Preparation of novel BiOBr/CeO₂ heterostructured photocatalysts and their enhanced photocatalytic activity

Abstract

Novel BiOBr/CeO₂ heterostructured photocatalysts were synthesized by a simple solvothermal method. The photocatalysts compared with pure BiOBr and pure CeO₂ are more photoactive in photocatalytic oxidation of rhodamine B (RhB), methylene blue (MB) and bisphenol A (BPA) under visible light irradiation. The photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) spectroscopy. The 5 : 1 BiOBr/CeO₂ shows the highest visible-light photocatalytic activity for degradation of RhB and is highly stable during photodegradation. The remarkably enhanced photocatalytic activities could be mainly attributed to the low recombination ability between photo-induced electrons and holes, which results from the formation of a heterojunction structure between BiOBr and CeO₂. The active species trapping experiments indicate that the primary oxidative species in the reaction system are the photogenerated hole (h⁺) and superoxide radicals (·O₂⁻). A possible mechanism for the photocatalytic activity enhancement was also put forward on the basis of the experimental results.

---

Introduction

Semiconductor photocatalysts have played an increasingly important role in pollution control and indoor air purification in recent years.^1–4 Photocatalysis oxidation has attracted wide attention and is considered a promising technique and optimal way to handle environmental problems. Among all the photocatalysts, cerium dioxide (CeO₂) as an important rare-earth metal oxide is acknowledged as a potential material for degradation of many organic pollutants because of its high photocatalytic activity, physiochemical stability, small size, environmental friendliness, and cost-effectiveness.^5–8 However, its further application into the visible light region is limited by the broad band-gap energy, which is close to that of TiO₂ (3.2 eV). The nanocomposites formed by CeO₂ and other narrow band gap semiconductors show excellent photocatalytic activity in the treatment of dye wastewater.^9–12 The visible-light-driven catalysts can be successfully prepared by modifying the band gap of CeO₂.

Meanwhile, many bismuth (Bi)-containing oxides with remarkable photocatalytic activity under visible light irradiation have been found, such as γ-Bi₂MoO₆,¹³ BiVO₄,¹⁴ Bi₂WO₆,¹⁵ α-Bi₂O₃,¹⁶ and especially BiOX (X = Cl, Br, I).^17–20 Bi oxyhalides are V-VI-VII ternary compounds with a tetragonal matlockite (PbFCl) structure, and possess a layer structure characterized by [Bi₂O₂] slabs interleaved by double slabs of halogen atoms.²¹ Owing to the advantages of intrinsic layer structure, high carrier mobility and small probability of recombination between photogenerated electrons and holes, BiOX is very photocatalytically active in degradation of organic pollutants.²² Among all the Bi oxyhalides, pure BiOBr with band-gap energy of 2.64–2.91 eV has drawn considerable attention owing to the relatively superior photocatalytic activity and high stability.²³ For instance, BiOBr compared with BiOCl and BiOI shows the highest photocatalytic activity in degradation of rhodamine B (RhB).²⁴ Nonetheless, the photocatalytic efficiency of BiOBr should be still further improved for practical applications. Up to now, many BiOBr-based heterostructured catalysts have been synthesized by various methods and the results shows all the photocatalysts exhibit significantly enhanced photocatalytic activity than pure BiOBr.^25–27 Therefore, this effective approach is promising for improving the photocatalytic property of BiOBr.

In this work, a series of BiOBr/CeO₂ composites were successfully synthesized by a simple solvothermal method. The BiOBr/CeO₂ composites with different molar ratios x (marked as x BiOBr/CeO₂; for instance, 1 : 5 BiOBr/CeO₂) were characterized by many methods. To evaluate the photocatalytic activity, we used the samples into degradation of rhodamine (RhB), methylene blue (MB) and bisphenol A (BPA) under visible light irradiation. Furthermore, the active species during the photocatalytic degradation and the possible photocatalytic mechanism of BiOBr/CeO₂ were also proposed.

Experimental

Chemicals and materials

All of the reagents for synthesis and analysis were chemical analysis grade and used without further purification or modification. Deionized water was used throughout the experiments.

