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Received 00th January 20xx, Accepted 00th January 20xx
Highly Exposed Surface Area of {001} Facets Dominated BiOBr Nanosheets with Enhanced Visible Light Photocatalytic Activity
DOI: 10.1039/x0xx00000x
Fang Duan, ab Xiaofeng Wang, a Tingting Tan, a and Mingqing Chen *ab
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Two Groups of BiOBr nanosheets with different sizes and similar exposure percentages of {001} facets were selectively synthesized through simple hydrothermal methods. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectroscopy (DRS). The photocatalytic activity was estimated by degradation of organic pollutants under visible light irradiation. The results indicated that BiOBr nanosheets with similar exposure percentage of {001} facets but smaller size exhibited higher photocatalytic activity. Besides, the size effect including the thickness and length of BiOBr nanosheets were also studied. The result showed that the impact of thickness was more significant than length. It was found that reducing the thickness of BiOBr nanosheets can obviously increase the exposed surface areas of {001} facets (S{001}) but not necessarily the exposure percentage of {001} facets. Moreover, in our experiment, the photocatalytic activity of BiOBr nanosheets increased linearly with the growth of S{001} in the range of 0.022 to 0.111 nm-1. Therefore, the photocatalytic activity of BiOBr nanosheets depended on the exposed surface areas of {001} facets rather than the exposure percentage of {001} facets. The enhancement of photocatalytic activity of ultrathin BiOBr nanosheets with high exposed surface areas of {001} facets can be mainly ascribed to the enhanced visible light absorption and improved separation efficiency of charge carriers. 2+
Introduction The environmental problems of organic pollutants and toxic water pollutants are more and more urgent in the area of 1,2 environmental remediation. As a result, photocatalytic technology is generally considered as one of the most effective energy-saving means to solve the environmental 3 pollution problems. Several photocatalytic semiconductor materials, such as TiO2 and ZnO, have been widely used, but they have the limitation in a certain extent that they can only absorb ultraviolet light, which accounts for just a tiny portion 4-8 of the sunlight. In order to realize the industrial application of photocatalytic technology, improvement of the utilization rate of the solar energy is an important direction in the photocatalytic field.9-18 Bismuth oxyhalides (BiOXs, X = F, Cl, Br, I), a new family of promising photocatalysts, have been attracting much attention because the separation of photo-induced electrons and holes in these materials can be promoted by the internal
a.
School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, PR China. E-mall: [email protected]; Tel: 86-510-85917019(O); Fax: 86-510-85917763 b. The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, Jiangnan University, Wuxi, Jiangsu 214122, PR China †Electronic Supplementary Informa)on (ESI) available: See DOI: 10.1039/x0xx00000x
static electric fields between the [Bi2O2] and anionic 19,20 halogen layers. These photocatalysts possess extraordinary photocatalytic activities under ultraviolet (UV) 21-23 or visible light irradiation. Among the BiOX photocatalysts, BiOBr has a desirable band gap (2.7 eV) for 24,25 visible light absorption. BiOBr reported has two separate valence bands that have different oxidation abilities. One 26 responds to UV and the other to visible light. Up to now, 27-29 various strategies such as doping, deposition of noble 30,31 32,33 metals, crystal facets control and so on, have been developed for the further improvement of BiOBr degradation efficiency under visible light irradiation. Especially, surface properties are vital to a photocatalyst’s photocatalytic performance, which sensitively depends on its exposed surfaces with distinct crystal facets. For example, recent progress made by Lu and coworkers on the synthesis of anatase single crystals with highly reactive {001} facets have paved a new way for the enhancement of photocatalytic 34 performance. It has been demonstrated theoretically and experimentally that the {001} facets of anatase are much more reactive than the thermodynamically more stable {101} 35 facets. Besides, Zhang’s research group has found that BiOCl single-crystalline nanosheets with exposed {001} facets exhibit higher activity for direct semiconductor photoexcitation pollutant degradation under UV light, while the counterparts with exposed {010} facets possess superior activity for indirect dye photosensitization degradation under
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visible light. For BiOBr photocatalyst, due to the layer structure and different layers stacking together via weak van der Waals interactions, it is easy to form a sheet-like shape and expose {001} active facets which are rich in Bi and O atoms. Under visible light irradiation, Bi–O square anti-prisms could generate many oxygen defects because of the unstable bonding between Bi and O, which will benefit for the 36 37 enhancement of photoactivity. In addition, Zhao et al has obtained {111} exposed rose-like BiOBr nanostructures through the assistant of an anionic surfactant, which exhibits excellent photocatalytic activity under both visible lightand monochromatic light. In recent years, nanostructure of a semiconductor material with a nanoscale dimension in thickness only, has attracted 38-41 great research interest. In such a unique anisotropic 2D structure, photoinitiated charge carriers experience two kinds of confinement. The strong confinement (thickness) is necessary to sufficiently increase the free energy of the conduction band electrons for the photocatalytic reaction. Meanwhile, the weak confinement (length and width) is needed to facilitate effective delocalization of longer-living 42 excitons and separated charges. Thereby, the probability of photoinduced electron–hole recombination is effectively minimized. Thus, exploration of facile and efficient methods for synthesis of 2D BiOBr nanomaterials with high percentage active facets may greatly improve their current photocatalytic 43 activity. Ye et al have synthesized BiOCl single-crystal nanosheets and disclosed its {001} facet dependent photoactivity. By comparing the photocatalytic degradation efficiency of rhodamine B on different percentages of the {001} facets of BiOCl, they have discovered that the {001} facets are photo-reactive in BiOCl crystals. Crystals with a high ratio of exposed active facets can functionalize better than those with low ratio of exposed active facets. Similar phenomenon can also be found in BiOBr photocatalyst, that is different exposing percentages of {001} facets result in 44 different photocatalytic performance. Therefore, it seems to be an effective way to enhance the photocatalytic performance by improving the ratio of exposed active facets. However, such reports investigating the influence of exposed active facet percentage on the photocatalytic performance have usually neglected the effects of other factors such as size, specific surface area and morphology. Furthermore, these factors may affect each other. If samples have the same exposure percentage of photoactive facets but different size, specific surface area or morphology, will their photocatalytic activities be same? This should be considered when exploring the influence of exposed active facet percentage on photocatalytic activity. Inspired by the aforementioned concepts, two groups of 2D BiOBr nanosheets with similar exposure percentage of {001} facets were synthesized by hydrothermal methods. The influence of sizes including thickness and length on photocatalytic activity were also systematically investigated. The results indicated that BiOBr nanosheets with similar exposed percentage of {001} facets but different length and thickness exhibited different photocatalytic activity, and the
thickness showed more significant influence on photocatalytic performance than length. It was found the exposed surface area of {001} facet was dominated by the thickness of BiOBr nanaosheets. The relationship between the exposed surface area of {001} facets and photocatalytic activity was discussed, and the main reason for the enhanced photocatalytic activity of BiOBr was also revealed.
