Parameters affecting electron transfer dynamics from semiconductors to molecular catalysts for the photochemical reduction of protons
James Durrant a, Anna Reynal a, Manuela Gross b, Erwin Reisner b, Fezile Lakadamyali b
a Imperial College London, South Kensington, London,, United Kingdom
b University of Cambridge, The Old Schools, Trinity Ln, Cambridge CB2 1TN, UK, Cambridge, United Kingdom
Poster, Anna Reynal, 009
Publication date: 31st March 2013

The photochemical reduction of water into H2 offers the possibility of harvesting sunlight and storing this energy in the form of chemical bonds. Most of such approaches combine a photoactive element to harvest solar energy with a catalytic component to catalyze the two electron reduction of protons to molecular hydrogen.[1]

In this conference, we aim to present the kinetics of electron transfer processes that take place in three different molecule-functionalized systems for the photochemical H2 evolution, with special emphasis on the parameters influencing the efficiency of the electron transfer from the TiO2 to a molecular catalyst.[2,3] All three approaches employ nanocrystalline TiO2 films functionalized by a molecular cobalt electrocatalyst (CoP) for the production of hydrogen in aqueous solution buffered at pH 7 under N2. The first approach is based upon these materials alone, excluding any sacrificial electron donor, and therefore relying upon the ability of TiO2 holes to oxidize water to molecular oxygen as the source of electrons for proton reduction. In the second approach, a sacrificial electron donor (triethanolamine) is used as a hole scavenger to remove photogenerated TiO2 holes. Finally, the third system involves the functionalisation of the TiO2 nanoparticles with a molecular ruthenium dye (RuP) and a cobalt catalyst anchored onto the surface, again using TEOA to regenerate the photosensitizer.

Our transient absorption spectroscopy (TAS) measurements indicate that, in the absence of a hole scavenger, the use of high excitation light densities increase the electron/hole recombination rate and decrease the electron transfer efficiency to the molecular catalyst. Nevertheless, removing the photoholes from the semiconductor by adding a sacrificial agent or a photosensitizer can attenuate this recombination reaction. The catalyst loading also plays a crucial role in the H2 evolution efficiency, since the electron transfer from the semiconductor to the catalyst shows a non-linear dependence upon surface coverage. We have assigned this behavior to the two-electron transfer reactions associated with H2 evolution and their thermodynamic properties. We believe that the control of electron transfer kinetics to reduce the molecular catalyst is specially important in the design of systems with two co-attached molecules such as a molecular catalyst and a photosensitizer.

Scheme 1. Illustration of the three experimental systems studied herein, and the functional processes underlying their function comprising (a) a TiO2 functionalized by a CoP proton reduction catalyst in water under UV excitation, with, (b), the addition of a sacrificial electron donor, and (c) the addition of a photosensitizer dye to enable visible light activity. The two reduction potentials of the catalyst (CoIII/II and CoII/I) relevant for the H2 evolution reaction are shown. Forward electron transfer processes are represented with solid black arrows, while re-combination reactions are drawn with dashed grey arrows.
[1] Tran, P. D.; Wong, L. H.; Barber, J.; Loo, J. S. C. Recent advances in hybrid photocatalysts for solar fuel production. Energy Env. Sci. 2012, 5, 5902-5918. [2] Lakadamyali, F.; Reynal, A.; Kato, M.; Durrant, J. R.; Reisner, E. Electron Transfer in Dye-Sensitised Semiconductors Modified with Molecular Cobalt Catalysts: Photoreduction of Aqueous Protons. E. Chem. Eur. J. 2012, 18, 15464-15475. [3] Reynal, A.; Lakadamyali, F.; Gross, M.; Reisner, E.; Durrant, J. R. Parameters affecting electron transfer dynamics from semiconductors to molecular catalysts for the photochemical reduction of protons. Manuscript in preparation.
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