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C3N4 composites
Journal Pre-proof The enhanced photodegradation of bisphenol A by TiO2/C3N4 composites Peng Mei, Huihui Wang, Han Guo, Ning Zhang, Sailun Ji, Yapeng Ma, Jiaqi Xu, Ying Li, Hamed Alsulami, Mohammed Sh Alhodaly, Tasawar Hayat, Yubing Sun PII:
S0013-9351(19)30886-2
DOI:
https://doi.org/10.1016/j.envres.2019.109090
Reference:
YENRS 109090
To appear in:
Environmental Research
Received Date: 29 September 2019 Revised Date:
25 November 2019
Accepted Date: 24 December 2019
Please cite this article as: Mei, P., Wang, H., Guo, H., Zhang, N., Ji, S., Ma, Y., Xu, J., Li, Y., Alsulami, H., Alhodaly, M.S., Hayat, T., Sun, Y., The enhanced photodegradation of bisphenol A by TiO2/C3N4 composites, Environmental Research (2020), doi: https://doi.org/10.1016/j.envres.2019.109090. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
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The enhanced photodegradation of bisphenol A by TiO2/C3N4 composites Peng Mei1, Huihui Wang1, Han Guo1, Ning Zhang1, Sailun Ji1, Yapeng Ma1, Jiaqi Xu1, Ying Li1, Hamed Alsulami2, Mohammed Sh. Alhodaly2, Tasawar Hayat2, Yubing Sun1,2* 1 MOE Key Laboratory of Resources and Environmental System Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR China 2.Department of Mathematics, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah, 21589, Saudi Arabia Abstract A new photocatalyst of TiO2/C3N4 composite (TiO2/g-C3N4) was synthesized by the hydrothermal method. The characterization showed that TiO2/g-C3N4 extended absorption light range and enhanced generation efficiency of photo-induced electron. Under the simulated solar irradiation, the photodegradation rate of bisphenol A (BPA) by TiO2/g-C3N4 was twice as fast as that of g-C3N4. Furthermore, TiO2/g-C3N4 presented the good stability and excellent selectivity for BPA degradation. The high degradation rate of BPA by TiO2/g-C3N4 was demonstrated to be superoxide radical (·O2-) and singlet oxygen (1O2) by radical quenching experiment, which was further evidenced by EPR, XPS, DRS and PL analysis. These findings revealed that TiO2/g-C3N4 can be used as a potential photocatalyst for removing organic pollutants in actual environmental remediation.
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Keywords: g-C3N4, TiO2, photocatalytic performance, BPA 1.
Introduction
With the rapid growth of various industries (e.g., chemical, printing/ dyeing and pharmaceutical plant) from all over the world, a large amount of organic pollutants were discharged into water bodies, which has become more serious concerns in recent decades due to their severe damage of ecological diversity and human beings [1]. Bisphenol A (BPA, endocrine-disrupting chemicals) as a typical organic pollutant has extensively detected in various wastewaters worldwide [2, 3]. Generally, these organic pollutants are difficult to naturally degrade into small organic molecules under actual environments. In order to remove BPA from the water system, many methods, including filtration, adsorption and phytoremediation, have been proposed in recent years [4-6]. However, these approaches are limited to actual environmental cleanup due to their low adsorption capacity, slow adsorption rate and/or high operation cost. Therefore, it is of great concern to seek a green, clean and efficient method for degradation of these refractory organic pollutants. As a typical advanced-oxidation method, photocatalysis has been extensively employed to degrade various organic pollutants owing to environmentally friendly, low-cost and high efficient approach [7-10]. Graphitic carbon nitride (g-C3N4), in recent years, has been demonstrated to be a promising active and stable metal-free photocatalyst [11, 12]. Liu et al. demonstrated that the degradation rate of BPA on g-C3N4 under 5.0 mmol/L persulfate increased from 72.5 to 100 % at 90 min irradiation [13]. Oh et al. also reported that the photodegradation rate of BPA on Ag
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and S-doped g-C3N4 was three times as high as that of pure g-C3N4 [14]. Recently, Wang et al. demonstrated that reaction kinetics of g-C3N4 for BPA photodegradation after adding U (VI) and Cr (VI) significantly decreased from 30 to 10 min due to their synergetic effect [15]. However, pure g-C3N4 has only narrow absorption visible-light range due to its relatively wide band gap (~ 2.7 eV) [16]. Therefore, many extensive researches have been conducted to modify g-C3N4 with other semiconductor materials to expand the range of adsorbed light in recent years [17-21]. For instance, TiO2/g-C3N4 heterojunction can improve its photocatalytic performance [22-24]. Jiang et al. realized collaborative removal of uranium (VI) and arsenic (III) by TiO2/g-C3N4 catalyst [25]. Approximately 40 % of As (III) and 83 % of U (VI) were removed after 4 h illumination due to the synergetic effect. The photocatalytic performance of TiO2/g-C3N4 was significantly enhanced due to the effective separation of photogenerated charge carriers from heterogamous structure, which suppressed the re-combination of electron and holes [26]. However, few studies concerning the removal mechanism of BPA on TiO2/g-C3N4 was available. In this study, the main objectives were 1) to synthesize TiO2/g-C3N4 by hydrothermal method and characterize it by SEM, TEM, XRD and FT-IR techniques; 2) to research the photocatalytical performance of TiO2/g-C3N4 for photodegradation of BPA under different reaction conditions; 3) to explore the photodegradation mechanism of BPA and TiO2/g-C3N4 using radical quenching experiment, XPS, diffuse reflection spectra (DRS), electron paramagnetic resonance (EPR) and photo-luminescence (PL) analysis. All in all, the focus of this research is the application of C3N4-based photocatalyst for
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photodegradation of organic pollutants into the actual environmental cleanup. 2.
