Chimica Fisica dei Materiali Avanzati Part 5 – Colloids and nanoparticles

1. Chimica Fisica dei Materiali AvanzatiPart 5 – Colloids and nanoparticles Laurea specialistica in Scienza e Ingegneria dei Materiali Curriculum Scienza dei Materiali Corso CFMA. LS-SIMat

2. 1nm<d<100nm d Increasing size molecule nanoparticle bulk material Nanoscale Materials Milan - Duomo Florence - S. Croce Single Electron Transistor Andres et al., Science, 1323, 1996 Corso CFMA. LS-SIMat

3. What are they? 0 dimensional nanomaterials: unique properties due to quantum confinement and very high surface/volume ratio Nanoparticles Nanorods 1 dimensional nanomaterials: extremely efficient classical properties Nanotubes Nanowires Corso CFMA. LS-SIMat

4. Properties of Metal Nanoparticles Optical Properties Nanoscale Materials have different properties when compared to their bulk counterparts! Melting point of Au vs. nanoparticle radius (Å) Corso CFMA. LS-SIMat Electronic Properties

5. A brief historical background Marie-Christine Daniel and Didier Astruc, Chem. Rev. 2004, 104, 293-346 • it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. • the Lycurgus Cup that was manufactured in the 5th to 4th century B.C. It is ruby red in transmitted light and green in reflected light, due to the presence of gold colloids. • The reputation of soluble gold until the Middle Ages was to disclose fabulous curative powers for various diseases, such as heart and venereal problems, dysentery, epilepsy, and tumors, and for diagnosis of syphilis. • the first book on colloidal gold, published by the philosopher and medical doctor Francisci Antonii in 1618. This book includes considerable information on the formation of colloidal gold sols and their medical uses, including successful practical cases. • In 1676, the German chemist Johann Kunckels published another book, whose chapter 7 concerned “drinkable gold that contains metallic gold in a neutral, slightly pink solution that exert curative properties for several diseases”. He concluded that “gold must be present in such a degree of communition that it is not visible to the human eye”. Corso CFMA. LS-SIMat

6. A brief historical background (cont’d) • A colorant in glasses, “Purple of Cassius”, is a colloid resulting from the heterocoagulation of gold particles and tin dioxide, and it was popular in the 17th century. • A complete treatise on colloidal gold was published in 1718 by Hans Heinrich Helcher. In this treatise, this philosopher and doctor stated that the use of boiled starch in its drinkable gold preparation noticeably enhanced its stability. • These ideas were common in the 18th century, as indicated in a French dictionary, dated 1769, under the heading “or potable”, where it was said that “drinkable gold contained gold in its elementary form but under extreme sub-division suspended in a liquid”. • In 1794, Mrs. Fuhlame reported in a book that she had dyed silk with colloidal gold. • In 1818, Jeremias Benjamin Richters suggested an explanation for the differences in color shown by various preparation of drinkable gold: pink or purple solutions contain gold in the finest degree of subdivision, whereas yellow solutions are found when the fine particles have aggregated. Corso CFMA. LS-SIMat

7. A brief historical background (cont’d) • In 1857, Faraday reported the formation of deep red solutions of colloidal gold by reduction of an aqueous solution of chloroaurate (AuCl4-) using phosphorus in CS2 (a two-phase system) in a well known work. • He investigated the optical properties of thin films prepared from dried colloidal solutions and observed reversible color changes of the films upon mechanical compression (from bluish-purple to green upon pressurizing). • The term “colloid” (from the French, colle) was coined shortly thereafter by Graham. Corso CFMA. LS-SIMat

