Untersuchung der Struktur von CTAB-Mizellen und deren Einfluss auf die Bildung und Stabilisierung von Goldnanopartikeln mittels Röntgen- und Neutronenkleinwinkelstreuung

Goldnanostäbchen (AuNRs) besitzen ein breites potentielles Anwendungsspektrum das von elektronischen und photonischen Anwendungen über den Einsatz in der (bio-)chemischen Sensorik, Krebsdiagnose und -therapie reicht. Zur Herstellung der AuNRs werden kleine Goldnanopartikel benötigt auf denen anschließend die Partikel anisotrop aufwachsen. Beide Partikelspezies werden mithilfe von Mizellen des kationischen Tensids Cetyltrimethylammonium Bromid (C19H42NBr, CTAB) stabilisiert. Jedoch ist die Stabilisierung der kleinen Goldnanopartikel (2nm im Durchmesser) unzureichend. Das wiederum wirkt sich negativ auf die anschließende Synthese der AuNRs aus. In dieser Arbeit wurden CTAB-Mizellen mithilfe von Röntgen- und Neutronenkleinwinkelstreuung strukturell charakterisiert. Morphology und chemische Zusammensetzung der Mizellen wurden systematisch durch die Zugabe von n-Alkoholen beeinflusst und durch die Kombination von SAXS und SANS quantifiziert. Eine drastische Stabilitätssteigerung der Goldnanopartikel konnte durch die Verwendung stark elongierter Mizellen erzielt werden. Mithilfe der klassischen Koagulationstheory konnte der vorgeschlagene Stabilisierungsmechanismus für Goldnanopartikel durch CTAB-Mizellen gestützt werden. Die Verwendung hoch-stabiler Keimpartikel für die anschließende Synthese von AuNRs führte zur erheblichen Verbesserung der morphologischen Selektivität und Reproduzierbarkeit der Reaktion. Ein fundamentales Verständnis über die Wechselwirkungen zwischen Goldnanopartikeln und Mizellen konnte erhalten werden.Gold nanorods (AuNRs) exhibit outstanding optical properties and can be used in a wide range of applications ranging from electronics and photonics over (bio-)chemical sensing, cancer diagnostics and therapeutics. Small gold nanoparticles are needed as seed particles for the subsequent growth to gold nanorods. Both particle species are stabilized by micelles of the cationic surfactant cetyltrimethylammonium bromide (C19H42NBr, CTAB). However, the stabilization of small gold particles in the lower nanometer regime (2 nm in diameter) is insufficient, which has a negative influence on the subsequent AuNR synthesis. In this work CTAB micelles were structurally characterized using small-angle X-ray and neutron scattering (SAXS, SANS). The morphology and chemical composition of the micelles was modified using n-alcohols and studied with the combination of SAXS and SANS. A drastic increase in the stability of the gold nanoparticles could be obtained using highly elongated micelles. Classical coagulation theory has been applied to validate the proposed stabilization mechanism of small gold nanoparticles by CTAB micelles. The use of highly stable gold nanoparticles lead to a significant increase of the morphological selectivity and reproducibility of the synthesis of AuNRs. A fundamental understanding about the interactions between gold nanoparticles and micelles could be gained

Concentration dependent morphology and composition of n -alcohol modified cetyltrimethylammonium bromide micelles

Cetyltrimethylammonium bromide (CTAB) is one of the most commonly used surfactants in nanoparticle synthesis and stabilization. Usually, CTAB is used in high concentrations besides co-surfactants leading to well defined products but the complex mesoscopic CTAB structures stay mostly unknown. N-alcohols for instance are widely used co-surfactants which modify the properties of native CTAB dispersions. In this paper we report about a detailed structure analysis of n-alcohol modified CTAB micelles. In particular, n-pentanol and n-hexanol exhibit a significantly different influence on the size, shape and composition of CTAB micelles. Using a combination of small-angle x-ray spectroscopy (SAXS) and neutron scattering spectroscopy (SANS), we applied a method for a complete structural characterization of such micelles. The incorporation of n-pentanol into CTAB micelles generally does not influence the morphology but enhances the number of micelles due to the volume of the added alcohol. N-hexanol, however, leads to an elongation of the micelles as a function of its concentration. It was found by extended contrast variation measurements that this difference is caused by a different distribution of the alcohols between the micellar core and shell. N-pentanol molecules are generally located at the core–shell interface of the CTAB micelles with not only the head group but also two additional methylene bridging groups located in the micellar shell. This leads to an increase of its effective head group volume. In comparison, in n-hexanol modified micelles the whole alkyl chain is located within the micellar core. The detailed structure for n-alcohol modified CTAB micelles is described herein for the first time. The knowledge of the structural details found is indispensable for an in-depth understanding of CTAB–n-alcohol–water interfaces in general which is relevant for the synthesis of many functional nanostructures like mesoporous silica and gold or silver nanoparticles

