Multi-functional Bi2O2(OH)(NO3) nanosheets with {001} active

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Multi-functional Bi2O2(OH)(NO3) nanosheets with {001} active exposing facets: Efficient photocatalysis, dye-sensitization and piezoelectric-catalysis Lin Hao, Hongwei Huang, Yuxi Guo, and Yihe Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03223 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Sustainable Chemistry & Engineering

Multi-functional Bi2O2(OH)(NO3) nanosheets with {001} active exposing facets: Efficient photocatalysis, dye-sensitization and piezoelectric-catalysis Lin Hao†, Hongwei Huang,*,† Yuxi Guo†, Yihe Zhang*,† †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid

Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, 29 Xueyuan Road, Haidian District, Beijing 100083, China

*Corresponding author. e-mail: [email protected] (Hongwei Huang); [email protected] (Yihe Zhang)

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ABSTRACT: Exploration for multi-responsive catalytic materials and synthesis of highly-active exposing crystal facets are challenging subjects for the catalysis research. In this work, well-defined Bi2O2(OH)(NO3) nanosheets (BON-S) with dominantly exposed {001} active facet were synthesized by a sodium dodecyl benzene sulfonate (SDBS)-assisted soft-chemical route. BON-S nanosheets present far superior photocatalytic activity compared to bulk materials as well as a universal performance for degradation of contaminants and antibiotic under UV light. The profoundly enhanced photocatalytic activity basically stems from the largely shortened diffusion pathway of photogenerated electrons (e-) and holes (h+), favoring their migration from bulk to surface of catalyst under the internal electric field between [Bi2O2(OH)]+ and NO3- layers along [001] direction. The photocatalytic active species production rates of BON-S are determined to be 3.14 µmol·L-1·min-1 for superoxide radicals (·O2−) and 0.03 µmol·L-1·min-1 for hydroxyl radicals (·OH), respectively. BON-S nanosheets also show an enhanced visible-light responsive dye-sensitization degradation activity with Rhodamine B (RhB) as a sensitized medium to provide photoinduced e-. Moreover, we for the first time unearth that Bi2O2(OH)(NO3) demonstrates an ultrasonic-assisted piezoelectric-catalytic performance for decomposition of methyl orange, bisphenol A and tetracycline hydrochloride, and ·OH dominates the piezoelectric-catalytic process with an evolution rate of 7.13 µmol·L1

·h-1, which far exceeds the photocatalytic-induced one. This study may cast

new inspirations on developing new microstructure-design strategy for high photocatalytic/dye-sensitization performance, and furnishes a novel piezoelectric-catalytic material for environmental applications.


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KEYWORDS:

Bi2O2(OH)(NO3);

photocatalytic;

dye-sensitization;

piezoelectric-

catalytic; active facet

INTRODUCTION Photocatalytic

semiconductors

can

degrade

organic

pollutants to non-

toxic small molecules through photo-induced oxidation reactions and are harmless to surroundings.1-5 Compared to conventional semiconductor photocatalysts, the layered bismuth-based materials, including Sillén-structured BiOX (X=Br, I, Cl),6-12 Aurivilliusstructured Bi2XO6 (X=Mo, W),13-16 Sillén-structure-related (BiO)2CO3,17-22 etc, showed powerful photooxidation ability in degradation of various contaminants. The specific layered structures of layered bismuth-based materials not only support the forming of an internal electric field to allow the charges diffusion between layers, but also provide abundant distance to polarize orbitals and atoms, enabling the electron-hole pairs separate efficaciously. Piezoelectric-catalysis is a novel concept proposed by our group lately, which is that the non-centrosymmetric (NCS) polar materials could utilize the piezoelectric-induced charges/potentials to initiate the molecular/ionic oxygen activation to produce abundant reactive oxygen species (ROS) and efficiently degrade diverse contaminants.23,24 It is demonstrated by us that the macroscopic polarization enhancement induced by the replacement of V5+ ions for I5+ in BiOIO3 largely facilitates the charge separation in the photocatalytic and piezoelectric-catalytic process, and thus gives rise to largely strengthened photo- and piezoelectric induced ROS evolution, e.g. superoxide radicals (·O2−) and hydroxyl radicals (·OH).23 The Aurivillius-structured Bi4Ti3O12 was also shown to display 3 ACS Paragon Plus Environment