Experimental Materials Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), potassium bromide (KBr), sodium hydroxide (NaOH), glucose, rhodamine B (RhB), Methyl orange (MO), phenol and N, Ndimethylformamide (DMF) were of A.R. grade. They were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P. R. China) and used without further purification. Water used in experiments was obtained from a Hitech-K flow water purification system (Hitech, Shanghai, P. R. China). Synthesis of BiOBr nanosheets 0.486 g Bi(NO3)3·5H2O (1 mmol) was dissolved into 10 mL DMF with vigorous stirring for 10 min. Then, 0.1 g glucose was dissolved in 10 mL distilled water, and 1mmol NaBr was dissolved in 5 mL distilled water, respectively. After that, both of them were slowly added to the above DMF solution, yielding a uniform white suspension after another 10 min of agitation. Finally, the mixture was transferred into a Teflonlined stainless steel autoclave of 45 mL capacity and heated at a temperature of 100 °C for 8 h, and then cooled to room temperature naturally. The resulting solid powders were collected by centrifugation and washed with deionized water to remove residual ions. The final products were then dried at 60 °C overnight for further characterization. The obtained sample was named as BOB-1, which referred to the ultrathin BiOBr nanosheets. The other BiOBr samples with different thickness were synthesized in a similar way. The sample obtained without adding glucose and by keeping other conditions unchanged was named as BOB-2. The sample obtained without adding glucose and by changing the pH value of solution to about 6.0 was named as BOB-3. The sample obtained without adding glucose and by changing the pH value of solution to about 10.0 was named as BOB-4. In order to specify the different affection of length and thickness to photocatalytic activity, BOB-5 and BOB-6 were obtained for comparison. BOB-5 was synthesized by prolonging the reaction time to 14 h without adding glucose and keeping other conditions same to BOB-1 sample. BOB-6 sample was obtained by grinding BOB-4 sample into smaller size. In addition, BOB-7 sample was synthesized by adding 0.05 g glucose and prolonging the reaction time to 15 h, and the other conditions were same to BOB-1 sample. Characterization Powder X-ray diffraction (XRD) was carried out using a D8 advance diffractometer (Bruker, Germany), with Cu Kα radiation (λ= 1.54184Å). UV-vis diffuse reflectance spectra
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(UV-vis DRS) were obtained on a DUV-3700 (Agilent, Japan), with a BaSO4 reference. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB250 X-ray photoelectron spectrometer, with Al Kα radiation (photoelectron energy 1486.6.6 eV). The C 1s peak at 284.8 eV was used to calibrate peak positions. Transmission electron microscopy (TEM, JEM2100, JEOL) with fast Fourier transformation (FFT) image was measured at an accelerating voltage of 200 kV. Morphologies were examined with a JEOL S4800 field-emission scanning electron microscope. Tapping-mode atomic force microscopy (AFM) images were obtained on a DI Innova Multimode SPM platform. Photocatalytic test The photocatalytic performances of the as-prepared catalysts were evaluated by the photodegradation of Rhodamine B （RhB）under a 300W Xe lamp irradiation with a light filter (λ > 420 nm). The 300 W Xe lamp was used as the light source to provide visible light. The distance between the lamp and glass tubes containing dye solutions was about 10 cm. In a typical process, 10 mg of photocatalyst was added to 100 mL of RhB (10 mg /L) solution. Prior to irradiation, the suspension was stirred in the dark for 0.5 h to reach adsorption-desorption equilibrium. After a given irradiation time, 4 mL aliquots were collected and centrifuged to remove the catalyst. The trapping experiments of active species were carried out by separately adding 1 mL isopropanol, 0.01g sodium oxalate, 0.01g K2Cr2O7 and 0.01 g p-benzoquinone into 100 ml RhB solutions with 10 mg photocatalyst. The absorbance of RhB was measured by a TU-1901 UV-vis spectrophotometer. The ratio of remaining RhB concentration to its initial concentration C/C0 was obtained by calculating the ratio of the corresponding absorbance intensity. Photoelectrochemical measurements Photoelectrochemical test systems were composed of a standard three-electrode configuration with the BiOBr nanosheets film electrode as a photoanode, a Pt counter electrode and an Ag/AgCl as reference electrode, which were immersed in a degassed 0.5 M Na2SO4 solution. The photoelectrochemical properties were measured on an electrochemical station under ambient conditions or illuminated by using a 300 W Xe lamp (HSX-F300/300UV, Trusttech Co., Ltd. Beijing).
Fig. 1 (a) XRD pattern, (b) SEM image, (c) TEM image, (d) the length distribution and the fitting results by a Gaussian distribution function, (e) HRTEM image and the corresponding fast Fourier transform pattern (FFT), and (f) AFM image of ultrathin BOB-1 sample.
revealed a highly [001] preferred orientation in the ultrathin BiOBr nanosheets. The morphology and structure of the BOB-1 sample were observed by SEM and TEM, as shown in Fig. 1b and 1c. From the image in Fig. 1b, it revealed that the obtained BiOBr nanosheets were obviously aggregated, which may be related to their small size or thickness. The TEM image in Fig. 1c clearly showed the morphology of the as-prepared BiOBr nanosheets with the thickness of nearly 3 nm. In Fig. 1d, the length distribution was obtained by measuring the length of BiOBr nanosheets, and the average length fitted by a Gaussian distribution function was about 106 nm. The clear lattice fringes in HRTEM image (Fig. 1e) indicated good crystallization of the ultrathin BiOBr nanosheets. The continuous lattice fringes with an interplanar lattice spacing of 0.277 nm corresponded to (110) atomic planes and an angle of 90° matched well with the interplanar lattice spacing of 0.196 nm corresponding to (200) atomic planes of the tetragonal BiOBr. The corresponding fast Fourier transform pattern (FFT) displayed a spot pattern, indicating the singlecrystalline characteristic of the obtained nanosheets. It meant that the set of diffraction spots can be indexed as the [001] zone axis, which agrees well with the XRD result.
Results and discussion XRD and size analysis
Fig.1a showed the XRD patterns of BOB-1 sample. The peaks of the sample could be well indexed to the tetragonal phase of BiOBr (JCPDS No.09-0393, a=b=3.92Å, c=8.10Å). The intense and clear diffraction peaks implied good crystallinity of the as-synthesized product. Fourthermore, no characteristic peak of any other phases and impurities was observed, indicating the high purity of the product. The enhanced relative intensity of the {001} peaks clearly
Fig. 2 XRD patterns of BOB-2, BOB-3 and BOB-4 samples.
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Fig. 3 The thickness and length estimated from the SEM images: (a, b) BOB-2, (c, d) BOB-3, and (e, f) BOB-4 samples.
More information on the thickness of the ultrathin BiOBr nanosheets was obtained from the AFM image and the corresponding height profile was shown in Fig.1f. As can be seen, the average thickness of these nanosheets was estimated to be 3.08 nm, which was consistent with the result from TEM image. The ultrathin BiOBr nanosheets were obtained by the assistant of glucose. As we known, glucose can form into polysaccharides with extensive number of hydroxyl groups in solution, which can provide H-bonding capability with oxygen atoms on the surface of BiOBr nanosheets. This effect in some extent acted as soft template and benefited the growth of ultrathin BiOBr nanosheets. Without adding glucose, BOB-2, BOB-3 and BOB-4 samples with different thicknesses were prepared by adjusting pH values and keeping other conditions unchanged, because pH value can affect the formation of nuclei and growth environment of BiOBr crystals. The obtained samples can be also well indexed to the tetragonal BiOBr (JCPDS No. 030393), as shown in Fig. 2. The morphologies of BiOBr samples with different thicknesses and lengths were revealed by SEM images in Fig. 3. The distribution of the thickness and length was not very narrow according to the SEM images. Due to the limited number of nanosheets exposing their thickness from the SEM images, the relatively medial size was used to represent the average thickness for each sample. The length of each sample was estimated by measuring dozens of nanosheets. As a result, the thicknesses of BOB-2, BOB-3 and
BOB-4 samples were estimated to be 33 nm, 54 nm and 92 nm, respectively, and the lengths of BOB-2, BOB-3 and BOB-4 samples were 392 nm, 706 nm and 2690 nm, respectively. The energy dispersive X-ray (EDX) spectroscopy (Fig. S1) was carried out to demonstrate the coexistence of Bi, O and Br elements in the BOB-2, BOB-3, and BOB-4 nanosheets, and the existence of C element was attributed to the substrate using for SEM observation. The Bi/Br molar ratios of BOB-2, BOB-3 and BOB-4 samples were about 1 on the basis of the EDX results, all of them were in accordance with the theoretical value. In order to determine the exposure of the crystal facets in BOB-2, BOB-3 and BOB-4 samples, HRTEM was also conducted. As shown in Fig. S2, BOB-4 sample, chosen as an example, had exposed {001} facets, which was as same as that of the ultrathin BOB-1 sample. Based on the scheme showed in Scheme S1, the exposure percentages of {001} facets of BOB-1, BOB-2, BOB-3 and BOB-4 samples can be estimated to be 94.6%, 85.6%, 86.7% and 93.6% from the equation of P{001} = 1/(1+2D/L)44. According to this equation, the exposure percentage of {001} facets (P{001}) was determined by the nanosheets’ length (L) and thickness (D) rather than the thickness only. In this article, by adjusting the ratio of D to L, two groups of BiOBr samples with similar P{001} were obtained. One was the BOB-1 sample and BOB-4 sample, the other was the BOB-2 sample and BOB-3 sample. As shown in Tab. 1, the sizes including the lengths and the thicknesses of BOB-1, BOB-2, BOB-3 and BOB-4 samples were increased in turn, which indicated that the length and thickness of the BiOBr samples can be well controlled by the adding of glucose or the adjusting of pH values. However, the P{001} of the BOB-1, BOB-2, BOB-3 and BOB-4 samples had no gradual increase or decrease change with the increase of length and thickness. Therefore, the exposure percentages of {001} facets showed no direct correlation with the change of length or thickness only, but can be dominated by the ratio of thickness to length. XPS analysis The surface chemical composition and structure of BiOBr nanosheets with different thicknesses and lengths (taking BOB1 and BOB-4 for examples) were analyzed by XPS. The XPS survey in Fig. 4a demonstrated that no peaks of other elements except C, O, Bi and Br appeared in both samples, indicating the high purity of the samples. The C 1s peaks in the survey scan spectra resulted from adventitious carbon.45 The highresolution XPS spectra of the Bi 4f, O 1s and Br 3d were showed in Fig. 4b, 4c and 4d, respectively. Peaks at 159.0 and 164.2 eV were ascribed to Bi 4f7/2 and Bi 4f5/2, respectively, which arose 2 from the Bi–O bonds in BiOBr. The O 1s peaks were fitted by two peaks at 529.9 and 531.4 eV, which were related to oxygen in BiOBr lattice and other components (such as –OH and H2O) 46 adsorbed on the surface of BiOBr samples, respectively. The peaks at 69.9 and 68.9 eV were attributed to Br 3d3/2 and Br 47 3d5/2, respectively. Besides, the spectra of the two samples exhibited similar peak intensities and binding energies,
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ARTICLE Tab. 1 Summary of lengths (L), thicknesses (D), exposure percentages of {001} facets (P{001}), zeta potentials (ζ), band gaps (Eg) and photocatalytic
Fig. 4 (a) Survey scan and high-resolution XPS spectra of the (b) Bi 4f, (c) O 1s and (d) Br 3d regions for BOB-1 and BOB-4 samples.
