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Preparation of silver nanopowders by a controlled wet-chemical synthesis
Materials Letters 173 (2016) 39–42
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of silver nanopowders by a controlled wet-chemical synthesis Lihui Wang a, Jingming Zhong a, Guolong Li a,b,c,n, Jian-Feng Chen c a
Northwest Rare Metal Material Reasearch Institute, Shizuishan City, Ningxia 753000, China Key Laboratory of Ningxia for Photovoltaic Materials, Ningxia University, Yin-Chuan, Ningxia 750021, China c College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 14 December 2015 Received in revised form 27 February 2016 Accepted 4 March 2016 Available online 5 March 2016
The synthesis of silver nanopowders prepared by the method of controlled wet-chemical reduction is reported. The effects of mixture ventilation of NO2 and O2 in process of silver nanopowders are investigated. It is found that the morphology and tap density of silver nanopowders can be influenced by the ventilation volume in the process. The ventilation effects are discussed on the basis of the aggregative growth theory. It is reasonable to speculate that ventilation has two effects on the silver nanopowders. On the one hand, ventilation chemically promotes the incorporation of silver nuclei, on another hand, the shearing action of ventilated gases physically improves the final tap density of nanopowders. & 2016 Elsevier B.V. All rights reserved.
Keywords: Silver nanoparticles Morphology Ventilation Tap density Shearing Aggregative growth theory
1. Introduction Silver nanopowders with ultra-fine size and well-distribution are widely used in the electronics industry due to their abilities of moving freely [1]. As is reported in various studies, electronic properties of these silver nanopowders are strongly dependent on their shapes, size, crystallization, composition and surface modification [2]. In this regard, nano-sized silver powders have attracted a great deal of attention in recent years. Generally, silver nanopowders can be prepared by numerous methods such as sonochemical deposition [3], spray pyrolysis [4], electron beam evaporation [5], photochemical reduction [6,7], and thermal plasma [8]. Compared with other methods, chemical reduction method usually does not require special equipment or rigorous conditions [9,10]. From a practical point of view, chemical reduction method is most preferable for obtaining good quality silver nanopowders [11,12]. Besides these, shapes and crystalline of silver powders can be well-controlled by altering the concentration of the reactants and constituents of the solutions [13]. In this study, we develop a simple wet-chemical synthesis route for preparing spherical silver nanopowders with high tap density. With ventilation of NO2 and O2 gas, large scale silver nanopowders are obtained from a reduction of silver nitrate with n Corresponding author at: Northwest Rare Metal Material Reasearch Institute, Shizuishan City, Ningxia 753000, China. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.matlet.2016.03.013 0167-577X/& 2016 Elsevier B.V. All rights reserved.
ascorbic aqueous solution at room temperature. Moreover, it is interesting to find that morphology of spherical silver nanopowders can be easily controlled by altering the mixture ventilation of NO2 and O2 .
2. Experimental To obtain non-agglomerated and high-tap-density silver nanopowders, the wet-chemical reduction in aqueous solutions of silver nitrate (SCRC 99.8% pure) is used. Ascorbic is widely used as a middle reducing agent, and gelatin aqueous is chosen as a dispersant in this work. Silver nanopowders are prepared by a controlled chemical reduction method, as presented in Eq. (1).
2AgNO3 + C6 H8 O6 = 2Ag + C6 H6 O6 + 2HNO3
(1)
4NO2 + O2 + 2H2 O = 4HNO3
(2)
The solution is stirred at 200 rpm in 30 min. The mixture of NO2 and O2 (volume ratio, 6:4) is simultaneously ventilated into the solution, which leads to an oxidation reduction reaction in solution as Eq. (2). The products are centrifuged at speed of 4000 r min 1 and dried in vacuum oven. Direct observation of the silver termination is made by X-ray diffraction (XRD, Rigaku, D/ MAX-RB) with a scanning rate of 0.02° s 1 in 2θ ranging from 30° to 80°. The crystal structure is investigated by scanning electron microscopy (JEOL, JSM-6060) and transmission electron
40
L. Wang et al. / Materials Letters 173 (2016) 39–42
microscopy (Hitachi, HT7700). Particle size is measured by laser diffraction particle size analyzer (Mastersizer, 3000). And the tap density is measured by a heap densitometer (Goodwill, JV2000).
