J. Phys. Chem. C 2011, 115, 23–30
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In Situ Crystallization and Growth Dynamics of Acentric Iron Iodate Nanocrystals in w/o Microemulsions Probed by Hyper-Rayleigh Scattering Measurements Yannick Mugnier,*,† Latifa Houf,† Moustafa El-Kass,† Ronan Le Dantec,† Rachid Hadji,‡ Brice Vincent,‡ Gnon Djanta,§ Laurent Badie,† Ce´cile Joulaud,† Julien Eschbach,‡ Didier Rouxel,‡ and Christine Galez† SYMME, UniVersite´ de SaVoie, BP 80439, 74944, Annecy Le Vieux Cedex, France, Institut Jean Lamour, UMR CNRS no. 7198, UniVersite´ Henri Poincare´, Nancy 1 BP 239, 54506 VandoeuVre-le`s-Nancy Cedex, France, and Faculte´ des Sciences, UniVersite´ de Lome´, BP 1515, Lome´, Togo ReceiVed: June 18, 2010; ReVised Manuscript ReceiVed: October 4, 2010
A detailed experimental investigation of the formation mechanisms of acentric iron iodate [Fe(IO3)3] nanocrystals in AOT-based reverse micelles according to the water-in-oil (w/o) microemulsion (ME) composition and temperature was performed. A low chemical reaction rate was first demonstrated by means of UV-vis absorption spectroscopy. Attainment of iron iodate crystalline nanorods was then attributed to an oriented aggregation mechanism of 10-20-nm amorphous primary nanoparticles. As for the intermicellar exchange of reactants, the aggregation mechanism was also found to be determined by the ME composition because clear effects of the surfactant and reactant concentrations, the nature of the oil phase, and the water droplet size were observed. Moreover, a combination of dynamic light scattering, X-ray diffraction, and transmission electron microscopy experiments revealed that hyper-Rayleigh scattering is a fast, valuable, and nondestructive alternative to probe, in real time, the crystallization and formation dynamics of acentric nanoparticles in microemulsions. This experimental approach can be extended to studies dealing with the formation of metal nanoparticles for which scattering of second-harmonic light is readily observed. 1. Introduction Because of their unique nonlinear optical properties, applications of acentric nanomaterials with tailored sizes and shapes are the subject of considerable current interest. Indeed, their potential use as new optical probes in biomedical imaging was recently proposed by different groups,1-4 as nonlinear optical processes such as second harmonic generation (SHG) and sum frequency generation (SFG) provide new opportunities in multiphoton microscopy compared to conventional biomarkers such as fluorescent dyes and quantum dots. The latter can suffer from poor long-term optical stability5 (bleaching) and strong fluorescence intensity variations6 (blinking), respectively, whereas recent investigations on KNbO3, ZnO, Fe(IO3)3, BaTiO3, and KTP nanocrystals1-4,7 have shown that the nonlinear optical probes are stable under high laser illumination. The optical detection of individual SHG probes, for instance, is based on the coherent nature of SHG emission in harmonic holographic microscopy,4 whereas the polarization properties of harmonic emission can be used to optically retrieve the spatial orientation of individual nanocrystals3,7,8 in classical SHG microscopy. Frequency-resolved optical gating (FROG) measurements at the focal point of a high-numerical-aperture (-NA) objective were also recently proposed as a novel phase-sensitive technique to probe phase and amplitude distortions induced by the local environment of SHG nanocrystals.9 Another unique optical property of SHG nanocrystals is that their subwavelength size results in the absence of phase-matching constraints, typical of * Corresponding author. Tel.: +33 (0) 450 096 516. Fax: +33 (0) 450 096 543. E-mail:
[email protected]. † Universite´ de Savoie. ‡ Universite´ Henri Poincare´. § Universite´ de Lome´.
