Hydrogen Donors in Anatase TiO2

Abstract

Two hydrogen‐related donors in anatase TiO2—interstitial hydrogen (normalHnormali) and hydrogen substituting for oxygen (normalHnormalO)—are addressed by means of ab initio density functional theory and vibrational mode spectroscopy. First‐principles calculations and infrared (IR) absorption measurements reveal that normalHnormali forms an OH bond lying in the basal plane of the crystal characterized by a stretch local vibrational mode (LVM) with a frequency of 3412 cm−1. According to the calculations, the vibrational spectrum of the threefold‐coordinated hydrogen in normalHnormalO includes two stretch LVMs with the frequencies of 1014 and 1294 cm−1. The theoretically predicted modes occur within the strong two‐phonon absorption band of TiO2, so the conclusions of theory cannot be verified by conventional IR absorption spectroscopy. In accordance with experimental findings, normalHnormali is shown to be unstable at room temperature, whereby formation of interstitial normalH2 should be a preferential sink for hydrogen in defect‐free anatase. It is also found that of normalH2, normalHnormali, and normalHnormalO, the latter is the most energetically favorable defect, which may account for the “hidden” hydrogen unavailable for the standard spectroscopic techniques.

Dominant hydrogen complex in natural anatase TiO2

“…24 Interstitial hydrogen has a low diffusion barrier resulting in high mobility already at room temperature. 27 After a few weeks of storage, most of H i becomes bound to various traps, which are always present in natural crystals. Many of these complexes (but not all) manifest themselves as IR absorption lines in the spectral range of 3200-3500 cm À1 resulting from the stretch local vibrational modes of the hydroxyl groups (O-H) preferentially aligned perpendicular to the c axis of the crystal.…”

Electric Field Manipulation for Improved Rates of Photocatalysis by Mesoporous TiO2

“…Hydrogen exists in several forms in anatase. , Hydrogen atoms present as interstitials or bonded to O vacancies act as shallow donors, while interstitial H 2 is electrically inactive. Hydrogen donors do not appear to dominate the behavior of V S here, as the increasing values signify movement of the surface Fermi energy toward the mid-gap instead of toward the conduction band.…”

“…The mild hydrogen reduction employed to reach this regime could have changed some aspects of surface reactivity. Interstitial hydrogen atoms are unstable during long-term storage at room temperature because a low diffusion barrier enables disappearance to form interstitial H 2 or to bond with other intrinsic or extrinsic traps. However, the treatment does affect light absorption as shown in Figure , which may propagate into the efficiency of hole loss at the sidewalls.…”

Photocorrosion of n‐ and p‐Type Semiconducting Oxide‐Covered Metals: Case Studies of Anodized Titanium and Copper

“…Electric fields also affect the stability of the dominating defects, [ 52 ] and are inevitably present at the solid/solution interface. In aqueous solution, hydrogen in defects plays an additional role, [ 53 ] and could also be involved in the dissolution reaction.…”