3. Results and discussion All dispersions are prepared by mixing aqueous solutions containing silver nitrate and gelatin aqueous with ascorbic acid solutions. By altering the mixture ventilation in the process, particles are formed in different shapes and sizes. The so-obtained silver nanoparticles are aggregated into the structures in Fig. 1. In Fig. 1, SEM and TEM of the silver nanopowders synthesized in different ventilation volumes are illustrated respectively. Using actually the same conditions, but increasing the mixture ventilation, the silver particles tend to separate self-evidently and
become spherical, and the tap density of silver powders monotonically increases from 4.2 g/cm3 to 4.8 g/cm3. Meanwhile, the particle size begins to increase then decreases and returns to increase again. It indicates the silver morphology and size are relevant to the ventilation volume, and they can be controlled by modifying the aggregation degree of silver nanoparticles. The particles described in this study shows X-ray patterns characteristic of silver, although the degree of crystallinity differs, as illustrated in Fig. 2. Fig. 2 shows the XRD patterns of the silver nanopowders prepared only in same conditions but varying the ventilation volume. Characters of a, b, c, d, e, f and h indicates the ventilation volume of 5, 8, 10, 12, 16, 18 and 20 mL respectively. From the pattern, the diffractogram exhibits the crystalline characteristic peaks of (111), (200), (220) and (311) crystal planes at 37.81°, 43.93°, 64.21° and 77.14° respectively. According to Debye–Scherre formula [14], the
Fig. 1. SEM of different silver nanopowders synthesized with the mixture ventilation volumes of (a) 8 mL, (b) 10 mL, (c) 16 mL, (d) 20 mL and (e) TEM of silver nanopowders.
41
(311)
(220)
(200)
a b c
(111)
Intensity (a.u.)
L. Wang et al. / Materials Letters 173 (2016) 39–42
d e f g h 30
Fig. 4. Schematic diagram of different mechanisms for silver nanopowders formation.
βij = 6πDij rij
40
50
60
70
80
2 (Degree) Fig. 2. XRD pattern from silver powder prepared with different ventilation volume.
Fig. 3. Grain and nanoparticle size (D50) distributions of silver powders.
calculated average sizes of the grains are 9.5, 17.5, 23.5, 18.1, 16.1, 16.5, 12.9, 11.9 nm, which are consistent with the measured data from a laser diffraction particle size analyzer, as illustrated in Fig. 3. As is observed in Fig. 3, the average grain size varies gradually from 9.3 to 23.5 nm by increasing the ventilation volume from 8 mL to 20 mL, and arrives at the maximum at 10 mL. In contrast, the nanoparticle size fluctuates from 80 nm to 210 nm alternately, which indicates the growth of silver particles sensitive to the mixture ventilation. As is well-known, silver ions can be completely converted into nanoparticles by ascobic acid as a valid reducing agent even in concentrated solutions where the amount of nitric acid, resulting from the above reaction is quite large. However, at sufficiently high concentration of nitric acid, the newly generated silver nanoparticles will re-dissolve, which is a critical factor that decreases the nanoparticle size at the beginning of ventilation. Consequently, the aggregation will not take place unless the surface of silver nanoparticle is protected to a certain extent. It has now been reported that the aggregation mechanism is dominant in yielding silver colloid particles [15]. Silver nanoparticle formation mechanism is schematically shown in Fig. 4 [16]. As given in the Aggregative Growth Theory by Hyning et al. [17], the binary aggregation rate for the aggregation of particles of size i and j is
(3)
