BiOBr with enhanced photocatalytic degradation performance under visible light

Journal Pre-proofs Full Length Article Direct Z-scheme hierarchical WO3/BiOBr with enhanced photocatalytic degradation performance under visible light Yulin Ling, Youzhi Dai PII: DOI: Reference:

S0169-4332(19)34018-8 https://doi.org/10.1016/j.apsusc.2019.145201 APSUSC 145201

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

21 September 2019 3 December 2019 26 December 2019

Please cite this article as: Y. Ling, Y. Dai, Direct Z-scheme hierarchical WO3/BiOBr with enhanced photocatalytic degradation performance under visible light, Applied Surface Science (2019), doi: https://doi.org/ 10.1016/j.apsusc.2019.145201

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Direct Z-scheme hierarchical WO3/BiOBr with enhanced photocatalytic degradation performance under visible light Yulin Ling Methodology Investigation Data curation Validation Writing original drafta,c, Youzhi Dai Conceptualization Resources Writing - review & editing Supervisionb,* aSchool

of Chemical Engineering, Xiangtan University, Xiangtan 411105, PR China of Environment and Resources, Xiangtan University, Xiangtan 411105, PR

bCollege

China cSchool

of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, PR China

Highlights    

A novel visible light-driven 3D hierarchical WO3/BiOBr was synthesised. BiOBr ultra-thin nanosheets grew on WO3 nanotube bundles by solvothermal method. WO3/BiOBr has enhanced catalytic activity and stability under visible-light. 3D hierarchical structure increased the active site and light absorption.



The direct Z-scheme mechanism of WO3/BiOBr was confirmed.

ABSTRACT A visible light-driven 3D hierarchical photocatalyst WO3/BiOBr (WB) was prepared by solvothermal method, through which 2D BiOBr ultra-thin nanosheets grew on 1D WO3 nanotube bundles. The as-prepared WB-0.5 (i.e. W:Bi with a mole ratio of 0.5:1) had the highest photocatalytic degradation efficiency of 94.7% for ciprofloxacin hydrochloride under visible light irradiation (λ > 400 nm) within 120 min; such efficiency was about 5.2 and 1.6 times higher than that of individual WO3 and BiOBr, respectively. Its removal rate of total organic carbon was 41.2%. Moreover, WB-0.5 showed high reusability and photostability, and its photocatalytic activity did not show any obvious decrease even after five cycles. The characterisation of WB-0.5’s crystal structure, morphology, and surface elemental state

1

confirmed its 3D hierarchical structure and formation of heterojunction. Its direct Z-scheme system was tested by analysing the active species, photocurrent characteristics, Mott–Schottky plots, UV-Vis diffuse reflection spectroscopy and in situ irradiation XPS. The 3D hierarchical structure may provide increased active sites and enhanced light absorption efficiency. The direct Z-scheme configuration is particularly beneficial for improving photocatalytic activity. Therefore, this work may provide a feasible strategy for designing a Z-scheme 3D hierarchical photocatalyst. Keywords: Photocatalysis; Heterojunction; WO3/BiOBr; Hierarchical photocatalyst; Z-scheme; Ciprofloxacin degradation 1. Introduction Photocatalysts use light energy to decompose water into hydrogen and oxygen, convert CO2 to produce organic matter and degrade and remove organic contaminants from water [1-3]; thus, they have great potential in solving the increasingly severe energy crisis and environmental pollution. At present, many photocatalysts, such as TiO2, CdS, WO3, C3N4, g-C3N4-agar [4], g-C3N4/Ag2CO3/GO [5], CdS/CoMoSx [6] and Bi2MoO6&Bi2S3 [7] have been

developed.

However,

the

performance

of

single-component

photocatalysts does not easily meet practical requirements because they are incapable of simultaneously possessing wide light absorption range and strong redox ability [8]. Constructing a heterojunction is one of the most effective ways to overcome these shortcomings, and various kinds of heterojunctions have been widely studied in recent years [9, 10]. Compared with traditional heterojunctions, Z-scheme photocatalytic systems have more efficient photogenerated carrier separation efficiency and greater capability of retaining a high oxidation reduction capacity [11, 12]. Thus, Z-scheme photocatalytic systems have become a research hotspot in recent years 2

