Optical Response of Porous Titania-Silica Waveguides to Surface Charging in Electrolyte Filled Pores

Abstract

Dedicated to Professor Dieter Seebach on the occasion of his 65th birthdayIn this work, we present a novel method for in situ investigation of surface charging and ion transport inside nanopores of titania-silica waveguide by means of the optical-waveguide-lightmode spectroscopy. Porous oxide waveguides show a strong optical response when exposed to electrolyte solutions, and this response is consistent with oxide surface charging due to changes in ionic strength and pH of the solution in contact with the waveguide. The optical response to pH or electrolyte concentration change is stabilized within several minutes when the solution ionic strength is sufficiently high (0.1m), while it takes two orders of magnitude longer to reach stable optical response at very low ionic strengths (< 0.1mm). The relaxation times at the high ionic strength are still by several orders of magnitude slower than expected from bulk diffusion coefficients of electrolytes in water. Our results indicate that diffusion of electrolytes is severely hindered (and more so with decreasing ionic strength) in charged pores inside waveguides.1. Introduction. ± Sol-gel-based silica-titania mixed oxides are important materials in catalysis [1] [2] and optical applications [3 ± 5]. Sol-gel synthesis typically involves one or more metal alkoxides undergoing hydrolysis in aqueous solutions and subsequent condensation eventually forming particles or gels [6]. Resulting materials are usually nanoporous with substantial internal surface areas on the order of 100 m 2 / cm 3 and even higher, when pore filling surfactants or molecular templates are used. Water can be removed from wet gels by supercritical drying to produce low-density aerogels or at ambient pressure with various heating protocols. Ambient-pressure drying causes further condensation and collapse of nanopores due to capillary pressure, resulting in densification and internal surface loss. Heat treatment promotes phase changes such as nucleation and growth of crystalline phases, and phase separation, also leading to densification and decreased porosity. Microstructure of mixed oxides is thus influenced in early stages, when small metal-oxide oligomers have a chance for heterocondensation to achieve molecular-scale homogeneity in the wet gel, as well as in later stages, when porosity and homogeneity can be severely modified during drying.Planar optical waveguides can be prepared by coating silica-titania sols on appropriate substrates to obtain wet gel films, into which diffraction gratings are embossed, followed by further heat treatment in order to achieve required film hardness and stability [7] [8]. The final waveguide thickness is on the order of few hundred nanometers. Depending on temperature and duration of drying, the film porosity can be as high as 20%. For standard drying procedures (few hours at 5008) typical porosity was found to be around 15% [9]. With temperatures as high as 9008, the porosity of silica-titania films can be reduced below 1% [10].

Effects of Temperature, pH, and Salt Concentration on β-Lactoglobulin Deposition Kinetics Studied by Optical Waveguide Lightmode Spectroscopy

“…Hydrolysis of metal-oxygen-metal bonds results in formation of highly polarizable metal-hydroxy groups, which then contribute to increasing the overall waveguide refractive index n F . 33 The same effect was also observed for silica-and niobia-coated waveguides. Coating of waveguide surfaces (i.e., by polymer or protein) can be used to effectively suppress this effect by providing a barrier between the waveguide and the liquid solution and thus slowing down the waveguide hydrolysis.…”

“…The observed drift in the waveguide refractive index may be due to gradual hydrolysis of metal−oxide−metal bonds of metal (silicon and/or titanium) oxide interfaces in internal waveguide pores when exposed to aqueous solutions at elevated temperatures. Hydrolysis of metal−oxygen−metal bonds results in formation of highly polarizable metal−hydroxy groups, which then contribute to increasing the overall waveguide refractive index n F . The same effect was also observed for silica- and niobia-coated waveguides.…”

Optical Waveguide Lightmode Spectroscopy (OWLS) as a Sensor for Thin Film and Quantum Dot Corrosion

“…QD-containing solution (pH 6.31) was injected at point E, which resulted in very rapid and extensive apparent mass loss at a rate similar to that in the initial part of the red trace. Correcting for pH change would increase the apparent mass loss slightly [18]; however, this correction was not necessary. QD-free MOPS buffer (pH 7) was reinjected at point F, creating conditions identical to the starting conditions.…”