Two Novel Bi-Based Borate Photocatalysts: Crystal Structure

Article pubs.acs.org/JPCC

Two Novel Bi-Based Borate Photocatalysts: Crystal Structure, Electronic Structure, Photoelectrochemical Properties, and Photocatalytic Activity under Simulated Solar Light Irradiation Hongwei Huang,*,† Ying He,† Zheshuai Lin,‡ Lei Kang,‡ and Yihe Zhang† †

National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, PR China ‡ Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China S Supporting Information *

ABSTRACT: Through the combination of Bi3+ and a large negative charge ion (BO3)3−, two novel Bi-based borate photocatalysts Bi4B2O9 and Bi2O2[BO2(OH)] with layered structure have been successfully developed. For the first time, the borates were investigated as photocatalysts. They were synthesized by solid-state reaction and hydrothermal method, respectively, and further characterized by XRD, SEM, TEM, HRTEM, and DRS. Bi4B2O9 and Bi2O2[BO2(OH)] possess direct and indirect transition optical band gaps of 3.02 and 2.85 eV, respectively. Density functional calculations revealed that the valence band (VB) and conduction band (CB) of both borates were composed of hybridized states of the O 2p and Bi 6p or 6s orbitals, and a large dispersion was observed in the energy band of Bi2O2[BO2(OH)]. The photodecomposition experiments demonstrated that Bi4B2O9 and Bi2O2[BO2(OH)] can be used as effective photocatalysts under simulated solar irradiation, and Bi2O2[BO2(OH)] exhibits the high photocatalytic activity, which is 2.5 and 3.2 times compared with that of P25 and Bi2O2CO3, respectively. Moreover, the photocurrent conversion further confirmed that Bi4B2O9 and Bi2O2[BO2(OH)] were potential photofunctional materials. The layered structure with (Bi2O2)2+ layer, hybridized and dispersion energy band, and large negative charge of (BO3)3− ion should be responsible for the high photocatalytic activity of Bi2O2[BO2(OH)].

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INTRODUCTION Photocatalysts have attracted much attention for solving the severe problems of energy shortages and environment crises as a potential solution over the past decades.1−3 It is because that they can be used to decompose organic contaminants for environmental purification and split water into hydrogen and oxygen gases for clean energy production and under UV and visible-light irradiation. Besides the focused work on TiO2 and its modifications,4−6 many efforts were made to develop other novel efficient photocatalysts, which can be generally classified as oxides,7 sulfides,8 oxysalts,9 and polymers.10 Among these, Bi-based compounds have drawn a lot of attention for their potential application as novel photocatalysts. Because of the lone pair electrons of Bi3+, the Bi-based compounds were often found to possess hybridized band structures. The hybridized states of band structures can not only decrease the effective masses of holes and electrons, to favor a longer traveling distance for excited carriers,11 but also effectively decrease the band gaps and increase the light absorption in the long-wavelength region, enhancing their high photocatalytic activities. Though bismuth-based photocatalysts crystallize in different structure types, they all exhibit high © 2013 American Chemical Society

efficiency in the degradation of organic pollutants, including perovskite-structured NaBiO312 and BiFeO3,13 Scheelitestructure BiVO4,14 pyrochlore-structure Bi2MNbO7 (M = Al, Ga, In, Fe, and Sm),15 Aurivillius structure Bi2MoO6,16 Bi2WO6,17 and Bi2SiO5,18 Sillén structure BiOX (X = Cl, Br, and I),19 and Sillén−Aurivillius structure Bi4NbO8Cl.20 Among these, the compounds with Aurivillius and Sillén structures display more excellent photooxidation ability and interesting structure−property relationships due to the existence of an active (Bi2O2)2+ layer. The Aurivillius structure is built up from alternate layers of (Bi2O2)2+ cations and perovskite-like (Am−1BmO3m+1)2‑ anionic blocks, with m being an integer corresponding to the number of cornershared octahedra forming the perovskite blocks. In the Sillén family expressed by [Bi2O2][Xm] (m = 1−3), the bismuth oxide-based fluoritelike layers, (Bi2O2)2+, are intergrown with single, double, or triple halogen layers to construct such compositions. These layered structure compounds were considered to promote the Received: August 22, 2013 Revised: October 10, 2013 Published: October 11, 2013 22986

dx.doi.org/10.1021/jp4084184 | J. Phys. Chem. C 2013, 117, 22986−22994

The Journal of Physical Chemistry C

Article

Figure 1. Crystal structure of (a) Bi4B2O9, (b) one-dimensional Bi2O2 chain in Bi4B2O9, and (c) Bi2O2[BO2(OH)].

