Molecular Precursors for Sol-Gel Hybrid Materials: Fundamentals. Ulrich Schubert Institute of Materials Chemistry

Transcription

1 olecular Precursors for Sol-Gel ybrid aterials: Fundamentals Ulrich Schubert Institute of aterials hemistry

2 Sol-Gel Process: etwork Formation by ondensation eactions Si Si Si Si + 2

3 Sol-Gel Processing: Development of the etwork Structure onomer Dimer yclic Particle Polymeric network structures Acidic conditions 1 nm 5 nm 10 nm Basic conditions olloidal Structures 30 nm Stöber particles 100 nm Three-dimensional gel networks Sols

4 Precursors 1. The precursors are the building blocks for the later materials. ulti-precursor mixtures allow combining different building blocks in a deliberate ratio and thus to prepare also materials with non-stoichiometric compositions ontrol of the materials properties through the chemical composition. 2. Whether gelation or precipitation occurs is a question of the overall reaction kinetics and is thus influenced by the precursors. Gels are kinetically controlled solids, while crystalline precipitates are formed under thermodynamic control ontrol of the materials properties through the reaction conditions. This includes the kind of employed precursor.

5 ybrid aterials: Building Blocks omposite aterial 1 aterial 2 acroscopic phases Inorganic Building Blocks Si ybrid material olecular building blocks echanical, optical, electrical, magnetical properties onnecting Blocks 2 Si X Y eduction of the crosslinking density, coupling sites between inorganic / organic components rganic Building Blocks A Functional groups, crosslinking, polymerizability Flexibility, elasticity, processability

6 Silica Precursors olloidal silica Si() 4 and Si() 3 preparation etal xide Precursors etal salts aqua, hydoxo and oxo species etal alkoxides Lewis-acidic properties chemical modification

7 Water Glass Solution (olloidal Suspension) 80% of particles in a 0.5 aqueous K 2 Si 2 solution: K 3 Si 4 ( = Si--Si)

8 ydrolysis and ondensation of Silicon Alkoxides Generation of reactive species: hydrolysis Si- + 2 Si- + etwork formation: condensation 2 Si- Si--Si + 2 Si- + -Si Si--Si +

9 Sol-Gel Process: rganically Substituted Precursors () 3 Si X A ydrolyzable group hemically inert spacer (Functional) rganic group early any group, except hydrolytically unstable groups Introduction of organic functionalities Polymerization and crosslinking sites

10 Preparation of Si() 3 (I) (a) l 3 Si- + 2 =-X-A l 3 Si X-A () 3 Si X-A or (b) () 3 Si- + 2 =-X-A () 3 Si X-A Examples: (a) () 3 Si l (b) () 3 Si () 3 Si () 3 Si

11 Preparation of Si() 3 (II) () 3 Si-( 2 ) n -A + A () 3 Si-( 2 ) n -A + A Examples: (Et) 3 Si( 2 ) 3 l + a 2 S 4 (Et) 3 Si( 2 ) 3 -S-S-S-S-( 2 ) 3 Si(Et) 3 (e) 3 Si( 2 ) 3 I + 2 (e) 3 Si () 3 Si 2 l + a-(e)= 2 () 3 Si

12 Preparation of Si() 3 (III) () 3 Si-( 2 ) n -A + Y-X-A () 3 Si-( 2 ) n -A(Y)-X-A Examples: (Et) 3 Si( 2 ) 3 S + Et (Et) 3 Si S Et (Et) 3 Si( 2 ) (Et) 3 Si 2 (Et) 3 Si( 2 ) 3 PPh 2 + hl()(pph 3 ) 2 (Et) 3 Si P Ph 2 l h P Ph 2 Si(Et) 3

13 Example: orrosion-protection oating for g Alloys In cooperation with EADS (1) leaning of the g substrate by pickling with an aqueous solution of 20% acetic acid and 5% a 3. (2) oating sol from esi(et) 3, Si(Et) 4, (e) 3 Si X (X = heterocyclic group with corrosion-inhibiting properties), water, 3 P 4, alcohol and a metal-organic compound as adhesion promotor Epoxy resin Sol-gel 2-3 m gaz31 orrosion test (WE34, 5% al solution) after 24 h untreated only pickling with sol-gel coating

14 Silica Precursors olloidal silica Si() 4 and Si() 3 preparation etal xide Precursors etal salts aqua, hydoxo and oxo species etal alkoxides Lewis-acidic properties chemical modification

15 Gels from etal ompounds z+ - + (z-1)+ - + (z-2)+ ydroxo complexes contain both groups with good nucleophilic properties ( or ) and good leaving groups ( 2 or - ). Therefore, it is generally necessary to be in the hydroxo domain to generate condensed species. This domain can be reached from either side by either changing the oxidation state of the metal (z) or the p. (1) (2) (3) (1) Adding a base to aquo complexes: l x + 3 (aqu.) u(ii), Zr, Ta, b oxide / hydroxide gels (2) eduction of metallate ions: n reductant n 2 gels (3) Acidifying solutions of metallate ions: Alkali or ammonium titanate, vanadate, niobate, tantalate, tungstate, or Au() - 4 solutions + +, V, b, Ta, W, Au oxide / hydroxide gels

