Boosting Visible-Light-Driven Photo-oxidation of BiOCl by Promoted

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Boosting Visible-light-driven Photooxidation of BiOCl by Promoted Charge Separation via Vacancy Engineering Li Wang, Dongdong Lv, Fan Dong, Xuelian Wu, Ningyan Cheng, Jason Scott, Xun Xu, Weichang Hao, and Yi Du ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04454 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Sustainable Chemistry & Engineering

Boosting Visible-light-driven Photooxidation of BiOCl by Promoted Charge Separation via Vacancy Engineering Li Wang,† Dongdong Lv,‡ Fan Dong,ǂ Xuelian Wu,‖ Ningyan Cheng,† Jason Scott,‖ Xun Xu,†,‡ Weichang Hao,‡* and Yi Du†,‡* †Institute

for Superconducting and Electronic Materials, Australian Institute for Innovative

Materials, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia. ‡UOW-BUAA

Joint Research Centre and School of Physics, Beihang University, 37 Xueyuan

Road, Haidian District, Beijing 100191, China ǂResearch

Center for Environmental Science & Technology, Institute for Fundamental and

Frontier Sciences, University of Electronic Science and Technology of China, No. 2006, Xiyuan Ave, West Hi-Tech Zone, Chengdu, Si Chuan, 611731, China. ‖Particles

and Catalysis Research Group, School of Chemical Engineering, The University of

New South Wales, Sydney, High Street, Kensington, NSW 2052, Australia

*To whom correspondence should be addressed. Email: [email protected] (Y.D.); [email protected] (W.H.) Keywords: oxygen vacancies, photocatalytic, NO removal, BiOCl, charge separation

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Abstract The separation of electron-hole pairs has an important influence on the photocatalytic process on semiconductors. In this work, BiOCl nanosheets with oxygen vacancies (BiOCl-OVs) have been prepared by reconstructing small hydrophobic BiOCl nanosheets. The transient photoresponse and the electron spin resonance (ESR) results prove that the separation of the charge carriers can be promoted by the oxygen vacancies via trapping the photoexcited electrons. Due to the improved charge separation and wide absorption of the solar spectrum, more photogenerated charge carriers are produced, as confirmed by the photocurrent response and the ESR measurements of the reactive oxygen species •O2- and •OH. Consequently, BiOClOVs present enhanced photocatalytic properties towards NO removal. Our study illustrates the importance of the construction of vacancies for improving photocatalytic performance. Introduction Photocatalysis represents a promising technology for addressing our current problems of environmental pollution and the energy crisis via degrading toxic pollutants, splitting water to produce H2, and photoreducing CO2 to generate hydrocarbon fuels by simply using solar light.1 In the typical photocatalytic process, the performance of the photocatalysts is determined by the light absorption, the photoinduced electron-hole separation, and the sequential chargecarrier migration to the catalytically reactive sites.2-5 Thus, tremendous efforts have been made towards widening the solar-light absorption spectrum, promoting electron-hole pair separation, and boosting the migration rate to pursue high photocatalytic performance.6,7 Some other researchers also explored the adoption of ligand protected metal clusters for improved photocatalytic properties.8-10 Recently, p-block bismuth oxyhalides (BiOX, X = Cl, Br, I) with their unique polar two-dimensional (2D) layered crystal structure have attracted considerable


