Iodine-Deficient BiOI Nanosheets with Lowered Valence Band

Apr 3, 2019 - The level of I 5p states and thus the valence band maximum shift .... The corresponding high-resolution TEM image (Figure 2d) suggests t...
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Iodine-Deficient BiOI Nanosheets with Lowered Valence Band Maximum To Enable Visible Light Photocatalytic Activity Xuewen Wang,† Chengxi Zhou,† Lichang Yin,‡ Rongbin Zhang,*,† and Gang Liu*,‡,§

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by QUEEN MARY UNIV OF LONDON on 04/03/19. For personal use only.

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Key Laboratory of Jiangxi Province for Environment and Energy Catalysis, the College of Chemistry, Nanchang University, 999# Xuefu Road, Nanchang 330031, P. R. China ‡ Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72# Wenhua Road, Shenyang 110016, P. R. China § School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang 110016, China S Supporting Information *

ABSTRACT: Bismuth oxyhalides with layered structures have emerged as an important class of photocatalysts but usually suffer from low activity largely due to their unfavorable band structures. Although point defects as typical electronic structure modifiers have been actively used to modify electronic structures of photocatalysts for high photocatalytic activity, the underlying role of halogen vacancies in improving activity of bismuth oxyhalides has been overlooked. In this study, we demonstrate the substantial role of iodine vacancies in enabling photocatalytic activity of BiOI as a model oxyhalide photocatalyst. It is found that iodine vacancies cause a 0.23 eV downward shift of the valence band maximum of BiOI but not much change in the bandgap. Such band structure modification leads to the formation of photogenerated holes with a stronger oxidative ability, which is favorable for photocatalysis with the holes induced half reaction as a ratedetermining step. The results obtained might provide an important implication in designing highly efficient oxyhalide-based photocatalysts by controlling halogen vacancies. KEYWORDS: Iodine vacancy, BiOI, Photocatalyst, Visible light

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INTRODUCTION Photocatalysis is considered as a potential solar conversion technology.1−4 Among various photocatalytic materials, layered materials with a short charge carrier transport length5−7 have obvious advantages in enabling high activity. Bismuth oxyhalide materials,8,9 including BiOCl, BiOBr, and BiOI, which are typical layered materials, have been widely used as photocatalysts. However, the large bandgap of BiOCl and BiOBr intrinsically limits the utilization of visible light.10,11 In contrast, BiOI with a bandgap of around 2 eV has the ability of fully capturing visible light. Moreover, the internal electric field in BiOI as a result of the strong bonding between [Bi2O2]2+ and I− boosts the separation of photoexcited carriers.12 Despite these favorable features, BiOI with a high valence band maximum shows a very limited photocatalytic activity due to the weak oxidative ability of photogenerated holes in its high valence band maximum.13 Thus, modifying the band structure of BiOI to improve the oxidative capability of photogenerated holes is desirable yet challenging. Various methods were developed to improve BiOI photocatalytic performance by introducing heteroatoms or defects, loading noble metal particles, and coupling with other semiconductors.14−17 Among these methods, introducing © XXXX American Chemical Society

heteroatoms and defects is an effective strategy of modifying electronic structures that intrinsically control the absorption range and/or the redox ability of the photogenerated charge carriers.18−20 For anion deficient photocatalysts, most cases focused on extending the absorption range, and impressive progress has been achieved.21−23 However, the possible modulation of band edges by anion vacancies, which is significant for many visible light responsive photocatalysts with unfavorable band edges, has been unfortunately overlooked. Considering the predominance of oxygen and iodine states in the BiOI valence band,24 it will be feasible to modulate the position of the valence band by introducing oxygen or iodine vacancies. The level of I 5p states and thus the valence band maximum shift upward with the percentage increase of iodine atoms in BiOI in the early report.25 Therefore, it is anticipated that the formation of iodine vacancies might reversely cause the downward shift of the valence band maximum of BiOI. Moreover, compared to other bismuth oxyhalides, BiOI tends to be the deficiency of iodine atoms because of the easy loss of Received: January 27, 2019 Revised: March 21, 2019

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DOI: 10.1021/acssuschemeng.9b00548 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering iodine upon to the thermal treatment.26 This feature makes it possible to produce iodine-deficient BiOI with a modulated valence band maximum that has not been achieved so far, to our best knowledge. In this study, abundant iodine vacancies were formed in BiOI nanosheets by heating the sample in air. The resultant iodine-deficient BiOI (BiOI1−x) has a lowered valence band and thus an enhanced oxidative ability of photoexcited holes. Compared with pristine BiOI, iodine-deficient BiOI1−x nanosheets as photocatalysts show considerably improved visible light photocatalytic performance in decomposing both methyl orange (MO) and phenol. Furthermore, the BiOI1−x photoanode has a superior activity in photoelectrochemical water oxidation. The results confirm this great promise of engineering iodine deficiency in boosting the activity of BiOI. The strategy developed here might be applicable to modify other photocatalysts.

