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Article Cite This: ACS Omega 2018, 3, 2820−2824
One-Step Synthesis of Calcium Sulfate Hemihydrate Nanofibers from Calcite at Room Temperature Satoru Fukugaichi*,† and Naoto Matsue‡ †
Paper Industry Innovation Center, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan Faculty of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan
‡
S Supporting Information *
ABSTRACT: In recent years, researchers have made significant progress in the development of inorganic nanofibers (including nanowires). Typically, inorganic nanofibers are synthesized via crystal growth in solution; however, a limited number of studies have focused on their preparation directly from solid raw materials (with no examples of synthesis conducted at room temperature and atmospheric pressure). In this work, we successfully synthesized nanofibers of calcium sulfate hemihydrate (bassanite, CaSO4·0.5H2O) at 20 °C and 1 atm by mixing calcite and dilute sulfuric acid in methanol. The bassanite nanofibers are concluded to be synthesized by the formation of calcium sulfate on the calcite surface and its simultaneous reaction with the generated H2O. Because bassanite exhibits useful physical properties that include high mechanical strength, high thermal stability, and excellent chemical stability, its nanofibers can be widely applied to rubber, plastics, antifriction materials, and paper as a strengthening agent, for heat-resistance, or as a flame retardant, or for creep resistance. can be easily obtained by mixing Ca2+ ions and sulfuric acid in organic solvents.21−23 Normally, gypsum (CaSO4·2H2O) is synthesized by the reaction of Ca2+ with SO42− in water, but bassanite is obtained by adding an organic solvent to a supersaturated aqueous solution of calcium sulfate, which quenches the gypsum formation reaction and produces a metastable aqueous bassanite phase.21 In this work, bassanite nanofibers have been successfully synthesized through the reaction of calcite and dilute sulfuric acid in methanol by combining Ca2+, SO42−, and H2O species on the calcite crystal surface.
1. INTRODUCTION Because of the various useful physical properties of inorganic nanofibers, researchers have actively explored new synthesis methods for these materials in recent years. Generally, inorganic nanofibers (including nanowires) are synthesized by growing crystals in solution. For example, nanofibers are fabricated using hydrothermal methods (for ZnS1 and CdS2), electrospinning (for TiO23), and photochemical etching (for GaN4). However, only TiO2 nanofibers have been produced from solid starting materials by a hydrothermal method.5 In our study, bassanite (calcium sulfate hemihydrate; CaSO4·0.5H2O) nanofibers have been successfully synthesized in one step at 20 °C and 1 atm using solid calcite (CaCO3) as a starting material and mixing it with dilute sulfuric acid in methanol. Bassanite potentially can be used in a wide range of applications due to its high mechanical strength, good thermal stability, and high chemical stability.7−9 For example, it can be added to rubber, plastics, antifriction materials, and paper as a strengthening agent, for heat-resistance, as a flame retardant, or for creep resistance.10−13 Gypsum has similar chemical and thermal properties as bassanite, but the synthesis of gypsum nanofibers has not been reported so far, and it is difficult to obtain gypsum in organic solvents. For these reasons, several studies have investigated the synthesis of bassanite whiskers.14−20 However, these whiskers cannot be considered nanofibers because of their large diameters and small aspect ratios. In addition, they were synthesized via crystal growth in solution. However, recent experiments indicate that bassanite whiskers © 2018 American Chemical Society
2. RESULTS AND DISCUSSION Both methanol and dilute sulfuric acid were added to calcite, and after stirring for 5 min at 20 °C and 1 atm, the scanning electron microscopy (SEM) observation revealed that the trigonal calcite crystals in Figure 1a were transformed into rods (sample M-5; Figure 1b). After 95 min, these rods were further transformed into the nanofibers with diameters ranging from 20 to 50 nm (sample M-95; Figure 1c), and only nanofibers with sizes ranging from 20 to 50 nm were observed in the sample after 425 min (sample M-425; Figure 1d). The X-ray diffraction (XRD) analysis confirmed the transformation of calcite to bassanite with increasing reaction time. In addition to the XRD peaks of calcite Received: December 15, 2017 Accepted: February 26, 2018 Published: March 8, 2018 2820
DOI: 10.1021/acsomega.7b01994 ACS Omega 2018, 3, 2820−2824
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Figure 1. Morphological change from trigonal calcite to nanofibers in methanol under a stirring condition. (a) Trigonal calcite crystals as a raw material. (b) Sample M-5: Small rods formed on the surface of the trigonal calcite crystals at a reaction time of 5 min. (c) Sample M-95: Most of the rods transform to nanofibers at 95 min. (d) Sample M-425: Only the nanofibers are observed, and the trigonal crystals are not recognized at 425 min. The presence of curved fibers indicates that the produced bassanite nanofibers are relatively flexible.