[13-15]. Z-scheme photocatalytic systems have three connection models. Two of these models require mediators to transfer electrons, whereas the other model (called the direct Z-scheme system) does not need any mediator [11]. In the direct z-scheme system, the contact interface between two catalysts serves as the recombination centre of photogenerated carriers, resulting in a simple structure and high photocatalytic performance [16, 17]. Therefore, the effective method for enhancing photocatalytic activity is to employ narrow band gap photocatalysts in constructing direct Z-scheme systems. WO3 is a visible light-driven (VLD) photocatalyst with a band gap of about 2.6–2.7 eV. The potential of its valence band (EVB) reaches 2.7–3.4 V (vs. NHE) [18], and thus, the photogenerated holes (h +) on its valence band have strong oxidation ability similar to TiO2 [19]. Unfortunately, WO3 has low photocatalytic activity because its conduction band potential (ECB) is positive and the recombination of photogenerated electron hole pairs is considerably easy [20]. Nevertheless, the advantages of high EVB and VLD characteristics give WO3 great potential to be used in constructing high performance heterojunction photocatalysts. Extensive research has aimed to develop a Z-scheme system with WO3 to obtain high performance catalysts, such as WO3/Bi2WO6 [21], CdS/WO3 [22], AgI/WO3 [23], WO3/g-C3N4 [24], SrTiO3(La,Cr)/WO3 [25], WO3/Ag3PO4 [26] and Ag/Ag3PO4/WO3 [27]. These composite photocatalysts are constructed with WO3 nanorods, nanoparticles, nanoplates, nanoblocks and hollow spheres. 1D WO3 nanotube bundles, another WO3 nanostructure material, are self-assembled by a large quantity of nanotubes that measure 10–20 nm in diameter and feature a high specific surface area and activity [28]; hence, this material can be used as ideal carriers for the construction of Z-scheme systems. However, 3

studies on this nanostructure have not been reported. Therefore, applying a suitable catalyst with WO3 nanotube bundles to build a direct Z-scheme system is a worthy research topic. BiOBr, a VLD photocatalyst with about 2.7 eV narrow band gap, is also a hot research topic. BiOBr can be prepared into ultra-thin nanosheets which have a large specific surface area [29]. BiOBr ultra-thin nanosheets are ideal materials for constructing hierarchical photocatalysts. Although a great deal of attention has been paid to the construction of a Z-scheme photocatalyst using BiOBr, most studies used BiOBr nanoplates, such as BiOBr/g-C3N4 [30], SnS2/BiOBr [31], BiOBrBi2MoO6 [32] and BiOBr/rGO/g-C3N4, as building materials [33]. In addition, AgI/BiOBr [34] has been constructed using a microsphere. The construction of a Z-scheme photocatalyst with BiOBr ultra-thin nanosheets has not been reported. Studies have shown that 1D linear nanostructures and 2D ultra-thin nanosheets can facilitate the development of a high performance 3D hierarchical photocatalyst with increased surface area, heterojunction density and improved light absorption efficiency [35-37]. Based on the above survey, it seems that one of the best ways to prepare efficient photocatalysts is to combine two methods, that is, constructing a direct Z-scheme system and constructing a 3D hierarchical structure [11, 16, 38, 39]. Zhang et al. reported a BiOBr/WO3 heterojunction photocatalyst, which is a flower-like structure powder with a diameter of 1 to 5 μm, and its photocatalytic activity is enhanced by the formation of p-n junction [40]. However, to our knowledge, studies about 2D BiOBr ultra-thin nanosheets and 1D WO3 nanotube bundles to form a 3D Z-scheme hierarchical photocatalyst have not been reported. In the current work, a novel VLD 3D hierarchical WO3/BiOBr (WB) photocatalyst was designed and prepared for the first time. 1D WO3 4

nanotube bundles were synthesised to provide support as 2D ultra-thin BiOBr nanosheets can firmly grow on their surfaces via the facile solvothermal method to form a scaly 3D hierarchical structure. The morphology, size, structure and also photocatalytic mechanism of the 3D hierarchical WO3/BiOBr are much different from the above-mentioned flower-like BiOBr/WO3 powder photocatalyst. The 3D hierarchical WO3/BiOBr has smaller size, larger surface area, more active sites and higher light utilization efficiency.The photocatalytic performance of the prepared catalyst was evaluated by the degradation of ciprofloxacin hydrochloride monohydrate (CIP). Furthermore, the optimal ratio of WO3 and BiOBr in the WB composite photocatalyst was determined. The WB composite photocatalyst was characterised by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) ，

UV–Vis diffuse

reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy and the electron paramagnetic resonance spectrometer（EPR）. In the study of the CIP degradation mechanism, the mode of photogenerated electrons and holes migration in the heterojunction of WB was found to be a direct Z-scheme mechanism. In addition, photocurrent characteristics and band potential of photocatalyst were studied by electrochemical method. 2. Experimental 2.1. Materials Sodium tungstate dihydrate (Na2WO4·2H2O), sodium hydrogen sulfate monohydrate

(NaHSO4·H2O),

hexadecyltrimethylammonium

bromide

(C19H42BrN, CTAB), 2-methoxyethanol (C3H8O2), ethylene glycol (C2H6O2) and Ciprofloxacin Hydrochloride Monohydrate (C17H18FN3O3·HCl·H2O,

