(00l) facets exposed plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders

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(00l) facets exposed plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders with single crystal grain for enhancement of photocatalytic activity Yifeng Zhang, Jing Yuan, Huihua Gong, Yue Cao, Kewei Liu, Hongmei Cao, Hongjian Yan, and Jianguo Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04181 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 10, 2018

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ACS Sustainable Chemistry & Engineering

(00l) facets exposed plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders with single crystal grain for enhancement of photocatalytic activity Yifeng Zhang,† Jing Yuan,† Huihua Gong,‡ Yue Cao,‡ Kewei Liu,‡ Hongmei Cao,‡ Hongjian Yan *‡ and Jianguo Zhu* † † College of Materials Science and Engineering, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu , China, 610065, P. R. China. E-mail: [email protected]; Fax: +028 85460353; Tel: +86 85412415 ‡ College of Chemistry, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu , China, 610065, P. R. China. E-mail: [email protected]; Tel: +86 1355-1341-892

KEYWORDS: (00l) facets, Aurivillius oxides, layered perovskite structure, molten salt method, single crystal, photocatalyst, hydrogen evolution, oxygen evolution.

ABSTRACT: (00l) facets exposed plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders with single crystal grain were obtained successfully by molten salt method. The plane-like grains were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, photoluminescence spectroscopy and X-ray

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photoelectron spectroscopy. With modification by platinum nanoparticles as cocatalyst, these materials were tested as photocatalysts for water splitting under full arc light irradiation. The H2 evolution activities of the plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) prepared by molten salt method were almost an order of magnitude higher than that of the ABi2Nb2O9 (A=Ca, Sr, Ba) prepared by traditional solid state method. The plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders also have higher O2 evolution activities than the ABi2Nb2O9 (A=Ca, Sr, Ba) powders prepared by traditional solid state method. The oxidation and reduction catalytic sites on the plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) were discussed in detail. With the temperature of molten salt increasing, the variation tendencies of the photocatalytic activities of CaBi2Nb2O9, SrBi2Nb2O9 and BaBi2Nb2O9 for water splitting were different from each other due to the different growth rhythm of the plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) grains. The CaBi2Nb2O9 and SrBi2Nb2O9 with orthorhombic lattice had a higher photocatalytic activity than BaBi2Nb2O9 with tetragonal lattice because of the strong distortion of NbO6 octahedra in the perovskite-like slabs of CaBi2Nb2O9 and SrBi2Nb2O9.

INTRODUCTION Since the first report of the Honda-Fujishima effect,1 hundreds of inorganic semiconductors have been discovered and used as photocatalysts for photocatalytic water splitting.2 The effective separation of electron-holes excited by light in the semiconductors is crucial to improve the photocatalytic activity of photocatalysts. It has been found that the unique layered K4Nb6O17 consisting of two kinds of interlayers realized the spatial separation of redox sites in molecular level natively.3 Hence, the inorganic materials with layered perovskite structure have been studied widely in the field of photocatalysis. The layered perovskite oxides (including the


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Ruddlesden-Popper, Dion-Jacobson and Aurivillius phase oxides) and their derivatives can be designed easily by exchanging the cations of the interlayer with other cations; the layered perovskite oxides nanosheets can also be fabricated by exfoliation4,5 and the spontaneous polarization of pseudo-perovskite slabs in these oxides favors the separation of photo-generated electron-hole pairs (PEHPs).6 Naturally, they are considered as the promising materials in photocatalytic field. Many layered perovskite oxides and their derivatives were used as photocatalysts for water splitting to generate hydrogen by utilizing solar energy, such as Ruddlesden-Popper phase Sr3Ti2O7 7 and A2La2Ti3O10 (A = K, Cs, Rb),8-11 Dion-Jacobson phase AB2Nb3O10, ALaNb2O7 (A = H, K, Rb, Cs; B = Ca, Sr)12-15 and RbLnTa2O7 (Ln = La, Pr, Nd, and Sm)16 and Aurivillius phase ABi2Ta2O9 (A = Ca, Sr, and Ba).17 The Aurivillius phase compounds can be written in a general formula (Bi2O2)2+(An-1BnO3n+1)2-. Such structure can be considered as (An-1BnO3n+1)2- pseudo-perovskite slabs inserted between (Bi2O2)2+ layers along caxis, where n is the number of octahedral layers in the pseudo-perovskite slab.18,19 Hence, the photocatalytic properties of Aurivillius phase oxides are closely related to the pseudo-perovskite slabs and (Bi2O2)2+ layers. A series of Aurivillius-type photocatalysts has attracted much attention for their excellent photocatalytic performance, such as Bi2AO6 (A = Mo, W),20-25 PbBi2Nb2O9,26 Bi3TiNbO927 and LiBi4M3O14 (M = Nb, Ta).28 Moreover, several new layered bismuth-based semiconductor photocatalysts with (Bi2O2)2+ layers also have been explored for photocatalytic water splitting.29-36 Recently, Li et al.37 reported the photocatalytic activities of Aurivillius-type ABi2Nb2O9 (ABN, A = Ca, Sr, Ba) powders for water splitting to generate hydrogen under UV-light irradiation. However, because of the limitation of traditional solid state reaction (SSR) for preparing ABi2Nb2O9 (A = Ca, Sr, Ba) oxides, the single crystal grains with good dispersion cannot be obtained easily. Therefore, there are few reports are available for the


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relationship between morphology and photocatalytic properties of ABi2Nb2O9 (A = Ca, Sr, Ba) oxides. The special morphology with active facets exposed is significant for improving the photocatalytic efficiency of the classic inorganic photocatalyts, such as TiO2,38 SrTiO3,39,40 BiVO4,41,42 and WO3.43-45 On the other hand, molten salts have been used as reaction media for various organic and inorganic reactions in research as well as in industry,46,47 because the molten salt allows faster mass transfer transport in the liquid phase by means of convection and diffusion. The molten salt method (MSM) which employs a molten inorganic salt as the medium emerges as an important complementary route to conventional liquid phase synthesis. For preparing the binary and multinary inorganic oxides, the MSM can facilitate better crystallinity and contribute to a special texture of the products, and the high temperature solvent route also enables the tuning of crystal habitus, exposed facets and microstructure by controlling processing parameters.48 Thus, we employed the molten salt method (MSM) to prepare the Aurivillius-type ABi2Nb2O9 (A=Ca, Sr, Ba) compounds with good dispersion and special morphology. The (00l) facets exposed plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders with single crystal grain were obtained. The relationship between the plane-like morphology of ABi2Nb2O9 (A=Ca, Sr, Ba) and the spatial separation of photo-excited electrons and holes on ABi2Nb2O9 (A = Ca, Sr, and Ba) was discussed in detail and the CaBi2Nb2O9 (CBN), SrBi2Nb2O9 (SBN) and BaBi2Nb2O9 (BBN) exhibited different performance in the catalytic test due to the effects of the different crystallinities, surface textures and crystal structures of these compounds.

