The evolution of crystalline ordering for ligand-ornamented zinc oxide nanoparticles

Recent total scattering experiments have opened up the possibility to study nanoparticle formation in situ and to observe the structural transformation from precursor clusters to adult particles. Organic ligand molecules interact with precursors of metal oxide nanoparticles, yet their influence onto the evolution of crystallinity during particle formation has not been addressed in detail; nor have in situ total scattering experiments ventured into the field of low-concentration, room-temperature syntheses in organic solvents to date. In this report, we follow the crystallization of ZnO nanoparticles in ethanol in the presence of different organic ligands. Low coordinated zinc precursor clusters rapidly polymerize upon base addition to particles of ca. 1 nm in diameter. In situ SAXS experiments reveal that the overall particle size increases to 2 to 4 nm with advancing reaction time. Complementary in situ PDF experiments show smaller crystalline domain sizes, which are only one third to half as large as the particle diameter. The ZnO particles thus feature a crystalline core surrounded by a disordered shell. Both, the core and the shell diameter are influenced by the different surface-bound organic ligands, which prevent an immediate relaxation to fully crystalline particles. A slow crystallization takes place in solution. We assume a dynamic equilibrium of the ligand and solvent molecules at the particle surface, which enables gradual bond restructuring. With suitably adjusted synthesis conditions, in our case by a continuous base addition, we show how to bypass the disordered intermediates, allowing the spontaneous nucleation of fully crystalline nanoparticles

Modellierung der Selbstorganisation nanopartikulärer Systeme: Wachstum und Keimbildung von Zinkoxid- und Silbernanopartikeln

Diese Arbeit befasst sich mit der Modellierung atomarer Vorgänge während der Keimbildung und des Wachstums von Zinkoxid- und Silbernanopartikeln. Die methodische Herangehensweise umfasst das Erarbeiten und Aufstellen von geeigneten Modellreaktionen, sowie die Entwicklung von Simulationsprotokollen und Analysemethoden. Hauptsächlich wird dabei auf Molekulardynamiksimulationen in Verbindung mit Kraftfeldern zurückgegriffen. Unterstützt werden diese Simulationen von statischen Berechnungen mittels der Dichtefunktionaltheorie und ab initio Methoden. Die untersuchte Modellreaktion für das Wachstum von Silbernanopartikeln ist die sogenannte „Polyol“-Synthese. Bei dieser wird ein Silbersalz in Ethylenglykol gelöst und anschließend reduziert und ausgefällt. Das Wachstum kleiner Silbernanopartikel (Atomzahl 13 bis 163) unter verschiedenen Redoxbedingungen konnte umfangreich studiert werden. Es zeigte sich ein systematischer Zusammenhang zwischen Ladung, Packung und Form der Cluster. Im zweiten Teil der Arbeit stand die Enstehung von Zinkoxidpartikeln in der Sol-Gel-Synthese im Mittelpunkt. Bei dieser Synthese reagiert ein Zinksalz (ZnAc2) in ethanolischer Lösung durch Zugabe einer Base (LiOH) zu Zinkoxid. Der erste untersuchte Schritt des Zinkoxidwachstums ist die Aggregation und Reifung von Zn4O(Ac)6 Clustern. Eine Studie mit systematischen Dichtefunktionaltheorierechnungen zeigte Reaktionsmöglichkeiten durch Ligandenaustauschreaktionen (Ac– gegen OH– ). Weiterhin wurde das Wachstum der wichtigsten Zinkoxidoberflächen – (10-10) und (000-1) – durch die Abscheidung der Ionen Zn2+ und OH– simuliert. Ergänzend zum „ungestörten“ Wachstum wurde der Einfluss verschiedener Additive (Acetat, 2-Ethylhexanoat, Citrat und Hexylamin) untersucht

A first-principles based force-field for Li+ and OH− in ethanolic solution

We report on the development of force-field parameters for accurately modeling lithium and hydroxide ions in ethanol in solution. Based on quantum calculations of small molecular clusters mimicking the solvent structure of individual ions as well as the solvated LiOH dimer, significant improvements of off-the-shelf force-fields are obtained. The quality of our model is demonstrated by comparison to ab initio molecular dynamics of the bulk solution and to experimental data available for ethanol/water mixtures

Molecular Mechanisms of ZnO Nanoparticle Dispersion in Solution: Modeling of Surfactant Association, Electrostatic Shielding and Counter Ion Dynamics

