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Article

Preparation of Hollow Silica Nanoparticles with Polyacrylic Acid and Their Moisture Sorption Properties

by
Quanyue Wen
,
Kento Ishii
and
Masayoshi Fuji
*
Advanced Ceramics Research Center, Nagoya Institute of Technology, 3-101-1, Honmachi, Tajimi 507-0033, Gifu, Japan
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(7), 829; https://doi.org/10.3390/coatings14070829
Submission received: 7 June 2024 / Revised: 24 June 2024 / Accepted: 29 June 2024 / Published: 3 July 2024
(This article belongs to the Special Issue New Developments of Protective Coatings, Organic Polymers, and Surface Analysis: 2nd Edition)
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Abstract

:
Hollow silica nanoparticles (HSNPs) have hygroscopic properties because of their high specific surface area and surface hydroxyl groups. However, compared with other hygroscopic materials, their hygroscopic properties are relatively weak, which limits the further application of HSNPs. One feasible method to enhance their hygroscopic properties is by combining highly hygroscopic materials with hollow silica nanoparticles. To take advantage of the high hygroscopicity of polyacrylic acid (PAA) when combined with the high specific surface area of the hollow particles, PAA was coated on the inner and outer surfaces of the silica shell of the nanoparticles in this study to prepare hollow nanoparticles with a PAA/silica/PAA multilayer structure. The size of the PAA/silica/PAA multi-layer nanoparticles is about 85 nm, and the shell thickness is 25 nm. The specific surface area of the multi-layer nanoparticles is 58 m2/g. The water vapor adsorption capacity of multi-layer structure hollow nanoparticles was increased by 160% compared with the HSNPs (increased from 45.9 cm3/m2 to 109.1 cm3/m2). Meanwhile, at the same content of PAA, the PAA/silica/PAA-structured particles will adsorb 9% more water vapor than the PAA/silica-structured particles. This indicates that the high specific surface area structure of the hollow particles will enhance the adsorption ability of PAA toward water vapor. This novel structure of PAA-HSNPs is expected to be used as a humidity-regulating material for filler in environmental and architectural applications.

1. Introduction

Hollow silica nanoparticles are a new type of silica nanoparticles that, due to their hollow structure compared to solid silica nanoparticles, not only provide more specific surface area and more potential reaction activity points to the material, but the hollow structure also allows more possibilities for the preparation of nanocomposite particles [1,2,3]. These features of hollow silica nanoparticles are currently attracting widespread attention. In past decades, porous materials have shown great application value and broad application prospects that include but are not limited to energy, biomedicine, and environmental protection [4,5,6]. Among them, because of their stable chemical properties, no biological toxicity, porous structure, and high specific surface area, porous silica nanoparticles have been widely used as catalyst carriers, drug carriers [7,8], in wastewater treatment [9,10], and in other fields [11,12,13,14]. In addition, due to the presence of the silicon hydroxyl group on the silica surface and their high specific surface area due to their porous structure, their affinity for water molecules is excellent; thus, there is a high number of applications in related fields such as humidity-sensing materials and hygroscopic materials [15]. The moisture sorption feature of porous silica depends on the porous structure and the number of adsorption hydroxyl groups on the surface of the silica. For these hollow particles, the specific surface area of the material can be significantly increased by microparticles. Moreover, the hollowing of the particles can further reduce the density, which can further improve the performance of the material without changing the porous structure. Nevertheless, due to the limitations of the silica material itself, its hygroscopic properties are difficult to improve. Compositing with other materials, especially highly absorbent polymer materials, is one method to further improve silica’s moisture absorption performance [16,17]. At the same time, because the thermal stability of silica is much better than that of polymer materials, combining a highly absorbent polymer with silica will significantly improve the heat resistance of the resulting composite material.
As an excellent hygroscopic polymer, polyacrylic acid (PAA), also known as poly (1-carboxy ethylene), has excellent water sorption capacity, based on its hydrophilicity due to the carboxyl groups [18,19]. Moreover, the hydrogen bonds formed between molecular chains will also attract water molecules, thus further improving its water absorption performance, a process that is widely used in the preparation of water-absorption materials [20]. Various studies have reported that the hygroscopicity of PAA comes from the carboxyl hydrophilic adsorption of PAA and the ability of vacancies between the cross-linked network of the polymer to accommodate water molecules [21,22,23,24,25,26].
Hollow particle synthesis methods are mainly divided into template methods and non-template methods. At present, there are many studies that have proposed a method of synthesizing hollow particles by using PAA emulsion as a template [27,28]. The hollow particles synthesized by the PAA emulsion method have the characteristics of lower particle agglomeration and low environmental pollution. However, with this method, it is necessary to remove the PAA nucleus to obtain the hollow particles, which will cause the waste of synthetic raw materials and reduce the water absorption characteristics of the hollow particles.
If the PAA in the hollow particle synthesis process can be unitized, it can not only reduce the waste of raw materials but also enhance the hygroscopicity performance of particles, creating novel composite nanoparticles.
In this study, a new nanocoating method was reported. This method can utilize the PAA during the HSNP synthesis process to coat the HSNP shells both inside and outside. The moisture adsorption characteristics of the PAA-HSNPs were evaluated. Moreover, the thermal stability between different particles was also evaluated.
At present, there is no report regarding the use of PAA/silica composite nanoparticles as moisture adsorption materials. Since this novel composite particle is a powder material, it is not necessary to consider the shape limitation factor in practical applications, such as humidity adjustment filler, etc. Furthermore, silica and PAA are non-toxic; in hollow silica nanoparticles coated with PAA, their biocompatibility will be further improved [29,30]. PAA composite hollow particles also have good application prospects in biomedicine; thus, they will have broad application potential.

