A BiOBr/Co–Ni layered double hydroxide nanocomposite with excellent adsorption and photocatalytic properties

Abstract

A BiOBr/Co–Ni–NO₃ layered double hydroxide (LDH) nanocomposite was prepared by an in situ growth method via a facile anion-exchange precipitation process. The resulting BiOBr/Co–Ni–NO₃ LDH nanocomposite was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible diffuse reflectance spectroscopy (UV-DRS), and Brunauer–Emmett–Teller (BET) surface areas. Results showed that the BiOBr nanosheets dispersed well on the surface of the LDHs in a vertical manner, like arrays. Moreover, compared to pure BiOBr nanosheets, Co–Ni–NO₃ LDHs and Co–Ni–Br LDHs, the BiOBr/Co–Ni–NO₃ LDH nanocomposite showed much higher adsorption and photocatalytic properties for organic dyes (methyl orange (MO), rhodamine B (RhB)) and phenol under UV-light irradiation. Furthermore, the mechanism of photocatalytic degradation of dyes under UV-light irradiation was investigated in detail. This work paves a way to design new types of LDH-based 2D–2D composite photocatalysts with high adsorption and photocatalytic activity.

---

1. Introduction

Layered double hydroxides (LDHs) are a kind of ionic lamellar compound, composed of positively charged brucite-like hydroxide layers with compensating anions between the layers. The general formula for this layered structure is written as [M_1−x²⁺M_x³⁺(OH)₂][A_x/n]ⁿ⁻·mH₂O. M²⁺ and M³⁺ represent divalent (Mg²⁺, Fe²⁺, Co²⁺, Cu²⁺, Ni²⁺, Zn²⁺) and trivalent (Al³⁺, Cr³⁺, In³⁺, Mn³⁺, Ga³⁺, Fe³⁺) metal ions, respectively; Aⁿ⁻ is an interlayer compensating anion which could be exchanged by inorganic or organic anions. The x value normally ranges between 0.2–0.4.^1–8 Due to the special layered structure and ability for flexible modification of chemical compositions, LDHs possess significant potential in different fields, such as catalysis, photocatalysis, adsorption and electrochemistry.^9–12 Specially, with the increasing development of photocatalysis, LDHs have aroused much attention in photocatalytic applications. With relatively high specific surface areas, a special structure and exchangeability for anions, LDHs not only work as the substrate for the photocatalysts growing on the layers, but also are employed as photocatalysts for direct photocatalytic degradation of pollutants. For example, Zhu et al. prepared a Ag₂WO₄/Zn–Cr layered double hydroxide composite with enhanced visible-light-driven photocatalytic activity.¹³ Xia et al. synthesized a series of Zn/M–NO₃ LDHs (M = Al, Fe, Ti, and Fe/Ti) for photodegradation of organic pollutants.¹⁴

Since 1976 when Carey et al. used TiO₂ to photocatalytically degrade chlorinated biphenyl and biphenyl,¹⁵ researchers have conducted extensive studies on enhancing semiconductor photocatalytic activity.^16–18 Semiconductor photocatalysts absorb sunlight when the energy of the incident photons is equal to or larger than the band gap (E_g) of the semiconductor. Then, electrons are excited from the valence band (VB) of the semiconductor and transfer into the conduction band (CB). The photoexcited electrons can perform reduction reactions on the photocatalyst surface. Meanwhile the remaining photogenerated “hole” in the VB of the semiconductor performs oxidation reactions.¹⁹ BiOBr is a semiconductor photocatalyst whose band gap is about 2.9 eV.^20,21 It can effectively absorb wavelengths less than 427 nm, and it is relatively active and stable under irradiation.^22,23 Also it has a layered tetragonal matlockite structure with [Bi₂O₂] slabs interleaved by halogen atoms.²⁴ The special structure and properties have aroused much attention in the field of photocatalytic degradation of organic pollutants.^25–28 It occurs to us that combining BiOBr flakes with layered double hydroxides could make the compounds stick more strongly and form heterojunctions. Especially, the formed heterojunction can increase the separation efficiency of the excited electrons and holes to improve the photocatalytic activity by inhibiting charge carrier recombination. To the best of our knowledge, there is no paper focused on the preparation and activity of such a type of LDH-based 2D–2D heterostructured composite photocatalyst.

In recent years, layered double hydroxides including Ni or Co ions have received increasing attention for adsorption and photocatalysis of environmental pollutants because of their relative stability and good selectivity.^29,30 The host layers composed of Ni and Co are also prepared by various methods such as homogeneous precipitation, co-precipitation, electro-precipitation and so on.^31–34 Moreover, it is reported that Co–Ni–Br LDHs could be fabricated by intercalating excess bromine into Co–Ni hydroxides which were synthesized through a topochemical method.^34,35

Therefore, in this paper, we find a facile method to synthesis a new LDH-based 2D–2D heterostructured composite photocatalyst: BiOBr/Co–Ni–NO₃ LDHs. Through anion-exchange and in situ deposition/growth, the BiOBr nanosheets dispersed uniformly and vertically into the layers of the Co–Ni LDHs, and the NO₃⁻ anions also entered into the interlamination for charge compensation. Afterwards, the BiOBr/Co–Ni–NO₃ LDH nanocomposites not only showed fine morphologies and structures, but also exhibited great adsorptive and photocatalytic properties for several types of organic pollutants.

