Anodic aluminum oxide membranes containing 70 nm diameter pores were grown by potentiostatic anodization of high-purity aluminum plates (0.2 mm thick, 99.99+ % purity) in an aqueous oxalic acid solution (4 %) in a water bath. After anodization, the remaining aluminum was etched by a mixed solution of HCl (20 %) and CuCl 2 (0.1 M), and the barrier layer was dissolved by H 3 PO 4 (5 %). For depositing the CNTs, the AAO templates were placed in a furnace with an argon flow of 100 sccm. After the reactor temperature was increased to 650 C, an ethyne flow of 30 sccm was introduced for 10 min, and then the template membrane was annealed at 650 C in argon for 10 h prior to cooling it to room temperature.
Electrodeposition of metal was performed potentiostatically in a three-electrode configuration in which a platinum plate, a saturated calomel electrode (SCE) and a Au/AAO template membrane served as the counter, reference, and working electrodes, respectively. Nickel was electrodeposited at a constant voltage of ±1.0 V versus SCE in an electrolyte of nickel sulfate (0.15 M) and boric acid (0.6 M). After the electrodeposition of nickel, the Ni/AAO membranes were oxidized at 400 C for 10 h and then heated at 600 C for 2 h in air. Using the annular nanochannel template as a working electrode, platinum nanotubes were electrodeposited at a constant voltage of ±0.8 V versus SCE in a mixed aqueous solution of H 2 PtCl 6 (1 g L ±1 ) and boric acid (30 g L ±1 ).
For electron microscopy measurements, the AAO template and NiO cores were dissolved by placing in NaOH (1 M) for 30 min and then in HCl (1 M) at 50 C for 10 h. The morphology and structure of the fabricated platinum nanotubes were analyzed by a scanning electron microscope (DB235 focused ion beam (FIB) system, FEI Company), a transmission electron microscope (JEM-2000FX TEM with X-ray energy dispersion analysis equipment, JEOL Company), and a high-resolution electron microscope (HRTEM, Tecnai F30, FEI Company).