A study of morphological properties of SiO2 aerogels obtained at different temperatures

In this paper, temperature dependence of nanoporous framework evolution process and variety of pore properties (pore volume, specific surface area (BET), and pore size) of SiO2 aerogels were characterized by FTIR, XPS, XRD, SEM, TEM, BET, BJH, etc. Results show that SiO2 aerogels treated at different temperatures all possess amorphous structure. With the increase of treated temperatures, BET values of SiO2 aerogels increase initially and then decrease, and it reaches the maximum value of 882.81 m2/g when treated at 600 °C for 2 h due to the addition of the nanopores and shrinkage skeleton of SiO2 aerogels. Higher temperatures may result in a framework transformation and particle growth; both factors could reduce the BET values of the aerogels. Nanoporous skeleton of SiO2 aerogels at room temperatures is composed of tetrahedron with a pore size of about 22.28 nm. Higher treated temperatures result in an increase of octahedron amount in nanoporous framework and a decrease of pore size. When treated at 1000 °C, an approximate dense SiO2 bulk via the framework collapse and particle growth is obtained. These varieties are derived from the formed extra bonds of Si–O–Si, higher local stress, and liquid phase between particles during heat treatment process.

the nanoparticles and special nanoporous framework.While the special skeleton of aerogel may be destroyed and nanoparticles may grow up at higher temperatures, both can acutely reduce the density and thermal properties of SiO 2 aerogel [6].Many researches concentrated on strengthening the thermal stability and strength of the aerogel via the preparation of hybrid aerogels [7][8][9][10][11].Kim et al. [9] synthesized a glass fiber/silica aerogel, which possessed bigger pore size and specific surface area than aerogel composites made of only colloidal silica.Wu et al. [10] fabricated aerogel composites using a multilayer aligned glass E-mail: gaopengzhao7602@hnu.edu.cnfiber as reinforcer, which owned a low thermal conductivity of 0.022-0.028W/(mK) and compressive strength of 1.71-3.70MPa.Fenech et al. [11] prepared ZrO 2 /Y 2 O 3 aerogels with BET surface area of 159 and 26 m 2 /g after heat treatment at 550 and 1000 ℃ respectively.
It is known that the performance of a porous material depends on the pore size and framework structure [12], and these will vary at high temperatures.There are a limited number of studies that report the relationship between the nanoporous framework formation and particle growth process with the pore properties of SiO 2 aerogels obtained at different temperatures [16].Zhou et al. [13] studied the framework structure of aerogels with two different granular sizes of 5 and 20 nm.They found that the small pores collapsed while the large pores expended after calcination at high temperatures.Huang et al. investigated the structural changes of silica aerogels in the 9501200 ℃ temperature range.Their results showed that the process consists of three steps [14,15].
The aim of this work is to study the process of SiO 2 aerogel nanoporous framework formation and particle growth at different temperatures with a series of physicochemical methods, among others with FTIR, XPS, and TEM, to propose a qualitative regular polyhedron model of the process.

1 Synthesis
The synthesis of silica aerogels involves two steps: (i) the preparation of an alcogel and (ii) the solvent removal from the alcogel by ethanol supercritical drying [17].The alcogel was prepared from tetraethoxysilane (TEOS) by a two-step method.Firstly, the designed amount of TEOS was dissolved in alcohol and then the mixture was stirred for 30 min at 35 ℃.Then, the designed amounts of water, alcohol, and catalyst were mixed and the mixture obtained was added dropwise to the TEOS solution under continuous stirring.In order to obtain alcogel with a stable structure, alcogel was submerged under alcohol for two days at room temperature.Finally, the alcogel was heated at a rate of 1 ℃/min to 300 ℃.When the temperature reached 300 ℃, the pressure was kept at 10 MPa for 2 h.This step was followed with a controlled cooling step to produce the aerogel [18].
The aerogels obtained were calcined under air atmosphere at different temperatures (400, 600, 700, 800, and 1000 ℃ ) to study the process of the nanoporous framework evolution, with a heating rate of 10 ℃/min.The holding time was designed as 2, 4, and 8 h.

