Facile hydrolysis synthesis of Bi₄O₅Br₂ photocatalyst with excellent visible light photocatalytic performance for the degradation of resorcinol

Abstract

A Bi₄O₅Br₂ photocatalyst had been synthesized successfully through a facile, one-step, and energy-saving hydrolysis route. The crystal structure, morphology, composition, optical property, and specific surface area of the as-synthesized material were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflection spectra and N₂ adsorption–desorption isotherms, respectively. Moreover, the photocatalytic activity of Bi₄O₅Br₂ for resorcinol degradation under visible light irradiation was investigated systematically. The results revealed that the as-prepared Bi₄O₅Br₂ photocatalyst exhibited strong visible light absorption (band gap energy 2.0 eV), and large specific surface area (62.25 m² g⁻¹), which resulted in the enhanced photocatalytic performance for resorcinol degradation. Compared with BiOBr and P25, the photocatalytic activity of Bi₄O₅Br₂ was appropriately 31 and 12 times higher, respectively. The active oxygen species of h⁺, ˙OH, and ˙O₂⁻ dominated the resorcinol degradation process over the Bi₄O₅Br₂ photocatalyst. In addition, the relatively high stability of Bi₄O₅Br₂ will benefit its practical application for the treatment of other organic pollutants.

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1. Introduction

To solve the energy crisis and environmental pollution problems, semiconductor-based photocatalytic technology has received increasing attention.^1–3 The preparation of inexpensive and eco-friendly photocatalysts is critical for the practical application of this technology. Up to now, many semiconductors including TiO₂,^4,5 CuO/TiO₂,⁶ Cu₂O/TiO₂,⁷ ZnO,^8–10 CuO,^11,12 and Bi₂O₃ ^13,14 have been extensively investigated as photocatalysts. However, the high band-gap energy and carrier recombination efficiency restrict the usage of solar energy and further limit their practical applications. Therefore, the development of a novel photocatalyst with a narrow band-gap, high photo-activity and chemical-stability is of great importance.

Recently, bismuth oxybromide (BiOBr) has attracted tremendous interest because of its unique layered structure, suitable band-gap, and excellent photocatalytic activity.^15–17 It has been reported that the layered structure is beneficial for the separation of photogenerated hole–electron pairs because of an internal electrostatic field perpendicular to each layer;^18,19 while the suitable band-gap benefits the absorption in the visible light region, resulting in good visible-light-induced photocatalytic performance. The density functional theory (DFT) calculation results show that the conduction band minimum (CBM) of BiOBr primarily consists of the Bi 6p orbital, whereas the valence band maximum (VBM) is mainly composed of the hybrid orbital of O 2p, and Br 4p.^20,21 Further, metal/nonmetal doping has also been applied to improve the photocatalytic activity of BiOBr.^22–27 Besides BiOBr, other Br-poor BiOBr (Br : O < 1) photocatalysts,^19,28 have been reported as well. It has been demonstrated that the Br-poor BiOBr photocatalysts have narrowed band gap than ordinary BiOBr, and exhibit stronger visible light absorption.²⁹ Thus, the Br-poor BiOBr may be used as visible-light-induced photocatalysts. Recently, Bi₄O₅Br₂ has been synthesized successfully by hydrothermal route, and can degrade ciprofloxacin³⁰ and bisphenol-A efficiently under visible light irradiation.^29,31 Nevertheless, due to the high cost of ionic liquid, energy-intensive and time-consuming of hydrothermal route, synthesizing method of Bi₄O₅Br₂ still faces a huge challenge. What's more, the influences of the composition on the physicochemical property, photocatalytic activity, and visible-light-induced reaction mechanism for Bi₄O₅Br₂ also require further investigation.

Resorcinol, as a typical environmental pollutant, exists widely in the industrial effluents. This compound is also listed as endocrine disrupting chemicals for it can interfere with tri-iodothyronine and thyroxine metabolism.³² Therefore, resorcinol-contained waste water needs to be eliminated. Since resorcinol has strong toxicity, high oxygen demand, and is poor degradable through biological degradation, how to effectively eliminate resorcinol from aqueous solution has drawn significant public concerns.

In this study, Bi₄O₅Br₂ photocatalyst was synthesized through hydrolysis reaction under room temperature; its structure, morphology, optical absorption, specific surface properties, and composition were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance (DRS), nitrogen sorption and X-ray photoelectron spectroscopy (XPS). Then, the photocatalytic activity of Bi₄O₅Br₂ towards resorcinol degradation was evaluated under visible light irradiation. Finally, the roles of photo-generated reactive species (including holes, hydroxyl radicals and superoxide radicals) on the degradation rate of resorcinol were clarified in detail, and possible photocatalytic mechanism was proposed.