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layers, all other atoms were relaxed in geometry optimization (energy convergence criterion was 10−6 eV). Photocatalytic Hydrogen Measurements. The visible light photocatalytic activity was measured via monitoring the decomposition of MO and phenol. In this measurement process, 50 mg of BiOI was dispersed to 100 mL of substrate molecule solution (phenol, 0.02 mmol L−1; MO, 0.04 mmol L−1). An adsorption equilibrium was obtained via 0.5 h of stirring in dark. Five milliliters of solution was sampled every 10 min for MO or 40 min for phenol to determine the concentration of MO or phenol. Photocatalysts were removed by centrifugation. The concentration of the remaining solution was tested using a UV−vis spectrophotometer (HITACHI U-4100). Photoelectrochemical performance was measured by an electrochemical workstation. BiOI1−x (or BiOI) nanosheets were coated on FTO glass as a working electrode. The counter electrode, reference electrode, and electrolyte were Pt foil and the Ag/AgCl electrode, 0.1 mol L−1 aqueous Na2SO4 solution, respectively. The light source was a solar simulator of 300 W. UV light was removed using a long-pass filter (λ ≥ 420 nm). The photoanode area was 1 cm2.

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RESULTS AND DISCUSSION The thermal stability of the BiOI nanosheets was explored using the TG curve (Figure S1). The slow weight-loss process occurred between 200 and 400 °C as a result of the light-loss of iodine atoms in the sheets without changing its phase, indicating that the content of iodine vacancies can be controlled via adjusting treatment temperature and time.37,38 The rapid weight-loss process occurred from 400 to 500 °C. This process was caused by the heavy loss of the iodine atoms and accompanied the phase transformation. These results suggest the feasibility of controlling the amount of iodine vacancies in the BiOI nanosheets by heating the nanosheets in air at different temperatures. The amount of iodine vacancies in the BiOI nanosheets was controlled by the thermal treatment in air. Figure 1 shows the

EXPERIMENTAL SECTION

Preparation of Iodine-Deficient BiOI1−x Nanosheets. Iodinedeficient BiOI1−x nanosheets were synthesized via the following method. First, the BiOI nanosheets were synthesized by a modified hydrothermal route.27 Bismuth nitrate (1 mmol) was dissolved in 38 mL of absolute ethanol. Then, 10 mL of solution containing 1 mmol NaI was dripped into the bismuth nitrate solution under stirring for 30 min. After that, the above solution was poured into an autoclave (100 mL) and treated for 12 h at 130 °C in an oven. The synthesized BiOI nanosheets were washed several times using ethanol and water. Products were dried at 80 °C. Second, iodine vacancies were introduced to produce BiOI1−x sheets by heating the sample at different temperatures (200, 250, 300, 350, 400, 450, and 500 °C) for 3 h in air. The ramping rate was five degrees per minute. Characterization and Computational Method. The phase compositions were analyzed by X-ray diffraction (XRD) equipment. A physical adsorbent (ASAP2010M) was used to measure BET surface areas. Scanning electron microscopy (SEM) images were recorded on a JSM 6701F. Energy dispersive X-ray (EDX) spectroscopy in Nova SEM 200 and JSM 6701F were used to analyze atomic ratios of samples. Transmission electron microscope (TEM) images were obtained using a JEOL-2100. An UV−vis spectrophotometer (U4100) was used to test UV−vis absorption spectra. The thermogravimetric (TG) curve was obtained using a TGA/DSC. A monochromatic Al Kα X-ray photoelectron spectroscopy (XPS) source (Thermo Escalab 250) was used to analyze chemical states. The binding energies were corrected by C 1s (284.6 eV). An electron spin resonance (ESR) spectrometer (JEOL-FA200) was used to obtain ESR spectra. The density functional theory (DFT) was calculated using the VASP28−31 code with the PBE functional32,33 for the exchangecorrelation term. The electron−ion interactions were described using PAW,34,35 and the cutoff energy of plane-wave was 400 eV. The 2s and 2p of O; 5s and 5p of I; and 5d, 6s, and 6p of Bi were taken as valence states. The calculated lattice constants of pristine BiOI are a = b = 4.011 Å snf c = 9.229 Å, which are very close to the experimental data (a = b = 3.985 Å and c = 9.129 Å)36 due to the well-known overestimation of lattice constants by PBE functional. A 2 × 2 × 1 supercell of tetrahedral BiOI was constructed to simulate the structures of BiOI1−x. For simplicity, we only consider two vacancy concentrations (Bi8O8I6 and Bi8O8I4) in this work. The Γ-centered 4 × 4 × 4 and 8 × 8 × 4 k-point meshes were used for sampling the Brillouin zones of pristine BiOI (Bi2O2I2) and defective BiOI with I vacancies (Bi8O8I6 and Bi8O8I4), respectively. Considering the layered structure of bulk BiOI, after full relaxation of bulk BiOI, Bi8O8I6, and Bi8O8I4, three slab models consisting four layers of Bi8O8I8, Bi8O8I6, and Bi8O8I4 atomic layers were constructed to calculate the electronic work functions. An 8 × 8 × 1 k-point mesh for all slab models was used as the Brillouin zone. Except for most bottom Bi−O−I atomic

Figure 1. XRD patterns of (a) the pristine BiOI (a1), BiOI treated at 300 (a2), and 500 °C (a3). (b) XRD patterns of BiOI treated at 200 (b1), 350 (b2), 400 (b3), and 450 °C (b4).