Experimental methods
2.1 Materials All chemicals were of analytic reagents without further purification. Urea, BPA and p-benzoquinone (p-BQ) were obtained from Shanghai Aladdin biochemical Technology Co., Ltd. Tertiary butanol (TBA), sodium azide (NaN3), methanol, 5, 5dimethyl-1-pyrroline-N-oxide (DMPO), 2, 2, 6, 6-tetramethylpiperidine (TEMP), sodium hydroxide, hydrogen chloride and tetrabutyl titanate were purchased from Sinopharm Chemical Reagent Co., Ltd. Phenol, 2.4-dichlorophenol (2, 4-DCP), methylene-blue (MB), methyl-orange (MO) and Rhodamine-B (RhB) were obtained from ACROS (Geel, Belgium). Diphenhydramine (DP) and Ibuprofen (IBU) were bought from Sigma-Aldrich Chemical Reagent Co., Ltd. 2.2 Preparation and characterization of TiO2/g-C3N4. TiO2/g-C3N4 was synthesized by hydrothermal method [27]. Firstly, g-C3N4 was gained by heating urea under 550
for 4 h. After grinding, the material was washed
using nitric acid (0.1 mol/L) and deionized water, and then freeze-dried for 12 h. Secondly, 1.36 g of C3N4 and 72 mL of anhydrous ethanol were ultrasonically treated for 0.5 h. Then 0.6 mL of HNO3 and equal amount of tetrabutyl titanate were sequentially provided under continuous stirring for 20 min [27, 28]. The mixture was placed into a Teflon stainless-steel autoclave and heated at 180
for 12 h. After
cooled to room temperature, the intermediates were alternatingly washed using ethanol and deionized water three times until to yellowish. Finally, TiO2/g-C3N4 was
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obtained by freeze-drying it for 12 h. TiO2 was prepared using the same method without adding g-C3N4. The morphology of as-prepared TiO2/g-C3N4 was characterized by SEM (SU8020, Hitachi, an accelerating voltage of 20 kV and detecting current of 10 mA) and TEM (JEOL-2010, JEOL, an accelerating voltage of 200 kV). X-ray diffraction (XRD) was carried out by a Scintag-XDS-2000 (Cu-Kα, λ = 1.54059 Å) at a working voltage of 40 kV and working current of 100 mA. XPS data were obtained by AXIS Ultra instrument (Kratos, 10 kV, 5 mV and 10-8 Pa residual pressure) with an Al Kα source (1486.5 eV). FT-IR was carried out using IRTracer-100 instrument with KBr (mass ratio of sample to KBr: 150: 1) at scanning range of 400 ~ 4000 cm-1 and scanning resolution of 4 cm-1. The optical properties of TiO2/g-C3N4 were characterized by DRS (LS 55, perkinelmer), PL (D8advance, Buker) and EPR (A300-10/12, Buker) analyses. 2.3 Photodegradation Experiments. All the photodegradation experiments were carried out in a photocatalytic reaction box (CEL-HXF 300, Beijing Aulight Co. Ltd.) within 100 mL quartz beaker and cooled with circulating water. Typically, 0.05 g of TiO2/g-C3N4 and 50 mL of BPA (20 ppm, pH 6.0) were stirred for 30 min under dark conditions to reach adsorption equilibrium. Then the suspensions were illuminated with a 450 W Xe arc lamp (simulated sunlight) under different irradiation time. The effect of pH (4, 6, 8 and 10) and initial BPA concentration (10, 20, 50 and 100 ppm) on BPA photodegradation was investigated by batch techniques. The regeneration experiments were conducted as
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followed: TiO2/g-C3N4 was filtered and centrifuged after the first photodegradation experiment, and then the samples were dried for 12 h in the oven in order to adsorbed water of TiO2/g-C3N4. The dried TiO2/g-C3N4 solid was used to conduct the second photodegradation experiment. The photodegradation time of five cycle experiments was set to 60 min. To compare the photocatalytic performance, the photodegradation of other organic refractory pollutants (2, 4-DCP, Phenol, IBU, DP) and organic dyes (MO, MB and RhB) on TiO2/g-C3N4 was also conducted in the same conditions. To explore the various oxygenated free radicals, the quenching experiments were also performed by adding TBA, p-BQ, NaN3 and methanol solution during photodegradation experiments. After the interval time of 0, 5, 10, 20, 30, 40, and 60 min, 1 mL aliquots of the suspension were separated by 0.45 µm nylon filter. The concentrations of BPA in the supernatant were measured by high performance liquid chromatography (HPLC, LC-2030, Shimadzu Instruments Co., Ltd., Japan). Detailed descriptions of the photodegradation experiments were shown in support information (SI). 3.
Results and discussion
3.1 Characterization The morphology of as-prepared TiO2/g-C3N4 was characterized by SEM and TEM images (Figure 1). As shown in Figure 1a, TiO2/g-C3N4 displayed the sheet-like heterostructure. Xu et al. demonstrated that heterostructure of catalyst was favorable for the separation of photo-generated electrons and holes [29]. According to EDX analysis, the main constitutes of TiO2/g-C3N4 included C (69.75 atomic %), N (20.54
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atomic %), O (6.76 atomic %) and Ti (2.93 atomic %). TEM observation showed that spherical TiO2 nanoparticles with ~ 20 nm size were uniformly dispersed on surface of g-C3N4 (Figure 1b). The BET specific surface area of TiO2/g-C3N4, g-C3N4 and TiO2 was 53.49, 46.37 and 222.11 m2/g, respectively. Compared to sheet-like g-C3N4, the high BET specific surface area of TiO2 could be attributed to its nano-spherical particle. As shown in Figure S1a of SI, the hysteresis loops of TiO2/g-C3N4, g-C3N4 and TiO2 samples showed the mesoporous features. The main pore diameter distributions of TiO2 (at ~3 nm) were significantly lower than that of TiO2/g-C3N4 and g-C3N4 (from 25 to 180 nm), indicating TiO2 with microporous nanoparticles while TiO2/g-C3N4
and
g-C3N4
with
macroporous
materials.
According
thermogravimetric analysis, the mass loss of TiO2/g-C3N4 (~ 10 %) at 500
to
could be
attributed to the evaporation of a small amount of water molecules in the sample and the decomposition of impurities. However, the main mass loss of TiO2/g-C3N4 (~84 %) at 500- 600
was ascribed to the decomposition of g-C3N4. The additional
characterizations of TiO2/g-C3N4, including SEM, TEM, XRD, FT-IR, DRS and thermogravimetric analysis, by adding diverse of HNO3 and TiO2 were provided in Figure S2-S6. Figure 1c shows XRD patterns of g-C3N4, TiO2 and TiO2/g-C3N4. For g-C3N4, characteristic peaks at 13 and 27.6 ° were designated as (100) plane of structure accumulation and (002) plane of interlayer accumulation, respectively [30]. For TiO2, characteristic peaks at 25.2, 37.9, 48.3, 53.8, 55.2, 62.7 ° corresponded to (101), (103), (200), (105), (204) and (213) plane of anatase (PDF card 00-001-0562), respectively.
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For TiO2/g-C3N4, the weak peaks of TiO2 indicated the dispersion of TiO2 nanoparticles on g-C3N4 surface. However, the relative peak intensities of TiO2 increased slightly with increasing TiO2 contents (Figure S3b). XRD patterns showed the uniform distribution of TiO2 on g-C3N4 nanometer sheets. As shown by FT-IR spectra in Figure 1d, the absorption IR peak at 812 cm-1 was attributed to the stretching vibration of triazine ring [24, 31]. The absorption peaks at 1243-1638 cm-1 were designated as the stretching vibration of C-N and C=N hetero-ring, and the wide IR peak at 3000-3500 cm-1 was related to amino group (N-H) and hydroxyl group (O-H) [24, 32]. In addition, the weak peaks of TiO2 were occurred due to the low doping content. FT-IR analysis showed TiO2/g-C3N4 with various oxygen and nitrogen-containing functional groups. Figure 2a shows DRS spectra of TiO2, g-C3N4 and TiO2/g-C3N4. For g-C3N4, the strong absorption peaks were observed in UV (200-400 nm) and visible (400-420 nm) regions, whereas the absorption spectra of TiO2 was mainly concentrated in ultraviolet region (200-400 nm). For TiO2/g-C3N4, the strong absorption peaks in visible and UV regions were significantly widened, which facilitated BPA photodegradation due to the prolonged adsorption range of spectrum. The bandgap of TiO2, g-C3N4 and TiO2/g-C3N4 was calculated to 2.94, 2.7 and 2.72 eV, respectively, which was consistent with the theoretical values [33]. The narrow bandgap of TiO2/g-C3N4 improved the photocatalytic efficiency. The addition of TiO2 into g-C3N4 reduced the probability of electron and hole recombination. Figure 2b compares PL spectra of TiO2, g-C3N4 and TiO2/g-C3N4 at different