raw materials of Bi2O3 and H3BO3 were mixed in stoichiometric proportions and then gradually elevated to sintering temperatures of 600 °C and kept at this temperature in air for 10 h. The calcination procedure was repeated another three times after grinding to ensure a complete reaction. Bi2O2[BO2(OH)] nanosheets were prepared via a hydrothermal method using Bi4B2O9 as the precursor. Typically, a powder of 1 g of Bi4B2O9 and 5 mL of water was added in a 15 mL Teflon autoclave and maintained at 200 °C for 24 h. After being cooled to room temperature, the yellowish products were washed with ethanol and distilled water several times and dried at 80 °C for 4 h. The Bi2O2CO3 sample as a reference was synthesized by a hydrothermal method.24 Characterization. The crystal structures of the obtained samples were examined by X-ray diffraction (XRD) using a D8 Advance X-ray diffractometer (Bruker AXS, Germany) with Cu Kα radiation (λ = 1.5418 Å). The scanning step width of 0.02° and the scanning rate of 0.2° S−1 were applied to record the patterns in the 2θ range of 1070°. The morphology and microstructure were obtained by a S-4800 scanning electron microscope (SEM) and a transmission electron microscope (TEM and HRTEM; JEM-2100F). UV−vis spectra were performed with sample powder from a PerkinElmer Lambda 35 UV−vis spectrometer. The spectra were collected at 200− 1000 nm referenced to BaSO4. Room temperature excitation and emission spectra were measured on a JOBIN 10 YVON FluoroMax-3 fluorescence spectrophotometer with a photomultiplier tube 11 operating at 400 V, and a 150 W Xe lamp was used as the excitation lamp. Specific surface areas of samples were characterized by the nitrogen adsorption BET method with a Micromeritics 3020 instrument. Electrochemical and photoelectrochemical measurements were performed in three-electrode quartz cells with a 0.1 M Na2SO4 electrolyte solution. Platinum wire was used as the counter electrode, and saturated calomel electrodes (SCE) were used as the reference electrodes. Bi4B2O9 and Bi2O2[BO2(OH)] film electrodes on ITO served as the working electrode. The photoelectrochemical experiment results were recorded using an electrochemical system (CHI-660B, China). The intensity of light was 1 mW/ cm2. Potentials are given with reference to the SCE. The photoresponses of the photocatalysts as UV light on and off were measured at 0.0 V. Electrochemical impedance spectra

generation and separation of the charge carriers, and to exhibit high photocatalytic activity for water splitting and degradation of pollutants under light irradiation. Recently, bismuth subcarbonate (Bi2O2CO3) with Sillénrelated structure has been found to be an efficient photocatalyst for decomposing organic contaminants under UV−vis light irradiation.21 Different from the Aurivillius and Sillén structures, the crystal structure of Bi2O2CO3 is composed of alternate (Bi2O2)2+ and (CO3)2− layers, presenting a new structural type of photocatalyst. Intrigued by the synergistic effects of layer structure and hybridized energy band, it is of great interest and importance to develop new Bi-based, especially Sillén-related, compounds for photocatalysis application. In addition, the newly found phosphate photocatalysts BiPO422 and Ag3PO423 were reported to possess excellent photooxidation properties. It is mainly because that (PO4)3− ions have a large negative charge which maintains a large dipole in these phosphates preferring the photogenerated charge separation. This effect is called an inductive effect described as the action of one group to affect electrostatically the electron distribution in another group. Herein, through the combination of Bi3+ and another large negative charge ion (BO3)3−, we successfully developed two novel Bi-based borate photocatalysts Bi 4 B 2 O 9 and Bi2O2[BO2(OH)]. So far, to our best knowledge, there is no report on the borates used as photocatalysts. In this paper, Bi4B2O9 and Bi2O2[BO2(OH)] were synthesized by solid-state reaction and hydrothermal method, respectively. Bi2O2[BO2(OH)] was found possessing a Sillén-related structure similar to Bi2O2CO3. The photocatalytic performances of Bi4B2O9 and Bi2O2[BO2(OH)] were investigated systematically. The photochemical properties of Bi4B2O9 and Bi2O2[BO2(OH)] were demonstrated by the photocatalytic decomposition of MB under simulated solar irradiation and photocurrent measurements under UV light. The origin of high photocatylytic activities was also suggested on the basis of the understanding of the structure−property relationship.