Synthesis of CeO₂ nanoparticles

The CeO₂ nanocrystallines were prepared using ammonium bicarbonate (NH₄HCO₃) as a precipitant. In a typical experimental procedure, 3.257 g of Ce(NO₃)₃·6H₂O and 2.469 g of NH₄HCO₃ were dissolved separately in 25 mL of deionized water to form two homogeneous solutions. Then, the Ce(NO₃)₃ solution was dripped slowly into the NH₄HCO₃ solution under violent stirring. Subsequently, an appropriate amount of PEG 400 was added into the above suspension, followed by stirring for 1 h at 70 °C and then centrifugation. The above precipitate was washed with deionized water and absolute ethanol for several times, and then dried in a microwave oven for 5 min. Then this precursor was calcined in a muffle furnace at 450 °C for 45 min and finally, CeO₂ nanoparticles were prepared.

Preparation of BiOBr/CeO₂ composites

The BiOBr/CeO₂ composites (Bi/Ce molar ratio = 10 : 1, 5 : 1, 2.5 : 1, 1 : 1 and 5 : 1) were synthesized via a solvothermal method. First, the 2.5 mmol Bi (NO₃)₃·5H₂O was dissolved in a 40 wt% CH₃COOH aqueous solution to form a Bi (NO₃)₃ solution. Second, 2.5 mmol cetyltriethylammnonium bromide (CTAB) was dissolved in deionized water at 40 °C, followed by addition of 2.5 mmol CeO₂. Subsequently, the mixtures were stirred for 1 h. In addition, the Bi (NO₃)₃ solution was dripped into the above mixtures. Finally, the resulting precursor suspension was transferred into a 100 mL Teflon-lined autoclave after 45 min of stirring, and kept at 160 °C for 8 h. After cooling down to room temperature, the precipitate was washed by absolute ethanol and distilled water for several times, and then vacuum-dried at 80 °C for 5 h. Other samples were prepared by the same procedure with addition of different weights of CeO₂. For comparison, pure BiOBr powder was also prepared by the same method without addition of CeO₂.

Characterization

The crystalline phase structure of the catalysts was evaluated by X-ray diffraction (XRD, TD-3500, λ = 1.5406 Å, 30 kV, 20 mA). The morphology was investigated using a scanning electron microscope (SEM, Hitachi SU-1510) and a transmission electron microscope (TEM, Hitachi) equipped with an energy-dispersive X-ray spectroscope (EDS) operated at an acceleration voltage of 10 kV. The chemical states were measured by X-ray photoelectron spectroscopy (XPS, PHI5300) with an Al Kα X-ray source (1486.6 eV). The UV-vis absorption spectra over a range of 200–800 nm were recorded on an HP8453 spectrophotometer using BaSO₄ as a reference. The photoluminescence (PL) spectra were measured with an excitation wavelength of 354 nm using an RF-5301PC spectrofluorophotometer (Shimadzu Corporation, Japan).

Photocatalytic activity measurement

The photocatalytic activity was evaluated by using the samples into dyes degradation (RhB, MB and BPA) under the irradiation from a 300 W Xe lamp. In each experiment, 0.2 g of a photocatalyst was added into 200 mL of a 10 mg L⁻¹ dye solution. Before illumination, the suspension was magnetically stirred for 30 min in the dark to achieve the absorption–desorption equilibrium. During the irradiation, about 3 mL of the suspension was taken out periodically and centrifuged to remove the suspended particles. Finally, the absorbance changes in the solutions as-collected were analyzed by a UV-vis spectrophotometer (Shimadzu, UV-2450).

Results and discussion

Catalyst characterization

The XRD spectra of the samples as-synthesized are showed in Fig. 1. Clearly, all the diffraction peaks of the BiOBr samples can be readily indexed to the tetragonal phase of BiOBr (JCPDS no. 09-0393), while the characteristic peaks of CeO₂ correspond to the fluorite structure of pure CeO₂ (JCPDS no. 43-1002). This pattern does not show any characteristic peak of impurity, which indicates a high purity and single-phase of the composites. In addition, the intense and sharp diffraction peaks imply the well-crystalline structure of the composites. In the diffraction patterns of BiOBr/CeO₂, with decrease of the CeO₂ content, the intensity of the CeO₂ peaks is reduced or even undetectable (Fig. 1c), whereas the intensity of the BiOBr peaks is enhanced gradually (Fig. 1c–g). Some diffraction peaks of BiOBr also disappear except for the stronger peaks (Fig. 1g). These results indicate that the samples as-prepared are composite materials and the addition of CeO₂ does not affect the formation of BiOBr crystals.

