Amorphous NiCu Thin Films Sputtered on TiO₂ Nanotube Arrays: A Noble-Metal Free Photocatalyst for Hydrogen Evolution

In this work, NiCu co-catalysts on TiO₂ are studied for photocatalytic hydrogen evolution. NiCu co-catalyst films are deposited at room temperature by argon plasma sputtering on high aspect-ratio anodic TiO₂ nanotubes. To tune the Ni : Cu atomic ratio, alloys of various compositions were used as sputtering targets. Such co-catalyst films are found to be amorphous with small nanocrystalline domains. A series of parameters is investigated, i. e., i) Ni : Cu relative ratio in the sputtered films, ii) NiCu film thickness, and iii) thickness of the TiO₂ nanotube layers. The highest photocatalytic activity is obtained with 8 μm long TiO₂ nanotubes, sputter-coated with a 10 nm-thick NiCu films with a 1 : 1 Ni : Cu atomic ratio. This photocatalyst reaches a stable hydrogen evolution rate of 186 μL h⁻¹ cm⁻², 4.6 and 3 times higher than that of Ni- and Cu-TiO₂, respectively, demonstrating a synergistic co-catalytic effect of Ni and Cu in the alloy co-catalyst film.