Liquid-Phase Exfoliation into Monolayered BiOBr Nanosheets for

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10499-10508

Liquid-Phase Exfoliation into Monolayered BiOBr Nanosheets for Photocatalytic Oxidation and Reduction Hongjian Yu,† Hongwei Huang,*,† Kang Xu,‡ Weichang Hao,‡ Yuxi Guo,† Shuobo Wang,† Xiulin Shen,† Shaofeng Pan,† and Yihe Zhang*,† †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China ‡ Center of Materials Physics and Chemistry and Department of Physics, Beihang University, Beijing 100191, China S Supporting Information *

ABSTRACT: Monolayered photocatalytic materials have attracted huge research interests in terms of their large specific surface area and ample active sites. Sillén-structured layered BiOX (X = Cl, Br, I) casts great prospects owing to their strong photo-oxidation ability and high stability. Fabrication of monolayered BiOX by a facile, low-cost, and scalable approach is highly challenging and anticipated. Herein, we describe the large-scale preparation of monolayered BiOBr nanosheets with a thickness of ∼0.85 nm via a readily achievable liquid-phase exfoliation strategy with assistance of formamide at ambient conditions. The as-obtained monolayered BiOBr nanosheets are allowed diverse superiorities, such as enhanced specific surface area, promoted band structure, and strengthened charge separation. Profiting from these benefits, the advanced BiOBr monolayers not only show excellent adsorption and photodegradation performance for treating contaminants, but also demonstrate a greatly promoted photocatalytic activity for CO2 reduction into CO and CH4. Additionally, monolayered BiOI nanosheets have also been obtained by the same synthetic approach. Our work offers a mild and general approach for preparation of monolayered BiOX, and may have huge potential to be extended to the synthesis of other single-layer two-dimensional materials. KEYWORDS: BiOBr, Monolayered nanosheets, Liquid-phase exfoliation, Photodegradation, CO2 reduction

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INTRODUCTION Layered materials are of considerable interest and are widely applied in the fields of electronics,1 catalysis,2 energy generation and storage,3,4 biology,5 and so on. The layered feature enables these layered materials, such as graphene,6 layered double hydroxides (LDHs),7 bismuth semiconductors,8 C3N4,9 and transition metal dichalcogenides,10 to be monolayered nanosheets by a variety of chemical methods. In particular, the preparation of single-layer or few-layer two-dimensional (2D) photocatalytic materials is considered as a new promising method to improve the photocatalytic efficiency. Because of the substantial exposed surfaces and numerous uncoordinated surface atoms, the monolayered nanosheets are endowed with large surface area and abundant catalytic sites for photocatalytic reactions. The ultrathin structure can also enable the photogenerated carriers to migrate fast from the inside of the particles to the surface, which greatly decreases the bulk recombination and promotes the photocatalytic redox reaction.11,12 As a series of layered ternary oxide semiconductors, Sillén structured BiOX (X = Cl, Br, I) has gained widespread attention, owing to their high activity, high stability, nontoxicity, and other advantages.13−16 Despite a great deal of © 2017 American Chemical Society

efforts having been made to improve the photocatalytic activity of BiOX (X = Cl, Br, I), such as elemental doping,17 noble metal deposition,18 and heterojunction fabrication,19,20 it is still far from sufficient for potential applications, and it is necessary to further improve the photocatalytic activity to meet the potential industrial applications. The crystal structure of BiOX (X = Cl, Br, I) belongs to the PbFCl-type tetragonal system with a space group of P4/nmm,21 where the layered structure is built by [Bi2O2] slabs stacked together by the weak nonbonding (van der Waals) interaction and slabs of double halogen atoms along the c-axis direction. Because of the weak interaction between layers, it is possible to exfoliate BiOX into ultrathin nanosheets or even monolayered sheets.22 Though monolayer or few-layer BiOX (X = Cl, Br, I) materials have been obtained,23−27 the reported synthetic techniques always require high temperature, high pressure, and complex procedures, which greatly confine their large-scale practical applications. Herein, we report the preparation of monolayered BiOBr nanosheets via a facile and scalable liquid-phase exfoliation Received: July 24, 2017 Revised: September 26, 2017 Published: October 12, 2017 10499

