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Synthesis of monodisperse gold nanoparticles via electrospray-assisted chemical reduction method in cyclohexane
Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 148–153
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of monodisperse gold nanoparticles via electrospray-assisted chemical reduction method in cyclohexane Katarzyna Soliwoda, Marcin Rosowski, Emilia Tomaszewska, Beata Tkacz-Szczesna, Grzegorz Celichowski, Maciej Psarski, Jaroslaw Grobelny ∗ Department of Materials Technology and Chemistry, Faculty of Chemistry, University of Lodz, Pomorska 163, 90-236 Lodz, Poland
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
• Gold
nanoparticles synthesized via electrospray-assisted chemical reduction method directly in non-polar solvent. • The mechanism of gold nanoparticles formation discussed. • Two methods of reagents incorporation to the reaction bath compared. • Highly monodisperse gold nanoparticles in cyclohexane obtained.
a r t i c l e
i n f o
Article history: Received 19 January 2015 Received in revised form 9 April 2015 Accepted 22 April 2015 Available online 5 May 2015 Keywords: Monodisperse gold nanoparticles Organic colloids Chemical reduction Electrospray
a b s t r a c t We present a fast and versatile method of monodisperse gold nanoparticles (AuNPs) synthesis, directly in non-polar solvent at room temperature, by electrospray-assisted chemical reduction method. The electrospray technique was used to spray a dispersed aerosol of gold precursor [gold (III) chloride hydrate] into the reaction bath containing reducing agent (octadecylaminomethanol) and non-polar solvent (cyclohexane). To find the best homogeneity in the size and low polydispersity of synthesized AuNPs, two methods of reagent incorporation into the reaction bath were compared: incorporation by continuous stream flow using a capillary, and by continuous fine droplet flow using electrospray technique. The obtained AuNPs were characterized by UV–vis spectroscopy, dynamic light scattering (DLS) and scanning transmission electron microscopy (STEM). Results confirmed the formation of AuNPs in both cases and showed that the size of NPs synthesized via chemical reduction method in non-polar solvent depends on the method of reagents incorporation. The continuous incorporation of precursor resulted in polydisperse colloid with NPs size of about: 4.4 ± 0.6 nm and 10.9 ± 1.4 nm, whereas incorporation of fine droplets to the reaction bath resulted in a highly monodisperse colloid with the NPs size of about 5.7 ± 0.5 nm. © 2015 Elsevier B.V. All rights reserved.
1. Introduction ∗ Corresponding author. Tel.: +48 42 6355837; fax: +48 42 6355832. E-mail address: [email protected] (J. Grobelny). http://dx.doi.org/10.1016/j.colsurfa.2015.04.040 0927-7757/© 2015 Elsevier B.V. All rights reserved.
Nanoparticles (NPs) as objects with unique properties, determined by their nanoscale size, attract great attention in catalysis
K. Soliwoda et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 148–153
[1,2], optics [3], electronics [4,5], as well as in biology [6,7] and tribology [8,9]. Because of unusual optical, electrical and catalytic properties of NPs, they can be used to modify the physical and chemical properties of different materials [8–11]. Expected results can be obtained even by application of polydisperse NPs, whereas for some specific applications highly monodisperse NPs, with defined size and uniform shape, have to be applied. Fine control over physical, chemical and structural properties of NPs is crucial for expanding their applicability, especially in optoelectronics, e.g., in organic–inorganic hydride materials like transistors [4,5] or memory elements [12–14]. The incorporation of metallic NPs in polymeric semiconductors (such as poly(3-hexylthiophene), poly(2-methoxy-5-(2 ethl-hexyloxy)1,4-phenylenevinlylene)) is one of the methods to modify polymer physical and chemical properties. It is already known that the electrical conductance between two electrodes in the device depends on the charge stored or trapped on NPs [4,5,14]. The charge trapping mechanism is affected by NPs size [4,5], thus the quality of NPs incorporated in these materials – their size monodispersity and shape uniformity – is crucial to their performance. Moreover, the changes in the field-effect mobility depend mainly on the NPs size [5,15]. Hence, to fully exploit the unique properties of NPs, it is extremely important to elaborate reproducible preparation methods of highly monodisperse NPs with defined and fully controlled shape and size. Nowadays, extensive studies are still being carried out on controlling NPs parameters in various synthesis methods, in both polar [16–19] and non-polar solvents [20,21]. The size of NPs can be controlled by adjusting the synthesis conditions, e.g., solvent