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Soft-template strategy enables the fabrication of composite nanomaterials with desired functionalities and structures. In this thesis, soft templates, including poly(ionic liquid) nanovesicles (PIL NVs), self-assembled polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) particles, and glycopeptide (GP) biomolecules have been applied for the synthesis of versatile composite particles of PILs/Cu, molybdenum disulfide/carbon (MoS2/C), and GP-carbon nanotubes-metal (GP-CNTs-metal) composites, respectively. Subsequently, their possible applications as efficient catalysts in two representative reactions, i.e. CO2 electroreduction (CO2ER) and reduction of 4-nitrophenol (4-NP), have been studied, respectively.
In the first work, PIL NVs with a tunable particle size of 50 to 120 nm and a shell thickness of 15 to 60 nm have been prepared via one-step free radical polymerization. By increasing monomer concentration for polymerization, their nanoscopic morphology can evolve from hollow NVs to dense spheres, and finally to directional worms, in which a multi-lamellar packing of PIL chains occurred in all samples. The obtained PIL NVs with varied shell thickness have been in situ functionalized with ultra-small Cu nanoparticles (Cu NPs, 1-3 nm) and subsequently employed as the electrocatalysts for CO2ER. The hollow PILs/Cu composite catalysts exhibit a 2.5-fold enhancement in selectivity towards C1 products compared to the pristine Cu NPs. This enhancement is primarily attributed to the strong electronic interactions between the Cu NPs and the surface functionalities of PIL NVs. This study casts new aspects on using nanostructured PILs as novel electrocatalyst supports in efficient CO2 conversion.
In the second work, a novel approach towards fast degradation of 4-NP has been developed using porous MoS2/C particles as catalysts, which integrate the intrinsically catalytic property of MoS2 with its photothermal conversion capability. Various MoS2/C composite particles have been prepared using assembled PS-b-P2VP block copolymer particles as sacrificed soft templates. Intriguingly, the MoS2/C particles exhibit tailored morphologies including pomegranate-like, hollow, and open porous structures. Subsequently, the photothermal conversion performance of these featured particles has been compared under near infrared (NIR) light irradiation. When employing the open porous MoS2/C particles as the catalyst for the reduction of 4-NP, the reaction rate constant has increased by 1.5-fold under light illumination. This catalytic enhancement mainly results from the open porous architecture and photothermal conversion performance of the MoS2 particles. This proposed strategy offers new opportunities for efficient photothermal-assisted catalysis.
In the third work, a facile and green approach towards the fabrication of GP-CNTs-metal composites has been proposed, which utilizes a versatile GP biomolecule both as a stabilizer for CNTs in water and as a reducing agent for noble metal ions. The abundant hydrogen bonds in GP molecules bestow the formed GP-CNTs with excellent plasticity, enabling the availability of polymorphic CNTs species ranging from dispersion to viscous paste, gel, and even dough by increasing their concentration. The GP molecules can reduce metal precursors at room temperature without additional reducing agents, enabling the in situ immobilization of metal NPs (e.g. Au, Ag, and Pd) on the CNTs surface. The combination of excellent catalytic property of Pd NPs with photothermal conversion capability of CNTs makes the GP-CNTs-Pd composite a promising catalyst for the efficient degradation of 4-NP. The obtained composite displays a 1.6-fold increase in conversion under NIR light illumination in the reduction of 4-NP, mainly owing to the strong light-to-heat conversion effect of CNTs. Overall, the proposed method opens a new avenue for the synthesis of CNTs composite as a sustainable and versatile catalyst platform.
The results presented in the current thesis demonstrate the significance of using soft templates for the synthesis of versatile composites with tailored nanostructure and functionalities. The investigation of these composite nanomaterials in the catalytic reactions reveals their potential in the development of desired catalysts for emerging catalytic processes, e.g. photothermal-assisted catalysis and electrocatalysis.
