Research Article www.acsami.org
Hydroxyl-Dependent Evolution of Oxygen Vacancies Enables the Regeneration of BiOCl Photocatalyst Sujuan Wu,*,†,# Jiawei Xiong,† Jianguo Sun,† Zachary D. Hood,‡,§ Wen Zeng,† Zhenzhong Yang,∥ Lin Gu,∥ Xixiang Zhang,⊥ and Shi-Ze Yang#
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†
Electron Microscopy Center of Chongqing University, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China ‡ School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Center for Nanophase Materials Sciences and #Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ⊥ Division of Physical Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia S Supporting Information *
ABSTRACT: Photoinduced oxygen vacancies (OVs) are widely investigated as a vital point defect in wide-band-gap semiconductors. Still, the formation mechanism of OVs remains unclear in various materials. To elucidate the formation mechanism of photoinduced OVs in bismuth oxychloride (BiOCl), we synthesized two surface hydroxyl discrete samples in light of the discovery of the significant variance of hydroxyl groups before and after UV light exposure. It is noted that OVs can be obtained easily after UV light irradiation in the sample with surface hydroxyl groups, while variable changes were observed in samples without surface hydroxyls. Density functional theory (DFT) calculations reveal that the binding energy of Bi−O is drastically influenced by surficial hydroxyl groups, which is intensely correlated to the formation of photoinduced OVs. Moreover, DFT calculations reveal that the adsorbed water molecules are energetically favored to dissociate into separate hydroxyl groups at the OV sites via proton transfer to a neighboring bridging oxygen atom, forming two bridging hydroxyl groups per initial oxygen vacancy. This result is consistent with the experimental observation that the disappearance of photoinduced OVs and the recovery of hydroxyl groups on the surface of BiOCl after exposed to a H2O(g)rich atmosphere, and finally enables the regeneration of BiOCl photocatalyst. Here, we introduce new insights that the evolution of photoinduced OVs is dependent on surface hydroxyl groups, which will lead to the regeneration of active sites in semiconductors. This work is useful for controllable designs of defective semiconductors for applications in photocatalysis and photovoltaics. KEYWORDS: BiOCl, regeneration, hydroxyl, oxygen vacancy, photocatalysis
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modulated by their defect disorder.11,12 Among all the defects, oxygen vacancies (OVs) are among the most prevalent defects in many wide-band-gap oxide semiconductors.11 It has been reported that OVs can behave as important adsorption and active sites for heterogeneous catalysis, which are able to strongly influence the reactivity of oxide semiconductors.13 Moreover, the electronic structure, charge transport, and surface properties are strongly related to the OVs, presenting a profound effect on the resulting photocatalytic properties.2,3,11 Thus, controlling the content of OVs in wide-band-
INTRODUCTION Wide-band-gap semiconductors have attracted extensive attention because of the powerful oxidation−reduction abilities of photoinduced electron−hole pairs in solar energy conversion systems.1−3 Nonetheless, the ineffective utilization of visible light and low quantum efficiency in photocatalytic reactions ultimately limits their applications in different photosystems. Considerable strategies have been proposed to narrow the band gap and increase the lifetime of photogenerated electron−hole pairs,2,3 such as chemical doping,4,5 coupling two or more semiconductors,6,7 photosensitization,6,8 noble metal deposition,6,9 hybridization with carbon materials,10 etc. In particular, recent studies have revealed that light absorption and photocatalytic reactivity of different semiconductors can be © 2017 American Chemical Society
Received: February 5, 2017 Accepted: May 2, 2017 Published: May 2, 2017 16620
DOI: 10.1021/acsami.7b01701 ACS Appl. Mater. Interfaces 2017, 9, 16620−16626
Research Article
ACS Applied Materials & Interfaces
under ambient atmosphere to obtain the pristine BiOCl-OH powder. BiOCl was crystallized from BiOCl-OH powder by annealing at 400 °C for 12 h under ambient conditions in a box furnace. The samples were irradiated under UV light (λ = 190 nm) for 15 min to study the evolution of the OVs. The recovery procedure was completed with oxygen gas (99.99%) in a tubular furnace at room temperature for 24 h. The recovery by H2O vapor was completed in H2O(g)-rich atmosphere with humidity of ∼90%−100% at room temperature for 2 h. To exclude the effect of the O2, we performed the recovery process in a vacuum chamber and pumped to 1 × 10−1 Pa using a remote controllable humidifier as the source of H2O(g). The humidity was monitored by the programmable hygrometer (GM1360A USB). Photocatalytic Activity of BiOCl-OH and BiOCl. The UV light driven photocatalytic activity was evaluated by photocatalytic degradation of 20 mg/L rhodamine B (RhB) with a 300 W mercury lamp (365 nm). In a typical reaction, 50 mg of photocatalyst was dispersed into a solution of 100 mL of distilled H2O and 