Titania/graphene nanocomposites from scalable gas-phase synthesis for high-capacity and high-stability sodium-ion battery anodes.

In sodium-ion batteries (SIBs), TiO2or sodium titanates are discussed as cost-effective anode material. The use of ultrafine TiO2particles overcomes the effect of intrinsically low electronic and ionic conductivity that otherwise limits the electrochemical performance and thus its Na-ion storage capacity. Especially, TiO2nanoparticles integrated in a highly conductive, large surface-area, and stable graphene matrix can achieve an exceptional electrochemical rate performance, durability, and increase in capacity. We report the direct and scalable gas-phase synthesis of TiO2and graphene and their subsequent self-assembly to produce TiO2/graphene nanocomposites (TiO2/Gr). Transmission electron microscopy shows that the TiO2nanoparticles are uniformly distributed on the surface of the graphene nanosheets. TiO2/Gr nanocomposites with graphene loadings of 20 and 30 wt% were tested as anode in SIBs. With the outstanding electronic conductivity enhancement and a synergistic Na-ion storage effect at the interface of TiO2nanoparticles and graphene, nanocomposites with 30 wt% graphene exhibited particularly good electrochemical performance with a reversible capacity of 281 mAh g-1at 0.1 C, compared to pristine TiO2nanoparticles (155 mAh g-1). Moreover, the composite showed excellent high-rate performance of 158 mAh g-1at 20 C and a reversible capacity of 154 mAh g-1after 500 cycles at 10 C. Cyclic voltammetry showed that the Na-ion storage is dominated by surface and TiO2/Gr interface processes rather than slow, diffusion-controlled intercalation, explaining its outstanding rate performance. The synthesis route of these high-performing nanocomposites provides a highly promising strategy for the scalable production of advanced nanomaterials for SIBs.

Simple and Reliable Method for Studying the Adsorption Behavior of Aquivion Ionomers on Carbon Black Surfaces.

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 Δ G calculated from the association constants of the adsorption isotherms agree well with Δ G values obtained by isothermal titration calorimetry (ITC) and thus validate the adsorption isotherm measurement method. From the values of adsorption enthalpy Δ H ((-7.3 ± 0.8) kJ/mol) and entropy Δ S (ca. 100 J/(mol K)), which were extracted from ITC, we conclude that the ionomer adsorption on carbon black is a spontaneous physisorption process.

Influence of Tail Groups during Functionalization of ZnO Nanoparticles on Binding Enthalpies and Photoluminescence.

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.

Adaptive Behavior of Dynamic Orthoester Cryptands.

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.

2D analysis of polydisperse core-shell nanoparticles using analytical ultracentrifugation.

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.

Spectra Library: An Assumption-Free In Situ Method to Access the Kinetics of Catechols Binding to Colloidal ZnO Quantum Dots.

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.

Simultaneous Identification of Spectral Properties and Sizes of Multiple Particles in Solution with Subnanometer Resolution.

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.

Simultaneous analysis of hydrodynamic and optical properties using analytical ultracentrifugation equipped with multiwavelength detection.

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.

On the mechanism of Zn4O-acetate precursors ripening to ZnO: How dimerization is promoted by hydroxide incorporation.

We report on a combined experimental and molecular modelling study on Zn4O 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 Zn4O(OAc)6-x(OH)x 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) Zn4O(OAc)6 and Zn4O(OAc)5 (+) clusters. Mass spectrometry and quantum calculations hint at the stability of (Zn4O)2(OAc)12-x(OH)x dimers with x = 1, 2. Therein, the hydroxide ions establish salt-bridges that allow for the formation of additional Zn3 motifs with the OH above the triangle center-a structural motif close to that of the ZnO-crystal. The association of Zn4O(OAc)6 clusters is thus suggested to involve OAc(-) → OH(-) substitution as an activation step, quickly followed by dimerization and the subsequent agglomeration of oligomers.

Ozonation of carbamazepine in the presence of sulfur-dopped graphene: Effect of process parameters and formation of main transformation products.

The stability of graphene structure in sulfur-doped graphene catalyst is demonstrated to be a key aspect during the ozonation process. Enhancing the stability of the sulfur-doped graphene structure is therefore important to improve its catalytic activity during the ozonation process. However, this has remained a challenge so far. Therefore, we adopted a low-energy microwave plasma technique to synthesize a high purity sulfur-doped graphene (S ⎯ Gr) catalyst for the ozonation process. The effect of S ⎯ Gr in the ozonation process was tested using carbamazepine (CBZ; 0.05 mM) as a probe compound. A complete CBZ removal was obtained at an ozone concentration of 0.08 mM while in comparison with single O3, ∼1.5 and 2.5 times decrease in the formation of the two important intermediate transformation products i.e., BQM (1-(2-benzaldehyde) - 4-hydroxy (1H, 3H)-quinazoline-2-one) and BQD (1-(2-benzaldehyde) - (1H, 3H)-quinazoline-2, 4-dione) was observed. Radical scavenging experiments confirmed the formation of HO. The XPS results showed that the activity of S ⎯ Gr towards the formation of HO was positively related to S-bearing carbon atoms at the edge of the graphene structure. Therefore, the addition of S ⎯ Gr is directly linked with the formation of HO, which further contributed to the improved elimination of intermediate transformation products. With a low sulfur loss of 1 %, the microwave plasma synthesized S ⎯ Gr catalyst remained stable during ozonation, implying its feasibility in practical application.

Cell-Membrane-Coated and Cell-Penetrating Peptide-Conjugated Trimagnetic Nanoparticles for Targeted Magnetic Hyperthermia of Prostate Cancer Cells.

