Bi2WO6–BiOCl heterostructure with enhanced photocatalytic activity for efficient degradation of oxytetracycline

The application of BiOCl in photocatalysis has been restricted by its low utilization of solar energy and fast recombination of charge carriers. In this study, zero-dimensional (0D) Bi2WO6 nanoparticles/two-dimensional (2D) layered BiOCl heterojunction composite was successfully constructed by facile hydrothermal and solvothermal methods. The most favorable sunlight photocatalytic activity was achieved for the as-prepared Bi2WO6–BiOCl composites with a ratio of 1%. The photocatalytic rate and mineralization efficiency of one typical antibiotic (i.e., oxytetracycline) over 1% Bi2WO6–BiOCl was about 2.7 and 5.3 times as high as that of BiOCl. Both experimental characterizations and density functional theory (DFT) calculations confirmed that the excellent photocatalytic performance mainly arised from the effective charge separation along the Bi2WO6 and BiOCl heterojunction interface. The effective electron transfer was driven by the internal electric field at the interfacial junction. In addition, 1% Bi2WO6–BiOCl exhibited excellent stability, and no apparent deactivation was observed after 4 test cycles. Therefore, the 0D/2D Bi2WO6–BiOCl heterojunction showed a great potential for the photocatalytic degradation of emerging organic pollutants.

In the current work, a 0D Bi 2 WO 6 nanoparticles/2D BiOCl nanosheets heterojunction was fabricated via a facile hydrothermal and solvothermal process. Their photocatalytic performance towards oxytetracycline (OTC) degradation under simulated sunlight irradiation was systematically evaluated, and the photocatalytic degradation of model contaminant (i.e., phenol) was also conducted for comparison. Mechanism of the enhanced photocatalytic activity of Bi 2 WO 6 -BiOCl heterojunction was comprehensively explained by instrumental characterizations (such as photo-luminescence spectra, valence band X-ray photoelectron spectroscopy, and UV-Vis diffuse reflectance spectra) and density functional theoretical (DFT) calculations.

Experimental section
Synthesis of Bi 2 WO 6 -BiOCl heterojunction. Bi 2 WO 6 nanoparticles were prepared by a modified solvothermal method 28 . Briefly, 1 mmol Bi(NO 3 ) 3 ·5H 2 O and 0.5 mmol Na 2 WO 4 ·2H 2 O were dispersed in 10 mL ethylene glycol (EG), respectively. These two solutions were ultra-sonicated for 30 min to form uniform tungsten source and bismuth source. The tungsten source was then slowly added to bismuth source with continuous stirring. The mixture was transferred into 50 mL Teflon-lined stainless steel autoclave, and heated at 160 °C for 10 h. After being cooled to room temperature, the precipitate was washed with ethanol for three times and dried at 60 °C for 12 h in air.
The Bi 2 WO 6 -BiOCl composites were fabricated by a previously reported method 3 . Briefly, 1 mmol Bi(NO 3 ) 3 ·5H 2 O and aliquots (i.e. 0.005 mmol, 0.01 mmol, 0.02 mmol, 0.04 mmol) of Bi 2 WO 6 were added in 10 mL distilled water at room temperature with vigorous stirring and ultra-sonication for 1 h. After that, 10 mL of 0.1 mol L −1 KCl aqueous solution was added to Bi(NO 3 ) 3 ·5H 2 O solution by dropwise. The solution was vigorously stirred for 30 min, and 1 M NaOH was then added to adjust pH value to 6.0. The mixture was stirred for 1 h, poured into a 50 mL Teflon-lined stainless steel autoclave, and heated up to 160 °C for 24 h. After being cooled to room temperature, the white precipitate was collected and washed with distilled water for three times. The washed precipitate was then dried at 60 °C for 12 h in air to get the Bi 2 WO 6 -BiOCl composites. According to Bi 2 WO 6 mass in the reaction system, Bi 2 WO 6 -BiOCl samples were denoted as 0.5%, 1%, 2%, and 4% Bi 2 WO 6 -BiOCl. The pure BiOCl were also obtained without the addtion of Bi 2 WO 6 under the same condition. The 1% mixture were prepared by simply mechanical blending of Bi 2 WO 6 and BiOCl at a ratio of 1%.
