Intercalation Effect of Attapulgite in gC - ACS Publications

Sep 26, 2017 - (MBT) under visible light. The improved photocatalytic performance is mainly attributed to the intercalation effect of attapulgite (ATP...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10614-10623

Intercalation Effect of Attapulgite in g‑C3N4 Modified with Fe3O4 Quantum Dots To Enhance Photocatalytic Activity for Removing 2‑Mercaptobenzothiazole under Visible Light Zhi Zhu,† Yang Yu,† Hongjun Dong,† Zhi Liu,‡ Chunxiang Li,† Pengwei Huo,*,† and Yongsheng Yan*,† †

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Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301, Zhenjiang City 212013, P.R. China ‡ School of the Environment and Safety Engineering, Liaoning Normal University, Shahekou District, No. 850, Dalian City 116021, P.R. China S Supporting Information *

ABSTRACT: A novel magnetic intercalation Fe3O4-QDs@gC3N4/ATP photocatalyst was first prepared by a combined eutectic method with deposition technology; it shows superior degradation efficiency for removing 2-mercaptobenzothiazole (MBT) under visible light. The improved photocatalytic performance is mainly attributed to the intercalation effect of attapulgite (ATP) in g-C3N4 together with the quantum effect of Fe3O4 quantum dots (QDs) and the better conductivity between ATP and g-C3N4 resulting in the enhanced separation efficiency of photogenerated electron−hole pairs in the light absorption range. Moreover, insight into this mechanism indicates that the holes and superoxide radicals are the major active species in the MBT removal procedure. This work provides an efficient and promising approach to construct new high-performance g-C3N4-based photocatalytic materials for wastewater treatment. KEYWORDS: Intercalation effect of attapulgite, Fe3O4-QDs, g-C3N4, 2-Mercaptobenzothiazole, Photocatalytic degradation

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INTRODUCTION With the development of the chemical industry, mercaptans, as one class of important chemical products, have been widely applied in many fields,1,2 such as medicine, rubber vulcanization, synthetic fungicides, etc. 2-Mercaptobenzothiazole (MBT), acting as a kind of mercaptan, has been widely used in the manufacturing of tires, rubber shoes, and other rubber products. Moreover, MBT can also serve as a sensitive reagent for testing metal and an intermediate for synthesis of herbicides and cephalosporins. But it is worth noting that MBT has a certain toxicity and is hard to remove, which will cause nausea and headaches if inhaled at low concentrations of MBT; higher concentrations will bring about fatal respiratory paralysis.4 Therefore, removing and reducing MBT to limit damage to the environment and human health have aroused extensive attention.3 Nowadays, many traditional methods have been investigated to remove the MBT pollutants from the aquatic environment, and photocatalytic technology has been applied significantly, in which the photocatalyst can degrade the pollutants into lowrisk or nonpoisonous small molecules or mineralize them into CO2 and H2O under mild reaction conditions.5 Recently, the semiconductor graphite-like carbon nitride (g-C3N4) has become the research hotspot in the field of hydrogen production and photodegradation, due to its high stability in © 2017 American Chemical Society

aqueous solution, easy preparation, and narrow band gap, about ∼2.7 eV. Unfortunately, the high recombination rate of electron−hole pairs and insufficient light utilization efficiency of bulk g-C3N4 limit its practical application.6 Moreover, gC3N4 also exists high temperature calcinations and hydrothermal synthesis, which results in small specific surface area and less active sites. From the current research, layered materials with large interlayer space are beneficial for the formation of intercalation materials with promising properties. At present, Zou et al. have constructed alkali metal salts intercalated g-C3N4, which showed an efficient separation rate of electron−hole pairs and thus improved photocatalytic performance.7 Moreover, Dong et al. have also prepared the K-intercalated g-C3N4 photocatalyst for improving removal efficiency of NO.8 The above results proved that building intercalated g-C3N4 is a smart way to improve the photocatalytic activity of bulk g-C3N4. To our knowledge, building a composite photocatalyst could be an efficient strategy to promote the performance of a single semiconductor and multiplex composites were constructed based on attapulgite (ATP, [Mg 5 Si 8 O 20 (OH) 2 (OH 2 ) 4 · Received: July 30, 2017 Revised: August 23, 2017 Published: September 26, 2017 10614

DOI: 10.1021/acssuschemeng.7b02595 ACS Sustainable Chem. Eng. 2017, 5, 10614−10623

