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A new method to synthesize super-small nanoparticles in glucose aqueous solution
Materials Chemistry and Physics 205 (2018) 480e486
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A new method to synthesize super-small nanoparticles in glucose aqueous solution Nan Jing, An-Nan Zhou, Ying-Qiao Xiang, Qing-Hong Xu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box. 98, 15 Beisanhuan Donglu, Beijing 100029, PR China
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Glucose was introduced to realize green synthesis of nanomaterials. “Molecular cage” method was applied to synthesis on nanomaterials. Super-small hydroxyapatite nanoparticles with 7 nm average size were synthesized. Some other nanoparticles with super-small sizes were synthesized in this way.
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Article history: Received 14 June 2017 Received in revised form 20 September 2017 Accepted 27 November 2017 Available online 28 November 2017
The chemical precipitation reaction of Ca2þ and phosphate aqueous solution was used to prepare supersmall hydroxyapatite (HA) nanoparticles in the presence of glucose. A hypothetical “molecular cage” was built in glucose alkaline solution to limit the chemical reaction in a tiny space to control the size and morphology of HA nanoparticles. A possible reaction mechanism of HA nanoparticles confined space synthesis was proposed in this work. Compared with the previous research of the preparation of HA nanoparticles, our work in the first time successfully obtained the surper-small HA nanoparticles which had a 7 nm average size with homogeneous globular morphology. The mild and green synthetic method achieve the low-cost and controllable preparation of super-small nanoscale HA particles. In addition, Zn(OH)2, Ni(OH)2, Ag and SiO2 nanoparticles (<10 nm) were also successfully synthesized by this method, indicating “molecular cage” can be appropriate for various nanoparticle synthesis and could become a universally method for the synthesis of nanoparticles. © 2017 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatite Nanoparticles Glucose
1. Introduction With the establishment of colloidal chemistry in 1861, nanoparticles have received more and more attention. Due to small particle size and high surface area, nano materials can be used in many fields, such as catalytic [1e3], energy storage materials [4,5],
* Corresponding author. E-mail address: [email protected] (Q.-H. Xu). https://doi.org/10.1016/j.matchemphys.2017.11.059 0254-0584/© 2017 Elsevier B.V. All rights reserved.
luminescent material [6], antibacterial material [7,8], heavy metal adsorption [9,10], nanoparticles drug delivery system [11,12]. Although traditional synthesis methods of nanoparticles could effectively control the particle size and morphology, the environmental pollution problems and price cost, technical restrictions still limited the mass production of nano materials. Moreover, the synthesis of nanoparticles is still lack a universally method in the present day. Using one way to synthesize diverse nanoparticles is still difficult to achieve. Hydroxyapatite (HA, Ca10(PO4)6(OH)2), which consists mainly of
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calcium and phosphate at a molar ratio of 1.67, is an important inorganic component of bones and teeth of the vertebrates [13]. Because of excellent bioactivity, HA is the most useful material in the inorganic phosphate family. Due to the high osteoconduction and biocompatibility [14], nanoscale HA is one of the most promising bone tissue engineering materials. In addition, the high specific surface area and strong surface adsorption abilities also lead HA materials have been successful used in drug delivery [15e18], proteins adsorption [19e21], removal of heavy mental ions [22e25] etc. However, the bioactivity of HA is always limited by the size. Traditional HA preparation methods, such as microwave-assisted [26e29], hydrothermal reaction [30e32], emulsion method [33e37] and sol-gel synthesis [38e40], usually need high temperature, high pressure or rigorous pH, which is high cost and energy use, and hardly obtained the super-small (<10 nm) HA nanoparticles. In our work, a facile and green method has been successfully used to synthesize super-small HA nanoparticles. In the first time, the 7 nm HA nanoparticles with globular morphology have been prepared in room temperature and atmospheric pressure without catalyst and organic solvent. Glucose was used to build “molecular cage” in aqueous solution to limit the ionic reaction process in a tiny space in order to control the size of HA. In addition, the same method was also successfully used to synthesize Zn(OH)2, Ni(OH)2, Ag and SiO2 nanoparticles (<10 nm), indicating the “molecular cage” is a blanket method for different inorganic nanoparticles. 2. Experiment 2.1. Chemicals For synthesis HA nanoparticles, glucose, CaCl2, (NH4)2HPO4 and NaOH were purchased from Beijing Chemical Reagent Company (Beijing, PR China). For synthesis other nanoparticles, Zn(NO3)2, Ni(NO3)2, AgNO3, sodium citrate, ethanol and ammonium hydroxide were also purchased from Beijing Chemical Reagent Company (Beijing, PR China). Ethyl orthosilicate was purchased from aladdin Reagent Co. (Shanghai, China). All chemicals were analytical grade and used without further purification. Deionized water was used throughout the experiments. 2.2. Synthesis of HA nanoparticles A chemical precipitation method was used to prepare the HA nanoparticles. In a typical procedure, 100 mL of CaCl2 (0.01 mol/L) and 100 mL of (NH4)2HPO4 (0.006 mol/L) solutions were mixed together and adjusted to weak acidity with HCl solution (0.1 M). A certain amount of glucose (0 g, 5 g, 10 g, 20 g or 40 g) and 1.5 g of NaOH were dissolved with 100 mL of deionized water. Then the mixed solution of CaCl2 and (NH4)2HPO4 was added drop by drop into glucose alkaline solution and the HA nanoparticles were formed immediately. The products are noted as 0GHA, 5GHA, 10GHA, 20GHA and 40GHA, respectively.
