Bi2O2CO3 heterojunction towards enhanced photocatalytic activity

Journal Pre-proofs Full Length Article Facile Fabrication of 2D/2D Step-Scheme In2S3/Bi2O2CO3 Heterojunction towards Enhanced Photocatalytic Activity Hongxia Fan, Hualei Zhou, Wenjun Li, Shaonan Gu, Guowei Zhou PII: DOI: Reference:

S0169-4332(19)33167-8 https://doi.org/10.1016/j.apsusc.2019.144351 APSUSC 144351

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

15 July 2019 23 September 2019 9 October 2019

Please cite this article as: H. Fan, H. Zhou, W. Li, S. Gu, G. Zhou, Facile Fabrication of 2D/2D Step-Scheme In2S3/Bi2O2CO3 Heterojunction towards Enhanced Photocatalytic Activity, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144351

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

Facile Fabrication of 2D/2D Step-Scheme In2S3/Bi2O2CO3 Heterojunction towards Enhanced Photocatalytic Activity Hongxia Fana, Hualei Zhoua,*, Wenjun Lia, Shaonan Gua,b,*, Guowei Zhoub a

Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline

Materials, University of Science and Technology Beijing, 100083 Beijing, China b

Key Laboratory of Fine Chemicals in Universities of Shandong, School of Chemistry and

Pharmaceutical Engineering, Qilu University of Technology (Shandong Academy of Sciences), 25053 Jinan, China

Abstract To fabricate step scheme (S-scheme) heterojunction is one of the newly effective approaches to promote the photocatalytic performance of coupled semiconductors. Herein, a two-dimension (2D)/2D In2S3/Bi2O2CO3 S-scheme heterojunction photocatalyst was designed and fabricated through a facile two-step precipitation method. The as-synthetized composite presented in face-toface hierarchical structure with enhanced photocatalytic performance towards the degradation of Rhodamine B (RhB) and tetracycline (TC) under light irradiation. The 2D/2D In2S3/Bi2O2CO3 heterojunction not only enlarges the light utilization region but also effectively restrains the recombination of useful photogenerated electron-hole pairs, which was demonstrated by diffuse reflection spectra (DRS), photoluminescence spectra (PLS) and active species trapping experiments. Particularly, the optimal performance was achieved using 10% In2S3/Bi2O2CO3 heterojunction with 91% degradation capacity towards RhB in 60 min, which was about 5 and 3 times higher than that of pure Bi2O2CO3 and In2S3, respectively. Moreover, the cycling performance of the photocatalysts further confirmed the excellent stability of the heterojunction. Based upon the characterization and experimental results, the mechanism of S-scheme heterojunction was employed to illustrate the effectively enhanced photocatalytic performance of the 2D/2D In2S3/Bi2O2CO3 heterojunction.

Keywords: S-scheme; Heterojunction; Photocatalysis; Degradation Corresponding author: Dr. Shaonan GU, E-mail: [email protected]; [email protected]

1

1. Introduction The semiconductor-based photocatalysis technology arouses considerable attentions for treating nocuous organic pollutants in water and air [1-13]. However, the intrinsic drawbacks of high recombination rate of photogenerated electrons and holes, limited redox capability, and unsatisfied solar utilization efficiency hamper the practical application of single-semiconductor photocatalysis system. Significant progress has been achieved to construct suitable heterojunction by two kinds (or two crystal phases) of semiconductor photocatalysts to address the above mentioned issues, which is an effective approach to suppress the recombination of photogenerated electrons and holes through the charge transfer in heterojunction [14-22]. Bismuth-based semiconductor photocatalysts, such as Bi2O3 [23-25], BiVO4 [26-28], BiOX (X = Cl, Br, I) [29-31], Bi2WO6 [32,33], Bi2S3 [34,35] and Bi2MoO6 [36,37], exhibit considerable potential in photodegradation of toxic pollutions [38], photocatalytic H2 production [39], CO2 photocatalytic reduction [40] and so on, due to their hybrid valence band by O 2p and Bi 6s [41]. Among various Bi-based semiconductors, Bi2O2CO3 attracts more attention in recent decade because of the satisfied oxidation ability of its valence band [42,43]. Bi2O2CO3 belongs to the Sillen phase with the alternating [Bi2O2]2+ sheets and CO32− groups [44-46], which is beneficial for forming 2D nanostructures and for separating photogenerated electron-hole pairs more efficiently. However, the practical photocatalytic performance of Bi2O2CO3 is still far from the requirement. Plenty of the second-semiconductors have been used to couple with Bi2O2CO3 to improve its photocatalytic activity, such as Bi2O2CO3/BiOX [29-31], Bi2O2CO3/CdS [47], Bi2O2CO3/Ag2CrO4 [48], Bi2O2CO3/MoS2 [49], Bi2O2CO3/BiVO4 [44], Bi2O2CO3/graphene [43]. All these type-II heterojunctions successfully improved the photocatalytic activity of Bi2O2CO3 through driving the photogenerated charges by internal electric field. However, in the traditional type-II heterojunction, the photoexcited electrons transfer from the higher conduction band to lower conduction band. Analogously, the holes in valence band transfer from the lower valence band to the higher valence band. Although this kind of charges transfer can prevent the recombination of electrons and holes, it still exists some obvious limitations, such as the re-arranged electrons and holes under internal electric field sacrificed the redox capability, and the electrons (holes) in conductive band (valence band) of different photocatalyst show Coulomb electrostatic repulsive force which hinder the kinetics of charge transfer. Very recently, Yu and his co-workers proposed a step-scheme (S-scheme) heterojunction [50] on the basis of well-known Z-scheme photocatalysts [26-28,32,33,51-54] to further enhance the photocatalytic capability of heterojunction photocatalysts. Generally, S-scheme heterojunction consists of two n-type semiconductors which act as oxidation photocatalyst I and reduction

