Bi2WO6 photocatalyst with direct Z-scheme heterojunction for efficient salicylic acid removal

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Facet-engineered surface and interface design of WO3/Bi2WO6 photocatalyst with direct Z-scheme heterojunction for efficient salicylic acid removal Xi Chena,b, Yixuan Lia, Li Lia,c,

⁎,1

a

College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, Heilongjiang 161006, PR China College of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150090, PR China c College of Materials Science and Engineering, Qiqihar University, Qiqihar, Heilongjiang 161006, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Z-scheme heterojunction Facet exposure Visible-light photocatalysis Interfacial carriers transfer Salicylic acid removal

WO3 (0 0 1) and WO3 (0 0 1)&(1 1 0) were initially grown on the surface of Bi2WO6 (0 1 0) nanoplates via simple hydrothermal route, respectively. Compared to WO3 (0 0 1)/Bi2WO6, the developed WO3 (0 0 1)&(1 1 0)/Bi2WO6 was endowed with the stronger interfacial interaction property. Moreover, the developed composite could present a broad visible light response at an absorption edge of 440 nm. Not only that, the optimized interfacial carriers were newly achieved, leading to the enhanced separation and transfer of photogenerated electron-hole pairs. The resulting WO3 (0 0 1)&(1 1 0)/Bi2WO6 was implemented to salicylic acid removal (up to 74.5%) under visible light irradiation, demonstrating a kinetics of about 2.4 times faster than that of WO3 (0 0 1)/Bi2WO6. The significantly improved photocatalysis of developed composite was attributed to the conversion from traditional heterojunction to Z-scheme heterojunction, thus, promoting the oxidation ability of photogenerated holes so as to engender abundant ·OH. Ultimately, based on the analysis of energy band structures, the migration pathways of photogenerated carriers in composites was explained. Noticeably, it may open a new door towards the Zscheme strategy of compound photocatalyst without any electronic mediators via controllable facet exposure engineering for highly photocatalysis.

1. Introduction Salicylic acid (SA) as one of the momentous raw material has been widely applied in the dyestuff and pharmaceutical industries. However, SA in wastewater can be an environmental pollutant to seriously injure human’s health [1–3]. Among many strategies of energy conservation and emission reduction for environmental problems, semiconductor photocatalysis has been regarded as the most promising solution, which can readily mineralize diverse refractory organic pollutants into nontoxic substances only driven by light irradiation [4–6]. Hence, seeking high-efficiency photocatalytic materials is a critical task and urgent demand. Actually, single-component photocatalysts are usually out of consideration by scholars owing to their essential drawbacks, such as low sunlight utilization and fast recombination of photogenerated electronhole pairs [7,8]. Thus, coupling two different semiconductors to evolve into heterojunction can be a highly regarded approach to dramatically improve the performance of photocatalyst [9,10]. A great deal of researches has been focused on such traditional heterojunction for many

years [11–13], even though there still have some deficiencies, such as plummeting redox capacity and low migration efficiency of photogenerated carriers. In recent years, a bran-new semiconductor heterostructure has be discovered, called Z-scheme heterojunction, which not only features the spatial isolation of photogenerated carriers, but also advances their thermodynamic redox potentials in the system [14–19]. Moreover, it is found that the formation of Z-scheme heterojunction always depends on the introduction of electronic media with high conductivity at the interface, such as Ag/AgBr/BiOBr [20], RGO/FeWO4/g-C3N4 [21], BiOBr/CDs/g-C3N4 [22] and Ag/Ag2S/AgI-Bi2S3/BiOI [23], in which the photogenerated electron-hole pairs can be separated effectively, thereby realizing an obvious improvement for photocatalysis. Currently, it's worth noting that some literatures have pointed out that the Z-scheme heterojunction can be constructed without electronic mediator [24–26]. For example, Peng et al. designed a three-dimensional hierarchical photocatalyst of CdS nanowires integrated Co-benzimidazole coordination polymers with a novel Z-scheme two-photon excitation for catalyzing H2 evolution [27]. Xu reported a direct Z-scheme

