Gold nanorod@iron oxide core-shell heterostructures: synthesis, characterization, and photocatalytic performance

Iron oxides are directly coated on the surface of cetyl-trimethylammonium bromide (CTAB)-capped gold nanorods (AuNRs) in aqueous solutions at room temperature, which results in AuNR@Fe₂O₃, AuNR@Fe₃O₄, and AuNR@Fe₂O₃@Fe₃O₄ core-shell heterostructures. The iron oxide shells are uniform, smooth, with characteristic porous structure, and their thickness can be readily tuned. The shell formation is highly dependent on the reaction parameters including pH and CTAB concentration. The Fe₂O₃ shell is amorphous and exhibits nearly zero remanence and coercivity, while the Fe₃O₄ shell is ferromagnetic with a low saturation magnetization of about 0.5 emu g^-1 due to its low crystallinity and the porous structure. At elevated temperatures achieved by plasmonic heating of the Au core, the Fe₂O₃ shell transforms from amorphous to γ-Fe₂O₃ and α-Fe₂O₃ phases, while the Fe₃O₄ phase disappears because of the oxidation of Fe²⁺. A 1.4-fold increase of photocatalytic performance is observed due to the plasmonic resonance provided by the Au core. The photocatalytic efficiency of Fe₃O₄ is about 1.7-fold higher than Fe₂O₃ as more surface defects are present on the Fe₃O₄ shell, promoting the adsorption and activation of reagents on the surface during the catalytic reactions. This approach can be readily extended to other nanostructures including Au spherical nanoparticles and nanostars. These highly uniform and multifunctional core-shell heterostructures can be of great potential in a variety of energy, magnetic, and environment applications.