Growth of AlPO₄-5 and CoAPO-5 films from amorphous seeds

The growth of aluminophosphate AlPO₄-5 (AFI structure with one-dimensional parallel and straight channels of 7.3 Å in cross section) and cobalt-substituted AlPO₄-5 (CoAPO-5) films by hydrothermal treatment is investigated. In situ growth on bare (unmodified) silicon substrates using different growth temperatures, times, and precursor mixture compositions was carried out first and yielded films that exhibited either preferred orientation and poor coverage or good coverage but random orientation. Introducing seeded growth, which involved functionalization and seeding of the substrate with the amorphous fraction of an AlPO₄-5-containing powder followed by hydrothermal seed growth, c-oriented, well-intergrown films were obtained with a variety of morphologies depending on the synthesis mixture composition and growth conditions. The effect of two different structure directing agents (SDAs), namely triethylamine and tripropylamine, was also examined. Controlled experiments by systematically varying secondary growth duration were performed in order to find the optimal growth time that yields complete coverage while minimizing the degree of misorientation. Seeded growth using dilute precursor mixtures favors c-out-of-plane growth yielding films with a thickness of several micrometers, while reducing the water/Al₂O₃ molar ratio and executing secondary growth under the appropriate hydrothermal conditions favored in-plane growth yielding well-intergrown films with sub-micron thickness. SEM, XRD, and UV-vis spectroscopy were used to characterize the synthesized AlPO₄-5 and CoAPO-5 films.