Defect engineering in TiO2 thin films: Recent advances in optical and electronic properties and sensing applications

Abstract

Variable Amount of Nitrogen Doping of TiO₂ To create a Variable amount of defects (05 by oxidizing nitrogen, and the resulting N-defect pairs can significantly modulate both its physical and functional properties, especially for gas sensor applications. Here, we critically evaluate progress reported in the literature between 2024 and 2026 which investigate features such as oxygen gaps, Ti³⁺ states and doping defects and how these phenomena can affect crystal structure, optical attributes and electronic properties. Defects can be modulated through the manipulation of deposition parameters, heat treatment, iron and non-metal doping, plasma or hydrogen treatment and heterojunction fabrication. Low interstitial and substitutional defect concentrations can alter phase stability, affect the stresses at a microscale level, determine crystal sizes, establish midgap states in between the bands gap or broaden its absorption response towards wavelengths ranging from 400 to 950 nm (visible light), as well as shift position of the Fermi level and control the charge carrier density/transport of carriers. Gas Sensors: Oxygen vacancies and defects on the sensor surface are potential active sites where oxygen and gas molecules gets adsorbed, increasing resistivity change when exposed to reducing or oxidizing gases. Increasing control of membrane thickness, surface porosity, photoactivation and interface geometry is also helpful for enhanced sensitivity and a lower operational temperature. On the contrary, an excessive defect density is responsible for borrowing oxygen atoms from the lattice, resulting in secondary phases, deep-level charge trapping centers and instability in general. The aim of the review is to identify defect types, concentrations, locations and charge states with optimal performance but also focuses on developing reproducible and quantitative techniques for accurate characterization of the defects and testing them under real-life conditions including humid environment, interfered gases and long-run operation.