Article pubs.acs.org/JPCC
Enhanced Photocatalytic Properties in BiOBr Nanosheets with Dominantly Exposed (102) Facets Haijun Zhang,†,‡ Yuxiao Yang,† Zhen Zhou,*,† Yaping Zhao,*,# and Lu Liu*,† †
Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China ‡ School of Physics and Materials Science, Anhui University, Hefei 230039, China # School of Ecological and Environmental Science, East China Normal University, Shanghai 200241, China ABSTRACT: Through first-principles computations, we compared the photocatalytic properties of (102) and (001) facets within BiOBr. Due to the surface states, the (102) facets of BiOBr have lower conduction band minimum and higher valence band maximum, compared with the (001) facets. Therefore, the (102) facets have more efficient electron injection, higher redox potential of photoinduced hole, and smaller band gap, which may result in better photocatalytic performances. Also, we prepared BiOBr-102 and BiOBr-001 samples with dominantly exposed (102) and (001) facets, respectively, and found red-shift absorption, and enhanced photodegradation rate of Rhodamine B in BiOBr-102, which agree well with the computations. Therefore, BiOBr samples with dominantly exposed (102) facets are superior in photocatalysis, and the results demonstrate the critical role of facet orientation in photocatalyst design.
(RhB).17 Additionally, the {001} facet-dependent photoactivity of BiOX (X = Cl, Br, I) single-crystal nanosheets has also been disclosed.11,12,18−22 By employing the density functional theory (DFT), we have compared the photocatalytic properties of the {001}, {110}, and {010} facets within BiOXs.23 Without surface dangling bonds and surface states, the {001} facets have higher thermodynamic stability and more efficient carrier separation. However, the thermodynamically stable facets are usually less reactive than the facets with higher surface energies.13,15 Furthermore, the previous theoretical and experimental reports simply compared {001} with {110} and {010} facets, which are symmetrical and perpendicular to each other. How about the photocatalytic properties of other asymmetrical facets within BiOX photocatalysts? Taking BiOBr as an example, we checked the photocatalytic properties of (102)-dominant BiOBr both computationally and experimentally. The band gap of BiOBr was estimated as ∼2.54 eV,4 which indicates the attractive utilization of visible light for photocatalysis. The photocatalytic properties of different facets within BiOBr are still unclear. The standard X-ray diffraction (XRD) patterns of tetragonal BiOBr (JCPDS No. 09−0393) have main peaks of (102), (110), (001), and (101), indicating the corresponding facets exposed. Considering the above reasons, we herein synthesized BiOBr nanosheets with dominant (102) and (001) facets and compared the photoactivities of these
1. INTRODUCTION Bismuth oxyhalides (BiOXs, X = F, Cl, Br, I), a new family of promising photocatalysts, have been attracting much attention because the separation of photoinduced electrons and holes in these materials can be promoted by the internal static electric fields between the [Bi2O2]2+ and anionic halogen layers.1,2 These photocatalysts possess extraordinary photocatalytic activities under ultraviolet (UV) or visible light irradiation.3−6 For example, BiOCl presented higher photocatalytic activities for the decomposition of methyl orange (MO) as compared with TiO2 (P25, Degussa) in the same conditions under UV light irradiation, as a result of its layered structure and high chemical stability.1 Under BiOBr catalysis, a very stable guanidine group of L-arginine that is nonreactive with TiO2 photocatalysts is converted to an amino group and subsequently oxidized to a nitro group during the decomposition of microcystin-LR.7 Many strategies have been explored for further enhancing the photocatalytic activities of BiOXs, including alloying,2 selfdoping,8 metal (or semiconductor) coupling,9,10 and facetcontrolled fabrication.11,12 As one of the promising methods for improving the photocatalytic performances, the effect of facets on photoactivity has been studied widely in photocatalysts over the past years. By tailoring the specific surface configurations and atomic structures, the electro- and photocatalytic properties of semiconductors can be promoted accordingly.13,14 For example, the {001} facets are reactive facets for anatase, while {110} for rutile TiO2.15,16 Ye and co-workers demonstrated that BiVO4 nanoplates with exposed {110} facets exhibited better photocatalytic activity in degrading the rhodamine B © 2014 American Chemical Society
Received: April 9, 2014 Revised: May 28, 2014 Published: June 10, 2014 14662
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Figure 1. Structures of BiOBr unit cell, (001) facet, passivated (102) facet, and RhB.
