Ab Initio Study of the Hydroxylated Surface of Amorphous Silica: A Representative Model Frederik Tielens,* ,† Christel Gervais, ‡ Jean Franc ¸ois Lambert, † Francesco Mauri, § and Dominique Costa | Laboratoire de Re ´actiVite ´ de Surface and Laboratoire de Chimie de la Matie `re Condense ´e, UniVersite ´ Pierre et Marie Curie-Paris6, 4, Place Jussieu, F-75252 Paris Cedex 05, France, Institut de Mine ´ralogie et Physique des Milieux Condense ´s, UniVersite ´ Pierre et Marie Curie-Paris6, Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France, and Laboratoire de Physico-Chimie des Surfaces, Ecole Nationale Supe ´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, F-75231 Paris cedex, France ReceiVed January 11, 2008. ReVised Manuscript ReceiVed March 4, 2008 A new complete, representative model for the hydroxylated surface of amorphous silica is presented and characterized by means of periodic DFT calculations. This model accounts for the experimentally encountered ring size distribution, Si-O-Si and O-Si-O angles, silanols density, and repartition (isolated, associated, geminals). Properties such as NMR shifts, dehydrogenation energies, OH vibrational frequencies, and the interaction with water are investigated. The results are compared with former experimental and theoretical results. This new representative model for this complex surface would probably help the investigation of its reactivity toward amino acids or other organic molecules, opening new perspectives in the understanding of the chemistry of amorphous materials. Introduction Silica, the most common mineral on Earth, is used for applications in catalysis and chromatography and is a key component in electronic devices, solar cells, and optical fibers. 1 Its chemical properties have been widely studied and reviewed. 2,3 Silica polymorphs are composed of SiO 4 tetrahedra which polymerize forming different structures. The flexibility of the Si-O-Si bond explains the great number of existing polymorphs, either natural or synthetic: several crystalline forms such as quartz, cristobalite, tridymite, diatomite, and edingtonite, a number of non- crystalline glasses or sol-gel phases, and micro/mesopo- rous materials. When created by cleavage, the silica surface exhibits undercoordinated atoms such as three coordinated Si atoms, terminal (nonbridging) oxygens, and strained Si-O-Si bridges in small size rings. The silica defects readily react with water in ambient conditions to form surface hydroxyl groups named silanols 3–6 which can make the surface hydrophilic (vide infra). In contrast, a nondefective surface is hydrophobic. 7–10 Due to the noncrystalline nature of silica, the classical diffraction techniques cannot be used to give structural information. Yet its surface seems to exhibit a quite rich diversity of chemical groups. The first important distinction that must be made here is between the terminal and geminal silanols. Terminal silanols are bound to a Si atom involved in three Si-O-Si siloxane groups, whereas geminal silanols complete the coordination sphere of a Si atom involved in two siloxane groups. Sometimes, longer-range interactions are taken into account: two silanols on Si atoms connected by a siloxane bridge are vicinal; they are adjacent if the Si atoms are separated by an O-Si-O bridge. A terminal silanol could be engaged in either vicinal or adjacent relations, both, or neither, and the same holds for a silanol in a geminal pair. In addition to this distinction based on through-bond interactions, silanols may differ according to through-space neighboring relations: they may be H bonded to other silanols (in which case they are said to be associated) or not: they are then isolated. The methods available to characterize silica surfaces give information on either of these distinctions. The silanols on the surface of amorphous silica have been characterized * Author to whom correspondence should be sent. E-mail: tielens@ ccr.jussieu.fr. † Laboratoire de Re ´activite ´ de Surface, Universite ´ Pierre et Marie Curie-Paris6. ‡ Laboratoire de Chimie de la Matie `re Condense ´e, Universite ´ Pierre et Marie Curie-Paris6. § Institut de Mine ´ralogie et Physique des Milieux Condense ´s, Universite ´ Pierre et Marie Curie-Paris6. | Ecole Nationale Supe ´rieure de Chimie de Paris. (1) Bakos, T.; Rashkeev, S. N.; Pantelides, S. T. Phys. ReV. Lett. 2002, 88, 055508. (2) Legrand, A. P. The Surface Properties of Silicas; Wiley: New York, 1998. (3) Iler, R. K. The Chemistry of Silicas; Wiley: New York, 1979. (4) KnozingerH. The Hydrogen Bond; North Holland: Amsterdam, 1976; Vol. 3. (5) Zhuravlev, L. T. Langmuir 1987, 3, 316. (6) Nishijima, M.; Edamoto, K.; Kubota, Y.; Tanaka, S.; Onchi, M. J. Phys. Chem. 1986, 84, 6458. (7) Bolis, V.; Fubini, B.; Marchese, L.; Martra, G.; Costa, D. J. Chem. Soc., Faraday Trans. 1991, 87, 497. (8) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 1487. (9) D’Souza, A. S.; Pantano, C. G. J. Am. Ceram. Soc. 2002, 85, 1499. (10) Wendt, S.; Frerichs, M.; Wei, T.; Chen, M. S.; Kempter, V.; Goodman, D. W. Surf. Sci. 2004, 565, 107. A Chem. Mater. XXXX, xxx, 000–000 10.1021/cm8001173 CCC: $40.75  XXXX American Chemical Society PAGE EST: 8.8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 ohio1/ycm-ycm/ycm-ycm/ycm99907/ycm3676d07z xppws 23:ver.3 4/12/08 14:14 Msc: cm-2008-001173 TEID: tsb00 BATID: cm6a14