Susana Mihaiu

Nanostructured SnO2–ZnO composite gas sensors for selective detection of carbon monoxide

A series of SnO2–ZnO composite nanostructured (thin) films with different amounts of SnO2 (from 0 to 50 wt %) was prepared and deposited on a miniaturized porous alumina transducer using the sol–gel and dip coating method. The transducer, developed by our research group, contains Au interdigital electrodes on one side and a Pt heater on the other side. The sensing films were characterized using SEM and AFM techniques. Highly toxic and flammable gases (CO, CO2, CH4, and C3H8) were tested under lab conditions (carrier gas was dry air) using a special gas sensing cell developed by our research group. The gas concentrations varied between 5 and 2000 ppm and the optimum working temperatures were in the range of 210–300 °C. It was found that the sensing performance was influenced by the amount of oxide components present in the composite material. Improved sensing performance was achieved for the ZnO (98 wt %)–SnO2 (2 wt %) composite as compared to the sensors containing only the pristine oxides. The sensor response, cross-response and recovery characteristics of the analyzed materials are reported. The high sensitivity (R S = 1.21) to low amounts of CO (5 ppm) was reported for the sensor containing a composite sensitive film with ZnO (98 wt %)–SnO2 (2 wt %). This sensor response to CO was five times higher as compared to its response to CO2, CH4, and C3H8, thus the sensor is considered to be selective for CO under these test conditions.

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)

Electrical characterization of In-N codoped p-type ZnO films grown by chemical methods

Preliminary results of the studies of electrical properties of ZnO films, codoped with donor and acceptor pair In-N are reported. The films were grown on Si substrates by chemical route procedures involving a sol-gel followed by hydrothermal treatment at 500?C. For electrical characterization, current-voltage and capacitance-voltage measurements were conducted on MIS structures with embedded ZnO(In,N) films. The estimated doping concentration in the studied films is higher than 1?1016 cm?3 corresponding to a semiconductor with good conductivity. However, the obtained large values of the differential specific resistivity (? = 3.2?105 ?cm) suggest that the nitrogen acceptors in these films are compensated by some kind of donor-type defects.

On the electrical conductivity in Al:ZnO layers; experimental investigation and a theoretical approach

Multi-layered Al:ZnO thin films, with wurtzite - type structure and thickness up to 120 nm, as determined by x-ray diffraction and HRTEM, were grown on Si-SiO2 and glass substrates by the sol-gel method. Fluorescence spectroscopy measurements show that 0.5 at. % Al doping determines a blue shift of the emission band observed at 387nm in the undoped material. The room temperature conductivity increases when the number of layers increases, to reach a value of 3.70 (Ω·m)−1 for a ten layer film. Results obtained by total energy first principles calculations performed on systems with chemical disorder are discussed in relationship with experimental data to account for the effect of Al on the conductivity.

CERAMIC PROPERTIES OF THE SINTERED MATERIALS IN THE Sn-Sb-Cu-O SYSTEM

Sintered SnO2-based ceramics were considered to be promising materials for manufacturing the stable electrodes for the various technological applications, as aluminium electrolysis 1 (inert anodes) or glass industry 2,3. For such applications, a high electrical conductivity and high sintering ability ensuring a high density and, consequently low porosity of the final product are necessary.

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.

Nanoscale analysis of the cross-sectional structure and composition of ZnO: Al active channel layer in thin film transistors

High spatial and analytical resolution HR SEM-EDX and STEM-EDX microscopy techniques were applied for characterization of thin films transistors (TFT) with ZnO: Al active channel layer. The structure and chemical composition of thin films and their interfaces were investigated at the nanometer scale using cross section samples from TFT devices. We evidenced that cross sectional chemical elements spectra and elemental mapping are very powerful tools for analysis of chemical elements distribution in the films and at the interfaces.

Micro-nanostructural and composition characterization of ZnO: Al doped films by SEM-EDX and HRTEM-EDX

We present the results of a structural and compositional investigation of Al-doped thin films obtained by sol-gel deposition on Si/SiO2 substrates. Taking into consideration the layer by layer deposition process and that the films contain 1–10 successive layers, the evolution of structure and elemental composition, especially Al concentration in the films were investigated by energy dispersive X-ray analyses in the scanning and high resolution transmission electron microscopy. Qualitative composition of the structures observed in the films have been evidenced in the elemental maps.