Crystal Defect Engineering of Aurivillius Bi2MoO6 by Ce Doping for

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Crystal Defects Engineering of Aurivillius Bi2MoO6 by Ce Doping for Increased Reactive Species Production in Photocatalysis Zan Dai, Fan Qin, Huiping Zhao, Jie Ding, Yunling Liu, and Rong Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00490 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016

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ACS Catalysis

Crystal Defects Engineering of Aurivillius Bi2MoO6 by Ce Doping for Increased Reactive Species Production in Photocatalysis

Zan Daia, Fan Qina, Huiping Zhaoa, Jie Dinga, Yunling Liub, and Rong Chen*a

a

Key Laboratory for Green Chemical Process of Ministry of Education and School of

Chemistry and Environmental Engineering, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan, 430073, PR China b

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, Changchun, 130012, PR China

* Corresponding author: Prof. R. Chen, E-mail: [email protected] Tel.: (+86)13659815698; fax: (+86)2787195680

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ABSTRACT Crystal defects have been extensively proved to have great influence on semiconductor photocatalysis. To optimize the reactivity of crystalline photocatalysts and achieve ideal solar energy conversion, crystal defects engineering has initiated a considerable interest in real catalysts. Herein, we develop a general strategy to manufacture and mediate crystal defects in the host Bi2MoO6 lattice by varying the cerium dopant content, resulting in the greatly improved visible-light-driven photocatalytic performance for the degradation of highly toxic nerve agent simulants (NAS) and organic dyes, as well as bacterial photoinactivation. After careful examination of crystal defects structure and charge carrier dynamics, it was evidently proved that the Ce-doping mediated crystal defects are crucial for controlling the photocatalytic efficiency of Aurivillius Bi2MoO6. More importantly, the well-engineered crystal defects not only exert a beneficial influence on the electron dynamics and band structure, but also facilitate the one-electron and two-electron reactions by introducing the Ce3+/Ce4+ and Mo4+/Mo6+ redox couples, which results in a significant enhancement in reactive oxygen species (ROS) photogeneration.

KEYWORDS: Crystal defects, Bi2MoO6, Redox couple, Ce-doping, ROS


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1. INTRODUCTION Generally, crystal defects exist ubiquitously in materials but are undesired, owing to the induced unpredictability and difficulty in application. The defects could alter the local bond and electronic properties, including the bond length, bond energy, potential trap depth, electroaffinity, charge and energy density through the atomic coordination.1,2 Particularly, it brings some unpredictable properties, such as the catalytic, electronic, dielectric, optic, magnetic and thermal properties at the specific atomic site.3,4 However, if properly being controlled, it could be an economical and convenient technique to substantially modify the electronic dynamics and phonon transport, or even mediate the band gap and work function, which would have much contribution to the photocatalytic activity.5-9 Subsequently, the question naturally occurs: how to fabricate or engineer the defects to an ideal state? Doping is a fundamental strategy to introduce impurities into a crystal structure, which could result in crystal defects and mediate the properties of bulk semiconductors.10-13 More importantly, two kinds of doping (i.e., n-type and p-type) could either donate extra electrons or provide extra holes.14 These electrons or holes are then available as carriers of electrical current, which is essential to improve electrochemical and photocatalytic properties. Nevertheless, the crystal structural changes caused by doping are not intrinsic, which hinder the applications of doped nanocrystalline materials.15,16 Therefore, the matching of matrix and doped impurities is crucially important and lies at the heart of this technology.17,18 Recently, it would be realized to generate or even regulate distortion (i.e., defect) in the crystal structure via different doping processes, which also suggests that a variety of defect-engineered nanocrystals for photocatalytic applications can be anticipated.19 Aurivillius-phase Bi2MoO6 belongs to the bismuth oxide family with a layered structure


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consisting of perovskite layers (Am-1BmO3m+1) and bismuth oxide layers (Bi2O22+), which is also an important photocatalyst in bismuth-based semiconductor.20-23 Both bismuth oxide layer with the fluorite Bi-O structure and perovskite layer with corner-shared octahedral Mo-O structure facilitate good electron-conductivity, which contributes to its outstanding dielectric, ion-conductive, luminescent and catalytic properties.24,25 Therefore, Aurivillius Bi2MoO6 structure and related compounds have evoked great research interests.26,27 However, the visible-light-response photocatalytic application of Bi2MoO6 is currently limited by its low overall efficiency due to its rapid electron-hole recombination, slow carrier migration, and poor surface chemical states. Motivated by these concerns, a general strategy of fabricating and engineering crystal defects via cerium doping was developed to mediate the photocatalytic performance. By varying the dopant content, the crystal structural distortion could be engineered via ions substitution, charge compensation and oxygen vacancy generation. Moreover, the wellengineered crystal defects were proved to be able to boost electron dynamics by trapping effect and to mediate the band structure by introducing new energy levels, leading to the improvement of electrochemical properties and the enhancement of photocatalytic activities. In particular, the redox couples of Ce3+/Ce4+ and Mo4+/Mo6+ generated in Ce-doping process could evidently promote the reactive oxygen species (ROS) production by one-electron and two-electron reaction, also resulting in the enhancement of photocatalytic ability towards biochemical pollutants, bacteria and organic dye. To the best of our knowledge, no crystal defects engineering study on doped photocatalysts has been reported.