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Corresponding author at: College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, Heilongjiang 161006, PR China. E-mail addresses: [email protected], [email protected] (L. Li). 1 Li Li, female, Ph.D., Professor, doctoral tutor, mainly engaged in the preparation and photocatalytic properties of nanocomposites. https://doi.org/10.1016/j.apsusc.2019.144796 Received 13 August 2019; Received in revised form 6 November 2019; Accepted 18 November 2019 Available online 28 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Xi Chen, Yixuan Li and Li Li, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144796

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Scheme 1. (0 0 1) exposed and (0 0 1)&(1 1 0) exposed WO3 were respectively in situ growth on Bi2WO6 nanoplates with (0 1 0) facet via facile hydrothermal process. Thereinto, the WO3 (0 0 1)/ Bi2WO6 was yielded using PVP as structure-directing agent, and WO3 (0 0 1)&(1 1 0)/Bi2WO6 was yielded both using PVP and ammonium acetate.

cooling to the room temperature, the obtained product was washed with deionized water and ethanol several times, and finally calcined at 300 °C for 4 h to dislodge the redundant acetic acid. Synthesis of WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6. The (0 0 1) exposed WO3 nanosheets or (0 0 1)&(1 1 0) exposed WO3 nanoparticles were grown on the surface of (0 1 0) exposed Bi2WO6. Typically, 0.042 g of Na2WO6·2H2O and 0.05 g of PVP were jointly dissolved in a mixed solution contained 1 mL acetic acid and 6 mL deionized water. Then, 0.5 g of as-prepared Bi2WO6 was dispersed in above mixture through ultrasonic treatment. After stirring for 30 min, the resulting suspension was transferred into a 12 mL of Teflon-lined stainless steel autoclave, and heated at 200 °C for 8 h. The obtained hoary precipitate was repeatedly centrifuged and washed with ethanol, and then dried at 60 °C for 12 h. Moreover, the WO3 (0 0 1)&(1 1 0)/ Bi2WO6 was obtained similar to the procedure with WO3 (0 0 1)/Bi2WO6 except newly adding 0.1 g of CH3COONH4 as the structure directing agent. Thereonto, PVP restrained the (1 1 0) facet of WO3, leading to the single (0 0 1) facet, whereas CH3COONH4 facilitated the growth of its (1 1 0) facet.

BiOI/g-C3N4 photocatalyst via the simple room-temperature in situ fabrication for enhanced phenol removal [28]. Dai's group found that the developed Z-scheme WO3/CdS-diethylenetriamine photocatalyst conducted the direct Z-scheme heterojunction for high photocatalytic H2 evolution under visible light irradiation [29]. However, some similar reports in this regard are just proposed in the results without deeper or explicit explanation [30–36]. Therefore, directionally developing a direct Z-scheme heterojunction without any electronic mediators will be a attractive challenge to pursue. Accordingly, for the first time, we reported that Z-scheme heterojunction can be simply designed through interface engineering of specific facet exposure. To be specific, the Bi2WO6 nanoplates with (0 1 0) facet were firstly synthesized, then (0 0 1) exposed and (0 0 1)&(1 1 0) exposed WO3 were separately growth on its surface, as illustrated in Scheme 1. By contrast, after exposing the (1 1 0) facet of WO3, the optical absorption capacity and interfacial carrier behaviors of resultant composite were significantly improved, which subsequently displayed an excellent removal ability of SA under visible light illumination. Base on the scavenging experiments and DMPO electron spin resonance, it was confirmed that the conspicuous enhancement of photocatalytic activity was ascribed to the conversion of traditional heterojunction into Z-scheme heterojunction, resulting in the abundant generation of ·OH instead of h+ as active species. This transformation was further explained by energy band structure analysis. Ultimately, we inferred that the strong facet interaction and prominent interfacial carrier migration might be the key for designing direct Z-scheme heterojunction.

