Colloidal copper-based nanocrystals: synthesis and mechanistic studies.

The growing interest in nanotechnology stems from its promise of developing new materials and devices by taking advantage of unique phenomena emerging at the nanoscale. In this size regime, parameters like size and shape strongly impact the physico-chemical properties of materials and, therefore, their performance in a particular application. In view of this intimate structure-property-function relationship, significant research efforts have been going on to synthesize nanocrystals (NCs) of various materials, with a high degree of control. Over the years, beautiful works have been reported on the preparation of high-quality NCs with well-defined morphologies, mainly using colloidal chemistry as synthetic tool. However, the predictive character of these approaches is scarce, making the NC synthesis essentially empirical. In order to move towards a rational synthesis design, it is crucial to unravel the mechanisms behind the formation of NCs. The chemistry-sensitivity of in-situ X-rays spectroscopy makes it suitable to probe the key stages of the dynamic processes involved during the NCs nucleation and growth at synchrotron facilities. While great progress has been accomplished for noble metal NCs, the current synthetic state of non-noble metal NCs is less developed; nevertheless they are extremely important for a wide array of technological applications. For this class of materials the challenge becomes to perform in-situ experiments in harsher conditions, such as high temperatures and use of organic solvents, than those required for non-noble metals. Hence, the scope of this thesis is to advance the general synthetic chemistry of colloidal NCs by investigating the underlying formation mechanisms and by developing more predictive synthetic approaches. The system of focus are Cu-based NCs, considering their pivotal role in many fields. We start by exploring the high-temperature syntheses of single-crystalline Cu NCs, using a variety of techniques, including X-ray absorption and scattering at synchrotron facilities. We discover that the synthesis of such relatively simple systems proceed via non-classical nucleation pathways, with the formation of pre-nucleation structures occupying a local minimum in the reaction energy landscape. As the next step, we provide unique insights into their role in determining the final reaction product and achieve a superior monodisperisty for Cu NCs of different sizes as well as a shape not previously reported for these NCs. Turning to more complex systems, we report on the synthetic development of novel ternary copper-based transition metal chalcogenides. Firstly, we focus on the synthesis of colloidal Cu3VS4 NCs, which is an intermediate band gap semiconductor. Thanks to the achieved size-control, we could investigate its optical properties which we found to fall in a weak quantum confinement regime. Moving forward, we contribute to define guidelines towards the rational synthesis design of phase-pure colloidal Cu-M-S NCs (M = V, Cr, Mn), by studying their reaction mechanisms. In particular, exploring the precursor chemistry allowed us to define the same conditions whereby these materials can be prepared. Overall, the work discussed in this thesis contribute to advance the general knowledge of NC formation and furthermore it may also inspire the synthesis of new nanomaterials (e.g. based on non-noble metal NCs) with improved or target properties for the desired application

Colloidal Nanocrystals as Precursors and Intermediates in Solid State Reactions for Multinary Oxide Nanomaterials

Polyelemental compounds with dimensions in the nanosized regime are desirable in a large variety of applications, yet their synthesis remains a general challenge in chemistry. One of the major bottlenecks to obtaining multinary systems is the complexity of the synthesis itself. As the number of elements to include in one single nano-object increases, different chemical interactions arise during nudeation and growth, thus challenging the formation of the targeted product. Choosing the reaction conditions and identifying the parameters which ensure the desired reaction pathway are of the uttermost importance. When, in addition to composition, the simultaneous control of size and shape is sought after, the development of new synthetic strategies guided by the fundamental understanding of the formation mechanisms becomes crucial.In this Account we discuss the use of colloidal chemistry to target multinary oxide nanomaterials, with focus on light absorbers which can drive chemical reactions. We propose the combination of soft and solid-state chemistries as one successful strategy to target this family of polyelemental compounds with control on composition and morphological features. To start with, we highlight studies where in situ forming nanoparticles act as reaction intermediates, which we found in both oxide (i.e., Bi-V-O) and sulfide (Cu-M-S, with M = V, Cr, Mn) nanocrystals (NCs). Examples of ternary sulfides are mentioned only with the purpose of showing that similar mechanisms can apply to different families of multinary nanomaterials. Using this new knowledge, we demonstrate that reacting pre-synthesized NCs with well-defined composition and size with molecular precursors allows significant control of these same property-dictating features (i.e., composition and grain size) in the resulting ternary and quaternary compounds. For example, nanostructured BiV1-xSbxO4 thin films with tunable composition and nanostructured beta-Cu2V2O7 with tunable grain size were accessed from colloidally synthesized Bi1-xSbxNCs (0 < x < 1) and sizecontrolled Cu NCs reacted with a vanadium molecular precursor, respectively. The analysis of reaction aliquots revealed that the formation of these materials occurs via a solid-state reaction between the NC precursors and V-containing amorphous nanoparticles, which form in situ from the molecular precursors. With the aim to achieve better control on the reaction product, we finally propose the use of colloidally synthesized NCs as reactants in solid state reactions. As the first proof of concept, ternary metal oxide NCs, including CuFe2O4, CuMn2O4, and CuGa2O4 with defined size and shape regulated by the NC precursors were obtained. Considering the huge library of single component and binary NCs accessible by colloidal chemistry, the extension of this synthetic concept, which combines soft and solid-state chemistries, to a larger variety of polyelemental nanomaterials is foreseen. Such an approach will contribute to facilitate a more rapid translation of design principles to materials with the desired composition and structural features