XRD patterns of the pristine BiOI and iodine-deficient BiOI1−x treated at different temperatures. When the thermal treatment temperature was below 400 °C, all the diffraction peaks of BiOI1−x were well indexed to the tetragonal phase BiOI. This result implied the good retention of the crystal structure of BiOI during the thermal treatment process. With increasing the treatment temperature, the diffraction peak at 30° of BiOI1−x gradually shifted toward the small angle. This result indicated that the lattice constant increases due to the absence of iodine atoms.39 However, when the temperature reached 450 °C, a new diffraction peak at 28° was formed and assigned to the Bi5O7I phase (JCPDS: No. 40-0548). This indicated the phase transformation from BiOI to Bi5O7I upon the thermal B

DOI: 10.1021/acssuschemeng.9b00548 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering treatment. When the treatment temperature reached 500 °C, BiOI was almost completely converted into Bi5O7I. This result was consistent with the result from the TG spectra. SEM images in Figures 2 and S2 compared the morphology of the BiOI1−x nanosheets obtained at different temperatures.

The chemical composition of the pristine BiOI and iodinedeficient BiOI1−x was tested by EDX analysis in Figure S6 and Table 1. The Bi/O atomic ratio in BiOI1−x treated from 200 to Table 1. Atomic Ratio of Bi to I Determined from EDX Spectra Analysis of the Pristine BiOI Nanosheets and Iodine-Deficient BiOI1−x Nanosheets Prepared at 200, 300, 350, and 400 °C in Air samples ratio

BiOI

200

250

300

350

400

Bi:I

1:0.96

1:0.85

1:0.74

1:0.66

1:0.5

1:0.4

400 °C is very close to that of the BiOI nanosheets (Table S2). This indicated that the O atoms had difficulty escaping from BiOI below 400 °C in air. On the contrary, the intensity of the characteristic I peaks (Figure S6) at 3.9 keV gradually decreased as the treatment temperature increased. This result indicated the easy escaping of I atoms from BiOI. The Bi/I atomic ratio gradually increased from 1:0.96 in the pristine BiOI nanosheets to 1:0.40 of BiOI1−x at 400 °C, with increasing treatment temperature, suggesting the formation of I-vacancies. Compared to BiOI, an amount of around 34% iodine atoms is lost in the BiOI1−x nanosheets treated at 300 °C. Thus, the heavy loss of I atoms causes the formation of abundant I-vacancy defects in the BiOI1−x nanosheets, which can greatly change electronic structures. Figure 3a shows UV−vis absorption spectra of BiOI1−x nanosheets treated at different temperature. The pristine Figure 2. SEM images of (a) BiOI nanosheets and iodine-deficient BiOI1−x nanosheets prepared at 300 °C in air, respectively. TEM images of (c) iodine-deficient BiOI1−x nanosheets treated at 300 °C and (d) its corresponding high-resolution TEM image.

The pristine BiOI nanosheets have an averaged thickness of about 30 nm and are aggregated into flower-like structures (Figure S3). No obvious shape change was observed in the BiOI1−x before the temperature was lower than 350 °C (Figure S4). Further increasing the temperature led to the collapse of the BiOI1−x flowers. When the thermal treatment temperature reached 500 °C, the sheets were broken. This result was attributed to the heavy loss of the iodine atoms and phase transformation from BiOI1−x to Bi5O7I, as confirmed by the XRD patterns. The BET surface areas of iodine-deficient BiOI1−x nanosheets treated from 200 to 400 °C (Table S1) were 12.1, 12.6, 13.3, 14.1, and 13.4 m2 g−1, respectively, which were close to the 12.5 m2 g−1 of the pristine BiOI. On the one hand, the vacancy formation might increase the surface areas. On the other hand, the thermal treatment generally decreases the surface areas. Thus, no obvious increase was found in the BET surface area of BiOI1−x. These results also indicated that the morphology and structure were not destroyed during the vacancy formation process. The TEM images of the iodinedeficient BiOI1−x shown in Figure 2c were used to detect the microstructure and crystal lattice spacing. The corresponding high-resolution TEM image (Figure 2d) suggests that the BiOI1−x nanosheets with a high concentration of iodine vacancies still exhibited high crystallinity. The lattice spacing of 0.284 nm in Figure 2d corresponds to the BiOI (110) plane.40 High-resolution TEM images from Figures 2d and S5 show that the lattice fringes of BiOI1−x widened progressively with the treatment temperature increase, which was in accord with the result from XRD.

Figure 3. (a) UV−vis spectra of the pristine BiOI and iodine-deficient BiOI1−x obtained at 200, 250, 300, 350, 400, and 450 °C for 3 h. (b) Kubelka−Munk-transformed reflectance spectra of the pristine BiOI and iodine-deficient BiOI1−x heated at 300 °C.