bulk nonlinear materials, which provides the possibility to double any incoming spectrum within the crystal transparency range. Tailoring the excitation source to the sample absorption and scattering properties (by using a near-infrared wavelength for excitation or detection) limits energy deposition in the embedding medium and considerably increases the imaging depth in multiphoton biomicroscopy.10,11 A second field of interest for acentric nanomaterials is optoelectronic and photonic devices, where nonlinear optical materials perform many functions such as electrooptical modulation, switching, rectification, and frequency generation. In particular, nanocomposite materials based on inclusions of acentric nanocrystals in an amorphous matrix have recently received much attention because of their transparency, good nonlinear optical properties, and low cost compared to conventional crystal technologies.12-16 Although SHG conversion is obviously lower in nanocomposite materials, this intrinsic drawback could be compensated by the ease in forming the material itself. Moreover, the absence of a strict phase-matching condition, allowing frequency doubling over very broad wavelength ranges, has recently been shown in random media by using three-wave mixing.17 In the literature, synthesis of acentric nanocrystals has been addressed through different chemical and physical routes. Hydrothermal conditions were used for KNbO3 nanowires,1 a sol-gel process in the case of ZnO2 and BaTiO318 nanocrystals, centrifugation from raw powder for KTP,7 reduction of niobium salts and subsequent hydrolysis with LiH for LiNbO3,19 and coprecipitation in aqueous solution for iron iodate20 Fe(IO3)3 (hereafter denoted as FIO). Unfortunately, although experimental parameters such as temperature, reactant concentration, and acidity are known to influence the nanoparticle size, the abovementioned techniques often give agglomerated nanopowders that
10.1021/jp105638s 2011 American Chemical Society Published on Web 12/06/2010
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J. Phys. Chem. C, Vol. 115, No. 1, 2011
are then difficult to redisperse even under powerful sonication treatments. A subsequent centrifugation step is then performed to separate nanoparticles according to their size. Dispersion of nanocrystals, in terms of size and sometimes shape, can be more easily controlled in water-in-oil microemulsions. Recently proposed as a convenient route to prepare ferroelectric nanocrystals,21 these ternary systems have been used for many years in the synthesis of metal, semiconductor, oxide, and hybrid22 nanomaterials (see, for example, refs 23 and 24 for reviews). When the oil phase of the ternary system is replaced by a liquid monomer, this route has also proven to be successful in the preparation of transparent polymer nanocomposites with adjustable linear optical properties (luminescence, UV absorption, refractive index, etc.) given by the concentration of embedded nanocrystals.25,26 In such micellar systems, dynamic light scattering (DLS), the time-dependent fluctuation of scattered light intensity monitored at the fundamental optical frequency of the input laser beam, is routinely used to measure the hydrodynamic diameter of reverse micelles. A first in situ estimation of the nanocrystals size is then obtained, but DLS is not sensitive to the structural properties of dispersed nanoparticles in microemulsions. Only ex situ measurements such as X-ray diffraction or transmission electron microscopy (TEM) can reveal the amorphous or crystalline nature of such nanomaterials. In addition, hyperRaleigh scattering (HRS) measurements, monitoring of the intensity of the scattered light at twice the optical frequency of the input laser beam, have been widely used to assess the quadratic hyperpolarizability of molecular compounds27 and metal28 and semiconductor29 nanoparticles. Polarization-resolved HRS studies have also recently appeared as a powerful, noninvasive tool to probe spatial arrangements of molecular aggregates.30 In this article, we show that HRS is also a valuable tool to follow time-dependent organization of “molecules”, that is, crystallization, of polar acentric materials (FIO, in this case) and their subsequent growth according to the microemulsion composition and temperature. This in situ diagnostic of FIO nanoparticle crystallization in microemulsions is nondestructive regarding the synthesis process and is of primary importance because the time needed to obtain crystallized nanoparticles strongly depends on the experimental parameters (temperature, reactant and surfactant concentrations, and molar water-tosurfactant ratio). Whereas real-time monitoring of the growth of ZnO nanoparticles in homogeneous solutions was recently published,31 to our knowledge, this is the first time that hyperRaleigh scattering measurements have been used to probe precipitation reactions in microemulsions. 2. Experimental Section The yield of crystallized iron iodate nanoparticles was found to be strongly influenced by experimental conditions such as the temperature and the aging time during which the micellar solutions were left under stirring, as well as the concentrations of reactants that affect the stability of the initial Fe3+- and IO3-based microemulsions. Here, only experimental conditions allowing the formation of stable and transparent colloidal solutions of reactants are discussed. They were obtained as follows: Iron nitrate (Fe(NO3)3 · 9H2O), iodic acid (HIO3), AOT [sodium bis(2-ethylhexyl) sulfosuccinate, C20H37NaO7S, with >99% purity], and the oil solvents (isooctane, heptane, and decane) were purchased from Sigma-Aldrich (Lyon, France) and used without further purification. Iron nitrate (or iodic acid) was first dissolved in distilled deionized water (Simplicity, Millipore,
Mugnier et al. F ≈ 18.2 MΩ · cm) at a typical molar concentration of 0.033 M (0.1 M), whereas the AOT concentration in the two separate oil solvents solutions was chosen in the range of 0.05-0.5 M. The molar concentrations of the reactants in water were chosen to be higher than the room-temperature solubility of iron iodate measured at 6.2 × 10-4 M.32 Then, two distinct initial microemulsions for which the water-to-AOT molar ratio, W, was adjusted in the range of 4-10 were prepared containing either iron nitrate or iodic acid. They were mixed together under vigorous stirring at room temperature or 50 or 80 °C. UV-vis absorption spectra of the transparent microemulsions are obtained with a UV-vis spectrophotometer (Cary 50, Varian), and dynamic light scattering (Zetasizer Nano-ZS, Malvern Instruments) was used to estimate the size of the reverse micelles containing reactants and/or FIO nanoparticles. Phase identification of agglomerated powders was determined by X-ray diffraction with Co KR radiation (INEL CPS 120 instrument with a position-sensitive detector) after centrifugation of the microemulsions (Sigma 2-16 centrifuge), and the size and morphologies of FIO nanoparticles were characterized by transmission electron microscopy (Philips CN200). Nanoparticles of iron iodate were dispersed in ethanol, and a drop of the solution was then placed on a carbon-film-coated copper grid and dried in air at room temperature. HRS measurements were performed with a Q-switched Nd:YAG laser (Wedge HB, Bright Solutions) operating at 1064 nm with a pulse width of