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. The concentration of RhB was then measured to monitor the photocatalytic degradation of RhB by UV−visible (UV−vis) spectrophotometer. Characterization Methods. X-ray diffraction (XRD) patterns of each sample were collected using a PANalytical Empyrean diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). UV− vis absorption spectra were performed in the range of 220−800 nm with a Shimadzu UV-2100 spectrometer (Tokyo, Japan) with BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) spectra were measured on the Thermo ESCALAB 250Xi system with Al Kα (hυ = 1486.6 eV) as the excitation source. The spectra were referenced to adventitious C1s at a binding energy (BE) of 284.5 eV. Electron paramagnetic resonance (EPR) spectra were collected on a JEOL JESFA200 EPR spectrometer (8.750−9.650 GHz). Fourier transform infrared (FT-IR) spectra were collected from 4000 to 400 cm−1 with a NICOLET FT-IR spectrometer. As-synthesized samples were embedded in KBr tablets at room temperature for all FT-IR measurements. Scanning transmission electron microscopy (STEM) images were collected using a FEI Titan 80−300 Super Twin electron microscope operated at 300 kV. All samples were drop cast on carboncoated copper grids for STEM analysis. Computational Methods. Density function theory (DFT) calculations were performed using the projected augmented wave method (PAW)19,20 as implemented in Vienna ab initio simulation package (VASP).21,22 For the structural relaxation and electronic structure calculations generalized gradient approximation (GGA) method is used with Perdew−Burke−Ernzerhof (PBE) exchangecorrelation functional.23 The wave-plane cutoff energy was 300 eV and a regular Monkhorst−Pack grid of 3 × 3 × 1 k points were used in all calculations. The fluctuation of atomic positions were permitted when the energy and force were less than 10−4 eV and 0.05 eV/Å, respectively. In the case of atomic slabs, a vacuum space of 15 Å was chosen. The adsorption and dissociation energy between target molecules (water or oxygen) and the (001) surface of BiOCl were defined as ΔEad(H2O) = E(surface-H2O) − E(surface) − E(H2O), ΔEdis(H2O) = E(surface−OH) − ΔEad(H2O), respectively. The E(surface−OH), E(surface-H2O), E(surface), and E(H2O) delegates were calculated for the (001) surface of BiOCl and the H2O system.
gap semiconductors is expected to play an important role in the development of different photocatalytic systems. Bismuth oxychloride (BiOCl), with a layered structure, consists of [Bi2O2]2+ layers and Cl layers along the c-axis, which has attracted a great deal of interest for photocatalytic applications, especially in defect-dependent investigations.14−17 Ye et al. have recently reported the generation of OVs by ultraviolet (UV) light irradiation, which enhanced the visible light photocatalytic activity by extending the visible light absorption of the semiconductor.15,16 It was also demonstrated that defective BiOCl (with OVs) can effectively convert N2 to NH3 and CO2 to CH4,2,3 while the OVs could be refreshed during the process of photocatalysis under UV light irradiation.17 Nevertheless, the formation mechanism of photoinduced OVs in BiOCl is still unclear. In previous reports, the generation of OVs is ascribed to the long bond length and low bond energy of the Bi−O bond where OVs can be repaired after thermal annealing under ambient conditions.15 Still, this explanation is not comprehensive since the OVs can be refilled in deionized water during photocatalysis processes,17 and the number of OVs in BiOCl would be saturated with sustained exposure to the UV light.18 Thus, it is critical to understand the formation mechanism of photoinduced OVs in order to further boost the photoactivity of semiconductors within the visible light region. Herein, BiOCl with and without surface hydroxyl groups have been designed to investigate the evolution of UV lightinduced oxygen defects. A black color change, which is attributed to the existence of OVs, occurs in the BiOCl with surface hydroxyl groups after UV light exposure, while BiOCl maintains a white color when the surface is free of hydroxyl groups. Furthermore, we demonstrate that the color change in BiOCl is reversible, where the OVs disappear and hydroxyl groups return to the surface when exposing black BiOCl in a H2O(g)-rich atmosphere at room temperature (instead of an oxygen-rich environment). The recovery of the binding energy of the Bi reveals that hydroxyl groups were incorporated into the OVs, suggesting that the Bi coordination shell underneath the vacancy is refilled, resulting in the regeneration of BiOCl. Density functional theory (DFT) calculations demonstrate that the bond length of Bi−OH is much longer than the Bi−O bond in the bismuth oxychloride system, which is energetically favorable to break and form OVs under UV light exposure. Photoinduced OVs are introduced by surface hydroxyl desorption from BiOCl, which can then be repaired under water vapor. This understanding corroborates that the hydroxyl takes a predominant function in the reversible evolution of OVs produced from UV light exposure in BiOCl and simultaneously enables the regeneration of BiOCl photocatalyst.
