Jong Hyeon Lee

Surface Selective Deposition of PMMA on Layered Double Hydroxide Nanocrystals Immobilized on Solid Substrates

2009 WILEY-VCH Verlag Gm Assembly of nanometer-sized particles on various solid substrates has been the focus of intense interest in the development of new integrated functional materials. Layered double hydroxides (LDHs), known as anionic or hydrotalcite-like clays, have been investigated as a multifunctional inorganic material, for example, host materials, catalysts, sorbents, and bioinorganic and polymer–inorganic composites. To date in this area, most of the work performed has been on powder samples in colloidal solutions, where the bulk properties of the randomly assembled nanocrystals predominate over the contribution of the individual ones. LDH particles in the form of powders are considered one of the strongly correlated systems in the field of strong interparticle interactions involving electrostatic forces as well as hydrogen bonding. These hydrophilic ensembles of LDH particles are expected to be less reactive toward incoming reactants, especially organic anions such as carboxylates. In this context, we recently introduced a novel method of controlling the face-to-face assembly of [Mg4Al2(OH)12]CO3 nH2O (MgAl-LDH) nanocrystals on Si substrates in closely packed arrays with a highly-ordered orientation, which can be used for solvothermal anion exchange to give a drastic anisotropic size change that can be observed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Multilayer LDH nanocrystals on solid substrates not only make chemical reactions much more reliable, but also open up a new platform of other useful chemical interfaces difficult to achieve in a bulk system. Motivated by the need to assemble functional nanomaterials based on hybrid thin films, we demonstrate in the present study that the LDH nanocrystal organization method can provide a tunable reactive inorganic interface, depending on the liquid media and the surface characteristics of the applied substrates. We were able to precisely modify the surface potentials of the LDH nanocrystals in colloid solutions by changing the solvents, leading to the well-oriented LDH monolayer films acting as a reactive inorganic interface for the fabrication of polymer–LDH hybrid films. The hydroxide groups of LDH provide a facile route to produce the additional surface modification required to develop nanoscale inorganic composite thin films, such as superhydrophobic and polymer–inorganic hybrid films. Among the existing synthetic approaches to the preparation of polymer–inorganic hybrid nanocomposites, surface-initiated polymerization (SIP) allows for the high affinity of the graft polymer by employing the surface modification of the initiator on the surface of layered inorganic compounds. Graft polymers generated on clay surfaces by SIP have especially attracted considerable interest because of their practical applications involving improved mechanical, thermal, and barrier properties. The density of the grafting surfaces could be adjusted by employing different initiators. However, in most systems based on silicate materials reported to date it has been proved that the grafting polymer films have low polymer densities because of the stepwise generation of the initiator molecules to form a monolayer, the difficulty in introducing initiators and monomers into the clay surfaces, and the occurrence of unnecessary side reactions. Herein, we present a precise control method to increase the grafting polymer density on hydroxyl-rich LDH surfaces by using self-assembled monolayers (SAMs) to create a uniform initiator monolayer, in which we were able to change the area coverage of the assembled LDH nanocrystals on the substrates. To the best of our knowledge, this is the first example of the graft density control of polymer films by adjusting the area coverage of an immobilized LDH monolayer with a highly oriented structure on oxide, metal, and polymer substrates. Specifically, the incorporation of poly(methyl methacrylate) (PMMA) on the immobilized LDH surface provides us with new polymer–LDH hybrid films as well as a nanoscale reaction platform, which is extremely difficult in bulk systems. We investigated the orientation and area coverage of MgAl-LDH depending upon the applied substrates and solvents. Figure 1 shows that the tile-like LDH crystals were bound in parallel to the substrate planes on Si. Protic solvents gave a monolayer of MgAl-LDH with higher coverage of at least 50%, whereas nonprotic solvents such as toluene resulted in double and triple layers in some parts with a lower coverage of about 20%. Additional ultrasonic treatment in clean organic solvents for 30min produced no distinguishable changes in the particle assembly, implying that the adhesion is strong enough to resist the ultrasound-induced vibration. Figure 2a presents the surface coverage ratios, namely the percentage areas covered by the MgAl-LDH nanocrystals with respect to the whole Si surface, which are governed by the degree of attraction of MgAl-LDH to the substrates. Alcohols gave higher values of the lateral packing among the solvents. Most of the alcohols, denoted as Group I, gave ratios of around 70% to 90%.

Anthraquinone sulfonate modified, layered double hydroxide nanosheets for dye-sensitized solar cells.

