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BiOI/BiVO4 Two-Dimensional Hetero-Nanostructures for Visible Light Photocatalytic Degradation of Rhodamine B Shining Ni, Tiantian Zhou, Haonan Zhang, Yongqiang Cao, and Ping Yang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01161 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Applied Nano Materials

BiOI/BiVO4 Two-Dimensional Hetero-Nanostructures for Visible Light Photocatalytic Degradation of Rhodamine B Shining Ni, Tiantian Zhou, Haonan Zhang, Yongqiang Cao*, Ping Yang* School of Material Science and Engineering, University of Jinan, 250022 Jinan, PR China

ABSTRACT In virtue of facile anion exchange technique, smart BiOI/BiVO4 two-dimensional (2D) hetero-nanostructures (HNSs) photocatalysts targeted for the degradation of organic pollutants have been developed by in situ growth of BiVO4 on the scaffolds of BiOI nanoplates. The as-obtained BiOI/BiVO4 HNSs exhibited excellent 2D structure as templates of BiOI nanoplates and well structure stability due to in situ growth pattern of BiVO4 on BiOI. The 2D HNSs exhibited the superior photocatalytic activities, especially, the V/I8 2D HNSs (V : I = 80%) exhibited the highest Rhodamine B (RhB) and salicylic acid (SA) degradation rate, which is 29.8 times (RhB) and 15.7 times (SA) of individual BiVO4, and 6.4 times (RhB) and 10 times (SA) of BiOI, respectively. Such excellent photocatalytic activities should be attributed to the organic combination of 2D structure and heterojunction, verified by the detailed morphology and composition characterizations. The well-known unique advantages of 2D nanostructure such as efficient light harvesting, quick reactants transport, exposure of plentiful surface reactive sites as well as significantly reduced perpendicular migration distance of photogenerated carriers from the interior to surface of materials, coupled with the enhanced carriers separation behavior owing to the built-in field at heterointerface endow the 2D HNSs with the ultrastrong photocatalytic abilities. It is hoped that our work could offer a paradigm for developing smart 2D HNSs optoelectronic functional materials used in photosynthesis, solar cells, sensors, catalysis, and so on, in the future. KEYWORDS: Two-dimensional, Hetero-nanostructure, In Situ Growth, Photocatalysis, BiOI nanoplates, BiVO4

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INTRODUCTION Photocatalytic technology is regarded as one of the most promising solutions to solve energy and environmental crisis, such as via water splitting to produce hydrogen and photodegradation of organic pollutants, etc.[1] However, most of semiconductor photocatalysts usually exhibited too lower activity to apply in practice, due to the narrow wavelength range of light absorption, sluggish photogenerated carriers transport, lower separation efficiency of carriers[2]. Tremendous efforts have been made by scientists to reduce and even eliminate the above negative influence by the methods of morphology control[3-6], formation of heterostructure[7-11], doping ion[12-14], exposure of specific crystal facet[15-17], and so on. Among the extensive research on morphology control of photocatalysts, the synthesis and application of two-dimensional (2D) nanomaterials have been becoming one of the research hotspots and attracted much more attentions in recent years[18-22]. In contrast with bulk counterparts, 2D nanomaterials demonstrate markedly enlarged surface area which is of great benefit for light harvesting, reactants transport, and exposure of plentiful surface reactive sites[23,24]. Moreover, the extremely thickness of 2D nanomaterials could significantly reduce the perpendicular migration distance of photogenerated carriers from the interior to surface of materials, leading to the enhanced separation of photogenerated carriers[23,25]. All the aforementioned characteristics of 2D materials make them show much higher photocatalytic activity over the corresponding bulk counterparts. Compounding with the other proper semiconductors and forming the heterostructures is another efficient way to improve the activities of photocatalysts. The built-in electric field and potential difference at the interface of heterostructure could inhibit the recombination of photogenerated carriers by accelerating the separation of carriers, improving the quantum yields of photocatalysis[7-9]. If two

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appropriate 2D nanoplates (nanosheets) were coupled together to fabricate a 2D hetero-nanostructures (HNSs) photocatalyst, the created novel hybrid structure will simultaneously have the particular strengths of 2D nanomaterial and heterostructure and thus probably display the super high photocatalytic performance. Moreover, the much larger interface region formed by face-to-face contact in 2D HNSs compared with the point-to-face contact in the 0D/2D heterostructures and the line-to-face contact in the 1D/2D heterostructures would result in the fast interfacial charge separation and thus the higher photocatalytic activity of 2D HNSs[26-28]. More recently, some novel and smart 2D HNSs with unprecedented optoelectronic properties have been reported[23,29-31]. Using the n-type Ca2Nb3O10 and p-type NiO ultrathin nanoplates as the elementary entity, Ca2Nb3O10/NiO 2D HNSs combined with van der Waals force have been fabricated using the Langmuir-Blodgett method by Ida et al.[32]. This ultra thin 2D p-n junction structure exhibited much higher photocatalytic H2 production rate than n-type Ca2Nb3O10 or p-type NiO nanoplates. BiOI(001)/BiOCl(001) and BiOI(001)/BiOCl(010) 2D HNSs were rationally constructed with BiOI and BiOCl nanoplates exposing the different crystal planes by the mixing and subsequent heat treatment at lower temperature, and both the heterojunctions were photocatalytically more active than their individual components[33]. 2D-2D structured SnNb2O6 nanosheets/graphene (SnNb2O6/GR) nanocomposite was developed via a simple surface charge modified self-assembly approach. Such nanocomposite showed a distinctly enhanced visible light photocatalytic performance as compared to blank SnNb2O6 nanosheets. The intimate interfacial contact and unique 2D-2D morphology made the goals of facilitating the transfer and separation of photogenerated carriers, and thereby contributing to the photoactivity enhancement was achieved[34]. However, the weak combination force of two components in many cases of 2D HNSs, such as van der Waals force and electrostatic attractive interaction, usually caused the unstability of nanostructure, the

