C60 with enhanced photocatalytic properties

Materials Research Bulletin 122 (2020) 110668

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One-step microwave synthesis of covalently bonded OeC3N4/C60 with enhanced photocatalytic properties

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Shujuan Liua,b,*, Changrui Jianga,b, Yunpeng Gaoa,b, Jiaxin Hea,b, Kexin Lia,b, Yan Fenga a b

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China Key Laboratory of Micro-Systems and Micro-Structures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin 150001, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: OeC3N4/C60 Oxygen doping In-situ Covalent bonding Photocatalytic

Covalently bonded OeC3N4/C60 hybrids with enhanced photocatalytic properties are fabricated by a facile onestep microwave synthesis method. Analyses confirm that in the system oxygen doping, in-situ formation of C60, covalent bonding and creation of porous structures are achieved simultaneously in one step. Only samples prepared at temperature higher than 170℃ and time longer than 10 min can the C60 be found in O-C3N4/C60. XPS spectra analyses confirm that after microwave treating C60 is formed in situ and the new doping oxygen element exists with the eOH form. The O-C3N4/C60 sample exhibits a hierarchical porous structure and the BET surface area increases from 53.4 m2/g of g-C3N4 to 104.5 m2/g. A chemical bond of C–N forms between oxygen doped g-C3N4 and C60 to act an electron transfer bridge. Photocatalytic performances analyses indicate the sample with the best DRS absorption exhibits the best photocatalytic performance and the degradation rate has been increased a lot.

1. Introduction As one promising metal-free photocatalyst polymeric graphitic carbon nitride (g-C3N4) has received great attention due to its easy preparation, good chemical stability and potential for pollution degradation and water reduction/oxidation under visible light [1–4]. However, the photocatalytic efficiency of pure g-C3N4 is far from satisfaction considering the limited utilization of solar energy (λ < 460 nm) and the high recombination rate of photogenerated electron-hole pairs [5–7]. To improve the photocatalytic performances of g-C3N4 various kinds of methods have been implemented by researchers. Among them, fabricating heterojunctions [8,9], doping with hetero elements [10–16] and introducing carbon nanomaterials [17] are generally considered as some representative effective methods to broaden the light absorption scope, prevent the recombination of photogenerated holes and electrons, and hence improve the photocatalytic properties. Oxygen doping in g-C3N4 framework is widely recognized as an effective means to modulate the band gap and hence improve the separation efficiency of carriers in the photocatalytic reactions. Jianghua Li et al. have ever reported that through the effective heteroatomic introduction of O element the absorbance edge of g-C3N4 extends up to 498 nm and consequently an enhanced visible-light photoactivity is obtained finally [18]. Chengyin Liu et al. reported the oxygen atoms ⁎

substitution not only broadens the visible-light response of g-C3N4 to 800 nm but also greatly promotes the bulk charge carrier separation efficiency, resulting in a significantly 9-fold enhancement of photocatalytic hydrogen evolution activity [19]. Fangyan Wei et al. found that oxygen doping can tune the intrinsic electronic state and band structure of g-C3N4 and thus unprecedentedly enhanced photocatalytic performance can be obtained from the oxygen self-doping g-C3N4 nanospheres [20]. Meanwhile, inclusion of some carbon materials such as carbon nanotubes [21], nanodots [22], graphene [23–27] or fullerene [28,29] is regarded as attractive means for the further promotion of charge carrier transfer and separation rate and hence the improved photocatalytic performances. Among them, C60 has a closed-shell configuration consisting of 30 bonding molecular orbital with 60 electrons. Such a unique structure of C60 makes it act as one efficient carbon electron acceptor and subsequently give rise to a rapid photoinduced charge separation rate. Including ZnO [30,31], TiO2 [32], SnO2 [33] and so on, a lot of nanomaterials are reported to couple with C60 to promote photocatalytic properties. Taking g-C3N4 as example, including physical blended and chemical bonded g-C3N4/C60, researchers have investigated a series of g-C3N4/C60 photocatalysts [34,35]. Chai et al. prepared C60/g-C3N4 composites by physically adsorbing C60 on the surface of g-C3N4 and attributed the enhanced photocatalytic performance to the efficient separation of photo-generated electrons and

Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. E-mail address: [email protected] (S. Liu).

https://doi.org/10.1016/j.materresbull.2019.110668 Received 11 April 2019; Received in revised form 10 October 2019; Accepted 10 October 2019 Available online 18 October 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.

