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.

Surface photovoltage spectroscopy (SPS) was used to observe photochemical charge separation and oxidation reactions on Fe 2 O 3 nanorod arrays under zero applied bias. Nanorod films were grown from FeCl 3 under hydrothermal conditions followed by calcination at 550 ºC. A negative photovoltage of up -130 mV is observed under 2.0 -4.5 eV (0.1 mW cm -2 ) illumination, confirming 2.0 eV as the effective bandgap of the material, and electrons as majority carriers. SPS in the presence of air, nitrogen, water, oxygen, and under vacuum suggest that the photovoltage is associated with the oxidation of surface water and with reversible surface hole trapping on the 1 min time scale and de-trapping on the 1 h time scale. O 2 promotes water oxidation by increasing the concentration of surface holes. Sacrificial donors KI, H 2 O 2 or potassium hydroxide increases the voltage to -240 and -400 mV, due to improved hole transfer. Cobalt oxide and Co-Pi cocatalysts quench the voltage, which is tentatively attributed to the removal of surface states and enhanced e/h recombination. An energy diagram is used to relate the experimental photovoltage to the built-in potentials at the respective interfaces.

The t-CNx layer functions as (1) a protection layer to isolate CdS from the electrolyte and (2) as a tunnel junction to promote charge separation.

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