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Surface reorganization leads to enhanced photocatalytic activity in defective BiOCl Sujuan Wu, Weiwei Sun, Jianguo Sun, Zachary D. Hood, ShiZe Yang, Lidong Sun, Paul R.C. Kent, and Matthew F. Chisholm Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01629 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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Chemistry of Materials

Surface reorganization leads to enhanced photocatalytic activity in defective BiOCl

Sujuan Wu†,§,+*, Weiwei Sun‡,+, Jianguo Sun†, Zachary D. Hood‡, ║, Shi-Ze Yang§, Lidong Sun†, Paul R. C. Kent‡, ∇, Matthew F. Chisholm§

†

Electron Microscopy Center of Chongqing University, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China

§

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA

‡

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ║ School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ∇ Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States +

These authors are both first author. *Corresponding author: Sujuan Wu, E-mail: [email protected]

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ABSTRACT Introducing defects in semiconductor photocatalysts has been identified as an effective approach to extend the visible light absorption and achieve high efficiency solar energy conversion. However, the band gap model system of defect states may not truly describe the evolutions in real materials as the narrower band gap would limit the photocatalytic activity via suppressing the charge separation. Here, we report that reorganizing the surface termination in a defective semiconductor plays a key role in determining the photocatalytic performance. We directly observed that the surface reorganizations are accompanied with the formation of defects in layered structured bismuth oxychloride (BiOCl). Both experimental and theoretical results demonstrate that varying terminations have strong effects on the electronic structure and electron-hole pair recombination, which is shown to be the driving force of the promotion of visible light photocatalytic activity in BiOCl. We also reveal that the surface reorganization induces a novel transfer path and high dielectric surface to prevent the trapping of charge carriers, highlighting an efficient way of improving the photocatalytic activity by surface reorganization.


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Introduction Solar energy conversion via semiconductor-based photocatalysis has attracted wide attention in the last decade due to the various applications in photocatalytic hydrogen generation and removal of environmental pollution1,2. In the photocatalytic process, the semiconductor photocatalyst absorbs light and produces charge carriers, which migrate to the surface and generate desirable reactive intermediates or chemical products3. In order to develop photocatalysts with high solar energy conversion efficiency, many research efforts have focused on extending their optical absorption to the visible light region while suppressing the electron-hole recombination, since the visible light accounts for a greater percentage of the solar spectrum and only surficial separated charges take part in the photocatalytic reaction. So far, introducing defects into semiconductors usually extends the light response into the visible region with enhanced photocatalytic activity and has been widely studied4-7. The corresponding band gap model system of defect state is established to describe the impact of defects in semiconductors. Nonetheless, the defects induced mid-gap states would narrower band gap and limit the photocatalytic activity via suppressing the charge separation. Moreover, the surface plays an important role in the photocatalytic reaction due to its close relationship with the electron-hole recombination processes, and surface defects are more general in defective semiconductors and even spontaneous in some cases8,9. However, the surface microstructure in the defective semiconductor is rarely reported in detail and thus the impact of defects on the high visible light photocatalytic activity is still largely unknown. 3 ACS Paragon Plus Environment


