Temperature Controlled Evolution of Silver Polypods from Biomass Encapsulated Nano-Droplets

Silver nanoparticles are produced by a novel biosynthesis technique using fungus Penicillium purpurogenum NPMF (MTCC 7356). The biomass when subjected to heat treatment up to 700 °C produces the nanoparticles of silver due to the presence of free carbon which acts as encapsulation for silver nanoparticles. Increase in temperature above 700 °C causes a reduction in concentration of free carbon so that the nanoparticles come close to each other and form clusters in the shape of tetrapods to polypods. The micro-Raman analysis supports the formation of mesoporous carbon with increase in the calcined temperature. The formation of Ag nanostructures has been explained in terms of free energy diagram i.e., the crystal favors more to grow on (100) facet or (110) facet rather than on (111) facet by the addition of silver atoms and thereby, it releases more energy. The release of energy has been confirmed from the differential thermo gravimetric analysis. In fact, it is the dynamics of the minimization of the free energy of the system which governs the reorientation and rearrangement of Ag themselves leading to formation of various types of polypods. A formation mechanism is presented schematically.