English

• 0.   Introduction

  Various organic dyes have been used in industrial processes, such as leather, textile, press, food, drug, and other areas, which cause the generation of water pollution. It is reported that dyeing wastewater is harmful to humans owing to its toxicity, mutagenicity, and carcinogenicity^[1-2]. Dyeing wastewater has the characteristics of large water volume, high organic pollutant content, deep color, and high water solubility, making it difficult to treat. How to deal with dyeing wastewater effectively and economically has always been a topic in water pollution treatment^[3-4]. Many technologies such as adsorption^[5], coagulation and precipitation^[6], activated sludge process^[7-8], membrane filtration^[9], and photo- catalytic oxidation^[10-11] were used to control wastewater emission. Among them, photocatalytic technology has attracted more attention than other methods due to its high efficiency, simple operation, and low environmen- tal side effects. Furthermore, an increasing number of photocatalysts are being developed. TiO₂ is the first semiconductor photocatalytic material discovered and studied, but due to its wide bandgap, it can only respond to ultraviolet light, which only accounts for 5% of sunlight, greatly limiting the utilization of the material^[12-13]. In recent years, bismuth - based oxides have been extensively studied, among which bismuth oxybromide (BiOBr) has attracted more attention, demonstrating excellent performance in catalytic degradation of organic pollutants, photocatalytic reduction of carbon dioxide, and other fields. However, many studies showed that BiOBr has a high rate of hole - electron pairs recombination and is difficult to recover and separate, which decrease its photocatalytic activity and limit its practical applications. Researchers have adopted many methods, such as doping^[14-15], precious metal deposition^[16], and formation of heterostructures^[17-18], to improve the photocatalytic efficiency of BiOBr parti- cles. Guo et al. synthesized transition metal in - situ doped BiOBr by selecting different metals (M=Co, Ni, Cu, Fe) and confirmed that transition metal doping modification can induce the generation of oxygen vacancy defects, which lead to the generation of a large number of hydroxyl radicals with stronger oxidative capacity^[19]. Leticia et al. have prepared n - p BiOBr/ FeWO₄ heterojunctions, which exhibited excellent photodegradation performance towards organic dyes under visible light^[20]. Han et al. described the synthesis of C@BiOBr with excellent photodegradation performance towards phenol using glucose as the carbon pre- cursor. It is attributed to the good conductivity of carbon and enhanced separation of the photocarriers by carbon coating^[21]. Zhong et al. used green tea as a raw material to obtain carbon dots, which introduced the BiOBr/C₃N₄ heterostructure, and the composite exhibit- ed good photocatalytic performance for rhodamine B (RhB) degradation by enhancing electron transfer and band regulation^[22]. Wang et al. embedded carbon quantum dots in BiOBr/Ti ₃C₂ heterojunction, which exhibited excellent photocatalytic degradation for multiple quinolone antibiotics^[23].

  Corn straws are a rich resource as a common waste in agricultural production. The corn straw production is about 216 per year in China and accounts for about 20% to 30% of the global total^[24]. The large accumulation of corn straws occupies land, and their disposal is also a headache. The usual incineration method wastes fuel and electricity and releases smoke and exhaust gases that pollute the atmosphere. How to effectively utilize this agricultural waste and turn it into treasure has always been a topic in solid waste treatment. For example, Feng et al. have used sulfonated corn stalk (SCS) and acrylic acid to prepare the double network hydrogel SCS-gel, which exhibited a stable adsorption capacity for heavy metals from metal mine gallery effluent^[25]. Ding et al. have prepared corn strawbased carbon-doped Nb₂O₅ photocatalysts, which showed excellent photocatalytic performance for organic dye degradation under visible light^[26]. Corn straw has a unique biological structure and chemical properties, which can affect the structure and properties of BiOBr, thereby improving its photocatalytic effi- ciency. In addition, corn straw shows significant properties such as high chemical stability, tunable surface loading features, low cost, eco-friendly properties, easy accessibility, abundance, and catalytic active sites.

  To improve the photocatalytic activity and recy-clability of BiOBr nanoparticles and reuse agricultural waste, a novel corn stalk - derived carbon/BiOBr (CS/ BiOBr) composite was successfully fabricated. The photocatalytic degradation performance of CS/BiOBr for RhB dye was studied. The possible mechanisms of the enhanced photocatalytic performance of CS/BiOBr were also explored.