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Bismuth oxychloride (BiOCl) is a novel ternary oxide semiconductor with a layered structure composed of alternating [Bi2O2]2+ slabs and double halogen atoms slabs10-12, and has wide applications in pigments13,14, optics15,16 , thermocatalysis17, and energy storage18,19. The surface atomic structure, especially the surface terminations, has a significant effect on the photo-response and determines the formation of the different kind of defects. Most publications report that the surface termination is Oxygen (O), as the surface O atoms would be easily evacuated and creating oxygen vacancies (VO) under UV light irradiation, and the VO enables the absorption of visible light and relatively high photocatalytic reactivity20-25. It has also been reported that disordered structures cover on the surface of BiOCl along with Vo23, the area of which generally increases with the concentration of Vo33. While the existence of Bismuth (Bi) point defects and possible formation of Bi-O-Bi and Bi-O point defects from BiOCl may suggest that the Bi termination would be exposed26,27, the Bi-terminated defective BiOCl also present higher activity under visible light26. In contrast, Hao11,28,29 et al. recently reported that the surface termination is Chlorine (Cl) atoms in BiOCl, and Chen17 et al. found that the surface Cl ions would be dissociated via reaction with photogenerated holes and spontaneously recover the BiOCl structure upon photocatalytic reaction. Ganose30 et al. have theoretically predicted that the halide anion plays a key role in determining the electronic structure and properties of BiOCl. The atomic surface structure in defective BiOCl remains unclear, largely limiting the assessment of its impact on the photocatalytic activity. Herein, we report defective BiOCl via ethylene glycol treatment and our direct observations of the atomic structure, particularly the terminated surfaces of defective BiOCl 4 ACS Paragon Plus Environment

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with scanning transmission electron microscopy. Our results indicate that the reorganized surfaces include localized Bi-Bi, Bi-O-Bi and O-Bi terminated surfaces that are accompanied with the formation of a large concentration of defects. The various terminations determine the electronic structure and surface charge state of BiOCl. The findings pave a new way for the generation and transport of photo-excited electrons and holes that create a new feature in the photoluminescence spectra. The reorganization of the surface structure also enhances the photocatalytic ability of BiOCl by simultaneous extending the visible light absorption and increasing the mobility of carriers. Experimental Section Synthesis of BiOCl and defective BiOCl. All reagents were commercially available analytical grade chemicals and were used as received without further purification. The phase pure BiOCl was synthesized using a hydrothermal method. In short, 0.972 g Bi(NO3)3·5H2O was dissolved into 50 mL of 0.1 M mannitol solution under vigorous ultrasonication for 10 minutes. Subsequently, 10 mL of an aqueous saturated NaCl solution was slowly injected to the above mixture under magnetic stirring at 800 rpm, and a white precipitate was obtained and washed by deionized water until pH=7. The as-obtained powder was finally dried at 100 °C for 4 hours. To synthesize defective BiOCl, the as-prepared BiOCl powder was reduced via ethylene glycol etching. In a typical process, 200 mg BiOCl powder was added to ethylene glycol at 180 °C for 20 min. The mixture was then rapidly cooled to room temperature using deionized water, and a precipitate was obtained by filtration. The precipitate was finally dried in a vacuum oven at 100 °C for 4 hours.



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Characterization. Powder x-ray diffraction (XRD) patterns were recorded by an Empryean X-ray diffractometer with monochromatic Cu-Kα radiation (λ=1.5418 Å). Scanning transmission electron microscopy (STEM) images were collected using a FEI Titan 80-300 Super Twin electron microscope operated at 300 kV and a Nion Ultra STEM 200 with sub-Ångstrom resolution at 200 kV. Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) were collected by using a field emission JEOL-SEM (JSM-7800F) equipped with energy spectrometer. X-ray photoelectron spectra (XPS) were collected with a Thermo K-Alpha XPS system with Al-Kα (hυ = 1486.6 eV) as the excitation source, a spot size of 400 µm, and a resolution of 0.1 eV. Raman spectra were recorded in the region of 40-2000 cm-1 using a confocal Laser Micro-Raman Spectrometer (LabRAM HR Evolution) with a laser set at 532 nm and 10 mW. The optical absorption spectra were measured from 240 to 800 nm using a Shimadzu UV-2100 UV-Vis spectrophotometer. Unpaired electron spins in specimens were characterized by electron paramagnetic resonance (EPR) spectroscopy (JEOL JES-FA200 EPR spectrometer, 77 K, 9.062 GHz). Photoluminescence (PL) spectra were collected by HR800 at room temperature with 325 nm He–Cd laser as exciting sources. Photocatalytic activity. The visible-light-driven photocatalytic activity was evaluated by photocatalytic degradation of an aqueous solution of Rhodamine B (RhB, 20 mg/L) under a 500 W Xe lamp with a 420 nm cut off filter. In a typical reaction, 50 mg of each photocatalyst was dispersed into a solution of 100 mL of RhB (20 mg/ L). Prior to illumination, the solution was continuously stirred for 2 h at 800 rpm in the dark to establish the adsorption−desorption equilibrium. For comparison, a 100 mL solution of 20 mg/L 6 ACS Paragon Plus Environment

