Hydrothermal synthesis of oriented ETS-4 films on porous α-alumina substrates

Preparation of ETS-4 films on porous α-alumina substrates was investigated using gel-forming synthesis mixtures (3.6SiO2:1TiO2:5.5Na2O:xH2SO4:230.2H2O, x = 4.4 or x = 3.6). The procedure involved two steps: direct in situ hydrothermal crystallization of seed layers, followed by their secondary growth. ETS-4 seed layers (thickness ∼ 3.0–4.0 μm) were obtained using gels with x = 4.4 when the autoclaves were rotated (72 RPM). The seed layers consisted of submicron plate-like crystals and were partially b-out-of-plane oriented. Films, obtained in the second step under static synthesis conditions, had a columnar microstructure and were highly (x = 4.4) or less preferentially (x = 3.6) oriented (b-out-of-plane). This orientation is desirable for many applications of ETS-4 films. The observable dimensions of crystals in films increased more than twice from these in seed layers. Film thickness increased up to ∼10.0–11.0 μm. Incorporation of new crystals into the film was not detected. Thus, these films were obtained by direct, evolutionary growth of seed crystals. Secondary growth conditions that effectively decouple nucleation from growth of seeds, and provide the highest growth rate along the b-direction, were established for ETS-4.