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EXPERIMENTAL SECTION Synthesis. Bi2O3 and H3BO3 are all in analytic grade purity and used as received, without further purification. Bi4B2O9 microparticles were synthesized by a solid-state reaction. The 22987



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(EIS) were measured at 0.0 V. A sinusoidal ac perturbation of 5 mV was applied to the electrode over the frequency range 0.05−105 Hz. Photocatalytic Evaluation. Photocatalytic activities of Bi4B2O9 and Bi2O2[BO2(OH)] were evaluated by degradation of methylene blue under simulated solar light irradiation of a 1000 W Xe lamp. Powder photocatalyst (50 mg) was dispersed into 100 mL of dye solution (10−5 mol/L). Before illumination, the photocatalyst powder and dye solution were vigorously stirred in the dark for 0.5 h to achieve the adsorption− desorption equilibrium of suspensions. After that, the light was turned on, and 2 mL of the suspension was taken at certain intervals and separated through centrifugation. The UV−vis spectra of the centrifuged solution were recorded using a U3010 spectrophotometer. Theoretical Calculation. The electronic structures, as well as total and partial densities of states (DOS), of Bi4B2O9 and Bi2O2[BO2(OH)] were obtained by the plane-wave pseudopotential method.25 The calculations were carried out using the local density approximation (LDA) with a very high kinetic energy cutoff of 500 eV adopted. The Monkhorst−Pack k-point with a density of (2 × 2 × 2) points in the Brillouin zone of the unit cell is chosen.26

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RESULTS AND DISCUSSION Crystal Structures of Bi4B2O9 and Bi2O2[BO2(OH)]. Bi4B2O9 crystallizes in the monoclinic space group P2/c with the unit cell parameters a = 11.127(9) Å, b = 6.641(5) Å, c = 11.058(9) Å, and β = 90.91(9)°.27 The crystal structure was shown in Figure 1a. It was constructed by BO3 triangles, BiO4 tetrahedron, and BiO5 polyhedra. In the asymmetric units, there are four crystallographically independent Bi atoms, among which two Bi atoms form a BiO4 tetrahedron and another two constitute BiO5 polyhedra with the Bi−O bond length ranging from 2.123 to 2.4794 Å. In this structure, two neighboring BiO5 polyhedra connect each other through edge sharing to form a Bi2O2 chain, as displayed in Figure 1b. Figure 1c illustrates the crystal structure of Bi2O2[BO2(OH)], which crystallizes in the monoclinic space group Cm with the lattice parameters a = 5.4676 Å, b = 14.6643 Å, c = 3.9058 Å, and β = 135.59°.28 In the asymmetric units, there is only one crystallographically independent Bi atom, one independent B atom, and three independent O atoms. Bi2O2[BO2(OH)] possesses the Sillénrelated crystal structure composed of (Bi2O2)2+ layers and (BO3)3− layers, as shown in Figure 1c. In Bi2O2[BO2(OH)], the Bi3+ cation links to oxygen atoms by five short and three long Bi−O bonds, and then, the lone electron pair of Bi3+ faces toward the open space around Bi3+, that is, the upper and lower sides of the (Bi2O2)2+ layers. This is very similar to that of bismuth oxycarbonates, Bi2O2CO3, and hydroxynitrates, Bi2O2(OH)(NO3),29 which were also reported to have Sillénrelated structures, built by (Bi2O2)2+, (CO3)2−, and (NO3)− layers, respectively. The XRD patterns of Bi4B2O9 and Bi2O2[BO2(OH)] were presented in Figure 2a and b, respectively. Obviously, all of the observed peaks of the patterns can be indexed to pure monoclinic Bi4B2O9 (ICSD#2796) and Bi2O2[BO2(OH)] phases (simulated from the single crystal model). No other peaks are found, suggesting the high purity and crystallinity of the two samples. Moreover, the strongest peaks of Bi4B2O9 and Bi2O2[BO2(OH)] are attributed to (−302) and (130) planes, respectively, which are in good agreement with the following HRTEM analyses.

