Pore structure of mesoporous silica (KIT-6) synthesized at different temperatures using positron as a nondestructive probe

Highlights

• Pore size of mesoporous KIT-6 can be tuned from 3.8 nm to 18.6 nm.

• The pore size estimated by o-Ps lifetime is consistent with N₂ adsorption results.

• The presence of micropore in pore wall was found.

• The second lifetime component ${\tau }_2$ can be also used to detect the pore size.

Abstract

Ordered mesoporous SiO₂ (KIT-6) was synthesized using triblock copolymer P123 as the structure template and tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si) as silica source. Small-angle X-ray scattering and high resolution electron microscope measurements indicate the 3d cubic Ia3d symmetry of the pore structure of KIT-6 synthesized at 30–120 $°$C. When the synthesis temperature increases to 180 $°$C, the order of pores was deteriorated. The pore size was estimated by nitrogen adsorption/desorption measurements, which increases from 3.8 nm to 18.5 nm as the synthesis temperature increases from 30 $°$C to 180 $°$C. With the increase of mesopore size, the pore wall thickness shows continuous decrease. Positron annihilation lifetime spectra was measured for the synthesized KIT-6. The lifetime spectra can be resolved to two short and two long lifetime components. The two long lifetimes ${\tau }_3$ and ${\tau }_4$ correspond to o-Ps lifetime in micropore and mesopores, respectively. The size of mesopores was estimated from the o-Ps lifetime by using Goworeck’s model for cylindrical pores, which shows continuous increase with synthesis temperature, and is consistent with the N₂ adsorption/desorption measurements. The size of micropore has no change with increasing synthesis temperature. However due to the decrease in wall thickness, the number of micropores in the wall shows subsequent decrease. In addition, the second lifetime component ${\tau }_2$ is also found to be a sensitive parameter for the pore size, which shows the same trend as lifetime ${\tau }_4$ with increasing synthesis temperature.

Introduction

In the last decades, mesoporous material has attracted much attention due to their wide applications in many fields, such as adsorption, separation, catalytic reaction, drug deliver, sensors, low-k dielectrics, energy storage and so on [1], [2], [3], [4]. Among the mesoporous materials, silica-based materials were most frequently studied because of their tunable pore size, large surface area and controllable structure [5], [6], [7], [8], [9]. A series of mesoporous silica, such as MCMn, SBAn, FDUn, KITn, etc. have been synthesized by different templating schemes [7], [10], [11], [12], [13]. KIT-6 silica has bicontinuous cubic structure of Ia3d symmetry and contains interpenetrating cylindrical pores, which is more suitable for the hard template and support of catalysts [14]. It was shown that the pore size of mesoporous silica could be controlled through delicate regulation of the reaction temperature, surfactant type or concentration, and swelling agent concentration [5], [7]. The pore structure of other mesoporous materials synthesized by hard-templating method can thus be controlled by adjusting the pore diameter of the templates. Therefore, it is important to tailor the pore size of KIT-6 in order to produce more abundant target materials.

Comparing with the tuning of pore size, characterization of the pore size and pore structure of the porous material is more important. Different techniques have been used to measure pore size and pore structure. The most popular methods are small-angle X-ray scattering (SAXS), gas adsorption/desorption and high-resolution transmission microscopy. SAXS can provide the unit cell dimensions and is more effective for materials with long range order. Gas adsorption/desorption provides information on pore size, pore volume, and surface area, but it is mainly effective for open mesopores. It cannot detect the closed-pores. High-resolution transmission microscopy is a powerful tool to elucidate the pore geometry. It is desirable to have more experimental techniques which can provide more comprehensive information about the pore size and pore structure.

Positron annihilation spectroscopy (PAS) has been widely used to study vacancy defects in various materials such as metal and alloys, semiconductors, polymers and porous materials [15], [16], [17]. Positrons will be preferentially trapped by vacancy-type defects such as monovacancies, divacancies, vacancy clusters and voids. Due to a lower electron density, the positron will live much longer at vacancy sites. In many porous materials, positron will also combine one electron to form a metastable hydrogen-like atom called positronium (Ps), which has a radius of 1.06 $\AA$ and binding energy of 6.8 eV. According to quantum mechanics, positronium can have either the spin anti-parallel para-positronium (p-Ps) (S = 0, ${\text{m}}_s$ =  0) or the spin parallel ortho-positronium (o-Ps) (S = 1, ${\text{m}}_s$ = 0, $\pm$1). In vacuum, the positronium will undergo self annihilation. The self-annihilation lifetime of p-Ps via 2$\gamma$-annihilation is 125 ps, while that of o-Ps via 3$\gamma$-annihilation is 142 ns. The ratio of p-Ps to o-Ps formation probability is 1:3.