2 Characterization
The phase composition of the aerogels was analyzed by powder X-ray diffraction using a D5000 apparatus (Siemens).The scanning rate was 2 (°)/min using nickel filtered Cu K radiation produced at 30 kV and 30 mA.The oxidation state was investigated by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS Ultra spectrometer, equipped with a monochromatic Al Kα X-ray source, a delay-line detector, and an Al anode (Al Kα = 1486.71eV) operated at 225 W. The XPS spectra collected were analyzed using an XPSpeak software and calibrated using the adventitious O 1s peak with a fixed value of 533.3 eV.A Shirley-type background subtraction was used to fit the curve using a 20% Gaussian-80% Lorentzian function.The framework structure of the aerogels was characterized by Fourier transform infrared spectroscopy (FTIR, Iraffinity-1, Shimadzu, Japan) in the wavenumber range of 4000-400 cm 1 .The morphology was examined by scanning electron microscopy (SEM, Jeol, JSM-6700F).The porous framework and the nanoparticle size were characterized by transmission electron microscopy (Philips, Tecnan F20 operated at 20 kV).
N 2 adsorption isotherms were recorded with a TriStar 3000 apparatus (Quantachrome).The pore size distribution of the aerogels was obtained from the desorption branch of N 2 adsorption isotherms using BerrettJoynerHallenda (BJH) method.The specific surface area was obtained by Brunauer-Emmet-Teller (BET) method.

1 Effect of calcination temperature on morphology of SiO 2 aerogels
Figure 1 shows the FTIR spectra of the SiO 2 aerogels calcined at different temperatures.The asymmetric, symmetric, and rocking bending vibrations of Si-O-Si bonds are observed at 1100, 800, and 468 cm 1 , respectively [19,20].The peak at 975 cm 1 is assigned respectively).The binding energy (BE) and the fraction (mol%) of different elements are listed in Table 1.This estimation was obtained from the deconvolution of the corresponding XPS bands following the method described in Refs.[23,24].The peak at 533.18-533.50eV originates from the Si-O-Si groups [25,26], the peak at 532.62-532.93 eV originates from the Si-OH groups [27], while the peak at 532.31-532.46eV originates from the Si-OC 2 H 5 groups [28].
As the calcination temperature increases, the amount of Si-O-Si groups increases from 51.8% to 96.6%, while that of Si-OH and Si-OC 2 H 5 groups decreases, indicating the formation of additional bonds of Si-O-Si via the condensation reaction, in agreement with the results obtained from the FTIR study.The formation of the additional bonds of Si-O-Si results in the shrinkage of the aerogel framework structure.This process is responsible for an increase of the total pore volume and the specific surface area.The Si-OH groups disappear at 800 ℃ while the amount of SiOC 2 H 5 groups decreases to 3.1% , suggesting that the framework is preferentially formed by the Si-O-Si bonds.to a small amount of Si-OH groups.The peak at 2983 cm 1 is assigned to vibrations of the CH 3 groups, suggesting the presence of Si-OC 2 H 5 groups.The presence of these groups suggests an incomplete condensation during gelation process [19,21,22].As the calcination temperature increases, the peak at 975 cm 1 slowly disappears and the intensity of the peak at 2983 cm 1 also decreases, while that of the peak at 800 cm 1 increases.This suggests the formation of additional Si-O-Si bonds by the condensation reaction between Si-OH and Si-OC 2 H 5 .The peak at 1710 cm 1 belongs to vibrations of water molecules [19].Its intensity deceases with calcination temperature due to removal of water from the framework structure.
Figure 2 shows a schematic view of the formation of additional Si-O-Si bonds after the thermal treatment.This process determines the morphological properties of aerogels.
Figure 4 shows the XRD patterns of four SiO 2 aerogels calcined at different temperatures.Wide peaks are observed in all samples indicating an amorphous structure.The intensity of the main peak increases with the calcination temperature which confirms ongoing crystallization process.
Table 2 shows the specific surface area, pore volume, and pore size of the as-synthesized aerogels and those thermally treated.Additionally, curves of specific surface area and average pore diameter of the SiO 2 aerogels obtained from different conditions are shown in Fig. 5.
Figure 3 shows O 1s XPS spectra of the as-synthesized aerogel (sample A) and three samples calcined for 2 h at 400, 600, and 800 ℃ (samples B, C, and D, It can be seen in Table 2 that the surface area of   framework structure shrinks due to the formation of the new bonds.This process is responsible for the increase of the specific surface area and it reduces the average pore diameter.The largest particle size increases and the shrinkage of the framework occurs at the highest calcination temperature.This reduces both  the surface area and the average pore diameter.The surface area of aerogels decreases dramatically after a long exposure to high temperature due to the continuous framework shrinkage of the SiO 2 aerogels.Figure 6 shows typical N 2 adsorption/desorption isotherms and the corresponding pore size distribution (PSD) plots of the aerogels before and after thermal treatment.The PSD plots were obtained from the desorption branch of N 2 isotherms by the method.The presence of hysteresis loop is due to the capillary condensation occurred in the mesoporous [31].The nitrogen adsorption follows type-IV isotherm indicating that a rather high amount of mesopores is present in the structure of SiO 2 aerogel [30].The number of mesoporous decreases at higher temperatures due to the accumulation of larger secondary particles.The average mesopore size of the as-synthesized aerogels is in the range of 20-40 nm.The SiO 2 aerogel treated at 600 ℃ for 2 h has a broader PSD with an average diameter of 14.7 nm.After the thermal treatment for 4 h, the mean pore size shifts to a lower value (6.7 nm).The PSD becomes more uniform, which can be caused by a complete transformation of the remaining Si-OC 2 H 5 groups to Si-OH groups by oxidation [32].However, the samples calcined for 8 h have a wider PSD than those calcined for 4 and 2 h.The latter could be due to the fact that the framework shrinks and the particles grow even after a time period of 4 h.This plays an important role in the stabilization of the porous framework.A surface area of 727 m 2 /g and an average pore diameter of 7.9 nm were observed after the treatment at 800 ℃ for 2 h, The number of pores reduces and the pore size distribution becomes non-uniform.Moreover, the surface area reduces dramatically after the treatments with a duration of more than 4 h.This reveals that the particles keep growing and the framework finally collapses.A further increase in the temperature to 1000 ℃ results in a complete collapse of porous structure.The samples transform to a semitransparent glass with little porosity [33].  .It can also be seen that the pore shape changes from a tetrahedron to an octahedron.
Figure 8 shows TEM images of the SiO 2 aerogels calcined at different temperatures.All samples show a randomly interconnected framework, which is composed of irregular spherical nanoparticles.As it can be seen in Figs.8(a), 8(b), and 8(c), the particle size of these samples ranges between 5 and 10 nm, while much bigger particles of 40-50 nm are observed at 1000 ℃ (Fig. 8(d)).This indicates that the neck growth results in the collapse of the tunnel structure above 800 ℃.

