Colloidal Synthesis of Ultrathin Monoclinic BiVO4 Nanosheets for Z

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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, Peng-Fei An, Yi Liu, Bai Yang, and Hao Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01645 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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ACS Catalysis

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,† 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. § ǁ

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. *Address correspondence to [email protected]

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Abstract Recently, ultrathin 2D photocatalysts have attracted people’s attention due to their performances in the area of solar energy conversion. However, synthesis of ultrathin 2D photocatalysts with non-layered 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 the thickness thinner than 3 nm, but the 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 umol 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, an Z-scheme overall water splitting system is successfully constructed. Under visible light irradiation, our Z-scheme photocatalytic system presents the 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.

Key words: 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 the thickness less than 5 nm 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 Till now, many top-down synthetic strategies, such as mechanical and liquid exfoliation,14-16 ion-intercalation and exfoliation,17 are 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 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 proved to have the capability to precisely control the size and morphology of nanomaterial 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 towards the low-cost, high-yield, and mass production of ultrathin 2D photocatalysts.20 Among various kinds of photocatalysts, BiVO4 exhibits tremendous potential due to their abundant resources and low toxicity.21-23 It is well known that BiVO4 exists in three polymorphs of tetragonal zircon, tetragonal scheelite, and monoclinic scheelite structures. Monoclinic BiVO4 with the band gap of 2.4 eV is widely studied as one of the most prominent photocatalysts for water oxidation due to their excellent photocatalytic activity under visible light.24,25 However, different from other layered materials including graphene, molybdenum disulphide, and boron nitride, monoclinic BiVO4 with a non-layered crystal structure is lack of intrinsic driving force for 2D anisotropic growth.26 Despite enormous progresses have been achieved on the preparation of BiVO4 with different morphologies,27,28 the synthesis of ultrathin 2D BiVO4 nanosheets (NSs) with monoclinic structure has never 3


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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 And 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 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 ~1.2 µm. Compared with the conventional synthesis strategies including hydrothermal method and co-precipitation method,38 our method is emerging as a promising route due to its intrinsic advantages, such as time saving, low cost, and mild reaction conditions. More importantly, the distinctive growth conditions endow our ultrathin BiVO4 NSs with 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 the 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 umol h-1, which is roughly three 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 H2-evolving 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. Results and Discussion In a typical synthesis, Bi(NO3)3 was first dissolved in the mixture of oleic acid (OA), oleylamine (OLA), and octadecene (ODE) under heating. Then NH4VO3 dissolved in HNO3 4


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ACS Catalysis

aqueous solution was added and the mixed solution was stirred and refluxed at 100 oC for 40 min. After that, the flask was rapidly cooled down to room temperature, and the products were washed by hexane and ethanol (or acetone) for three times and redispersed in hexane for further characterizations and applications. It is note that HNO3 play an important role in the dissolution of NH4VO3 at room temperature. Otherwise, NH4VO3 can only be dissolved in the 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 are quasi-circular-disc morphology with the average lateral size as large as ~1.2 µm. The flower-like patterns over the NSs are attributed to the electron diffraction 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 with 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, S3 and 1f). Besides, the enhanced (040) diffraction peak in XRD pattern is quantity-dependent, suggesting the preferential parallel orientation between the {010} planes of BiVO4 NSs and the substrate (Figure S4). The aberration-corrected high-angle annular dark field image-scanning transmission electron microscopy (HAADF-STEM) image in Figure 1c exhibits the atom distribution of BiVO4 NSs, which is precisely coincided 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 and 1h), corresponding to about less than 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 is determined by energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and inductively coupled 5