piezoelectric-catalytic activity for degradation of a range of industrial contaminants and antibiotic, including methyl orange, bisphenol A and tetracycline hydrochloride under ultrasonic treatment.24 Recently, two new Sillén-structure-related photocatalysts, Bi2O2[BO2(OH)]25 and Bi2O2(OH)(NO3),26 have been reported by our group. Interestingly, they not only contain layered structures that are constructed by [Bi2O2]2+/[Bi2O2(OH)]+ layers and alternately interleaved BO2(OH)2-/NO3- along z-axis, but also crystallize in NCS polar crystal structures with monoclinic space group Cm and orthorhombic space group Cmc21.27-29 The absence of a symmetric center in local structure could induce a large spontaneous macroscopic polarization to promote the separation of photogenerated electron-hole pairs.30,31 Particularly, Bi2O2(OH)(NO3) (donated as BON) was demonstrated to possess superior UV photocatalytic activity, exceeding P25 and some other benchmark photocatalysts, with generation of ·O2− and ·OH as reactive species. Currently, the investigations on BON are very absent, and there are only two other papers reported on BON. Han et al reported the synthesis of flower-like BON hierarchical microstructures by incomplete hydrolysis of anhydrous bismuth nitrate (Bi(NO3)3) after adsorption of glacial acetic acid,32 and Zscheme BiPO4-Bi2O2(OH)(NO3) heterojunction was reported, which shows enhanced photocatalytic performance for degradation of 2,4-DCP.33 But the progress in the research of facet-dependent photocatalytic activity of BON has not been achieved. In view of the exposure of specific facet of layered bismuth-based semiconductors could induce strong internal electric field favorable for charge separation, synthesis of BON nanosheets with dominantly exposed specific facet, herein {001} facet, is of great significance. In addition, broadening the photoabsorption region of BON photocatalyst to visible light region 4 ACS Paragon Plus Environment

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by using dye as a sensitization medium, seems highly meaningful from the viewpoint of solar-energy conversion. More importantly, considering the NCS polar crystal structure of BON, exploration for piezoelectric-catalytic performance of BON is high desirable and meaningful. Herein, Bi2O2(OH)(NO3) nanosheets with {001} active exposing facets are synthesized via a sodium dodecyl benzene sulfonate (SDBS)-assisted one-pot hydrothermal method. The photocatalytic activity of obtained BON sample with nanosheet morphology features show large enhancement than that of bulk BON for degrading methyl orange (MO) under UV light irradiation. It also shows an universal photocatalytic performance for degradation of antibiotic and phenolic contaminants, such as tetracycline hydrochloride, 2,4-dichlorophenol (2,4-DCP), phenol and bisphenol A (BPA). To extend the photoresponsive range of BON, Rhodamine B (RhB) was used as an efficacious sensitizer to enable BON nanosheets an enhanced degradation activity via dye-sensitization. More interestingly, the piezoelectric-catalytic performance of BON was firstly reported here to extend the application of the photocatalyst, and diverse pollutants are successfully degraded in the piezoelectric-catalytic process. Additionally, the reactive species generated in the photocatalytic and piezoelectric-catalytic process are inspected and quantificationally determined by active species trapping and ·O2−/·OH quantification experiments. Our work paves a new way for crystalline structure design and novel piezoelectricity application on layered bismuth compounds.

EXPERIMENTAL SECTION



Preparation. The chemical reagents used were of analytical grade and without further purification. 3 mmol Bi(NO3)3·5H2O was dissolved into 30 ml deionized water and under ultrasonic for 30 min at room temperature. Then, the solution was subsequently stirred for another 30 min and transferred into a 50 mL Teflon lined stainless steel autoclave, sealed and maintained at 150℃ for 24 h. After cooling, the resulting white products were collected by filtration and washed repeatedly with deionized water and ethanol and then dried at 80℃ for 10h to obtain bulk BON sample. The Bi2O2(OH)(NO3) nanosheets were synthesized by adding 0.15 mmol SDBS into the above Bi(NO3)3 suspension before heating, with the other conditions unchanged. And the final products were named as BON-S. 3 mmol Bi2O3 was first dispersed in 60 ml deionized water, and then 6 ml of 1M HNO3 water solution was dropwise added into the above Bi2O3 suspension. After being stirred for 30min, the resulting white suspension was transferred into a 100 ml Teflon autoclave and heated at 150℃ for 24 h. Subsequently, the autoclave was cooled down to room temperature naturally, and the solid product was washed with ethanol and deionzied water for several times. At last, the final product was dried at 80℃ for 10 h and named as BON-O. Characterization. The crystal structures of the as-prepared samples were determined by X-ray diffraction (XRD) using a Cu Kα radiation Bruker D8 Advance diffractometer. X-ray photoelectron spectroscopy (XPS) was conducted for determining the elementary composition and the surface states on a Perkin-Elmer PHI 5000C X-ray photoe6 ACS Paragon Plus Environment

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lectron spectroscopy. Scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, high-resolution TEM, JEM-2100F) were carried out to confirm the microstructure and morphology. UV–vis diffuse reflectance spectra (DRS) were recorded on a Varian Cary 5000 UV−vis spectrophotometer. The specific surface areas were performed on 3020 Micromeritics instrument through the BET nitrogen adsorption method. Surface photovoltage (SPV) spectra were recorded by a self-made instrument. A 500 W xenon lamp (CHF XQ500W, Global xenon lamp power made in China) coupling with a double-prism monochromator (Hilger and Watts, D 300 made in England) to provide the monochromatic light. A lock-in amplier (SR830-DSP, made in US) synchronized with a light chopper (SR540, made in the US) was used to amplify the photovoltage signal.34 The photoluminescence emission (PL) spectra were obtained on a Hitachi F-4600 fluorescence system with a xenon lamp (400 V, 150 W) as an excitation light. Timeresolved fluorescence emission spectra were recorded at room temperature with a fluorescence spectrophotometer (Edinburgh Instruments, FLSP-920). The total organic carbon (TOC) removal of MO was determined on a total organic carbon analyzer (TOC, Shimadzu 500). Photocatalytic activity test. The photocatalytic activities of samples were assessed via degrading varieties of industrial pollutants and antibiotic, including methyl orange (MO, 0.03 mM), phenol (10 mg/L), 2,4-dichlorophenol (2,4-DCP, 10 mg/L), tetracycline hydrochloride (10 mg/L) and bisphenol A (BPA, 10 mg/L), under UV light irradiation (a 300 W mercury lamp). The detailed degradation process was displayed as follows: 50 mg of catalysts was dispersed into 50 ml aqueous solution containing the pollutants. The suspension was stirred in the darkness for 1 h to accomplish absorption-desorption equilibri-



