Effects of synthesis conditions on particle size and pore size of spherical mesoporous silica

The particle size and pore size of spherical mesoporous silica materials play significant roles in their application. However, relatively limited systematic research has been conducted on how preparation conditions influence these properties. In particular, the effects of some important factors have not been adequately studied, including reaction time, reaction temperature, and organic solvent type. In this work, octane and water were used as solvents, and tetraethyl orthosilicate was used as the silicon source to systematically study the effects of reaction time, reaction temperature, different organic solvents, octane/water mass ratio, styrene template concentration, and surfactant (cetyltrimethylammonium bromide, CTAB)/H2O mass ratio on the particle morphology, particle size, and pore size of silica. The results suggest that the above-mentioned neglected factors exert a substantial influence on both particle size and pore size. In the experimental temperature range, the pore diameter decreases and the particle size increases with increasing temperature. The maximum particle size and pore size are achieved after a reaction time of 3 h, and a further increase in reaction time leads to a smaller particle size and pore size. As the number of carbon atoms in the organic solvent decreases, the pore size also gradually increases. Styrene and organic solvents that dissolve in CTAB micelles are crucial factors in pore formation, while the aggregation of the swollen CTAB micelles influences the particle size. The changes in the pore structure stability and hydroxyl density of the synthesized samples in water were also studied. After undergoing water treatment at temperatures ranging from 20 to 60 °C for 72 h, both the pore structure and morphology remain relatively unchanged. When the temperature increases, the surface hydroxyl density exhibits a more pronounced increase in the presence of water. After water treatment for 5 h, the surface hydroxyl density reaches saturation.


Introduction
Mesoporous silica is widely utilized in catalysis [1,2], bioengineering [3,4], medical [5,6], and environmental [7,8] applications due to its adjustable pore size, high specific surface area, and easy surface hydroxyl group functionalization.Currently, mesoporous silica materials with the film [9][10][11], fiber [12,13], spherical [4,14], core-shell [15,16], and ellipsoid [17] morphologies can be obtained by controlling the synthesis conditions.Among them, spherical mesoporous silica particles have been used in various applications such as biomedicine [5,6], chromatography [18][19][20][21], and catalysis [3,22,23] due to their advantages of favorable monodispersity, regular morphology, and easily tunable pore and particle sizes.For example, Zhang et al found that mesoporous silica nanoparticles with smaller particle sizes have a more significant tumor inhibition effect when used for tumor-targeted drug delivery [24].Kim et al and Zhang et al showed that spherical mesoporous silica with larger pores could deliver protein-coding plastids more effectively, and this silica material also had a higher drug loading capacity and drug dissolution rate [25,26].Therefore, the preparation conditions used to obtain spherical mesoporous silica materials, which strongly influence particle size and pore size, have attracted extensive attention.
Previous studies have mainly focused on the use of different synthesis methods to investigate the effects of individual reaction conditions on the morphology, particle size, and pore size of spherical mesoporous silica.For example, Polshettiwar et al adopted a microemulsion method with CTAB as the template, urea as the catalyst, cyclohexane-pentanol-water as the reaction system, and tetraethyl orthosilicate (TEOS) as the silicon source.The morphology, particle size, and dispersibility of the prepared silica were investigated by changing the amount of template, solvent, and catalyst [13].By using CTAB as a template, Yu et al investigated the effect of preparation conditions on particle size by adjusting the amount of Pluronic F 127 , and the effect of the alkyl chain length of cosurfactant imidazolium ionic liquids (ILs) on the structure and morphology was studied [27].Wang et al employed a 3-dimensional macroporous carbon as the template and nonionic surfactant Pluronic copolymers (P123) as the co-template to change the morphology and pore structure of a silica material by controlling the TEOS to P123 ratio.Additionally, they adjusted the particle size by changing the pore size of the 3-dimensional macroporous carbon [28].Nandiyanto et al prepared spherical mesoporous silica nanoparticles by using CTAB as the surfactant, octane and water as solvents, L-lysine as the catalyst, styrene (which was polymerized into polystyrene during preparation) as the template, 2,2′-azobis (2-methylpropionamide) dihydrochloride (AIBA) as the initiator, and TEOS as the silicon source.The effects of styrene concentration and octane/water ratio on the pore size and particle size of the products were investigated [29].
Despite the many studies investigating various reaction systems for the preparation of spherical silica particles, there is still a lack of systematic research on the factors affecting the particle size and pore size of silica.The spherical silica preparation method proposed by Nandiyanto et al has attracted widespread attention because it provides excellent monodispersity and uniform pore sizes [30,31].However, only the effects of styrene concentration, octane/water mass ratio, CTAB mass, and different surfactants on particle size and pore size have been reported [26,29,32,33].The effects of reaction temperature, reaction time, and solvent type on the pore structure, particle morphology, and particle size of spherical mesoporous silica are not yet well-understood.
In this study, the preparation method reported by Nandiyanto et al was employed to systematically study the effects of reaction temperature, reaction time, and use of different organic solvents on the particle morphology, particle size, and pore size of spherical mesoporous silica.The results revealed substantial influences of the above rarely studied factors on both particle size and pore size.Furthermore, the effects of styrene concentration, CTAB/H 2 O mass ratio, and octane/water mass ratio were also investigated, which was consistent with previous research.The pore structure and surface hydroxyl group density of silica materials are crucial parameters for their application [34,35].Considering that water is one of the most commonly used solvents in various applications (such as adsorption, catalysis, etc.) [7,8,36], the changes in the morphology, pore structure, and surface hydroxyl group density of the synthesized silica samples after mixing with water were also studied.We believe that this work contributes to enriching and refining the synthetic methodology, and is expected to provide a valuable reference for the application of silica in related fields.