2. Experimental

2.1 Materials and methods

The bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) and potassium bromide (KBr) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ethylene glycol (EG) was obtained from Tianjin Tianli Chemical Reagents Ltd. Ammonia solution (NH₃, 25%) was bought from Tianjin Guangfu Technology Development Co., Ltd. Resorcinol (C₆H₆O₂) was provided by Tianjin Fuchen Chemical Reagents Factory. TiO₂ (P25, Evonik Corp.) was commercially available from Germany. All of the chemicals were of analytical grade and used without further purification.

The Bi₄O₅Br₂ was synthesized through hydrolysis route. In a typical synthesis, 10 mmol of Bi(NO₃)₃·5H₂O and 10 mmol of KBr were dissolved in 40 mL EG with magnetic stirring at room temperature (named solution A). Meanwhile, 4 mL of ammonia solution was diluted with 16 mL distilled water (named solution B). Then, solution B was poured into solution A under vigorous stirring, leading to the formation of precipitates. After keeping on reaction for 60 min, the precipitates were collected by filtration, washed several times with distilled water, and finally dried at 50 °C. The resulting powder was ground by mortar. The schematic diagram of preparation process of Bi₄O₅Br₂ was depicted in Fig. 1. For comparison, a BiOBr sample was prepared through the same process. Under magnetic stirring, 10 mmol of Bi(NO₃)₃·5H₂O and 10 mmol of KBr were dissolved in 40 mL EG. Then, 20 mL distilled water was added into this solution. Before use, the resultant precipitates were filtered, washed, and finally dried at 50 °C.

Fig. 1 Schematic diagram of the preparation process of Bi₄O₅Br₂ photocatalyst.

2.2 Characterization

The crystalline structures of as-synthesized samples were characterized by powder X-ray diffractometer (XRD, Shimadu, Japan) with Cu Kα radiation. The morphology of photocatalyst was detected by scanning electron microscopy (SEM, JEOL JSM-7001F, Japan), and the microstructure was tested by high-resolution transmission electron microscope (HRTEM, JEOL-100CX, Japan). Sample's surface composition was analyzed by using X-ray photoelectron spectroscopy (XPS, Thermo Scientific ES-CALAB 250, USA). The specific surface areas were measured by nitrogen adsorption–desorption isotherms at 77 K according to the Brunauer–Emmett–Teller analysis (Norcross, GA). The pore size distribution of Bi₄O₅Br₂ and BiOBr was determined by using desorption isotherm through Barrett–Joyner–Halenda (BJH) method. The UV-vis diffuse reflection spectra (DRS) of samples were obtained from a UV-vis spectrophotometer (UV-3600, Shimadzu, Japan), and BaSO₄ was used as the reference. The concentration of resorcinol solution was measured by UV-visible spectrophotometer (U-3900, Hitachi).

2.3 Measurement of photocatalytic activity

The photocatalytic degradation experiments were performed in a photochemical reactor equipped with a 350 W Xe lamp combined with a 420 nm cutoff filter as the light source. The reactor was placed in a thermostatic water bath to keep the temperature constant. All photocatalytic degradations were performed under the same experimental conditions: the as-prepared photocatalyst (varying from 0.5 to 2.0 g L⁻¹) was dispersed in 100 mL resorcinol solution at various initial concentrations (from 9 to 100 mg L⁻¹) under magnetic stirring. Before light irradiation, the reaction system was stirred for 60 min in dark to allow the establishment of adsorption–desorption equilibrium between catalyst and resorcinol. Besides, during the photocatalytic process, the distance between light and liquid level was kept in 20 cm. The absorbance of supernatant centrifuged from the extracted 3 mL liquid was examined every 30 min of light irradiation.

3. Results and discussion

3.1 Characterization of Bi₄O₅Br₂

3.1.1 Structure and composition. Fig. 2 presents the XRD patterns of as-synthesized samples. From Fig. 2, it can be seen that the prepared Bi₄O₅Br₂ and BiOBr are the pure phase with excellent degree of crystallinity, and all the peaks can be accurately indexed to the below standard cards (PDF # 97-041-2591 and PDF # 09-0393). The emergence of other impurity phase is not observed. The XRD patterns indicate that Bi₄O₅Br₂ and BiOBr photocatalysts have been synthesized successfully. Further, the average size of photocatalyst crystallites has been assessed through Scherrer equation and the results show that the average size of Bi₄O₅Br₂ and BiOBr is 17.1 nm and 29.6 nm, respectively.

Fig. 2 XRD patterns of as-synthesized Bi₄O₅Br₂ and BiOBr.

