We report unprecedented superomniphobic characteristics of nanotube-structured TiO(2) surface fabricated by electrochemical etching and hydrothermal synthesis process, with the wettability contact angles for water and oil both being ∼174° or higher. A tangled forest of ∼8-nm-diameter, multiwalled nanotubes of TiO(2) was produced on the microtextured Ti surface, with the overall nanotube length controlled to 150 nm by adjusting the processing time. Wettability measurements indicate that the nanotube surface is extremely nonwettable to both water and oil. The contact angle of the 8 nm TiO(2) nanotube surface after perfluorosilane coating is extremely high (178°) for water droplets indicating superhydrophobic properties. The contact angle for oil, measured using a glycerol droplet, is also very high, about 174°, indicating superoleophobic characteristics. These dual nonwetting properties, superomniphobic characteristics, are in sharp contrast to the as-made TiO(2) nanotubes which exhibit superhydrophilic properties with a contact angle of essentially ∼0°. Such an extreme superomniphobic material made by a simple and versatile method can be useful for a variety of technical applications. It is interesting to note that all three properties can be obtained with identical nanotube structures. A nanometer-scaled structure introduced by hydrothermally grown TiO(2) nanotubes is an effective air trapping nanostructure in enhancing the amphiphobic (superomniphobic) wettability.

Typical dye sensitized solar cells (DSSCs) exhibit a severe reduction of power conversion efficiency when the cell size is increased. In order to cope with this issue, we have investigated the use of anodized TiO(2) nanotubes on Ti foil in combination with the standard TiO(2) nanoparticle paste coated anode structure. The presence of nanotubes in the anode structure enabled a significant mitigation of the size-dependent deterioration of the DSSC performance, with a trend of much milder decrease of the efficiency as a function of the cell dimension up to 9 cm(2). The observed improvement is partly attributed to the elimination of fluorine-doped tin oxide glass in the anode structure, as well as the enhanced charge collection via the nanotube coated Ti substrate, resulting from enhanced mechanical and electrical connections and possibly improved light trapping. The introduction of TiO(2) nanotubes on the Ti foil substrate led to a substantial improvement of the J(sc) current density.

In this study, we report the fabrication and characterization of a novel PEC tandem cell, consisting of p-Si/TiO 2 /Fe 2 O 3 core/shell/hierarchical nanowire (csh-NW) array photocathode and TiO 2 /TiO 2 core/shell nanotube (cs-NT) array photoanode, for overall solar water splitting in a neutral pH water. The p-Si/n-TiO 2 /n-Fe 2 O 3 csh-NWs, made mainly by solution-processed methods, offer significantly improved performance in the neutral pH water with a low (positive) onset potential and photoactivity at zero bias, due to the increased reaction surface area, effective energy band alignment among p-Si, n-TiO 2 and n-Fe 2 O 3 enhancing the charge separation, improved optical absorption, and enhanced gas evolution. Nitrogen modification (annealing under N 2 ) is used to further enhance the csh-NWs photocathodic performance. The PEC tandem cell is then able to handle overall solar water splitting in the neutral pH water with a solar-to-hydrogen (STH) efficiency of $ 0.18%. The achieved results demonstrate initial steps toward the realization of full PEC devices using earth-abundant materials for solar hydrogen generation suggesting competitive performance when solar matched photoanode core material and co-catalysts are used.

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