- Email: [email protected]
Ag nanoparticles and their surface-enhanced Raman scattering properties
Journal of Molecular Structure 1035 (2013) 471–475
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis of polymer-stabilized monometallic Cu and bimetallic Cu/Ag nanoparticles and their surface-enhanced Raman scattering properties Danhui Zhang a,b,⇑, Xiaoheng Liu b a b
School of Mechanical Engineering, Linyi University, Linyi 276005, China Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China
h i g h l i g h t s " Advantages of the synthetic method include production of copper nanoparticles at room temperature under no inert atmosphere. " The Ag/Cu bimetallic nanoparticles were also synthesized successfully. " The SERS of copper and silver/copper bimetallic nanoparticles were also discussed.
a r t i c l e
i n f o
Article history: Received 9 October 2012 Accepted 12 December 2012 Available online 21 December 2012 Keywords: Gelatin Copper nanoparticles Bimetallic nanoparticles SERS
a b s t r a c t The present study demonstrates a facile process for the production of spherical-shaped Cu and Ag nanoparticles synthesized and stabilized by hydrazine and gelatin, respectively. Advantages of the synthetic method include its production of water dispersible copper and copper/silver nanoparticles at room temperature under no inert atmosphere. The resulting nanoparticles (copper or copper/silver) are investigated by X-ray diffraction (XRD), UV–vis spectroscopy, and transmission electron microscopy (TEM). The nanometallic dispersions were characterized by surface plasmon absorbance measuring at 420 and 572 nm for Ag and Cu nanoparticles, respectively. Transmission electron microscopy showed the formation of nanoparticles in the range of 10 nm (silver), and 30 nm (copper). The results also demonstrate that the reducing order of Cu2+/Ag+ is important for the formation of the bimetallic nanoparticles. The surface-enhanced Raman scattering effects of copper and copper/silver nanoparticles were also displayed. It was found that the enhancement ability of copper/silver nanoparticles was little higher than the copper nanoparticles. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction There is currently immense interest in the synthesis and characterization of mono- and bimetallic nanoparticles [1]. This field presents a broad scope of research, as nanoparticles (NPs) can exhibit unusual chemical, physical and electrical properties that are not apparent in bulk materials [2], as well as possessing size-dependent properties and the potential for constructing nanoand micro assemblies [3]. Recently, considerable efforts have been devoted to bimetallic nanoparticles since they are of great interest from both scientific and technological perspective for the modification of physical and chemical properties of metal nanoparticles [4,5]. Bimetallic colloids, in which two kinds of metals are assembled in one entity, have well different catalytic, electronic and optical properties distinct from those of the corresponding monometallic nanoparticles ⇑ Corresponding author at: School of Mechanical Engineering, Linyi University, Linyi 276005, China. Tel./fax: +86 539 8766260. E-mail address: [email protected] (D. Zhang). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.12.023
[6]. Bimetallic colloids can be prepared by simultaneous co-reduction of two kinds of metal ions with or without protective agent (usually polymer or surfactant) or by successive reduction of one metal over the nuclei of another. Many methods for the preparation of bimetallic nanoparticles have been reported, such as alcohol reduction [7], citrate reduction, hydrothermal [8], sonochemical method [9], co-precipitation [10] and reverse micelles [11–15]. In general, their stability and sizes are controlled by the addition of protective agents, such as soluble polymers, surfactants, organic ligands. Moreover, the size distribution, structure and composition of the bimetallic nanoparticles have also been affected by the preparation conditions. Ag–Cu alloys were first produced by rapid quenching in 1960 [16,17]. Since then, a lot of methods have been used to prepare Ag/Cu bimetallic nanoparticles due to their bactericida, catalytic activities and their well-studied structure [18]. Gelatin is the thermally and hydrolytically denatured product of collagen, which has been extensively applied as the immobilization matrix for the preparation of biosensors [19]. It has a triple-helical structure and offers distinctive advantages such as good
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biocompatibility, nontoxicity, remarkable affinity to proteins, and excellent gel-forming ability [20]. To the best of our knowledge, there is no report on the use of gelatin as stabilizing agent to prepare Cu or Cu/Ag NPs-embedded gelatin nanocomposites. In this study, Cu or Cu/Ag NPs were prepared in the gelatin solution without using any other additional stabilizing agents. The major advantage of gelatin as a stabilizing agent is that it can be used to tailor the nanocomposite properties and also provide long-term stability of the nanoparticles by preventing particle agglomeration. Herein, we report a simple method to synthesize colloidal Cu or bimetal Cu/Ag nanoparticles by reducing copper and silver ions with hydrazine using gelatin as a stabilizing agent. The advantages of this method are that the use of atmospheric air (or no use of inert atmosphere) for the synthesis of nanoparticles. The reducing order of Cu2+/Ag+ is important for the formation of the bimetallic nanoparticles. The surface-enhanced Raman scattering effects of copper and copper/silver nanoparticles were also displayed. 2. Experimental section 2.1. Reagents Copper (II) nitrate trihydrate was obtained from Guangdong Chem. Co.; hydrazine hydrate solution (80%) and gelatin were gotten from Kermel; Silver nitrate (AgNO3) and methylene blue were obtained from Nanjing Chem. Co. All the reagents were used as received, without further purification, and the water was deionized.