Materials Research Bulletin 122 (2020) 110668

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holes in the C60/g-C3N4 composite [36]. C60/g-C3N4 was prepared by Zhu et al. through a facile thermal treatment of dicyandiamide at 550 ℃ in the presence of C60, whose photocatalytic degradation activities on phenol and MB, as well as photocurrent response, is about 2.9, 3.2 and 4.0 times as high as those of bulk g-C3N4 [37]. Yang et al. has ever reported a covalently bonded g-C3N4/C60 hybrid was synthesized via the covalent bonding of C60 onto the edges of g-C3N4 nanosheets through a solid-state mechanochemical route [38]. Note that in these reports whether those g-C3N4/C60 hybrids based on physically blended or those based on covalently bonded, all the C60 used in these hybrids are included externally. Considering the dimension divergence of zerodimensional (0D) fullerene and 2D g-C3N4, unlike 2D graphenes analogous to g-C3N4, the intimate hybridization between C60 and g-C3N4 is much more difficult and thus the interaction force is relatively weak. Xu et al. used first-principles calculations to get that in the C60/g-C3N4 nanocomposites the unsaturated nitrogen atoms of g-C3N4 and C60 molecule interact with each other strongly through dominating the valence band maximum and conduction band minimum [39]. And a more intimate interaction will result in a much stronger charge transfer between the two involved constituents and a much better photocatalytic properties. There is no doubt that covalently bonded g-C3N4C60 hybrid is more conducive to the formation of a tight bond than the physical blended ones, thus facilitating electrons transport. Meanwhile, in comparison with C60 molecules brought in from outside those originated from g-C3N4 framework itself are more likely to form tight connection with g-C3N4. Hence, constructing a chemical bonded gC3N4-C60 hybrid with in situ formed C60 and so as to strengthen the intermolecular interactions between g-C3N4 and C60 toward photocatalytic activity enhancement becomes highly desirable. As one facile method, microwave synthesis is well-known for its green energy saving due to the fast heating speed. Compared with other conventional heating methods the specific heating mechanism makes microwave synthesis be completed often in several seconds or minutes [40,41]. If some easy decomposing molecules are inserted into the lamellar g-C3N4 interlayers the microwave heating speed is fast enough to make the decomposition reaction occur in a very short time. Such an outburst will weaken the Van Der Waals’ force between g-C3N4 layers and therefore increase the BET surface area subsequently. As we know H2O2 molecules are easy to decompose to generate O2 gas under high temperature. In the meantime, the radius of H2O2 molecules are small enough to permeate into g-C3N4 interlayers under the room temperature stirring. In addition, it could be used as an effective oxygen doping source of g-C3N4. Considering the fast heating of microwave irradiation the inserted H2O2 molecules cannot escape from g-C3N4 interlayers before decomposing occurs. The sudden released O2 gas will rush out from the g-C3N4 interlayer in a short time. Subsequently, destroying the original laminar structures of g-C3N4 and doping oxygen atoms may be completed simultaneously. Hence here we used 30% H2O2 solution as reaction media, through room temperature stirring to make the H2O2 molecules diffuse into the g-C3N4 interlayers and then we used the microwave irradiation to heat the system to a high temperature in a very short time. O-C3N4/C60 is produced here easily through the facile microwave treating of g-C3N4 in the 30% H2O2 solution. Both the oxygen doping and formation of hybrid with in situ formed C60 are achieved simultaneously, which shed new light on the development of a series of nanomaterials with enhanced photocatalytic performance in the future.

2.1.2. Preparation of g-C3N4 Pristine graphitic carbon nitride (g-C3N4) was prepared through a direct thermal condensation method by using urea as precursor. 10 g urea in a covered crucible was calcined in a tube furnace at 550 °C for 3 h with a ramp rate of 3 °C min−1. After the sample was naturally cooled to room temperature, the resulting yellow powder was collected for use without any further treatment. 2.1.3. Preparation of O-C3N4/C60 CEM Discover SP ring-focus single-mode microwave synthesis system is used to synthesize O-C3N4/C60 composite through the microwave treatment of g-C3N4 in the H2O2 solution (30%wt.). A certain amount of pristine g-C3N4 obtained above was dispersed in 4 mL hydrogen peroxide solution (30%wt.) by magnetic stirring for 2 h at room temperature first. Then the solution was transferred to a 35 mL silica reaction tube and treated in the microwave synthesis system for a certain time at different temperatures. After the microwave treatment the obtained O-C3N4/C60 products need to be centrifuged, washed with water and ethanol alternatively for five times and then dried in an oven at 60℃ for 12 h. 2.1.4. Preparation of O-C3N4/C60-CS2 Initially, 100 mg O-C3N4/C60 sample is immersed and stirred in 4 mL of CS2 at room temperature for 72 h. Then, the products need to be centrifuged and washed with water and ethanol alternatively for six times. After drying in an oven at 60℃ for 12 h O-C3N4/C60-CS2 is obtained finally. 2.1.5. Preparation of g-C3N4-M The g-C3N4-M sample is prepared with the same conditions with OC3N4/C60 sample except water is used to substitute 30%wt. H2O2 solution. First 45 mg of pristine g-C3N4 was dispersed in 4 mL distilled water by magnetic stirring for 2 h at room temperature. Then the solution was transferred to a 35 mL silica reaction tube and treated in the microwave synthesis system for a 10 min at 170℃. After microwave treatment the obtained product g-C3N4-M needs to be centrifuged, washed with water and ethanol alternatively for five times and then dried in an oven at 60℃ for 12 h. 2.1.6. Photocatalytic tests Photocatalytic activities of O-C3N4/C60 samples were evaluated by degradation of methylene blue under simulated sunlight irradiation using 500 W Xe lamp (Institute of Electric Light source, Beijing). The reaction cell whose top was open to provide irradiation was placed in a sealed block box. The system was cooled by circulating water and maintained at 20℃. In each experiment, 50 mg of as-prepared photocatalyst was added into 100 mL of methylene blue solution (1 × 10−5 mol/L). Prior to illumination, the suspensions were stirred in the dark for 30 min to establish the adsorption/desorption equilibrium between photocatalyst and methylene blue. Then, the solutions were stirred and exposed to the simulated sunlight irradiation. At given time intervals, 5 mL of aliquots were sampled and centrifuged to remove photocatalyst. The supernatant of methylene blue was analyzed by recording at 663 nm using a UV–vis spectrophotometer (VARIAN Cary 50 conc). 2.2. Characterization part

