Defect Engineering of Titanium Dioxide Nanostructures: From Fundamentals to Applications

Abstract

Titanium dioxide (TiO2) is widely used in photocatalytic and photoelectrochemical applications due to its low cost, stability, and abundance. However, its wide band gap limits its light absorption to the ultraviolet region, and fast electron-hole recombination reduces its efficiency. Strategies to overcome these challenges include doping, heterojunction formation, and defect engineering.

Recent studies have shown that creating controlled defects in TiO2 can improve its optical and electrical properties. One promising defect engineering approach is laser based modification, which can precisely introduce defects into TiO2 structures without the use of high temperatures, pressures, or toxic chemicals. While laser processing has shown potential to enhance optical and electrical properties, the specific influence of laser parameters and defect locations on performance remains unclear. Additionally, hydrogen doping is known to introduce beneficial shallow states, yet in situ hydrogen doping during thin film growth has not been fully explored.

This research explores defect engineering of TiO2 nanostructures through controlled laser treatments and hydrogen doping to enhance their photocatalytic, photoelectrochemical, and electronic performance. Two different laser systems, femtosecond and nanosecond lasers, are used to systematically study the effects of laser parameters on defect formation, band structure modification, and performance improvement.

Femtosecond laser irradiation is applied to TiO@ nanoparticles to create localized surface defects without damaging the bulk structure or causing phase transformation. The introduction of oxygen vacancies and Ti3+ states result in improved photocatalytic activity under UV illumination by reducing electron-hole recombination and increasing surface active sites.

In contrast, nanosecond laser irradiation produces black TiO2 with significant band gap narrowing, phase transformation from anatase to rutile, and bulk defect formation. This material shows enhanced photocatalytic performance under visible light due to extended light absorption and modified electronic properties. The effect of defect location, i.e. surface versus bulk, on charge separation and recombination is also investigated, revealing that controlled surface defects promote better photoelectrochemical performance under UV light.

In addition, an in-situ hydrogen doping method is developed using plasma-assisted atomic layer deposition (PAALD) to produce hydrogen-doped TiO$_2$ thin films. This approach introduces shallow defect states, modifies the electronic structure, and improves the electrical conductivity of TiO2. The doped films were integrated into metal-insulator-metal (MIM) diodes, where significant reductions in resistance and improvements in responsivity are achieved, showing their potential for high-frequency applications.

This work provides new insights into laser-based and deposition-based defect engineering strategies for TiO2, highlighting their potential to improve solar-driven processes and electronic device performance.