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orthosilicate was used to synthesize nano SiO2 by following the steps: 20% glucose aqueous solution and ethanol were mixed together by volume ratio of 1:6, then a small amount of ammonium hydroxide was added as catalyst. Ethyl orthosilicate was added drop by drop into the above solution. The SiO2 nanoparticles were received after 4 h later. 2.4. Sample characterizations Nicolet 8700 Fourier transform infrared spectrometer (FTIR, Thermo Electron, USA) was used to characterize the functional groups of the samples. Crystallinities of the samples were investigated by X-ray diffraction (XRD, Bruker D8 Advance, Germany). The changes of ultraviolet absorption of HA samples were obtained by Shimadzu UV-3600 ultraviolet and visible spectrophotometer (UVevis, Shimadzu, Japan). S-4700 field emission scanning electron microscope (SEM, Hitachi, Japan) and J-3010 high resolution transmission electron microscopy (HRTEM, Hitachi J-3010, Japan) were used to observe the morphology and size of the synthesized nanoparticles. 3. Results and discussion 3.1. Synthesis and characterizations of HA nanoparticles In 2000, Nauta et al. [41] proved that several water molecules could be bonded by hydrogen bonds to form tiny clusters in liquid helium droplets (Fig. 1A). The extraneous water molecules could enter into the established clusters, which involved in numerous rearrangements of hydrogen bonds. Cheng et al. [42] pointed out that the distribution of ethanol and water molecules were not randomly in the ethanol/water system. Most water molecules existed in tiny aggregate form so that there was no isolated water molecule (Fig. 1B). The above thoughts offer a line of thinking to synthesize super-small nanoparticles. If the reaction to form the target compound can be finished in the limited molecular cage area that was formed by oxy-compounds, the super-small nanoparticles will be synthesized. In this paper, ethanol was used as limited domain reagents to synthesize super-small HA nanoparticles in the first. SEM image (shown in Fig. S1, in supporting information) indicates that most of the products are nanoparticles with diameter about 6e7 nm, however some nano rod products (length is about 100 nm) were also found when the ethanol (concentration is about 50%) was used as reagent to form the molecular cage. The reason on the formation of nano rod products possibly come from the lesser hydrogen bonds between water and ethanol for only one hydroxyl in each ethanol molecule, which leads some cages were not tight junction when the reaction ions diffused into the clusters formed between the water
2.3. Synthesis of Zn(OH)2, Ni(OH)2, Ag and SiO2 nanoparticles Similar to the above steps, 20 g of glucose and 1.5 g of NaOH were dissolved with 100 mL of deionized water. Then 100 mL of nitrate (Zn(NO3)2 or Ni(NO3)2, 0.01 mol/L) was added drop by drop into glucose alkaline solution to prepare Zn(OH)2 and Ni(OH)2 nanoparticles. In order to obtain Ag nanoparticles, 20 g of glucose was added into 100 mL of sodium citrate aqueous solution (0.01 mol/L). Then 100 mL of AgNO3 (0.01 mol/L) was added drop by drop into the above glucose sodium citrate solution under 94 C thermostatic waterbath to prepare Ag nanoparticles. Ethyl
Fig. 1. The structure of water molecules in liquid helium droplets [41] (A) and the model of ethanol/water system [42] (B).
