Sol-Gel Methods Sol-gel process: Hydrolysis Condensation Gelation Ageing Drying Densification Powders: microcrystalline, nanocrystaUine, amorphous Monoliths, Coatings, Films, Fibers Aerogels Glasses, Ceramics, Hybrid materials Sol-Gel Methods Sol-Gel Methods 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, van der Walls, hydrogen bonds, polymeric chain entanglement Sol-Gel Methods 2 Sol-Gel Process Solution \ spin or dip coating films Sol t, 4 j $> ITS •) drawing extrusion fibers SC drying Gel drying Aerogel \$ Sp f* X...A .H yl at,t) o,) A goo*« ""0. ÍÍÍÍ* Xerogel ^"ífltffij« gam casting sintering ceramics, glass t Monolith UBS Sol and Gel RjŕacEams Solvents Colloidal particle n _ Q D Ü ft Catalyst Colloidal S-dlmenMoriEul grid Colloidal suspension of volume V [M(H20)blOH](zl)+ + H + Condensation-polymerization M(H20)bz+ +> [(H20)blM(OH)2M(H20)bl](2Z2)+ + 2H+ z +7 - +6 - o2- +5 - +4 —"""'*"■-"*, HO +3 - +2 — H20 +1 i i 0 14 PH Colloid Route 3© B HB+ OH, H20-H201 ■^ yx OH- AF—OH OH, 2© pH = 4-5 [Al,A(OH);d(H:0)l2r+ • ti-OH2 o(x4-0 • [x2-OH T o ji2-OH tT the Keggin cation [AI1304(OH)24(OH2)12P Sol-Gel Methods [AI(H20)4(OH)2]- pH = 5-7 AI(OH); [AI(OH)4]- Fe2+(aq) + C032 Colloid Route Fe3+ (aq) + co 2 Reduction [Mn04]- [Al(OII2)Ďl3+ Base addition tFe(OH2)ü]2+ Oxidation t) Acid addition [V04]u 14 pH Sol-Gel Methods Sol - Gel Procedure Stirring for 10 h at 60-65 °C Stirring for 3 h at 60-65 °C Stirring for 3 h at 60-65 °C Evaporation and drying at 110 °C Calcination for 2 h bei 800 CC Annealing for 10 h at 1000 °C Y203| 0.2 M CH3COOH AI(N03)3, aq. '' Transparent solution 1 Nd(N03)jaq., or Ho(CH3COO)3 aq. ' Transparent solution Complexing agent 1.2 ethanediol.... 1 ' Sol ' • Amorphous gel precursor i ' Preheated mixture < ' Polycrystalline single-phase Major components Dopants Gelling agent Removal of solvents Removal of organics Doped YAG product 8 Metal-Oragnic (Alkoxide) Route Hydrolysis Metal Aikoxides [M(OR)x]n + H20 -> ROH + M-O-H Metal Amides [M(NR2)x]n + H20 -> R2NH + M-O-H Condensation 2 M-O-H-> MOM +H20 OXIDE Sol-Gel Methods 9 Metal Alkoxides and Amides mH Metaí center Organic rest Organic rest Ligand Metal center Ligand Metal Alkoxides [M(OR)x]n formed by the replacement of the hydroxy lie 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 10 Metal Alkoxides and Amides Homometallic Alkoxides General Formula: [M(OR)x]n Heterometallic Alkoxides General Formula: MJVľb(OR)x]n Metal Amides 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 General Formula: [M(NR2)x]n Sol-Gel Methods 11 Metal Coordination Í-M-Q 0 «K V 0 0 o.? 0 3 c e g i <^%> 0 0 * o^ o Oxygen Coordination M=0 M—0—U ",1 r/ ■f- rerttUiiaL linear trigonal planar telrabídral bipyrainidal m-% M-CK—M bent trigonal pyramidal butterfly octahedral M—O—M M2t«-0) linear M M \/ O M20í/-O>3 planar M M bent M —M folded MzCu-0)., «I I I o-. M M* M ■a*" M «r0-«. Metal-Oxide Clusters ÍAAl o 0 o 1 M O I M °^M-*° -M / MjlŕO)* H-o-/ U-......'"Oh MiLuj-OJí ň Ö PŕA. M "€-4^" o^o M^-OJ* ^M--------O ' kt.....V MJjb-Ol, Sol-Gel Methods 14 Sol-gel in Silica Systems Silicate (aq) / ^^^^^^^H \ e -------Si-----0 / \H+ Hydrolysis HO- —Si------- \ -H20 "^\ Condensation -------Si-----0-----Si—------ \ -------Si- —OH \ H20/< /^ROH / -------Si-----OR -RQ\\^^^ / / Alkoxide (nonaq) RO Si \ Sol-Gel Methods 15 Metal-Oragnic Route é^Ŕ Metal-organic Route metal alkoxide in alcoholic solution, water addition Acid catalysed hydrolysis RO ?R OR \ „—A © /h \ © / h / © /C/-< — >......-;t.....\ — --^„ *-•■ RO RÖ \D OR UK Base catalysed hydrolysis Sol-Gel Methods 16 OR Fď^ H,0 cat. OR Metal-Organic Route *ifK™ OH ™^Sl^OH RO" OR OR I HO' mo* OR I h Si ¥ OH OH io^Si^ HO<° -OH OR \ RO' RO"* OH OR I I ., OR tR EO^ OH cŕ ,Si SL RO"i RO1* OR I rSi ř"' ""OH OR I Si. ■OR tiE ™?"'^y ^ OE I. EOv^rS\ EO* TO EO ď d*f' SV OH OE RďK°' ^JOR OE OE I I OR ^RE I R(f OR I ■OR OR °" %T OR \ O HO-^./ R0-/i^°R Oligomers formed by hydrolysis-condensation process -linear -branched -cyclic -polyhedral *ďK°» EO-^./ l\ OE EO^^* RO-^/ 17 GC of TMOS hydrolysis products Si(OMe)4 + H20 Sol-Gel Methods 18 100 565 t>0 77 -------------1------- I "T HUh^Wj^Xi^W ^y. + —I------------------1------------------]------------------1--------- 0.0 -5.0 -10.0"15.0 -20.0 -25-0 -30.0 -35.0 <----- S 413 J/% '37 S5 275 197 J3 173 155 yLyJi^L 215 A& 233 353 335 293 aJ|M llfiiWl^lJ ji Ü.Ü 551 533 ^s i 427 MS SigO^OH^OMe)1 S79 SiB013(0H>5(OMe£ CSS S33 SisOia(OH)4(OMe)3e cos ů/n/z73 AwSr73 O29 ■TMA® 0* ■2TMA* 49 3TMA® 697 770 UkllLlh Lkpl,..* ^ ■ ■■.- Jhfli ň ^——*" ^ ■'- *■ 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 m/z---► 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) *^ >- A M = OSiR D = 02SiR2 T = 03SiR Q = 04Si Q° = 04Si Q1 = 03SiOSi Q2 = 02Si(OSi)2 Q3 = OSi(OSi)3 Q4 = Si(OSi)4 i ;Q0; 4* •—• VÍQV n> ä n M VII VIII IX JtíQM ái<>-^^<5 XI W XII Xlll XIV XV n *v& XVI XVII ((P, > XVflT 1& A \[\ XX m xxi xxii mmol,) xxiv (Q1^) Sol-Gel Methods >^\\ 20 The Electrical Double Layer The electrical double layer at the interface of silica and a diluted KCl solution v|/, local potential OHP, outer Helmholtz plane w, 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 © (^ © O © £jl © © © ----------------v; ■------■ Shear plane OuieľMelmhoÍĽ piatie -(£)---------------4 Sol-Gel Methods The Electrical Double Layer -Particle surface ■ Slern plene ■Surface of sheer " Diffuse Jc-yer ^S(ern layer I A \fc OiiľiinŕE Sol-Gel Methods Sol-Gel Methods Isoelectronic point: zero net charge pH = 2.2 for silica /.ítin í hiiri£i; I'■I'll i i'■ ľ ■ NilitA UíkhiItľm *---------------* . 1 1 , 1 , 1 . 