BiOCl heterojunctions with enhanced separation efficiency of photo-generated charge pairs and photocatalytic performance

Inorganic Chemistry Communications 113 (2020) 107806

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Ionic liquid-assisted hydrothermal preparation of BiOI/BiOCl heterojunctions with enhanced separation efficiency of photo-generated charge pairs and photocatalytic performance

T

Huanhuan Liua, Cai Yanga, Jiao Huanga, Jiufu Chena, Junbo Zhonga,b, , Jianzhang Lia, ⁎

⁎

a

Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environmental Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China b College of Chemical Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: BiOI/BiOCl Heterojunction Charge pairs separation efficiency Ionic liquid

In this contribution, BiOI/BiOCl heterojunctions were successfully synthesized with the assistance of reactable ionic liquid (IL) 1-propyl-3-methylimidazolium iodide ([PrMIm]I) via a hydrothermal method. The crystal structures, morphologies, chemical compositions, optical properties and separation efficiency of photo-generated charge pairs of the as-prepared BiOI/BiOCl heterojunctions were characterized by various analytic methods. Surface photovoltage spectroscopy (SPS) and photoelectrochemical measurements reveal that the BiOI/BiOCl heterojunctions exhibit higher separation efficiency of photo-generated charge pairs than the reference BiOCl and BiOI. XPS results reveal that I element exists in the as-prepared samples with a chemical valence of −1, and when the mass ratio of [PrMIm]I/BiOCl is 8%, the sample holds the highest surface hydroxyl content. Photocatalytic performance of the as-fabricated photocatalysts was evaluated by abatement of rhodamine B (RhB) aqueous solution under simulated sunlight irradiation. BiOI/BiOCl heterojunctions exhibit enhanced photocatalytic performance; the 8% sample displays the highest photocatalytic activity. Trapping experiments certify that the superoxide radicals (%O2−) is the predominant active free radicals in the photocatalytic degradation process of RhB over the BiOI/BiOCl heterojunctions. In light of experimental results, a possible Z-

⁎ Corresponding authors at: Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environmental Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China (J. Zhong). E-mail addresses: [email protected] (J. Zhong), [email protected] (J. Li).

https://doi.org/10.1016/j.inoche.2020.107806 Received 14 November 2019; Received in revised form 16 January 2020; Accepted 20 January 2020 Available online 21 January 2020 1387-7003/ © 2020 Elsevier B.V. All rights reserved.

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scheme photocatalytic degradation mechanism was discussed to elaborate the separation and migration of the photo-induced electron-hole pairs. This study provides a useful reference for ILs assisted fabrication of high efficiency photocatalytic materials.

1. Introduction

purchased from Chengdu Kelong Chemical Reagents Factory (Chengdu, China). Ionic liquid 1-propyl-3-methylimidazolium iodide ([PrMIm]I) was provided by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. For synthesis of BiOI/BiOCl heterojunctions, desired [PrMIm]I was dissolved into 40 mL glacial acetic acid to obtain solution A. 0.02 mol Bi(NO3)3·5H2O was added into solution A and magnetic stirring to acquire suspension solution B. 0.02 mol KCl was dissolved into 10 mL deionized water to obtain solution C. Solution C was dropwise added into suspension solution B under intensely stirring, then the suspension system was continuously stirred for 1 h. Finally, the suspension system was transferred into a 100 mL Teflon-lined stainlesssteel autoclave, and treated at 453 K for 24 h. After hydrothermal treatment, the autoclave was taken out and naturally cooled to room temperature. The samples were acquired by filtration and washed with deionized water and absolute ethanol for several times, and then the asprepared products were dispersed into absolute ethanol and dried in an oven at 333 K for 12 h to obtain BiOI/BiOCl heterojunctions. The BiOI/ BiOCl heterojunctions with different mass ratios of [PrMIm]I/BiOCl were denoted as 0% (BiOCl), 2%, 4%, 6%, 8% and 10%, respectively.

