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Complex-mediated synthesis of tantalum oxyfluoride hierarchical nanostructures for highly efficient photocatalytic hydrogen evolution Leilei Xu, Haotian Gong, Li Deng, Fei Long, Yu Gu, and Jianguo Guan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02622 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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ACS Applied Materials & Interfaces

Complex-mediated synthesis of tantalum oxyfluoride hierarchical nanostructures for highly efficient photocatalytic hydrogen evolution Leilei Xu, Haotian Gong, Li Deng, Fei Long, Yu Gu, and Jianguo Guan* State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, P. R. China.

*

Corresponding author. Tel: +86-27-87218832; Fax: +86-27-87879468. E-mail: [email protected]

ACS Paragon Plus Environment


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Abstract: This work has for the first time developed a facile wet-chemical route to obtain a novel photocatalytic material of tantalum oxyfluoride hierarchical nanostructures composed of amorphous cores and single crystalline TaO2F nanorod shells (ACHNs) by regulating the one-step hydrothermal process of TaF5 in a mixed solution of isopropanol (i-PrOH) and H2O. In this approach, elaborately controlling the reaction temperature and volume ratio of i-PrOH and H2O enabled TaF5 to transform into intermediate coordination complex ions of [TaOF3·2F]2- and [TaF7]2-, which subsequently produced tantalum oxyfluoride ACHNs via a secondary nucleation and growth due to a stepwise change in hydrolysis rates of the two complex ions. Because of the unique chemical composition, crystal structure and micro-morphology, the as-prepared tantalum oxyfluoride ACHNs show a more negative flat band potential, an accelerated charge transfer, and a remarkable surface area of 152.4 m2 g-1 contributing to increased surface reaction sites. As a result, they exhibit a photocatalytic activity for hydrogen production up to 1.95 mmol h-1 g-1 under the illumination of a simulated solar light without any assistance of cocatalysts, indicating that the as-prepared tantalum oxyfluoride ACHNs are a novel promising photocatalytic material for hydrogen production. Key words: Tantalum oxyfluoride, Complex-mediated synthesis, Hierarchical structure, Photocatalytic hydrogen production, Stepwise hydrolysis, Core-shell particles


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Introduction Sustainable and green energy production from solar energy conversion has attracted much attention in the past decades due to the ever-increasing environmental contamination and energy demand.1-5 Among so far available means, photocatalytic hydrogen production from water splitting can accomplish the direct conversion of solar energy into chemical energy without any recrements.6-9 In order to photocatalytically produce hydrogen with a high efficiency, the employed photocatalysts are necessarily required to have large surface areas and/or high crystallinities, which can increase surface active sites and/or accelerate charge separation.10-15 Besides, the light absorption and charge transfer resistance of photocatalysts as well as the reduction abilities of photoexcited electrons are also especially pivotal for enhancing photocatalytic performances.16-18 Introducing dopant atoms into photocatalysts can to some extent modify their light absorptions, but there seems still no way to effectively improve charge transfer and reduction abilities of photoexcited electrons, both of which closely depend on chemical compositions, electronic structures and crystal structures of photocatalysts. Therefore, developing new photocatalytic materials with relatively negative flat-band potentials and/or specific crystal structures is an alternative strategy for highly efficient hydrogen production. Semiconductors with d0 electron configurations, such as tantalum-, niobium- and titanium-based oxides, etc., possess suitable energy band structures, superior chemical stabilities and photocorrosion resistances, and thus are widely used as photocatalytic