2. Experimental

2.1. Materials

Ethanol (99.5%) was purchased from Wako Pure Chemical. Ammonia water (25 wt %), polyacrylic acid (PAA, Mw = 5000), and ethyl silicate (TEOS, >95%) were also purchased from Wako Pure Chemical Industries (Osaka, Japan).

2.2. Experimental Process

The first step involved preparing the emulsion template solution of polyacrylic acid (PAA), whereby 0.6 g of PAA was added to 20 mL of ammonia water. The polyacrylic acid was stirred until fully dissolved and then stored for 24 h. After that, the prepared PAA emulsion was added to 350 mL of ethanol and stirred for 30 min until the PAA solution was completely dispersed. The prepared PAA emulsion was then added to 350 mL of ethanol and stirred for 30 min until the PAA solution was evenly dispersed. Then, 20 ml of TEOS was added dropwise to the ethanol solution at a rate of 200 μL/min. After this addition was completed, the solution was stirred for 6 h to generate a silica shell on the surface of the PAA emulsion particles by the sol-gel process [28].
The solution was then centrifuged at 15,000 rpm to separate the silica particles. Finally, the separated particles were washed with ethanol to remove any unreacted PAA. The specific method used is to add the particles to ethanol and disperse them, and the ratio of ethanol to the particles is 30 mL of ethanol/1 g of particles. The particle dispersion solution was then centrifuged to obtain the particles. This process was carried out twice, after which washed silica particles were finally obtained. To obtain hollow particles with different PAA contents, we used water to clean the particles again. The cleaning method was the same as for ethanol cleaning. Finally, to obtain hollow silica particles without a PAA, the HSNPs were heated in a muffle furnace at 700 °C for 5 h.

2.3. Characterization and Equipment

The structural information of the particles was obtained by transmission electron microscopy (TEM, JEOL, Japan). The particle size and shell thickness distribution were observed by scanning electron microscopy (SEM, JEOL, Tokyo, Japan) using the transmission electron diffraction (TED) mode. A Fourier transform infrared spectrometer (FT-IR, JASCO, Tokyo, Japan) was used to characterize the changes in the particle composition. Samples were fully dried before testing and formed into 5% particle content KBr tablet samples for testing. The test spectrum range was 350~4000 cm−1. The particles’ thermal stability and PAA content changes were obtained by thermogravimetric analysis (TG, Shimadzu, Kyoto, Japan). The gas atmosphere was air at a gas flow rate of 200 mL/min; before the test, the particles were dried in the equipment at 120 °C for 2 h to remove any adsorbed water from the particles. The gas adsorption test used a specific surface area analyzer (Microtrac BEL Corp, Osaka, Japan). N2 and water vapor were used as an adsorbent at temperatures of 77 k and 298 k, respectively. We used a zeta potential analyzer (Malvern Instruments, Ltd., Worcestershire, UK) to measure the zeta potential of the particles, which was used to characterize the surface charge distribution of the silica nanoparticles. Before each test, the particles were vacuum dried at 150 °C for more than 10 h to remove any water from the particles.