2

photocatalyst II respectively. The internal electric field of S-scheme heterojunction could eliminate the excited electrons and holes from the lower conduction band in photocatalyst I and from higher valence band in photocatalyst II, therefore, the strong oxidative holes in lower valence band (photocatalyst I) and the strong reductive electrons in higher conduction band (photocatalyst II) are separated. This strategy not only efficiently restrain the recombination of photogenerated charges but also enlarge the redox capability of the heterojunction in photocatalysis process. To utilize the satisfied oxidative valence band of Bi2O2CO3 and improve its photocatalytic degradation activity, herein, we chose another n-type semiconductor In2S3 with well reductive conduction band to couple with Bi2O2CO3 to form 2D/2D ultrathin In2S3/Bi2O2CO3 (IS/BOC) Sscheme heterojunction. Moreover, In2S3 in the suggested S-scheme heterojunction can not only act as a reduction photocatalyst but also a light collector with the narrow band gap of about 2.0 eV [55,56] further improving the photocatalytic activity of the heterojunction. Moreover, the face-toface 2D/2D structure can facilitate the charges transfer between two semiconductors and also the charges transfer throughout the composite. The result reveal that the 2D/2D IS/BOC S-scheme heterojunction showed very enhanced photocatalytic degradation towards RhB and tetracycline (TC). Meanwhile, the active species trapping experiments provide a solid evidence to the formation the S-scheme heterojunction with strong redox capability. 2. Experimental 2.1. Preparation of 2D/2D IS/BOC composites All the reagents used in this work are analytical grade without further purification before operation. Bi2O2CO3 nanosheets were synthesized as described followings. Firstly, Na2CO3 solution was prepared by dissolving 2.82 mmol Na2CO3 and 1.38 mmol CTAB (cetyltrimethyl ammonium bromide) in 100 mL deionized water. Subsequently, 3.52 mmol Bi(NO3)3·5H2O was added into 10 mL of 1 mol L−1 HNO3 to obtain Bi(NO3)3 solution using ultra sonication, followed by adding Bi3+ into the solution of Na2CO3 dropwise under continuous stirring for 3 h. Then, Bi2O2CO3 nanosheets were collected after washing with deionized water and ethanol, centrifugation, and drying at 80 °C overnight. Afterwards, 5 mmol Bi2O2CO3 nanosheets was dispersed into 50 mL deionized water which contains 1.35 mmol citric acid under continues stirring at 55 ◦C for 2 h. Then, 25 mL InCl3 aqueous solution was added into the above mixture and kept stirring for 2 h, followed by instilling 25 mL Na2S aqueous solution slowly (mole ratio of InCl3 : Na2S was 2:3). The products were obtained by washing with deionized water and ethanol, and drying overnight. The theoretical mass ratio of In2S3 in the prepared IS/BOC heterojunction was 5 wt%, 10 wt%, 20 wt%, 30 wt% and were denoted as

3

5% IS/BOC, 10% IS/BOC, 20% IS/BOC, 30% IS/BOC. For contrast, pure In2S3 was also obtained under the same process without introducing of Bi2O2CO3. 2.2. Characterization The crystalline phases of the samples were recorded in the range of 2θ = 10~70° by X-ray diffraction analysis (XRD, D/Max-RB, Rigaku, Japan). The morphologies and microstructures of the products were measured by using Scanning Electron Microscope (SEM, SU8010, Hitachi, Japan). The transmission electron microscopy (TEM) and the high-resolution transmission electron microscopy (HRTEM) images were obtained with a transmission electron microscope (F-20, FEI, USA) at an accelerating voltage of 200 kV. UV-vis spectra were obtained from a T9s (Persee, China) spectrometer with BaSO4 as reference. The elemental valence states and compositions were examined by X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi, USA). The photoluminescence (PL) spectra with a Xe lamp as the excitation light source were acquired using a fluorescence spectrophotometer (F-4500, Hitachi, Japan). 2.3. Photocatalytic experiment The degradation experiments of RhB (10 mg L−1) and TC (10 mg L−1) were conducted to evaluate the photocatalytic performance of IS/BOC heterojunction photocatalysts and the light source was a 400 W Xe lamp. For the details, 30 mg photocatalyst was dispersed in 30 mL RhB solution under vigorous stirring in dark to obtain the adsorption and desorption equilibrium between photocatalyst and target molecular. After turning the light on, the suspension was sampled at a determined period and the UV-vis spectrophotometer was used to assess the corresponding degradation rate (C/C0) at the maximal absorption wavelength of 553 nm. The degradation experiment of TC was performed under the same condition, only the degradation rate was measured by scanning the whole wavelength at 359 nm wavelength. 3. Result and discussion 3.1. Morphology and structure

4

Fig. 1. XRD spectra of Bi2O2CO3, In2S3 and IS/BOC composites The powder X-ray diffraction was used to confirm the crystalline phase of the samples first. Fig. 1 compares the XRD patterns of Bi2O2CO3, In2S3 and IS/BOC composites with different In2S3 percentage. The XRD peaks of Bi2O2CO3 nanosheets located at 23.9°, 30.2°, 32.7°, 42.3°, 46.9°, 56.9° are indexed to the crystal planes of (0 1 1), (0 1 3), (1 1 0), (1 1 4), (0 2 0) and (1 2 3) (JCPDS 41-1488). Meanwhile, the XRD pattern of In2S3 synthesized using the same method with that of IS/BOC composites exhibits the typical peaks located at 28.7°, 33.4° and 47.7°, manifesting its phase-pure property of In2S3 (JCPDS 32-0456). For the XRD patterns of IS/BOC composites, no apparent peaks of In2S3 can be observed, probably ascribing to the relatively low intensity and broad width of the In2S3 diffraction peaks and its high dispersity in the composites [57]. Furthermore, the relative intensity of the diffraction peaks for Bi2O2CO3 nanosheets gradually weaken with the increasing In2S3 contents, which suggested the successful combination of Bi2O2CO3 and In2S3 as further demonstrated subsequently by HR-TEM.

