Sol-Gel Methods 1 Hydrolysis Polycondensation Gelation Ageing Drying Densification Sol-gelprocess Powders: microcrystalline, nanocrystalline, amorphous Monoliths, Coatings, Films, Fibers Aerogels Glasses, Ceramics, Hybrid materials Sol-Gel Methods PRECURSOR Sol-Gel Methods 2 Sol = a fluid system of stable suspension of colloidal (1 nm – 1 m) solid particles or polymeric molecules in a liquid (Below 1 m – Brownian motion, above – sedimentation) Gel = nonfluid, porous, three-dimensional, continuous solid network (elastic or rigid) surrounded by a continuous liquid phase Colloidal (particulate) gels = agglomeration of dense colloidal particles Polymeric gels = agglomeration of polymeric particles made from subcolloidal units Agglomerate = assemblage of particles rigidly joined together, as by partial fusion (sintering) or by growing together, covalent bonds, hydrogen bonds, polymeric chain entanglement Aggregate = assemblage of particles which are loosely coherent, van der Walls forces Sol-Gel Methods Sol-Gel Methods 3 Sol and Gel Gel point = point of incipient network formation Sol-to-Gel transition is difficult to define Rheological methods = viscosity increases Sol-Gel Methods 4 Sol-Gel Methods 5 Sol-Gel Methods Sol-Gel Methods 6 Aqueous • Colloid Route – inorganic salts, water glass, pH, hydrolysis, polycondensation • Metal-Oragnic Route – metal alkoxides, amides, hydrolysis, polycondensation • Pechini and Citrate Gel Method – inorganic metal salts, complexing agent, chelate formation, polyesterification with polyfunctional alcohol Nonaqueous • Hydroxylation • Heterofunctional Condensations Sol-Gel Chemistry Sol-Gel Methods 7 Colloid Route Metal salts in aqueous solution, pH and temperature control Solvation – water molecule becomes more acidic Mz+ + :OH2  [M  OH2]z+ For transition metal cations, charge transfer occurs from the filled bonding orbital of the water molecule to the empty d orbitals of the transition metal. Therefore, the partial positive charge on the H of water molecule increases, making the water molecule more acidic. Hydrolysis [M(H2O)b]Z+ ⇄ [M(H2O)b-1OH](Z-1)+ + H+ Condensation-polymerization [M(H2O)b]Z+ ⇄ [(H2O)b-1M(OH)2M(H2O)b-1](2Z-2)+ + 2H+ Sol-Gel Methods 8 Colloid Route OH2 AlH2O OH2 OH2 H2O OH2 3 OH2 AlH2O OH OH2 H2O OH2 2 pH < 3 pH = 4 - 5 B HB+ pH = 5 - 7 Al(OH)3 [Al(OH)4]- [Al(H2O)4(OH)2]the Keggin cation [Al13O4(OH)24(OH2)12]7+ Colloid Route Sol-Gel Methods 9 [M(OH2)]z+ ⇄ [M–OH](z-1)+ + H+ ⇄ [MO](z-2)+ + 2 H+ Depending on the water acidity and the charge transfer, the following equilibria are established: Aqua Hydroxo Oxo Only hydroxo can condense Sol-Gel Methods 10 Colloid Route Reduction Oxidation Acid addition Base addition Fe2+(aq) + CO3 2  ? Fe3+ (aq) + CO3 2  ? Sol-Gel Methods 11 Pechini Sol-Gel Route The transesterification reaction between citric acid and ethylene glycol Sol-Gel Methods 12 Pechini Sol-Gel Route Major components Dopants Gelling agent Doped YAG product Removal of organics Removal of solvents Sol-Gel Methods 13 Pechini Sol-Gel Route EG : CA : M Sol-Gel Methods 14 Metal Alkoxides [M(OR)x]n + H2O  ROH + M-O-H Metal Amides [M(NR2)x]n + H2O  R2NH + M-O-H 2 M-O-H  M-O-M + H2O Hydrolysis Polycondensation OXIDE Metal-Organic (Alkoxide) Route Sol-Gel Methods 15 Metal-Organic (Alkoxide) Route Oligomers formed by hydrolysis-condensation process -linear -branched -cyclic -polyhedral Never goes to pure SiO2 n Si(OR)4 + 2n+(ab)/2 H2O  SinO2n(a+b)/2(OH)a(OR)b + (4nb) ROH Sol-Gel Methods 16 Metal Alkoxides and Amides as Precursors