Colloidal Synthesis of Ultrathin Monoclinic BiVO4 Nanosheets for Z

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Research Article Cite This: ACS Catal. 2018, 8, 8649−8658

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Colloidal Synthesis of Ultrathin Monoclinic BiVO4 Nanosheets for Z‑Scheme Overall Water Splitting under Visible Light Chunwei Dong,† Siyu Lu,‡ Shiyu Yao,†,§ Rui Ge,† Zidong Wang,† Ze Wang,† Pengfei An,∥ Yi Liu,*,† Bai Yang,† and Hao Zhang† †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001 People’s Republic of China § College of Physics, Jilin University, Changchun 130012, People’s Republic of China ∥ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

ACS Catal. 2018.8:8649-8658. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 01/11/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Recently, ultrathin 2D photocatalysts have attracted people’s attention due to their performances in the area of solar energy conversion. However, the synthesis of ultrathin 2D photocatalysts with a nonlayered crystal structure is still full of challenges. Herein, ultrathin 2D BiVO4 nanosheets (NSs) with monoclinic crystal structure are synthesized through a convenient colloidal two-phase method. The as-prepared BiVO4 NSs possess a thickness of less than 3 nm but a diameter larger than 1.2 μm. Furthermore, the presence of HNO3 facilitates the growth of BiVO4 NSs with nearly naked surfaces, largely exposed {010} planes, and widely distributed oxygen vacancies (VO) inside the crystalline structure, which are of great benefit to their photocatalytic activity under visible light irradiation. As a result, our ultrathin 2D BiVO4 NSs exhibit an impressive photocatalytic performance for water oxidation. The O2 evolution rate is 107.4 μmol h−1, and the apparent quantum yield (AQY) is as high as 26.1% (420 nm). Furthermore, by employing our ultrathin 2D BiVO4 NSs as the O2-evolving photocatalyst, Ru-SrTiO3:Rh and Fe3+/Fe2+ as the H2-evolving photocatalyst, and the redox mediator, respectively, a Z-scheme overall water splitting system is successfully constructed. Under visible light irradiation, our Z-scheme photocatalytic system presents high H2 and O2 evolution rates (16.7 and 8.0 μmol h−1) with an AQY of 1.88% (420 nm) and good photocatalytic stability. KEYWORDS: BiVO4, 2D nanosheets, photocatalysis, Z-scheme, water splitting

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INTRODUCTION Since the discovery of photocatalytic water splitting by photoelectrocatalysis technology in 1972, the efficient utilization of solar energy via photocatalytic processes, for example water splitting, CO2 reduction, and N2 fixation, has been considered as one of the most promising pathways to solve the urgent global energy shortage and environmental pollution issues.1−3 Along with the development of photocatalytic technology, ultrathin 2D photocatalysts with a thickness of less than 5 nm have attracted more and more attention because of their high specific surface area, vast number of unsaturated coordinated surface atoms, and short migration distance for the charge carriers.4−13 Up to now, many top-down synthetic strategies, such as mechanical and liquid exfoliation14−16 and ion intercalation and exfoliation,17 have been explored for synthesizing ultrathin 2D photocatalysts with impressive catalytic activities. However, most of them are only applicable to the materials with layered crystal structures. Some bottom-up methods including hydro-/ solvothermal synthesis and templated synthesis can overcome © 2018 American Chemical Society

this obstacle, but the reaction time and energy consumption are still very high, and the residual template on products may not be favorable for the subsequent photocatalytic applications.18,19 Since colloidal synthesis has been proven to have the capability to precisely control the size and morphology of nanomaterials in a cost-effective and easily scaled up way, it is undoubtedly believed that exploring an efficient colloidal synthesis strategy is a very promising alternative toward the low-cost, high-yield, and mass production of ultrathin 2D photocatalysts.20 Among various kinds of photocatalysts, BiVO4 exhibits tremendous potential due to its abundant resources and low toxicity.21−23 It is well-known that BiVO4 exists in the three polymorphs tetragonal zircon, tetragonal scheelite, and monoclinic scheelite structures. Monoclinic BiVO4 with a band gap of 2.4 eV has been widely studied as one of the most Received: April 26, 2018 Revised: August 4, 2018 Published: August 8, 2018 8649

DOI: 10.1021/acscatal.8b01645 ACS Catal. 2018, 8, 8649−8658

Research Article

ACS Catalysis

Figure 1. (a) TEM, (b) HRTEM, and (c) aberration-corrected HAADF-STEM images of ultrathin 2D BiVO4 NSs. (d) 3D lattice model along the [010] direction of monoclinic BiVO4. (e) SAED and (f) XRD patterns of ultrathin 2D BiVO4 NSs. (g) AFM image and (h) the corresponding height profiles of ultrathin 2D BiVO4 NSs. (i−l) EDS elemental mapping images of a typical ultrathin 2D BiVO4 NS. Inset in (a): lateral size distribution of ultrathin 2D BiVO4 NSs.

