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Nanocrystal Symmetry Breaking and Accelerated Solid-State Diffusion in the Lead–Cadmium Sulfide Cation Exchange system
Chemistry of Materials ( IF 7.2 ) Pub Date : 2018-12-28 00:00:00 , DOI: 10.1021/acs.chemmater.8b04490 Andrew Nelson 1 , Shreyas Honrao 1, 2 , Richard G. Hennig 1, 2 , Richard D. Robinson 1
Chemistry of Materials ( IF 7.2 ) Pub Date : 2018-12-28 00:00:00 , DOI: 10.1021/acs.chemmater.8b04490 Andrew Nelson 1 , Shreyas Honrao 1, 2 , Richard G. Hennig 1, 2 , Richard D. Robinson 1
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The phenomenology of solid-state transformations in nanoparticles is important for applications utilizing their reactivity and for investigations into how nearby interfaces interact with the defects responsible for mass transport. We directly interrogate the structure and reaction kinetics of lead sulfide (PbS) nanocrystals undergoing cation exchange in organic solution to cadmium sulfide (CdS) via X-ray diffraction. The epitaxial relationship of zincblende CdS to rocksalt PbS breaks the overall symmetry of the core–shell nanocrystal without requiring the loss of unit cell symmetry, leading to anomalous peak shifts in the diffraction pattern. Conversion occurs in three stages: (1) surface exchange to form a metastable rocksalt CdS shell, (2) crystallization of this shell to zincblende, and (3) diffusive transport of ions through the completed shell. The interdiffusion coefficient, D̃, for ions diffusing through the shell follows the Arrhenius relationship with an activation energy of 160–180 kJ mol–1, which exceeds that observed in many other experiments in diffusion in nanoparticles and is similar to values measured in bulk solids, suggesting the barrier to exchange is dominated by the energies of point defect formation rather than by surface-bound reactions. However, the magnitude of D̃ is greater by a factor of 104 or more relative to the self-diffusion coefficients of the slowest component in our system (Cd in CdS). This surprising result suggests that interdiffusion is much faster in nanocrystals. Cation exchange illustrates that the distinction between chemical diffusion in a potential gradient and diffusion at thermodynamic equilibrium has not been fully appreciated. Acceleration of interdiffusion in core–shell nanoparticles due to large chemical potential gradients will be important for understanding nanoscale heterostructure formation and stability.
中文翻译：
铅-镉硫化物阳离子交换系统中的纳米晶体对称性破坏和加速的固态扩散
纳米粒子中固态转变的现象学对于利用其反应性的应用以及研究附近的界面如何与负责质量传输的缺陷相互作用非常重要。我们通过X射线衍射直接询问在有机溶液中进行阳离子交换的硫化铅（PbS）纳米晶体到硫化镉（CdS）的结构和反应动力学。闪锌矿CdS与岩盐PbS的外延关系在不需要损失晶胞对称性的情况下打破了核-壳纳米晶体的整体对称性，从而导致衍射图谱中出现异常的峰偏移。转化过程分为三个阶段：（1）表面交换以形成亚稳的岩盐CdS壳层；（2）将该壳层结晶为闪锌矿；（3）离子通过完成的壳的扩散传输。互扩散系数，D̃，对于穿过外壳扩散的离子，遵循阿伦尼乌斯（Arrhenius）关系，其活化能为160–180 kJ mol –1，这超出了许多其他实验在纳米颗粒中的扩散所观察到的值，并且与在散装固体中测得的值相似，这表明存在障碍交换的能量主要由点缺陷形成的能量决定，而不是由表面结合反应决定。但是，D̃的大小要大10 4倍或相对于我们系统中最慢成分（CdS中的Cd）的自扩散系数而言。这一令人惊讶的结果表明，相互扩散在纳米晶体中要快得多。阳离子交换表明，尚未完全认识到电势梯度中的化学扩散与热力学平衡时的扩散之间的区别。由于较大的化学势梯度，在核-壳纳米颗粒中相互扩散的加速对于理解纳米级异质结构的形成和稳定性至关重要。
更新日期：2018-12-28
中文翻译：
铅-镉硫化物阳离子交换系统中的纳米晶体对称性破坏和加速的固态扩散
纳米粒子中固态转变的现象学对于利用其反应性的应用以及研究附近的界面如何与负责质量传输的缺陷相互作用非常重要。我们通过X射线衍射直接询问在有机溶液中进行阳离子交换的硫化铅（PbS）纳米晶体到硫化镉（CdS）的结构和反应动力学。闪锌矿CdS与岩盐PbS的外延关系在不需要损失晶胞对称性的情况下打破了核-壳纳米晶体的整体对称性，从而导致衍射图谱中出现异常的峰偏移。转化过程分为三个阶段：（1）表面交换以形成亚稳的岩盐CdS壳层；（2）将该壳层结晶为闪锌矿；（3）离子通过完成的壳的扩散传输。互扩散系数，D̃，对于穿过外壳扩散的离子，遵循阿伦尼乌斯（Arrhenius）关系，其活化能为160–180 kJ mol –1，这超出了许多其他实验在纳米颗粒中的扩散所观察到的值，并且与在散装固体中测得的值相似，这表明存在障碍交换的能量主要由点缺陷形成的能量决定，而不是由表面结合反应决定。但是，D̃的大小要大10 4倍或相对于我们系统中最慢成分（CdS中的Cd）的自扩散系数而言。这一令人惊讶的结果表明，相互扩散在纳米晶体中要快得多。阳离子交换表明，尚未完全认识到电势梯度中的化学扩散与热力学平衡时的扩散之间的区别。由于较大的化学势梯度，在核-壳纳米颗粒中相互扩散的加速对于理解纳米级异质结构的形成和稳定性至关重要。
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