All-in-One Derivatized Tandem pn-Silicon-SnO/TiO Water Splitting Photoelectrochemical Cell.

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 μm thick SnO films with an outer 4.3 nm shell of TiO added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and pn type silicon substrates. Modified electrodes were derivatized by addition of the water oxidation catalyst, [Ru(bda)(4-O(CH)POH)-pyr)], 1, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate), and chromophore, [Ru(4,4'-POH-bpy) (bpy)], RuP, (bpy = 2,2'-bipyridine), which form 2:1 RuP/1 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; white light source) perform efficient water oxidation with a photocurrent of 1.5 mA/cm with an 88% Faradaic efficiency (FE) for O production at an applied bias of 600 mV versus RHE ( ACS Energy Lett. , 2016 , 1 , 231 - 236 ). The SnO/TiO-chromophore-catalyst assembly was integrated with the Si electrodes by a thin layer of titanium followed by an amorphous TiO (Ti/a-TiO) coating as an interconnect. In the integrated electrode, pn-Si-Ti/a-TiO-SnO/TiO|-2RuP/1, the pn-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 were obtained for water splitting, 2HO → 2H + O. The FE for water oxidation was 79% with a hydrogen efficiency of ∼100% at the Pt cathode.