalyst synthesized at 450 °C. Finally, the possible photodegradation pathway of 2-naphthol by g-C3N4 was proposed.

Keywords: g-C3N4; Calcination temperature; Band structure; Visible-light photocatalysis; 2-Naphthol.

CONTENTS Abstract ..................................................................................................................... 2 1. Introduction ........................................................................................................... 4 2. Experimental ......................................................................................................... 6 2.1. Synthesis of g-C3N4 ..................................................................................... 6 2.2. Material characterizations ............................................................................ 6 2.3. Photocatalytic degradation ........................................................................... 7 3. Results and discussion ........................................................................................... 8 3.1. Crystalline structure ..................................................................................... 8 3.2. Morphology and porosity ............................................................................ 11 3.3. Chemical and electronic structure .............................................................. 12 3.4. Chemical composition ............................................................................... 13 3.5. UV–vis DRS analysis ................................................................................ 14 3.6. Enhancement of photocatalytic activity ...................................................... 16 3.7. Kinetics ..................................................................................................... 17 3.8. Possible photocatalytic mechanism ............................................................ 18 3.9. Stability test ............................................................................................... 21 4. Conclusions ......................................................................................................... 22 Acknowledgments ................................................................................................... 23 References ............................................................................................................... 23

3

1. Introduction With the development of the global economy and society, various environmental pollution problems have gradually emerged, and the harmless treatment of pollutants has become a common problem. In recent years, semiconductor photocatalysis using sunlight as the energy source has attracted wide attention in the field of pollutants treatment. Compared with traditional physical and chemical methods, photocatalysis has the advantages of mild reaction conditions, facile and environment-friendly operation, energy conservation and no secondary pollution. Hence, it has been widely used in the treatment of pollutants in the water environments, such as dyes, antibiotics and polycyclic aromatic hydrocarbons (PAHs) [1–10]. However, there are still few photocatalysts with high visible light response activity and high quantum efficiency at the same time, and some inorganic semiconductor photocatalysts using expensive rare metals, which limits the large-scale application of photocatalysis technology [11–13]. Therefore, efficient, inexpensive, sustainable and environment-friendly photocatalysts are the focus of current research. A metal-free organic polymeric photocatalyst, named as graphitic carbon nitride (g-C3N4), was recently reported by Xinchen Wang et al [14]. g-C3N4 possesses excellent properties, such as controllable semiconductor photonic band-gap, high chemical and thermal stability, as well as low cost and facile preparation process. Generally, g-C3N4 can be synthesized by pyrolysis of nitrogen-rich precursors, such as cyanamide, dicyandiamide, melamine, urea and thiourea [15–18]. The C-N bonds are condensed during the pyrolysis to form the tri-s-triazine sheets of g-C3N4. Moreover, it is found that the species and calcination conditions of precursors have a significant effect on the material properties and photocatalytic performance of g-C3N4 [19]. Therefore, the band gap of g-C3N4 can be controlled by means of doping and 4

modifying its morphology, so as to improve its visible light response activity and quantum efficiency. At present, g-C3N4 has been applied in many fields of photocatalytic research, such as photoinduced hydrogen production from water, photocatalytic organic synthesis and photocatalytic degradation of pollutants [20–24]. As previously reported, many contaminants can be degraded by g-C3N4 photocatalysts, such as antibiotics [25–27]，acarbose [28]，organic dyes [29–31]，sulfonamides [32]， herbicides [33,34]，steroids [35] and PAHs [36]. Polycyclic aromatic hydrocarbons and their derivatives have been identified as carcinogenic, mutagenic and teratogenic substances, which are widely distributed in the natural environment (atmosphere, water and soil), and also are of great concern due to their harmful effects on humans and organisms [37,38]. Among typical PAHs and their derivatives, 2-naphthol is one of the persistent organic pollutants with significant toxicity and has been identified as a hazardous pollutant [39]. Up to now, 2-Naphthol mainly comes from the wastewater produced in many chemical industries, such as dyes, pharmaceuticals, pesticides, plastics and synthetic rubbers [40–43]. Since 2-naphthol has serious adverse effects on the natural environment and human health, the effective removal of 2-naphthol has become one of the key tasks in wastewater treatment. Currently, there are many technologies that can remove and degrade 2-naphthol from the water. However, compared with conventional water purification technologies (adsorption, membrane separation, etc.), photocatalytic oxidation for removing 2-naphthol has the advantages of strong oxidation ability, high degradation efficiency, complete mineralization, handy operation and low cost [44,45]. Thus, photocatalysis is considered as one of the most effective methods for removing toxic and refractory pollutants in wastewater. In addition, the photodegradation of 2-naphthol by g-C3N4 is less reported so far. 5

This work aimed to prepare g-C3N4 photocatalysts by controlling the thermal polycondensation of melamine at different temperatures and to evaluate their activity in photocatalytic degradation of 2-naphthol under visible light. Meanwhile, the intermediates in the photodegradation of 2-naphthol were analyzed by the gas chromatograph mass spectrometer (GC-MS), and the possible degradation pathway of 2-naphthol was discussed. In addition, the mechanism of enhancing photocatalytic activity was elucidated through the analysis of crystalline structure, morphology, chemical and optical properties of the prepared photocatalysts.

2. Experimental 2.1. Synthesis of g-C3N4 The g-C3N4 photocatalysts were prepared by thermal polycondensation of melamine in a tube furnace. In brief, 5 g melamine was put into a crucible and then calcined at different temperatures (450 ℃, 500 ℃, 550 ℃, 600 ℃ and 650 ℃) with a heating rate of 2.2 ℃/min. The tube furnace was kept at this temperature for 4 h, then naturally cooled to room temperature. Nitrogen flow was used throughout the experiment to remove oxygen from the tube furnace. The prepared coarse g-C3N4 at different temperatures was grounded into powder and denoted as CN450, CN500, CN550, CN600 and CN650. Additionally, the samples in this paper were prepared at a slower heating rate to a predetermined temperature and maintained for a relatively longer period of time, compared to other methods of preparing g-C3N4 with melamine as the precursor (listed in Table S1). 2.2. Material characterizations X-ray diffraction (XRD) analysis of the prepared g-C3N4 was performed using a Rigaku Ultima IV-285E X-Ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) in the range of 2θ = 10° ~ 80°. The elemental analysis (C/H/N) was performed on a 6

Thermo Fisher Flash 2000 elemental analyzer. The morphology and structure of the as-prepared samples were studied by scanning electron microscope (SEM) and transmission electron microscope (TEM). The SEM images were taken by FEI Quanta 200 FEG, and the TEM investigations were operated with JEOL JEM-2010 at 200 kV. The Brunauer-Emmett-Teller (BET) specific surface area of the samples was measured on Micromeritics ASAP 2020. The samples were degassed at 180 ℃ before the analysis. The BET specific surface area was calculated with the adsorption data in the relative pressure (P/P0) range of 0.05 ~ 0.30. The pore size distribution was estimated following the Barret-Joyner-Halender (BJH) method. The structural information of the samples was measured by Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer Spectrum One). The elemental compositions and oxidation states of the photocatalysts were analyzed by X-ray photoelectron spectroscopy (XPS) analysis on an Escalab 250 X-ray photoelectron spectrometer. The ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) of the samples were measured by UV-vis spectrophotometer (Shimadzu UV-2550) in the range of 200 to 800 nm with BaSO4 used as the standard material. The electron spin resonance (ESR) signals were detected at ambient temperature on Bruker A300 spectrometer. 2.3. Photocatalytic degradation The photocatalytic activity of the as-prepared photocatalysts was evaluated by the photocatalytic degradation of the 2-naphthol solution under visible light irradiation. A 300 W Xe lamp was used as the light source and placed above the photocatalytic reactor. A 420 nm cut-off filter was set on the lamp to provide the visible light for photocatalytic reactions. In each experiment, 0.0200 g photocatalysts were added into the 200 mL 2-naphthol solution (100 mg/L). Before irradiation, the suspension was magnetically stirred in the dark for 30min to ensure an 7

adsorption-desorption equilibrium of 2-naphthol on the surface of the photocatalyst. 5 mL suspension was taken at predetermined irradiation intervals and filtered through a 0.22 μm nylon 66 membrane. The filtrates were analyzed at the maximum absorption wavelength of 2-naphthol with a characteristic absorption peak of 224 nm on a UV-vis spectrophotometer (Shimadzu UV-2550). The photocatalytic degradation efficiency (R) of the photocatalysts was calculated by the following formula: R = ( 1 – Ct /C0 ) × 100%

