Heterostructured α-Bi2O3/BiOCl Nanosheet for Photocatalytic Applications

Photocatalytic degradation of organic pollutants in wastewater is recognized as a promising technology. However, photocatalyst Bi2O3 responds to visible light and suffers from low quantum yield. In this study, the α-Bi2O3 was synthetized and used for removing Cl− in acidic solutions to transform BiOCl. A heterostructured α-Bi2O3/BiOCl nanosheet can be fabricated by coupling Bi2O3 (narrow band gap) with layered BiOCl (rapid photoelectron transmission). During the degradation of Rhodamine B (RhB), the Bi2O3/BiOCl composite material presented excellent photocatalytic activity. Under visible light irradiation for 60 min, the Bi2O3/BiOCl photocatalyst delivered a superior removal rate of 99.9%, which was much higher than pristine Bi2O3 (36.0%) and BiOCl (74.4%). Radical quenching experiments and electron spin resonance spectra further confirmed the dominant effect of electron holes h+ and superoxide radical anions ·O2− for the photodegradation process. This work develops a green strategy to synthesize a high-performance photocatalyst for organic dye degradation.


Introduction
Current industrial productions generate large amounts of pollutants. Hazardous organic contaminants that exist in industrial wastewater can endanger the whole ecosystem and human health [1]. Rhodamine B (RhB, C 28 H 31 ClN 2 O 3 ) is one of the most commonly used cationic dyes applied in the textile, painting, medicine and cosmetic industries [2]. Due to its high solubility in aqueous solutions and organic solvents, RhB compound has strong irritation to the skin, eyes and respiratory tract, and can even have potential carcinogenic effects [3,4]. The common removal methods of RhB include adsorption, ion exchange, photocatalytic degradation and biological treatment [5][6][7][8]. Among these methods, photocatalytic degradation is recognized as an effective technology with a high removal rate [9].

Preparation of α-Bi 2 O 3
All reagents were of analytical grade and purchased from Sinopharm Group Co. Ltd., Shanghai, China. On the basis of previous literature [19], 12.13 g of Bi(NO 3 ) 3 ·5H 2 O (25 mmol) was dissolved in 50 mL of deionized H 2 O and 4 mL of HNO 3 (68 wt%) under continuous stirring of 300 RPM for 10 min. NH 3 ·H 2 O (28 wt%) was added dropwise into the above solution until pH was~7, and then the solution was stirred under ultrasonic radiation at 80 • C for 10 h. After filtration and drying, the residue was calcined at 500 • C for 2 h to obtain the yellow α-Bi 2 O 3 product.

Photocatalytic Degradation Measurement
The photocatalytic degradation performances of the materials were evaluated by using RhB solution. First, 20 mg Bi 2 O 3 or Bi 2 O 3 /BiOCl was added into 100 mL RhB solution (20 mg/L) under stirring of 300 RPM at room temperature for 30 min in the dark, and then a 300 W Xe lamp (Asahi Spectra, MAX-303, Tokyo, Japan) was used for photocatalytic irradiation. The control experiments of adsorption were conducted as the same as photocatalytic degradation, but without light radiation during the whole process. The RhB concentration in the solution was analyzed using the ultraviolet-visible (UV-Vis) spectrophotometer (Shimadzu, UV-3600 PIUS, Kyoto, Japan) at wavelength of~550 nm.
Radical quenching experiments were conducted to detect the dominant active species for RhB degradation. Isopropanol (IPA, 10 mmol/L), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, 5 mmol/L) and 1,4-benzoquinone (BQ, 1 mmol/L) were employed to quench hydroxyl radical (·OH), hole (h + ) and superoxide radical (·O 2 − ), respectively [34]. All the experiments were conducted under the same conditions as in the photocatalytic degradation test except for the addition of a separate scavenger.
Removal rate (R t ) of pollutant was calculated using Equation (2).
where C 0 and C t are the concentration of pollutant at contact time of 0 and t, respectively. Reaction kinetic study was evaluated using the pseudo first-order model of Equation (3) [17].
where k is the apparent rate constant of equation.