Hybrid nanomaterials offer the combination of individual properties of different types of nanoparticles. Some strategies for the development of new nanostructures in larger scale rely on the self-assembly of nanoparticles as a bottom-up approach. The use of templates provides ordered assemblies in defined patterns. In a typical soft-template, nanoparticles and other surface-active agents are incorporated into non-miscible liquids. The resulting self-organized dispersions will mediate nanoparticle interactions to control the subsequent self-assembly. Especially interactions between nanoparticles of very different dispersibility and functionality can be directed at a liquid-liquid interface.
In this project, water-in-oil microemulsions were formulated from quasi-ternary mixtures with Aerosol-OT as surfactant. Oleyl-capped superparamagnetic iron oxide and/or silver nanoparticles were incorporated in the continuous organic phase, while polyethyleneimine-stabilized gold nanoparticles were confined in the dispersed water droplets. Each type of nanoparticle can modulate the surfactant film and the inter-droplet interactions in diverse ways, and their combination causes synergistic effects. Interfacial assemblies of nanoparticles resulted after phase-separation. On one hand, from a biphasic Winsor type II system at low surfactant concentration, drop-casting of the upper phase afforded thin films of ordered nanoparticles in filament-like networks. Detailed characterization proved that this templated assembly over a surface is based on the controlled clustering of nanoparticles and the elongation of the microemulsion droplets. This process offers versatility to use different nanoparticle compositions by keeping the surface functionalization, in different solvents and over different surfaces. On the other hand, a magnetic heterocoagulate was formed at higher surfactant concentration, whose phase-transfer from oleic acid to water was possible with another auxiliary surfactant in ethanol-water mixture. When the original components were initially mixed under heating, defined oil-in-water, magnetic-responsive nanostructures were obtained, consisting on water-dispersible nanoparticle domains embedded by a matrix-shell of oil-dispersible nanoparticles.
Herein, two different approaches were demonstrated to form diverse hybrid nanostructures from reverse microemulsions as self-organized dispersions of the same components. This shows that microemulsions are versatile soft-templates not only for the synthesis of nanoparticles, but also for their self-assembly, which suggest new approaches towards the production of new sophisticated nanomaterials in larger scale.
Solar photocatalysis is the one of leading concepts of research in the current paradigm of sustainable chemical industry. For actual practical implementation of sunlight-driven catalytic processes in organic synthesis, a cheap, efficient, versatile and robust heterogeneous catalyst is necessary. Carbon nitrides are a class of organic semiconductors who are known to fulfill these requirements.
First, current state of solar photocatalysis in economy, industry and lab research is overviewed, outlining EU project funding, prospective synthetic and reforming bulk processes, small scale solar organic chemistry, and existing reactor designs and prototypes, concluding feasibility of the approach.
Then, the photocatalytic aerobic cleavage of oximes to corresponding aldehydes and ketones by anionic poly(heptazine imide) carbon nitride is discussed. The reaction provides a feasible method of deprotection and formation of carbonyl compounds from nitrosation products and serves as a convenient model to study chromoselectivity and photophysics of energy transfer in heterogeneous photocatalysis.
Afterwards, the ability of mesoporous graphitic carbon nitride to conduct proton-coupled electron transfer was utilized for the direct oxygenation of 1,3-oxazolidin-2-ones to corresponding 1,3-oxazlidine-2,4-diones. This reaction provides an easier access to a key scaffold of diverse types of drugs and agrochemicals.
Finally, a series of novel carbon nitrides based on poly(triazine imide) and poly(heptazine imide) structure was synthesized from cyanamide and potassium rhodizonate. These catalysts demonstrated a good performance in a set of photocatalytic benchmark reactions, including aerobic oxidation, dual nickel photoredox catalysis, hydrogen peroxide evolution and chromoselective transformation of organosulfur precursors.
Concluding, the scope of carbon nitride utilization for net-oxidative and net-neutral photocatalytic processes was expanded, and a new tunable platform for catalyst synthesis was discovered.