  1.   Experimental

  1.1   Materials

  Corn straws were collected from Jiangsu Province, China. First, corn straws were pulverized using a mill, sieved through a 200 - mesh screen, then soaked with deionized water, dried in an oven at 80 ℃, and calcined in a muffle furnace at 400 ℃ for 2 h. The product was cooled naturally to room temperature to obtain powder CS. All other chemicals used in the experiment are analytical pure, obtained from Sinopharm Group Chemical Reagent Co., Ltd.

  1.2   Preparation of the samples

  CS/BiOBr was fabricated by a hydrothermal method. Firstly, 0.02 mol of bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) was dissolved in 20 mL of ethylene glycol under continuous stirring for 30 min. And 0.02 mol of sodium bromide (NaBr) was dissolved in 20 mL of ethanol under continuous stirring. The Bi precursor solution was added dropwise to the Br precursor solution with stirring to obtain solution A. Then, 1 g of CS was dispersed into distilled water to form suspension B. Next, solution A was slowly dropped into sus- pension B via constant stirring to yield suspension C. After stirring intensely for 1 h, suspension C was transferred into a 100 mL Teflon-lined stainless-steel autoclave at 180 ℃ for 6 h. After the autoclave was cooled to room temperature, the product was filtered and washed several times with deionized water and ethanol. Finally, the products were dried at 80 ℃. Pure BiOBr was prepared with the same method except for the addition of CS.

  1.3   Characterization

  X-ray diffraction (XRD) patterns of photocatalysts were filed by using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ=0.154 nm), the workning rate of 10 (°)·min-1, and the scanning range (2θ) of 5°-70°. The molecular structure was determined by a Nicolet 5700 infrared spectrometer (FTIR). The surface morphology of the samples was determined by scanning electron microscope (SEM, Sigma 500 of the Carl Zeiss company). An energy dispersive X-ray spectrum (EDS) microanalysis probe (IE250, Oxford Instru- ments, UK) was used to analyze the elemental composition of CS/BiOBr. The Brunauer-Emmett-Teller specific surface area (S_BET) and pore size were analyzed by N₂ adsorption - desorption isotherm analysis (Tristar Ⅱ 3020, USA). Surface element compositions were determined by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000, Japan). Diffuse reflectance spectra (DRS) were performed using a UV-Vis spectrophotometer (Shimadzu UV - 2450, Japan) and BaSO₄ was used as the blank reference. The photoelectro-chemical experiment was conducted on an electrochem- ical workstation (Chenhua CHI 760E, China) in a conventional three - electrode configuration with a Pt wire as the counter electrode and a saturated calomel electrode as the reference.

  1.4   Photocatalytic experiment

  The photocatalytic efficiencies of as - synthesized samples were measured by photocatalytic degradation of RhB solution (10 mg·L^-1), utilizing a 300 W Xe lamp operating cut - off filter (λ >420 nm). Before the photocatalytic process, 100 mg (0.5 g·L^-1) photocatalysts was added to RhB solution and maintained in the dark under stirring for 30 min to reach adsorption- desorption equilibrium. Then, it was exposed to the visible - light source. After a given irradiation time, 6 mL of dispersion was removed and separated from the photocatalyst. RhB content was analyzed by UV-visible absorbance at its characteristic peaks (554 nm) to determine the content remaining.

  2.   Results and discussion

  2.1   XRD analysis

  Crystal structures of BiOBr, CS, and CS/BiOBr photocatalysts were characterized by XRD. As shown in Fig. 1, the wide peak between 20° and 30° can corre-spond to the non-crystalline structure of biochar^[27]. The diffraction peaks with 2θ values at 10.90°, 21.93°, 25.16°, 32.69°, 33.10°, 39.38°, 44.69°, 46.21°, 50.67°, 53.38°, 57.12°, 61.90°, and 67.40° correspond to the crystal planes of (001), (002), (101), (102), (110), (112), (004), (200), (104), (211), (212), (105), and (220) of tetragonal phase BiOBr (PDF No.52-0084). After the introduction of CS, the diffraction peaks of the composites did not shift compared with those of pure BiOBr. The above phenomenon suggests that the addition of CS did not change the crystal structure of BiOBr^[28]. However, no characteristic diffraction peaks of CS were observed in the composites. The main reason might be that CS entered the laminates of BiOBr and was highly dispersed in the composites. The existence of CS in the composite has been further identified using FTIR, EDS, and XPS characterization^[29-30].

