Single- and Double-Site Substitutions in Mixed-Metal Oxides

Article pubs.acs.org/JPCC

Single- and Double-Site Substitutions in Mixed-Metal Oxides: Adjusting the Band Edges Toward the Water Redox Couples Jonathan Boltersdorf,† Brandon Zoellner,† Chris M. Fancher,§ Jacob L. Jones,§ and Paul A. Maggard*,† †

Department of Chemistry and §Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: New mixed-metal oxide solid solutions, i.e., the single-metal substituted Na2Ta4−yNbyO11 (0 ≤ y ≤ 4) and the double-metal substituted Na2−2xSnxTa4−yNbyO11 (0 ≤ y ≤ 4; 0 ≤ x ≤ 0.35), were investigated and used to probe the impact of composition on their crystalline structures, optical band gaps, band energies, and photocatalytic properties. The Na2Ta4O11 (y = 1) phase was prepared by flux-mediated synthesis, while the members of the Na2Ta4−yNbyO11 solid solution (1 ≤ y ≤ 4) were prepared by traditional high-temperature reactions. The Sn(II)-containing Na2−2xSnxTa4−yNbyO11 (0 ≤ y ≤ 4) solid solutions were prepared by flux-mediated ion-exchange reactions of the Na2Ta4−yNbyO11 solid solutions within a SnCl2 flux. The crystalline structures of both solid solutions are based on the parent Na2B4O11 (B = Nb, Ta) phases and consist of layers of edge-shared BO7 pentagonal bipyramids that alternate with layers of isolated BO6 octahedra surrounded by Na(I) cations. Rietveld refinements of the Na2Ta4−yNbyO11 solid solution showed that Nb(V) cations were disordered equally over both the BO7 and BO6 atomic sites, with a symmetry-lowering distortion from R3̅c to C2/c occurring at ∼67−75% Nb (y = ∼2.7−3.0). A red-shift in the optical band gaps from ∼4.3 to ∼3.6 eV is observed owing to a new conduction band edge that arises from the introduction of the lower-energy Nb 4d-orbitals. Reactions of these phases within a SnCl2 flux yielded the new Na2−2xSnxTa4−yNbyO11 solid solution with Sn-content varying from ∼11% to ∼21%. However, significant red-shifting of the band gap is found with increasing Nb-content, down to ∼2.3 eV for Na1.4Sn0.3Nb4O11, because of the higher energy valence band edge upon incorporation of Sn(II) into the structure. Aqueous suspensions of the particles irradiated at ultraviolet−visible energies yielded the highest photocatalytic hydrogen production rates for Na1.3Sn0.35Ta1.2Nb2.8O11 (∼124 μmol H2·g−1·h−1) and Na1.4Sn0.3Ta3NbO11 (∼105 μmol H2·g−1·h−1), i.e., for the compositions with the highest Sn(II)-content. Further, polycrystalline films show n-type anodic photocurrents under ultraviolet−visible light irradiation. These results show that the valence and conduction band energies can be raised and lowered, respectively, using single-metal and double-metal substituted solid solutions. Thus, a novel approach is revealed for achieving smaller visible-light bandgap sizes and a closer bracketing of the water redox couples in order to drive total water splitting reactions that are critical for efficient solar energy conversion.

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INTRODUCTION Growing world energy consumption has led to an increase in the demand for an economy based upon clean and renewable energy sources. Major research efforts have targeted the advance of solar energy utilization, which necessitates the discovery and development of new materials that enable the construction of efficient systems for solar energy conversion.1 Metal-oxide semiconductors can be used as earth-abundant light absorbers within artificial photosynthetic systems that function to produce fuels from sunlight and water. In these systems, the photogenerated electron−hole pairs within a metal-oxide semiconductor can be separated by the electric field that forms at the semiconductor−liquid interface.1−4 In addition, the metal-oxide should be able to absorb a significant fraction of the visible region with an optical band gap of ≥1.23 eV, which is the minimum required in order to thermodynami© XXXX American Chemical Society

cally drive the water splitting reaction. The conduction band must also be more negative than that of the proton reduction potential, and the valence band must be more positive than that of the water oxidation potential.2,3,5 Metal-oxide photocatalysts with suitable visible-light band gaps typically exhibit short-lived stability under irradiation in solution (e.g., Cu2O), while those with larger ultraviolet band gaps have shown to be significantly more stable, but are limited in efficiency owing to the lower solar intensities at these wavelengths (e.g., Nb2O5, Ta2O5).3 The discovery of new photocatalysts is known to be possible through the synthesis of metal-oxides that introduce posttransition metals with filled ns2 states (e.g., Sn(II), Pb(II), Received: June 7, 2016 Revised: July 29, 2016

A

DOI: 10.1021/acs.jpcc.6b05758 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

and found to be a sensitive function of the mixed-metal composition.

Bi(III)) that serve to increase the valence band energy and reduce the optical band gap.3,4,6−8 For example, recent studies by the Maggard group have focused on the family of tantalatebased structures with the general formula Am+(n+1)/mB(3n+1)O(8n+3) (e.g., A = Pb(II), Bi(III); B = Ta(V)). These studies have shown that the incorporation of Pb(II) and Bi(III), i.e., in PbTa4O11 and BiTa7O19, respectively, results in red-shifted band gaps and photocatalytic activity for hydrogen and/or oxygen production under ultraviolet−visible irradiation.6 However, all compositions within this family have relatively large bandgap sizes, with the exception of the Cu(I) containing phases, including Cu2Ta4O11, Cu3Ta7O19, and Cu5Ta11O30.9−14 While these phases show band gaps of around ∼2.4 to ∼2.6 eV, as well as significant p-type photocurrents under visible-light irradiation as polycrystalline films, the stability of these Cu(I)-containing phases in their unprotected form is currently limited. Alternatively, the incorporation of Sn(II) cations into these structures may facilitate increased visible-light absorption for these photocatalysts. The only known Sn(II)-containing niobates/tantalates are the SnB2O6 and Sn2B2O7 (B = Nb, Ta) phases that exhibit visible-light band gaps and photocatalytic activity for hydrogen production.15−19 A complementary approach is the synthesis of mixed-metal solid solutions that can be formed when two or more types of metals are disordered over the same atomic sites, which can result in lower-lying conduction band energies and/or higherlying valence band energies that reduce their bandgap sizes. For example, previous investigations of the NaCuTa4−yNbyO11 solid solution probed changes in the optical properties with increasing substitution of niobium for tantalum, resulting in a lower energy conduction band.12,20−22 The concomitant incorporation of the Sn(II) cation into these structures is desirable for its potential to raise the energies of the valence bands of these structures and effectively lower their band gaps into the visible-light range. Synthetic challenges associated with the incorporation of Sn(II) into the structural framework can be addressed with the aid of a molten-salt flux that can enable low-temperature stabilization (e.g., Cu2Ta4O11)10 and fluxmediated ion-exchange of parent structures (e.g., PbTa4O11).6 Flux-mediated synthetic methods have been shown to enable facile ion-exchange of interlayer A-site cations, as well as conservation of the particle morphology of the precursor.4,23−25 Therefore, the combination of Sn(II) and Ta(V)/Nb(V) cations in a metal-oxide solid solution is desirable for its potential to exhibit a visible-light band gap and photocatalytic activity. Reported herein are the solid-state and flux-mediated syntheses and characterization of the single-metal substituted Na2Ta4−yNbyO11 and double-metal substituted Na2−2xSnxTa4−yNbyO11 solid solutions in order to study the changes in the optical and photocatalytic properties as a function of their mixed-metal compositions. The Na2−2xSnxTa4−yNbyO11 solid solutions are the first examples of Sn(II) cations incorporated into members of the Am+(n+1)/mB(3n+1)O(8n+3) structural family. The products were characterized by powder X-ray diffraction (PXRD) as well as by high-resolution synchrotron powder diffraction experiments and subsequent Rietveld refinement methods. The particle sizes and morphologies were characterized by scanning electron microscopy (SEM), and elemental analyses of the particles were performed using energy dispersive X-ray spectroscopy (EDS). Their optical band gaps, band energies, and photocatalytic activities for hydrogen production were characterized