Over the past few years, water pollution problem is an enormous threat for human being [1–6]. As a promising and environmentalfriendly technology, photocatalysis has been widely applied in elimination of organic contaminants [7–15]. Among the photocatalysts, as a novel semiconductor photocatalyst, bismuth oxychloride (BiOCl) has attracted considerable attention owing to its low cost, non-toxicity and excellent photocatalytic degradation efficiency toward organic pollutants [16,17]. More important, BiOCl possesses a unique layered structure, forming an internal static electric field between the [Bi2O2]2+ layer and double [Cl]− layers, which is enable to effectively separate and migrate the photo-induced electron-hole pairs, resulting in excellent photocatalytic performance [18–22]. However, the activity of BiOCl photocatalyst is far from to meet the practical application owing to the high recombination rate of photo-generated electron-hole pairs [23–25]. To address this bottleneck, it is necessary to develop effective strategy to improve the photocatalytic performance of BiOCl [26]. Up to now, tremendous approaches, such as, controlling morphologies [27,28], introducing oxygen vacancies [29,30], doping of ions [31,32], depositing of metals [33,34] and fabricating of heterojunctions [35,36] have been employed to facilitate the photocatalytic activity. Coupling BiOCl with other semiconductors to form heterojunctions is deemed as a favorable strategy to boost the separation efficiency of photo-generated charge pairs [37–43]. Recently, various BiOCl-based heterojunctions have been reported, such as TiO2/BiOCl [44,45] and Bi2S3/BiOCl [46], built of heterojunctions can effectively promote the separation rate of photo-induced charge pairs and photocatalytic performance of BiOCl. Nowadays, ionic liquids (ILs) have been broadly used to synthesis diverse inorganic nanomaterials due to their high thermal stability, ionic conductivity, low vapor pressure and excellent dissolving ability as well as transport properties [47–51]. ILs can be served as solvent, template, reactant, capping agent and dispersing agent [52–55]. Xia and co-workers successfully synthesized BiOCl with uniform flower-like microspheres and porous nanospheres structure in the presence of ILs through a one-pot solvothermal method, the results demonstrate that BiOCl porous nanospheres exhibit higher photocatalytic activity than BiOCl synthesized by traditional method [56]. Our previous work has well certified that BiOBr/BiOCl with a flower-like structure synthesized with the aid of ILs can dramatically enhance the photocatalytic performance [57]. Therefore, it is anticipated that the ionic liquid assisted hydrothermal preparation of BiOI/BiOCl can ameliorate photocatalytic performance of BiOCl. In this work, a hydrothermal method was employed to fabricate BiOI/BiOCl heterojunction photocatalysts with the assistance of reactable ionic liquid 1-propyl-3-methylimidazolium iodide ([PrMIm]I). During the preparation process, [PrMIm]I acts as solvent and provides I source for the heterojunctions. Photocatalytic performances of BiOI/ BiOCl heterojunctions were evaluated using RhB as target pollutants under simulated sunlight irradiation. SPS and photoelectrochemical results demonstrate that separation efficiency of photo-generated charge pairs of the BiOI/BiOCl heterojunctions has been greatly improved compared with the bare BiOCl. The leading active free radicals is %O2−, which was determined by the trapping experiments.

2.2. Characterization X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.15406) was used to characterize the crystal structures of the as-prepared photocatalysts. Scanning electron microscopy (SEM) and elemental mapping were operated on a VEGA-3-SBU instrument with an accelerating voltage of 15 kV. A trapping-mode atomic force microscopy (AFM) was conducted on an E-Sweep instrument. X-ray photoelectron spectroscopy (XPS) was determined on an XSAM 8000 with Mg Kα. UV–Vis diffuse reflectance spectra (UV–Vis DRS) were executed on a UH 4150 UV–Vis spectrophotometer using BaSO4 as a reflectance. SPS used to study the separation rate of photo-generated charge pairs on a home-made instrument as the procedure described in the Ref. [58]. In order to investigate the photoelectrochemical properties of the as-fabricated BiOI/ BiOCl heterojunctions, electrochemical impedance spectroscopy (EIS) and transient photocurrent response were conducted on an electrochemical workstation with three-electrode system (CHI 760E) using Pt and saturated calomel electrode as counter electrode and reference electrode, respectively. Light source is a 500 W Xe lamp, 0.1 mol/L Na2SO4 was used as electrolyte solution. Trapping experiments were performed to clarify the role of active radicals during the photocatalysis process. 2 mL (5 mmol/L) isopropyl alcohol (IPA), ammonium oxalate (AO) and benzoquinone (BQ) were added into photocatalytic reaction system to trapping hydroxyl radicals (%OH), holes (h+) and superoxide radicals (%O2−), respectively. The reaction condition is the same as the photocatalytic evaluation experiment. 2.3. Photocatalytic performance evaluation Photocatalytic activity of the as-fabricated BiOI/BiOCl heterojunctions was assessed by degradation of RhB aqueous solution under a 500 W Xe lamp illumination. 50 mg photocatalyst was dispersed into a quartz tube containing 50 mL RhB aqueous solution, the initial concentration of RhB was 10 mg·L−1. Before illumination, the suspension system was magnetically stirred for 30 min to reach the absorption–desorption equilibrium. At a given interval, 8 mL suspension was sampled, and then was centrifuged to remove the powder. The photocatalytic experiments were executed on a Phchem III photochemical reactor (Beijing NBET Technology Co., Ltd, China). All the photocatalytic reactions were carried out with a circulating system of water-

