Fto substrates

Photoelectrochemical
measurements were performed on FTO substrates (Sigma-Aldrich) with
a conductivity of ca. 7 Ω sq^–1 that were cut
into 0.5 × 2.5 cm² pieces and cleaned as described
above. Kapton tape was used to mask off a portion of the substrate
for electrode contact. Photoelectrochemical responses for both polymorphs
of Cu₂SnS₃ were performed using a BASi Epsilon-EC
potentiostat. A 3-neck flask was used with a Pt-wire counter electrode
and a Pt-wire pseudoreference electrode. The working electrodes were
tetragonal and orthorhombic Cu₂SnS₃ films drop-cast
on FTO-coated glass and annealed to 330 °C. The tetragonal Cu₂SnS₃ electrode was further annealed at 550 °C
to improve the rigidity of the working electrode and prevent delamination
during measurements. An aqueous 0.1 M Na₂S/0.01 M sulfur
electrolyte was made from nitrogen-sparged deionized water. For photoelectrochemical
experiments, a standard laboratory white light placed ca. 15 cm from
the samples was used to illuminate the working electrode. The total
illumination areas of the tetragonal and orthorhombic Cu₂SnS₃ working electrodes were ∼0.75 cm².

For the sdH and rgH syntheses, β-FeOOH was first grown on fluorine-doped tin oxide (FTO) substrates (∼7 Ω sq^–1, Sigma) in a solution containing 0.15 M iron(iii) chloride hexahydrate (FeCl₃, 97%, Alfa Aesar) and 1 M sodium nitrate (NaNO₃, 99%, Alfa Aesar). The deposition was carried out at 100 °C for 1 h. After rinsing, the electrodes were annealed in a tube furnace at 800 °C for 5 minutes to convert β-FeOOH into hematite. For the rgH electrodes they were subjected to the same procedure one more time.
For aH (ALD) samples a Cambridge Nanotech, Savannah 100 atomic layer deposition apparatus was used. Iron tert-butoxide (heated to 125 °C) and water (25 °C) were used as the precursors for hematite and were pulsed alternatingly into the deposition chamber (heated to 180 °C) with a 10 cm³ min^–1 flow of N₂ as carrier gas. 500 cycles was used to produce films between 20–30 nm thick, grown onto FTO.26 Following the deposition, the samples were annealed at 500 °C for 15 minutes in air to assure all samples were completely converted into hematite.