Electrospun nanofiber

1. Electrospunnanofiber

2. Electrospinning is… Process used to produce fiber from solution by using high static voltage Advantages Easy to produce large amounts of nanofibers Inexpensive process High temperature is not required Various composite can be made Operation parameters Molecular Weight and Architecture of the polymer Solution properties (viscosity, conductivity & and surface tension) Electric potential, Flow rate & Concentration Distance between the capillary and collection screen Ambient parameters (temperature, humidity and air velocity in the chamber) Motion of target screen (collector) Schematic of an electrospinning setup, shown without a syringe pump

3. Basic of electrospinning If high voltage is applied to a liquid droplet Body of the liquid becomes charged Electrostatic repulsion counteracts the surface tension Liquid drop distorted into a Taylor cone If strength of electric field surpass a threshold value Electrostatic force overcome surface tension Liquid jet eject from nozzle Bending and whiping caused by electrostatic interactions between external electric field and surface charge on the jet The elongation and thinning of the fibre resulting from this bending instability As liquid jet is elongated and solvent is evaporated its diameter is decreased to submicron size These nanofibers is attracted and deposited on collector plate as a random-oriented, non-woven mat

4. Electrospunnanofibers Generally deposit on collection target planar random nonwovens Properties Micron to submicron diameter Nonwovens are highly porous High surface to volume ratio Bead formation

5. Electrospunnanofibers - Materials Generally polymers– Simply made by spinning of polymer solution Metals– Spinning of polymer/precursor (like metal nitrate) solution followed by calcination and annealing in H2 atmosphere Metal oxides– Spinning of polymer/metal oxide gel (from sol-gel precursor) followed by calcination PVA Fe ZnO

6. Electrospunnanofibers - Morphology Core-sheath– 1) Deposit shell on electrospunnanofiber template 2) Coaxial electrospinning Hollowed– 1) Spinning of core-shell nanofibers followed by removal of core 2) Core material deposited on shell when core solution evaporates Porous– 1) Phase seperation of polymer blends 2) Spinning in high humidity 3) Spinning onto very cold substrates Schematic: coaxial electrospinning

7. Electrospunnanofibers – Fiber alignment Aligning by using electrostatic force Collecting the electrospunnanofibers over a gap Formed between two grounded conductive substrates Uniaxially aligned nanofiber arrays

8. Electrospunnanofibers – Fiber alignment Electric field splitting Electric field lines split and head to grounded electrodes Electrostatic force F1: from the splitting electric field F2: Coulombic force between charged fiber and image charge induced on surfaces of electrodes

9. Electrospunnanofibers – Fiber alignment Aligning by using rotating collector Aligned nanofibers as a result Collector can be A insulated cylinder rotating at high speed or Wire drum collector rotating at low speed using electrostatic force (F2 of previous one)

10. Electrospunnanofibers – Applications Nanofiber-reinforced composites Membranes and smart cloths Catalysts Sensors Scaffolds for tissue engineering Battery electrodes Sacrificial templates

11. Electrospunnanofibers – Applications In our laboratory? Scattering polarizer Light scattering shutter Transparent paper LCD OPV ZnOnanofiber Oxide TFT Sacrificial template Nanofiber patterning Nanoimprinting & patterning

12. Transparent nanofiber paper Transparent Substrates for electronic devices Polymers – Large coefficients of thermal expansion (CTE) Glass – Inflexible and fragile Flexible, transparent nanofiber paper Made from wood fiber Freeze-drying and 160MPa compressing of cellulose nanofibers Low CTE of 0.1ppm/K 2-3 GPa strength Supression of light scattering Dense packing of nanofibers Smooth surface

13. Scattering polarizer using nanofiber Polarizer consists Anisotropic nanofibers and birefringement LC Refractive index of nanofibers matches on of the RI of LC and mismatches another Light scattering occurs when RI of materials mismatch Polarizer using nanofiber Maximum ΔT ~ 0.72 T⊥ ~ 89% T∥ ~ 14% PE ~ 0.80 Tsp > 0.54

14. Light scattering shutter using nanofiber Cell fabrication Nanofibers deposited on two ITO glasses Cell is assembled and annealing at 110℃ for 30 min. Nematic LC was inserted by capillary rise On With nanofiber Cell thickness is reduced → Turn-on voltage is reduced → Turn-on time is decreased Turn-on voltage: half of former device (30~100V by thickness) Response times in the ms range Off

15. Electrospinning of inorganic nanofiber General routes Making aqueous solution of polymer and sol-gel precursor Electrospinning of solution to form nanofibers Calcination of nanofibers at high temperature Ex) ZnOnanofiber Zinc acetate (0.5g) + 10wt% PVA aqueous solution (2.5g) Electrospinning Calcination at 500℃ for 4h Electrospun oxide nanofibers exhibit poor performance because of impurities from polymer It is desirable polymers to be excluded, but most inorganic materials have limited solubility to be electrospun Figure: I-V output characteristic of electrospunZnO TFT

16. Electrospinning of inorganic nanofiber Soluble inorganics Metal chalcogenides can be deposited by spin-coating via “dimensional reduction” approach David B. Mitzi IBM T.J. Watson Research Center (C6H5C2H4NH3)2SnI4 Organic-inorganic hybrid TFT Science (1999) SnS2-xSex from hydrazinium precursor TFT application Nature (2004) CIGS from hydrazinium precursor Photovoltaic application Advanced materials (2008)

17. Electrospinning of inorganic nanofiber Dimensional reduction approach 1) Breaking up the insoluble extended inorganic framework into more soluble-isolated anionic species, which are separated by some small and volatile cationic species 2) Solution-processing thin films of the precursor 3) Heating the precursor films such that the cationic species and corresponding chalcogen anions are dissociated, leaving behind the targeted inorganic semiconductor Dimensional reduction Solution process film Apply heat Small volatile cation (Hydrazine)

18. Electrospinning of inorganic nanofiber Dimensional reduction approach Organic-inorganic hybrid perovskite Tin halide: sensitive to humidity and difficult to handle Hydrazinium precursor Hydrazine: highly explosive and toxic material Although hydrazine has the advantage of being small, volatile, and non-carbon-containing, other solubilizing species are required

19. Electrospinning of inorganic nanofiber Approach without hydrazine Procedure: 1) 1.5 mmol (NH4)2MoS4 + 1.2ml methylamine (40wt%, aq. sol.) 2) Constant stirring in an argon atmosphere at RT 3) Dry in a vacuum oven at RT for 12h 4) (CH3NH3)2MoS4 is collected Solubility of (CH3NH3)2MoS4: Up to 70% at RT, 1atm Figure: Molecular crystal structure of (CH3NH3)2MoS4

20. Electrospinning of inorganic nanofiber Further toward ZnOnanofiber 1) Oxidation of sulfide nanofiber - Make hybrids like ZnS/N2H4, ZnS/cyclohexylamine, ZnS/n-butylamine - Find appropriate solvent - Spin to nanofiber and annealing in oxygen atmosphere 2) Direct formation of oxide nanofiber - There are hybrids like WO2.72(N6C123H136O22)0.04 or GeOx/Ethylenediamine - Some possibility for soluble ZnO-containing hybrid

21. KIER 0.5% DPL, Yonsei Univ. Purdue Univ. 3.2% Purdue Univ. 5.5%

22. (112) (204)/(220) (116)/(312) (200) Mo (110) (008)/(400) (316)/(332) (228)/(424) (211) Mo Mo