Effect of pH in the hydrothermal preparation of monoclinic tungsten oxide

This paper presents the preparation of monoclinic WO3 by a one-step hydrothermal method. The effect of very acidic pH (0.1) and the significance of various additives (CH3COOH, NaClO4, Na2SO4) were investigated. To clarify the role of pH on the obtained crystal structure and morphology, every synthesis using pH 1 were repeated, and the effect of temperature, using 180 and 200 °C, was also studied. All samples prepared at pH 0.1 were pure, well crystallized monoclinic WO3 independently from the temperature, the presence and the quality of the additives. At 180 and 200 °C, applying CH3COOH and NaClO4 resulted nanosheets similar in size. With Na2SO4 additive at 180 °C sheets, at 200 °C sheets and also rods formed indicating that SO42− was a capping agent only at 200 °C. For comparison, at pH 1 at both temperatures the crystalline phases and the morphologies varied depending on the type of the additive. Hydrothermal preparation of m-WO3(Na2WO4, HCl, 180 °C, pH 0.1, additive*). [Display omitted] •The role of pH 0.1 in the hydrothermal reaction of Na2WO4 and HCl was investigated.•Monoclinic WO3 was prepared for the first time by one-step hydrothermal method.•The very acidic (0.1) pH has only decisive role in the formation of m-WO3.•Only Na2SO4 additive was a capping agent.