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  • Mid-Term Report Summary - APHOTOREACTOR (Entirely Self-organized: Arrayed Single-Particle-in-a-Cavity Reactors for Highly Efficient and Selective Catalytic/Photocatalytic Energy Conversion and Solar Light Reaction Engineering)
ERC

APHOTOREACTOR Report Summary

Project ID: 340511
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term Report Summary - APHOTOREACTOR (Entirely Self-organized: Arrayed Single-Particle-in-a-Cavity Reactors for Highly Efficient and Selective Catalytic/Photocatalytic Energy Conversion and Solar Light Reaction Engineering)

A photocatalytic process is based on interaction of light with a semiconductor catalyst that is in contact with the environment. The absorption of light by the semiconductor leads to separation of charge carriers (electrons and holes) that can diffuse through the catalyst, reach the solid/fluid interface and react with suitable species in the environment. From a thermodynamic point of view, the main prerequisite to enable a photocatalytic reaction is that the energy of photogenerated electrons and holes are situated favorably for a desired redox process.
The most investigated photocatalytic material is titanium dioxide (TiO2), owing mainly to its chemical stability, low cost and intrinsic energetic properties (i.e. band gap and band edge positions). TiO2-based photocatalysis has extremely wide application to tackle several contemporary global challenges such as the ecologic conversion and storage of solar energy. TiO2 can catalyze the generation of H2, the “clean fuel” of the future, from water under irradiation with light of suitable wavelengths. However, obstacles to this are the bandgap of TiO2 that allows for absorption of only UV light (which represents only a minor portion – 5% – of the solar spectrum at the sea level), charge carrier recombination in the semiconductor, sluggish kinetic of charge transfer towards the reactants in the environment, catalyst/cocatalyst deactivation and poisoning.
Thus, essential is a precise engineering of the photocatalyst (TiO2 but also metal/TiO2 combinations) on several levels to design a (composite) material with physicochemical and morphological features that will enable efficient solar energy-into-hydrogen conversion. In this context, a strategy to enhance TiO2 photocatalytic efficiency implies the surface modification of the catalyst with cocatalytic materials such as noble metal nanoparticles (Pt, Au, Pd). The noble metal facilitates the extraction of charge carriers and mediates their transfer to the reactants. This leads overall to a more efficient light management as parasitic processes (e.g., charge recombination) are thereby limited.
In the frame of this project we have developed an entirely novel photocatalytic platform that consists of functional self organized metal oxide semiconductor, namely TiO2 in the form of highly-defined one-dimensional nanostructures such as nanotube layers. This platform is fabricated by an innovative electrochemical anodization pathway of a simple piece of Ti metal carried out under self-organizing conditions: a finely tuned equilibrium between the metal oxidation and the dissolution of the formed oxide leads to the formation of a layer of vertically aligned TiO2 nanotube arrays coating the Ti metal surface. This newly developed method forms tubes with a short aspect-ratio (tube length ca. 150 nm, tube diameter ca. 80 nm), almost ideally-hexagonally packed and thus showing an unprecedented level of self-ordering.
Such a defined and ordered geometry is crucial for achieving fully controllable metal cocatalyst decoration by metal dewetting on TiO2 nanotube surfaces. Thermal dewetting of metal originates from the fact that a thin metal film is unstable when heated up to certain temperatures, and hence it splits up into metal particles. This process, when carried out on a flat substrate, initiates at random location and thus eventually leads to metal islands and beads of various size and spacing. We demonstrated that the use of periodic topography such as that of the highly regular TiO2 nanotubes as substrate for sputter-coating thin metal cocatalyst films allows for achieving long range ordered metal dewetting. This is a first example of synergistic overlay of self-ordering principles. By such an approach we reached a geometric control at the nanoscale, and fabricated metal/TiO2 systems where the metal can be selectively deposited in precise amounts and in the form of particles of adjustable size, density and placement at the TiO2 surface.
These defined metal/TiO2 structures have shown improved H2 generation efficiency compared to photocatalyst fabricated by conventional approaches since each individual metal/nanotube cavity represents an ideal “reaction vessel geometry” for UV-based photocatalysis. The depth of each nanotube cavity is in the order of the UV light penetration depth in titania and thus grants an optimized light absorption/management. The thickness of the TiO2 tube walls (ca. 10 nm) is comparable to solid-state diffusion length of holes, and allows for their efficient transfer to the environment. The virtual volume of the reaction phase in each cavity matches well the typical diffusion lengths of radicals generated in the liquid phase (by reaction of the charge carriers with water and dissolved oxygen).
A further example of overlaid self-ordering principles is the combined dewetting-dealloying of metal films. We developed a strategy based on the deposition (on the nanotubes) of e.g. Au or Pt in form of alloys with less noble elements, e.g., Ag, Al. A following (electro)chemical treatment has been optimized to achieve selective dissolution/removal of only the less noble (sacrificial) element leaving being porous noble metal nanoparticles decorating the TiO2 nanotube arrays. In more general terms, this strategy allows for defined placement of porous noble metal decorations at the TiO2 nanotubes surface (but potentially at other catalytic surfaces too) and these porous noble metal particles show large interfacial surface with the oxide nanotubes and with the fluid reaction environment. Owing to this, the photocatalytic structures have shown improved efficiency per unit amount of cocatalyst compared to classic noble metal deposition techniques (e.g., impregnation, photodeposition) – in other words, we identified and developed a cost effective fabrication approach based on sequential dewetting-dealloying pathway for efficient photocatalysis in spite of using only minimized amounts of expensive noble metals.
There is still an enormous potential regarding the tailoring of TiO2 nanotube structure and physicochemical properties, and we anticipate that electrochemical anodization and metal dewetting processes will even provide a higher degree of designed functionalities. We predict that additional self-ordering processes (e.g. spinodal decomposition, site-selective functionalization, annealing post-treatments in controlled/reactive atmosphere) are worth investigating pathways that can be interlaced to form advanced hierarchical assemblies, such as core-shell and nano-sponge structures, not only for efficient photocatalysis, but also for sensors, plasmonic platforms, magnetic devices, etc..

Reported by

FRIEDRICH-ALEXANDER-UNIVERSITAT ERLANGEN NURNBERG
Germany
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