um. After illumination, 3 ml solution was collected at some time intervals and centrifuged to remove the powder. The absorption spectrum and absorption peak of centrifuged solution were analyzed using a UV–vis spectrometer (Hitachi U-3310). Ultrasonic-assisted piezoelectric-catalytic degradation. 200 mg catalysts were dispersed into 150 ml methyl orange solution (MO, 0.01 mM) in a 250 ml, threenecked, round-bottomed flask with a mechanical stirrer placing in an ultrasonic cleaner. To keep from the temperature and light interference, the reaction system was maintained at 0 ℃ in the ice-water bath, and they were placed in darkness. Similar to the photocatalytic degradation experiment, the suspension was first stirred without ultrasonic wave to reach equilibrium of absorption and desorption. Then the ultrasonic cleaner was switched on to provide stress to the reaction system and about 4 ml of MO suspension was taken at specific intervals. The blank experiment under ultrasonic irradiation without catalysts was conducted in order to exclude influence of ultrasonic. Active species detection and •O2− and •OH quantification experiments. In order to detect the reactive species, different species quenchers were introduced into the process of photocatalytic degradation of pollutants using method similar to the photocatalytic experiment. Hydroxyl radical (·OH), holes (h+) and superoxide radical (·O2−) could be investigated through adding iso-propanol (IPA, 1 mM), ethylene diamine tetraacetic acid disodium salt (EDTA-2Na, 1 mM) and benzoquinone (BQ, 1 mM), respectively. ·O2− quantitative test was conducted through adding NBT as a chemical probe to detect the amount of ·O2− generated from BON catalysts during the reaction process and its maximum characteristic absorption peak was recorded at 259 nm.35 And the concentration variation of NBT was inspected on a UV–vis spectrophotometer. Terephthalic acid 8 ACS Paragon Plus Environment

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(TA) was a specific probe to measure the content of ·OH radicals. TA would transform into 2-hydroxyterephthalic acid with a particular maximum fluorescence emission at the wavelength of 425 nm by reacting with ·OH.36 By surveying the fluorescence intensity (F97XP, China) with the excitation wavelength of 315 nm, ·OH radicals concentrations were attained depending on the homologous linear relation between fluorescence spectrum intensity and concentration. The above ·O2− and ·OH quantification experiments are the same as MO degradation one except for replacing MO with NBT or TA. Dye-sensitized photodegradation. Rhodamine B (RhB, 0.02mM) was applied herein to be a sensitization medium so as to enhance the visible-light induced degradation performance of white BON samples. The photocatalytic degrading experiment was similar to the former photocatalytic experiment with the addition of RhB in the presence of pollutants under visible light illumination (λ>420 nm). Photoelectrochemical measurements. The electrochemical impedance spectra (EIS) and photocurrent density measurements were performed here using an electrochemical analyzer (CHI 660E, Shanghai) equipped with a systematic three-electrode pattern. The BON powders coated on ITO glass serve as the working electrode. The platinum (Pt) wire was set as the counter electrode and the saturated calomel electrode was used as the reference electrode. The working electrode was sampled by the method of dip-coating: 50 mg photocatalyst powder was dispersed into 5 mL ethanol to obtain uniform suspending liquid. And then the slurry was dropwise added on an indium–tin oxide (ITO) glass with size of 15 mm × 30 mm. Then the working electrode was dried at 373 K for 10 h to remove eth-



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anol. The illuminant source was simulated solar light provided by a Xe lamp (300 W) and the electrolyte used in this measurement was 0.1 M NaSO4 solution.

RESULTS AND DISCUSSION Structure,

composition

and

morphology.

The

crystalline

phase

of

Bi2O2(OH)(NO3) samples was analyzed by the XRD patterns, which are shown in Fig. 1. The diffraction peaks of all the BON samples could be indexed to the orthorhombic Bi2O2(OH)(NO3) (ICSD #15-4359). With the addition of the surfactant of SDBS and using Bi2O3 as a precursor, no impurity peaks were found in XRD patterns of BON-S and BON-O, reflecting that the three BON samples are all pure phase. The sharp and the strong peaks of (002) (112) and (114) indicated that they have high crystallinity. The particle size was estimated using the Scherrer’s formula:

D=

Kγ βcosθ

(1)

