Article pubs.acs.org/Langmuir
Effect of Mg2+ on Hydrothermal Formation of α‑CaSO4·0.5H2O Whiskers with High Aspect Ratios Sichao Hou,† Jing Wang,† Xiaoxue Wang,‡ Haoyuan Chen,† and Lan Xiang*,† †
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Department of Chemical Engineering, Massachusetts Institute of Technology, 25 Ames Street, Cambridge, Massachusetts 02139, United States
‡
ABSTRACT: The effect of Mg2+ on hydrothermal formation of α-CaSO4·0.5H2O whiskers with high aspect ratios was investigated in this paper. α-CaSO4·0.5H2O whiskers with a preferential growth along the c axis and an average aspect ratio up to 370 were synthesized using hydrothermal treatment of CaSO4·2H2O precursor in the presence of 1.97 × 10−3 mol·L−1 MgCl2. The preferential adsorption of Mg2+ on the negative (200), (400), and (020) facets was confirmed by EDS, XPS, and zeta potential measurements. ATR-FTIR analysis revealed the ligand adsorption of Mg2+ on the surface of α-CaSO4·0.5H2O. The doping of Mg2+ in α-CaSO4·0.5H2O whiskers was confirmed by the XRD analysis. The experimental results indicated that the adsorption and doping of Mg2+ promoted the 1-D growth of α-CaSO4·0.5H2O whiskers, leading to the formation of whiskers with high aspect ratios. of 21% in flexural strength and 22% in the impact strength.14 The tensile strength and the elongation of polypropylene (PP) reached up to 39.18 MPa and 125.4%, respectively, after filling 5 wt % of CaSO4 whiskers.15,16 The preparation of CaSO4 whiskers with high aspect ratios and homogeneous morphology has drawn much attention in recent years. CaSO4 whiskers were often prepared by calcinating α-CaSO4·0.5H2O whiskers at temperature above 300 °C. α-CaSO4·0.5H2O whiskers were usually prepared from commercial CaSO4·2H2O,17 the natural gypsum18 or the impurity-bearing materials as phosphogypsum,19 flue gas desulfurization gypsum,20 carbide slag,21 and so on by the methods of the hydrothermal conversions, the microwaveassisted reactions, the reverse microemulsion methods,18−24 and so on. The anisotropic −Ca−SO4−Ca−SO4−Ca− chains in α-CaSO4·0.5H2O favored its 1-D growth along the c axis.25 The morphology of α-CaSO4·0.5H2O whiskers can be controlled by many methods, including the activation of the raw materials, the alteration of the process parameters, the use
1. INTRODUCTION One-dimensional materials with high aspect ratios have drawn much attention in recent years due to their excellent properties in optic, electric, and mechanic applications.1−4 The mechanical properties of 1-D materials as CaSO4 whiskers are usually improved with an increase in the aspect ratios.5−7 According to the continuum theory,6 the modulus of elasticity (E) of homogeneous rod-shaped increases with an increase in aspect ratios and can be expressed as follows ⎛ 8πvL2 ⎞ E = ρ⎜⎜ 2i ⎟⎟ ⎝ βi D ⎠ where βi is a constant and E, ρ, vi, L, and D represent the modulus of elasticity, the density, the frequency of resonation, the length, and the diameter of the material, respectively. CaSO4 whiskers with high aspect ratios are promising reinforcing materials widely used in plastics, ceramics, rubber, paper making, and so on.8−12 It was reported that the frictional wear resistance of nitrile butadiene rubber (NBR)-modified phenol formaldehyde (PF) was improved by adding fibers of CaSO4 and aramid.13 The presence of 15 wt % of CaSO4 whiskers in the polycaprolactone composite led to the increase © 2014 American Chemical Society
Received: March 22, 2014 Revised: July 31, 2014 Published: August 4, 2014 9804
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Figure 1. Influence of MgCl2 on morphology (a−e) and XRD patterns (f) of hydrothermal products MgCl2 (mol·L−1): (a) 0, (b) 9.84 × 10−4, and (c−e) 1.97 × 10−3; ●: α-CaSO4·0.5H2O.
