Stable
Solar-Driven Water Oxidation to O<sub>2</sub>(g) by Ni-Oxide-Coated
Silicon Photoanodes
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Abstract
Semiconductors
with small band gaps (<2 eV) must be stabilized
against corrosion or passivation in aqueous electrolytes before such
materials can be used as photoelectrodes to directly produce fuels
from sunlight. In addition, incorporation of electrocatalysts on the
surface of photoelectrodes is required for efficient oxidation of
H<sub>2</sub>O to O<sub>2</sub>(g) and reduction of H<sub>2</sub>O
or H<sub>2</sub>O and CO<sub>2</sub> to fuels. We report herein the
stabilization of np<sup>+</sup>-Si(100) and n-Si(111) photoanodes
for over 1200 h of continuous light-driven evolution of O<sub>2</sub>(g) in 1.0 M KOH(aq) by an earth-abundant, optically transparent,
electrocatalytic, stable, conducting nickel oxide layer. Under simulated
solar illumination and with optimized index-matching for proper antireflection,
NiO<sub><i>x</i></sub>-coated np<sup>+</sup>-Si(100) photoanodes
produced photocurrent-onset potentials of −180 ± 20 mV
referenced to the equilibrium potential for evolution of O<sub>2</sub>(g), photocurrent densities of 29 ± 1.8 mA cm<sup>–2</sup> at the equilibrium potential for evolution of O<sub>2</sub>(g),
and a solar-to-O<sub>2</sub>(g) conversion figure-of-merit of 2.1%