which indicated that they consisted of the same elemental composition and surface structure. UV-vis diffuse reflectance spectroscopy The optical properties of the BiOBr samples were measured by UV-vis spectroscopy. Fig 5a showed the diffuse reflectance spectra of the obtained BiOBr samples with different thickness and length. All the samples exhibited significant increase in the photoabsorption at wavelengths lower than about 470 nm due to the band-gap transition, which implied the possibility of photocatalytic activity over these materials under visible-light irradiation. It was well known that the optical absorption near the band edge followed the formula below: n/2 αhv = A(hv- Eg) (1)
Fig. 5 (a) UV-vis diffuse reflectance spectra of BOB-1, BOB-2, BOB-3 and BOB-4 samples. (b) (αhv)1/2 versus photon energy (hv) curves of the asprepared samples. (c) XPS valence-band spectra of BOB-1 and BOB-4 samples. (d) Schematic illustration of the band structures of BOB-1 and BOB-4 samples.
Sample
L (nm)
D (nm)
P{001} (%)
BOB-1 BOB-2 BOB-3 BOB-4
106 392 706 2690
3 33 54 92
94.6 85.6 86.7 93.6
(mV)
Eg (eV)
k x 102 (min-1)
24.99 3.32 4.76 6.12
2.59 2.67 2.71 2.72
22.640 6.061 2.351 0.770
ζ
where α, h, Eg and A were absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. Among them, n depended on the characteristics of the transition in a semiconductor. For BiOBr, the value of n was 4 for the indirect transition. Thus, the band gap energies (Eg) of BiOBr samples can be estimated from the tangent line of (αhv)1/2 versus photon energy (hv) curve, as shown in Fig. 5b. Based on this calculation method, the band gap of BiOBr samples with different thicknesses and lengths were 2.59 eV, 2.67 eV, 2.71 eV and 2.72 eV, respectively. The XPS valence band (VB) spectra were employed to determine the band edges, as illustrated in Fig. 5c. Compared with the thicker or larger nanosheets, the VB maximum of the BOB-1 nanosheets was up-shifted from 2.49 eV to 1.69 eV. The CB band edge can be determined by ECB = EVB - Eg, where the EVB was the valence band edge potential and Eg was the band gap energy. From this, the CB band edges of BOB-1 and BOB-4 were calculated to be -0.9 eV and -0.23 eV, respectively. According to the calculated results, we speculated a simultaneous upward shift of the CB minimum of ultrathin BiOBr nanosheets by 0.67 eV. Therefore, BiOBr nanosheets with different thicknesses and lengths had different band gap, VB maximum and CB minimum energy. It was well known that reducing the particle size was a generic approach to tune the band gap energy by changing the band edges. The reason for ultrathin BiOBr nanosheets with the narrowest band gap can be ascribed to the smallest length and thickness. From the viewpoint of kinetic and thermodynamic requirements for photocatalytic reactions, two features were worthy of noting: the valence band width and the conduction band (CB) minimum energy.48 As shown in Fig. 5d, the increased VB width was also beneficial for the separation of charge carriers since the VB width intrinsically controlled the mobility of holes. A wider VB can result in higher mobility and better photo-oxidation of the photo-generated holes. Elevation of the CB minimum can also be crucial in the photoreaction for inhibiting electron-hole recombination due to the rapid transfer of photoexcited electrons to reactants. It was interesting to note that the elevated CB minimum of BOB-1 sample was much higher than the redox potential of E0 − 49 (O2/•O2 = − 0.046 eV vs NHE). Therefore, BOB-1 would − show higher photocatalytic activity for •O2 and •OH photocatalytic production via one-electron and two-electron routes, respectively. Photocatalytic activity
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activities of BOB-1, BOB-2, BOB-3 and BOB-4 samples.
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Fig. 6 (a) Time profiles of photocatalytic degradation of RhB under different BiOBr samples. (b) Adsorption capacity of RhB on different BiOBr samples. (c) Reaction rate constants for photocatalytic degradation of RhB on BiOBr samples. (d) Cycle runs of photocatalytic degradation of RhB with ultrathin BOB-1 sample.
The effects of size and exposure percentages of {001} facet on the photocatalytic activity of BiOBr nanosheets were studied by RhB degradation in an aqueous solution under visible light irradiation. As shown in Fig. 6a, the different degradation times of RhB solution were observed when the four BiOBr samples with different thicknesses and lengths were used as photocatalysts. Before degradation, the photocatalysts were dispersed in the RhB solution by stirring in dark for 30 min without visible light irradiation. After that, the photocatalysts can reach the adsorption equilibrium. The adsorption capacity for each sample was different as shown in Fig. 6a and 6b. The adsorption capacities order of the BOBr samples was BOB-1 > BOB-2 > BOB-3 > BOB-4. It can be seen that the adsorption capacity of ultrathin BiOBr nanosheets was much higher than the others. According to the previous HRTEM analysis, the ultrathin BiOBr nanosheets were highly exposed {001} facet, and the surface of {001} facet was terminated with negatively charged oxygen ions, which would be beneficial for adsorbing RhB cations in aqueous solution via electrostatic interactions. In fact, the zeta potential result showed that the charge of BOB-1 sample was more negative (-24.99 mV) on surface than that of other samples as shown in Tab. 1. Both of the reasons may be responsible for the higher adsorption capacity of ultrathin BiOBr nanosheets. After reaching the adsorption equilibrium, the RhB solution was degraded under visible light irradiation by BiOBr samples. As shown in Fig. 6a, the RhB solution was completely decomposed by the ultrathin BiOBr nanosheets within 20 min, while only 52%, 33% and 22% of RhB solution were degraded by the BOB-2, BOB-3, and BOB-4 samples within 20 min, respectively. When the time was prolonged to 50 min, the RhB solution was degraded nearly 99%, 68% and 50% in the presence of BOB-2, BOB-3, and BOB-4 samples, respectively. In order to quantitatively investigate the reaction kinetics of RhB photodegradation by the BiOBr
samples, the experimental data was fitted to the pseudofirst-order model, as expressed by the following equation: - ln(C/C0) = kt (2) where k was the pseudo-first-order rate constant. As shown in Fig. S3, all plots of ln(C/C0) against t exhibited linear trends, indicating that RhB photodegradation was described well by the pseudo-first-order model. The k values for RhB degradation by BOB-1, BOB-2, BOB-3, and BOB-4 were -1 0.22640, 0.06061, 0.02351 and 0.00770 min , respectively, as shown in Fig. 6c. The k value of BOB-1 was nearly 30 times higher than that of BOB-4 sample, indicating that the asprepared ultrathin BiOBr nanosheets with smaller thickness and length showed higher visible-light photocatalytic activity than the other BiOBr samples. Comparing the photocatalytic activities of BOB-1 and BOB-4, BOB-2 and BOB-3, it can be found that BiOBr samples with similar exposure percentages of {001} facet showed different photocatalytic performances. Besides, more organic pollutants such as methyl orange (MO) and phenol were also used to further evaluate the photocatalytic activity of the obtained four samples, as shown in Fig. S4 and Fig. S5. The adsorption capacity of MO for each sample was slightly different as shown in Fig. S4, and all samples were within 6% adsorption. The adsorption capacities order of the BiOBr samples was BOB-1 < BOB-2 < BOB-3 < BOB-4. It can be seen that the adsorption capacity of MO by ultrathin BOB-1 nanosheets was lower than other samples, and also much lower than that of RhB with nearly 40 % adsorption, which was mainly ascribed to the contrary charge of MO and RhB in solution. The adsorption capacity of phenol for each sample was shown in Fig. S5. It can be seen that the adsorption capacities of four samples were very similar and low, which could be ascribed to phenol existing as the form of molecules not ions in water. Although the adsorption capacity of RhB, MO and phenol for each sample was different, the photocatalytic activity of the four BiOBr samples toward RhB, MO or phenol was always BOB-1 > BOB2 > BOB-3 > BOB-4. In order to confirm the stability of the high photocatalytic performance of BiOBr nanosheets, recycling experiments for the photodegradation of RhB were conducted by choosing ultrathin BOB-1 sample and BOB-4 sample as examples. As shown in Fig. 6d, the photodegradation ratio remained above 99% over 5 cycles, indicating that the as-prepared ultrathin BiOBr sample had good recycling photocatalytic performance under visible light irradiation. After 5 cycles, as shown in Fig. S6, the photodegradation ratio of BOB-4 sample remained above 96%, lower than that of BOB-1 sample. But it still showed relatively good stability, which was especially important for its practical application. To further investigate the influence of the length and thickness of BiOBr nanosheets, another two samples were synthesized for comparison. One was the BOB-5 sample with length of 716 nm similar to BOB-3 sample, but different thickness to BOB-3 sample, as shown in Fig. 7a and 7b; the other was the BOB-6 sample with thickness of 93 nm similar to BOB-4 sample, but different length to BOB-4 sample, as
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ARTICLE Tab. 2 The S{001} of BiOBr nanosheets and the corresponding
Sample
S{001} (nm-1)
k x 102 (min-1)
BOB-1 a BOB-7 BOB-2 BOB-5 BOB-3 BOB-4
0.667 0.111 0.061 0.044 0.037 0.022
22.640 12.217 6.061 b 3.157 2.351 0.770
the relevant information of BOB-7 sample can be found in Fig. S7; the pseudo-first-order rate of BOB-5 was showed in Fig. S8
b
Fig. 7 The lengths, thicknesses and photocatalytic performances of (a, b, c) BOB-5 sample and (d, e, f) BOB-6 sample.
