The thesis for for a candidate of technical science degree in speciality 05.17.01 «Technology of inorganic substances». – National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute", Kyiv, 2018.
The dissertation is devoted to the obtaining of tin (IV) oxide nanostructures of various morphologies, the establishment of technological features of their synthesis and the study of their physical and chemical properties for further use as sensitive materials for metal oxide chemoresistive gas sensors.
The influence of methods (thermal, sol-gel, vapor transport) on the structure and morphology of tin (IV) oxide powders is considered in the paper. Investigation of tin (IV) oxide samples, synthesized by different methods, shows that the used methods allow to obtain nano-sized particles of SnO2 and, in accordance with the results of X-ray analysis, the formation of a pure phase of tin (IV) oxide occurs in all samples. However, the most promising is the vapor transport method (CVD), which produces single crystals SnO2 of high degree of crystallinity, and powders based on them have higher values of the specific surface area and characterized by better adsorption properties. Therefore, the vapor transport method was chosen for synthesis of SnO2 nanostructured powders.
As a result of the studies on the influence of temperature, composition of the gas medium and the heating rate when using the CVD method, technological features of the synthesis of nanosized and one-dimensional SnO2 nanostructures were invented. It was established that: the pure SnO2 phase is formed at 1123 K; dosage of oxygen to the carrier gas in the amount of 5% vol. leads to a change in the morphology of SnO2 from circular particles to the elongated lamellar form; reducing the heating rate of the furnace from 80 deg/min to 20 deg/min changes the morphology of the tin (IV) oxide from nanosized to one-dimensional nanostructures; with a rise in temperature from 1123 to 1323 K the thickness of the filaments SnO2 increases.
The comparison of the characteristics of nanosized and one-dimensional SnO2 has been carried out. The influence of morphology and modification on optical, electrical characteristics and sensitivity of SnO2 nanostructures is established.
It has been shown that SnO2 nanostructures differ not only visually, but also are vary in physical and chemical properties. In particular, they are dissimilar by intensity of peaks on diffractograms, by values of the specific surface area, and in the case of one-dimensional SnO2, differ in the presence of a strong band of oscillations with a maximum for the value of a wave number of 563 cm-1, which is characteristic of infrared spectra of one-dimensional SnO2 structures.
The study of optical characteristics of modified and unmodified samples of nanosized tin (IV) oxide and one-dimensional SnO2 nanostructures shows that one-dimensional tin (IV) oxide samples much stronger absorb UV radiation in comparison with nanosized SnO2 samples. The determined values of the band gap width for nanosized and one-dimensional SnO2 nanostructures are in the range from 3.85 to 4.2 eV and from 2.8 to 3.4 eV, respectively.
The comparison of the volt-ampere characteristics of the nanosized and one-dimensional tin (IV) oxide indicates a different nature of the curves due to differences in the morphology of SnO2 nanostructures. For nanosized SnO2 at all temperatures, there are non-linear dependencies that are characteristic of semiconductor materials. While the one-dimensional SnO2 is characterized by ohmic (linear) volt-ampere curves that are typical for substances with metallic properties.
In the study of electrical properties of tin (IV) oxide samples modified by argentum, a lack of a linear relationship between the value of the electrical resistance and the amount of the introduced modifier was found for both nanosized and one-dimensional SnO2. Calculated values of the sensitivity of SnO2 samples indicate that among unmodified tin (IV) oxide powders the higher response to acetone has a one-dimensional SnO2. Among the modified SnO2 powders, the highest sensor respond has nanosized tin (IV) oxide with argentum content of 10% by weight. Modification of one-dimensional SnO2 samples results in deterioration of sensitivity towards acetone.
A technological scheme for the synthesis of tin (IV) oxide nanostructures was developed, according to which the choice of equipment was made, material and thermal balance was calculated. Feasibility studies for obtaining of SnO2 nanostructures of different morphologies by CVD method are done. Experimental-industrial tests of SnO2 nanostructures in the conditions of existing production are carried out.
Keywords: tin (IV) oxide, 0D and 1D nanostructures, CVD method, semiconductors, gas sensor, modification, bandgap width, current-voltage curves, sensitivity.
