Optical Limiting Characteristics of Core-Shell Nanoparticles

TiO2 nanoparticles were synthesized by hydrothermal method at 180 ̊C from TiOSO4 aqueous solution with1m/l concentration. The obtained products were coated with silica by means of a seeded polymerization technique for a coating time of 1440 minutes to obtain well defined TiO2@SiO2 core-shell structure. The uncoated and coated nanoparticles were characterized by using X-Ray diffraction technique (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) to study their physico-chemical properties. Evidence from XRD and FTIR results show that SiO2 is homogenously coated on the surface of titania particles. FTIR spectra show that there exists an interaction between TiO2 and SiO2 and results in the formation of Ti-O-Si chemical bonds at the interface of TiO2 particles and SiO2 coating layer. The non linear optical limiting properties of TiO2 and TiO2@SiO2 nanoparticles dispersed in ethylene glycol were studied at 532nm using 5ns Nd:YAG laser pulses. Three-photon absorption is responsible for optical limiting characteristics in these nanoparticles and it is seen that the optical nonlinearity is enhanced in core-shell structures when compared with single counterparts. This effective three-photon type absorption at this wavelength, is of potential application in fabricating optical limiting devices. Keywords—hydrothermal method, optical limiting devices seeded polymerization technique, three-photon type absorption


I. INTRODUCTION
HE development of modern optical technology demands the ability to control the intensity of light in a predetermined and predictable manner. In this aspect optical limiters have received significant attention. The search for efficient optical limiters has lead to the study of various materials [1,2]. Bulk semiconductors [3][4][5] were first applied to optical limiting because of their diverse optical nonlinearities, such as two-photon absorption (TPA) [4][5], free-carrier non linearities [3] and the Kerr effect [6]. Among the various nonlinear optical materials, direct band gap semiconductors especially TiO 2 and ZnO have attractive non linear properties that make them ideal candidates for NLO based devices. The rapid development of Nanoscience and nanotechnology injects new opportunities into the synthesis of optical materials, as well as the research and design of optical limiters. Nanoscale composite materials containing TiO 2 are interesting because of their potential applications in G.Vinitha is with the B.S.Abdur Rahman University, Chennai, India (corresponding author to provide phone: 044-22751347; fax: 044-22351369; e-mail: svini2005@yahoo.co.in).
A.Ramalingam is with the Department of Physics, Anna University, Chennai, India (e-mail: ramalingam@yahoo.com). optoelectronic devices and the bulk TiO 2 has a direct band gap of 3.2eV [7]. A great deal of effort on research has been focused on the synthesis and optical linear properties of TiO 2 nanocomposites. Recently the non linear optical properties of such materials have also received attention. Nanoscale silica nanoparticles have also been extensively studied, due to its CMOS compatibility and enhancement of its non linear effects due to quantum confinement [8]. In the case of silica, the non linearities are usually attributed to two photon related absorption and χ (3) variation related to quantum confinement effects [9][10][11][12].
In this paper, we have reported the optical limiting studies on TiO 2 and TiO 2 @SiO 2 nanoparticles in ethylene glycol using nanosecond laser pulses at 532nm wavelength. The effect of silica coating on the optical limiting is also investigated. It has been observed that Silica coated particles show relatively better optical limiting compared to as prepared TiO 2 nanoparticles.

A. Chemicals
Special grade reagents Tetraethylorthosilicate (TEOS) (95%), Titanium Oxysulfate (TiOSO 4 ) were purchased from Sigma Aldrich Chemicals Ltd., Ethanol (99.5%) and ammonia (25% aqueous solution) purchased from (SISCO Research Laboratories, India) were used for silica coating and as catalysts. All other chemicals were reagent grade and used without further purification. Ultrapure deionized water was used in all the preparations.

B. Preparation of titania particles
Anatase-type TiO 2 nanoparticle powder was prepared by the following method. Initially, 18ml of 1m/l of TiOSO 4 aqueous solution was taken into a 25ml container of Teflon lined stainless steel autoclave. The container was tightly sealed and was heated at 180°C for 5 hrs with a constant rotation of 1.5 rpm, where hydrolysis of TiOSO 4 occurred under autogenous hydrothermal condition. After the completion of the reaction, the precipitate formed was washed several times using double distilled water and decantation. The product was collected at high-speed centrifuge and dried in air at 90°C. The powder thus prepared was heated in an alumina crucible at 700°C in air.