3 Nanoporous framework evolution of SiO 2 aerogels calcined at different temperatures
No literature reports on the evolution of mesoporous framework of the aerogels during the calcination process.However, the understanding of this process is important for prediction of the morphology and the thermal stability of the aerogels.According to the electrostatic valence rule, the cells of SiO 2 aerogel possess [SiO 4 ] tetrahedron structure, and vertexes of this structure are occupied by the same structure.Being connected together, they form a three-dimensional network structure, a spherical granular-secondary particle [34].These secondary particles connect with each other to form the polyhedron-nanoporous framework.Thus, we assume that the microstructure of the aerogel is composed of different regular polyhedrons, and the pore size is controlled by the diameter of face inscribed circle.In turn, the pore volume equals to the  sum of polyhedron volumes.The following equations are used to estimate the key parameters of the mesoporous framework evolution.
Firstly, the weight of each secondary particle is calculated by Eq. ( 1): The number of secondary particles is calculated by Eq. ( 2): where m is the weight of secondary particles, d is its diameter (nm) (taken from TEM images), M is the sample weigth, and  aerogel is the apparent density of the silica aerogel (2.1 g/cm 3 ) [35].
The number of particles on each edge of polyhedron (remove both ends) is calculated by Eq. ( 3): The total number of particles of each regular polyhedron is calculated by Eq. ( 4): The number of regular polyhedron is calculated by Eq. ( 5): Combining Eqs. ( 1), ( 2), ( 3), (4), and (5), the total volume of polyhedrons in the aerogel can be calculated by Eq. ( 6): where a represents the polyhedron edge length, β is the space volume percentage occupied by vertex, h is the number of edge of regular polyhedron, e is the vertex number, and  is the dihedral angle.The estimated framework and calculated pore volume of SiO 2 aerogels are listed in Table 3.The calculated results are It can be seen in Table 3 that the calculated values of pore volume are in good agreement with the experimental results.The porous framework of as-synthesized samples consists of tetrahedrons and that calcined at 1000 ℃ of octahedrons.A tetrahedron to octahedron ratio of 1 is observed in samples calcined at 600 ℃, while this ratio decreases to 0.5 after calcination at 800 ℃.
Figure 9 presents a schematic evolution of SiO 2 aerogel framework as a function of calcination temperature.Figure 10 shows further geometrical interpretation of the model proposed.As for SiO 2 aerogels at room temperature in Fig. 9, the nanoporous framework totally is composed of tetrahedron with a pore size of about 22.28 nm; as the temperature reaches to 600 ℃, the framework is composed of tetrahedron and octahedron with a ratio of 1:1.It becomes more compact, being due to the formation of extra Si-O-Si bond via the condensation reaction between Si-OH and/or Si-OC 2 H 5 on the secondary particles, as shown in Fig. 2, and it is also consistent with the SEM (Fig. 6(b)).When the temperature reaches to 700 ℃, the quantity ratio of tetrahedron and octahedron reaches up to 1:2.The pore size of aerogel is 11.04 nm at 700 ℃.With the heat treatment temperature increases to 800 ℃, the nanoporous framework totally is composed of octahedron with a pore size of about 7.91 nm.In addition, the newly formed Si-O-Si bonds can produce a shrinkage of nanoporous framework and result in a decrease of pore size.A high local stress and the liquid in the pores between the particles result in the deformation of individual particles.This process can eventually cause a rearrangement of particles during the thermal treatment process [36].Above 600 ℃, the newly formed Si-O-Si bonds and the rearrangement of particles result in a variety of pore shapes and particles sizes.The aerogels are stabilized by reducing the surface energy via the pore collapse and particle growth after the treatment at 1000 ℃, as shown in Fig. 7(d).
In reality, the geometric figure shown in Fig. 9 should be a twisted shape as the silica aerogels mainly consist of Si-O-Si groups whose bond angle is 109°28.Due to the calculation requirement, the model was described by tetrahedron and octahedron.