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high energy barrier, and long transfer distance for photogenerated carriers transfer in the interface of hetero-components, and thereby the detrimental influence on the improved photocatalytic activity[35]. Some 2D HNSs built by the strong atomic binding force such as covalent bond, overcoming the aforementioned shortcomings, have been reported accordingly. For example, Li and Zhang et al[35] have assembled MoS2 monolayers with the Bi12O17Cl2 monolayers to craft a new 2D Janus (Cl2)/(Bi12O17)/(MoS2) bilayer junction. The Bi-S bonds at the interface of two monolayers resulted in more close and firm combination of monolayers, and thus the decreased energy barrier and transfer distance for the photogenerated electrons transfer from Bi12O17Cl2 to MoS2, implying the energetically and spatially more favorable electron transfer along the Bi-S bonds. Gong and Zhou et al[36] reported that the vertically stacked WS2/MoS2 bilayers via the epitaxial growth of WS2 on the top of MoS2 monolayer were achieved with preferred stacking order at high temperature of 850 oC in a single-step vapour phase growth process. The WS2/MoS2 bilayers showed a much higher estimated mobility as a field-effect transistor than the WS2/MoS2 bilayer made by the mechanical transfer method and the MoS2 monolayer as well as bilayer structures. Here, we have designed and successfully manufactured the BiOI/BiVO4 2D HNSs by means of in situ growth of BiVO4 on the top of BiOI nanoplates through a partial anion exchange reaction under the facile hydrothermal condition. BiOI nanoplates, with a narrow bandgap of 1.7 ~ 1.9 eV[37,38], has a layered Aurivillius structure consisting of doubie I- and [Bi2O2]2+ alternated layers[33,39]. The internal static electric fields between the I- and [Bi2O2]2+ layers enable effective separation of photogenerated electron-hole pairs and thereby the high photocatalytic ability[33,39]. Monoclinic BiVO4 is another attractive visible-light responded semiconductor photocatalyst with suitable band structure (bandgap, 2.4 ~ 2.5 eV), excellent conductivity and good chemical stability[40]. The distortion of VO43- tetrahedron in monoclinic BiVO4

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leads to an internal electric field, which is useful for the separation of electron-hole pairs[40,41]. Owing to the above excellent layered lattice structure, beneficial internal electric field, and high performance in photocatalysis, BiOI and BiVO4 have been selected to fabricate the composite photocatalysts with high activities by some research groups recently.[42-45] For example, Huang and Zhang et al[42] have assembled the BiOI nanosheets vertically on the surface of the BiVO4 cores by the deposition route for the fabrication of BiVO4/BiOI core/shell heterostructure, and the composite catalyst showed an improved performance in contrast with the individual BiVO4 and BiOI. Xiang[43] have prepared the BiOI/BiVO4 (BiOI molar percentage: 30%) with the nanoflower morphology by a coprecipitation method, and the obtained composite showed a higher visible activity in the degradation methyl blue and killing P. aeruginosa. The BiVO4 powder crystals randomly coupled with BiOI nanosheets have been synthesized by Shan and He et al[44,45], and the composites indicated a better photoinduced redox ability comparing with the corresponding pure components. Considering the aforementioned super high performance of 2D HNSs, in this work, BiOI and BiVO4 were selected and attempted to assemble the BiOI/BiVO4 2D HNSs, applied to photodagrade the organic pollutants, such as Rhodamine B (RhB) and salicylic acid (SA). The common Bi atoms in both components and in situ growth method by anion exchange resulted in the super high stability of 2D HNSs. The chemical binding between BiOI and BiVO4 assisted by the more stronger covalent bonds in 2D HNSs could enable the improved electrons transfer behavior at the interface of HNSs. Besides of the advantages of 2D individual component nanoplates, the extra unique characteristics in BiOI/BiVO4 2D HNSs, such as much larger heterointerface region of junction, efficient separation of photogenerated carriers via built-in field at the interface, as well as widened light-response region compared with pure BiVO4 component, made the 2D HNSs show significantly superb visible light photocatalytic performance in the applications for the degradations of RhB and SA in contrast with

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individual BiVO4 and BiOI nanoplates. EXPERIMENTAL SECTION Materials The bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) was purchased from Aladdin biopharmaceutical Co., Ltd. Potassium iodide (KI), ammonium metavanadate (NH4VO3), HNO3, Na2SO4, RhB and SA were all purchased from Sinopharm Chemical Reagent Co., Ltd (China). Commercial TiO2 photocatalyst (P25) was obtained from Degussa Co. (Germany). All of the chemical reagents were analytical grade and used as received without any further purification. Ultrapure water (ρ >18 M Ω ·cm) was used for all experiments. Preparation of BiOI nanoplates BiOI nanoplates were first prepared via a facile method as reported elsewhere[46]. Typically, 2.8 mmol solid Bi(NO3)3·5H2O was added into 80 mL KI solution (0.035 mol/L) with magnetic stirring. After ultrasonic dispersing for 2 min and subsequently magnetic stirring for 5 h at room temperature, the BiOI precipitate was centrifuged and washed with ultrapure water and then dried at 60 oC for 8 h. Preparation of BiOI/BiVO4 2D HNSs BiOI/BiVO4 2D HNSs were prepared via a simple anion exchange method using BiOI as scaffold. In detail, 1 mmol BiOI nanoplates powder was dispersed into 30 mL H2O by ultrasonic for 20 min. Simultaneously, a certain amount of NH4VO3 was dissolved into another 30 mL H2O with magnetic stirring for 20 min. Then, the NH4VO3 solution was dropwisely added into the BiOI suspension with magnetic stirring for 15 min at room temperature. The suspension was transferred into Teflon-lined autoclave and heated at 160 oC for 12 h. After being cooled down to room temperature naturally, the resulting BiOI/BiVO4 2D HNSs were collected by centrifuging and washing with absolute ethanol and

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ultrapure water for several times, then dried at 60 oC for 8 h. By changing the amount of NH4VO3 (0.3, 0.5, 0.7, 0.8, 0.9, 1.1mmol), BiOI/BiVO4 2D HNSs with different composition molar ratios of BiOI and BiVO4 were obtained and denoted as V/I3, V/I5, V/I7, V/I8, V/I9, and V/I11, respectively. For comparison, pure BiVO4 was synthesized similarly except that the BiOI and ultrapure water were respectively replaced by Bi(NO3)3·5H2O and 2M HNO3 aqueous solution. The amounts of Bi(NO3)3·5H2O and NH4VO3 were both 1 mmol. In addition, in order to compare with the V/I8 2D HNS sample, the conventional BiOI/BiVO4 composite (denoted as V/I8-C) was also synthesized by dispersing the as-obtained pure BiVO4 in the BiOI growth solution with the molar ratio of 8: 2 (BiVO4 : BiOI) and subsequently magnetic stirring for 5 h at room temperature. The V/I8-C precipitate was centrifuged and washed with ultrapure water and then dried at 60 oC for 8 h. Characterizations The crystalline structures of various photocatalysts were characterized by X-ray powder diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation. The XRD phases of the samples were identified with the help of JCPDS data files. UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded on a U-4100 UV-vis spectrophotometer (Hitachi, Japan) with an integrating sphere attachment. BaSO4 was used as a reflectance standard. The surface morphologies of samples were examined by the field-emission scanning electron microscopy (SEM) on a QUANTA FEG 250 (FEI, USA) instrument. The energy-dispersive X-ray spectroscopy (EDS) images of 2D HNSs were acquired with EDS spectrometer fitted on the microscopy. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were conducted using a Tecnai G2 F20 (FEI, USA) with the accelerating voltage of 200 kV. The atomic force microscope (AFM) characterization was taken with a MultiMode-8 system (Bruker, Germany) using the silica wafer as the substrate. The surface composition, surface states, and valence