Dye-sensitized solar cells (DSCs) have been extensively investigated for solar energy conversion by using various combinations of inorganic semiconductors and organic sensitizers because of their low cost, easy production, and high efficiency. For efficient visible-light absorption, various organic dyes have been intensively exploited because of their advantages, such as their high molar extinction coefficients and tunable optical bands, in which they are anchored on mesoscopic TiO2 semiconductors. Recently, inorganic semiconducting materials, such as quantum dots (CdSe, CdS,) and organometal perovskites, have been proposed as inorganic sensitizers in photovoltaic cells to overcome some drawbacks of the organic dyes such as their relatively low heat stability and narrow absorption bands. One possible approach to improving the inherent light-harvesting ability of the organic dyes is to hybridize them with nanosized multifunctional inorganic materials such as layered double hydroxides (LDHs), which can provide a stable chemical environment, higher heat or photostability, and are environmentally friendly. The LDHs, also known as anionic or hydrotalcite-like clays, are useful in new multifunctional systems such as biological carriers, catalysts, and hybrid optical layers. Recently, Duan et al. reported an ultrathin hybrid film consisting of LDH nanosheets and luminescent polyanions, in which the LDH nanosheets induced a welldefined photoluminescence of polymer monolayers that are individually separated by the exfoliated LDH nanosheets. Moreover, the LDHs provide a stable chemical environment to increase the photochemical function and thermal stability of the intercalated organic photochromic dyes. In this study, LDH nanosheets are suggested as the inorganic matrix in an attempt to induce an intense photochromic function of the organic photochromic dyes, anthraquinone sulfonate anion (AQS), that are chemically immobilized on the surface of the LDH nanosheets. Herein, we report a new hybrid light sensitizer for DSCs, in which the AQS anion is selected as the organic sensitizer and the LDH nanosheets as the inorganic host. This is believed to be the first example of hybrid LDH/organic nanosheets used as a light sensitizer in photovoltaic devices. The chemical structure and photochromic behavior of the LDH AQS nanosheets in formamide are shown in Figure 1a. The powder X-ray diffraction (XRD) pattern of LDH AQS microcrystals indicated a well-crystallized rhombohedral hydrotalcite-like, 3R1 phase with lattice parameters of a=3.05 and c=60.0 . An antiparallel arrangement of the AQS would be the best model by assuming that the length of the AQS anion was 12.9 . Notably, a transparent solution was obtained by ultrasound treatment for 10 min, indicating the successful exfoliation of the platelike LDH AQS microcrystals. Typical Tyndall light scattering of the resulting solution demonstrated the presence of exfoliated LDH nanosheets as shown in Figure 1a. Interestingly, the suspension showed a strong photoinduced coloration that was not seen in the AQS–formamide solution. In (4) of Figure 1b, the UV/Vis absorption spectra for the irradiated suspension of the LDH AQS nanosheets show extremely enhanced absorption bands in the range of 400–600 nm, where the absorption bands at 435 and 525 nm are characteristic signals for the reduction state of anthraquinone sulfonate (AQS ) that have a long-term stability in a high pH condition, whereas formamide (the solvent) might be oxidized during the photoreaction. Under continuous irradiation of the exfoliated LDH AQS solution shown in (2) of Figure 1b, the band intensities gradually increased as a function of the irradiation time and then decreased after the light was cut off. The rate constants for photocoloration and decay were 0.072 and 0.059 min , respectively. This may be [a] Dr. J. H. Lee, J. Chang, J.-H. Cha, Prof. D.-Y. Jung, S. S. Kim, Prof. J. M. Kim Department of Chemistry-BK21 and Sungkyunkwan Advanced Institute of Nanotechnology Institute of Basic Sciences, Sungkyunkwan University Suwon, 440-746 (Korea) Fax: (+82)31-290-7075 E-mail : dyjung@skku.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000703.

Enhanced Catalytic Activity of Platinum Nanoparticles by Exfoliated Metal Hydroxide Nanosheets

A novel 2‐dimensional catalytic system was developed in which platinum nanoparticles (Pt NPs) were immobilized on exfoliated MgAl‐layered double hydroxide (LDH) nanosheets through an electrostatic self‐assembly between negatively charged Pt NPs and positively charged LDH nanosheets. The LDH nanosheets effectively provided the large double sides of hydroxide functionality to absorb the Pt NPs, as well as fast diffusion rates of the incoming reactants into catalyst surfaces. This new nanostructure improved the rate of reaction, turnover frequency and reaction durability of Pt NPs on LDH nanosheet without significant loss in conversion efficiencies for the reduction of p‐nitrophenol into p‐aminophenol by NaBH4, maintaining more than 97% of catalytic conversions compared to free Pt NPs as well as commercial Pt/C catalyst.