5

98.0%, CIP) were purchased from Sinopharm Chemical Reagent Co. Ltd; bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) was purchased from Xilong Scientific Co., Ltd; ITO glass was purchased from Zhuhai Kaivo Optoelectronic Technology Co. Ltd. All Chemicals and reagents were of analytical grade. Deionized water was used throughout this study. 2.2. Synthesis of photocatalyst WO3 nanotube bundles were synthesised as reported [28], and WB composites at various mole ratios of W to Bi were prepared by solvothermal method. In a typical synthesis, 2 mmol CTAB was dissolved in 50 mL 2-methoxyethanol and added with 0.4 mmol WO3 nanotube bundles to form suspension A after ultrasonic dispersion for 30 min. Solution B was obtained by dissolving 2 mmol Bi(NO3)3·5H2O in 20 mL ethylene glycol. Suspension A and solutions B were mixed, stirred for 30 min using a magnetic stirrer, sealed in a 100 mL Teflon-lined stainless steel autoclave to react at 180 ℃ for 2 h, and cooled to room temperature. The precipitate was collected by centrifugation, washed alternately with deionised water and ethanol thoroughly for several times, and dried at 60 °C for 12 h. A photocatalyst marked as WB-0.2 was obtained. Change the dosage of WO3 nanotube bundles of 1 mmol or 1.6 mmol and the photocatalysts WB-0.5 or WB-0.8 were prepared as the aforementioned steps. Pure BiOBr was also prepared as above mentioned method except no addition of WO3 nanotube bundles. 2.3. Characterisation X-ray diffraction (XRD) was applied to characterize the purity and crystallinity

of

the

photocatalysts

on

Bruker

D8-advance

X-ray

diffractometer. Scanning electron microscopy (SEM, JSM-6360LV, JEOL) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20 S-TWIN) with the energy-dispersive X-ray spectroscopy (EDS) was 6

applied to characterize the morphologies of the samples. The X-ray photoelectron spectroscopy (XPS) was obtained by ESCALAB 250Xi (Thermo Scientific) with Al Kɑ radiation (hv = 1486.6 eV). The spectra were recorded in the fixed analyzer transmission mode with pass energy 120 eV for survey and 30 eV for region scan. The C1s peak at 284.8 eV was used to calibrate peak positions. UV–Vis diffuse reflectance spectra (DRS) were obtained by a Shimadzu UV-2550. The total organic carbon (TOC) of the reacted solution was determined by the Shimadzu TOC-VCPH analyzer. Fluorescence Spectrophotometer (Hitachi, F-7000) was applied to determine Photoluminescence (PL) spectra. 2.4. Photocatalytic degradation of CIP The photocatalyst (50 mg) was added with 100 mL 20 mg/L CIP solution and subjected to ultrasonic treatment in the dark for 5 min to evenly disperse the catalyst in the solution. Magnetic stirring was then continued in the dark for 40 min to reach adsorption equilibrium. Photocatalytic degradation was processed in a photochemical reaction instrument, and a 500 W Xe lamp with an ultraviolet cutoff filter UVcut400 served as the light source (λ>400 nm). Before illumination, 5 mL of the suspension was extracted for centrifugation. The liquid supernatant was analysed by UV spectrometer with CIP peak absorbances at 276.4 nm to determine residual CIP concentration (C0). After illumination, 5 mL of the suspension was extracted every 20 min, and the residual concentration of CIP (C) was determined with the above method. The photostability and reusability of the prepared photocatalysts was tested as follows: After each reaction, the photocatalyst was collected and washed as mentioned in section 2.2. 2.5. Active species detection and photoelectrochemical measurements In the active species detection, the scavengers (1 mmol/L) of 7

triethanolamine (TEOA), benzoquinone (BQ) and tert-Butanol (TBA) were applied for hole (h+), superoxide radical (•O2 −) and hydroxyl radical (•OH) respectively. Furthermore, the •OH and •O2− was detected by the electron paramagnetic resonance spectrometer （ EPR, JES FA200 ， Japanese electronics JEOL) (Shown in Text S.1). Photoelectrochemical measurements were performed by electrochemical workstation (CHI760E, Shanghai Chenhua, China) with three-electrode cell in 0.1 M of Na2SO4 solution. A platinum plate electrode and Ag/AgCl electrode were used as the counter and reference electrode respectively. The working electrode was prepared as follows: 20 mg photocatalyst was suspended in mixed solution containing 0.2 ml glycol and 0.4 ml nafion (0.5%) ， and then sonicated for 10 min to form homogeneous emulsion; Afterwards, 200 μL slurry was slowly added to the surface of ITO glass electrode (2.0 cm×3.0 cm), then dried for 8 h at 70℃. Electrochemical impedance spectroscopy (EIS) was performed at an open circuit potential, and the ac disturbance was 5 mv in the frequency range of 0.01~10 kHz. The Mott-Schottky curve was obtained through measurement of impedancepotential in dark state with set frequency 1 KHz. All the potentials in this paper were referred to Ag/AgCl if without specified 3. Results and discussion 3.1 Catalyst characterisation 3.1.1 Crystal structure analysis The crystal structures of pure WO3 nanotube bundles, pure BiOBr and WB composites were detected by X-ray diffraction (XRD), as shown in Fig. 1. The high intensity and sharp diffraction peak indicated that the synthesised product had good crystallinity. The diffraction peaks of pure