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attention.11-15 Their characteristic electronic s-p hybridization can induce highly dispersive valence and conduction band structures, which result in small effective mass and consequently high mobility of the photoexcited electrons/holes.16 Even though the BiOX photocatalysts possess superior properties in terms of their charge-carrier migration, their wide band-gap and poor charge recombination characteristics suppress their photocatalytic efficiency. In this case, exploring strategies to induce wide absorption of the solar spectrum and promote the charge separation process is necessary for enhancing the photocatalytic properties of the BiOX photocatalysts. Crystal defects are common in semiconductors, especially in nanosized or microsized semiconductor particles.17-19 Atomic vacancies, such as the oxygen vacancy (OV) in TiO2,20-22 the ternary defects of VBi'''VO●●VBi''' in ultrathin BiOCl nanosheets,23 and the oxygen vacancy in BiOBr,24 are very common in semiconductor photocatalysts. The atomic vacancies are proposed to assist in the photocatalytic process through several mechanisms: (i) enhancing the absorption and activation of inert gas molecules; (ii) inhibiting the charge carrier recombination via trapping electrons or holes on defect states and accelerating the migration of trapped charge carriers to the adsorbates; and (iii) lowering the energy barrier for interfacial charge transfer.25,26 Investigations of the effects of vacancies on the photocatalytic properties have still been quite limited, however. Here, BiOCl nanosheets with oxygen vacancies (BiOCl-OVs) were prepared via reconstructing small hydrophobic BiOCl nanosheets. The oxygen vacancies can help to induce wide solar-light absorption of large-scale BiOCl nanosheets and produce more charge carriers. The photoexcited charge separation is enhanced by oxygen vacancies via trapping the photogenerated electrons. Benefiting from its increased density of charge carriers and excellent charge separation properties, the defective BiOCl with oxygen vacancies has demonstrated superior photo-oxidization properties towards NO removal, both under illumination by both



simulated solar light and visible light with wavelengths longer than 420 nm. Our study reveals the correlations between the atomic defects and the photocatalytic performance, which can provide inspiration for the future material design and fabrication. Experimental Section Chemicals Bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, 99.999%), bismuth chloride (BiCl3, 98%), sodium chloride (NaCl, 99.5%), oleic acid (OA, 90%), oleylamine (OM, 98%), iron (III) acetylacetonate (Fe(acac)3, 97%), nitric acid (HNO3, 70%), and 1-octadecene (ODE, 90%) were purchased from Sigma-Aldrich. All chemicals were used without further purification. Materials preparation Synthesis of BiOCl without vacancies (BiOCl) In the typical process,27 15 ml aqueous solution containing 1 mmol Bi(NO3)3·5H2O and 1 mmol NaCl was prepared at room temperature under continuous magnetic stirring for 30 min, and then transferred to a 50 ml Teflon-lined stainless steel autoclave. The autoclave was heated at 170 oC for 16 h and then naturally cooled to room temperature. The resulting products were collected and washed with ethanol and distilled water several times, and then dried at 80 oC for 24 h in air. Synthesis of BiOCl with vacancies (BiOCl-OVs) The small hydrophobic BiOCl nanosheets were prepared according to the previous reports28 and then redispersed into 2 ml cyclohexane. The BiOCl cyclohexane solution was added to 30 ml distilled water under magnetic stirring. The temperature of the mixture was increased to 80 oC

to remove the cyclohexane, which has a low-boiling point, and then decreased to room


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temperature. Concentrated nitric acid was used to set the pH of the mixture to 0.8. The resulting solution was transferred to a 50 ml autoclave and then kept at 170 oC for 16 h. The final powders were obtained and then washed with water several times. The final products were dried at 80 oC

in air.

Characterization The X-ray diffraction (XRD) patterns of the as-prepared samples were obtained on a GBC MMA X-ray powder diffractometer with Cu-Kα1 irradiation (40 kV, 25 mA, λ = 0.15418 nm). The scanned detection angle (2θ) range was from 10o to 90o. The morphologies and structures of the saples were characterized on a JEOL JSM-7500 scanning transition microscope (SEM) and a JEOL JEM-2010 transmission electron microscope (TEM). High-resolution X-ray photoelectron spectroscopy was conducted on the Photoelectron Spectroscopy Station (Beamline 4W9B). Ultraviolet-visible (UV-vis) diffuse reflectance spectra were collected on a Shimadzu UV-3600. Electron spin resonance (ESR) spectra were collected, and the reactive species were detected on a JES-FA200 ESR Spectrometer. The photoluminescence (PL) spectra were characterized with a Horiba Fluoromax-4 spectrofluorometer. The photoelectrochemical characterization was conducted on a VSP-300 electrochemical analyser using the standard three-electrode set-up with a working electrode (as-prepared samples), platinum foil as the counter electrode, and a standard Ag/AgCl electrode as the reference electrode. 0.5 M Na2SO4 solution was selected as the electrolyte solution. At 300 W Xe lamp at the distance of 10 cm from the working electrode was utilized as the full-spectrum light source in the photocurrent-time response studies. The fixed frequency for Mott-Schottky measurements was 1 kHz. The frequency range for the potentiostatic electrochemical impedance (PEIS) measurements was from 10 Hz to 200 kHz.