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Fig. 2. Schematic structure of “molecular cage” built by water and glucose.
and ethanol molecules. Glucose, a kind of small multi-hydroxyl molecule, was used as limited domain reagent in the further research. In aqueous solution, hydrogen bonds were formed easily between glucose and water molecules due to the polyhydric structure of glucose, which is shown in Fig. 2. When concentration of the glucose reached a certain value, some water molecules would be limited into a small space possibly, and the water molecules were packed by glucose
and formed a “molecular cage”. In this work, glucose was used to build the “molecular cage” in aqueous solution to control the size of the synthesized HA nanoparticles, and the SEM images of the products are shown in Fig. 3. A possible reaction mechanism of HA confined space synthesis by glucose was shown in Fig. 4. With the mixed solution of PO3 4 and Ca2þ was added drop by drop into the glucose alkaline solution, PO3 and Ca2þ entered into the cage and HA was formed 4 immediately. Because of the limited space of the cage, the size of the synthesized HA could be limited into a narrow value and the maximum distribution size is approximately 7 nm when 20 g of glucose was used in the reaction. The BET analysis show the surface area of the synthesized 20GHA nanoparticles is about 113 m2/g while the commercial HA nano rod (length is about 50 nm) only is 44 m2/g. FTIR spectra (Fig. 5) show that all the synthesized HA nanoparticles have similar absorptions. The vibrations of -OHs in HA are found at 3569 and 630 cm1. The absorptions at 1093 and 1034 cm1 are the stretching vibrations of P¼O, 565 and 603 cm1 are the bending vibrations of P-O in PO3 4 . In addition, the weak bands at 1452 and 1418 cm1 are attributed to the CO2 3 , which came from the adsorbed CO2 during the preparation [43]. X-ray diffractions (Fig. 6) indicate that the diffractions of (002), (211), (202), (310), (222), (213) and (004) planes in all the synthesized HA are located at 26.05 , 31.95 , 34.01, 39.68 , 47.56 , 49.45
Fig. 3. SEM images and the corresponding statistical particle size results of the synthesized HA with various glucose concentrations during the reaction: 0GHA (A), 5GHA (B), 10GHA (C), 20GHA (D) and 40GHA (E).
Fig. 4. The formation process of super-small HA nanoparticles.
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Fig. 5. FTIR spectra of the HA nanoparticles with various glucose concentrations during the synthesis: 0GHA (a), 5GHA (b), 10GHA (c), 20GHA (d) and 40GHA (e).
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Fig. 7. UVevis spectra of the synthesized HA with various glucose mass added in the reaction.
Fig. 8. HRTEM and SAED results of 20GHA: HRTEM image of 20GHA (A); lattice fringes of 20GHA (B); SAED pattern result of 20GHA (C).
Fig. 6. XRD diffractions of the synthesized HA with various glucose concentrations during the reaction: 0GHA (a), 5GHA (b), 10GHA (c), 20GHA (d) and 40GHA (e).
and 53.18 , respectively, which coincided with the standard data of JCPDS Card No.09e0432 for the crystalline HA, confirming the formation of pure HA in both methods. In addition, the broad and weak diffractions indicate that the crystallinities of the samples were low, for the perfect crystal had not formed and slight distortions of the crystal lattice in progress of crystallization [14]. The UVevis spectrum (Fig. 7) indicates that the 0GHA has two absorptions at 272 nm and 228 nm. With the increase of glucose in reaction, the corresponding absorptions occurred blue shift. For example, they shifted to 256 nm and 226 nm in 20GHA and their intensities had an obvious enhancement, which suggests the size decrease of HA [44]. However, when the glucose mass increased to 40 g, there was no obvious change in positions and intensities of the two characteristic absorptions. SEM images (Fig. 3) show that the HA nanoparticles are dispersive globular particles. With increase of the glucose concentration in the reaction, the diameters of HA nanoparticles
gradually decreased and the morphology of these particles tended to be uniform. 200 nanoparticles of each sample were chosen randomly to obtain the relevant statistical results of the particle size. The sample of 0GHA shows an atypical morphology and the particle size could not be control (Fig. 3A). Fig. 3B shows inhomogeneous globular particles and the diameters are range from 6 nm to 30 nm with the concentrated distribution in 8 nm and 14 nm when 5 g glucose was added during the reaction process. When mass of the glucose was increased to 10 g (Fig. 3C), the max distributions of particle size were 7 nm and 11 nm, for the glucose concentration was still too low to limited the HA particles size in a narrow scope. When 20 g glucose were added in the reaction (Fig. 3D), the sample shows homogeneous globular particles and the size of these nanoparticles were further reduced. The statistical result shows the diameters of these HA nanoparticles are range from 4 nm to 13 nm with a concentrated distribution and the average diameter is approximately 7 nm. However, when the glucose mass more than 20 g used, there was no obvious size reduce of the HA. When mass of the glucose was low, only parts of the reactant ions were deemed to be limited into a small space
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Fig. 9. SEM images and the corresponding statistical particle size analysis, EDS results and FTIR spectrum of the synthesized Ni(OH)2 (A), SiO2 (B), Zn(OH)2 (C) and Ag (D).