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wavelengths. The separation and recombination rates of photo-induced conductive loads in TiO2/g-C3N4 were significantly faster than those in g-C3N4 and TiO2. It could be found that the wavelength of TiO2/g-C3N4 had a slight blue-shift from 460 (g-C3N4) to 442 nm, which was associated with the wide bandgap. PL analysis shows that the addition of TiO2 on g-C3N4 improved the absorption efficiency of excited photoelectrons. 3.2 Effect of concentration and pH Figure 3a shows the effect of initial BPA concentrations on the photodegradation of BPA under irradiation conditions. The photodegradation rate constants significantly decreased from 0.17 to 0.04 min−1 with increasing BPA concentrations from 10 to 100 ppm. After irradiation 30 min, approximate 100, 99 and 87.5 % of BPA were photodegraded by TiO2/g-C3N4 at 10, 20 and 50 ppm, respectively. As summarized in Table 1, the photodegradation performance of TiO2/g-C3N4 was significantly higher than that of other photocatalysts such as Bi12O15Cl6 (100 %, 6 h) [34], g-C3N4 in the presence of persulfate (100 %, 1.5 h) [13], MOF-derived g-C3N4 (97 %, 1 h) [35] and Ag2O/ZnFe2O4 (92.5 %, 1 h) [36]. The decrease of photodegradation rate at high BPA concentration could be due to the limited number of free radicals produced under identical concentration of catalyst, although the increased probability of effective collision between BPA molecule and photocatalyst under high BPA amount. Figure 3b shows the effect of pH on BPA photodegradation by TiO2/g-C3N4. BPA photodegradation increased slightly when pH increased from 4.0 to 6.0, whereas the photodegradation performance decreased gradually at pH > 8.0. After irradiation 10
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min, 68.5, 79, 67.4 and 60.3 % of BPA were photodegraded by TiO2/g-C3N4 at pH 4.0, 6.0, 8.0 and 10.0, respectively, whereas approximate 100 % of BPA can be photodegraded by TiO2/g-C3N4 at pH 4.0-10.0 after irradiation 30 min. The decrease of BPA photodegradation at pH > 8.0 could be due to the inhibited migration of photo-reduced electron (negative charge) to the surface of catalyst at high pH, leading to increased recombination rate of e- and h+ [37]. These observations revealed TiO2/g-C3N4 with excellent photocatalytic activity and fast photodegradation rate at neutral pH. 3.3 Effect of catalyst For
comparison
of
photodegradation
performance
of
TiO2/g-C3N4,
the
photodegradation of BPA on g-C3N4 and TiO2 were also examined at 20 ppm BPA and pH 6.0. As shown in Figure 3c, the photodegradation rate of BPA on TiO2/g-C3N4 under simulated sunlight irradiation was significantly faster than that of TiO2 and g-C3N4, although 100 % of BPA was photodegraded by catalysts after irradiation 45 min. After irradiation 10 min, 52.3, 33 and 86.3 % of BPA were photodegraded by TiO2, g-C3N4 and TiO2/g-C3N4, respectively. The further evidence was provided in Figure S7-S9. As shown in Figure S9, the removal of total organic carbon (TOC) by TiO2/g-C3N4 +BPA (54.2 %) after 1 h irradiation was remarkably higher than that of g-C3N4 (38.3 %) and TiO2 (20.5 %), indicating that TiO2/g-C3N4 displayed the high efficient photodegradation of BPA compared to single TiO2 and g-C3N4. These results indicated that TiO2/g-C3N4 significantly increased the photodegradation rate of BPA. 3.4 Regeneration of TiO2/g-C3N4.
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Figure 3d shows the cycling performance of BPA photodegradation by TiO2/g-C3N4 under five cycles. After irradiation 30 min, ~ 100 % of BPA was photodegraded by TiO2/g-C3N4 at each photocatalytic experiment. After five cycles, TiO2/g-C3N4 remained stable photocatalytic performance demonstrating that TiO2/g-C3N4 can serve as a promising catalyst for highly efficient treatment of organic wastewater in environmental remediation. 3.5 Comparison of photocatalytic performance The photodegradation of various organic refractory pollutants and organic dyes on TiO2/g-C3N4 was investigated under neutral conditions for evaluating the versatility of TiO2/g-C3N4 (Figure 4a). After irradiation 20 min, ~ 48 % of DP, 62 % of 2, 4-DCP, 87 % of phenol and 88 % of IBU were photodegraded by TiO2/g-C3N4, which was significantly lower than that of BPA (~ 100 %). Figure 4b shows the photodegradation of MO, MB and RhB on TiO2/g-C3N4. The photodegradation of MO, MB and RhB decreased significantly with increasing irradiation time. After irradiation 20 min, ~ 28 % of MO, 52 % of RhB and 89 % of MB were photodegraded by TiO2/g-C3N4. Approximate 100 % of MB and RhB were completely photodegraded, but 68.2 % MO was degraded by TiO2/g-C3N4 under irradiation 60 min. These results indicated that TiO2/g-C3N4 had a versatile photocatalytic degradation capability and had degradation effects on a variety of refractory pollutants and organic dyes. 3.6 Photo-degradation mechanism The photodegradation mechanism of BPA on TiO2/g-C3N4 was demonstrated by EPR and XPS analysis. Figure 5a and 5b show EPR signals of DMPO/·O2− and TEMP/1O2,
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respectively, on TiO2/g-C3N4 in various methanol dispersions and under the simulated sunlight. The blank experiment (DMPO) and BPA system displayed four characteristic peaks of DMPO/·O2− and three of TEMP/1O2, whereas no characteristic peaks were observed under dark conditions, indicating superoxide radicals (·O2 −) and singlet oxygen radicals (1O2) were generated under illumination conditions. The relative intensities of the DMPO/·O2− and TEMP/1O2 peak for BPA system were weaker than that of DMPO, whereas the characteristic peaks of hydroxyl radicals (·OH) were not significantly changed in the two systems (Figure S10a). These results showed that the highly efficient photodegradation of BPA on TiO2/g-C3N4 was ascribed to ·O2− and 1O2 rather than ·OH radicals. Therefore, the interaction mechanism can be described as Eqns. (1-5): TiO2/g-C3N4 + hν → TiO2/g-C3N4 (h+ + e-)
(1)
TiO2/g-C3N4 (e-) + O2 → TiO2/g-C3N4 + ·O2-
(2)
TiO2/g-C3N4 (h+) + ·O2- → TiO2/g-C3N4 + 1O2
(3)
BPA + ·O2- → CO2 + H2O
(4)
BPA + 1O2 →CO2 + H2O
(5)
The electric potential (vs NHE) of ·O2− derived from TiO2/g-C3N4 (− 1.17 eV at CB) was more negative than that of O2/·O2− (− 0.33 eV) [34]. Moreover, ·O2- was partly oxidized to 1O2 by h+ [38], therefore ·O2− and 1O2 could be produced for BPA degradation. The photodegradation mechanism was further demonstrated by quenching experiments under simulated sunlight. In this study, p-BQ, NaN3, TBA and methanol were served as scavengers to remove ·O2 −, 1O2, ·OH and holes. As shown
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in Figure 5c, BPA was barely photodegraded by TiO2/g-C3N4 after adding p-BQ agents, indicating that ·O2- radicals played the main role in BPA photodegradation. Compared to blank experiment, the slow photodegradation rate of BPA after adding NaN3 suggested that 1O2 radicals had the slight effect of BPA photodegradation, whereas the slight change in BPA photodegradation after adding TBA and menthol revealed that ·OH and h+ radicals had no effect of BPA photodegradation. Figure 5d, 5e and 5f show the high-resolution XPS spectra of O 1s, C 1s and N 1s, respectively. O 1s XPS spectra can be divided into four peaks at ~ 529.8, 531.0, 532.2 and 533.5 eV, which belong to the lattice oxygen of Ti-O, C=O, N/C-OH and adsorbed H2O, respectively [28, 39]. The peak of C=O (531.4 eV) and the peak of N/C-OH (532.2 eV) for TiO2/g-C3N4 after reaction were slightly shifted to high binding energy, indicating that the various oxygen-containing radicals (e.g., ·OH, ·O2and 1O2 radicals) played a vital role in BPA photodegradation during irradiations. The further evidence was supported by C 1s XPS spectra (Figure 5e), which can be divided at 284.9, 286.3 and 288.3 eV, corresponding to C-C, C-O and N-C=N groups, respectively. The occurrence of C-O could be derived from the CO2 adsorbed by materials and/or the incomplete oxygen-containing intermediate formed during heating [24, 40, 41]. After reaction, the low binding energies of C-O (285.8 eV) and C-C (284.7 eV) were surveyed. As shown in Figure 5f, N 1s XPS spectra can be deconvoluted at 398.9, 399.8 and 401.1 eV, which can belong to pyridinic N, Ti-N-O/Ti-O-N and pyrrolinic N, respectively [42-44]. Figure S10b shows the high resolution Ti 2p XPS spectra of TiO2/g-C3N4. The position and intensity of Ti 2p3/2
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(458.7 eV) and Ti 2p1/2 (464.3 eV) of TiO2/g-C3N4 remained unchanged before and after the reaction [39, 45]. XPS analysis proved that the uniform dispersion of TiO2 on g-C3N4 formed the heterogeneous structure, increasing the excitation rate of photoelectron. 4.