(1)

where Ct was the concentration of 2-naphthol solution at irradiation time t and C0 was the concentration after stirring 30 min in the dark. In order to determine the intermediates of photocatalytic degradation, 50 mL samples were taken at 10 min, 20 min and 40 min. Following centrifugation (10000 rpm, 15 min), the samples were extracted in the separatory funnels with 10 mL dichloromethane. The extracted organic phase was treated with anhydrous Na2SO4 to remove water and then filtered with a 0.22 μm polytetrafluoroethylene (PTFE) membrane. The treated solvent was analyzed for organic compounds on a GC–MS system (Shimadzu GCMS-QP2010) with the SH-Rxi-5Sil column (30.0 m × 0.25 mm × 0.25 μm). The GC was operated in the scan mode, and the samples were injected in the split mode with a split ratio of 30.0. The inlet temperature was 250.0 °C, and the oven temperature was started at 50.0 °C (2 min hold), then increased to 260.0 °C at a ramp rate of 20.00 °C/min and held for 2 min.

3. Results and discussion 3.1. Crystalline structure XRD patterns were used to determine the crystal structure of the as-prepared g-C3N4. The XRD patterns of g-C3N4 samples prepared at different temperatures and 8

melamine were shown in Fig. 1, indicating the samples synthesized at 650 °C, 600 °C and 550 °C had two characteristic peaks at 27.3° and 13.1° respectively. The strong characteristic peak at 27.3° could be indexed as (002) plane of g-C3N4 (JCPDS #87-1526), which was corresponding to the interplanar stacked aromatic structure [46]. Meanwhile, the weak peak at 13.1° could be put down to (100) plane, which was attributed to the packing motif of the in-plane structure [15,47]. Nevertheless, it could be found out from the XRD patterns that the characteristic peaks of the samples prepared at 500 °C and 450 °C were different from others, except for the peaks of 27.3° and 13.1°. For those two samples, there were other diffraction peaks which were associated with melem and its derivatives [48]. This result indicated that the crystalline structure of the composites was not consistent with that of g-C3N4 when the calcination temperature was not more than 500 °C, which means the sample being synthesized was not g-C3N4, but the intermediates in the deamination of melamine during the synthesis of g-C3N4. Moreover, the peaks of (002) plane moved from 27.2° towards a higher diffraction angle of 27.7° as the temperature rose up, indicating that the interplanar stacking distance was reduced, and the stacking order was optimized to make the samples prepared at a higher temperature had a more stable crystal structure [49,50]. Furthermore, the peaks of (002) plane became narrower and stronger with the increase of the calcination temperature, implying that the crystallinity of the samples was enhanced with the temperature [48]. Therefore, the genuine g-C3N4 with layered structure was synthesized only when the calcination temperature was greater than 500 °C, at which time the further polycondensation reaction took place. The elemental composition and C/N atomic ratio of the photocatalysts synthesized at different temperatures were analyzed by the elemental analysis to reveal the crystallinity of the samples (Table 1). The as-prepared photocatalysts were 9

mainly composed of C and N with a trace amount of H and O. Usually, the H of the samples came mainly from H2O adsorbed on the surface and its own –NH or -NH2 groups, while O was attributed to the adsorbed H2O, CO2, O2 and oxygen-containing impurities generated during the calcination, such as ammelide and ammeline [49,51]. Moreover, CN450 had the highest amount of H and O compared with others, especially the O, which indicated that g-C3N4 synthesized at 450 °C was not completely generated and contained an amount of oxygen-containing impurities. Meanwhile, the contents of H and O were significantly reduced when the calcination temperature was greater than 500 °C, indicating that the oxygen-containing impurities began to transform and the g-C3N4 actually began to form at this point, which was consistent with the XRD results. Furthermore, the C/N atomic ratio of the as-prepared photocatalysts gradually increased as the calcination temperature rose up, indicating that the degree of polymerization was enhanced and the crystallinity of g-C3N4 was improved. Theoretically, the C/N atomic ratio of the precursor melamine (C 3H6N6) was 0.500, and melam (C6H9N11), melem (C6H6N10) and melon (C6N9H3), as the intermediates and impurities during the synthesis, had a C/N ratio of 0.545, 0.600 and 0.667, respectively. Nevertheless, the C/N atomic ratio of the synthesized photocatalysts was less than the theoretical value of g-C3N4 (0.750), indicating that the amino groups in the samples did not completely polycondense to form a perfect periodic structure of tri-s-triazine units and there still contained certain impurities [48,52]. Noticeably, CN650 with the maximum C/N atomic ratio compared to others had a relatively low content of N, while the content of O was relatively high, implying that the highest calcination temperature of 650 °C leaded to the further condensation of amino groups in the CN650, and more oxygen-containing impurities were generated during the process [49,51]. Therefore, the increase of the calcination 10

temperature could effectively improve the crystallinity of g-C3N4, and the content of impurities in the photocatalysts decreased as the temperature rose up, while the oxygen-containing impurities relatively increased at the highest temperature of 650 °C. 3.2. Morphology and porosity SEM and TEM images of g-C3N4 calcined at different temperatures were shown in Fig. 2, which were used to investigate the morphology of the samples. From the SEM images, CN650 (Fig. 2a) showed a large bulk and CN600 (Fig. 2b) had a relatively smooth lamellar structure, while CN450 (Fig. 2e) had not yet fully formed the lamellar structure of g-C3N4, with a large number of solid rod structures. Thus, with the increase of calcination temperature, g-C3N4 photocatalysts gradually took shape, forming the lamellar structure from the solid rods, which could effectively increase the specific surface area. It was well known that the larger specific surface area could provide the more active sites and adsorption sites, which could improve the efficiency of the photocatalytic reaction [47,53]. In addition, the layered structure of g-C3N4 synthesized at 600 ℃ was found out to be the smoothest and more evenly distributed. This might be due to that when the calcination temperature was below 600 ℃, the layered structure had not fully grown yet, and as the temperature rose up to 650 ℃, the pores of g-C3N4 begun to collapse with the lamellar structure growing into large blocky structures. As could be seen from TEM images of g-C3N4, all of CN650, CN600 and CN550 had obvious lamellar structure, and the layered structure distribution of CN600 was the most uniform, which was consistent with SEM images. The nitrogen adsorption-desorption isotherms and the corresponding pore-size distribution curves of g-C3N4 synthesized at different temperatures were shown in Fig. 3, and Table S2 presented the BET surface area, pore volume and average pore width. 11

The BET specific surface areas of g-C3N4 calcined at 650 °C, 600 °C, 550 °C, 500 °C and 450 °C were respectively 29.7 m2/g, 21.5 m2/g, 12.0 m2/g, 4.3 m2/g and 3.8 m2/g, in which the specific surface area of CN650 and CN600 was about 7.8 and 5.7 times of the CN450. The result indicated that the surface area of g-C3N4 could be significantly increased with the calcination temperature rising up, which was consistent with what was observed in SEM and TEM images. Generally, the larger surface area facilitated the adsorption of more contaminants on the surface of the catalysts and correspondingly provided more active sites, which both could significantly enhance the photocatalytic degradation performance. Moreover, the nitrogen absorption-desorption isotherm of g-C3N4 was the typical type