because of the limited amount of the “molecular cage” and thus most of the synthesized HA particles have bigger diameter and their size could not be controlled very well. With the glucose addition, the number of the “molecular cage” was increased and more super-small HA particles appeared in narrow size distribution. When the glucose added was increased to 20 g, the number of “molecular cage” reached saturation and all the reactions between Ca2þ and PO3 4 were limited in the cages, so the synthesized HA shows super-small homogeneous globular particles with a concentrated distributed. However, the size of the synthesized HA could not be decreased when the glucose addition was further increased because the amounts of “molecular cages” were no longer increased. The HRTEM images indicate that the average diameter of HA nanoparticles is approximately 7 nm (Fig. 8A). The detailed structure of the synthesized HA was further observed by selected area electron diffraction (SAED). As shown in Fig. 8B, the lattice fringes of HA samples were clearly observed. The interplanar distance of 0.46 nm and 0.277 nm present the (110) and (112) planes of HA, respectively [42]. The SAED pattern result was shown in Fig. 8C. The clearly SAED pattern rings indicated the synthesized HA nanoparticles were crystalline. In addition, the distinct diffraction rings could be vested in (002) and (211) planes of HA [30]. 3.2. Synthesis and characterization of Zn(OH)2, Ni(OH)2, Ag and SiO2 nanoparticles In the further research, the same method was also successfully used to synthesize Ni(OH)2, SiO2, Zn(OH)2 and Ag nanoparticles
(<10 nm). According to the above results, 20% glucose aqueous solution was used as the reaction medium to prepare these nanoparticles. The results of Ni(OH)2 were shown in Fig. 9A. SEM picture reveal the Ni(OH)2 were homogeneous globular particles and the distribution size is approximately 5 nm according to the relevant statistical analysis. The element weight ratio and atomic ration of O/Ni from EDS test in accordance with Ni(OH)2. FTIR spectra show the typical adsorption bands of Ni(OH)2. The strong adsorption at 3639 cm1 and 524 cm1 belongs to the stretching vibrations and lattice vibration of -OH, respectively. The weak adsorption at 464 cm1 attribute to the lattice vibration of Ni-O. Fig. 9B indicates that the synthesized SiO2 nanoparticles show spherical-shape distribution and its average size was less than 10 nm. EDS result shows the weight ratio and atomic ratio of Si/O was 0.10 and 0.57, which corresponding to the structure characteristics of SiO2. FTIR spectra show a strong adsorption band at 1106 cm1, which attribute to the antisymmetric stretching vibrations of Si-O-Si. The adsorption at 956 cm1 belongs to the bending vibration of Si-OH. The weak bands at 799 cm1 and 473 cm1 were corresponding to the symmetrical stretching vibrations of Si-O. Zn(OH)2 nanoparticles were successfully prepared in this method and the results were shown in Fig. 9C. SEM picture and statistical analysis result indicate the Zn(OH)2 nanoparticles had globular morphology with average size of 8e9 nm. FTIR spectra show a typical adsorption at 487 cm1 and 417 cm1, which corresponding to the characteristic vibration of Zn-OH. EDS result shows the element ratio in accordance with Zn(OH)2. Moreover, Ag nanoparticles with an average size of 6 nm were
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also received (Fig. 9D). SEM picture suggest the synthesized Ag were homogeneous globular particles. EDS result indicates a typical characteristic of Ag elementary substance, the few element of O was from the oxidation during the EDS sample preparation process. Our work suggests the “molecular cage” method could be used to prepare various nanoparticles in aqueous solution. This method could become a universally method for the synthesis of nanoparticles, which was cheap and environmental friendly and may bring breakthrough in preparation of nanoparticles.
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4. Conclusions
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A universally method was used to prepare HA and other various nanoparticles. A “molecular cage” model was proposed to describe the confined space process during the reaction. In the first time, the 7 nm average size surper-small HA nanoparticles had successfully obtained. Moreover, Zn(OH)2, Ni(OH)2, SiO2 and Ag nanoparticles (<10 nm) were also prepared by this method, which indicate it could become a universally method for the synthesis of nanoparticles in aqueous solution. The low cost, easy and environmental friendly synthetic process lead this method could be used in mass production, and the method may bring breakthrough in preparation of nanoparticles.
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Notes
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The authors declare no competing financial interest. Acknowledgments
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We are grateful to the financial support from Project of National Natural Science Foundation of China (Project No.: U1362113). And we are also grateful to Prof. Pudun Zhang for the kindly advices.