2. Experimental part and characterization XRD patterns were recorded with a German BRUKER D8-ADVANCE using Cu Kα radiation (=0.15406 nm) at 40 kV and 40 mA. Fieldemission scanning electron microscope (FESEM) was carried out on Sirion 200 using an acceleration voltage of 10 kV. UV–vis diffuse reflectance spectra (DRS) were determined by a UV–vis spectrophotometer (Hitachi U-3100 spectrometer) using an integrating sphere accessory. The photoluminescence (PL) measurements were carried out

2.1. Materials preparation 2.1.1. Chemicals All reagents were of analytical grade and used without further purification, including urea, H2O2 (30%wt.) and ethanol. All these chemicals were purchased from the Sinopharm Chemical Reagent Co. 2

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Fig. 1. XRD patterns of a) C60, g-C3N4, g-C3N4-M and O-C3N4/C60; b–d) samples prepared at different reaction conditions.

temperature lower than 170℃ and time shorter than 10 min, no characteristic diffraction peaks of C60 exist. Moreover, it is found that in the system the formation of C60 is not influenced by the relative ratio ranging from 30 mg/4 mL to 50 mg/4 mL of g-C3N4 to H2O2. Such a result indicates reaction temperatures and times play key roles in the formation of C60 in O-C3N4/C60 sample. Close observation of the exact position of diffraction peaks shows that after hybrids formation the diffraction peak of (002) plane of g-C3N4 at 27.4° gradually shifts to a larger 2Θr value (27.7°) in O-C3N4/C60 [41]. And the shift extent to a larger degree is increased gradually as H2O2 treating time and temperature increases. Chen et al. reported that hydrothermal treating of gC3N4 in H2O2 solution could result in oxygen doping in g-C3N4 framework [17]. Considering the larger electronegativity of oxygen element when nitrogen atoms are substituted by oxygen the interaction between g-C3N4 layers will be strengthened, in consequence giving rise to a shortened interplanar distance. As doping amount of oxygen atoms increases the interaction between g-C3N4 layers should increase simultaneously. As a consequence an increased 2Θ shift extent will be observed in the corresponding XRD patterns due to the further shortened interplanar distance. Hence the gradual increased extent of 2Θ value shift at higher reaction temperature and time can be attributed to the gradual increased oxygen doping content here. All the results above confirm that through a facile microwave method a hybrid of O-C3N4/ C60 is formed easily and treating temperature and time play significant roles in the C60 formation and oxygen doping. FESEM images in Fig. 2 show the microstructure transformation process of the samples treating in H2O2 solution for different temperatures, which shows striking contrast to the images of g-C3N4-M sample (Figure S1 in Supplemental information). Fig. 2a and its inset are FESEM images for pristine g-C3N4 and an aggregated morphology with a large size and typical lamellar structure can be observed [44]. After treated in H2O2 solution under microwave irradiation the g-C3N4 lamellar structures began to break into fragments with smaller sizes at high temperatures. TEM images (Figure S2 in Supplemental information) of O-C3N4/C60 prepared at 170℃ prove that the structure of g-

at room temperature with a Fluoromax-4 spectrofluorometer using 350 nm as the excitation wavelength. Fourier transform infrared spectra (FTIR) were tested on the Nicolet 380 F spectrometer. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5700 ESCA spectrometer with an Al Kα (1486.6 eV) source operated at 13 kV and 300 W. The wide scan：Pass energy 187.85 eV and narrow scan: Pass energy 29.35 eV. Nitrogen adsorption-desorption measurements were conducted at 77.35 K on a Micromeritics Tristar 3000 analyzer after the samples were degassed at 150℃ for 6 h.

