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Contact Angle Relaxation on Amorphous, Mixed-Phase (Anatase + Rutile), and Anatase TiO₂ Films and Its Mechanism

Son, Yohan and Lee, Min-Kyo and Park, Young-Chun (2021) Contact Angle Relaxation on Amorphous, Mixed-Phase (Anatase + Rutile), and Anatase TiO₂ Films and Its Mechanism. Langmuir, 37 (5). pp. 1850-1860. ISSN 0743-7463. https://resolver.caltech.edu/CaltechAUTHORS:20210129-160040407

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Abstract

TiO₂ films generally undergo contact angle relaxation in the dark. It has been suggested that carbon contamination and the loss of surface OH generated by UV may be the major causes. However, the mechanisms for the long-lasting hydrophilicity have not been fully understood. Here, we studied contact angle relaxation of amorphous, mixed-phase, and anatase, and a new mechanism is proposed. After UV exposure and oxygen plasma treatment, the films’ relaxation was observed over short-term (1 day) and long-term (>30 days) scales with XPS analysis using two quantitative parameters: relative amount and binding energy (B.E.) shifting. One day after plasma treatment, we observed that the donor–acceptor complex (DAC) and Ti–OH peaks of anatase shifted toward lower B.E., while the other films showed no shift or positive B.E. shifting. Interestingly, the relaxation of the amorphous and mixed-phase TiO₂ occurred over time despite the large number of total OH groups (I_(OH)/I_(bulk) > 75%) and DAC (I_(DAC)/I_(bulk) > 110%), and only the anatase film showed superhydrophilicity (∼10°) for 90 days. Also, the B.E. of all OH peaks increased over time, indicating that polarizable hydroxyls relaxed in the dark. Although the greater binding strength of Ti–OH and DAC on the anatase surface maintains long-lasting hydrophilicity, the loss of polarizable OH causes relaxation on the less-reactive TiO₂ films. Carbon contamination can also contribute to the relaxation over time. Taken together, we conclude that the surface energy, polarizable OH, and contaminants are the major factors affecting relaxation; this study gives a full picture of the mechanism integrated over some of the previously reported models.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/acs.langmuir.0c03259DOIArticle
ORCID:
AuthorORCID
Lee, Min-Kyo0000-0002-2714-179X
Park, Young-Chun0000-0002-3848-398X
Alternate Title:Contact Angle Relaxation on Amorphous, Mixed-Phase (Anatase + Rutile), and Anatase TiO2 Films and Its Mechanism
Additional Information:© 2021 American Chemical Society. Received: November 11, 2020; Revised: January 23, 2021; Published: January 29, 2021. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2020R1I1A3A04036543). Author Contributions. Y.S. and M.-K.L. contributed equally to this work. Y.S., M.-K.L., and Y.-C.P. conceived the idea and designed experiments. Y.S. performed all experiments. Y.S. and M.-K.L. analyzed the data. M.-K.L. prepared all figures for publication. Y.S. and M.-K.L. wrote the original draft. M.-K.L. and Y.-C.P. reviewed and edited the manuscript for publication. Y.-C.P. supervised the project. The authors declare no competing financial interest.
Funders:
Funding AgencyGrant Number
National Research Foundation of KoreaNRF-2020R1I1A3A04036543
Subject Keywords:Hydroxyls, Oxides, Contact angle, Plasma, Minerals
Issue or Number:5
Record Number:CaltechAUTHORS:20210129-160040407
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20210129-160040407
Official Citation:Contact Angle Relaxation on Amorphous, Mixed-Phase (Anatase + Rutile), and Anatase TiO2 Films and Its Mechanism. Yohan Son, Min-Kyo Lee, and Young-Chun Park. Langmuir 2021 37 (5), 1850-1860; DOI: 10.1021/acs.langmuir.0c03259
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107827
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:01 Feb 2021 15:06
Last Modified:11 Feb 2021 23:24

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