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materials for hydrogen production.19 In particular, tantalum-based oxides with a relatively negative conduction band composed of Ta 5d, which enables the photoexcited electrons to possess a much stronger reduction ability, have been recognized as a promising material for photocatalytic hydrogen production from water splitting.20-31 In order to further improve photocatalytic activities for hydrogen production by improving specific surface areas or the crystallinities, past efforts mainly focus on the fabrication of tantalum oxide micro- or nanostructures, such as mesoporous structures,21 nanorods23-25, nanospheres26,27 and hierarchical structures (HNs) composed of single crystalline nanorods,30,31 etc. The recent demonstration of a new efficient tantalum oxide photocatalyst TaO2.18Cl0.64 for hydrogen production indicates that changing chemical compositions and crystal structures of tantalum oxides is an alternative means to enhance photocatalytic activities, which plays an important role in improving charge transfer and making the flat-band potential more negative.16 As reported, F- ions doping not only increases the charge separation efficiencies, but also improves the light harvest in UV-Vis region under the condition of almost remaining the reduction abilities of photogenerated electrons.30,32-35 For tantalum oxyfluorides involving Ta3O7F and TaO2F, the incorporation of F- ions may improve the photocatalytic activity by modulating the crystal structure and composition. Between them, TaO2F is more desirable for photocatalytic hydrogen production due to the layered ABO3 Perovskite structure with vacant A sites36,37 and the crystallographic Ta-O(F)-Ta bond angles close to 180o.38 Herein, we have for the first time demonstrated a facile, eco-friendly wet-chemical


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route to fabricate tantalum oxyfluoride HNs composed of amorphous cores and crystalline TaO2F shells (ACHNs) for photocatalytic hydrogen production. In our protocol, TaF5 as the precursor is hydrothermally treated in a mixed solution of isopropanol and H2O to yield tantalum oxyfluoride ACHNs via a stepwise hydrolysis of the mediated [TaOF3·2F]2- and [TaF7]2- complex ions. This strategy is dramatically different from the previous preparation methods of TaO2F irregular particles, where corrosive HF is needed to dissolve Ta/Ta2O5 powders at a higher temperature.36,37 The as-prepared tantalum oxyfluoride ACHNs have a dramatically increased BET surface area up to 152.4 m2 g-1 and a unique micro-morphology, endowing them with increased surface reaction sites, accelerated charge transfer and enhanced photoexcited electron reduction ability. Consequently, they exhibit a far greater photocatalytic hydrogen production activity than the corresponding calcined tantalum oxide CHNs, the reported F-Ta2O5 HNs30 and commercial tantalum oxide, promising a novel potential photocatalytic material for hydrogen production. Experimental Section Synthesis of tantalum oxyfluoride ACHNs In a typical procedure, 40 mg of TaF5 (Sigma-Aldrich, 98 %, AR) white powders was dissolved in H2O under ultrasonic for 5 min to obtain a transparent solution followed by adding isopropanol (i-PrOH). The volume ratio of i-PrOH into H2O (λi-PrOH/H2O) is 23 : 7. The aforementioned solution was homogeneously mixed by stirring for 1 h and then transferred into a 50 mL Teflon-lined autoclave, which was subsequently sealed and heated at 220 oC for 24 h. After the autoclave was naturally



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cooled to room temperature, the white precipitates obtained were separated through centrifugation, washed with ethanol and H2O for three times, and then dried in a vacuum oven at 60 oC for 24 h to obtain tantalum oxyfluoride ACHNs. For comparison, the contrastive samples were also prepared by changing λi-PrOH/H2O, the reaction temperature (T) and time (t), besides the calcined product of tantalum oxide CHNs, which is obtained by calcining the tantalum oxyfluoride ACHNs at 700 o

C for 3 h in a tube furnace. The commercial tantalum oxide was purchased from

Aladdin Industrial Inc. without any treatments. Characterization of tantalum oxyfluoride ACHNs X-ray diffraction (XRD) patterns were obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (λ=1.5418 Å) with a resolution of 0.02° of 2θ from 10o to 80o. The accelerating voltage and applied current were 15 kV and 20 mA, respectively. The morphologies of the samples were characterized by using a field emission scanning electron microscope (FE-SEM, HITACHI, S-4800, Japan). The elemental composition was analyzed using a Horiba EX250 X-ray energy-dispersive spectrometer (EDS) attached to the FE-SEM. X-ray photoelectron spectra (XPS) were performed on a VG-ADES 400 instrument with a MgKα-ADES source at a residual gas pressure of below 10-8 Pa. All of the binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. TEM and high-resolution TEM (HRTEM) images were obtained on an H-600 STEM/EDX PV9100 (HITACHI). The BET surface areas of the samples were analyzed by nitrogen adsorption on an ASAP 2020 nitrogen-adsorption apparatus (Micromeritics