Characterization. The powder X-ray diffraction (XRD) patterns were collected by an X-ray diffractometer (Bruker D-8 Advance) with Cu Ka radiation (λ = 0.15406 nm). The 2θ ranged from 10° to 80° with a scanning rate of 8° min −1 . The morphologies and compositions of Bi 2 WO 6 , BiOCl, and Bi 2 WO 6 -BiOCl composites were performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) on a FEI Nova-450 scanning electron microscopy. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) images, and selected-area electron diffraction (SAED) pattern were recorded in a JEM-200CX instrument with an accelerating voltage 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out by a PHI 5000 Versa Probe spectrometer with an Mg Kα ray source, and the binding energies were calibrated to the C1s peak at 284.6 eV. The specific surface areas of samples were examined by nitrogen adsorption and desorption apparatus (NOVA, Quantachrome, USA) with Brunauer-Emmett-Teller (BET) method. The light absorption properties of the samples were recorded by UV-Vis diffuse reflectance spectra (UV-Vis-DRS, Hitachi UV-3600) with BaSO 4 as the reference. The photoluminescence spectra (PL) were obtained using Fluorescence spectrometry (HORIBA fluoromax-4) with excitation at 315 nm. The electron spin resonance (ESR) signals of radicals ( · OH and · O 2 − ) were tested on the X-band Bruker A-200 spectrometer (Germany).
Photocatalytic activity measurements. The photocatalytic activities for degradation of oxytetracycline (OTC) and phenol were tested by using a 500 W Xe lamp (CEL-HXF500, AULTT, China) at ambient temperature, and the optical power density was maintained about 42 mW cm −2 measured by a radiometer (CEL-NP2000, AULTT). In a typical process, 30 mg of the photocatalyst was dispersed in 30 mL OTC or phenol aqueous solution with a concentration of 20 ppm. Before irradiation, the solution was continuously stirred in the dark for 2 h to ensure the establishment of adsorption-desorption equilibrium. During the degradation test, 1 mL of suspension were taken at given time intervals, centrifuged, and filtered to obtain the suspernatant for OTC or phenol analysis. The concentrations of OTC and phenol were determined by high performance liquid chromatography (HPLC; Waters, e2695, Ireland) with a XBridge™ C18 column (5 μm, 4.6 × 250 mm) and a UV detector operated at 355 nm and 270 nm, respectively. The mobile phase consisted of methanol and water (volume ratio: 60/40) at a flow rate of 1 mL min −1 for phenol analysis 29 , and 0.005 mol L −1 oxalic acid solution/methanol/acetonitrile (60/20/20, v/v/v) with a flow rate of 0.4 mL min −1 for OTC analysis 30 . The mineralization of OTC and phenol were determined by the change of total organic carbon (TOC) in supernant on a total organic carbon analyser (Vario TOC, Elementar).
(DFT), as implemented in the Vienna Ab-initio Simulation Package (VASP) 31,32 within the framework of the projector augmented wave (PAW) method 33 . The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was utilized to describe the exchange correlation interactions 34 . A 500 eV cutoff for the plane wave basis set was adopted in all computations. Structural relaxations were carried out until the residual forces on atoms less than 0.01 eV Å −1 . The convergence criterion of self-consistent calculations for ionic relaxations was set to 1 × 10 -5 eV atom −1 . The corresponding lattice parameter of unit cell for Bi 2 WO 6 was calculated as a = 5.55 Å, b = 16.85 Å and c = 5.58 Å and that for BiOCl was a = b = 3.91 Å and c = 7.84 Å, which were in good agreement with previous reports 35,36 . For the Bi 2 WO 6 (020)/BiOCl (010) interface model, a 3 × 4 Bi 2 WO 6 (020) surface slab was used to match a 7 × 3 BiOCl (010) surface slab. The lattice mismatch between 3 × 4 Bi 2 WO 6 (020) and 7 × 3 BiOCl (010) surface slabs is 3.82%.

Results and discussion
Characterization of the catalysts. The phase structure and purity of the as-prepared Bi 2 WO 6 , BiOCl, 0.5%, 1%, 2% and 4% Bi 2 WO 6 -BiOCl were characterized by power X-ray diffraction (XRD). As shown in Fig. 1a, the peak of all the photocatalysts were sharp and narrow, indicating that the samples possess single-phase and well-identified crystalline structures (Bi 2 WO 6 : JCPDS card no.39-0256; BiOCl: JCPDS card no. 85-0861). For pure Bi 2 WO 6 , a sharp peak at 28.3° was assigned to (131) plane. While a series of typical peaks located in 12.0°, 24.1°, 25.8°, 32.5° and 33.5° were indexed as (001), (002), (101), (110) and (102) of BiOCl. The magnified part of XRD data from 27° to 39° are shown in Fig. 1b. With the augment of Bi 2 WO 6 proportions, the peak intensity of the Bi 2 WO 6 (28.3°) gradually increased, and the relative peak intensity of the BiOCl was almost unchange. This indicated that compositing of Bi 2 WO 6 cannot influence the crystal structure of BiOCl.
