. First Principles Studies of Water and Ice on Oxide Surfaces Xiaoliang Hu Department of Chemistry University College London Thesis submitted for the degree of Doctor of Philosophy January 2010 I, Xiaoliang Hu, confirm that the work presented in this thesis is my own. Where in- formation has been derived from other sources, I confirm that this has been indicated in the thesis. Abstract The interaction of water (and ice) with oxide surfaces has been studied with first prin- ciples density functional theory. To begin, an extensive series of studies on the clay mineral kaolinite (Al2Si2O5(OH)4) are reported with a view to understanding the effi- cacy of kaolinite as a heterogeneous ice nucleating agent. The main conclusions are: (i) water clustering is disfavoured on the kaolinite surface; (ii) a stable two dimensional ice-like water overlayer can form; (iii) water covered kaolinite is itself “hydrophobic” compared to the bare “hydrophilic” kaolinite surface; and (iv) it is shown that ampho- terism (the ability to accept and donate H bonds) of the hydroxylated kaolinite surface is key to its many properties with regard to water adsorption and ice nucleation. Fol- lowing the water adsorption study, the perfect basal surface of kaolinite was investi- gated and it (and in general the entire class of so-called 1:1 clay surfaces) is shown to be polar. Extending the water adsorption studies beyond kaolinite the interaction of water with a range of rocksalt (alkaline earth metal) oxide surfaces was examined with a view to better understanding the fundamental properties of water adsorption, dissociation, and proton transfer on oxide surfaces. These studies reveal that the water adsorption energy and the tendency to dissociate both increase as one moves down the alkaline earth series of oxides. Finally, it is observed that water on MgO(001) under- goes rapid proton transfer within clusters of just two water molecules, made possible by facile dissociation and recombination of the water molecules within the dimers. Overall, it is hoped that these studies shed new light on several fundamental aspects of the interaction of water with oxide surfaces at the molecular level. 3 Contents Abstract 3 1 Introduction 10 2 A brief introduction to water-oxide interactions 15 3 Theoretical background 20 3.1 Electronic structure theory . 20 3.1.1 Born-Oppenheimer approximation . 20 3.1.2 Density functional theory . 23 3.1.3 Exchange-correlation functionals . 26 3.2 Periodicity . 28 3.2.1 Bloch’s theorem . 28 3.2.2 Brillouin zone sampling . 29 3.2.3 Plane wave basis sets . 31 3.2.4 Supercells . 32 3.3 Pseudopotentials . 33 3.4 Molecular dynamics . 37 3.4.1 Equation of motion . 37 3.4.2 Ensemble . 39 4 Water and ice on kaolinite 42 4.1 Introduction . 43 4.2 Computational details . 46 4.3 Bulk kaolinite . 47 4 4.3.1 Crystal structure . 47 4.3.2 Electronic structure . 50 4.4 Single layer of kaolinite . 53 4.5 Water and ice adsorption . 56 4.5.1 Monomer adsorption . 56 4.5.2 Cluster and 1D chain adsorption . 58 4.5.3 2D overlayer adsorption . 60 4.5.4 Adsorption beyond the 2D overlayer . 64 4.5.5 Electronic structure analysis . 65 4.6 Discussion and conclusions . 69 5 Polar (001) basal plane of kaolinite 73 5.1 Introduction . 73 5.2 Computational details . 76 5.3 Polar basal surface . 76 5.3.1 Perfect surface structures . 76 5.3.2 Defective surface structures . 83 5.3.3 Mixed mineral structures . 86 5.3.4 Foreign atom or molecule adsorbed structures . 88 5.4 Conclusions . 92 6 Towards a general understanding of water monomer adsorption on rock- salt oxide surfaces 93 6.1 Introduction . 94 6.2 Computational details . 95 6.3 Adsorption and dissociation trends . 97 6.3.1 Adsorption structure and energy . 97 6.3.2 Intact water monomer analysis . 100 6.3.3 Dissociated monomer analysis . 103 6.3.4 Surface relaxation and lattice constant . 106 6.4 Conclusions . 110 5 7 Proton transfer in adsorbed water dimers on MgO 112 7.1 Introduction . 112 7.2 Computational details . 113 7.3 Monomer and dimer adsorption structures . 115 7.4 Proton transfer in the dimer . 116 7.5 Conclusions . 121 8 Summary and Perspectives 122 A Tests of pseudopotentials 125 Bibliography 129 Acknowledgements 139 Publications 140 6 List of Figures 3.1 Schematic illustration of a supercell . 33 3.2 Schematic illustration of a pseudopotential . 35 4.1 Bulk kaolinite crystal structure . 45 4.2 DOS and PDOS of bulk kaolinite . 52 4.3 Clean surface and water monomer adsorption structures . 54 4.4 Cleavage (free) energy of a single layer of kaolinite . 55 4.5 Water cluster adsorption structures . 59 4.6 2D overlayer adsorption structures . 61 4.7 Electron density rearrangement of monomer and overlayer . 66 4.8 PDOS of monomer and overlayer . 66 4.9 Adsorption energy versus coverage . 70 5.1 Kaolinite bulk and schematic illustration of dipole moment . 74 5.2 Local potential in the [001] direction of kaolinite . 77 5.3 Calculated properties of perfect structures versus thickness . 