Where, D is the size of the particle, K=0.9 is the size factor, β is the full width of half maximum, γ is the wavelength of X-ray radiation used and 2θ is the angle at which the maximum intensity was observed. As nanocrystalline materials generally display broaden XRD peak relative to their bulk materials, the FWHM (full width at half maximum) of (204) peak for BON and BON-S was calculated for comparison. The FWHM of BON-S (0.24°) is slightly larger than that of BON (0.2°), demonstrating that nanosheet-structured BON-S shows broaden peak. The X-ray photoelectron spectroscopy (XPS) of the BON-S was conducted to investigate its chemical composition and surface state. As shown in Fig. 2a, the constituent el10 ACS Paragon Plus Environment

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ements Bi, O and N could precisely be probed. Fig. 2 b-d were the high resolution XPS spectra of Bi 4f, O 1s and N 1s, respectively. The binding energies of Bi 4f7/2 and Bi 4f5/2 were 159.4 and 164.7 eV (Fig. 2b), respectively, which are attributed to the Bi3+.37 As shown in Fig. 2c, the O 1s peak could be deconvoluted into two different peaks at 531.9 and 530.1 eV, which were separately assigned to the O-H bond and lattice Bi-O bond,38 respectively. The peak for N 1s located at about 406.5 eV (Fig. 2d). The XPS analysis of BON-S further confirmed the successful synthesis of pure Bi2O2(OH)(NO3). The microstructures of BON, BON-O and BON-S were investigated by scanning electron microscopy (SEM), as shown in Fig. 3. As observed from Fig. 3a-b, bulk BON synthesized from Bi(NO3)3·5H2O were irregular thick sheets with a diameter of 400-1000 nm. The microstructure of BON-O (synthesized from Bi2O3) sample was similar to that of BON (Fig. 3c-d). The synthetic strategy by using different raw materials seems to have no significant effect on crystal morphology of Bi2O2(OH)(NO3). Upon introducing SDBS as the surfactant into Bi(NO3)3·5H2O suspension, the morphology of BON become very regular and the surface of BON became smooth. Fig. 3e-f showed that the BON-S products were composed of loosely packed nanosheets. This morphology features might not only provide large specific surface area, but also offer some other advantages, like more efficient charge separation, which would be discussed later. The TEM patterns of BON-S (Fig. 4a) further demonstrated that the surface of pristine BON became smooth and regular with the addition of SDBS. The selected area electronic diffraction (SAED) pattern of BON-S (Fig. 4b) confirmed its single-crystal nature. As shown in Fig.4b, there were four sets of diffraction spots which could be indexed to



-

the (110)/(110) and (200)/(020) lattice planes of BON-S, respectively.26 Thus, the main exposing facet of BON-S was {001} facet. Fig. 4c displayed the three-dimensional crystal structure of BON, which possesses a Sillén-related layered structure composed of [Bi2O2(OH)]+ layers (Fig. 4d) with NO3- slices interleaved inside. It is obvious that the stacking direction of [Bi2O2(OH)]+ layers is along the c axis, namely, the self-built electric field direction. Thus, the dominant exposure of {001} facet of BON is expected to be very favorable for the separation of photogenerated electron-hole pairs along [001] direction.26 The nitrogen adsorption BET measurement was conducted here to measure the specific surface area of the BON samples. The specific surface areas of BON, BON-O and BON-S were 4.7, 3.0 and 6.8 m2/g, respectively. In comparison with the bulk BON and BON-O, the specific surface area is moderately increased by construction of BON-S nanosheets, which would favor the adsorption of pollutants and subsequently promote the photocatalytic/dye-sensitization reactions as discussed in the following. Optical absorption and energy band structure. The light absorption of the three Bi2O2(OH)(NO3) samples was investigated through UV-Vis diffuse reflectance spectroscopy (DRS). As shown in Fig. 5a, the absorption spectra of BON, BON-O and BON-S did not vary significantly and displayed sharp absorption in the UV region. The absorption edges of BON and BON-O are approximately 375 and 370 nm, respectively. Comparatively, BON-S shows the shortest absorption edge located at 365 nm. The optical energy band gap could be calculated from the plot of (ah υ )1/2 versus photon energy (h υ ),


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and the band-gap energy (Eg) of BON, BON-O and BON-S are estimated to be 3.25, 3.30 and 3.35 eV, respectively (Fig. 5b).39 As the type of a semiconductor and its band edge positions are of significance to the photocatalytic degradation activity of catalysts, Mott-Schottky (M-S) measurements were conducted to identify the semiconductor type and the flat band (FB) energy potential.40 The Mott-Schottky curves of BON, BON-O and BON-S were illustrated in Fig. 6a-c. The as-prepared BON samples are all n-type semiconductors and the FB energy potentials (Ef) of BON, BON-O and BON-S were about -0.62, -0.79 and -0.68 eV, respectively. For ntype semiconductor, the conduction band (CB) position was located quite close to the FB, which was 0.1-0.3 eV above FB.41 Therefore, the potentials of CB/valence band (VB) for BON, BON-O and BON-S were estimated to be -0.48/ 2.77 eV, -0.65/ 2.65 eV and -0.54/ 2.81 eV, respectively (Fig. 6d). The most positive VB position was observed for BON-S, which could allow the photogenerated holes to show stronger oxidative ability. Photocatalytic performance. The photocatalytic activity of the as-prepared BON samples was first evaluated by measuring the degradation of methyl orange (MO) under UV irradiation. As shown in Fig. 7a, for bulk Bi2O2(OH)(NO3) samples (BON and BON-O), both exhibited weak photocatalytic performance, and merely 38% of MO was degraded under 25 min illumination. With regard to the BON-S nanosheets synthesized with the assistance of SDBS, it displayed highly improved photocatalytic degradation, where about 90% of MO was photo-degraded under the same period and same other conditions. Fig. 7b showed that BON-S sample gradually destroyed the molecular structure of MO with increasing time. Based on the pseudo-first-order kinetics, experimental results showed that the apparent rate constants of BON, BON-C and BON-S were 0.021, 0.022 13 ACS Paragon Plus Environment


