Sol–Gel Synthesis and Characterization of Coatings of Mg-Al Layered Double Hydroxides

In this study, new synthetic approaches for the preparation of thin films of Mg-Al layered double hydroxides (LDHs) have been developed. The LDHs were fabricated by reconstruction of mixed-metal oxides (MMOs) in deionized water. The MMOs were obtained by calcination of the precursor gels. Thin films of sol–gel-derived Mg-Al LDHs were deposited on silicon and stainless-steel substrates using the dip-coating technique by a single dipping process, and the deposited film was dried before the new layer was added. Each layer in the preparation of the Mg-Al LDH multilayers was separately annealed at 70 °C or 300 °C in air. Fabricated Mg-Al LDH coatings were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and atomic force microscopy (AFM). It was discovered that the diffraction lines of Mg3Al LDH thin films are sharper and more intensive in the sample obtained on the silicon substrate, confirming a higher crystallinity of synthesized Mg3Al LDH. However, in both cases the single-phase crystalline Mg-Al LDHs have formed. To enhance the sol–gel processing, the viscosity of the precursor gel was increased by adding polyvinyl alcohol (PVA) solution. The LDH coatings could be used to protect different substrates from corrosion, as catalyst supports, and as drug-delivery systems in medicine.


Experimental
The Mg 3 Al LDH specimens were prepared by the sol-gel technique using metal nitrates Mg(NO 3 ) 2 ·6H 2 O (99.9%, Fluka, Saint Louis, MO, USA) and Al(NO 3 ) 3 ·9H 2 O (99.9%, Fluka, Saint Louis, MO, USA) dissolved in 50 mL of deionized water as starting materials. To the obtained mixture, a 0.2 M solution of citric acid (C 6 H 8 O 7 , 99.0%, Alfa Aesar, Haverhill, MA, USA) was added. The resulting solution was additionally stirred for 1 h at 80 • C. Finally, 2 mL of ethylene glycol (C 2 H 6 O 2 , 99.0%, Alfa Aesar, Haverhill, MA, USA) was added with continued stirring at 150 • C. During the evaporation of solvent, the transformations from the sols to the gels occurred. The synthesized precursor gels were dried at 105 • C for 24 h. The MMOs were obtained by heating the gels at 650 • C for 4 h. LDHs were fabricated by reconstruction of MMOs in deionized water at 80 • C for 6 h. The relative humidity of the atmosphere was about 50%. Mg 3 Al LDH coatings were synthesized using the sol-gel method in different solutions. In the first attempt, only an aqueous solution of LDH was used. Secondly, 0.5 g of LDH was mixed with 1 g of polyvinyl alcohol (PVA) (PVA7200, 99.5%, Aldrich, Saint Louis, MO, USA) in distilled water. LDH suspensions were deposited on silicon and stainless-steel substrates using the dip-coating technique by a single dipping process, and the deposited film was dried before the new layer was added.
X-ray diffraction (XRD) analysis of synthesized compounds was performed with a MiniFlex II diffractometer (Rigaku, The Woodlands, TX, USA) using primary-beam Cu Kα radiation (λ = 1.541838 Å). The 2θ • angle of the diffractometer was tuned from 8 to 80 • in steps of 0.02 • , with the measuring time of 0.4 s per step. The surface morphological features were characterized using a scanning electron microscope (SEM) (Hitachi SU-70, Tokyo, Japan). The roughness of the Mg 3 Al LDH films was estimated using an atomic force microscope (AFM) (BioscopeII/Catalyst, Karlsruhe, Germany). The ScanAsyst, operated in the peak-force tapping mode and equipped with a wafer of silicon nitride probe Asyst at the air AFM tip, was used for imaging. Surface root mean square (RMS) values were calculated using the MATLAB R2015b programme.

Results and Discussion
The XRD patterns of Mg 3 Al LDHs obtained from the silicon and steel substrates are presented in Figures 1 and 2, respectively. Evidently, the intensity of Si reflection originating from the substrate is much higher in comparison to the main reflection of LDH samples. However, after eliminating silicon reflection from the XRD patterns (see the insertion in Figure 1), the main reflections clearly represent the formation of an LDH structure after 15 dipping procedures. The formation of Mg 3 Al LDH thin films on stainless-steel substrate ( Figure 2) was also observed. The diffraction lines of Mg 3 Al LDH thin films are sharper and more intense for the sample obtained from the silicon, confirming higher crystallinity of synthesized Mg 3 Al LDH. In both cases, the single-phase crystalline LDHs have formed [4][5][6].
Materials 2019, 12, x FOR PEER REVIEW 3 of 13 silicon nitride probe Asyst at the air AFM tip, was used for imaging. Surface root mean square (RMS) values were calculated using the MATLAB R2015b programme.