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band spectra were determined by X-ray photoelectron spectroscopy (XPS) on a ESCALAB 250Xi X-ray photoelectron spectrometer (ThermoFisher Scientific, USA). All binding energies were referenced to the C1s peak (284.8 eV) arising from the adventitious carbon. The specific surface area and pore size distribution of samples were obtained at 77 K by using MFA-140 multifunction adsorption instrument ( Builder Electronic Technology, China). Photoluminescence (PL) spectra were detected using F-4600 PL spectrophotometer (Hitachi, Japan) with excitation wavelength of 280 nm. Photocurrent, electrochemical impedance spectroscopy (EIS), and Mott-Schottky curves measurements were all recorded on CHI 600e electrochemical workstation (CH Instruments, China) using a standard three-electrode cell with a platinum wire as the counter electrode, Ag/AgCl as the reference electrode, and the as-prepared samples deposited on ITO conducting glass (1×5 cm) as the working electrode. Na2SO4 aqueous solution (1 M) was used as the electrolyte. The 300 W Xe arc lamp (PLS-SXE300, Trusttech, China) equipped with an ultra violet cut-off filter (λ > 420 nm) was used as the light source in the photocurrent measurement. Photocatalytic activity measurements The photocatalytic activities of as-prepared samples were estimated by the degradation of RhB dye aqueous solution (5 mg/L) and colorless SA aqueous solution (5×10-5 mol/L) under visible light irradiation at room temperature. A sunlamp (Philips HPA 400/30S, Germany) equipped with an ultra violet cut-off filter (λ > 400 nm) to remove the UV light was used as the light source. 30 mg (20 mg for SA) photocatalyst sample was dispersed in 50 mL RhB (35 mL for SA) solution and magnetically stirred for 30 min in the dark to reach an adsorption-desorption equilibrium before the photocatalysis. During the photocatalytic process, 2 mL RhB (or SA) solution was withdrawn from the catalytic vessel every 15 min (1 h for SA), and the variations of RhB (or SA) concentration under illumination were monitored by a U-4100 UV-vis spectrophotometer (Hitachi, Japan). Total organic carbon (TOC) for the photocatalysis of

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RhB and SA were measured with a TOC-LCPH analyzer (Shimadzu, Japan). Reactive Species Detection In order to determine the roles of various radicals in photocatalytic reaction, the active species trapping experiments were conducted by introducing the corresponding trapping agents in the photocatalytic process. Superoxide radicals (•O2−), hydroxyl radicals (•OH), and holes (h+) can be respectively consumed by adding p-benzoquinone (p-BQ), isopropanol (IPA), and ethylenediamine tetraacetic acid disodium salt (EDTA-2Na) in RhB solution. The reactive species detecting experiments were conducted similarly with the photodegradation reactions of RhB except that 0.05 mmol of additional trapping agents were added into the photocatalytic reaction system. RESULTS XRD Analysis

Figure 1. XRD patterns (a) and their partial enlargements (b) for the samples of BiOI/BiVO4 2D HNSs, BiOI nanoplates, and BiVO4.

The crystal phase structures and purities of BiOI/BiVO4 2D HNSs, BiOI nanoplates, and BiVO4 were

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determined by XRD technique, and the patterns are shown in Figure 1a. Firstly, all the patterns exhibit the sharp and narrow peaks, indicating the high crystallinities of samples. The control samples of BiVO4 and BiOI exhibit the standard monoclinic phase BiVO4 (JCPDS File No. 14-0688) and tetragonal phase BiOI (JCPDS File No. 10-0445), and no impurity peaks could be observed in the patterns. As for the BiOI/BiVO4 2D HNSs, all characteristic diffraction peaks could be well indexed to monoclinic BiVO4 and tetragonal BiOI, and no other impurity peaks are observed, indicating that all the composites are only composed of BiVO4 and BiOI phases. The sharp and narrow BiVO4 peaks in BiOI/BiVO4 imply that the new transformed BiVO4 component in 2D HNSs still maintained the high crystallinity as BiOI. From the magnified XRD patterns in Figure 1b, it can be clearly seen that the intensities of diffraction peaks at around of 2θ = 18.7°, 19.0°, 28.8° and 30.5° (corresponding to (110), (011), (-121) and (040) peaks of BiVO4 component) enhanced gradually from V/I3 to V/I11, while the peaks at 19.4°, 29.6° and 31.7° (corresponding to (002), (102) and (110) peaks of BiOI component) appeared to be more and more weaker. Such XRD results demonstrate that more and more BiOI component has been successfully transformed into BiVO4 with the increase addition of NH4VO3 in the synthesis of BiOI/BiVO4 2D HNSs. As illustrated in equation 1 and Scheme 1, due to the lower solubility of BiVO4 than BiOI, part of BiOI thermodynamically favored to be transformed into BiVO4 by means of anion exchange of I- with VO3- in the NH4VO3 solution during the hydrothermal treatment[47]. BiOI nanoplates should act as the Bi source and morphology templates of BiVO4 (hereinafter verified by the SEM, TEM and AFM characterizations) during the formation of BiOI/BiVO4 2D HNSs. The schematic diagram for the fabrication of BiOI/BiVO4 2D HNSs is also illustrated in Scheme 2. The XRD pattern of control sample V/I8-C was also obtained as shown in Figure S1. Besides of BiOI and BiVO4, there is no other phase can be found in the pattern, furtherly verifying the components of V/I8-C composite.

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BiOI + NH4VO3 → BiVO4 + NH4I

(1)

Scheme 1. Schematic illustration of the hydrothermal anion exchange in the transformation process from BiOI to BiVO4.

Scheme 2. Schematic illustration for the fabrication of BiOI/BiVO4 2D HNSs.