Figure 2. Mineralogical change from calcite to bassanite in methanol under a stirring condition. (a) Calcite as a raw material. (b) Sample M-5: Small XRD peaks ascribed to bassanite in addition to the peaks of calcite and a small unknown peak at 11.4° at a reaction time of 5 min. (c) Sample M-95: The peaks of bassanite become stronger and those of calcite weaker at 95 min. A small unknown peak is observed at 11.3°. (d) Sample M-425: At 425 min, only the peaks of bassanite are observed except for the shoulder peak at 29.4°. (e) Bassanite standard sample.
that the trigonal calcite crystals directly transformed to the bassanite nanofibers. From the results of thermogravimetry/differential thermal analysis (TG/DTA) obtained for the M-425 sample (see Figure S1), bassanite content was estimated at about 93 mass %. Hence, mixing calcite with dilute sulfuric acid in methanol with the subsequent stirring at 20 °C and 1 atm can produce high-purity bassanite nanofibers. In addition, the structural analysis of the M425 sample was conducted using a high-resolution transmission
(Figure 2a), M-5 showed small peaks corresponding to a bassanite phase (Figure 2b), which suggested that the rods observed in Figure 1b also contained some bassanite phase. The XRD peaks of the bassanite phase increased with the reaction time. Sample M-95 (Figure 2c) showed a predominance of the bassanite peaks, and M-425 (Figure 2d) mostly consisted of the bassanite peaks, whereas only a small calcite peak was detected at 29.4° in addition to the peaks of a standard bassanite sample shown in Figure 2e. The above SEM and XRD results revealed 2821
DOI: 10.1021/acsomega.7b01994 ACS Omega 2018, 3, 2820−2824
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Figure 3. TEM results for M-425 showing the growth of a bassanite nanofiber along its c axis. (a) A HR-TEM microphotograph and (b) an FFT pattern corresponding to the reciprocal bassanite two-dimensional lattice structure along the c axis. The samples for the TEM observations were prepared by dispersing the M-425 powder on a carbon−copper grid without using solvents.
electron microscope. The d-spacing values obtained from the high-resolution transmission electron microscopy (HR-TEM) images (Figure 3a) and their fast Fourier transform (FFT) analysis (Figure 3b) revealed that the synthesized nanofibers grew along the c axis direction. This result is in agreement with previous studies of bassanite nanorods6 and whiskers,19 in which elongation along the c axis was also observed. To evaluate the effect of stirring on bassanite nanofiber synthesis, we also prepared nanofibers using the same procedure but without stirring. When the mixture of calcite, methanol, and dilute sulfuric acid was stored at room temperature under normal pressure, similar nanofibers were produced. The SEM image of the product obtained at a reaction time of 5 min (M′-5) is shown in Figure 4a, which confirms the presence of fibers with diameters of 30−100 nm on the surface of the calcite crystal. The sample obtained after 95 min (M′-95) predominantly consisted of the nanofibers that were formed directly from the calcite crystals (see the SEM images depicted in Figure 4b,c). The content of the nanofibers increased with increasing reaction time, and the sample obtained after 425 min (M′-425) was almost exclusively composed of nanofibers (Figure 4d). The XRD pattern of M′-5 (see Figure S2a) contained strong calcite peaks and weak bassanite peaks, and M′-95 contained more calcite peaks than bassanite ones (Figure S2b), whereas the XRD pattern of M′-425 (Figure S2c) was almost identical to that of M′-95. The results indicate that mixing calcite with dilute sulfuric acid in methanol and storing the resulting mixture at 20 °C and 1 atm without stirring generates nanofibers consisting of both the bassanite and calcite phases. As indicated by the TG/ DTA curves (shown in Figure S3), the estimated bassanite content in M′-425 was about 64 mass %. Nevertheless, when calcite was added to an “aqueous” solution of sulfuric acid and stored for 425 min, it produced only the gypsum phase without bassanite (W-425; see Figure S2d). The SEM image of W-425 (Figure 4e) displays the formation of rodshaped crystals with dimensions of about 1 μm × 30 μm. The reason for the formation of gypsum when water is used as a solvent is the instantaneous reaction between the calcite and H+ species of the sulfuric acid, which leads to the interaction between Ca2+ and SO42− ions and crystallization of the obtained product after the dissolution of calcite.24
In the reaction of calcite and sulfuric acid in methanol, we also conducted experiments at a sulfuric acid concentration of 10 M (0.05 mL) instead of 1 M (0.5 mL). After 425 min of reaction, nanofibers containing bassanite were produced (Figure S4). When 0.5 mL of 1 M sulfuric acid was added to 50 mL of methanol, the content of H2O in the solvent is 1.23 mass %, and the value for the case of 10 M is 0.07 mass %. The effect of H2O content in a methanol/water system on the formation of bassanite is investigated by Tritschler et al.,25 where CaCl2 and sulfuric acid react in methanol/water while changing the methanol/water ratio to systematically produce calcium sulfate anhydrite, bassanite, and gypsum. They show that bassanite forms between 0.2 and 33 mass % H2O, bassanite content in the product is almost 0 at 0.2 mass % H2O, it increases with increasing H2O content to reach 100% at about 2 mass % H2O. The change in the products with changing methanol/water ratio may be caused by the change in the dielectric constant of the solvent; with