Confined Crystallization of Single Crystalline Gold Nanowire with Unusual Polymorphism
One dimensional (1D) nanostructures of noble metals, such as Au, Ag, and Pt, have attracted much attention due to their novel physical and chemical properties. In this study, nanotubular structure of T i02, which is widely used in photocatalytic applications as supports, was used as a key material to synthesis single crystalline Au NWs with high aspect ratio, without adding any surfactant and reducing agent. Crystalline anatase Ti02 nanotubes (NTs) were prepared by atomic layer deposition (ALD) with replications of anodic aluminum oxides (AAO) membranes.
After ALD process, T i02 NTs was removed from AAO membranes and dispersed on Si substrates, and then were irradiated by UV light in the solution of HAuCI4. As a result, maximum aspect ratio of - 500 single crystalline Au NWs with the length of 20 iJm and the diameter of 40 nm were obtained by perfectly filling in the confined inner cores of Ti02 NTs. An experimental investigation was also conducted to observe the influence of pH of the solutions. At low pH 4, multi-twinned Au NWs are formed, however, around pH 4.5, twin free or longitudinally twinned Au NWs are formed. Interestingly, when the pH below 6, Au are nucleated and grow as NWs only inside of Ti02 NTs. When pH above 6, Au are nucleated both inside and outside of Ti02 NTs.
The number of necessary OH· ions, which are assumed to be directly responsible for the reduction of Au ions, will increase locally only inside of Ti02 NTs. Nanoscale confinement in the Ti02 NTs leads to OH ion production and subsequently the growth of Au NWs along the pores. Even though the growth mechanism of one dimensional structure is difficult to ascertain, it is apparent that the nucleation event and the subsequent changes in surrounding media and confined structures of Ti02 NTs are crucial to the formation of single crystalline Au NWs with very high aspect ratio. And we observe HCP (Hexagonally Close Packing) as well as BCT (Body Centered Tetragonal) Au nanowires by TEM, which have different crystal structure from the bulk gold.