Computational modelling of Silica nanoparticle formation in a flame - PowerPoint PPT Presentation

Computational modelling of Silica nanoparticle formation in a flame reactor S. Shekar, M. Sander, A. J. Smith, M. Kraft 26 April, 2010

Introduction Mesoporous silica nanoparticles Precursor (TEOS) Aim: To answer the following questions • What happens in the gas-phase? • How do gas-phase precursors form the particles? • How do these particles grow? • How to describe the overall system from first-principles? Shraddha Shekar ss663@cam.ac.uk

Product : Silica nanoparticles Mesoporous Silica Nanoparticles: network of Si-O bonds such that Si:O = 1:2 Applications: •Support material for functional/composite nanoparticles. •Optics, optoelectronics, photoelectronics •Catalysis •Bio-medical applications, drug delivery Shraddha Shekar ss663@cam.ac.uk

Industrial Flame Reactor Product (Silica nanoparticles) Aim: To describe the flame synthesis of silica nanoparticles P ≥ 1 atm Flame Spray T ≈ 1100 - Experiments Model Reactor 1500 K (Herzler et al / Seto et al / Pratsinis et al ) Precursor(TEOS) Fuel(ethanol+air) Shraddha Shekar / Inerts(Ar) ss663@cam.ac.uk

Ab initio modelling Species generation Quantum Chemistry calculations Statistical Mechanics Thermochemistry Overall Model calculation H(T) S(T) C p (T) Equilibrium Chemical Population calculation Kinetics Balance Model Shraddha Shekar ss663@cam.ac.uk

Equilibrium Plot Ref: W. Phadungsukanan, S. Shekar, R. Shirley, M. Sander, R. H. West, and M. Kraft. Shraddha Shekar First-principles thermochemistry for silicon species in the decomposition of tetraethoxysilane. J. Phys. Chem. A , 113 , 9041–9049, 2009 ss663@cam.ac.uk

Reaction kinetics • 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

Gas-phase mechanism 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 H O 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 HO OH 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 CH 2 O H 3 C O CH 2 O CH 2 O O O O Si Si Si CH 3 O O O O -C 2 H 4 O -C 2 H 4 Si -C 2 H 4 O O CH 3 O H H H 2 C H CH 2 H 2 C O CH 2 H 2 C CH 3 CH 3 CH 3 Shraddha Shekar ss663@cam.ac.uk

Reactor Plot Shraddha Shekar ss663@cam.ac.uk

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 O O INCEPTION 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 Model 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

The Data Structure Shraddha Shekar ss663@cam.ac.uk

Particle-gasphase reactions 1. Inception 2. Surface growth Shraddha Shekar ss663@cam.ac.uk

The Algorithm 1. Set start time t ← t 0 and the initial system x ← x 0. 2. Calculate an exponentially distributed waiting time where U is a uniformly distributed random number, U Є (0; 1), and R tot is the total rate of all processes (surface reaction, coagulation and inception) defined for rates R i , i Є {coag,, inception, surfrxn} Ref: M. Sander, R. H. West, M. S. Celnik, and M. Kraft. A Detailed Model for Shraddha Shekar the Sintering of Polydispersed Nanoparticle Agglomerates, Aerosol Sci. Tech ., ss663@cam.ac.uk 43 , 978-989, 2009

The Algorithm 3. Increment time variable t ← t+dt . 4. If t > t stop then end. 5. Update the sintering level for the time dt for all the particles. 6. Choose a process i according to the probability: 7. Perform process i . 8. Go to step 2. Ref: M. Sander, R. H. West, M. S. Celnik, and M. Kraft. A Detailed Model for Shraddha Shekar the Sintering of Polydispersed Nanoparticle Agglomerates, Aerosol Sci. Tech ., ss663@cam.ac.uk 43 , 978-989, 2009

Experimental Setup of Seto et al. Ref: T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Shraddha Shekar Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech ., 27 , 422- ss663@cam.ac.uk 438, 1997

Model Validation Ref: T. Seto, A. Hirota, T. Fujimoto, M. Shimada, and K. Okuyama. Sintering of Shraddha Shekar Polydisperse Nanometer-Sized Agglomerates, Aerosol Sci. Tech ., 27 , 422- ss663@cam.ac.uk 438, 1997

Model produced TEM-like images at 0.1 s, T = 1300 K Shraddha Shekar ss663@cam.ac.uk

Overall mechanism for particle formation s n o i t c a The gas-phase and e r e s a particle model h p - s a described above are G coupled using an n o i operator splitting t a m r o technique to generate f e l c i t r a the overall model. P h t w o r g e l c i t r a P 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

Acknowledgements Thank you! Shraddha Shekar ss663@cam.ac.uk