The Applications of Nano Materials

1. The Applications of Nano Materials Department of Chemical and Materials Engineering San Jose State University Zhen Guo, Ph. D.

2. Fundamentals of • Nano Material Science • Session II: Atomic Structure/Quantum Mechanics • Session III: Bonding / Band Structures • Session IV: Computational Nano Materials Science • Session V: Surface / Interface Properties

3. Session V: Surface Energy and Related Properties • What Is Bulk and Surface? • The Source of Surface Energy • Surface Reconstruction • Adsorption and Desorption • Wetting (Hydrophilic vs. Hydrophobic) • Examples and Case Studies on Nano Materials • -- Surface Energy verse Nano particle size • -- Surface Energy verse Nano particle shape • -- Surface Energy verse Nano particle melting temperature

4. Crystal Structures • Materials can be divided into Amorphous, Single Crystalline, Poly Crystalline according to their crystal structure. • Bulk Material composed a repeat pattern of unit cell and thus their properties are determined by their crystal structures Ball-stick model: Ball – Atom, Stick -- Bonding FCC Unit cell BCC Unit cell HCP Unit cell

5. Crystalline direction and plane

6. What is Surface and Surface Energy Wetting Hydrophilic Surface Reconstruction Absorption Surface is the place where atoms terminating bulk, i.e. the termination of bulk crystalline structure (Repeating pattern) Surface Energy is coming from unbonded electrons

7. What is the surface / interface • Atoms at surface / Interface has unbonded electrons which will cause extra energy compared with atoms in bulk • This extra energy per area is defined as surface / interface energy f • The total energy associated with surface / interface E = f * S • This energy becomes significant in nano material system

8. Why surface properties are important for Nano materials d d H Total Volume V=H*A Total Surface Area S=A*H/d=V/d Thin Films

9. Why surface properties are important for Nano materials Nano Particles Total Volume V=6*4/3pR3=8pR3 Total Surface Area S=6*4pR2=24pR2=3V/R

10. What happen in real material system? • Surface reconfiguration will impact about 2-3 atomic layers in depth. (or 6-8A) • In general, atoms in those regions are different than their cousins in bulk and therefore included as surface. • For Si particle, assume surface region are 2 atomic layers or roughly 5A in depth => Another reason that surface properties are more important for nano materials as most of atoms are at surface

11. Surface Energy in BCC and FCC Structure • Surface energy is determined by how many bonds we have to break in order to create a fresh surface (Cleavage) and thus depending upon crystalline planes? • Which crystalline plane shall have lowest surface energy? Why? Closed packed plane has most atoms in plane and thus the out plane bonding is less and weak.

12. Surface Energy in Diamond Structure such as Silicon Diamond Structures in Cubic (Si, C) Which one is close packed plane? Courtesy from Dabrowski and Mussig: Si Surface and Formation of Interface

13. Surface Reconstruction Surface atoms are re-arranging their position and bonding to minimize the surface energy Courtesy from Dabrowski and Mussig: Si Surface and Formation of Interface

14. Adsorption and Desorption • The adsorption of molecules on to a surface is a necessary prerequisite to any surface mediated chemical process. • In general, surface reaction can be broken down into the following steps : • Transferring of reactants to the active surface • Adsorption of one or more reactants onto the surface • Surface reaction • Desorption of products from the surface • Transferring of products away from the surface • Absorption and Desorption are equally important.

15. Physical and Chemical Absorption • The basis of distinction is the nature of the bonding between the molecule and the surface. With ... • Physical Adsorption : the only bonding is by weak Van der Waals - type forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface. • Chemisorption : a chemical bond, involving substantial rearrangement of electron density, is formed between the adsorbate and substrate. The nature of this bond may lie anywhere between the extremes of virtually complete ionic or complete covalent character.

16. Typical Characteristics of Adsorption Processes http://www.chem.qmul.ac.uk/surfaces/scc/

17. Adsorbate Geometry NH3 -> NH2 (ads) + H (ads) -> NH (ads) + 2 H (ads) -> N (ads) + 3 H (ads) As the number of hydrogens bonded to the nitrogen atom is reduced, the adsorbed species will tend to move into a higher co-ordination site on the surface (thereby tending to maintain the valence of nitrogen).

