  Layered transition metal hydroxides (LTMHs) with transition metal centers
sandwiched between layers of coordinating hydroxide anions have attracted
considerable interest for their potential in developing clean energy sources
and storage technologies. However, two dimensional (2D) LTMHs remain largely
unstudied in terms of their physical properties and the applications in
electronic devices. Here, directed by the relationship of the spin state of 3d
transition metal (TM) ions such as Ni, Co, Cu, and the corresponding geometry
of the crystal field, we discover that Ni2+ with perfect Oh symmetry is ideal
for intraplanar growth, leading to the achievement of >20 {\mu}m
{\alpha}-Ni(OH)2 2D crystals with high yield, which are the largest 2D domains
reported so far. We also report the successful synthesis of 2D Co(OH)2 crystals
(>40 {\mu}m) with less yield due to the slight geometry distortion resulted
from uneven number of electrons. Moreover, the detailed structural
characterization of synthesized {\alpha}-Ni(OH)2 are performed; the optical
band gap energy is extrapolated as 2.54 eV from optical absorption measurements
and is measured as 2.50 eV from reflected electrons energy loss spectroscopy
(REELS), suggesting the potential as insulating 2D dielectric material for
electronic devices. Furthermore, key parameters of the hydrothermal reaction
including soaking temperature, starting pH and cooling rate, are systematically
tuned to understand their effects on morphological and crystallographic
perspectives, allowing the establishment of a 2D growth mechanism. This work
demonstrates a scalable pathway to synthesize large 2D LTMHs from simple
methods, paving the way for the study of fundamental physical properties and
device applications of 2D LTMHs.
