Intercalation Effect of Attapulgite in g-C3N4 Modified with Fe3O4

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Intercalation effect of ATP in g-C3N4 modified with Fe3O4-QDs 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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02595 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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ACS Sustainable Chemistry & Engineering

Intercalation effect of ATP in g-C 3 N 4 modified with Fe 3 O 4 -QDs to enhance photocatalytic activity for removing 2-Mercaptobenzothiazole under visible light

Zhi Zhua, Yang Yua, Hongjun Donga, Zhi Liub, Chunxiang Lia, Pengwei Huoa*, Yongsheng Yana*

a. Institute of the Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301, Zhenjiang City, P.R. China b. School of the Environment and Safety Engineering, Liaoning Normal University, Shahekou District, No. 850, Dalian City, P.R. China

E-mail address: Zhi Zhu: [email protected], Yang Yu: [email protected] Hongjun Dong: [email protected], Zhi Liu: [email protected] Chunxiang Li: [email protected] *

Corresponding authors:

Pengwei Huo: [email protected], Yongsheng Yan: [email protected] 1

ACS Paragon Plus Environment


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Abstract: A novel magnetic intercalation Fe 3O 4 -QDs@g-C 3N 4 /ATP photocatalyst was firstly prepared by combined eutectic method with deposition technology, which shows the superior degradation efficiency for removing 2-Mercaptobenzothiazole (MBT) under the visible light. The improved photocatalytic performance mainly attributed to the intercalation effect of ATP in g-C 3 N 4 together with the quantum effect of Fe 3 O 4 -QDs and the better conductivity between ATP and g-C 3 N 4 resulting in the enhanced separation efficiency of photogenerated electron-hole pairs and the light absorption range. Moreover, the insight into mechanism indicates that the holes and superoxide radicals are major active species in MBT removal procedure. This work provides an efficient and promising approach to construct the new high-performance g-C 3 N 4 based photocatalytic materials for wastewater treatment.

Keywords: Intercalation effect of attapulgite; Fe 3 O 4 -QDs; g-C 3 N 4; 2-Mercaptobenzothiazole; photocatalytic degradation

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Introduction With the development of the chemical industry, mercaptans, as one class of important chemical products, were widely applied in many fields[1, 2], such as medicine, antidote, rubber vulcanization accelerator, raw material of synthetic fungicides etc. 2-Mercaptobenzothiazole (MBT), acted as a kind of mercaptans, was widely used in manufacture of tires, rubber shoes and other rubber products. Moreover, the MBT can also serve as the sensitive reagents for testing metal and the intermediates for synthesis herbicides and cephalosporins. But it is worth noting that the MBT has a certain toxicity and is hard to remove, which will cause nausea and headache if inhalation the low concentration of MBT, and the higher concentrations will bring about the fatal respiratory paralysis[4]. Therefore, removing and reducing the harm of MBT to ecological environment and human health has aroused extensive attention[3]. Nowadays, many traditional methods have been investigated to remove the MBT pollutants from the aquatic environment, photocatalytic technology has received significant application, in which the photocatalysis as a green technology has received significant application because it can degrade the pollutants into low-/non-poisonous small molecules or mineralize them into CO 2 and H 2 O under the mild reaction conditions[5]. Recently, the semiconductor graphite-like carbon nitride (g-C 3 N 4 ) has become the research hotspot in the field of hydrogen production and photodegradation, due to its high stability in 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-C 3 N 4 limit its practical application[6]. Moreover, the g-C 3 N 4 also exist the phenomenon that reunite together through high temperature calcinations and hydrothermal synthesis, which resulted in small specific surface area and less active sites. From the current research situation, the layered materials with large interlayer space are beneficial for the formation of intercalation material with promising properties. At present, Zou et al. have constructed the alkali metal salts intercalated g-C 3 N 4 , which showed efficiently separation rate of electron-hole pairs and thus improved the photocatalytic performance[7]. Moreover, Dong et al. have also prepared the K-intercalated g-C 3 N 4 photocatalyst for improving removal efficient of NO[8]. The above results proved that building of intercalation g-C 3 N 4 is a smart way to improve the photocatalytic activity of bulk g-C 3 N 4 . To our knowledge, building composite photocatalyst could be an efficient strategy to promote the performance of single semiconductor, and multiplex composites were constructed based on attapulgite (ATP, [Mg 5 Si 8 O 20 (OH) 2 (OH 2 ) 4 .34H 2 O]), such as Cu/TiO 2 /attapulgite, attapulgite-BiOCl-TiO 2 , attapulgite-CeO 2 /MoS 2 , attapulgite/Ag 3 PO 4 etc. showed an enhanced photocatalytic performance than 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 3