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plasma-optical emission spectrometer (ICP-OES) (Figure S5, S6 and Table S1). The Bi:V atomic ratio of BiVO4 NSs is calculated to be close with the stoichiometric ratio of 1:1, though a large excess of NH4VO3 is added during the synthesis process. The element distribution of BiVO4 NSs is characterized by EDS elemental mapping. All of Bi, V, and O are uniformly distributed throughout entire BiVO4 NSs (Figure 1i-f). 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, Figure S7 and S8). The ligand-free surface endows BiVO4 NSs with excellent dispersibility both in 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 is investigated by UV-vis diffuse reflectance spectrum and ultraviolet photoelectron spectroscopy (UPS). The band gap of BiVO4 NSs is identified to be 2.38 eV based on the plot of (αhν)2 versus photon energy (Figure 2a), which is consisted with the bandgap 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 Femi level of BiVO4 should locate near CB but between CB and VB. However, the Femi 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 defect leading to the upward movement of Femi level.39-41 To prove the existence of VO, the energy band structure of BiVO4 NSs under annealing at 400 oC in air is characterized. After annealing, the Femi level of BiVO4 NSs moves back to the energy level between CB and VB, implying the elimination of VO (Figure S10). XPS is 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, 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, 6


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ACS Catalysis

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 the change of V-O bond length, which is consistent with the characterization of VO in monoclinic BiVO4.41 On the other hand, both the ultrathin BiVO4 NSs before and after annealing exhibit the signals at g = 2.003 in their EPR spectra, corresponding to the presence of V4+ and VO. While the different signal intensity imply that the concentration of V4+ and VO decrease obviously after annealing, consisting 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 are 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 distance of V-O does not change significantly, the reduced coordination number strongly indicates a severe structural distortion of V cations in BiVO4 NSs, consisting 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 than that of the bulk counterpart. Based on 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 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 is further proved by dynamic light scattering (DLS) measurement (Figure S13). HNO3 plays a critical role on 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 7


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2D growth of BiVO4 NSs. As the formation of hydroxyl group 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 high 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 prefer to be reduced into VO2+ at 100 oC 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 people claimed that the presence of VO can promote the separation of photogenerated electrons and holes, then enhance the photocatalytic efficiency of BiVO4.45,46 Other people believed that the photoelectron conversion efficiency of BiVO4 is suppressed by both radiative and nonradiative recombination since the existence of VO.47 We speculate that the location of VO determine their functions. VO inside BiVO4 act as the charge recombination center, while VO on the surface of BiVO4 facilitate the charge separation, then accelerate the photocatalytic reactions.34 Because the thickness of our BiVO4 NSs is less than 3 nm, most VO can be seen as the surface VO. In order to prove our speculation, the first-principles calculation is 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 molecule from 4.562 to 2.455 eV, promoting the splitting of H2O molecules into H and O atoms, resulting in high photo conversion efficiency. The evolution of total charge and H-O bond length of H2O molecule upon adsorption on the surface of BiVO4 show 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 bond length from 0.973 to 0.999 (0.995) Å. Under the same condition but eliminating VO, only 0.028 electrons transferred from H2O to BiVO4 and the elongation of H-O bond is reduced (Table 2). 8


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Density-functional-theory (DFT) calculations are 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 conduction band minimum (CBM) and valence band maximum (VBM). These alterations remarkably promote the excitation, separation of photogenerated electrons and holes, and their subsequent migration to participate the water splitting reactions. The spatial distribution of the charge density at Fermi level are shown in Figure 4c as well. From which it can be seen that the majority of charges are concentrated around VO. Due to the ultrathin 2D structure, most of 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 is firstly evaluated by monitoring O2 production from water in the presence of AgNO3 as sacrificial agents. Thanks to their ligand-free 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 co-catalysis exhibit an impressive photocatalytic activity under visible light illumination (λ > 420 nm). The average evolution O2 rate is as high as 107.4 umol h-1, and the AQY at 420 nm is determined to be 26.1%, which is much higher than previous reports.21,48 Vigorous bubbles of O2 can be observed even after irradiation for 1 h. The temporary gentle 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 are evaluated as well. With increasing the annealing temperature from 200 to 400 oC, the O2 evolution rate of BiVO4 NSs decrease from 96.2 to 57.8 umol h-1. However, despite under annealing at high temperature, the average evolution O2 rates of BiVO4 NSs are still much higher than that of BiVO4 NPs (2.2 umol h-1). What is more, as to BiVO4 NPs, extra ligand-exchange processes are still needed to replace the original hydrophobic ligands with KI before using as the photocatalyst for water splitting (Figure S18 and S19). For the better comparison purposes, two kinds of currently well-recognized BiVO4 samples with monoclinic structure are also prepared through conventional hydrothermal and co-precipitation methods. The O2 evolution rates of them under the same experimental conditions are 31.2 and 27.6 umol h-1 respectively (Figure S20), 9