EXPERIMENTAL SECTION Synthesis of ABi2Nb2O9 (A=Ca, Sr, Ba) powders by MSM. The samples were synthesized according to a previously reported procedure.49 Bi2O3(99.999%), CaCO3


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(99%) and Nb2O5(99.5%), SrCO3(99%) and BaCO3(99%) were used as raw materials and were mixed according to the chemical ingredient ratio. NaCl(99.8%)-KCl(99%) (1:1 mass ratio) were added as the molten salt. The mass ratio of ABi2Nb2O9 (A=Ca, Sr, Ba) to the molten salt was 1:1. Ethanol was used as the medium during milling. After 24 h milling, the mixture was placed into an oven at 80 °C to remove the ethanol. The dried powders were then put into an Alumina crucible and then were heat-processed under the temperature of 850-1000 °C for 4 h. After that, the obtained powders were washed several times with deionized water to get rid of the NaCl and KCl. Finally, the ABi2Nb2O9 (A=Ca, Sr, Ba) powders were obtained and dried. The samples were labeled as CBN(MSM)-X, SBN(MSM)-X and BBN(MSM)-X (where X represents the temperature of molten salt), respectively. Synthesis of ABi2Nb2O9 (A=Ca, Sr, Ba) powders by traditional solid state reaction. As a comparison, the ABi2Nb2O9 (A=Ca, Sr, Ba) powders were also synthesized by the traditional solid state reaction. The raw materials were mixed according to the chemical ingredient ratio. These powders were milled for 24 h using the ball-milling after mixed. After that, the mixtures were place into an oven at 80 °C to remove the ethanol and then the powders were calcined at 850 °C for 4 h. After being cooled down to room temperature, the mixtures were milled again under the same condition and dried. The samples

were labeled as CBN(SSR)-850, SBN(SSR)-850 and BBN(SSR)-850,

respectively. The loading of Pt cocatalyst on ABi2Nb2O9 (A=Ca, Sr, Ba) powders. (1) in-situ photodeposition method. Typically, 0.300 g ABi2Nb2O9 (A = Ca, Sr, Ba) powder was suspended in 200 mL aqueous methanol (10% in volume) solution containing appropriate amount of


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H2PtCl6·6H2O. The suspension was magnetically stirred in the dark for at least an hour to achieve adsorption/desorption equilibrium between the photocatalyst and the [PtCl6]2ions. Then the mixed solution was illuminated for 30 min with a Xenon lamp after removing the dissolved oxygen completely. The product was collected by filtration and washed with deionized water for several times, and then dried at 60 °C in air overnight. (2) ethylene glycol reduction method. 0.300 g ABi2Nb2O9 (A = Ca, Sr, Ba) powder, appropriate amount of H2PtCl6·6H2O and 100 mL ethylene glycol were added into a round-bottom flask with a capacity of 300 mL and the suspension was vigorously stirred for 12 h. Then, the mixed solution was refluxed under vigorous stirring at 120 °C for 4 h. After cooling to room temperature, the precipitate was collected, washed with deionized water for several times, and then dried at 60 ºC in air overnight. Electrochemical and photoelectrochemical measurements. All electrochemical tests were carried out at room temperature on a computer-controlled Autolab PGSTAT 12 potentiostat / galvanostat (Metrohm, Switzerland) with conductive fluorine-doped tin oxide (FTO) glass with photocatalysts powder as working electrode, Ag/AgCl as reference electrode and platinum foil as counter electrode. Catalyst inks for electrochemical testing were prepared by adding 5 mg ABi2Nb2O9 (A=Ca, Sr, Ba) catalyst powder to a mixture of 500 µL distilled water / isopropyl alcohol (3:1, v/v) and 10 µL Nafion solution (5% wt, Dupont, USA). After ultrasonically dispersion to homogeneous, 40 µL fresh catalyst ink was spread onto FTO glass (1 cm2). The electrode was dried at 80 ℃ for 30 min in oven to improve adhesion after air drying. The electrolyte was 0.1 M Na2SO4 aqueous solution without additive. The photocurrent densities were measured under simulated sunlight irradiation using a 500 W Xe lamp equipped with an AM 1.5G filter (CEAulight, China). Before testing, the incident light intensity was determined


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and controlled at 100 mWcm-2 by a digital photo-power meter. Besides, a cubic quartz cell was used for all photoelectrochemical measurements. Mott-Schottky (MS) analysis was carried out at a DC potential range 1.2 ~ 1.6 V vs. RHE, and the perturbation signal were 50 mV with the frequency 5KHz. Photocatalytic reactions of ABi2Nb2O9 (A=Ca, Sr, Ba) powders. The photocatalytic properties of ABi2Nb2O9 (A=Ca, Sr, Ba) were also evaluated on the basis of the test reactions: the photo-reduction/photo-oxidation of H2O to H2/O2. Accordingly, the tests were carried out using triethanolamine (TEA, 10 vol%, 200 mL) as a sacrificial electron donor and Ag+ as a sacrificial electron acceptor, respectively. The photocatalytic reactions were carried out in a top irradiation-type pyrex reaction cell at room temperature with a closed circulation system. Xenon lamp (300 W, Ceralux 300BF) with full arc light was used as light source. Prior to irradiation, the reactant solution was evacuated to remove dissolved air completely. The hydrogen evolution reaction was performed in aqueous TEA solution (10 vol%, 200 mL) containing 0.3 g catalyst. The oxygen evolution was determined in aqueous solution (200 ml) containing catalyst (0.3g) and AgNO3 (0.01 M). The evolved amounts of H2 and O2 were detected by gas chromatography (SPSIC, GC112AT, argon carrier). Characterization. The crystallinities of the products were determined with a DX2700 X-ray diffractometer (Dandong, China) with Cu-Kα radiation (λ = 1.5406 Å) at a scan step of 0.03°. The crystal morphologies of the samples were observed by SEM with a JSM-7500F scanning electron microscope (JEOL, Japan). The transmission electron microscopy (TEM) images were obtained with a transmission electron microscopy (TEM2010F, JEOL). Photoluminescence (PL) measurements were obtained with a fluorescence


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spectrophotometer (Hitachi F-7000) and operated at room temperature. The excitation wavelength was 210 nm. The UV/Vis diffuse reflection spectra of the samples were obtained with a UV/Vis spectrophotometer (UV3600, Shimadzu) and converted from reflection to absorbance by the Kubelka–Munk method. BaSO4 was used as a reflectance standard in the UV/Vis diffuse reflectance experiments. X-ray photoelectron spectroscopy (XPS) spectra were collected using a V4105 instrument (Thermo Electron, USA) with a Mn Kα radiation source. Specific surface area determination was based on BET formalism according to nitrogen adsorption/desorption isotherms obtained on a Quantachrome Autosorb Station 2.

