Hydrogen in the Near-Surface Region of Materials Studied by Nuclear Reaction Analysis and Thermal Desorption Spectroscopy

Hydrogen (H) in the near-surface region of solids is of great relevance in science and technology. Hydrogen diffusion across the gas/solid interface of H-absorbing metals is exploited routinely in metal hydride H storage and H₂ permeation membranes to produce purified H₂ for fuel cell utilization. While palladium (Pd) absorbed hydrogen enables olefin hydrogenation catalysis [1], H in the interior of metals may also cause embrittlement and pose other problems, such as the retention of hydrogen isotopes in plasma-facing walls of fusion reactor devices. This talk introduces our unique combination of ¹H-specific depth profiling via ¹H(¹⁵N,aγ)¹²C nuclear reaction analysis (NRA) [2] with thermal desorption spectroscopy (TDS) as a powerful technique to identify and evaluate the thermal stability of H states in the near-surface region of solids in a highly depth-resolved fashion. I describe our recent research applying this method and isotope-labeling (H, D) of surface hydrogen to structurally well-defined Pd single crystal surfaces, which clarifies the atomic-level H transportation mechanisms at H₂-exposed Pd surfaces during H₂ absorption and thermal desorption [3]. I will further demonstrate that the insight thus obtained into the surface structure-sensitive H ingress can be used to achieve control over the thermal desorption of Pd-dissolved H in a wide range of temperatures [4] and discuss implications for industrial olefin hydrogenation over Pd catalysts [1, 5]. The NRA(/TDS) technique is equally applicable to nanostructured metals [6], ultrathin oxide films, and semiconductor/dielectric material stacks relevant to the microelectronics industry [7]. (References: [1] M. Wilde, K. Fukutani, W. Ludwig, B. Brandt, J.H. Fischer, S. Schauermann, H.J. Freund, Angew. Chem. Int. Ed., 47 (2008) 9289-9293. [2] M. Wilde, K. Fukutani, Surf. Sci. Rep., 69 (2014) 196-295. [3] S. Ohno, M. Wilde, K. Fukutani, J. Chem. Physics, 140 (2014) 134705. [4] S. Ohno, M. Wilde, K. Fukutani, J. Phys. Chem. C, 119 (2015) 11732-11738. [5] S. Ohno, M. Wilde, K. Mukai, J. Yoshinobu, K. Fukutani, J. Phys. Chem. C, 120 (2016) 11481-11489. [6] M. Wilde, K. Fukutani, M. Naschitzki, H.J. Freund, Phys. Rev. B, 77 (2008) 113412. [7] Z. Liu, S. Fujieda, H. Ishigaki, M. Wilde, K. Fukutani, ECS Transactions, 35 (2011) 55-72.)