The effect of feedstock concentration on the crystal phase, morphology, and optical properties of WO₃ nanostructures

In this study, the effect of feedstock concentration on the synthesis of WO₃ nanostructures in a one-step hydrothermal process was investigated. According to our experiments, when titrating aqueous Na₂WO₄$2H₂O with HCl solutions of different concentrations to a constant pH of 1.5, after identical hydrothermal treatments at 180 °C, the morphology, crystal size and phase composition as well as the optical properties of the products could be tuned. Furthermore, by applying Na₂SO₄ in the precursor solution, we get a further degree of freedom to alter the structural and optical properties of the products. The precursor concentration dependent optical band gap energy values (that vary from ∼2.5 eV to ∼2.9 eV) are in good agreement with the associated hexagonal (from 32.7% to 65.5%) and orthorhombic (from 67.3% to 34.5%) phases of nanostructured tungsten oxide products. The calculated Urbach energies can be well explained by the oxygen deficient crystals. The latter is also coherent with measured photoluminescence (multiple peaks between 400 and 580 nm) that indicate abundant defect states in the tungsten oxide products.