Molecular-Cage Method: An Improvement of the Precipitation Method

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Molecular-cage method: an improvement of precipitation method in synthesizing nanoparticles Yu Liu, Weidong Chi, Donglin Zhao, Hui Liu, and Yu Deng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01714 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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Industrial & Engineering Chemistry Research

Molecular-cage method: an improvement of precipitation method in synthesizing nanoparticles Yu Liu*, Weidong Chi, Donglin Zhao, Hui Liu, Yu Deng Key Laboratory of Carbon Fiber and Functional Polymers, Beijing University of Chemical Technology, National Carbon Fiber Engineering Research Center， Beijing 100029, China E-mail: [email protected] Abstract: Molecular-cage method is a convenient and universal way of synthesizing nanoparticles in aqueous phase. In this paper, hydroxyapatite (HA) and Zirconium hydroxide nanoparticles are made by molecular-cage method with hydroxyl-rich solute. Structure, morphology and particle size of the product were analyzed by differential scanning calorimetric analysis (DSC) and scanning electron microscopy (SEM). The chemical structures of the particles are not affected by the molecular-cage effect. The sizes of nanoparticles and the concentration of solute have corresponding relationship in a limited range. The higher the concentration of solute, the smaller the nanoparticles. Besides, HA nanoparticles are also made by molecular-cage method in a water/ethanol mixture. Keywords: nanoparticles, molecular-cage, precipitation reaction 1. Introduction Nanoparticles have a lot of advantages than ordinary particles, such as, high surface to volume ratio [1], high reactivity, high surface energy and high absorbability. These advantages make them applicable in following areas: catalysts [2,3], drug delivery [4], wave absorbing [5], nano sensors [6,7], and so on. So far, various techniques, such as emulsion and microemulsion method [8], template method [9], mechanochemical method [10], polymer micelle method [11], sol-gel method [12] and microwave method [13] have been used for the synthesis of nanoparticles. But these methods, especially emulsion and microemulsion method, are too complex and costly. Additives like catalysts and surfactants have to be accurately used in the synthesis of nanoparticles by these methods. In another case, although chemical co-precipitation

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method is a traditional and widely used way of synthesizing particle materials, but the conditions like reaction temperature, solution concentration, stirring speed and pH value [14] must be accurately controlled for a narrow and uniform particle size. All in all, different reaction system and products need different control conditions, therefore it is not universal. In this paper, we find a universal way to synthesis nanoparticles by using molecular-cage effect.

The main advantage of this method is not to need to

design every reaction system. And the concentration of reactants of this method could be higher for the same-size particles than others. In former studies, synthetic templates have been used as molecular cages in the synthesizing of nanoparticles[15-18]. Cages formed by covalent organic frameworks or fiber networks can efficiently block the reactions in nanoscaled regions. These molecular cages have stable configurations. In this paper, we use saccharides as molecular cages. Cages formed by saccharides are dynamic in the reaction system. The reactants and products are blocked by the interaction forces of saccharides.

2. Experimental 2.1. Materials Diammonium hydrogen phosphate ((NH4)2HPO4), calcium chloride (CaCl2), sodium hydroxide (NaOH), glucose (C6H12O6) and sucrose (C12H22O11) were purchased from Beijing Chemical Works. Zirconium oxychloride octahydrate (ZrOCl2·8H2O) was purchased from Aladdin Industrial Corporation. Deionized water was used in all experiments.


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2.2. Synthesis of nanoparticles In order to synthesis hydroxyapatite (HA) nanoparticles, a aqueous mixture of 0.1 mol L-1 CaCl2 and 0.06 mol L-1 (NH4)2HPO4 was added dropwise to excessive NaOH solution with no additives, 185 g L-1 sucrose and 50/75/100/150/150 g L-1 glucose under continuous stirring respectively. In another method, 0.1 mol L-1 CaCl2 and 0.06 mol L-1 (NH4)2HPO4 was dissolved in 68 mol% ethanol-water solution and added dropwise to excessive NaOH 68 mol% ethanol-water solution under continuous stirring. Zirconium hydroxide (Zr(OH)4) nanoparticles were synthesized by adding 0.1 mol L-1 ZrOCl2 solution dropwise to excessive NaOH solution with no additives, 185 g L-1 sucrose and 200 g L-1 glucose under continuous stirring. 2.3. Characterization Powder X-ray diffraction (XRD, Bruker D8 Advance) were used to identify the crystal structure of the HA nanoparticles. The scans were taken within the 2Θ between 5° and 90°, and operated at an accelerating voltage of 40 kV and an emission current of 40 mA. Morphological characteristic of the nanoparticles were analyzed using scanning electron microscopy (SEM, HITACHI S-4700). The resistance of 18 g L-1 NaOH solution with 0/50/75/100/150/200 g L-1 glucose was measured in a one-meter-long container.

