Torben Schindler

A Combined SAXS/SANS Study for the in Situ Characterization of Ligand Shells on Small Nanoparticles: The Case of ZnO.

ZnO nanoparticles (NPs) have great potential for their use in, e.g., thin film solar cells due to their electro-optical properties adjustable on the nanoscale. Therefore, the production of well-defined NPs is of major interest. For a targeted production process, the knowledge of the stabilization layer of the NPs during and after their formation is of particular importance. For the study of the stabilizer layer of ZnO NPs prepared in a wet chemical synthesis from zinc acetate, only ex situ studies have been performed so far. An acetate layer bound to the surface of the dried NPs was found; however, an in situ study which addresses the stabilizing layer surrounding the NPs in a native dispersion was missing. By the combination of small angle scattering with neutrons and X-rays (SANS and SAXS) for the same sample, we are now able to observe the acetate shell in situ for the first time. In addition, the changes of this shell could be followed during the ripening process for different temperatures. With increasing size of the ZnO core (d(core)) the surrounding shell (d(shell)) becomes larger, and the acetate concentration within the shell is reduced. For all samples, the shell thickness was found to be larger than the maximum extension of an acetate molecule with acetate concentrations within the shell below 50 vol %. Thus, there is not a monolayer of acetate molecules that covers the NPs but rather a swollen shell of acetate ions. This shell is assumed to hinder the growth of the NPs to larger macrostructures. In addition, we found that the partition coefficient μ between acetate in the shell surrounding the NPs and the total amount of acetate in the solution is about 10% which is in good agreement with ex situ data determined by thermogravimetric analysis.

In Situ Study on the Evolution of Multimodal Particle Size Distributions of ZnO Quantum Dots: Some General Rules for the Occurrence of Multimodalities.

Properties of small semiconductor nanoparticles (NPs) are strongly governed by their size. Precise characterization is a key requirement for tailored dispersities and thus for high-quality devices. Results of a careful analysis of particle size distributions (PSDs) of ZnO are presented combining advantages of UV/vis absorption spectroscopy, analytical ultracentrifugation, and small-angle X-ray scattering (SAXS). Our study reveals that careful cross-validation of these different methods is mandatory to end up with reliable resolution. PSDs of ZnO NPs are multimodal on a size range of 2-8 nm, a finding that is not yet sufficiently addressed. In the second part of our work the evolution of PSDs was studied using in situ SAXS. General principles for the appearance of multimodalities covering a temperature range between 15 and 45 °C were found which are solely determined by the aging state indicated by the size of the medium-sized fraction. Whenever this fraction exceeds a critical diameter, a new multimodality is identified, independent of the particular time-temperature combination. A fraction of larger particles aggregates first before a fraction of smaller particles is detected. Fixed multimodalities have not yet been addressed adequately and could only be evidenced due to careful size analysis.

Mesoscopic structures of triglyceride nanosuspensions studied by small-angle X-ray and neutron scattering and computer simulations.

Aqueous suspensions of platelet-like shaped tripalmitin nanocrystals are studied here at high tripalmitin concentrations (10 wt % tripalmitin) for the first time by a combination of small-angle X-ray and neutron scattering (SAXS and SANS). The suspensions are stabilized by different lecithins, namely, DLPC, DOPC, and the lecithin blend S100. At such high concentrations the platelets start to self-assemble in stacks, which causes interference maxima at low Q-values in the SAXS and SANS patterns, respectively. It is found that the stack-related interference maxima are more pronounced for the suspension stabilized with DOPC and in particular DLPC, compared to suspensions stabilized by S100. By use of the X-ray and neutron powder pattern simulation analysis (XNPPSA), the SAXS and SANS patterns of the native tripalmitin suspensions could only be reproduced simultaneously when assuming the presence of both isolated nanocrystals and stacks of nanocrystals of different size in the simulation model of the dispersions. By a fit of the simulated SAXS and SANS patterns to the experimental data, a distribution of the stack sizes and their volume fractions is determined. The volume fraction of stacklike platelet assemblies is found to rise from 70% for S100-stabilized suspensions to almost 100% for the DLPC-stabilized suspensions. The distribution of the platelet thicknesses could be determined with molecular resolution from a combined analysis of the SAXS and SANS patterns of the corresponding diluted tripalmitin (3 wt %) suspensions. In accordance with microcalorimetric data, it could be concluded that the platelets in the suspensions stabilized with DOPC, and in particular DLPC, are significantly thinner than those stabilized with S100. The DLPC-stabilized suspensions exhibit a significantly narrower platelet thickness distribution compared to DOPC- and S100-stabilized suspensions. The smaller thicknesses for the DLPC- and DOPC-stabilized platelets explain their higher tendency to self-assemble in stacks. The finding that the nanoparticles of the suspension stabilized by the saturated lecithin DLPC crystallize in the stable β-tripalmitin modification with its characteristic platelet-like shape is surprising and can be explained by the fact that the main phase transformation temperature for DLPC is, as for unsaturated lecithins like DOPC and S100, well below the crystallization temperature of the supercooled tripalmitin emulsion droplets.