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at https://doi.org/10.1016/j.matchemphys.2017.11.059.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A new method to synthesize super-small nanoparticles in glucose aqueous solution Nan Jing, An-Nan Zhou, Ying-Qiao Xiang, Qing-Hong Xu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box. 98, 15 Beisanhuan Donglu, Beijing 100029, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Glucose was introduced to realize green synthesis of nanomaterials. “Molecular cage” method was applied to synthesis on nanomaterials. Super-small hydroxyapatite nanoparticles with 7 nm average size were synthesized. Some other nanoparticles with super-small sizes were synthesized in this way.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2017 Received in revised form 20 September 2017 Accepted 27 November 2017 Available online 28 November 2017
The chemical precipitation reaction of Ca2þ and phosphate aqueous solution was used to prepare supersmall hydroxyapatite (HA) nanoparticles in the presence of glucose. A hypothetical “molecular cage” was built in glucose alkaline solution to limit the chemical reaction in a tiny space to control the size and morphology of HA nanoparticles. A possible reaction mechanism of HA nanoparticles confined space synthesis was proposed in this work. Compared with the previous research of the preparation of HA nanoparticles, our work in the first time successfully obtained the surper-small HA nanoparticles which had a 7 nm average size with homogeneous globular morphology. The mild and green synthetic method achieve the low-cost and controllable preparation of super-small nanoscale HA particles. In addition, Zn(OH)2, Ni(OH)2, Ag and SiO2 nanoparticles (<10 nm) were also successfully synthesized by this method, indicating “molecular cage” can be appropriate for various nanoparticle synthesis and could become a universally method for the synthesis of nanoparticles. © 2017 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatite Nanoparticles Glucose
1. Introduction With the establishment of colloidal chemistry in 1861, nanoparticles have received more and more attention. Due to small particle size and high surface area, nano materials can be used in many fields, such as catalytic [1e3], energy storage materials [4,5],
* Corresponding author. E-mail address: [email protected] (Q.-H. Xu). https://doi.org/10.1016/j.matchemphys.2017.11.059 0254-0584/© 2017 Elsevier B.V. All rights reserved.
luminescent material [6], antibacterial material [7,8], heavy metal adsorption [9,10], nanoparticles drug delivery system [11,12]. Although traditional synthesis methods of nanoparticles could effectively control the particle size and morphology, the environmental pollution problems and price cost, technical restrictions still limited the mass production of nano materials. Moreover, the synthesis of nanoparticles is still lack a universally method in the present day. Using one way to synthesize diverse nanoparticles is still difficult to achieve. Hydroxyapatite (HA, Ca10(PO4)6(OH)2), which consists mainly of
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calcium and phosphate at a molar ratio of 1.67, is an important inorganic component of bones and teeth of the vertebrates [13]. Because of excellent bioactivity, HA is the most useful material in the inorganic phosphate family. Due to the high osteoconduction and biocompatibility [14], nanoscale HA is one of the most promising bone tissue engineering materials. In addition, the high specific surface area and strong surface adsorption abilities also lead HA materials have been successful used in drug delivery [15e18], proteins adsorption [19e21], removal of heavy mental ions [22e25] etc. However, the bioactivity of HA is always limited by the size. Traditional HA preparation methods, such as microwave-assisted [26e29], hydrothermal reaction [30e32], emulsion method [33e37] and sol-gel synthesis [38e40], usually need high temperature, high pressure or rigorous pH, which is high cost and energy use, and hardly obtained the super-small (<10 nm) HA nanoparticles. In our work, a facile and green method has been successfully used to synthesize super-small HA nanoparticles. In the first time, the 7 nm HA nanoparticles with globular morphology have been prepared in room temperature and atmospheric pressure without catalyst and organic solvent. Glucose was used to build “molecular cage” in aqueous solution to limit the ionic reaction process in a tiny space in order to control the size of HA. In addition, the same method was also successfully used to synthesize Zn(OH)2, Ni(OH)2, Ag and SiO2 nanoparticles (<10 nm), indicating the “molecular cage” is a blanket method for different inorganic nanoparticles. 2. Experiment 2.1. Chemicals For synthesis HA nanoparticles, glucose, CaCl2, (NH4)2HPO4 and NaOH were purchased from Beijing Chemical Reagent Company (Beijing, PR China). For synthesis other nanoparticles, Zn(NO3)2, Ni(NO3)2, AgNO3, sodium citrate, ethanol and ammonium hydroxide were also purchased from Beijing Chemical Reagent Company (Beijing, PR China). Ethyl orthosilicate was purchased from aladdin Reagent Co. (Shanghai, China). All chemicals were analytical grade and used without further purification. Deionized water was used throughout the experiments. 2.2. Synthesis of HA nanoparticles A chemical precipitation method was used to prepare the HA nanoparticles. In a typical procedure, 100 mL of CaCl2 (0.01 mol/L) and 100 mL of (NH4)2HPO4 (0.006 mol/L) solutions were mixed together and adjusted to weak acidity with HCl solution (0.1 M). A certain amount of glucose (0 g, 5 g, 10 g, 20 g or 40 g) and 1.5 g of NaOH were dissolved with 100 mL of deionized water. Then the mixed solution of CaCl2 and (NH4)2HPO4 was added drop by drop into glucose alkaline solution and the HA nanoparticles were formed immediately. The products are noted as 0GHA, 5GHA, 10GHA, 20GHA and 40GHA, respectively.
