Open-ended inorganic nanotubes: band-gap and bands-engineering of a photo-catalyst

Winton teatime discussion on Photocatalysis for converting sunlight into industrially viable forms of energy or fuels

A discussion hosted by Prof David MacKay and Dr Gilberto Teobaldi. Dr Teobaldi is based at the Stephenson Institute for Renewable Energy at the University of Liverpool.

Link to presentation.

Abstract: As the costs of energy production increase, due to both the limitation of resources and the impact of an oil-fuelled economy on the climate, the significance of materials and technologies capable of converting sunlight into industrially viable forms such as electrical power or fuels has been steadily growing. Crucial to this energy conversion is the presence of photo-catalysts (PCs) i.e. substances capable of generating, upon light absorption, highly reactive excited electron-hole (e*-h) pairs which may eventually transfer excited state energy or, following their separation, enter an electric circuit or being transferred to reactants. These processes can then be used to produce electrical current, fuels and chemical feedstocks, to decompose pollutants, and even for viruses or bacteria disinfection.

Together with costs and scalability considerations, the development of a viable PC for a given reaction rests critically on good adsorption properties in the visible part of the spectrum (which the sun radiates for free), appropriate energy-alignment of the PC and reactant electronic states, and favourable kinetics for both e*-h separation and their transfer to reactants. Although it is recognised that all these factors are governed by the instantaneous atomic composition and structure of the PC-reactant(s) interfaces in a given medium, the current understanding of the atomistic requirements to maximise the efficiency of a given PC for a given reaction is far from exhaustive. As a result, great efforts have started to be directed towards atomically-and time-resolved characterization of PCs and their interfaces with reactants in different media. Improved atomistic understanding of PCs would greatly expedite the development of novel photo-catalytic solutions.

The experimental challenges (and costs) in time- and atom-resolved characterization of photo-catalytic interfaces on the one hand, and improved accuracy-viability compromises of first principles simulation methods on the other, have made modelling of PCs and their interfaces an important component in fundamental research in photo-catalysis. This is because modelling can relatively inexpensively access the atomic and electronic factors underpinning the (mal-)functioning of PCs. Furthermore, as presented and discussed here, modelling can be also used to highlight elements hardly accessible to experimental analysis, which could be used to develop novel photo-catalytic strategies.

Targeting optimisation of the initial electron-hole separation and their eventual transfer to reactants, I will report the marked effects that permanent polarisations in photo-active open-ended inorganic nanotubes generate on the nanotubes electronic structure, and ensuing redox chemistry towards energy- and environmental-relevant reactants. Finally, the real-space separation of the nanotube valence-band and conduction-band on different side of the nanotube cavity, and its dependence on the nanotube core and periphery regions, will be presented and discussed in view of the generality of these findings and their possible use in developing novel photo-catalytic strategies.