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The assembly of nanocrystals (NCs) into three-dimensional network structures is a recently established strategy to produce macroscopic materials with nanoscopic properties. These networks can be formed by the controlled destabilization of NC colloids and subsequent supercritical drying to obtain NC-based aerogels. Even though this strategy has been used for many different semiconductor NCs, the emission of NC-based aerogels is limited to the ultraviolet and visible and no near-infrared (NIR) emitting NC-based aerogels have been investigated in literature until now. In the present work we have optimized a gelation route of NIR emitting PbS and PbS/CdS quantum dots (QDs) by means of a recently established gel formation method using trivalent ions to induce the network formation. Thereby, depending on the surface ligands and QDs used the resulting network structure is different. We propose, that the ligand affinity to the nanocrystal surface plays an essential role during network formation, which is supported by theoretical calculations. The optical properties were investigated with a focus on their steady-state and time resolved photoluminescence (PL). Unlike in PbS/CdS aerogels, the absorption of PbS aerogels and their PL shift strongly. For all aerogels the PL lifetimes are reduced in comparison to those of the building blocks with this reduction being especially pronounced in the PbS aerogels.

In this work, the influence of two different types of cations on the gel formation and structure of mixed gel networks comprised of semiconductor (namely CdSe/CdS nanorods NR) and Au nanoparticles (NP) as well as on the respective monocomponent gels is investigated. Heteroassemblies built from colloidal building blocks are usually prepared by ligand removal or cross-linking, thus, both the surface chemistry and the destabilising agent play an essential role in the gelation process. Due to the diversity of the composition, morphology, and optical properties of the nanoparticles, a versatile route to fabricate functional heteroassemblies is of great demand. In the present work, the optics, morphology, and gelation mechanism of pure semiconductor and noble metal as well as their mixed nanoparticle gel networks are revealed. The influence of the gelation agents (bivalent and trivalent cations) on the structure-property correlation is elucidated by photoluminescence, X-ray photoelectron spectroscopy, and electron microscopy measurements. The selection of cations drastically influences the nano- and microstructure of the prepared gel network structures driven by the affinity of the cations to the ligands and the nanoparticle surface. This gelation technique provides a new platform to control the formation of porous assemblies based on semiconductor and metal nanoparticles.

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, 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 Su

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.

In the large field of research on nanoplatelets (NPLs), their strong tendency to self-assemble into ordered stacks and the resulting changes in their properties are of great interest. The assembly reveals new characteristics such as the charge carrier transport through the NPL assembly or altered optical properties. In particular, a reduced distance should enhance the charge carrier transport due to higher electronic coupling of neighboring NPLs, and therefore, is the focus of this work. To modify the inter-particle distances, the straightforward method of ligand exchange is applied. Various CdSe and CdSe/CdX (hetero-) NPLs serve as building blocks, which not only display different material combinations but also different types of hetero-structures. The surface-to-surface distance between the stacked NPLs can be reduced to below 1 nm, thus, to less than the half compared to assemblies of pristine NPLs. Moreover, for certain NPLs stacking is only enabled by the ligand exchange. To characterize the ligand exchanges and to investigate the influences of the reduced distances, photo-electrochemical measurements, fluorescence spectroscopy, energy dispersive X-ray spectroscopy, nuclear magnetic resonance, and X-ray photoelectron spectroscopy are performed. It is possible to show higher photocurrents for smaller distances, indicating enhanced charge transport ability within those stacks.

Room-temperature sodium–sulfur (RT Na–S) batteries are arousing great interest in recent years. Their practical applications, however, are hindered by several intrinsic problems, such as the sluggish kinetic, shuttle effect, and the incomplete conversion of sodium polysulfides (NaPSs). Here a sulfur host material that is based on tungsten nanoparticles embedded in nitrogen-doped graphene is reported. The incorporation of tungsten nanoparticles significantly accelerates the polysulfides conversion (especially the reduction of Na2S4 to Na2S, which contributes to 75% of the full capacity) and completely suppresses the shuttle effect, en route to a fully reversible reaction of NaPSs. With a host weight ratio of only 9.1% (about 3–6 times lower than that in recent reports), the cathode shows unprecedented electrochemical performances even at high sulfur mass loadings. The experimental findings, which are corroborated by the first-principles calculations, highlight the so far unexplored role of tungsten nanoparticles in sulfur hosts, thus pointing to a viable route toward stable Na–S batteries at room temperatures.

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.

In this paper, it is shown that the nanoscopic design of combining semiconductors and noble metals has a direct impact on the macroscopic (electrochemical) properties of their assembled, hyperbranched, macroscopic gel networks. Controlled and arbitrary deposition of gold domains on CdSe/CdS nanorods leads to tipped and randomly decorated heteroparticles, respectively. Structural and optical properties of the gel networks depend upon assembling the hybrid particles by means of oxidative or ionic routes. Additionally, the impact of different building block designs on the charge carrier separation processes is investigated from spectroelectrochemical point of view. A more efficient charge carrier separation is revealed in the tipped design manifesting in higher negative photocurrent efficiencies compared to the arbitrary decoration, where the charge recombination processes are more remarkable. This work sheds light on the importance of the nanostructuring on the spectroelectrochemical properties at the macroscale paving the way towards their use in photochemical reactions.

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.