2. EXPERIMENTAL SECTION ACS Paragon Plus Environment

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2.1 Materials Bismuth

nitrate

pentahydrate

(Bi(NO3)3∙5H2O),

cerium

nitrate

hexahydrate

(Ce(NO3)3·6H2O), sodium molybdate (Na2MoO4·2H2O) and sodium tungstate dehydrate (Na2WO4·2H2O) were purchased from Aladdin (Shanghai, China). Nitroblue tetrazolium (NBT), horseradish peroxide (POD) and N,N-diethyl-p-phenylenediamine (DPD) were purchased from Sigma-Aldrich. Nitric acid (HNO3), methylene blue (MB) and rhodamine B (RhB) were obtained from Sinopharm Chemical Reagent Co. (China). Methyl paraoxon (MP), bis(4-nitrophenyl)phosphate (b-NPP) and 4-nitrophenol were purchased from J&K Chemical Reagent Co. (China). The strain of S. aureus was purchased from Southern Biological. And all the reagents were analytical grade and used directly without further purification. 2.2 Synthesis Ce-doped Bi2MoO6 (Ce-BMO) with different Ce/Mo molar ratio (5%, 10%, 20% and 40%) were prepared through a facile one-step hydrothermal process. The details are as follows: 1.94 g Bi(NO3)3·5H2O (4 mmol), 0.484 g Na2MoO4·2H2O (2 mmol) and different amount of Ce(NO3)3·6H2O (0.043, 0.087, 0.174 and 0.347 g for 5%, 10%, 20% and 40% Ce-doped Bi2MoO6, respectively) were added to 140 mL deionized water under vigorously stirring. After stirring for 2 h at room temperature, the result pale-yellow suspensions were added into 200 mL Teflon-lined stainless autoclave and heated at 180 °C for 24 h. Finally, the products were collected and washed with deionized water and ethanol for five times and dried at 60 °C for several hours. For comparison, undoped Bi2MoO6 was prepared under identical conditions in the absence of Ce(NO3)3·6H2O. Ce-doped Bi2WO6 was also prepared via the same hydrothermal procedure by using Na2WO4·2H2O as tungsten precursor. 2.3 Characterization


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The composition and crystal phase of obtained samples were characterized by powder Xray diffraction (XRD, Bruker axs D8 Discover) with Cu Kα radiation of 1.5406 Å. The relative content of cerium element in Ce-doped Bi2MoO6 samples was analyzed by using an inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500ce). The morphology and structure of obtained samples were characterized by scanning electron microscope (SEM, Hitachi S4800) operating at 5.0 kV and transmission electron microscope (TEM, Philips Tecnai G2 20) operating at 200 kV. UV-vis diffuse reflectance spectra (DRS) were recorded on a UV-vis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference and were converted from reflection to absorbance by the Kubelka-Munk method. Brunauer-EmmettTeller (BET) specific surface area was analyzed by nitrogen adsorption on a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). X-ray photoelectron spectra (XPS) were performed on a VG Multilab2000 spectrometer by using Al K α (1486.6 eV) radiation as the source.

Room

temperature

photoluminescence

spectra

(PL)

and

time-resolved

photoluminescence spectra (TR-PL) were detected with a HORIBA FL-TCSPC fluorescence spectrophotometer. Raman spectra were recorded by using a Horiba Jobin-Yvon LabRam HR800 Raman microspectrometer, with an excitation laser at 320 nm. 2.4 Photocatalytic Activity Test The photocatalytic activities of Ce-BMO samples were evaluated by the photodegradation of b-NPP (Co=6.8 μM), MP (Co=5.1 μM), 4-NP (Co=1.25 μM), RhB (Co=10 μM) and MB (Co=10 μM). 500 W Xe lamp with a 420 nm cut-off filter (Bilon Co. Ltd., Shanghai) was used as the visible light source. In a typical photocatalytic experiment, 0.02 g photocatalyst was dispersed in 40 mL contaminated solution under magnetic stirring. The suspension was stirred in dark for 1 h to ensure an adsorption-desorption equilibrium before irradiation. Then, the


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solution was exposed to visible light irradiation under stirring. At each given time interval, 3 mL suspension was sampled and centrifuged to remove the solid photocatalysts. The concentration of nerve agent simulants and organic dyes during the photodegradation was monitored by colorimetry using a Shimadzu UV2800 spectrophotometer. All the photocatalytic reactions were carried out at room temperature. 2.5 Analysis of intermediates and final products in photocatalysis The intermediates analysis was also performed by a high-performance liquid chromatography-mass spectrometry (HPLC/MS) system (Agilent 1260/6310, Ion Trap LC/MS) equipped with a Poroshell 120 SB-C18 column (150 mm × 4.6 mm i.d., 2.7 m). The mobile phase was composed of water (50%) and methanol (50%) and the flow rate was 0.2 mL/min. The gradient started with 20% B, increased to 30% within 2.5 min, to 80% within 3.5 min, then returned to initial composition within 5 min and equilibrated within 4 min. The injection volume was 10 L and the column temperature was 30 C. MS was performed by operating in the positive ion mode using ESI under the following conditions: capillary, 27 nA; nebulization pressure, 30 psi; temperature of drying gas, 300 C; drying gas flow, 10 L/min. MS was scanned by mass range from m/z 50 to 1000. Organic carbon mineralization was determined by total organic carbon (TOC) analysis, performed on a Dohrmann Phoenix 8000 UV-persulfate TOC analyzer. 2.6 Photocatalytic inactivation The photocatalytic inactivation of S. aureus over Ce-BMO samples were conducted under visible light irradiation by using a 500 W Xenon lamp with a 400 nm cut-off filter as light source (Chang Tuo, Beijing). The bacterial cells were incubated in Luria-Bertani (LB) agar plates at 37 °C for 24 h with shaking at 150 rpm, and then re-suspended in sterilized saline


