Bismuth Vacancy Tuned Bismuth Oxybromide Ultrathin Nanosheets

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Energy, Environmental, and Catalysis Applications

Bismuth Vacancy Tuned Bismuth Oxybromide Ultrathin Nanosheets towards Photocatalytic CO2 Reduction Jun Di, Chao Chen, Chao Zhu, Pin Song, Jun Xiong, Mengxia Ji, Jiadong Zhou, Qundong Fu, Manzhang Xu, Wei Hao, Jiexiang Xia, Shuzhou Li, Hua-ming Li, and Zheng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08109 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

Bismuth Vacancy Tuned Bismuth Oxybromide Ultrathin Nanosheets towards Photocatalytic CO2 Reduction Jun Di1,2‡, Chao Chen2,‡, Chao Zhu2,‡, Pin Song2,‡, Jun Xiong1, Mengxia Ji1, Jiadong Zhou2, Qundong Fu2, Manzhang Xu2, Wei Hao2, Jiexiang Xia1,*, Shuzhou Li2, Huaming Li1, Zheng Liu2,*

1

School of Chemistry and Chemical Engineering, Institute for Energy Research,

Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, P. R. China 2

Center for Programmable Materials, School of Materials Science & Engineering,

Nanyang Technological University, Singapore 639798, Singapore

‡ These authors contributed equally to this work.

*Corresponding author: [email protected]; [email protected]

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Surface defects in semiconductor has significant role to tune the photocatalytic reactions. However, the dominant studied defect type is oxygen vacancy and metal cation vacancies is seldom explored. Herein, bismuth vacancies are engineered into BiOBr through ultrathin structure control and employed to tune the photocatalytic CO2 reduction. The VBi-BiOBr ultrathin nanosheets delivers a high selective CO generation rate of 20.1 μmol g-1 h-1 in pure water without any co-catalyst, photosensitizer and sacrificing reagent, roughly 3.8 times higher than that of BiOBr nanosheets. The increased CO2 reduction activity is ascribed to the tuned electronic structure, optimized CO2 adsorption, activation and CO desorption process over VBi-BiOBr ultrathin nanosheets. This work offers new opportunities for designing of surface metal vacancies to optimize the CO2 photoreduction performances.

Keywords: Bismuth vacancies; BiOBr; Ultrathin nanosheets; Electronic structure; CO2 photoreduction

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1. INTRODUCTION Photocatalytic CO2 reduction to value-added carbon products have been regarded as an attractive approach to assuage current energy and environmental issues.1-6 However, since the dissociation energy of C-O bond is as high as 750 kJ mol-1, the CO2 molecule is extraordinarily stable, which greatly limit the catalytic conversion activity.7 Moreover, the multiple electron involved processes for the reduction of CO2 will result in the multifarious products and this process will also suffer from H2 evolution side reaction from H2O.8 Thus, it is desirable to design efficient photocatalyst to enable high activity and selectivity simultaneously. Recent studies found that engineering surface defects into semiconductors can effective adjust the surface microstructure, electronic structure, charge separation and charge density, so as to tune the photoelectric parameters of photocatalysts.9-11 Oxygen vacancy is the mainly studied defect type in photocatalysts due to easy to create and prevalence in oxide materials. For example, the Zhang’s group, Xie’s group, and Xiong’s group found the oxygen vacancy on the surface of semiconductors are beneficial to the adsorption and activation of specific molecules (such as N2, H2O, O2) due to the formed abundant localized electrons.12-14 However, there is seldom reports regarding metal vacancies for activity improvement since it is comparative difficult to operate metal defects via a credible process. Owing to the diversified electron configuration and orbit of metal cations, the engineered surface metal vacancies will give rise to conspicuous electronic-structure variation relative to perfect ones.15 Thus, it is desirable to engineered surface metal vacancies into semiconductors and study the roles for CO2 interfacial catalytic conversion process. To engineer surface vacancies into materials, decreasing the thickness of materials to 2D atomic thick is an effective means due to the significantly reduced formation energy of surface metal vacancies at atom-scale thickness.16 Take the nontoxic BiOBr as an example, since it is a van der Waals layered structured material and favorable photocatalyst for CO2 reduction.17 The BiOBr possess a suitable conduction band (CB) potential for CO2 reduction, while the H2 3



