Flux Synthesis, Optical and Photocatalytic Properties of n-type Sn2TiO4

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Flux Synthesis, Optical and Photocatalytic Properties of ntype SnTiO: Hydrogen and Oxygen Evolution under Visible Light 2

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Jonathan Boltersdorf, Ian Sullivan, Timothy L Shelton, Zongkai Wu, Matthew Gray, Brandon Zoellner, Frank E Osterloh, and Paul A. Maggard Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02003 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016

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Chemistry of Materials

Flux Synthesis, Optical and Photocatalytic Properties of n-type Sn2TiO4: Hydrogen and Oxygen Evolution under Visible Light Jonathan Boltersdorf,† Ian Sullivan,† Timothy L. Shelton,

§

Zongkai Wu,§ Matthew Gray,

ǁ

Brandon

Zoellner,† Frank E. Osterloh, § and Paul A. Maggard*,† †

Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204

§

Department of Chemistry, University of California Davis, Davis, CA 95616

ǁ

Department of Chemistry, Allegheny College, Meadville, PA 16335

E-mail of corresponding author: [email protected] Office phone of corresponding author: (919) 515-3616

ABSTRACT The n-type Sn2TiO4 phase was synthesized using flux methods and found to have one of the smallest visible-light bandgap sizes known that also maintains suitable conduction and valence band energies for driving photocatalytic water-splitting reactions. The Sn2TiO4 phase was synthesized using either a SnCl2 flux or a SnCl2/SnF2 peritectic flux in a 2:1 flux-to-precursor ratio heated at 600 ˚C and 400 ˚C for 24 h, respectively. The two types of salt fluxes resulted in large rod-shaped particles at 600 ˚C and smaller tetragonal prism-shaped particles at 400 ˚C. Surface photovoltage spectroscopy measurements produced a negative photovoltage under illumination >1.50 eV, confirming electrons as the majority charge carriers and ~1.50 eV as the effective band gap. Mott-Schottky measurements at pH 9.0 show the conduction (-0.54 V vs. NHE) and valence band (+1.01 V vs. NHE) positions meet the critical thermodynamic requirements for total water splitting. The Sn2TiO4 particles were deposited and annealed as polycrystalline films on FTO slides, which exhibited photoanodic currents in aqueous solutions under visible-light irradiation. The Sn2TiO4 particles were also suspended in aqueous methanol solutions and

1 ACS Paragon Plus Environment


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irradiated with visible and ultraviolet light. The larger rod-shaped Sn2TiO4 particles had the highest rates of photocatalytic hydrogen production (~11.6 µmol H2·h-1), in comparison to the smaller tetragonal prism-shaped Sn2TiO4 particles (~3.4 µmol H2·h-1). Conversely, for photocatalytic oxygen production, the rates for the smaller tetragonal prism-shaped particles in aqueous AgNO3 solution were slightly higher (~16.3 µmol O2·h-1) than for the larger rod-shaped particles (~11.9 µmol O2·h-1).

INTRODUCTION A prominent challenge in the field of water-splitting photocatalysis has been the discovery of a metal oxide semiconductor that can absorb a significant fraction of the visible-light region for solar energy conversion. Application of an efficient artificial photosynthetic system has been of increasing importance owing to the global need for developing clean and renewable energy sources. When irradiated by sunlight, electron-hole pairs are photogenerated and separated in the depletion region of a metal-oxide semiconductor photocatalyst in order to oxidize and/or reduce adsorbed water at its surfaces. A material with an optical band gap of ≥1.23 eV is required in order to thermodynamically drive the water splitting reaction, with a lowest-energy conduction-band edge more negative than that of the proton reduction potential (-0.41 V vs. NHE at pH = 7) and a highest-energy valence band edge more positive than that of the water oxidation potential (+0.82 V vs. NHE at pH = 7).1–3 Photocatalysts reported with small band gaps typically exhibit low stability under irradiation in solution (e.g. Cu2O), while those with larger bandgap sizes have shown to be significantly more stable. However the latter are limited in efficiency owing to either very limited or no absorption of visible-light energies (e.g. TiO2).3 Titanium dioxide (TiO2) is an n-type wide band gap semiconductor that is the most widely studied material for solar energy conversion applications. Intense research efforts have been made in order to successfully develop a visible-light sensitive TiO2 catalyst with the appropriate energy band positions. The research approaches have included the incorporation of nitrogen, doping of transition metals, the anchoring of chromophoric dyes at its surfaces, the controlled growth of nano-TiO2, and many other methods.4–9 An alternative approach is the investigation of mixed-metal oxide semiconductors that


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introduce post-transition metals with filled ns2 states to increase the valence band energy and reduce the optical band gap (e.g., using Sn(II), Pb(II), Bi(III)).3,10–13 Recent investigations have shown that the incorporation of Sn(II), Pb(II), and Bi(III) has been successful in introducing higher energy valence band in order to shift the band gaps of metal-oxide photocatalysts into the visible-light energy region.11,12,14 Stannous oxide (SnO) has also been the subject of recent attention owing to its hole-mediated electrical conductivity (p-type) and its small band gap.15 While SnO has a relatively smaller bandgap size, it has been reported to fall within a relatively wide range (~2.5-3.4 eV), likely depending upon the quality/purity of the material.15–17 The smaller band gap energy for SnO is the result of the occupied 5s2 states hybridized with O 2p and empty Sn 5p states that causes a local distortion in the electron density and coordination environment, resulting in the layered litharge structure (i.e., PbO). In contrast, the rutile SnO2 phase exhibits electron-mediated electrical conductivity (n-type) and a larger optical band gap (~3.6 eV).16,18 The combination of Sn(II) with filled ns2 states and transition metal cations with unfilled nd0 states (e.g, Ti(IV)) in ternary metal-oxides, e.g., as found in SnM2O6 and Sn2M2O7 (M = Nb, Ta),14,19–23 has shown much promise as a route to new small bandgap semiconductors for driving visible-light photocatalytic water splitting. Unfortunately, a majority of the attempts to grow new phases in the ternary Sn/Ti oxide system (e.g., SnTiO3) have been synthetically challenging, such as resulting in the formation of rutile Ti1-xSnxO2 solid solutions and elemental Sn owing to the disproportionation of the Sn(II) cations.13,24–26 However, Kumada et al. reported the successful synthesis of Sn2TiO4 single crystals from potassium titanate precursors using a SnCl2 molten salt, with a recent report of the microwave assisted synthesis of Sn2TiO4 by Ohara et al.13,27 The optical and photocatalytic properties of Sn2TiO4 have not yet been reported. Reported herein is the flux-mediated synthesis and characterization of Sn2TiO4 and a detailed investigation of its optical, photocatalytic, and photoelectrochemical properties.