The structure and morphology of the samples were characterized with scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) (Fig. 2).
SEM revealed that pure Bi 2 WO 6 were nanoparticales with size of 5-10 nm, and BiOCl consisted of nanosheets with size of 0.5-1 μm (Fig. 2a,b). On the other hands, the morphology of 1% Bi 2 WO 6 -BiOCl heterostructure is shown in Fig. 2c. The surface element dispersion states of 1% Bi 2 WO 6 -BiOCl was measured by EDS technology. Strong signals from Bi, Cl, O and W elements can be observed in EDS spectra (Fig. 2d). The atomic ratio of Bi:O:Cl:W was 45:36:29:1 in the 1% Bi 2 WO 6 -BiOCl, indicating that Bi 2 WO 6 was successfully incorporated with BiOCl, which was consistent with XRD data.
Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) analyses were applied to further investigate the phase structure of Bi 2 WO 6 nanoparticles, BiOCl nanosheets, and 1% Bi 2 WO 6 -BiOCl heterojunctions. As shown in Fig. 3a, the Bi 2 WO 6 sample exhibited nanoparticles with a size of 5-10 nm. The SAED pattern indicated the single-crystalline characteristic of the Bi 2 WO 6 sample (Fig. 3b). The angle labled in the SAED pattern was 45°, which was in agreement with the theoretical value of the angle between the (200) and (202) planes. The set of diffraction spots can be indexed as the [010] zone axis of orthorhombic Bi 2 WO 6 . Figure 3c is the high-resolution TEM (HR-TEM) of Bi 2 WO 6 sample, the interplanar lattice spacing of 0.273 nm corresponded to the (200) planes of Bi 2 WO 6 . Thus, it can be found that the main exposed facet of Bi 2 WO 6 was {020} facets. The TEM image of BiOCl sheets is illustrated in Fig. 3d, and the width of BiOCl was estimated to be 0.5-1 μm. The corresponding SAED pattern (Fig. 3e), indexed as the [010] zone, showed (200) and (102) planes with an interfacial angle of 46.6°, which was identical to the theoretical value. In HR-TEM image (Fig. 3f) of BiOCl nanosheet, the distances between adjacent lattice fringes were measured as 0.194 and �ρ = �ρBi 2 WO 6 (020)/BiOCl (010) − �ρBiOCl (010) − �ρBi 2 WO 6 (020)  (200) and (102), respectively. Therefore, these BiOCl nanosheets can be considered as enclosed by dominat {010} facets. In theTEM and HR-TEM images of 1% Bi 2 WO 6 -BiOCl (Fig. 3g,h), it can be clearly observed that some Bi 2 WO 6 nanoparticles were anchored on the surface of BiOCl nanosheets. This result further demonstrated that Bi 2 WO 6 -BiOCl heterojunctions was formed in the composite. The formation of intimate interface contact is significant for promoting the charge separation to achieve high photocatalytic activity. The surface element compositions, metal oxidation states, and valence state of 1% Bi 2 WO 6 -BiOCl as well as the pure BiOCl were further characterized by XPS. All of the data were calibrated by C as reference (284.6 eV). As shown in Fig. 4a, the XPS spectra of pure BiOCl was composed of Bi, O, Cl peaks, and the 1% Bi 2 WO 6 -BiOCl samples was comprised of Bi, O, Cl and W. The high-resolution XPS spectra of 1% Bi 2 WO 6 -BiOCl are further displayed in Fig. 4b-e. The Bi 4f. XPS spectra of 1% Bi 2 WO 6 -BiOCl was deconvoluted in two peaks with binding energies (BE) of 159.1 and 164.3 eV, which were assigned to the Bi 3+ 4f 7/2 and Bi 3+ 4f 5/2 signals 37,38 . In Fig. 4c, O 2− 1 s peak located at 530.0 and 531.8 eV can be attributed to the surface lattice oxygen of 1% Bi 2 WO 6 -BiOCl and binding hydroxyls of the water attached onto the surface 39,40 . In addition, the Cl 2p peaks associated with the bingding energy at 199.3 eV and 197.9 eV were all indexed to Cl − 2p 3/2 41 . While the peaks in Fig. 4e at 35.1 and 37.2 eV were attributed to the surface W 6+ 4f 7/2 and W 6+ 4f 5/2 , indicating the existence of W 6+ oxidation state 42 .
