2, Department of Quantum Engineering, Graduate School of Engineering, Nagoya University, Nagoya, , Japan
3, Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka, , Japan
Platinum (Pt) and Pt-based (e.g. Pt/Cu, Pt/Au, Pt/Ag) alloy nanoparticles (NPs) have been demonstrated as high performance catalysts. Usually Pt and Pt-based alloy NPs have been synthesized via chemical reduction methods. However, impurity and incomplete removal of byproducts and toxic reductants can hinder catalytic properties. On the other hand, due to the difference in reduction potential of metal salts and/or difference in decomposition temperature of metal complexes, core-shell structure or phase segregation were observed in the formed bimetallic NPs. This is often seen in bimetallic NPs of immiscible elemental components in the bulk state such as Pt/Au. In our research, we have proposed to prepare highly uniform Pt and Pt alloy NPs, such as Pt/Cu and Pt/Au alloy NPs, by sputtering at room temperature onto a low vapor pressure liquid, polyethylene glycol (PEG, Mw 600). The method combines the advantages of sputtering to produce atoms and clusters for any metals from the bulk counterparts and the suppression and control of particle growth by the liquid medium in vacuum sputtering chamber. Thus varying the sputtering parameters allows for particle size control and tunable alloy NPs’ composition.
Sputtering was performed onto PEG and TEM grid. Sputtering current applied to each magnetron was separately controlled and varied in order to tune the composition in the resulting bimetallic NPs. Various characterization methods such as UV-Vis, XRD, XPS, TEM, HRTEM and STEM-HAADF and STEM-EDX mapping have been used to analyze the obtained NPs. TEM, HRTEM and STEM showed that Pt NPs with tunable sizes from 0.9 nm to 1.4 nm and narrow size distribution were produced. In addition, we found that negligible particle aggregation happened in PEG and Pt NPs were stable even after keeping in the dark at room temperature for several months. The slight growth of Pt NPs in PEG during storage was found accompanied with the consumption of free Pt atoms in PEG. The method was applied for a Pt/Cu alloy target and the effect of sputtering parameters such as sputtering time, the rotation speed of PEG on particle size has been studied. Pt29Cu71 alloy NPs have been synthesized by sputter deposition for the first time. Furthermore, we showed that we were able to produce Pt/Au alloy NPs using a double-target sputtering. The target design allowed us to finely control Pt/Au alloy NPs’ composition via simultaneous sputter deposition onto PEG. The results showed that particle size and composition are strongly correlated and can be tailored by varying the sputtering current. Increasing Pt content resulted in smaller particle size and particles with same composition had similar sizes. Moreover, the agglomeration of NPs is dependent on the Pt content. Our findings in the relation among particle size, particle composition, and aggregation state of the formed NPs with respect to its composition can shed light into the formation mechanism of Pt-based alloy NPs.