High Surface Area to Volume Ratio 3D Nanoporous Nb2O5 for Enhanced Humidity Sensing

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.