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functionalized composites due to its rod-like morphology. Moreover, the ATP possesses many hydrophilic group and active sites, which beneficial for adsorption of many pollutants such as heavy metals[13], phenol[14] and dyes[15] from wastewater. The previous studies only regard the ATP as support for degradation of contaminants. However, the intercalation effect of ATP which was embedded in g-C 3 N 4 for degradation of MBT is rarely reported. On the one hand, if insert the ATP into g-C 3 N 4 , it can not only play a supporting role in preventing g-C 3 N 4 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, constructing intercalation structure of g-C 3 N 4 /ATP may be an effective approach to overcome the disadvantages of g-C 3 N 4 for better removing MBT. It is well known that the quantum dots (QDs) materials have been extensively investigated in building blocks of photocatalyst due to their unique physical and chemical properties[16-18], including the special quantum size effect, electron transfer properties, and the efficient absorption to visible light[19-21]. In fact, the magnetic QDs showed both the characteristic of QDs and the magnetic separation performance, which is very important to practical value. For example, Han et al. prepared the magnetic QDs modified ZnO nanorods for the degradation of RhB, which showed a better photocatalytic activity[22]. Cao et al. has fabricated magnetic QDs-graphene nanocomposites for extraction of dye from aqueous solution[23]. So it can be infer that, if introduce the magnetic QDs in the intercalation structure g-C 3 N 4 /ATP, the photocatalytic activity and the recycle ability of the photocatalysts will be enhanced. Therefore, inspired by the above principles, a novel composite photocatalyst of Fe 3 O 4 -QDs modification on g-C 3 N 4 /ATP was developed and applied in removing MBT. In this work, it is the first time that, the ATP was incorporated into the g-C 3 N 4 interlamination through the eutectic method, and then deposited Fe 3 O 4 -QDs on g-C 3 N 4 /ATP surface by water bath deposition strategy to obtain the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP photocatalyst. We have an insight into the influence of ATP, morphology, structure, optical and electronic properties of Fe 3 O 4 -QDs@g-C 3 N 4 /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 as-prepared photocatalysts exhibit superior photocatalytic activity for MBT degradation under the visible light. At last, the possible photocatalytic reaction mechanism is also systematically investigated.

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Experimental Section Materials. Urea (AR), iron (II) chloride tetrahydrate (FeCl 2 ·4H 2 O, AR), iron (III) chloride hexahydrate (FeCl 3 ·6H 2 O, AR) and 2-mercaptobenzothiazole (MBT) are all supported by Aladdin Chemistry Co., Ltd. NH 3 H 2 O solution (25.0 %), HCl solution (38.0 %), p-benzoquinone (BQ, AR), isopropanol (IPA, AR), triethanolamine (TEOA, AR), ethanol (C 2 H 5 OH, AR) and 5, 5-dimethyl-L-pyrroline N-oxide (DMPO) are all purchased from Sinopharm Chemical Reagent Co., Ltd. Attapulgite (ATP) is purchased from Jiangsu Da Yu Development Co., Ltd of China, the distilled water is used in the whole 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, collecting the products and drying at 60 ℃. Then, different ratios of the modified ATP were diffused in 20 ml urea solution (Vethanol /V water =1:3) within ultrasonic 5 h (5 g urea). Then, heating and stirring 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 ℃ with the heating rate of 5 ℃ min-1 and kept for 2 h at 500 ℃. After that, the powder of g-C 3 N 4 /ATP was obtained. Finally, FeCl 3 .6H 2 O (0.2 mmol) and FeCl 2 .4H 2 O (0.1 mmol) was dissolved into 20 ml g-C 3 N 4 /ATP suspension with stirring for 30 min at 80 ℃. Afterwards, 2 ml NH 3 .H 2 O was quickly injected into the above reaction mixture and stirred for another 30 min, collecting the product by magnet and washing with ethanol for several times to obtain the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP photocatalyst. The different content of Fe 3 O 4 -QDs was only changed by the original addition of FeCl 2 .4H 2 O and FeCl 3 .6H 2 O.

Characterization. The crystal properties of the as-prepared samples are characterized by powder X-ray diffractometer (XRD) with Ni-filtrated Cu Ka radiation (40 kV, 200 mA) by a scan rate (2θ) of 0.05° S-1. The morphological measurement is examined by transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM). The Fourier transform infrared (FT-IR) spectrometer is collected on Nicolet Magna-IR 550 within wavelength range of 400-4000 cm−1 and used KBr 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 N 2 adsorption-desorption and Brunauer-Emmett-Teller (BET) method and porosity analyzer (NDVA-2000e). The magnetic 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

. The Ultraviolet visible diffused reflectance spectra (UV-vis DRS) are ℃

obtained via a UV-vis spectrophotometer (A Shimadzu UV-3600) using BaSO 4 as the reference. The photoluminescence spectra (PL) and transient fluorescence (FL) are obtained on a F4500 5



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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-L-pyrroline N-oxide (DMPO) is carried on an electron paramagnetic resonance spectrometer (A300-10/12, Bruker) at room temperature. And the resistivity is test on a resistivity system HALL8800. Photocatalytic and trapping experiments. The photocatalytic activities of the as-prepared various photocatalysts are measured by 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 irradiation start, 0.05 g photocatalyst is suspended in MBT solution to reach adsorption/desorption equilibration in 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 C 0 -C/C 0 , here, C 0 and C are the initial and final concentrations of MBT, respectively. Photo-electrochemical measurements.

The

photo-electrochemical

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 cm2) and used as corresponding working electrodes. A Pt electrode is used as the counter electrode and an Ag/AgCl electrode in saturated KCl solution is employed as the reference electrode.

Result and discussion Structure and morphology characteristics. The crystalline structure of the as-prepared ATP, Fe 3 O 4 , g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP are investigated by XRD analysis, which showed in Figure 1. For the ATP, the peak 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 g-C 3 N 4 , the peak at 13.1◦ and 27.4◦ can be indexed to the (100) and (002) diffraction planes of g-C 3 N 4 , which agree well with the PDF#50-1250 data. The XRD pattern of Fe 3 O 4 shows the characteristic peaks at 2θ= 30.2◦, 35.5◦, 57.3◦, 62.6◦ and 27.4◦, respectively, which is consistent with the PDF #75-0033 data[25]. Interestingly, the diffraction peaks of ATP also present in the XRD pattern of g-C 3 N 4 /ATP with the lower intensity than that of pure ATP, and the peak of g-C 3 N 4 (2θ= 27.8◦) in g-C 3 N 4 /ATP has smaller warp compared with pure g-C 3 N 4 [26], this shifting of peak position due to interaction between g-C 3 N 4 and ATP. Thus, the results illustrated that the ATP may be interspersed into 6


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g-C 3 N 4 sheets. For the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP, the main peaks of Fe 3 O 4 are obvious, and the peaks of ATP are slightly lower than that of g-C 3 N 4 /ATP, but it is enough to imply the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP composites have been successfully synthesized.

