Phase Changes in WO₃ Films upon Heating and Cooling

Abstract

WO₃ is a promising photocatalyst for drinking water and wastewater treatment. A reproducible crystal structure would be necessary to investigate and optimise the catalytic properties, as amorphous WO₃ layers are less suitable for photocatalysis. Temperatures between 300 and 700 °C are often used to produce crystalline WO₃ layers. During heating, several phase transformations can occur, which are more or less reversible. Studies on powders have shown that at least seven modifications of WO₃ can exist in a temperature range from room temperature to 1000 °C [1, 2]. At room temperature, WO₃ can exist in monoclinic, orthorhombic and triclinic structures. However, determining the crystal structure of films is considerably more difficult than for powders, as transfor-mations can take place over larger temperature ranges which can depend primarily on the layer thick-ness, the temperature treatment, the substrate, the manufacturing method and the starting products. In this work, the structural changes during heating of amorphous layers to 750 °C were investigated by in situ XRD and compared with the changes during cooling of the calcined layers. Both fused silica and FTO-coated borosilicate glass were used as substrates [3]. The coating was applied using spray pyrolysis of peroxotungstic acid with a substrate temperature of 250 °C. In the range from 650 to 730 °C, a phase transformation could be clearly observed both during heating and cooling, which in powders was attributed to the transformation of orthorhombic (Pbcn) to monoclinic (P21/c). A transformation from the monoclinic to the orthorhombic or the triclinic phase could not be demonstrated. The slight difference in the diffraction peaks did not allow a distinction to be made using this method for polycrystalline layers, in contrast to powders. A clear specification of the crystal structure of WO₃ layers is therefore not possible with the X-ray examination described and requires more specific methods.