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ACS Sustainable Chemistry & Engineering

dimethyl-L-pyrroline N-oxide (DMPO) were all purchased from Sinopharm Chemical Reagent Co., Ltd. Attapulgite (ATP) was purchased from Jiangsu Da Yu Development Co., Ltd. of China; distilled water is used in all of the experiments. Synthesis. The modified ATP was synthesized by a typical way, 10 g original ATP was added into 20 mL HCl (1.0 M) and stirred for 5 h. Subsequently, the products were collected and dried at 60 °C. Then, different ratios of the modified ATP were diffused in 20 mL urea solution (Vethanol/Vwater = 1:3) ultrasonically for 5 h (5 g urea). Then, heating and stirring was done for some time to get a well-dispersed mixture. After that, 5.0 g of the above mixture was put into an open crucible for heating in muffle furnace from room temperature to 500 °C with the heating rate of 5 °C min−1 and kept for 2 h at 500 °C. After that, the powder of g-C3N4/ATP was obtained. Finally, FeCl3· 6H2O (0.2 mmol) and FeCl2·4H2O (0.1 mmol) was dissolved into 20 mL g-C3N4/ATP suspension with stirring for 30 min at 80 °C. Afterward, 2 mL NH3·H2O was quickly injected into the above reaction mixture and stirred for another 30 min, collecting the product by magnet and washing with ethanol several times to obtain the Fe3O4-QDs@g-C3N4/ATP photocatalyst. The different content of Fe3O4-QDs was only changed by the original addition of FeCl2·4H2O and FeCl3·6H2O. Characterization. The crystal properties of the as-prepared samples are characterized by powder X-ray diffractometer (XRD) with Ni-filtrated Cu Kα radiation (40 kV, 200 mA) by a scan rate (2θ) of 0.05°/s. The morphological measurement is examined by transmission electron microscope (TEM) and high-resolution TEM (HRTEM). The Fourier transform infrared (FT-IR) spectrometer is collected on Nicolet Magna-IR 550 within the wavelength range of 400−4000 cm−1, and KBr was used as the reference. The scanning electron microscopy (SEM) was performed with an F20S-TWIN electron microscope (SEM, Hitachi). The specific surface areas are characterized via N2 adsorption−desorption, the Brunauer−Emmett− Teller (BET) method, and a porosity analyzer (NDVA-2000e). The magnetic analysis is carried out by using a vibrating sample magnetometer (VSM; HH-15, Jiangsu University). Thermogravimetric analysis (TGA) was carried out using a thermal analyzer (NETZSCHGeratebau GmbH, Germany) in air, from room temperature up to 700 °C. The ultraviolet−visible diffuse reflectance spectra (UV−vis DRS) are obtained via a UV−vis spectrophotometer (A Shimadzu UV-3600) using BaSO4 as the reference. The photoluminescence spectra (PL) and transient fluorescence (FL) are obtained on a F4500 photoluminescence detector (Hitachi, Japan). Transient photocurrent and electrochemical impedance spectroscopy (EIS) and Mott−Schottky are investigated by an electrochemical workstation (CHI 852C, Germany). Electron spin resonance (ESR) signals of radicals spin-trapped by spin-trapped reagent 5,5-dimethyl-Lpyrroline N-oxide (DMPO) are carried on an electron paramagnetic resonance spectrometer (A300-10/12, Bruker) at room temperature. And, the resistivity is tested on a resistivity system HALL8800. Photocatalytic and Trapping Experiments. The photocatalytic activities of the as-prepared various photocatalysts are measured by the decomposition of MBT (100 mL, 20 mg L−1) under visible-light (300 W xenon lamp covered with a UV filter λ > 420 nm) irradiation. Before the start of irradiation, 0.05 g photocatalyst is suspended in MBT solution to reach adsorption/desorption equilibration in the dark. During the illumination process, 6 mL of the suspension is withdrawn in 15 min interval. The trapping experiments are the same as photocatalytic experiment, only add additional triethanolamine (TEOA, 1 mM), isopropanol (IPA, 1 mM), and benzoquinone (BQ, 1 mM) before the photocatalytic degradation process. The absorbance of MBT solution is monitored by UV−vis spectrophotometer. The degradation rate (Dr) is calculated using C0 − C/C0; here, C0 and C are the initial and final concentrations of MBT, respectively. Photoelectrochemical Measurements. The photoelectrochemical performance of the photocatalyst is investigated by the photocurrent response and electrochemical impedance spectroscopy (EIS) in a CHI 852C electrochemical station. Briefly, 0.005 g photocatalyst is dispersed in 1.5 mL ethanol and 1.0 mL ethanol glycol, the dispersion mixture is dipcoated onto FTO substrates (1.0

