Mechanical milling of hydrothermally obtained CaTiO3 powders—morphology and photocatalytic activity

Abstract

Nanosized calcium titanate (CaTiO3) powders were prepared for the first time by combination of two synthesis methods: hydrothermal treatment and mechanical milling (MM). The samples were analyzed by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy and High-Resolution Transmission Electron Microscopy (TEM/HRTEM), Diffuse-reflectance spectroscopy (DRS), infrared spectroscopy (IR), Differential Thermal analysis (DTA-TG) and Brunauer–Emmett–Teller (BET) method. The crystallite sizes and degree of crystallinity were decreasing with the increasing of MM time. SEM and TEM analyses have revealed that the mechanical milling results in changes in the size and shape of the particles — from regular prismatic shape to polygonal form with more oval borders. The increased milling time leads to increased number of agglomerates, which consist of smaller particles. The shifts to higher wavenumbers in the IRS bands were observed, which corresponded to Ti-O and Ti-O-Ti bridging stretching vibration modes. These effects could be assigned to structural distortion of the lattice. It was established that the mechanical milling influences the band gap energy of the samples: it is decreasing upon increasing the MM time. This study proved that the milled CaTiO3 exhibited increased photocatalytic activity in the Malachite Green (MG) discoloration under UV light with the increasing of milling time. The highest degree of discoloration of MG was observed with the sample, activated for 45 min (97% after 1 h of irradiation). The increased photocatalytic activity of MM samples could be due to increased number of defects (oxidation active sites) and lattice disorder, lower degree of crystallization, decreased particle size.