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METHODS Syntheses. The flux synthesis of the layered natrotantite, Na2Ta4O11,26,27 was performed according to previously reported methods by grinding stoichiometric amounts of Na2CO3 (Alfa Aesar, 98%) and Ta2O5 (Acros Organics, 99.99%) within a 50% K2SO4 (Fisher Scientific, 99.8%) and 50% Na2SO4 (Alpha Aesar, 99.0%) eutectic flux in a 12:1 fluxto-reactant molar ratio. The mixture was ground together in a mortar and pestle and heated for 2 h at 1000 °C inside a box furnace, and then quenched in air. The excess K2SO4/Na2SO4 eutectic flux was removed by washing with deionized water, followed by drying at 80 °C for 12 h. The Na2Ta4−yNbyO11 (1 ≤ y ≤ 4) solid solutions were prepared by the solid-state method by combining and grinding stoichiometric mixtures of reagent grade Na2CO3, Nb2O5 (Alfa Aesar, 99.9985%), and Ta2O5. The reaction mixtures for 1 ≤ y ≤ 2.8 were pressed into a pellet, heated for 30 min at 1000 °C within alumina crucibles inside a box furnace, quenched in air, reground, pelletized, and then heated for an additional 2 h at 1000 °C. The layered Na2NbyO11 (y = 4) was prepared using the previously reported reaction conditions in which it was heated at 800 °C for 24 h.28,29 The Na2−2xSnxTa4−yNbyO11 phases were prepared using flux-mediated ion-exchange synthesis methods by heating Na2Ta4−yNbyO11 within a SnCl2 (Alfa Aesar, 99%) flux in a 10:1 flux-to-precursor molar ratio. The Na2Ta4−yNbyO11 and the SnCl2 flux were ground under an argon atmosphere in a glovebox, and then loaded into a fused-silica tube. The reaction mixtures were dried under dynamic vacuum (∼50 mTorr) at 200 °C for 1 h to remove any excess water. The tube was flamesealed and then heated for 4 h at 300 °C in a box furnace. The products were washed and centrifuged using a sequence of acetone and deionized water to remove excess byproducts to yield powders that ranged from white to yellow in color. Characterization. Powder X-ray diffraction (PXRD) data were collected on an INEL diffractometer using Cu Kα1 (λ = 1.54056 Å) radiation from a sealed-tube X-ray source (35 kV, 30 mA) equipped with a curved position-sensitive detector (CPS120). UV−vis diffuse reflectance spectra were collected on a Shimadzu UV-3600 equipped with an integrating sphere. A pressed barium sulfate disc was used as the background. UV− vis DRS measurements were taken in order to obtain the reflectance of the samples, which were subsequently transformed using the Kubelka−Munk function. The Kubelka− Munk remission function (F(R)), shown in eq 1, relates reflectivity (R) of a sample to its absorption coefficient (α) and scattering coefficient (S). F (R ) =

(1 − R ∞)2 α = 2R ∞ S

(1)

Assuming a constant scattering coefficient (S), F(R) is proportional to α and represents the absorbance of the sample.30−32 The measured reflectance can then be related to the band gap (Eg) of the material, where the absorption edge of the sample and the incident photon energy are related by the Tauc equation, shown in eq 2. (αhν)n = A(hν − Eg )

(2)

where h is Planck’s constant, ν is frequency, A is a proportionality constant, and n denotes the nature of the B


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Table 1. Rietveld Refinement Results for (a) Na2Ta4O11, (b) Na2Ta3NbO11, (c) Na2Ta1.2Nb2.8O11, and (d) Na2Nb4O11 loaded formula

(a) Na2Ta4O11

(b) Na2Ta3NbO11

(c) Na2Ta1.2Nb2.8O11

(d) Na2Nb4O11

refined formula(s) formula weight crystal system(s) space group(s), Z a (Å) b (Å) c (Å) V (Å3) weight (%) data/parameters R-values (%) X2 goodness of fit

Na2Ta4O11/NaTaO3 945.76/251.93 rhombohedral/orthorhombic R3̅c (167), 6/Pcmn (62) 6.218566(8) 6.218566(8) 36.66849(5) 1228.016(3) 94.8/5.2 58996/61 wRp = 8.90; Rp = 7.25 2.50 1.58

Na2Ta3.2Nb0.8O11 874.09 rhombohedral R3̅c (167), 6 6.208776(8) 6.208776(8) 36.62979(6) 1222.861(4) 100.0 46137/37 wRp = 8.68; Rp = 6.71 1.75 1.32

Na2Ta1.35Nb2.65O11/Na2Ta1Nb3O11 712.59/680.71 rhombohedral/monoclinic R3̅c (167), 6/C2/c (15), 4 6.20900(1)/10.83780(7) 6.20900(1)/6.16720(3) 36.6350(1)/12.75002(4) 1223.124(6)/818.401(6) 69.7/30.3 46100/82 wRp = 9.11; Rp = 7.40 1.33 1.15

Na2Nb4O11 593.59 monoclinic C2/c (15), 4 10.84406(3) 6.16423(1) 12.75061(2) 818.594(3) 100.0 48996/65 wRp = 11.11; Rp = 9.12 1.99 1.41