The present thesis focuses on the synthesis of nanostructured iron-based compounds by using β-FeOOH nanospindles and poly(ionic liquid)s (PILs) vesicles as hard and soft templates, respectively, to suppress the shuttle effect of lithium polysulfides (LiPSs) in Li-S batteries. Three types of composites with different nanostructures (mesoporous nanospindle, yolk-shell nanospindle, and nanocapsule) have been synthesized and applied as sulfur host material for Li-S batteries. Their interactions with LiPSs and effects on the electrochemical performance of Li-S batteries have been systematically studied.
In the first part of the thesis, carbon-coated mesoporous Fe3O4 (C@M-Fe3O4) nanospindles have been synthesized to suppress the shuttle effect of LiPSs. First, β-FeOOH nanospindles have been synthesized via the hydrolysis of iron (III) chloride in aqueous solution and after silica coating and subsequent calcination, mesoporous Fe2O3 (M-Fe2O3) have been obtained inside the confined silica layer through pyrolysis of β-FeOOH. After the removal of the silica layer, electron tomography (ET) has been applied to rebuild the 3D structure of the M-Fe2O3 nanospindles. After coating a thin layer of polydopamine (PDA) as carbon source, the PDA-coated M-Fe2O3 particles have been calcinated to synthesize C@M-Fe3O4 nanospindles. With the chemisorption of Fe3O4 and confinement of mesoporous structure to anchor LiPSs, the composite C@M-Fe3O4/S electrode delivers a remaining capacity of 507.7 mAh g-1 at 1 C after 600 cycles.
In the second part of the thesis, a series of iron-based compounds (Fe3O4, FeS2, and FeS) with the same yolk-shell nanospindle morphology have been synthesized, which allows for the direct comparison of the effects of compositions on the electrochemical performance of Li-S batteries. The Fe3O4-carbon yolk-shell nanospindles have been synthesized by using the β-FeOOH nanospindles as hard template. Afterwards, Fe3O4-carbon yolk-shell nanospindles have been used as precursors to obtain iron sulfides (FeS and FeS2)-carbon yolk-shell nanospindles through sulfidation at different temperatures. Using the three types of yolk-shell nanospindles as sulfur host, the effects of compositions on interactions with LiPSs and electrochemical performance in Li-S batteries have been systematically investigated and compared. Benefiting from the chemisorption and catalytic effect of FeS2 particles and the physical confinement of the carbon shell, the FeS2-C/S electrode exhibits the best electrochemical performance with an initial specific discharge capacity of 877.6 mAh g-1 at 0.5 C and a retention ratio of 86.7% after 350 cycles.
In the third part, PILs vesicles have been used as soft template to synthesize carbon nanocapsules embedded with iron nitride particles to immobilize and catalyze LiPSs in Li-S batteries. First, 3-n-decyl-1-vinylimidazolium bromide has been used as monomer to synthesize PILs nanovesicles by free radical polymerization. Assisted by PDA coating route and ion exchange, PIL nanovesicles have been successfully applied as soft template in morphology-maintaining carbonization to prepare carbon nanocapsules embedded with iron nitride nanoparticles (FexN@C). The well-dispersed iron nitride nanoparticles effectively catalyze the conversion of LiPSs to Li2S, owing to their high electrical conductivity and strong chemical binding to LiPSs. The constructed FexN@C/S cathode demonstrates a high initial discharge capacity of 1085.0 mAh g-1 at 0.5 C with a remaining value of 930.0 mAh g-1 after 200 cycles.
The results in the present thesis demonstrate the facile synthetic routes of nanostructured iron-based compounds with controllable morphologies and compositions using soft and hard colloidal templates, which can be applied as sulfur host to suppress the shuttle behavior of LiPSs. The synthesis approaches developed in this thesis are also applicable to fabricating other transition metal-based compounds with porous nanostructures for other applications.
In dieser Dissertation konnten erfolgreich mechanisch stabile Hydrogele über eine freie radikalische Polymerisation (FRP) in Wasser synthetisiert werden. Dabei diente vor allem das Sulfobetain SPE als Monomer. Dieses wurde mit dem über eine nukleophile Substitution erster bzw. zweiter Ordnung hergestellten Vernetzer TMBEMPA/Br umgesetzt.
