The direct conversion of solar energy to hydrogen is considered as a promising method to produce carbon neutral hydrogen fuel. The mechanism of water splitting involves the chemical breakdown of water into hydrogen and oxygen by photonic energy. In 1972 Fujishima and Honda described a first photo electrochemical system capable of generating H2 and O2 using thin-films of TiO2.
Nowadays, the major goal is to improve the solar to hydrogen conversion efficiency by doping, coupling and modifying the catalysts. The selection rules of a suitable material are well established and the major challenges are now to maximize the quantum yield, to improve the electrons-holes mobility and to understand the interfacial reactions driving the water oxidation reduction.
Our research focuses on the mechanistic understanding of the water splitting at the interface between the catalyst and water. Using the surface specific vibrational spectroscopy sum frequency generation (SFG), our final goal is to describe the water splitting process by observing the oxygen-hydrogen bonds conformation along the reaction. In this spectroscopic method an infrared beam in resonance with a molecular vibration and a visible laser beam are combined at the interface from where the sum frequency is generated. As no SFG signal is generated in centrosymmetric media like bulk water, this method provides information about the water alignment at the interface. Moreover, the frequency of the resonance reports on the hydrogen bonding strength.
We have over the last month intensively studied the interface of the water splitting active photocatalyst Strontium titanate (SrTiO3). By studying the conformation of the water molecules at the strontium-water interface we have been able to describe the water orientation at the interface, the point of zero charge of the catalyst and to observe changes of the hydrogen bonding strength as a function of pH. This study shows a first chemical characterization of the Strontium-water interface by SFG spectroscopy and it is a first stepping stone to dynamically study the water splitting mechanism.
: Akira Fujishima and Kenichi Honda, Nature, 1972, vol 238, 7 July 1972, p. 37, 10.1038/238037a0
: Si Yin Tee, Khin Yin Win, Wee Siang Teo, Leng-Duei Koh, Shuhua Liu, Choon Peng Teng and Ming-Yong Han, Advanced Science, 2017, vol 4, 13 January 2017, p. 1600337, 10.1002/advs.201600337
: Alex G. Lambert, Paul B. Davies & David J. Neivandt, Applied Spectroscopy Reviews, 2007, vol 40, 2 February 2007, p. 103, 10.1081/ASR-200038326