The specific surface area and porosity of BiOCl and Bi 2 WO 6 -BiOCl were measured by the N 2 adsorption-desorption method. Compared with BiOCl (26.6 m 2 g −1 ), surface area of 1% Bi 2 WO 6 -BiOCl composite was relatively lower with a value of 14.8 m 2 g −1 , probably due to the deposition of Bi 2 WO 6 . The UV-Vis diffuse reflectance spectra (UV-Vis DRS) were conducted to determine the band gap energies of as-prepared samples. The pure BiOCl was a typical wide-band-gap semiconductor with an absorbtion edge about 415 nm (Fig. 5). A similar blue-shift of the absorption edge of 0.5%, 1%, 2% Bi 2 WO 6 -BiOCl nanosheets could be observed when comparing with BiOCl, and the absorbtion edge of 4% Bi 2 WO 6 -BiOCl showed a obvious red-shift. Based on UV-Vis DRS, the band gap of as-prepared samples can be estimated according to the Kubelka-Munk equation 43 : where α, h, ν, A, and E g were absorption coefficient, Planck's constant, light frequency, a constant, and band gap energy, respectively. The value of n was 4 for BiOCl due to its indirect transition 44 , and n = 1 for Bi 2 WO 6 since it was a direct-gap semiconductor 45 . By extrapolating the linear portion of the (αhν) 2/n versus hν curves to the energy axis at (αhν) 2/n = 0, the corresponding E g value were calculated to be 2.99, 2.80, 3.39, 3.36, 3.35 and 2.37 eV for BiOCl, Bi 2 WO 6 , 0.5%, 1%, 2%, and 4% Bi 2 WO 6 -BiOCl, respectively. www.nature.com/scientificreports/ Based on the valence band X-ray photoelectron spectra (Fig. 4f), the valance band maximum (VBM) of the BiOCl and Bi 2 WO 6 were estimated to be 1.17 and 1.85 eV vs NHE, respectively. According to the relation of E CB = E VB − E g , the conduction band minimum (CBM) of BiOCl and Bi 2 WO 6 were estimated to be − 1.82 and − 0.95 eV vs NHE, respectively. These data indicated that the CBM and VBM of BiOCl and Bi 2 WO 6 were at suitable positions to construct a heterojunction structure.
Photocatalytic activity and stability. The photocatalytic activities of the as-prepared samples were evaluated by degrading oxytetracycline (OTC) and phenol (20 ppm for each compound) under simulated sunlight irradiation. The 1% Bi 2 WO 6 -BiOCl exhibited the highest photocatalytic acticity with 98.6% removal of OTC after 80 min, while removal rates of 77.2%, 83.4%, 98.5%, and 88.3% can be observed for BiOCl, 0.5%, 2%, and 4% Bi 2 WO 6 -BiOCl composites, respectively (Fig. 6a). More excellent performace can be observed for the degradation of phenol (Fig. 6b,d). No phenol degradation could be found in blank treatment under the simulated sunlight irradiation, indicating the stability of phenol. The 1% Bi 2 WO 6 -BiOCl exhibited the highest photocatalytic activity with 93.4% removal of phenol after 5 h. In addition, Bi 2 WO 6 and BiOCl were simply mechanical mixed with a ratio of 1% (denoted as 1% mixture) for comparsion, and the phtocatalytic activity toward OTC declined to 79.4%. This further implied that the heterojunction was formed between the interfaces of Bi 2 WO 6 and BiOCl, thus leading to the favorable photocatalytic performance.