and 0.074 h-1, respectively (Fig. 7c). The optimal photocatalytic degradation rate presented by BON-S was 3.5 and 3.4 times that of bulk BON and BON-O, respectively. To investigate the generation and separation of photoinduced charge carries of BON samples in the photocatalytic process, transient photocurrent response measurements were performed.25,42 As shown in Fig. 8a, all the three as-prepared samples exhibited stable and reversible response of photocurrent. The immediate current signals of BON were approximately the same as BON-O with light on. BON-S sample displayed enhanced photocurrent response compared to the two bulk BON samples, which were 7.5 times as high as those of BON and BON-O, respectively. The electrochemical impedance spectra (EIS) showed in Fig. 8b indicate the interfacial charge transfer process. BON-S sample exhibited a distinctly smaller arc radius in the EIS Nynquist plots comparing to the two bulk BON, proving a much smaller interfacial resistance between electrolyte and electrode. The smaller arc radius and the improved photocurrent response revealed a strongly elevated separation and transfer of photogenerated charge carriers in BON-S.25,42 Photoluminescence (PL) is a useful method to investigate the recombination efficiency of photo-generated electrons and holes generated from the semiconductor photocatalyst.8 PL emission spectra of BON, BON-O and BON-S were shown in Fig. 9. The maximum emission peak is situated at 410 nm, and BON-S shows obviously lower PL emission intensity compared to BON and BON-O samples, indicating the low recombination and superior separation efficiency of electron-hole pairs in BON-S. To investigate the photo-excited charge carriers transfer dynamics of BON and BON-S, the nanosecondlevel time-resolved fluorescence decay spectroscopy was conducted and shown in Fig. 9b. The lifetime of BON and BON-S could be determined through fitting the decay curves 14 ACS Paragon Plus Environment

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with the analytical parameters summarized in the inset table. The short lifetime (τ1) and long lifetime (τ2) are 18.63 and 110.34 ns for BON and 21.59 and 131.61 ns for BON-S, respectively. It is obvious that both the short and long lifetimes are increased for BON-S. The prolonged lifetime demonstrated that the efficiencies of the carriers transfer and separation in nanosheet-structured BON-S are greatly enhanced, and the photoexcited charge carriers of BON-S are more prone to participate in the redox reactions instead of recombination. This result was in good accordance with the above MO degradation experiments under UV light and photocurrent observation. Surface photovoltage spectroscopy (SPV) was another effective method to reflect the electron-hole separation efficiency of photocatalysts.34 As shown in Fig. 10, one could clearly see that both BON and BON-S showed photovoltage signal in the UV light range of 275-400 nm. And the signal intensity of BON-S was significantly higher than that of BON sample, indicating that BON-S possessed the strong photo-induced voltage response and hence improved the separation efficiency of photogenerated carries. Thus, it was further confirmed that the good photo-oxidation abilities of BON-S was mainly contributed by the profoundly strengthened separation efficiency of photo-induced electronhole pairs. Based on the above results, the photocatalytic activity enhancement mechanism over BON-S nanosheets is proposed here: The modestly higher specific surface area of BON-S than BON and BON-O would enhance adsorption of pollutant on its surface to promote the photocatalytic activity. But the increased level of specific surface area is not big enough to provide so large photodegradation improvement of BON-S (3.5 and 3.4 times that of bulk BON and BON-O, respectively). According to the largely strengthened pho15 ACS Paragon Plus Environment


toelectrochemical properties of BON-S, the efficient charge separation and transfer derived from the dominant exposure of {001} reactive facet in BON-S nanosheets is responsible for the profoundly promoted photocatalytic performance. Active species trapping experiments were conducted for the purpose of distinguishing the primary active species produced in the degradation process under UV light, and Benzoquinone (BQ), EDTA-2Na and isopropyl alcohol (IPA) were authoritative scavengers employed to trap superoxide radicals (·O2−), holes (h+) and hydroxyl radicals (·OH), respectively. As shown in Fig. 11a, the photocatalytic degradation behaviors of BON and BON-S were both strongly inhibited by adding EDTA-2Na and benzoquinone (BQ), which demonstrated the active species generated during the irradiation process were h+ and ·O2−. When trapping ·OH, for the BON the addition of IPA still strongly inhibited the degradation of MO. However, with the adding of IPA the BON-S demonstrated distinctive degradation of MO, which indicated that ·OH was not a dominated active species for BON-S. Moreover, NBT was introduced as a molecular probe to quantify ·O2− production.43 As shown in Fig. 11b, the great decrease of NBT absorbance spectra of BON-S centering at 259 nm indicates that large amount of O2 has transformed into ·O2−. According to the reaction equation that NBT (1 mol) can react chemically with ·O2− (4 mol),43 the evolution concentration curves of ·O2− over BON-S nanosheets and BON particles are shown in Fig. 11d, and the average generation rates of ·O2− were 3.14 and 0.96 µmol·L-1·min-1, respectively. Terephthalic acid (TA)-photoluminescence (PL) method was employed to inspect the ·OH concentration, as TA could react with ·OH in the same stoichiometric ratio to produce a highly fluorescent 2-hydroxyterephthalic acid with emission peak center16 ACS Paragon Plus Environment