  Figure 1

  

  Figure 1.  XRD patterns of CS, BiOBr, and CS/BiOBr

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  2.2   FTIR analysis

  The chemical structure of the samples was analyzed using FTIR. FTIR spectrum of CS is shown in Fig. 2. The peak at 3 427 cm^-1 is attributed to the stretching vibration of the O—H bond and adsorbed water on its surface. The absorption peaks appeared at 1 696, 1 587, 1 376, 1 092, and 797 cm^-1, which belong to the stretching vibration of C=O, the stretching vibration of the COOH group, the C=C bond on the aromatic ring, the stretching vibration of the C— O—C bond, and the bending vibration of C—H bond, respectively^[29]. Furthermore, in the FTIR spectra of BiOBr and CS/BiOBr, the peak at 514 cm^-1 is associated with the Bi—O stretching mode, proving the exis- tence of BiOBr^[30-31]. The characteristic peaks located at 1 613 cm^-1 correspond to the stretching vibration of C=O in carboxyl groups^[32]. The observation confirmed that the CS/BiOBr composites were successfully prepared

  Figure 2

  

  Figure 2.  FTIR spectra of CS, BiOBr, and CS/BiOBr

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  2.3   SEM and EDS analysis

  In Fig. 3a, it can be seen that raw corn straw had a natural vascular plant skeleton with a rough surface^[33]. After carbonization at 400 ℃, the vascular wall morphology of corn straw was not broken. The CS prepared by high - temperature pyrolysis had a porous structure with a few fragments attached to the surface, as shown in Fig. 3b. The presence of pores on the CS surfaces provides suitable sites for BiOBr particles to settle^[34-35]. In Fig. 3c and 3d, the single BiOBr displayed a tightly stacked sheet - like structure. With the introduction of CS, the morphology of BiOBr underwent a significant transformation, forming a floral spherical structure. Due to the presence of carboxyl and hydroxyl function- al groups in CS, these groups interact with Bi³⁺ during the hydrothermal process, and the cross-linking of the sheets resulted in the formation of a flower-like structure, as shown in Fig. 3e and 3f. It can provide more active points for the adsorption of organic pollutants and help enhance the photocatalytic activity of the sample^{[29, 36]}.

  Figure 3

  

  Figure 3.  SEM images of (a) raw corn straws, (b) CS, (c, d) BiOBr, and (e, f) CS/BiOBr

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  The corresponding EDS mappings of the sample were analyzed to identify the elemental composition of the sample (Fig. 4). As shown in Fig. 4b-4f, the elements Bi, Br, O, C, and P were all distributed uniformly in the composite. The mass fractions of C, O, P, Br, and Bi for CS/BiOBr were 16.67%, 6.43%, 1.31%, 18.94%, and 56.65%, respectively. Introducing the C element is believed to make for adsorbing more visible light and pro- moting the photogenerated charge separation^[37-38]. In addition, P is a component of many important biomolecules such as nucleic acids and phospholipids in corn straw. Due to its low content, it was not reflected in XPS and IR.

  Figure 4

  

  Figure 4.  SEM image (a) and the corresponding elemental EDS-mappings (b-f) of CS/BiOBr

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  2.4   Surface area and pore size analysis

  The surface area and pore size parameters of CS, BiOBr, and CS/BiOBr are listed in Table 1. Compared to BiOBr, the synthesized CS/BiOBr composite possessed a higher BET surface area (33 m²·g^-1) and micropore volume (0.005 cm³·g^-1). The floral spherical structure prevents excessive stacking of BiOBr layers, resulting in a high specific surface area^[39]. Moreover, a high specific surface area can provide more active sites and adsorption centers for the photocatalytic process^[40].