DOI: 10.1021/acssuschemeng.7b02508 ACS Sustainable Chem. Eng. 2017, 5, 10499−10508

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) Crystal structure of bulk BiOBr. (b) Surface energy of BiOBr with different surface-exposed atoms.(c) Crystal structure of monolayered BiOBr with different surface-exposed atoms. (Figure S1), and 0.2 g of bulk BiOBr can produce about 30−40 mg of monolayered BiOBr nanosheets. Characterization. AFM images were recorded using an Oxford MFP-3D AFM instrument (Oxford MFP-3D). XRD of bulk BiOBr and monolayered BiOBr was carried out on a D8 Advance X-ray diffractometer (Bruker AXS) with Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) images were obtained by an electron microscope (Hitachi H-8100). High-resolution transmission electron microscopy (HRTEM) images were acquired by a Tecnai F20 electron microscope. The chemical compositions of the obtained materials were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi ThermoFisher). The room-temperature Xray absorption fine structure (XAFS) data on the Bi L3-edge (13.419 keV) of bulk BiOBr and monolayered BiOBr were measured at the 16BM-D station of the High-Pressure Collaborative Access Team (HPCAT) at the Advanced Phonon Source of Argonne National Laboratory. The data were processed and fitted with the program Athena.28 Brunauer−Emmett−Teller (BET) surface area was recorded by nitrogen adsorption with a Micromeritics 3020 instrument. ζ potential measurements were determined by a ζ potential instrument (Malvern ZS90). The UV−vis diffuse reflectance spectra (DRS) of prepared samples were determined on a UV−vis spectrometer (Cary 5000 Varian) which can measure the optical properties of photocatalysts. The reflectance spectra of photocatalysts were collected at the range 200−800 nm referenced to BaSO4. The PL spectra of the prepared materials were determined using a fluorescence spectrometer (F-4600 Hitachi). The photoelectrochemical properties of the obtained materials were measured by a three-electrode system and an electrochemical workstation (CHI660E, Chenhua Instruments Co., Shanghai, China). In this system, a saturated calomel electrode (SCE) was utilized as the reference electrode, and platinum wire was the counter electrode; the electrolyte solution was 0.1 M Na2SO4 solution. The sample films were coated on indium−tin oxide (ITO) sheet glass as the working electrode. A 300 W xenon lamp was used as the visible

strategy with assistance of formamide at room temperature. Formamide molecules can react with the surface OH on bulk BiOBr, thereby rendering the formation of single-layer BiOBr. These BiOBr monolayers are found to possess large specific surface area, abundant active sites and oxygen vacancies, optimized band structure, as well as promoted charge separation. Because of these advantages, the monolayered BiOBr nanosheets show high adsorption capability, and efficient photocatalytic degradation and CO 2 reduction performance. The proposed strategy has huge potentials to be extended to the preparation of other layered materials.

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EXPERIMENTAL SECTION

Materials Synthesis. All of the reagents used in this work were analytical grade and used as received without further purification. The bulk BiOBr was obtained by a sample hydrolysis method. An 8 mmol portion of Bi(NO3)3·5H2O (Sinopharm) was added to 50 mL of deionized water, and dispersed by ultrasonic bath and magnetic stirring for 20 min. Meanwhile, 8 mmol of KBr (Sinopharm) was dissolved in 50 mL of deionized water. Then, the KBr solution was dropwise added into the Bi(NO3)3·5H2O suspension. The mixture was stirred for 8 h at room temperature. After that, the pale-yellow products were washed with deionized water several times and dried at 60 °C for 12 h in a vacuum drying oven. For synthesis of monolayered BiOBr nanosheets, 0.2 g of as-prepared BiOBr was added into 100 mL of formamide (XiLong Scientific) solution in a three-mouth flask, and then, the mixture was stirred at a speed of 260 rpm for 6 h under ultrasonic conditions. Then, the suspension was centrifuged at the speed of 4000 rpm for 10 min to remove nonexfoliated materials. Finally, the supernatants were collected and centrifuged at the speed of 10 000 rpm for 20 min, and the precipitates are monolayered BiOBr nanosheets. The color of monolayered BiOBr nanosheets is white 10500


