Bi4Ti3O12 heterojunction with enhanced photocatalytic performance for sulfamethoxazole degradation

Journal Pre-proofs Direct Z-scheme Ag3PO4/Bi4Ti3O12 heterojunction with enhanced photocatalytic performance for sulfamethoxazole degradation Chenlu Liu, Jingjing Xu, Junfeng Niu, Mindong Chen, Yonghui Zhou PII: DOI: Reference:

S1383-5866(19)35335-3 https://doi.org/10.1016/j.seppur.2020.116622 SEPPUR 116622

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Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

20 November 2019 23 January 2020 24 January 2020

Please cite this article as: C. Liu, J. Xu, J. Niu, M. Chen, Y. Zhou, Direct Z-scheme Ag3PO4/Bi4Ti3O12 heterojunction with enhanced photocatalytic performance for sulfamethoxazole degradation, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116622

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Direct Zscheme Ag3PO4/Bi4Ti3O12 heterojunction with enhanced photoca talytic performance for sulfamethoxazole degradation Chenlu Liua, Jingjing Xua *, Junfeng Niub, Mindong Chena, Yonghui Zhoua

a

Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of

Environmental Science and Engineering, Collaborative Innovation Center of Atmospheric Environment and

Equipment Technology, Jiangsu Engineering Technology Research Center of Environmental Cleaning Materials,

Nanjing University of Information Science and Technology, Nanjing, China

b

Research Center for Eco-Environmental Engineering, Dongguan University of Technology, Dongguan,

Guangdong 523808, P.R. China

Abstract This paper introduces a Z-scheme Ag3PO4/Bi4Ti3O12 heterojunction composite photocatalyst prepared by an in-situ growth method. The photocatalytic properties of the prepared samples under visible light irradiation were studied by degrading sulfamethoxazole (SMX). The results of photocatalytic experiments and cycle experiments showed that Ag3PO4/Bi4Ti3O12-20% composite exhibited much better photocatalytic activity and stability than pure Ag3PO4 and pure Bi4Ti3O12. The capture experiment results showed that h+ is the main active substance in the process of SMX

*

Corresponding author. Tel./fax: +86 25 58731090

E-mail address: [email protected], [email protected] (JingJing Xu)

1

degradation. In addition, we proposed a possible photocatalytic mechanism based on the results of experiment and characterization. The formation of a direct Z-Scheme photocatalytic system promoted the separation of carriers, thereby improving the stability and activity of the composite samples. Keywords: Photocatalysis, Ag3PO4, Bi4Ti3O12, Z-scheme, Stability

1. Introduction

In recent years, along with the rapid development of the economy, an obvious side effect has been the increasing environmental pollution[1-4]. In the medical, industrial profession etc., a large amount of organic wastewater and dye wastewater are generated every year. To solve these problems, several conventional method have been developed [5-13]. How to effectively solve these environmental problems has become a major problem facing humanity. Photocatalytic technology, which can utilize the inexhaustible solar energy, has been widely regarded as one of the effective ways [14-18]. Traditional TiO2 based photocatalytic technology is limited to function only under ultraviolet light. However, ultraviolet light accounts for only 4% of the solar spectrum, while visible light accounts for most of the total solar energy (about 43%)[19-22]. Consequently, how to make full use of solar light and develop efficient photocatalysts capable of responding visible light is urgent. Silver phosphate, as a highly efficient visible light photocatalyst, has been studied by a large number of scholars [23-42]. It has been demonstrated that Ag3PO4 semiconductor has a high photooxidation ability and can effectively degrade 2

