Flux Synthesis, Optical and Photocatalytic Properties ofn-type Sn₂TiO₄: Hydrogen and Oxygen Evolution under Visible Light

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 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, which confirmed electrons as the majority charge carriers and ∼1.50 eV as the effective band gap. Mott–Schottky measurements at pH 9.0 showed 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, and exhibited photoanodic currents in aqueous solutions under visible-light irradiation. The Sn2TiO4 particles were also suspended in aqueous methanol solutions and irradiated with visible and ultraviolet light. The larger rod-shaped Sn2TiO4 particles had the higher 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). Apparent quantum yields of 0.995% and 0.0098% were measured for O2 and H2 production, respectively, under 435 nm light.

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