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Failure modes of protection layers produced by atomic layer deposition of amorphous TiO₂ on GaAs anodes

Buabthong, Pakpoom and Ifkovits, Zachary P. and Kempler, Paul A. and Chen, Yikai and Nunez, Paul D. and Brunschwig, Bruce S. and Papadantonakis, Kimberly M. and Lewis, Nathan S. (2020) Failure modes of protection layers produced by atomic layer deposition of amorphous TiO₂ on GaAs anodes. Energy and Environmental Science, 13 (11). pp. 4269-4279. ISSN 1754-5692. https://resolver.caltech.edu/CaltechAUTHORS:20201007-073929719

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Abstract

Amorphous titanium dioxide (a-TiO₂) films formed by atomic layer deposition can serve as protective coatings for semiconducting photoanodes in water-splitting cells using strongly alkaline aqueous electrolytes. Herein, we experimentally examine the mechanisms of failure for p⁺-GaAs anodes coated with a-TiO₂ films (GaAs/a-TiO₂). Galvanic displacement of exposed GaAs by Au allowed imaging of pinholes in the a-TiO₂ coatings, and enabled collection of quantitative and statistical data associated with pinhole defects. A combination of imaging, electrochemical measurements, and quantitative analyses of corrosion products indicated that extrinsic pinholes were present in the a-TiO₂ films before electrochemical operation. During electrochemical operation these pinholes led to pitting corrosion of the underlying GaAs substrate. The dominant source of pinholes was the presence of atmospheric particulate matter on the GaAs surface during deposition of the a-TiO₂ layer. The pinhole density decreased substantially when the thickness of the a-TiO₂ coating increased beyond 45 nm, and approached zero when the thickness of the film exceeded 112 nm. The density of pinholes in films thinner than 45 nm decreased when the a-TiO₂ coating was deposited in an environmentally controlled cleanroom. Pinhole-free GaAs/a-TiO₂ devices were also tested via chronoamperometry to quantify the rate of pinhole formation during electrochemistry. The time-to-failure increased with thickness, suggesting that the failure mechanism may involve diffusion or migration through the film. However, other mechanisms may also contribute to the degradation of thicker films (>112 nm). Nevertheless, as previously hypothesized, extrinsic pinhole defects formed during deposition and testing control the short-term protective performance of the a-TiO₂ film for GaAs anodes evolving O₂ from water.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1039/d0ee02032jDOIArticle
http://www.rsc.org/suppdata/d0/ee/d0ee02032j/d0ee02032j1.pdfPublisherSupplementary Information
ORCID:
AuthorORCID
Buabthong, Pakpoom0000-0001-5538-138X
Kempler, Paul A.0000-0003-3909-1790
Chen, Yikai0000-0002-2955-9671
Nunez, Paul D.0000-0001-7039-0516
Brunschwig, Bruce S.0000-0002-6135-6727
Papadantonakis, Kimberly M.0000-0002-9900-5500
Lewis, Nathan S.0000-0001-5245-0538
Additional Information:© 2020 The Royal Society of Chemistry. Submitted 26 Jun 2020; Accepted 23 Sep 2020; First published 23 Sep 2020. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. We gratefully acknowledge a gift from the Lam Research Unlock Ideas program. Deposition of a-TiO₂ in a cleanroom was performed in the Kavli Nanoscience Institute (KNI) at Caltech, and we thank the KNI staff for their assistance during fabrication. There are no conflicts of interest to declare.
Group:JCAP, Kavli Nanoscience Institute
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-SC0004993
Issue or Number:11
Record Number:CaltechAUTHORS:20201007-073929719
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20201007-073929719
Official Citation:Failure modes of protection layers produced by atomic layer deposition of amorphous TiO₂ on GaAs anodes. Energy Environ. Sci., 2020, 13, 4269-4279; doi: 10.1039/d0ee02032j
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105852
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:07 Oct 2020 15:19
Last Modified:24 Nov 2020 22:51

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