RESULTS AND DISCUSSIONS Phase structures, morphologies and surface chemical states of products. The crystalline phases and compositions of the as-prepared samples were identified by XRD. The XRD patterns of the ABi2Nb2O9 (A=Ca, Sr, Ba) powders prepared via the molten slat method (MSM) and the solid state reaction (SSR) are shown in Fig. 1. The XRD patterns of CaBi2Nb2O9 (Fig. 1(a)) and SrBi2Nb2O9 (Fig. 1(b)) could be indexed to an orthorhombic lattice with the A21am space group corresponding to the JCPDS card No. 49-0608 and No. 49-0607, respectively. The diffraction patterns of BaBi2Nb2O9 (Fig. 1(c)) could be indexed to a tetragonal cell with space group I4/mmm (JCPDS No. 40-0355). The XRD results are in accordance with the previous reports.50-52 Through comparison with the XRD patterns of ABi2Nb2O9 (A=Ca, Sr, Ba) prepared by traditional solid state reaction at 850 °C, ABi2Nb2O9 (A=Ca, Sr, Ba) obtained by molten salt method at the same temperature have the sharper diffraction peaks, which means the molten salt method contributes to obtaining ABi2Nb2O9 (A=Ca, Sr, Ba) crystal grains with better crystallinities than solid state reaction. As shown in Fig. 1(a), the intensities of (002), (004),


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(008) and (0010) diffraction peaks of CBN(MSM)-X become more and more sharper with the increase of temperature, whereas the intensities of dominant (115) diffraction peaks for CBN(MSM)-X samples at 29° have no obvious changes. This indicates the temperature of molten salt did not have a clear effect on the crystallinities of CBN(MSM)-X, but impacted on the growth of (00l) facets significantly. The change of (00l) diffraction peaks with the increase of molten salt temperature suggests that the as-synthesized CBN(MSM)-X had a preferred orientation along the (00l) planes. As shown in Fig. 1(b) and (c), the intensities of (00l) diffraction peaks of SBN(MSM)-X and BBN(MSM)-X samples also heighten significantly in comparison with those of the samples prepared by solid state reaction. However, the (00l) diffraction peaks of SBN(MSM)-X and BBN(MSM)-X have a slight decrease at the high temperature, indicating that the growth along the (00l) planes of SBN(MSM)-X and BBN(MSM)-X crystal grains was hindered at the high temperature.


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Figure 1. XRD patterns of ABi2Nb2O9 (A = Ca, Sr, Ba) prepared by molten slat method (MSM) and solid state reaction (SSR). (a) CaBi2Nb2O9; (b) SrBi2Nb2O9; (c) BaBi2Nb2O9.

Figure 2. SEM image (a), TEM image (b) and SAED (c) of CBN(SSR)-850; SEM image (d), TEM image (e) and SAED (f) of CBN(MSM)-850.


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The morphologies of ABi2Nb2O9 (A=Ca, Sr, Ba) samples prepared by the two different methods were observed by SEM and TEM. As shown in Fig. 2(a), the CaBi2Nb2O9 prepared by solid state reaction at 850 °C (CBN(SSR)-850) has an irregular morphology with particle size of ca. 3 µm. The TEM image of CBN(SSR)-850 (Fig. 2(b)) shows that the big grain is assembled by small grains and the boundaries of the small grains are clear. The corresponding selective area electron diffraction (SAED) pattern of the grain (Fig. 2(c)) shows indistinct and annular distributed ED spots. The TEM image and SAED pattern imply that the crystal grains of CBN(SSR)-850 had a polycrystalline structure. The CaBi2Nb2O9 prepared by molten salt method at 850 °C (CBN(MSM)-850) possesses regular square plane-like morphology, and the surfaces and edges of the particles are smooth, as shown in Fig. 2(d). The widths and lengths of the grains are ca. 2–3 µm. The TEM image of CBN(MSM)-850 (Fig. 2(e)) further shows that the grain has a well-defined plane-like structure with a rectangular outline. The inset of Fig. 2(e) shows a HRTEM image recorded from the white framed area indicated in Fig. 2(e). The fringe spacing 0.274 and 0.272 nm agree well with the lattice distances of the (200) and (020) crystal planes of CBN respectively, demonstrating that the major plane is characterized by (00l) facets. The SAED pattern of a single grain (Fig. 2(f)) shows well-defined ED spots indicating the single crystal nature of the plane-like grain, and the ED pattern could be indexed to the diffraction pattern of [00l] zone axis indicating a preferred (00l) surface orientation of the grain, in agree with the HRTEM image and XRD pattern of CBN(MSM)-850. Considering the different processes of SSR and MSM, it can be speculated that the grains of SBN(SSR) and BBN(SSR) were polycrystalline structure, and the plane-like grains of SBN(MSM) and BBN(MSM) were also single crystal structure. Fig. 3 shows the SEM images of CBN(MSM)-X, SBN(MSM)-X and BBN(MSM)-X. As shown in Fig. 3(a-d), the widths, lengths and thicknesses of the CBN(MSM)-


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X grains increase gradually as the temperature of molten salt rises. The widths and lengths of CBN(MSM)-850 grains are ca. 2–3 µm, while the size of CBN(MSM)-1000 grains has grown to ca. 10 µm. That is, the grains are inclined to grow along this major planes ((00l) facets) preferentially, which is in accordance with the XRD patterns of CBN(MSM)-X. The SBN(MSM)-X (Fig. 3(e-h)) and BBN(MSM)-X (Fig. 3(i-l)) also have the plane-like morphologies, indicating that the crystal particles of SBN(MSM)-X and BBN(MSM)-X have the same characteristic of the preferential growth of certain facets like that of CBN(MSM)-X. Considering the XRD patterns of SBN(MSM)-X and BBN(MSM)-X in the Fig. 1(b) and (c), it can be deduced that the major planes in the SEM images of SBN(MSM)-X and BBN(MSM)-X are also the (00l) facets. As shown in Fig. 3(e-h), the small crystal grains of SBN(MSM)-X are ca. 3-5 µm, and the big grains are ca. 10-12 µm. The surface evenness of the SBN(MSM)-X grains has changed significantly with the temperature increasing. It can be seen that the grains of SBN(MSM)-850 (Fig. 3(e)) has a lot of steps on the major planes and the edges of the plane-like grains are very regular. The high magnified SEM image of the steps (see in Fig. S1) shows that the steps are stacked with a series of parallel crystal faces with different sizes and the edges of the square parallel crystal faces are very regular. The inset of Fig. 3(e) shows the layer structure of parallel crystal faces from the lateral side of the grain clearly. However, the Fig. 3(h) shows that the SBN(MSM)-1000 grains have the very smooth major planes but zigzag edges. This indicates that the steps on the major planes disappeared gradually, but the edges of the plane-like grains became irregular by degrees as the temperature rose. That is, the parallel crystal planes with different sizes grew along the major plane and trended to uniformity and the edges of the plane-like grains were destroyed at the high temperature. The destroyed edges of SBN(MSM)-X grains indicate the size of the major planes of the SBN(MSM)-X grains decreased slightly, in


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accordance with the XRD patterns of SBN(MSM)-X. The widths, lengths and thicknesses of BBN(MSM) grains all increased, but the growth degree of the thickness was not obvious as that of the widths and lengths (Fig. 3(i)-(l)). Although the sizes of BBN(MSM) grains increased with the temperature increasing, the major planes of grains started to crack at the high temperature, as shown in Fig. 3(k) and (l). A lot of grains fractured and the inside chips fell off at high temperature (see in Fig. S2). Different from the diffusion mechanism of crystal growth in traditional solid state reaction, the mechanism of crystal growth in molten salt method is the interface control.53 This indicates that there is a stress difference between the inside and the surface of crystal grains in the liquid state molten salt and the stress difference of BBN(MSM) crystal grains broke the major plane. The fragmentation of the BBN(MSM) grains destroyed the (00l) planes and this is why the (00l) diffraction peaks of BBN(MSM) had a slight decrease at the high temperature. The decrease of (00l) peaks of BBN(MSM)-X prepared at high temperature in XRD patterns does not contradict the result of SEM images of BBN(MSM) because the XRD peaks reveal the total variation tendency of grain sizes and the SEM images exhibit the sizes of certain grains at local area. That is, the high temperature improved the growth of BBN(MSM) along (00l) orientation, but fragmented the BBN(MSM) grains at the same time.