8

WO3 were exclusively ascribed to the hexagonal WO3 crystalline phase (JCPDS card No. 85-2460), and the diffraction peaks of pure BiOBr were ascribed to the pure tetragonal phase of BiOBr (JCPDS card No. 73-2061) [29]. For the WB composites with different ratios, the characteristic peaks of WO3 and BiOBr indicated that the BiOBr nanosheets were successfully deposited on the surface of the WO3 nanotube bundles. Meanwhile, the diffraction peak intensity of WO3 enhanced gradually with increasing WO3 content, whereas that of BiOBr decreased gradually. This observation indicated the decrease of BiOBr generated on the surface of WO3 in the composite materials as the mole ratio of W to Bi increased from 0.2 to 0.8. The absence of other diffraction peaks implied that the as-prepared WB composites were of high purity. 3.1 .2 Morphology and heterostructure interface analysis As shown in Fig. 2a, the WO3 nanotube bundles, which were self-assembled by nanotubes with a diameter of 10–20 nm, had a diameter and length of about 200–300 nm and 2–5 μm, respectively (Fig. S1) [28]. As shown in Fig. 2b–d, the BiOBr ultra-thin nanosheets (about 5–10 nm thick, Fig. S2) grew on the surface of the 1D WO3 nanotube bundles and formed a 3D hierarchical structure. As the proportion of W in the WB composite material increased, the thickness of BiOBr on the surface of W decreased. The thick BiOBr nanosheet layer completely covered the WO3 nanotube bundles and even formed some flower-like BiOBr microspheres because of the few WO3 nanotube bundles in WB-0.2 (Fig. 2b), which could have weaken the function of the heterojunctions. For WB-0.5 (Fig. 2c), the WO3 nanotube bundles were packaged with a layer of BiOBr ultra-thin nanosheets with suitable thickness. As a result, many heterojunctions were exposed in 9

the photocatalytic process. The resulting hierarchical structure could also increase the absorption of photons via multiple reflections at the photocatalyst interface [41] and thereby benefit the improvement of photocatalytic performance. For WB-0.8 (Fig. 2d), the BiOBr nanosheet layers on the surface of the WO3 nanotube bundles were sparse. Some of the WO3 nanotube bundles were bare on the outside because of their relatively large number, resulting in few heterojunctions. Some possible chemical reactions that occurred during the formation of the WB composite materials with 3D hierarchical structure could be proposed as follows [42]: Bi(NO 3 ) 3 → Bi 3 + + 3NO 3 −

(1)

Bi 3 + + HOCH 2 CH 2 OH → Bi(OCH 2 CH 2 OH) 2 + + H +

(2)

Bi(OCH 2 CH 2 OH) 2 + + H 2 O → BiO + + HOCH 2 CH 2 OH + H +

(3)

Bi(OCH 2 CH 2 OH) 2 + → BiO + + CH 3 CHO + H +

(4)

C 1 6 H 3 3 (CH 3 ) 3 N–Br + BiO + → BiOBr + C 1 6 H 3 3 (CH 3 ) 3 N +

(5)

As shown in Formula (1) and (2), Bi(NO3)3·5H2O was completely dissolved in ethylene glycol, and the coordination of ethylene glycol with Bi3+ could generate alkoxides Bi(OCH2CH2OH)2+, which can be adsorbed on the surface of the WO3 nanotube bundles. During the solvothermal process, there are two routes (3) and (4) to form BiO+. The function of the CTAB surfactant is both Br source and template. Subsequently, the combination of Br− in CTAB with BiO+ induced the formation of BiOBr nuclei. large amounts of BiOBr nuclei finally transformed into ultra-thin nanosheets under the action of the template [43]. As shown in Fig. 3a, the BiOBr thin nanosheets intimately adhered on the surface of the WO3 nanotube bundles in the WB-0.5 composites even after ultrasonic dispersion, indicating the strong connection between them. 10

Moreover, WO3 and BiOBr crossed fringes and knitted tightly in their interfaces (Figs. 3b–c). The lattice spacing of 0.282 and 0.201 nm can be ascribed to the (012) and (020) crystal planes of BiOBr, respectively, while the lattice spacing of about 0.389 nm can be attributed to the (001) crystal plane of WO3 (Fig. 3c), which were in good agreements with the previous XRD results. In addition, the EDS pattern (Fig. 3d) showed that WB-0.5 contained the elements Bi, O, Br and W without other impurities, whereas the signals for Cu and C might have been brought by the instrument. 3.1.3 Surface elements composition and atomic chemical state in nanometer range of thickness of lateral cross section analysis The elemental composition and the surface chemical state difference of the catalyst were investigated by X-ray photoelectron spectroscopy (XPS). All peak positions were corrected using C1s as reference (284.8 eV). The survey spectrum of the sample (Fig. 4a) showed Bi, O and Br components in pure BiOBr and W and O components in pure WO3. Sample WB-0.5 had the same characteristic peaks for the elements Bi, Br, O and W as BiOBr and WO3, whereas the peak of W was relatively weak, indicating the minimal content of W. As confirmed by the scanning electron microscopy and transmission electron microscopy images (Figs. 2 and 3, respectively), the surface of the WO3 nanotube bundles was covered with BiOBr ultra-thin sheet, which hindered the detection of W. High-resolution XPS was employed to thoroughly investigate the interaction between WO3 and BiOBr in the WB-0.5 composites. In pure BiOBr, two peaks at 159.39 and 164.69 eV were ascribed to Bi 4f7/2 and Bi 4f5/2(Fig.4b), and two peaks at 68.44 and 69.48 eV were assigned to Br3d5/2 and Br33/2(Fig.4c), respectively. The binding energies of W4f7/2 and W4f5/2 orbital of pure WO3 were 35.85 and 37.98 eV (Fig.4c), respectively. However, for WB-0.5 composite, the peaks