Physical Properties of Materials
According to Figure S1, Bi 2 O 3 presented excellent adsorption capacity of Cl − in the acidic solution. Under the optimal conditions of pH = 1, initial Cl − concentration = 2500 mg/L, Bi 3+ :Cl − ratio = 1:1, contact time = 180 min and the removal rate of Cl − could be reached to >95%, which suggests that there is a low concentration of residual Cl − (<120 mg/L) [30]. The dechlorination products of BiOCl and Bi 2 O 3 /BiOCl were further used for photocatalysts.
The XRD patterns of Bi2O3, BiOCl and Bi2O3/BiOCl are illustrated in Figure 1a. For the Bi2O3, two sharp peaks located at around 27.5° and 33.3° could be attributed to the (120) and (200) planes of monoclinic α-Bi2O3 (JCPDS #72-0398), respectively [20]. Several diffraction peaks at approximately 11.9°, 25.8°, 32.5° and 33.4° corresponded to the (001), (101), (110) and (102) planes of tetragonal BiOCl (JCPDS #85-0861), respectively [35]. The Bi2O3/BiOCl composite displayed a hybrid pattern with characteristic peaks of α-Bi2O3 and BiOCl, which suggests that it maintained the original crystal structure of the two compositions. All materials showed sharp diffraction peaks, which indicated their good crystallinity [19].  Figure 1b presented the FTIR spectra of three materials. For the Bi2O3, two broad absorption peaks at ~3450 and ~1620 cm −1 could be classified as the stretching and bending vibrations of the O−H bond, respectively [20]. A peak located at ~850 cm −1 was attributed to the stretching vibration of Bi−O−Bi bond, and a sharp peak at ~520 cm −1 was assigned to the stretching vibration of Bi−O bond [36]. With regard to the BiOCl, the Bi−O−Bi peak disappeared and a new peak appeared at ~1380 cm −1 (stretching vibration of Bi−Cl bond) [37]. The Bi2O3/BiOCl simultaneously contained the Bi−O−Bi and Bi−Cl peaks, which indicated the successful preparation of such composite material.
The porosities of Bi2O3, BiOCl and Bi2O3/BiOCl were characterized by the liquid nitrogen adsorption-desorption isotherms in Figure 1c. All materials exhibited type IV isotherms with a hysteresis loop, which suggests the existence of mesopores (2−50 nm) [38]. Among the three samples, the Bi2O3/BiOCl expressed the largest N2 adsorption volume, which suggests its maximum specific surface area. Using the BET method, the surface area of Bi2O3/BiOCl was calculated as 11.2 m 2 /g, which was much higher than the 2.3 m 2 /g of Bi2O3 and 6.8 m 2 /g of BiOCl. In addition, Figure S2 reveals the Barrett−Joyner−Halenda (BJH) pore size distributions of three samples [39]. Similarly, the Bi2O3/BiOCl composite delivered the largest pore volume with an average pore diameter of 20.7 nm (Table S1). The large surface area and pore volume of Bi2O3/BiOCl can expose  Figure 1b presented the FTIR spectra of three materials. For the Bi 2 O 3 , two broad absorption peaks at~3450 and~1620 cm −1 could be classified as the stretching and bending vibrations of the O-H bond, respectively [20]. A peak located at~850 cm −1 was attributed to the stretching vibration of Bi-O-Bi bond, and a sharp peak at~520 cm −1 was assigned to the stretching vibration of Bi-O bond [36]. With regard to the BiOCl, the Bi-O-Bi peak disappeared and a new peak appeared at~1380 cm −1 (stretching vibration of Bi-Cl bond) [37]. The Bi 2 O 3 /BiOCl simultaneously contained the Bi-O-Bi and Bi-Cl peaks, which indicated the successful preparation of such composite material.
The porosities of Bi 2 O 3 , BiOCl and Bi 2 O 3 /BiOCl were characterized by the liquid nitrogen adsorption-desorption isotherms in Figure 1c. All materials exhibited type IV isotherms with a hysteresis loop, which suggests the existence of mesopores (2-50 nm) [38]. Among the three samples, the Bi 2 O 3 /BiOCl expressed the largest N 2 adsorption volume, which suggests its maximum specific surface area. Using the BET method, the surface area of Bi 2 O 3 /BiOCl was calculated as 11.2 m 2 /g, which was much higher than the 2.3 m 2 /g of Bi 2 O 3 and 6.8 m 2 /g of BiOCl. In addition, Figure S2 reveals the Barrett-Joyner-Halenda (BJH) pore size distributions of three samples [39]. Similarly, the Bi 2 O 3 /BiOCl composite delivered the largest pore volume with an average pore diameter of 20.7 nm (Table S1). The large surface area and pore volume of Bi 2 O 3 /BiOCl can expose more active site, which is beneficial for subsequent adsorption and photocatalysis [15,40].
