Formation of oxygen vacancies and Ti3+ state in TiO2 thin film and enhanced optical properties by air plasma treatment

TLDR

The study used air‑plasma treatment of Fe‑ and Co‑doped TiO₂ films, varying exposure from 0 to 60 s, and characterized surface species (Ti³⁺, Ti⁴⁺, O²⁻, vacancies, OH) by XPS and UV‑Vis to investigate how doping influences charge trapping and recombination. Air‑plasma treatment of Fe‑ and Co‑doped TiO₂ films increases Ti³⁺ and oxygen vacancies, boosting optical absorbance and shifting the absorption edge—red‑shifted in Fe‑doped and blue‑shifted in Co‑doped films—while preserving transparency.

Abstract

Abstract This is the first time we report that simply air plasma treatment can also enhances the optical absorbance and absorption region of titanium oxide (TiO 2 ) films, while keeping them transparent. TiO 2 thin films having moderate doping of Fe and Co exhibit significant enhancement in the aforementioned optical properties upon air plasma treatment. The moderate doping could facilitate the formation of charge trap centers or avoid the formation of charge recombination centers. Variation in surface species viz. Ti 3+ , Ti 4+ , O 2− , oxygen vacancies, OH group and optical properties was studied using X-ray photon spectroscopy (XPS) and UV-Vis spectroscopy. The air plasma treatment caused enhanced optical absorbance and optical absorption region as revealed by the formation of Ti 3+ and oxygen vacancies in the band gap of TiO 2 films. The samples were treated in plasma with varying treatment time from 0 to 60 seconds. With the increasing treatment time, Ti 3+ and oxygen vacancies increased in the Fe and Co doped TiO 2 films leading to increased absorbance; however, the increase in optical absorption region/red shift (from 3.22 to 3.00 eV) was observed in Fe doped TiO 2 films, on the contrary Co doped TiO 2 films exhibited blue shift (from 3.36 to 3.62 eV) due to Burstein Moss shift.