High Temperature Anatase TiO 2 Stabilization in TiO 2 /Si Multilayer Structures
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High Temperature Anatase TiO2 Stabilization in TiO2/Si Multilayer Structures Helmut Karl, Martina Schaedler, Eugen Ruff and Bernd Stritzker Institut für Physik, Universität Augsburg, D-86135 Augsburg, Germany
ABSTRACT In this work TiO2/Si multilayer structures have been grown by sputtering. After rapid thermal annealing in pure inert gas or inert gas with oxygen atmosphere the multilayers have been investigated by high resolution transmission electron microscopy, µ-Raman and dynamic secondary ion mass spectrometry for their structure and anatase/rutile phase composition. It has been found that the photocatalytically more active anatase TiO2 is stabilized and that interdiffusion and chemical reaction processes were strongly hindered up to 1100°C annealing temperature in oxygen containing atmosphere. These findings are of particular importance since only at this high temperature simultaneous formation of embedded Si nanocrystallites can be achieved.
INTRODUCTION Direct splitting of water into hydrogen and oxygen by sunlight represents a very preferable renewable energy source. However materials criteria which have to be achieved simultaneously are highly challenging. The photo-electrochemically (PEC) active material should have an energy gap in the range of 1.5 to 2.5 eV, a high carrier mobility and high quantum yield. Moreover it must have a low price and has to be compatible to existing highvolume production technologies. In addition, the materials should be stable in aqueous electrolytes and withstand photo corrosion. In particular the band edge positions have to fit to the redox potentials of water. TiO2 meets some of these criteria and hence a promising material for photocatalytic water splitting. Nanostructured silicon has unique properties with respect to its bulk material counterpart. Dipole-forbidden transitions in bulk crystalline silicon become allowed for nanocrystalline clusters of silicon [1]. Additionally, the band gap increases due to quantum confinement when the size of the semiconductor nanocrystal becomes comparable or smaller to the exciton Bohr diameter. In parallel, the oscillator strength increases leading to an enhanced optical transition probability [1]. Apart from electron-hole pair generation in single nanocrystallites charge carrier excitation can take place across the wide energy band gap of the host material by absorption of two sub-band gap photons via embedded nanocrystallites. This concept has been theoretically modeled for intermediate band solar cells [2]. Promising PEC materials are oxides. However, no single oxide material meets all the physical and chemical requirements. Among the different approaches to tailor single oxide properties by doping, nanocomposite materials on the basis of titanium dioxide (TiO2) thin films embedded with optically active semiconductor [3] or metal nanocrystallites (NC´s) [4; 5] might allow independent and separate adaption and optimization of the properties.
EXPERIMENT In this work we focus on (TiO2/Si)10/TiO2 multilayers in which 10 (TiO2/Si) bila
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