Photovoltaic characteristics of dye-sensitized solar cells fabricated from branched titania nanotube arrays are compared with those obtained from unbranched ones. Branched titania nanotubes result in increased efficiency and short circuit current density without any discernible increases in dark current, than in devices with purely unbranched tubes. Adsorption isotherms show that increased inter-tube porosity exposes the outer surfaces of the branched tubes, providing increased access and area for dye adsorption. Our findings indicate that branched titania nanotubes could be attractive for use in many applications.

Titania nanotubes are attractive for many applications such as energy generation, storage and delivery, gas sensing, and water purification. Here, we demonstrate branched titania nanotube formation during potentiostatic anodization of titanium films or foils in a single electrochemical bath by stepping down the anodization voltage V-onod below a threshold value. The linear dependence on the titanium nanotube diameter with V-onod and the lack of nanotube formation for V-anod<20 V constrains homogeneous branching to occur only V-2 <= V-1/root 2-V0, where V-1 and V-2 are the initial and final anodization voltages and V-0 is a voltage offset dependent on the anodization bath chemistry. Our technique circumvents the constraints of multi-bath and multi-temperature methods for branching, and provides a versatile means for creating hierarchically sized and/or interconnected titania nanotubes for applications. (C) 2011 Elsevier B.V. All rights reserved.

We report the kinetics of titania nanotube length evolution during anodization of titanium films. Our results show that the nanotube length increase is thermally activated, and governed by voltage-dependent activation energy 0.6 eV ≤ Eeff ≤ 1.1 eV expressed by Eeff = E0-αVanod where α is a constant and E0 = 1.6 eV is a voltage-independent term. The proximity of E0 to that of oxygen diffusion in titania suggests that oxygen transport across the titania walls at the pore bottoms is the rate-limiting step. These results provide insights into the mechanism of titania nanotube formation and a framework for their rational synthesis for applications.

Coral-like branched architectures comprised of single-crystal copper nanocrystals were synthesized at room temperature through a galvanic displacement reaction between aqueous CuCl2 and Al foil in the presence of a cationic double-chain surfactant diocta-decyl-dimethyl-ammonium bromide. The corals are monolithic single crystals consisting of nanorod stems with an axis along ⟨001⟩ and orthogonal branches along ⟨110⟩. The branch diameters fall in a narrow range between 80 and 100 nm, and the branch lengths vary between 200 and 800 nm. The branch density is controllable by adjusting the surfactant/metal-ion ratio in solution and reaction time. These branched structures could serve as attractive building blocks for creating interconnected nanorod networks or porous materials for diverse applications, including catalysts, sensors, solid-state refrigerators, and nanodevices.

In this paper, the effect of ion irradiation on nanoscale hardness of ZnO microcones is reported. The hardness of ZnO cones determined by nanoindentation using atomic force microscope initially increases from 4.7±1.4 to 9.5±1.6 GPa after irradiation with 1.2 MeV Ar+8 ions at an ion fluence of 1015 ions cm−2 and then decreases with increasing ion fluence. This change in mechanical hardness has been correlated with the residual stress of the sample revealed by Raman peak shift in the E2(H) mode. These results show that the generally reported radiation-hard nature of ZnO depends critically on irradiation conditions, especially the irradiation temperature.

Platinum catalyst layers with Pt loadings w = 0.05-0.40 mg/cm(2) were deposited by magnetron sputtering from a variable deposition angle alpha onto gas diffusion layer (GDL) substrates and tested as cathode electrodes in proton exchange membrane (PEM) fuel cells using Nafion 1135 membranes and Teflon-bonded Pt-black electrode (TBPBE) anodes. Layers deposited at normal incidence (alpha = 0 degrees) are continuous and approximately replicate the rough surface morphology of the underlying GDL. In contrast, glancing angle deposition (GLAD) with alpha = 87 degrees and continuous substrate rotation yields highly porous layers consisting of vertically oriented Pt particles, 100-500 nm high and 100-300 nm wide, that are separated by 20-100 nm. The particle electrodes exhibit a higher (lower) mass-specific performance than the continuous-layer electrodes for a high (low) current density i. This is attributed to a higher porosity but lower overall electrochemically active surface area for the particles compared to the continuous layer. Increasing w in particle cells from 0.05 to 0.10 to 0.18 mg/cm(2) yields increasing potentials, but w = 0.40 mg/cm(2) causes a voltage drop at i > 0.4 A/cm(2), associated with the reduced pore density at large w. Comparison cells with a TBPBE cathode exhibit comparatively low Tafel slopes but a lower Pt mass specific performance than the sputtered catalysts. Quantitative analyses of kinetic and mass-transport losses in the polarization curves suggest a competing microstructural effect, favoring mass-transport performance and an efficient oxygen reduction reaction for particle and continuous layer electrodes, respectively. The overall results suggest that in addition to the well-known promise of sputter-deposited Pt catalysts as an approach to increase Pt utilization at low loading, GLAD provides the unique ability to control Pt porosity and to achieve efficient reactant flow for high-current-density operation. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3097188] All rights reserved.