Faceted alloy nanoparticles are crucial in heterogeneous catalysis due to their unique structure-property relationships, while plenty of open questions still remain, e.g. facet formation mechanism, bulk/surface atom distribution etc. Understanding these questions would benefit the morphology control and property tuning in catalysis. However, this is a very challenging research topic considering the growth is a complex chemical reaction process and technique limitations / difficulties in atomic level characterizations. To make it possible in understanding the growth pathway of faceted bimetallic nanoparticles at atomic level, we selected the octahedral Pt3Ni growth as a starting point and collaborated with several experienced groups to meet the challenge.
The synthesis method of the Pt3Ni octahedra is a solid state approach developed by Peng’s lab (J. Am. Chem. Soc., 2014, 136 (22): 7805–7808) that using CO as “surfactant” and H2 as “reductant” which aimed for realizing scalable production of fuel cell catalysts. This method is pretty suitable for in situ study due to the facile procedure, good uniform facet morphology and high octahedra yield, which are the reasons to the system choice. Once we settle down the right system to study, it’s time to start the engine.
We established the collaboration with Dr. Pan from University of California-Irvine (professor experienced on in situ TEM research), Dr. Miller from Purdue University (professor specialized in XAS study at Advanced Photon Source) and Dr. Waluyo from Brookhaven National Laboratory (scientist on AP-XPS at National Synchrotron Light Source II). Though it’s a long story on the collaboration process (e.g. we encountered quite a lot of difficulties on the experimental setup and troubleshooting), we finally made it thanks to our collaborators’ experience and support.
The current results indicates a “synergic effect” between Pt and Ni atoms that contribute to the formation of octahedral shaped nanoparticles, which is pretty interesting. We found that during the growth process, Pt was first reduced and forms the Pt nuclei. And then the Ni was catalytically reduced by the prior reduced Pt (control experiments confirm that pure Ni precursor can not be reduced at the same condition). With continuing growth, a Ni enriched surface was revealed and we found that this enriched surface plays a significant role in the octahedra formation: the segregated Ni tuned the CO preferential adsorption facet from (100) to (111) that slowed down the growth rate of (111) facet and led to the (111) dominated morphology, i.e. octahedra. On contrary, for a pure Pt or non-Ni segregated condition, the CO preferential adsorption facet was still (100) that led to cubic nanoparticles. In a nut shell, Pt helps the reduction of Ni while Ni helps to guide the nanoparticle to become octahedral shaped, just like a good collaboration work from Pt and Ni atoms.
The final story we came up for the Pt3Ni octahedral growth is quite interesting, you can read the full story here. One last thing, this work have been lasting for nearly 3 years, we have to thank our group and collaborators for their strong support, persistence and nice job on this research topic.