Abstract
The local structure of water on chemically and structurally different surfaces is a subject of ongoing research. In particular, confined spaces as found in mesoporous silica have a pronounced effect on the interplay between the adsorbate-adsorbate and adsorbate-surface interactions. Mid-infrared spectroscopy is ideally suited to quantitatively and qualitatively study such systems as the probed molecular vibrations are highly sensitive to intermolecular interactions. Here, the quantity and structure of water adsorbed from the gas phase into silica mesopores at different water vapor pressures was monitored using mid-infrared attenuated total reflection (ATR) spectroscopy. Germanium ATR crystals were coated with different mesoporous silica films prepared by evaporation-induced self-assembly. Quantitative analysis of the water bending vibration at 1640 cm-1 at varying vapor pressure allows for retrieving porosity and pore size distribution of the mesoporous films. The results were in excellent agreement with those obtained from ellipsometric porosimetry. In addition, different degrees of hydrogen bonding of water as reflected in the band position and shape of the stretching vibrations (3000-3800 cm-1) were analyzed and attributed to high-density, unordered bulk, low-density, and surface-induced ordered water. Thereby, the progression of surface-induced ordered water and bulk water as a function of water vapor pressure was studied for different pore sizes. Small pores of 5 nm diameter showed a number of two-ordered monolayers, whereas for pores >12 nm diameter, the number of ordered monolayers is significantly larger and agrees with the number observed on planar SiO2 surfaces.
Highlights
All applications of mesoporous materials in industry and research, such as catalysis,[1,2,3] separation,[4] electrochemistry,[5] orsensing[6,7] benefit from the materials high-surface area and defined porosity
The quantity and structure of water adsorbed from the gas phase into silica mesopores at different water vapor pressures was monitored using mid-infrared attenuated total reflection (ATR) spectroscopy
Scheme 1: Mesoporous silica film coated on ATR crystal (Germanium, 20 active bounces) placed in a flow cell that is inserted into the beam path of an FTIR spectrometer and purged with humidified air
Summary
All applications of mesoporous materials in industry and research, such as (photo) catalysis,[1,2,3] separation,[4] electrochemistry,[5] or (bio-)sensing[6,7] benefit from the materials high-surface area and defined porosity. Measurements during adsorption/desorption from the gas phase at controlled humidity are needed to study the effect of the degree of pore filling on water’s local structure While such measurements have been reported for planar, dense SiO2 films, the strong effects of confined spaces on the structure of adsorbed water sparks interest in a corresponding study on mesoporous materials that is yet pending.[15,18,19,20] In order to discuss the water adsorption/desorption process quantitatively and qualitatively, characteristic parameters such as porosity and pore size distribution (PSD) are needed. Scheme 1: Mesoporous silica film coated on ATR crystal (Germanium, 20 active bounces) placed in a flow cell that is inserted into the beam path of an FTIR spectrometer and purged with humidified air In this contribution, we present a method for determining porosity and PSD of mesoporous films using ATR FTIR spectroscopy non-destructively and at ambient conditions. We calculate the number of surface-induced ordered monolayers of water for two different pore sizes and demonstrate that the number is similar to planar SiO2 surfaces only for big pores, while for smaller pores the number is significantly reduced
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