Nanoscopic Insight into Sol-Gel Chemical Kinetics of Oriented Attachment Crystal Growth in Anisotropic Copper Hydroxide Nanowires

The reaction kinetics and the oriented attachment (OA) crystal growth mechanism of anisotropic metal oxide/ hydroxide nanowire formation in a sol-gel colloidal system have not been well understood. Herein, the kinetics of a base-catalyzed sol-gel synthesis of anisotropic copper hydroxide nanowires was studied to gain an in-depth understanding of the OA-directed crystal growth mechanism and its chemical kinetic reaction pathways in a quasi-homogeneous colloidal system. The OAdirected sol-gel synthesis process developed in this work followed the initial stage of salt metathesis and nucleation by base-catalyzed hydrolysis, then sol formation by hydrolysis and condensation stages, and finally nanowire formation by the polycondensation process. A novel chemical kinetic model that governs the crystal growth at each stage of this sol-gel process was elucidated from the nanoscopic insight provided by high-resolution transmission electron microscopy (HR-TEM) and UV-vis absorbance kinetic plots. The time-dependent HR-TEM analysis revealed the initiation step of the OA-directed crystal growth mechanism that began from the sol formation. The nanocrystals' volume growth analysis showed sigmoidal growth behavior, confirming a second-order sigmoidal Boltzmann kinetic growth model with a growth rate constant of 0.243 +/- 0.867 min-1 for the hydrolysis and condensation stage. The time-dependent HR-TEM images, collected during the polycondensation process at ambient temperature, exhibited longitudinal crystal growth of nanoarrays by facet-specific alignment and fusion of nanocrystals to form single-crystal nanowires. In the subsequent low-temperature polycondensation step, these nanowires showed further growth via directional elongation along the [020] crystal facet to form fully grown nanowires. The respective kinetic growth models for these two subsequent polycondensation steps supported the propagation step of the OAdirected crystal growth and followed a sigmoidal Boltzmann zeroth-order growth model, with mean growth rates of 0.197 +/- 0.064 nm/min and 2.448 +/- 0.633 nm/h, respectively. These experimentally derived multistep kinetic models using a simple but versatile analytical approach could be used to understand the OA mechanism of metal hydroxides'/oxides' nanowire growth in a sol-gel colloidal system. Furthermore, this study tests and verifies a robust anisotropic single-crystal growth process to make size- and shapecontrolled nanowires with a spatial lattice orientation.

Automated summary

The kinetics of anisotropic copper hydroxide nanowires synthesis in a sol-gel colloidal system were studied to understand the oriented attachment (OA) crystal growth mechanism. Different stages of crystal growth were elucidated and characterized using high-resolution transmission electron microscopy and UV-vis absorbance kinetic plots. The study provides insights into the growth process of metal hydroxide/oxide nanowires and presents a versatile analytical approach to understand the OA-directed crystal growth.