AuCu bimetallic nanocluster-modified titania nanotubes for photoelectrochemical water splitting: composition-dependent atomic arrangement and activity

Abstract

The photoelectrochemical (PEC) water splitting reaction of bimetallic Au_xCu_1−x (x = 1, 0.75, 0.5, 0.25 and 0) nanocluster-decorated TiO₂ nanotube (TNT) photoanodes was investigated using a solar simulator. A strong enhancement in the anodic photocurrent relative to pristine TNTs was found with clear composition-dependent PEC activity, increasing with the Cu content and peaking at Au_0.25Cu_0.75. Electron microscopy and X-ray absorption fine structure spectra recorded at both Au and Cu edges identified a clear composition-dependent atomic arrangement of the spherical nanoclusters on anatase TNTs, resulting mostly from a time-dependent restructuring of the original metallic nanoalloys in the ambient environment. With time, Cu segregates from the alloy to form a surface oxide layer surrounding a pure gold metallic core in the gold-rich nanoclusters (x = 0.75 and 0.50) or a face centered tetragonal (fct)-intermetallic Au_0.5Cu_0.5 nanoalloy in copper-rich (x = 0.25) particles. In pure Cu nanoclusters, a metallic Cu core is stabilized by surrounding Cu₂O and CuO. The enhanced PEC activity is attributed to a synergy between Au and Cu that upon segregation produces bifunctional catalytic sites consisting of a metallic Au/AuCu alloy and copper oxide at the surface of the nanoclusters. The photoactivity under solar light illumination is boosted by the plasmonic response of the metal. The ordered structure of the fct-AuCu alloy present in the most active Au_0.25Cu_0.75 may explain its higher stability and photocatalytic performance. Hence, this work provides insight into the relationship between the atomic-level structure of Au_xCu_1−x nanoalloys on TNTs and their PEC activity.