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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. While the existence of VO has a positive effect on the photocatalytic activity of ultrathin 2D BiVO4 NSs. By employing our ultrathin 2D BiVO4 NSs as the O2-evolving 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 Ru-SrTiO3:Rh and BiVO4 NSs on photocatalytic water splitting under visible light irradiation. Different from most of previous experiences implying that the dosage of BiVO4 should be higher than Ru-SrTiO3:Rh,49,50 the optimized weight ratio of BiVO4:SrTiO3 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 is also 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), it still remains nearly 80% of the photocatalytic activity. These impressive photocatalytic performances indicate that our ultrathin 2D BiVO4 NSs are very potential candidates for photocatalytic water oxidation, and even overall water splitting via Z-scheme strategy by integrating with appropriate H2-evolving 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 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 molecular to O2, and the electrons in CB of BiVO4 NSs are captured by Fe3+ to form Fe2+: 10


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ACS Catalysis

2H2O + 4h+VB (BiVO4) → O2 + 4H+

Equation 1

Fe3+ + e-CB (BiVO4) → Fe2+

Equation 2

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 molecular to H2, and the holes in the donor levels of Ru-SrTiO3:Rh will re-oxidize Fe2+ to Fe3+: 2H+ + 2e-CB (Ru-SrTiO3:Rh) → H2

Equation 3

Fe2+ + h+Donor (Ru-SrTiO3:Rh) → Fe3+

Equation 4

Because of the excellent photocatalytic water oxidation activity, only a small amount of ultrathin BiVO4 NSs is enough to balance the cycle during overall water splitting process. Based on the analysis above, the superior photocatalytic activity of our ultrathin 2D BiVO4 NSs can be ascribed as follow: 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 shorten 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 enhances the separation efficiency between photogenerated electrons and holes, ensuring higher photocatalytic activity. Most importantly, VO play an important role on the enhancement of BiVO4 NSs’ photocatalytic activity. The presence of VO not only dramatically increases the DOS at CBM and VBM, ensuring higher photo conversion efficiency and charge separation efficiency under irradiation,18,39 but also contributes to stronger interaction between H2O and BiVO4, easier charge transfer from H2O to BiVO4, and lower dissociation energy of H2O. Conclusion In conclusion, we demonstrate a simple and facile two-phase strategy to synthesize ultrathin 2D BiVO4 NSs with monoclinic crystal structure. The as-prepared BiVO4 NSs possess the thickness thinner than 3 nm, but the diameter larger than 1.2 µm. The presence of 11


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HNO3 provide 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 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 umol h-1, which is nearly three times higher than that of BiVO4 samples prepared by conventional hydrothermal and co-precipitation methods. The corresponding AQY at 420 nm is as high as 26.1%. Through combining with Ru-SrTiO3:Rh and Fe2+/Fe3+ as the H2-evolving photocatalyst and the redox mediator, a visible-light-driven Z-scheme overall water splitting system with an AQY of 1.88% (420 nm) can be further constructed, which displays excellent H2 and O2 evolution rates (16.7 and 8.0 µmol h-1) and photocatalytic stability. Since the two-phase method is low cost, time saving, and mild reaction conditions, it is believed that our study not only discloses the effects of morphology, crystal phase, VO on the photocatalytic activity of BiVO4, but also proposes an effective method to design and synthesis of photocatalysts with ultrathin 2D morphology. 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., 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 added to one three-neck flask. Then the flask was heated to 170 oC under N2 atmosphere until Bi(NO3)3 was completely dissolved. In another vessel, NH4VO3 (1 mmol) was dissolved in the mixture of HNO3 (2 mL) and H2O (10 mL). Afterward, the 12


