Control of Surface Defects and Agglomeration Mechanism of Layered

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Control of Surface Defects and Agglomeration Mechanism of Layered Double Hydroxide Nanoparticles Yongshan Zhou, Xiaoming Sun, Kai Zhong, David G. Evans, Yanjun Lin,* and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Agglomeration is a common problem in the production and application of nanoparticles. The degree of agglomeration of layered double hydroxide (LDH) nanoparticles is difficult to control in its industrial production. The properties of industrial scale MgAl−CO3−LDH products obtained using an aging reactor composed of a 1 m3 kettle with a water cooling jacket have been compared with MgAl−CO3−LDHs prepared using different aging times in a model laboratory scale reactor [a 500 mL flask]. The effect of varying aging times on the agglomeration of the LDH nanoparticles has been studied experimentally. The crystallinity, surface defects, and surface zeta potential of the LDHs have been studied in an effort to understand the mechanism of agglomeration of the nanoparticles. The results show that in poorly crystalline LDHs, accumulation of Al3+ cations at different points in the layers results in an increase in local charge density. Consequently, the zeta potential and the electrostatic repulsion between particles decrease, resulting in serious agglomeration of LDH nanoparticles. In contrast, for LDHs with higher crystallinity produced with extended aging times, the layer cations become uniformly distributed resulting in an increase in zeta potential and increased electrostatic repulsion between the particles. As a result, the degree of agglomeration is reduced. galleries.9,10 The chemical composition and charge density of the host layers, as well as the type, amount, and arrangement of guest interlayer anions can be readily tailored and controlled, and new materials with enhanced properties can be obtained. LDHs have become a large class of functional materials with many actual and potential applications, including use as catalysts, biomaterials, electronic materials, environmental materials, optical materials, and polymer additives.11 LDHs are traditionally synthesized by coprecipitation reactions from an aqueous solution either at variable pH at high supersaturation, or at constant pH at low supersaturation. It is difficult to control the particle size and distribution of LDHs using either of these methods since the primary particle formation processesnucleation and agingtake place simultaneously during the prolonged addition process.12 We have recently reported a method for the preparation of LDHs which involves a very rapid mixing and nucleation process in a modified colloid mill followed by a separate aging process.13 This results in very small crystal nuclei which have an equal length of time to grow prior to aging, affording products with a narrow range of primary particle diameter. We describe this process as the separate nucleation and aging steps (SNAS) method. We have employed the SNAS method in the industrial production of LDHs for several years using a 1 m3 reactor in a batch process. However, we have found that the degree of agglomeration of the resulting LDH nanoparticles varies rapidly with synthesis conditionssuch as aging time and rate of cooling of the reactor from the aging temperature to room temperature after aging is complete. The resulting varying

1. INTRODUCTION Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. The properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. The resulting nanoparticles have potential uses in many fields including electronics, magnetics and optoelectronics, biomedicine, pharmaceuticals, cosmetics, energy, and catalytic and environmental detection and monitoring.1 However, nanoparticles tend to spontaneously agglomerate, which is a very important problem that cannot be ignored in their production, storage, and application. Agglomeration of nanoparticles is wellknown as the result of their large surface area and surface energy which make them thermodynamically unstable with respect to agglomerated species.2 Elimination or reduction of agglomeration of nanoparticles has therefore become the primary problem in their production and application. Many methods have been employed to reduce nanoparticle surface energy in an attempt to prevent agglomeration, such as adding dispersants,3,4 using stabilizing agents,5,6 or surface modification methods,7,8 etc. Because of the large percent of surface atoms, the surface atom arrangement and crystal defect must have a marked effect on the properties of nanoparticles, including agglomeration. However, the effect of the surface defect of nanoparticles on the agglomeration has never been reported, and much less effort has been focused on the agglomeration reduction by controlling surface properties of the nanoparticles. Layered double hydroxides (LDHs), also known as hydrotalcite-like materials, are a family of two-dimensional anionic clays with the general chemical composition [M2+1‑xM3+x(OH)2]x+(An−x/n)·mH2O. M2+ and M3+ are respectively divalent and trivalent cations; x is the molar ratio M2+/ (M2++ M3+); An− represents the interlayer anion; and m represents the amount of water located in the interlayer © 2012 American Chemical Society

Received: Revised: Accepted: Published: 4215

October 7, 2011 February 27, 2012 February 28, 2012 February 28, 2012 dx.doi.org/10.1021/ie202302n | Ind. Eng. Chem. Res. 2012, 51, 4215−4221

Industrial & Engineering Chemistry Research

Article

Figure 1. SEM images of industrial samples of MgAl−CO3−LDH obtained using (a) natural cooling and (b) rapid cooling.