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EXPERIMENTAL SECTION
Preparation of BiOCl and BiOCl-OH. All reagents included commercially available analytical grade chemicals and were used as received without any further purification. In a typical synthesis of BiOCl-OH, 100 mmol Bi2O3 (≥99.0%, Chengdu Kelong Chemical Reagent Factory) were placed into a round-bottomed flask and 20 mL of concentrated hydrochloric acid (35.5 wt %, Guangdong Deshu Chemical Co., Ltd.) was added with continuous stirring until a transparent solution emerged. This mixture was then transferred to a hot plate to evaporate excess hydrochloric acid acquire BiCl3 powder. Next, 200 mL of deionized water was poured to the round-bottomed flask for hydrolyzing BiCl3 to BiOCl. After 0.5 h, the resulting precipitates were collected after filtration and washed with deionized water until the filtrate reached a pH 7, followed by drying at 100 °C
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RESULTS AND DISCUSSION After UV irradiation, the color of BiOCl changed from white to black. The O1s X-ray photoelectron spectroscopy (XPS) spectra of BiOCl, as shown in Figure S1, demonstrate that after UV irradiation, the peak at 532.0 eV disappears and the peak at 530.0 eV is maintained. The peak at 530.0 eV corresponds to lattice oxygen in the crystal structure of BiOCl, whereas the peak at 532.0 eV corresponds to surface hydroxyl groups, which may originate as a result of a hydrolysis process 16621
DOI: 10.1021/acsami.7b01701 ACS Appl. Mater. Interfaces 2017, 9, 16620−16626
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Normalized FT-IR spectra in the range 400−2000 cm−1; the inset is the relative band intensity of I2/I1 and I3/I1. (b) XRD patterns of pristine BiOCl (black) and BiOCl-OH (red).
Figure 2. (a) UV−vis absorption and (b) EPR spectra for BiOCl (black) and BiOCl-OH (red) after UV light exposure.
from surface water or moisture in the air.24 As such, it was suggested that UV light irradiation causes the disappearance of surface hydroxyl groups, and the surface hydroxyl groups play a vital role during the irradiation process. Still, it is unclear as to the role the surface hydroxyl groups play in the formation of OVs, which, in turn, induce color change in BiOCl. The BiOCl including surface hydroxyl groups (referred to as BiOCl-OH) and excluding surface hydroxyl groups (referred to as BiOCl) were synthesized by a wet-chemistry method to fathom the relationship between the photoinduced formation of OVs and surface hydroxyl groups. Fourier transform infrared spectroscopy (FT-IR) allowed for direct detection of surface hydroyl groups in BiOCl (Figure 1a). Three dominant vibration bands at 510, 1634, 3440 were observed in both spectra, which are marked by the corresponding absorption intensities as I1, I2, and I3. The absorption peak at 510 cm−1 is assigned to the Bi−O stretching vibration in BiOCl, while the bands at 1634 and 3440 cm−1 correspond to the stretching vibration modes of acutely adsorbed water molecules and surface hydroxyl groups, respectively.25,26 Previously, it was reported that annealing above 490 K leads to the disappearance of hydroxyl species in BiOCl.27 However, owing to atmospheric moisture, the weak influence of surface hydroxyls in the surface water molecules on BiOCl cannot be prevented. When comparing absorption intensities for BiOCl and BiOCl-OH, the ratio of I3/I1 and I2/I1 are greater for BiOCl-OH, indicating that BiOCl-OH holds a higher concentration of surface hydroxyls than BiOCl. Figure 1b presents the XRD patterns of each sample, where all diffraction peaks can be indexed to the tetragonal phase of bismuth oxychloride (cell constants: a = 3.89 Å, c = 7.37 Å; JCPDS card No. 85−0861), without any
