SOL-GEL TECHNIQUES:DEFINITIONS, GENERAL MECHANISM, INORGANIC ROUTE

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Chapter - 5

SOL-GEL TECHNIQUES

L. Hima Kumar

INTRODUCTION

Sol-gel processing generally refers to the hydrolysis and condensation of alkoxide-based

precursors. It can produce ceramic and glasses with better purity and homogeneity than high

temperature conventional process. Sol-gel process can be used to produce a wide range of

oxides in various forms, including powders, fibers, coatings and thin films, monoliths and

porous membranes. Organic/inorganic hybrids can also be made, where a gel, usually silica,

is impregnated with polymer or organic dyes with specific properties. The advantage of sol-

gel method is, metal oxides which are difficult to attain by conventional methods can be

produced by using the sol-gel process. Another benefit is that the mixing level of the solution

is retained in the final product, often in the molecular scale.

Sol-gel processing proved useful in the manufacturing of stained glass and for the

preparation of oxide materials from sol-gel precursors. Sol-gel derived products have

numerous applications. One of the promising application areas is for coatings and thin films

used in electronics, optical and electro-optical components and devices, such as substrates,

capacitors, memory devices, infrared (IR) detectors and wave guides. Antireflection coatings

are also used for automotive and architectural applications. Submicron particle size powders

of single and multicomponent composition can be made for structural, electronic, dental, and

biomedical applications. Composite powders can also be used as agrochemicals or herbicides.

Optical and refractory fibers are used for fiber optics sensors and thermal insulation,

respectively. In addition, sol-gel techniques can be used to infiltrate fiber performs to make

composites. Glass monoliths and coating and organic/inorganic hybrids are under

development for lenses, mirror substrates, graded index optics, optical filters, chemical

sensors, passive and nonlinear active waveguides, and lasers. Membranes for separation and

filtration processes also are being investigated, as well as catalysts. Biomolecules (such as

proteins, enzymes, antibodies, etc.) are incorporated into sol gel matrices, which can be used

for the monitoring of biochemical processes, environmental testing, food processing, and

drug delivery for medicine or agriculture.

Sol-gel process, starting with a metal alkoxide solution and going through various

processes and operations until a final product is obtained, such as a dense film, an aerogel, a

ceramic fiber was shown in Fig. 5.1.

5.2

Sol-Gel Methods

Fig.

5.1

Schematic

representation

the

sol-gel

process

(reproduced

from

http://www.chemat.com/assets/images/Flowchat72.jpg)

DEFINITIONS

A colloid is a suspension in which the dispersed phase is so small (~1-1000 nm) that the

gravitational forces are negligible and interactions are dominated by short-range forces, such

as van der Waals attraction and surface charges.

A sol is a colloidal suspension of solid particles in a liquid.

An aerosol is a colloidal suspension of particles in a gas.

A gel consists of a three dimensional continuous network, which encloses a liquid phase, in a

colloidal gel, the network is built from agglomeration of colloidal particles and is limited by

the size of container. In a polymer gel the particles have a polymeric sub-structure made by

aggregates of sub-colloidal particles. Generally, the sol particles may interact by van der

Waals forces or hydrogen bonds. A gel may also be formed from linking polymer chains.

SOL-GEL SYNTHESIS

Sol-gel synthesis is a particular approach to the preparation of glasses and ceramics at low

temperatures, which may precede either by the metallorganic route, with metal alkoxides in

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5.3

organic solvents, or by the inorganic route, with metal salts (chlorides, oxychlorides, nitrates,

etc) in aqueous solution.

GENERAL MECHANISM

Disregarding the nature of the precursors, the sol-gel process can be characterized by a series

of distinct steps.

Step 1: Formation of the `sol' i.e. stable solutions of the alkoxide or solvated metal precursor.

Fig. 5.2. Formation of `sol' (reproduced form ref. 3)

Step 2: Gelation resulting from the formation of an oxide- or alcohol-bridged network (the

gel) by a polycondensation or polyesterification reaction that results in a dramatic increase in

the viscosity of the solution. If so desired, the gel may be cast into a mold during this step.

