Photochemical Charge Transfer and Trapping at the Interface between an Organic Adlayer and an Oxide Semiconductor

Abstract

Identification of charge transfer and trapping sites on semiconducting oxide surfaces is of fundamental importance in furthering the field of heterogeneous photocatalysts. Using scanning tunneling microscopy, electron energy loss spectroscopy, and photodesorption, we observed both electron trapping and hole transfer events on the (110) surface of TiO2 rutile. UV irradiation of a saturated monolayer of trimethyl acetate (TMA) on TiO2(110) at room temperature resulted in hole transfer to the carboxylate group, followed by (CH3)3C-COO bond cleavage and desorption of CO2 and isobutene/isobutane. Hole transfer to TMA proceeded in the absence of a gas-phase electron scavenger (which is typically O2) because the accompanying photogenerated electrons could be trapped at the surface as Ti3+ cations bound to bridging OH groups. The extent of electron trapping, gauged by electron spectroscopy, correlated directly with the yields of photodesorption fragments resulting from the hole transfer channel. Charge at the Ti3+ sites was titrated in the dark via a reaction between O2 and the Ti3+-OH groups.

Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

“…23,55,243 The decay of excitonic modes is the main source for the emission of photons 131,132,133 and hot-electrons from the oxide surface. 244,245,246 Not surprisingly, the properties and abundance of lowcoordinated surface sites is therefore tightly connected with the optical and photochemical activity of oxide materials.…”

“…359 The molecule decomposes under UV-irradiation into trimethyl acetate and a surface hydroxyl group. The electrons released during the decomposition process become trapped at the Ti ion located below the OH-groups and produce an increasing Ti 3+ signal in electron-energy-loss spectra taken as a function of irradiation time.…”

Single electron states in polyethylene

“…Electron trapping (carrier localisation) processes are of fundamental interest in the chemistry and physics of a wide range of technologies, for example, in photocatalysis, 1 photovoltaics, 2,3 organic thin film transistors (OTFTs), lightemitting diodes (OLEDs), 4 and electrical insulation. 5 More generally, such trapping can profoundly influence electron transfer processes that are fundamental to energy transfer in nature.…”

Surface photochemistry probed by two-photon photoemission spectroscopy

“…Various spectroscopic techniques have been used to probe chemical reactions at interfaces. Since surface chemical reactions involve structural changes, many spectroscopic techniques, such as electron energy loss spectroscopy (EELS), [5][6][7][8] infrared (IR) spectroscopy, 9,10 Raman spectroscopy 9,11,12 and sum frequency generation (SFG) spectroscopy [12][13][14] have been widely used to probe vibrational structures and their changes in surface chemical reactions. In addition, scanning tunnelling microscopy (STM) has also been utilized to study surface chemical reactions by exploiting its ability to probe changes in electronic structure and large geometric changes of species reacting at a surface.…”