where Dij is the relative diffusion coefficient, and rij is the radius of interaction of particles i and j . The monomeric particles accounts for the particle growth, and the final grain and nanoparticle size distributions are determined by the binary aggregation rate βij , which is interpreted in terms of pair potentials involving a synthesis of van der Waals attraction and electrostatic repulsion [17]. The continuous ventilation decreases pair potential of silver nanoparticles, which results in the rise of βij . Consequently, the aggregation proceeds along preferential directions combined with a relatively slow generation of silver nuclei in the system, and allows for the incorporation of these nuclei into the larger, anisotropic and crystal-like aspherical grains and nanoparticles. As the ventilation continues, the vast majority of silver nuclei is reduced, and the sizes of grain and nanoparticle decrease as illustrated in Fig. 3. This also suggests that subsequent growth is dominated by aggregation and coalescence. Meanwhile, the influence of shearing action from the ventilation gas on the radius of interaction solution can not be neglected. When the ventilation amounts to 18 mL, rij increases significantly, which means the aggregation probability of adjacent crystal grains is enhanced, and as a result, the nanoparticle size increases. Moreover, the physical shearing action can also play a role in determining the final tap density of silver nanopowders. Due to the increasingly shearing action, silver nanoparticles are easily to form regularly and spherically, which improves the final tap density. Therefore, an effective way to promote the tap density of silver nanopowders in wet-chemical synthesis is to control strictly the ventilation volume, optimize the aggregative growth process and improve the uniformity and shapes of silver nanopowders.
4. Conclusion In conclusion, a controlled reduction process by the mixture ventilation has influences on the morphology and size of silver nanopowders. On the one hand, the ventilation decreases pair potential of silver nanoparticles, thus chemically promotes the incorporation of silver nuclei. On another hand, the physical shearing action enhances the aggregation probability of adjacent crystal grains, increases the nanoparticle size and improves the final tap density of nanopowders. As a result, shapes and sizes of silver nanopowders can be further well-controlled by varying the ventilation volume.
Acknowledgments This work was financially supported by the Natural Science Foundation of Ningxia Province, China (Grant no. NZ15036).
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L. Wang et al. / Materials Letters 173 (2016) 39–42
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.03. 013.
References [1] J.L.H. Chau, M.K. Hsu, C.C. Hsieh, et al., Microwave plasma synthesis of silver nanopowders, Mater. Lett. 59 (2005) 905–908. [2] M. Auffan, J. Rose, J.Y. Bottero, et al., Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective, Nat. Nanotechnol. 4 (2009) 634–641. [3] V.G. Pol, D.N. Srivastava, O. Palchik, et al., Sonochemical deposition of silver nanoparticles on silica spheres, Langmuir 18 (2002) 3352–3357. [4] K.H. Lee, S.C. Rah, S.G. Kim, Formation of monodisperse silver nanoparticles in poly (vinylpyrrollidone) matrix using spray pyrolysis, J. Sol–Gel Sci. Technol. 45 (2008) 187–193. [5] A.I. Korchagin, N.K. Kuksanov, A.V. Lavrukhin, et al., Production of silver nanopowders by electron beam evaporation, Vacuum 77 (2005) 485–491. [6] J.M. Gorham, R.I. MacCuspie, K.L. Klein, et al., UV-induced photochemical transformations of citrate-capped silver nanoparticle suspensions, J. Nanopart. Res. 14 (2012) 1–16. [7] H. Li, W. Lu, J. Tian, et al., Synthesis and study of plasmon-induced carrier behavior at Ag/TiO2 nanowires, Chem. – A Eur. J. 18 (2012) 8508–8514. [8] I. Reinhold, C.E. Hendriks, R. Eckardt, et al., Argon plasma sintering of inkjet printed silver tracks on polymer substrates, J. Mater. Chem. 19 (2009)
3384–3388. [9] J. Tian, S. Liu, Y. Zhang, et al., Environmentally friendly, one-pot synthesis of Ag nanoparticle-decorated reduced graphene oxide composites and their application to photocurrent generation, Inorg. Chem. 51 (2012) 4742–4746. [10] X. Qin, W. Lu, Y. Luo, et al., Preparation of Ag nanoparticle-decorated polypyrrole colloids and their application for H2O2 detection, Electrochem. Commun. 