2.3. Characterization of photocatalytic materials X-ray diffraction (XRD) patterns of the products were received on a diffractometer (Bruker-AXS/D8) using Cu Kα radiation in the range of 10°–70°. X-ray photoelectron spectra (XPS) measurements were done on a spectrometer (ESCALAB/250Xi) with Al Kα source using C1s peak at 284.6 eV as reference. The morphological analysis of the samples were performed by transmission electron microscopy (TEM, JEOL/JEM2100F) and scanning electron microscopy (SEM, HitachiS-4300). The BET specific surface areas and pore distributions of the samples were recorded by a surface area instrument (3H-2000PS2) at 77 K. The photoluminescence spectra of different samples were performed by a spectrophotometer (PL, F-7000FL). The UV–vis diffuse reflection absorption spectra (DRS) were acquired by a spectrophotometer (Purkinje/TU-1901) using BaSO4 as reference. Electron spin resonance (ESR) tests of the samples were achieved by a spectrometer (JEOL/JESFA200) in the presence of DMPO.

2. Experimental section 2.1. Reagents Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 99.0%), sodium tungstate dihydrate (Na2WO4·2H2O, 99.5%), ammonium acetate (CH3COONH4, 98.0%) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Polyvinylpyrrolidone (PVP ≈ WM 58000, 99.5%) and acetic acid (C2H4O2, 99.5%) were purchased from Sigma-Aldrich Co. Ltd. (Shanghai, China). Salicylic acid (C7H6O3), and ethanol (C2H5OH) were used as-bought without further purification. Double distilled water was used for all experiments.

2.4. Electrochemical and photocatalytic tests Electrochemical tests were determined by a electrochemical workstation (Shanghai Chenhua Technology Co./CHI 760D) in a conventional three-electrode system using a Pt wire and Ag/AgCl electrode as the counter electrode and reference electrode. The working electrodes were fabricated by dip-coating a 10 μL of sample slurry (5 mg/mL) on glassy carbon electrodes, followed by air drying at room temperature. 0.1 M KCl aqueous solution was used as electrolyte. The salicylic acid (SA) removal experiments were carried out in a photocatalytic online analysis system (Beijing PerfectLight Technology Co./LabSolar-IIIAG) [38,39]. The visible light source was a 300 W Xe-

2.2. Synthesis of photocatalytic materials Synthesis of (0 1 0) exposed Bi2WO6. The synthetic procedure of (0 1 0) exposed Bi2WO6 was similar to our previous report with some alteration [37]. Typically, 1.455 g of Bi(NO3)3·5H2O and 0.485 g of Na2WO6·2H2O were separately dissolved in 3 mL acetic acid and 27 mL deionized water. Afterward, the Bi(NO3)3 solution was slowly added into the Na2WO6 solution under vigorous stirring, forming the white precursor precipitate. Then, the product was transferred into a 70 mL of Teflon-lined stainless steel autoclave, and heated at 140 °C for 8 h. After 2

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Fig. 1. (a) XRD patterns of different samples; Williamson-Hall plots of (b) Bi2WO6, (c) WO3

(0 0 1)/Bi2WO6,

and (d) WO3

(0 0 1)&(1 1 0)/Bi2WO6.

(−0.38%), revealing a less interfacial stress and lattice mismatch between that of (0 0 1) and (0 1 0) facets. However, a strong strain was acquired while WO3 (0 0 1)&(1 1 0) nanoparticles were grown on Bi2WO6 (−0.01%) (Fig. 1d). This emerging lattice strain might generate lots of dislocation defects at the interface, which induced the carriers to migrate towards, and in favor of the construction of efficient heterojunction. The results indicate that WO3 (0 0 1)&(1 1 0) has an advantage to combine with Bi2WO6 towards heterojunction than that of WO3 (0 0 1).

lamp with 420 nm cut-off (PLS-SXE300/300UV), and 20 cm apart from the reaction solution. Typically, an aliquot of 0.05 g photocatalyst was added to 50 mL of SA solution (5 mg L−1). The temperature of reaction was kept at 15 ± 2 °C. Before the illumination, the solutions were stirred for 30 min in darkness to reach the adsorption-desorption equilibrium. At given time intervals, 3 mL suspension was collected and centrifuged to remove the photocatalyst particles. The concentrations of SA were then determined by measuring the absorbance values at λmax = 296 nm using UV − vis spectrophotometer (TU-1901).

