D. CRYSTALLIZATION OF AMORPHOUS STARTING MATERIALS

Solvothermal Synthesis



51



the products. Since hydrous gel is highly hydrophilic, solvents miscible with

water are usually favored, and lower alcohols and glycols are usually used.

Ethylenediamine was also used.181 Inert organic solvents together with a surfactant

to disperse the precursor gel may also work,183 and may control the morphology

of the agglomerated particles. Solvothermal crystallization of sonochemically

prepared hydrous yttrium-stabilized zirconia (YSZ) colloid was also reported.187

A semicontinuous process for BaTiO3 synthesis was reported by Bocquet et al.176

The first step is the hydrolysis of the alkoxide BaTi(O-iPr)6 in isopropanol at 100

to 200°C. The second step is a thermal treatment of the formed solids under the

supercritical state of the solvent.

An important point is that the precursor gels thus prepared contain significant amounts of water, even if the gels are dried by some suitable method.

One of the weak points of solvothermal crystallization may be difficulty in

controlling the water content in the precursor gel. Water facilitates hydrothermal

crystallization of the precursor gel, and therefore the essential chemistry here

may be hydrothermal. Actually, solvothermal crystallization usually requires

higher temperatures and more prolonged reaction times than hydrothermal

crystallization.175 Moreover, in the solvothermal crystallization of stabilized

zirconia, the presence of an adequate amount of water was reported to be so

critical in dissolving the oxide powder that no crystallization occurred in absolute alcohol.184

Solvothermal crystallization usually produces products with smaller sizes

than those obtained by the hydrothermal method. The formation of polymorphs that cannot be obtained by the hydrothermal method is frequently

reported: PbTiO 3 with pyrochlore structure/perovskite PbTiO 3, 175 cubic

BaTiO3/tetragonal-phase BaTiO3,176,178 cubic ZrO2/tetragonal ZrO2.177 Formation of these phases may be due to the smaller crystallite size of the solvothermal products.

For the hydrothermal crystallization of amorphous gel, the dissolutionrecrystallization mechanism usually, although not always, takes place in which

dissolution of amorphous particles occurs followed by nucleation of the product

in the solution (homogeneous nucleation) and crystal growth. Ostwald ripening

occurs when the product has enough solubility. For solvothermal crystallization,

similar dissolution-recrystallization mechanisms are frequently reported to

occur.175,178,183,184 However, since a limited amount of water is present in the

reaction system, microscopic chemistry may differ from that of hydrothermal

chemistry. Water is adsorbed on the surface of the amorphous particles and

dissolves a part of the surface having higher energies. Water molecules transfer

the solute species to other parts of the particle surface that have lower energies,

which can act as the nucleation site of the product (heterogeneous crystallization). Crystal growth takes place by diffusion of the component with the aid

of adsorbed water, finally converting whole amorphous particles into crystals.

This mechanism is proposed because solvothermal crystallization of the amorphous precursors usually leads to nanocrystals, in spite of the fact that nucleation



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of oxide in organic solvent or aqueous organic solvent is difficult as compared

with aqueous systems. If homogeneous nucleation were to occur, one should

find some reason for the higher nucleation frequency in organic solvent than

in the aqueous system. In fact, our experience is that in the glycothermal

synthesis of metal oxides, nucleation of oxides is quite difficult or even does

not occur in the solvent, whereas in the presence of seed crystals, rapid crystal

growth takes place.

Because dissolution of the whole particle is not expected and because one

amorphous particle is expected to transform into a crystalline particle, formation

of homogeneous precursor gel is essential. On the other hand, if the real dissolution-recrystallization mechanism takes place, synthesis of homogenous gel does

not have any meaning, since preferential dissolution of one component may be

expected to occur in the solvothermal reaction.

