Thermal-structural relationship of individual titania nanotubes

Effect of Anodized TiO2-Nb2O5-ZrO2 Nanotubes with Different Nanoscale Dimensions on the Biocompatibility of a Ti35Zr28Nb Alloy

Some important factors in the design of biomaterials are surface characteristics such as surface chemistry and topography, which significantly influence the relationship between the biomaterial and host cells. Therefore, nanotubular oxide layers have received substantial attention for biomedical applications due to their potential benefits in the improvement of the biocompatibility of the substrate. In this study, a nanotubular layer of titania-niobium pentoxide-zirconia (TiO₂-Nb₂O₅-ZrO₂) was developed via anodization on a β-type Ti35Zr28Nb alloy surface with enhanced biocompatibility. Scanning electron microscopy (SEM) and surface profilometry analysis of the anodized nanotubes indicated that the inner diameter (D_i) and wall thicknesses (W_t) increased with an increase in the water content of electrolyte and the applied voltage during anodization, while the nanotube length (L_n) increased with increasing the anodization time. TiO₂-Nb₂O₅-ZrO₂ nanotubes with different D_i, W_t, and L_n showed different surface roughnesses (R_a) and surface energies (γ), which affected the biocompatibility of the base alloy. MTS assay results showed that the TiO₂-Nb₂O₅-ZrO₂ nanotubes with the largest inner diameter (D_i) of 75.9 nm exhibited the highest cell viability of 108.55% due to the high γ of the surface, which led to high adsorption of proteins on the top surface of the nanotubes. The second highest cell viability was observed on the nanotubular surface with D_i of 33.3 nm, which is believed to result from its high γ as well as the optimum spacing between nanotubes. R_a did not appear to be clearly linked to cellular response; however, there may exist a threshold value of surface energy of ∼70 mJ/m², below which the cell response is less sensitive and above which the cell viability increases with increasing γ. This indicates that the TiO₂-Nb₂O₅-ZrO₂ nanotubes provided a suitable environment for enhanced attachment and growth of osteoblast-like cells as compared to the bare Ti35Zr28Nb alloy surface.

Thermal conductivity of TiO2 nanotube: a molecular dynamics study

The thermal conductivity of anatase TiO₂ nanotubes was investigated using equilibrium molecular dynamics simulations based on Green-Kubo formalism. The calculated thermal conductivity of [Formula: see text] for anatase crystal at room temperature agrees well with experimental value of ~8.5 W K^-1 · m^-1, demonstrating that the method used in our calculation can provide a good description for the thermal transport of TiO₂. The dependence of the thermal conductivity of TiO₂ nanotubes with temperature, tube size and chirality were studied in detail. The relationship between the thermal conductivity and the vibrational density-of-states of the nanotubes was also investigated.

Effect of Morphology and Crystal Structure on the Thermal Conductivity of Titania Nanotubes

Titania nanotubes (TNTs) with different morphology and crystal structure are prepared by chemical processing and rapid breakdown anodization (RBA) methods. The nanotubes are studied in terms of thermal conductivity. The TNTs with variable wall thickness below 30 nm have significantly reduced thermal conductivity than bulk titania, due to the phonon confinement, smaller phonon mean free path, and enhanced phonon boundary scattering. The amorphous nanotubes (TNT_Amor) have comparatively thicker walls than both crystalline nanotubes. The TNT_Amor has a thermal conductivity of 0.98 W m^-1 K^-1, which is slightly less than the thermal conductivity of crystalline anatase nanotubes (TNT_A; 1.07 W m^-1 K^-1). However, the titania nanotubes with mixed structure (TNT_A,T) and the smallest dimensions have the lowest thermal conductivity of 0.75 W m^-1 K^-1, probably due to the phonon confinement. The experimental results are compared with the theoretical study considering the size confinement effect with different wall dimensions of TNTs and surface scattering. The results agree well with the surface roughness factor (p) of 0.26 for TNT_A,T, 0.18 for TNT_A, and 0.65 for TNT_Amor, indicating diffusive phonon scattering and rougher surfaces for TNT_A. Interestingly, the present results together with those presented in literature suggest that thermal conductivity reduction with respect to the wall thickness occurs also for the amorphous nanotubes. This is ascribed to the role of propagons in the thermal transport of disordered structures.

