Characterization of zirconium phosphate polycation thin films grown by sequential adsorption reactions

Monolayer and multilayer thin films consisting of anionic alpha-zirconium phosphate (alpha-ZrP) sheets and polycations (poly(allylamine hydrochloride) (PAH), cytochrome c) were characterized by transmission electron microscopy (TEM), ellipsometry, UV-visible absorbance spectroscopy, reflectance FT-IR, XPS, and X-ray diffraction. Titration and powder X-ray diffraction experiments confirm that exfoliation of alpha-ZrP begins to occur when enough tetra-(n-butylammonium) hydroxide (TBA(+)OH(-)) has been added to exceed single-layer packing of TBA(+) ions (x approximate to 0.50) in the intercalation compound Zr(HPO4)(2-x)(TBA(+)PO(4)(-))(x) . nH(2)O. The identical contrast of many sheets in TEM micrographs suggests that the suspension is unilamellar. Alternately dipping cationic substrates into alpha-ZrP-containing suspensions and aqueous PAH gives a multilayer film that resembles the corresponding bulk intercalation compound. X-ray photoelectron spectra of multilayer films show that they are Zr-rich, relative to alpha-ZrP, consistent with some corrosion during the exfoliation reaction. The alpha-ZrP/PAH layer pair thickness is 13/14.7 Angstrom, as measured by ellipsometry/X-ray diffraction, respectively. A 13-layer pair film is sufficiently well-ordered in the stacking direction to give a Bragg peak in the diffraction pattern. The agreement between the bilayer thickness and the total film thickness, measured from Kiessig fringes in the low-angle part of the diffraction pattern, confirms that only a single dense alpha-ZrP or PAH monolayer is deposited in each adsorption step.