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Effect of different polymers on morphology and particle size of silver nanoparticles synthesized by modified polyol method
Superlattices and Microstructures 98 (2016) 267e275
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Effect of different polymers on morphology and particle size of silver nanoparticles synthesized by modified polyol method Zeinab Fereshteh a, *, Ramin Rojaee b, Ali Sharifnabi b a
Department of Ceramic, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, 76315117, Kerman, Iran b Biomaterials Research Group, Department of Materials Engineering, Isfahan University of Technology, Isfahan, 8415683111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2016 Received in revised form 20 August 2016 Accepted 20 August 2016 Available online 23 August 2016
In this work, simple, economic, eco-friendly modified method with high efficiency was applied for synthesis of silver nanoparticles (Ag NPs) by using polyethylene glycol (PEG) as a capping agent, reductant, and media agent. In order to preparation uniform and small Ag NPs, the reaction parameters such as type of polymer, AgNO3/Polymer weight concentration ratio, and AgNO3/NaBH4 molar concentration ratio were modified. The best condition was optimized in concentration ratio of AgNO3: PEG: NaBH4 where are 1:10:0.01, respectively with 82% efficiency and 98.95% purity. Therefore, this modified polyol method can also be scaled up for synthesis of Ag NPs appropriately. Due to polymeric coating on the Ag NPs, they can be employed as a promising candidate for drug delivery systems. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Silver nanoparticles (Ag NPs) Modified polyol Characterization
1. Introduction In the recent years, silver nanoparticles (Ag NPs) is one of the most important nanoparticles due to its widespread applications in many different areas such as catalysts, photocatalysis, lubricating materials, and anti-bacterial materials [1e4]. The most widely used and known applications of Ag NPs are in the medical applications including medical device, implants, antibacterial gels/creams and other similar items [5,6]. Therefore, synthesis of size-controlled Ag NPs with high purity and high yield via a simple and cost effective method is interested by many researchers [5e7]. There are various methods to prepare Ag NPs including microwave irradiation, sonoelectrochemical methods, atom beam sputtering, chemical reduction, biological methods etc [5,8e12]. Among of these methods, wet chemical procedures are the most prevalent methods for synthesis Ag NPs regarding to be simple and rapid. However, toxic chemicals and surfactants are used in many of which. On account of using Ag NPs in biotechnology contacted directly with human life, there is an essential require to produce silver nanoparticles via environmentally friendly methods. It should be noticed that green ingredients especially solvent and surfactant must be applied for synthesis of nanoparticles [13e15]. Polyol method is an easy, simple, and environmentally friendly method to synthesis of nanoparticles which applies polymers as green surfactants. By using different type or weight molecules of polymer, synthesis of nanoparticles with different shapes and sizes can be accomplished in the versatile single step polyol method [2,11e15].
* Corresponding author. E-mail address: [email protected] (Z. Fereshteh). http://dx.doi.org/10.1016/j.spmi.2016.08.034 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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Zhao's group synthesized Ag NPs with different shapes and sizes by controlling the mass ratios of polyvinyl pyrrolidone (PVP; MW: 40,000) in AgNO3 solutions. Owing to increasing the viscosity of the reaction system, when the mass ratio of PVP to AgNO3 was above 10:1, the particle size was not dramatically decreased and it was independent of the PVP concentration. By employing ammonia as a complexing agent, the reduction of [Ag (NH3)2]þ to silver metal is harder Agþ which in turn leads to prepare smaller particle size of Ag NPs [2]. In addition, by ethylene glycol (EG) and PVP, silver nanowires were synthesized in two steps [16]. Lin et al. indicated that several factors; including silver nitrate concentration, temperature, reaction rate, and PVP/AgNO3 molar concentration ratio; had an influence on size and shape of the silver nanowires. A high aspect ratios of PVP employed as a capping agent which guided the growth orientation of the silver nanowires in 1D [16]. Moreover, by thermal decomposition of Ag2C2O4 - polyvinyl alcohol (PVA), Navaladian's group synthesized silver nanoparticles in different sizes. PVA, similar to PVP in the former study, played as a capping agent thereby the particle size decreased. They also illustrated increasing PVA concentration causes to decline the particle size; it was approximately decreased twice when the concentration was doubled [17]. Despite attractive properties of polyethylene glycol (PEG) in Ag NPs production process, it is occasionally employed as a capping agent. Chen et al. synthesized Ag NPs by PEG, plying role as a reducing as well as a capping agent, and Dimethylacetamide, as a solvent. By increasing molecule weight of PEG, there is a slight descending tendency of the mean diameter, being sufficiently high for the protection of Ag NPs in Mw~1000 [18]. In this study, size-controlled silver nanoparticles were synthesized by an elegant and simple modified polyol process applying PVP, PVA, and PEG as a surfactant and effect of different varieties of polymer on shape and size of Ag NPs were investigated. In order to achieving of a rapid, single step, and economical method with the highest yield, NaBH4 employed as a reductant. In addition, the effective parameters of synthesizing Ag NPs and its formation mechanism have been studied in details. 