Fig. 3a and b show the SEM and TEM images of Bi₄O₅Br₂ sample. It can be seen that well-defined Bi₄O₅Br₂ photocatalyst is about 5 nm in thickness and 30–50 nm in in-plane size. Fig. 3c exhibits the HRTEM image recorded on an individual Bi₄O₅Br₂ nanosheet. The observed clear lattice fringes with interplanar distances around 0.281 nm indicate the high crystallinity of Bi₄O₅Br₂ nanosheet.

Fig. 3 SEM (a), TEM (b), and HRTEM (c) images of as-synthesized Bi₄O₅Br₂.

The elements and chemical state data present on Bi₄O₅Br₂ surfaces are supplied by XPS technique, and the spectra are shown in Fig. 4. The binding energies obtained in the XPS analysis are corrected for specimen charging by referencing C 1s to C 284.6 eV. According to Fig. 4a, the observation of transition peaks involving in the C 1s, Bi 4f, Br 3d, and O 1s orbitals reveals that the as-synthesized sample is composed with Bi, O, and Br elements. Two strong peaks centered at 159.2 and 164.5 eV could attribute to Bi 4f_5/2 and Bi 4f_7/2, demonstrating the trivalent oxidation state of the Bi element in Bi–O bonding (Fig. 4b). Fig. 4c shows the high-resolution XPS spectra for the O 1s region, which can be fitted into two peaks. The main peaks at 530.0 and 531.3 eV are assigned to the Bi–O bonds in [Bi₂O₂]²⁺ slabs, and the hydroxyl groups on the surface, respectively.^33,34 The binding energy of 68.5 and 69.4 eV exhibited in Fig. 4d is referred to Br 3d_3/2 and Br 3d_5/2, indicating the monovalent oxidation state of the Br element.

Fig. 4 High resolution XPS spectra of as-prepared Bi₄O₅Br₂ photocatalyst (a) total survey; (b) Bi 4f; (c) O 1s; (d) Br 3d.

3.1.2 Optical absorption property. The optical absorption property of as-synthesized BiOBr and Bi₄O₅Br₂ photocatalysts is characterized by UV-vis diffuse reflectance spectroscopy and the results are shown in Fig. 5. The band gap energy E_g of these two samples is determined by extrapolating the straight portion of (αhν)^1/2-vs.-(hν) plot to the α = 0 point. The results show that the E_g values of BiOBr and Bi₄O₅Br₂ samples are 2.75 and 2.0 eV. It can be seen clearly that the E_g of Bi₄O₅Br₂ is obviously lower than that of BiOBr sample, further convincing that the Br-poor BiOBr can exactly narrow the band gap than ordinary BiOBr. The optical property of the as-synthesized Bi₄O₅Br₂ sample implies that it can degrade organic pollutants under both ultraviolet and visible light irradiation in theory.

Fig. 5 UV-vis diffuse reflectance spectra (DRS) of BiOBr and Bi₄O₅Br₂ samples; inset is the plot of (αhν)^1/2 versus (hν).

3.1.3 BET surface area and pore structure. In general, the specific surface area and pore diameter distribution of a photocatalyst are also important characteristics, and exhibit significant influences on its photocatalytic activity. Therefore, the Brunauer–Emmett–Teller (BET) specific surface area and the pore diameter distribution of as-synthesized Bi₄O₅Br₂ and BiOBr samples were investigated by using a specific surface analyzer. As shown in Fig. 6, the nitrogen adsorption–desorption isotherm curves of Bi₄O₅Br₂ and BiOBr display the IVth isothermal curve, which is typical characteristics of mesoporous material.³⁵ The BET specific surface area of Bi₄O₅Br₂ is calculated to be 62.25 m² g⁻¹, much larger than that of BiOBr, which is 3.477 m² g⁻¹. Upon observing the Barrett–Joyner–Halenda (BJH) pore diameter distribution diagram obtained from desorption isotherm, pore diameter of Bi₄O₅Br₂ mainly ranges from 2 nm to 25 nm, and the most probable distribution appears at 10 nm. The pore volume is calculated to be 0.229 cm³ g⁻¹. However, the pore size distribution of BiOBr is more spread out, and its pore volume is 0.0094 cm³ g⁻¹.

Fig. 6 N₂ absorption–desorption isotherm curve for Bi₄O₅Br₂ and BiOBr photocatalyst (inset is pore diameter distribution).

3.2 Photocatalytic activity of Bi₄O₅Br₂

To investigate whether Bi₄O₅Br₂ photocatalyst can be excited by visible light and exhibit photocatalytic activity, the photocatalytic degradation of resorcinol under visible light irradiation was examined. It can be observed clearly from Fig. 7 that the resorcinol could hardly be degraded without any photocatalyst under visible light irradiation. After 180 min light irradiation, the degradation percentage of resorcinol over Bi₄O₅Br₂, BiOBr, and P25 is about 93.8%, 8.7%, and 20.4%, respectively. The high photocatalytic activity of Bi₄O₅Br₂ may be ascribed to its lower E_g than that of BiOBr and P25. The narrower energy gap makes the visible light utilization of Bi₄O₅Br₂ more efficiently than the latter. Additionally, the high specific surface area may also contribute to its high photoactivity.