The as-prepared samples were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500 X-ray diffractometer) with Cu Kα radiation (λ = 1.54056 Å) at 40 kV and 100 mA. The data were recorded at a scan rate of 0.02 deg s−1 in the 2θ range from 3° to 70°. The morphology and structure of the samples were observed with use of a field emission scanning electron microscope (FESEM, JEOL JSM-6700) and a highresolution transmission electron microscope (HRTEM, JEOL2010) with an acceleration voltage of 200 kV. Additionally, ultraviolet−visible (UV−vis) diffuse reflectance spectra (DRS) were measured with a Shimadzu UV-3600 UV−vis spectrophotometer. 2.2. Photocatalytic Activity Evaluation. A widely used dye, RhB, was selected to evaluate the degradation efficiency. The photocatalytic activity of BiOBr was monitored through the degradation of RhB under visible light irradiation in a cylindrical quartz reactor with water circulation facility. A 0.02 g sample of BiOBr and 200 mL of RhB solution with a concentration 2 × 10−5 mol/L were added into the reactor and the mixture was stirred in the dark for 30 min. Absorption and desorption of RhB on the surface of BiOBr reached equilibrium during stirring. During irradiation with a 350 W Xe lamp equipped with an ultraviolet cutoff filter to provide visible light, an ∼3 mL aliquot was taken at the time interval of 4 min, and
samples. To clarify the underlying mechanism of these photocatalysis, we also investigated the structural and electronic properties of the (102) and (001) facets in BiOBr, by employing DFT computations with a slab model, and then compared their photocatalytic properties. To the best of our knowledge, this is the first report that focused on the photocatalytic properties of the (102) facet within BiOBr, and disclosed its unique superiority.
2. EXPERIMENTAL SECTION 2.1. Material Preparation and Characterization. A 0.485 g sample of Bi(NO3)3·5H2O and 0.8 g of cetyltrimethylammonium bromide (CTAB) were added into a 30 mL Teflon lined autoclave, and then 20 mL of deionized water and 1 mL of ethanol were also added. After the mixture was stirred for 20 min, the autoclave was sealed and heated at 180 °C for 24 h. After the autoclave was cooled to room temperature naturally, white precipitate was collected and washed with distilled water and absolute ethanol three times separately and dried at 80 °C for 2 h. For comparison, deionized water or ethanol was used as the sole solvent. To investigate the influence of reaction time, another experiment was conducted with the reaction time shortened to 12 h and other experimental conditions unchanged. 14663
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synthesized the assembled BiOBr nanosheets with dominantly exposed (001) and (102) facets, which are denoted as BiOBr001 and BiOBr-102, respectively. The XRD patterns of the asprepared samples agree well with the standard data of tetragonal BiOBr (JCPDS No. 09−0393) and have the most intensive peak of (102) and (001), respectively. The assynthesized BiOBr-102 and BiOBr-001 have the dominant (102) and (001) diffraction peaks (Figure 2, panels a and b),
then the photocatalyst was separated from solution by centrifugation. The concentration of RhB was determined under its characteristic absorption wavelength of 553 nm on a UV−vis spectrophotometer (UV2550, Shimadzu). 2.3. Acute Toxicity of Photodegradation Intermediates. The photodegradation samples were diluted into onefourth of the initial concentration with 0.85% NaCl (w/v). The luminous bacteria (Vibroqinghaiensis) provided as freeze-dried powder (1 mL each bottle) was first reactivated in 1 mL of 0.85% NaCl solution. Then 100 μL of freshly prepared reactivated bacteria was added to 2 mL of diluted samples. A 2 mL sample of 0.85% NaCl solution with 100 μL of reactivated bacteria were used as the blank. Acute toxicity of the bioluminescent bacterium Vibrioqinghaiensis was measured at 15 min incubation times with luminescence at 490 nm by a BHP9514-Drinking water safety detector. All samples were tested three times in parallel. The relative luminosity of luminous bacteria is the ratio of the bioluminescence intensities when luminous bacteria are exposed in the samples or not: relative luminosity = E/E0 × 100%, where E0 and E are normalized bioluminescence intensities in the absence and presence of samples after 15 min of exposure, respectively. 