Multistep, low-temperature pseudomorphic transformations of nanostructured silica to titania via a titanium oxyfluoride intermediate

Synthetic silica preforms with an inverse opal or three-dimensionally ordered macroporous (3DOM) structure were converted to 3DOM TiOF₂ and subsequently to 3DOM TiO₂ by solid-gas pseudomorphic transformation reactions, reactions which maintain the shape and structural features of the original material. 3DOM SiO₂ preforms with periodic arrays of macropores and hierarchical feature sizes (e.g., macropore separation 334 nm, average wall thickness 59 nm) were prepared by colloidal crystal templating. They were reacted with TiF₄ in sealed steel pipes at 190, 235, and 300 °C. At 190 °C no conversion took place, while at 300 °C the material was converted mostly to crystalline TiOF₂ with an irregular structure. However, at 235 °C the periodic macroporous structure of the preform was maintained with little change in average pore separation. In these samples, the initially smooth wall structure of 3DOM SiO₂ was largely replaced by interconnected TiOF₂ cubes with edge lengths of 133 nm. The X-ray diffraction (XRD) pattern showed sharp lines of TiOF₂. The product exhibited opalescence similar to that of the preform, giving a visual confirmation of the success of the pseudomorphic transformation on an extended length scale. An analogous transformation was also investigated with spherical silica preforms. Different stages of transformation were observed by scanning electron microscopy, permitting a discussion of critical parameters in these conversions. The macroporous TiOF₂ product was subsequently converted to TiO₂ (anatase) by reaction with moist air at 300 °C. In this reaction, pseudomorphism was observed on the scale of tens of micrometers, on the submicrometer macropore scale, and on the scale of the cubic particles forming the wall skeleton. The sample was still composed of interconnected cubes with similar edge lengths, and the pore spacing was nearly maintained. XRD showed only TiO₂ anatase reflections. The synthetic paradigms demonstrated for the silica to anatase conversion may be transferable to other 2D or 3D material shapes within the applicable range of feature sizes.