Metal Alkoxides [M(OR)x]n formed by the replacement of the hydroxylic hydrogen of an alcohol (ROH) through a metal atom Most frequently used precursor for sol-gel: TEOS = Si(OEt)4 Metal Amides [M(NR2)x]n formed by the replacement of one of the hydrogen atoms of an amine (R2NH) through a metal atom Sol-Gel Methods 17 Metal Alkoxides and Amides as Precursors Homometallic Alkoxides General Formula: [M(OR)x]n Heterometallic Alkoxides General Formula: MaM’b(OR)x]n Metal Amides General Formula: [M(NR2)x]n M = Metal or metalloid of valency x O = Oxygen Atom N = Nitrogen atom R = simple alkyl, substituted alkyl or aryl group n = degree of molecular association Sol-Gel Methods 18 Modified Silicon Alkoxides as Precursors Terminal groups Bridging groups Polymerizable groups Functional groups Silsesquioxanes = RSiO1.5 ( = 3/2) Hybrid Inorganic-Organic Materials Sol-Gel Methods 19 Organization in Xerogels of Bridged Silicon Alkoxide Precursors Sol-Gel Methods 20 Self-Assembly of Bridged Silsesquioxanes Sol-Gel Methods 21 Nanostructuring of hybrid silicas through a Self-Recognition Process - the crystallization of the hydrolyzed species by H-bonding followed by their polycondensation in solid state 1,4-Bis(triethoxysilyl)propylureidobenzene Templating Porosity in Bridged Polysilsesquioxanes Sol-Gel Methods 22 Polyhedral Oligomeric Silsesquioxanes (POSS) Sol-Gel Methods 23 Polymers and Copolymers of POSS Sol-Gel Methods 24 Polymers and Copolymers of POSS Sol-Gel Methods 25 a certain degree of crystallinity Sol-Gel Methods 26 Sol-Gel in Silica Systems Si O Si OR H+ H2O -ROH Si OH SiHO SiRO -H2O -ROH Si O Si Hydrolysis PolycondensationSilicate (aq) Alkoxide (nonaq) Sol-Gel Methods 27 H O H RO Si O RO RO R H OR Si O OR RO R H OR SiO OR OR H H O H + ROH + H Acid catalysed hydrolysis H O RO Si OR RO RO OR Si O OR RO R OR SiO OR OR HH O + RO Base catalysed hydrolysis  Metal-organic Route metal alkoxide in alcoholic solution, water addition Metal-Oragnic (Alkoxide) Route Metal-Oragnic (Alkoxide) Route Sol-Gel Methods 28 Isotope labelling experiments Sol-Gel Methods 29 Metal-Oragnic (Alkoxide) Route Oligomers formed by hydrolysis-condensation process -linear -branched -cyclic -polyhedral Never goes to pure SiO2 n Si(OR)4 + 2n+(ab)/2 H2O  SinO2n(a+b)/2(OH)a(OR)b + (4nb) ROH Sol-Gel Methods 30 GC of TMOS hydrolysis products Si(OMe)4 + H2O Sol-Gel Methods 31 Neg. ion ESI-MS and 29Si NMR of silicate aq with TMA ions Q - notation Sol-Gel Methods 32 Q0 = O4Si Q1 = O3SiOSi Q2 = O2Si(OSi)2 Q3 = OSi(OSi)3 Q4 = Si(OSi)4 the notation of Qa b, ‘‘Q’’ stands for the maximum 4 siloxane bonds for each silicon, ‘‘a’’ is the actual number of siloxane bonds on each Si, and ‘‘b’’ is the number of Si in the unit Sol-Gel Methods 33 Silicate anions in aqueous alkaline media (detected by 29Si-NMR) M = OSiR3 D = O2SiR2 T = O3SiR Q = O4Si Q0 = O4Si Q1 = O3SiOSi Q2 = O2Si(OSi)2 Q3 = OSi(OSi)3 Q4 = Si(OSi)4 Sol-Gel Methods 34 Silicate anions Sol-Gel Methods 35 Silicate anions Sol-Gel Methods 36 Si50O75(OH)50 three-dimensional clusters formed by (A) four-rings (B) six-rings Oligomers formed by hydrolysis-condensation Sol-Gel Methods 37 IR spectrum of silica , cm-1 460800 1100 Sol-Gel Methods 38 The electrical double layer at the interface of silica and a diluted KCl solution  = local potential OHP = outer Helmholtz plane u = local electroosmotic velocity Negative surface charge stems from deprotonated silanols Shielding of this surface charge occurs due to adsorbed ions inside the OHP and by mobile ions in a