Figure 2. XRD pattern of as-prepared samples of (a) Bi4B2O9 and (b) Bi2O2[BO2(OH)].

Morphologies and microstructure of Bi4B2O9 and Bi2O2[BO2(OH)] Products. The surface morphologies and particle sizes of Bi4B2O9 and Bi2O2[BO2(OH)] were observed by SEM. Figure 3a and b showed that the Bi4B2O9 products are

Figure 3. SEM images of as-prepared samples of (a, b) Bi4B2O9 and (c, d) Bi2O2[BO2(OH)]. 22988



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facets, as shown in Figure S1 (Supporting Information). For Bi2O2[BO2(OH)], the lattice fringe measurements with an interplanar spacing of 0.194 and 0.305 nm (Figure 4j and l) can be assigned to the corresponding (−222) and (130) planes, respectively. The angle indicated in the corresponding FFT image (Figure 4i) is 80.9°, which is in accordance with the theoretical values between the {−222} and {130} facets, as displayed in Figure S2 (Supporting Information). Optical Properties and Band Gaps. Figure 5 displays the UV−vis diffuse reflectance absorption spectra (DRS) of the asprepared nanocrystals of Bi4B2O9 and Bi2O2[BO2(OH)]. In semiconductors, the square of the absorption coefficient is linear with energy for direct optical transitions in the absorption edge region, whereas the square root of the absorption coefficient is linear with energy for indirect transitions.22 Data plots of absorption2 versus energy and absorption1/2 versus energy in the absorption edge region are shown in the upper inset of Figure 5a and b. From Figure 5a, the absorption2 versus energy plot is nearly linear for Bi4B2O9, while the absorption1/2 versus energy deviates from the fitted straight line. For Bi2O2[BO2(OH)], the absorption1/2 versus energy plot is nearly linear, while the absorption2 versus energy deviates from the fitted straight line from Figure 5b. These features suggest that the absorption edges of Bi4B2O9 and Bi2O2[BO2(OH)] are caused by direct and indirect transitions, respectively. Band gaps of Bi4B2O9 and Bi2O2[BO2(OH)] are determined by optical absorption near the band edge by the following equation:

composed of irregular microparticles, and the dimension of the particles was estimated to be 500 nm ∼ 2 μm. The SEM micrographs of Bi2O2[BO2(OH)] were illustrated in Figure 3c and d. It reveals that products of Bi2O2[BO2(OH)] consist of a large quantity of rectangular nanosheets with uniform cut edges. The length and width are about 1 μm and 200−400 nm, respectively. This nanosheet morphology is very similar to those of other (Bi2O2)2+ layers containing compounds.30 The obtained Bi4B2O9 and Bi2O2[BO2(OH)] products were further characterized by TEM and HRTEM. The low magnification TEM image in Figure 4a confirmed the particle

αhν = A(hν − Eg )n /2

(1)

where α, hν, A, and Eg are the optical absorption coefficient, photonic energy, proportionality constant, and band gap, respectively.31 In this equation, n decides the type of the transition in a semiconductor (n = 1, direct absorption; n = 4, indirect absorption). By applying n = 1, the direct band gap of Bi4B2O9 is determined from the plot of absorption2 versus energy, as indicated in Figure 5c, and by applying n = 4, the indirect band gap of Bi2O2[BO2(OH)] is determined from the plot of absorption1/2 versus energy, as presented in Figure 5d. By extrapolating the straight line to the x-axis in this plot, the Eg values of Bi4B2O9 and Bi2O2[BO2(OH)] were estimated to be 3.02 and 2.85 eV. Furthermore, we can calculate their conduction and valence band positions through the following equations: Figure 4. (a) TEM and (c) HRTEM images, (e, g) FFT (fast Fourier transition) patterns, and (f, h) magnified HRTEM images of the lattice fringe of the Bi4B2O9. (b) TEM and (d) HRTEM images, (i, k) FFT (fast Fourier transition) patterns, and (j, l) magnified HRTEM images of the lattice fringe of the Bi2O2[BO2(OH)].