Particle sizes and

morphologies have been characterized by scanning electron microscopy (SEM), and elemental analyses of the particles were performed using energy dispersive X-ray spectroscopy (EDS). The optical, energetic, and photocatalytic properties of the metal-oxide semiconductors were investigated using UV-Vis diffuse



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reflectance spectroscopy (DRS), surface photovoltage (SPV) spectroscopy, Mott-Schottky measurements, suspended particle photocatalytic measurements of hydrogen and oxygen production in aqueous solutions, and photoelectrochemical (PEC) measurements.

EXPERIMENTAL METHODS Flux Synthesis of Sn2TiO4. The Sn2TiO4 phase was prepared using flux-mediated synthesis methods according to a previously reported procedure that utilizes the reaction of a potassium titanate precursor within a SnCl2 (Alfa Aesar, 99%) flux in a 2:1 flux-to-precursor molar ratio.13 First, the solidstate synthesis of the potassium titanate precursor, K2Ti2O5, was performed by combining reagent grade K2CO3 (Alfa Aesar, 99.9997%) and TiO2 (Alfa Aesar, 99.9%, anatase) in a 1.2:1 molar ratio. The reactants were ground in a mortar and pestle for 30 min, heated for 6 h at 700 ˚C within an alumina crucible in a box furnace, and radiatively cooled to room temperature to yield a white powder. The potassium titanate precursor and the SnCl2 flux were ground together under an argon atmosphere in a glovebox. The reactant mixture was loaded into a fused-silica tube and dried under dynamic vacuum at 200 ˚C.13 The evacuated and flame-sealed fused-silica reaction tube was subsequently heated for 24 h at 600 ˚C. The product was washed and sonicated using a sequence of acetone and deionized water to remove excess byproducts, and then dried at 80 ˚C for 12 h, resulting in black Sn2TiO4 single crystals with a ~55% yield. In order to obtain higher yields, new flux-synthesis reactions were also performed using a 50% SnCl2 and 50% SnF2 (Alfa Aesar, 97.5%) peritectic flux in a 2:1 flux-to-precursor molar ratio. The potassium titanate precursor and the SnCl2/SnF2 peritectic flux were ground together under an argon atmosphere in a glovebox. The reactant mixture was loaded into an evacuated and flame-sealed fusedsilica reaction tube that was subsequently heated for 24 h at 400 ˚C. The products were washed and centrifuged using a sequence of acetone and deionized water to remove excess byproducts, and then dried at 80 ˚C for 12 h, resulting in a black Sn2TiO4 powder with a >90% yield. Characterization. High-resolution powder X-ray diffraction (PXRD) data were collected on an


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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 and the data were transformed using the Kulbelka-Munk, F(R), function.28 Tauc plots were plotted as (F(R)×hν)n vs. hν (eV), where n = 2 for direct and n = ½ for indirect band gap transitions, which were approximated from the inflection point tangent to the linear portion of the absorption curve.29 High-resolution 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). X-ray photoelectron spectroscopy (XPS) measurements (SPECS System with PHOIBOS 150 Analyzer) were carried out in an ultra-high vacuum chamber. Al/Ag dual anode monochromatic Kα radiation (10-14 kV) was used as the excitation source. Spectra were analyzed using commercial CasaXPS software (version 2.3.16).30,31 Film Preparation. Polycrystalline films of Sn2TiO4 were prepared on TEC-15 fluorine-doped tin oxide (FTO) slides (Pilkington Glass Inc.). The FTO slides were first cleaned by sonicating in deionized water, followed by sonication in ethanol, and then finally acetone for 30 mins each. A 1 cm2 area was taped off using Scotch tape on the conductive side and the polycrystalline Sn2TiO4 powder was deposited by the doctor blade method. A solution of water/tert-butanol was used as the dispersant as previously described.32 Films were annealed under dynamic vacuum at 500 ˚C for 3 h (P ≤ 50 mTorr) and allowed to cool to room temperature. Surface Photovoltage (SPV) measurements. Surface photovoltage (SPV) measurements were performed using a Kelvin control 07 / probe (3 mm diameter, Delta PHI Besocke) system with a sensitivity of 1 mV. Sample films were mounted inside a home-built vacuum chamber and illuminated through the probe with monochromatic light from a 300 W Xe lamp filtered through an Oriel Cornerstone 130 monochromator (light intensity range: 0.1–0.3 mW/cm2). The vibrating probe-sample distance was kept constant at ca. 1 mm. SPV spectra were corrected for drift by subtracting a dark background. Measurements were performed under 10−4 mbar. In the study of thickness dependence, film thickness was



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measured by a Veeco Dektak profilometer. Mott-Schottky Measurements. Electrochemical impedance spectroscopy (EIS) was used to measure the flat band potential and conduction and valence band energies of Sn2TiO4 using a frequency of 1 kHz, an AC amplitude of 10 mV, and a potential range of -1.0 V to 0.2 V. Measurements were taken on a Princeton Applied Research potentiostat (Parstat-2263) in a custom Teflon cell using a 0.5 M Na2SO4 solution at pH 9.0, purged with N2 gas 30 min prior to, and during the experiments. Polycrystalline films of Sn2TiO4 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. Photoelectrochemical Measurements. After annealing, the films were mounted onto a window of a custom made Teflon electrochemical cell for photoelectrochemical (PEC) measurements in freshly prepared solutions of 0.5 M Na2SO4, or 0.5 M Na2SO4 and 0.05 M K4Fe(CN6), both at a pH ~9.0. The polycrystalline films 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. All solutions were purged with nitrogen gas for 30 min prior to, and during the experiments. Linear sweep voltammetry (LSV) measurements were taken from the open circuit voltage to +1.20 V at a scan rate of 20 mVs-1. Chronoamperometric measurements were obtained with an applied bias of +1.0 V for 1000 s. A high pressure Xenon lamp (Newport Corporation) was used as the light source, with either an ultraviolet cutoff filter (1000 nm > λ > 420 nm) or AM 1.5 G filter. The radiant power of the lamp was set to 100 mW/cm2 in both cases. Suspended Particle Photocatalysis Measurements. Photocatalytic hydrogen generation measurements of Sn2TiO4 samples were performed with and without a surface cocatalyst, i.e., a 1 wt% platinum cocatalyst deposted using the photochemical deposition (PCD) method.11,33,34 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 onto its surfaces. The suspension was irradiated in an outer-irradiation type fused-silica reaction cell using an 800 W Xenon arc-lamp under constant stirring. Platinum serves as a catalyst that


