Study of the Effect of Nitric Acid in Electrochemically Synthesized Silicon Nanocrystals: Tunability of Bright and Uniform Photoluminescence.

In this work, we show a correlation between the composition and the microstructural and optical properties of bright and uniform luminescent porous silicon (PSi) films. PSi films were synthesized by electrochemical etching using nitric acid in an electrolyte solution. PSi samples synthesized with nitric acid emit stronger (up to six-fold greater) photoluminescence (PL) as compared to those obtained without it. The PL peak is shifted from 630 to 570 nm by changing the concentration ratio of the HF:HNO3:(EtOH-H2O) electrolyte solution, but also shifts with the excitation energy, indicating quantum confinement effects in the silicon nanocrystals (Si-NCs). X-ray photoelectron spectroscopy analysis shows a uniform silicon content in the PSi samples that emit the strongest PL. High-resolution transmission electron microscopy reveals that the Si-NCs in these PSi samples are about ~2.9 ± 0.76 nm in size and are embedded in a dense and stoichiometric SiO2 matrix, as indicated by the Fourier transform infrared analysis. On the other hand, the PSi films that show PL of low intensity present an abrupt change in the silicon content depth and the formation of non-bridging oxygen hole center defects.

Lateral size reduction of graphene oxide preserving its electronic properties and chemical functionality.

Graphene oxide (GO) is widely considered as a graphene precursor when chemically reduced. Nevertheless, through the precise control of two parameters: lateral size and oxidation degree, GO can be useful in many applications as modified graphene oxide or functional reduced graphene oxide. Commonly, the decrease in GO lateral size, involves a change in the C/O ratio and therefore a modification in a large number of characteristics. Here, a simple but effective approach to synthesize GO with lateral dimensions below 100 nm and without modification of its chemical, optical and electronic features is presented. The use of a sonifier at low temperature allows to rapidly reduce the lateral size in ∼82% while preserving the C/O ratio and consequently the chemical stability, the band gap, the electronic energy levels and the functionality. This method will allow several applications from biomedicine to energy, where reliable reduced size of GO is required.

Functionalization Effect on Polymer Nanocomposite Coatings Based on TiO2⁻SiO2 Nanoparticles with Superhydrophilic Properties.

In this study, a novel binary nanocomposite system based on TiO2-SiO2 was functionalized with trimethylolpropane triacrylate (TMPTA) and characterized by XPS and XRD. Results revealed that TiO2-SiO2 nanoparticles were covalently functionalized. Functionalized nanoparticles at low concentrations (0.1 wt % and 0.5 wt %) were dispersed in acrylic acid acting as a polymer matrix. Nanocomposite coatings analysis demonstrate to achieve superhydrophilic properties as well as very good optical characteristics. Water contact angle characterization showed the functionalization effect by achieving a superhydrophilic behavior with a contact angle less than 5°. UV-Vis measurements demonstrated high optical transmittance above 95% for the coatings. Based on the obtained results a mechanism describing the chemical interactions of the constituents responsible for the synergy in the nanocomposite as well as the morphological play role in the behavior are presented.

Immobilization of pectinesterase in genipin-crosslinked chitosan membrane for low methoxyl pectin production.

In this study, an environmentally friendly method for preparation of chitosan crosslinked membranes with different genipin concentrations (0, 125, 250, and 500 mg/L) was developed. Genipin-crosslinked chitosan membranes were used for the immobilization of fungal pectinesterase (PE). PE was efficiently immobilized in chitosan membranes and used for modification of high-methoxylated pectins into low-methoxylated pectins. The charge density (ζ-potential), infrared spectroscopy (FTIR), and ion-exchange chromatography (IEC) revealed the modification of pectin for immobilized PE in genipin-crosslinked chitosan membrane. The ζ-potential results indicated a decrease of -5.2 mV in the pectin control solution, whereas using the PE immobilized in chitosan and genipin-chitosan membranes, the obtained values were -30.45 mV and -36.38 mV, respectively. Genipin-crosslinked chitosan membrane jointly with the immobilized PE represents an eco-friendly support to prepare tailor-made low-methoxylated lime pectin.