Synthesis of silica samples
The detailed synthesis conditions for obtaining different silica samples are shown in table 1. Taking sample S7 as an example, 1.52 g CTAB and 465 ml deionized water were added to a three-necked flask and stirred in an oil bath at 60 °C for 1 h to achieve complete dissolution of the CTAB.Next, 0.34 g L-lysine and 346 ml octane were continuously added to the mixture under stirring at constant temperature for 20 min.Finally, 0 g AIBA, 0 ml styrene (no styrene was used to prepare sample S7), and 16.2 ml TEOS were added to the mixture, which was reacted under an Ar atmosphere for 3 h.The styrene monomer was pre-washed with 2.5 M NaOH to remove the stabilizer before the reaction.After reacting, the reaction liquid was naturally cooled overnight.Then, the product was separated by centrifugation and washed with ethanol 4 times.The obtained white solid was dried at 50 °C for 2 h, then calcined at 550 °C for 6 h under an air atmosphere to obtain the silica sample.The total volume of the reaction solution was controlled between 816 ml and 840 ml.

Interaction of silica samples with water
The synthesis of representative sample S7 was amplified to 3000 ml to explore its pore structure stability and changes in surface hydroxyl density upon exposure to water (denoted as sample S).Sample S was mixed with

Characterization
Transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TWINTMP) was used to characterize the morphology of the samples, and particle size was measured using ImageJ software.N 2 adsorption-desorption curves were obtained with an automatic specific surface area and pore size analyzer (Autosorb-iQ2, Quantachrome, USA) at 77 K.The Brunauer-Emmett-Teller (BET) model was used to analyze the specific surface areas of the samples, and the nonlocal density functional theory (NLDFT) model was used to analyze their pore sizes and pore volumes.Fourier transform infrared spectrometer (FT-IR, Bruker, Germany) was adopted to test the samples in the spectral range of 400-4000 cm −1 .A simultaneous thermal analyzer (STA 449F3, Netzsch, Germany) was used to analyze the samples before and after interaction with water, and the surface hydroxyl density was calculated based on the obtained weight loss.During measurement, each sample was heated at 50 °C-1200 °C in an air atmosphere at a heating rate of 10 °C min −1 .X-ray diffraction (XRD, PANalytical Empyream, Netherlands) was used to evaluate the structures of the samples.