5°/min in the 2θ range from 3 to 70° using a count time of 4 s per step. High-resolution transmission electron microscopy (HRTEM) images were recorded with a JEOL JEM-2100 transmission electron microscope using an accelerating voltage of 200 kV. The specimens were prepared as follows: MgAl− CO3−LDH was suspended in ethanol and ultrasonicated for 0.5 h. A drop of the resulting suspension was then deposited onto a thin Si3N4 film. Scanning electron microscope (SEM) images were recorded with a Zeiss SUPRA 55 microscope. The agglomerate particle size distributions were determined using a Malvern Mastersizer 2000 laser particle size analyzer. Samples were dispersed in deionized water under ultrasonication for 1 min prior to measurements. Zeta potentials were determined using a Malvern Zetasizer 3000HS nano-granularity analyzer at the original pH value of the suspended colloid of LDHs, which decreased slightly with increasing aging time from 9.79 to 9.23. Samples were prepared by thoroughly dispersing MgAl−CO3− LDH powders in deionized water under ultrasonication to prepare a suspended colloid of concentration 0.50 g/L.

degree of agglomeration of the primary particles affects the quality of the product markedly. There have been very few studies of the effect of secondary particle formation processes on aggregation and product quality.14 In this work, MgAl−CO3−LDHs with different aging times have been prepared in model laboratory scale reactor [a 500 mL flask], and the effect of varying the aging time on the agglomeration of the LDH nanoparticles has been studied experimentally. The results are compared with materials obtained using an aging reactor composed of a 1 m3 kettle with a water cooling jacket. The surface properties and the zeta potential of the LDH nanoparticles have also been characterized in an effort to understand the mechanism of agglomeration of the nanoparticles in a new way.

2. EXPERIMENTAL METHOD 2.1. Materials. MgSO4·7H2O, Al2(SO4)3·18H2O, NaOH, and Na2CO3 were all of A.R. grade and purchased from the Beijing Chemical Co. Ltd. 2.2. Preparation of MgAl−CO3−LDH. MgAl−CO3−LDH was synthesized by a method involving separate nucleation and aging steps developed in our laboratory. A mixed salt solution containing Mg2+ and Al3+ was prepared with a Mg2+/Al3+ molar ratio of 2.0 in which the concentration of Mg2+ was fixed at 0.8 M. A corresponding mixed base solution containing NaOH and Na2CO3 was prepared with n(CO32−)/n(Al3+) = 2 and n(NaOH)/[n(Mg2+) + n(Al3+)] = 1.6. These two solutions were simultaneously added to a colloid mill at the same rate by means of metering pumps. The resulting slurry was aged at reflux temperature for different times in a 500 mL roundbottomed glass flask. The resulting mixture was then rapidly cooled in a low-temperature bath at −30 °C which reduced the slurry temperature to 20 °C within 25 min. The resulting precipitate was then filtered, washed, and finally dried at 70 °C in a desiccator in order to obtain the MgAl−CO3−LDH powder. 2.3. Industrial Manufacture of MgAl−CO3−LDH. MgAl−CO3−LDH was prepared in an industrial plant using an aging reactor composed of a 1 m3 kettle with a water cooling jacket. The products were aged for 4 h at reflux temperature and then cooled to room temperature by either allowing heat to radiate naturally to the air, or in 0.5 h using cooling water in the cooling jacket. 2.4. Characterization. Powder XRD patterns of the samples were recorded using a Shimadzu XRD-6000 powder diffractometer under the following conditions: 40 kV, 30 mA, Cu Kα radiation. The samples were step-scanned in steps of