composition [22], reactant types [23] and their concentrations [24], temperature of the reaction [25], etc. Another parameter affecting NPs homogeneity is the method of reactants addition into the reaction bath: continuously, progressively, drop by drop, with a constant or variable flow rate, etc. One of the methods that can be used for addition of the precursor solution into the reaction bath is the electrospray technique [26–28]. The electrospray technique is a method of aerosol formation by electric field [29–31]. The droplet size generated via electrospray ranges from hundreds of micrometers to tens of nanometers [30,31]. The droplets are highly charged and, because of the repulsive forces, the coagulation process between droplets during the electrospray does not occur. Electrospray technique can be used to atomize the precursor solution in the chemical reduction method, to obtain higher precursor dispersion in the reaction bath [26–28]. The precursor droplet size, its velocity and acceleration can be controlled by adjusting of the electrospray parameters, such as flow rate, bias voltage, distance between needle and sample, inner diameter of the needle, etc. [32,33]. In this study, we present a procedure of synthesis of highly monodisperse AuNPs, directly in non-polar solvent (cyclohexane), via electrospray-assisted chemical reduction method. The presented method consists of two steps: first, the generation of precursor solution aerosol under the electric field, and second, the chemical reduction of dispersed gold ions with carbinolamine in the reaction bath. Spherical AuNPs with defined size and very narrow size distribution are obtained in cyclohexane and can be directly used in hybrid organic-inorganic materials, e.g., in polymeric thinfilm transistors or memory elements.
Lach-Ner, 36–38%), octadecylaminomethanol (ODAM) prepared based on the procedure described by Chen and Wang [34], isopropanol (POCH, 99%) and cyclohexane (POCH, 99%) were used without further purification. 2.2. Synthesis of AuNPs AuNPs were synthesized by gold (III) chloride hydrate reduction with octadecylaminomethanol (ODAM) in cyclohexane. ODAM was used as both reductive and stabilizing agent of NPs. Precursor solution was prepared by dissolving HAuCl4 ·H2 O (9.5 × 10−3 g) in isopropanol (5.500 g). The reductive solution was prepared based on the procedure described by Chen and Wang [34], but with octadecylamine instead of dodecylamine. The procedure of reductive solution preparation was as follows: octadecylamine (1.077 g) was dissolved in the mixture of formaldehyde (6.438 g, Lach-Ner, 36–38%) and cyclohexane (16.348 g) and vigorously stirred for 10 min at room temperature. Next, the cyclohexane phase containing octadecylaminomethanol was separated out and placed in the reaction bath. Precursor solution was incorporated using two methods: by fine droplets generated via electrospray (Method 1) and continuously, through a capillary (Method 2). During the addition of the precursor, the solution in the reaction bath was stirred at the same rate in both cases. The use of the electrospray and capillary for incorporation of the precursor solution into the reaction bath allows to examine the influence of the method of reagent addition on the shape, size and size distribution of synthesized AuNPs. The NPs weight concentration of gold in both cases was about 60 ppm. 2.2.1. Method 1 (Colloid 1) Electrospray was the first method used for incorporation of precursor solution to the reaction bath. The process consists in simultaneous charged droplet generation by means of electric field. A typical system for electrospray comprises a high voltage power supply, a glass syringe, a syringe pump, a hose with a nozzle, a ring as a counter and a reaction bath. A scheme of home-made equipment used for electrospray in this study is presented in Fig. 1. In the electrospraying process the precursor solution is pumped at a constant flow rate to the reaction bath, by a syringe pump, through an electrified capillary. High voltage is applied between the stainless nozzle and the platinum ring, which is dipped in the reaction bath. The electric field between the needle and the platinum ring causes the atomization of the precursor solution into small droplets and formation of a spray (aerosol), which is incorporated into the reaction bath containing a reducing solution.
2. Materials and methods 2.1. Materials Gold (III) chloride hydrate (HAuCl4 ·H2 O, Aldrich, ≥49% Au), octadecylamine (C18 NH2 , Aldrich, ≥99.0%), formaldehyde (HCOH,
149
Fig. 1. Scheme of the equipment used for synthesis of AuNPs via electrospray.
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Fig. 2. The image of colloid tubes and UV–vis spectra of Colloid 1 and Colloid 2, with the maxima of the absorption bands recorded at 527 nm and 521 nm, respectively.