∼1.2 μm. In comparison with the conventional synthesis strategies including hydrothermal methods and coprecipitation methods,38 our method is emerging as a promising route due to its intrinsic advantages, such as time savings, low cost, and mild reaction conditions. More importantly, the distinctive growth conditions endow our ultrathin BiVO4 NSs with a nearly naked surface, large-scale exposed {010} planes, and uniformly distributed VO in the crystalline structure, which are extremely beneficial for exciton separation and subsequent transfer to participate in water-splitting reactions. As a result, our ultrathin 2D BiVO4 NSs exhibit a superior photocatalytic activity for water oxidation under visible light irradiation. The rate of O2 evolution is 107.4 μmol h−1, which is roughly 3 times higher than that of BiVO4 samples prepared by conventional methods. The corresponding apparent quantum yield (AQY) is as high as 26.1% (420 nm). Upon further integration with Ru-SrTiO3:Rh and Fe3+/Fe2+ as the H2evolving photocatalyst and the redox mediator, an efficient visible-light-driven Z-scheme overall water splitting system with an AQY of 1.88% (420 nm) can be achieved. The H2 and O2 evolution rates can reach 16.7 and 8.0 μmol h−1, respectively.

prominent photocatalysts for water oxidation due to its excellent photocatalytic activity under visible light.24,25 However, different from other layered materials, including graphene, molybdenum disulfide, and boron nitride, monoclinic BiVO4 with a nonlayered crystal structure has a lack of intrinsic driving force for 2D anisotropic growth.26 Despite enormous progress having been achieved in the preparation of BiVO4 with different morphologies,27,28 the synthesis of ultrathin 2D BiVO4 nanosheets (NSs) with a monoclinic structure has never been reported. Furthermore, previous works have indentified that the photogenerated electrons and holes in monoclinic BiVO4 preferentially migrate to {010} and {110} facets respectively resulting from the different band energies of these two facets.29,30 In addition, the photocatalytic activity of monoclinic BiVO4 is mostly determined by the percentage of exposed active {010} planes.31,32 At the same time, different from counterparts with bulk structures or other nanostructures, the defects in ultrathin 2D photocatalysts can directly affect the molecule adsorption, exciton separation and transfer, and the activation processes of catalytic reactions due to their atomic scale thickness.33−37 Thus, synthesis of ultrathin 2D monoclinic BiVO4 NSs with high percentages of exposed {010} planes and oxygen vacancies (VO) must possess an impressive photocatalytic activity under visible light irradiation. Herein, we develop a convenient two-phase approach for synthesizing ultrathin 2D BiVO4 NSs with a monoclinic crystal phase. The thickness of the as-prepared ultrathin 2D BiVO4 NSs is only ∼2.9 nm, while the lateral dimension is as large as

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RESULTS AND DISCUSSION In a typical synthesis, Bi(NO3)3 was first dissolved in a mixture of oleic acid (OA), oleylamine (OLA), and octadecene (ODE) with heating. Then NH4VO3 dissolved in HNO3 aqueous solution was added and the mixed solution was stirred and refluxed at 100 °C for 40 min. After that, the flask was rapidly 8650

DOI: 10.1021/acscatal.8b01645 ACS Catal. 2018, 8, 8649−8658

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ACS Catalysis

Figure 2. (a) UV−vis diffuse reflectance spectrum, (b, c) UPS spectra and (d) band positions of ultrathin 2D BiVO4 NSs. (e) V K-edge EXAFS oscillation function k3χ(k) and (f) the corresponding Fourier transforms FT(k3χ (k)) for ultrathin 2D BiVO4 NSs. Inset in (a): plot of (αhν)2 versus the photon energy.

phenomenon, which has been observed on other large NSs (Figure S2). The high-resolution TEM (HRTEM) image shows well-formed monocrystal with the interplanar spacings of 0.260, 0.255, and 0.180 nm, consisting of the (200), (002), and (202) planes of monoclinic BiVO4, respectively (Figure 1b). The selected area electron diffraction (SAED), Raman spectrum, and X-ray diffraction (XRD) pattern further demonstrate the pure monoclinic phase of BiVO4 NSs (JCPDS File No. 14-0688) (Figure 1e, Figure S3, and Figure 1f, respectively). In addition, the enhanced (040) diffraction peak in the XRD pattern is quantity-dependent, suggesting a

cooled to room temperature, and the products were washed three times with hexane and ethanol (or acetone) and redispersed in hexane for further characterizations and applications. It is noted that HNO3 plays an important role in the dissolution of NH4VO3 at room temperature. Otherwise, NH4VO3 can only be dissolved in boiling water (Figure S1). Figure 1a exhibits the transmission electron microscopy (TEM) image of ultrathin 2D BiVO4 NSs, from which it can be seen that BiVO4 NSs have a quasi-circular-disk morphology with an average lateral size as large as ∼1.2 μm. The flowerlike patterns over the NSs are attributed to the electron diffraction 8651

