Controlling polaron formation at hematite surfaces by molecular functionalization probed by XUV reflection-absorption spectroscopy

Abstract

Small polaron formation is known to limit the photocatalytic charge transport efficiency of hematite via ultrafast carrier self-trapping. While small polaron formation is known to occur in bulk hematite, a complete description of surface polaron formation in this material is not fully understood. Theoretical predictions indicate that the kinetics and thermodynamics of surface polaron formation are different than those in bulk. However, to test these predictions requires the ability to experimentally differentiate polaron formation dynamics at the surface. Near grazing angle extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy is surface sensitive and provides element and oxidation state specific information on a femtosecond time scale. Using XUV-RA, we provide a systematic comparison between surface and bulk polaron formation kinetics and energetics in photoexcited hematite. We find that the rate of surface polaron formation (250 ± 40 fs) is about three times slower than bulk polaron formation (90 ± 5 fs) in photoexcited hematite. Additionally, we show that the surface polaron formation rate can be systematically tuned by surface molecular functionalization. Within the framework of a Marcus type model, the kinetics and energetics of polaron formation are discussed. The slower polaron formation rate observed at the surface is found to result from a greater lattice reorganization relative to bulk hematite, while surface functionalization is shown to tune both the lattice reorganization as well as the polaron stabilization energies. The ability to tune the kinetics and energetics of polaron formation and hopping by molecular functionalization provides the opportunity to synthetically control electron transport in hematite.

Ultrafast Spin Crossover in a Room-Temperature Ferrimagnet: Element-Specific Spin Dynamics in Photoexcited Cobalt Ferrite

“…This process occurs via fast electron-phonon scattering (tens of fs), followed by coupling of the free electron state to an optical phonon to form a small electron polaron (hundreds of fs) and has been studied in detail in α-Fe 2 O 3 (hematite). 55,58,59 Here we observe similar spectral evolution at the Fe M2,3-edge occurring on the fs time scale for CFO, indicating that electron polarons also form in this material. By similar logic, oxidation of the Co metal center will increase attraction between photoexcited Co 3+ and the oxygen anions, leading to bond length contraction and formation of a hole polaron.…”

Polaronic structure of excess electrons and holes for a series of bulk iron oxides

“…As these iron oxides are generally considered native n-type semiconductors, 13 and are frequently doped with electron donors, 7,14 the structure of the electron polaron has received much attention. In the small electron polaron model, supported by experiment, 4 a conduction electron induces a localised lattice distortion that lowers its energy to the point that the electron becomes self-trapped. This reorganisation energy therefore acts to reduce the mobility of the electron.…”

Element-specific electronic and structural dynamics using transient XUV and soft X-ray spectroscopy

“…Bulk and surface polaron formation occur on slightly different timescales, with bulk polaron formation occurring with a time constant of 90 fs and surface polarons forming with a time constant of 250 fs. 4,46 Molecular functionalization of the hematite surface has also been shown to change the lattice reorganization and polaron stabilization energies, but polarons still form with rates between 146 fs for OMe-PPA-Fe2O3 and 250 fs for hydroxyl-Fe2O3. 46 Furthermore, comparing small polaron formation in hematite and goethite (a-FeOOH) has shown that small changes in ligands and crystal structure have a significant effect on formation rates, with polarons forming in 180  30 fs in goethite nanorods and 90  5 fs in a hematite thin film.…”

“…4,46 Molecular functionalization of the hematite surface has also been shown to change the lattice reorganization and polaron stabilization energies, but polarons still form with rates between 146 fs for OMe-PPA-Fe2O3 and 250 fs for hydroxyl-Fe2O3. 46 Furthermore, comparing small polaron formation in hematite and goethite (a-FeOOH) has shown that small changes in ligands and crystal structure have a significant effect on formation rates, with polarons forming in 180  30 fs in goethite nanorods and 90  5 fs in a hematite thin film. 47 Engineering materials to remove polaronic effects remains outstanding questions for photocatalysis and photoelectrochemistry.…”