Solvothermal Growth of Bismuth Chalcogenide Nanoplatelets
by the Oriented Attachment Mechanism: An in Situ PXRD Study
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Abstract
Ultrathin two-dimensional bismuth
chalcogenide materials have received
substantial research attention due to their potential applications
in electronics and optoelectronics. While solvothermal synthesis is
considered to be one of the most promising methods for large-scale
production of such materials, the mechanisms that govern the crystallization
during solvothermal treatment are still poorly understood. In this
work, the solvothermal syntheses of Bi<sub>2</sub>Se<sub><i>x</i></sub>Te<sub>3–<i>x</i></sub> (<i>x</i> = 0–3) hexagonal nanoplatelets were monitored by synchrotron-based
in situ powder X-ray diffraction, which enabled investigation of crystallization
curves, lattice parameters, and crystal size evolution under a variety
of synthesis conditions. On the basis of the crystallization curves
and crystal size evolution, a general 3-step crystallization process
has been deduced: (1) An induction period for the dissolution of the
precursor and nucleation of Bi<sub>2</sub>Se<sub><i>x</i></sub>Te<sub>3–<i>x</i></sub>, followed by (2) rapid
growth of planar crystals through the oriented attachment, and finally
(3) a diffusion-controlled slow growth step consuming the remaining
precursor from the solution. Oriented attachment is very effective
for the growth of binary composites, resulting in a high yield of
large planar crystals; however, it is much less effective for the
growth of ternary composites due to lattice mismatch of the nuclei
formed at the induction period, producing much smaller crystals accompanied
by a limited yield of large planar crystals. Additionally, three intermediate
phases (Bi<sub>2</sub>TeO<sub>5</sub>, Bi<sub>2</sub>SeO<sub>5</sub>, and Na<sub>2</sub>SeO<sub>3</sub>) were observed that played an
important role in controlling the phase separation as well as the
composition of the final ternary compounds