DOI: 10.1021/acscatal.8b01645 ACS Catal. 2018, 8, 8649−8658

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ACS Catalysis

the O 1s lines shift from 529.9 eV to a lower binding energy (529.8 eV) and the fwhm of the peak becomes narrower after annealing. We speculate that it is caused by the decrease of the shoulder peak at 530.8 eV, which can be ascribed to the existence of VO.41 Raman and Electron Paramagnetic Resonance (EPR) spectra are further used to verify the presence of VO in ultrathin BiVO4 NSs (Figure S12). On one hand, the peak in Raman spectra shifts from 826 to 824 cm−1 after annealing, suggesting a change in V−O bond length, which is consistent with the characterization of VO in monoclinic BiVO4.41 On the other hand, the ultrathin BiVO4 NSs both before and after annealing exhibit signals at g = 2.003 in their EPR spectra, corresponding to the presence of V4+ and VO. The different signal intensities imply that the concentrations of V4+ and VO decrease obviously after annealing, consistent with the above result that annealing can efficiently eliminate VO (Figure S10). In addition, extended X-ray absorption fine structure spectroscopy (EXAFS) measurements at the V K-edge have been performed to probe the local atomic arrangements in ultrathin 2D BiVO4 NSs. As shown in Figure 2e, the BiVO4 NSs and their bulk counterpart exhibit similar V K-edge oscillation curves, suggesting their same tetrahedral V−O coordination. The corresponding Fourier transform (FT)k3[χ(k)] functions in R space and the structural parameters between V and O atoms are shown in Figure 2f and Table 1. Although the average V−O distance does not change

preferential parallel orientation between the {010} planes of BiVO4 NSs and the substrate (Figure S4). The aberrationcorrected high-angle annular dark field image-scanning transmission electron microscopy (HAADF-STEM) image in Figure 1c exhibits the atom distribution of BiVO4 NSs, which precisely coincides with the 3D lattice model along the [010] direction of monoclinic BiVO4 (Figure 1d), further proving the concept that the surfaces of the BiVO4 NSs are mainly composed of {010} planes. The thickness of BiVO4 NSs is verified to be 2.9 nm by atomic force microscopy (AFM) image and height profiles (Figure 1g,h), corresponding to less than about three unit cells along the [010] direction of monoclinic BiVO4. Previous works have reported that the spatial heterojunction between {010} and {110} planes of monoclinic BiVO4 can promote the separation between photogenerated electrons and holes.29,30 Therefore, our ultrathin 2D BiVO4 NSs with large-scale exposed {010} planes can be seen as an ideal nanomaterial for photocatalysis. The composition of ultrathin 2D BiVO4 NSs has been determined by energy-dispersive X-ray spectroscopy (EDS), Xray photoelectron spectroscopy (XPS), and inductively coupled plasma-optical emission spectrometry (ICP-OES) (Figures S5 and S6 and Table S1). The Bi:V atomic ratio of BiVO4 NSs is calculated to be close to the stoichiometric ratio of 1:1, though a large excess of NH4VO3 is added during the synthesis process. The element distribution of BiVO4 NSs has been characterized by EDS elemental mapping. Bi, V, and O are all uniformly distributed throughout the entire BiVO4 NSs (Figure 1i−l). The elemental analysis, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) characterizations indicate that the ultrathin 2D BiVO4 NSs are covered by only a tiny trace of ligands (OA and OLA) (Table S2 and Figures S7 and S8). The ligand-free surface endows BiVO4 NSs with excellent dispersibility in both aqueous and organic solvents (Figure S9), which is of great benefit to their future applications in field of photocatalysis. The energy band structure of ultrathin 2D BiVO4 NSs has been investigated by UV−vis diffuse reflectance spectroscopy and ultraviolet photoelectron spectroscopy (UPS). The band gap of BiVO4 NSs is identified to be 2.38 eV on the basis of the plot of (αhν)2 versus photon energy (Figure 2a), which consists of the band gap of monoclinic BiVO4 (2.4 eV), perfectly suitable for their photocatalysis under visible light irradiation. The Fermi level, valence band (VB), and conduction band (CB) of ultrathin 2D BiVO4 NSs are identified as −3.68, −6.84, and −4.46 eV, respectively (Figure 2b−d). As a typical n-type semiconductor, the Fermi level of BiVO4 should be located near CB but between CB and VB. However, the Fermi level of BiVO4 NSs is far above the CB, suggesting the existence of VO in BiVO4 NSs. It has been reported that VO in oxides can act as the shallow defects leading to the upward movement of the Fermi level.39−41 To prove the existence of VO, the energy band structure of BiVO4 NSs on annealing at 400 °C in air is characterized. After annealing, the Fermi level of BiVO4 NSs moves back to the energy level between CB and VB, implying the elimination of VO (Figure S10). XPS has been employed to characterize the chemical state variations of elements in BiVO4 NSs before and after annealing. As shown in Figure S11, annealing does not affect the XPS spectra of Bi 4f. However, the V 2p lines slightly shift to higher binding energy after annealing. The difference spectrum of V 2p clearly indicates the presence of V4+ caused by VO in as-prepared ultrathin 2D BiVO4 NSs. Furthermore,