shown in Fig. 7d and 7e. By comparing their photocatalytic performances in Fig. 7c and 7f, the results were different between sample BOB-3 and BOB-5, but similar between sample BOB-4 and BOB-6. It can be concluded that BiOBr nanosheets with similar length but different thickness showed different photocatalytic activity, and samples with different length but similar thickness showed similar photocatalytic activity. Therefore, the thickness of BiOBr nanosheets exhibited more significant affection to photocatalytic performance. Considering that the photocatalytic reactions mainly took place at the surface of photocatalysts rather than in the bulk, and the main exposed facet of BiOBr nanosheets was {001} facets, much more surface areas of {001} facets exposed would be important to the photocatalytic activity. According to the schematic illustration in Scheme S1, reducing the thickness of nanosheets can greatly improve the exposed surface area of {001} facets for certain amount of nanosheets, which can be clearly revealed by the following equations: 2 L2 2 (3) S{001} = 2 = LD D
S{001} =
P{001} ⋅ (2L2 +4DL) L2 D
=(
1 2 + ) ⋅ 2P{001} D L
higher exposure percentage of {001} facets, but larger thickness and length, may not get higher exposed surface areas of {001} facets, e.g. BOB-4 sample, and cannot exhibit higher photocatalytic activity. Therefore, the enhanced photocatalytic activity of BiOBr nanosheets mainly depended on the large exposed surface areas of {001} facets rather than the high exposure percentage of {001} facets. In addition, the relationship between the exposed surface areas of {001} facets and photocatalytic activity can be revealed by Tab. 2 and showed in Fig. 8, from which pseudofirst-order rate constant k increased linearly with the increase -1 of S{001} in the range of 0.022 to 0.111 nm . The formula of this linear relationship was: (5) k = 1.335S{001} - 0.023 When reducing the thickness of BiOBr nanosheets, more and more active {001} facets were exposed and the S{001} was increased accordingly, which could make more active atoms exposed on the surface to participate the photocatalytic reaction. Thus, the k value can be greatly improved. However, -1 when increasing the S{001} to 0.667 nm by reducing the -1 thickness to 3 nm, the k value was just 0.2264 min , which was much lower than the value calculated from the fitting formula. This may be caused by the agglomeration of BiOBr
(4) 0.25
k = 1.335 S{001}- 0.023
where S{001} was the exposed surface areas of {001} facets per volume for BiOBr photocatalyst; L was the average length or width of the BiOBr nanosheets; D was the thickness of the BiOBr nanosheets; P{001} was the exposure percentage of {001} facets. From the equation (3), a reverse relationship can be found between S{001} and D, which meant the higher exposed surface areas of {001} facets can be obtained by decreasing the thickness of BiOBr nanosheets. As shown in equation (4), the exposed surface areas of {001} facets cannot only be determined by P{001} but also affected by the thickness and length of the BiOBr nanosheets. BiOBr nanosheets with higher exposure percentage of {001} facets, smaller thickness and length can get higher exposed surface areas of {001} facets, e.g. BOB-1 sample, and can show a higher photocatalytic activity. While the BiOBr nanosheets with
0.20
2
R =0.9939
0.15 0.10 0.05 0.00 0.0
0.2
0.4
S{001} (nm-1)
0.6
Fig. 8 The relationship between pseudo-first-order rate constant k and the exposed surface areas of {001} facets S{001}.
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Fig. 9 Photocatalytic degradation of RhB over the BiOBr samples with different scavengers: (a) BOB-1 and (b) BOB-4.
nanosheets with small thickness. Small thickness with high surface energy can make BiOBr nanosheets easy to aggregate to reduce the surface energy. In fact, the aggregation of BOB1 sample was revealed by SEM image in Fig. 1b. This aggregation would lead to the actual S{001} much lower than theoretical value, and finally resulted in k value much lower than the calculated value. When the thickness was relatively larger, such as 18 nm, 33 nm, 45 nm, 54 nm and 92 nm from the obtained samples, the BiOBr nanosheets were dispersed well as showed in the SEM images. In this condition, the exposed S{001} was much close to the theoretical S{001}, which would show a good linear relationship with k. Therefore, reducing the difference between actual S{001} and theoretical S{001} of BiOBr nanosheets would make a better linear relationship of k to S{001} . In order to further understand the mechanism of photodegradation of RhB by BiOBr nanosheets, the trapping experiments of active species were carried out, and the results were shown in Fig. 9. Isopropanol (a quencher of •OH), + sodium oxalate (a quencher of h ), K2Cr2O7 (a quencher of e ) − and benzoquinone (a quencher of •O2 ) were generally used to detect the main active species in the photocatalytic 50 reaction. From Fig. 9a, the addition of isopropanol did not change the photodegradation performance of RhB by using the ultrathin BOB-1 nanosheets, indicating that the hydroxyl radicals were not the main active species for the degradation of RhB. While sodium oxalate was added, the photodegradation activity of BOB-1 sample was suppressed to some extent, indicating that holes played an important role in the phtocatalytic reaction. Besides, the photocatalytic degradation of RhB decreased obviously with the addition of − benzoquinone (a quencher of •O2 ) and K2Cr2O7 (a quencher of e ). In Fig. 9b, the results of the BOB-4 were similar with − BOB-1. Therefore, it can be concluded that •O2 and e were the two main active species for photodegradation of RhB. BiOBr samples with similar exposure percentage of {001} facets but different exposed surface areas of {001} facets had similar active species in the photocatalytic process. The efficient separation of photogenerated electron–hole pairs was confirmed by comparing the photocurrent responses of BiOBr films. The photocurrent was carried out by irradiating the films of BiOBr samples on FTO substrates under Xe lamp irradiation at a potential of 0 V in 0.5 M Na2SO4 solution. As shown in Fig. S9, four electrodes generated photocurrent promptly. The current densities
under irradiation and dark conditions were stable during the whole time, which indicated the stability of structures of the BiOBr products during the visible light irradiation. Contrast to the thicker BiOBr samples film electrode, the photocurrent density of the ultrathin BOB-1 nanosheet film electrode was much higher, indicating the more efficient separation and faster transfer of photo-induced charge of ultrathin BiOBr nanosheets. This may be caused by the difference in thickness of BiOBr samples. The ultrathin nanosheets would benefit for the transfer of photo-induced charge, and the electrons or holes can quickly get to the surface of nanosheets, along with the enhanced visible light absorption, thus the ultrathin BOB-1 sample with large S{001} can exhibit higher photocatalytic activity.
Conclusions Two groups of BiOBr nanosheets with similar P{001} but different sizes were successfully synthesized. The size including length and thickness showed different affects to the photocatalytic activity of BiOBr nanosheets. The relationship between S{001} and P{001} was also clearly revealed, from which we can find that high S{001} can be obtained by reducing the thickness but the P{001} cannot. Besides, it was found that the photocatalytic activity of BiOBr nanosheets increased linearly -1 with the growth of S{001} in the range of 0.022 to 0.111 nm . Therefore, photocatalytic activity of BiOBr nanosheets can be effectively enhanced by increasing the S{001}. The main reasons for the improved photocatalytic activities of BiOBr nanosheets with high S{001} were the enhanced visible light absorption and the increased separation efficiency of charge carriers. Based on the results, we can expect that the photocatalytic activity of 2D nano-photocatalyst can be controlled by adjusting the exposed surface areas of certain active facets.
Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (No. 51302108), Natural Science Foundation of Jiangsu Province (BK20130151) and the Fundamental Research Funds for the Central Universities (JUSRP51408B).
