A 'mix and match' ionic-covalent strategy for self-assembly of inorganic multilayer films

Multilayer thin films consisting of anionic α-zirconium phosphate (α- ZrP) sheets, tetrameric zirconium hydroxide cations [Zr₄(OH)₈(H₂O)₁₆]⁸⁺ (Zr₄⁸⁺), and alkanediylbis(phosphonic acid) (C(n)BPA) have been grown on silicon and gold surfaces by sequential adsorption reactions. The thin films were characterized by ellipsometry, X- ray diffraction, reflectance infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Alternately dipping cationic substrates into exfoliated α-ZrP-containing suspensions, aqueous zirconium oxychloride, and ethanolic C₁₆BPA solutions generates a mixed ionic/covalent multilayer structure. The tetrameric Zr₄⁸⁺ cation adsorbs onto the α-ZrP surface, providing a covalent anchoring point for the growth of the C₁₆BPA layer. Adsorbing a second layer of zirconium ions onto the C₁₆BPA layer allows one to continue the layer growth sequence using either covalent (metal/phosphonate) or ionic (α-ZrP/polycation) interlayer connections. A multilayer film with a repeating α- ZrP/Zr₄⁸⁺/C₁₆BPA/Zr₄⁸⁺ sequence is sufficiently well-ordered in the stacking direction to give a Bragg peak in the diffraction pattern. The intensities of infrared absorbances in the symmetric and asymmetric C-H stretching regions, which arise from C₁₆BPA, are linear with the C₁₆BPA layer number. This 'mix and match' approach provides a versatile means of assembling multilayer heterostructures from both ionic and covalent building blocks, with essentially any desired sequence of layers.