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Figure 2. (a−d) AFM images and corresponding height profiles of monolayered BiOBr nanosheets. (e, f) TEM image and the corresponding SAED pattern of superposition of multiple monolayer BiOBr. (g, h) TEM image and the corresponding HRTEM image of monolayered BiOBr. (i) XRD patterns of bulk BiOBr and monolayered BiOBr. (j) Crystal structure of monolayered BiOBr. uniform slurry. After that, the suspension was dropped on an indium−

light source. There was no applied voltage between the electrodes, and all the measurements were collected at room temperature. The working electrode was sampled by a dip-coating method: Photocatalyst powder (30 mg) was dispersed in 2 mL of ethanol to obtain a

tin oxide (ITO) glass with a size of 10 mm × 20 mm. Then, this working electrode was dried at 373 K for 10 h to eliminate ethanol. 10501


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Figure 3. (a) Normalized Bi L-edge XAFS spectra and (b) Fourier transformed profiles for Bi coordination environments of bulk and monolayered BiOBr. XPS spectra of (c) survey, (d) high-resolution Bi 4f, (e) O 1s, and (f) Br 3d of bulk and monolayered BiOBr.

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Photodegradation Experiments. The photocatalytic activities of as-prepared products were investigated by the degradation of methyl orange (MO) under simulated sunlight irradiation provided by a 500 W xenon lamp. During the test, 30 mg of sample powders was dispersed into 50 mL of 2 × 10−5 M methyl orange (MO) aqueous solution in a quartz tube. To ensure the adsorption−desorption equilibrium between solution and powder, all the suspensions were stirred in the dark for 60 min before irradiation. After the Xe lamp was turned on, 3.0 mL of suspensions was taken at a certain interval of time. For removal of the catalyst, the obtained liquids needed to be filtrated by a high-speed centrifuge. Finally, the filtrates were tested on a Varian UV−vis spectrophotometer (Shimadzu UV-5500PC) at a wavelength of 464 nm. Photocatalytic CO2 Reduction Test. Photocatalytic reduction of CO2 experiments were conducted in a closed PLS-SXE300 LabsolarIIIAG gas system with a 300 W Xe lamp (PLS-SXE300) as the light source. Then, the reactor was filled with CO2 or argon to conduct the reaction with 200 mg of photocatalyst powder. Afterward, the produced gases were analyzed by gas chromatography (GC7900, Tianmei, Shanghai, China; TCD).

RESULTS AND DISCUSSION

Figure 1a shows the layered structure of bulk BiOBr built up by [Bi2O2] layers and Br slabs.29 When bulk BiOBr is exfoliated into monolayers, the BiOBr monolayer has three possible structures with different surface-exposed atoms, namely, Bi exposed surface, O exposed surface, and Br exposed surface, as presented in Figure 1c. According to density functional theory (DFT) calculations, the surface energies of the above three possible structures are calculated (Figure 1b). It is demonstrated that the surface energy of the crystal surface for the monolayered BiOBr structure with Br-exposed atoms is the lowest among the three structures, indicating the higher thermodynamic stability. Therefore, the surface-exposed atoms of monolayered BiOBr should be Br atoms. Atomic force microscopy (AFM) is employed to determine the thickness of BiOBr samples. The AFM images (Figure 2a− d) show that the thickness of exfoliated BiOBr samples is 10502


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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration for Formation of Monolayered BiOBr Nanosheets