Notes and references 1 2 3 4 5 6
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Highly Exposed Surface Area of {001} Facets Dominated BiOBr Nanosheets with Enhanced Visible Light Photocatalytic Activity
DOI: 10.1039/x0xx00000x
Fang Duan, ab Xiaofeng Wang, a Tingting Tan, a and Mingqing Chen *ab
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Two Groups of BiOBr nanosheets with different sizes and similar exposure percentages of {001} facets were selectively synthesized through simple hydrothermal methods. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectroscopy (DRS). The photocatalytic activity was estimated by degradation of organic pollutants under visible light irradiation. The results indicated that BiOBr nanosheets with similar exposure percentage of {001} facets but smaller size exhibited higher photocatalytic activity. Besides, the size effect including the thickness and length of BiOBr nanosheets were also studied. The result showed that the impact of thickness was more significant than length. It was found that reducing the thickness of BiOBr nanosheets can obviously increase the exposed surface areas of {001} facets (S{001}) but not necessarily the exposure percentage of {001} facets. Moreover, in our experiment, the photocatalytic activity of BiOBr nanosheets increased linearly with the growth of S{001} in the range of 0.022 to 0.111 nm-1. Therefore, the photocatalytic activity of BiOBr nanosheets depended on the exposed surface areas of {001} facets rather than the exposure percentage of {001} facets. The enhancement of photocatalytic activity of ultrathin BiOBr nanosheets with high exposed surface areas of {001} facets can be mainly ascribed to the enhanced visible light absorption and improved separation efficiency of charge carriers. 2+
Introduction The environmental problems of organic pollutants and toxic water pollutants are more and more urgent in the area of 1,2 environmental remediation. As a result, photocatalytic technology is generally considered as one of the most effective energy-saving means to solve the environmental 3 pollution problems. Several photocatalytic semiconductor materials, such as TiO2 and ZnO, have been widely used, but they have the limitation in a certain extent that they can only absorb ultraviolet light, which accounts for just a tiny portion 4-8 of the sunlight. In order to realize the industrial application of photocatalytic technology, improvement of the utilization rate of the solar energy is an important direction in the photocatalytic field.9-18 Bismuth oxyhalides (BiOXs, X = F, Cl, Br, I), a new family of promising photocatalysts, have been attracting much attention because the separation of photo-induced electrons and holes in these materials can be promoted by the internal
a.
School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, PR China. E-mall: [email protected]; Tel: 86-510-85917019(O); Fax: 86-510-85917763 b. The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, Jiangnan University, Wuxi, Jiangsu 214122, PR China †Electronic Supplementary Informa)on (ESI) available: See DOI: 10.1039/x0xx00000x
static electric fields between the [Bi2O2] and anionic 19,20 halogen layers. These photocatalysts possess extraordinary photocatalytic activities under ultraviolet (UV) 21-23 or visible light irradiation. Among the BiOX photocatalysts, BiOBr has a desirable band gap (2.7 eV) for 24,25 visible light absorption. BiOBr reported has two separate valence bands that have different oxidation abilities. One 26 responds to UV and the other to visible light. Up to now, 27-29 various strategies such as doping, deposition of noble 30,31 32,33 metals, crystal facets control and so on, have been developed for the further improvement of BiOBr degradation efficiency under visible light irradiation. Especially, surface properties are vital to a photocatalyst’s photocatalytic performance, which sensitively depends on its exposed surfaces with distinct crystal facets. For example, recent progress made by Lu and coworkers on the synthesis of anatase single crystals with highly reactive {001} facets have paved a new way for the enhancement of photocatalytic 34 performance. It has been demonstrated theoretically and experimentally that the {001} facets of anatase are much more reactive than the thermodynamically more stable {101} 35 facets. Besides, Zhang’s research group has found that BiOCl single-crystalline nanosheets with exposed {001} facets exhibit higher activity for direct semiconductor photoexcitation pollutant degradation under UV light, while the counterparts with exposed {010} facets possess superior activity for indirect dye photosensitization degradation under
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visible light. For BiOBr photocatalyst, due to the layer structure and different layers stacking together via weak van der Waals interactions, it is easy to form a sheet-like shape and expose {001} active facets which are rich in Bi and O atoms. Under visible light irradiation, Bi–O square anti-prisms could generate many oxygen defects because of the unstable bonding between Bi and O, which will benefit for the 36 37 enhancement of photoactivity. In addition, Zhao et al has obtained {111} exposed rose-like BiOBr nanostructures through the assistant of an anionic surfactant, which exhibits excellent photocatalytic activity under both visible lightand monochromatic light. In recent years, nanostructure of a semiconductor material with a nanoscale dimension in thickness only, has attracted 38-41 great research interest. In such a unique anisotropic 2D structure, photoinitiated charge carriers experience two kinds of confinement. The strong confinement (thickness) is necessary to sufficiently increase the free energy of the conduction band electrons for the photocatalytic reaction. Meanwhile, the weak confinement (length and width) is needed to facilitate effective delocalization of longer-living 42 excitons and separated charges. Thereby, the probability of photoinduced electron–hole recombination is effectively minimized. Thus, exploration of facile and efficient methods for synthesis of 2D BiOBr nanomaterials with high percentage active facets may greatly improve their current photocatalytic 43 activity. Ye et al have synthesized BiOCl single-crystal nanosheets and disclosed its {001} facet dependent photoactivity. By comparing the photocatalytic degradation efficiency of rhodamine B on different percentages of the {001} facets of BiOCl, they have discovered that the {001} facets are photo-reactive in BiOCl crystals. Crystals with a high ratio of exposed active facets can functionalize better than those with low ratio of exposed active facets. Similar phenomenon can also be found in BiOBr photocatalyst, that is different exposing percentages of {001} facets result in 44 different photocatalytic performance. Therefore, it seems to be an effective way to enhance the photocatalytic performance by improving the ratio of exposed active facets. However, such reports investigating the influence of exposed active facet percentage on the photocatalytic performance have usually neglected the effects of other factors such as size, specific surface area and morphology. Furthermore, these factors may affect each other. If samples have the same exposure percentage of photoactive facets but different size, specific surface area or morphology, will their photocatalytic activities be same? This should be considered when exploring the influence of exposed active facet percentage on photocatalytic activity. Inspired by the aforementioned concepts, two groups of 2D BiOBr nanosheets with similar exposure percentage of {001} facets were synthesized by hydrothermal methods. The influence of sizes including thickness and length on photocatalytic activity were also systematically investigated. The results indicated that BiOBr nanosheets with similar exposed percentage of {001} facets but different length and thickness exhibited different photocatalytic activity, and the
thickness showed more significant influence on photocatalytic performance than length. It was found the exposed surface area of {001} facet was dominated by the thickness of BiOBr nanaosheets. The relationship between the exposed surface area of {001} facets and photocatalytic activity was discussed, and the main reason for the enhanced photocatalytic activity of BiOBr was also revealed.