34H2O]), such as Cu/TiO2/attapulgite, attapulgite-BiOClTiO2, attapulgite-CeO2/MoS2, attapulgite/Ag3PO4, etc. These showed enhanced photocatalytic performance over that of pure photocatalyst.9−12 ATP, as a kind of natural silicate clay widely distributed in Jiangsu Province in China, has been extensively used to form functionalized composites due to its rodlike morphology. Moreover, ATP possesses many hydrophilic group and active sites, which are beneficial for adsorption of many pollutants such as heavy metals,13 phenols,14 and dyes15 from wastewater. The previous studies only regard the ATP as a support for degradation of contaminants. However, the intercalation effect of ATP which was embedded in g-C3N4 for degradation of MBT has been rarely reported. On the one hand, if ATP is inserted into g-C3N4, it can not only play a supporting role in preventing g-C3N4 agglomeration but also increase the active sites. On the other hand, ATP in the intercalation structure will effectively promote the separation of electron−hole pairs and enhance the photocatalytic activity. Therefore, construction of an intercalated structure of g-C3N4/ATP may be an effective approach to overcome the disadvantages of g-C3N4 for better removal of MBT. It is well-known that quantum dots (QDs) have been extensively investigated as building blocks of photocatalysts due to their unique physical and chemical properties,16−18 including the special quantum size effect, electron transfer properties, and efficient absorption to visible light.19−21 In fact, magnetic QDs showed both the characteristic of QDs and magnetic separation performance, which is of very important practical value. For example, Han et al. prepared magnetic QD modified ZnO nanorods for the degradation of RhB, which showed good photocatalytic activity.22 Cao et al. has fabricated magnetic QDgraphene nanocomposites for extraction of dye from aqueous solution.23 So it can be infered that, if magnetic QDs are introduced into the intercalation structure of g-C3N4/ATP, the photocatalytic activity and the recycling ability of the photocatalyst will be enhanced. Therefore, inspired by the above principles, a novel composite photocatalyst of Fe3O4QDs modified on g-C3N4/ATP was developed and applied in removing MBT. In this work, it is the first time that ATP was incorporated into the g-C3N4 interlamination through the eutectic method and then Fe3O4-QDs were deposited on the g-C3N4/ATP surface by a water bath deposition strategy to obtain the Fe3O4QDs@g-C3N4/ATP photocatalyst. We have examined the influence of ATP, morphology, structure, and optical and electronic properties of Fe3O4-QDs@g-C3N4/ATP photocatalyst by high-resolution transmission electron microscopy (HRTEM), photoluminescence spectra (PL), transient fluorescence (FL), transient photocurrent and electrochemical impedance spectroscopy (EIS), etc. As expected, the asprepared photocatalyst exhibited superior photocatalytic activity for MBT degradation under the visible light. Finally, the possible photocatalytic reaction mechanism is also systematically investigated.

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EXPERIMENTAL SECTION

Materials. Urea (AR), iron(II) chloride tetrahydrate (FeCl2·4H2O, AR), iron(III) chloride hexahydrate (FeCl3·6H2O, AR), and 2mercaptobenzothiazole (MBT) were all supplied by Aladdin Chemistry Co., Ltd. NH3H2O solution (25.0%), HCl solution (38.0%), p-benzoquinone (BQ, AR), isopropanol (IPA, AR), triethanolamine (TEOA, AR), ethanol (C2H5OH, AR), and 5,510615

DOI: 10.1021/acssuschemeng.7b02595 ACS Sustainable Chem. Eng. 2017, 5, 10614−10623

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ACS Sustainable Chemistry & Engineering cm2) and used as corresponding working electrodes. A Pt electrode is used as the counterelectrode, and an Ag/AgCl electrode in saturated KCl solution is employed as the reference electrode.

27.8°) in g-C3N4/ATP has smaller warp compared with that of pure g-C3N4;26 this shifting of peak position is due to the interaction between g-C3N4 and ATP. Thus, the results illustrated that the ATP may be interspersed into g-C3N4 sheets. For the Fe3O4-QDs@g-C3N4/ATP, the main peaks of Fe3O4 are obvious, and the peaks of ATP are slightly lower than those of g-C3N4/ATP, but it is enough to imply that the Fe3O4QDs@g-C3N4/ATP composites have been successfully synthesized. The morphologies of the prepared ATP, g-C3N4, g-C3N4/ ATP, and Fe3O4-QDs@g-C3N4/ATP are investigated by TEM. As shows in Figure 2a, the ATP presents different length rodlike structures with diameters ranging from 20 to 30 nm. Notably, in Figure 2b, the g-C3N4 is curled and stacked together with some pores on the irregular lamellar surface. We analyzed that this may be the main reason for g-C3N4 having less active sites. It is worth noting that the TEM image of gC3N4/ATP (Figure 2c) displays a relatively flat intercalation structure through the pores on the surface, but the two materials of the rodlike ATP and sheets of g-C3N4 interspersed together can be easily distinguished from each other. More importantly, the curling and stacking phenomenon of g-C3N4 has disappeared, and many pores are still on the g-C3N4 surface. In the following measurement, the N2 adsorption−desorption isotherm further proves that g-C3N4 has mesoporous structure. Additionally, Figure 2d displays the TEM image of Fe3O4QDs@g-C3N4/ATP, in which a large number of small Fe3O4QDs on the g-C3N4/ATP surface as well as the rodlike ATP interspersed in g-C3N4 sheets can also be seen clearly. In the HRTEM image of Fe3O4-QDs (Figure 2e), the crystal plane spacing of 0.205 nm is well assigned to the (311) facet. And the SAED pattern of Figure 2f demonstrates that the Fe3O4-QDs possess better crystallinity, which is in accordance with the results of XRD analysis. Remarkably, the diameter distribution curve of Figure 2g shows that the Fe3O4-QDs are uniformly dispersed on g-C3N4 with an average diameter of 2−4 nm.

