Prof. Dr. Doris Segets

Process Technology for Electrochemical Functional Materials
Institute for Combustion and Gas Dynamics – Reactive Fluids
University of Duisburg-Essen

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  • Chromatographic property classification of narrowly distributed ZnS quantum dots
    Süβ, S. and Bartsch, K. and Wasmus, C. and Damm, C. and Segets, D. and Peukert, W.
    Nanoscale 12 (2020)
    Although optimized synthesis methods for nanoparticles (NPs) on small scale can lead to narrow particle size distributions (PSDs) and thus defined optical properties, in particular during scale-up, an additional classification step must be applied to adjust the particle properties according to the needs of the later application. NP chromatography is a promising separation method, which can be potentially transferred to preparative and industrial scale. Herein, we demonstrate that remarkable classification of ZnS quantum dots (QDs) with respect to the fundamental band gap energy is achieved by chromatography although the PSD of the feed material is already very narrow (1.5-3.0 nm). We investigated the interactions of ZnS QDs with stationary and mobile phase materials in order to select a proper material couple so that irreversible NP adhesion, agglomeration, decomposition or dissolution of the ZnS QDs during the chromatographic experiments are avoided and highly reproducible chromatograms are obtained. Using a fraction collector, the already narrowly size distributed feed material was separated into coarse and fine fractions with distinct band gap energies. For characterization of the chromatographic fractionation, quantities known from particle technology, i.e. separation efficiency, cut size and yield, were adapted to the band gap energy distributions accessible from UV/Vis spectroscopy. The optimization of process conditions (flow rate, temperature, switching time of the fraction collector) allows fine-tuning of the property classification and therefore of the optical properties within the narrow distribution of the ZnS QDs. Our study shows the strength and high potential of chromatography for preparative and continuous separation of NPs even in case of narrow size-distributed sub-10 nm semiconductor QDs.
    view abstract10.1039/d0nr03890c
  • Diffusion of Gold Nanoparticles in Inverse Opals Probed by Heterodyne Dynamic Light Scattering
    Giraudet, C. and Knoll, M.S.G. and Galvan, Y. and Süß, S. and Segets, D. and Vogel, N. and Rausch, M.H. and Fröba, A.P.
    Transport in Porous Media 131 (2020)
    The diffusive behavior of nanoparticles inside porous materials is attracting a lot of interest in the context of understanding, modeling, and optimization of many technical processes. A very powerful technique for characterizing the diffusive behavior of particles in free media is dynamic light scattering (DLS). The applicability of the method in porous media is considered, however, to be rather difficult due to the presence of multiple sources of scattering. In contrast to most of the previous approaches, the DLS method was applied without ensuring matching refractive indices of solvent and porous matrix in the present study. To test the capabilities of the method, the diffusion of spherical gold nanoparticles within the interconnected, periodic nanopores of inverse opals was analyzed. Despite the complexity of this system, which involves many interfaces and different refractive indices, a clear signal related to the motion of particles inside the porous media was obtained. As expected, the diffusive process inside the porous sample slowed down compared to the particle diffusion in free media. The obtained effective diffusion coefficients were found to be wave vector-dependent. They increased linearly with increasing spatial extension of the probed particle concentration fluctuations. On average, the slowing-down factor measured in this work agrees within combined uncertainties with literature data. © 2019, Springer Nature B.V.
    view abstract10.1007/s11242-019-01364-1
  • Effects of filter structure, flow velocity, particle concentration and fouling on the retention efficiency of ultrafiltration for sub-20 nm gold nanoparticles
    Lee, H. and Segets, D. and Süß, S. and Peukert, W. and Chen, S.-C. and Pui, D.Y.H.
    Separation and Purification Technology 241 (2020)
    Ultrafiltration techniques with membranes of pore sizes under 100 nm have been widely applied in drinking water, wastewater, semiconductor and pharmaceutical process water treatments for nanoparticle (NP) and pathogen removal. The most direct way to evaluate the membrane performance is to experimentally obtain the size fractional retention efficiency. However, the real-life performance of the membrane in terms of fouling (or loading) characteristics and the effects of the concentration of challenging particles and rate of flux (or filtration velocities) on the filtration efficiency during fouling have not been well understood. In this study, systematic filtration experiments for filtration efficiency at clean and loaded conditions were conducted for three different 50 nm rated membrane filters, including PTFE (Polytetrafluoroethylene), PCTE (Polycarbonate Track-Etched) and MCE (Mixed Cellulose Ester) membranes, against 5, 10 and 20 nm Au NPs at different feed concentrations and fluxes. The results showed that the effects of feed concentration and flux are significant. This study provides important insights of retention mechanisms and efficiency for different ultrafiltration membrane structures at varied filtration velocities and fouling characteristics giving clear directions of future NP ultrafiltration research. © 2020 Elsevier B.V.
    view abstract10.1016/j.seppur.2020.116689
  • Flowsheet simulation of integrated precipitation processes
    Michaud, M. and Haderlein, M. and Segets, D. and Peukert, W.
    Dynamic Flowsheet Simulation of Solids Processes (2020)
    This work presents the fundamentals and exemplary applications of a generalized model for precipitation, aggregation and ripening processes including the formation of solid phases with two dimensions. The particle formation is governed by awidely applicable population balance approach. Solid formation processes are described via the numerically efficient Direct Quadrature Method of Moments (DQMOM), which can calculate the evolution of multiple solid phases simultaneously. The particle size distribution (PSD) is approximated by a summation of delta functions while the moment source term is approximated by a two-point quadrature. The moments to calculate the multivariate distributions are chosen carefully to represent the second order moments. Solid formation is based on themodel of Haderlein et al. (2017) and is extended by a multidimensional aggregation model. Now, the influences of mixing, complex hydrochemistry and particle formation dynamics including nucleation, growth and aggregation on multiphase precipitation processes are modelled and simulated along independent dimensions with high efficiency. © Springer Nature Switzerland AG 2020.
    view abstract10.1007/978-3-030-45168-4_8
  • Microstructure characteristics of non-monodisperse quantum dots: On the potential of transmission electron microscopy combined with X-ray diffraction
    Neumann, S. and Menter, C. and Mahmoud, A.S. and Segets, D. and Rafaja, D.
    CrystEngComm 22 (2020)
    Although the concept of quantum confinement was introduced more than thirty years ago, a wide application of quantum dots is still limited by the fact that monodisperse quantum dots with controlled optoelectronic properties are typically synthesized on a relatively small scale. Larger scale synthesis techniques are usually not able to produce monodisperse nanoparticles yet. In this contribution, we illustrate the capability of the combination of transmission electron microscopy and X-ray diffraction to reveal detailed and scale-bridging information about the complex microstructure of non-monodisperse quantum dots, which is the first step towards further upscaling of the techniques for production of quantum dots with controlled properties. As a model system, CdSe quantum dots synthesized using an automated robotic hot-injection method at different temperatures were chosen. The combined microstructure analytics revealed the size and shape of the CdSe nanocrystals and the kind, density and arrangement of planar defects. The role of the planar defects in the particle coarsening by oriented attachment and the effect of the planar fault arrangement on the phase constitution, on the crystallographic coherence of the counterparts and on the optoelectronic properties are discussed. © The Royal Society of Chemistry 2020.
    view abstract10.1039/d0ce00312c
  • Model-Based Optimization of Ripening Processes with Feedback Modules
    Spinola, M. and Keimer, A. and Segets, D. and Leugering, G. and Pflug, L.
    Chemical Engineering and Technology 43 (2020)
    In order to obtain high-quality particulate products with tailored properties, process conditions and their evolution in time must be chosen appropriately. Although the efficiency of these products depends on their dispersity in several dimensions, in established processes the particle size is usually the decisive variable to adjust. As part of the synthesis of these products, feedback modules are often incorporated so that a time-dependent ratio of the obtained product can flow back into the system. Moreover, the synthesis should be an energy- and resource-efficient process. To provide a means of ensuring this requirement, a model- and gradient-based, numerically efficient optimization tool for particle synthesis is presented which was developed to describe population balance equations incorporating feedback terms. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ceat.201900515
  • Modeling, simulation and optimization of process chains
    Spinola, M. and Keimer, A. and Segets, D. and Pflug, L. and Leugering, G.
    Dynamic Flowsheet Simulation of Solids Processes (2020)
    Processes in the field of chemical engineering do not consist of one single step, but typically a high number of strongly interconnected unit operations linked with recycling streams. This inherent complexity further exacerbates when distributed particle properties, i.e., dispersity, must be considered, noteworthy being the case whenever particulate products are in focus. Out of all five possible dimensions of dispersity (size, shape, composition, surface and structure) particle size most often determines the efficiency of particulate products. Thus, its optimization is key to reach tailored handling and end product properties. In this work, a model-based optimization tool for particle synthesis was elaborated which is often the first step of a process chain. It is described by population balance equations relying on the method of characteristics for the numerical simulation and on the usage of gradient information to enhance the performance of the optimization. The presented scheme to optimize time-dependent process conditions in a time efficient manner is applicable for a wide range of particle syntheses. © Springer Nature Switzerland AG 2020.
    view abstract10.1007/978-3-030-45168-4_16
  • Rapid Characterization and Parameter Space Exploration of Perovskites Using an Automated Routine
    Reinhardt, E. and Salaheldin, A.M. and Distaso, M. and Segets, D. and Peukert, W.
    ACS Combinatorial Science 22 (2020)
    Hybrid, e.g., organic inorganic, perovskites from the type methylammonium lead iodide CH3NH3PbI3 are promising solar cell materials. However, due to the large parameter space spanned by the manifold combinations of divalent metals with organic cations and anions, an efficient approach is needed to rapidly test and categorize new promising materials. Herein, we developed a high throughput approach for the automated synthesis of perovskite layers with different precursor ratios at varying annealing temperatures. The layers were analyzed by optical absorption and photoluminescence (PL) spectroscopy as well as X-ray diffraction (XRD) and evaluated using two different procedures. The first one is a stepwise exclusion of nonperforming reactant ratios and synthesis conditions by using both spectroscopic techniques, followed by a final validation of the procedure by XRD. In the second procedure, only PL results were consulted in combination with high throughput screening using design of experiments (DoE) to reduce the total number of experiments needed and compared to the manual cascade approach. Noteworthy, by simple PL screening, it was possible to identify the best ratio of perovskite to byproducts and annealing temperature. Thus, only with PL, more detailed results as with the manual protocol were reached, while at the same time the effort for characterization was significantly reduced (by 60% of the experimental time). In conclusion, our approach opens a way toward fast and efficient identification of new promising materials under different reaction and process conditions. © 2019 American Chemical Society.
    view abstract10.1021/acscombsci.9b00068
  • Sedimentation Dynamics of Colloidal Formulations through Direct Visualization: Implications for Fuel Cell Catalyst Inks
    Bapat, S. and Segets, D.
