Humboldt-Universität zu Berlin - Mathematisch-Naturwissenschaftliche Fakultät - Prof. Rademann

Methods and Materials Science

Research in the group of Rademann at the Humboldt-Universität in the Physical Chemistry Department (since 1993) is strongly focused on antinomy, bismuth, tellurium and noble metals (copper, silver, gold) with relevance to structure, function and stability for the study of energy conversion mechanisms. In the thermoelectric series of elements according to Seebeck-potentials antimony and bismuth have the highest, positive or negative values amongst the metals. Therefore we studied the properties and stabilities of these two elements and compounds formed thereof (more than 29 publications). A great number of our articles are also related to the stabilisation of nanoparticles of copper, silver, and gold as well as their respective oxides.  According to a Web-of-Science bibliometric study by now appeared 33 publications. Our strategy is to enhance the electric conductivity and simultaneously decrease the thermal conductivity of these materials.


As for applications of functionalized nanoparticles of copper, silver, gold and palladium in the research field and environmental sciences the specific environments of the nanoparticles become of utmost importance: We study nanoparticles (i) at surfaces, (ii) in glassy matrices (SIMO glasses), or (iii) in solutions (colloidal nanoparticles). 

Klaus Rademann is interested in the properties of metal nanoparticles exactly at the interface between chemical bonding (localized electrons) and delocalized electrons (plasmonics). This interface has been detected systematically for copper, silver, gold and mercury, always when the particles’ diameter is around 1 nm (about 30 to 40 atoms). Experimentally, it is still a great challenge for chemists or physicists to synthesize, stabilize, analyze and fully characterize these particles. Generally speaking, isolated or functionalized nanoparticles with a typical diameter of 1 nm show extremely fascinating electronic, magnetic, optical, chemical and thermoelectric properties. Materials with dimensions of about one nanometer are expected to be superior for advanced applications in energy conversion devices. We fabricate and study (i) prototypes of gold based catalysts for efficient chemical energy conversion processes, (ii) long term stable 3 UV/VIS-IR up or down converters based on soda-lime silicate glasses, which constitute a perfect stabilizing matrix for functionalized nanoparticles, and (iii) bismuth-copper based thermoelectric converters for transferring heat directly into electric current. This research is conducted in cooperation with BAM and HZB in Berlin.

1. Polymorphism (in co-operation with the Federal Institute for Materials Research and Testing, BAM, Dr. F. Emmerling)
Synchrotron X-Ray diffraction, crystal structure analysis through X-ray diffraction and Raman spectroscopy are useful tools to investigate polymorphism of pharmaceutical substances such as caffeine, nifedipine, o-terphenyl and ROY. Our research is focused on the identification of new metastable crystal phases. New insights are gained by time-resolved in-situ investigations using a broad range of analytical methods (AFM, STM, ESEM, WAXS, SAXS at μ-spot beamline at BESSY, DSC, dielectric spectroscopy and NMR).

2. Metal clusters in Glassy Matrices: Formation Mechanisms, Nucleation and Growth
Metal clusters in glassy matrices are of interest in the field of plasmonics, in particular sensing. The aim of our studies is the preparation of defined metal cluster doped layers within a glass matrix. One focus is the development of quantitative procedures which enables the generation of high concentrations of neutral metal clusters localized in layers close to phase boundaries. In particular, relevant transition metals are screened concerning differences in their growth mechanisms. For a directed design of optical systems a fundamental understanding of growth mechanisms of transition metals in glass is required. Systematic studies of the nucleation and growth processes are conducted in order to control and optimize those and generate localized plasmonic structures in glass. Time-resolved small angle x-ray scattering experiments can elucidate the nucleation mechanisms of transition metals in glassy matrices. The measurement of small angle scattering enables the deduction of particle sizes and size distributions and thus the elucidation of mechanisms.

3. Growth Mechanisms of Metal Colloids (in cooperation with Dr. Polte)
Colloidal nanoparticles have attracted much attention due to their unique properties and their potential applications. Nevertheless, a comprehensive picture of nanoparticle growth mechanisms does not exist yet, and the influence of synthesis parameters on the final particle size distribution remains unclear. As a consequence, the common approach for the design of synthetic procedures is based on trial and error. In contrast, profound understanding of nanoparticle growth processes can enable a directed design of synthetic procedures for nanoparticles on demand. The main goal of our research group is to identify general principles of nanoparticle growth and the development of theoretical models.
The investigation of nanoparticle growth mechanisms demands appropriate experimental techniques for the time-resolved in-situ characterization of particles. Therefore, we develop and optimize various setups which can provide information about the particle size distribution and concentration as well as about their optical properties. These setups are applied to study a whole range of systems, from metal nanoparticles in water to metal clusters in glass. The knowledge of the growth mechanism often enables the design of synthetic procedures for tailored particles which can be used e.g. for catalytic applications.