4f, and O 1s all can be found in the survey XPS spectra as displayed in Figure 3c. For bulk BiOBr, the Bi 4f XPS spectra contain two peaks at binding energies of 159.5 and 164.8 eV, which belong to Bi 4f5/2 and Bi 4f7/2 in the [Bi2O2]2+ layer (Figure 3d), respectively. Compared to that in the bulk BiOBr, the Bi 4f peak in the BiOBr monolayer exhibits an obvious shift with binding energies of 4f5/2 and 4f7/2 at 158.8 and 164.3 eV, respectively. This difference showed that the chemical environment of surface Bi atoms in the monolayered BiOBr has changed, mainly because of the absence of some lattice oxygen atoms. Figure 3e shows the O 1s XPS spectra. The two peaks at 529.9 and 531.3 eV occurred for both bulk BiOBr and monolayered BiOBr, which correspond to lattice oxygen and surface hydroxyls, respectively. In comparison with bulk BiOBr, monolayered BiOBr has an evidently higher OH amount. This may be due to the absence of parts of surface Br atoms, resulting in more exposure of Bi atoms on the surface. The peak at 533.3 eV that appeared in monolayered BiOBr is assigned to adsorbed water. This is because the surrounding water has strong interaction with open metal sites on the monolayer surfaces.11,32 The peaks located at 68.6 eV belong to Br 3d, which are well-consistent with those in BiOBr11 (Figure 3f). In contrast to that of bulk BiOBr, the Br 3d peak of monolayered BiOBr also shifted, which further suggests the exposure of the surface Bi atom. Obviously, these observations are in good agreement with the above XAFS results. According to the above observations, the formation mechanism for monolayered BiOBr was proposed as exhibited in Scheme 1. Similar to the LDH exfoliation process, it is easy for formamide molecules to enter into the interlayers of bulk BiOBr and combine with the hydroxyl groups via hydrogen bonds.33 With the increase of the amount of formamide molecules, the hydrogen bond between formamide molecules enlarges the interlayer distance of BiOBr, which weakens the interaction between layers. Under the action of mechanical agitation from stirring and ultrasonic conditions, the BiOBr lamellas undergo lateral sliding, and bulk structures collapse; finally, a large amount of monolayered BiOBr nanosheets are obtained. For inspection of the change of optical absorption capability and band gap, UV−vis diffuse reflectance spectra (DRS) were measured and displayed in Figure 4a. Compared to that of bulk BiOBr, the absorption edge of monolayered BiOBr nanosheets shows an obvious blue shift, which corresponds well to the nanosheet nature. The band gap energies of the as-obtained samples are calculated by the Kubelka−Munk (KM) expression:34

approximately 0.85 nm, which agrees well with the thickness of monolayered BiOBr along the [001] direction. Clearly, this result illustrates that monolayered BiOBr nanosheets are successfully prepared by this simple liquid-phase exfoliation route. The transmission electron microscopy (TEM) images (Figure 2e,g) also demonstrate that the transparent BiOBr nanosheets are observed, which confirms the ultrathin structure and supports the AFM result. The selected area electron diffraction pattern displayed in Figure 2f reveals the [200] and [110] zone axis. The marked interplanar spacing of 0.28 nm corresponds to the (110) crystal plane of BiOBr (Figure 2h), which indicates that the exposed facet of monolayered BiOBr is the (001) facet. X-ray diffraction patterns of the bulk BiOBr and monolayer BiOBr samples are displayed in Figure 2i. All of the diffraction peaks can be indexed to the tetragonal-phase BiOBr with lattice parameters a = 3.915 Å, b = 3.915 Å, and c = 8.11 Å (JCPDS 73-2061). Notably, disappearance of the diffraction peak corresponds to the (001) plane of BiOBr surface, which reveals the drastic decrease in diffraction intensity of the (001) direction, suggesting the successful exfoliation of BiOBr along this direction. The above results demonstrate that the monolayered BiOBr nanosheets were successfully obtained. In addition, the monolayered BiOI nanosheets have been prepared by the same synthetic route. The thickness of BiOI nanosheets is in the range 0.8−1 nm (Figure S2), which is consistent with the theoretical monolayered BiOI thickness (0.91 nm). This indicates the universality of the current synthetic strategy. To further reveal the interaction between atoms and obtain more information on the local structure evolution, XAFS data on the Bi L3-edge (13.419 keV) of bulk BiOBr and monolayered BiOBr was analyzed at room temperature. As shown in Figure 3a, the Bi L-edge oscillation curves of bulk BiOBr and monolayered BiOBr present different profiles in the energy range 13 200−14 200 eV, which illustrates the difference of local atomic arrangements in the catalysts. Figure 3b displays the in situ Fourier transformed profiles of XAFS. It is obvious that there are three peaks in the range 1−4 Å, which correspond to the Bi−O, Bi−Br, and Bi−Bi distances, respectively. The radial distance of Bi−O in monolayered BiOBr is 0.04 Å shorter than that in bulk BiOBr, which revealed the presence of O vacancy.30,31 The larger Bi−Br bond length in monolayered BiOBr than that in bulk BiOBr should be attributed to the exposed of surface Br atoms derived from the monolayered structure. Compared to those in bulk BiOBr, the shorter Bi−Bi bonds in monolayer BiOBr should also result from the lattice local contraction due to O vacancy. X-ray photoelectron spectroscopy (XPS) of the bulk BiOBr and monolayered BiOBr was conducted to analyze the surface composition and chemical states of related elements. Br 3d, Bi