Fig. 1 The XRD patterns of as-prepared samples: (a) CeO₂ (b) BiOBr (c) 10 : 1 BiOBr/CeO₂ (d) 5 : 1 BiOBr/CeO₂ (e) 2.5 : 1 BiOBr/CeO₂ (f) 1 : 1 BiOBr/CeO₂ (g) 1 : 5 BiOBr/CeO₂.

The structure and morphology of the photocatalysts as-synthesized were examined by SEM, and the typical SEM images are presented in Fig. 2. The pure CeO₂ nanoparticles show an irregular coarse-surface shape, and have agglomerated modestly (Fig. 2A). Fig. 2B clearly shows the smooth surface of BiOBr. Moreover, the pure BiOBr shows a lamellar structure, which is composed of a large quantity of aggregated nanosheets with the particle size from 0.5 to 2.0 μm (edge length). After modification with CeO₂ (Fig. 2C–F), BiOBr/CeO₂ composites possess the similar morphology as BiOBr, except that the CeO₂ particles are uniformly distributed on the surfaces of BiOBr plates. Obviously, the BiOBr/CeO₂ composites were successfully prepared. Moreover, the structure of BiOBr was not destroyed by the addition of CeO₂, but its surfaces were wrapped by CeO₂ with further addition of CeO₂ (1 : 5 BiOBr/CeO₂; Fig. 2G).

Fig. 2 SEM patterns of different samples: (A) CeO₂ (B) BiOBr (C) 10 : 1 BiOBr/CeO₂ (D) 5 : 1 BiOBr/CeO₂ (E) 2.5 : 1 BiOBr/CeO₂ (F) 1 : 1 BiOBr/CeO₂ (G) 1 : 5 BiOBr/CeO₂.

In order to further investigate the heterostructure of the samples as-obtained, we used HRTEM to characterize the BiOBr, CeO₂ and 5 : 1 BiOBr/CeO₂. The TEM image of CeO₂ exhibits an irregular structure and the particle size less than 30 nm (Fig. 3A). Fig. 3B confirms the anomalous crystal structure and the smooth surface of BiOBr nanosheets. The selected area electron diffraction (SAED) pattern in the inset of Fig. 3A and B reveal the polycrystalline nature of the CeO₂ nanostructure and single-crystalline nature of BiOBr, respectively. Furthermore, the CeO₂ nanoparticles are uniformly located on the surface of BiOBr (Fig. 3C), which corresponds to the SEM results. Furthermore, clear lattice fringes can be observed in the high resolution TEM image of the 5 : 1 BiOBr/CeO₂ (Fig. 3D). The fringes with interplanar spacing of 0.283 nm is in agreement with the (102) lattice plane of BiOBr, while the fringes with interplanar spacing of 0.317 nm can be matched with (111) planes of CeO₂. The EDS analyses of 5 : 1 BiOBr/CeO₂ shows that the sample is composed of Bi, O, Br and Ce (Fig. 3E). Those evidences clearly show that the sample prepared was BiOBr/CeO₂ composite material.

Fig. 3 TEM patterns and inserted SAED images of the samples: (A) CeO₂ (B) BiOBr, (C) TEM patterns 5 : 1 BiOBr/CeO₂, (D) HRTEM patterns 5 : 1 BiOBr/CeO₂, (E) EDS of the 5 : 1 BiOBr/CeO₂.

