Investigation of charge carriers in nanocrystal-based aerogels and the influence of micro- and nanostructuring

The transformation of intricate nanocrystals (NCs) available through colloidal chemical synthesis into branched network structures (referred to as gels) presents a promising pathway to macroscopic solids exhibiting properties of their NC building blocks. In this work, the possibilities to influence these properties at various stages during the gel synthesis sequence will be explored and expanded on. Additionally, the nanoscopic (opto-)electronic properties of networks build up from semiconducting, photoluminescent (PL) NCs will be investigated to gather insights into the electronic structure of such networks and the dynamics of excited charge carriers therein. In chapter 5 the investigation of gel networks based on CdSe/CdS dot/rod NC building blocks is shown. The PL properties of such structures are analysed in regards to their temperature and time dependent emission (down to 4K). It is shown that at cryogenic temperature the emission splits into two discernible processes correlating to dark and bright exciton states. While this observation is comparable to the results obtained for individual NCs, at all temperatures gel networks exhibit a longer PL decay most likely caused by increased electron delocalization. The investigation also includes a look at differently sized CdSe cores used for synthesis of the CdSe/CdS dot/rod building blocks. It is found that the combination of different core sizes and therefore different emission color building blocks allows for straight-forward color tuning of the final networks. The modification of NC-based gels with thin metal oxide shells is shown in chapter 6. Importantly, NC-based aerogels can be influenced not only at the point of NC building block synthesis but also after the gel formation (post-gelation). In this work, the application of wet-chemical methods based on colloidal seeded-growth processes to a NC-based gel network substrate is presented. This results in uniform, thin shells over the whole of the gel network. The approach can be shown to work on different gel substrates (semiconducting and noble metal) and different shell materials (silica and titania) with only minor modifications. The mechanical stability which is one of the common problems in the applicability of NC-based gels is considerably improved by this shell reinforcement. Building on these developed processes the optical properties of modified CdSe/CdS dot/rod gels are investigated in chapter 7. The silica modification of either the individual NC building blocks before gel assembly or the continuous gel after assembly yield two comparable systems only differing in one crucial point. In gels based on silica modified building blocks, these NC building blocks are isolated and thereby decoupled in the final gel network. For a modification after the gel assembly the building blocks are connected first and therefore coupled in the final gel. The crucial influence on charge carrier dynamics in such networks is shown by time-resolved PL spectroscopy and supported by theoretical calculations. Driving this process further, by incorporation of ZnS at the tip of the CdSe/CdS dot/rod building blocks using a cation exchange procedure a large bandgap material of variable length is introduced to the connecting point of the building blocks. Through the length of the ZnS segment separating theCdSe/CdS building blocks the interaction between these building blocks can not simply be coupled and decoupled but even influenced in its strength as also shown by optical spectroscopy.Die Transformation von komplexen Nanokristallen (NC), hergestellt durch kolloid-chemische Synthese, in verzweigte Netzwerkstrukturen (Gele genannt) repräsentiert einen vielversprechenden Weg zu makroskopischen Feststoffen, die die Eigenschaften ihrer NC Baueinheiten zeigen. In dieser Arbeit werden Möglichkeiten untersucht und erweitert, die Eigenschaften dieser Netzwerke an verschiedenen Punkten ihrer Herstellung zu beeinflussen. Zudem werden die (opto-)elektronischen Eigenschaften von Netzwerken basierend auf halbleitenden, lumineszenten NC untersucht, um ein tieferes Verständnis der elektronischen Struktur dieser Netzwerke und der Vorgänge der angeregten Ladungsträger zu generieren. Die Untersuchung von Gelnetzwerken aus stäbchenförmigen CdSe/CdS NC-baueinheiten ist in Kapitel 5 beschrieben. Die Photolumineszenz-(PL-)Eigenschaften dieser Strukturen werden temperaturabhängig (bis zu 4K) in Hinblick auf ihre zeitaufgelöste Emission untersucht. Dabei wird gezeigt, dass im kryogenen Bereich zwei Prozesse zur Emission beitragen, die sich hellen und dunklen Excitonen zuordnen lassen, ein von einzelnen NC bekanntes Verhalten. Die PL-lebenszeit ist allerdings bei allen Temperaturen in Netzwerkstrukturen länger, vermutlich aufgrund einer stärkeren Delokalisierung der angeregten Elektronen. Zusätzlich ist die Untersuchung von Netzwerken, bei denen die Bausteine auf unterschiedlichen Größen an CdSe Quantenpunkten in den CdSe/CdS Stäbchen basieren, gezeigt. Aus unterschiedlichen Größen resultieren unterschiedliche PL-Farben und ein unkomplizierter Weg zur Farbeinstellung in diesen Netzwerkstrukturen wird aufgezeigt. Die Modifikation von Nanokristallgelen mit dünnen Metalloxidschalen wird in Kapitel 6 gezeigt. NC-basierte Aerogele können nicht nur über die gezielte Synthese der Baueinheiten beeinflusst werden, sondern auch nach der Netzwerkbildung (post-Gelierung). In diesem Abschnitt wird die Adaption bekannter nasschemischer Prozesse zur Anwendung an Nanokristallgelen als Substrat beschrieben. Dabei werden Gelnetzwerke vollständig mit einer einheitlichen, dünnen Schale ummantelt. Dieser Prozess kann auf verschiedene Substrate (Gele aus Halbleiter oder Edelmetall NC) und für verschiedenen Schalenmaterialien (Siliziumdioxid und Titandioxid) mit minimalen Änderungen angepasst werden. Die mechanische Stabilität, eine bedeutende Hürde in der Anwendbarkeit von Nanokristallgelen, wird durch die Verstärkung mit einer solchen Schale deutlich erhöht. Darauf aufbauend werden die PL Eigenschaften modifizierter Netzwerke aus CdSe/CdS Stäbchen in Kapitel 7 untersucht. Die Modifikation mit einer Silikatschale wird entweder an einzelnen kolloiden Nanokristallbaueinheiten vor der Netzwerkbildung oder an dem bereits verbundenen Netzwerk durchgeführt. Werden zuerst die Baueinheiten mit einer Silikatschale umschlossen, so sind diese im finalen Netzwerk isoliert bzw. entkoppelt voneinander, während Baueinheiten, die zuerst in ein Netzwerk überführt werden, auch nach dem Wachstum der Silikatschale miteinander kontaktiert bzw. gekoppelt sind. Der entscheidende Einfluss dieser Verbindung auf die Ladungsträgervorgänge wird durch zeitaufgelöste PL-spektroskopie gezeigt und von theoretischen Berechnungen unterstützt. Dieser Ansatz wird durch den gezielten Einbau von ZnS an die Spitzen der CdSe/CdS Stäbchen weiter verfolgt. Durch einen Kationenaustauschprozess ist es möglich, verschiedene Mengen ZnS (ein Material mit großer Bandlücke) so in die Baueinheiten einzubringen, dass es sich im Netzwerk an den Kontaktpunkten befindet. Über die Länge dieser ZnS Abschnitte zwischen den CdSe/CdS Einheiten, können die Einheiten nicht nur gekoppelt und entkoppelt werden, sondern die Stärke der Wechselwirkung zwischen den Baueinheiten, welche sich in den optischen Eigenschaften widerspiegelt, gesteuert werden.ERC/Horizon 2020/714429/E