When positronium is formed in porous materials, it will be localized in the pores. Due to the very short lifetime, p-Ps is weakly affected by the surrounding environment. However, the o-Ps has relatively much longer lifetime, then it will move around and collide with the pore wall back and forth. During the collision, it has chance to pick off one electron from pore wall and undergo 2$\gamma$-annihilation [18]. The pick-off annihilation lifetime of o-Ps will be reduced to a few ns, which depends on the pore size. A semi-empirical relationship between o-Ps lifetime and pore size R was established by Tao and Eldrup et al. [19], [20] assuming that positronium is trapped in a spherical pore, which can be expressed as follows:

$${\tau }_{o-\mathit{Ps}}^{-1}=2\left[1-\frac{R}{R+\mathrm{\Delta }R}+\frac{1}{2\pi }\sin \left(2\pi \frac{R}{R+\mathrm{\Delta }R}\right)\right]\text{.}$$

where $\mathrm{\Delta }$R is the thickness of the electron layer on the surface of pores which is determined to be 1.656 $\AA$ for many materials, and was also suggested to be 1.8 $\AA$ for silica [21], [22], [23]. The above equation is based on spherical pores, and for pores with irregular shape, we can get equivalent dimension of the pores, such as the radius of cylinder, length of the cube and so on.

The above Tao-Eldrup model works for micropores with diameter less than 2 nm (corresponding to o-Ps lifetime $\sim 20\mathit{ns}$). For larger pores, the chance of positron to pick electron from the wall is reduced, so the self-annihilation of o-Ps should be considered. There are many modified models to evaluate pore size from the o-Ps lifetime for large pores [24], [21], [25]. In addition, Goworeck [26] also extended Tao-Eldrup models to correlate o-Ps lifetime with pore size for the cylindrical pores. Therefore those well established models enable positronium to be a good probe for evaluation of pores in a wide size range covering micropores and mesopores. On the other hand, the intensity of o-Ps lifetime component can also reflect the relative number of pores. Moreover, it is worth noting that positronium probe is not limited to open pores, it can also detect any closed pores. Thus positronium probe can provide more comprehensive information about the pore structures than other methods [27], [28], [29], [30], [31], [32].

It should be noted that the formation and annihilation of o-Ps is also affected by other factors such as the chemical environment on the wall of the pores [33]. Some chemical agent will cause quenching of o-Ps lifetime and most of the time also prohibit positronium formation. If there are paramagnetic molecules in the surrounding of the pore, spin conversion of positronium will also occur, which causes decrease of both o-Ps lifetime and its intensity, since some o-Ps are converted to p-Ps and then annihilate by 2$\gamma$ emission. Those chemical quenching and spin conversion will bring uncertainties in evaluating the pore size and pore volume. In addition, the positronium formation is forbidden in porous materials which are electrically conductive [34]. If the positronium formation is forbidden, we will even fail to probe the pore structure through o-Ps lifetime. Jean et al. [35] proposed a new idea to measure the free volume size in polymer materials by using the second lifetime component. This is the positron annihilation lifetime in free volume holes without formation of positronium. They have successfully extended the Tao-Eldrup equation to measure the free volume hole with radius R $<$ 5 $\AA$ by using ${\tau }_2$. For free volume hole radius $>$ 5 $\AA$, their equation is no longer applicable and should be further modified. But this give hints that the positron lifetime ${\tau }_2$ can be also used to detect the pores in porous materials.

In this paper, we synthesized the mesoporous KIT-6 template at different temperatures from 30 $°$C to 180 $°$C. The pore structure was studied by positron lifetime spectroscopy together with SAXS, HRTEM and N₂ adsorption/desorption measurements. Our results show that positronium is a very sensitive probe for not only mesopores but also micropores, and the mesopore size increases with increasing synthesis temperature. The second lifetime component ${\tau }_2$ can be also used to detect the pore size.