Plasmonic Modulation in Fe2O3/Au/TiO2 Heterojunctions Using Au Nanoparticles for Enhanced Photoelectrochemical Response

Abstract The incorporation of size-controlled Au nanoparticles (NPs) within metal oxide heterojunctions offers an effective approach to enhance photoelectrochemical (PEC) water splitting performance. In this work, Fe2O3/Au/TiO2 heterojunctions containing two distinct Au nanoparticle sizes (5 nm and 20 nm) were fabricated using RF sputtering, followed by a differential mobility analyzer (DMA) technique to achieve precise size control. The optical analysis revealed that the Fe2O3/Au (5 nm)/TiO2 photoanode exhibited significantly enhanced visible light absorption compared to pristine Fe2O3, TiO2, and Fe2O3/Au (20 nm)/TiO2, accompanied by a red shift in the optical absorption, indicating improved band structure modulation. Among all samples, the Fe2O3/Au (5 nm)/TiO2 heterojunction delivered the highest photocurrent density of 2.4 mA cm−2 at 0.7 V vs Ag/AgCl, which is significantly higher than that of pristine Fe2O3 and the 20 nm Au-modified sample, demonstrating a clear size-dependent enhancement in PEC performance. The superior performance of the 5 nm Au nanoparticles is attributed to the enhanced interfacial plasmon-induced charge transfer and improved light-harvesting efficiency arising from localized surface plasmon resonance (LSPR). Mott−Schottky analysis further revealed a negative shift in the flat-band potential and reduced interfacial charge-transfer resistance, confirming improved charge separation and transport dynamics. The enhanced PEC activity arises from the synergistic effects of improved electrical conductivity, Schottky junction formation at the Fe2O3/Au/TiO2 interface, and plasmon-mediated processes, including hot electron transfer (HET) and plasmon-induced resonant energy transfer (PIRET). Furthermore, the smaller Au nanoparticles (5 nm) provide a higher surface-to-volume ratio and stronger plasmonic coupling compared to 20 nm particles, leading to more efficient charge carrier generation and separation. These results highlight the critical importance of nanoparticle size optimization in engineering plasmonic metal-semiconductor heterojunctions for efficient solar energy conversion and environmental remediation applications.