    ACS Applied Nano Materials 3 (2020)
    Quantitative appraisal of real-world colloidal systems in their native state is key for knowledge-based nanoparticle formulations. Analytical centrifugation (AC) is touted as a quantitative methodology examining settling and creaming of dispersions, but an understanding of the main transmission readout is often nonintuitive and complex. Herein, we introduce a new visualization technique, called transmittograms, that readily depicts the time-resolved settling behavior of solid-liquid dispersions, measured by AC. We validate the utility of transmittogram analysis using silica particles. Further, we demonstrate the strength of the approach to study the nanoscale dynamics in complex fuel cell inks. © 2020 American Chemical Society.
    view abstract10.1021/acsanm.0c01467
  • Tailored SiNx-based Anode Processing for Li-Ion Batteries
    Bapat, S. and Oezcan, F. and Kilian, S.O. and Wiggers, H. and Segets, D.
    ECS Transactions 97 (2020)
    In order to successfully implement promising new battery materials at industrial production rates, it is important to provide suitable recipes for information. Typically, active materials are processed as dispersions in liquid-phase together with further additives aiming at the improvement of battery slurry properties. Interactions between particles and dispersing liquid(s) are decisive for electrode manufacturing and performance. Hence, we combined Hansen parameter approach with analytical centrifugation for dispersing SiNx nanoparticles as promising next generation battery material. Suitable probe liquids were chosen for the identification of beneficial dispersion properties. Electron microscopy was employed for a first qualitative plausibility analysis. Diacetone alcohol showed favorable dispersion properties for SiNx particles while toluene was found to be not suitable. Our approach and findings are an excellent starting point for the systematic characterization and evaluation of new battery materials with regard to processability. © 2020 ECS - The Electrochemical Society.
    view abstract10.1149/09707.0185ecst
  • Tailoring of Electrocatalyst Inks for Performance Enhancement in Proton Exchange Membrane Fuel Cells
    Bapat, S. and Fricke, S. and Kohsakowski, S. and Goessling, S. and Peinecke, V. and Segets, D.
    ECS Transactions 97 (2020)
    Solvent-based processing is the most favored method for the fabrication of electrocatalyst layers of proton exchange membrane fuel cells. In this process, the catalyst supported on carbon and stabilized by an ionomer is dispersed in a continuous solvent medium, forming a catalyst ink. Although the importance of ink processing has been realized, it is still governed by empiricism. To this end, we systematically investigated the effects of catalyst ink processing on the colloidal stability and the electrochemical performance of membrane electrode assemblies (MEAs). Analytical centrifugation and electron microscopy were employed to characterize the catalyst inks. It was found that high-pressure homogenization produced the most stable ink with maximum MEA performance in comparison to dispersion by ultrasonication. This study paves the way for a better understanding of the process-structure-property relationships towards the development of knowledge-based electrocatalyst inks. © 2020 ECS - The Electrochemical Society.
    view abstract10.1149/09707.0651ecst
  • Unraveling Complexity: A Strategy for the Characterization of Anisotropic Core Multishell Nanoparticles
    Lin, W. and Greve, C. and Härtner, S. and Götz, K. and Walter, J. and Wu, M. and Rechberger, S. and Spiecker, E. and Busch, S. and Schmutzler, T. and Avadhut, Y. and Hartmann, M. and Unruh, T. and Peukert, W. and Segets, D.
    Particle and Particle Systems Characterization 37 (2020)
    In this work, a widely applicable routine to characterize the core, surface, stability, and optical properties of CdSe/CdS/ZnS core–shell–shell nanorods after multiple growth steps is established. First, size, shape, and shell thickness of the nanorods are characterized by transmission electron microscopy (TEM), analytical ultracentrifugation (AUC), and small angle X-ray/neutron scattering (SAXS/SANS). In the next step, Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), and SANS measurements are applied to determine the surface species of nanorods. Then, the colloidal stability of the nanorods is investigated by UV–vis spectroscopy and dynamic light scattering (DLS) after different washing cycles. Finally, photoluminescence quantum yield (PLQY) of the nanorods during washing and sample storage is determined. With this highly complementary routine for particle characterization, the core, surface, stability, and optical properties of nanorods after multiple growth steps are resolved. The results demonstrate the importance of the developed toolbox to characterize such highly complex, anisotropic nanorods for a technical environment. This is of major importance for the handling of colloidal quantum materials and their quality control in industrial applications. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/ppsc.202000145
  • A solution-based ALD route towards (CH3NH3)(PbI3) perovskite: Via lead sulfide films
    Koch, V.M. and Barr, M.K.S. and Büttner, P. and Mínguez-Bacho, I. and Döhler, D. and Winzer, B. and Reinhardt, E. and Segets, D. and Bachmann, J.
    Journal of Materials Chemistry A 7 (2019)
    We present a procedure to grow thin films of lead sulfide (PbS) with 'solution Atomic Layer Deposition' (sALD), a technique which transfers the principles of ALD from the gas phase (gALD) to liquid processing. PbS thin films are successfully deposited on planar and porous substrates with a procedure that exhibits the unique ALD characteristics of self-limiting surface chemistry and linear growth at room temperature. The polycrystalline p-type PbS films are stoichiometric and pure. They are converted to the hybrid perovskite methylammonium iodoplumbate (methylammonium lead iodide, MAPI, CH3NH3PbI3) by annealing to 150 °C in the presence of vapors from methylammonium iodide (MAI). © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9ta09715e
  • Cleaning Matters!
    Mahmoud, A.S. and Segets, D.
    ACS Combinatorial Science 21 (2019)
    Translation of a manual process to high throughput for research and development requires special consideration. One important and often unreported aspect is the establishment of an efficient cleaning routine. This becomes significant, as precious time and, in particular, material would be lost, that is, when low-quality high-throughput experimentation is involved. We present a fully automated cleaning routine of the challenging synthesis of cadmium selenide quantum dots. Manual, semiautomated, and fully automated cleaning protocols were executed and compared in terms of spectral similarities of the synthesized colloids. Only the fully automated protocol enabled true 24/7 operation. © 2019 American Chemical Society.
    view abstract10.1021/acscombsci.9b00122
  • Evolution of the Ligand Shell Around Small ZnO Nanoparticles During the Exchange of Acetate by Catechol: A Small Angle Scattering Study
    Schindler, T. and Lin, W. and Schmutzler, T. and Lindner, P. and Peukert, W. and Segets, D. and Unruh, T.
    ChemNanoMat 5 (2019)
    The core-shell structure of colloids surrounded by ligands is of great importance for their later application as it can significantly alter the chemical and physical properties of the nanoparticles (NPs). A combination of small angle X-ray and neutron scattering (SAXS/SANS) in the native solution with additional ex situ measurements (titration-UV) was applied to study the NP/ligand interface of ZnO NPs after functionalization with catechol derivatives. Based on SAXS data, it was found that within the multimodal particle size distribution the fraction of agglomerates is shifted to smaller sizes and nearly disappeared upon the binding of ethyl 3,4-dihydroxybenzoate (CAT) molecules. This is ascribed to improved stabilization at the primary particle level by CAT molecules. By combining the neutron scattering contrast with the input of bound CAT molecules from a previously developed titration-UV method, the heterogeneous composition of the ligand shell became accessible for the first time. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cnma.201800465
  • Predicting collision efficiencies of colloidal nanoparticles in single spherical and fibrous collectors using an individual particle tracking method
    Lee, H. and Kim, S.C. and Chen, S.-C. and Segets, D. and Pui, D.Y.H.
    Separation and Purification Technology 222 (2019)
    We investigate the deposition of colloids onto granular and fibrous collectors by computational fluid dynamics (CFD) simulations. In particular the collision efficiency under unfavorable conditions, i.e., like-charged surfaces, was in focus. Particle trajectories were analyzed in a Lagrangian reference frame using a discrete phase model (DPM). By user-defined functions (UDFs) we incorporated interception as important deposition mechanism and calculated interaction energies between particle and collector surfaces utilizing the extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) theory. Adhesive and hydrodynamic torques acting on deposited particles were compared through the developed UDFs to consider particle detachment. Within each DPM process, all abovementioned calculations on every particle are performed continuously, allowing to understand particle deposition under different physico-chemical conditions. Simulated data on collision efficiencies for the granular collector were in good agreement with theory and experiments. Simulations for the fibrous collector showed that with increasing fluid velocity the hydrodynamic torque acting on particles attached to smaller fibers was increased. This enhanced the detachment and significantly lowered the collision efficiency, especially for larger particles. In conclusion, the developed CFD methods for predicting the collision efficiency on granular and fibrous collectors provide a powerful tool for examining the deposition behaviors of colloidal particles in porous media. © 2019 Elsevier B.V.
    view abstract10.1016/j.seppur.2019.04.025
  • Scalable classification of nanoparticles: A proof of principle for process design
    Menter, C. and Segets, D.
    Advanced Powder Technology 30 (2019)
    Nanoparticles like quantum confined ZnS semiconductor nanocrystals, exhibit unique structure-property relationships. Narrow particle size distributions (PSDs) become one of the most important factors to tailor product performance. Size selective precipitation has already been proven to be an effective post processing strategy for ZnS nanoparticles. It is based on the titration of a poor liquid into a stable dispersion, which leads to the preferred flocculation of larger particles. Afterwards, these flocks must be separated from the continuous phase. While on lab scale the formed flocks can be easily separated by centrifugation from the fine fraction, for larger scale production using continuous processes, new concepts are urgently needed. Herein we developed a filtration process for flock removal that allows the handling of larger quantities. For process design, we first investigated the flock properties in order to know how stable the generated flocks are and how the flock properties can be controlled. Then, we replaced the classical flock separation by centrifugation through separation by surface filtration under the constraint that the underlying separation efficiency was not affected. By the future use of properly controlled, alternating filtration modules, our work opens the door for establishing an urgently needed, scalable post-processing for sub-10 nm nanoparticles. © 2019 Society of Powder Technology Japan
    view abstract10.1016/j.apt.2019.08.027
  • Determination of Hansen parameters for particles: A standardized routine based on analytical centrifugation
    Süß, S. and Sobisch, T. and Peukert, W. and Lerche, D. and Segets, D.
