Changes in Polymorph Composition in P25-TiO2 during Pretreatment Analyzed by Differential Diffuse Reflectance Spectral Analysis

Elucidating structural and compositional poly morph changes of multiphase semiconductors in a quick and quantitative manner is important for their manufacturing and applications in catalysis alike. Derivative peak fitting of diffuse reflectance UV-visible spectra (DPR) is presented as an inexpensive, fast, and quantitative method to estimate both the composition of a multiphase semiconductor sample as well as the band gap energies of each component semiconductor in the mixture. Degussa P25 TiO2, a widely used catalyst support and semiconductor photocatalyst, is a mixture of anatase and rutile polymorphs. The as-received P25 samples were subjected to grinding, sieving, and calcination or a combination of these operations. Samples were analyzed via DPR, X-ray diffraction (XRD), and Raman spectroscopy to quantify the percentage of anatase and rutile polymorphs present in each sample. Compositional measurements from DPR were in good quantitative agreement with XRD and Raman analysis. The application of in situ UV-visible spectroscopy DPR allowed for the on-stream determination of the onset temperature for rutilization during calcination, which occurred just above 823 K. In situ analysis of the ground and sieved samples during calcination revealed that grinding and sieving (between 45 and 53 mu m) lowered the onset of rutilization to 673 K, leading to an increase in the formation of rutile when calcined at 773 K compared to as-received P25 calcined at the same temperature. However, the same process reduced the overall extent of rutilization at a higher temperature of 973 K. Changes in the exposed facets and shapes of the anatase crystallite, as indicated by XRD, were implicated in both reducing the rutilization onset temperature as well as leading to an overall greater resistance to rutilization. The quantitative success of the DPR method in this study may lead to application in other systems of mixed semiconductors.