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\DOI{10.5802/crchim.249}
\datereceived{2023-02-09}
%\datererevised{2023-06-15}
\daterevised{2023-06-15}
\dateaccepted{2023-07-21}
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\dateposted{2023-11-20}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{CNRS Cavitation Research Network}
%\def\cdr@specialtitle@french{GDR Cavitation}
%\makeatother

\RubricEF{Review article}{Article de revue}

\title{Review on the contribution of ultrasounds in layered~double
hydroxides synthesis and in their~performances}

\alttitle{Apport des ultrasons pour la synth\`ese et les performances
des hydroxydes doubles lamellaires}

\author{\firstname{Hamed} \lastname{Kalawoun}}
\address{Unit\'{e} de Chimie Environnementale et Interactions sur le
Vivant, UR 4492, Univ.~Littoral C\^{o}te d'Opale, Dunkerque, France}
\email[H. Kalawoun]{hamed.kalawoun@etu.univ-littoral.fr}

\author{\firstname{Michel} \lastname{Obeid}}
\addressSameAs{1}{Unit\'{e} de Chimie Environnementale et Interactions
sur le Vivant, UR 4492, Univ. Littoral C\^{o}te d'Opale, Dunkerque,
France}
\address{Laboratory of Physical Chemistry of Materials (LCPM/PR2N),
Faculty of Sciences, Lebanese University, Fanar, Lebanon}
\email[M. Obeid]{michel.obeid@univ-littoral.fr}

\author{\firstname{Carmen} \lastname{Ciotonea}\CDRorcid{0000-0002-7711-356X}}
\addressSameAs{1}{Unit\'{e} de Chimie Environnementale et Interactions
sur le Vivant, UR 4492, Univ. Littoral C\^{o}te d'Opale, Dunkerque,
France}
\email[C. Ciotonea]{carmen.ciotonea@eilco.univ-littoral.fr}

\author{\firstname{Muriel} \lastname{Chaghouri}}
\addressSameAs{1}{Unit\'{e} de Chimie Environnementale et Interactions
sur le Vivant, UR 4492, Univ. Littoral C\^{o}te d'Opale, Dunkerque,
France}
\email[M. Chaghouri]{Muriel.Chaghouri@univ-littoral.fr}

\author{\firstname{Christophe} \lastname{Poupin}\CDRorcid{0000-0002-7940-5128}}
\addressSameAs{1}{Unit\'{e} de Chimie Environnementale et Interactions
sur le Vivant, UR 4492, Univ. Littoral C\^{o}te d'Opale, Dunkerque,
France}
\email[C. Poupin]{Christophe.Poupin@univ-littoral.fr}

\author{\firstname{Samer} \lastname{Aouad}\CDRorcid{0000-0003-2576-5707}}
\address{Department of Chemistry, Faculty of Arts and Sciences,
University of Balamand, Koura, Lebanon}
\email[S. Aouad]{Samer.Aouad@balamand.edu.lb}

\author{\firstname{Madona} \lastname{Labaki}}
\addressSameAs{2}{Laboratory of Physical Chemistry of Materials
(LCPM/PR2N), Faculty of Sciences, Lebanese University, Fanar, Lebanon}
\email[M. Labaki]{mlabaki@ul.edu.lb}

\author{\firstname{C\'{e}dric} \lastname{Gennequin}\CDRorcid{0000-0002-9105-2265}}
\addressSameAs{1}{Unit\'{e} de Chimie Environnementale et Interactions
sur le Vivant, UR 4492, Univ. Littoral C\^{o}te d'Opale, Dunkerque,
France}
\email[C. Gennequin]{cedric.gennequin@univ-littoral.fr}

\author{\firstname{Edmond} \lastname{Abi-Aad}}
\addressSameAs{1}{Unit\'{e} de Chimie Environnementale et Interactions
sur le Vivant, UR 4492, Univ. Littoral C\^{o}te d'Opale, Dunkerque,
France}
\email[E. Abi-Aad]{edmond.abiaad@univ-littoral.fr}

\author{\firstname{Fran\c{c}ois} \lastname{Delattre}\CDRorcid{0000-0003-4039-8474}\IsCorresp}
\addressSameAs{1}{Unit\'{e} de Chimie Environnementale et Interactions
sur le Vivant, UR 4492, Univ. Littoral C\^{o}te d'Opale, Dunkerque,
France}
\email{Francois.delattre@univ-littoral.fr}
\email[F. Delattre]{delattre@univ-littoral.fr}

\shortrunauthors

\keywords{\kwd{Layered double
hydroxides}\kwd{Ultrasounds}\kwd{Heterogeneous
catalysis}\kwd{Environmental application}\kwd{Wastewater treatment}}

\altkeywords{\kwd{Hydroxydes doubles
lamellaires}\kwd{Ultrasons}\kwd{Catalyse
h\'et\'erog\`{e}ne}\kwd{Application environnementale}\kwd{Traitement
des eaux us\'ees}}

\def\thanksname{Note}

\thanks{This article is part of the CNRS Cavitation Research Network
project. More information at \url{https://gdr-cavitation.cnrs.fr/}
(available in several languages).}

%\thanks{Note. Cet article fait partie du projet GDR Cavitation. Plus
%d'informations sur \url{https://gdr-cavitation.cnrs.fr/}.}

\begin{abstract} 
Layered double hydroxides (LDHs) are promising because of some of their
several interesting characteristics. LDHs can be synthesized using
different preparation methods. The application of ultrasounds (US)
in the preparation method leads to reduced synthesis times and
better dispersion of particles characterized by small size and narrower
particle size distribution. Consequently, the specific surface area is
higher in US-prepared samples compared to materials
prepared by classical methods. These modified characteristics are
responsible for improving the performances of LDHs used as catalyst
precursors, adsorbents of dyes and heavy metals, fire suppressants, and
other applications.
\end{abstract} 

\begin{altabstract}
Les hydroxydes doubles lamellaires (HDLs) sont des compos\'{e}s
prometteurs en raison de caract\'{e}ristiques int\'{e}ressantes. Les
LDHs peuvent \^{e}tre pr\'{e}par\'{e}s par diff\'{e}rentes
m\'{e}thodes. L'application des ultrasons comme m\'{e}thode de
pr\'{e}paration aboutit \`{a} la r\'{e}duction des temps de
synth\`{e}se et une meilleure dispersion des particules qui sont
caract\'{e}ris\'{e}es par des tailles plus petites et une meilleure
distribution. Par voie de cons\'{e}quence, les \'{e}chantillons
pr\'{e}par\'{e}s sous ultrason pr\'{e}sentent des surfaces
sp\'{e}cifiques plus importantes compar\'{e}s \`{a} ceux obtenus par
voie classique. Ces caract\'{e}ristiques sont \`{a} l'origine de
l'augmentation des performances des LDH appliqu\'{e}s notamment comme
pr\'{e}curseurs de catalyseurs, adsorbants de pigments ou de m\'{e}taux
lourds, compos\'{e}s coupe-feu et d`autres applications.
\end{altabstract}

