Hierarchically structured multi-shell nanotube arrays by self-assembly for efficient water oxidation.

Photosynthesis in plants occurs at structures which form by self-assembly under ambient conditions, while catalysts used for artificial photosynthesis normally need special conditions like high pressure or temperature. Herein, a facile and cost effective way for the synthesis of a highly complex and efficient oxygen evolution reaction (OER) catalyst, formed solely by self-assembly in solution, is presented. Without the need for any instrumentation except for a glass beaker, highly active nickel-iron-copper multi-shell nanotube arrays are produced by immersion of a copper plate in three different solutions. Cu(OH) nanowires are first self-grown on a copper substrate in a basic solution and subsequently converted to novel iron-copper hydroxide nanotubes by immersion in an Fe solution by a sacrificial template-accelerated hydrolysis mechanism. Finally, an additional layer of nickel nanosheets is added by treating in a nickel chemical bath. The resulting electrode shows a current density as high as 100 mA cm at an overpotential of 320 mV with a Tafel slope of 32 mV dec, while also exhibiting long time stability. The use of inexpensive first-row transition metals, simple preparation methods with no energy consumption, the unique hierarchical structure of the nanosheet covered nanotubes, and the high catalytic performance are remarkable, and this study may therefore lead to more convenient and competitive routes for water splitting.