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.

In photocatalytic transition metal oxides, the surface electronic structure and carrier kinetics are complicated by both charge trapping at defects and self-trapping by small polaron formation. These effects are specific to both the material and the type of defect or surface states involved. Here we review recent studies on ultrafast photoexcited charge carrier dynamics at the surfaces of α-Fe 2 O 3 (hematite) and NiO and at the interface of a α-Fe 2 O 3 /NiO heterojunction using extreme ultraviolet (XUV) reflection−absorption spectroscopy. We study the dynamics of small polaron formation at hematite surfaces showing that the rate of carrier self-trapping is slower than in the bulk due to a greater lattice reorganization energy required for surface polaron formation. We also show that surface dynamics of hematite can be systematically tuned to control carrier transport properties through surface functionalization. To better understand the impact of defects on carrier events, we study charge carrier recombination of NiO. It is shown that O vacancies have no detrimental effect on carrier lifetime while grain boundaries cause fast recombination. Finally, we study a model heterojunction consisting of α-Fe 2 O 3 and NiO and find that interfacial charge transfer across these two materials occurs by a two-step mechanism where the interfacial electric field drives fast exciton dissociation followed by hole injection from α-Fe 2 O 3 to NiO. These examples illustrate future opportunities to observe surface electron dynamics in photocatalytic materials with element and chemical state resolution using ultrafast XUV spectroscopy.

Directly observing active surface intermediates represents a major challenge in electrocatalysis, especially for CO 2 electroreduction on Au. We use in-situ, plasmonenhanced vibrational sum frequency generation spectroscopy, which has detection limits of <1% of a monolayer and can access the Au/electrolyte interface during active electrocatalysis in the absence of mass transport limitations. Measuring the potentialdependent surface coverage of atop CO confirms that the rate-determining step for this reaction is CO 2 adsorption. An analysis of the interfacial electric field reveals the formation of a dense cation layer at the electrode surface, which is correlated to the onset of CO production. The Tafel slope increases in conjunction with the field saturation due to active site blocking by adsorbed cations. These findings show that CO 2 reduction is extremely sensitive to the potential-dependent structure of the electrochemical double layer and provides direct observation of the interfacial processes that govern these kinetics.

Here we present plasmon-resonant vibrational sum frequency generation spectroscopy for use in electrochemical measurements. Using surface plasmon resonance we couple light through a CaF 2 prism to Au films of >50 nm in order to reach the buried Au/ electrolyte interface. The approach enables us to use bulk electrolyte, and high current densities (>1 mA/cm 2 ), and therefore is suitable to probe active intermediates under relevant electrochemical reaction conditions. Fresnel factor modeling of the plasmon resonance for a three layer system (CaF 2 /Au/electrolyte) shows good agreement with experimental data. Off-angle momentum-matching to the surface plasmon resonance allows us to measure functional groups (−CH, −CD, −CN, −NO 2 ) across a wide range of infrared frequencies by simply scanning the infrared wavelength without any angular realignment. Additionally we report a detection limit <1% of a monolayer for the Au/electrolyte interface. Using this method we observe an active intermediate during CO 2 reduction on Au at catalytic currents. Consequently, we believe that this method will provide mechanistic understanding of electrochemical reactions.

Anatase TiO 2 is an efficient water splitting photocatalyst using UV light, but solar energy harvesting requires the presence of midgap states to increase visible light absorption. Despite numerous studies, important questions remain regarding the photophysics in O vacancy doped TiO 2 . By employing extreme ultraviolet reflection−absorption (XUV-RA) spectroscopy at the Ti M 2,3 -edge, spectral signatures of both large and small polaron states are identified, allowing ultrafast electron and hole dynamics in these states to be independently resolved. Results show that visible light absorption occurs via promotion of an electron from the small polaron state to the TiO 2 conduction band. In contrast, absorption of UV light results in direct band gap excitation followed by carrier relaxation during which hot holes trap as small polarons in 45 ± 42 fs, and hot electrons couple to polar optical phonons leading to vibrational coherence and large polaron formation in 945 ± 92 fs.