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Periodic Report Summary - NOBLETITANIA (Modification of titanium(IV) oxide with metal compounds for photocatalysis under visible light irradiation)

The NOBLETITANIA project aims to develop novel photocatalysts with high photoactivity under both ultraviolet (UV) and visible light irradiation (overall solar spectrum) and high stability (resistance to photocorrosion).

Though environmental application of semiconductor photocatalysis has become increasingly interest among scientists in recent years, wide range application is still limited by few reasons:

1. fast recombination of charge carriers (e-/h+),
2. absorption below 400 nm by the most active photocatalysts, e.g. titanium dioxide (TiO2) and
3. impossibility of water splitting by visible light active semiconductors with a narrow band-gap.

Doping of S, N, C, B (narrowing band-gap) or surface modification by coloured compounds (antenna) can be applied to improve photoactivity of wide band-gap semiconductor photocatalysts. Though doped titanias show photoactivity under visible light irradiation, dopants are often recombination centers in the crystalline lattice resulting in quantum yield decrease under UV irradiation. In this regard, surface modification seems to be more advisable, but special care must be taken in selection of modifiers.

Noble metals have proved to be excellent modifiers, because they accelerate the transfer of photoexcited electrons to substrates and due to their ability of photoabsorption of visible-light irradiation either by metal complexes fixed on semiconductor or by photoexcitation of localised surface plasmon resonance (LSPR) of nanoparticles (NPs) of noble metal.

In the presented proposal the research on three systems of semiconductor photocatalysis has been examined:

1. plasmonic activation of wide band-gap semiconductors, i.e. TiO2, tungsten trioxide (WO3), cerium oxide (CeO2) by surface modification with gold and silver NPs
2. adsorption of noble metal complexes (e.g. Ru) on wide band-gap semiconductors as photocentres with simultaneous deposition of gold and silver (Au/Ag) NPs (as electron sink or additional antenna)
3. preparation of coupled semiconductors of narrow band-gaps.

For three examined systems the selection of semiconductor and noble metal NPs with appropriate surface properties was the most important in preparation of photoactive materials with high photoactivity and stability.

We have found that, in contrast with catalytic active gold nanoparticles, where nano-sized gold is recommended, the polydispersity in the size and shape of deposited gold on a semiconducting support is beneficial for photocatalytic activity under visible light irradiation. It is thought that wide size/shape distribution of gold nanoparticles and thus ability of absorption of light in a wide wavelengths range is responsible for the high level of photoactivity. However these photocatalysts lose their activity due to formation of large gold deposits (not NPs) as a result of gold aggregation. In this regard we have prepared novel photocatalysts with large gold NPs (high photoactivity) surrounded by fine titania NPs (to hinder gold aggregation) by modified photodeposition method. These new photocatalysts show high stability (inhibition of gold aggregation) and higher level of photoactivity due to larger interface between gold and titania and enhancement of plasmonic field.

These results allow to find the interaction between nano-blocks and to give guidance for future development of solar heterogeneous photocatalysts which might be applied in environmental purification and generation of solar energy (solar electricity and solar fuel).