Effect of sodium bicarbonate solution on methyltrimethoxysilane-derived silica aerogels dried at ambient pressure

Here we present an economical ambient pressure drying method of preparing monolithic silica aerogels from methyltrimethoxysilane precursor while using sodium bicarbonate solution as the exchanging solvent. We prepared silica aerogels with a density and a specific surface area of 0.053 g·cm−3 and 423 m2·g−1, respectively. The average pore diameter of silica aerogels is 23 nm as the pore specific volume is 1.11 cm3·g−1. Further, the contact angle between water droplet and the surface of silica aerogels in specific condition can be as high as 166°, which indicates a super-hydrophobic surface of aerogels.


Introduction
Silica aerogels (SAs) are highly porous nanomaterials with extremely low densities, high specific surface areas and high porosities [1]. Due to these properties, SAs have various potential applications, e.g., drug delivery systems [2], energy storage systems [3], cosmic dust capture [4], paints [5], food packaging [6], and insulation in buildings [7]. Despite the advantages of SAs, there are still some practical limitations. For example, supercritical drying, freeze drying and ambient pressure drying (APD) methods are commonly used in the manufacturing process of SAs during the drying stage. However, the most conventional supercritical drying method requires expensive equipment with high-pressure operational conditions [8,9]. Freeze drying method applies sublimation of pore liquid but may break the network of aerogel [10]. The less energyintensive alternative is APD method, which usually relies on low surface tension solvents, e.g., hexane, heptane, xylene, etc. [11]. Thus, this is still uneconomical because of large quantities of organic low surface tension solvents used in scaling up SAs production in APD method. However recently, it was suggested that a low-cost combination of sodium bicarbonate with trimethylchlorosilane (TMCS) due to in-situ generation of CO 2 can prevent wet gels from shrinkage and collapse in this APD method [12].
Furthermore, conventional SAs made from precursor tetraethoxysilane and tetramethoxysilane are usually moisture-sensitive because of the hydrophilic surface [9]. Studies of hydrophobic SAs prepared from methyltrimethoxysilane (MTMS) as the precursor, which has a methyl group '-CH 3 ' attached to 'Si-O' framework, generally focus on dilution of precursor [13,14], the concentration of catalyst [14,15], effect of surfactant [15,16]. To the best of our knowledge, there has been no report on using sodium bicarbonate solution as the exchanging solvent in APD method of MTMS-derived SAs, hence it is the aim of this study. In addition, we here present comparison in regard to properties of the MTMSbased SAs using deionized water, hexane and different concentration of sodium bicarbonate solutions during the APD process.

Preparation
Wet gels were prepared via an acid-base catalysis sol-gel process [14]. Firstly, precursor MTMS, methanol and half of deionized water (in form of 0.01 mol•L -1 oxalic acid solution) were mixed in a 40 mL vial at a molar ratio of 1:32:8 while consistently stirring at 350 r•min -1 for 30 min. After 24 h, another half of water in form of 10 mol•L -1 ammonia solution was slowly dropped while stirring at 350 r•min -1 for 30 min. Gelation happened within 24 h, and then extra methanol was filled into the vial in case of shrinkage. This ageing process lasted 3 d, and 4 wet gels that are obtained were labelled as G1, G2, G3 and G4. At the solvent exchange stage, G1 was exchanged by deionized water, while G2 was first exchanged by ethanol and then by hexane. G3 and G4 were exchanged with 0.044 g•mL -1 and 0.022 g•mL -1 sodium bicarbonate solution, respectively. After full solvent exchange, G1 and G2 were dried at ambient pressure for 1 d. G3 and G4 were first poured with mixed solvent of TMCS and ethanol at volume ratio of 1:4, and then extra ethanol was filled into vials [12]. After washed by the mixed solvent of deionized water and ethanol at a volume ratio of 1:1, G3 and G4 were dried at ambient pressure for 1 d. Finally, four aerogel samples obtained were labelled as A1, A2, A3 and A4.

