DOI:
10.1039/C6RA14155B
(Paper)
RSC Adv., 2016,
6, 64617-64625
Facile fabrication of BiOIO3/BiOBr composites with enhanced visible light photocatalytic activity
Received
31st May 2016
, Accepted 29th June 2016
First published on 4th July 2016
Abstract
BiOIO3/BiOBr composite photocatalysts were successfully fabricated through a hydrothermal and subsequent chemical precipitation method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), Brunauer–Emmett–Teller (BET) surface area, photoluminescence (PL) and photocurrents to investigate the structures, crystallinity, morphology and optical properties. Compared with pure BiOIO3 and BiOBr, BiOIO3/BiOBr composites exhibited significantly enhanced visible light photocatalytic activity towards the degradation of organic dyes. Specifically, the 1I/6Br composite was found to show the maximum value of the activity and maintained good stability in the recycling process. The enhanced photocatalytic activity could be attributed to the formation of a p–n heterojunction between BiOIO3 and BiOBr, which resulted in the effective separation and transfer of photogenerated electron–hole pairs. Moreover, the active species trapping experiment confirmed that ˙O2− and h+ were the main active species during the photocatalytic process.
1. Introduction
Nowadays, semiconductor photocatalysts have attracted considerable attention due to their promising applications in organic pollutant remediation.1–3 Among them, Bi-based layered structure compounds, within the Aurivillius family, such as Bi2WO6,4 BiVO4,5 Bi2O2CO3,6 etc., have been extensively investigated as efficient photocatalysts due to their unique layered structure. Recently, as a new bismuth-based material, bismuth oxide iodate (BiOIO3) has been found to be an efficient photocatalyst and displays promising photocatalytic activity for the degradation of organic pollutants.7,8 It possesses a polar and layered crystal structure composed of (Bi2O2)2+ and (IO3)− layers, which is supposed to be very favorable for the separation of photogenerated charge carriers.9 Nevertheless, BiOIO3 can only be excited by ultraviolet light that occupies about 5% of the solar light due to the wide band-gap (3.08 eV),10 which restricts its practical application. Therefore, it is urgent to direct efforts towards extending the photo-response wavelength range of BiOIO3 to obtain an improved photocatalytic performance.
It is generally accepted that combining two or more semiconductors with appropriate band positions could promote the separation of the photo-generated electrons.11 In particular, p–n heterojunction structure has been demonstrated as a very efficient method to separate photo-induced electron–hole pairs for the existence of the internal electric field and thus favor the photocatalytic reaction.12 As a lamellar-structured p-type semiconductor, BiOBr with small band gap (Eg = 2.64–2.91 eV)13 has received remarkable interest in solar energy conversion.14–19 Some BiOBr-containing p–n junction photocatalysts, such as BiOBr/ZnFe2O4,20 Bi2WO6/BiOBr,21–23 BiOBr/ZnO,24 BiOBr/α-Fe2O3,25 BiOBr/BiPO4
11,12,26 and carbon/BiOBr/AgBr27 have been developed. The results indicate that these p–n junction photocatalysts display much higher photocatalytic performance than the individual compound. By investigating the energy levels of n-type BiOIO3 and p-type BiOBr,28,29 it is fortunate to find that the energy levels of BiOIO3 and BiOBr are well-matched overlapping band-structures. Thus, it is expected to prepare p–n BiOBr/BiOIO3 heterostructure with improved photocatalytic performance.
Herein, a series of heterostructured BiOIO3/BiOBr composites were prepared by a simple chemical precipitation method at room temperature. The photocatalytic experiments under visible light irradiation showed that the present BiOIO3/BiOBr heterojunction possessed excellent photocatalytic activity for degrading organic dyes under visible light irradiation, which was much higher than those of either individual BiOIO3 or BiOBr. In addition, the recycling performance of the samples was evaluated for the long-term application. The possible mechanisms for the enhanced photocatalytic performance of p–n BiOBr/BiOIO3 heterostructure were also discussed in detail on the basis of its energy band structure and the measurement of reactive species.
