ACTIVIDAD ¿CUANTO TARDA EN DEGRADARSE?

Nitrogen sorption is commonly used in the characterization of porous solids, allowing one to determine the specific surface area, pore volume, and pore size distribution10. With the discovery of the ordered mesoporous materials, the theoretical models for estimation of the aforementioned parameters have been successfully proven11. A result of the gas sorption measurement is a plot of the volume of gas absorbed/desorbed vs relative pressure P/Po (P is

the absolute pressure, Po is the saturation vapor pressure) at constant temperature, called

sorption isotherm. Nitrogen-sorption instruments present an important characterization tool for many laboratories dealing with nanoporous materials.

Experimental gas-adsorption isotherms usually fall into six categories according to the IUPAC classification12 (Figure 4.5.A). The most important for gas absorption in mesoporous solids are Type IV, V, and IVc isotherms. The gas absorption proceeds via

Chapter 4. Characterization techniques for nanostructured materials

Capillary condensation and capillary evaporation often do not take place at the same relative pressures, which leads to the appearance of hysteresis loops. This phenomenon is usually attributed to thermodynamic or pore connectivity problems; the latter may be more prominent10, 13. Gold surface Quartz crystal Mesoprous film B) A)

Figure 4.5. (A) Classification of gas adsorption isotherms and (B) schematic representation of the QCM device with a mesoporous film.

The thermodynamic effects are related to the metastability of adsorption or desorption (or both) branches of the isotherm. Namely, the capillary condensation or evaporation may be delayed and take place at higher or lower pressures, respectively, in comparison to the pressure of coexistence between the gas-like or liquid-like phases in the pore. Pore- connectivity (network) effects play an important role when, for example, larger pores have access to the surrounding only through narrower pores, the former cannot be emptied at the relative pressures corresponding to their capillary evaporation since the latter are still filled with the condensed adsorbate. So the larger pores may actually be emptied at a relative pressure corresponding to the capillary evaporation in the smaller pores (or at the relative pressure corresponding to the lower limit of adsorption-desorption). However, it was

Chapter 4. Characterization techniques for nanostructured materials

confirmed many times that capillary condensation/evaporation can also be reversible as in Type IVc isotherms14.

The adsorption isotherm may be used to calculate the surface area of the sample. The procedure known as the BET model was introduced by Brunauer, Emmett, and Teller10. It is based on the evaluation of the monolayer capacity (the number of the absorbed molecules in the monolayer on the surface of a material). The monolayer capacity is multiplied by the cross-sectional area of the absorbed molecules in the monolayer formed on a given surface. The derivation of the BET model is based on the Langmuir equation relating the number of molecules adsorbing on the surface with the number of molecules evaporating (desorbing) from the sample, involving several assumptions, such as a flat surface, equivalence of all the adsorption sites, and the absence of lateral interactions between absorbed molecules. In the case of adsorption on real solids, these assumptions often do not hold. Therefore caution must by exercised regarding the interpretation of the specific surface area of the solids derived from the BET model. Despite all of these issues, the BET model is currently a standard method for the specific surface area evaluation, and relative comparisons provide valuable information.

For the evaluation of the pore size distribution, a number of models have been suggested that are usually based on the Kelvin equation. Assuming that the vapor side of the meniscus formed in a pore behaves as an ideal gas at constant temperature, the general form of the Kelvin equation for the determination of the radius of the hemispherical meniscus may be derived

ln[P/Po] = -2σVL / RkRT (4.4.)

where VL is the molar volume, R is the gas constant, T is the temperature, Rk is Kelvin

radius (the mean radius of curvature of the meniscus at which capillary condensation occurs). The most commonly used model to calculate pore size distribution that is often integrated in the sorption apparatus software is the BJH model (Barett, Joyner, Halenda method)15. It has recently been confirmed using MCM-41 with approximately cylindrical pore geometry that this model reflects the actual nature of adsorption in mesopores16. More theory on the sorption of gases in porous hosts can be found elsewhere10, 17.

Chapter 4. Characterization techniques for nanostructured materials

In this thesis, the nitrogen-sorption measurements on powdered mesoporous materials were performed with a NOVA 4000e Surface area & Pore Size Analyser after evacuation of the samples at 120°C. Unfortunately, nitrogen-sorption experiments on thin films are very difficult or even impossible to obtain with this technique because of the small amount of the adsorbent (less than 0.1 µg/cm2 of the film surface). Here, an alternative technique to estimate the amount of the sorbed gas that relies on the high gravimetric sensitivity of the quartz crystal microbalance (QCM) is described.

The QCM detection is a variant of acoustic wave microsenseors that is capable of ultrasensitive mass measurements. Under favorable conditions, a typical QCM can measure a mass change of 1-10 ng/cm2. The QCM oscillates in a mechanically resonant shear mode under the influence of a high-frequency AC electric field that is applied across the thickness of the crystal. The central portions of the top and bottom sides of the crystal are coated with thin films of gold or other metals that are typically of disk shape. The mass sensitivity of the QCM originates from the relationship between the oscillation frequency on the total mass of the crystal and the adlayers of material residing on the metal-coated crystal faces, the Sauerbrey equation, as shown below.

∆f = -2f02∆m / [A sqrt(µρ)] (4.5.)

where f0 is the resonant frequency of the fundamental mode of the crystal, A is the area of the

gold disk coated onto the crystal, ρ is the density of the crystal (= 2.684 g/cm3), and µ is the shear modulus of quartz (= 2.947 x 1011 g/cm.s2). Using a crystal with a 10 MHz fundamental frequency (as used in our measurements), a net change of 1 Hz corresponds to 26.4 ng of materials adsorbed or desorbed onto a crystal surface of 1 cm2.

In a typical measurement, the gold surface of the QCMs modified with linking molecules (see Chapter 6.2.) were covered with mesoporous film (Figure 4.5B) and after calcination the mass of the film was measured by recording the frequency of the QCM before and after deposition of the film. The mesoporous film was dehydrated in flowing helium at 120°C for 24 h and the measurement chamber was cooled to liquid-nitrogen temperature. A gas flow of water-free nitrogen in helium was conducted by a computer-controlled gas-flow

Chapter 4. Characterization techniques for nanostructured materials

system (MKS Instruments) in the range of 0 – 95 %. Using digital mass-flow controllers and a calibrated gas-flow system it was possible to finely control the partial pressure of the sorbed gas (N2) at every sorption step. The nitrogen adsorption/desorption isotherm was

obtained by measuring the frequency of the QCM at every adsorption/desorption step (every partial pressure of N2) and recalculating the amount of the nitrogen adsorped by using the

Sauerbrey equation.