Die entstandenen Netzwerke wurden im Gleichgewichtsquellzustand im Wesentlichen mittels Niederfeld-Kernresonanzspektroskopie, Röntgenkleinwinkelstreuung (SAXS), Rasterelektronenmikroskopie mit Tieftemperaturtechnik (Kryo-REM), dynamisch-mechanische Analyse (DMA), Rheologie, thermogravimetrische Analyse (TGA) und dynamische Differenzkalorimetrie (DSC) analysiert.
Das hierarchisch aufgebaute Netzwerk wurde anschließend für die matrixgesteuerten Mineralisation von Calciumphosphat und –carbonat genutzt. Über das alternierende Eintauchverfahren (engl. „alternate soaking method“) und der Variation von Mineralisationsparametern, wie pH-Wert, Konzentration c und Temperatur T konnten dann verschiedene Modifikationen des Calciumphosphats generiert werden. Das entstandene Hybridmaterial wurde qualitativ mittels Röntgenpulverdiffraktometrie (XRD), abgeschwächte Totalreflexion–fouriertransformierte Infrarot Spektroskopie (ATR-FTIR), Raman-Spektroskopie, Rasterelektronenmikroskopie (REM) mit energiedispersiver Röntgenspektroskopie (EDXS) und optischer Mikroskopie (OM) als auch quantitative mittels Gravimetrie und TGA analysiert.
Für die potentielle Verwendung in der Medizintechnik, z.B. als Implantatmaterial, ist die grundlegende Einschätzung der Wechselwirkung zwischen Hydrogel bzw. Hybridmaterial und verschiedener Zelltypen unerlässlich. Dazu wurden verschiedene Zelltypen, wie Einzeller, Bakterien und adulte Stammzellen verwendet. Die Wechselwirkung mit Peptidsequenzen von Phagen komplettiert das biologische Unterkapitel.
Hydrogele sind mannigfaltig einsetzbar. Diese Arbeit fasst daher weitere Projektperspektiven, auch außerhalb des biomedizinischem Anwendungsspektrums, auf. So konnten erste Ansätze zur serienmäßige bzw. maßgeschneiderte Produktion über das „Inkjet“ Verfahren erreicht werden. Um dies ermöglichen zu können wurden erfolgreich weitere Synthesestrategien, wie die Photopolymerisation und die redoxinitiierte Polymerisation, ausgenutzt. Auch die Eignung als Filtermaterial oder Superabsorber wurde analysiert.
Layered structures are ubiquitous in nature and industrial products, in which individual layers could have different mechanical/thermal properties and functions independently contributing to the performance of the whole layered structure for their relevant application. Tuning each layer affects the performance of the whole layered system.
Pores are utilized in various disciplines, where low density, but large surfaces are demanded. Besides, open and interconnected pores would act as a transferring channel for guest chemical molecules. The shape of pores influences compression behavior of the material. Moreover, introducing pores decreases the density and subsequently the mechanical strength. To maintain defined mechanical strength under various stress, porous structure can be reinforced by adding reinforcement agent such as fiber, filler or layered structure to bear the mechanical stress on demanded application.
In this context, this thesis aimed to generate new functions in bilayer systems by combining layers having different moduli and/or porosity, and to develop suitable processing techniques to access these structures.
Manufacturing processes of layered structures employ often organic solvents mostly causing environmental pollution. In this regard, the studied bilayer structures here were manufactured by processes free of organic solvents.
In this thesis, three bilayer systems were studied to answer the individual questions.
First, while various methods of introducing pores in melt-phase are reported for one-layer constructs with simple geometry, can such methods be applied to a bilayer structure, giving two porous layers?