Results and Discussion
The XRD patterns of Mg3Al LDHs obtained from the silicon and steel substrates are presented in Figures 1 and 2, respectively. Evidently, the intensity of Si reflection originating from the substrate is much higher in comparison to the main reflection of LDH samples. However, after eliminating silicon reflection from the XRD patterns (see the insertion in Figure 1), the main reflections clearly represent the formation of an LDH structure after 15 dipping procedures. The formation of Mg3Al LDH thin films on stainless-steel substrate ( Figure 2) was also observed. The diffraction lines of Mg3Al LDH thin films are sharper and more intense for the sample obtained from the silicon, confirming higher crystallinity of synthesized Mg3Al LDH. In both cases, the single-phase crystalline LDHs have formed [4][5][6]. The surface morphology of the representative Mg3Al LDH film sample obtained from the Si substrate is presented in Figure 3. The surface of the substrate is covered with a monolithic layer of agglomerated plate-like particles that are 5-10 µm in size. However, the SEM micrographs obtained at a higher magnification clearly show that these plate-like particles are composed of hexagonally shaped nanoparticles that are characteristic of LDH structures [6]. An almost identical surface morphology was observed for the LDH coatings on the stainless-steel substrate.
The sol-gel synthesis processing route for high-quality calcium hydroxyapatite coatings on silicon substrate when PVA was used as a gel-network-forming agent [67] was a source of inspiration for this study. Therefore, to enhance the sol-gel processing, the viscosity of the precursor gel was increased by adding PVAsolution, and the drying temperature was also increased.  The XRD patterns of the LDH coatings obtained from Si ( Figure 4) and stainless-steel ( Figure 5) substrates, however, were almost the same as without the addition of PVA. The surface morphology of the representative Mg 3 Al LDH film sample obtained from the Si substrate is presented in Figure 3. The surface of the substrate is covered with a monolithic layer of agglomerated plate-like particles that are 5-10 µm in size. However, the SEM micrographs obtained at a higher magnification clearly show that these plate-like particles are composed of hexagonally shaped nanoparticles that are characteristic of LDH structures [6]. An almost identical surface morphology was observed for the LDH coatings on the stainless-steel substrate. inspiration for this study. Therefore, to enhance the sol-gel processing, the viscosity of the precursor gel was increased by adding PVAsolution, and the drying temperature was also increased.  The XRD patterns of the LDH coatings obtained from Si ( Figure 4) and stainless-steel ( Figure 5) substrates, however, were almost the same as without the addition of PVA. The sol-gel synthesis processing route for high-quality calcium hydroxyapatite coatings on silicon substrate when PVA was used as a gel-network-forming agent [67] was a source of inspiration for this study. Therefore, to enhance the sol-gel processing, the viscosity of the precursor gel was increased by adding PVAsolution, and the drying temperature was also increased. The XRD patterns of the LDH coatings obtained from Si ( Figure 4) and stainless-steel ( Figure 5) substrates, however, were almost the same as without the addition of PVA.      The XRD patterns show the formation of the same crystallinity LDH phase on the Si substrate. With the dip-coating in PVA solution and drying at 300 • C (PVA's melting point is ∼266 • C), the LDH phase did not form. The LDH sample with higher crystallinity was obtained from the steel substrate. Interestingly, no side iron oxide (Fe 2 O 3 and Fe 3 O 4 ) phases were formed during the synthesis, as was observed in the case of sol-gel synthesis of calcium hydroxyapatite on stainless-steel substrate [63].
The SEM micrographs of Mg 3 Al LDH films obtained from Si and stainless-steel substrates using a precursor in the PVA solution are shown in Figure 6. The formation of nanograins of LDH is evident when PVA solution was used in the sol-gel processing. Moreover, these nanograins show a tendency to form cloudy agglomerates. The XRD patterns show the formation of the same crystallinity LDH phase on the Si substrate. With the dip-coating in PVA solution and drying at 300 °C (PVA's melting point is ~266 °C), the LDH phase did not form. The LDH sample with higher crystallinity was obtained from the steel substrate. Interestingly, no side iron oxide (Fe2O3 and Fe3O4) phases were formed during the synthesis, as was observed in the case of sol-gel synthesis of calcium hydroxyapatite on stainless-steel substrate [63].
The SEM micrographs of Mg3Al LDH films obtained from Si and stainless-steel substrates using a precursor in the PVA solution are shown in Figure 6. The formation of nanograins of LDH is evident when PVA solution was used in the sol-gel processing. Moreover, these nanograins show a tendency to form cloudy agglomerates. The amount of water, hydroxide, and carbonate in the formula of synthesized bulk LDH samples can be calculated from the results of thermogravimetric analyses [6,68]. For example, the composition was defined in our previous study to be [Mg0.75Al0.25(OH)2](CO3)0.125·4H2O [6]. However, the experimental procedure could be more complicated in the case of thin films and should be redefined in the future.
In Figures 7-10, atomic force microscopy images of different Mg3Al LDH films prepared before and after modification on silicon and stainless-steel substrates are represented. The atomic force microscopy (AFM) data of LDH profiles were filtered with a mathematical procedure implemented in the MATLAB software. This software computes several roughness parameters at different "walks" of axes x (vertical) and y (horizontal) positions (see Table 1). For this reason, the AFM images were reduced and cut off from the middle 10 µm 2 square for a better comparison. Figures 7  and 8 show the Mg3Al LDH films dip-coated on the silicon and stainless-steel substrates, respectively.
The average RMS parameter obtained by AFM was determined to be 186.14(8) nm for the Mg3Al LDH surface on the silicon substrate (64.89(7) nm for raw Si substrate) and 352.62(9) nm for the Mg3Al LDH surface on the stainless-steel substrate (112.54(8) nm for raw Fe substrate). However, using the PVA (Figures 9 and 10) solution for the modification of the Mg3Al LDH synthesis, the roughness increased to 733.30 (8) and 1181.12(7) nm on the silicon and stainless-steel substrates, respectively. This might be because the higher concentration of polymers resulted in the formation of larger micelles of the monomer in the solution and larger polymer aggregates on the surface. As we can see from the AFM images and the calculated RMS values, the synthesized LDH coatings can be characterized as nanometer-size thin films. It was observed that the Mg3Al LDH film that formed on silicon substrate in the distilled water had the smoothest surface. The synthesized coatings could be applied for future work for the investigation of anticorrosive properties. The amount of water, hydroxide, and carbonate in the formula of synthesized bulk LDH samples can be calculated from the results of thermogravimetric analyses [6,68]. For example, the composition was defined in our previous study to be [Mg 0.75 Al 0.25 (OH) 2 ](CO 3 ) 0.125 ·4H 2 O [6]. However, the experimental procedure could be more complicated in the case of thin films and should be redefined in the future.
In Figures 7-10, atomic force microscopy images of different Mg 3 Al LDH films prepared before and after modification on silicon and stainless-steel substrates are represented. The atomic force microscopy (AFM) data of LDH profiles were filtered with a mathematical procedure implemented in the MATLAB software. This software computes several roughness parameters at different "walks" of axes x (vertical) and y (horizontal) positions (see Table 1). For this reason, the AFM images were reduced and cut off from the middle 10 µm 2 square for a better comparison. Figures 7 and 8 show the Mg 3 Al LDH films dip-coated on the silicon and stainless-steel substrates, respectively.

Conclusions
Mg3Al LDH coatings were successfully fabricated on silicon and stainless-steel substrates using the sol-gel processing route for the first time, to the best of our knowledge. The LDHs were fabricated by reconstruction of MMOs in deionized water. The MMOs were obtained by calcination of the precursor gels. The XRD patterns demonstrated the high crystallinity of the synthesized Mg3Al1 LDH coatings. The SEM micrographs clearly showed that the plate-like particles that formed The average RMS parameter obtained by AFM was determined to be 186.14(8) nm for the Mg 3 Al LDH surface on the silicon substrate (64.89(7) nm for raw Si substrate) and 352.62(9) nm for the Mg 3 Al LDH surface on the stainless-steel substrate (112.54 (8) nm for raw Fe substrate). However, using the PVA (Figures 9 and 10) solution for the modification of the Mg 3 Al LDH synthesis, the roughness increased to 733.30 (8) and 1181.12(7) nm on the silicon and stainless-steel substrates, respectively. This might be because the higher concentration of polymers resulted in the formation of larger micelles of the monomer in the solution and larger polymer aggregates on the surface. As we can see from the AFM images and the calculated RMS values, the synthesized LDH coatings can be characterized as nanometer-size thin films. It was observed that the Mg 3 Al LDH film that formed on silicon substrate in the distilled water had the smoothest surface. The synthesized coatings could be applied for future work for the investigation of anticorrosive properties.

Conclusions
Mg 3 Al LDH coatings were successfully fabricated on silicon and stainless-steel substrates using the sol-gel processing route for the first time, to the best of our knowledge. The LDHs were fabricated by reconstruction of MMOs in deionized water. The MMOs were obtained by calcination of the precursor gels. The XRD patterns demonstrated the high crystallinity of the synthesized Mg 3 Al 1 LDH coatings. The SEM micrographs clearly showed that the plate-like particles that formed on the surface are composed of hexagonally shaped nanoparticles, which are characteristic of LDH structures. The average RMS parameter obtained by AFM was determined to be 186.14(8) nm for the Mg 3 Al LDH surface on the silicon substrate and 352.62(9) nm for the Mg 3 Al LDH surface on the stainless-steel substrate. The roughness of the coatings increased to 733.30 (8) and 1181.12(7) nm on the silicon and stainless-steel substrates, respectively, using the PVA solution for the modification of the Mg 3 Al LDH. The phase purity of coatings obtained from Si and stainless-steel substrates, however, was almost the same with or without the addition of PVA.