Wolfgang Peukert

Facile synthesis and post-processing of eco-friendly, highly conductive copper zinc tin sulphide nanoparticles

Cu2ZnSnS4 (CZTS) nanoparticles have shown promising properties to be used as an energy harvesting material. They are usually synthesised under inert atmosphere or vacuum, whereas the subsequent step of film formation is carried out under an atmosphere of sulphur and/or Sn in order to avoid the decomposition of CZTS nanoparticles into binary and ternary species as well as the formation of the corresponding oxides. In the present paper we show that both the synthesis of CZTS nanoparticles and the film formation from the corresponding suspension can be considerably simplified. Namely, the synthesis can be carried out without controlling the atmosphere, whereas during the film annealing a nitrogen atmosphere is sufficient to avoid the depletion of the CZTS kesterite phase. Furthermore, an integrated approach including in-depth Raman analysis is developed in order to deal with the challenges associated with the characterization of CZTS nanoparticles in comparison to bulk systems. The formation of competitive compounds during the synthesis such as binary and ternary sulphides as well as metal oxides nanoparticles is discussed in detail. Finally, the as-produced films have ten times higher conductivity than the state of the art.

Breakage behaviour of different materials—construction of a mastercurve for the breakage probability

Abstract An approach to quantify the grinding behaviour of different materials is presented. Based on a dimensional analysis and on fracture mechanical considerations, two material parameters, fMat. and Wm,min, are derived theoretically. fMat. characterises the resistance of particulate material against fracture in impact comminution. fMat. comprises the effect of particle size on fracture and the resistance against the external load which is quantified by the specific impact energy. Wm,min characterises the specific energy which a particle can take up without fracture. Values for fMat. and Wm,min which determine the grindability of different materials are given. Both the dimensional analysis and the fracture mechanical considerations lead to the same influence of initial particle size, mass-specific impact energy and newly derived material parameters on the comminution result. Experimentally, the material parameters are determined by single-particle impact tests. Together with the initial particle size and the impact energy, they allow for a quantitative description of the breakage probability of different materials in the form of a mastercurve and can also be applied for the qualitative description of the breakage function. The results for five polymers, limestone and glass spheres of different sizes are shown.

Mechanical production and stabilization of submicron particles in stirred media mills

A joint research project between the Technical University of Braunschweig and the Technical University of Munchen investigates the possibilities for the production of stable product suspensions in a particle size range smaller than 100 nm. This paper shows the experimental setup which allows the measurement of the most important electrochemical properties and the analysis of the particle size distribution of the product suspension as well as an adjustment of the pH value for stabilization during the comminution process. Results for comminution of fused corundum with different grinding media materials and grinding media sizes are shown. In addition, results showing the influence of the electrostatic stabilization on the grinding progress are presented. Further, the rheology of the product suspension is examined depending on grinding progress and suspension stability.

Carbon nanodots: toward a comprehensive understanding of their photoluminescence.

We report the characterization of carbon nanodots (CNDs) synthesized under mild and controlled conditions, that is, in a microwave reactor. The CNDs thus synthesized exhibit homogeneous and narrowly dispersed optical properties. They are thus well suited as a testbed for studies of the photophysics of carbon-based nanoscopic emitters. In addition to steady-state investigations, time-correlated single-photon counting, fluorescence up-conversion, and transient pump probe absorption spectroscopy were used to elucidate the excited-state dynamics. Moreover, quenching the CND-based emission with electron donors or acceptors helped shed light on the nature of individual states. Density functional theory and semiempirical configuration-interaction calculations on model systems helped understand the fundamental structure-property relationships for this novel type of material.

Analysis of Optical Absorbance Spectra for the Determination of ZnO Nanoparticle Size Distribution, Solubility, and Surface Energy

We present a model to calculate particle size distributions (PSDs) of colloidal ZnO nanoparticles from their absorbance spectra. Using literature values for the optical properties of bulk ZnO and correlating the measurement wavelengths in the UV-visible regime with distinct particle sizes by a tight binding model (TBM), an algorithm deconvolutes the absorbance spectra into contributions from size fractions. We find an excellent agreement between size distributions determined from TEM images and the calculated PSDs. For further validation, bimodal PSDs have been investigated and an approach to determine not only particle size but also solid concentration is introduced. We will show the applicability of our model by the determination of temperature-dependent ripening rates, which enables the calculation of solubilities, surface tensions, and the activation enthalpy of ripening. In principle, our methodology is applicable to different semiconductor nanoparticles in various solvents as long as their bulk properties are known and scattering is negligible.