Fe3O4-QDs@g-C3N4/ATP g-C3N4/ATP

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


g-C3N4 Fe3O4 ATP g-C3N4 PDF #50-1250 Fe3O4 PDF #75-0033

10

20

30

40 50 2θ (deg)

60

70

80

Figure 1. XRD patterns of ATP, Fe 3 O 4 , g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP.

The morphology of the prepared ATP, g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP are investigated by TEM. As shows in Figure 2a, the ATP presents different length rod-like structure with the diameter ranging from 20-30 nm. Notably, in Figure 2b, the g-C 3 N 4 is curled and stacked together with some porous on the irregular lamellar surface. We analyzed that this may be a main reason of g-C 3 N 4 having the less active sites. It is worth noting that, the TEM image of g-C 3 N 4 /ATP (Figure 2c) displays a relatively flat intercalation structure through the porous on the surface, but the two materials of the rod-like ATP and sheets g-C 3 N 4 interspersed together can be easily distinguished to each other. More importantly, the curling and stacking phenomenon of g-C 3 N 4 has disappeared, and many porous are still on g-C 3 N 4 surface. In the following measurement, the N 2 adsorption-desorption isotherm further proves that g-C 3 N 4 has the mesoporous

structure.

Additionally,

Figure

2d

displays

the

TEM

image

of

Fe 3 O 4 -QDs@g-C 3 N 4 /ATP, in which a large number of small Fe 3 O 4 -QDs on g-C 3 N 4 /ATP surface as well as the rod-like ATP interspersed in g-C 3 N 4 sheets can also be seen clearly. In the HRTEM image of Fe 3 O 4 -QDs (Figure 2e), the crystal plane spacing of 0.205 nm is well assigned to (311) facet. And the SAED pattern of Figure 2f demonstrates that the Fe 3 O 4 -QDs possess a better crystallinity, which is in accordance with the result of XRD analysis. Remarkably, the diameter distribution curve of Figure 2g shows that the Fe 3 O 4 -QDs are uniformly dispersed on g-C 3 N 4 with an average diameter of 2-4 nm. Therefore, the TEM measurement further proving the intercalation structure of ATP in g-C 3 N 4 modified by Fe 3 O 4 -QDs 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-C 3 N 4 /ATP, the elements O and Fe also existed in the 7



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Fe 3 O 4 -QDs@g-C 3 N 4 /ATP, indicating that the magnetic ATP intercalation structure g-C 3 N 4 is successful prepared. Moreover, we also employ the TG-DTA measurement, which showed in Figure S2. From the TG-DTA curves, the ATP is stable from the room temperature up to 650 0C. Because of there is a slight weight loss that only 3 %. Therefore, the ATP cannot be decomposed under our reaction temperature (500 ℃).

Figure 2. The TEM of ATP (a), g-C 3 N 4 (b), g-C 3 N 4 /ATP (c), and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d), HRTEM image of Fe 3 O 4 -QDs (e), SAED patterns (f), and diameter distribution of Fe 3 O 4 -QDs select from d (g).

Surface chemical composition and group analysis. Figure 3 is the FT-IR spectra of the as-prepared samples. The ATP (Figure 3a) presents the absorption bands at 3417 cm−1 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. At the low frequency region of 950-1100 cm−1 are attribute to the bend vibrations of Si (or Al)-O tetrahedra and M-OH (M= Mg, Al, Fe) octahedral[27]. This result indicates that the surface hydroxyl will enhance the adsorption capacity of ATP. As shown in Figigure 3b, the absorption peaks in FT-IR spectra of g-C 3 N 4 attribute to the stretching vibrational modes of =NH and -NH amines (3400 cm−1 to 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]. It means that the g-C 3 N 4 is successfully synthesized. Compared with Fig. 3b, the main characteristic peaks of g-C 3 N 4 in g-C 3 N 4 /ATP (Figure 3c) are almost no change after inserting ATP, but the peaks at 1035 cm−1 and 985 cm−1 become weaker, and the peaks around 3400 cm−1 to 3000 cm−1 become stronger, which illustrates that the ATP inserted in g-C 3 N 4 sheet is not covered on its surface, in accordance with XRD results. However, the FT-IR characteristic peaks of g-C 3 N 4 and ATP are all decreased in Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (Figure 3d), which implies that the Fe 3 O 4 -QDs must deposit on 8


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g-C 3 N 4 /ATP surface.

a

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1635

3282

3417 b

985 1035

c 1035

985

d 4000

3500

3000 2500 2000 -1 1500 Wavelength (cm )

808 1000 500

Figure 3. FT-IR spectra of ATP (a), g-C 3 N 4 (b), g-C 3 N 4 /ATP (c), and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d).

Specific surface areas analysis. The specific surface areas and pore volumes of the prepared samples are further analyzed by N 2 adsorption-desorption isotherms measurement. According to the insert image of Figure 4, it could be seen that all the samples have mesoporous pore size (about 10 nm). Moreover, according to the IUPAC classification, the prepared photocatalysts display an H3-type hysteresis loop (P/P 0 >0.4) and a type IV of N 2 adsorption-desorption isotherms, which indicates that these samples both have mesoporous structure. In addition, the specific surface areas of g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP are 45.1, 60.9 and 73.6 m2 g-1, respectively. It is worth noting that the specific surface area of g-C 3 N 4 /ATP is higher than g-C 3 N 4 , which implies that the ATP is inserted into g-C 3 N 4 sheet and further provides an interior space. Apparently, the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP still possesses a relatively higher specific surface area than g-C 3 N 4 /ATP, so Fe 3 O 4 -QDs@g-C 3 N 4 /ATP will possess more reaction sites than that of pure g-C 3 N 4 and g-C 3 N 4 /ATP, thus enhancing the photocatalytic activity. The enhanced specific surface area of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP must be caused by the uniformly dispersed Fe 3 O 4 -QDs on g-C 3 N 4 /ATP surface. Furthermore, the pore volume and pore size are showed in Table. S1.