Table 1. EXAFS Curve-Fitting Results for the Structural Parameters around V Atoms in Bulk BiVO4 and Ultrathin 2D BiVO4 NSs species

shell

N

R (Å)

σ2 (Å)

ΔE

S02

bulk BiVO4 BiVO4 NSs

V−O V−O

4.00 3.68

1.69 1.69

0.0052 0.0036

4.72 4.72

0.9 0.9

significantly, the reduced coordination number strongly indicates a severe structural distortion of V cations in BiVO4 NSs, consistent with the formation of abundant VO (Table 1). It is noted that the Fourier -transformed curves of ultrathin 2D BiVO4 NSs exhibit a higher and narrower main peak at 1.37 Å in comparison to that of the bulk counterpart. On the basis of the crystal structure of monoclinic BiVO4, there are four O atoms around each V atom, which can be divided into two species according to the V−O bond length (1.679 and 1.803 Å).42 Because of the structural distortion induced by VO, the difference between two species of V−O bond lengths may become weakened. As a result, the main peak reflecting the delicate structures between V and O becomes higher and narrower due to the overlap of the original two peaks belonging to V−O bonds with different bond lengths. Upon the analysis above, the crystal structure of ultrathin 2D BiVO4 NSs can be speculated as shown in Figure 3. Because of the ligand-free surface and low surface free energy of the {010} planes,43 BiVO4 NSs are terminated by O atoms along the {010} planes, which has been further proved by dynamic light scattering (DLS) measurements (Figure S13). HNO3 plays a critical role oin the growth of ultrathin 2D BiVO4 NSs and the formation of VO. On one hand, HNO3 can efficiently reduce the energy of the {010} planes, promoting the 2D growth of BiVO4 NSs. As the formation of hydroxyl groups on the surface of bismuth-contained oxide NSs can reduce their surface energy,44 it is believed that our ultrathin 2D BiVO4 NSs with {010} planes exposed under a high 8652

DOI: 10.1021/acscatal.8b01645 ACS Catal. 2018, 8, 8649−8658

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ACS Catalysis

due to the existence of VO.47 We speculate that the locations of VO determine their functions. VOs inside BiVO4 act as charge recombination centers, while VOs on the surface of BiVO4 facilitate charge separation and then accelerate the photocatalytic reactions.34 Because the thickness of our BiVO4 NSs is less than 3 nm, most VOs can be seen as the surface VOs. In order to prove our speculation, first-principles calculations have been performed to disclose the influence of surface VO on the photocatalytic activity of ultrathin 2D BiVO4 NSs (Figure S17). As shown in Figure 4a, the existence of VO on the surface of {010} planes can efficiently lower the dissociation energies of H2O molecules from 4.562 to 2.455 eV, promoting the splitting of H2O molecules into H and O atoms, resulting in high photoconversion efficiency. The evolution of total charge and H−O bond length of H2O molecules upon adsorption on the surface of BiVO4 shows the same tendency. When H2O molecules are adsorbed on the surface of BiVO4 composed of {010} planes with VO, there are 0.083 (0.084) electrons transferred from H2O to BiVO4. The charge transfer weakens the intramolecular interaction and elongates the H−O bonds from 0.973 to 0.999 (0.995) Å. Under the same conditions but with elimination of VO, only 0.028 electron is transferred from H2O to BiVO4 and the elongation of the H−O bond is reduced (Table 2).

Figure 3. (a) Scheme of ultrathin 2D BiVO4 NSs. (b) Threedimensional structure model of ultrathin 2D BiVO4 NSs. (c) [100] direction and (d) [010] direction projection of (b).

Table 2. Calculated Total Charge and the H−O Distance of the Water Molecule

concentration of HNO3 (∼2.4 mol/L) have the lowest surface free energy and the most stable morphology (Figure S14). As a comparison, only BiVO4 nanoparticles (NPs) with an average diameter of 4 nm are obtained in the absence of HNO3 (Figure S15). On the other hand, HNO3 facilitates the formation of VO (V4+) in BiVO4 NSs. It has been demonstrated that VO3− or VO43− mainly exists in the form of VO2+ in acid solution, which prefers to be reduced into VO2+ at 100 °C in the presence of ODE, OA, and OLA (Figure S16). Up to now, the influence of VO on the photocatalytic activity of BiVO4 is still contradictory and ambiguous. Some have claimed that the presence of VO can promote the separation of photogenerated electrons and holes, thus enhancing the photocatalytic efficiency of BiVO4.45,46 Others have believed that the photoelectron conversion efficiency of BiVO4 is suppressed by both radiative and nonradiative recombination

species

total charge

H−O distance (Å)

free H2O molecule H2O on BiVO4 surface H2O on BiVO4 surface with VO

8.000 7.972 7.917 7.916

0.973 0.992 0.999 0.995

Density functional theory (DFT) calculations have been employed to study the effect of VO on the electronic structure of ultrathin 2D BiVO4 NSs. The electronic densities of states (DOS) of BiVO4 with and without VO are shown in Figure 4b. The presence of VO not only extends the edge of CB across the Fermi level but also increases DOS at the conduction band minimum (CBM) and valence band maximum (VBM). These alterations remarkably promote the excitation, separation of