3.2. Surface element composition 3. Results and discussion

The contributed surface elements of product were characterized by X-ray photoelectron spectroscopy. As shown in Fig. 2a, the WO3/ Bi2WO6 composite is composed of C, O, W and Bi elements. Therein to, C is derived from the instrument itself. Three divided peaks of oxygen signal found from Fig. 2b belong to the adsorbed oxygen, lattice oxygen and hydroxy oxygen, respectively. Two split peaks located at 36.9 and 34.7 eV are associated to W4f5/2 and W4f7/2, suggesting that W is in the form of W6+ (Fig. 2c) [42]. The binding energies of 163.9 and 158.6 eV assigned to Bi4f5/2 and Bi4f7/2 can be confirmed that the chemical state of Bi is trivalent (Fig. 2d) [43]. Not only that, the real ratios of WO3 and Bi2WO6 in composites are given in Table S1, indicating that their ratios of WO3 and Bi2WO6 are similar.

3.1. Phase structure and interfacial lattice strain X-ray diffraction was firstly applied for examining the crystal structure of as-prepared samples, with the results shown in Fig. 1a. The diffraction peaks at 2θ values of 28.4°, 32.9°, 47.3°, 56.0° and 58.5° correspond to (1 3 1), (2 0 0), (2 0 2), (1 3 3) and (2 6 2) facets, indicating that the phase structure is well indexed to orthorhombic Bi2WO6 (JCPD standard card No.39–0256) [40]. It is noticeable that no significant diffraction peaks of WO3 can be found in the composites, which might attributes to the WO3 (0 0 1) or WO3 (0 0 1)&(1 1 0) are highly distributed on Bi2WO6 in the state of amorphism, thus, availing to eliminate the effect of the bulk phase structure in the subsequent research. This results show that we have successfully synthesized the desired composite materials. As is well-known, different facets with specific atomic arrangement can render the lattice stress of interfacial contact at the interface. In order to explore the interfacial interaction between WO3 and Bi2WO6, the lattice stress analysis were performed according to Williamson and Hall equation: B cosθ/λ = η sinθ/λ + D [23,41]. As seen in Fig. 1b, Bi2WO6 (0 1 0) nanoplates exhibited a negative stress in their essential features (-0.60%). Notably, when WO3 (0 0 1) nanosheets were grown on Bi2WO6, a small variation of lattice strains could be observed in Fig. 1c

3.3. Surface morphologies and exposed facets The microstructures of WO3/Bi2WO6 composites were first investigated by scanning electron microscopy (SEM), with the results shown in Fig. S1. As seen, the pure Bi2WO6 was consisted of lots of microplates towards a flower-like structure. After the WO3 (0 0 1) and WO3 (0 0 1)&(1 1 0) grown on, the Bi2WO6 still sustained this 3D structure, presenting a good structural stability. Furthermore, transmission electronic microscopy (TEM) was conducted to study the morphologies of WO3 (0 0 1) and WO3 (0 0 1)&(1 1 0), 3

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Fig. 2. (a) XPS spectra of WO3

(0 0 1)&(1 1 0)/Bi2WO6

composite, (b) O1s, (c) W4f, and (d) Bi4f.

WO3 was grown on the surface of Bi2WO6. However, the pore diameters and pore volumes of the composites were enhanced owing to the accumulate of WO3 on the whole. It is worth noting that compared to WO3 (0 0 1)/Bi2WO6, the WO3 (0 0 1)&(1 1 0)/Bi2WO6 possesses the broader average pore diameters and pore volumes, which can increase the accommodations of organic molecules in the pore wall, thus, boosting the rate of photocatalytic reaction.

