Photoinduced “Stick−Slip” on Superhydrophilic Semiconductor Surfaces

Abstract

Transparent mesoporous TiO2 (M-TiO2) thin films were prepared on quartz via a reverse micelle, sol-gel, spin-coating technique. Films were characterized by atomic force microscopy (AFM) and Raman and UV-vis spectroscopies and were found to be mostly anatase with low surface roughness (Rt approximately 5 nm). The time dependence of film photoinduced superhydrophilicity (PISH) was measured by observation of the spreading of a sessile water drop using a new, continuous measurement technique wherein the drop was first applied to the semiconductor surface and then was filmed while it and the underlying substrate were illuminated by 315 nm ultraband gap light. Results obtained at 100% relative humidity (RH) at 293 K showed that drops on M-TiO2 surfaces exhibited a photoinduced "stick-slip" behavior, the first time such an effect has been observed. The thermodynamic driving force for this photoinduced stick-slip was the departure of the system from capillary equilibrium as, with increasing illumination time, the concentration of surface Ti-OH groups increased and the equilibrium contact angle of the drop, theta0, decreased. A simple theoretical description of photoinduced stick-slip is derived and is used to calculate a value of the potential energy barrier associated with surface inhomogeneities that oppose onset of movement of the triple line, U = 6.63 x 10(-6) J m(-1). This is the first time that U has been quantified for a surface with photoinduced superhydrophilicity. Triple line retreat measurements on an evaporating drop on M-TiO2 in the dark, RH = 60%, T = 293 K, gave a value of U = 9.4 x 10(-6) J m(-1), indicating that U decreases upon UV illumination and that U in the light is primarily associated with inhomogeneities that are unaffected by an increase in the surface Ti-OH population, such as the physical roughness of the surface. In the dark evaporation experiment, the drop was found to retreat with an areal velocity of 1.48 x 10(-8) m2 s(-1). However, under UV illumination, the drop was found to spread at a substantially faster velocity of 2.33 x 10(-5) m2 s(-1), the latter being of the order of the velocities of 10(-4) m2 s(-1) observed in (dark) drop-spreading experiments conducted in the presence of trisiloxane surfactant superspreaders. This suggests that, once slip has started, the triple line processes over a thin precursor film of condensed water whose formation has been promoted by the photoinduced increase in the Ti-OH population at the semiconductor surface.

A surface science perspective on TiO2 photocatalysis

“…These effects are reversed if the light is turned off or if O 2 is removed from the ambient. As can be glean from the Fujishima et al review [2], there is considerable controversy as to the main source of this effect, with groups falling into two camps: (1) those attributing the effect to UV light induced modifications of the TiO 2 surface that facilitate better wetting [1783][1784][1785][1786][1787][1788][1789][1790][1791]; and (2) those attributing the effect to the well-known photooxidation properties of TiO 2 in removing hydrophobic organic adlayers from the TiO 2 surface [740,[1779][1780][1781][1782]. Both sides of this issue have been presented in the Fujishima et al review [2], and need not be repeated here.…”

Thermo-responsive stick-slip behavior of advancing water contact angle on the surfaces of poly(N-isopropylacrylamide)-grafted polypropylene membranes

“…Therefore, CA measurements on real surfaces that are rough, heterogeneous, reconstructive, or stimuli-responsive are of great importance, though they are complicated. For example, a stick-slip phenomenon has been occasionally observed on various surfaces and with different working liquids [6][7][8][9][10][11][12][13][14][15]. During the measurement of an advancing CA, with the continuous addition of the working liquid, the volume of the droplet increases, whereas the three-phase line sticks, thus the CA increases steadily.…”

Titanium Dioxide Derived Materials with Superwettability

“…The water CAs release to less than 5 • quickly, in 160 ms. With the introduction of TiO 2 onto the glass substrate, the coating showed obvious anti-fogging performance. Denison et al [47] reported ultra-fast spreading behavior of water droplets on a transparent mesoporous superhydrophilic TiO 2 film prepared by reverse micelle, sol-gel, and spin coating technology. The photo-induced stick-slip behavior on mesoporous TiO 2 can be observed by investigating the spreading of water droplets on the semiconductor surface irradiated by ultraviolet light.…”