3. Results and Discussion

3.1. Morphology of Composite Particles

The electron microscopy images (TEM) and the EDS results for the HSNPs are shown in Figure 1. The PAA emulsion size was determined using dynamic light scattering (DLS) via a nano sizer. The measurement result of the average PAA emulsion size was 120.9 nm, while the EDS silicon distribution results revealed that the inner diameter of the silica ranged from 118 to 120 nm, which is consistent with the PAA emulsion particle size. These findings suggest that a silica layer forms on the surface of the PAA droplet. For the changes of PAA seen in the particles in Figure 1a, the PAA in the hollow particles did not exist inside the particle in the form of particle nuclei but were attached to the inner wall of the silica shell, so that the hollow particles formed the double-layer composite structure of SiO2-PAA. At the same time, carbon was observed on the outside layer of the particle. This was caused by the residual PAA on the particle surface and also by the impurities that had been adsorbed on the particle surface. When the particles were washed, parts of the PAA layer became thinner, indicating that some PAA had been removed; as shown in Figure 1b–d, in the particles that were roasted, the PAA layer completely disappeared, and the particles exhibited a single-layer structure in the silica shell. At the same time, as shown by the EDS results for the distribution of the C and Si elements, the particles had a silica shell layer and an inner shell layer of PAA, giving the particles a double-layer hollow composite structure. When the particles were roasted, the inner PAA layer disappeared, making the particles appear as a single layer of silica. The analysis results also showed that with the increased washing times, while the proportion of the C element decreased, the proportion of Si increased. This result is the same as that shown in the TEM image. At the same time, there was a distribution of C elements on the outer part of the particle’s silica shell. It is suggested that particle washing was caused by the PAA molecular chains that were discharged from the particles and were partly wrapped in the silica shell; for the no-PAA hollow silica particles, this may have been caused by particles adsorbing a small number of organic impurities from the air.
The nano-coating technology in this research is different from traditional coating techniques. The traditional nano-coating process utilizes the particles’ surface functional group graft with the modifier to coat the material surface with the modifier, which is a chemical coating method [31]. In this research, PAA was transferred to the inner and outer surface of the particle through the pores of the hollow nanoparticle shell to coat the nanoparticles, resulting in a physical coating.
Figure 2 shows the composite nanoparticle SEM images (TED mode) and size and shell distribution results. For the SEM sample preparation, the nanoparticles were dispersed in ethanol; during the drying process, the removal of ethanol gradually reduced the spacing of particles, and when the particle concentration was high, some particles were agglomerated. This can be seen in Figure 2. The calculated statistical number of each particle was 100 particles. The average results of the particles’ outer diameter, inner diameter (hollow diameter), and shell thickness are shown in Table 1. After the hollow particles were washed and centrifuged, the inner PAA was transferred to the surface of the hollow particles through pores on the silica shell, which made the outer diameter and inner diameter of the particles increase. At the same time, it can also be seen that the overall change of shell thickness was very small, which indicates that the PAA loss was small and that the PAA only transferred to the hollow particles and basically did not separate from the hollow particles.