Figure 4. (a) Dissociation energies of H2O molecule: (1) dissociation of free H2O molecule; (2) dissociation of H2O molecule on the BiVO4 surface of (010) plane; (3, 4) dissociation of H2O molecule on the BiVO4 surface of (010) plane with VO. (b) Calculated DOS of BiVO4 without and with VO on the surface of the (010) plane. The Fermi energy level was adjusted to 0 and is marked as a pink dashed line. (c) Charge density distribution at the Fermi level of BiVO4 with VO on the surface of the (010) plane. 8653

DOI: 10.1021/acscatal.8b01645 ACS Catal. 2018, 8, 8649−8658

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generation of O2 in the first 30 min is attributed to the slow diffusion of O2 in the recycle gas circuit. The photocatalytic performances of annealed BiVO4 NSs have been evaluated as well. With an increase in the annealing temperature from 200 to 400 °C, the O2 evolution rate of BiVO4 NSs decreases from 96.2 to 57.8 μmol h−1. However, despite annealing at high temperature, the average evolution O2 rates of BiVO4 NSs are still much higher than that of BiVO4 NPs (2.2 μmol h−1). What is more, for BiVO4 NPs, extra ligand-exchange processes are still needed to replace the original hydrophobic ligands with KI before use as a photocatalyst for water splitting (Figures S18 and S19). For better comparison purposes, two kinds of currently well recognized BiVO4 samples with a onoclinic structure were also prepared through conventional hydrothermal and coprecipitation methods. Their O2 evolution rates under the same experimental conditions are 31.2 and 27.6 μmol h−1, respectively (Figure S20), obviously lower than that of our ultrathin 2D BiVO4 NSs. These results imply that the ultrathin 2D structure is significantly superior to other nanostructures for photocatalysis. However, the existence of VO has a positive effect on the photocatalytic activity of ultrathin 2D BiVO4 NSs. By employment of our ultrathin 2D BiVO4 NSs as the O2evolving photocatalyst, Ru-SrTiO3:Rh (Figure S21) and Fe3+/ Fe2+ as the H2-evolving photocatalyst, and the redox mediator, respectively, an efficient visible-light-driven Z-scheme overall water-splitting system can be constructed as well. Figure S22 exhibits the influence of the weight ratio between RuSrTiO3:Rh and BiVO4 NSs on photocatalytic water splitting under visible light irradiation. Different from most previous experiences implying that the dosage of BiVO4 should be higher than that of Ru-SrTiO3:Rh,49,50 the optimized BiVO4:SrTiO3 weight ratio in our system is only 1:6 (Figure 5c and Table 3). Under this condition, the H2 and O2 production rates are as high as 16.7 and 8.0 μmol h−1 with the ideal stoichiometric ratio of 2:1. The AQY of our Z-scheme system is determined to be 1.88% at 420 nm, which is comparable to that of present suspension particulate systems for overall water splitting.13,32,49−51 The low content of BiVO4 NSs strongly proves the superior photocatalytic performance of our ultrathin 2D BiVO4 NSs. The photocatalytic durability of the Z-scheme system has also been investigated by cyclic experiments with periodical evacuation of generated gas. Figure 5d shows the stable increase of H2 and O2 with the irradiation time in each cycle. Although the gas generation rate decreases after a long-term reaction (20 h), nearly 80% of the photocatalytic activity is still retained. These impressive photocatalytic performances indicate that our ultrathin 2D BiVO4 NSs are very good potential candidates for photocatalytic water oxidation and even for overall water splitting via a Z-scheme strategy by integrating with appropriate H2evolving photocatalysts and redox mediators. According to the energy band positions of BiVO4 NSs and Ru-SrTiO3:Rh, the Z-scheme water splitting mechanism is proposed (Figure 5e). The VB of BiVO4 NSs is more positive than the oxidation potential of 2H2O/O2 (1.08 V, pH 2.5). Upon excitation under visible light, the holes in VB of BiVO4 NSs will oxidize H2O molecules to O2, and the electrons in CB of BiVO4 NSs are captured by Fe3+ to form Fe2+:

photogenerated electrons and holes, and their subsequent migration to participate in the water-splitting reactions. The spatial distribution of the charge density at the Fermi level is shown in Figure 4c as well. From this it can be seen that the majorities of charges are concentrated around VO. Due to the ultrathin 2D structure, most of the VO can be involved in photocatalytic reactions directly to improve the photocatalytic activity of ultrathin 2D BiVO4 NSs. The photocatalytic activity of ultrathin 2D BiVO4 NSs was first evaluated by monitoring O2 production from water in the presence of AgNO3 as sacrificial agents. Thanks to their ligandfree property, BiVO4 NSs can disperse in aqueous solution directly to trigger the water splitting for O2 generation. As shown in Figure 5a, ultrathin 2D BiVO4 NSs without any cocatalysis exhibit an impressive photocatalytic activity under visible light illumination (λ >420 nm). The average O2 evolution rate is as high as 107.4 μmol h−1, and the AQY at 420 nm is determined to be 26.1%, which is much higher than that in previous reports.21,48 Vigorous bubbles of O2 can be observed even after irradiation for 1 h. The temporary gentle