2.4. Models and Computational Details. Our firstprinciples computations were performed by employing DFT method as implemented in the Vienna ab initio simulation package (VASP).24 A plane wave basis set with the projector augmented plane wave (PAW)25,26 was used to model the ion− electron interaction. The generalized gradient approximation (GGA) with the PW91 functional27 and a 480 eV cutoff for the plane wave basis set were adopted in all computations. The tolerance for energy and ionic relaxation convergence was set to be 10−4 eV and 10−3 eV/Å, respectively. The surface simulations were performed by using the slab model, in which a finite number of crystal layers in a three-dimensional periodic-boundary-condition (PBC) cell is used to generate two surfaces through the introduction of a vacuum gap perpendicular to the surface. The structure of BiOBr belongs to the tetragonal space group P4/nmm (No. 129) and the RhB dye has the molecular formula of C28H31N2O3Cl (Figure 1). Considering the experimental feasibility, the bare surface of the slab models in the computations may not truly reflect the entirely or partially saturated facets of the synthesized materials. Our recent computations have disclosed that the (001) facets have no surface dangling bonds, due to their bulk-like configuration.23 Therefore, a slab model, in which the surface dangling bonds are saturated by hydrogen atoms or hydroxyl groups, was employed to simulate the (102) facets within BiOBr. For the cleaved slab models, the (102) and (001) facets were considered and only the atomic positions are allowed to be relaxed, as shown in Figure 1. With a vacuum gap of 20 Å, slab models having 10 and 12 atomic layers were used to simulate the (102) and (001) surface, respectively. Monkhorst−Pack generated 4 × 4 × 1 (5 × 5 × 5) and 6 × 6 × 1 (7 × 7 × 7) k-point grids were used for the surface (bulk) relaxation and the computations of density of states (DOS), respectively. Band structures of full relaxed slabs were computed along the special lines connecting the following high-symmetry points: G (0, 0, 0), M (0.5, 0.5, 0), X (0, 0.5, 0), and G (0, 0, 0) in the k-space.
Figure 2. XRD patterns of the as-prepared (a) BiOBr-102 and (b) BiOBr-001 products. (c) Standard XRD patterns of tetragonal BiOBr crystals.
implying their preferred orientations along the (102) and (001) plane, respectively. This is further demonstrated by SEM and HRTEM, as shown in Figure 3. The typical SEM images show that the samples are composed of assembled nanosheets. As represented in Figure 3c,f, the fringe spacing of 0.173 and 0.198 nm agrees well with that of the (2̅11) and (200) lattice planes (0.171 and 0.196 nm) of the tetragonal BiOBr crystals. Therefore, the BiOBr-102 and BiOBr-001 samples have predominantly exposed (102) and (001) facets, which are perpendicular to (2̅11) and (200), respectively. The light absorption properties of as-synthesized samples were investigated through the UV−visible diffuse reflectance spectra (Figure 4). The absorption light wavelength of BiOBr102 (445 nm) is a little longer than that of BiOBr-001 (437 nm), implying the smaller band gap and red-shift absorption of the former, which may result from the different surface configurations of these BiOBr nanosheets. Additionally, the photocatalytic activity of BiOBr was monitored through the degradation of RhB under visible light irradiation. For comparison, the photosensitized degradation of RhB by BiOBr-102 and BiOBr-001 were both performed. The loss of RhB over BiOBr-102 was more rapid than that in BiOBr-001. In the photodegradation of BiOBr-102, the color of the dispersion disappeared (after 16 min of irradiation),
3. RESULTS AND DISCUSSION 3.1. Enhanced Photocatalytic Properties of BiOBr with (102) facets. Through a facile hydrothermal route, we 14664
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Figure 3. SEM images for (a) BiOBr-102 and (b) BiOBr-001 samples. Panels c−f represent the corresponding HRTEM images.
photocatalysts have the highest activity, as they could completely degrade RhB dye in 16 min under visible-light irradiation. The photo-oxidation of organic compounds usually induces the toxicity variation of the contaminants. Some cases reported that part of the intermediate products were less toxic than the parent organic compounds while some reported the opposite results.28,29 During the photocatalytic reaction, the generation of intermediates in the reaction solution subsequently leads to the variation of toxicity of the recipient organism. Luminous bacteria are commonly used as an indicator of biological acute toxicity in a simple and quick way. For example, Molkenthin et al. successfully described the inhibitory effect of bisphenol A (BPA) in photo-Fenton like systems by using the bioluminescence of marine bacteria Vibrio f ischeri.30 Luminous bacteria are a very good way to characterize the toxicity of different photodegradation intermediates. The acute toxicity of RhB photodegradation samples with dye removal rate was assessed by using luminous bacteria (Vibrioqinghaiensis), as shown in Figure 5b. The luminosity of the blank was normalized to 1. The luminosity rate of the photodegradation sample after irradiation was basically the same as that of the blank, reflecting no toxicity increase or decrease. This indicated
Figure 4. UV−visible diffuse reflectance spectra of the BiOBr-001 and BiOBr-102 samples. Green lines are guides for the eyes.