5

Fig. 2. SEM images of (a) Bi2O2CO3 nanosheets, (b) 10% IS/BOC composite, (c) 20% IS/BOC composite and (d) In2S3 nanosheets; (e~j) elemental mapping results from the 10% IS/BOC composite shows in (e), the element labels are marked inset the figures; TEM and HRTEM images of (k) IS/BOC heterojunction and (i, m) crystal fringes information of the heterojunction circled in the (k). The morphology of the samples was studied using SEM. As shown in Fig. 2a and d, clear nanosheets structure are observed for the Bi2O2CO3 (Fig. 2a) and In2S3 (Fig. 2d). The lateral size of the nanosheets measured as bout 150~250 nm with irregular shape, and the thickness of the nanosheets is about 3~5 nm. Fig. 2b and 2c show the aggregation of 10% IS/BOC and 20% IS/BOC composites respectively, which are clear that Bi2O2CO3 nanosheets and In2S3 nanosheets were stacked by face-to-face together randomly to form 2D/2D heterostructrue. And the elemental mapping results in Fig. 2e~2j confirm that all the components distributed uniformly in the composite. TEM analysis is generally an effective approach to study nanosheets heterojunction. As shown in Fig. 2k, the TEM image of 10% IS/BOC exhibited obviously that the In2S3 nanosheets and Bi2O2CO3 nanosheets combined in the 2D orientation. The HRTEM images corresponding to the

6

part in the red circles in Fig. 2k shows that the lattice spacing measured as 0.253 nm and 0.273 nm were corresponded to the (4 1 1) plane of In2S3 and the (1 1 0) plane of Bi2O2CO3 respectively, further indicating the formation of 2D/2D IS/BOC heterojunction. 3.2 Chemical state analysis

Fig. 3. (a) Overall XPS spectra of Bi2O2CO3, In2S3 and the 10% IS/ BOC composite; high-resolution XPS spectra of Bi2O2CO3, In2S3 and the 10% IS/ BOC: (b) Bi 4f, (c) In 3d,

(d) O 1s, (e) C 1s

(f) S 2p. The chemical state of each element in IS/BOC heterojunction was further investigated using XPS and the results are shown in Fig. 3. All the XPS spectra were calibrated to the peak at 284.6 eV for C 1s. As shown in Fig. 3a, the XPS survey spectrum of Bi2O2CO3, In2S3 and the 10% IS/BOC composite display the characteristic peaks of Bi, In, S, C and O elements. Fig. 3b-f demonstrate high-resolution XPS spectra for five primary elements. The two distinct peaks located at 159.2/158.5 eV and 164.5/163.8 eV which are attributed to Bi 4f7/2 and Bi 4f5/2 of Bi3+, respectively (Fig. 3b) [58,59]. For the element of In in the heterojunction, the signals at 444.9/444.2 eV and 452.4/451.9 eV could be assigned to In 3d5/2 and In3d3/2 of In3+ [60]. Compared with bare Bi2O2CO3 and In2S3 crystal powder, the peaks of Bi 4f and In 3d in 10% IS/BOC heterojunction showed slight shift towards to lower binding energies, which attributed to the interfacial charge transfer of Bi2O2CO3 and In2S3[61]. Peaks at 530.2, 531.3 and 533.0 eV were obtained from the O 1s spectrum after fitting, and they could be fitted by the characteristic peaks of Bi-O bonds in [Bi2O2] layers, CO32- and O-H bonds, respectively [62]. In Fig. 3(e), the peak located at 284.5 eV was caused by adventitious carbon, and the fitted peaks at 285.8 and 288.5 eV corresponded to CO32- in Bi2O2CO3 [63,64]. As shown in Fig. 3(f), the peaks at 161.2 and 162.5 eV could be contributed to the binding energies of the S 2p3/2 and S 2p in In2S3 [65]. The XPS results further displayed the strong interaction 7

between In2S3 and Bi2O2CO3, demonstrating the formation of IS/BOC heterojunction. 3.3 Optical properties

Fig. 4. (a) UV-vis DRS spectra of Bi2O2CO3, In2S3, and IS/BOC photocatalysts; (b) Band gaps (Eg) of Bi2O2CO3 and In2S3 calculated from the DRS data. The UV-vis diffraction reflectance spectra of the different samples were compared in Fig. 4 (a). The visible light response of pure Bi2O2CO3 was limited with a clear absorption edge at about 380 nm, whereas In2S3 exhibited wider optical response range with the absorption edge at about 600 nm. It can be seen that the IS/BOC composites exhibit obvious red-shift in comparison with pure Bi2O2CO3, which was primarily attributed to In2S3 nanosheets loaded on the surface of Bi2O2CO3 nanosheets. The narrow band gap semiconductors always play the role of light harvesting. Additionally, the band gap energy of Bi2O2CO3 and In2S3 could be calculated by [66]: Ahν = α (hν – Eg) n/2 Where A is the absorption coefficient, h is the Planck’s constant, ν, α and Eg are the light frequency, proportionality constant and band gap energy, respectively. According to previous report, In2S3 and Bi2O2CO3 were direct and indirect band gap semiconductor and their values of n is 1 and 4, respectively. Thus, the band gap of Bi2O2CO3 was about 3.2 eV according to a plot of (αhν)1/2 against hν to the energy axis. The band gap of In2S3 is found to be approximate 1.9 eV extracted from the plots of (αhν)2 against hν to the energy axis (Fig. 4b). This results were quite consistent with the previous results [67,68]. Furthermore, the adsorption edge in diffraction reflectance spectrum generally means the transition of charges from ground state to excited state. The results in Fig. 4a shows that the transition energy of charges in IS/BOC composites is smaller than both Bi2O2CO3 and In2S3, implied that the charges transfer is not only existed in individual semiconductor which was further discussed in the following sections.

8

Fig. 5. PL spectra of Bi2O2CO3 and IS/BOC heterojunction. Photoluminescence (PL) was introduced to demonstrate the separation of the photogenerated electron-hole pairs of the samples. In general, the higher PL intensity implies the larger recombination rate. The PL spectra for the Bi2O2CO3 nanosheets and IS/BOC heterojunction were obtained with an excitation light of 280 nm (Fig. 5). The peak appeared at around 370 nm attributed to the recombination of charges in the valance band of Bi2O2CO3, which decreased gradually with the increase of In2S3 mass percentage. However, 10% IS/BOC composites exhibited the lowest peak intensity and the PL intensity raised again with the further increasing of the In2S3 content, suggesting that the recombination of electron-hole pairs can be efficiently suppressed by the 2D/2D IS/BOC heterojunction that is helpful to improve the photocatalytic activity showed afterwards.