2. Experimental procedure Silver nitrate (AgNO3, 99.9%) (Merck), polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), with molecular weight (Mw), 30,000, 70,000, and 1000, respectively, ethylene glycol (EG), acetone and ethanol were purchased from Merck and used without further purification. AgNO3 (0.1 M) was dissolved in distilled water and heated up to 160 C in an oil bath with a stirring speed of 250 rpm; afterwards, an appropriate amount of polymer solved in EG was added dropwise into the solution, and the reaction continued for 4 h. Emerging a brown colloidal dispersion indicated the formation of the silver nanoparticles (Ag-NPs). The solution was cooled down to room temperature; then, it was admixed with acetone to precipitate Ag-NPs. The nanoparticles were centrifuged by acetone in 8000 rpm for 30 min until the supernatant solution was clear; they were followed by drying at 60 C for 24 h in a vacuum oven. In the obtained samples by using NaBH4, NaBH4 dissolved in ethanol and add into solution after mixing polymer solution. Finally, the obtained sediment decomposed at 450 C for 30 min, in order to eliminate leftover polymer and achieve Ag-NPs with high purity. The schematic diagram of the silver nanoparticles synthesis process is presented in Fig. 1. Table 1 shows the modified reaction parameters and their effect on the particle size and process yield in the synthesis process of the silver nanoparticles by the modified polyol method. The functional groups of the samples were identified by Fourier transform infrared spectroscopy (FT-IR: Bruker-Tensor 27, Jasco-680 spectrophotometer, Japan) in the range of 400e4000 cm1. The phase composition of the samples was analyzed by X-ray diffraction patterns (XRD, Philips XPert) with the voltage of 40 kV, and Cu Ka radiation (l ¼ 0.15,406 nm). The XRD patterns were recorded in 2q range of 5e100 (step size of 0.02 and time per step of 1s). The morphology and size of NPs were examined using transmission electron microscopy (TEM: EM 109, ZEISS, Germany, with the accelerating voltage of 100 kV), scanning electron microscope (SEM: LEO 435VP), and field emission scanning electron microscopy (FE-SEM: Hitachi, S-4160).
Fig. 1. The schematic diagram of the synthesis process of the Ag NPs by the modified polyol method.
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Table 1 Details of the reaction parameters used in the silver nanoparticles synthesis by the modified polyol method. S.No.
Reductant
AgNO3/NaBH4
Polymer
AgNO3/Polymer
Yield (%)
Particle size (nm)
S1 S1 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
e e e e e e e e NaBH4 NaBH4 NaBH4 NaBH4
e e e e e e e e e (1:1) (10:1) (100:1)
e PVP PVA PEG PEG PEG PEG PEG e PEG PEG PEG
e (1:1) (1:1) (1:1) (1:5) (1:10) (1:15) (1:20) (1:1) (1:1) (1:1) (1:1)
3 5 8 10 17 23 15 e 40 36 54 82
524 ± 87 (agglomerated) 4 1 ± 2 0.5 mm 413 ± 62 352 ± 40 powder powder powder/colloid colloid agglomerated 481 ± 53 385 ± 64 220 ± 30
The SEM, FE-SEM, and TEM images were analyzed using image analysis software (NIH Image J), in order to determine the size of prepared NPs. The thermal gravimetric analysis (TGA: Rheometric scientific 1998, USA) was used to determine the weight loss of the samples from room temperature to 800 C by the heating rate of 10 C min1. The inductive coupled plasma-optical emission spectroscopy (ICP-OES: Perkin Elmer, Optima 7300DV) was demonstrated purity of nanoparticles. The efficiency of the Ag NPs production process, the ratio between the actual yield and the theoretical yield, was calculated by Equation (1), where WA and WT are weights of actual and theoretical reaction, respectively.
Yeild ð%Þ ¼
WA 100 WT
(1)
3. Results and discussion In order to optimize process of the silver nanoparticles synthesis, polymer type, concentration of AgNO3, and ratio of AgNO3/NaBH4 were adjusted so that size distribution and particle size adapted narrower and smaller, respectively. Regarding to the aim of preparing uniform silver nanoparticles, several type of polymer were tried to determine the optimum polymer as a capping agent. The AgNO3 and polymer concentrations were fixed, while the type of polymer varied in EG, PVP, PVA, and PEG. By using EG, Ag nanoparticles were synthesized in a reduction reaction. In this way, EG lost water and changed into acetaldehyde which in turn led to acetaldehyde reduced silver cation to silver metal [2,14]. Fig. 2 shows SEM images of polymer capped Ag nanoparticles prepared with various polymers. When there was only EG as a solvent and capping agent,
Fig. 2. SEM micrographs of different samples with different magnification. S1: EG, S2: PVP/EG, S3: PVA/EG, S4: PEG/EG (capping agent/solvent).
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Fig. 3. The XRD patterns of the obtained samples in conditions: S1: EG, S2: PVP/EG, S3: PVA/EG, S4: PEG/EG (as a capping agent/solvent).
