Mesoporous metal
oxide film electrodes consisting of derivatized 5.5 μm thick
SnO<sub>2</sub> films with an outer 4.3 nm shell of TiO<sub>2</sub> added by atomic layer deposition (ALD) have been investigated to
explore unbiased water splitting on p, n, and p<sup>+</sup>n type
silicon substrates. Modified electrodes were derivatized by addition
of the water oxidation catalyst, [Ru(bda)(4-O(CH<sub>2</sub>)<sub>3</sub>PO<sub>3</sub>H<sub>2</sub>)-pyr)<sub>2</sub>], <b>1</b>, (pyr = pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate),
and chromophore, [Ru(4,4′-PO<sub>3</sub>H<sub>2</sub>-bpy)
(bpy)<sub>2</sub>]<sup>2+</sup>, <b>RuP</b><sup>2+</sup>, (bpy
= 2,2′-bipyridine), which form 2:1 <b>RuP</b><sup>2+</sup>/<b>1</b> assemblies on the surface. At pH 5.7 in 0.1 M acetate
buffer, these electrodes with a fluorine-doped tin oxide (FTO) back
contact under ∼1 sun illumination (100 mW/cm<sup>2</sup>; white
light source) perform efficient water oxidation with a photocurrent
of 1.5 mA/cm<sup>2</sup> with an 88% Faradaic efficiency (FE) for
O<sub>2</sub> production at an applied bias of 600 mV versus RHE (ACS Energy Lett., 2016, 1, 231−236). The SnO<sub>2</sub>/TiO<sub>2</sub>–chromophore–catalyst
assembly was integrated with the Si electrodes by a thin layer of
titanium followed by an amorphous TiO<sub>2</sub> (Ti/<i>a-</i>TiO<sub>2</sub>) coating as an interconnect. In the integrated electrode,
p<sup>+</sup>n-Si–Ti/<i>a</i>-TiO<sub>2</sub>–SnO<sub>2</sub>/TiO<sub>2</sub>|-2<b>RuP</b><sup>2+</sup>/<b>1</b>, the p<sup>+</sup>n-Si junction provided about 350 mV in added potential
to the half cell. In photolysis experiments at pH 5.7 in 0.1 M acetate
buffer, bias-free photocurrents approaching 100 μA/cm<sup>2</sup> were obtained for water splitting, 2H<sub>2</sub>O → 2H<sub>2</sub> + O<sub>2</sub>. The FE for water oxidation was 79% with
a hydrogen efficiency of ∼100% at the Pt cathode