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evolution performance is usually poor.18 The outer exposed Bi atoms in BiOBr crystal can build powerful adsorption of hydrogen, result in hydrogen poisoning effect and thus the poor H2 evaluation rate. Inspired by above-mentioned advantages, it is desirable to tune BiOBr thickness to atom-scale and create bismuth metal vacancies on the surface, so as to disclose the atomic-level relationship between metal vacancies with CO2 photoreduction. Herein, BiOBr ultrathin nanosheets with abundant surface Bi vacancies (VBi-BiOBr

UNs)

are

successfully

prepared

by

reactive

ionic

liquids

[C16mim]Br-assisted synthesis at room temperature. The Bi vacancies can adjust the band structure of BiOBr, increase the carrier concentration and promote the charge separation. Take the advantage of optimized CO2 adsorption, activation and CO desorption, the VBi-BiOBr UNs can deliver a 3.8 times improved CO formation rate relative to BiOBr nanosheets.

2. EXPERIMENTAL SECTION 2.1. Synthesis of VBi-BiOBr UNs In a typical synthesis procedure, two solutions are prepared at first. Solution A: 9 mL of H2O, 1 mL of acetic acid and 0.4850 g of Bi(NO3)3·5H2O are mixed and stirred. Solution B: 0.3873 g of 1-hexadecyl-3-methylimidazolium bromide ([C16mim]Br) is dissolved into 10 mL of alcohol. The sample are achieved via add solution B into solution A within 5 min under continue stirring and the mixture is further stirred for 60 min. The initial formed sample is centrifuged at about 1000 rpm to remove the large-size nanosheets. The VBi-BiOBr UNs are obtained via 12000 rpm centrifugation and washed by water and ethanol for further characterization. For comparison, BiOBr nanosheets samples are prepared by hydrothermal treatment. 0.3873 g of [C16mim]Br is added into 20 ml deionized water and then 0.4850 g of Bi(NO3)3·5H2O is added under stirring. The pH value of the solution is regulated to 1. The mixture is stirred for 30 min and then transferred into 25 mL autoclave, treated at 140 oC for 24 h. 4


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2.2. Characterization XRD was performed on a Shimadzu XRD-6000 X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å). X-ray photoelectron spectroscopy (XPS) is tested on a PHI5300 with a monochromatic Mg Kα source. The scanning electron microscopy (SEM) images were taken on a JEOL JSM-7001F. The transmission electron microscopy (TEM) images were obtained on a JEOL-2100F at an acceleration voltage of 200 kV. STEM-HAADF images were acquired from a JEOL JEM-ARM200F STEM with a spherical aberration corrector operated at 80 kV. Electrochemical performance was studied in a three-electrode system at an electrochemical station (CHI 660B). A ThermoFisher Nicolet iZ10 was employed to measure the in-situ FTIR spectra with co-adsorption of CO2 and H2O vapor under Xe lamp irradiation. CO temperature-programmed desorption (TPD) was carried out on Quantachrome autosorb-iQ-C chemisorption analyzer coupled with TCD detector. 2.3. Calculation Method The first-principles simulations are performed through the Vienna ab initio simulation package (VASP),19, 20 the projector augmented wave (PAW) potentials are employed as pseudopotentials to describe the interactions between valence electrons and ions. The Perdew–Burke–Ernzerhof (PBE) functional of generalized gradient approximation (GGA) was used to describe the exchange-correlation of valence electrons.21 The lattice parameters of bulk BiOBr was calculated first with the plane wave cutoff energy set as 500 eV and the k-point mesh set as 11×11×3. The optimized lattice parameter for bulk BiOBr was 3.929 Å × 3.929 Å × 8.126 Å. To calculate the BiOBr (001) surface, a 2×2×1 supercell was created and a 20 Å vacuum layer was added on top of the supercell to avoid interlayer interplay. The surface model contains totally 4 Bi atomic layers, and the outmost atomic layer are Bi atoms bonded with O atoms underneath them. Thus, the lattice parameter of the layer slab model was 7.858 Å × 7.858 Å × 30.66 Å, with the plane wave cutoff energy set as 500 eV and the k-point mesh set as 7×7×1. In surface relaxation simulations, the outermost 3 layers of atoms (2 Bi layers and 1 O layer) were free to move while atoms of inner layers were fixed. The convergence criteria were kept unchanged. For the case of CO2 molecule 5