Step 3: Aging of the gel, during which the polycondensation reactions continue until the gel

transforms into a solid mass, accompanied by contraction of the gel network and expulsion of

solvent from the gel pores. Ostwald ripening and phase transformations may occur

concurrently with syneresis. The aging process of gels can exceed 7 days and is critical to the

prevention of cracks in gels that have been cast.

Step 4: Drying of the gel is, loss of water, alcohol and other volatile components, first as

syneresis (expulsion of the liquid as the gel shrinks), then as evaporation of liquid from with

in the pore structure with associated development of capillary stress which frequently leads to

cracking. This also includes super critical drying, in which capillary stress is avoided by the

use of supercritical fluids, such as CO2, in conditions where there are no liquid/vapor

densities. Drying process is complicated due to fundamental changes in the structure of the

gel. The drying process has itself been broken into four distinct steps:

5.4

Sol-Gel Methods

(i) Constant rate period, (ii) the critical point, (iii) the first falling rate period, and (iv) the

second falling rate period. If isolated by thermal evaporation, the resulting monolith is termed

a xerogel. If the solvent is extracted under supercritical or near supercritical conditions, the

product is an aerogel.

Fig. 5.3. Formation of gel (reproduced form ref. 3)

Fig. 5.4. Aging of gel (reproduced form ref. 3)

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5.5

Step 5: Dehydration, during which surface-bound M-OH groups are removed, thereby

stabilizing the gel against rehydration. This is normally achieved by calcining the monolith at

temperatures up to 800 °C.

Step 6: Densification and decomposition of the gels at high temperatures (T > 800 °C). The

pores of the gel network are collapsed, and remaining organic species are volatilized. This

step is normally reserved for the preparation of dense ceramics or glasses.

Fig. 5.5. Densification (reproduced form ref. 3)

INORGANIC ROUTE

The growth of metal-oxo polymers in a solvent inorganic polymerization reactions through

hydrolysis and condensation of metal alkoxides M(OR)Z, where M = Si, Ti, Zr, Al, Sn, Ce,

OR is an alkoxy group and Z is the valence or the oxidation state of the metal, occur in two

steps

First step: hydrolysis, Metal cations, Mz+, are formed by dissolving metal salts in water, are

solvated by water molecules as follows:

Mz+ + hH2O

[M(OH2)h]z+

---------------------------------------------------------(1)

where h is the coordination number of the cation. The electron-transfer weakens the O-H

interactions of bound water, and depending on the pH of the solution, various degrees of

hydrolysis (deprotonation) can be induced:

[M(OH2)]z+

[M-OH](z-1)+ + H+

[M=O](z-2)+ + 2H+ ------------------------------(2)

In general, hydrolysis reaction is facilitated by increase in the charge density on the metal, the

number of metal ions bridged by a hydroxo or oxo ligand, and the number of hydrogens

contained in the ligand. It also affected by the pH. The typical effects of charge and pH are

shown in Fig. 5.6, where three domains corresponding to aquo, hrdroxy and oxo ions are

5.6

Sol-Gel Methods

defined. Acidic conditions force the equilibria (Eq. 2) to the left and favor the formation of

hydroxo ligands, whereas basic conditions will force the equilibria to the right and favor oxo

ligands. The charge on the metal, however, also plays a significant role in the above

equilibria with respect to pH, given that highly positive charges on the metals tend to

substantially weaken the O-H bonds and favor the formation of oxo ligands.

Fig. 5.6. Relationship between charge, pH and hydrolysis equilibrium of cations (reproduced

form ref. 1)

Second step: polycondensation process leading to the formation of branched oligomers and

polymers with a metal-oxo based skeleton and reactive residual hydroxo and alkoxy groups

M-OH + XO-M

M-O-M + XOH --------------------------------------------------------(3)

This condensation step occurs by either of the following ways

1. Olation, is a condensation in which a hydroxyl bridge is formed.

2. Oxolation, is condensation in which a oxo (-O-) bridge is formed.

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5.7

Chemical Reactivity of Metal Alkoxides

The chemical reactivity of metal alkoxides can be examined under acidic and basic

conditions, as shown in Fig. 5.7.