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Figure 3. (a-d) SEM images of CBN(MSM)-X; (e-h) SEM images of SBN(MSM)-X; (i-l) SEM images of BBN(MSM)-X. The inserts show the thicknesses of the plane-like grains and the X represents the temperature of molten salt. The SEM and TEM images indicated that the ABi2Nb2O9 (A = Ca, Sr, Ba) powders prepared by the two different methods show different morphologies and exposed facets. Hence, the XPS was applied to characterize the elemental and chemical state of the sample surfaces. Fig. 4 shows the XPS spectra of CBN(MSM)-850 and CBN(SSR)-850. The survey scan indicated that there is no Na, K, or Cl residue in the sample prepared by MSM. The C 1s peak at 284.8 eV (shown in Fig. 4(b)) is attributed to adventitious hydrocarbons generated by the XPS instrument and used as reference. The banding energies of the Ca 2p of CBN(MSM)-850 and CBN(SSR)-850 are similar as shown in Fig. 4(c), indicating the A site atom of the (CaNb2O7)2- pseudo-perovskite slabs has the stable chemical valence state. The slight shifts of Bi 4f, Nb 3d and O 1s towards high binding energies indicate that the polarities of Bi-O band in the (Bi2O2)2+ layers and Nb-O band in NbO6 octahedra of the sample surface. The Bi 4f (Fig. 4(d) and of Nb 3d (Fig. 4(e)) are


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all consistent with those reported in literature for the valence states of Bi3+ and Nb5+.54,55 The O 1s XPS measurements (Fig. 4(f)) reveal two signals at around 529.8 eV and 531.3 eV which can be attributed to the oxygen in CBN crystal structure and O-H adsorbed at the surface, respectively.25,36 The XPS spectra of SBN and BBN prepared by different methods also exhibited the same change rule as CBN (Fig. S3 and Fig. S4). It is worth noting that the one of O 1s peaks of the SBM(MSM)-850 sample which belonged to adsorbed O-H at 531.44 eV shifted to the high banding energy obviously in comparison with SBM(SSR)-850. The O-H bond in H2O molecules on the surface of SBM(MSM)-850 may be activated for oxidation.

Figure 4. XPS spectra of the CBN(MSM)-850 and CBN(SSR)-850. (a) survey, (b) C 1s, (c) Ca 2p, (d) Bi 4f, (e) Nb 3d and (f) O 1s. Optical and electrochemical properties of plane-like ABi2Nb2O9(A=Ca, Sr, Ba). The UV/Vis diffuse reflectance spectra of ABi2Nb2O9 (A = Ca, Sr, Ba) prepared by molten salt


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method at 850 °C are shown in Fig. 5(a). As shown in Fig. 5(a), the absorption edges of these photocatalysts are in the order CBN(MSM)-850 < SBN(MSM)-850 < BBN(MSM)-850. As a crystalline semiconductor, the relationship of band edge and optical absorption can be determined by the following equation: αhν = A(hν − Eg)n/2 where hν, α, Eg, and A represent the photon energy, optical absorption coefficient, band gap, and proportionality constant, respectively. 56, 57 Additionally, n = 1 and 4 means that the material is direct-gap semiconductor and indirect-gap semiconductor, respectively.30,31 The CBN and SBN are direct-gap semiconductors, whereas the BBN is indirect-gap semiconductor.37 Hence, n values of CBN, SBN are 1and n value of BBN is 4. The Eg of CBN and SBN are estimated from the plot of (αhν)2 versus (αhν)2, whereas that of BBN is calculated from the plot of (αhν)1/2 versus (hν). By extrapolating the linear portion of (αhν)2 versus (hν) curves and (αhν)1/2 versus (hν) curves, the Eg of CBN(MSM)-850, SBN(MSM)-850, BBN(MSM)-850 is evaluated to be 3.53, 3.42 and 2.82 eV, respectively. Moreover, the Eg of CBN(SSR)-850, SBN(SSR)-850, BBN(SSR)-850 is estimated to be 3.51, 3.48 and 3.08 eV, respectively (Fig. S5). The estimated band gaps of ABi2Nb2O9 (A = Ca, Sr, Ba) compounds decreased gradually with the increase of the alkaline earth metal ionic radius of Ca2+, Sr2+, Ba2+, in good agreement with the previous report.37 The ABi2Nb2O9 (A = Ca, Sr) prepared by different methods have the approximate band gap values, whereas the estimated Eg of BBN(MSM)-850 is narrower than that of BBN(SSR)850. The reason may be that the stress difference during the crystal grains growth led to the lattice distortion which affected the energy band of BBN(MSM). The UV/Vis spectra of the CBN prepared by the two different methods shown in Fig. 5(b) are used to explore the effect of different preparation methods on the UV/Vis diffuse reflectance spectra of the photocatalyst. The


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UV/Vis spectra of CBN(MSM)-850 (curve a) and CBN(SSR)-850 (curve b) have the same absorption edges, but the absorption intensity of CBN(SSR)-850 is stronger than that of CBN(MSM)-850. This is mainly because the irregular morphology and boundaries in polycrystalline CBN(SSR)-850 grains enhanced the reflection times of the incident photons in the grains. The SBN(SSR)-850 and BBN(SSR)-850 also have the stronger UV/Vis absorption intensities than SBN(SSR)-850 and BBN(SSR)-850, respectively (Fig. S6). After loading the Pt cocatalyst, the absorption intensities of CBN(MSM)-850 and CBN(SSR)-850 increase, indicating the Pt cocatalyst promoted the light absorption of the photocatalysts.