Efficient drug-delivery using magnetic nanoparticles — biodistribution and therapeutic effects in tumour bearing rabbits

UNLABELLED To treat tumours efficiently and spare normal tissues, targeted drug delivery is a promising alternative to conventional, systemic administered chemotherapy. Drug-carrying magnetic nanoparticles can be concentrated in tumours by external magnetic fields, preventing the nanomaterial from being cleared by metabolic burden before reaching the tumour. Therefore in Magnetic Drug Targeting (MDT) the favoured mode of application is believed to be intra-arterial. Here, we show that a simple yet versatile magnetic carrier-system (hydrodynamic particles diameter <200nm) accumulates the chemotherapeutic drug mitoxantrone efficiently in tumours. With MDT we observed the following drug accumulations relative to the recovery from all investigated tissues: tumour region: 57.2%, liver: 14.4%, kidneys: 15.2%. Systemic intra-venous application revealed different results: tumour region: 0.7%, liver: 14.4 % and kidneys: 77.8%. The therapeutic outcome was demonstrated by complete tumour remissions and a survival probability of 26.7% (P=0.0075). These results are confirming former pilot experiments and implying a milestone towards clinical studies. FROM THE CLINICAL EDITOR This team of investigators studied drug carrying nanoparticles for magnetic drug targeting (MDT), demonstrating the importance of intra-arterial administration resulting in improved clinical outcomes in the studied animal model compared with intra-venous.

Experimental Investigation into the Influence of Mixing on Nanoparticle Precipitation

Precipitation is a promising method for the economic production of commercial amounts of nanoparticles because it is fast, and operable at ambient temperature. However, process control – due to the rapidity of the involved processes of mixing, nucleation, growth, and agglomeration – and stabilization against agglomeration represent challenges. This paper shows how these challenges can be successfully handled. The focus of this work is therefore set on how to tailor the particle-size distribution in continuous precipitation. Precipitation experiments with barium sulfate in a T-mixer are presented. It was found that the size of the precipitated primary particles is strongly dependent on the mixing intensity. On increasing the mixing intensity, it was possible to generate particles of approximately 50 nanometers in diameter. The second challenge, to stabilize the particles against agglomeration, was successfully met by adsorbing potential-determining ions on the particle surfaces, i.e., by increasing repulsive particle interactions. Thus, stable suspensions of barium sulfate nanoparticles were obtained.

Dispersive forces of particle–surface interactions: direct AFM measurements and modelling

Abstract Fundamentals of particle–particle interaction are of great interest in agglomeration processes. Particle adhesion depends on dispersive forces (van der Waals force), local chemical bindings, Coulomb force and capillary attractions. Additionally, surface properties like roughness, adsorption layers and surface chemistry strongly affect adhesion forces. van der Waals interactions are poorly understood because popular ab initio force calculations for molecules like density functional theory (DFT) often do not lead to proper results. van der Waals forces are difficult to measure directly. We present direct measurements of particle–particle and particle–surface interactions in the gas phase carried out with an atomic force microscope (AFM). Special emphasis is given to a proper statistical treatment of the data. For modelling of particle adhesion, we use computer-assisted empirical potential methods. Parameters like adsorbed water and surface roughness are considered. We extract parameters for weak interactions from the Lifshitz theory and gas adsorption data. Adsorbing molecules can be used as probes to measure dispersive forces. Studying surface and particle properties combined with computer-assisted modelling is a basic requisite to reach the aim of predicting particle–particle interactions in industrial processes.

Industrial separation of fine particles with difficult dust properties

This paper describes possibilities to separate particles with difficult dust properties from gases. Difficult dust properties are related to extreme values of particle size and shape and to the flowability, the adhesion properties or the reactivity of the particles. Special emphasis is given to submicron particles. In cyclones, conductive particles such as diesel soot can be removed by means of additional electrostatic forces. Experimental investigations into wet tubular electrostatic precipitators show that the measured separation efficiencies are much higher than theoretically anticipated. This result is explained by higher particle charges than predicted by the existing charging models. For small flow rates where electrostatic precipitators are economically not feasible, a new type of wet scrubber may be an alternative. The critical issue of surface filters is the adhesion of the dust cake at the surface of the filter medium. Regeneration of the dust cake as well as trends for the separation efficiency can be determined by small coupon testers which can be used for lab investigations and for field tests. Surface filters are widely used in industry for the separation of nanoparticles. Even extremely sticky particles such as tar particles can be removed by surface filters if a precoat layer protects the filter medium. It is shown that by looking at the physical fundamentals of particle separation, new and innovative solutions can be discovered. Guidelines for the separation of particles with difficult dust properties are given.

Determination of the Quantum Dot Band Gap Dependence on Particle Size from Optical Absorbance and Transmission Electron Microscopy Measurements

This work addresses the determination of arbitrarily shaped particle size distributions (PSDs) from PbS and PbSe quantum dot (QD) optical absorbance spectra in order to arrive at a relationship between band gap energy and particle size over a large size range. Using a modified algorithm which was previously developed for ZnO, we take only bulk absorption data from the literature and match the PSDs derived from QD absorbance spectra with those from transmission electron microscopical (TEM) image analysis in order to arrive at the functional dependence of the band gap on particle size. Additional samples sized solely from their absorbance spectra with our algorithm show excellent agreement with TEM results. We investigate the influence of parameters of the TEM image analysis such as threshold value on the final result. The band gap versus size relationship developed from analysis of just two samples lies well within the bounds of a number of published data sets. We believe that our methodology provides an attractive shortcut for the study of various novel quantum-confined direct band gap semiconductor systems as it permits the band gap energies of a broad size range of QDs to be probed with relatively few synthetic experiments and without quantum mechanical simulations.