3. Results and discussion 3.1. Structures and compositions of O-C3N4/C60 As presented in Fig. 1a, X-ray diffraction (XRD) patterns are shown to analyze the crystal structure of the prepared O-C3N4/C60 sample in comparison with that of pristine g-C3N4, C60 and the g-C3N4-M. The XRD pattern of g-C3N4 can be identified as pure phase g-C3N4 [42] and that of C60 can be well indexed to C60 (JCPDS NO. 47-0787). The sample of g-C3N4-M, which has been treated under the same microwave conditions except substituting H2O2 solution with distilled water, shows the similar XRD pattern with g-C3N4, indicating the g-C3N4 essence of gC3N4-M. Analyses of the XRD pattern of O-C3N4/C60 indicate both characteristic diffraction peaks of g-C3N4 and C60 can be found, proving their coexistence in this microwave treating sample. Among them the peaks with solid rhombus are from the diffraction of g-C3N4 and those with solid dots come from that of C60. Comparative analyses of the peak positions indicate that after microwave treating in the H2O2 solution the peaks of (100) and (002) of g-C3N4 in O-C3N4/C60 will shift to a larger degree, meaning a shortened interplanar distance. To investigate the influences of reaction conditions the XRD patterns of samples prepared with different temperatures, times and g-C3N4/H2O2 ratios are shown in Fig. 1b-d. It is found that in the samples prepared at the temperature higher than 170℃ and time longer than 10 min can the diffraction peak of C60 be found. While in those samples prepared at the 3

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Fig. 2. FESEM images of samples: a) g-C3N4; b–d) O-C3N4/C60 samples prepared at different temperatures: b)160℃;c) 170℃; d) 180℃.

distribution curves are further recorded (Fig. 3). Both the nitrogen adsorption-desorption isotherms of g-C3N4 and O-C3N4/C60 display IV type, proving the existence of mesopores (2–50 nm). Moreover, both the hysteresis loops are of H3 type, meaning slit-shaped pores are formed arising from the aggregation of platelike particles. Compared with gC3N4, the hysteresis loop of O-C3N4/C60 shifts to the region of lower pressure and the area covered becomes larger, indicating relative larger amount of mesopores in O-C3N4/C60. Pore size distribution curves in Fig. 3b further confirm that more mesopores and macropores are introduced into the O-C3N4/C60 structure after microwave treating in H2O2 solution. It is well-known that the reactants transmitting in the macropores (> 50 nm) is much easier and more mesopores (2–50 nm) is beneficial for the increase of photocatalytic reaction sites. Hence a perfect combination of mesopores and macropores plays a significant role in the improvement of photocatalytic performance. At the same time after microwave irradiation the BET specific surface area is increased from 53.4m2/g of g-C3N4 to 104.5 m2/g of O-C3N4/C60. In view of the H2O2 decomposing reaction due to the fast microwave heating up speed, it should be rational to deduce that the increased BET surface area and the formation of mesopores and macropores are due to the

C3N4 has been destructed and many pores form after microwave treating in the H2O2 solution. In addition, on the g-C3N4 substrate some nanoparticles with sizes less than 10 nm can be observed and the interplanar spacings are 3.20 Å, which could be corresponded to the (114) planes of C60. Considering the fast heating up speed of microwave irradiation most H2O2 molecules can’t escape from the interlayers before decomposing takes place. When the temperature is high enough a large amount of O2 gas will burst out suddenly. On one way it will destruct the g-C3N4 structure to form pores and on the other way it will oxidize g-C3N4 to form some new chemical bonds. And the higher the temperature, the more intense the H2O2 decomposing reaction, and subsequently the more severe the destruction to the g-C3N4 framework. XRD patterns analyses above confirm higher temperature is beneficial for oxygen atoms doping and C60 formation, which means a higher damage degree for the g-C3N4 framework. Thus as temperature rises it is rational to observe the gradual size decrease and morphology transformation of O-C3N4/C60 as illustrated in Fig. 2. To investigate the actual structure transformation of O-C3N4/C60 due to the introduction of oxygen element and C60 N2 adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size

Fig. 3. a) Nitrogen adsorption-desorption isotherms; b) Pore size distribution curves of g-C3N4 and O-C3N4/C60. 4

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Fig. 4. XPS spectra of g-C3N4 and O-C3N4/C60: a) Survey spectra; b) O1s high resolution spectra; c) C1s high resolution spectra; d) N1s high resolution spectra.