2. Experimental details

Materials: nickel(II) chloride hexahydrate (NiCl₂·6H₂O, AR), cobalt(II) chloride hexahydrate (CoCl₂·6H₂O, AR), D-mannitol (C₆H₁₄O₆, AR), acetonitrile, phenol and rhodamine B were purchased from Sinopharm Chemical Reagent Co., Ltd. Hexamethylenetetramine (HTM, AR) and bromine were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Bismuth nitrate (Bi(NO₃)₃·5H₂O, ≥99.0%, AR) was obtained from Guangdong Guanghua Chemical Factory Co., Ltd.

Synthesis of brucite-like Co–Ni layered double hydroxide: the Co–Ni LDH and Co–Ni–Br LDH samples were synthesized by the method reported in a previous paper with a little modification.^34,35 In a typical synthesis process, 4 mmol of cobalt chloride, 2 mmol of nickel chloride and 36 mmol of HMT were added into 400 ml ultrapure water in order. The mixed solution was well-mixed and transferred into five Teflon-lined stainless steel autoclaves with a capacity of 100 mL. The autoclaves were sealed and maintained at 95 °C for 5 h, then cooled to room temperature naturally. Finally, the products were filtered and washed with ethanol and deionized water several times respectively, and dried at room temperature.

Synthesis of Br⁻ ion inserted Co–Ni hydroxide: 0.372 g of Co–Ni hydroxide was added into 200 mL of acetonitrile (previously 13.34 mmol of bromine was added into the brown bottle). Nitrogen was filled into the bottle for 15 min, and then the bottle was covered in case of leakage. The whole reaction was magnetically stirred at room temperature for 24 h. Finally, the products were centrifugally separated and washed with massive amounts of anhydrous ethanol to remove the excess bromine.

Preparation of BiOBr/Co–Ni LDH nanocomposite: to prepare the BiOBr/Co–Ni–NO₃ LDH nanocomposite, 0.15 g of the as-prepared Co–Ni–Br powder was dispersed in 30 mL ultrapure water by sonicating for 15 min, the solution was abbreviated as A. 0.9701 g of Bi(NO₃)₃ was dissolved in 60 mL of D-mannitol to form solution B. Then, solution A was added into solution B slowly under vigorous stirring, the reaction was maintained at 40 °C for 1 h in an oil bath. The resulting products were centrifuged and washed with ethanol and deionized water several times, and dried at 60 °C. The chemical formula of the nanocomposite is calculated to be (BiOBr)_0.42/Co _0.67Ni _0.33(OH)₂ (NO₃)_0.67.

Preparation of BiOBr and Co–Ni–NO₃ LDH: for comparison, pure BiOBr was also prepared as follows. 2 mmol of sodium bromide was dissolved in 100 mL ultrapure water and 2 mmol of Bi(NO₃)₃ was added into the above solution under vigorous stirring. Then, the pH was adjusted to 10 using 1 M ammonia. The mixed solution was magnetically stirred for 12 h at room temperature. The final solution was filtered and washed with ethanol and deionized water several times respectively, and dried at room temperature. The Co–Ni–NO₃ LDH was synthesized by a traditional co-precipitation method as per the literature.³⁶ The amount of the Co(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O was 6.67 and 3.33 mmol, respectively. The composition was the same as that of the BiOBr/Co–Ni LDH, just without BiOBr.

Adsorption activity of the nanocomposite: the adsorption activity of the as-prepared BiOBr/Co–Ni–NO₃ LDH nanocomposite was evaluated with a MO (an anion dye) adsorption experiment. At the same time, we also studied the adsorptive capabilities of the nanocomposite with a cationic dye (RhB) and an organic pollutant (phenol). The reaction system contained 100 mL of a pollutant aqueous solution with different concentrations (MO 20, RhB 10, phenol 20 mg L⁻¹, respectively) and the photocatalyst sample (20 mg). The mixture was magnetically stirred in the dark to study the optimal time of the adsorption equilibrium. The concentration of the pollutant was monitored using the absorption of the pollutant via a UV/vis spectrophotometer. The whole reaction was carried out at room temperature.