which were in-situ grown on the surface of Bi2WO6, with the results shown in Fig. 3. Bi2WO6 were initially formed by the hydrothermal process showing the 3D structure composing of massive (0 1 0) exposed nanoplates. After that, WO3 (0 0 1) nanosheets and WO3 (0 0 1)&(1 1 0) nanoparticles were respectively grown on the surface of Bi2WO6 by controlling the addition of structure-directing agents (Fig. 3a and b). To be specific, WO3 nanosheets were fabricated using PVP as structuredirecting agent. Owing to the PVP could be absorbed on the (0 0 1) facet of WO3 so as to extremely suppress the growth along [1 1 0] orientation, resulting in the single (0 0 1) facet (Fig. 3c and S2). Nevertheless, when the CH3COONH4 was newly added as another structuredirecting agent, the growth of (1 1 0) facet could be facilitated owing to NH4+ cation mediation, and ultimately towards the WO3 nanoparticles with (0 0 1)&(1 1 0) facets (Fig. 3d) (more information for the magnified HRTEM images can be seen in Fig. S3) [44]. Fig. 3e–f verified the exposed facets of WO3 (0 0 1) and WO3 (0 0 1)&(1 1 0) on Bi2WO6. The measured lattice fringe from the selected areas in Fig. 3e illustrated the single spacing of 0.39 nm that correspond to the (0 0 1) facet of WO3. Moreover, a new measured lattice spacing of 0.37 nm was found from the selected areas in Fig. 3f, which could be indexed to the (1 1 0) facet of WO3.

3.5. Optical absorption properties and energy band location The UV–Vis diffuse reflectance spectra of WO3/Bi2WO6 were studied in Fig. 5a. Compared to WO3 (0 0 1)/Bi2WO6, one can be noted that the absorption edge of WO3 (0 0 1)&(1 1 0)/Bi2WO6 presents an obvious red shift, which caused by the exposure of (1 1 0) facet. Thus, the presence of (1 1 0) facet not only decreases the band gap of WO3, but also enhances the light utilization efficiency of composites. In addition, the band gaps of WO3 (0 0 1), WO3 (0 0 1)&(1 1 0), and Bi2WO6 were estimated base on the formula: αhv = A(hv − Eg)2, with the results of 2.89, 2.68 and 2.81 eV shown in Fig. 5b [45]. Moreover, Mott-Schottky test was conducted to calculate the conduction band potential of WO3 (0 0 1), WO3 (0 0 1)&(1 1 0), and Bi2WO6 according to Nernst equation: ERHE = EAg/AgCl + 0.05916 pH + E0Ag/AgCl (Fig. 5c and d) [46]. As a matter of fact, the positive slope of plots indicates they are n-type semiconductors, of which conduction bands almost verge on the flat band. Hence, the obtained conduction band potential (ECB) of WO3 (0 0 1), WO3 (0 0 1)&(1 1 0), and Bi2WO6 are calculated to be 0.58 , 0.53, and 0.30 eV (vs. NHE), respectively. Also, the energy band data of above semiconductors are listed in Table S2. These data attest that efficient heterojunction can be established between WO3 and Bi2WO6 by this well-matched band structure.

3.4. BET specific surface area and pore size distribution The N2 adsorption–desorption isotherms and BJH pore size distributions were conducted for Bi2WO6, WO3 (0 0 1)/Bi2WO6, and WO3 (0 0 1)&(1 1 0)/Bi2WO6 to investigate their surface physical properties. Fig. 4 presents that all these samples well feature the type of IV isotherms with a H3 type hysteresis loop, which subsequently indicates the favorable mesoporous distribution. As shown in Table 1, the Bi2WO6 shows a uniform pore size distribution with the average value at 15.89 nm, which means that most of the particles have been packed tightly. Although the surface areas was decreased obviously, when the 4

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Fig. 3. TEM images of WO3 (0 0 1)/Bi2WO6 (a and c) and WO3 (0 0 1)&(1 1 0)/Bi2WO6 (b and d); TEM images of WO3 (0 0 1) (e) and WO3 (0 0 1)&(1 1 0) (f) with exposed facets on Bi2WO6.

3.6. Interfacial carriers behaviors

Table 1 BET surface areas (SBET), average pore diameters (D), and pore volumes (Vtotal) of different samples.