Temperature and Composition Dependent Optical Properties of CdSe/CdS Dot/Rod-Based Aerogel Networks

Employing nanocrystals (NCs) as building blocks of porous aerogel network structures allows the conversion of NC materials into macroscopic solid structures while conserving their unique nanoscopic properties. Understanding the interplay of the network formation and its influence on these properties like size-dependent emission is a key to apply techniques for the fabrication of novel nanocrystal aerogels. In this work, CdSe/CdS dot/rod NCs possessing two different CdSe core sizes were synthesized and converted into porous aerogel network structures. Temperature-dependent steady-state and time-resolved photoluminescence measurements were performed to expand the understanding of the optical and electronic properties of these network structures generated from these two different building blocks and correlate their optical with the structural properties. These investigations reveal the influence of network formation and aerogel production on the network-forming nanocrystals. Based on the two investigated NC building blocks and their aerogel networks, mixed network structures with various ratios of the two building blocks were produced and likewise optically characterized. Since the different building blocks show diverse optical response, this technique presents a straightforward way to color-tune the resulting networks simply by choosing the building block ratio in connection with their quantum yield

Nanocrystal Aerogels with Coupled or Decoupled Building Blocks

The influence of interparticle contact in nanoparticle-based aerogelnetwork structures is investigated by selectively connecting or isolating the buildingblocks inside of the network, thereby coupling and decoupling them in regards to theiroptical and electronic properties. This is achieved by tuning the synthesis sequence andexchanging the point of shell growth and the point of particle assembly, leading to twodistinctly different structures as examined by electron microscopy. By thoroughexamination of the resulting optical properties of the generated structures, the clearcorrelation between nanoscopic/microscopic structure and macroscopic optical proper-ties is demonstrated. Temperature-dependent measurements and effective massapproximation calculations support ourfindings