13 (2011) 785–787. [11] Y. Zhang, L. Wang, J. Tian, et al., Ag@ poly (m-phenylenediamine) core–shell nanoparticles for highly selective, multiplex nucleic acid detection, Langmuir 27 (2011) 2170–2175. [12] S. Liu, J. Tian, L. Wang, et al., Stable aqueous dispersion of graphene nanosheets: noncovalent functionalization by a polymeric reducing agent and their subsequent decoration with Ag nanoparticles for enzymeless hydrogen peroxide detection, Macromolecules 43 (2010) 10078–10083. [13] L. Wang, H. Li, J. Tian, et al., Monodisperse, micrometer-scale, highly crystalline, nanotextured Ag dendrites: rapid, large-scale, wet-chemical synthesis and their application as SERS substrates, ACS Appl. Mater. Interfaces 2 (2010) 2987–2991. [14] S.Y. Zhao, R. Qiao, X.L. Zhang, et al., Preparation and reversible phase transfer of CoFe2O4 nanoparticles, J. Phys. Chem. C 111 (2007) 7875–7878. [15] S.P. Wu, S.Y. Meng, Preparation of ultrafine silver powder using ascorbic acid as reducing agent and its application in MLCI, Mater. Chem. Phys. 89 (2005) 423–427. [16] L. Suber, I. Sondi, E. Matijević, et al., Preparation and the mechanisms of formation of silver particles of different morphologies in homogeneous solutions, J. Colloid Interface Sci. 288 (2005) 489–495. [17] D.L. Van Hyning, W.G. Klemperer, C.F. Zukoski, Silver nanoparticle formation: predictions and verification of the aggregative growth model, Langmuir 17 (2001) 3128–3135.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of silver nanopowders by a controlled wet-chemical synthesis Lihui Wang a, Jingming Zhong a, Guolong Li a,b,c,n, Jian-Feng Chen c a
Northwest Rare Metal Material Reasearch Institute, Shizuishan City, Ningxia 753000, China Key Laboratory of Ningxia for Photovoltaic Materials, Ningxia University, Yin-Chuan, Ningxia 750021, China c College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 14 December 2015 Received in revised form 27 February 2016 Accepted 4 March 2016 Available online 5 March 2016
The synthesis of silver nanopowders prepared by the method of controlled wet-chemical reduction is reported. The effects of mixture ventilation of NO2 and O2 in process of silver nanopowders are investigated. It is found that the morphology and tap density of silver nanopowders can be influenced by the ventilation volume in the process. The ventilation effects are discussed on the basis of the aggregative growth theory. It is reasonable to speculate that ventilation has two effects on the silver nanopowders. On the one hand, ventilation chemically promotes the incorporation of silver nuclei, on another hand, the shearing action of ventilated gases physically improves the final tap density of nanopowders. & 2016 Elsevier B.V. All rights reserved.
Keywords: Silver nanoparticles Morphology Ventilation Tap density Shearing Aggregative growth theory
1. Introduction Silver nanopowders with ultra-fine size and well-distribution are widely used in the electronics industry due to their abilities of moving freely [1]. As is reported in various studies, electronic properties of these silver nanopowders are strongly dependent on their shapes, size, crystallization, composition and surface modification [2]. In this regard, nano-sized silver powders have attracted a great deal of attention in recent years. Generally, silver nanopowders can be prepared by numerous methods such as sonochemical deposition [3], spray pyrolysis [4], electron beam evaporation [5], photochemical reduction [6,7], and thermal plasma [8]. Compared with other methods, chemical reduction method usually does not require special equipment or rigorous conditions [9,10]. From a practical point of view, chemical reduction method is most preferable for obtaining good quality silver nanopowders [11,12]. Besides these, shapes and crystalline of silver powders can be well-controlled by altering the concentration of the reactants and constituents of the solutions [13]. In this study, we develop a simple wet-chemical synthesis route for preparing spherical silver nanopowders with high tap density. With ventilation of NO2 and O2 gas, large scale silver nanopowders are obtained from a reduction of silver nitrate with n Corresponding author at: Northwest Rare Metal Material Reasearch Institute, Shizuishan City, Ningxia 753000, China. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.matlet.2016.03.013 0167-577X/& 2016 Elsevier B.V. All rights reserved.