2. Experiment
2.1 Materials
All chemicals used in this study were analytical grade and were used without further purification. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), potassium iodate (KIO3), potassium bromide (KBr), acetic acid (HAc), rhodamine B (RhB), crystal violet (CV), methyl blue (MB) and methyl orange (MO) were purchased from Sinopharm Chemical Reagent Co., Ltd.
2.2 Synthesis of BiOIO3 precursor
BiOIO3 was obtained through a simple hydrothermal method. In a typical procedure, 1 mmol of Bi(NO3)3·5H2O was added to 60 mL deionized water, and stirred vigorously for 30 min. Then, 1 mmol of KIO3 was dissolved in 60 mL deionized water to obtain a clear solution and the solution was dropped into the Bi(NO3)3·5H2O suspension. The resulting suspension was transferred into a Teflon-lined stainless autoclave and maintained at 150 °C for 6 h. After being cooled down to room temperature naturally, the resulting products were separated by filtration, washed several times with deionized water and dried at 60 °C for 12 h.
2.3 Preparation of a series of BiOIO3/BiOBr heterojunction
BiOIO3/BiOBr heterostructured composites were synthesized by a simple chemical precipitation method at room temperature. Firstly, 1.2 mmol of Bi(NO3)3·5H2O was dissolved in 89 mL aqueous solution containing 80 mL H2O and 9 mL acetic acid (HAc). Secondly, a certain amount of BiOIO3 was added into the above aqueous solution with magnetically stirring. After stirring for 10 min, the solution was ultrasonicated for another 10 min. Then, 30 mL aqueous solution containing 1.2 mmol of KBr was added dropwise into the solution and stirred for 2 h. After the stirring was completed, the resulted suspension was aged for 2 h. Finally, the resulting precipitate was filtered, rinsed thoroughly with deionized water and dried at 60 °C for 12 h. According to I/Br molar ratio, the as-prepared photocatalysts were marked as 0I/1Br, 1I/8Br, 1I/6Br, 1I/4Br, 1I/3Br, 1I/2Br, 1I/1Br, and 1I/0Br, respectively.
2.4 Characterization
The crystal phases of the samples were examined by X-ray diffraction (XRD) analysis using a Bruker D8-advance X-ray diffractometer with Cu Kα radiation (40 kV/250 mA). The scanning step width of 0.02° and the scanning rate of 0.15° s−1 were applied to record the patterns in the 2θ range of 5–70°. The morphologies and microstructures of the products were observed by a FESEM-4800 field emission scanning electron microscope (SEM, Hitachi) with 5.0 kV scanning voltages. To further evaluate the morphology and crystallinity of the samples, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were taken on a transmission electron microscope (TEM, FEI Tecnai G20) with an accelerating voltage of 200 kV. A UV-vis spectrophotometer (Hitachi U-4100) was utilized to record the UV-vis diffuse reflectance spectra (DRS) in the range of 200 to 800 nm with BaSO4 as a reference sample. The Brunauer Emmett Teller (BET) specific surface area and total pore volume of the samples were measured by nitrogen adsorption–desorption analysis with an automatic Micromeritics 3 Flex instrument. The photoluminescence (PL) emission spectra were obtained using a Hitachi F-4600 fluorescence spectrophotometer.
2.5 Evaluation of photocatalytic activity
Rhodamine B (RhB), crystal violet (CV), methyl orange (MO), and methylene blue (MB) were used as model pollutants to evaluate the photocatalytic performance. A 300 W Xe lamp equipped with a UV cutoff filter (λ > 420 nm), was used as the light source. The detailed experiments were performed as follows: 0.05 g photocatalyst was dispersed in RhB aqueous solution (100 mL, 20 mg L−1). Before irradiation, the suspension was magnetically stirred in dark for 50 min to get an adsorption–desorption equilibrium. In the process of light irradiation, about 3 mL suspensions were taken out and centrifuged (8000 rpm, 5 min) to remove the catalyst particles. Photocatalytic activity of the sample was also evaluated by the degradation of CV, MB and MO (100 mL, 10 mg L−1) in aqueous solution under the same conditions. The concentration of dyes was analyzed by measuring the absorbance of supernatants at absorption band maximum by using a UV-vis spectrophotometer (Shimadzu 2550, Japan), and deionized water was adopted as a reference sample. To further test the stability and repeatability of the photocatalyst, the cycle experiments were also carried out under the same condition.