for the C1s high resolution XPS spectra (Fig. 4c) they can be deconvoluted into two peaks for g-C3N4 and three peaks for O-C3N4/C60. Among them the peaks at 284.6 eV and 288.0 eV can be attributed to CeC and NCNe] bonds of g-C3N4 framework. While for O-C3N4/C60 besides the peaks of 284.6 eV and 288.1 eV which can be assigned to CeC and NCNe] bonds, a new peak at 289.3 eV from the binding energy of CeO bond can be also obtained. In addition, the peak area ratio of CeC to NCNe] analyses show that the value for O-C3N4/C60 sample is much larger than that in the pristine g-C3N4. It means CeC bond takes up a much larger part in O-C3N4/C60 than in g-C3N4. Hence, excluding the effects of adventitious carbon the dramatic increase of relative amount of CeC bond in the O-C3N4/C60 sample should be due to the introduction of C60 moiety in the system [38]. For the high resolution XPS spectra of N1s (Fig. 4d) the peaks at 398.1 eV (398.2 eV), 398.6 eV (398.8 eV) and 400.3 eV (400.0 eV) can be ascribed to sp2hybridized nitrogen C–NC], sp3-hybridized nitrogen N-(C)3 and NeH bonds, respectively. Analyses of N1s spectra in depth (Table S1 in supplemental Information) indicate the area ratio of N(sp2)/N(sp3) increases from 0.356 to 0.539. And meanwhile the C/N atomic ratio increases from 0.74 to 1.28 (Table S2). Combined with the XRD results above, the dramatic increase of C/N atomic ratio here should be due to two reasons. First, C60 is formed in situ using g-C3N4 as raw materials and part of N atoms runs off from the system. Although the actual formation mechanism of C60 from g-C3N4 is not clear, the increased N (sp2)/N(sp3) ratio here may suggest us most N atoms loss comes from

sudden rush out of the decomposing product of O2. As we know for an excellent photocatalyst it not only should have a larger BET specific surface area value which means larger reaction sites, but also should have a perfect combination of mesopores and macropores. Hence, both the BET surface area increase and the perfect combination of mesopores and macropores here suggest O-C3N4/C60 should exhibit an excellent photocatalytic performance. To analyze the actual chemical environment of the elements X-ray photoelectron spectroscopy (XPS) is used to test the detailed information of the samples. In Fig. 4, XPS survey spectra and high resolution spectra of the pristine g-C3N4 and O-C3N4/C60 are illustrated. Except C1s, N1s and O1s peaks three peaks below 200 eV coming from the additives in the conductive adhesive can also be observed in the OC3N4/C60 survey spectrum. And the conductive adhesive is usually used to fix the powder sample on the substrate in the XPS test. The peak positions in all XPS spectra are calibrated with C1s at 284.6 eV. The survey scans confirm that an obvious larger amount of O can be found in O-C3N4/C60, proving the successful oxygen introduction in the system. Further XPS high resolution spectra of O1s in Fig. 4b show the detailed chemical environment of oxygen element in O-C3N4/C60. The peaks at 531.9 eV and 532.0 eV correspond with the oxygen atoms in the adsorbed H2O. While the new peak at 531.4 eV found in the OeC3N4/C60 sample can be attributed to the binding energy of NeCOeH according to the report. [20] It means the new doping oxygen element exist with the form of eOH in the O-C3N4/C60 system. While 5

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Fig. 5. a) FTIR spectra of C60, g-C3N4, O-C3N4/C60 and O-C3N4/C60-CS2; b) XRD pattern of sample O-C3N4/C60-CS2.

in FTIR should come from the modified eCOOH on the C60 surface. According to the XPS and FTIR results above, a small amount of -NH2 residual remains on the surface of oxygen doped g-C3N4. The new formed eCOOH from C60 will react with the residual -NH2 from the oxygen doped g-C3N4 and hence the amido bond forms. Here in the FTIR spectrum the new band at 1540 cm−1 should due to the new formed amino bond. Considering the relative smaller amount of residual -NH2 on the g-C3N4 so not all the eCOOH on the C60 can transform into amido bonds. When the sample is soaked in CS2 for enough time those C60 which did not form amido bonds with oxygen doped g-C3N4 will dissolve into the CS2. Consequently after CS2 treatment, the 1540 cm−1 band remains on the surface while the band at 1734 cm−1 band disappears. In conclusion, after microwave treating of g-C3N4 in H2O2 solution not only oxygen doping and C60 formed in situ are achieved meanwhile but also a stable chemical bond forms between oxygen doped g-C3N4 and C60, predicating an excellent photocatalytic performance of the O-C3N4/C60 sample.

