Influence of intermolecular hydrogen bonding on water dissociation at the MgO„001… surface Jun-Hyung Cho,* Jung Mee Park, and Kwang S. Kim Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Received 14 March 2000 The adsorption of water on the MgO001 surface is studied by using density-functional theory calculations within the generalized gradient approximation. Our calculations show that coupled three and four water mol- ecules are partly dissociated, indicating that the intermolecular hydrogen bonding plays an important role in water dissociation on MgO001. Especially, four water molecules are found to be significantly stabilized due to the increase in the number of the intermolecular hydrogen bonds. This hydrogen-bonding unit can explain experimental observations of the c (4 2) phase and its transition to the p (3 2) phase composed of three water molecules. The interaction of water with oxide surfaces is interesting because of its importance for understanding various funda- mental phenomena such as heterogeneous catalysis, corro- sion, ground-water contaminants, and atmospheric chemistry. 1 As a model system, the adsorption of water on the MgO001 surface has been intensively studied both experimentally 2–8 and theoretically. 9–16 Despite many stud- ies, there has been a controversy concerning whether the type of water adsorption on MgO001 is molecular or dissocia- tive. Earlier theoretical calculations 9–12 predicted that water dissociation on a clean MgO001 surface is energetically unfavorable, but at a defective surface the dissociation can be stabilized. Thus, the vibrational spectra of the hydroxyl group by infrared spectroscopy 4,5 were tentatively ascribed to water dissociation at steps and defects. However, unlike earlier experiments 2–4 performed on powdered MgO substrates containing various surface de- fects, recent experiments 5–8 determined the structure of wa- ter adsorbed on a single-crystal MgO001 surface. A spot profile analysis low-energy electron diffraction SPA-LEED experiment 5 showed that the adsorbed water molecules form a c (4 2) phase at about 150 K. Subsequently, LEED and helium-atom scattering HAS experiments 6 also showed that the water adsorption at T =100–180 K forms a layer with the c (4 2) phase and annealing at T =185–221 K leads to the formation of a new ordered phase of the p (3 2) symmetry. The experimental observations of the p (3 2) phase stimu- lated theorists to determine its atomic structure. Recent two first-principles calculations by Giordano, Goniakowski, and Suzanne 15 and by Odelius 16 reported that the p (3 2) water monolayer contains H 2 O in both molecular and dissociated forms. However, there is no theoretical study for the low- temperature phase of c (4 2), and its phase transition to p (3 2) as well. Thus, an understanding of the water ad- sorption on MgO001 is still lacking. In this paper we study the energetical and geometrical properties of water molecules adsorbed on MgO001 by means of the density-functional theory 17 within the general- ized gradient approximation GGA. 18 We find that the dis- sociation of either a water molecule or a water dimer is en- ergetically unfavored, but in the presence of three or four water molecules, the mixture of molecular and dissociative adsorption is stabilized by the intermolecular hydrogen bonding. Especially, four water molecules are more stable than three water molecules which are the basic unit of the p (3 2) water monolayer in the previous first-principles calculations. 15,16 This higher stability of four water mol- ecules is likely to form a well-ordered island with the c (4 2) structure, supporting the experimental observation of the c (4 2) phase at low temperature. With the hydrogen- bonding units of three and four water molecules, we will explain the phase transition from c (4 2) to p (3 2) ac- companied by water desorption. 6,7 Our total-energy calculations have been performed using the plane-wave-basis pseudopotential method within the GGA. We use the exchange-correlation functional of Per- dew, Burke, and Ernzerhof 18 for the GGA calculations. The Mg and H atoms are described by the norm-conserving pseudopotentials of Troullier and Martins 19 and the O atom is described by the ultrasoft pseudopotential of Vanderbilt. 21 The MgO001 surface is modeled using a periodic slab ge- ometry consisting of three atomic layers and a vacuum re- gion of seven such layers. 20 Water molecules are adsorbed on both sides of the slab. For the c (4 2) structure, we employ the equivalent p (4 2) cell whose surface area is twice as large as that of the c (4 2) cell. The electronic wave functions are expanded in a plane-wave basis set using a cutoff energy of 25 Ry, and the electron density is obtained from the wave functions calculated at four k points in the surface Brillouin zone of the p (3 2) and p (4 2) unit cells. 22 Here, the positions of all atoms, except the innermost one MgO layer held at their theoretical bulk positions ( a 0 =4.23 Å, are allowed to relax along the calculated Hellmann-Feynman forces until all the residual force com- ponents are less than 0.02 eV/Å. We first determine the atomic structure of water mol- ecules adsorbed on MgO001 with increasing number of molecules within the p (3 2 ) cell. The top and side views of the optimized structures of one, two, three, and four water molecules, corresponding to coverage  =1/6, 1/3, 1/2, and 2/3 ML, are given in Fig. 1, together with the calculated adsorption energy. We find that the adsorption energy in- creases with increasing coverage, indicating the attractive in- teraction between the adsorbed water molecules. Note that in PHYSICAL REVIEW B 15 OCTOBER 2000-I VOLUME 62, NUMBER 15 PRB 62 0163-1829/2000/6215/99814/$15.00 9981 ©2000 The American Physical Society