of the specific additives, and so on. For example, α-CaSO4· 0.5H2O whiskers with an aspect ratio up to 325 were prepared by hydrothermal treatment of the active CaSO4·2H2O precursor (formed by sintering CaSO4·2H2O at 150 °C for 6.0 h followed by hydration at room temperature) at 135 °C for 4.0 h;26 the morphology of α-CaSO4·0.5H2O can be adjusted by changing the pH or the solution composition;26−28 the presence of 23.5-35.5 wt % of CaCl2 led to the change of the morphology of α-CaSO4·0.5H2O whiskers from long-and-slim hexagonal rods to fat-and-short hexagonal columns;25 the presence of ethanol, potassium sodium tartrate, and sodium citrate led to the increase in the aspect ratios of CaSO4·0.5H2O whiskers from 1.7 to 4.8.27,29,30 The preferential adsorption and inhibition of cetyltrimethyl ammonium bromide (CTAB) on the side facets and sodium dodecyl sulfonate (SDS) on the top facets of α-CaSO4·0.5H2O whiskers led to the increase in the aspect ratios from 2−7 to 180−250 in the reversed microemulsion system.22 A facile method was developed to synthesize α-CaSO4· 0.5H2O whiskers with high aspect ratios by hydrothermal treatment of CaSO4·2H2O precursor in the presence of minor amount of MgCl2. The adsorption and doping of Mg2+ in the hydrothermal formation of α-CaSO4·0.5H2O whiskers were revealed, and the corresponding mechanisms were studied.
2.2. Characterization. The morphology and element composition of the samples were characterized with the field-emission scanning electron microscopy (SEM, JSM 7401F, JEOL, Japan) and the highresolution transmission electron microscopy (HRTEM, JEM-2010, JEOL, Japan) equipped with the selected area electron diffraction (SAED). The structures of the samples were identified by powder Xray diffractometer (XRD, D8 advanced, Bruker, Germany) using Cu Kα radiation (λ = 1.54178 Å), with a scanning rate of 5° min−1 and a scanning 2θ range of 5 to 90°. The surfaces of the whiskers were characterized by X-ray photoelectron spectroscopy (XPS, model PHI5300, PHI, USA) with a Mg−Kα photon energy of 1253.6 eV. The concentration of Mg2+ was analyzed by EDTA complexometric titration method.31 The surface electric potential was measured with the Zeta Potential (ZETAPALS, Brookhaven Instrument Corporation) in the pH range of 2.0 to 13.0. The interaction between Mg2+ and SO42− was investigated by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The average diameters and the lengths of the whiskers for each sample were estimated by direct measuring about 200 whiskers from the typical FESEM images with the magnifications of 250−5000.
3. RESULTS AND DISCUSSION 3.1. Effect of MgCl2 on the Formation of α-CaSO4· 0.5H2O Whiskers. The influence of MgCl2 on morphology and XRD patterns of the hydrothermal products formed at 135 °C are shown in Figure 1, and the distributions of the diameters and the aspect ratios of the products are shown in Figure 2. αCaSO4·0.5H2O whiskers with a length of 20−80 μm, an average width of 1.35 μm, and an average aspect ratio of about 94 were synthesized in the absence of MgCl2 (Figure 1a). The increase in MgCl2 from 9.84 × 10−4 to 1.97 × 10−3 mol·L−1 led to the increase in the lengths of the whiskers from 110−235 μm to 210−500 μm and the decrease in the average diameters from 1.10 to 0.91 μm (Figure 1b,c). The average aspect ratios of the whiskers reached up to 370 in the presence of 1.97 × 10−3 mol· L−1 MgCl2. TEM and HRTEM patterns Figure 1d,e revealed
2. EXPERIMENTAL SECTION 2.1. Experimental Procedure. Commercial chemicals with analytical grade were used in the experiments. CaSO4·2H2O with a purity of 99.0% was sintered at 150 °C for 3.0 to 6.0 h, then mixed with deionized water and minor amount of MgCl2 with a purity of 98.0% at room temperature to get the suspensions containing 1.0−5.0 wt % CaSO4·2H2O and 0 to 1.970 × 10−3 mol·L−1 MgCl2. The slurries were then treated under hydrothermal condition (135 °C, 4.0 h), filtrated, and dried at 105 °C for 6.0 h. 9805
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Figure 2. Diameter (a) and aspect ratio (b) distributions of hydrothermal products MgCl2 (mol·L−1): blue ■, 0; red ■, 9.84 × 10−4, green ■, 1.97 × 10−3.