This was addressed with Bilayer System 1. Two porous layers were obtained from melt-blending of two different polyurethanes (PU) and polyvinyl alcohol (PVA) in a co-continuous phase followed by sequential injection molding and leaching the PVA phase in deionized water. A porosity of 50 ± 5% with a high interconnectivity was obtained, in which the pore sizes in both layers ranged from 1 µm to 100 µm with an average of 22 µm in both layers. The obtained pores were tailored by applying an annealing treatment at relevant high temperatures of 110 °C and 130 °C, which allowed the porosity to be kept constant. The disadvantage of this system is that a maximum of 50% porosity could be reached and removal of leaching material in the weld line section of both layers is not guaranteed. Such a construct serves as a model for bilayer porous structure for determining structure-property relationships with respect to the pore size, porosity and mechanical properties of each layer. This fabrication method is also applicable to complex geometries by designing a relevant mold for injection molding.
Secondly, utilizing scCO2 foaming process at elevated temperature and pressure is considered as a green manufacturing process. Employing this method as a post-treatment can alter the history orientation of polymer chains created by previous fabrication methods. Can a bilayer structure be fabricated by a combination of sequential injection molding and scCO2 foaming process, in which a porous layer is supported by a compact layer?
Such a construct (Bilayer System 2) was generated by sequential injection molding of a PCL (Tm ≈ 58 °C) layer and a PLLA (Tg ≈ 58 °C) layer. Soaking this structure in the autoclave with scCO2 at T = 45 °C and P = 100 bar led to the selective foaming of PCL with a porosity of 80%, while the PLA layer was kept compact. The scCO2 autoclave led to the formation of a porous core and skin layer of the PCL, however, the degree of crystallinity of PLLA layer increased from 0 to 50% at the defined temperature and pressure. The microcellular structure of PCL as well as the degree of crystallinity of PLLA were controlled by increasing soaking time.
Thirdly, wrinkles on surfaces in micro/nano scale alter the properties, which are surface-related. Wrinkles are formed on a surface of a bilayer structure having a compliant substrate and a stiff thin film. However, the reported wrinkles were not reversible. Moreover, dynamic wrinkles in nano and micro scale have numerous examples in nature such as gecko foot hair offering reversible adhesion and an ability of lotus leaves for self-cleaning altering hydrophobicity of the surface. It was envisioned to imitate this biomimetic function on the bilayer structure, where self-assembly on/off patterns would be realized on the surface of this construct.
In summary, developing layered constructs having different properties/functions in the individual layer or exhibiting a new function as the consequence of layered structure can give novel insight for designing layered constructs in various disciplines such as packaging and transport industry, aerospace industry and health technology.
Im Rahmen dieser Arbeit wurden Energie induzierte Nanopartikel-Substrat Interaktionen untersucht. Dazu wurden Goldnanopartikelanordnungen (AuNPA) auf verschiedenen Silizium-basierten Substraten hergestellt und der Einfluss eines Energieeintrages, genauer gesagt einer thermischen Behandlung oder des Metall-assistierten chemischen Ätzens (MaCE) getestet. Die Nanopartikelanordnungen, welche für die thermische Behandlung eingesetzt wurden, wurden nass-chemisch in Toluol synthetisiert, mit Thiol-terminiertem Polystyrol funktionalisiert und mittels Schleuderbeschichtung auf verschiedenen Substraten (drei Gläser und ein Siliziumwafer) in quasi-hexagonalen Mustern angeordnet. Diese AuNP-Anordnungen wurden mit Temperaturen zwischen 475 °C – 792 °C über verschiedene Zeiträume thermisch behandelt. Generell sanken die Nanopartikel in die Substrate ein, und es wurde festgestellt, dass mit Erhöhung der Glasübergangstemperatur der Substrate die Einsinktiefe der Nanopartikel abnahm. Die AuNPA auf Siliziumwafern wurden auf Temperaturen von 700 °C – 900 °C erhitzt. Die Goldnanopartikel sanken dabei bis zu 2,5 nm in das Si-Substrat ein. Ein Sintern der Nanopartikel fand ab einer Temperatur über 660 °C statt. Welcher Sintermechanismus der dominante ist konnte abschließend nicht eindeutig geklärt werden.
