Oxidation-state-specific dynamics at the Fe M 2,3 -edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe 2 O 3 thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at ∼57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe 3+ and photoinduced Fe 2+ state confirm this assignment and exclude the alternative d−d excitation. The Fe 2+ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.
The identities of photoexcited states in thin-film Co 3 O 4 and the ultrafast carrier relaxation dynamics of Co 3 O 4 are investigated with oxidation-state-specific pump−probe femtosecond core level spectroscopy. A thin-film sample is excited near the 2.8 eV optical absorption peak, and the resulting spectral changes at the 58.9 eV M 2,3edge of cobalt are probed in transient absorption with femtosecond highorder harmonic pulses generated by a Ti/sapphire laser. The initial transient state shows a significant 2 eV redshift in the absorption edge compared to the static ground state, which indicates a reduction of the cobalt valence charge. This is confirmed by a charge transfer multiplet spectral simulation, which finds the experimentally observed extreme ultraviolet (XUV) spectrum matches the specific O 2− (2p) → Co 3+ (e g ) charge-transfer transition, out of six possible excitation pathways involving Co 3+ and Co 2+ in the mixed-valence material. The initial transient state has a power-dependent amplitude decay (190 ± 10 fs at 13.2 mJ/cm 2 ) together with a slight redshift in spectral shape (535 ± 33 fs), which are ascribed to hot carrier relaxation to the band edge. The faster amplitude decay is possibly due to a decrease of charge carrier density via an Auger mechanism, as the decay rate increases when more excitation fluence is used. This study takes advantage of the oxidation-state-specificity of time-resolved XUV spectroscopy, further establishing the method as a new approach to measure ultrafast charge carrier dynamics in condensed-phase systems.
The emerging molybdenum disulfide (MoS ) offers intriguing possibilities for realizing a transformative new catalyst for driving the hydrogen evolution reaction (HER). However, the trade-off between catalytic activity and long-term stability represents a formidable challenge and has not been extensively addressed. This study reports that metastable and temperature-sensitive chemically exfoliated MoS (ce-MoS ) can be made into electrochemically stable (5000 cycles), and thermally robust (300 °C) while maintaining synthetic scalability and excellent catalytic activity through physical-transformation into 3D structurally deformed nanostructures. The dimensional transition enabled by a high throughput electrohydrodynamic process provides highly accessible, and electrochemically active surface area and facilitates efficient transport across various interfaces. Meanwhile, the hierarchically strained morphology is found to improve electronic coupling between active sites and current collecting substrates without the need for selective engineering the electronically heterogeneous interfaces. Specifically, the synergistic combination of high strain load stemmed from capillarity-induced-self-crumpling and sulfur (S) vacancies intrinsic to chemical exfoliation enables simultaneous modulation of active site density and intrinsic HER activity regardless of continuous operation or elevated temperature. These results provide new insights into how catalytic activity, electrochemical-, and thermal stability can be concurrently enhanced through the physical transformation that is reminiscent of nature, in which properties of biological materials emerge from evolved dimensional transitions.
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