Ag nanoparticles were adhered and several of them were needle-like and non-uniform. It was found that size and shape of the Ag nanoparticles obviously changed by employing polymer as a capping agent, as shown in Fig. 2 (the particle size of S1, S2, S3, and S4 are 524 ± 87 nm (agglomerated), 4 1 ± 2 0.2 mm, 413 ± 62 nm, 352 ± 40 nm, respectively). The polymer molecules could adsorb on the nanoparticles surface and prevented them from growing nucleuses during process. It is should be mentioned that capping agent molecules must have active site/s to connect on surface of nanoparticles, leading to produce smaller nanoparticles [19e30]. Owing to the fact that polarity of polymer increases, it is suggested that these polymers can connect stranger in order PVP < PVA < PEG. In the presence of alcoholic functional groups of the polymer molecules, they can interact on the surface of nanoparticles. Generally, these groups are adsorbed on the surface of the nanoparticle in a certain temperature and pH rang. According to the results, PEG enhanced the stability of nanoparticles in suspension due to high solubility in polar and non-polar solvents and therefore had a good connection with nanoparticles [26,27].
Fig. 4. FE-SEM micrographs of the Ag NPs using NaBH4 without polymer (S9), 1 M (S10), 0.1 M (S11), and 0.01 M (S12) of NaBH4 in the AgNO3/PEG solution.
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Fig. 5. TEM micrographs of Ag NPs a,b) synthesized in AgNO3: PEG ¼ 1:10 wt concentration ratio, and c-e) synthesized in optimum condition (AgNO3: PEG: NaBH4 ¼ 1:10: 0.01 M concentration ratio) (the arrows show polymer coating on the surface of nanoparticles).
XRD patterns of the obtained products by different polymer are showed in Fig. 3. No peaks of byproducts are detected, indicating the high purity of the products. All samples represented high crystallinity excluding S1 because of the presence of organic materials on the surface of the nanoparticles probably. The six diffraction peaks of (111), (200), (220), (311), (222), and (400) of cubic Ag can be easily observed (JCPDS card #: 004-0783 and Space group: Fm3m). Enhancing concentration of PEG as a capping agent can be employed for decreasing the particle size [12,31]. On the other hand, a colloid silver nanoparticle was produced in this condition which is able to gather them up by centrifuge or other typical methods. As shown in Table 1, increasing PEG concentration reduced particle size of silver nanoparticles caused to protect them from clotting in addition of playing as a reducing agent [18]. The TEM images of S6 are shown in Fig. 5a and b. In the former procedure, NaBH4 was used as a stronger reductant leading smaller and monodisperse nanoparticles. Using a strong reductant, such as NaBH4, helps to synthesis smaller and more uniform nanoparticles with higher efficiency due to decomposition of NaBH4 and accelerate nucleation of nanoparticles, at the same time. By free electrons created in NaBH4 þ decomposition step (e.g., BH 4 þ 2H2O / BO2 þ 8H þ 8e ), the silver ions are converted to the silver nanocluster which can
Fig. 6. The XRD patterns of the samples synthesized by NaBH4 without polymer (S9), 1 M (S10), 0.1 M (S11), and 0.01 M (S12) of NaBH4 in the AgNO3/PEG solution.
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Fig. 7. FT-IR of spectrum of the Ag NPs synthesized in optimum condition before and after calcination at 450 C for 30 min.
Fig. 8. TGA analysis of as-prepared Ag NPs synthesized in optimum condition.
Fig. 9. The suggested interactions between the Ag NPs and PEG as a capping agent and surfactant.
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Fig. 10. a) The XRD patterns of the Ag NPs before and after thermal treatment at 450 C for 30 min, b) SEM micrographs of the Ag NPs after thermal treatment at 450 for 30 min.