impurity peaks. From XRD analysis, there was no observed change in the structure or crystal orientation for BiOCl due to the presence of an increased concentration of surface hydroxyls. Even the morphology and composition has no obvious change, as shown by SEM, EDS (Figure S2) and XPS (Figure S3) analyses. After UV irradiation for 30 min, BiOCl-OH exhibits a significant visible light absorption (Figure 2a) and its color converts from white to black. On the other hand, BiOCl maintains a white color and no obvious absorption in the visible light region (Figure 2a). Previously, the extended visible-light absorbance in BiOCl was ascribed to the presence of surface OVs.17,28 In the current study, the different color change between BiOCl and BiOCl-OH may indicate that a different content of OVs appeared after serious UV-light exposure. Electron paramagnetic resonance (EPR) spectroscopy was used to confirm the existence of OVs in BiOCl and BiOCl-OH, as shown in Figure 2b. EPR spectra were collected from 318 to 340 mT, which allowed for direct detection of OVs in BiOCl and BiOCl-OH; the EPR peak intensities were found to be 269 and 1155 for BiOCl and BiOCl-OH, respectively. There was no observed signal before UV-light irradiation (Figure S4), which demonstrates that UV irradiation is critical in triggering OVs and the presence of surface hydroxyls increases the concentration of OVs after UV light exposure in BiOCl. Still, our results raise the following question: If the presence of hydroxyls could sway the formation of UV light-induced OVs in BiOCl, is it then possible that the surface hydroxyl groups could repair the OVs? To explore this question, we performed experiments to evaluate whether the formation of OVs was reversible and whether hydroxyls influenced the reverse process 16622
DOI: 10.1021/acsami.7b01701 ACS Appl. Mater. Interfaces 2017, 9, 16620−16626
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ACS Applied Materials & Interfaces
Figure 3. (a) UV−vis absorption and (b) EPR spectra after oxygen vacancy repair experiments for BiOCl-OH after UV light and O2 exposure (blue) and H2O(g) (orange).
Figure 4. (a) ABF-STEM images of UV-BiOCl-OH samples and FFT patterns with some example OVs noted by red circles (b) XPS spectra of Bi4f in BiOCl-OH, UV-light-irradiated BiOCl-OH, and water-repaired BiOCl.
for the formation of OVs. As displayed in Figure S4, we observed no apparent change in the color of the photoinduced black BiOCl after 24 h in an oxygen-rich atmosphere, but the black color faded drastically when exposed to a humid atmosphere, where oxygen was exchanged with water molecules. To accurately substantiate that the return of the color denotes the reverse process for the formation of OVs, we again utilized UV−vis and EPR spectroscopy. As displayed in Figure 3a, the UV−vis absorption of BiOCl-OH exposed under UV light (referred to as BiOCl-OH-UV) and after water exposure is appreciably lower than that of BiOCl−OH-UV exposed to the oxygen-rich environment. Furthermore, the EPR signal was found to be 116 and 591 for BiOCl-OH exposed to water and oxygen, respectively (Figure 3b). Obviously, the intensity of the signal for BiOCl-OH-UV after water exposure is completely different than the BiOCl-OH-UV after oxygen exposure. These data demonstrate that oxygen alone could not induce the recovery of OVs at ambient temperatures, but on the other hand, the water molecule was found to restore OVs in BiOCl. We further used FT-IR to reveal the difference in hydroxyl group concentration in the BiOCl-OH-UV after water exposure (Figure S5), where the surface hydroxyls were also recovered. Collectively, these studies demonstrate that the surface hydroxyl groups dominate the reversible evolution of UV light-induced OVs in BiOCl.