\maketitle
\clearpage

\twocolumngrid

\end{noXML}

\section{Introduction} \label{sec1}
\vspace*{-.1pc}

Layered (or lamellar) double hydroxides (LDHs)-type materials are also known 
as anionic clays or hydrotalcite-like compounds and have attracted
a lot of \mbox{attention} in recent years. The mineral hydrotalcite
[Mg$_6$Al$_2${\ubreak}(OH)$_{16}$](CO$_3$)$\cdot$4(H$_2$O) was the first
LDH to be identified in Sweden in 1842~\cite{bib1}. Because of its
water content (hydro), and similarity to talc (talcite), the substance
was named hydrotalcite~\cite{bib1}, which typically exists in nature
under the form of sheets or as a \mbox{fibrous} mass and can also be
easily manufactured. The general formula for LDHs is:
{\[
[\text{M(II)}_{1-x}\text{M(III)}_{x}(\text{OH})_{2}]^{x+}
(\text{A}^{n-})_{x/n}\cdot \text{mH}_2\text{O}
\]}\unskip
with M(II): bivalent cation, M(III): trivalent cation, $x$: molar
fraction, A$^{n-}$: anion, and m:~degree of \mbox{hydration}.

Figure~\ref{fig1} shows the schematic of the LDH structure. The
metallic active site can either be confined at the intermediate levels
of the double-layer structure or disseminated uniformly within the
layered structure by cation substitution. Bivalent and trivalent
cations M(II) and M(III) create a positively charged layer when they
are octahedrally coordinated into six hydroxyl groups. Divalent cations
($\text{M}^{2+} = \text{Zn}^{2+}$, $\text{Ni}^{2+}$,{\break}
$\text{Cu}^{2+}$, $\text{Co}^{2+}$, etc.) and trivalent cations
($\text{M}^{3+} = \text{Fe}^{3+}$, $\text{Cr}^{3+}$, etc.) can either
partially or completely replace $\text{Mg}^{2+}$ and $\text{Al}^{3+}$
respectively in the hydrotalcite structure~\cite{bib3}. Anions
($\text{A}^{n-}$) and water molecules form interstitial layers, which
neutralize the metal hydroxide layers' positive charge. There are no
strict restrictions on the nature of the interlayer anions. For
example, simple inorganic anions, such as carbonate, and
nitrate~\cite{bib4} can be used as interlayer anion, as well as organic
anions, such as terephthalate, acrylate, and lactate~\cite{bib5}.
Furthermore, coordination compounds, polyoxometalates, and biomolecules
like nucleoside monophosphates have been successfully
\mbox{intercalated}~\cite{bib6}.

\begin{figure}
{\vspace*{.1pc}}
\includegraphics{fig01}
{\vspace*{2pt}}
\caption{\label{fig1}Schematic structure and different preparation
methods of LDHs.}
\end{figure}

Due to their adaptability, compositional flexibility, fundamental
makeup, enormous surface area, redox characteristics, and
biocompatibility~\cite{bib7}, LDHs received extensive academic research
for use in practically all scientific domains such as
catalysis~\cite{bib8}, wastewater treatment~\cite{bib9}, drug
{delivery}
\cite{bib10}, biomedicine~\cite{bib11}. LDHs can be used as
synthesized maintaining their lamellar structure~\cite{bib12}, as
catalyst support~\cite{bib13} and as catalyst precursor~\cite{bib14}.
The LDH precursors allow to design a variety of catalysts including
(1)~homogenized mixed oxides when the LDH structure is calcined, and
(2)~reconstituted LDH obtained from the rehydration of mixed oxides
with anionic solutions~\cite{bib15}.

Oxides produced by calcining LDHs exhibit a wide range of
characteristics, uniform elemental dispersion, and persistence even
after thermal treatments or reduction. They have been used as catalyst
in different reactions, such as methanation, dry reforming of methane,
total oxidation of volatile organic compounds (VOCs),
hydrodesulfurization of fluid catalytic 
{gasoline cracking}, NO reduction,
and removal of $\text{SO}_2$ and $\text{NO}_x$~\cite{bib16}. There are
several traditional methods to prepare LDHs including urea hydrolysis,
sol--gel method, hydrothermal synthesis, and
co-precipitation~\cite{bib17}. Nowadays, research is still being
conducted to develop materials with increased surface areas, smaller
particle sizes, and tunable pore diameters.\looseness=-1

Among the existing synthesis routes, ultrasound (US) technology has gained
a lot of attention due to some advantages including the ability to
create high-temperature and high-pressure regions through the formation
and collapse of microbubbles (cavitation), increase mass transfer
processes as a result of system pressure variation, and apply large
amounts of vibrational energy to small volumes with little
heating~\cite{bib18}. The combined consequences of those three
processes induce a reduction of synthesis time, enhanced solid textural
characteristics, and greater and stronger active basic
sites~\cite{bib18}. In order to highlight the interest of 
ultrasound-assisted synthesis (UAS) of LDHs, we summarize the most recent
advances in the synthesis of LDHs with ultrasound-assisted methods 
(including co-precipitation, urea hydrolysis, hydrothermal and
US-assisted methods) and their \mbox{parameters}. Finally, we 
{will consider} the effect of
ultrasounds in the application of these materials in different fields.