To increase the crystallization rate and to alter the product phase, an alkaline

mineralizer is sometimes added to the solvothermal reaction. Some researchers

believe that, compared with the hydrothermal process, solvothermal synthesis

allows the product to be free from foreign ions because the organic solution,

having a low relative permittivity, is free from ionic species. When precursor gels

are prepared from alkoxide, one can prepare products free of foreign ions. However, when the precursor gel is prepared by precipitation from salt solutions, or

when alkali/acid mineralizer or ionic surfactant is added to the solvothermal

crystallization system, the above statement is a myth. In fact, ions are easily

adsorbed or occluded in the product particles because of the low dielectric

constant of the organic solvent.

Zhao et al.177 prepared a translucent sol of cubic zirconia particles with a size

of 5 nm by addition of ethanol-water to a solution of Zr(OPr)4 in a mixture of

ethanol and diethylene glycol followed by solvothermal treatment. Feldmann and

Jungk179 used essentially the same technique and reported the formation of a

variety of colloidally stable oxide particles. Their method is as follows: a solution

of acetylacetonate or alkoxide in diethylene glycol was heated to 140°C. Subsequently deionized water was added and the mixture was heated to 180°C for 2 h.

Kominami et al.103 reported that the amorphous product obtained by solvothermal decomposition of La(OiPr)3 and Fe(OBu)3 in toluene crystallized into

perovskite-type LaFeO3 using glycothermal treatment, while direct glycothermal reaction of the two starting materials yielded a mixture of La(OH)3 and

magnetite.

Yin et al.188,189 and Inoue et al.190 reported that fine crystals of anatase prepared

with solvothermal treatment of an amorphous gel in methanol possessed much

better sinterability and photocatalytic activity than those fabricated by hydrothermal reactions or calcinations in air.

Lu et al.191 prepared dense ceramic fibers of Pb1xLaxTiO3 (PLT, x = 0–0.2)

by a sol-gel method from lead acetate trihydrate, lanthanum acetate, and titanium

isopropoxide in triethanolamine and acetic acid, followed by solvothermal treatment in a mixture of xylene and triethylamine at 200°C for 12 h. They reported



© 2005 by Taylor & Francis Group, LLC



Solvothermal Synthesis



53



that removal of the organic residue prior to the gel-to-ceramic fiber transformation

(heat treatment) is a key step in achieving dense ceramic fibers with a reduced

grain size, although solvothermal treatment does not give the crystalline material.

This result shows that removal of organic residue is an interesting application of

the solvothermal method in relation with sol-gel chemistry.



E. HYDROTHERMAL CRYSTALLIZATION



IN



ORGANIC MEDIA



Since primary alkoxide of zirconium does not decompose in toluene, and therefore hydrolysis of the alkoxide in inert organic solvent followed by hydrothermal

crystallization of the hydrolyzed product is examined.192 In this method, the

alkoxide solution in an inert organic solvent is placed in a test tube, which is

then placed in an autoclave. The desired amount of water is placed between the

test tube and the autoclave wall. When the autoclave is heated, the water evaporates and is dissolved into the toluene solution from the gas phase, where hydrolysis of the alkoxide takes place, followed by hydrothermal crystallization of the

hydrolyzed products.

Using this method, monoclinic zirconia with a large surface area was prepared.192 Kominami et al.193–195 further extended the application of this method

and found that titania prepared by this method has a high photocatalytic activity

because the product has a large surface area as well as a low number of crystal

defects, which can act as the recombination sites for holes and electrons generated

by photoactivation. They called this method the HyCOM method.194

Using this method, amorphous Nb2O5 powder with a large surface area

(greater than 200 m2/g) was obtained from Nb(OBu)5 at 300°C.196 An increase

in the amount of water induced crystallization of Nb2O5 into the TT phase and

reduced the surface area of the product.196 Amorphous Ta2O5 with a large surface

area (greater than 200 m2/g) was also synthesized from Ta(OBu)5 at 200 to 250°C,

while β phase of Ta2O5 was obtained at 300°C.197 Microcrystalline α-Fe2O3

(hematite) powders were obtained from Fe(III) acetylacetonate, which then was

reduced to magnetite after a prolonged reaction time.102 Note that direct decomposition of the same precursor yields magnetite.102

This method may provide a convenient route for the synthesis of nanocrystalline oxide materials from alkoxide using only one reaction vessel; however, because this method uses water dissolved from the gas phase, configuration of the reaction apparatus, that is, the ratio of the surface area of the

liquid to the bulk volume of the liquid, may affect the physical properties of

the product.