    Advanced Powder Technology 29 (2018)
    The Hansen Solubility Parameters (HSP) are powerful descriptors to evaluate interactions of (polymer) molecules and their solubility in different liquids. Although approaches do exist to transfer the HSP-concept to the question of dispersibility of particles, HSP determination of slowly sedimenting (nano)particles (NPs) is time consuming and depends on the subjective evaluation of the experimenter. Herein, we introduce a new method for HSP determination for colloidal systems using analytical centrifugation (AC) which was applied to standardize and accelerate the experimental procedure. However, as not dissolution but dispersion is in focus, we propose to use the term Hansen Dispersibility Parameters (HDP) instead of HSP whenever dispersibility and stability of particles against agglomeration/flocculation are to be discussed. First, we implemented a standard dispersion routine for the well-known, industrially highly relevant pigment carbon black (CB). Then, a standardized method for the evaluation of measured AC profiles and appropriate ranking of NPs dispersibility in different media was developed. We demonstrate outstanding reproducibility of our results by comparing HDP derived for the same CB material from independent experiments performed at two different affiliations. Finally, we show the predictive power of HDP and the accuracy of our approach by evaluating the dispersibility of CB in additional liquids and mixtures of so-called “good” and “poor” liquid media crossing the border from stable to unstable medium conditions. Our study evidences the enormous potential of AC to determine the HDP of colloidal systems using a standardized and non-subjective method to access particle interactions and colloidal stability. © 2018 The Society of Powder Technology Japan
    view abstract10.1016/j.apt.2018.03.018
  • Efficient adsorption and sustainable degradation of gaseous acetaldehyde and o-xylene using rGO-TiO2 photocatalyst
    Lin, W. and Xie, X. and Wang, X. and Wang, Y. and Segets, D. and Sun, J.
    Chemical Engineering Journal 349 (2018)
    Two types of volatile organic chemicals (VOCs), acetaldehyde and o-xylene, were selected to probe the different adsorption and photodegradation mechanism of gaseous photocatalysis. Reduced graphene oxide (rGO)-TiO2 nanocomposites were prepared by facile solvothermal process to perform the photocatalytic reactions. In the experiments, the removal efficiencies of the acetaldehyde and o-xylene at 80 mL·min−1 flow rate were only 15% and 12% when P25 was applied, while the efficiencies were sharply increased to 42% and 54% by using 0.5 wt% rGO-TiO2 as the photocatalyst, respectively. Interestingly, it is notable that the removal efficiency of o-xylene was higher than that of acetaldehyde with identical rGO-TiO2 photocatalyst. Experiments suggested that there were possibly two reasons. Firstly, the adsorbance of o-xylene was more than that of acetaldehyde owing to the π-π conjugation between rGO and aromatic compounds, which was proved by adsorption equilibrium and TPD tests. ESR tests proved that rGO can promote the generation of surface [rad]OH radicals and depress the [rad]O2 − radicals formation. Compared with the dominant role played by [rad]O2 − radicals in the degradation of acetaldehyde, an almost equal position of [rad]O2 − and [rad]OH radicals was observed in the degradation of o-xylene according to the subsequent radical scavenger experiments. Moreover, the optimized rGO-TiO2 exhibited sustainable photocatalytic activity at 40–120 mL·min−1 flow rate through 160 min tests, while P25 was deactivate only after 25 min. This work demonstrated the different adsorption and degradation characteristics of two types of VOCs, which could propel the target orientation design of photocatalyst in VOCs removal applications. © 2018 Elsevier B.V.
    view abstract10.1016/j.cej.2018.05.107
  • Quantifying Surface Properties of Silica Particles by Combining Hansen Parameters and Reichardt's Dye Indicator Data
    Stauch, C. and Sebastian, S. and Luxenhofer, R. and Binks, B.P. and Segets, D. and Mandel, K.
    Particle and Particle Systems Characterization 35 (2018)
    To obtain quantitative understanding of the effects of a chemisorbed organic modification on the surface of particles, the use of Reichardt's dye (RD) and Hansen solubility parameter (HSP) is discussed, whereby the S should be understood in terms of “similarity” rather than solubility as dispersibility is in focus. Silica nanoparticles modified to different extents with a medium chain silane including completely hydrophilic and hydrophobic particles are chosen. During spray-drying such particles form fully redispersible micro-raspberry superstructures. After qualitative estimations of the particles' polarity based on measuring both immersion time and ability of modified particles to stabilize oil–water emulsions, surface properties are quantified by HSP and RD. With increasing hydrophobicity, i.e., increasing amount of silane at the surface, all three contributions to HSP change. At the same time, RD analysis reveals that the normalized solvent polarity parameter decreases progressively. HSP and RD analysis are in good agreement, giving strong confidence on each method applied individually. This work demonstrates that after noticeable attempts for combined solubility parameters in case of molecules, carbon allotropes, and gelators, such studies can be extended toward functional (nano)particles and that a full picture of particle surface properties is possible via the combination of different, quantitative techniques. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ppsc.201800328
  • Quantitative evaluation of nanoparticle classification by size-exclusion chromatography
    Süß, S. and Metzger, C. and Damm, C. and Segets, D. and Peukert, W.
    Powder Technology 339 (2018)
    Although few attempts for classification of nanoparticles (NPs) in labscale do exist, the transfer to industrial applications is still challenging. One promising separation method, which is already established for biological molecules, is chromatography. Herein, we study the classification of differently sized gold NPs (AuNPs) by size-exclusion chromatography (SEC). First, we investigated the interactions of AuNPs with potential stationary phases in order to identify a suitable material for the chromatographic process where irreversible NP adhesion is excluded. Then, we demonstrate the high reproducibility of our SEC experiments by multiple sample injections that lead to constant peak areas. In particular, we show the size-dependent elution behavior of AuNP mixtures resulting in bimodal elution peaks, where size separation was confirmed by inline measured UV/Vis spectra. Finally, NP classification results by using a fraction collector are characterized by retention time, mass balances and size-dependent separation efficiencies. The adjustment of the particle size distributions (PSDs) is demonstrated by changing the switching time of the fraction collector. Our study evidences the high potential of SEC for preparative and continuous separation of NPs. © 2018 The Authors
    view abstract10.1016/j.powtec.2018.08.008
  • Retention mechanisms of 1.7 nm ZnS quantum dots and sub-20 nm Au nanoparticles in ultrafiltration membranes
    Lee, H. and Segets, D. and Süß, S. and Peukert, W. and Chen, S.-C. and Pui, D.Y.H.
    Journal of Membrane Science 567 (2018)
    Membrane processes are considered to be a very effective and promising method for drinking water and wastewater treatments. However, particle removal mechanisms have not been fully elucidated due to complex surface interactions between colloids and membranes, especially for very small colloidal particles. In this study, a series of systematic filtration tests for eight different types of membrane filters, having nominal pore sizes from 0.005 to 0.1 µm, against 1.7 nm ZnS quantum dots (QDs) and 5, 10 and 20 nm Au nanoparticles (NPs) was performed to understand their retention mechanisms, including rejection in front of the filter surface and adsorption inside the filter. By comparing rejection, adsorption and recovery, it was found that the predominant retention mechanisms for retaining small NPs varied from filter to filter. For instance, electrostatic repulsion played a significant role for the rejection of NPs, i.e. impeding them entering the membrane pores in most membranes. In comparison, the Nylon membrane had a significant adsorption retention ability for Au NPs due to electrostatic attraction. Besides, it was found that filtration flow rate, or flux, was also an important parameter for the final retention because the enhanced hydrodynamic drag could trigger the detachment of deposited NPs or press NPs flowing through the superficial entrance leading to penetration. Tests of 10 nm Au NP retention using five different membranes with the same nominal pore size of 0.1 µm showed large variation of NP retention efficiencies demonstrating that pore size should not be used as the only criterion for rating filter performance, especially for small NPs. Our results provide not only detailed insights into the retention mechanisms of various membranes but also suggestions on how to select membrane filters for different filtration purposes. © 2018 Elsevier B.V.
    view abstract10.1016/j.memsci.2018.09.033
  • Simple and Reliable Method for Studying the Adsorption Behavior of Aquivion Ionomers on Carbon Black Surfaces
    Thoma, M. and Lin, W. and Hoffmann, E. and Sattes, M.-M. and Segets, D. and Damm, C. and Peukert, W.
    Langmuir 34 (2018)
    A better understanding of the interactions of carbon black and perfluorinated sulfonic acid (PFSA) ionomer helps to improve the effectiveness of polymer electrolyte membrane fuel cells. We present a simple and fast method for quantitative PFSA ionomer analysis based on suspension density measurements. After validation of the reliability of our method by thermogravimetric analysis and isothermal titration calorimetry (ITC), we investigate the adsorption equilibrium of short-side-chain PFSA ionomers of different equivalent weights (EW) and polarities on carbon black. The measured adsorption isotherms exhibit a plateau in the ionomer surface concentration for ionomer equilibrium concentrations ≤2 g/L. In this concentration range, the adsorption isotherms are described by the Langmuir model, whereby the surface concentrations in the plateau region are between 0.041 and 0.070 g/g. The plateau value of the ionomer surface concentration increases with EW and therefore with decreasing number of side chains with terminal sulfonic acid group per ionomer molecule, while the amount of adsorbed sulfonic acid groups remains constant for all investigated ionomers, resulting in similar ζ-potentials and sedimentation stability of the suspensions. The free energies of adsorption ΔG calculated from the association constants of the adsorption isotherms agree well with ΔG values obtained by isothermal titration calorimetry (ITC) and thus validate the adsorption isotherm measurement method. From the values of adsorption enthalpy ΔH ((-7.3 ± 0.8) kJ/mol) and entropy ΔS (ca. 100 J/(mol K)), which were extracted from ITC, we conclude that the ionomer adsorption on carbon black is a spontaneous physisorption process. © 2018 American Chemical Society.
    view abstract10.1021/acs.langmuir.8b02726
  • Suspension- and powder-based derivation of Hansen dispersibility parameters for zinc oxide quantum dots
    Süß, S. and Lin, W. and Getmanenko, O. and Pflug, L. and Sobisch, T. and Peukert, W. and Lerche, D. and Segets, D.
    Particuology (2018)
    For most particle-based applications, formulation in the liquid phase is a decisive step, and thus, particle interactions and stability in liquid media are of major importance. The concept of Hansen solubility parameters (HSP) was initially invented to describe the interactions of (polymer) molecules and their solubility in different liquids and is increasingly being used in particle technology to describe dispersibility. Because dispersions are not thermodynamically stable, the term Hansen dispersibility parameters (HDP) is used instead of HSP (Süß Sobisch, Peukert, Lerche, & Segets, 2018). Herein, we extend a previously developed standardized and non-subjective method for determination of Hansen parameters based on analytical centrifugation to the important class of quantum materials. As a technically relevant model system, zinc oxide quantum dots (QDs) were used to transfer our methodology to nanoparticles (NPs) with sizes below 10 nm. The results obtained using the standard procedure starting from a dried powder were compared with those obtained through redispersion from the wet sediment produced during the typical washing procedure of QDs, and drying was observed to play an important role. In conclusion, our study reveals the high potential of HDP for quantifying the interfacial properties of NPs as well as their link to dispersibility. © 2018 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences
    view abstract10.1016/j.partic.2018.05.010
  • The effect of mixing on silver particle morphology in flow synthesis
    Yang, T. and Segets, D. and Thajudeen, T. and Han, Y. and Peukert, W.