\section{Layered double hydroxides preparation{\hfill\break} methods}
\label{sec2}
\subsection{Urea hydrolysis} \label{sec2.1}

Urea (CO(NH$_2$)$_{2}$) is a weak Bronsted base that hydrolyzes slowly
and is easily soluble in water. In the production of LDHs, urea can be
utilized as a substitute precipitating agent. In comparison to
Na$_2$CO$_3$, urea has the advantage of hydrolyzing slowly, resulting
in a gradual degree of supersaturation during precipitation. It is very
soluble in water and can produce ammonium cyanate or its ionic form
(\ref{eq1}){\break} ($\text{NH}_{4}^{+}$, NCO$^{-}$) through controlled
hydrolysis in aqueous solutions. Prolonged hydrolysis produces
$\text{CO}_{3}^{2-}$ in basic medium (\ref{eq2}) or carbonic acid
H$_2$CO$_3$ in acidic medium,
eventually evolving to CO$_2$ (\ref{eq3}) according to the
following reactions~\cite{bib20}:
{\begin{eqnarray}
& \text{CO}(\text{NH}_2)_{2} \rightarrow \text{NH}_4^{+}+
\text{NCO}^{-} \label{eq1}\Seqnsplit
& \text{NCO}^{-} + 2\text{H}_2\text{O} \rightarrow \text{NH}_{4}^{+}
+ \text{CO}_{3}^{2-} \label{eq2}\Seqnsplit
& \text{NCO}^{-} + 2\text{H}^{+} + 2\text{H}_{2}\text{O} \rightarrow
\text{NH}_{4}^{+} + \text{H}_2\text{CO}_{3} \label{eq3}
\end{eqnarray}}\unskip
During a hydrothermal reaction, a homogeneous solution of urea, Mg, and
Al nitrates may tend to go through the following processes, which will
lead to the creation of hydrotalcite compounds.
A hypothetical reaction scheme may be proposed as: 
{\begin{eqnarray*}
&& \text{Mg}(\text{H}_2\text{O})_{n}^{2+} + \text{H}_2\text{O}
\rightarrow \text{Mg(OH)}(\text{H}_2\text{O})^{+}_{n-1} +
\text{H}_{3}\text{O}^{+}\\
&& \text{Al}(\text{H}_2\text{O})_{n}^{3+} + \text{H}_2\text{O}
\rightarrow \text{Al(OH)}(\text{H}_2\text{O})^{2+}_{n-1} +
\text{H}_3\text{O}^{+}\\
&& \text{Mg(OH)}(\text{H}_2\text{O})^{+}_{n-1} + \text{Al(OH)}
(\text{H}_2\text{O})^{2+}_{n-1} + \text{OH}^{-}+ \text{CO}_{3}^{2-} \\
&&\qquad \rightarrow \text{MgAl(OH)}_3
\text{CO}_3\cdot (2n-2)\text{H}_2\text{O}
\end{eqnarray*}}\unskip

The simplicity of washing is another benefit of employing urea. Due to
the predominance of charge-balancing $\text{CO}_{3}^{2-}$ as a
by-product of urea decomposition, the biggest disadvantage of urea
hydrolysis is the inability to select the major counter ion
\mbox{intercalated} in the LDH structure~\cite{bib21}. Urea {hydrolysis} becomes
a simpler process than co-precipitation if $\text{CO}_{3}^{2-}$ is the
desired intercalated anion as neither extra NaCO$_3$ nor strict
atmospheric control are needed~\cite{bib22}.

{\vspace*{-1pt}}

\subsection{Sol--gel method} \label{sec2.2}

{\vspace*{-1pt}}

The sol--gel method allows for the synthesis of very pure
LDHs~\cite{bib23}. In this process, room-temperature water or organic
solvents are used to hydrolyze the metal precursor salts or their
organic constituents. These materials are sometimes made soluble using
a different solvent. It is necessary to add either an acid or a base to
the hydrolysis solution to speed up the precipitation process to
produce highly distributed metals in the solution. Detailed work on the
synthesis of magnesium and aluminum LDH by the sol--gel method was
performed by Prinetto {\etal}~\cite{bib24}. The composition of the
precursors, temperature, and aging time have direct effects on the
structural properties of LDHs~\cite{bib25}. For instance, it is possible
to enhance the surface area or particle size of synthetic LDHs by
lowering the reaction temperature~\cite{bib23}. This 
preparation method has a long processing time~\cite{bib27}.
\vspace*{-.25pc}

\subsection{Hydrothermal method} \label{sec2.3}

{\vspace*{-1pt}}

Typically, this synthesis is carried out in autoclaves by starting with
a basic solution containing the metals that would make up the LDH and
using the autogenous pressure produced by heating at temperatures
between 60 and 200~{\textdegree}C. Aging is the same process but when
the temperature is lower than 60~{\textdegree}C. The combination is
held in hydrothermal conditions for a period of time that could range
from hours to many days. Using hydrothermal techniques, it is possible
to create bigger particles with more crystallinity~\cite{bib3}. For
instance, the conditions to obtain the largest crystals of 
magnesium and aluminum LDH (1653~{\AA}) are 200~{\textdegree}C for
24~h~\cite{bib29}.
\vspace*{-.25pc}

\subsection{Co-precipitation method} \label{sec2.4}

{\vspace*{-1pt}}

The technique with the highest usage rate for producing LDHs is
co-precipitation. By evaporation or by altering the solution's pH,
inorganic salt solutions are supersaturated with an alkaline solution
in this approach. The degree of solution supersaturation affects the
structural morphology and particle size~\cite{bib30}. Not all ions will
precipitate out of the solution at low pH levels, and metal ions may
dissolve at very high pH levels. It is important to choose the
saturation pH carefully, which is determined by the cation types
employed~\cite{bib23}. This approach involves progressively adding an
aqueous solution of bivalent and trivalent metal salts into an aqueous
solution of the desired interlayer anion (usually carbonates), along
with an alkaline solution with a set pH to promote precipitation. By
altering the ratio of metal salts, it is simple to change the
proportion of divalent and trivalent ions in the hydroxyl layers when
using this method~\cite{bib32}. The type of anions and cations, pH, the
ratio of different metals, temperature, and aging time all have a
significant impact on how the metal-LDH material precipitates. One of
the major drawbacks of this method is the difficulty to control the
size of precipitating particles and their agglomeration~\cite{bib33}.

\subsection{Ultrasound-assisted synthesis (UAS)} \label{sec2.5}

US irradiations have been recently used during the
co-precipitation method either in the maturation phase~\cite{bib34} or
from the beginning of the co-precipitation, instead of 
magnetic stirring~\cite{bib35, bib36}. When synthesizing
materials like LDHs, the medium is heterogeneous,
constituted from a liquid--solid phase, causing a violent jet of 
liquid to emerge towards the surface. Hence the name of asymmetric
cavitation whose effects cause surface cleaning, destruction of limit
layers and enhancement of mass and heating transfer~\cite{bib37}. This
last effect is due to turbulent mixing and acoustic streaming. The
collision of microjets on the surface creates erosion and defective
sites and consequently leads to particle fragmentation. In addition,
US causes dispersion of small groups of LDH layers 
and reduce agglomeration during the crystal growth step, giving rise to
small crystal sizes. Consequently, the enhancement of chemical
reactions in a liquid--solid system is due to interparticle collisions
generation and to increase in specific surface area by
fragmentation of friable solids~\cite{bib38}.