band gap transition. Since α is proportional to F(R), the Tauc equation can be expressed as a function of F(R). Therefore, Tauc plots of (F(R) × hν)n versus hν (eV) can be used to obtain the energies of the allowed direct (n = 2) and indirect (n = 1/2) band gap transitions from the inflection point tangent to the linear portion of the absorption curve.27,33−35 Highresolution images and elemental analysis were obtained on an FEI Verios 460L field emission scanning electron microscope (FESEM) equipped with an Oxford energy dispersive X-ray spectrometer (EDS). High-Resolution Synchrotron Powder Diffraction and Rietveld Refinements. Synchrotron powder diffraction data were collected on beamline 11-BM at the Advanced Photon Source (APS), Argonne National Laboratory, with an average wavelength of 0.459169 Å. Discrete detectors covering an angular range from −6 to 16° 2θ were scanned over a 34° 2θ range, with data points collected every 0.001° 2θ with a scan speed of 0.1°/s. The polycrystalline samples of Na2Ta4−yNbyO11 (y = 0, 1, 2.8, and 4) were packed into Kapton tubes with an inner diameter of 0.80 mm. Samples were submitted using the 11-BM mail-in program, and the data were collected at room temperature. Crystal structures were refined from the synchrotron diffraction data by the Rietveld method using the General Structure Analysis System (GSAS) and the graphical user interface (EXPGUI) software packages.36,37 The implemented refinement strategy was conducted as follows for each data set. Corrections for the constant wavelength (CW) sample X-ray absorption (μR) were computed and fixed in the refinements. All fractional site occupancies were initially set to the loaded compositions, and the isotropic thermal parameters were initially set to 0.025. Equivalence constraints for the fractional atomic coordinates and isotropic thermal parameters were placed on the Ta and Nb atoms. First, the scale factor and background were refined using the Chebyshev polynomial with a maximum of 15 terms for several cycles. Next, the lattice constants followed by the atomic coordinates and isotropic thermal parameters (Uiso) were damped and independently refined in order of decreasing scattering length, and the damping was progressively decreased after several refinement cycles. The “zero” shift parameter and the constant wavelength (CW) peak profile shape functions for the Lorentzian isotropic and the anisotropic crystallite size and microstrain terms were refined for several cycles. The scale factor, background, lattice parameters, atomic coordinates, Uiso, fractional site occupancies, zero, and peak profile shapes were all refined together for several cycles. The damping factors for

the atomic coordinates, Uiso, and fractional site occupancies were all removed, and all parameters were simultaneously refined to completion. Refinement results are listed in Table 1. Film Preparation. For Mott−Schottky measurements, polycrystalline films of the Na 2 Ta 4 − y Nb y O 1 1 and Na2−2xSnxTa4−yNbyO11 powders were prepared on TEC-15 fluorine-doped tin oxide (FTO) slides (Pilkington Glass Inc.). FTO slides were first cleaned by sonicating in deionized water, followed by ethanol, then finally acetone for 30 min each. A 1 cm2 area was taped off using Scotch tape on the conductive side, and the polycrystalline powders were deposited by the doctor blade method. A solution of water/tert-butanol was used as the dispersant, as previously described.14 Films were annealed under dynamic vacuum (P ≤ 50 mTorr) at 500 °C for Na2Ta4−yNbyO11 and 300 °C for Na2−2xSnxTa4−yNbyO11 for 3 h and allowed to cool to room temperature. For linear-sweep voltammetry measurements, a TiO2 blocking layer was first deposited onto the FTO slides using a previously reported procedure.38 Briefly, a ∼50 mM TiCl3 solution (20% TiCl3 solution in HCl, Fisher Scientific) was bubbled with N2 gas for 15 min prior to the electrodeposition. The pH was then adjusted to ∼2.5 using a saturated Na2CO3 solution. A clean FTO slide was submerged into the solution along with the reference electrode (SCE, Sat KCl) and a platinum counter electrode. An applied potential of 0 V vs SCE was applied for 10 min under gentle stirring. The slide was rinsed with 1 M HCl followed by ethanol before annealing at 450 °C for 1 h in air. Subsequent deposition of the polycrystalline films of the solid solutions on top of the blocking layer were performed as described above. Photoelectrochemical Measurements. Electrochemical impedance spectroscopy (EIS) was used to investigate the flat band potentials of the Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11 phases using a frequency of 1 kHz and an AC amplitude of 10 mV. Linear sweep voltammetry measurements were also performed within a potential range of −0.4 to +0.6 V versus SCE and under chopped UV−vis or visible-light only irradiation. Measurements were taken on a Princeton Applied Research potentiostat (Parstat-2263) in a custom Teflon cell using a 0.1 M KCl solution at pH ≈ 5.8 and purged with N2 gas 30 min prior to the experiments. Polycrystalline films of the solid solutions served as the working electrode, Pt foil served as the counter electrode, and a saturated calomel electrode (SCE sat. KCl) served as the reference electrode. The flat-band potentials (Vfb) of the semiconductors were determined for each material by C


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chromatograph (SRI MG #2; thermal conductivity detector).6,23

measuring the capacitance of the depletion region of the semiconductor at the solid-electrolyte interface with an appliedpotential. The inverse capacitance squared is related to the flatband potential through eq 3 ⎞⎛ ⎛ 1 2 kT ⎞ ⎟ ⎟⎜V − Vfb − =⎜ 2 2 ⎝ o e ⎠ C ⎝ εrεoeNDA ⎠

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RESULTS AND DISCUSSION Synthesis. The flux-mediated synthesis of Na2Ta4O11 and the solid-state synthesis of Na2Nb4O11 have previously been performed at 1000 °C.6,28,29,47,48 Initial attempts at the fluxsynthesis of the Na2Ta4−yNbyO11 (y > 0) solid solutions utilized the previously reported reaction conditions for preparing Na2Ta4O11,6,27 but instead yielded the NaTaO3 and B2O5 (B = Ta, Nb) phases. However, the Na2Ta4−yNbyO11 (y > 0) solid solutions could be obtained in high purity by heating the reactants at 1000 °C for 30 min, quenching in air, and regrinding and calcining the product for an additional 2 h at 1000 °C for improved crystallinity. Gradual heating of the samples up to the reaction temperature and/or prolonged heating resulted instead in the formation of the NaTaO3 phase in large quantities. The Na2Ta4−yNbyO11 (0 ≤ y ≤ 2.8) solid solutions containing 100% to 30% Ta were prepared in high purity in a nondistorted rhombohedral structure, while the Nbricher solid solutions containing ≤30% Ta (2.8 < y < 4) were found to crystallize in a distorted monoclinic polymorph, as described below. These results are consistent with a previous synthetic investigation of the NaCuTa4−yNbyO11 solid-solution that showed that the rhombohedral form occurs up to only y = 2.8, or ≤30% Ta.20 Various reaction conditions were utilized in an attempt to exchange the Na(I) cations for Sn(II) cations in the layered Na2Ta4−yNbyO11 solid solution in high yields. Reaction temperatures above 300 °C, reaction times longer than 4 h, and reactions in air resulted in the rapid conversion of the targeted “SnTa4−yNbyO11” phases into the SnB2O6 (B = Nb, Ta) phases.18,49 The SnB2O6 (B = Nb, Ta) phases form quickly, within as little as 5 min, at temperatures of 400−1000 °C. These phases are the only Sn(II)-containing tantalates/niobates that have been reported and subsequently studied for their optical and photocatalytic properties.15−19 The lack of structural diversity is the result of the synthetic challenges of stabilizing Sn(II)-containing structures, but which can be addressed with the aid of a molten-salt flux. Longer reaction times at temperatures below 300 °C resulted in significantly less tin-content, and the use of other salts as the flux solvent was unsuccessful. Therefore, the reaction conditions were restricted to ≤300 °C with reaction times ≤4 h. All samples were heated under static vacuum conditions with a SnCl2 flux, resulting in the Na2−2xSnxTa4−yNbyO11 solid solution. Structural Characterization. The structural family of niobate- and tantalate-based ternary phases, with the general formula Am+(n+1)/mB(3n+1)O(8n+3) (e.g., A = Na, Ag, Cu, Pb, Bi; B = Nb, Ta) are based on the stacking of α-U3O8 type of edgeshared pentagonal bipyramidal BO7 layers, where n defines the average thickness (1 ≤ n ≤ 2) of the BO7 layers. A wide range of A-site metal cations can occupy the interlayer sites and affords a broad compositional diversity within this structural family.6,10,12,13,27,29,48,50−54 The Am+(n+1)/mB(3n+1)O(8n+3) phases with n = 1 consist of single layers of edge-sharing BO7 pentagonal bipyramids that alternate with isolated BO6 octahedra. This structure is known to crystallize in rhombohedral or monoclinic forms, as shown in Figure 1. Each BO6 octahedral environment is respectively surrounded by six 7coordinate Na(I) ions, six 6-coordinate Ag(I) ions, six 2coordinate Cu(I) ions, or three 7-coordinate Pb(II) ions, as previously described for this family of structures.6,10,48,50 The