2.6 Photoelectrochemical measurements
The photoelectrochemical property was measured in a standard three-electrode system with using an electrochemical workstation (CHI660E, China). Platinum wires and saturated calomel electrodes (SCE) were utilized as the counter electrode and reference electrode, respectively. The 0.1 M Na2SO4 solution was used as electrolyte solution. The working electrode was prepared by coating the sample film on indium-tin oxide (ITO) sheet glass. A 300 W xenon lamp with a 420 nm cutoff filter was employed as the visible light source.
3. Results and discussion
3.1 Characterization
3.1.1 XRD. The detailed information on the phase structure and crystallinity of the as-prepared samples were obtained by using X-ray diffraction (XRD). Fig. 1 showed the XRD patterns of the pure BiOIO3, BiOBr and BiOIO3/BiOBr (1I/4Br and 1I/6Br) composites. The pristine BiOIO3 showed a series of sharp diffraction peaks, which can be indexed into the orthorhombic BiOIO3 (ICSD #262019).30 The strongest peak at about 27.42° was attributed to the (121) crystal plane of BiOIO3. For the pure BiOBr, the characteristic diffraction peaks were detected at 2θ angles of 10.84, 25.15, 31.68, 32.20, 46.20, and 57.12°, attributed to the (001), (101), (102), (110), (200), and (212) crystal planes, respectively. This was well in accordance with the standard diffraction data of the tetragonal BiOBr phase (JCPDS file no. 09-0393).31 In the case of the BiOIO3/BiOBr composites, the XRD pattern exhibited two kinds of diffraction peaks, besides the diffraction peaks assigned to BiOIO3 (marked with “♥”), all the additional ones were well matched to the BiOBr. With the amount of BiOIO3 component in BiOIO3/BiOBr composites increasing from 1
:
6 to 1
:
4, the intensity of corresponding diffraction peaks of BiOIO3 were gradually increased, whereas that of BiOBr were decreased. No other phases were found in the composites, suggesting that the obtained photocatalysts were of high purity.
 |
| Fig. 1 XRD patterns of the as-prepared pure BiOBr, 1I/4Br composite, 1I/6Br composite and pure BiOIO3. | |
3.1.2 SEM. The morphology of the as-prepared BiOBr, BiOIO3 and BiOIO3/BiOBr (1I/6Br) photocatalyst was investigated by scanning electron microscopy, and the result was shown in Fig. 2. It can be seen from Fig. 2a and b that the as-synthesized pristine BiOIO3 were composed of nanoplates and nanoparticles with very smooth surface and the size was about several hundred nanometers. As shown in Fig. 2c and d, the pure BiOBr exhibited a flower-like hierarchical microsphere structure with an average diameter of 8 μm, which was assembled by plenty of nanosheets. As for the BiOIO3/BiOBr (1I/6Br) composites, it displayed a morphology with incomplete microspheres and nanoplates coexisting. Also, there was an intimate interfacial contact between BiOBr and BiOIO3, the small BiOIO3 nanoparticles were firmly assembled on the surface of the BiOBr nanosheets (Fig. 2e and f). It may reveal that the heterojunction between BiOBr and BiOIO3 was formed. This kind of formation can be proven by the HR-TEM analysis below.
