Ultrafast Carrier Dynamics in Exfoliated and Functionalized Calcium Niobate Nanosheets in Water and Methanol

Carroll, Elizabeth C.; Compton, Owen C.; Madsen, Dorte; Osterloh, Frank E.; Larsen, Delmar S.

doi:10.1021/jp077427d

Cited by 73 publications

(72 citation statements)

References 58 publications

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“…5,12,[24][25][26][27][28] As we reported before, dispersions of individual nanosheets are able to evolve H 2 from water under UV light, but no oxygen. [29][30][31] We now find that hydrogen evolution with the niobate sheets is coupled with stoichiometric formation of peroxide, according to eq 2. This peroxide does not form via reduction of O 2 (eq 3), since irradiation experiments conducted in the presence of O 2 mostly lead to free H 2 O 2 and only to small amounts of coordinated peroxide.…”

Section: Introductionmentioning

confidence: 70%

Niobate Nanosheets as Catalysts for Photochemical Water Splitting into Hydrogen and Hydrogen Peroxide

2008

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Tetrabutylammonium-stabilized Ca 2 Nb 3 O 10 nanosheets catalyze photochemical splitting of water into hydrogen and hydrogen peroxide under UV irradiation. The peroxide forms on the surface of the catalyst and adsorbs to it, fully deactivating the catalyst after 48 h of irradiation. For the Pt-modified niobate, complete deactivation occurs within 24 h due to higher H 2 evolution rate with this system. The peroxide was identified via Raman and IR spectroscopy. In vibrational spectra, the irradiated niobate exhibits bands at 580, 778, and 940 cm -1 , which suggest that the peroxide is present as a side-on ligand coordinated to the Nb 5+ ions on the nanosheet surface. The same species can be formed by treating the nanosheets with 30% aqueous H 2 O 2 . If the catalyst is irradiated in H 2 18 O, the bands shift to lower energy, indicating that the peroxide species is formed from water. Room-temperature storage of the nanosheets in H 2 18 O also leads to isotopic exchange of all Nb-O atoms over the course of 24 h. Peroxide amounts measured by titration of the catalyst with o-tolidine after 6, 12, and 24 h of irradiation form in a 1:1 stoichiometry with hydrogen. Infrared data shows that the peroxide is partially desorbed by evacuating the catalyst and by purging with argon gas. Under catalytic conditions, this treatment restores up to 60% of the original activity. For the H 2 O 2 -treated catalyst, near quantitative H 2 O 2 desorption occurs within 20 min at 450°C. Separate irradiation experiments of the catalyst in the presence of air reveal that H 2 evolution is diminished and that O 2 uptake occurs from the atmosphere. Most of the hydrogen peroxide formed under these conditions is subsequently detected in the supernatant and only small amounts on the catalyst. NMR spectra show that the tetrabutylammonium cations are decomposed. These findings suggest that there are two distinct pathways for peroxide formation, one involving one-electron reduction of O 2 and one involving two-electron reduction of water (in the absence of O 2 ).

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Section: Introductionmentioning

confidence: 70%

Niobate Nanosheets as Catalysts for Photochemical Water Splitting into Hydrogen and Hydrogen Peroxide

2008

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“…On the basis of transient absorption spectroscopy, photogenerated charge carriers in the sheets recombine on a subnanosecond time scale following second-order kinetics. 16 In extension of our previous work on Ca 2 2 ) that are supported by γ-aminoalkyltrimethoxysilanes as organic linkers. Platinum nanoparticles are commonly employed as a cocatalysts for water reduction, 5 and IrO 2 is one of the most active water oxidation catalysts.…”

Section: Introductionmentioning

confidence: 87%

A Building Block Approach to Photochemical Water-Splitting Catalysts Based on Layered Niobate Nanosheets

