Abstract
Understanding how adsorbates influence polaron behavior is of fundamental importance in describing the catalytic properties of TiO2. Carboxylic acids adsorb readily at TiO2 surfaces, yet their influence on polaronic states is unknown. Using UV photoemission spectroscopy (UPS), two-photon photoemission spectroscopy (2PPE), and density functional theory (DFT) we show that dissociative adsorption of formic and acetic acids has profound, yet different, effects on the surface density, crystal field, and photoexcitation of polarons in rutile TiO2(110). We also show that these variations are governed by the contrasting electrostatic properties of the acids, which impacts the extent of polaron–adsorbate coupling. The density of polarons in the surface region increases more in formate-terminated TiO2(110) relative to acetate. Consequently, increased coupling gives rise to new photoexcitation channels via states 3.83 eV above the Fermi level. The onset of this process is 3.45 eV, likely adding to the catalytic photoyield.
Highlights
Understanding how adsorbates influence polaron behavior is of fundamental importance in describing the catalytic properties of TiO2
A minority carboxylate component is present, which is a monodentate species oriented perpendicular to [001] and accounts for up to 1/3 of the interface.[14,18−20] Formic and acetic acid adsorption saturates at ∼0.5 ML in ultrahigh vacuum (UHV) at 298 K, where a monolayer corresponds to the number of surface unit cells
The energy levels of the electron polarons represent what are commonly referred to as the band gap states (BGS) of reduced TiO2, which are detectable at ∼1.0 eV binding energy (BE) in UV photoelectron spectroscopy (UPS).[22,23]
Summary
Understanding how adsorbates influence polaron behavior is of fundamental importance in describing the catalytic properties of TiO2. This increases when the scattering plane is perpendicular to the [001] crystal azimuth, (p-[001]).[28−30,32] In contrast, a weaker feature is observed when the scattering plane is parallel to the [001] azimuth, (s[001]).[29,30] it has been shown that water and methanol adsorption influences this channel, altering the orbital character and resulting in an enhancement t2g excitation oscillator strength.[27,33−35] Despite of the t2g → these recent advances, the impact of carboxylates on electron polaronic
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