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ACS Catalysis

aqueous solution was injected into the above flask containing dissolved Bi(NO3)3. The solution was kept at 100 oC for 40 min under N2 atmosphere. Then the reaction solution was naturally cooled to room temperature follow 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 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 non-polar solvent such as chloroform, toluene, and hexane. Ligand exchange with KI for BiVO4 NPs. The ligand exchange process was typically carried out under atmospheric environment. BiVO4 NPs with organic ligands were dissolved in non-polar 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. The color of hexane changed to colorless and DMF changed to yellow. The DMF phase was separated out and washed for 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 two-step 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 balance the concentration of Fe3+ and Fe2+, the solution was irradiated for 1h under vacuum. The amount of generated O2 and H2 was analyzed using an online gas chromatography (CEAULIOHT; 13


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GC-7920, Ar as carrier gas). AQY experiment was carried out in a similar procedure except that a bandpass 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 Equation 5: AQY % =

×

× 100 =

! × "#λ

100

Equation 5

For the Z-scheme system, the AQY was calculated by following Equation 6: AQY % =

× $

× 100 =

! × "#λ

100

Equation 6

where n is the total amount of evolved O2 or H2 during the irradiation, NA (6.022 x 1023 mol-1) is the Avogadro’s constant, h (6.626 x 10-34 J s) is Planck’s constant, c (2.998 x 108 m s-1) is the speed of light and λ (420 x 10-9 m) is the wavelength of the incident monochromatic light, P (35.5 W m-2) is the power density, S (1.9625 x 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 x 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 x 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 fix-exit Si (111) double crystal monochromator. The absorption edge of standard metal foils is used to calibrate the X-ray energy. Samples were ground into fine powers and then pressed into thin disks of 10 mm in diameter. V K-edge XANES/EXAFS spectra were collected at room temperature in transmission mode. The data were processed using IFEFFIT package.52 Computational Details. The total energy and electronic structure calculations were performed using the 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 exchange-correlation functional. In all calculations, an energy cutoff of 400 eV for the plane-wave expansion of the wavefunctions was used. The model of the pristine surface of BiVO4 used here contains 72 atoms (12 BiVO4 14


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ACS Catalysis

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 layers 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 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 B.V.) 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 an 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 N2 atmosphere. Dynamic light scattering (DLS) measurements were performed on Zetasizer NanoZS (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). Supporting Information. Additional TEM images, structural characterizations, XRD patterns, EDS spectra, FTIR and XPS spectra of BiVO4 NSs, BiVO4 NPs and Ru-SrTiO3:Rh are provided. Corresponding Author Fax: +86 431 85193423. E-mail: [email protected]. Notes The authors declare no competing financial interest. 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 15


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(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 would like to thank the staff of 1W1B for their technical support in XAFS measurement and guide for data analysis. References (1) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37-38. (2) Montoya, J. H.; Seitz, L. C.; Chakthranont, P.; Vojvodic, A.; Jaramillo, T. F.; Noørskov, J. K. Materials for solar fuels and chemicals. Nat. Mater. 2016, 16, 70-81. (3) Lewis, N. S. Developing a scalable artificial photosynthesis technology through nanomaterials by design. Nat. Nanotechnol. 2016, 11, 1010-1019. (4) Luo, B.; Liu, G.; Wang, L. Recent advances in 2D materials for photocatalysis. Nanoscale 2016, 8, 6904-6920. (5) Li, Y.; Li, Y.-L.; Sa, B.; Ahuja, R. Review of two-dimensional materials for photocatalytic water splitting from a theoretical perspective. Catal. Sci. Technol. 2017, 7, 545-559. (6) Di, J.; Xiong, J.; Li, H.; Liu, Z. Ultrathin 2D Photocatalysts: Electronic-Structure Tailoring, Hybridization, and Applications. Adv. Mater. 2018, 30, 1704548. (7) Han, G.; Jin, Y.-H.; Burgess, R. A.; Dickenson, N. E.; Cao, X.-M.; Sun, Y. Visible-Light-Driven Valorization of Biomass Intermediates Integrated with H2 Production Catalyzed by Ultrathin Ni/CdS Nanosheets. J. Am. Chem. Soc. 2017, 139, 15584-15587. (8) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015, 347, 970-974. (9) Zhou, Y.; Zhang, Y.; Lin, M.; Long, J.; Zhang, Z.; Lin, H.; Wu, J. C-S; Wang, X. Monolayered Bi2WO6 nanosheets mimicking heterojunction interface with open surfaces for photocatalysis. Nat. Commun. 2015, 6, 8340.