adsorption, all the parameters were remained same as that of BiOBr surface model, except that the convergence criteria for ionic relaxation was changed to 0.02 eV/Å instead. DFT-D3 method with Becke-Jonson damping was introduced to account for vdW corrections for all the simulations mentioned above.22 2.4. Photocatalytic Experiments Photocatalytic activity of VBi-BiOBr UNs is evaluated by CO2 photoreduction, in which the photocatalysts is tested in a Labsolar-6A closed gas system, and reactor volume is 500 ml. 30 mg sample is dispersed in 50 mL pure water, without addition of any other agents. A 300 W Xe lamp (Microsolar300, Perfectlight, China) is employed as the light source. The system is subjected to thorough vacuum treatment and pumped into high-purity CO2 with pressure of 0.08 MPa. Full-automatic on line gas chromatograph (Cotrun GC2002, FID) is used to analyze the products. CH4 is tested by a FID and CO is converted to CH4 via methanation reactor.

3. RESULTS AND DISCUSSION The morphology of the as-prepared sample is visualized by SEM and TEM. The VBi-BiOBr UNs displays a laminar sheet-like structure (Figure 1a, b), in which the near-transparency nature suggest the ultrathin thickness. The contrastive BiOBr nanosheets shows similar sheet-like structure with thicker thickness (Figure S1). High-resolution TEM image shows the lattice spacing of 0.277 nm in VBi-BiOBr UNs, corresponds to the (110) planes of tetragonal BiOBr (Figure 1c). According to the vertical angle between (110) and (1-10) planes as well as the crystal structure of BiOBr, it can be deduced the (001) facet exposure in VBi-BiOBr UNs. To further study the surface microstructure, aberration-corrected STEM-HAADF operated at low voltage is employed. The near-transparency ultrathin sheets structure can also be observed (Figure 1d), and thickness determined from cross-sectional is ca. 2 nm. Atomic force microscopy is used to further explore the thickness of VBi-BiOBr UNs (Figure 1h, i). From the corresponding height profile, the thickness is determined to be around 1.2 nm, further revealing the ultrathin nature of VBi-BiOBr UNs. By tuning 6


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the Br source to other ionic liquid [Bmim]Br, [Omim]Br or KBr (with other conditions invariability), the ultrathin structure can not be obtained, suggesting the crucial role of long carbon chain [C16mim]Br for thickness controlling (Figure S2-S5). View from STEM image, the lattice spacing of 0.277 nm can also be found in VBi-BiOBr UNs (Figure 1e), further indicate the (001) facet exposure. It is worth noting that partial surface bismuth atoms missing can be observed, revealing the existence of Bi vacancies (Figure 1f).23 However, the contrastive BiOBr nanosheets displays ordered lattice and surface atomic arrangement, suggesting the deficiency of Bi vacancies (Figure 1g). To discriminate the possible O vacancies, the EPR and XPS O 1s spectra are performed. No obvious signal at g = 2.001 can be found, suggesting no oxygen vacancy is created in VBi-BiOBr UNs (Figure 2a). The XPS O 1s spectra also do not show distinct peak attributable to oxygen vacancy (Figure 2b). Therefore, the dominating existed defect type in VBi-BiOBr UNs is isolated Bi vacancies. The purity and crystallinity of the as-prepared samples are analyzed by XRD (Figure 2c, S6). All the peaks for the VBi-BiOBr UNs sample are readily indexed to the tetragonal phase of BiOBr (JCPDS No. 73-2061). The intensity of (00l) peak presents a distinctly higher value than other peaks, which indicate the [001] orientation is highly preferred of VBi-BiOBr UNs. No peaks of the other impurities are found. The surface chemical states of the VBi-BiOBr UNs and BiOBr nanosheets are measured by XPS technique, in which the two samples show similar composition and chemical states (Figure S7). The peak of Bi 4f5/2 of VBi-BiOBr UNs showed positive shift relative to BiOBr nanosheets, suggesting the difference of local atomic structure (Figure S7b). Based on the XPS results, the atom ratio of Bi to O is 1.11 for the VBi-BiOBr UNs, which is slightly lower than that (1.19) of BiOBr nanosheets, further revealing the existence of Bi vacancies in VBi-BiOBr UNs. The Raman spectrum of the VBi-BiOBr UNs and BiOBr nanosheets are shown in Figure 2d. The Raman bands at 112.0 and 160.8 cm-1 are ascribed to the A1g internal Bi-Br stretching and E1g internal Bi-Br stretching mode, respectively.24 As is well known, the Raman shift and full width at half maximum (fwhm) are sensitive to the number of layers in layered materials. Compared to the BiOBr nanosheets, the external A1g Bi-Br stretching mode 7



