Tin(II) Doped Anatase (TiO2) Nanoparticles: A Potential Route to "Greener" Yellow Pigments

During our exploration of compounds in the Sn-II-Ti-IV-O system, we discovered that hydrolysis of titanium alkoxide solution in the presence of Sn-II salts resulted in stable deep-yellow colored anatase nanoparticles. The samples were characterized by X-ray powder diffraction, electron microprobe, thermal analysis, transmission electron microscopy, and Sn-119 Mossbauer spectroscopy. Mossbauer data of the yellow colored samples showed the presence of both Sn-II and Sn-IV in a distorted environment as expected in the anatase structure. It is suggested that the cationic charge imbalance is compensated by oxygen vacancies and/or hydroxyl groups as evidenced by Mossbauer data which show two types of Sn-II environments. When heated in air to 300 degrees C the samples changed color to completely white and Sn-119 Mossbauer data of these samples showed only the presence of Sn-IV. These observations indicate that the origin of the yellow color in our Sri doped anatase nanoparticles arises from filled Sn 5s states just above the O 2p band, thus decreasing the band gap. The Sn-II doped anatase TiO2 nanoparticles reported here can potentially lead to environmentally benign yellow pigments. The simplistic nature of the synthetic procedure could easily be adapted to large-scale industrial manufacture.