Fundamental Insights into the Nucleation and Growth of Mg–Al

Communication pubs.acs.org/crystal

Fundamental Insights into the Nucleation and Growth of Mg−Al Layered Double Hydroxides Nanoparticles at Low Temperature Anup P. Tathod and Oz M. Gazit* Faculty of Chemical Engineering, Technion, Israel Institute of Technology, Haifa, Israel S Supporting Information *

ABSTRACT: Nucleation and growth of the Mg−Al layered double hydroxide nanoparticles are controlled by varying the nucleation temperature in conjunction with a fast addition of metal precursor solution. Nanoparticles of the size between 20 and 200 nm are obtained, while avoiding the use of organic structure directing agents and forgoing aging process. Nanoparticles self-assembly in solution is discussed and correlated to the agglomeration process.

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agreement with agglomeration theory, we show that following the aging process sample with smaller LDH NPs stack together to form larger agglomerates. Samples are prepared by the coprecipitation method with a Mg/Al ratio of 4:1. Compositional analysis by ICP-AES is in agreement with theoretical calculations; see Table S1 in Supporting Information. During synthesis the metal precursor is either added by a fast syringe injection (∼3 s), by dropwise addition, or by a combination of the two methods; see Supporting Information for a complete synthesis procedure. As can be seen in Figure 1 and Figure S1 the size of the NPs, determined using dynamic light scattering (DLS), before and after the aging step, is strongly affected by the method of addition. The size of the NPs decreases with the increase in the fraction of metal precursor solution added by the syringe method (indicated in percent as subscript in the sample name). The largest NPs of 200 nm are obtained for sample S00, in which the entire metal precursor solution (100 mL) is added dropwise (∼1 mL/min). The smallest NPs size of 55 nm is measured for sample S100, in which the entire 100 mL solution is injected quickly by syringe. The NPs size decreases exponentially with the increase in the fraction of metal precursor solution added by syringe and follows S00> S25 > S50 > S75 > S100. Both the native metal precursor solution and the base solution containing the intercalating ions showed a zero response in DLS. This trend in NPs size can be explained in terms of classical nucleation and growth theory. When the metal precursor solution is added dropwise, as in the conventional method, the first drop forms a relatively small number of LDH nuclei (low

ayered double hydroxides (LDH), commonly known as hydrotalcites, are widely used as catalysts in various basecatalyzed reactions,1−5 as anion exchangers, as catalyst supports, as adsorbents, and as fillers for polymeric materials.6−8 LDH materials have a brucite-like layered structure in which some of the divalent metal ions are isomorphically substituted by trivalent metal ions. This substitution forms positively charged LDH layers compensated by intercalating anions. LDH versatile composition and the ability to synthetically control its basicity and crystallinity are partly what make them attractive as catalytic materials. Similar to many catalytic materials, reduction to the nanoscale dimensions of LDH nanoparticles (NPs) can affect their catalytic performance, by creating an increased number of low-coordination sites such as corners and edges.9−11 Hence, there is great interest in obtaining LDH NPs in a controlled and reproducible manner.10,12−20 The most common method for the synthesis of LDH crystallites is by coprecipitation at room temperature, a process governed by a nucleation and growth mechanism.21,22 In this method the metal precursor solution is added dropwise to a base solution containing the intercalating ion (e.g., nitrate, carbonate). This initiates a simultaneous nucleation and growth process dictated by synthesis parameters such as metal concentration, pH, rate of addition, and mixing efficiency. In a subsequent step the LDH material is aged at a slightly elevated temperature (i.e., 60−150 °C) to enhance particle growth and increase the number of stacked layers.8,13,14,23 In this work we demonstrate a simple, efficient, and highly reproducible method for the synthesis of Mg−Al LDH NPs with tunable lateral dimension. We show that by controlling the synthesis temperature in conjunction with the method of precursor addition we can promote the nucleation and restrict growth. This enables us to make highly dispersed LDH NPs of 20−200 nm with a relatively narrow size distribution. In © XXXX American Chemical Society

Received: August 28, 2016 Revised: October 13, 2016 Published: October 27, 2016 A

DOI: 10.1021/acs.cgd.6b01272 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 1. LDH NPs size, determined using DLS, as a function of the fraction of metal precursor solution added using syringe; samples are indicated as Sx, where x = the percent of metal precursor solution added using a syringe. Particle size error is calculated to be within ±1%.

