Bi24O31Br10 Type II

Article pubs.acs.org/JPCC

Facile Fabrication of Bi12O17Br2/Bi24O31Br10 Type II Heterostructures with High Visible Photocatalytic Activity Yin Peng,† Pian-Pian Yu,† Qing-Guo Chen,† Hai-Yan Zhou,† and An-Wu Xu*,‡ †

The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Key Laboratory of Molecular Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China ‡ Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: One-dimensional (1D) Bi12O17Br2/Bi24O31Br10 type II heterostructures were synthesized by calcining BiOBr/ Bi(OHC2O4)·2H2O heterostructures in air at 400 °C. The photocatalytic activity of the as-prepared products was evaluated by the degradation of phenol and Rhodamin B (RhB) under visible light irradiation. The Bi12O17Br2/Bi24O31Br10 hierarchical heterostructures show enhanced visible light catalytic activity with the increase of the loaded/Bi24O31Br10 content, which results from the efficient separation of photogenerated charge carriers due to the staggered band potentials of the two materials. Radical scavenger experiments confirm that photogenerated holes (h+) are the main active species for oxidizing RhB molecules during the photocatalytic processes.

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INTRODUCTION The controlled synthesis of semiconductor photocatalysts with narrower band gap and higher quantum efficiency has attracted considerable interest for clean hydrogen energy production and environmental decontamination.1−3 To date, TiO2 and ZnO as well-known semiconductors have display advantages of nontoxic nature, low cost, high photosensitivity, and environmentally friendly features in the field of photocatalytic applications.4 However, one of their most shortcomings is that they can only absorb ultraviolet irradiation (only 4% of the solar spectrum) due to their wide band gap.4,5 Therefore, it is urgent to fabricate novel visible-light-driven photocatalysts5−9 and study their catalytic performances.10−12 Many bismuth-based compounds possessing related Sllén structure, such as BiOX (X = Cl, Br, I),13−15 as a class of ternary oxide photocatalysts,16−18 have exhibited favorable catalytic properties and potential applications. BiOX has unique layered structures characterized by [Bi2O2]2+ slabs interleaved with halogen atoms. An internal static electric field perpendicular to [Bi2O2]2+ and halogen layers will be generated due to nonuniform charge distribution between each layer, which facilitates to improve the separation of photogenerated charge carriers and enhance the photoelectrochemical/photocatalytic performances.19,20 © XXXX American Chemical Society

However, we are unfamiliar with Br-poor bismuth oxybromides, for example, Bi 1 2 O 1 7 Br 2 , 2 1 , 2 2 Bi 5 O 7 Br, 2 3 Bi24O31Br10,24,25 and Bi4O5Br226,27 due to few information on their preparation, photocatalytic andoptical properties. Very recently, it is also found that bismuth oxyhalides have shown excellent photocatalytic performances in indoor air purification and wastewater treatment.28−31 For example, Shang and coworkers30 synthesized plate-like Bi24O31Br10 powders by a chemical precipitation method, and this compound displayed good photocatalytic activities for the decomposition of the Cr (VI) in wastewater. Recently, Li et al.31 synthesized BiOBr/ Bi24O31Br10 heterojunctions by ionic liquid self-combustion method. They found that the heterostructures exhibit better photocatalytic activities than BiOBr in the decomposition rhodamine B (RhB) and methylene orange (MO). However, to the best of our knowledge, type II Bi12O17Br2/Bi24O31Br10 heterostructures has not been reported. According to the electronic affinity and bandgaps of semiconductors, semiconductor heterostructures have three kinds of cases: type-I, type-II, and type-III band alignment. In terms of type-I band Received: March 4, 2015 Revised: May 14, 2015