The TG (25-900 • C) analysis was used to investigate the thermal stability of Bi 2 O 3 /BiOCl. From Figure S3, the small weight loss on the TG curve below 700 • C could be assigned to the absorbed water. Until the temperature rose to 700 • C, an obvious decline started to Nanomaterials 2022, 12, 3631 5 of 14 appear at 700-800 • C, which can be attributed to the pyrolysis of the BiOCl composition [35]. The result revealed the excellent thermal stability of the Bi 2 O 3 /BiOCl material.
The XPS survey spectra of Bi 2 O 3 , BiOCl and Bi 2 O 3 /BiOCl is presented in Figure S4. For the Bi 2 O 3 , some obvious peaks at around 159, 285, 442 and 531 eV were attributed to the Bi 4f, C 1s, Bi 4d and O 1s, respectively [41]. Two new peaks appeared at 197 and 268 eV in BiOCl and Bi 2 O 3 /BiOCl spectra could correspond to the Cl 2p and Cl 2s. It confirmed the successful introduction of Cl − [37]. For the high-resolution Bi 4f spectra in Figure 1d, all samples displayed two spin-orbit doublet peaks of Bi 4f 7/2 (159.2 eV) and Bi 4f 5/2 (164.5 eV). The above doublet peaks could be attributed to the presence of Bi 3+ [38]. The O 1s spectra ( Figure 1e) could be deconvoluted into two compositions. The peak at 529.8 eV was assigned to the lattice oxygen of Bi-O-Bi and the peaks at 531.3 eV originated from adsorbed oxygen in the water molecule (H-O-H) [27]. The Cl 2p high-resolution spectra ( Figure 1f) also expressed two spin-orbit doublet peaks of Cl 2p 3/2 (198.0 eV) and Cl 2p 1/2 (199.5 eV) for the characteristic of Cl − [39]. Figure 2 shows the SEM images of three samples. As observed in Figure 2a, the as-prepared Bi 2 O 3 was stacked by interlaced nanorods with irregular shapes [19,42]. These irregular nanorods had the particle size of 100-300 nm ( Figure 2b). After chlorination, the nanorods disappeared and there were only irregular nanoflakes for the BiOCl (Figure 2c,d) [27,43]. For the Bi 2 O 3 /BiOCl composite, Figure 2e displayed a distinctive structure different from both Bi 2 O 3 and BiOCl. The Bi 2 O 3 /BiOCl was assembled by numerous staggered nanosheets with a diameter of 80-200 nm (Figure 2f). Such a distinctive nanosheet array structure of Bi 2 O 3 /BiOCl can devote a large surface area and provide more active sites for photocatalytic reactions [26,33]. The mapping images (Figure 2g) detected the uniform distribution of Bi, Cl and O elements on Bi 2 O 3 /BiOCl [37,44]. more active site, which is beneficial for subsequent adsorption and photocatalysis [15,40].
The TG (25−900 °C) analysis was used to investigate the thermal stability of Bi2O3/BiOCl. From Figure S3, the small weight loss on the TG curve below 700 °C could be assigned to the absorbed water. Until the temperature rose to 700 °C, an obvious decline started to appear at 700−800 °C, which can be attributed to the pyrolysis of the Bi-OCl composition [35]. The result revealed the excellent thermal stability of the Bi2O3/BiOCl material.