    Chemical Engineering Science 192 (2018)
    Silver particles, prepared in a T-mixer under different flow rates, were selected to study the influences of mixing on particle shape evolution. Mixing effects on particle growth were visualized by scanning electron microscopy (SEM) and analysed quantitatively by sedimentation coefficient distributions derived from analytical centrifugation (AC). The mixing time under different flow rates was determined by the Villermaux-Dushman method to quantify the mixing quality. Based on the finding of a mixing-induced shape transformation from plates to dendrites, an extended growth mechanism involving mixing effects was proposed. Slow mixing leads to a non-uniform distributed reactant mixture and low effective supersaturation. This causes preferential growth of high-energy facets resulting in plate-like particles with broad, multimodal sedimentation distributions. In contrast, fast mixing, corresponding to uniform reactant mixture and thus high effective supersaturation and nucleation rate, leads to dendritic products, and narrow but bimodal sedimentation distributions. © 2018
    view abstract10.1016/j.ces.2018.07.043
  • Ultrastable photodegradation of formaldehyde under fluorescent lamp irradiation by anti-reflection structure SnS2/TiO2 composite
    Lu, G. and Xie, X. and Wang, X. and Shi, G. and Zeng, Q. and Segets, D. and Sun, J.
    Journal of Photochemistry and Photobiology A: Chemistry 364 (2018)
    In this work, mesoporous titanium dioxide sphere Mp-TiO2/SnS2 composites have been synthesized for the photodegradation of formaldehyde with fluorescent lamp. SnS2 nanosheets/nanoparticles were deposited on the surface of Mp-TiO2 sphere uniformly through an in-situ method. The morphology of SnS2 can be modulated through adjusting the Sn/Ti molar ratio. SnS2 nanosheet turned into nanoparticle when the Sn/Ti molar ratio changed from 0.05 and 0.10 to 0.15 and 0.20. The physical and chemical properties of as-prepared composite catalysts were thoroughly studied by X-ray diffraction (XRD), Raman, scanning electron microscope (SEM), Brunauer Emmett Teller (BET), energy-dispersive X-ray spectroscopy (EDS), photoluminescence (PL) and UV–vis spectrophotometer. The photocatalytic performance of Mp-TiO2/SnS2 in the degradation of HCHO was assessed in a smog chamber with the irradiation of fluorescent light. A degradation efficiency of about 63% was achieved when the molar ratio of SnS2/TiO2 was 0.10. Through extending the light absorption range and hindering the recombination of photo-generated electron-hole pairs, the combination with SnS2 enabled the fabrication of hybrid photocatalysts with high photocatalytic activity and long-term stability. © 2018 Elsevier B.V.
    view abstract10.1016/j.jphotochem.2018.06.043
  • 2D analysis of polydisperse core-shell nanoparticles using analytical ultracentrifugation
    Walter, J. and Gorbet, G. and Akdas, T. and Segets, D. and Demeler, B. and Peukert, W.
    Analyst 142 (2017)
    Accurate knowledge of the size, density and composition of nanoparticles (NPs) is of major importance for their applications. In this work the hydrodynamic characterization of polydisperse core-shell NPs by means of analytical ultracentrifugation (AUC) is addressed. AUC is one of the most accurate techniques for the characterization of NPs in the liquid phase because it can resolve particle size distributions (PSDs) with unrivaled resolution and detail. Small NPs have to be considered as core-shell systems when dispersed in a liquid since a solvation layer and a stabilizer shell will significantly contribute to the particle's hydrodynamic diameter and effective density. AUC measures the sedimentation and diffusion transport of the analytes, which are affected by the core-shell compositional properties. This work demonstrates that polydisperse and thus widely distributed NPs pose significant challenges for current state-of-the-art data evaluation methods. The existing methods either have insufficient resolution or do not correctly reproduce the core-shell properties. First, we investigate the performance of different data evaluation models by means of simulated data. Then, we propose a new methodology to address the core-shell properties of NPs. This method is based on the parametrically constrained spectrum analysis and offers complete access to the size and effective density of polydisperse NPs. Our study is complemented using experimental data derived for ZnO and CuInS2 NPs, which do not have a monodisperse PSD. For the first time, the size and effective density of such structures could be resolved with high resolution by means of a two-dimensional AUC analysis approach. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c6an02236g
  • A widely applicable tool for modeling precipitation processes
    Haderlein, M. and Güldenpfennig, A. and Segets, D. and Peukert, W.
    Computers and Chemical Engineering 98 (2017)
    This work presents a generalized tool for modeling precipitation processes for the parallel formation of multiple solid phases. A symmetric mixing model for T-mixers is presented which mimics the mixing process in a numerically highly efficient way. This model can easily be extended to other reactor types such as stirred tank reactors following the implementation given in this work. Modelling ion activities and ion complexation is strongly accelerated by the analytical formulation of the Jacobian of the corresponding system of equations. Solid formation processes are described via the numerically efficient Direct Quadrature Method of Moments (DQMOM) which is parallelized for treating multiple solid phases simultaneously. Expressions for agglomeration of multiple solid phases and for particle transfer between different mixer zones are given. Both the models of the individual processes and the entire precipitation tool are validated and tested in multiple scenarios proving the flexibility of the tool. © 2016 Elsevier Ltd
    view abstract10.1016/j.compchemeng.2016.12.007
  • Adaptive Behavior of Dynamic Orthoester Cryptands
    Shyshov, O. and Brachvogel, R.-C. and Bachmann, T. and Srikantharajah, R. and Segets, D. and Hampel, F. and Puchta, R. and von Delius, M.
    Angewandte Chemie - International Edition 56 (2017)
    The integration of dynamic covalent bonds into macrocycles has been a tremendously successful strategy for investigating noncovalent interactions and identifying effective host–guest pairs. While numerous studies have focused on the dynamic responses of macrocycles and larger cages to various guests, the corresponding constitutionally dynamic chemistry of cryptands remains unexplored. Reported here is that cryptands based on orthoester bridgeheads offer an elegant entry to experiments in which a metal ion selects its preferred host from a dynamic mixture of competing subcomponents. In such dynamic mixtures, the alkali metal ions Li+, Na+, K+, Rb+, and Cs+exhibit pronounced preferences for the formation of cryptands of certain sizes and donor numbers, and the selection is rationalized by DFT calculations. Reported is also the first self-assembly of a chiral orthoester cryptate and a preliminary study on the use of stereoisomers as subcomponents. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201609855
  • Automated synthesis of quantum dot nanocrystals by hot injection: Mixing induced self-focusing
    Salaheldin, A.M. and Walter, J. and Herre, P. and Levchuk, I. and Jabbari, Y. and Kolle, J.M. and Brabec, C.J. and Peukert, W. and Segets, D.
    Chemical Engineering Journal 320 (2017)
    The hot injection technique for the synthesis of quantum dots (QDs) is a well-established and widely used method in the lab. However, scale-up rules do not exist. One reason is that in particular the role of process parameters like mixing on particle formation is largely unknown, as systematic examination of the latter is impossible for the laborious and complex manual synthesis. Herein we studied the mixing induced self-focusing of particle size distributions (PSDs) of CdSe QDs using automation in combination with a defined stirrer geometry. Basis for our study is a platform that allows parallelization with inline temperature monitoring, defined injection rate, accurate sampling times as well as controlled stirring. Reproducibility in terms of optical product properties was analyzed by absorption and emission whereas reproducibility in terms of the PSD was verified by deconvolution of UV/Vis absorbance spectra and especially by analytical ultracentrifugation (AUC) complemented by transmission electron microscopy (TEM). In line with previous results, AUC confirmed that even QDs made by hot injection in an automated setup are polydisperse with multimodal size distributions. Finally, reproducibility in combination with early stage sampling and controlled mixing allowed us for the first time to analyze the influence of stirring on focusing and defocusing of PSDs, that has been expressed in terms of the evolution of the relative standard deviation (RSD). Our work paves the way to gain in-depth understanding of often forgotten process-structure relationships of colloidal nanoparticles which eventually is a first step in the direction of the development of scalable synthesis and reliable application of high-quality QDs in technical applications. © 2017 Elsevier B.V.
    view abstract10.1016/j.cej.2017.02.154
  • Changes within the stabilizing layer of ZnO nanoparticles upon washing
    Schindler, T. and Schmutzler, T. and Schmiele, M. and Lin, W. and Segets, D. and Peukert, W. and Appavou, M.-S. and Kriele, A. and Gilles, R. and Unruh, T.
    Journal of Colloid and Interface Science 504 (2017)
    ZnO nanoparticles (NPs) are highly relevant for various industrial applications, however, after synthesis of the NPs residual chemicals need to be removed from the colloidal raw product by washing, as they may influence the performance of the final device. In the present study we focus on the effect of washing by antisolvent flocculation with subsequent redispersion of the NPs on the stabilizing acetate shell. Purification of the ZnO nanoparticles is reported to be optimal with respect to zeta potential that has a maximum after one washing cycle. In this work, we will shed light on this observation using small angle X-ray and neutron scattering (SAXS, SANS) by demonstrating that after the first washing cycle the content of acetate in the ligand shell around the ZnO NPs increases. In detail, it was observed that the diffuse acetate shell shrinks to the size of a monolayer upon washing but the acetate content of this monolayer is higher than within the diffuse shell of the particles of the native dispersion. A second washing cycle reduces the acetate concentration within the stabilizing shell and the stability of the dispersion drops accordingly. After another (third) washing cycle strong agglomeration was observed for all investigated samples. © 2017 Elsevier Inc.
    view abstract10.1016/j.jcis.2017.05.059
  • Choosing the right nanoparticle size-designing novel ZnO electrode architectures for efficient dye-sensitized solar cells
    Pfau, M.W. and Kunzmann, A. and Segets, D. and Peukert, W. and Wallace, G.G. and Officer, D.L. and Clark, T. and Costa, R.D. and Guldi, D.M.