Table 1 The parameters of electrospraying used in synthesis of AuNPs (Colloid 1). Mode of electrospray
V [kV]
F [ml h−1 ]
ød [mm]
d [mm]
t [min]
Multi-jet
8
2.00
0.3
5
35
V – voltage; F – flow rate; ød – nozzle inner diameter; d – distance between nozzle and reaction bath; t – time of the process.
The parameters of the electrospraying process (voltage, flow rate, nozzle inner diameter, distance between nozzle and reaction bath and time of the process), used in synthesis of AuNPs via chemical reduction method, are presented in Table 1.
detector located under the sample allows detection of electrons passing through the sample as in the case of transmission electron microscopy (TEM). Samples for STEM investigations were prepared as follows: 4 l of the colloid were deposited onto carbon-coated copper grids (300 mesh). The suspension was left for 1 h for solvent evaporation. Then, the samples were cleaned under vacuum (60 min) to remove the excess of the stabilizer and other postsynthesis products. The ImageJ software (Wayne Rasband, National Institute of Health USA) was used for the measurements of AuNPs, the size and the size distribution histogram was obtained based on the selection of at least 2000 NPs. 3. Results and discussion
2.2.2. Method 2 (Colloid 2) The synthesis of AuNPs was also carried out by incorporation of the precursor solution directly into the reaction bath through a capillary tube connected to a syringe (the inner capillary diameter was 0.3 mm). The end of the capillary was immersed in the reaction bath and the precursor solution was added at the constant flow rate of 2 ml h−1 (the same as in the case of electrospray technique), by a syringe pump. 2.3. AuNPs characterization AuNPs were characterized using the UV–vis spectroscopy, dynamic light scattering (DLS) and scanning transmission electron microscopy (STEM) techniques. UV–vis spectra of AuNPs were recorded using the Ocean Optics DH-2000-S (deuterium and tungsten source, 210–1700 nm). The USB2000+ detector (a miniature fiber optic spectrometer) was used to collect the UV–vis spectra. The NPs hydrodynamic diameter and the agglomeration state of AuNPs colloids were measured using the DLS technique (Nano ZS Zetasizer system, Malvern Instruments, laser wavelength of 633 nm (He–Ne), scattering angle 173◦ , measurement temperature 25 ◦ C, medium viscosity 0.98 mPa s, medium refractive index 1.4262). All measurements were performed in quartz microcuvette. Both colloids were filtered before measurements (0.2 m polyvinylidene fluoride membrane) to remove the residues of amine crystals, formed after the synthesis process. The shape, size and size distribution of NPs were investigated using scanning electron microscopy (SEM, Nova NanoSEM 450, FEI, accelerating voltage of 30 kV) equipped with a detector for scanning transmission electron microscopy (STEM II). The STEM
The colloids synthesized via Method 1 and 2 had a red color, characteristic for AuNPs (Fig. 2). The maxima of the UV–vis absorption bands for Colloid 1 and Colloid 2 were recorded at 527 nm and 521 nm, respectively (Fig. 2). These values are within the range attributed to AuNPs smaller than 30 nm [35]. The agglomeration state of colloids was investigated using the DLS technique (Fig. 3). The hydrodynamic sizes of AuNPs were 12 nm ± 2 nm and 15 nm ± 6 nm for Colloid 1 and Colloid 2, respectively. The polydispersity index (PdI) was 0.096 and 0.210 for Colloid 1 and Colloid 2, respectively (PdI given represents the size ranges present in the solution, is calculated from the statistics of the distribution and is expressed by the square of the width/mean). The size and shape of AuNPs in Colloid 1 and Colloid 2 were investigated using STEM technique (Fig. 4). AuNPs synthesized via Method 1 (Colloid 1) are spherical and monodisperse with the mean particle size of 5.7 ± 0.5 nm. In case of Method 2 (Colloid 2), spherical NPs were also obtained, but with less homogeneity in size. Both smaller (4.4 ± 0.6 nm) and larger (10.9 ± 1.4 nm) particle populations are present in Colloid 2. These results correlate with the changes in the positions of the UV–vis absorption band maxima of AuNPs (Fig. 2): 521 nm in case of Colloid 2, compared to 527 nm in Colloid 1. The presence of small NPs causes the shift of the absorption band maximum to the shorter wavelengths in case of Colloid 2. The overall results obtained for colloids synthesized via methods 1 and 2 are presented in Table 2. Such results indicate that the method of precursor addition to the reaction bath highly influences the size of AuNPs, synthesized via chemical reduction method in cyclohexane. All other process parameters such as flow rate, inner diameter of the needle, and time of the reaction were kept the same in both methods, thus
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Fig. 3. The size distribution histograms of AuNPs: Colloid 1 (a) and Colloid 2 (b).
