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Exposure of the mixed-terminated surface to atomic hydrogen at room temperature is found to lead to drastic changes of the electrical properties. The insulator surface is found to become metallic. By employing several experimental techniques (electron energy loss spectroscopy, He-atom scattering, and scanning tunneling microscopy) together with ab initio electronic structure calculations we demonstrate that a low-temperature (1 x 1) phase with two H atoms in the unit cell transforms upon heating to another (1 x 1) phase with only one H atom per unit cell. The odd number of electrons added to the surface per unit cell gives rise to partially filled surface states and thus a metallization of the surface.

The adsorption of atomic hydrogen on a single crystal ZnO(1010) surface has been studied by scanning tunneling microscopy (STM) under ultrahigh vacuum conditions at room temperature and at elevated temperatures. High resolution STM images indicate that a well-ordered (1x1) H adlayer is formed on the ZnO(1010) surface. The STM data strongly indicate that the hydrogen adsorbs on top of the oxygen atoms forming hydroxyl species. Scanning tunneling spectroscopy (STS) studies reveal a H atom induced metallization at room temperature. In contrast to the clean surface for the hydrogen-covered surface distinct defects structures consisting of missing O and Zn atoms could be identified.

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The adsorption of hydrogen on oxides leading to the formation of OH species and the subsurface diffusion of H atoms are topics of pronounced interest with regard to understanding oxide surface chemistry. In a recent Letter by Enevoldsen et al. [1], the authors use ab initio density functional theory (DFT) calculations to determine the activation energy for subsurface diffusion of H atoms. The value obtained by them, 2.4 eV, is rather high and would imply that this process can be basically neglected at room temperature. In fact, this value lies more than 1 eV above the results reported by us in a previous publication [2], 1.11 eV. Enevoldsen et al. explain this difference by the different H-atom coverage in the two cases. We would like to point out that this explanation is not correct; the rather large difference arises from the fact that in [1] an inadequate diffusion mechanism has been considered. The path taken by the H atom to enter the bulk considered in [1] starts at a surface bridging site O b and from there leads directly to a subsurface position O sub , whereas the optimum path described in [2] starts from a threefold surface oxygen O 3f site. The two different paths are depicted in Fig. 1. We repeated the DFT calculations for the nonoptimal path considered in [1] and were able to recover a value of 2.6 eV very close to that of 2.4 eV, reported by Enevoldsen et al. As described above, this value is unphysically high and is also not consistent with He-atom scattering experiments [3] where it was found that the hydrogenated surface undergoes a structural rearrangement at a temperature of 388 K. This temperature would correspond to an activation energy of $0:97 eV.Recent DFT calculations reported by Kowalski et al.[4] also support the threefold to subsurface diffusion path proposed by us [2]. Table I compares the values reported for H diffusion paths from similar DFT calculations. The barriers calculated for O b -O sub diffusion are in the order of 2.4-2.56 eV, while the O 3f -O sub barriers are lower, 0.93-1.11 eV. The surface diffusion O b -O 3f is 0.63 eV. The hydrogen coverage , considered by the authors of [1] to play a crucial role, has a rather limited impact on the calculated barriers, in the case of the direct diffusion 2.4 eV ( ¼ 1=8, Ref. [1]) versus 2.56 eV ( ¼ 1, Ref. [4]).In conclusion, to correctly obtain a theoretical value for the activation energy governing subsurface diffusion of H atoms, the minimum-energy path has to be considered. For H atoms adsorbed on rutile TiO 2 ð110Þ, this path starts from a threefold hollow site, possibly after an easy diffusion O b -O 3f , and not from a bridge site. For this minimumenergy path the activation energy amounts to 1.1 eV. This value is fully consistent with previous experimental data and clearly demonstrates that subsurface diffusion of H atoms is a process occurring at room temperature, with important implications for H-atom doping and for hydrogenation processes occurring at metal oxide surfaces [5].COST D-36 and D-41 actions are acknowledged.