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methylene orange (MO) was also used to demonstrate the photocatalytic degradation under a 500 W Xe lamp with a 420 nm cut off filter. The concentration of RhB and MO was then measured using a UV−visible (UV−vis) spectrophotometer. Computational methods. Density functional theory (DFT) calculations were performed using the projected augmented wave (PAW)32 method as implemented in Vienna ab initio simulation package (VASP)33. The plane-wave cutoff energy was 500 eV and a Monkhorst−pack grid of 5 × 5 × 1 k points were used in geometry optimizations, with a finer 7 × 7 × 1 k point grid used for static electronic structure analysis including born effective charges. The atomic positions were relaxed until the energy and forces were converged better than 10−6 eV and 0.01 eV/Å, respectively. The van der Waals density functional (vdW-DF) of optB86b34,35 was employed for all calculations due to the van der Waals interactions in the layered structure12,36,37. To avoid the interactions between periodic images, a total length of 65 Å along the vacuum direction was fixed for all calculations, giving a separation of 30 Å for pristine BiOCl. Band alignment between calculations was performed by aligning the average potentials. The Born effective charge tensors were computed using linear response techniques.38



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Results and Discussion

Figure 1. UV-Vis absorbance spectra (a), low-temperature EPR spectra (b), Bi 4f XPS (c) and Raman spectra (d) of black and white BiOCl. The defective BiOCl was prepared by ethylene glycol treatment, which has been reported as an efficient method to remove Cl ions39 and create large amount of Vo resulting in the black feature28. As illustrated in Figure 1a, the diffuse reflectance UV-Vis spectra of white BiOCl exhibits a sharp absorption edge located at 365 nm, featured by an intrinsic bandgap 8 ACS Paragon Plus Environment

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absorption edge around 3.4 eV. Black BiOCl has extremely high photon-absorption throughout the entire visible light region (>400 nm). The extension to visible light absorbance in BiOCl is attributed to the surface VO, which is characterized by the signals at lower temperature electron paramagnetic resonance (EPR) spectra.16,40 Here, the crystalline structure of black BiOCl is similar to that of white BiOCl (see XRD patterns in Figure S1). Still, we observe significant unpaired electron signals (at magnetic field B=320-340 mT) in black BiOCl, as shown in Figure 1b. These spectra confirm the existence of VO in black BiOCl, consistent with the previous reports.23,24 However, the ultrahigh absorbance maintains over the full visible light region and the further high-resolution X-ray photoelectron spectroscopy (XPS) spectra demonstrate that the black color change in BiOCl cannot be merely attributed to the VO. As shown in Figure 1c, each bismuth spectrum features two intense peaks at 159.4 and 164.7 eV, assigned to trivalent oxidation states of bismuth: Bi3+ 4f7/2 and Bi3+ 4f5/2, respectively. At lower binding energies, an additional spin–orbital doublet overlapped the major Bi 4f photoemission at 157.9 and 163.2 eV, which is similar to that in BiOCl with VO40, suggesting a surface environment involving VO. Another two deconvoluted peaks for Bi with a lower binding energy at 156.8 eV and 162.1 eV can be assigned to the metallic bismuth states. This suggests that the Bi with various chemical states and Vo both exist on the surface of black BiOCl. Besides, the binding energy of Cl 2p becomes lower and the surface adsorbed hydroxyl groups almost disappear in black BiOCl, as shown in Figure S2. These results demonstrate a verified surface structure in black BiOCl. Raman spectroscopy further confirms the breakage of crystal structure in black BiOCl (Figure 1d). The predominant bulk signals at 58.3 cm-1, 143.3 and 196.9 cm-1, correspond to 9 ACS Paragon Plus Environment