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RESULT AND DISCUSSION Structure and Morphology Characteristics. The crystalline structures of the as-prepared ATP, Fe3O4, g-C3N4, gC3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP are investigated by XRD analysis, which is shown in Figure 1. For the ATP, the

Figure 1. XRD patterns of ATP, Fe3O4, g-C3N4, g-C3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP.

peaks at 19.9°, 27.6°, 35.3°, 42.7°, and 61.9° are according to previous report.24 In the XRD patterns of the obtained pure gC3N4, the peaks at 13.1° and 27.4° can be indexed to the (100) and (002) diffraction planes of g-C3N4, which agree well with the PDF no. 50-1250 data. The XRD pattern of Fe3O4 shows the characteristic peaks at 2θ = 30.2°, 35.5°, 57.3°, 62.6°, and 27.4°, respectively, which is consistent with the PDF no. 750033 data.25 Interestingly, the diffraction peaks of ATP also present in the XRD pattern of g-C3N4/ATP with the lower intensity than that of pure ATP, and the peak of g-C3N4 (2θ =

Figure 2. TEM of ATP (a), g-C3N4 (b), g-C3N4/ATP (c), and Fe3O4-QDs@g-C3N4/ATP (d). HRTEM image of Fe3O4-QDs (e). SAED patterns (f) and diameter distribution of Fe3O4-QDs selected from part d (g). 10616

DOI: 10.1021/acssuschemeng.7b02595 ACS Sustainable Chem. Eng. 2017, 5, 10614−10623

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ACS Sustainable Chemistry & Engineering Therefore, the TEM measurement further proves that the intercalation structure of ATP in g-C3N4 modified by Fe3O4QDs was successfully constructed. Obviously, according to the EDS spectroscopy analyses (Figure S1), in addition to the elements Mg, Si, Al, C, and N from g-C3N4/ATP, the elements O and Fe also existed in the Fe3O4-QDs@g-C3N4/ATP, indicating that the magnetic ATP intercalation structure g-C3N4 is successful prepared. Moreover, we also employ TG-DTA measurement, which is shown in Figure S2. From the TG-DTA curves, the ATP is stable from room temperature up to 650 °C because there is a slight weight loss of only 3%. Therefore, ATP cannot be decomposed at our reaction temperature (500 °C). Surface Chemical Composition and Group Analysis. Figure 3 is the FT-IR spectra of the as-prepared samples. The

Figure 4. BET analysis of g-C3N4, g-C3N4/ATP, and Fe3O4-QDs@gC3N4/ATP.