11

of Bi 4f and Br 3d slightly shifted towards lower binding energy, while the binding energy of W4f showed a positive shift. These binding energy shifts clearly suggest that some electrons in WO3 have transferred to BiOBr upon hybridization [44]. The reason for this result is p-type BiOBr has lower Fermi level than n-type WO3, resulting in electrons migrated from WO3 to BiOBr until their Fermi levels are equalized [45]. As shown in Figure 4e, the O 1s peak at 530.50 and 532.01 eV of pure WO3 corresponds to its lattice oxygen (W-O) and hydroxyl groups, respectively. While in pure BiOBr, two peaks at 530.30 and 531.94 eV were ascribed to lattice oxygen (Bi-O) and hydroxyl groups, respectively. The O 1s spectrum of WB-0.5 can be fitted into three peaks at 520.18 (Bi-O), 530.70 (W-O) and 531.98 eV (hydroxyl groups). The binding energy of Bi-O and W-O showed a negative and positive shift, respectively, which also indicated that electrons migrated from WO3 to BiOBr. In summary, these binding energy shifts in WB-0.5 indicated the strong interaction between BiOBr and WO3 at their interfaces, further confirmed that they form a heterojunction rather than a simple mixture [46]. 3.1.4 Optical properties and band structures of the photocatalyst UV-Vis diffuse reflectance spectra (DRS) were used to study the photo-response range of the prepared samples. As shown in Fig. 5a, the absorption edges of BiOBr, WO3, WB-0.2, WB-0.5 and WB-0.8 were about 430, 440, 432, 435 and 437 nm, respectively. This result indicated that WB composite photocatalyst exhibited photocatalysis in visible light. The band gap energies were estimated according to the following equation: αhν =A(hν-Eg)n/2, where n is 4 for the indirect semiconductor WO3 or BiOBr and α, A, h and Eg are the absorption coefficient, constant, photonic energy and absorption band gap energy, respectively. Therefore, the band gap energies 12

of WO3 and BiOBr were deduced to be 2.62 and 2.75 eV, respectively (Fig. 5b). Transient

photocurrent

and

Mott–Schottky

(M-S)

plots

were

determined to further clarify the relative positions of conduction band (CB) and valence band (VB) on the prepared samples. The photocurrent for n-type semiconductor (SC) working electrodes is anodic current under illumination, whereas that for p-type SCs is cathodic current. If the cathode is set to positive, the positive current is cathode current, and the negative current is anode current; this method is reliable in distinguishing n- or p-type photocatalysts [47-49]. As shown in the inset image of Fig. 6a, the photocurrent of the WO3 electrode was a negative photocurrent under illumination; thus, it is a classic anodic photocurrent and is indicative of WO3 being an n-type SC. Meanwhile, BiOBr displayed a positive photocurrent (cathodic current), indicating that BiOBr is a p-type SC (inset image of Fig. 6b) The positions of the CB of WO3 or VB of BiOBr were obtained by determining the M-S plot. The M-S plot slope was positive for n-type SCs, whereas it was negative for p-type SCs. The flat band potential (Efb) of SC can be obtained as the intercept by the extrapolation of the linear region of the M-S plot to the potential x-axis. Generally, the value of Efb is approximately equal to ECB (for n-type) or EVB (for p-type) of SCs [50, 51]. As shown in Fig. 6a and 6b, the Efb values of pure WO3 and BiOBr were estimated to be −0.16 and 2.10 V respectively; thus, the ECB of WO3 and EVB of BiOBr were −0.16 and 2.10 V, respectively. A potential conversion between Ag/AgCl (in 3 M KCl) and NHE is given by E(NHE) = E(Ag/AgCl)+0.212 V. Therefore, the ECB of WO3 was about 0.05 V, 13