indicating that at least the chromophoric structure of the dye was completely destroyed, as shown in the inset of Figure 5a. The residual percent of RhB with radiation is presented in Figure 5a. It can be clearly seen that all of these BiOBr photocatalysts exhibited excellent photocatalytic activities for the RhB degradation reaction. Among them, the BiOBr-102 14665
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Figure 5. Normalized residual concentration (a) and normalized luminosity change (b) of RhB with radiation time. The inset in panel a shows the UV−visible spectral changes of RhB in aqueousBiOBr-102 dispersions as a function of irradiation time.
that no other more toxic intermediates than RhB occurred during the photodegradation in the BiOBr/UV system. Also, the photodegradation of RhB by BiOBr-102 was compared with the case by TiO2 (P25) under visible light irradiation (Figure 6a). BiOBr-102 exhibited much better performance in the photodegradation of RhB even than P25, and also its cycling stability was excellent, as shown in Figure 6b. Therefore, (102) facet-dominant BiOBr samples are
promising photocatalysts with superior photocatalytic activity, good cycling stability, and no second pollution. Compared with BiOBr-001, the higher photocatalytic acvivity in BiOBr-102 may result from the broader light absorption and specific electronic structure. The photocatalytic properties of photocatalysts are determined by the light absorption spectrum, redox potential of carriers, efficiency of electron injection, surface activity, etc. As a result of different surface configurations, BiOBr with different exposed facets should have various physicochemical characteristics, indicating some unique thermodynamics, as well as electronic and/or optical properties. To further clarify the underlying mechanism for the higher photocatalytic activity of BiOBr with dominantly exposed (102) facets, we investigated the electronic structures of the BiOBr-RhB system and compared the electronic properties of BiOBr (001) and (102) planes through DFT computations. 3.2. Electron Injection of BiOBr-RhB Systems. Dyesensitized TiO2 has been widely investigated for dye-sensitized solar cells (DSSCs). In DSSCs, dye molecules adsorbed on the TiO2 surface are excited by absorbing visible light and subsequently inject electrons into the conduction band (CB) of TiO2 to generate photocurrent via an external circuit.31,32 If the injected electrons are not transferred to the external circuit but alternatively to the reductive substrates on the surface, a photoreductive conversion can be achieved under visible light. This procedure is a basis of developing sensitized photocatalysts with a broadened absorption spectrum.33 Therefore, the sensitization of BiOBr by RhB may also improve the performance of photocatalysts. To confirm the speculation, we computed the DOS and partial DOS (PDOS) for the isolated BiOBr crystal and RhB molecule (Figure 7). The valence band maximum (VBM) of BiOBr mainly comes from the 4p electrons of Br and 2p electrons of O, while the conduction band minimum (CBM) consists of Bi-6p and O-2p orbitals (Figure 7a). The energy of the lowest unoccupied molecular orbital (LUMO) of RhB is 0.2 eV above the CBM of BiOBr (Figure 7b). As emphasized in the previous studies, the higher LUMO energy level of the dye, relative to the semiconductor’s CBM, is beneficial for electron injection.33−35 The photoexcitation of RhB also results in efficient injection of electrons into the conduction band of BiOBr, leaving the dye in its oxidized state. Additionally, the LUMO and highest occupied molecular orbitals (HOMO) of
Figure 6. (a) Concentration percent of RhB in the presence of different photocatalysts under visible light irradiation. (b) Cycling runs in the photocatalytic degradation of RhB by BiOBr-102. 14666
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Figure 7. (a) DOS and PDOS for bulk BiOBr and (b) illustration of electron injection of the RhB-BiOBr system.