Conclusions
In this study, TiO2/g-C3N4 composites were synthesized by solvothermal method. The characterization results confirmed that the spherical TiO2 nanoparticles were uniformly distributed on g-C3N4 surface with heterogeneous structure, which extended the optical absorption range and accelerated the separation/migration of electrons and holes. The batch photodegradation experiments showed TiO2/g-C3N4 displayed
the
highly
efficient
photodegradation
rate
of
BPA
(complete
photodegradation within 20 min), good stability at pH 4.0-10.0 and high selectivity (superior to other organic pollutants). The ·O2- radicals were responsible for highly efficient photodegradation of BPA on TiO2/g-C3N4 by quenching experiments, EPR and XPS analysis. These findings are critical to the removal of organic contaminants by using C3N4-based composites in environmental remediation. Acknowledgement Financial support from the National Natural Science Foundation of China (21822602) is acknowledged. References [1] M.A. Ashraf, Persistent organic pollutants (POPs): a global issue, a global challenge, Environ. Sci. Pollut. Res. Int., 24 (2017) 4223-4227. [2] E. Diamanti-Kandarakis, J.P. Bourguignon, L.C. Giudice, R. Hauser, G.S. Prins,
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Figure Captions Figure 1. Characterization of TiO2/g-C3N4, (a) and (b): SEM and TEM images, respectively; (c) and (d): XRD and FT-IR patterns, respectively. Figure 2. DRS (a) and PL spectra (b) of TiO2, g-C3N4 and TiO2/g-C3N4. Figure 3. Photocatalytic degradation of BPA by TiO2/g-C3N4 under different conditions, (a): effect of initial BPA concentrations, m/v = 1.0 g/L, pH = 6.0; (b): pH effect, m/v = 1.0 g/L, C0 = 20 ppm; (c): effect of different photocatalysts, m/v = 1.0 g/L, pH = 6.0; (d): regeneration experiments, m/v = 1.0 g/L, C0 = 20 ppm, pH = 6.0. Figure 4. Comparison of photodegradation of BPA by TiO2/g-C3N4 with other organic refractory pollutants (a) and organic dyes (b), m/v = 1.0 g/L, C0 = 20 ppm, pH = 6.0. Figure 5. (a): DMPO spin-trapping EPR spectra for DMPO/·O2- on TiO2/g-C3N4 (simulated sunlight); (b): TEMP spin-trapping EPR spectra for TEMP/1O2 on TiO2/g-C3N4 in various methanol dispersions (simulated sunlight); (c) Radical quenching experiment; (d), (e) and (f): O 1s, C 1s and N 1s XPS spectra, respectively.
10
20
40
50
(213)
(103) 30
(200) (105) (204)
g-C3N4 TiO2/g-C3N4 TiO2
(101)
(100)
Intensity (a.u.)
(c)
60
(d) transmitance (a.u.)
(002)
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70
TiO2/g-C3N4, 1st cycle TiO2/g-C3N4 g-C3N4
3000-3500 g-C3N4 TiO32N4 N-TiO2/g-C
1638
812 1243
TiO2/g-C3N4 N-TiO2 3500 3000 2500 2000 1500 1000
2 theta (degree) Figure 1.
wavenumbers (cm-1)
500
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0.4 g-C3N4 TiO2/g-C3N4 TiO2
0.3
0.2
0.1
0.0 200
400
600
wavelength (nm)
TiO2/g-C3N4 g-C3N4 TiO2
(b) Intensity (a.u.)
absorbance (a.u.)
(a)
800
Figure 2.
400
450
500
wavelenth (nm)
550
600
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(b)
(a)
1.0
4 6 8 10
10ppm 20ppm 50ppm 100ppm
BPA C/C0
0.8 0.6 0.4 0.2 0.0 0
15
30
45
60
0
15
30
45
60
Time (min)
Time (min)
(d)
(c)
1.0
g-C3N4 TiO2 TiO2/g-C3N4
BPA C/C0
0.8
1st
2nd
3th
4th
5th
0.6 0.4 0.2 0.0 0
15
30
Time (min)
45
60
0
60
120
180
Time (min) Figure 3.
240
300
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(a)
1.0
MO MB RhB
DP 2， 4-DCP Phenol IBU
BPA C/C0
0.8 0.6
(b)
0.4 0.2 0.0 0
15
30
45
60
Time (min) Figure 4.
0
15
30
Time (min)
45
60
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(a)
·O2-
3460
3480
3500
3520
3540
3560
3460
3480
TBA/OH· P-BQ/·O2-
BPA C/C0
NaN3/1O2
0.4
Methanol/h+ Blank
0.2
Intensity(a.u.)
(d)
0.6
3520
532.2 533.5
3540
3560
529.8
O 1s
531.0
TiO2/g-C3N4
532.4 531.4 after reaction
0.0
533.2 0
15
30
45
538
60
536
Time (min)
(e)
290
Intensity(a.u.)
285.8 284.7
288
286
530
528
526
284
398.7 N 1s
TiO2/g-C3N4
286.3
292
532
(f)
C 1s 284.9
after reaction
534
Binding energy (eV)
288.3
TiO2/g-C3N4
Intensity(a.u.)
3500
Magnetic Filed (G)
(c)
0.8
O2
BPA TEMP/1O2
Magnetic Filed (G) 1.0
1
Intensity (a.u.)
Intensity (a.u.)
BPA DMOP/·O2-
(b)
282
406
Binding energy (eV)
Figure 5.
399.9 401.2
after reaction
404
402
400
398
Binding energy (eV)
396
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Table 1. Photocatalytic degradation of BPA by other catalysts Photocatalysts
Efficiency
Ref.
Bi12O15Cl6
100 %, 6 h
[34]
Persulfate-bearing g-C3N4
100 %, 1.5 h
[13]
MOF-derived g-C3N4
97 %, 1 h
[35]
Ag2O/ZnFe2O4
92.5 %, 1 h
[36]
TiO2/g-C3N4
100 %, 0.5 h
This study
Highlight
Heterogeneous TiO2/g-C3N4 extended absorption-light range and photoelectron production
TiO2/g-C3N4 display the good stability and high selectivity for BPA degradation
BPA degradation was attributed to ·O2- and 1O2 radicals by EPR and XPS analysis
C3N4-based composite was a promising photocatalyst for removing organic pollutants
Conflict of Interest Statement Hereby, the authors state no conflicting financial interest.