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(19) Xiao, M.; Luo, B.; Lyu, M.; Wang, S.; Wang, L. Single-Crystalline Nanomesh Tantalum Nitride Photocatalyst with Improved Hydrogen-Evolving Performance. Adv. Energy Mater. 2018, 8, 1701605. (20) Tan, C.; Zhang, H. Wet-chemical synthesis and applications of non-layer structured two-dimensional nanomaterials. Nat. Commun. 2015, 6, 7873. (21) Kudo, A.; Omori, K.; Kato H. A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO4 powder from layered vanadates at room temperature and its photocatalytic and photophysical properties. J. Am. Chem. Soc. 1999, 121, 11459-11467. (22) Li, P.; Chen, X.; He, H.; Zhou, X.; Zhou, Y.; Zou, Z. Polyhedral 30-Faceted BiVO4 Microcrystals Predominantly Enclosed by High-Index Planes Promoting Photocatalytic Water-Splitting Activity. Adv. Mater. 2018, 30, 1703119. (23) Ye, M.-Y.; Zhao, Z.-H.; Hu, Z.-F.; Liu, L.-Q.; Ji, H.-M.; Shen, Z.-R.; Ma, T.-Y. 0D/2D Heterojunctions of Vanadate Quantum Dots/Graphitic Carbon Nitride Nanosheets for Enhanced Visible‐Light‐Driven Photocatalysis. Angew. Chem. Int. Ed. 2017, 56, 8407-8411. (24) Iwase, A.; Kudo, A. Photoelectrochemical water splitting using visible-light-responsive BiVO4 fine particles prepared in an aqueous acetic acid solution. J. Mater. Chem. 2010, 20, 7536-7542. (25) Kim, T. W.; Choi, K. S. Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting. Science 2014, 343, 990-994. (26) Du, Y.; Yin, Z.; Zhu, J.; Huang, X.; Wu, X.-J.; Zeng, Z.; Yan, Q.; Zhang, H. A general method for the large-scale synthesis of uniform ultrathin metal sulphide nanocrystals. Nat. Commun. 2012, 3, 1177. (27) Zhang, L.; Chen, D.; Jiao, X. Monoclinic structured BiVO4 nanosheets: hydrothermal preparation, formation mechanism, and coloristic and photocatalytic properties. J. Phys. Chem. B 2006, 110, 2668-2673. (28) Xi, G.; Ye, J. Synthesis of bismuth vanadate nanoplates with exposed {001} facets and enhanced visible-light photocatalytic properties. Chem. Commun. 2010, 46, 1893-1895.