Photocatalytic activity of the products: the photocatalytic performances of different products were evaluated by degradation of MO, RhB and phenol in aqueous solution under UV-light irradiation after saturated adsorption. A CEL-HXUV300 Ultra-Violet Light Source was used for the UV light illumination. The lamp was turned on above 30 min before the photocatalytic degradation experiments to make sure of the stability of the light intensity. The photocatalyst sample (20 mg) was dispersed in a 100 mL solution of the different pollutants and stirred under a specific speed by a magnetic stirrer. The concentrations were 20, 10 and 20 mg L⁻¹ for MO, RhB and phenol, respectively. The reactor was kept at room temperature by cooling with flowing water. Then, 1 ml of the reaction solution was withdrawn by a syringe every 5 min for the determination of concentration. In order to eliminate the influence of adsorption on the photocatalytic results for photodegrading MO, 1.5 mol L⁻¹ of NaNO₃ solution (dissolved in ethanol/water (1/1 v/v)) was added to the withdrawn solution to desorb the adsorbed pollutants. Then, the mixed solution was analyzed using a UV/vis spectrophotometer.

Characterization: the X-ray diffraction (XRD) patterns of the samples were identified on a SiemensD5005 powder diffractometer in the range of 3° ≤ 2θ ≤ 80°. The morphologies and microstructures were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800). A TEM image was recorded on a Hitachi H-7650 transmission electron microscope. The UV/vis diffuse reflectance spectra (DRS) of the catalysts were recorded using a UV/vis spectrometer (UV3600, SHIMADZU). Brunauer–Emmett–Teller (BET) surface areas were measured using nitrogen adsorption at 77 K (ASAP2020, HD88).

3. Results and discussion

3.1 Characterization of as-prepared samples

The formation mechanism of the BiOBr/Co–Ni–NO₃ LDH nanocomposite is shown in Scheme 1. Br⁻ ions were inserted into the interlayers of Co–Ni hydroxides by simple addition of excess bromine.³⁵ Then Bi(NO₃)₃ was dissolved into the Co–Ni–Br suspension. The solution was stirred in an oil bath at 40 °C for 1 h. The inserted Br⁻ ions reacted with the Bi³⁺ ions and generated BiOBr flakes which regularly and vertically anchored on the Co–Ni–NO₃ layers. Then, NO₃⁻ anions entered into the interlayers to maintain the charge balance.

Scheme 1 The formation mechanism of the BiOBr/Co–Ni–NO₃ LDH nanocomposite.

Fig. 1 shows the XRD patterns of Co–Ni–Br, BiOBr nanosheets and BiOBr/Co–Ni–NO₃ LDH. The characteristic peaks of Co–Ni–Br (Fig. 1a) appear at 2θ = 11.1° and 22.4°, which are consistent with what is reported in the literature.³⁵ It illustrates that the Co–Ni–Br is successfully synthesized with a basal spacing (d₀₀₃) of 0.79 nm.

Fig. 1 XRD patterns of the different samples: (a) Co–Ni–Br, (b) BiOBr nanosheets and (c) BiOBr/Co–Ni–NO₃ LDH.

The peaks of pure BiOBr in Fig. 1b are in good agreement with the crystalline plane of the BiOBr phase (JCPDS 09-0393).²⁵Fig. 1c is the XRD pattern of BiOBr/Co–Ni–NO₃ LDH which contains the diffraction peaks of Co–Ni–Br and BiOBr. It illustrates that the growth of BiOBr nanosheets on the Co–Ni–NO₃ LDH has no apparent influence on the phase structure of the LDH. Furthermore, it is observed that the first characteristic peak becomes wide at about 10° (Fig. 1c inset). The interlayer spacing (d₀₀₃ = 0.83 nm) of the composites shows a little shift to a lower 2 theta (10.6°) in the XRD patterns, suggesting some NO₃⁻ anions have intercalated into Co–Ni LDHs. The changes of the interlayer distance are similar to the literature where the d-spacings of the nitrate intercalated LDHs are observed among 8.5 to 8.9.^37–39 In addition, the peak intensities of BiOBr/Co–Ni–NO₃ LDHs are obviously weaker than that of Co–Ni–Br and pure BiOBr. The results indicate that the deposition of BiOBr nanosheets onto LDHs induces smaller crystal sizes. It also implies that the BiOBr nanosheets load uniformly on the surface of Co–Ni LDHs and would provide active sites for the photocatalytic reaction.

The morphologies and structures of as-prepared products were characterized using SEM. Fig. 2a shows the SEM image of a pure BiOBr sample. It clearly exhibits that the pure BiOBr flakes gather together like flowers. Also, a TEM image of the sample proves that the as-prepared BiOBr is composed of 2D nanosheets (inset in Fig. 2a). The SEM image of Co–Ni–Br is given in Fig. 2b. It can be seen that different sizes of Co–Ni–Br materials accumulate together and the surfaces of the sheets are smooth. The low magnification image of the BiOBr/Co–Ni–NO₃ LDH nanocomposite is shown in Fig. 2c. It can be seen that the sample exhibits a plate like structure with a coarse surface. Fig. 2d is the high-magnification SEM image of the BiOBr/Co–Ni–NO₃ LDH nanocomposite. It can be seen more clearly that the overlapped BiOBr nanosheets are distributed widely, vertically and uniformly on the surfaces of Co–Ni LDHs. It indicates that the formed heterojunction combines closely. Therefore, it would be beneficial for the separation of electron and hole pairs. Furthermore, it can be seen from the figure that the sizes of BiOBr nanosheets are much smaller than the pure BiOBr. The results are in agreement with the XRD results discussed in the former section. It also indicates that the BiOBr/Co–Ni–NO₃ LDH sample would provide more active sites for the photocatalytic reaction. According to the observed morphologies and structures, we draw a schematic of the idealized structure of the BiOBr/Co–Ni–NO₃ LDH nanocomposite, shown in Fig. 2e.