ascorbic aqueous solution at room temperature. Moreover, it is interesting to find that morphology of spherical silver nanopowders can be easily controlled by altering the mixture ventilation of NO2 and O2 .
2. Experimental To obtain non-agglomerated and high-tap-density silver nanopowders, the wet-chemical reduction in aqueous solutions of silver nitrate (SCRC 99.8% pure) is used. Ascorbic is widely used as a middle reducing agent, and gelatin aqueous is chosen as a dispersant in this work. Silver nanopowders are prepared by a controlled chemical reduction method, as presented in Eq. (1).
2AgNO3 + C6 H8 O6 = 2Ag + C6 H6 O6 + 2HNO3
(1)
4NO2 + O2 + 2H2 O = 4HNO3
(2)
The solution is stirred at 200 rpm in 30 min. The mixture of NO2 and O2 (volume ratio, 6:4) is simultaneously ventilated into the solution, which leads to an oxidation reduction reaction in solution as Eq. (2). The products are centrifuged at speed of 4000 r min 1 and dried in vacuum oven. Direct observation of the silver termination is made by X-ray diffraction (XRD, Rigaku, D/ MAX-RB) with a scanning rate of 0.02° s 1 in 2θ ranging from 30° to 80°. The crystal structure is investigated by scanning electron microscopy (JEOL, JSM-6060) and transmission electron
40
L. Wang et al. / Materials Letters 173 (2016) 39–42
microscopy (Hitachi, HT7700). Particle size is measured by laser diffraction particle size analyzer (Mastersizer, 3000). And the tap density is measured by a heap densitometer (Goodwill, JV2000).
3. Results and discussion All dispersions are prepared by mixing aqueous solutions containing silver nitrate and gelatin aqueous with ascorbic acid solutions. By altering the mixture ventilation in the process, particles are formed in different shapes and sizes. The so-obtained silver nanoparticles are aggregated into the structures in Fig. 1. In Fig. 1, SEM and TEM of the silver nanopowders synthesized in different ventilation volumes are illustrated respectively. Using actually the same conditions, but increasing the mixture ventilation, the silver particles tend to separate self-evidently and
become spherical, and the tap density of silver powders monotonically increases from 4.2 g/cm3 to 4.8 g/cm3. Meanwhile, the particle size begins to increase then decreases and returns to increase again. It indicates the silver morphology and size are relevant to the ventilation volume, and they can be controlled by modifying the aggregation degree of silver nanoparticles. The particles described in this study shows X-ray patterns characteristic of silver, although the degree of crystallinity differs, as illustrated in Fig. 2. Fig. 2 shows the XRD patterns of the silver nanopowders prepared only in same conditions but varying the ventilation volume. Characters of a, b, c, d, e, f and h indicates the ventilation volume of 5, 8, 10, 12, 16, 18 and 20 mL respectively. From the pattern, the diffractogram exhibits the crystalline characteristic peaks of (111), (200), (220) and (311) crystal planes at 37.81°, 43.93°, 64.21° and 77.14° respectively. According to Debye–Scherre formula [14], the
Fig. 1. SEM of different silver nanopowders synthesized with the mixture ventilation volumes of (a) 8 mL, (b) 10 mL, (c) 16 mL, (d) 20 mL and (e) TEM of silver nanopowders.
