Hydrophilicity transition of the clean rutile TiO2 (110) surface

Room temperature bilayer water structures on a rutile TiO2(110) surface: hydrophobic or hydrophilic?

The lack of understanding of the molecular-scale water adsorbed on TiO2 surfaces under ambient conditions has become a major obstacle for solving the long-time scientific and applications issues, such as the photo-induced wetting phenomenon and designing novel advanced TiO2-based materials. Here, with the molecular dynamics simulation, we identified an ordered water bilayer structure with a two-dimensional hydrogen bonding network on a rutile TiO2(110) surface at ambient temperature, corroborated by vibrational sum-frequency generation spectroscopy. The reduced number of hydrogen bonds between the water bilayer and water droplet results in a notable water contact angle (25 ± 5°) of the pristine TiO2 surface. This surface hydrophobicity can be enhanced by the adsorption of the formate/acetate molecules, and diminishes with dissociated H2O molecules. Our new physical framework well explained the long-time controversy on the origin of the hydrophobicity/hydrophilicity of the TiO2 surface, thus help understanding the efficiency of TiO2 devices in producing electrical energy of solar cells and the photo-oxidation of organic pollutants.

Photo-induced hydrophilicity at the ZnO(112̄0) surface: an evolutionary algorithm-aided density functional theory study

Evolutionary algorithm-aided density functional theory calculations were utilized to determine the stable adsorption structures of H₂O at ZnO(112̄0) extensively under different coverages. By decomposing the adsorption energetics, we illustrate that H₂O dissociation to a large extent is actually hampered by the barrier for induced distortion of the ZnO surface, and once the surface becomes less difficult to be distorted it will exhibit higher hydrophilicity or even superhydrophilicity. Specifically, photo-stimulation modelling suggests that the surface Zn-O bonds can be weakened by photo-excitation, and the layer of fully dissociated H₂O can be then facilitated to form. Accordingly, a novel mechanism for photo-induced superhydrophilicity is proposed.

Substrate mediated interaction of terbium(III) double-deckers with the TiO2(110) surface

A terbium(iii)-bis(phthalocyaninato) neutral complex was deposited on the rutile TiO2(110) surface, and their interaction was studied by Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). It was found that the TiO2 rutile surface favours the adsorption of isolated molecules adopting a lying down configuration with the phthalocyanine planes tilted by about 30° when they lie in the first layer. The electronic and chemical properties of the molecules on the surface were studied by XPS as a function of the TiO2(110) substrate preparation. This study evidences that strong molecule-substrate interactions are present and a charge transfer process occurs from the molecule to the surface.

Mirror twin boundaries in MoSe2 monolayers as one dimensional nanotemplates for selective water adsorption

Water adsorption on transition metal dichalcogenides and other 2D materials is generally governed by weak van der Waals interactions. This results in a hydrophobic character of the basal planes, and defects may play a significant role in water adsorption and water cluster nucleation. However, there is a lack of detailed experimental investigations on water adsorption on defective 2D materials. Here, by combining low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we study in that context the well-defined mirror twin boundary (MTB) networks separating mirror-grains in 2D MoSe2. These MTBs are dangling bond-free extended crystal modifications with metallic electronic states embedded in the 2D semiconducting matrix of MoSe2. Our DFT calculations indicate that molecular water also interacts similarly weak with these MTBs as with the defect-free basal plane of MoSe2. However, in low temperature STM experiments, nanoscopic water structures are observed that selectively decorate the MTB network. This localized adsorption of water is facilitated by functionalization of the MTBs by hydroxyls formed by dissociated water. Hydroxyls may form by dissociating of water at undercoordinated defects or adsorbing of radicals from the gas phase in the UHV chamber. Our DFT analysis indicates that the metallic MTBs adsorb these radicals much stronger than on the basal plane due to charge transfer from the metallic states into the molecular orbitals of the OH groups. Once the MTBs are functionalized with hydroxyls, molecular water can attach to them, forming water channels along the MTBs. This study demonstrates the role metallic defect states play in the adsorption of water even in the absence of unsaturated bonds that have been so far considered to be crucial for adsorption of hydroxyls or water.

The role of hydrophobic, aromatic and electrostatic interactions between amino acid residues and a titanium dioxide surface

Understanding the nature of interactions between inorganic surfaces and biomolecules, such as amino acids and peptides, can enhance the development of new materials.

Surface residual stress dependence on photoinduced highly hydrophilic conversion and back-reaction in the dark of rutile single crystals

The wettability changes of TiO(2) surface, photo-induced hydrophilic conversion and back-reaction in the dark, were evaluated using rutile (100) and (001) single crystals with diamond polishing (DP) and chemical mechanical polishing (CMP) treatments. Dynamic hardness measurements indicated that the DP surface had a residual compressive stress; however, the CMP surface did not. The rate of hydrophilic conversion was greatly suppressed (approximately one fourteenth) on the DP surface compared to the CMP surface, showing that the photoinduced hydrophilicity was greatly suppressed on the surface with compressive stress although the number of photogenerated carriers at the DP surface was estimated to decrease to only ca. half that at the CMP surface. In addition, when a heat treatment relaxed the compressive stress, the hydrophilicity was greatly increased. The back-reaction in the dark, i.e., the degradation of the photo-produced hydrophilic state, was ca. two times faster on the DP surface with compressive stress. The results strongly suggest that the pressure effect caused by the compressive stress could be the main reason for the degradation of the hydrophilicity and acceleration of the back reaction.