3. Results and discussion 3.1. Mechanism and effect of molecular-cage



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Molecular-cage method is a variant of traditional template method in which template is always solid or micelle. The structures of traditional template are stable and tangible micro-region and that limits the space of reaction. On the other, in molecular-cage method, micro-region is structured by the solute in a homogeneous system. The nanoscale cages are made by the solute with hydroxyl. The state of the solute and solvent is dynamic; therefore, the molecular-cages are intangible. Fig. 1(a) shows the formation and growth of the products in molecular-cage system. The yellow molecules are solutes that contain hydroxyl (saccharide). The motions of water and reactant molecules are blocked by Van der Wals forces and hydrogen bonds; therefore, dynamic and homogeneous micro-regions are structured in the molecular-cage method. The reactions of synthesizing particles can be blocked in nanoscale spaces so that nanoparticles can be made. Fig. 1(b) is the schematic diagram of molecular-cage method and ordinary method. Blocked by the cages, the particles synthesized by molecular-cage method are small and uniform. The growth of the particles are limited in the molecular-cage system. Without the limit, the products made by ordinary method are less likely to have nanoscale and uniform shapes. In order to investigate the effect of molecular-cage, HA and Zr(OH)4 nanoparticles are synthesized by precipitation reaction. Fig. 2 shows the XRD patterns of HA particles. It confirms that final product of the precipitation reaction is HA and its chemical structure was not affected by the presence of additives of sucrose or glucose. Saccharides are added in the excessive NaOH solutions. Saccharides and the products are stable and will not react in alkaline environments; therefore, the


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excessive NaOH can insure the stability of the product system. Fig. 3 shows the morphology and particle sizes of HA particles. The size of HA particles synthesized by ordinary precipitation method is larger than 50 nm, and it reduced to 20-30 nm and 5-15 nm while HA particles were synthesized by adding sucrose and glucose, respectively. In ordinary precipitation reaction, without the controlling of pH, reactant concentration, adding rate and stirring speed, the size of HA particles are uneven, due to the changing of pH and reactant concentration in the process of the reaction. These changes directly affected the morphology of the HA particles. In the method of adding sucrose or glucose, the effect of molecular-cage limits the precipitation reaction in nanoscale spaces. The effect of molecular-cage becomes the main influencing factor of the morphology and size of synthesized nanoparticles, that is why the HA particles are nano-sized and uniform. The particle size distributions can be seen in Fig. 4. The above phenomenon happened when Zr(OH)4 particles were synthesized by precipitation reactions. Fig. 5 shows the morphology and particle size of Zr(OH)4 particles. The size of Zr(OH)4 particles made by ordinary precipitation reaction is larger than 50 nm, some of them is larger than 100 nm. Also the morphology and particle size made by ordinary method with no additives is uneven. On the contrary, the particles are much smaller and uniform when they are synthesized by molecular-cage method. Comparison of the size of the particles made by ordinary precipitation method and molecular-cage method are shown in Fig. 6. The results show that the molecular cages can significantly make the particle size smaller. And the method of adding glucose can



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form smaller nanoparticle than adding sucrose when the two systems have equal molar ratio of hydroxyl. The smaller the hydroxyl-rich solute the smaller the micro-region. Glucose has smaller steric hindrance than sucrose; therefore, the glucose can more efficiently limit the movement of reactants. 3.2. Effect of solute concentration Influence of solute concentration was investigated by adding different quantity of glucose in the precipitation reaction of synthesizing HA particles. As is shown in Fig. 7, the average size of HA particles is 21 nm, 18 nm, 15 nm, 12nm and 10nm when the concentration of glucose is 50 g L-1, 75 g L-1, 100 g L-1 150 g L-1 and 200 g L-1, respectively. Fig. 8(a) shows the variation of NaOH solution’s resistance, in a one-meter-long container, with the concentration of glucose above. The higher the glucose concentration, the larger the resistance of the solution. This phenomenon indicates that the ions’ motion ability and range are limited in the reaction of synthesizing HA particles with the increase of glucose concentration. In this molecular-cage model, the micro-regions can be approximately considered as a sphere. The concentration of glucose is inversely proportional to the surface area of the sphere within limits. The relation of radius of particles (r) and concentration of solute (c) is indicated by r2=k/c

(1)

where k is a constant. This relation is also verified by the HA particles synthesized by adding different quantity of glucose. Fig. 8(b) shows the relation of r2 to 1/c as a linear relation. This linear relation can only be verified in a limited range. When the