The detail chemical compositions and states of the photocatalysts were investigated by X-ray photoelectron spectroscopy (XPS) and the results are shown in Fig. 4. The typical survey XPS spectra (Fig. 4A) shows that only C, Bi, O, Br signals in BiOBr and C, Bi, O, Br, Ce signals in 5 : 1 BiOBr/CeO₂ were observed. The presence of C 1s peak mainly ascribe to the hydrocarbon of the XPS instrument itself. In Fig. 4B, two sharp peaks located at 159.0 eV and 164.4 eV were observed in pure BiOBr, which were assigned to the Bi (4f_7/2) and Bi (4f_5/2) of Bi³⁺ signals, respectively.²⁸ While the Bi 4f spectra of BiOBr/CeO₂ exhibited peaks at 159.3 eV and 164.6 eV, have a slight shift to high binding energy. Six peaks of Ce 3d spectra in Fig. 4C with banding energies of 882.4, 889.0, 898.1, 900.8, 907.3, 916.7 eV in BiOBr/CeO₂ confirmed the Ce⁴⁺ states in the composite,²⁹ indicates the presence of CeO₂ species in heterostructured BiOBr/CeO₂. A slight shift to low banding energy than those of CeO₂ can also be observed. Those shifts can also certify the interaction between BiOBr and CeO₂. The Br 3d peaks of the samples centering at 68.5 eV were in agreement with Br⁻ in the composites (Fig. 4D). In Fig. 4E, the O 1s banding energy of 529.8 eV was characteristic of lattice oxygen in the materials. Those results clearly confirmed that the prepared products are BiOBr/CeO₂ composites.

Fig. 4 XPS survey spectra (A) and the high resolution XPS spectrum of (B) Bi 4f, (C) Ce 3d, (D) Br 3d and (E) O 1s over various samples.

Fig. 5A and B displays the ultraviolet-visible (UV-vis) absorption spectra of the as-synthesized samples. The optical properties of the catalysts were then studied using the Kubelka–Munk function F(R),³⁰ which is expressed as follows:

Fig. 5 (A) UV-vis absorption spectra of the as-obtained samples: (a) CeO₂ (b) BiOBr (c) 5 : 1 BiOBr/CeO₂; (B) UV-vis absorption spectra of other samples: (a) 1 : 5 BiOBr/CeO₂ (b) 1 : 1 BiOBr/CeO₂ (c) 2.5 : 1 BiOBr/CeO₂ (d) 10 : 1 BiOBr/CeO₂; (C) the plotting of [F(R)hν]^1/2 versus photo energy (hν) of (a) CeO₂ (b) BiOBr (c) 5 : 1 BiOBr/CeO₂; (D) the plotting of [F(R)hν]^1/2 versus photo energy (hν) of other BiOBr/CeO₂ composites: (a) 1 : 5 BiOBr/CeO₂ (b) 1 : 1 BiOBr/CeO₂ (c) 10 : 1 BiOBr/CeO₂ (d) 2.5 : 1 BiOBr/CeO₂.

The relations between reflectance R and absorbance A, Planck constant h, light frequency v, and absorption wavelength λ are described as follows:

A = −log R (2)

Then the band-gap energy (E_g) of each catalyst is computed as follows:

where C is a constant, and the parameter n is decided by the type of the optical transition of a semiconductor (n = 4 for direct transition and n = 1 for indirect transition).³¹ n is equal to 1 for both BiOBr and CeO₂.^9,32 The E_g values of the catalysts were studied as follows: first, the corresponding R under each A was calculated by eqn (2); second, F(R) and hν were expressed using eqn (1) and (3); finally, E_g was estimated from the intercept of the tangents to the x-axis to the plot of vs. hν. Therefore, the E_g of the samples as-prepared was calculated (Fig. 5C and D), and the results are exhibited in Table 1. Clearly, the pure CeO₂ has wide E_g (3.04 eV), which means it can absorb little light in the visible range. In addition, all the heterostructured BiOBr/CeO₂ composites show the E_g about 2.64–2.81 eV. The 5 : 1 BiOBr/CeO₂ with the narrowest E_g (2.64 eV) indicates the strongest visible light response.

Table 1 The band gap energy (E_g) of prepared samples

Sample	CeO2	BiOBr	10 : 1	5 : 1	2.5 : 1	1 : 1	1 : 5
Eg/eV	3.04	2.72	2.76	2.64	2.77	2.68	2.81

---

Furthermore, the valence band (VB) and conduction band (CB) edge positions of both BiOBr and CeO₂ were also calculated as follows:³³

E_VB = X − E^e + 0.5E_g (5)

E_CB = E_VB − E_g (6)

where E_VB is the VB edge potential; E_CB is the CB edge potential; X is the electronegativity of the semiconductor, which is the geometric mean electronegativity of the constituent atoms; E^e is the energy of free electrons on the hydrogen scale (about 4.5 eV). Accordingly, the E_VB and E_CB of BiOBr, and the E_VB and E_CB of CeO₂ were calculated, which were 3.04, 0.32, 2.58, and −0.46 eV, respectively (Table 2).