Fig. 7 Degradation curves of resorcinol with Bi₄O₅Br₂, BiOBr, and P25 under visible light irradiation (experiment condition: c_resorcinol = 18 mg L⁻¹, m_Bi4O5Br2 = 1.5 mg L⁻¹).

The effects of Bi₄O₅Br₂ dosage (from 0.5 to 2 g L⁻¹) and resorcinol initial concentration (in the range of 9–100 mg L⁻¹) on photocatalytic degradation of resorcinol were investigated at the natural pH of the suspension (6.2, without adjustment). The results are illustrated in Fig. 8. According to Fig. 8a, the degradation efficiency of resorcinol increases significantly as the Bi₄O₅Br₂ dosage increases from 0.5 to 1.5 mg L⁻¹, and then nearly the same with a further increase in Bi₄O₅Br₂ dosage to 2.0 mg L⁻¹. It is easy to understand that a lower photocatalytic activity is exhibited with low catalyst dosage for the smaller amount of catalytic active sites. When the catalyst dosage is much higher than the optimal value, the competition between the increase of active sites and light scattering of the catalyst will not result in the continued increase of photocatalytic activity. The effect of resorcinol initial concentration on the degradation efficiency is illustrated in Fig. 8b. It is clearly observed that the resorcinol degradation rate with Bi₄O₅Br₂ decreased following the increase of initial resorcinol concentration. The number of active sites and active species available for photocatalytic reaction remains the same at a fixed Bi₄O₅Br₂ dosage; these active sites and species could not meet the requirement of increased resorcinol molecules. Besides, the partial decomposition products of resorcinol could adsorb more strongly at the surface of Bi₄O₅Br₂ than resorcinol itself, and these products could occupy the active sites, consume the active species, and further impede the photocatalytic process.

Fig. 8 Effect of catalyst dosage (a), and resorcinol initial concentration (b) on the photocatalytic performances of Bi₄O₅Br₂.

3.3 Photocatalytic degradation mechanism

In order to further understand the photocatalytic mechanism of resorcinol over Bi₄O₅Br₂ sample, the possible active oxygen species involving in the photocatalytic reaction process are identified by adding scavengers, i.e. h⁺ scavenger (ammonium oxalate, AO), ˙OH scavenger (isopropanol, IPA), and ˙O₂⁻ scavenger (ascorbic acid, AA). The result of these experiments is shown in Fig. 9. It can be observed that with the addition of AO, IPA, and AA, about 43.45%, 61.83%, and 36.77% of resorcinol is degraded after 180 min irradiation. These results indicate that h⁺, ˙OH, and ˙O₂⁻ are all responsible for resorcinol degradation by Bi₄O₅Br₂, which is different from the results on photodegradation of resorcinol with ZnO photocatalyst.³⁶

Fig. 9 Effects of scavengers in the photodegradation process of resorcinol over Bi₄O₅Br₂.

3.4 Stability of Bi₄O₅Br₂

The XRD of Bi₄O₅Br₂ after photocatalytic reaction and the recycling properties of Bi₄O₅Br₂ are performed to investigate the stability of Bi₄O₅Br₂. As indicated in Fig. 2, the structure of Bi₄O₅Br₂ after photocatalytic reaction is the same as that of unused samples. Moreover, Bi₄O₅Br₂ maintains high photocatalytic activity during four reaction cycles (Fig. 10). These features further convince the stability of Bi₄O₅Br₂ synthesized by this facile hydrolysis method, which is significant for its practical application.

Fig. 10 Recycling property of Bi₄O₅Br₂ photocatalyst.

4. Conclusion

In summary, Bi₄O₅Br₂ photocatalyst has been synthesized successfully through a facile, one-step, and fast hydrolysis method. Owing to its relatively narrow band gap, and large specific surface area, the as-synthesized sample exhibits good visible-light-induced photocatalytic activity towards resorcinol degradation. The degradation efficiency of resorcinol attains 93.8% after 180 min visible light irradiation. In addition, the as-prepared Bi₄O₅Br₂ exhibits excellent stability in resorcinol degradation, and maintains high photocatalytic activity during four reaction cycles, which will benefit its practical application for the treatment of other organic pollutants.

Acknowledgements

This project was financially supported by the University Science and Technology Innovation Project of Shanxi Province (2013159).

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