Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3

Carneiro, Lucas M.; Cushing, Scott K.; Liu, Chong; Su, Yude; Yang, Peidong; Alivisatos, A. Paul; Leone, Stephen R.

doi:10.1038/nmat4936

Cited by 206 publications

(371 citation statements)

References 51 publications

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“…Time-zero and the instrument response function of 105 fs FWHM were determined using concurrent transient absorption of α-Fe 2 O 3 , which displays IRF-limited formation of a ligand-to-metal charge transfer state. 23,24 Transient spectra at a range of pumpprobe delay times are shown in Figure 4A, with kinetic slices at selected energies in shown in Figure 4B. These spectra are analyzed using both single-energy kinetic fits and a global fit using a sequential These time constants are taken from the global fit (τ=1/k), and are summarized with fit uncertainties in Table 1.…”

Section: Transient M-edge Xanesmentioning

confidence: 99%

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

2019

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Nickel porphyrins have been extenstively studied as photosensitizers due to their long-lived metal-centered excited states. The multiplicity of the (d,d) state, and/or the rate of intersystem crossing between singlet and triplet metal-centered states, has remained uncertain due to the spin-insensitivity of many spectral probes. In this work, we directly probe the metal 3d shell occupation of nickel(II) octaethylporphyrin (NiOEP) using femtosecond M 2,3 -edge X-ray absorption near-edge structure (XANES). A tabletop high-harmonic source is used to perform 400 nm pump, extreme-ultraviolet probe transient absorption spectroscopy with ~100 fs time resolution. Photoexcitation produces a (π,π*) state that evolves with a time constant of 48 fs to a vibrationally hot metal-centered triplet 3 (d,d) excited state with a lifetime of 595 ps. The spin sensitivity of M-edge XANES allows the 3 (d,d) state to be distinguished from a potential 1 (d,d) state, as shown by charge transfer multiplet simulations and comparison to triplet nickel(II) oxide. Vibrational cooling of the hot triplet state occurs over tens of ps, with minimal change in the electronic structure of the nickel(II) center. No evidence of an LMCT or MLCT intermediate state is seen within the time resolution of the instrument, suggesting that if such a state exists in NiOEP it depopulates in <25 fs. Finally, this study demonstrates the ability of table highharmonic XUV sources to measure excited-state spin transitions in molecular transition metal complexes. 3

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Section: Transient M-edge Xanesmentioning

confidence: 99%

“…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]…”

Section: Introductionmentioning

confidence: 99%

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

2019

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“…To help resolve this dilemma, time‐dependent relaxation measurements are a promising way to elucidate the nature of the carriers and their lifetime. Hematite charge carrier dynamics have been studied extensively by transient absorption spectroscopy (TAS), 4D electron energy loss spectroscopy, and extreme ultra‐violet (XUV) spectroscopy . These different studies have shown relaxation dynamics in hematite spanning a wide range of timescales, with no consensus on the attribution of extracted time constants to specific processes such as charge carrier recombination, trapping, polaronic state formation, lattice expansion and cooling, or long lived holes on the surface …”

Section: Introductionmentioning

confidence: 99%

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

Kay

Fiegenbaum‐Raz

Müller

et al. 2019

Adv Funct Materials

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The charge carrier dynamics of epitaxial hematite films is studied by timeresolved microwave (TRMC) and time-resolved terahertz conductivity (TRTC). After excitation with above bandgap illumination, the TRTC signal decays within 3 ps, consistent with previous reports of charge carrier localization times in hematite. The TRMC measurements probe charge carrier dynamics at longer timescales, exhibiting biexponential decay with characteristic time constants of ≈20-50 ns and 1-2 µs. From the change in photoconductance, the effective carrier mobility is extracted, defined as the product of the charge carrier mobility and photogeneration yield, of differently doped (undoped, Ti, Sn, Zn) hematite films for excitation wavelengths of 355 and 532 nm. It is shown that, unlike in conventional semiconductors, donor doping of hematite dramatically increases the effective mobility of the photogenerated carriers. Furthermore, it is shown that all hematite films possess higher effective mobility for 355 nm excitation than for 532 nm excitation, although the time dependence of the photoconductance decay, or charge carrier lifetime, remains the same. These results provide an explanation for the wavelength dependent photoelectrochemical behavior of hematite photoelectrodes and suggest that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.

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“…However, it has some critical drawbacks that should be overcome to obtain a high h STH , including a short hole diffusion length, poor conductivity, and a large overpotential for water oxidation. 2,3 A multitude of strategies have been developed to improve the PEC performance of hematite by addressing these shortcomings. Making a nanostructure (especially one-dimensional, small-diameter nanorods) can reduce the electron-hole recombination by shortening the actual diffusion distance of charge carriers.…”

Section: Introductionmentioning

confidence: 99%

Activating the surface and bulk of hematite photoanodes to improve solar water splitting

et al. 2019

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A simple electrochemical activation treatment is proposed to improve effectively the photoelectrochemical performance of Nb,Sn co-doped hematite nanorods. The activation process involves an initial thrice cathodic scanning (reduction) and a subsequent thrice anodic scanning (oxidation), which modifies both the surface and bulk properties of the Nb,Sn:Fe 2 O 3 photoanode. First, it selectively removes the surface components to different extents endowing the hematite surface with fewer defects and richer Nb-O and Sn-O bonds and thus passivates the surface trap states. The surface passivation effect also enhances the photoelectrochemical stability of the photoanode. Finally, more Fe 2+ ions or oxygen vacancies are generated in the bulk of hematite to enhance its conductivity. As a result, the photocurrent density is increased by 62.3% from 1.88 to 3.05 mA cm À2 at 1.23 V RHE , the photocurrent onset potential shifts cathodically by $70 mV, and photoelectrochemical stability improves remarkably relative to the pristine photoanode under simulated sunlight (100 mW cm À2 ). Fig. 3 XPS spectra of the pristine and activated Nb,Sn:Fe 2 O 3 photoanode before and after surface etching ($5 nm), respectively. (a 0 and a 1 ) Fe 2p. (b 0 and b 1 ) O 1s. (c 0 and c 1 ) Nb 3d. (d 0 and d 1 ) Sn 3d.This journal is

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Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3

Cited by 206 publications

References 51 publications

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

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

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

Activating the surface and bulk of hematite photoanodes to improve solar water splitting

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