The influence of n-hexanol on the morphology and composition of CTAB micelles

The effect of the addition of n-hexanol as co-surfactant on the structure of cetyltrimethylammonium bromide (CTAB) micelles has been studied using small-angle X-ray and neutron scattering (SAXS, SANS). Contrast variation neutron scattering experiments were performed to determine the structure of both pure CTAB and n-hexanol modified CTAB micelles. The incorporation of n-hexanol leads to an elongation of the ellipsoidal CTAB micelles. The scattering length density of the micellar shell linearly depends on the degree of deuteration of the dispersion medium water and revealed the existence of substantial amounts of water in the micellar shell. The water content in the shell increased from 20 vol-% observed for pure CTAB micelles to 44 vol-% found for n-hexanol modified CTAB micelles. The amount of n-hexanol in the micellar shell was determined by varying the amount of fully deuterated and protonated n-hexanol. These experiments revealed a volume fraction of 26 vol-% of n-hexanol molecules in the micellar core which equals a molar fraction of 50 % n-hexanol within the CTAB micelles. The total composition of micellar core and shell was estimated. The packing density of headgroups, water molecules and bromide ions turned out to drastically increase in n-hexanol modified CTAB micelles. These findings contribute to a fundamental understanding of the stabilization mechanism of micelles by alcoholic co-surfactants and the resulting alteration of the morphology and interface composition. These results will facilitate the optimization of processes where CTAB and other comparable surfactants are used as phase transfer catalysts, structure directing agents or stabilizers in colloidal dispersions or emulsions

n -Hexanol Enhances the Cetyltrimethylammonium Bromide Stabilization of Small Gold Nanoparticles and Promotes the Growth of Gold Nanorods

Gold nanorods (AuNRs) are of interest for many applications, since their absorption in the regime of visible light can easily be tuned by their exact shape. To produce these AuNRs, a two-step synthesis that starts from small seed particles is used. These seed particles are stabilized by cetyltrimethylammonium bromide (CTAB), which forms micelles at the used concentration (0.1 mol/L). In this work, the influence of the micelle morphology on the stabilization of these seed particles and the consequences on the formation of AuNRs is reported. The elongation of CTAB micelles by the addition of n-hexanol leads to much more stable seed particle dispersions and thus less polydisperse AuNRs. In contrast, a higher number of micelles compared to pure CTAB dispersions result from the addition of n-pentanol. This promotes the formation of larger seed particles and leads to lower yields of AuNRs. The gold nanoparticles are characterized by UV–vis–NIR absorption spectroscopy, transmission electron microscopy, and small-angle X-ray scattering (SAXS). The morphology of the micelles has been determined by a combination of SAXS and small-angle neutron scattering (SANS). The experimental results were used to calculate the collision kinetics of seed particles by using an improved approach of classical coagulation theory to consider the anisotropy of the micelles. The combination of these experiments with the calculations strongly supports the mechanistic model—that these gold seed particles are not stabilized by a CTAB bilayer but by the micelles itself. For the first time, the influence of the micellar size and shape on the stabilization mechanism of noble metal nanoparticles could be clarified. Theses findings contribute to the development of targeted design routes for distinct nanoparticle morphologies by the use of suitable dispersions

Changes within the stabilizing layer of ZnO nanoparticles upon washing

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

Real‐Time Study on Structure Formation and the Intercalation Process of Polymer: Fullerene Bulk Heterojunction Thin Films

Fullerene intercalation between the side chains of conjugated polymers has a detrimental impact on both charge separation and charge transport processes in bulk heterojunction (BHJ) organic photovoltaic cells (OPVs). In situ grazing incidence X‐ray scattering experiments allow to characterize the structure formation, drying kinetics, and intercalation in blends of phenyl‐c61‐butyric acid methyl ester (PC60BM) and poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) named (pBTTT‐C14) from their 1,2‐orthodichlorobenzene (oDCB) solutions with different volume fractions of dodecanoic acid methyl ester (Me12) as a solvent additive. The structure formation process during evaporation of the solvent:additive mixture can be described by five periods, which are correlated to a multistep contraction of the lamellar stacking of the bimolecular crystals. The onset of crystallization is delayed by increasing the additive volume fraction in the coating solution leading to a promoted crystallinity. A conclusive picture of fullerene intercalation and additive‐tuned structural evolution during the drying of thin films of the polymer:fullerene BHJ blends will be presented