The XPS survey spectra of Bi2O3, BiOCl and Bi2O3/BiOCl is presented in Figure S4. For the Bi2O3, some obvious peaks at around 159, 285, 442 and 531 eV were attributed to the Bi 4f, C 1s, Bi 4d and O 1s, respectively [41]. Two new peaks appeared at 197 and 268 eV in BiOCl and Bi2O3/BiOCl spectra could correspond to the Cl 2p and Cl 2s. It confirmed the successful introduction of Cl − [37]. For the high-resolution Bi 4f spectra in Figure 1d, all samples displayed two spin-orbit doublet peaks of Bi 4f7/2 (159.2 eV) and Bi 4f5/2 (164.5 eV). The above doublet peaks could be attributed to the presence of Bi 3+ [38]. The O 1s spectra (Figure 1e) could be deconvoluted into two compositions. The peak at 529.8 eV was assigned to the lattice oxygen of Bi−O−Bi and the peaks at 531.3 eV originated from adsorbed oxygen in the water molecule (H−O−H) [27]. The Cl 2p high-resolution spectra (Figure 1f) also expressed two spin-orbit doublet peaks of Cl 2p3/2 (198.0 eV) and Cl 2p1/2 (199.5 eV) for the characteristic of Cl − [39]. Figure 2 shows the SEM images of three samples. As observed in Figure 2a, the as-prepared Bi2O3 was stacked by interlaced nanorods with irregular shapes [19,42]. These irregular nanorods had the particle size of 100−300 nm (Figure 2b). After chlorination, the nanorods disappeared and there were only irregular nanoflakes for the BiOCl (Figure 2c,d) [27,43]. For the Bi2O3/BiOCl composite, Figure 2e displayed a distinctive structure different from both Bi2O3 and BiOCl. The Bi2O3/BiOCl was assembled by numerous staggered nanosheets with a diameter of 80−200 nm (Figure 2f). Such a distinctive nanosheet array structure of Bi2O3/BiOCl can devote a large surface area and provide more active sites for photocatalytic reactions [26,33]. The mapping images ( Figure  2g) detected the uniform distribution of Bi, Cl and O elements on Bi2O3/BiOCl [37,44]. lattice fringes. These lattice fringes exhibited different arrangement directions with various crystalline domains, and could be attributed to Bi 2 O 3 and BiOCl nanocrystallines [42]. For example, the lattice fringes with d-spacing of 0.33 nm were assigned to the (120) plane of Bi 2 O 3 , and the lattice fringes with d-spacing of 0.74 nm were assigned to the (001) plane of BiOCl (Figure 3d) [30,45]. A fast Fourier transform (FFT) image (inset in Figure 3d) illustrates regular lattice patterns for monoclinic crystal (Bi 2 O 3 ) and tetragonal crystal (BiOCl), verifying two compositions of Bi 2 O 3 /BiOCl [30]. The heterogeneous interface of the material can modify electronic structure and improve the photocatalytic performances [46]. The TEM images were used to further reveal the microstructure of Bi2O3/BiOCl. From Figure 3a,b, the Bi2O3/BiOCl presented a thin nanosheet structure, which was consistent with the SEM results. The high resolution TEM (HR-TEM) in Figure 3c showed clear lattice fringes. These lattice fringes exhibited different arrangement directions with various crystalline domains, and could be attributed to Bi2O3 and BiOCl nanocrystallines [42]. For example, the lattice fringes with d-spacing of 0.33 nm were assigned to the (120) plane of Bi2O3, and the lattice fringes with d-spacing of 0.74 nm were assigned to the (001) plane of BiOCl (Figure 3d) [30,45]. A fast Fourier transform (FFT) image (inset in Figure 3d) illustrates regular lattice patterns for monoclinic crystal (Bi2O3) and tetragonal crystal (BiOCl), verifying two compositions of Bi2O3/BiOCl [30]. The heterogeneous interface of the material can modify electronic structure and improve the photocatalytic performances [46].

Photocatalytic Performances
UV-Vis DRS were conducted to characterize the optical absorption properties of Bi2O3, BiOCl and Bi2O3/BiOCl. As illustrated in Figure 4a, Bi2O3 and Bi2O3/BiOCl possessed strong absorption in the UV light region compared with that of BiOCl [47]. The absorption edges of Bi2O3, Bi2O3/BiOCl and BiOCl were 440, 380 and 360 nm, respectively. Band gap energy (Eg) of the semiconductor photocatalyst can be calculated according to the Beer-Lambert law (Equation (S1)) based on the absorption edge [17,48]. The Eg values of Bi2O3 and BiOCl were calculated as 2.85 eV (Figure 4b) and 3.52 eV (Figure 4c). In addition, the positions of the valence band edge (EVB) and conduction band edge (ECB) for Bi2O3 and BiOCl were also estimated according to Equations (S2) and (S3) [17,49].
