posted on 2018-04-18, 00:00authored byGeorges Siddiqi, Zhenya Luo, Yujun Xie, Zhenhua Pan, Qianhong Zhu, Jason A. Röhr, Judy J. Cha, Shu Hu
Accomplishing acid-stable water oxidation
is a critical matter
for achieving both long-lasting water-splitting devices and other
fuel-forming electro- and photocatalytic processes. Because water
oxidation releases protons into the local electrolytic environment,
it becomes increasingly acidic during device operation, which leads
to corrosion of the photoactive component and hence loss in device
performance and lifetime. In this work, we show that thin films of
manganese-modified titania, (Ti,Mn)Ox,
topped with an iridium catalyst, can be used in a coating stabilization
scheme for acid-stable water oxidation. We achieved a device lifetime
of more than 100 h in pH = 0 acid. We successfully grew (Ti,Mn)Ox coatings with uniform elemental distributions
over a wide range of manganese compositions using atomic layer deposition
(ALD), and using X-ray photoelectron spectroscopy, we show that (Ti,Mn)Ox films grown in this manner give rise to
closer-to-valence-band Fermi levels, which can be further tuned with
annealing. In contrast to the normally n-type or intrinsic TiO2 coatings, annealed (Ti,Mn)Ox films
can make direct charge transfer to a Fe(CN)63–/4– redox couple dissolved in aqueous electrolytes. Using the Fe(CN)63–/4– redox, we further demonstrated
anodic charge transfer through the (Ti,Mn)Ox films to high work function metals, such as iridium and gold,
which is not previously possible with ALD-grown TiO2. We
correlated changes in the crystallinity (amorphous to rutile TiO2) and oxidation state (2+ to 3+) of the annealed (Ti,Mn)Ox films to their hole conductivity and electrochemical
stability in acid. Finally, by combining (Ti,Mn)Ox coatings with iridium, an acid-stable water-oxidation anode,
using acid-sensitive conductive fluorine-doped tin oxides, was achieved.