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Fundamentals Of Photocatalytic Splitting of Water

Final Report Summary - FOPS-WATER (Fundamentals Of Photocatalytic Splitting of Water)

The limited amount of fossil fuels and the increasing levels of CO2 emission are prompting the development of environmentally friendly energy sources. A well-established alternative source of energy is the sun. The energy of the sun can be stored in chemical energy: sunlight can be used to directly produce hydrogen. Hydrogen is one of the most promising energy carriers, especially since the discovery of Fujishima and Honda more than 45 years ago that hydrogen can be produced by photocatalytic splitting of water on a titanium dioxide (TiO2) electrode. Much research on photocatalytic water splitting is aimed at determining and improving the efficiency of the process. Remarkably little is known about the molecular mechanism of the process. The overall goal of this project was to provide molecular-level insights into the binding of water to TiO2, the structural dynamics of water at that interface, and the water splitting reaction on the photocatalyst TiO2.
To study solely the monolayer of water at the interface we used the surface sensitive spectroscopy technique sum-frequency generation (SFG) resulting in basically the vibrational spectrum of the molecules at the interface. As this is a second order nonlinear technique it is forbidden in centrosymmetric media (e.g. bulk water) making the technique sensitive to an interface where the symmetry is per definition broken. If one of the two incoming beams is an infrared beam in resonance with a molecular vibration, molecular sensitivity can be obtained. High time resolution, can be achieved by using femtosecond laser pulses and a pump-probe geometry: a femtosecond pump pulse initiates a process which will be probed at a certain time delay with the SFG probe pair.
After constructing the SFG setup in an empty lab, we focused on the first part of the project: unravelling the structure of water at the interface with the model catalyst titanium dioxide. Understanding the binding is the first step in understanding the reaction mechanism. We focused on thin films of anatase TiO2, thin films of amorphous TiO2, and single crystalline rutile TiO2. From a combination of the experimental results and molecular dynamics simulations, we could assign how the water molecules bind to the interface for these different cases. We showed that the anatase surface is hydroxylated with strong hydrogen bonded water molecules in the second layer and weakly hydrogen bonded groups sticking out from the TiO2 into the water. For the amorphous layer the water structure depends on the surface charge controlled by the pH of the aqueous solution.
Besides structure also dynamics might be an important factor in the reaction mechanism. Therefore, we determined in a pump-probe experiment how energy could dissipate out of the interfacial molecules, after exciting with IR light, resulting in timescales of several 100s of femtoseconds. Moreover, we mimic the sunlight to initiate the water dissociation by a laser pulse and follow subsequently in time the behavior of the molecules at the interface with SFG. We observe a change in the water response on sub 100 ps timescales as a result of the change in the surface charge. The response is dependent on the pH of the solution. These results show mechanistic insights in the first steps of the photocatalytic water dissociation process.
In parallel to the studies on TiO2 we studied simpler systems like calcium fluoride-water and SiO2-water interface. For the SiO2-water interface we found that the dissolution is an autocatalytic process. In a combined study with theoreticians we showed the presence of hydrophobic pockets on a macroscopic hydrophilic interface. Moreover, we studied the orientational distribution of water molecules at the water-air interface. In this way we developed the SFG method and enhanced our understanding of complex spectra.