79 5.4 PDOS of multilayer kaolinite slabs . 81 5.5 Calculated properties of defective structures versus thickness . 84 5.6 Mixed mineral structures and their calculated properties versus thickness 87 5.7 Atom adsorbed surface structures and their calculated properties ver- sus thickness . 89 6.1 Structures of monomer on rocksalt oxide (001) surfaces . 98 6.2 Adsorption energy of monomer on rocksalt oxide (001) surfaces . 99 6.3 PDOS of intact water . 101 7 6.4 Electron density rearrangement of intact water on rocksalt oxide (001) surfaces . 102 6.5 PDOS of dissociated water . 105 7.1 Dimer structures on MgO(001) . 114 7.2 Proton transfer simulations of dimers on MgO(001) . 118 7.3 Illustration and energy profile of one sequence of proton transfer events 120 A.1 Test of cutoff energy . 126 8 List of Tables 4.1 Lattice parameters for bulk kaolinite . 49 4.2 Internal bond lengths of bulk kaolinite . 51 4.3 Adsorption energy of overlayer and cohesive energy of ice . 63 5.1 Cleavage energies of selected kaolinite (001) surfaces . 83 6.1 Electron decomposition of intact water on rocksalt oxide (001) surfaces 103 6.2 Bond length and angle in intact water adsorption structure . 104 6.3 Bond length in dissociated water adsorption structure . 106 6.4 Water adsorption energy on relaxed and unrelaxed rocksalt oxide (001) surfaces . 107 6.5 Surface rumpling and displacement of surface ions of rocksalt oxide (001) surfaces . 108 6.6 Adsorption structure on rumpled CaO(001) . 108 6.7 Adsorption energy on rumpled surface . 109 6.8 Adsorption energy of water monomer on strained surfaces . 110 7.1 Adsorption energies of dimers and monomer on MgO(001) . 115 A.1 Pseudopotential tests for atomization energy of small molecules . 127 A.2 Pseudopotential tests for bulk Al and Si . 128 9 Chapter 1 Introduction Theoretical chemistry and physics based on quantum mechanical electronic structure theories have become an important and powerful complement to experiment in the atomic scale study of materials. Such simulations have extended and broadened our understanding in a wide variety of areas and are considered by many as virtual exper- iments. These computational experiments have been incredibly useful, for example, helping to understand important aspects of high pressure and high temperature physics of the earth’s core where conditions are so extreme that they can not readily be ex- plored in experiments (e.g. [1]). Or, in surface science, the chemical reactivity of catalyst surfaces has been unravelled (e.g. [2, 3]). There are many other examples in other fields too. In the electronic structure theories which underpin theoretical chemistry and physics, the central task is to solve the Schrodinger¨ or Schrodinger-like¨ equation. There are many approaches to do this. Density functional theory (DFT) [4, 5] can often be a good option, because it has proven capable of computing a host of properties of con- densed matter, their surfaces, and the interactions between surfaces and adsorbates to a reasonable accuracy with an acceptable computational cost. DFT combined with other practical computational techniques is the main approach used in this thesis to investi- gate water-oxide interfaces. 10 Water is one of the most common and important substances on earth. The many fas- cinating molecular properties of water and its almost ubiquitous appearance on solid surfaces, in particular oxide surfaces, makes it a topic of central importance to a sur- prisingly wide variety of scientific disciplines, including atmospheric chemistry, het- erogeneous catalysis and photocatalysis, chemical sensing, corrosion science, environ- mental chemistry and geochemistry, soil science, semiconductor manufacturing and cleaning, and biology, to name but a few [6, 7]. In this thesis, several key aspects of water at oxide surfaces will be examined with the aid of DFT simulations. Specif- ically, this involves a treatment of water adsorption and ice nucleation on a layered oxide surface, the examination of proton transfer dynamics in adsorbed water clusters, and a systematic study aimed at understanding adsorption trends and the fundamental factors that decide the adsorption structure and state (intact or dissociated) of water on a range of simple rocksalt oxide surfaces. The interest in water adsorption on layered oxide surfaces, specifically layered alu- minosilicate clays1, stems from the fact that the adsorption of water on clay mineral surfaces is often the first step in heterogeneous ice nucleation2, and subsequently cloud formation and precipitation [8]. This happens in the atmosphere on clay aerosol parti- cles, with the aerosol particles being the ice forming nuclei. Chemical analysis of par- ticles found at the center of snow crystals reveals that clay minerals as well as micro- organisms, combustion products, and some other natural and anthropogenic products commonly act as sources of ice forming nuclei.