Control Over Structure and Properties in Nanocrystal Aerogels at the Nano-, Micro-, and Macroscale

The assembly of individual colloidal nanocrystals into macroscopic solvogels and aerogels introduced a new exciting type of material into the class of porous architectures. In these so-called nanocrystal gels, the structure and properties can be controlled and fine-tuned to the smallest details. Recently it was shown that by employing nanocrystal building blocks for such gel materials, the interesting nanoscopic properties can be conserved or even expanded to properties that are available neither in the nanocrystals nor in their respective bulk materials. In general, the production of these materials features the wet-chemical synthesis of stable nanocrystal colloids followed by their carefully controlled destabilization to facilitate arrangement of the nanocrystals into highly porous, interconnected networks. By isolation of the synthesis of the discrete building blocks from the assembly process, the electronic structure, optical properties, and structural morphology can be tailored by the myriad of procedures developed in colloidal nanocrystal chemistry. Furthermore, knowledge and control over the structure-property correlation in the resulting gel structures opens up numerous new ways for extended and advanced applications. Consequently, the amount of different materials converted to nanocrystal-based gel structures is rising steadily. Meanwhile the number of methods for assembly initiation is likewise increasing, offering control over the overall network structure and porosity as well as the individual nanocrystal building block connection. The resulting networks can be dried by different methods to obtain highly porous air-filled networks (aerogels) or applied in their wet form (solvogels). By now a number of different applications profiting from the unique advantages of nanocrystal-based gel materials have been realized and exploited in the areas of photocatalysis, electrocatalysis, and sensing.In this Account, we aim to summarize the efforts undertaken in the structuring of nanocrystal-based network materials on different scales, fine-tuning of the individual building blocks on the nanoscale, the network connections on the microscale, and the macroscale structure and shape of the final construct. It is exemplarily demonstrated how cation exchange reactions (at the nanoscale), postgelation modifications on the nanocrystal networks (microscale), and the structuring of the gels via printing techniques (macroscale) endow the resulting nanocrystal gel networks with novel physicochemical, mechanical, and electrocatalytic properties. The methods applied in the more traditional sol-gel chemistry targeting micro- and macroscale structuring are also reviewed, showing their future potential promoting the field of nanocrystal-based aerogels and their applications. Copyright © 2020 American Chemical Society

Influencing the coupling between network building blocks in CdSe/CdS dot/rod aerogels by partial cation exchange

The assembly of CdSe/CdS dot/rod nanocrystals (NCs) with variable length of ZnS tips into aerogel networks is presented. To this end, a partial region selective cation exchange procedure is performed replacing Cd by Zn starting at the NC tip. The produced aerogel networks are investigated structurally and optically. The networks of tip-to-tip connected NCs have an intricate band structure with holes confined to the CdSe cores while electrons are delocalized within the CdS also within connected building blocks. However, the ZnS tips act as a barrier of variable length and strength between the NC building blocks partly confining the electrons. This results in NC based aerogel networks with tunable strength of coupling between building blocks

Hypericum perforatum L.-Mediated Green Synthesis of Silver Nanoparticles Exhibiting Antioxidant and Anticancer Activities

This contribution focuses on the green synthesis of silver nanoparticles (AgNPs) with a size < 100 nm for potential medical applications by using silver nitrate solution and Hypericum Perforatum L. (St John’s wort) aqueous extracts. Various synthesis methods were used and compared with regard to their yield and quality of obtained AgNPs. Monodisperse spherical nanoparticles were generated with a size of approximately 20 to 50 nm as elucidated by different techniques (SEM, TEM). XRD measurements showed that metallic silver was formed and the particles possess a face-centered cubic structure (fcc). SEM images and FTIR spectra revealed that the AgNPs are covered by a protective surface layer composed of organic components originating from the plant extract. Ultraviolet-visible spectroscopy, dynamic light scattering, and zeta potential were also measured for biologically synthesized AgNPs. A potential mechanism of reducing silver ions to silver metal and protecting it in the nanoscale form has been proposed based on the obtained results. Moreover, the AgNPs prepared in the present study have been shown to exhibit a high antioxidant activity for 2, 2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) radical cation, and super oxide anion radical and 2,2-diphenyl-1-picrylhydrazyl. Synthesized AgNPs showed high cytotoxicity by inhibiting cell viability for Hela, Hep G2, and A549 cells

A Versatile Route to Assemble Semiconductor Nanoparticles into Functional Aerogels by Means of Trivalent Cations

