Gaidhani Nikita G, Patra Sabyasachi, Chandwadkar Hemant S, Sen Debasis, Majumder Chiranjib, Ramagiri Shobha V, Bellare Jayesh R
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India.
Department of Chemistry, Sandip University, Nashik 422213, Maharashtra, India.
Langmuir. 2021 Feb 9;37(5):1637-1650. doi: 10.1021/acs.langmuir.0c02311. Epub 2021 Jan 26.
Galvanic replacement between metals has received notable research interest for the synthesis of heterometallic nanostructures. The growth pattern of the nanostructures depends on several factors such as extent of lattice mismatch, adhesive interaction between the metals, cohesive forces of the individual metals, etc. Due to the difficulties in probing ultrafast kinetics of the galvanic replacement reaction and particle growth in solution, real-time mechanistic investigations are often limited. As a result, the growth mechanism of one metal on the surface of another metal at the nanoscale is poorly understood so far. In the present work, we could successfully probe the galvanic replacement of silver ions with nickel nanoparticles, stabilized in a polymer membrane, using two complementary methods, namely, small-angle X-ray scattering (SAXS) and radiolabeling, and the results are supported by density functional theory (DFT) computations. The silver-nickel system has been chosen for the present investigation because of the high degree of bulk immiscibility caused by the large lattice mismatch (15.9%) and the weak adhesive interaction, which makes it a perfect model system for immiscible metal pairs. Membrane, as a host medium, plays a crucial role in retarding the kinetics of atomic and particle rearrangements (nucleation and growth) due to slower mobility of the atoms (monomers) and particles within the polymer network. This allowed us to examine the real-time concentration of silver monomers during galvanic replacement of silver ions with nickel nanoparticles and evolution of Ni/Ag nanoparticles. From combined experiment and DFT computations, it has been demonstrated, for the first time to the best of our knowledge, that the majority of silver atoms, which are produced on the nickel nanoparticle surface by galvanic reactions, do not form traditional core-shell nanostructures with nickel and undergo a self-governing sequential nucleation and growth of silver nanoparticles via formation of intermediate prenucleation silver clusters, leading to the formation of mixed metallic nanoparticles in the membrane. The surface of NiNPs has a heterogeneous effect on the silver nucleation pathway, which is evident from the reduced critical free energy barrier of nucleation (Δ). The present work establishes an original mechanistic pathway based on a sequential nucleation model for formation of mixed metallic nanoparticles by the galvanic replacement route, which opens up future possibilities for size-controlled synthesis in mixed systems.
金属之间的电置换反应在异质金属纳米结构的合成方面受到了显著的研究关注。纳米结构的生长模式取决于几个因素，如晶格失配程度、金属之间的粘附相互作用、各金属的内聚作用力等。由于探测电置换反应和溶液中粒子生长的超快动力学存在困难，实时机理研究往往受到限制。因此，迄今为止，对于一种金属在另一种金属纳米尺度表面上的生长机制仍知之甚少。在本工作中，我们能够使用两种互补方法，即小角X射线散射（SAXS）和放射性标记，成功探测在聚合物膜中稳定的镍纳米颗粒与银离子之间的电置换反应，并且密度泛函理论（DFT）计算支持了该结果。选择银 - 镍体系进行本研究是因为晶格失配度大（15.9%）导致体相高度不混溶以及粘附相互作用较弱，这使其成为不混溶金属对的理想模型体系。膜作为主体介质，由于聚合物网络内原子（单体）和粒子的迁移率较低，在延缓原子和粒子重排（成核和生长）的动力学方面起着关键作用。这使我们能够研究在用镍纳米颗粒进行银离子电置换反应过程中银单体的实时浓度以及镍/银纳米颗粒的演变。据我们所知，首次通过结合实验和DFT计算表明，通过电化学反应在镍纳米颗粒表面产生的大多数银原子不会与镍形成传统的核壳纳米结构，而是通过形成中间预成核银簇经历银纳米颗粒的自调控顺序成核和生长，从而在膜中形成混合金属纳米颗粒。镍纳米颗粒表面对银的成核途径具有非均匀效应，这从成核临界自由能垒（Δ）的降低中可以明显看出。本工作基于顺序成核模型建立了通过电置换途径形成混合金属纳米颗粒的原始机理途径，为混合体系中的尺寸控制合成开辟了未来的可能性。