  Table 1

  

  Table 1.  Texture parameters of the samples

  下载: 导出CSV

  Sample	SBET / (m2·g-1)	Micropore volume / (cm3·g-1)	Pore diameter / nm
  CS	101	0.03	3.2
  BiOBr	9	0.000 6	16.5
  CS/BiOBr	33	0.005	10.4

  2.5   XPS analysis

  Fig. 5a shows that the XPS survey spectra provided C1s peaks for CS and CS/BiOBr, as well as Bi4f, Br3d, and O1s peaks for BiOBr and CS/BiOBr, which were consistent with the chemical composition of the photocatalysts. The characteristic peaks of Bi4f at the bind- ing energies of 158.7 and 164.0 eV (Fig. 5b) are almost attributed to Bi³⁺ in BiOBr^[28]. The peaks located at 67.7 and 68.6 eV in Fig. 5c are attributed to the coupling of Br3d_5/2 and Br3d_3/2, respectively, indicating that the Br element exists in the form of Br^- in BiOBr. In Fig. 5d, two peaks at 529.5 and 532.3 eV in BiOBr originated from lattice oxygen (Bi—O) and adsorbed oxygen^[41]. In Fig. 5e, the peaks with binding energies at 284.3 and 285.1 eV in CS can correspond to the characteristic peaks of C—C and C—O, respectively^[32]. The binding energies of Bi4f, Br3d, and O1s of CS/BiOBr were all higher than those of BiOBr, however, the binding energies of C1s of CS/BiOBr was lower than that of CS. Such results could be similarly ascribed to the interaction between CS and BiOBr^[42].

  Figure 5

  

  Figure 5.  XPS spectra of the samples: (a) Survey; (b) Bi4f; (c) Br3d; (d) O1s; (e) C1s

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  2.6   Optical and photoelectrical properties

  The optical properties of the samples were checked using UV-Vis DRS. All samples exhibited good light absorption ability in the range of visible light (Fig. 6a). CS/BiOBr exhibited stronger light adsorption within the range of 250 to 700 nm than BiOBr, and the absorption edges underwent a red shift (The absorption edges of BiOBr and CS/BiOBr were 459 and 483 nm, respectively). The better optical properties the photocatalyst possesses, the better photocatalytic performance it has^[43]. This result can be verified from the fol- lowing band gap (E_g) of the samples determined by the Kubelka-Munk formula:

  $ \alpha h \nu=A\left(h \nu-E_{\mathrm{g}}\right)^{n / 2} $

  (1)

  Figure 6

  

  Figure 6.  (a) UV-Vis DRS and (b) Tauc curves of BiOBr and CS/BiOBr

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  where α, h, ν, and A are denoted as the absorption coefficient, Planck′s constant, optical frequency, and con- stant, respectively. As for BiOBr, the value of n is 4^[44]. Fig. 6b shows the relationship between the variation of (αhν)^1/2 with hν, and band gaps of BiOBr and CS/BiOBr were 2.64 and 2.47 eV, indicating a reduction in band gap of the sample. Typically, a smaller band gap can enhance light utilization efficiency, thus providing favorable conditions for improving the efficiency of photocatalytic reactions.

  To explore the charge generation and transfer properties of the photocatalyst, a transient current analysis was carried out under visible light illumination. Fig. 7a shows the photocurrent density versus time characteristics for BiOBr and CS/BiOBr under five cycles of intermittent illumination. It can be seen that CS/ BiOBr exhibited a higher photocurrent density than that of BiOBr; This highlights its efficient separation and photo-generated charge transportation. In addition, the charge transfer resistance of the photocatalytic materials was investigated by electrochemical impedance spectroscopy (EIS). The results are shown in Fig. 7b. The arc radius of the EIS Nyquist plot for the CS/BiOBr electrode was smaller than that of the BiOBr electrode. A smaller arc radius indicated a lower electric charge transfer resistance of the sample. Thus, in the case of CS/BiOBr, the photo-induced electron-hole pairs were more easily separated and transferred to the sample surface, subsequently enhancing the photocatalytic activity^[29].