whereas EVB of BiOBr was about 2.31 V (vs. NHE). ECB and EVB were calculated from the equation as follows: EVB = ECB+ Eg, where Eg represents the band gap energies of WO3 and BiOBr, which were 2.62 and 2.75 eV, respectively. Hence, we can calculate that the EVB of WO3 was 2.67 V and that the ECB of BiOBr was −0.44 V (vs. NHE). The Eg, ECB and EVB values of WO3 and BiOBr are summarised and illustrated in Fig. 6c. 3.2 Photocatalytic activity and stability CIP photodegradation under visible light irradiation was employed to evaluate the photocatalytic activity of the photocatalysts. Fig. 7a shows that without a catalyst, CIP degradation is not obvious even after illumination for 120 min, indicating the non-photolysis of CIP in visible light. The degradation capacity of the catalysts followed the sequence: WB-0.5 > WB-0.8 > WB-0.2 > pure BiOBr > WO3+BiOBr (mole ratio of W:Bi is 0.5:1, physical mixture) > pure WO3. The CIP solution added with pure WO3 or BiOBr dropped to less than 18.3% and 59.1%, respectively. The lowest photocatalytic performance of WO3 was mainly due to the easy recombination of photogenerated carriers [20], which indicated by the highest PL intensity. BiOBr has higher photocatalytic performance than WO3, which might be ascribed to its ultra-thin nanosheet structure and lower electron-hole recombination rate (lower PL intensity, Fig.9a). WB composite photocatalysts exhibited higher catalytic performance than WO3 and BiOBr or the mixture alone. The CIP degradation efficiency of WB-0.5 was the highest and reached 94.7%, which is 5.2 and 1.6 times higher than that of pure WO3 and BiOBr, respectively. Obviously, the CIP degradation efficiency of the mixture (WO3+BiOBr, 0.5:1) was only 45.5%, which was about half of the degradation efficiency of WB-0.5. This phenomenon 14

explains the fact that in the WB composite catalyst, a heterogeneous junction is formed between WO3 and BiOBr instead of a simple physical mixture, which greatly improves the separation efficiency of photogenerated electron-hole pairs, so the photocatalytic performance is greatly enhanced compared to WO3 and BiOBr alone [52, 53]. It can be seen that the pL intensity of the WB composite catalyst is lower than that of WO3 and BiOBr, confirming this conclusion. Fig. 7b shows the CIP degradation efficiencies of the WB composite catalysts with different molar ratios of W to Bi after 120 min of illumination. As the mole ratio of W to Bi increased from 0.1 to 0.5, the photocatalytic performance continued to increase. When the ratio of W to Bi was 0.5, the corresponding catalytic performance was the best, but when the ratio increased to 0.8, the degradation efficiency decreased. This result indicated that in WB composites, the optimal ratio between W and Bi was approximately 0.5. As shown in Fig. 7c, the related kinetic curves and the process of photodegradation of CIP accord with the pseudo-first-order kinetics equation: -ln(C/C0) = kt. The rate constant (k) values (Fig. 7d) of pure WO3, WO3+BiOBr, pure BiOBr, WB-0.2, WB-0.5 and WB-0.8 were 0.0016, 0.0051, 0.0078, 0.0134, 0.0258 and 0.02 min−1, respectively. WB-0.5 had the fastest photodegradation rate constant, which was 16.1 and 3.3 times greater than those of WO3 and BiOBr, respectively. Its removal rate of TOC for CIP reached 41.2% (Fig. 7e), which was significantly higher than those of WO3 and BiOBr, respectively, indicating an obvious improvement

in

mineralisation

ability.

WB-0.5

has

the

highest

photocatalytic activity due to the appropriate thickness and density of BiOBr ultrathin nanosheets on the surface of WO3 nanotube bundles (Fig. 2c). Thus, a maximum amount of heterojunctions can be formed and exposed, which means higher photo-generated electron-hole separation efficiency and more 15

active sites. In addition, this structure has a relatively high light absorption utilization rate [17]. In WB-0.2, due to the low molar ratio of W to Bi, BiOBr nanosheets cover WO3 nanotube bundles densely (Fig. 2b), which leads to insufficient exposure of heterojunction for the degradation reaction, resulting in a decrease in the efficiency of photo-generated carrier separation, so the catalytic performance is low. The high W content in WB-0.8 resulted in only a portion of the surface of the WO3 nanotube bundle being covered by the BiOBr nanosheet (Fig. 2d). Therefore, the quantities of heterojunctions and active sites decreased, leading to the lower of photocatalytic performance.

Fig.8. (a) Recycling experiments for CIP degradation by WB-0.5 and (b) XRD patterns of WB-0.5 before and after photocatalytic degradation.