<div><p>Molecular models of 5 nm sized ZnO/Zn(OH)2 core-shell nanoparticles in ethanolic solution were derived as scale-up models (based on an earlier model created from ion-by-ion aggregation and self-organization) and subjected to mechanistic analyses of surface stabilization by block-copolymers. The latter comprise a poly-methacrylate chain accounting for strong surfactant association to the nanoparticle by hydrogen bonding and salt-bridges. While dangling poly-ethylene oxide chains provide only a limited degree of sterical hindering to nanoparticle agglomeration, the key mechanism of surface stabilization is electrostatic shielding arising from the acrylates and a halo of Na⁺ counter ions associated to the nanoparticle. Molecular dynamics simulations reveal different solvent shells and distance-dependent mobility of ions and solvent molecules. From this, we provide a molecular rationale of effective particle size, net charge and polarizability of the nanoparticles in solution.</p></div

Illustration of the colloid model II highlighting the pathways of Na⁺ counterions (yellow) within 0.5 ns.

<p>While the solvent is not shown for clarity, the effective colloid dimensions including the halo is illustrated as a surrounding surface. Atom colors: Zn (cyan), O (red), H(white), Na(yellow). Polymer colors: EO (pink), MA (green).</p

Number of block-copolymers and sodium ions bound as surfactants to the nanoparticles, number of charge carriers and thickness of the halo of counterions, effective radius and colloid charge, net dipole moment and polarizability as assessed for all model systems investigated.

<p>Number of block-copolymers and sodium ions bound as surfactants to the nanoparticles, number of charge carriers and thickness of the halo of counterions, effective radius and colloid charge, net dipole moment and polarizability as assessed for all model systems investigated.</p

Occurrence profile a) of the dipole moment as sampled from the solvated colloid model II.

<p>The width of the Gaussian fit (black curve) is used to estimate the polarizability of the halo of counterions. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125872#pone.0125872.s002" target="_blank">S2 Fig</a> for analogous plots of models I and III. The dipole moment b) of the ZnO/Zn(OH)₂ core-shell nanoparticle (red curve) and the colloid including surfactants and the halo of counterions (blue curve) are shown as functions of time. In agreement with ab initio calculations related to bulk ZnO crystal we observe that surface passivation with hydroxides already reduces the dipole moment by a factor of almost 10 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125872#pone.0125872.ref010" target="_blank">10</a>]. Surfactants and counterions lead to further reduction by about 50%.</p

Left: Diffusion coefficient (blue) of the Na⁺ ions as a function of the distance to the colloid and fitted switching function (black) used for quantification of the degree of ion association to the colloid (model II).

<p>Right: same as left, but additionally illustrating the number of Na⁺ ions (red curve). The vertical blue line denotes the surface distance delimiter discriminating halo and bulk counter ions. The horizontal red lines point out the number of counter ions directly associated with the particle (58.7), and the total number of Na⁺ ions including the halo (74.9). All data was averaged over 100 ns. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125872#pone.0125872.s001" target="_blank">S1 Fig</a> for analogous plots of models I and III.</p

Ratio of surface areas normal to the polar axis of ZnO with respect to lateral faces and absolute numbers of surfactants bound to the top, bottom and lateral faces.

<p>For comparison of association to the top and to lateral faces, the numbers of surfactants were put in relation to the corresponding surface areas.</p><p>Ratio of surface areas normal to the polar axis of ZnO with respect to lateral faces and absolute numbers of surfactants bound to the top, bottom and lateral faces.</p

The evolution of crystalline ordering for ligand-ornamented zinc oxide nanoparticles

Recent total scattering experiments have opened up the possibility to study nanoparticle formation in situ and to observe the structural transformation from precursor clusters to adult particles. Organic ligand molecules interact with precursors of metal oxide nanoparticles, yet their influence onto the evolution of crystallinity during particle formation has not been addressed in detail; nor have in situ total scattering experiments ventured into the field of low-concentration, room-temperature syntheses in organic solvents to date. In this report, we follow the crystallization of ZnO nanoparticles in ethanol in the presence of different organic ligands. Low coordinated zinc precursor clusters rapidly polymerize upon base addition to particles of ca. 1 nm in diameter. In situ SAXS experiments reveal that the overall particle size increases to 2 to 4 nm with advancing reaction time. Complementary in situ PDF experiments show smaller crystalline domain sizes, which are only one third to half as large as the particle diameter. The ZnO particles thus feature a crystalline core surrounded by a disordered shell. Both, the core and the shell diameter are influenced by the different surface-bound organic ligands, which prevent an immediate relaxation to fully crystalline particles. A slow crystallization takes place in solution. We assume a dynamic equilibrium of the ligand and solvent molecules at the particle surface, which enables gradual bond restructuring. With suitably adjusted synthesis conditions, in our case by a continuous base addition, we show how to bypass the disordered intermediates, allowing the spontaneous nucleation of fully crystalline nanoparticles