Synthesis methods

Several synthetic techniques have been developed and reported in literature for LDHs preparation. The most commonly employed are the simple coprecipitation and hydrothermal methods, but other methods include urea hydrolysis, glycine assisted hydrothermal synthesis, topochemical oxidation and reconstruction, which is based on the so-called “memory effect”. Furthermore, some other synthetic approaches such as

33 microemulsion method and electrochemical synthesis have been successfully employed. In the following the methods will be briefly described.

3.2.1 Coprecipitation at a constant pH

This synthesis is one of the most commonly employed and consists in the following procedure [7]: typically the solutions of two different metal salts (commonly nitrates or chlorides) are prepared with a total cationic concentration of 0.1 M in order to keep a constant M(II)/M(III) ratio of about 2-3. The solution is then added dropwise to an aqueous solution containing organic guest species. Throughout the addition, the pH is maintained constant at a value of ~9 by the simultaneous addition of a 0.2 M NaOH solution in order to induce the co-precipitation. The reaction is carried out under N2 atmosphere to avoid carbonate contamination, with vigorous stirring. The addition of the salts solution is completed within about 3 hours and then the suspensions is immediately centrifuged at 4500 rpm without any ageing in order to quench the crystal growth and, therefore, obtain small platelets. The solids recovered by centrifugation are washed a few times with deionized water and dried in air at 30◦C overnight.

Otherwise, in order to obtain bigger particles, the precipitate is aged at room temperature for 24 h, filtered, washed carefully with decarbonated water and finally dried under vacuum.

3.2.2 Urea hydrolysis method

Urea has a number of unique properties such as its very weak Brǿnsted base characteristic (pKb = 13.8), high solubility in water, and a hydrolysis rate that can be easily controlled, making it an attractive agent to precipitate several metal ions as hydroxides or as insoluble salts in the presence of a appropriate anion. The hydrolysis of urea consists in two steps, the formation of ammonium cyanate (rate determining step) and the fast hydrolysis of the cyanate to ammonium carbonate:

(3.1) _(3.2)

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The acid-base reactions of ammonium and carbonate to give ammonia and hydrogen carbonate lead to a pH of about 9, depending on the temperature. This pH is suitable for precipitating a large number of metal hydroxides. Costantino et al. [8] investigated the best conditions in terms of temperature, total metal cations concentration, molar fraction Al/Al+M(II) and molar fraction urea/M(II)+Al to prepare Mg, Zn or Ni/Al-carbonate LDHs by this method.

A well crystalline LDH phase can be prepared in a relatively short time using the homogeneous precipitation method based on urea hydrolysis, dissolving solid urea in an aqueous solution of the chosen metal chlorides to give the chosen urea to ion ratio [9]. The precipitation of the LDH is achieved by thermal hydrolysis of urea with formation of ammonium carbonate. The obtained solid is separated from the mother solution, washed with deionized water and 0.1 M sodium carbonate in order to exchange residual chloride ions incorporated during the synthetic procedure, with carbonate anions. Eventually, the solid is dried at room temperature overnight.

Alternatively, the so obtained precipitate can be submitted to a hydrothermal treatment, at 90 °C for 24-48 hours at pH 9.

The compounds prepared using this method display homogeneous sizes and platelet-like primary particles with well-defined hexagonal shapes, which may be very interesting from the viewpoint of nanotechnology since LDHs provide nano-size two-dimensional spaces for the creation of functional materials.

3.2.3 Hydrotermal method

The hydrothermal method is usually employed when organic guest species with low affinity for LDHs are required to be intercalated into the interlayers or when the anion- exchange reactions or the coprecipitation using soluble metal salts cannot be applied. This method is particularly interesting because only the desired organic anions can occupy the interlayer space under the hydrothermal condition since insoluble hydroxides are used as precursors and no other competing species are present [10].

The following is a typical synthetic procedure: the hydroxides precursors and the organic species are mixed and stirred in de-ionized water at room temperature. Then the suspension is transferred into an autoclave and heated at 100-150 °C for a few days. The

35 solid precipitate is collected by filtration using a membrane filter and subsequently washed with ethanol.