S0013-9351(19)30886-2
DOI:
https://doi.org/10.1016/j.envres.2019.109090
Reference:
YENRS 109090
To appear in:
Environmental Research
Received Date: 29 September 2019 Revised Date:
25 November 2019
Accepted Date: 24 December 2019
Please cite this article as: Mei, P., Wang, H., Guo, H., Zhang, N., Ji, S., Ma, Y., Xu, J., Li, Y., Alsulami, H., Alhodaly, M.S., Hayat, T., Sun, Y., The enhanced photodegradation of bisphenol A by TiO2/C3N4 composites, Environmental Research (2020), doi: https://doi.org/10.1016/j.envres.2019.109090. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
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The enhanced photodegradation of bisphenol A by TiO2/C3N4 composites Peng Mei1, Huihui Wang1, Han Guo1, Ning Zhang1, Sailun Ji1, Yapeng Ma1, Jiaqi Xu1, Ying Li1, Hamed Alsulami2, Mohammed Sh. Alhodaly2, Tasawar Hayat2, Yubing Sun1,2* 1 MOE Key Laboratory of Resources and Environmental System Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR China 2.Department of Mathematics, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah, 21589, Saudi Arabia Abstract A new photocatalyst of TiO2/C3N4 composite (TiO2/g-C3N4) was synthesized by the hydrothermal method. The characterization showed that TiO2/g-C3N4 extended absorption light range and enhanced generation efficiency of photo-induced electron. Under the simulated solar irradiation, the photodegradation rate of bisphenol A (BPA) by TiO2/g-C3N4 was twice as fast as that of g-C3N4. Furthermore, TiO2/g-C3N4 presented the good stability and excellent selectivity for BPA degradation. The high degradation rate of BPA by TiO2/g-C3N4 was demonstrated to be superoxide radical (·O2-) and singlet oxygen (1O2) by radical quenching experiment, which was further evidenced by EPR, XPS, DRS and PL analysis. These findings revealed that TiO2/g-C3N4 can be used as a potential photocatalyst for removing organic pollutants in actual environmental remediation.
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Keywords: g-C3N4, TiO2, photocatalytic performance, BPA 1.
Introduction
With the rapid growth of various industries (e.g., chemical, printing/ dyeing and pharmaceutical plant) from all over the world, a large amount of organic pollutants were discharged into water bodies, which has become more serious concerns in recent decades due to their severe damage of ecological diversity and human beings [1]. Bisphenol A (BPA, endocrine-disrupting chemicals) as a typical organic pollutant has extensively detected in various wastewaters worldwide [2, 3]. Generally, these organic pollutants are difficult to naturally degrade into small organic molecules under actual environments. In order to remove BPA from the water system, many methods, including filtration, adsorption and phytoremediation, have been proposed in recent years [4-6]. However, these approaches are limited to actual environmental cleanup due to their low adsorption capacity, slow adsorption rate and/or high operation cost. Therefore, it is of great concern to seek a green, clean and efficient method for degradation of these refractory organic pollutants. As a typical advanced-oxidation method, photocatalysis has been extensively employed to degrade various organic pollutants owing to environmentally friendly, low-cost and high efficient approach [7-10]. Graphitic carbon nitride (g-C3N4), in recent years, has been demonstrated to be a promising active and stable metal-free photocatalyst [11, 12]. Liu et al. demonstrated that the degradation rate of BPA on g-C3N4 under 5.0 mmol/L persulfate increased from 72.5 to 100 % at 90 min irradiation [13]. Oh et al. also reported that the photodegradation rate of BPA on Ag
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and S-doped g-C3N4 was three times as high as that of pure g-C3N4 [14]. Recently, Wang et al. demonstrated that reaction kinetics of g-C3N4 for BPA photodegradation after adding U (VI) and Cr (VI) significantly decreased from 30 to 10 min due to their synergetic effect [15]. However, pure g-C3N4 has only narrow absorption visible-light range due to its relatively wide band gap (~ 2.7 eV) [16]. Therefore, many extensive researches have been conducted to modify g-C3N4 with other semiconductor materials to expand the range of adsorbed light in recent years [17-21]. For instance, TiO2/g-C3N4 heterojunction can improve its photocatalytic performance [22-24]. Jiang et al. realized collaborative removal of uranium (VI) and arsenic (III) by TiO2/g-C3N4 catalyst [25]. Approximately 40 % of As (III) and 83 % of U (VI) were removed after 4 h illumination due to the synergetic effect. The photocatalytic performance of TiO2/g-C3N4 was significantly enhanced due to the effective separation of photogenerated charge carriers from heterogamous structure, which suppressed the re-combination of electron and holes [26]. However, few studies concerning the removal mechanism of BPA on TiO2/g-C3N4 was available. In this study, the main objectives were 1) to synthesize TiO2/g-C3N4 by hydrothermal method and characterize it by SEM, TEM, XRD and FT-IR techniques; 2) to research the photocatalytical performance of TiO2/g-C3N4 for photodegradation of BPA under different reaction conditions; 3) to explore the photodegradation mechanism of BPA and TiO2/g-C3N4 using radical quenching experiment, XPS, diffuse reflection spectra (DRS), electron paramagnetic resonance (EPR) and photo-luminescence (PL) analysis. All in all, the focus of this research is the application of C3N4-based photocatalyst for
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photodegradation of organic pollutants into the actual environmental cleanup. 2.