3.2. Thermal Analysis of Composite Particles

To characterize the content of the PAA inside the composite particles and the influence of the change of the PAA on the heat resistance of the particles, the composite particles underwent thermogravimetric (TG) analysis. Figure 3 shows the thermal analysis results of the hollow particles with different PAA residues. From the PAA thermal loss curve, we can identify three decomposition regions, which are at 160 °C, 224 °C, and 269 °C. The thermal degradation of the PAA causes these three separate decomposition regions. The first region is from a minor decarboxylation reaction [32]. The second decomposition stage is mainly caused by anhydride formation. The third decomposition region is from the thermal degradation of the polyacrylic anhydride [33]. It can also be seen that the decomposition temperature of the PAA composite particles was significantly higher than for pure PAA, indicating that the silica shell delayed the pyrolysis of PAA and the thermal stability was significantly improved.
The thermal mass loss results of the composite particles are shown in Table 2. Compared to the particles without washing and the washed hollow particles, the PAA content was minimally reduced, which is the same as seen in the particle shell thickness results (Table 1). This indicates that it is difficult to remove PAA from hollow particles because the PAA and silica both own hydrophilic groups (-COOH and Si-OH). The initial decomposition temperature of the particles was also measured, and the results are shown in Figure 3. The initial decomposition temperature of the particles was significantly higher than that of PAA, and the initial decomposition temperature of the washed particles was further improved. Due to the intermolecular entanglement of polyacrylic acid, which is a chain polymer, and the pore size of the hollow particle shell, it was not easy for PAA to be discharged from the particles [34]. Therefore, even after washing, there was still a large amount of PAA residue in the particles.

3.3. Chemical Structure of Composite Particles

The composition of the composite particles was determined by infrared spectrum analysis. The hollow-particle FT-IR test results of the different PAA residues are shown in Figure 4. The absorption peaks at 486 cm−1 and 814 cm−1 were for Si-O-Si, indicating the presence of a silica shell [35,36,37]. The stretching vibration peak of C=O at 1721 cm−1 indicates the presence of the -COOH group and proves the existence of the PAA [26,38]. Meanwhile, Si-O-Si and C-CH2 exhibited co-existing peaks at 1093 cm−1 [39,40,41]. With the increase in particle cleaning cycles, the PAA content decreased, and the intensity of the peak decreased. The peak at 1093 cm−1 was the lowest and the C=O peak completely disappeared, indicating that the PAA had been completely removed. The following results on the TG partial heat weight loss also proved that the internal PAA was completely removed after roasting.

3.4. Charge Distribution on the Composite Particle Surface

The effect of the particle structure change on the dispersion stability was evaluated by pH titration zeta potential measurement. The zeta potential and isoelectric point (IEP) results are shown in Figure 5. Due to the polarization effect on the hydration of the silica particles in water, oxygen atoms on the surface of the SiO2 shell attracted each other to the hydrogen in the water molecules, resulting in a negative charge on the surface of the particles [42]. In addition, due to the deionization of the PAA by water, the PAA attached to the surface of the silica layer was dissociated into COO-, which further increased the negative charge carried by the composite particles [31]. After PAA recombination, the particle was still negative.
Regarding the particle isoelectric points (IEP) results, although PAA is water-soluble, there is no significant difference shown in the change rate of the curves of all samples in Figure 5, indicating that the PAA content that was dissolved in the solvent (water) was very small, so the free COO- content was also low, which is consistent with the finding that PAA is hard to remove from hollow particles during the washing process (as shown by the TG results). Therefore, the ion concentration change caused by dissolved PAA was minimal, and the effect on the IEP determination results can be ignored. During the PAA dissociation process, due to the influence of PAA molecular chains and the hydrogen bond between molecular chains after water absorption, only the surface layer of the PAA molecules was fully dissociated, while the inner layer of PAA was limited and it was difficult for it to be completely dissociated. Therefore, the external surface area of the PAA layer had an impact on the degree of PAA dissociation. The larger the surface area of PAA, the higher the proportion of PAA that can be fully dissociated, meaning that a higher concentration of hydrogen ions is required to reach IEP. In summary, the IEP of the SiO2/PAA/SiO2 three-layer particles was smaller than that of SiO2/PAA two-layer particles (data marked a in Figure 1), while that of the single-layer SiO2 particles had the least negative charge; hence, the IEP was the largest.

