Focus on Fundamental Science: What are layered double hydroxides (LDHs)?
In this new feature in the 6th Biannual Research Report ‘Focus on Fundamental Science’, the purpose of which is to explain the important underlying scientific concepts of our programme to an interested layman, WANG Jing Jing (PhD student, IRP1, NUS) explains what layered double hydroxides are and how they can be applied.
Layered double hydroxides (LDHs) are a large family of two-dimensional (2D) inorganic solid materials which can be found not only in laboratories and man-made products but occur foremost in nature in the form of clays. They consist of positively-charged host layers and exchangeable interlayer anions, which can be expressed by the formula [M2+1-xM3+x (OH)2](An-)x/n ·mH2O (M2+ and M3+are divalent and trivalent metals, respectively; An- is the interlayer anion). The value of x is equal to the molar ratio M2+ / M2+ + M3+ and is generally in the range 0.2–0.33; water and anions are present in the interlayer galleries. Each hydroxyl group in the LDH layers is oriented toward the interlayer region and may be hydrogen bonded to the interlayer anions and water molecules, as shown in the figure.
LDHs can be seen as derived from hydroxides of divalent cations, by oxidation or cation replacement in the metal layers, so as to give them an excess positive electric charge and intercalation of extra anion layers between the hydroxide layers to neutralize that charge. LDHs can be formed with a wide variety of anions in the intercalated layers, such as Cl−, Br−, NO3−, CO22− and SO42−.
The idealized structure of carbonate-intercalated LDHs with different M2+/M3+ molar ratios showing the metal hydroxide octahedra stacked along the crystallographic c-axis, as well as water and anions present in the interlayer region.[1]
Properties of layered double hydroxides
1. The MII and MIII cations are distributed in a uniform manner in the hydroxide layers and it’s easy to tune the types and ratios of these metal ions without altering the structure. The precisely-controlled chemical composition provides great potential to disperse and tune active sites at the atomic scale. [2]
2. The anions located in the interlayer regions can be replaced easily and a wide variety of anions can be incorporated, ranging from simple inorganic anions (e.g. CO32−) through organic anions (e.g. benzoate, succinate) to complex biomolecules (e.g. DNA).
3. LDH microcrystals can be exfoliated into positively-charged 2D nanosheets, which can serve as building blocks for assembly with various catalytically active anions.
4. The LDH morphology on the micro-nanoscale can be easily manipulated by various facile synthetic strategies, like in-situ growth, biological template, and electrochemical synthesis. [3]
5. The transformation of the LDHs precursors to mixed metal oxides (MMOs) or metal/metal oxide composites upon calcination in an air or hydrogen atmosphere, can further improve the structure modulation strategies and thus expand the applied range of LDH materials as catalysts and adsorbents. [4]
Applications of layered double hydroxides
Based on the inherent merits of easy tunability of chemical composition, anion exchange properties, various synthesis approaches as well as the unique topotactic transformation behavior, LDH-based materials become promising alternatives in a wide variety of fields, including water treatment, energy conversion and storage, magnetism, pharmaceutics, thin film devices, ion exchange materials, elastomer composites, and fire-retardant additives. In particular, LDHs have been widely employed in heterogeneous catalysis and adsorption.
For heterogeneous catalysis, LDH materials can serve as supports for active components such as metal nanoparticles, which prevent the sintering/aggregation of them by means of an exterior confinement effect from the LDH layers.[5] Moreover, the topological transformation of the LDH precursors containing VIII element (e.g., Fe, Co, Ni) or some noble metals (e.g., Pd) offers a facile strategy to prepare supported metal nanocatalysts.[6]
For the application to serve as adsorbents, LDH materials have various structural units, such as positive ion or basic sites, to provide specific active sites for many adsorbates with strong chemical affinity. In terms of CO2 capture, the MMO (mixed metal oxides) materials derived from the calcination of LDHs have been identified as the most suitable one in the high temperature range. The alkaline component (e.g., MgO, CaO) in MMOs can serves as the active species for the adsorption of acidic CO2 molecule.[7]
Conclusion
Due to the versatility and unique features, LDH-based nanomaterials have been widely applied to various fields. It is believed that with the rapid advance in synthetic and characteristic strategies of nanoscience and nanotechnology, the design and controlled synthesis of LDH-based materials would lead to more and more practical applications.
Reference:
[1] J. A. Gursky, S. D. Blough, C. Luna, C. Gomez, A. N. Luevano, E. A. Gardner, J. Am. Chem. Soc. 2006, 128, 8376;
[2] Q. Wang, D. O’Hare, Chem. Rev. 2012, 112, 4124.
[3] a) C. Li, J. Zhou, W. Gao, J. Zhao, J. Liu, Y. Zhao, M. Wei, D. G. Evans, X. Duan, J. Mater. Chem. A 2013, 1, 5370. b) Y. Zhao, M. Wei, J. Lu, Z. L. Wang, X. Duan, ACS Nano 2009, 3, 4009. c) L. Tian, Y. Zhao, S. He, M. Wei, X. Duan, Chem. Eng. J. 2012, 184, 261.
[4] M.-Q. Zhao, Q. Zhang, W. Zhang, J.-Q. Huang, Y. Zhang, D. S. Su, F. Wei, J. Am. Chem. Soc. 2010, 132, 14739.
[5] J. Wang, L. Zhao, H. Shi, J. He,
Angew. Chem. Int. Ed. 2011, 50, 9171.
[6] P. Li, L. Wen, J. S. Dennis, H. C. Zeng, Adv. Funct. Mater. 2016, 31, 5658.
[7] Z. Yang, S. Ji, W. Gao, C. Zhang, L. Ren, W. W. Tjiu, Z. Zhang, J. Pan, T. Liu, J. Colloid Interf. Sci. 2013, 408, 25.