(3)

where C is the capacitance (F), e is the elementary charge (C), εr is the dielectric of the material, εo is the permittivity of free space (F/m), ND is the donor density (m−3) in the semiconductor, A is the geometrical area of the electrode (m2), k is Boltzmann’s constant (eV/K), and T is temperature (K).39 The flat-band potential of the semiconductor was calculated by extrapolating the linear portion of the plot to the x-ordinate to find Vo and thus Vfb, using eq 4. Vfb = Vo −

kT e

(4)

The slopes of the linear regions of the plots yield the donor density (ND), as calculated using eq 5. slope =

2 εrεoeNDA2

(5)

Finally, the conduction band energy can be determined using eq 6 ⎛N ⎞ Ec = Vfb − kT ln⎜ D ⎟ ⎝ NC ⎠

(6)

using an estimated value of 1019 cm−3 for the effective density of states in the conduction band (NC). Suspended Particle Photocatalysis Measurements. Measurements of photocatalytic hydrogen generation were performed with a 1 wt % platinum cocatalyst that was photodeposited using the photochemical deposition (PCD) method.6,25,40 Each sample was immersed in an aqueous solution with the appropriate amount of dihydrogen hexachloroplatinate (IV) (H2PtCl6·6H2O; Alfa Aesar, 99.95%; 1 mg/mL), methanol, and deionized water in order to photodeposit 1 wt % Pt. The suspension was irradiated in an outerirradiation type fused-silica reaction cell using an 800 W xenon arc-lamp under constant stirring. Platinum serves as a cocatalyst for proton reduction on the surfaces of metal-oxide photocatalysts.6,23,41−44 Methanol was added as a sacrificial reagent to aid platinum deposition on the particles’ surfaces.41 After irradiation, the samples were washed with deionized water and dried at 80 °C for 12 h. The photocatalytic rates for hydrogen production were measured in an outer-irradiation type fused-silica reaction cell irradiated using a xenon arc-lamp with a photon flux of ∼300 mW/cm2, equipped with an IR filter, under ultraviolet−visible (λ > 230 nm) or only visible-light (λ > 420 nm) irradiation. All suspensions were degassed by purging with N2(g) and sonication.6,23 Measurements utilized ∼50 mg of each sample dispersed in a 20% aqueous methanol solution as the sacrificial reagent.45 Methanol acts as a hole-scavenger, photo-oxidizing to CO2, allowing for the measurement of hydrogen without the potentially rate-limiting step of water oxidation.46 Measurements of the photocatalytic hydrogen generation were recorded at 30 min intervals and calculated in units of μmol H2·g−1·h−1. The amount of evolved gases produced were measured volumetrically, and the products were identified using a gas D


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Information. Refinements of the lattice constants for the Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11 solid solutions before and after tin-exchange show that as the amount of Nb(V) increases from y = 0 to y = 2.8, the a/b lattice parameters slightly decrease, while there is no statistically significant change in the c lattice parameter. There is a decrease in the unit cell volume with an increase in the Nb-content from y = 0 to y = 1, but no statistically significant change in the volume for y > 1. Lattice parameters for Na2Nb4O11 (y = 4) are not shown in Figure 4 owing to the fact that it undergoes a symmetry-lowering distortion to a monoclinic space group. After the tin-exchange reactions there is a slight increase in the unit cell volume, as shown in Figure 4c. As the chemical composition of the solid-solution changes, the lattice parameters and volumes of the unit cells showed only very minor changes. High-resolution synchrotron powder diffraction data were used in order to determine the changes in the lattice constants, atomic site coordinates, and fractional site occupancies. Rietveld structural refinements of the Na2Ta4−yNbyO11 (y = 0, 1, 2.8, and 4) solid solutions were performed starting from models obtained from the related n = 1 members of this structural family, i.e., starting from either Na 2 Ta 4 O 11 (R3̅c)20,27,47,48,51 or Na2Nb4O11 (R3̅c,C2/c,Cc).28,29,57 The best refinement fits for the Na2Ta4−yNbyO11 (y < 2.8) compositions were obtained using the higher-symmetry Na 2 Ta 4 O 11 (R3̅ c ) structural model. 20 , 27, 47 ,4 8, 51 The Na2Ta4−yNbyO11 (y = 2.8) solid-solution was refined as a two-phase mixture of the higher-symmetry Na2Ta4O11 (R3̅c) and the monoclinic Na2Nb4O11 (C2/c) structural models. The Na2Nb4O11 phase was refined based on the Na2Nb4O11 (C2/c) structural model. The solid solutions were refined with the fractional occupancies of the Ta and Nb atoms statistically distributed over each type of B-site, with equivalence constraints placed on the atomic coordinates and thermal parameters that resulted in the best fit of the experimental data. The statistical distribution of Nb(V) in the solid-solution refinements showed no preferential occupation for either of the two crystallographic sites corresponding to the BO6 and BO7 coordination environments. In the rhombohedral structural model, the BO7 pentagonal bipyramids and BO6 octahedra

Figure 1. Polyhedral views of the (a) overall unit cell of the rhombohedral (R3̅c) and (f) monoclinic (C2/c) structures of Na2−2xSnxTa4−yNbyO11 with (b,d) alternating layers of edge-sharing BO7 (B = Nb, Ta) pentagonal bipyramids and (c,e) isolated BO6 octahedra surrounded by Na(I) or Sn(II) cations that stack along the c axis.

varied coordination preferences are the result of the different crystal radii and coordination preferences of the A-site cations, with Na(I), Ag(I), Cu(I), Pb(II), and Sn(II) exhibiting crystal radii of 1.26, 1.29, 0.6, 1.37, and 1.18 Å, respectively.55 However, members of this structural family have not yet been found that contain Sn(II) cations in the interlayer spaces, i.e., analogous to the incorporation of Pb(II) cations in the PbTa4O11 structure.6,50 Members of the Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11 solid solutions (0 ≤ y ≤ 4; 0 ≤ x ≤ 0.35) were characterized by PXRD and the lattice constants were refined using LATCON,56 plotted in Figures 2, 3, and 4. The powder XRD patterns of the Na2Ta4−yNbyO11 (y < 4) solid solutions and Na2Nb4O11 closely matched that for the rhombohedral Na2Ta4O11 and the monoclinic Na2Nb4O11 phases, respectively, shown in Figure 2. The patterns after the tin-exchange reactions of the Na2Ta4−yNbyO11 solid solutions yielded the Na2−2xSnxTa4−yNbyO11 phases that showed no significant change in the peak positions or intensities, as shown in Figure 3. Given the very similar ionic radii of Nb(V) and Ta(V) cations in an octahedral coordination environment (0.64 Å), the changes in the lattice constants and the unit cell volume were found to be relatively small with increasing Nb-content, as shown in Figure 4 and listed in Table S1 in the Supporting

Figure 2. Calculated and experimental powder X-ray diffraction patterns of the (a) Na2Ta4−yNbyO11 (0 ≤ y < 4) solid-solutions and (b) Na2Nb4O11. The most intense peaks are labeled with their Miller indices (hkl) on the calculated patterns. E


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Figure 3. Calculated and experimental powder X-ray diffraction patterns of the (a) Na2−2xSnxTa4−yNbyO11 (0 ≤ y < 4) solid-solutions and (b) Na2−2xSnxNb4O11. The most intense peaks are labeled with their Miller indices (hkl) on the calculated patterns.