that the interplanar spacing of the lattice fringes parallel to the growth direction of the whiskers was 0.598 nm, quite similar to the spacing of (002) plane (d(002) = 0.599 nm) of α-CaSO4· 0.5H2O, indicating the possible preferential orientation of the whiskers along the c axis. The diffraction spots in the SAED pattern (Figure 1e) could be indexed to the [11̅0] zone axis of α-CaSO4·0.5H2O, reconfirming the preferential growth of the α-CaSO4·0.5H2O whiskers along the c axis. The influence of MgCl2 on the XRD patterns of α-CaSO4·0.5H2O whiskers is shown in Figure 1f. Al2O3 was used as the reference sample to eliminate the experimental deviation. The same location of the Al2O3 sample (2θ = 35.251° for (104) peak) in all cases confirmed the accuracy of the XRD spectra. All of the XRD peaks were attributed to the sole existence of α-CaSO4·0.5H2O. Most of the occurred planes as (200), (020), and (400) were parallel to the c axis, reconfirming the preferential growth of the α-CaSO4·0.5H2O whiskers along the c axis. Figure 3 shows the influence of MgCl2 on EDS patterns of αCaSO4·0.5H2O whiskers. The atomic ratios were labeled at the corresponding peaks. Compared with the blank experiment without MgCl2 (curve a), 3.35 and 5.46% of Mg were detected in the presence of 9.84 × 10−4 and 1.97 × 10−3 mol·L−1 MgCl2, respectively, while no Cl was detected in all cases. The atomic ratios of Ca/S/O were 1:1.03:4.29, 1:1.04:4.39, and 1:1.06:4.72 in the presence of 0, 9.84 × 10−4, and 1.97 × 10−3 mol·L−1 MgCl2, respectively. The higher atomic ratios of Ca/S/O of αCaSO4·0.5H2O whiskers formed in the presence of MgCl2
Figure 3. Influence of MgCl2 on EDS patterns of α-CaSO4·0.5H2O whiskers MgCl2 (mol·L−1): (a) 0, (b) 9.84 × 10−4, and (c) 1.97 × 10−3.
indicated the possible doping of Mg2+ in α-CaSO4·0.5H2O or the adsorption of Mg2+ on the surface of α-CaSO4·0.5H2O. 3.2. Doping of Mg2+ on α-CaSO4·0.5H2O Whiskers. Figure 4 shows the influence of MgCl2 on the peak shift of the XRD patterns of the α-CaSO4·0.5H2O whiskers. The presence of 0, 9.84 × 10−4, and 1.97 × 10−3 mol·L−1 MgCl2 led to the shift of 2θ for (200) peaks from 14.756 to 14.775° and 14.816°, (020) peaks from 25.643 to 25.654° and 25.663°, and (400) peaks from 29.745 to 29.764° and 29.782°, respectively, corresponding to the decrease in the lattice spacing from 9806
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Figure 5. Influence of MgCl2 on XPS spectra of α-CaSO4·0.5H2O whiskers MgCl2 (mol·L−1): (a) 0, (b) 9.84 × 10−4, and (c) 1.97 × 10−3. (I) Mg 1s peaks and (II) S 2p peaks.
banding energy of 170.26 eV in the absence of MgCl2, while double S 2p peaks occurred in the presence of MgCl2: one peak was located at 169.25 eV, which indicated the interaction between Ca2+ and SO42−, and another peak was located at 170.26 eV, which should be related to the interaction between Mg2+ and SO42−.32 The occurrence of the double S 2p peaks should be attributed to the partial substitution of Ca by Mg. The stronger combining tendency or higher binding energy between Mg and S than those between Ca and S should be attributed to the different electronegativities of Mg (1.39) and Ca (1.00), which led to the shift of the S 2p peak to the right side.33 Figure 6 shows the variation of the soluble Mg2+ ([Mg2+]) in hydrothermal solutions and the aspect ratios of whiskers with reaction time. The gradual decrease in [Mg2+] with reaction time revealed the continuous adsorption or doping of Mg2+ on/ in the whiskers. [Mg2+] decreased from (9.84 to 6.30) × 10−4 mol·L−1in curve b and 1.97 × 10−3 to 9.03 × 10−4 mol·L−1 in curve c after 4.0 h of hydrothermal reaction, corresponding to the theoretic bulk atomic ratios of Mg to Ca being 1:78 and 1:26, respectively, which were much smaller than the atomic ratio of Mg to Ca on the whisker surface (1:36 and 1:8) deduced from the XPS data shown in Table 1. The different atomic ratios in the bulk and on the surface should be attributed to the adsorption of Mg2+ on whisker surfaces. The rapid decrease in [Mg2+] and the fast increase in the aspect ratios of the whiskers within 2.0 h in Figure 6 also indicated that the adsorption of [Mg2+] on whisker surface favored the 1D growth of the whiskers. The influence of [Mg2+] on the zeta potentials of α-CaSO4· 0.5H2O whiskers is shown in Figure 7. The data in curve a indicated the absence of Mg2+, and the zeta potential declined from −8.23 to −20.92 mV as pH increased from 3.1 to 11.3,
Figure 4. Influence of MgCl2 on peak shift of XRD patterns of αCaSO4·0.5H2O whiskers [MgCl2] (mol·L−1): (a) 0, (b) 9.84 × 10−4, and (c) 1.97 × 10−3.
