English

• 0.   Introduction

  The rapid development of the human economy and society not only improves people's living conditions but also brings about a worldwide problem of environmental pollution. Among them, water pollution is particularly serious^[1-2], which seriously threatens people's health and survival. There are many reasons for water pollution, and organic dyes are one of the important pollutants.

  To solve such problems, many kinds of research have been carried out to develop advanced water purification methods^[3-4], such as physical methods, chemical methods, and biological methods, but the effect is often unsatisfactory and prone to other problems. As a green and environmentally friendly new technology, semiconductor photocatalysis technology has been tried to be applied to water pollution control in recent years. Because photocatalysis technology uses environmentally friendly and clean solar energy, it will not bring other pollution. However, a single photocatalyst still has problems such as low utilization of visible light and serious photogenerated electron recombination, which directly affects the promotion and application of photocatalytic technology^[5-7]. To further improve the performance of photocatalysts, many measures including element doping^[8], heterostructure construction^[9], and special morphology design^[10] have been tried.

  Among the newly developed photocatalytic materials, Bi-based materials have attracted extensive attention due to their excellent performance and wide application in removing organic dyes^[11-16]. Bismuth oxyhalides (BiOX, X=Cl, Br, I) have received more attention due to their excellent optoelectronic properties. Firstly, the band gap of BiOX is generally smaller than that of traditional photocatalytic materials. With the increase of the halogen atomic number, the band gap gradually decreases, and the smallest band gap of BiOI is only about 1.7-2.1 eV, which indicates their absorption efficiency for visible light far beyond the general photocatalytic materials. Secondly, BiOX is a highly anisotropic layered structure similar to the graphite structure, which is formed by the alternating arrangement of double X-ion layers and (Bi₂O₂)²⁺ layers along the c-axis direction^[17]. Other halogen atoms are readily incorporated into their lattices, thus substituting each other to form BiOX solid solutions^[18-20]. The formation of solid solutions may be an ideal strategy to achieve continuous energy level regulation. Up to now, a series of BiOX solid solutions have been obtained as efficient photocatalysts with tunable band gaps and visible light response.

  Therefore, the design and synthesis of multi-component solid solutions is an important means to develop new photocatalysts. The formation of solid solutions will regularly change the band gap, crystal structure, and internal electric field of BiOX, thus affecting its photocatalytic activity. The internal relationship between the crystal chemical characteristics of BiOX solid solution and the enhanced photocatalytic activity needs to be further understood.

  This research aimed to investigate the solid solution as the research object. In this work, Bi₃OX_y(WO₆)_1-y composites were synthesized by hydrothermal method, and the morphology of the composites showed a special erythrocyte-like shape. The as-prepared materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-Vis) spectroscopy and photoluminescence (PL). Using the removal rate of rhodamine B (RhB) as an index, the adsorption and photocatalytic properties of the as-prepared materials were investigated. Our research findings unveil the novel synthesis of erythrocyte-like materials, namely Bi₃OX_y(WO₆)_1-y (X=Cl, Br, I) solid solution. This study pioneers the application of Bi₃OX_y(WO₆)_1-y as a dual-functional adsorbent-photocatalyst, presenting a promising avenue in the field. Additionally, it establishes the fundamental groundwork for the development of bismuth-based solid solution materials.

  1.   Experimental

  All chemicals were of reagent‐grade quality obtained from commercial sources and used as supplied unless otherwise stated. Deionized water was prepared in our laboratory.

  1.1   Synthesis

  BiOCl was synthesized by a typical hydrothermal method. 4 mmol Bi(NO₃)₃·5H₂O was dissolved in 15 mL 0.3 mol·L^-1 HNO₃ with continuous stirring at room temperature. 4 mmol of KCl was dissolved in 50 mL of deionized water, and it was slowly added drop by drop into the Bi(NO₃)₃ solution. The resulting solution was stirred at room temperature for 30 min and then transferred to a 100 mL hydrothermal reactor, where it was hydrothermally treated at 170 ℃ for 8 h. After cooling to room temperature, the product was separated by centrifugation, washed, and dried at 60 ℃ for 8 h. Other BiOX samples were prepared under the same conditions by changing the precursor (KI, KBr).