The hydrothermal synthesis is particularly useful when LDH-like materials are prepared using powders as starting materials and it has been found out that the temperature influences the crystal structure of the resulting material. Structures synthesized at different temperatures, despite having identical main layers, show a different arrangement of water molecules and carbonate groups forming the interlayer region.

3.2.4 Glycine assisted hydrothermal synthesis

LDHs are usually prepared by glycine assisted hydrothermal method, according to the following procedure [11]. In a typical experiment, glycine (NH2CH2COOH; pKa1 = 2.3, pKa2 = 9.6) and sodium sulfate are dissolved in a solution of Ni(NO3)2 and Al(NO3)3 in a fixed molar ratio. After a few minutes of stirring a clear green solution is obtained. Then 5 M NaOH is added under stirring, to induce a pH increase from ~3.5 to ~13.5. Afterward, the solution is transferred into an autoclave and heated at 120 °C for 24 h. After the hydrothermal treatment, the autoclave is allowed to cool to room temperature, the resulting precipitate is recovered by centrifugation, and the supernatant is removed. There is no change in the solution pH before and after the hydrothermal treatment. The precipitate is then washed a few times with de-ionized water and dried at room temperature.

Depending on the reactants ratio in this method (the metal salt and the glycine amounts) it is possible to obtain different shapes and sizes of the particles.

3.2.5 Topochemical oxidation

Taking the example of a Co-based LDH, through a topochemical oxidative reaction under air, a β-Co(OH)2 brucite-like structure can be converted into a monometallic Co(II)/Co(III)–CO3 layered double hydroxide [12].

The synthesis of the LDH consists in the partial oxidation of -Co(OH)2 and is carried out for 60 hours at 40 °C, in 0.5 M NaOH. The modification of the electrodes can be performed by casting the colloidal suspension and drying it at ambient temperature obtaining stable and homogeneous films.

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3.2.6 Reconstruction method

Another common method to produce LDHs is rehydration/reconstruction using the structural “memory effect”. This method involves calcination of LDHs to remove the interlayer water, interlayer anions, and structural water, resulting in mixed metal oxides. It is interesting to note that the calcined LDHs are able to regenerate the layered structure when they are exposed to water and anions [13]. Again in addition, the included anions do not need to be the same species originally present in the interlayer of the uncalcined LDHs, and, therefore, this is an important method to synthesize LDHs with the desired inorganic or organic anions in order to accomplish specific application requirements [14]. Metal salts are calcined at 500°C for 4 h, under nitrogen, at a heating rate of 5 °C/min. The solid is then added to a solution containing decarbonated water with the guest molecule. The desired pH value (7-8) is adjusted by adding NaOH. Then, the precipitate is aged at room temperature, filtered, washed with decarbonated water thoroughly, and finally dried under vacuum.

3.2.7 Microemulsion method

Colloidal aqueous dispersions of LDHs can be prepared by the double-microemulsion method according to literature data [15]. This method utilizes two water-in-oil microemulsions containing cetyl trimethylammonium bromide as surfactant, n-butanol as cosurfactant and isooctane as oil phase, mixed together. One solution contains the reactants M(II) and M(III) salt precursors in aqueous phase and the other one a precipitating agent such as NH3. In this case, the intermicellar exchange of solutes is essential to the formation and precipitation of the product.

3.2.8 Electrochemical synthesis

The electrodeposition of LDHs is an appealing route of synthesis since it allows the simultaneous synthesis and modification of the electrode surface [16]. The electrochemical synthesis can be performed starting from a solution containing the nitrate salts of the metal M(II) and M(III) in a proper molar ratio and KNO3 as supporting

37 electrolyte, then a cathodic potential is applied for a fixed time, usually between 30 and 90 seconds [17].

The method is based on the electrochemical production of hydroxide anions necessary to induce the LDH precipitation on the electrode surface by the cathodic reduction of nitrate ions [18].

During the cathodic reduction of nitrates the following reactions occur near and on the electrode surface, leading to the disappearance of H+ and to the generation of OH-,and contribute to the precipitation of the LDH.