Electrochemical impedance spectroscopy (EIS) was carried out to evaluate the carrier transfer capacities of different facets [47]. As shown in Fig. 6a, the Nyquist arc radius of the composites are less than that of pure Bi2WO6, manifesting that efficient heterojunctions have been indeed constructed between WO3 and Bi2WO6. Moreover, WO3 (0 0 1)&(1 1 0)/Bi2WO6 exhibits the smaller transfer resistance than that of WO3 (0 0 1)/Bi2WO6. The results implies that the photogenerated carriers can transfer easier at the interface between (1 1 0) facet of WO3

Samples Bi2WO6 WO3 (0 0 WO3 (0 0

1)/Bi2WO6 1)&(1 1 0)/Bi2WO6

Fig. 4. N2 adsorption–desorption isotherms and BJH pore size distributions of Bi2WO6 (a), WO3 5

(0 0 1)/Bi2WO6

SBET/m2·g−1

D/nm

Vtotal/cm3·g−1

41.22 31.19 31.57

15.89 24.12 31.29

0.1638 0.1881 0.2470

(b), and WO3

(0 0 1)&(1 1 0)/Bi2WO6

(c).

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Fig. 5. (a) UV–Vis DRS spectra of WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6; (b) plots of (αhv)1/2 versus energy (hv) of WO3 Bi2WO6; (c) Mott-Schottky plots of Bi2WO6 at 1000 Hz; (d) Mott-Schottky plots of WO3 (0 0 1) and WO3 (0 0 1)&(1 1 0) at 1000 Hz.

and (0 1 0) facet of Bi2WO6 rather than the (0 0 1) of WO3 and (0 1 0) of Bi2WO6. The similar consequences also can be found in their transient photocurrent response [48]. As seen in Fig. 6b, WO3 (0 0 1)&(1 1 0)/ Bi2WO6 composite shows the highest photocurrent density than those of other samples, suggesting that the construction of such heterojunction has dramatically promoted the separation efficiency of photogenerated electron-hole pairs. More importantly, an evident cathode photocurrent and anode overshoot are occurred in composite after exposing the (1 1 0) facet, which further demonstrates that a gradual conversion from n-type WO3 (0 0 1)/Bi2WO6 to p-type WO3 (0 0 1)&(1 1 0)/Bi2WO6, owing to the excellent hole migration ability of (1 1 0) facet in WO3 (this event is also certified by Lin et al. [49]). Not only that, the results reveal the active site of the photocatalysis has been transferred from (0 1 0) facet of Bi2WO6 to (1 1 0) facet of WO3, thus, embodies the characteristic of Z-scheme heterojunction. Furthermore, photoluminescence (PL) spectra were conducted by comparing with WO3

(0 0 1),

WO3

(0 0 1)&(1 1 0),

and

(0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6 so as to understand the influence of their heterojunctions on the photogenerated carriers (Fig. S5). It can be seen that WO3 (0 0 1)&(1 1 0)/Bi2WO6 presents the lowest photoluminescence, indicating the highest separation efficiency of photo-induced carriers in all of the samples. These supplied data imply that the WO3 (0 0 1)&(1 1 0)/Bi2WO6 should be the proposed Z-scheme mechanism for SA removal.

3.7. Photocatalytic performances To assess the photocatalytic performances of as-prepared samples, salicylic acid removal experiments were performed under visible light irradiation, with the results shown in Fig. 7a. Bi2WO6 and WO3 both present the noticeable abilities for SA photo-removal. After exposing the (1 1 0) facet of WO3, the WO3 (0 0 1)&(1 1 0)/Bi2WO6 composite shows the highest photocatalytic activity of all, even far superior to WO3

Fig. 6. (a) Electrochemical impedance spectra and (b) transient photocurrent responses of different samples. 6

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Fig. 7. (a) Photocatalytic SA removal of different samples under visible light irradiation; (b) kinetic experiments of different samples under visible light irradiation; (c) photostability test of WO3 (0 0 1)&(1 1 0)/Bi2WO6 for four runs; (d) XRD patterns of WO3 (0 0 1)&(1 1 0)/Bi2WO6 before and after photocatalytic SA removal.

