Among several classes of porous materials, metal-organic frameworks (MOFs) are particularly attractive due to their unprecedented internal surface areas (up to 7800 m2/g),[1] easy chemical tunability, and strong, selective binding of a host of guest species. Through judicious selection of MOF building blocks, which include metal ions and organic ligands, one can readily modify their properties for a variety of potential applications. Despite these attractive features, there are still challenges in the field that limit our ability to use MOFs as a solution for a wide range of industrial problems. For instance, some MOFs have limited mechanical and chemical stability, particularly in highly humid, acidic or basic environments. Overcoming this problem could lead to extended lifetimes and hence increased feasibility for their use in areas where such conditions are required.

In response to these needs, we have recently begun to combine MOFs and polymers in an effort to boost MOF performance and extend their stability. Our recent work demonstrates that selected polymers can significantly enhance MOF performance in a number of important liquid and gas separations[2-4] as well as extend catalyst lifetimes in selected reactions.[5] In addition to this, controlled polymerization processes were employed to enhance the mechanical stability[6] of large pore frameworks and extend the chemical stability of a number structurally diverse MOFs not only in humid environments, but also in acidic and basic media.[7] We hope such work can help bring these frameworks a few steps closer to their deployment into a range of ecologically and economically important applications. In this presentation, our recent work, devoted to the development of novel MOF polymer composites, will be outlined.

A naive comparison between nature and chemists on production capacities, e.g. biopolymers and plastics productions, respectively, marks nature as clear winner. The same accounts for the required feedstocks, because nature uses CO₂ directly from air whereas the chemist relies on crude oil. Although both parties have been independent actors for such a long time, the development of Green Chemistry has helped chemists to learn nature´s biodegradation strategies in materials design and to synthesize novel bio-inspired materials. Within this respect, I will demonstrate how the chemist can dress chemistry in green on examples covering metal-free catalysts^1-2, sustainable methods for Li-ion batteries recycling (Figure 1)^3-4, functional lignin-based coatings^5-6, electrocatalytic denitrification⁷, and bio-inspired fibres⁸.

Figure 1. Green MOF for sustainable separation of cobalt and nickel from leaching solutions.

(1) T.Zhang, A.Slabon, S.Das et al. ACS Catalysis 2021 in print

(2) I. Szewczyk, A. Slabon, P. Kuśtrowski et al. Chem. Mater. 2020, 32, 7273

(3) J. Piątek, A. Slabon et al. Adv. Energ. Mater. 2021 in print

(4) J.Piątek, A.Slabon et al. ACS Sustainable Chem. Eng. 2021, 9, 9770

(5) A.Moreno, A.Slabon, M.H.Sipponen et al. Angew. Chem. Int. Ed. 2021, 60, 20897

(6) T.M. Budnyak, A. Slabon et al. ACS Sustain. Chem. Eng. 2020, 8, 16262

(6) Z. Ma, A. Slabon et al. ACS Sustain. Chem. Eng. 2021, 9, 3658

(7) W.F. de Deus, A. Slabon, B.V.M. Rodrigues et al. ACS App. Mater. Interfaces 2021, 13, 24493

Modern syntheses of colloidal nanocrystals yield extraordinarily narrow size distributions that are believed to result from a rapid “burst of nucleation” [1] followed by diffusion limited growth and size distribution focusing [2] Using a combination of in situ X-ray scattering, optical absorption, and ¹³C nuclear magnetic resonance (NMR) spectroscopy, we monitor the kinetics of PbS solute generation, nucleation, and crystal growth from three thiourea precursors whose conversion reactivity spans a 2-fold range. In all three cases, nucleation is found to be slow and continues during > 50% of the precipitation. A population balance model based on a size dependent growth law (1/r) fits the data with a single growth rate constant (kG) across all three precursors. However, the magnitude of the kG and the lack of solvent viscosity dependence indicates that the rate limiting step is not diffusion from solution to the nanoparticle surface.  Several surface reaction limited mechanisms and a ligand penetration model that fits data our experiments using a single fit parameter are proposed to explain the results.

