Crina Anastasescu

Optical and Structural Properties of SnO2-Based Sol-Gel Thin Films

SnO2 thin films have been deposited on glass from Sn (II)-2-ethylhexanoate precursor using the dip-coating sol-gel method. Based on spectroscopic ellipsometry (SE) measurements optical and structural properties of the SnO2 films have been obtained. It was found that the concentration of tin oxide sols used for deposition strongly influences the thickness and the optical properties of the film, such as optical constants (n and k) or optical conductivity (sigma)

Tubular and Spherical SiO2 Obtained by Sol Gel Method for Lipase Immobilization and Enzymatic Activity

A wide range of hybrid biomaterials has been designed in order to sustain bioremediation processes by associating sol-gel SiO2 matrices with various biologically active compounds (enzymes, antibodies). SiO2 is a widespread, chemically stable and non-toxic material; thus, the immobilization of enzymes on silica may lead to improving the efficiency of biocatalysts in terms of endurance and economic costs. Our present work explores the potential of different hybrid morphologies, based on hollow tubes and solid spheres of amorphous SiO2, for enzyme immobilization and the development of competitive biocatalysts. The synthesis protocol and structural characterization of spherical and tubular SiO2 obtained by the sol gel method were fully investigated in connection with the subsequent immobilization of lipase from Rhizopus orizae. The immobilization is conducted at pH 6, lower than the isoelectric point of lipase and higher than the isoelectric point of silica, which is meant to sustain the physical interactions of the enzyme with the SiO2 matrix. The morphological, textural and surface properties of spherical and tubular SiO2 were investigated by SEM, nitrogen sorption, and electrokinetic potential measurements, while the formation and characterization of hybrid organic-inorganic complexes were studied by UV-VIS, FTIR-ATR and fluorescence spectroscopy. The highest degree of enzyme immobilization (as depicted from total organic carbon) was achieved for tubular morphology and the hydrolysis of p-nitrophenyl acetate was used as an enzymatic model reaction conducted in the presence of hybrid lipase–SiO2 complex.

Particularities of photocatalysis and formation of reactive oxygen species on insulators and semiconductors: cases of SiO2, TiO2 and their composite SiO2-TiO2

Highly defected tubular SiO2 is found to outperform the activity of TiO2-P25 and the SiO2–TiO2 composite in photocatalytic H2 generation from methanol–water solution under simulated solar light AM 1.5. The enhanced performances of SiO2 come from the particularities of the reaction mechanism and ROS (reactive oxygen species) generation. The SiO2 exposed to light generates solely 1O2 (singlet oxygen). The TiO2-P25 produces uniquely large quantities of ˙OH radicals whereas the formation of O2− is evidenced only over SiO2–TiO2, along with small amounts of ˙OH. The TiO2 works as a photocatalyst by intermediation of ˙OH radicals. In contrast, the organic substrate is activated on the surface of SiO2 by the intra-band gap, isolated, surface quantum defects. Distinct reaction mechanisms, involving the participation of photogenerated charges and ROS, are proposed. The material-related ROS production can be of great practical importance in fields such as biology (germ inactivation), medicine (photodynamic therapy by 1O2), and synthesis of oxygenated organic compounds of great added value.

Synthesis of Oxide Nanotubes/Nanorods by Hydrothermal Method

Titanate nanotubes and ZnO nanorods are the most studied 1D oxide nanostructures, due to their specific structure, morphology, and special properties. Several physical and chemical methods of preparation have been developed for their preparation. Among the chemical methods, the mostly used are the template-assisted sol–gel method, hydrothermal method, and anodic oxidation. In the following chapter, a short review of the results obtained by hydrothermal preparation of titanate nanotubes and ZnO nanorods and their structural, morphological, and thermal characterization is presented. In the case of titanate nanotubes, the factors influencing the hydrothermal synthesis as well as factors influencing the post-hydrothermal treatment of resulted titanate were discussed. The influence of the microwaves on the hydrothermal preparation of titanate nanotubes is also presented. In the case of ZnO nanorods/nanotubes, the different experimental conditions for their preparation by hydrothermal method were established. Among the different precursors and reagents involved in the hydrothermal procedure for the ZnO nanorod/nanotube preparation, the zinc nitrate–hexamethylenetetramine system is one of the mostly used, and it was approached in more details. The obtaining of ZnO nanorods and their transformation into nanotubes in the mentioned system was discussed. At the same time, the possibility of doping ZnO nanotubes and their growth as aligned ZnO nanorods on the different substrates was presented. Besides titanate nanotubes and ZnO nanorods/nanotubes, several other oxides (V2O5, MoO3, WO3, SnO2, Fe2O3) that could be obtained by hydrothermal method were discussed.

Synthesis of Oxide Nanotubes by Sol–Gel Method

The great interest for the 1D oxide nanostructures, due to their specific and sometimes unexpected properties, led to the development of a great number of physical and chemical preparation methods. In the following chapter, the preparation of several oxide nanotubes by template-assisted sol–gel method is presented. The preparation, characterization, and properties of the SiO2 nano-/microtubes are approached in more details. The preparation was realized by sol–gel method in the presence of organic templates, namely, DL-tartaric acid, introduced in the reaction mixture. The resulted samples have been characterized from structural, morphological, and thermal point of view, and their specific catalytic and photocatalytic properties have been investigated. The resulted samples were nanometric in diameter, but micrometric in length and proved to be amorphous. The significant results have been obtained by testing the catalytic and photocatalytic activity of the un-doped and Pt-doped SiO2 nano-/microtubes for catalytic mineralization of formic acid and photocatalytic oxidation of oxalic acid to CO2, respectively. The behavior of the nanotubes could be considered as morphology dependent, the catalyst acting in fact as membrane-type microreactors. The preparation of some other 1D oxides nanostructures (TiO2, ZrO2, Nb2O5, Ta2O5, WO3, ZnO, Fe2O3, V2O5, ThO2 Eu2O3, Eu-doped ThO2, MnO2, ZnO, Co3O4, In2O3, Ga2O3, Al2O3, PbTiO3, BaTiO3) were also discussed.

Introduction (General Considerations on the 1D Oxide Nanostructures)

Since the discovery of the carbon nanotubes, the interest for obtaining analogue nanostructures based on inorganic materials increased considerably. Besides the layered d-metal dichalcogenides, followed by boron nitrides, boron carbides or boron carbonitrides, metallic nanotubes and nanowires, and so on, a wide range of data concerning the p-(Si, Al, Mg), d-(Ge, Ti, Zn, Nb, Ta, Zr, V, Mo), and f-(Dy, Tb, Eu) oxide 1D structure synthesis were reported. The book is focused on wet chemical methods of 1D-type oxide nanostructure preparation, as sol–gel and hydrothermal methods which are versatile, not expensive techniques, and thus appropriate for obtaining a wide range of oxide materials with tailored morphology and properties. Three specific oxides (SiO2, TiO2, ZnO) were selected in order to describe the principle of the sol–gel and hydrothermal preparation of the 1D oxide nanostructures, followed by discussion of other oxides synthesized by the mentioned method. The correlation between the tubular structure and the physicochemical properties of studied 1D oxide nanostructures was driving in unexpected and valuable results.