Niobium pentoxide (Nb2O5 or niobia) exhibits several key properties that make it an excellent optical material. These include exceptional stability, resistance to acidic and basic environments, high optical transmission and refractive index, and minimal light scattering, which further enhance its performance in optical applications. Among the various methods for depositing Nb₂O₅ thin films, sol–gel stands out as a particularly promising approach due to its versatility, scalability, and the ability to precisely tailor material properties for a broad range of applications.
In the current study single- and multi-layered niobia coatings were prepared by spin-coating a niobium sol, which was synthesized using niobium chloride as the precursor and ethanol and water as solvents, followed by annealing at 320oC to form the niobia film. Doped niobia films were prepared by incorporating commercially available SiO2 (Ludox) and Au nanoparticles (NPs) into the sol before spin-coating of the films. After annealing the silica-doped films, they were subjected to chemical etching for varying durations to remove the silica phase. This process generated porosity within the films, which in turn enabled the tailoring of both their optical and sensing properties.
The morphology of the films was investigated using transmission electron microscopy (TEM). The optical parameters and film thicknesses were determined by nonlinear curve fitting of the reflection spectra, and the results were cross-validated through additional ellipsometric measurements. Effective medium approximation was used to assess the degree of porosity. The sensing properties of the films were evaluated by using both quartz crystal microbalance (QCM) and optical reflectance spectra measurements, recorded prior to and during exposure to the analyte (acetone vapors).
The study demonstrated that silica NPs enhanced the porosity of the niobia coatings, resulting in vapor-sensitive films with tunable optical and sensing properties. The gold NP doping further improved the sensing performance of the studied films.
