Engineering Surface Structure of Pt Nanoshells on Pd Nanocubes to Preferentially Expose Active Surfaces for ORR by Manipulating the Growth Kinetics

作者：Weicong Wang, Xiang Li, Tianou He, Yaming Liu, and Mingshang Jin*

杂志：Nano Letters, 2019, 19, 1743-4720.

摘要：Synthesis of Pt nanoshells on substrates can increase the utilization efficiency of Pt atoms and reduce the amount of Pt used in the applications. However, it is still an enormous challenge in tailoring the required crystal facets of Pt nanoshells on a given substrate. In this work, we demonstrate a facile and convenient approach capable for generating Pt octahedral islands with tunable sizes and densities on Pd nanocubes by manipulating the deposition rate. The key to this synthesis is the fine control over the deposition rate of Pt on Pd seeds. Due to the different reactivities at the surface sites, the deposition of Pt can be controlled at a certain site by carefully tuning the deposition rate. With a low concentration of reductant (8.33 mg/mL of glucose), surface diffusion dominates the process, and thus the Pt cubic shells form on Pd cubic seeds. In contrast, when a higher amount of the reductant (16.67 mg/mL of glucose) is added, the deposition starts to dominate the growth of Pt shells. In this case, the deposition would be controlled at the corners, forming eight large Pt octahedra on a cubic Pd seed. Further increasing the deposition rate can induce much higher deposition rates, in which case, the deposition of Pt would like to take place not only at the corners, but also the edge and surface sites of the seeds. Not surprisingly, this growth habit can result in the formation of high-density octahedral islands on Pd cubic seeds. With the same amount of precursor supply, the higher densities of Pt islands, the smaller the size of the octahedral islands on Pd nanocubes. Unlike other synthetic methods, the size of the octahedral islands on Pd seeds can be even controlled to be smaller than 3 nm by controlling the amount of the Pt precursor. Considering the excellent performance of {111} facets of Pt catalysts toward ORR, the Pt nanocages with small octahedral islands on the surfaces can exhibit a high activity, with a mass activity 0.68 A/mg, as high as 5.2 times of that of commercial Pt/C.

        随着全球化石能源的日益枯竭和生存环境的严重污染，人们将注意力逐渐投入到清洁能源技术的研发中以缓解未来的能源之忧和环境恶化。在燃料电池的研究中，铂金属一直以来都是极其重要的阴、阳极反应的催化剂。但由于其储量少，价格高，所以通过构筑催化剂的结构以提高铂原子的利用率是十分必要的。夏幼南老师于2015年在Science上首次报道借助钯纳米立方体和八面体为模板构筑铂纳米笼结构，以降低铂的用量且能够大幅度地提高其催化活性。此工作报道后，涌现出了利用不同形貌的模板来构建铂纳米笼结构的研究工作，其中包括铂十面体纳米笼、铂二十面体纳米笼的构筑。在已报导的工作中，纳米笼的结构往往是由模板的形貌所决定的，因此无法在给定形貌的模板上实现铂纳米笼的表面结构的自由调控，而表面结构是影响铂纳米笼催化性能的主要因素之一。
        最近，西安交通大学前沿科学技术研究院的金明尚教授针对铂纳米笼催化剂的研究有了全新的发现：该研究团队发现，通过动力学调控可以在钯纳米立方体表面构筑具有不同表面结构的铂壳层（例如{100}表面和{111}表面），该研究成功打破了模板形貌在铂纳米笼催化剂制备中对其表面结构所具有的决定性作用，有助于铂纳米笼催化剂的规模化制备和商业化应用。

        如何实现铂原子在钯纳米晶体表面生长过程中的动力学调控是该研究的关键问题。铂壳层沉积到钯纳米晶体表面的过程中，影响壳层表面结构的因素的主要有两个，分别是：铂原子沉积到钯纳米晶体表面的沉积速率和铂原子在钯纳米晶体表面的迁移速率。当沉积速率小于迁移速率时，铂原子在钯纳米晶体表面的生长就会受到模板材料表面结构的影响。以钯纳米立方体为模板的情况为例，在其表面就会形成铂立方体纳米笼。以前报道过的铂纳米笼的合成主要就是这类情况，因此传统上认为模板形貌对铂纳米笼表面结构具有决定性影响。而当沉积速率大于迁移速率的时候，被还原的铂原子来不及在钯纳米晶体表面进行扩散就被后续沉积的铂原子所束缚。在这种情况下，铂原子首先会优先沉积在钯立方体的八个角上，同样以钯纳米立方体为模板的情况为例，在钯纳米立方体的八个角上形成八个铂八面体。同时，随着沉积速率的加快，铂纳米岛逐渐出现在立方体的棱和面上，在钯纳米立方体表面形成高密度的铂八面体纳米岛，通过进一步调控可以获得尺寸在3 nm以下的铂八面体纳米岛。刻蚀去除模板后，就得到了由高密度、超小尺寸八面体岛构成的铂纳米笼催化剂。这种特殊结构的铂纳米笼所具有的高密度{111}晶面在燃料电池关键步骤——氧还原反应中表现出极其优异的性能。该论文已发表在 Nano Letters上。

文章标题为“Engineering Surface Structure of Pt Nanoshells on Pd Nanocubes to Preferentially Expose Active Surfaces for ORR by Manipulating the Growth Kinetics”。西安交通大学前沿科学技术研究院为该系列文章第一作者及通讯作者单位。
全文链接：https://pubs.acs.org/doi/10.1021/acs.nanolett.8b04735

金明尚教授课题组主要从事的是金属纳米材料的结构调控和功能化研究，主要包括金属纳米颗粒的结构调控、催化性能研究、金属复合催化剂制备等。欢迎相关院系的学生报考。近年来金明尚课题组在金属纳米材料合成和催化方面已发表系列工作：ChemSusChem, 2013, 6, 1883-1887; J. Mater. Chem. A, 2013, 1, 7316-7320; J. Mater. Chem. A, 2014, 2, 902-906; Nanoscale, 2014, 6, 3518-3521; Chem. Sci., 2015, 6, 5197-5203; ACS Nano, 2015, 9, 3307-3313; ACS Nano, 2016, 10, 4559-4564; J. Mater. Chem. A, 2016, 4, 13033-13039; Nano Lett., 2016, 16, 5669-5674; ACS Nano, 2017, 11, 163-170; J. Mater. Chem. A, 2017, 5, 10150-10153; Mater. Horiz., 2017, 4, 584-590; Nat. Commun., 2017, 8, 1261；Nano Lett., 2019, 19, 1743-1748等。