Fig. 5.7. Polymer structures formed in acid or basic environments (reproduced from ref. 1)

As with initial hydrolysis, condensation reactions may be either acid catalyzed or base

catalyzed and either cases the reaction proceeds via a rapid formation of charged intermediate

by a reaction with a proton or hydroxide ion, followed by slow attack of second neutral

silicon species.

Under acidic conditions (e.g. with mineral acids), hydrolysis is faster than condensation, as

shown in the following reaction:

5.8

Sol-Gel Methods

For Si(OR)4-n(OH)n, under acidic conditions (pH < 4), the rate of hydrolysis will always be

faster than the rate of condensation due to the ability of -OR groups to better stabilize the

transition states. Condensation involves the attack of silicon atoms carrying protonated

silanol species on neutral Si-OH nucleophiles. A bushy network of weakly branched

polymers is obtained under acidic conditions.

followed by

Fig. 5.8. Effect of pH on particle morphology in sol-gel reactions (reproduced from ref. 6)

The result of basic catalysis is an aggregation (monomer cluster) that leads to more compact

highly branched silica networks that are not interpenetrable before drying and thus behave as

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5.9

discrete species. The differences between acid and base catalyzed reactions and the

consequences for particle morphology are conceptually represented in Fig. 5.8.

Gelation

Gelation is freezing in a particular structure (i.e. may be considered as a rapid solidification

process). Gelation occurs when links form between silica sol particles produced by hydrolysis

and condensation, to such an extent that a giant spanning cluster reaches across the

containing vessel. In other words, as the sol particles grow and collide, lead to the

condensation and then forming macroparticles. The sol becomes a gel when it can support a

stress elastically. This is typically defined as the gelation point or gelation time. All

subsequent stages of processing depend on the initial structure of the wet gel formed in the

reaction bath during gelation. Under acid catalysis polymeric gels are gradually formed, as

depicted in Fig. 5.8. Under basic conditions and/or with higher additions of water, more

highly branched clusters are formed, which behave as discrete species. If the total

concentration of alkoxysilane is low, e.g., < ~0.3 M, gelation leads to formation of colloidal

silica (Stoeber process).

Aging

When a gel is maintained in its pore liquid, its structure and properties continue to change

long after the gel point. This process is called aging. During aging, four processes can occur,

singly or simultaneously, including polycondensation, syneresis, coarsening, and phase

transformation.

Polycondensation reactions occur continuously, within the gel network as long as

neighboring silanols are close enough to react. This increases the connectivity of the network

and its fractal dimension. Usually in alkoxide-based gels, the chemical hydrolysis reaction is

rapid, especially, when the sol is acid catalyzed, and is completed in the early stages of sol

preparation. Since the chemical reaction is faster at higher temperatures, aging can be

accelerated by hydrothermal treatment, which increases the rate of the condensation reaction.

Syneresis is the spontaneous shrinkage of the gel and resulting expulsion of liquid from

the pores. Syneresis in alcoholic gel systems is generally attributed to formation of new

bonds through condensation reactions, which increases the bridging bonds and causes

contraction of the gel network. In aqueous gel systems or colloidal gels, the structure is

controlled by the balance between electrostatic repulsion and attractive van der Waals forces.

Therefore, the extent of shrinkage is controlled by additions of electrolyte.

5.10

Sol-Gel Methods

Coarsening or Ostwald ripening is the irreversible decrease in surface area through

dissolution and reprecipitation processes. Structural changes attributed primarily to surface

energy effects. It is well known that surfaces exhibiting positive radii of curvature dissolve

more readily than surfaces exhibiting negative radii of curvature. If a gel is immersed in a

liquid in which it is soluble, dissolved material will tend to precipitate into regions of

negative curvature. Therefore, as the dissolution rate increases, dissolution redeposition

results in neck formation, causing the gel structure to become fibrillar and the pore formation.

Further, when dissolution is extensive, the gel network would break down and ripen to form a

colloidal sol.

