Article Cite This: Chem. Mater. 2018, 30, 5128−5136
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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,§,⊥ and Matthew F. Chisholm‡
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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, Tennessee 37830, United States § 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 S Supporting Information *
ABSTRACT: Introducing defects into 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 by 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 a great deal of attention in the past decade because of the various applications in photocatalytic hydrogen generation and removal of environmental pollution.1,2 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 products.3 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 electron−hole recombination, because 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 studied.4−7 The corresponding band gap model system of defect state is established to describe the impact of defects in semiconductors. Nonetheless, the defect-induced midgap states would produce a narrower band gap and limit the photo© 2018 American Chemical Society
catalytic activity by suppressing charge separation. Moreover, the surface plays an important role in the photocatalytic reaction because of its close relationship with the electron− hole recombination processes, and surface defects are more general in defective semiconductors and even spontaneous in some cases.8,9 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. Bismuth oxychloride (BiOCl) is a novel ternary oxide semiconductor with a layered structure composed of alternating [Bi 2O 2 ]2+ slabs and double halogen atom slabs10−12 and has wide applications in pigments,13,14 optics,15,16 thermocatalysis,17 and energy storage.18,19 The surface atomic structure, especially the surface terminations, has a significant effect on the photoresponse and determines the formation of the different kinds of defects. Most publications report that the surface termination is oxygen Received: April 18, 2018 Revised: July 15, 2018 Published: July 17, 2018 5128
DOI: 10.1021/acs.chemmater.8b01629 Chem. Mater. 2018, 30, 5128−5136
Article
Chemistry of Materials
Figure 1. (a) UV−vis absorbance spectra, (b) low-temperature EPR spectra, (c) Bi 4f XPS spectra, and (d) Raman spectra of black and white BiOCl.
(O), as the surface O atoms would be easily evacuated and create oxygen vacancies (VO) under ultraviolet (UV)-light irradiation, and the VO enable the absorption of visible light and relatively high photocatalytic reactivity.20−25 It has also been reported that disordered structures cover the surface of BiOCl along with VO,23 the area of which generally increases with the concentration of VO.33 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 exposed,26,27 the Bi-terminated defective BiOCl also presents higher activity under visible light.26 In contrast, Hao et al.11,28,29 recently reported that the surface termination is chlorine (Cl) atoms in BiOCl, and Chen et al.17 found that the surface Cl ions would be dissociated via reaction with photogenerated holes and spontaneously restore the BiOCl structure upon photocatalytic reaction. Ganose et al.30 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 with scanning transmission electron microscopy (STEM). Our results indicate that the reorganized surfaces include localized Bi−Bi-, Bi−O−Bi-, and O−Bi-terminated surfaces that are accompanied by the formation of a high 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 photoexcited 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
simultaneously extending the visible-light absorption and increasing the mobility of carriers.
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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 of Bi(NO3)3·5H2O was dissolved in 50 mL of a 0.1 M mannitol solution under vigorous ultrasonication for 10 min. Subsequently, 10 mL of an aqueous saturated NaCl solution was slowly injected into the mixture described above under magnetic stirring at 800 rpm, and a white precipitate was obtained and washed with deionized water until the pH reached 7. The as-obtained powder was finally dried at 100 °C for 4 h. To synthesize defective BiOCl, the as-prepared BiOCl powder was reduced via ethylene glycol etching. In a typical process, 200 mg of 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 h. Characterization. Powder X-ray diffraction (XRD) patterns were recorded by an Empryean X-ray diffractometer with monochromatic Cu Kα radiation (λ = 1.5418 Å). STEM images were collected using an FEI Titan 80-300 Super Twin electron microscope operated at 300 kV and a Nion Ultra STEM 200 instrument with subangstrom resolution at 200 kV. Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) images were collected by using a field emission JEOL-SEM instrument (JSM-7800F) equipped with an energy spectrometer. X-ray photoelectron spectra (XPS) were recorded with a Thermo K-Alpha XPS system with Al Kα radiation (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 ultraviolet−visible (UV−vis) spectrophotometer. Unpaired electron spins in specimens were characterized by electron paramagnetic resonance (EPR) spectroscopy (JEOL JES-FA200 EPR 5129
DOI: 10.1021/acs.chemmater.8b01629 Chem. Mater. 2018, 30, 5128−5136
Article
Chemistry of Materials
Figure 2. Scanning transmission electron microscopy (STEM) images of black BiOCl. HAADF-STEM images of (a) (001) facets and the atomic structure images of (b) the outer layer and (c) the grain interior. (d) Schematic diagram of the BiOCl crystal structure. HAADF-STEM images of (e) (100) facets and the atomic structure images of (f) the outer layer and (g) the grain interior.
for removing Cl ions39 and creating a large amount of VO resulting in the black feature.28 As illustrated in Figure 1a, the diffuse reflectance UV−vis spectra of white BiOCl exhibit a sharp absorption edge located at 365 nm, featured by an intrinsic bandgap 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 are characterized by the signals in lower-temperature EPR spectra.16,40 Here, the crystalline structure of black BiOCl is similar to that of white BiOCl (see the 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 persists 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 VO,40 suggesting a surface environment involving VO. Another two deconvoluted peaks for Bi with a lower binding energy at 156.8 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. In addition, 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.
spectrometer, 77 K, 9.062 GHz). Photoluminescence (PL) spectra were recorded with the HR800 instrument at room temperature with a 325 nm He−Cd laser as the exciting source. 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 cutoff 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 methylene orange (MO) was also used to demonstrate the photocatalytic degradation under a 500 W Xe lamp with a 420 nm cutoff filter. The concentration of RhB and MO was then measured using a UV−vis spectrophotometer. Computational Methods. Density functional theory (DFT) calculations were performed using the projected augmented wave (PAW)31 method as implemented in the Vienna ab initio simulation package (VASP).32 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 because of the van der Waals interactions in the layered structure.12,36,37 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 The defective BiOCl was prepared by ethylene glycol treatment, which has been reported as an efficient method 5130
DOI: 10.1021/acs.chemmater.8b01629 Chem. Mater. 2018, 30, 5128−5136
Article
Chemistry of Materials
Figure 3. (a) 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. The inset shows the corresponding degradation rate. (c) Recyclability of black BiOCl toward degradation of RhB. (d) Transient photocurrent response of black and white BiOCl.
surface of black BiOCl, confirming again the formation of large amounts of defects and the 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 below. Annular bright field (ABF) STEM of the (110) facets in Figure 2g further confirms the migration of Bi atoms and formation of tremendous defects. When a high concentration of defects exists in the layered structures, the outermost surface would thereby undergo reorganization 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 a bond length of 2.32 Å each, forming two square faces on opposite sides,5 as shown in Figure 2d. Therefore, the migration of Bi atoms should be accompanied by breakage 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 atom slabs, the (001) facets have a 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 a metallic state that stayed on the surface, which is also confirmed by in situ TEM studies in
Raman spectroscopy further confirms the breakage of the crystal structure in black BiOCl (Figure 1d). The predominant bulk signals at 58.3, 143.3, and 196.9 cm−1 correspond to the A1g internal Bi−Cl stretching, the Eg external stretching, and the Eg 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 symmetry,41 confirming the partial reorganization of chemical bonds in black BiOCl. Taken collectively, a large amount of defects have been introduced into black BiOCl by straightforward treatment with ethylene glycol. Atomic-resolution STEM images were collected to characterize the microstructure of black BiOCl. The nanosheet structure with a thickness of