IV

isotherm

with

an

H3

hysteresis

loop

according

to

the

Brunauer-Deming-Deming-Teller (BDDT) classification [46,54], indicating that there were mesoporous structures of the sample. Meanwhile, the mesopores could be confirmed from the pore-size distributions of g-C3N4. It could be found out from Fig. 3f that the pore-size distributions of CN650, CN600 and CN550 had an obvious peak around 3 nm, and all of the samples’ distributions were mainly dominant around 2 ~ 50 nm which was consistent with that of mesoporous materials. In addition, g-C3N4 gradually formed distinct pore structures as the calcination temperature rising up, and the pore volume also increased significantly (Table S2), where the BJH pore volume of CN650 and CN600 was 5.7 and 5.3 times that of CN450 (or CN500). Hence, the result indicated that the increasing calcination temperature facilitated to generate more pore structures of g-C3N4, increasing the specific surface area and promoting the adsorption and mass transfer of photodegradation reactions. 3.3. Chemical and electronic structure The chemical structures of g-C3N4 were further analyzed using FT-IR 12

spectroscopy (Fig. 4). All of the g-C3N4 samples calcined at different temperatures had a sharp absorption peak at 810 cm-1, corresponding to the breathing mode of triazine units [48,55]. The absorption peaks at 1640 cm-1, 1564 cm-1, 1412 cm-1, 1326 cm-1 and 1241 cm-1were mainly assigned to the stretching modes of C - N and C = N heterocycles in the range of 1225 ~ 1646 cm-1 [50]. Within this range, it could be found that the g-C3N4 photocatalysts synthesized at not less than 500 ℃ had a high similarity in the absorption peaks, while CN450 was different from the others with two characteristic peaks at 1460 cm-1 and 1610 cm-1 corresponding to the characteristic peaks of tri-s-triazine units, indicating that there might be melem [56]. It indicated that when the calcination temperature was below 500 °C, the synthesized g-C3N4 had not yet been molded, and there might be intermediates during the conversion of melamine to g-C3N4. Only when the calcination temperature was greater than 500 °C, did the actual formation of g-C3N4 begin, which was consistent with the previous results from XRD patterns. At last, the broad absorption peaks in the range of 3000 ~ 3600 cm-1 were mainly assigned to the N–H stretching of amino groups and the -OH band from water molecules adsorbed on the samples [26,57]. 3.4. Chemical composition XPS spectra could determine the surface elemental composition and chemical states of as-prepared g-C3N4 photocatalysts (Fig. 5). From the survey XPS spectra (Fig. 5a), all of the g-C3N4 samples showed similar peak structures. The C and N peaks were dominant in all these samples, of which the peak positions were very close. Apart from the C and N peaks, the small O peaks were observed in the spectra, which were usually attributed to oxygen-containing substances such as water or oxygen adsorbed during the measurement [50], and no peaks of other elements had been observed. The high-resolution C 1s spectra of the g-C3N4 samples had similar peak 13

structures and peak positions (Fig. 5b). The C 1s spectra could be further separated into two peaks at approximately 288.03 eV and 284.77 eV, corresponding to the sp 2 bond of C in the N-C=N groups of the samples and the sp2-bonded carbon of the organic impurities in the instrument [50,55], respectively. Similar to the C 1s spectra, the high-resolution N 1s spectra of all samples presented the same peak structures and positions of the main peaks (Fig. 5c), which could be deconvoluted into three peaks at 398.57 eV, 399.68 eV and 401.07 eV. The main peak at 398.57 eV was attributed to the sp2-bonded nitrogen of C=N-C groups, while the peaks at 399.68 eV and 401.07 eV corresponded to the nitrogen in the N–(C)3 groups and the C–N–H groups [50,55]. Therefore, all the g-C3N4 photocatalysts had similar peak structures and positions in the survey and high-resolution XPS spectra without obvious binding energy shift, indicating that the surface elements of g-C3N4 calcined at different temperatures were totally identical and so were the chemical states of all. 3.5. UV–vis DRS analysis The optical properties of g-C3N4 calcined at different temperatures were studied by UV-vis diffuse reflectance spectra. The g-C3N4 samples could be roughly divided into 3 groups according to the absorption edge of these nanomaterials (Fig. 6a). The first group consisted of CN650 and CN600. The absorption edge of these two catalysts had a large degree of redshift compared with CN450, and they had the strongest absorption of visible light in the range of 400 ~ 600 nm. The second group included CN550 and CN500, both of which had a far less absorbing edge than the first group, but there was still a certain redshift in comparison with CN450. And so was the absorption of visible light within the 400 ~ 600 nm. At last, the third group was CN450, which had a blueshift absorption edge compared to the others. Although there was absorption in visible light, CN450 was obviously the weakest. This result might 14

be due to that g-C3N4 synthesized at 450 °C had not been fully formed and it contained intermediates of the synthesis process, according to the previous XRD analysis. Moreover, the result of UV-vis DRS implied that the slower heating rate and the longer holding time might play a crucial role in the spectral grouping of the prepared g-C3N4 [48–50], which significantly affected the visible light absorption performance of g-C3N4 and enabled the controllable adjustment of the samples’ band gap. Therefore, the calcination temperature had a very significant effect on the optical properties of g-C3N4 based on the UV-vis DRS analysis. As the temperature increased, the absorption of visible light by g-C3N4 was enhanced, among which CN650 and CN600 had the best absorption performance. In addition, the band gaps of the CN650 and CN600 in the first group were the narrowest due to the maximum redshift of the absorption edge, which could make the catalysts most easily stimulated by visible light and have the best response-ability. The band gap (Eg) of the catalyst could be calculated by the following formula: αhν = k ( hν – Eg ) n/2

(2)

where, α，h，ν，k，Eg were the adsorption coefficient, Planck constant, photon frequency, constant and band gap, respectively. The value of n was determined by the optical transition type of the semiconductor. For the direct absorption n was 1, and for the indirect absorption n was 4. The n of g-C3N4 was 4 because of the indirect absorption. Then the approximate band gap value of g-C3N4 could be obtained by mapping (αhν)0.5 to hν, as shown in Fig. 6b. The results showed that the band gaps of g-C3N4 synthesized at 650 ℃, 600 ℃, 550 ℃, 500 ℃ and 450 ℃ were respectively about 2.47 eV, 2.31 eV, 2.58 eV, 2.61 eV and 2.68 eV, indicating that increasing calcination temperature could effectively narrow the band gap of g-C3N4, and enhance the absorption of light and the photocatalytic activity. Moreover, g-C3N4 synthesized 15

at 600 ℃ had the smallest band gap, which was most responsive to visible light correspondingly. 3.6. Enhancement of photocatalytic activity The photodegradation activity of g-C3N4 calcined at different temperatures was evaluated by the degradation of 2-naphthol under visible light (Fig. 7a). Before the photodegradation experiment, the mixture was stirred in the dark to make the catalysts and pollutants reach the adsorption equilibrium. From the adsorption results, the removal rate of 2-naphthol was about 4.1% ~ 6.5%, indicating that g-C3N4 had certain adsorption effects on 2-naphthol. Then the photodegradation experiment was carried out under visible light after stirring in the dark. The photodegradation efficiency of CN650, CN600, CN550, CN500 and CN450 were 82.9%, 86.6%, 69.7%, 64.5% and 52.4%, after 60 min irradiation (Fig. 7b). The degradation of 2-naphthol by the g-C3N4 samples could be broadly divided into 3 groups (Fig. 7a), similar to the results obtained in UV-vis DRS. The first group consisted mainly of CN650 and CN600, both of which significantly improved photodegradation performance by about 1.58 and 1.65 times compared to CN450. Furthermore, the photodegradation of CN600 was optimal compared with others. The second group included CN550 and CN500, both of which had a 1.33 and 1.23 times-fold increase in the photodegradation relative to CN450. Meanwhile, CN550 had better photodegradation performance than CN500, which might be due to CN500 still contained intermediates during the synthesis according to the XRD analysis. Finally, the third group was CN450, which had the worst adsorption and photodegradation performance, mainly because of that g-C3N4 synthesized at 450 ℃ had not yet been fully formed based on the previous analysis. Hence, the photocatalytic activity of g-C3N4 was enhanced with the increase of calcination temperature, and CN600 had the best degradation effect on 2-naphthol, 16

indicating that the photocatalytic activity could be controlled by the calcination temperature and 600 °C was the most reasonable calcination temperature for g-C3N4. 3.7. Kinetics The mechanism of photocatalytic degradation of 2-naphthol by g-C3N4 could be simulated by the zero-order and first-order kinetic models [58]: C0 – Ct = k0 t