The photocatalytic properties of the as-prepared powders were estimated by investigating photocatalytic NO removal and the photocatalytic dye degradation process. In testing the photooxidation of NO, a continuous flow reactor, which was made of stainless steel and covered with Saint-Glass, was utilized. A 100 mW commercial tungsten halogen lamp vertically placed outside the reactor was used as the light source, and a 420 nm cut-off filter was used to simulate visible light. The NO gas was provided via diluting a 100 ppm compressed gas cylinder to 500 ppb with an air stream. A humidification chamber was adopted to keep the humidity level of the flowing NO at 50%. The rate of NO flow is controlled at 2.4 L min-1 by a mass flow controller. 0.2 g of as-prepared photocatalyst was mixed with 15 ml ethanol. The dispersion was coated onto two culture dishes. After vacuum drying, the dishes were located at the centre of the reactor. When the absorption-desorption equilibrium of the sample was achieved, the lamp was turned on. A chemiluminescence NO analyser with a sampling rate of 1.0 L min-1 was used to test the NO concentration. In the photodegradation test, 5 mg of photocatalyst was dispersed in 5 ml of 10 mg/L methyl orange (MO) aqueous solution in a 20 ml glass vial surrounded by cooling water. The light source was a 300 W Xe lamp, and filter glass was used to obtain the visible light (λ > 420 nm). After 1 h of dark absorption, 1.5 ml of solution was withdrawn for UV-vis testing at defined time intervals under the light illumination and then put back in the vial. Results and discussion The BiOCl nanosheets with and without oxygen vacancies were prepared via the hydrothermal method. The morphology and structural characterizations are displayed in Figure 1a-c and Figure 1e-g, respectively. The scanning electron microscope (SEM) images in Figure 1a and e indicate that the two different types of nanosheets possess square-like shapes several micrometres in size. The X-ray diffraction (XRD) patterns of the as-prepared nanosheets are presented in Figure S1 in the Supporting Information. The positions of the diffraction peaks


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for the two samples are similar and they can be indexed to the tetragonal phase of BiOCl. Transmission electron microscopy (TEM) was adopted to acquire detailed structural information on the samples. As shown in the high-resolution TEM images in Figure 1b and f, the lattice fringes with the lattice spacing of 0.28 and 0.38 nm correspond to the (110) and (100) atomic planes of BiOCl, respectively. The corresponding selected-area electron diffraction (SAED) patterns in the insets indicate the single-crystalline nature of the asprepared nanosheets. The angle between two series of adjacent spots labelled in the SAED patterns for the two different particles is 45o, which is consistent with the theoretical values of the angle between (200) and (110) planes and the angle between (100) and (110) planes. Hence, the two sets of diffraction patterns in Figure 1b and f can be indexed to the [001] zone axis, revealing that the exposed facets of both samples are (001) facets. The cross-sectional TEM images in Figure 1c and h shows the layered structures of the two BiOCl samples. The distances between the interlayers for BiOCl and BiOCl-OVs are both about 0.75 nm. The above results indicate that the oxygen vacancies do not have a big impact on the structure of BiOCl. Schematic illustrations of the crystal structures of the two nanosheets from the top view are presented in Figure 1d and h.