  Figure 7

  

  Figure 7.  (a) Transient photocurrent response curves and (b) EIS Nyquist plots of the as-prepared samples

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  2.7   Photocatalytic performance

  The catalytic properties of the prepared catalysts were tested by degrading water-soluble dye RhB using a 300 W xenon lamp as a light source. After a continuing stirring for 30 min in darkness to reach the adsorp- tion - desorption equilibrium, the pure CS particles could absorb the RhB molecules, and the absorption percentage for RhB was 58% of the dark reaction. Fig. 8a shows the adsorption and photocatalytic degradation curves of different samples. The total removal efficiencies of RhB by CS, BiOBr, and CS/BiOBr were 69%, 66%, and 97%, respectively. Among them, the CS/BiOBr catalyst showed better photocatalytic performance than that of the pure BiOBr and CS. The photocatalytic degradation efficiencies of BiOBr and CS/ BiOBr in RhB solution were 57% and 94% within 60 min of visible light irradiation, however, the value of CS was less than 30%. As shown in Fig. 8c, the absorption of RhB in the visible light region significantly decreased with increasing irradiation time. The maxi- mum absorption wavelength exhibited a blue shift, and the main absorption peak of RhB gradually shifted from 554 to 498 nm, corresponding to the stepwise formation of a series of intermediates^[29].

  Figure 8

  

  Figure 8.  (a) Photocatalytic performance and (b) pseudo-first-order kinetic fitting curves of RhB degradation by as-prepared samples under visible light; (c) Absorption spectra of RhB solution after different irradiation times in the presence of CS/BiOBr

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  The enhanced photocatalytic performance is attributed to the interaction between CS and BiOBr, which accelerates the photoexcited charge carrier separation and migration. Fig. 8b presents the fitted first - order kinetic plot for degradation of RhB on the photocatalysts, where the slope of the fitted line represents the degradation rate constant (k) of RhB on the photocatalysts. The k value of CS/BiOBr was 0.043 7 min^-1, which was three times that of BiOBr (0.014 6 min^-1).

  The accumulation of small crystal grains into a flower-like structure, which makes CS/BiOBr processing a high specific surface area and improves the material′s ability to adsorb pollutants. The band gap of CS/ BiOBr was smaller than that of BiOBr, which enhances visible light utilization efficiency. Transient current analysis and EIS show that the interaction between CS and BiOBr accelerates charge carrier separation and transfer, thereby reducing the recombination population of charge carriers. These may be factors for the photoactivity enhancement.

  In addition, the stability of photocatalytic materials has an important impact on their practical application. To study the reusability of the as- prepared sam- ples, five photocatalytic cycles were carried out with CS/BiOBr as an example. After each cycle, the catalyst was filtered, washed, and dried, and then added to the fresh RhB solution to continue the experiment. As shown in Fig. 9a, no obvious loss occurred in the photocatalytic efficacy after five consecutive runs. From the analysis of the XRD patterns of CS/BiOBr before and after the cycle experiments (Fig. 9b), it was almost the same as the fresh sample, and the CS/BiOBr composites after cycle experiments still maintained the charac- teristic peak pattern of BiOBr. The results indicated that the composites had high and stable activity for the photocatalytic decomposition of dye in water.

  Figure 9

  

  Figure 9.  (a) Cycling experiments of CS/BiOBr under visible light irradiation; (b) XRD patterns of CS/BiOBr before and after the cycle experiments of RhB

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  2.8   Possible photocatalytic mechanism

  To investigate the effect of active species in the photocatalytic degradation process, free radical trapping experiments were carried out. In this work, ethyl-enediaminetetraacetic acid (EDTA), benzoquinone (BQ), and isopropanol (IPA) were used as scavengers for holes (h⁺), superoxide radicals (·O₂^-), and hydroxyl radicals (·OH), respectively^[42]. The concentration of the scavengers was 0.005 mol·L^-1. As shown in Fig. 10, compared with the absence of scavenger reagent (97.8% of RhB photodegradation efficiency), the degradation of RhB on the photocatalyst is inhibited to various degree with the addition of IPA, BQ, and EDTA, which the photodegradation efficiency at 60 min decreased to 89.4%, 53.1% and 20.7%, respectively, indicating that ·OH, ·O₂^-, and h⁺ were all taken part in the RhB photodegradation process. EDTA had the greatest inhibition (97.8% to 20.7%), indicating that h⁺ is the important active species in the catalytic process, followed by BQ (97.8% to 53.1%), and finally IPA (97.8% to 89.4%). Therefore, the high degradation efficiency of RhB on the CS/BiOBr might be ascribed to ·O ₂ ^- and h⁺ active species during the photodegrada- tion process.