3. RESULTS AND DISCUSSION 3.1. Agglomeration of Nanoparticles in Industrial Samples. The reactor used in the industrial production of MgAl−CO3−LDH was cooled to room temperature using two different methods: (a) allowing heat to radiate naturally to the air over 12 h (natural cooling method); (b) in 0.5 h using cooling water in the cooling jacket (rapid cooling method). SEM micrographs of the resulting two products are shown in Figure 1. The primary particle sizes of the LDH platelets prepared using the two different cooling methods were similar, mostly being within the range 50−150 nm. However the secondary particle size distributions of the agglomerated products measured by dynamic light scattering with a laser particle size analyzerdiffered markedly as shown in Figure 2. These small primary particles have an essentially spherical shape, unlike larger LDH crystallites which adopt a hexagonal platelet structure. The agglomerate particle size distribution of LDHs prepared using the natural cooling method was mainly within the range 0.04−0.60 μm with 90% of the particle size distribution by volume being below 0.50 μm [d(0.9) = 0.50 μm], showing that little aggregation of the primary particles occurred when the reaction mixture was allowed to cool naturally. In contrast, significant aggregation was observed on rapid cooling of the mixture, with the agglomerate particle size distribution of the 4216

dx.doi.org/10.1021/ie202302n | Ind. Eng. Chem. Res. 2012, 51, 4215−4221


Article

Figure 2. Agglomerate particle size distribution of industrial samples of MgAl−CO3−LDH obtained using (a) natural cooling and (b) rapid cooling.

Figure 4. Agglomerate particle size distribution of laboratory samples of MgAl−CO3−LDH obtained using (a) natural cooling and (b) rapid cooling.

LDHs mainly within the range 1.5−15 μm, with d(0.9) = 21.7 μm. 3.2. Investigation of Agglomeration of LDHs in Laboratory Samples. MgAl−CO3−LDH was synthesized in the laboratory using the SNAS method and samples were aged at 100 °C for 0.5, 1.0, 1.5, or 2.0 h. The flask was then cooled naturally, or placed in a low temperature bath which reduced the slurry temperature to 20 °C within 25 min. 3.2.1. Agglomeration of LDHs Prepared in Laboratory with Different Cooling Speed. The resulted product slurry aged for 0.5 h was cooled by a natural cooling method (over 2 h) or a rapid cooling method (within 25 min). The SEM micrographs in Figure 3 show that the primary particle sizes of the two samples are similar, being within the range of 20−40 nm. The laser particle size distributions of the two samples are shown in Figure 4. The agglomerate particle size distribution of LDHs prepared using the natural cooling method was mainly within the range 0.04−0.60 μm [d(0.9) = 0.48 μm], showing little aggregation. In contrast, significant aggregation was observed when using the rapid cooling method, with the agglomerate particle size distribution of the LDHs mainly within the range 1−10 μm and 0.03−1 μm [d(0.9) = 7.54 μm]. 3.2.2. SEM Images of LDHs Prepared with Different Aging Times Followed by Rapid Cooling. The SEM images in Figure

5 show that the primary particle size increased slowly from about 30 to 80 nm as the aging time at 100 °C was increased from 0.5 to 2.0 h. 3.2.3. Agglomerate Particle Size Distribution of LDHs Prepared with Different Aging Times Followed by Rapid Cooling. The laser particle size distributions for the samples prepared with different aging times are shown in Figure 6. The particle sizes of LDH samples prepared with short crystallization times have a bimodal distribution over a wide range with peaks in the particle size distribution falling in the approximate ranges 1−10 μm and 0.03−1 μm. The relative proportion of the larger agglomerated particles decreased gradually with increasing aging time. For a longer aging time of 2 h, the particle size distribution showed a single peak at 0.03− 0.5 μm. The results in Figure 6 clearly indicate that the agglomeration phenomenon becomes less serious with increasing aging time, and the agglomeration has been essentially fully eliminated after aging for 2 h. The particle size distribution of the product aged for 0.5 h after dispersion in deionized water by ultrasonication for 1 min (Figure 6a) was compared with those of samples after ultrasonication in deionized water for 5 min, and after ultrasonication in aqueous sodium hexametaphosphate as a dispersant for 1 min. The resulting agglomerate particle size distributions are shown in Figure 7.

Figure 3. SEM images of laboratory samples of MgAl−CO3−LDH obtained using (a) natural cooling and (b) rapid cooling. 4217



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Figure 5. SEM images of LDHs prepared by rapid cooling after aging at 100 °C for (a) 0.5, (b) 1.0, (c) 1.5, and (d) 2.0 h.

Figure 6. Agglomerate particle size distribution of LDHs prepared with aging times of (a) 0.5, (b) 1.0, (c) 1.5, and (d) 2.0 h, followed by rapid cooling.

