טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentPasternak Sagi
SubjectNovel Materials for Photocatalytic Splitting of Water
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor Professor Yaron Paz
Full Thesis textFull thesis text - English Version


Abstract


The use of light-absorbing semiconductors catalysts to drive various chemical reactions has attracted considerable scientific attention in the last several decades and is generally referred to as semiconductor photocatalysis. The two main branches of photocatalysis are storage of solar energy and degradation of pollutants.

The first chapter of this dissertation deals with the similarity and dissimilarity between photocatalytic degradation of pollutants and photocatalytic water splitting, taken as a representative energy storage reaction. The discussion encompasses all the various processes which inherently take place for every photocatalytic reaction and the various thermodynamic and light-utilization aspects. The different, often conflicting, requirements from photocatalysts lead to the inevitable conclusion that photocatalysts suitable for one branch are likely to be unsuitable for the other.

The following chapters deal with the study of BiYWO6 and other Bi2-xYxWO6 compounds. BiYWO6 was synthesized by hydrothermal and sol-gel methods for the first time. These methods yielded polycrystalline powders with high specific surface area and tunable morphology. The powders were characterized and their photocatalytic activity under visible-light irradiation was tested by following the degradation of a resazurin dye. It was found that the method of preparation had a considerable effect not only on the reaction rates but also on the tendency to form the intermediate resorufin. The reason for this effect was explained by changes in the charge separation capability of the photocatalyst and by changes in the locations of the energy bands.

The various optical and structural properties of the Bi2-xYxWO6 solid solution were studied using diffuse-reflectance spectroscopy, Raman spectroscopy and Rietveld analysis. The approximate limits of this solid solution were found to be 0.3<x<1. All the members of this group had a monoclinic unit cell and a layered structure, consisting of edge-sharing WO6 octahedrons and (Bi,Y)O2 layers.

The developed sol-gel synthesis route has also enabled the preparation of thin films of Bi2-xYxWO6 grown on ITO-covered glass substrates for the first time. These electrodes were characterized by methods including: HR-SEM, XRD, UV-Vis spectroscopy, Mott-Schottky analysis and Kelvin probe measurements. The activity of the electrodes for photoelectrochemical water splitting was studied. It was found that increasing the yttrium content in the compound led to a cathodic shift of the flatband potential according to a theoretical analysis and Mott-Schottky results. This can be explained by the increasing amount of Y4d orbitals contributing to the formation of the conduction band. Although high yttrium content is thermodynamically favorable for the water splitting reaction, it was found to have a deleterious effect on the visible-light absorption and crystallinity of the films. This, in turn, led to a decrease in the photocurrent density of the films both due to poor absorption and increased rate of recombination.  

Surface photovoltage measurements revealed an unexpected p-type-like behavior in response to irradiation in the indirect optical transition range. As the photon energy increased further, the expected decrease in the measured contact potential difference, typical for n-type materials, was measured. These results suggest the existence of preferential trapping of charge carriers on the surface of the films.