2. Experiment section 2.1. Fabrication of BiOI/BiOCl heterojunctions All chemicals used in this paper were analytical reagent and 2

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cooling to keep room temperature. The concentration of supernatant was measured at 554 nm on a GBC UV–Vis 916 spectrophotometer.

1s high resolution XPS spectra of the bare BiOCl, which can be allocated to oxygen of BieO on [Bi2O2]2+ slabs and the chemisorbed H2O or surface adsorbed eOH [65,66], respectively. Fig. 4E–I show the surface hydroxyl content of the 0%, 4%, 8%, 10% and bare BiOI samples. O 1s region curve-fitting results were presented in Table.1. As shown in Table.1, the BiOI/BiOCl heterojunction photocatalysts possess higher surface hydroxyl content than that of the bare BiOCl and BiOI. Commonly, surface hydroxyl of BiOCl can be oxidized by hole to form %OH radicals, thereby leading to boosted photocatalytic activity [67,68]. Consequently, the 8% sample exhibits the highest content of surface hydroxyl, corresponding to the highest photocatalytic activity, which is in good accordance well with the photocatalytic evaluation experiment. UV–Vis DRS spectra were employed to reveal the optical properties of BiOI/BiOCl heterojunctions. As displayed in Fig. 5A, the results imply that the existence of BiOI can effectively improve the visible light absorption ability of the heterojunctions. The absorption edges of the samples appear obvious red-shift after forming of BiOI/BiOCl heterojunctions. The bandgaps of the as-prepared samples can be estimated by the following formula: (αhυ)1/2 = A(hυ − Eg) [69], where α, h, υ, A, and Eg are the absorption coefficient, Planck’s constant, light frequency, constant and bandgap energy, respectively. The plots of (αhυ)1/2 versus hυ were depicted in Fig. 5B, the calculated Eg of BiOCl and BiOI are 3.25 eV and 1.44 eV, respectively, the Eg of the 8%BiOI/BiOCl heterojunctions is approximately 1.63 eV, which is narrower than that of the pure BiOCl, indicating a stronger visible light absorption ability. SPS was measured to elaborate the separation efficiency of photogenerated electron-hole pairs, the results were shown in Fig. 6. It is can be seen that the bare BiOI has SPS response signal from 300 nm to 700 nm due to its narrow band gap. The SPS response intensities of BiOI/BiOCl heterojunctions gradually boost as the loading content of BiOI increasing, the 8% sample holds the highest response intensity, and then the SPS signal tends to drop as the loading amount of BiOI further elevating. According to the SPS measure principle, the stronger SPS response corresponds to higher separation efficiency of photogenerated charge pairs and photocatalytic activity. The results can be confirmed by the photoelectrochemical experiments. Transient photocurrent responses and electrochemical impedance spectrum (EIS) were used to further investigate the separation and transfer behavior of photo-generated charge pairs of the as-prepared samples. The stronger photocurrent response corresponds to higher separation efficiency of photo-induced carries and higher interfacial charge transfer rate [70]. As displayed in Fig. 7A, the BiOI/BiOCl heterojunctions hold stronger photocurrent responses compared with the reference BiOCl and BiOI. Wherein, the 8% sample possesses the highest photocurrent intensity, which is approximately 7.5 and 5.8 times of that of the bare BiOCl and BiOI, respectively. The results illustrate that the existence of heterostructures between BiOI and BiOCl is conducive to the separation of photo-generated electron-hole pairs. Moreover, obvious photocurrent at the initial time of irradiation was