Figure 2. Agglomerate size, determined using DLS, of LDH NPs after aging as a function of the initial LDH NPs size; inset is a TEM image of sample ag-S100. Particle size error is calculated to be within ±1%.

supersaturation). The metal precursor in the second drop can either contribute to the growth of the existing nuclei, form new nuclei, or both. This leads to the formation of larger particles with a wider particle size distribution (PD), as seen in sample S00. In contrast, in sample S100 when the entire metal precursor solution is added at once the nucleation process, being kinetically favored, forms a large number of nucleation sites (high supersaturation), and growth is suppressed by the lack of free metal precursor. In this case smaller particles with narrow PD are formed. While nucleation rate increases exponentially with supersaturation, forming smaller nuclei, growth of the crystallites is mostly a linear function.24 Therefore, the shape of the curve obtained in Figure 1 is determined by the number of initially formed nuclei or more practically by how much of the metal precursor solution is added with a syringe. Results are consistent for samples prepared using a 10-fold more concentrated metal precursor solution as in sample S25 °C as opposed to sample S100; see Figure S2 in Supporting Information. The effect of aging, at 60 °C for 18 h, on the particle size is studied as a function of the initial LDH NPs size. During the aging process crystal growth, agglomeration or stacking of the LDH sheets can all occur simultaneously. During the aging step, crystal growth is known to occur by the Ostwald ripening mechanism, in which smaller particles dissolve back to solution and contribute to the growth of larger particles.23 The effect of the aging process on the NPs size is studied by comparing DLS and TEM data. TEM analysis of sample ag-S100, which is sample S100 after aging, seen in the inset of Figure 2, shows that most of the NPs are in the range of 60−70 nm, which is slightly greater than the 55 nm found for sample S100 before aging. This correlates well with known reports in literature, which show that if the aging is performed at a higher temperature and for a longer time the obtained LDH NPs are larger in lateral dimension and have more layers.13,23,25 Under the conditions here the enhanced stacking in the aged samples is evident from the XRD analysis, while as shown above growth in the lateral dimensions is minor. This is attributed to the relatively low aging temperature and short time used here. In contrast to the TEM results, which depict the average size of the primary NPs, the particle size measured using DLS for aged sample ag-S100 is ∼500 nm, which is significantly larger than the size measured by TEM. This indicates that the larger particle size observed by

DLS is due to NPs agglomeration during the aging treatment. The results for DLS analysis of aged samples are summarized in Figure 2. It can clearly be seen that the LDH NPs of smaller initial size form larger agglomerates after aging. Plotting the size of the NPs after aging as a function of initial particle size, we find the trend to be ag-S00< ag-S25< ag-S50 < ag-S75 < ag-S100. This can be explained by the higher surface energy of the smaller NPs as compared to the larger NPs, which is consistent with the formation of more low coordination surface sites in the smaller LDH NPs. An important characteristic of LDH materials is the quality of their crystal structure. Characteristic peaks for LDH (JCPDS file no. 01-89-0461) are clearly visible in all samples, indicating the formation of a high quality crystalline structures.26 Most importantly, within the range of the XRD sensitivity all the materials do not show any indication of metal hydroxides species. All the raw XRD spectra for samples before and after aging are provided in Figure S3 in Supporting Information. Careful examination of the XRD patterns shows that the intensities of the (003) and (006) peaks are 1.5 ± 0.1 fold higher in samples S100 and S50 with respect to the same peaks in sample S00, while all other peaks are the same. These two peaks correspond to the ‘c’ direction of the LDH unit cell (i.e., perpendicular to LDH layers) and is associated with the stacking of the LDH layers.21 Calculating the average crystallite size using the Scherrer equation for the (003) and (006) peaks we find that samples S00, S50, and S100 are 9.9, 13.1, and 13.9 nm thick, respectively. Similarly, the average crystallite size in the stacking direction for aged samples ag-S00, ag-S50 and ag-S100 is found to be 11.5, 15.8, and 15.9 nm, respectively. The consistency in the trend obtained for the stack sizes before and after aging despite the reverse trend obtained in NPs size measured by DLS proves that the improved stacking is not an artifact of XRD sample preparation. A summary of all XRD based calculations can be found in Table S2 in SI. It is worth mentioning that in agreement with known literature the stacking in all samples is enhanced after aging regardless of the synthesis procedure. In all above-mentioned experiments, the nucleation temperature was kept constant at 25 ± 1 °C. With the aim of reducing particle size even below the 55 nm, we decided to explore the effect of nucleation temperature on particle size. Though much B