8. Synthesis of Metal Nanoparticles The citrate method (J. Turkecitch et al., 1951) • It is used to produce modestly monodisperse spherical gold nanoparticles suspended in water of around 10–20 nm in diameter. • Take 5.0×10−6 mol of HAuCl4, dissolve it in 19 ml of deionized water (should be a faint yellowish solution) • Heat it until it boils • While stirring vigorously, add 1 ml of 0.5% sodium citrate solution; keep stirring for the next 30 minutes • The colour of the solution will gradually change from faint yellowish to clear to grey to purple to deep purple, until settling on wine-red. • Add water to the solution to bring the volume back up to 20 ml (to account for evaporation). The sodium citrate first acts as a reducing agent, and later the negative citrate ions are adsorbed onto the gold nanoparticles and introduce the surface charge that repels the particles and prevents them from aggregating. To produce bigger particles, less sodium citrate should be added (possibly down to 0.05%, after which there simply would not be enough to reduce all the gold). The reduction in the amount of sodium citrate will reduce the amount of the citrate ions available for stabilizing the particles, and this will cause the small particles to aggregate into bigger ones (until the total surface area of all particles becomes small enough to be covered by the existing citrate ions). • It is easy • It requires only water • It requires skills • It has reproducibility issues Corso CFMA. LS-SIMat

9. Definition: A colloid is a dispersion of small particles in a medium. The particles may be solid, liquid or gas, and the medium is normally liquid but may be a gas. • Types of Colloid • Sol: Dispersion of very small solid particles in solution. Example: Faraday gold sol (15nm) • Aerosol: Dispersion of droplets or particles in air. (Example: steam) • Emulsion: Dispersion of oil in water or water in oil. (Example: salad dressing) What is a Colloid? Corso CFMA. LS-SIMat

10. Colloids and nanoparticles • If only VdW forces were operating, we might expect all dissolved particles to stick together (coagulate) immediately and precipitate out of solution as a mass of solid material. • Fortunately, there are repulsive forces that prevent coalescence, like electrostatic, solvation and steric forces. • Particles suspended in water or any other liquid of high dielectric constant are usually charged • Charge can arise from several chemical processes: • Dissolution of the lattice E.g., AgI(s)  Ag+(aq) + I-(aq) • Adsorption of ions, polymers & detergents E.g., humic acid (a poly-carboxylic acid) onto soil particles • Surface Acid Base reactions E.g., Ti-OH(s)  TiO- + H+ T.W. Healy and L.R. White Adv. Coll. Interface Sci., 9, 303 (1978). • Electron Transfer E.g., (CH3)2COH + Agn (CH3)2CO + Agn- + H+ Corso CFMA. LS-SIMat

11. amphoteric pH COOH Only surface Point of Zero Charge A surface charge is at its point of zero charge (pzc) when the surface charge density is zero. It is a value of the negative logarithm of the activity in the bulk of the charge-determining ions. Typically a particular material has a characteristic pH at which it is neutral. This is called the point of zero charge (pzc) The pzc is sensitive to adsorbed ions and molecules, crystallinity of the particles and ionic strength Corso CFMA. LS-SIMat

12. Electrostatic stabilization of metal colloids Corso CFMA. LS-SIMat

13. The electric double layer Independent of the charging mechanism, the surface charge is balanced by an equal but oppositely charged region of counterions. Some of them are transiently bound to the surface and form the Stern or Helmoltz layer. Other form a diffuse layer in rapid thermal motion: the electric double layer. Corso CFMA. LS-SIMat

14. Basic relations determining the ionic distribution • The chemical potential of an ion, of valency zi and number density at a distance x from the surface, is constant • In the bulk, where the ionic concentration is , the potential is • Therefore, at the surface (x = 0) • The two unknown quantities are related by the Poisson equation Corso CFMA. LS-SIMat

15. Poisson-Boltzmann equation • It is a non-linear second-order differential equation • Subjected to the boundary condition of overall neutrality of the system, i.e., the total charge of the counterions must counterbalance the charge on the surface s. Thus whence • A further general relation can be worked out Corso CFMA. LS-SIMat