Structural Characterization of Lecithin-Stabilized Tetracosane Lipid Nanoparticles. Part I: Emulsions

The structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-stabilized colloidal tetracosane emulsions was investigated by photon correlation spectroscopy and small-angle X-ray and neutron scattering, using emulsions with different neutron scattering contrasts. Special emphasis was placed on the structure of the DMPC stabilizer layer covering the emulsion droplets. A monolayer, structurally similar to a half DMPC bilayer, with a thickness of 16 Å is found. Thereby, the phosphocholine headgroups arrange flat at the oil-water interface. A deep penetration of the tetracosane oil into the stabilizer layer can be ruled out.

Changes within the stabilizing layer of ZnO nanoparticles upon washing

ZnO nanoparticles (NPs) are highly relevant for various industrial applications, however, after synthesis of the NPs residual chemicals need to be removed from the colloidal raw product by washing, as they may influence the performance of the final device. In the present study we focus on the effect of washing by antisolvent flocculation with subsequent redispersion of the NPs on the stabilizing acetate shell. Purification of the ZnO nanoparticles is reported to be optimal with respect to zeta potential that has a maximum after one washing cycle. In this work, we will shed light on this observation using small angle X-ray and neutron scattering (SAXS, SANS) by demonstrating that after the first washing cycle the content of acetate in the ligand shell around the ZnO NPs increases. In detail, it was observed that the diffuse acetate shell shrinks to the size of a monolayer upon washing but the acetate content of this monolayer is higher than within the diffuse shell of the particles of the native dispersion. A second washing cycle reduces the acetate concentration within the stabilizing shell and the stability of the dispersion drops accordingly. After another (third) washing cycle strong agglomeration was observed for all investigated samples.

From evaporation-induced self-assembly to shear-induced alignment.

The functionality of compact nanostructured thin films depends critically on the degree of order and hence on the underlying ordering mechanisms during film formation. For dip coating of rigid nanorods the counteracting mechanisms, evaporation-induced self-assembly (EISA) and shear-induced alignment (SIA) have recently been identified as competing ordering mechanisms. Here, we show how to achieve highly ordered and homogeneous thin films by controlling EISA and SIA in dip coating. Therefore we identify the influences of the process parameters including temperature, initial volume fraction and nanorod aspect ratio on evaporation-induced convective flow and externally applied shear forces and evaluate the resulting films. The impact of evaporation and shear can be distinguished by analysing film thickness, surface order and bulk order by careful in situ SAXS, Raman and SEM-based image analysis. For the first time we derive processing guidelines for the controlled application of EISA and SIA towards highly ordered thin nematic films.

Influence of Tail Groups during Functionalization of ZnO Nanoparticles on Binding Enthalpies and Photoluminescence

We report on the tailoring of ZnO nanoparticle (NP) surfaces by catechol derivatives (CAT) with different functionalities: tert-butyl group (tertCAT), hydrogen (pyroCAT), aromatic ring (naphCAT), ester group (esterCAT), and nitro group (nitroCAT). The influence of electron-donating/-withdrawing properties on enthalpy of ligand binding (ΔH) was resolved and subsequently linked with optical properties. First, as confirmed by ultraviolet/visible (UV/vis) and Fourier transform infrared (FT-IR) spectroscopy results, all CAT molecules chemisorbed to ZnO NPs, independent of the distinct functionality. Interestingly, the ζ-potentials of ZnO after functionalization shifted to more negative values. Then, isothermal titration calorimetry (ITC) and a mass-based method were applied to resolve the heat release during ligand binding and the adsorption isotherm, respectively. However, both heat- and mass-based approaches alone did not fully resolve the binding enthalpy of each molecule adsorbing to the ZnO surface. This is mainly due to the fact that the Langmuir model oversimplifies the underlying adsorption mechanism, at least for some of the tested CAT molecules. Therefore, a new, fitting-free approach was developed to directly access the adsorption enthalpy per molecule during functionalization by dividing the heat release measured via ITC by the amount of bound molecules determined from the adsorption isotherm. Finally, the efficiency of quenching the visible emission caused by ligand binding was investigated by photoluminescence (PL) spectroscopy, which turned out to follow the same trend as the binding enthalpy. Thus, the functionality of ligand molecules governs the binding enthalpy to the particle surface, which in turn, at least in the current case of ZnO, is an important parameter for the quenching of visible emission. We believe that establishing such correlations is an important step toward a more general way of selecting and designing ligand molecules for surface functionalization. This allows developing strategies for tailored colloidal surfaces beyond empirically driven formulation on a case by case basis.