at about 56.5 cm-1 shift towards higher wavenumber about 58.4 cm-1 in the VBi-BiOBr UNs. At the same time, the fwhm of A1g and E1g vibration peaks in VBi-BiOBr UNs increased relative to BiOBr nanosheets. Benefiting from the ultrathin thickness, VBi-BiOBr UNs displayed higher than the specific surface areas than that of BiOBr nanosheets (16.55 vs. 4.10 m2 g-1) (Figure 2e, f), allowing it adsorb more CO2 molecule to involve the subsequent interfacial catalytic process. To investigate the light absorption and electronic structure of VBi-BiOBr UNs and BiOBr nanosheets, further characterizations are carried out. UV-Vis diffuse reflection spectra dislpay that the absorption edges of BiOBr are around 430 nm, in which the VBi-BiOBr UNs is apparently red shifted relative to BiOBr nanosheets (Figure 3a). Furthermore, the VBi-BiOBr UNs shows tail peak from 430 to 500 nm, which may derived from the Bi vacancies. The bandgaps of VBi-BiOBr UNs and BiOBr nanosheets are estimated to be 2.65 and 2.77 eV, respectively (Figure 3b). The valence band (VB) of BiOBr samples are measured by XPS valence band spectra. 25 The VB potential shifts from 2.01 eV of BiOBr nanosheets to 1.57 eV of VBi-BiOBr UNs (Figure 3c). Thus, the conduction band (CB) potentials of BiOBr nanosheets and VBi-BiOBr UNs can be calculated to be -0.76 and -1.08 eV, respectively. It should be noted that both the CB potentials of BiOBr can meet the thermodynamic demands for CO2 reduction (E0(CO2/CO) = -0.51 V vs. NHE).26 This result is further certified by the density functional theory (DFT) calculation, in which the calculated density of states (DOS) at the VB edge of VBi-BiOBr is much higher than that of perfect BiOBr (Figure 3d, 3e, S8), suggesting the engineered Bi vacancy can increase charge density near the Fermi level. The increased DOS can endow the VBi-BiOBr more charge carriers to involve in the interfacial CO2 photoreduction process. Moreover, the more negative CB potential of VBi-BiOBr UNs enables the formed electrons with more reductibility, favors the CO2 reduction process (Figure 3f). Due to the diverse local atomic structure of VBi-BiOBr UNs and BiOBr nanosheets, the electronic structure display differentiated results, and thus lead to an interesting phenomenon that plentiful singlet oxygen can be produced over VBi-BiOBr UNs while it is difficult to produce over BiOBr nanosheets (Figure S9). 8


