Characterization of Nb-doped WO3 thin films produced by Electrostatic Spray Deposition

In this paper we provide full characterization of Nb-doped WO3 thin films produced by Electrostatic Spray Deposition on glass substrates. The films were produced by electrospraying a tungsten isopropoxide in 2-propanol precursor solution in a temperature-controlled environment. Film morphologies and crystal structures were characterized by Scanning Electron Microscopy, Transmission Electron Microscopy, and X-Ray Diffraction, whereas band gap values, composition, and oxidation state of the materials were determined by Ultraviolet–Visible Spectroscopy, Energy Dispersive X-Ray, and X-Ray Photoemission Spectroscopy, respectively. It is shown that highly-porous tree-like morphologies can be achieved by adjusting the deposition temperature to 325 °C. Under these conditions, the size of the primary particles of the films ranges from 150 to 200 nm, and their crystallinity can be varied from completely amorphous for the as-deposited samples to hexagonal structures for the annealed samples. The measured Nb content of the doped samples was found to be similar to that of the precursor solution, leading to an increase of the band gap of 0.23 eV as compared to the pure WO3 samples.

► Nb-doped WO3 nanostructured layers were produced by Electrostatic Spray Deposition.
► Layer morphologies can be controlled by altering the deposition temperature.
► A qualitative model is provided to describe a tree-like morphology formation.
► Annealed Nb-doped and undoped samples are organized in a WO3 hexagonal structure.