Hydrogen: A metastable donor in TiO2 single crystals

Abstract

The properties of TiO 2 ͑110͒ single crystals are greatly changed after hydrogen is doped into them through electrochemical hydrogen charging, in which the crystals have been placed in 0.01M NaOH solution to deposit hydrogen on their electrodes through the electrolysis of water. The changes in properties, including great increases in leakage current, strong dielectric dispersions over 10 2 -10 6 Hz, and a strong aging phenomenon suggest that hydrogen is a donor in TiO 2 while ionized hydrogen is not bound to an oxygen site by a strong O-H bond. This behavior of hydrogen, which can be characterized as a metastable donor, is different from the usual donor behavior of hydrogen in oxides and should be interesting for a better understanding of the behavior of hydrogen in oxides. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2900957͔The behavior of hydrogen in oxides has been a topic of extensive investigations. On one hand, hydrogen is a ubiquitous impurity in various oxides. It can be introduced into oxides from precursors, from annealing gases, 1 or even from aqueous vapor in ambient atmosphere.2 On the other hand, hydrogen behaves quite differently from one oxide to another. In some oxides, hydrogen forms deep gap states in all of its three charge states, H + , H 0 , and H − ; while in other oxides, hydrogen gives rise to a shallow level at the conduction band edge and acts as a donor. [3][4][5] As an example, hydrogen has been predicted theoretically 6 and proven experimentally 7 as a shallow donor in ZnO. It must possess a high stability in ZnO to survive high-temperature sintering or crystal growth. As a matter of fact, it is generally believed that as a donor, hydrogen ionizes to H + with the electron being delocalized in the oxide conduction band and then H + forms a single strong O-H bond to an O site; in contrast, when hydrogen is a deep impurity, it will only form some longer, weaker bonds to more than one neighboring oxygen and exist as a metastable interstitial.5 Metastable interstitial hydrogen has been observed in proton-exchanged LiNbO 3 . A sharp peak at around 3500 cm −1 can always be observed in the infrared absorption spectra of proton exchange processtreated LiNbO 3 , which is the characteristic absorption peak for the stretch mode of the O-H bond, as hydrogen has replaced some lithium ions in LiNbO 3 . When the acidity of the melt in the proton exchange process is high enough, another broad peak at 3280 cm −1 can also be observed, which is only metastable and will disappear through an annealing process. Obviously, these two peaks should correspond to two different states of hydrogen with quite different stabilities. The broad peak should indicate the existence in LiNbO 3 of hydrogen with a much lower stability, which is possibly a deep impurity forming only long and weak bonds to neighboring oxygen sites. In a previous investigation, hydrogen has been doped into TiO 2 single crystals through an electrochemical method.9 Presently, a study on the behavior of hydrogen in TiO 2 single crysta...

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