Figure 5. (a) Time courses and (b) O2 generation rates of BiVO4 NSs, BiVO4 NPs, and BiVO4 NSs after annealing at different temperatures in air in AgNO3 aqueous solution (50 mM). (c) Gas generation rates of Z-scheme overall water-splitting systems composed of BiVO4 NSs and Ru-SrTiO3:Rh photocatalyst with different weight ratios in FeCl3 aqueous solution (2 mM). (d) Cycle stability test on the Z-scheme overall water-splitting system composed of BiVO4 NSs and Ru-SrTiO3:Rh photocatalyst (SrTiO3:BiVO4 = 6:1) in FeCl3 aqueous solution (2 mM). (e) Mechanism of the Zscheme overall water-splitting system (pH 2.5). Conditions: 20 mg of catalyst in 50 mL of aqueous solution, light source 300 W Xe lamp (λ >420 nm). 8654

2H 2O + 4h+ VB(BiVO4 ) → O2 + 4H+

(1)

Fe3 + + e−CB(BiVO4 ) → Fe 2 +

(2) DOI: 10.1021/acscatal.8b01645 ACS Catal. 2018, 8, 8649−8658

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ACS Catalysis

Table 3. H2 and O2 Evolution Rates of Z-Scheme Overall Water Splitting Systems Composed by BiVO4 NSs and Ru-SrTiO3:Rh Photocatalyst with Different Weight Ratios in FeCl3 Aqueous Solution (2 mM)a SrTiO3:BiVO4 H2 (μmol h−1) O2 (μmol h−1)

SrTiO3

9:1

6:1

3:1

2:1

1:1

1:2

BiVO4

1.0 0.4

12.8 6.6

16.7 8.2

11.4 6.2

9.3 4.5

6.6 3.0

5.1 3.3

0 9.1

a

Conditions: 20 mg of catalyst in 50 mL of aqueous solution, light source 300 W Xe lamp (λ >420 nm).

excellent H2 and O2 evolution rates (16.7 and 8.0 μmol h−1) and photocatalytic stability. Since the two-phase method features low cost, time savings, and mild reaction conditions, it is believed that our study not only discloses the effects of morphology, crystal phase, and VO on the photocatalytic activity of BiVO4 but also proposes an effective method to design and synthesize photocatalysts with ultrathin 2D morphology.

At the same time, since the CB of Ru-SrTiO3:Rh is more negative than the reduction potential of H2/H2O (−0.15 V, pH 2.5), excited electrons in CB of Ru-SrTiO3:Rh will reduce H2O molecules to H2, and the holes in the donor levels of RuSrTiO3:Rh will reoxidize Fe2+ to Fe3+: 2H+ + 2e−CB(Ru‐SrTiO3:Rh) → H 2

(3)

Fe 2 + + h+donor(Ru‐SrTiO3:Rh) → Fe3 +

(4)

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EXPERIMENTAL SECTION Materials. Octadecene (ODE, 90%), oleylamine (OLA, 70%), and oleic acid (OA, 90%) were purchased from Aldrich. Bulk BiVO4 was purchased from Alfa Aesar. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 99.99%), ammonium metavanadate (NH4VO3, 99.9%), silver nitrate (AgNO3, 99.8%), strontium hydroxide octahydrate (Sr(OH)2·8H2O, 99.5%), rhodium chloride hydrate (RhCl3·3H2O, 38.5−42.5% Rh), ruthenium chloride hydrate (RuCl3·xH2O, 35.0−42.0% Ru), and iron chloride hexahydrate (FeCl3·6H2O, AR) were purchased from Aladdin. P25 TiO2 was purchased from Sinopharm Chemical Reagent Co., Ltd. Ammonium hydroxide (NH3·H2O, 25%, AR), urea (AR), hexane (AR), methanol (AR), ethanol (AR), N,N-dimethylformamide (DMF, AR) and acetone (AR) were purchased from Beijing Chemical Reagent Ltd., People’s Republic of China. All of the reagents were used as received. Synthesis of Ultrathin 2D BiVO4 NSs. A typical synthetic procedure of ultrathin 2D BiVO4 NSs is briefly described below. First, Bi(NO3)3·5H2O (0.5 mmol), OA (1 mL), OLA (1 mL), and ODE (10 mL) were placed in a three-neck flask. Then the flask was heated to 170 °C under an N2 atmosphere until Bi(NO3)3 was completely dissolved. In another vessel, NH4VO3 (1 mmol) was dissolved in a mixture of HNO3 (2 mL) and H2O (10 mL). Afterward, an aqueous solution was injected into the above flask containing dissolved Bi(NO3)3. The solution was kept at 100 °C for 40 min under an N2 atmosphere. Then the reaction solution was naturally cooled to room temperature followed by the addition of hexane and ethanol (or acetone). After the solution was stratified, the aqueous solution at the lower layer was discarded and the organic solution was centrifuged to discard the unreacted precursors. Then hexane and ethanol (or acetone) were added to the products and the solution was centrifuged for total of three times. Synthesis of BiVO4 NPs. The synthetic procedure of BiVO4 NPs was similar to that of ultrathin 2D BiVO4 NSs except that NH4VO3 (1 mmol) was dissolved in boiling water (10 mL) and the reaction time was only 5 min. The purified BiVO4 NPs can be well dissolved in nonpolar solvents such as chloroform, toluene, and hexane. Ligand Exchange with KI for BiVO4 NPs. The ligand exchange process was typically carried out in an atmospheric environment. BiVO4 NPs with organic ligands were dissolved

