Determination of crystal phases in mixed TiO2 paint films by non-invasive optical spectroscopies

Modern pigments have been synthetically produced starting from the 19th century. Frequently, achievements in the synthesis technology have changed the physical characteristics of these inorganic pigments in order to improve their stability and coloring power. A clear example of this trend is represented by titanium white (TiO2) that evolved from the less performant anatase to a highly pure rutile crystalline structure. In this work, we propose a non-invasive method based on two complementary optical techniques, time-resolved photoluminescence and Raman spectroscopy, for determining the presence of these two polymorphs of titanium oxide in paint films. Results demonstrate that Raman spectroscopy is highly sensitive to anatase, whereas time-resolved photoluminescence allows the easy identification of rutile even when present in very low concentrations, up to a minimum value of 1.5 wt%. The complementarity and flexibility of the two spectroscopy techniques make the proposed approach promising for dating titanium white paints in modern and contemporary paintings on the basis of the presence of rutile and anatase polymorphs.