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facilitates high rates of proton reduction on the surfaces of metal oxides.11,35–38 Methanol was added as a sacrificial reagent to aid in the platinum formation on the particles’ surfaces.35 After irradiation, the samples were washed with deionized water to remove excess byproducts and then dried in an oven at 80 ˚C for 12 h. Photocatalytic oxygen generation measurements of Sn2TiO4 samples were performed without a cocatalyst, with a MnOx cocatalyst, and with a RuO2 cocatalyst. The MnOx cocatalysts were loaded by photochemical deposition (PCD) using the appropriate amount of Mn(NO3)2 (Alfa Aesar, 99.98%; 1 mg/mL).33,34 Each sample was immersed within an aqueous NaIO3 solution (Alfa Aesar, 99%, 0.05 M) in order to photodeposit 1 wt% MnOx water oxidation cocatalyst. The suspensions were irradiated in an outer-irradiation type fused-silica reaction cell using an 800 W Xe arc-lamp under constant stirring. The NaIO3 acts as an oxidizing agent and a sacrificial reagent to aid in the metal-oxide nano-island formation on the surfaces of the particles.39 After irradiation, the samples were washed with deionized water and dried in an oven at 80 ˚C for 12 h. The RuO2 cocatalysts were loaded via incipient wetness impregnation (IWI) by immersing each sample in an aqueous solution with the appropriate amount of RuCl3·xH2O (Strem, 99.9%; 1 mg/mL). The solutions were stirred at 100 ˚C until dry and subsequently annealed in air at 400 ˚C for 1 h in order to load 1 wt% RuO2 water oxidation cocatalyst.37,40–42 MnOx and RuO2 are known surface cocatalysts for water oxidation.37,39,42 The photocatalytic rates of hydrogen and oxygen production were measured for all Sn2TiO4 samples. An outer-irradiation type fused-silica reaction cell was irradiated using a Xenon arc-lamp with a photon flux of ~200 mW/cm2, equipped with an IR filter, under either ultraviolet-visible (λ > 230 nm) or only visible-light (λ > 420 nm) irradiation. All suspensions were degassed by purging with N2(g) and sonicated.11,38 Photocatalytic hydrogen generation measurements utilized ~50 mg of each sample suspended in a 20% aqueous methanol solution.43 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.44 Measurements of the photocatalytic hydrogen generation were recorded at 30 min intervals and calculated in units of µmol H2·h-1. Photocatalytic oxygen rates were measured using an aqueous



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AgNO3 (Alfa Aesar, 99.9%, 0.050 M) solution as the sacrificial reagent, which results in reduced Ag(s) at the particle’s surface.1,3

Oxygen production was measured every 10 min for the first hour in order to

record the initial rate before coverage of the reaction sites with Ag(s), and then every 30 min after the first hour. Additionally, photocatalytic oxygen production was measured using an aqueous NaIO3 solution (Alfa Aesar, 99%, 0.05 M) solution. The amount of evolved gases produced were measured volumetrically and the products were identified using a gas chromatograph (SRI MG #2; thermal conductivity detector).11,38 Apparent Quantum Yield Measurements. Apparent quantum yield measurements were performed with Pt-Sn2TiO4 (600 ˚C) and Sn2TiO4 (600 ˚C) for hydrogen and oxygen evolution, respectively. For hydrogen evolution, 100 mg of Pt-Sn2TiO4 particles were dispersed in 80 mL deionized water and 20 mL methanol. The water was purified to 18 MΩ·cm resistivity by a Nanopure II system. For oxygen evolution, 100 mg of Sn2TiO4 particles were dispersed in 100 mg deionized water with AgNO3 (0.050M). Both samples were dispersed in 100 mL quartz flask. The solutions were dispersed and sonicated for 30 minutes and then purged with N2 for 30 minutes to remove oxygen before irradiation. The flask was then purged with argon and the solution mixture was irradiated with a 435 nm LED lamp. The photon flux was measured by an International Light IL1400BL photometer equipped with a GaAsP detector for 280 to 660 nm sensitivity range at the flask surface to be ~800-810 mW·cm-2. The irradiation area of the LED is 2.54 cm2. The airtight irradiation system was connected to an SRI 8610C gas chromatograph to identify and measure the evolved gaseous products. Apparent quantum yield (AQY) was calculated using the following Equation 1 and 2 for hydrogen evolution and oxygen evolution, respectively: AQY% = AQY% =

,

,

Where , and

,

× 100

(1)

× 100

(2)

(mol) is the amount of hydrogen or oxygen evolved over the duration of the

irradiation, NA is Avogadro’s constant, h (m2·kg/s) is Planck’s constant, c (m/s) is the speed of light, P


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(W/m2) is the power density of the incident monochromatic light, S (m2) is the irradiation area, !" (m) is the wavelength of the incident monochromatic light and t (s) is the duration of incident light exposure.

RESULTS AND DISCUSSION Synthesis and Characterization. The Sn2TiO4 phase was characterized by powder X-ray diffraction (Figure 1) and the lattice constants were refined using the LATCON software (Sn2TiO4, P42/mbc (no. 135), a = 8.483(2) Å, c = 5.931(2) Å). The previously reported flux synthetic method was initially utilized in the preparation of Sn2TiO4. In this procedure, a potassium titanate precursor and a SnCl2 flux in a 2:1 flux-to-precursor molar ratio was heated for 24 h at 700 ˚C, which resulted in black rod-shaped single crystals of Sn2TiO4.13 Initial attempts using this synthetic method resulted in Sn2TiO4 single crystals, as well as high amounts of SiO2, TiO2, Ti1-xSnxO2, and Sn(s) side products. The large Sn2TiO4 single crystals could be manually isolated from the other products. The SiO2 resulted from heating the SnCl2 flux above its boiling point, causing the mixture to react with the fused-silica tube. Therefore, the reaction temperature was decreased below the boiling point of the SnCl2 flux at 600 ˚C. However, the TiO2 (anatase), Ti1-xSnxO2 (rutile), and Sn(s) side products were still present in the temperature range of 500-700 ˚C using the SnCl2 flux. These impurities could be washed out using a series of acetone and water washings with intermittent sonication. This purification method resulted in a ~55% yield of Sn2TiO4 single crystals with a minor TiO2 impurity (~5 wt%), as shown in Figure 1. Various reaction conditions and fluxes were utilized in an attempt to obtain a pure phase Sn2TiO4 in a greater yield. The Sn2TiO4 yield decreased as the 2:1 SnCl2 flux-to-reactant ratio was decreased to 1:1, and the reaction yielded only Ti1-xSnxO2 and Sn(s) as the flux ratio was increased to 5:1. The sensitivity of the flux ratio indicates that the reaction is not purely an ion-exchange reaction, but rather the SnCl2 flux acts as a reactant and must be loaded stoichiometrically. Solid-state reactions and fluxmediated reactions using SnO and TiO2, rather than the precursor, in a 2:1 stoichiometric ratio in air and under vacuum conditions yielded only Ti1-xSnxO2 and Sn(s). Flux-mediated reactions of the potassium titanate precursor with a SnF2 flux under similar conditions yielded only Ti1-xSnxO2 and Sn(s). Finally, a