et al. 2008

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We present a modular approach to the synthesis of nanostructured catalysts for photochemical splitting of water into hydrogen and oxygen. The catalysts are built from exfoliated, semiconducting niobate nanosheets derived from the layered perovskite HCa 2 Nb 3 O 10 . The latter is a catalyst for photochemical evolution of hydrogen from water under UV irradiation. After chemical modification with 3-aminopropyltrimethoxysilane (APS), IrO 2 or Pt particles can be attached to the nanosheets to produce various two-component nanostructures that were fully characterized with transmission electron microscopy and ultraviolet and infrared spectroscopy. Cyclic voltammetry was used to determine the onset potentials for O 2 and H 2 evolution. At pH ) 14, the observed values are in the range +0.61 to +1.24 V (NHE, water oxidation) and -1.36 to -1.62 V (NHE, water reduction). Under UV irradiation, all catalysts evolve hydrogen from water without any sign of deactivation for 5 h. The highest quantum efficiency of 3.49% is observed for a structure with Pt directly grown onto the nanosheets. No O 2 is evolved, which we attribute to the adsorption of O 2 to the catalyst surface. For Pt-[HCa 2 Nb 3 O 10 ], this process starts to shut down H 2 evolution after 9 h of constant irradiation, but the activity can be restored to >60% by evacuating the catalyst dispersion and purging it with Ar. Catalysts assembled from preformed citrate-coated Pt nanoparticles are slightly less active for H 2 evolution and so are catalysts that use the linker aminoethyl-aminoundecanetrimethoxysilane (AEAUS) instead of APS. The activity of IrO 2 -APS-[Ca 2 Nb 3 O 10 ] is lowest among two component catalysts, near the activities of the pure or APSmodified nanosheets. On the basis of XPS data, IrO 2 in this catalyst undergoes photochemical reduction to Ir(0) upon UV irradiation.

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“…The dispersed-probe transient absorption setup was constructed from an amplified Ti:sapphire laser system (Spectra Physics Spitfire Pro + Tsunami) operating at 1 kHz, which produced 2.25 mJ pulses of 800 nm fundamental output with a 40 fs (full width at half-maximum) duration. 49 The 800 nm fundamental pulse train from the amplifier was split into multiple paths. One path generated the dispersed white-light probe continuum (440−750 nm) by focusing the laser pulses into a slowly translating CaF 2 crystal.…”

Section: ■ Materials and Methodsmentioning

confidence: 99%

Femtosecond Photodynamics of the Red/Green Cyanobacteriochrome NpR6012g4 from Nostoc punctiforme. 1. Forward Dynamics

et al. 2012

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Phytochromes are well-known red/far-red photosensory proteins that utilize the photoisomerization of a linear tetrapyrrole (bilin) chromophore to detect the ratio of red to far-red light. Cyanobacteriochromes (CBCRs) are related photosensory proteins with a bilin-binding GAF domain, but much more diverse spectral sensitivity, with five recognized subfamilies of CBCRs described to date. The mechanisms that underlie this spectral diversity have not yet been fully elucidated. One of the main CBCR subfamilies photoconverts between a red-absorbing ground state, like the familiar P(r) state of phytochromes, and a green-absorbing photoproduct (P(g)). Here, we examine the ultrafast forward photodynamics of the red/green CBCR NpR6012g4 from the NpR6012 locus of the nitrogen-fixing cyanobacterium Nostoc punctiforme. Using transient absorption spectroscopy with broadband detection and multicomponent global analysis, we observed multiphasic excited-state dynamics that induces the forward reaction (red-absorbing to green-absorbing), which we interpret as arising from ground-state heterogeneity. Excited-state decays with lifetimes of 55 and 345 ps generate the primary photoproduct (Lumi-R), and the fastest decay (5 ps) did not produce Lumi-R. Although the photoinduced kinetics of Npr6012g4 is comparable with that of the Cph1 phytochrome isolated from Synechocystis cyanobacteria, NpR6012g4 exhibits a ≥2-3-fold higher photochemical quantum yield. Understanding the structural basis of this enhanced quantum yield may prove to be useful in increasing the photochemical efficiency of other bilin-based photosensors.

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Ultrafast Carrier Dynamics in Exfoliated and Functionalized Calcium Niobate Nanosheets in Water and Methanol

Cited by 73 publications

References 58 publications

Niobate Nanosheets as Catalysts for Photochemical Water Splitting into Hydrogen and Hydrogen Peroxide

Niobate Nanosheets as Catalysts for Photochemical Water Splitting into Hydrogen and Hydrogen Peroxide

A Building Block Approach to Photochemical Water-Splitting Catalysts Based on Layered Niobate Nanosheets

Femtosecond Photodynamics of the Red/Green Cyanobacteriochrome NpR6012g4 from Nostoc punctiforme. 1. Forward Dynamics

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