Abstract
Sensors based on metal oxide platforms offer ease of device fabrication and simple sensing operation. As a metal oxide platform, highly nanoporous niobium oxide (Nb
2
O
5
) films consisting of unique three-dimensional vein-like structures can be efficiently used for developing humidity sensors. In this work, nanoporous Nb
2
O
5
films (with different thicknesses of ∼ 1
μ
m, ∼ 2
μ
m, and ∼ 4
μ
m) were prepared by anodization of niobium foil for 30 min, 60 min, and 120 min. Electron, x-ray, atomic, and vibrational microscopies and spectroscopies were used for characterizing the morphological and structural properties of the Nb
2
O
5
films. The analysis revealed that the nanoporous Nb
2
O
5
exhibited hierarchical vein-like structures with orthorhombic crystalline orientation, and their surface roughness showed a proportional increase with the anodization duration. Metal–semiconductor–metal humidity sensors based on nanoporous Nb
2
O
5
with platinum electrodes were tested in a humidity chamber under conditions of 40% to 90% relative humidity (RH) and different bias voltages. According to the obtained results, the ∼ 4-
μ
m-thick nanoporous Nb
2
O
5
presented the highest relative sensitivity of 216.5 under a bias voltage of 5 V, taking advantage of its extremely porous structure. These sensors provide high surface area to volume ratio, leading to highly effective affinity and interactions between surface-active sites and water molecules.