During aging, there are changes in most physical properties of the gel. Time, temperature,

solvent and pH conditions affect the aging process and lead to structural modifications.

Heating the gel in water at 80-100 °C can strengthen the weak gel structure and generally

brings about reinforcement, but does not modify the pore structure. The pH of the wash water

during the washing of pore liquor out of gel is critical in the case of gels made from acid-

catalyzed silicate precursors. The final properties of such gels depend on both the pH at

which the gel was formed and the pH at which it was aged before drying.

Drying

Drying is nothing but removing of the solvent phase. The method is influenced by the

intended use of the dried material. If powdered ceramics are desired, no special care need be

exercised to prevent fragmentation; if monoliths from colloidal gels are desired, the drying

procedures are largely determined by the need to minimize internal stresses associated with

the volume changes on drying and the capillary forces in the gel pores.

There are three stages of drying:

During the first stage of drying, the volume change of the gel equals the volume of

evaporated liquid. The gel network is still flexible and can rearrange to accommodate the

decreasing volume. The compliant gel network is deformed by the large capillary forces,

which causes shrinkage of the object. All pores are filled with solvent and no liquid-air

interfaces are present. In classical large-pore systems, this first stage of drying is called "the

constant rate period" because the evaporation rate per unit area of the drying surface is

independent of time. First stage of drying ends when shrinkage stops.

Second stage of drying begins when the "critical point" is reached (Fig .8). The packing

density of the solid phase increases with increasing the strength of the network, and resists

further shrinkage. This stage is called critical point. As drying proceeds, the gel network

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5.11

becomes more restricted and the removal of liquid leads to the formation of such interfaces

and the development of capillary stresses. The liquid flows to the surface where evaporation

takes place. The flow is driven by the gradient in capillary stress. Because the rate of

evaporation decreases in second stage, classically this is termed "the first falling rate period'.

Fig. 5.9. Schematic representation of gel surface at the end of first stage (critical

point) (reproduced from ref. 4)

When the pores have substantially emptied, surface films along the pores cannot be

sustained. This is third stage of drying, which is also called as "second falling rate period".

The remaining liquid can escape only by evaporation from within the pores and diffusion of

vapor to the surface. During this stage, there are no further dimensional changes but just a

5.12

Sol-Gel Methods

slow progressive loss of weight until equilibrium is reached, determined by the ambient

temperature and partial pressure of water.

The capillary pressure, P, developed in a cylindrical capillary of radius r, partially filled

with a liquid of wetting angle α, can be expressed by:

2γ

cos α

ĆP =

where γ is the surface tension. When evaporation leads to the formation of menisci, the

different radii of the pores cause unequal capillary pressures to generate differential stresses.

If the stress difference locally exceeds the strength of the gel network, a crack will result

(drying failure). When the gel has had insufficient aging and strength, does not possess the

dimensional stability to withstand the increasing compressive stress and leads to cracking in

the first stage. Most failure of drying occurs during the early part of second stage, the point at

which the gel stops shrinking. This is the point at which the meniscus falls below the surface.

Generally, fractures associated with excessive capillary forces can be eliminated or

reduced by one or more of the following proceedings:

strengthening the gel by reinforcement

enlarging the pores

reducing the surface tension of the liquid with surfactants

making the interior surfaces hydrophobic

evacuating the solvent by freeze-drying

Operating under hypercritical conditions.

Xerogel and Aerogel Formation

As a gel is evaporated to dryness either by thermal evaporation or supercritical solvent

extraction, the gel structure changes in some cases substantially. During thermal drying or

room-temperature evaporation, capillary forces induce stresses on the gel that increases the

coordination numbers of the particles and induces collapse of the network. The increase in

particle coordination numbers results in the formation of additional linkages that strengthen

the structure against further collapse and eventually leads to the formation of a rigid pore

structure. The structure of the resulting xerogel can therefore be considered a collapsed,

highly distorted form of the original gel network.

The supercritical extraction of solvent from a gel does not induce capillary stresses due to

the lack of solvent-vapor interfaces. As a result, the compressive forces exerted on the gel

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5.13

network are significantly diminished relative to those created during formation of a xerogel.