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solution (0.9% NaCl) to about 1×105 CFU/mL. In each experiment, 0.01 g photocatalyst was dispersed into 30 mL bacterial cell suspension under magnetic stirring. At different time intervals, aliquots of mixture was collected and then immediately spread on LB agar plates, which were incubated at 37 °C for 24 h to determine the number of viable cells. All glass apparatuses and materials used in this experiment were autoclaved at 121 oC for 20 min to ensure sterility. 2.7 Reactive Oxygen Species Analysis The formation of hydroxyl radicals and superoxide radical anions upon irradiation of CeBMO aqueous suspensions was investigated by the EPR spin-trapping technique, using 5,5dimethyl-1-pyrroline N-oxide (DMPO) as spin-trapping agent. The mixture of 50 μL Ce-BMO suspensions (1 g·L−1) and 25 μL DMPO (25 g·L−1) were freshly prepared in different solvent based on the targeted radical, together with 125 μL deionized water. Water and DMSO (Merck Seccosolv max 0.05% H2O) were used for hydroxyl radical and superoxide radical anions trapping, respectively. The carefully mixed suspensions were saturated in air with a syringe and then irradiated five minutes under visible light. 500 W Xe lamp (λ≥ 400 nm cutoff; ChangTuo, Beijing) was used as an irradiation source. The EPR spectra were recorded on an EMX X-band EPR spectrometer (Bruker, Germany). The hydrogen peroxide was analyzed at 551 nm through the POD-catalyzed oxidation of DPD.28 2.8 Photoelectrochemical Experiments The

photocurrent

measurement,

electrochemical

impedance

spectroscopy (EIS)

measurement, and Mott−Schottky measurement were performed on a CHI660E electrochemistry workstation (Chenhua, Shanghai) at room temperature. All the experiments were carried out in a standard three-electrode cell containing 0.5 mol/L Na2SO4 aqueous


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solution with a platinum foil and a saturated calomel electrode as the counter electrode and the reference electrode, respectively. To prepare working electrodes, ITO glass were ultrasonically cleaned in soap-suds, deionized water and acetone, successively. The electrodes were prepared by mixing a slurry containing 80% as-prepared photocatalysts, 10% dimethylformamide (DMF) and 10% nafion on ITO glass and then dried in the air at 60 °C for 6 h. The area of electrodes is about 1×1 cm2. A 500W Xe lamp with a 420 nm cut off filter was used as light source.

3. RESULTS AND DISCUSSTION A series of Ce-doped Bi2MoO6 (Ce-BMO) samples with different cerium contents were successfully prepared. As shown in Figure 1a, Ce-doped Bi2MoO6 exhibit similar XRD patterns compared with that of pure Bi2MoO6. However, a slightly shift of (131) peak could be found in the amplifying XRD patterns after cerium doping, as illustrated in the inset of Figure 1a. Noticeably, the peak shift to high degree was firstly observed with the increase of cerium content to 20%, followed by a peak shift to low degree with further increasing cerium amount to 40%. In addition, weak diffraction peak at around 31º ascribed to (200) plane of metallic bismuth were detected with the increase of doped cerium content. It was proposed that the doping could result in the generation of crystal defects and nice crystal structure distortion in Ce-doped Bi2MoO6 crystal. To further understand the crystal defects and crystal structure distortion in Ce-doped Bi2MoO6 nanostructures, Raman spectra of Ce-doped Bi2MoO6 samples were performed to detect the crystal distortion of MoO6 octahedron. As shown in Figure 1b, all the Ce-BMO samples with different cerium doping content exhibit typical vibration mode of Bi2MoO6. Nevertheless, tiny variations in peak width


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and location of Mo-O stretching motions were observed. In the magnified Raman spectra (Figure 1c and 1d), it is found that the peak broadening and low wavenumber shift occurs with increase the doping amount of cerium, which is in accordance with the XRD results. Besides, to investigate the degree of crystal distortion, the empirical equation of RMo-O=0.48239 ln (32895/) was used to calculate the Mo-O bond length,29 where  is the Raman stretching frequency in wavenumbers and R is the metal-oxygen bond length in angstroms. The calculated results of Mo-O bond length of apical and equatorial oxygen are listed in Table S1 (Supporting Information), illustrating that Ce-doped BMO exhibits distinct distortion and symmetry breaking of MoO6 octahedron, especially for 40% Ce-BMO.

Figure 1. XRD patterns (a) and Raman spectra (b-d) of Ce-BMO samples: full-spectrum (b); magnified spectra of Mo-O vibration (c); evolution of Mo-O vibration intensity (d). To confirm the successful cerium doping into Bi2MoO6 crystal, ICP-MS measurement was


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conducted to determine the doping amount of cerium in different Ce-BMO samples. As depicted in Figure S1a (Supporting Information), the relative content of cerium in Ce-BMO samples was consistent with the amount of added cerium, illustrating the success of cerium doping into Bi2MoO6 phase. As shown in Figure S1b~1d (Supporting Information), the content of Bi readily decreases with the increase of cerium from 5% to 40% in Ce-BMO samples, implying the substitution of Bi by Ce in the Ce-BMO samples. On the other hand, the content of Mo almost unchanged with the doping content from 5% to 10%, but decreased dramatically with further increasing the doping content to 20% and 40%. However, the amount of bismuth decreased only equal to the amount of cerium increased in 5% and 10% Ce-BMO samples, as listed in the table of variation values of Ce, Bi and Mo content (Table S2, Supporting Information). More importantly, the increased Ce amount was almost the same as the sum amount of Bi and Mo decreased. It verified that the substitution of Mo by Ce only occurred in Ce-BMO samples with high doping content, which is different with the doping process of Ce-BMO with low doping content. This result confirms that the two stages substitution doping process happened in both 20% and 40% Ce-BMO samples. Undoubtedly, the crystal structure distortion and increased defect density of Ce-BMO samples mainly originate from the doping of cerium ions. To further understand the function of cerium ions doping, XPS spectra were performed to investigate the chemical states of doped cerium ions, bismuth and molybdenum atom in the distorted crystal, as shown in Figure 2a-c. It was found that the peaks of Mo 3d and Bi 4f shifted to low binding energy region after the cerium doping due to the smaller electronegativity of cerium atoms, compared with that of bismuth and molybdenum atoms. According to the reported literature, Ce 3d spectrum could be de-convoluted into four pairs of spin-orbital bands (v/u, v′/u′, v″/u″ and v′′′/u′′, where v and