Figure 1. Structural characterization of the synthesized powders. a, e, SEM images of BiOCl



(a) and BiOCl-OVs (e). b, f, High-resolution TEM images of BiOCl (b) and BiOCl-OVs (f) (top-view). The insets show the corresponding SAED patterns. c, g, High-resolution crosssectional TEM images of BiOCl (c) and BiOCl-OVs (g). d, h, Crystal structures of BiOCl (d) and BiOCl-OVs (h). The UV-Vis diffuse reflectance spectra in Figure 2a indicate that the absorption band edge for BiOCl and BiOCl-OVs is at about 360 nm, which corresponds to a band gap of about 3.4 eV. Compared with BiOCl, BiOCl-OVs presents an additional continuous and exponentially decaying absorption tail extended to 600 nm, which is also reflected by the changes in colour of the BiOCl and BiOCl-OVs powders, as shown in the Figure 2a insets. This absorption tail is attributed to the absorption of defect states induced by the oxygen vacancies.26,29 X-ray photoelectron spectroscopy (XPS) is generally adopted to analyse the surface states of materials. For BiOCl-OVs, there are two additional peaks centred at 163.38 eV and 157.97 eV that appear in the high-resolution core-level XPS spectrum of Bi 4f (Figure 2b left), indicating that oxygen vacancies exist in the material and that the OV-connected Bi atoms have been partially reduced.26 The peaks in the high-resolution XPS spectrum of O 1s for BiOCl are located at 530.30 eV and 531.53 eV (Figure 2b right) which correspond to lattice oxygen and chemisorbed hydroxide, respectively. Compared with BiOCl, the two deconvolved peaks of O 1s for BiOCl-OVs are downshifted to 530.15 eV and 531.26 eV. The downshifts are caused by structural relaxation, and they further verify the presence of oxygen vacancies in BiOCl-OVs. Figure 2c displays the Raman spectroscopy results for the samples. Compared with BiOCl, the Raman spectrum of BiOCl-OVs shows two new bands at 69 cm-1 and 98 cm-1, which are assigned to the Eg and A1g vibration modes of Bi.30 They reflect the existence of a small amount of reduced Bi in BiOCl-OVs and the presence of oxygen vacancies in this sample. To further confirm that there are vacancies in the Bi-connected oxygen in BiOCl-OVs, electron spin resonance (ESR) spectroscopy was conducted.24,27 The BiOCl-OVs possesses an obvious


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signal at g-factor, g = 2.007. As the typical value of the g-factor for the electrons is about 2.0, the ESR signal reflects the existence of unpaired electrons in BiOCl-OVs. Figure S2 shows that no big difference inthe high-resolution peaks of Cl 2p can be observed in BiOCl and BiOCl-OVs. Thus, Cl vacancy in BiOCl-OVs can be excluded. In addition, there was no UV illumination over BiOCl-OVs during the ESR measurement, and thus, the detected ESR signal of BiOCl-OVs must have originated from the intrinsic oxygen vacancies. In the case of BiOCl, no detectable ESR signals can be found, indicating that no oxygen vacancies exist in BiOCl.

Figure 2. Characterization of oxygen vacancies in the samples. a, TUV-visible absorption spectra of BiOCl and BiOCl-OVs, with photographs of the respective powders in the insets. b, High-resolution XPS core level spectra of Bi 4f (left) and O 1s (right) for BiOCl and BiOClOVs. c, Raman spectra of BiOCl and BiOCl-OVs. d, ESR spectra of BiOCl and BiOCl-OVs, with the respective crystal structures shown in the insets. The photocurrents and the transient photoresponses of BiOCl and BiOCl-OVs were examined to study their respective abilities to separate the photo-generated charge carriers. As



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shown in Figure 3a, both the BiOCl and the BiOCl-OVs electrodes can promptly generate a reproducible photocurrent in each light on/off cycle under illumination by simulated solar light. The photocurrent density of BiOCl-OVs is about -0.35 μA/cm2 which is nearly 2 times larger than that of BiOCl. As BiOCl cannot absorb visible light (λ > 420 nm), almost negligible photocurrent is observed over BiOCl under visible light illumination (Figure 3b). In contrast, a photocurrent density of about -0.05 μA/cm2 is observed over BiOCl-OVs in the same process. These photocurrent responses reveal that the oxygen vacancies promote the generation of more charge carriers in BiOCl-OVs. The transient photoresponse was used to analyse the dynamic properties of the photogenerated carriers under illumination, and thus confirm the charge separation in the photocatalytic process. Generally, the following exponential functions (1) and (2) can be used to describe the rise and decay processes of the photocurrent, respectively.31 ―𝑡 𝛾