diffuse layer The shear plane = where hydrodynamic motion becomes possible Zeta = potential at the shear plane The Electrical Double Layer Silica surface Solution Zeta Sol-Gel Methods 39 Isoelectronic point: zero net charge pH = 2.2 for silica Sol-Gel Methods SiOH SiOH2 + SiO Sol-Gel Methods 40 Rate of H+ catalyzed TEOS hydrolysis (gel time) as a function of pH Effects on hydrolysis rate: pH substituents solvent water Sol-Gel Methods Longest TEOS gel time = the slowest reaction Sol-Gel Methods 41 Precursor substituent effects: Steric effects: branching and increasing of the chain length LOWERS the hydrolysis rate Si(OMe)4 > Si(OEt)4 > Si(OnPr )4 > Si(OiPr)4 > Si(OnBu)4 Inductive effects: electronic stabilization/destabilization of the transition state (TS). Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi Sol-Gel Methods Partial Charge Model (Livage and Henry) Sol-Gel Methods 42 Electron transfer occurs when atoms combine to give a molecule. Charge transfer causes each atom to acquire a partial positive or negative charge, i. This transfer mainly depends on the electronegativity difference between atoms. The electronegativity i of an atom varies linearly with its partial charge i . i = o i + k i Electron transfer must stop when all electronegativities have the same value called the mean electronegativity. Partial Charge Model (Livage and Henry) Sol-Gel Methods 43 The mean electronegativity of a molecule z = the electric charge for ions k = a constant that depends on the electronegativity scale (k = 1.36 in Pauling's units). The partial charge i on an element in the molecule Partial Charge Model (Livage and Henry) Sol-Gel Methods 44 Alkoxide Zr(OEt)4 Ti(OEt)4 Nb(OEt)5 Ta(OEt)5 VO(OEt)3 W(OEt)6 Si(OEt)4 (M) +0.65 +0.63 +0.53 +0.49 +0.46 +0.43 +0.32 The hydrolysis rate depends on the (M): kh  5·10-9 mol-1s-1 for Si(OEt)4 kh  10-3 mol-1s-1 for Ti(OEt)4 Partial Charge Model Sol-Gel Methods 45 the number of valence electrons n* on the central atom of a radical AB N = the number of valence electrons on the free atom A p = the number of valence electrons supplied by B when forming the A–B bond. m = the number of bonds between A and B s = the number of resonance contributions from A B+ Group electronegativity g rA = the covalent radius of atom A in the radical AB. Partial Charge Model Sol-Gel Methods 46 Sol-Gel Methods 47 H O H RO Si O RO RO R H OR Si O OR RO R H OR SiO OR OR H H O H + ROH + H Acid catalysed hydrolysis Hydrolysis Transition State Acidic conditions: Hydrolysis reaction rate decreases as more alkoxy groups are hydrolyzed Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi TS (+) is destabilized by increasing number of electron withdrawing OH groups (wrt OR) The reaction at terminal Si favored, as there is only one electron withdrawing SiO group Linear polymer products are favored, leading to fibers RSi(OR)3 is more reactive than Si(OR)4 Sol-Gel Methods 48 Hydrolysis H O RO Si OR RO RO OR Si O OR RO R OR SiO OR OR HH O + RO Base catalysed hydrolysis Transition State Basic conditions: Hydrolysis reaction rate increases as more alkoxy groups are hydrolyzed Electron density at Si decreases: RSi > ROSi > HOSi > SiOSi TS () is stabilized by increasing number of electron withdrawing OH groups (wrt OR) The reaction at central Si favored, as there is more electron withdrawing SiO groups Branched polymer products are favored, spherical particles, powders RSi(OR)3 less reactive than Si(OR)4 Sol-Gel Methods 49 Si-OH becomes more acidic with increasing number of