Shaping non-noble metal nanocrystals via colloidal chemistry

Non-noble metal nanocrystals with well-defined shapes have been attracting increasingly more attention in the last decade as potential alternatives to noble metals, by virtue of their earth abundance combined with intriguing physical and chemical properties relevant for both fundamental studies and technological applications. Nevertheless, their synthesis is still primitive when compared to noble metals. In this contribution, we focus on third row transition metals Mn, Fe, Co, Ni and Cu that are recently gaining interest because of their catalytic properties. Along with providing an overview on the state-of-the-art, we discuss current synthetic strategies and challenges. Finally, we propose future directions to advance the synthetic development of shape-controlled non-noble metal nanocrystals in the upcoming years

Structural Sensitivities in Bimetallic Catalysts for Electrochemical CO2 Reduction Revealed by Ag-Cu Nanodimers

Understanding the structural and compositional sensitivities of the electrochemical CO2 reduction reaction (CO2RR) is fundamentally important for developing highly efficient and selective electrocatalysts. Here, we use Ag/Cu nanocrystals to uncover the key role played by the Ag/Cu interface in promoting CO2RR. Nanodimers including the two constituent metals as segregated domains sharing a tunable interface are obtained by developing a seeded growth synthesis, wherein preformed Ag nanoparticles are used as nucleation seeds for the Cu domain. We find that the type of metal precursor and the strength of the reducing agent play a key role in achieving the desired chemical and structural control. We show that tandem catalysis and electronic effects, both enabled by the addition of Ag to Cu in the form of segregated nanodomain within the same catalyst, synergistically account for an enhancement in the Faradaic efficiency for C2H4 by 3.4-fold and in the partial current density for CO2 reduction by 2-fold compared with the pure Cu counterpart. The insights gained from this work may be beneficial for designing efficient multicomponent catalysts for electrochemical CO2 reduction

Colloidal Synthesis of Cu-M-S (M = V, Cr, Mn) Nanocrystals by Tuning the Copper Precursor Reactivity

New ternary and higher order inorganic materials are needed for a large variety of applications, yet their synthesis still represents a chemistry challenge. Herein, we focus on the synthesis of Cu-M-S nanocrystals (where M = V, Cr, Mn) via a wet-chemistry route and investigate their formation mechanisms. We reveal that the interplay between the copper precursor and the thiophilicity of the transition metal M is the key for the synthesis of pure phase Cu-M-S nanocrystals under the same reaction conditions. In particular, we observe that the interdiffusion kinetics of the intermediate species is crucial, and the extent of nucleation of the ternary product can be controlled by the copper precursor reactivity. The insights provided by this work contribute to open up new avenues toward the design of improved synthesis strategies to multinary nanocrystalline compounds

Synthesis and Size-Dependent Optical Properties of Intermediate Band Gap Cu3VS4 Nanocrystals

Intermediate band gap semiconductors are an underex-plored class of materials with unique optical properties of interest for photovoltaic and optoelectronic applications. Herein, we synthesize highly crystalline cubic Cu3VS4 nanocrystals with tunable edge length of 9, 12, and 18 nm. Because size control is achieved for the first time for this semiconductor, particular emphasis is laid on the structural/compositional analysis, the formation mechanism, and the size dependent optical properties. The corresponding UV-vis spectra reveal three absorption peaks in the visible range, resulting from the intermediate band gap electronic structure of Cu3VS4, which blue shift with decreasing size. Density functional theory reveals these size dependent optoelectronic properties to result mostly from weak quantum confinement. The reported results pave the way toward further fundamental investigations of the physicochemical properties of intermediate band gap semiconductors in the nanoscale regime for solar energy harvesting

Transport of CLCA2 to the nucleus by extracellular vesicles controls keratinocyte survival and migration

Chloride channel accessory 2 (CLCA2) is a transmembrane protein, which promotes adhesion of keratinocytes and their survival in response to hyperosmotic stress. Here we show that CLCA2 is transported to the nucleus of keratinocytes via extracellular vesicles. The nuclear localization is functionally relevant, since wild-type CLCA2, but not a mutant lacking the nuclear localization signal, suppressed migration of keratinocytes and protected them from hyperosmotic stress-induced cell death. In the nucleus, CLCA2 bound to and activated β-catenin, resulting in enhanced expression of Wnt target genes. Mass-spectrometry-based interaction screening and functional rescue studies identified RNA binding protein 3 as a key effector of nuclear CLCA2. This is of likely relevance in vivo because both proteins co-localize in the human epidermis. Together, these results identify an unexpected nuclear function of CLCA2 in keratinocytes under homeostatic and stress conditions and suggest a role of extracellular vesicles and their nuclear transport in the control of key cellular activities.ISSN:2001-307

Nanocrystals as Precursors in Solid-State Reactions for Size- and Shape-Controlled Polyelemental Nanomaterials

Solid-state reactions between micrometer-size powders are among the oldest, simplest, and still widely used methods for the fabrication of inorganic solids. These reactions are intrinsically slow because, although the precursorsare "well mixed" at the macroscale, they are highly inhomogeneous at the atomic level. Furthermore, their products are bulk powders that are not suitable for device integration. Herein, we substitute micrometersize particles with nanocrystals. Scaling down the size of the precursors reduces the reaction time and temperature. More importantly, the final products are nanocrystals with controlled size and shape that can be used as active materials in various applications, including electro- and photocatalysis. The assembly of the nanocrystal precursors as ordered close-packed superlattices enables microscopy studies that deepen the understanding of the solid-state reaction mechanism. We learn that having only one of the two nanocrystal precursors dissolving and diffusing toward the other is crucial to obtain a final nanocrystalline product with homogeneous size and shape. The latter are regulated by the nanocrystal precursor that is the most stable at the reaction temperature. Considering the variety of controlled nanocrystals available, our findings open a new avenue for the synthesis of functional and tunable polyelemental nanomaterials