III. IDEAL POWDERS AND REAL POWDERS

Hydrothermal Synthesis of Ceramic Oxide Powders



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REFERENCES

1. Veale, C.R., Fine Powders: Preparation, Properties and Uses, Applied Science

Publishers, London, 147, 1972.

2. Kato, A., and Yamaguchi, T., New Ceramic Powder Handbook, Tokyo Science

Forum, Tokyo, 558, 1983.

3. Vincenzini, P., Ed., Ceramic Powders, Elsevier Scientific, Amsterdam, 1025, 1983.

4. Segal, D., Chemical Synthesis of Advanced Ceramic Materials, Cambridge University Press, Cambridge, 182, 1989.

5. Ganguli, D., and Chatterjee, M., Ceramic Powder Preparation: A Handbook,

Kluwer Academic, Dordrecht, The Netherlands, 1997, 221.

6. Somiya, S., and Akiba, T., Trans. MRS-J, 24, 531, 1999.

7. Morey, G.W., Hydrothermal synthesis, J. Am. Ceram. Soc., 36, 279, 1953.

8. Walker, A.C., J. Am. Ceram. Soc., 36, 250, 1953.

9. Eitel, W., Silicate Science, vol. IV, Academic Press, New York, 149, 1966.

10. Laudise, R.A., Hydrothermal Growth: The Growth of Single Crystals, Prentice

Hall, Englewood Cliffs, NJ, 275, 1970.

11. Lobachev, A.N., Ed., Hydrothermal Synthesis of Crystals, Consultant Bureau, New

York, 1971, 152.

12. Somiya, S., Ed., Proceedings of the First International Symposium on Hydrothermal Reactions, Gakujutsu Bunken Fukyu Kai, Tokyo, 965, 1983.

13. Somiya, S., Ed., Hydrothermal Reactions for Material Science and Engineering:

An Overview of Research in Japan, Elsevier Applied Science, London, 505, 1989.

14. Somiya, S., Advanced Materials: Frontiers in Materials Science and Engineering,

vol. 19B, Elsevier Science, Amsterdam, 1105, 1993.

15. Rabenau, A.A., Chem. Int. Ed. Engl., 24, 1026, 1985.

16. Brice, L.C., Hydrothermal Growth, Crystal Growth Processes, Blackie Halsted

Press, Glasgow, 194, 1986.

17. Dawson, W.J., Hydrothermal synthesis of advanced ceramic powders, Am. Ceram.

Soc. Bull., 67, 1673, 1988.

18. Byrappa, K., Ed., Hydrothermal Growth of Crystals, Progress in Crystal Growth

and Characterization of Materials, Pergamon Press, Oxford, 1991.

19. Johnson, D.W., Jr., Advances in Ceramics, vol. 21, Innovations in Ceramic Powder

Preparation, G.L. Messing et al., Eds., American Ceramic Society, Westerville,

OH, 3, 1987.

20. Ismail, M.G.M.U., and Somiya, S., Proceedings of the International Symposium

on Hydrothermal Reactions, Gakujutsu Bunken Fukyu Kai, Tokyo, 669, 1983.

21. Yoshimura, M., and Somiya, S., Rep. Res. Lab. Eng. Mat. Tokyo Inst. Technol.,

9, 53, 1984.

22. Toraya, H. et al., Advances in Ceramics, vol. 12, Science and Technology of

Zirconia II, American Ceramic Society, Westerville, OH, 806, 1984.

23. Yoshimura, M. et al., Rep. Res. Lab. Eng. Mat. Tokyo Inst. Technol., 12, 59, 1987.

24. Tani, E., Yoshimura, M., and Somiya, S., Hydrothermal preparation of ultrafine

monoclinic ZrO2 powder, J. Am. Ceram. Soc., 64, C181, 1981.

25. Nishizawa, H. et al., J. Am. Ceram. Soc., 65, 343, 1982.

26. Komarneni, S. et al., Advanced Ceramic Materials, 1, 87, 1986.

27. Haberko, K. et al., J. Am. Ceram. Soc., 74, 2622, 1991.

28. Haberko, K. et al., J. Am. Ceram. Soc., 78, 3397, 1995.



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Chemical Processing of Ceramics, Second Edition

29. Riman, R., The Textbook of Ceramic Powder Technologies, American Ceramic

Society, Westerville, OH, 1999.