 |
| Fig. 2 SEM images of (a) and (b) BiOIO3; (c) and (d) BiOBr; (e) and (f) 1I/6Br. | |
3.1.3 TEM and HRTEM analysis. In order to further investigate the microstructure of the BiOIO3/BiOBr (1I/6Br) composite, TEM and HRTEM observations were carried out. The TEM images of BiOIO3/BiOBr (1I/6Br) (Fig. 3a and b) clearly showed that incomplete microspheres and nanoplates coexisted in the sample and this was in accordance with the results of the SEM images. Furthermore, Fig. 3c and d presented the HRTEM images of the BiOIO3/BiOBr (1I/6Br) composite. The clear lattice fringes indicated that the as-synthesized composites were highly crystallized. It was clear in Fig. 3d that the distinct lattice fringes was 0.277 nm and 0.326 nm, which coincided with the planes of BiOBr (110) (JCPDS file no. 09-0393) and BiOIO3 (121) (ICSD #262019), respectively. It can also be found some other lattice fringes in Fig. 3d, the lattice fringe of 0.282 nm corresponded to the (102) crystal plane of BiOBr (JCPDS file no. 09-0393), whereas the fringe of 0.289 nm matched well with the (002) plane of BiOIO3 (ICSD #262019). The above results further demonstrate that a heterojunction structure is formed between BiOIO3 and BiOBr, which is in well agreement with the XRD results.
 |
| Fig. 3 TEM images of (a) and (b) 1I/6Br; HRTEM images of (c) and (d) 1I/6Br. | |
3.1.4 DRS. It is well known that the photoinduced charge-transfer property and enhanced light photocatalytic activity of semiconductors in the photocatalytic reaction are closely related to their optical properties.32 Thus, UV-vis DRS spectral were performed at wavelengths of 200–800 nm. As shown in Fig. 4a, the pure BiOIO3 had slight light absorption in the visible region with an absorption edge around 400 nm, while the pure BiOBr displayed strong light absorption at wavelength lower than 440 nm. This was in consistent with the results previously reported.33,34 Comparatively, the BiOIO3/BiOBr (1I/6Br) composite presented similar absorption characteristics to pure BiOBr, which assured the light absorption of 1I/6Br on visible light region. The result indicated that the BiOBr could serve as an effective visible-light sensibilizer for BiOIO3, which was beneficial for the photoexcitation process.
 |
| Fig. 4 (a) DRS spectra of the as-obtained samples and (b) the band gap energies (Eg) of BiOIO3, BiOBr and 1I/6Br. | |
In addition, the band gap energies (Eg) of BiOIO3 and BiOBr were calculated according to the following equation and the figures were shown in Fig. 4b:35
where
α,
h,
ν,
Eg and
A stand for the absorption coefficient, Planck's constant, the light frequency, band gap energy and a constant, respectively. In this equation,
n is determined by the type of the optical transition of a semiconductor (
n = 1 for a direct transition and
n = 4 for an indirect transition).
36 For BiOIO
3 and BiOBr, they all possessed an indirect transition band gap, and the value of
n equals 4.
13,28 As shown in
Fig. 4b, the corresponding
Eg values of BiOIO
3, BiOBr and 1I/6Br could be obtained from the plots of (
αhν)
1/2 versus energy (
hν), which were calculated to be 3.06 eV, 2.76 eV and 2.70 eV, respectively.