(3)

ln ( C0 /Ct ) = k1 t

(4)

where Ct (mg/L) represented the concentration of 2-naphthol at irradiation time t, C0 (mg/L) represented the concentration of 2-naphthol after stirring in the dark, and k0 (mg/(L h)) and k1 (h-1) represented rate constants of zero-order and first-order models, respectively. The rate constants and relative coefficients (R2) of the zero-order and first-order kinetic models were summarized in Table S3. The kinetics models of the photodegradation of 2-naphthol by g-C3N4 were more close to the first-order model according to the results (Fig. 8a). And rate constants of the first-order reaction for g-C3N4 calcined at 650 ℃, 600 ℃, 550 ℃, 500 ℃ and 450 ℃ were respectively 1.767 h-1, 1.949 h-1, 1.176 h-1, 1.033 h-1 and 0.730 h-1, as shown in Fig. 8b. Obviously, the rate constant of CN600 was the largest, which was 2.67 times that of CN450, while CN650, CN550 and CN500’s rate constants were respectively 2.42 times, 1.61 times and 1.41 times that of CN450, consistent with the results of photodegradation experiments. It was well known that the narrower the band gap of a semiconductor, the easier it was to be stimulated by the irradiated visible light [46,47], thus producing more active radicals, which could effectively enhance the photodegradation activity and increase the reaction rate. Therefore, as the temperature increased, the band gap of g-C3N4 correspondingly decreased, and the band gap reached the minimum value 17

when the temperature was 600 ℃ according to the UV-vis DRS analysis, at which time the catalyst was most easily stimulated correspondingly and its photodegradation activity was the best with the largest rate constant. 3.8. Possible photocatalytic mechanism In general, the relatively narrow band gap of photocatalyst contributed to improving the response activity and degradation efficiency of photocatalytic reactions. Additionally, the higher calcination temperature could significantly make the band gap of the g-C3N4 catalysts decrease through the previous analysis. In addition to the effect of the band gap on the activity of the photocatalytic reactions, the conduction band (CB) and valence band (VB) positions of the photocatalysts also had an effect [48,50], which could influence the type of photo-induced active radicals. The valence band positions of g-C3N4 calcined at different temperatures could be estimated by the valence band X-ray photoelectron spectroscopy (VB XPS). The VB potentials of g-C3N4 synthesized at 650 °C, 600 °C, 550 °C, 500 °C and 450 °C were respectively 1.74 eV, 1.64 eV, 1.77 eV, 1.83 eV and 1.85 eV, as shown in Fig. 9 a. Then, the CB potentials of CN650, CN600, CN550, CN500 and CN450 were respectively calculated to be -0.73 eV, -0.67 eV, -0.81 eV, -0.78 eV and -0.83 eV, according to the formula ECB = EVB - Eg. Hence, the band structures of the g-C3N4 samples could be obtained in accordance with the potentials of VB and CB (Fig. 9b). It could be found that the band gap of the catalysts synthesized in the temperature range of 650 °C and 600 °C was obviously narrower than that of other temperature ranges, and the band gap of CN600 was the narrowest with the best response activity to visible light. In addition, the CB potentials of all g-C3N4 samples were more negative than E(O2/O2• −) [59], so the catalysts were able to reduce dissolved oxygen in the solution to O2• − radicals. And the contaminants were eventually degraded by a series of radicals’ chain 18

reactions and redox reactions. To determine the source of photooxidation ability and explore the mechanism of photodegradation, reactive oxygen radicals produced by CN600 were identified by ESR using DMPO as a spin-trapping reagent under visible light irradiation. In the dark environment, no obvious ESR signals of superoxide radicals (O 2• −) and hydroxyl radicals (•OH) were found, indicating that visible light irradiation made a great difference in the generation of reactive oxygen radicals [60,61]. Meanwhile, the characteristic peaks of O2• − and •OH were observed typically under visible light, and the intensities of both were enhanced as the irradiation time prolonged, indicating that •OH could be indirectly generated by a series of chain reactions of O 2•

−

in the

solution, while O2• − directly photoreduced from dissolved oxygen on the surface of photocatalysts [62–64]. Therefore, both of O2• − and •OH with strong oxidative ability exerted indispensable roles in the photocatalytic degradation of pollutants by g-C3N4. The mechanism of photocatalytic degradation of 2-naphthol by g-C3N4 was illustrated in Fig. 11 and Eqs. 5 ~ 11 depicted the reactions of 2-naphthol during the photocatalytic process. The g-C3N4 photocatalysts were stimulated under the visible light irradiation, with the photo-induced electrons (e-) and holes (h+) generated on the CB and VB of the catalysts, respectively (Eqs. 5). Due to the large potential difference between photoelectrons on the conduction band and dissolved oxygen in the solution (Fig. 9b), the dissolved O2 could be reduced to O2• − radicals (Eqs. 6), which could further generated •HO2 radicals and H2O2 through chain reactions (Eqs. 7 and 8). In addition, there would be theoretically a small amount of •OH radicals generated by the reaction of H2O2 [58] (Eqs. 9). Eventually, 2-naphthol could be oxidized to degrade by these free radicals and oxidizing substances (Eqs. 10). Moreover, the photo-induced holes h+ could also play a role in the photodegradation of 2-naphthol 19

[65] (Eqs. 11). g-C3N4 + hν → h+ + e–

(5)

O2 + e– → O2• −

(6)

O2• − + H+ → •HO2

(7)

O2• − + e– + 2H+ → H2O2

(8)

H2O2 + O2• − → •OH + OH– + O2

(9)

2-naphthol + O2• − (or •HO2, H2O2, •OH) → products

(10)

2-naphthol + h+ → products

(11)

GC-MS spectra of photodegradation byproducts of 2-naphthol by CN600 at various reaction times was shown in Fig. 12, and Fig. S1 presented the mass spectra of 2-naphthol and degradation byproducts during the photocatalytic reaction. It could be seen that 5 peaks were detected by GC-MS at the reaction time of 10 min and 20 min, and the strongest peak (D) was subjected to 2-naphthol, while the other four peaks were mainly straight-chain alkanes (A, B, C and E). However, the peak strength of 2-naphthol (D) was significantly weakened and the peak structure of straight-chain alkanes (A, B, C and E) was more pronounced when the photodegradation time reached 40 min. Furthermore, in addition to these 5 peaks, the new peaks of F and G were

detected,

corresponding

to

(E)-hex-3-en-1-yl

propyl

carbonate

and

2H-chromen-2-one, respectively. Straight-chain alkanes might be the main phased end products of the photodegradation process, thus making the peak strength at the initial stage too strong and masking other intermediates of the photodegradation. When the photodegradation reaction was carried out to 40 min, the concentration of 2-naphthol was reduced significantly, so the intermediates during the photodegradation reaction had relatively distinct peak structures. The possible degradation pathway during the photodegradation of 2-naphthol by 20