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Instruments, USA). All of the samples were degassed at 150 oC for 4 h prior to the measurements. The desorption isotherm was used to determine the pore-size distribution by using Barret–Joyner–Halender (BJH) method. UV/Vis diffusion reflectance spectra (DRS) were obtained on an UV/Vis spectrophotometer (UV-2550, Shimadzu, Japan). BaSO4 was used as a reflectance standard in the UV/Vis diffuse-reflectance experiments. Photocatalytic performances of tantalum oxyfluoride ACHNs The photocatalytic performances for hydrogen evolution were performed in a closed gas circulation system Labsolar-I (Beijing Perfectlight Co., Ltd., China) with an external-irradiation Pyrex cell. A xenon lamp (PLS-SXE300, Beijing Trusttech Co., Ltd., China) with an average light intensity of 5 mW cm-2 was used as the light source after equipped with an IR filter. As shown in Figure S1, it has a spectrum similar to that of solar light covering UV and visible light regions. In the whole test process, the photocatalytic reactor was cooled by circulating water to rule out the possible thermal effect deriving from the light irradiation. The sample put in the Pyrex cell was made by ultrasonically dispersing 20 mg of photocatalysts in a 20 vol% methanol aqueous solution for 10 min and subsequently stirring for 12 h. Prior to irradiation, both the reaction cell and the closed gas-circulation system were evacuated. The photocatalytic hydrogen production activities were detected in-situ with a 7890-II gas chromatograph equipped with an MS-5A column with a N2 carrier and a thermal-conductivity detector that was connected to the closed gas-circulating line. Electrochemical measurement



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The electrochemical impedance spectra (EIS) and Mott–Schottky plots were both performed on an electrochemical workstation (Autolab PGSTAT302N) with a standard three-electrode cell. Platinum foil and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. 3 M NaCl aqueous solution was used as the electrolyte. For preparing the working electrodes, 0.05 g sample powders and 0.015 g PVDF were mixed together uniformly, and then pressed onto a piece of Ni porous foil under 10 MPa pressure for 10 min. All the working electrodes investigated in this study were of similar thickness and area (2 cm2). The frequency range was from 0.1 Hz to 105 Hz with ac amplitude of 10 mV. Results and discussion Morphology, structure and composition characterizations of the typical sample are shown in Figure 1. The SEM image shows that the as-prepared sample is obviously thorn spherical hierarchical nanostructures (HNs) with an average size of approximately 300 nm (Figure 1a). They are composed of inner spherical cores and outer nanorods. The TEM image in Figure 1b reveals a distinct contrast of a single particle, indicating a high porosity of the inner spherical core formed by interconnection nanoparticles. The inner spherical core has a diameter of about 200-300 nm while the outer nanorods display an average diameter of ~ 20 nm and a length of 20-50 nm. The HRTEM image of the outer nanorod shown in Figure 1b-A exhibits a representative interplanar spacing of 0.382 nm assigned to plane (100) of cubic TaO2F (JCPDS card No. 76–2370), revealing its single-crystalline nature and growth direction along [100]. Figure 1b-B demonstrates the co-existence of


