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

Abstract

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.

2D analysis of polydisperse core–shell nanoparticles using analytical ultracentrifugation

“…34 Moreover, small-angle X-ray (SAXS) and small-angle neutron scattering (SANS) studies conducted in cooperation with the Chair for Crystallography and Structural Physics at FAU also showed that scattering data for ZnO colloids can only be well fitted when assuming polydisperse spherical NPs. 17, 35 In contrast, data could not be fitted when using models for shape anisotropic NPs.…”

“…A combined SANS/SAXS study revealed slightly larger shell thicknesses reaching from 0.7 nm to 1.3 nm. 35 In contrast to AUC, the shell thickness provided by SANS/SAXS is not derived from the hydrodynamic properties of the NP but it describes the native stabilizing layer of the NPs and how far it extends into the dispersion medium. Thus, also acetate molecules exceeding the shear plane of the NP are considered by SANS/SAXS, which is the reason why slightly larger values in case of scattering methods are clearly expected.…”

Safe‐by‐Design Ligand‐Coated ZnO Nanocrystals Engineered by an Organometallic Approach: Unique Physicochemical Properties and Low Toxicity toward Lung Cells

“…[4][5][6] Ak ey role in advancing ZnO NCs has been playedb yt he classical sol-gelm ethodology, [7,8] providing accesst oamultitude of tailoredZ nO nanostructures. Moreover,i nadequate controlo ver the nucleation and growth of quantum-sized ZnO NCs causes low reproducibility and lacking uniformity of the composition of the inorganic core/organics hell interface of the particles, which are associatedw ith ac orrugated surface structure, instability, and permeability of the "swollen" organic coating [10] (Scheme 1). Moreover,i nadequate controlo ver the nucleation and growth of quantum-sized ZnO NCs causes low reproducibility and lacking uniformity of the composition of the inorganic core/organics hell interface of the particles, which are associatedw ith ac orrugated surface structure, instability, and permeability of the "swollen" organic coating [10] (Scheme 1).…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…The very dynamic nature of the NC surface and the strong interplay between capping ligands and the inorganic core [16] are also likely important factors determining charge carrier trapping. Unfortunately, the organic surface ligands are invisible in electron microscopy and can currently be investigated only on the ensemble level using infrared absorption [249, 265, 266], nuclear magnetic resonance spectroscopy [249, 267, 268], or neutron scattering [269, 270]. …”