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Hou et al. Microstructures 2023;3:2023039 https://dx.doi.org/10.20517/microstructures.2023.37 Page 11 of 17

Figure 5. MOF derivatives treatment as flame retardants.

re-coordination with other ligands or functional groups, leading to a new MOF with altered properties .
[73]
The acid-base theory of ligand removal in MOFs has found diverse applications, ranging from the synthesis
of novel MOFs to the functionalization of existing ones with guest molecules and the recovery of metals
from them [74-76] . The judicious selection of acids and solvents can significantly impact both the extent and
selectivity of ligand removal, rendering this approach highly versatile and tunable.

Previously, Pan s group have done a series of work to derive LDH by etching ZIF [Figure 6] [77-82] .
'
We incorporated mesoporous zinc hydroxystannate (ZHS) nanoparticles into NiCo-LDH nanocages that
were derived from ZIF-67. The resulting composite material was then tested for its flame retardancy
properties, and it was found that the LOI value of an epoxy composite containing 6 wt.% of these fillers
increased to 27.2%. This improvement was significant enough to meet the UL-94 V-0 level, which is a
widely recognized standard for flame retardancy. Subsequently, polyphosphazenes (PZS) is often used
in modification and compounding of flame retardants because of its rich phosphorus and nitrogen,
ZIF@LDH@PZS core-shell structures and LDH@PZS@NH trishell structures were synthesized through
surface polycondensation on ZIF-67 using PZS, followed by ligand etching via nickel brine
acidification. The interface of LDH was analyzed by adjusting the reaction time to enhance
compatibility with the resin matrix. Recently, we have synthesized several hollow LDH nanocages
with high thermal stability, featuring single-yolk shell nanostructures (s-CBC@LDH) and multi-
yolk shell nanostructures (m-CBC@LDH). By incorporating phosphorus-based flame retardants, we
have further enhanced their flame-retardant properties.

In the context of flame retardancy, LDHs are often used as additives to reduce the flammability of polymers.
When LDHs are added to a polymer, they act as a physical barrier that prevents the diffusion of oxygen and
other flammable gases to the surface of the material, thereby reducing the rate of combustion. One issue
with the physical barrier effect of LDHs is that the thickness and width of the barrier layer are limited by the
amount of the LDH that can be added to the polymer matrix without affecting its properties. Thicker
barriers may be required to achieve adequate flame retardancy in some applications, but a small length-
diameter ratio of LDH may not meet the flame-retardant demand. One of the main challenges in using
LDHs as pure inorganic flame retardants is achieving proper dispersion in the polymer matrix. Inadequate
dispersion can lead to the formation of voids or weak points in the barrier layer, which can compromise its