30. Hishinuma, K. et al., Advances in Ceramics, vol. 24, Science and Technology of

Zirconia III, Somiya, S., Yamamoto, N., and Hanagida, H., Eds., American

Ceramic Society, Westerville, OH, 201, 1988.

31. Somiya, S. et al., Hydrothermal Growth of Crystals, vol. 21, Progress in Crystal

Growth and Characterization of Materials, Byrappa, K., Ed., Pergamon Press,

Oxford, 195, 1991.

32. Yoo, S.E., Yoshimura, M., and Somiya, S., Preparation of BaTiO3 and LiNbO3

powders by hydrothermal anodic oxidation, in Sintering ’87, vol. l, 4th International Symposium on the Science and Technology of Sintering, Nov. 4–6, Somiya

S. et al., Eds., Elsevier, New York, 108, 1988.

33. Yoshimura, M. et al., Rep. Res. Lab. Eng. Mat. Tokyo Inst. Technol., 14, 21, 1989.

34. Kumar, A., and Roy, R., J. Mater. Res., 3, 1373, 1988.

35. Kumar, A., and Roy, R., J. Am. Ceram. Soc., 72, 354, 1989.

36. Yoshimura, M. et al., J. Ceram. Soc. Jap. Int. Ed., 97, 14, 1989.

37. Komarneni, S., Roy, R., and Li Q.H., Mater. Res. Bull., 27, 1393, 1992.

38. Komarneni, S., Li, Q.H., Stefasson, K.M. and Roy, R., J. Mater. Res., 8, 3176,

1993.

39. Komarneni, S., and Li, Q.H., J. Mater. Chem., 4, 1903, 1994.

40. Komarneni, S., Pidugu, R., Li, and Roy, R., J. Mater. Res., 10, 1687, 1995.

41. Komarneni, S., Hussen, M.Z., Liu, C., Breval, E., and Malla, P.B., Eur. J. Solid

State Inorg. Chem., 32, 837, 1995.

42. Komarneni, S., Novel microwave–hydrothermal processing for synthesis of

ceramic and metal powders, in Novel Techniques in Synthesis and Processing of

Advanced Materials, Singh, J., and Copley, S.M., Eds., Minerals, Metals, and

Materials Society, Warrendale, PA, 103, 1995.

43. Komarneni, S., and Menon, V.C., Mater. Letts., 27, 313, 1996.

44. Katsuki, H., Furuta, S., and Komarneni, S., J. Am. Ceram. Soc., 82, 2257, 1999.

45. Komarneni, S., Rajha, P.K., and Katsuki, H., Mater. Chem. Phys., 61, 50, 1999.

46. Katsuki, H., Furuta, S., and Komarneni, S., J. Porous Mater., 8, 5, 2001.

47. Katsuki, H., and Komarneni, S., J. Am. Ceram. Soc., 84, 2313, 2001.

48. Komarneni, S., Li, D., Newalkar, B. Katsuki, H., and Bhalla, A.S., Microwavepolyol process for Pt and Ag nanoparticles, Langmuir, 18, 5959, 2002.

49. Milia, A.M., Sonochemistry and Cavitation, Gordon and Breach Publishers, Luxembourg, 543, 1995.

50. Meskin, P.E., Barantchikov, A.Y., Ivanov, V.K., Kisterev, E.V., Burukhin, A.A.,

Churagulov, B.R., Oleynikov, N.N., Komarneni, S., Tretyakov, Yu. D., Doclady

Chem., 389, 207, 2003.



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Solvothermal Synthesis

Masashi Inoue



CONTENTS

I. Introduction .............................................................................................22

II. Choice of the Reaction Medium.............................................................23

A. Inorganic Medium............................................................................23

1. Water .........................................................................................23

2. Ammonia...................................................................................24

3. Other Inorganics .......................................................................24

B. Organic Medium ..............................................................................24

1. General Considerations.............................................................24

2. Paraffins ....................................................................................25

3. Aromatic Hydrocarbons ...........................................................26

4. Alcohols ....................................................................................27

5. Glycols ......................................................................................28

6. Cyclic Ethers.............................................................................28

7. Carboxylic Acids and Esters ....................................................28

8. Amines ......................................................................................29

9. Other Nitrogen-Containing Compounds ..................................30

10. Dipolar Aprotic Solvents ..........................................................30

III. Solvothermal Synthesis of Metal Oxides ...............................................31

A. Solvothermal Dehydration ...............................................................31

1. Solvothermal Dehydration of Aluminum Hydroxide

in Alcohols ................................................................................32