Because of the excellent photocatalytic water oxidation activity, only a small amount of ultrathin BiVO4 NSs is enough to balance the cycle during the overall water-splitting process. On the basis of the analysis above, the superior photocatalytic activity of our ultrathin 2D BiVO4 NSs can be ascribed as follows. First, the ultrathin 2D structure significantly enlarges the specific surface area of BiVO4 NSs, supplying more surface atoms as active sites to accelerate reaction progress. At the same time, the atomic thickness greatly shortens the migration distance of the charge carriers generated inside BiVO4 NSs to their surface, efficiently preventing the recombination of photogenerated charge carriers. Second, because of the spatial heterojunction between {010} and {110} planes of monoclinic BiVO4, BiVO4 NSs with large-scale {010} planes exposed strongly enhance the separation efficiency between photogenerated electrons and holes, ensuring higher photocatalytic activity. Most importantly, VOs play an important role in the enhancement of BiVO4 NS photocatalytic activity. The presence of VO not only dramatically increases the DOS at CBM and VBM, ensuring higher photoconversion efficiency and charge separation efficiency under irradiation18,39 but also contributes to stronger interaction between H2O and BiVO4, easier charge transfer from H2O to BiVO4, and lower dissociation energy of H2O.

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CONCLUSION In conclusion, we have demonstrated a simple and facile twophase strategy to synthesize ultrathin 2D BiVO4 NSs with monoclinic crystal structure. The as-prepared BiVO4 NSs possess a thickness of less than 3 nm but a diameter of larger than 1.2 μm. The presence of HNO3 provides a distinctive environment for the growth of ultrathin 2D BiVO4 NSs with ligand-free surfaces, large-scale exposed {010} planes, and widely distributed VO in a crystalline structure. These unique features endow BiVO4 NSs with a superior photocatalytic activity for water oxidation. The O2 evolution rate of our ultrathin 2D BiVO4 NSs is 107.4 μmol h−1, which is nearly 3 times higher than that of BiVO4 samples prepared by conventional hydrothermal and coprecipitation methods. The corresponding AQY at 420 nm is as high as 26.1%. Through combination with Ru-SrTiO3:Rh and Fe2+/Fe3+ as the H2evolving photocatalyst and the redox mediator, a visible-lightdriven Z-scheme overall water-splitting system with an AQY of 1.88% (420 nm) can be further constructed, which displays 8655

DOI: 10.1021/acscatal.8b01645 ACS Catal. 2018, 8, 8649−8658

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ACS Catalysis

Si (111) double-crystal monochromator. The absorption edge of standard metal foils was used to calibrate the X-ray energy. Samples were ground into fine powders and then pressed into thin disks 10 mm in diameter. V K-edge XANES/EXAFS spectra were collected at room temperature in transmission mode. The data were processed using the IFEFFIT package.52 Computational Details. The total energy and electronic structure calculations were performed using density functional theory as implemented in the Vienna ab initio simulation package (VASP), with projected augmented wave (PAW) formalism for the electron−ion interactions. The generalized gradient approximation formulated by Perdew, Burke, and Ernzerhof (PBE) was employed for the exchange-correlation functional. In all calculations, an energy cutoff of 400 eV for the plane-wave expansion of the wave functions was used. The model of the pristine surface of BiVO4 used here contains 72 atoms (12 BiVO4 formula units) and has 6 layers of atoms. A vacuum layer of 15 Å in the Z direction was created to avoid the interactions between the adjacent surfaces. The bottom 3 of atoms were fixed during the geometry optimization; only the atoms in the top 3 layers were allowed to optimize. Characterization. Transmission electron microscopy (TEM) was conducted on a Hitachi H-800 electron microscope and JEM-2100F electron microscope. UV−visible diffuse reflectance spectra were obtained using a PerkinElmer Model Lambda950 UV−vis−NIR spectrophotometer. Raman spectra were collected on a Horiba Raman spectrometer (LabRAM HR Evolution). X-ray powder diffraction (XRD) patterns were obtained using an Empyrean diffractometer (PANalytical BV) with Cu K radiation (λ = 1.5418 Å). The contents of C, H, and N were determined by an Elementar elemental analyzer (Vario micro cube). X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were carried out on a PREVAC XPS/UPS system with Al K excitation (1486.7 eV). AFM measurement was conducted with a Bruker atomic force microscope in tapping mode. Thermogravimetric (TG) measurement was performed on a Model TGA Q500 unit (TA Instruments) under an N2 atmosphere. Dynamic light scattering (DLS) measurements were performed on a Zetasizer NanoZS apparatus (Malvern Instruments). Fourier-transform infrared (FTIR) spectra were obtained on a Bruker IFS66 V instrument. Electron paramagnetic resonance (EPR) spectra were recorded on a E500 CW-EPR spectrometer (Bruker ELEXSYSII) at low temperature (100 K).