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crystalline and amorphous phases in the as-prepared sample. A sectioned particle is used to characterize the inner structure of the as-prepared sample (Figure 1c). The SAED pattern showing diffuse rings indicates the amorphous nature of the inner core. From the XRD pattern in Figure 1d, the diffraction peaks appearing at about 22.8o, 32.5 o, 46.6o and 52.5o should come from the outer nanorods, and are respectively well indexed to (100), (110), (200) and (210) crystal planes of cubic TaO2F. The other arch peaks located among 20o ~ 40o and 45o ~ 60o stem from the amorphous character of tantalum oxides, which is consistent with the previous reports.17,27 Combining the above characterizations, we can reasonably deduce that the as-prepared sample is composed of amorphous cores and single crystalline TaO2F nanorod shells growing along [100]. The elemental mapping of a typical ultra-thin sectioned particle in Figure S2 reveals even distributions of Ta, O and F elements in the inner core. The EDS analysis in Figure 1d indicates that the F and Ta atom ratio of the typical sample is 1.6 : 1, larger than that in TaO2F and Ta3O7F. This is because besides lattice F, the as-prepared samples contain some surface F, as confirmed by the following XPS analysis in Figure 2. Although it is hard to measure the accurate content of lattice F and O in the inner core due to the testing limitation, we can reasonably deduce that the inner cores have the similar crystal structure and composition to TaO2F rather than Ta3O7F. Nevertheless, the as-prepared sample may be certainly denoted as tantalum oxyfluoride ACHNs. Figure 1f displays the N2 adsorption-desorption isotherm of the tantalum oxyfluoride ACHNs. It shows a type IV pattern with a H2 hysteresis loop.



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The capillary condensation occurred at P/P0 = 0.4 – 0.8 suggests its mesoporosity, which is identical with the TEM image (the inset of Figure 1b). The BET surface area (SBET) of the as-prepared tantalum oxyfluoride ACHNs is up to 152.4 m2 g-1, which is far larger than the previously reported HNs and would benefit the enhancement of photocatalytic activity.30,31 The inset of Figure 1f implies that the as-prepared tantalum oxyfluoride ACHNs contain uniform pores with an average size of ~ 5 nm.

Figure 1

(a) SEM image of the as-prepared tantalum oxyfluoride ACHNs; (b) TEM

image of a tantalum oxyfluoride ACHN. A and B are the HRTEM images of the outer nanorod as well as the interface between nanorods and cores labelled with squares, respectively; (c) TEM image of an ultra-thin section of a tantalum oxyfluoride ACHN.


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Inset: the corresponding SAED image; (d) XRD pattern, (e) EDS analysis and (f) N2 absorption-desorption analysis of the as-prepared tantalum oxyfluoride ACHNs. Figure 2 presents the detailed XPS spectra of the typical tantalum oxyfluoride ACHNs. The XPS full spectrum confirms the existence of Ta, O, and F elements (Figure 2a), which is consistent with the result of Figure 1d. In Figure 2b, the doublet peaks centered at 26.3 and 28.2 eV are assigned to Ta4f7/2 and Ta4f5/2 orbitals, respectively, revealing that the oxidation state of Ta is identical to that of tantalum oxides.16 The XPS spectrum of F 1s core electrons has a peak centered at 684.5 eV, originated from Ta-F bonds on the surface of tantalum oxyfluoride ACHNs.29,39 The other F 1s peak at 689.0 eV is assigned to the substitutional F atoms that occupied oxygen sites in the lattice and formed Ta-F-Ta bonds.40 This supports that both surface F and lattice F atoms co-exist in the as-prepared tantalum oxyfluoride ACHNs. Figure 2d shows O 1s signals at 530.5 and 531.9 eV, which correspond to lattice O and surface hydroxyl groups, respectively. The existences of surface F and hydroxyl groups result in the larger atom percentages of O or F than those in TaO2F (Figure 1d).