prepare smaller and monodisperse nanoparticles, providing nanoparticles are prevented from aggregation at this stage by using capping agents [31,32] Consequently, NaBH4 solutions in several concentrations were sluggishly dropped into solution containing silver ions stabilized by PEG. In this way, NaBH4 without PEG (S9), 1, 0.1, and 0.01 M of NaBH4 with PEG (labeled S9, S10, S11, and S12, respectively) were employed to obtain the minimum particle size. As can be seen in Fig. 4, the silver particles agglomerated in sample 5 an account of lack of polymer as a capping agent. On the other hand, using NaBH4 with PEG caused to diminish the size distribution and particle size owing to quick nucleation of the silver clusters by using NaBH4 and protection against growing them by PEG. Decreasing NaBH4 concentration leads to minimize particle size (the particle size of S9, S10, S11, and S12 are 200 ± 25 nm (agglomerated), 481 ± 53 nm, 385 ± 64 nm, 120 ± 30 nm, respectively). Fig. 5 shows TEM images of Ag NPs synthesized in 0.01 M of NaBH4 and 1:10 AgNO3:PEG weight concentration ratio; the particle size of nanoparticles are 32 ± 6 nm and 185 ± 9 nm, respectively. This is similar to the results proposed by Refs. [1,33] concerning the effect of NaBH4 concentration in synthesis of silver nanoparticles. The reason is the silver ions enclosed by PEG should destabilize after adding NaBH4 owing to the fact that the PEG molecules replaced by ions and/or the free electrons produced during its decomposition, which in turn leads to agglomerate the initial nucleus and increase the particle size. Consequently, it should be mention that low concentrations of NaBH4 should sluggishly drop into the silver nitrate/PEG solution in order to decelerate nucleation and achieve smaller particle size [1,33]. On the other hand, it can be seen in TEM images, the around the Ag NPs were enclosed by shadow which is suggested that is a polymer coating on the surface of Ag NPs (Fig. 5e). The PEG coating presented as a barrier on the surface of Ag NPs to prevent them from agglomeration as well as produce nanoparticles with more uniform size distribution. Additionally, this coating provides a promising surface for uniform dispersion of Ag NPs in the polymeric matrix as a result of effectively adhering on the NPs surface which is applicable to any polymeric materials with antibacterial properties [17,34,35]. Moreover, the reaction mass efficiency improved up to 82% in these samples, along with increasing PEG concentration. The XRD patterns of all samples are represented in Fig. 6; all diffraction patterns of all samples display the typical diffraction peaks of the cubic structure of the silver nanoparticles with high crystallinity and no impurity. To obtain silver nanoparticles with high purity, the prepared silver nanoparticles by the modified polyol method thermal treated at 450 C for 30 min which leads to decomposition of the adsorbed polymer on the nanoparticles surface. According to ICP-OES results, the purity of the silver nanoparticles slightly increased up to 98.95% which was 95.83% before thermal treatment. Inorganic impurity of nanoparticles is less than 1% and the rest of that is organic materials adsorbing on the nanoparticles surface such as PEG or EG confirmed by FT-IR or TAG. Therefore, it can be concluded that no byproducts or impurities had been in the Ag NPs as a purpose product. According to the FT-IR spectra of nanoparticles before and after calcination in Fig. 7, PEG molecules adsorbed on the surface nanoparticles in small quantities, and calcination of them in 450 C for 30 min is not able to expurgate PEG on the surface of Ag NPs and produce very high pure nanoparticles. As can be seen in Fig. 8, as-synthesized Ag NPs lost weight ~2.4% within the temperature range of ~120 up to 650 C. The peaks at ~3700, 2800, 1600, and 1500 cm1 are characteristic of the OeH stretching, CeH stretching, CeH bending, OeH and CeOeH stretching [7]. The broad peak in <500 cm1 attributed to Ag….O which is related to hydroxyl groups of PEG chains banding on the surface of the Ag nanoparticles by van der Waals interactions [4,15]. The FT-IR spectra peaks reveal that a chemical reaction has occurred between the active positions of PEG and Ag nanoparticles surface, along with efficiently modifying the Ag nanoparticles by polymer molecules. According to FT-IR and TEM observations, the proposed schematic of interactions between PEG and the Ag nanoparticles is illustrated in Fig. 9. The PEG molecules can be connected on the Ag NPs surface by the first functional group (Fig. 9a) or several alcoholic groups of polymer chain (Fig. 9 b). In high concentration of PEG, the NPs can fixedly place in polymer net resulting in smaller uniform nanoparticles [32,34]. XRD patterns and SEM images of the silver nanoparticles after thermal treatment were shown in Fig. 10. By calcination of the silver nanoparticles, the crystallinity was dramatically improved and the intensity of peaks was doubled [11]. Owing to the