3. Results and discussion 3.1. Characterization of the BiOI/BiOCl heterojunctions XRD patterns of the reference BiOCl, BiOI and BiOI/BiOCl heterojunctions were shown in Fig. 1. The as-prepared BiOCl photocatalysts exhibit diffraction peaks at 2θ = 11.98°, 24.10°, 25.86°, 32.50°, 33.45°, 36.54°, 49.70° and 55.11°, corresponding to the (0 0 1), (0 0 2), (1 0 1), (1 1 0), (1 0 2), (0 0 3), (1 1 3) and (1 0 4) planes, respectively. All the diffraction peaks can be perfectly indexed as the tetragonal structure of BiOCl (JCPDS card No. 06-0249). No diffraction peaks of BiOI were detected in the BiOI/BiOCl heterojunctions, which can be ascribed to high dispersion or low amount of BiOI [59,60]. With the presence of BiOI in the composites, it is evident that the full width at half maximum (FWHM) of (0 0 1) crystal plane for BiOI/BiOCl heterojunctions gradually becomes wider compared with the reference BiOCl, and the 8% sample has the widest FWHM. The results certify that the existence of BiOI has a significant effect on the nucleation and growth of BiOCl crystal. According to the Debye-Scherrer formula, the average crystal sizes of the bare BiOCl and 8%BiOI/BiOCl heterojunctions were calculated to be 58 nm and 53 nm, respectively. Generally, wider FWHM corresponds to smaller crystal size, which is favorable for the photocatalytic activity [61–63]. The results can be proved by the photocatalytic assessment results. SEM images of the bare BiOCl and 8% sample were presented in Fig. 2. As displayed in Fig. 2A, the pure BiOCl exhibits thick sheet-like shape, while the 8% sample has thinner and smaller sheet-like shape (Fig. 2B), resulting in smaller crystal sizes. The results imply that presence of BiOI has considerable influence on the growth of BiOCl crystal, which is consistent with the XRD results. Elemental mapping distribution of the 8% sample from the selected region in Fig. 2C was investigated by EDS. As depicted in Fig. 2D–H, O, Cl, I and Bi elements were observed and four elements were evenly dispersed in the sample. Fig. 3 shows the AFM images and the corresponding thickness profiles of the bare BiOCl, 8%BiOI/BiOCl heterojunctions and the bare BiOI. It is apparent that the thicknesses of the bare BiOCl (Fig. 3A and B) and BiOI (Fig. 3E and F) were tested to be 6.91 nm and 6.45 nm, respectively. The thickness of the 8%BiOI/BiOCl composite was measured to be 2.26 nm (Fig. 3C and D), which is much smaller than that of the bare BiOCl and BiOI. The results are in good accordance with SEM observations. XPS spectra were carried out to investigate the surface chemical compositions and valence states. The XPS spectra of BiOCl, 8%BiOI/ BiOCl heterojunctions and BiOI were depicted in Fig. 4A, Bi, Cl and O elements were detected in BiOCl and the 8% samples. Moreover, I element was observed in the 8% sample, in generally, the I3d peaks for BiOI/BiOCl heterojunctions are in good consistent with that of the bare BiOI (Fig. 4A). The results certify that the chemical valence of I element is −1 in the BiOI/BiOCl heterojunctions [64]. High resolution XPS spectrum of I 3d were exhibited in Fig. 4B, the peaks situated at 630.5 eV and 619.1 eV of the bare BiOI can be assigned to the binding energies of I 3d3/2 and I 3d5/2, respectively. The enlargement of I3d peaks of the 4%, 8% and 10% samples were shown in Fig. 4B (inset), it can be seen that the I 3d3/2 and I 3d5/2 of the 8% sample gradually shift to a lower value of binging energies due to the powerful interaction between BiOI and BiOCl. Fig. 4C shows the high resolution XPS spectroscopy of Bi 4f, two strong peaks with binding energies of 164.4 eV and 159.1 eV correspond to Bi 4f5/2 and Bi 4f7/2, respectively, confirming that the chemical valence of Bi element is +3. The high resolution XPS of Cl 2p was displayed in Fig. 4D, the intensity of Cl 2p peaks gradually weakens as the BiOI content increases, and Cl 2p peaks also shift to lower binding energy as I 3d. As displayed in Fig. 4E, two oxygen peaks located at 532.9 eV and 530.2 eV were observed in the O

BiOI 10%

Intensity (a.u.)