16. Charged surfaces in electrolyte solutions • If, as in most practical cases, the charged surfaces interact across a solution that already contains electrolyte ions, the total ionic concentration is and the net charge density is . • The total concentration of ions at an isolated surface of charge density s is given by this is known as Grahame equation. Once we replace on the LHS , it provides a link between potential and surface charge density. • For low potentials it simplifies to where Corso CFMA. LS-SIMat

17. The Debye length • The diffuse electric double layer near a charged surface has a characteristic length or ‘thickness’ known as the Debye length . • The magnitude of the Debye length depends solely on the properties of the liquid and not on the charge or potential of the surface • Values Corso CFMA. LS-SIMat

18. Potential and ionic concentrations • Worked out from the general relation • For 1:1 electrolytes (e.g., NaCl) where . For high potentials . For low potentials Gouy-Chapman theory Debye-Hükel equation Corso CFMA. LS-SIMat

19. Interaction free energies • From the above results, the interaction free energy per unit area is estimated (at low surface potentials) • For two planar surfaces at distance D • For two spheres of radius R Corso CFMA. LS-SIMat

20. DLVO theory Derjaguin & Landau (1937); Verwey & Overbeek (1944) • Consider equal sized spheres and low potentials. • The total interaction must also include the VdW attraction Corso CFMA. LS-SIMat

21. DLVO theory, colloid stability and coagulation • Depending on the electrolyte concentration and surface charge density or potential, one of the following may occur: • (a) – For highly charged surfaces and dilute electrolytes (long Debye length): strong long range repulsion peaking at the energy barrier (some 1 – 4 nm away from the surface) • (b) – For higher electrolyte conc. A significant secondary minimum appears before the energy barrier. The potential energy minimum at constant is called primary minimum. The colloids are kinetically stable because overcoming the energy barrier is a slow process and the particles either sit in the secondary minimum or remain dispersed. • (c) – For surfaces of low charge density or potential, the energy barrier is much lower leading to slow aggregation, known as coagulation or floculation. • (d) – Above some concentration of electrolyte the energy barrier falls below zero and the particles coagulate rapidly: the colloids are now unstable. • (e) – As the surface charge approaches zero the interaction curve approaches the pure VdW curve and the two surfaces attract each other at all separations. Corso CFMA. LS-SIMat

22. Effect of Hamaker Constant • DLVO Theory was a seminal moment in solution chemistry and physics. By assuming an attractive force between particles balanced by the repulsive charge, can quantitatively predict particle coagulation, settling, adhesion. Corso CFMA. LS-SIMat

23. Effect of Surface Potential • When the particle charge is low enough the van der Waals attraction will lead to coagulation. • For a metal oxide particle, we can approximate: • When salt is added the repulsive curve decays more quickly, and the maximum repulsion decreases. Corso CFMA. LS-SIMat

24. Metal Salt (AuHCl4) + + Reducing Agent (NaBH4) Direct mixed ligands reaction** Ligand exchange reaction* Metal Nanoparticles Synthesis Corso CFMA. LS-SIMat

25. Place Exchange Reactions Corso CFMA. LS-SIMat

26. Ligands on Nanoparticles Corso CFMA. LS-SIMat

27. liquid decomposition DH deinterdigitation Solubility Issue liquid state Solid interdigitated state Solid de-interdigitated state DHde-int DHsol DSC - Differential Scanning Calorimetry Pradeep et al, Phys. Rev. B, 2(62), R739,2000. Badia et. al. Chem. Europ. J., 2(3), 359,1996. Corso CFMA. LS-SIMat

28. Control of interdigitation • The shorter the ligand the smaller the interdigitation enthalpy • The larger the nanoparticle the smaller the interdigitation enthalpy, probably because of surface curvature effect • Mixture of ligands lower the interdigitation enthalpy Corso CFMA. LS-SIMat

29. 10nm Order/Disorder Transition @1220C @200C Corso CFMA. LS-SIMat Deinterdigitated Interdigitated

30. Characterizing Metal Nanoparticles 3 nm TEM shows atoms in the core STM shows ligands in the shell Corso CFMA. LS-SIMat 2.7 nm