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120

90

8

dV/dD(1*10-2cm3/g.nm)

Adsorbed volume (cm3/g STP)

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60

g-C3N4 g-C3N4/ATP Fe3O4-QDs/g-C3N4/ATP

6

4 2

0 0

50

100

Pore size (nm)

150

200

30

0

0.2

0.4

0.6 0.8 Relative pressure (p/p0)

1.0

Figure 4. The BET analysis of g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP.

Optical properties. The optical properties of the prepared g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP are investigated by UV-vis DRS spectra. As shows of Figure 5a, the pure g-C 3 N 4 shows the typical absorption edge at about 450 nm and 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-C 3 N 4 /ATP (ATP inserted g-C 3 N 4 ), the light response has a significant red-shift towards longer wavelength, the absorption intensities also enhanced. The results should be ascribed to intercalation effect of ATP in g-C 3 N 4 /ATP, which can make the visible light multiple refract in the interior space to enhance the light absorption, further leading to a better photocatalytic activity[29]. Significantly, Fe 3 O 4 -QDs@g-C 3 N 4 /ATP shows a red-shift wavelength in visible light response region (550 nm). And there is also an absorption peak standout at ranged from 450 nm to 750 nm. The significantly enhanced absorption intensity may be attributed to the quantum effect of Fe 3 O 4 -QDs which will promote the visible light spread and absorption efficiency in the magnetic

intercalation

system[30].

Therefore,

the

wider

light

adsorption

region

of

Fe 3 O 4 -QDs@g-C 3 N 4 /ATP is able to make the most of visible light and produce more effectively 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-C 3 N 4 is n-type semiconductor[5], therefore, the potential of CB is -1.21 eV. Moreover, the VB of pure g-C 3 N 4 is 1.57 eV, which is calculated from the result of in the Figure 5b.

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g-C3N4 g-C3N4/ATP Fe3O4-QDs@g-C3N4/ATP

g-C3N4 a g-C3N4/ATP Fe3O4-QDs@g-C3N4/ATP

b

(Ahυ)2(ev)2

Absorbance (a. u.)

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2.78 200

300

400 500 600 700 800 2 3 hυ (ev) 4 Wavelength (nm) 2 Figure 5. (a) The UV-vis DRS, (b) the plots of (ahυ) versus (hυ) of pure g-C 3 N 4 , g-C 3 N 4 /ATP and

5

Fe 3 O 4 -QDs@g-C 3 N 4 /ATP.

Photo-electrochemical charge carrier separation and decay lifetime analysis. To better understand the photo-response 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 Fe 3 O 4 -QDs@g-C 3 N 4 /ATP is nearly 3 times higher than that of pure g-C 3 N 4 and 2 times than g-C 3 N 4 /ATP. It suggests the more efficient separation of electrons and holes in Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. Noticeably, the ATP also has the photocurrent responses, and the intensity is higher than that of g-C 3 N 4 . Therefore, it gives further an evidence to support the ATP can promote separation of the generated 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 Fe 3 O 4 -QDs@g-C 3 N 4 /ATP showed a considerably smaller arc radius than those of g-C 3 N 4 /ATP, g-C 3 N 4 and ATP. It implies the faster interfacial charge transfer in the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP and also suggests that the incorporation of ATP and Fe 3 O 4 -QDs can effectively improve the photogenerated carriers transfer and accelerate the charge separation during the MBT degradation process[31]. Additionally, the photoluminescence 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 showed in Figure 7.

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1.0

120

a b c d

B

A off

0.8

on

0.6

90

-Z''(KΩ)

Photocurrent (µA)

d c

0.4 b

60

30

0.2 a 0.0 0

25

50

75 100 Time (sec)

125

0 0

150

20

40

60 Z'(KΩ)

80

100

120

Figure 6. Transient photocurrent responses (A), electrochemical impedance spectra (B) of g-C 3 N 4 (a), ATP (b), g-C 3 N 4 /ATP (c) and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d).

As 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-C 3 N 4 /ATP reduces obviously than that of g-C 3 N 4 . Moreover, peak intensity of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP is further takes place more seriously quenching phenomenon compared than that of g-C 3 N 4 /ATP. The two different groups of comparison confirmed that the Fe 3 O 4 -QDs and the ATP inserted in g-C 3 N 4 can effectively promote the separation of electron-hole pairs. In addition, the time-resolved fluorescence decay spectra also characterized. Through fitting the decay spectra in Figure 7b, the decay lifetime of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (7.1 ns) is longer than that of g-C 3 N 4 /ATP (4.3 ns) and g-C 3 N 4 (3.1 ns). Comparing with the pure g-C 3 N 4 , the lifetime of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP is significantly prolonged when ATP and Fe 3 O 4 -QDs are introduced simultaneously. The longer decay lifetime indicates the faster interfacial electron transfer occurring at Fe 3 O 4 -QDs@g-C 3 N 4 /ATP surface[34, 35]. The result also demonstrates that the inserted ATP and the loaded Fe 3 O 4 -QDs raised the separation efficiency and increased more charge carriers to take part in the photocatalytic reaction. Besides, the spectra of FL further proves the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP composite has lower recombination rate of photogenerated electron-hole pairs, as is showed in Figure S4. g-C3N4 g-C3N4/ATP Fe3O4-QDs@g-C3N4/ATP