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u represents 3d3/2 and 3d5/2, respectively).30 Among them, v/u, v″/u″ and v′′′/u′′′ peaks are associated with the characteristic states of Ce4+ 3d, and v′/u′ corresponds to that of Ce3+ 3d. Based on the de-convoluted Ce 3d XPS spectra of 10% and 40% Ce-BMO (Figure 2a), two cerium species of Ce3+ and Ce4+ are identified in the Ce-BMO samples. The faction of Ce3+ species could be estimated by using relative areas of v′/u′ peaks according to the equation:31 [Ce3+ ]=

Sv' +Su' ∑ (Sv +Su )

where S is the integrated area of peak v or u. The calculated ratio of Ce3+ are 38.67% and 24.03% for 10% and 40% Ce-BMO samples, respectively. It indicates that the relative content for Ce3+ in Ce3+/Ce4+ couple decreases with the increase of cerium dopant amount, which implies that the increase of oxidized cerium species with a valence of 4 would lead to charge imbalance. However, partial reduction of Mo6+ to Mo4+ could make the charge compensation, which was observed in the de-convoluted Mo 3d XPS spectra, as shown in Figure 2b. In this case, the substitution of Ce4+ for Mo4+ becomes possible and results in the increase of the ratio of Ce4+ in Ce3+/Ce4+ couples.32 When cerium ions were introduced into the crystal lattice of Bi2MoO6, the doping process undergoes two stages: 1) the substitution of Ce3+ (ionic radius = 0.102 nm) for Bi3+ (ionic radius = 0.103 nm); 2) the substitution of Ce4+ (ionic radius = 0.087 nm) for Mo4+ (ionic radius = 0.065 nm), as illustrated in Figure 2d. However, the substitution of Ce4+ for charge compensation generated Mo4+ is only evident upon increasing the amount of doped cerium ions. It is probably because that there was only little Mo4+ produced when small amount of cerium introduced. The relative intensity of Mo4+ increased with the increase of the cerium dopant, thus resulting in the substitution of Ce4+ for Mo4+. The detailed ICP-MS results also demonstrates this two stages substitution doping process in high doping ratio Ce-


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BMO samples. Apparently, the cerium doping process in the second stage with obvious ionic radius difference would exert greater influence on the crystal structure of Bi2MoO6. Moreover, the solid state ESR experiment was also carried out to detect the defects state in the crystal structure. Figure 2e shows the ESR spectra of Ce-BMO samples at room temperature. The two signals (g1=1.98 and g2=1.93) corresponds to oxygen vacancy and low valence state Mo ions in the solid, respectively.33,34 Together with the XPS results, it is reasonable that this signal is ascribed to electron trapped in defects coupled to Mo ions or the interaction between conduction electrons and 4f orbitals of Mo4+ ions in the Bi2MoO6 matrix. Apparently, the amount of oxygen vacancy and Mo4+ increases with the doped cerium concentration. Usually, Ce3+ ions ([Xe]4f15d06s0, 2F5/2) present in the sample are undetected by ESR measurements above liquid nitrogen temperature. Therefore, ESR experiment in liquid argon was carried out to detect Ce3+ signals. As shown in Figure 2f, three signals (g1=2.00, g2=2.02 and g3=2.03) were observed in this spectrum, which are attributed to the presence of Ce3+ sites in the solid.35


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Figure 2. High-resolution Ce 3d (a), Mo 3d (b), and Bi 4f (c) XPS spectra of Ce-BMO samples, illustration of doping sites in Bi2MoO6 crystal structure (d) and ESR spectra of Ce-BMO samples at room temperature (e) and in liquid argon (f). The morphologies of Ce-doped BMO samples were also characterized by SEM and TEM images, as shown in Figure S2 (Supporting Information). All the Ce-BMO samples display a


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sheet-like morphology, similar to pure Bi2MoO6 sample. Noticeably, the average thickness of Ce-BMO nanosheets decreased from 35 to 20 nm with the increase of cerium doping amount into Bi2MoO6 crystal lattice. HRTEM images of Ce-BMO samples reveal two kinds of perpendicularly arranged lattice fringes, which corresponds to the vertically crossed (002) and (060) planes of orthorhombic Bi2MoO6, indicating that the nanosheets were exposed with (200) crystal planes. The calculated BET surface area also suggests that the cerium doping into Bi2MoO6 nanosheets benefits the increase of BET surface area due to the decrease of its thickness (Figure S3, Supporting Information). Based on the results, the crystal defects engineering of Aurivillius Bi2MoO6 after cerium doping was summarized in Scheme 1. As shown in Scheme 1, Aurivillius Bi2MoO6 possesses unique layered structures sandwiched between the corner-sharing MoO6 octahedron and the (Bi2O2)2+ layers. Cerium doping process could result in different types of crystal defects, such as symmetry disturbance, redox couple and oxygen vacancy. At the beginning of doping process, cerium species, including Ce3+ and Ce4+, were introduced into Bi3+ sites, which resulted in subtle variation in (Bi2O2)2+ layers due to the different ion radius. On the other hand, the presence of oxidized cerium species (Ce4+) disturbed the charge balance of Bi-Mo-O system, leading to the generation of Mo4+ and metallic bismuth for the charge compensation. It is the formation of Mo4+ that enables the substitution of Ce4+ for Mo4+, which distorted the Mo-O layer evidently. Besides, oxygen vacancies were also produced in Ce-BMO samples during the cerium doping process. Therefore, both the ion substitution and generation of oxygen vacancies after cerium doping could destroy the corner-sharing octahedral structure for the lack of equatorial oxygen and low coordination of molybdenum ions.


