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.
Tandem structured spectrally selective coating layer of copper oxide nanowires combined with cobalt oxide nanoparticles, Nano Energy, http://dx.Abstract Increasing the light absorption across the wide solar spectrum has important implications for applications in solar-thermal and photovoltaic energy conversion. Here, we report novel tandem structures combing two different materials with complementary optical properties and microstructures: copper oxide (CuO) nanowires (NWs) and cobalt oxide (Co 3 O 4 ) nanoparticles (NPs). Copper oxide NWs of 100-200 nm in diameter and 5µm long are grown thermally on copper foil in air and cobalt oxide NPs of 100-200 nm in diameter are synthesized hydrothermally. Tandem structures of spectrally selective coating (SSC) layer are built with three different methods: spray-coating, dip-coating of cobalt oxide NPs into copper oxide NWs forest, and transferring of copper oxide NWs layer onto cobalt oxide NPs layer. The tandemstructured SSC layers fabricated from the spray-coating, dip-coating and transferring methods exhibit figure of merit (FOM) values of 0.875, 0.892 and 0.886, respectively, which are significantly higher than that of the starting copper oxide NWs (FOM = 0.858) and cobalt oxide NPs (FOM= 0.854). Our results demonstrate the efficacy of using novel tandem structures for enhanced light absorption of solar spectrum, which will find broad applications in solar energy conversion.
We report here for the first time a successful distribution and attachment of fine Au nanoparticles on ∼8 nm diameter TiO2 nanotubes having significantly increased surface area. Au thin film deposition onto hydrothermally grown TiO2 nanotube arrays followed by thermal annealing breaks up the Au film into desired, uniformly distributed nanoparticles. Visible light absorption spectra of the gold nanoparticles on TiO2 nanotubes indicate that the Au nanoparticles are photo-excited due to plasmon resonance, and charge separation is accomplished by the transfer of photoexcited electrons from the gold particle to the TiO2 conduction band, thereby enhancing photoelectrochemical performance. By virtue of substantially increased surface area with the 8 nm TiO2 nanotube substrate in combination with the plasmonic effect of distributed Au nanoparticles, significantly increased photocurrent density was obtained with extended light absorbance from the UV regime to the visible spectrum region. Such gold nanoparticle decorated, fine TiO2 nanostructures fabricated by a simple and versatile method can be useful for hydrogen generation by water splitting, CO oxidation and various other types of photocatalysts and photovoltaic fuel cells.
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