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Figure 2 (a) XPS full, high magnified (b) Ta 4f, (c) F 1s and (d) O 1s spectra of the tantalum oxyfluoride ACHNs. In water, most inorganic or organic tantalum precursors such as TaCl5 and Ta(OC2H5)5 are rapidly hydrolyzed to generate Ta(OH)5 precipitates, whereas TaF5 is dissolvable to produce water-soluble coordination complex ions of fluorine and oxygen ([TaOF3·2F]2-).41 As an alternative to the widely used etching strategy, this can be employed to directly prepare the tantalum oxyfluoride ACHNs. Figure 3 shows the SEM images and XRD patterns of the samples obtained at different solvothermal time (t). At preliminary stage of t = 20 min, the obtained product consists of some nanoparticles and spherical cores with multiple sizes (Figure 3a). Prolonging t to 30 min, the nanoparticles disappear and the spherical cores obviously grow up with a smooth surface and a uniform size of 250 - 350 nm (Figure 3b). When t is increased to 1 h, small nanoparticles appear on the surface of the spherical cores, indicating a secondary nucleation (Figure 3c). Further increasing t to 3 h enables the small


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nanoparticles to grow anisotropically along [100], forming nanorods with a diameter of ~10 nm and a length of ~20 nm. At t = 12 h, the length of the nanorods increases to ~ 50 nm, and the sample exhibits a similar HNs morphology to the typical tantalum oxyfluoride ACHNs.

Figure 3 SEM images of the samples obtained at different t of (a) 20 min，(b) 30 min, (c) 1 h, (d) 3 h and (e) 12 h, as well as (f) the corresponding XRD patterns. Figure 3f shows the XRD patterns of the samples obtained at different t. It is obvious that the spherical particles obtained at t = 20 or 30 min are both amorphous. The XPS spectra of the amorphous spherical particles obtained at 30 min are shown in Figure S3, which reveals that they are composed of Ta, O and F elements, and possess similar atom percentages and chemical states to the as-prepared ACHNs. In view of the similar amorphous nature and chemical composition as well as uniform size (250 350 nm) of the product obtained at 30 min to the inner cores, we use it as a contrastive



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sample for evaluating the photocatalytic performances of the as-prepared tantalum oxyfluoride ACHNs and denote it as tantalum oxyfluoride amorphous microspheres (AMSs). When t is further prolonged to 1 h and 3h, the distinct diffraction peaks corresponding to TaO2F (JCPDS card No. 76–2370) begin to appear and their intensities increase gradually, suggesting the secondary nucleation and growth of crystalline TaO2F on the surface of the ASCs. The formation mechanism of the as-prepared tantalum oxyfluoride ACHNs is assumed to be a two-step hydrolysis of the generated coordination complex ions of [TaOF3·2F]2-. The possible chemical reactions are listed as follows: TaF 5 + H 2 O → [TaOF 3 ·2F] 2- + 2H +

(1)

[TaOF3·2F]2- + 2H+ +2H2O → TaO 2 F↓ + 4HF

(2)

[TaOF 3 ·2F] 2- + 2H + +2HF → H 2 TaF 7 + H 2 O

(3)

H 2 TaF 7 + 2H 2 O →TaO 2 F↓ + 6HF

(4)

Water-soluble [TaOF3·2F]2- or [TaF7]2- complex ions are respectively formed by the reaction of TaF5 and H2O with different hydrolysis rates in non-fluorine or fluorine enrichment environments.41 In our protocol, TaF5 was firstly dissolved in a mixed solvent of isopropanol (i-PrOH) and H2O to obtain a transparent solution containing [TaOF3·2F]2- ions at room temperature following eqn. (1). When the sealed solution is heated, [TaOF3·2F]2- ions are rapidly hydrolyzed into TaO2F accompanied with HF according to eqn. (2). Since eqn. (2) occurs too fast compared to the crystallization of TaO2F, there form uniform amorphous tantalum oxyfluoride inner cores through a ripening process to reduce the surface energy (Figures 3a and b). Subsequently, the


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significantly increased concentration of HF induces eqn. (3) to generate H2TaF7,41 which would be hydrolyzed at a rate slow enough to match with the crystallization rate of TaO2F at a high temperature. As a result, anisotropic nanorods of TaO2F grow along [100] on the surfaces of the uniform amorphous tantalum oxyfluoride inner cores. In such a process, adding i-PrOH can stabilize the surface adsorption of F to reduce the surface energy and thus stimulate the preferred anisotropic growth of TaO2F,30,42 leading to the formation of the tantalum oxyfluoride ACHNs composed of amorphous tantalum oxyfluoride inner cores and crystalline TaO2F nanorod shells.

