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 molecule • Many possible ways of bond breaking, many possible reactions 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 Shraddha Shekar ss663@cam.ac.uk
Physical system Silica nanoparticles Precursor(TEOS) Flame reactor Industrial P ≥ 1 atm Macroscopic level questions: •Optimal process conditions? Scale •Final product properties? •Final particle size distribution? T ≈ 1100 - 1500 K 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? Shraddha Shekar ss663@cam.ac.uk
Methods: Ab initio modelling Species generation Quantum Chemistry calculations Statistical Mechanics Thermochemistry Overall Model calculation H(T) S(T) C p (T) Equilibrium Chemical Population Balance Model calculation Kinetics Shraddha Shekar Ref: W. Phadungsukanan, S. Shekar, R. Shirley, M. Sander, R. H. West, and M. ss663@cam.ac.uk Kraft. First-principles thermochemistry for silicon species in the decomposition of tetraethoxysilane. J. Phys. Chem. A , 113 , 9041–9049, 2009
Step1: Species Generation Branch 4 10 TEOS: Symmetric molecule C State 1: No : 4-ethoxy groups attached to Si removal 11 : 4-possible states for each branch C State 2: Terminal C : Combinations for all 4 branches removed produced using 4-nested loops 12 O Branch 1 O C C Si O C C 7 8 9 3 2 1 O Branch 3 6 State 3: Penultimate C C removed 5 State 4: O removed C Branch 2 Shraddha Shekar 4 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 Shraddha Shekar ss663@cam.ac.uk
Step 3: Thermochemistry S Vs T Frequency Partition Data from H Vs T Thermochemistry Functions (q) Gaussian C p Vs T 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 Shraddha Shekar tetraethoxysilane. J. Phys. Chem. A , 113 , 9041–9049, 2009 ss663@cam.ac.uk
Step 5: Kinetic model • Kinetics • Equilibrium – Reaction set generated – Hints towards the to include all existence of stable intermediates and intermediates & products from products. equilbrium. – Intermediates – Reactions obey Si(OH) x (OCH 3 ) 4-x Arrhenius law rate Si(OH) y (OC 2 H 5 ) 4-y constant k = AT β e -Ea/RT – Main Product Si(OH) 4 – 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 Shraddha Shekar decomposition of tetraethoxysilane and Related Compounds. J. Phys. Chem. A , 101 , ss663@cam.ac.uk 5500-5508, 1997
Flux and Sensitivity Analyses Reaction number Main Reaction Pathway 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 1: Low discrepancy series To perform a pre-scan of parameters for 18 Si reactions. Step 2: Sensitivity Analysis To identify the 4 most sensitive parameters Step 3: Response Surface Methodology Uncertainties in model parameters for reactions R1 and R15 To estimate model uncertainties Shraddha Shekar 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: ss663@cam.ac.uk 428–438, 2010. doi:10.1021/ie901230u
Gas-phase mechanism Shraddha Shekar ss663@cam.ac.uk
Reactor Plot Conclusion from kinetic model: Si(OH) 4 is the predominant gas-phase precursor Shraddha Shekar ss663@cam.ac.uk
Main reaction pathway H 2 Reaction Pathway 1 C H 3 C H 3 C H H H 3 C H 2 C H 3 C CH 2 H 3 C H 3 C O O O CH 2 O O O O Si Si Si O O O O Si -C 2 H 4 -C 2 H 4 -C 2 H 4 H 2 C O O O O CH 3 CH 3 H O H H 2 C H 3 C H 3 C CH 2 -C 2 H 4 H 2 C CH 3 CH 3 OH HO Si OH HO Reaction Pathway 2 -C 2 H 4 H 2 H H H C H 3 C H 3 C H H 3 C H 2 C CH 2 O CH 2 O H 3 C CH 2 O CH 2 O O O O Si Si Si CH 3 O O O O -C 2 H 4 -C 2 H 4 Si -C 2 H 4 O O O CH 3 O H H H 2 C CH 2 H H 2 C O CH 2 H 2 C CH 3 CH 3 CH 3 Shraddha Shekar ss663@cam.ac.uk
Step 6: Particle Model H H H H O O H H H O O H O O O O Si Si -H 2 O Si Si O H H O O INCEPTION O O O O H H H H SURFACE -nH 2 O GROWTH O Si Si Si Si O O O O Si Si Si(OH) 4 molecules in gas-phase undergo O O O inception to form a dimer (-Si-O-Si). This Si Si n(-O-Si-O-Si-) dimer is considered to be the first particle. Particle growth then proceeds by subsequent removal of hydroxyl groups. Shraddha Shekar ss663@cam.ac.uk
Particle Processes P = P(p 1 , p 2 , .....p n , C, I, S) p = p(v i ) Inception Particle ineption Condensation Coagulation Surface reaction Surface growth Sintering Particle rounding due to surface growth New inception and surface growth steps have been incorporated in a previously developed stochastic particle model developed by Sander et al. [1]. [1]: M. Sander, R. H. West, M. S. Celnik, and M. Kraft. A Detailed Model for the Shraddha Shekar Sintering of Polydispersed Nanoparticle Agglomerates, Aerosol Sci. Tech ., 43 , ss663@cam.ac.uk 978-989, 2009
Individual Processes and Rates 1. Inception 2. Surface Reaction Shraddha Shekar ss663@cam.ac.uk
Individual Processes 3. Coagulation P j P k P i + 4. Sintering p k p j p j p i p i Complete Sintering No Sintering Partial Sintering Shraddha Shekar ss663@cam.ac.uk
Model Optimisation Material dependent sintering parameters are optimised Optimisation method: LD series and RSM Primary diameter (d p ) and collision diameters (d c ) fitted to experimental values at different temperatures. Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Shraddha Shekar Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech ., ss663@cam.ac.uk 27 , 422-438, 1997
Particle size distribution Solid lines: Model Circles: Experiments (a) Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Shraddha Shekar Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech ., ss663@cam.ac.uk 27 , 422-438, 1997
Model produced TEM-like images Shraddha Shekar Ref (a): T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. ss663@cam.ac.uk Sintering of Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech ., 27 , 422-438, 1997
Step 7: Overall mechanism CH 3 Gas-phase reactions H H 3 C H 2 C H 3 C H O CH 2 H 3 C O O O O O Si Si Si -2C 2 H 4 -2C 2 H 4 O H O CH 3 O CH 2 O O O The gas-phase and H H 2 C H 3 C CH 3 [monomer] particle model CH 3 H O H H H described above are H Particle formation H O O H O H O Si O O coupled using an Si Si O H O O Si -2H 2 O O H O O O H operator splitting O O H H H H H technique to generate [primary particle] H H the overall model. H O H O H H O O O Si Si O O H Si Si Particle growth O H O Si H O Si O O O O O H Si Si H H O O O Si Si H O O H O O H H H -nH 2 O O O O O O Si Si Si H Si O H O O H O H (-O-Si-O-) n [Silica particle] H 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. Shraddha Shekar ss663@cam.ac.uk
Thank you! Please visit our website: http://como.cheng.cam.ac.uk Shraddha Shekar ss663@cam.ac.uk
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