Fig. 4. STEM images with corresponding size distribution histograms of AuNPs: Colloid 1 (a) and Colloid 2 (b).
Fig. 5. Scheme of the AuNPs synthesis via Method 2 (precursor solution incorporated by continuous stream flow, using capillary): incorporation of precursor solution into the reaction bath (a); local increase in the reagent concentration results in reaching of the critical supersaturation level and generation of nuclei centers (b); growth of already existing nuclei and formation of new crystal nuclei as a result of multiple local reaching of the critical supersaturation level (c); obtaining of polydisperse colloid (d).
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Fig. 6. Scheme of the AuNPs synthesis via Method 1 (aerosol of precursor solution incorporated by continuous fine droplet flow using electrospray): homogenous dispersion of precursor solution (aerosol) by electrostatic forces to the large surface area to the reaction bath (a); reaching of the critical supersaturation level and generation of nuclei centers (b); growth of already generated nuclei centers (c); obtaining of monodisperse colloid (d).
Table 2 The comparison of the results obtained for Colloid 1 and Colloid 2. UV–vis
DLS
max [nm]
Hydrodynamic size [nm]
PdI
Size [nm]
Colloid 1
527
12 ±2
0.096
5.7 ± 0.5
Colloid 2
521
15 ±6
0.210
4.4 ± 0.6; 10.9 ± 1.4
STEM
one would expect very similar results for in both cases. In the case of capillary flow the precursor solution is incorporated into the reaction bath by continuous stream, hence the local concentration of reagents near the capillary nozzle can be higher than in the other areas of the reaction bath (despite continuous stirring of the solution) (Fig. 5a). This in turn may result in reaching locally the critical solution supersaturation level and generation of nuclei centers (Fig. 5b). The critical supersaturation level can be reached multiple times in case of continuous precursor incorporation through the capillary. Further addition of the precursor leads, first, to growth of already existing nuclei and, second, to the formation of new crystal nuclei (Fig. 5c). This in turn results in formation of both populations of smaller and larger NPs and consequently in obtaining a polydisperse colloid (Fig. 5d). In case of the electrospray technique the precursor solution is dispersed into very small droplets (aerosol) by electrostatic forces and consequently incorporated into the reaction bath by continuous fine droplet flow. Thus, only very small and controlled volume of precursor solution is added into the reaction bath (Fig. 6a). Moreover, the precursor aerosol is dispersed homogeneously into a larger volume of reaction solution, compared to capillary method, therefore there is no local increase in reagent concentration. Thus, the nucleation process occurs only once in the entire volume of the solution after critical supersaturation level is reached (Fig. 6b). Further incorporation of the precursor results in growth of already formed nuclei and formation of NPs (Fig. 6c) as the diffusion process of reduced atoms to the metallic clusters is faster than the formation of new nucleation centers, and as a consequence monodisperse NPs are formed (Fig. 6d).
4. Conclusions In this paper we presented the chemical reduction method of AuNPs synthesis in non-polar solvent (cyclohexane). The homogeneity in size (polydispersity) of AuNPs synthesized via two methods of reagent incorporation into the reaction bath was compared. First method employed continuous fine droplet flow using electrospray technique, second method employed a continuous stream flow through a capillary. The results clearly indicate that the size of NPs synthesized via chemical reduction method in non-polar solvent depends on the method of reactant dispersion. The incorporation of reagents continuously to the reaction bath results in polydisperse colloid with the NPs size of about: 4.4 ± 0.6 and 10.9 ± 1.4, while fine droplet flow by electrospray provided monodisperse colloid with the NPs size of about 5.7 ± 0.5. The electrospray technique turned out to be an effective method of incorporation of small and fully controlled amounts of reagents into the reaction bath. Moreover, the presented method of AuNPs synthesis allows to obtain highly monodisperse NPs directly in non-polar solvent, suitable for electronic applications.
Acknowledgements This work was supported by FP7-NMP-2010-SMALL-4 program (‘Hybrid organic/inorganic memory elements for integration of electronic and photonic circuitry’ – HYMEC), project number 263073. Scientific work was supported by the Polish Ministry of Science and Higher Education, funds for science in 2011–2014 allocated for the co-funded international project.