BiOI nanosheets have a strong light absorption band to 670 nm. The absorption edges of the BiOI1−x nanosheets with the concentration of I-vacancies below 34% have a certain blueshift compared with that of the pristine BiOI nanosheets. However, when the temperature further increased from 350 to 450 °C, the absorption edges showed a great increase in blue shifts because the numerous I defects modified the band structure of BiOI1−x. When the temperature reached 450 °C, the absorption edges were located at less than 500 nm in the visible-light region. The corresponding Kubelka−Munk-transformed reflectance spectra in Figure 3b were used to estimate the bandgaps of iodine-deficient BiOI1−x nanosheets. The bandgap of the BiOI was 1.977 eV, while the bandgap of the BiOI1−x nanosheets treated at 300 °C was slightly increased to 1.984 eV. C

DOI: 10.1021/acssuschemeng.9b00548 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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observation indicates that no oxygen vacancy was formed in the BiOI1−x in air. Valence band spectra of the BiOI and BiOI1−x nanosheets are shown in Figure 4b. The VBM position of the iodinedeficient BiOI1−x was 1.56 eV, which was higher than the 1.33 eV of the BiOI. The CBM positions of the BiOI and BiOI1−x were −0.647 and −0.424 eV, respectively. A more positive VBM position provides a stronger oxidative capability of photogenerated holes for photocatalysis. These vacancies can lower the VBM position. To uncover the role of I-vacancies on BiOI1−x electronic structure, the electronic density of states (DOS) of pristine BiOI, Bi8O8I6, and Bi8O8I4 was studied by using the standard DFT calculation. As we can see in Figure S8a, the calculated bandgap of pristine BiOI is 1.57 eV and about 0.4 eV lower than the experimental value (1.977 eV). It should be noted, that the standard DFT calculation normally underestimates the semiconductor bandgap. After introducing 25 and 50% Ivacancies, both Bi8O8I6 and Bi8O8I4 show an obviously ndoping feature. For both cases, the Fermi levels move upward into the conduction band. Interestingly, the bandgaps (1.58 eV) of Bi8O8I6 and Bi8O8I4 are almost the same as that (1.57 eV) of the pristine BiOI. This is in good agreement with the experimental results. To explore the VBM positions relative to the vacuum energy level for BiOI, Bi8O8I6, and Bi8O8I4, we further calculated the electronic work functions (WF) of the (001) surfaces of BiOI, Bi8O8I6, and Bi8O8I4. As given in Figure S8b, the calculated WFs of the (001) surfaces of BiOI, Bi8O8I6, and Bi8O8I4 are 6.27, 4.53, and 4.22 eV, respectively. In combination with the results of Fermi level positions and bandgap values, the VBM positions relative to the vacuum energy level for BiOI, Bi8O8I6, and Bi8O8I4 are −6.27, −6.72, and −6.49 eV, respectively. The results are schematically summarized in Figure 5. After introducing I-vacancies, both the VBM and CBM of iodine-deficient BiOI1−x nanosheets shift to a lower energy position compared with the pristine BiOI. This result indicates that the photogenerated holes in BiOI1−x possess lower energy and thus a stronger oxidative capability compared with those in pristine BiOI. Phenol and MO pollutants were used to evaluate visible light photocatalytic performance of iodine-deficient BiOI1−x nanosheets. Iodine-deficient BiOI1−x nanosheets effectively photodegraded MO molecules (Figure 6a). The photocatalytic

Surface structure and chemical state can affect photocatalytic activity.41 The XPS spectra of the pristine BiOI and iodinedeficient BiOI1−x nanosheets shown in Figure S7 are recorded to study the surface composition and chemical states. Compared to those of pristine BiOI, the Bi 4f5/2 and 4f7/2 peaks (Figure S7a) of the iodine-deficient BiOI1−x at 164.4 and 159.1 eV shifted to the high energy. The slight shift in the + BiOI1−x is caused by Bi(3−x) chemical state defects.42 The O 1s peaks at 529.6 and 531.1 eV (Figure S7b) are attributed to the − lattice oxygen (O2 ) in BiOI and adsorbed oxygen on the BiOI surface,43 respectively. No obvious shift was detected in the O 1s peaks of the BiOI and BiOI1−x. This result indicated that the [Bi2O2]2+ structures in the BiOI1−x nanosheets were retained at 300 °C. However, the I 3d5/2 and 3d3/2 peaks (Figure S7c) in the BiOI1−x nanosheets appeared at 630.7 and 619.2 eV, respectively, which indicated a distinct shift to the high energy compared to the peaks at 630.0 and 618.6 eV in the pristine BiOI nanosheets. The results suggested that the I−Bi bonds were substantially modified in the iodine-deficient BiOI1−x nanosheets due to the formation of I-vacancies. Additional evidence for I-vacancies can be observed from the ESR spectra (Figure 4a). Compared with the pristine BiOI, the iodine-

Figure 4. ESR (a) and valence band (b) spectra of the pristine BiOI and iodine-deficient BiOI1−x nanosheets heated at 300 °C.

deficient BiOI1−x nanosheets exhibited a strong ESR signal at g = 3.0, which should correspond to the signal of iodine vacancies. However, the typical signals of oxygen vacancies,44 at around g = 2.0 were not obvious in the BiOI1−x. This

Figure 5. Schematic structures of (001) surfaces and the energy band structures of the pristine BiOI, Bi8O8I6, and Bi8O8I4. D

DOI: 10.1021/acssuschemeng.9b00548 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. (a) Light absorption spectra of MO over the iodine-deficient BiOI1−x nanosheets with the increase of visible light irradiation time. (b) Visible light degradation curves of MO over no catalyst; BiOI nanosheets; and iodine-deficient BiOI1−x nanosheets heated at 200, 250, 300, 350, 400, and 450 °C for 3 h. (c) Photodegradation cycles of MO over the iodine-deficient BiOI1−x nanosheets irradiated by visible light. (d) Their corresponding visible-light photodegradation curves of phenol.

