Sol-Gel Methods 1 Hydrolysis Condensation Gelation Ageing Drying Densification Sol-gel process: Powders: microcrystalline, nanocrystalline, amorphous Monoliths, Coatings, Films, Fibers Aerogels Glasses, Ceramics, Hybrid materials Sol-Gel Methods Sol-Gel Methods 2 Sol = a stable suspension of colloidal solid particles or polymers in a liquid Gel = porous, three-dimensional, continuous solid network surrounding a continuous liquid phase Colloidal (particulate) gels = agglomeration of dense colloidal particles Polymeric gels = agglomeration of polymeric particles made from subcolloidal units Agglomeration = covalent bonds, hydrogen bonds, polymeric chain entanglement Aggregation = van der Walls forces Sol-Gel Methods Sol-Gel Methods 3 Sol and Gel Sol-Gel Methods 4 Sol-Gel Methods 5 Sol-Gel Methods Sol-Gel Methods 6 Aqueous •Colloid Route – inorganic salts, 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 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+ Colloid Route 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+ Sol-Gel Methods 9 Colloid Route Reduction Oxidation Acid addition Base addition Fe2+(aq) + CO3 2− → ? Fe3+ (aq) + CO3 2− → ? Sol-Gel Methods 10 Pechini Sol-Gel Route Major components Dopants Gelling agent Doped YAG product Removal of organics Removal of solvents Sol-Gel Methods 11 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 Condensation OXIDE Metal-Oragnic (Alkoxide) Route Sol-Gel Methods 12 Metal Alkoxides and Amides Metal Alkoxides [M(OR)x]n formed by the replacement of the hydroxylic hydrogen of an alcohol (ROH) through a metal atom 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 13 Metal Alkoxides and Amides 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 14 Sol-gel in Silica Systems Si O Si OR H+ H2O -ROH Si OH SiHO SiRO -H2O -ROH Si O Si Hydrolysis Condensation Silicate (aq) Alkoxide (nonaq) Sol-Gel Methods 15 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 Route Sol-Gel Methods 16 Metal-Organic 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 17 GC of TMOS hydrolysis products Si(OMe)4 + H2O Sol-Gel Methods 18 Neg. ion ESI-MS and 29Si NMR of silicate aq with TMA ions Sol-Gel Methods 19 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 20 Si50O75(OH)50 three-dimensional clusters formed by (A) four-rings (B) six-rings Sol-Gel Methods 21 Sol-Gel Methods 22 The electrical double layer at the interface of silica and a diluted KCl solution ψ, 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 Potential and EOF velocity profiles are shown at right The shear plane is where hydrodynamic motion becomes possible; z is the potential at this plane The Electrical Double Layer Sol-Gel Methods 23 The Electrical Double Layer Sol-Gel Methods 24 Isoelectronic point: zero net charge pH = 2.2 for silica Sol-Gel Methods Sol-Gel Methods 25 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 26 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 > Si(OHex)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 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 Hydrolysis Transition State Acidic conditions: Hydrolysis reaction rate decreases as more alkoxy groups are hydrolyzed TS (+) is destabilized by increasing number of electron withdrawing OH groups The reaction at terminal Si favored, as there is only one electron withdrawing SiO group Linear polymer products are favored, fibers RSi(OR)3 is more reactive than Si(OR)4 Sol-Gel Methods 28 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 TS (−) is stabilized by increasing number of electron withdrawing OH groups 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 29 Si-OH becomes more acidic with increasing number of Si-O-Si bonds Sol-Gel Methods Nucleophilic catalysis: F- HMPA N-methylimidazol N,N-dimethylaminopyridin Sol-Gel Methods 30 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 31 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 32 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 33 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 34 Reaction limited monomer cluster growth (RLMC) or Eden growth Reaction limited cluster aggregation (RLCA) Acid catalysed Base catalysed Sol-Gel Methods 35 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 36 Gelation 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-Gel Methods 37 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 Sol-Gel Methods Sol-Gel Methods 38 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 θ) into the pores (radius r), surface tension γ 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 θγ cos2 = Sol-Gel Methods 39 1. Supercritical drying 2. Freeze-drying 3. Drying control chemical additives 4. Ageing 5. Large pore gels Drying Methods r Pc θγ cos2 = To avoid cracking: •No meniscus •Decrease surface tension •Increase wetting angle (isopropanol) •Increase pore size •Make a stiff gel Sol-Gel Methods 40 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 41 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 42 Supercritical Drying Cold supercritical drying path in the Pressure (P) Temperature (T) phase diagram of CO2 Sol-Gel Methods 43 Supercritical Drying Sol-Gel Methods 44 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 45 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, vacancies Sintering mechanisms Sol-Gel Methods 46 Densification Densification Sol-Gel Methods 47 Densification Sol-Gel Methods 48 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 49 HT-XRD of the phase transitions g = Gibbsite γ-Al(OH)3 b = Boehmite γ-Al(O)OH γ = γ-Al2O3 alumina α = α-Al2O3 Corundum Sol-Gel Methods 50 Gibbsite to Boehmite to Gamma Gibbsite γ-Al(OH)3 to Boehmite γ-Al(O)OH to γ-Al2O3 alumina (defect spinel) CCP Sol-Gel Methods 51 27Al Solid-State NMR spectra Sol-Gel Methods 52 Bayerite to Diaspore to Corundum Bayerite α-Al(OH)3 to Diaspore α-Al(O)OH to α-Al2O3 Corundum HCP Sol-Gel Methods 53 Oxygen Coordination Metal Coordination Sol-Gel Methods 54 Metal-Oxide Clusters Sol-Gel Methods 55 Metal-Oxide Clusters