Lithium-sulfur (Li-S) batteries have attracted widespread attention due to their high theoretical energy density. However, their practical application is still hindered by the shuttle effect and the sluggish conversion of lithium polysulfides (LiPSs). Herein, monodisperse molybdenum (Mo) nanoparticles embedded onto nitrogen-doped graphene (Mo@N-G) were developed and used as a highly efficient electrocatalyst to enhance LiPS conversion. The weight ratio of the electrocatalyst in the catalyst/sulfur cathode is only 9%. The unfilled d orbitals of oxidized Mo can attract the electrons of LiPS anions and form Mo–S bonds during the electrochemical process, thus facilitating fast conversion of LiPSs. Li-S batteries based on the Mo@N-G/S cathode can exhibit excellent rate performance, large capacity, and superior cycling stability. Moreover, Mo@N-G also plays an important role in room-temperature quasi-solid-state Li-S batteries. These interesting findings suggest the great potential of Mo nanoparticles in building high-performance Li-S batteries.

In this work, mixing and co-gelation of Au nanoparticles (NPs) and highly luminescent CdSe/CdS core/shell nanorods (NRs) are used as tools to obtain noble metal particle-decorated macroscopic semiconductor gel networks. The hybrid nature of the macrostructures facilitates the control over the optical properties: while the holes are trapped in the CdSe cores, the connected CdSe/CdS NRs support the mobility of excited electrons throughout the porous, hyperbranched gel networks. Due to the presence of Au NPs in the mixed gels, electron trapping in the gold NPs leads to a suppressed radiative recombination, namely, quenches the fluorescence in certain fragments of the multicomponent gel. The extent of fluorescence quenching can be influenced by the quantity of the noble metal domains. The optical properties are monitored as a function of the NR:NP ratio of a model system CdSe/CdS:Au. By this correlation, it demonstrates that the spatial extent of quenching initiated by a single Au NP exceeds the dimensions of one NR, which the Au is connected to (with a length of 45.8 nm ± 4.1 nm) and can reach the number of nine NRs per Au NP, which roughly corresponds to 400 nm of total electron travel distance within the network structure.

Noble-metal-based electrocatalysts usually contain small nanoparticle building blocks to ensure a high specific surface area as the scene for the surface processes. Here, we show that relatively large noble-metal nanorods are also promising candidates to build up functional macrostructures with prominent electrocatalytic activity. After optimizing and upscaling the syntheses of gold nanorods and gold bipyramid-templated silver nanorods, cryoaerogels are fabricated on a conductive substrate via flash freezing and subsequent freeze drying. The versatile cryoaerogelation technique allows the formation of macrostructures with dendritic, open-pore structure facilitating the increase of the accessible nanorod surfaces. It is demonstrated via electrochemical oxidation and stripping test experiments that noble-metal surface sites are electrochemically active in redox reactions. Furthermore, gold nanorod cryoaerogels offer a platform for redox sensing, ethanol oxidation reaction, as well as glucose sensing. Compared to their simply drop-cast and dried counterparts, the noble-metal nanorod cryoaerogels offer enhanced activity due to the open porosity of the fabricated nanostructure while maintaining structural stability.

Hybrid semiconductor-based nanocrystals (NCs) are generally synthesized in organic media prior to their assembly into catalytically promising nanostructures via multistep methods. Here, a tunable, easy-to-adapt and versatile approach for the preparation of hybrid nanoparticle networks from aqueous nanocrystal solutions is demonstrated. The networks consist of interconnected semiconductor NC backbones (made of CdSe/CdS dot-in-rods or core/crown nanoplatelets) decorated with noble metal (Au and Pt) or with metal-based domains (Co2+ and Ni2+) demonstrating a powerful synthetic control over a variety of hybrid nanostructures. The deposition of the domains and the formation of the network take place simultaneously (one-step method) at room temperature in dark conditions without any external trigger. Beside the in-depth structural characterization of the gel-like hybrid networks, the wavelength-dependent optical features are studied to reveal an efficient charge carrier separation in the systems and a controllable extent of fluorescence quenching through the domain sizes. Photoluminescence quantum yields and decay dynamics highlight the importance of fine-tuning the conduction band/Fermi level offset between the semiconductors and the various deposited metals playing central role in the electron–hole separation processes. This procedure provides a novel platform toward the preparation of photo(electro)catalytically promising hybrid nanostructures (acetogels and xerogels) without the need of presynthetic hybrid particle design.

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

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.

Over the past decades, cation exchange reactions applied to nanoparticles have opened up synthetic pathways to nanocrystals, which were not accessible by other means before. The limitation of cation exchange on the macroscopic scale of bulk materials is given by the limited ion diffusion within the crystal structure. Lyogels or aerogels are macroscopic, highly voluminous, porous materials composed of interconnected nanoscopic building blocks and hence represent a type of bridge between the macroscopic and the nanoscopic world. To demonstrate the feasibility of cation exchange on such macroscopic nanomaterials, the cation exchange on CdSe/CdS core/shell and CdS nanorod based lyogels to Cu2−xSe/ Cu2−xS and Cu2−xS and the reversible exchange back to CdSe/CdS and CdS lyogels is presented. These copper-based lyogels can also be used as an intermediate state on the way to other metal chalcogenidebased macroscopic structures. By reversed cation exchange back to cadmium an additional proof is given, that the crystal structures remain unchanged. It is shown that cation exchange reactions can also be transferred to macroscopic objects like aerogels or lyogels. This procedure additionally allows the access of aerogels which cannot be synthesized via direct destabilization of the respective colloidal solutions.