    Journal of Materials Chemistry A 5 (2017)
    A novel concept for constructing optimized ZnO-based photoanodes as integrative components of dye-sensitized solar cells (DSSCs) is realized by deploying differently sized nanoparticles, ranging from 2 to 10 nm, together with commercially available 20 nm nanoparticles. The 2 nm nanoparticles were used to construct an efficient buffer layer for transparent electrodes based on 10 nm nanoparticles, resulting in a relative increase of device efficiency from 1.8 to 3.0% for devices without and with a buffer layer, respectively. A mixture of 10 and 20 nm nanoparticles was optimized to maximize the diffuse reflection and to minimize the charge transport resistance in a light-scattering layer. This optimization resulted in a homogenous layer of more than 15 μm that provided a device efficiency of 3.3%. The buffer layer, transparent electrode, and light-scattering electrode, were then combined to give an overall efficiency of around 5%. Thus, this work demonstrates that varying the electrode architecture with nanoparticles of different diameters is a powerful strategy for improving the overall efficiency of ZnO-based DSSCs. © The Royal Society of Chemistry.
    view abstract10.1039/c6ta11012f
  • Enhanced Crystallization of Lysozyme Mediated by the Aggregation of Inorganic Seed Particles
    Weichsel, U. and Segets, D. and Thajudeen, T. and Maier, E.-M. and Peukert, W.
    Crystal Growth and Design 17 (2017)
    We show that aggregation plays a major role in seeded growth of protein crystals. The seeded batch approach provides the opportunity to set the starting conditions for protein crystallization by adding a defined amount of well-characterized seed particles. The experimental observations for tetragonal hen egg-white lysozyme (LSZ) confirm the concept of the oriented aggregation of larger building blocks to form a protein crystal. It was shown that the aggregation of the seed particles/bioconjugates is advantageous for the product quality in terms of larger and more defined LSZ crystals and in terms of accelerated reaction kinetics. We present a population balance (PB) model for the seeded batch crystallization of LSZ considering the aggregation of growth units to form protein crystals. For the modeling of crystal growth, evolving particle size distributions (PSDs) of agglomerating LSZ molecules were measured by dynamic light scattering (DLS). Moreover, the aggregation of seed particles in LSZ solutions under crystallization conditions was investigated by DLS. In line with our expectations, the number of seeds was found to be important as it strongly affects the collision frequency in the aggregation term of our PB model. Finally, the applied model gives trends of the supersaturation depletion curves and orders of magnitude of the measured CSDs in particle size correctly, ranging from only a few nanometers up to micrometer-sized particles/crystals. Thus, by the combination of PB modeling and experimentally determined crystallization parameters, insights into the crystal formation mechanism were obtained. To the best of our knowledge, this is the first attempt to model growth within a crystal population by an aggregation mechanism induced by seeding with foreign particles. © 2016 American Chemical Society.
    view abstract10.1021/acs.cgd.6b01026
  • Influence of Tail Groups during Functionalization of ZnO Nanoparticles on Binding Enthalpies and Photoluminescence
    Lin, W. and Schmidt, J. and Mahler, M. and Schindler, T. and Unruh, T. and Meyer, B. and Peukert, W. and Segets, D.
    Langmuir 33 (2017)
    We report on the tailoring of ZnO nanoparticle (NP) surfaces by catechol derivatives (CAT) with different functionalities: tert-butyl group (tertCAT), hydrogen (pyroCAT), aromatic ring (naphCAT), ester group (esterCAT), and nitro group (nitroCAT). The influence of electron-donating/-withdrawing properties on enthalpy of ligand binding (ΔH) was resolved and subsequently linked with optical properties. First, as confirmed by ultraviolet/visible (UV/vis) and Fourier transform infrared (FT-IR) spectroscopy results, all CAT molecules chemisorbed to ZnO NPs, independent of the distinct functionality. Interestingly, the ζ-potentials of ZnO after functionalization shifted to more negative values. Then, isothermal titration calorimetry (ITC) and a mass-based method were applied to resolve the heat release during ligand binding and the adsorption isotherm, respectively. However, both heat- and mass-based approaches alone did not fully resolve the binding enthalpy of each molecule adsorbing to the ZnO surface. This is mainly due to the fact that the Langmuir model oversimplifies the underlying adsorption mechanism, at least for some of the tested CAT molecules. Therefore, a new, fitting-free approach was developed to directly access the adsorption enthalpy per molecule during functionalization by dividing the heat release measured via ITC by the amount of bound molecules determined from the adsorption isotherm. Finally, the efficiency of quenching the visible emission caused by ligand binding was investigated by photoluminescence (PL) spectroscopy, which turned out to follow the same trend as the binding enthalpy. Thus, the functionality of ligand molecules governs the binding enthalpy to the particle surface, which in turn, at least in the current case of ZnO, is an important parameter for the quenching of visible emission. We believe that establishing such correlations is an important step toward a more general way of selecting and designing ligand molecules for surface functionalization. This allows developing strategies for tailored colloidal surfaces beyond empirically driven formulation on a case by case basis. © 2017 American Chemical Society.
    view abstract10.1021/acs.langmuir.7b03079
  • Liquid filtration of nanoparticles through track-etched membrane filters under unfavorable and different ionic strength conditions: Experiments and modeling
    Lee, H. and Segets, D. and Süß, S. and Peukert, W. and Chen, S.-C. and Pui, D.Y.H.
    Journal of Membrane Science 524 (2017)
    Nanoparticle deposition experiments under unfavorable conditions were conducted experimentally and theoretically. The 0.2 and 0.4 µm rated track-etched membrane filters were challenged with 60, 100, 147, 220, 350 and 494 nm polystyrene latex (PSL) particles with different ionic strengths ranging from 0.005 to 0.05 M. The capillary tube model, with replacing the viscosity of air to water, was used to estimate the initial efficiency, or the transport efficiency of the particles to the filter surface, which was corrected in a second step by allowing the detachment of the nanoparticles according to the sum of adhesive and hydrodynamic torques. The adhesive torques were derived from surface interactions accessed by the extended DLVO theory. Calculation results showed that the adhesive torque of a particle located in the calculated primary minimum was slightly larger than the hydrodynamic torque, resulting in particle deposition. However, experimental data clearly indicated that detachment occurred. This could only be explained by the presence of additional hydration forces, leading to a larger separation which became relevant at high ionic strengths. By including hydration into our theoretical framework, experiment and theory were in very good agreement under all different ionic strength conditions. The findings allow a basic understanding of surface interactions between nanoparticles and membranes in micro- and ultra-filtration applications for drinking water production, wastewater treatment and particle free water production in industries. © 2016 Elsevier B.V.
    view abstract10.1016/j.memsci.2016.11.023
  • Mechanochemically induced sulfur doping in ZnO via oxygen vacancy formation
    Daiko, Y. and Schmidt, J. and Kawamura, G. and Romeis, S. and Segets, D. and Iwamoto, Y. and Peukert, W.
    Physical Chemistry Chemical Physics 19 (2017)
    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. © the Owner Societies 2017.
    view abstract10.1039/c7cp01489a
  • An experimental study of ultrafiltration for sub-10nm quantum dots and sub-150 nm nanoparticles through PTFE membrane and Nuclepore filters
    Chen, S.-C. and Segets, D. and Ling, T.-Y. and Peukert, W. and Pui, D.Y.H.
    Journal of Membrane Science 497 (2016)
    Ultrafiltration techniques (pore size of membrane below 100nm) are widely used in chemical engineering, semiconductor, pharmaceutical, food and beverage industries. However, for small particles, which are more and more attracting interests, the pore size often does not correlate well with sieving characteristics of the ultra-membranes. This may cause serious issues during modeling and prediction of retention efficiencies. Herein, a series of liquid filtration experiments with unfavorable conditions were performed. PTFE membranes (50, 100nm) and Nuclepore filters (50, 400nm) were challenged with 1.7nm manganese doped ZnS and 6.6 nm ZnO quantum dots (QDs), 12.4, 34.4 and 50 nm Au and 150 nm SiO2 nanoparticles. For larger and medium sized particles, sieving and eventually pore blockage phenomena were observed. In comparison, for small QDs, a high initial retention efficiency (>0.4) in both filters was monitored, followed by a reduced efficiency with ongoing particle loading. This high initial retention of small nanoparticles was attributed to diffusion deposition rather than to sieving since the ratio of pore size to particle size was significantly high (up to 58). Our experimental results allow a basic understanding of the deposition mechanism of small nanoparticles (diffusion vs. sieving) in different filter structures. © 2015 Elsevier B.V.
    view abstract10.1016/j.memsci.2015.09.022
  • Analysis of particle size distributions of quantum dots: From theory to application
    Segets, D.
    KONA Powder and Particle Journal 2016 (2016)
    Small, quantum-confined semiconductor nanoparticles, known as quantum dots (QDs) are highly important material systems due to their unique optoelectronic properties and their pronounced structure-property relationships. QD applications are seen in the emerging fields of thin films and solar cells. In this review, different characterization techniques for particle size distributions (PSDs) will be summarized with special emphasis on strategies developed and suggested in the past to derive data on the dispersity of a sample from optical absorbance spectra. The latter use the assumption of superimposed individual optical contributions according to the relative abundance of different sizes of a colloidal dispersion. In the second part, the high potential of detailed PSD analysis to get deeper insights of typical QD processes such as classification by size selective precipitation (SSP) will be demonstrated. This is expected to lead to an improved understanding of colloidal surface properties which is of major importance for the development of assumption-free interaction models. © 2016 Hosokawa Powder Technology Foundation.
    view abstract10.14356/kona.2016012
  • Classification of Nanoparticles by Size-Selective Precipitation: The Role of Solubility Parameters
    Süß, S. and Sobisch, T. and Lerche, D. and Mori, Y. and Peukert, W. and Segets, D.
    Chemie-Ingenieur-Technik 88 (2016)
    view abstract10.1002/cite.201650451
  • Ein verallgemeinertes Modellierungswerkzeug für die numerisch effiziente Beschreibung mehrphasiger Fällungsprozesse
    Haderlein, M. and Sterbak, M. and Segets, D. and Peukert, W.
    Chemie-Ingenieur-Technik 88 (2016)
    view abstract10.1002/cite.201650358
  • Extension of the Deep UV-Capabilities in Multiwavelength Spectrometry in Analytical Ultracentrifugation: The Role of Oil Deposits
    Walter, J. and Segets, D. and Peukert, W.
    Particle and Particle Systems Characterization 33 (2016)
    The optical performance of an analytical ultracentrifuge equipped with a fiber-coupled detector is investigated with special emphasis on the influence of oil leakage on the long-term deep ultraviolet stability. Two strategies for enhanced optical performance are presented. The first includes regeneration of fiber transmittance under atmospheric conditions, whereas the second utilizes an optimized vacuum system consisting of a turbomolecular pump. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ppsc.201500212
  • Multidimensional Nanoparticle Characterization by Means of Analytical Ultracentrifugation and Multiwavelength Detection
    Walter, J. and Nacken, T.J. and Thajudeen, T. and Segets, D. and Sherwood, P.J. and Stafford, W.F. and Peukert, W.