3.1.5 Nitrogen adsorption–desorption analysis. Fig. 5 showed the nitrogen adsorption–desorption isotherms of BiOIO3, BiOBr and BiOIO3/BiOBr (1I/6Br) photocatalysts. The recorded isotherms can be identified as a type III isotherm with an indistinct H3 hysteresis loop according to IUPAC classification. More adsorbed volumes of N2 were observed at the relative pressure (P/P0) range of 0.8–1.0. The pore size distribution of the samples was calculated using the Barrette–Joynere–Halenda (BJH) method (inset of Fig. 5). It can be observed that the pore size distribution of the photocatalysts confirms a mesoporous structure. Table 1 listed the BET surface area, total pore volume and pore diameter of as-synthesized samples calculated on the basis of the isotherms. It can be seen that the BiOIO3/BiOBr composite had higher specific surface area than the pure BiOIO3 and BiOBr, respectively. During the photocatalytic process, high specific surface area might provide more active sites, which could improve the adsorption of dye molecules and help enhancing the photocatalytic activity.37,38
 |
| Fig. 5 N2 adsorption–desorption isotherms of the BiOBr, BiOIO3 and 1I/6Br samples; inset: the corresponding pore-size distribution. | |
Table 1 BET surface areas (SBET), total pore volume (VBJH) and pore diameter of samples
Catalyst |
SBET (m2 g−1) |
VBJH (cm3 g−1) |
Pore diameter (nm) |
BiOIO3 |
12.004 |
0.074 |
12.661 |
BiOBr |
9.597 |
0.050 |
8.815 |
BiOIO3/BiOBr |
13.180 |
0.054 |
7.927 |
3.2 Photocatalytic activity
The photo-degradation performance of the as-prepared samples were evaluated by photodecomposition of organic dyes in aqueous solution under visible light (λ > 420 nm). Fig. 6 showed the photocatalytic activities of the BiOBr samples with different BiOIO3 molar ratio for degrading RhB. As shown in Fig. 6, the direct photolysis of RhB from the blank experiment could almost be neglected. The pure BiOIO3 almost had no ability to decompose RhB meanwhile the degradation rate of RhB over BiOBr was nearly 80% after 40 min irradiation. It can be found that all BiOIO3/BiOBr samples exhibited improved photocatalytic activities than bare BiOBr or BiOIO3. In particular, the maximum value of the degradation rates was obtained by 1I/6Br heterojunction and approximately 99% of RhB was degraded within 40 min. Increasing the amount of BiOIO3 resulted in decreasing photocatalytic activity of the photocatalyst. The reason for this decrease may be that the excess BiOIO3 will create some isolated BiOIO3 nanoparticles, which were out-of-commission for the formation of BiOIO3/BiOBr heterojunction. Moreover, these excessed BiOIO3 may hinder the visible light capture of BiOBr.
 |
| Fig. 6 Photocatalytic degradation of RhB of the samples under visible light irradiation. | |
To understand the reaction kinetics of the photo-degradation process, the degradation rates were further calculated according to the following apparent pseudo-first-order kinetics equation:11
where
C0 is the initial concentration (mg L
−1),
Ct is the concentration at time
t (mg L
−1),
kapp is the apparent pseudo-first-order rate constant (min
−1). As shown in
Fig. 7b, the corresponding
kapp values of the composites were calculated to be 0.0440, 0.1296, 0.1329, 0.1287, 0.1071, 0.0690, and 0.0679 min
−1 respectively. All BiOIO
3/BiOBr composites exhibited higher photocatalytic activity than pure BiOBr and BiOIO
3, indicating that the coupling of BiOIO
3 and BiOBr improved the photocatalytic performance. 1I/6Br catalyst exhibited the maximum value of the reaction rate constant, which was almost three times higher than that of bare BiOBr.
 |
| Fig. 7 (a) ln(C0/Ct) versus irradiation time for degradation of RhB in the presence of different catalysts; (b) the rate constant k of the degradation of RhB in the presence of different catalysts. | |
In order to evaluate the extensive adaptability of the obtained sample, 1I/6Br composite was further used to degrade crystal violet (CV), methyl blue (MB) and methyl orange (MO) under the same experiment conditions. As shown in Fig. 8a, the degradation efficiency of CV, MB and MO reached to 97%, 91%, and 84% after 120 min, 180 min and 270 min in the presence of 1I/6Br composite. It can be seen from Fig. 8b that the corresponding apparent pseudo-first-order rate constant (kapp) values were calculated to be 0.0222, 0.0132 and 0.0062 min−1 for the degradation of CV, MB, and MO. The results revealed that the 1I/6Br composite can be used as a catalyst in a relatively wide range for practical applications.