Characterization of silica samples
The FT-IR spectrum of the representative sample S7 is shown in figure 1(a).A typical FT-IR spectrum of silica is obtained, in which the absorption peaks at 445, 810, and 1066 cm −1 are attributed to the Si-O-Si bending vibration peak, the Si-O-Si symmetric stretching vibration peak, and the asymmetric stretching vibration of Si-O-Si bridges, respectively [37].The XRD pattern of sample S7 is shown in figure S1.This sample does not show any clear diffraction peaks, indicating its amorphous structure.To determine whether the pores are arranged in an ordered manner, we performed a low-angle XRD test (figure 1(b)).There are no distinguishable diffraction peaks at low angles, suggesting a lack of structural ordering within the internal pores of the sample [38].The N 2 adsorption-desorption isotherm of sample S7 is shown in figure 1(c).This silica sample has a type IV(a) isotherm, and a steep and narrow hysteresis loop is observed in the relative pressure range of p/p 0 = 0.9-1.This hysteresis loop is similar to a type H1 hysteresis loop, indicating that the sample has a relatively concentrated mesoporous structure.The gas adsorption analysis results of all samples, including their specific surface areas, pore volumes, and pore diameters, are summarized in table 2. The average particle sizes of the samples measured from their TEM images are also reported in table 2.

Influence of synthesis conditions 3.2.1. Synthesis mechanism
Based on our findings and previous studies, the proposed formation mechanism of the silica samples is shown in figure 2. Initially, when the surfactant (CTAB) is dissolved in water, it exists as individual dissolved molecules.When the CTAB concentration exceeds the critical micelle concentration, the CTAB will aggregate in the water phase to form inner-hydrophobic and outer-hydrophilic micelles (step a in figure 2).The non-polar styrene monomer and octane have a strong affinity with the oleophilic groups of CTAB, leading to their solubilization in the micelle cores.Consequently, the micelles will expand into swollen micelles [39] (step b in figure 2).When the initiator is added to the system, the free radicals dispersed in the water phase diffuse into the swollen micelles, where styrene polymerization is triggered.. Thus, the swollen micelles become polymer micelles [40,41] (step c in figure 2).At the same time, under agitation, the TEOS gradually diffuses to the oil-water interface in the organic solvent, where it hydrolyzes and condenses under the action of the alkaline catalyst L-lysine to form negatively charged silicic acid monomers and oligomers.Due to the hydrophilic Si−OH groups, these reactions tend to continuously occur in the hydrophilic region of the water-oil interface.When the silicic acid monomer and oligomer reach the critical nucleation concentration, they enter the aqueous phase and assemble with the positively charged CTA + of the micelle surfaces to form composite micelles composed of SiO 2 and CTAB (step d in figure 2) [4,14].These composite micelles then gather together to form spherical silica particles (step e in figure 2) [42,43].The micelles in this system serve as templates to support the formation of silica pore walls.Finally, calcination is performed to remove the micelles and form mesoporous pore channels (step f in figure 2).As the styrene polymerizes, the polymer micelles gradually grow and the surface area increases.Consequently, more CTAB is required to stabilize the micelles in the water phase.With increasing m C:W , the growth of the polymer micelles can be adequately supported by increasing the amount of CTAB, leading to a larger micelle swelling volume.On the other hand, with increasing surfactant concentration, the solubilization effect of the organic solvents in micelles is more significant [44].This means that larger micelles cause the eventual formation of larger pores.When m C:W increases from 0.5:310 to 1:310, the micelle volume increases, and the assembly of these micelles causes the formation of larger particles.However, when m C:W is further increased, the number of formed micelles also increases and the viscosity of the solution increases, which hinders the aggregation of polymer micelles and leads to a decrease in particle size.