Figure 4 SEM images of the samples obtained at t = 24 h and different T of (a) 100 oC, (b) 140 oC, (c) 200 oC, as well as (d) the corresponding XRD patterns. According to the above proposed formation mechanism of the as-prepared tantalum oxyfluoride ACHNs, solvothermal temperature (T) and volume ratio of i-PrOH and H2O (λi-PrOH/H2O) should have significant effects on the structures and morphologies of the resultant samples. Figure 4a shows that the sample obtained at T = 100 oC and t = 24 h is also amorphous microspheres of 250 – 350 nm in diameter, similar to that obtained at T = 220 oC and t = 30 min (Figure 3b). Moreover, the morphology is



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almost invariable even when t is prolonged to 48 h. This indicates that the secondary nucleation and growth of crystalline TaO2F nanorods hardly occur at a low T (eg. 100 o

C). When T is increased to 140 oC, there are some small and uniform nanoparticles

appearing on the surface of amorphous microspheres (Figure 4b). With T further increasing to 200 oC, the surface-attached nanoparticles seem to grow in an orientational way, indicating the occurrence of a secondary nucleation and growth (Figure 4c). The XRD patterns in Figure 4d evidence that the product prepared at 100 o

C is fully amorphous. The crystalline TaO2F only appears in the products obtained at

T of 140 or 200 oC, and its content increases with increasing T. This result indicates that at a low T (e.g. 100 oC), the product only derives from the hydrolysis of [TaOF3·2F]2- ions (eqn. 2), and the hydrolysis of H2TaF7 (eqn. 4) does not occur. Consequently, only ASCs form. When T rises up, eqn. 4 is induced with an extremely low rate, resulting in a slowly secondary nucleation and growth of the crystalline nanorods. Figure 5 reveals that the product obtained at λi-PrOH/H2O of 15 : 15 is composed of ~100 nm-sized irregular particles with small nanoparticles attached on the surface and nanorods with an average length of ~100 nm (Figure 5a), while that obtained at λi-PrOH/H2O of 20 : 10 is aggregated particles with an average diameter of ~100 nm and some small thorns on the surface (Figure 5b). Further increasing λi-PrOH/H2O to 26 : 4 makes aggregated HNs with an overall size of ~400 nm (Figure 5c). This suggests that increasing the H2O content favors the generation of irregular particles and separated nanorods by accelerating TaF5 dissolution and improving the reaction rates of eqns. (2)


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and (4). Increasing the i-PrOH content to a certain value can adjust the hydrolysis rates of eqns. (2) and (4) to generate amorphous inner cores and crystalline outer nanorods in sequence, forming ACHNs. When TaF5 is introduced in the system without H2O, only trace products are available as shown in Figure 5d, while in pure water only a transparent solution is obtained under the same condition. This implies that in a non-H2O system, only a trace product is generated due to the almost complete suppression of the hydrolysis of [TaOF3·2F]2- or [TaF7]2- ions. Whereas in pure water, TaF5 has a high solubility to obtain a clear and transparent acidic solution after reaction (pH ≈ 2). All these results support our proposed formation mechanism of the as-prepared tantalum oxyfluoride ACHNs.

