2.1. LDH Bulk Synthesis

The typical procedures to perform the bulk synthesis are based on the co-precipitation, the ion-exchange, the calcination recovery, the hydrothermal synthesis, the microemulsion method, and the layer-by-layer deposition.

Among them, the most commonly employed method is the co-precipitation in low oversaturation conditions [6], which is based on the dropwise slow addition of a base (usually NaOH) to an aqueous solution, containing the metals at a proper molar ratio and concentration, kept under stirring. The pH is controlled during the whole synthesis to a target value, the precipitate is aged in contact with the mother solution until it is filtered, washed, and dried.

The ion-exchange method starts from an already synthesized LDH containing an easily exchangeable interlayer anion ^[1][7][1,7], and consists in the replacement of the original anion with a new guest species to obtain a different LDH. A drawback of such a procedure is that the exchange reaction sometimes is not complete, and consequently the method exhibits a low efficiency.

Both the co-precipitation and the ion exchange methods can be employed to prepare LDHs of variable composition, using different combinations of bivalent and trivalent cations, but often lead to poorly crystalline materials; in order to improve crystallinity a thermal ageing step can be conducted after the co-precipitation or ion-exchange occurrence [8].

Another well-known method for LDHs synthesis is calcination recovery which is based on the “memory effect” [9]. The interlayer anion is removed throughout a calcination process that converts the LDH into a layered double oxide (LDO) with the layered structure collapsing to a cubic phase. When the LDO is added to an aqueous solution the layered structure is rebuilt again, with a different intercalated anion, thus obtaining another LDH.

More recently, other synthesis methods have been proposed to improve LDH crystallinity and to obtain LDHs with a more uniform and controlled morphology. For example, using urea as the hydrolysis agent, LDHs with large, thin platelets and narrow particle size distributions can be easily obtained. These characteristics are achievable by co-precipitation only after extensive ageing. The method is mainly restricted to Al-based LDHs, but more recently tri-metal LDHs with composition Mg/M/Al (with M = Fe, Co, Ni, Cu or Zn) have been synthesized, where Mg is partly substituted with another bivalent cation, but even Al can be substituted by Fe [10]. The urea hydrolysis method has been further improved by adding sodium citrate as a chelating reagent to the synthesis solution, and this procedure has been successfully employed to obtain Ni/Fe LDHs with high crystallinity and well-defined hexagonal-shaped crystallites [11].

Another preparation procedure, which allows for the synthesis of micrometer scale LDH particles, is the hydrothermal method which consists in heating a mixed homogeneous slurry of a bivalent and a trivalent metal (usually in the form of oxides or nitrate salts) to obtain the corresponding LDH. The morphology and the size of the LDH particles can be varied by controlling the heating temperatures and the metals ratio. According to this procedure, the formation of Mg/Al LDH has been extensively investigated [12], but, more recently, composites based on Ni/Fe LDHs and carbon nanotubes (CNTs) or reduce graphene oxide (RGO) have been prepared and characterized for the development of supercapacitors [13].

One of the main drawbacks of physical deposition methods is that the adhesion between LDH crystallites and the underlying conductor is weak, and sometimes some detachments can occur, especially when the electrode is polarized. An improvement in film adhesion can be obtained by decreasing the particles but this is a challenging topic. Several strategies have been implemented to this aim when performing the synthesis by co-precipitation.

A fine control of co-precipitation conditions can narrow the particles size down to 40–300 nm [14], and separating the nucleation and ageing steps can further adjust the sizes in a narrower region (60–80 nm) [8]. Another strategy involves the vigorous stirring of the mixed solution of metal nitrates and sodium hydroxide in methanol medium to form a LDH slurry, which evolves into LDH nanoparticles (∼40 nm) after a further solvothermal treatment [15].

LDHs can be also synthesized by a sol gel like process that is based on the hydrolysis and condensation of alkoxide precursors in alcohol [16] or on the hydrolysis of acetate precursors in polyol medium [17]. This method has several advantages over other synthetic procedures. It is a simple way to obtain nanoscaled particles with high specific surface area and narrow pore size distribution and allows for an accurate control over structural and textural properties of the products which are obtained with high-purity. Several LDHs have been successfully synthesized by the sol gel procedure, e.g., those containing Mg/Al ^[18][19][18,19], Mg/Ga [19], Mg/In [19], Ni/Al [18], and Zn/Al [20].

Another promising synthesis that enables the dimensions of LDHs particles to be decreased to a nanometer level is the microemulsion method, where the synthesis is carried out in a solution typically composed of oil, water, and a surfactant (occasionally with a co-surfactant). The process exploits the confined environment of the water pools of reverse microemulsions that can be considered as nanosized reaction chambers, and thus constitutes an efficient tool to control the nucleation and growth of inorganic precipitates. Recently, O’Hare and Davis’ group [21] prepared Mg/Al LDH nanocrystals in a water-in-oil microemulsion of the anionic surfactant sodium dodecyl sulfate, isooctane, and water. More recently, the production of several LDHs (Ni/Al, Zn/Al, Ni/Cr, Zn/Cr, Ni/Fe and Mg/Fe) by the double microemulsion technique with a cationic surfactant has been reported ^[22][23][22,23]. Bellezza et al. employed a quaternary water-in-oil microemulsion of cetyltrimethylammonium bromide/n-butanol/isooctane/water, and obtained LDH nanoparticles whose mean size was in the 10–30 nm range [22].

For electrochemical applications, whichever the procedure employed for the synthesis, the LDH must be deposited on a conductive surface. LDH-modified electrodes are usually prepared by coating the surface of the working electrode (metal or carbon based) by a film of controlled thickness [24].