The previous results inspired us to gain deeper insights into the hydroxyl dependent generation and the recovery of OVs. The photoinduced OVs in the UV irradiated BiOCl was directly visualized using annular bright-field STEM (ABFSTEM) imaging (Figure 4a). On the (001) facets of BiOCl, the contrast of Bi atoms and O atoms can be clearly distinguished based on the crystal structure, in which the dark atoms are assigned to Bi atoms and the slightly bright contrast to crossbridging oxygen atoms.4,5,17 However, several additional brighter dots appear at the position of O atoms, some of which are denoted as red circles in Figure 4a, which are ascribed to OVs. The distribution of these additional brighter dots is similar to OVs observed in rutile TiO2 (110) facets by the atomically resolved scanning tunneling microscopy (STM).27,29 It is reported that the OVs acted as an active site responsible for the dissociation of water molecules adsorbed on TiO2 via proton transfer to a neighboring bridging oxygen atom, creating two bridging hydroxyl groups per initial oxygen vacancy,27,29 a reason similar to the regeneration of BiOCl-OH when exposed to the humid atmosphere. Similar as the TiO2, the change in the valence state of the Bi ions (Bi (3−x)+) could be corroborated with the formation of OVs. X-ray photoelectron spectroscopy (XPS) analysis revealed that two additional peaks with lower binding energies at 163.9 and 158.5 eV appeared in the Bi4f high resolution spectra of 16623
DOI: 10.1021/acsami.7b01701 ACS Appl. Mater. Interfaces 2017, 9, 16620−16626
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ACS Applied Materials & Interfaces
Figure 5. (a) Photocatalytic degradation of RhB under simulated solar light. (b) FT-IR spectra of BiOCl-OH with RhB absorption (pink) and after degradation (green).
Figure 6. (a) Schematic view of the Bi−O bond length and representation of charge density distribution along the O-terminated (001) surface of BiOCl: clean surface (left) and hydroxyl-rich surface (right). (b) Structure and total energy variation of the reactions involving H2O(g) on the oxygen defective (001) surface of BiOCl.
BiOCl-OH-UV, which arose from Bi(3−x)+.17,30 As the lower charged Bi ions corresponded to the oxygen atoms loss, the OVs concentrations in BiOCl-OH-UV were proportional to the percentages of Bi(3−x)+. These XPS results demonstrated that the concentration of OVs is decreased by water exposure, consistent with the EPR results. These results also suggest the recovery of OVs and valence state of Bi under humidity atmosphere. It is consistent with the report by Schaub et al.27that hydroxyl groups were incorporated into OVs, enabling a coordination shell of the Bi underneath the vacancy to be refilled, resulting in the regeneration of BiOCl. This electronic effect is very similar to the electronic effect found in the adsorption of various adsorbates on metal surface defects or rutile TiO2 (110)27,31 The photocatalytic activity has also been evaluated by degradation of RhB dye under UV irradiation (Figure 5a). Compared with pure BiOCl, the hydroxyl-rich BiOCl possesses higher activity, which was attributed to the presence of UV light induced OVs. An et al. reported18 that BiOCl, similar to the preparation method for BiOCl-OH in the current work, could achieve visible-light-driven photodegradation of isopropanol with a 300W Xe lamp, and the white color of the BiOCl photocatalyst turned gray after photocatalysis. Nonetheless, the pure BiOCl without vacancies17,28 shows the negligible photocatalytic activity under visible light. This difference can be thereby attributed to the UV light irradiation, which causes the formation of OVs in BiOCl-OH. The FT-IR spectra of BiOCl-OH with RhB exposure and after RhB degradation were collected to determine the existence of hydroxyl groups, as
shown in Figure 5b. The higher concentration of peaks corresponding to hydroxyl groups appeared in the BiOCl-OH after RhB degradation than in the adsorption one, indicating that the regeneration of BiOCl via hydroxyl groups during photocatalytic degradation of RhB. To better understand the hydroxyl-dependent evolution of OVs in BiOCl, we investigated the structure, energy, and charge-density distribution of BiOCl and BiOCl-OH by DFT calculations. Models display that the Bi−O bond length is 2.196 Å in the perfect O-terminated (001) surface of BiOCl, but these bond lengths became 2.293 and 2.315 Å on both sides of the optimized hydroxylated surface (Figure 6a and Figure S6). Moreover, a strong overlapping of electrons between Bi and O can be seen in the optimized (001) crystal plane, indicating a strong Bi−O bond. By contrast, the overlapping of Bi and O was greatly suppressed at the hydroxyl group sites of BiOCl, which