organic pollutants under visible light[43]. Nevertheless, Ag3PO4 semiconductors also have certain limitations. It is well known that Ag3PO4 photocatalyst is unstable under illumination conditions and is easily corroded by photogenerated electrons[44]. If Ag3PO4 is left in a non-dark condition long-term, yellow photocatalyst will turn black slowly. Of course, in addition to photolysis, the dissolution in water is also one of the reasons for the poor stability of Ag3PO4. In order to break through this limitation, scholars have done quantities of work, in which construction of heterojunction composite materials between Ag3PO4 and other semiconductors is widely tried. For instance, TiO2, g-C3N4, AgX (X = Cl, Br, I), Fe3O4, graphene, Bi2MoO6, BiPO4[45-52] have been coupled with Ag3PO4. In this type of composite materials photo-generated electrons can be moved from the surface of Ag3PO4 to the acceptor material before Ag+ is reduced to metal Ag0[53]. Bismuth titanate is a wide bandgap semiconductor, and its structure is realized by alternating layers of (Bi2O2)2+ and (Bi2Ti3O10)2- layers along the C-axis direction[54-57]. The unique layered crystal structure promotes the separation of electron-hole pairs and enhances photocatalytic activity. Like other photocatalysts, the photocatalytic performance of a single bismuth titanate is still low, which is difficult to meet the requirements of practical applications. Therefore, we consider using silver phosphate to modify bismuth titanate. They probably can form Z-scheme structure considering their suitable band structure. On the one hand, it can reduce the electron-hole recombination. On the other hand, it can maintain the strong reducing ability of the bismuth titanate conduction band electrons and the strong oxidizing 3

ability of the silver phosphate valence band holes. In addition, it can effectively reduce the photocorrosion phenomenon of the silver phosphate. In this paper, we prepared Ag3PO4/Bi4Ti3O12 heterojunction composites. The photocatalytic activity and stability of the prepared Ag3PO4/Bi4Ti3O12 heterojunction composites were studied by the degradation of antibiotic sulfamethoxazole. The following consequences were obtained. Under visible light, the photocatalytic activity of the composite was better than that of pure Ag3PO4 and Bi4Ti3O12, and the stability of the composite was much better than that of pure Ag3PO4. Studies have expressed that Ag3PO4 and Bi4Ti3O12 formed a direct Z-scheme photocatalytic system, which promotes the separation of carriers. Thus, it can improve the stability and activity of the composite samples.

2. The experimental part

2.1. Preparation method of Bi4Ti3O12 4 mmol of bismuth oxide (Bi2O3) and 6 mmol of P25 were weighed and mixed evenly. Then, 100 mmol of NaCl and KCl were weighed and grinded evenly (at least 30 minutes). Afterwards, they were calcined at 800 ℃ in muffle furnace for 2 hours (the heating rate is 5 ℃/min). After cooling, the products were washed several times with a large amount of water, and finally dried at 60 ℃. 2.2. Synthesis of Ag3PO4/Bi4Ti3O12 composites

4

0.315 g of Bi4Ti3O12 (BTO) was added to 30 mL of water and ultrasonicated for 30 minutes to form solution A. 0.0574 g (or 0.0765 g, 0.1148 g) of silver nitrate (AgNO3) was dispersed in 50 mL of water to form solution B. Then, solution B was poured into solution A and was stirred for 1 hour (wrap the beaker with tin foil). Then 20 mL of Na2HPO4 (0.0240 g, 0.0319 g, 0.0479 g) was added to above mixture drop by drop. The suspension was stirred for 1 hour after completion of the addition. The products were washed with a large amount of water and dried at 60 ℃. (The composite samples are recorded as AP/BTO-15%, AP/BTO-20%, AP/BTO-30%, respectively). In addition, pure Ag3PO4 (AP) was prepared by the same method just without the addition of Bi4Ti3O12 nanosheets. (The above agents are of analytical grade and are purchased from Sinopharm Chemical Regent Co, Ltd, China. ) 2.3. Sample characterization The prepared Bi4Ti3O12, Ag3PO4 and Ag3PO4/Bi4Ti3O12 composites were characterized by powder X-ray diffractometry (XRD) (XRD-6100, Shimadzu, Japan). A projection electron microscope (TEM) image was obtained by using an American FEI Tecnai G2 F20 electron microscope. X-ray photoelectron spectroscopy (XPS) was obtained on an X-ray energy spectrometer (Thermo ESCALAB 250Xi). The electron spin resonance (ESR) signal of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) trapping free radicals in water was studied by using the Jes FA200 electronic resonance spectrometer. UV-visible diffuse reflectance spectroscopy (DRS) was studied using a spectrophotometer (UV-3600, Shimadzu, Japan). Electrochemical 5

