A facile synthesis of 1D nano structured selenium and Au decorated nano selenium: catalysts for the clock reaction

Elemental selenium nanowires (Se NWs) are obtained from the easily available, less-toxic, low-cost reagents sodium selenite and ethylenediamine (en) through a modified hydrothermal reaction (MHT). The reaction supports the habitual growth of selenium nanowires and the amine molecules act as a reducing agent. Then, an interesting galvanic exchange reaction deposits Au nanoparticles on the as-synthesized Se evolving an Au decorated composite material. But a similar galvanic exchange produces Ag2Se because of the lower diffusion activation energy of Ag. Selenium nanowires, even as metalloids, have been found to catalyze the reduction of methylene blue (MB) using NaBH4 as a reducing agent. The blue colour of MB vanishes due to the formation of leucomethylene blue (LMB). On shaking the reaction mixture, the blue colour reappears by aerial oxidation. This oscillation of reversible colour change is taken as a clock reaction. The reversible reaction is monitored by a UV-visible spectrophotometer with respect to the absorbance of MB at lambda(max) = 663 nm. Common reducing agents such as hydrazine hydrate, ammonium thiocyanate and glucose do not show this stunning phenomenon. Sodium borohydride fails to assist the reduction process in the presence of bulk selenium. The reduction process proceeds through two steps. Initially, the rate is slower due to an 'induction period' and then it becomes facile. A higher catalyst dose reduces the time lag, i.e., the induction period. The Se NWs performed well for about 30-50 cycles for a clock reaction set up. Finally, the composite materials have been found to exhibit a faster clock reaction cycle in comparison to neat Se NWs.