in nonpolar hexane, while KI was dissolved in polar DMF. Typically, 20 mL of BiVO4 NP solution (∼40 mg/mL) was mixed with 20 mL of KI solution (0.1 mmol/mL). The mixture was vigorously stirred until BiVO4 NPs were completely transferred from hexane to the DMF phase. Hexane turned colorless, and DMF turned yellow. The DMF phase was separated out and washed three times with water to remove excess KI. Photocatalytic O2 Evolution Test. The photocatalytic tests were carried out in a sealed gas circulation and evacuation system using a 300 W xenon lamp (CEL-HXF300) with a UV cutoff filter (λ >420 nm) as light source. In a typical test, 20 mg of catalyst was dispersed in 50 mL of 0.05 M AgNO3 aqueous solution in a reaction cell by sonication. Before irradiation, the suspension solution was degassed under vacuum with stirring for 20 min and then irradiated from the top of the reaction cell through a quartz window. The temperature of the photocatalysis system was maintained at ∼16 °C with cooling water during the reaction. For the twostep water-splitting reaction, 20 mg of Ru-SrTiO3:Rh and BiVO4 NSs with different ratios was suspended in 50 mL of FeCl3 aqueous solution (2 mM). The pH of the solution was adjusted to 2.5 by adding a small amount of HCl aqueous solution. In order to balance the concentration of Fe3+ and Fe2+, the solution was irradiated for 1 h under vacuum. The amount of generated O2 and H2 was analyzed using an online gas chromatograph (CEAULIOHT; GC-7920, Ar as carrier gas). The AQY experiment was carried out by a similar procedure except that a band-pass filter (420 ± 10 nm) was used rather than a UV cutoff filter. The average power density was about 3.55 mW cm−2, and the irradiation area was 19.625 cm2. The AQY for BiVO4 NSs in the presence of AgNO3 was calculated by following eq 5: 4 × number of O2 molecules × 100 number of incident photons 4nNAhc = × 100 PSλt

AQY (%) =

(5)

For the Z-scheme system, the AQY was calculated by following eq 6: 4 × number of H 2 molecules × 100 number of incident photons 4nNAhc = × 100 PSλt

AQY (%) =

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(6)

ASSOCIATED CONTENT

S Supporting Information *

where n is the total amount of evolved O2 or H2 during the irradiation, NA (6.022 × 1023 mol−1) is Avogadro’s constant, h (6.626 × 10−34 J s) is Planck’s constant, c (2.998 × 108 m s−1) is the speed of light, λ (420 × 10−9 m) is the wavelength of the incident monochromatic light, P (35.5 W m−2) is the power density, S (1.9625 × 10−3 m2) is the irradiation area, and t (1800 s) is the irradiation time. The amount of evolved O2 for BiVO4 NSs in the presence of AgNO3 was 2.88 × 10−5 mol, and the AQY was determined to be 26.1%. The amount of evolved H2 for the Z-scheme system was 2.07 × 10−6 mol, and the AQY was determined to be 1.88%. EXAFS Measurement. XAFS experiments were performed at the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF). The storage ring runs at 2.5 GeV with a maximum electron current of about 250 mA. The energy range of the incident X-ray is tunable from 4 to 25 keV by a fixed-exit

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01645.

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Additional TEM images, structural characterizations, XRD patterns, EDS spectra, and FTIR and XPS spectra of BiVO4 NSs, BiVO4 NPs, and Ru-SrTiO3:Rh (PDF)

AUTHOR INFORMATION

Corresponding Author

*Y.L.: fax, +86 431 85193423; e-mail, [email protected]. ORCID

Shiyu Yao: 0000-0002-6988-898X Yi Liu: 0000-0003-0548-6073 Bai Yang: 0000-0002-3873-075X Hao Zhang: 0000-0002-2373-1100 8656

DOI: 10.1021/acscatal.8b01645 ACS Catal. 2018, 8, 8649−8658

Research Article

ACS Catalysis Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (No. 2016YFB0401701), NSFC (Nos. 21773088, 51425303), the 973 Program of China (2014CB643503), JLU Science and Technology Innovative Research Team (2017TD-06), and the Special Project from MOST of China. The XAFS beam time was granted by the 1W1B beamline of Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences. The authors thank the staff of 1W1B for their technical support in XAFS measurement and guide for data analysis.

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