Moreover, according to the IUPAC classification, the prepared photocatalysts display an H3-type hysteresis loop (P/P0 > 0.4) and a type IV N2 adsorption−desorption isotherm, which indicates that these samples both have mesoporous structure. In addition, the specific surface areas of g-C3N4, g-C3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP are 45.1, 60.9, and 73.6 m2/g, respectively. It is worth noting that the specific surface area of g-C3N4/ATP is higher than that of g-C3N4, which implies that the ATP is inserted into the g-C3N4 sheet and further provides an interior space. Apparently, the Fe3O4-QDs@g-C3N4/ATP still possesses a relatively higher specific surface area than gC3N4/ATP, so Fe3O4-QDs@g-C3N4/ATP will possess more reaction sites than pure g-C3N4 and g-C3N4/ATP, thus enhancing its photocatalytic activity. The enhanced specific surface area of Fe3O4-QDs@g-C3N4/ATP must be caused by the uniformly dispersed Fe3O4-QDs on the g-C3N4/ATP surface. Furthermore, the pore volume and pore size are shown in Table. S1. Optical properties. The optical properties of the prepared g-C3N4, g-C3N4/ATP and Fe3O4-QDs@g-C3N4/ATP are investigated by UV−vis DRS spectra. As shown in Figure 5a, the pure g-C3N4 shows the typical absorption edge at about 450 nm and the band gap is about ∼2.78 eV (Figure 5b), which is calculated by the plots of (αhυ)2 versus hυ. As expected, for the g-C3N4/ATP (ATP inserted g-C3N4), the light response has a significant red-shift toward longer wavelength, and the absorption intensities also enhanced. The results should be ascribed to intercalation effect of ATP in g-C3N4/ATP, which can make the visible light refract multiple times in the interior space to enhance the light absorption, further leading to better photocatalytic activity.29 Significantly, Fe3O4-QDs@g-C3N4/ ATP shows a red-shift wavelength in the visible light response region (550 nm). And there is also an absorption peak from 450 to 750 nm. The significantly enhanced absorption intensity may be attributed to the quantum effect of Fe3O4-QDs which will promote the visible light spread and absorption efficiency in the magnetic intercalation system.30 Therefore, the wider light adsorption region of Fe3O4-QDs@g-C3N4/ATP is able to make the most of visible light and produce more effective photogenerated charge carriers, resulting in the higher photocatalytic activity. According to the Mott−Schottky curve of Figure S3 and based on these plots, the flat band potentials are estimated to be −1.21 eV. As the g-C3N4 is an n-type semiconductor,5 therefore, the potential of CB is −1.21 eV.

Figure 3. FT-IR spectra of ATP (a), g-C3N4 (b), g-C3N4/ATP (c), and Fe3O4-QDs@g-C3N4/ATP (d).

ATP (Figure 3a) presents absorption bands at 3417 and 3282 cm−1, which are attributed to the stretch vibrations of structural Si−OH−Si and Si−(OH)−Al, respectively. Moreover, the region of 1700−1600 cm−1 is caused by the bend vibration of adsorbed hygroscopic water or structural −OH. The low frequency region of 950−1100 cm−1 is attributable to the bend vibrations of Si (or Al)−O tetrahedra and M−OH (M = Mg, Al, Fe) octahedra.27 This result indicates that the surface hydroxyl will enhance the adsorption capacity of ATP. As shown in Figure 3b, the absorption peaks in FT-IR spectra of gC3N4 are attributed to the stretching vibrational modes of  NH and −NH amines (3400−3000 cm−1). The typical stretching modes of the CN heterocycles (1251, 1325, 1419, 1575, and 1639 cm−1), and even the typical bending vibration of s-triazine units (808 cm−1) can also be seen.28 This means that the g-C3N4 is successfully synthesized. Compared with Figure 3b, the main characteristic peaks of g-C3N4 in g-C3N4/ ATP (Figure 3c) are almost not changed after insertion of ATP, but the peaks at 1035 and 985 cm−1 become weaker and the peaks around 3400−3000 cm−1 become stronger, which illustrates that the ATP inserted into the g-C3N4 sheet is not covered on its surface, in accordance with XRD results. However, the FT-IR characteristic peaks of g-C3N4 and ATP are all decreased in Fe3O4-QDs@g-C3N4/ATP (Figure 3d), which implies that the Fe3O4-QDs must be deposited on the gC3N4/ATP surface. Specific Surface Areas Analysis. The specific surface areas and pore volumes of the prepared samples are further analyzed by N2 adsorption−desorption isotherm measurement. According to the inset image of Figure 4, it could be seen that all the samples have mesoporous pore size (about 10 nm). 10617

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Figure 5. (a) UV−vis DRS and (b) plots of (ahυ)2 versus (hυ) of pure g-C3N4, g-C3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP.

Figure 6. Transient photocurrent responses (A) and electrochemical impedance spectra (B) of g-C3N4 (a), ATP (b), g-C3N4/ATP (c), and Fe3O4QDs@g-C3N4/ATP (d).

Figure 7. PL spectra (a) and time-resolved fluorescence decay curves (b) of pure g-C3N4, g-C3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP.

C3N4. Therefore, this gives further an evidence to support the idea that ATP can promote separation of the photogenerated carriers. Meanwhile, to gain deeper insights into the charge transport behavior of the as-prepared samples, the electrochemical impedance spectroscopy (EIS) measurement is also carried out, and the results are showed in Figure 6b. Comparing with the diameter of arc radius, the electrode of Fe3O4-QDs@gC3N4/ATP showed a considerably smaller arc radius than those of g-C3N4/ATP, g-C3N4, and ATP. This implies faster interfacial charge transfer in the Fe3O4-QDs@g-C3N4/ATP and also suggests that the incorporation of ATP and Fe3O4QDs can effectively improve the photogenerated carrier transfer and accelerate the charge separation during the MBT degradation process.31 Additionally, the photoluminescence