Fig. 2 (a) SEM image of pure BiOBr nanosheets and an inserted TEM image, (b) Co–Ni–Br, (c) low-magnification and (d) high-magnification SEM images of BiOBr/Co–Ni–NO₃ LDHs, and (e) the schematic structure of BiOBr/Co–Ni–NO₃ LDHs.

To further analyze the distribution of the elements (Bi, Br, Co and Ni) on the Co–Ni LDH surface, element mapping images were recorded using a SEM equipped for energy dispersive spectroscopy. The obtained results are shown in Fig. 3. It can be seen that the elements Bi, Br, Co and Ni are all distributed uniformly. It indicates that the BiOBr nanosheets indeed distribute on the surface of Co–Ni LDHs.

Fig. 3 Element mapping images of (a) Bi, (b) Br, (c) Co and (d) Ni in the BiOBr/Co–Ni–NO₃ LDH nanocomposite (the scale bar are all 5 μm).

Fig. 4 shows the UV/vis diffuse reflectance spectra of Co–Ni–Br, BiOBr nanosheets and BiOBr/Co–Ni–NO₃ LDH. The absorbance of Co–Ni–Br and BiOBr–Co–Ni–NO₃ LDH is almost the same in the range of 400 to 800 nm, while the BiOBr nanosheets have little absorbance in this region. But it is obvious that both BiOBr nanosheets and BiOBr/Co–Ni–NO₃ LDH have a higher absorption peak than Co–Ni–Br in the UV range. In addition, the absorption curve of Co–Ni–Br gradually declines in the range of 400 to 200 nm. The high absorbance of BiOBr/Co–Ni–NO₃ LDH is ascribed to the existence of BiOBr nanosheets which are well distributed on the surface of the Co–Ni LDHs. Therefore, we conducted the photocatalytic experiments under UV-light irradiation to emphasize the photocatalytic properties of BiOBr/Co–Ni–NO₃ LDH.

Fig. 4 UV/vis diffuse reflectance spectra of Co–Ni–Br, BiOBr nanosheets and BiOBr/Co–Ni–NO₃ LDH nanocomposite.

3.2 Properties of as-prepared samples

We chose a typical anionic dye, methyl orange (MO), and studied the adsorptive activity of the products in detail. First, BiOBr/Co–Ni–NO₃ LDH nanocomposite powder was well-dispersed in MO dye by ultrasonication and stirring. Then, the reactor was covered with tin foil in the dark for 30 min. The absorption spectra in Fig. 5a show briefly that the sample quickly reached the adsorption equilibrium within 10–15 min. The initial MO concentration decreased to 5.08 mg L⁻¹ after 15 min. The adsorption rate is about 75%. The high adsorption ability is probably attributed to the fact that MO anions can exchange with the NO₃⁻ on the surfaces or interlayers of the BiOBr/Co–Ni–NO₃ LDH. It is reported that the molecular size of MO is 1.19 nm × 0.67 nm × 0.38 nm.^40,41 Therefore, some MO anions can enter into the interlayers in parallel because there are two sides of the MO molecules shorter than the interlayer distance of the nanocomposite, which is about 0.83 nm. Some MO anions adsorb on the outer surfaces of the BiOBr/Co–Ni–NO₃ LDH and combine with the cationic layers. Furthermore, the BET surface areas of BiOBr nanosheets, Co–Ni–Br and BiOBr LDHs are 20.3 m² g⁻¹, 57.6 m² g⁻¹ and 71.9 m² g⁻¹ respectively. The obtained nanocomposites have relatively higher surface areas that may also contribute to the efficient adsorption. In order to prove these assumptions, we also performed the adsorption experiments of nanocomposites with non-anionic pollutants (RhB dye and phenol) under the same conditions. Fig. 5b is the adsorbance curve of the nanocomposite for RhB dye adsorption. The result obviously indicates that the nanocomposite quickly reached adsorption equilibrium within 10 min. The adsorption rate of the RhB dye on the nanocomposite is about 38% which is apparently lower than that of the nanocomposite for MO adsorption. Meanwhile, Fig. 5c shows the adsorption of phenol on the nanocomposite. It can be seen that there is no apparent adsorption within 60 min. Finally, we can conclude that the BiOBr/Co–Ni–NO₃ LDH shows greater adsorptive ability for an anionic dye, compared to a cationic dye and phenol.