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large specific surface area and high surface energy of nanoparticles, they extremely tend to agglomerate especially in high temperature [35], nanoparticles were grown more than 1 mm up in this case. 4. Conclusion In summary, we modified a facile large-scale, green method to synthesis silver nanoparticles by applying three different polymers as capping agents which were PVP, PVA, and PEG. By adjusting type of polymer, AgNO3/Polymer concentration, and AgNO3/NaBH4, the size and morphology of Ag NPs could be controlled. Regards to our results, the Ag NPs fabricated by PEG was the smallest nanoparticles in this study, owing to have highest polarity among the polymers, resulted from presence of alcoholic functional groups of the polymer molecules. In order to decrease the particle size and increase the efficiency of reaction, NaBH4 was employed as a strong reductant which accelerated nucleation of nanoparticles, leading to decrease the particle size and increase the number of nucleus. However, the smallest nanoparticles were synthesized in the low concentrations of NaBH4, due to replacement of PEG molecules by ions and/or the free electrons. Eventually, the smallest Ag NPs with the highest efficiency (82%) and purity in 98.95% were synthesized in the optimal condition where AgNO3: PEG: NaBH4 concentration ratio was 1:10:0.01. FT-IR data and TEM images showed that PEG molecules adsorbed on the surface of nanoparticles as a barrier to prevent them from agglomeration along with improve of size distribution which gives Ag NPs an opportunity to apply as a drug delivery system or polymeric materials with antibacterial applications. Acknowledgment The financial support of Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology is gratefully acknowledged (Grant Number: 95/403). References [1] S. Wojtysiak, A. 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Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Effect of different polymers on morphology and particle size of silver nanoparticles synthesized by modified polyol method Zeinab Fereshteh a, *, Ramin Rojaee b, Ali Sharifnabi b a
Department of Ceramic, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, 76315117, Kerman, Iran b Biomaterials Research Group, Department of Materials Engineering, Isfahan University of Technology, Isfahan, 8415683111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 July 2016 Received in revised form 20 August 2016 Accepted 20 August 2016 Available online 23 August 2016
In this work, simple, economic, eco-friendly modified method with high efficiency was applied for synthesis of silver nanoparticles (Ag NPs) by using polyethylene glycol (PEG) as a capping agent, reductant, and media agent. In order to preparation uniform and small Ag NPs, the reaction parameters such as type of polymer, AgNO3/Polymer weight concentration ratio, and AgNO3/NaBH4 molar concentration ratio were modified. The best condition was optimized in concentration ratio of AgNO3: PEG: NaBH4 where are 1:10:0.01, respectively with 82% efficiency and 98.95% purity. Therefore, this modified polyol method can also be scaled up for synthesis of Ag NPs appropriately. Due to polymeric coating on the Ag NPs, they can be employed as a promising candidate for drug delivery systems. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Silver nanoparticles (Ag NPs) Modified polyol Characterization
1. Introduction In the recent years, silver nanoparticles (Ag NPs) is one of the most important nanoparticles due to its widespread applications in many different areas such as catalysts, photocatalysis, lubricating materials, and anti-bacterial materials [1e4]. The most widely used and known applications of Ag NPs are in the medical applications including medical device, implants, antibacterial gels/creams and other similar items [5,6]. Therefore, synthesis of size-controlled Ag NPs with high purity and high yield via a simple and cost effective method is interested by many researchers [5e7]. There are various methods to prepare Ag NPs including microwave irradiation, sonoelectrochemical methods, atom beam sputtering, chemical reduction, biological methods etc [5,8e12]. Among of these methods, wet chemical procedures are the most prevalent methods for synthesis Ag NPs regarding to be simple and rapid. However, toxic chemicals and surfactants are used in many of which. On account of using Ag NPs in biotechnology contacted directly with human life, there is an essential require to produce silver nanoparticles via environmentally friendly methods. It should be noticed that green ingredients especially solvent and surfactant must be applied for synthesis of nanoparticles [13e15]. Polyol method is an easy, simple, and environmentally friendly method to synthesis of nanoparticles which applies polymers as green surfactants. By using different type or weight molecules of polymer, synthesis of nanoparticles with different shapes and sizes can be accomplished in the versatile single step polyol method [2,11e15].
* Corresponding author. E-mail address: [email protected] (Z. Fereshteh). http://dx.doi.org/10.1016/j.spmi.2016.08.034 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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Zhao's group synthesized Ag NPs with different shapes and sizes by controlling the mass ratios of polyvinyl pyrrolidone (PVP; MW: 40,000) in AgNO3 solutions. Owing to increasing the viscosity of the reaction system, when the mass ratio of PVP to AgNO3 was above 10:1, the particle size was not dramatically decreased and it was independent of the PVP concentration. By employing ammonia as a complexing agent, the reduction of [Ag (NH3)2]þ to silver metal is harder Agþ which in turn leads to prepare smaller particle size of Ag NPs [2]. In addition, by ethylene glycol (EG) and PVP, silver nanowires were synthesized in two steps [16]. Lin et al. indicated that several factors; including silver nitrate concentration, temperature, reaction rate, and PVP/AgNO3 molar concentration ratio; had an influence on size and shape of the silver nanowires. A high aspect ratios of PVP employed as a capping agent which guided the growth orientation of the silver nanowires in 1D [16]. Moreover, by thermal decomposition of Ag2C2O4 - polyvinyl alcohol (PVA), Navaladian's group synthesized silver nanoparticles in different sizes. PVA, similar to PVP in the former study, played as a capping agent thereby the particle size decreased. They also illustrated increasing PVA concentration causes to decline the particle size; it was approximately decreased twice when the