the A1g internal Bi-Cl stretching, Eg external and internal stretching modes of Bi-Cl, respectively. An additional 121.7 cm−1 peak, assigned to the surface or interface modes of BiOCl, is induced by breakage of lattice periodicity and symmetry41, confirming the partly reorganization of chemical bonds in black BiOCl. Taken collectively, a large amount of defects have been introduced into the black BiOCl by straightforward treatment with ethylene glycol.

Figure 2. Scanning transmission electron microscopy (STEM) images of the black BiOCl. HAADF-STEM image of (001) facets (a) and the atomic structure images of the outer layer (b) and the grain interior (c). Schematic diagram of the BiOCl crystal structure (d). HAADF-STEM images of (100) facets (e) and the atomic structure images of the outer layer (f) and the grain interior (g). Atomic-resolution scanning transmission electron microscopy (STEM) images were collected to characterize the microstructure of the black BiOCl. The nanosheet structure with the thickness of less than 20 nm was observed by high angle annular dark field (HAADF) STEM, as shown in Figure 2a and 2e. Due to the unique layered structures of BiOCl (Figure 10 ACS Paragon Plus Environment

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2d), consisting of [Cl-Bi-O-Bi-Cl] sheets stacked together by the weak van der Waals interaction through the Cl atoms along the c-axis, the strong intralayer bonding and the weak interlayer van der Waals interaction give rise to the crystallization into sheets or platelets with a high aspect ratio. Even after EG treatment, the nanosheet structure and uniform distribution of the elements were also maintained (Figure S3). The detailed atomic structure of the (001) facets of BiOCl are shown in Figure 2b, in which bright spots represent Bi atoms. We observed obvious surface steps at outer edges on the (001) facets. Moreover, an uneven distribution of Bi atoms was observed for the (001) facets as highlighted by the brighter regions in Figure 2b. More detailed atomic microstructures in Figure 2c demonstrate significant bismuth migration and surface reorganization. Atomic arrangements expected for Bi metal (shown as red dots in the inset of Figure 2c) were observed by means of fast Fourier transform (FFT). This observation suggests that the migrated Bi atoms prefer to exist in a metallic state on the surface of black BiOCl, confirming again the formation of large amounts of defects and observation of various chemical states in XPS results. On the (100) facets of black BiOCl, the edge of the surface of black BiOCl is sharp and the outside extracted Bi bilayer structure is likely to have various surface terminations after EG treatment, compared with the original one in Figure S4. The direct atomic-resolution images indicate that the reorganized surface on the (001) facet of black BiOCl may include localized Bi-O-Bi, O-Bi, and Bi-Bi terminations after removal of surface Cl ions by EG treatment and will be discussed later. The annular bright field (ABF) STEM of the (110) facets in Figure 2g further confirms the migration of Bi atoms and formation of tremendous defects.



