Reversible surface wettability conversion on titanium dioxide (TiO2) single crystals has been achieved, and its mechanism has been examined by means of contact angle measurement and X-ray photoelectron spectroscopy (XPS). A UV light illuminated TiO2 single-crystal surface exhibited a 0° contact angle for both water and oil, indicative of a highly amphiphilic surface against its native hydrophobic surface. This was ascribed to photoreduction of surface Ti4+ to Ti3+ at definite sites, leading to preferential adsorption of dissociative water on top. A long-term storage of the highly amphiphilic surface resulted in reconversion of the surface wettability. It was found that the amphiphilic-to-hydrophobic reconversion is due to the replacement of the adsorbed hydroxyl groups by oxygen, which returns the surface geometric and electronic structures similar to the native TiO2 surface. The result of angle-resolved XPS measurement revealed that the surface reactions occurred at the uppermost layers of the single crystals. By comparing the reactivities of (110), (100), and (001) single-crystal surfaces, it was concluded that oxygen bridging sites played an important role in the surface wettability conversions.

Kinetics on the photoinduced hydrophilic conversion processes of the TiO2 surface was investigated. First, we show that there exist linear relationships between the reciprocal of the contact angle and the UV irradiation time and use the slope of this straight line as the hydrophilic conversion rate, which is independent of the initial value of the contact angle. Second, we examine the relationships between the reciprocal of the contact angle and the reconstruction of the surface hydroxyl groups and show that the reciprocal of the contact angle corresponds to the density of the surface hydroxyl groups reconstructed by UV irradiation. Finally, the dependence of the hydrophilic conversion on various parameters such as the incident UV intensity, the wavelength of irradiated light, and the concentration of the hole scavenger are quantitatively investigated with the proposed hydrophilic conversion rate. These results show that the photoinduced hydrophilic conversion proceeds competitively with the photocatalytic oxidation process on the TiO2 surface under UV irradiation. The comparison of the hydrophilic conversion rate with the rate for the reverse process clarifies that the critical contact angle is obtained when the reconstruction of the surface hydroxyl groups and their relaxation process are equilibrated under UV irradiation.

Electrochemical and photoelectrochemical studies were conducted on self-assembled multilayer films of titania nanosheets on a conductive ITO substrate. Cyclic voltammogram (CV) curves indicated that the titania nanosheet electrode underwent insertion/extraction of Li(+) ions into/from the nanosheet galleries, associated with reduction/oxidation of Ti(4+)/Ti(3+). These processes accompanied reversible changes in UV-vis absorption of the titania nanosheet electrodes. Applying a negative bias of -1.3 V (vs Ag/Ag(+)) and lower brought about absorption reduction where the wavelength is shorter than 323 nm, and vice versa, indicating a flat-band potential of (approximately) -1.3 V and a band gap energy of 3.84 eV. Photocurrents were generated from the titania nanosheet electrodes under a positive bias. The onset potential for photocurrent generation from the titania nanosheet electrodes was around -1.27 V, and the band gap energy estimated from the photocurrent action spectra was 3.82 eV, in excellent agreement with the values obtained from the spectroelectrochemical data. The lack of difference in the band gap energies for titania nanosheet electrodes with different numbers of layers suggests that a nanosheet is electronically isolated in multilayer assemblies without affecting the electronic state of neighboring nanosheets. Similar measurements on the anatase-type TiO(2) electrode revealed that the lower edge of the conduction band for the titania nanosheet is approximately 0.1 V higher than that for anatase, while the upper edge of the valence band is 0.5 V lower.

Glutathione‐protected Au25 as well as Aun (n = 15, 18, 22, 29, 33, 39) clusters adsorbed on TiO2 electrodes exhibit anodic photocurrents and negative shifts of photopotential in response to visible and/or near‐infrared light (400 < λ < 900 nm) on the basis of HOMO−LUMO and similar transitions, indicating that the electrodes are applicable to the conversion of light to electricity (see figure).

Changes in hydrophilicity of TiO2 film electrode surfaces by UV light irradiation in an aqueous solution were investigated under potential-controlled conditions. Although no change was observed when the electrode potential of TiO2 was set around its flat-band potential E fb, the photoinduced hydrophilic conversion proceeded at potentials positive of E fb. The hydrophilic conversion rate increased with larger positive potentials, but the addition of hole scavengers decreased the rate, suggesting that the diffusion of photogenerated holes to the surface is the important process for the hydrophilic conversion. Based on these results, we propose a model in which the photoinduced hydrophilicity is initiated by two-hole trapping by a surface lattice oxygen, producing an oxygen defect, followed by the dissociative adsorption of a water molecule at the defect site.

Glass surfaces coated with polycrystalline titanium dioxide (TiO2) films were found to exhibit a 0° water contact angle when the surfaces were illuminated with UV light in the air. This highly hydrophilic surface was maintained for more than 1 week in the dark in air. However, ultrasonic treatment in pure water decreased the degree of surface hydrophilicity, yielding a contact angle of approximately 11°. X-ray photoelectron spectroscopic measurements indicated that hydroxyl groups and molecular water adsorption, which govern the surface wettability, were partially removed from the surface by the ultrasonic treatment. The effect of ultrasonic treatment was ascribed to the generation of OH radicals that reoxidized the photoreduced surface, accompanied by the removal of surface-adsorbed water. This has been confirmed by adding acrylamide, a typical OH radical scavenger, to pure water to effectively suppress the hydrophilic-to-hydrophobic reconversion on the TiO2 surface.

Unilamellar metallic nanosheets possess superiority for electrochemical energy storage and conversion applications compared to the few-layered bulk and semiconducting counterparts. Here, we report the utilization of unilamellar metallic 1T phase MoS 2 nanosheets for efficient sodium storage and hydrogen evolution through a MoS 2 /graphene superlattice. The superlattice-like assembly composed of alternately restacked unilamellar MoS 2 and modified reduced graphene oxide nanosheets was prepared by a facile solution-phase direct restacking method. As an anode for sodium storage, the MoS 2 /graphene superlattice anode exhibited an excellent rate capability of ∼240 mA h g −1 at 51.2 A g −1 and a stable reversible capacity of ∼380 mA h g −1 after 1000 cycles at 10 A g −1 . In addition, a low onset potential of ∼88 mV and a small Tafel slope of 48.7 mV decade −1 were attained for the hydrogen evolution reaction. Our findings are important for further developing the potential of 2D nanosheets for energy storage and conversion.

Unilamellar nanosheet crystallites of manganese oxide generated the anodic photocurrent under visible light irradiation (lambda < 500 nm), while the nanosheets themselves were stable as revealed by in-plane XRD and UV-visible absorption spectra. The band gap energy was estimated to be 2.23 eV on the basis of the photocurrent action spectrum. The molecular thickness of approximately 0.5 nm may facilitate the charge separation of excited electrons and holes, which is generally very difficult for strongly localized d-d transitions. The monolayer film of MnO2 nanosheets exhibited the incident photon-to-electron conversion efficiency of 0.16% in response to the monochromatic light irradiation (lambda = 400 nm), which is comparable to those for sensitization of monolayer dyes adsorbed on a flat single-crystal surface. The efficiency declined with increasing the layer number of MnO2 nanosheets, although the optical absorption was enhanced. The recombination of the excited electron-hole pairs may become dominant when the carriers need to migrate a longer distance than 1 layer through multilayered nanosheets.