The Raman spectra of tetragonal
TbPO4
(zircon-type I41/amd
structure, D4h19
space group) have been measured at ambient conditions and under variable pressure up to 15.5 GPa
inside a diamond anvil cell (DAC). Assignment of the Raman active modes of the tetragonal
phase has been carried out based on polarized measurements from a single oriented crystal of
TbPO4
at ambient conditions. The abrupt Raman mode discontinuities
and the appearance of numerous new Raman peaks at a pressure
Pc≈9.5 GPa have provided strong evidence for a first-order phase transition to a lower crystal
symmetry, most likely monoclinic. The high-pressure structure appears to be more compact
compared to the tetragonal one, and is retained upon bringing the crystal to ambient
pressure.
Self-organized TiO(2) nanotubes with packed, vertically aligned morphology and different lateral characteristics were grown on Ti metal substrates by controlled electrochemical anodization in phosphate/HF and ethylene glycol/HF electrolytes. The wetting, photo-induced superhydrophilicity, and photocatalytic activity of the nanotubular materials were investigated under ultraviolet irradiation. The photoactivity of the TiO(2) nanotube arrays was analysed in terms of their morphological characteristics that were determined by means of scanning electron microscopy and atomic force microscopy in conjunction with geometrical modelling. The wetting and the UV-induced superhydrophilicity could be accordingly modelled by the Cassie-Baxter mode arising from the large scale roughness of the nanotubular arrays in combination with the Wenzel mode due to the small scale roughness induced by ridges at the outer tube surface. The photocatalytic activity of the TiO(2) nanotube arrays was further found to correlate quantitatively with the variation of the geometric roughness factor, verifying the strong impact of morphology on the photo-induced properties of the vertically oriented TiO(2) tubular architecture.
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