Für die Untersuchung des Einflusses des zweiten Energieeintrages mittels MaCE wurden AuNPA sowie Goldkern-Silberschale-Anordnungen auf Siliziumsubstraten genutzt. Die AuNPA wurden mit Hilfe von Poly-N-Isopropylacrylamid Mikrogelen und Natriumcitrat-stabilisierten Goldnanopartikeln (Na-AuNP) bzw. Tetrachloridogoldsäure (TCG) präpariert. Es ergaben sich Nanopartikelanordnungen mit hemisphärischen Partikeln (aus Na-AuNP) und zum anderen Nanopartikelanordnungen mit sphärischen Partikeln (aus TCG). Durch eine anschließende Silberwachstumsreaktion konnten dann die dazugehörigen Goldkern-Silberschale Nanopartikelanordnungen erhalten werden. Beim MaCE konnten signifikante Unterschiede im Verhalten dieser vier Nanopartikelanordnungen festgestellt werden, z.B. mussten bei den hemisphärischen Partikelanordnungen höhere Wasserstoffperoxidkonzentrationen (0,70 M – 0,91 M) als bei den sphärischen Partikelanordnungen (0,08 M – 0,32 M) für das Ätzen eingesetzt werden, um ein Einsinken der Nanopartikel in das Substrat zu erreichen.
Im Rahmen dieser Dissertation wurden die erstmaligen Totalsynthesen der Arylnaphthalen-Lignane Alashinol D, Vitexdoin C, Vitrofolal E, Noralashinol C1 und Ternifoliuslignan E vorgestellt. Der Schlüsselschritt der entwickelten Methode, basiert auf einer regioselektiven intramolekularen Photo-Dehydro-Diels-Alder (PDDA)-Reaktion, die mittels UV-Strahlung im Durchflussreaktor durchgeführt wurde. Bei der Synthese der PDDA-Vorläufer (Diarylsuberate) wurde eine Synthesestrategie nach dem Baukastenprinzip verfolgt. Diese ermöglicht die Darstellung asymmetrischer komplexer Systeme aus nur wenigen Grundbausteinen und die Totalsynthese einer Vielzahl an Lignanen. In systematischen Voruntersuchungen konnte zudem die klare Überlegenheit der intra- gegenüber der intermolekularen PDDA-Reaktion aufgezeigt werden. Dabei stellte sich eine Verknüpfung der beiden Arylpropiolester über einen Korksäurebügel, in para-Position, als besonders effizient heraus. Werden asymmetrisch substituierte Diarylsuberate, bei denen einer der endständigen Estersubstituenten durch eine Trimethylsilyl-Gruppe oder ein Wasserstoffatom ersetzt wurde, verwendet, durchlaufen diese Systeme eine regioselektive Cyclisierung und als Hauptprodukt werden Naphthalenophane mit einem Methylester in 3-Position erhalten. Mit Hilfe von umfangreichen Experimenten zur Funktionalisierung der 4-Position, konnte zudem gezeigt werden, dass die Substitution der nucleophilen Cycloallen-Intermediate, während der PDDA-Reaktion, generell durch die Zugabe von N-Halogen-Succinimiden möglich ist. In Anbetracht der geringen Ausbeuten haben diese intermolekularen Abfangreaktionen, jedoch keinen präparativen Nutzen für die Totalsynthesen von Lignanen. Mit dem Ziel die allgemeinen photochemischen Reaktionsbedingungen zu optimieren, wurde erstmalig die triplettsensibilisierte PDDA-Reaktion vorgestellt. Durch die Verwendung von Xanthon als Sensibilisator wurde der Einsatz von effizienteren UVA-Lichtquellen ermöglicht, wodurch die Gefahr einer Photozersetzung durch Überbestrahlung minimiert wurde. Im Vergleich zur direkten Anregung mit UVB-Strahlung, konnten die Ausbeuten mit indirekter Anregung durch einen Photokatalysator signifikant gesteigert werden. Die grundlegenden Erkenntnisse und die entwickelten Synthesestrategien dieser Arbeit, können dazu beitragen zukünftig die Erschließung neuer pharmakologisch interessanter Lignane voranzutreiben.