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50% SnCl2 and 50% SnF2 peritectic flux in a 2:1 flux-to-precursor molar ratio resulted in a black Sn2TiO4 powder with a >90% yield. The flux mixture containing the potassium titanate precursor and the SnCl2/SnF2 peritectic flux was heated for 24 h at 400 ˚C, which yielded Sn2TiO4 with a minor TiO2 impurity (~5 wt%), as shown in Figure 1. This SnCl2/SnF2 flux mixture facilitated the low-temperature crystal growth of Sn2TiO4 in higher yields as a result of using a peritectic flux. Similar to eutectic reactions, a 50% SnCl2 and 50% SnF2 mixture reacted to form solid SnClF, which is a melt above its peritectic melting point of 200 ˚C.45,46 Metal-halide fluxes can act as mineralizers, with the halides and cations determining the melting point, viscosity, and supersaturation of the flux solvent system.10 Wada et al previously reported the decomposition of Sn2TiO4 into Ti1-xSnxO2 in air above 600 ˚C using thermogravimetric analysis;13 similar to the decomposition and disproportionation of Sn2TiO4 into Ti1-xSnxO2 and Sn under static vacuum reaction conditions. Higher yields of the Sn2TiO4 phase are attributed to the low temperature crystal growth conditions facilitated by the peritectic flux, which inhibits decomposition and disproportionation of the Sn(II) cations. The Sn2TiO4 products obtained using the SnCl2 and SnCl2/SnF2 peritectic fluxes in a 2:1 flux-to-precursor ratio heated at 600 and 400 ˚C for 24 h, respectively, were compared for all subsequent measurements. The Sn2TiO4 samples prepared by these two methods will be referred to in the ensuing results and discussion by their synthesis temperature, i.e., Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C). Structural Descriptions. The crystal structure for Sn2TiO4 was previously determined by Kumada et al., and forms in the tetragonal system with the space group P42/mbc. The structure of Sn2TiO4 is composed of one-dimensional chains of edge-sharing distorted TiO6 octahedra along the c-axis that are connected via two Sn(II) cations, as shown in Figure 2. Each TiO6 octahedra is bonded to eight cornersharing SnO3 with a pyramidal coordination geometry. The SnO3 pyramids are formed by the three nearest-neighbor oxygens from two adjacent chains of TiO6 octahedra. There are notable structural cavities within the crystal structure stabilized by the SnO3 pyramids via Van der Waals interactions of the stereoactive lone pairs on Sn(II).13,16,47 The crystal structure of Sn2TiO4 is isostructural to the low-


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temperature form of Pb3O4, which has only been reported for a few other materials including Sb2CoO4, Pb2SnO4, and Sb2ZnO4.16,48 The crystal growth of Sn2TiO4 is stabilized in the low-temperature Pb3O4 structure, as it simultaneously satisfies the coordination preferences of both the Sn(II) and Ti(IV) cations.13,16 Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The particle morphologies and sizes of Sn2TiO4 were investigated by high-resolution SEM imaging, while EDS was used to confirm the elemental compositions of Sn2TiO4 prepared by the two different preparation methods. The morphologies of Sn2TiO4 prepared using a SnCl2 flux at 600 ˚C and a SnCl2/SnF2 peritectic flux at 400 ˚C were observed to be rod-shaped and tetragonal prism-shaped particles, respectively, as shown in Figure 3. The particles with the rod-shaped morphologies are wellfaceted at the ends of the crystals, with flat smooth sides. The rod-shaped particles have an average length of ~20 µm with an average diameter of ~5 µm. The tetragonal prism-shaped particles are highly faceted and much smaller in size compared to the rod-shaped particles. The tetragonal prism-shaped particles have an average length of ~3.6 µm and an average width of ~2.3 µm. The morphological and size differences between the two samples are attributed to the different solubilities and crystal growth rates of the Sn2TiO4 crystallites in the two different molten solutions. The pertitectic SnClF phase forms at the interface between the two salts, forming a diffusion barrier, and generally causing the crystal growth of Sn2TiO4 to proceed at a slower rate than crystal growth in a single salt or eutectic flux.45,46 Utilization of the peritectic SnCl2/SnF2 flux decreases the reaction temperature and results in the lower solubility of the Sn2TiO4 phase, resulting in slower transport through the melt and the formation of many smaller particles. The tin-halide peritectic fluxes acted as mineralizers, with the two anion sizes effecting the melting point, viscosity, and supersaturation of the solvent system in order to yield the two distinct well-defined particle morphologies and sizes. The effects of the particle morphology and size of Sn2TiO4 on the photocatalytic properties will be examined in subsequent sections. Energy dispersive X-ray spectroscopy (EDS) was utilized to confirm the elemental composition of the two Sn2TiO4 particle morphologies. Comparison of the EDS of Sn2TiO4 prepared using a SnCl2



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flux at 600 ˚C and a SnCl2/SnF2 peritectic flux at 400 ˚C showed that there was no compositional difference between the two samples. No residual potassium, chloride, or fluoride ions were observed after the flux reactions. The elemental analysis resulted in a Sn:Ti ratio that matched the reported 2:1 composition for both samples.13 Additional EDS elemental analysis graphs and the quantitative analysis of the compositions calculated from the atomic percent can be found in Figure S1 and Tables S1-S2 in the Supporting Information. Optical Properties. The optical properties of Sn2TiO4 were measured by UV-Vis diffuse reflectance spectroscopy (DRS) in order to compare the band gaps and absorption profiles. UV-Vis DRS measurements were taken in order to obtain the reflectance of the Sn2TiO4 samples, which was subsequently transformed using the Kulbelka-Munk function. The Kubelka-Munk remission function (F(R)), shown in Equation 3, relates reflectivity (R), of a sample to its absorption coefficient (α), and scattering coefficient (S). FR =

&'() * = + ()

(3)

Assuming a constant scattering coefficient (S), F(R) is proportional to α and represents the absorbance of the sample.28,49,50 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 Equation 4. αhν = Ahν − Eg

(4)

where h is Planck’s constant, ν is frequency, A is a proportionality constant, and n denotes the nature of the bandgap 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 = ½) band gap transitions from the inflection point tangent to the linear portion of the absorption curve.29,51–53 Tauc plots of Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C) revealed the same indirect transition of ~1.55 eV, and slightly larger direct transitions of ~1.70 eV, as plotted in Figure 4.


