Hydrothermal Synthesis of Nanomaterials

Andrew van BommelJanuary 18th, 2006

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.

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)

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 temperatureBarrer, R.M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982

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 15

15

20135514

350 15

15

25152515

400 15

15

35155560

450 15

15

35163560

Barrer, R.M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982

Next Slides

• Examples:– Nanoparticles– Nanowires/tubes– Nanoflowers

• Control of Shape

BaTiO3 Nanoparticles

• Ba(OH)2 + TiO2 BaTiO3 nanoparticles

• 300 - 450°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, 840-846

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, 973-977.

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.

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.

Tube/wire Formation

• TiO2 +NaOHtitanate nanomaterial

• Crystal flakes rolled along [001] direction

Chen, Q., Du, G.H., Zhang, S., Peng, L.M. Acta Cryst. 2002, B58, 587-593.

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, 1497-1502.

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. 2004, 108, 3955-3958

Hydrothermal Route

• Pros:– New materials– Easy, relatively cheap

• Cons:– Difficult to control morphology, size– Not for all materials– May obtain variation in size

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

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