Charge Properties of TiO₂ Nanotubes in NaNO₃ Aqueous Solution

Charging of material surfaces in aqueous electrolyte solutions is one of the most important processes in the interactions between biomaterials and surrounding tissue. Other than a biomaterial, titania nanotubes (TiO₂ NTs) represent a versatile material for numerous applications such as heavy metal adsorption or photocatalysis. In this article, the surface charge properties of titania NTs in NaNO₃ solution were investigated through electrophoretic mobility and polyelectrolyte colloid titration measuring techniques. In addition, we used high-resolution transmission electron microscopy imaging to determine the morphology of TiO₂ NTs. A theoretical model based on the classical density functional theory coupled with the charge regulation method in terms of mass action law was developed to understand the experimental data and to provide insights into charge properties at different physical conditions, namely, pH and NaNO₃ concentration. Two intrinsic protonation constants and surface site density have been obtained. The electrostatic properties of the system in terms of electrostatic potentials and ion distributions were calculated and discussed for various pH values. The model can quantitatively describe the titration curve as a function of pH for higher bulk salt concentrations and the difference in the equilibrium amount of charges between the inner and outer surfaces of TiO₂ NTs. Calculated counterion (NO₃^–) distributions show a pronounced decrease of NO₃^– ions for high bulk pH (both inside and outside TiO₂ NT) because of the strong electric field. With the decrease of bulk pH or the increase of the salt concentration, NO₃^– is able to accumulate near the TiO₂ NTs surfaces

Atomic-Level Response of the Domain Walls in Bismuth Ferrite in a Subcoercive-Field Regime

The atomic-level response of zigzag ferroelectric domain walls (DWs) was investigated with in situ bias scanning transmission electron microscopy (STEM) in a subcoercive-field regime. Atomic-level movement of a single DW was observed. Unexpectedly, the change in the position of the DW, determined from the atomic displacement, did not follow the position of the strain field when the electric field was applied. This can be explained as low mobility defect segregation at the initial DW position, such as ordered clusters of oxygen vacancies. Further, the triangular apex of the zigzag wall is pinned, but it changes its shape and becomes asymmetric under electrical stimuli. This phenomenon is accompanied by strain and bound charge redistribution. We report on unique atomic-scale phenomena at the DW level and show that in situ STEM studies with atomic resolution are very relevant as they complement, and sometimes challenge, the knowledge gained from lower resolution studies

Atomically Resolved Anisotropic Electrochemical Shaping of Nano-electrocatalyst

Catalytic properties of advanced functional materials are determined by their surface and near-surface atomic structure, composition, morphology, defects, compressive and tensile stresses, etc; also known as a structure–activity relationship. The catalysts structural properties are dynamically changing as they perform via complex phenomenon dependent on the reaction conditions. In turn, not just the structural features but even more importantly, catalytic characteristics of nanoparticles get altered. Definitive conclusions about these phenomena are not possible with imaging of random nanoparticles with unknown atomic structure history. Using a contemporary PtCu-alloy electrocatalyst as a model system, a unique approach allowing unprecedented insight into the morphological dynamics on the atomic-scale caused by the process of dealloying is presented. Observing the detailed structure and morphology of the same nanoparticle at different stages of electrochemical treatment reveals new insights into atomic-scale processes such as size, faceting, strain and porosity development. Furthermore, based on precise atomically resolved microscopy data, Kinetic Monte Carlo (KMC) simulations provide further feedback into the physical parameters governing electrochemically induced structural dynamics. This work introduces a unique approach toward observation and understanding of nanoparticles dynamic changes on the atomic level and paves the way for an understanding of the structure–stability relationship

Corrosion Protection of Platinum-Based Electrocatalyst by Ruthenium Surface Decoration

A comprehensive insight into the electrochemical performance of PtCu₃ electrocatalyst nanoparticles with and without Ru decoration is provided. The online dissolution investigation using the highly sensitive online analytical methodology of electrochemical flow cell coupled to inductively coupled plasma mass spectrometry reveals that the addition of Ru nanoparticles inhibits Pt dissolution presumably because of three effects: (i) suppression of Pt oxide formation, (ii) sacrificial corrosion of Ru, and (iii) lowering of local surface pH. The Ru nanoparticles, however, also lead to a decrease of the amount of crystal structure ordering, which in turn is one of the reasons for the increase of the corrosion of Cu. By measuring the potential of total zero charge it is shown that Ru decoration does not alter the electrochemical properties of the native Pt surface. Finally, Ru decoration of the Pt-based electrocatalyst is shown to present a viable approach to enhance the platinum corrosion resistance, which is confirmed by thin-film rotating disc electrode accelerated degradation tests

Hindered Disulfide Bonds to Regulate Release Rate of Model Drug from Mesoporous Silica

With the advancement of drug delivery systems based on mesoporous silica nanoparticles (MSNs), a simple and efficient method regulating the drug release kinetics is needed. We developed redox-responsive release systems with three levels of hindrance around the disulfide bond. A model drug (rhodamine B dye) was loaded into MSNs’ mesoporous voids. The pore opening was capped with β-cyclodextrin in order to prevent leakage of drug. Indeed, in absence of a reducing agent the systems exhibited little leakage, while the addition of dithiothreitol cleaved the disulfide bonds and enabled the release of cargo. The release rate and the amount of released dye were tuned by the level of hindrance around disulfide bonds, with the increased hindrance causing a decrease in the release rate as well as in the amount of released drug. Thus, we demonstrated the ability of the present mesoporous systems to intrinsically control the release rate and the amount of the released cargo by only minor structural variations. Furthermore, an <i>in vivo</i> experiment on zebrafish confirmed that the present model delivery system is nonteratogenic