Ph.D Thesis


Ph.D StudentBenisti Itamar
SubjectUltra-fast FTIR Spectroscopy of Photoactive Materials
DepartmentDepartment of Chemical Engineering
Supervisor PROF. Yaron Paz
Full Thesis textFull thesis text - English Version


Abstract

Photocatalysis is a sustainable way for purification of air and water, decontamination of surfaces, and for water splitting. Over the years, the spotlight was given to the thermodynamic parameters of photocatalysts such as the location of their energy bands. The vibrational spectroscopy right after excitation was hardly studied. The aim of this work was to determine how transient IR spectroscopy may elucidate the interplay between the photocatalysts and the photoexcited carriers right after excitation. For this, three photocatalysts were chosen: BiOCl, BiVO4 and g-C3N4.

            BiOCl was modified to expose the [001] facets on its surface. Exposed [001] planes showed a clear tendency of electrons accumulation manifested by photo-deposition of Pt nanoparticles. The activity of BiOCl particles for photoreduction of Cr(VI) versus the ratio of (001)/(101)  showed a “volcano” plot behavior. This was explained by a “bottle-neck” stage, originated from the parallel nature of the two redox reactions. The transient FTIR measurements of BiOCl showed changes in the intensity of the Bi-O symmetric absorption band during the first 100 nanoseconds from excitation. Negative correlation was found between the increased intensity of the transient signal and the kinetic constant of Cr(VI) photoreduction. This was explained by deep traps probably located at the sidewalls. 

            Bismuth vanadate particles were found to be very different in their activity; monoclinic particles with prismatic structure, as well as tetragonal phase particles were more active for Cr(VI) reduction than a mixed phase of monoclinic and tetragonal or octahedron-shaped monoclinic particles. UV-vis photoluminescence measurements following UV/vis pulsed excitation did not show any correlation with the activity. Consequently, they are quite silence in terms of explaining the activity. A negative correlation was found between the activity and the concentration of oxygen vacancies near the surface. This could indicate a shortage in holes at the surface of the less-active types of BiVO4. Transient FTIR measurements following pulsed excitation revealed temporal changes in the spectra prevailing for dozens of nanoseconds. These variations differed in terms of duration and wavelength-dependence among the various types of BiVO4. A negative correlation between the duration of the transient signal and the photocatalytic activity was found.

Transient IR spectral changes in hollow spheres of graphitic carbon nitride (g-C3N4), with and without deposited nano-islands of Pt were studied as well. These measurements suggested that the deposition of nano-islands of noble metals may assist photocatalysis not only by forming electron sinks, but also by assisting the shallow trapping of holes in the vicinity of the islands, most likely in N-H, NH2 moieties, thus extending the lifetime of the (trapped) carriers to 150 ns or so. Unfortunately, the benevolent effect of the deposited platinum was hampered overtime either due to detaching of the metallic islands or due to reduction of Pt2 species, co-existing in the photo-deposited platinum islands.

 To summarize, the potential of the step-scan FTIR technique as a tool for understanding, rationalizing, or even predicting the activity of photocatalytic materials was demonstrated.