Asymmetric In3+/Nb5+ co–doping strategy for optimized dielectric and humidity sensing properties in rutile TiO2 ceramics

The long-held belief that balanced aliovalent In 3+ /Nb 5+ co-doping optimizes rutile TiO 2 is challenged here, as we demonstrate that Nb 5+ -rich compositions provide superior dielectric and sensing properties. Nb x In 0.05 Ti 0.95− x O 2 ( x = 0.01–0.20; x %NbInTO) ceramics were synthesized and sintered at 1400°C for 5 h. Under this sintering condition, structural analyses confirmed the presence of single-phase rutile for x ≥ 0.025 with gradual lattice expansion and grain growth as the Nb 5+ concentration increased, while Raman and X-ray photoelectron spectral analyses verified the existence of Ti 3+ states together with oxygen vacancy defects in the x %NbInTO series. The dielectric response was highly composition-dependent. The 5%NbInTO achieved an optimal balance of high permittivity (ε′ = ∼3.1 × 10 3 ) and low dielectric loss (tanδ = ∼0.052), while further Nb 5+ enrichment (10% and 20%NbInTO ceramics) resulted in an ultrahigh ε′ of >10 4 at the expense of higher dielectric loss. Remarkably, humidity sensing was enhanced in Nb 5+ -rich ceramics, with x = 10%–20% exhibiting large capacitance variations (172%–367%), high sensitivity (∼27.4 nF/%RH at 1 kHz), and ultralow hysteresis errors (0.5%–1.4%). Fast dynamic sensing behavior was also observed, including response/recovery times as short as 1.1–1.6/0.1–1.7 min. These findings redefine the role of asymmetric In 3+ /Nb 5+ ratio in rutile TiO 2 , establishing Nb 5+ -rich compositions as promising candidates for high-performance dielectric devices and capacitive humidity sensors.