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When a large concentration of defects exists in the layered structures, the outmost surface would thereby undergo reorganization in order to maintain the structural stability and/or reduce potential lattice distortion. Considering the anion coordination environment in BiOCl, the bismuth centers adopt a distorted square antiprismatic coordination geometry, where the Bi3+ ion is coordinated by four Cl atoms with a bond length of 3.06 Å each, and four O atoms with bond length of 2.32 Å each, forming two square faces in opposite side5, as shown in Figure 2d. Therefore, the migration of Bi atoms should be accompanied by breaking of the Bi-Cl and Bi-O tetrahedral coordination structure. Generally, the loss of atoms proceeds through the facets with the lowest cleavage energy during contact surface chemical reactions. In BiOCl, which is composed of alternating [Bi2O2]2+ slabs and double halogen atoms slabs, the (001) facets have the lowest cleavage energy of 0.026 J/m2, while the (010) facets have the second lowest cleavage energy of 0.532 J/m2.42 Extraction of surface Cl layers and Bi layers would be expected to proceed along [001] directions via EG treatment, and the migration of extracted Bi atoms would finally develop into to metallic state stayed on the surface, which is also confirmed by in situ TEM studies in our recent work50. Due to the existence of this uneven extraction and migration of surface atom layers, surface reorganization of BiOCl along [001] direction is thus introduced. These surface terminations of reorganized BiOCl, introduced by ethylene glycol, would thereby vary from pristine Cl, via intermediate Bi-O-Bi and O-Bi, to Bi-Bi surface structures (Figure 5a). The physical properties that are strongly related to the surface termination will thereby be affected.


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Figure 3. (a) The temporal evolution of the photodegradation and photographs of the RhB solutions under visible light irradiation (λ>420 nm) at different points during the reaction. (b) Photodegradation curve of white and black BiOCl and the inset shows the corresponding degradation rate. (c) Recyclability of black BiOCl towards degradation of RhB. (d) Transient photocurrent response of black and white BiOCl. The visible-light-driven photocatalytic activity of the black BiOCl was evaluated by using Rhodamine B (RhB) and methylene orange (MO) as probe molecules in aqueous solutions, which is consistent with the catalysis performed in Ref [23,40]. As shown in Figure 3a and b, the black BiOCl produces very high photocatalytic activity for the degradation of RhB under visible light irradiation. The decoloration rate of RhB in the presence of black BiOCl reaches 90% after 50 min, but less than 10% in the presence of white BiOCl for the same period. Corresponding degradation rates of 0.039 min-1 vs 0.001 min-1 in Figure 3(b) demonstrate the visible light photocatalytic performance of black BiOCl, which is superior to that of white BiOCl. No significant difference can be observed in Brunauer-Emmett-Teller (BET) surface areas (Figure S6) of black and white BiOCl, and the surface area induced 13 ACS Paragon Plus Environment


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improvement of photocatalytic activity can be therefore ruled out. Moreover, the enhancement of visible light driven photocatalytic activity by black BiOCl was determined by evaluating MO degradation (Figure S5). It is obvious that the photocatalytic activity of black BiOCl is highly enhanced and does not significantly decline after five cycles, indicating the excellent stability for environmental applications (Figure 3c). The degradation of dyes under visible light is through direct semiconductor photoexcitation and indirect dye photosensitization.26 However, the photocurrent density under the visible light excitation in Figure 3d demonstrates that the promoted visible light photocatalytic activity could derive from more direct photoexcitation, not only in the indirect dye photosensitization way. These results demonstrate that the BiOCl with EG treatment could improve both its UV and visible light photocatalytic activity.

Figure 4. (a) Photodegradation rate of RhB and MO dye by BiOCl with VO and black BiOCl with surface reorganization; (b) PL emission spectra and (c) X-ray valance band spectra of white and black BiOCl.

It is noted that the photocatalytic activity of this black BiOCl is much higher than that of BiOCl with only VO, compared with our previous study40 and the work by Ye et al.23, as shown in Figure 4 a. Under UV light irradiation, the photocatalytic activity of black BiOCl 14 ACS Paragon Plus Environment