concentration was doubled [17]. Despite attractive properties of polyethylene glycol (PEG) in Ag NPs production process, it is occasionally employed as a capping agent. Chen et al. synthesized Ag NPs by PEG, plying role as a reducing as well as a capping agent, and Dimethylacetamide, as a solvent. By increasing molecule weight of PEG, there is a slight descending tendency of the mean diameter, being sufficiently high for the protection of Ag NPs in Mw~1000 [18]. In this study, size-controlled silver nanoparticles were synthesized by an elegant and simple modified polyol process applying PVP, PVA, and PEG as a surfactant and effect of different varieties of polymer on shape and size of Ag NPs were investigated. In order to achieving of a rapid, single step, and economical method with the highest yield, NaBH4 employed as a reductant. In addition, the effective parameters of synthesizing Ag NPs and its formation mechanism have been studied in details. 2. Experimental procedure Silver nitrate (AgNO3, 99.9%) (Merck), polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), with molecular weight (Mw), 30,000, 70,000, and 1000, respectively, ethylene glycol (EG), acetone and ethanol were purchased from Merck and used without further purification. AgNO3 (0.1 M) was dissolved in distilled water and heated up to 160 C in an oil bath with a stirring speed of 250 rpm; afterwards, an appropriate amount of polymer solved in EG was added dropwise into the solution, and the reaction continued for 4 h. Emerging a brown colloidal dispersion indicated the formation of the silver nanoparticles (Ag-NPs). The solution was cooled down to room temperature; then, it was admixed with acetone to precipitate Ag-NPs. The nanoparticles were centrifuged by acetone in 8000 rpm for 30 min until the supernatant solution was clear; they were followed by drying at 60 C for 24 h in a vacuum oven. In the obtained samples by using NaBH4, NaBH4 dissolved in ethanol and add into solution after mixing polymer solution. Finally, the obtained sediment decomposed at 450 C for 30 min, in order to eliminate leftover polymer and achieve Ag-NPs with high purity. The schematic diagram of the silver nanoparticles synthesis process is presented in Fig. 1. Table 1 shows the modified reaction parameters and their effect on the particle size and process yield in the synthesis process of the silver nanoparticles by the modified polyol method. The functional groups of the samples were identified by Fourier transform infrared spectroscopy (FT-IR: Bruker-Tensor 27, Jasco-680 spectrophotometer, Japan) in the range of 400e4000 cm1. The phase composition of the samples was analyzed by X-ray diffraction patterns (XRD, Philips XPert) with the voltage of 40 kV, and Cu Ka radiation (l ¼ 0.15,406 nm). The XRD patterns were recorded in 2q range of 5e100 (step size of 0.02 and time per step of 1s). The morphology and size of NPs were examined using transmission electron microscopy (TEM: EM 109, ZEISS, Germany, with the accelerating voltage of 100 kV), scanning electron microscope (SEM: LEO 435VP), and field emission scanning electron microscopy (FE-SEM: Hitachi, S-4160).
Fig. 1. The schematic diagram of the synthesis process of the Ag NPs by the modified polyol method.
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Table 1 Details of the reaction parameters used in the silver nanoparticles synthesis by the modified polyol method. S.No.
Reductant
AgNO3/NaBH4
Polymer
AgNO3/Polymer
Yield (%)
Particle size (nm)
S1 S1 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
e e e e e e e e NaBH4 NaBH4 NaBH4 NaBH4
e e e e e e e e e (1:1) (10:1) (100:1)
e PVP PVA PEG PEG PEG PEG PEG e PEG PEG PEG
e (1:1) (1:1) (1:1) (1:5) (1:10) (1:15) (1:20) (1:1) (1:1) (1:1) (1:1)
3 5 8 10 17 23 15 e 40 36 54 82
524 ± 87 (agglomerated) 4 1 ± 2 0.5 mm 413 ± 62 352 ± 40 powder powder powder/colloid colloid agglomerated 481 ± 53 385 ± 64 220 ± 30
The SEM, FE-SEM, and TEM images were analyzed using image analysis software (NIH Image J), in order to determine the size of prepared NPs. The thermal gravimetric analysis (TGA: Rheometric scientific 1998, USA) was used to determine the weight loss of the samples from room temperature to 800 C by the heating rate of 10 C min1. The inductive coupled plasma-optical emission spectroscopy (ICP-OES: Perkin Elmer, Optima 7300DV) was demonstrated purity of nanoparticles. The efficiency of the Ag NPs production process, the ratio between the actual yield and the theoretical yield, was calculated by Equation (1), where WA and WT are weights of actual and theoretical reaction, respectively.
Yeild ð%Þ ¼
WA 100 WT
(1)
3. Results and discussion In order to optimize process of the silver nanoparticles synthesis, polymer type, concentration of AgNO3, and ratio of AgNO3/NaBH4 were adjusted so that size distribution and particle size adapted narrower and smaller, respectively. Regarding to the aim of preparing uniform silver nanoparticles, several type of polymer were tried to determine the optimum polymer as a capping agent. The AgNO3 and polymer concentrations were fixed, while the type of polymer varied in EG, PVP, PVA, and PEG. By using EG, Ag nanoparticles were synthesized in a reduction reaction. In this way, EG lost water and changed into acetaldehyde which in turn led to acetaldehyde reduced silver cation to silver metal [2,14]. Fig. 2 shows SEM images of polymer capped Ag nanoparticles prepared with various polymers. When there was only EG as a solvent and capping agent,
Fig. 2. SEM micrographs of different samples with different magnification. S1: EG, S2: PVP/EG, S3: PVA/EG, S4: PEG/EG (capping agent/solvent).
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Fig. 3. The XRD patterns of the obtained samples in conditions: S1: EG, S2: PVP/EG, S3: PVA/EG, S4: PEG/EG (as a capping agent/solvent).
