Non-Band-Gap Photoexcitation of Hydroxylated TiO₂

Abstract

The photochemistry of TiO2 has been studied intensively since it was discovered that TiO2 can act as a photocatalyst. Nevertheless, it has proven difficult to establish the detailed charge-transfer processes involved, partly because the excited states involved are difficult to study. Here we present evidence of the existence of hydroxyl-induced excited states in the conduction band region. Using two-photon photoemission, we show that stepwise photoexcitation from filled band gap states lying 0.8 eV below the Fermi level of rutile TiO2(110) excites hydroxyl-induced states 2.73 eV above the Fermi level that has an onset energy of ∼3.1 eV. The onset is shifted to lower energy by the coadsorption of molecular water, which suggests a means of tuning the energy of the excited state.

Creating Excess Electrons at the Anatase TiO2(101) Surface

“…Measurements employed an instrument (base pressure *4 9 10 -10 mbar) and laser system described elsewhere [41]. 2PPE (hm = 3.10-3.54 eV, 400-350 nm, 0.3-0.5 mW, spot diameter *0.5 mm) and UPS (He-I, hm = 21.2 eV, 58 nm and He-II, hm = 40.8 eV, 30 nm) spectra were recorded using a pass energy of 10 eV.…”

Polaron-Adsorbate Coupling at the TiO2(110)-Carboxylate Interface

“… 28 − 30 At reduced and hydroxylated TiO 2 (110) surfaces, 2PPE spectra are dominated by a t 2 g → t 2 g excitation feature where the excited state lies ∼2.6–2.8 eV above E F . 28 , 30 − 32 The oscillator strength of this excitation is strongly dependent on the orientation of the electric field vector. This increases when the scattering plane is perpendicular to the [001] crystal azimuth, ( p- [001]).…”

“…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 Furthermore, it has been shown that water and methanol adsorption influences this channel, altering the orbital character and resulting in an enhancement of the t 2 g → t 2 g excitation oscillator strength.…”

“… 36 In reduced and hydroxylated TiO 2 (110) (see SI for preparation methods), the resonant photon energy for the t 2 g → t 2 g excitation is known to be ∼3.54 eV (350 nm). 28 In Figure 1 (b), this resonance was monitored ( p -[001], 3.54 eV, 350 nm) as a reduced rutile TiO 2 (110) sample (R-R110) partially hydroxylated in UHV (H p -R110, ∼0.05 ML) and was sequentially exposed to gas-phase acid. This was performed in situ until the saturation level (∼0.5 ML) was reached.…”

Global and local aspects of the surface potential landscape for energy level alignment at organic-ZnO interfaces

“…The above-mentioned difficulties to predict and utilize energy level alignment at semiconductor surfaces have proven to be particularly tedious for metal oxides such as ZnO or TiO 2 [24,25,26,27]. While one of the most promising wide band gap semiconductors for optoelectronic applications [28], ZnO surfaces are notoriously difficult to prepare and characterize, both experimentally and theoretically [29,30,31,32,33,34,35,36,37].…”