Erzsébet Hild

Characterisation of silica nanoparticulate layers with scanning-angle reflectometry

Scanning-angle reflectometry using p-polarized laser light is a sensitive in situ method to characterize the structure of very thin surface layers when the reflectance is scanned around the Brewster angle of the substrate. Monolayers of silica particles (of size 40 nm and of different hydrophobicity) on the water surface were investigated with this method. We used both a uniform-layer model to evaluate the measured reflectance curves and a “gradient refractive index layer” approximation. The uniform-layer approximation yields both an effective refractive index and a layer thickness, but it breaks down when the layer is strongly inhomogeneous, i.e. the size of particles is comparable with the wavelength and/or the refractive index of the particles differs much from that of the host material. At the air–water interface, the particles are partially submerged in the liquid phase and partially surrounded by air, so there is a steep refractive index change inside the layer. We elaborated a model which takes in depth inhomogeneity into account, and the structure parameters, such as the immersion depth, the particle diameter, the nearest-neighbour distance and the surface coverage, were determined by fitting the theoretical reflectivity to the measured reflectance curves. The effective refractive index of the hydrophilic layers was considerably higher than that of the hydrophobic ones, and that proved to correlate with the immersion depth of the particles. The parameters also changed with the applied surface pressure.

Langmuir and Langmuir−Blodgett Films of Bidisperse Silica Nanoparticles

We present the studies on the structure and optical properties of bidisperse Stöber silica nanoparticulate Langmuir films prepared at the air/water interface in a Wilhelmy film balance and transferred onto glass slides using the Langmuir-Blodgett technique. Three different compositions (covered area ratios: 4:1; 1:1, and 1:4) of two bidisperse systems were used in the experiments. Bidisperse samples (B1 and B2) were prepared by mixing the appropriate amount of monodisperse sols of particles with 61 and 100 nm diameters (B1) and those with 37 and 100 nm diameters (B2). By surface pressure-area isotherms and (transmission and scanning) electron microscopy images we provide information about the structure of the films. Optical properties of the supported films were measured with UV-vis spectroscopy and the transmittance spectra were evaluated in terms of an optical model which allows monotonous in-depth variation of the refractive index across the film. (1) We have found that the refractive index decreased from the substrate-layer interface toward the air-layer interface when the smaller particles were in majority, and increased otherwise. That would suggest that the smaller particles of each bidisperse system can be positioned at the air side of the film if they are in minority in the sample and they can be situated on the substrate if they are in majority. The scanning electron microscope images of bidisperse films supported the in-depth film structure suggested by optical studies.

Use of the optical admittance function and its WKB approximation to simulate and evaluate transmittance spectra of graded-index colloidal films

A method is presented for the simulation of transmittance spectra of graded-index dielectric layers deposited on a transparent substrate of finite thickness, and the simulation process is applied to films of silica and zinc oxide nanoparticles. The method is based on the optical admittance by solving its differential equation for an appropriate refractive index profile. When the refractive index changes with depth z according to the function N(z) = N(0)/(1+z/ζ)2, an exact solution exists which is equivalent to the WKB (geometric optical) approximate solution derived for the electromagnetic field in weakly inhomogeneous media. Using this solution allows us to characterize transparent graded-index films by two optical parameters—the mean refractive index and the grade of inhomogeneity, in addition to the film thickness through a fast curve fitting process.

Complex Langmuir−Blodgett Films of SiO2 and ZnO Nanoparticles with Advantageous Optical and Photocatalytical Properties

Multifunctional Langmuir-Blodgett (LB) films were fabricated on the surface of glass substrates using sol-gel derived ZnO and SiO2 particles. ZnO particles of 6 and 110 nm diameter were synthesized according to the methods of Meulenkamp and Seelig et al. (Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566; Seelig, E. W.; Tang, B.; Yamilov, A.; Cao, H.; Chang, R. P. H. Mater. Chem. Phys. 2003, 80, 257). Silica particles of 37 and 96 nm were prepared by the Stober method (Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62). Alternate deposition of monoparticulate Langmuir films of SiO2 and ZnO nanoparticles provided complex (six- and nine-layered) LB films with both antireflective and photocatalytic properties. The LB films were investigated with scanning electron microscopy (morphology and structure) and UV-vis spectroscopy (optical properties and stability). The photocatalytic activity was measured by immersing the UV-irradiated films into an aqueous solution of Methyl Orange and following the photodegradation of the dye by optical spectroscopy. Adding ZnO particles to the silica films slightly lowered the antireflection property but ensured strong photocatalytic activity. Both the photocatalytic activity and antireflection properties were proved to be sensitive to the sequence of the silica and ZnO layers, with optimum properties in the case of nine-layered films with a repeated (SiO2-ZnO-ZnO) structure.

CONTACT ANGLE DETERMINATION OF NANOPARTICLES: REAL EXPERIMENTS AND COMPUTER SIMULATIONS

Monolayers of Stöber-silica nanoparticles (ca. 40 nm diameters) at the water–air interface have been studied in a Wilhelmy film balance. Scanning angle reflectometry and molecular dynamics computer simulation have been used to assess the contact angles of the particles. Our results indicate that the traditional film balance method of contact angle determination overestimates the real contact angles even in the lower range of particle hydrophobicities.

Characterisation of solid supported nanostructured thin films by scanning angle reflectometry and UV-Vis spectrometry

Nanostructured Langmuir-Blodgett (LB) films of Stöber-silica nanoparticles have been prepared on silicon and quartz glass substrates. The deposited layers were investigated using scanning angle reflectometry and UV-Vis spectroscopy. The reflectivity and the transmittance spectra of the LB films were evaluated using a model based on thin layer optics. Film thickness and effective refractive index of the films were determined. From the refractive index values the volume fraction of the particles in the film was estimated by effective medium approach.