Influence of different organic solvents
Silica samples were prepared with different organic solvents (1-octadecene, decahydronaphthalene, octane, and cyclohexane) without adding styrene to study the influence of the solvent type on particle and pore sizes.TEM images of these samples are shown in figures 3(e)-(h), confirming that all the evaluated organic solvents result in the formation of regular spherical particles.The pore size distribution curves of these samples are shown in figure 4(c), and the changes in particle size and pore size are shown in figure 4(d).The samples prepared in different organic solvents show different pore sizes and particle sizes.Using 1-octadecene (sample S5), decahydronaphthalene (sample S6), octane (sample S7), or cyclohexane (sample S8) leads to pore sizes of 4.9, 7.0, 7.6, and 11.7 nm and particle sizes of 31, 34, 42, and 71 nm, respectively.The swelling behavior of micelles may exert a predominant influence.
Under the same conditions, the solubilization of micelles on the hydrocarbons increases with decreasing number of carbon atoms [45].Therefore, as the number of carbon atoms of 1-octadecene (C 18 H 36 ), decahydronaphthalene (C 10 H 18 ), octane (C 8 H 18 ), and cyclohexane (C 6 H 12 ) successively decreases, the solubilization of the micelles on the solvent molecules increases, the volume of the swollen micelles increases, and the pore size of the samples also increases.The presence of larger micelles results in the formation of larger particles.Nandiyanto et al showed that the pore size of silica can be regulated by changing the styrene concentration, but they did not study the influence of solvent choice on pore size.The results obtained by changing the organic solvent without adding styrene in this work show that in addition to using styrene as a template for pore formation, large mesopores can also be obtained by using organic solvents, and the sizes of these mesopores are influenced by the carbon number of the organic solvent.These results confirm that both the presence of styrene and the choice of organic solvent are important factors affecting pore formation.Thus we have refined our mechanism (figure 2) based on our findings and previous studies.process, causing the polymer micelles to have larger and more uniform swelling volumes.Thus the particle size increases and a regular spherical shape is observed (figure 5(b)).As the reaction time continues to increase, the polymer micelles continue to grow.We speculate that the CTAB monomers might not be able to completely cover the surfaces of the polymer micelles, which negatively affects the stability of the emulsion system [46].Therefore, polystyrene and octane will diffuse out of the polymer micelles, causing these micelles to shrink and the emulsion system to stabilize again.After polystyrene diffuses out of the micelles, reentry into the micelles is difficult.Consequently, the pore size and particle size decrease after a reaction time of 5 h (figure 5(c)).The restabilization of the emulsion system means that the micelle volume does not significantly change with a further increase in reaction time, so the pore size and particle size of the sample obtained after 8 h (figure 5(d)) of reaction are similar to those obtained after 5 h.With increasing reaction time from 3 h to 8 h, the pore size decreases from 10.5 nm to 6.8 nm and the particle size decreases from 90 nm to 50 nm (table 2).

Effect of reaction temperature
TEM images of silica samples prepared at different reaction temperatures (60, 70, 80 °C, correspond to samples S2, S13, and S14, respectively) are shown in figures 5(e)-(g).All samples show regular spherical shapes.The pore size distribution curves and changes in particle size and pore size of these samples are displayed in figures 6(c), (d).With increasing temperature, the pore size decreases from 9.4 nm to 6.1 nm and the particle size increases from 84 nm to 148 nm (table 2).A higher reaction temperature will accelerate the free radical formation rate, and the diffusion rate of free radicals to the polymer micelles will also be accelerated.The additional free radicals entering the polymer micelles will collide with the free radicals originally participating in the polymerization process within the micelles.This leads to radical-radical termination [46], resulting in the accelerated chain termination of the styrene polymerization process.Consequently, shorter polystyrene chains are obtained.This reduces the volume of the polymer micelles, leading to smaller pore sizes.However, as the reaction temperature increases, the movement of the micelles may be promoted, which enhances the collision and aggregation of the micelles, leading to formation of larger particles.When the styrene concentration is increased from 0 to 50 mg ml −1 , the particle size and pore size both showed an increasing trend, from 42 nm to 90 nm and from 7.6 nm to 10.5 nm, respectively (table 2).The styrene polymerization process occurs within the micelles.With increasing styrene concentration, the degree of polymerization increases, leading to the formation of larger swollen polymer micelles.Consequently, the resulting pores also exhibit a significant increase.Moreover, larger polymer micelles aggregate and grow to form larger particles.Overall, the influence of styrene monomer concentration is consistent with the results of Nandiyanto et al.