4. Mechanisms of Pulsed Laser Ablation of Suspensions for the Generation of Nanostructured Thermoelectric Materials
Pulsed laser ablation in liquids (PLAL) is a versatile route towards stable colloids without the need for stabilizing agents. The use of suspensions instead of bulk targets further simplifies the experimental set-up and even improves the productivity. However, the utilization of this approach is hindered by limited knowledge about the underlying mechanisms of the nanoparticle formation. Therefore, we investigate different chemical precursor materials which either follow a fragmentation mechanism or a reductive ablation mechanism. The generated nanomaterials are characterized by energy filtered transmission electron microscopy and electron energy loss spectroscopy in cooperation with the Helmholtz-Zentrum Berlin. The knowledge gained from these model systems will be used for the synthesis of nanostructured materials for improved thermoelectric devices.

5. In-situ Investigations of Formation and Growth of Iron Oxide Nanoparticles by Microwave-Assisted Solvothermal Synthesis
Solvothermal syntheses are one of most applied methods for the preparation of highly crystalline and monodisperse metal oxide nanoparticles. Nevertheless the drawbacks of this method are long reaction times, expensive autoclaves and the impossibility of investigations of nanoparticle growth. As a consequence of these drawbacks, microwave-assisted solvothermal syntheses (MWASS) have become subject of renewed fundamental and applied interests. Main features of MWASS are distinguished control and exact on-line determination of pressure and temperature inside the sealed vessel as a reactor. The advantage in contrast to conventional heating is the efficient internal volumetric “in-core” heating by direct coupling of MW energy to the reaction molecules. Our work includes innovative in-situ investigations of the formation and growth mechanisms of iron oxide nanoparticles by an innovative MWASS-system. The system allows exact addition of precursor solutions into the sealed vessel reactor and withdrawals of colloid solution at any time online. Thereby we are able to investigate and characterize the nanoparticles by TEM, UV-Vis spectroscopy, small-angle X-ray scattering, and EXAFS. With the help of these results we provide concepts for the formation and growth mechanisms.

6. Copper(I) oxide based thermoelectric powders and pastes with high Seebeck coefficients
Copper(I) oxide based compounds are investigated as earth abundant, environmentally friendly thermoelectric materials. Copper(I) oxide powders from different vendors are first examined, resulting in Seebeck coefficients of approximately 650 μV/K. Pastes of the powders with two different polymer matrices, polychlorotrifluoroethene and polydimethylsiloxane, in various mass ratios produce values reaching 567 μV/K. Small quantities of transition metal oxides added to the already mentioned compounds produce further enhancements up to 729 μV/K in case of the powders and 606 μV/K for the polymers. The results offer motivation for further long-term research in the field of thermoelectric pastes. This research field is still in its foundation stage.

7. Nanoparticle catalysis
Gold nanoparticles have been extensively studied during recent years because of their catalytic activity. CTAB-stabilized gold nanoparticles are synthesized by applying the seeding-growth approach in order to gain information about the size dependence of the catalytic reduction of p-nitrophenol to p-aminophenol with sodium borohydride. Colloidal solutions of stabilized gold nanoparticles are characterized by TEM, AFM, UV-Vis, SAXS, and DLS for their particle size distributions. Gold nanoparticles (mean sizes: 3.5, 10, 13, 28, 56 nm diameter) are tested for their catalytic efficiency. Kinetic data are acquired by UV-Vis spectroscopy at different temperatures between 25 and 45 °C. By studying the p-nitrophenol to p-aminophenol reaction kinetics we determine the nanoparticle size which is needed to gain the fastest conversion under ambient conditions in the liquid phase. Unexpectedly, CTAB-stabilized gold nanoparticles with a diameter of 13 nm are catalytically most efficient.

8. Aerogels
Mixed metal–semiconductor nanocrystal aerogels are fabricated, which are light-emitting and highly porous macroscopic monoliths. Thiol-stabilized CdTe and noble metal nanoparticles (Au, Pd and bimetallic Au/Pd) from aqueous synthesis act as building blocks for the hybrid material. The metal colloids undergo a surface-modification to enhance the particle stability and achieve thiol functionalities. Addition of H2O2 or a photochemical treatment is applied for the gelation process which is found to be reversible by subsequent addition of thiol molecules. Via supercritical drying aerogels are formed. The variation of the initial CdTe to metal nanoparticle ratio permits a facile tuning of the content, the work function and the resulting properties of the aerogels. The obtained structures are characterized by means of optical spectroscopy, electron microscopy, elemental analysis, and nitrogen physisorption. Materials of this type combining the optical and catalytic properties of their nano-building blocks with very high porosity of the gel structure are of special interest for applications in nanosensing, photocatalysis and water oxidation reactions.