3.5. Pore Structure of Composite Particles

To establish the relationship between particle structure and adsorption properties and to confirm the specific structure of the composite particles, nitrogen adsorption tests were carried out on the particles. Figure 6 shows the nitrogen adsorption results for the hollow particles. Table 3 shows the particle measurement parameters in more detail. The unwashed particles had the highest specific surface area; after washing by water, the specific surface area decreased, to a certain extent, but showed signs of increasing after the PAA had been completely removed. The reason for this is that the PAA inside the hollow particles provided a certain specific surface area. After the PAA was completely removed from the particles, the specific surface area provided by PAA no longer existed, and at the same time, the PAA in the micropores of the hollow particles was also removed, so that the micropore size became larger. At that moment, the micropores provided a larger surface area than when the PAA was present, resulting in a certain increase in the overall specific surface area of the particles.
To study the micropore properties further, the adsorption data of the nonporous nano silica particles were plotted as a standard adsorption curve to draw the t-plot curve of the composite hollow particles. The calculation result from the t-plot transformed by the isothermal adsorption curve is shown in Figure 7. It can be seen that the t-plot curve has an obvious inflection point, indicating the micropore structure of the hollow particles. According to the t-plot curve, the external surface area, the volume of the micropores, and the micropore diameter of the particles can be obtained, and are shown in Table 4. The results show that after the particles were washed, the PAA was discharged from the micropores to the outside of the particles, while the PAA filled the micropores so that the micropore volume decreased.
Once the PAA had been completely removed by roasting, the micropore size became the greater cause of removal of the PAA. The shell thickness of the particle also became thinner because there was only one silica layer in the particles. Meanwhile, although there was no longer any PAA left in the micropores, the micropore volume still remained relatively low because of the influence of the thinness of the particle (only the silica shell remained).

3.6. Water Molecular Sorption Characteristics of the Composite Particles

To explore the sorption characteristics of the composite hollow particles under water vapor and exclude the adsorption effect of other gas components in the air, water isothermal adsorption-desorption was tested, and the isothermal adsorption curve is shown in Figure 8. It can be seen that all the adsorption isotherms do not appear to be closed, indicating that it is difficult for water vapor to achieve desorption in silica and PAA. Due to the hydrophilic groups of the silica and the PAA, the particles had good hygroscopicity, and it is difficult for water molecules to separate, so the isothermal adsorption-desorption curve is not closed [43]. At the same time, the hydrophilicity of silica made it difficult for water molecules to be desorbed due to the presence of the silicon hydroxyl group. Moreover, in lower partial pressure conditions, the water vapor adsorption of the nanoparticle’s capacity is also significantly different. At lower partial pressures (e.g., P/P0 = 0–0.2), the adsorption capacity of water molecules is significantly higher than that of the simple silica particles. Afterward, with the increase in the partial pressure, the growth rate of the adsorption capacity continuously decreases. This shows that when the humidity is low, the affinity of the polymers, such as PAA, with water molecules is significantly stronger than that of silica so that the particles have a strong ability to capture water molecules in very low humidity. This suggests that composite particles can improve the adsorption sensitivity of particles to water vapor, especially in low-humidity conditions.
When the adsorption substance is water vapor, the hydrophilicity of PAA and Si-OH and the hydrogen bond ensure that the water molecules can easily adsorb more than one layer of water molecules on the surface of hollow particles. Therefore, the specific surface area calculated using the BET model is larger than the result of nitrogen adsorption. The PAA can adsorb more water molecules than Si-OH. Therefore, the PAA composite hollow silica particles had a higher surface area than hollow silica particles. Furthermore, the washed composite hollow particles had a PAA coating both inside and outside of the particle shell. As the PAA area increased in the hollow particles, they could adsorb more water molecules, so its specific surface area increased significantly. This pattern is the same as that of the results shown in Table 5.
To evaluate the influence of the PAA addition’s content on the water adsorption properties of the composite particles, the comparison results of the water vapor adsorption properties of particles with different PAA contents are shown in Figure 9.
The water sorption capacity of the nanoparticles containing PAA was significantly higher than that of hollow silica particles. This is due to the high hydrophilicity of COOH in the PAA molecules, which adsorbs more water vapor [44]. At the same time, water molecules were also absorbed between the molecular chains of the polymer, which further improved the water absorption of PAA. Therefore, the water sorption of the composite particles containing PAA was improved [45].
In addition, the washed composite particles with a reduced PAA content had better water molecule adsorption capacity than the unwashed composite particles, which had a higher PAA content. This indicates that there are factors other than the PAA content that affected the water adsorption performance of the particles. Moreover, the enhanced water adsorption effect due to such factors is significantly stronger than the enhanced water adsorption performance due to the presence of PAA alone, under certain conditions. The TG and TEM characterization results show that particle washing can not only slightly reduce the PAA content but may also significantly change the PAA distribution of the composite particles, with these becoming a three-layer composite structure of PAA/silica/PAA. At this point, part of the PAA will be transferred to the surface of the particle, and this relocated PAA, unhindered by the silica layer, can adsorb more water molecules, so the water adsorption of the composite particles increased after washing. Furthermore, at low partial pressure, water molecules are more easily adsorbed on the surface of the PAA and SiO2 shell; therefore, a larger external surface area can offer greater adsorption capacity. When the partial pressure increases, the water molecules have enough pressure to enter through the intertwined PAA molecular chain, so the influence of the adsorption capacity affecting the outer area will gradually weaken. This change is also reflected in Figure 9.
The water molecule adsorption capacity and particle size information of the nanoparticles in this work have been compared with other research results and are shown in Table 6.
The adsorption capacity of water vapor per unit area obtained in Figure 9 needs to be converted to compare it to other results. The conversion method is as follows.
First, based on P/P0 = 0.6 and the saturated vapor pressure of water vapor at 298 K, the partial pressure of water vapor at this temperature is calculated. Based on the ideal gas law, where PV = nRT, the molar volume of water vapor under a specific partial pressure can be obtained. Then, according to the water vapor molar volume, the adsorption mass of water molecules per unit area of the nanoparticles can be calculated, and, finally, the water molecule adsorption mass per unit mass of the nanoparticles can be converted.