Lee et al.198 reported that a colloidal solution containing nano-sized TiO2

(anatase) particles was obtained by solvothermal treatment of titanium isopropoxide in 1-butanol at 200°C in the presence of a small amount of aqueous HCl.

They found that the particles coated on γ-alumina showed excellent performances

for photocatalytic decomposition of CHCl3.



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Chemical Processing of Ceramics, Second Edition



F. SOLVOTHERMAL ION EXCHANGE



AND INTERCALATION



Ion exchange and intercalation are synthetic routes for a variety of oxide materials

from starting materials with tunneled and layered structures.199 Aqueous media

are usually used for this purpose, however, organic solvents may produce novel

products because ions are less solvated in organic media. Although there are many

host lattices that can undergo ion exchange reactions, solvothermal methods have

been sparingly investigated.

Li et al.200 reported that the reaction of MnO2 with LiOH · H2O in the presence

of NaOH in ethanol at 200°C yielded microcrystalline Li1-xMn2O4-σ. (180 nm).

Wang et al.201 pointed out the importance of the tunnel structure of the starting

MnO2 in their solvothermal reaction with LiCl in 1-pentanol at 170°C: MnO2

with the 1 × 1 tunnel structure yields the spinel lithium manganese oxide, whereas

a novel structural LixMnO2 is formed from MnO2 with the 2 × 1 tunnel structure.

In both reactions, alcoholic media acted as the reducing agent.

A new modification of LiFeO2 with a corrugated layer structure is synthesized

by the ion exchange reaction between γ-FeOOH and LiOH · H2O under hydrothermal conditions.202 On the other hand, solvothermal ion exchange reaction of

these two starting materials in various alcohols at the reflux temperature yields

another modification of LiFeO2 with a hollandite-type structure.203 It was reported

that lithium cells consisting of cathodes of this compound and lithium anodes

showed good charge and discharge reversibility.203

Tabuchi et al.204 reported that the solvothermal reaction of a mixture of αNaFeO2 (obtained by hydrothermal reaction of α-FeOOH with NaOH) and LiCl

in ethanol at 220°C for 96 h yielded an Fe2+-containing compound with the

formula of Li1-xFe5+xO8 with an inverse spinel structure.

Although a variety of solvents can be used for solvothermal reactions, only

alcoholic solvents have been investigated for solvothermal ion exchange. This

may be due to the fact that both cation and counteranion are required to be

solvated. As for another possible solvent system, a binary system composed of

a solvent with a low basicity and a high dielectric constant together with a crown

ether that can selectively solvate a cation with a specific size may be designed

which poorly solvates the exchanging ions and can capture exchangeable cations

from the host lattice into the solvent.

Intercalation of organic molecules into layered host lattice produces a variety

of organic-inorganic hybrid materials. The solvothermal method provides a reaction system that allows application of high temperatures and therefore is a powerful technique for preparation of intercalation compounds. Exfoliation of layers

may occur because of applied high temperatures. For example, exfoliated polyethylene/montmorillonite nanocomposites were reported to be prepared by solvothermal reaction of organophilic montmorillonite with polyethylene in toluene

at 170°C for 2 h.205

Pillared inorganics are prepared by intercalation of suitable metalorganics

into host layers followed by heat treatment to decompose the guest molecules,



© 2005 by Taylor & Francis Group, LLC