The optical properties of the materials are summarized in Table S2 and Figure 4d. Based on analysis above, the Bi2O3/BiOCl composite was a hybrid photocatalyst with

Photocatalytic Performances
UV-Vis DRS were conducted to characterize the optical absorption properties of Bi 2 O 3 , BiOCl and Bi 2 O 3 /BiOCl. As illustrated in Figure 4a, Bi 2 O 3 and Bi 2 O 3 /BiOCl possessed strong absorption in the UV light region compared with that of BiOCl [47]. The absorption edges of Bi 2 O 3 , Bi 2 O 3 /BiOCl and BiOCl were 440, 380 and 360 nm, respectively. Band gap energy (E g ) of the semiconductor photocatalyst can be calculated according to the Beer-Lambert law (Equation (S1)) based on the absorption edge [17,48]. The E g values of Bi 2 O 3 and BiOCl were calculated as 2.85 eV (Figure 4b) and 3.52 eV (Figure 4c). In addition, the positions of the valence band edge (E VB ) and conduction band edge (E CB ) for Bi 2 O 3 and BiOCl were also estimated according to Equations (S2) and (S3) [17,49]. Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 14 two compositions. Compared to pristine Bi2O3 or BiOCl, these hybrid compositions can modify band structure and enhance the photocatalytic activity [41,45]. RhB was employed as the target pollutant to investigate the photocatalytic performances of the materials. Figure 5a showed the adsorption effect of three samples in darkness. The RhB can exist stably in the solution during the whole process without any photocatalyst (blank curve) [37]. Adding the materials, all three curves achieved the adsorption-desorption equilibrium at a contact time of 30 min. In addition, the Bi2O3/BiOCl exhibited the maximum adsorption capacity (minimum Ct/C0 value). The reason could be attributed to the large surface area of Bi2O3/BiOCl [38]. The optical properties of the materials are summarized in Table S2 and Figure 4d. Based on analysis above, the Bi 2 O 3 /BiOCl composite was a hybrid photocatalyst with two compositions. Compared to pristine Bi 2 O 3 or BiOCl, these hybrid compositions can modify band structure and enhance the photocatalytic activity [41,45].
RhB was employed as the target pollutant to investigate the photocatalytic performances of the materials. Figure 5a showed the adsorption effect of three samples in darkness. The RhB can exist stably in the solution during the whole process without any photocatalyst (blank curve) [37]. Adding the materials, all three curves achieved the adsorptiondesorption equilibrium at a contact time of 30 min. In addition, the Bi 2 O 3 /BiOCl exhibited the maximum adsorption capacity (minimum C t /C 0 value). The reason could be attributed to the large surface area of Bi 2 O 3 /BiOCl [38]. The photodegradation behavior of RhB was displayed in Figure 5b. After the adsorption process in darkness for 30 min, there was no obvious degradation for RhB by using the Bi2O3. The Bi2O3/BiOCl composite presented significant degradation of its performance, with a total removal rate of ~99.9% at an irradiation time of 60 min (determined from the UV-Vis absorbance spectra of RhB in Figure 5c) [20]. By contrast, the total removal rates of Bi2O3 and BiOCl were only recorded as 36.0% and 74.4%, respectively. Accordingly, the removal rate of RhB increased as Bi2O3/BiOCl > BiOCl > Bi2O3 [41,42]. The surface area values of the catalysts exhibited the positive correlation with the removal rate of RhB, which suggests that the larger surface area can provide more active sites for photocatalytic reaction. Figure 5d illustrated the pseudo first-order kinetic model of RhB photodegradation [17]. Using the linear fitting, the Bi2O3/BiOCl showed the steepest straight line with the largest slope value among three samples, which indicates its excellent photocatalytic activity [35,37]. The apparent rate constant k of kinetic equation was recorded in Figure 5e. As expected, the Bi2O3/BiOCl catalyst had the maximum k value of 0.1061 min −1 . The k value was almost 100 times higher than that of Bi2O3 (0.0013 min −1 ).