𝐼 = 𝐼0 ― 𝐼0𝑒

(𝜏 )

―𝑡 𝛾

𝐼 = 𝐴𝐼0𝑒

(𝜏 ) 𝑑

(1)

𝛾

―𝑡 𝛾′

+𝐵𝐼0𝑒

(𝜏 ) ′𝑑

(2)

Here, I is the current density; t is time; A and B are constants; τγ and τd represents the relaxation time constants of the rise and decay processes, respectively; γ is used to describe the stretching property of the single-exponential functions, and its value is in the range of 0 to 1. For BiOCl and BiOCl-OVs, the fitted value of γ in the rise process is 1, revealing that the separation of the photoinduced electron-hole pairs is the dominant factor in the generation of the photocurrent for these two materials. It should be noted that the overshoot in the rise process of BiOCl originates from the charge carrier recombination and that the fitting of the rise process only takes the photocurrent response into account. For the decay process, two singleexponential decay components are necessary to describe the photocurrent decay, which suggests that multiple photorelaxation processes occur during the decay process. Here, the


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fitted values of γ for the two decay processes of the two materials are smaller than 1, indicating that stretched exponential functions should be used to describe the processes. The two decay processes include a fast one and a slow one. It is well documented that the fast decay process is determined by the photoinduced electron-hole recombination. The slow process is caused by the local potential fluctuations, which can trap and spatially separate the photoexcited carriers, and thus suppress the electron-hole recombination. The fitted relaxation times for BiOCl and BiOCl-OVs in the fast decay process (Figure 3c and d) are 0.3 s and 0.3 s, respectively, indicating that the photoexcited electron-hole recombination of BiOCl-OVs is comparable with that of BiOCl. For the slow decay process, the relaxation time is estimated to be 0.53 s for BiOCl and 0.71 s for BiOCl-OVs. This means that the local potential fluctuation of BiOClOVs is larger than that of BiOCl. Thus, the oxygen vacancies are proved to facilitate trapping and spatially separation of the photoexcited charge carriers of BiOCl-OVs. By combining the fast and slow photodynamic decay processes, BiOCl-OVs possesses better charge separation properties. The proposed separation processess of the photoinduced electron-hole pairs are illustrated in Figure 3e and f for BiOCl and BiOCL-OVs, respectively.



Figure 3. a, Photocurrent response of BiOCl and BiOCl-OVs under illumination by simulated solar light. b, Photocurrent response of BiOCl and BiOCl-OVs under illumination by visible light (λ > 420 nm). c, Acquiring the rise and decay time of BiOCl by fitting the on/off curve. d, Acquiring the rise and decay time of BiOCl-OVs by fitting the on/off curve. e, Crystal structure of BiOCl with the proposed photoexcited charge carriers. f, Crystal structure of BiOCl-OVs with the proposed photoexcited charge carriers. The ESR spectrum was detected to quantitatively analyse the photocatalytic active species.32 Typically, the •O2- is derived from the O2 reduction by electrons, and it can be detected by probing the 5,5-dimethyl-pyrroline N-oxide (DMPO)-•O2- complex in the methanol medium.33 The •OH originates from the OH-/H2O oxidation in water by holes, and it can be detected by probing the DMPO-•OH in the water medium.34 Under illumination by simulated solar light, the ESR signals observed in Figure 4a and c can be ascribed to DMPO-•O2- and DMPO-•OH, indicating that both electrons and holes were generated over BiOCl and BiOCl-OVs. Compared with BiOCl, the ESR signals for DMPO-•O2- and DMPO-•OH are stronger over BiOCl-OVs. As the square of the intensity of the ESR signal is approximately proportional to the amount of free radicals, more photoexcited electrons and holes are proved to be generated in BiOCl-OVs than in BiOCl.35 Figure 4b and d show that there are no ESR signals of DMPO-•O2- and DMPO•OH for BiOCl under visible light irradiation. This indicates that the visible-light-driven activity of BiOCl is negligible, which is in accordance with the absorption properties of BiOCl. For BiOCl-OVs under illumination by visible light, no obvious ESR signal of DMPO-•O2- was detected, as shown in Figure 4b, while four-line ESR signals of DMPO-•OH with the intensity ratio of about 1:2:2:1 can be observed in Figure 4d. As the absorption of visible light for BiOClOVs takes place in the defect states caused by oxygen vacancies, the oxygen vacancies are considered to consume or trap the electrons with the holes left behind. Combined with the photodynamic results in Figure 3c and d, the oxygen vacancies are believed to promote charge