DOI: 10.1021/acs.cgd.6b01272 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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three times, and the results showed extremely good reproducibility with a standard deviation of ≤2 nm. The trend found in Figure 3 indicates that the smallest LDH NPs are obtained at the nucleation temperature ranging between 5 to −15 °C. Particle sizes obtained by DLS analysis are corroborated using TEM and cryo-TEM techniques; see Figure 4. We find

has been done exploring the conditions of LDH synthesis to our knowledge nucleation temperature has never been explored. LDH NPs were prepared at various nucleation temperatures ranging between 60 and −55 °C, keeping all other synthesis parameters constant. 30% ethanol in water (vol/vol) is used as a solvent for the synthesis at −7.5 and −15 °C, while 80% ethanol in water is used for the synthesis at −55 °C to avoid freezing of the reaction mixtures; see Supporting Information for more details on the synthesis procedure. DLS results, shown in Figure 3, indicate that the particle size of the

Figure 3. Effect of nucleation temperature on LDH particle size, determined using DLS. *30% or 80% Ethanol is used as a solvent to prevent the freezing of the reaction mixture at a particular temperature; particle size error is calculated to be within ±1%.

Figure 4. HR-TEM images of LDH samples, (A) sample S60 °C; (B) sample S25 °C; (C) sample S−55 °C; (D) cryo-TEM image of sample S5 °C; arrows indicate edge-to-edge arrangement of NPs.

LDH decreases significantly with decrease in the nucleation temperature. Our data shows that the size of the NPs, before aging, nucleated at 60 °C is ∼145 nm, which is over 2 fold larger and 2 fold more polydispersed than that of the NPs nucleated at 25 °C. From 25 °C to 10 °C a more gradual decrease in the NPs size from 58 to 41 nm is observed. Upon further reduction of nucleation temperature to 5, −7.5, and down to −15 °C the particle size is observed to gradually level out going from 35 to 33 and 30 nm, respectively. When LDH synthesis is carried out at −55 °C, a slight increase in the size of NPs to 37 nm is observed; Figure S4 in SI contains a summary of the particle size and polydispersity index. As mentioned above, different solvent systems were used at different temperature ranges. Therefore, the minor change observed in the particle size between 5 and −15 °C can be attributed to the combined effect of temperature and addition of ethanol. To assess the effect of ethanol addition on the particle size, experiments at various temperatures are repeated for all three solvent systems, and the results are shown in Figure 3. Average particle size by the DLS at 25 °C using water, 30% ethanol, and 80% ethanol as a solvent is found to be 58, 66, and 52 nm, respectively. At 5 °C NPs size made in water and 30% ethanol is measured to be 35 and 39 nm, respectively. It is clear from the above results that the effect of ethanol addition on the NPs size is only marginal, at the temperatures tested. Moreover, irrespective of the solvent system the results show a significant decrease in the NPs size as a function of the nucleation temperature. To validate our results LDH synthesis at nucleation temperatures 25, 5, and −15 °C was repeated

that the sizes obtained from TEM analysis are a close match to those obtained from the DLS. Using TEM we find that the average particle size for the sample synthesized at 60 °C is 90− 100 nm, for 25 °C it is 50−60 nm, and for 5 °C size it is 20−30 nm. Samples nucleated at −55 °C show an average particle size of 40−50 nm. Histograms representing the particle size distribution for the samples nucleated at various temperatures are given in Figure S5 in SI. These results reinforce the findings of the DLS analysis, showing that the samples nucleated between 5 and −15 °C have the smallest NPs size of 20−30 nm. Interestingly, close observation of the cryo-TEM image taken for the sample nucleated at 5 °C, see Figure 4D, suggests that the LDH NPs self-assemble in an edge-to-edge arrangement when in suspension. The validity of this observation is confirmed by examining several cryo-TEM images, see Figure S6 in SI. This qualitative observation can explain why during the aging step agglomeration is favored rather than stacking and is more prominent as the LDH NPs become smaller in the their lateral size. The effect of the nucleation temperature on morphological properties such as crystallite size and surface area is well documented.27 The synthetic approach described here appears to demonstrate similar mechanistic trends.27 According to classical nucleation theory, nucleation rate reaches its maximum at a particular temperature (Tmax) and decreases at the temperature above or below this point. At temperatures higher than Tmax the critical size of the nucleus and growth rate increased and nucleation is kinetically less favored, the C