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concentration of saccharide is too low, the sphere-shaped micro-region cannot be structured. And when the concentration of saccharide is too high, the micro-region cannot lock the reactants and products in it, so that the effect of molecular-cage cannot increase with the concentration increase. 3.3. Special molecular cage Apart from the situations above, there is another kind of molecular cage. In the water/ethanol mixture, the water molecules are not independent or continuous, but micro-cluster distributed [19]. Fig. 9 shows the distribution in water/ethanol mixture. The reaction of synthesizing HA particles is an ionic reaction. It cannot happen in the ethanol; therefore, the ethanol molecules can also generate molecular-cage effect. Fig. 10 shows the particle size of HA synthesized in water/ethanol mixture. The particle size is around 15-20 nm.

4. Conclusions Nanoparticles can be made by molecular-cage method. Reactions happen in homogeneous systems. The particle size can be as small as 5-15 nm. Within limits, the smaller the saccharides and the higher the concentration the saccharides, the smaller the nanoparticles. The water/ethanol mixture can also form molecular-cage system to synthesize nanoparticles.



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Fig. 1( (a). a) Simulation diagram of molecular-cage.

Fig. 1(b). Schematic diagram of molecular-cage method and ordinary method.



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Fig. 2. XRD patterns of HA particles synthesized by precipitation method with (a) no additives; (b) additives of sucrose; (c) additives of glucose.

Fig. 3(a). SEM images of HA particles synthesized by precipitation method with no additives.


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Fig. 3(b). SEM images of HA particles synthesized by precipitation method with additives of sucrose

Fig. 3(c). SEM images of HA particles synthesized by precipitation method with additives of glucose.



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Fig. 4(a). Particle size distribution of HA particles synthesized by precipitation method with additives of sucrose.

Fig. 4(b). Particle size distribution of HA particles synthesized by precipitation method with additives of glucose.


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Fig. 5(a). SEM images of Zr(OH)4 particles synthesized by precipitation method with no addditives.

Fig. 5(b). SEM images of Zr(OH)4 particles synthesized by precipitation method with additives of sucrose.



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Fig. 5(c). SEM images of Zr(OH)4 particles synthesized by precipitation method with additives of glucose.

Fig. 6. Size of the particles synthesized by (a) ordinary precipitation method; (b) molecular-cage method with additives of sucrose; (c) molecular-cage method with additives of glucose.


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Fig. 7(a). SEM images of HA particles synthesized by precipitation method with glucose of 50 g L-1

Fig. 7(b). SEM images of HA particles synthesized by precipitation method with glucose of 75 g L-1



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Fig. 7(c). SEM images of HA particles synthesized by precipitation method with glucose of 100 g L-1

Fig. 7(d). SEM images of HA particles synthesized by precipitation method with glucose of 150 g L-1


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Fig. 7(e). SEM images of HA particles synthesized by precipitation method with glucose of 200 g L-1

Fig. 8(a). Variation of NaOH solution’s resistance with different concentration of glucose.



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Fig. 8(b). Particles radius vs. solute concentration plot of reactions of synthesizing HA particles.

Fig. 9. Simulation diagram of water/ethanol mixture.


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Fig. 10. SEM images of HA particles synthesized in water/ethanol mixture.



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Figure captions Fig. 1. (a) Simulation diagram of molecular-cage; (b) Schematic diagram of molecular-cage method and ordinary method. Fig. 2. XRD patterns of HA particles synthesized by precipitation method with (a) no additives; (b) additives of sucrose; (c) additives of glucose. Fig. 3. SEM images of HA particles synthesized by precipitation method with (a) no additives; (b) additives of sucrose; (c) additives of glucose. Fig. 4(a). Particle size distribution of HA particles synthesized by precipitation method with additives of (a) sucrose; (b) glucose. Fig. 5. SEM images of Zr(OH)4 particles synthesized by precipitation method with (a) no addditives; (b) additives of sucrose ; (c) additives of glucose. Fig. 6. Size of the particles synthesized by (a) ordinary precipitation method; (b) molecular-cage method with additives of sucrose; (c) molecular-cage method with additives of glucose. Fig. 7. SEM images of HA particles synthesized by precipitation method with glucose of (a)50 g L-1; (b) 75 g L-1; (c) 100 g L-1; (d) 150 g L-1; (e) 200 g L-1. Fig. 8. (a)Variation of NaOH solution’s resistance with different concentration of glucose; (b) Particles radius vs. solute concentration plot of reactions of synthesizing HA particles. Fig. 9. Simulation diagram of water/ethanol mixture. Fig. 10. SEM images of HA particles synthesized in water/ethanol mixture.


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Molecular-Cage Method: An Improvement of the Precipitation Method

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