Investigation on the photocatalytic and sonophotocatalytic activities of {002} facets of ZnO nanoparticles synthesized through template/surfactant-free hydrothermal method at different temperatures and time durations

Template/surfactant-free ZnO nanoparticles were synthesized by hydrothermal process at 120 °C, 150 °C, 180 °C and 210 °C for 3-h duration and were annealed at 400 °C for 3 h. Since the ZnO nanoparticles synthesized at 150 °C for 3 h and annealed at 400 °C for 3 h showed improved photocatalytic activities, the ZnO nanoparticles were further synthesized at 150 °C for 6-h, 9-h, and 12-h durations and were annealed at 400 °C for 3 h. All the synthesized ZnO nanoparticles were characterized for their structural, optical, and morphological properties. X-ray diffraction analysis confirmed that the ZnO nanoparticles belong to the hexagonal wurtzite system. Transmission electron microscopy and high-resolution transmission electron microscopy analyses revealed that the hydrothermally synthesized ZnO nanoparticles at 150 °C for 3 h and annealed at 400 °C for 3 h acquired spherical and hexagonal morphologies with size ~ 50 nm. Variation in the hydrothermal temperatures and time durations with annealing at 400 °C for 3 h acquired nearly hexagonal, spheroidal, and hexagonal morphology of the ZnO nanoparticles. Optical band gap of synthesized ZnO nanoparticles was slightly influenced by the different synthesis parameters. The photocurrent measurements revealed that the ZnO nanoparticles synthesized at 150 °C for 3 h and annealed at 400 °C for 3 h possess relatively enhanced of photocurrent of about 3.58 μA than that of the other synthesized samples. The ZnO nanoparticles synthesized at 150 °C for 3 h and annealed at 400 °C for 3 h showed ~ 78% and 36% of photodegradation efficiency against Rhodamine B dye and 4-chlorophenol, respectively, in a time period of 60 min. The sonophotocatalytic activity process enhanced degradation efficiency to 99% in 60 min against Rhodamine B and 80% in 60 min against 4-chlorophenol.