tests were performed on an electrochemical workstation (CHI-660D Chenhua Instrument company, Shanghai, China) such as Mott-Schottky (MS), photocurrent (PC), and impedance (EIS). Specifically, as follows, a certain amount of photocatalyst was added thereto to add anhydrous ethanol and a binder (5% nafion solution) for 30 minutes, and then the resulting suspension was evenly spread on a conductive glass (1 × 1 cm), and finally placed in an drying oven at 120 ℃ for 2 h. The electrolyte used 0.5M Na2SO4 aqueous solution, the light source was 300 W Xe arc lamp filter (λ>400nm) (CEL‐HXF300, Beijing AuLight), the reference electrode was Ag/AgCl, and the counter electrode was Pt (Platinum). 2.4. Photocatalytic experiment In this section, we tested the activity of the prepared samples by degradation of sulfamethoxazole (SMX) (analytical grade, Aladdin Industrial Corporation, Shanghai, China). The light source used in this experiment was a 300 W xenon lamp with a filter (λ > 400 nm). First, a certain amount of photocatalyst was added to a 50 mL 5 ppm sulfamethoxazole (SMX) solution, and dark agitation was carried out for 30 minutes before illumination in order to achieve an adsorption-desorption equilibrium between the contaminant and the sample. After the dark treatment, the suspension was placed under the above light source for visible light irradiation while stirring was continued and a part of the suspension was collected at intervals of time, and the collected suspension was centrifuged to test the concentration. The reaction vessel was placed in a water cooling system to maintain the reaction solution at a certain temperature throughout the illumination process. The absorbance was measured by an ultraviolet6

visible spectrophotometer (UV-6100, Shimadzu, Japan) to determine the concentration, and the characteristic peak of SMX was 257.0 nm. The degradation rate of different samples can be obtained by using the formula [1-(C/C0)]×100%, where C is the concentration of SMX during the illumination and C0 is the initial concentration. In addition, TOC analyzer (muti N/C 3100, ANALYTIK JENA, Germany) was used to measure the mineralization degree of pollutants. 2.5. Active species trapping The purpose of this test was to determine the primary active species that caused the degradation of the contaminants by adding different scavengers to the SMX solution during the photocatalytic process. In this experiment, EDTA-2Na, pbenzoquinone (BQ), and isopropanol (IPA) were used as h+, ·O2-, ·OH scavengers, respectively. 2.6. Cycling experiment In order to verify the stability of the photocatalyst, four repeated experiments were conducted on the degradation of SMX. The experimental method was the same as the photocatalytic experimental procedure, except that the sample used was the photocatalyst after the last photocatalytic process, which was filtered, washed with water and ethonal.

3. Results and discussion

3.1. Characterization of the as-prepared samples

7

The as-prepared samples were characterized by XRD, and the obtained results are shown in Fig. 1. The figure shows the XRD patterns of Ag3PO4, Bi4Ti3O12, Ag3PO4/Bi4Ti3O12-15%, Ag3PO4/Bi4Ti3O12-20%, Ag3PO4/Bi4Ti3O12-30% respectively, including the standard card number of the Ag3PO4 (JCPDS No. 060505) [58] and Bi4Ti3O12 (JCPDS No. 35-0795) [59]. It can be seen that both Ag3PO4 and Bi4Ti3O12 correspond to their standard card number diffraction peaks, indicating that the prepared samples are pure Ag3PO4 and pure Bi4Ti3O12. In the XRD pattern of the composite sample, it is not difficult to find that the diffraction peaks corresponding to the two pure samples are included. Furthermore, as the content of Ag3PO4 increases, the intensity of the diffraction peak also increases, and it can be clearly seen that the diffraction peaks at 33.3° and 36.6° are corresponding to (210) and (211) planes. No other diffraction peaks were found in the composite sample, and no other possible impurities were considered. The average crystal size of particles can be calculated by Debye-Scherrer formula （ D  k /  cos  ）, where D is the crystal size, K is a constant of 0.89, λ is the wavelength of X-ray (0.15406 nm), β is the half height width, and θ is the Bragg’s angle.[71] The calculated crystal sizes of AP and BTO in AP/BTO-20% composite sample are 13.6 nm and 39.7 nm, respectively. The morphology and structure of the sample were characterized by TEM. Fig. 2(a) shows a TEM image of pure Ag3PO4. It is apparent that it consists of spherical particles. Fig. 2(b) is a TEM image of pure Bi4Ti3O12 with a sheet structure. Fig. 2(c) shows the TEM image of Ag3PO4/Bi4Ti3O12 composite. A large amount of Ag3PO4 8