Figure 4. Refined lattice parameters of the Na2Ta4−yNbyO11 (black) and Na2−2xSnxTa4−yNbyO11 (red) solid-solutions for the (a) a lattice constant, (b) c lattice constant, and (c) unit cell volume as a function of the Nb-content (y), with error bars indicating the standard deviations.

two-phase mixture consisting of ∼70% (by weight) Na 2 Ta 1 . 3 5 Nb 2 . 6 5 O 1 1 (R3̅ c ) and ∼30% (by weight) Na2Ta1Nb3O11 (C2/c). The monoclinic Na2Ta1Nb3O11 exhibited a Nb-richer content with ∼75% Nb (i.e., y = 3) in comparison to the rhombohedral Na2Ta1.35Nb2.65O11 with a Nb-poorer content of ∼59% Nb (i.e., y = 2.65). The monoclinic distortion of Na2Ta1Nb3O11 is the result of the Nb-richer content (y = 3), which is outside the reported homogeneity region of the A2Ta4−yNbyO11 solid-solutions. These results compare to previously reported structural refinements of the NaCuTa4−yNbyO11 (y = 2.8) solid solution, which found the solid solution forms with up to ∼70% Nbcontent. This disparity in the homogeneity of the solid solutions at ∼70% Nb is attributed to the different reaction conditions and possibly the 50% Cu-content for the NaCuTa4−yNbyO11 (y = 2.8) solid solution. The monoclinic distortions observed in the Nb-richer Na2TaNb3O11 (y = 2.8) and Na2Nb4O11 (y = 4) are a result of the off-center displacement of the Nb atoms in the NbO7 pentagonal bipyramids toward the apical oxygens and in the NbO6 octahedra toward the octahedral faces, as shown in Figures S2 and S3 in the Supporting Information. Previous reports have shown that the niobate members of this family of structures tend to undergo rhombohedral to monoclinic structural distortions, e.g., as found for Ag2Nb4O11 and Na2Nb4O11. In general the niobate structures more commonly exhibit symmetry-lowering structural distortions compared to

occupy the respective 6b and 18e Wyckoff positions for those atomic sites. While in the monoclinic structural model, the BO7 pentagonal bipyramids are located on the 8f and 4e Wyckoff positions and the BO6 octahedra are located on the 4d Wyckoff positions. Previous studies of tantalate/niobate solid solutions have reported the disorder of Nb(V) and Ta(V) cations on the same sites within several structures owing to their nearly identical sizes, e.g., as found in NaCuTa 4−y Nb y O 11 , SnNb4−yTayO11, CuNb1−yTayO3, etc.18,20,22 Results of trial refinements in alternate possible atomic site configurations and lower-symmetry space groups for these structures (i.e., R3c, R3, and Cc) resulted in relatively poorer fits of the experimental data. Rietveld refinements results are given in Table 1 and shown in Figure S1 in the Supporting Information. Selected refinement parameters, atomic coordinates, thermal parameters, site occupancies, and nearest-neighbor interatomic distances are listed in Tables S2−S8 in the Supporting Information. The refined structures of the rhombohedral Na2Ta4−yNbyO11 (y ≤ 2.8) solid-solutions were generally similar to that of the reported Na2Ta4O11 structure, while the refined Na2Nb4O11 (C2/c) matched the reported monoclinic crystal structure, as shown in Figures S2 and S3 in the Supporting Information. The refined chemical compositions were found to be consistent with the loaded composition, i.e., Na 2 Ta 4 O 11 (y = 0), Na2Ta3.2Nb0.8O11 (y = 1), 70% Na2Ta1.35Nb2.65O11/30% Na2TaNb3O11 (y = 2.8), and Na2Nb4O11 (y = 4). In contrast, the Na2Ta4−yNbyO11 (y = 2.8) solid-solution was refined as a F


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hexagonal-platelet and truncated tetrahedron shaped morphologies with relatively well-defined facets with smooth hexagonal faces, with an average size of ∼1.5 μm. By contrast, the solid solution Na2Ta4−yNbyO11 (y > 1) samples prepared by the traditional solid-state synthesis method resulted in irregularly shaped particle morphologies that aggregated into large clusters, with individual particles varying in size from ∼1 to ∼3 μm, as shown in Figure 5. The irregularly shaped particles of the Na2Ta4−yNbyO11 (y > 1) particles also exhibited concentric growth rings on the smooth single-particle surfaces, indicating layer-by-layer particle growth. After Sn(II) exchange within a SnCl2 flux, the Na2−2xSnxTa4−yNbyO11 particles maintained either the hexagonal-platelet and truncated tetrahedron shaped (y = 1) or irregularly shaped (y > 1) particle morphologies, sizes, and surface features as observed for the Na2Ta4−yNbyO11 precursors, shown in Figure 6. The surfaces of the particles were smooth and did not show any visible defects or etching of the surface by the flux-exchange reactions. Optical Properties. The optical properties were measured by UV−vis diffuse reflectance spectroscopy (DRS) in order to compare the effects of the single-metal (i.e., Nb/Ta) and double metal (i.e., Nb/Ta and Na/Sn) substitutions. The lowest-energy indirect band gaps were measured for Na2Ta4O11 (∼4.3 eV), Na2Ta3NbO11 (∼3.8 eV), Na2Ta2Nb2O11 (∼3.7 eV), Na2Ta1.2Nb2.8O11 (∼3.6 eV), and Na2Nb4O11 (∼3.6 eV), shown in Figure 7 and given in Table 4. Tauc plots of the Na2Ta4−yNbyO11 solid-solutions exhibited a red-shift in the absorption edge with an increase in the Nb-content. The largest red-shift (∼0.7 eV) was observed between the Ta-richest Na2Ta4O11 and Nb-richest Na2Nb4O11. The red-shift is the result of the new conduction band formed by the lower-energy Nb 4d orbitals, as described below. There are two band gap transitions that can occur in the Na2Ta4−yNbyO11 solid solutions, either the higher-energy O 2p to Ta 5d transition or the lower-energy O 2p to Nb 4d transition.6,20,27,59−62 Therefore, the conduction band edge is delocalized over Ta 5d and Nb 4d orbitals and red-shifted to lower energies with an increase in the Nb-content from y = 0 to y = 4. The lowest-energy indirect band gaps for the Sn(II)exchanged Na2−2xSnxTa4−yNbyO11 solid-solutions were measured to be ∼3.4 eV for Na1.8Sn0.1Ta4O11, ∼2.6 eV for Na1.4Sn0.3Ta3NbO11 (75% Ta), ∼2.5 eV for Na1.6Sn0.2Ta2 Nb2O11 (50% Ta), ∼2.4 eV for Na1.3Sn0.35Ta1.2 Nb2.8O11 (30% Ta), and ∼2.3 eV for Na1.4Sn0.3Nb4O11, as shown in Figure 8 and given in Table 4. A visible change in the color of the Na2−2xSnxTa4−yNbyO11 powders from white to yellow was observed with an increase in the Nb-content and with Sn(II)exchange, as shown in the inset image in Figure 8. Similar to the Na2Ta4−yNbyO11 solid solutions, the largest red-shift (∼1.1 eV) was observed between the Ta-richest Na1.8Sn0.1Ta4O11 and Nb-richest Na1.4Sn0.3Nb4O11. The direct and indirect band gap

