Bismuth titanate nanobelts through a low-temperature nanoscale solid-state reaction

In this study, an effective low-temperature method was developed, for the first time, for the synthesis of bismuth titanate nanobelts by using Na2Ti3O7 nanobelts as the reactants and templates. The experimental procedure was based on ion substitution followed by a nanoscale solid-state reaction. In the first step, Na2Ti3O7 nanobelts were soaked in a bismuth nitrate solution where ion substitution at the nanobelt surfaces led to the formation of a bismuth compound overlayer. The resulting bismuth-modified nanobelts were then subject to a calcination process at controlled temperatures. At the calcination temperature of 400 °C, the top layer was converted to Bi2O3 whereas the interior was converted to TiO2(B), forming TiO2(B)@Bi2O3 core–shell nanobelts. When the calcination temperature was increased to 450 °C, a metastable interphase Bi20TiO32 was produced on the nanobelt surface whereas the interior structure remained virtually unchanged, and the nanobelts now exhibited a TiO2(B)@Bi20TiO32 core–shell structure. At calcination temperatures higher than 550 °C, the shell of the nanobelts became Bi4Ti3O12. At even higher temperatures (600–700 °C), no TiO2(B) was found and the nanobelts exhibited single-crystalline characteristics that were consistent with those of Bi4Ti3O12. Such a structural evolution was manifested in X-ray diffraction, Raman and Fourier transform infrared spectroscopic measurements, and scanning electron microscopic and transmission electron microscopic studies showed that the belt-like surface morphology was maintained without any apparent distortion or destruction. A mechanism based on nanoscale solid-state reactions was proposed to account for the structural evolution. Photoluminescence measurements showed that the core–shell nanobelts exhibited a markedly suppressed emission intensity, suggesting impeded recombination of photogenerated carriers as compared to the single-phase counterparts. Such a unique feature was found to be beneficiary to photocatalysis, as exemplified by the photodegradation of methyl orange under UV irradiation, where TiO2(B)@Bi20TiO32 core–shell nanobelts were found to exhibit the best performance among the series.