3.4. Photocatalytic property and mechanism

9

Fig. 6. Photocatalytic performance of Bi2O2CO3 nanosheets, In2S3 nanosheets and IS/BOC heterojunctions. The degradation rates of (a) RhB for different samples in degradation percentage and (b) Kinetics of the RhB decomposition over different samples. The degradation rates of (c) TC for different samples in degradation percentage and (d) Kinetics of the TC decomposition over different samples. (e) Cycling test results of 10% IS/BOC for the degradation of RhB. Table 1. Specific surface area of In2S3/ Bi2O2CO3 composite catalysts.

Photocatalysts

Bi2O2CO3

5% IS/BOC

10% IS/BOC

20% IS/BOC

30% IS/BOC

In2S3

Surface area (m2.g-1)

25.84

23.10

23.21

20.72

20.1243

15.92

10

The photocatalytic degradation of RhB and TC was employed to evaluate the photocatalytic performance of the samples. Fig. 6a compares the RhB photodegradation curves over the In2S3, Bi2O2CO3 and 2D/2D IS/BOC heterojunction. As it can be seen in this plots, the adsorption of RhB in dark shows that the adsorption ability towards RhB molecular of the tested samples did not exhibit significant difference. After turn on the light, Bi2O2CO3 nanosheets seldom showed efficient photocatalytic activity, whereas the bare In2S3 sample could remove 47% RhB by photodegradation. However, all the heterojunction samples clearly exhibited the enhanced photocatalytic capability towards the photocatalytic degradation of RhB. Particularly, the sample of 10% IS/BOC sample exhibited more excellent photocatalytic activities and the RhB molecular could be almost eliminated in 60 min. The kinetics of the RhB decomposition over the samples were also discussed. A pseudo-firstorder kinetic model was used to fit the degradation data using In(C0/C) =kt + a, where k is the apparent reaction rate constant. The k values of the corresponding samples for the degradation of RhB were calculated and listed( Fig. 6b). It is clear that the 10% IS/BOC sample exhibited the fastest photodegradation rate which was approximately 8 times in comparison with that of pure Bi2O2CO3. The BET results (table 1) revealed that the specific area of IS/BOC composites had no distinctly change than that of pure Bi2O2CO3. It is considered that the specific area in the amount of absorbed RhB was not the main reason but the formation of heterojunction structures between In2S3 and Bi2O2CO3 contributes most to its outstanding performance. Moreover, further increasing the In2S3 on the Bi2O2CO3 led to decrease of the photocatalytic performance. This finding may implied the following aspects: Firstly, excess In2S3 could cover the active sites on the Bi2O2CO3 surface and hinder the charge-separation efficiency. Secondly, excessive In2S3 might act as the recombination centers of the photogenerated electron-holes. TC was also used to evaluate the photocatalytic performance of the prepared samples. As shown in Fig. 6c, the 10% IS/BOC heterojunction also showed the best photocatalytic activity. The result showed after 3h light irradiation, about 70% TC was decomposed. The findings indicated that 2D/2D heterojunction formed by Bi2O2CO3 and In2S3 nanosheets effectively promoted the photocatalytic capability of each individual. The kinetics of the TC decomposition over the samples (Fig. 6d) displayed the 10% IS/BOC sample had best photocatalytic activity, which was about 9 times in comparison with that of pure Bi2O2CO3 and further increasing the In2S3 on the Bi2O2CO3 led to decrease photocatalytic performance, the reason might be the same as RhB degradation. Moreover, the stability of the photocatalyst is also as important as the excellent photocatalytic activity. To evaluate the durable performance of IS/BOC heterojunction, recycling experiments were carried out for RhB degradation, and the results are shown in Fig. 6(e). After four successive cycles, it still maintained satisfied activity, suggesting that the IS/BOC heterojunction photocatalyst was

11

stability and durability.

Fig. 7. Active species trapping experiments of 10% IS/BOC for the photodegradation of RhB Although the results of PL already demonstrated the restricted photogenerated electron-hole pairs, it is not enough to reveal the mechanism of enhanced photocatalytic activity of IS/BOC heterojunction. Many of photo induced reactive species such as •O2−, •OH and h+ would be generated during the photocatalytic process. To better understand the detail function of each reactive species produced in the photocatalysis system, 1 mM benzoquinone (BQ), 10 mM isopropanol (IPA,) and 5 mM Na2C2O4 was introduced as the quenchers for •O2−, OH, and h+, respectively, in the RhB degradation experiment under light irradiation. As showed in Fig. 7, the photodegradation rate of RhB is 91% in absence of any trapping agent, and the degradation rate of RhB is obviously suppressed after the addition of IPA (28%), implying that •OH is one of the active species in the photodegradation process. Particularly, photocatalytic efficiency declined dramatically after BQ and Na2C2O4 were added to the reaction solution, conforming that both •O2− and h+ play significant roles in the photodegradation process. Therefore, •OH, •O2− and h+ are all the main active species after IS/BOC heterojunction being excited by light irradiation. Meanwhile, the chemical potential of conductive band (CB) and valance band (VB) of In2S3 and Bi2O2CO3 can be calculated as the following formula [69]. EVB = - Ee + 0.5 Eg ECB = EVB - Eg Where ECB and EVB are the edge potentials of the conduction and valence band, respectively, Ee is the free electron energy in the hydrogen scale (about 4.5 eV), Eg and  are the band gap (obtained from DRS in Fig. 4) and electronegativity of the semiconductor, respectively, and the 12