In addition, the liner relationship of − ln(C/C 0 ) vs irradiation time (t) (− ln(C/C 0 ) = kt) was investigated to simulate the degradation knietics. In this equation, C t , C 0 , and k were the OTC concentration after a certain reaction time (t), initial OTC concentration, and apparent rate constant (min −1 ), respectively. The photocatalysis of OTC were fitted well with pseudo first order reaction kinetics model. As shown in Fig. 6c, The rate constants (k) was 0.018, 0.017, 0.021, 0.049, 0.049, and 0.026 min −1 for BiOCl, 1% mixture, 0.5%, 1%, 2% and 4% Bi 2 WO 6 -BiOCl. To further explore the intrinstic photoreactivity, apparent reaction rate constant (k) was normalized to the surface area, referred to k s . The OTC degradation normalized reaction rate constant (k s ) of 1% Bi 2 WO 6 -BiOCl ( 33.1 × 10 -4 min −1 g m −2 ) was 3.9 folds greater than that of BiOCl (6.8 × 10 -4 min −1 g m −2 ).  www.nature.com/scientificreports/ Similarly, the k s for phenol degradation of 1% Bi 2 WO 6 -BiOCl (39.9 × 10 -3 h −1 g m −2 ) was also 10.1 folds higher than that of BiOCl (3.6 × 10 -3 h −1 g m −2 ). The excellent photocatalytic performance of 1% Bi 2 WO 6 -BiOCl under simulated sunlight irradiation can be also visualized through the time-dependent HPLC of OTC and phenol (Fig. 6e,f). The mineralization rate, which was evaluated as the removal of total organic carbon (TOC), is crucial for evaluating photocatalyst performance. Similar with the degradation efficiency, the highest mineralization rate was measured for 1% Bi 2 WO 6 -BiOCl with 52.2% of TOC romoval after 80 min under sunlight irradiation, which was 5.3-fold relative to that of pure BiOCl and 1.9 times that of 1% mixture (Fig. 6a). Similarly, the TOC values of phenol solution dramatically decreased from 18.63 to 1.98 mg L −1 after 5 h of sunlight irradiation, corresponding to 79.4% TOC removal, which was 2.8 times of that of BiOCl and 2.3 times that of 1% mixture (Fig. 6b). In addition, the photocatalytic degradation performance of phenol in related works is summarized for comparison (Table 1). Based on the parameters of degradation rate, it is obvious that 1% Bi 2 WO 6 -BiOCl sample in the current study exhibited favorable photocatalytic degradation and mineralization performance.
The stability of the 1% Bi 2 WO 6 -BiOCl nanosheets was investigated by cyclings of photodegradation tests. No apparent deactivation could be observed for OTC and phenol degradation after four test cycles (Fig. 7a,b). www.nature.com/scientificreports/ Morever, there was no significant changes between the fresh and used 1% Bi 2 WO 6 -BiOCl samples through XRD pattern diffraction peaks (Fig. 7c) and SEM images (Fig. 7d), suggesting its favorable stability for the photocatalytic decomposition of environmental contaminants.
Mechanism of photocatalytic activity enhancement. The recombination of photogenerated electron-hole pairs is the primary cause for the emission of photoluminescence (PL). A higher PL intensity indicates a higher recombination rate of photoexcited electron-hole pairs 53 . As shown in Fig. 8, the pure BiOCl exhibited the strongest photoluminescence intensity, while the PL intensity of Bi 2 WO 6 -BiOCl samples decreased with the introduction of Bi 2 WO 6 . The weakest PL intensity was observed for 1% Bi 2 WO 6 -BiOCl sample. It therefore can be concluded that the Bi 2 WO 6 -BiOCl heterojunctions could efficiently inhibit the recombination of photoexcited charge carriers. On the basis of the HR-TEM results, a theoretical heterojunction model through contacting Bi 2 WO 6 (020) plane and BiOCl (010) plane were constructed to investigate the charge transfer mechanism between Bi 2 WO 6 and BiOCl interface. A 3 × 4 Bi 2 WO 6 (020) surface slab and a 7 × 3 BiOCl (010) surface slab were matched to build the optimized Bi 2 WO 6 (020)/BiOCl (010) interface model with an energy minimization (Fig. 9a)   www.nature.com/scientificreports/ To visualize the electron distribution the electron distributions, the electron difference density (EDD) maps and Bader charge analysis for Bi 2 WO 6 (020)/BiOCl (010) interface were performed. The EDD mappings showed that there was an interlacing behavior between the electron rich and deficient areas. Figure 9b showed that electrons were deplected from Bi and O atoms of Bi 2 WO 6 (as shown in cyan region), while they were accumulated in the BiOCl (as shown in yellow region), which indicated that electrons from Bi 2 WO 6 could flow into BiOCl.
Combining with Bader charge analysis (Table 2), it can be summarized that the electron density of Bi 2 WO 6 (020) plane was more negative than that of BiOCl (010) plane in the Bi 2 WO 6 (020)/BiOCl (010) interface. This electron distribution resulted in a heterojunction interface electric field pointed from Bi 2 WO 6 (020) to BiOCl (010) along the (010) direction. The built-in electric field at the interface served as a driving force to rapidly separate the photo-generated electrons and enhance photocatalytic activity.