Norfloxacin and tetracycline hydrochloride (TCHC) were used as targets to further estimate the photodegradation performances of Bi2O3/BiOCl. Similarly, after adsorption in darkness for 30 min, both the norfloxacin and TCHC expressed obvious degradation (Figure 5f). At irradiation time of 120 min, the total removal rates of norfloxacin and TCHC were recorded as 59% and 63%, respectively. The high removal rates of organic pollutants (e.g., RhB, norfloxacin and TCHC) demonstrated the availability of our Bi2O3/BiOCl photocatalyst [36,48].
The stability for the Bi2O3/BiOCl catalyst after 60 min photocatalytic degradation of RhB was evaluated. As shown in Figure 6a, the catalytic Bi2O3/BiOCl sample presented a similar XRD pattern to pristine Bi2O3/BiOCl, with significant characteristic peaks of monoclinic Bi2O3 and tetragonal BiOCl. Such results confirmed the good structure stability of Bi2O3/BiOCl material [49,50]. The photodegradation behavior of RhB was displayed in Figure 5b. After the adsorption process in darkness for 30 min, there was no obvious degradation for RhB by using the Bi 2 O 3 . The Bi 2 O 3 /BiOCl composite presented significant degradation of its performance, with a total removal rate of~99.9% at an irradiation time of 60 min (determined from the UV-Vis absorbance spectra of RhB in Figure 5c) [20]. By contrast, the total removal rates of Bi 2 O 3 and BiOCl were only recorded as 36.0% and 74.4%, respectively. Accordingly, the removal rate of RhB increased as Bi 2 O 3 /BiOCl > BiOCl > Bi 2 O 3 [41,42]. The surface area values of the catalysts exhibited the positive correlation with the removal rate of RhB, which suggests that the larger surface area can provide more active sites for photocatalytic reaction. Figure 5d illustrated the pseudo first-order kinetic model of RhB photodegradation [17]. Using the linear fitting, the Bi 2 O 3 /BiOCl showed the steepest straight line with the largest slope value among three samples, which indicates its excellent photocatalytic activity [35,37]. The apparent rate constant k of kinetic equation was recorded in Figure 5e. As expected, the Bi 2 O 3 /BiOCl catalyst had the maximum k value of 0.1061 min −1 . The k value was almost 100 times higher than that of Bi 2 O 3 (0.0013 min −1 ).
Norfloxacin and tetracycline hydrochloride (TCHC) were used as targets to further estimate the photodegradation performances of Bi 2 O 3 /BiOCl. Similarly, after adsorption in darkness for 30 min, both the norfloxacin and TCHC expressed obvious degradation (Figure 5f). At irradiation time of 120 min, the total removal rates of norfloxacin and TCHC were recorded as 59% and 63%, respectively. The high removal rates of organic pollutants (e.g., RhB, norfloxacin and TCHC) demonstrated the availability of our Bi 2 O 3 /BiOCl photocatalyst [36,48].
The stability for the Bi 2 O 3 /BiOCl catalyst after 60 min photocatalytic degradation of RhB was evaluated. As shown in Figure 6a, the catalytic Bi 2 O 3 /BiOCl sample presented a similar XRD pattern to pristine Bi 2 O 3 /BiOCl, with significant characteristic peaks of  . Compared to the pristine Bi2O3/BiOCl, the peak corresponding to the O was still maintained [41,45]. A slight shift toward a lower binding energy (~0.3 eV) of the Bi−O peak was ascribed to the lower oxidation state of Bi atoms [43].
As observed in Figure 6d, the catalytic Bi2O3/BiOCl sample still displayed the nanosheet shape. The results further demonstrated the potential application opportunity of the Bi2O3/BiOCl photocatalyst [50].