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DOI: 10.1021/acs.jpcc.5b02132 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C alignment, the position of conduction band (CB) of semiconductor A is higher than that of semiconductor B and the position of valence band (VB) of semiconductor A is lower than that of semiconductor B, while for the type-III one, the position of VB of semiconductor A is higher than the CB of semiconductor B. And in a type-II structure, the position of VB and CB of semiconductor A is higher or lower than that of semiconductor B, and the steps in the CB and VB go in the same orientation. Importantly, band bending at the interface is generated by the different chemical potential between semiconductor A and B, which results in the formation of a built-in field. As a result, the move of the photogenerated charge carrier into opposite directions is enhanced, and the electron−hole spatial separation at the interface of junction is correspondingly improved. In this current work, we successfully synthesized onedimensional (1D) Bi12O17Br2/Bi24O31Br10 heterostructures by calcining BiOBr/Bi(OHC2O4)·2H2O precursors. The asprepared products exhibit higher photocatalytic performances than Bi12O17Br2 in degradating RhB (or phenol) under visible light irradiation. Due to unique heterostructure, the products possess excellent photocatalytic by the synergistic effects: (a) effective absorbing visible light; (b) the formation of Bi12O17Br2/Bi24O31Br10 heterostructures with two different energy-level systems can enhance the separation of photogenerated electrons and holes; (c) 1D ordered nanostructure is in favor of directional and high efficient separation and transport of charge carriers.

Figure 1. XRD patterns of (a) standard card of Bi12O17Br2, (b) S1, (c) S2, (d) S3, (e) Standard Card of Bi24O31Br10, (f) calcined product from Bi(OHC2O4)·2H2O, (g) Standard Card of Bi2O3, (h) standard card of BiOBr, (i) calcined product from BiOBr, and (j) standard card of Bi4O5Br2.

from Bi(OHC2O4)·2H2O can be attributed to the monoclinic structure of Bi2O3 (JCPDS No. 76−1730; Figure 1g), and Figure 1i obviously displays two sets of XRD diffraction peaks of tetragonal BiOBr (JCPDS No. 09−0393; Figure 1h) and monoclinic Bi4O5Br2 (JCPDS No. 37−0699; Figure 1j). The above results clearly reveal that the chemical reactions occurred between Bi(OHC2O4)·2H2O and BiOBr during calcination. Bi2O3 can be obtained directly by calcining Bi(OHC2O4)·2H2O nanorods (the reaction eq 1). At the same time, Bi2O3 can react with the loaded-BiOBr to generate Bi12O17Br2 phase at high temperature. Moreover, when the loaded-BiOBr content further increases in the BiOBr-Bi(OHC2O4)·2H2O precursors, according to the reaction eq 2, Bi24O31Br10/Bi12O17Br2 heterostructures can be obtained. m mBi(OHC2O4 )· 2H 2O + O2 2 m 3 → Bi 2O3 + 2mCO2 + mH 2O (1) 2 2

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RESULTS AND DISCUSSION BiOBr/Bi(OHC2O4)·2H2O precursors were synthesized by a hydrothermal method using Bi(OHC2O4)·2H2O nanorods, Bi(NO3)3·5H2O and KBr as raw materials. The obtained samples with the nominal weight percentages of 20, 40, and 60 wt % BiOBr in BiOBr/Bi(OHC2O4)·2H2O were labeled as SP1, SP2, and SP3, respectively. The X-ray powder diffraction (XRD) patterns of the as-made BiOBr/Bi(OHC2O4)·2H2O precursors are given in Figure S1 (Supporting Information). The diffraction peaks shown in Figure S1a can be attributed to bismuth oxalate (Bi(OHC2O4)·2H2O) precursor. The new diffraction peaks indexed to tetragonal structure of BiOBr (Figure S1b−d) appear and their intensities become stronger and stronger with the loaded/BiOBr content increasing. These results show that the BiOBr/Bi(OHC2O4)·2H2O heterostructures are formed. The S1, S2, and S3 samples were obtained by calcining BiOBr/Bi(OHC2O4)·2H2O precursors at 400 °C for 2 h in air, and the corresponding calcined SP1, SP2, and SP3 products were labeled S1, S2, and S3, respectively. Their XRD patterns are shown in Figure 1b-d. Figure 1b shows the diffraction peaks of S1 sample, which matches well with the tetragonal structure of Bi12O17Br2 (JCPDS No. 37−0701; Figure 1a). The narrow and sharp diffraction peaks of Bi12O17Br2 show the high degree of crystallinity. With the loaded-BiOBr content increase, a new monoclinic-phase Bi24O31Br10 (JCPDS No. 75−0888; Figure 1e) can be found in the XRD pattern of S2 sample (Figure 1c) except Bi12O17Br2 phase. Further increasing the loaded-BiOBr content in the reaction system, the diffraction peaks of Bi 24 O 31 Br 10 become obviously stronger, and those of Bi12O17Br2 are gradually weaker. The XRD patterns of obtained samples by calcining pure Bi(OHC2O4)·2H2O and BiOBr are shown in Figure 1f and i, respectively. It can be seen from Figure 1f that all of the diffraction peaks of the calcined product

