Hydrogen depth profiling using ¹H(¹⁵N, αγ)¹²C resonant nuclear reaction on water-treated olivine surfaces and characterization of hydrogen species

抄録

Natural olivine crystals (Fo₉₁) were subjected to water treatment under different temperatures covering 25 to 800°C and different pH covering 2 to 12. Hydrogen analysis using ¹H(¹⁵N, αγ)¹²C resonant nuclear reaction was carried out and semi-quantitative hydrogen concentration depth profiles were obtained. The hydrogen depth profiles reveal the presence of high hydrogen concentration at the surface region. The depth of the hydrogen-rich region is less than 20 nm under medium to high pH conditions at 25°C. In contrast, hydrogen penetrates into olivine as deep as 200 nm under low pH conditions at 25°C. The penetration depth is independent of temperature up to 800°C under near neutral conditions. Combining this technique and XPS analysis, the speciation of hydrogen at the near surface region has been examined. The thickness of the leached surface layer is less than 20 nm and seems to be independent of the treatment conditions such as temperature and pH, however, the surface species vary with the treatment conditions. Mg and Fe are selectively leached out under low to medium pH conditions at 25°C. H⁺ or H₃O⁺ occupies the vacant octahedral sites in this case. The high hydrogen concentration within 200 nm depth in the low pH region arises from infiltration of molecular water. Hydrogen is highly localized at etch pits, etch hole, and dislocations in this case. On the contrary, Si is selectively leached out and hydroxyl species are formed under a high pH condition.