Experimental methods
2.1 Materials All chemicals were of analytic reagents without further purification. Urea, BPA and p-benzoquinone (p-BQ) were obtained from Shanghai Aladdin biochemical Technology Co., Ltd. Tertiary butanol (TBA), sodium azide (NaN3), methanol, 5, 5dimethyl-1-pyrroline-N-oxide (DMPO), 2, 2, 6, 6-tetramethylpiperidine (TEMP), sodium hydroxide, hydrogen chloride and tetrabutyl titanate were purchased from Sinopharm Chemical Reagent Co., Ltd. Phenol, 2.4-dichlorophenol (2, 4-DCP), methylene-blue (MB), methyl-orange (MO) and Rhodamine-B (RhB) were obtained from ACROS (Geel, Belgium). Diphenhydramine (DP) and Ibuprofen (IBU) were bought from Sigma-Aldrich Chemical Reagent Co., Ltd. 2.2 Preparation and characterization of TiO2/g-C3N4. TiO2/g-C3N4 was synthesized by hydrothermal method [27]. Firstly, g-C3N4 was gained by heating urea under 550
for 4 h. After grinding, the material was washed
using nitric acid (0.1 mol/L) and deionized water, and then freeze-dried for 12 h. Secondly, 1.36 g of C3N4 and 72 mL of anhydrous ethanol were ultrasonically treated for 0.5 h. Then 0.6 mL of HNO3 and equal amount of tetrabutyl titanate were sequentially provided under continuous stirring for 20 min [27, 28]. The mixture was placed into a Teflon stainless-steel autoclave and heated at 180
for 12 h. After
cooled to room temperature, the intermediates were alternatingly washed using ethanol and deionized water three times until to yellowish. Finally, TiO2/g-C3N4 was
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obtained by freeze-drying it for 12 h. TiO2 was prepared using the same method without adding g-C3N4. The morphology of as-prepared TiO2/g-C3N4 was characterized by SEM (SU8020, Hitachi, an accelerating voltage of 20 kV and detecting current of 10 mA) and TEM (JEOL-2010, JEOL, an accelerating voltage of 200 kV). X-ray diffraction (XRD) was carried out by a Scintag-XDS-2000 (Cu-Kα, λ = 1.54059 Å) at a working voltage of 40 kV and working current of 100 mA. XPS data were obtained by AXIS Ultra instrument (Kratos, 10 kV, 5 mV and 10-8 Pa residual pressure) with an Al Kα source (1486.5 eV). FT-IR was carried out using IRTracer-100 instrument with KBr (mass ratio of sample to KBr: 150: 1) at scanning range of 400 ~ 4000 cm-1 and scanning resolution of 4 cm-1. The optical properties of TiO2/g-C3N4 were characterized by DRS (LS 55, perkinelmer), PL (D8advance, Buker) and EPR (A300-10/12, Buker) analyses. 2.3 Photodegradation Experiments. All the photodegradation experiments were carried out in a photocatalytic reaction box (CEL-HXF 300, Beijing Aulight Co. Ltd.) within 100 mL quartz beaker and cooled with circulating water. Typically, 0.05 g of TiO2/g-C3N4 and 50 mL of BPA (20 ppm, pH 6.0) were stirred for 30 min under dark conditions to reach adsorption equilibrium. Then the suspensions were illuminated with a 450 W Xe arc lamp (simulated sunlight) under different irradiation time. The effect of pH (4, 6, 8 and 10) and initial BPA concentration (10, 20, 50 and 100 ppm) on BPA photodegradation was investigated by batch techniques. The regeneration experiments were conducted as
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followed: TiO2/g-C3N4 was filtered and centrifuged after the first photodegradation experiment, and then the samples were dried for 12 h in the oven in order to adsorbed water of TiO2/g-C3N4. The dried TiO2/g-C3N4 solid was used to conduct the second photodegradation experiment. The photodegradation time of five cycle experiments was set to 60 min. To compare the photocatalytic performance, the photodegradation of other organic refractory pollutants (2, 4-DCP, Phenol, IBU, DP) and organic dyes (MO, MB and RhB) on TiO2/g-C3N4 was also conducted in the same conditions. To explore the various oxygenated free radicals, the quenching experiments were also performed by adding TBA, p-BQ, NaN3 and methanol solution during photodegradation experiments. After the interval time of 0, 5, 10, 20, 30, 40, and 60 min, 1 mL aliquots of the suspension were separated by 0.45 µm nylon filter. The concentrations of BPA in the supernatant were measured by high performance liquid chromatography (HPLC, LC-2030, Shimadzu Instruments Co., Ltd., Japan). Detailed descriptions of the photodegradation experiments were shown in support information (SI). 3.
Results and discussion
3.1 Characterization The morphology of as-prepared TiO2/g-C3N4 was characterized by SEM and TEM images (Figure 1). As shown in Figure 1a, TiO2/g-C3N4 displayed the sheet-like heterostructure. Xu et al. demonstrated that heterostructure of catalyst was favorable for the separation of photo-generated electrons and holes [29]. According to EDX analysis, the main constitutes of TiO2/g-C3N4 included C (69.75 atomic %), N (20.54
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atomic %), O (6.76 atomic %) and Ti (2.93 atomic %). TEM observation showed that spherical TiO2 nanoparticles with ~ 20 nm size were uniformly dispersed on surface of g-C3N4 (Figure 1b). The BET specific surface area of TiO2/g-C3N4, g-C3N4 and TiO2 was 53.49, 46.37 and 222.11 m2/g, respectively. Compared to sheet-like g-C3N4, the high BET specific surface area of TiO2 could be attributed to its nano-spherical particle. As shown in Figure S1a of SI, the hysteresis loops of TiO2/g-C3N4, g-C3N4 and TiO2 samples showed the mesoporous features. The main pore diameter distributions of TiO2 (at ~3 nm) were significantly lower than that of TiO2/g-C3N4 and g-C3N4 (from 25 to 180 nm), indicating TiO2 with microporous nanoparticles while TiO2/g-C3N4
and
g-C3N4
with
macroporous
materials.
According
thermogravimetric analysis, the mass loss of TiO2/g-C3N4 (~ 10 %) at 500
to
could be
attributed to the evaporation of a small amount of water molecules in the sample and the decomposition of impurities. However, the main mass loss of TiO2/g-C3N4 (~84 %) at 500- 600
was ascribed to the decomposition of g-C3N4. The additional
characterizations of TiO2/g-C3N4, including SEM, TEM, XRD, FT-IR, DRS and thermogravimetric analysis, by adding diverse of HNO3 and TiO2 were provided in Figure S2-S6. Figure 1c shows XRD patterns of g-C3N4, TiO2 and TiO2/g-C3N4. For g-C3N4, characteristic peaks at 13 and 27.6 ° were designated as (100) plane of structure accumulation and (002) plane of interlayer accumulation, respectively [30]. For TiO2, characteristic peaks at 25.2, 37.9, 48.3, 53.8, 55.2, 62.7 ° corresponded to (101), (103), (200), (105), (204) and (213) plane of anatase (PDF card 00-001-0562), respectively.
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For TiO2/g-C3N4, the weak peaks of TiO2 indicated the dispersion of TiO2 nanoparticles on g-C3N4 surface. However, the relative peak intensities of TiO2 increased slightly with increasing TiO2 contents (Figure S3b). XRD patterns showed the uniform distribution of TiO2 on g-C3N4 nanometer sheets. As shown by FT-IR spectra in Figure 1d, the absorption IR peak at 812 cm-1 was attributed to the stretching vibration of triazine ring [24, 31]. The absorption peaks at 1243-1638 cm-1 were designated as the stretching vibration of C-N and C=N hetero-ring, and the wide IR peak at 3000-3500 cm-1 was related to amino group (N-H) and hydroxyl group (O-H) [24, 32]. In addition, the weak peaks of TiO2 were occurred due to the low doping content. FT-IR analysis showed TiO2/g-C3N4 with various oxygen and nitrogen-containing functional groups. Figure 2a shows DRS spectra of TiO2, g-C3N4 and TiO2/g-C3N4. For g-C3N4, the strong absorption peaks were observed in UV (200-400 nm) and visible (400-420 nm) regions, whereas the absorption spectra of TiO2 was mainly concentrated in ultraviolet region (200-400 nm). For TiO2/g-C3N4, the strong absorption peaks in visible and UV regions were significantly widened, which facilitated BPA photodegradation due to the prolonged adsorption range of spectrum. The bandgap of TiO2, g-C3N4 and TiO2/g-C3N4 was calculated to 2.94, 2.7 and 2.72 eV, respectively, which was consistent with the theoretical values [33]. The narrow bandgap of TiO2/g-C3N4 improved the photocatalytic efficiency. The addition of TiO2 into g-C3N4 reduced the probability of electron and hole recombination. Figure 2b compares PL spectra of TiO2, g-C3N4 and TiO2/g-C3N4 at different
9 / 26