E VB = X − E e + 0.5Eg

(2)

ECB = E VB − Eg

(3)

where X is the absolute electronegativity of the semiconductors, which is defined as the geometric average of the absolute electronegativity of the constituent atoms, Ee is the energy of free electrons on the hydrogen scale (≈4.5 eV), and Eg is the band gap.32 For Bi4B2O9, X is calculated to be 6.16 eV, consequently. ECB and EVB are estimated to be 0.15 and 3.17 eV, respectively. The X of Bi2O2[BO2(OH)] is calculated to be 6.34 eV, and ECB and EVB are estimated to be 0.41 and 3.26 eV, respectively. Band Structures and Density of States. The electronic structures of Bi4B2O9 and Bi2O2[BO2(OH)] were calculated by using the ab initio density functional theory (DFT) calculations. Although the bandgaps from DFT calculations are usually underestimated, they nonetheless often provide

size of Bi4B2O9. From Figure 4b, it can be clearly seen that the thicknesses of these nanosheets of Bi2O2[BO2(OH)] are in the range 40−50 nm. The HRTEM image and fast Fourier transform (FFT) images (Figure 4c−l) confirm the single crystal nature of Bi4B2O9 and Bi2O2[BO2(OH)]. The highresolution transmission electron microscopy (HRTEM) image of Bi4B2O9 (Figure 4f and h) shows two sets of lattice fringes with spacings of 0.309 and 0.302 nm, which can be indexed to (−302) and (121) planes of Bi4B2O9, respectively. The angle indicated in the corresponding fast-Fourier transform (FFT) image (Figure 4e) is 85.6°, which is identical to the theoretical values obtained for the angles between the {−302} and {121} 22989



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Figure 5. UV−vis diffuse reflectance spectra of (a) Bi4B2O9 and (b) Bi2O2[BO2(OH)]. The upper inset shows the plots of absorption2 vs energy and absorption1/2 vs energy in the absorption edge region (circles for experimental data and the line for a linear fit). (c) The absorption2 vs energy in the absorption edge region of Bi4B2O9. (d) The absorption1/2 vs energy in the absorption edge region of Bi2O2[BO2(OH)].

also a little of the Bi 6s and 6p orbitals, which varies from those in other oxides or oxygen compounds.33,34 These results are also obviously different from foreign elements creating impurity levels in the forbidden band in doped oxides. Meanwhile, we also found that the bottoms of the CB were mainly composed of the hybridized Bi 6p and 6s orbitals. The contribution of O 2p to the CB seemed to be much smaller than that in the oxides whose conduction electron was O 2p.35 Photocatalytic Activities of Bi4B2O9 and Bi2O2[BO2(OH)] Samples. Figure 7a showed the photocatalytic performance of the as-prepared Bi4 B2O9 and Bi2O2[BO2(OH)] samples evaluated by the degradation of MB. For comparison purposes, the MB photodegradated by TiO2 (P25) and Bi2O2CO3 was also performed. The XRD and TEM images of as-prepared Bi2O2CO3 were shown in Figures S3 and S4 (Supporting Information), respectively. It can be obviously seen that MB was almost photodecomposed and catalyzed by Bi2O2[BO2(OH)] and TiO2 (P25) in 80 and 160 min, and the photodegradation efficiency of Bi2O2CO3 and Bi4B2O9 reached 91 and 88% after 180 min of reaction, respectively. The MB photolysis without the photocatalyst can almost be neglected. As shown in Figure 6b and c, the main absorption peak of MB molecules at 664 nm decreases with irradiation time, and almost disappears after about 80 min for Bi2O2[BO2(OH)] and 180 min for Bi4B2O9. The insets in

