Computational Approaches to the Electronic Properties of Noble Metal Nanoclusters Protected by Organic Ligands

Organometallic nanoparticles composed by metal cores with sizes under two nanometers covered with organic capping ligands exhibit intermediate properties between those of atoms and molecules on one side, and those of larger metal nanoparticles on the other. In fact, these particles do not show a peculiar metallic behavior, characterized by plasmon resonances, but instead they have nonvanishing band-gaps, more along molecular optical properties. As a consequence, they are suitable to be described and investigated by computational approaches such as those used in quantum chemistry, for instance those based on the time-dependent density functional theory (TD-DFT). Here, I present a short review of the research performed from 2014 onward at the University of Modena and Reggio Emilia (Italy) on the TD-DFT interpretation of the electronic spectra of different organic-protected gold and/or silver nanoclusters.

Observation of ordered organic capping ligands on semiconducting quantum dots via powder X-ray diffraction

Powder X-ray diffraction is one of the key techniques used to characterize the inorganic structure of colloidal nanocrystals. The comparatively low scattering factor of nuclei of the organic capping ligands and their propensity to be disordered has led investigators to typically consider them effectively invisible to this technique. In this report, we demonstrate that a commonly observed powder X-ray diffraction peak around $$q=1.4{\AA}^{-1}$$ q = 1.4 Å − 1 observed in many small, colloidal quantum dots can be assigned to well-ordered aliphatic ligands bound to and capping the nanocrystals. This conclusion differs from a variety of explanations ascribed by previous sources, the majority of which propose an excess of organic material. Additionally, we demonstrate that the observed ligand peak is a sensitive probe of ligand shell ordering. Changes as a function of ligand length, geometry, and temperature can all be readily observed by X-ray diffraction and manipulated to achieve desired outcomes for the final colloidal system.

Unwrap Them First: Operando Potential- induced Activation Is Required when Using PVP-Capped Ag Nanocubes as Catalysts of CO2 Electroreduction

Metallic nanoparticles of different shape can be used as efficient electrocatalysts for many technologically and environmentally relevant processes, like the electroreduction of CO2. Intense research is thus targeted at finding the morphology of nanosized features that best suits catalytic needs. In order to control the shape and size distribution of the designed nanoobjects, and to prevent their aggregation, synthesis routes often rely on the use of organic capping agents (surfactants). It is known, however, that these agents tend to remain adsorbed on the surface of the synthesized nanoparticles and may significantly impair their catalytic performance, both in terms of overall yield and of product selectivity. It thus became a standard procedure to apply certain methods (e.g. involving UV-ozone or plasma treatments) for the removal of capping agents from the surface of nanoparticles, before they are used as catalysts. Proper design of the operating procedure of the electrocatalysis process may, however, render such cleaning steps unnecessary. In this paper we use poly-vinylpyrrolidone (PVP) capped Ag nanocubes to demonstrate a mere electrochemical, operando activation method. The proposed method is based on an observed hysteresis of the catalytic yield of CO (the desired product of CO2 electroreduction) as a function of the applied potential. When as-synthesized nanocubes were directly used for CO2 electroreduction, the CO yield was rather low at moderate overpotentials. However, following a potential excursion to more negative potentials, most of the (blocking) PVP was irreversibly removed from the catalyst surface, allowing a significantly higher catalytic yield even under less harsh operating conditions. The described hysteresis of the product distribution is shown to be of transient nature, and following operando activation by a single 'break-in' cycle, a truly efficient catalyst was obtained that retained its stability during long hours of operation.

CsPbBr3 and CsPbI3 rhombic dodecahedra and nanocubes displaying facet-dependent optical properties

CsPbBr3 rhombic dodecahedra, octahedra, and nanocubes have been synthesized using typical reagents but by adjusting the amounts of organic capping molecules used and the reaction temperature. Although some Cs4PbBr6 has...