 |
| Fig. 8 (a) Photocatalytic activity of 1I/6Br for CV, MB and MO under visible light irradiation; (b) ln(C0/Ct) versus irradiation time for degradation of CV, MB and MO. | |
The recycle experiment of 1I/6Br for photocatalytic reaction was carried out under the same reaction conditions. As shown in Fig. 9, the photocatalytic activity of BiOIO3/BiOBr still remained well after 4 recycling runs. The decrease of the efficiency was thought to be due to some catalyst washout during the recovery steps, which could be minimized through the use of centrifugation between runs. Therefore, the BiOIO3/BiOBr composite could be a kind of active and stable photocatalyst in favor of the practical application.
 |
| Fig. 9 Recycling test on 1I/6Br composite for the degradation of RhB under visible light irradiation. | |
3.3 Photocatalytic mechanism discussion
In order to understand the possible photocatalytic mechanism of the degradation in detail, active species trapping measurements, photoluminescence and photoelectrochemistry were performed to survey the charge movement behavior as well as the photocatalytic mechanism.
Three different radical scavengers were added into the 1I/6Br photocatalytic system to detect the active species. In this work, benzoquinone (BQ), sodium oxalate (Na2C2O4) and iso-propyl alcohol (IPA) were adopted as the scavengers of ˙O2−, h+ and ˙OH, respectively.25 The result of the trapping experiment was shown in Fig. 10. It can be seen that the addition of IPA (10 mM) had almost no influence upon this reaction, demonstrating ˙OH was not the major factor. However, the degradation efficiency was significantly decreased in the presence of Na2C2O4 (10 mM) and BQ (10 mM). The results suggested that both h+ and ˙O2− were the dominating active oxygen species during the degradation process under visible light irradiation.
 |
| Fig. 10 Trapping experiment of active species during the photocatalytic reaction under visible light irradiation. | |
It is well known that the photocatalytic reaction is usually completed by photo-generated electrons and holes, so to further clarify the separation efficiency of charge carriers, photoluminescence (PL) emission spectra was measured. Generally, high recombination of electron–hole pairs leads to the high fluorescence intensity and low photocatalytic activity.39 Fig. 11a presents the PL spectra with an excitation wavelength of 300 nm. The emission intensity of the 1I/6Br heterojunction was obviously lower than that of the pure BiOBr and BiOIO3 sample, indicating the lower recombination rate of photogenerated charge carriers in the 1I/6Br heterojunction. The result might reveal that the fabrication of 1I/6Br heterojunction could effectively suppress the recombination of electron hole pairs.
 |
| Fig. 11 (a) PL spectra of BiOIO3, BiOBr and 1I/6Br materials; (b) transient photocurrent of the samples under visible light. | |
Transient photocurrent analysis is another widely used typical method to investigate the electron–hole separation and transition effect for a material. The high photocurrent response usually signified the high separation efficiency of electrons and holes.9 As shown in Fig. 11b, the photocurrent of BiOIO3, 1I/6Br and BiOBr were recorded for several on–off cycles with a pulse of 10 s under visible light irradiation (λ > 420 nm) to give further evidence to support the proposed photocatalytic mechanism. The result indicated that the photocurrents of the BiOIO3 were negligible under the visible light irradiation. It might be explained that the BiOIO3 cannot be excited by the irradiation of visible light owing to its high band-gap energy. Comparatively, when 1I/6Br and BiOBr photoelectrodes were exposed to visible light, rapid photocurrent response was observed for both of them. Obviously, the photocurrent density generated by 1I/6Br is much higher than that by BiOBr. Therefore, the result of photocurrent–time measurement further suggested that the 1I/6Br composite photocatalyst has a stronger ability to separate electron–hole pairs than pure BiOBr and BiOIO3.