With the extremely fast growth of electronics and optoelectronics market for devices with a transparency window in the visible range, the need for proper electrodes and the related raw materials has increased exponentially¹. State of the art devices for photovoltaics, lighting and energy-related applications make use, in many cases, of Transparent Conductive Oxides (TCOs) thin films². These materials present peculiar optical and electronic properties, tailorable to meet specific requirements.

Even more desirable is the use of TCOs in the form of Nanocrystals (NCs), considering for example their solution processability. This enables a wider range of low cost and versatile implementations, such as the use of flexible, polymeric substrates³.

Nonetheless, to consider them as valid alternative materials in optoelectronics, it’s essential to have a firm control over synthesis parameters⁴. Process scalability holds a crucial value, considering future applications and greater production volumes.

In this contribution, we investigated methods for composition control and dopant placement in the NCs structure through colloidal synthesis. In particular, we analyzed the effects of the variation of precursor concentration injected overtime. This applies to a wide variety of doped and undoped TCOs. Among them, we focused on Indium Tin Oxide (ITO) NCs, with different composition profiles. Moreover, we present some preliminary analysis on Zinc Iron Oxide (ZIO) NCs. Both materials are interesting in energy storage applications, owing to light-driven charge accumulation.

The self-assembly of nanocrystals into binary superlattices enables the targeted integration of orthogonal physical properties, like photoluminescence and magnetism, into a single superstructure, unlocking a vast design space for multifunctional materials. Yet, the formation of binary nanocrystal superlattices remains poorly understood, restricting the use of simulation to predict structure and properties of the final superlattices. Here, we use in situ scattering experiments to unravel the time-dependent self-assembly of nanocrystals into 3D binary superlattices, and molecular dynamics simulations to obtain interparticle interactions consistent with experimental observations. We show definitively that short-ranged, attractive interparticle forces are necessary to obtain the binary crystalline phases observed in experiment. The short-ranged attraction stabilizes these crystalline phases relative to fluid phases, dramatically enhancing their formation kinetics over the purely repulsive interactions of the hard-sphere model. In these conditions, the formation of binary nanocrystal superlattices proceeds through homogeneous nucleation in the absence of intermediate ordered structures. These results establish a robust correspondence between experiment and theory, paving the way towards a priori prediction of binary nanocrystal superlattices.

Nanoparticles and their assemblies offer unique properties, which are highly related to their composition, size, shape[1] and - for assemblies - to their nature of connection[2],[3]. By connecting CdSe/CdS (NR) dot-in-rod semiconductor nanoparticles into network structures, excited electrons can travel through the CdS shell while holes are trapped in the CdSe core region. In our work, we present an experimental approach to evaluate the spatial extent of fluorescence quenching due to the separation of the excited electrons and holes in the connected NR networks.[4] To extract this information, we perform photoluminescence spectroscopy (such as fluorescence lifetime and photoluminescence quantum yield measurements) on different types of materials, namely colloidal mixtures, hydrogels, lyogels and aerogels consisting of the semiconductor and a noble metal component. The ratio of the NRs and the Au nanoparticles (NP) attached to the NR semiconductor network is varied in a wide range. Thus, the scale of the electron travelling distances within the connected NR backbone upon using internal standards can be extracted from the macroscopic optical measurements.[4] One Au NP is able to quench the photoluminescence of several semiconductor NRs and can reach a number of nine NRs (roughly 400 nm connected CdS shell) quenched in the interconnected network.[4]

Nanoparticle thin films and polymer/nanoparticle composites show a wide range of applications in catalysis, electrochemistry, optoelectronics, sensors and nanomedicine. In all these applications, detailed knowledge of the nanoparticle size and aggregation behavior is necessary to understand the properties of the produced systems.