Optical Properties The optical responses of the as-prepared samples were detected by UV-vis DRS. Figure S2a displays the corresponding spectra of BiOI/BiVO4 2D HNSs, BiOI nanoplates, and BiVO4. BiOI exhibits the strong absorption in the region of λ < 650 nm, corresponding to the band transition of photogenerated electrons from the nonbonding states of O2p and I5p orbitals at the top of valence band (VB) to the nonbonding states of Bi6p orbitals at the bottom of conduction band (CB) in BiOI[48]. For the BiVO4, the strong absorption is observed at the range of λ < 530 nm, which is attributed to the electron transitions from the Bi6s orbitals at the top of VB to the V3d orbitals at the bottom of CB in BiVO4[49]. It is worth noted that, there is a obvious additional absorption peak in the range of 250 ~ 400 nm for BiVO4, which should be responded to the electron transitions from O2p located below the Bi6s to V3d orbitals[49]. With the Page 11



increase of NH4VO3 added in anion exchange step, the absorption edges of BiOI/BiVO4 composites display obvious and regular blueshift in DRS spectra, i.e., shifting from absorption edge of BiOI to that of BiVO4 gradually, and the additional peaks in the range of 250 ~ 400 nm also increased accordingly. This indicates that the amount of generated BiVO4 component in BiOI/BiVO4 2D HNSs increased gradually with the increase of added NH4VO3, which is well consistent with the XRD results. The absorption spectra of composite samples simultaneously exhibit both the features of BiOI and BiVO4, implying the wider spectra response of 2D HNSs than individual BiVO4 component. As crystalline semiconductors, the band edges of pure BiVO4 and BiOI can be estimated using the following equation: αhν = A(hν− Eg)n/2

(2)

where hν, α, Eg and A represent the photon energy, optical absorption coefficient, band gap, and proportionality constant, respectively. Moreover, n = 1 and 4 means that the material is direct semiconductor and indirect semiconductor, respectively[50,51]. Hence, as a direct semiconductor, the n value of BiVO4 is 1, while BiOI is 4 owing to its indirect transition feature[42,43]. As shown in Figure S2b, the Eg of BiVO4 and BiOI is 2.42 eV and 1.83 eV estimated from the plots of (αhv)2 versus (hv) and (αhv)1/2 versus (hv), respectively, which are quite consistent with the reported data[42,43,52,53]. The wide photoabsorption region composed by simultaneous contributions from BiOI and BiVO4 components in BiOI/BiVO4 enabled 2D HNSs to have the high photoabsorption and photoactivity under the irradiation. Morphology and Microstructure Figure 2 depicts the SEM images of BiOI nanoplates and V/I8 2D HNSs samples. Figures 2a and 2b display that the as-prepared BiOI has the plate-like structure with irregular shape and smooth surface. The lateral sizes of BiOI nanoplates are nonuniform and vary from several hundreds of nanometers to several micrometers. The thickness of nanoplates is in the range of ca. 5 ~ 70 nm (Figure S3a and AFM characterization there-in-after). After the anion exchange in hydrothermal process, the acquired V/I8 Page 12


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HNSs still maintained the perfect 2D structures like the template of BiOI as shown in Figures 2c and 2d, indicating that the anion exchange between I- and VO3- has not destroyed the 2D morphology of BiOI template. However, it is worth noting that the ion exchange and further epitaxial growth of BiVO4 on BiOI nanoplates led to the relative thickening of V/I8 2D HNSs (ca. 10 ~ 90 nm) comparing with BiOI nanoplates (Figure S3b and the following AFM images). The morphology of control sample of pure BiVO4 was also acquired by SEM as shown in Figure S4. The particulate BiVO4 shows the irregular polyhedral shapes with the sizes ranging from a few hundreds of nanometers to several micrometers. For further exploration of the space distribution of BiVO4 component in BiOI/BiVO4 2D HNSs, the EDS elemental mappings for V/I8 2D HNSs were conducted and shown in Figures 2e-2i. As we can see that the distributions of Bi, I, V and O elements in the plane of 2D HNS are very homogeneous. Considering that there are only two components of BiOI and BiVO4 in 2D HNSs as verified by XRD, the above EDS mapping results suggest that BiOI nanoplate template could hold its plate structure in the anion exchange reaction, and the simultaneously generated BiVO4 layer on the surface of BiOI nanoplate was uniform and existed in 2D form. These results imply that the obtained BiOI/BiVO4 composite samples had a well 2D heterojunction structure, which can also be verified by the following HRTEM measurements. The morphology of control sample V/I8-C was also characterized and shown in Figure S5. It can be observed that massive amounts of BiOI nanoplates were randomly grown on the surface of particulate BiVO4, indicating the formation of composite (Figures S5a and S5b). The EDS mappings can further verified the space distribution of BiOI and BiVO4 components in the composite (Figures S5c-S5g).

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Figure 2. SEM images of pure BiOI nanoplates (a, b) and V/I8 2D HNSs (c, d). EDS elemental mappings of V/I8 2D HNSs (e-i).

To further clarify the microstructure of V/I8 2D HNSs, TEM and HRTEM characterizations were implemented. The TEM observations of V/I8 2D HNSs as shown in Figures 3a and 3d suggest that the sample possessed a uniform 2D layer structure similar with SEM results. The circled square region in Figure 3a is selected and magnified with different multiples as shown in HRTEM images of Figures 3b and 3c. The clear lattice fringes could be observed easily in images, indicating the high crystalline feature of V/I8 2D HNSs in agreement with the XRD. After the measuring of fringe spacing, two sets of lattice fringes (divided by the yellow dash line in Figure 3c ) are identified in V/I8, which are assigned to the (020) crystallographic plane of BiVO4 (d020 = 5.971 Å) and (102) plane of BiOI (d102 = 2.988 Å), Page 14


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respectively. This confirms the generation of uniform layer-structure BiVO4 atop the upper surface of BiOI nanoplates. Figure 3d shows a relatively complete BiOI/BiVO4 2D HNS, and the dark area in 2D HNS is attributed to a larger thickness owing to the epitaxial growth of BiVO4 atop the BiOI substrate. The edge regions of 2D heterostructure marked with square dashed frame are further magnified and observed detailedly. The well-crystallized BiVO4 and BiOI, also divided by yellow dash lines in Figures 3e and 3f, are both confirmed by the measurement of fringe spacings. The fringe spacings of 3.139 Å and 3.077 Å in Figures 3e and 3f respectively are determined to be the (102) plane of BiOI, whereas the fringe spacings of 5.824 Å and 5.850 Å in Figures 3e and 3f are determined to be the (020) plane of BiVO4. All of these results suggest the achievement of perfect 2D heterostructure in V/I8 HNSs by coupling the in situ generated BiVO4 with BiOI nanoplates, which are quite consistent with the aforementioned SEM and EDS mapping informations.

Figure 3. TEM (a, d) and HRTEM (b, c, e, f) images of V/I8 2D HNSs.