Figure 5. (a) UV/Vis spectra of ABN(MSM)-850 (A = Ca, Sr, Ba); (b) UV/Vis spectra of a: CBN(MSM)-850, b: CBN(SSR)-850, c: CBN(MSM)-850/Pt and d: CBN(SSR)-850/Pt. Mott-Schottky plots was performed to determine the electronic band structure of ABi2Nb2O9 (A = Ca, Sr, Ba) prepared by different methods. The position of the flat-band (Efb) for ABi2Nb2O9 (A = Ca, Sr, Ba) was estimated by Mott-Schottky plot analysis. Mott-Schottky curves of as-prepared CBN(SSR)-850 and CBN(MSM)-850 was shown in Figure 6(a) and (b), respectively. The positive slope of the Mott-Schottky curves reveals the n-type semiconductor behavior of CBN(SSR)-850 and CBN(MSM)-850. As the conduction band minimum (CBM) potentials of n-type semiconductors are slightly higher than the flat potentials,29-33 the CBM of


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CBN(SSR)-850 can be obtained by extrapolating the x-axis intercept at −1.43 eV versus the reversible hydrogen electrode (RHE). Considering the optical band gap value of 3.51 eV gained from the UV–vis diffuse reflectance spectra, the valence band maximum (VBM) of CBN(SSR)850 is located at 2.08 eV (vs. RHE). In the same way, the VBM and the CBM of CBN(MSM)850 are estimated to be −1.34 and 2.19 eV (vs. RHE), respectively. Furthermore, Mott-Schottky plots also demonstrate comparable results of band structure for SBN and BBN prepared by the two different methods (Fig. S7). The energy band structures of all ABi2Nb2O9 (A = Ca, Sr, Ba) materials satisfy the essential thermodynamic requirements for photocatalytic water splitting (Fig. S8). The valence band of SBN (MSM)-850 is feasible for water oxidation and the conduction band is capable for proton reduction in spite of the difficulty in kinetical aspect.

Figure 6. Mott–Schottky plots of (a) CBN(SSR)-850 and (b) CBN(MSM)-850.

Photoluminescence spectral analysis was used to study the transfer and recombination processes of photogenerated electron–hole pairs (PEHPs) in the photocatalysts. Fig. 7(a) shows the PL spectra of ABi2Nb2O9 (A = Ca, Sr, Ba) prepared by molten salt method at 850℃ under incident light with a wavelength of 210 nm. All PL spectra of the samples have a major peak at ca. 400 nm, which could be attributed to the recombination of photo-excited electrons and holes of the band gaps of ABi2Nb2O9 (A = Ca, Sr, Ba). The PL intensity of CBN(MSM)-850 is much


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stronger than those of SBN(MSM)-850 and BBN(MSM)-850, indicating that recombination probability of PEHPs in CBN(MSM)-850 was higher than those in SBN(MSM)-850 and BBN(MSM)-850. The enhancement of the peak at 480 nm of SBN(MSM)-850 may be related to the steps of parallel crystal planes. The CaBi2Nb2O9 prepared by different methods were also studied as the example for exploring the effect of different methods on the PL spectra of the photocatalysts. As shown in Fig. 7(b), the PL intensity of the CBN(SSR)-850 (curve b) is lower than that of CBN(MSM)-850 (curve a), indicating the lower recombination rate of PEHPs in CBN(SSR)-850. The reason may be that the polycrystalline structure of CBN(SSR)-850 grains contributed to the non-radiative transition rather than the radiative transition in comparison with the single crystal structure of CBN(MSM)-850 grains.58-60 Significant PL quenching was observed in CBN(MSM)-850/Pt (curve c) and CBN(SSR)-850/Pt (curve d). The quenching could be attributed to the electron migration from CBN to the Pt particles, which was more conducive to the photo-reduction of H2O to H2.

Figure 7. (a) Photoluminescence spectra of ABN(MSM)-850 (A = Ca, Sr, Ba); (b) Photoluminescence spectra of a: CBN(MSM)-850, b: CBN(SSR)-850, c: CBN(MSM)-850/Pt and d: CBN(SSR)-850/Pt.


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Photocatalytic activity and photocatalytic mechanism. Photocatalytic activities for water splitting of ABi2Nb2O9 (A = Ca, Sr, Ba) were evaluated under full arc light irradiation. The H2 evolution rates (HERs) and O2 evolution rates (OERs) of ABi2Nb2O9 (A = Ca, Sr, Ba) prepared by two different methods are listed in the Table 1. It can be seen that after loading Pt cocatalyst, the ABi2Nb2O9 (A = Ca, Sr, Ba) prepared by molten salt method show much higher photocatalytic activities for water splitting than the samples prepared by solid state reaction at 850 °C. The HERs of CBN(MSM)-850/Pt and BBN(MSM)-850/Pt are 15.5 times of those of CBN(SSR)-850/Pt and BBN(SSR)-850/Pt, and the H2 evolution rate (HER) of SBN(MSM)850/Pt is 9.8 times of that of SBN(SSR)-850/Pt. The O2 evolution rate (OER) of CBN(MSM)850/Pt is 2.3 times of that of CBN(SSR)-850/Pt; the OER of SBN(MSM)-850/Pt is 9.1 times of that of SBN(SSR)-850/Pt and the OER of BBN(MSM)-850/Pt increases slightly in comparison with that of BBN(SSR)-850/Pt. The nitrogen adsorption/desorption measurement indicated that the BET surface area of samples prepared by MSM was higher than that of samples prepared by SSR (Table S1). Usually, higher specific surface area of photocatalysts can contribute more active sites on the surface for the reactant contacting ， and resulting higher photocatalytic activity. However, the increment of the BET surface areas of plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) prepared by MSM was no more than 50% of the BET surface area of the samples prepared by SSR. It means that for the plane-like ABi2Nb2O9 (A = Ca, Sr, Ba), the significant increment of photocatalytic H2 evolution rate was not mainly attributed to the increasement of specific surface area. The higher photocatalytic activities of the samples prepared by MSM manifest the PEHPs in plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) grains with single crystal structure have better separation and utilization of photogenerated electrons and holes than that in the polycrystalline grains prepared by traditional solid state reaction. To confirm this, the photocurrent densities


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were measured. As shown in Fig.8(a), the photocurrent densities of CBN(MSM)-850 was higher that of CBN(SSR)-850. Moreover, the CBN(MSM)-850 showed higher photocurrent densities than that of SBN(MSM)-850 and BBN(MSM)-850 (Fig. 8(b)), which is agreement with the photocatalytic activity for H2 evolution. Although the CBN(MSM)-850 has the stronger PL intensity than SBN(MSM)-850 and BBN(MSM)-850, the stronger photocurrent response of CBN(MSM)-850 indicates that more PEHPs have been separated effectively in CBN(MSM)-850 and utilized for water splitting after loading the Pt cocatalyst. That is why the HER of CBN(MSM)-8/Pt (104.1 µmol·h-1·g-1) is much higher than those of SBN(MSM)-850/Pt (8.9 µmol·h-1·g-1) and BBN(MSM)-850/Pt (6.2 µmol·h-1·g-1). The hydrogen and oxygen evolutions of plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) without Pt cocatalyst have also been tested to clarify whether the oxidation or reduction catalytic sites exist on the plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) and the results are also shown in Table 1. No hydrogen generated for these powders without Pt cocatalyst, but the oxygen evolutions had been detected. This means the surfaces of plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) lacks the reduction active sites for H2O reduction reaction, but possesses the oxidation sites for O2 generation. The OERs of CBN(MSM)-850 and SBN(MSM)850 are even higher than those of the corresponding samples with Pt cocatalyst, whereas OERs of BBN(MSM)-850 and BBN(MSM)-850/Pt are approximately the same. To confirm oxidation active sites or reduction active sites on which face of the plane-like grain, the Pt was loaded on the surface of CBN(MSM)-850 by different methods.