Metallic and metal oxide nanoparticles can be produced chemically first and later deposited in thin films or included inside a polymeric matrix. Despite the chemical route allows great control of the nanoparticle shape and the size dispersity, the use of insulating ligands to passivate the nanoparticle surface and enhance nanoparticle stability and solubility is not advantageous for electronic applications. Alternatively, ligand-free nanoparticles can be produced by physical methods in the gas phase and be transported via aereosol to a substrate to form thin films or composites.

In this talk, the production of different metallic nanoparticles (Au, Pt and Sn) using spark ablation technique is discussed. Using X-ray scattering, information on the nanoparticle synthesis and their deposition in thin and ultra-thin films are obtained. Quantitative analysis is demonstarted, allowing the measurement of size polydispersity and the aggregation behavior when deposited on silicon substrated. The quantitative analysis can be conducted on well-defined nanoparticles (Pt and Au) as well as on reactive (Sn/SnO_x) nanoparticles.

A recent example of the use of these aereosol-generated nanoparticles in conducting polymer nanocomposites is also illustrated. Low energy deposition methods was employed to gentle deposite nanoparticle on a thin films of a conducting, thermoelectric polymer such as PEDOT:PSS. Own to the naked nanoparticle nature and the interaction between the nanoparticle and the polymer, the thermoelectric properties of PEDOT:PSS can be drastically enhanced.[1]

The ability to organize functional nanoscale components into the targeted architectures promises to enable a broad range of nanotechnological applications, from designed biomaterials to photonic devices and information processing systems. However, we are currently lacking an adaptable and broadly applicable methodology for the 3D bottom-up nanomaterials fabrication with ability to prescribe a structure of organizations and to integrate different types of components. The talk will discuss our progress in establishing a versatile platform for the fabrication of designed large-scale and finite-size nano-architectures from diverse nanocomponents through the DNA-programmable assembly. The recent advances in creating periodic and hierarchical organizations from inorganic nanoparticles, proteins and enzymes will be presented. The use of the developed assembly approaches for generating functional nanomaterials with nano-optical, electrical, mechanical and biochemical functions will be demonstrated.

Our team works on developing ultrathin optoelectronic devices for biomedical implants that can stimulate biophysical processes. All of these devices rely on near infrared irradiation in the tissue transparency window to actuate nanoscale organic semiconductor components. Our motivation is to provide a minimalistic wireless implant which can perform the duty of standard implantable electrodes, but without invasive wiring. The devices we fabricate are not only wireless, but also 100-1000 times thinner than most existing technologies. Making implants have as small as possible mechanical footprint improves the efficacy of bioelectronic medical treatments by minimizing the risk for inflammation and making surgical implantation less invasive.

Organic semiconductor thin films can afford charging of electrolytic double layers or faradaic reactions. The magnitude of these two effects will depend on the thermodynamics of the materials used in the devices, in particular the nature of the cathodic and anodic components of the device, as well as the capacitance. Through judicious selection of materials one can obtain high photovoltages which can either drive efficient charging of double layer capacitors or faradaic reactions. The former is used to generate displacement currents which can capacitively couple with the cell membrane potential of nearby cells – this can be used to stimulate action potentials. Our experiment and model converge to create a detailed picture of how such devices, known as organic electrolytic photocapacitors, work to affect the gating of ion channels. This device can mimic biphasic current-pulse neurostimulation and thus transduces an optical signal into directly-evoked action potentials in neurons. On the other hand, the other block of our research efforts is directed at devices which, when stimulated with light, perform faradaic chemistry. We focus on the delivery of controlled amounts of reactive oxygen species (mostly peroxide). We aim to study the effects of photoelectrochemically-generated peroxides on physiological processes, with the hope of developing novel therapeutic approaches to neurodegenerative diseases.

