1 Layered Compounds • Graphite and Graphene and the likes • Clay Minerals, Mica • Layered Double Hydroxides (LDHs) • Layered Zirconium Phosphates and Phosphonates • Layered Metal Oxides • Layered Metal Chalcogenides - TiS2, MoS2, WS2, MPS3 (M = Ti, V, Mo, W, Mn, Fe, Co, Ni, Zn) • Alkali Silicates and Crystalline Silicic Acids 2D = Two-dimensional layers 2 Layered Compounds Intralayer bonding - strong (covalent, ionic) Interlayer bonding - weak (H-bonding, van der Waals) 3 Host-Guest Structures Host dimensionality 3D 2D 1D 0D Topotactic reactions = modifying existing solid state structures while maintaining the integrity of the overall structure 4 Intercalation Intercalation = Insertion of guest molecules between layers 5 Intercalation Mechanisms Exfoliation Host + Guest Staging Intercalate APB = advancing phase boundary HT = Hendricks-Teller effect Galleries are filled randomly 6 Exfoliation Decrease attractive forces between layers Separate layers H-bonding 7 Graphite ABABAB Hexagonal graphite = two-layer ABAB stacking sequence Graphite C-C sp2 sigma-bonding in-plane and out-of-plane p-orbital pi-bonding The pi-type orbitals create the valence and conduction bands of graphite, very small band gap, metallic conductivity properties inplane, 104 times that of out-of-plane electric conductivity 8 Graphite Intercalation G (s) + K (melt or vapor)  KC8 (bronze) KC8 (vacuum, heat)  KC24  KC36  KC48  KC60 KC8 potassium graphite ordered structure Ordered K guests between the sheets of G Intercalates KC8 AAAA stacking sequence K nesting between parallel eclipsed hexagonal planar carbon six-rings K to G charge transfer Reduction of graphite sheets Electrons enter the conduction band Ionic bonding K+ C8 KC8 KC24 Intercalation in Li-ion Cells 9 LiC6 Li to G charge transfer Li+ C6 - Graphene • 1962 H.-P. Boehm monolayer flakes of reduced graphene oxide • 2004 Andre Geim and Konstantin Novoselov - Graphene produced and identified • Exotic properties: Firm structure Inert material Hydrofobic character Electric and thermal conductivity High mobility of electrons Specific surface area (theor.): 2630 m2g-1 • Application: diodes, sensors, solar cells, energy storage, composites, … K. Novoselov A. Geim 10 2010 Nobel Prize in Physics 11 Graphene Properties High electric conductivity (metallic) Optically transparent – 1 layer absorbs 2.3% of photons High mechanical strength HRTEM 12 Preparation of Graphene Top-Down Mechanical exfoliation of graphite - Scotch tape – layer peeling, flaking Chemical exfoliation of graphite (chemical, sonochemical) Bottom-Up SiC pyrolysis – epitaxial graphene layer on a SiC crystal CVD from CH4, CH2CH2, or CH3CH3 on Ni (111), Cu, Pt surfaces 13 Scotch Tape – Layer Peeling Mechanical exfoliation 14 Exfoliation Chemical exfoliation (surfactant) Sonochemical exfoliation 15 SiC Pyrolysis • Annealing of the SiC crystal in a vacuum furnace (UHV 10-10 Torr) • Sublimation of Si from the surface at 1250 - 1450 °C • The formation of graphene layers by the remaining carbon atoms 16 CVD from CH4 / H2 on Metal Surfaces (A) SEM - graphene on a Cu foil (B) High-resolution SEM - Cu grain boundary and steps, two- and three-layer graphene flakes, and graphene wrinkles. Inset (B) TEM images of folded graphene edges 1L = one layer; 2L = two layers Graphene transferred onto (C) a SiO2/Si substrate (D) a glass plate Graphene Family 17 Graphene Graphene oxide Fluorographene Graphitic Carbon Nitride C3N4 Phosphorene Graphene Oxide • More reactive than graphene • Presence of oxygen groups: -OH, -COOH, =O, -O- hydrophilic character • Electric insulator • Specific SA (theoretically): 1700-1800 m2g-1 • Hummers method 18 inhomogeneous Fluorographene / Graphene Fluoride • Monolayer of graphite fluoride (CF) • Synthesis: – Graphene + XeF2 (70 °C) – Mechanical exfoliation of carbon monofluoride (CF)n – Liquid-phase exfoliation of graphite fluoride with sulfolane • High-quality insulator, resistivity > 1012 Ω, an optical gap = 3 eV • Mechanical strength - a Young’s modulus = 100 N m−1 • Inert and stable up to 400 °C in air, similar to Teflon 19 A puckered cyclohexane-ring Chair conformation Each carbon bears a fluorine alternately above and below the ring Graphitic Carbon Nitride 20 Temperature- induced condensation Dicyandiamide NH2C(=NH)NHCN In a LiCl/KCl melt 1834 Berzelius, Liebig g-C3N4 Band gap 1.6 - 2.0 eV Small band gap semiconductors Si (1.11 eV), GaAs (1.43 eV), and GaP (2.26 eV) Graphitic Carbon Nitride 21 (a) triazine and (b) tri-s-triazine (heptazine) Phosphorene 22 Semiconductor - direct band gap bulk black P 0.3 eV monolayer phosphorene 1.5 