400

450

500 Wavelength (nm)

550

g-C3N4 g-C3N4/ATP Fe3O4-QDs@ g-C3N4/ATP

b

PL intensity (a. u.)

a

Intensity(a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 29

600

120

A2

τ2

1913 1.5

495

9.1 3.1

1800 1.8

455

14

4.3

326

152

18

7.1

A1

140

τ1

2.0

160 180 Time (ns)

200

τ

220

Figure 7. The PL spectra (a) and Time-resolved fluorescence decay curves (b) of pure g-C 3 N 4 , g-C 3 N 4 /ATP and 12


Page 13 of 29

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Photocatalytic performance of different photocatalysts. The content of ATP is important to affect the performance of g-C 3 N 4 /ATP photocatalysts, so the effect of different initial ATP content ing-C 3 N 4 /ATP on degradation of MBT has been investigated. Figure 8a is the degradation dynamics curves on degradation MBT over g-C 3 N 4 /ATP with different content of ATP. When the initial contentof ATP is 4.0 wt % (0.01 g), the degradation rate reached to 70.1% and when the content of ATP increased to 0.1 g, the degradation rate is only 19%. It means that the higher content of ATP covered on g-C 3 N 4 surface decreased the light transmission opportunity, so as to cause negative effects on the photocatalytic process. As lower content of ATP, they are not enough ATP inserting into g-C 3 N 4 sheet tocreatthe intercalation structureg-C 3 N 4 /ATP, which will reduce the penetration rate and refraction effect of the irradiation light. So make less photon migrate to the surface of the photocatalyst and cause the relative low removal efficiency. Moreover, different contents of Fe 3 O 4 -QDs coupled with g-C 3 N 4 /ATP influencing on degradation of MBT is also studied and the significant degradation rates are showed in Figure 8b. With 10 wt % of Fe 3 O 4 , the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP exhibits the highest activity (90.6%). With further increasing Fe 3 O 4 -QDs amount, the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP activity is decreased. The result indicates that the less Fe 3 O 4 -QDs will be incompletely dispersed on g-C 3 N 4 /ATP surface and results in weak transmitting ability of photogenerated electrons. If large amount of Fe 3 O 4 -QDs are completely covered on the surface of g-C 3 N 4 /ATP, the g-C 3 N 4 could not be excited by light, thereby leading to worse photocatalytic activity. Figure 8c displays the photocatalytic degrading MBT capability of the prepared g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. The Fe 3 O 4 -QDs@g-C 3 N 4 /ATP has higher photocatalytic activity than the others photocatalysts, which further demonstrates that the intercalation effect of ATP in g-C 3 N 4 together with the quantum effect of Fe 3 O 4 -QDs will effectively increase the utilization of visible light and the separate rate of photogenerated carriers. Figure 8d describes the absorbance variation of MBT solutions over the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP sample during photodegradation process. It is notable that the absorbance of MBT decreased obviously along with the irradiation time increasing, which indirectly proved the MBT molecules are destroyed and decomposed completely, and even mineralized into CO 2 , H 2 O or other smaller molecules[36, 37].

13



1.0

1.0

b

a

0.6

0.6 0.100 g ATP 0.050 g ATP 0.008 g ATP 0.005 g ATP 0.010 g ATP

0.4 0.2 0.0

C/C0

0.8

C/C0

0.8

0

15

30 wt % Fe3O4 20 wt % Fe3O4 1.0 wt % Fe3O4 5.0 wt % Fe3O4 10 wt % Fe3O4

0.4 0.2

30

45 60 Time (min)

75

90

0.0

0

15

30 45 Time (min)

60

75

90

1.0

c

d

0.8

0.4

g-C3N4

0.2

g-C3N4/ATP

0 min 15 min 30 min 45 min 60 min 75 min 90 min

Absorbance (a. u.)

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

Fe3O4-QDs@g-C3N4/ATP 0.0

0

15

30

45 60 Time (min)

75

90

200

250

300 350 Wavenumber (nm)

400

450

Figure 8. The degradation dynamics curves with different content ATP over g-C 3 N 4 /ATP (a), the degradation dynamics curves with different content Fe 3 O 4 over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (b), the degradation dynamics curves over g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (c), The UV-Vis spectral of MBT changes with reaction time of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d).

Intermediates and mineralization ability test. Taking into account the intermediates during degradation of MBT, the HPLC-MS experiments are carried out. As showed in Figure 9a-c, the peak at m/z=167 of MBT decreased obviously during the degradation process, and almost disappeared after 90 min. The results directly prove the MBT was decomposed into small molecules or CO 2 , H 2 O. 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 showed in Figure 10.

14


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Figure 9. m/z of degrading MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP: initial solution (a), degradation in 45 min (b), degradation in 90 min (c).

As can be seen in Figure 10, the MBT A (m/z= 167) is fragmented into B (m/z= 171) by addition reaction. At the meantime, the C (m/z= 139) is formed by losing the group of -SH. As the reaction proceeded, D (m/z= 125) is fragmented by losing -CH 3 . Subsequently, D is further decomposed to E (m/z= 110) by losing -NH 2 . Then, the F and G are generated by removing -SH and addition reaction. Finally, the small intermediate products may further be degraded into CO 2 and H 2 O.

Figure 10. The possible intermediate products of degradation of MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. 15



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Page 16 of 29

Recyclability and magnetic performance. The recyclability of the photocatalyst is very important for realistic applications. As shown in Figure 11a, the photocatalytic activity of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP towards MBT degradation showed hardly reducing after repeated for five runs. The degradation rate still kept 89%, which suggests that the fabrication of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP presents superior stability and good performance during the photocatalytic degradation. Besides, the magnetic separation performance is further studied. Form the magnetic hysteresis loops of Figure 11b, Fe 3 O 4 -QDs@g-C 3 N 4 /ATP exhibits a distinctly symmetric hysteresis loop with a satisfactory magnetization saturation value (~12.8 emu g−1), because it can be easily separated by a magnet from the reaction solution, as shown in the insert photograph

of

Figure

11b.