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for degradation of RhB is 0.12 min-1, while it is merely 0.04 min-1 by BiOCl with VO. The degradation rate slows down to 0.039 and 0.026 min-1 under simulated visible light irradiation, respectively. This difference is also verified by evaluating the degradation of MO, revealing that VO could improve the visible light photocatalytic activity of BiOCl23,26, and further enhancement of the photocatalytic activity is mainly attributed to the migration of Bi induced surface reorganization. Moreover, the enhancement of the UV-light-driven photocatalytic activity is much higher than that of visible light activity. The photocatalytic activity related photoexcitation and recombination of electron-hole (e--h+) pairs were further investigated by photoluminescence (PL) spectroscopy with a 325 nm laser used as the excitation source (Figure 4b). There is a broad green emission at 540 nm (∼ 2.29 eV), which is attributed to the trapping of free electrons in the conduction band by recombination centers originating from VO as mentioned in a previous work15. Whereas for the black BiOCl (red curve), an extra strong PL peak at 440 nm (∼ 2.8 eV, peak I) emerges, which is a result of a new recombination path43,44. The highly improved UV-light-driven photocatalytic activity is thus attributed to this new peak I, which is expected to be caused by the surface reorganization and further investigated by using density functional theory involving different surface terminations as discussed at the end of this section. The Vo has not been detected in the white BiOCl, but there is still a wide peak in the visible light range, which means that the surface adsorbed hydroxyls induced Vo are generated under laser irradiation40. Nonetheless, the concentration of the hydroxyls induced Vo are much lower than that in black BiOCl, and it is therefore the intensity of PL peak in black BiOCl is thus much higher than that of white BiOCl. All these clues indicate that more defect states exist in the black BiOCl. To 15 ACS Paragon Plus Environment


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understand the effect of surface reorganization on the electronic structure, the corresponding valance band spectra is shown in Figure 4c. In the valance band XPS of white BiOCl, the valence band edge is located at about 2.1 eV, while it is 1.8 eV in the BiOCl with only VO. For this surface reorganized black BiOCl, the valence band edge blue-shifts towards the vacuum level by approximately 0.8 eV. In contrast to the sharp decay of the valence band edge observed in the BiOCl with only VO, a broader extension of the tail states through Fermi energy (EF) at 0 eV are responsible for extending the absorption to visible light and suggests a metallic feature of the black BiOCl surface. To date, most of the studied structural models of black BiOCl are primarily focused on the O terminated surface having VO defects, which tends to produce both mid-gap states and levels near the conduction band minimum in the O-Bi terminated (001) facet.20-24 However, our atomic-scale STEM images provide clear evidence that the surface reorganized black BiOCl consists of various terminations synergistically contributing to the high photocatalytic activity, potentially altering the previously reported recombination mechanism and electronic structures. We propose prototype surface models starting from the Cl terminated pristine surface (I), and two intermediate Bi-O-Bi (II) and O-Bi (III) terminated surfaces, to the Bi-Bi metallic layer terminated surface (IV), as illustrated in Figure 5a. We use electron localization functional (ELF) of each surface structure is to assess the changes in electronic structure (Figure 5b). The covalent feature of the near termination Bi-O/Bi-Cl bonding in Bi-O-Bi and O-Bi terminated surfaces is weakened compared with the pristine BiOCl surface reflecting by as the weaker localization (see color bar) of a bond, consistent with the lower binding energy of Cl 2p in experiment; the most pronounced ionic feature is characterized in 16 ACS Paragon Plus Environment

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the Bi-Bi metallic surface, visible as the significantly enhanced electronic localization (red center sphere) on the surface.

Figure 5. (a) The top and side view of structural models of white pristine BiOCl (I), Bi-O-Bi termination surface (II), O-Bi terminated surface (III), as well as the Bi-Bi metallic layer terminated surface (IV). Atom types are indicated by red (Bi), blue (Cl), and green (O). (b) 2D plot of electron localization functional of each termination surface. (c) The total and atom projected DOS of the above structure mode, in which Bi (s, p, d) is displayed in red, O (s, p) is displayed in green, Cl (s, p) is displayed in blue and the Fermi energy is set to 0 eV. (d) The schematic of the band alignment and two proposed mechanisms of carrier transfer. We next consider the electronic structure of different surface models, as shown in Figure 5. On the basis of Density of States (DOS) of the pristine surface, there is a band gap with the value of 2.3 eV. Note that DFT often underestimates the value of the band gap. The band structure (Figure S7) reveals the band gap as of an indirect characteristic. The valence band maximum (VBM) edge has contributions from all three elements, while the conduction band 17 ACS Paragon Plus Environment


