Web Release Date: August 25,
Multistep, Low-Temperature Pseudomorphic Transformations of Nanostructured Silica to Titania via a Titanium Oxyfluoride Intermediate
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455
Received March 25, 2004
Revised Manuscript Received July 8, 2004
Abstract:
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.
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