particles adhere to the surface of Bi4Ti3O12 nanosheets. Fig. 2(d) is the HRTEM of AP/BTO-20%，it clearly shows the lattice fringes of the two substances, with the spacing of 0.263 nm and 0.192 nm, corresponding to the (191) plane of BTO and the (310) plane of AP, respectively. The EDS mapping is used to further analyze the element distribution on AP/BTO-20% composite, as shown in Fig. 2(e-i). It is confirmed that Ag, P, O, Bi, Ti are the constituent elements in AP/BTO-20%. The experimental results have proved that the Ag3PO4/Bi4Ti3O12 heterojunction composites have been successfully prepared. X-ray photoelectron spectroscopy (XPS) can determine the elemental composition and chemical state of the material. Fig. 3 shows the XPS spectra of the prepared samples. Fig. 3(a) is the XPS measurement spectra of pure Bi4Ti3O12, pure Ag3PO4 and Ag3PO4/Bi4Ti3O12-20%. It is not difficult to see that the Ag3PO4/Bi4Ti3O12-20% composite contains two pure samples constituent elements. Fig. 3(b) is the O 1s XPS spectrum of the sample. For pure Bi4Ti3O12 and Ag3PO4, two peaks were fitted at 529.6 eV, 531.6 eV and 530.8 eV, 532.9 eV, respectively due to lattice oxygen and adsorbed oxygen[60]. For Ag3PO4/Bi4Ti3O12-20% composite, there are simultaneous binding energies at 529.4 eV and 530.8 eV, corresponding to the lattice oxygen of Bi4Ti3O12 and Ag3PO4, respectively, and the presence of adsorbed oxygen is beneficial to the activation of oxygen[43]. Fig. 3(c) is the Bi 4f XPS spectrum of the sample. The Bi 4f7/2 and Bi4f5/2 binding energies of pure Bi4Ti3O12 are 159.2 eV and 164.5 eV, respectively. The Ag3PO4/Bi4Ti3O12-20% composite showed peaks at 158.9eV and 164.1eV, belonging to Bi 4f7/2 and Bi4f5/2, 9

respectively. This shows that Bi exists in the +3 valence state[56]. Fig. 3(d) shows the Ti 2p XPS spectrum of the sample. In the pure Bi4Ti3O12, two peaks of 457.9 eV and 465.5 eV appeared, corresponding to Ti 2p3/2 and Ti 2P1/2, respectively. In the Ag3PO4/Bi4Ti3O12-20% composite, the binding energies of Ti 2p3/2 and Ti 2P1/2 are located at 457.7 eV and 465.3 eV, respectively. This indicates that Ti is present in the form of Ti4+[56]. Fig. 3(e) shows the Ag 3d XPS spectra of pure Ag3PO4 and Ag3PO4/Bi4Ti3O12-20% composite, respectively. Two distinct peaks at 368.1eV and 374.1eV appear in the Ag3PO4 spectrum, respectively. In the Ag3PO4/Bi4Ti3O12-20% composite spectrum, there are two peaks of 368.2 eV and 374.2 eV. These peaks belong to the binding energy of Ag 3d5/2 and Ag 3d3/2, respectively[51]. Fig. 3(f) is the P 2p XPS spectrum of the sample. A peak at 133.2 eV was observed in pure Ag3PO4, and a peak at 133.3 eV was observed in Ag3PO4/Bi4Ti3O12-20% composite. Although the binding energy slightly changed, it could be attributed to the binding energy of P5+ in Ag3PO4. It is not difficult to find that the binding energy of Ag3PO4/Bi4Ti3O12 composite exhibits a shift, in which the binding energy of Bi 4f and Ti 2p is decreased, and the binding energy of Ag 3d and P 2p is increased. This transfer can be ascribed to the interaction between Bi4Ti3O12 and Ag3PO4, indicating successful formation of heterojunctions between the corresponding two components [61]. 3.2. Photocatalytic performance In order to study the photocatalytic properties of the prepared samples, degradation of sulfamethoxazole (SMX) was carried out under visible light 10