AFM images of V/I8 2D HNSs and their corresponding height profiles are also illustrated in Figure 4. Page 15



The results demonstrate that V/I8 2D HNSs are indeed crafted by coupling BiOI nanoplates with BiVO4 layers epitaxially grown on the upper surface of BiOI. As shown by the height profiles, both BiOI and BiVO4 component layers had the uniform thicknesses (BiOI: 5.1 ~ 6.0 nm, BiVO4: 4.2 ~ 5.6 nm) and smooth surfaces. These results together with the SEM and TEM (HRTEM) observations demonstrate the perfect two-dimensional heterojunction structure of V/I8 sample.

Figure 4. AFM images and the corresponding height profiles of V/I8 2D HNSs (a, b).

The N2 adsorption-desorption isotherms of typical samples were also tested accordingly as shown in Figure S6a. Obviously, all the samples demonstrate the type III isotherms (BDDT classification). The Brunauer-Emmett-Teller (BET) surface areas for BiOI, BiVO4, V/I3, V/I7, V/I8 and V/I9 are determined to be 6.52, 1.23, 28.94, 57.42, 53.53 and 53.62 m2/g, respectively. All the BiOI/BiVO4 composites show a increscent BET specific surface areas than pure BiOI, which should be attributed to the decrease in the lateral size of 2D HNSs and increase in surface roughness of 2D HNSs owing to the anion exchange of VO3- with I- ions and epitaxial growth of BiVO4 on BiOI nanoplates during the fabrication process of BiOI/BiVO4 2D HNSs. Moreover, V/I7, V/I8 and V/I9 show the larger specific surface areas than V/I3 due to the high degree of anion exchange and epitaxial growth. In addition, the Barrett-Joyner-Halenda (BJH) pore size distributions of the samples are also illustrated in Figure S6b. The results indicate that the diameters of pores of all measured samples are mainly distributed in the range of 2−50 nm. Page 16


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XPS Analysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The XPS analysis of BiOI, BiVO4, and V/I8 2D HNSs were performed for the elementary composition and chemical states analysis of the samples. The XPS survey spectra of BiOI, BiVO4, and V/I8 2D HNSs as shown in Figure S7 demonstrate that, the composite sample was composed of five elements of Bi, V, I, O and C, and the pure samples were composed of Bi, I, O and C, or Bi, V, O and C, respectively. The C1s peak (284.8 eV) from the adventitious hydrocarbons is used for calibration to determine other elements. In order to explore the details regarding bonding nature of elements in the composite with respect to its neat counterparts, the obtained XPS data were plotted and deconvoluted accordingly. The doublet peak corresponding to the characteristic spin-orbit splitting of Bi4f7/2 and Bi4f5/2 is observed from the high-resolution XPS spectrum of Bi4f for BiOI, BiVO4, and V/I8 in Figure 5a. The Bi4f for V/I8 could be deconvoluted into two doublet peaks, indicating the existence of two types of Bi3+ ions in the composite. The doublet peak located at 158.38 and 163.63 eV is assigned to the Bi3+ in BiOI, whereas the doublet peak at 159.16 and 164.45 eV should be attributed to the Bi3+ in BiVO4. Apparently, both the Bi3+ ion binding energies of BiOI and BiVO4 components in V/I8 have a lower-energy shift in contrast with those in the neat counterparts. This should be attributed to the loss of chemiadsorbed oxygen in both the components of BiOI and BiVO4 in V/I8 during the process of anion exchange[54], which could be verified by the O1s spectra in Figure 5b. Obviously, the O1s spectra of both neat BiOI and BiVO4 can be divided into two peaks, assigned to the lattice oxygen (OL-Bi, 530.09 and 530.20 eV) and hydroxyl oxygen and adsorbed oxygen (OL-Bi-OH(Oads), 532.30 and 532.01 eV) in BiOI and BiVO4[43, 54-56]. In contrast, the O1s of V/I8 shows only a main peak consisted by the lattice oxygen of BiOI (529.27 eV) and BiVO4 (530.07 eV), accompanying the disappear of hydroxyl oxygen and adsorbed oxygen. Generally, the increase or decrease in electron concentration could enhance or reduce the electron screening effect, leading to weakening or strengthening the binding energy[57]. The loss of hydroxyl oxygen and adsorbed Page 17



oxygen for V/I8 would enhance the electron concentrations of Bi3+ ions as well as those of lattice oxygen OL binding with Bi3+, and accordingly the decrease of their binding energies. It is easily to be observed that the binding energies of lattice oxygen of BiOI and BiVO4 components in V/I8 also have a lower-energy shift in contrast with the neat counterparts. In Figure 5c, the doublet peaks of V2p3/2 and V2p1/2 for BiVO4 (517.07 and 524.46 eV) and V/I8 (516.86 and 524.22 eV) are observed and attributed to the V5+ in BiVO4[58]. The lower V2p binding energy value of V/I8 than BiVO4 should also be attributed to the increase of electron concentration of V owing to the aforementioned similar increase of OL in V-OL bonds. The loss of chemiadsorbed oxygen and subsequent binding energy shift of Bi, O, and V elements confirm the formation of heterostructure between the BiOI and BiVO4 in V/I8. As for the I3d doublet peaks for BiOI and V/I8 in Figure 5d, the binding energies of I3d5/2 and I3d3/2 confirm the -1 valence of I in the samples, which is in accordance with other XPS results in BiOI[43,59]. However, there is no shift of I binding energy of V/I8 compared with that of neat BiOI, which may be due to the weaker van der Waals force between I- and Bi3+ ions in BiOI. The XPS results demonstrate the coexistence of BiVO4 and BiOI in V/I8 2D HNSs, which is in good accordance with the XRD and HRTEM analysis.

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Figure 5. The high resolution XPS spectra of Bi4f (a), O1s (b), V2p (c), and I3d (d) for BiOI, BiVO4, and V/I8 2D HNSs samples.