Conclusions
(1) SiO 2 aerogels treated at different temperatures and holding time all possess amorphous structure; with the increase of treated temperatures, it has an obvious trend of crystallization.
(2) With the increase of treated temperatures, BET values of SiO 2 aerogels increase initially and then decrease; it reaches maximum value of 882.81 m 2 /g when treated at 600 ℃ for 2 h, while the values of average pore diameter simply decrease.These varieties may be due to the newly formed Si-O-Si bonds via condensation reaction between the secondary particles during heat treatment process; it can not only add the amount of nanopores but also help to shrink the skeleton; the former can improve the BET values and the latter helps to reduce the average pore diameter of aerogels.A higher temperature may result in the skeleton transformation and particle size growth; both factors lead to the reduction of BET and average pore diameter values.
(3) Nanoporous skeleton of SiO 2 aerogels at room temperatures is mainly composed of tetrahedron with a pore size of about 22.28 nm, and when treated at 600 ℃, its microstructure is mainly composed of tetrahedron and octahedron with a ratio of 1:1 and the relative pore size is about 14.65 nm.A higher temperature results in an increase of octahedron amount in the nanoporous framework and a decrease of the pore size in aerogels.Finally, treating at 1000 ℃ results in an approximate dense SiO 2 bulk via the framework collapse and particle growth.The newly formed Si-O-Si bonds, the high local stress, and liquid  phase between particles can cause the particle growth and rearrangement of nanoporous framework.

Fig. 2
Fig. 2 Schematic view of the formation of additional Si-O-Si bonds in aerogels after a thermal treatment.

Fig. 5
Fig. 5 Specific surface area and average pore diameter of SiO 2 aerogels treated at different temperatures.

Fig. 6
Fig. 6 N 2 adsorption/desorption isotherms and the PSD of SiO 2 aerogels before and after thermal treatment.

3. 2 Figure 7
Figure7presents typical SEM photographs of SiO 2 aerogels before and after the thermal treatment.It can be seen that the secondary particles are aggregated and condensed to form clusters.The crosslinking among these clusters a porous framework.As the temperature increases, the amount of mesopores first increases (in Fig.7(b)) and then decreases (in Figs.7(c) and 7(d)).The morphology of the porous framework in aerogels changes from loose (in Fig.7(a)) to compact (in Fig.7(b)), and then dense (in Fig.7(c)).Finally the formation of larger particles results in a collapse of porous structure (in Fig.7(d)).It can also be seen that the pore shape changes from a tetrahedron to an octahedron.Figure8shows TEM images of the SiO 2 aerogels calcined at different temperatures.All samples show a randomly interconnected framework, which is composed of irregular spherical nanoparticles.As it can be seen in Figs.8(a), 8(b), and 8(c), the particle size of these samples ranges between 5 and 10 nm, while much

Fig. 9
Fig. 9 Models of the transformation mechanism of pore structure during heat treatment.