irradiation. Before illumination, we first conducted dark treatment. After dark treatment for 30 min, AP/BTO-15%, AP/BTO-20%, AP/BTO-30%, AP and BTO adsorbed 2.48%, 3.91%, 1.15%, 2.56% and 0.12% of pollutants respectively. Fig. 4(a) shows the photocatalytic degradation of SMX by pure Bi4Ti3O12, pure Ag3PO4 and Ag3PO4/Bi4Ti3O12 composites with different Ag3PO4 content under visible light irradiation. It can be clearly seen from the figure that the activity of pure BTO is poor, while pure AP exhibits good photocatalytic activity. When the Ag3PO4/Bi4Ti3O12 heterojunction composites are formed according to different proportions of AP content, the photocatalytic ability is also improved. More importantly, as the photocatalytic time increases, the degree of degradation of SMX by each photocatalyst also increases. As the AP content in the composite increases proportionally, the photocatalytic activity also increases. When the ratio reaches 20%, the best photocatalytic ability is exhibited. When the ratio reaches 30%, the photocatalytic ability begins to decrease. We think that AP will agglomerate and can’t form effective contact or form heterojunction with BTO when the content of AP increases. Therefore, the activity of AP/BTO-30% is worse than that of AP/BTO-20%. It is worth mentioning that the degradation of SMX by pure AP after 20 min is very small. It can be considered that after this, its degradation ability is saturated, and the prolongation time does not increase the degradation rate of SMX. In addition, degradation experiments were carried out on SMX solution without photocatalyst. The experimental results show that the SMX hardly degraded, so the photolysis of SMX can be neglected. In addition, we also studied the kinetics of 11

photocatalytic degradation of SMX under visible light irradiation, and Fig. 4(b) shows the kinetic rate constant of the prepared sample. The rate constant can be obtained from the first-order kinetic equation, ie ln[C]=-kt+ln[C0], where C is the concentration during degradation, C0 is the initial concentration, and k is the first-order reaction rate constant[62]. As can be seen from the figure, the average rate constants of pure BTO, pure AP, composite samples AP/BTO-15%, AP/BTO-20% and AP/BTO-30% are 0.0003, 0.0281, 0.0246, 0.0372 and 0.0318 min-1, respectively. Among them, the average value of the rate constant of the composite sample AP/BTO-20% showed the highest value (0.0372 min-1), which was 1.3 times higher than that of the pure AP (0.0281 min-1). In addition, the total organic carbon (TOC) analysis can be used to determine the mineralization degree of SMX.[72-73] The removal rate of TOC of AP / BTO-20% composite sample in SMX solution reaches 43.01% after 40 minutes of illumination. In order to study the main active substances of photocatalytic degradation of SMX in Ag3PO4/Bi4Ti3O12-20% composite, we carried out free radical trapping experiments. EDTA-2Na, p-benzoquinone (BQ), and isopropanol (IPA) were used as the scavengers of h+, ·O2-, ·OH, respectively[43,61,63]. The concentration of scavengers are all 10-4 mol/L. There was no change in the position of the characteristic peak of UV-vis absorption spectrum before and after adding scavengers. As shown in Fig. 5(a), EDTA-2Na has a significant inhibitory effect on photocatalysis compared to BQ and IPA. In the process of degradation of SMX by Ag3PO4/Bi4Ti3O12-20% composite, h+ plays a more important role than ·O2- and ·OH. 12

As we all know, besides the photocatalytic activity of high efficiency photocatalysts is important, the stability is also an extremely important part of the research.[74] In order to prove that Ag3PO4/Bi4Ti3O12 composites have better stability, we used both as photocatalysts to carry out cycling experiments on the degradation of SMX. The experimental results are shown in Fig. 6(a). The results show that the photocatalytic activity of the Ag3PO4/Bi4Ti3O12-20% composite remains above 70% after 4 cycles. However, the activity of pure Ag3PO4 was significantly reduced after 4 cycles, and the degradation efficiency was reduced from 70% to 30%. Fig. 6(b) is the XRD contrast diagram before and after AP/BTO-20% cycle experiment. It can be seen that the diffraction peak is basically the same, and the crystal structure is basically unchanged, indicating that it has high stability. In addition, Fig. 6(c) shows AP/BTO-20% TEM after photocatalytic reaction, which is consistent with that before use. These results show that the Ag3PO4/Bi4Ti3O12-20% composite has better stability and recyclability. These results are consistent with previous reports[43,53,64]. 3.3. Photocatalytic mechanism For the sake of studying the light absorption of the prepared photocatalyst, we used UV-visible absorption spectroscopy to characterize it. Fig. 7 shows the UVvisible diffuse reflectance spectra of pure Ag3PO4, pure Bi4Ti3O12 and Ag3PO4/Bi4Ti3O12-20% composite. It can be seen that pure AP has a distinct absorption band in the visible region with a wavelength of less than 540 nm, and pure BTO exhibits a strong absorption capacity at a wavelength of less than 450 nm. This 13