3.7. Mechanism Elucidation of the Relationship between Structural Absorption and the Structure of the Composite Particles

After obtaining the SEM, TEM, and EDS observation results and combining them with the t-plot curve results, we can obtain the change rule of the particle composite structure; its structural change is depicted in Figure 10.
According to the structural changes of the silica nanoparticles, the change can be divided into three stages: the double-layer structure of the PAA/silica of the composite particles when they are not washed, the three-layer composite structure of PAA/silica/PAA after washing, and the single-layer silica structure with the PAA completely removed by roasting.
In the single-layer silica stage, only the silica exists, so when water vapor is adsorbed, the water molecule adsorption of its particles lacks the adsorption capacity of PAA, so its hygroscopicity is the weakest, and, because only the silica shell is available, its nitrogen adsorption amount is also the lowest.
In the PAA/silica two-layer stage, the gas adsorption of the particles mainly occurs on the surface of the silica shell, as well as on the inner layer of PAA. The PAA layer can facilitate the additional adsorption of water vapor; therefore, the water molecule adsorption capacity of PAA/silica two-layer composite particles is significantly higher than that of single-layer hollow silica particles.
In the PAA/silica/PAA three-layer stage, the partial PAA inside the hollow particles moves to the outer surface of the silica shell through the micropores on the silica shell to form the PAA layer. When water molecule adsorption is carried out, the outside layer of PAA is not affected by the silica shell. In addition, when the total amount of PAA is unchanged, the single-layer PAA structure is converted into a double-layer PAA structure, which will significantly increase the outer area of the PAA and expose more hydrophilic groups. Therefore, the particles can adsorb more water molecules, which further enhances the hygroscopicity of composite particles. This structural change will enhance the water absorption capacity of the particles, which is also demonstrated by the results in Figure 9. It can be seen that the composite particles with a three-layer multi-structure have the best hygroscopic properties.
At the same time, it should be pointed out that no matter what kind of nanoparticle multilayer structure is produced, its hygroscopicity performance is weaker than that of pure PAA. Nonetheless, compared to pure PAA, the composite particles will be more thermally stable; they can be heated to accelerate the dehydration rate when the water molecules are desorbed without worrying about the thermal decomposition of the material. Thus, this can expand their operating temperature range.