Fig. 5 (a) UV/vis spectra of MO solution after being adsorbed by BiOBr/Co–Ni–NO₃ LDH nanocomposite for different adsorption times, (b) UV/vis spectra of RhB solution after being adsorbed by BiOBr/Co–Ni–NO₃ LDH nanocomposite for different adsorption times. (c) UV/vis spectra of phenol solution after being adsorbed by BiOBr/Co–Ni–NO₃ LDH nanocomposite for different adsorption times.

Then, we investigated the photocatalytic properties of the BiOBr/Co–Ni–NO₃ LDH nanocomposite for the degradation of MO dye under UV-light irradiation. But the high adsorption of MO dye interfered with the results of the photocatalytic process: the MO molecules exchanged with the NO₃⁻ on the layers. Therefore, we conducted a desorption experiment to probe the actual amount of degraded MO. Every five minutes, 1 mL of the solution was withdrawn and added to a 10 mL centrifuge tube with 1.5 mol L⁻¹ NaNO₃ solution which was dissolved in ethanol/water (1/1 v/v) for a period in the dark. Then the tube was centrifuged and analyzed using a UV/Vis spectrophotometer (Shimadzu UV3600). Therefore, we could eliminate the interference of adsorption and estimate the photocatalytic capability directly. Fig. 6 shows that the MO molecules are almost all desorbed from the surface of the nanocomposite in the adsorption process after adding 1.5 mol L⁻¹ of NaNO₃. In the irradiation process, the curves indicate that the MO molecules are really photodegraded after 30 minutes of UV-light illumination.

Fig. 6 Concentration variation of MO by different photocatalysts in different processes: (a) adsorption, (b) after being released by 1.5 mol L⁻¹ solution of NaNO₃ in ethanol/water (1/1 v/v) and (c) after being irradiated for different times and then released by 1.5 mol L⁻¹ solution of NaNO₃.

To make the results more convincing, we conducted a series of comparison tests. The photocatalytic degradation of MO in the presence of Co–Ni–NO₃ LDH, Co–Ni–Br and BiOBr nanosheets or in the absence of any photocatalyst (photolytic) was conducted under the same experimental conditions. In the adsorption process, BiOBr/Co–Ni–NO₃ LDH shows the highest adsorption ability in accord with the special layered structures and the BET surface areas. The Co–Ni–NO₃ LDH, BiOBr nanosheets and Co–Ni–Br are also layered structures and display similarly good adsorption. In the photo-degradation process, BiOBr/Co–Ni–NO₃ LDH exhibits an excellent photocatalytic ability. Almost all MO molecules are degraded within 30 minutes, while Co–Ni–Br, Co–Ni–NO₃ LDH and BiOBr nanosheets degraded only 17%, 31% and 73% of the MO dye, respectively, within 30 minutes. Thus, we assert that the BiOBr/Co–Ni–NO₃ LDH nanocomposite has an enhanced adsorptive and phototcatalytic capacity, mainly attributed to the BiOBr nanosheets sticking closely on the Co–Ni LDH. This is because the formed heterojunction makes the excited electrons and holes effectively separated and transferred, which inhibits charge carrier recombination and improves photocatalytic activity.

To further evaluate the photocatalytic activity of the BiOBr/Co–Ni–NO₃ LDH, we performed photodegradation experiments with the cationic dye RhB and the typical non-dye pollutant phenol. First, we demonstrated that the optimal amount of catalyst with respect to the 10 mg L⁻¹ of RhB dye is 20 mg, displayed in Fig. 7a. With the amount of the catalyst increasing from 5 to 20 mg, the efficiency of the photocatalysis is gradually enhanced. While when the amount of the catalyst reaches as much as 25 mg, it presents a downward trend. It is probably that the excess catalyst increases the turbidity of the suspension, which disturbs the penetration of UV light into the suspension. The decrease of UV light penetration causes an increased scattering effect. Consequently, the photo-activated volume of the suspension decreases.^42,43 So we chose 20 mg as the appropriate amount in the following experiments. Fig. 7b shows that the BiOBr/Co–Ni–NO₃ LDH has great photodegradation efficiency with RhB, which is degraded completely within 15 minutes, while Co–Ni–Br and Co–Ni–NO₃ LDHs show no ability for RhB dye degradation. Furthermore, the same mass ratio of BiOBr and Co–Ni–NO₃ LDH as that in the BiOBr/Co–Ni–NO₃ LDH composite were physically mixed and used to conduct the photocatalytic activity. It shows that the physical mixture exhibits much lower activity than the BiOBr/Co–Ni–NO₃ LDH composite. The results verified the high efficiency of the BiOBr/Co–Ni–NO₃ LDH in the photocatalytic degradation of RhB. It indicates that heterojunctions have been formed in the composite. Then, we investigated the photocatalytic activity of as-prepared samples for phenol (non-dye pollutant) degradation. The results are given in Fig. 7c. The concentration curves of the phenol in aqueous solution tend to increase after 30 min of the UV-light illumination, which is ascribed to the polymerization of phenol caused by the UV light irradiation. The results are in good agreement with previous publications.^44–46 After the combination of BiOBr and Co–Ni LDH, it makes great progress in the photo-reaction observed in Fig. 7c. The catalyst speeds up the opening of the aromatic ring and leads to the formation of aliphatic acids, and finally mineralizes gradually to carbon dioxide and water. It obviously shows that 50% of the phenol is degraded efficiently by BiOBr/Co–Ni–NO₃ LDH in 30 min. The results reveal that the BiOBr/Co–Ni–NO₃ LDH nanocomposite has a promising application for organic pollutant degradation. Especially, the nanocomposite can be used for the selective degradation of anionic dyes with outstanding capabilities.