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The electronic structure of Sn2TiO4 has been previously investigated by Walsh et al in order to characterize the atomic contributions to the conduction and valence bands and the nature of the band gap transitions.16 The band gaps calculated using DFT and hybrid-DFT methods were 1.2 eV and 2.1 eV, respectively. The highest-energy valence-band states consisted of filled Sn 5s-orbitals mixed with O 2porbitals, while the lowest-energy conduction-band states all primarily consist of empty Ti 3d-orbitals delocalized over the chains of edge-sharing TiO6 octahedra. Interactions of the O 2p-orbitals with the filled 5s and empty 5p orbitals of Sn(II) resulted in positively-shifted valence band states and a reduction in the bandgap size. The direct and indirect band gap transitions that differ by less than 5 meV were reported to occur at the Γ-point and between the X-point and the Γ-point, respectively.16,18 The band structure exhibited a high degree of band dispersion that is indicative of small effective masses and high mobility of the charge carriers.11,16,18,38,54 Excited electrons are thus predicted to diffuse along the titanate chains to reach the surface. Band Positions of Sn2TiO4. Mott-Schottky experiments were performed in order to determine the flat band potential (Vfb) and the valence and conduction band energies of Sn2TiO4. For n-type semiconductors, the inverse capacitance squared is related to the flat band potential in Equation 5, & 2

= 34

: 3;< 4 7 5 6 89

− ;=> −

?@ : 7

(5)

where C is the capacitance (F), e is the elementary charge (C), εr is the dielectric of the material (unitless), εo is the permittivity of free space (F/m), ND is the donor density (m-3), A is the geometrical area of the electrode (M2), Vo is the extrapolated applied voltage, k is Boltzmann’s constant (eV/K), and T is temperature (K).55 The dielectric constant for Sn2TiO4 has not been previously reported in the literature; therefore, dielectric constants based on similar materials (e.g., SnO, TiO2, etc) were used in the calculations.56,57 Shown in Figure 5 are the Mott-Schottky plots obtained from polycrystalline films of Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C) in 0.5 M Na2SO4 solution at pH 9.0. The inverse square of the capacitance is plotted against applied bias, and the flat band potential (Vfb) is determined by extrapolating the linear



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portion of the curve to the x-ordinate in order to give the value Vo. Table 1 lists the obtained values for the flatband, valence, and conduction band positions, with each showing very similar values. The positive slope of the Mott-Schottky plot indicates that Sn2TiO4 is an n-type material, in agreement with surface photovoltage and photoelectrochemical measurements (see below). A sample calculation of these values follows. The value obtained for Vo of Sn2TiO4 (400 ˚C) was -0.675 V vs. SCE. Converting this to the NHE yields a Vo value of -0.433 V, and thus a value of -0.46 V was obtained for the Vfb using Equation 6, ;=> = ;< −

?@ 7

(6)

The conduction band energy can be determined using Equation 7,58

A = ;=> − BC ∙ E 3 8 : F

(7)

where NC is the effective density of states (cm-3) in the conduction band approximated from previously reported values.55,59,60 The donor concentration (ND) can be determined from the slope of the linear portion of the graph using Equation 8, GEHIJ = 4

(8)

5 46 78 9

Using a calculated value of 2.4 x 1020 cm-3 for ND and 1.0 x 1019 cm-3 for NC, the conduction band was calculated to be at a potential of -0.54 +/-0.01 V vs. NHE. The difference between the conduction and valence band is equal to the band gap (1.55 eV), thus the valence band position is estimated at +1.01+/0.01 V vs. NHE, approximately 300 mV positive of the water oxidation potential at pH 9. The conduction and valence band energies are graphically shown in Figure 6 with reference to the electrochemical scale, vacuum scale, and redox potentials of water. As both band edges straddle the redox potentials of water, Sn2TiO4 is found to meet the thermodynamic requirements for water splitting, with a slightly greater overpotential for water oxidation than for water reduction. These experimental results compare to DFT calculations reported by Burton and Walsh, yielding a possible valence band position at -6.8 eV vs. vacuum (+2.3 V vs. NHE), and conduction band at -4.7 eV vs. vacuum (-0.2 V vs. NHE).16 Surface Photovoltage (SPV) Measurements. In order to identify the majority carrier type and to confirm the effective band gap of Sn2TiO4, surface photovoltage spectra were obtained for Sn2TiO4


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particle films on FTO substrates.61 The SPV and optical absorption spectra for Sn2TiO4 synthesized at 600˚C (Figure 7a) and 400˚C (Figure 7b) are very similar. Both show a negative photovoltage signal and an optical absorption onset under illumination above ~1.50 eV, confirming this value as the effective band gap of the material. The negative sign of the photovoltage is caused by majority charge carrier injection into the FTO substrate, as shown in the Figure 7a insert, and confirms that Sn2TiO4 is an n-type semiconductor.9,62–64 The SPV measurements were performed with a 31.5 +/-5.25 µm thick film of Sn2TiO4 (600 ˚C) on FTO and (b) an 18.6 +/-2.6 µm thick film of Sn2TiO4 (400 ˚C) on FTO. In both samples, the maximum voltage occurs near the peak of the optical absorption (2.7 eV). At higher photon energies the signal declines as a result of the lower light penetration depth and the decreasing photon flux available from the Xe lamp. Interestingly, the rod-shaped Sn2TiO4 (600°C) produce up to -215 mV, whereas for the Sn2TiO4 (400°C) particles the maximum voltage is -355 mV. The larger voltage value for the smaller Sn2TiO4 (400°C) particles is likely due to more efficient trapping of photoholes at the larger particle surface. This interpretation also agrees with the residual photovoltage of 0.14 V (at 5 eV) for this sample, when the illumination intensity approaches zero. Under these conditions, the residual voltage is from trapped charge carriers (~40% in this case). In comparison, the Sn2TiO4 (600°C) rods only trap 17% of the photoholes, based on the 0.037 V residual voltage. It is insightful to compare the measured voltage values to the theoretical maximum theoretical voltage (Vbi) of the Sn2TiO4-FTO contact, which can be estimated as the difference between the Sn2TiO4 conduction band edge (from Mott-Schottky) and the tabulated Fermi energy of the conducting FTO substrate (Equation 9).62 A voltage of 1.1 V should be possible according to ;>" = A2K,LMNOP − EQ,Q@ = −0.54 V − +0.56 ; = −1.10 ;

(9)

However, only 0.215 V (20% of the theoretical) is observed for the films of the Sn2TiO4 (600˚C)

particles, while 0.355 V (32 % of the theoretical) is observed for the films of the Sn2TiO4 (400˚C) particles. The disparity is likely a result of the low ~0.1 mW cm-1 illumination power density, and slow charge transport through the films, which prevents electrons from the top layer to diffuse to the substrate