Using Mosaicity to Tune Thermal Transport in Polycrystalline Aluminum Nitride Thin Films

The effect of controlling the c-axis alignment (mosaicity) to the cross-plane thermal transport in textured polycrystalline aluminum nitride (AlN) thin films is experimentally and theoretically investigated. We show that by controlling the sputtering conditions we are able to deposit AlN thin films with varying c-axis grain tilt (mosaicity) from 10° to 0°. Microstructural characterization shows that the films are nearly identical in thickness and grain size, and the difference in mosaicity alters the grain interface quality. This has a significant effect to thermal transport where a thermal conductivity of 4.22 vs 8.09 W/mK are measured for samples with tilt angles of 10° versus 0° respectively. The modified Callaway model was used to fit the theoretical curves to the experimental results using various phonon scattering mechanisms at the grain interface. It was found that using a non-gray model gives an overview of the phonon scattering at the grain boundaries, whereas treating the grain boundary as an array of dislocation lines with varying angle relative to the heat flow, best describes the mechanism of the thermal transport. Lastly, our results show that controlling the quality of the grain interface provides a tuning knob to control thermal transport in polycrystalline materials.

Observation of a low temperature n-p transition in individual titania nanotubes

Manipulating the transport properties of titania nanotubes (NTs) is paramount in guaranteeing the material's successful implementation in various solid state applications. Here we present the unique semiconducting properties of individual titania NTs as revealed from thermoelectric and structural studies performed on the same individual NTs. The NTs were in the anatase phase fabricated by anodic oxidation and doped with intrinsic defects created by reducing the lattice thermally. Despite their polycrystalline nature and nanoscale walls, the doped NTs were found to be 4-5 orders of magnitude more electrically conducting than TiO₂ nanowires and thin films, with values approaching the bulk single crystal conductivity. The reason for the high conductivity was found to be the high carrier concentration on the order of 10²² cm^-3, which counteracted the low mobility values ∼0.006 cm² V^-1 s^-1. Furthermore, this high level of carrier concentration transitioned the NTs to a degenerate state, which is the first such example in thermally doped titania NTs. More importantly, our study showed the creation of acceptor states along with donor states in individual nanotubes upon lattice reduction. These acceptor levels were found to be active at low temperatures when donor states were not ionized, shifting the Fermi level (E_f) from the conduction band to the valence band.

Anodically Grown Titania Nanotube Induced Cytotoxicity has Genotoxic Origins

Nanoarchitectures of titania (TiO₂) have been widely investigated for a number of medical applications including implants and drug delivery. Although titania is extensively used in the food, drug and cosmetic industries, biocompatibility of nanoscale titania is still under careful scrutiny due to the conflicting reports on its interaction with cellular matter. For an accurate insight, we performed in vitro studies on the response of human dermal fibroblast cells toward pristine titania nanotubes fabricated by anodic oxidation. The nanotubes at low concentrations were seen to induce toxicity to the cells, whereas at higher concentrations the cell vitality remained on par with controls. Further investigations revealed an increase in the G₀ phase cell population depicting that majority of cells were in the resting rather than active phase. Though the mitochondrial set-up did not exhibit any signs of stress, significantly enhanced reactive oxygen species production in the nuclear compartment was noted. The TiO₂ nanotubes were believed to have gained access to the nuclear machinery and caused increased stress leading to genotoxicity. This interesting property of the nanotubes could be utilized to kill cancer cells, especially if the nanotubes are functionalized for a specific target, thus eliminating the need for any chemotherapeutic agents.

Physisorbed versus chemisorbed oxygen effect on thermoelectric properties of highly organized single walled carbon nanotube nanofilms

The effect of physisorbedvs.chemisorbed oxygen on highly organized single walled carbon nanotube (SWCNT) ultrathin films is investigated by correlating the thermoelectric properties measured by a suspended micro-device to the SWCNT structure.

Improving the stability of titania nanosheets by functionalization with polyelectrolytes

Highly stable suspensions of titanate nanosheets were designed by surface functionalization with P(AAm-co-DADMAC) copolymer.