Si-O-Si bonds Sol-Gel Methods Nucleophilic catalysis: F- Si-F bonds HMPA N-methylimidazol N,N-dimethylaminopyridin Sol-Gel Methods 50 Water:alkoxide ratio (Rw) effect stoichiometric ratio for complete hydrolysis = 4 Si(OR)4 + 4 H2O Si(OH)4 + 4 ROH additional water from condensation Si-OH + HO-Si Si-O-Si + H2O Small amount of water = slow hydrolysis due to the reduced reactant concentration Large amount of water = slow hydrolysis due to the reactant dilution Sol-Gel Methods Sol-Gel Methods 51 Hydrophobic effect Si(OR)4 are immiscible with water cosolvent ROH is used to obtain a homogeneous reaction mixture polarity, dipole moment, viscosity, protic behavior alcohol produced during the reaction alcohols - transesterification sonication drying Sol-Gel Methods Sol-Gel Methods 52 Acid catalysed condensation fast protonation, slow condensation RO Si O RO RO H H OR SiO OR OR H RO Si O RO RO OR Si OR OR + + H3O Condensation Positively charged transition state, fastest condensation for (RO)3SiOH > (RO)2Si(OH)2 > ROSi(OH)3 > Si(OH)4 TS (+) is destabilized by increasing number of electron withdrawing OH groups Hydrolysis fastest in the first step, i.e. the formation of (RO)3SiOH Condensation for this species also fastest, the formation of linear chains TS Sol-Gel Methods 53 Condensation Base catalysed condensation fast deprotonation, slow condensation RO Si O RO RO RO Si OH RO RO RO Si O RO RO OR Si OR OR + + OH Negatively charged transition state, fastest condensation for (RO)3SiOH < (RO)2Si(OH)2 < ROSi(OH)3 < Si(OH)4 TS () is stabilized by increasing number of electron withdrawing OH groups Hydrolysis speeds up with more OH, i.e. the formation of Si(OH)4 Condensation for the fully hydrolysed species fastest, the formation of highly crosslinked particles TS Sol-Gel Methods 54 Reaction limited monomer cluster growth (RLMC) or Eden growth Reaction limited cluster aggregation (RLCA) Acid catalysed Base catalysed Sol-Gel Methods 55 Base catalysed condensation • condensation to highly crosslinked particles • large primary particles • mesoporosity, Type IV isotherms Acid catalysed condensation • condensation to linear chains • small primary particles • microporosity, Type I isotherms Sol-Gel Methods 56 Gel point - a spannig cluster reaches across the container, sol particles, oligomers and monomer still present a sudden viscosity increase at the gel point further crosslinking - increase in elasticity Gelation = Sol-to-Gel Transition Bond Percolation Sol-Gel Methods 57 p = the fraction of created links sav(p) = average cluster size lav(p) = average spanning length P(p) = percolation probability = a bond is added to a spanning cluster Sol-to-Gel Transition Sol-Gel Methods 58 Sol-to-Gel Transition Sol-Gel Methods 59 c = condensation degree, max 83 % Sol-Gel Methods 60 Ageing Crosslinking condensation of the OH surface groups, stiffening and shrinkage Syneresis shrinkage causes expulsion of liquid from the pores Coarsening materials dissolve from the convex surfaces and deposits at the concave surfaces: necks Rippening Smaller particles have higher solubility thean larger ones Phase separation Fast gelation, different miscibility, isolated regions of unreacted precursor, inclusions of different structure, opaque, phase separation Ageing of Gel Sol-Gel Methods 61 1. The constant rate period the gel is still flexible and shrinks as liquid evaporates 2. The critical point the gel becomes stiff and resists further shrinkage, the liquid begins to recede (contact angle ) into the pores (radius