Role of water adsorption in photoinduced superhydrophilicity on TiO2 thin films

Lee, Fuk Kay; Andreatta, Gaëlle; Benattar, J. J.

doi:10.1063/1.2737132

Cited by 27 publications

(33 citation statements)

References 28 publications

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“…The phenomenon has been attributed to production of surface radical groups (1,(3)(4)(5) or photo-oxidation of hydrophobic surface contaminants (6)(7)(8), but its origin remains controversial (9,10) in part because conventional contact-angle analysis alone cannot fully account for the complex surface wettability (11). To address the fundamental mechanism responsible for superwetting, one has to go beyond the contact-angle measurement that reflects the specific surface properties and to probe the temporal growth dynamics of the water layer itself adsorbed on TiO 2 under wide-spectrum illumination of visible and near-infrared (NIR) in addition to UV light.…”

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confidence: 99%

Superwetting of TiO ₂ by light-induced water-layer growth via delocalized surface electrons

Lee

Kim

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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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 TiO 2 we observe light-induced enhancement of the rate and height, which decrease after O 2 annealing. The results lead us to confirm that the long-range attraction between water molecules and TiO 2 , which is mediated by delocalized electrons in the shallow traps associated with O 2 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 photoadsorbed water layers is responsible for the light-induced superwetting of TiO 2 . Similar mechanism may be applied for better understanding of the hydrophilic conversion of doped TiO 2 or other photo-catalytic oxides.

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confidence: 99%

Superwetting of TiO ₂ by light-induced water-layer growth via delocalized surface electrons

Lee

Kim

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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“…It is acknowledged that the presence of water is important for photocatalysis, [10][11] but its exact role in the process is poorly understood. [12][13][14][15] Numerous models 13,[16][17][18][19] have been proposed for the influence of adsorbed water on charge carrier trapping but consensus has still to be reached.…”

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confidence: 99%

Apparent Semiconductor Type Reversal in Anatase TiO₂ Nanocrystalline Films

et al. 2007

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The electronic properties of anatase and rutile TiO 2 polycrystalline particle films have been studied using surface photovoltage (SPV) and infrared (IR) spectroscopies. The films were prepared from aqueous suspensions using a range of particle sizes (8-400 nm) and were examined under different ambient conditions. The results show that all of the examined films exhibited the expected n-type semiconductor characteristics in dry nitrogen ambient. Films created from the 30 and 400 nm anatase particles exhibited the largest surface photovoltage and also a broad mid-IR absorption attributed to shallowly trapped electrons. The latter was absent from films made from rutile particles. When examined under as-prepared "wet" conditions, the SPV was reduced in magnitude and the IR signal was absent. Further, the films formed from 8 and 30 nm particles exhibited an apparent p-type behavior in their SPV spectra. Interestingly, small anatase particles are known to exhibit enhanced photocatalytic activity when compared to larger anatase particles. These correlations indicate that water, adsorbed on TiO 2 particles of nanodimensions, induce surface states and enable redistribution of photogenerated charge carriers, which is conducive to photocatalysis.Photocatalysis at semiconductor surfaces has been extensively investigated over recent decades. 1-3 Titanium dioxide has usually been the photocatalyst of choice and Degussa P25, which contains about 80% anatase and 20% rutile, has emerged as the benchmark material. 3 Photocatalysis studies have spanned well-characterized, single-crystal (usually rutile) surfaces under ultrahigh vacuum (UHV) conditions 4-6 to polycrystalline powders under catalysis operational conditions. 7-9 The latter have generally involved the catalyst in contact with an aqueous solution or water vapor. It is acknowledged that the presence of water is important for photocatalysis, 10-11 but its exact role in the process is poorly understood. 12-15 Numerous models 13,[16][17][18][19] have been proposed for the influence of adsorbed water on charge carrier trapping but consensus has still to be reached.Surface photovoltage (SPV) spectroscopy has been used to study both polycrystalline and single crystal (110) rutile TiO 2 . [20][21][22] For both types of material, it was reported that water modified the surface work function of rutile TiO 2 surfaces. There appear to have been no corresponding studies of anatase TiO 2 surfaces. It has been observed that adsorbed water influences a broad mid-infrared spectral absorption between 4000 and 700 cm -1 , which arises when TiO 2 polycrystalline films are irradiated with UV light 13,17,23,24 and that has been assigned to excitation of shallowly trapped electrons. 24 Panyatov and Yates 17 observed that this signal decreases when water is chemisorbed to the surface of a thermally reduced P25 TiO 2 film. They propose that the water depletes conduction band electrons during the formation of Ti-OH groups in the early stages of chemisorption. In this paper, we report the results...

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“…Correspondingly, a strong and broad OH band was observed in the Raman spectra of samples exposed to UV, as shown in Figure 5. Superhydrophilicity induced by UV irradiation is a well-known phenomenon, [20] and our findings are therefore not surprising. Superhydrophilicity was, however, a transient effect which disappeared after a few days (or a few hours if the samples were heated at 80 8C).…”

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confidence: 55%

Densification of Oxide Nanoparticle Thin Films by Irradiation with Visible Light

Bertino

Smarsly

Stocco

et al. 2009

Adv Funct Materials

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A technique is presented that allows for altering of the physical characteristics of films of TiO2 nanoparticles by exposure to visible light. In this technique, dye‐sensitized oxide nanoparticles are deposited on a substrate by dip‐coating. Photodissociation of the organic ligand layer leads to cross‐linking of the nanoparticles. Consequently, irradiated films have a decreased porosity, an increased index of refraction and an increased hydrophobicity. Films irradiated with green light are compared to films irradiated with UV light. Within experimental error, visible‐ and UV‐illumination induces the same changes in the films. The mechanism of surfactant elimination in dye‐sensitized oxide particles is discussed, patterning is demonstrated, and prospective applications of the technique are considered.

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Role of water adsorption in photoinduced superhydrophilicity on TiO2 thin films

Cited by 27 publications

References 28 publications

Superwetting of TiO ₂ by light-induced water-layer growth via delocalized surface electrons

Superwetting of TiO ₂ by light-induced water-layer growth via delocalized surface electrons

Apparent Semiconductor Type Reversal in Anatase TiO₂ Nanocrystalline Films

Densification of Oxide Nanoparticle Thin Films by Irradiation with Visible Light

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Role of water adsorption in photoinduced superhydrophilicity on TiO2 thin films

Cited by 27 publications

References 28 publications

Superwetting of TiO 2 by light-induced water-layer growth via delocalized surface electrons

Superwetting of TiO 2 by light-induced water-layer growth via delocalized surface electrons

Apparent Semiconductor Type Reversal in Anatase TiO2 Nanocrystalline Films

Densification of Oxide Nanoparticle Thin Films by Irradiation with Visible Light

Contact Info

Product

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Superwetting of TiO ₂ by light-induced water-layer growth via delocalized surface electrons

Superwetting of TiO ₂ by light-induced water-layer growth via delocalized surface electrons

Apparent Semiconductor Type Reversal in Anatase TiO₂ Nanocrystalline Films