Controlling over the shape and thus the surface structure of Pt nanocrystals is an effective way to optimize their catalytic activities for various applications. So far, a variety of protocols has been used to produce Pt nanocrystals with different shapes. However, it is difficult to directly compare their catalytic activities due to the complication of surface contamination by different protocols. Recently, the Xia group at the Georgia Institute of Technology has paved a way to overcome the difficult by developing a general method to produce Pt nanocrystals with distinctive shapes, including those with low- or high-index facets. [Qian et al., Materials Today (2018), doi: org/10.1016/j.mattod.2018.08.005]
Generally, the researchers have to rely on the use of different protocols/methods to generate these nanocrystals with distinctive shapes. “Under this situation, one cannot directly compare the ORR activities of the two low-index facets by employing the reported octahedral and cubic nanocrystals, because their surfaces are likely covered by very different species due to the involvement of different protocols or chemical environments.” Says Younan Xia, the corresponding author of the study.
Now researchers can use the same protocol to generate diversified Pt nanocrystals exposing either low- or high-index facets. This is different from all previously reported syntheses where different shapes were typically achieved using drastically different experimental conditions, including modifications to the precursors, reductants, surfactants, additives, reactant concentrations, and reaction temperatures, among others. “This method uses Na2PtCl6 as a precursor, glucose as a reductant, hexadecyltrimethylammonium bromide (CTAB) as a surfactant, together with the use of oleyamine as a solvent, a surfactant, and a co-reductant.” Says Jing Qian, the first author of the report.
By simply manipulating the concentration of glucose, Pt nanocrystals with a variety of different shapes were readily produced at different stages of the synthesis. When the synthesis was carried out with glucose at 100 mM, they obtained truncated cubes, cuboctahedrons, truncated octahedrons, octahedrons, and enlarged octahedrons after 1, 2, 3, 4, and 5 h, respectively, into the synthesis (Fig. 1). When 167 mM of glucose was used, they obtained Pt cubes, larger cubes, and concave cubes after 1, 2, and 4 h, respectively, into the synthesis. The researchers argued that the reduction kinetics, as determined by the concentration of glucose, was largely responsible for the variations in terms of both shape and size. These nanocrystals are beneficial to a direct comparison of the specific activities of the Pt electrocatalysts without concerning about the possible difference in terms of surface capping. The researchers found that the high-index facets on Pt concave cubes possessed a specific activity of 6.3 and 1.3 times greater than those of Pt cubes and octahedrons enclosed by {100} and {111} facets, respectively (Fig. 2). This study not only offers a general method for the synthesis of Pt nanocrystals with diverse shapes and thus different types of facets but also highlights the significance of the reduction kinetics in controlling the structure evolution of metal nanocrystals.