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separation in BiOCl-OVs via trapping the electrons. The effect of oxygen vacancies on the charge carrier behaviours is further confirmed by the steady-state photoluminescence (PL) spectra and the potentiostatic electrochemical impedance spectra (PEIS) (Figure S3).

Figure 4. a, b, ESR signals for DMPO-•O2- over BiOCl and BiOCl-OVs under illumination by simulated solar light and visible light (λ > 420 nm) for 4 min, respectively. c, d, ESR signals for DMPO-•OH over BiOCl and BiOCl-OVs under illumination by simulated solar light and visible light (λ > 420 nm) for 4 min, respectively. The effects of the oxygen vacancies were further investigated by examining the photocatalytic NO removal capabilities of BiOCl and BiOCl-OVs. As shown in Figure 5a, nearly 38% of NO was removed over BiOCl-OVs in 30 min under illumination by simulated solar light, while only 27% of NO was removed over BiOCl in the same period. This reveals that the superior charge separation of BiOCL-OVs facilitates the generation of more oxygen active species under solar light irradiation. Figure 5b demonstrates that the removal rate of NO over BiOCl-OVs is about 11% in 30 min under irradiation by visible light (λ > 420 nm). As BiOCl cannot utilize the visible light, its visible-light-driven photocatalytic NO removal is negligible. The photodegradation of methyl orange (MO) displays similar results (Figure S4).



This proves that the oxygen vacancy can help to produce holes, which are utilized in the photooxidation reaction. By characterizing the SEM images and the XRD patterns for BiOClOVs, as shown in Figure S5, it is clear that the structure of BiOCl-OVs is stable during the photocatalytic testing. In-situ Fourier transform infrared spectroscopy (FTIR) was used to investigate the products of photooxidized NO over BiOCl-OVs (Figure 5c). The absorption band located at 1025 cm-1 is attributed to NO.36,37 The absorption bands of monodentate nitrates (1280 cm-1)38 and bridge NO3- (1628 cm-1)39 can also be detected. The absorption peaks at 2884 cm-1 and 2945 cm-1 are assigned to NO2.40 Therefore, the products of the photooxidation reaction are proposed to be HNO3 and NO2. According to Figure S6, the quantum yield of NO oxidation into HNO3 and NO2 is estimated to be 5.86%. Figure 5d shows the cycling test of photocatalytic NO removal over BiOCl-OVs under irradiation by simulated solar light. It can be clearly observed that the photooxidation property of BiOCl-OVs is stable after five durability tests.

Figure 5. a, TPhotocatalytic removal of NO in the presence of BiOCl and BiOCl-OVs under illumination by simulated solar light. b, Photocatalytic removal of NO in the presence of BiOCl and BiOCl-OVs under illumination by visible light (λ > 420 nm). c, In-situ FTIR spectra of the