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of monodisperse gold nanoparticles via electrospray-assisted chemical reduction method in cyclohexane Katarzyna Soliwoda, Marcin Rosowski, Emilia Tomaszewska, Beata Tkacz-Szczesna, Grzegorz Celichowski, Maciej Psarski, Jaroslaw Grobelny ∗ Department of Materials Technology and Chemistry, Faculty of Chemistry, University of Lodz, Pomorska 163, 90-236 Lodz, Poland
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
• Gold
nanoparticles synthesized via electrospray-assisted chemical reduction method directly in non-polar solvent. • The mechanism of gold nanoparticles formation discussed. • Two methods of reagents incorporation to the reaction bath compared. • Highly monodisperse gold nanoparticles in cyclohexane obtained.
a r t i c l e
i n f o
Article history: Received 19 January 2015 Received in revised form 9 April 2015 Accepted 22 April 2015 Available online 5 May 2015 Keywords: Monodisperse gold nanoparticles Organic colloids Chemical reduction Electrospray
a b s t r a c t We present a fast and versatile method of monodisperse gold nanoparticles (AuNPs) synthesis, directly in non-polar solvent at room temperature, by electrospray-assisted chemical reduction method. The electrospray technique was used to spray a dispersed aerosol of gold precursor [gold (III) chloride hydrate] into the reaction bath containing reducing agent (octadecylaminomethanol) and non-polar solvent (cyclohexane). To find the best homogeneity in the size and low polydispersity of synthesized AuNPs, two methods of reagent incorporation into the reaction bath were compared: incorporation by continuous stream flow using a capillary, and by continuous fine droplet flow using electrospray technique. The obtained AuNPs were characterized by UV–vis spectroscopy, dynamic light scattering (DLS) and scanning transmission electron microscopy (STEM). Results confirmed the formation of AuNPs in both cases and showed that the size of NPs synthesized via chemical reduction method in non-polar solvent depends on the method of reagents incorporation. The continuous incorporation of precursor resulted in polydisperse colloid with NPs size of about: 4.4 ± 0.6 nm and 10.9 ± 1.4 nm, whereas incorporation of fine droplets to the reaction bath resulted in a highly monodisperse colloid with the NPs size of about 5.7 ± 0.5 nm. © 2015 Elsevier B.V. All rights reserved.
1. Introduction ∗ Corresponding author. Tel.: +48 42 6355837; fax: +48 42 6355832. E-mail address: [email protected] (J. Grobelny). http://dx.doi.org/10.1016/j.colsurfa.2015.04.040 0927-7757/© 2015 Elsevier B.V. All rights reserved.
Nanoparticles (NPs) as objects with unique properties, determined by their nanoscale size, attract great attention in catalysis
K. Soliwoda et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 148–153
[1,2], optics [3], electronics [4,5], as well as in biology [6,7] and tribology [8,9]. Because of unusual optical, electrical and catalytic properties of NPs, they can be used to modify the physical and chemical properties of different materials [8–11]. Expected results can be obtained even by application of polydisperse NPs, whereas for some specific applications highly monodisperse NPs, with defined size and uniform shape, have to be applied. Fine control over physical, chemical and structural properties of NPs is crucial for expanding their applicability, especially in optoelectronics, e.g., in organic–inorganic hydride materials like transistors [4,5] or memory elements [12–14]. The incorporation of metallic NPs in polymeric semiconductors (such as poly(3-hexylthiophene), poly(2-methoxy-5-(2 ethl-hexyloxy)1,4-phenylenevinlylene)) is one of the methods to modify polymer physical and chemical properties. It is already known that the electrical conductance between two electrodes in the device depends on the charge stored or trapped on NPs [4,5,14]. The charge trapping mechanism is affected by NPs size [4,5], thus the quality of NPs incorporated in these materials – their size monodispersity and shape uniformity – is crucial to their performance. Moreover, the changes in the field-effect mobility depend mainly on the NPs size [5,15]. Hence, to fully exploit the unique properties of NPs, it is extremely important to elaborate reproducible preparation methods of highly monodisperse NPs with defined and fully controlled shape and size. Nowadays, extensive studies are still being carried out on controlling NPs parameters in various synthesis methods, in both polar [16–19] and non-polar solvents [20,21]. The size of NPs can be controlled by adjusting the synthesis conditions, e.g., solvent composition [22], reactant types [23] and their concentrations [24], temperature of the reaction [25], etc. Another parameter