    Chemie-Ingenieur-Technik 88 (2016)
    view abstract10.1002/cite.201650214
  • Simultaneous Identification of Spectral Properties and Sizes of Multiple Particles in Solution with Subnanometer Resolution
    Karabudak, E. and Brookes, E. and Lesnyak, V. and Gaponik, N. and Eychmüller, A. and Walter, J. and Segets, D. and Peukert, W. and Wohlleben, W. and Demeler, B. and Cölfen, H.
    Angewandte Chemie - International Edition 55 (2016)
    We report an unsurpassed solution characterization technique based on analytical ultracentrifugation, which demonstrates exceptional potential for resolving particle sizes in solution with sub-nm resolution. We achieve this improvement in resolution by simultaneously measuring UV/Vis spectra while hydrodynamically separating individual components in the mixture. By equipping an analytical ultracentrifuge with a novel multi-wavelength detector, we are adding a new spectral discovery dimension to traditional hydrodynamic characterization, and amplify the information obtained by orders of magnitude. We demonstrate the power of this technique by characterizing unpurified CdTe nanoparticle samples, avoiding tedious and often impossible purification and fractionation of nanoparticles into apparently monodisperse fractions. With this approach, we have for the first time identified the pure spectral properties and band-gap positions of discrete species present in the CdTe mixture. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201603844
  • Spectra library: An assumption-free in situ method to access the kinetics of catechols binding to colloidal ZnO quantum dots
    Lin, W. and Haderlein, M. and Walter, J. and Peukert, W. and Segets, D.
    Angewandte Chemie - International Edition 55 (2016)
    Assumption-free and in situ resolving of the kinetics of ligand binding to colloidal nanoparticles (NPs) with high time resolution is still a challenge in NP research. A unique concept of using spectra library and stopped-flow together with a "search best-match" Matlab algorithm to access the kinetics of ligand binding in colloidal systems is reported. Instead of deconvoluting superimposed spectra using assumptions, species absorbance contributions (ligand@ZnO NPs and ligand in solution) are obtained by offline experiments. Therefrom, a library of well-defined targets with known ligand distribution between particle surface and solution is created. Finally, the evolution of bound ligand is derived by comparing in situ spectra recorded by stopped-flow and the library spectra with the algorithm. Our concept is a widely applicable strategy for kinetic studies of ligand adsorption to colloidal NPs and a big step towards deep understanding of surface functionalization kinetics. Well-established libraries of target spectra that are derived by means of careful offline analysis and identification of equilibrium data within larger kinetic datasets can be used for any particle-ligand system. Kinetics of ligand binding to nanoparticles can be derived free of assumption, in situ, and with high time resolution. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201508252
  • A Combined SAXS/SANS Study for the in Situ Characterization of Ligand Shells on Small Nanoparticles: The Case of ZnO
    Schindler, T. and Schmiele, M. and Schmutzler, T. and Kassar, T. and Segets, D. and Peukert, W. and Radulescu, A. and Kriele, A. and Gilles, R. and Unruh, T.
    Langmuir 31 (2015)
    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<inf>core</inf>) the surrounding shell (d<inf>shell</inf>) 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. © 2015 American Chemical Society.
    view abstract10.1021/acs.langmuir.5b02198
  • A general approach to study the thermodynamics of ligand adsorption to colloidal surfaces demonstrated by means of catechols binding to zinc oxide quantum dots
    Lin, W. and Walter, J. and Burger, A. and Maid, H. and Hirsch, A. and Peukert, W. and Segets, D.
    Chemistry of Materials 27 (2015)
    A general strategy to study the thermodynamics of ligand adsorption to colloidal surfaces was established. The versatility of our approach is demonstrated by means of catechols binding to ZnO quantum dots (QDs). First, isothermal titration calorimetry (ITC) was used to extract all relevant thermodynamic parameters, namely association constant, enthalpy, entropy, and free energy of ligand binding. Noteworthy, the determined δG of -20.3 ± 0.4 kJ mol-1 indicates a strong, reproducible, and exothermic interaction between the catechol anchor group and the oxide particle surface. To confirm the characterization of ligand binding by measuring the heat of adsorption, the free energy was cross-validated by mass-based adsorption isotherms. A combination of inductively coupled plasma optical emission spectroscopy (ICP-OES) and UV/vis spectroscopy was developed to quantitatively determine the mass of bound catechols with respect to the available particle surface. The association constant K was determined by a Langmuir fit to be 2618 M-1 which leads to δG = -19.50 kJ mol-1 according to δG = -RTln K. To close the mass balance, analytical ultracentrifugation (AUC) was applied to detect the amount of the free, unbound catechol in solution. Finally, Raman spectroscopy and nuclear magnetic resonance spectroscopy (NMR) were performed to quantify the amount of remaining acetate from particle synthesis and to distinguish bound (chemisorbed) and unbound (physisorbed) catechol. Our results reveal that approximately 65 wt % of acetate is replaced, and physisorbed catechol will not affect the amount of remaining acetate on the ZnO surface. Moreover, no pronounced chemical shift peak as it would be expected for free catechol is observed by NMR at all. This indicates a highly dynamic adsorption-desorption equilibrium between the free and the physisorbed state of catechol on the particle surface. Our concept of combined analytics is seen to be a generally applicable strategy for particle-ligand interfacial studies. It gives detailed insight into thermodynamics, binding states, and ligand composition and is thus considered as an important step toward tailored colloidal surface properties. © 2014 American Chemical Society.
    view abstract10.1021/cm504080d
  • Classification of zinc sulfide quantum dots by size: Insights into the particle surface-solvent interaction of colloids
    Segets, D. and Lutz, C. and Yamamoto, K. and Komada, S. and Süß, S. and Mori, Y. and Peukert, W.
    Journal of Physical Chemistry C 119 (2015)
    We present a detailed study on the classification of ZnS quantum dots (QDs) by size selective precipitation (SSP). SSP allows the postsynthetic narrowing of a given feed distribution and is usually realized by titration of a poor solvent into a suspension of dispersed particles. Thereby preferred flocculation of larger structures is induced. Our results confirm that SSP is a highly robust process, following a law of mass action. That means a certain solvent composition always leads to the same ratio between coarse and fines particles with respect to a specific particle size xi. This ratio is independent of the particle size distribution (PSD) of the feed, the washing history of the particles, and the solid concentration of the particles. Regarding the illustration of our findings, we established a combined approach that takes Hansen solubility parameters (HSP) of solvent mixtures as well as changing van der Waals interactions into account. Relating both to each other, a size-dependent region of enhanced solubility is clearly identified. Our concept allows a differentiation between volume-related effects like van der Waals interactions and surface-related effects like the interaction of a ligand with a solvent mixture. A comprehensive interpretation of classification results obtained with different good solvents and poor solvents enables to deduce a general strategy for the demanding determination of HSP for small colloids. Our work makes an important contribution to the design of an appropriate colloidal postprocessing which is applicable to larger quantities. © 2015 American Chemical Society.
    view abstract10.1021/jp508746s
  • Enhanced Nucleation of Lysozyme Using Inorganic Silica Seed Particles of Different Sizes
    Weichsel, U. and Segets, D. and Janeke, S. and Peukert, W.
    Crystal Growth and Design 15 (2015)
    In this work we investigate the impact of differently sized plain silica nanoparticles (NPs) between 10 and 200 nm on the crystallization of lysozyme (LSZ). In the first part of our work we investigate the electrostatic interactions between LSZ and NPs by zeta potential measurements and place special emphasis on the adsorption behavior of LSZ@SiO<inf>2</inf>. The determined adsorption isotherms - derived from UV-vis spectroscopy - indicate that with increasing particle size more LSZ is adsorbed per NP surface area probably due to a size-dependent surface chemistry and the variation of surface curvature. Second, seeded crystallization experiments both at the microliter and milliliter scale and thus close to a technically relevant scale were performed. A clearly extended crystallization window upon the addition of seed particles toward lower protein and salt concentrations was found. Moreover, induction times of crystal formation and crystallization times were considerably reduced. These effects were intensified with the addition of larger seed particles. In general, with the addition of silica seed particles, a shift of the final crystal size distribution to larger structures is observed. © 2015 American Chemical Society.
    view abstract10.1021/cg501681g
  • FIMOR: An efficient simulation for ZnO quantum dot ripening applied to the optimization of nanoparticle synthesis
    Haderlein, M. and Segets, D. and Gröschel, M. and Pflug, L. and Leugering, G. and Peukert, W.
    Chemical Engineering Journal 260 (2015)
    This work presents the application of a Fully Implicit Method for Ostwald Ripening (FIMOR) for simulating the ripening of ZnO quantum dots (QDs). Its stable numerics allow FIMOR to employ the full exponential term of the Gibbs-Thomson equation which significantly outperforms the common Taylor-approximation at typical QD sizes below 10. nm. The implementation is consistent with experimental data for temperatures between 10 and 50. °C and the computational effort is reduced by a factor of 100-1000 compared to previous approaches. This reduced the simulation time on a standard PC from several hours to a few minutes. In the second part, we demonstrate the high potential and accuracy of FIMOR by its application to several challenging studies. First, we compare numeric results obtained for ripening of ZnO QDs exposed to temperature ramps with experimental data. The deviation between simulation and experiment in the mean volume weighted particle size was as small as 5%. Second, a map for the process parameter space spanned by ripening time and temperature is created based on a large number (>50) of FIMOR runs. From this map appropriate process parameters to adjust a desired dispersity are easily deduced. Further data analysis reveals in agreement with literature findings that the particle size distribution converges towards a self-preserving stable shape. Equations describing the time dependent particle size distribution with high accuracy are presented. Finally, we realized the transfer from low volume batch experiments to continuous QD processing. We modeled the continuous ZnO synthesis in a fully automated microreaction plant and found an excellent agreement between the numeric prediction and the experimental results by considering the residence time distribution. © 2014 Elsevier B.V.
    view abstract10.1016/j.cej.2014.09.040
  • From in situ characterization to process control of quantum dot systems
    Segets, D. and Peukert, W.