The

above

results

demonstrate

that

the

as-prepared

Fe 3 O 4 -QDs@g-C 3 N 4 /ATP has excellent stability and better cycle ability in photocatalytic reaction.

Figure 11. The cycle runs (a) and Hysteresis loops (b) of MBT solutions over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. The Fe 3 O 4 -QDs@g-C 3 N 4 /ATP was attracted by magnet (inset in b).

Trapping experiments and ESR. To further detection the predominant active species during Fe 3 O 4 -QDs@g-C 3 N 4 /ATP degradation process, the trapping agent benzoquinone (BQ) is used to capture the superoxide radical (.O 2 -), while TEOA is for hole (h+) and IPA is for hydroxyl radical (·OH) in MBT solution[38-40]. The results are showed in Figure 12a, the MBT degradation rate decreased obviously (~39 %) after adding the capture agent of TEOA compared with no scavenger, it means that the h+ played an intensively effect on 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 the .O 2 − is the dominant species and the .OH has weaker effect on photodegradation of MBT. For affirming the existence of the radical species mentioned above, the ESR technique is conducted on Fe 3 O 4 -QDs@g-C 3 N 4 /ATP and the ESR pattern is showed in Figure 12b. It can be seen that there are no ESR signals in the dark. Interestingly, the characteristic peaks of the DMPO-.O 2 − are observed while the characteristic signals of the DMPO-.OH are relatively weak in the measure system under visible light irradiation. This phenomenon also suggested that .O 2 - is generated in the photo-catalytic reaction system as well as small amount .OH. To sum up, all the three kinds of species are generated in the reaction and the influence order in 16


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the process of MBT degradation is .O 2 − > h+ > .OH.

Figure 12. The degradation efficiencies of MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP with different substrates (a), ESR spectra of DMPO-.OH and DMPO-.O 2 − resulting from with visible light irradiation (b).

Possible photocatalytic reaction mechanism. Based on above analyses, the macroscopic diagrammatic sketch and the degradation mechanism over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP are extracted and showed in Figure 13. When the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP is exposed to visible-light, the g-C 3 N 4 is excited and generate electrons-holes pairs (Eq. (1)) [41] and the light will occur multiple refraction and reflection in the intercalation structure through the mesoporous of g-C 3 N 4 [42]. On the basis of the experimental results, the intercalation effect of g-C 3 N 4 /ATP and the quantum effect of Fe 3 O 4 -QDs cannot only enhance the light absorption, but also improve the reducing capacity of electrons. The probable photocatalytic mechanism is illustrated by Figure 13b. Firstly, the excited g-C 3 N 4 will generate electrons and holes. Subsequently, the electrons on the g-C 3 N 4 valence band (VB, +1.57 eV) will rapidly transfer to the surface of Fe 3 O 4 -QDs (Eq. (2)), and the holes on the g-C 3 N 4 valence band (CB, -1.21 eV ) will direct participate in oxidation reaction, which owing to the better conductivity of Fe 3 O 4 -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 Fe 3 O 4 -QDs and ATP enhanced the electronic transport, prolonged electron lifetime, lowered recombination of charge carrier. At the same time, the holes directly react with MBT through oxidation reaction, electrons react with O 2 in reaction system to produce .O 2 − (Eq. (3)), (O 2 /.O 2 −, -0.33 eV vs NHE)[45]. In addition, due to the VB of g-C 3 N 4 (+1.57 eV) possessing more negative than .OH/OH− (+2.38 eV vs NHE), the few contents of .OH may come from the e− react with O 2 and H+ in the solution and generate .OH (Eq. (4)) [46,47]. Finally, the MBT is gradually destroyed by the active species (Eq. (5)) from the intercalation structure Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. g-C 3 N 4 + hv→ g-C 3 N 4 (e−+h+)

(1)

transfer

g-C 3 N 4 (e-)�⎯⎯⎯⎯�Fe 3 O 4 (e−)

(2)

O 2 + e- →.O 2 -

(3)

O 2 −+ 2H+ → 2.OH

(4)

.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

O 2 − + .OH + h+ + MBT→ small molecules/ions

.

Page 18 of 29

(5)

Figure 13. The diagrammatic sketch of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (a) and the mechanism schematic illustration for degradation of MBT under visible light irradiation (b).

Conclusion In summary, the magnetic intercalation Fe 3 O 4 -QDs@g-C 3 N 4 /ATP is successfully prepared by

the

means

of

eutectic

method

and

deposition

technology.

The

as-prepared

Fe 3 O 4 @g-C 3 N 4 /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-C 3 N 4 (46%) under the same conditions when the optimum mass ratios of ATP, Fe 3 O 4 and g-C 3 N 4 is 1:2.5:25. The considerable photocatalytic activity and stability are attributed to intercalation effect of ATP and the quantum effects of Fe 3 O 4 -QDs enhancing the light absorption and promoting transfer and separation of the photogenerated electron-holes. Moreover, the introduced ATP can also inhibit g-C 3 N 4 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 intercalation structure photocatalysts for removing organic pollutant molecules according to practical requirements.

Acknowledgements 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 (Nos BK20150536), China Postdoctoral Science Foundation Funded Project (Nos. 2015M571683, 1501102B, 2016M590418), and the Postgraduate Innovation Programs Foundation of Jiangsu Province (No. KYCX17_1794).