affecting NPs homogeneity is the method of reactants addition into the reaction bath: continuously, progressively, drop by drop, with a constant or variable flow rate, etc. One of the methods that can be used for addition of the precursor solution into the reaction bath is the electrospray technique [26–28]. The electrospray technique is a method of aerosol formation by electric field [29–31]. The droplet size generated via electrospray ranges from hundreds of micrometers to tens of nanometers [30,31]. The droplets are highly charged and, because of the repulsive forces, the coagulation process between droplets during the electrospray does not occur. Electrospray technique can be used to atomize the precursor solution in the chemical reduction method, to obtain higher precursor dispersion in the reaction bath [26–28]. The precursor droplet size, its velocity and acceleration can be controlled by adjusting of the electrospray parameters, such as flow rate, bias voltage, distance between needle and sample, inner diameter of the needle, etc. [32,33]. In this study, we present a procedure of synthesis of highly monodisperse AuNPs, directly in non-polar solvent (cyclohexane), via electrospray-assisted chemical reduction method. The presented method consists of two steps: first, the generation of precursor solution aerosol under the electric field, and second, the chemical reduction of dispersed gold ions with carbinolamine in the reaction bath. Spherical AuNPs with defined size and very narrow size distribution are obtained in cyclohexane and can be directly used in hybrid organic-inorganic materials, e.g., in polymeric thinfilm transistors or memory elements.
Lach-Ner, 36–38%), octadecylaminomethanol (ODAM) prepared based on the procedure described by Chen and Wang [34], isopropanol (POCH, 99%) and cyclohexane (POCH, 99%) were used without further purification. 2.2. Synthesis of AuNPs AuNPs were synthesized by gold (III) chloride hydrate reduction with octadecylaminomethanol (ODAM) in cyclohexane. ODAM was used as both reductive and stabilizing agent of NPs. Precursor solution was prepared by dissolving HAuCl4 ·H2 O (9.5 × 10−3 g) in isopropanol (5.500 g). The reductive solution was prepared based on the procedure described by Chen and Wang [34], but with octadecylamine instead of dodecylamine. The procedure of reductive solution preparation was as follows: octadecylamine (1.077 g) was dissolved in the mixture of formaldehyde (6.438 g, Lach-Ner, 36–38%) and cyclohexane (16.348 g) and vigorously stirred for 10 min at room temperature. Next, the cyclohexane phase containing octadecylaminomethanol was separated out and placed in the reaction bath. Precursor solution was incorporated using two methods: by fine droplets generated via electrospray (Method 1) and continuously, through a capillary (Method 2). During the addition of the precursor, the solution in the reaction bath was stirred at the same rate in both cases. The use of the electrospray and capillary for incorporation of the precursor solution into the reaction bath allows to examine the influence of the method of reagent addition on the shape, size and size distribution of synthesized AuNPs. The NPs weight concentration of gold in both cases was about 60 ppm. 2.2.1. Method 1 (Colloid 1) Electrospray was the first method used for incorporation of precursor solution to the reaction bath. The process consists in simultaneous charged droplet generation by means of electric field. A typical system for electrospray comprises a high voltage power supply, a glass syringe, a syringe pump, a hose with a nozzle, a ring as a counter and a reaction bath. A scheme of home-made equipment used for electrospray in this study is presented in Fig. 1. In the electrospraying process the precursor solution is pumped at a constant flow rate to the reaction bath, by a syringe pump, through an electrified capillary. High voltage is applied between the stainless nozzle and the platinum ring, which is dipped in the reaction bath. The electric field between the needle and the platinum ring causes the atomization of the precursor solution into small droplets and formation of a spray (aerosol), which is incorporated into the reaction bath containing a reducing solution.
2. Materials and methods 2.1. Materials Gold (III) chloride hydrate (HAuCl4 ·H2 O, Aldrich, ≥49% Au), octadecylamine (C18 NH2 , Aldrich, ≥99.0%), formaldehyde (HCOH,
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Fig. 1. Scheme of the equipment used for synthesis of AuNPs via electrospray.
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Fig. 2. The image of colloid tubes and UV–vis spectra of Colloid 1 and Colloid 2, with the maxima of the absorption bands recorded at 527 nm and 521 nm, respectively.
