Manuel A. Pérez

Silver oxide particles/silver nanoparticles interconversion: susceptibility of forward/backward reactions to the chemical environment at room temperature{

The thermal stability of the silver oxide particles (Ag2O)/metallic silver nanoparticles (AgNPs) system in aqueous and gaseous environments is investigated with UV-Visible spectroscopy, TEM, SEM and DLS as characterisation techniques, and with calculations using electromagnetic theory. Thermal decomposition of aqueous Ag2O colloids to produce AgNPs is conclusively demonstrated and used as a base reaction to produce clean AgNPs without any external reducing agent. Such a spontaneous character of Ag2O decomposition in alkaline aqueous/water-enriched environments at room temperature makes the formation of silver oxide films on silver nanoparticles/nanostructures unlikely, keeping the silver surface oxide-free, a crucial feature in determining the silver catalytic and Raman enhancing properties. The synthetic suitability of this reaction to develop new routes to produce AgNPs is explored by analyzing the effect of temperature, complexing agents, and environment polarity on the AgNPs size/shape control. Thermal decomposition of Ag2O colloids in aqueous/water-enriched environments offers the possibility to produce AgNPs at low cost, with easy, clean, safe and green chemistry procedures.

A pH-Sensitive Supramolecular Switch Based on Mixed Carboxylic Acid Terminated Self-Assembled Monolayers on Au(111)

We show that homogeneously mixed self-assembled monolayers (SAMs) of mercaptoalkanoic acids of different chain lengths can be used to build up a pH-sensitive supramolecular switch. The acids with short and long alkyl chains interact via the strong hydrogen bond between carboxylic acid groups. The pH acts as a trigger by breaking or restoring the hydrogen bond interaction in basic or acidic solutions, respectively. The corresponding changes in the monolayer structure were determined by ellipsometry, surface-enhanced Raman spectroscopy, and contact angle measurements. Density functional theory (DFT) calculations were performed to elucidate the structures of interacting molecules compatible with the surface coverage obtained from electrochemical reductive desorption experiments. The simplicity of the preparation procedure assures a high reproducibility whereas the stability of the homogeneous mixed SAM guarantees the reversibility of the switching process.

An Ellipsometric Study of Manganese Oxide Films In Situ Characterization of the Deposition and Electroreduction of MnO 2

Inst. de Ciencias Quimicas Fac. de Agronom. y Agroindustrias Univ. Nac. de Santiago del Estero, 4200 Santiago del Estero

Gold nucleation inhibition by halide ions: a basis for a seed-mediated approach

In the present work, we examine the effect of halide ions on gold nucleation, a typical synthetic variable in the wet-chemical production of gold nanostructures. It was found that the homogeneous nucleation of gold by the chemical reduction of aqueous gold ions is kinetically quenched by an increase in the concentration of halide ions, and this effect grows stronger as the Au–halide complex stability increases. The nucleation quenching is not exclusively related to a specific reducing agent, but appears to be a more general behavior, and is affected by the pH of the media. While no nucleation is observed, Au(I) metastable species coexist together with the reducer, constituting metastable solutions. It is demonstrated that nucleation inhibition by halide ions can be employed as a basis for a seed-mediated approach to produce gold nanostructures. The metastable solutions are proved to function as growth baths, where Au(I) reduction is triggered on the surface of previously synthesized gold nanoparticles, driving their growth in the absence of secondary nucleation. It is also shown how, with this approach, the synthesis conditions can be rationally designed to obtain gold nanoparticles with the desired properties in a controlled and reproducible fashion.

One-step/one-pot decoration of oxide microparticles with silver nanoparticles.

HYPOTHESIS Heterogeneous nucleation of silver oxide (Ag2O) onto oxide microparticles (OMPs) followed by spontaneous thermal decomposition produce nanostructures made of OMPs decorated with silver nanoparticles (OMP|AgNPs). EXPERIMENTS Colloidal chemistry methods have been used to produce the decoration of OMPs with silver nanoparticles (AgNPs), by carrying out the Ag2O precipitation/thermal decomposition. The process is driven in water enriched acetone medium containing NaOH, NH3, AgNO3 and SiO2MPs as substrate. Optical and morphological properties of OMP|AgNPs were characterized by using STEM, EDS, HRTEM and Raman spectroscopy. FINDINGS A new synthetic method to decorate OMPs (TiO2, SiO2) with metallic AgNPs in a single step/single pot reaction is proven effective to produce OMP|AgNPs either in aqueous or water enriched media.

