Production of Nanoparticles by Electrosprays

1. Production of Nanoparticles by Electrosprays Apple group Kelly Tipton, Yechun Wang, Matthew McHale, Brendan Hoffman

2. Introduction to Electrosprays of Nanoparticles • Nanoparticles are of interests because their chemical and physical behavior is unprecedented, making way for applications in electronics, chemical and mechanical industries, drug delivery, magnetic materials, as well as a variety of others • Monodispersed Particles preferred • Goal: Want a process in which particles having controlled characteristics such as size, morphology and composition can be produced, at the lowest cost and highest yield.

3. Use of the Aerosol Process and Methods of Preparation • There are at least tow routes for the preparation of ultrafine particles by aerosol process. • Gas to Particle Conversion (Build up Method) • Advantages: Small particle size, narrow distribution, high purity of particles produced • Disadvantages: Formation of hard agglomerates in the gas phase, difficulty in separating multi-component materials, non-uniform composition • Liquid to Solid Conversion (Break Down Method) • Advantages: Multicomponent materials prepared well, comparatively low cost • Disadvantages: Not very well understood

4. CVD vs. PVD • For Gas to Particle Conversion • CVD: Chemical Vapor Deposition • The vapor evaporated from the solution precursors is thermally decomposed or reacted w/ another precursor vapor or surrounding gas. Solids are produced during nucleation, condensation and coagulation • PVD: Physical Vapor Deposition • Evaporation of a solid or liquid is the source of vapor • In the cooling stage nucleation and condensation of the saturated vapor take place and solid particles are formed

5. Electrospray Physics Taylor cone Electrospray jet Capillary To colloid source E-field Voltage Src. Colloid solution (electrolyte) Under normal laminar flow Collector/ voltage sink Solvent Particle From colloid Solution, no particle yet formed

6. The Taylor Cone-Jet Accumulation of excess surface charges Formation of charged droplets Electric field required for onset of electrospray: Surface tension Capillary radius Vacuum permittivity Taylor cone forms along direction of electric field under the competing influences of charge accumulation and surface tension Smith, 1989 • Liquids with high surface tension (e.g. water) require an enormous electric field • Leads to problems with discharging • Cone-jet mode for low flow rates and small electrolyte concentrations • Charged droplet formation is statistical (no electrochemistry needed)

7. Droplet Evolution Radius of an evolved droplet: Charge of an evolved droplet: Qf vol. flow rate K  solution conductivity Colloid pathway To reaction Chamber / drier Acceleratedto counterelectrode Newly formed droplet Coulombic explosion (droplet fission) Solvent Evaporation ~ 100 ms to Raleigh limit

8. \ Rayleigh Limit Rayleigh limit: the equilibrium state at which further addition of charge will cause the drop to become unstable and break nm range • Daughter droplets not equal! • Offspring usually carry 2% of mass and 15% • of charge Excess charge Coulomb fission As large drops evaporate new Rayleigh limits arereached until drops of the nanosize are predominant When all solvent evaporates, we are left with our particles (charged solid residues) E-field required for fission (as opposed toion evaporation) For very small drops, ion evaporation occurs. This is more favorable than successive fissions past some critical drop size.

9. Limits of Electrospray Particles produced by complete evaporation of droplets(solid residue formation) require VERY LOW concentrations forsmall sized particles: 10 mm 100 nm Example: Produce a 100 nm particle of density r=2000 g/cm3by evaporating drops of 10 mm. What is the necessary solution concentration (in g/mL) of solute? Assume the solute is nonvolatile. This is a fairly low concentration. Electrosprays can produce droplets from about 200 nm to 10 mm. Concentrations can become prohibitivelylow for smaller particles (10 nm2e-6 g/cm3).