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orthosilicate was used to synthesize nano SiO2 by following the steps: 20% glucose aqueous solution and ethanol were mixed together by volume ratio of 1:6, then a small amount of ammonium hydroxide was added as catalyst. Ethyl orthosilicate was added drop by drop into the above solution. The SiO2 nanoparticles were received after 4 h later. 2.4. Sample characterizations Nicolet 8700 Fourier transform infrared spectrometer (FTIR, Thermo Electron, USA) was used to characterize the functional groups of the samples. Crystallinities of the samples were investigated by X-ray diffraction (XRD, Bruker D8 Advance, Germany). The changes of ultraviolet absorption of HA samples were obtained by Shimadzu UV-3600 ultraviolet and visible spectrophotometer (UVevis, Shimadzu, Japan). S-4700 field emission scanning electron microscope (SEM, Hitachi, Japan) and J-3010 high resolution transmission electron microscopy (HRTEM, Hitachi J-3010, Japan) were used to observe the morphology and size of the synthesized nanoparticles. 3. Results and discussion 3.1. Synthesis and characterizations of HA nanoparticles In 2000, Nauta et al. [41] proved that several water molecules could be bonded by hydrogen bonds to form tiny clusters in liquid helium droplets (Fig. 1A). The extraneous water molecules could enter into the established clusters, which involved in numerous rearrangements of hydrogen bonds. Cheng et al. [42] pointed out that the distribution of ethanol and water molecules were not randomly in the ethanol/water system. Most water molecules existed in tiny aggregate form so that there was no isolated water molecule (Fig. 1B). The above thoughts offer a line of thinking to synthesize super-small nanoparticles. If the reaction to form the target compound can be finished in the limited molecular cage area that was formed by oxy-compounds, the super-small nanoparticles will be synthesized. In this paper, ethanol was used as limited domain reagents to synthesize super-small HA nanoparticles in the first. SEM image (shown in Fig. S1, in supporting information) indicates that most of the products are nanoparticles with diameter about 6e7 nm, however some nano rod products (length is about 100 nm) were also found when the ethanol (concentration is about 50%) was used as reagent to form the molecular cage. The reason on the formation of nano rod products possibly come from the lesser hydrogen bonds between water and ethanol for only one hydroxyl in each ethanol molecule, which leads some cages were not tight junction when the reaction ions diffused into the clusters formed between the water
2.3. Synthesis of Zn(OH)2, Ni(OH)2, Ag and SiO2 nanoparticles Similar to the above steps, 20 g of glucose and 1.5 g of NaOH were dissolved with 100 mL of deionized water. Then 100 mL of nitrate (Zn(NO3)2 or Ni(NO3)2, 0.01 mol/L) was added drop by drop into glucose alkaline solution to prepare Zn(OH)2 and Ni(OH)2 nanoparticles. In order to obtain Ag nanoparticles, 20 g of glucose was added into 100 mL of sodium citrate aqueous solution (0.01 mol/L). Then 100 mL of AgNO3 (0.01 mol/L) was added drop by drop into the above glucose sodium citrate solution under 94 C thermostatic waterbath to prepare Ag nanoparticles. Ethyl
Fig. 1. The structure of water molecules in liquid helium droplets [41] (A) and the model of ethanol/water system [42] (B).
