Vertically aligned amorphous titania (TiO2) nanotubes are produced by anodizing Ti foils at various applied potentials in a neutral electrolyte solution containing fluoride ions. Pore size and wall thickness are tuned in the range from 30 to 70 nm and 17 to 35 nm, respectively, by adjusting the applied potential, in addition to tuning the tube length from 355 to 550 nm. Utilizing all of these films as negative electrode materials in lithium-ion batteries delivers stable capacities of 130–230 mAh g–1 and 520–880 mAh cm–3 up to 200 cycles. Microstructural analysis shows that there is no structural change or mechanical degradation in the active material, and the amorphous active material maintains good contact with the substrate/current collector. A continuum elasticity model for the tubular geometry is presented to understand the diffusion-induced stresses, fracture tendency, and stability in TiO2 nanotubes. Modeling results indicate that the fracture tendencies of nanotubes with the dimensions in this work are very small; stable reversible capacity retention results from the high ratio of inner to outer diameter of the tubes. In other words, tubes with thinner walls more easily accommodate expansion or contraction during the lithiation/delithiation process. A guideline for designing lithium-ion battery nanotube electrodes is given such that under specific conditions the fracture tendency is small and volumetric charge density is high.

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A method is reported to generate mesoporous titania thin films with large pores and large surface areas by adding poly(propylene glycol), PPG, to the coating sol of films prepared by templating with Pluronic surfactant P123. Because P123 has an average structure (PEO)20(PPO)70(PEO)20 where PEO is a poly(ethylene oxide) unit and PPO is a poly(propylene oxide) unit, the hypothesis underlying this work is that PPG segregates to the PPO domains to induce swelling of the template micelles. With the introduction of PPG, the final calcined films are observed to have well-defined mesoporous structures, although the degree of long-range order diminishes with added PPG. The mesopore size of the films increases with increasing amounts of PPG, but PPG also introduces smaller mesopores into the films under our synthesis conditions to generate a bimodal pore size distribution. The size of these smaller pores remains nearly constant at ∼2.4 nm over a large range of PPG:P123 mass ratios, even as the larger mesopore size continues to increase. To understand the effect of PPG on mesostructured titania, the behavior in aqueous solution of the PPG polymer used for swelling is investigated and modeled with Flory–Huggins theory to estimate the full two-phase envelope. The phase diagram reveals that PPG (M n ≈ 3500) and water have a lower consolute temperature of about −9 °C, so at the temperature used for aging of the films immediately after coating (4 °C), the composition is well within the two-phase region. Therefore, repulsion between PPG molecules and polar species is expected to drive PPG into the hydrophobic cores of P123 micelles, thus leading to final products with large pore size. However, because of partitioning of some PPG into the polar phase, a bimodal pore size distribution and decreasing long-range pore order are observed as the amount of PPG increases.