A. I. Gavrilov

Hydrothermal/microwave and hydrothermal/ultrasonic synthesis of nanocrystalline titania, zirconia, and hafnia

We compare the physical-chemical properties (X-ray diffraction (XRD), powder X-ray diffraction, TGA, TEM, and BET) of titania, zirconia, and hafnia nanopowders (d = 10–15 nm) synthesized from amorphous titanyl hydroxide TiO2 · nH2O, zirconyl hydroxide ZrO(OH)2 · nH2O, and hafnyl hydroxide HfO(OH)2 · nH2O using hydrothermal (HT), hydrothermal/microwave (HT-MW), and hydrothermal/ultrasonic (HT-US) methods at 150, 180, and 250°C with treatment lasting 0.5–24 h. Titania, zirconia, and hafnia crystallization from amorphous hydroxides is substantially enhanced, and the percentage of the thermally stable zirconia phase (m-ZrO2) in the HT-MW and HT-US processes increases compared to conventional HT synthesis. The observed similar effects have completely different causes. A common factor in both cases is likely the uniformity of heating of treated suspensions. Local overheating in the reaction mixture, which appears during both ultrasonication and microwave treatment, can also play a role in accelerating the hydrothermal processes.

Synthesis and photocatalytic activity of anatase-based aerogels

VxTi1–xO2 (x = 5 and 10 mol %) solid solutions have been synthesized through supercritical drying in isopropanol at t = 250°C and p = 10 MPa. Their physicochemical properties and photocatalytic performance have been compared to those of an earlier synthesized ZnxTi1–xO2 aerogel containing 10 mol % Zn [1]. It has been shown that increasing the vanadium content of VxTi1–xO2 from 5 to 10 mol % leads to a decrease in hydrogen evolution rate in methanol/water splitting reaction under UV irradiation from 190 to 32 µL/(min gcatal), whereas in the case of the anatase-based aerogel solid solution containing 10 mol % Zn an opposite picture is observed: the hydrogen evolution rate in methanol/water splitting reaction under UV irradiation increases sharply to 700 µL/(min gcatal).

Specifics of hydrothermal synthesis of oriented zinc oxide nanorods on metallic zinc substrates

Zinc oxide nanostructures are prepared hydrothermally in the presence of ethylenediamine (EDA). The morphology and photoluminescent properties of final products are studied as functions of synthesis temperature, synthesis time, and EDA concentration. A decrease in EDA concentration to 30% favors the formation of more perfect and more ordered structures. Blank experiments show that hydrothermal synthesis without organic reagents does not produce nanostructures. When samples are sheltered from convective flows in the cell, the rod growth direction is dictated by the grain orientation in the foil. When nanorods are formed under low supersaturations (in the absence of convective flows), oxygen nonstoichiometry arises in the nanorods and appears in photoluminescence spectra as increased peak intensities in the green spectral range.