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Table 1. The H2 and O2 Evolution Rates of ABi2Nb2O9 (A = Ca, Sr, Ba) Prepared by Different Methods Photocatalyst

H2 evolution rate

O2 evolution rate

(µmol·h-1·g-1)

(µmol·h-1·g-1)

CBN(SSR)-850/Pt

6.7

29.8

CBN(MSM)-850

Not detected

112

CBN(MSM)-850/Pt

104.1

69.4

SBN(SSR)-850/Pt

0.9

15.7

SBN(MSM)-850

Not detected

276.5

SBN(MSM)-850/Pt

8.9

143.3

BBN(SSR)-850/Pt

0.4

10.9

BBN(MSM)-850

Not detected

11.2

BBN(MSM)-850/Pt

6.2

13.3

Reaction time: 5 h. The content of the deposited Pt was 0.5 wt%. 200 mL TEA solution (10 vol%,) and 20 ml AgNO3 solution were used as sacrificial reagents for photoreduction and photooxidation, respectively

Figure 8. The photocurrent densities of (a) CBN (SSR)-850 and CBN(MSM)-850, (b) the ABN(MSM)-850 (A = Ca, Sr, Ba). The electrolyte was 0.1 M Na2SO4 and +0.8 V bias versus Ag/AgCl was applied.


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Figure 9. (a) SEM image and (b) TEM image of CBN(MSM)-850/Pt-IP; (c) SEM image and (d) TEM image of CBN(MSM)-850/Pt-EG. As shown in Fig. 9(a), the SEM image of CBN(MSM)-850 with Pt loaded by in-situ photodeposition (CBN(MSM)-850/Pt-IP) shows the Pt nanoparticles are loaded on the side faces of the plane-like grains, and the major planes ((00l) facets) do not have the Pt particles on them. The TEM image of CBN(MSM)-850/Pt-IP shown in Fig. 9(b) indicated that the Pt nanoparticles are inclined to aggregate. The SEM image of the CBN(MSM)-850 with Pt loaded by ethylene glycol reduction method (CBN(MSM)-850/Pt-EG) is shown in Fig. 9(c). The Pt nanoparticles could not be observed at the sides or on the major planes of the grains due to the low resolution of SEM. Thus, the TEM image was also employed to character the sample CBN(MSM)-850/PtEG. The TEM image (Fig. 9(d)) shows that, different from the CBN(MSM)-850/Pt-IP, dispersive Pt nanoparticles were deposited on the major plane ((00l) facets), but almost no Pt particles were loaded on the side faces of the grain. Photocatalytic activities of CBN(MSM)-850/Pt-IP and


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CBN(MSM)-850/Pt-EG were also measured and are shown in Table 2. The HER and OER of CBN(MSM)-850/Pt-IP are 104 and 69.4 µmol·h-1·g-1 respectively. The capability of CBN(MSM)-850/Pt-EG for hydrogen production was very weak in comparison with CBN(MSM)-850/Pt-IP and the HER was only 1.1 µmol·h-1·g-1. Considering that the CBN(MSM)-850 without Pt cocatalyst did not show an activity for hydrogen generation, it can be speculated that the Pt nanoparticles at the sides of plane-like CBN(MSM) grains were the reduction active sites for hydrogen generation and the reduction reaction was conducted on the side surfaces of the grains. Both the CBN(MSM)-850/Pt-IP and CBN(MSM)-850/Pt-EG have good oxygen production activity, and the OER of CBN(MSM)-850/Pt-EG is double of that of CBN(MSM)-850/Pt-IP, reaching 143 µmol·h-1·g-1. This result indicates the oxidation active sites were on the major planes ((00l) facets), and although the Pt nanoparticles on the (00l) facets contributed to H2O oxidation reaction, the Pt nanoparticles on the (00l) facets were not the necessary condition for H2O oxidation reaction in comparison with H2O reduction reaction that the Pt nanoparticles were the active sites. It has been well known that the layer structure of ABi2Nb2O9 (A=Ca, Sr, Ba) can be considered as the structure which the (An-1BnO3n+1)2- pseudoperovskite slabs are inserted between (Bi2O2)2+ layers along c-axis. Thus, the (Bi2O2)2+ layer is the outermost layer which contacts with the H2O molecules. Therefore, it can be speculated that the oxidation active sites were located in the (Bi2O2)2+ layers and the photo-generated holes were inclined to gather in (Bi2O2)2+ layers, in other words, the (Bi2O2)2+ layers were positively charged. The Pt nanoparticles loaded by in-situ photo-deposition attached at the side of plan-like grain, which indicates the sides of the plane-like grains are negatively charged. Because of the sandwich structure of ABi2Nb2O9 (A = Ca, Sr, Ba)，the sides of the plane-like grains exposed the edges of pseudo-perovskite slabs. Therefore, the (An-1BnO3n+1)2- pseudo-perovskite slabs


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were negatively charged. The Pt nanoparticles on the SBN(MSM)-850 and BBN(MSM)-850 loaded by in-situ photo-deposition also attached at the side of the grains (see in Fig. S9). Considering the gas production results of SBN(MSM) and BBN(MSM) in Table 1, the reduction and oxidation active sites on SBN(MSM) and BBN(MSM) crystal grains are the same as that on CBN(MSM). Table 2. The H2 and O2 Evolution Rates of CBN(MSM)-850 with Different Pt Loading Methods. Pt loading method

H2 evolution rate (µmol·h-1·g-1)

O2 evolution rate (µmol·h-1·g-1)

in-situ photo-deposition

104

69.4

ethylene glycol

1.1

143

Reaction time: 5 h. The content of the deposited Pt was 0.5 wt%. The scheme of the polycrystalline ABi2Nb2O9 (A = Ca, Sr, Ba) grains prepared by solid state reaction is shown in Fig. 10(a). Because a lot of crystal boundaries existed in the polycrystalline particles, the diffusion of photon-generated carriers was affected severely by the grain boundaries. Firstly, the potential barriers of crystal boundaries would hinder the mobility of photon-generated carriers in the crystal. Then considering the special layer structure of the Aurivillius-type ABi2Nb2O9 (A = Ca, Sr, Ba), the mobility and recombination processes of photon-generated carriers in the polycrystalline grains of ABi2Nb2O9 (A = Ca, Sr, Ba) were quiet complicated. If the (Bi2O2)2+ layers (or pseudo-perovskite slabs) of two grains contact each other at the crystal boundary as shown in Circle 1 in Fig. 10(a), this situation will enable the holes (or electrons) to diffuse in the (Bi2O2)2+ layers (or pseudo-perovskite slabs) from one crystal grain to another. Thus, this progress could prolong the life-span of photon-generated carriers in the