Experimental Materials Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), potassium bromide (KBr), sodium hydroxide (NaOH), glucose, rhodamine B (RhB), Methyl orange (MO), phenol and N, Ndimethylformamide (DMF) were of A.R. grade. They were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P. R. China) and used without further purification. Water used in experiments was obtained from a Hitech-K flow water purification system (Hitech, Shanghai, P. R. China). Synthesis of BiOBr nanosheets 0.486 g Bi(NO3)3·5H2O (1 mmol) was dissolved into 10 mL DMF with vigorous stirring for 10 min. Then, 0.1 g glucose was dissolved in 10 mL distilled water, and 1mmol NaBr was dissolved in 5 mL distilled water, respectively. After that, both of them were slowly added to the above DMF solution, yielding a uniform white suspension after another 10 min of agitation. Finally, the mixture was transferred into a Teflonlined stainless steel autoclave of 45 mL capacity and heated at a temperature of 100 °C for 8 h, and then cooled to room temperature naturally. The resulting solid powders were collected by centrifugation and washed with deionized water to remove residual ions. The final products were then dried at 60 °C overnight for further characterization. The obtained sample was named as BOB-1, which referred to the ultrathin BiOBr nanosheets. The other BiOBr samples with different thickness were synthesized in a similar way. The sample obtained without adding glucose and by keeping other conditions unchanged was named as BOB-2. The sample obtained without adding glucose and by changing the pH value of solution to about 6.0 was named as BOB-3. The sample obtained without adding glucose and by changing the pH value of solution to about 10.0 was named as BOB-4. In order to specify the different affection of length and thickness to photocatalytic activity, BOB-5 and BOB-6 were obtained for comparison. BOB-5 was synthesized by prolonging the reaction time to 14 h without adding glucose and keeping other conditions same to BOB-1 sample. BOB-6 sample was obtained by grinding BOB-4 sample into smaller size. In addition, BOB-7 sample was synthesized by adding 0.05 g glucose and prolonging the reaction time to 15 h, and the other conditions were same to BOB-1 sample. Characterization Powder X-ray diffraction (XRD) was carried out using a D8 advance diffractometer (Bruker, Germany), with Cu Kα radiation (λ= 1.54184Å). UV-vis diffuse reflectance spectra
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(UV-vis DRS) were obtained on a DUV-3700 (Agilent, Japan), with a BaSO4 reference. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB250 X-ray photoelectron spectrometer, with Al Kα radiation (photoelectron energy 1486.6.6 eV). The C 1s peak at 284.8 eV was used to calibrate peak positions. Transmission electron microscopy (TEM, JEM2100, JEOL) with fast Fourier transformation (FFT) image was measured at an accelerating voltage of 200 kV. Morphologies were examined with a JEOL S4800 field-emission scanning electron microscope. Tapping-mode atomic force microscopy (AFM) images were obtained on a DI Innova Multimode SPM platform. Photocatalytic test The photocatalytic performances of the as-prepared catalysts were evaluated by the photodegradation of Rhodamine B （RhB）under a 300W Xe lamp irradiation with a light filter (λ > 420 nm). The 300 W Xe lamp was used as the light source to provide visible light. The distance between the lamp and glass tubes containing dye solutions was about 10 cm. In a typical process, 10 mg of photocatalyst was added to 100 mL of RhB (10 mg /L) solution. Prior to irradiation, the suspension was stirred in the dark for 0.5 h to reach adsorption-desorption equilibrium. After a given irradiation time, 4 mL aliquots were collected and centrifuged to remove the catalyst. The trapping experiments of active species were carried out by separately adding 1 mL isopropanol, 0.01g sodium oxalate, 0.01g K2Cr2O7 and 0.01 g p-benzoquinone into 100 ml RhB solutions with 10 mg photocatalyst. The absorbance of RhB was measured by a TU-1901 UV-vis spectrophotometer. The ratio of remaining RhB concentration to its initial concentration C/C0 was obtained by calculating the ratio of the corresponding absorbance intensity. Photoelectrochemical measurements Photoelectrochemical test systems were composed of a standard three-electrode configuration with the BiOBr nanosheets film electrode as a photoanode, a Pt counter electrode and an Ag/AgCl as reference electrode, which were immersed in a degassed 0.5 M Na2SO4 solution. The photoelectrochemical properties were measured on an electrochemical station under ambient conditions or illuminated by using a 300 W Xe lamp (HSX-F300/300UV, Trusttech Co., Ltd. Beijing).
Fig. 1 (a) XRD pattern, (b) SEM image, (c) TEM image, (d) the length distribution and the fitting results by a Gaussian distribution function, (e) HRTEM image and the corresponding fast Fourier transform pattern (FFT), and (f) AFM image of ultrathin BOB-1 sample.
revealed a highly [001] preferred orientation in the ultrathin BiOBr nanosheets. The morphology and structure of the BOB-1 sample were observed by SEM and TEM, as shown in Fig. 1b and 1c. From the image in Fig. 1b, it revealed that the obtained BiOBr nanosheets were obviously aggregated, which may be related to their small size or thickness. The TEM image in Fig. 1c clearly showed the morphology of the as-prepared BiOBr nanosheets with the thickness of nearly 3 nm. In Fig. 1d, the length distribution was obtained by measuring the length of BiOBr nanosheets, and the average length fitted by a Gaussian distribution function was about 106 nm. The clear lattice fringes in HRTEM image (Fig. 1e) indicated good crystallization of the ultrathin BiOBr nanosheets. The continuous lattice fringes with an interplanar lattice spacing of 0.277 nm corresponded to (110) atomic planes and an angle of 90° matched well with the interplanar lattice spacing of 0.196 nm corresponding to (200) atomic planes of the tetragonal BiOBr. The corresponding fast Fourier transform pattern (FFT) displayed a spot pattern, indicating the singlecrystalline characteristic of the obtained nanosheets. It meant that the set of diffraction spots can be indexed as the [001] zone axis, which agrees well with the XRD result.
Results and discussion XRD and size analysis
Fig.1a showed the XRD patterns of BOB-1 sample. The peaks of the sample could be well indexed to the tetragonal phase of BiOBr (JCPDS No.09-0393, a=b=3.92Å, c=8.10Å). The intense and clear diffraction peaks implied good crystallinity of the as-synthesized product. Fourthermore, no characteristic peak of any other phases and impurities was observed, indicating the high purity of the product. The enhanced relative intensity of the {001} peaks clearly
Fig. 2 XRD patterns of BOB-2, BOB-3 and BOB-4 samples.
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Fig. 3 The thickness and length estimated from the SEM images: (a, b) BOB-2, (c, d) BOB-3, and (e, f) BOB-4 samples.
More information on the thickness of the ultrathin BiOBr nanosheets was obtained from the AFM image and the corresponding height profile was shown in Fig.1f. As can be seen, the average thickness of these nanosheets was estimated to be 3.08 nm, which was consistent with the result from TEM image. The ultrathin BiOBr nanosheets were obtained by the assistant of glucose. As we known, glucose can form into polysaccharides with extensive number of hydroxyl groups in solution, which can provide H-bonding capability with oxygen atoms on the surface of BiOBr nanosheets. This effect in some extent acted as soft template and benefited the growth of ultrathin BiOBr nanosheets. Without adding glucose, BOB-2, BOB-3 and BOB-4 samples with different thicknesses were prepared by adjusting pH values and keeping other conditions unchanged, because pH value can affect the formation of nuclei and growth environment of BiOBr crystals. The obtained samples can be also well indexed to the tetragonal BiOBr (JCPDS No. 030393), as shown in Fig. 2. The morphologies of BiOBr samples with different thicknesses and lengths were revealed by SEM images in Fig. 3. The distribution of the thickness and length was not very narrow according to the SEM images. Due to the limited number of nanosheets exposing their thickness from the SEM images, the relatively medial size was used to represent the average thickness for each sample. The length of each sample was estimated by measuring dozens of nanosheets. As a result, the thicknesses of BOB-2, BOB-3 and
BOB-4 samples were estimated to be 33 nm, 54 nm and 92 nm, respectively, and the lengths of BOB-2, BOB-3 and BOB-4 samples were 392 nm, 706 nm and 2690 nm, respectively. The energy dispersive X-ray (EDX) spectroscopy (Fig. S1) was carried out to demonstrate the coexistence of Bi, O and Br elements in the BOB-2, BOB-3, and BOB-4 nanosheets, and the existence of C element was attributed to the substrate using for SEM observation. The Bi/Br molar ratios of BOB-2, BOB-3 and BOB-4 samples were about 1 on the basis of the EDX results, all of them were in accordance with the theoretical value. In order to determine the exposure of the crystal facets in BOB-2, BOB-3 and BOB-4 samples, HRTEM was also conducted. As shown in Fig. S2, BOB-4 sample, chosen as an example, had exposed {001} facets, which was as same as that of the ultrathin BOB-1 sample. Based on the scheme showed in Scheme S1, the exposure percentages of {001} facets of BOB-1, BOB-2, BOB-3 and BOB-4 samples can be estimated to be 94.6%, 85.6%, 86.7% and 93.6% from the equation of P{001} = 1/(1+2D/L)44. According to this equation, the exposure percentage of {001} facets (P{001}) was determined by the nanosheets’ length (L) and thickness (D) rather than the thickness only. In this article, by adjusting the ratio of D to L, two groups of BiOBr samples with similar P{001} were obtained. One was the BOB-1 sample and BOB-4 sample, the other was the BOB-2 sample and BOB-3 sample. As shown in Tab. 1, the sizes including the lengths and the thicknesses of BOB-1, BOB-2, BOB-3 and BOB-4 samples were increased in turn, which indicated that the length and thickness of the BiOBr samples can be well controlled by the adding of glucose or the adjusting of pH values. However, the P{001} of the BOB-1, BOB-2, BOB-3 and BOB-4 samples had no gradual increase or decrease change with the increase of length and thickness. Therefore, the exposure percentages of {001} facets showed no direct correlation with the change of length or thickness only, but can be dominated by the ratio of thickness to length. XPS analysis The surface chemical composition and structure of BiOBr nanosheets with different thicknesses and lengths (taking BOB1 and BOB-4 for examples) were analyzed by XPS. The XPS survey in Fig. 4a demonstrated that no peaks of other elements except C, O, Bi and Br appeared in both samples, indicating the high purity of the samples. The C 1s peaks in the survey scan spectra resulted from adventitious carbon.45 The highresolution XPS spectra of the Bi 4f, O 1s and Br 3d were showed in Fig. 4b, 4c and 4d, respectively. Peaks at 159.0 and 164.2 eV were ascribed to Bi 4f7/2 and Bi 4f5/2, respectively, which arose 2 from the Bi–O bonds in BiOBr. The O 1s peaks were fitted by two peaks at 529.9 and 531.4 eV, which were related to oxygen in BiOBr lattice and other components (such as –OH and H2O) 46 adsorbed on the surface of BiOBr samples, respectively. The peaks at 69.9 and 68.9 eV were attributed to Br 3d3/2 and Br 47 3d5/2, respectively. Besides, the spectra of the two samples exhibited similar peak intensities and binding energies,
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ARTICLE Tab. 1 Summary of lengths (L), thicknesses (D), exposure percentages of {001} facets (P{001}), zeta potentials (ζ), band gaps (Eg) and photocatalytic
Fig. 4 (a) Survey scan and high-resolution XPS spectra of the (b) Bi 4f, (c) O 1s and (d) Br 3d regions for BOB-1 and BOB-4 samples.