n−m 3n − m Bi 2O3 + mBiOBr → Bi nO Brm (2) 2 2 Here, m is the mole of Bi(OHC2O4)·2H2O added, and n is the initial mole of Bi3+. The O-rich bismuth oxybromides can be obtained when m < n.32 Figure 2 displays the morphologies of the obtained products. The as-made BiOBr/Bi(OHC2O4)·2H2O precursors are 1D rod-like structures, and thin BiOBr nanosheets grow horizontally onto the surface of Bi(OHC2O4)·2H2O nanorods (Figure 2a). Meanwhile, the more the loaded-BiOBr content is, the more BiOBr nanosheets grow onto the Bi(OHC2O4)·2H2O rods (Figure 2c), finally each Bi(OHC2O4)·2H2O rod is nearly covered by BiOBr nanosheets (Figure 2e). After calcining BiOBr/Bi(OHC2O4)·2H2O precursors at 400 °C for 2 h, we can clearly seen that the shapes of the obtained Bi12O17Br2/ Bi24O31Br10 heterostructures (Figure 2b,d,f) are still kept as rod-like structures. A lot of pores can be found in these rods. Few ultrathin nanosheets grow out from the surface of the S1 nanorod and are perpendicular to the surface of the nanorod (highlighted as red circles in Figure 2b). With the loadedBiOBr content increase, these ultrathin nanosheets vertically growing onto the surface of nanorods become more and more, which can be clearly seen from Figure 2d,f. Combined with the XRD results, we draw a conclusion that these ultrathin B


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The Journal of Physical Chemistry C

Figure 3. (a) TEM, (b) HRTEM image, and (c) EDS spectrum of the S3 heterostructures.

copy (XPS).33 The full survey and high-resolution spectra of Br 3d, O 1s, and Bi 4f peaks are shown in Figure 4. O, Bi, and Br peaks can be observed (Figure 4a), which proves the compositions of S3. The peaks centered at 159.2 and164.5 eV are indexed to Bi 4f7/2 and Bi 4f5/2, respectively (Figure 4b), proving the existence of Bi3+ in S3 sample. O 1s signals at 530.1 and 531.1 eV imply that there are two kinds of oxygen species (Figure 4c),34 which are attributed to the Bi−O chemical bonding in the Bi24O31Br10 and Bi12O17Br2. The binding energies located at 67.9−68.5 eV and 68.8−69.4 eV are ascribed to Br 3d5/2 and 3d3/2, respectively, which could be related to Br in the −1 oxidation state (Figure 4d).35 According to above analysis, a possible formation mechanism for the formation of Bi12O17Br2/Bi24O31Br10 heterostructures can be proposed as shown in Scheme 1. The BiOBr nanosheets first grow horizontally on the Bi(OHC2O4)·2H2O nanorods and form BiOBr/Bi(OHC2O4)·2H2O heterostructures via a hydrothermal process at 150 °C. During calcination process, Bi(OHC2O4)·2H2O nanorod is decomposed to Bi2O3 at high temperature. At the same time, these Bi2O3 nanorods react with BiOBr to form Bi12O17Br2/Bi24O31Br10 hierachical heterostructures. We carried out the nitrogen adsorption and desorption experiments at 77 K. The nitrogen sorption isotherm of the S3 is shown in Figure 5a. A type IV isotherm with a type H3 hysteresis loop at high relative pressures can be found according to the IUPAC classification, which shows that the S3 sample is a mesoporous structure with a pore diameter of 2−50 nm.36,37 This result can be further proved by the corresponding pore size distribution (Figure 5b). Table 1 displays the pore volume and BET surface area of different samples. It can be seen that the BET specific surface areas of obtained products increase gradually from S1 to S3, which could be ascribed to the formation of more Bi24O31Br10 ultrathin nanosheets. Additionally, S3 has the larger pore volume than S1 and S2 samples. As we know, light waves can penetrate deep into the photocatalysts if the pore volume is suitable, which will result in high mobility of electrons and holes.38−40 It is predicted that S3 sample will exhibit the highest

