# Water on Actinide Dioxide Surfaces: A Review of Recent Progress
## 3 Water on AnO\({}_{2}\) Surfaces
### Theoretical Calculations
#### 3.2.1 Adsorption and dissociation of H\({}_{2}\)O on AnO\({}_{2}\) surfaces  
The adsorption and dissociation of H\({}_{2}\)O on AnO\({}_{2}\) surfaces have been extensively studied in several papers [44; 53; 54; 64; 65]. H\({}_{2}\)O molecules can adsorb on surfaces in the molecular form or the dissociated form. For the molecular adsorption (Figure 8a,b), the oxygen atom from the H\({}_{2}\)O molecule binds via its lone pair with the positive actinide atom on the surface, while, at the same time, one of the hydrogen atoms forms a hydrogen bond with a nearby oxygen atom on the surface. If the H\({}_{2}\)O dissociates at the surface into OH' and H\({}^{+}\), the hydroxyl group bonds with one surface actinide atom, and the proton bonds with one surface oxygen atom (Figure 8c,d). On the basis of these studies [44; 53; 54; 64], the following general conclusions have been reached on the adsorption and dissociation of H\({}_{2}\)O on AnO\({}_{2}\) surfaces:  
1. A mixture of molecular and dissociated adsorption of H\({}_{2}\)O occurs on the (111) surface of AnO\({}_{2}\). 2. Dissociative adsorption of H\({}_{2}\)O preferentially occurs on the (110) and (100) surfaces of AnO\({}_{2}\). 3. The adsorption energies of H\({}_{2}\)O are correlated with the surface energies; a stronger adsorption energy is expected on the surface with a higher surface energy. 4. The presence of oxygen vacancies on the surface of AnO\({}_{2}\) favors the dissociative adsorption mechanism. The distribution of excess electrons on reduced AnO\({}_{2}\) surfaces is different upon the adsorption of H\({}_{2}\)O. [PERSON] et al. first studied the adsorption configurations of H\({}_{2}\)O on ThO\({}_{2}\) surfaces using both accurate calorimetric measurements and first-principle calculations [38]. They observed coverage-dependent adsorption energies and demonstrated a mixture of molecular and dissociative adsorption of H\({}_{2}\)O on ThO\({}_{2}\) (111) surface. They also showed that the adsorption energy of water molecules is correlated with the surface energies; a higher surface energy results in a stronger water adsorption on ThO\({}_{2}\) surfaces [38]. [PERSON] et al. [22] found that, for a single water molecule on UO\({}_{2}\) (111) surface, the two adsorption configurations, molecular adsorption and dissociative adsorption, exhibit comparable adsorption energies (0.61 vs. 0.68 eV, as shown in Table 5). However, the adsorption in the molecular form, on (110) and (100) surfaces, is far less stable than the dissociative adsorption (0.62 vs. 1.27 eV and 1.02 vs 1.71 eV, respectively) [22]. [PERSON]'s results are consistent with those of  
Figure 7: Temperature programed desorption of H\({}_{2}\) after H\({}_{2}\)O adsorption at 300 K over Ar\({}^{+}\) sputtered UO\({}_{2}\) (111) single crystal [58]. The inset shows the computed H\({}_{2}\) peak area as well as the H\({}_{2}\)/H\({}_{2}\)O peak area ratios. [PERSON] et al. [66] and [PERSON] et al. [67], who also found that the molecular and dissociative adsorptions have similar adsorption energies on UO\({}_{2}\) and PuO\({}_{2}\) (111) surfaces, while there is a clear preference for dissociative adsorption on (110) surfaces. Both groups reported the mixture of molecular and dissociative adsorption to be the most stable adsorption configuration at a higher coverage of H\({}_{2}\)O on (111) surfaces [22; 66; 67]. The calculated adsorption energies of H\({}_{2}\)O on UO\({}_{2}\) surfaces increase from (111), (110), to (100) surface, as summarized in Table 5, which is consistent with the observation by [PERSON] et al. that stronger H\({}_{2}\)O adsorption energies are expected on ThO\({}_{2}\) surfaces with higher surface energies [38]. [PERSON] et al. further studied the adsorption of up to five layers of H\({}_{2}\)O on PuO\({}_{2}\) and UO\({}_{2}\) surfaces, and found significant variation in the adsorption energies as a result of intra- and interlayer hydrogen bonding network [68].