Moreover, the VB of pure g-C3N4 is 1.57 eV, which is calculated from the results of Figure 5b. Photoelectrochemical Charge Carrier Separation and Decay Lifetime Analysis. To better understand the photoresponse properties and charge carriers transfer ability of the prepared samples, the photocurrent−time measurement and the electrochemical impedance spectroscopy (EIS) have been conducted. As shown in Figure 6a, the four samples exhibit sharp different photocurrent responses in a light on-and-off cycle mode. Notably, the photocurrent intensity for Fe3O4QDs@g-C3N4/ATP is nearly 3 times higher than that of pure g-C3N4 and 2 times higher than that of g-C3N4/ATP. This suggests the more efficient separation of electrons and holes in Fe3O4-QDs@g-C3N4/ATP. Noticeably, ATP also has photocurrent responses, and the intensity is higher than that of g10618

DOI: 10.1021/acssuschemeng.7b02595 ACS Sustainable Chem. Eng. 2017, 5, 10614−10623

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Figure 8. Degradation dynamics curves with different content of ATP over g-C3N4/ATP (a), degradation dynamics curves with different content of Fe3O4 over Fe3O4-QDs@g-C3N4/ATP (b), degradation dynamics curves over g-C3N4, g-C3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP (c), UV−vis spectra of MBT which changes with reaction time of Fe3O4-QDs@g-C3N4/ATP (d).

PL spectra and time-resolved fluorescence decay spectra are all employed to further provide information about the charge transfer, separation, and decay lifetime, which are shown in Figure 7. It is well-known that the peak intensity of PL emission spectroscopy indirectly reflects the recombination degree of photogenerated carriers.32,33 As shown in Figure 7a, the PL peak intensity of g-C3N4/ATP reduces obviously compared to that of g-C3N4. Moreover, the peak intensity of Fe3O4-QDs@gC3N4/ATP shows better quenching phenomenon compared that of g-C3N4/ATP. The comparison of the two different groups confirmed that the Fe3O4-QDs and the ATP inserted in g-C3N4 can effectively promote the separation of electron−hole pairs. In addition, the time-resolved fluorescence decay spectra was also observed. Through fitting the decay spectra in Figure 7b, the decay lifetime of Fe3O4-QDs@g-C3N4/ATP (7.1 ns) is longer than those of g-C3N4/ATP (4.3 ns) and g-C3N4 (3.1 ns). Compared with the pure g-C3N4, the lifetime of Fe3O4QDs@g-C3N4/ATP is significantly prolonged when ATP and Fe3O4-QDs are introduced simultaneously. The longer decay lifetime indicates the faster interfacial electron transfer occurring at the Fe3O4-QDs@g-C3N4/ATP surface.34,35 The result also demonstrates that the inserted ATP and the loaded Fe3O4-QDs raised the separation efficiency and increased the number of charge carriers to take part in the photocatalytic reaction. Besides, the spectra of FL further proves that the Fe3O4-QDs@g-C3N4/ATP composite has lower recombination rate of photogenerated electron−hole pairs, as is shown in Figure S4. Photocatalytic Performance of Different Photocatalysts. The content of ATP affects the performance of g-C3N4/ ATP photocatalysts, so the effect of different initial ATP

content in g-C3N4/ATP on the degradation of MBT has been investigated. Figure 8a is the degradation dynamics curves on degradation of MBT over g-C3N4/ATP with different contents of ATP. When the initial content of ATP is 4.0 wt % (0.01 g), the degradation rate reached 70.1%, and when the content of ATP increased to 0.1 g, the degradation rate is only 19%. This means that the higher content of ATP on the g-C3N4 surface decreased the light transmission opportunity, so as to cause negative effects on the photocatalytic process. With a lower content of ATP, there is not enough ATP inserted into the gC3N4 sheet to create the intercalation structure g-C3N4/ATP, which will reduce the penetration rate and refraction effect of the irradiation light. So less photons migrate to the surface of the photocatalyst and cause the relatively low removal efficiency. Moreover, the influence of different contents of Fe3O4-QDs coupled with g-C3N4/ATP on the degradation of MBT is also studied, and the significant degradation rates are shown in Figure 8b. With 10 wt % of Fe3O4, the Fe3O4-QDs@g-C3N4/ ATP exhibits the highest activity (90.6%). Further increasing the Fe3O4-QD amount decreases the Fe3O4-QDs@g-C3N4/ ATP activity. This result indicates that the Fe3O4-QDs will be incompletely dispersed on the g-C3N4/ATP surface, resulting in the weak transmitting ability of photogenerated electrons. If a large amount of Fe3O4-QDs are completely covered on the surface of g-C3N4/ATP, the g-C3N4 could not be excited by light, thereby leading to worse photocatalytic activity. Figure 8c displays the photocatalytic capability of the prepared g-C3N4, gC3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP. The Fe3O4-QDs@ g-C3N4/ATP has higher photocatalytic activity than the other photocatalysts, which further demonstrates that the intercalation effect of ATP in g-C3N4 together with the quantum effect 10619