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Scheme 1. Illustration of the possible formation process of crystal defects in Ce-doped Aurivillius Bi2MoO6 structure. It is believed that crystal defects could influence the charge carrier dynamics of Ce-BMO samples, including excitation, separation, trap and transfer, which acts as the initial charge carriers acceptor to inhibit electron-hole recombination, as well as promotes the interfacial charge transfer from the excited Ce-BMO nanosheets to adsorbed molecules.36 Figure 3a shows the transient photocurrent responses of as-prepared Ce-BMO samples. The distinguished photocurrent density implies that the crystal defects in Bi2MoO6 facilitate good excitation of carrier, and 10% Ce-BMO displays almost twice as high photocurrent density as that of pure BMO. The weakest photoluminescence emission intensity of 10% Ce-BMO was also observed in the photoluminescence spectra of Ce-BMO samples (Figure 3b), illustrating that 10% Ce-BMO processed the best electron-hole separation property. However, 20% CeBMO sample with higher defects density shows relatively stronger emission intensity and


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lower photocurrent density, compared with 10% Ce-BMO. It is probably because the excessive existence of electron trapping in 20% Ce-BMO, such as oxygen vacancy and redox couples (Ce3+/Ce4+ and Mo4+/Mo6+), acts as recombination centers to inhibit the charge separation, thus resulting in the decrease of electrical current carriers and therefore a low photocurrent density.37 The electrochemical impedance spectroscopy (EIS) Nyquist plots also demonstrate that 10% Ce-doped BMO sample (10% Ce-BMO) shows the fastest interfacial charge transfer characteristics, compared with pure BMO and other Ce-doped BMO samples (Figure 3c). While 20% Ce-BMO sample with more crystal defects exhibits a higher impedance than that of 10% Ce-BMO sample, which is probably owing to the excess electron trapping effect and the breaking of corner-sharing structure in Mo-O layer of Bi2MoO6. It implies that an appropriate amount of crystal defects could cause reduction in impedance, facilitating the migration of photo-induced carriers. To further understand the recombination process in Ce-BMO samples, we performed timeresolved photoluminescence experiments, and the normalized decay profiles were shown in Figure 3d. The photoluminescence peak decay was fitted by using the curve-fitting tool cftool (Matlab) to retrieve both PL amplitudes and lifetimes with a nonlinear least-squares interpolation, as depicted in Figure S4.38,39 Double-exponential decay curves, assumed as model functions, were found to adequately fit the signal of decay profiles. More specifically, the following model function was used to fit the normalized PL decay profiles:

I(t )  A1 exp( t / 1 )  A 2 (t /  2 ) where I(t) is the intensity of PL signal, τ1 and τ2 are the lifetimes and A1 and A2 are the corresponding magnitudes. As listed in Table 1, the relevant parameters reveal a very fast (nanoseconds) relaxation, which could be separated into two exponential decay components


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of the fast recombination of excitons and a relatively long-lasting emission.39,40 Among them, 10% Ce-BMO shows much slower PL decay and the longest average lifetime (40.22 ns) among Ce-BMO samples, which is almost four times longer than that of pure BMO (11.51 ns), indicative of an increased permanence of long living excited states. It may originate from the electrons trapping by crystal defects.41 It illustrates that appropriate amount of crystal defects caused by cerium doping is beneficial for increasing the number and persistence of long-living charged trapped species.

Figure

3.

Transient

photocurrent

response

under

visible

light

irradiation

(a),

photoluminescence spectra (b), electrochemical impedance spectroscopy (EIS, c) and the normalized decay profiles (d) of Ce-BMO samples.


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Table 1. Parameters obtained from time-resolved PL decay curves according to a doubleexponential decay. Sample

1 (ns)

2 (ns)

A1 (%)

A2 (%)

Average Lifetime (ns)

Pure BMO

3.89

39.95

26.69

73.31

11.51

5% Ce-BMO

4.62

81.70

19.52

80.48

19.19

10% Ce-BMO

7.55

210.60

15.75

84.25

40.22

20% Ce-BMO

6.49

133.06

19.75

80.25

27.44

The surface work function (φ) was measured by scanning Kelvin probe (SKP), which provided with the information about the surface electronic properties.42 Figure 4a-d shows the work function maps of Ce-BMO samples, which display the distinct potential changes, i.e. remarkable work function changes. The obtained surface work functions (φ) are 5.12, 5.06, 5.01 and 4.93 eV for pure BMO and 5%, 10% and 20% Ce-doped BMO samples respectively. Based on the work function, we could determine the Fermi level and the built-in electric field of the BMO samples. It was found that the Fermi level was escalated with the incorporation of cerium dopants, indicating that new energy levels, i.e. defects states, were formed on the bottom of the conduction band, as illustrated in Figure 4e. As an n-type semiconductor, the flat band potential is equal to its Fermi level. Hence, Mott-Schottky measurement was performed to calculate the flat band potential of Ce-BMO samples to confirm the variation of Fermi level (Figure 4f). The increased flat potential, i.e. Fermi level, with the incorporation of cerium dopants is approximately in accordance with SKP results. In this case, the electrons are more easily excited from defect state energy level to the conduction band of Bi2MoO6, resulting in a better electron excitation, as confirmed by the transient photocurrent response results (Figure 3a). Moreover, lower surface work function weakens the restriction to


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photoelectron and increases the bending of surface band, which facilitates the carrier migration.43

Figure 4. The relative work function maps (a-d), schematic illustration of Fermi level variation (e) and Mott-Schottky plots (f) of Ce-doped BMO samples.

