A “Mix and Match” Ionic−Covalent Strategy for Self-Assembly of Inorganic Multilayer Films

Abstract

Multilayer thin films consisting of anionic α-zirconium phosphate (α-ZrP) sheets, tetrameric zirconium hydroxide cations [Zr 4 (OH) 8 (H 2 O) 16 ] 8+ (Zr 4 8+ ), 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 C 16 BPA solutions generates a mixed ionic/covalent multilayer structure. The tetrameric Zr 4 8+ cation adsorbs onto the α-ZrP surface, providing a covalent anchoring point for the growth of the C 16 BPA layer. Adsorbing a second layer of zirconium ions onto the C 16 BPA 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/Zr 4 8+ /C 16 BPA/Zr 4 8+ 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 C 16 BPA, are linear with the C 16 BPA 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.