Phase evolution and optical properties of nanometric Mn-doped TiO2 pigments

Abstract

This paper aims to evaluate the effects of calcination temperature and manganese doping on optical properties and phase evolution of Mn-doped TiO2 pigments. The powders were prepared by the polymeric precursor method at Mn contents of 0, 1, 2 and 3 % and calcined at 600, 700 and 800 ◦C. The results demonstrated that crystalline powders were produced in all conditions evaluated. It was further noted that the samples calcined at 600 ◦C were composed of rutile and anatase TiO2 polymorphs. However, the anatase phase was completely converted into rutile at temperatures above 700 ◦C. The particle size increased with increasing temperature. The same behavior was observed for the measured crystallite sizes. The Mn3+ ion charge was determined as the main valence for manganese ions in the TiO2 host. Oxygen vacancies are the probable crystal defects formed for charge balancing in the Mn-doped rutile structure. The results obtained for the powder with 3 % Mn and calcined at 800 ◦C demonstrated dopant cation segregation to form Mn2O3. It is important to emphasize that the absorbance values of the powders increase in the visible range when adding manganese compared to the absorbance values of pure TiO2. The band gap of the powders was reduced in accordance with the content of doping: from 3.04 eV (0 % Mn) to 2.05–2.20 eV (3 % Mn), with small differences depending on the calcination temperature. The increase in visible light absorbance led to a brown color in Mn-doped materials. Thus, the color intensity varied with the Mn content, ranging from light brown to dark brown powders. In conclusion, the temperature of 700 ◦C was established in this paper as the best condition to produce Mn-doped TiO2 pigments, based on the absence of anatase polymorph, small contents of residual carbon, and undetectable amounts of Mn2O3 due to segregation.