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A novel pathway for gas phase synthesis of silica nanoparticles - - PowerPoint PPT Presentation

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A novel pathway for gas phase synthesis of silica nanoparticles Shraddha Shekar, Markus Sander, Markus Kraft 21 September 2010 Tetraethoxysilane TEOS Central silicon attached to 4-ethoxy branches Vibrations and rotations within the

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Shraddha Shekar, Markus Sander, Markus Kraft 21 September 2010

A novel pathway for gas phase synthesis of silica nanoparticles

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Shraddha Shekar ss663@cam.ac.uk

Tetraethoxysilane

TEOS

  • Central silicon attached to 4-ethoxy

branches

  • Vibrations and rotations within the

molecule

  • Many possible ways of bond

breaking, many possible reactions

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Shraddha Shekar ss663@cam.ac.uk

Silica nanoparticles

Silica Nanoparticles: network of Si-O bonds such that Si:O = 1:2 Applications:

  • Support material for

functional/composite nanoparticles, catalysis

  • Bio-medical applications, drug delivery
  • Optics, optoelectronics,

photoelectronics

  • Fabrics, clothes
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Shraddha Shekar ss663@cam.ac.uk

Physical system

Precursor(TEOS) Flame reactor P ≥ 1 atm T ≈ 1100 - 1500 K Silica nanoparticles

Macroscopic level questions:

  • Optimal process conditions?
  • Final product properties?
  • Final particle size

distribution?

Industrial Scale Molecular Scale

Answers from molecular level studies:

  • How to determine the thermochemistry of the system?
  • What happens in the gas-phase?
  • How do gas-phase precursors form the particles?
  • How to describe the overall system from first-principles?
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Shraddha Shekar ss663@cam.ac.uk

Methods: Ab initio modelling

Quantum Chemistry calculations Statistical Mechanics

H(T) S(T) Cp(T)

Thermochemistry calculation Chemical Kinetics Equilibrium calculation Overall Model Species generation Population Balance Model

Ref: W. Phadungsukanan, S. Shekar, R. Shirley, M. Sander, R. H. West, and M.

  • Kraft. First-principles thermochemistry for silicon species in the decomposition of
  • tetraethoxysilane. J. Phys. Chem. A, 113, 9041–9049, 2009
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Shraddha Shekar ss663@cam.ac.uk

Step1: Species Generation

Si O C C O C C O C C C O C Branch 3 Branch 4 Branch 2 1 2 3 4 5 6 7 8 9 10 11 12 Branch 1 TEOS: Symmetric molecule : 4-ethoxy groups attached to Si : 4-possible states for each branch : Combinations for all 4 branches produced using 4-nested loops State 1: No removal State 2: Terminal C removed State 3: Penultimate C removed State 4: O removed

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Shraddha Shekar ss663@cam.ac.uk

Step 2: Quantum Calculations

  • Relative positions of nuclei and electrons given by
  • Gaussian-03 package used to perform quantum

calculations

  • Output from quantum calculations:

– Optimised Geometries (minimum energy configuration) – Frequencies

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Shraddha Shekar ss663@cam.ac.uk

Step 3: Thermochemistry

Frequency Data from Gaussian

Partition Functions (q) Thermochemistry

S Vs T H Vs T Cp Vs T

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Shraddha Shekar ss663@cam.ac.uk

Step 4: Equilibrium Calculation

Ref: W. Phadungsukanan, S. Shekar, R. Shirley, M. Sander, R. H. West, and M. Kraft. First-principles thermochemistry for silicon species in the decomposition of

  • tetraethoxysilane. J. Phys. Chem. A, 113, 9041–9049, 2009
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Shraddha Shekar ss663@cam.ac.uk

Step 5: Kinetic model

  • Equilibrium

– Hints towards the existence of stable intermediates & products. – Intermediates Si(OH)x(OCH3)4-x Si(OH)y(OC2H5)4-y – Main Product Si(OH)4

  • Kinetics

– Reaction set generated to include all intermediates and products from equilbrium. – Reactions obey Arrhenius law rate constant k = ATβe-Ea/RT – Rate parameters (A, β, Ea) fitted to experimental vaues (a)

(a) J. Herzler, J. A. Manion, and W. Tsang. Single-Pulse Shock Tube Study of the decomposition of tetraethoxysilane and Related Compounds. J. Phys. Chem. A, 101, 5500-5508, 1997

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Shraddha Shekar ss663@cam.ac.uk

Flux and Sensitivity Analyses

Main Reaction Pathway

Reaction number

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Shraddha Shekar ss663@cam.ac.uk

Model Optimisation

The rate parameters have been fitted to shock-tube experimental data provided by Herzler et al

Step 2: Sensitivity Analysis To identify the 4 most sensitive parameters Step 1: Low discrepancy series To perform a pre-scan of parameters for 18 Si reactions. Step 3: Response Surface Methodology To estimate model uncertainties