Figure 5 SEM images of the samples obtained at different λi-PrOH/H2O (a) 15 : 15; (b) 20 : 10; (c) 26 : 4; (d) 30 : 0. The as-prepared tantalum oxyfluoride ACHNs demonstrate an excellent photocatalytic activity for hydrogen production in 20 % methanol solution without any cocatalysts. Under the illumination of a 5 mW cm-2 xenon lamp as simulated solar



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light, the photocatalytic hydrogen production efficiency reaches up to 1.95 mmol h-1 g-1, as shown in Figure 6a. This value is superior to those of our previously reported F-Ta2O5 and Nb3O7F HNs under the same testing conditions. 15,30 In the absence of light or the tantalum oxyfluoride ACHNs, no hydrogen is detected, indicating that the hydrogen production is a photocatalytic process. In order to elucidate the originality of the excellent photocatalytic hydrogen production activity of the tantalum oxyfluoride ACHNs, the tantalum oxyfluoride AMSs and the tantalum oxide CHNs which possess a morphology similar to the as-prepared tantalum oxyfluoride ACHNs but a better crystallinity (Figure S4), as well as commercial tantalum oxide were also tested as the contrastive samples. Figure 6a indicates that among the 4 listed samples, the as-prepared tantalum oxyfluoride ACHNs exhibit the highest photocatalytic activity for hydrogen production, which is twice more than that of tantalum oxyfluoride AMSs and is nearly four times as much as that of the tantalum oxide CHNs. In general, photocatalytic activities for hydrogen production of photocatalysts strongly depend on the specific surface area (SBET), charge separation and transfer efficiencies, as well as reduction ability of photogenerated electrons. To clear the contributions for the enhanced photocatalytic activity, we have estimated the SBET of the tantalum oxyfluoride AMSs and tantalum oxide CHNs to be 85.6 and 37.8 m2 g-1 (Figure S5) and calculated the specific surface area activities of the tantalum oxyfluoride AMSs, tantalum oxyfluoride ACHNs and tantalum oxide CHNs to be 12.0, 12.5 and 10.7 µmol h-1 g-1 m-2, respectively. This suggests that excluding SBET,


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the as-prepared tantalum oxyfluoride ACHNs also show the highest photocatalytic activity. It is reasonable to attribute this phenomenon to the enhanced charge transfer and electron reduction abilities, as well as the synergistic effects between amorphous cores and crystalline nanorod shells, which, however, is hardly approved due to the difficulty in the preparation of separated amorphous cores and nanorods. The charge transfer properties of the tantalum oxyfluoride ACHNs and the tantalum oxide CHNs were characterized by EIS Nyquist plots over the frequency range of 105 – 10-1 Hz. As shown in Figure 6b, the tantalum oxyfluoride ACHNs show a smaller semicircle than the tantalum oxide CHNs, suggesting that the tantalum oxyfluoride ACHNs possess a smaller charge transfer resistance than the tantalum oxide CHNs. The small charge transfer resistance should benefit the separation efficiencies of photogenerated electron−hole pairs,43-45 and be possibly ascribed to the unique cubic ReO3 crystal structure of TaO2F, which is more favorable to accelerate charge transfer.38



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Figure 6 (a) Photocatalytic hydrogen production activities of the as-prepared tantalum oxyfluoride ACHNs and the contrastive samples under the xenon lamp irradiation; (b) EIS Nyquist plots, (c) UV-Vis DR spectra, and Mott-Schottky plots of the tantalum oxyfluoride ACHNs (d) and the tantalum oxide CHNs (e). As shown in Figure 6c, the tantalum oxyfluoride ACHNs have an absorption onset around 310 nm, suggesting that the efficient light irradiation for photocatalysis mainly derives from the UV light below 310 nm. The blue-shifted absorption edge compared with the tantalum oxide CHNs indicates an increase of the band gap. According to the Kubelka–Munk function, the band gap energy (Eg) of the as-prepared tantalum oxyfluoride ACHNs is estimated to be 3.91 eV, wider than that of the contrastive tantalum oxide CHNs (the inset of Figure 6c). This phenomenon is also found in F-Ta2O5,30 but different from the decreased Eg of TaON by N-doping, in which the