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ACS Sustainable Chemistry & Engineering

Figure 9. m/z of degrading MBT over Fe3O4-QDs@g-C3N4/ATP: initial solution (a), degradation in 45 min (b), and degradation in 90 min (c).

of Fe3O4-QDs will effectively increase the utilization of visible light and the separation rate of photogenerated carriers. Figure 8d describes the absorbance variation of MBT solutions over the Fe3O4-QDs@g-C3N4/ATP sample during the photodegradation process. It is notable that the absorbance of MBT decreased obviously along with the increase of irradiation time, which indirectly proved that MBT molecules are destroyed and decomposed completely, and even mineralized into CO2, H2O, or other smaller molecules.36,37 Intermediates and Mineralization Ability Test. Taking into account the intermediates during the degradation of MBT, the HPLC-MS experiments are carried out. As shown in Figure 9a−c, the peak at m/z = 167 of MBT decreased obviously during the degradation process and almost disappeared after 90 min. These results prove that MBT was decomposed into small molecules, CO2, or H2O. In order to investigate the photodegradation process in depth, the possible intermediate products were analyzed according to the change of the measured mass, which is shown in Figure 10. As can be seen in Figure 10, the MBT A (m/z = 167) is fragmented into B (m/z = 171) by addition reaction. In the meantime, C (m/z = 139) is formed by losing the group −SH. As the reaction proceeded, D (m/z = 125) is fragmented by losing −CH3. Subsequently, D is further decomposed to E (m/ z = 110) by losing −NH2. Then, the F and G are generated by removing −SH and an addition reaction. Finally, the small intermediate products may further be degraded into CO2 and H2O. Recyclability and Magnetic Performance. The recyclability of the photocatalyst is very important for realistic applications. As shown in Figure 11a, the photocatalytic activity of Fe3O4-QDs@g-C3N4/ATP toward MBT degradation hardly reduced after recycling for five runs. The degradation rate was maintained at 89%, which suggests that the fabrication of Fe3O4-QDs@g-C3N4/ATP presents superior stability and good

Figure 10. Possible intermediate products of degradation of MBT over Fe3O4-QDs@g-C3N4/ATP.

performance during the photocatalytic degradation. Also, the magnetic separation performance is further studied. Form the magnetic hysteresis loops of Figure 11b, Fe3O4-QDs@g-C3N4/ ATP exhibits a distinctly symmetric hysteresis loop with a satisfactory magnetization saturation value (∼12.8 emu/g), because it can be easily separated by a magnet from the reaction solution, as shown in the inset photograph of Figure 11b. The above results demonstrate that the as-prepared Fe3O4-QDs@gC3N4/ATP has excellent stability and better cycle ability in the photocatalytic reaction. Trapping Experiments and ESR. To further detection the predominant active species during the Fe3O4-QDs@g-C3N4/ ATP degradation process, the trapping agent benzoquinone (BQ) is used to capture the superoxide radical (·O2−), while TEOA is used for holes (h+), and IPA is used for hydroxyl radicals (·OH) in MBT solution.38−40 The results are showed in Figure 12a; the MBT degradation rate decreased obviously 10620

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ACS Sustainable Chemistry & Engineering

Figure 11. Cycle runs (a) and hysteresis loops (b) of MBT solutions over Fe3O4-QDs@g-C3N4/ATP. The Fe3O4-QDs@g-C3N4/ATP was attracted by a magnet (inset in b).

Figure 12. Degradation efficiencies of MBT over Fe3O4-QDs@g-C3N4/ATP with different substrates (a) and ESR spectra of DMPO-·OH and DMPO-·O2− resulting from visible light irradiation (b).

Figure 13. Diagram of Fe3O4-QDs@g-C3N4/ATP (a) and the mechanism for degradation of MBT under visible light irradiation (b).

generated in the photocatalytic reaction system as well as a small amount of ·OH. To sum up, all three of these species are generated in the reaction, and the influence order in the process of MBT degradation is ·O2− > h+ > ·OH. Possible Photocatalytic Reaction Mechanism. Based on above analyses, the macroscopic sketch and the degradation mechanism over Fe3O4-QDs@g-C3N4/ATP are extracted and shown in Figure 13. When the Fe3O4-QDs@g-C3N4/ATP is exposed to visible light, g-C3N4 is excited and generates electron−hole pairs (eq 1)41 and the light will refract and reflect multiple times in the intercalated structure of g-C3N4.42 On the basis of the experimental results, the intercalation effect of g-C3N4/ATP and the quantum effect of Fe3O4-QDs not only enhance the light absorption but also improve the reducing