Effect of the octane/water mass ratio
TEM images of samples prepared with different octane/water mass ratios (m O:W = 0.17, 0.3, 0.5, 1) are shown in figures 7(e)-(h).Samples S16 (m O:W = 0.17) and S17 (m O:W = 0.3) have small particle sizes and poor sphericity (figures 7(e), (f)).In contrast, samples S7 (m O:W = 0.5) and S18 (m O:W = 1) are significantly larger and more spherical (figures 7(g), (h)).The pore size distribution curves of these samples are shown in figure 8(c).The changes in the pore size and particle size are shown in figure 8(d).The pore size and particle size both show an increasing trend with increasing m O:W .Specifically, the pore size increases from 4.9 nm to 9.1 nm and the particle size increases from 20 nm to 60 nm (table 2).From the synthesis mechanism, a larger m O:W will lead to a greater solubilization effect, causing the formation of micelles with larger volumes and the formation of larger pores.Moreover, the particles formed by micelles aggregation are also larger.Nandiyanto et al reported that regulating the octane/water mass ratio can effectively adjust the particle size of silica particles, but they did not investigate the influence of m O:W on pore size [29].This study confirms that changing the octane/water mass ratio influences both particle size and pore size.
The stirring speed also affects the pore size of the silica samples.For sample S2, different stirring speeds were evaluated.Increasing the stirring speed from 240 r min −1 to 280 r min −1 causes the pore size to increase from 8.1 nm to 9.4 nm.With increasing stirring speed, the styrene monomers are dispersed into smaller monomer droplets, providing a larger specific surface area [46].Thus, more CTAB is adsorbed on the surface of these monomer droplets, leading to the formation of a smaller number of micelles.If the initial styrene monomer content is fixed, the number of styrene monomers per unit micelle involved in polymerization will increase, resulting in the growth of micelles with larger volumes.Consequently, silica with a larger pore size is produced.

Pore structure and surface hydroxyl group density after interaction with water
To investigate the pore structure and surface hydroxyl group density of the synthesized spherical mesoporous silica in water, the synthesis of sample S7 was scaled up (represented by S).Then, the morphology, pore size, and surface hydroxyl group density of samples after mixing with water under different conditions were investigated.Considering that conventional adsorption processes generally do not exceed temperatures of 60 °C, sample S was mixed with water at 20 °C, 40 °C, and 60 °C.change in sample morphology is observed after 72 h of interaction with water at different temperatures, and the particle size remains stable at around 42 nm.
The N 2 adsorption-desorption isotherms and pore size distribution curves of samples S, S-20-72, S-40-72, and S-60-72 are shown in figure 10.The scaled-up procedure used to prepare sample S causes the pore size to increase from 7.6 nm (sample S7 in tables 1) to 8.1 nm, which is likely because the volume of the reaction solution and the stirring speed (after changing the stir bar) influence the pore size.However, sample S is still suitable for studying the influence of water treatment on textural properties and hydroxyl group density.The synthesized silica was calcined at a high temperature (550 °C) for template removal, meanwhile the surface hydroxyl groups were partially removed, forming siloxane.The interaction between siloxane and water can restore hydroxyl groups to some extent, and the number of formed hydroxyl groups is related to temperature and time [47].The thermogravimetric (TG) curves of samples S-20, S-40, and S-60 after interaction with water for different lengths of time are shown in figure 11.It is generally believed that weight loss below 200 °C is related to the removal of physically adsorbed water, while weight loss above 200 °C is related to the removal of surface hydroxyl groups [47].Therefore, weight loss in the temperature range of 200 °C-1200 °C and the specific surface areas of the samples were used to investigate the surface hydroxyl density [34].The surface hydroxyl density was calculated according to the following formula: here M is the weight loss (%) in the range of 200 °C-1200 °C, W H2O is the molecular weight of water, N A is the Avogadro constant, n OH is the surface hydroxyl density, and S BET is the BET specific surface area.The weight loss and calculation results are summarized in table 3, and the relationship between the surface hydroxyl density and interaction time with water is shown in figure 11(d).After interacting with water, the surface hydroxyl density gradually increases, and the rate of increase is higher at higher temperatures.After 5 h of interaction with water, the surface hydroxyl density reaches saturation, and the maximum hydroxyl density is 3.7 OH/nm 2 .