wavelengths. The separation and recombination rates of photo-induced conductive loads in TiO2/g-C3N4 were significantly faster than those in g-C3N4 and TiO2. It could be found that the wavelength of TiO2/g-C3N4 had a slight blue-shift from 460 (g-C3N4) to 442 nm, which was associated with the wide bandgap. PL analysis shows that the addition of TiO2 on g-C3N4 improved the absorption efficiency of excited photoelectrons. 3.2 Effect of concentration and pH Figure 3a shows the effect of initial BPA concentrations on the photodegradation of BPA under irradiation conditions. The photodegradation rate constants significantly decreased from 0.17 to 0.04 min−1 with increasing BPA concentrations from 10 to 100 ppm. After irradiation 30 min, approximate 100, 99 and 87.5 % of BPA were photodegraded by TiO2/g-C3N4 at 10, 20 and 50 ppm, respectively. As summarized in Table 1, the photodegradation performance of TiO2/g-C3N4 was significantly higher than that of other photocatalysts such as Bi12O15Cl6 (100 %, 6 h) [34], g-C3N4 in the presence of persulfate (100 %, 1.5 h) [13], MOF-derived g-C3N4 (97 %, 1 h) [35] and Ag2O/ZnFe2O4 (92.5 %, 1 h) [36]. The decrease of photodegradation rate at high BPA concentration could be due to the limited number of free radicals produced under identical concentration of catalyst, although the increased probability of effective collision between BPA molecule and photocatalyst under high BPA amount. Figure 3b shows the effect of pH on BPA photodegradation by TiO2/g-C3N4. BPA photodegradation increased slightly when pH increased from 4.0 to 6.0, whereas the photodegradation performance decreased gradually at pH > 8.0. After irradiation 10
10 / 26
min, 68.5, 79, 67.4 and 60.3 % of BPA were photodegraded by TiO2/g-C3N4 at pH 4.0, 6.0, 8.0 and 10.0, respectively, whereas approximate 100 % of BPA can be photodegraded by TiO2/g-C3N4 at pH 4.0-10.0 after irradiation 30 min. The decrease of BPA photodegradation at pH > 8.0 could be due to the inhibited migration of photo-reduced electron (negative charge) to the surface of catalyst at high pH, leading to increased recombination rate of e- and h+ [37]. These observations revealed TiO2/g-C3N4 with excellent photocatalytic activity and fast photodegradation rate at neutral pH. 3.3 Effect of catalyst For
comparison
of
photodegradation
performance
of
TiO2/g-C3N4,
the
photodegradation of BPA on g-C3N4 and TiO2 were also examined at 20 ppm BPA and pH 6.0. As shown in Figure 3c, the photodegradation rate of BPA on TiO2/g-C3N4 under simulated sunlight irradiation was significantly faster than that of TiO2 and g-C3N4, although 100 % of BPA was photodegraded by catalysts after irradiation 45 min. After irradiation 10 min, 52.3, 33 and 86.3 % of BPA were photodegraded by TiO2, g-C3N4 and TiO2/g-C3N4, respectively. The further evidence was provided in Figure S7-S9. As shown in Figure S9, the removal of total organic carbon (TOC) by TiO2/g-C3N4 +BPA (54.2 %) after 1 h irradiation was remarkably higher than that of g-C3N4 (38.3 %) and TiO2 (20.5 %), indicating that TiO2/g-C3N4 displayed the high efficient photodegradation of BPA compared to single TiO2 and g-C3N4. These results indicated that TiO2/g-C3N4 significantly increased the photodegradation rate of BPA. 3.4 Regeneration of TiO2/g-C3N4.
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Figure 3d shows the cycling performance of BPA photodegradation by TiO2/g-C3N4 under five cycles. After irradiation 30 min, ~ 100 % of BPA was photodegraded by TiO2/g-C3N4 at each photocatalytic experiment. After five cycles, TiO2/g-C3N4 remained stable photocatalytic performance demonstrating that TiO2/g-C3N4 can serve as a promising catalyst for highly efficient treatment of organic wastewater in environmental remediation. 3.5 Comparison of photocatalytic performance The photodegradation of various organic refractory pollutants and organic dyes on TiO2/g-C3N4 was investigated under neutral conditions for evaluating the versatility of TiO2/g-C3N4 (Figure 4a). After irradiation 20 min, ~ 48 % of DP, 62 % of 2, 4-DCP, 87 % of phenol and 88 % of IBU were photodegraded by TiO2/g-C3N4, which was significantly lower than that of BPA (~ 100 %). Figure 4b shows the photodegradation of MO, MB and RhB on TiO2/g-C3N4. The photodegradation of MO, MB and RhB decreased significantly with increasing irradiation time. After irradiation 20 min, ~ 28 % of MO, 52 % of RhB and 89 % of MB were photodegraded by TiO2/g-C3N4. Approximate 100 % of MB and RhB were completely photodegraded, but 68.2 % MO was degraded by TiO2/g-C3N4 under irradiation 60 min. These results indicated that TiO2/g-C3N4 had a versatile photocatalytic degradation capability and had degradation effects on a variety of refractory pollutants and organic dyes. 3.6 Photo-degradation mechanism The photodegradation mechanism of BPA on TiO2/g-C3N4 was demonstrated by EPR and XPS analysis. Figure 5a and 5b show EPR signals of DMPO/·O2− and TEMP/1O2,
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respectively, on TiO2/g-C3N4 in various methanol dispersions and under the simulated sunlight. The blank experiment (DMPO) and BPA system displayed four characteristic peaks of DMPO/·O2− and three of TEMP/1O2, whereas no characteristic peaks were observed under dark conditions, indicating superoxide radicals (·O2 −) and singlet oxygen radicals (1O2) were generated under illumination conditions. The relative intensities of the DMPO/·O2− and TEMP/1O2 peak for BPA system were weaker than that of DMPO, whereas the characteristic peaks of hydroxyl radicals (·OH) were not significantly changed in the two systems (Figure S10a). These results showed that the highly efficient photodegradation of BPA on TiO2/g-C3N4 was ascribed to ·O2− and 1O2 rather than ·OH radicals. Therefore, the interaction mechanism can be described as Eqns. (1-5): TiO2/g-C3N4 + hν → TiO2/g-C3N4 (h+ + e-)
(1)
TiO2/g-C3N4 (e-) + O2 → TiO2/g-C3N4 + ·O2-
(2)
TiO2/g-C3N4 (h+) + ·O2- → TiO2/g-C3N4 + 1O2
(3)
BPA + ·O2- → CO2 + H2O
(4)
BPA + 1O2 →CO2 + H2O
(5)
The electric potential (vs NHE) of ·O2− derived from TiO2/g-C3N4 (− 1.17 eV at CB) was more negative than that of O2/·O2− (− 0.33 eV) [34]. Moreover, ·O2- was partly oxidized to 1O2 by h+ [38], therefore ·O2− and 1O2 could be produced for BPA degradation. The photodegradation mechanism was further demonstrated by quenching experiments under simulated sunlight. In this study, p-BQ, NaN3, TBA and methanol were served as scavengers to remove ·O2 −, 1O2, ·OH and holes. As shown
13 / 26
in Figure 5c, BPA was barely photodegraded by TiO2/g-C3N4 after adding p-BQ agents, indicating that ·O2- radicals played the main role in BPA photodegradation. Compared to blank experiment, the slow photodegradation rate of BPA after adding NaN3 suggested that 1O2 radicals had the slight effect of BPA photodegradation, whereas the slight change in BPA photodegradation after adding TBA and menthol revealed that ·OH and h+ radicals had no effect of BPA photodegradation. Figure 5d, 5e and 5f show the high-resolution XPS spectra of O 1s, C 1s and N 1s, respectively. O 1s XPS spectra can be divided into four peaks at ~ 529.8, 531.0, 532.2 and 533.5 eV, which belong to the lattice oxygen of Ti-O, C=O, N/C-OH and adsorbed H2O, respectively [28, 39]. The peak of C=O (531.4 eV) and the peak of N/C-OH (532.2 eV) for TiO2/g-C3N4 after reaction were slightly shifted to high binding energy, indicating that the various oxygen-containing radicals (e.g., ·OH, ·O2and 1O2 radicals) played a vital role in BPA photodegradation during irradiations. The further evidence was supported by C 1s XPS spectra (Figure 5e), which can be divided at 284.9, 286.3 and 288.3 eV, corresponding to C-C, C-O and N-C=N groups, respectively. The occurrence of C-O could be derived from the CO2 adsorbed by materials and/or the incomplete oxygen-containing intermediate formed during heating [24, 40, 41]. After reaction, the low binding energies of C-O (285.8 eV) and C-C (284.7 eV) were surveyed. As shown in Figure 5f, N 1s XPS spectra can be deconvoluted at 398.9, 399.8 and 401.1 eV, which can belong to pyridinic N, Ti-N-O/Ti-O-N and pyrrolinic N, respectively [42-44]. Figure S10b shows the high resolution Ti 2p XPS spectra of TiO2/g-C3N4. The position and intensity of Ti 2p3/2
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(458.7 eV) and Ti 2p1/2 (464.3 eV) of TiO2/g-C3N4 remained unchanged before and after the reaction [39, 45]. XPS analysis proved that the uniform dispersion of TiO2 on g-C3N4 formed the heterogeneous structure, increasing the excitation rate of photoelectron. 4.