(∼39%) after adding the capture agent of TEOA compared with no scavenger. This means that the h+ played a large role in the MBT degradation. Moreover, when BQ and IPA are added into the MBT solution, the degradation rate is 16% and 85%, respectively. The result illustrates that ·O2− is the dominant species and ·OH has a weaker effect on the photodegradation of MBT. For affirming the existence of the radical species mentioned above, the ESR technique is conducted on Fe3O4QDs@g-C3N4/ATP, and the ESR pattern is shown in Figure 12b. It can be seen that there are no ESR signals in the dark. Interestingly, the characteristic peaks of the DMPO-·O2− are observed while the characteristic signals of the DMPO-·OH are relatively weak in the measured system under visible light irradiation. This phenomenon also suggests that ·O2− is 10621

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ACS Sustainable Chemistry & Engineering capacity of electrons. The probable photocatalytic mechanism is illustrated by Figure 13b. First, the excited g-C3N4 will generate electrons and holes. Subsequently, the electrons on the g-C3N4 valence band (VB, +1.57 eV) will rapidly transfer to the surface of Fe3O4-QDs (eq 2), and the holes on the g-C3N4 valence band (CB, −1.21 eV) will directly participate in the oxidation reaction, owing to the better conductivity of Fe3O4-QDs and ATP.43,44 The calculated resistivity of ATP by the U−I curves are showed in Figure S5, while the small resistivity is about 11.0 Ω·m. Therefore, synergistic effects of Fe3O4-QDs and ATP enhanced the electronic transport, prolonged electron lifetime, and lowered recombination of charge carriers. At the same time, the holes directly react with MBT through an oxidation reaction, and electrons react with O2 in the reaction system to produce ·O2− (eq 3), (O2/·O2−, −0.33 eV vs NHE).45 In addition, due to the VB of g-C3N4 (+1.57 eV) being more negative than ·OH/OH− (+2.38 eV vs NHE), the small amount of ·OH that may come from the e− reacts with O2 and H+ in the solution and generates ·OH (eq 4).46,47 Finally, the MBT is gradually destroyed by the active species (eq 5) from the intercalated structure of Fe3O4-QDs@g-C3N4/ATP. g‐C3N4 + hv → g‐C3N4(e− + h+)

■

−

O2 + e → ·O2

■

−

*E-mail: [email protected] (P.H.) *E-mail: [email protected] (Y.Y.). ORCID

Yongsheng Yan: 0000-0002-5430-4881 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (nos. U1662125, 21546013, and 21606114), the Natural Science Foundation of Jiangsu Province (no. BK20150536), China Postdoctoral Science Foundation Funded Project (nos. 2015M571683, 1501102B, 2016M590418), and the Postgraduate Innovation Programs Foundation of Jiangsu Province (no. KYCX17_1794).

■

(1) (2)

(4)

·O2− + ·OH + h+ + MBT → small molecules/ions

(5)

CONCLUSION In summary, magnetically intercalated Fe3O4-QDs@g-C3N4/ ATP is successfully prepared by means of the eutectic method and deposition technology. The as-prepared Fe3O4@g-C3N4/ ATP photocatalyst presents the best performance on degrading MBT (20 mg L−1), the removal efficiency is 90.6% within 90 min, which is two times higher than that of pristine g-C3N4 (46%) under the same conditions when the optimum mass ratio of ATP, Fe3O4, and g-C3N4 is 1:2.5:25. The considerable photocatalytic activity and stability are attributed to the intercalation effect of ATP, and the quantum effects of Fe3O4-QDs enhanced the light absorption and promoted transfer and separation of the photogenerated electrons and holes. Moreover, the introduced ATP can also inhibit g-C3N4 stacking and lead to a larger specific surface. It should be noted that this work provides a new avenue for the development of other high performance and cost-effective intercalated photocatalysts for removing organic pollutant molecules according to practical requirements.

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(3)

·O2− + 2H+ → 2·OH

AUTHOR INFORMATION

Corresponding Authors

transfer

g‐C3N4(e−) ⎯⎯⎯⎯⎯⎯→ Fe3O4 (e−)

ATP. Table S1: BET pore volume analysis of g-C3N4, gC3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02595. Figure S1: EDS of g-C3N4/ATP (a) and Fe3O4-QDs@gC3N4/ATP (b). Figure S2: Thermogravimetric analysis of ATP. Figure S3: Mott−Schottky plots of g-C3N4. Figure S4: FL spectra of g-C3N4, g-C3N4/ATP, and Fe3O4-QDs@g-C3N4/ATP. Figure S5: U−I curves of 10622

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