αhv = A(hv − Eg )n /2 10503

(1) DOI: 10.1021/acssuschemeng.7b02508 ACS Sustainable Chem. Eng. 2017, 5, 10499−10508

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Figure 4. (a) UV−vis diffuse reflectance spectra (DRS) and band gap (inset) of bulk and monolayered BiOBr. (b) Mott−Schottky plots of bulk and monolayered BiOBr at a frequency of 1000 Hz (0.1 M Na2SO4). (c) Band structure diagrams of bulk and monolayered BiOBr. (d, e) Electronic band structures and (f, g) densities of states (DOS) of bulk and monolayered BiOBr, respectively.

where α, hv, A, and Eg represent the optical absorption coefficient, photonic energy, proportionality constant, and band gap, respectively. BiOBr is an indirect semiconductor, so n is 4.35 The bandgap energy of monolayered BiOBr is 2.82 eV,

which is 0.9 eV higher than that of the bulk BiOBr (2.73 eV; inset of Figure 4a). Mott−Schottky plots are measured to provide the band structure information. As seen from Figure 4b, the flat band potentials of bulk and monolayered BiOBr 10504


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Figure 5. (a, b) Adsorption and photodegradation of MO over bulk and monolayered BiOBr materials under simulated sunlight. (c, d) CO and CH4 production curves and (e, f) corresponding rates over bulk and monolayered BiOBr under simulated sunlight.

samples are −0.60 and −0.66 eV versus SCE, respectively. As the conduction band (CB) position is near the flat band potential, the CB position of monolayered BiOBr is approximately −0.06 eV higher than that of bulk BiOBr. Combined with UV−vis DRS results, it is easy to know that the monolayered BiOBr has a more positive valence band (VB) position than bulk BiOBr by 0.03 eV. For confirmation of the alteration in band energy levels, electronic band structures of the bulk and monolayer BiOBr were analyzed theoretically. As exhibited in Figure 4d,e, the band structure evolution results from the upshift of the CB position and downshift of the VB position, which agree well with the above experimental data. In general, the bandgaps calculated by DFT are smaller than those measured from experimental determination,11,36,37 because of the band gap underestimation of DFT calculations. The plots of densities of states are illustrated in Figure 4f,g. For bulk and monolayered BiOBr, the top of the VB and the bottom of the CB are both mainly composed of a hybrid O 2p−Br 3p orbital and Bi 6p orbital, respectively. These results revealed that the monolayered BiOBr has a stronger oxidation and reduction

driving force,38,39 which would contribute to the photocatalytic oxidation and reduction performance. For a systematic assessment of the monolayered BiOBr, the photocatalytic oxidation and reduction performance are investigated. First, the as-prepared materials were monitored by testing the adsorption and photodegradation properties of MO. As presented in Figure 5a, compared to bulk BiOBr, monolayered BiOBr shows a stronger adsorption capacity to MO. After the irradiation for 4 h (Figure 5b), 33% of MO can be removed by monolayered BiOBr, while, for bulk BiOBr, the removal of MO was only 16.5%. This indicates that the monolayered BiOBr demonstrates more efficient photocatalytic oxidation ability. For further evaluation of the photocatalytic activity of the monolayered BiOBr, CO2 reduction performance was monitored, as displayed in Figure 5c−f. It is obvious that the generated amounts of CH4 and CO increase with irradiation time, and the production of CH4 and CO of monolayered BiOBr is almost 3 times higher than that of the bulk BiOBr. No gaseous products can be detected with purging argon, which 10505