2. Alcohothermal Dehydration of Hydroxides of Metals

Other Than Aluminum..............................................................34

3. Solvothermal Dehydration of Aluminum Hydroxide

in Glycols and Related Solvents ..............................................35

4. Glycothermal Synthesis of α-Alumina ....................................36

B. Solvothermal Decomposition of Metal Alkoxides ..........................37

1. Metal Alkoxide in Inert Organic Solvents ...............................37

2. Metal Alkoxides in Inert Organic Solvent: Synthesis

of Mixed Oxides .......................................................................40

3. Metal Acetylacetonate in Inert Organic Solvent......................41

4. Metal Carboxylates...................................................................41

5. Cupferron Complexes ...............................................................42



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6.



Solvothermal Decomposition of Alkoxide Followed

by Removal of Organic Media in a Supercritical

or Subcritical State ...................................................................42

7. Metal Alkoxide in Alcohols .....................................................43

8. Reaction of Alkoxide in Secondary Alcohols..........................44

9. Reaction of Alkoxide in Glycols..............................................45

C. Glycothermal Synthesis of Mixed Metal Oxides............................46

1. Rare Earth Aluminum Garnets .................................................46

2. Rare-Earth (Nd-Lu) Gallium Garnets ......................................47

3. Metastable Hexagonal REFeO3 ................................................48

4. Other Mixed Oxides .................................................................48

5. Reaction in Ethylene Glycol ....................................................50

D. Crystallization of Amorphous Starting Materials ...........................50

E. Hydrothermal Crystallization in Organic Media.............................53

F. Solvothermal Ion Exchange and Intercalation ................................54

G. Solvothermal Oxidation of Metals ..................................................55

H. Solvothermal Reduction...................................................................56

References ...........................................................................................................56



I. INTRODUCTION

Metal oxides are usually prepared by calcinations of suitable precursors such as

hydroxides, nitrates, carbonates, carboxylates, etc. This process usually gives

oxides with pseudomorphs of the starting materials. When large amounts of

thermal energy are applied for the decomposition of the precursors, it facilities

sintering of the product particles and therefore aggregated particles are obtained.

When mixed oxides such as spinel, perovskite, and pyrochlore are the desired

products, heat treatment at higher temperatures is required.

For the preparation of inorganic materials with well-defined morphologies,

liquid phase syntheses are preferred. These synthetic reactions proceed at relatively lower temperatures and therefore require lower energies. The sol-gel (alkoxide) method is one of these methods;1,2 however, this method usually gives

amorphous products, and calcination of the products is required to obtain crystallized products. In this chapter, solvothermal methods are dealt with, which are

convenient for the synthesis of a variety of inorganic materials. General considerations for solvothermal reactions are discussed first and then the solvothermal

synthesis of metal oxides is reviewed.

Recently, use of organic media for inorganic synthesis has garnered much

attention. Since 1984, we have been exploring the synthesis of inorganic materials

in organic media at elevated temperatures (200 to 300°C) under autogenous

pressure of the organics.3 This technique is now generally called the “solvothermal” method.4 The term “solvothermal” means reactions in liquid or supercritical

media at temperatures higher than the boiling point of the medium. Hydrothermal

reactions are one type of solvothermal reaction. To carry out reactions at temperatures higher than the boiling point of the reaction medium, pressure vessels

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(autoclaves) are usually required. Some researchers favor the use of sealed

ampoules of glass or silica, but these experiments should be carried out with great

care because the ampoules are easily broken by the internal pressure of the

reaction medium. To avoid explosion of the ampoules, they may be placed in an

autoclave together with a suitable medium to create a vapor pressure to balance

the internal pressure of the ampoule.

It must be noted that the liquid structure of the solvent is essentially

unchanged at above or below the boiling point because the compressibility of the

liquid is quite small. (Note that near the critical point, the structure of the solvent

is drastically altered by changes in the solvent density.) Higher pressure may

increase or decrease the reaction rate; it depends on the relative volume of the

activated complex at the transition state to the volume of the starting molecule(s).