  1.6 mmol of the BiOX sample prepared in the previous step was added to 40 mL of ethylene glycol and stirred. 2 mmol of Na₂WO₄·2H₂O was dissolved in 20 mL of water and added dropwise to the above solution, which was then stirred at room temperature for 30 min. The solution was transferred into a 100 mL hydrothermal reactor and hydrothermally treated at 180 ℃ for 12 h. After cooling the hydrothermal reactor to room temperature, the product was separated by centrifugation and washed three times with deionized water to remove excess reagents. The obtained samples were dried overnight at 60 ℃. The samples containing BiOX (X=Cl, Br, I) were respectively denoted as BX (X=Cl, Br, I). Pure Bi₂WO₆ was synthesized without adding BiOX and named BW.

  1.2   Characterization

  XRD analysis was performed using Bruker D8 advanced diffractometer at 40 kV and 40 mA with Cu Kα radiation (λ=0.154 18 nm) in a scanning range of 20° to 90°. The morphology and microstructure were characterized by using the SU8010 scanning electron microscope produced by Hitachi, Japan and the operating voltage was 15 kV. UV-Vis diffuse reflectance spectrum (DRS) was measured using an Agilent Cary 5000 spectrometer in the United States. XPS (thermo escalab 250 XPS system) was used to analyze the surface state of the catalyst, where Al Kα radiation was used as a source of excitation. Brunauer-Emmett-Teller (BET) surface area and pore size distribution were analyzed by nitrogen adsorption.

  1.3   Adsorption and photocatalytic performance test

  The photocatalytic performance of the samples was evaluated by decomposing RhB under the visible light source. 0.1 g of photocatalyst was dispersed in 50 mL of 20 mg·L^-1 RhB solution. The mixture was magnetically stirred in a dark environment for 30 min to achieve adsorption equilibrium. At regular intervals of 30 min, 3 mL of the solution was sampled and then centrifuged to remove the catalyst powder. The changes in the maximum absorption wavelength (554 nm) were recorded using a UV-Vis spectrophotometer to detect the variations in RhB concentration.

  The adsorption properties of the samples were measured by adsorbing high-concentration RhB solutions under dark conditions. 0.1 g of the sample was dispersed in 100 mL of 30 mg·L^-1 RhB solution. The mixture was stirred magnetically in the dark. Every 5 min, 3 mL of the solution was withdrawn. After centrifugation to remove the catalyst, the changes in its absorbance were measured using a UV-Vis spectrophotometer.

  2.   Results and discussion

  2.1   Structure and morphology

  The morphologies of BiOX and BX were analyzed by SEM. As can be seen from Fig. 1a, BiOCl was composed of many nanosheets of different sizes, with an average thickness of about 10 nm. The nanosheets were self-assembled to form a flower-like structure. BiOBr and BiOI (Fig. 1b and 1c) also showed similar morphology, and the whole was a flower-like structure formed by nanosheet self-assembly. The morphology of BCl is shown in Fig. 1d, showing a special erythrocyte-like structure. Each erythrocyte-like structure was separated from the other, with a diameter of about 1.5 μm. It was self-assembled from two-dimensional nanosheets. The morphologies and diameter of BBr and BI were similar to that of BCl (Fig. 1e and 1f). The thickness of the nanosheets of each sample was different. The thickness of BI nanosheets was the largest, about 10 nm, the thickness of BBr nanosheets was the second, and the thickness of BCl nanosheets was the smallest. However, the thickness change trend of each erythrocyte-like structure was opposite, and the thickness of BI was only 0.3 μm. The thickness of BBr and BCl was close to 0.8 μm. The concave region in the center of the erythrocyte-like structure also expands with the increase of halogen atomic number.