CN600 was proposed (Fig. 13). The hydroxyl addition reaction was taken on the benzene ring of 2-naphthol by the radicals (Step 1 in Fig. 13), and then 2H-chromen-2-one (Peak G in Fig. 12) was formed by the ketonization reaction [58] (Steps 2 and 3). Moreover, the unstable intermediate was produced via the radicals’ attack on the aromatic ring of 2-naphthol (Step 4), which could be further oxidized to naphthalene-1,4-dione [66] (Step 5). Then, the aromatic ring cleavage could generate phthalic acid with carboxylic acid on the cleaved bond, and the phthalic anhydride was formed due to the dehydration of phthalic acid [67,68] (Step 6). The new peak F in Fig. 12 arose after 40 minutes of photodegradation, attributed to (E)-hex-3-en-1-yl propyl carbonate, which was generated by the ring-opening of benzene rings and the alkylation via photoactivated alkanes (•R) [58] (Step 7). Finally, straight-chain alkanes generated by a series of radical reactions could be the main phased end products (Step 8), and the intermediates of the photodegradation would be mineralized into CO2 and H2O with the increase of reaction time (Step 9). 3.9. Stability test The stability of photocatalysts was an important aspect of the performance of catalysts and an important factor affecting its practical application. Thus, the stability of the g-C3N4 samples during the photocatalytic degradation was studied by the cycling experiments. CN600 was selected as an example in the cyclic experiments and each photodegradation experiment was performed under the same experimental conditions. Moreover, the catalysts were recovered at the end of each experiment and then reused for the next adsorption-photodegradation cycle. The photodegradation of 2-naphthol by CN600 in 5 consecutive runs was presented in Fig. 14, and the first-order reaction rate constants (k1) as well as the removal efficiency (R) of 2-naphthol after 60-min visible light irradiation in each experiment were listed in 21

Table S4. The results showed that the removal efficiency of 2-naphthol and the reaction rate were gradually decreasing, while the removal efficiency of 2-naphthol remained at 75.1% in the final cycle, indicating that CN600 was relatively stable during the cycling experiments.

4. Conclusions In this summary, g-C3N4 was successfully synthesized by one-step calcination at different temperatures for enhanced photodegradation of 2-naphthol under visible light. XRD, SEM, TEM, and FT-IR characterizations indicated that the formation of genuine g-C3N4 with layered structures actually began only when the calcination temperature was higher than 500 ℃. Based on the UV-vis DRS analysis, the calcination temperature had great impacts on the optical properties of g-C3N4, which could be divided into three stages, the primary stage (450 °C), the intermediate stage (500 °C and 550 °C) and the advanced stage (600 °C and 650 °C). And g-C3N4 synthesized at the advanced stage had not only the strongest absorption of visible light but also the narrowest band gap that significantly improved the sensitivity to visible light, enabling controllable adjustment of the catalysts’ band structure. Meanwhile, the enhancement of the optical properties of g-C3N4 was obvious for the improvement of the photodegradation of 2-naphthol under visible light, and the g-C3N4 synthesized at 600 ℃ performed the best, of which the first-order kinetic rate constant was 2.67 times that of the 450 °C. GC-MS indicated that the phased end products could be straight-chain alkanes during the photodegradation of 2-naphthol. All in all, g-C3N4 appears to be a high-efficiency photocatalyst with a controllable band structure, promising for photodegrading PAHs (eg., 2-naphthol) under the visible light irradiation.

22

Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 21777034).

References [1]

X. Yang, H. Cai, M. Bao, J. Yu, J. Lu, Y. Li, Insight into the highly efficient degradation of PAHs in water over graphene oxide/Ag3PO4 composites under visible light irradiation, Chem. Eng. J. 334 (2018) 355–376. doi:https://doi.org/10.1016/j.cej.2017.09.104.

[2]

Z. Xing, J. Zhang, J. Cui, J. Yin, T. Zhao, J. Kuang, Z. Xiu, N. Wan, W. Zhou, Recent advances in floating TiO2-based photocatalysts for environmental application, Appl. Catal. B Environ. 225 (2018) 452–467. doi:https://doi.org/10.1016/j.apcatb.2017.12.005.

[3]

U. Shanker, V. Jassal, M. Rani, Degradation of toxic PAHs in water and soil using potassium zinc hexacyanoferrate nanocubes, J. Environ. Manage. 204 (2017) 337–348. doi:https://doi.org/10.1016/j.jenvman.2017.09.015.

[4]

N. Vela, M. Martínez-Menchón, G. Navarro, G. Pérez-Lucas, S. Navarro, Removal of polycyclic aromatic hydrocarbons (PAHs) from groundwater by heterogeneous photocatalysis under natural sunlight, J. Photochem. Photobiol. A Chem. 232 (2012) 32–40. doi:https://doi.org/10.1016/j.jphotochem.2012.02.003.

[5]

P. Madhusudan, J. Ran, J. Zhang, J. Yu, G. Liu, Novel urea assisted

23

hydrothermal synthesis of hierarchical BiVO4/Bi2O2CO3 nanocomposites with enhanced visible-light photocatalytic activity, Appl. Catal. B Environ. 110 (2011) 286–295. doi:https://doi.org/10.1016/j.apcatb.2011.09.014. [6]

J. Ding, Z. Dai, F. Qin, H. Zhao, S. Zhao, R. Chen, Z-scheme BiO1-xBr/Bi2O2CO3 photocatalyst with rich oxygen vacancy as electron mediator for highly efficient degradation of antibiotics, Appl. Catal. B Environ. 205 (2017) 281–291. doi:https://doi.org/10.1016/j.apcatb.2016.12.018.

[7]

H. Huang, K. Xiao, Y. He, T. Zhang, F. Dong, X. Du, Y. Zhang, In situ assembly of BiOI@Bi12O17Cl2p-n junction: charge induced unique front-lateral surfaces coupling heterostructure with high exposure of BiOI {001} active facets for robust and nonselective photocatalysis, Appl. Catal. B Environ. 199 (2016) 75–86. doi:https://doi.org/10.1016/j.apcatb.2016.06.020.

[8]

H. Huang, X. Han, X. Li, S. Wang, P.K. Chu, Y. Zhang, Fabrication of Multiple Heterojunctions with Tunable Visible-Light-Active Photocatalytic Reactivity in BiOBr–BiOI Full-Range Composites Based on Microstructure Modulation and Band Structures, ACS Appl. Mater. Interfaces. 7 (2015) 482–492. doi:10.1021/am5065409.

[9]

H. Huang, K. Xiao, T. Zhang, F. Dong, Y. Zhang, Rational design on 3D hierarchical bismuth oxyiodides via in situ self-template phase transformation and phase-junction construction for optimizing photocatalysis against diverse contaminants, Appl. Catal. B Environ. 203 (2017) 879–888. doi:https://doi.org/10.1016/j.apcatb.2016.10.082. 24

[10] H. Huang, F. Chen, A.H. Reshak, S. Auluck, Y. Zhang, Insight into crystal-structure dependent charge separation and photo-redox catalysis: A combined experimental and theoretical study on Bi(IO3)3 and BiOIO3, Appl. Surf. Sci. 458 (2018) 129–138. doi:https://doi.org/10.1016/j.apsusc.2018.07.054. [11] A. Lebedev, F. Anariba, J.C. Tan, X. Li, P. Wu, A review of physiochemical and photocatalytic properties of metal oxides against Escherichia coli, J. Photochem. Photobiol. A Chem. 360 (2018) 306–315. doi:https://doi.org/10.1016/j.jphotochem.2018.04.013. [12] Z. Zeng, S. Chen, T.T.Y. Tan, F.-X. Xiao, Graphene quantum dots (GQDs) and its derivatives for multifarious photocatalysis and photoelectrocatalysis, Catal. Today. (2018). doi:https://doi.org/10.1016/j.cattod.2018.01.005. [13] M.J. Muñoz-Batista, A. Kubacka, A.B. Hungría, M. Fernández-García, Heterogeneous photocatalysis: Light-matter interaction and chemical effects in quantum efficiency calculations, J. Catal. 330 (2015) 154–166. doi:https://doi.org/10.1016/j.jcat.2015.06.021. [14] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2008) 76. doi:10.1038/nmat2317https://www.nature.com/articles/nmat2317#supplementa ry-information. [15] Z. Sun, H. Wang, Z. Wu, L. Wang, g-C3N4 based composite photocatalysts for 25

