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

The n -type Sn 2 TiO 4 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 Sn 2 TiO 4 phase was synthesized using either a SnCl 2 flux or a SnCl 2 /SnF 2 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 Sn 2 TiO 4 particles were deposited and annealed as polycrystalline films on FTO slides, and exhibited photoanodic currents in aqueous solutions under visible-light irradiation. The Sn 2 TiO 4 particles were also suspended in aqueous methanol solutions and irradiated with visible and ultraviolet light. The larger rod-shaped Sn 2 TiO 4 particles had the higher rates of photocatalytic hydrogen production (∼11.6 μmol H 2 h –1 ) in comparison to the smaller tetragonal prism-shaped Sn 2 TiO 4 particles (∼3.4 μmol H 2 h –1 ). Conversely, for photocatalytic oxygen production, the rates for the smaller tetragonal prism-shaped particles in aqueous AgNO 3 solution were slightly higher (∼16.3 μmol O 2 h –1 ) than for the larger rod-shaped particles (∼11.9 μmol O 2 h –1 ). Apparent quantum yields of 0.995% and 0.0098% were measured for O 2 and H 2 production, respectively, under 435 nm light.