indicates that the AP sample has strong UV and visible light absorption capacity, while the BTO sample is active in the ultraviolet light and narrow visible light range[65-66]. In the Ag3PO4/Bi4Ti3O12-20% composite, the absorption band shift may be due to the light absorption interaction between them[61]. By Kubelka-Munk method, we draw the Fig. 7(b) and Fig. 7(c) respectively. It can be seen that the band gap energy (Eg) of Ag3PO4 is about 2.45 eV, and the band gap energy (Eg) of Bi4Ti3O12 is about It is 2.97 eV. In order to study the separation and transfer behavior of electrons and holes in photocatalysts, we conducted electrochemical experiments, including photocurrent response and EIS testing. Fig. 8 shows the transient photocurrent response of Bi4Ti3O12, Ag3PO4 and Ag3PO4/Bi4Ti3O12-20% composite samples. It can be seen that the Ag3PO4/Bi4Ti3O12-20% composite sample clearly shows higher photocurrent values than the other two pure samples, indicating that the composite sample has higher electron hole separation and transfer efficiency. In addition, the AP sample and the AP/BTO-20% composite sample confirmed the stability of the AP in several intermittent switching illumination cycles, and the composite sample had better stability, which is consistent with the results of the cycling experiment (Fig. 6). EIS can be used to study the conductivity properties of materials. It is well known that the lower the impedance is, the higher the photo-generated carrier separation efficiency is and the lower the interface resistance is[53]. Fig. 9 shows the EIS spectrum of pure Bi4Ti3O12, pure Ag3PO4 and Ag3PO4/Bi4Ti3O12-20% composite. It can be clearly seen that the Ag3PO4/Bi4Ti3O12-20% composite has the lowest 14

interfacial impedance, which means it has better electron transfer ability and higher photo-generated carrier separation efficiency than pure Bi4Ti3O12 and pure Ag3PO4. The flat band potential (Efb) of Ag3PO4 and Bi4Ti3O12 was determined using a Mott-Sckottky (MS) pattern, as shown in Fig. 10. Taking Ag/AgCl as the reference electrode, the flat band potentials of Ag3PO4 and Bi4Ti3O12 are about 0.42eV and 1.13eV, respectively. From the tangent line, it can be roughly judged that Ag3PO4 and Bi4Ti3O12 are N-type semiconductors， which was consistent with previous reports[57,65,66]. Pass the following equation[67]:

 ENHE  E Ag/AgCl  EAg/AgCl The potential relative to the Ag/AgCl can be converted to a potential relative to a standard hydrogen electrode (NHE), wherein E A g / AgCl =0.197 ev. The data shows that the minimum value of the conduction band (CB) is about 0.1 eV higher than the Efb of the N-type semiconductor[68]. Therefore, according to the calculation, we have found that the CB positions of Ag3PO4 and Bi4Ti3O12 are 0.52 eV and -1.03 eV (vs. NHE), respectively. In order to study the charge transfer mechanism of AP/BTO composite, ESR experiment was carried out to detect the formed free radicals.[7577]

As shown in Fig. 11, the signals of ·O2- and ·OH were observed after

illumination, indicating that superoxide radicals and hydroxyl radicals were produced in the process of photocatalysis. There are two possible ways of photogenerated charge separation for AP/BTO composites. If the traditional type II heterojunction transfer mechanism is followed, the electrons on CB of 15