  Figure 1

  

  Figure 1.  SEM images of BiOCl (a), BiOBr (b), BiOI (c), BCl (d), BBr (e), and BI (f)

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  Because the morphologies of the three samples had high similarity, it was speculated that they had the same growth mechanism. We took BI as an example to conduct time-dependent experiments to investigate its formation process. The structures of BI with different hydrothermal times are shown in Fig. 2. Fig. 2a-2b showed that only irregular nanoclusters were obtained in the 3 h hydrothermal reaction. As the reaction time was extended to 6 h, erythrocyte-like structures began to appear, but a large number of nanosheets still existed (Fig. 2c-2d). When the reaction time was extended to 12 h, a complete erythrocyte-like structure with a diameter of about 1.5 μm and a thickness of about 0.6 μm was formed, and the thickness of the nanosheets was about 10 nm (Fig. 2e-2f). With the prolongation of reaction time, the concave curvature continued to increase until the densely assembled layered erythrocyte-like BI of nanosheets was obtained after 24 h of reaction (Fig. 2g-2h).

  Figure 2

  

  Figure 2.  SEM images of BI obtained when the reaction time was 3 h (a, b), 6 h (c, d), 12 h (e, f), and 24 h (g, h)

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  The phase structure and purity of the prepared samples were characterized by XRD. As shown in Fig. 3, the strong and sharp peaks in the BiOX samples indicated excellent crystallinity and high purity of the products. This suggests that the preparation method employed does not compromise the crystallinity of the products. The diffraction peaks of orthorhombic Bi₂WO₆ (PDF No.079-2381) can be found in the images, and the diffraction peaks at 2θ values of 28.30°, 32.93°, 47.16°, and 55.83° correspond to those of Bi₂WO₆ (113), (020), (220) and (313) crystal planes. With the increase of the halogen atomic number, the diffraction peak intensity of Bi₂WO₆ gradually increases, while the diffraction peak intensity of BiOX gradually weakens. In the BI image, it was even impossible to find the diffraction peak of BiOI. In comparison with BW, the diffraction peak of BiOI in BI had not shifted. This statement indicates that element I has been incorporated into the [Bi₂O₂]²⁺ layer, forming a solid solution material, rather than a conventional composite material of Bi₂WO₆ and BiOI. This reveals the successful preparation of a solid solution photocatalyst. In general, the crystal structures of the solvent (host crystal) and the solute (impurity) are similar, and the atomic or ionic radius is not much different (not more than 10%-15%), which is suitable and favorable for the formation of substituted solid solutions^[21-23]. The ionic radius of WO₄^2- (0.19 nm) and the ionic radius of Cl^- (0.18 nm), Br^- (0.19 nm), and I^- (0.22 nm) are all within a suitable range, so theoretically a substituted solid solution can be formed, and for I^- with a larger ionic radius, WO₄^2- is more easily substituted. So the diffraction peak of BiOI was not observed in the XRD pattern of BI, but for Cl^- with a smaller ionic radius, the substitution of WO₄^2- became difficult. As a result, there were still distinct diffraction peaks of BiOCl present in the XRD patterns.

  Figure 3

  