Typically, the coating is obtained by depositing a fixed amount of a colloidal suspension of a chemically synthesized LDH, sometimes with the help of a spin-coater, and allowing it to dry in air. To improve the mechanical adhesion a binder (polytetrafluoroethylene, acetylene black, or Nafion) is often added.

In order to obtain LDH thin films the layer-by-layer (LbL) deposition can be employed, which exploits the electrostatic self-assembly technique. Sasaki and co-workers have deeply studied this procedure [25] consisting of the delamination of LDHs into single positively charged nanosheets that are then alternated to negatively charged species and assembled on the electrode surface [26].

The adherence of the film to the electrode substrate can be also improved by means of methods that are based on the one step synthesis of the LDH directly on the conductive substrate (in situ growth methods). These procedures are very rapid and allow, at the same time, for the synthesis of the LDH and for the electrode modification, and they are not limited by the shape of the substrate. The main procedures developed for the realization of electrode coatings to be employed in the fields of electrocatalysis are described and compared in the following paragraph.

2.2. In Situ Growth Methods

One of the most interesting in situ growth methods to realize the one step modification of a conductive surface is the electrochemical deposition. This procedure, firstly reported by Kamath’s group [27] to synthesize LDHs containing Co(II) or Ni(II) and Al(III), is based on the cathodic reduction of nitrate ions in a solution containing a bivalent and a trivalent metal in order to generate a basic solution that allows the precipitation of the LDH. Starting from those results, Tonelli’s group conducted an extensive study aimed at finding the best experimental conditions to modify several electrode materials by electrosynthesis of a Ni/Al-NO₃ LDH ^[28][29][28,29]. The initially-adopted procedure was based on a potentiostatic deposition, where a cathodic potential was applied to the working electrode for fixed time lengths; the pulse length (usually between 30 and 120 s) determined the thickness of the LDH coating that ranged between 100 and 800 nm. The procedure was applied to synthesize LDHs having Al as the trivalent metal and Ni or Co as the bivalent one, and later the substitution of Fe with Al was taken into account to increase the conductivity properties of the LDHs ^[4][30][31][4,30,31]. The electrochemical deposition also allowed the synthesis of LDHs with a more complex composition (containing Mg(II) and variable amounts of Rh(III)) on FeCrAlY alloy foams to be used as precursors of Rh-based catalysts for the catalytic partial oxidation of methane [32]. Recently, an alternative synthesis protocol based on a potentiodynamic deposition has been studied and optimized to better and finely control the composition of the LDH materials. In particular, one of the main issues of the potentiostatic or galvanostatic deposition lies in the possibility to control the ratio Me(II)/Me(III) of the LDH. In fact, the ratio of the two metals in the electrodeposited compound is not strictly the same as present in the electrolytic solution because of the concentration gradients originating during the synthesis, especially when a long potential pulse is applied. Our group ^[33][34][35][33,34,35] has demonstrated that the potentiodynamic approach, which is based on the application of few CV cycles in the cathodic potential range, provides a reproducibility of the deposit which is much better than the one achieved with either the galvanostatic or potentiostatic methods. This is due to the strong reduction of cations concentration gradients in the diffusion layer, which are typical of the potentiostatic approach, thus allowing the restoration of the initial concentrations at the electrode surface. The result is that the LDH Me(II)/Me(III) ratio is the same as the one present into the electrolytic bath.

The advantages of the electrochemical deposition are the short time needed to obtain the LDH film, the possibility of coating complex geometry supports and to easily modulate both the electrolytic bath composition and the electrosynthesis time to tune the film thickness and the M(II)/Me(III) molar ratios. One drawback is the low crystallinity exhibited by the LDH which is due to the short time of the synthesis so that the material does not have time to reorganize its structure. Moreover, the electrochemical deposition does not allow the orientation of the LDH material and the morphology of the film to be finely controlled.

Recently, some authors have proposed one step methods to directly synthesize more oriented LDHs with the aim to enhance their specific surface area and increase their conductivity. One of the most interesting procedures has been proposed by Chang Yu and coworkers [36] who reported a simple and efficient strategy for assembling vertically oriented Ni/Co-LDH nanoarrays (Ni/Co-LDH-NA) on carbon fiber papers (CFP) to be employed for OER in water splitting. The CFP, being a current collector and a robust substrate, is capable both of promoting fast electron transport and to modulate the LDH assembly. Briefly, the procedure consists of the addition of cobalt and nickel nitrates, and of cetyltrimethylammonium bromide to a solution of ethanol and water containing the CFP substrate, followed by ultrasonication for 30 min, and a hydrothermal treatment at 180 °C for 12 h.

LDHs with a vertical orientation have been also synthesized by Qian Xiang and coworkers [37] who prepared Fe/Ni LDH nanosheet arrays on various metal foils by a facile hydrothermal method. The synthesis consisted of the precipitation of iron and nickel on the metal foil in aqueous solution, which was induced by urea hydrolysis, in alkaline solution, upon hydrothermal treatment at 120 °C for 12 h. The LDH grows in a vertical direction with respect to the metal foil because CO₃²⁻ anions, produced by urea decomposition, adsorb on the (001) surface and passivate this plane so that to suppress the LDH growth along this direction, and causing the stacking of the crystals in the direction vertical to the substrate surface.

Vertically grown LDH nanosheet arrays are particularly interesting, because on one hand many nanoscale channels are created, thus enabling an easy access for the reaction intermediates to the catalytic active sites, and on the other hand they ensure the direct contact of each individual sheet with the conductive substrates, thus promoting the intralayer electron transfer.