Generally, reactive oxygen species (ROS), such as hydroxyl radicals (•OH), superoxide radical anions (•O2-) and hydrogen peroxide (H2O2), were considered as the main reactive species involved in photocatalytic reactions.44-46 To investigate the influence of cerium doping on the ROS production capacity, ESR-DMPO and colorimetric DPD method were used to determine the produced ROS in photocatalysis. The formation of •DMPO-OH was firstly studied in water upon visible light irradiation of the aerated Ce-BMO suspensions. As shown in Figure 5a, in the presence of DMPO, typical fourline EPR spectra were detected, which were attributed to the hydroxyl radical added to DMPO (•DMPO-OH). Obviously enhanced ESR signal was observed in the presence of Ce-BMO samples, demonstrating the increase of


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hydroxyl radicals under visible light irradiation. Moreover, the hydroxyl radical concentrations generated by different Ce-BMO samples were quantitatively determined, as shown in Figure 5c. It illustrates that the production of •OH was greatly enhanced after cerium doping. The hydroxyl radical concentration generated by 10% Ce-BMO sample (6.51 M) is 4 times higher than that of pure BMO sample (1.91 M). However, further increase of the doping content to 20% is not beneficial for • OH generation because of the deteriorated carrier dynamics character. Superoxide radical anions (•O2-) generated by different Ce-BMO samples were also detected in aprotic solvent (DMSO) in the presence of DMPO, as shown in Figure 5b. The concentrations of the spin adducts •DMPO-O2- in the different Ce-BMO suspensions are determined and illustrated in Figure 5d. 10% Ce-BMO sample with proper amount of crystal defects also exhibits the highest •DMPO-O2- concentration, which is approximately 5 times stronger •O2- production ability than that of undoped Bi2MoO6. The formation of H2O2 was examined by colorimetric DPD method under visible light irradiation, as shown in Figure 5e. It is found that BMO samples could promote the photo-generation of H2O2, especially for Cedoped BMO samples. More importantly, the concentration of produced H2O2 increased from 6 µM (pure BMO) to 20 µM (10% Ce-BMO) within 1 h. To exclude the effect of surface area, the values of ROS production rate per unit surface area of the catalysts (k/SBET) were calculated to evaluate the photo-generated ROS efficiency of Ce-BMO samples. Figure 5f shows the volcano-type k/SBET values curves of different ROS over Ce-BMO samples. 10% Ce-BMO sample with optimal charge carrier dynamic property possesses the highest k/SBET values for ROS species. Further increasing cerium dopant to 20% leads to a decrease of the k/SBET values, although 20% Ce-BMO possesses the highest BET surface area.


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Figure 5. ESR spectra of DMPO-•OH (a) and DMPO-•O2- (b) of Ce-BMO samples under visible-light irradiation upon a given period of time (3 min for •DMPO-OH and 10 min for •DMPO-O2-); measured •DMPO-OH (c) and •DMPO-O2- (d) spin adduct concentration and the production of H2O2 (e) in the presence of Ce-BMO samples under visible light; production -

rate per unit surface area of •OH, •O2 and H2O2 of Ce-BMO samples under visible light irradication (f).


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It was proposed that the redox couples generated by crystal defects of Ce-doped Bi2MoO6 played a crucial role in the production of ROS species via one-electron reaction and twoelectron reaction pathway. The redox couple of Ce3+/Ce4+ and Mo4+/Mo6+ could facilitate the one-electron and two-electron reaction, separately. The superoxide radical anions trapped were originated from the interaction between molecular oxygen and photo-generated electrons according to eqs 1-2. Ce-BMO + h → h+ + e− O2 + e− → •O− 2

(1) (2)

The redox couples of Ce3+/Ce4+ could act as an electron scavenger to facilitate the one-electron reaction in the generation of superoxide radical anions according to eqs 3-4.47 4+ Ce3+ + O2 → •O− 2 + Ce

(3)

Ce4+ + e− → Ce3+

(4)

It is believed that H2O2 is mainly originated from two pathway, i.e., one-electron reaction (eqs 5-7) and two-electron reaction (eq 8).26 H2 O + h+ → •OH + H+

(5)

+ •O− 2 + H → •O2 H

(6)

2 •O2 H → H2 O2 + O2

(7)

2H+ + O2 + 2e− → H2 O2

(8)

Obviously, the one-electron pathway could be strengthened by the redox couple of Ce3+/Ce4+ in the as-synthesized Ce-BMO samples. More importantly, the two-electron reaction is considered as a more effective way to generate H2O2, which occurs in extremely small probability due to the lack of double electron sites in most photocatalysts. It is proposed that the presence of Mo4+/Mo6+ redox couples in Ce-BMO samples could promote the


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generation of H2O2 through the two-electron reduction of O2. As illustrated in Figure S5A (Supporting Information), the photo-generated electrons could migrate efficiently in Mo-O layers via the corner-sharing MoO6 octahedral structure. However, the disconnection of MoO octahedron caused by the generation of Mo4+ would break the electron transfer pathway, leading to the accumulation of electrons at the breaking corner. It is believed that the electrons accumulation in the Mo4+/Mo6+ redox couple sites could facilitate the H2O2 production through the following pathway (eqs 9-10). 2H+ + O2 + Mo4+ → Mo6+ + H2 O2

(9)

Mo6+ + 2e− → Mo4+

(10)