41
(311)
(220)
(200)
a b c
(111)
Intensity (a.u.)
L. Wang et al. / Materials Letters 173 (2016) 39–42
d e f g h 30
Fig. 4. Schematic diagram of different mechanisms for silver nanopowders formation.
βij = 6πDij rij
40
50
60
70
80
2 (Degree) Fig. 2. XRD pattern from silver powder prepared with different ventilation volume.
Fig. 3. Grain and nanoparticle size (D50) distributions of silver powders.
calculated average sizes of the grains are 9.5, 17.5, 23.5, 18.1, 16.1, 16.5, 12.9, 11.9 nm, which are consistent with the measured data from a laser diffraction particle size analyzer, as illustrated in Fig. 3. As is observed in Fig. 3, the average grain size varies gradually from 9.3 to 23.5 nm by increasing the ventilation volume from 8 mL to 20 mL, and arrives at the maximum at 10 mL. In contrast, the nanoparticle size fluctuates from 80 nm to 210 nm alternately, which indicates the growth of silver particles sensitive to the mixture ventilation. As is well-known, silver ions can be completely converted into nanoparticles by ascobic acid as a valid reducing agent even in concentrated solutions where the amount of nitric acid, resulting from the above reaction is quite large. However, at sufficiently high concentration of nitric acid, the newly generated silver nanoparticles will re-dissolve, which is a critical factor that decreases the nanoparticle size at the beginning of ventilation. Consequently, the aggregation will not take place unless the surface of silver nanoparticle is protected to a certain extent. It has now been reported that the aggregation mechanism is dominant in yielding silver colloid particles [15]. Silver nanoparticle formation mechanism is schematically shown in Fig. 4 [16]. As given in the Aggregative Growth Theory by Hyning et al. [17], the binary aggregation rate for the aggregation of particles of size i and j is
(3)
where Dij is the relative diffusion coefficient, and rij is the radius of interaction of particles i and j . The monomeric particles accounts for the particle growth, and the final grain and nanoparticle size distributions are determined by the binary aggregation rate βij , which is interpreted in terms of pair potentials involving a synthesis of van der Waals attraction and electrostatic repulsion [17]. The continuous ventilation decreases pair potential of silver nanoparticles, which results in the rise of βij . Consequently, the aggregation proceeds along preferential directions combined with a relatively slow generation of silver nuclei in the system, and allows for the incorporation of these nuclei into the larger, anisotropic and crystal-like aspherical grains and nanoparticles. As the ventilation continues, the vast majority of silver nuclei is reduced, and the sizes of grain and nanoparticle decrease as illustrated in Fig. 3. This also suggests that subsequent growth is dominated by aggregation and coalescence. Meanwhile, the influence of shearing action from the ventilation gas on the radius of interaction solution can not be neglected. When the ventilation amounts to 18 mL, rij increases significantly, which means the aggregation probability of adjacent crystal grains is enhanced, and as a result, the nanoparticle size increases. Moreover, the physical shearing action can also play a role in determining the final tap density of silver nanopowders. Due to the increasingly shearing action, silver nanoparticles are easily to form regularly and spherically, which improves the final tap density. Therefore, an effective way to promote the tap density of silver nanopowders in wet-chemical synthesis is to control strictly the ventilation volume, optimize the aggregative growth process and improve the uniformity and shapes of silver nanopowders.
4. Conclusion In conclusion, a controlled reduction process by the mixture ventilation has influences on the morphology and size of silver nanopowders. On the one hand, the ventilation decreases pair potential of silver nanoparticles, thus chemically promotes the incorporation of silver nuclei. On another hand, the physical shearing action enhances the aggregation probability of adjacent crystal grains, increases the nanoparticle size and improves the final tap density of nanopowders. As a result, shapes and sizes of silver nanopowders can be further well-controlled by varying the ventilation volume.
Acknowledgments This work was financially supported by the Natural Science Foundation of Ningxia Province, China (Grant no. NZ15036).