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over the 30 min time scale of the photovoltage scan. Photoelectrochemical (PEC) Properties. While the synthesis and electronic structure of Sn2TiO4 have been reported,13,16 there have been no investigations of its PEC properties. Polycrystalline films of Sn2TiO4 were deposited onto FTO slides using the doctor blade method and annealed under vacuum at 500 ˚C for 3 h. After annealing Sn2TiO4 films, a minute amount of TiO2 was detected by powder X-ray diffraction (XRD), shown in Figure S3 in the Supporting Information. SEM images taken from a polycrystalline film of Sn2TiO4, Figure S6 in the Supporting Information, show the average film thickness to be ~55 µm with relatively large crystallites and high porosity. Films prepared from both Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C) exhibited similar photocurrent responses. Anodic photocurrents were observed for polycrystalline films of Sn2TiO4 during linear sweep voltammetry measurements under simulated sunlight (AM 1.5 G) and using a UV cutoff filter, shown in Figure 8. The observed photocurrents when using the UV cutoff filter are ~10% less than the photocurrents observed when using the AM 1.5 G filter. The UV cutoff filter was used in all subsequent measurements to measure the PEC response of Sn2TiO4 alone. Based on the photo onset potential for the anodic currents, the quasi Fermi level of Sn2TiO4 at pH = 9 can be estimated at -0.06 V NHE (-0.3 V SCE), i.e. at a ~0.40 V more positive potential than the flatband value (-0.46) from the Mott-Schottky analysis. The difference likely arises from an overpotential needed to drive water oxidation at the surfaces.51 The photoanodic currents in the LSV measurements increase with an increasing positive applied potential, indicating that Sn2TiO4 is an n-type material in agreement with Mott-Schottky and SPV experiments. Photocurrents of up to ~20 µA/cm2 are observed at +1.20 V (NHE) using a UV filter, Figure 8a. Shown in Figure 8b, chronoamperometric data in 0.5 M Na2SO4 at an applied bias of +1.00 V show a decay in the photocurrent over 1000 s, decreasing from ~12 µA/cm2 to ~2 µA/cm2. The facile redox mediator, [Fe(CN)6]3-/4-, which has a redox potential of +0.36 V (vs. NHE),65 was used to probe the origin of the photocurrents produced. The valence band energy of Sn2TiO4 is ~600 mV below the redox potential of [Fe(CN)6]3-/4-, and thus a greater overpotential to oxidize the mediator. Shown in Figure 8 are the chronoamperometric traces of Sn2TiO4 films with and without the redox mediator. Larger anodic


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photocurrents ranging from ~20 µA/cm2 to ~7 µA/cm2 are observed with the redox mediator. This is twice as stable as without the redox mediator, showing that the charge carriers are diffusing to and reacting at the surface/electrolyte interface. Thus, the increased photocurrents arise from the oxidation of water and/or [Fe(CN)6]3-/4-, and not from the self-oxidation of Sn2TiO4. These results are consistent with the photocatalysis measurements showing the production of oxygen, as described below. As there is twice the amount of overpotential for the [Fe(CN6)]3-/4- couple, the relative stability of these films calculated from the ratio of the final current density (J) and the initial current density (Jo) increased by a factor of 2. Dark LSV measurements, shown in Figure 5, show no oxidation or reduction peaks, indicating that no dark degradation of the film occurs owing to the applied bias. For these n-type films, the majority charge carriers (electrons) must diffuse from particle-to-particle over micrometer length scales in order to reach the backside FTO contact. By contrast, the minority charge carriers (holes) must only reach the surface/electrolyte interface of an individual particle of the film owing to the porosity of the film. Thus, irradiation of the films should yield a higher overall photocurrent when irradiated from the backside, i.e., from the side of the FTO/Sn2TiO4 interface, owing to the shorter distances for the excited electron to reach the FTO contact. In non-porous films this is not necessarily the case, as both carriers must diffuse the full width of the film. The Sn2TiO4 films were irradiated from both the back-side and the front-side, given in Figure S5 in the Supporting Information, with larger photocurrents occurring in the backside irradiated films. In addition, LSV measurements taken at pH values of 4 and 12 show relatively little change in the size of the photoanodic currents, as shown in Figure S4 in the Supporting Information. Further photoelectrochemical studies are necessary in order to investigate the long-term stability and the

efficiencies of the Sn2TiO4 films. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed for Sn2TiO4 (600 ˚C), Sn2TiO4 (400 ˚C), after heat treatment under dynamic vacuum (at 500 ˚C), and after photoelectrochemical (PEC) measurements. Survey scans of Sn2TiO4 confirm the presence of Sn 3d peak doublets, Ti 2p peak doublets, and the O 1s peak at the appropriate energies, as shown in Figure 9a and in Table S3 of the Supporting Information. Comparison of the detailed scans in Figure 9b-d of the Sn 3d, Ti



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2p, and O 1s binding energy peaks for each sample show no significant change in the binding energies between different preparation methods and after annealing. XPS measurements were also taken on a film that was scanned several times from -0.25 V to +1.2 V under chopped simulated sunlight (AM 1.5 G, 100 mW/cm2), and then irradiated for 1000 s with an applied bias of +1.0 V vs. SCE. Some small peak asymmetry was observed in the Sn 3d peak profiles, shown in Figure S7 in the Supporting Information. Deconvolution and fitting of the overlapping profiles of the 3d5/2 peak indicate that the Sn(II) was partially reduced at the surface of the films during PEC measurements, with an ~80% Sn(II) and an ~20% Sn0 contribution.31

While the reduction potential of Sn/Sn2+ (-0.14V) is more negative than the water

reduction potential of H+/H2, this result indicates that the reduced Sn0 is a result of photogenerated electrons not reaching the backside FTO contact in the films. This is also consistent with the higher photocurrents from the backside irradiated films. By contrast, no Sn(IV) peaks are observed in the XPS scan at higher binding energies, which indicates that Sn2TiO4 does not undergo self-oxidation during photoelectrochemical measurements under an anodic bias. Suspended Particle Photocatalysis. Suspended particle photocatalysis of metal-oxide semiconductors can be described as a three step process involving 1) the absorption of photons greater than the energy of the bandgap, 2) charge separation and diffusion of the photogenerated carriers to the surface active sites, and 3) reaction of the charge carriers with adsorbed chemical species at those active sites.10 The two flux-synthesized Sn2TiO4 particles with differing particle characteristics were each investigated for their photocatalytic activities for hydrogen and oxygen production from aqueous solutions, as shown in Figures 10-12, as well as listed in Table 2. The photocatalytic hydrogen activities were measured for Sn2TiO4 without surface cocatalysts and with a 1 wt% Pt water reduction cocatalyst in an aqueous solution of 20% methanol as the sacrificial reagent. SEM images of the Sn2TiO4 particles’ surfaces after the photochemical deposition of the Pt surface cocatalyst show that they deposit uniformly over its smooth faces, as shown in Figure S8. Prior studies have shown that the photodeposition of the Pt cocatalyst effectively maps the active reduction sites on the surfaces of metal oxides particles.35–37,39 The highest photocatalytic rate for hydrogen product was observed for the rod-shaped Sn2TiO4 (600 ˚C)