4. Conclusions

In this study, composite hollow silica nanoparticles were synthesized with a PAA template. Moreover, these hollow particles could be nano-coated to obtain composite hollow nanoparticles with a double- or triple-layered structure by controlling the PAA distribution. Information on the basic chemical and physical properties of composite hollow nanoparticles is shown in Table 7.
When the hollow nanoparticle shell was coated with two layers of PAA, creating a PAA/SiO2/PAA shell structure, the water vapor adsorption capacity reached a maximum of 109.9 cm3/m2, which is 9% higher than that reached by a one-layer PAA coating, and 160% higher than for nanoparticles without the PAA layer.
The materials used in the synthesis method are easy to obtain (ethanol, PAA, TEOS, and ammonia water) and the preparation conditions are simple (room temperature, atmospheric pressure), so this method has good reproducibility and potential for scalability.
Although the moisture sorption properties of composite nanoparticles are discussed in this research, there are still some limitations. For example, the biocompatibility and toxicity of composite nanoparticles need to be discussed.

Author Contributions

Conceptualization, Q.W. and M.F.; methodology, Q.W. and M.F.; validation, Q.W. and M.F.; investigation, Q.W.; resources, M.F.; data curation, Q.W.; writing—original draft preparation, Q.W.; writing—review and editing, M.F. and K.I.; visualization, Q.W.; supervision, M.F.; project administration, M.F.; funding acquisition, M.F. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partly supported by the Aichi Knowledge Center Priority Research Program, Phase IV (2022–2024), and JSPS KAKENHI, grant number JP 23H01801.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Coatings 14 00829 g001
Figure 1. Transmission electron microscope images (TEM) and energy spectrum scanning (EDS) images of hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
Figure 1. Transmission electron microscope images (TEM) and energy spectrum scanning (EDS) images of hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
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Figure 2. Statistical results of composite particle diameter and the shell thickness distribution of composite hollow particles ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
Figure 2. Statistical results of composite particle diameter and the shell thickness distribution of composite hollow particles ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
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Figure 3. Thermogravimetry test results for the hollow particles and pure polyacrylic acid (PAA) under different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
Figure 3. Thermogravimetry test results for the hollow particles and pure polyacrylic acid (PAA) under different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
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Figure 4. Infrared spectral analysis results (FT-IR) for hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
Figure 4. Infrared spectral analysis results (FT-IR) for hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
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Figure 5. Zeta potential titration results for the composite particles ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
Figure 5. Zeta potential titration results for the composite particles ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
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Figure 6. Nitrogen isothermal adsorption curves of hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
Figure 6. Nitrogen isothermal adsorption curves of hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
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Figure 7. The t-plot curves under the nitrogen adsorption of composite hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
Figure 7. The t-plot curves under the nitrogen adsorption of composite hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
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Figure 8. Water vapor isothermal adsorption curves of hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
Figure 8. Water vapor isothermal adsorption curves of hollow particles with different PAA contents ((a) = not washed, (b) = washed once, (c) = washed twice, (d) = PAA completely removed).
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Figure 9. Comparison of the adsorption capacity of water molecules in the different shell structures of the hollow nanoparticles (silica shell, silica/PAA shell, and PAA/silica/PAA shell).
Figure 9. Comparison of the adsorption capacity of water molecules in the different shell structures of the hollow nanoparticles (silica shell, silica/PAA shell, and PAA/silica/PAA shell).
