Hydrothermal Synthesis of Tungsten Trioxides Using Citric Acid as Controlling Agent

Nano-scaled orthorhombic tungsten trioxide rectangular flakes were synthesized by hydrothermal method using citric acid as controlling agent. The phase composition of the products strongly depends on the reaction temperature. XRD results show that the product is tungsten trioxide monohydrate (WO3·H2O) mixed with a small quantity of monoclinic tungsten trioxide (γ-WO3) at 393 K, γ-WO3 at 413 K, and orthorhombic WO3 (β-WO3) at 433 K and 453 K, respectively. SEM images demonstrate that the products obtained at different reaction temperatures are all rectangular flakes with lamellar structure. All the products have almost the same specific surface areas between 8~9 m2·g-1. The phase composition, morphology and specific surface area of the products can be controlled by the molar ratio of citric acid to tungstic acid (nCit/nTA). When nCit/nTA=0, the product is the mixture of h-WO3 (hexagonal system WO3) and WO3·1/3H2O, and have a morphology of nanorods; when nCit/nTA=0.5 or nCit/nTA=1, the product is γ-WO3, and have a morphology of rectangular flakes; when nCit/nTA=2, the product is β-WO3, with the same morphology of rectangular flakes. In the case of adding citric acid, the specific surface area of the products decreases with the increase of nCit/nTA.