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ing at 425 nm.23 With an excitation wavelength of 315 nm, the PL peak intensity increase occurs at 425 nm for BON-S and BON with the continuous UV light irradiation, demonstrating the production of ·OH for both samples (Fig. 11c). Based on the linear formula on the relation between the PL intensity and concentration of TA,24 the ·OH evolution rates of BON and BON-S were determined to be 0.033 and 0.029 µmol·L-1·min-1, respectively (Fig. 11e). Scheme 1 displayed the illustration for detailed charge separation of BON-S and its degradation mechanism of MO under UV light irradiation. With irradiation, the electronhole pairs are generated from BON-S nanosheets. The internal electric field between the positively charged [Bi2O2(OH)]+ layers and negatively charged NO3- slices would propel the migration of electrons and holes to the CB and VB along [001] direction, respectively. The electrons in CB reduce oxygen molecules adsorbed on BON-S into ·O2− and holes oxidize H2O/OH- into ·OH. Due to the thin nanosheet structure with large exposure of {001} facet, the diffusion pathway of photogenerated electrons (e-) and holes (h+) from BON-S is shortened. The e- and h+ could rapidly migrate to the surface of catalyst and then participate in the redox reactions, thus showing drastically enhanced photodegradation and photoelectrochemical performance. In order to confirm the general photooxidation capability of BON-S sample, different varieties of industrial pollutants and pharmaceutical, such as phenol, 2,4-DCP, tetracycline hydrochloride and BPA, were introduced to be the target containments. Fig. 12b-e showed the UV-Vis absorption spectra of phenol, 2,4-DCP, tetracycline hydrochloride and BPA over BON-S under UV light, respectively. With the time went on, the characteristic absorption peaks of phenol (275 nm), 2,4-DCP (285 nm), tetracycline hydrochloride 17 ACS Paragon Plus Environment


(270 and 350 nm) and BPA (276 nm) apparently declined with photooxidation degradation efficiencies of 31.97%, 45.69%, 97.75% and 53.57%, respectively (Fig. 12a). In view of the unselected excellent photocatalytic performance in diverse pollutants degradation, BON-S photocatalysts will show great potential in the actual application of environmental purification. To confirm if MO was fully oxidized or partly oxidized, total organic carbon (TOC) analysis was conducted to determine the mineralization ratio of MO over BON-S sample. Fig. 13 shows the degradation efficiency of MO solution and TOC data of BON-S composite with 25 min light irradiation. As revealed by the experimental results, both the removal efficiency of TOC and the photodegradation efficiency were approximately the same, which indicated that all the degraded MO solution was mineralized into CO2 and H2O. Considering the stability of a photocatalyst is important for practical application, three-cycle degradation test was conducted to precisely investigate the photochemical stability of BON-S sample. As shown in Fig. 14a, only a slight decline of photodegradation activity was seen after 3 successive cycles, and the apparent rate constant was approximately from 0.074 to 0.069. Furthermore, the XRD patterns displayed in Fig. 14b demonstrated that no significant difference was seen before and after the photoreaction. The above results indicated the high photochemical stability of BON-S under UV light irradiation, suggesting its great promise in environmental remediation. Dye-sensitization photodegradation. Dye sensitization is a kind of main strategy that help photocatalysts to extend their range of spectrum response from UV light to visible 18 ACS Paragon Plus Environment

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light region.8 Due to the weak visible light absorption of the BON samples, Rhodamine B (RhB) was adopted here being a sensitization medium to respond to visible light. In Fig. 15a, BON, BON-O and BON-S all exhibited photocatalytic activity for degradation of RhB under visible light illumination (λ>420 nm). Similar to the performance on MO photocatalytic degradation under UV light, BON-S sample displayed higher dye-sensitization abilities compared to BON and BON-O. As shown in Fig. 15b, the absorption spectra of RhB demonstrated that RhB molecules are gradually decomposed by BON-S with occurrence of N-demethylation and de-ethylation in the photocatalytic processes. Scheme 2 reveals the RhB dye-sensitization process and the enhanced photocatalytic mechanism of BON-S under visible light. The larger specific surface area of BON-S nanosheets than bulk BON and BON-O allows it an improved ability in absorbing organic pollutants, especially dyes (e.g. RhB). RhB is strongly absorbed on the reactive sites of BON-S nanosheets and thus expand their absorption region to visible range. Upon visible light, photogenerated electrons are released from RhB molecules and then transferred onto the CB of BON-S. Due to the larger specific surface area of BON-S nanosheets, more electrons are produced, and then reduce O2 absorbed on the surface of catalyst into abundant ·O2−, leading to efficient dye-sensitization photodegradation. Piezoelectric-catalytic activity and mechanism investigation. The NCS polar crystal structure of Bi2O2(OH)(NO3) with an orthorhombic space group Cmc21 attracted us to explore its piezoelectric-catalytic activity, which might provide new insight into catalysts application. Meanwhile, to study the morphology effect on the piezoelectric-catalysis, both BON and BON-S samples are selected as the catalysts. Fig. 16a shows schematic illustration for ultrasonic-assisted piezoelectric-catalytic degradation of MO. Without ad19 ACS Paragon Plus Environment