18. Wetting and Contact Angle • The “Young Equation” determines the “contact angle” • Balance of forces at the periphery of a drop on a rigid surface • The wetting angle,  ranges from 0 (wetting) to  (de-wetting) • The engineering importance of surfaces • Wetting -- Frying pans and car waxes, Detergents, Lubricants • Bonding – Glues, Solders • Catalysis -- Adsorption • Capillarity -- Tree sap and blood vessels

19. V L S Film Formation (Spreading) • Spreading: LV+SL interfaces have lower energy than SV • Want for painting, coating, soldering, etc. • To promote spreading • Raise SV: e.g., clean the interface • Flux in soldering removes oxides from surface • Clean the substrate interface prior to deposit thin film • Lower SL: e.g., include reactive species in L • Sn in solder forms intermetallic compounds with Cu, Ni or Au • Flow catalyst or reactant to form an intermediate layer and prepare for deposition • Lower LV: e.g., add surfactant (species that adsorbs at LV interface) • Flux in solder coats surface, lowers LV

20. L V S De-Wetting • De-wetting: film of vapor preferred between S and L • LV+SV interfaces have lower energy than SL • Want for “non-stick” coatings (frying pans, car wax). • Passivation Layers to stop moisture. • To promote de-wetting • Lower SV: e.g., add surfactants or low- coatings to solid • Teflon on frying pans • Lower LV: e.g., add surfactant (species that adsorbs at LV interface) • Raise SL: e.g., remove any possible surfactants or reactive species

21. Hydrophilic vs. Hydrophobic • Hydrophilic (Water Loving): Materials exhibit an affinity for water The surface chemistry allows these materials to absorb water and be wetted forming a water film. Hydrophilic materials also possess a high surface tension and have the ability to form "hydrogen-bonds" with water. • Hydrophobic (Water hating): have little tendency to adsorb water and water tends to "bead" on their surfaces. Hydrophobic materials possess low surface tension values and lack active groups in their surface chemistry for formation of "hydrogen-bonds" with water.

22. Case Study I – Surface Energy verse Nano Particle Size Homogeneous Nucleation of Nano Particles Liquid Liquid Solid G1=V*GvL=(Vs+VL) *GvL G2=VL*GvL+Vs*Gvs+ASL*fSL DG=G2-G1=-Vs(GvL-Gvs)+ASL*fSL= -VsDGv+AS*fSL For a given undercooling, DG=(LvDT)/Tm

23. DG Interface Energy r2 DGr* r r* DGr Volume Free Energy r3 Critical Nucleus Size • Assume sphere shape DG = -VsDGv+AS*fSL = -(4/3pr3)DGv+(4pr2)fSL əDG/ər (r*) = 0 at peak point (4pr*2)DGv=(8pr*)fSL r*=2fSL/DGv=(2fSLTm)/(Lv*DT)

24. Case Study II – Surface Energy verse Nano Particle Shape • Atomic configuration on the three closest packed plans Broken Bonds per atom: 3 4 5 Inter-atomic Layer Spacing: 0.577a 0.5a 0.354a Surface Energy Per atom: 3*0.5e 4*0.5e 5*0.5e Courtesy from Porter and Easterling Phase Transformation in Metals and Alloys

25. Any Arbitrary Crystalline plane in cubic Broken Bonds per unit length: Cosq/a + Sinq/a=(Conq+Sinq)/a Broken Bonds per atom: [(Cosq+Sinq)/a]*1/a=(Cosq+Sinq)/a2 Surface Energy per atom: (Cosq+Sinq)/a2*0.5e Courtesy from Porter and Easterling Phase Transformation in Metals and Alloys

26. g plot and Wulff Constructions This theoretically determined the equilibrium shape and crystallographic orientation of Nano particles Courtesy from Porter and Easterling Phase Transformation in Metals and Alloys

27. Case Study III – Melting Temperature of Nano Particle Shape Blue Sheet activity: Giving the particle size of d, what is the relationship between melting temperature and d? How about vapor pressure?