calculation results are illustrated in Fig. 8. The observation of h+ and •OH as main active species suggests that most of the photogenerated holes still stay in the VB of Bi2O2CO3 because of its strong oxidation capability. Meanwhile, due to the weak reduction ability of Bi2O2CO3 conductive band, the generation of •O2− species are derived from the reduction of In2S3 conductive band and seldom photogenerated electrons transfer to the CB of Bi2O2CO3, which does not follow the conventional type II heterojunction [70,71]. Contrarily, the S-scheme heterojunction mechanism discussed below can better explain the enhancement of photocatalytic capability of IS/BOC heterojunction. Fig. 8 shows the detail charge transfer mechanism of S-scheme heterojunction between Bi2O2CO3 and In2S3. Generally, In2S3 is a n-type semiconductor with higher Fermi level and act as a reduction photocatalyst in the heterojunction. The heterojunction irradiated by light, both Bi2O2CO3 and In2S3 can be excited and the photogenerated electrons jump from VB to CB as shown in Fig. 8. whereas Bi2O2CO3 act as an oxidation photocatalyst with lower Fermi level [72,73]. After Bi2O2CO3 and In2S3 are in close contact and form the heterojunction, the electron which excited from the VB of In2S3 transfer to the VB of Bi2O2CO3 spontaneously until their Fermi level are equal, an internal electric field established across the interface via the variation process of Fermi level [50]. Subsequently, driven by the above suggested internal electric field, the electrons in Bi2O2CO3 (from CB) transfer to In2S3 and recombine with the holes in the VB of In2S3. As a result, this kind of Sscheme heterojunction eliminate the weak reductive electrons in CB of Bi2O2CO3 and the weak oxidative holes in VB of In2S3 and suppress the recombination of the more active photogenerated electrons and holes. Meanwhile, the 2D/2D structure further provide an effective contact modality of two photocatalysts, leading excellent photocatalytic capability towards RhB and TC degradation in water.

13

Fig. 8. Sketch of the S-scheme heterojunction mechanism towards the charge transfer between Bi2O2CO3 and In2S3 under light irradiation.

4. Conclusion In summary, 2D/2D IS/BOC S-scheme heterojunction photocatalyst was designed and fabricated through a facile two-step precipitation method. The photocatalytic degradation of RhB and TC was improved due to the fabrication of S-scheme heterojunction which effectively suppress the recombination of useful electrons and holes and eliminate the useless electrons and holes. The heterojunction of 10 wt% IS/BOC exhibited 91% degradation capacity towards RhB in 60 min, which was 5 and 3 times higher than that of pure Bi2O2CO3 and In2S3, respectively. The current work may pave a new way to fabricate this type highly efficient S-scheme heterojunction with enhanced photocatalytic activity.

Acknowledgment We gratefully acknowledge the financial support provided by the Project of the National Natural Science Foundation of China (Grant No. 21271022) and the Program for Scientific Research Innovation Team in Colleges and Universities of Jinan (No. 2018GXRC006).

14

References [1] Z.L. Wang, T.P. Hu, K. Dai, J.F. Zhang, C.H. Liang, Construction of Z‐ scheme Ag3PO4/Bi2WO6 composite with excellent visible‐ light photodegradation activity for removal of organic Contaminants, Chin. J. Catal. 38 (2017) 2021-2029. [2] P.F. Xia, B.C. Zhu, B. Cheng, J.G. Yu, J.S. Xu, 2D/2D g‑ C3N4/MnO2 nanocomposite as a direct Z‑ Scheme photocatalyst for enhanced photocatalytic activity, ACS Sustainable Chem. Eng. 6 (2018) 965-973. [3] L.P. Zhang, M. Jaroniec, Toward designing semiconductor-semiconductor heterojunctions for photocatalytic applications, Appl. Surf. Sci. 430 (2018) 2-17. [4] B.T. Jiang, S.Y. Zhang, X.Z. Guo, B.T. Jin, Y.P. Tian, Preparation and photocatalytic activity of CeO2/TiO2 interface composite film, Appl. Surf. Sci. 255 (2009) 5975-5978. [5] J. Luo, X.S. Zhou, L. Ma, X.Y. Xu, J.X. Wu, H.P. Liang, Enhanced photodegradation activity of methyl orange over Ag2CrO4/SnS2 composites under visible light irradiation, Mater. Res. Bull. 77 (2016) 291-299. [6] X.K. Zeng, Z.Y. Wang，G. Wang，T.R. Gengenbach, D.T. McCarthy, A. Deletic, J.G. Yu, X.W. Zhang, Highly dispersed TiO2 nanocrystals and WO3 nanorods on reduced graphene oxide: Zscheme photocatalysis system for accelerated photocatalytic water disinfection, Appl. Catal. B: Environ. 218 (2017) 163-173. [7] T. Ahmed, H.L. Zhang, Y.Y. Gao, H.B. Xu, Y. Zhang, Surfactant-free synthesis of m-BiVO4 nanoribbons and enhanced visible-light photocatalytic properties, Mater. Res. Bull. 99 (2018) 298-305. [8] Y. Ye, Z.G. Zang, T.W. Zhou, F. Dong, S.R. Lu, X.S. Tang, W. Wei, Y.B. Zhang, Theoretical and experimental investigation of highly photocatalytic performance of CuInZnS nanoporous structure for removing the NO gas, J. Catal. 357 (2018) 100-107. [9] W.P. Zhang, X.Y. Xiao, Y. Li, X.Y. Zeng, L.L. Zheng, C.X. Wan, Liquid-exfoliation of layered MoS2 for enhancing photocatalytic activity of TiO2/g-C3N4 photocatalyst and DFT study, Appl. Surf. Sci. 389 (2016) 496-506. [10] B.C. Zhu, P.F. Xia, Y. Li, W.K. Ho, J.G. Yu, Fabrication and photocatalytic activity enhanced mechanism of direct Z-scheme g-C3N4/Ag2WO4 photocatalyst, Appl. Surf. Sci. 391 (2017) 175-183. [11] A.M. Yang, Y. Han, S.S. Li, H.W. Xing, Y.H. Pan, W.X. Liu, Synthesis and comparison of photocatalytic properties for Bi2WO6 nanofifibers and hierarchical microspheres, J. Alloys Compd. 695 (2017) 915-921. [12] Z.M. Al-Amshany, M.A. Hussein, Novel Pd/ZnWO4 nanocomposite materials for photocatalytic degradation of atrazine, Appl. Nanosci. 8 (2018) 527-536. 15

