Bimetallic AuPd alloy nanoparticles on TiO₂ nanotube arrays: a highly efficient photocatalyst for hydrogen generation

Decoration of TiO₂nanotube (TNT) arrays by AuPd nanoparticles (NPs) produces a dramatic enhancement in the rate of hydrogen generation through photocatalytic water-splitting under solar illumination. XRD and TEM confirmed alloy formation in bimetallic AuPd NPs while XPS ruled out a core-shell architecture in the AuPd NPs. Well-dispersed, size-controlled AuPd NPs were formed by sequential physical vapor deposition of Au and Pd on TNTs followed by spontaneous thermal dewetting (TNT-AuPd). TNT-AuPd samples were characterized by small tensile microstrains. For comparison purposes and to derive physical insights, an identical method was used to form TNT-Au and TNT-Pd samples wherein TNTs were decorated by monometallic Au and Pd NPs respectively. In every case, an accumulation-type heterointerface between TiO₂and the metallic/bimetallic NPs was indicated by binding energy shifts in the Ti2p high-resolution x-ray photoelectron spectra (HR-XPS). Initial and final state effects in the Au4f HR-XPS pointed to a large number of Au atoms in low coordinate sites such as edges, kinks and corners as well as a slower excited atom relaxation in the alloy. A similar preponderance of Pd atoms at low coordinate sites was found along with the presence of a small amount of palladium oxide. The alloying of Au with a low Pd content on TNT yields significant enhancement in hydrogen production under UV-visible light in aqueous triethanolamine solutions. TNT-AuPd demonstrated the highest photocatalytic H₂production rate of 2920<i>µ</i>mol g^-1h^-1, which is 8.9 times higher than that of TNTs, 2.1 times that of TNT-Au, and 1.69 times that of TNT-Pd under solar illumination. We studied H₂generation under UV-filtered solar illumination with TNT-AuPd outperforming monometallic Au- and Pd-NP decorated TNTs, which is attributed to the enhancement of the catalytic activity of Pd in an Au environment, the presence of Pd and Au atoms at low coordinate sites, and photoinduced electron transfer between TNTs and AuPd alloy NPs, where AuPd acts as an efficient electron sink, in turn reducing carrier recombination losses. AuPd bimetallic nanoparticles on TNTs, prepared via a simple anodization and vapor deposition method, exhibit excellent stability across multiple cycles and offer valuable insights for the development of efficient photocatalysts with promising potential for emerging energy applications.