the tantalate structures, owing to the differences in the interatomic B−O bond distances. For example, the c lattice parameter, unit cell volume, and Ta−O interatomic distances of Ag2Ta4O11 are relatively shorter in comparison to its niobate analogue, Ag2Nb4O11.11,29,52,57,58 Results of the Rietveld refinements show that the Ta1/Nb1−O3 octahedron distances are shorter for the solid solutions (y = 1, 2.8) in comparison to the Ta1−O3 octahedron distances in the Na2Ta4O11 (y = 0) composition. The incorporation of Nb into the solid solutions also resulted in shorter Na1−O2 interatomic distances. Chemical Composition and Particle Characterization. Energy dispersive X-ray spectroscopy (EDS) was used to characterize the elemental compositions of t he Na2Ta4−yNbyO11 solid solutions, both before and after tinexchange. As shown in Table 2, the bulk elemental analyses of Table 2. Elemental Analysis of the Na2Ta4−yNbyO11 (1 ≤ y ≤ 3) Solid-Solutions by Energy Dispersive Spectroscopya

a

loaded composition

% Ta

% Nb

Ta =4−y

Nb =y

composition determined by EDS

Na2Ta3NbO11 Na2Ta2Nb2O11 Na2Ta1.2Nb2.8O11

84.4 58.0 36.5

15.6 42.0 63.5

3.4 2.3 1.5

0.6 1.7 2.5

Na2Ta3.4Nb0.6O11 Na2Ta2.3Nb1.7O11 Na2Ta1.5Nb2.5O11

Data were averaged from three separate sampling sites.

the Na2Ta4−yNbyO11 solid-solutions were relatively consistent with the loaded reaction compositions. The Ta/Nb ratios determined by EDS were slightly Nb-deficient compared to the loaded reaction compositions, but consistent with the refined compositions in Table 1. The Na2−2xSnxTa4−yNbyO11 samples showed variable amounts of Sn-content as a result of the partial exchange for sodium within each of the Na2Ta4−yNbyO11 solidsolutions. The Na/Sn ratios and the resulting compositions were calculated for each sample, with the percent of Sn(II) exchange ranging from ∼4.9% to ∼17.7%, as listed in Table 3. T h e h i g h e s t S n ( I I ) - c o n t e n t w a s fo u n d f o r t h e Na1.3Sn0.35Ta1.5Nb2.5O11 (∼17.7% Sn; 30% Ta) composition. The next highest Sn(II)-content was observed for Na1.4Sn0.3Nb4O11 (∼15.1% Sn) followed by Na1.4Sn0.3Ta3.4Nb0.6O11 (∼14.2% Sn; 75% Ta), Na1.6Sn0.2Ta2.3Nb1.7O11 (∼10.3% Sn; 50% Ta), and finally Na1.8Sn0.1Ta4O11 (4.9% Sn). No residual chloride was observed after the flux-mediated exchange reactions. Additional quantitative analysis of the compositions can be found in Tables S9− S16 and Figures S4−S5 in the Supporting Information. The particle morphologies, sizes, and surface features of the Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11 samples were investigated by SEM. The flux-mediated growth of Na2Ta4O11 particles with hexagonal-platelet to truncated tetrahedron morphologies has been reported to occur with the use of a K2SO4/Na2SO4 flux.6,27 The Na2Ta4O11 particles exhibited

Table 3. Elemental Analysis of the Na2‑2xSnxTa4−yNbyO11 Solid Solutions by Energy Dispersive Spectroscopya

a

compound

% Na

% Sn

Na = 2 − 2x

Sn = x

composition determined by EDS

Na2−2xSnxTa4O11 Na2−2xSnxTa3NbO11 Na2−2xSnxTa2Nb2O11 Na2−2xSnxTa1.2Nb2.8O11 Na2−2xSnxNb4O11

90.2 71.5 79.3 64.6 69.9

4.9 14.2 10.3 17.7 15.1

1.8 1.4 1.6 1.3 1.4

0.1 0.3 0.2 0.35 0.3

Na1.8Sn0.1Ta4O11 Na1.4Sn0.3Ta3.4Nb0.6O11 Na1.6Sn0.2Ta2.3Nb1.7O11 Na1.3Sn0.35Ta1.5Nb2.5O11 Na1.4Sn0.3Nb4O11

Data were averaged from three separate sampling sites. G


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Figure 5. SEM images of the Na2Ta4−yNbyO11 particles for the (a,f) Na2Ta4O11, (b,g) Na2Ta3NbO11 (75% Ta), (c,h) Na2Ta2Nb2O11 (50% Ta), (d,i) Na2Ta1.2Nb2.8O11 (30% Ta), and (e,j) Na2Nb4O11 compositions.

Figure 6. SEM images of the Na2−2xSnxTa4−yNbyO11 particles for the (a,f) Na1.8Sn0.1Ta4O11, (b,g) Na1.4Sn0.3Ta3NbO11 (75% Ta), (c,h) Na1.6Sn0.2Ta2 Nb2O11 (50% Ta), (d,i) Na1.3Sn0.35Ta1.2 Nb2.8O11 (30% Ta), and (e,j) Na1.4Sn0.3Nb4O11 compositions.

Figure 7. UV−vis diffuse reflectance spectra plotted as Tauc plots of (F(R) × hν)n vs hν (eV) for the (a) direct (n = 2) and (b) indirect (n = 1/2) band gap transitions of Na2Ta4−yNbyO11.

of the changes in the orbital contributions to both the conduction and valence band. The lower-energy conduction band is formed by the lower-energy Nb 4d orbitals and the higher-energy valence band is formed by the higher energy Sn 5s orbitals. Band Energies of Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11. Changes in the energies of the valence and conduction bands were investigated as a function of chemical compositions of the solid solutions using the Mott− Schottky electrical impedance technique. Shown in Figure 9, each sample exhibited a positive slope over a similar potential range, which is indicative of an n-type semiconductor. The flatband potential of each sample was calculated by extrapolating the linear portion of the plot to the x-ordinate to find Vo, allowing for the determination of the conduction and valence

transitions of the Na2−2xSnxTa4−yNbyO11 compositions all exhibit a single uniform absorption edge. The lowest-energy transition for the Na2−2xSnxTa4−yNbyO11 (1 ≤ y ≤ 4) compositions is the Sn 5s to Nb 4d band gap transition, followed by the higher-energy O 2p to Nb 4d band gap transition. Thus, the lowest-energy band gap excitations are the result of a metal-to-metal charge transfer between Sn(II) and Nb(V), such as previously reported for the SnNb2−yTayO6 solid solutions.18 The introduction of Sn(II) cations into these structures causes the formation of a new higher-energy valence band comprising the higher-energy Sn 5s orbitals, thereby lowering their band gaps to promote the absorption of visiblelight energies. The larger red-shift for the Na2−2xSnxTa4−yNbyO11 compositions, as compared to the Na2Ta4−yNbyO11 compositions, is the result of the combination H


