Synthesis and thermoelectric properties of noble metal ternary chalcogenide systems of Ag-Au-Se in the forms of alloyed nanoparticles and colloidal nanoheterostructures

The optimization of a material functionality requires both the rational design and precise engineering of its structural and chemical parameters. In this work, we show how colloidal chemistry is an excellent synthetic choice for the synthesis of novel ternary nanostructured chalcogenides, containing exclusively noble metals, with tailored morphology and composition and with potential application in the energy conversion field. Specifically, the Ag–Au–Se system has been explored from a synthetic point of view, which leads to a set of Ag2Se-based hybrid and ternary nanoparticles including the room temperature synthesis of the rare ternary Ag3AuSe2 fischesserite phase. An in-depth structural and chemical characterization of all nanomaterials has been performed, which proofed especially useful for unravelling the reaction mechanism behind the formation of the ternary phase in solution. The work is complemented with the thermal and electric characterization of a ternary Ag–Au–Se nanocomposite with promising results: we found that the use of the ternary nanocomposite represents a clear improvement in terms of thermoelectric energy conversion as compared to a binary Ag–Se nanocomposite analogue.Peer ReviewedPostprint (author's final draft

Rare Earth-Ion/Nanosilicon Ultrathin Layer: A Versatile Nanohybrid Light-Emitting Building Block for Active Optical Metamaterials

We fabricate an Er³⁺/nano-Si ultrathin (≈ 4 nm) layer and explore its optical response from the near-UV to the near-IR, in the linear and nonlinear regimes. This nanohybrid layer combines the tunable broad-band light harvesting properties of nano-Si with the robust and sharp Er³⁺ light emission. Its unique nanostructure enables efficient nanometer-range transfer of the harvested energy to the Er³⁺ ions. Therefore, clear 1.54 μm Er³⁺ photoluminescence (PL) is observed under excitation at any photon energy (<i>E</i>_exc) from the visible to the near-UV, despite the small amount of Er³⁺ ions in the layer (<2.5% of atomic monolayer). In the linear regime, the Er³⁺ PL intensity can be tuned to a maximum by setting the amount of nano-Si (<i>Q</i>_Si) in the layer at a suitable value, independent of <i>E</i>_exc. In the nonlinear regime, adjustment of <i>Q</i>_Si allows the dependence of the Er³⁺ PL intensity on <i>E</i>_exc to be tuned and achievement of nonconventional saturation properties not reported so far in Er³⁺:nano-Si systems. Based on this characteristic tunability, at sufficiently low <i>Q</i>_Si the nanohybrid layer is an ideal candidate for efficient near-IR emission under intense near UV–visible broad-band excitation. Furthermore, the nanohybrid layers with high enough <i>Q</i>_Si show an interesting potential for the optical modulation of the PL intensity by using UV light in a pump–probe configuration. Therefore, this nanohybrid layer is an outstanding candidate as a pure-color light-emitting building block for the development of advanced multiscale active optical metamaterials

Galvanic Replacement onto Complex Metal-Oxide Nanoparticles: Impact of Water or Other Oxidizers in the Formation of either Fully Dense Onion-like or Multicomponent Hollow MnO_<i>x</i>/FeO_<i>x</i> Structures

Multicomponent metal-oxide nanoparticles are appealing structures from applied and fundamental viewpoints. The control on the synthetic parameters in colloidal chemistry allows the growth of complex nanostructures with novel morphologies. In particular, the synthesis of biphase metal-oxide hollow nanoparticles has been reported based on galvanic replacement using an organic-based seeded-growth approach, but with the presence of H₂O. Here we report a novel route to synthesize either fully dense or hollow core/shell metal-oxide nanoparticles (MnO_<i>x</i>/FeO_<i>x</i>) by simply adding or not oxidants in the reaction. We demonstrate that the presence of oxidants (e.g., O₂ carried by the not properly degassed H₂O or (CH₃)₃NO) allows the formation of hollow structures by a galvanic reaction between the MnO_<i>x</i> and FeO_<i>x</i> phases. In particular, the use of (CH₃)₃NO as oxidant allows for the first time a very reliable all-organic synthesis of hollow MnO_<i>x</i>/FeO_<i>x</i> nanoparticles without the need of water (with a somewhat unreliable oxidation role). Oxidants permit the formation of MnO_<i>x</i>/FeO_<i>x</i> hollow nanoparticles by an intermediate step where the MnO/Mn₃O₄ seeds are oxidized into Mn₃O₄, thus allowing the Mn³⁺ → Mn²⁺ reduction by the Fe²⁺ ions. The lack of proper oxidative conditions leads to full-dense onion-like core/shell MnO/Mn₃O₄/Fe₃O₄ particles. Thus, we show that the critical step for galvanic replacement is the proper seed oxidation states so that their chemical reduction by the shell ions is thermodynamically favored