Fig. 7 (a) Effect of BiOBr/Co–Ni–NO₃ LDH amount on photodegradation efficiency of RhB at a UV-light irradiation time of 30 min ([RhB]₀ = 10 ppm), and photodegradation of RhB (b) and phenol (c) by different photocatalysts under UV-light irradiation after reaching adsorption equilibrium.

3.3 Photocatalytic mechanism

The above photocatalytic experiments show that the BiOBr/Co–Ni–NO₃ LDH nanocomposite has superior photocatalytic properties. To further evaluate what kind of radical plays the main role in the degradation process, we designed and conducted reactive species trapping experiments for RhB dye degradation under UV-light irradiation. MO molecules can be easily adsorbed on the interlayers which affect the apparent photocatalytic activity of the BiOBr/Co–Ni–NO₃ LDH. In addition, the efficiency of degrading phenol is not as good as that of degrading RhB. Finally, we chose RhB dye as the goal pollutant to study the photocatalytic mechanism of the nanocomposite. Moreover, the experiments were conducted under the same conditions except for adding different radical scavengers. Before carrying out photocatalytic experiments, the BiOBr/Co–Ni–NO₃ LDH nanocomposite was added into a RhB aqueous solution and stirred in the dark for 40 minutes to reach adsorption equilibrium. Then ethylenediaminetetraacetic acid disodium salt (EDTA–2Na, 5 mM) was added to trap photoexcited holes (h⁺). The results are shown in Fig. 8: the concentration of the RhB dye at 5 min was higher than that of the RhB dye at 0 min (adsorption equilibrium). This is probably because the RhB molecules are desorbed from the nanocomposite. After 30 min, the remaining concentration of RhB almost has no change under these conditions. This indicates that the existence of EDTA–2Na leads to fast deactivation of the BiOBr/Co–Ni–NO₃ LDH nanocomposite for RhB photocatalytic degradation. Therefore, photoexcited holes are the major active species for RhB photocatalytic degradation. It also shows the results of the experiment which was conducted by adding p-benzoquinone (BZQ, 1 mM). The photocatalytic degradation efficiency of BiOBr/Co–Ni–NO₃ LDH for RhB is decreased by 75% after 30 min. It reveals that superoxide anion radicals (O₂⁻) also play an important role in the photocatalytic degradation of RhB. Finally, we added tert-butyl alcohol (5 mM) to remove OH radicals: the result is also shown in Fig. 8. It is obvious that the curve of RhB degradation by adding an OH scavenger almost coincides with the curve of that without adding any radical scavengers. The results indicate that OH has little effect on the degradation of RhB under the present conditions. In summary, the result verifies the holes photogenerated from the BiOBr/Co–Ni–NO₃ LDH nanocomposite are the predominant active species in the photocatalytic degradation of RhB dye. Furthermore, it also demonstrates that O₂⁻ plays an important role in the degradation of RhB dye by the as-prepared BiOBr/Co–Ni–NO₃ LDH nanocomposite.

Fig. 8 The variation of RhB concentration during the photocatalytic degradation process as a function of irradiation time with the addition of different scavengers (a) EDTA–2Na, (b) C₆H₄O₂, (c) C₄H₉OH.

On the basis of the above results, we drew the possible schematic diagram of the photocatalytic mechanism for organic pollutant degradation by BiOBr/Co–Ni–NO₃ LDHs under UV-light illumination. The scheme is shown in Fig. 9. The BiOBr nanosheets, which are distributed widely and uniformly on the surface of Co–Ni–NO₃ LDH, produce electrons and holes under the UV-light irradiation. The electrons transfer from the valence band (VB) of BiOBr to the conduction band (CB). The photoexcited electrons react with O₂ molecules which are accumulated on the surface of BiOBr nanosheets to produce O₂⁻. The produced O₂⁻ has great oxidation ability for organic pollutant degradation. Meanwhile, the photoexcited holes in the VB not only have strong oxidation ability for the degradation of organic pollutants, the photoexcited holes can also react with hydroxy to produce OH which is one kind of strong oxidant. What’s more, the photogenerated holes and electrons have realized an excellent photocatalytic activity owed to the formed heterojunction between the special layered structure. The heterojunction would inhibit the recombination of charge carriers, thus more holes or electrons can be captured to induce the photocatalytic reaction. Therefore, the as-prepared BiOBr/Co–Ni–NO₃ LDH nanocomposite exhibits excellent photocatalytic activity for organic pollutant degradation.