  Figure 3.  XRD patterns of the samples

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  To study the surface composition and chemical state of different samples, XPS detection was performed on different samples. As shown in Fig. 4a, all four samples showed similar XPS signals, indicating that the BCl, BBr, and BI samples had similar elements, and both elements of Bi, O, and W were present. The Bi4f and O1s peaks located at approximately 159 and 530 eV are associated with Bi—O bonds in BiOX^[24]. For the spectrum of BCl, the peak around 200 eV was assigned to Cl2p. Similarly, the peaks around 70 eV in BBr and 619 eV in BI were assigned to the Br3d and I3d signals, respectively. Combined with the previous XRD data, it can be inferred that BiOI is substituted under this reaction condition, and the generated I^- is mainly adsorbed on Bi₂WO₆ in the form of adsorption, while the substitution rate of BCl and BBr is low, and only part of the halide ions are decomposed. Fig. 4b presented the high-resolution spectrum of Bi4f, with two peaks between 158-166 eV assigned to Bi4f_7/2 and Bi4f_5/2^[25]. Through further observation, the binding energies of Bi4f changed differently, mainly caused by the presence of different halogen atoms. Because I^- in BI was mainly adsorbed on the surface, it had the greatest influence, and the Bi4f peak of BI had the smallest binding energy. The influence of halogen atoms in BCl and BBr was relatively small, so the peak shifted to the direction of high binding energy. This indicates that the interaction between Bi and halogen atoms differs in different [X-Bi-O-Bi-X] layered structures. The O1s peak^[26] and W4f peak^[27] in Fig. 4c and 4d showed the same shifting trend as Bi4f. This change in binding energy can be attributed to the change in electronegativity of different halogen atoms^[28]. In Fig. 4e, the Cl2p_1/2 and Cl2p_3/2 peaks of Cl in BCl were located at 199.8 and 198.1 eV. For BBr (Fig. 4f), the peaks at 69.6 and 68.6 eV were assigned to Br3d_3/2 and Br3d_5/2. For I3d peaks (Fig. 4g), doublets with binding energies of 619.8 and 631.3 eV correspond to I3d_5/2 and I3d_3/2, respectively^[29]. The binding energies of I3d_5/2 and I3d_3/2 are significantly different from those of I₂ at 618.3 eV, I⁵⁺ at 624.7 eV, or I⁷⁺ at 627.1 eV^[30-31], indicating the absence of other iodine species.

  Figure 4

  

  Figure 4.  XPS spectra: Survey (a), Bi4f (b), O1s (c), W4f (d), Cl2p (e), Br3d (f), and I3d (g)

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  Based on the above characterization results, the following speculations are made on the formation mechanism of the erythrocyte-like structure. The formation of Bi₂WO₆ and BiOX solid solution is a typical hydrothermal growth mechanism. These small particles will continue to grow into Bi₂WO₆ nanosheets. In detail, BiOX will be hydrolyzed to form Bi₂O₂²⁺ after being dissolved in water. In the presence of WO₄^2-, Bi₂WO₆ nuclei are preferentially formed. According to Gibbs-Thomson equation, in a supersaturated solution, to reduce the surface energy of the tiny nuclei, it will continue to grow and will soon grow into small particles^[32]. Larger particles grow at the expense of smaller particles and grow into nanosheets through dissolution-recrystallization (Ostwald ripening process)^[33-34]. The crystal growth of ordinary Bi₂WO₆ is anisotropic, but the halide ions will be adsorbed on the surface of the nanosheets, thereby inhibiting its inherent anisotropic growth of crystals, and at the same time, due to the existence of steric hindrance, the longitudinal direction of Bi₂WO₆ has prevented growth, thus leading to the creation of ultrathin nanosheets. The adsorption of halide ions will also cause edge interactions between Bi₂WO₆ nanosheets and overlap along the vertical direction. As more halide ions adsorb on the surface of Bi₂WO₆ nanosheets, larger steric hindrance will be generated, and the nanosheets will preferentially stack along the direction perpendicular to the c-axis, which easily generates curved nanostructures. During the self-assembly of nanosheets, the growth rate of the c-axis is lower than the packing velocity perpendicular to the c-axis, which is due to the weak adsorption force between the halide ions on the nanosheets, which may be a non-equilibrium kinetic growth process^[35]. Meanwhile, when the nanosheet units are stacked to form a certain roundness, further stacking will tend to stack outward due to the internal steric hindrance caused by the adsorbed halide ions, and the larger the ionic radius, the greater the steric hindrance, which also explains why the erythrocyte-like structure of BI has the largest depression in the middle.