Au-Assisted Growth of Anisotropic and Epitaxial CdSe Colloidal Nanocrystals via in Situ Dismantling of Quantum Dots

Metallic nanocrystals have been revealed in the past years as valuable materials for the catalytic growth of semiconductor nanowires. Yet, only low melting point metals like Bi have been reported to successfully assist the growth of elongated CdX (X = S, Se, Te) systems in solution, and the possibility to use plasmonic noble metals has become a challenging task. In this work we show that the growth of anisotropic CdSe nanostructures in solution can also be efficiently catalyzed by colloidal Au nanoparticles, following a preferential crystallographic alignment between the metallic and semiconductor domains. Noteworthy, we report the heterodox use of semiconductor quantum dots as a homogeneous and tunable source of reactive monomer species to the solution. The mechanistic studies reveal that the in situ delivery of these cadmium and chalcogen monomer species and the formation of Au_<i>x</i>Cd_<i>y</i> alloy seeds are both key factors for the epitaxial growth of elongated CdSe domains. The implementation of this method suggests an alternative synthetic approach for the assembly of different semiconductor domains into more complex heterostructures

Galvanic Replacement onto Complex Metal-Oxide Nanoparticles: Impact of Water or Other Oxidizers in the Formation of either Fully Dense Onion-like or Multicomponent Hollow MnO_<i>x</i>/FeO_<i>x</i> Structures

Multicomponent metal-oxide nanoparticles are appealing structures from applied and fundamental viewpoints. The control on the synthetic parameters in colloidal chemistry allows the growth of complex nanostructures with novel morphologies. In particular, the synthesis of biphase metal-oxide hollow nanoparticles has been reported based on galvanic replacement using an organic-based seeded-growth approach, but with the presence of H₂O. Here we report a novel route to synthesize either fully dense or hollow core/shell metal-oxide nanoparticles (MnO_<i>x</i>/FeO_<i>x</i>) by simply adding or not oxidants in the reaction. We demonstrate that the presence of oxidants (e.g., O₂ carried by the not properly degassed H₂O or (CH₃)₃NO) allows the formation of hollow structures by a galvanic reaction between the MnO_<i>x</i> and FeO_<i>x</i> phases. In particular, the use of (CH₃)₃NO as oxidant allows for the first time a very reliable all-organic synthesis of hollow MnO_<i>x</i>/FeO_<i>x</i> nanoparticles without the need of water (with a somewhat unreliable oxidation role). Oxidants permit the formation of MnO_<i>x</i>/FeO_<i>x</i> hollow nanoparticles by an intermediate step where the MnO/Mn₃O₄ seeds are oxidized into Mn₃O₄, thus allowing the Mn³⁺ → Mn²⁺ reduction by the Fe²⁺ ions. The lack of proper oxidative conditions leads to full-dense onion-like core/shell MnO/Mn₃O₄/Fe₃O₄ particles. Thus, we show that the critical step for galvanic replacement is the proper seed oxidation states so that their chemical reduction by the shell ions is thermodynamically favored

Metal Oxide Aerogels with Controlled Crystallinity and Faceting from the Epoxide-Driven Cross-Linking of Colloidal Nanocrystals

We present a novel method to produce crystalline oxide aerogels which is based on the cross-linking of preformed colloidal nanocrystals (NCs) triggered by propylene oxide (PO). Ceria and titania were used to illustrate this new approach. Ceria and titania colloidal NCs with tuned geometry and crystal facets were produced in solution from the decomposition of a suitable salt in the presence of oleylamine (OAm). The native surface ligands were replaced by amino acids, rendering the NCs colloidally stable in polar solvents. The NC colloidal solution was then gelled by adding PO, which gradually stripped the ligands from the NC surface, triggering a slow NC aggregation. NC-based metal oxide aerogels displayed both high surface areas and excellent crystallinity associated with the crystalline nature of the constituent building blocks, even without any annealing step. Such NC-based metal oxide aerogels showed higher thermal stability compared with aerogels directly produced from ionic precursors using conventional sol–gel chemistry strategies

Seeded Growth Synthesis of Au–Fe₃O₄ Heterostructured Nanocrystals: Rational Design and Mechanistic Insights