Radical quenching experiments were performed to explore the potential photodegradation mechanism of RhB using Bi2O3/BiOCl. IPA, EDTA-2Na and BQ were employed as scavengers to quench hydroxyl radicals (· OH), electron holes (h + ) and superoxide radical anions (· O2 − ), respectively [51]. From Figure 7a, adding the IPA into the system made almost no difference to the entire photocatalytic degradation rate. However, EDTA-2Na and BQ (especially EDTA-2Na) greatly inhibited Bi2O3/BiOCl from degrading RhB. In other words, · OH did not play a role here while h + and · O2 − dominated the photocatalytic degradation of RhB by Bi2O3/BiOCl [39,52]. Using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a capture agent, the ESR electron spin capture technology was performed to further detect the generation of ·OH and ·O 2 -. No ⋅OH signals appeared whether or not there was light ( Figure S5). As for the ·O 2 -active free radical, no signal was generated in the dark, while the obvious characteristic peak of the ·O 2 -spectrum was observed after light irradiation (Figure 7b) [35,53]. Therefore, ·O2 -   [41,45]. A slight shift toward a lower binding energy (~0.3 eV) of the Bi-O peak was ascribed to the lower oxidation state of Bi atoms [43].
As observed in Figure 6d, the catalytic Bi 2 O 3 /BiOCl sample still displayed the nanosheet shape. The results further demonstrated the potential application opportunity of the Bi 2 O 3 /BiOCl photocatalyst [50].
Radical quenching experiments were performed to explore the potential photodegradation mechanism of RhB using Bi 2 O 3 /BiOCl. IPA, EDTA-2Na and BQ were employed as scavengers to quench hydroxyl radicals (·OH), electron holes (h + ) and superoxide radical anions (·O 2 − ), respectively [51]. From Figure 7a, adding the IPA into the system made almost no difference to the entire photocatalytic degradation rate. However, EDTA-2Na and BQ (especially EDTA-2Na) greatly inhibited Bi 2 O 3 /BiOCl from degrading RhB. In other words, ·OH did not play a role here while h + and ·O 2 − dominated the photocatalytic degradation of RhB by Bi 2 O 3 /BiOCl [39,52]. Using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a capture agent, the ESR electron spin capture technology was performed to further detect the generation of ·OH and ·O 2 − . No ·OH signals appeared whether or not there was light ( Figure S5). As for the ·O 2 − active free radical, no signal was generated in the dark, while the obvious characteristic peak of the ·O 2 − spectrum was observed after light irradiation (Figure 7b) [35,53]. Therefore, ·O 2 − was proven to be produced and ·OH was absent during the light reaction, which was inconsistent with the result of radical quenching experiments. was proven to be produced and ·OH was absent during the light reaction, which was inconsistent with the result of radical quenching experiments. The main degradation route of RhB using Bi2O3/BiOCl was deduced according to LC-MS analysis, as shown in Figure 7c. The probable degradation pathway was present in Figure 7d based on the detected intermediates in Figure 7c. Generally, RhB degradation included five main steps: N-de-ethylation, chromophore cleavage, de-carboxylation, ring opening, and mineralization [35,52]. RhB was decomposed by Bi2O3/BiOCl into various molecular fragments step by step under light irradiation gradually, and was completely degraded into CO2, H2O, and finally NH4 + [52,54]. Figure 8 illustrates the synthesis process of the Bi2O3/BiOCl nanosheet and its photocatalytic degradation mechanism of RhB. The as-prepared α-Bi2O3 was used as an adsorbent to remove Cl − in acidic solution. The adsorbed Bi2O3 could transform the BiOCl and be further assembled into the heterostructured Bi2O3/BiOCl nanosheet [41]. Under visible light irradiation, the electrons on VB of Bi2O3/BiOCl were excited and transferred to CB of the material, leaving h + on the VB. These photogenerated e − can react with adsorbed O2 on the Bi2O3/BiOCl surface to produce abundant · O2 − [55,56]. Such highly oxidizing · O2 − and h + can further react with RhB during the degradation process to generate small molecular substances (e.g., CO2 and H2O) [52,54]. The main degradation route of RhB using Bi 2 O 3 /BiOCl was deduced according to LC-MS analysis, as shown in Figure 7c. The probable degradation pathway was present in Figure 7d based on the detected intermediates in Figure 7c. Generally, RhB degradation included five main steps: N-de-ethylation, chromophore cleavage, de-carboxylation, ring opening, and mineralization [35,52]. RhB was decomposed by Bi 2 O 3 /BiOCl into various molecular fragments step by step under light irradiation gradually, and was completely degraded into CO 2 , H 2 O, and finally NH 4 + [52,54]. Figure 8 illustrates the synthesis process of the Bi 2 O 3 /BiOCl nanosheet and its photocatalytic degradation mechanism of RhB. The as-prepared α-Bi 2 O 3 was used as an adsorbent to remove Cl − in acidic solution. The adsorbed Bi 2 O 3 could transform the BiOCl and be further assembled into the heterostructured Bi 2 O 3 /BiOCl nanosheet [41]. Under visible light irradiation, the electrons on VB of Bi 2 O 3 /BiOCl were excited and transferred to CB of the material, leaving h + on the VB. These photogenerated e − can react with adsorbed O 2 on the Bi 2 O 3 /BiOCl surface to produce abundant ·O 2 − [55,56]. Such highly oxidizing ·O 2 − and h + can further react with RhB during the degradation process to generate small molecular substances (e.g., CO 2 and H 2 O) [52,54].   [57][58][59][60][61]. It could be found that our Bi2O3/BiOCl material exhibited a relatively short reaction time and high removal rate. Due to the distinctive structure and good photocatalytic activity, the α-Bi2O3/BiOCl nanosheet could be considered as a promising photocatalyst of organic dye.