photocatalytic CO2 reduction, Catal. Today. 300 (2018) 160–172. doi:https://doi.org/10.1016/j.cattod.2017.05.033. [16] L. Zhang, H. Wang, W. Shen, Z. Qin, J. Wang, W. Fan, Controlled synthesis of graphitic carbon nitride and its catalytic properties in Knoevenagel condensations, J. Catal. 344 (2016) 293–302. doi:https://doi.org/10.1016/j.jcat.2016.09.023. [17] Z. Li, B. Li, S. Peng, D. Li, S. Yang, Y. Fang, Novel visible light-induced g-C3N4 quantum dot/BiPO4 nanocrystal composite photocatalysts for efficient degradation of methyl orange, RSC Adv. 4 (2014) 35144–35148. doi:10.1039/C4RA05749J. [18] Z. Li, S. Yang, J. Zhou, D. Li, X. Zhou, C. Ge, Y. Fang, Novel mesoporous g-C3N4 and BiPO4 nanorods hybrid architectures and their enhanced visible-light-driven photocatalytic performances, Chem. Eng. J. 241 (2014) 344–351. doi:https://doi.org/10.1016/j.cej.2013.10.076. [19] Q. Liu, X. Wang, Q. Yang, Z. Zhang, X. Fang, Mesoporous g-C3N4 nanosheets prepared by calcining a novel supramolecular precursor for high-efficiency photocatalytic hydrogen evolution, Appl. Surf. Sci. 450 (2018) 46–56. doi:https://doi.org/10.1016/j.apsusc.2018.04.175. [20] H. Zhao, H. Zhang, G. Cui, Y. Dong, G. Wang, P. Jiang, X. Wu, N. Zhao, A photochemical synthesis route to typical transition metal sulfides as highly efficient cocatalyst for hydrogen evolution: from the case of NiS/g-C3N4, Appl. Catal. B Environ. 225 (2018) 284–290. 26

doi:https://doi.org/10.1016/j.apcatb.2017.11.083. [21] W. Wang, Z. Shu, J. Zhou, T. Li, P. Duan, Z. Zhao, Y. Tan, C. Xie, S. Cui, Halloysite-derived mesoporous g-C3N4 nanotubes for improved visible-light photocatalytic hydrogen evolution, Appl. Clay Sci. 158 (2018) 143–149. doi:https://doi.org/10.1016/j.clay.2018.03.018. [22] J. Wu, Y. Zhang, T. Wang, Y. Xin, D. Ma, Au nanoparticles and graphene oxide co-loaded graphitic carbon nitride: Synthesis and photocatalytic application, Mater. Res. Bull. 100 (2018) 282–288. doi:https://doi.org/10.1016/j.materresbull.2017.12.040. [23] K. Li, M. Sun, W.-D. Zhang, Polycyclic aromatic compounds-modified graphitic carbon nitride for efficient visible-light-driven hydrogen evolution, Carbon N. Y. 134 (2018) 134–144. doi:https://doi.org/10.1016/j.carbon.2018.03.089. [24] C. Liu, H. Huang, W. Cui, F. Dong, Y. Zhang, Band structure engineering and efficient charge transport in oxygen substituted g-C3N4 for superior photocatalytic hydrogen evolution, Appl. Catal. B Environ. 230 (2018) 115–124. doi:https://doi.org/10.1016/j.apcatb.2018.02.038. [25] Y. Wang, F. Wang, Y. Feng, Z. Xie, Q. Zhang, X. Jin, H. Liu, Y. Liu, W. Lv, G. Liu, Facile synthesis of carbon quantum dots loaded with mesoporous g-C3N4 for synergistic absorption and visible light photodegradation of fluoroquinolone antibiotics, Dalt. Trans. 47 (2018) 1284–1293. doi:10.1039/c7dt04360k. 27

[26] Y. Wang, M. Qiao, J. Lv, G. Xu, Z. Zheng, X. Zhang, Y. Wu, g-C3N4/g-C3N4 isotype heterojunction as an efficient platform for direct photodegradation of antibiotic, Fullerenes Nanotub. Carbon Nanostructures. 26 (2018) 210–217. doi:10.1080/1536383x.2018.1427737. [27] X. Lu, Y. Wang, X. Zhang, G. Xu, D. Wang, J. Lv, Z. Zheng, Y. Wu, NiS and MoS2 nanosheet co-modified graphitic C3N4 ternary heterostructure for high efficient visible light photodegradation of antibiotic, J. Hazard. Mater. 341 (2018) 10–19. doi:10.1016/j.jhazmat.2017.07.004. [28] T. Muhmood, M. Xia, W. Lei, F. Wang, Erection of duct-like graphitic carbon nitride with enhanced photocatalytic activity for ACB photodegradation, J. Phys. D-Applied Phys. 51 (2018). doi:10.1088/1361-6463/aaa278. [29] D. Chen, S. Wu, J. Fang, S. Lu, G. Zhou, W. Feng, F. Yang, Y. Chen, Z. Fang, A nanosheet-like alpha-Bi2O3/g-C3N4 heterostructure modified by plasmonic metallic Bi and oxygen vacancies with high photodegradation activity of organic pollutants, Sep. Purif. Technol. 193 (2018) 232–241. doi:10.1016/j.seppur.2017.11.011. [30] Q. Wang, J. Zhang, W. Mao, Y. Zhu, J. Yang, Y. Min, X. Li, Efficient Organic Dyes Photodegradation Catalyzed by Nickel-Species Loaded Graphitic Carbon Nitride, J. Inorg. Organomet. Polym. Mater. 27 (2017) 1177–1189. doi:10.1007/s10904-017-0564-2. [31] L. Song, Y. Pang, Y. Zheng, L. Ge, Hydrothermal synthesis of novel g-C3N4/BiOCl heterostructure nanodiscs for efficient visible light 28

photodegradation of Rhodamine B, Appl. Phys. a-Materials Sci. Process. 123 (2017). doi:10.1007/s00339-017-1091-2. [32] Y. Song, J. Tian, S. Gao, P. Shao, J. Qi, F. Cui, Photodegradation of sulfonamides by g-C3N4 under visible light irradiation: Effectiveness, mechanism and pathways, Appl. Catal. B-Environmental. 210 (2017) 88–96. doi:10.1016/j.apcatb.2017.03.059. [33] T. Muhmood, M. Xia, W. Lei, F. Wang, M.A. Khan, Efficient and stable ZrO2/Fe modified hollow-C3N4 for photodegradation of the herbicide MTSM, Rsc Adv. 7 (2017) 3966–3974. doi:10.1039/c6ra26227a. [34] X. Liu, C. Li, Y. Zhang, J. Yu, M. Yuan, Y. Ma, Simultaneous photodegradation of multi-herbicides by oxidized carbon nitride: performance and practical application, Appl. Catal. B-Environmental. 219 (2017) 194–199. doi:10.1016/j.apcatb.2017.07.007. [35] A. Kumar, A. Kumar, G. Sharma, M. Naushad, R.C. Veses, A.A. Ghfar, F.J. Stadler, M.R. Khan, Solar-driven photodegradation of 17-beta-estradiol and ciprofloxacin from waste water and CO2 conversion using sustainable coal-char/polymeric-g-C3N4/RGO metal-free nano-hybrids, New J. Chem. 41 (2017) 10208–10224. doi:10.1039/c7nj01580a. [36] X. Yang, H. Cai, M. Bao, J. Yu, J. Lu, Y. Li, Highly Efficient Photocatalytic Remediation of Simulated Polycyclic Aromatic Hydrocarbons (PAHs) Contaminated Wastewater under Visible Light Irradiation by Graphene Oxide Enwrapped Ag3PO4 Composite, Chinese J. Chem. 35 (2017) 1549–1558. 29

