1. Hydrothermal Synthesis of Nanomaterials
Andrew van Bommel
January 18th, 2006

2. Hydrothermal Synthesis
The reactants are dissolved (or placed) in water or another solvent (solvothermal) in a closed vessel
Bomb is heated above BP
Conventional or MW oven
Commercially:
Tons of zeolites daily
Nazari, G.A., Pistoria, G. Lithium Batteries; Klumer Academic Pubs.: Boston, 2004.

3. Conditions Solvent above boiling point (supercritical water)
Usually basic conditions
Useful for Nano!
Oxides (TiO2, VOx, MxMnO2)
Layered oxides: nanowires/tubes
C-nanotubes can be formed this way
Some elemental nanostructures (eg: Bi)

4. Mechanism for HT rxn Usually follows a liquid nucleation model
Differs from solid-state- reaction mechanism from diffusion of atoms or ions between reactants
Due to enhanced solubility:
Solubility of water increases with temperature, but alkaline solubility increases dramatically with temperature
Barrer, R.M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982

5. Solubility Solubility of SiO2 (quartz)
T (°C)
Sol (g/L)
100 -
150
0.0030
200
0.024
250
0.049
300
0.068
350
0.070
374
0.023
T (°C)
NaOH wt%
Sol (g/L)
300 1 5 15 20
135
514
350 25
152
515
400 35
155
560
450
163
Barrer, R.M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982

6. Next Slides Examples: Control of Shape Nanoparticles Nanowires/tubes
Nanoflowers
Control of Shape

7. BaTiO3 Nanoparticles Ba(OH)2 + TiO2  BaTiO3 nanoparticles
°C, HT
Two proposed mechanisms:
Dissolution-recrystallization
In situ crystallization
Hakuta, R., Ura, H. Hayashi, H, and Arai, K. Ind. Eng. Chem. Res. 2005, 44,

8. Dissolution-Recrystallization
BaTiO3 formation at 150°C
Stated:
“when varying the water/isopropanol ratio in synthesis at 150°C, the grain size of barium titanate decreases when the amount of alcohol increases, i.e. when the solubility of the precursors decreases”
“TEM observations of incompletely reacted powders showed that the grains are either amorphous or entirely crystalline BaTiO3, which means that homogeneous nucleation and growth is occurring instead of heterogeneous nucleation
“high resolution TEM observations of fully reacted powders revealed the presence of necks between particles. These three experimental observations in the same reaction system provide strong evidence of dissolution-precipitation as the primary reaction mechanism.”
Pinceloup, R., Courtoisa, C., Vicensb, J., Lerichea, A., and Thierry, B. J. Eur. Ceram. Soc. Res. 1999, 19,

9. In-situ crystallization
Non-dissolved TiO2 reacts with barium and hydroxide ions
Eckert, J.O., Hung-Houston, C.C., Gersten, B.L., Lencka, M.M., Riman, R.E., J. Am. Ceram. Soc. 1996, 79, 2939.

10. Why Nano?
For in-situ transformation, formation of a porous product facilitates the transfer of the reacting species, leading to a facile reaction
For dissolution-precipitation reaction, dissolution (and subsequent precipitation) must be fast to ensure a steady flow of reactants
Eckert, J.O., Hung-Houston, C.C., Gersten, B.L., Lencka, M.M., Riman, R.E., J. Am. Ceram. Soc. 1996, 79, 2939.

11. Tube/wire Formation TiO2 +NaOHtitanate nanomaterial
Crystal flakes rolled along [001] direction
Chen, Q., Du, G.H., Zhang, S., Peng, L.M. Acta Cryst. 2002, B58,

12. Why nanowires/tubes?
Two-dimensional crystal flakes have low resistance to bending: hydrothermal energy curls these flakes
Why tubes?
When diameter grows, the strain in the tubes is outweighed by the minimizing of energy with nanotubes’ self-closed layers (no dangling bonds)
Remskar, M. Adv. Mater. 2004, 16,

13. More Nano!
Nanoflower ZnO- CTAB assisted hydrothermal reaction of ZnO (in NaOH solution)
Zhang, H., Yang, D., Ji, Y., Ma, X., Xu, J., Que, D. J. Phys. Chem. B , 108,

14. Hydrothermal Route Pros: Cons: New materials Easy, relatively cheap
Difficult to control morphology, size
Not for all materials
May obtain variation in size

15. Summary
Hydrothermal synthesis involves the chemical reaction of materials in aqueous solution heated (usually above BP) in a sealed vessel (bomb)
Alkaline solution used to increase solubility (for a dissolution-precipitation reaction)
It is difficult to predict the resultant morphology of the product

16. Summary
Nanomaterials are apt to form in HT synthesis as the formation of these structures allows for facile diffusion of reactants
Hydrothermal synthesis is used for oxide nanoparticle synthesis as the solubility is high in the alkaline medium