Pseudomorphic Transformations

Turning lead into gold? Not quite! Instead, members of Professor Andreas Stein's group, graduate student Justin Lytle with postdoc Hongwei Yan and undergraduate student Ryan Turgeon, recently developed a solid-gas transformation method that allows the conversion of specially shaped silica into titania, while preserving some of the structural features of the original silica. Titanium dioxide is a well-known photooxidation catalyst, dielectric, and optical material. However, it cannot be shaped into intricate structures as easily as silica can. In particular on the micrometer and nanometer scales, the ability to structure ceramic compositions with the complex architectures found in silica would benefit many materials and device applications. A possible approach to transcribe shapes is by pseudomorphic transformations, reactions in which preforms are converted to other compositions while maintaining the shape and structural features of the original material. In order to evaluate the applicability of pseudomorphic transformations to nanostructures, a synthetic silica preform with hierarchical feature sizes was chosen in this study: a three-dimensionally ordered macroporous (3DOM) structure. 3DOM silica prepared by colloidal crystal templating consists of smooth amorphous silica walls (typically tens of nm in thickness) that encompass three-dimensionally interconnected spherical voids hundreds of nanometers in diameter. These materials have relatively large specific surface areas and appear opalescent due to the diffraction of visible light by the periodic structure.

Multistep gas-solid reactions at relatively low temperatures were employed to convert the amorphous 3DOM silica skeleton to nanocrystalline titanium oxyfluoride and then to titania (Figure 1). 3DOM silica preforms were reacted with gaseous titanium tetrafluoride at 190-300 deg. C. Under appropriate conditions, 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 silica was largely replaced by interconnected titanium oxyfluoride cubes. The product exhibited similar opalescence as the preform, giving a visual confirmation of the success of the pseudomorphic transformation on an extended length scale. The macroporous titanium oxyfluoride product was subsequently converted to titania by reaction with moist air at 300 deg. 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. 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. This work is described in an article in Chemistry of Materials 2004, 16, 3829-3837.

Pseudomorphic Transformation

Fig. 1. Schematic diagram and scanning electron microscopy image illustrating the pseudomorphic transformation from 3DOM silica (left) to titanium oxyfluoride (middle) and titania (right).