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minimum (CBM) edge is mostly defined by Bi derived states. Moreover, the main orbital of each atoms is shown in Figure S8. The smallest direct gap is 2.85 eV, while the lowest energy indirect excitation path is from the high symmetry point X (VBM) to Γ (CBM) (Figure S7). The Bi-O-Bi terminated surface generates defect states crossing the EF near the CBM indicating metallicity. In contrast, the O-Bi terminated surface structure generates defect states near the VBM of pristine surface, which is governed by O derived states. The character of these band edges implies the likely dominant excitation may be from O to Bi. The Bi-Bi terminated surface is a strongly metallic in comparison with the other surfaces due to Bi derived bands. While going from surface (I) to (IV), it is clear that the BiOCl surface experiences a gradual reduction of band gaps and a shift of valance band edge. Note that localized energy levels excited from point defects cannot produce such drastic changes20,45, i.e. transforming from semiconducting to metallic states by surface reorganization, and the predicted evident band shifts are in accord with the measured energy blue-shifts shown in Figure 4(b). Combining DFT calculations as well as XPS measurements, the surface reorganization does provide very unique effects and extensively modify the electronic properties, particularly the surface states. Owing to the coexistence of localized metallic and insulating surface areas, the transport of photo-excited e--h+ pairs would differ from that of the white BiOCl, as well as the VO dominant BiOCl surface45 confirmed by the distinct observed PL spectra23,40. We make use of the band alignment in Figure 5d to address the different surface reorganized electronic structure, and on the basis of that we propose a new mechanism of the recombination of e--h+ pairs. When e--h+ pairs are created by light absorption, the transfer of electrons would be 18 ACS Paragon Plus Environment

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initialized from localized terminated pristine surface to Bi-O-Bi terminated surface; meanwhile the holes would be easily transferred from pristine surface to O-Bi terminated surface, both due to the extended defects band and the mixture of different local surfaces. This spontaneous carrier transfer would efficiently suppress the recombination of e--h+ pairs in the presence of various surface terminations, similar as the charge transfer process at semiconductor heterojunctions. It is worth noting that the non-mid gap states lead to this unique electronic structure. Remarkably, the surface reorganization results in a lower e--h+ recombination energy than the DFT calculated intrinsic bandgap barrier of ~2.3 eV. The recombination events occurred on the Bi-O-Bi terminated surface (II) and on the O-Bi terminated surface (III), after removal of surface Cl ions, are both about 2.1 eV in a lateral binding of various surface terminations circumstance. This recombination energy, which is close to the intrinsic emission energy, corresponds to the newly found emission peak feature I. In contrast, both prior theoretical findings and electronic structure of BiOCl containing VO and Bi point defects (VBi) visible in Figure S9 demonstrate that VO created mid-gap states that actually serve as recombination centers corresponding to emission energy of 1.5 eV contributing to the wide PL peak II, while VBi being capable of further extending the CBM that favors the electron jumping to lower energy level, which also strengthens and broadens the peak II. Note that the absence of intrinsic emission peaks at 360 nm (~3.45 eV) from the direct recombination of e--h+ from CBM to VBM in BiOCl does support the rapid carrier transfer between local Bi-O-Bi and O-Bi terminated surfaces.