which indicated that they consisted of the same elemental composition and surface structure. UV-vis diffuse reflectance spectroscopy The optical properties of the BiOBr samples were measured by UV-vis spectroscopy. Fig 5a showed the diffuse reflectance spectra of the obtained BiOBr samples with different thickness and length. All the samples exhibited significant increase in the photoabsorption at wavelengths lower than about 470 nm due to the band-gap transition, which implied the possibility of photocatalytic activity over these materials under visible-light irradiation. It was well known that the optical absorption near the band edge followed the formula below: n/2 αhv = A(hv- Eg) (1)
Fig. 5 (a) UV-vis diffuse reflectance spectra of BOB-1, BOB-2, BOB-3 and BOB-4 samples. (b) (αhv)1/2 versus photon energy (hv) curves of the asprepared samples. (c) XPS valence-band spectra of BOB-1 and BOB-4 samples. (d) Schematic illustration of the band structures of BOB-1 and BOB-4 samples.
Sample
L (nm)
D (nm)
P{001} (%)
BOB-1 BOB-2 BOB-3 BOB-4
106 392 706 2690
3 33 54 92
94.6 85.6 86.7 93.6
(mV)
Eg (eV)
k x 102 (min-1)
24.99 3.32 4.76 6.12
2.59 2.67 2.71 2.72
22.640 6.061 2.351 0.770
ζ
where α, h, Eg and A were absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. Among them, n depended on the characteristics of the transition in a semiconductor. For BiOBr, the value of n was 4 for the indirect transition. Thus, the band gap energies (Eg) of BiOBr samples can be estimated from the tangent line of (αhv)1/2 versus photon energy (hv) curve, as shown in Fig. 5b. Based on this calculation method, the band gap of BiOBr samples with different thicknesses and lengths were 2.59 eV, 2.67 eV, 2.71 eV and 2.72 eV, respectively. The XPS valence band (VB) spectra were employed to determine the band edges, as illustrated in Fig. 5c. Compared with the thicker or larger nanosheets, the VB maximum of the BOB-1 nanosheets was up-shifted from 2.49 eV to 1.69 eV. The CB band edge can be determined by ECB = EVB - Eg, where the EVB was the valence band edge potential and Eg was the band gap energy. From this, the CB band edges of BOB-1 and BOB-4 were calculated to be -0.9 eV and -0.23 eV, respectively. According to the calculated results, we speculated a simultaneous upward shift of the CB minimum of ultrathin BiOBr nanosheets by 0.67 eV. Therefore, BiOBr nanosheets with different thicknesses and lengths had different band gap, VB maximum and CB minimum energy. It was well known that reducing the particle size was a generic approach to tune the band gap energy by changing the band edges. The reason for ultrathin BiOBr nanosheets with the narrowest band gap can be ascribed to the smallest length and thickness. From the viewpoint of kinetic and thermodynamic requirements for photocatalytic reactions, two features were worthy of noting: the valence band width and the conduction band (CB) minimum energy.48 As shown in Fig. 5d, the increased VB width was also beneficial for the separation of charge carriers since the VB width intrinsically controlled the mobility of holes. A wider VB can result in higher mobility and better photo-oxidation of the photo-generated holes. Elevation of the CB minimum can also be crucial in the photoreaction for inhibiting electron-hole recombination due to the rapid transfer of photoexcited electrons to reactants. It was interesting to note that the elevated CB minimum of BOB-1 sample was much higher than the redox potential of E0 − 49 (O2/•O2 = − 0.046 eV vs NHE). Therefore, BOB-1 would − show higher photocatalytic activity for •O2 and •OH photocatalytic production via one-electron and two-electron routes, respectively. Photocatalytic activity
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activities of BOB-1, BOB-2, BOB-3 and BOB-4 samples.
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Fig. 6 (a) Time profiles of photocatalytic degradation of RhB under different BiOBr samples. (b) Adsorption capacity of RhB on different BiOBr samples. (c) Reaction rate constants for photocatalytic degradation of RhB on BiOBr samples. (d) Cycle runs of photocatalytic degradation of RhB with ultrathin BOB-1 sample.
The effects of size and exposure percentages of {001} facet on the photocatalytic activity of BiOBr nanosheets were studied by RhB degradation in an aqueous solution under visible light irradiation. As shown in Fig. 6a, the different degradation times of RhB solution were observed when the four BiOBr samples with different thicknesses and lengths were used as photocatalysts. Before degradation, the photocatalysts were dispersed in the RhB solution by stirring in dark for 30 min without visible light irradiation. After that, the photocatalysts can reach the adsorption equilibrium. The adsorption capacity for each sample was different as shown in Fig. 6a and 6b. The adsorption capacities order of the BOBr samples was BOB-1 > BOB-2 > BOB-3 > BOB-4. It can be seen that the adsorption capacity of ultrathin BiOBr nanosheets was much higher than the others. According to the previous HRTEM analysis, the ultrathin BiOBr nanosheets were highly exposed {001} facet, and the surface of {001} facet was terminated with negatively charged oxygen ions, which would be beneficial for adsorbing RhB cations in aqueous solution via electrostatic interactions. In fact, the zeta potential result showed that the charge of BOB-1 sample was more negative (-24.99 mV) on surface than that of other samples as shown in Tab. 1. Both of the reasons may be responsible for the higher adsorption capacity of ultrathin BiOBr nanosheets. After reaching the adsorption equilibrium, the RhB solution was degraded under visible light irradiation by BiOBr samples. As shown in Fig. 6a, the RhB solution was completely decomposed by the ultrathin BiOBr nanosheets within 20 min, while only 52%, 33% and 22% of RhB solution were degraded by the BOB-2, BOB-3, and BOB-4 samples within 20 min, respectively. When the time was prolonged to 50 min, the RhB solution was degraded nearly 99%, 68% and 50% in the presence of BOB-2, BOB-3, and BOB-4 samples, respectively. In order to quantitatively investigate the reaction kinetics of RhB photodegradation by the BiOBr
samples, the experimental data was fitted to the pseudofirst-order model, as expressed by the following equation: - ln(C/C0) = kt (2) where k was the pseudo-first-order rate constant. As shown in Fig. S3, all plots of ln(C/C0) against t exhibited linear trends, indicating that RhB photodegradation was described well by the pseudo-first-order model. The k values for RhB degradation by BOB-1, BOB-2, BOB-3, and BOB-4 were -1 0.22640, 0.06061, 0.02351 and 0.00770 min , respectively, as shown in Fig. 6c. The k value of BOB-1 was nearly 30 times higher than that of BOB-4 sample, indicating that the asprepared ultrathin BiOBr nanosheets with smaller thickness and length showed higher visible-light photocatalytic activity than the other BiOBr samples. Comparing the photocatalytic activities of BOB-1 and BOB-4, BOB-2 and BOB-3, it can be found that BiOBr samples with similar exposure percentages of {001} facet showed different photocatalytic performances. Besides, more organic pollutants such as methyl orange (MO) and phenol were also used to further evaluate the photocatalytic activity of the obtained four samples, as shown in Fig. S4 and Fig. S5. The adsorption capacity of MO for each sample was slightly different as shown in Fig. S4, and all samples were within 6% adsorption. The adsorption capacities order of the BiOBr samples was BOB-1 < BOB-2 < BOB-3 < BOB-4. It can be seen that the adsorption capacity of MO by ultrathin BOB-1 nanosheets was lower than other samples, and also much lower than that of RhB with nearly 40 % adsorption, which was mainly ascribed to the contrary charge of MO and RhB in solution. The adsorption capacity of phenol for each sample was shown in Fig. S5. It can be seen that the adsorption capacities of four samples were very similar and low, which could be ascribed to phenol existing as the form of molecules not ions in water. Although the adsorption capacity of RhB, MO and phenol for each sample was different, the photocatalytic activity of the four BiOBr samples toward RhB, MO or phenol was always BOB-1 > BOB2 > BOB-3 > BOB-4. In order to confirm the stability of the high photocatalytic performance of BiOBr nanosheets, recycling experiments for the photodegradation of RhB were conducted by choosing ultrathin BOB-1 sample and BOB-4 sample as examples. As shown in Fig. 6d, the photodegradation ratio remained above 99% over 5 cycles, indicating that the as-prepared ultrathin BiOBr sample had good recycling photocatalytic performance under visible light irradiation. After 5 cycles, as shown in Fig. S6, the photodegradation ratio of BOB-4 sample remained above 96%, lower than that of BOB-1 sample. But it still showed relatively good stability, which was especially important for its practical application. To further investigate the influence of the length and thickness of BiOBr nanosheets, another two samples were synthesized for comparison. One was the BOB-5 sample with length of 716 nm similar to BOB-3 sample, but different thickness to BOB-3 sample, as shown in Fig. 7a and 7b; the other was the BOB-6 sample with thickness of 93 nm similar to BOB-4 sample, but different length to BOB-4 sample, as
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ARTICLE Tab. 2 The S{001} of BiOBr nanosheets and the corresponding
Sample
S{001} (nm-1)
k x 102 (min-1)
BOB-1 a BOB-7 BOB-2 BOB-5 BOB-3 BOB-4
0.667 0.111 0.061 0.044 0.037 0.022
22.640 12.217 6.061 b 3.157 2.351 0.770
the relevant information of BOB-7 sample can be found in Fig. S7; the pseudo-first-order rate of BOB-5 was showed in Fig. S8
b
Fig. 7 The lengths, thicknesses and photocatalytic performances of (a, b, c) BOB-5 sample and (d, e, f) BOB-6 sample.