Figure 2. SEM images of (a) SP1, (b) S1, (c) SP2, (d) S2, (e) SP3, (f) S3, (g) Bi(OHC2O4)·2H2O, (h) calcined products from Bi(OHC2O4)· 2H2O, (i) BiOBr, and (j) calcined product from BiOBr.

nanosheets should be Bi24O31Br10, and all the calcined products are Bi24O31Br10/Bi12O17Br2 heterostructures even though we do not obviously observed the diffraction peaks of Bi24O31Br10 in S1 sample due to very low content of Bi24O31Br10/Bi12O17Br2 phase. The FE-SEM images of Bi(OHC2O4).2H2O, BiOBr and their calcined products are shown in Figure 2g−j, respectively. Bi(OHC2O4)·2H2O displays 1D rod-like structure with smooth surface (Figure 2g). After calcining at 400 °C for 2 h, Bi(OHC2O4)·2H2O transformed to Bi2O3 porous rods due to gas removal (Figure 2h). BiOBr nanosheets with the thickness of 50−60 nm are observed (Figure 2i), BiOBr by calcination transformed to the mixture of Bi4O5Br2 nanorods and BiOBr nanosheets (Figure 2j). Transmission electron microscopy (TEM) and highresolution transmission electron microscope (HRTEM) images were measured in order to obtain further information about the structure of S3 sample. It can found that there are some Bi24O31Br10 nanosheets growing on the surface of the Bi12O17Br2 nanorods (Figure 3a), consistent with XRD and SEM results. HRTEM image of S3 taken from the edge of Bi24O31Br10 nanosheet (see arrow in Figure 3a) is shown in Figure 3b. The spacing of the lattice fringes is 0.280, which matches well with the (117) atomic planes of Bi24O31Br10. The energy dispersive spectroscopy (EDS) analysis of nanosheets (Figure 3c recorded from a marked box in Figure 3a) further proves that only Bi, O, and Br elements is contained in these nanosheets, and Bi/Br molar ratio calculated is 45.68:19.31, close to 2.4:1 ratio in Bi24O31Br10. To further determine the chemical states and compositions, the S3 sample was measured by X-ray photoelectron spectrosC


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Figure 4. Survey XPS spectrum (a), high-resolution XPS spectra of (b) Bi 4f, (c) O 1s, and (d) Br 3d for S3 sample.