  Figure 10

  

  Figure 10.  Trapping experiment of active species during the photocatalytic degradation of RhB over CS/BiOBr under visible light irradiation

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  According to the above discussion, the probable reaction mechanisms are presented. Under visible light irradiation, BiOBr absorbs light energy, acquiring excited electrons that migrate from the valence band to the conduction band, generating photogenerated electrons (e^-) and holes (h⁺). CS had excellent interfacial charge transfer capability, which can boost the separation efficiency of photogenerated electron-hole pairs. The electrons react with dissolved oxygen to generate ·O₂^- radi- cals. And h⁺ reacts with water to generate ·OH radicals. The h⁺, ·O₂^-, and ·OH can degrade RhB into small molecules. The involved photocatalytic reaction processes are described as^{[28, 45]}

  $ \begin{aligned} & \mathrm{CS} / \mathrm{BiOBr}+h \nu \rightarrow \mathrm{~h}^{+}+\mathrm{e}^{-} \\ & \mathrm{e}^{-}+\mathrm{O}_2 \rightarrow \cdot \mathrm{O}_2^{-} \\ & \mathrm{h}^{+}+\mathrm{H}_2 \mathrm{O} \rightarrow \cdot \mathrm{OH}+\mathrm{H}^{+} \\ & \mathrm{h}^{+} / \cdot \mathrm{O}_2^{-} / \cdot \mathrm{OH}+\mathrm{RhB} \rightarrow \rightarrow \text { Degradation products } \end{aligned} $

  3.   Conclusions

  A novel CS/BiOBr composite photocatalyst was successfully prepared by the hydrothermal method. The SEM images indicated that the introduction of CS promotes the formation of a unique flower - like structure in BiOBr, which not only optimizes the efficiency of light capture but also increases the SBET of BiOBr. The CS/BiOBr composite exhibited enhanced photodegradation efficiency for RhB compared with BiOBr, which resulted from a reduced photogenerated electronhole recombination. The radical trap experiments showed that the degradation of RhB was driven mainly by the participation of holes (h⁺) via the direct hole oxidation process and partly by the action of ·O₂^- and ·OH radicals. The CS/BiOBr photocatalyst exhibited excellent stability, and a less than 10% decline in activity was observed after five photocatalytic cycles. This composite catalyst is a promising candidate in dye wastewater treatment.

  
  
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  Figure 1  XRD patterns of CS, BiOBr, and CS/BiOBr

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  Figure 2  FTIR spectra of CS, BiOBr, and CS/BiOBr

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  Figure 3  SEM images of (a) raw corn straws, (b) CS, (c, d) BiOBr, and (e, f) CS/BiOBr

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  Figure 4  SEM image (a) and the corresponding elemental EDS-mappings (b-f) of CS/BiOBr

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  Figure 5  XPS spectra of the samples: (a) Survey; (b) Bi4f; (c) Br3d; (d) O1s; (e) C1s

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  Figure 6  (a) UV-Vis DRS and (b) Tauc curves of BiOBr and CS/BiOBr

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  Figure 7  (a) Transient photocurrent response curves and (b) EIS Nyquist plots of the as-prepared samples

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  Figure 8  (a) Photocatalytic performance and (b) pseudo-first-order kinetic fitting curves of RhB degradation by as-prepared samples under visible light; (c) Absorption spectra of RhB solution after different irradiation times in the presence of CS/BiOBr

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  Figure 9  (a) Cycling experiments of CS/BiOBr under visible light irradiation; (b) XRD patterns of CS/BiOBr before and after the cycle experiments of RhB

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  Figure 10  Trapping experiment of active species during the photocatalytic degradation of RhB over CS/BiOBr under visible light irradiation

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  Table 1.  Texture parameters of the samples

  Sample	SBET / (m2·g-1)	Micropore volume / (cm3·g-1)	Pore diameter / nm
  CS	101	0.03	3.2
  BiOBr	9	0.000 6	16.5
  CS/BiOBr	33	0.005	10.4

  下载: 导出CSV
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