Supplementary Information Novel core-shell (TiO 2 @Silica) nanoparticles for scattering medium in a random laser: higher efficiency, lower laser threshold and lower photodegradation. Ernesto Jimenez-Villar* a , Valdeci Mestre b , Paulo C. de Oliveira b and Gilberto F. de Sá c a Instituto de Ciencia Molecular, Universitat de València. C/ Catedrático José Beltrán 2, Paternan46980, Spain. E-mail: Ernesto.Jimenez@uv.es 5 b Departamento de Física, Universidade Federal da Paraíba, João Pessoa, Paraíba58051-970, Brasil c Departamento de Química Fundamental,Universidade Federal de Pernambuco, Recife 50670-901, Brazil Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Materials 10 R6G laser dye (C 28 H 31 N 2 O 3 Cl), with molecular weight 479.02 g/mol, supplied by Fluka: Ethanol alcohol (C 2 H 5 OH) with spectroscopic grade purity, supplied by Alphatec: Tetra-ethyl- ortho-silicate (TEOS), supplied by Sigma-Aldrich. The titanium dioxide (TiO 2 , nanoparticles 410 nm) of rutile crystal structure 15 was acquired from DuPont Inc (R900). Chemical synthesis of silica-coating on TiO 2 nanoparticles In the first stage, 2 g of TiO 2 Np were diluted in 250 ml of absolute ethanol. The solution of TiO 2 nanoparticles was then 20 divided into two equal portions of 125 ml. One of the parts was placed in a bath at 5 °C and 1.1 ml of TEOS, previously diluted in 11 ml of ethanol, was added. The 10% diluted solution of TEOS was added in 110 portions of 100 μl during the course of 1 hour. The solution was stirred during the TEOS addition. The other 25 portion was stored and used as a reference in every experiment. Previously, before adding the TEOS, the solution of TiO 2 Np was placed in an ultrasound bath for 20 minutes to disperse the particles. The TEOS hydrolysis and subsequent condensation of 30 silica on the TiO 2 surface was provoked, taking advantage of the catalytic effect in the near surroundings of TiO 2 Np. 1,2 In addition, the possible accumulation of water molecules around the nanoparticles would favor TEOS hydrolysis. Probably, the electric potential associated with the surface of the nanoparticles 35 themselves, in conjunction with the higher dielectric permittivity of the water, should promote the accumulation and polarization of water molecules in the vicinity of TiO 2 Np. 3 Self superficial hydrolysis of TiO 2 nanoparticles 4 would also favor the TEOS hydrolysis. The superficial irregularities of the TiO 2 nanoparticles 40 must cause intensification of superficial electric potential in the regions with a smaller curvature radius. This phenomenon should increase the medium polarization, water accumulation and catalytic effects in these regions, leading to silica coating irregularities. 45 Characterization and sample preparation: Transmission electron microscopy (TEM) was used on a 100kV JEOL, model 1200EX, microscope. The commercial carbon- coated Cu TEM grid was immersed in the solution of TiO 2 @Silica nanoparticles that had previously been diluted 50- 50 fold and then left to dry, before being introduced into the microscope. The stoichiometric ratio (Ti/Si) was determined by Energy Dispersive X-Ray fluorescence (ED-XRF) using an X-ray spectrometer SIEMENS D5000. The sample was prepared in three steps: precipitation, washing and drying. The nanoparticle 55 powder (TiO 2 @Silica) was pressed into a tablet form with a 12 mm diameter. Experimental setup of random laser measurement Supplementary Figure 1S shows a schematic diagram of the random laser (RL) experimental setup. The pumping source of 60 the random laser was the second harmonic of a Q-switched Nd:YAG Continuum Minilite II (25 mJ, λ = 532 nm, with a pulse width of ~6 ns, repetition rate up to 15 Hz and spot size of 3 mm). The pump laser beam was incident upon the sample at 15 degrees. The laser power was regulated through neutral density 65 filters (NDF), a polarizer and a half wave plate. The samples were accommodated in a 2 mm path length quartz cuvette. The emission spectra were collected through a multimode optical fiber (200 μm), coupled to a spectrometer HR4000 UV-VIS (Ocean Optics) with a 0.36 nm 70 spectral resolution (FWHM). The collection angle was ~60 degrees with respect to the sample surface, that is, 45 degrees with respect to the incident pumping beam. The liquid samples were placed in an ultrasound bath for about 10 minutes before recording the spectrum, in order to obtain the same dispersion of 75 nanoparticles (initial conditions) in all measurements. Supplementary Figure 1S. Schematic diagram of the RL experimental setup; NDF neutral density filter. Electronic Supplementary Material (ESI) for Nanoscale This journal is © The Royal Society of Chemistry 2013