Specifically, the valence band of WO3 is much positive than that of Bi2WO6, so that it's easy to create %OH on WO3 but create h+ on Bi2WO6. After (1 1 0) facet exposing, the main active species of composites for photocatalytic SA removal has been changed absolutely, suggesting that photo-reactive sites have been transferred from (0 1 0) facet of Bi2WO6 to (1 1 0) facet of WO3, that is, realizing the conversion from traditional heterojunction to Z-scheme heterojunction. To ulteriorly validate the conjecture above, DMPO electron spin resonance (ESR) was characterized to probe into the existence of %OH in photocatalysis [24,52]. It was found in Fig. 8b, four weak characteristic peaks of DMPO-·OH was perceived while WO3 (0 0 1)/Bi2WO6 was illuminated by visible light for 10 min. However, four such strong peaks of DMPO-%OH were observed under the same condition, implying that abundant %OH could also be continually generated by WO3 (0 0 1)&(1 1 0)/ Bi2WO6 during the photocatalytic reaction. Hence, the results of ESR are in good agreement with those of scavenging experiments, manifesting the developed WO3 (0 0 1)&(1 1 0)/Bi2WO6 can feature the Zscheme route for efficient photocatalytic SA removal. Combining with the above experiment consequences and band structure data (Table S2), the photocatalytic mechanisms of WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6 were proposed with the illustration given in Fig. 8c. Under visible light irradiation, the photogenerated electrons in WO3 (0 0 1)/Bi2WO6 migrate from the CB of Bi2WO6 to that of WO3, while the holes migrate from the VB of WO3 to that of Bi2WO6. The ECB of WO3 is more positive than E(O2/%O2−) [53], thus, O2 cannot be reduced by photogenerated electrons into %O2−. Moreover, the photogenerated holes of Bi2WO6 just only oxidizes SA directly instead of converting H2O into %OH due to its relatively positive EVB, resulting in such inefficient pathway for SA degradation. But for WO3 (0 0 1)&(1 1 0)/Bi2WO6, the photogenerated electrons can straightway transfer from CB of WO3 to VB of Bi2WO6, and then combine with the holes of Bi2WO6, leading to the high-efficiency carriers

(0 0 1)/Bi2WO6. Moreover, the analysis of kinetic experiments was given in Fig. 7b, so as to discuss the reaction kinetics of the SA removal. According to the equation: −ln (Ct/C0) = kt + b [50], the fitting rate constants of direct photolysis, pure Bi2WO6, WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6 under visible light irradiation can be 0.00249, 0.0638, 0.107 and 0.261 h−1, respectively. It is displayed that the established heterojunctions of WO3 (0 0 1)&(1 1 0)/Bi2WO6 enjoy more efficient than that of WO3 (0 0 1)/Bi2WO6, resulting in remarkably improved photocatalytic activity. In addition, the photostability of WO3 (0 0 1)&(1 1 0)/Bi2WO6 photocatalyst was investigated by four recycle experiments of SA removal under visible light irradiation. As shown in Fig. 7c, the photocatalytic removal efficiency just declines from 74.5% to 69.6% after four runs experiments, representing a stable photo-removal capacity. Furthermore, the XRD pattern was conducted to explore the chemical stability of WO3 (0 0 1)&(1 1 0)/Bi2WO6. The used composite shows no apparent changes compared to the fresh one, confirming a high stability of crystal structure (Fig. 7d). The results prove that WO3 (0 0 1)&(1 1 0)/Bi2WO6 can be a promising photocatalyst for the removal of refractory organic pollutants, such as salicylic acid.

