Control of defect concentrations within a semiconductor through adsorption

Influence of interstitial cluster families on post-synthesis defect manipulation and purification of oxides using submerged surfaces

Injection of interstitial atoms by specially prepared surfaces submerged in liquid water near room temperature offers an attractive approach for post-synthesis defect manipulation and isotopic purification in device structures. However, this approach can be limited by trapping reactions that form small defect clusters. The compositions and dissociation barriers of such clusters remain mostly unknown. This communication seeks to address this gap by measuring the dissociation energies of oxygen interstitial traps in rutile TiO2 and wurtzite ZnO exposed to liquid water. Isotopic self-diffusion measurements using 18O, combined with progressive annealing protocols, suggest the traps are small interstitial clusters with dissociation energies ranging from 1.3 to 1.9 eV. These clusters may comprise a family incorporating various numbers, compositions, and configurations of O and H atoms; however, in TiO2, native interstitial clusters left over from initial synthesis may also play a role. Families of small clusters are probably common in semiconducting oxides and have several consequences for post-synthesis defect manipulation and purification of semiconductors using submerged surfaces.

Titania coating of mesoporous silica nanoparticles for improved biocompatibility and drug release within blood vessels

Blood vessel disease is a major contributor to cardiovascular morbidity and mortality and is hallmarked by dysfunction of the lining endothelial cells (ECs). These cells play a significant role in vascular homeostasis, through the release of mediators to control vessel diameter, hence tissue perfusion. Mesoporous silica nanoparticles (MSNs) can be used as potential drug delivery platforms for vasodilator drugs. Here, using an ex vivo model of vascular function, we examine the use of titania coating for improved biocompatibility and release dynamics of MSN loaded sodium nitroprusside (SNP). MSNs (95 ± 23 nm diameter; pore size 2.7 nm) were synthesised and fully characterised. They were loaded with SNP and coated with titania (TiO₂), using the magnetron sputtering technique. Pre-constricted aortic vessels were exposed to drug loaded MSNs (at 1.96 × 10¹² MSN mL^-1) and the time course of vessel dilation observed, in real time. Exposure of viable vessels to MSNs lead to their internalization into the cytoplasm of ECs, while TiMSNs were also observed in the elastic lamina and smooth muscle cell layers. We demonstrate that titania coating of MSNs significantly improves their biocompatibility and alters the dynamics of drug release. A slow and more sustained relaxation was evident after uptake of TiMSN-SNP, in comparison to uncoated MSN-SNP (rate of dilation was 0.08% per min over a 2.5 h period). The use of titania coated MSNs for drug delivery to the vasculature may be an attractive strategy for therapeutic clinical intervention in cardiovascular disease.

STATEMENT OF SIGNIFICANCE

Cardiovascular disease is a major cause of mortality and morbidity worldwide, with a total global cost of over $918 billion, by 2030. Mesoporous silica nanoparticles (MSNs) have great potential for the delivery of drugs that can treat vessel disease. This paper provides the first description for the use of titania coated MSNs with increased vascular penetration, for the delivery of vasodilator drugs, without compromising overall vessel function. We demonstrate that titania coating of MSNs significantly improves their biocompatibility and uptake within aortic blood vessels and furthermore, enables a slower and more sustained release of the vasodilator drug, sodium nitroprusside within the vessel, thus making them an attractive strategy for the treatment of vascular disease.

First-principles description of oxygen self-diffusion in rutile TiO2: assessment of uncertainties due to enthalpy and entropy contributions

Properties related to transport such as self-diffusion coefficients are relevant to fuel cells, electrolysis cells, and chemical/gas sensors. Prediction of self-diffusion coefficients from first-principles involves precise determination of both enthalpy and entropy contributions for point defect formation and migration. We use first-principles density functional theory to estimate the self-diffusion coefficient for neutral O0i and doubly ionized Oi2- interstitial oxygen in rutile TiO2 and compare the results to prior isotope diffusion experiments. In addition to formation and migration energy, detailed estimates of formation and migration entropy incorporating both vibrational and ionization components are included. Distinct migration pathways, both based on an interstitialcy mechanism, are identified for O0i and Oi2-. These result in self-diffusion coefficients that differ by several orders of magnitude, sufficient to resolve the charge state of the diffusing species to be Oi2- in experiment. The main sources of error when comparing computed parameters to those obtained from experiment are considered, demonstrating that uncertainties due to computed defect formation and migration entropies are comparable in magnitude to those due to computed defect formation and migration energies. Even so, the composite uncertainty seems to limit the accuracy of first-principles calculations to within a factor of ±103, demonstrating that direct connections between computation and experiment are now increasingly possible.

Microkinetic model for reaction and diffusion of titanium interstitial atoms near a TiO2(110) surface

Semiconductor surfaces provide efficient pathways for injecting native point defects into the underlying bulk. In the case of interstitial atoms in rutile, the TiO₂(110) surface exemplifies this behavior, although extended defects in the bulk such as platelets and crystallographic shear planes act as net sources or sinks depending upon specific conditions. The present work constructs a quantitative microkinetic model to describe diffusion and based upon isotopic gas-solid exchange experiments. Key activation barriers for are 0.55 eV for surface injection, 0.50 eV for site-to-site hopping diffusion, and 3.3 eV for dissociation of titanium interstitials from extended defects.

Manipulation of native point defect behavior in rutile TiO2 via surfaces and extended defects

Semiconductor surfaces offer efficient pathways for exchanging native point defects with the underlying bulk. For rutile TiO₂(1 1 0), isotopic self-diffusion studies of oxygen have suggested that the surface may act as a source for O_i while simultaneously acting as a sink for titanium interstitials Ti_i. Through self-diffusion measurements with labeled Ti as well as O, the present work develops a more complete picture of the diffusion-reaction network involving O_i and Ti_i, complete with the surface acting as a source for whichever elements are available from the gas phase and a sink for elements that are not. The picture points to the importance of extended defects such as platelets and crystallographic shear planes as reservoirs of O_i and Ti_i, acting as net sources or sinks of these species depending upon specific conditions. The results exemplify the combined roles of surfaces and extended defects in regulating point defect behavior even in macroscopic metal oxide crystals, and point to specific strategies for manipulating that behavior intentionally.

Reversible oxygen scavenging at room temperature using electrochemically reduced titanium oxide nanotubes

A material capable of rapid, reversible molecular oxygen uptake at room temperature is desirable for gas separation and sensing, for technologies that require oxygen storage and oxygen splitting such as fuel cells (solid-oxide fuel cells in particular) and for catalytic applications that require reduced oxygen species (such as removal of organic pollutants in water and oil-spill remediation). To date, however, the lowest reported temperature for a reversible oxygen uptake material is in the range of 200-300 °C, achieved in the transition metal oxides SrCoOx (ref. 1) and LuFe₂O(4+x) (ref. 2) via thermal cycling. Here, we report rapid and reversible oxygen scavenging by Ti(2-x) nanotubes at room temperature. The uptake and release of oxygen is accomplished by an electrochemical rather than a standard thermal approach. We measure an oxygen uptake rate as high as 14 mmol O₂ g(-1) min(-1), ∼2,400 times greater than commercial, irreversible oxygen scavengers. Such a fast oxygen uptake at a remarkably low temperature suggests a non-typical mechanistic pathway for the re-oxidation of Ti(2-x). Modelling the diffusion of oxygen, we show that a likely pathway involves 'exceptionally mobile' interstitial oxygen produced by the oxygen adsorption and decomposition dynamics, recently observed on the surface of anatase.