Table 1 The parameters of electrospraying used in synthesis of AuNPs (Colloid 1). Mode of electrospray
V [kV]
F [ml h−1 ]
ød [mm]
d [mm]
t [min]
Multi-jet
8
2.00
0.3
5
35
V – voltage; F – flow rate; ød – nozzle inner diameter; d – distance between nozzle and reaction bath; t – time of the process.
The parameters of the electrospraying process (voltage, flow rate, nozzle inner diameter, distance between nozzle and reaction bath and time of the process), used in synthesis of AuNPs via chemical reduction method, are presented in Table 1.
detector located under the sample allows detection of electrons passing through the sample as in the case of transmission electron microscopy (TEM). Samples for STEM investigations were prepared as follows: 4 l of the colloid were deposited onto carbon-coated copper grids (300 mesh). The suspension was left for 1 h for solvent evaporation. Then, the samples were cleaned under vacuum (60 min) to remove the excess of the stabilizer and other postsynthesis products. The ImageJ software (Wayne Rasband, National Institute of Health USA) was used for the measurements of AuNPs, the size and the size distribution histogram was obtained based on the selection of at least 2000 NPs. 3. Results and discussion
2.2.2. Method 2 (Colloid 2) The synthesis of AuNPs was also carried out by incorporation of the precursor solution directly into the reaction bath through a capillary tube connected to a syringe (the inner capillary diameter was 0.3 mm). The end of the capillary was immersed in the reaction bath and the precursor solution was added at the constant flow rate of 2 ml h−1 (the same as in the case of electrospray technique), by a syringe pump. 2.3. AuNPs characterization AuNPs were characterized using the UV–vis spectroscopy, dynamic light scattering (DLS) and scanning transmission electron microscopy (STEM) techniques. UV–vis spectra of AuNPs were recorded using the Ocean Optics DH-2000-S (deuterium and tungsten source, 210–1700 nm). The USB2000+ detector (a miniature fiber optic spectrometer) was used to collect the UV–vis spectra. The NPs hydrodynamic diameter and the agglomeration state of AuNPs colloids were measured using the DLS technique (Nano ZS Zetasizer system, Malvern Instruments, laser wavelength of 633 nm (He–Ne), scattering angle 173◦ , measurement temperature 25 ◦ C, medium viscosity 0.98 mPa s, medium refractive index 1.4262). All measurements were performed in quartz microcuvette. Both colloids were filtered before measurements (0.2 m polyvinylidene fluoride membrane) to remove the residues of amine crystals, formed after the synthesis process. The shape, size and size distribution of NPs were investigated using scanning electron microscopy (SEM, Nova NanoSEM 450, FEI, accelerating voltage of 30 kV) equipped with a detector for scanning transmission electron microscopy (STEM II). The STEM
The colloids synthesized via Method 1 and 2 had a red color, characteristic for AuNPs (Fig. 2). The maxima of the UV–vis absorption bands for Colloid 1 and Colloid 2 were recorded at 527 nm and 521 nm, respectively (Fig. 2). These values are within the range attributed to AuNPs smaller than 30 nm [35]. The agglomeration state of colloids was investigated using the DLS technique (Fig. 3). The hydrodynamic sizes of AuNPs were 12 nm ± 2 nm and 15 nm ± 6 nm for Colloid 1 and Colloid 2, respectively. The polydispersity index (PdI) was 0.096 and 0.210 for Colloid 1 and Colloid 2, respectively (PdI given represents the size ranges present in the solution, is calculated from the statistics of the distribution and is expressed by the square of the width/mean). The size and shape of AuNPs in Colloid 1 and Colloid 2 were investigated using STEM technique (Fig. 4). AuNPs synthesized via Method 1 (Colloid 1) are spherical and monodisperse with the mean particle size of 5.7 ± 0.5 nm. In case of Method 2 (Colloid 2), spherical NPs were also obtained, but with less homogeneity in size. Both smaller (4.4 ± 0.6 nm) and larger (10.9 ± 1.4 nm) particle populations are present in Colloid 2. These results correlate with the changes in the positions of the UV–vis absorption band maxima of AuNPs (Fig. 2): 521 nm in case of Colloid 2, compared to 527 nm in Colloid 1. The presence of small NPs causes the shift of the absorption band maximum to the shorter wavelengths in case of Colloid 2. The overall results obtained for colloids synthesized via methods 1 and 2 are presented in Table 2. Such results indicate that the method of precursor addition to the reaction bath highly influences the size of AuNPs, synthesized via chemical reduction method in cyclohexane. All other process parameters such as flow rate, inner diameter of the needle, and time of the reaction were kept the same in both methods, thus
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Fig. 3. The size distribution histograms of AuNPs: Colloid 1 (a) and Colloid 2 (b).