As shown in Fig. 8a, the reusability and photostability of WB-0.5 were evaluated by several cycles of CIP photocatalytic degradation. After five cycles, the photocatalytic performance of WB-0.5 remained satisfactory, with only a small loss of photocatalytic activity. Moreover, no obvious change was observed in the XRD peaks of the recycled samples (Fig. 8b), indicating the good reusability and stability of the crystal structures of WB-0.5. The (110) diffraction peaks of the used BiOBr decreased slightly 16

possibly because the catalyst was ultrasonically dispersed in each use and some of the BiOBr ultra-thin sheets on the WO3 surface were ultrasonically detached after five cycles. 3.3. Mechanism considerations of photocatalytic performance enhancement The separation efficiency of the photogenerated electron–hole pair was obtained by fluorescence and electrochemical tests. The diagram of the photoluminescence emission spectra shows that the emission peak of WB-0.5 was the lowest (Fig. 9a), indicating that when the photogenerated electron–hole pair recombination was the lowest, the carrier separation efficiency was the highest[54]. The electrochemical impedance spectra (EIS) Nyquist plots of pure WO3, BiOBr and WB were used to study the process of electron transfer. The small radius of the arc denoted that the charge transfer resistance of the photoelectrode was small, indicating the rapid separation of the photogenerated electron–hole pairs[55]. The arc radius of the EIS Nyquist plot of WB-0.5 was the smallest, indicating the fastest rate of interfacial charge transfers to the electron acceptor (Fig. 9b). As shown in the LSV curve (Fig. 9c), the photocurrent intensity increased with increasing electrode potential, and the increase slowed down after the potential reached about 0.9 V. This observation was due to the generated photoelectrons having been effectively transferred through this applied potential (commonly known as the bias voltage). Therefore, transient photocurrent responses (i-t curve) were employed at the 0.9 V bias voltage (Fig. 9d). In the dark, the current was about 0 and remained almost constant, indicating that the current was generated by photocatalytic materials excited by visible light. The photocurrent intensity of WB-0.5 was the greatest, whereas that of WB-0.2 and WB-0.8 were relatively low because under the irradiation of visible light, 17

their generated photoelectron concentrations were different. A high photoelectron concentration resulted in a large current. In other words, the photoelectron–hole separation efficiency was the highest. The above photocurrent analysis explained the results of the CIP degradation experiment and indicated that WB-0.5 had the best catalytic performance and was the reason for the high efficiency of the optical photocarrier separation. In general, •OH, •O2− and h+ are the main active species of the photocatalytic degradation of organic contaminants[56]. BQ, TBA and TEOA as scavengers of •O2−, •OH and h+, respectively, were explored by capture experiments to identify which active species react towards CIP degradation. When BQ, TBA or TEOA exists in the CIP solution, a significant suppression of CIP degradation was observed (Fig.10a), implying the presence of •O2−, •OH and h+ in the process of degrading CIP by WB-0.5. To further confirm the existence of •OH and •O2− during the process, we applied the electron paramagnetic resonance (EPR) technique with DMPO as the trapping agent under visible light. No peak was observed in the dark, but six characteristic peaks of •O2− were noted under visible light (Fig. 10b), indicating the generation of a large amount of •O2− by WB-0.5. In addition, four peaks appeared in the reaction system with spectral line intensities of 1:2:2:1 (Fig. 10c), which were the typical spectra of DMPO-•OH adduct, indicating the generation of •OH. The EPR experimental results are consistent with those of the capture experiments. In conclusion, •O2−, •OH and h+ are the main active species in the photocatalytic degradation of CIP by WB-0.5.

18

According to the above results and discussion, a possible catalytic mechanism of WB-0.5 was proposed and deduced from the following analysis. The Fermi energy level of n-type WO3 is close to that of conduction band (CB), whereas that of p-type BiOBr is close to that of valence band (VB). Once these two kinds of SCs form p-n heterostructures, the Fermi levels tend to align (Fig. 11a). If a photogenerated charge carrier transfers in WB-0.5 via a conventional p-n heterojunction mechanism (Fig. 11b), the photogenerated electrons on the CB of BiOBr transfer to that of WO3 because the CB of WO3 is more positive than that of BiOBr. Therefore, the photogenerated holes on the VB of WO3 flow to that of BiOBr because the VB of BiOBr is more negative than that of WO3. The standard redox potentials (vs. NHE, pH=7) of O2/•O2- and OH-/•OH- are −0.33 and 2.29 V, respectively[38]. The electrons on CB of WO3 cannot reduce O2 into •O2because of the 0.05 V potential of CB of WO3, as explained in previous studies[57]. Although the EVB of BiOBr (2.31 V) is more positive than the redox potentials of OH- /•OH, most existing research concluded that BiOBr mainly uses •O2− and h+, not •OH, in the degradation of organic compounds[58, 59]. Therefore, if the photogenerated charge carrier transfer follows the conventional p-n heterojunction mechanism, only h+ plays a main role in CIP degradation. However, the active species experiment proves that •O2−, •OH and h+ play crucial roles in the photocatalytic degradation of CIP (Fig. 10) by WB-0.5. This result is contradictory to the mechanism shown in Fig. 11b. Therefore, we can conclude that a novel separation pathway is needed for the photogenerated charge carrier in WB-0.5. As shown in Fig. 11c, the photogenerated electrons on the CB of WO3 and holes on the VB of BiOBr directly recombine at the interface. Consequently, the photogenerated electrons and h+ separate effectively and 19

