The main motivation of the work presented herein is understanding how photochemical reactions that are difficult to run (e.g. CO2 reduction) can be facilitated by the surface of a solid-state material. Referred to as a heterogeneous catalyst, its purpose is to lower the energetic barrier to take the reactants over to the desired products by providing an alternative, energetically more favorable mechanism to drive the transformation. In order to provide detailed understanding of the light-induced charge carrier dynamics in these solid state materials, it is necessary to have a spectroscopic probe capable of: 1) following the dynamics with chemical state speci ficity to identify the location of the excited charges; 2) having ultrafast time resolution to follow the reactions in real time; and 3) providing surface sensitivity because it is at the surface where these catalytic reactions occur. While optical spectroscopies using fs laser pulses are capable of following charge carrier dynamics on the ultrafast time scale, often X-ray based techniques are desirable because X-ray spectroscopy is element speci c as well as extremely sensitive to oxidation and spin state of a system.
To address these challenges, we have designed and built a tabletop, femtosecond, XUV spectrometer capable of working in a transmission as well as reflection geometry. All of the work presented here has been performed in reflection for the purpose of gaining surface sensitivity and the ability to probe actual heterogeneous catalysts without regard for the sample thickness. Details of the experimental apparatus are given in Chapter 2.
Having developed XUV reflection-absorption (XUV-RA) spectroscopy, we demonstrate, using a series of fi rst row transition metal oxides, the element and oxidation state speci city of XUV-RA spectroscopy. Additionally, this new method requires data interpretation algorithms which are also presented in Chapter 3. Since reflection geometry samples both the real and imaginary part of the refractive index, we have developed a general method for simulating experimental spectra.
After demonstrating the element speci ficity of XUV-RA spectroscopy, we have performed the first time-resolved study on α-Fe2O3 in Chapter 4. The results show
that poor charge carrier mobility and surface trapping in this material are due to small polaron formation around a photoexcited Fe2+ metal center. The lattice expansion during small polaron formation is the driving force for ultrafast electron localization to the surface. We further show that these kinetics are negligibly influenced by grain boundaries and surface defects by obtaining nearly identical results for single crystalline and polycrystalline samples.
We also show the ability to detect the photoexcited hole and electron directly which paves the way for measuring ultrafast hole localization in CuFeO2, an active
CO2 reduction photocatalyst in Chapter 5. Upon photoexcitation, the electron localizes
on a Fe center whereby the hole resides in a highly hybridized Cu 3d O 2p orbital. The layered structure of delafossite allows for such charge separation which is likely responsible for the observed photocurrent present for CuFeO2 in CO2 saturated electrolyte.
Finally, measurements on anatase TiO2 have shown the ability of XUV-RA to
directly observe quantum beating between the conduction band states and large polaron states after above band gap photoexcitation. The presence of reduced Ti3+ defects in the material serves as trap sites just below the conduction band edge and the observed quantum beating corresponds to the dynamic equilibrium between the free charge carriers in the conduction band and the large polaronic state below the conduction band edge. Understanding the transient kinetics of TiO2 provides important foundation for studying plasmon mediated charge transfer dynamics of Au nanoparticles supported on TiO2.
This dissertation extends the ability of X-ray and XUV spectroscopy to investigate electron dynamics in catalytically relevant solid-state materials with chemical state speci city, and ultrafast time resolution. Understanding the self-trapping mechanism in hematite, highly localized nature of excitons in metal oxides, ultrafast charge separation in delafossite, and role of defects in TiO2 provides important design parameters for next generation materials. Having a direct spectroscopic probe of the carrier dynamics in these solid-state systems promises to advance the fi eld of photocatalysis to address the most challenging questions regarding clean energy generation and storage as well as the sustainability of our planet.