11:15 AM - ES11.06.03
Metastable Intermediates in Amorphous Titanium Oxide—A Hidden Role Leading to Ultra-Stable Photoanode Protection
Lazarus German1,Yanhao Yu1,2,Xudong Wang1
University of Wisconsin-Madison1,Harvard University2
Among many functional materials, amorphous metal oxides, particularly the thin-film morphology synthesized by atomic layer deposition (ALD), are critical components in many modern energy systems, such as batteries,[1,2] catalysts,[3–5] solar cells,[6,7] and photoelectrochemical (PEC) electrodes.[8,9] One representative example is the use of ALD amorphous TiO2 films to protect Si PEC electrodes for fuel generation. However, a wide variety of performances have been obtained from ALD TiO2 by varying the Ti precursor, growth temperature, and thickness despite a longstanding, yet likely overlooked expectation that ALD films are homogeneous. Our recent observations have shown structural homogeneities in amorphous ALD TiO2 films identified as metastable intermediates: kinetically trapped structures between the metastable amorphous phase and the equilibrium crystalline phase.[11,12] These intermediates enable new and enhanced properties by which their presence significantly change the performance of the overall film.
Metastable intermediates represent a nonequilibrium state of matter that impose profound impacts to materials properties beyond our understandings of monolithic and equilibrium systems. We have discovered hidden metastable intermediates in amorphous TiO2 thin films and their critical role in electrochemical damage. These intermediates have a non-bulk crystal-like structure and exhibit significantly higher electrical conductivity than both the amorphous and the crystalline phases. When these TiO2 films are applied to protect Si PEC photoanodes, the intermediates can induce localized high electrochemical currents that largely accelerate the etching of the TiO2 film and the Si electrode underneath. The intermediates can be effectively suppressed by raising their nucleation barrier via reducing the film thickness from 24 to 2.5 nm. The homogeneous amorphous TiO2-film-coated Si photoanodes achieved more than 500 h of PEC water oxidation at a steady photocurrent density of over 30 mA/cm2.
These results reveal an additional pathway for optimizing amorphous oxides grown by ALD: controlling the phase composition of the amorphous and intermediate phases is crucial for optimizing the performance of oxide protected electrodes. In addition to thickness, manipulating other deposition parameters such as temperature, precursor choice, pulse and purge times, and doping offer multiple avenues for achieving greater protection performance by impeding the formation of intermediates.
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