[13] A. Phuruangrat, P. Dumrongrojthanath, S. Thongtem, T. Thongtem, Influence of Dy dopant on photocatalytic properties of Dy-doped ZnWO4 nanorods, Mater. Lett. 166 (2016) 183-187. [14] S. Zarezadeh, A. Habibia-Yangjeh, M. Mousavi, BiOBr and AgBr co-modified ZnO photocatalyst: A novel nanocomposite with p-n-n heterojunctions for highly effective photocatalytic removal of organic contaminants, J. Photochem. Photobiol. A 379 (2019) 11-23. [15] Y.T. Liu, T. Cai, L.L. Wang, S.Q. Zhang, G.Z. Zhang, X.N. Xia, Hollow microsphere TiO2/ZnO p-n heterojunction with high photocatalytic performance for 2,4-Dinitropheno Mineralization, Nano 12 (2017) 1750076. [16] M.Y. Lu, J.H. Song, M.P. Lu, C.Y. Lee, L.J. Chen, Z.L. Wang, ZnO-ZnS heterojunction and ZnS nanowire arrays for electricity generation, ASC Nano 3 (2009) 357-362. [17] P.Y.Kuang, L.Y. Zhang, B. Cheng, J.G. Yu, Enhanced charge transfer kinetics of Fe2O3/CdS composite nanorod arrays using cobalt-phosphate as cocatalyst, Appl. Catal. B 218 (2017) 570580. [18] M. Yan, Y.Q. Hua, F.F. Zhu, W. Gu, J.H. Jiang, H.Q. Shen, W.D. Shi, Fabrication of nitrogen doped graphene quantum dots-BiOI/MnNb2O6 p-n junction photocatalysts with enhanced visible light efficiency in photocatalytic degradation of antibiotics, Appl. Catal. B 202 (2017) 518-527. [19] R. Saravanan, S. Karthikeyan, V.K. Gupta, G. Sekaran, V. Narayanan, A. Stephen, Enhanced photocatalytic activity of ZnO/CuO nanocomposite for the degradation of textile dye on visible light illumination, Mater. Sci. Eng. C 33 (2013) 91-98. [20] Y.H. Yan, H.Y. Guan, S. Liu, R.Y. Jiang, Ag3PO4/Fe2O3 composite photocatalysts with an n-n heterojunction semiconductor structure under visible-light irradiation, Ceram. Int.

40

(2014) 9095-9100. [21] T.X. Liu, B.X. Li, Y.G. Hao, Z.Y. Yao, MoO3-nanowire membrane and Bi2Mo3O12/MoO3 nano-heterostructural photocatalyst for wastewater treatment, Chem. Eng. J. 244 (2014) 382390. [22] P. Junploy, A. Phuruangrat, N. Plubphon, S. Thongtem, T. Thongtem, Photocatalytic degradation of methylene blue by Zn2SnO4-SnO2 system under UV visible radiation, Mater. Sci. Semicon. Procond. Process. 66 (2017) 56-61. [23] R.A. He, J.Q. Zhou, H.Q. Fu, S.Y. Zhang, C.J. Jiang, Room-temperature in situ fabrication of Bi2O3/g-C3N4 direct Z-scheme photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 430 (2018) 273-282. [24] Z.F. Bian, J. Zhu, S.H. Wang, Y. Cao, X.F Qian, H.X. Li, Self-Assembly of Active Bi2O3/TiO2 Visible Photocatalyst with Ordered Mesoporous Structure and Highly Crystallized Anatase. J. Phys. Chem. C 112 (2008) 6258-6262.

16

[25] S. Balachandran, M. Swaminathan, Facile Fabrication of Heterostructured Bi2O3-ZnO Photocatalyst and Its Enhanced Photocatalytic Activity, J. Phys. Chem. C 116 (2012) 2630626312. [26] Z.L. Wang, J.L. Lv, J.F. Zhang, K. Dai, C.H. Liang, Facile synthesis of Z-scheme BiVO4/porous graphite carbon nitride heterojunction for enhanced visible-light-driven photocatalyst, Appl. Surf. Sci. 430 (2018) 595-602. [27] N. Tian, H.W. Huang, Y. He, Y.X. Guo, T.R. Zhang, Y.H. Zhang, Mediator-free direct Zscheme photocatalytic system: BiVO4/g-C3N4 organic–inorganic hybrid photocatalyst with highly efficient visible-light-induced photocatalytic activity. Dalton Trans. 44 (2015) 4297-4307. [28] F.Q. Zhou, J.C. Fan, Q.J. Xu, Y.L. Min, BiVO4 nanowires decorated with CdS nanoparticles as Z-scheme photocatalyst with enhanced H2 generation. Appl. Catal. B 201 (2017) 77-83. [29] L.Q. Ye, L. Zan, L.H. Tian, T.Y. Peng, J.J. Zhang, The {001} facets-dependent high photoactivity of BiOCl nanosheets, Chem. Commun. 47 (2011) 6951-6953. [30] F.Z. Qiu, W.J. Li, F.Z. Wang, H.D. Li, X.T. Liu, C.J. Ren, Preparation of novel p-n heterojunction Bi2O2CO3/BiOBr photocatalysts with enhanced visible light photocatalytic activity, Colloids Surf. A: 517 (2017) 25-32. [31] L. Chen, S.F. Yin, S.L. Luo, R. Huang, Q. Zhang, T. Hong, Peter C.T. Au, Bi2O2CO3/BiOI photocatalysts with heterojunctions highly efficient for visible-light treatment of dyecontaining wastewater, Ind. Eng. Chem. Res. 51 (2012) 6760-6768. [32] Y.N. Su, G.Q. Tan, T. Liu, L. Lv, Y. Wang, X.L Zhang, Z.W. Yue, H.J. Ren, A. Xia, Photocatalytic properties of Bi2WO6/BiPO4 Z-scheme photocatalysts induced by double internal electric fields, Appl. Surf. Sci. 457 (2018) 104-114. [33] D. Ma, J. Wu, M.C. Gao, Y.J. Xin, Y.Y. Sun, Fabrication of Z-scheme g-C3N4/RGO/Bi2WO6 photocatalyst with enhanced visible-light photocatalytic activity, Chem. Eng. J. 290 (2016) 136-146. [34] N. Liang, J.T. Zai, M. Xu, Q. Zhu, X. Wei, X.F. Qian, Novel Bi2S3/Bi2O2CO3 heterojunction photocatalysts with enhanced visible light responsive activity and wastewater treatment, J. Mater. Chem. A 2 (2014) 4208-4216. [35] A. Helal, F.A. Harraz, A.A. Ismail, T.M. Sami, I.A. Ibrahim, Hydrothermal synthesis of novel heterostructured Fe2O3/Bi2S3 nanorods with enhanced photocatalytic activity under visible light, Appl. Catal. B 213 (2017) 18-27. [36] T.P. Hu, Y. Yang, K. Dai, J.F. Zhang, C.H. Liang, A novel Z-scheme Bi2MoO6/BiOBr photocatalyst for enhanced photocatalytic activity under visible light irradiation, Appl. Surf. Sci. 456 (2018) 473-481. [37] Y.J. Chen, G.H. Tian, Y.H. Shi, Y.T. Xiao, H.G. Fu, Hierarchical MoS 2 /Bi 2 MoO 6