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To explore the photocatalytic CO2 reduction performance of VBi-BiOBr UNs, the solar CO2 reduction experiments are carried out in water with 0.08 M high-purity gas (Figure 4a). The dominant reduction product over VBi-BiOBr UNs is CO with a trace methane. The CO-evolving rate over VBi-BiOBr UNs can reach 20.1 μmol g-1 h-1 with a 98 % selectivity under the condition without cocatalyst, sacrifice reagent or extra photosensitizer, roughly 3.8 times as high as that of BiOBr nanosheets (5.3 μmol g-1 h-1). Simultaneously, the O2 generation can be acquired over VBi-BiOBr UNs with an average O2 generation rates of about 9 μmol g-1 h-1 during a 5 h process. The generation rates ratio of CO : O2 is 2.2, approach to the stoichiometric ratio of 2. Control experiments in dark or in Ar atmosphere did not show any CO formation, suggesting the CO is indeed produced via CO2 reduction over BiOBr. 13CO2 labelling experiment is performed in our VBi-BiOBr UNs system to track the authentic carbon source. The peak at m/z = 29 in the mass spectra can be attributed to 13CO, confirming that the C source of CO is come from the CO2 (Figure S10). Under UV light irradiation, the VBi-BiOBr UNs can display a CO formation rate of 16.5 μmol g-1 h-1, lower than that of 20.1 μmol g-1 h-1 in Xe light full spectrum irradiation (Figure S11). It is worth noting that the CO-evolving rate over VBi-BiOBr UNs is comparable or superior to our previous results (Figure S12) or other reported single photocatalyst systems under the comparable conditions (Table S1). Moreover, after 4 times cycling test, the VBi-BiOBr UNs can still maintain the efficient photoreduction activity (Figure 4b), revealing the stability of VBi-BiOBr UNs. The XRD and HAADF-STEM are employed to study the morphology and structure of VBi-BiOBr UNs after long-term testing. As shown in Figure S13, the materials after long-term testing is still BiOBr and the surface Bi vacancies can also be observed, suggesting the stability of VBi-BiOBr UNs. To investigate the reason of the improved CO-evolving rate, the adsorption, carrier separation, CO2 activation and CO desorption process are carefully studied. Firstly, the CO2 adsorption isotherms of BiOBr are measured since larger adsorption capacity can ensure more CO2 involve the interfacial conversion process. As shown in Figure 4c, a higher CO2 adsorption amount can be achieved on VBi-BiOBr UNs, 9



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demonstrating the defective ultrathin structure is beneficial to the CO2 adsorption, which is a prerequisite for CO2 photoreduction.27 Moreover, the adsorption energy of CO2 over the surface of BiOBr is calculated through DFT (Figure S14). The CO2 molecule prefer to horizontal structure rather than vertical structure, in which adsorption energy on the VBi-BiOBr UNs is -9.45 eV, greatly lower than that of perfect BiOBr (-0.16 eV). To explore the charge separation and study the surface-reaching electrons able to participate in the catalytic process, time-resolved fluorescence emission decay spectra are carried out (Figure 4d). Benefiting from the ultrathin thickness to lower the bulk carrier recombination and surface Bi vacancies to serve as separation center, the VBi-BiOBr UNs displays greatly longer average lifetime than that of BiOBr nanosheets.28 As such, more photogenerated electrons can arrive to the surface and activate the adsorbed CO2 molecules to trigger the conversion process. The increased charge separation efficiency of VBi-BiOBr UNs can also be certified by electrochemical impedance spectra and steady state PL spectra (Figure S15, S16). To further explore the CO2 activation and reaction intermediates, in situ Fourier transform infrared spectroscopy (FTIR) over VBi-BiOBr UNs is performed. Thermochemical analysis for the two-electron CO2-to-CO conversion follows the reaction pathway with COOH* and CO* intermediates: CO2 + * + (H+ + e−) → COOH*

(1)

COOH* + (H+ + e−) → CO* + H2O

(2)

CO* → CO

(3)

where “*” represents the surface adsorption state.29, 30 As shown from Figure 4e, the bands at 1248, 1338 and 1395 cm-1 can be ascribed to CO2−, symmetric OCO stretches of bidentate carbonate (b-CO32−) and monodentate carbonate (m-CO32−) groups, respectively.31,

32

It is noted that the new band at ca 1542 cm-1 can be

attributed to the COOH* group, a crucial intermediate during photocatalytic CO2-to-CO conversion. Once the CO2* molecules reacted with the surface protons, the COOH* will be produced gradually, whereas the further reaction of COOH* intermediate with protons will generate the CO* molecules. Finally, the adsorbed CO* molecules will deviate from the catalysts surface to yield CO molecules. To 10


