Hydrogen Donors in Anatase TiO2

Two hydrogen-related donors in anatase TiO2-interstitial hydrogen (Hi) and hydrogen substituting for oxygen (HO)-are addressed by means of ab initio density functional theory and vibrational mode spectroscopy. First-principles calculations and infrared (IR) absorption measurements reveal that Hi 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 HO 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, Hi is shown to be unstable at room temperature, whereby formation of interstitial H2 should be a preferential sink for hydrogen in defect-free anatase. It is also found that of H2, Hi, and HO, the latter is the most energetically favorable defect, which may account for the hidden hydrogen unavailable for the standard spectroscopic techniques.

Automated summary

Two types of hydrogen-related donors in anatase TiO2, interstitial hydrogen (Hi) and hydrogen substituting for oxygen (HO), were studied using density functional theory and vibrational mode spectroscopy. Experimentally, Hi was found to be unstable at room temperature, while HO was the most energetically favorable defect. However, the predicted vibrational modes were not able to be verified through conventional IR absorption spectroscopy due to their occurrence within the strong two-phonon absorption band of TiO2.