Figure 7. Agglomerate particle size distribution of LDHs aged for 0.5 h followed by rapid cooling and ultrasonication for (a) 1 min in deionized water, (b) for 1 min in aqueous sodium metaphosphate dispersant, and (c) 5 min in deionized water.

A comparison of Figure 7 panels a and b shows that in the case of ultrasonication for 1 min, the proportion of the smaller aggregates with particle size distribution in the range 0.03−1 μm increased significantly after the addition of the dispersant. After being dispersed under ultrasonication in deionized water for longer times (Figure 7c), the particle size distribution become almost unimodal with most of the agglomerated particles having a size within the range 0.04−0.41 μm. Since addition of a dispersant and longer ultrasonication times both lead to a reduction in the degree of agglomeration, this suggests the secondary particles are weak agglomerates of primary particles.

3.2.4. XRD Patterns of LDHs Prepared with Different Aging Times Followed by Rapid Cooling. XRD patterns of LDHs prepared with different aging times are shown in Figure 8. With increasing aging time, the relative intensities of the characteristic diffraction peaks increased markedly while the peak widths at half height gradually decreased. These results are consistent with the increasing size of the primary LDH particles observed by SEM (Figure 5). 3.2.5. HRTEM of LDH Samples Prepared with Different Aging Times Followed by Rapid Cooling. HRTEM images of LDH samples prepared with different aging times are shown in 4218



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regular and the long-term order becomes enhanced with increasing aging time. This shows that the atoms in the crystal surface rearrange during the aging process and gradually approach a regular array. In Figure 9d, the atom array on the surface of the LDH aged for 2 h is clear and regular, as shown by the long-range order of the lattice fringes. 3.2.6. Zeta Potential of LDH Samples Prepared with Different Aging Times Followed by Rapid Cooling. The zeta potential (ξ), which is defined as the electric potential of a particle at the surface of shear (the boundary between the immobilized layer of fluid on the particle and the mobile fluid), is closely associated with the stability of colloid suspensions in aqueous systems.15,16 As the zeta potential approaches zero, particles tend to agglomerate.17 The effect of varying the pH, adding inorganic and organic salts, and varying the interlayer anion on the zeta potential of LDHs has been recently investigated,18,19 but the effect of particle size on the zeta potential has not been previously considered. MgAl−CO3− LDH powders prepared with different aging times followed by rapid cooling were thoroughly dispersed in deionized water under ultrasonication. The measured zeta potentials, shown in Figure 10, gradually increased from 14.6 to 21.5 mV as the aging time was extended from 0.5 to 2 h. This indicates that the positive charge of the LDH micelle surfaces increase with aging time, which enhances the electrostatic repulsion between particles and reduces the extent of agglomeration of the particles.

Figure 8. XRD patterns of LDHs prepared with aging times of (a) 0.5, (b) 1.0, (c) 1.5, and (d) 2.0 h, followed by rapid cooling.

Figure 9. For the LDH aged for 0.5 h (Figure 9a), the lattice fringes are very irregular, short, and oriented in various directions. This indicates that the LDH crystal aged for a short time is very imperfect and the array of atoms on the surface is seriously disordered. Therefore, there are many point defects on the crystal surface, especially at the borders of two lattice fringe areas in different crystal domains. As shown in the micrographs in Figure 9b−9d, the lattice fringes became more

Figure 9. HRTEM images of LDHs prepared with different aging times of (a) 0.5, (b) 1.0, (c) 1.5, and (d) 2.0 h, followed by rapid cooling. 4219



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which has close relationship with the distribution of Al3+, will decrease rapidly, and the electrostatic repulsion between particles decreases, and makes particle agglomeration more likely. A schematic illustration of the diffuse double layer in this situation is shown in Figure 11b. We can therefore conclude that extending the aging time results in the layered cations being more uniformly dispersed and closer to the ideal model, giving the LDH particles a larger zeta potential and lower agglomeration tendency. As mentioned above, the crystals grown in the 1 m3 reactor in the industrial process (Figure 2b) are uniform while those grown in the small flask in the laboratory experiment (Figure 4b) are much less uniform. After the aging process, some crystals aged in the small flask are almost perfect and give small agglomerate particle sizes, while others are imperfect and give larger agglomerate particle sizes. Therefore, the agglomerate particle size distribution of the laboratory samples in Figure 4 has a bimodal distribution.

Figure 10. Zeta potentials of LDH samples prepared with different aging times followed by rapid cooling.

