Serendipity following attempts to prepare C-doped rutile TiO2

• Forming C-doped rutile TiO2 from a sol–gel is not as facile as forming C-doped anatase TiO2.
• This indicates that aspects of the preparation are important in carbon doping.
• Melamine borate in a rutile-directed sol–gel forms undoped rodlike materials.
• These are more catalytically active rutile crystallites.

Attempts to mimic the band gap narrowing seen in anatase TiO2 following C-doping of the lattice where the C arose from a melamine borate precursor were made in situations where the sol–gel mixture was directed towards rutile formation. The formed materials were characterised using XRD, BET, UV–vis spectroscopy, XPS and TEM and their activities in promoting the photo-degradation of 4-chlorophenol were analysed. It was found that carbon was not doped into the lattice (in contrast to the situations where the sol–gel mixture was directed towards the precipitation of anatase TiO2). In spite of how common reports of the preparation of C-doped TiO2 using sol–gel processes have been, the presence of carbon dopant precursors in a crystallising sol does not necessarily result in the incorporation of C dopants within the final crystalline material, i.e. the nature of the condensing sol is also important.The presence of melamine borate did however increase the proportion of rutile in the final mixture (indeed in the presence of melamine borate the pure rutile phase was formed) and also resulted in materials with higher surface areas (as measured using BET). Furthermore, TEM has shown that rutile TiO2 condensed in the presence of melamine borate had a much more distinct rod-like shape than that condensed in its absence (the latter being more spherical in shape).These materials, notwithstanding the absence of any dopant effect, demonstrated enhanced photocatalytic activity when compared with analogous materials prepared in the absence of melamine borate and this effect is ascribed to both their relatively larger surface areas and their specific shape. Therefore, we have serendipitously come across a method for improving the performance of rutile photocatalysts while searching for a method to generate C-doped rutile TiO2.

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