1 , ' I ■ t 2 4 6 N pH 10 12 Sol-Gel Methods Sol-Gel Methods Effects on hydrolysis rate: pH substituents solvent water Longest TEOS gel time = the slowest reaction pH Rate of H+ catalyzed TEOS hydrolysis (gel time) as a function of pH Sol-Gel Methods 24 Sol-Gel Methods Precursor substituent effect Steric effects: branching and increasing of the chain length LOWERS the hydrolysis rate Si(OMe)4 > Si(OEt)4 > Si(OnPr )4 > Si^Pr^ > Si(OnBu)4 > Si(OHex)4 Inductive effects: electronic stabilization/destabilization of the transition state. Electron density at Si decreases: R-Si > RO-Si > HO-Si > Si-O-Si Sol-Gel Methods 25 Hydrolysis Acid catalysed hydrolysis Base catalysed hydrolysis Sol-Gel Methods Sol-Gel Methods Acidic conditions: reaction rate decreases as more alkoxy groups are hydrolyzed reaction at terminal Si favored, linear polymer products, fibers RSi(OR)3 more reactive than Si(OR)4 Basic conditions: reaction rate increases as more alkoxy groups are hydrolyzed reaction at central Si favored, branched polymer products, spherical particles, powders RSi(OR)3 less reactive than Si(OR)4 Si-OH becomes more acidic with increasing number of Si-O-Si bonds Sol-Gel Methods 27 Sol-Gel Methods Water ralkoxide ratio (Rw) effect stoichiometric ratio for complete hydrolysis = 4 Si(OR)4 + 4 H20 Si(OH)4 + 4 ROH additional water from condensation Si-OH + HO-Si Si-O-Si + H20 100 4. io H* EtOH:TEOS 3 2 1 _L J_ 12 4 8 16 WATER:TEOS Small amount of water = slow hydrolysis due to the reduced reactant cone. Large amount of water = slow hydrolysis due to the reactant dilution Sol-Gel Methods 28 Sol-Gel Methods Hydrophobic effect Si(OR)4 are immiscible with water cosolvent ROH to obtain a homogeneous reaction mixture polarity, dipóle moment, viscosity, protic behavior alcohol produced during the reaction alcohols - transesterification sonication drying / Miscible TEOS Immiscible \ ALCOHOL WATER Sol-Gel Methods 29 Condensation Acid catalysed condensation fast protonation, slow condensation Base catalysed condensation fast deprotonation, slow condensation Sol-Gel Methods Condensation Acid catalysed condensation positively charged transition state, fastest condensation for (RO)3SiOH > (RO)2Si(OH)2 > ROSi(OH)3 > Si(OH)4 hydrolysis fastest in the first step, i.e. the formation of (RO)3SiOH condensation for this species also fastest, the formation of linear chains Base catalysed condensation negatively charged transition state, fastest condensation for (RO)3SiOH < (RO)2Si(OH)2 < ROSi(OH)3 < Si(OH)4 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 Sol-Gel Methods 31 Reaction limited cluster aggregation (RLCA) cljster-busier RLCA monorfler-clusler RLMC Eden growth Reaction limited monomer cluster growth (RLMC) or Eden growth Sol-Gel Methods 32 Acid catalysed condensation condensation to linear chains small primary particles microporosity, Type I isotherms Base catalysed condensation condensation to highly crosslinked particles large primary particles mesoporosity, Type IV isotherms Sol-Gel Methods Gelation Gelation gel point - a spannig cluster reaches across the container, sol particles, olig and monomer still present a sudden viscosity increase at the gel point 200 further crosslinking - increase in elasticity 1 Ä S íoo I 7 8 Time/104s Sol-Gel Methods Sol-Gel Methods 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 35 