17

Composites with synergistic effect for enhanced visible photocatalytic activity, Appl. Catal. B 164 (2015) 40-47. [38] H.B. Fu, L.W. Zhang, W.Q. Yao, Y.F. Zhu, Photocatalytic properties of nanosized Bi2WO6 catalysts synthesized via a hydrothermal process, Appl. Catal. B 66 (2016 ) 100-110. [39] Z.H. Sun, J.J. Guo, S.M. Zhu, L. Mao, J. Ma, D. Zhang, A high-performance Bi2WO6-graphene photocatalyst for visible light-induced H2 and O2 generation, Nanoscale 6 (2014) 2186-2193. [40] J.S. Chen, S.Y. Qin, G.X. Song, T.Y. Xiang, F. Xin, X.H. Yin, Shape-controlled solvothermal synthesis of Bi2S3 for photocatalytic reduction of CO2 to methyl formate in methanol, Dalton Trans. 42 (2013) 15133-15138. [41] R.A. He, S.W. Cao, J.G. Yu, Recent advances in morphology control and surface modification of Bi-based photocatalysts, Acta Phys. Chim. Sin. 32 (2016) 2841-2870. [42] J. Chen, W.G. Mei, Q.J. Huang, N.N. Chen, C.L. Lu, H.J. Zhu, J. Chen, W.H. Hou, Highly efficient three-dimensional flower-like AgI/Bi2O2CO3 heterojunction with enhanced photocatalytic performance, J. Alloys Compd. 688 (2016) 225-234. [43] P. Madhusudan, J. G, W.G. Wang, B. Cheng, G. Liu, Facile synthesis of novel hierarchical graphene-Bi2O2CO3 composites with enhanced photocatalytic performance under visible light, Dalton Trans. 41 (2012) 14345-14353. [44] P. Madhusudana, J.R. Ran, J. Zhang, J.G. Yu, G. Liu, Novel urea assisted hydrothermal synthesis of hierarchical BiVO4/Bi2O2CO3 nanocomposites with enhanced visible-light photocatalytic activity, Appl. Catal. B 110 (2011) 286-295. [45] H.J. Lu, L.L. Xu, B. Wei, M.Y. Zhang, H. Gao, W.J. Sun, Enhanced photosensitization process induced by the p–n junction of Bi2O2CO3/BiOCl heterojunctions on the degradation of Rhodamine B, Appl. Surf. Sci. 303 (2014) 360-366. [46] N. Tian, H.W. Huang, Y.X. Guo, Y. He, Y.H. Zhang, Ag-C3N4/Bi2O2CO3 composite with high visible-light-driven photocatalytic activity for Rhodamine B degradation, Appl. Surf. Sci. 322 (2014) 249-254. [47] Y.H. Ao, L.Y. Xu, P.F. Wang, C. Wang, J. Hou, J. Qian, Preparation of CdS nanoparticle loaded flower-like Bi2O2CO3 heterojunction photocatalysts with enhanced visible light photocatalytic activity, Dalton Trans. 44 (2015) 11321-11330. [48] H.X. Fan, H.L. Zhou, H.D.Li, X.T. Liu, C.J. Ren, F.Z. Wang, W.J. Li, Novel Ag2CrO4/Bi2O2CO3 heterojunction: Simple preparation, wide visible-light absorption band and excellent photocatalytic activity, Chem. Phys. 517 (2019) 60-66. [49] Q. Z. Wang, G.X. Yun, Y. Bai, N. An, J.h. Lian, H.H. Huang, B.T. Su, Photodegradation of Rhodamine B with MoS2/Bi2O2CO3 composites under UV light irradiation, Appl. Surf. Sci. 313 (2014) 537-544.

