Nano Science…. Some Interesting Facts of Coinage Metals Tarasankar Pal Department of Chemistry, Indian Institute of Technology, Kharagpur, Kharagpur 712302 Email: [email protected] Mendeleev designed the periodic table (1869) with a vision. His focus was very important that was supported by the discoveries of scientists in the later stage. He placed heavier nickel before lighter cobalt even though he was arranging the elements as per their increasing atomic weights (at. wt.). This apparent anomalous arrangement was justified by the discovery of atomic number (at. no.) of the elements at a later stage. Thus the position of cobalt (at. no. 27) and nickel (at. no. 28) was justified. Again, he did not place any element instead kept a void position in between manganese and rhenium in the manganese group. He himself realized that the element was not discovered by then. But below manganese, under the void place, he intelligently placed rhenium (Re). His realization gave birth to the first technically prepared element technetium (Tc). Thus manganese group was completed. Scientists realized that Tc, for its small half life, T1/2 (211,000 yrs) value, disappeared from earth’s crust and first man made element was born. This discovery filled a gap in the periodic table, and the fact that no stable isotopes of technetium exist. This explains its natural absence on Earth (and the gap). He also located the positions (one room for lanthanide and beneath that are actinide elements) of lanthanide and actinide elements below Sc and Y. In so doing he completed the group of alkaline earth metal with the representative element Sc. Like many other groups in his periodic table group 11 or one 15 Pal, T.: Some Interesting Facts …. can say I B was reserved for copper, silver and gold and we call the elements in this group as coinage metals. These sub-grouping, i.e., B signify less electropositive character of these elements as compared to the alkali metal belonging to subgroup A. Thus sub-grouping ‘A’ (for more electropositive) and ‘B’ (for less electropositive) of group I is justified as is evident in recent times. The metallic colours of these three metals attracted the attention not only of the scientists but also for common mass. These metals in their bulk structure have distinctive colours: copper is reddish brown, silver metal is white and gold is fascinating yellow while viewed using visible light. Among them gold is most noble. Because of the nobility (corrosion resistance), availability and of the fascinating colour price of gold controls the market price. Thus people grab gold by fair means or foul. In Swiss Bank alone ~400, 000 tons of gold have been deposited by the people of different countries. Nobility of the metals can be described for their standard reduction potential value at room temperature. Thus metallic gold (AuIII/Au, E0, +1.50 V) can easily be found in Earth’s crust. Silver (AgI/Ag, E0, +0.79V) and copper (CuII/Cu, E0, +0.34V) are mainly present as compounds again because of there lower reduction potential values. The lower reduction potential values stand for their less noble character in comparison to gold. So one can easily reduce Au(III) ions in solution. The reduction reaction is progressively less facile for the other two coinage metal ions in solution. Gold is a metal that lures many. Primarily because of its easy liquidity, and is also used by women for adorning themselves. Now gold has become a symbol of perfection even in research. An interesting fact emerges out when you take a metal (bulk) to the nano (10-9 meter) meter size which is called a nanoparticle of the metal. So is true for any other object. 16 Nano Science…. Figure 1: Shape of nanoparticles It can be a nanowire, nanosphere, nanoalloy etc. (Fig.1) if any constituent or dimension of the object lie in the ~10-9 nm range. Nanoparticle can be prepared by ‘Top Down’ or ‘Bottom Up’ approach. The physicists use the top down (i.e., physical) method but chemists mainly prefer bottom up technique. A nanoparticle of a metal comprises of many atoms. The assembly with lowest energy depicts a sphere. It has one atom in the centre and many atoms in the surface. (Fig.2). Figure 2: Nanoparticle composed of many atoms The central atom is co-ordinatively saturated (fully surrounded by other atoms from different sides) whereas the surface atoms are co- ordinatively unsaturated (partly surrounded). Thus the surface atoms become marginally electron deficient and find an avenue to get stabilized by negative ions/groups (Fig. 3). Now the negative ion-linked nanoparticles become a negatively charged assembly. Citrate stabilized gold nanoparticles are negatively charged and are known as gold hydrosol. Such negatively charged species in solution does not come closer to coalesce together because of 17 Pal, T.: Some Interesting Facts …. electrostatic repulsion and get stabilized under