dition of catalysts, the concentration of MO was nearly invariable with 6 h continuous ultrasonic irradiation, which excludes the possibility that stress could degrade MO directly. Therefore, any slight concentration change of MO would be attributed to the piezoelectric-catalytic effect of Bi2O2(OH)(NO3) catalysts. During the piezoelectric-catalytic process, the concentration of MO decreased almost 40% or 45% with the addition of BON samples or BON-S, respectively, and BON-S showed a slightly better catalytic performance compared to BON (Fig. 16b). It demonstrated that morphology has not an obvious impact on the piezoelectric-catalytic degradation, which is consistent with the conclusions obtained with Bi4Ti3O12.24 Fig. 16c-d displayed the temporal absorption spectra of MO over BON and BON-S with 5 h piezoelectric-catalytic treatment, respectively, confirming the decomposition of MO in the above process. To detect the universal application on piezoelectric-catalytic performance of BON-S, BPA and tetracycline hydrochloride as representative contaminant and antibiotic, were chosen here as target pollutants. Fig. 16e-f display the absorbance spectra of BPA and tetracycline hydrochloride, which show the gradual degradation with irradiation time, and the removal efficiencies achieved 30.3% and 56.4% for BPA and tetracycline hydrochloride after 5 h ultrasonic treatment, respectively. It illustrates that BON-S possesses powerful and unselective piezoelectric-catalytic performance, which can decompose diverse contaminants and antibiotics. Radicals quantification experiments were conducted to study the piezoelectriccatalytic degradation mechanism of BON-S. Terephthalic acid (TA) was also employed here to quantify ·OH using the similar method to the photocatalytic active species trapping experiment.23 As shown in Fig. 17a, BON-S generated strong PL peak at 425 nm 20 ACS Paragon Plus Environment

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under 315 nm exciting wavelength with increasing the ultrasonic irradiation time. Fig. 17b showed the plot of ·OH concentration vs. the ultrasonic time, and the production rate of ·OH was determined to be 7.13 µmol·L-1·h-1. NBT serves as a molecular probe to quantify ·O2−.43 It was seen that there was no obvious decrease in NBT absorption (Figure S1), which should be due to the tiny amount of ·O2− generated in the piezoelectriccatalytic degradation process. As a consequence, the primary active species generated in ultrasonic-assisted piezoelectric-catalytic reactions were quite different from that in photocatalytic process, and ·OH dominates the piezoelectric-catalytic process of Bi2O2(OH)(NO3). Based on above measurements and analyses, the possible piezoelectric-catalysis degradation mechanism of Bi2O2(OH)(NO3) is proposed and displayed in Scheme 3. Due to the piezoelectricity of Bi2O2(OH)(NO3), the stress introduced by ultrasonic can produce positive and negative polarization charges on the opposite sides of Bi2O2(OH)(NO3) as well as induce asymmetrical distribution on its surface. In the process of piezoelectric catalysis, e-, H+ and ·OH are produced at the anode.23,44 As the piezoelectric polarization negative-charges will aggregate at the anode of BON, and simultaneously cathode is electropositive, the piezoelectric polarization field can be formed directing from the cathode to anode. According to the charge interaction, e- would migrate from anode to cathode. The polarization of Bi2O2(OH)(NO3) may be not as strong as that of BiOIO3 to separate e- and H+, here few ·O2− are produced. Thus, the generated ·OH as the principal active radicals initiate the piezoelectric-catalytic reaction. Consequently, BON-S is an advantageous material not only in photodegrading multiple contaminants and antibiotics but also in universal ultrasonic-assisted piezoelectric-catalytic performance. 21 ACS Paragon Plus Environment


CONCLUSION In conclusion, uniformly nanosheet-structured Bi2O2(OH)(NO3) photocatalyst with {001} exposing facet was successfully prepared through adding SDBS as surfactant in a one-pot hydrothermal preparation process. The dominant exposure of {001} reactive facet allows Bi2O2(OH)(NO3) nanosheets (BON-S) greatly boosted charge separation and transfer efficiency, which results in the far higher MO photodegradation activity than bulk materials as well as powerful photocatalytic performance for decomposing various toxic contaminants and antibiotic under UV light. The photocatalytic active species production rates of BON-S are determined to be 3.14 µmol·L-1·min-1 for powerful superoxide radicals (·O2−) and 0.029 µmol·L-1·min-1 for hydroxyl radicals (·OH), respectively. The photoabsorption range of as-prepared Bi2O2(OH)(NO3) samples was also extended to visible light by using RhB as a sensitization medium, and enhanced dye-sensitization photodegradation was obtained for BON-S. Furthermore, Bi2O2(OH)(NO3) was demonstrated showing piezoelectric-catalytic performance for degradation of methyl orange, bisphenol A and tetracycline hydrochloride under ultrasonic irradiation, and the piezoelectric-induced ·OH evolution rate is 7.13 µmol·L-1·h-1. The current work provide a reference for designing highlyreactive exposing facet of bismuth-based materials, and shed new light on development of new multi-responsive catalytic material for environmental applications.