r), surface tension  creates large capillary pressures Pc, stress, cracking 3. The first falling-rate period a thin liquid film remains on the pore walls, flows to the surface and evaporates, the menisci first recede into the largest pores only, as these empty, the vapor pressure drops and smaller pores begin to empty 4. The second falling-rate period liquid film on the walls is broken, further liquid transport by evaporation Drying r Pc  cos2  Drying Sol-Gel Methods 62 Sol-Gel Methods 63 1. Supercritical drying 2. Freeze-drying 3. Drying control chemical additives 4. Ageing 5. Large pore gels Drying Methods r Pc  cos2  To avoid cracking: •No meniscus •Decrease surface tension •Increase wetting angle (isopropanol) •Increase pore size •Make a stiff gel Sol-Gel Methods 64 25 mmol (5.2 g) of tetraethoxysilane (TEOS) TEOS:H2O:HNO3:ammonium acetate molar ratio of the solution is 1 : 10 : 0.002 : 0.02 Sol-Gel Methods 65 Aerogels 1931 Steven S. Kistler J. Phys. Chem. 34, 52, 1932 Aerogels = materials in which the typical structure of the pores and the network is largely maintained while the pore liquid of a gel is replaced by air The record low density solid material - 0.001 g/cm3 density of air 1.2 mg/cm3 Sol-Gel Methods 66 Aerogels - Supercritical Drying Silica aerogel From sodium silicate – 3 steps •Salt washing •Water replacement •Supercritical drying From silicon alkoxides – 1 step •Supercritical drying Sol-Gel Methods 67 Supercritical Drying Cold supercritical drying path in the Pressure (P) Temperature (T) phase diagram of CO2 Sol-Gel Methods 68 Supercritical Drying Sol-Gel Methods 69 Densification Stage I. Below 200 C, weight loss, no shrinkage pore surface liquid desorption Stage II. 150 - 700 C, both weight loss and shrinkage loss of organics - weight loss further condensation - weight loss and shrinkage structural relaxation - shrinkage Stage III. Above 500 C, no more weight loss, shrinkage only close to glass transition temperature, viscous flow, rapid densification, large reduction of surface area, reduction of interfacial energy, termodynamically favored Densification Sol-Gel Methods 70 Densification - Sintering Sol-Gel Methods 71 Densification Densification - Sintering Sol-Gel Methods 72 Sintering mechanisms - solid, liquid, gas phase 1. Evaporation-condensation and dissolution-precipitation 2. Volume diffusion 3. Surface diffusion 4. Grain boundary diffusion 5. Volume diffusion from grain boundaries 6. Volume diffusion from dislocations – plastic flow Volume diffusion from dislocations vacancies Viscous flow Sintering mechanisms Sol-Gel Methods 73 Sintering Mechanisms Sol-Gel Methods 74 Sintering Mechanisms Sol-Gel Methods 75 Sintering Mechanisms Sol-Gel Methods 76 Sintering Mechanisms Sol-Gel Methods 77 Sintering Mechanisms Sol-Gel Methods 78 Dehydration sequence of hydrated alumina in air Path (b) is favored by moisture, alkalinity, and coarse particle size (100µm) path (a) by fine crystal size (<10µm) Sol-Gel Methods 79 HT-XRD of the phase transitions g = Gibbsite -Al(OH)3 b = Boehmite -Al(O)OH  = -Al2O3 alumina  = -Al2O3 Corundum Sol-Gel Methods 80 Gibbsite to Boehmite to Gamma Gibbsite -Al(OH)3 to Boehmite -Al(O)OH to -Al2O3 alumina (defect spinel) CCP Sol-Gel Methods 81 27Al Solid-State NMR spectra Sol-Gel Methods 82 Bayerite to Diaspore to Corundum Bayerite -Al(OH)3 to Diaspore -Al(O)OH to -Al2O3 Corundum HCP Sol-Gel Methods 83 Oxygen Coordination Metal Coordination Sol-Gel Methods 84 Metal-Oxide Clusters Sol-Gel Methods 85 Metal-Oxide Clusters