Figure 7. (a) Current curves of the pristine BiOI nanosheets and iodine-deficient BiOI1−x nanosheets heated at 250, 300, 350, and 400 °C for 3 h under visible light. (b) Photocurrent curves of the iodine-deficient BiOI1−x nanosheets treated at 300 °C with time at 0 V under visible light. (c) Electrochemical impedance spectra of the pristine BiOI sheets and the iodine-deficient BiOI1−x sheets treated at 300 °C.

photodegradation efficiency began to reduce with increased amount of I-vacancies in BiOI1−x treated from 300 to 450 °C, despite the stronger dark absorption capability. The possible reason is that the excessive I-vacancies destroyed the surface atomic structures that are originally favorable for photocatalysis. The photodegradation reaction kinetics of MO are presented in Figure S9a to compare the photodegradation rates of BiOI1−x nanosheets with different amounts of I-vacancies. The degradation process is in accord with the first-order kinetics (−ln(C/C0) =kt). The photodegradation rate of the BiOI1−x nanosheets treated at 300 °C was 159 times that of the pristine BiOI. The photodegradation stability of the iodinedeficient BiOI1−x nanosheets was measured by using threecycling photochemical reactions (Figure 6c). After three cycles of degradation, the BiOI1−x nanosheets still exhibited effective visible light photodegradation activity of MO. No significant

degradation curves of MO by the pristine BiOI and iodinedeficient BiOI1−x nanosheets are shown in Figure 6b. The suspensions were stirred in dark to achieve adsorption equilibrium. The pristine BiOI nanosheets exhibited the low photodegradation activity of MO, and the degradation percentage was less than 10%. The low photocatalytic activity of the pristine BiOI also indicated that the sensitization effect of organic dyes was weak in the photodegradation process. On the contrary, all the iodine-deficient BiOI1−x nanosheets showed higher visible light photodegradation rates for MO than the pristine BiOI nanosheets. The photocatalytic activity of BiOI1−x nanosheets was gradually enhanced due to the formation of additional I-vacancies. When the amount of Ivacancies reached 34%, the iodine-deficient BiOI1−x nanosheets exhibited the highest visible light photodegradation activity and fully degraded MO within 1 h. However, the E

DOI: 10.1021/acssuschemeng.9b00548 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering attenuation of catalytic activity was observed in the third cycle. In addition, no obvious structural change was found after photodegradation cycles (Figure S10). This suggested the high stability of iodine-deficient BiOI1−x nanosheets as photocatalysts. Phenol is a colorless, toxic, and refractory organic compound used to evaluate the activity of photocatalysts.45 The photodegradation curves of phenol are displayed in Figures 6d and S11. All the samples showed a low absorption in dark, which suggested that the phenol molecules were difficult to absorb on the BiOI1−x sheet surface. Unlike the pristine BiOI nanosheets that exhibited no obvious photodegradation activity, all the iodine-deficient BiOI1−x nanosheets effectively degraded phenol under visible light irradiation. The iodinedeficient BiOI1−x nanosheets heated at 300 °C achieved the highest photodegradation rate. The nanosheets fully degraded phenol within 4 h under visible light. The corresponding degradation rate constant k of phenol (Figure S9b) was 0.0142. The effective photodegradation of phenol over the iodinedeficient BiOI1−x nanosheets suggested that the photooxidative capability of the photogenerated holes was significantly improved after I-vacancies were introduced. This effect was also confirmed by the XPS and DOS calculations. Photoelectrochemical measurements can be used to explore the photoelectrochemical water splitting capability and transport rate of photoexcited carriers in the photocatalytic materials.46 Therefore, the photoelectrochemical behaviors of the pristine BiOI nanosheets and iodine-deficient BiOI1−x nanosheets loaded on the FTO glass were estimated in a conventional three-electrode system (Figure 7a). The pristine BiOI nanosheets exhibited a low visible light photocurrent intensity. The current intensity of iodine-deficient BiOI1−x nanosheets gradually increased with the formation of Ivacancies. Herein, the BiOI1−x nanosheets treated at 300 °C exhibited the strongest photocatalytic water splitting capability. The photocurrent intensity was 235 times that of the BiOI nanosheets at 0.25 V. The improved current intensity of iodine-deficient BiOI1−x indicated that photoexcited electrons and holes were easily transported to the surface. The arc radius on the spectrum of iodine-deficient BiOI1−x (Figure 7c) was smaller than that of the pristine BiOI in internal resistance measurements. This result indicated that the resistivity decreased after the introduction of I-vacancies. Thus, the photogenerated carrier transport is better in the iodinedeficient BiOI1−x nanosheets. Introducing vacancies may affect the photocatalytic stability of materials. Therefore, a long-term measurement of visible light photoelectrochemical water splitting over the iodine-deficient BiOI1−x nanosheets at 0 V is shown in Figure 7b. No obvious decay of photocurrent intensity was observed within 8000 s of the photoelectrochemical process. This result suggests that the iodine-deficient BiOI1−x nanosheets are stable in photoelectrochemical reaction. The photocatalytic activity of iodine-deficient BiOI1−x nanosheets was significantly improved by introducing Ivacancies through a facile heat treatment method. The enhanced photooxidative ability and stability of the iodinedeficient BiOI1−x nanosheets are attributed to the following reasons. (i) Photodegradation of organic molecules is mainly an oxidation reaction process.47 Hence, the oxidative capability of photogenerated holes is critical for photodegradation reactions. After I-vacancies were introduced, the VBM position (Figure 8) was lowered from 1.33 eV of the pristine BiOI to

Figure 8. Scheme of energy band structures of the pristine BiOI and iodine-deficient BiOI1−x nanosheets.

