Layered Double Hydroxides and the Environment: An Overview - Advanced Materials for Agriculture, Food and Environmental Safety - page 9

In the other case, LDHs are synthesized at low temperatures and undergo a
process of aging. Aging involves rel uxing the precipitate at a set tempera-
ture for 18 hours.

1.4.3 Urea Hydrolysis Method

Urea is a weak Bronsted base highly soluble in water which has been used
for the precipitation of metal ions as hydroxides. h e hydrolysis of urea
gives a pH of about 9 depending on the temperature of the mixture, which
allows its use as precipitating agent in the synthesis of LDHs. h e crystal-
linity degree of LDHs has been observed to depend on the temperature
of synthesis and the aging time. Larger particles are formed at low tem-
peratures [52, 53] due to the lower nucleation rate which depends on the
decomposition rate of the urea. However, this method is not indicated for
the preparation of LDHs with low charge density, but allows the prepara-
tion of compounds with high charge density which are dii cult to obtain
with other procedures [52].

1.4.4 Sol-Gel Method

h e sol-gel method involves the formation of a sol by hydrolysis and par-
tial condensation of metallic precursor and followed by the gel formation.
As metallic precursor, metallic alkoxides, acetates, acetylacetones and
many inorganic salts are used. h e properties of the LDH depends on the
hydrolysis and condensation rates of the metallic precursors [54], which
are modii ed by dif erent parameters of the reaction like pH, nature and
concentration of the precursor, solvent and temperature of the synthesis.
h e LDHs obtained by this method exhibit specii c surface area larger than
those obtained by co-precipitation method [55-57], but with controver-
sial results regarding the basicity and the trivalent and divalent metal ion
molar ratios [56-58].

1.4.5 Ion-Exchange Method

In this method, there is an exchange of the interlayer anions with anionic
guest molecules, i.e., molecules one wishes to introduce into the LDH
structure in order to produce the desired LDH-guest compound. h e
ion exchange in LDHs depends on several factors according to He et
al. [59], ai nity for the incoming guest anion, the medium in which
the exchange occurs, the pH and the chemical composition of the LDH
brucite-like layers.