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Fig. 2. Schematic structure of “molecular cage” built by water and glucose.
and ethanol molecules. Glucose, a kind of small multi-hydroxyl molecule, was used as limited domain reagent in the further research. In aqueous solution, hydrogen bonds were formed easily between glucose and water molecules due to the polyhydric structure of glucose, which is shown in Fig. 2. When concentration of the glucose reached a certain value, some water molecules would be limited into a small space possibly, and the water molecules were packed by glucose
and formed a “molecular cage”. In this work, glucose was used to build the “molecular cage” in aqueous solution to control the size of the synthesized HA nanoparticles, and the SEM images of the products are shown in Fig. 3. A possible reaction mechanism of HA confined space synthesis by glucose was shown in Fig. 4. With the mixed solution of PO3 4 and Ca2þ was added drop by drop into the glucose alkaline solution, PO3 and Ca2þ entered into the cage and HA was formed 4 immediately. Because of the limited space of the cage, the size of the synthesized HA could be limited into a narrow value and the maximum distribution size is approximately 7 nm when 20 g of glucose was used in the reaction. The BET analysis show the surface area of the synthesized 20GHA nanoparticles is about 113 m2/g while the commercial HA nano rod (length is about 50 nm) only is 44 m2/g. FTIR spectra (Fig. 5) show that all the synthesized HA nanoparticles have similar absorptions. The vibrations of -OHs in HA are found at 3569 and 630 cm1. The absorptions at 1093 and 1034 cm1 are the stretching vibrations of P¼O, 565 and 603 cm1 are the bending vibrations of P-O in PO3 4 . In addition, the weak bands at 1452 and 1418 cm1 are attributed to the CO2 3 , which came from the adsorbed CO2 during the preparation [43]. X-ray diffractions (Fig. 6) indicate that the diffractions of (002), (211), (202), (310), (222), (213) and (004) planes in all the synthesized HA are located at 26.05 , 31.95 , 34.01, 39.68 , 47.56 , 49.45
Fig. 3. SEM images and the corresponding statistical particle size results of the synthesized HA with various glucose concentrations during the reaction: 0GHA (A), 5GHA (B), 10GHA (C), 20GHA (D) and 40GHA (E).
Fig. 4. The formation process of super-small HA nanoparticles.
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Fig. 5. FTIR spectra of the HA nanoparticles with various glucose concentrations during the synthesis: 0GHA (a), 5GHA (b), 10GHA (c), 20GHA (d) and 40GHA (e).
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Fig. 7. UVevis spectra of the synthesized HA with various glucose mass added in the reaction.
Fig. 8. HRTEM and SAED results of 20GHA: HRTEM image of 20GHA (A); lattice fringes of 20GHA (B); SAED pattern result of 20GHA (C).
Fig. 6. XRD diffractions of the synthesized HA with various glucose concentrations during the reaction: 0GHA (a), 5GHA (b), 10GHA (c), 20GHA (d) and 40GHA (e).
and 53.18 , respectively, which coincided with the standard data of JCPDS Card No.09e0432 for the crystalline HA, confirming the formation of pure HA in both methods. In addition, the broad and weak diffractions indicate that the crystallinities of the samples were low, for the perfect crystal had not formed and slight distortions of the crystal lattice in progress of crystallization [14]. The UVevis spectrum (Fig. 7) indicates that the 0GHA has two absorptions at 272 nm and 228 nm. With the increase of glucose in reaction, the corresponding absorptions occurred blue shift. For example, they shifted to 256 nm and 226 nm in 20GHA and their intensities had an obvious enhancement, which suggests the size decrease of HA [44]. However, when the glucose mass increased to 40 g, there was no obvious change in positions and intensities of the two characteristic absorptions. SEM images (Fig. 3) show that the HA nanoparticles are dispersive globular particles. With increase of the glucose concentration in the reaction, the diameters of HA nanoparticles
gradually decreased and the morphology of these particles tended to be uniform. 200 nanoparticles of each sample were chosen randomly to obtain the relevant statistical results of the particle size. The sample of 0GHA shows an atypical morphology and the particle size could not be control (Fig. 3A). Fig. 3B shows inhomogeneous globular particles and the diameters are range from 6 nm to 30 nm with the concentrated distribution in 8 nm and 14 nm when 5 g glucose was added during the reaction process. When mass of the glucose was increased to 10 g (Fig. 3C), the max distributions of particle size were 7 nm and 11 nm, for the glucose concentration was still too low to limited the HA particles size in a narrow scope. When 20 g glucose were added in the reaction (Fig. 3D), the sample shows homogeneous globular particles and the size of these nanoparticles were further reduced. The statistical result shows the diameters of these HA nanoparticles are range from 4 nm to 13 nm with a concentrated distribution and the average diameter is approximately 7 nm. However, when the glucose mass more than 20 g used, there was no obvious size reduce of the HA. When mass of the glucose was low, only parts of the reactant ions were deemed to be limited into a small space
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Fig. 9. SEM images and the corresponding statistical particle size analysis, EDS results and FTIR spectrum of the synthesized Ni(OH)2 (A), SiO2 (B), Zn(OH)2 (C) and Ag (D).