To further confirm the contribution of the two-electron reaction to the production of H2O2, nitroblue tetrazolium (NBT) was used as the •O2- scavenger to eliminate the influence of oneelectron reaction pathway on the H2O2 production. As shown in the absorption spectra of the DPD/POD reacted with H2O2 in the presence of 10% Ce-BMO and NBT (line a), pure BMO and NBT (line b) under visible light irradiation for 30 min (Figure S5B, Supporting Information), pure BMO sample almost lost the capability of H2O2 production, whereas 10% Ce-BMO still could produce H2O2 upon the addition of NBT. It indicates that the improved two-electron reaction in Ce-BMO samples caused by the redox couple of Mo4+/Mo6+ benefits the production of H2O2. Moreover, we performed XPS spectra of 10% Ce-BMO after 6 h irradiation in water to confirm that the Mo4+ and Mo6+ were only a couple of electron mediator, instead of reactants. As shown Figure S6 (Supporting Information), the relative content of Mo4+ and Mo6+ in the redox couples unchanged before and after visible light irradiation, implying that the Mo4+/Mo6+ was only an electron mediator and could recycle without depletion.


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On the basis of above-mentioned analysis, it is considered that the enhanced ROS production efficiency of Ce-doped BMO sample could significantly improve their redox capability in photocatalysis, especially for degradation-resistant pollutants. Therefore, highly toxic nerve agent simulants (NAS), including methyl paraoxon (MP) and bis(4-nitrophenyl) phosphate (b-NPP) were selected as the targets to evaluate the photodegradation ability of the crystal-defects-engineered Ce-BMO samples, which are considered the most nefarious synthetic chemical compounds that threaten humans and the environment because of their phosphorylating mode of action.48 Upon visible light irradiation, the photodegradation efficiency of MP and b-NPP over Ce-BMO samples is much higher than that of pure BMO sample and strongly dependent on the amount of cerium dopant, as shown in Figure 6a and 6b. 10% Ce-BMO with the highest ROS production ability possesses the highest photocatalytic activity, which indicates that excessive cerium doping amount could lead to a decrease of NAS photodegradation efficiency. In order to confirm the complete mineralization of MP and b-NNP, the photodegradation of 4-nitrophenol (a hydrolysis product of MP and bNPP) was performed under identical conditions, illustrating that the decomposition process occurs primarily through redox reactions and promotes complete mineralization, rather than hydrolysis (Figure S7, Supporting Information). To further understand the ROS-induced photodegradation process of MP and b-NPP over Ce-BMO photocatalysts, HPLC-MS analysis was employed to investigate the photodegradation pathway. It is considered that the photocatalytic degradation of nerve agent simulants molecules (MP and b-NPP) begins with cleavage of the P-O-C bond and produces secondary products (4-NP).49,50 Therefore, 4-NP was selected as the target reactant to detect the degradation intermediates by HPLC-MS during photocatalytic degradation. The representative HPLC chromatogram of the solution before


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and after irradiation for 15, 30 and 45 min, together with the corresponding mass spectra were shown in Figure 6c and Figure S8 (Supporting Information). Obviously, the peak intensity of 4-NP decreased gradually with the increase of irradiation time. And new peaks (retention time around 1 to 3 min) were observed during the photodegradation process, which could be indexed to different degradation intermediates. It illustrated that 4-NP was mineralized through an oxidation step-reaction. Based on the identified intermediates and the reported literatures,51,52 the possible degradation pathway of MP and b-NPP in the Ce-BMO photocatalytic system is proposed and summarized in Figure 6d. In addition, the TOC removal results further confirmed that both MP and b-NPP could be completely mineralized in a given period of time (Figure S9, Supporting Information).

It is believed that the complete

mineralization of nerve agent simulants and phenolic compounds is ascribed to the redox reaction by the highly active ROS species generated on the Ce-BMO surface.


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Figure 6. Photodegradation efficiency of MP (a) and b-NPP (b) of Ce-BMO samples under visible light irradiation; representative HPLC chromatograms of 4-NP during the photodegradation (c) and the proposed degradation pathway of MP and b-NPP over Ce-BMO upon visible-light-irradiation (d). Ce-doped BMO samples also exhibit significantly enhanced photocatalytic disinfection efficiency against S. aureus, a gram positive bacteria, compared with pure BMO samples. Figure 7a shows the photoinactivation ability of the Ce-BMO samples against S. aureus, also


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demonstrating that 10% Ce-BMO sample possesses the highest photoinactivation efficiency among Ce-BMO samples. 20% Ce-BMO sample with excess crystal defects shows relatively lower bactericidal efficiency than 10% Ce-BMO. For comparison, direct S. aureus photolysis exhibits negligible disinfection activity under visible irradiation. Moreover, it is found that the photocatalytic activity of Ce-BMO sample is much higher than that of commercial Degussa P25, indicative of considerably promising prospects in practical photocatalysis. In addition, the versatility of the crystal-defects-engineered Ce-BMO samples was also demonstrated by the degradation of organic dyes. As shown in Figure 7b and 7c, the photodegradation efficiency of MB and RhB over Ce-BMO samples is much higher than that of pure BMO sample and strongly dependent on the amount of cerium dopant. The highest photocatalytic activity and largest apparent rate constant per unit surface area (k/SBET, 28.5 m2

•h-1 for MB and 5.4 m-2•h-1 for RhB) are also achieved on 10% Ce-BMO with the best charge

carrier dynamic property (Figure S10, Supporting Information). An excessive amount of cerium dopant (20% Ce-BMO) also results in a decrease of dyes photodegradation efficiency. The complete mineralization and photodegradation pathway of MB and RhB over 10% CeBMO photodegradation was also confirmed by TOC removal results (Figure S11, Supporting Information) and HPLC/MS analysis (Figure S12 and S13, Supporting Information). The possible photodegradation pathway of MB and RhB was summarized in Figure S14 (Supporting Information). 53,54


