Atom Probe Tomography of Encapsulated Hydroxyapatite Nanoparticles

Silica-embedded Gold Nanoparticles Analyzed by Atom Probe Tomography

Nanoparticles are utilized in a multitude of applications due to their unique properties. Consequently, characterization of nanoparticles is crucial, and various methods have been employed in these pursuits. One such method is Atom Probe Tomography (APT). However, existing sample preparation techniques for APT generally involve embedding of the nanoparticles in a matrix different from their environment in solutions or at solid-liquid interfaces. In this work, we demonstrate a methodology based on silica embedding and explore how it can be utilized to form a matrix for nanoparticles suitable for APT analysis. Through chemisorption to a surface, gold nanoparticles were densely packed, ensuring a high probability of encountering at least one particle in the APT analyses. The nanoparticle-covered surface was embedded in a silica film, replacing the water and thus making this method suitable for studying nanoparticles in their hydrated state. The nanoparticle's silver content and its distribution, originating from the nanoparticle synthesis, could be identified in the APT analysis. Sodium clusters, possibly originating from the sodium citrate used to stabilize the particles in solution, were observed on the nanoparticle surfaces. This indicates the potential for silica embedding to be used for studying ligands on nanoparticles in their hydrated state.

In Situ Metallic Coating of Atom Probe Specimen for Enhanced Yield, Performance, and Increased Field-of-View

Atom probe tomography requires needle-shaped specimens with a diameter typically below 100 nm, making them both very fragile and reactive, and defects (notches at grain boundaries or precipitates) are known to affect the yield and data quality. The use of a conformal coating directly on the sharpened specimen has been proposed to increase yield and reduce background. However, to date, these coatings have been applied ex situ and mostly are not uniform. Here, we report on the controlled focused-ion beam in situ deposition of a thin metal film on specimens immediately after specimen preparation. Different metallic targets e.g. Cr were attached to a micromanipulator via a conventional lift-out method and sputtered using Ga or Xe ions. We showcase the many advantages of coating specimens from metallic to nonmetallic materials. We have identified an increase in data quality and yield, an improvement of the mass resolution, as well as an increase in the effective field-of-view. This wider field-of-view enables visualization of the entire original specimen, allowing to detect the complete surface oxide layer around the specimen. The ease of implementation of the approach makes it very attractive for generalizing its use across a very wide range of atom probe analyses.

Preparation of atom probe tips from (nano)particles in dispersion using (di)electrophoresis and electroplating

The behavior of catalytic particles depends on their chemical structure and morphology. To reveal this information, the characterization with atom probe tomography has huge potential. Despite progresses and papers proposing various approaches towards the incorporation of particles inside atom probe tips, no single approach has been broadly applicable to date. In this paper, we introduce a workflow that allowed us to prepare atom probe specimens from Ga particles in suspension in the size range of 50 nm up to 2 μm. By combining dielectrophoresis and electrodeposition in a suitable way, we achieve a near-tip shape geometry, without a time-consuming FIB lift-out. This workflow is a simple and quick method to prepare atom probe tips and allows for a high preparation throughput. Also, not using a lift-out allowed us to use a cryo-stage, avoiding melting of the Ga particles, while ensuring a mechanical stable atom probe tip. The specimen prepared by this workflow enable a stable measurement and low fracture rates. RESEARCH HIGHLIGHTS: Enabling cryo-preparation of (nano)particles for the atom probe. Characterization of surface and bulk elemental distribution of GaPt model SCALMS.

Permanently polarized hydroxyapatite, an outstanding catalytic material for carbon and nitrogen fixation

Hydroxyapatite (HAp) is a well-known ceramic material widely used in the biomedical field. This review summarizes the very recent developments on permanently polarized HAp (pp-HAp), a HAp variety with tuned electrical properties that confer remarkable catalytic activity. pp-HAp is obtained by applying a thermal stimulation polarization process (TSP), which consists on a DC electric voltage of 500 V at 1000 °C, to previously sintered HAp. The TSP not only increases the crystallinity, reducing the defects in the crystal lattice, but also creates charges that accumulate at the crystalline boundaries and at the surface of microscopic grains, boosting the electrical conductivity. Finally, the successful utilization of pp-HAp in the catalytic fixation of carbon and nitrogen from CO₂ and N₂ gases, respectively, is reported and the formation of different products of chemical interest (e.g. amino acids, ethanol and ammonium) as a function of the reaction conditions (i.e. feeding gases and presence/absence of UV illumination) and catalyst plasticity is discussed. pp-HAp exhibits important advantages with respect to other consolidated catalysts, which drastically increases the final energetic net balance of the reactions.

Internalization of Fluoride in Hydroxyapatite Nanoparticles

Hydroxyapatite (HAP) is a cost-effective material to remove excess levels of fluoride from water. Historically, HAP has been considered a fluoride adsorbent in the environmental engineering community. This paper substantiates an uptake paradigm that has recently gained disparate support: assimilation of fluoride to bulk apatite lattice sites in addition to surface lattice sites. Pellets of HAP nanoparticles (NPs) were packed into a fixed-bed media filter to treat solutions containing 30 mg-F/L (1.58 mM) at pH 8, yielding an uptake of 15.97 ± 0.03 mg-F/g-HAP after 864 h. Solid-state ¹⁹F and ¹³C magic-angle spinning nuclear magnetic resonance spectroscopy demonstrated that all removed fluoride is apatitic. A transmission electron microscopy analysis of particle size distribution, morphology, and crystal habit resulted in the development of a model to quantify adsorption and total fluoride capacity. Low- and high-estimate median adsorption capacities were 2.40 and 6.90 mg-F/g-HAP, respectively. Discrepancies between experimental uptake and adsorption capacity indicate the range of F^- that internalizes to satisfy conservation of mass. The model was developed to demonstrate that F^- internalization in HAP NPs occurs under environmentally relevant conditions and as a tool to understand the extent of F^- internalization in HAP NPs of any kind.