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particles with a Pt cocatalyst under ultraviolet-visible (~11.6 µmol H2·h-1) and visible-light irradiation (~2.5 µmol H2·h-1). In all cases the samples with the Pt cocatalyst had higher photocatalytic rates (~1.711.6 µmol H2·h-1) compared to those without a cocatalyst (~0-1.2 µmol H2·h-1). The photocatalytic hydrogen production rates were significantly lower overall for the tetragonal prism-shaped Sn2TiO4 (400 ˚C), as shown in Figure 10 and listed in Table 2. The photocatalytic oxygen activities were similarly measured for Sn2TiO4 without cocatalysts, with a 1 wt% MnOx water oxidation cocatalyst, and with a 1 wt% RuO2 water oxidation cocatalyst in an aqueous solution of 0.05 M AgNO3 as the sacrificial reagent. The rate of oxygen production sharply decreases over time as Ag(s) deposits onto the particles’ surfaces and acts to inhibit oxygen generation after the initial hour, as previously described by Domen et al.66–68 Therefore, the photocatalytic rates for oxygen production were evaluated from the initial rates, as shown in Figure 11 and given in Table 2. The rod-shaped Sn2TiO4 (600 ˚C) particles functioned most effectively without a cocatalyst for oxygen production under ultraviolet-visible and visible-light irradiation with initial rates of ~11.4 and ~11.9 µmol O2·h-1, respectively. The total amount of oxygen produced was more than double (~2.7-4.4 µmol O2·h-1) the total oxygen produced in the presence of the MnOx cocatalyst (~0.6-2.1 µmol O2·h-1). The RuO2 cocatalyst resulted in higher initial (~4.8-9.6 µmol O2·h-1) and total (~1.8-2.8 µmol O2·h-1) oxygen production rates in comparison to the MnOx cocatalyst, but was less effective than without a cocatalyst. Similarly, the tetragonal prism-shaped Sn2TiO4 (400 ˚C) particles functioned most effectively without a cocatalyst under visible-light irradiation with an initial rate ~16.3 µmol O2·h-1. In all cases the MnOx cocatalyst resulted in lower initial (~2.9-9.6 µmol O2·h-1) and total (~0.6-2.5 µmol O2·h-1) oxygen production rates. Oxygen production without a cocatalyst and with a RuO2 cocatalyst under ultravioletvisible light irradiation resulted in comparable initial (~9.3-9.9 µmol O2·h-1) and total (~2.4 µmol O2·h-1) rates. The average quantum yield (AQY) for the oxygen production measurements with Sn2TiO4

(600 ˚C) varied from 0.995% within the first 10 minutes to 0.123% after 6 hours of irradiation, as shown in Figure 13. The decrease in the AQY during oxygen evolution in a 0.050 M AgNO3



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aqueous solution was due to the deposition of Ag(s) on the Sn2TiO4 particles. Additional apparent quantum yield measurements and results are given in Figure S11 and Table S5 in the Supporting Information. Additionally, photocatalytic oxygen production using an aqueous 0.05 M NaIO3 solution as the sacrificial reagent without surface cocatalysts was used to examine oxygen production without the deposition of Ag(s) on the particles’ surfaces that acted to inhibit oxygen generation after the initial hour. Surprisingly, the total oxygen production rates of the rod-shaped Sn2TiO4 (600 ˚C) and the tetragonal prism-shaped Sn2TiO4 (400 ˚C) particles were very similar under ultraviolet-visible (~3.8 µmol O2·h-1) and visible-light (~1.6-1.9 µmol O2·h-1) irradiation, as shown in Figure 12 and given in Table 2. The total amount of oxygen produced using a NaIO3 sacrificial reagent without a cocatalyst (~1.6-3.8 µmol O2·h-1) was greater than the total amount of oxygen produced using a AgNO3 sacrifical reagent with a MnOx (~0.6-2.1 µmol O2·h-1) and a RuO2 cocatalyst (~1.6-2.8 µmol O2·h-1). Notably, the photocatalytic rates for oxygen production are similar to or greater than that for hydrogen production, i.e., 11.9 µmol O2·h-1 versus 11.6 µmol H2·h-1 for Sn2TiO4 (600oC). Given that the overall splitting of water produces oxygen and hydrogen in a 2:1 ratio, i.e. 2H2O(l) → 2H2(g) + O2(g), these photocatalytic data indicate that the rate of hydrogen production is the limiting rate owing to the relatively poorer electron mobility as found in the polycrystalline films.

Furthermore, total water splitting was attempted in deionized water, 0.5 M

Na2SO4 pH 9.0, and under higher illumination intensity (~500 mW/cm2) that showed no presence of hydrogen gas. However, photocatalytic oxygen gas production was observed during the reactions and confirmed by GC. The average apparent quantum yield (AQY) of Pt-Sn2TiO4 (600 ˚C) for

hydrogen evolution was measured to be 0.0098% under irradiation with a 435 nm LED lamp, listed in Table S4. The AQY was consistent during the 6 hours of hydrogen evolution under irradiation, with the exception of the first 50 minutes (AQY = 0.0073%), which is attributed to residual oxygen in the aqueous solution. Additional AQY measurements for hydrogen production are given in Figure S10 in the Supporting Information. These experiments confirm 20 ACS Paragon Plus Environment

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that hydrogen evolution is the rate limiting step for total water splitting using Sn2TiO4 as a photocatalyst.

The Sn2TiO4 samples were characterized by PXRD in order to investigate their photostability after prolonged irradiation during suspended particle photocatalysis measurements for hydrogen and oxygen production. The Sn2TiO4 photocatalysts did not show any degradation of their crystallinity for any of the samples under ultraviolet/visible-light irradiation for prolonged periods of time under various testing conditions, as shown in Figure S12 in the Supporting Information. Further investigations of the Sn2TiO4 phase and similar materials that incorporate Sn(II) and Ti(IV) cations will be key to determining the influence of the structural features, choice of cocatalyst, and synthesis methods on the photocatalytic activity.

CONCLUSIONS The Sn2TiO4 phase was prepared using the SnCl2 flux and SnCl2/SnF2 peritectic flux in a 2:1 fluxto-precursor ratio heated at 600 and 400 ˚C for 24 h, respectively. The two preparation methods resulted in larger rod-shaped Sn2TiO4 (600 ˚C) particles and smaller tetragonal prism-shaped Sn2TiO4 (400 ˚C) particles. Films of Sn2TiO4 produce negative photovoltage under illumination >1.50 eV, which confirms electrons as the majority charge carriers and ~1.50 eV as the effective band gap. The conduction (-0.54 V vs. NHE) and valence band (+1.01 V vs. NHE) positions at pH 9 meet the thermodynamic requirements for water splitting, allowing for an overpotential of ~300 mV for water oxidation. LSV measurements show anodic photocurrents at applied potentials positive of -0.25 V (SCE), indicating that Sn2TiO4 is an n-type material. This agrees with Mott-Schottky and SPV experiments, with photocurrents of up to ~20 µA/cm2 at an applied bias of +1.0 V using [Fe(CN6)]3-/4- as an electron donor. The larger rod-shaped Sn2TiO4 (600 ˚C) particles had the highest suspended particle photocatalytic hydrogen production rates (~11.6 µmol H2·h-1) in comparison to the smaller tetragonal prism-shaped Sn2TiO4 (400 ˚C) particles (~3.4 µmol H2·h-1). However, the photocatalytic oxygen production rates for the smaller tetragonal prism-



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shaped particles (~16.3 µmol O2·h-1) were much higher than the larger rod-shaped particles (~11.9 µmol O2·h-1) overall using a AgNO3 sacrificial reagent with no cocatalyst. Both the higher photocatalytic rates for oxygen production and the higher photocurrents for backside irradiation reveal that the electron mobility within the particles and through the films is the limiting factor in their photocatalytic activities and photoanodic currents, respectively, under visible-light irradiation.