Aerogels consequently retain a stronger resemblance to their original gel network structure

compared to xerogels (Fig. 5.10).

Fig. 5.10. Structural relationship between a sol-gel precursor, a xerogel, and an aerogel

(reproduced from ref. 6)

Densification

Densification is the last treatment process of gels during which the dried gel is heated to

convert into a dense ceramic. The sintering of gels, where the pore network is collapsed and

organic species are volatilized, is a critical factor in determining the size and morphology of

the sol-gel product. For silica gels, the following reactions occur during the densification:

desorption of physically adsorbed solvent and water from the walls of micropores (100-

200 °C)

decomposition of residual organic groups into CO2 + H2O (300- 500 °C)

collapse of small pores (400-500 °C)

collapse of larger pores (700-900 °C)

continued polycondensation (100-700 °C)

If powdered ceramics are desired, no need to take care to prevent fragmentation. Attention

must, however, be directed to the removal of organics to avoid undesirable bloating, foaming

or blackening. Where as, for monoliths preparation, special care must be taken to ensure

complete removal of water, organic groups or decomposition products, prior to micropore

collapse to avoid the development of stresses leading to fragmentation for the production of

large monoliths of SiO2.

5.14

Sol-Gel Methods

Sintering and densification phenomena also take place, via typical sintering mechanisms such

as evaporation condensation, surface diffusion, and grain boundary and bulk diffusion. The

small particle size of the powders lead to high reactivities and enhanced sintering and/or

coarsening rates (the principal process involved in densification is often viscous sintering).

As illustrated in Fig. 5.11, densification of a gel network occurs between 1000 and 1700 °C

depending upon the radii of the pores and the surface area. Fig. 5.11, describes the overall

development of a sol-gel process for glass in relative time and temperature scales. This

schematic representation summarizes from establishment of a sol condition to formation of a

dense material.

Fig. 5.11. Time vs. Temperature dependence in a sol-gel process

(reproduced from ref. 4)

APPLICATIONS

Monoliths

The physical properties of gel-derived glasses are usually closely similar to those of

glasses obtained from the melt. Gel-derived powders are used as batch ingredients for

glass melting primarily because of the high degree of chemical homogeneity they offer.

This leads to shorter melting times and lower melting temperature as well as

compositional uniformity. The key to glass formation is the development of an

appropriate heat treatment schedule to remove the residual organic groups and achieve

pore collapse without inducing crystallization in the sample. The most attractive feature is

the development of novel glass compositions: CaO-SiO2 or Na2O-ZrO2-SiO2 with high

ZrO2 content, which is impossible to obtain from the melt, because the cooling rate must

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5.15

be high to avoid detectable crystallization. Monoliths are defined as bulk gels cast to

shape and processed without cracking. Monolithic gels are potentially interesting because

complex shapes may be formed at room temperature and consolidated at rather low

temperatures with out melting. The principle applications of monoliths are optical ones:

fiber optic preforms, lenses and other near-net-shape optical components, graded

refractive index glasses and transparent foams (Aerogels) used as Cherenkov detectors

and as super insulation.

Fibers

Fibers can be drawn directly from polymer sols by controlling the viscosity of acid catalyzed

alkoxide gels. A key to prevent premature gelation is to maintain water at a lower level. High

viscosity stabilizes the fibers against spheroidization during the drawing operation. Both

silica and titania fibers have been obtained at near room temperature, but without sintering,

they remain porous and contain some unreacted alkoxide. The porosity decreases with

increasing temperature. Sol-gel method using metal alkoxide as precursor has been applied to

the preparation of oxide glass fibers other than SiO2. Fibers produced by sol-gel processing

are an interesting application for the manufacture of optical waveguides (A device that

constrains or guides the propagation of electromagnetic radiation along a path defined by the

physical construction of the guide is called waveguide). The dispersion-casting technique

produces silica fibers with optical loses of about 3.5 dB/km.