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Figure 10. Structural change mechanism diagram of composite particles.
Figure 10. Structural change mechanism diagram of composite particles.
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Table 1. The average value results of hollow particle diameter, shell thickness, and inner diameter (a = not washed, b = washed once, c = washed twice, d = PAA completely removed).
Table 1. The average value results of hollow particle diameter, shell thickness, and inner diameter (a = not washed, b = washed once, c = washed twice, d = PAA completely removed).
SampleParticle Diameter, nmShell Thickness, nmParticle Inner Diameter, nm
a79.7127.7551.96
b81.8626.0855.78
c86.3225.5760.35
d77.1915.5561.64
Table 2. Thermal weight-loss results for composite nanoparticles under different wash conditions (a = not washed, b = washed once, c = washed twice, d = roasted to remove the PAA).
Table 2. Thermal weight-loss results for composite nanoparticles under different wash conditions (a = not washed, b = washed once, c = washed twice, d = roasted to remove the PAA).
SampleWeight Loss, %
a17.74%
b14.07%
c13.45%
d0.28%
Table 3. Determination results of specific surface area and pore volume analyses of hollow particles with different PAA contents under nitrogen adsorption mass (a = not washed, b = washed once, c = washed twice, d = PAA completely removed).
Table 3. Determination results of specific surface area and pore volume analyses of hollow particles with different PAA contents under nitrogen adsorption mass (a = not washed, b = washed once, c = washed twice, d = PAA completely removed).
SampleSpecific Surface Area (BET), m2/gTotal Full Fine Pore Volume, cm3/g
a97.6250.4114 (P/P0 = 0.982)
b58.1470.3863 (P/P0 = 0.982)
c57.4450.3614 (P/P0 = 0.98)
d71.6010.5547 (P/P0 = 0.986)
Table 4. External surface area, micropore volume, and average micropore diameter of hollow particles with different PAA contents (a = not washed, b = washed once, c = washed twice, d = PAA completely removed).
Table 4. External surface area, micropore volume, and average micropore diameter of hollow particles with different PAA contents (a = not washed, b = washed once, c = washed twice, d = PAA completely removed).
SampleExternal Surface Area, m2/gMicropore Volume, cm3/gMicropore Diameter, nm
a14.0317.590.84
b13.286.560.80
c13.856.630.80
d19.196.940.90
Table 5. Determination results of the specific surface area and pore volume of hollow particles with different PAA contents under water vapor adsorption mass (a = not washed, b = washed once, c = washed twice, d = PAA completely removed).
Table 5. Determination results of the specific surface area and pore volume of hollow particles with different PAA contents under water vapor adsorption mass (a = not washed, b = washed once, c = washed twice, d = PAA completely removed).
SampleSpecific Surface Area (BET), m2/gTotal Full Fine Pore Volume, cm3/g
a184.780.1154 (P/P0 = 0.983)
b206.750.1141 (P/P0 = 0.990)
c220.000.1166 (P/P0 = 0.990)
d77.220.0851 (P/P0 = 0.981)
Table 6. Features and moisture sorption properties of various composite silica nanoparticles.
Table 6. Features and moisture sorption properties of various composite silica nanoparticles.
Composite Silica Nanoparticle TypeParticle Size, nmMoisture Sorption Capacity, wt %Reference
PE/silica nanoparticles120.9 ± 0.03 [46]
Cellulose fibers/silica nanoparticles 192 ± 4512.3 [47]
PEG/silica nanoparticles174.7 ± 18.98 [48]
Mesoporous silica gel 8 ± 2 [49]
HSNPs77.1975.8 (P/P0 = 0.6)this work
PAA/HSNPs86.32145.9 (P/P0 = 0.6)this work
Table 7. Summary of the physical and chemical properties of PAA composite hollow nanoparticles.
Table 7. Summary of the physical and chemical properties of PAA composite hollow nanoparticles.
Layer Number of Hollow NanoparticlesParticle Size, nmShell Thickness, nmShell StructurePAA Content,
%
177.1915.55SiO20
279.7127.75PAA/SiO217.46
386.3225.57PAA/SiO2/PAA13.17
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Wen, Q.; Ishii, K.; Fuji, M. Preparation of Hollow Silica Nanoparticles with Polyacrylic Acid and Their Moisture Sorption Properties. Coatings 2024, 14, 829. https://doi.org/10.3390/coatings14070829

AMA Style

Wen Q, Ishii K, Fuji M. Preparation of Hollow Silica Nanoparticles with Polyacrylic Acid and Their Moisture Sorption Properties. Coatings. 2024; 14(7):829. https://doi.org/10.3390/coatings14070829

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Wen, Quanyue, Kento Ishii, and Masayoshi Fuji. 2024. "Preparation of Hollow Silica Nanoparticles with Polyacrylic Acid and Their Moisture Sorption Properties" Coatings 14, no. 7: 829. https://doi.org/10.3390/coatings14070829

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