Prostate malignancy represents the second leading cause of cancer-specific death among the male population worldwide. Herein, enhanced intracellular magnetic fluid hyperthermia is applied in vitro to treat prostate cancer (PCa) cells with minimum invasiveness and toxicity and highly specific targeting. We designed and optimized novel shape-anisotropic magnetic core-shell-shell nanoparticles (i.e., trimagnetic nanoparticles - TMNPs) with significant magnetothermal conversion following an exchange coupling effect to an external alternating magnetic field (AMF). The functional properties of the best candidate in terms of heating efficiency (i.e., Fe3O4@Mn0.5Zn0.5Fe2O4@CoFe2O4) were exploited following surface decoration with PCa cell membranes (CM) and/or LN1 cell-penetrating peptide (CPP). We demonstrated that the combination of biomimetic dual CM-CPP targeting and AMF responsiveness significantly induces caspase 9-mediated apoptosis of PCa cells. Furthermore, a downregulation of the cell cycle progression markers and a decrease of the migration rate in surviving cells were observed in response to the TMNP-assisted magnetic hyperthermia, suggesting a reduction in cancer cell aggressiveness.

Hansen parameter evaluation for the characterization of titania photocatalysts using particle size distributions and combinatorics.

Titania photocatalysts have great potential as remediators of air pollution. Although various aspects of photocatalyst synthesis, adsorption and photoactivity have been investigated, a thorough understanding of the particle surface behavior has not yet been fully realized. In order to learn more about the principles behind the surface behavior, we investigate the Hansen solubility/similarity parameters (HSPs) for analyzing and evaluating three photocatalysts synthesized by the gas phase method, solvothermal reaction and sol-gel method, respectively. A particle size distribution-based categorization scheme is introduced for characterizing each material's Hansen parameters based on its interaction with a list of selected probe liquids. The latter was deduced from particle size distributions assessed by analytical centrifugation. Subsequent comparison of the Hansen parameters of the investigated materials shows how HSPs can potentially be used as a model for predicting the pollutant adsorption behavior on the photocatalyst surface. This serves as a first step in heading towards an improved understanding of the particle behavior and translating it into a knowledge-based design, i.e., synthesis and hybridization of novel photocatalysts.

Shape transformation mechanism of silver nanorods in aqueous solution.

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.

Towards a framework for evaluating and reporting Hansen solubility parameters: applications to particle dispersions.

A thorough understanding of complex interactions within particulate systems is a key for knowledge-based formulations. Hansen solubility parameters (HSP) are widely used to assess the compatibility of the dispersed phase with the continuous phase. At present, the determination of HSP is often based on a liquid ranking list obtained by evaluating a pertinent dispersion parameter using only one pre-selected characterization method. Furthermore, one cannot rule out the possibility of subjective judgment especially for liquids for which it is difficult to decipher the compatibility or underlying interactions. As a result, the end value of HSP might be of little or no information. To overcome these issues, we introduce a generalized and technology-agnostic combinatorics-based procedure. We discuss the principles of the procedure and the implications of evaluating and reporting particle HSP values. We demonstrate the procedure by using SiN x particles synthesized in the gas phase. We leverage the analytical centrifugation data to evaluate stability trajectories of SiN x dispersions in various liquids to deduce particle-liquid compatibility.

Calcium Oxalate Crystallization: Influence of pH, Energy Input, and Supersaturation Ratio on the Synthesis of Artificial Kidney Stones.

The removal of kidney stones can lead to small residual fragments remaining in the human body. Residual stone fragments can act as seeds for kidney stone crystallization and may necessitate another intervention. Therefore, it is important to create a consistent model with a particle size comparable to the range of kidney stone fragments. Thus, the size-determining parameters such as supersaturation ratio, energy input, and pH value are examined. The batch crystallizations were performed with supersaturation ratios between 5.07 and 6.12. The compositions of the dried samples were analyzed with Raman spectroscopy, infrared spectroscopy, and X-ray diffraction (XRD). The samples were identified as calcium oxalate monohydrate with spectroscopic analysis, while calcium oxalate dihydrate being the most prominent crystalline species at all supersaturation ratios for the investigated conditions. The aggregate size, obtained with analytical centrifugation, varied between 2.9 and 4.3 µm, while the crystallite domain size, obtained from XRD, varied from 40 to 61 nm. Our results indicate that particle sizes increase with increasing supersaturation, energy input, and pH. All syntheses yield a high particle heterogeneity and represent an ideal basis for reference materials of small kidney stone fragments. These results will help better understand and control the crystallization of calcium oxalate and the aggregation of such pseudopolymorphs.

Crossing the Valley of Death: From Fundamental to Applied Research in Electrolysis.

The growing societal and political focus on the use of environmentally friendly technologies has led to an ever-increasing interest in electrolysis technologies in the scientific communities. This development is reflected by the plethora of candidate catalysts for the hydrogen and oxygen evolution reactions, as well as the CO2 reduction reaction, reported in the literature. However, almost none of them entered the stage of application yet. Likewise, the reports on process engineering inadequately address the utilization of these catalysts, as well as electrode and cell concepts, that might be suitable for the market. Evidently, a closer collaboration between chemists and engineers from industry and academia is desirable to speed up the development of these disruptive technologies. Herein, we elucidate the critical parameters and highlight the necessary aspects to accelerate the development of industrially relevant catalysts capable of fulfilling the forthcoming challenges related to energy conversion and storage. The aim of this Perspective, composed by industrial and academic partners, is to critically question current undertakings and to encourage researchers to strike interdisciplinary research pathways.

Chromatographic property classification of narrowly distributed ZnS quantum dots.

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.

Rapid Characterization and Parameter Space Exploration of Perovskites Using an Automated Routine.

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.

Cleaning Matters!

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.