Breaking the symmetry: Designing colloidal microswimmers

Breaking symmetry is at the very core of achieving propulsion at the microscale, where viscous forces dominate. Nature has perfected a range of different strategies to reach this goal for swimming microorganisms, which scientists have taken inspiration from to produce artificial micro-swimmers [1]. A common way to achieve propulsion at the colloidal scale is to produce artificial particles that have asymmetric shapes and surface properties. I will show a new fabrication strategy to create microswimmers with full control on their geometrical and compositional asymmetry. The method is based on the sequential deposition of microspheres on topographical templates, where we independently define the swimmers’ shape by defining the shape of template, and we program their composition by fixing the deposition sequence [2]. I will show how we can use this fabrication strategy to design and obtain particles that translate, rotate, switch between these two modes of motion and even perform drag-and-drop tasks in crowded environments, propelled by uniform AC electric fields [3,4]. These results show how the design of microswimmers can enable the development of active components for the realization of autonomous systems working in complex environments.

Nanocrystals (NCs) of intermetallic compounds (IMCs, long-range ordered alloys) are a large family of emerging materials. They offer attractive characteristics originating from well-defined crystallinity, synergistically combined properties of the incorporated metals and unique size- and surface-related physical and chemical phenomena exhibited by NCs. Intermetallic NCs have recently acquired a large amount of research interest due to their extensive applications in catalysis, electronics, e.g. thermoelectric devices and memory technologies, superconductivity, energy storage and conversion technologies, photonics and in life sciences and medicine. Nevertheless, a general synthetic method towards uniform intermetallic nanocrystals was still lacking.

We have developed a wet-chemical colloidal synthesis method towards uniform intermetallic nanocrystals based on the amalgamation of monometallic nanocrystal seeds with low-melting point metals [1]. We use this approach to achieve crystalline and compositionally uniform intermetallic nanocrystals of Au-Ga, Ag-Ga, Cu-Ga, Ni-Ga, Pd-Ga, Pd-In and Pd-Zn compounds. We demonstrate both compositional tunability across the phase spaces (e.g. AuGa2, AuGa, Au7Ga2 and Ga-doped Au), size tunability (e.g. 14.0, 7.6 and 3.8 nm AuGa2) and size uniformity (e.g. 5.4 % size deviations). In summary, the developed amalgamation seeded growth approach makes it possible to systematically achieve size and composition controlled intermetallic nanocrystals of excellent quality, opening up a multitude of possibilities for these materials.

Intermetallic nanoparticles (NPs), defined by an atomically ordered structure with high stability, have shown enhanced catalytic properties as compared to their disordered alloy counterparts.^1,2 The enhancement in catalytic properties is partially attributed to changes in the nature of available surface sites, which are induced by changes in the crystal structure.^3,4, To advance green energy solutions and pave the way for new improved catalytic materials comprised of intermetallic NPs, it is therefore crucial that we understand what controls the formation of intermetallic NPs to synthetically promote these structures. By carefully selecting the additives used in a PdCu NP synthesis, we show that monodisperse, intermetallic PdCu NPs can be synthesized in a controllable manner. In this synthesis, the additives iron(III) chloride and ascorbic acid are used to control both the morphology and polymorph of the intermetallic NPs. Ascorbic acid provides a fast reduction of the ionic metal precursor species, while iron(III) chloride facilitates ligand exchange with the metal ion complexes and can assist in oxidative etching. Combined, ascorbic acid and iron(III) chloride provide a synergetic effect resulting in precursor reduction and defect-free growth; ultimately leading to monodisperse intermetallic PdCu NPs. Using in situ X-ray total scattering and pair distribution function analysis, we follow the disorder-order transformation all the way from the initial precursor structure to the formation of intermetallic PdCu NPs. We report a hitherto unknown transformation pathway that diverges from the commonly reported co-reduction disorder-order transformation. A Cu-rich structure initially forms, followed by incorporation of Pd(0) into the structure to obtain the disordered alloy PdCu structure. The formation of a Cu-rich structure suggests that it is not essential that the metallic species reduce at the same rate to ultimately form stoichiometric intermetallic PdCu NPs, as previously believed. These findings underpin the importance and strength of performing further combined multi-technique studies to uncover the driving force of intermetallic NP formation. When we understand how intermetallic NPs are formed, these mechanistic insights might open new opportunities to expand our library of intermetallic NPs by exploiting synthesis by design.