Uncertainties in model parameters for reactions R1 and R15

  • A. Braumann, P. L. W. Man, and M. Kraft. Statistical approximation of the inverse

problem in multivariate population balance modelling. Ind. Eng. Chem. Res., 49: 428–438, 2010. doi:10.1021/ie901230u

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Shraddha Shekar ss663@cam.ac.uk

Gas-phase mechanism

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Shraddha Shekar ss663@cam.ac.uk

Reactor Plot

Conclusion from kinetic model: Si(OH)4 is the predominant gas-phase precursor

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Shraddha Shekar ss663@cam.ac.uk

Main reaction pathway

Si O O O O H2C CH2 CH2 H2C

  • C2H4

H3C H2 C CH3 H Si O O O O H2C CH2 CH3 H3C H3C CH3 Si O O O O H3C H3C CH3 H3C Si OH HO HO OH H2C H

  • C2H4

Si O O O O H3C H3C H H

  • C2H4

Si O O O O H2C CH2 CH2 H2C

  • C2H4

H3C H2 C CH3 H CH3

  • C2H4

Si O O O O H2C CH2 CH2 H H3C CH3 CH3

  • C2H4

Si O O O O H2C CH2 H H H3C CH3 Si O O O O H CH2 H H H3C

  • C2H4
  • C2H4

Reaction Pathway 1 Reaction Pathway 2

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Shraddha Shekar ss663@cam.ac.uk

Step 6: Particle Model

Si O O O O H H H H Si O O O O H H H H

  • H2O

Si O O O O H H H Si O O O H H H

INCEPTION

Si O O O O Si O O Si

  • nH2O

SURFACE GROWTH

O Si Si Si Si Si O

n(-O-Si-O-Si-)

Si(OH)4 molecules in gas-phase undergo inception to form a dimer (-Si-O-Si). This dimer is considered to be the first particle. Particle growth then proceeds by subsequent removal of hydroxyl groups.

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Shraddha Shekar ss663@cam.ac.uk

Particle Processes

[1]: M. Sander, R. H. West, M. S. Celnik, and M. Kraft. A Detailed Model for the Sintering of Polydispersed Nanoparticle Agglomerates, Aerosol Sci. Tech., 43, 978-989, 2009

New inception and surface growth steps have been incorporated in a previously developed stochastic particle model developed by Sander et al. [1].

Particle ineption Surface growth Coagulation Particle rounding due to surface growth

P = P(p1, p2, .....pn, C, I, S) p = p(vi)

Sintering Surface reaction Condensation Inception

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Shraddha Shekar ss663@cam.ac.uk

Individual Processes and Rates

1. Inception

2. Surface Reaction

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Shraddha Shekar ss663@cam.ac.uk

Individual Processes

3. Coagulation

Pi Pj Pk +

pi pj pi pj

pk

No Sintering Partial Sintering Complete Sintering

  • 4. Sintering
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Shraddha Shekar ss663@cam.ac.uk

Model Optimisation

Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech., 27, 422-438, 1997

Material dependent sintering parameters are optimised Optimisation method: LD series and RSM Primary diameter (dp) and collision diameters (dc) fitted to experimental values at different temperatures.

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Shraddha Shekar ss663@cam.ac.uk

Particle size distribution

Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech., 27, 422-438, 1997

Solid lines: Model Circles: Experiments (a)

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Shraddha Shekar ss663@cam.ac.uk

Model produced TEM-like images

Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech., 27, 422-438, 1997

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Shraddha Shekar ss663@cam.ac.uk

Step 7: Overall mechanism

Si O O O O H H H H Si O O O O H H H H Si O O O O H H H Si O O O H H H O H H Si O O O O H2C CH2 CH2 H2C CH3 CH3 CH3 H3C Si O O O O H3C H3C CH3 H3C Si O O O O H H H H Si O O O O H H H Si O O O H H H Si O O O O H H H H Si O O O O H H H H Si O O O O H H H H Si O O O O H H H H Si O O O O Si O O Si O Si Si Si Si Si O Gas-phase reactions Particle formation Particle growth

  • 2C2H4
  • 2C2H4
  • 2H2O
  • nH2O

[monomer] [primary particle] (-O-Si-O-)n [Silica particle]

The gas-phase and particle model described above are coupled using an

  • perator splitting

technique to generate the overall model.

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Shraddha Shekar ss663@cam.ac.uk

Conclusion

1. New kinetic model proposed which postulates silicic acid Si(OH)4 as the main product of TEOS decomposition. 2. A novel pathway proposed for the formation of silica nanoparticles via the interaction of silicic acid monomers. 3. Feasibility of using first-principles to gather a deeper understanding of complex particle synthesis processes.

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Shraddha Shekar ss663@cam.ac.uk

http://como.cheng.cam.ac.uk

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