Scheme 1. Formation Process of Bi12O17Br2/Bi24O31Br10 Heterostructures

Table 1. Surface Area, Pore Volume, and Band Gap of Different Samples

photocatalytic performance among all the samples due to large BET surface area and pore volume. UV−vis diffuse-reflectance spectra (DRS) was used to measure the optical properties of S1, S2, and S3. It is found that these three samples exhibit visible light absorption and the absorption band edge blue-shifts with increasing content of Bi24O31Br10 in the Bi12O17Br2/Bi24O31Br10 heterostructures (S1 → S3), which is attributed to the wider band gap of Bi24O31Br10 (2.79 eV)32 than Bi12O17Br2 (2.59 eV; inset in Figure 6). According to Figure 2b, the S1 sample is Bi12O17Br2 phase containing a spots of Bi24O31Br10. So, the S1 optical band gap could be determined by eq 3.41,42

samples

BET surface area (m2·g−1)

porous volume (cm3·g−1)

band gap (eV)

S1 S2 S3

9.32 10.81 11.14

0.070744 0.070858 0.071861

2.59 2.67 2.75

Figure 6. UV−vis diffuse reflectance spectra of S1, S2, and S3 samples, plot of (αhν)1/2 vs photon energy (hν) for S1 inset in Figure 6.

Figure 5. (a) Nitrogen adsorption−desorption isotherm and (b) the pore size distribution of the S3. D


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Figure 7. (a) UV−vis absorption spectra of RhB solution under visible light (λ ≥ 400 nm) irradiation in the presence of S3 sample. Inset in (a) is photographs of RhB solutions by different reaction times. (b) The degradation curves of RhB (10 mg/L) in the presence of different photocatalysts.

Figure 8. (a) Cycling times of the photodegrading RhB using S3 as photocatalyst under visible light (λ ≥ 400 nm) irradiation and (b) XRD patterns of the S3 before and after six repeated cycles.

αℏν = A(ℏν − Eg )n /2

photoactivity of Bi12O17Br2/Bi24O31Br10 gradually enhances with the loaded-Bi24O31Br10 content increase, and S3 exhibits the highest photocatalytic activity. It can degrade 97% of RhB solution in 1 h under visible light irradiation. However, only 45% RhB molecules can be degraded in 1 h using S1 as visible light photocatalyst. Obviously, the formation of Bi12O17Br2/ Bi24O31Br10 heterostructures is favorable for enhancing photocatalytic activity. Moreover, 1D ordered nanostructure is beneficial for increasing the path length of light, directional transport and efficient separation of carriers during photocatalysis. The S3 sample was reused six times to decompose RhB dye in order to explore the photostability in visible light irradiation. It is can be seen from Figure 8 that S3 sample displays high photostability during the process of photocatalysis (Figure 8a), and its photocatalytic efficiency only reduces 2% after six repeated cycles. XRD patterns (Figure 8b) of S3 sample before and after irradiation show that the peaks are all attributed to tetragonal phase Bi12O17Br2 and monoclinic-phase Bi24O31Br10. No other impurity peaks appear. These results demonstrate that S3 has good stability during the cycling photocatalytic reaction. We carry out the trapping experiments of the active species in photocatalysis for exploring the photocatalytic mechanism of S3 in detail. The scavengers tertbutyl alcohol (TBA), benzoquinone (BQ) and ammonium oxalate (AO) were used to trap hydroxyl radical (•OH), superoxide radical (•O2−) and h+, respectively.46−50 Figure 9 displays the relationship of different scavengers and the photodegradation rate of RhB solution in the presence of S3 photocatalyst. It can be found that introduction of TBA or BQ does not cause significant deactivation of S3 photocatalyst, and only a slight decrease in degradation of RhB from 99.7 to 84.6% or 94.7% in 2 h,

(3)

in which ℏ, α, A, ν, and Eg are Planck constant, the absorption coefficient, a constant, light frequency, and band gap, respectively, and n is related to the characteristics of the transition in a semiconductor. Here, n = 4 for BiOX due to the indirect transition. The calculated band gap of the S1 is about 2.59 eV (inset in Figure 6). The band gap values of S2 and S3 samples were calculated according to the bandgap relation and listed in Table 1: Eg (x) = (1 − x)Eg (Bi12O17 Br2) + xEg(Bi 24O31Br10) − bx(1 − x)