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modified Mg-Al hydrotalcite: a solid base as catalyst useful in synthetic organic chemistry. Chem. Commun. 1998, 1033−1034. (2) Ebitani, K.; Motokura, K.; Mori, K.; Mizugaki, T.; Kaneda, K. Reconstructed Hydrotalcite as a Highly Active Heterogeneous Base Catalyst for Carbon-Carbon Bond Formations in the Presence of Water. J. Org. Chem. 2006, 71, 5440−5447. (3) Evans, D. G.; Duan, X. Preparation of layered double hydroxides and their applications as additives in polymers, as precursors to magnetic materials and in biology and medicine. Chem. Commun. 2006, 485−496. (4) Jinesh, C. M.; Antonyraj, C. A.; Kannan, S. Allylbenzene isomerisation over as-synthesized MgAl and NiAl containing LDHs: Basicity-activity relationships. Appl. Clay Sci. 2010, 48, 243−249. (5) Fraile, J. M.; Garcia, J. I.; Mayoral, J. A.; Figueras, F. Comparison of Several Heterogeneous Catalysts in the Epoxidation of alphaIsophorone with Hydroperoxides. Tetrahedron Lett. 1996, 37, 5995− 5996. (6) Itaya, K.; Chang, H. C.; Uchida, I. Anion-exchanged clay (hydrotalcite-like compounds) modified electrodes. Inorg. Chem. 1987, 26, 624−626. (7) Tathod, A.; Kane, T.; Sanil, E. S.; Dhepe, P. L. Solid base supported metal catalysts for the oxidation and hydrogenation of sugars. J. Mol. Catal. A: Chem. 2014, 388−389, 90−99. (8) Cavani, F.; Trifirò, F.; Vaccari, A. Hydrotalcite-type anionic clays: Preparation, properties and applications. Catal. Today 1991, 11, 173− 301. (9) Roeffaers, M. B. J.; Sels, B. F.; Uji-i, H.; De Schryver, F. C.; Jacobs, P. A.; De Vos, D. E.; Hofkens, J. Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting. Nature 2006, 439, 572−575. (10) Roelofs, J. C. A. A.; Lensveld, D. J.; van Dillen, A. J.; de Jong, K. P. On the Structure of Activated Hydrotalcites as Solid Base Catalysts for Liquid-Phase Aldol Condensation. J. Catal. 2001, 203, 184−191. (11) Abello, S.; Medina, F.; Tichit, D.; Perez-Ramirez, J.; Cesteros, Y.; Salagre, P.; Sueiras, J. E. Nanoplatelet-based reconstructed hydrotalcites: towards more efficient solid base catalysts in aldol condensations. Chem. Commun. 2005, 1453−1455. (12) Abello, S.; Mitchell, S.; Santiago, M.; Stoica, G.; Perez-Ramirez, J. Perturbing the properties of layered double hydroxides by continuous coprecipitation with short residence time. J. Mater. Chem. 2010, 20, 5878−5887. (13) Oh, J. M.; Hwang, S.-H.; Choy, J.-H. The effect of synthetic conditions on tailoring the size of hydrotalcite particles. Solid State Ionics 2002, 151, 285−291. (14) Wang, Q.; Tay, H. H.; Guo, Z.; Chen, L.; Liu, Y.; Chang, J.; Zhong, Z.; Luo, J.; Borgna, A. Morphology and composition controllable synthesis of Mg−Al−CO3 hydrotalcites by tuning the synthesis pH and the CO2 capture capacity. Appl. Clay Sci. 2012, 55, 18−26. (15) Li, L.; Shi, J. In situ assembly of layered double hydroxide nanocrystallites within silica mesopores and its high solid base catalytic activity. Chem. Commun. 2008, 996−998. (16) Winter, F.; van Dillen, A. J.; de Jong, K. P. Supported hydrotalcites as highly active solid base catalysts. Chem. Commun. 2005, 3977−3979. (17) Wang, C. J.; O’Hare, D. Topotactic synthesis of layered double hydroxide nanorods. J. Mater. Chem. 2012, 22, 23064−23070. (18) Wang, Q.; O’Hare, D. Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124−4155. (19) Lee, W.; Kim, E.; Choi, J.; Lee, K. B. Kinetic Analysis of Secondary Crystal Growth for Hydrotalcite Film Formation. Cryst. Growth Des. 2015, 15, 884−890. (20) Lü, Z.; Zhang, F.; Lei, X.; Yang, L.; Evans, D. G.; Duan, X. Microstructure-controlled synthesis of oriented layered double hydroxide thin films: Effect of varying the preparation conditions and a kinetic and mechanistic study of film formation. Chem. Eng. Sci. 2007, 62, 6069−6075.