■ ASSOCIATED CONTENT Supporting Information Absorption spectra of NBT over BON-S with 5 h ultrasonic-assisted irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. 22 ACS Paragon Plus Environment

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■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel: +86-010-82332247. *E-mail: [email protected]. ORCID Hongwei Huang: 0000-0003-0271-1079 Yihe Zhang: 0000-0002-1407-4129 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was jointly supported by the National Natural Science Foundations of China (No. 51672258 and No. 51572246), the Fundamental Research Funds for the Central Universities (2652015296).

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Fig. 1. XRD patterns of BON, BON-O and BON-S.



Fig. 2. XPS spectra of the BON-S: (a) survey, (b) Bi 4f, (c) O 1s and (d) N 1s.


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Fig. 3. SEM images of (a, b) BON, (c, d) BON-O, (e, f) BON-S.



Fig. 4. (a) HR-TEM image and (b) selected area electronic diffraction (SAED) pattern of BON-S; (c) Crystal structure illustration of Bi2O2(OH)(NO3); (d) [Bi2O2]2+ layers along {001} facet.


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Fig. 5. (a) DRS and (b) band gaps of BON, BON-O and BON-S.



Fig. 6. Mott-Schottky curves of (a) BON; (b) BON-O and (c) BON-S; (d) Schematic band structures of BON, BON-O and BON-S.


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Fig. 7. (a) Photocatalytic degradation curves of MO over BON, BON-O and BON-S under UV light; (b) Absorption spectra of MO catalyzed by BON-S; (c) Apparent rate constants of MO degradation.



Fig. 8. (a) Transient photocurrent responses and (b) EIS Nynquist plots for BON, BON-O and BON-S under simulated solar light irradiation ([NaSO4]=0.1 M).


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Fig. 9. (a) PL spectra of BON, BON-O and BON-S; (b) Nanosecond-level time-resolved fluorescence spectra monitored under 248 nm excitation at room temperature for BON and BON-S.



Fig. 10. Surface photovoltage (SPV) spectra of BON and BON-S.


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Fig. 11. (a) Photodegradation efficiencies of MO over BON and BON-S with different scavengers within 25 min UV light illumination; (b) Absorption spectra of NBT and (c) fluorescence spectra of TAOH solution over BON-S within 25 min UV light illumination; Concentration rate curves of (d) ·O2− and (e) ·OH produced by BON and BON-S.



Scheme 1. Schematic illustration for charge separation and photocatalytic generationof active species over BON-S under UV light irradiation.


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Fig. 12. (a) Photodegradation efficiencies of different contaminants and antibiotics over BON-S under UV light irradiation; Time-resolved absorption spectra of (b) phenol, (c) 2,4-dichlorophenol (2,4-DCP), (d) tetracycline hydrochloride and (e) bisphenol A (BPA) degraded by BON-S under UV light. 43 ACS Paragon Plus Environment


Fig. 13. Photocatalytic degradation efficiency and TOC percentage of MO over BON-S photocatalyst under UV-light illumination.


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Fig. 14. (a) Cycling runs for UV light photodegradation of MO; (b) XRD patterns of BON-S photocatalyst after different cycling times.



Fig. 15. (a) RhB sensitized photodegradation on BON, BON-O and BON-S under visible light (λ>420nm); (b) Absorption spectra of RhB over BON-S during the sensitized photodegradation process.


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Scheme 2. Schematic illustration for mechanism of dye-sensitization degradation over BON-S under visible light illumination.



Fig. 16. (a) Schematic illustration for ultrasonic-assisted piezoelectric-catalytic degradation of MO; (b) Degradation curves of MO over BON and BON-S with ultrasonic (40 kHz, 600 W) irradiation; Temporal absorption spectra of MO over (c) BON and (d) BON-S; Temporal absorption spectra of (e) bisphenol A and (f) tetracycline hydrochloride over BON-S with ultrasonic time (40 kHz, 600 W). 48 ACS Paragon Plus Environment

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Fig. 17. (a) Temporal fluorescence spectra of TAOH solution and (b) ·OH concentration evolution rate over BON-S with ultrasonic (40 kHz, 600 W) irradiation.



Scheme 3. Schematic illustration for piezoelectric-catalytic mechanism of BON-S.


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For Table of Contents Use Only

Well-defined Bi2O2(OH)(NO3) nanosheets with dominantly exposed {001} facets exhibit efficient photocatalysis, dye-sensitization and piezoelectric-catalysis performance for decomposition of diverse industrial contaminants.


Multi-functional Bi2O2(OH)(NO3) nanosheets with {001} active

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