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the Sn(II)-exchange reactions owing to the higher-energy Sn 5s orbital contributions. In all cases, the valence bands maintain significant overpotentials for water oxidation at pH 5.8 (+0.89 V). Generally, the valence band energy of each of the solid solutions shifts to more positive potentials with increasing Nbcontent. The small variation in the Sn(II)-content has a less pronounced effect on the energy of the valence band. As illustrated in Figure 10, the overall impact of the single-metal and double-metal solid solutions, i.e., Nb/Ta and Na/Sn, is to enable a significantly closer bracketing of the water splitting redox couples; enabling photocatalytic water splitting at lowerenergy visible-light wavelengths. Photocatalytic Hydrogen Production. Suspended particle photocatalysis of metal-oxide semiconductors occurs with the absorption of photons greater than that of the band gap energy, resulting in photogenerated electron−hole pairs that are separated at the surfaces and used to drive water reduction and/or oxidation reactions.4 Members of the Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11 solid solutions were investigated for their photocatalytic activities for hydrogen production as suspended powders in aqueous solutions, as shown in Figures 11 and 12, as well as listed in Table 4. The rates of photocatalytic gas production were measured under ultravioletvisible irradiation (>230 nm). The samples were prepared by photodeposition of a 1 wt % Pt cocatalyst, and the activities were measured in an aqueous solution of 20% methanol used as the sacrificial reagent. The Na 2 Ta 4 − y Nb y O 1 1 and Na2−2xSnxTa4−yNbyO11 samples showed good photostability with no bulk degradation observed by powder XRD after irradiation at ultraviolet/visible-light energies under various testing conditions, as shown in Figure S8 in the Supporting Information. All solid solution compositions were photocatalytically active for hydrogen production at relatively dilute concentrations, wherein the rates are dependent on the mass of the sample.63 The highest rate for the Na2Ta4−yNbyO11 solid-solution was observed for Na2Nb4O11 (∼84 μmol H2·g−1·h−1). The next highest photocatalytic rate was found for Na2Ta1.2Nb2.8O11 (∼75 μmol H2·g−1·h−1) followed by Na2Ta3NbO11 (∼70 μmol H2·g−1·h−1), Na2Ta2Nb2O11 (∼56 μmol H2·g−1·h−1), and finally

Table 4. Direct and Indirect Band Gaps Extrapolated from Tauc Plots, and Photocatalytic Rates of Hydrogen Production under Ultraviolet−Visible (λ > 230 nm) Irradiation composition

direct band gap (eV)

indirect band gap (eV)

rates (μmol H2·g−1·h−1)

Na2Ta4O11 Na2Ta3NbO11 Na2Ta2Nb2O11 Na2Ta1.2Nb2.8O11 Na2Nb4O11 Na1.8Sn0.1Ta4O11 Na1.4Sn0.3Ta3NbO11 Na1.6Sn0.2Ta2Nb2O11 Na1.3Sn0.35Ta1.2Nb2.8O11 Na1.4Sn0.3Nb4O11

4.5 4.0 3.9 3.7 3.8 3.7 2.9 2.75 2.65 2.6

4.3 3.8 3.7 3.6 3.6 3.4 2.6 2.5 2.4 2.3

34 70 56 75 84 27 105 46 124 31

band energies versus the water splitting redox couples. Plots of the linear regions for each sample can be found in Figures S6 and S7 in the Supporting Information section. As shown in Figure 10 and Table 5, the conduction band energies follow a trend to more positive potentials as the percentage of niobium was increased owing to the lower-energy Nb 4d orbitals. All potentials are given versus RHE. As the composition changes from Na2Ta4O11 to Na2Nb4O11, the conduction band shifted from −0.996 to −0.786 V while maintaining a significant overpotential for water reduction at pH 5.8 (−0.34 V). The shift in the conduction band energy for the Na2−2xSnxTa4−yNbyO11 solid solutions follows a similar trend with increasing Nb content. The largest change in the conduction band energy was found for the Na2Ta4O11 (100% Ta) and Na2Nb4O11 (0% Ta) samples with conduction band positions of −0.955 and −0.701 V, respectively. In the case for Na2Ta3NbO11 (75% Ta), Na2Ta2Nb2O11 (50% Ta), and Na2Ta1.2Nb2.8O11 (30% Ta), the conduction band positions were found to be −0.756, −0.779, and −0.786 V, respectively. The valence band energy of each of the Na2−2xSnxTa4−yNbyO11 samples were found to shift to more negative potentials after

Figure 8. UV−vis diffuse reflectance spectra plotted as Tauc plots of (F(R) × hν)n vs hν (eV) for the (a) direct (n = 2) and (b) indirect (n = 1/2) band gap transitions of Na2−2xSnxTa4−yNbyO11. Inset: image of the color change from white to yellow powders with increasing Nb-content and after Sn(II)-exchange. I


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Figure 9. Plots of Mott−Schottky data taken at 1 kHz for members of the (a) Na2Ta4−yNbyO11 and (b) Na2−2xSnxTa4−yNbyO11 solid solutions.

Table 5. Measured Flatband Potentials and Calculated Valence and Conduction Band Energies (versus RHE) Determined from Mott−Schottky Measurements composition

flatband potential (V)

valence band position (V)

conduction band position (V)

Na2Ta4O11 Na2Ta3NbO11 Na2Ta2Nb2O11 Na2Ta1.2Nb2.8O11 Na2Nb4O11 Na1.8Sn0.1Ta4O11 Na1.4Sn0.3Ta3NbO11 Na1.6Sn0.2Ta2Nb2O11 Na1.3Sn0.35Ta1.2Nb2.8O11 Na1.4Sn0.3Nb4O11