Hybrid Al2O3-CH3NH3PbI3 Perovskites towards Avoiding Toxic Solvents

We report the synthesis of organometal halide perovskites by milling CH3NH3I and PbI2 directly with an Al2O3 scaffold to create hybrid Al2O3-CH3NH3PbI3 perovskites, without the use of organic capping ligands that otherwise limit the growth of the material in the three dimensions. Not only does this improve the ambient stability of perovskites in air (100 min versus 5 min for dimethylformamide (DMF)-processed material), the method also uses much fewer toxic solvents (terpineol versus dimethylformamide). This has been achieved by solid-state reaction of the perovskite precursors to produce larger perovskite nanoparticles. The resulting hybrid perovskite–alumina particles effectively improve the hydrophobicity of the perovskite phase whilst the increased thermal mass of the Al2O3 increases the thermal stability of the organic cation. Raman data show the incorporation of Al2O3 shifts the perovskite spectrum, suggesting the formation of a hybrid 3D mesoporous stack. Laser-induced current mapping (LBIC) and superoxide generation measurements, coupled to thermogravimetric analysis, show that these hybrid perovskites demonstrate slightly improved oxygen and thermal stability, whilst ultra-fast X-ray diffraction studies using synchrotron radiation show substantial (20×) increase in humidity stability. Overall, these data show considerably improved ambient stability of the hybrid perovskites compared to the solution-processed material.

Biosynthesized Quantum Dots as Improved Biocompatible Tools for Biomedical Applications

: Quantum dots (QDs), whose diameters are often limited to 10 nm, have been of interest to researchers for their unique optical characteristics, which are attributed to quantum confinement. Following their early application in the electrical industry as light-emitting diode materials, semiconductor nanocrystals have continued to show great potential in clinical diagnosis and biomedical applications. The conventional physical and chemical pathways for QD syntheses typically require harsh conditions and hazardous reagents, and these products encounter non-hydrophilic problems due to organic capping ligands when they enter the physiological environment. The natural reducing abilities of living organisms, especially microbes, are then exploited to prepare QDs from available metal precursors. Low-cost and eco-friendly biosynthesis approaches have the potential for further biomedical applications which benefit from the good biocompatibility of protein-coated QDs. The surface biomass offers many binding sites for modifying substances or targeting ligands and so achieving multiple functions through simple and efficient operations. Biosynthetic QDs could function as bioimaging and biolabeling agents because of their luminescence properties similar to those of chemical QDs. In addition, extensive research has been carried out on the antibacterial activity, metal ion detection and bioremediation. As a result, this review details the advanced progress of biomedical applications of biosynthesized QDs and illustrates these principles as clearly as possible.

Green synthesis and spectral analysis of surface encapsulated copper (II) oxide nanostructures

Nanostructures of copper (II) oxide were synthesized through chemical reduction of copper (II) sulfate pentahydrate using phytochemicals present in leaf extracts of Leucas aspera. The crystalline phases and size were assessed by X-ray diffraction data analysis. From the Bragg reflection peaks, existence of monoclinic end-centered phase of copper (II) oxide along with presence of cubic primitive phase of copper (I) oxide and traces of cubic face centered lattices of zero valent copper was revealed. The three Raman active modes corresponding to CuO phase were identified in the sample with permissible merging of characteristic bands due to nanostructuring and organic capping. The surface topography measurement using field emission scanning electron microscope evidenced the occurrence of cylindrical rod shaped morphological structures along with a number of unshaped aggregates in the sample. The effective crystallite size and lattice strain were estimated from Williamson-Hall analysis of Bragg reflection data. Tauc plot analysis of UV-Vis-NIR absorption data in direct transition mode provided an estimation of band gap, viz. 1.83 eV and 2.06 eV respectively, for copper (II) oxide and copper (I) oxide. Thermal degradation study using thermogravimetric curve analysis could reveal the amount of moisture content, volatile components as well as the polymer capping over nanorods present in the sample. It could be seen that upon heating, inorganic core crystals undergo oxidation process and at temperature above 464 °C, the sample was found to be composed solely of inorganic crystallite phase of copper (II) oxide.