Data published on LDH materials prepared by US-assisted methods 
are summarized in Table~\ref{tabI}. In most papers, the
technology used is an ultrasonic bath, which is mainly employed for
cleaning laboratory instruments and in analytical grade solvent
degassing. Transducers are located at the bottom of the tank, which
limits cavitation activity. Consequently, in this device type, the
acoustic field generated is not homogeneously distributed in the total
volume~\cite{bib39, bib40}. In comparison to other US devices,
the power density generated by a bath is very low~\cite{bib41}. In
order to increase the acoustic density in the medium, some authors have
worked LDH synthesis using an ultrasonic
horn~\cite{bib42}. This device is a direct irradiation method because 
the horn is partly immersed in the reaction medium and can produce a
high quantity of power in a small volume. Regarding the operational
parameters, it is interesting to note that the authors of different
articles describe them in an incomplete way which suggests that
they have little knowledge of US technology. The US
frequencies used are between 20 to 80~kHz, which means that mechanical
effects are preferentially sought for the synthesis of LDH materials.
However, we need a study on the synthesis of LDH materials at high
frequency (${>}$100~kHz) in order to analyze the impact of chemical
effects of US on the physicochemical properties of these
materials. This parameter may play an important role in the synthesis
of LDHs, since as shown in Table~\ref{tabI} there are no studies on
the effect of this factor on the characteristics of the resulting
material. The irradiation time varies from 10 to 600~min but the
delivered powers are often omitted or described without indication of
volume which does not allow access to power density. Another
important factor mentioned in most papers is temperature. 
We observe that temperatures vary
between room temperature (25~{\textdegree}C) and 65~{\textdegree}C.
Only three papers have applied a temperature higher than
65~{\textdegree}C. This temperature range is considered low compared to
the temperatures used in several conventional methods. This factor is
one of the advantages of using US in the synthesis of
inorganic materials.

\begin{table*} %tab1
\caption{\label{tabI}Published articles on LDHs prepared by UAS\vspace*{2pt}}
\tabcolsep4.7pt
\fontsize{8.4}{10.8}\selectfont
\begin{tabular}{cccccccc}
\thead
LDH & Device & \parbox[t]{1.15cm}{\centering Frequency (kHz)} & Power
(W) & Time (min) & \parbox[t]{1.15cm}{\centering Temperature
({\textdegree}C)\vspace*{2pt}} & Application & Reference\\
\endthead
Co--Al & Horn & 20 & --- & --- & 25 & Toluene oxidation &
\cite{bib36}\\
Mg--Co--Al & Bath & --- & 100 & --- & 80 & Water treatment &
\cite{bib44}\\
Mn--Ni & Horn & 20 & 100 & 150 & --- & Supercapacitor &
\cite{bib45}\vspace*{1.5pt}\\
Ni--Al & Bath & --- & --- & 60 & 65 & \parbox{4.1cm}{\centering
Removal of the reactive azo dye (Remazol Brilliant Violet
RBV-5R)\vspace*{3.5pt}} & \cite{bib46}\\
Mg--Al diacid & --- & --- & --- & 60 & --- & Intercalation of diacid in
LDH & \cite{bib47}\vspace*{1.5pt}\\
Zn--Mg--Al & Bath & 50 & --- & 30, 120, 600 & 25 &
\parbox{4cm}{\centering LDH activated by ultrasounds induced
reconstruction\vspace*{3.5pt}} & \cite{bib48}\\
Fe--Al & Bath & 80 & 160 & 15, 30, 60, 90 & 65 & Reduction of bromate &
\cite{bib49}\\
Ca--Al & Bath & --- & --- & 40 & 55 & Adsorption of U(VI) and Cr(VI)
& \cite{bib50}\vspace*{1.5pt}\\
Mg--Al & Horn & --- & --- & --- & 0, 10, 25, 50 &
\parbox{4.1cm}{\centering Knoevenagel and aldol
condensations} & \cite{bib35}\vspace*{5pt}\\
Mg--Co--Al & Bath & 42 & --- & 90 & 65 & \parbox{4.1cm}{\centering
Hydrogen production by oxidative steam reforming of ethanol
(OSRE)\vspace*{3.5pt}} & \cite{bib52}\\
Ni--Mg--Al--Ce & Bath & --- & --- & 260 & 25, 60, 90 &
\parbox{4.1cm}{\centering Incorporation of cerium in NiMgAl
LDH\vspace*{3.5pt}} & \cite{bib53}\\
Ni--Co--Mg--Al & Bath & 50 & --- & 60 & --- &
\parbox{4.1cm}{\centering Oxidative steam reforming of ethanol
(OSRE)\vspace*{3.5pt}} & \cite{bib34}\\
Co--Mg--Al & Bath & 50 & --- & 10 & 25 & VOCs oxidation & \cite{bib55}\\
Ni--Al & Bath & 45 & 200 & --- & 50 & Carbonyl sulfide removal &
\cite{bib56}\\
Mg--Al & Bath & 25 & 140 & 60 & 65 & Fluoride removal & \cite{bib57}\\
Fe--Al & Bath & 80 & 160 & 30 & 65 & U(VI) removal & \cite{bib58}\\
Mg--Al & Horn & 20 & --- & --- & 25 & \parbox{4.1cm}{\centering
Toluene and CO total oxidation\vspace*{3.5pt}} & \cite{bib59}\\
Zn--Al & --- & 40, 59 & 250, 175, 88 & --- & --- &
\parbox{4.1cm}{\centering Synthesis of small particle size of Zn--Al
LDH\vspace*{3.5pt}} & \cite{bib60}\\
Mg--Al & Bath & 40 & --- & 480 & 95 & Fire suppression agent &
\cite{bib61}\vspace*{1pt}
\botline
\end{tabular}
\vspace*{6pt}
\end{table*}

There are many advantages to this preparation method like 
time saving, reduction of crystallite size, increase in specific
surface area, better active phase dispersion in the
catalysts~\cite{bib62}. Those advantages are mentioned in the synthesis
of several types of inorganic materials but few articles have studied
the preparation of LDHs by US irradiation.

As mentioned above, LDHs can be synthesized by several preparation
methods. Each of those has its advantages and
limitations. The advantages vary according to the principle of applied
method while limitations range from long preparation times to
difficulty in controlling particle size. US~assistance 
\mbox{appears} to be an alternative to those methods, combining several
advantages and limiting their drawbacks. In the following section, we
focus on the effect of US on the properties and performance of
LDHs in different \mbox{applications}.