Conclusions
The as-prepared α-Bi2O3 was used to remove Cl − in acidic solutions to generate Bi-OCl. The α-Bi2O3/BiOCl heterostructured photocatalyst could be synthetized by coupling Bi2O3 with layered BiOCl. The composite material displayed the staggered nanosheet shape with a diameter of 80-200 nm. Both heterogeneous interface and nanosheet structure enhanced the photocatalytic activity of the material. For example, the Bi2O3/BiOCl binary catalyst delivered an excellent degradation rate of 99.9% for RhB at 60 min of visible light irradiation, which was much higher than 36.0% of Bi2O3 and 74.4% of BiOCl. Radical quenching experiments and ESR spectra confirmed that h + and · O2 − dominated the photocatalytic degradation process of RhB. The probable intermediates of RhB during the photodegradation process were further investigated using the LC-MS analysis. This work constructed a heterostructured Bi2O3/BiOCl photocatalyst with high photocatalytic performances, which provided a new opportunity for potential commercial applications.

Supplementary Materials:
The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Calculation of band gap energy Eg, valence band edge EVB and conduction band edge ECB; removal rate of Cl − onto Bi2O3 ( Figure S1), BJH pore size distributions ( Figure S2), TG curve of Bi2O3/BiOCl ( Figure S3), XPS survey spectra ( Figure S4), ESR spectra for capturing ·OH ( Figure S5); pore texture parameters of three samples (Table S1), optical properties of Bi2O3 and BiOCl (Table S2).  Table 1 listed the photocatalytic performances of RhB for some recent representative catalysts in literature [57][58][59][60][61]. It could be found that our Bi 2 O 3 /BiOCl material exhibited a relatively short reaction time and high removal rate. Due to the distinctive structure and good photocatalytic activity, the α-Bi 2 O 3 /BiOCl nanosheet could be considered as a promising photocatalyst of organic dye.

Conclusions
The as-prepared α-Bi 2 O 3 was used to remove Cl − in acidic solutions to generate BiOCl. The α-Bi 2 O 3 /BiOCl heterostructured photocatalyst could be synthetized by coupling Bi 2 O 3 with layered BiOCl. The composite material displayed the staggered nanosheet shape with a diameter of 80-200 nm. Both heterogeneous interface and nanosheet structure enhanced the photocatalytic activity of the material. For example, the Bi 2 O 3 /BiOCl binary catalyst delivered an excellent degradation rate of 99.9% for RhB at 60 min of visible light irradiation, which was much higher than 36.0% of Bi 2 O 3 and 74.4% of BiOCl. Radical quenching experiments and ESR spectra confirmed that h + and ·O 2 − dominated the photocatalytic degradation process of RhB. The probable intermediates of RhB during the photodegradation process were further investigated using the LC-MS analysis. This work constructed a heterostructured Bi 2 O 3 /BiOCl photocatalyst with high photocatalytic performances, which provided a new opportunity for potential commercial applications.