3.029 to3.023 Å and 3.014 Å for (200), from 1.782 to 1.781 Å and 1.780 Å for (020) and from 1.554 to 1.553 Å and 1.552 Å for (400). The previous phenomena illustrated the possible doping of Mg2+ on those planes because the radius of Mg2+ (0.66 Å) was smaller than that of Ca2+ (1.00 Å) and the substitution of Ca2+ by Mg2+ would lead to the increase in the interplanar spacing of α-CaSO4·0.5H2O. The doping of Mg2+ on the side facets of the α-CaSO4·0.5H2O whiskers as (200), (020), and (400) may inhibit the growth of the whiskers along the radial direction and promote the 1-D growth of the whiskers along the c axis, leading to the formation of α-CaSO4· 0.5H2O whiskers with high aspect ratios. 3.3. Adsorption of Mg on Surface of α-CaSO4·0.5H2O Whiskers. Figure 5 displays the influence of MgCl2 on XPS patterns of α-CaSO4·0.5H2O whiskers. The Mg 1s peaks occurred in the presence of MgCl2, implying the possible doping or adsorption of Mg2+ on whisker surface. The data in Figure 4II shows that a single S 2p peak was detected at the 9807
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Figure 7. Influence of [Mg]2+ on zeta potential of α-CaSO4·0.5H2O [Mg2+](mol·L−1): (a) 0, (b) 9.84 × 10−4, and (c) 1.97 × 10−3.
Figure 6. Variation of [Mg2+] in hydrothermal solutions (I) and the aspect ratios of whiskers (II) with reaction time [MgCl2] (mol·L−1): (a) 0, (b) 9.84 × 10−4, and (c) 1.97 × 10−3.
indicating that the whisker surfaces were negatively charged under the experimental conditions. The data in curves b and c showed that the presence of Mg2+ led to the increase in the zeta potentials, while the surfaces of α-CaSO4·0.5H2O whiskers were still negatively charged. The increase in the zeta potentials with the increase in [Mg2+] indicated that the adsorption of Mg2+ on the surface of α-CaSO4·0.5H2O whiskers, especially on the side facets as (200), (020), and (400), was enhanced with the increase in Mg2+ in solutions, which favored the preferential growth of α-CaSO4·0.5H2O whiskers along the c axis. One of the reasons for the preferential adsorption of Mg2+ on whisker surfaces may be related to the different properties of the planes of α-CaSO4·0.5H2O whiskers. Figure 8 shows the molecular structure of α-CaSO4·0.5H2O deduced by Material Studio 5.0 software.36 The 3D model of α-CaSO4·0.5H2O in Figure 8a was built in a box (a = 12.0317 Å, b = 6.9269 Å, c = 12.6712 Å), while the facet models in Figure 8b−d were the top-view charts of (200), (400), (020), and (001) planes cut from the 3D structure with a thickness of 0.5 Å. The data in Figure 8a showed that α-CaSO4·0.5H2O consisted of −Ca− SO4−Ca−SO4−Ca− chains, where S atom was covalently bonded to four O atoms. The −Ca−SO4−Ca−SO4−Ca− chains formed a framework with continuous channels parallel to the c axis, which favored the 1-D growth of α-CaSO4·0.5H2O along the c axis.7 Table 2 shows the statistics of Ca2+ and SO42−
Figure 8. Molecular model of α-CaSO4·0.5H2O (a) 3D model and (b−d) facet model: (b) (200) and (400), (c) (020), and (d) (001); yellow ●, S; red ●, O; green ●, Ca; white ●, H.