decreasing H2O content, the dielectric constant of the solvent decreases and less hydrated species may form.21,25 These observations indicate that, in our experiments in methanol, H2O of the bassanite nanofibers is not derived from H2O in the sulfuric acid because at least in the case with 10 M sulfuric acid, the content of H2O in the solvent (0.07 mass %) is far below the range for bassanite formation (0.2−33 mass %). Therefore, the observed formation of bassanite nanofibers in a methanol solvent can be described as follows. Initially, the reaction between calcite and H+ species occurs on the surface of calcite crystals to produce Ca2+, H2O, and CO2; the content of H2O on the surface of the calcite crystals becomes higher than that of the bulk phase; then on the surface of the calcite crystals, the produced Ca2+ and H2O react with SO42− to form bassanite nanofibers. This study showed that stirring of the calcite in the methanol solvent affected the results. Without stirring, we believe that the H2CO3 and CO2 species produced during the neutralization reaction remain on the calcite crystal surface, which decreases the reactivity of its inner part with H+ and suppresses subsequent bassanite formation. In contrast, stirring moves H2CO3 and CO2 molecules into the bulk solution, thus promoting the neutralization of calcite and the formation of bassanite. In reality, the amount of the released CO2 measured inside a sealed 2822
DOI: 10.1021/acsomega.7b01994 ACS Omega 2018, 3, 2820−2824
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Figure 4. Products in methanol without stirring (a−d), and a product in water without stirring (e). (a) Sample M′-5: Small fibers formed on the surface of the trigonal calcite crystals at a reaction time of 5 min. (b, c) Sample M′-95: At 95 min, many nanofibers are formed (b), and the nanofibers are directly formed from the trigonal crystals (c). (d) Sample M′-425: Only the nanofibers are recognized at 425 min. (e) Sample W-425: When water was used as a solvent, the trigonal calcite crystals transformed to rod-shaped crystals at 425 min.
dispersed in rubber or/and plastic to create promising new materials.
reaction vessel was very small before stirring and significantly increased after stirring (see Figure S5). Because stirring significantly affects the bassanite content in the final product, it is likely that these synthesis conditions can be optimized to obtain high-purity bassanite nanofibers.
4. METHODS 4.1. Synthesis of Bassanite Nanofibers. A weight of 50 mg of calcium carbonate (Wako Pure Chemical Industries, Ltd.) and 50 mL of methanol (Wako Pure Chemical Industries, Ltd.) was added to a 130 mL Pyrex glass container, which was subsequently sealed and purged with nitrogen to remove any traces of reactive gases. After that, 500 μL of 1 M sulfuric acid was added to the container using a micropipette, and the resulting mixture was stirred using a magnetic stirring bar on a stir plate. The reaction was allowed to proceed for 425 min at a temperature of 20 °C. For comparison, the same experiment was conducted using distilled water as the solvent. The samples synthesized in methanol were filtered and washed with methanol, and the samples synthesized in distilled water were filtered and washed with distilled water. 4.2. Sample Characterization. The calcite, bassanite (Wako Pure Chemical Industries, Ltd.), and produced samples were subjected to XRD mineralogical analysis with Cu Kα
3. CONCLUSIONS In this study, high-purity bassanite nanofibers were synthesized in one step at 20 °C and 1 atm using inexpensive calcite as a raw material. The bassanite content in the sample with a stirring condition in methanol at a reaction time of 425 min was about 93 mass %. In addition to its use as a strengthening agent for rubber, plastics, antifriction materials, and paper, it can be also fabricated in a transparent form due to its fine fiber diameter of 20−50 nm. Hence, the produced bassanite nanofibers potentially can be utilized in more specialized applications, such as glassstrengthening products. Further, because nanofibers and whiskers in previous works were obtained in aqueous solutions, incorporating them into a piece of rubber or plastic in a disperse form was difficult. In contrast, the bassanite nanofibers in this study were synthesized in methanol, which can be more easily 2823
DOI: 10.1021/acsomega.7b01994 ACS Omega 2018, 3, 2820−2824
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radiation (Rigaku; Ultima IV), whereas their morphologies were investigated using a scanning electron microscope (JEOL JSM6335F). Crystal structure analysis was performed using a transmission electron microscope (FEI Titan 80-300). Additionally, to evaluate the thermal characteristics of the obtained bassanite nanofibers, their TG/DTA analysis (Rigaku; Thermo Plus Evo TG8120) was performed from room temperature to 900 °C at a heating rate of 10 °C/min in an air flow of 100 cm3/ min.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01994. Thermal analysis data, XRD patterns, SEM images, and the amount of CO2 released in the reaction (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Satoru Fukugaichi: 0000-0003-1847-4231 Author Contributions
S.F. designed the study, carried out the experiments, and wrote the manuscript. N.M. designed the study and wrote the manuscript. All authors contributed to the discussion of the results and revision of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank Editage (www.editage.jp) for English language editing. REFERENCES
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