\section{Applications of LDHs obtained from UAS} \label{sec3}

As mentioned above, LDHs are used in many
fields (catalysis, adsorption, pharmaceutics, electrochemistry, etc.)
in their initial form or as oxides after thermal treatment. This is due
to their characteristics like simplicity in synthesis, distinctive
structure, uniform distribution of various metal cations in the brucite
layer, surface \mbox{hydroxyl} groups, flexibility, and high chemical
and thermal stability. Moreover, the capacity to intercalate a
\mbox{variety} of anions \mbox{(inorganic,}\break \mbox{organic,} biomolecules, and even
genes), sustained delivery of intercalated anions, intercalated anions
with interlayer spaces, swelling properties, oxo-bridged linkage, and
high biocompatibility are all desirable characteristics. Here, we will
discuss the effect of US in the synthesis of LDHs on the
physicochemical properties and performances of those materials in
different areas. 

\subsection{Remediation processes} \label{sec3.1}

In the field of water treatment, the removal of pollutants such as heavy
metals, bromate, reactive dyes, and fluoride from wastewater is a topic
of great interest. When released into the environment,
they seriously harm both ecological
security and human health.

\subsubsection{Elimination of heavy metals} \label{sec3.1.1}

Due to the low cost of LDHs, tailored properties, and their affinity for
anions like dichromate and uranyl, they can be used as anions exchangers
and adsorbents for dangerous anions detected in
wastewater~\cite{bib63}. Many studies have shown that LDHs can have
more advantages if obtained by UAS. Li
{\etal}~\cite{bib50} prepared Ca--Al LDH by co-precipitation
method under US irradiation. They obtained the expected
material at 55~{\textdegree}C for 30~min with no aging. In those
conditions, higher specific surface area and narrower particle size
distribution were obtained. As a consequence, this material 
demonstrated its efficiency in removing uranium and chromium. In
addition, removal efficiency was not affected by the presence of
competing ions such as nitrate radical and chloride. Likewise, Xie
{\etal}~\cite{bib58} synthesized Fe--Al LDH for removing U(VI). They
also prepared the adsorbent material with the US assistance 
(50~kHz, 160~W, during 30~min in an ultrasonic bath at
65~{\textdegree}C). Scanning electron microscopy (SEM) evidenced that
the structure of US-prepared Fe--Al LDH (Fe--Al (30~min))
was characterized by better particle dispersion and narrower particle
size distribution than that prepared classically (Fe--Al (0~min)).
Figure~\ref{fig2} shows that U(VI) residual concentration  was lower after
using Fe--Al (30~min) rather than Fe--Al
(0~min). This indicates that the US-prepared material has a
removal capacity higher than that obtained by the conventional method.

\begin{figure}
{\vspace*{.2pc}}
\includegraphics{fig02}
\caption{\label{fig2}Effect of solid-to-solution ratio on U(VI) removal
by Fe--Al LDHs. Experimental conditions: initial concentration of U(VI)
10.0~mg/L, contact time 120~min, pH~6.0, 298~K~\cite{bib58}.}
\end{figure}

However, other materials have shown higher adsorption capacity such as
mesoporous silica prototype, nano-flake 
Fe-sludge chars, and Ca--Al LDH. The
partial oxidation of Fe$^{2+}$ into Fe$^{3+}$ leads to a decrease in
the number of reactive sites and contributes to losing the removal
efficiency after three experiments.\looseness=-1

\subsubsection{Elimination of bromate} \label{sec3.1.2}

The catalyst Fe--Al prepared using LDH precursors by the
US-assisted co-precipitation method was used as an adsorbent to
remove bromate from the solution~\cite{bib49}. Bromate
($\text{BrO}_3^{-}$) is an oxyhalide disinfection byproduct (DBP)
during chlorination or ozonation in bromide-containing water
treatment~\cite{bib68}. Due to high solubility, stability, and
non-biodegradability in water, bromate is difficult to eliminate once
formed~\cite{bib69}. Adsorption by LDH materials is one of the
methods used to remove bromate by ion exchange~\cite{bib70}. According
to Figure~\ref{fig3}, anion exchange adsorption and reduction
appeared to be two distinct processes involved in the potential
mechanism of bromate removal by Fe--Al LDHs. First, bromate in solution
was rapidly adsorbed onto Fe--Al LDHs by ion exchange between sulfate
and bromate. Then, adsorbed bromate was reduced by Fe$^{2+}$ and
the harmless reduced products, bromide, entered the solution. Thus,
Zhong {\etal}~\cite{bib49} confirmed ultrasound irradiation 
to be a simple and fast method to assist in the preparation of
Fe--Al LDHs. That study showed that when irradiation time increased
from 0 to 30~min, bromate removal efficiency of Fe--Al LDHs was
increased.

\begin{figure*}
{\vspace*{-1pt}}
\includegraphics{fig03}
{\vspace*{-1pt}}
\caption{\label{fig3}Schematic representation of bromate removal by
Fe--Al LDHs~\cite{bib49}.}
{\vspace*{-1pt}}
\end{figure*}

\subsubsection{Removal of reactive dyes from wastewater}
\label{sec3.1.3}

LDHs are also employed in removal of reactive dyes
present in wastewater generated by different industries (textile,
printing, plastic, cosmetics, etc.)~\cite{bib73}. Pahalagedara
{\etal} \cite{bib46} worked on the synthesis of Ni--Al LDH by
sonochemical method for removal of a reactive azo dye. The results show
that the US-prepared material exhibited 100\% dye removal
efficiency in 6~min, while the same material prepared by the
\mbox{conventional} co-precipitation method required 60~min. Bharali
{\etal}~\cite{bib75} prepared Ni--Al LDH using US and
evaluated it in the removal of congo red. Uniform and well crystalline
materials were obtained. In terms of activity, US irradiation
led to a 12.7\% improvement in the removal of congo
red. In addition, the as-prepared material could be used in three
successful adsorption cycles. Likewise, Kostic {\etal}~\cite{bib44}
have prepared Mg--Co--Al LDH by US-assisted co-precipitation
method (100~W, in an ultrasonic bath at 80~{\textdegree}C) and used
it for removal of another dye (RB19). UAS
led to an increase in specific surface area from 8.1 to
$47.7~\text{m}^2{\cdot}\text{g}^{-1}$ that results in significant
improvement of dye sorption capacity, from 57.5 to
$367.9~\text{mg}{\cdot}\text{g}^{-1}$. \looseness=1

