Synthesis and characterization of novel nanorod superstructures and twin octahedral morphologies of WO ₃ by hydrothermal treatment

Controlled WO ₃ morphologies, such as nanorods and octahedral structures, were synthesized by the hydrothermal technique using sulfate salts based structure directing agents (SDAs). The role of the sulfate salts' cation in controlling the shape, size and phase of WO ₃ nanomaterials was investigated by choosing sulfate salts whose cations are from d-bloc elements (FeSO ₄, (NH ₄) ₂Fe(SO ₄) ₂, CoSO ₄, CuSO ₄, ZnSO ₄), an alkaline earth metal (MgSO ₄) and a non-metal ((NH ₄) ₂SO ₄). In addition chloride (MnCl ₂) and acetate (Zn(CH ₃CO ₂) ₂) anion based SDAs were also used in order to clarify the role of sulfate ions in the growth of WO ₃ nanostructures. We controlled the pH of the reaction medium with oxalic acid. The obtained WO ₃ samples were investigated by SEM, micro-Raman, and XRD. At pH = 1, the WO ₃ samples exhibit novel superstructures consisting of aligned hexagonal nanorods, whereas at pH = 5.25, novel twin octahedral morphology with a cubic structure is obtained. The results demonstrate that the phase and morphology change is influenced by the pH and both the anion and the cation of the SDA. A growth mechanism for the obtained novel WO ₃ morphologies is presented. Graphical abstract: Highlights: Novel WO ₃ microstructures were obtained by SDAs mediated hydrothermal technique. At pH = 1, hexagonal WO ₃ nanorod superstructures were obtained. At pH = 5.25, cubic pyrochlore WO ₃ twin octahedral structures were obtained. First time experimental evidence of Raman spectrum of cubic pyrochlore WO _3.