Two-Step Grain-Growth Kinetics of Sub-7 nm SnO2 Nanocrystal under Hydrothermal Condition
Journal article, 2015
In this work, the grain growth kinetics of SnO2 quantum dots under hydrothermal conditions was investigated. By varying the reaction temperature and duration, SnO2 particle sizes were tuned from 2 to 7 nm. It is demonstrated that the growth behavior of subnanometer-sized SnO2 underwent two distinct processes: below the critical size of 5.5 nm, about double of Bohr radius, the grain growth kinetics obeys an Ostwald ripening mechanism, while above that, an oriented attachment process governs the particle growth. For the former cases, the activation energies were Ea1 = 61.94 kJ/mol at 200 °C and Ea1′ = 62.84 kJ/mol at 160 °C, which greatly differs from that of Ea2 = 131.32 kJ/mol for the latter case. High-resolution transmission electron microscope, X-ray diffraction as well as UV–vis diffuses reflectance, photoluminescence, Fourier transmission infrared, and Raman spectra were employed to reveal the size-dependent properties. As the particle size of SnO2 reduces, there occurred a lattice expansion, band gap broadening, and an abnormal blue shift. All these characteristics are closely related to the size changing in a narrow range from quantum dots to several nanometers. The findings reported here may shed light on further understanding the unique behaviors of quantum dots.