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

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 degrees C for 24 h, respectively. The two types of salt fluxes resulted in large rod-shaped particles at 600 degrees C and smaller tetragonal prism-shaped particles at 400 degrees C. Surface photovoltage spectroscopy measurements produced a negative photovoltage under illumination >1.50 eV, which confirmed electrons as the majority charge carriers and similar to 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 (similar to 11.6 mu mol H-2 h(-1)) in comparison to the smaller tetragonal prism-shaped Sn2TiO4 particles (similar to 3.4 mu mol H-2 h(-1)). Conversely, for photocatalytic oxygen production, the rates for the smaller tetragonal prism-shaped particles in aqueous AgNO3 solution were slightly higher (similar to 16.3 mu mol O-2 h(-1)) than for the larger rod-shaped particles (similar to 11.9 mu mol 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.