Energetics of Oriented Attachment of Mn-Doped SnO₂ Nanoparticles

Abstract

Growth of nanoparticles by oriented attachment (OA) can occur in electrolyte solutions, in vacuum, or even in air. The energy change in this event is determined by the difference between the excess interface energies of the free nanoparticles and the arrangement with attached grains. Therefore, low energy boundaries (highly coherent) are more likely to attach, and after attachment, atomic rearrangement will allow crystals to effectively grow by coalescing two or more grains. However, if the energy of the boundary formed by attachment is low but the activation energy for coalescence is high, it is possible for the system to enter a metastable state that disables nanocrystals' growth (enabling design of growth resistant nanocrystals). In this work, we test this hypothesis in manganese-doped SnO 2 nanoparticles in the absence of organic agents. We observe that manganese dramatically lowers the energies of (noncoherent) grain boundaries of SnO 2 , which enhances nanostability and slows grain growth due to the increased activation energy for OA.

Sintering and Nanostability: The Thermodynamic Perspective

“…To address the role of dopants on the energetics of OA, let's consider the case of Mn‐doped SnO 2 . A recent report presented calorimetric data on both surface and grain boundary of Mn‐doped SnO 2 nanoparticles . While the surface energy was observed to slightly decrease with increasing Mn content (from 1.2 to 1.1 J/m −2 ), the grain‐boundary energy dropped by roughly 0.3 J/m 2 from 0.7 to 0.4 J/m 2 .…”

“…Therefore, differently from the grain growth analysis, separating the growth dependences into energetic and kinetic terms by using an analytical model is a more complex task, save for specific cases where the growth mechanism is well‐defined. For instance, SnO 2 has been reported to grow by oriented attachment, making it a good case for study …”

“…Regardless if happening in liquid or in air, OA does not require large amounts of mass transport, being triggered at low temperatures. The energetics of OA has been described, and every step of the process releases energy. That is, excess energy is released due to the lower energy of the attached state as compared to free particles, and more excess energy is released once the grains merge and the particle–particle interface disappears.…”

“…By fitting data of OA of Mn‐doped SnO 2 , Chang et al have shown an increase in the activation energy for OA from 41 kJ/mol for undoped SnO 2 to 59.8 kJ/mol for 8 mol% doped SnO 2. This increase is expected, since in addition to the thermodynamics of the states of OA, segregated Mn will also affect the mass transport responsible for merging particles together. That is, if Mn is at the boundary, for the system to change from state II or III to V, Mn ions must migrate out of the boundary to allow SnO 2 crystal of the neighboring particles to blend.…”

Oriented Attachment: A Unique Mechanism for the Colloidal Synthesis of Metal Nanostructures

“…Figure 2A shows a schematic illustration of the energy states of two nanocrystallites during an OA process. [57] A significant decrease in surface energy is expected upon attachment of the particles due to the reduction in exposed surface and formation of low energy interface, no matter in a mirrorrelated fashion to generate a twin plane (denoted T) or in a low-angle misorientation (denoted LA) to form a grain boundary. Afterwards, the energy could be further lowered by the rotation and realignment of the particles to reduce the intrinsic energy of the grain boundary or even eliminate the defect.…”