However, it is known that to measure the effect of reaction pressure, GPa-scale

pressure is required. This means that the autogenous pressure created by the vapor

pressure of the solvent has only a minor effect on the reaction rate. Therefore

there is no need to differentiate the reactions at the temperatures above and below

the boiling point. Consequently “solvothermal” reaction should be defined more

loosely as the reaction in a liquid (or supercritical) medium at high temperatures.

Reactions in a closed system using autoclaves or sealed ampoules and in an open

system using a flask equipped with a reflux condenser sometimes give completely

different results, especially when a low boiling point byproduct such as water is

formed.

Various compounds have been prepared by solvothermal reactions: metals,5,6

metal oxides,7,8 chalcogenides,9,10 nitrides,4,9,11 phosphides,12 open-framework

structures,13,14 oxometalate clusters,15,16 organic-inorganic hybrid materials,14,17,18

and even carbon nanotubes.19,20 Most of the solvothermal products are nano- or

microparticles with well-defined morphologies. The distribution of the particle

size of the product is usually quite narrow, and formation of monodispersed

particles is frequently reported.21,22 When the solvent molecules or additives are

preferentially adsorbed on (or have a specific interaction with) a certain surface

of the products, growth of the surface is prohibited and therefore products with

unique morphologies may be formed by the solvothermal reaction.9,23,24 Thus

nanorods,24 wires,25 tubes,26 and sheets27 of various types of products have been

obtained solvothermally.



II. CHOICE OF THE REACTION MEDIUM

A. INORGANIC MEDIUM

1. Water

Water (boiling point [bp], 100°C; critical temperature [Tc], 374°C; critical pressure [Pc], 218 atm) is the most widely examined reaction medium for solvothermal reactions. Geochemists first applied this technique to explore the formation

mechanism of minerals and thus quite long reaction periods were applied to

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Chemical Processing of Ceramics, Second Edition



examine the equilibrium conversion of minerals. Today researchers seek more

rapid conversion to synthesize materials, and therefore adequate synthesis of the

precursors by, for example, the coprecipitation method and the sol-gel method

become more important. Addition of salt, acid, or base may facilitate the reaction

or alter the morphology of the products. These materials are called mineralizers.

Fluoride ions sometimes have a drastic effect on the hydrothermal synthesis of

materials. Besides the excellent article by Somiya, Roy, and Komarneni in this

book, many review articles have appeared on hydrothermal synthesis;28–32 therefore this technique will not be discussed further in this chapter.

2. Ammonia

Besides water for hydrothermal reactions, liquid ammonia (bp, 78°C; Tc, 132°C;

Pc, 113 atm) is also used for the solvothermal synthesis of nitrides. Metastable

or otherwise unobtainable nitride materials were reported to be formed by this

method.33–35 Ammonium and amide (NH2) ions are the strongest acid and base,

respectively, for the liquid ammonia system, and therefore ammonium salt acts

as the acid mineralizer,36 while amide ion can be prepared by addition of alkali

metals to the solvent. Since ammonia has a low boiling point, the reaction pressure

is usually quite high.

3. Other Inorganics

Sulfur dioxide (bp, −10°C; Tc, 157.5°C; Pc, 78 atm) is another interesting inorganic solvent. This compound has a high dielectric constant and low basicity

(actually, it acts as an acid). To the best of my knowledge, there have been no

articles that apply this solvent for the solvothermal synthesis of inorganic materials. However, the highly corrosive nature of this solvent may limit its use in

autoclaves.

Hydrofluoric acid (bp, 19.5°C; Tc, 188°C; Pc, 64 atm), nitrogen dioxide (bp,

21°C; Tc, 158.2°C; Pc, 100 atm; in equilibrium with N2O4), sulfuric acid (decomposition at 280°C), and polyphosphoric acid are candidates for solvents in solvothermal reactions, and the reactions of these solvents will produce a variety of

products that cannot be prepared by any other methods. For example, Bialowons

et al.37 reported that solvothermal treatment of (O2)2Ti7F30 in anhydrous HF at

300°C yielded single crystals of TiF4. Solvothermal reactions in these solvents

may produce fruitful results and a new field seems to be awaiting many researchers.



B. ORGANIC MEDIUM

1. General Considerations

Various organic solvents have been applied for the synthesis of inorganic materials. Because most of inorganic synthesis researchers are not familiar with

organic solvents, some important features are summarized here.



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