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Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation

Science  30 May 2014:
Vol. 344, Issue 6187, pp. 1005-1009
DOI: 10.1126/science.1251428

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Keeping semiconductors safe from harm

Solar cells harvest the energy of sunlight to create electricity, but electricity is hard to store. Solar cells could also be used to make hydrogen from water, which can be stored as a fuel. Separating water into hydrogen and oxygen, however, presents challenges, especially if this is done directly by illuminating the anode that oxides water. Under the acidic or alkaline conditions needed for practical devices, semiconducting anode materials corrode during operation. Hu et al. now show that amorphous titanium dioxide coatings can protect semiconductors from alkaline corrosion while still allowing light through.

Science, this issue p. 1005

Abstract

Although semiconductors such as silicon (Si), gallium arsenide (GaAs), and gallium phosphide (GaP) have band gaps that make them efficient photoanodes for solar fuel production, these materials are unstable in aqueous media. We show that TiO2 coatings (4 to 143 nanometers thick) grown by atomic layer deposition prevent corrosion, have electronic defects that promote hole conduction, and are sufficiently transparent to reach the light-limited performance of protected semiconductors. In conjunction with a thin layer or islands of Ni oxide electrocatalysts, Si photoanodes exhibited continuous oxidation of 1.0 molar aqueous KOH to O2 for more than 100 hours at photocurrent densities of >30 milliamperes per square centimeter and ~100% Faradaic efficiency. TiO2-coated GaAs and GaP photoelectrodes exhibited photovoltages of 0.81 and 0.59 V and light-limiting photocurrent densities of 14.3 and 3.4 milliamperes per square centimeter, respectively, for water oxidation.

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