Hydrothermal synthesis of Co-doped willemite powders with controlled particle size and shape

The preparation of uniform elongated (axial ratio ∼2) particles with mean length from 0.2 to 4.5 μm of Co-doped willemite (Zn2SiO4) blue pigments is reported. The procedure is based on the hydrothermal treatment at 225 °C for 5 h of the dispersions obtained by the precipitation with ammonia of Zn(II) and Co(II) sulphate aqueous solutions containing Ludox silica. The pH and Co concentration of the starting dispersions were varied to analyse the effects of these parameters on particle size, shape and composition. The crystallochemical, morphological and colour changes produced by the thermal treatment up to 1200 °C of the hydrothermally prepared powders were also studied by mainly using scanning electron microscopy, thermogravimetric analysis and optical and X-ray photoelectron spectroscopies.