    Procedia Engineering 102 (2015)
    Quantum confined semiconductor nanoparticles (quantum dots, QDs) are promising candidates for various applications in emerging fields like electronics, solar cells, sensors and diagnostics. However, a larger scale production of QDs at high product quality is still missing. One of the key requirements to address this issue in the near future was identified to be a fast and in situ applicable characterization method. Suitable characterization requires knowledge on the full shape of the particle size distributions (PSDs) under investigation. Thus, determination of a mean particle size together with the width of the PSD is not sufficient. In the following, a method will be presented that allows the derivation of arbitrary shaped PSDs for QDs with direct band gap based on their optical absorbance spectra. After validation of the technique by means of ZnO nanoparticles the transfer of the concept to other QD materials like PbS and PbSe will be proven. Therefore we will extend our methodology and show how our approach can be used to derive spectral properties like the size dependent band gap energy. This is realized by proper calibration of the calculation results against PSDs determined by an independent analysis technique like transmission electron microscopy (TEM). © 2015 The Authors.
    view abstract10.1016/j.proeng.2015.01.129
  • In Situ Study on the Evolution of Multimodal Particle Size Distributions of ZnO Quantum Dots: Some General Rules for the Occurrence of Multimodalities
    Schindler, T. and Walter, J. and Peukert, W. and Segets, D. and Unruh, T.
    Journal of Physical Chemistry B 119 (2015)
    Properties of small semiconductor nanoparticles (NPs) are strongly governed by their size. Precise characterization is a key requirement for tailored dispersities and thus for high-quality devices. Results of a careful analysis of particle size distributions (PSDs) of ZnO are presented combining advantages of UV/vis absorption spectroscopy, analytical ultracentrifugation, and small-angle X-ray scattering (SAXS). Our study reveals that careful cross-validation of these different methods is mandatory to end up with reliable resolution. PSDs of ZnO NPs are multimodal on a size range of 2-8 nm, a finding that is not yet sufficiently addressed. In the second part of our work the evolution of PSDs was studied using in situ SAXS. General principles for the appearance of multimodalities covering a temperature range between 15 and 45 °C were found which are solely determined by the aging state indicated by the size of the medium-sized fraction. Whenever this fraction exceeds a critical diameter, a new multimodality is identified, independent of the particular time-temperature combination. A fraction of larger particles aggregates first before a fraction of smaller particles is detected. Fixed multimodalities have not yet been addressed adequately and could only be evidenced due to careful size analysis. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcb.5b08005
  • Investigation of the size-property relationship in CuInS2 quantum dots
    Akdas, T. and Walter, J. and Segets, D. and Distaso, M. and Winter, B. and Birajdar, B. and Spiecker, E. and Peukert, W.
    Nanoscale 7 (2015)
    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. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5nr04291g
  • New possibilities of accurate particle characterisation by applying direct boundary models to analytical centrifugation
    Walter, J. and Thajudeen, T. and Süß, S. and Segets, D. and Peukert, W.
    Nanoscale 7 (2015)
    Analytical centrifugation (AC) is a powerful technique for the characterisation of nanoparticles in colloidal systems. As a direct and absolute technique it requires no calibration or measurements of standards. Moreover, it offers simple experimental design and handling, high sample throughput as well as moderate investment costs. However, the full potential of AC for nanoparticle size analysis requires the development of powerful data analysis techniques. In this study we show how the application of direct boundary models to AC data opens up new possibilities in particle characterisation. An accurate analysis method, successfully applied to sedimentation data obtained by analytical ultracentrifugation (AUC) in the past, was used for the first time in analysing AC data. Unlike traditional data evaluation routines for AC using a designated number of radial positions or scans, direct boundary models consider the complete sedimentation boundary, which results in significantly better statistics. We demonstrate that meniscus fitting, as well as the correction of radius and time invariant noise significantly improves the signal-to-noise ratio and prevents the occurrence of false positives due to optical artefacts. Moreover, hydrodynamic non-ideality can be assessed by the residuals obtained from the analysis. The sedimentation coefficient distributions obtained by AC are in excellent agreement with the results from AUC. Brownian dynamics simulations were used to generate numerical sedimentation data to study the influence of diffusion on the obtained distributions. Our approach is further validated using polystyrene and silica nanoparticles. In particular, we demonstrate the strength of AC for analysing multimodal distributions by means of gold nanoparticles. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c5nr00995b
  • On the mechanism of Zn4O-acetate precursors ripening to ZnO: How dimerization is promoted by hydroxide incorporation
    Milek, T. and Kirschbaum, R.W. and Gernler, M.S.V. and Lübbert, C. and Segets, D. and Drewello, T. and Peukert, W. and Zahn, D.
    Journal of Chemical Physics 143 (2015)
    We report on a combined experimental and molecular modelling study on Zn<inf>4</inf>O ion clusters stabilized by acetate molecules (OAc). In particular, ab initio calculations of acetate substitution by hydroxide ions are compared with mass spectrometry data. Though quantum calculations in the gas phase indicate strong energetic preference, no experimental evidence of stable Zn<inf>4</inf>O(OAc)<inf>6-x</inf>(OH)<inf>x</inf> clusters in ethanolic solutions could be observed. This apparent contradiction is rationalized by identifying the supportive role of hydroxide ions for the association of (OAc- → OH- substituted) Zn<inf>4</inf>O(OAc)<inf>6</inf> and Zn<inf>4</inf>O(OAc)<inf>5</inf>+ clusters. Mass spectrometry and quantum calculations hint at the stability of (Zn<inf>4</inf>O)<inf>2</inf>(OAc)<inf>12-x</inf>(OH)<inf>x</inf> dimers with x = 1, 2. Therein, the hydroxide ions establish salt-bridges that allow for the formation of additional Zn<inf>3</inf> motifs with the OH above the triangle center - a structural motif close to that of the ZnO-crystal. The association of Zn<inf>4</inf>O(OAc)<inf>6</inf> clusters is thus suggested to involve OAc- → OH- substitution as an activation step, quickly followed by dimerization and the subsequent agglomeration of oligomers. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4928190
  • Process engineering of nanoparticles below 20 nm—a fundamental discussion of characterization, particle formation, stability and post processing
    Segets, D. and Peukert, W.
    Colloid Process Engineering (2015)
    In the following chapter fundamental aspects that have to be considered during the processing of small nanoparticles will be addressed. We investigated quantum confined manganese doped ZnS, ZnO, PbS and PbSe semiconductor nanoparticles, so-called quantum dots (QDs) and silver noble metal nanorods. All materials were chosen due to their technical relevance for future applications in the emerging fields of solar cells, sensors and diagnostics as well as due to the possibility of their in situ characterization by UV/Vis absorbance spectroscopy. After a brief introduction to the specific prospects and challenges of these materials we will focus on the important processing issues that need to be solved for producing these particles at high quality on a larger scale: (i) the modelling of particle formation including nucleation, growth and ripening based on a mechanistic understanding and on experimentally derived data on solubility and surface energies, (ii) the stabilization of nanoparticles not only against agglomeration but also against shape changes and (iii) classification. The latter is realized by size selective precipitation which allows surprisingly sharp separations (κ= 0.75) of particles with only a few nm in diameter. Although the extremely small particle sizes (feed PSD between 1.5 and 3 nm), classification results were successfully analyzed by well-known concepts from particle technology. Our results are seen to be an essential contribution to colloidal processing. They enable a future optimization of process parameters by a knowledge-based design strategy that can be applied within continuous as well as automatized batch reactor concepts. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-15129-8_12
  • Simultaneous Analysis of Hydrodynamic and Optical Properties Using Analytical Ultracentrifugation Equipped with Multiwavelength Detection
    Walter, J. and Sherwood, P.J. and Lin, W. and Segets, D. and Stafford, W.F. and Peukert, W.
    Analytical Chemistry 87 (2015)
    Analytical ultracentrifugation (AUC) has proven to be a powerful tool for the study of particle size distributions, particle shapes, and interactions with high accuracy and unrevealed resolution. In this work we show how the analysis of sedimentation velocity data from the AUC equipped with a multiwavelength detector (MWL) can be used to gain an even deeper understanding of colloidal and macromolecular mixtures. New data evaluation routines have been integrated in the software SEDANAL to allow for the handling of MWL data. This opens up a variety of new possibilities because spectroscopic information becomes available for individual components in mixtures at the same time using MWL-AUC. For systems of known optical properties information on the hydrodynamic properties of the individual components in a mixture becomes accessible. For the first time, the determination of individual extinction spectra of components in mixtures is demonstrated via MWL evaluation of sedimentation velocity data. In our paper we first provide the informational background for the data analysis and expose the accessible parameters of our methodology. We further demonstrate the data evaluation by means of simulated data. Finally, we give two examples which are highly relevant in the field of nanotechnology using colored silica and gold nanoparticles of different size and extinction properties. (Figure Presented). © 2015 American Chemical Society.
    view abstract10.1021/ac504649c
  • Unified Design Strategies for Particulate Products
    Peukert, W. and Segets, D. and Pflug, L. and Leugering, G.
    Advances in Chemical Engineering 46 (2015)
    Unit operations and product design are the two most important pillars of chemical engineering. Product design is the formation, formulation, handling, manufacturing, and characterization of complex multiphase products with specific properties and is thus at the core of mesoscale science and engineering. The applications define the required product properties which cover both classical fields of process technology in the chemical industry as well as new emerging fields of electronics, energy and environmental technologies, life sciences, materials science and engineering, nanotechnology, and photonic technologies highlighting the broad relevance of mesoscale science. Unifying principles of product design are proposed which are widely applicable to many different kinds of products including solid, liquid, and even gaseous particles. Results from the Erlangen Cluster of Excellence "Engineering of Advanced Materials" show that the joint venture of chemical engineering with materials science in concert with the basic sciences opens new prospects for all involved disciplines. In particular, chemical and biochemical engineering expands through particle technologies also in physics-related fields of technology such as electronics, photonics, or 3D printing. Rigorous mathematical optimization methods based on predictive models for products, structures, and processes catalyze new possibilities for true design of particulate products which is at the core of mesoscale science and technology. © 2015 Elsevier Inc.
    view abstract10.1016/bs.ache.2015.10.004
  • Mixed layers of β-lactoglobulin and SDS at air-water interfaces with tunable intermolecular interactions
    Engelhardt, K. and Weichsel, U. and Kraft, E. and Segets, D. and Peukert, W. and Braunschweig, B.