Conclusions
In this study, TiO2/g-C3N4 composites were synthesized by solvothermal method. The characterization results confirmed that the spherical TiO2 nanoparticles were uniformly distributed on g-C3N4 surface with heterogeneous structure, which extended the optical absorption range and accelerated the separation/migration of electrons and holes. The batch photodegradation experiments showed TiO2/g-C3N4 displayed
the
highly
efficient
photodegradation
rate
of
BPA
(complete
photodegradation within 20 min), good stability at pH 4.0-10.0 and high selectivity (superior to other organic pollutants). The ·O2- radicals were responsible for highly efficient photodegradation of BPA on TiO2/g-C3N4 by quenching experiments, EPR and XPS analysis. These findings are critical to the removal of organic contaminants by using C3N4-based composites in environmental remediation. Acknowledgement Financial support from the National Natural Science Foundation of China (21822602) is acknowledged. References [1] M.A. Ashraf, Persistent organic pollutants (POPs): a global issue, a global challenge, Environ. Sci. Pollut. Res. Int., 24 (2017) 4223-4227. [2] E. Diamanti-Kandarakis, J.P. Bourguignon, L.C. Giudice, R. Hauser, G.S. Prins,
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Figure Captions Figure 1. Characterization of TiO2/g-C3N4, (a) and (b): SEM and TEM images, respectively; (c) and (d): XRD and FT-IR patterns, respectively. Figure 2. DRS (a) and PL spectra (b) of TiO2, g-C3N4 and TiO2/g-C3N4. Figure 3. Photocatalytic degradation of BPA by TiO2/g-C3N4 under different conditions, (a): effect of initial BPA concentrations, m/v = 1.0 g/L, pH = 6.0; (b): pH effect, m/v = 1.0 g/L, C0 = 20 ppm; (c): effect of different photocatalysts, m/v = 1.0 g/L, pH = 6.0; (d): regeneration experiments, m/v = 1.0 g/L, C0 = 20 ppm, pH = 6.0. Figure 4. Comparison of photodegradation of BPA by TiO2/g-C3N4 with other organic refractory pollutants (a) and organic dyes (b), m/v = 1.0 g/L, C0 = 20 ppm, pH = 6.0. Figure 5. (a): DMPO spin-trapping EPR spectra for DMPO/·O2- on TiO2/g-C3N4 (simulated sunlight); (b): TEMP spin-trapping EPR spectra for TEMP/1O2 on TiO2/g-C3N4 in various methanol dispersions (simulated sunlight); (c) Radical quenching experiment; (d), (e) and (f): O 1s, C 1s and N 1s XPS spectra, respectively.
10
20
40
50
(213)
(103) 30
(200) (105) (204)
g-C3N4 TiO2/g-C3N4 TiO2
(101)
(100)
Intensity (a.u.)
(c)
60
(d) transmitance (a.u.)
(002)
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70
TiO2/g-C3N4, 1st cycle TiO2/g-C3N4 g-C3N4
3000-3500 g-C3N4 TiO32N4 N-TiO2/g-C
1638
812 1243
TiO2/g-C3N4 N-TiO2 3500 3000 2500 2000 1500 1000
2 theta (degree) Figure 1.
wavenumbers (cm-1)
500
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0.4 g-C3N4 TiO2/g-C3N4 TiO2
0.3
0.2
0.1
0.0 200
400
600
wavelength (nm)
TiO2/g-C3N4 g-C3N4 TiO2
(b) Intensity (a.u.)
absorbance (a.u.)
(a)
800
Figure 2.
400
450
500
wavelenth (nm)
550
600
23 / 26
(b)
(a)
1.0
4 6 8 10
10ppm 20ppm 50ppm 100ppm
BPA C/C0
0.8 0.6 0.4 0.2 0.0 0
15
30
45
60
0
15
30
45
60
Time (min)
Time (min)
(d)
(c)
1.0
g-C3N4 TiO2 TiO2/g-C3N4
BPA C/C0
0.8
1st
2nd
3th
4th
5th
0.6 0.4 0.2 0.0 0
15
30
Time (min)
45
60
0
60
120
180
Time (min) Figure 3.
240
300
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(a)
1.0
MO MB RhB
DP 2， 4-DCP Phenol IBU
BPA C/C0
0.8 0.6
(b)
0.4 0.2 0.0 0
15
30
45
60
Time (min) Figure 4.
0
15
30
Time (min)
45
60
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(a)
·O2-
3460
3480
3500
3520
3540
3560
3460
3480
TBA/OH· P-BQ/·O2-
BPA C/C0
NaN3/1O2
0.4
Methanol/h+ Blank
0.2
Intensity(a.u.)
(d)
0.6
3520
532.2 533.5
3540
3560
529.8
O 1s
531.0
TiO2/g-C3N4
532.4 531.4 after reaction
0.0
533.2 0
15
30
45
538
60
536
Time (min)
(e)
290
Intensity(a.u.)
285.8 284.7
288
286
530
528
526
284
398.7 N 1s
TiO2/g-C3N4
286.3
292
532
(f)
C 1s 284.9
after reaction
534
Binding energy (eV)
288.3
TiO2/g-C3N4
Intensity(a.u.)
3500
Magnetic Filed (G)
(c)
0.8
O2
BPA TEMP/1O2
Magnetic Filed (G) 1.0
1
Intensity (a.u.)
Intensity (a.u.)
BPA DMOP/·O2-
(b)
282
406
Binding energy (eV)
Figure 5.
399.9 401.2
after reaction
404
402
400
398
Binding energy (eV)
396
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Table 1. Photocatalytic degradation of BPA by other catalysts Photocatalysts
Efficiency
Ref.
Bi12O15Cl6
100 %, 6 h
[34]
Persulfate-bearing g-C3N4
100 %, 1.5 h
[13]
MOF-derived g-C3N4
97 %, 1 h
[35]
Ag2O/ZnFe2O4
92.5 %, 1 h
[36]
TiO2/g-C3N4
100 %, 0.5 h
This study
Highlight
Heterogeneous TiO2/g-C3N4 extended absorption-light range and photoelectron production
TiO2/g-C3N4 display the good stability and high selectivity for BPA degradation
BPA degradation was attributed to ·O2- and 1O2 radicals by EPR and XPS analysis
C3N4-based composite was a promising photocatalyst for removing organic pollutants
Conflict of Interest Statement Hereby, the authors state no conflicting financial interest.