(4)

in which b is the bowing parameter, and x is the mole fraction of Bi24O31Br10 [Bi24O31Br10/(Bi24O31Br10 + Bi12O17Br2)].43,44 Here, the bowing parameter is negligible in our approximate calculation. The rhodamine B (RhB) evolution of UV−vis spectrum over time using S3 sample as photocatalyst under visible light irradiation (λ ≥ 400 nm) is given in Figure 7a. All the absorption peaks of the RhB gradually decrease with irradiation time increase and almost disappear in 1.5 h. The maximum absorption peak is blue-shift from 554 to 494 nm during the whole photocatalytic process, which indicates that the removal of the ethyl groups is the first stage and the cleavage of the whole chromophore structure (cycloreversion) is the second stage in RhB molecules under the visible light irradiation.45 The color of the RhB solution changes from initial red to transparent as the irradiation time increases (inset in Figure 7a). The correlation curves between the concentration of RhB solution and the irradiation time is shown in Figure 7b using S1, S2, and S3 as photocatalysts. It is found that the E


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eV. Thus, the calculated band gap of Bi24O31Br10 is about 2.79 eV. Based on the band positions of Bi24O31Br10 and Bi12O17Br2, the schematic description for enhanced photocatalytic mechanism of the Bi12O17Br2/Bi24O31Br10 heterostructures under visible light irradiation are easily illustrated (Figure 11).

Figure 9. The degradation curve of RhB in the presence of different scavengers using S3 as photocatalyst under visible light (λ ≥ 400 nm) irradiation.

respectively. But, the photocatalytic activity of S3 significantly decreases when the AO was added. These results suggest that h+ is the main active species in the photocatalytic process under visible light irradiation. In order to further assess the photocatalytic performance of the as-made samples, we chose toxic organic phenol as model pollutant to degrade under visible light irradiation. Figure 10

Figure 11. Schematic description of the enhanced photocatalytic mechanism of Bi12O17Br2/Bi24O31Br10 type II heterostructures.

Bi12O17Br2 and Bi24O31Br10 can be excited by visible light due to their narrow band gaps, then photogenerated carriers are formed. When Bi12O17Br2 and Bi24O31Br10 semiconductors are in contact to form Bi12O17Br2/Bi24O31Br10 heterostructures, the excited electrons in the CB of Bi24O31Br10 can transfer to the CB of Bi12O17Br2 due to their different CB position, while the holes stay in the VB of Bi24O31Br10. Meanwhile, the photogenerated holes in the VB of Bi12O17Br2 transfer to the VB of Bi24O31Br10 due to different VB position. Thus, the photogenerated electron−hole pairs in Bi24O31Br10/Bi12O17Br2 type II heterostructures can be effectively separated. As a result, the photocatalytic activity is remarkably improved because of the formation of Bi12O17Br2/Bi24O31Br10 type II heterostructures. From Figure 11, we can see that the CB potential (0.48 eV) of Bi12O17Br2 is positive enough over E0 (O2/•O2−) (−0.046 eV vs NHE), O2 which adsorb on the surface of Bi12O17Br2 cannot be reduced to •O2− through one electron reduction reaction by the photogenerated electrons left in the CB of Bi12O17Br2. The oxidation potential of Bi24O31Br10 is 2.93 eV, which implies that the water molecules or hydroxyl groups adsorbed on the surface of Bi24O31Br10 can be directly oxidized by photogenerated holes to form •OH radicals (2.7 V vs NHE). So, in combination with the results shown in Figure 9, a conclusion can be drawn that the holes are not only the main active species and but also directly take part in reaction with RhB molecules in the solution. When, the nominal weight percentage of BiOBr in BiOBr/ Bi(OHC2O4)·2H2O heterostructures reaches 80 wt %, the obtained BiOBr/Bi(OHC2O4)·2H2O precursor (labeled SP4) is still 1D rod-like structure, some BiOBr nanosheets grow horizontally onto the surface of Bi(OHC2O4)·2H2O nanorod, but some do not (Figure S2a). This implies that some BiOBr nanosheets do not contact with Bi(OHC2O4)·2H2O nanorods to form BiOBr/Bi(OHC2O4)·2H2O heterostructure. SEM image of the corresponding calcined product (labeled S4) from SP4 precursor is shown in Figure S2b. We can clearly see that the shapes of the obtained S4 sample are the mixture of nanosheets and nanorods. According to XRD pattern (Figure S3), S4 sample is the mixture of Bi3O4Br nanorods (JCPDS No. 84−0793) and BiOBr nanosheets (JCPDS No. 09−0393). Further study on the morphology of S4 (highlighted as a red circle in Figure S2b), it is found that a new bismuth oxybromide compound grows from Bi3O 4Br nanorods.

