Visible light guided manipulation of liquid wettability on photoresponsive surfaces

Photoresponsive titania surfaces are of great interest due to their unique wettability change upon ultraviolet light illumination. However, their applications are often limited either by the inability to respond to visible light or the need for special treatment to recover the original wettability. Sensitizing TiO2 surfaces with visible light-absorbing materials has been utilized in photovoltaic applications. Here we demonstrate that a dye-sensitized TiO2 surface can selectively change the wettability towards contacting liquids upon visible light illumination due to a photo-induced voltage across the liquid and the underlying surface. The photo-induced wettability change of our surfaces enables external manipulation of liquid droplet motion upon illumination. We show demulsification of surfactant-stabilized brine-in-oil emulsions via coalescence of brine droplets on our dye-sensitized TiO2 surface upon visible light illumination. We anticipate that our surfaces will have a wide range of applications including microfluidic devices with customizable wettability, solar-driven oil–water clean-up and demulsification technologies.

To qualify the porosity of the nanoporous TiO 2 film we calculate the ratio of the total internal surface area of the nanoporous structure per unit projected area. The absorbance of N3 dyesensitized TiO 2 surface at 478 nm is 0.1736 (see Fig. 1a in the main text). The extinction coefficient of N3 dye at 478 nm is e 478nm = 1.88 × 10 7 cm 2 mol −1 1 . The dye concentration on the surface can be calculated by dividing the absorbance with the extinction coefficient. This yields 9.23 × 10 −9 mol cm −2 . Considering that each dye molecule occupies an area of ≈ 1 nm 2 1 , the internal surface area is estimated to be 56 cm 2 for each 1 cm 2 projected area.

Supplementary Note 2. Measured contact angles for KI and KCl droplets with various concentrations on an N3 dye-sensitized TiO 2 surface under visible light illumination.
We measured the advancing contact angles for aqueous KI and KCl droplets with a range of different concentrations on an N3 dye-sensitized TiO 2 surface upon visible light illumination.
Supplementary Figure 1 shows the contact angles for aqueous KI droplets with 0.5 wt%, 5 wt%, 10 wt% and 20 wt% KI as a function of illumination time. It shows that KI droplets with higher ionic concentration exhibit a more rapid decrease in the contact angles under optical illumination. On the other hand, the contact angles for KCl droplets remained almost unchanged during illumination and this negligible change in the contact angles remained invariant of the ionic concentration (see Supplementary Table 1).

Supplementary Note 3. X-ray photoelectron spectroscopy (XPS) analysis of an N3 dyesensitized TiO 2 surface after visible light illumination.
To verify that the surface chemistry of our N3 dye-sensitized TiO 2 surface remains unaffected after visible light illumination, XPS measurements were conducted using a PHI 5600 ESCA multi-detection system with a base pressure of 1 × 10 -10 Torr. The X-ray radiation was the monochromatic Al Kα line (1486.7 eV); the X-ray spot size and the take-off angle were 0.8 mm and 45°, respectively.

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The survey spectra of the N3 dye-sensitized TiO 2 before and after multiple cycles of wetting under visible light illumination are shown in Supplementary Figure 2. The photoelectron peaks in the spectra can be attributed to titanium (Ti), oxygen (O) and ruthenium (Ru). The origin of ruthenium is due to N3 dye molecules adsorbed on the TiO 2 surface. The surface compositions were obtained by normalizing the area under the curve. We found that the atomic concentration ratio of titanium (Ti2p) and ruthenium (Ru3d) of our N3 dye-sensitized TiO 2 surface remained unchanged (72.1:27.9, atomic%:atomic%) even after multiple cycles of wetting under visible light illumination. This indicates that our N3 dye-sensitized TiO 2 surface is robust against through optical illumination.

Supplementary Note 4. Prediction of voltages between the contacting liquid droplets and
our N3 dye-sensitized TiO 2 surfaces.
As discussed in the main text, the electrolytic double layer formed at the liquid-solid interface under incident illumination can be considered as a capacitor. We predict the evolution in the phase which leads also to some resistive characteristic. When such an 'imperfect' capacitor is discharged, the voltage can be characterized by a stretched exponential function,

Supplementary Note 5. Measured contact angles for a KI droplet on an N3 dye-sensitized TiO 2 surface upon intermittent visible light illumination.
We measured the contact angles for a KI droplet (10 wt% in water) on an N3 dye-sensitized TiO 2 surface upon intermittent visible light illumination. Supplementary Figure 3 Figure 5a). This is because the HOMO energy level of a D149 dye is lower (less positive) than the reduction potential of bromide which allows the regeneration process of oxidized D149 dye 3 (see main text). In contrast, the measured voltage between the KCl droplet and the surface decreases rapidly and reaches zero within a minute of illumination. As expected, this is because the reduction potential of chloride is higher (more positive) than the HOMO energy level of D149 dye which hinders effective regeneration of oxidized dye (see main text).
Eqn. (1) in the main text describes the measured voltages well with various values of τ d and α (see Supplementary Table 2).
(iii) Chlorin dye-sensitized TiO 2 surface: Supplementary Figure 5c shows the measured in situ voltages across the contacting liquids and the Chlorin dye-sensitized TiO 2 surface. As the HOMO energy level of a Chlorin dye is higher (more positive) than the reduction potential of all contacting liquids (K 2 S 2 O 3 , KI, KBr and KCl), we observed that voltages for all contacting liquid droplets decrease gradually with increasing illumination time. This leads to spreading of all S13 contacting liquids including KCl droplets (see Fig. 3c in the main text).
Supplementary Table 2 lists the values of τ d and α found in the voltage predictions using our fractional RC circuit model.

Supplementary Note 8. Measured contact angles for K 2 S 2 O 3 and KBr droplets on an N3
dye-sensitized TiO 2 surface under visible light illumination.
We also measured the in situ contact angles for two liquid droplets: K 2 S 2 O 3 and KBr (10 wt% in water) on an N3 dye-sensitized TiO 2 surface. Supplementary Figure 6 shows the evolution in the macroscopic contact angles for K 2 S 2 O 3 and KBr droplets as a function of illumination time. The contact angles for the K 2 S 2 O 3 droplet decrease from θ t=0 * = 119° with increasing illumination time before it approaches θ t=120 min * = 60° while those for KBr remain almost constant during illumination ( Δθ * ≈ 2° where Δθ * = θ t=0 * − θ t=120 min