3D nanoparticle assemblies offer a unique platform to enhance and extend the functionality and optical/electrical properties of individual nanoparticles. Especially, a self-supported, voluminous, and porous macroscopic material built up from interconnected semiconductor nanoparticles provides new possibilities in the field of sensing, optoelectronics, and photovoltaics. Herein, a method is demonstrated for assembling semiconductor nanoparticle systems containing building blocks possessing different composition, size, shape, and surface ligands. The method is based on the controlled destabilization of the particles triggered by trivalent cations (Y3+, Yb3+, and Al3+). The effect of the cations is investigated via X-ray photoelectron spectroscopy. The macroscopic, self-supported aerogels consist of the hyperbranched network of interconnected CdSe/ CdS dot-in-rods, or CdSe/CdS as well as CdSe/CdTe core-crown nanoplatelets is used to demonstrate the versatility of the procedure. The non-oxidative assembly method takes place at room temperature without thermal activation in several hours and preserves the shape and the fluorescence of the building blocks. The assembled nanoparticle network provides longer exciton lifetimes with retained photoluminescence quantum yields, that make these nanostructured materials a perfect platform for novel multifunctional 3D networks in sensing. Various sets of photoelectrochemical measurements on the interconnected semiconductor nanorod structures also reveal the enhanced charge carrier separation

Versatile Route for Multifunctional Aerogels Including Flaxseed Mucilage and Nanocrystals

Preparation of low density monolithic and free-standing organic-inorganic hybrid aerogels of various properties is demonstrated using green chemistry from a biosafe natural source (flaxseed mucilage) and freeze-casting and subsequent freeze drying. Bio-aerogels, luminescent aerogels, and magneto-responsive aerogels are obtained by combination of the flaxseed mucilage with different types of nanoparticles. Moreover, the aerogels are investigated as possible drug release systems using curcumin as a model. Various characterization techniques like thermogravimetric analysis, nitrogen physisorption, electron microscopy, UV/Vis absorption, and emission spectroscopy, bulk density, and mechanical measurements, as well as in vitro release profile measurements, are employed to investigate the obtained materials. The flaxseed-inspired organic-inorganic hybrid aerogels exhibit ultra-low densities as low as 5.6 mg cm−3 for 0.5% (w/v) the mucilage polymer, a specific surface area of 4 to 20 m2 g−1, high oil absorption capacity (23 g g−1), and prominent compressibility. The natural biopolymer technique leads to low cost and biocompatible functional lightweight materials with tunable properties (physicochemical and mechanical) and significant potential for applications as supporting or stimuli responsive materials, carriers, reactors, microwave- and electromagnetic radiation protective (absorbing)-materials, as well as in drug delivery and oil absorption

Nanosecond Pulsed Laser-Heated Nanocrystals Inside a Metal-Organic Framework Matrix

Investigations on gold and gold-zinc oxide nanocrystals encapsulated in a matrix of a metal-organic framework (ZIF-8) upon plasmonic heating with nanosecond laser pulses are presented. Irradiation of Au@ZIF-8 composite particles leads to heating of the gold core and decomposition of surrounding matrix acting as temperature probe. Cavities inside the ZIF-8 matrix are found on TEM images after irradiation. Their size is determined dependent on laser energy density and the generated heat at the gold core after absorption of a laser pulse approximated. The surrounding of the gold cores can be heated up to ZIF-8 decomposition over a distance up to 60 nm. This represents a method to visualize heat transfer from the gold cores to the ZIF-8 matrix in three dimensions. Studies on ZIF-8 encapsulated Au@ZnO dot-rod particles give insight in heat transfer between the particle components and show the applicability of the method to different, more complex systems. © 2022 The Authors. ChemNanoMat published by Wiley-VCH GmbH

Methanol‐to‐Olefins in a Membrane Reactor with in situ Steam Removal – The Decisive Role of Coking

The reaction of methanol to light olefins and water (MTO) was studied in a fixed bed tubular membrane reactor using commercial SAPO-34 catalyst. In the fixed bed reactor without membrane support, the MTO reaction collapsed after 3 h time on stream. However, if the reaction by-product steam is in situ extracted from the reactor through a hydrophilic tubular LTA membrane, the reactor produces long-term stable about 60% ethene and 10% propene. It is shown that the reason for the superior performance of the membrane-assisted reactor is not the prevention of catalyst damage caused by steam but the influence of the water removal on the formation of different carbonaceous residues inside the SAPO-34 cages. Catalytically beneficial methylated 1 or 2 ring aromatics have been found in a higher percentage in the MTO reaction with a water removal membrane compared to the MTO reaction without membrane support