Morphological evolution of noble metal nanoparticles in chloroform: mechanism of switching on/off by protic species

The morphological stability/morphological reshaping of noble metal nanoparticles are studied experimentally in order to unravel the chemical mechanisms lying beneath. Gold and silver nanoparticles (AuNPs and AgNPs, respectively) formed in chloroformic environment are used, as model synthetic systems, to study phenomena of morphological change. The morphological evolution of NPs that follows their formation, is characterized by spectroscopy (UV-Visible, Raman and FTIR) and TEM (Transmission Electron Microscopy). The change of NP morphology involves the increase of the average NP size and the broadening of size distribution, in a close resemblance with the effect characteristically obtained from the Ostwald ripening. The effect of the poor solvating properties of chloroform in stabilizing small charged species (H+, Ag+, Au+) as well as the principle of electroneutrality of matter are analyzed in order to formulate a feasible reaction scheme consisting of a three-step processes: the generation of soluble intermediary species by corrosion of nanoparticles, the diffusion of intermediary species from one nanoparticle to another, and the re-deposition process involving the reduction of intermediary species. This basic reaction scheme is used as hypothesis to plan and perform experiments, which reveal that molecular oxygen dissolved in the dispersive medium can drive NP corrosion, however, protic species are also required as co-reactant. The polarity of the hydrogen bond and the ligand properties of the anions produced by deprotonation are feature of the protic species that enable/disable the corrosion and, in turn, the NP morphological evolution.

Au@ZnO hybrid nanostructures: correlation between morphology and optical response

Au@ZnO hybrid nanostructures (HNs) have been synthesized in aqueous media by implementing a simple chemical methodology. These HNs consist of 40 nm mean size almost spherical Au cores, over which the heterogeneous formation of ZnO is observed. By increasing the concentration of ZnO precursors in the system, it is possible to change the morphology of the material formed over the Au core from branched to shell like structures, which, in turn, significantly modifies the extinction properties of the naked Au core nanoparticles. This effect has been rationalized by means of electrodynamics simulations based on two different approaches: the Mie theory for coated spheres and the Discrete Dipole Approximation (DDA). The changes measured in the extinction spectra as the amount of ZnO formed around the Au cores increases are properly described by both methodologies, while the good correlation between experimental and theoretical spectra suggest that the ZnO material includes a significant amount of water. Furthermore, based on the Mie theory results, a graphical method was implemented which allows us to predict the main morphological parameters of the Au@ZnO HNs. In addition, the combination of optical measurements, morphological characterization and DDA modeling allowed us to estimate that the water content of the shell surrounding the metallic core is 65%. The methodology presented in this work provides a useful tool to characterize the structural properties of HNs and can be straightforwardly generalized to other systems.

Gold decoration of silica by decomposition of aqueous gold(III) hydroxide at low temperatures

The decomposition of gold hydroxide to give metallic gold is known to take place around 300 °C in dry environments. However, little information about the gold hydroxide stability in wet environments has been recorded. Here, we present experimental evidence which shows that aqueous/water-enriched gold(III) hydroxide colloids decompose spontaneously to form gold nanoparticles at temperature values above the freezing point of water. Based on this reaction, we developed a method to decorate silica spheres with gold nanoparticles by precipitation and decomposition of gold(III) hydroxide onto the silica surface in wet media by a simple one-pot/one-step protocol. The silica|gold nanostructures are prepared in high yield and with a low level of by-products.

A novel one-pot room-temperature synthesis route to produce very small photoluminescent silicon nanocrystals

AbstractA novel strategy to synthesize photoluminescent silicon nanocrystals (SiNCs) from a reaction between tetraethylorthosilicate (TEOS) and trimethyl-hexadecyl-ammonium borohydride (CTABH4) in organic solvent is presented. The formation reaction occurs spontaneously at room temperature in homogeneous phase. The produced silicon nanocrystals are characterized by using their photoluminescent properties and via HRTEM. In addition, theoretical calculations of the optical absorption spectrum of silicon quantum dots in vacuum with different sizes and surface moieties were performed in order to compare with the experimental findings. The new chemical reaction is simple and can be implemented to produce silicon nanocrystal with regular laboratory materials by performing easy and safe procedures. Graphical abstractᅟ