Sol-Gel Methods Drying 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 into the pores, surface tension creates large pressures, capillary 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 Sol-Gel Methods 36 Sol-Gel Methods Drying methods 1. Supercritical drying 2. Freeze-drying 3. Drying control chemical additives 4. Ageing 5. Large pore gels Sol-Gel Methods 37 Aerogels Aerogels = materilas 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 density is only three times that of air Aerogels - Supercritical Drying Gas inlet 'lliermoDOLple Ethanol —id Pressure iaujfc Vapor oullct Teflon ring ■|j.i:oľ 'Al» gel Autoclave Sol-Gel Methods Supercritical Drying P (MPs) A 7.37 Supercritical drying path ť1 ■ y. * Triple point Vapor *T ČřSTíaL poinl :-■ 31.0 T(*C) Cold supercritical drying path in the Pressure (P) Temperature (T) phase diagram of C02 Sol-Gel Methods 40 Supercritical Drying fluid formula TC(JC) Fc (MFa) water H20 374.1 22.04 carbon dioxide C02 31.0 737 Freon 116 (CF3)Z 19.7 2.97 acetone ÍCH,)20 235.0 4.66 nitrous oxide NzQ 36.4 7.24 methanol CH3OH 239.4 S.09 ethanol C2HäOH 243.0 6.3 Solvent tc[*q /?,[Mpa] ^[onW]-1] methanol 240 79 118 ethanoL 243 6.3 167 acetone 235 4.7 209 2-propanoI 235 4.7 HnO 374 22.1 56 CO* 31 7.1 94 NnO 37 7.3 97 Densiflcation Densiflcation 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 densiflcation, large reduction of surface area, reduction of interfacial energy, termodynamically favored Sol-Gel Methods Sintering mechanisms 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 Sol-Gel Methods 43 Densifícation Densifícation 2mm lili 2min 9 min l4min 30min x,;n=aie x;a=q;e x/o=[is3 x/oíO.vb Sol-Gel Methods 44 Densifícation U) U) o U) "53 10 - 15 _L - 10 CS V) 75 o 20 Ü 30 200 400 Temperature [°C] Sol-Gel Methods 600 45 Dehydration sequence of hydrated alumina in air i i : Gibbsite E Chi Boehmite _ Bayerite I * r~ i Eta Kappa Alpha Gamma Delta Theta Alpha -> J_____L Theta AJpha Diaspóre i--------r ■> Alpha AJumina J_____L 0 200 400 600 000 1000 Temperature f C\ Hydrated aluminas Transitional aluminas Aluminium oxide (corundum) Path (b) is favored by moisture, alkalinity, and coarse particle size (lOOum) path (a) by fine crystal size (<10um) Sol-Gel Methods 46 HT-XRD of the phase transitions g = Gibbsite y-Al(OH)3 b = Boehmite y-Al(0)OH y = y-Al203 alumina a = a-Al203 Corundum 12ÖOüC,ö L100°C>a?Sfe 1000cCra,3re 9OO°CřY,5f0 800°C> y 700°C, y 600DC, y 500°C, 7 400DC, b,% 300DC, b,x 200ůCř &b 100°C,g V ii** ***••+« '"——■'—■ - — '■ ■—»"■■ ■ľ"! 60 29 (deft) Gibbsite to Boehmite to Gamma Gibbsite y-Al(OH)3 to Boehmite y-Al(0)OH to y-Al203 alumina (defect spinel) CCP Sol-Gel Methods 48 27A1 Solid-State NMR spectra 3D0°C 20EľC 100ÜC _A—■—t—■—t—>—i------>—I—■____I____ 5,8 a .80 40 0-40-80 ppm doi-ut;i ivieinoas i___.___i___.___j___i___i___i___i___i___t_ 80 40 0-40-80 ppm 49 Bayerite to Diaspore to Corundum Bayerite oc-Al(OH)3 to Diaspoře (x-Al(O)OH to a-Al203 Corundum HCP Sol-Gel Methods 50