    Journal of Physical Chemistry B 118 (2014)
    Mixtures of β-lactoglobulin (BLG) and sodium dodecyl sulfate (SDS) were studied at pH 3.8 and 6.7 under equilibrium conditions. At these pH conditions, BLG carries either a positive or a negative net charge, respectively, which enables tunable electrostatic interactions between anionic SDS surfactants and BLG proteins. For pH 3.8, vibrational sum-frequency generation (SFG) and ellipsometry indicate strong BLG-SDS complex formation at air-water interfaces that is caused by attractive electrostatic interactions. The latter complexes are already formed in the bulk solution which was confirmed by a thermodynamic study of BLG-SDS mixtures using isothermal titration calorimetry (ITC). For acidic conditions we determine from our ITC data an exothermal binding enthalpy of -40 kJ mol-1. Increasing SDS/BLG molar ratios above 10 leads to a surface excess of SDS and thus to a charge reversal from a positive net charge with BLG as the dominating surface adsorbed species to a negatively charged layer with SDS as the dominating surface species. The latter is evidenced by a pronounced minimum in SFG intensities that is also accompanied by a phase change of O-H stretching bands due to a reorientation of H2O within the local electric field. This phase change which occurs at SDS/BLG molar ratio between 1 and 10 causes a polarity change in SFG intensities from BLG aromatic C-H stretching vibrations. Conclusions from SFG spectra are corroborated by ellipsometry which shows a dramatic increase in layer thicknesses at molar ratios where a charge reversal occurs. The formation of interfacial multilayers comprising SDS-BLG complexes is, thus, caused by cancellation of electrostatic interactions which leads to agglomeration at the interface. In contrast to pH 3.8, behavior of BLG-SDS mixtures at pH 6.7 is different due to repulsive electrostatic interactions between SDS and BLG which lead to a significantly reduced binding enthalpy of -17 kJ mol-1. Finally, it has to be mentioned that SFG spectra show a coexistence of BLG and SDS molecules at the interface for BLG-SDS molar ratios &gt; 2. © 2014 American Chemical Society.
    view abstract10.1021/jp501541q
  • Quantitative evaluation of size selective precipitation of Mn-doped ZnS quantum dots by size distributions calculated from UV/Vis absorbance spectra
    Segets, D. and Komada, S. and Butz, B. and Spiecker, E. and Mori, Y. and Peukert, W.
    Journal of Nanoparticle Research 15 (2013)
    We demonstrate the quantitative evaluation of the sharp classification of manganese-doped zinc sulfide (ZnS:Mn) quantum dots by size selective precipitation. The particles were characterized by the direct conversion of absorbance spectra to particle size distributions (PSDs) and high-resolution transmission electron micrographs (HRTEM). Gradual addition of a poor solvent (2-propanol) to the aqueous colloid led to the flocculation of larger particles. Though the starting suspension after synthesis had an already narrow PSD between 1.5 and 3.2 nm, different particle size fractions were subsequently isolated by the careful adjustment of the good solvent/poor solvent ratio. Moreover, due to the fact that for the analysis of the classification results the size distributions were available, an in-depth understanding of the quality of the distinct classification steps could be achieved. From the PSDs of the feed, as well as the coarse and the fine fractions with their corresponding yields determined after each classification step, an optimum after the first addition of poor solvent was identified with a maximal separation sharpness κ as high as 0.75. Only by the quantitative evaluation of classification results leading to an in-depth understanding of the relevant driving forces, a future transfer of this lab scale post-processing to larger quantities will be possible. © 2013 Springer Science+Business Media Dordrecht.
    view abstract10.1007/s11051-013-1486-8
  • Synthesis of silver nanoparticles in melts of amphiphilic polyesters
    Vasylyev, S. and Damm, C. and Segets, D. and Hanisch, M. and Taccardi, N. and Wasserscheid, P. and Peukert, W.
    Nanotechnology 24 (2013)
    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. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/24/11/115604
  • A population balance model of quantum dot formation: Oriented growth and ripening of ZnO
    Segets, D. and Hartig, M.A.J. and Gradl, J. and Peukert, W.
    Chemical Engineering Science 70 (2012)
    The precipitation of zinc oxide (ZnO) semiconductor quantum dots was investigated throughout the whole particle formation process, namely reaction, nucleation, growth and ripening and described by means of population balance equations (PBE). Regarding nucleation, the simulation revealed that the mechanism for the solid formation is by orders of magnitude lower than predicted by classical homogeneous nucleation theory. Thus, the earliest stages of particle formation were described by a combination of reaction kinetics determined by experiments for the formation of preformed clusters and subsequent oriented cluster aggregation. Finally, slow Ostwald ripening, i.e. the growth of larger structures at the expense of smaller particles, was modeled in good agreement with the already experimentally determined particle sizes for ripening temperatures between 10 and 50 °C. © 2011 Elsevier Ltd.
    view abstract10.1016/j.ces.2011.04.043
  • Determination of the quantum dot band gap dependence on particle size from optical absorbance and transmission electron microscopy measurements
    Segets, D. and Lucas, J.M. and Klupp Taylor, R.N. and Scheele, M. and Zheng, H. and Alivisatos, A.P. and Peukert, W.
    ACS Nano 6 (2012)
    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. © 2012 American Chemical Society.
    view abstract10.1021/nn303130d
  • Tuning the size and the optical properties of ZnO mesocrystals synthesized under solvothermal conditions
    Distaso, M. and Segets, D. and Wernet, R. and Taylor, R.K. and Peukert, W.
    Nanoscale 4 (2012)
    Mesocrystals are a promising class of nanomaterials enabling new optical and mechanical properties due to their three dimensional organization of primary crystallites sharing a common crystallographic orientation. In the present article, the influence of process parameters such as temperature profile and stirring on the primary and secondary size of ZnO mesocrystals synthesized under solvothermal conditions has been investigated. In general, small but noticeable lattice strain is introduced to the particles during the synthesis process. Additionally, with increasing mass transport the fusion of primary crystallites due to coarsening is enhanced. A closer analysis revealed an influence of the polymer chain length on the final particle structure throughout different hierarchical levels. Based on our findings a reaction mechanism with nucleation and growth taking place embedded in poly-N-vinylpyrrolidone (PVP), followed by a polymer-mediated step of oriented aggregation and subsequent coarsening is proposed. In consequence, the careful control throughout all hierarchical levels of particle synthesis allows fine-tuning of the optical properties of ZnO mesocrystals which show a high UV absorption and minimal scattering in the visible region. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c1nr11226k
  • Experimental and theoretical studies of the colloidal stability of nanoparticles?a general interpretation based on stability maps
    Segets, D. and Marczak, R. and Schäfer, S. and Paula, C. and Gnichwitz, J.-F. and Hirsch, A. and Peukert, W.
    ACS Nano 5 (2011)
    The current work addresses the understanding of the stabilization of nanoparticles in suspension. Specifically, we study ZnO in ethanol for which the influence of particle size and reactant ratio as well as surface coverage on colloidal stability in dependence of the purification progress was investigated. The results revealed that the well-known 〈-potential determines not only the colloidal stability but also the surface coverage of acetate groups bound to the particle surface. The acetate groups act as molecular spacers between the nanoparticles and prevent agglomeration. Next to DLVO calculations based on the theory of Derjaguin, Landau, Verwey and Overbeek using a core-shell model we find that the stability is better understood in terms of dimensionless numbers which represent attractive forces as well as electrostatic repulsion, steric effects, transport properties, and particle concentration. Evaluating the colloidal stability in dependence of time by means of UV-vis absorption measurements a stability map for ZnO is derived. From this map it becomes clear that the dimensionless steric contribution to colloidal stability scales with a stability parameter including dimensionless repulsion and attraction as well as particle concentration and diffusivity of the particles according to a power law with an exponent of ?0.5. Finally, we show that our approach is valid for other stabilizing molecules like cationic dendrons and is generally applicable for a wide range of other material systems within the limitations of vanishing van der Waals forces in refractive index matched situations, vanishing 〈-potential and systems without a stabilizing shell around the particle surface. © 2011 American Chemical Society.
    view abstract10.1021/nn200465b
  • Shape transformation mechanism of silver nanorods in aqueous solution
    Damm, C. and Segets, D. and Yang, G. and Vieweg, B.F. and Spiecker, E. and Peukert, W.
    Small 7 (2011)
    The spontaneous shape transformation of silver nanorods with an initial length of several hundred nanometers towards spherical particle shapes in aqueous solution is investigated by means of scanning electron microscopy, UV-vis absorption spectroscopy, anodic stripping voltammetry, and high-resolution transmission electron microscopy (HRTEM). The consolidation of the results reveals an increase in the particle number density with time. Moreover, HRTEM image analysis along the cross section of the rods evidences the presence of fivefold twinning defects which extend along the whole rod length. According to the analytical model of Monk et al. this kind of rod structure is only thermodynamically stable if the rod length is below a critical value at a given diameter. The rods investigated in the present work do not fulfill the stability criterion as they exceed the critical length. Thus, the rods decay into smaller "nanobuns" and defective as well as defect-free spheres. A mechanism based on findings from the literature, HRTEM image analysis of former rods, transition states, and the final particle structures is proposed. The defects along the surface are seen as starting points for the dissolution of material, which is reintegrated into the solid phase by homogeneous as well as heterogeneous nucleation and growth. The decay process of silver nanorods in aqueous suspension is investigated. During ageing the aspect ratio decreases with time whereas the absolute particle number increases. Defects play a decisive role in rod decay and underline how crystal structure influences particle shape. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/smll.201001600
  • Optimum between purification and colloidal stability of ZnO nanoparticles
    Marczak, R. and Segets, D. and Voigt, M. and Peukert, W.
    Advanced Powder Technology 21 (2010)
    Crystalline ZnO quantum dots have been synthesized by hydrolysis of zinc acetate dihydrate with lithium hydroxide in ethanolic solution. By varying different parameters of the synthesis process, the size of the ZnO particles can be controlled. Detailed investigation of the ripening of the nanoparticles evidenced that despite of the well-known influence of ageing temperature and time, the presence of the reaction byproduct lithium acetate strongly affects the ripening behaviour. In particular, the particle size can be almost completely arrested by the removal of this byproduct via reversible flocculation of the ZnO nanoparticles using heptane as an antisolvent. A closer analysis of the repeated washing process shows an initial improvement of the colloidal stability of the ZnO nanoparticles during the first purification cycle as it mainly removes the lithium acetate from the suspension and not the stabilizing acetate groups directly bound to the particle surface. With further washing the remaining acetate ligands are unable to maintain the stabilization against agglomeration of the ZnO nanoparticles. Thus, there exists an optimum between purification progress and colloidal stability. These findings are also confirmed by calculations according to the DLVO theory, which show that there exists nearly no primary minimum of small ZnO nanoparticles below 5 nm in the presence of stabilizing acetate ions whereas the decrease in acetate ions bound to the particle surface leads to a more and more pronounced primary minimum. The present work is of particular significance for the preparation of purified colloidal ZnO nanoparticles for studies of their electrical and optical properties with respect to their wide range of potential applications. © 2009 The Society of Powder Technology Japan.
    view abstract10.1016/j.apt.2009.10.005
  • nanocrystals

  • nanoparticles

  • particle size analysis

  • quantum dots

  • semiconductors

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