The photochromic effect of bismuth vanadate pigments: Investigations on the photochromic mechanism

We report on investigations of the photochromic effect of BiVO4 pigments focussing on pigment powder samples. Results are compared to those of the corresponding pigment coatings, and we discuss the differences and similarities concerning photochromism. Besides impurity levels of Fe and Pb, the specific surface area of pigment particles as well as interactions between the particles and their surroundings turn out to be important factors that determine the degree of color change. The outcomes of illumination experiments in different atmospheres and powder EPR spectroscopy suggest formation of VIV and photochemical red-ox disproportionation of FeIII to be the central processes in the so-called V-type and Fe-type mechanisms, respectively, but details of a further mechanism (Pb-type) are not yet well understood.