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polycrystalline particles. This progress would result in the non-radiative transition and that is reason why the polycrystalline CBN(SSR)-850 grains had the lower PL intensity than plane-like CBN(MSM)-850 grains. If the (Bi2O2)2+ layers of one grain come into contact with the pseudoperovskite slabs of another grain at the crystal boundary just like the situation shown in Circle 2 in Fig. 10(a), the photon-generated holes in the (Bi2O2)2+ layers of one grain will recombine with the photon-generated electrons in the pseudo-perovskite slabs of another grain. This recombination in the polycrystalline grains would significantly reduce the effective number of photon-generated carriers that diffused to the active sites on the surface for water splitting. Another situation that could lead to the severe recombination of the PEHPs is that the side of the layer structure of one grain contacted with the front of the layer structure of another grain, as shown in Circle 3 in Fig. 10(a). This contact also results in that the PEHPs in one grain recombined with each other at the boundary or with the opposite charges on the surface of another gain severely. The Fig. 10(b) shows the scheme of single crystal structure and the photocatalytic water splitting process of the plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) grains prepared by molten salt method. The plane-like grains with single crystal structure ensured that the macro-morphology of the crystal grains was in accord with the micro layer structure in the grains and there were no crystal boundaries in the grains or the intimate contact between the grains like the polycrystalline ABi2Nb2O9 (A = Ca, Sr, Ba) grains. Thus, the morphology contributed to not only the water contacting with more active surfaces, but also the transportation of the photon-generated carriers along the layer structure. That is why the ABi2Nb2O9 (A = Ca, Sr, Ba) prepared by molten salt method had a better photocatalytic activity than that prepared by solid state method. In conclusion, the plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) grains with single crystal structure improved the photocatalytic activities of the photocatalysts.


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Figure 10. (a) Schematic of the polycrystalline ABi2Nb2O9 (A = Ca, Sr, Ba) grains; (b) the photocatalytic water splitting process of plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) grains with single crystal structure；(c) the CFSs and photocatalytic water splitting process of SBN(MSM) prepared at low molten salt temperature.

Figure 11. (a, d) HERs and OERs of CBN(MSM)-X; (b, e) HERs and OERs of CBN(MSM)-X; (c, f) HERs and OERs of BBN(MSM)-X.


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The effects of the molten salt temperature on the photocatalytic activities of ABi2Nb2O9 (A = Ca, Sr, Ba) for water splitting were also studied. As shown in Fig. 11, the variation tendencies of the photocatalytic activities of ABi2Nb2O9 (A = Ca, Sr, Ba) with the temperature increasing are different from each other. The HERs and OERs of CBN(MSM)-X are shown in the Fig. 11(a) and (d), respectively. Both HERs and OERs of CBN(MSM)-X decrease with the increase of the temperature. This is mainly because the increasing size of the crystal grains (including the width, length and thickness) made it more and more difficult for the photo-generated electrons and holes in the interior of grains to diffuse to the active surfaces as the temperature increased. The results are in accordance with the SEM images of CBN(MSM)-X. The stability test results (Fig S10) indicated that the photocatalyst had poor stability. Comparing with HER, the OER of CBN(MSM)-850 had a slightly decrease as the photocatalytic reaction progressed, which indicated CBN(MSM)-850 exhibited better stability for OER than for HER. For the oxygen evolution of CBN(MSM)-850, the active sites are located on the (00l) facets of the plane-like grains and the Pt cocatalyst is not necessary for this reaction. Hence, although the reduced Ag2O covering the sides of the plane-like grains would decrease the activity,44,45 the active sites on the (00l) facets were not been affected and kept good catalytic oxidation activity for O2 evolution. The H2 evolution and O2 evolution of CBN(SSR)-850 also had the same variation tendency as those of CBN(MSM)-850 (Fig. S10 (c) and (d)). Fig. 11(b) and (e) show the HERs and OERs of SBN(MSM)-X respectively. The HERs of SBN(MSM)-X increase with the temperature increasing, whereas the OERs of SBN(MSM)-X decrease first and then increase. This may be closely related to the growth process of the crystal face steps (CFSs) on the major plane of SBN(MSM)-X and the schematic of the CFSs is shown in Fig. 10(c). Because a lot of the CFSs


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existed at the side or on the major plane of SBN(MSM) grains prepared at low temperature and the Pt nanoparticles loaded by in-situ photo-deposition method tended to aggregate at the side of grains, the agglomerated Pt nanoparticles would connect the (Bi2O2)2+ layers with the (SrNb2O7)2- pseudo-perovskite slabs easily at the CFSs. The SEM image of agglomerated Pt nanoparticles at the CFSs is shown in Fig. S9(a). The Pt particles acted as a conductive medium for photo-generated electrons in the (SrNb2O7)2- pseudo-perovskite slabs transferring to the (Bi2O2)2+ layers and then the electrons recombined with the holes in (Bi2O2)2+ layers. Thus, a large number of effective electrons on the surface of Pt nanoparticles for hydrogen generation were consumed by this recombination. As the temperature increased, the small crystal faces on the major plane grew up and then the amount of CFSs reduced gradually. Thus, the function of Pt nanoparticles for electron conduction weakened and the consumption of electrons by recombination decreased. Consequently, the effective electrons on the surface of Pt nanoparticles for hydrogen production increased. That is why the HERs of SBN(MSM)-X improved with the increase of the temperature. The electron conduction action of agglomerated Pt nanoparticles at the side of grains is also the reason why the OERs of CBN(MSM)-850 and SBN(MSM)-850 decreased after loading Pt cocatalyst. However, the approximate OERs of BBN(MSM)-850 and BBN(MSM)-850/Pt indicates the Pt cocatalyst did not exert an obvious separation for PEHPs or electron conduction action on BBN(MSM)-850/Pt. The weak electron conduction action of Pt cocatalyst on BBN(MSM)-850 was attributed to the high dispersion of the Pt nanoparticles (see in Fig. S9(b)). Although the CFSs on the (00l) facets of SBN(MSM) counted against the hydrogen evolution after loading Pt cocatalyst, they improved the oxygen evolution because the steps exposed plenty of inner layer crystal faces of the grains. It has been already known that the oxidation active sites for oxygen evolution were on the (00l) facets and the inner layer crystal