1 Bisher ist nur die semisynthetische Darstellung von Noralashinol C ausgehend von Hydroxymatairesinol literaturbekannt.
Lithium-ion capacitors (LICs) are promising energy storage devices by asymmetrically combining anode with a high energy density close to lithium-ion batteries and cathode with a high power density and long-term stability close to supercapacitors. For the further improvement of LICs, the development of electrode materials with hierarchical porosity, nitrogen-rich lithiophilic sites, and good electrical conductivity is essential. Nitrogen-rich all-carbon composite hybrids are suitable for these conditions along with high stability and tunability, resulting in a breakthrough to achieve the high performance of LICs. In this thesis, two different all-carbon composites are suggested to unveil how the pore structure of lithiophilic composites influences the properties of LICs. Firstly, the composite with 0-dimensional zinc-templated carbon (ZTC) and hexaazatriphenylene-hexacarbonitrile (HAT) is examined how the pore structure is connected to Li-ion storage property as LIC electrode. As the pore structure of HAT/ZTC composite is easily tunable depending on the synthetic factor and ratio of each component, the results will allow deeper insights into Li-ion dynamics in different porosity, and low-cost synthesis by optimization of the HAT:ZTC ratio. Secondly, the composite with 1-dimensional nanoporous carbon fiber (ACF) and cost-effective melamine is proposed as a promising all-carbon hybrid for large-scale application. Since ACF has ultra-micropores, the numerical structure-property relationships will be calculated out not only from total pore volume but more specifically from ultra-micropore volume. From these results above, it would be possible to understand how hybrid all-carbon composites interact with lithium ions in nanoscale as well as how structural properties affect the energy storage performance. Based on this understanding derived from the simple materials modeling, it will provide a clue to design the practical hybrid materials for efficient electrodes in LICs.
Technologically important, environmentally friendly InP quantum dots (QDs) typically used as green and red emitters in display devices can achieve exceptional photoluminescence quantum yields (PL QYs) of near-unity (95-100%) when the-state-of-the-art core/shell heterostructure of the ZnSe inner/ZnS outer shell is elaborately applied. Nevertheless, it has only led to a few industrial applications as QD liquid crystal display (QD–LCD) which is applied to blue backlight units, even though QDs has a lot of possibilities that able to realize industrially feasible applications, such as QD light-emitting diodes (QD‒LEDs) and luminescence solar concentrator (LSC), due to their functionalizable characteristics.
Before introducing the main research, the theoretical basis and fundamentals of QDs are described in detail on the basis of the quantum mechanics and experimental synthetic results, where a concept of QD and colloidal QD, a type-I core/shell structure, a transition metal doped semiconductor QDs, the surface chemistry of QD, and their applications (LSC, QD‒LEDs, and EHD jet printing) are sequentially elucidated for better understanding. This doctoral thesis mainly focused on the connectivity between QD materials and QD devices, based on the synthesis of InP QDs that are composed of inorganic core (core/shell heterostructure) and organic shell (surface ligands on the QD surface). In particular, as for the former one (core/shell heterostructure), the ZnCuInS mid-shell as an intermediate layer is newly introduced between a Cu-doped InP core and a ZnS shell for LSC devices. As for the latter one (surface ligands), the ligand effect by 1-octanethiol and chloride ion are investigated for the device stability in QD‒LEDs and the printability of electro-hydrodynamic (EHD) jet printing system, in which this research explores the behavior of surface ligands, based on proton transfer mechanism on the QD surface.
Chapter 3 demonstrates the synthesis of strain-engineered highly emissive Cu:InP/Zn–Cu–In–S (ZCIS)/ZnS core/shell/shell heterostructure QDs via a one-pot approach. When this unconventional combination of a ZCIS/ZnS double shelling scheme is introduced to a series of Cu:InP cores with different sizes, the resulting Cu:InP/ZCIS/ZnS QDs with a tunable near-IR PL range of 694–850 nm yield the highest-ever PL QYs of 71.5–82.4%. These outcomes strongly point to the efficacy of the ZCIS interlayer, which makes the core/shell interfacial strain effectively alleviated, toward high emissivity. The presence of such an intermediate ZCIS layer is further examined by comparative size, structural, and compositional analyses. The end of this chapter briefly introduces the research related to the LSC devices, fabricated from Cu:InP/ZCIS/ZnS QDs, currently in progress.