In a hybrid composite, the process of generation, migration and recombination of the photogenerated electrons and holes largely lies on the band positions of the individual components, which is also important to understand the mechanism of the dye photo-degradation. The valence band (VB) edge position and the conduction band (CB) edge position of BiOBr and BiOIO3 were theoretically calculated using the atom's Mulliken electronegativity definition:39
where
Evb is the valence band potential,
Ecb is the conduction band potential,
Ec is the energy of free electrons on the hydrogen scale (
ca. 4.5 eV),
X is the absolute electronegativity of the semiconductor (geometric mean of the absolute electronegativity of the constituent atoms), and
Eg is the band gap energy of the semiconductor. Based on the result derived from UV-vis diffuse reflectance spectra (
Fig. 4), the band gap energies of BiOBr and BiOIO
3 were found to be 2.76 and 3.06 eV, respectively. The electronegativity
X of BiOBr and BiOIO
3 was 6.17 and 6.84 eV, respectively.
31,35 According to the above equation, the valance band (
Evb) and conduction band (
Ecb) of BiOBr were calculated to be 3.05 and 0.29 eV. The
Evb and
Ecb of BiOIO
3 were calculated to be 3.87 and 0.81 eV, respectively.
According to the above results, a possible photocatalytic mechanism of dyes over BiOIO3/BiOBr under visible light was proposed as follows. First, BiOIO3 and BiOBr can form a p–n heterojunction for efficient charge separation. BiOIO3 is an n-type semiconductor whose Fermi energy level is close to conduction band (CB), whereas BiOBr is a p-type semiconductor with the Fermi energy level adjacent to the valence band (VB). When BiOBr contacts with BiOIO3 to form the p–n junction, the Fermi levels of BiOIO3 and BiOBr tend to shift downward and upward to achieve an equilibrium state, and an inner electric field will be generated in the interface of the composite (right of Fig. 12). The inner electric field will be beneficial to the separation of photo-induced carriers. Under visible light illumination, BiOBr can be activated due to its narrow band gap energy to generate electron–hole pairs. The inner electric field at p–n junction interface will push the electrons on the CB bottom of p-BiOBr toward that of n-BiOIO3, and are further captured by oxygen molecules to generate reactive ˙O2−. Meanwhile, the photo-induced holes will be clustered on the VB of BiOBr. Then both the h+ and ˙O2− will react with the dyes adsorbed on the surface of the photocatalyst, in according with the results of the radical scavenger experiments. In this case, the photoinduced electrons and holes are efficiently separated, which is consistent with the PL and photocurrent result. What's more, the dye-sensitized photocatalysis would also exist in the RhB degradation process. According to the previous reports,25,40 it is reasonable to believe that the adsorbed RhB can be excited by visible light irradiation. Since the ECB of BiOIO3 is much higher than the LUMO potential (−1.42 eV)41 of the RhB molecule, the excited electrons can transfer to the CB of BiOIO3 to react with O2 to produce ˙O2− and further participating in the degradation of RhB. Based on the above discussion, a possible pathway for the photocatalytic degradation of dyes can be described as follows:
BiOIO3/BiOBr + hν → BiOBr (e−) + BiOBr (h+) |
BiOBr (e− + h+) + BiOIO3 → BiOBr (e−) + BiOIO3 (e−) + BiOBr (h+) |
˙O2−/h+ + dyes → degradation products |
 |
| Fig. 12 Schematic diagram illustrating the proposed mechanism. | |
4. Conclusions
To sum up, a series of novel BiOIO3/BiOBr composites were prepared through a hydrothermal and chemical precipitation method by combining BiOIO3 with BiOBr. Under visible light irradiation (λ > 420 nm), the as-prepared BiOIO3/BiOBr heterostructures displayed higher photocatalytic performance on the degradation of dyes than that of pure BiOBr and BiOIO3. The highest photocatalytic activity was obtained on the 1I/6Br component and it also maintained excellent recyclability and stability. The enhanced photocatalytic activity could be mainly attributed to the formation of p–n heterojunction between BiOIO3 and BiOBr, which facilitated the separation of photoinduced charge carriers and inhibited electron–hole recombination. This work may provide a new perspective for the design and development of efficient and stable visible light photocatalysts.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (51541801, 51521006), the Hunan Provincial Natural Science Foundation of China (14JJ2045), and a Project Supported by Scientific Research Fund of Hunan Provincial Education Department (13k017).
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