The reactive species trapping experiments were conducted to further explore the possible photocatalytic mechanisms over the 1% Bi 2 WO 6 -BiOCl sample. Ascorbic acid (AA, 5 mM), isopropanol (IPA, 5 mM), and sodiun oxalate (SO, 5 mM) were used as scavengers for superoxide radical ( · O 2 − ), hydroxyl radical ( · OH), and holes (h + ) 54 , respectively. The OTC conversion slightly decreased with the addition of IPA and SO (Fig. 10a). In   (Fig. 10b). This demonstrated that the photocatalytic process was mainly governed by the · O 2 − , and the role of h + cannot be ignored in this system. ESR spin trap technique was employed to study the main reactive oxygen species generated by 1% Bi 2 WO 6 -BiOCl. The 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used to capture the · O 2 − and · OH. After irradiation for 3 min and 10 min, the special spectrum with an intensity ratio of 1:1:1:1 quartet signal was obviously observed in 1% Bi 2 WO 6 -BiOCl (Fig. 10c), which was ascribed to the characteristic spectrum of DMPO-· O 2 − adduct 55 . In addition, the ESR signal increased with irradiation time prolonging from 3 to 10 min. At the same time weak DMPO-· OH adduct ESR signals with the relative intensities of 1:2:2:1 were detected over the 1% Bi 2 WO 6 -BiOCl under the irradiation of sunlight (Fig. 10d). Considering the results of reactive species trapping experiments and ESR characterization, it can be inferred that · O 2 − was the main active species in the photocatalytic process over the 1% Bi 2 WO 6 -BiOCl.
The schematic diagrams for CBM and VBM electrochemical potentials of BiOCl and Bi 2 WO 6 as well as the possible charge separation process of 1% Bi 2 WO 6 -BiOCl are shown in Fig. 11. The CBM potentials (− 1.82 and − 0.95 eV vs. NHE) of BiOCl and Bi 2 WO 6 were more negative than the standard redox potential of O 2 / · O 2 − (− 0.33 eV vs NHE, pH 7) 56 . The more negative potential than · O 2 − radical allowed the yield of · O 2 − via reduction of adsorbed O 2 by photogenerated e − . The VBM potentials for Bi 2 WO 6 and BiOCl (1.85 and 1.17 eV vs. NHE) were more negative than the standard redox potential edge of · OH/OH − (+ 1.99 eV vs NHE, pH 7) and · OH/H 2 O (+ 2.27 eV eV vs NHE, pH 7), indicating that the photogenerated holes cannot directly oxide H 2 O molecules to · OH radicals 57 .
Based on the above characterizations and DFT calculation results, a possible photocatalytic mechanism for 1% Bi 2 WO 6 -BiOCl is depicated in Fig. 11. The BiOCl and Bi 2 WO 6 were exicited simultaneously under simulated sunlight irradiation, the electrons in the VBM were excited into the CBM, and the same amount of holes radicals. While the photogenerated holes concentrated on the surface of BiOCl, achieving the efficient separation of the photoinduced electrons and holes on the heterojunction. Moreover the existence of internal electric field could further promote the efficient transfer of photogenerated carriers, thus leading to enhanced photocatalytic activity. The whole process can be described by the following equations:

Conclusions
The BiOCl nanosheets and Bi 2 WO 6 -BiOCl composites were successfully synthesized by the facile hydrothermal and solvothermal process. The photocatalytic activities and mineralization rates of 1% Bi 2 WO 6 -BiOCl for OTC and phenol were superior to individual BiOCl under simulated sunlight irradiation. The OTC and phenol degradation rates was almost 2.7 and 6.1 times as that of BiOCl, and the mineralization rate of OTC and phenol was 5.3 and 2.8 folds relative to that of BiOCl. The favorable photocatalytic performance was attributed to the synergistic effect of proper bandgap matching, and efficient separation of photogenerated charge carriers as a result of heterojuntion interface effect between BiOCl and Bi 2 WO 6 , which was verified by the experimental characterizations and DFT calculations. Further experiments demonstrated that the photocatalysis degradation of OTC was due to the oxidation of superoxide radical. Cyclic sunlight irradiation experiments demonstrated the reusability and stability of 1% Bi 2 WO 6 -BiOCl. Therefore, it can be concluded that the Bi 2 WO 6 -BiOCl heterojunction photocatalyst is a promising candidate for photocatalytic decomposition of organic contaminants.