8% 6% 4% 2% 0%

10

20

10 (1 ) 02 )

(1

(0

(0

01

)

02 (1) 01

)

BiOCl JCPDS NO. 06-0249

30

40

2 Theta (degree) Fig. 1. XRD patterns of the samples. 3

50

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Fig. 2. SEM images of the pure BiOCl (A) and the 8% sample (B); EDS image of 8%BiOI/BiOCl heterojunctions (C); Elemental mapping distribution of O, Cl, I and Bi elements of the 8% sample (D-H).

detected in BiOI/BiOCl heterojunctions, which can be attributed to the accumulation of photo-generated holes on the surface of heterojunctions [71,72]. The photocurrent value gradually decreases with time increasing, then reaches a stable current, indicating that equilibrium was established between photo-generated electron-hole pairs recombination and separation processes [71,73]. In addition, the EIS signals of all samples present the same tendency. In general, the smaller

arc radius of EIS Nyquist corresponds to higher separation efficiency of photo-induce charge pairs, resulting in enhanced photocatalytic activity [74]. As given in Fig. 7B, the BiOI/BiOCl heterojunctions exhibit smaller arc radius of the EIS Nyquist plot than the bare BiOCl and BiOI, the 8% sample has the smallest arc radius. Consequently, the 8%BiOI/ BiOCl heterojunctions possesses the highest separation rate of photoinduced charge pairs, thereby it is anticipated that the 8% sample will 4

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Fig. 3. AFM images and the measured thickness of the bare BiOCl (A, B), 8%BiOI/BiOCl heterojunction (C, D), bare BiOI (E, F).

demonstrate the highest photocatalytic activity.

process of photocatalytic degradation RhB over the BiOI/BiOCl heterojunctions, trapping experiments were performed by adding three scavengers into the photocatalytic reaction system, the results were shown in Fig. 9. It can be seen that addition IPA and AO into the photocatalytic reaction system executes relative minor influence on the photocatalytic performance, while the presence of BQ in the photocatalytic system significantly inhibits the abatement of RhB, suggesting that the %O2− radicals plays a leading role in photocatalytic degradation of RhB over the BiOI/BiOCl heterojunctions, while %OH and h+ act as secondary role in the photocatalytic degradation of RhB. The energy gap (Eg) of BiOCl and BiOI are 3.25 eV and 1.44 eV, respectively. Therefore, the energy band structure of BiOCl and BiOI can be calculated as the following formula [76,77]:

3.2. Photocatalytic activities of the BiOI/BiOCl heterojunctions The photocatalytic performance of all the samples was evaluated by photocatalytic degradation of RhB under simulated sunlight illumination. The decolorization efficiencies of RhB over the bare BiOCl, bare BiOI and BiOI/BiOCl heterojunctions are below 25% after 30 min in dark (Fig. 8A), which can be calculated as the following equation: Dt(%) = [(C0 − Ct)/C0] × 100%, where Dt is the adsorption efficiency of RhB over the samples, C0 and Ct are the RhB concentration at time is t = 0 and t = 30. Under solar light illumination, the decay of RhB was depicted in Fig. 8B. It can be seen that all the BiOI/BiOCl heterojunctions exhibit higher photocatalytic activity than the bare BiOCl and BiOI, and the 8% sample possesses the highest degradation efficiency toward RhB. The results illustrate that construction of BiOI/BiOCl heterojunctions can remarkably promote the photocatalytic performance. Photocatalytic degradation of RhB over all the samples follows a first-order kinetics: ln(C0/C) = kt [70,75], where C0 and C is concentration of RhB at the time of 0 and t, k is the rate constant, the plots of ln(C0/C) vs. t was displayed in Fig. 8C. The decay rate constants of RhB over all the samples were displayed in Fig. 8D. Photocatalytic degradation rate constant of RhB over the 8% sample is 0.023 min−1, which is approximately 7.7 and 5.8 times of that over the bare BiOCl (0.003 min−1) and BiOI (0.004 min−1), respectively.