Photocatalytic Results The photodegradations of RhB and SA under visible light (λ > 400 nm) were carried out to evaluate the optoelectronic performances of the as-prepared samples. Figure 6a and Table S1 illustrate the photocatalytic degradation efficiencies of RhB upon BiOI, BiVO4, V/I8-C, and BiOI/BiVO4 2D HNSs photocatalysts. Obviously, the pure BiOI and BiVO4 showed the relatively lower photoactivities among the catalysts. However, excitedly, the coupling of BiOI with BiVO4 in the BiOI/BiVO4 2D HNSs enabled the photocatalysts had a great improvement in the activities. The activities of 2D HNSs increased gradually with the increase of V/I ratio, and the V/I8 sample demonstrated the highest degradation efficiency of 97%, which is nearly 9 times that of BiVO4, and 2.3 times of BiOI. Nevertheless, the further increase of V/I ratio from 80% to 110% made the photocatalytic performances of 2D HNSs become deteriorated gradually from the efficiency of 97% to 25.2%. For comparison, the photolysis of RhB without catalysts was also tested, and only 3% of dyes was decomposed under the same condition with photocatalysis. In addition, the absorption of RhB by V/I8 sample was also investigated, and the result suggested that only 12.7 % of dyes was transferred from the solution to the surface of catalyst in the absorption process (Figure S8a and Table S1). All the above control experiments indicate that the high RhB degradation efficiency upon BiOI/BiVO4 2D HNSs is mainly attributed to the superior Page 19



photocatalytic performances of catalysts. The Langmuir-Hinshelwood (L-H) kinetics model (ln(C0/C) = kt) is used to determine the pseudo-first-order kinetics of photocatalysis[60,61]. Figure 6b shows the linear fitting of ln(C0/C) ~ t curves of all the samples for the decomposition of RhB. The photocatalytic dynamics of the samples are all accord with the pseudo-first-order kinetic characteristic. The apparent degradation rate constants (k) are fitted and illustrated in Figure 6c and Table S1. The change rule of rate constants k is very similar with that of degradation efficiencies. Moreover, it is worthy noted that the rate constant k of V/I8 is much higher than the other samples, which is 29.8 times that of pure BiVO4 and 6.4 times that of BiOI nanoplates, respectively. These results further demonstrate that the 2D HNSs had a significant improvement in photocatalytic abilities compared with the individual components owing to the coupling of BiOI nanoplates with BiVO4 nanolayers. For comparison, the degradation of RhB by commercial TiO2 photocatalyst (P25) under the same photocatalytic experimental condition was also carried out (Figure S9). Comparing with the degradation efficiency (95.7 %) and rate constant (406.7×10-4 min-1) of P25, the V/I8 sample still demonstrates a relatively higher photocatalytic activity than P25. The stability of V/I8 2D HNSs was also measured by the successive photodegradation of RhB for 4 recycles (Figure 6d), and the catalyst only exhibited a small amount of activity loss (from 97% to 74%), demonstrating the excellent reuse ability of V/I8 2D HNSs. The activity loss should be attributed to the reduction of active sites due to residual RhB absorbed on the catalyst surface and the mass loss of catalyst during the recovery process in every cycle. In order to rule out the dye-sensitized effect of RhB in the visible photocatalytic process, the colorless SA were selected as the another target pollutant to test the visible activities of catalysts. The degradation results of SA upon the samples under visible light irradiation are illustrated in Figure S10 and Table S1. There is nearly no reduction of SA in the photolysis and absorption experiments for V/I8 sample (Figures S10 and S8b). Similar with the case of RhB, all the

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BiOI/BiVO4 2D HNSs samples showed the enhanced photocatalytic decomposition abilities of SA than the neat BiOI and BiVO4, and the V/I8 sample demonstrated the highest degradation efficiency of 68.2%, which is 8.2 times that of BiVO4, and 6.6 times of BiOI. The degradation efficiency curves were also fitted by the L-H kinetics model as shown in Figure S10b. The V/I8 still retained the highest apparent degradation rate constant k, which is 15.7 times that of BiVO4, and 10 times of BiOI. In addition, the V/I8 also showed an excellent reusability in the case of SA degradation (from the 1st of 68.2% to 4th of 62%) as shown in Figure S10d. It is noteworthy that V/I8 sample still demonstrated much higher activities than the control sample of V/I8-C in whether the case of RhB (2.1 times for efficiency and 5.5 times for rate k) or SA (2.2 times for efficiency and 3.1 times for rate k) degradation, suggesting the super advantages of the 2D HNS than the conventional BiOI/BiVO4 composite. Comparing with the BiOI/BiVO4 photocatalysts with the other morphologies reported in all the latest literatures, our BiOI/BiVO4 2D HNSs still showed much better photocatalytic performance as shown in Table S2, further demonstrating the significance of two-dimensional hetero-nanostructure morphology for composite photocatalyst. In addition to assessing the photocatalytic degradation ability of samples, the degree of mineralization of organic contaminants during the photocatalysis process was also tested by the TOC technique. Figure S11 and Table S3 show the TOC removal rates of the RhB and SA over the BiOI, BiVO4, V/I8-C, and V/I8 2D HNSs samples at different degradation times. In the case of RhB, the TOC remove rate of V/I8 2D HNSs is 2.5 times of BiOI, 10.5 times of BiVO4, and 6.2 times of V/I8-C, respectively. For the SA degradation, the TOC remove rate of V/I8 2D HNSs is 2.4 times of BiOI and V/I8-C, and much higher than that of BiVO4 (TOC remove rate: ~ 0). Obviously, the V/I8 2D HNSs showed the much higher TOC remove rate than the neat BiOI, BiVO4, and V/I8-C, demonstrating the super mineralization ability of the V/I8 2D HNSs.

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The morphology and composition of V/I8 2D HNSs after the photocatalytic degradation RhB experiment were also characterized by SEM, EDS mapping, and XRD as shown in Figures S12 and S13. It can be observed that the catalyst after the photocatalysis still remained the favorable nanoplate shape, and all the Bi, I, V, and O elements distributed homogeneously in the plane of 2D HNS, which are similar with the case before the photocatalysis. Moreover, there is also no change comparing with the XRD patterns of V/I8 2D HNSs before and after the photocatalytic experiment, as illustrated in Figure S13. All the above characterization results demonstrated the excellent physical and chemical stabilities of BiOI/BiVO4 2D HNSs.

Figure 6. Photocatalytic degradation efficiencies (a), kinetics (b), and rate constants (c) of RhB by BiOI, BiVO4, V/I8-C, and BiOI/BiVO4 2D HNSs photocatalysts under visible light irradiation. (d) Cycling performance of photocatalytic degradation RhB over the sample of V/I8 2D HNSs.