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(29) Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li, C. Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4. Nat. Commun. 2013, 4, 1432. (30) Tan, H. L.; Wen, X.; Amal, R.; Ng, Y. H. BiVO4 {010} and {110} Relative Exposure Extent: Governing Factor of Surface Charge Population and Photocatalytic Activity. J. Phys. Chem. Lett. 2016, 7, 1400-1405. (31) Wang, D.; Jiang, H.; Zong, X.; Xu, Q.; Ma, Y.; Li, G.; Li, C. Crystal facet dependence of water oxidation on BiVO4 sheets under visible light irradiation. Chem. Eur. J. 2011, 17, 1275-1282. (32) Zhu, M.; Sun, Z.; Fujitsuka, M.; Majima, T. Z-Scheme Photocatalytic Water Splitting on a 2D Heterostructure of Black Phosphorus/Bismuth Vanadate Using Visible Light. Angew. Chem. Int. Ed. 2018, 57, 2160 –2164. (33) Li, H.; Li, J.; Ai, Z.; Jia, F.; Zhang, L. Oxygen Vacancy-Mediated Photocatalysis of BiOCl: Reactivity, Selectivity, and Perspectives. Angew. Chem. Int. Ed. 2018, 57, 122-138. (34) Kong, M.; Li, Y.; Chen, X.; Tian, T.; Fang, P.; Zheng, F.; Zhao, X. Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency. J. Am. Chem. Soc. 2011, 133, 16414-16417. (35) Di, J.; Chen, C.; Yang, S.-Z.; Ji, M.; Yan, C.; Gu, K.; Xia, J.; Li, H.; Li, S.; Liu, Z. Defect engineering in atomically-thin bismuth oxychloride towards photocatalytic oxygen evolution. J. Mater. Chem. A 2017, 5, 14144-14151. (36) Li, H.; Shang, J.; Ai, Z.; Zhang, L. Efficient Visible Light Nitrogen Fixation with BiOBr Nanosheets of Oxygen Vacancies on the Exposed {001} Facets. J. Am. Chem. Soc. 2015, 137, 6393-6399. (37) Jiao, X.; Chen, Z.; Li, X.; Sun, Y.; Gao, S.; Yan, W.; Wang, C.; Zhang, Q.; Lin, Y.; Luo, Y.; Xie, Y. Defect-Mediated Electron-Hole Separation in One-Unit-Cell ZnIn2S4 Layers for Boosted Solar-Driven CO2 Reduction. J. Am. Chem. Soc. 2017, 139, 7586-7594. (38) Huang, Z.-F.; Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Nanostructured bismuth vanadate-based materials for solar-energy-driven water oxidation: a review on recent progress. Nanoscale 2014, 6, 14044-14063. 19


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(39) Wang, G.; Ling, Y.; Lu, X.; Qian, F.; Tong, Y.; Zhang, J. Z.; Lordi, V.; Rocha Leao, C.; Li, Y. Computational and Photoelectrochemical Study of Hydrogenated Bismuth Vanadate. J. Phys. Chem. C 2013, 117, 10957-10964. (40) Wang, S.; Chen, P.; Bai, Y.; Yun, J. H.; Liu, G.; Wang, L. New BiVO4 Dual Photoanodes with Enriched Oxygen Vacancies for Efficient Solar-Driven Water Splitting. Adv. Mater. 2018, 30, 1800486. (41) Tan, H. L,; Suyanto, A.; De Denko, A. T.; Saputera, W. H.; Amal, R.; Osterloh, F. E.; Ng, Yu. H. Enhancing the Photoactivity of Faceted BiVO4 via Annealing in Oxygen-Deficient Condition. Part. Part. Syst. Charact. 2017, 34, 1600290. (42) Granzin, J.; Pohl, D. Refinement of pucherite, BiVO4. Z. Kristallogr. 1984, 169, 289-294. (43) Kim, C. W.; Son, Y. S.; Kang, M. J.; Kim, D. Y.; Kang, Y. S. (040)-Crystal Facet Engineering of BiVO4 Plate Photoanodes for Solar Fuel Production. Adv. Energy Mater. 2016, 6, 1501754. (44) Zhao, K.; Zhang, L.; Wang, J.; Li, Q.; He, W.; Yin, J. J. Surface structure-dependent molecular oxygen activation of BiOCl single-crystalline nanosheets. J. Am. Chem. Soc. 2013, 135, 15750-15753. (45) Wang, S.; Chen, P.; Yun, J-H.; Hu, Y.; Wang, L. An Electrochemically Treated BiVO4 Photoanode for Efficient Photoelectrochemical Water Splitting. Angew. Chem. Int. Ed. 2017, 56, 8500–8504. (46) Qin, D-D.; Wang, T.; Song, Y-M.; Tao, C-L. Reduced monoclinic BiVO4 for improved photoelectrochemical oxidation of water under visible light. Dalton Trans., 2014, 43, 7691– 7694. (47) Kho, Y. K.; Teoh, W. Y.; Iwase, A.; Mädler, L.; Kudo, A.; Amal, R. Flame Preparation of Visible-Light-Responsive BiVO4 Oxygen Evolution Photocatalysts with Subsequent Activation via Aqueous Route. ACS Appl. Mater. Interfaces 2011, 3, 1997–2004. (48) Kong, H. J.; Won, D. H.; Kim, J. Woo, S. I. Sulfur-Doped g-C3N4/BiVO4 Composite Photocatalyst for Water Oxidation under Visible Light. Chem. Mater. 2016, 28, 1318−1324.