because of the limited amount of the “molecular cage” and thus most of the synthesized HA particles have bigger diameter and their size could not be controlled very well. With the glucose addition, the number of the “molecular cage” was increased and more super-small HA particles appeared in narrow size distribution. When the glucose added was increased to 20 g, the number of “molecular cage” reached saturation and all the reactions between Ca2þ and PO3 4 were limited in the cages, so the synthesized HA shows super-small homogeneous globular particles with a concentrated distributed. However, the size of the synthesized HA could not be decreased when the glucose addition was further increased because the amounts of “molecular cages” were no longer increased. The HRTEM images indicate that the average diameter of HA nanoparticles is approximately 7 nm (Fig. 8A). The detailed structure of the synthesized HA was further observed by selected area electron diffraction (SAED). As shown in Fig. 8B, the lattice fringes of HA samples were clearly observed. The interplanar distance of 0.46 nm and 0.277 nm present the (110) and (112) planes of HA, respectively [42]. The SAED pattern result was shown in Fig. 8C. The clearly SAED pattern rings indicated the synthesized HA nanoparticles were crystalline. In addition, the distinct diffraction rings could be vested in (002) and (211) planes of HA [30]. 3.2. Synthesis and characterization of Zn(OH)2, Ni(OH)2, Ag and SiO2 nanoparticles In the further research, the same method was also successfully used to synthesize Ni(OH)2, SiO2, Zn(OH)2 and Ag nanoparticles
(<10 nm). According to the above results, 20% glucose aqueous solution was used as the reaction medium to prepare these nanoparticles. The results of Ni(OH)2 were shown in Fig. 9A. SEM picture reveal the Ni(OH)2 were homogeneous globular particles and the distribution size is approximately 5 nm according to the relevant statistical analysis. The element weight ratio and atomic ration of O/Ni from EDS test in accordance with Ni(OH)2. FTIR spectra show the typical adsorption bands of Ni(OH)2. The strong adsorption at 3639 cm1 and 524 cm1 belongs to the stretching vibrations and lattice vibration of -OH, respectively. The weak adsorption at 464 cm1 attribute to the lattice vibration of Ni-O. Fig. 9B indicates that the synthesized SiO2 nanoparticles show spherical-shape distribution and its average size was less than 10 nm. EDS result shows the weight ratio and atomic ratio of Si/O was 0.10 and 0.57, which corresponding to the structure characteristics of SiO2. FTIR spectra show a strong adsorption band at 1106 cm1, which attribute to the antisymmetric stretching vibrations of Si-O-Si. The adsorption at 956 cm1 belongs to the bending vibration of Si-OH. The weak bands at 799 cm1 and 473 cm1 were corresponding to the symmetrical stretching vibrations of Si-O. Zn(OH)2 nanoparticles were successfully prepared in this method and the results were shown in Fig. 9C. SEM picture and statistical analysis result indicate the Zn(OH)2 nanoparticles had globular morphology with average size of 8e9 nm. FTIR spectra show a typical adsorption at 487 cm1 and 417 cm1, which corresponding to the characteristic vibration of Zn-OH. EDS result shows the element ratio in accordance with Zn(OH)2. Moreover, Ag nanoparticles with an average size of 6 nm were
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also received (Fig. 9D). SEM picture suggest the synthesized Ag were homogeneous globular particles. EDS result indicates a typical characteristic of Ag elementary substance, the few element of O was from the oxidation during the EDS sample preparation process. Our work suggests the “molecular cage” method could be used to prepare various nanoparticles in aqueous solution. This method could become a universally method for the synthesis of nanoparticles, which was cheap and environmental friendly and may bring breakthrough in preparation of nanoparticles.
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4. Conclusions
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A universally method was used to prepare HA and other various nanoparticles. A “molecular cage” model was proposed to describe the confined space process during the reaction. In the first time, the 7 nm average size surper-small HA nanoparticles had successfully obtained. Moreover, Zn(OH)2, Ni(OH)2, SiO2 and Ag nanoparticles (<10 nm) were also prepared by this method, which indicate it could become a universally method for the synthesis of nanoparticles in aqueous solution. The low cost, easy and environmental friendly synthetic process lead this method could be used in mass production, and the method may bring breakthrough in preparation of nanoparticles.
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Notes
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The authors declare no competing financial interest. Acknowledgments
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We are grateful to the financial support from Project of National Natural Science Foundation of China (Project No.: U1362113). And we are also grateful to Prof. Pudun Zhang for the kindly advices.
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at https://doi.org/10.1016/j.matchemphys.2017.11.059.
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