shown in Fig. 7d and 7e. By comparing their photocatalytic performances in Fig. 7c and 7f, the results were different between sample BOB-3 and BOB-5, but similar between sample BOB-4 and BOB-6. It can be concluded that BiOBr nanosheets with similar length but different thickness showed different photocatalytic activity, and samples with different length but similar thickness showed similar photocatalytic activity. Therefore, the thickness of BiOBr nanosheets exhibited more significant affection to photocatalytic performance. Considering that the photocatalytic reactions mainly took place at the surface of photocatalysts rather than in the bulk, and the main exposed facet of BiOBr nanosheets was {001} facets, much more surface areas of {001} facets exposed would be important to the photocatalytic activity. According to the schematic illustration in Scheme S1, reducing the thickness of nanosheets can greatly improve the exposed surface area of {001} facets for certain amount of nanosheets, which can be clearly revealed by the following equations: 2 L2 2 (3) S{001} = 2 = LD D
S{001} =
P{001} ⋅ (2L2 +4DL) L2 D
=(
1 2 + ) ⋅ 2P{001} D L
higher exposure percentage of {001} facets, but larger thickness and length, may not get higher exposed surface areas of {001} facets, e.g. BOB-4 sample, and cannot exhibit higher photocatalytic activity. Therefore, the enhanced photocatalytic activity of BiOBr nanosheets mainly depended on the large exposed surface areas of {001} facets rather than the high exposure percentage of {001} facets. In addition, the relationship between the exposed surface areas of {001} facets and photocatalytic activity can be revealed by Tab. 2 and showed in Fig. 8, from which pseudofirst-order rate constant k increased linearly with the increase -1 of S{001} in the range of 0.022 to 0.111 nm . The formula of this linear relationship was: (5) k = 1.335S{001} - 0.023 When reducing the thickness of BiOBr nanosheets, more and more active {001} facets were exposed and the S{001} was increased accordingly, which could make more active atoms exposed on the surface to participate the photocatalytic reaction. Thus, the k value can be greatly improved. However, -1 when increasing the S{001} to 0.667 nm by reducing the -1 thickness to 3 nm, the k value was just 0.2264 min , which was much lower than the value calculated from the fitting formula. This may be caused by the agglomeration of BiOBr
(4) 0.25
k = 1.335 S{001}- 0.023
where S{001} was the exposed surface areas of {001} facets per volume for BiOBr photocatalyst; L was the average length or width of the BiOBr nanosheets; D was the thickness of the BiOBr nanosheets; P{001} was the exposure percentage of {001} facets. From the equation (3), a reverse relationship can be found between S{001} and D, which meant the higher exposed surface areas of {001} facets can be obtained by decreasing the thickness of BiOBr nanosheets. As shown in equation (4), the exposed surface areas of {001} facets cannot only be determined by P{001} but also affected by the thickness and length of the BiOBr nanosheets. BiOBr nanosheets with higher exposure percentage of {001} facets, smaller thickness and length can get higher exposed surface areas of {001} facets, e.g. BOB-1 sample, and can show a higher photocatalytic activity. While the BiOBr nanosheets with
0.20
2
R =0.9939
0.15 0.10 0.05 0.00 0.0
0.2
0.4
S{001} (nm-1)
0.6
Fig. 8 The relationship between pseudo-first-order rate constant k and the exposed surface areas of {001} facets S{001}.
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k (min-1)
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photocatalytic activities.
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Fig. 9 Photocatalytic degradation of RhB over the BiOBr samples with different scavengers: (a) BOB-1 and (b) BOB-4.
nanosheets with small thickness. Small thickness with high surface energy can make BiOBr nanosheets easy to aggregate to reduce the surface energy. In fact, the aggregation of BOB1 sample was revealed by SEM image in Fig. 1b. This aggregation would lead to the actual S{001} much lower than theoretical value, and finally resulted in k value much lower than the calculated value. When the thickness was relatively larger, such as 18 nm, 33 nm, 45 nm, 54 nm and 92 nm from the obtained samples, the BiOBr nanosheets were dispersed well as showed in the SEM images. In this condition, the exposed S{001} was much close to the theoretical S{001}, which would show a good linear relationship with k. Therefore, reducing the difference between actual S{001} and theoretical S{001} of BiOBr nanosheets would make a better linear relationship of k to S{001} . In order to further understand the mechanism of photodegradation of RhB by BiOBr nanosheets, the trapping experiments of active species were carried out, and the results were shown in Fig. 9. Isopropanol (a quencher of •OH), + sodium oxalate (a quencher of h ), K2Cr2O7 (a quencher of e ) − and benzoquinone (a quencher of •O2 ) were generally used to detect the main active species in the photocatalytic 50 reaction. From Fig. 9a, the addition of isopropanol did not change the photodegradation performance of RhB by using the ultrathin BOB-1 nanosheets, indicating that the hydroxyl radicals were not the main active species for the degradation of RhB. While sodium oxalate was added, the photodegradation activity of BOB-1 sample was suppressed to some extent, indicating that holes played an important role in the phtocatalytic reaction. Besides, the photocatalytic degradation of RhB decreased obviously with the addition of − benzoquinone (a quencher of •O2 ) and K2Cr2O7 (a quencher of e ). In Fig. 9b, the results of the BOB-4 were similar with − BOB-1. Therefore, it can be concluded that •O2 and e were the two main active species for photodegradation of RhB. BiOBr samples with similar exposure percentage of {001} facets but different exposed surface areas of {001} facets had similar active species in the photocatalytic process. The efficient separation of photogenerated electron–hole pairs was confirmed by comparing the photocurrent responses of BiOBr films. The photocurrent was carried out by irradiating the films of BiOBr samples on FTO substrates under Xe lamp irradiation at a potential of 0 V in 0.5 M Na2SO4 solution. As shown in Fig. S9, four electrodes generated photocurrent promptly. The current densities
under irradiation and dark conditions were stable during the whole time, which indicated the stability of structures of the BiOBr products during the visible light irradiation. Contrast to the thicker BiOBr samples film electrode, the photocurrent density of the ultrathin BOB-1 nanosheet film electrode was much higher, indicating the more efficient separation and faster transfer of photo-induced charge of ultrathin BiOBr nanosheets. This may be caused by the difference in thickness of BiOBr samples. The ultrathin nanosheets would benefit for the transfer of photo-induced charge, and the electrons or holes can quickly get to the surface of nanosheets, along with the enhanced visible light absorption, thus the ultrathin BOB-1 sample with large S{001} can exhibit higher photocatalytic activity.
Conclusions Two groups of BiOBr nanosheets with similar P{001} but different sizes were successfully synthesized. The size including length and thickness showed different affects to the photocatalytic activity of BiOBr nanosheets. The relationship between S{001} and P{001} was also clearly revealed, from which we can find that high S{001} can be obtained by reducing the thickness but the P{001} cannot. Besides, it was found that the photocatalytic activity of BiOBr nanosheets increased linearly -1 with the growth of S{001} in the range of 0.022 to 0.111 nm . Therefore, photocatalytic activity of BiOBr nanosheets can be effectively enhanced by increasing the S{001}. The main reasons for the improved photocatalytic activities of BiOBr nanosheets with high S{001} were the enhanced visible light absorption and the increased separation efficiency of charge carriers. Based on the results, we can expect that the photocatalytic activity of 2D nano-photocatalyst can be controlled by adjusting the exposed surface areas of certain active facets.
Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (No. 51302108), Natural Science Foundation of Jiangsu Province (BK20130151) and the Fundamental Research Funds for the Central Universities (JUSRP51408B).
Notes and references 1 2 3 4 5 6
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