Solid-state compounds have historically been prepared through high-temperature solid-solid reactions. New mechanistic understanding of these reactions suggests possible routes to metastable compositions and structures as well as to thermodynamically stable, low-temperature phases that decompose at higher temperatures. Intermediate-temperature synthetic techniques, including flux and hydrothermal methods, as well as low-temperature intercalation and coordination reactions, have recently been developed and have been used to prepare unprecedented materials with interesting electronic, optical, and catalytic properties. The trend in modern solid-state synthesis resembles increasingly the approach used in small-molecule chemistry, in the sense that attention to reaction mechanism and the use of molecular building blocks result in an ability to prepare new materials of designed structure.
Monolayer and multilayer thin films consisting of anionic
α-zirconium phosphate (α-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 α-ZrP begins to
occur when enough tetra(n-butylammonium) hydroxide
(TBA+OH-) has been added to exceed
single-layer packing
of TBA+ ions (x ≈ 0.50) in the intercalation
compound
Zr(HPO4)2-x
(TBA+PO4
-)
x
·nH2O.
The
identical contrast of many sheets in TEM micrographs suggests that the
suspension is
unilamellar. Alternately dipping cationic substrates into
α-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 α-ZrP, consistent with some corrosion during the
exfoliation reaction. The α-ZrP/PAH layer pair thickness is 13/14.7 Å, 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 α-ZrP or PAH
monolayer is deposited
in each adsorption step.
Two integrated systems for light-induced vectorial electron transfer are described. Both utilize photosensitized semiconductor particles grown in linear channel zeolites as components of the electron transfer chain. One system consists of internally platinized zeolites L and mordenite containing TiO 2 particles and methylviologen ions, with a size-excluded photosensitizer, tris(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium (RuL 3 2+ ), adsorbed on the external surface of the zeolite/TiO 2 composite. In the other system, Nb 2 O 5 replaces TiO 2 . The kinetics of photochemical electron transfer reactions and charge separation were studied by diffuse reflectance flash photolysis. Despite very efficient initial charge separation, the TiO 2 system does not generate hydrogen photochemically in the presence of an electrochemically reversible, anionic electron donor, methoxyaniline N,N′-bis(ethyl sulfonate). Only the Nb 2 O 5 -containing composites evolved hydrogen photochemically under these conditions. These results are interpreted in terms of semiconductor band energetics and the irreversibility of electron transfer from Nb 2 O 5 to intrazeolitic platinum particles.
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