High–performance capacitive humidity sensors based on (Sb5+/In3+) co–doped TiO2 ceramics with low hysteresis and rapid recovery

Abstract

In this study, (Sb⁵⁺/In³⁺)_xTi_1-xO₂ ceramics (x = 2.5–10 %) were developed to evaluate their potential for capacitive humidity sensing applications. All compositions exhibited a single–phase rutile TiO₂ structure with expanded lattice parameters due to substitution of Ti⁴⁺ by larger In³⁺ ions. The ceramics showed dense microstructures with high relative densities (93.5–98.9 %) and homogeneous dispersion of dopants. Increasing dopant concentrations promoted the formation of Ti³⁺ species and oxygen vacancies, which contributed to improved dielectric and sensing performance. The optimized composition, (Sb⁵⁺/In³⁺)_0.05Ti_0.95O₂, exhibited a high dielectric permittivity (>10³) and low dielectric loss (∼0.01 at 1 kHz). Under 30–90 % RH, this sample achieved a capacitance variation of ∼564 %, a low maximum hysteresis error (γ_H∼1.2 %), and excellent repeatability across multiple humidity cycles. It exhibited a practical 63 % response within ∼0.7 min and a rapid recovery of ∼0.4 min, which are considerably faster than those of conventional bulk giant–dielectric ceramics. Notably, the sensor maintained stable operation across 20–40 °C with minimal γ_H of 1.2–1.4 % and good linearity (nonlinearity <0.1 %). In particular, the (Sb⁵⁺/In³⁺)_0.05Ti_0.95O₂ showed high linearity (R² = 0.985) in the 50–90 % RH range, making it well–suited for agricultural and greenhouse humidity monitoring. These findings highlight the promise of chemically stable TiO₂–based ceramics as high–performance capacitive humidity sensors for practical environmental applications.