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DISCUSSION Determination of Band Structures of BiOI/BiVO4 2D HNSs For crystal semiconductor compounds, the positions of VB and CB could be evaluated by equations 3 and 4[42,62 ]. EVB = χ − Ee + 0.5Eg

(3)

ECB = EVB – Eg

(4)

Where χ is the electronegativity of the semiconductor calculated from electronegativity of constituent atoms, Eg is the band gap of semiconductor, and Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV). The χ values of pure BiVO4 and BiOI are 6.04[50] and 5.943 eV[51], respectively, and the Eg values of BiVO4 and BiOI are obtained to be 2.42 and 1.83 eV by the UV-vis DRS. EVB and ECB of BiVO4 are calculated to be 2.75 and 0.33 eV, respectively, and those of BiOI are 2.36 and 0.53 eV, respectively. From the valence band XPS spectra of neat BiOI and BiVO4 as shown in Figure S14, the Fermi level is located above the VB of BiOI and BiVO4 for approximately 1.41 and 2.23 eV, respectively. The positions of Fermi levels closer to CB demonstrate that both the BiOI and BiVO4 are n-type semiconductors, which are also verified by the positive slope of Mott-Schottky plots as illustrated in Figure S15[63,64]. All the calculated results of band structures are illustrated in Scheme 3a. Charge Transfer As illustrated in Scheme 3a, pure BiOI and BiVO4 have the nested band structure positions before contact, apparently, which is not conducive to the effective separation of photogenerated carriers. However, when the BiOI and BiVO4 are coupled together forming a heterostructure, resulting in the generation of internal static electric field at the interface. Meanwhile, the energy levels of BiOI shift upward, whereas those of BiVO4 shift downward along the shifting of Fermi levels until the Fermi levels of BiOI and BiVO4 reach an energetic equilibration[52,65]. Eventually, a type II heterojunction at the

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interface of BiOI/BiVO4 2D HNSs is generated, and the schematic diagram of band alignment in such heterostructure is shown in Scheme 3b. Under visible light irradiation, electrons could be excited from VB to CB in both individual components of BiOI and BiVO4 owing to their strong visible light response. With the help of internal electric field at heterointerface, the excited CB electrons in BiOI can easily migrate to the CB of BiVO4, and meanwhile, the generated VB holes in BiVO4 will transfer to the VB of BiOI oppositely. Eventually, the photogennerated electrons will be accumulated in BiVO4 and holes will be accumulated in BiOI. Such migrations of photogenerated electrons and holes in BiOI/BiVO4 2D HNSs would make an impactful inhibition on the recombination of photogenerated carriers in heterostructure and lead to the significantly enhanced photocatalytic activity.

Scheme 3. Schematic diagrams for the band edge positions of BiVO4 and BiOI before contact (a) and the charge carriers transfer in BiOI/BiVO4 2D HNSs under visible light irradiation (b).

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Photocatalytic Mechanism 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Because of the narrow band gap of BiOI (1.83eV) and BiVO4 (2.42 eV ), the photogenerated carriers could be obtained in BiOI and BiVO4 by visible light excitation. Thus, both the neat BiOI and BiVO4 showed the modest visible light photocatalytic activities in the experiment. As for BiOI/BiVO4 2D HNSs samples, the obvious improvement in photocatalytic performance in contrast with individual components should be attributed to the efficient separation of photogenerated carriers on account of the internal electric field at heterointerface (Scheme 3b). At initial stage of the increase of V/I ratios, the more BiOI in nanoplates were converted into BiVO4 with the increase of V/I ratios, the more BiOI/BiVO4 2D HNSs were obtained. The increasing 2D heterostructures enabled the more efficiently separating behavior of carriers. Moreover, the increased specific surface areas of BiOI/BiVO4 2D HNSs than BiOI verified by the above nitrogen adsorption analysis also had a promoting effect on the improvement of photocatalytic activities of 2D HNSs. However, when the ratios of V/I exceeded that in V/I8 sample, the 2D HNSs samples presented the deteriorative photocatalytic performances. This should be attributed to the excessive conversion from BiOI to BiVO4 (verified by XRD and UV-vis DRS) leading to the decrease of heterointerface of junctions and thus the weaker ability to separate carriers. The V/I8 2D HNSs also showed much higher photocatalytic ability comparing with the conventional composite sample of V/I8-C, which could further verify the structural advantages of 2D HNSs, such as the larger specific surface area and heterointerface region in heterostructure, the fast interfacial charge separation owing to the reduced perpendicular migration distance of photogenerated carriers, and so on. It is well known that active species play an important role in a photocatalytic process. In order to confirm which is crucial active species in the reaction, the trapping experiments were conducted during the photocatalytic reactions in presence of V/I8 2D HNSs. Figure S16 shows that the RhB degradation rates decrease apparently with the addition of 1 mM EDTA-2Na and 1mM p-BQ, while only a slight Page 25



decline with adding 1 mM IPA. The inhibition efficiencies of EDTA-2Na, p-BQ, and IPA are about 90%, 52%, and 13%, respectively. Thus, it can be deduced that h+ and •O2− played a major role in the photodegradation rather than •OH. It is known to all that active radicals are generated via redox reactions of photogenerated electrons and holes on the surface of photocatalyst. A possible pathway for photocatalytic reactions can be proposed as follows: BiOI/BiVO4 + hv → BiOI/BiVO4 + e− + h+ e− + O2 → •O2−

(6)

h++ OH− → •OH

(7)

(5)

RhB + h+ /•O2− /•OH → degradation products

(8)

Photocurrent and EIS Analysis To study the interfacial generation and separation dynamics of photogenerated carriers in samples, photocurrent measurements were carried out on the as-prepared BiOI, BiVO4, V/I8-C, and V/I8 2D HNSs samples under visible light (λ>420nm) irradiation. As shown in Figure S17, in comparison with BiOI, BiVO4 and V/I8-C, V/I8 2D HNSs exhibits much higher photocurrent density, indicating the significantly improved separation of electron-hole pairs in 2D heterostructure. EIS spectroscopy can also be employed to monitor the charge migration process in the semiconductor photocatalysts[66,67]. Figure 7a shows the EIS Nynquist plots of the BiOI, BiVO4, V/I8-C, and V/I8 2D HNSs samples, and the arc radius of V/I8 2D HNSs is much smaller than those of pure BiOI, BiVO4, and V/I8-C, reflecting the lower transfer resistance of photogenerated electrons and holes in the 2D HNSs. The lifetime (τ) of the injected electrons can be determined by the following expression[66-68]: τ = 1 / (2πf)

(9)

where f indicates the inverse minimum frequency. As indicated by the Bode-phase spectra in Figure 7b, the inverse minimum frequencies of BiVO4, BiOI, V/I8-C, and V/I8 2D HNSs are 35244, 19605, 14700, Page 26


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and 13264 Hz, respectively. Thus, the electron lifetime of V/I8 2D HNSs is calculated to be 12.0 µs, which is 2.67, 1.48, and 1.11 times of the BiVO4 (4.5 µs), BiOI (8.1 µs), and V/I8-C (10.8 µs), respectively. The greatly prolonged electron lifetime of V/I8 indicates that the 2D heterojunction could effectively promote the separation and migration of electron-hole pairs in composite catalysts.

