Particle Formation, Growth, and Agglomeration

This field of research has recently been introduced into our activities and is intended to be expanded in the future. Co-operations with several groups of the cluster of excellence “Engineering of Advanced Materials” (EAM) are already established. In the following some current projects are introduced.

Growth of Metal Oxide Nanoparticles

In our working group we investigate the growth and ripening of ZnO semiconductor nanoparticles (NPs). These exhibit promising electro-optical properties which can be adjusted for different applications due to the quantum size effect. Thus, they can be integrated into electronic devices as well as in thin film solar cells where they act as an electron transfer system. For these applications stable and well-defined NPs have to be designed, and knowledge about the nucleation and growth processes as well as the stabilization is crucial.

Precursor

Fig. 1: Structure of the tetrahedral precursor molecule for the production of ZnO nanoparticles; black: Zn, red: O, blue: C, white: H.

To synthesize these nanoparticles we use a standard ansatz developed by Meulenkamp and Spanhel in which an ethanolic solution of zinc acetate dehydrate is heated for 3 h under solvothermal conditions to form a tetrahedral precursor (cf. Fig. 1). These precursor molecules consist of a tetragonal Zn4O unit and are stabilized by six surrounding acetate molecules.   The addition of an ethanolic solution of the strong base LiOH to the precursor solution starts the ZnO formation.

The formed ZnO NPs ripen in dependence of time and temperature to larger sizes. We study this ripening process by in-situ small angle X-ray scattering (SAXS) and absorption spectroscopy (UV-Vis) simultaneously. Typical time resolved SAXS and UV-Vis measurements of freshly prepared ZnO nanoparticles are displayed in figure 2. From these measurements the temporal evolution of the size and shape distributions of the nanoparticles can be determined and allow to get a detailed understanding of the kinetics of the underlying formation, growth and aging processes.

UV-Vis spectra

Fig. 2: Time resolved UV-Vis measurements of ZnO nanoparticles during their ripening at 20°C

 

SAXS measurements

Fig. 3: Time resolved SAXS measurements  of ZnO nanoparticles during their ripening at T = 50 °C

In addition to this well established sample system we recently started to study the influence of different bases on the synthesis of the ZnO nanoparticles as well as the effect of the addition of different stabilizer agents to the solution. Due to the compexity of these systems our SAXS studies need to be complemented by further techniques like small angle neutron scattering (SANS) which is in contrast to SAXS highly sensitive to the organic stabilizer molecules also in presence of the inorganic nanoparticles. To get insight into the native solution we also use cryo –TEM techniques.

Further topics in this area which are worked on in our group:

Formation mechanism of metal nanoparticles in solution