A Single-step Process of Generating Hollow and Porous TiO2 Nanoparticles by Picosecond Laser Ablation in Deionised Water

Over the last two decades, hollow nanoparticles have received considerable attention from researchers due to their specific properties that do not appear in other forms of nanoparticles. It has been recognised that TiO2 hollow nanoparticles have superior photocatalytic properties than solid TiO2 nanoparticles. Previous methods of producing hollow TiO2 nanoparticles typically involve multiple-steps. In this paper, we report the production of hollow and porous TiO2 nanoparticles in a single step via high-repetition rate picosecond laser ablation in deionised water. The absorption spectra of the colloidal nanoparticles were obtained by UV-VIS spectroscopy. The size distribution and morphology were characterised by transmission electron microscopy (TEM). The morphology and chemical composition of the nanoparticles were characterised using a High-Angle Annular Dark-Field - Scanning Transmission Electron Microscope (HAADF-STEM) and Energy Dispersive X-ray Spectroscopy (EDS). In addition, the crystalline structures were investigated using X-ray diffraction (XRD). The results show that a higher ratio of crystalline hollow and porous TiO2 nanoparticles (20 - 160 nm in size with majority of hollow nanoparticles at 20 nm and an average size of 37 nm) of mixed anatase, rutile and brookite phases, was produced at lower laser energy; the yield was increased from 8% to 25% by reducing the laser power from 9.12 W to 3.35 W respectively. High laser powers result in reduced energy gap in the nanoparticles produced. The work shows that laser power can be used to control the yield of hollow nanoparticles.