10. \ Colloid Sprays Electrosprays can atomize colloid solutions down to 1 particle/drop resolution! Dispersed particles (colloid) Evaporation Maintains integrity of colloidal particle Drop size dependent on pre-reaction, not size of droplet (below resolution) Coulombic explosion Sample Dry Particle Generator Reactants Heat Collector Particle sizedepends on residence time in reactor and reactioninside drop Colloidal mixture Voltage

11. Reactive Sprays Sometimes we wish to produce small droplets of reactants, then ‘turn on’ the reaction when the droplets are small enough. Example: Production of ZnS particles via electrospray (de la Mora)

12. Reactive Sprays (continued) Particle size depends on reactant concentration (rate), temperature, pressure, gas phasecomposition, drop size, residence time in reactor… and many more variables. In this example:

13. Applications of Electrosprays in Nanoparticle Production • Ionization for mass spectrometry of large biomolecules • Fine metal powder production • Deposition of ceramics • Electrospray pyrolysis for metal salts production

14. Mass spectrometry Transform individual molecules into ions in vacuo and measure the trajectories in electric or magnetic fields. Classical methods of ionization Based on gas-phase encounters of molecule to be ionized with electrons, photons or other ions. Difficulties in large biomolecules Vaporization together with extensive, catastrophic decomposition ES ionization for MS Ionization for mass spectrometry of large biomolecules John B. Fenn et al, Science 1989

15. Ionization for mass spectrometry of large biomolecules (continued)

16. Classical method Impacting a liquid metal stream by either gas or liquid jets Other methods Rotating electrode, centrifugal atomization, gas evaporation Advantages of ES method Sufficient yields and efficiency Fine metal powder production John F. Mahoney et al, IEEE transactions on industry applications. 1987

17. Aim of ceramic manufacturing process: Fine particles will be deposited one at a time at high speed Comparison of ES method with jet printers w.r.t volumetric resolution: Deposition of Ceramics W.D. Teng et al, J. Amer. Ceram. Soc. 1997

18. Electrospray pyrolysis for metal salt nanoparticle production General spray pyrolysis process in metal salt nanoparticle production: Conventional spray pyrolysis: Very hard to get particles with diameter less than 100 nm. Electrospray pyrolysis: A method to generate ultrafine droplets. Solvent containing metal salt Size determination Spray methods Droplets Evaporation, diffusion, drying, precipitaion, reaction or sintering Introduced into furnace Final particles

19. Analyzing sizes of Colloidal Nanoparticles • Transmission/Scanning electron microscopy (TEM/SEM) • Scanning near-field optical microscopy • Scanning probe microscopy • Atomic force microscopy

20. What is a DMA? • Two charges concentric cylinders with an inlet slit & sampling slit • Separates particles based on their electrical mobility • Aerosol particles are inserted and carried by clean air through the annular region

21. Concerns with Analysis techniques • Sampling quality • Operators ability • Time spent on procedures • Characterizing the size of isolated nanoparticles imbedded within an oxide layer or substrates

22. New techniques for Sizing • Transferring the colloidal particles from the liquid into gas phase using electrosprays • Particle sizing in the gas phase using an inertial impactor or a differential mobility analyzer (DMA)

23. Need to use the new techniques in series Need for a high charge on the particles Need to account for the large particle losses in the flow lines between the detector & electrospray Need less deformation of the size distribution Need a method for “unknown” sizes as well as smaller ones Assessed by analyzing colloids’ spheres with diameters from 21 to 74 nanometers Adjustment proposal

24. Determination of the Stability Domain • Measure the typical curves of the electric current vs. the applied voltage of the spraying solutions • Increase the voltage gradually • Reduce the liquid meniscus at the capillary tip to a dripping mode, then a pulsating mode

25. Size distribution of Naturally Dried Colloids • Raw material dries naturally prior to TEM/SEM analysis • An average is taken with the collected nanoparticles • Some particles are encapsulated by a large amount of surfactants • Surfactants generate a residue around the particles during the solvent evaporation

26. Particle Size Distribution of Electrosprayed Colloids • Measured using the ES-DMA system • Ambient temperature of 25oC & liquid flow rate at its minimum • Initial concentration influences final particle size distribution • At high concentrations, produces more than one particle per droplet • At low concentrations, changes electrical conductivity but size of particles barely change • At medium concentrations, provides proper concentration per droplet ( defines conditions of 1 particle/droplet)

27. Homework Problem Why is the particle distribution the same over all tested concentration ranges?? Assume that the only variable changed is the reactant concentration.