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

Abstract

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 OA-directed 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 OA-directed 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 shape-controlled nanowires with a spatial lattice orientation.

Topics & Concepts