Hydrothermal synthesis of rare earth and transition metal co-doped BiFeO3 and investigation of their photocatalytic performance

Abstract

This investigation reports the synthesis of undoped and 10% rare earth (Gd, Dy) doped BiFeO3 (BFO) nanomaterials via a facile hydrothermal (HT) technique. Later on, 10% rare earth (Gd, Dy) and 10% transition metal (Cr, Co) co-doped BFO nanomaterials were also synthesized using the same technique. This study also highlights the application of the synthesized nanomaterials toward the degradation of industrial effluents, such as rhodamine B (RhB), methylene blue (MB), and pharmaceutical pollutants ciprofloxacin (CIP), levofloxacin (LFX) under solar irradiation. Finally, 10% Dy-doped BFO (BDFO) sample was also prepared through the widely used sol-gel (SG) method to compare the influence of HT and SG synthesis routes on the physico-chemical properties of materials at the nanoscale. Rietveld refined X-ray diffraction patterns of undoped and 10% Gd-doped BFO demonstrated the formation of rhombohedral perovskite structure. On the contrary, 10% Gd and 10% Cr co-doped BFO (referred to as S-BFO) samples revealed the formation of sillenite phase at the reaction temperature of 120 to 160 °C. This outcome eventually ensured the development of a one-step low-temperature route to reproducibly fabricate sillenite bismuth ferrite. Notably, 10% Dy and 10% Co co-doped BFO did not form any defined structure using the low-temperature hydrothermal approach. However, the 10% Dy mono-doped BDFO sample revealed the formation of sillenite dominating phase through the HT synthesis at an optimized temperature of 160 °C, whereas the SG method resulted in perovskite phase following its standard synthesis conditions. The morphology of S-BFO gradually changed from irregular shapes to spherical powder nanomaterials with the increase in reaction temperature from 120 to 160 °C. Both scanning and transmission electron microscopy imaging demonstrated a mixed nanopowder and rod-like morphology of the BDFO materials synthesized by the HT technique, however, semi-spherical BDFO with a particle size of ∼100 nm were produced via the SG technique. Among all the S-BFO samples, the materials synthesized at 160 °C reaction temperature, demonstrated the highest photocatalytic efficiency due to the homogeneous phase distribution, regular powder-like morphology, lowest band gap, and quenching of electron-hole pair recombination. The HT-synthesized BDFO exhibited better photocatalytic activity with almost 100% removal of RhB within 90 min than SG-synthesized BDFO, due to their nanopowder and rod-like mixed morphology and higher number of oxygen vacancies. Moreover, the photodegradation of colorless antibiotics ensured that the reactions were truly photocatalytic, not dye-sensitized. Based on the band structures, a plausible mechanism was proposed to comprehend the rationale behind the promising photocatalytic performance of the synthesized nanomaterials.