4. CONCLUSIONS When LDH samples have good crystallinity, the layer cations Mg2+ and Al3+ are dispersed uniformly in the sheets making the charge density uniform over the whole layer. The counterions adsorbed around the LDH particles have uniform concentration at a given distance from the surface forming a uniform shear plane. However, when the crystals are imperfect, the cation array is disordered and Al3+ will accumulate especially at the borders of crystal domains, which leads to an increased charge density at these points and adsorption of more counterions in the multilayer. Although an earlier solid-state NMR study concluded that the layers in an LDH are constructed from Mg2AlOH and Mg3OH units with no MgAl2OH units,20 a more recent solid-state NMR study by Cadars et al. recently reported that there are also Al-rich MgAl2OH defects in the LDH structure.21 The accumulation of Al3+ and the formation of the surface defects described in our paper are consistent with the results of Cadars et al. Consequently, the zeta potential decreases rapidly and the electrostatic repulsion between particles is reduced, resulting in serious agglomeration of the original particles. Therefore, methods of ensuring the layer cations are uniformly dispersed,

MgAl−CO3−LDHs have a brucite-like (Mg(OH)2) layered structure. Some divalent Mg2+ cations are isomorphously replaced by trivalent cations of Al3+, resulting in positively charged host layers. Anions such as CO32− are intercalated in the interlayer galleries to balance the positively charged layers. In well-crystallized LDH materials, divalent and trivalent cations in the layers are separated from each other and uniformly dispersed on the sheets without the formation of “lakes” of like cations, which results in an even distribution of layer charge density. Therefore, the counterions (OH− and CO32−) adsorbed around the LDH particles have a uniform concentration at given distance from the surface, forming a uniform diffuse double layer and shear plane, as shown in Figure 11a. However, when crystal growth is imperfect, Mg2+ and Al3+ cations in the layers will deviate from their ideal positions resulting in many point defects in the crystals. At certain points on the surface, Al3+ will accumulate resulting in a change in the local Mg2+/Al3+ molar ratio and a high local charge density of the layers. Therefore, more counterions will be adsorbed near this high local charge density area forming a multilayer, which broadens the local diffuse double layer and shifts the shear plane outward. As a result, the zeta potential,

Figure 11. Sketch of the diffuse double layer when (a) Al3+ is uniformly dispersed within the layers in well-crystalline materials and (b) there are local high concentrations of Al3+ in some areas as a result of poor crystallinity. 4220



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modified Gouy−Chapman models in a 0.1 M NaCl-montmorillonite system. J. Colloid Interface Sci. 2009, 339, 533−541. (17) Vergouw, J. M.; Difeo, A.; Xu, Z.; Finch, J. A. An agglomeration study of sulphide minerals using zeta potential and settling rate. Part II: Sphalerite/pyrite and sphalerite/galena. Miner. Eng. 1998, 11, 605− 614. (18) Rojas, R.; Bruna, F.; dePauli, C. P.; Ulibarri, M. A.; Giacomelli, C. E. The effect of interlayer anion on the reactivity of Mg−Al layered double hydroxides: Improving and extending the customization of anionic clays. J. Colloid Interface Sci. 2011, 359, 136−141. (19) Xu, Z. P.; Jin, Y. G.; Liu, S. M.; Hao, Z. P.; Lu, G. Q. Surface charging of layered double hydroxides during dynamic interactions of anions at the interfaces. J. Colloid Interface Sci. 2008, 326, 522−529. (20) Sideris, P. J.; Nielsen, U. G.; Gan, Z. H.; Grey, C. P. Mg/Al ordering in layered double hydroxides revealed by multinuclear NMR spectroscopy. Science 2008, 321, 113−117. (21) Cadars, S.; Layrac, G.; Gerardin, C.; Deschamps, M.; Yates, J. R.; Tichit, D.; Massiot, D. Identification and quantification of defects in the cation ordering in Mg/Al layered double hydroxides. Chem. Mater. 2011, 23, 2821−2831.

such as extending the aging time, can increase the particle zeta potential and prevent unwanted agglomeration phenomena.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-64412125. Fax: +86-10-64425385. E-mail: linyj@ mail.buct.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Technologies R&D Program (Grant No. 2011BAE28B01) and the National Natural Science Foundation (Grant No. 21036001).

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REFERENCES

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Control of Surface Defects and Agglomeration Mechanism of Layered

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