ACKOWLEDGEMENTS The authors acknowledge support of this research from the Chemistry Scholars Graduate Research Assistantship (GSGRA) and the Research Experience for Undergraduates at the Interface of Computations and Experiments, funded by the National Science Foundation (CHE-1359377), from the Department of Chemistry at North Carolina State University. The authors acknowledge the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation. FEO thanks the Department of Energy for support (grant DE-SC0015329). SUPPORTING INFORMATION AVAILABLE Powder X-ray diffraction patterns of Sn2TiO4 products before and after photocatalysis measurements, surface photovoltage spectra, quantitative analysis of XPS data, initial rates of oxygen production, and additional apparent quantum yield measurements and results can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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of Graphitic Carbon Nitride Powder for Water Reduction and Oxidation under Visible Light. J. Phys. Chem. C 2009, 113, 4940–4947.



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Table 1. Flatband potential, conduction and valence band energies from Mott-Schottky calculations. Parameter Vfb EC EV

Values (V) Sn2TiO4 (600˚C) Sn2TiO4 (400˚C) -0.45 -0.46 -0.53 -0.54 +1.02 +1.01

Table 2. Suspended particle photocatalytic rates of hydrogen and oxygen production for Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C) obtained under ultraviolet-visible and visible-light irradiation.

Sample

Photocatalytic Conditions

UV-Vis (> 230 nm) Gas Production (µmol gas·h-1)

Visible (> 420 nm) Gas Production (µmol gas·h-1)

Initial Total Initial Total Sacrificial Cocatalyst H2 H2 a.b a.b a.b Reagent O2 O2 O2 O2a.b Sn2TiO4 20% 1 wt% Pt 11.6 0 0 2.5 0 0 (600˚C) MeOH 20% No 0.9 0 0 0 0 0 MeOH Cocatalyst 0.05 M 1 wt% 0 6.9 2.1 0 2.9 0.6 AgNO3 MnOx 1 wt% 0.05 M 0 9.6 2.8 0 4.8 1.8 RuO2 AgNO3 0.05 M No 0 11.4 4.4 0 11.9 2.7 AgNO3 Cocatalyst 0.05 M No 0 N/A 3.8 0 N/A 1.9 NaIO3 Cocatalyst 20% Sn2TiO4 1 wt% Pt 3.5 0 0 1.7 0 0 (400˚C) MeOH 20% No 1.2 0 0 0 0 0 MeOH Cocatalyst 0.05 M 1 wt% 0 7 1.8 0 9.6 2.5 AgNO3 MnOx 0.05 M 1 wt% 0 9.9 2.4 0 6.1 1.6 AgNO3 RuO2 0.05 M No 0 9.3 2.4 0 16.3 3.4 AgNO3 Cocatalyst 0.05 M No 0 N/A 3.8 0 N/A 1.6 NaIO3 Cocatalyst a The rates are given for O2 generation as initial gas produced within first 1 h and total gas produced. b The initial rate of O2 evolution was calculated based on the first 1 h, as the concurrent photodeposition of Ag(s) at the surfaces decreased the rate over time.


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Figure 1. Calculated (black) and experimental powder X-ray diffraction patterns of (a) Sn2TiO4 (600 ˚C) and (b) Sn2TiO4 (400 ˚C) prepared using a (a) SnCl2 flux (red) and a (b) SnCl2/SnF2 peritectic flux (green). The TiO2 anatase peaks are indicated by an asterisk. The most intense peaks are labeled with their Miller indices (hkl) on the calculated pattern of Sn2TiO4.

Figure 2. Polyhedral views of Sn2TiO4 along the (a) a-axis and the (b) c-axis with one-dimensional chains of edge-sharing TiO6 octahedra surrounded by Sn(II) cations. The unit cell is outlined in black with the atoms indicated in blue for titanium, purple for tin, and red for oxygen.



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Figure 3. Scanning electron microscopy images of the Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C) particles synthesized using a (a-c) SnCl2 flux and a (d-f) SnCl2/SnF2 peritectic flux that exhibit rod- and tetragonal prism-shaped morphologies, respectively.

Figure 4. UV-Vis diffuse reflectance spectra as Tauc plots of (F(R)×hν)n vs. hν (eV) for the (a) direct (n = 2) and (b) indirect (n = ½) band gap transitions of the Sn2TiO4 samples. Inset: image of black Sn2TiO4 crystals.


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Figure 5. Mott-Schottky plots collected at 1 kHz for (a) Sn2TiO4 (600 ˚C) and (b) Sn2TiO4 (400 ˚C). Insets: Linear portion of the Mott-Schottky plots extrapolated to the x-ordinate.

Figure 6. Band positions obtained from Mott-Schottky measurements relative to the electrochemical and vacuum scales.



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Figure 7. SPV spectrum (black) for films (a) of Sn2TiO4 (600 ˚C) and (b) Sn2TiO4 (400 ˚C) on FTO. Diffuse reflectance spectra (blue) are shown for comparison. (Inset in a) Schematic mechanism for charge generation and injection under bandgap illumination of the Sn2TiO4 films.


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Figure 8. Linear sweep voltammetry of polycrystalline films under AM 1.5 G irradiation (black) and under a UV cutoff filter (red) in 0.5 M Na2SO4 (a). Chronoamperometric curve of Sn2TiO4 in 0.5 M Na2SO4 (black) and in 0.5 M Na2SO4 with 0.05 M [Fe(CN)6]3-/4- (red), with an applied bias of +1.0 V vs. SCE (b).



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Figure 9. XPS (a) survey scan of the Sn2TiO4 products showing the Sn 3d, Ti 2p, and O 1s binding energy peaks, and (b-d) detailed scans for the Sn 3d, Ti 2p, and O 1s binding energy peaks.


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Figure 10. Photocatalytic hydrogen production (µmol H2) versus time (h) for Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C) under ultraviolet-visible (λ > 230 nm) and visible-light (λ > 420 nm) irradiation.

Figure 11. Photocatalytic oxygen production (µmol O2) versus time (h) for Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C) under (a) ultraviolet-visible (λ > 230 nm) and (b) visible-light (λ > 420 nm) irradiation in the presence of 0.050 M AgNO3.



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Figure 12. Photocatalytic oxygen production (µmol O2) versus time (h) for Sn2TiO4 (600 ˚C) and Sn2TiO4 (400 ˚C) under (a) ultraviolet-visible (λ > 230 nm) and (b) visible-light (λ > 420 nm) irradiation in the presence of 0.050 M NaIO3.


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Figure 13. Apparent quantum yield (%) versus time (h) for Sn2TiO4 (600 ˚C) under 435 nm LED irradiation in the presence of 0.050 M AgNO3 for oxygen evolution.



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Table of Contents Graphic:


Flux Synthesis, Optical and Photocatalytic Properties of n-type Sn2TiO4

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