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valence band consists of a hydrobridization of N 2p and O 2p. For tantalum oxyfluoride ACHNs, as evidenced by the following Mott-Schottky plots in Figure 6d and e, the introduction of F ions cannot endow any impurity states to narrow the band gap, but to some extent can lift the conduction band,46,47 which determines the reduction abilities of the transferred electrons and play an important role in the enhancement of photocatalytic activity. Figures 6d and e display the Mott-Schottky plots of the tantalum oxyfluoride ACHNs and the tantalum oxide CNNs obtained at three different frequencies, respectively. The linear regions between -1.3 and -0.2 V with positive slopes clearly reveals their n-type characters.48,49 The flat-band potentials of the tantalum oxyfluoride ACHNs and the tantalum oxide CNNs are estimated to be -1.26 V and -1.01 V versus normal hydrogen electrode (NHE), respectively, by extrapolating the linear part of the data to capacitance C-2 = 0. Since the flat-band potentials of n-type semiconductors are adjacent to their CB bottoms,16 it is reasonably believed that the CB bottom of tantalum oxyfluoride is much negative than that of tantalum oxide, indicating its stronger reduction abilities of the transferred electrons on the CB. On the basis of the above analyses, the enhanced photocatalytic activity of the as-prepared TaO2F ACHNs is reasonably attributed to the following three reasons: (1) the unique ACHNs endow them with an outstandingly large SBET for providing more active sites; (2) the cubic ReO3 crystal structure benefits the photogenerated charge carriers separation and charge transfer; (3) the more negative CB bottom of tantalum oxide than tantalum oxide brings about a much stronger reduction ability for the



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photoexcited electrons.

Figure 7 Photocatalytic stability for hydrogen production of the as-prepared tantalum oxyfluoride ACHNs. The photocatalytic stability of the as-prepared tantalum oxyfluoride ACHNs was carried out by cycling the photocatalytic test in each 8 h under the xenon lamp irradiation in 20 vol.% methanol aqueous solution without any cocatalysts. As displayed in Figure 7, no noticeable decrease of the photocatalytic hydrogen production efficiencies in each cycle demonstrates the good stability of the as-prepared tantalum oxyfluoride ACHNs. Conclusions In summary, we have for the first time developed a one-pot liquid-phase chemical synthesis route to prepare a novel tantalum oxyfluoride hierarchical nanostructure (HNs) photocatalyst via regulating the stepwise hydrolysis of in-situ generated coordination complex ions. The as-obtained tantalum oxyfluoride HNs show unique crystal and electronic structures, and are composed of amorphous tantalum


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oxyfluoride cores and crystalline TaO2F nanorod shells (ACHNs), endowing them with much strong charge transfer and electron reduction abilities as well as a remarkably large specific surface area (SBET) for photocatalytic hydrogen production. As a result, they in the absence of any co-catalysts show an excellent photocatalytic activity for hydrogen production up to 1.95 mmol h-1 g-1 and good photocatalytic stability. This is also the first time to report a direct strategy to fabricate tantalum-based HNs in a non-HF environment. Furthermore, in view of their large SBET and small charge transfer resistance, the as-prepared tantalum oxyfluoride ACHNs may have potential applications in photoelectro- or electro- chemistry fields. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21474078, 51521001 and 51002111), the Natural Science Foundation of Hubei Province (2014CFB163 and 2015CFA003), the Top Talents Lead Cultivation Project of Hubei Province, the Yellow Crane talent plan of Wuhan municipal government and the Fundamental Research Funds for the Central Universities (WUT: 2014-IV-131). References 1.

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Complex-mediated synthesis of tantalum oxyfluoride hierarchical nanostructures for highly efficient photocatalytic hydrogen evolution Leilei Xu, Haotian Gong, Li Deng, Fei Long, Yu Gu, and Jianguo Guan*

Regulating the one-step hydrothermal process of TaF5 in an isopropanol aqueous solution can obtain a tantalum oxyfluoride hierarchical nanostructure composed of amorphous cores and single crystalline TaO2F nanorod shells, which exhibit an excellent photocatalytic activity for hydrogen production under the illumination of a simulated solar light.


Complex-Mediated Synthesis of Tantalum ... - ACS Publications

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