Microstructural Coarsening Evolution Of Nanoporous Gold By Solution Treatment

Nanoporous gold (NPG) can be fabricated by selective dissolution of silver from silver-gold alloys, where the silver atoms were selectively dissolved into the etching solution. NPG is a stable open-cellular structure with a bicontinuous network of pores and ligaments whose sizes are approximately10nm or smaller. It has been found that the pore and ligament sizes can be markly coarsened by annealing or immersing in some acid solutions, and we found NPG can be coarsened in some salt solutions.Two kinds of NPG were prepared by free corrosion and electrochemically corrosion, the pore sizes were10nm and39nm, respectively. The microstructures were quantitative characterization by SEM image anlysis. The mean pore size, ligament width, porosity, pore density, pore roundness of the two NPG samples were determined and compared. The procedures of image pro-cessing and measurement were combined into a macro program, by which we could process these images semi-automatically, the standard deviation of the determination results are smaller than5%.The microstruture of the NPG sample with10nm pore size was coarsened by immersing in silver ion complexing agent, such as Na2S2O3, NaSCN, EDTA and citric acid solution. With increasing the immersing time, the pore size, ligament width and porosity increased, the pore density decreased, after24hours, the pore structure was stable and changed littlely. The bigger the concentration is, the stronger the coarsening phenomenon is. The NPG with39nm pore size was not coarsened in the silver ion complexing agent solutions.The coarsened phenomenon of NPG in FeCl3solution was first studied. By increasing the immersing time, the pore size, ligament width and porosity increased, the pore density and pore roundness decreased, after24hours, the pore size and porosity decreased, and if countine coarsening, the pores will be closed and collapsed. Compared with the thermal treatment, the NPG which was immersed in FeCl3solution has a bigger porosity and pore density, the distribution of pore sizes is wider.The coarsen mechanism was preliminarily studied. The pore and ligament sizes are at nanoscale, so we presume that nanascal Ag was dissolved in solutions and nanoscal Au was diffusion and self-assembly that coarsened the microstructure in the end.