Characterization of Photo-Induced Charge Transfer and Hot Carrier Relaxation Pathways in Spinel Cobalt Oxide (Co₃O₄)

Abstract

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.

Operando Spectroscopic Analysis of an Amorphous Cobalt Sulfide Hydrogen Evolution Electrocatalyst

“…The pre-edge feature at 58 eV and rising edge at 61 eV have previously been assigned to Co 2+ ions. 37 The M-edge position in the CoS x spectrum suggests that the main oxidation state of cobalt is Co 2+ , unlike Co 3 O 4 , which is a 1:2 mixture of Co 2+ and Co 3+ and has relatively more absorption features at higher energies.…”

Rapid Thermal Treatment of Reactive Sputtering Grown Nanocrystalline Co3 O4 for Enhanced All-Oxide Photovoltaics

“…The Co 3 O 4 film exhibited an α value > 2 × 10 4 cm −1 at λ = 826 nm, and an α value > 6 × 10 4 cm −1 at λ = 563 nm. The RTP treatment effectively reduces the absorptions in the infrared (IR) range attributed to the free carrier modulation …”

Sub-100 fs Intersystem Crossing to a Metal-Centered Triplet in Ni(II)OEP Observed with M-Edge XANES

“…18 This technique has been used recently to resolve ultrafast photophysics of heme model complex iron(III)tetraphenylporphyrin chloride 19 as well as carrier photophysics in metal oxide semiconductors such as α-Fe 2 O 3 , Co 3 O 4 , and NiO. [20][21][22][23][24]…”