18

[50] J.W. Fu, Q.L. Xu, J.X. Low, C.J. Jiang, J.G. Yu, Ultrathin 2D/2D WO3/g-C3N4 step-scheme H2production photocatalyst, Appl. Catal. B 243 (2019) 556-565. [51] W.L. Yu, J.X. Chen, T.T. Shang, L.F. Chen, L. Gu, T.Y. Peng, Direct Z-scheme g-C3N4/WO3 photocatalyst with atomically definedjunction for H2 production, Appl. Catal. B 219 (2017) 693-704. [52] W.L. Yu, S. Zhang, J.X. Chen, P.F. Xia, M.H. Richter, L.F. Chen, W. Xu, J.P. Jin, S.L. Chen, T.Y. Peng, Biomimetic Z-scheme photocatalyst with a tandem solid-state electron flow catalyzing H2 evolution, J. Mater. Chem. A 6 (2018) 15668-15674. [53] T.M. Di, Q.L. Xu, W. Ho, H. Tang, Q.J. Xiang, J.G. Yu, Review on Metal Sulphide-based Zscheme Photocatalysts, ChemCatChem 11(2019)1394-1411. [54] J.X. Low, B.Z. Dai, T. Tong, C.J. Jiang, J.G. Yu, In Situ Irradiated X-Ray photoelectron spectroscopy investigation on direct Z-scheme TiO2/CdS composite film photocatalyst, Adv. Mater. 30 (2019)1802981. [55] W.M. Qiu, M.S. Xu, X. Yang, F. Chen, Y.X. Nan, J.L. Zhang, H. Iwai, H.Z. Chen, Biomoleculeassisted hydrothermal synthesis of In2S3 porous films and enhanced photocatalytic properties, J. Mater. Chem. 21 (2011) 13327-13333. [56] J. Chen, W.X. Liu, W.W. Gao, Tuning photocatalytic activity of In2S3 broadband spectrum photocatalyst based on morphology, Appl. Surf. Sci. 368 (2016) 288-297. [57] F.Z. Wang, W.J. Li, S.N. Gu, H.D. Li, H.L. Zhou, X.B. Wu, Novel In2S3/ZnWO4 heterojunction photocatalysts: facile synthesis and high-efficiency visible-light-driven photocatalytic activity, RSC Adv. 5 (2015) 89940-89950. [58] F. Dong, Y.J. Sun, M. Fu, W.K. Ho, S.C. Lee, Z.B. Wu, Novel in situ N- doped (BiO)2CO3 hierarchical microspheres self-assembled by nanosheets as efficient and durable visible light driven photocatalyst, Langmuir 28 (2012) 766-773. [59] L.M. Yang, G.Y. Zhang, H.R. Wang, X. Bai, X.Q. Shen, J.W. Liu, D.Z. Gao, Mild synthesis of {001} facet predominated Bi2O2CO3 clusters with outstanding simulated sunlight photocatalytic activities, CrystEngComm. 18 (2016) 3683-3695. [60] X.L. Fu, X.X. Wang, Z.X. Chen, Z.Z. Zhang, Z.H. Li, Dennis Y.C. Leung, L. Wu, X.Z. Fu, Photocatalytic performance of tetragonal and cubic β-In2S3 for the water splitting under visible light irradiation, Appl. Catal. B 95 (2010) 393-399. [61] L. Li, J.M. Cai, Y.H. Yan, F.Y. Zhao, J.G. Zhou, Flower-like direct Z-scheme WS2/Bi2O2CO3 photocatalyst with enhanced photocatalytic activity, J. Alloys Compd. 810 (2019) 151872. [62] L.H. Yu, X.Y. Zhang, G.W. Li, Y.T. Cao, Y. Shao, D.Z. Li, Highly efficient Bi2O2CO3/BiOCl photocatalyst based on heterojunction with enhanced dye-sensitization under visible light, Appl. Catal. B 187 (2016) 301-309. 19

[63] Y.Y. Zhang, L.L. Wang, F.Y. Dong, Q. Chen, H.Y. Jiang, M. Xu, J.S. Shi, Non-additional carbon source one-step synthesis of Bi2O2CO3-based ternary composite for effificient Zscheme photocatalysis, J. Colloid Interf. Sci. 536 (2019) 575-585. [64] G.Y. Zhang, J.J. Wang, X.Q. Shen, J.J. Wang, B.Y. Wang, D.Z. Gao, Br-doped Bi2O2CO3 nanosheets with improved electronic structure and accelerated charge migration for outstanding photocatalytic behavior, Appl. Surf. Sci. 470(2019) 63-73. [65] S. Rengaraj, S. Venkataraj, C.W. Tai, Y. Kim, E. Repo, M. Sillanpaa. Self-assembled mesoporous hierarchical-like In2S3 hollow microspheres composed of nanofibers and nanosheets and their photocatalytic activity, Langmuir 27 (2011) 5534-5541. [66] X.S. Wu, Y.D. Hu, Y. Wang, Y.S. Zhou, Z.H. Han, X.L. Jin, G. Chen, In-situ synthesis of Zscheme Ag2CO3/Ag/AgNCO heterojunction photocatalyst with enhanced stability and photocatalytic activity, Appl. Surf. Sci. 464 (2019) 108-114. [67] Y.C. Huang, W.J. Fan, B. Long, H.B. Li, F.Y. Zhao, Z.L. Liu, Y.X. Tong, H.B. Jia, Visible light Bi2S3/Bi2O3/Bi2O2CO3 photocatalyst for effective degradation of organic pollutions, Appl. Catal. B 185 (2016) 68-76. [68] X. Yang, J. Xu, T.L. Wong, Q.D. Yang, C.S. Lee, Synthesis of In2O3-In2S3 core–shell nanorods with inverted type-I structure for photocatalytic H2 generation, Phys. Chem. Chem. Phys. 15 (2013) 12688-12693. [69] H.S. EL-Sheshtawy, H.M. El-Hosainy, K.R. Shoueir, I.M. El-Mehasseb, M. El-Kemary, Facile immobilization of Ag nanoparticles on g-C3N4/V2O5 surface for enhancement of postillumination, catalytic, and photocatalytic activity removal of organic and inorganic pollutants, Appl. Surf. Sci. 467- 468 (2019) 268-276. [70] P. Zhou, J.G. Yu, M. Jaroniec, All-Solid-State Z-Scheme Photocatalytic Systems, Adv. Mater. 26 (2014) 4920-4935. [71] Q.L. Xu, L.Y. Zhang, J.G. Yu, S. Wageh, A.A. Al-Ghamdi, M. Jaroniec, Direct Z-scheme photocatalysts: Principles, synthesis, and applications, Materials Today. 21 (2018) 1042-1063. [72] M. Bär, N. Barreau, F. Couzinié-Devy, L. Weinhardt, R.G. Wilks, J. Kessler, C. Heske, Impact of Annealing-Induced Intermixing on the Electronic Level Alignment at the In2S3/Cu (In, Ga) Se2 Thin-Film Solar Cell Interface, ACS Appl. Mater. Interf. 8 (2016) 2120-2124. [73] L.L. Zhang L, C. Hu, H.H. Ji, p-AgI anchored on {001} facets of n-Bi2O2CO3 sheets with enhanced photocatalytic activity and stability. Appl. Catal. B 205 (2017) 34-41.

20

Photocatalytic mechanism and electron transfer mode of 2D/2D StepScheme In2S3/Bi2O2CO3 Heterojunction.

21

Highlights 1. Step scheme (S-scheme) heterojunction is one of the newly effective approaches to promote the photocatalytic performance of coupled semiconductors 2. 2D/2D In2S3/Bi2O2CO3 S-scheme heterojunction photocatalyst was designed and fabricated. 3. The internal electric field of S-scheme heterojunction not only efficiently restrain the recombination of photogenerated charges but also enlarge the redox capability of the 2D/2D In2S3/Bi2O2CO3. 4. The designed S-scheme heterojunction showed very enhanced photocatalytic degradation towards RhB and tetracycline (TC).

22

The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

23