the sp3-hybridized nitrogen. Second, some nitrogen atoms are substituted by oxygen atoms. And both of these two aspects will result in an increase of C/N atom ratios. To detect the actual chemical interaction environment due to the introduction of C60 and oxygen element, the prepared O-C3N4/C60 sample is characterized by FTIR spectroscopy in detail in comparison with that of g-C3N4 and C60 (Fig. 5). As shown in the FTIR spectrum of pristine g-C3N4 (Fig. 5a) the vibrational bands at 810 cm−1 and 1637 cm−1 come from the s-triazine moiety and C]N stretching vibration, respectively. The broad absorption band from 3100 to 3300 cm−1 can be assigned to the stretching mode of NeH in the g-C3N4 framework. While the bands at 1237, 1316, 1409, 1457 and 1568 cm−1 can be attributed to the aromatic C–N stretching of pristine g-C3N4. And all these results are consistent with the FTIR reports of pristine g-C3N4 [20,36,38]. While in the spectrum of O-C3N4/C60 a much broader peak ranging from 3000 to 3400 cm−1 can be observed, which should be due to the overlap of OeH and NHe stretching vibration modes [20,45]. Besides the characteristic vibrational peaks of g-C3N4, two intense vibrational peaks at 527 cm−1 and 591 cm−1 corresponding to that of pristine C60 can also be found in the O-C3N4/C60 sample. And the vibrational peak of 591 cm−1 in the O-C3N4/C60 sample has a 16 cm−1 blue shift in comparison with the pristine C60 at 575 cm−1. Furthermore, it needs to be noted that two new vibrational peaks occurs at 1734 cm−1 and 1540 cm−1 in the O-C3N4/C60 sample. Among them the one at 1734 cm−1 can be attributed to the vibration of C]O while the one at 1540cm−1 can be assigned to the CeN of amides. Such a band can’t be found from both pristine C60 and g-C3N4. Considering both the shift and the new appearance of C]O and CNe peaks, it should be rational to deduce that in O-C3N4/C60 a new chemical bond forms between C60 and oxygen doped g-C3N4. To prove this, considering the soluble essence of C60 in CS2 the prepared O-C3N4/C60 sample is stirred in CS2 at room temperature for about 72 h and this sample is named as O-C3N4/C60-CS2. After the treatment we tested again the FTIR spectrum and XRD (Fig. 5b) pattern of the sample (O-C3N4/C60-CS2). It is found that albeit with a little decrease of the relative intensity the characteristic peaks of C60 still exist in the corresponding FTIR spectrum and diffraction pattern. According to the report [36], if no chemical bond forms between C60 and g-C3N4 then simple immersion in CS2 solvent for only about 2 h will dissolve all the C60. Obviously, the results here demonstrate that the interaction between C60 and O-C3N4 is not simple physical adsorption but formation of a stable chemical bond. Manickam et al. reported that pristine fullerene (C60) can transfer into potential fullerenol moieties [C60(OH)n·mH2O] after ultrasound-assisted treatment in H2O2 aqueous media [46]. That means a H2O2 treatment can introduce eOH and CeOOH into the surface of the C60. Here considering the in-situ formation of C60 in H2O2 reaction media it is reasonable to deduce that the new formed C60 will be modified with eCOOH and OeH simultaneously. And most of the band of 1734 cm−1

3.2. Photocatalytic properties of O-C3N4/C60 samples Fig. 6 shows the typical UV–vis diffuse reflectance spectra (DRS) of a series of samples. As revealed from Fig. 6 the absorption thresholds of all O-C3N4/C60 samples are extended to the visible light region. Test of samples prepared at different reaction conditions (Fig. 6a–c) shows that the largest absorption of UV–vis light comes from the sample prepared at 170℃ for 10 min with the ratio of 45 mg in 4 mL H2O2. Taking it as a representative O-C3N4/C60 photocatalyst, we compare its DRS with those of C60 and g-C3N4. As indicated in Fig. 6d the absorption edge of pure g-C3N4 occurs at about 450 nm, while the pure C60 has a much greater absorption from 200 to 800 nm. In comparison with that of pristine g-C3N4, the absorption of O-C3N4/C60 at visible light region is increased a lot and the absorption edge is extended to almost 700 nm. According to the reports the band gap can be estimated through the Eq. (1) using the DRS data:

(αhν )2 = A (hν − Eg )n

(1)

where α, ν , Eg and A are absorption coefficient, light frequency, band gap energy and a constant, respectively. For direct gap semiconductor the value of n is 1 and 4 for indirect gap semiconductor. As a direct gap semiconductor the estimated band gap of O-C3N4/C60 and g-C3N4 are 2.48 eV and 2.64 eV, respectively (Figure S3). Obviously, the semiconductor of O-C3N4/C60 has a much narrower band gap and it can harvest much more visible light. According to the reports [17,18,43], doping with proper amount of oxygen element could induce the visible light absorption to increase. Moreover, considering the more intense visible light absorption of pure C60 it can be concluded that the improvement of O-C3N4/C60 in the visible region is due to both the in situ formation of C60 and oxygen doping. The obvious DRS improvement of 6

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Fig. 6. DRS spectra of a–c) samples prepared at different reaction conditions; d) C60, g-C3N4 and O-C3N4/C60.