\subsubsection{Elimination of fluoride} \label{sec3.1.4}

Fluoride excess in drinking water can cause dental fluorosis,
skeletal fluorosis, etc.~\cite{bib77}. Among different methods used
for fluoride removal from water, adsorption technology is
the most employed because of several advantages like high efficiency,
simplicity, and profitability~\cite{bib78}. Chang {\etal}~\cite{bib57}
studied the synthesis of magnetic magnesium--aluminum LDHs
by a combination of magnetic Fe$_{3}$O$_{4}$ nanoparticles
(MNPs) and Mg--Al LDH. In that work, US technology was
applied in two steps. Firstly, the Fe$_3$O$_4$ MNPs were synthesized by
\mbox{US-assisted} \mbox{reverse-co-precipitation} and then redispersed in
water. Secondly, two different solutions were prepared: one containing
aluminum and magnesium nitrates and the other containing NaOH and
Na$_2$CO$_3$. Both solutions were added dropwise to the dispersion of
Fe$_3$O$_4$ MNPs under US irradiation, in order to obtain the
composite Fe$_3$O$_4$--Mg--Al-LDH. The average particle size of the
US-prepared calcined composite was 80~nm, while that of the
sample obtained by conventional co-precipitation was around 120~nm. In
addition, there was an increase of approximately 14\% in specific
surface area. The material prepared with US irradiation showed
an adsorption capacity 60\% higher than that of the composite prepared
without US assistance. Moreover, the same authors
studied the effect of different US parameters on the particle
size of the composite before calcination. They concluded that an
optimal time of 30 to 60~min is required. Indeed, if the time was
prolonged to 120~min, the average particle size increased. In addition,
higher power density leads to more cavitation bubbles which generate
more shock waves and microjets and consequently decrease the level of
agglomeration.

\subsection{Preparation of fire suppressants} \label{sec3.2}

LDH materials were also used as fire suppression agents \cite{bib80}.
The researchers have demonstrated the benefits of particles with
spherical shapes and large surface areas because they are easily
spotted and 
{remain suspended} for
a long time in the flame zone. Several methods
have been used to prepare LDHs with various shapes and good dispersion
such as reverse microemulsions or using a polar solvent/surfactant
system \cite{bib81}. However, those methods are complicated, because in
some cases it is necessary to use extra templates. Ni
{\etal} \cite{bib61} synthesized microspheres of Mg--Al LDH by
simple sonication in a mix of ethylene glycol and
water using an ultrasonic bath operated at 40~kHz during 480~min
at 95~{\textdegree}C. X-ray diffraction (XRD) patterns showed 
formation of pure LDH structure without any other 
{diffraction peak.}
Moreover, the
authors observed a significant increase in specific surface area
for the sample submitted to US irradiation
(164.7 $\text{m}^{2}{\cdot} \text{g}^{-1}$) compared to that prepared
in silent mode (43.8 $\text{m}^{2}{\cdot} \text{g}^{-1}$). In this
work, Ni {\etal} studied the effect of frequency and sonication
time on particle shape. They remarked that the optimal frequency
and sonication duration were 40--50~kHz and 8--10~h, respectively. The
fire suppression experiments showed that microspheres obtained
with US assistance enable a decrease in extinguishing time.
\looseness=1

\subsection{LDHs as inorganic fillers in nanocomposites} \label{sec3.3}

LDHs can be used as nanofillers in a polymer matrix. However, it is
difficult to delaminate LDHs into a polymer matrix~\cite{bib83} because
of their high charge density and high anion content which lead to 
strong electrostatic interaction between sheets. One of the 
strategies used is surface modification~\cite{bib84}. Dinari
{\etal}~\cite{bib47} prepared a chiral diacid-modified LDH which was
employed in the preparation of polyvinylpyrrolidone (PVP)/modified LDH
nanocomposites using US in three steps. SEM and TEM
(transmission electron microscopy) photographs showed  no
aggregation in the different synthesized materials. Thermal analyses
revealed that the introduction of small amounts of the modified LDH improves
thermal stability of the nanocomposites at higher temperatures.
\looseness=1

\subsection{LDHs as catalyst precursors} \label{sec3.4}

The main challenge in catalyst development is then to improve active
phase dispersion in order to enhance activity and
stability~\cite{bib86}. Catalysts elaborated by the LDH route may be an
alternative solution because they show high thermal resistance and
high dispersion of active phase as well as high specific surface
area~\cite{bib87}. Indeed, transition metal catalysts prepared by the
LDH route have demonstrated their efficiency in many applications. In
literature, the elaboration of catalysts by the LDH route with US
assistance is carried out for a variety of \mbox{applications}.

\subsubsection{Total oxidation of volatile organic compounds}
\label{sec3.4.1}

VOCs are classified as dangerous for human health and environment
because of their properties like volatility, toxicity, and
diffusivity~\cite{bib88}. One of the techniques used for VOCs removal
is catalytic oxidation~\cite{bib89}. LDHs can be used as precursors to
synthesize catalysts for total oxidation of volatile organic
compounds (VOCs).

P\'{e}rez {\etal}~\cite{bib55} prepared Co--Mg--Al LDH by
conventional co-precipitation and by sonication method during aging
step for only 10~min at room temperature using an ultrasonic bath
at 50~kHz. US use led to a 22.5\% increase
in specific surface area as well as to higher catalyst reducibility
(36.6\% increase in H$_2$ consumption). In addition, pore volume
increased from $0.2209~\text{cm}^{3}{\cdot}\text{g}^{-1}$ to
$0.4552~\text{cm}^{3}{\cdot}\text{g}^{-1}$ upon US irradiation. 
Furthermore, US-prepared samples exhibited
narrowest pore size distribution in comparison to the conventionally
prepared ones. Those characteristics led to better catalytic activity
in terms of butanol conversion as shown in Figure~\ref{fig4} as well as
to higher selectivity towards CO$_2$ and H$_2$O, the desired products.
It is then inferred that US use during the aging step
leads to catalysts with better characteristics and performances, with a
shorter preparation time.