EVB = X ECB = EVB

E e + 0.5Eg Eg

where EVB and ECB are the valence band (VB) and conduction band (CB) edge potential, respectively. X is the absolute electronegativity of the semiconductor, Ee is the energy of free electrons on the hydrogen scale (about 4.50 eV). X value of the BiOCl and BiOI are 6.34 eV and 5.94 eV [46,78]. Consequently, the EVB and ECB of BiOCl were calculated to be 3.47 eV and 0.22 eV, respectively. The EVB and ECB of BiOI were 2.16 eV and 0.72 eV, respectively. According to the above experimental results, a Z-scheme mechanism was proposed to elaborate the separation and migration of photo-generated electrons and holes of the BiOI/BiOCl heterojunctions. As shown in Fig. 10, under simulated sunlight irradiation, BiOCl and BiOI can be excited to yield electron-hole charge pairs, the photo-induced electrons

3.3. Photocatalytic enhancement mechanism In order to elaborate the role of active free radicals during the 5

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Bi4p

BiOI

Bi4f

O1s Bi4d C1s

I3d

Bi4f

Bi4d

8% Bi4p

0% 1000

800

O1s

C1s

600

400

200

I3d

I3d5/2 10%

8%

4% 632

Bi5d

628

632

624

620

616

Binding Energy (eV)

I3d5/2

BiOI

I3d3/2

0

Bi4f7/2

Bi4f5/2

I3d3/2

Cl2p

Binding Energy (eV)

(C)

Intensity (a.u.)

I3p

Intensity (a.u.)

(B)

I3d

Intensity (a.u.)

(A)

628

624

Binding Energy (eV)

620

616

(D)

Bi4f

Cl 2p

10%

Intensity (a.u.)

Intensity (a.u.)

10%

8% 4%

0%

8% 4% 0%

168

164

160

156

Binding Energy (eV)

202

200

198

Binding Energy (eV)

(F)

(E)

O1s (Bi-O)

Intensity (a.u.)

Intensity (a.u.)

O1s (Bi-O)

O1s (-OH)

536

534

532

530

528

Binding Energy (eV)

526

O1s (-OH)

536

534

532

530

O1s (Bi-O)

526

O1s (Bi-O)

Intensity (a.u.)

Intensity (a.u.)

528

Binding Energy (eV)

(H)

(G)

O1s (-OH)

536

196

534

532

530

528

526

Binding Energy (eV)

O1s (-OH)

536

534

532

530

Binding Energy (eV)

528

526

(I)

Intensity (a.u.)

O1s (Bi-O) O1s (-OH)

536

534

532

530

Binding Energy (eV)

528

526

Fig. 4. Survey XPS spectrum of the pure BiOCl, 8%BiOI/BiOCl heterojunctions and bare BiOI (A); High resolution XPS spectra of I3d of the bare BiOI (inset is the enlargement of I3d of the 4%, 8% and 10% samples) (B); Bi4f (C); Cl2p (D); High resolution XPS spectra of O1s of the photocatalysts: BiOCl (E), 4%BiOI/BiOCl (F), 8%BiOI/BiOCl (G), 10%BiOI/BiOCl (H), pure BiOI (I).

6

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Table 1 Curve-fitting results of the high resolution XPS spectra for the O 1s region (ri (%) is the ratio/ΣAi (Ai is the area of each peak).

(A)

0.8 0.6

(B)

0%

0% 2% 4% 6% 8% 10% BiOI

2%

1.5

(ah v ) 1 /2 (eV ) 1 /2

A bsorbance (a.u.)

1.0

0.4 0.2

4% 6% 8% 10%

1.0

BiOI 1.44

0.5 3.25

1.63

0.0

300

400

500

600

0.0

700

Wavelength (nm)

1.6

2.0

2.4

2.8

3.2

3.6

hv (eV)

Fig. 5. UV–Vis diffuse reflectance spectra of BiOCl, BiOI and BiOI/BiOCl heterojunctions (A); plots of (αhυ)1/2 versus hυ of all samples (B).

4

Photovaltage (µV)

formed %O2− radicals will degrade the organic pollutants adsorbed on the surface of the photocatalysts. On the other hand, the holes yielded in the VB of BiOCl is more positive than the %OH/OH− potential (1.99 eV vs NHE) and %OH/H2O potential (2.68 eV vs NHE), the holes can react with OH− and H2O adsorbed on the surface of the photocatalysts to generate %OH radical, the produced %OH will involve in the degradation of pollutants [81]. In addition, photo-induced holes in the VB of BiOCl have strong oxidation capacity, photo-induced holes in the VB of BiOCl can directly react with organic pollutants to produce CO2, H2O and other inorganic small molecules. In conclusion, the generation of heterostructure between BiOCl and BiOI is favorable for the separation and migration of the photo-generated electrons and holes, which is a primary ingredient for the boosted photocatalytic activity of BiOI/BiOCl heterojunctions.