Multifunctional hybrid nanoparticles comprising two or more entities with different functional properties are gaining ample significance in industry and research. Due to its combination of properties, a particularly appealing example is Au–Fe₃O₄ composite nanoparticles. Here we present an in-depth study of the synthesis of Au–Fe₃O₄ heterostructured nanocrystals (HNCs) by thermal decomposition of iron precursors in the presence of preformed 10 nm Au seeds. The role of diverse reaction parameters on the HNCs formation was investigated using two different precursors: iron pentacarbonyl (Fe­(CO)₅) and iron acetylacetonate (Fe­(acac)₃). The reaction conditions promoting the heterogeneous nucleation of Fe₃O₄ onto Au seeds were found to significantly differ depending on the precursor chosen, where Fe­(acac)₃ is considerably more sensitive to the variation of the parameters than Fe­(CO)₅ and more subject to homogeneous nucleation processes with the consequent formation of isolated iron oxide nanocrystals (NCs). The role of the surfactants was also crucial in the formation of well-defined and monodisperse HNCs by regulating the access to the Au surface. Similarly, the variations of the [Fe]/[Au] ratio, temperature, and employed solvent were found to act on the mean size and the morphology of the obtained products. Importantly, while the optical properties are rather sensitive to the final morphology, the magnetic ones are rather similar for the different types of obtained HNCs. The surface functionalization of dimer-like HNCs with silica allows their dispersion in aqueous media, opening the path to their use in biomedical applications

Assessing Oxygen Vacancies in Bismuth Oxide through EELS Measurements and DFT Simulations

Pioneering electron energy loss spectroscopy (EELS) measurements of α-Bi₂O₃ are performed on three samples obtained through different synthesis methods. Experimental low-loss and core-loss EELS spectra are acquired. By combining them with detailed structural characterization and Density Functional Theory (DFT) simulations, we are able to detect and evaluate the presence of oxygen vacancies in the samples. This type of information has not been accessed previously from EELS data in bismuth oxide, because high-resolution EELS spectra or how vacancies reflect in Bi₂O₃ spectra were unreported. This novel measurement is further validated through comparison with photoluminescence data. Therefore, the technique has the ability to probe oxygen vacancies in Bi₂O₃ at an unprecedented resolution, which might allow solving material science and technological issues related to this material

Acetate-Induced Disassembly of Spherical Iron Oxide Nanoparticle Clusters into Monodispersed Core–Shell Structures upon Nanoemulsion Fusion

It has been long known that the physical encapsulation of oleic acid-capped iron oxide nanoparticles (OA–IONPs) with the cetyltrimethylammonium (CTA⁺) surfactant induces the formation of spherical iron oxide nanoparticle clusters (IONPCs). However, the behavior and functional properties of IONPCs in chemical reactions have been largely neglected and are still not well-understood. Herein, we report an unconventional ligand-exchange function of IONPCs activated when dispersed in an ethyl acetate/acetate buffer system. The ligand exchange can successfully transform hydrophobic OA–IONP building blocks of IONPCs into highly hydrophilic, acetate-capped iron oxide nanoparticles (Ac–IONPs). More importantly, we demonstrate that the addition of silica precursors (tetraethyl orthosilicate and 3-aminopropyltriethoxysilane) to the acetate/oleate ligand-exchange reaction of the IONPs induces the disassembly of the IONPCs into monodispersed iron oxide–acetate–silica core–shell–shell (IONPs@acetate@SiO₂) nanoparticles. Our observations evidence that the formation of IONPs@acetate@SiO₂ nanoparticles is initiated by a unique micellar fusion mechanism between the Pickering-type emulsions of IONPCs and nanoemulsions of silica precursors formed under ethyl acetate buffered conditions. A dynamic rearrangement of the CTA⁺–oleate bilayer on the IONPC surfaces is proposed to be responsible for the templating process of the silica shells around the individual IONPs. In comparison to previously reported methods in the literature, our work provides a much more detailed experimental evidence of the silica-coating mechanism in a nanoemulsion system. Overall, ethyl acetate is proven to be a very efficient agent for an effortless preparation of monodispersed IONPs@acetate@SiO₂ and hydrophilic Ac–IONPs from IONPCs

Tailoring Staircase-like Hysteresis Loops in Electrodeposited Trisegmented Magnetic Nanowires: a Strategy toward Minimization of Interwire Interactions

A new strategy to minimize magnetic interactions between nanowires (NWs) dispersed in a fluid is proposed. Such a strategy consists of preparing trisegmented NWs containing two antiparallel ferromagnetic segments with dissimilar coercivity separated by a nonmagnetic spacer. The trisegmented NWs exhibit a staircase-like hysteresis loop with tunable shape that depends on the relative length of the soft- and hard-magnetic segments and the respective values of saturation magnetization. Such NWs are prepared by electrodepositing CoPt/Cu/Ni in a polycarbonate (PC) membrane. The antiparallel alignment is set by applying suitable magnetic fields while the NWs are still embedded in the PC membrane. Analytic calculations are used to demonstrate that the interaction magnetic energy from fully compensated trisegmented NWs with antiparallel alignment is reduced compared to a single-component NW with the same length or the trisegmented NWs with the two ferromagnetic counterparts parallel to each other. The proposed approach is appealing for the use of magnetic NWs in certain biological or catalytic applications where the aggregation of NWs is detrimental for optimized performance