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photocatalytic NO oxidation process over BiOCl-OVs under simulated solar light irradiation. d, Cycling test of the NO removal property over BiOCl-OVs under illumination by simulated solar light. Conclusion In conclusion, BiOCl-OVs nanosheets with oxygen vacancies have been prepared via reconstructing small hydrophobic BiOCl nanosheets. The UV-Vis absorption spectrum suggests that BiOCl-OVs can utilize visible light via the defect states induced by the oxygen vacancies. Analysis of the dynamic properties of the photoresponse and the reactive species proves that the oxygen vacancies can facilitate the dissociation of photogenerated charge carriers via trapping the electrons. The photocurrent density indicates that the oxygen vacancies can promote the generation and transportation of the charge carriers. Benefiting from the improved charge carrier generation, separation, and transportation, the defect-rich BiOCl-OVs sample exhibits excellent performance in the photooxidation reactions towards NO removal. This work demonstrates the importance of vacancies in pursuing efficient photocatalysts. Supporting information The Supporting Information includes XRD patterns, high-resolution XPS spectra of Cl, photoluminescence spectra, electrochemical impedance spectra, methyl orange degradation, stability testing of BiOCl-OVs, and the evolution process of HNO3 and NO2 over BiOCl-OVs. Author Information Corresponding authors *Email: [email protected] (Y.D.) *Email: [email protected] (W.H.)



Author contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the Australian Research Council (ARC) through Discovery Projects (DP160102627 and DP170101467), the National Natural Science Foundation of China (51672018, 51472016 and 51272015), and the Fundamental Research Fund for the Central Universities. The authors would like to acknowledge support by the University of Wollongong through an AIIM for GOLD 2018 grant, and use of facilities within the UOW Electron Microscopy Centre. The authors thank Dr. T. Silver for her valuable comments on this work. References (1)

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For Table of Contents Use Only

BiOCl-OVs, which is fabricated via reconstructing hydrophobic BiOCl nanosheets, is proved to facilitate charge separation via trapping electrons and possesses superior NO oxidation properties.


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Figure 1. Structural characterization of the as-prepared powders. a, e, The SEM image of BiOCl (a) and BiOCl-OVs (e). b, f, The high-resolution TEM images of BiOCl (b) and BiOCl-OVs (f) from the top view. The insets show the corresponding SAED. c, g, The high-resolution TEM images of BiOCl (c) and BiOCl-OVs (g) from the side view. d, h, The crystal structure of BiOCl (d) and BiOCl-OVs (h). 256x120mm (150 x 150 DPI)



Figure 2. The characterization of oxygen vacancies. a, The UV-visible absorption spectra of BiOCl and BiOClOVs. b, The high-resolution XPS core level spectra of Bi 4f (left) and O 1s (right) for BiOCl and BiOCl-OVs. c, The Raman spectra of BiOCl and BiOCl-OVs. d, The ESR spectra of BiOCl and BiOCl-OVs. 112x95mm (220 x 220 DPI)


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Figure 3. a, Photocurrent response of BiOCl and BiOCl-OVs under the illumination of simulated solar light. b, Photocurrent response of BiOCl and BiOCl-OVs under the illumination visible light (λ > 420 nm). c, Acquiring the rise and decay time of BiOCl by fitting the on/off curve. d, Acquiring the rise and decay time of BiOClOVs by fitting the on/off curve. e, The crystal structure of BiOCl with the proposed photoexcited charge carriers. f, The crystal structure of BiOCl-OVs with the proposed photoexcited charge carriers. 262x139mm (150 x 150 DPI)



Figure 4. a, b, The ESR signals for DMPO-•O2- over BiOCl and BiOCl-OVs under the illumination of simulated solar light and visible light (λ > 420 nm) for 4 min, respectively. c, d, The ESR signals for DMPO-•OH over BiOCl and BiOCl-OVs under the illumination of simulated solar light and visible light (λ > 420 nm) for 4 min, respectively. 165x133mm (150 x 150 DPI)


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Figure 5. a, The photocatalytic activity of the removal of NO in the presence of BiOCl and BiOCl-OVs under the illumination of the simulated solar light. b, The photocatalytic activity of the removal of NO in the presence of BiOCl and BiOCl-OVs under the illumination of visible light (λ > 420 nm). c, In situ FT-IR spectra of photocatalytic NO oxidation process over BiOCl-OVs under the simulated solar light irradiation. d, The cycling test of the NO removal property over BiOCl-OVs under the illumination of the simulated solar light. 177x138mm (150 x 150 DPI)


Boosting Visible-Light-Driven Photo-oxidation of BiOCl by Promoted

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