Synthesis of hybrid organic–inorganic nanocomposite materials based on CdS nanocrystals for energy conversion applications

Efficient solar energy conversion is strongly related to the development of new materials with enhanced functional properties. In this context, a wide variety of inorganic, organic, or hybrid nanostructured materials have been investigated. In particular, in hybrid organic–inorganic nanocomposites are combined the convenient properties of organic polymers, such as easy manipulation and mechanical flexibility, and the unique size-dependent properties of nanocrystals (NCs). However, applications of hybrid nanocomposites in photovoltaic devices require a homogeneous and highly dense dispersion of NCs in polymer in order to guarantee not only an efficient charge separation, but also an efficient transport of the carriers to the electrodes without recombination. In previous works, we demonstrated that cadmium thiolate complexes are suitable precursors for the in situ synthesis of nanocrystalline CdS. Here, we show that the soluble [Cd(SBz)₂]₂·(1-methyl imidazole) complex can be efficiently annealed in a conjugated polymer obtaining a nanocomposite with a regular and compact network of NCs. The proposed synthetic strategies require annealing temperatures well below 200 °C and short time for the thermal treatment, i.e., less than 30 min. We also show that the same complex can be used to synthesize CdS NCs in mesoporous TiO₂. The adsorption of cadmium thiolate molecule in TiO₂ matrix can be obtained by using chemical bath deposition technique and subsequent thermal annealing. The use of NCs, quantum dots, as sensitizers of TiO₂ matrices represents a very promising alternative to common dye-sensitized solar cells and an interesting solution for heterogeneous photocatalysis.     

Preparation and characterization of close-packed nanostructured sol–gel ceria thin films prepared using cerium-sec-butoxide as precursor

Polycrystalline, close-packed, homogeneous nanostructured ceria thin films were prepared by sol–gel process via dip-coating technique on soda-lime glass and (100)-oriented Si substrates. To produce the films, a sol was prepared using, as precursor, a home made cerium sec-butoxide dissolved in secondary butanol. The chemical composition, the microstructural/morphological characteristics and the optical properties of the coatings were investigated in detail. The experimental results clearly demonstrate that the ceria films are nanocrystalline (CeO₂, cubic phase \textFm[`3]\textm {\text{Fm}}\bar{3}{\text{m}} ) with an average grain size of about 2–3 nm for the samples grown on glass and of about 4–5 nm for the samples grown on silicon. The analyses of ceria layers grown on silicon show that the ceria coatings are free from organic residues and that a Si-oxide layer is formed at the film/substrate interface. The optical results evidence a red shift of the energy gap of about 0.5 eV that can be ascribed to conversion of relevant Ce⁴⁺ sites to Ce³⁺ sites and a consequent creation of oxygen vacancy at the surface of the ceria grains.     

Controlled nucleation and growth of CdS nanoparticles in a polymer matrix

In-situ synchrotron X-ray diffraction (XRD) was used to monitor the thermal decomposition (thermolysis) of Cd thiolates precursors embedded in a polymer matrix and the nucleation of CdS nanoparticles. A thiolate precursor/polymer solid foil was heated to 300 degrees C in the X-ray diffraction setup of beamline W1.1 at Hasylab, and the diffraction curves were each recorded at 10 degrees C. At temperatures above 240 degrees C, the precursor decomposition is complete and CdS nanoparticles grow within the polymer matrix forming a nanocomposite with interesting optical properties. The nanoparticle structural properties (size and crystal structure) depend on the annealing temperature. Transmission electron microscopy (TEM) and photoluminescence (PL) analyses were used to characterize the nanoparticles. A possible mechanism driving the structural transformation of the precursor is inferred from the diffraction features arising at the different temperatures.     

Detection of additional impurities in the UV-chromatogram of <Emphasis Type="SmallCaps">l</Emphasis>-[<Emphasis Type="Italic">S</Emphasis>-methyl-<Superscript>11</Superscript>C]methionine

Samples of l-[S-methyl-¹¹C]methionine prepared by on-column [¹¹C]methylation on a C18 cartridge were analyzed by HPLC under different conditions in order to explain some anomalies observed in the UV-chromatograms. By so doing, two new unlabelled impurities were found and identified as iodide and homocystine. Their amounts are still compatible with the safe human use of the radiotracer solution. Nevertheless, if needed, iodide can be totally removed by elution through an anion-exchange resin, while homocystine can be reduced by either decreasing the amount of precursor used in the synthesis or preventing its air oxidation.     

Pd nanoparticles catalyzed stereospecific synthesis of beta-aryl cinnamic esters in ionic liquids

Reaction of the catalyst 1 or Pd(OAc)(2) with tetrabutylammonium acetate, dissolved in tetrabutylammonium bromide, leads to a fast formation of Pd nanoparticles which efficiently catalyze the stereospecific reaction of cinnamates with aryl halides to give beta-aryl-substituted cinnamic esters. The role of tetrabutylammonium acetate is crucial in determining the formation of nanoparticles and stereospecificity of the C-C coupling process.     

Dynamics of spray and swirling flame under acoustic oscillations : A joint experimental and LES investigation

The dynamics of spray swirling flames is investigated by combining experiments on a single sector generic combustor and large eddy simulations of the same configuration. Measurements and calculations correspond to a self-sustained limit cycle operation where combustion coupled by an axial quarter wave acoustic mode induces large amplitude oscillations of pressure in the system. A detailed analysis of the mechanisms controlling the process is carried out first by comparing the measured and calculated spray and flame dynamics. Considering in a second stage that the spray and flame are compact with respect to the acoustic wavelength the analysis can be simplified by defining state variables that are obtained by taking averages over the combustor cross section and representing the behavior of these average quantities as a function of the axial coordinate and time. This reveals a first region in which essentially convective processes prevail. The convective heat release rate then couples further downstream with the pressure field giving rise to positive Rayleigh source terms which feed energy in the axial acoustic mode. In the convective region, the swirl number features oscillations around its mean value with an impact on the flow aerodynamics and flame radial displacement. Fluctuations in the fuel flow rate are initiated at the injector exhaust and likewise convected downstream. The total mass flow rate that exhibits strong convective disturbances is dominated further downstream by the acoustic motion. This information provides new insights on the convective-acoustic coupling that controls the heat release rate disturbances and reveals the time delays governing the combustion oscillation process.