Figure 10. Degradation curves of phenol (10 mg/L) aqueous in the presence of different photocatalysts under visible light (λ ≥ 400 nm) irradiation.

shows the degradation curves of phenol solution using S1, S2, and S3 samples as photocatalysts under the visible light irradiation. It is found that the photocatalytic activity of Bi12O17Br2/Bi24O31Br10 heterojunctions enhances with the loaded-Bi24O31Br10 content increase, and the S3 sample also displays the highest photocatalytic activity to degrade phenol among three samples, and 60% phenol molecules can degraded within 4 h. The band positions of Bi12O17Br2 and Bi24O31Br10 compounds were calculated according to the empirical eq 5:51 E VB = X − E e + 0.5Eg

(5)

where EVB is the VB edge potential, X is the electronegativity of the semiconductor, namely, the geometric average of the absolute electronegativity of the constituent atoms, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), and Eg is the band gap. The value of the VB edge is calculated to 3.07 eV for the S1 sample (Bi12O17Br2). The CB edge of S1 is 0.48 eV according to the equation of ECB = EVB − Eg. The relative positions of VB and CB edges of Bi24O31Br10 are referred to the data reported in ref 32. The VB edge of Bi24O31Br10 is 2.93 eV, and the CB edge of Bi24O31Br10 is 0.14 F


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PerkinElmer RBD upgraded PHI-5000C ESCA system. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images, the selected area electron diffraction (SAED) patterns and energy-dispersive X-ray spectroscopy (EDS) analysis were recorded on a JEOL-2010 microscope with an accelerating voltage of 200 kV. UV-2450 spectrophotometer in the wavelength range of 200−800 nm was used to measure UV− vis diffuse-reflectance spectra at room temperature, and the reflectance standard material was BaSO4. Micromeritics Tristar II 3020 M analyzer was used to measure Nitrogen adsorption/ desorption experiment of samples at 77 K after degassing at 180 °C for 6 h. Using adsorption data in a relative pressure range from 0.05 to 0.3 to estimate the Brunauer−Emmett−Teller (BET) surface area. Photocatalytic Experiments. RhB (or phenol) was chosen to measure photocatalytic performance of the obtained S1, S2, and S3 samples under visible light irradiation. The 500 W Xe lamp (PLS-SXE500/500UV, Trusttech Co., Ltd., Beijing) acted as the light source. The reaction was maintained at room temperature by a cooling water circulation. 100 mg of the photocatalyst mixed with 10 mg/L of RhB (or phenol) (100 mL) and formed a suspension for the following degradation reaction at room temperature. Prior to irradiation, stir the suspension in the dark for 30 min to reach an adsorption−desorption equilibrium. Then illuminate the suspension using the Xe lamp coupled with a 400 nm UV cutoff filter under magnetic stirring. At appropriate intervals, withdraw 4 mL of suspension, centrifuge, and remove the photocatalyst. Monitor the concentration of RhB (or phenol) solution using UV−vis spectrophotometer. The photocatalyst was centrifuged and used directly for the next experiment after each cycle in order to measure the stability of photocatalyst. The experiments of trapping active species are similar to the photocatalytic tests. Scavengers t-butanol, p-benzoquinone (BQ), and ammonium oxalate (AO) were added into RhB solution to trap hydroxyl radicals (•OH), the superoxide radicals (•O2−), and hole (h+), respectively, followed by the photocatalytic tests.