combination of which will cause the formation of larger particles. At temperatures lower than Tmax both nucleation and growth are slower and fewer nucleation sites are formed, which results in the formation of larger particles.28,29 Generally higher nucleation rates form larger number of nuclei and a smaller crystallite size.30 To conclude, this work demonstrates a fast route for the synthesis of LDH NPs with a controlled size, using low solution volumes while avoiding the use of organic structure directing agents and a lengthy aging process. Herein, we show for the first time, the effect of nucleation temperature and method of addition on the size of LDH NPs. The quick addition of the metal precursor solution to the base solution provides a means to make smaller nanoparticles with lower polydispersity. We show here that the effect of aging at 60 °C mainly causes agglomeration and the effect on lateral growth is limited. The trend observed for the NPs size with varying nucleation temperature is shown to be in line with theoretical expectations. The smallest particle size, i.e., highest nucleation rate, resides at a temperature range between 5 to −15 °C. Finally the method described here in detail for Mg−Al LDH can successfully be implemented for the synthesis of other types of LDH with different metal combinations. To demonstrate this we synthesized Ni−Al LDH (Ni/Al = 4) by following the quick addition method. Ni−Al LDH shows good crystallinity and purity of the LDH phase similar to what is described above for the Mg−Al system. Synthesis procedures and XRD data for the Ni−Al LDH samples synthesized by conventional dropwise addition and quick addition are provided in Figure S7 in SI.

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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/acs.cgd.6b01272. Experimental section, characterization techniques and methods, DLS analysis to study the effect of addition method, concentration effect and effect of nucleation temperature, XRD analysis and cryo-TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Ms. Yehudit Schmidt for assistance with the cryo-TEM measurements and Dr. Yaron Kaufmann for the HR-TEM images. GTEP and ISF-ICORE are acknowledged for their financial support under Grant No. 152/11.

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ABBREVIATIONS LDH: layered double hydroxide; ICP-AES: inductively coupled plasma-atomic emission spectroscopy; DLS: dynamic light scattering; HR-TEM: high resolution transmission electron microscopy; cryo-TEM: cryogenic transmission electron microscopy; XRD: X-ray diffraction

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(21) Zhao, Y.; Li, F.; Zhang, R.; Evans, D. G.; Duan, X. Preparation of Layered Double-Hydroxide Nanomaterials with a Uniform Crystallite Size Using a New Method Involving Separate Nucleation and Aging Steps. Chem. Mater. 2002, 14, 4286−4291. (22) Thanh, N. T. K.; Maclean, N.; Mahiddine, S. Mechanisms of Nucleation and Growth of Nanoparticles in Solution. Chem. Rev. 2014, 114, 7610−7630. (23) Hickey, L.; Kloprogge, J. T.; Frost, R. L. The effects of various hydrothermal treatments on magnesium-aluminium hydrotalcites. J. Mater. Sci. 2000, 35, 4347−4355. (24) de Jong, K. P. Synthesis of Solid Catalysts; WILEY-VCH Verlag GmbH & Co: Weinheim, 2009. (25) Zhao, R.; Yin, C.; Zhao, H.; Liu, C. Synthesis, characterization, and application of hydotalcites in hydrodesulfurization of FCC gasoline. Fuel Process. Technol. 2003, 81, 201−209. (26) Bellotto, M.; Rebours, B.; Clause, O.; Lynch, J.; Bazin, D.; Elkaïm, E. A Reexamination of Hydrotalcite Crystal Chemistry. J. Phys. Chem. 1996, 100, 8527−8534. (27) Schüth, F.; Hesse, M.; Unger, K. K. Precipitation and Coprecipitation. In Handbook of Heterogeneous Catalysis; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2008. (28) Porter, D. A.; Easterling, K. E., Sherif, M. Y. Phase Transformations in Metals and Alloys; CRC Press: Boca Raton, FL, 2009. (29) Markov, I. V. Nucleation. In Crystal Growth for Beginners, 2nd ed.; World Scientific: Singapore, 2011; pp 77−179. (30) Söhnel, O.; Nývlt, J.; Broul, M.; Matuchová, M. The Kinetics of lndustrial Crystallization; Elsevier: Amsterdam, 1985; Vol. 19.

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DOI: 10.1021/acs.cgd.6b01272 Cryst. Growth Des. XXXX, XXX, XXX−XXX