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Figure 6. (a) Nitrogen absorption−desorption isotherms; (b) BET surface areas and ζ potential; (c) steady-state PL spectra; and (d) photocurrent density under simulated sunlight (0.1 M Na2SO4) of bulk BiOBr and monolayered BiOBr.

higher than that of bulk BiOBr. This result further implies that more efficient charge separation occurred in the monolayered BiOBr.

eliminates the influence of residual organic impurities on the surface of monolayered BiOBr and demonstrates that CH4 and CO are truly produced in the photocatalytic CO2 reduction process. The photocatalytic activity of a photocatalyst is always closely associated with band structure, specific surface area, and charge separation. Brunauer−Emmett−Teller (BET) N2 adsorption methods are employed to determine the surface area and porosity of the obtained materials, and the results are shown in Figure 6a,b. The surface areas are 9.27 and 33.37 m2/g for the bulk BiOBr and monolayered BiOBr, respectively. Obviously, the monolayered BiOBr photocatalysts with higher BET surface area can absorb more reactants and generate more active species on their surface, enhancing the photocatalytic activity.12 Meanwhile, the ζ potentials of bulk BiOBr and monolayered BiOBr are measured, which are +1.76 and +19.2 mV, respectively. Their ζ potential difference is consistent with that of their surface areas. For photocatalytic materials, separation efficiency of charge carriers is a very important factor to affect the photocatalytic performance. As shown in Figure 6c, monolayered BiOBr shows a photoluminescence emission peak with obviously lower intensity than bulk BiOBr, which illustrates that the recombination of photogenerated electrons and holes is depressed in monolayered BiOBr. Meanwhile, because of the nanosheet nature,39,40 the PL peak of monolayered BiOBr samples also displays a clear blue shift, in accordance with the enlarged band gap. For further inspection of the improved charge separation efficiency of monolayered BiOBr, photocurrent measurements are conducted as shown in Figure 6d.41,42 The photocurrent intensity of monolayered BiOBr is

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CONCLUSIONS

In summary, monolayered BiOBr nanosheets with a thickness of ∼0.85 nm have been synthesized by facile liquid-phase exfoliation of bulk BiOBr under stirring and ultrasonic, and ambient conditions. Compared to bulk BiOBr, monolayered BiOBr shows enhanced specific surface area, allowing more active sites. Monolayered BiOBr also possesses a more negative conduction band position and a more positive valence band position, offering stronger reduction and oxidation abilities, respectively. More importantly, a well-strengthened charge separation efficiency was also achieved in monolayered BiOBr. The photocatalytic tests uncover that the monolayered BiOBr presents higher performance for adsorption, photodegradation, and photocatalytic CO2 reduction into CO and CH4. Combining the photoelectrochemical and photocatalytic measurement results, enhanced charge separation in monolayered BiOBr should be the dominant factor for the increased photoactivity. Via the same synthetic route, monolayered BiOI nanosheets have been prepared as well, demonstrating the universality of the current strategy. Our study illustrates an easily handled and efficient protocol for preparation of monolayered materials for environmental and energy applications. 10506


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02508. Color of monolayered BiOBr, AFM images, and the corresponding height profiles of monolayered BiOI nanosheets (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-10-82322247. *E-mail: [email protected]. ORCID

Hongwei Huang: 0000-0003-0271-1079 Weichang Hao: 0000-0002-1597-7151 Yihe Zhang: 0000-0002-1407-4129 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by the National Natural Science Foundations of China (51672258 and 51572246), and the Fundamental Research Funds for the Central Universities (2652015296).

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REFERENCES

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