dispersion (Fig. 4) in polar solvent. Figure 3: Negatively charged nanoparticles Figure 4: Electrostatic repulsion Interestingly long chain polymer, surfactant (charged or uncharged), thiols amines etc. can bind with the nanoparticle surface thorough their charge re-distribution. This time also the particles get stabilized. Here the particles cannot coalesce together. This is due to long chain barrier related to steric factor. This type of nanoparticle stabilization is called steric stabilization (Fig. 5). Figure 5: Steric stabilization This type of phenomenon is prevalent in colloid chemistry. The surface charge of particle with long chain molecule may not acquire any surface charge. So they easily dissolve in non-polar solvents. It is known 18 Nano Science…. as ligand stabilized nanoparticle or organosol1 In this case, the long chain stabilized particles can be obtained as solid powder just evaporating the solvent. Again, the solid may be dissolved reversibly in non-polar solvents. Thus they find applications as reagent or catalyst. Coinage metal nanoparticles can easily be produced in solution as done by Michel Faraday2 with gold way back in 1857 and then in modern times Fren’s3 produced citrate stabilized gold in 1972. In the nanostage, gold, silver and copper may exhibit fascinating colour under dispersion. In the language of physics, it said that coinage metal nanoparticles (< 100 nm in size) has rich plasmon band (Fig. 6) and the peak position lies in the visible region. Figure 6: Plasmon band of gold (A) and silver (B) This band appears because of the resonance (excited) of all the conduction band electrons of the tiny particles with the incoming electromagnetic radiation. The electrons get excited all at a time. The excitation of all the electrons is believed to follow a gas model. Popularly it is known that gold depicts pink, and silver and copper nanoparticles are yellow in colour under dispersed condition. The colour shades differ with the change in size and shape of the particle as also the peak position. Solvent (dispersion medium) plays a dominant role for peak shifting thus 19 Pal, T.: Some Interesting Facts …. solvent property can be evaluated using this solvent dependent peak shifting. Here we would confine our discussion to coinage metal nanoparticles only for a particular reaction which is hitherto unknown. Among the three different coinage metal nanoparticles gold deserves a special mention not because of its rich plasmon band but because of its novel applications in different fields. Innumerable papers are published each month involving gold nanoparticle. However, a small number of papers appear for silver. Copper nanoparticle work is very difficult to reproduce. This information relates to gradation in nobility of the coinage metals. It is important to discuss that the nobility of the metals can be altered by (i) down sizing a bulk metal to the atomic (size dependent redox potential) stage and (ii) inducting a strong nucleophile onto a metal surface. The latter case is the simplistic and over expressed idea of gold silver extraction by cyanide. Cyanide is a pseudo halide, a reducing agent and a strong complexing agent. But it is seldom uttered that cyanide is a strong nucleophile. In nanoparticle chemistry of metal, the density of states are well defined unlike the bulk metal (Fig.7). It is well described that a strong nucleophile shifts the Fermi level of metal to a more negative region. Figure 7: Energy levels in metal particle 20 Nano Science…. It is analogous to the change in reduction potential value while one bulk metal enters successively into nanoparticle region which relates to the aggregation number of nanoparticle of metal (Fig. 8). Thus small metal particle together with a nucleophile induces much more pronounced Femi level shift of reduction potential value of metal particle.4 Figure 8: Reduction Potential of metallic silver with agglomeration number (Ref 4 and other cited reference therein) Consequence is the facile oxidation of metal nanoparticle in the presence of a nucleophile like potassium cyanide or sodium borohydride 0 2- even by dissolved oxygen (E , O2/O =1.23 V, acidic pH). So one can reversibly dissolve gold, silver etc. metals in cyanide/borohydride solution under ambient condition. 21 Pal, T.: Some Interesting Facts …. Figure 9: Reversible formation and dissolution of silver in aqueous surfactant solution (Ref. 4) However, some surfactant has to be there in the aqueous medium to dissolve out the newly formed/wrappedoxide layer onto metal surface to make the surface fresh for further reaction in steps.4 References 1. Nath, S.; Jana, S.; Pradhan, M.; Pal, T. Journal of Colloid and Interface Science. 2009, 341(2), 333-352. 2. Faraday. M. Philosophical Transactions of the Royal Society of London, 1847, 147, 159. 3. Frens, G. Nature (London) Physical Science. 1973, 241(105), 20-22.