1.56 eV of the BiOI 1−x . The iodine-deficient BiOI 1−x nanosheets with a more positive VBM position provided stronger oxidative holes for photodegradation reactions. This observation was confirmed by the XPS spectral results and first-principles calculation. (ii) The transport of photoexcited carriers is a key process in photocatalytic reactions.48 The promoted carrier transport process (Figure 7c) will substantially increase the chance of photogenerated carrier participation in photocatalytic reactions.

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CONCLUSIONS I-vacancies were introduced into the BiOI1−x nanosheets by a thermal treatment method. The morphology and crystal structure of BiOI1−x were not obviously destroyed by numerous I-defects that formed at high temperatures. Nevertheless, introducing I-vacancies elevates the VBM position from 1.33 to 1.56 eV and then significantly improves the oxidative ability of photogenerated holes from the iodinedeficient BiOI1−x. The surface atom structure is suitable for photocatalytic reactions, and smooth transport channels with electron scavengers are formed by I-vacancies. Moreover, the iodine-deficient BiOI1−x nanosheets do not only exhibit a significantly improved photodegradation rate of MO but can also effectively degrade phenol under visible light. Furthermore, the enhanced and highly stable photoelectrochemical performance can be achieved in the iodine-deficient BiOI1−x nanosheets due to its low resistivity.

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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/acssuschemeng.9b00548. TG curve, EDX spectra, XPS spectra, atomic ratio statistics of Bi to O, calculated electronic density of states and electrostatic potential results, high-resolution TEM images, photodegradation reaction kinetics of MO and phenol, photodegradation spectra of phenol, and electrochemical impedance spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. F

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(13) Ye, L. Q.; Su, Y. R.; Jin, X. L.; Xie, H. Q.; Zhang, C. Recent advances in BiOX (X = Cl, Br and I) photocatalysts: synthesis, modification, facet effects and mechanisms. Environ. Sci.: Nano 2014, 1 (2), 90−112. (14) Huang, H. W.; Han, X.; Li, X. W.; Wang, S. C.; Chu, P. K.; Zhang, Y. H. Fabrication of Multiple Heterojunctions with Tunable Visible-Light-Active Photocatalytic Reactivity in BiOBr-BiOl FullRange Composites Based on Microstructure Modulation and Band Structures. ACS Appl. Mater. Interfaces 2015, 7 (1), 482−492. (15) Dong, Y. Q.; Pang, H. C.; Yang, H. B.; Guo, C. X.; Shao, J. W.; Chi, Y. W.; Li, C. M.; Yu, T. Carbon-Based Dots Co-doped with Nitrogen and Sulfur for High Quantum Yield and ExcitationIndependent Emission. Angew. Chem., Int. Ed. 2013, 52 (30), 7800−7804. (16) Yu, C. L.; Cao, F. F.; Li, G.; Wei, R. F.; Yu, J. C.; Jin, R. C.; Fan, Q. Z.; Wang, C. Y. Novel noble metal (Rh, Pd, Pt)/BiOX(Cl, Br, I) composite photocatalysts with enhanced photocatalytic performance in dye degradation. Sep. Purif. Technol. 2013, 120, 110−122. (17) Ye, K. H.; Chai, Z. S.; Gu, J. W.; Yu, X.; Zhao, C. X.; Zhang, Y. M.; Mai, W. J. BiOI-BiVO4 photoanodes with significantly improved solar water splitting capability: p−n junction to expand solar adsorption range and facilitate charge carrier dynamics. Nano Energy 2015, 18, 222−231. (18) Huang, Y. C.; Li, H. B.; Fan, W. J.; Zhao, F. Y.; Qiu, W. T.; Ji, H. B.; Tong, Y. X. Defect Engineering of Bismuth Oxyiodide by OI3− Doping for Increasing Charge Transport in Photocatalysis. ACS Appl. Mater. Interfaces 2016, 8 (41), 27859−27867. (19) Hsu, C. L.; Li, Y. J.; Jian, H. J.; Harroun, S. G.; Wei, S. C.; Ravindranath, R.; Lai, J. Y.; Huang, C. C.; Chang, H. T. Green synthesis of catalytic gold/bismuth oxyiodide nanocomposites with oxygen vacancies for treatment of bacterial infections. Nanoscale 2018, 10 (25), 11808−11819. (20) Zhang, G. Q.; Cai, L.; Zhang, Y. F.; Wei, Y. Bi5+, Bi(3‑x)+, and Oxygen Vacancy Induced BiOClxI1‑x Solid Solution toward Promoting Visible-Light Driven Photocatalytic Activity. Chem. - Eur. J. 2018, 24 (29), 7434−7444. (21) Liu, G. G.; Wang, T.; Ouyang, S. X.; Liu, L. Q.; Jiang, H. Y.; Yu, Q.; Kako, T.; Ye, J. H. Band-structure-controlled BiO(ClBr)((1-x)/ 2)I-x solid solutions for visible-light photocatalysis. J. Mater. Chem. A 2015, 3 (15), 8123−8132. (22) Yu, H. J.; Shi, R.; Zhao, Y. X.; Bian, T.; Zhao, Y. F.; Zhou, C.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C. H.; Zhang, T. R. AlkaliAssisted Synthesis of Nitrogen Deficient Graphitic Carbon Nitride with Tunable Band Structures for Efficient Visible-Light-Driven Hydrogen Evolution. Adv. Mater. 2017, 29 (16), 1605148. (23) Xiong, Z. G.; Zhao, X. S. Nitrogen-Doped Titanate-Anatase Core-Shell Nanobelts with Exposed {101} Anatase Facets and Enhanced Visible Light Photocatalytic Activity. J. Am. Chem. Soc. 2012, 134 (13), 5754−5757. (24) Zhao, L. J.; Zhang, X. C.; Fan, C. M.; Liang, Z. H.; Han, P. D. First-principles study on the structural, electronic and optical properties of BiOX (X = Cl, Br, I) crystals. Phys. B 2012, 407 (17), 3364−3370. (25) Zhang, X.; Zhang, L. Z. Electronic and Band Structure Tuning of Ternary Semiconductor Photocatalysts by Self Doping: The Case of BiOI. J. Phys. Chem. C 2010, 114 (42), 18198−18206. (26) Li, J.; Yu, Y.; Zhang, L. Z. Bismuth oxyhalide nanomaterials: layered structures meet photocatalysis. Nanoscale 2014, 6 (15), 8473−8488. (27) Liu, Y. F.; Yao, W. Q.; Liu, D.; Zong, R. L.; Zhang, M.; Ma, X. G.; Zhu, Y. F. Enhancement of visible light mineralization ability and photocatalytic activity of BiPO4/BiOI. Appl. Catal., B 2015, 163, 547−553. (28) Kresse, G.; Hafner, J. Abinitio Molecular-Dynamics for LiquidMetals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47 (1), 558− 561. (29) Kresse, G.; Hafner, J. Ab-Initio Molecular-Dynamics Simulation of the Liquid-Metal Amorphous-Semiconductor Transition in