eV N-methyl-2-pyrrolidone Black phosphorus Orthorhombic a = 3.31 Å, b = 4.38 Å, c = 10.50 Å = 90 Space group Bmab Exfoliation Phosphorene 23 Height-mode AFM images single-layer phosphorene ca. 0.9 nm 24 Zirconium Phosphates (a) α-zirconium phosphate = Zr(HPO4)2.H2O interlayer spacing 7.6 Å (b) γ-zirconium phosphate = Zr(PO4)(H2PO4)2H2O interlayer spacing 12.2 Å Brucite - Mg(OH)2 25 trioctahedral H-bonds Shares 6 edges Bayerite and Gibbsite - Al(OH)3 26 Opposite faces of a single layer Al(OH)3 (A and B sides, respectively) dioctahedral Shares 3 edges Bayerite and Gibbsite - Al(OH)3 27 Gibbsite is stacked by AB-BA sequence CCP of oxides Bayerite and Gibbsite phases have an identical single layer as the building block Bayerite is stacked by AB-AB sequence HCP of oxides 28 Clay Minerals 2:1 1:1 kaolinitemontmorillonite [Mg6O12]12- trioctahedral sheet of octahedral units [Al4O12]12- dioctahedral sheet of octahedral units [Si4O10]4- tetrahedral sheet 29 Montmorillonite (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2ꞏ10H2O Phyllosilicate Minerals 30 T = tetrahedral sheet O = octahedral sheet Phyllosilicate Minerals 31 Mica 32 Layered Double Hydroxides The layered structure of LDH is closely related to brucite Mg(OH)2 A brucite layer, Mg2+ ions octahedrally surrounded by six OHthe octahedra share 6 edges and form an infinite two-dimensional layer the brucite-like layers stack on top of one another either rhombohedral (3R) or hexagonal (2H) sequence Natural mineral Hydrotalcite Mg6Al2(OH)16CO3.4H2O - 3R stacking Brucite layers, Mg2+ substituted partially by Al3+ Layers have positive charge [MII 1-xMIII x (OH)2]x+(Am-)x/m]ꞏnH2O x = 0.25 Mg6Al2(OH)16CO3 x = 0 Mg(OH)2 33 Hydrotalcite Hydrotalcite Mg6Al2(OH)16CO3.4H2O - 3R stacking Unit cell parameters: a = 0.305 nm c = 3 d(003) = 2.281 nm The interlayer spacing: d(003) = 0.760 nm the spacing occupied by the anion (gallery height) = 0.280 nm a thickness of the brucite-like layer = 0.480 nm Interlayer (basal) spacing d(003) = 0.760 nm The brucite-like layer 0.480 nm Gallery height 0.280 nm 34 Layered Double Hydroxides - Hydrotalcites Brucite layers, M2+ substituted partially by M3+ Layers have positive charge (a) [Ca2Al(OH)6]2SO4.6H2O (b) [LiAl2(OH)6]Cl (c) [Mg2.25Al0.75(OH)6]OH 35 Intercalation to LDH The intercalation of methylphosphonic acid (MPA) into Li/Al LDH (a) [LiAl2(OH)6]Cl.H2O (b) second-stage intermediate, alternate layers occupied by Cl and MPA anions (c) first-stage product with all interlayer regions occupied by MPA 36 The Anionic Exchange Capacity (AEC) 37 LDH Composite Structures Li Intercalation Compounds 38 39 Li Intercalation Compounds x Li + TiS2  LixTiS2 40 Li Intercalation Compounds Li/C  e + Li+ + C Li+ + e + Fe(III)PO4  LiFe(II)PO4 Molybdenum Disulfide (MoS2) 41 Mineral molybdenite Hydrodesulfurization catalyst at edges Lubricant Polymorphs of MoS2 42 semiconductors metallic MoS6 octahedralMoS6 trigonal prismatic 2H phase - thermodynamically stable 1T and 3R polymorphs - metastable Polymorphs of MoS2 43 Digit = number of monolayers in the unit cell Letters: T = trigonal, H = hexagonal, R = rhombohedral 1T - MoS6 octahedral2H, 3R - MoS6 trigonal prismatic Monolayer of Molybdenum Disulfide 44 Fermi level An indirect band gap 1.29 eV A direct band gap 1.9 eV (2H) Conduction band Valence band photoluminescence Monolayer of Molybdenum Disulfide 45 (b,c) infraredand (d−f) Raman-active Frequency of A1g band is increasing while that of E1 2g is decreasing with increase in number of layers MoS2 nanosheets - all sulfur atoms exposed on surfaces S = a soft Lewis base - a high affinity for heavy metal ions (e.g., Hg2+ and Ag+) = soft Lewis acids MoS2 nanosheets • high adsorption capacity - abundant sulfur adsorption sites • fast kinetics - easy access to adsorption sites Synthesis of Molybdenum Disulfide 46 Nature Reviews Materials volume 2, Article number: 17033 (2017) 47 3D Intercalation Compounds Tungsten trioxide WO3 Structure = WO6 octahedra joined at their corners = ReO3 = the perovskite structure of CaTiO3 with all the calcium sites vacant The color and conductivity changes are due to the intercalation of protons into the cavities in the WO3 structure, and the donation of their electrons to the conduction band of the WO3 matrix The material behaves like a metal, with both its conductivity and color being derived from free electron behavior The coloration reaction used in electrochromic displays for sun glasses, rear view mirrors in cars Zn + 2 HCl  2 H + ZnCl2 WO3 + x H  HxWO3 48 0D Intercalation Compounds C60 = FCC K3 C60 Octahedral voids (N) Tetrahedral voids (2N) K