BiOBr with (001) facets has an indirect band gap of ∼2.22 eV, which is in good accordance with our previous report.23 The (102) faceted BiOBr is a semiconductor with an indirect band gap of 1.44 eV, indicating the red-shift of the light absorption. However, the curvature of the band near the CBM and VBM for (102) facets seems much smaller than that of the (001) facets, which results in lower mobility of the excitons within the former. For quantification, we computed the curvature of the parabolic portions of the band (aCB and aVB) and the relative ratio of effective mass (D value), by using the approach in our recent work.23 The aCB values (aVB) of (001) and (102) facets are 18.31 (5.92) and 1.12 (0.38), respectively, implying the higher mobility of carriers within the former.23,37 However, the (001) and (102) facets have the D value of 3.09 and 2.95, respectively, corresponding to the comparative separation rate of the electron−hole pairs. Moreover, the absolute energy levels of CBM and VBM within (102) facets are lower and higher than those of the (001) facets, respectively, which indicates the higher efficiency of electron injection and redox potential of photoinduced holes for the former.35,38 3.4. Surface States of (102) Facets. The origin of the unique electronic structures for (102) facets was studied by computing the PDOS of surface and central atoms within the investigated facets. We can see from Figure 9, panels a and b that the surface Bi and O atoms contribute to the gap state above the VBM, respectively, which results from their special geometric and electronic conditions. Also, the gap states above the VBM and the states below the CBM are caused by the
RhB are higher than the CBM and VBM of BiOBr, respectively, indicating the type-II band alignment36 of the BiOBr-RhB system and accordingly the red-shift of the absorption spectrum (Figure 7b). Therefore, the surface sensitization of BiOBr via RhB adsorption can effectively expand the photoabsorption range and contribute to high-efficiency electron injection. 3.3. Band Structures of (001) and (102) Facets. We also computed the band structures of slab models (Figure 8) to simulate the different facets within the BiOBr photocatalysts.
Figure 8. Band structures of (a) (001) and (b) hydrogenated (102) facets within BiOBr photocatalysts. Red dashed lines represent the Fermi level at 0 eV. 14667
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Figure 9. PDOS of the (a) Bi, (b) O, (c) Br, and (d) passivating atoms, as well as (e) total DOS of the (120) facet within BiOBr. Green dashed lines represent the Fermi level at 0 eV.
4. CONCLUSIONS
surface Br atoms (Figure 9c). The surface Bi, O, and Br atoms predominantly determine the band edge levels of (102) facets, corresponding to the surface states. As a result of surface states, the energy levels of VBM and CBM are elevated and depressed, respectively, which causes the reduction of band gap and accordingly the red-shift of absorption. We additionally computed the PDOS of hydrogen atoms and hydroxyl groups used for the saturation of surface atoms (Figure 9d) and found that the electronic structures of (102) facets are practically not affected by the saturating atoms or groups. All in all, our DFT computations suggest the surface states of (102) facets lead to the depressed CBM, elevated VBM, and reduced band gap (red-shift), indicating the more efficient electron injection, higher redox potential of hole, broader absorption spectrum of (102) facets within BiOBr. Despite the underestimated band gaps of semiconductors by standard DFT method, the variation tendency is reliable and valuable for predicting the facet-dependent photocatalytic properties of BiOBr. Due to the smaller band gap, more efficient electron injection and higher redox potential of hole within the (102) facets of BiOBr, BiOBr-102 are promising photocatalysts for degradation of RhB.
We compared the photocatalytic properties of (102) and (001) facets within BiOBr, by employing DFT computations. As compared with the (001) facets, the (102) facets of BiOBr have lower CBM and higher VBM level, respectively, resulting from the surface states of (102) facets. Therefore, the (102) facets have more efficient electron injection, higher redox potential of hole, and the red-shift absorption, indicating better photocatalytic properties. To validate, we synthesized the assembled BiOBr nanosheets with dominant (102) and (001) facets, and found the longer wavelength of absorbed light, and higher photodegradation rate of RhB for the former, which is in good accordance with the computations. All results suggest the superiority of the photocatalytic BiOBr with exposed (102) facets.
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AUTHOR INFORMATION
Corresponding Authors
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[email protected]. Notes
The authors declare no competing financial interest. 14668
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21271108 and 21377039), 2011 Science Foundation of Tianjin (11JCZDJC24800), and China-US Center for Environmental Remediation and Sustainable Development. The computations were performed on Magic Cube at Shanghai Supercomputer Center.
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