Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published February 9, 2021 | public
Journal Article

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

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.

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.

Additional details

Created:
August 20, 2023
Modified:
October 23, 2023