Characterisations
A Thermo-Scientific Surfer system was used to determine specific surface areas and pore size distributions in all samples. The specific surface area was found by measuring the adsorption-desorption isotherms at 77 K of gaseous N 2 using a Brunaer-Emmett-Teller analysis, while the pore size distributions were obtained from the same isotherms by the Barrett-Joyner-Halenda method.
The porosities of all samples were calculated by Eq. (1): where bulk is determined by mass over volume. skeletal is the skeletal density of MTMS-derived SAs, and it usually is 1.9 g•cm -3 [14]. X-ray diffraction (XRD) analysis was undertaken via a PANalytical X'Pert Pro multipurpose diffractometer by using Cu Kα X-rays. Samples were mounted on a lowbackground silicon substrate and diffraction scans covered a 2θ range of 5°to 90°. Fourier-transform infrared (FT-IR) spectroscopy, an IRAffinity-1s (Shimadzu Scientific Instruments), was used to study chemical bonds of SAs in the range of 500 to 4000 cm -1 , taken after 1-hour heating of samples at 120°C in the oven in order to remove moisture. Furthermore, an angle meter CAM100 (KSV Instrument Ltd.) was used to measure the contact angle between deionized water droplet and surface of SAs. After coated with gold, microstructure morphology of SAs was measured with a scanning electron microscopy (SEM) (JEOL 7800F Prime). Transmission electron microscopy (TEM) experiments were carried out by a HT7800 (Hitachi High-Technologies Corporation) at Newcastle University after ultrasonication of SAs in ethanol until solution was dropped on the copper grid.
3 Results and discussion Figure 1 presents the observation of all samples. We see that in Fig. 1(a) SAs are powder after solvent exchange step with deionized water while in Fig. 1(b) SAs in solvent exchange step with hexane and in Figs. 1(c,d) different concentration of sodium bicarbonate solution remain monolithic after the APD. Drying process causes more shrinkage of wet gels when solvent exchange was with hexane than sodium bicarbonate solution. This can be explained from previous work [12], which shows that SAs could stay monolithic because CO 2 produced from reaction between TMCS and sodium bicarbonate is trapped within the gels and stops collapse of porous structure of SAs. Also, wet gels exchanged with 0.044 g•mL -1 sodium bicarbonate solution (Fig. 1(c)) have less shrinkage than those with 0.022 g•mL -1 sodium bicarbonate solution ( Fig. 1(d)) after APD process. In general, XRD 2θ scans obtained from all SAs (Fig. 2) present a wider peak at 22°that is characteristic of amorphous silica [17]. The narrower peak at 2θ < 10°has been observed in many previous studies with MTMS, tetraethoxysilane, PhTEOS, etc. precursors, see for example references [7,[17][18][19]. It has been emphasised, that the origin of this peak is still controversial in literature, because peak has been assigned to the presence of fourfold siloxane rings or to a discrete structural unit in matrix based on an octametric silicon arrangement [19]. Additional recent hypothesis is that this peak at 2θ < 10°is due to structure that consists of a siloxane network and organic layers containing ordered arrangements [19]. Peaks at 32°, 45°, 56°, 67°and 75°of sample A3 and A4 (see Figs. 2(c,  d)) are corresponding to sodium chloride (NaCl) at (200), (220), (222), (400) and (420), respectively (PDF 00-005-0628). In addition, NaCl can be removed by extra wash with mixed solvent of deionized water and ethanol ( Fig. 2(e)).
Physical properties of samples are presented in Table 1. Wet gels exchanged with sodium bicarbonate solution after APD process maintain the lower average pore diameter of 17 nm and pore specific volume of 0.53 cm 3 •g -1 whereas hexane offers sample a high specific surface area of 480 cm 2 •g -1 . In addition, sodium bicarbonate provides SAs with the lowest bulk density of 0.053 g•cm -3 . In other words, the porosity of samples using sodium bicarbonate solution in the solvent exchange step based on calculation was obtained as high as 97.2%. However, there are reductions on specific surface area and pore specific volume of SAs using sodium bicarbonate solution in solvent exchange step after extra wash of wet gels prior to drying. Figure 3 shows the SEM micrographs of SAs synthesised with deionized water (a), hexane (b), 0.044 and 0.022 g•mL -1 sodium bicarbonate solution (c,d) in the solvent exchange step. All samples display a threedimensional nanoporous structure. However, there is a visible coarsening of the structure with decreasing sodium bicarbonate concentration, reflecting in an increase in the average feature size observed in the micrographs together with a small porosity, which correlates well with the pore size increase for lower concentration of sodium bicarbo-   nate with TMCS, which generates more CO 2 gas bubbles within a gel during pre-drying step, hence improving overall porosity as well as suppressing collapse of the holes, which then results in smaller pore size. FTIR spectra of all SAs are presented in Fig. 4. It is noticeable that backbone of SAs 'Si-O-Si' is located at 1100 cm -1 and 770 cm -1 which are asymmetric and symmetric modes, respectively [20]. Strong peaks at 1271 and 852 cm -1 correspond to 'Si-C' bond [21,22]. Peaks at 2950 and 1450 cm -1 are stretching and bending modes of 'C-H' bonding, respectively [20]. TEM images (Fig. 5) show the microstructure of the MTMS-derived aerogels. It is clearly that wet gels exchange with deionized water and hexane (Figs. 5(a,b)) after APD process have a more compact structure when compared to those exchanged with sodium bicarbonate solution (Figs. 5(c,d)). This is also correlated with reduced shrinkage of A3 and A4 samples observed in Fig. 1.
Investigation of hydrophobicity is shown in Fig. 6. We find that all samples are hydrophobic and float on water surface even after 15 d (see Figs. 6(a-c)). For further investigation of hydrophobicity, contact angle measurement between deionized water droplet and aerogels surface are presented in Figs. 6(d-g). The SAs using 0.022 g•mL -1 sodium bicarbonate solution in the solvent exchange step have the highest value of 166°, indicating a super-hydrophobicity [23]. Sample A3 using 0.044 g•mL -1 sodium bicarbonate solution has the lowest contact angle of 108°, which is likely due to the presence of residual NaCl that is by-product of reaction of sodium bicarbonate and TMCS. However, this contact angle increases to 141°after the A3 has been extra washed with mixed solvent of ethanol and deionized water (see Fig. 6(g)), NaCl has been removed from SAs (as confirmed by XRD in Fig. 2). The A2 sample, where solvent exchanged in wet gels with hexane, has a contact angle of 143°.

Conclusions
Super-hydrophobic SAs with the specific surface area of 423.6 m 2 •g -1 , pore specific volume of 1.11 cm 3 •g -1 , and density of 0.053 g•cm -3 were prepared from precursor MTMS via APD method by using sodium bicarbonate solution with TMCS in solvent exchange step. Comparing to deionized water, sodium bicarbonate solution prevents wet gels from collapse during APD process and reduces shrinkage of the wet gels due to generated CO 2 gas within the pores of those. Although high concentration of sodium bicarbonate solution improves porosity and reduces the shrinkage of the wet gels, it also increases the yield of byproduct NaCl that requires extra washing step. This then still delivers SAs similar contact angle to samples using hexane in solvent exchange step. However, lower concentration sodium bicarbonate solution leads to superhydrophobic SAs sample without any extra wash, with a contact angle as high as 166°, whereas hexane used SAs has a contact angle of 143°.
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