important insight into the physicochemical behavior of the materials investigated. Parts a and c of Figure 6 show the band structures of Bi4B2O9 and Bi2O2[BO2(OH)]. The Fermi energy, defined as the highest occupied energy level, has been taken as the valence band maximum (VBM), and the lowest unoccupied occupied state is the conduction band minimum (CBM). For Bi4B2O9, the VBM and CBM are all situated at the G point, indicating the direct bandgap property, whereas the VBM and CBM of Bi2O2[BO2(OH)] are located at the point between L and M and the G point, respectively, which confirms the fact that Bi2O2[BO2(OH)] is an indirect bandgap semiconductor. These results are all consistent with those revealed from the absorption spectra. It can be found that the energy gaps between the VB maximum and CB minimum given from band structures are 2.8 and 2.34 eV for Bi4B2O9 and Bi2O2[BO2(OH)], respectively, which are in good agreement with the experimental values. Besides, dispersed energy bands were observed in Bi2O2[BO2(OH)], as shown in Figure 6c, and the width of dispersion of VBM and CBM is about 1.5 and 0.5 eV, respectively. Parts b and d of Figure 6 show the total density of states (TDOS) and main partial density of states (PDOS), corresponding to the energy regions in Figure 6a and c. The tops of the valence bands of Bi4B2O9 and Bi2O2[BO2(OH)] were not only found to be composed of the O 2p orbital but 22990



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Figure 6. Electronic band structures of (a) Bi4B2O9 and (c)Bi2O2[BO2(OH)]. Total and partial DOS of (b) Bi4B2O9 and (d) Bi2O2[BO2(OH)].

firmed that Bi4B2O9 and Bi2O2[BO2(OH)] were two potential photofunctional materials. As the electrochemical impedance spectra (EIS) Nyquist plots are supposed to indicate the charge separation and transfer process in the electrode−electrolyte interface region,37 the EIS technology was employed to study the photocatalytic performance. Figure 8b shows Nyquist plots of Bi4B2O9 and Bi2O2[BO2(OH)] before and after UV light irradiation. It can be observed that the arc radius of Bi2O2[BO2(OH)] is smaller than that of Bi4B2O9, which indicates that Bi2O2[BO2(OH)] exhibits a higher separation and transfer efficiency of photogenerated e−h pairs. Structure−Property Relationship. The photocatalytic oxidation of the organic contaminants is closely correlative to two factors: efficient photoinduced electron−hole separation and transfer and the structure of the band gap in the photocatalyst.11 In the process of photocatalytic degradation, charge separation is important and necessary to prevent recombination of the photoinduced electrons and holes. In the crystal structure of Bi2O2[BO2 (OH)], the layered configuration was considered to be very beneficial for the high photocatalytic activity. On one hand, oxidation and reduction sites in photocatalytic reaction locate at the surface and edge position of two-dimensional layered structure, respectively. Thus, the photogenerated holes only travel a very short distance (sub-nanometer) to reach the surface layer structure, and then were trapped by the hydroxyls in the layer gap. This rapid hole-trapping process allows more photogenerated electrons to more easily move to the edge of the layered structure, reducing the recombination probability of photogenerated carriers. On the other hand, the internal

Figure 7b and c showed the corresponding color changes of MB solution from the blue starting solution gradually to colorless with increasing light irradiation time. In order to compare the degradation rate quantitatively, the pseudo-first-order kinetic curves of MB photodegradation were also plotted (Figure 7d). The experimental data obviously show the apparent rate constant k is 0.0381, 0.0152, 0.0122, and 0.0115 min−1 for Bi2O2[BO2(OH)], P25, Bi2O2CO3, and Bi4B2O9, respectively. In other words, Bi2O2[BO2(OH)] exhibits the highest photocatalytic activity, which is 2.5 and 3.2 times compared with that of P25 and Bi2O2CO3, respectively, though the surface area of Bi2O2[BO2(OH)] (1.25 m2/g) and Bi4B2O9 (0.84 m2/g) is much smaller than P25 (48.6 m2/g) and Bi2O2CO3 (3.24 m2/g). The photocatalytic activity of Bi4B2O9 is relatively low but very close to that of Bi2O2CO3. Photoelectrochemical Properties. The mobilities of the electrons generated in the photocatalyst can be directly monitored by the photocurrent, and the rate should directly correlate with the photocatalytic activity of the material.36 Figure 8a shows the photocurrent of Bi 4 B 2 O 9 and Bi2O2[BO2(OH)] samples generated in electrolyte under UV light. When the light was on, the photocurrent of Bi4B2O9 and Bi2O2[BO2(OH)] was generated immediately, and the photocurrent generated by Bi4B2O9 was about 1/5 times that of Bi2O2[BO2(OH)], which is consistent with the order of their photocatalytic activities, showing the photocurrent was positively relevant to the photocatalytic activity. The generation of photoelectrons was the critical initial step of the photocatalytic reaction, and the rate directly governed the photocatalytic activity. The photocurrent conversion further con22991



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Figure 7. (a) Photocatalytic degradation curves of MB under the irradiation of simulated solar light. Temporal absorption spectral patterns of MB during the photodegradation process over (b) Bi4B2O9 and (c)Bi2O2[BO2(OH)]. (the insets in parts b and c demonstrate the color changes of MB). (d) Kinetic curves for the photocatalytic degradation of MB.