Thin films and coatings

Thin films and coatings were the first commercial application of sol-gel processing

technology. Films are applied either by dip coating or spin coating techniques, using

polymeric alkoxide sols. By controlling the precursor chemistry and deposition conditions,

pore size, porosity, surface area, and refractive index of the films can be controlled. Large

substrates may be accommodated and it is possible to uniformly coat both sides of planar and

axially symmetric substrates such as pipes, tubes, rods and fibers.

Generally, ceramics are more resistant than metals to oxidation, corrosion, erosion and

wear. Moreover, they can have good thermal and electrical properties that make them

particularly interesting as coating materials. Ceramic coatings are usually deposited on metals

for improving their performances in high temperature aggressive environments. Some

important applications, include improving resistance against gas, solid, condensed, and

molten-phase corrosion, to localized overheating and melting; decreasing fretting and wear;

decreasing heat losses and/or reflecting radiations in high temperature systems.

5.16

Sol-Gel Methods

A major development was the formation of multiple transparent layers that produce a gradual

change in refractive index from the air interface to that of the substrate. Antireflection

coatings are also a vast application of the sol-gel technique, for example, in store front

windows, to allow light transmission but reduced glare. Such coatings are also used in solar

cells and laser optics. A head up display is a unique application of these coatings: by

controlling the reflectance to transmission ratio, the speed of the automobile can be displayed

at eye level on the windshield, without distorting the field of view. The driver no longer

needs to shift his eyes to the instrument panel.

Electrochromic devices

Active electronic thin films include high temperature superconductors, conductive indium tin

oxide (ITO) and vanadium pentoxide, and ferro electric barium and lead titanates,

electrochromic tungsten oxide, and titania films used as phtotoanodes. Solid state

electrochromic (EC) devices for smart windows, large area displays and automotive rearview

mirrors are of considerable technological and commercial interest.

Fig. 5.12. Schematics of an EC window device deposited on one substrate. The

electrochromic layer is anode, cathode or both (reproduced from ref .7).

Controlled modulation of solar radiation through smart windows has a large potential in

buildings for energy savings and in automobiles to increase occupant comfort while reducing

air conditioning requirements. Typically these devices consist of multiple layer stacks that are

sequentially vacuum deposited or are laminate constructions. Sol-gel processing offers

advantage in depositing films containing multiple cations and controlling the microstructure.

These parameters can influence the kinetics, durability, coloring efficiency and charge

storage in the electrochromic films (electrodes). A typical structure of a solid state

electrochromic window is shown in Fig. 5.12.

Other applications of sol-gel techniques include

Synthesis of carbides sulphides and nitrides

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5.17

Synthesis of nanocomposites

Synthesis of powders, grains and spheres

Catalysis ( Mesoporous materials)

Preparing solid electrolytes

Absorbing coatings

Filters for lighting and optical purposes

Semiconducting coatings

Protective layers (both chemical and thermal)

Scratch resistant coating

Embedding organic molecules (hybrid organic-inorganic materials)

Flammable gas sensors

Porous gels and membranes

REFERENCES

1. C. J. Brinker, and G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel

Processing, Academic Press Inc., New York, 1989.

2. J. D. Wright and N.A.J.M Sommerdijk, Sol-gel materials chemistry and applications, vol.4,

Taylor & Francis Books Ltd., London, 2001.

3. R. D. Gonzalez, T. Lopez and R. Gomez, Sol-Gel preparation of supported metal catalysts.

Catalysis Today, 35 (1997) 293.

4. L. C. Klein, (Ed.), Sol-Gel Optics: Processing and Applications, Kluwer Academic

Publishers, Boston, 1993.

5. L.L. Hench and J.K. West, The sol-gel process. Chem. Rev., 90 (1990) 33.

6. B. L. Cushing, V.L. Kolesnichenko and C.J. O'Connor, Recent Advances in the Liquid-

Phase Syntheses of Inorganic Nanoparticles. Chem. Rev., 104 (2004) 3893.

7. A. Agrawal, J.P. Cronin and R. Zhang, Review of solid state electrochromic coatings

produced using sol-gel techniques. Solar Energy Materials and Solar Cells, 31 (1993) 9.

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