Conclusion
The effects of octane/water mass ratio, styrene concentration, CTAB/H 2 O mass ratio, reaction temperature, reaction time, and use of different organic solvents on the morphology, particle size, and pore size of spherical silica particles were investigated.The solubilization of styrene and organic solvent into CTAB micelles is a pivotal step in this synthesis process.The size of the swollen CTAB micelles directly affects the particle size and pore size of the silica particles.In the absence of styrene, organic solvents can also be used as pore-forming templates.The amount of octane affects both the particle size and the pore size.The effects of reaction time on particle size and pore size are complicated, so further studies should be performed to clarify this relationship.In addition, the pore structure is also affected by the volume and stirring rate of the reaction solution.After 72 h of interaction with water, the pore structure of the silica remains unaffected, enabling its possible application in many fields.The saturation of surface hydroxyl groups after 5 h of interaction with water also provides a reference for the application of these spherical silica particles.Based on our comprehensive understanding of the effects of different synthesis conditions on particle and pore sizes, in future work, we can strategically select appropriate conditions to tailor the desired silica materials.
/H 2 O mass ratio TEM images of samples prepared using different CTAB/H 2 O mass ratios (m C:W = 0.5:310, 1:310, 3:310, 8:310) are shown in figures 3(a)-(d).All particles show regular spherical shapes with porous structures and excellent monodispersity.With increasing m C:W from 0.5:310 (sample S1) to 1:310 (sample S2), the silica particles slightly increase in size (figures 3(a), (b)).When m C:W is further increased to 3:310 (sample S3), the particle size does not significantly change (figure 3(c)).With a further increase in m C:W to 8:310 (sample S4), the silica particles become smaller (figure3(d)).The pore size distribution curves and the changes in the particle size and pore size of the samples are shown in figures 4(a), (b).With increasing m C:W , the pore size increases from 8.1 nm to 11.7 nm, and particle sizes of 79, 84, 84, and 70 nm are respectively obtained.

3. 2 . 4 .
Impact of reaction time TEM images of silica samples prepared at different reaction times (2, 3, 5, 8 h) are shown in figures 5(a)-(d).The pore size distribution curves of these samples are shown in figure 6(a).The changes in particle size and pore size of these samples are shown in figure 6(b).Sample S9 (2 h) does not have a regular spherical morphology and is easily agglomerated (figure 5(a)).Based on our understanding of the synthesis mechanism, a short reaction time (2 h) causes the polymer micelles to have uneven swelling volumes, leading to a wide pore size distribution.Due to the short reaction time, the particles formed by aggregation are small, and agglomeration of particles easily occurs.After a reaction time of 3 h (sample S10), more styrene monomers participate in the polymerization

Figure 4 .
Figure 4. Pore size distribution and particle size and pore size of silica samples with (a) and (b) different CTAB/H 2 O mass ratios and (c) and (d) different organic solvents.

3. 2 . 6 .
Effect of styrene concentration TEM images of silica samples prepared with different styrene concentrations (0, 5, 20, 50 mg ml −1 , correspond to samples S7, S15, S2, and S10 respectively) are shown in figures 7(a)-(d).Spherical silica particles are obtained under all the evaluated styrene concentrations, figure 8(a) shows the pore size distribution curves of these samples.The changes in pore size and particle size with styrene concentration are shown in figure 8(b).

Figure 6 .
Figure 6.Pore size distribution and particle size and pore size of silica samples with (a) and (b) different reaction time and (c) and (d) different reaction temperature.

Figure 8 .
Figure 8. Pore size distribution and particle size and pore size of silica samples with (a) and (b) different styrene concentrations and (c) and (d) different octane/waters mass ratios.

Figure 11 .
Figure 11.(a)-(c) TG curves and (d) surface hydroxyl density of the samples mixed with water for differ time.

Table 1 .
Synthesis conditions of mesoporous silica (All samples were using the mass ratio of H 2 O/TEOS/L-lysine of 310:10:0.22,and 0.84 mg ml −1 of AIBA).

Table 2 .
The analysis results of N 2 adsorption-desorption and particle size of silica samples.

Table 3 .
TG mass loss from 200 °C to 1200 °C and surface hydroxyl density for all samples.