C60 photocatalysts after illumination for about 90 min. Among them, the sample prepared at 170℃ for 10 min with the ratio of 45 mg in 4 mL H2O2 shows the best photocatalytic performance. And this result is consistent with the DRS analyses above, which means the sample with the largest visible light absorption shows the best photocatalytic properties. In addition, we compare the photocatalytic properties with that of the pure g-C3N4, g-C3N4-M and commercial TiO2 of P25 (Fig. 7d). After illumination for about 90 min, the O-C3N4/C60 photocatalyst has degraded about 90% of the methylene blue solution, while the degradation rates of pristine g-C3N4, g-C3N4-M and P25 are only

O-C3N4/C60 predicts that O-C3N4/C60 should have an excellent photocatalytic performance and the one with the best DRS should have the best photocatalytic properties. The photocatalytic performances of the series of O-C3N4/C60 samples are tested through degradation of methylene blue solution under the illumination of Xe lamp. Prior to illumination the methylene blue solution containing photocatalyst is stirred in dark for half an hour to achieve the adsorption and desorption equilibrium. The photocatalytic results are shown in Fig. 7 in detail. As indicated in Fig. 7a-c, most part of methylene blue solution has been degraded by the series of O-C3N4/

Fig. 7. Photodegradation curves of a–c) samples prepared at different reaction conditions; d) P25, g-C3N4, g-C3N4-M and O-C3N4/C60. 7

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Fig. 8. a) Photocatalytic recycle test of O-C3N4/C60; b) Photodegradation rate of g-C3N4, O-C3N4/C60, O-C3N4/C60-CS2 and O-C3N4/C60-CS2+C60.

the organic molecules. Third, in situ formation of C60 originated from the g-C3N4 framework. As superior electron acceptors C60 can promote the effective separation of electron-holes pairs. And more intimate connection between photocatalyst and electron acceptors of C60 much easier the electron transfer. Different from previous reports C60 here is not introduced from outside oppositely it is formed in situ taking gC3N4 as raw materials. According to FTIR, XPS and XRD analyses above, amide bonds form between C60 and oxygen doped g-C3N4 framework. Such a chemical bond connection works as the electron transfer bridge and makes the electron transfer much easier. Subsequently the electronhole pairs can be separated timely and hence an improved photocatalytic performance is obtained. A series of control photocatalytic experiments are carried out in detail to make clear the roles of doping oxygen element and in situ formed C60 in the photocatalytic process. The corresponding photocatalytic degradation efficiencies of these reference experiments are shown in Fig. 8b. After 90 min illumination the degradation rate of O-C3N4/C60 can reach 90.0%. The sample O-C3N4/ C60-CS2 which is prepared by immersing O-C3N4/C60 photocatalyst in CS2 for about 72 h to dissolve those no chemical bonded C60 can degrade 72.3% methylene blue solution. It is about twice of the degradation rate of g-C3N4 (37.0%), which confirms the significance of chemical bonded C60 and oxygen doping in the system. And the 17.7% decrease relative to that of O-C3N4/C60 proves the significance of physical adsorbed C60 in the photocatalytic process. Then we used the same amount of C60 to mix mechanically with the sample O-C3N4/C60CS2 and its degradation rate of 73.3% is almost the same with the 72.3% of O-C3N4/C60-CS2. Such a result means the in situ formed C60 has much better electron transfer ability than those introduced outside due to the closer connection. To analyze the separation efficiency of photogenerated charge carriers, the PL spectra of the g-C3N4, O-C3N4/C60, O-C3N4/C60-CS2 and OC3N4/C60-CS2+C60 samples are provided in Fig. 9. Higher PL emissions mean higher recombination efficiency of electrons and holes. As indicated, the sample of g-C3N4 shows the highest PL intensity, which means it shows the highest recombination efficiency, subsequently the worst photocatalytic performance. While for O-C3N4/C60 it shows the lowest PL intensity and its separation efficiency is the highest. And this result is consistent with its best photocatalytic performance above. In comparison with O-C3N4/C60 the PL intensity of O-C3N4/C60-CS2 increases to an extent due to the dissolved physical adsorbed C60. After addition of C60 the PL intensity of O-C3N4/C60-CS2+C60 decreases a little compared with the sample of O-C3N4/C60-CS2, indicating the positive effect of C60 in the O-C3N4/C60 sample. A schematic diagram in Figure S5 in Supplemental Information is used to show the synergetic photocatalytic mechanism of O-C3N4/C60 clearly. As illustrated oxygen doping brings the band gap decrease from