Colloidal heterostructure nanocrystals (NCs) exhibit enhanced physical and electronic properties due to the synergistic effect of chemically distinguished domains.^{[1], [2]} Among them, multi-pods are more active in electrochemical processes than core-shell structures accredited to the more accessible core with adequate shielding from electrochemical reaction-related adverse structural deformations.³ However, the synthesis and development of colloidal multipods lack a direct growth approach. Here, we synthesize colloidal Bi-Cu_2-xS multipods, and single pod NCs in a solution-liquid-solid (SLS) growth of Cu_2-xS on insitu formed Bi NCs (Figure 1). We reveal that controlling the state (liquid or solid) of Bi seed and influx rate of Cu⁺ cations can alter the number of pods formation during hetero-nucleation of Bi seeded Cu_2-xS heterostructures. The ex-situ mechanistic investigation of the growth process reveals that the equivalent amount of diphosphonic acid (DPA) stabilizes the Cu-thiolate complex, thus lowering the free Cu⁺ concentration and increasing the induction time. Therefore, Bi NCs transition into solid faceted NCs, forming a single Cu_2-xS pod. In contrast, a low DPA (0 to 0.5 mmol) concentration cannot stabilize the Cu-thiolate complex, resulting in high free Cu⁺ concentration and multipods of Cu_2-xS on Bi. Furthermore, we studied the electrochemical performance of Bi-Cu_2-xS as anode material for potassium ion batteries.   

Iron oxide and hafnium oxide nanocrystals are two of the few successful examples of inorganic nanocrystals used in a clinical setting. Although crucial to their application, their aqueous surface chemistry is not fully understood. The literature contains conflicting reports regarding the optimum binding group. To alleviate these inconsistencies, we set out to systematically investigate the interaction of carboxylic acids, phosphonic acids and catechols to metal oxide nanocrystals in polar media. Here [1], we use Nuclear Magnetic Resonance spectroscopy and Dynamic Light Scattering to map out the pH-dependent binding affinity of the ligands towards hafnium oxide nanocrystals (an NMR compatible model system). Carboxylic acids easily desorb in water from the surface and only provide limited colloidal stability from pH 2 – 6. Phosphonic acids on the other hand provide colloidal stability over a broader pH range but also feature a pH-dependent desorption from the surface. They are most suited for acidic to neutral environments (pH < 8). Finally, nitrocatechol derivatives provide a tightly bound ligand shell and colloidal stability at physiological and basic pH (6-10). While dynamically bound ligands (carboxylates and phosphonates) do not provide colloidal stability in phosphate buffered saline, the tightly bound nitrocatechols provide long term stability. We thus shed light on the complex ligand binding dynamics on metal oxide nanocrystals in aqueous environments. Finally, we provide a practical colloidal stability map, guiding researchers to rationally design ligands for their desired application.

Colloidal InAs quantum dots (QDs) are considered as an infrared material candidate for many applications. Yet, there is not enough realization of the surface chemistry of these QDs. Here, 1H NMR has been used to analyze the surface chemistry of  recently reported InAs QDs produced using In(I)Cl as both the reducing agent and the precursor. After exposing QDs surface to acids, we realize that Oleylammonium Chloride salt is removed from the surface, suggesting InAs QDs are terminated by Oleylamine used as the sole coordinating solvent and Cl anions. In fact, there is only a partial displacement between the initially present Oleylamine and the added acid. Comparison of initial and final concentrations of bound Oleylamine  results an exchanged fraction of about 56% after titration with excess carboxylic acid.  Furthermore, addition of excess alkylthiol proved to result in a similar ligand exchange process in which it binds as a thiolate together with the desorption of Oleylammonium Chloride salt proved by H NMR and X-ray Photoelectron Spectroscopy analyses.