Fig. 9 Schematic diagram of photocatalytic mechanism of BiOBr/Co–Ni–NO₃ LDH under UV-light illumination.

The stability and recycling of the BiOBr/Co–Ni–NO₃ LDH was also investigated. Fig. 10 shows the repetitive photodegradation of RhB during three consecutive cycles with the same dye concentration under 15 min of UV-light illumination. After each cycle, the catalyst was washed with double-distilled water, and a fresh solution of RhB was refilled in the next photocatalytic run. It is observed that the photocatalytic efficiency of the BiOBr/Co–Ni–NO₃ LDH exhibits a little decrease after three cycles. The results reveal that the nanocomposite not only has an excellent photocatalytic activity, but also presents good stability in the photocatalytic oxidation of RhB molecules under UV-light illumination. This is significant for its practical application.

Fig. 10 Photodegradation of RhB with BiOBr/Co–Ni–NO₃ LDH under three recycling runs.

4. Conclusions

We have proposed a facile method to synthesize a new 2D–2D heterojunction photocatalyst: a BiOBr/Co–Ni–NO₃ LDH nanocomposite. Characterization results show that the BiOBr nanosheets are dispersed well on the layers of the Co–Ni LDH. The obtained nanocomposite showed superior adsorptive capacity. What’s more, the BiOBr/Co–Ni–NO₃ LDH nanocomposite exhibited excellent photocatalytic properties with anionic/cationic dyes and phenol degradation under UV-light irradiation. The combination of these special 2D layered structures helps to form a heterojunction, which increases the separating efficiency of photo-generated electron–hole pairs and thus improves the photocatalytic activity. This advantage has broadened the usage of pure BiOX sheets and layered double hydroxides in the field of wastewater remediation.

Acknowledgements

We are grateful for grants from the National Science Funds for Creative Research Groups of China (no. 51421006), the National Science Foundation of China for Excellent Young Scholars (no. 51422902), the National Science Fund for Distinguished Young Scholars (no. 51225901), the Program for Changjiang Scholars and Innovative Research Team in University (no. IRT13061), the Key Program of National Natural Science Foundation of China (no. 41430751), the National Natural Science Foundation of China (no. 51479065), Natural Science Foundation of Jiangsu Province (BK20141417), the Fundamental Research Funds for the Central Universities (2013B32114 and 2013B14114), and PAPD.