However, its crystal phase does not be identified at present. S4 sample also displays good visible light photocatalytic activity, and can completely degrade RhB in 2 h (Figure S4a).

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CONCLUSION In summary, one-dimensional Bi12O17Br2/Bi24O31Br10 heterostructures have been prepared for the first time by calcining one-dimensional BiOBr/Bi(OHC2O4)·2H2O heterostructures at 400 °C in air. The Bi12O17Br2/Bi24O31Br10 heterostructures exhibit high photocatalytic activity to degrade RhB under visible light irradiation. The synergistic effects originating from the stagger band potentials between Bi12O17Br2 and Bi24O31Br10 can explain this enhanced photocatalytic activity. Furthermore, 1D ordered nanostructure is beneficial for highly directional and efficient transport and separation of charge carriers. The photogenerated holes (h+; main active species) directly react with RhB molecules under the visible light irradiation. The obtained Bi12O17Br2/Bi24O31Br10 heterostructures exhibit good photostability after six repeated cycles. This work has developed a novel path to well-defined semiconductor type II heterostructures with high photocatalytic performance, which highlights the promising optoelectronic applications by rational nanostructure designs.

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EXPERIMENTAL SECTION

Samples Preparation. All the reagents were analytical grade in our experiment and used as received without further purification. Bi(OHC2O4)·2H2O nanorods were obtained according to ref 43. A total of 2.911 g of Bi(NO3)3·5H2O and 1.206 g of Na2C2O4 were dissolved in 20 mL of distilled water, respectively. The Bi(NO3)3 solution was added into the Na2C2O4 solution under vigorous magnetic stirring, and then transferred into a stainless steel autoclave with a Teflon liner. After heating at 120 °C for 40 h, use deionized water and anhydrous ethanol to wash sample for several times, and then dry at 60 °C for 6 h. BiOBr/Bi(OHC2O4)·2H2O heterostructures were prepared by hydrothermal route. A total of 0.1 g of Bi(OHC2O4)·2H2O nanorods and Bi(NO3)3·5H2O with different amounts were added into the solution which contains acetic acid (1 mL) and distilled water (19 mL) under magnetic stirring for 30 min. Then, KBr aqueous solution (20 mL) was added drop by drop under magnetic stirring for another 30 min. The mixture was transferred into a stainless steel autoclave with a Teflon liner and treated at 150 °C for 24 h. Cool to room temperature, collect and wash products with deionized water and absolute ethanol for several times, then dry at 60 °C for 4 h. The nominal weight percentages of BiOBr in BiOBr/Bi(OHC2O4)· 2H2O heterostructures were 20, 40, and 60 wt %, respectively. The respective obtained samples were labeled as SP1, SP2, and SP3. Finally, calcine the obtained BiOBr/Bi(OHC2O4)·2H2O heterostructures at 400 °C for 2 h in air. And the corresponding calcined SP1, SP2, and SP3 products were labeled S1, S2, and S3, respectively. Sample Characterization. X-ray powder diffraction (XRD) was performed on a Rigaku (Japan) D/max-γA X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). Field emission scanning electron microscopy (FE-SEM) images were carried out on a Hitachi S-4800 microscope. The X-ray photoelectron spectroscopy (XPS) was carried out on a

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ASSOCIATED CONTENT

S Supporting Information *

The X-ray powder diffraction (XRD) patterns of the BiOBr/ Bi(OHC2O4)·2H2O heterostructures, SEM images, XRD pattern, and the degradation curves of RhB using S4 sample as photocatalyst under visible light irradiation. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02132.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21271165, 21101006). G


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Bi24O31Br10 Type II

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