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(49) Kato, H.; Sasaki, Y.; Shirakura, N.; Kudo, A. Synthesis of highly active rhodium-doped SrTiO3 powders in Z-scheme systems for visible-light-driven photocatalytic overall water splitting. J. Mater. Chem. A 2013, 1, 12327. (50) Melo, M. A., Jr.; Wu, Z.; Nail, B. A.; De Denko, A. T.; Nogueira, A. F.; Osterloh, F. E. Surface Photovoltage Measurements on a Particle Tandem Photocatalyst for Overall Water Splitting. Nano Lett. 2018, 18, 805-810. (51) Chen, S.; Takata, T.; Domen, K. Particulate photocatalysts for overall water splitting. Nat. Rev. Mater. 2017, 2, 17050. (52) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Rad. 2005, 12, 537-541.

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Figure 1. (a) TEM, (b) HTEM and (c) aberration-corrected HAADF-STEM image 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 panel (a): lateral size distribution of ultrathin 2D BiVO4 NSs.

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Figure 2. (a) UV-vis diffuse reflectance spectrum, (b, c) UPS spectra and (d) band position 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 panel (a): plot of (αhν)2 versus the photon energy.

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

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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 (010) plane. The Fermi energy level was adjusted to 0 and marked as the pink dashed line. (c) The charge density distribution at the Fermi level of the BiVO4 with VO on the surface of (010) plane.

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Figure 5. (a) Time courses and (b) O2 generation rates of BiVO4 NSs, BiVO4 NPs and BiVO4 NSs after annealed at different temperatures in air in AgNO3 aqueous solution (50 mM). (c) Gas generation 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). (d) Cycle stability test on the Z-scheme overall water splitting system composed by BiVO4 NSs and Ru-SrTiO3:Rh photocatalyst (SrTiO3:BiVO4 = 6:1) in FeCl3 aqueous solution (2 mM). (e) Mechanism of the Z-scheme 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).

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Table 1. EXAFS curve-fitting results for the structural parameters around V atoms in bulk BiVO4 and ultrathin 2D BiVO4 NSs. Name

Shell

N

R (Å)

σ2 (Å)

△E

S02

Bulk BiVO4

V-O

4.00

1.69

0.0052

4.72

0.9

BiVO4 NSs

V-O

3.68

1.69

0.0036

4.72

0.9

Table 2. Calculated total charge and the H-O distance of the water molecule. Name

Total charge

H-O distance

Free H2O molecule

8.000

0.973

H2O on BiVO4 surface

7.972

0.992

7.917a

0.999a

7.916b

0.995b

H2O on BiVO4 surface with VO

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). Conditions: 20 mg of catalyst in 50 mL of aqueous solution. Light source: 300 W Xe lamp (λ > 420 nm). SrTiO3/BiVO4

SrTiO3

9:1

6:1

3:1

2:1

1:1

1:2

BiVO4

H2 (µmol h-1)

1.0

12.8

16.7

11.4

9.3

6.6

5.1

0

O2 (µmol h-1)

0.4

6.6

8.2

6.2

4.5

3.0

3.3

9.1

27


ACS Catalysis 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

TOC

28


Page 28 of 28

Colloidal Synthesis of Ultrathin Monoclinic BiVO4 Nanosheets for Z

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