42
L. Wang et al. / Materials Letters 173 (2016) 39–42
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.03. 013.
References [1] J.L.H. Chau, M.K. Hsu, C.C. Hsieh, et al., Microwave plasma synthesis of silver nanopowders, Mater. Lett. 59 (2005) 905–908. [2] M. Auffan, J. Rose, J.Y. Bottero, et al., Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective, Nat. Nanotechnol. 4 (2009) 634–641. [3] V.G. Pol, D.N. Srivastava, O. Palchik, et al., Sonochemical deposition of silver nanoparticles on silica spheres, Langmuir 18 (2002) 3352–3357. [4] K.H. Lee, S.C. Rah, S.G. Kim, Formation of monodisperse silver nanoparticles in poly (vinylpyrrollidone) matrix using spray pyrolysis, J. Sol–Gel Sci. Technol. 45 (2008) 187–193. [5] A.I. Korchagin, N.K. Kuksanov, A.V. Lavrukhin, et al., Production of silver nanopowders by electron beam evaporation, Vacuum 77 (2005) 485–491. [6] J.M. Gorham, R.I. MacCuspie, K.L. Klein, et al., UV-induced photochemical transformations of citrate-capped silver nanoparticle suspensions, J. Nanopart. Res. 14 (2012) 1–16. [7] H. Li, W. Lu, J. Tian, et al., Synthesis and study of plasmon-induced carrier behavior at Ag/TiO2 nanowires, Chem. – A Eur. J. 18 (2012) 8508–8514. [8] I. Reinhold, C.E. Hendriks, R. Eckardt, et al., Argon plasma sintering of inkjet printed silver tracks on polymer substrates, J. Mater. Chem. 19 (2009)
3384–3388. [9] J. Tian, S. Liu, Y. Zhang, et al., Environmentally friendly, one-pot synthesis of Ag nanoparticle-decorated reduced graphene oxide composites and their application to photocurrent generation, Inorg. Chem. 51 (2012) 4742–4746. [10] X. Qin, W. Lu, Y. Luo, et al., Preparation of Ag nanoparticle-decorated polypyrrole colloids and their application for H2O2 detection, Electrochem. Commun. 13 (2011) 785–787. [11] Y. Zhang, L. Wang, J. Tian, et al., Ag@ poly (m-phenylenediamine) core–shell nanoparticles for highly selective, multiplex nucleic acid detection, Langmuir 27 (2011) 2170–2175. [12] S. Liu, J. Tian, L. Wang, et al., Stable aqueous dispersion of graphene nanosheets: noncovalent functionalization by a polymeric reducing agent and their subsequent decoration with Ag nanoparticles for enzymeless hydrogen peroxide detection, Macromolecules 43 (2010) 10078–10083. [13] L. Wang, H. Li, J. Tian, et al., Monodisperse, micrometer-scale, highly crystalline, nanotextured Ag dendrites: rapid, large-scale, wet-chemical synthesis and their application as SERS substrates, ACS Appl. Mater. Interfaces 2 (2010) 2987–2991. [14] S.Y. Zhao, R. Qiao, X.L. Zhang, et al., Preparation and reversible phase transfer of CoFe2O4 nanoparticles, J. Phys. Chem. C 111 (2007) 7875–7878. [15] S.P. Wu, S.Y. Meng, Preparation of ultrafine silver powder using ascorbic acid as reducing agent and its application in MLCI, Mater. Chem. Phys. 89 (2005) 423–427. [16] L. Suber, I. Sondi, E. Matijević, et al., Preparation and the mechanisms of formation of silver particles of different morphologies in homogeneous solutions, J. Colloid Interface Sci. 288 (2005) 489–495. [17] D.L. Van Hyning, W.G. Klemperer, C.F. Zukoski, Silver nanoparticle formation: predictions and verification of the aggregative growth model, Langmuir 17 (2001) 3128–3135.