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Matthias Graf1 2 Dirk Jalas2 Etienne Blandre2 Alexander Petrov2 3 Joerg Weissmueller4 1 Manfred Eich2 1

1, Helmholtz-Zentrum Geesthacht, Geesthacht, , Germany
2, Hamburg University of Technology, Hamburg, , Germany
3, ITMO University, St. Petersburg, , Russian Federation
4, Hamburg University of Technology, Hamburg, , Germany

Conventionally, semiconductors are employed for photocatalysis. An incident photon creates an electron-hole pair which then proceeds to engage in the desired chemical reaction. Inherent to this approach is the fact that only photons above the bandgap energy can be utilized. This has the drawback that photocatalytic highly active and stable materials such as titania can convert UV-photons into carriers. Thus, such photocatalysts cannot utilize the solar spectrum effectively.
It has been shown that the photocatalytic active wavelength spectrum of a semiconductor can be expanded by bringing it in contact with gold nanoparticles. These particles can be designed such that photons with energy below the semiconductor bandgap are absorbed resonantly. This process can create hot electrons which can be injected into the semiconductor’s conduction band and then take part in the catalytic reaction. While this approach is very promising, it has two drawbacks: First, the metal particles must be close to the semiconductor surface in order to inject electrons before these either lose their energy by electron-electron scattering or recombine with holes. Only those carriers which are successfully injected into the semiconductor and that reach its surface can take part in the catalytic reaction. This calls for short carrier diffusion paths, i.e. an essentially thin absorber layers that absorb a large fraction of the incoming light. Second, the nanoparticles absorb only at their resonance frequencies and thus the optical bandwidth of the system remains limited.
We will present a novel approach to mitigate both drawbacks. We replace the gold nanoparticles on the semiconductor surface with a nanoporous gold (NPG) network with extremely high specific surface area that is conformally coated with a thin layer of semiconductor such as titania. NPG represents a broad-band absorber (with tunable absorption under applied electric fields) and a surfactant-free surface. NPG manufacturing creates curved surfaces with a high number of low-coordinated atoms that reside far from their crystallographic equilibrium. Apart from that, residual elements are also currently discussed as possible origins for the found high catalytic activity. Consequently, we discuss to perspectively invert the classical approach with few NPs over a semiconductor electrode, such that a NPG carrier with homogeneous semiconductor coverage is envisaged as potentially more efficient water splitting electrode. Based on theoretical calculations (of hot electron mean free path and carrier insertion into a titania layer) we discuss structural parameters for water splitting electrodes to be introduced into synthesis approaches (involving thin film dealloying and atomic layer deposition) as well as possible methods for a direct prove of hot-electron (or hot hole) injection into the semiconductor (and the active electrolyte species) and the efficient utilization in electro-chemical conversion.

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