A novel series of photocatalysts with an ion-exchangeable layered structure of niobate

Domen, Kazunari; Yoshimura, Juji; Sekine, Toshimori; Tanaka, Akira; Onishi, Takaharu

doi:10.1007/bf00765319

Cited by 178 publications

(131 citation statements)

References 7 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.…”

Section: Introductionsupporting

confidence: 57%

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: Introductionsupporting

confidence: 57%

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

2008

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“…Correcting for a ω 4 scattering component revealed that the nanosheets have an absorption edge at 3.53 ( 0.05 eV (∼350 nm). This represents an increase in the band gap from both KCa 2 Nb 3 O 10 (E g ) 3.35 eV) 1 and the parent bulk (TBA,H)Ca 2 Nb 3 O 10 (E g ) 3.33 eV) 32 and indicates a relatively localized excited state in the exfoliated sheet. A two-dimensional quantum confinement argument is also supported by the sheet thickness (1.16 nm) and may be related to the reduced coordination of corner-shared NbO 6 octahedra at the sheet surface.…”

Section: Steady-state Characterizationmentioning

confidence: 92%

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

et al. 2008

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We have characterized the initial charge carrier trapping and recombination processes in suspensions of exfoliated calcium niobate nanosheets derived from the Dion Jacobsen phase KCa 2 Nb 3 O 10 . Niobate nanosheets have previously been found to catalyze hydrogen generation in photochemical water-splitting reactions. Photogenerated charge carriers relaxed with second-order kinetics on a sub-nanosecond time scale that depended on the nanosheet size. Methanol was used as a sacrificial electron donor to separate the electron and hole dynamics, and the photo-oxidation dynamics was found to directly compete with electron-hole recombination. Absorption spectra for trapped carriers were determined and used to separately analyze the trapped electron and hole dynamics in [H 1-x Ca 2 Nb 3 O 10 ]x-(x ) 0.15-0.20) sheets in water. Surface modification by 3-aminopropyl(trimethoxy)silane was found to quench trapped carrier absorptions, resulting in an observed optical gain.

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“…25 The photolysis of HI at sensitized LMOS particles is shown schematically in Figure 3. The layered architecture of an LMOS, such as K 4 Nb 6 O 17 , allows for selective internal platinization 26 such that the hydrogen evolving Pt catalyst sites are segregated by charge exclusion from the triiodide that accumulates in solution.…”

Section: Visible Light Water Splittingmentioning

confidence: 99%

Visible Light Water Splitting Using Dye-Sensitized Oxide Semiconductors

et al. 2009

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R esearchers are intensively investigating photochemical water splitting as a means of converting solar to chemical energy in the form of fuels. Hydrogen is a key solar fuel because it can be used directly in combustion engines or fuel cells, or combined catalytically with CO 2 to make carbon containing fuels. Different approaches to solar water splitting include semiconductor particles as photocatalysts and photoelectrodes, molecular donor-acceptor systems linked to catalysts for hydrogen and oxygen evolution, and photovoltaic cells coupled directly or indirectly to electrocatalysts.Despite several decades of research, solar hydrogen generation is efficient only in systems that use expensive photovoltaic cells to power water electrolysis. Direct photocatalytic water splitting is a challenging problem because the reaction is thermodynamically uphill. Light absorption results in the formation of energetic charge-separated states in both molecular donor-acceptor systems and semiconductor particles. Unfortunately, energetically favorable charge recombination reactions tend to be much faster than the slow multielectron processes of water oxidation and reduction. Consequently, visible light water splitting has only recently been achieved in semiconductor-based photocatalytic systems and remains an inefficient process.This Account describes our approach to two problems in solar water splitting: the organization of molecules into assemblies that promote long-lived charge separation, and catalysis of the electrolysis reactions, in particular the four-electron oxidation of water. The building blocks of our artificial photosynthetic systems are wide band gap semiconductor particles, photosensitizer and electron relay molecules, and nanoparticle catalysts. We intercalate layered metal oxide semiconductors with metal nanoparticles. These intercalation compounds, when sensitized with [Ru(bpy) 3 ] 2+ derivatives, catalyze the photoproduction of hydrogen from sacrificial electron donors (EDTA 2-) or non-sacrificial donors (I -). Through exfoliation of layered metal oxide semiconductors, we construct multilayer electron donor-acceptor thin films or sensitized colloids in which individual nanosheets mediate light-driven electron transfer reactions. When sensitizer molecules are "wired" to IrO 2 · nH 2 O nanoparticles, a dye-sensitized TiO 2 electrode becomes the photoanode of a water-splitting photoelectrochemical cell.Although this system is an interesting proof-of-concept, the performance of these cells is still poor (∼1% quantum yield) and the dye photodegrades rapidly. We can understand the quantum efficiency and degradation in terms of competing kinetic pathways for water oxidation, back electron transfer, and decomposition of the oxidized dye molecules. Laser flash photolysis experiments allow us to measure these competing rates and, in principle, to improve the performance of the cell by changing the architecture of the electron transfer chain.

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A novel series of photocatalysts with an ion-exchangeable layered structure of niobate

Cited by 178 publications

References 7 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

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

Visible Light Water Splitting Using Dye-Sensitized Oxide Semiconductors

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