0% 2% 4% 6% 8% 10% BiOI

3

2

1

0 300

350

400

Wavelength (nm)

450

500

4. Conclusion

Fig. 6. SPS response of the pure BiOCl, pure BiOI and BiOI/BiOCl heterojunctions.

In this contribution, BiOI/BiOCl heterojunction photocatalysts were successfully synthesized via a hydrothermal method with the assistant of reactable ionic liquid [PrMIm]I. SPS and photoelectrochemical measurements results imply that the as-prepared BiOI/BiOCl heterojunctions exhibit higher separation efficiency of photo-generated electron-hole pairs as compared to the bare BiOCl and BiOI. Photocatalytic performance evaluation results demonstrate that the activity of 8% BiOI/BiOCl is 7.7 and 5.8 times of that of the bare BiOCl and BiOI, respectively. A possible Z-scheme mechanism was proposed to elaborate the separation and migration of photo-induced electrons and holes

in the CB of BiOI can be easily excited to a higher potential edge (-0.56 eV) [79]. The electrons in the CB of BiOCl recombine with the photo-generated holes in the VB of BiOI, thereby resulting in effective space separation of photo-generated electron-hole charge pairs. Photoinduced electrons in the CB of BiOI (−0.56 eV) is more negative than % O2−/O2 potential (−0.33 eV vs NHE) [79,80], the photo-induced electrons of BiOI (−0.56 eV) can reduce O2 to form %O2− radicals, the 7

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(A)

8

0% 6% BiOI

Light on

2% 8%

4% 10%

(B)

40

0% 2% 4%

6 4

6%

30

-Z '' (K ohm)

Current density (µ A/cm 2 )

10

8% 10% BiOI

20

10

2 Dark

0

40

80

120

160

0

200

0

5

10

Time (s)

15

20

25

Z' (Kohm)

Fig. 7. Photocurrent responses (A); EIS spectra (B) of the pure BiOCl, pure BiOI and BiOI/BiOCl samples under simulated sunlight irradiation.

(A)

23.46

22.48

1.0 0.8

C /C 0

Adsorption rate (%)

16.92

10 6.99

6.78

4.00

0

0%

0.4 0.2

2%

4%

6%

8%

10%

0.0

BiOI

(C)

2.0

Rate constant (min -1 )

1.0 0.5

0

-20

0

20

40

Illumination time (min)

(D) 0% 2% 4% 6% 8% 10% BiOI

1.5

RhB 0% 2% 4% 6% 8% 10% BiOI

0.6

Photocatalysts

ln (C 0 /C)

Light on

19.54

20

0.0

(B)

20

40

60

Illumination time (min)

80

0.023

0.02 0.014

0.013

0.01

0.008 0.003

0.00

80

60

0%

0.004

2%

0.004

4%

6%

Photocatalysts

8%

10%

BiOI

Fig. 8. The adsorption efficiency of RhB over all samples for 30 min in dark (A); Photocatalytic decay of RhB under simulated sunlight illumination (B); kinetic curves for the RhB degradation over all the samples (C); the corresponding rate constant of RhB over the samples (D).

as well as the enhanced photocatalytic activity. This work may provide a meaningful reference for the ionic liquid-assisted preparation of BiOCl-based photocatalysts with largely enhanced photocatalytic activity.

Acknowledgements This project was supported financially by National Natural Science Foundation of China (No. 21777168), the program of Science and Technology Department of Sichuan Province (No. 2019YJ0457, 2019ZYZF0069), the Project of Zigong City (No. 2018YYJC10), Graduate student Innovation Fund of Sichuan University of Science and Engineering (Y2018060), Opening Project of Jiangsu Key Laboratory for Environment Functional Materials (SJHG1805), Opening Project of Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province (No. CSPC201903).

Declaration of Competing Interest 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. 8

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100 88.40 78.03

Decolorization (%)

80

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76.44

[11]

60

[12]

38.03

40 20 0

[13]

Blank

IPA

AO

BQ

[14]

Scavenger Fig. 9. Influence of scavengers on the degradation of RhB over the 8% sample (illumination time = 80 min, scavenger dosage = 0.2 mmol/L).

[15]

[16] [17] [18] [19] [20] [21] [22] [23] [24]

Fig. 10. A possible Z-scheme mechanistic diagram of the photodegradation process for the BiOI/BiOCl heterojunction under simulated sunlight irradiation.

[25] [26]

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