Xuewen Wang: 0000-0001-6269-8086 Gang Liu: 0000-0002-6946-7552 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the funding support from the National Science Fund of China (NSFC; 51662030, 51862023, 51472249, and 51825204) and the Natural Science Foundation of Jiangxi Province (20171BAB206014). The theoretical calculations were performed on a TianHe-1(A) at the National Supercomputer Center in Tianjing and a Tianhe2 at the National Supercomputer Center in Guangzhou. We also acknowledge the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant U1501501.

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REFERENCES

(1) Chen, X. B.; Liu, L.; Huang, F. Q. Black titanium dioxide (TiO2) nanomaterials. Chem. Soc. Rev. 2015, 44 (7), 1861−1885. (2) Moniz, S. J. A.; Shevlin, S. A.; Martin, D. J.; Guo, Z. X.; Tang, J. W. Visible-light driven heterojunction photocatalysts for water splitting - a critical review. Energy Environ. Sci. 2015, 8 (3), 731−759. (3) Wang, Q.; Hisatomi, T.; Jia, Q. X.; Tokudome, H.; Zhong, M.; Wang, C. Z.; Pan, Z. H.; Takata, T.; Nakabayashi, M.; Shibata, N.; Li, Y. B.; Sharp, I. D.; Kudo, A.; Yamada, T.; Domen, K. Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%. Nat. Mater. 2016, 15 (6), 611−612. (4) Yang, J. H.; Wang, D. G.; Han, H. X.; Li, C. Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis. Acc. Chem. Res. 2013, 46 (8), 1900−1909. (5) Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y. 25th Anniversary Article: MXenes: A New Family of Two-Dimensional Materials. Adv. Mater. 2014, 26 (7), 992−1005. (6) Butler, S. Z.; Hollen, S. M.; Cao, L. Y.; Cui, Y.; Gupta, J. A.; Gutierrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J. X.; Ismach, A. F.; Johnston-Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; Windl, W.; Goldberger, J. E. Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7 (4), 2898−2926. (7) Sun, Z. Y.; Talreja, N.; Tao, H. C.; Texter, J.; Muhler, M.; Strunk, J.; Chen, J. F. Catalysis of Carbon Dioxide Photoreduction on Nanosheets: Fundamentals and Challenges. Angew. Chem., Int. Ed. 2018, 57 (26), 7610−7627. (8) Ren, K. X.; Liu, J.; Liang, J.; Zhang, K.; Zheng, X.; Luo, H. D.; Huang, Y. B.; Liu, P. J.; Yu, X. B. Synthesis of the bismuth oxyhalide solid solutions with tunable band gap and photocatalytic activities. Dalton T. 2013, 42 (26), 9706−9712. (9) Cheng, H. F.; Huang, B. B.; Dai, Y. Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications. Nanoscale 2014, 6 (4), 2009−2026. (10) Jiang, J.; Zhao, K.; Xiao, X. Y.; Zhang, L. Z. Synthesis and Facet-Dependent Photoreactivity of BiOCl Single-Crystalline Nanosheets. J. Am. Chem. Soc. 2012, 134 (10), 4473−4476. (11) Jiang, G. H.; Wang, X. H.; Wei, Z.; Li, X.; Xi, X. G.; Hu, R. B.; Tang, B. L.; Wang, R. J.; Wang, S.; Wang, T.; Chen, W. X. Photocatalytic properties of hierarchical structures based on Fe-doped BiOBr hollow microspheres. J. Mater. Chem. A 2013, 1 (7), 2406− 2410. (12) Zhang, X.; Ai, Z. H.; Jia, F. L.; Zhang, L. Z. Generalized onepot synthesis, characterization, and photocatalytic activity of hierarchical BiOX (X = Cl, Br, I) nanoplate microspheres. J. Phys. Chem. C 2008, 112 (3), 747−753. G