Fig. 4. STEM images with corresponding size distribution histograms of AuNPs: Colloid 1 (a) and Colloid 2 (b).
Fig. 5. Scheme of the AuNPs synthesis via Method 2 (precursor solution incorporated by continuous stream flow, using capillary): incorporation of precursor solution into the reaction bath (a); local increase in the reagent concentration results in reaching of the critical supersaturation level and generation of nuclei centers (b); growth of already existing nuclei and formation of new crystal nuclei as a result of multiple local reaching of the critical supersaturation level (c); obtaining of polydisperse colloid (d).
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Fig. 6. Scheme of the AuNPs synthesis via Method 1 (aerosol of precursor solution incorporated by continuous fine droplet flow using electrospray): homogenous dispersion of precursor solution (aerosol) by electrostatic forces to the large surface area to the reaction bath (a); reaching of the critical supersaturation level and generation of nuclei centers (b); growth of already generated nuclei centers (c); obtaining of monodisperse colloid (d).
Table 2 The comparison of the results obtained for Colloid 1 and Colloid 2. UV–vis
DLS
max [nm]
Hydrodynamic size [nm]
PdI
Size [nm]
Colloid 1
527
12 ±2
0.096
5.7 ± 0.5
Colloid 2
521
15 ±6
0.210
4.4 ± 0.6; 10.9 ± 1.4
STEM
one would expect very similar results for in both cases. In the case of capillary flow the precursor solution is incorporated into the reaction bath by continuous stream, hence the local concentration of reagents near the capillary nozzle can be higher than in the other areas of the reaction bath (despite continuous stirring of the solution) (Fig. 5a). This in turn may result in reaching locally the critical solution supersaturation level and generation of nuclei centers (Fig. 5b). The critical supersaturation level can be reached multiple times in case of continuous precursor incorporation through the capillary. Further addition of the precursor leads, first, to growth of already existing nuclei and, second, to the formation of new crystal nuclei (Fig. 5c). This in turn results in formation of both populations of smaller and larger NPs and consequently in obtaining a polydisperse colloid (Fig. 5d). In case of the electrospray technique the precursor solution is dispersed into very small droplets (aerosol) by electrostatic forces and consequently incorporated into the reaction bath by continuous fine droplet flow. Thus, only very small and controlled volume of precursor solution is added into the reaction bath (Fig. 6a). Moreover, the precursor aerosol is dispersed homogeneously into a larger volume of reaction solution, compared to capillary method, therefore there is no local increase in reagent concentration. Thus, the nucleation process occurs only once in the entire volume of the solution after critical supersaturation level is reached (Fig. 6b). Further incorporation of the precursor results in growth of already formed nuclei and formation of NPs (Fig. 6c) as the diffusion process of reduced atoms to the metallic clusters is faster than the formation of new nucleation centers, and as a consequence monodisperse NPs are formed (Fig. 6d).
4. Conclusions In this paper we presented the chemical reduction method of AuNPs synthesis in non-polar solvent (cyclohexane). The homogeneity in size (polydispersity) of AuNPs synthesized via two methods of reagent incorporation into the reaction bath was compared. First method employed continuous fine droplet flow using electrospray technique, second method employed a continuous stream flow through a capillary. The results clearly indicate that the size of NPs synthesized via chemical reduction method in non-polar solvent depends on the method of reactant dispersion. The incorporation of reagents continuously to the reaction bath results in polydisperse colloid with the NPs size of about: 4.4 ± 0.6 and 10.9 ± 1.4, while fine droplet flow by electrospray provided monodisperse colloid with the NPs size of about 5.7 ± 0.5. The electrospray technique turned out to be an effective method of incorporation of small and fully controlled amounts of reagents into the reaction bath. Moreover, the presented method of AuNPs synthesis allows to obtain highly monodisperse NPs directly in non-polar solvent, suitable for electronic applications.
Acknowledgements This work was supported by FP7-NMP-2010-SMALL-4 program (‘Hybrid organic/inorganic memory elements for integration of electronic and photonic circuitry’ – HYMEC), project number 263073. Scientific work was supported by the Polish Ministry of Science and Higher Education, funds for science in 2011–2014 allocated for the co-funded international project.
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