Ag nanoparticles were adhered and several of them were needle-like and non-uniform. It was found that size and shape of the Ag nanoparticles obviously changed by employing polymer as a capping agent, as shown in Fig. 2 (the particle size of S1, S2, S3, and S4 are 524 ± 87 nm (agglomerated), 4 1 ± 2 0.2 mm, 413 ± 62 nm, 352 ± 40 nm, respectively). The polymer molecules could adsorb on the nanoparticles surface and prevented them from growing nucleuses during process. It is should be mentioned that capping agent molecules must have active site/s to connect on surface of nanoparticles, leading to produce smaller nanoparticles [19e30]. Owing to the fact that polarity of polymer increases, it is suggested that these polymers can connect stranger in order PVP < PVA < PEG. In the presence of alcoholic functional groups of the polymer molecules, they can interact on the surface of nanoparticles. Generally, these groups are adsorbed on the surface of the nanoparticle in a certain temperature and pH rang. According to the results, PEG enhanced the stability of nanoparticles in suspension due to high solubility in polar and non-polar solvents and therefore had a good connection with nanoparticles [26,27].
Fig. 4. FE-SEM micrographs of the Ag NPs using NaBH4 without polymer (S9), 1 M (S10), 0.1 M (S11), and 0.01 M (S12) of NaBH4 in the AgNO3/PEG solution.
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271
Fig. 5. TEM micrographs of Ag NPs a,b) synthesized in AgNO3: PEG ¼ 1:10 wt concentration ratio, and c-e) synthesized in optimum condition (AgNO3: PEG: NaBH4 ¼ 1:10: 0.01 M concentration ratio) (the arrows show polymer coating on the surface of nanoparticles).
XRD patterns of the obtained products by different polymer are showed in Fig. 3. No peaks of byproducts are detected, indicating the high purity of the products. All samples represented high crystallinity excluding S1 because of the presence of organic materials on the surface of the nanoparticles probably. The six diffraction peaks of (111), (200), (220), (311), (222), and (400) of cubic Ag can be easily observed (JCPDS card #: 004-0783 and Space group: Fm3m). Enhancing concentration of PEG as a capping agent can be employed for decreasing the particle size [12,31]. On the other hand, a colloid silver nanoparticle was produced in this condition which is able to gather them up by centrifuge or other typical methods. As shown in Table 1, increasing PEG concentration reduced particle size of silver nanoparticles caused to protect them from clotting in addition of playing as a reducing agent [18]. The TEM images of S6 are shown in Fig. 5a and b. In the former procedure, NaBH4 was used as a stronger reductant leading smaller and monodisperse nanoparticles. Using a strong reductant, such as NaBH4, helps to synthesis smaller and more uniform nanoparticles with higher efficiency due to decomposition of NaBH4 and accelerate nucleation of nanoparticles, at the same time. By free electrons created in NaBH4 þ decomposition step (e.g., BH 4 þ 2H2O / BO2 þ 8H þ 8e ), the silver ions are converted to the silver nanocluster which can
Fig. 6. The XRD patterns of the samples synthesized by NaBH4 without polymer (S9), 1 M (S10), 0.1 M (S11), and 0.01 M (S12) of NaBH4 in the AgNO3/PEG solution.
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Fig. 7. FT-IR of spectrum of the Ag NPs synthesized in optimum condition before and after calcination at 450 C for 30 min.
Fig. 8. TGA analysis of as-prepared Ag NPs synthesized in optimum condition.
Fig. 9. The suggested interactions between the Ag NPs and PEG as a capping agent and surfactant.
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Fig. 10. a) The XRD patterns of the Ag NPs before and after thermal treatment at 450 C for 30 min, b) SEM micrographs of the Ag NPs after thermal treatment at 450 for 30 min.