BTO will migrate to CB of AP due to potential difference, and the holes on VB of AP will migrate to VB of BTO. However, due to the relatively positive CB potential of AP compared to O2/·O2 - (-0.33 eV vs. NHE), the electrons on CB potential of AP have no ability to reduce O2 to form ·O2- radical. The VB potential of BTO is lower than the standard oxygen reduction potential of OH/·OH (2.40 eV vs. NHE) and H2O/·OH (2.72 eV vs. NHE), so the hole of BTO cannot oxidize OH- or H2O to produce ·OH. That is to say, superoxide radicals and hydroxyl radicals can’t be formed if the conventional type II heterojunction mechanism is used[70]. Therefore, we believe that the separation and transfer of photogenerated electron-hole pairs should follow the direct Z-scheme mechanism. To sum up, the photocatalytic reaction mechanism of Ag3PO4/Bi4Ti3O12-20% composite degrading SMX under visible light irradiation is obtained, as shown in Fig. 12(a). This is a typical direct Z-scheme photocatalytic system[69]. It can be seen from the DRS spectrum that both AP and BTO have visible light response capability. As a photo-oxidation system, the AP has a CB potential lower than the VB potential of the BTO as a photoreduction system and higher than its CB potential. When the two combine to form a direct Z-scheme photocatalytic system, the electrons on the CB of the AP are transferred to the VB of the BTO and compounded with the holes on the VB of the BTO. Due to the electron transfer of the composite sample, there is no extra electrons on the CB of the AP to convert Ag+ into Ag0, which inhibits the photocorrosion of AP. Therefore, AP/BTO-20% shows better stability than pure 16

AP. At the same time, the generation of h+ on the VB of the AP is directly used to degrade the SMX. The formation of a direct Z-scheme photocatalytic system promotes the separation efficiency of carriers, and more electrons and holes participate in the photocatalytic reaction, thereby improving the stability and activity of the composite sample.

4. Conclusion

We have successfully prepared Ag3PO4/Bi4Ti3O12-20% composite, which have better photocatalytic activity than pure Ag3PO4 and pure Bi4Ti3O12. Through the cycle experiment, it is proved that the stability of Ag3PO4/Bi4Ti3O12-20% composite is much higher than that of pure Ag3PO4. The formation of a direct Z-scheme photocatalytic system promotes the separation of carriers, thereby improving the stability and activity of the composite sample.

Acknowledgements We are grateful for grants from the Natural Science Foundation of China (51978342), the funding of the Jiangsu Provincial Graduate Research and Innovation Program (SJKY19-0976), Guangdong Innovation Team Project for Colleges and Universities (No. 2016KCXTD023), and Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2017) the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Figure captions Fig. 1 XRD patterns of Ag3PO4, Bi4Ti3O12, and Ag3PO4/Bi4Ti3O12 photocatalysts Fig. 2 TEM images of the as prepared (a) Ag3PO4, (b) Bi4Ti3O12, (c) Ag3PO4/Bi4Ti3O12, (d) HRTEM image of Ag3PO4/Bi4Ti3O12-20%, (e-j) EDS elemental mapping images of

Ag, P, O, Bi, Ti in Ag3PO4/Bi4Ti3O12-20% Fig. 3 (a) Full XPS survey spectra, XPS spectra of (b) O 1s, (c) Bi 4f, (d) Ti 2p, (e) Ag 3d, and (f) P 2p of Bi4Ti3O12, Ag3PO4 and Ag3PO4/Bi4Ti3O12-20% Fig. 4 (a) The degradation curves of SMX by the as-prepared samples under visible light irradiation. (b) Kinetic rate constant for SMX degradation by as-prepared samples. Fig. 5 (a) Effects of different scavengers on the SMX photo-degradation by Ag3PO4/Bi4Ti3O12-20%. Fig. 6 (a) Recycling experiments of Ag3PO4 and Ag3PO4/Bi4Ti3O12-20% for photodegradation of SMX under visible light illumination for 4 cycles. (b) XRD patterns of Ag3PO4/Bi4Ti3O12-20% before and after 4 cycles. (c) TEM of Ag3PO4/Bi4Ti3O12-20% after photocatalytic reaction.

