Interfacial chemistry and electronic structure of quantum-confined oxide heteronanostructures for solar water splitting

Lionel Vayssieres^a

^aIRCRE-Xian Jiaotong University, 28 Xianning West Rd, Xian, 710049, China

Proceedings of International Conference on New Advances in Materials Research for Solar Fuels Production (SolarFuel14)

Montréal, Canada, 2014 June 25th - 26th

Organizer: Thomas Hamann

Invited Speaker, Lionel Vayssieres, presentation 022
Publication date: 16th April 2014

The major innovation of this work is the ability to develop novel metal oxide semiconductor heteronanostructures consisting of highly pure quantum dots and rods entirely made from aqueous chemistry without surfactant and at low temperature with controlled surface chemistry and having intermediate bands for high visible light energy conversion and highly quantized band structure for bandgap and band edge engineering. Such unique characteristics, combined with in-depth investigation of their electronic structure, optical, structural and transport properties provide substantial advances in the fundamental understanding of the energetics and structure-property relationships as well as for efficiency optimization of nanodevices. For instance, quantum dot-sensitized iron oxide quantum rods which by intermediate band effects enable a full visible light absorption profile while still being stable against photo-corrosion for efficient and low cost solar hydrogen generation by direct water splitting at neutral pH were fabricated allowing therefore the use of the largest free natural resource on Earth, that is seawater, as unique electrolyte.Moreover, the effect of size on the surface chemistry of metal oxides was demonstrated by the reversal of the surface acidity from acidic to neutral to basic by changing the size from 12 to 7.5 to 3.5 nm, respectively. The size dependence (i.e. 2-200nm) on the orbital character of the conduction band of anatase TiO₂ nanocrystals was investigated and provided the first experimental evidence supported by calculation of such an effect. The appearance and predominance of unoccupied states derived from the hybridization of antibonding Ti4s and O2p band is observed when the nanoparticle size approaches the exciton radius (ca 1 nm). Such extended new hydridization compared to narrow directional 3d is of great importance not only from the fundamental point of view of demonstrating a confinement effect in anatase TiO₂ nanocrystals but also for device applications. In fact, the presence of s-hybridized bandgap states may improve not only the overall carrier dynamics and electron mobility (i.e. by reducing the electron effective mass and electron-hole recombination) but should also contribute to a significant improvement of the overall efficiency of dye-sensitized solar cells by controlling the interfacial electron transfer and reducing the back reaction. Finally, we also studied the interfacial electronic structure of various TCO related interfaces and the origin of the widely reported order-of-magnitude photoanodic current increase upon short high temperature annealing as well as the experimental observation of spontaneous electron enrichment of metal d orbitals in new transition metal oxide heteronanostructures.

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