Photogeneration of Highly Amphiphilic TiO2 Surfaces

Abstract

The discovery of photoinduced water splitting on TiO 2 electrodes [1] has prompted extensive research on TiO 2 and other semiconductor materials, which have been widely adopted as potential substances for solar energy conversion and environmental purification. Most work has focused on improving the efficiency of energy conversion [2±5] or photocatalytic reactions. [6±12] Little research has been reported to clarify the effect of light on the properties of TiO 2 surfaces. Very recently, we found that UV illumination of TiO 2 materials can generate surfaces that display 0 contact angle for both water and oily liquids. [13] Following this finding, intensive research has been performed to explicate the mechanism of this unique amphiphilic surface character. In this communication, we report the details of the photoconvertible surface wettability. The formation of a microstructured composite between hydrophilic and oleophilic phases, which results from the photogenerated Ti 3+ defects at definite sites, is considered to account for this unique feature.The observation of the amphiphilic surfaces was initiated by the contact angle measurements of TiO 2 anatase thin films. The water contact angle for a freshly prepared film averaged 15±1. After the sample had been stored in the dark for 2 months, the water contact angle increased to 72±1. When a water droplet touched the UV-illuminated film, it spread immediately, leaving an irregular shape on the surface with a contact angle of 0±1. [13] The contact angle of glycerol trioleate (GT), a main ingredient of edible oil, for the TiO 2 surface was also measured. Prior to UV illumination, the GT contact angle averaged 10±1, indicating that the surface is hydrophobic and oleophilic. Surprisingly, after UV illumination a GT droplet also spread out, resulting in a contact angle of 0±1 when it touched the TiO 2 surface. Parallel experiments were performed using other liquid species, e.g., hexadecane, ethylene glycol, tetralin. Distinct contact angles resulted for the hydrophobic TiO 2 surface. However, all of the liquids spread completely on a UV-illuminated TiO 2 surface, with a contact angle of 0±1. This leads to the tremendous conclusion that UV illumination has created a surface that is both highly hydrophilic and highly oleophilic. The wettability change was observed on both anatase and rutile TiO 2 surfaces in the form of either polycrystals or a single crystal, independent of their photocatalytic activities. Even after the TiO 2 had been stored in the dark for a few days, the high amphiphilicity of the TiO 2 surface was maintained. A longer storage period induced a gradual increase in the water contact angle, revealing a surface wettability trend towards hydrophobicity. However, the high amphiphilicity was repeatedly regenerated by UV illumination.Surface wettability is generally denoted by the contact angle. According to Young's equation, the contact angle of a liquid drop on a solid surface results from the balance between the cohesive forces in the liquid and the adhesiv...

Reversibly Light‐Switchable Wettability of Hybrid Organic/Inorganic Surfaces With Dual Micro‐/Nanoscale Roughness

“…As reported previously, [20][21][22][23][24][25][26][27][28][29][30][31][32] the OH groups newly implanted onto the TiO 2 surfaces upon illumination are neither thermodynamically nor kinetically stable. In fact, once UV irradiation is halted, O 2 substitution for the newly implanted hydroxyl moieties becomes the prevalent chemical reaction under air in the dark, leading to progressive attenuation of the hydrophilicity gained under light.…”

“…[20][21][22][23][24][25][26][27][28][29][30][31][32] In addition, under the specific irradiation conditions used in this work (see the Experimental Section), a remarkable increase in hydrophilicity can be observed, which is correlated with the increase in the degree of surface hydroxylation of TiO 2 , while the surfactant molecules anchored to the NR surfaces are not subjected to noticeable photocatalytic degradation. [31,32] Therefore, the porous TiO 2 NRs-film is expected to embody a distribution of alternating highly hydrophilic and quite hydrophobic domains, the former being related to the newly introduced -OH groups, and the latter being associated with surfactantprotected TiO 2 areas.…”

A Nanosegregant Approach to Superwettable and Water‐Attracting Surfaces

“…Although there are quite a number of superhydrophilic materials reported in the literature, [19][20][21][22][23][24][25][26] to the best of our knowledge, the nanosegregant in Figure 3 is the first one exhibiting of the water-attracting function. Due to their superwettability, the nanosegregants also show anti-fogging effect as illustrated in Figure 3e.…”

“…[11][12][13][14][15][16][17][18] Compared to the technological success in superhydrophobicity, [1][2][3][4][5][6][11][12][13][14][15][16][17][18] methods to obtain superhydrophilicity are very limited. In the literature, superhydrophilicity is mainly achieved by the UV-irradiation of oxide semiconductor films such as TiO 2 (titanium oxide), [19][20][21] ZnO (zinc oxide), [22] and WO 3 . [23] In this case, the superhydrophilicity is induced by photon-generated short-lived charges, which will gradually disappear without UV illumination.…”

Plasma-Assisted Chemical Vapor Deposition of TiO<sub>2</sub> Thin Films for Highly Hydrophilic Performance

“…Commonly, anatase-phase TiO 2 is preferred for the photo-induced applications because of the efficient surface reaction by UV-irradiation compared to another crystal phases (brookite, rutile) [5]. In addition, the defects in the crystal should be also taken into account to control the reaction because it is recognized that the Ti 3+ sites reduced from Ti 4+ at the surface by photoexcited electrons accompanying oxygen vacancies generated by the photoexcited holes play an important role in the photo-induced surface reaction [6]. Therefore, a low of wet or dry process to fabricate anatase-TiO 2 layer has been advanced to control the crystallinity such as stoichiometric composition, impurity concentration, surface morphology, crystal orientation and so on.…”