3.8. Photocatalytic mechanism The scavenging experiments were firstly carried out to understand the active species during photocatalytic SA removal (Fig. 8a) [51]. Owing to the low conduction band position of WO3 and Bi2WO6, the WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6 both could not generate large amounts of %O2−, thus, the removal efficiency were not markedly changed after adding BQ. In contrast, the SA removal of WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6 were seriously suppressed by EDTA and IPA, indicating that h+ and %OH played major roles in photocatalysis, respectively. As a consequence, we can infer that the photo-reaction centers of these two composites are different. 7

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Fig. 8. (a) Reactive species trapping experiments of WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6, 1.0 mM p-benzoquinone (BQ), edetic acid (EDTA), and isopropanol (IPA) were separately used as the scavengers for ·O2−, h+ and ·OH; (b) DMPO spin-trapping ESR spectra of WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/ Bi2WO6 in water for ·OH before and after 10 min visible light illumination; (c) Comparison of the constitute heterojunctions in WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)& (1 1 0)/Bi2WO6 for photocatalytic SA removal.

the dislocation defects for boosted carriers transfer. The electrochemical impedance spectroscopy and transient photocurrent response further confirmed the improved interfacial carriers behaviors of composite after exposing the (1 1 0) facet of WO3. Also, the generated cathode photocurrent and anode overshoot of WO3 (0 0 1)&(1 1 0)/ Bi2WO6 were featured the formation of Z-scheme heterojunction, which subsequently verified by active species experiments and DMPO electron spin resonance. On the whole, the so constructed WO3 (0 0 1)/Bi2WO6 and WO3 (0 0 1)&(1 1 0)/Bi2WO6 heterojunctions could perform as the traditional heterojunction and Z-scheme heterojunction during the photocatalysis, respectively. Moreover, the Z-scheme strategy of developed composite had significantly converted the transfer pathway of photogenerated electrons, thus elevating the photo-oxidization ability of the photogenerated holes. As a result, compared to WO3 (0 0 1)/ Bi2WO6, the developed WO3 (0 0 1)&(1 1 0)/Bi2WO6 could yield numerous % OH instead of h+ for efficient photocatalytic SA removal, which allowed for the removal rate of 74.5% after six-hour visible light irradiation. Importantly, this work demonstrates a promising and convenient strategy to design a direct Z-scheme heterojunction without any electronic mediators via exposing specific facets for efficient photocatalytic removal of refractory organic pollutants.

separation in composite. Subsequently, the H2O can be readily transformed into %OH by photogenerated holes of WO3, which possess a strong oxidizability to ultimately mineralize organic pollutants into CO2 and H2O, achieving a preeminent SA removal efficiency under visible light irradiation. Therefore, WO3 (0 0 1)/Bi2WO6 performs traditional heterojunction with the (0 1 0) facet of Bi2WO6 as activity site for photocatalytic SA removal, whereas WO3 (0 0 1)&(1 1 0)/Bi2WO6 conducts the novel Z-scheme heterojunction with the (1 1 0) facet of WO3 as activity site. The results point out that the Z-scheme heterojunction can be established without any electronic mediators via the interfacial design towards specific facet exposure for efficient photocatalytic organic pollutant removal. Importantly, it is inferred that the construction of direct Z-scheme heterojunction may be derived from several vital factors at the interface: Stronger interfacial interaction of facets, the exposure of high active facets, special facets of ultrathin structure, which can serve as the interfacial reasons to boost the carrier transfer between different facets, guiding us to excavate more outstanding composite photocatalysts with direct Z-scheme heterojunction.

4. Conclusion To summarize, we creatively grown (0 0 1)&(1 1 0) exposed WO3 insitu on (0 1 0) exposed Bi2WO6 towards Z-scheme heterojunction via simple hydrothermal route using ammonium acetate as structure-directing agent. It was discovered that the newly generated (1 1 0) facet of WO3 could not only red shift the absorption edge of WO3 (0 0 1)& (1 1 0)/Bi2WO6 for sufficient visible light utilization, but also played a strong interfacial influence on the (0 1 0) facet of Bi2WO6, resulting in

CRediT authorship contribution statement Xi Chen: Methodology, Formal analysis, Investigation, Writing original draft, Visualization. Yixuan Li: Data curation, Writing - original draft. Li Li: Conceptualization, Validation, Resources, Supervision, Funding acquisition. 8

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Declaration of Competing Interest

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