suggests that the energy of the Bi−O bond is much lower in sites adjacent to hydroxyl groups when considering the binding energy is intensely correlated with the interaction of electrons.32 This increased bond length and reduced bonding energy on the hydroxyl-rich surface of BiOCl illustrates that the BiOCl-OH can be more easily excited by photons in the UV− visible region, leading to the formation of OVs on the surface of BiOCl.33,34 The total energy evolution of OVs by water molecules is presented in Figure 6b. The optimized system with water adsorption at the oxygen vacancy site presents a much lower energy state with an overall 0.919 eV energy reduction. Moreover, the total energy of the system was further decreased 16624
DOI: 10.1021/acsami.7b01701 ACS Appl. Mater. Interfaces 2017, 9, 16620−16626
Research Article
ACS Applied Materials & Interfaces by 0.839 eV when the water molecule dissociated into two adjacent hydroxyl groups near the oxygen vacancy. These calculations support the preceding interpretation in that the photoinduced OVs in BiOCl are energetically favorable over the dissociation of water into hydroxyl groups when surface water molecules are present on BiOCl, enabling the regeneration of BiOCl. Additionally, these calculations suggest that water prefers to dissociate via proton transfer to a nearby oxygen atom on the BiOCl surface, forming two bridging hydroxyl groups per initial vacancy along the (001) surface of BiOCl (Figure 6b and Figure S7), in agreement with ref 27. Both the experimental and theoretical results suggest that the evolution of photoinduced OVs is strongly dependent on the hydroxyl groups, which are generated by the dissociation of adsorbed water molecules onto the active OVs. This explanation also clarifies why hydroxyls exist in pristine BiOCl.
Xixiang Zhang: 0000-0002-3478-6414 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (51302329) and the Fundamental Research Funds for the Central Universities (106112015CDJXY130010). We gratefully acknowledge the help from Mr. Yuqi Zhang and Yuan Yuan for helpful discussions. The preparation of this manuscript was also supported by the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. ZDH gratefully acknowledges a graduate fellowship from the National Science Foundation under Grant DGE-1148903 and the Georgia TechORNL Fellowship. Lin Gu acknowledges the National Program on Key Basic Research Project (2014CB921002), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB07030200), and National Natural Science Foundation of China (51522212, 51421002).
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CONCLUSION Overall, we clarify that the hydroxyl group plays an essential role in the evolution process of photoinduced oxygen vacancies (OVs) in bismuth oxychloride (BiOCl) (Figure S8). The hydroxyl-rich BiOCl preferentially forms OVs and converts to a black color under UV light irradiation. The photoinduced OVs, which act as active surface sites responsible for the dissociation of adsorbed water molecules, can be coordinated to hydroxyl groups, leading to the regeneration of BiOCl under a water-rich atmosphere. First-principles calculations suggest that a longer Bi−OH bond with a weak bonding energy leads to the formation of OVs under UV light irradiation. Moreover, DFT calculations suggest that the dissociation of water molecules via transfer of proton to a nearby oxygen atom is energetically favored, forming two bridging hydroxyl groups per oxygen vacancy along the (001) axis, which then participate in the repair of OVs and recovery of black BiOCl. Collectively, these results indicate that surface hydroxyls are critical to changing the chemistry of BiOCl. It is expected that our understanding of hydroxyl-dependent evolution of photoinduced OVs within different semiconductors will help elevate the activity of photoresponsive materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01701. High-resolution XPS spectra of the O1s peak of fresh BiOCl and UV-irradiated black BiOCl; SEM images of the original BiOCl, UV-irradiated samples, and the UVirradiated samples exposed to oxygen and water vapor; FT-IR spectra of pristine BiOCl-OH and black BiOCl recovered by H2O in the range 400−2000 cm−1; simulated structure and calculated surface energy of BiOCl with OH adsorption and H2O dissociation, respectively; proposed mechanism of hydroxyl-dependent reversible evolution of OVs in BiOCl (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Sujuan Wu: 0000-0003-4390-2082 Lin Gu: 0000-0002-7504-031X 16625
DOI: 10.1021/acsami.7b01701 ACS Appl. Mater. Interfaces 2017, 9, 16620−16626
Research Article
ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.7b01701 ACS Appl. Mater. Interfaces 2017, 9, 16620−16626