prepare smaller and monodisperse nanoparticles, providing nanoparticles are prevented from aggregation at this stage by using capping agents [31,32] Consequently, NaBH4 solutions in several concentrations were sluggishly dropped into solution containing silver ions stabilized by PEG. In this way, NaBH4 without PEG (S9), 1, 0.1, and 0.01 M of NaBH4 with PEG (labeled S9, S10, S11, and S12, respectively) were employed to obtain the minimum particle size. As can be seen in Fig. 4, the silver particles agglomerated in sample 5 an account of lack of polymer as a capping agent. On the other hand, using NaBH4 with PEG caused to diminish the size distribution and particle size owing to quick nucleation of the silver clusters by using NaBH4 and protection against growing them by PEG. Decreasing NaBH4 concentration leads to minimize particle size (the particle size of S9, S10, S11, and S12 are 200 ± 25 nm (agglomerated), 481 ± 53 nm, 385 ± 64 nm, 120 ± 30 nm, respectively). Fig. 5 shows TEM images of Ag NPs synthesized in 0.01 M of NaBH4 and 1:10 AgNO3:PEG weight concentration ratio; the particle size of nanoparticles are 32 ± 6 nm and 185 ± 9 nm, respectively. This is similar to the results proposed by Refs. [1,33] concerning the effect of NaBH4 concentration in synthesis of silver nanoparticles. The reason is the silver ions enclosed by PEG should destabilize after adding NaBH4 owing to the fact that the PEG molecules replaced by ions and/or the free electrons produced during its decomposition, which in turn leads to agglomerate the initial nucleus and increase the particle size. Consequently, it should be mention that low concentrations of NaBH4 should sluggishly drop into the silver nitrate/PEG solution in order to decelerate nucleation and achieve smaller particle size [1,33]. On the other hand, it can be seen in TEM images, the around the Ag NPs were enclosed by shadow which is suggested that is a polymer coating on the surface of Ag NPs (Fig. 5e). The PEG coating presented as a barrier on the surface of Ag NPs to prevent them from agglomeration as well as produce nanoparticles with more uniform size distribution. Additionally, this coating provides a promising surface for uniform dispersion of Ag NPs in the polymeric matrix as a result of effectively adhering on the NPs surface which is applicable to any polymeric materials with antibacterial properties [17,34,35]. Moreover, the reaction mass efficiency improved up to 82% in these samples, along with increasing PEG concentration. The XRD patterns of all samples are represented in Fig. 6; all diffraction patterns of all samples display the typical diffraction peaks of the cubic structure of the silver nanoparticles with high crystallinity and no impurity. To obtain silver nanoparticles with high purity, the prepared silver nanoparticles by the modified polyol method thermal treated at 450 C for 30 min which leads to decomposition of the adsorbed polymer on the nanoparticles surface. According to ICP-OES results, the purity of the silver nanoparticles slightly increased up to 98.95% which was 95.83% before thermal treatment. Inorganic impurity of nanoparticles is less than 1% and the rest of that is organic materials adsorbing on the nanoparticles surface such as PEG or EG confirmed by FT-IR or TAG. Therefore, it can be concluded that no byproducts or impurities had been in the Ag NPs as a purpose product. According to the FT-IR spectra of nanoparticles before and after calcination in Fig. 7, PEG molecules adsorbed on the surface nanoparticles in small quantities, and calcination of them in 450 C for 30 min is not able to expurgate PEG on the surface of Ag NPs and produce very high pure nanoparticles. As can be seen in Fig. 8, as-synthesized Ag NPs lost weight ~2.4% within the temperature range of ~120 up to 650 C. The peaks at ~3700, 2800, 1600, and 1500 cm1 are characteristic of the OeH stretching, CeH stretching, CeH bending, OeH and CeOeH stretching [7]. The broad peak in <500 cm1 attributed to Ag….O which is related to hydroxyl groups of PEG chains banding on the surface of the Ag nanoparticles by van der Waals interactions [4,15]. The FT-IR spectra peaks reveal that a chemical reaction has occurred between the active positions of PEG and Ag nanoparticles surface, along with efficiently modifying the Ag nanoparticles by polymer molecules. According to FT-IR and TEM observations, the proposed schematic of interactions between PEG and the Ag nanoparticles is illustrated in Fig. 9. The PEG molecules can be connected on the Ag NPs surface by the first functional group (Fig. 9a) or several alcoholic groups of polymer chain (Fig. 9 b). In high concentration of PEG, the NPs can fixedly place in polymer net resulting in smaller uniform nanoparticles [32,34]. XRD patterns and SEM images of the silver nanoparticles after thermal treatment were shown in Fig. 10. By calcination of the silver nanoparticles, the crystallinity was dramatically improved and the intensity of peaks was doubled [11]. Owing to the
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large specific surface area and high surface energy of nanoparticles, they extremely tend to agglomerate especially in high temperature [35], nanoparticles were grown more than 1 mm up in this case. 4. Conclusion In summary, we modified a facile large-scale, green method to synthesis silver nanoparticles by applying three different polymers as capping agents which were PVP, PVA, and PEG. By adjusting type of polymer, AgNO3/Polymer concentration, and AgNO3/NaBH4, the size and morphology of Ag NPs could be controlled. Regards to our results, the Ag NPs fabricated by PEG was the smallest nanoparticles in this study, owing to have highest polarity among the polymers, resulted from presence of alcoholic functional groups of the polymer molecules. In order to decrease the particle size and increase the efficiency of reaction, NaBH4 was employed as a strong reductant which accelerated nucleation of nanoparticles, leading to decrease the particle size and increase the number of nucleus. However, the smallest nanoparticles were synthesized in the low concentrations of NaBH4, due to replacement of PEG molecules by ions and/or the free electrons. Eventually, the smallest Ag NPs with the highest efficiency (82%) and purity in 98.95% were synthesized in the optimal condition where AgNO3: PEG: NaBH4 concentration ratio was 1:10:0.01. FT-IR data and TEM images showed that PEG molecules adsorbed on the surface of nanoparticles as a barrier to prevent them from agglomeration along with improve of size distribution which gives Ag NPs an opportunity to apply as a drug delivery system or polymeric materials with antibacterial applications. Acknowledgment The financial support of Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology is gratefully acknowledged (Grant Number: 95/403). References [1] S. Wojtysiak, A. 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