Table 2 The electronegativity (X), valence band (VB) gap, conduction band (CB) gap and band gap energy (E_g) of different BiOBr and CeO₂

Catalysts	Electronegativity (X) (eV)	Conduction band (CB) (eV)	Valence band (VB) (eV)	Band gap energy (Eg) (eV)
BiOBr	6.18	0.32	3.04	2.72
CeO2	5.56	−0.46	2.58	3.04

---

The separation efficiency of photogenerated charge carriers in the samples as-synthesized was investigated by photoluminescence (PL) spectra and the results are presented in Fig. 6. Clearly, the main emission peaks of BiOBr and BiOBr/CeO₂ composites are similar (about 448 nm). The emission intensity of BiOBr/CeO₂ composites is gradually weakened and minimized in the 5 : 1 BiOBr/CeO₂. The lowest fluorescent emission intensity in the 5 : 1 BiOBr/CeO₂ indicates a much lower recombination rate of photo-generated charge carriers. We can also conclude that the recombination between photogenerated electrons and holes was inhibited and the photocatalytic activity of the BiOBr/CeO₂ composites was improved by the introduction of CeO₂.

Fig. 6 Photoluminescence (PL) spectra of different samples (λ_em = 354 nm).

To investigate the photocatalytic activities, we used the samples as-prepared to degrade RhB in aqueous solutions under visible irradiation. As shown in Fig. 7a, the self-photodegradation of RhB is very low with the absence of any catalyst, indicating that the stability and photolysis of RhB can be ignored. The RhB photodegradation ability of the 5 : 1 BiOBr/CeO₂ composite was significantly enhanced compared to both pure BiOBr and pure CeO₂.

Fig. 7 Photocatalytic activities in the degradation of RhB (a), MB (b) and BPA (c) under visible light irradiation.

During the RhB degradation process, other BiOBr/CeO₂ composites (e.g. 10 : 1 BiOBr/CeO₂ and 1 : 1 BiOBr/CeO₂) also exhibit excellent photocatalytic activity. RhB can also be thoroughly degraded by pure BiOBr. However, after 60 min of visible light irradiation, only a part of RhB was removed by the 1 : 5 BiOBr/CeO₂ composite. The RhB photodegradation activity of the catalysts as-prepared changes with the BiOBr/CeO₂ moral ratio as follows: 5 : 1 > 2.5 : 1 > 10 : 1 > 1 : 1 > 1 : 0 > 1 : 5 > 0 : 1. This result indicates that too low and too large content of CeO₂ could both weaken the photocatalytic activity of the BiOBr/CeO₂ composites under visible light.

Fig. 8 shows the time-dependent absorption spectra of RhB (Fig. 8a and b) and BPA (Fig. 8c) vs. irradiation time with the presence of 5 : 1 BiOBr/CeO₂ (Fig. 8a and c) and pure BiOBr (Fig. 8b). As can be seen, the adsorbability of 5 : 1 BiOBr/CeO₂ is stronger than pure BiOBr, and the adsorbed RhB molecules in neither case could be degraded without irradiation. The results also show that the intensity of the RhB absorption peak was diminished evidently. Furthermore, the maximum absorption blue-shifted slightly from 554 to 500 nm, which accords with the color change and intermediates form of the RhB suspension during the visible light irradiation. This phenomenon is mainly attributed to the N-deethylation processes of RhB.³⁴ After a period of irradiation (200 min for pure BiOBr and 140 min for 5 : 1 BiOBr/CeO₂), the RhB dye solution was degraded completely, suggesting that the intermediates of N-deethylation could also be photodegraded by the catalysts as-synthesized. Fig. 8c shows that after 300 min irradiation, 80% BPA solution were degraded by 5 : 1 BiOBr/CeO₂, indicates that heterostructured BiOBr/CeO₂ also have excellent photocatalytic activity in degradation colourless pollutant.

Fig. 8 Temporal UV-vis absorption spectral change during the photocatalytic degradation of RhB in the presence of (a) 5 : 1 BiOBr/CeO₂ and (b) BiOBr and BPA in the presence of (c) 5 : 1 BiOBr/CeO₂.

