ABSTRACT
Figure 12.20 Typical SEM images of (a) the pristine MgAl-LDH powder sample, (b, c) a monolayer assembly of MgAl-LDH nanoplates on a Si substrate, (d) a DC-MgAl-LDH nanoplate film after the anion exchange reaction through a solvothermal treatment, and (e) AFM images of selected MgAl-LDH nanoplates on Si for MgAl-LDH and DC-MgAl-LDH before and after the anion exchange reaction. Reproduced from Chem. Mater., 108, pp. 64-170 (2004). Copyright © 2004, American Chemical Society [20]. One of the most exploited properties of LDH materials is the so-called “reconstruction effect”: calcination of LDHs at moderate temperatures leads to the formation of mixed metal oxides (MMOs) with a porous structure, and rehydration of MMOs results in spontaneous structural reconstruction of the LDHs. Recently, Duan et al. revealed the application of the reconstruction effect for Mg-Al-LDH nanoplates on antireflection (AR) films for optical lenses [22]. As illustrated in Fig. 12.21a, the precursor films were fabricated by assembly of LDH nanoplates with the polyanion poly(sodium styrene-4-sulfonate), denoted as PSS, by an electrostatic layer-by-layer method, yielding LDH/PSS multilayer films, as shown in Fig. 12.21b. Then nanoporous films, as shown in Fig. 12.21c, were obtained by calcination of LDH/PSS films by utilizing the so-called sacrificial porogen (pore generator) approach: the selective removal of the porogen (PSS and the interlayer anions of LDH) resulted in the formation of MMO films with a nanoporous structure. This versatile process was particularly amenable to the creation of large-area uniform AR coatings on nonflat surfaces with precise control over thickness and optical properties. The transmission spectrum of the MMOn (n: layer number) film-coated quartz substrate showed an apparent increase in transmittance (98.6%, 650 nm) compared to the bare quartz (92%, 350-800 nm). The AR properties of the MMO film were also readily apparent in Fig. 12.21e. The letters below the quartz with the AR coating (left side) were much clearer than those below bare quartz (right side), demonstrating the increased transparency and reduced reflection of the MMO-coated quartz
slide. More significantly, after rehydration of the MMO films, the transformation to the nonporous LDH structure (shown in Fig. 12.21d) could be achieved by the structural “reconstruction effect” of LDH plates. By cycling the calcination-rehydration process, the AR properties of the coating could be switched on and off between porous and nonporous states (Fig. 12.21f).
