טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBenisti Itamar
SubjectThe Effect of Faceting on the Photocatalytic Activity of
Bismuth-Oxides
DepartmentDepartment of Chemical Engineering
Supervisor Professor Yaron Paz
Full Thesis textFull thesis text - English Version


Abstract


Surface properties are a critical factor affecting the physical and chemical performants of photocatalysts. Many studies, especially in TiO2, have proven that faceting can increase the photocatalytic activity (PCA). At the same time, there is an increased interest in bismuth oxides, probably because of their ability to separate electrons and holes due to their layered and polar structure. Along these lines, this work studied the effect of faceting on three types of bismuth oxide photocatalysts: BiOCl, BiYWO6 and BiVO4.

BiOCl was prepared in a batch reactor using Sodium Dodecyl Benzene Sulfonate (SDBS), as a capping agent. This resulted in a faceted photocatalyst whose shape was depended on a surfactant concentration. Photo-deposition of platinum nanoparticles was observed only on the (001) facet, indicating tendency for collecting negative charge carriers. Photoreduction of Cr (VI) under UV radiation showed a negative correlation between the relative area of the (001) planes and the reduction kinetics. These results were in contrast to literature, which reported opposite tendency. It is suggested that the reason for the discrepancy originated from the difference in the rate-limiting step between the two cases. For most cases reduction is the bottle neck, hence increasing the (001) area promotes PCA. For the Cr (VI) reduction, oxidation is the bottle-neck, hence the opposite behavior. 

BiYWO6 was reported as a photocatalyst that, potentially, has the ability to split water. No work has been done so far regarding the effect of its faceting on its photocatalytic activity. In our research, we use a sol-gel method under various calcination temperatures as well as a combination of a sol-gel process and hydrothermal process followed by calcination at 10000C. Here, again  SDBS was used to induce faceting. This synthesis has been producing faceted particles which differ in their orientation. The particles that were prepared with 0.144 SDBS/Bi showed better PCA compared with other particles that have been synthesized by the same method but with different concentration of surfactant. 

The last chapter presents a new approach of studying the excitation of photocatalytic materials. The method is based on measuring time-resolved (5 nanoseconds in resolution) IR spectroscopy of the photocatalytic materials upon excitation a 355 nm pulsed laser. The time-resolved FTIR spectra were obtained by using a “step-scan” configuration. The technic is demonstrated by measuring temporal changes in the FTIR spectrum of BiVO4. Fast (100-1000 nanoseconds) changes in the FTIR spectrum of BiVO4 following excitation, particularly in the 740 cm-1 peak, representing the V-O vibrational mode, were recorded. The time profile of these changes differed between monoclinic BiVO4 and a tetragonal BiVO4, demonstrating the strength of this technique, reported by us for the first time, as a tool for obtaining detailed information on photocatalysis.