Videoconference

10.12.2021

Prof. Dr. Hicham Idriss

Electron transfer reactions in the photocatalytic hydrogen production rely on the presence of metals of cluster or nanoparticle nature dispersed on top of a semiconductor.  The complexity of multi-component photo-catalysts hinders accurate measurements dictating the use of simplified methods.  In order to explore part of this complex kinetics, H2 production rates of an electron donor, such as ethanol, over Au clusters with different sizes and coverage deposited on single crystal rutile TiO2(110) were studied by scanning tunneling microscopy, online mass spectrometry and complemented by femto second pump probe spectroscopy.  It was also found that there is a non-linear increase of the H2 production rate with increasing gold coverage.  The key determining factor appears to be the Au inter-particle distance.  Increasing this distance resulted in an increase in the normalized reaction rate.  These results are explained in terms of competition between particles for excited electrons to reduce H+ (of surface OH groups) to H2. 

A new atomic scale anisotropy in the photoreaction of surface carboxylates on rutile TiO2(110) induced by gold clusters will also be presented. STM and DFT+U were used to study this phenomenon by monitoring the photoreaction of the prototype hole-scavenger molecule over s-TiO2, Au9/s-TiO2, and Au9/r-TiO2. STM results show that adsorption displaces a large fraction of Au clusters from the terraces towards their edges. DFT calculations explains that Au9 clusters on stoichiometric TiO2 are distorted upon adsorption.  More importantly, the photoreaction rate appears to be directional.  A roughly three-fold higher depletion rate is observed in the [001] direction. This is linked to the anisotropic conduction of excited electrons along [001], with subsequent trapping by Au clusters leaving a higher concentration of holes and thus an increased decomposition rate.  To our knowledge this is the first-time atomic scale directionality is reported upon photo-excitation of an adsorbate on a semiconductor.