37.0%, 46.5% and 44.6%, respectively. The result indicates that the photocatalytic performance of O-C3N4/C60 sample here is indeed improved a lot due to the chemical connection of oxygen doped g-C3N4 and in situ formed C60. As an excellent photocatalyst, recycle stability in the photocatalytic testing process plays an essential role in the application of pollution degradation. In Fig. 8a, five times photocatalytic recycle test results are shown clearly. From the results we can observe that after one cycle the photocatalytic property has achieved a stable state and the photocatalytic degradation efficiency can maintain at about 80% after every 90 min illumination recycle. In comparison with the first cycle degradation efficiency of commercial TiO2 P25 (44.6%) and g-C3N4 (37.0%) this value is still about twice as much as them, proving the excellent photocatalytic performance of the O-C3N4/C60. To investigate the dominant reactive species in the photocatalytic reaction, the radicals and holes trapping experiments were carried out (Figure S4). According to the reports hydroxyl radical (%OH), superoxide radical (%O2−) and holes (h+) are three main photoactive species in the photodegradation process. In this work, methanol (MA), N2 and ammonium oxalate (AO) are added to remove the effects of %OH, %O2− and holes, respectively. The degradation efficiency of blank one is added to compare with the ones with scavengers added. As shown in Figure S4 the introduction of MA has the greatest inhibition influences for the photodegradation efficiency of all hybrids, which means in these systems %OH plays the most important roles in the photocatalytic process. While for the introduction of N2 a small decrease of degradation efficiency is observed, indicating a relative small influence of %O2− for the system. AO was conducted as the holes scavenger of the O-C3N4/C60 sample. And the decrease of photodegradation efficiency with addition of AO can be even ignored, which means the effect of holes is nearly negligible for this system.

3.3. The mechanism for improved photocatalytic performance of O-C3N4/ C60 To investigate the actual promoting mechanism three different aspects are put forward to explain the improved photocatalytic performance. First, the hierarchical porous structure and the enhanced BET surface area due to the sudden gas release of H2O2 decomposition under the fast microwave heating condition. With the hierarchical porous structure the transmission of reactants to the active reaction sites becomes easier and more active reaction sites due to the larger BET surface area make the photodegradation take place much quicker. Second, the successful oxygen element doping creates a tunable band structure which can be excited by visible light. That means more light in the sunlight spectrum can be used to activate the photocatalyst to degrade 8

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interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement We acknowledge the support from National Natural Science Foundation of China (Grant No. 21101043). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.materresbull.2019. 110668. References [1] 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 (2009) 76–80. [2] P. Niu, L.C. Yin, Y.Q. Yang, G. Liu, H.M. Cheng, Increasing the visible light absorption of graphitic carbon nitride (melon) photocatalysts by homogeneous selfModification with nitrogen vacancies, Adv. Mater. 26 (2015) 8046–8052. [3] D. Dontsova, S. Pronkin, M. Wehle, Z. Chen, C. Fettkenhauer, G. Clavel, M. 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Fig. 9. PL spectra of the samples under excitation of 350 nm light: A) g-C3N4; B) O-C3N4/C60; C) O-C3N4/C60-CS2; D) O-C3N4/C60-CS2+C60.

2.64 eV of g-C3N4 to 2.48 eV, which makes more visible light can be absorbed. After light absorption the electrons will be excited from valence band to the conductance band and hence holes and electrons are generated. According to the reports, the delocalized π structure of C60 makes it easy transfer electrons and act as excellent electron acceptors. Considering the extraordinary electrons transfer bridge of chemical bond here the photoinduced electrons will quickly transfer from the framework of oxygen doped g-C3N4 to the C60 under visible light illumination. The dissolved O2 in the solution will react with photogenerated electrons to produce ∙O2− and in the following part it will transform into the oxidative species of ∙OH . Therefore the recombination of holes and electrons are inhibited effectively and hence an enhanced photocatalytic performance is resulted. 4. Conclusions In summary, O-C3N4/C60 hybrids with excellent photocatalytic properties are synthesized through a facile microwave heating method. The oxygen doping and C60 formation are verified by the XRD, FTIR and XPS analyses. XRD patterns analyses indicate treating temperature and time in H2O2 solution play significant roles in the formation of O-C3N4/ C60 hybrid. Compared with pristine g-C3N4 the O-C3N4/C60 sample exhibit a hierarchical porous structure and the BET surface area increases from 53.4 m2/g of g-C3N4 to 104.5 m2/g. A new peak at 531.4 eV is found in the XPS O1s high resolution spectrum of O-C3N4/ C60 sample, which means the new doping oxygen element exist with the form of −OH in the system. In the C1s spectrum of O-C3N4/C60 due to the introduction of C60 moiety CeC bond takes up a much larger part. The new peak at 1540 cm−1 of FTIR spectra shows a chemical bond forms between oxygen doped g-C3N4 and C60 in the O-C3N4/C60 sample. Photocatalytic performances analyses indicate the sample with the best DRS absorption exhibit the best photocatalytic degradation activities. The O-C3N4/C60 photocatalyst can degrade about 90% of methylene blue solution after 90 min illumination, while the degradation rates of pristine g-C3N4 and P25 are only 37.0% and 44.6%, respectively. The mechanism analyses attribute the enhanced photocatalytic performance to several aspects, including the creation of hierarchical porous structure, enhanced BET surface area, oxygen element doping and covalent bonding between g-C3N4 and in situ formed C60. The convenient preparation of O-C3N4/C60 sample provides an inspiration for the scalable production of photocatalysts with enhanced performances. Declaration of Competing Interest The authors declare that they have no known competing financial 9

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