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Fig. 7 (a) UV–vis diffuse-reflectance spectra of Bi4Ti3O12, Ag3PO4 and Ag3PO4/Bi4Ti3O1220%, and band gap energy values estimated from UV−vis DRS of (b) Ag3PO4 and (c) Bi4Ti3O12 Fig. 8 Transient photocurrent responses of Bi4Ti3O12, Ag3PO4 and Ag3PO4/Bi4Ti3O12-20% Fig. 9 EIS spectra of Bi4Ti3O12, Ag3PO4 and Ag3PO4/Bi4Ti3O12-20% Fig. 10 Mott‐Schottky plots of (a) Ag3PO4 and (b) Bi4Ti3O12 Fig. 11 Spin-trapping ESR spectra for (a) Ag3PO4/Bi4Ti3O12-20% DMPO- ·O2 – and (b) Ag3PO4/Bi4Ti3O12-20% DMPO- ·OH. Fig. 12 (a) Possible mechanism for the photocatalytic degradation of SMX by Ag3PO4/Bi4Ti3O12-20%, (b) Charge carrier migration mechanism in type-II heterojunction photocatalysts

Fig. 1

31

Fig. 2 32

Bi 4f

(b) O 1s Ti 2p

Bi4Ti3O12

Ag 3d

Intensity(a.u.)

529.6

O 1s

O 1s P 2p

Ag3PO4

531.6

Intensity(a.u.)

(a)

Bi4Ti3O12

530.8 532.9 Ag3PO4

Bi 4f Ti 2p O 1s

Ag 3d

530.8 531.6

529.4

0

200

400

600

800

525

530

Binding energy(eV)

535

Ti 2p1/2

(d) Ti 2p Ti 2p3/2

Bi 4f5/2

164.5

Bi4Ti3O12

Bi 4f5/2

465.5

457.9

Intensity(a.u.)

Intensity(a.u.)

159.2

Bi 4f7/2

540

Binding energy(eV)

Bi 4f7/2

(c) Bi 4f

532.9 Ag3PO4/Bi4Ti3O12-20%

Ag3PO4/Bi4Ti3O12-20%

Bi4Ti3O12

Ti 2p1/2

Ti 2p3/2 465.3 457.7

158.9 164.1

Ag3PO4/Bi4Ti3O12-20% Ag3PO4/Bi4Ti3O12-20% 156

158

160

162

164

166

450

455

Binding energy(eV) Ag 3d5/2

Ag3PO4

Ag 3d3/2

Intensity(a.u.)

368.1

374.1

368.2 365

Ag 3d5/2

374.2

370

465

470

Ag3PO4 Ag3PO4/Bi4Ti3O12-20%

(f) P 2p

Ag3PO4/Bi4Ti3O12-20%

133.2

Intensity(a.u.)

(e) Ag 3d

460

Binding energy(eV)

133.3

Ag 3d3/2 375

125

Binding energy(eV)

130

135

Binding energy(eV)

Fig. 3

33

140

Fig. 4

34

1.0

C/C0

0.8

0.6

EDTA-2Na BQ IPA No scavengers

0.4

0.2 0

10

20

Time(min)

Fig. 5

35

30

40

Fig. 6

36

Fig. 7

37

38

Fig. 8

39

Fig. 9

40

Fig. 10

41

(a) ·O 2

(b) ·OH Light on 10 min

Light on 5 min

Intensity(a.u.)

Intensity(a.u.)

Light on 10 min

Light on 5 min

Dark

320

322

324

Dark

326

320

B(mT)

322

324

B(mT)

Fig. 11

42

326

Fig. 12

43

Highlight Novle Z-scheme Ag3PO4/Bi4Ti3O12 composite photocatalysts were synthesized. The photocatalyst exhibited high performance for the degrdation of sulfamethoxazole. The rate constant improved by a factor of 124 and 1.3 compared to Bi4Ti3O12 and Ag3PO4. The mechanism for the enhanced activity was discussed deeply.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

44

Chenlu Liu: Methodology, Software, Validation, Visualization, Writing - Original Draft, Jingjing Xu: Conceptualization, Writing - Review & Editing, Supervision, Funding acquisition, Data Curation, Validation, Formal analysis Junfeng Niu: Formal analysis, Writing - Review & Editing Mindong Chen: Writing - Review & Editing Yonghui Zhou: Validation

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