Femtosecond diffuse reflectance spectroscopy of TiO2 powders

Colombo, D.; Bowman, R. M.

doi:10.1021/j100030a020

Cited by 139 publications

(124 citation statements)

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“…Bowman and coworkers first reported pump-probe diffuse-reflectance spectroscopic measurements of heterogeneous photocatalytic reactions [4][5][6][7]. Decay kinetics of electrons trapped in levels located below the CB bottom (electron trap = ET) or electrons in the CB after photoexcitation by femtosecond ultraviolet pump pulses was observed in the picosecond time region and analyzed.…”

Section: Direct Observation Of Electron-hole Recombinationmentioning

confidence: 99%

Titania Photocatalysis beyond Recombination: A Critical Review

2013

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Section: Direct Observation Of Electron-hole Recombinationmentioning

confidence: 99%

Titania Photocatalysis beyond Recombination: A Critical Review

2013

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“…The TRDR technique [34][35][36][37] has also been used to study the decay of the electron in TiO 2 powders. 38,39 The absorption of the electron was followed at 600 nm ( 600 ) 1200 M -1 cm -1 ). 32,33 The transient absorption spectrum in Figure 8 for D-TKP 102-A powder undergoing a laser pulse with λ ) 450 nm showed the existence of a maximum around 550 nm after 5 s of laser pulse and extending to the limit of the oscilloscope of 2 ms due to electron trapping on TiO 2 dehydrated shallow traps.…”

Section: -33mentioning

confidence: 99%

Synthesis, Characterization, and Photocatalytic Activities of Nanoparticulate N, S-Codoped TiO₂ Having Different Surface-to-Volume Ratios

et al. 2010

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An efficient, visible light active, N, S-codoped TiO 2 -based photocatalyst was prepared by reacting thiourea with nanoparticulate anatase TiO 2 . Commercial anatase powders were manually ground with thiourea and annealed at 400°C in two crucibles with different surface-to-volume ratios (S/V ) 20 and 1.5) to prepare two N, S-codoped TiO 2 materials. The differentiated aeration conditions during the catalyst annealing on the crucibles allowed for different amounts of O 2 to reach the catalyst surface. The first material, with S/V ) 20, herein referred to as D-TKP 102-A, was clear beige colored. The second material, with S/V ) 1.5, herein referred to as D-TKP 102-B, was darker and revealed a markedly lower efficiency in Escherichia coli inactivation. The D-TKP 102-A powder presented visible light absorption due to the nitrogen (N) and sulfur (S) doping. X-ray photoelectron spectroscopy signals for this catalyst were observed for N 1s peaks at binding energies of 399.2 and 400.7 eV due to interstitial N-doping or Ti-O-N species. The S 2p were due to SO 4 -2 signals with BE >168 eV and signals at 162.8 and 167.2 eV due to anionic and cationic S-doping, respectively. By fast kinetic spectroscopy, the decay of the electron induced by pulsed light at λ ) 450 nm (∼8 ns/laser pulse) was followed for the D-TKP 102-A catalyst. Undoped D-TKP 102 catalyst did not promote the electron in the visible range, and consequently no signal decay could be observed in the latter case. Low-temperature electron spin resonance measurements at 8 K provided evidence for electrons trapped in shallow traps, such as oxygen vacancies, V o , induced by N, S doped on D-TKP 102-A. The ESR measurements implementing the reactive scavenging with singlet oxygen scavenger, TMP-OH, revealed the production of singlet oxygen ( 1 O 2 ).

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“…3 Defects are thought to play a critical role in the trapping process by acting as recombination centers. For example, n-type doping of anatase reduces the photocatalytic activity of experimental samples, 4 and reduced anatase is less efficient than stoichiometric TiO 2 in the trapping of photogenerated holes.…”

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Polaronic trapping of electrons and holes by native defects in anataseTiO2

2009

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We have investigated the formation of native defects in anatase TiO 2 using density functional theory ͑DFT͒ modified with on-site Coulomb terms ͑DFT+ U͒ applied to both Ti d and O p states. Oxygen vacancies and titanium interstitials are found to be deep donors that trap two and four electrons, with transition levels that explain the two features seen in deep level transient spectroscopy experiments. Titanium vacancies are deep acceptors accommodating four holes. Self-trapping of both electrons and holes is also predicted. In all cases both donor and acceptor trap states correspond to strongly localized small polarons, in agreement with experimental EPR data. Variation in defect formation energies with stoichiometry explains the poor hole-trapping of reduced TiO 2 . DOI: 10.1103/PhysRevB.80.233102 PACS number͑s͒: 71.38.Ht, 71.55.Ϫi, 82.50.Ϫm In semiconductors used in photocatalysis and photovoltaics, the primary process is the light-induced production of charge carriers.1 Photocatalysis proceeds by photoexcitation of electrons to unoccupied bands, producing free electronhole pairs. These charge carriers then typically undergo trapping, followed by diffusion to the surface where they can initiate reactions of adsorbed molecules. Electron-hole recombination is often a major competing process, and it is highly desirable to promote charge separation and inhibit subsequent charge-carrier-recombination in order to improve the quantum efficiency of photocatalytic processes.TiO 2 has been widely studied as a photocatalyst for the degradation of environmental pollutants, 1 and for water splitting. 2 Nanocrystalline anatase TiO 2 is often used, since it is more photocatalytically active than the ground-state polymorph, rutile, and the diffusion pathways of photogenerated charge carriers to the surface are shortened, resulting in increased quantum efficiencies.Following photoexcitation, trapping of charge carriers occurs on a nanosecond time scale.3 Defects are thought to play a critical role in the trapping process by acting as recombination centers. For example, n-type doping of anatase reduces the photocatalytic activity of experimental samples, 4 and reduced anatase is less efficient than stoichiometric TiO 2 in the trapping of photogenerated holes.5 Understanding the interaction of defects with charge carriers is essential for the optimization of TiO 2 samples for photocatalysis. Electronand hole traps have both been observed in EPR spectra. [5][6][7][8] These data have been interpreted as charge localization at single atoms to give small polarons: hole trapping has been associated with O − sites; O O • , and electron trapping with Ti 3+ species; Ti Ti Ј . Additional evidence for charge localization at Ti 3+ states in n-type TiO 2 comes from core-level XPS shifts, and characteristic gap-state features observable in UPS spectra, with samples annealed to produce oxygen vacancies giving identical EPR signals to those seen following uv irradiation. Deep traps 0.9 and 0.5 eV below the conduction band edge have also bee...

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Femtosecond diffuse reflectance spectroscopy of TiO2 powders

Cited by 139 publications

References 7 publications

Titania Photocatalysis beyond Recombination: A Critical Review

Titania Photocatalysis beyond Recombination: A Critical Review

Synthesis, Characterization, and Photocatalytic Activities of Nanoparticulate N, S-Codoped TiO₂ Having Different Surface-to-Volume Ratios

Polaronic trapping of electrons and holes by native defects in anataseTiO2

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Femtosecond diffuse reflectance spectroscopy of TiO2 powders

Cited by 139 publications

References 7 publications

Titania Photocatalysis beyond Recombination: A Critical Review

Titania Photocatalysis beyond Recombination: A Critical Review

Synthesis, Characterization, and Photocatalytic Activities of Nanoparticulate N, S-Codoped TiO2 Having Different Surface-to-Volume Ratios

Polaronic trapping of electrons and holes by native defects in anataseTiO2

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Synthesis, Characterization, and Photocatalytic Activities of Nanoparticulate N, S-Codoped TiO₂ Having Different Surface-to-Volume Ratios