Chapter 4 mainly deals with ligand effect in 1-octanethiol passivation of InP/ZnSe/ZnS QDs in terms of incomplete surface passivation during synthesis. This chapter demonstrates the lack of anionic carboxylate ligands on the surface of InP/ZnSe/ZnS quantum dots (QDs), where zinc carboxylate ligands can be converted to carboxylic acid or carboxylate ligands via proton transfer by 1-octanethiol. The as-synthesized QDs initially have an under-coordinated vacancy surface, which is passivated by solvent ligands such as ethanol and acetone. Upon exposure of 1-octanethiol to the QD surface, 1-octanthiol effectively induces the surface binding of anionic carboxylate ligands (derived from zinc carboxylate ligands) by proton transfer, which consequently exchanges ethanol and acetone ligands that bound on the incomplete QD surface. The systematic chemical analyses, such as thermogravimetric analysis‒mass spectrometry and proton nuclear magnetic resonance spectroscopy, directly show the interplay of surface ligands, and it associates with QD light-emitting diodes (QD‒LEDs).
Chapter 5 shows the relation between material stability of QDs and device stability of QD‒LEDs through the investigation of surface chemistry and shell thickness. In typical III–V colloidal InP quantum dots (QDs), an inorganic ZnS outermost shell is used to provide stability when overcoated onto the InP core. However, this work presents a faster photo-degradation of InP/ZnSe/ZnS QDs with a thicker ZnS shell than that with a thin ZnS shell when 1-octanethiol was applied as a sulfur source to form ZnS outmost shell. Herein, 1-octanethiol induces the form of weakly-bound carboxylate ligand via proton transfer on the QD surface, resulting in a faster degradation at UV light even though a thicker ZnS shell was formed onto InP/ZnSe QDs. Detailed insight into surface chemistry was obtained from proton nuclear magnetic resonance spectroscopy and thermogravimetric analysis–mass spectrometry. However, the lifetimes of the electroluminescence devices fabricated from InP/ZnSe/ZnS QDs with a thick or a thin ZnS shell show surprisingly the opposite result to the material stability of QDs, where the QD light-emitting diodes (QD‒LEDs) with a thick ZnS shelled QDs maintained its luminance more stable than that with a thin ZnS shelled QDs. This study elucidates the degradation mechanism of the QDs and the QD light-emitting diodes based on the results and discuss why the material stability of QDs is different from the lifetime of QD‒LEDs.
Chapter 6 suggests a method how to improve a printability of EHD jet printing when QD materials are applied to QD ink formulation, where this work introduces the application of GaP mid-shelled InP QDs as a role of surface charge in EHD jet printing technique. In general, GaP intermediate shell has been introduced in III–V colloidal InP quantum dots (QDs) to enhance their thermal stability and quantum efficiency in the case of type-I core/shell/shell heterostructure InP/GaP/ZnSeS QDs. Herein, these highly luminescent InP/GaP/ZnSeS QDs were synthesized and applied to EHD jet printing, by which this study demonstrates that unreacted Ga and Cl ions on the QD surface induce the operating voltage of cone jet and cone jet formation to be reduced and stabilized, respectively. This result indicates GaP intermediate shell not only improves PL QY and thermal stability of InP QDs but also adjusts the critical flow rate required for cone-jet formation. In other words, surface charges of quantum dots can have a significant role in forming cone apex in the EHD capillary nozzle. For an industrially convenient validation of surface charges on the QD surface, Zeta potential analyses of QD solutions as a simple method were performed, as well as inductively coupled plasma optical emission spectrometry (ICP-OES) for a composition of elements.
Beyond the generation of highly emissive InP QDs with narrow FWHM, these studies talk about the connection between QD material and QD devices not only to make it a vital jumping-off point for industrially feasible applications but also to reveal from chemical and physical standpoints the origin that obstructs the improvement of device performance experimentally and theoretically.