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faces were all parallel to the (00l) facets. Considering the periodic layer structure of SBN(MSM) along the c-axis, it is easy to imagine that a large amount of oxidation active sites also existed on the exposed inner layer crystal faces. Thus, the SBM(MSM)-X prepared at low temperature had a high photocatalytic activity for oxygen evolution and this oxygen evolution activity of SBM(MSM)-X decreased with the decrease of CFSs at high temperature. The oxygen evolution activity of the SBN(MSM)-1000 increased slightly than that of SBN(MSM)-950 on account of the serrated edges of SBN(MSM)-1000 crystal grains which were beneficial to the reaction of more Ag+ electron acceptor with the electrons on the Pt nanoparticles at the edges of the grains. The HERs and OERs of BBN(MSM)-X are shown in Fig. 11(c) and (f) respectively. The HERs of BBN(MSM)-X decrease with the temperature increasing due to the growth of widths and lengths of the BBN(MSM)-X grains which made it more difficult for photo-generated electrons to reach the edges of the grains. The OERs of BBN(MSM) increases gradually with the increase of temperature mainly because the growth of BBN(MSM)-X grains along (00l) facets provided more oxidation active sites for oxygen evolution. However, the growth of (00l) facets of CBN(MSM)-X did not improve the photocatalytic activity for oxygen evolution in comparison with the that of BBN(MSM)-X. The major reason is that the restriction abilities of the thicknesses of CBN(MSM)-X and BBN(MSM)-X grains are different for the diffusion of photogenerated holes along c-axis. The thicknesses of CBN(MSM)-X grains increased from ca. 109 nm to ca. 737 nm, whereas the thicknesses of BBN(MSM)-X grains increased from ca. 0.32 µm to ca. 0.49 µm as the temperature rose from 850 to 1000 °C (see in inserts of Fig. 3). The growth rate of the thicknesses of CBN(MSM) grains was much higher than that of BBN(MSM) grains. In addition, the PL spectra shew the recombination probability of photo-generated electron hole pairs in CBN(MSM) was higher than that of BBN(MSM). Thus, although the growth of (00l)


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facets increased the oxidation active sites, the short life photo-generated holes in CBN(MSM) grains were unable to diffuse to the active surfaces because of the overlong diffusion distance along c-axis. The low recombination probability of PEHPs and the relatively short diffusion distance of holes along c-axis in BBN(MSM) grains contributed to the holes reaching at the oxidation sites on (00l) facets. Therefore, the OERs of BBN(MSM)-X show a reverse tendency to that of CBN(MSM)-X with the temperature increasing. Although the BBN(MSM)-X had low recombination probability of PEHPs and the relatively short diffusion distance of holes along caxis, the highest HER and OER of BBN(MSM)-X were very low in comparison with those of CBN(MSM)-X and SBN(MSM)-X. This is mainly because the crystal lattice structure and space group of BBN(MSM)-X are different from those of CBN(MSM)-X and SBN(MSM)-X. Fig. 11 shows the crystal structures of ABi2Nb2O9 (A = Ca, Sr, and Ba). The crystal lattice of the Aurivillius-type ABi2Nb2O9 (A = Ca, Sr, Ba) consists of pseudo-perovskite layers with composition (ANb2O7)2- interspersed between (Bi2O2)2+ layers and the divalent A-type cations (Ca2+, Sr2+, or Ba2+) are located between the corner-sharing NbO6 octahedra within the perovskite-like layer. It has been known from XRD patterns above that the CBN(MSM)-X and SBN(MSM)-X adopted the orthorhombic structure and space group A21am, while the BBN(MSM)-X was tetragonal and space group I4/mmm. The space group I4/mmm of BBN(MSM)-X has a higher symmetry than the space group A21am of CBN(MSM)-X and SBN(MSM)-X. Susan et al.50 reported that the result of the BBN structure refinements indicated no significant distortions from idealized symmetry occurred, whereas the structure refinements of CBN and SBN manifested that a tilting of the NbO6 octahedra with respect to the c-axis was required for the stabilization of the pseudo-perovskite layers and bonding requirements at perovskite A site, as shown in Fig. 12(a) and (b). The structure refinements also indicated that


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the orthorhombic distortion increased with the decreasing A2+ cation size. That is to say, the NbO6 octahedra of CBN has a larger distortion than that of SBN because of the smaller Ca2+. The distortion of NbO6 octahedra in the perovskite-type slabs could induce strong local spontaneous polarization field which improved the excitation and separation of PEHPs.6,33,34 Although the BBN had a narrower band gap than CBN and SBN, the higher symmetrical crystal structure of BBN resulted in weak spontaneous polarization field and the low mobility of electrons along the pseudo-perovskite layers. That is why the highest HER was in the order CBN(MSM)-X>SBN(MSM)-X≫BBN(MSM)-X. The tilting of NbO6 octahedra in SBN made the formation of the fifth strong bond between Bi in the [Bi2O2]2+ layers and O from pseudoperovskite layer which is absent in BBN,52 and the fifth Bi-O bond contributed to the spontaneous polarization which improved the photocatalytic activity of SBN.61 This conclusion could also apply to CBN because of the similar crystal structure of CBN and SBN.

Figure 12. The crystal structure schematics of (a) ABi2Nb2O9 (A = Ca, Sr) and (b) BaBi2Nb2O9.


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CONCLUSIONS (00l) facets exposed plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders with single crystal grain were obtained successfully by molten salt method. The H2 evolution activities of the plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) prepared by molten salt method were almost an order of magnitude higher than those of the ABi2Nb2O9 (A=Ca, Sr, Ba) powders prepared by traditional solid state method. The plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders also have the higher O2 evolution activities than the ABi2Nb2O9 (A=Ca, Sr, Ba) grains prepared by traditional solid state method. The Pt nanoparticles loaded at the edges of plane-like grains were the active sites for H2 evolution and the oxidation active sites were on the (00l) facets of plane-like ABi2Nb2O9 (A = Ca, Sr, Ba) grains. With the temperature of molten salt increasing, the variation tendencies of the photocatalytic activities of CBN(MSM), SBN(MSM) and BBN(MSM) for water splitting were different from each other. The photocatalytic water splitting activities of CBN(MSM) decreased with the increasing size of the plane-like grains. Although the crystal face steps on (00l) facets of SBN(MSM) counted against the H2 evolution after loading Pt cocatalyst, they contributed to the O2 evolution of SBN(MSM). The increasing size of BBN(MSM) grains had an adverse impact on the H2 evolution, but the (00l) facets growth of BBN(MSM) grains improved O2 evolution at high temperature. The strong distortion of NbO6 octahedra in the perovskite-like slabs of CaBi2Nb2O9 and SrBi2Nb2O9 contributed to their photocatalytic activities. This work provides an idea that the good matching of the macro-morphology and microstructure of the photocatalysts is beneficial to exert the best possible performance of the materials, which offers a new thought for studying the structural and morphological design of the layered perovskite oxide photocatalysts.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ? brief description (file type, i.e., PDF) SEM images, XPS, Mott–Schottky plots, UV/Vis spectra, Energy band structures diagram, BET surface area, and stability test results of samples. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: +028 85460353; Tel: +86 85412415 *E-mail: [email protected]; Fax: +86 2885221339; Tel: +86 13551341892

Present Addresses † College of Materials Science and Engineering, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu , China, 610065, P. R. China. ‡ College of Chemistry, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu , China, 610065, P. R. China. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21273157 and 51332003) for financial support. We thank Sichuan University Analytical & Testing Centre for the analysis of


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SYNOPSIS TOC

(a) Schematic of the polycrystalline ABi2Nb2O9 (A = Ca, Sr, Ba) grains; (b) the photocatalytic water splitting process of monocrystalline ABi2Nb2O9 (A = Ca, Sr, Ba) grains with single crystal structure.


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(00l) facets exposed plane-like ABi2Nb2O9 (A=Ca, Sr, Ba) powders

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