References

1. Y. Kuang, L. N. Zhao, S. Zhang, F. Z. zhang, M. D. Dong and S. L. Xu, Materials, 2010, 3, 5220–5235.
2. X. Y. Xu, Y. J. Lin, D. G. Evans and X. Duan, Sci. China: Chem., 2010, 53(7), 1461–1469.
3. Z. P. Xu, J. Zhang, M. O. Adebajo, H. Zhang and C. H. Zhou, Appl. Clay Sci., 2011, 53, 139–150.
4. Q. Wang and D. O’ Hare, Chem. Rev., 2012, 112, 4124–4155.
5. Z. Chang, C. Y. Wu, S. Song, Y. Kuang, X. D. Lei, L. Wang and X. M. Sun, Inorg. Chem., 2013, 52, 8694–8698.
6. C. M. Li, M. Wei, D. G. Evans and X. Duan, Small, 2014, 10, 4469–4486.
7. G. Fan, F. Li, D. G. Evans and X. Duan, Chem. Soc. Rev., 2014, 43(20), 7040–7066.
8. X. Xiang, F. Li and Z. Q. Huang, Reviews in Advanced Sciences and Engineering, 2014, 14, 158–171.
9. M. Q. Zhao, Q. Zhang, Q. Zhang, J. Q. Huang, J. Q. Nie and F. Wei, Carbon, 2010, 48, 3260–3270.
10. L. Mohapatra and K. M. Parida, Sep. Purif. Technol., 2012, 91, 73–80.
11. Z. C. Li, B. J. Yang, S. N. Zhang, B. N. Wang and B. Xue, J. Mater. Chem. A., 2014, 2, 10202–10210.
12. L. J. Zhang, J. Wang, J. J. Zhu, X. G. Zhang, K. S. Hui and K. N. Hui, J. Mater. Chem. A., 2013, 1, 9046–9053.
13. J. Y. Zhu, H. Fan, J. C. Sun and S. Y. Ai, Sep. Purif. Technol., 2013, 120, 134–140.
14. S. J. Xia, F. X. Liu, Z. M. Ni, J. L. Xue and P. P. Qian, J. Colloid Interface Sci., 2013, 45, 195–200.
15. J. H. Carey, J. Lawrence and H. M. Tosine, Bull. Environ. Contam. Toxicol., 1976, 16, 697–701.
16. W. X. Li, J. Aust. Ceram. Soc., 2013, 49(2), 41–46.
17. H. Yan, X. D. Wang, M. Yao and X. J. Yao, Prog. Nat. Sci., 2013, 23(4), 402–407.
18. H. L. Wang, L. S. Zhang, Z. Q. Chen and J. Q. Hu, et al., Chem. Soc. Rev., 2014, 43, 5234–5244.
19. R. Marschall, Adv. Funct. Mater., 2014, 24(17), 2421–2440.
20. H. Z. An, Y. Du, T. M. Wang, C. Wang, W. C. Hao and J. Y. Zhang, Rare Met., 2008, 27, 243–250.
21. Z. Jiang, F. Yang, G. D. Yang, L. Kong, M. O. Jones, T. C. Xiao and P. P. Edwards, J. Photochem. Photobiol., A, 2010, 212, 8–13.
22. M. Shang, W. Z. Wang, L. Zhang and J. Hazard, Mater, 2009, 167, 803–809.
23. J. Zhang, F. J. Shi, J. Lin, D. F. Chen, J. M. Gao, Z. X. Huang, X. X. Ding and C. C. Tang, Chem. Mater., 2008, 20, 2937–2941.
24. W. L. Huang and Q. S. Zhu, Comput. Mater. Sci., 2008, 43, 1101–1108.
25. M. Shang, W. Z. Wang, L. Zhang and J. Hazard, Mater, 2009, 167, 803–809.
26. J. Cao, B. Y. Xu, B. D. Luo, H. L. Lin and S. F. Chen, Catal. Commun., 2011, 13, 63–68.
27. L. Zhang, X. F. Cao, X. T. Chen and Z. L. Xue, J. Colloid Interface Sci., 2011, 354, 630–636.
28. Y. H. Ao, H. Tang, P. F. Wang, C. Wang, J. Hou and J. Qian, Composites: Part B, 2014, 59, 96–100.
29. F. B. D. Saiah, B. L. Su, N. Bettahar and J. Hazard, Mater, 2009, 165, 206–217.
30. D. Q. Li, L. L. Qian, Y. J. Feng, J. T. Feng, P. G. Tang and L. Yang, ACS Appl. Mater. Interfaces, 2014, 6, 20603–20611.
31. Y. Zhang, X. Cao, H. Yuan, W. Zhang and Z. Zhou, Int. J. Hydrogen Energy, 1999, 24, 529–536.
32. B. Cui, H. Lin, J. B. Li, X. Li, J. Yang and J. Tao, Adv. Funct. Mater., 2008, 18, 1440–1447.
33. D. Klissurski and E. Uzunova, Chem. Mater., 1991, 3, 1060–1063.
34. Y. Zhang, B. Cui, C. S. Zhao, H. Lin and J. B. Li, Phys. Chem. Chem. Phys., 2013, 15, 7363–7369.
35. J. B. Liang, R. Z. Ma, N. B. Iyi, Y. S. Ebina, K. Takada and T. Sasaki, Chem. Mater., 2010, 22, 371–378.
36. Z. A. Hu, Y. L. Xie, Y. X. Wang, H. Y. Wu, Y. Y. Yang and Z. Y. Zhang, Electrochim. Acta, 2009, 54, 2737–2741.
37. L. Lv, P. D. Sun, Z. Y. Gu, H. G. Du, X. J. Pang, X. H. Tao, R. F. Xu, L. L. Xu and J. Hazard, Mater, 2009, 161, 1444–1449.
38. F. P. de Sá, B. N. Cunha and L. M. Nunes, Chem. Eng. J., 2013, 215–216, 122–127.
39. J. C. Sun, Y. B. Zhang, J. Cheng, H. Fan, J. Y. Zhu, X. Wang and S. Y. Ai, J. Mol. Catal. A: Chem., 2014, 382, 146–153.
40. J. H. Huang, K. L. Huang, S. Q. Liu, A. T. Wang and C. Yan, Colloids Surf., A, 2008, 330, 55–61.
41. S. Hosseini, M. A. Khan, M. R. Malekbala, W. Cheah and T. S. Choong, Chem. Eng. J., 2011, 171, 1124–1131.
42. T. A. Gad-Allah, S. Kato, S. Satokawa and T. Kojima, Desalination, 2009, 244, 1–11.
43. V. K. Gupta, R. Jain, S. Agarwal, A. Nayak and M. Shrivastava, J. Colloid Interface Sci., 2012, 366, 135–140.
44. F. Tzompantzi, Y. Pina, A. Mantilla, O. Aguilar-Martine, F. Galindo-Hernandez, X. Bokhimi and A. Barrera, Catal. Today, 2014, 220–222, 49–55.
45. J. S. Valente, F. Tzompantzi, J. Prince, J. G. H. Cortez and R. Gomez, Appl. Catal., B, 2009, 90, 330–338.
46. A. Mantilla, F. Tzompantzi, J. L. Fernandez, J. A. Gongora and R. Gomez, Catal. Today, 2010, 150, 353–357.

---