accumulate on the CB of BiOBr and VB of WO3 respectively. The potential of CB of BiOBr is −0.44 V, which is more negative than that of O2/•O2(−0.33V, vs. NHE, pH=7). Thus, the photogenerated electrons in the CB of BiOBr can easily reduce O2 absorbed into •O2–. Moreover, the potential of VB of WO3 (2.67 V) is more positive than that of OH-/•OH (2.29 V, vs. NHE, pH=7), indicating that the photogenerated h+ in the VB of WO3 can oxidise OH- into •OH. Obviously, WB-0.5 can produce •O2-, •OH and h+ efficiently for the photocatalytic degradation of CIP. This theory is completely consistent with the experimental results. Therefore, we can conclude that the separation pathway of the photogenerated charge in WB-0.5 is a direct Z-scheme system, which may be ascribed to the number of interfacial defects between the contact surfaces of WO3 and BiOBr that allow electric charges to easily pass through and serve as centres for charge recombination [11, 60]. In order to further elucidate the direct Z-scheme photocatalytic mechanism of WB-0.5, in situ irradiation X-ray photoelectron spectroscopy (ISI-XPS) was performed (Text S.2). In the absence of light, two peaks of Bi 4f7/2 and Bi 4f5/2 of BiOBr appeared at 159.29 and 164.55 eV (Fig.11a), and the binding energies of W4f5/2 and W4f7/2 of WO3 were 38.16 and 36.04 eV (Fig.11b), respectively. Under light irradiation, the characteristic peak of Bi4f shifted slightly to the lower binding energy, while that of W4f shifted slightly to the higher in the opposite direction, which indicates that the electron density of BiOBr increased and the electrons density of WO3 reduced. In other words, the photo-generated electrons have migrated from WO3 to BiOBr across their interface[61], which is completely agreement with the previous mechanism analysis, further illustrating that the carrier separation pathway in WB-0.5 is a direct Z-scheme system[62]. 20

4. Conclusions A 3D hierarchical WB photocatalyst was successfully synthesised by solvothermal method. The optimised mole ratio of W and Bi in the WB composites is 0.5:1, and the corresponding WB-0.5 has the best photocatalytic

performance.

The

highly

enhanced

photocatalytic

performance is due to the direct Z-scheme mechanism and 3D hierarchical structure. The XRD results showed that the synthesized product has good crystallinity and purity. SEM and TEM images demonstrated WB-0.5 is 3D hierarchical structure. XPS results indicated that WO3 and BiOBr form a heterojunction structure. The results of EPR and ISI-XPS revealed that the carrier separation pathway in WB-0.5 is a direct Z-scheme system. Constructing a direct Z-scheme photocatalyst based on a 3D hierarchical structure may be a feasible strategy in designing high performance photocatalysts. Acknowledgements This study was supported by the Hunan Provincial Science and Technology Plan Project (No. 2018SK2021); and Hunan 2011 Collaborative Innovation Center

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https://doi.org/10.1039/c5cs00064e [61] J.X. Low, B.Z. Dai, T. Tong, C.J. Jiang, J.G. Yu, In Situ Irradiated X-Ray Photoelectron Spectroscopy Investigation on a Direct Z-Scheme TiO2/CdS Composite

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https://doi.org/10.1002/adma.201807920 [62] 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, Mater. Today 21 (2018) 1042-1063. https://doi.org/10.1016/j.mattod.2018.04.008 Fig.1. XRD patterns of pure BiOBr, pure WO3 and WB composites.

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Fig.2. SEM images of (a) WO3 nanotube bundles and (b) WB-0.2, (c) WB-0.5 and (d) WB-0.8 composites. Fig.3. (a-c) TEM images in scale of 500 nm, 20 nm and 10 nm, and (d) EDS pattern for WB-0.5 composites. Fig. 4. XPS photoelectron peak of survey scan and region scan for Bi4f, Br3d, W4f and O1s on the composites. Fig.5. (a) UV-Vis diffuse reflection spectra (DRS) and (b) plots of (ahv)1/2 vs. energy (hv) of pure BiOBr, WO3 nanotube bundles and WB composites. Fig.6. Mott–Schottky plot and transient photocurrent of (a) WO3 and (b) BiOBr and (c) energy level distributions of CB and VB of WO3 and BiOBr. Fig.7. (a) Photocatalytic activity of different photocatalysts to CIP degradation, (b) effect of ratio of W to Bi in WB composites on degradation rate, (c) plots of -ln(C/C0), (d) rate constant k and (e) TOC removal efficiency.

Fig.9. (a) Photoluminescence (PL) spectra, (b) electrochemical impedance spectra (EIS) Nyquist plots, (c) linear sweep voltammetry (LSV) and (d) transient photocurrent responses of WO3, BiOBr and WB composites. Fig.10. (a) Degradation efficiency of CIP with different scavengers by WB-0.5 and EPR test of (b) •O2− and (c) •OH with WB-0.5 under visible light. Fig.11. High-resolution XPS for Bi4f (a) and W4f (b) of WB-0.5 in the dark or under light irradiation Fig.12. Proposed schematic for charge separation of WB-0.5

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