Mechanochemically induced sulfur doping in ZnO via oxygen vacancy formation.

Surface defects of ZnO nanoparticles were induced via mechanical stressing using a Turbula shaker mixer and a planetary ball mill, and the possibilities for surface modification and functionalization of the ZnO nanoparticles were exemplified by sulfur doping of activated ZnO. Raman spectroscopy reveals that the formation of oxygen vacancies (VO) does not only occur under high stressing conditions in a planetary ball mill but even upon rather 'mild stressing' in the shaker mixer. The temporal evolution of the vacancy concentration in ZnO stressed under different conditions can be described by a model that accounts for stress number and vacancy diffusion with diffusion coefficients of VO of 3.7 × 10-21 m2 s-1 and 2.4 × 10-20 m2 s-1 for stressing in the shaker and the planetary ball mill, respectively. The thickness of the VO layer was estimated to be about 1 nm. Thiourea was mixed with defective ZnO particles, and then heated at various temperatures for sulfur-doping. A linear relationship between the amount of induced VO and the level of sulfur doping was found. Remarkably, mechanical activation is indispensable in order to control the level of sulfur doping quantitatively. High-angle annular dark field scanning transmission electron microscopy (HAADF STEM) observations with energy dispersive X-ray spectroscopy (EDX) analysis clearly revealed that the doped sulfur atoms are concentrated at the particle surface. Thus, ZnO (core)/ZnS (shell) structures are obtained easily via mechanochemical activation and subsequent thermal treatment.

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.

Synthesis of silver nanoparticles in melts of amphiphilic polyesters.

The current work presents a one-step procedure for the synthesis of amphiphilic silver nanoparticles suitable for production of silver-filled polymeric materials. This solvent free synthesis via reduction of Tollens' reagent as silver precursor in melts of amphiphilic polyesters consisting of hydrophilic poly(ethylene glycol) blocks and hydrophobic alkyl chains allows the production of silver nanoparticles without any by-product formation. This makes them especially interesting for the production of medical devices with antimicrobial properties. In this article the influences of the chain length of the hydrophobic block in the amphiphilic polyesters and the process temperature on the particle size distribution (PSD) and the stability of the particles against agglomeration are discussed. According to the results of spectroscopic and viscosimetric investigations the silver precursor is reduced to elemental silver nanoparticles by a single electron transfer process from the poly(ethylene glycol) chain to the silver ion.

Investigation of the size-property relationship in CuInS2 quantum dots.

In this work we investigated fundamental properties of CuInS2 quantum dots in dependence of the particle size distribution (PSD). Size-selective precipitation (SSP) with acetone as poor solvent was performed as an adequate post-processing step. Our results provide deep insight into the correlation between particle size and various optical characteristics as bandgap energy, absorption and emission features and the broadness of the emission signal. These structure-property relationships are only achieved due to the unique combination of different analytical techniques. Our study reveals that the removal of 10 wt% of smallest particles from the feed results in an enhancement of the emission signal. This improvement is ascribed to a decreased quenching of the emission in larger particles. Our results reveal the impact of PSDs on the properties and the performance of an ensemble of multicomponent QDs and anticipate the high potential of controlling PSDs by well-developed post-processing.