Table 2. Distribution of Ca2+ and SO42− on Facets of αCaSO4·0.5H2O facets
ions
vertex (1/8)
edge (1/4)
surface (1/2)
ratios (Ca:SO4)
(200), (400)
Ca2+ SO42− Ca2+ SO42− Ca2+ SO42−
4 0 2 0 1 0
3 2 1 3 1 0
3 6 4 4 4 4
0.79
(020) (001)
0.91 1.19
on the facets of α-CaSO4·0.5H2O deduced from the data in Figure 8. It was assumed that all ions were located on (200),
Table 1. Surface Composition of α-CaSO4·0.5H2O Whiskers Detected by XPS MgCl2 (mol·L−1)
Ca (%)
Mg (%)
S (%)
O (%)
C (background) (%)
0 9.84 × 10−4 1.97 × 10−3
11.74 11.29 11.64
0 0.31 1.46
13.39 12.74 13.27
58.89 55.91 57.92
15.98 19.75 15.71
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of a covering layer of Mg-SO4 owing to the adsorption of Mg2+ on the side facets of whiskers may inhibit the radial growth of whiskers and then promote the 1-D growth of the whiskers along the c axis. Figure 10 shows the schematic drawing of the adsorption and doping of Mg2+ on/in the surface of CaSO4.0.5H2O whiskers. The adsorption and the doping of Mg2+ on the side facets of αCaSO4·0.5H2O whiskers as (200), (020), and (400) inhibited the growth on these facets, which favored the selective growth of the whiskers along the c axis and promoted the formation of α-CaSO4·0.5H2O whiskers with high aspect ratios.
(400), (020), and (001) facets, ignoring the thickness of the facets. The data in Table 2 showed that the ratios of Ca2+ to SO42− on (200), (400), (020), and (001) facets were 0.79, 0.79, 0.91, and 1.19, respectively. The denser distribution of SO42− on the side facets as (200), (400), and (020) than that on the top facet as (001) indicated that compared with the top facet the side facets were more negatively charged. Therefore, the positive Mg2+ ions were easier to be adsorbed on the side facets than on the top facet, which favored the 1-D growth of αCaSO4·0.5H2O whiskers along the c axis. On the basis of the above fact that Mg2+ may be adsorbed preferentially on the side facets of α-CaSO4·0.5H2O whiskers that were negatively charged, the detail adsorption styles of Mg2+ were then investigated by ATR-FTIR spectroscopy. Figure 9 shows the ATR-FTIR spectra of α-CaSO4·0.5H2O
4. CONCLUSIONS A facile MgCl2-assisted hydrothermal method was developed to synthesize α-CaSO4·0.5H2O whiskers with high aspect ratios. The presence of 1.97 × 10−3 mol·L−1 MgCl2 led to the increase in the aspect ratios of α-CaSO4·0.5H2O whiskers from 50 to 400. The adsorption and doping of Mg2+ on the side surfaces of α-CaSO4·0.5H2O whiskers as (200), (020), and (400) inhibited the growth of these facets and promoted the 1-D growth of αCaSO4·0.5H2O whiskers along the c axis.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
Experiments and data analysis were conducted by S.H., J.W., and H.C. The manuscript was written by S.H. and revised by X.W. and L.X. All authors have given approval to the final version of the manuscript. Figure 9. ATR-FTIR spectra of α-CaSO4·0.5H2O whiskers formed in the presence of 9.84 × 10−4 mol·L−1 MgCl2.
Notes
whiskers formed in the presence of 9.84 × 10−4 mol·L−1 MgCl2. The triple peaks located at 1145, 1117, and 1099 cm−1 were connected to the active v3 band of SO42−. The peak located at 983 cm−1 should be attributed to the distorted symmetric stretching of the v1 band of SO42−, showing the existence of the outer-sphere complex structure.34,35 The occurrence of the outer-sphere structure should be connected with the ligand interaction between Mg2+ and SO42− when Mg2+ was adsorbed on the side facets of α-CaSO4·0.5H2O whiskers. The formation
ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (nos. 51234003 and 51174125) and National Hi-Tech Research and Development Program of China (863 Program, 2012AA061602).
The authors declare no competing financial interest.
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dx.doi.org/10.1021/la502451f | Langmuir 2014, 30, 9804−9810
