Controllable Morphology of Engelhard Titanium Silicates ETS-4: Synthetic, Photocatalytic, and Calorimetric Studies

Abstract

Titanosilicate ETS-4 materials with three different morphologies were synthesized hydrothermally by controlling the pH of the synthesis gel mixtures. Their morphology changed from thin rectangular plate monolithic crystals to double-fan-like polycrystalline aggregates as the pH decreased. The crystallization time depended on the initial pH. The effect of morphology on photocatalytic activity was investigated using the photodegradation reaction of Rhodamine B under visible light irradiation. The double-fan-like material exhibited significantly greater photodegradation activity than the other two ETS-4 samples and Degussa P25 titania. Powder X-ray diffraction, electron microprobe analysis, scanning electron microscopy, Raman spectroscopy, and diffuse reflectance ultraviolet−visible spectroscopy were performed to find possible reasons for the photocatalytic activity difference of the three samples. The enthalpies of formation from oxides for all the ETS-4 materials were investigated by high temperature oxide melt calorimetry. The results showed that the three samples with different morphologies have differences in their enthalpies of formation, which, although small, are bigger than likely to arise from surface area effects alone, and the product with most Ti−OH groups has the smallest thermodynamic stability. The calorimetric and spectroscopic data together provide evidence that the high photocatalytic activity of the double-fan-like aggregates is linked to their high Ti−OH content.