Figure 8. (a) Comparison of transient photocurrent responses of Bi4B2O9 and Bi2O2[BO2(OH)] under UV light irradiation (λ = 254 nm, [Na2SO4] = 0.1 M). (b) EIS Nynquist plots of Bi4B2O9 and Bi2O2[BO2(OH)] with light on/off cycles under the irradiation of UV light (λ = 254 nm, [Na2SO4] = 0.1 M).

electric fields are one of the important parameters to evaluate the ability of electron−hole separation and transport in the crystal lattice. Generally, the presence of internal electric fields between [Bi2O2] and [BO3] is favorable for the efficient photoinduced electron−hole separation and transfer, which is also propitious to a high photocatalytic efficiency of Bi2O2[BO2(OH)]. Because of the lone pair electrons of Bi3+, the VB structures of Bi4B2O9 and Bi2O2[BO2(OH)] are all hybridized by 0 2p, Bi 6s, and 6p orbitals, which can effectively decrease the band gaps

and increase the light absorption in the longer wavelength region, to further enhance their photocatalytic activity under simulated solar irradiation. Moreover, a large dispersion was observed in the hybridized orbitals in both the CB and VB of Bi2O2[BO2(OH)], suggesting that the photoexcited charges have a high mobility in the VB and CB, which should be beneficial for the transport of photoexcited electrons and holes.23,35 This in turn is likely to suppress the recombination of electron−hole pairs and thus account for the high photooxidative activity. 22992



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In addition, the large negative charge of (BO3)3− was also considered to be in favor of the photogenerated charge separation like (PO4)3−,22,23 which can maintain a large dipole in these compounds to affect electrostatically the electron distribution in cations, enhancing the photocatalytic activity.

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CONCLUSIONS In summary, two novel Bi-based borate phtocatalysts Bi4B2O9 and Bi2O2[BO2(OH)] have been successfully synthesized by solid-state reaction and hydrothermal method, respectively. The borates were investigated as photocatalysts for the first time. Morphologies and microstructures of Bi4B2O9 and Bi2O2[BO2(OH)] were characterized in detail, and they possess direct and indirect transition optical band gaps of 3.02 and 2.85 eV, respectively. The calculated electronic structures of Bi4B2O9 and Bi2O2[BO2(OH)] confirmed their optical transition types and that the VB and CB were occupied by hybridized states of the O 2p and Bi 6p or 6s orbitals. The photodegradation reaction revealed that Bi4B2O9 and Bi2O2[BO2(OH)] are effective photocatalysts, which can efficiently decompose methylene blue (MB), under simulated solar irradiation. Meanwhile, Bi2O2[BO2(OH)] exhibits the high photocatalytic activity, which is 2.5 and 3.2 times higher than that of P25 and Bi2O2CO3, respectively. Moreover, they all yield photocurrent density under ultraviolet (UV) light in the photocurrent conversion experiments. The layered structure with the (Bi2O2)2+ layer, hybridized and dispersion energy band, and large negative charge of the (BO3)3− ion should be very beneficial for the photoinduced electron−hole separation and transfer, resulting in the high photocatalytic activity of Bi2O2[BO2(OH)].

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ASSOCIATED CONTENT

S Supporting Information *

Pictures of crystal structures of Bi4B2O9 and Bi2O2[BO2(OH)] and XRD and TEM images of as-prepared Bi2O2CO3. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-10-82332247. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (2652013052), and the special coconstruction project of Beijing city education committee, Key Project of Chinese Ministry of Education (No. 107023).

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REFERENCES

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Two Novel Bi-Based Borate Photocatalysts: Crystal Structure

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