−0.932 −0.871 −0.780 −0.769 −0.747 −0.668 −0.750 −0.739 −0.705 −0.903

3.304 2.874 2.871 2.787 2.813 2.444 1.843 1.72 1.613 1.598

−0.996 −0.926 −0.829 −0.813 −0.787 −0.955 −0.756 −0.779 −0.786 −0.701

compositions, which may be indicative of shorter charge carrier diffusion path lengths across the disordered tantalate/niobate layers at the 50% composition. The highest photocatalytic hydrogen production rate for the Na2−2xSnxTa4−yNbyO11 solid solutions was observed for the Na1.3Sn0.35Ta1.2Nb2.8O11 composition (∼124 μmol H2·g−1·h−1). The next highest photocatalytic hydrogen production rate was for Na1.4Sn0.3Ta3NbO11 (∼105 μmol H2·g−1·h−1), followed by Na1.6Sn0.2Ta2Nb2O11 (∼46 μmol H2·g−1·h−1), Na1.4Sn0.3Nb4O11 (∼31 μmol H2·g−1·h−1), and Na1.8Sn0.1Ta4O11 (∼27 μmol H2· g −1 ·h −1 ). The Na 1.3 Sn 0.35 Ta 1.2 Nb 2.8 O 11 (30% Ta) and Na1.4Sn0.3Ta3NbO11 (75% Ta) compositions had the highest overall rates and also the highest Sn(II)-content. Conversely, the solid solutions with a lower Sn-content had lower photocatalytic rates than the Na-analogues. The hydrogen production rates decreased after Sn(II)-exchange for the Na 1.8 Sn 0.1 Ta 4 O 11 , Na 1.6 Sn 0.2 Ta 2 Nb 2 O 11 (50% Ta), and Na1.4Sn0.3Nb4O11 samples. These decreases may be due to the creation of lattice defects that can act as sites for electron− hole recombination. Recently, Takanabe et al. observed that the SnNb1.4Ta0.6O6 (30% Ta) solid-solution showed enhanced photocatalytic activity for hydrogen production compared to pure SnNb2O6.18 The higher activity for the 30% Ta sample was described as resulting from improved charge separation and a negative shift in the conduction band energy, similar to the results observed for the Na1.3Sn0.35Ta1.2Nb2.8O11 (30% Ta)

Figure 10. Band positions obtained from Mott−Schottky measurements for (a) Na2Ta4−yNbyO11 and (b) Na2−2xSnxTa4−yNbyO11, with values for the water redox couples shown as dashed lines on the electrochemical scale.

Na2Ta4O11 (∼34 μmol H2·g−1·h−1).6,27 Prior investigations have shown that the excited electrons are delocalized over the BO7 pentagonal bipyramidal layers within this structural family, enabling the diffusion of electrons to the particles’ surfaces.6,9−14,20,27,50,59−62 Therefore, photocatalytic rates for hydrogen production might be expected to decrease as a result of the Nb/Ta disorder, consistent with Na2Nb4O11 exhibiting the highest rates. However, the Na2Ta4−yNbyO11 solid solutions exhibit higher rates than for Na2Ta4O11, likely the result of the smaller band gaps and increased light absorption of the solid solution compositions. However, the photocatalytic hydrogen production rates were remarkably lower for the 50% Ta composition compared to the 75% Ta and 30% Ta J


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Figure 11. Photocatalytic hydrogen production versus time for (a) Na2Ta4−yNbyO11 and (b) Na2−2xSnxTa4−yNbyO11 under ultraviolet−visible (λ > 230 nm) irradiation.

nm), and with different surface cocatalysts (e.g., NiO, Ru). However, the highest and most consistent hydrogen production activity was observed for the 1 wt % Pt− Na1.3Sn0.35Ta1.2Nb2.8O11 samples under ultraviolet−visible irradiation (λ > 230 nm) in 20% methanol solution. Linear sweep voltammetry measurements of polycrystalline films of Na1.3Sn0.35Ta1.2Nb2.8O11 (30% Ta), plotted in Figure 13, confirmed n-type anodic photocurrents that were significantly larger under UV−vis irradiation (∼0.015 mA/cm2) than under visible-light irradiation alone (∼0.0007 mA/cm2). Control experiments show that a fraction of the anodic photocurrent, i.e., ∼25% to ∼50%, under ultraviolet irradiation arises from the TiO2 blocking layer alone, depending upon the applied potential, as shown in Figure S9 in the Supporting Information. Further investigations are necessary in order to determine the influence of the Sn(II)-exchanged Na2Ta4−yNbyO11 solid solution compositions on the long-term stability, efficiency of charge separation, and the diffusion of carriers to the surfaces for driving photocatalytic reactions.

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Figure 12. Plots of photocatalytic hydrogen production versus time for Na1.3Sn0.35Ta1.2Nb2.8O11 (30% Ta), including at different pH values, under UV−vis or visible-light only irradiation, and with a Rucocatalyst.

CONCLUSIONS The single-metal substituted Na2Ta4−yNbyO11 and doublemetal substituted Na2−2xSnxTa4−yNbyO11 solid solutions were prepared using solid-state and flux-mediated ion-exchange reactions within a SnCl2 flux, respectively. The structures of the Na2Ta4−yNbyO11 solid-solutions were investigated by Rietveld refinements and found to crystallize as either the rhombohedral Na2Ta4O11 or the monoclinic Na2Nb4O11, with a two-phase mixture occurring at y = 2.8, i.e., 70% Nb. The Nb(V) and Ta(V) cations were equally disordered over the crystallographic sites for the BO 6 octahedra and BO 7 pentagonal bipyramids. After Sn(II) exchange reactions, the amounts of Sn varied from x = 0.1 to x = 0.35. Both the Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11 solid-solutions showed a red-shift in their absorption edge with an increase in the Nb-content and after Sn(II)-exchange, with the largest red-shift of ∼2.0 eV between Na2Ta4O11 (∼4.3 eV) and Na1.4Sn0.3Nb4O11 (∼2.3 eV). The conduction band energies of Na2Ta4−yNbyO11 were shifted to more positive potentials with an increase in the Nb-content, while the valence band energies of Na2−2xSnxTa4−yNbyO11 were shifted to more negative potentials after the incorporation of Sn(II)-cations. All

composition (∼124 μmol H2·g−1·h−1).18 Previous reports of solid-solutions have shown no clear dependence of photocatalytic activity on variable compositions,18,19 but rather an activity that may fluctuate depending on composition, with an optimum composition yielding the highest activities, i.e., the Na1.3Sn0.35Ta1.2Nb2.8O11 (30% Ta) and Na1.4Sn0.3Ta3NbO11 (75% Ta) compositions. Photocatalytic activity for oxygen production was not detected for any of the Na2−2xSnxTa4−yNbyO11 solid-solutions, even with the aid of a 1 wt % RuO2 cocatalyst in an aqueous solution of 0.05 M AgNO3 as the sacrificial reagent. Total water splitting in deionized water also was not observed under ultraviolet−visible or visible-light irradiation with a photon flux of 300 mW/cm2. Shown in Figure 12, the most active photocatalyst, Na1.3Sn0.35Ta1.2Nb2.8O11 (30% Ta), was tested under several different reaction conditions in acidic solutions (pH ≈ 2) adjusted with HCl, basic solutions (pH ≈ 12) adjusted with NaOH, under visible-light irradiation (λ > 420 K


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Figure 13. Linear sweep voltammetry measurements of polycrystalline films of Na1.3Sn0.35Ta1.2Nb2.8O11 (30% Ta) under UV−visible or visible-light only irradiation. Conditions: TiO2 blocking layer between sample and FTO slide, aqueous 0.1 M KCl electrolyte, pH = 5.8.

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members of the Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11 solid solutions exhibited conduction and valence band energies to drive total water splitting and exhibited photocatalytic activity for hydrogen production.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05758. Additional Rietveld refinement data, EDS elemental analyses, and powder X-ray diffraction patterns of Na2Ta4−yNbyO11 and Na2−2xSnxTa4−yNbyO11 products before and after photocatalysis measurements (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (919) 515-3616. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support of this research from the Chemistry Scholars Graduate Research Assistantship (GSGRA) from the Department of Chemistry at North Carolina State University. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). L


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