Surfactant-free one-pot synthesis of Au-TiO₂ core-shell nanostars by inter-cation redox reaction for photoelectrochemical water splitting

We used a novel inter-cation redox reaction between two metal cations with different redox potentials to synthesize nanostar structures without the addition of toxic surfactants, surface-adsorbing polymers, or additives with intrinsic reducing power. Along a multistage reaction route, the nanostars were fabricated via a shift in mechanism from kinetic to thermodynamic control. Next, semiconducting TiO₂ shells were deposited on the nanostar surface, with controlled formation of tip-capped or closed core–shell structures depending on the reaction conditions. The Au-TiO₂ nanostar structure was subsequently applied in a photoanode for photoelectrochemical applications. Specifically, the photoanode was prepared by depositing the nanostars onto hydrothermally grown TiO₂ nanorods on fluorine-doped tin oxide. The deposition of Au-TiO₂ nanostars enhanced the photoelectrochemical performance of the water-splitting device owing to the localized surface plasmon resonance effect in the Au nanoparticles, which can be attributed to an enhanced efficiency of incident photon-to-electron conversion. Compared to the pristine TiO₂ nanorods, the Au-TiO₂ nanostar on TiO₂ nanorods exhibited a 33% increase in photocurrent density at 1.23 V vs. the reversible hydrogen electrode under AM 1.5 G simulated sunlight irradiation (100 mW cm⁻²). This minimal synthetic protocol enables the rapid and reproducible fabrication of nanostructures. It also provides new possibilities for nanostructure synthesis via spontaneous complex formation of independent end-products and applications in the photoelectrochemical water splitting system.