Silica coating on titania particles
The titania powder thus prepared was coated with silica by means of seeded polymerization technique, using TEOS as a precursor for a coating time of 1440 min at temperatures from 40°C. Titania nanoparticles and ethanol were charged in a 100ml flask and mixed by ultrasonication for 10min to ensure complete dispersion. After sonication predetermined amount of 29wt% aqueous ammonia solution and double distilled water were added. Then a proper amount of TEOS was added to the mixture to carry out the silica growth. The molar ratios of TEOS, ethanol, water and ammonia were 1,100,400 and 20 respectively. After synthesis, the slurry was washed with ethanol twice to remove the secondary silica particles. Finally the prepared sample was annealed to 800°C and characterized.

Characterizations
FT-IR spectra were recorded on a Perkin Elmer spectrum 2000. X-ray powder diffraction (XRD) measurement was made on (SEIFERT, Germany) with Cukα radiation. TEM image was obtained on a JEOL JEM -3010 Electron microscope, using an accelerating voltage of 300 kV. UV-Vis absorption spectra were obtained using Perkin Elmer Lambda 35 spectrophotometer equipped with an integrating sphere.
III. OPTICAL LIMITING Optical limiting measurements were carried out at 532nm in solution, using 5 ns laser pulses produced by a Q-switched, frequency-doubled Nd:YAG laser (Minilite, Continuum Inc.). Optical limiting data were extracted from open aperture z-scan experiments. In our experiment we used lens of 20cm focal length. The laser pulses were plane polarized with a Gaussian spatial profile. At each position z, the sample sees different laser intensity, and the position dependent (ie, intensitydependent) transmission is measured using an energy meter placed after the sample. Laser pulses are fired at a repetition rate of 1 Hz, and the data acquisition is automated. The low repetition rate is chosen for avoiding thermal effects in the samples during measurement. The pulse energy reaching the sample is approximately 150 μJ.

XRD analysis
The XRD pattern of the as prepared TiO 2 annealed at 700°C for 3 hrs and silica coated TiO 2 nanoparticles is shown in the fig (1). After calcination, the TiO 2 powders are identified to have sharpened diffraction peaks can be well indexed to tetragonal structure (lattice cell parameters a=3.777A JCPDS File NO. 89-4921). All the peaks can be assigned to anatase phase and no diffraction peaks due to other impurity phases are detected. The average crystallite size is calculated using the Scherrer equation, estimated from the XRD line broadening was around 16-19nm. For silica coated TiO 2 nanoparticles, peaks for TiO 2 are not clearly seen due to dense silica coating on its surface. The presence of amorphous silica effectively suppresses the sharpened peaks of TiO 2 . Moreover, the suppression is more remarkable with the introduction of higher silica content.  Fig (2) shows the FT-IR spectra of pure TiO 2 and Silica coated TiO 2 nanoparticles. The peaks at 1099 cm -1 , 800 cm -1 and 470 cm -1 correspond to the asymmetric, symmetric and anamorphic streches of Si-O-Si. The absorption peak at 3457 cm -1 is attributed to the O-H stretching vibration of free water and its corresponding O-H bending vibration occurs at 1636 cm -1 due to the chemically adsorbed water [13]. The IR band observed at 951 cm -1 indicates the existence of Ti-O-Si chemical bonding as the IR bands observed from 910 to 960 cm -1 are widely accepted as the characteristic vibration due to the formation of Ti-O-Si bond [14].

Optical Limiting Studies
Open-aperture z-scan measurement on the samples was performed with the intention of calculating their nonlinear absorption coefficients. Interestingly, the nonlinear transmission is found to arise from three-photon type absorption.
The experimental data fits well to the corresponding nonlinear transmission equation, given by where 'R' is the surface reflectivity and p 0 is given by 2γ(1-R) 2 I 0 2 L, where γ is the three photon absorption co-efficient and I 0 is the on-axis peak intensity. α is the linear absorption coefficient. The z-scan signatures for TiO 2 and Silica coated TiO 2 are shown in Figures 3 (a) and (b) respectively. The non linear absorption coefficient is higher in the coreshell nanoparticles as compared to uncoated nanoparticles, in confirmation with a previous report [15]. The three photon absorption data fitted well with the experimental curves, indicating that three-photon absorption is the basic mechanism of optical limiting.
V. CONCLUSIONS Titania nanoparticles were synthesized and coated with amorphous silica by a seeded polymerization technique. Evidence from XRD and FTIR results show that SiO 2 is homogenously coated on the surface of titania particles. The average particle size of TiO 2 @SiO 2 nanoparticles was roughly found out to be about 24nm. FTIR spectra show that there exists an interaction between TiO 2 and SiO 2 and results in the formation of Ti-O-Si chemical bonds at the interface of TiO 2 particles and SiO 2 coating layer. Nonlinear optical transmission measurements at 532 nm using 5 ns laser pulses on the TiO 2 as well as TiO 2 @SiO 2 core-shell nanoparticles show the existence of an effective three-photon type absorption at this wavelength, which may be of potential application in fabricating optical limiting devices.