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Temperature at 100\% butanol conversion of MOCo CT
and MOCo US~\cite{bib55}.}
\end{figure}

In addition, Genty {\etal} prepared the catalyst Co--Al using the LDH
route by conventional co-precipitation and US assistance
(ultrasonic horn, 20~kHz at room temperature) for applications in
toluene oxidation~\cite{bib36}. The results showed an increase in 
specific surface area of 26.75\%.
TEM analysis indicated larger and less regular size of
oxide particles for the solid resulting from conventional
synthesis. From XRD patterns, the crystallite size of oxides was
reduced between 25.53 and 61.21\% under US irradiation according to the
material composition. The H$_2$-TPR (Hydrogen temperature-programmed
reduction) analysis showed Co$_3$O$_4$ species reduction at lower
temperatures for the {US-prepared} sample. Finally, the catalytic test
of the US-prepared sample showed a 50\% toluene conversion at a lower
temperature than the sample prepared by co-precipitation (Figure~\ref{fig5}),
due to a higher amount of Co$^{2+}$ on the surface of
the US-prepared sample.\looseness=-1

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Temperature at 50\% toluene conversion of
Co$_6$Al$_2$HT500 and Co$_6$Al$_2$HTUS500~\cite{bib36}.}
\end{figure}

Likewise, the same authors worked on the UAS of another LDH
composition (Mg--Al) under the same
conditions~\cite{bib59}. Those materials were used as catalysts for 
total oxidation of toluene and CO. An increment of 51\% in specific
surface area of hydrotalcite precursors was observed. XRD results showed
an increase in the crystallite size of MgO from 4.5~nm for the 
US-prepared sample to 11.6~nm for the sample prepared by
co-precipitation. The activities of the calcined samples were compared
by T$_{50}$ (temperature at which 50\% of initial reactant is oxidized)
of CO and toluene. T$_{50}$ decreased from 479 to 423~{\textdegree}C 
with the US-prepared catalyst. Likewise, T$_{50}$ of CO
oxidation decreased from 285 to 273~{\textdegree}C.

\subsubsection{Knoevenagel and aldol condensations} \label{sec3.4.2}

Furthermore, LDHs can be used as precursors to synthesize catalysts for
Knoevenagel and aldol condensations. Climent {\etal}~\cite{bib35} 
prepared Mg--Al LDH with a Mg/Al ratio of 3 by three different methods.
One of those was US irradiation during the
precipitation step without the aging step. SEM showed that 
US-prepared samples present homogeneous morphology with highly
dispersed particles characterized by an average size of 80~nm, while 
average particle size varied between 100 and 540~nm
for the samples prepared by the two other methods (classical
co-precipitation and microwave-assisted method).
N$_2$ adsorption at 77~K showed
that the sample obtained by calcination of US-prepared LDHs
had the largest specific surface area (20\% increase in comparison with
the two other methods). The results show that the US-prepared mixed oxides 
lead to the best catalytic activity. In the Knoevenagel
condensation between benzaldehyde and malononitrile, yield
increased from 23 to 50\% in 15~min. In addition, in the Knoevenagel
condensation between benzaldehyde and ethyl cyanoacetate, an increase
from 31 to 60\% was detected in 5~min using the US-prepared
material. In the aldol condensation of citral with acetone, yield
increased from 68 to 93\% and selectivity to pseudo-ionones 
increased from 82 to 95\%.

\subsubsection{Oxidative steam reforming of ethanol} \label{sec3.4.3}

Moreover, oxidative steam reforming of ethanol (OSRE) is another
application of catalysts prepared using LDHs as precursors. OSRE is the
combination of two reactions, steam reforming of ethanol and
partial oxidation of {ethanol}~\cite{bib96}. OSRE {enables}
hydrogen production~\cite{bib97}. Espitia-Sibaja
{\etal}~\cite{bib52} prepared CoMgAl LDH by US-assisted
co-precipitation method (ultrasonic bath, 42~kHz, 90~min at
65~{\textdegree}C) with different percentages of cobalt and tested the
materials in OSRE. This study revealed that 
{Co--Mg--Al}
LDH can be synthesized by the US-assisted co-precipitation method and
{the resulting}
oxides can be used as catalysts in the OSRE reaction. The
average aggregate size determined by the Scherrer equation from 
XRD patterns of the mixed oxides was 4~nm. US irradiation
facilitates diffusion of Co particles into the brucite lattice.
This diffusion maintains the crystallite size after the calcination
process. Furthermore, Mu\~{n}oz {\etal}~\cite{bib34} \mbox{prepared} 
{Ni--Co--Mg--Al}
LDH by co-precipitation and US-assisted co-precipitation.
Sonochemically synthesized solids  had higher basicity 
(CO$_2$ adsorbed $=$ 268~$\upmu\text{mol}{\cdot}\text{g}^{-1}$) 
than the conventional \mbox{catalyst}\break
(CO$_2$~adsorbed~$=$~199~${\upmu}\text{mol}{\cdot}\text{g}^{-1}$). 
This fact may be
connected to the development of surface defects, such as exfoliation or
erosion of the material's outer layers, disintegration of agglomerates
into smaller particles, or formation of grain boundaries where the
oxygen atoms in metal--oxygen (M--O) bonds are not perfectly
coordinated~\cite{bib18}. Indeed, some researchers link the development
of isolated surface defects to increased basicity of the M--O bonds after
sonication of {LDHs}
precursors~\cite{bib35}. Another work involving
US in LDH synthesis was led by Macedo
Neto~\cite{bib53}. US irradiations were used to incorporate
large cations like cerium in Ni--Mg--Al LDHs. 
Cerium in LDHs offers several advantages such as
easy and high reducibility as well as high oxygen storage capacity. The
as-obtained materials showed better textural properties and an increase of
38\% in specific surface area. In addition, calculation of
lattice parameters revealed a better incorporation of cerium ions
within the LDH structure when US is\break applied.

\subsubsection{Removal of carbonyl sulfide} \label{sec3.4.4}

Another catalytic application of LDH-derived oxides
is the removal of carbonyl sulfide (COS) by desulfurization. COS
in petroleum gas and natural gas leads to
catalyst deactivation as well as corrosion of equipment used in
different treatments of petroleum fractions. Moreover, sulfur compounds
emissions in atmosphere produce acid rain~\cite{bib103}. Zhao
{\etal}~\cite{bib56} prepared 
{Ni--Al}
LDH using US
irradiation during the aging step in an ultrasonic bath
(45~kHz) with a power of 200~W at 50~{\textdegree}C. SEM images
(Figure~\ref{fig6}) showed that the US-prepared material
(NiAl-UHTLCs) was characterized by homogeneous morphology and highly
dispersed particles, whereas the sample prepared without US
(NiAl-HTLCs) showed a dense appearance.

\begin{figure}
{\vspace*{.1pc}}
\includegraphics{fig06}
\caption{\label{fig6}SEM image of NiAl-HTLCs~(a) and
NiAl-UHTLCs~(b)~\cite{bib56}.}
\end{figure}