Based on the above discussion, all the BiOBr/CeO₂ composites have a much narrow band gap than pure CeO₂. Owing to the narrow band gap, the BiOBr/CeO₂ composites can be easily excited by visible light. Fig. 7a illustrates how BiOBr/CeO₂ composites with different CeO₂ contents affect the RhB degradation. We know that the ratio of BiOBr/CeO₂ in degradation of organic dyes can be optimized. The 5 : 1 BiOBr/CeO₂ exhibits the highest photocatalytic activity for RhB degradation, mainly because the heterostructured BiOBr/CeO₂ composites were synthesized. When the CeO₂ content was too low, although a modest heterostructure of BiOBr/CeO₂ was obtained, the electrons and holes could not be separated efficiently. However, with excessive load of CeO₂, the absorption of visible light was hindered because CeO₂ wrapped the surface of BiOBr (Fig. 2G) and also decreased the photocatalytic activity of the BiOBr/CeO₂ composites. The PL spectra (Fig. 6) suggest that the surface modification of BiOBr by CeO₂ is beneficial for charge separation (all emission peaks of BiOBr/CeO₂ are weaker than those of pure BiOBr and pure CeO₂). The 5 : 1 BiOBr/CeO₂ composite shows significantly enhanced ability in separation between photogenerated electrons and holes, and it has the narrowest band gap among the samples as-prepared (Table 1). Therefore, 5 : 1 BiOBr/CeO₂ is the most photocatalytically active for degradation of RhB (Fig. 7a).

Furthermore, the photocatalytic activity of BiOBr/CeO₂ composites was further investigated using MB and BPA as other substrates. It is surprising that the self-photodegradation of MB is slightly higher than the photocatalytic degradation by CeO₂ (Fig. 7b), probably because the adsorption of photocatalysts in the dye solution decreased the direct photolysis of MB. In addition, 86.6% and 48.1% of the MB solution were photodegraded by the 5 : 1 BiOBr/CeO₂ and pure BiOBr respectively, after 210 min of irradiation (Fig. 7b). Fig. 7c shows that 80% and 65.5% of the BPA solution were photodegraded by the 5 : 1 BiOBr/CeO₂ and BiOBr respectively within 300 min irradiation. This result proves that the composites could degrade different organic dyes, and their photodegradation activities were improved greatly compared with pure BiOBr and pure CeO₂. Because the MB and BPA degradation rate is much slower than the RhB degradation rate, the RhB solution was chosen for further investigation.

As we know, the stability of photocatalysts is also an important factor for photocatalytic activity and practical application. Moreover, BiOBr is extremely stable during photodegradation. In this work, the 5 : 1 BiOBr/CeO₂ composite was chosen to study the stability of the samples by recycle degradation experiments. As showed in Fig. 9, the photocatalytic activity in RhB degradation after three recycles was only reduced slightly. The XRD patterns of 5 : 1 BiOBr/CeO₂ were measured after three cycles of photodegradation (Fig. 10). Compared to 5 : 1 BiOBr/CeO₂ without photodegradation, the crystal structure did not change largely. This result indicates that the heterostructured photocatalysts formed by BiOBr and CeO₂ are highly stable and can be used to degrade organic pollutants under visible light irradiation.

Fig. 9 Recycle experiments of RhB photocatalytic degradation in the presence of 5 : 1 BiOBr/CeO₂.

Fig. 10 XRD patterns of 5 : 1 BiOBr/CeO₂ (a) before photodegradation, (b) after three times photodegradation.

Possible photocatalytic mechanism of BiOBr/CeO₂ composites

Role of reactive species. In order to explain the reaction mechanism more clearly, we conducted active species trapping experiments by adding various scavengers. The active species include photogenerated holes (h⁺), superoxide radical (·O₂⁻) and hydroxyl radical (·OH), which were quenched by the scavengers triethanolamine (TEOA), benzoquinone (BQ) and isopropanol (IPA) during the degradation, respectively. The k_app was calculated by the following equation:

ln(C₀/C) = k_appt (7)

where k_app is the apparent pseudo-first-order rate constant (min⁻¹), C₀ is the initial concentration before illumination (t = 0) (mol L⁻¹), and C is the revised concentration at time t (mol L⁻¹).³⁵ The results are depicted in Fig. 11. Obviously, the RhB degradation efficiency was significantly reduced by the addition of TEOA or BQ, but not IPA. Therefore, these evidences demonstrate that h⁺ and ·O₂⁻ are the main oxidative species in the degradation, while ·OH plays a relatively minor role.

