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Superwetting of TiO2 by light-induced water-layer growth via delocalized surface electrons

  1. Wonho Jhea,1
  1. aInstitute of Applied Physics, Department of Physics and Astronomy, Seoul National University, Gwanak-gu, Seoul 151-747, Korea; and
  2. bPark Systems, Iui-Dong 906-10, Suwon 443-270, Korea

  1. Edited by Alenka Luzar, Virginia Commonwealth University, Richmond, VA, and accepted by the Editorial Board March 9, 2014 (received for review October 9, 2013)

Significance

TiO2, which is chemically stable, harmless, and inexpensive, has been widely used for industrial applications. Recently, TiO2-coated materials, exhibiting superwetting under sunlight, have been developed for environmental solutions. However, the mechanism responsible for superwetting of TiO2 is still in controversy despite many studies. We clarified its origin by performing tip-based in situ measurements of the growth dynamics of the photo-adsorbed water layers, as associated with delocalized surface electrons. Combined with molecular dynamics simulations, we provided conclusive clues that the “water wets water” process promotes water adsorption on the water layers, producing superwetting.

Abstract

Titania, which exhibits superwetting under light illumination, has been widely used as an ideal material for environmental solution such as self-cleaning, water–air purification, and antifogging. There have been various studies to understand such superhydrophilic conversion. The origin of superwetting has not been clarified in a unified mechanism yet, which requires direct experimental investigation of the dynamic processes of water-layer growth. We report in situ measurements of the growth rate and height of the photo-adsorbed water layers by tip-based dynamic force microscopy. For nanocrystalline anatase and rutile TiO2 we observe light-induced enhancement of the rate and height, which decrease after O2 annealing. The results lead us to confirm that the long-range attraction between water molecules and TiO2, which is mediated by delocalized electrons in the shallow traps associated with O2 vacancies, produces photo-adsorption of water on the surface. In addition, molecular dynamics simulations clearly show that such photo-adsorbed water is critical to the zero contact angle of a water droplet spreading on it. Therefore, we conclude that this “water wets water” mechanism acting on the photo-adsorbed water layers is responsible for the light-induced superwetting of TiO2. Similar mechanism may be applied for better understanding of the hydrophilic conversion of doped TiO2 or other photo-catalytic oxides.

Footnotes

  • ↵1To whom correspondence should be addressed. E-mail: whjhe{at}snu.ac.kr.

  • Author contributions: W.J. designed research; K.L., S.A., J.A., and W.J. performed research; Q.K. performed molecular dynamic simulations; K.L. and J.A. performed Kelvin probe force microscopy measurement; K.L., Q.K., J.K., B.K., and W.J. analyzed data; and K.L. and W.J. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission. A.L. is a guest editor invited by the Editorial Board.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1319001111/-/DCSupplemental.

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