Figure 7. EIS Nynquist (a) and Bode-phase (b) plots of the BiOI, BiVO4, V/I8-C, and V/I8 2D HNSs samples ([Na2SO4] =1 M).

PL Analysis To further confirm the inhibition effect on the recombination of photogenerated carriers in heterostructure, PL spectra of BiOI, BiVO4, V/I8-C, and V/I8 2D HNSs, regarded as a direct approach to understand the separation efficiency of carriers, have been recorded under the excitation wavelength of 280 nm. As shown in Figure S18, BiVO4 shows a wide emission peak centered at around 560 nm, corresponding to the recombination of photogenerated holes formed in Bi6s orbitals with the photoexcited electrons located at V3d orbitals[49]. The platform in the range of 300 ~ 500 nm should be attributed to the radiative recombination of holes at O2p band with V3d orbital electrons[49]. For the BiOI, a PL peak centered at around 670 nm and a plateau in the range of 300 ~ 550 nm are observed. In the VB top of BiOI, the predominant states of I5p are hybridized with the Bi6s and O2p orbitals, demonstrating σ bonding features. While in the CB bottom, the predominant states of Bi6p are respectively hybridized Page 27



with O2p and I5p orbitals, resulting in the generations of σ and π antibonding orbitals[69]. The broad PL peak at 670 nm corresponds to the band edge recombination from σ antibonding to σ bonding orbitals, while the wide plateau (300 ~ 550 nm) corresponds to the recombination of holes at σ bonding with the photoexcited electrons located at the deep energy levels of π antibonding in the CB of BiOI[48,69-73]. The V/I8 2D HNSs sample displayed much lower PL emission intensity than those of BiVO4 and BiOI, except in the range of 490 ~ 690 nm V/I8 had a relatively higher intensity than BiOI. The decrease of PL intensity in V/I8 2D HNSs demonstrates the significant inhibition of the recombination and thus effective separation of photoexcited electron-hole pairs by the BiOI/BiVO4 heterojunction. As for the higher intensity of V/I8 than BiOI in the range of 490 ~ 690 nm, which should be attributed to a relatively large proportion of high luminescent BiVO4 in the composite sample as demonstrated by the aforementioned XRD result in Figure 1. However, the control sample V/I8-C demonstrated a relatively lower PL intensity than BiOI. Besides the promoted separation of charge carriers by heterostructure, the lower PL intensity of V/I8-C should also be attributed to the complete encapsulation of BiVO4 particles by lots of BiOI nanoplates as shown in Figure S5, leading to the obviously weakened optical excitation of BiVO4 in the PL measuring process. CONCLUSION In summary, the novel BiOI/BiVO4 2D HNSs photocatalysts applied for the degradation of organic pollutants were successfully synthesized by a facile anion exchange method using the BiOI nanoplates as the scaffolds. The as-prepared BiOI/BiVO4 2D HNSs exhibited much superior photocatalytic performance in the degradation of RhB and SA than individual BiVO4 and BiOI components under visible light irradiation. Especially, the V/I8 2D HNS (V : I = 80%) exhibited the best photocatalytic activity, and its apparent degradation rate is 29.8 times (RhB) and 15.7 times (SA) of individual BiVO4,

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and 6.4 times (RhB) and 10 times (SA) of BiOI, respectively. Such super enhanced photocatalytic activities are mainly attributed to the integration of excellent 2D morphology structure and smart 2D heterointerface between BiVO4 and BiOI in composite samples. The 2D morphology structure could usually lead to the improved reactants transport, surface reactive sites exposure, as well as the reduced perpendicular migration distance of photogenerated carriers from the interior to surface of catalysts. While the 2D heterointerface in BiOI/BiVO4 could result in the significantly enhanced separation of photogenerated carriers, benefited from the much larger heterointerface area and built-in field in the heterojunction. Hopefully, the application of facile ion-exchange technique to prepare BiOI/BiVO4 2D HNSs in this work could promote and inspire the fabrication of other 2D HNSs functional nanomaterials in the fields of photosynthesis, solar cells, sensors, catalysis, and so on, in the future. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. XRD pattern of control sample V/I8-C. UV-vis DRS of BiOI/BiVO4 2D HNSs, BiOI nanoplates and BiVO4, and the bandgap energies of BiOI and BiVO4. SEM images for the side views of BiOI nanoplates and V/I8 2D HNSs. SEM images of the control sample of pure BiVO4. SEM images and EDS elemental mappings of control sample V/I8-C. N2 adsorption-desorption isotherms and BJH pore size distribution plots of BiOI, BiVO4, V/I3, V/I7, V/I8 and V/I9. The XPS survey spectra of the BiOI, BiVO4, and V/I8 2D HNSs samples. The absorption efficiency curves and the corresponding temporal absorption spectra changes of RhB and SA solutions after different absorption times for V/I8 2D HNSs sample. The result of photocatalytic degradation RhB by P25 photocatalyst under visible light illumination. Photocatalytic degradation efficiencies, kinetics and rate constants of SA by BiOI, BiVO4, V/I8-C, and BiOI/BiVO4 2D HNSs photocatalysts under visible light irradiation. Cycling performance of photocatalytic degradation SA over the sample of V/I8 2D HNSs. The TOC removal rates of RhB and SA over the BiOI, BiVO4, V/I8-C, and V/I8 2D HNSs samples at different degradation times. SEM images and EDS elemental mappings of V/I8 2D HNSs after the photocatalytic degradation RhB experiment. XRD patterns of V/I8 2D HNSs before and after the photocatalytic degradation RhB experiment. Valence band XPS spectra and Mott-Schottky plots for BiOI and BiVO4 samples. Photodegradation RhB on V/I8 2D HNSs in the Page 29



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presence of different active radical scavengers. Comparison of transient photocurrent response between 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

BiOI, BiVO4, V/I8-C, and V/I8 2D HNSs samples under visible light illumination. PL spectra of BiOI, BiVO4, V/I8-C, and V/I8 2D HNSs samples.

AUTHOR INFORMATION Corresponding authors *Tel: +86-531-89736225. E-mail: [email protected]. *Tel: +86-531-89736225. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the projects from National Natural Science Foundation of China (51202090, 51572109, 51772130), the program for Taishan Scholars, China Postdoctoral Science Foundation (2016M602138), Shandong Province Higher Educational Science and Technology Program (J17KA002), Outstanding Young Scientists Foundation Grant of Shandong Province (BS2012CL004), Doctoral Foundation of University of Jinan, China (XBS1027), and Natural Science Foundation of University of Jinan, China (XKY1601).

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