doi:10.1002/cjoc.201700202. [37] M.F. de Oliveira Galvão, N. de Oliveira Alves, P.A. Ferreira, S. Caumo, P. de Castro Vasconcellos, P. Artaxo, S. de Souza Hacon, D.A. Roubicek, S.R. Batistuzzo de Medeiros, Biomass burning particles in the Brazilian Amazon region: Mutagenic effects of nitro and oxy-PAHs and assessment of health risks, Environ. Pollut. 233 (2018) 960–970. doi:https://doi.org/10.1016/j.envpol.2017.09.068. [38] D.M. Agudelo-Castañeda, E.C. Teixeira, I.L. Schneider, S.R. Lara, L.F.O. Silva, Exposure to polycyclic aromatic hydrocarbons in atmospheric PM1.0 of urban environments: Carcinogenic and mutagenic respiratory health risk by age groups, Environ. Pollut. 224 (2017) 158–170. doi:https://doi.org/10.1016/j.envpol.2017.01.075. [39] C. Croera, D. Ferrario, L. Gribaldo, In vitro toxicity of naphthalene, 1-naphthol, 2-naphthol and 1,4-naphthoquinone on human CFU-GM from female and male cord blood donors, Toxicol. Vitr. 22 (2008) 1555–1561. doi:10.1016/j.tiv.2008.06.004. [40] Y. Kimura, A. Kashiwada, K. Matsuda, K. Yamada, Use of chitosan for removal of naphthols through tyrosinase‐ catalyzed quinone oxidation, J. Appl. Polym. Sci. 125 (2012) E42–E50. doi:10.1002/app.36661. [41] Z. Wang, C. Ye, L. Chen, H. Wu, Co-immobilization of 1-vinyl-3-octadecylimidazolium cations and p-styrenesulphonate anions on silica and their anti-interference performance for the adsorption of naphthols, 30

Rsc Adv. 7 (2017) 2650–2657. doi:10.1039/C6RA26028D. [42] L. Xu, S. Wang, J. Zhou, H. Deng, R.L. Frost, Column adsorption of 2-naphthol from aqueous solution using carbon nanotube-based composite adsorbent, Chem. Eng. J. 335 (2018) 450–457. doi:https://doi.org/10.1016/j.cej.2017.10.176. [43] S. Zang, B. Lian, Synergistic degradation of 2-naphthol by Fusarium proliferatum and Bacillus subtilis in wastewater, J. Hazard. Mater. 166 (2009) 33–38. doi:https://doi.org/10.1016/j.jhazmat.2008.10.117. [44] T. Huang, R. Zhou, J. Cui, J. Zhang, X. Tang, S. Chen, J. Feng, H. Liu, Fast and cost-effective preparation of antimicrobial zinc oxide embedded in activated carbon composite for water purification applications, Mater. Chem. Phys. 206 (2018) 124–129. doi:https://doi.org/10.1016/j.matchemphys.2017.11.044. [45] J. Gröttrup, F. Schütt, D. Smazna, O. Lupan, R. Adelung, Y.K. Mishra, Porous ceramics based on hybrid inorganic tetrapodal networks for efficient photocatalysis and water purification, Ceram. Int. 43 (2017) 14915–14922. doi:https://doi.org/10.1016/j.ceramint.2017.08.008. [46] K. Wang, Q. Li, B. Liu, B. Cheng, W. Ho, J. Yu, Sulfur-doped g-C3N4 with enhanced photocatalytic CO2-reduction performance, Appl. Catal. B-Environmental. 176 (2015) 44–52. doi:10.1016/j.apcatb.2015.03.045. [47] J. Wen, X. Li, H. Li, S. Ma, K. He, Y. Xu, Y. Fang, W. Liu, Q. Gao, Enhanced visible-light H-2 evolution of g-C3N4 photocatalysts via the synergetic effect 31

of amorphous NiS and cheap metal-free carbon black nanoparticles as co-catalysts, Appl. Surf. Sci. 358 (2015) 204–212. doi:10.1016/j.apsusc.2015.08.244. [48] I. Papailias, T. Giannakopoulou, N. Todorova, D. Demotikali, T. Vaimakis, C. Trapalis, Effect of processing temperature on structure and photocatalytic properties of g-C3N4, Appl. Surf. Sci. 358 (2015) 278–286. doi:10.1016/j.apsusc.2015.08.097. [49] P. Praus, L. Svoboda, M. Ritz, I. Troppova, M. Sihor, K. Koci, Graphitic carbon nitride: Synthesis, characterization and photocatalytic decomposition of nitrous oxide, Mater. Chem. Phys. 193 (2017) 438–446. doi:10.1016/j.matchemphys.2017.03.008. [50] Z. Mo, X. She, Y. Li, L. Liu, L. Huang, Z. Chen, Q. Zhang, H. Xu, H. Li, Synthesis of g-C3N4 at different temperatures for superior visible/UV photocatalytic performance and photoelectrochemical sensing of MB solution, Rsc Adv. 5 (2015) 101552–101562. doi:10.1039/c5ra19586a. [51] J. Xu, Y. Li, S. Peng, G. Lu, S. Li, Eosin Y-sensitized graphitic carbon nitride fabricated by heating urea for visible light photocatalytic hydrogen evolution: the effect of the pyrolysis temperature of urea, Phys. Chem. Chem. Phys. 15 (2013) 7657–7665. doi:10.1039/c3cp44687e. [52] M. Caux, F. Fina, J.T.S. Irvine, H. Idriss, R. Howe, Impact of the annealing temperature on Pt/g-C3N4 structure, activity and selectivity between photodegradation and water splitting, Catal. Today. 287 (2017) 182–188. 32

doi:10.1016/j.cattod.2016.11.007. [53] H. Jia, J. Zhao, X. Fan, K. Dilimulati, C. Wang, Photodegradation of phenanthrene on cation-modified clays under visible light, Appl. Catal. B Environ. 123–124 (2012) 43–51. doi:https://doi.org/10.1016/j.apcatb.2012.04.017. [54] Z. Li, L. Zhang, B. Li, Z. Liu, Z. Liu, H. Wang, Q. Li, Convenient and large-scale synthesis of hollow graphene-like nanocages for electrochemical supercapacitor application, Chem. Eng. J. 313 (2017) 1242–1250. doi:https://doi.org/10.1016/j.cej.2016.11.018. [55] F. Dong, L. Wu, Y. Sun, M. Fu, Z. Wu, S.C. Lee, Efficient synthesis of polymeric g-C3N4 layered materials as novel efficient visible light driven photocatalysts, J. Mater. Chem. 21 (2011) 15171–15174. doi:10.1039/C1JM12844B. [56] X. Wei, Y. Qiu, W. Duan, Z. Liu, Cathodic and anodic photocurrents generation from melem and its derivatives, Rsc Adv. 5 (2015) 26675–26679. doi:10.1039/C5RA02816G. [57] W. Feng, J. Fang, G. Zhou, L. Zhang, S. Lu, S. Wu, Y. Chen, Y. Ling, Z. Fang, Rationally designed Bi@BiOCl/g-C3N4 heterostructure with exceptional solar-driven photocatalytic activity, Mol. Catal. 434 (2017) 69–79. doi:10.1016/j.mcat.2017.03.004. [58] X. Zhao, Z. Cai, T. Wang, S.E. O’Reilly, W. Liu, D. Zhao, A new type of cobalt-deposited titanate nanotubes for enhanced photocatalytic degradation of 33

