Nonlinear optical properties and optical limiting measurements of graphene oxide – Ag@TiO2 compounds
Introduction
Nowadays nonlinear optical properties of materials are widely used in the industry. For this reason, measurement and evaluation of these properties are of importance which leads to identification of cheaper and more applicable materials. One of the most sensitive methods for measuring these properties is the z-scan method. In the z-scan method we can simultaneously measure nonlinear refractive index (n2) and nonlinear absorption coefficient (β). Also the Optical limiting properties are investigated with evaluating the changes in material transmittance with changing the incident light power. These properties are mostly used in applications such as optical switches and protection of the sensors. Studying of different optical limiting mechanisms is possible through z-scan technique [1], [2], [3].
Graphene, reduced graphene oxide (rGO) and graphene oxide (GO) have been studied widely for their nonlinear optical properties (NLO) and their unique optical limiting behavior [4], [5], [6], [7], [8]. Reduced graphene oxide samples exhibited stronger NLO and optical limiting responses than their GO precursors because of their increased crystallinity and conjugation [8]. They have found that an unusually efficient nonlinear optical-limiting behavior that occurs in graphene and alkyl-functionalized sub-GOx when they are dispersed as single sheets in appropriate solvents or film matrices. There are also reports on the optical limiting behavior of graphene oxide dispersed with organic solvents [3]. The research of Liaros et al. [9] on the broad band optical limiting of graphene oxide colloids has shown that the broad optical limiting behavior is comparable to C60 as well and dispersion in aqueous solution is an advantage of their report. There are also few reports on covalently bond graphene to metal oxide such as CuO [10], and NaYF4:Yb3+/Er3+ Nanoparticles [11]. Plasmonic materials such as Ag nanoparticles and its core-shells have been of great interest due to their novel optical behavior in visible range [12]. Among different core-shells of Ag nanoparticles, Ag@TiO2 has been studied widely due to its unique optical, photochemical and electronic behaviors of TiO2 which is widely used in many research areas such as solar cells and photonics [13], [14], [15]. Due to the production of electron–hole pairs on the surface of TiO2 under photon excitation accompanied by near field plasmon resonance of Ag cores, it shows also nonlinear optical behavior [16]. Since electron–hole pair on the surface of TiO2 is readily transferred to other materials with loosing bonds on their surface such as Graphene oxide, the proximity of Ag@TiO2 and Graphene oxide possibly would result in interesting optical limiting behavior in visible range of light. Recently, Karimipour et al. [17] reported facile synthesis of Ag@SiO2 core-shells using oleylamine with microwave irradiation. Thus, in this work we synthesized Ag@TiO2 core-shells with a similar method and thereafter Graphene Oxide (GO) and GO-Ag@TiO2 compounds were prepared and their nonlinear optical properties were investigated. Optical nonlinearity induced in samples by a CW diode laser at 532 nm was studied using Z-scan technique. The optical limiting effect was also investigated using the 2nd harmonics of a pulsed Nd-YAG laser at 532 nm.
Section snippets
Fabrication of graphene oxide (GO)
Graphene oxide nanosheets (GONs) were prepared from purified natural graphite powder using an improved Hummer's method reported by Marcano et al. [18]. Briefly, 1 g of graphite flakes and 6 g of KMnO4 were added slowly over 1 h into a 150 ml mixture of H2SO4/H3PO4 with 9:1 volumetric ratio. The resulting mixture was stirred at 45–50 °C for 2 h. Then, 150 mL water was added, and the solution was stirred for 15 min at 95 °C. Additional 150 mL water was added and followed by a slow addition of
Results and discussion
Fig. 1a shows the XRD pattern of GO and its corresponding absorbance spectrum (Fig. 1a-inset). XRD pattern shows that GO has been fabricated with Bragg peaks located at 2ɵ = 12.3, 43.1° with an interlayer spacing of 7.7 Å. The broad peak around 2ɵ = 26.4° is related to the presence of partially reduced graphene oxides (rGOs). Absorbance of GO shows the peak at 225 nm and a shoulder at 300 nm which are correspondent to π-π∗ and n-π∗ transition of the carbonyl groups [24], respectively. Fig. 1b
Conclusion
The nonlinear optical and optical limiting properties of suspensions of nanoparticles of Ag@TiO2, Graphene oxide (GO) and GO/Ag@TiO2 in ethanol has been investigated. Using a low power laser diode at a 532 nm wavelength, the magnitude and the sign of the thermal nonlinear refractive index and nonlinear absorption were determined by the Z-scan technique and the thermal lens model. It was observed that the nonlinear absorption of GO-Ag@TiO2 mixture was higher than pure GO. The optical limiting
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