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DOI.org/10.1016/j.apcatb.2016.01.054 (38) Huang, H.; Tu, S.; Zeng, C.; Zhang, T.; Reshak, A. H.; Zhang, Y. Macroscopic Polarization Enhancement Promoting Photo- and Piezoelectric-Induced Charge Separation and Molecular Oxygen Activation. Angew. Chem. 2017, DOI.10.1002/anie.201706549 (39) Huang, H.; Li, X.; Wang, J.; Dong, F.; Chu, P. K.; Zhang, T.; Zhang, Y. Anionic group self-doping as a promising strategy: band-gap engineering and multi-functional applications of high-performance CO 3 2–-doped Bi 2 O 2 CO 3 . ACS. Catal. 2015, 5, 4094-4103. DOI.10.1021/acscatal.5b00444 (40) Martha, S.; Nashim, A.; Parida, K. M. Facile synthesis of highly active g-C 3 N 4 for efficient hydrogen production under visible light. J. Mater. Chem. A. 2013, 1, 7816-7824. DOI.10.1039/C3TA10851A (41) Yang, L.; Huang, J.; Shi, L.; Jie, Y. A surface modification resultant thermally oxidized porous g-C 3 N 4 with enhanced

photocatalytic

hydrogen

production.

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DOI.org/10.1016/j.apcatb.2016.11.047 (42) Zhu, Z.; Tang, X.; Kang, S.; Song, M.; Yan, Y. Constructing of the magnetic photocatalytic nanoreactor MS@FCN for cascade catalytic degrading of tetracycline. J. Phys. Chem. C. 2016, 120, 27250-27258. DOI.10.1021/acs.jpcc.6b05642 (43) Pan, Z.; Ma, W.; Wang, L. Construction of a magnetic Z-scheme photocatalyst with enhanced oxidation/reduction abilities and recyclability for the degradation of tetracycline. RSC. Adv. 2016, 6, 114374-114382. DOI.10.1039/C6RA24096H (44) Akhundi, A.; Habibi-Yangjeh, A. Codeposition of AgI and Ag 2 CrO 4 on g-C 3 N 4 /Fe 3 O 4 nanocomposite: Novel magnetically separable visible-light-driven photocatalysts with enhanced activity. Adv. Powder Technol. 2016, 27, 2496-2506. DOI.org/10.1016/j.apt.2016.09.025 (45) Bai, X.; Wang, L.; Zong, R.; Zhu, Y. Photocatalytic activity enhanced via g-C 3 N 4 nanoplates to nanorods. J. Phys. Chem. C. 2013, 117, 9952-9961. DOI.10.1021/jp402062d (46) Ge, L., Han, C., Liu, J., Li, Y. Enhanced visible light photocatalytic activity of novel polymeric g-C 3 N 4 loaded with Ag nanoparticles. Appl. Catal. A-Gen. 2011, 409, 215-222. DOI.org/10.1016/j.apcata.2011.10.006 (47) Luo, B.; Xu, D.; Li, D.; Wu, G.; Wu, M.; Shi, W.; Chen, M. Fabrication of an Ag/Bi 3 TaO 7 plasmonic photocatalyst with enhanced photocatalytic activity for degradation of tetracycline. ACS. Appl. Mater. Inter. 2015, 7, 17061-17069. DOI.10.1021/acsami.5b03535 Supporting Information Figure S1 is the EDS of g-C 3 N 4 /ATP (a), and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (b), the different element can be detected. Figure S2 is the thermogravimetric analysis of ATP, it shows that the ATP has a better stability. Figure S3 is Mott-Schottky plots of g-C 3 N 4 , and the flat band potentials are estimated. Figure

S4

is

the

FL

spectra

of

g-C 3 N 4 ,

g-C 3 N 4 /ATP

and

Fe 3 O 4 -QDs@g-C 3 N 4 /ATP,

the

Fe 3 O 4 -QDs@g-C 3 N 4 /ATP shows the lowest response intensity. Figure S5 is three times of the U-I curves of ATP for calculation its resistivity under magnetic field (6800 G), it confirmed that the ATP has better conductivity. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table S1 is the BET, Pore Volume, Pore Volume analysis of g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP, the Fe 3 O 4 -QDs@g-C 3 N 4 /ATP possess the higher BET surface.

For Table of Contents Use Only Figure 1. XRD patterns of ATP, Fe 3 O 4 , g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. Figure 2. The TEM of ATP (a), g-C 3 N 4 (b), g-C 3 N 4 /ATP (c), and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d), HRTEM image of Fe 3 O 4 -QDs (e), SAED patterns (f), and diameter distribution of Fe 3 O 4 -QDs select from d (g). Figure 3. FT-IR spectra of ATP (a), g-C 3 N 4 (b), g-C 3 N 4 /ATP (c), and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d). Figure 4. The BET analysis of g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. Figure 5. (a) The UV-vis DRS, (b) the plots of (ahυ)2 versus (hυ) of pure g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. Figure 6. Transient photocurrent responses (A), electrochemical impedance spectra (B) of g-C 3 N 4 (a), ATP (b), g-C 3 N 4 /ATP (c) and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d) Figure 7. The PL spectra (a) and Time-resolved fluorescence decay curves (b) of pure g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. Figure 8. The degradation dynamics curves with different content ATP over g-C 3 N 4 /ATP (a), the degradation dynamics curves with different content Fe 3 O 4 over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (b), the degradation dynamics curves over g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (c), The UV-Vis spectral of MBT changes with reaction time of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d). Figure 9. m/z of degrading MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP: initial solution (a), degradation in 45 min (b), degradation in 90 min (c).