Hybrid organic–inorganic perovskite semiconductors have triggered a paradigm shift in the field of photovoltaics (PVs). Low dimensional 2D hybrid organic–inorganic lead halide semiconductors have recently attracted major attention [1] owing to the emergence of new photo-physical properties compared to the 3D perovskites, including exciton-phonon coupling, edge state based charge dissociation at grain boundaries and photon confinement [2]. The ionic nature of these layered materials also make them ideal candidates for solution processing into both thin films and nanostructured crystals [3]. Studying the wide range of process parameters in different solution processing techniques like spin coating [4], drop casting [5,6], meniscus coating [7] is important as they greatly influence the crystallization of 2D perovskites. However, this plethora of parameters in solution processing of 2D perovskites are not understood as well as their 3D counterparts.

Here we report the effects of different lead precursors in the crystallization process of 2D perovskite during a simple drop-cast based synthesis. We study the role of lead iodide (PbI₂), lead carbonate (PbCO₃) and lead dioxide (PbO₂) in the crystallization of BA₂PbI₄ perovskite in IPA-based solvent system. Our in-situ observations during the drop-casting process reveal the different crystallization kinetics based on the initial lead precursor. We also observe crystallization of the perovskite either on the substrate or in solution as floating micro-plates or both depending on the type of the precursor. We attribute this behavior to the formation of different co-ordinate complexes based on the initial lead precursor. Upon structural and optical investigation of the synthesized perovskite crystals (on substrate and in solution), we find them to be highly crystalline when compared against the traditionally grown and exfoliated 2D perovskites.

Semiconductor nanoplatelets have attracted interest due to their exceptionally narrow optical features. However, the direct synthesis of semiconductor nanoplatelets is limited to nanoparticles with thicknesses from 2 to 6 monolayers (N monolayers NPLs present N+1 planes of cations alternated with N planes of chalcogens in the [001] direction of the zinc blende crystal structure). It thus limits the wavelength range reached by such materials. Here, we show that it is possible to synthesize thicker NPLs by carefully tuning the NPLs surface energy. The native carboxylate ligands are exchanged with halides ligands which induce a further dissolution of the NPLs and a recrystallization of the released monomers on the top and bottom wide facets. This versatile method is applied to all three cadmium chalcogenide semiconductor. Thus, in NPLs, where the surface represents a high ratio of the particles, the surface chemistry is a key factor. In the second part of the presentation, we will show that the control of surface chemistry is also a way to tune the shape of the NPLs.

The conversion of thermal energy to electricity and vice versa through solid-state thermoelectric devices is extremely appealing for many applications. Greater than 60% of all the energy produced is lost as waste heat. However, thermoelectric materials have high production costs and low efficiency, limiting their use to niche applications. The problem is that thermoelectric materials require high electrical conductivity (s), high thermopower (S), and low thermal conductivity (k), three strongly counteracting properties.

Thermoelectric materials are often dense, polycrystalline inorganic semiconductors. Solution synthesis of nanoparticles emerged as an alternative to prepare thermoelectric materials with less demanding processing conditions than conventional solid-state synthetic methods. Moreover, it provides opportunities to synthesize nanoparticles with well-defined structures (size, shape, composition, and crystal structure), offering a broad range of tunable properties to target the desired polycrystalline material.

One of the most remarkable properties of a polycrystalline material, especially if the crystal domains go down to the nanoscale, is the high density of grain boundaries/interfaces and related defects. Such structural properties determine the material transport properties and, therefore, the thermoelectric performance. However, defect engineering in polycrystalline materials suffers the loss of control by the tendency of the crystals to grow and defects to annihilate, especially during high-temperature processes and operations. This limitation is particularly concerning for the bottom-up engineering of nanostructured thermoelectric materials. To date, very little attention has been paid to the grain growth of the original nanostructured powder during thermal processing or even during operation.

The work presented here report two important advances [1]:

- Crystalline grain growth inhibition in nanocomposites at high processing and operating temperatures obtained through a particle surface treatment.

- Scalable, simple, and economical method to produce high-performance polycrystalline thermoelectric materials through defect engineering.