16 Example: eflective oating Zr 2 / PVP. 210 nm Si 2 Zr 2 / PVP 210 nm Si 2 substrate 15 nm Zr 2 particles (from Zrl 2 ) in PVP binder (15 wt%); n D 1.70 Si 2 from Si(Et) 4 ; n D layers Si 2 / Zr 2 a: theoretical, b:experimental Pohl Institute of Solid State Physics, Shanghai

17 Silica Precursors olloidal silica Si() 4 and Si() 3 preparation etal xide Precursors etal salts aqua, hydoxo and oxo species etal alkoxides Lewis-acidic properties chemical modification

18 Lewis-Acidity of () x : ligomerization Through Bridges Structures of dimeric, trimeric and tetrameric aluminum alkoxides Al( t Bu) 3 dimer Al( s Bu) 3 trimer-tetramer equilibrium (in solution) Al Al Al Al Al Al Al Al Al

19 Lewis-Acidity of () x : Solvent Adducts () 4 as an example () 4 + L () 4 L Si( i Pr) 3 () 4 : = Zr, f: = i Pr = : fluorinated i Pr i Pr i Pr i Pr i Pr i Pr i Pr i Pr i Pr i Pr i Pr Et Et i Pr i Pr i Pr (Pr i ) 3 Si Stability of the base adducts is a balance between the strength of the - donor-acceptor interaction and the - hydrogen bond

20 Substitution of etal Alkoxides by Bi-(Tri-)dentate Ligands (BL) () x + y BL- () x-y (BL) y + y ' ' ' S ' S ' ' 2 ' P arboxylate Sulfonate ß-Diketonate -Amino- Phosphonate carboxylate

21 rganic odification of () 4 by Bidentate Ligands (BL) X X X = or () 4 + BL- () 3 BL + ' ' ' '. J. Errington et al., 1998 Pr Pr e Pr Pr e Pr Pr helating ligands: X Y Y X Et Et Et 2 Et 2 Et Et 2 2 ' '

22 rganic odification of () 4 by Bidentate Ligands (BL) X X X = or Bridging ligands: () 4 + BL- () 3 BL + - P e e e e e e P Ph Ph 2- - Y X X Y

23 Example: Preparation of Lead Zirconate tanate (PZT) Solid State eaction Sol-Gel Processing Pb Zr 2 Pb(Ac) (Pr) 4 + Zr(Pr) 4 + acetylacetone several h PZT Sol Pb(Zr ) / 2 min

24 Substitution of etal Alkoxides by Bi-(Tri-)dentate Ligands (BL) () x + y BL- () x-y (BL) y + y ' ' ' S ' S ' ' 2 ' P arboxylate Sulfonate ß-Diketonate -Amino- Phosphonate carboxylate hemical issues: - oordination equilibria - Degree of substitution (y) - Introduction of functional organic groups - rganic side reactions - Structural developments during hydrolysis

25 oordination Equilibria: ß-Diketonate Derivatives ' Al Al Al Al() 2 (acac) Proposed structures (from ) Al Al = acetylacetonate Al Al Al ' ' + ' - 2 ' () 4 ' ' ' + '() 2 ()' ' ' ' ' () 3 (acac) The metal : ligand ratio tells nothing about the molecular structures

26 oordination Equilibria: Aminoethanolate Derivatives ono-substitution product in benzene solution Solid state structures 2 + ( i Pr) 4 Pr i 2 i i Pr Pr Pr i i Pr 2 i Pr 2 i Pr i Pr i Pr Di-substitution product in gas phase 2 2 i Pr i Pr

27 Degree of Substitution: ximate Derivatives () 4 + = () 4-x (-= ) x Error " ' " ' ' ' " " degree of substitution

28 rganically Substituted etal Alkoxides () 3 Y X A ydrolyzable group (Functional) rganic group Bidentate group hemically inert spacer

29 etal Alkoxides with Functional rganic Ligands: Examples 2 () 3 2 =, Zr () 3 PPh 2 () 3 3 S () 3 S PPh 2 () 3 3 S o S 3 () 3 () 3 3 S

30 omparison of rganically Substituted Silicon and etal Alkoxides Silicon precursors: Si() 4, water glass rganically modified alkoxysilanes: etal precursors: () n, hydrolysable metal salts, e.g. acetates, nitrates rganically modified metal alkoxides: Examples: 3 Si() 3 () 3 Si X () n + BL- () n-1 (BL) + BL = X = functional group () 3 Si () 3 Si e

31 eaction etal Alkoxides with arboxylic Acids ' () 4 + ' - + ' () 2 (') 2 ' + 2 ' lusters a b (/) d (') e 6 4 (Et) 8 (methacrylate) 8

32 Partial ydrolysis Products ' ( i Pr) 12 (Phee) 4 ' 6 6 (Bu) 6 (= 5 8 ) 6 only bridging oximate ligands! ' 2 Same structure as 6 6 () 6 (- ) 6 with bridging carboxylate ligands

33 Library of Partially ydrolyzed Derivatives BL/ (degree of substitution, BL = bidentate ligand) () 4 (') () 6 (') () 8 (') 8 [ 4 3 (Et) 6 ( ) 4 ] () 6 (BL) 6 (L = or oximate) 4 4 () 4 (BL) () 8 (') () 12 (') () 10 (') / (degree of condensation)