Insights on the role of β-Bi2O3/Bi5O7NO3 heterostructures synthesized by a scalable solid-state method for the sunlight-driven photocatalytic degradation of dyes

A simple, solvent-free and scalable route for the synthesis of nanostructured Bi5O7NO3 and Bi2O3 polymorphs photocatalysts and their heterostructures is here presented. Morphology, porosity and crystal phases were tuned by controlling the thermal decomposition of Bi(NO3)3:5H2O, by using different calcination temperatures and cooling processes. Bi5O7NO3 layered nanosheets, highly porous β-Bi2O3/Bi5O7NO3 heterostructured nanoflakes and micrometric sheets of α-Bi2O3 were obtained at temperatures of 400 °C, 425–450 °C and 525 °C with a slow cooling rate, respectively. Moreover, in contrast to previous reports on Bi5O7NO3, the role on the visible-light driven photocatalytic activity of Bi5O7NO3 due to the formation of a stable heterojunction with β-Bi2O3, which here was identified to be also formed by a rapid cooling after calcination at 400 °C, was deeply investigated and demonstrated. All the samples were fully characterized by X-ray Diffraction, Field-emission scanning electron microscopy, UV–vis spectroscopy, N2 adsorption and X-ray Photoelectron Spectrometry analyses. The role of oxygen vacancies, which indicate structural defects and/or sub-stoichiometric forms of Bi5O7NO3 or Bi2O3 in the photocatalysts surface, was also analyzed and correlated to their photo-response. Simulated sunlight-driven degradation of organic dyes with different functionalities, i.e. a cationic dye (Rhodamine B) and an anionic one (Indigo Carmine), was investigated and proved to be optimized due to an efficient synergy between suitable amounts of Bi5O7NO3 and β-Bi2O3 in the heterostructured samples. An improved charge carriers separation at the heterojunction interphase and an enhanced formation of reactive oxygen species in these materials were additionally confirmed by both photocatalytic water oxidation experiments with O2 evolution and mineralization of a refractory anionic dye, i.e. Remazol Brilliant Blue R. The latter was reached with an optimum photocatalyst/ dye weight ratio of 33, which is about 2–3 times lower than previous literature results.