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Figure 7. Photocatalytic disinfection efficiency against S. aureus (a) and photodegradation efficiency of MB (b), RhB (c) of Ce-BMO samples under visible light irradiation. From the viewpoint of practical applications, the chemical and physical stability of photocatalyst is of significant importance. Therefore, recycling photocatalytic experiments over 10% Ce-BMO sample were also conducted. As shown in Figure S15 (Supporting Information), 10% Ce-BMO sample remains excellent photodegradation efficiency and its after five consecutive photodegradation cycles, implying that Ce-BMO photocatalyst possesses favorable stability and no photocorrosion occurs during the photodegradation process. The overall effect of crystal defects engineering by cerium doping on the structure and photocatatlytic abilities of Ce-BMO samples are summarized in Scheme 2. Proper amount of cerium doping could induce lattice distortion and crystal defects, and the generated redox couples and oxygen vacancies could facilitate the carriers dynamics and ROS production. Subsequently, it improves the photocatalytic detoxification performances.


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Scheme 2. Schematic representation of the photocatalytic degradation of dyes and photoinactivation of bacterium by Ce-doped Bi2MoO6 photocatalyst.

More importantly, the crystal defects engineering caused by Ce-doping was not only limited in Bi2MoO6, but also found in Ce-doped Aurivillius structured Bi2WO6 (Ce-BWO). The formation of Ce-BWO samples with different cerium doping contents was confirmed by XRD, as shown in Figure 8a. All the XRD patterns could be indexed to pure Bi2WO6, and amplifying patterns show a peak shift accompany with the variation of cerium contents, which is similar to that of Ce-BMO samples. The crystal defects and distortion formed in Bi2WO6 crystal structure were further examined by XPS spectra of 40% Ce-BWO (Figure 8b and 8c). Noticeably, the redox couples of Ce3+/Ce4+ and W4+/W6+ could be easily found in 40% CeBWO sample, verifying the successful crystal defects manipulation in Bi2WO6 by Ce-doping. Moreover, the contribution of crystal defects to photocatalytic activity of Ce-BWO samples was examined. As illustrated in Figure 8d and 8e, 10% Ce-BWO shows the best photocatalytic performance for organic dye degradation and bacterial inactivation under visible light irradiation. However, the enhancement of photocatalytic efficiency over 40% Ce-BWO is


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much smaller than that of 10% Ce-BWO, indicating that proper amount of crystal defects in Bi2WO6 lead to the best photocatalytic performance. Obviously, we could also easily mediate the crystal defects by varying the amount of cerium dopant to optimize the photocatalytic activity of Ce-BWO samples.

Figure 8. XRD patterns (a) and high-resolution Ce 3d (b) and W 4f (c) XPS spectra of CeBWO samples ; Photocatalytic degradation of MB (d) and disinfection efficiency against S. aureus (e) in the aerated aqueous suspensions of Ce-BWO samples under visible-light irradiation.

4. CONCLUSIONS ACS Paragon Plus Environment

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In surmmary, we developed a Ce-doping strategy to fabricate and mediate crystal defects in Aurivillius Bi2MoO6 for improving photocatalytic performance. By varying the dopant content, the two stage doping process involving ions substitution, charge compensation and structural distortion was demonstrated. The well-engineered crystal defects were proved to boost electron dynamics by trapping effect and mediate the band structure by introducing new energy levels, leading to the improvement of electrochemical properties and the resulted enhanced photocatalytic activity. In particular, the redox couples generated in Ce-doping process, including Ce3+/Ce4+ and Mo4+/Mo6+, could evidently promote the ROS production by one-electron and two-electron reaction, separately. The crystal defects engineering method by Ce-doping not only provides new opportunities for boosting the photocatalytic activity of Bi2MoO6-based materials, but also offers a novel strategy to fabricate high-performance photocatalysts by manipulating the type and concentration of crystal defects.

ASSOCIATED CONTENT Supporting Information Calculated Mo-O bond length and content variation of Ce, Bi and Mo ions in different CeBMO samples; Cerium relative content and molar quantities of Ce, Bi and Mo in Ce-BMO samples; SEM, TEM and HRTEM images of Ce-BMO samples; N2 adsorption-desorption isotherms and pore size distributions of Ce-BMO samples; Fitting on photoluminescence decay data of Ce-BMO samples; Illustration of electrons accumulation in Mo-O layer of CeBMO samples; Absorption spectra of the DPD/POD reacted with H2O2 in the presence of 10% Ce-BMO and NBT, pure BMO and NBT upon 30 min visible light irradiation; High-resolution Mo 3d and Ce 3d XPS spectra of 10% Ce-BMO samples before and after irradiation for 6 h


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under visible light in water; Absorbance spectra of 4-NP (Co=1.25 M) after 30 and 60 min treatment with 10% Ce-BMO under visible light irradiation; Pseudo-first-order kinetic constants per unit surface area of Ce-BMO samples under visible light irradiation; Recycle RhB and MB photodegradation results of 10% Ce-BMO under visible light irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Phone: (+86)13659815698; fax: (+86)2787195680 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21471121, 21171136), High-Tech Industry Technology Innovation Team Training Program of Wuhan Science and Technology Bureau (2014070504020243) and Open Research Fund of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (Jilin University, 2014-09).

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ACS Catalysis

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A general strategy was developed to manufacture and mediate crystal defects in Bi2MoO6 lattice by varying the cerium dopant content, leading to the greatly improved visible-light-driven photocatalytic performance for the degradation of highly toxic nerve agent simulants and organic dyes, as well as bacterial photoinactivation. 443x228mm (150 x 150 DPI)


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Crystal Defect Engineering of Aurivillius Bi2MoO6 by Ce Doping for

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