Picosecond laser generation of Ag-TiO2 nanoparticles with reduced energy gap by ablation in ice water and their antibacterial activities

Ag-TiO2 nanoparticles were synthesised in ice water using a picosecond laser with a 1064-nm wavelength, at a 200-kHz repetition rate, a laser pulse energy of 42-43.79 A mu J, and laser fluences of 0.342-0.357 J/cm(2), by ablation of solid Ag and Ti targets. The absorption spectra and size distribution of the colloidal nanoparticles were obtained by UV-Vis spectroscopy and transmission electron microscopy, respectively. The morphology and chemical composition of the nanoparticles were characterised using high-angle annular dark-field-scanning transmission electron microscope and energy-dispersive X-ray spectroscopy. The results show that the sizes of the Ag-TiO2 nanoparticles range from less than 10-130 nm, with some large particles above 130 nm, of which the predominant size is 20 nm. A significant reduction in the energy gap of TiO2 nanoparticles was obtained to 1.75 eV after the modification with Ag nanoparticles during co-ablation. The role of Ag nanoparticles in the reduction in the energy band gap of the TiO2 nanoparticles can only be seen during laser ablation in an ice environment but not in deionised water at room temperature. Furthermore, the TiO2 nanoparticles were produced in ice and deionised water under the same laser and experimental conditions; the results show that the nanoparticles in both media have the same energy gap (about 2.4 eV). The antibacterial activity of the Ag-TiO2 nanoparticles generated was then tested against E. coli bacteria under standard laboratory light conditions. The results show that the nanoparticles can effectively kill E. coli bacteria much more effectively than laser-generated TiO2 nanoparticles.