DOI: 10.1021/acssuschemeng.9b00548 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49 (20), 14251−14269. (30) Kresse, G.; Furthmuller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6 (1), 15−50. (31) Kresse, G.; Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54 (16), 11169−11186. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77 (18), 3865− 3868. (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple (vol 77, pg 3865, 1996). Phys. Rev. Lett. 1997, 78 (7), 1396−1396. (34) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50 (24), 17953−17979. (35) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59 (3), 1758−1775. (36) Wyckoff, R. W. G. Crystal Structures, 2 ed.; Interscience Publishers: New York, 1963; Vol. 1, pp 294−296. (37) Huang, Y. C.; Li, H. B.; Balogun, M. S.; Liu, W. Y.; Tong, Y. X.; Lu, X. H.; Ji, H. B. Oxygen Vacancy Induced Bismuth Oxyiodide with Remarkably Increased Visible-Light Absorption and Superior Photocatalytic Performance. ACS Appl. Mater. Interfaces 2014, 6 (24), 22920−22927. (38) Li, J. L.; Zhang, M.; Guan, Z. J.; Li, Q. Y.; He, C. Q.; Yang, J. J. Synergistic effect of surface and bulk single-electron-trapped oxygen vacancy of TiO2 in the photocatalytic reduction of CO2. Appl. Catal., B 2017, 206, 300−307. (39) Wang, Y. N.; Deng, K. J.; Zhang, L. Z. Visible Light Photocatalysis of BiOI and Its Photocatalytic Activity Enhancement by in Situ Ionic Liquid Modification. J. Phys. Chem. C 2011, 115 (29), 14300−14308. (40) Li, P.; Zhao, X.; Jia, C. J.; Sun, H. G.; Sun, L. M.; Cheng, X. F.; Liu, L.; Fan, W. L. ZnWO4/BiOI heterostructures with highly efficient visible light photocatalytic activity: the case of interface lattice and energy level match. J. Mater. Chem. A 2013, 1 (10), 3421−3429. (41) Hu, R. M.; Zhou, X. L.; Yu, J. The effect of surface structure on Ag atom adsorption over CuO (111) surfaces: A first principles study. Appl. Surf. Sci. 2017, 425, 1111−1117. (42) Kong, X. Y.; Ng, B. J.; Tan, K. H.; Chen, X. F.; Wang, H. T.; Mohamed, A. R.; Chai, S. P. Simultaneous generation of oxygen vacancies on ultrathin BiOBr nanosheets during visible-light-driven CO2 photoreduction evoked superior activity and long-term stability. Catal. Today 2018, 314, 20−27. (43) Ye, L. Q.; Jin, X. L.; Ji, X. X.; Liu, C.; Su, Y. R.; Xie, H. Q.; Liu, C. Facet-dependent photocatalytic reduction of CO2 on BiOI nanosheets. Chem. Eng. J. 2016, 291, 39−46. (44) Yang, C. M.; Gao, G. M.; Zhang, J. J.; Liu, R. P.; Fan, R. C.; Zhao, M.; Wang, Y. W.; Gan, S. C. Surface oxygen vacancy induced solar light activity enhancement of a CdWO4/Bi2O2CO3 core-shell heterostructure photocatalyst. Phys. Chem. Chem. Phys. 2017, 19 (22), 14431−14441. (45) Cong, Y. Q.; Li, Z.; Zhang, Y.; Wang, Q.; Xu, Q. Synthesis of alpha-Fe2O3/TiO2 nanotube arrays for photoelectro-Fenton degradation of phenol. Chem. Eng. J. 2012, 191, 356−363. (46) Guo, J.; Oshikiri, T.; Ueno, K.; Shi, X.; Misawa, H. Plasmoninduced photoelectrochemical biosensor for in situ real-time measurement of biotin-streptavidin binding kinetics under visible light irradiation. Anal. Chim. Acta 2017, 957, 70−75. (47) Nakanishi, Y.; Imae, T. Synthesis of dendrimer-protected TiO2 nanoparticles and photodegradation of organic molecules in an aqueous nanoparticle suspension. J. Colloid Interface Sci. 2005, 285 (1), 158−62. (48) Jiang, J.; Zhang, X.; Sun, P. B.; Zhang, L. Z. ZnO/BiOI Heterostructures: Photoinduced Charge-Transfer Property and Enhanced Visible-Light Photocatalytic Activity. J. Phys. Chem. C 2011, 115 (42), 20555−20564. H

DOI: 10.1021/acssuschemeng.9b00548 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX