How Density Functional Theory Surface Energies May Explain the Morphology of Particles, Nanosheets, and Conversion Films Based on Layered Double Hydroxides

Abstract

Conversion films based on layered double 13 hydroxides constitute an important and environmentally 14 friendly technology for the corrosion protection of aeronaut-15 ical structures. Unfortunately, the morphology of layered 16 double hydroxide (LDH) conversion films is still not well 17 understood. In the present work, the structure and driving 18 forces behind the morphology of zinc−aluminum LDH 19 conversion films on aluminum alloy 2024 (AA2024) are explained from the perspective of molecular modeling. Since LDH 20 particles are the core structures of LDH conversion films, the first step in this work was to understand the relation between 21 structure and morphology of the particles themselves and the single-layer nanosheets that constitute them. Results regarding 22 53 Atomic force microscopy (AFM), X-ray diffraction (XRD), 25 54 and dynamic light scattering (DLS) 25 results were elucidated in 55 terms of the energies of different exposed surfaces of LDH 56 particles, obtained computationally using periodic model 57 density functional theory (DFT) calculations. These con-58 clusions were used to comprehend the molecular interactions 59 ruling the growth pattern of LDH conversion films onto 60 aluminum substrates. 1,26 61 Generally, conversion films consist in a first coating layer 62 grown directly on top of a metallic substrate to provide 63 corrosion protection by barrier effect and to promote adhesion 64 between the metal and the following layers of a coating 65 system. 27 In a previous study, Tedim et al. 1 developed a 66methodology to grow in situ a nanostructured film of Zn−Al LDHs directly onto the surface of aluminum alloys, taking 68 advantage of the dissolution of Al 3+ from the surface. 26,28 The 69 second two-valent cation (Zn 2+ ) is supplied from the treatment 70 bath. This approach 1 makes possible to intercalate corrosion 71 inhibitors by a subsequent step of ion-exchange providing the 72 LDH conversion films with the same active protective 73 functionality as LDH particles 7 but with the advantage of 74 being near the metallic surface. LDH particles typically exhibit a 75 platelike morphology. 25 When LDHs are grown directly onto 76 aluminum alloys, these plates are assembled perpendicularly to f1 77 the surface 1,26,29,30 as illustrated in Figure 1. 78 Herein, the formation and growth of LDH particles is 79 investigated by means of AFM and scanning electron 80 microscopy (SEM) complemented with additional information 81 about the energies of different exposed surfaces from DFT 82 calculations using periodic structural models of LDH clusters 83 interacting with aluminum surfaces. 84 The material investigated in this work was zinc−aluminum 85 LDH with a Zn:Al ratio of 2:1 and the nitrate anion intercalated 86 (Zn(2)Al−NO 3 ), since it can be easily synthesized as individual 87 particles 25 and as conversion films onto aluminum substrates. 1 88 It is also a functional material on its own. 8 Moreover, the nitrate 89 anion can easily allow us to obtain single-layer nanosheets, 18−22 90 an...

Unveiling the local structure of 2-mercaptobenzothiazole intercalated in (Zn2Al) layered double hydroxides

“…Zn2Al(OH)6-NO3) and the Zn2Al(OH)6-MBT LDH synthesized at pH = 10, thermogravimetric analysis (TGA) was also performed to obtain insights into the amount of water molecules inside the interlayers (Figures S3 and S11), which are analyzed in view of the results from the MD simulations discussed above. For the precursor, it is possible to find computational studies in the literature where one (Galvão et al, 2017) and, more commonly, two (Galvão et al, 2016b;Pérez-Sánchez et al, 2018) water molecules per nitrate anion yielded structures with interlayer spacings that are in good agreement with those estimated from the experimental XRD results. From the Zn2Al(OH)6-NO3 TGA results presented in the Supporting Information (Figures S3 and S11), the mass loss at T = 170 ºC is associated to the release of a single water molecule per anion, whereas the mass loss at T = 230 ºC corresponds to the release of two water molecules per anion.…”

“…DFT calculations were also employed to analyze the morphology of ZnAl LDH conversion films on the aluminum alloy 2024, with the metal surface modelled with periodic Al(111) or -Al2O3(0001) slabs while the LDH considered a fragment with a single nitrate anion (Galvão et al, 2017). Galvão et al demonstrated Experimental data obtained in our laboratory or found in the literature were analyzed and discussed in the light of the results from the MD simulations.…”

Emerging switchable ultraviolet photoluminescence in dehydrated Zn/Al layered double hydroxide nanoplatelets

“…1c. Morphology and crystallization process of Zn/Al-LDHs grown on Al substrates are comprehensively discussed in previous studies 11,42,43 . Some Zn/Al-LDH nanoplatelets were scraped from the Al foils and elemental composition analysis by means of energy dispersive X-ray spectroscopy (EDX) revealed an average Zn/Al ratio equal to about 2.0 for all the prepared LDH samples (a summary of EDX data is given in Supplementary Fig.…”

Improving the functionality and performance of AA2024 corrosion sensing coatings with nanocontainers

“…Then, hydroxyl groups were added to the uppermost aluminum layer of the model until reaching a degree of hydroxylation of 75%, i.e., 3 hydroxyl groups per (2 × 2) supercell as suggested from the DFT studies in Ref. [27], and as applied in a previous work [28].…”