  2.2   Specific surface area and optical properties

  The BET specific surface areas of BCl, BBr, BI, and BW were studied by an N₂ adsorption-desorption experiment. As shown in Fig. 5, the adsorption isotherms of all samples were typical type Ⅳ isotherms, indicating that mesopores existed in all four samples. The surface area, average pore diameter, and pore volume of the samples were summarized in Table 1. Compared with BW, the specific surface areas of the three solid solutions were increased, in which BI had the largest specific surface area, which was significantly higher than BCl and BBr, which had more adsorption area and reaction sites; With the increase of halogen atomic number, the average pore size and pore volume show an increasing trend. Large pore size and pore volume are more conducive to the adsorption of organic pollutants. The characterization results obtained through ζ potential measurements also reveal significant differences. The ζ potential of BW was measured to be 2.0 mV, indicating a slightly positive surface charge. This positive ζ potential could lead to less than ideal adsorption effects. In contrast, the ζ potential of BI was measured to be -19.3 mV, indicating a strong negative surface charge. Consequently, BI exhibited a high affinity for RhB adsorption.

  Figure 5

  

  Figure 5.  Typical N₂ gas adsorption-desorption isotherm of samples

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  Table 1

  

  Table 1.  BET specific surface area, average pore diameter, and pore volume of the samples

  下载: 导出CSV

  Sample	BET surface area / (m2·g-1)	Average pore diameter / nm	Pore volume / (cm3·g-1)
  BCl	46	5.51	0.064 5
  BBr	48	5.95	0.073 4
  BI	55	6.53	0.090 9
  BW	43	4.64	0.085 9

  The optical properties and electronic band structure of samples were characterized by UV-Vis DRS. Fig. 6a shows the DRS spectra of BW, BCl, BBr, and BI. The four materials had strong absorption in the ultraviolet region, and the absorption edge bands of the three solid solutions gradually redshift with the increase of halogen atomic number. The absorption edge of BCl and BBr was slightly greater than 420 nm, the visible light absorption capacity of BI was the strongest, and the absorption edge reached 530 nm, which could be used for visible photocatalysis. The steep trend of the curve shows that the absorption of visible light is not caused by the transition of impurity energy level, but by the band gap transition^[36]. As we all know, the optical absorption of semiconductors near the band edge follows the formula: αhν=A(hν-E_g)^n/2, where α, h, ν, A, and E_g represent the absorption coefficient, Planck′s constant, light frequency, proportionality constant, and bandgap energy, respectively. The value of slope n for indirect bandgap materials is set to 4^[37]. As shown in Fig. 6b, the band gap widths of BW, BCl, BBr, and BI were 2.61, 2.67, 2.66, and 2.19 eV respectively. The band gap mainly reflects the characteristics of semiconductors. A low band gap can promote relatively low electron excitation energy, and more electrons in the conduction band can be used for photocatalysis. After forming solid solution with BW, the band gap widths of BCl and BBr were reduced to varying degrees compared with BiOCl (3.2 eV) and BiOBr (2.8 eV). Although the band gap of BI was wider than BiOI (1.9 eV), it was narrower than that of BW. The band gap values of the three solid solutions were between pure BiOX and Bi₂WO₆, which was related to the gradual substitution of halogen ions by WO₄^2-, which further proved the formation of solid solutions. This result shows that the band gap value of BiOX can be effectively adjusted by forming a solid solution with Bi₂WO₆. It is well known that improving the visible light absorption can improve the utilization of solar energy, to obtain good photocatalytic performance.

  Figure 6

  

  Figure 6.  UV-Vis DRS spectra (a) and the plots of (αhν)^1/2 versus energy (hν) (b) of the samples

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  The photocatalytic activity of photocatalysts depends in part on their quantum efficiency, which is usually estimated by the corresponding luminescence spectrum. As shown in Fig. 7, the PL spectra of four materials were obtained under the excitation wavelength of 350 nm, and the separation and recombination of the photogenerated electrons and holes of the synthesized composites were analyzed. The PL intensity of the samples from large to small was BW, BI, BCl, and BBr. The lower the peak, the higher the separation efficiency of photo-generated electrons and holes, indicating that the formation of the solid solution significantly inhibits the recombination of photo-generated electrons and holes. The higher PL intensity in BI can be attributed to the narrower bandgap width.

  Figure 7

  

  Figure 7.  PL spectra of the samples

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  2.3   Adsorption and photocatalytic properties

  The adsorption and photocatalytic properties of the five catalysts were tested by removing RhB at room temperature. First, it was stirred in the dark for 30 min to reach adsorption equilibrium. As shown in Fig. 8a, after the dark reaction, the RhB removal rates of BI and BBr had reached 65% and 61.5% respectively, and the BCl adsorption capacity was slightly poor, only 24.7%, but it was also higher than the unmodified BW (18.8%), which was consistent with the previous N₂ adsorption results, indicating that the adsorption capacity of the modified material was significantly enhanced. After the light source was turned on, the photodegradation reaction was carried out. Compared with BW, the degradation rate of the three modified materials was significantly improved. BI and BBr almost completely removed RhB in the solution, and the removal rate of BCl was 66.3%, much higher than 36.5% of BW. This indicates that the performance improvement is caused by the formation of a solid solution between BiOX and Bi₂WO₆, which leads to the synergistic effect of adsorption and photocatalysis.

  Figure 8

  

  Figure 8.  RhB removal rate curves (a) and first-order kinetic curves (b) of the samples

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  The photocatalytic reaction followed the first-order reaction kinetic equation, as shown in Fig. 8b. The linear fitting correlation value R of each group of data was greater than 0.9, indicating that the reliability of the trend line was very high. Fig. 9a shows the kinetic constants of different samples. The apparent rate constants of BCl, BBr, and BI were 2.72, 3.51, and 5.57 times that of BW, respectively. Fig. 9b shows the proportion of adsorption and photodegradation in the RhB removal rate of different samples. The photodegradation percentage of BCl was higher than that of adsorption, indicating that photodegradation occupied the main advantage in the removal process, which also corresponded to the results of PL. Compared to BCl and BBr, BI exhibited higher adsorption capacities, resulting in a relatively smaller proportion of photocatalytic degradation.

  Figure 9

  

  Figure 9.  Kinetic constants (a) of the photodegradation process and the percentage of adsorption and photodegradation (b)

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  Considering that BBr and BI had strong adsorption on RhB, the adsorption kinetics of the material was studied by measuring the change of RhB concentration with time. The adsorption process was carried out in the dark, and a higher concentration of RhB solution (30 mg·L^-1) was selected as the adsorption solution. The adsorption kinetics of BX are shown in Fig. 10. The amount Q_t (mg·g^-1) of RhB adsorbed on the photocatalyst at time t was calculated by the following formula:

  $ \begin{equation} Q_t=\frac{V\left(c_0-c_t\right)}{m} \end{equation} $

  (1)

  Figure 10

  

  Figure 10.  Adsorption kinetics curves (a) and pseudo-second-order kinetics plots (b) for RhB at an initial concentration of 30 mg·L^-1 on BWI-1.6

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  Where c₀ (mg·L^-1) and c_t (mg·L^-1) are the concentration of RhB at the beginning and at time t, respectively, while V (L) and m (g) are the volume of RhB solution, and the mass of photocatalyst, respectively. The adsorption capacity of photocatalyst at adsorption equilibrium is defined as Q_e.

  It could be seen from Fig. 10a that the adsorption of RhB on the catalyst was a rapid adsorption process in the first 5 min. When the time reached 20 min, no obvious adsorption was found with the extension of time, which means that the adsorption equilibrium was reached. After the adsorption experiment, the color of the composite photocatalyst changed from gray to dark purple. In addition, the pseudo-second-order kinetic model was used to study the adsorption behavior, and the equation is expressed as^[38-39]:

  $ \begin{equation} \frac{t}{Q_t}=\frac{1}{k_2 Q_{\mathrm{e}}^2}+\frac{t}{Q_{\mathrm{e}}} \end{equation} $

  (2)

  where k₂ (g·mg^-1·min^-1) is the rate constant of pseudo-second-order adsorption. Fig. 10b shows the linear graph of t/Q_t versus t of the experimental data in Fig. 10a, and Q_e and k₂ can be obtained from the slope and intercept, respectively. The results show that the adsorption data fit the pseudo-second-order kinetic model well, and the fitting correlation coefficient R was greater than 0.999, indicating that the adsorption of RhB on photocatalyst conforms to the second-order kinetic model.

  2.4   Mechanism of synergistic removal of RhB by adsorption and photocatalytic degradation

  Combined with the results of adsorption and photocatalytic degradation, BX may be an excellent material, which can be used as an adsorbent and visible light photocatalyst to remove RhB in water. BX first rapidly enriches RhB molecules from high-concentration wastewater by adsorption and then generates a low-concentration RhB solution. Under visible light irradiation, the remaining RhB in the solution was further removed by photocatalytic degradation. A possible adsorption enhancement mechanism is proposed, which may be caused by the increase in surface area and average pore size. The pseudo-second-order model well explains the kinetics of the adsorption process, indicating that the adsorption of RhB on BX will be more inclined to the interaction of chemical adsorption type. At the same time, the light absorption capacity of BX is significantly higher than that of Bi₂WO₆, the separation efficiency of photogenerated charges is also improved, and the photocatalytic efficiency is further improved. The synergistic effect of adsorption and photocatalysis greatly improves the removal efficiency of RhB.

  3.   Conclusions

  Bi₃OX_y(WO₆)_1-y (X=Cl, Br, I) composites were successfully synthesized by a simple two-step hydrothermal method. It was found that Bi₃OX_y(WO₆)_1-y (X=Cl, Br, I) solid solution materials had regular erythrocyte-like morphology and were self-assembled by many ultra-thin nanosheets. A possible formation mechanism of the morphology was proposed by analyzing the composition and surface chemical state of the material. The specific surface area and pore size distribution of the three materials were compared by BET, and their optical properties were evaluated by UV-Vis DRS and PL. Through the degradation of RhB, the adsorption and photocatalytic degradation abilities of several materials were compared. BI showed the strongest RhB removal ability. Through adsorption at high concentration, the adsorption kinetics of the composites was studied. It was found that the adsorption of the three materials accorded with the second-order kinetic equation. Therefore, we conclude that the removal process of RhB is a process of synergy between adsorption and photocatalysis. Therefore, Bi₃OX_y(WO₆)_1-y (X=Cl, Br, I) solid solution materials are a promising adsorption photodegradation integrated catalyst. At the same time, this work provides an idea for the construction of Bi solid solution.

  
  
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  Figure 1  SEM images of BiOCl (a), BiOBr (b), BiOI (c), BCl (d), BBr (e), and BI (f)

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  Figure 2  SEM images of BI obtained when the reaction time was 3 h (a, b), 6 h (c, d), 12 h (e, f), and 24 h (g, h)

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  Figure 3  XRD patterns of the samples

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  Figure 4  XPS spectra: Survey (a), Bi4f (b), O1s (c), W4f (d), Cl2p (e), Br3d (f), and I3d (g)

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  Figure 5  Typical N₂ gas adsorption-desorption isotherm of samples

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  Figure 6  UV-Vis DRS spectra (a) and the plots of (αhν)^1/2 versus energy (hν) (b) of the samples

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  Figure 7  PL spectra of the samples

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  Figure 8  RhB removal rate curves (a) and first-order kinetic curves (b) of the samples

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  Figure 9  Kinetic constants (a) of the photodegradation process and the percentage of adsorption and photodegradation (b)

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  Figure 10  Adsorption kinetics curves (a) and pseudo-second-order kinetics plots (b) for RhB at an initial concentration of 30 mg·L^-1 on BWI-1.6

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  Table 1.  BET specific surface area, average pore diameter, and pore volume of the samples

  Sample	BET surface area / (m2·g-1)	Average pore diameter / nm	Pore volume / (cm3·g-1)
  BCl	46	5.51	0.064 5
  BBr	48	5.95	0.073 4
  BI	55	6.53	0.090 9
  BW	43	4.64	0.085 9

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