Quantitative and site-sensitive analysis of surface deuterium on TiO2(001) by low-energy ERDA

Quantification of hydrogen in the surface atomic layers is essential for understanding hydrogen-related surface reactions and for their practical applications. However, surface-selective quantification of hydrogen remains a major challenge even for advanced analytical techniques. In the present study, we demonstrate the quantification of deuterium in the surface atomic layers of rutile TiO₂(001) by low-energy time-of-flight elastic recoil detection analysis (low-energy TOF-ERDA). We show that surface D produces an isolated recoil peak that is well separated from both the H recoil peak and the scattered-ion peak. This separation was achieved using a low-emittance focused ion beam with a suppressed halo component. Layer-resolved full-trajectory simulations show that the recoil D signal, D(r), is dominated by D atoms in the upper eight atomic layers, whereas the recoil O signal, O(r), mainly originates from the outermost two atomic layers. The surface D concentration, averaged over the upper eight atomic layers, was estimated to be 3.3 at.% by comparing the D(r)/O(r) intensity ratio with full-trajectory simulations. These results demonstrate low-energy TOF-ERDA as a quantitative method for analyzing deuterium in the surface atomic layers. The estimated D concentration indicates that atomic-D irradiation introduces deuterium as a minor but non-negligible near-surface species in TiO₂(001).

The present study further demonstrates that simultaneous TOF and energy analyses (TOF+EA) are useful for separating recoil hydrogen, recoil deuterium, and the large scattering component.[1] Since TOF+EA detects only ions that survive neutralization near the sample surface, TOF+EA-ERDA is extremely surface sensitive. Previous low-energy H⁺ ion-scattering studies have shown that low-energy D⁺ ions are strongly neutralized, so that the detected D⁺ signal is expected to originate predominantly from the outermost atomic layer. [2]

We applied TOF+EA-ERDA to atomic deuterium-irradiated rutile TiO₂(001) surfaces. Periodic variations in the intensity of the direct recoil D peak were observed as a function of the azimuthal angle of the sample, revealing the incorporation of irradiated deuterium into the outermost atomic layer of TiO₂ at a specific crystallographic site. The detailed analysis of the azimuthal angle scan measurements using full trajectory simulation was consistent with a structural model involving both Ti and D interstitials.

TOF-ERDA gives quantitative near-surface D concentration over several atomic layers, whereas TOF+EA-ERDA selectively probes ionized D recoils from the outermost atomic layer because of strong neutralization of low-energy D⁺ near the surface.

References

[1] Combined energy and TOF analyses of low-energy elastic recoil detection for analyzing the topmost surface of atomic deuterium-irradiated rutile TiO₂ (001) crystals. T.T. Suzuki, I. Sakaguchi, S. Iimura, T. Nishimura, B. Tsuchiya, Surf. Sci. 769 (2026) 122967.

[2] Neutralization and electronic excitation during low-energy H and He scattering from LiF, R. Souda, T.T. Suzuki, K. Yamamoto, Surf. Sci. 397 (1998) 63.