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Table 1. The Born effective charge (Z*) of Bi atoms, surface atoms, and atoms beneath the surface atoms in the three surfaces (a-c) in Figure 5. The static dielectric constants are also computed and listed. Born effective charge (Z*) of Bi Atom type

Static dielectric constant ( )

Direction a(b)

(I)Bi beneath the Cl termination

5.44(5.44)

(I) averaged value for the rest Bi

5.37(5.37)

(II) Bi surface

5.02(5.02)

(II) Bi atoms beneath surface

5.37(5.37)

(III) Bi beneath the termination

6.79(6.79)

(III) averaged value for the rest Bi

5.41(5.41)

Direction a(b)

Direction c

4.57(4.57)

2.08

4.68(4.68)

1.59

27.62(27.62)

1.80

We also assess the role of polarization by computing the Born effective charge and static dielectric constants. In Table 1, significantly enhanced Born effective charges at Bi sites in all surface structures are displayed. Note that the calculated Z* of Bi in a(c) direction in BiOCl is similar to the value of 5.62 for BiOBr51. The most pronounced enhancement is observed in the Bi atoms beneath the O layer in the O-Bi terminated surface, where Z* is 6.79. The strong lattice polarization in the O-Bi terminated surface may lead to anomalously large static dielectric constants (εst).46,48 For O-Bi terminated surface, is increased by seven times with respect to the bare and Bi-O-Bi terminated surfaces. The O-Bi terminated surface with largely enhanced would efficiently reduce carrier scattering, trapping and promote their mobility on the surface.47,49 We emphasize that the low recombination rate induced by the enhanced results in free e-/h+ accumulation at the BiOCl surface and further promotes photocatalytic activity as experimentally measured. Conclusions In summary, we report that surface reorganization is responsible for increased UV- and visible-light-driven photocatalytic activity in defective BiOCl. The varied surface 20 ACS Paragon Plus Environment

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terminations lead to an up-shift in the valance band position and a novel transfer path for photoelectrons. Additionally, the predicted large dielectric constants from ab initio calculations suggests reduction of carriers scattering and trapping in the surface reorganized BiOCl, which ultimately promotes the efficient charge transport and stimulates the enhancement of the visible light photocatalytic activity. This work extensively elucidates the significant role of surface termination in defective BiOCl and paves the way to control the surface reorganization, thereby allowing for the tunability of the photocatalytic properties of the material. This study is also beneficial to the rational design and synthesis of defective photocatalyst that perform under visible light irradiation.

Associated Content Supporting Information The Supporting Information is available free of charge on the website. Comparison of XRD, detailed XPS, STEM image, BET of white and black BiOCl. SEM and EDS mapping of black BiOCl. Photocatalytic activity characterization by degradation MO. Total DOS and orbital resolved DOS of BiOCl with various surface terminations. Comparison of the DOS of BiOCl with VO and VBi. Author Information S.W. and W. S. are co-first authors. Corresponding author: Sujuan Wu, E-mail: [email protected] S.W. and J.S. synthesized the nanocatalysts and carried out sample physical characterizations, photocatalytic and electrochemical measurements. The theoretical calculations are performed 21 ACS Paragon Plus Environment


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by S.W. and P. R. C.K. Z.D.H. carried out the XPS characterizations. S.W., S.Z.Y and M. F. C. did the HAADF-STEM characterization. S. W., W. S., L. S and M. F. C. discussed the results and co-wrote the paper. All of the authors have revised the manuscript. Notes The authors declare no competing financial interest. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (No.51302329, 51501024), the Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2018jcyjAX0408, No. cstc2015jcyjA90004), the Fundamental Research Funds for the Central Universities (No. 2018CDQYCL0027) and China Scholarship Council (No. 201606055013). The authors gratefully acknowledge the valuable experimental support from Mr. Yuqi Zhang and Dr. Yuan Yuan and helpful editing form Mr. Arashdeep Singh Thind. Theoretical calculations (W.S., P.K.) were supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Z.D.H. gratefully acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1650044 and the Georgia Tech-ORNL Fellowship. A portion of this research was completed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User facility. S.Z.Y. and M.F.C gratefully acknowledge support from the 22 ACS Paragon Plus Environment

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U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

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