Fig. 11 The k_app changes in the photodegradation of RhB by adding various quenchers.

Since ·O₂⁻ is generated via e⁻ + O₂ →·O₂⁻, we also used N₂ bubbling test to further investigate the role of dissolved oxygen (Fig. 11). We found that the degradation of RhB was suppressed under the N₂ purging condition (k_app decreased from 0.0685 to 0.0572), implying that the dissolved oxygen plays an important role in the photodegradation.

Possible mechanism of photocatalytic activity enhancement

The possible charge separation process of the BiOBr/CeO₂ heterostructure is depicted in Fig. 12. Both BiOBr and CeO₂ can be excited under the visible light irradiation. Owing to the unique heterostructure formed by BiOBr and CeO₂, the excited electrons on the CB of CeO₂ would transfer to the CB of BiOBr. Meanwhile, the BiOBr-produced photogenerated holes would cross the interface and inject into the VB of CeO₂. This process improved the separation efficiency but weakened the recombination between electrons and holes. Therefore, the photocatalytic activity of BiOBr/CeO₂ was enhanced compared with pure BiOBr and pure CeO₂. In addition, the standard redox potential of Biv/BiIII (+1.59 eV) is more negative than that of ·OH/OH⁻ (+1.99 eV),³³ so the photogenerated holes could not oxidize OH⁻ to ·OH and would decompose the dyes directly. Furthermore, the electrons on the CB of CeO₂ could reduce O₂ to ·O₂⁻ because the CB potential of CeO₂ (−0.46 eV) is more negative than the standard redox potential of O₂/·O₂⁻ (−0.28 eV). In general, h⁺ and ·O₂⁻ are the main active species in the degradation process, which accords with the results of active species trapping experiments.

Fig. 12 Proposed mechanism of photocatalytic reaction in BiOBr/CeO₂ system.

Conclusions

A series of BiOBr/CeO₂ heterostructured photocatalysts were successfully synthesized via a solvothermal method. The investigations indicate that CeO₂ is uniformly distributed on the surface of BiOBr, and BiOBr/CeO₂ is strongly absorptive in the visible region. The photocatalytic degradation tests with rhodamine B, methylene blue and bisphenol A reveal that the degradation efficiency of BiOBr/CeO₂ composites (especially 5 : 1 BiOBr/CeO₂) was tremendously increased through the heterostructure system. In addition, h⁺ and ·O₂⁻ are the dominant active species in the photodegradation of RhB over BiOBr/CeO₂ composites. The enhanced photocatalytic activity of BiOBr/CeO₂ composites could be attributed to the unique heterostructure, the narrow band-gap, and the low recombination between photo-generated electrons and holes. The cycling tests show the excellent stability of BiOBr/CeO₂ composites during the photodegradation, which indicates a wide application prospect of these photocatalysts.