Figure 10. The possible intermediate products of degradation of MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. Figure 11. The cycle runs (a) and Hysteresis loops (b) of MBT solutions over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. The Fe 3 O 4 -QDs@g-C 3 N 4 /ATP was attracted by magnet (inset in b). Figure 12. The degradation efficiencies of MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP with different substrates (a), ESR spectra of DMPO-.OH and DMPO-.O 2 − resulting from with visible light irradiation (b). Figure 13. The diagrammatic sketch of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (a) and the mechanism schematic illustration for degradation of MBT under visible light irradiation (b).

22


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Fe3O4-QDs@g-C3N4/ATP g-C3N4/ATP Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


g-C3N4 Fe3O4 ATP g-C3N4 PDF #50-1250 Fe3O4 PDF #75-0033

10

20

30

40 50 2θ (deg)

60

70

80

Figure 1. XRD patterns of ATP, Fe 3 O 4 , g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP.

Figure 2. The TEM of ATP (a), g-C 3 N 4 (b), g-C 3 N 4 /ATP (c), and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d), HRTEM image 23



of Fe 3 O 4 -QDs (e), SAED patterns (f), and diameter distribution of Fe 3 O 4 -QDs select from d (g).

a

Transmittance (%)

3417 b

1635

3282

985 1035

c 1035

985

d 4000

3500

3000 2500 2000 -1 1500 Wavelength (cm )

808 1000 500

Figure 3. FT-IR spectra of ATP (a), g-C 3 N 4 (b), g-C 3 N 4 /ATP (c), and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d).

120

90

8

dV/dD(1*10-2cm3/g.nm)

Adsorbed volume (cm3/g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

g-C3N4 g-C3N4/ATP Fe3O4-QDs/g-C3N4/ATP

6

4 2

0 0

50

100

Pore size (nm)

150

200

30

0

0.2

0.4

0.6 0.8 Relative pressure (p/p0)

Figure 4. The BET analysis of g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. 24


1.0

Page 25 of 29


b

(Ahυ)2(ev)2

Absorbance (a. u.)

g-C3N4 a g-C3N4/ATP Fe3O4-QDs@g-C3N4/ATP

2.78 200

300

400 500 600 700 800 2 3 hυ (ev) 4 Wavelength (nm) Figure 5. (a) The UV-vis DRS, (b) the plots of (ahυ)2 versus (hυ) of pure g-C 3 N 4 , g-C 3 N 4 /ATP and

5


1.0

120

a b c d

B

A off

0.8

on

0.6

90

-Z''(KΩ)

Photocurrent (µA)

d c

0.4 b

60

30

0.2 a 0.0 0

25

50

75 100 Time (sec)

125

0 0

150

20

40

60 Z'(KΩ)

80

100

120

Figure 6. Transient photocurrent responses (A), electrochemical impedance spectra (B) of g-C 3 N 4 (a), ATP (b), g-C 3 N 4 /ATP (c) and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d).


400

450

500 Wavelength (nm)

550

g-C3N4 g-C3N4/ATP Fe3O4-QDs@ g-C3N4/ATP

b

PL intensity (a. u.)

a

Intensity(a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

120

A2

τ2

1913 1.5

495

9.1 3.1

1800 1.8

455

14

4.3

326

152

18

7.1

A1

140

τ1

2.0

160 180 Time (ns)

200

τ

220

Figure 7. The PL spectra (a) and Time-resolved fluorescence decay curves (b) of pure g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP.

25



1.0

1.0

b

a

0.6

0.6 0.100 g ATP 0.050 g ATP 0.008 g ATP 0.005 g ATP 0.010 g ATP

0.4 0.2 0.0

C/C0

0.8

C/C0

0.8

0

15

30 wt % Fe3O4 20 wt % Fe3O4 1.0 wt % Fe3O4 5.0 wt % Fe3O4 10 wt % Fe3O4

0.4 0.2

30

45 60 Time (min)

75

90

0.0

0

15

30 45 Time (min)

60

75

90

1.0

c

d

0.8

0.4

g-C3N4

0.2

g-C3N4/ATP

0 min 15 min 30 min 45 min 60 min 75 min 90 min

Absorbance (a. u.)

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

Fe3O4-QDs@g-C3N4/ATP 0.0

0

15

30

45 60 Time (min)

75

90

200

250

300 350 Wavenumber (nm)

400

450

Figure 8. The degradation dynamics curves with different content ATP over g-C 3 N 4 /ATP (a), the degradation dynamics curves with different content Fe 3 O 4 over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (b), the degradation dynamics curves over g-C 3 N 4 , g-C 3 N 4 /ATP and Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (c), The UV-Vis spectral of MBT changes with reaction time of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (d).

Figure 9. m/z of degrading MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP: initial solution (a), degradation in 45 min (b), degradation in 90 min (c). 26


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. The possible intermediate products of degradation of MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP.

Figure 11. The cycle runs (a) and Hysteresis loops (b) of MBT solutions over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP. The Fe 3 O 4 -QDs@g-C 3 N 4 /ATP was attracted by magnet (inset in b).

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 12. The degradation efficiencies of MBT over Fe 3 O 4 -QDs@g-C 3 N 4 /ATP with different substrates (a), ESR spectra of DMPO-.OH and DMPO-.O 2 − resulting from with visible light irradiation (b).

Figure 13. The diagrammatic sketch of Fe 3 O 4 -QDs@g-C 3 N 4 /ATP (a) and the mechanism schematic illustration for degradation of MBT under visible light irradiation (b).

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TOC for the manuscript

The magnetic intercalation Fe 3 O 4 -QDs@g-C 3 N 4 /ATP is successfully prepared by the means of eutectic method and deposition technology, which exhibits high-efficient photodegradation performance for 2-Mercaptobenzothiazole (MBT), and also showed a superior stability and reusability.

29


Intercalation Effect of Attapulgite in g-C3N4 Modified with Fe3O4

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