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deeply insight the CO desorption process, the CO TPD is performed (Figure 4f). Compared with BiOBr nanosheets, the VBi-BiOBr UNs displays lower onset desorption temperature and increased overall amount of detected CO, meaning the CO* can depart from the surface of VBi-BiOBr UNs much easier.33 Consequently, all the above results forcefully suggested that the bismuth vacancy-rich ultrathin structure completely optimize the key processes during CO2 photoreduction.

4. CONCLUSIONS In summary, Bi vacancy-rich BiOBr ultrathin nanosheets are controlled prepared via long carbon chain ionic liquid assisted synthesis at room temperature. The Bi vacancies can be directly observed via low voltage STEM-HAADF. DFT calculations suggest that the existence of Bi vacancies ensures the higher carrier concentration and enhanced carrier transport, as testified by fluorescence decay spectra. At the same time, the defect-rich ultrathin configuration favors the adsorption, activation of CO2 molecule and CO desorption. As a result, the VBi-BiOBr UNs can deliver a high selective CO formation rate of 20.1 μmol g-1 h-1 at pure water, much higher than that of BiOBr nanosheets (5.3 μmol g-1 h-1). This work will give inspiration on the reasonable design of metal vacancy tuned photocatalysts towards CO2 photoreduction.

ASSOCIATED CONTENT Experimental section, SEM images and XRD of various BiOBr materials, XPS spectra, calculated DOS, ESR spectra, electrochemical impedance spectra, photocatalytic activity comparison

Author Contributions J. Di, J. X. Xia and Z. Liu conceived and designed the experiments. J. Di prepared and characterized the photocatalysts and performed the photocatalytic experiments. P. Song, J. Xiong and M. X. Ji performed part of the experiments. C. Zhu carried out STEM. C. Chen, W. Hao and S. Z. Li provided theoretical calculation and discussion. 11



All authors discussed the results and comments the paper. J. Di wrote the paper, J. X. Xia and Z. Liu revised the paper.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21676128, 21606113 and 21576123), Singapore National Research Foundation under NRF RF Award No. MOE2016-T2-1-131, Tier 1 2017-T1-001-075, MOE2018-T3-1-002. The computational work for this article was fully carried out on National Supercomputing Centre, Singapore (https://www.nscc.sg)

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Figure 1. (a) SEM image, (b, c) TEM images, (d-f) aberration-corrected HAADFSTEM images of VBi-BiOBr UNs, scale bar in f, 1 nm. (g) Aberration-corrected HAADF-STEM image of BiOBr nanosheets. (h) AFM image and (i) the corresponding height profiles of VBi-BiOBr UNs.



Figure 2. (a) EPR of VBi-BiOBr UNs, (b) XPS O 1s spectra, (c) XRD pattern of VBiBiOBr UNs, (d) Raman spectra of VBi-BiOBr UNs and BiOBr nanosheets, nitrogen absorption-desorption isotherms of (e) VBi-BiOBr UNs and (f) BiOBr nanosheets.


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Figure 3. (a) UV-Vis diffuse reflection spectra, (b) Tauc plot, (c) XPS valence spectra of VBi-BiOBr UNs and BiOBr nanosheets, (d) calculated density of states of perfect BiOBr and VBi-BiOBr UNs, (e) structure model of VBi-BiOBr UNs, (f) schematic illustration of the band structure of BiOBr nanosheets and VBi-BiOBr UNs.



Figure 4. (a) Photocatalytic CO evolution over VBi-BiOBr UNs and BiOBr nanosheets under 300 W Xe lamp irradiation, (b) cycling stability test for VBi-BiOBr UNs, (c) CO2 adsorption isotherms, (d) fluorescence emission decay spectra, (e) In situ FTIR spectra for the CO2 reduction process on VBi-BiOBr UNs and (f) CO TPD spectra.


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Bismuth Vacancy Tuned Bismuth Oxybromide Ultrathin Nanosheets

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