References

1. J. Di, J. Xia, M. Ji, S. Yin, H. Li, H. Xu, L. Xu, Q. Zhang and H. Li, J. Mater. Chem. A, 2015, 3, 15108.
2. M. A. Gondal, X. Chang, M. A. Ali, Z. H. Yamani, Q. Zhou and G. Ji, Appl. Catal., A, 2011, 397, 192.
3. W. Yao, B. Zhang, C. Huang, C. Ma, X. Song and Q. Xu, J. Mater. Chem., 2012, 22, 192.
4. G. Huang, R. Shi and Y. Zhu, J. Mol. Catal. A: Chem., 2011, 348, 100.
5. I. T. Liu, M. H. Hon and L. G. Teoh, Ceram. Int., 2014, 40, 4019.
6. F. Chen, Y. Cao and D. Jia, Appl. Surf. Sci., 2011, 257, 9226.
7. H. I. Chen and H. Y. Chang, Ceram. Int., 2005, 31, 795.
8. L. Yin, Y. Wang, G. Pang, K. Yuri and G. Aharon, J. Colloid Interface Sci., 2002, 246, 78.
9. N. Wetchakun, S. Chaiwichain, B. Inceesungvorn, K. Pingmuang, S. Phanichphant, A. I. Minett and J. Chen, ACS Appl. Mater. Interfaces, 2012, 4, 3718.
10. J. Zhang, B. Wang, C. Li, H. Cui, J. Zhai and Q. Li, Environ. Sci., 2014, 26, 1936.
11. N. Wetchakun, S. Chaiwichain, K. Wetchakun, W. Kangwansupamonkon, B. Inceesungvorn and S. Phanichphant, Mater. Lett., 2013, 113, 76.
12. Z. Yang, G. Huang, W. Huang, J. Wei, X. Yan, Y. Liu, C. Jiao, Z. Wan and A. Pan, J. Mater. Chem. A, 2014, 2, 1750.
13. F. Zhou, R. Shi and Y. Zhu, J. Mol. Catal. A: Chem., 2011, 340, 77.
14. P. Madhusudan, M. V. Kumar, T. Ishigaki, K. Toda, K. Uematsu and M. Sato, Environ. Sci. Pollut. Res., 2013, 20, 6638.
15. L. Zhang, Y. Wang, H. Cheng, W. Yao and Y. Zhu, Adv. Mater., 2009, 21, 1286.
16. J. Hu, H. Li, C. Huang, M. Liu and X. Qiu, Appl. Catal., B, 2013, 142–143, 598.
17. G. Li, F. Qin, R. Wang, S. Xiao, H. Sun and R. Chen, J. Colloid Interface Sci., 2013, 409, 43.
18. J. Di, J. Xia, Y. Ge, L. Xu, H. Xu, M. He, Q. Zhang and H. Li, J. Mater. Chem. A, 2014, 2, 15864.
19. J. Yang, X. Wang, X. Lv, X. Xu, Y. Mi and J. Zhao, Ceram. Int., 2014, 40, 8607.
20. J. Di, J. Xia, S. Yin, H. Xu, L. Xu, Y. Xu, M. He and H. Li, J. Mater. Chem. A, 2014, 2, 5340.
21. X. Wei, H. Cui, S. Guo, L. Zhao and W. Li, J. Hazard. Mater., 2013, 263, 650.
22. Z. Liu, Y. Bi, Y. Zhao, X. Huang and Y. Zhu, Mater. Res. Bull., 2014, 49, 167.
23. X. Chang, M. A. Gondal, A. A. Al-Saadi, M. A. Ali, H. Shen, Q. Zhou, J. Zhang, M. Du, Y. Liu and G. Ji, J. Colloid Interface Sci., 2012, 377, 291.
24. H. An, Y. Du, T. Wang, C. Wang, W. Hao and J. Zhang, Rare Met., 2008, 27, 243.
25. J. Di, J. Xia, Y. Ge, L. Xu, H. Li, H. Xu, J. Chen, M. He and H. Li, J. Mater. Chem. A, 2014, 43, 15429.
26. W. Cui, W. An, L. Liu, J. Hu and Y. Liang, Appl. Surf. Sci., 2014, 319, 298.
27. J. Di, J. Xia, S. Yin, H. Xu, M. He, H. Li, L. Xu and Y. Jiang, RSC Adv., 2013, 3, 19624.
28. J. Xia, J. Di, S. Yin, H. Li, L. Xu, Y. Xu, C. Zhang and H. Shu, Ceram. Int., 2014, 40, 4607.
29. S. H. Hsieh, A. Manivel, G. J. Lee and J. J. Wu, Mater. Res. Bull., 2013, 48, 4174.
30. R. Kavitha and L. G. Devi, J. Environ. Chem. Eng., 2014, 2, 857.
31. M. Rusu, M. Baia, Z. Pap, V. Danciu and L. Baia, J. Mol. Struct., 2014, 1073, 150.
32. J. Xia, J. Di, S. Yin, H. Li, H. Xu, L. Xu, H. Shu and M. He, Mater. Sci. Semicond. Process., 2014, 24, 96.
33. J. Cao, X. Li, H. Lin, R. Xu, S. Chen and Q. Guan, Appl. Surf. Sci., 2013, 266, 294.
34. L. Ye, J. Chen, L. Tian, J. Liu, T. Peng, K. Deng and L. Zan, Appl. Catal., B, 2013, 130–131, 1.
35. J. Cao, B. Xu, H. Lin and S. Chen, Chem. Eng. J., 2013, 228, 482.

---