This is because bubbles coming from the acoustic cavitation phenomenon
cause erosion when collapsing. The
temperature-programmed desorption (TPD) of CO$_2$ showed that 
US irradiation led to a higher number of weak and
moderate basic sites than the catalyst prepared by the conventional
method. US-prepared
{Ni--Al}
mixed oxides led to better
\mbox{catalytic} activity (100\%~of COS \mbox{removal} in 90~min) compared to
that obtained by the conventional method (100\%~of COS \mbox{removal} in
{only 90~min).} 
This difference is due to better particle dispersion and
formation of well-developed pore structures. In addition, in
this type of application, the removal rate increased with the number
of basic sites~\cite{bib106}. The results showed that US irradiation
 leads to a sample with more weak and moderate basic
sites. As shown in Figure~\ref{fig7}, moderate basic sites (M--O) lead
to the formation of H$_2$S by hydrolysis: COS $+$ H$_2$O
${\rightarrow}$ H$_2$S $+$ CO$_2$. Then, H$_2$S is oxidized into
elemental sulfur by oxygen-containing functional groups (M--O pairs).
This is the possible mechanism of COS removal when 
using US in the preparation method.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig07}
{\vspace*{-2pt}}
\caption{\label{fig7}Possible enhancing mechanism of
ultrasound assistance in the synthesis of {Ni--Al}
hydrotalcite for carbonyl sulfide
removal (where M is a metal)~\cite{bib56}.}
{\vspace*{-3pt}}
\end{figure*}

\subsubsection{Oxygen evolution reaction} \label{sec3.4.5}

Munonde and Zheng~\cite{bib108} prepared Ni--Fe LDH for oxygen
evolution reaction (OER) which is applied in electrochemical water
splitting. The \mbox{objective} was the generation of clean
energy in the form of oxygen and hydrogen fuels. The electrochemical
performance of LDHs was enhanced by exfoliation into single or thinner
nanosheets. This \mbox{process} increases mass transfer, exposing more
active sites and accelerating electron transfer~\cite{bib109}. 
US irradiation decreases the size of stacked nanosheets
from 70--800 to 30--300~nm and the thickness from 25--40~nm to 12--0~nm
in an optimal sonication duration of 15~min. Moreover, XRD patterns
show that diffraction peaks became sharper with increased 
intensities after US exfoliation. This indicated the increase in
periodicity of crystallographic directions. The US-exfoliated 
{Ni--Fe}
LDH catalyst showed advanced performance with 250~mV
overpotential at a current density of $10~\text{mA}{\cdot}
\text{cm}^{-1}$, that is 100~mV less than the initial 
{Ni--Fe} LDH catalyst.

\subsubsection{Carbonylation of glycerol} \label{sec3.4.6}

LDHs can also be used as catalysts using the ``memory effect''. This
effect is defined as the reconstruction of a mixed oxide obtained by 
thermal decomposition of LDHs immersed in an aqueous
solution~\cite{bib110}. Generally, the reconstruction of LDHs is
performed by hydrothermal treatment~\cite{bib111}. {\'A}lvarez
{\etal}~\cite{bib48} studied the reconstruction of 
{Zn--Al}\break
and 
{Zn--Al--Mg}
LDH
by mechanical stirring and{\break} ultrasonication during rehydration.
Those materials have been applied in the carbonylation of glycerol with
urea. Firstly, LDH structure is not completely reconstructed
because ZnO is detected in the samples treated by the two methods.
Moreover, there are no significant differences in LDH specific surface
area. But a 30\% decrease in surface area by comparison
to the parent hydrotalcite was observed for both treatments. 
Higher total basicity was obtained in the reconstructed material
prepared by ultrasonication for 10~h. Likewise, after adding magnesium,
the number of basic sites achieved a maximum in samples US-treated
for 2~h. In this study, the turnover frequencies (TOF) were
used for comparing the activity of reconstructed LDHs in the
carbonylation of glycerol with urea. The results showed that 
US irradiation increases TOF values.

As shown above, US irradiation offers many
advantages over non-US methods. Those {characteristics} are
very important in many fields such as catalytic applications. For
example, high basicity, well-dispersed active sites, and small
particles are very important features in a catalyst for methanation
reaction ($\text{CO}_2 + 4\text{H}_2 \rightarrow \text{CH}_4 + 2
\text{H}_2\text{O}$, $\Delta\text{H}_{298} =
-165~\text{kJ}{\cdot}\text{mol}^{-1}$) \cite{bib113}. High basicity
allows more CO$_2$ to be captured, and small, well-dispersed active
site particles allow higher contact area between reactants and
catalyst, leading to better activities.

\section{Conclusion} \label{sec4}

Layered double hydroxides (LDHs) are promising materials for a wide range of
applications. LDHs offer significant benefits over the use of
homogeneous alkali hydroxides or alkoxides as well as in heterogeneous
catalysis. In each of those applications, the form of the material
{(original form}
or as mixed oxides obtained after thermal treatment), the
composition and required properties vary. In this review, the
impact of ultrasound (US) use in the synthesis of LDHs has been discussed.
First of all, the review of the literature devoted to the
US-assisted \mbox{synthesis} of LDHs shows that in the majority
of cases the experimental conditions are poorly or insufficiently
described to permit comparison of the results obtained. As a result,
experimental designs are not well described and this does not allow for
optimization of the syntheses and of the performance induced by 
US irradiation. US has been used for a variety of
applications and very satisfying results 
were obtained compared to silent methods. US irradiation can be done
in one or more steps of the synthesis process and enables reduction of the
overall synthesis time compared to conventional preparation methods
from minimum 19~h to only 30~min. This is very important from
an industrial point of view because it leads to producing materials
with less energy. Furthermore, acoustic cavitation leads to
better dispersion of particles characterized by a small size and a
narrower particle size distribution since US leads to a
25--33\% particle size reduction in
comparison to the same materials prepared by classical method.
Moreover, US irradiation leads to an
increase in specific surface area from 22\% to more than 100\% in
comparison to the conventional methods. This is due to the brutal
collapse of cavitation bubbles 
resulting in shockwaves and microjets toward the
surface of solid particles causing local defects, 
surface erosion and particle fragmentation. Eventually, there is no doubt
that future research on these US-prepared materials  will
broaden their range of application and therefore increase the potential
of LDHs. Moreover, using high US frequency
is an important choice for studying the impact of one of the parameters
related to US irradiations on the properties of the prepared
materials. In addition, companies working on the development of
efficient new US devices have a role to play in scaling up LDH
synthesis from small-scale (laboratory scale) to large-scale. This
transition requires cooperation between laboratory researchers and
companies to improve the production of LDH materials using US.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

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