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Hydrothermal Synthesis of Nanomaterials

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

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Hydrothermal Synthesis of Nanomaterials

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

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

  7. 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

  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, 973-977.

  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, 587-593.

  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, 1497-1502.

  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. 2004, 108, 3955-3958

  14. Hydrothermal Route • Pros: • New materials • Easy, relatively cheap • Cons: • 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

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