phenanthrene, Appl. Catal. B-Environmental. 187 (2016) 134–143. doi:10.1016/j.apcatb.2016.01.010. [59] T. Ma, J. Wu, Y. Mi, Q. Chen, D. Ma, C. Chai, Novel Z-Scheme g-C3N4/C@Bi2MoO6 composite with enhanced visible-light photocatalytic activity for beta-naphthol degradation, Sep. Purif. Technol. 183 (2017) 54–65. doi:10.1016/j.seppur.2017.04.005. [60] H. Huang, A.H. Reshak, S. Auluck, S. Jin, N. Tian, Y. Guo, Y. Zhang, Visible-Light-Responsive Sillén-Structured Mixed-Cationic CdBiO2Br Nanosheets: Layer Structure Design Promoting Charge Separation and Oxygen Activation Reactions, J. Phys. Chem. C. 122 (2018) 2661–2672. doi:10.1021/acs.jpcc.7b08661. [61] H. Huang, X. Li, J. Wang, F. Dong, P.K. Chu, T. Zhang, Y. Zhang, Anionic Group Self-Doping as a Promising Strategy: Band-Gap Engineering and Multi-Functional Applications of High-Performance CO32–-Doped Bi2O2CO3, ACS Catal. 5 (2015) 4094–4103. doi:10.1021/acscatal.5b00444. [62] H. Huang, S. Tu, C. Zeng, T. Zhang, A.H. Reshak, Y. Zhang, Macroscopic Polarization Enhancement Promoting Photo- and Piezoelectric-Induced Charge Separation and Molecular Oxygen Activation, Angew. Chemie Int. Ed. 56 (2017) 11860–11864. doi:10.1002/anie.201706549. [63] H. Huang, Y. He, X. Li, M. Li, C. Zeng, F. Dong, X. Du, T. Zhang, Y. Zhang, Bi2O2(OH)(NO3) as a desirable [Bi2O2]2+ layered photocatalyst: strong intrinsic polarity, rational band structure and {001} active facets co-beneficial 34

for robust photooxidation capability, J. Mater. Chem. A. 3 (2015) 24547–24556. doi:10.1039/C5TA07655B. [64] H. Huang, K. Xiao, N. Tian, F. Dong, T. Zhang, X. Du, Y. Zhang, Template-free precursor-surface-etching route to porous, thin g-C3N4 nanosheets for enhancing photocatalytic reduction and oxidation activity, J. Mater. Chem. A. 5 (2017) 17452–17463. doi:10.1039/C7TA04639A. [65] T.N. Das, P. Neta, Reduction Potentials of Naphthoxyl and Pyridoxyl Radicals in Aqueous Solutions, J. Phys. Chem. A. 102 (1998) 7081–7085. doi:10.1021/jp981930w. [66] O.T. Woo, W.K. Chung, K.H. Wong, A.T. Chow, P.K. Wong, Photocatalytic oxidation of polycyclic aromatic hydrocarbons: Intermediates identification and toxicity testing, J. Hazard. Mater. 168 (2009) 1192–1199. doi:https://doi.org/10.1016/j.jhazmat.2009.02.170. [67] X. Wang, C. Chen, J. Li, X. Wang, Ozone degradation of 1-naphthol on multiwalled carbon nanotubes/iron oxides and recycling of the adsorbent, Chem. Eng. J. 262 (2015) 1303–1310. doi:https://doi.org/10.1016/j.cej.2014.10.107. [68] S. Xia, L. Zhang, X. Zhou, M. Shao, G. Pan, Z. Ni, Fabrication of highly dispersed Ti/ZnO–Cr2O3 composite as highly efficient photocatalyst for naphthalene degradation, Appl. Catal. B Environ. 176–177 (2015) 266–277. doi:https://doi.org/10.1016/j.apcatb.2015.04.008.

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Table 1. The elemental composition and atomic ratio of C to N for the g-C3N4 samples. Fig. 1. XRD patterns of as-prepared g-C3N4 samples. Fig. 2. SEM images of g-C3N4 samples: (a) CN650, (b) CN600, (c) CN550, (d) CN500 and (e) CN450; TEM images of g-C3N4 samples: (f) CN650, (g) CN600 and (h) CN550. Fig. 3. Nitrogen adsorption-desorption isotherms of g-C3N4 samples: (a) CN650, (b) CN600, (c) CN550, (d) CN500, (e) CN450, and the corresponding pore-size distribution curves (f). Fig. 4. FT-IR spectra of g-C3N4 samples. Fig. 5. XPS spectra of g-C3N4 samples: (a) survey and high resolution of (b) C 1s and (c) N 1s. Fig. 6. (a) UV-vis diffuse reflectance spectra and (b) estimated band gap energy of g-C3N4 samples. Fig. 7. (a) Photocatalytic degradation and (b) removal efficiency of 2-naphthol by the g-C3N4 samples (Initial 2-naphthol = 100 mg/L, solution volume = 200 mL, catalysts dosage = 0.0200 g, visible light λ ≥ 420 nm). Fig. 8. (a) Linear first-order model fitting for the g-C3N4 samples and (b) rate constants of the first-order model. Fig. 9. (a) VB XPS spectra and (b) schematic illustration of the band structures of the g-C3N4 samples. Fig. 10. ESR spectra of the (a) O2• − and (b) •OH for CN600 under visible light. Fig. 11. Schematic illustration of photodegradation of 2-naphthol by g-C3N4. Fig. 12. GC-MS analysis of photodegradation byproducts of 2-naphthol at 10 min, 20 36

min and 40 min by CN600. Fig. 13. Proposed photodegradation pathway of 2-naphthol by CN600 under visible light. Fig. 14. Reusing the same CN600 in 5 consecutive runs of photodegradation of 2-naphthol.

Table 1. The elemental composition and atomic ratio of C to N for the g-C3N4 samples. Sample

C (wt. %)

N (wt. %)

H (wt. %)

O (wt. %)

C/N

CN450

31.02

55.44

3.19

10.35

0.653

CN500

34.66

60.81

1.81

2.71

0.665

CN550

35.14

60.90

1.78

2.17

0.673

CN600

35.05

60.87

1.58

2.50

0.672

CN650

35.36

59.80

1.61

3.23

0.690

37

Fig. 1. XRD patterns of as-prepared g-C3N4 samples.

38

Fig. 2. SEM images of g-C3N4 samples: (a) CN650, (b) CN600, (c) CN550, (d) CN500 and (e) CN450; TEM images of g-C3N4 samples: (f) CN650, (g) CN600 and (h) CN550. 39

Fig. 3. Nitrogen adsorption-desorption isotherms of g-C3N4 samples: (a) CN650, (b) CN600, (c) CN550, (d) CN500, (e) CN450, and the corresponding pore-size distribution curves (f).

40

Fig. 4. FT-IR spectra of g-C3N4 samples.

41

Fig. 5. XPS spectra of g-C3N4 samples: (a) survey and high resolution of (b) C 1s and (c) N 1s. 42

Fig. 6. (a) UV-vis diffuse reflectance spectra and (b) estimated band gap energy of g-C3N4 samples.

43

Fig. 7. (a) Photocatalytic degradation and (b) removal efficiency of 2-naphthol by the g-C3N4 samples (Initial 2-naphthol = 100 mg/L, solution volume = 200 mL, catalysts dosage = 0.0200 g, visible light λ ≥ 420 nm).

44

Fig. 8. (a) Linear first-order model fitting for the g-C3N4 samples and (b) rate constants of the first-order model.

45

Fig. 9. (a) VB XPS spectra and (b) schematic illustration of the band structures of the g-C3N4 samples.

46

Fig. 10. ESR spectra of the (a) O2• − and (b) •OH for CN600 under visible light.

47

Fig. 11. Schematic illustration of photodegradation of 2-naphthol by g-C3N4.

48

Fig. 12. GC-MS analysis of photodegradation byproducts of 2-naphthol at 10 min, 20 min and 40 min by CN600.

49

Fig. 13. Proposed photodegradation pathway of 2-naphthol by CN600 under visible light.

50

Fig. 14. Reusing the same CN600 in 5 consecutive runs of photodegradation of 2-naphthol.

51

52

Highlights

(1) g-C3N4 was obtained at different temperatures by one-step calcination. (2) The structure and optical property of g-C3N4 could be controlled by the temperature. (3) The photodegradation of 2-naphthol by g-C3N4 synthesized at 600 °C was the best.

53