Impact of Temperature on the Fusion Growth of Bimetallic Au–Pt Nanoparticles from Each Nanocluster Conjugated with a Thermoresponsive Polymer

Fusion growth from each nanocluster applying the random coil–globule transition of a thermoresponsive polymer was explored for alloy nanoparticle (NP) formation for the first time. The fusion growth of bimetallic Au–Pt alloy NPs was examined at 100, 150, and 200 °C via the structural collapse caused by the transition of a thermoresponsive polymer, poly­(N-isopropylacrylamide). A significant temperature dependence was revealed in the formation path, even in the narrow range of reaction temperatures studied here. Structural investigations for the products were performed using transmission electron microscopy, energy-dispersive X-ray spectrometry, elemental mapping, X-ray absorption fine structure, and X-ray diffraction. At 100 °C, Au NPs slightly alloyed with Pt atoms rather than form neat Au NPs. The element ratio was determined to be Au:Pt = 12:1 or less of Pt atoms. Small Pt particles coated the surface of the central NPs. A nonmetallic component of Pt atoms was mainly detected in the product solution on the basis of the number of unoccupied Pt 5d orbitals. Aggregates comprising small particles several nanometers in size and void parts were generated at 150 °C. All of the characterization results indicate that the hydrothermal condition at 200 °C led to the production of Au–Pt alloy NPs having a solid-solution-type structure. The reaction path of the fusion growth of the bimetallic NPs is discussed on the basis of the effect of the temperature applied during the reaction.