on the metallic nanoparticles, the metallic substrates were mixed with 105 M of methyl blue in 10 mL water solution. After 24 h of adsorption, the sample was measured. 3. Results and discussion 3.1. The formation of monometallic nanoparticles 3.1.1. The UV–vis spectra of monometallic nanoparticles After addition of hydrazine hydrate to the AgNO3 and Cu(NO3)2 solutions led to the appearance of yellowish brown and dark red color in solutions after 10 h of reaction, indicating the formation of silver and copper nanoparticles, respectively. These colors arise due to light absorption by surface plasmons in the metal nanoparticles. Fig. 1a and b shows the UV–vis spectra recorded from the aqueous silver nitrate-gelatin and copper (II) nitrate-gelatin reaction medium, as a function of time of reaction. The silver surface plasmon resonance (SPR) band occurred at 414 nm and steadily increased in intensity as a function of time of reaction without any shift in the peak wavelength. In copper ion reduction, the SPR band occurred at 572 nm [21]. The gelatin mediated syntheses of metal nanoparticles (silver and copper) were observed to be stable in solution even 2 months after their synthesis. 3.1.2. The XRD patterns The formation of Ag and Cu nanoparticles were confirmed by using XRD patterns, respectively. Fig. 2a showed diffraction peaks
2.2. Synthesis of metal nanoparticles In a typical synthesis of copper or copper/silver nanoparticles, about 0.5 g of gelatin were completely dissolved in H2O (30 mL) under magnetic stirring at 60 °C for about 30 min, and then cool to room temperature. At this time, copper (II) nitrate trihydrate or the mixture of copper (II) nitrate trihydrate and silver nitrate were added in. After stirring for 30 min, 8 mL of hydrazine hydrate solution was dropped into the above solution under constant stirring. The reactor was kept at room temperature without needing any inert atmosphere. It should be pointed out that hydrazine hydrate can also increase the pH of the solution. The reaction was monitored by UV–vis spectroscopy until no change of the absorbance spectrum was observed. The metallic nanoparticles in this work were obtained from the redox reaction between metal and hydrazine in the presence of gelatin as capping agent. Another method is similar to the above description, the difference is that copper (II) nitrate trihydrate was first added in, and reduced by hydrazine hydrate. And then the silver nitrate was injected. 2.3. Characterization UV–vis spectra were recorded on a Shimadzu UV-2500 spectrophotometer in a 1 cm optical path quartz cuvette over a 300–800 nm range at room temperature. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advanced X-ray diffractometer using Cu Ka radiation (k = 0.1542 nm). Nanoparticle size was analyzed by TEM using a JEOL 2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Raman spectra were recorded on a Renishaw Invia Raman microscope excited by an argon ion laser beam (514.5 nm, 20 mW). The laser power at the sample position was 10 mW with a spot size of ca. 1–2 lm. The data acquisition time used in the measurement was 25 s. Replicate measurements on different areas were made three times to verify that the spectra were a true representation of each experiment. To record the Raman spectrum of methylene blue adsorbed
Fig. 1. UV–vis spectra recorded as a function of time of reaction of aqueous solution of silver nitrate (a) and copper (II) nitrate (b) with hydrazine hydrate.
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3.2. The formation of Cu/Ag bimetallic nanoparticles 3.2.1. The XRD pattern of gelatin-encapsulated Ag–Cu bimetallic nanoparticles The formed Ag–Cu bimetallic nanoparticles are also characterized by the XRD. In Fig. 4, the Bragg diffraction peaks of silver and copper can be seen at the same time, which confirms the formation of Ag–Cu bimetallic nanoparticles.
Fig. 2. The XRD patterns of silver (a) and copper (b) nanoparticles.
for Ag nanoparticles, we can see four Bragg diffraction peaks at 38.2°, 44.3°, 64.5° and 77.6° which pertains to (1 1 1), (2 0 0), (2 2 0), (3 1 1) planes of a face centered cubic (fcc) lattice of silver (JCPDS File No. 04–0783), respectively [22]. In Fig. 2b, all Bragg’s reflections due to the metallic copper are observed at 43.6°, 50.7° and 74.4° representing (1 1 1), (2 0 0) and (2 2 0) planes of fcc crystal structure of copper. An estimation of particle measurement by use of Scherrer’s equation reveals a crystallite size of about 30 nm in such power.
3.2.2. The influence of reducing order of Cu2+/Ag+ Fig. 5 shows the UV–vis spectra of gelatin-encapsulated Ag–Cu bimetallic nanoparticles synthesized under different conditions. The surface plasmon resonance (SPR) bands of silver and copper are also displayed under different conditions. In the first condition, Cu(NO3)2 was added in and reduced at the beginning. Then AgNO3 was added. The reactions occurring in this condition are shown in the following:
2Cu2þ þ N2 H4 þ 4OH ! 2Cu þ N2 þ 4H2 O
ð1Þ
Cu þ Agþ ! Cu2þ þ Ag
ð2Þ
4Agþ þ N2 H4 þ 4OH ! 4Ag þ N2 þ 4H2 O
ð3Þ
2Cu2þ þ N2 H4 þ 4OH ! 2Cu þ N2 þ 4H2 O
ð4Þ
However, under the second condition, due to the standard electrode potentials of Ag and Cu which are shown below:
Agþ þ e;Ag þ 0:337 3.1.3. TEM studies Fig. 3 shows the TEM images of Ag and Cu colloidal samples taken from a typical synthesis, in which AgNO3 and Cu(NO3)2 were reduced respectively, in the presence of gelatin as a capping agent. Fig. 3a shows Ag colloid samples taken after 10 h of reaction. It can be seen from the Fig. 3a that silver nanoparticles with mean size are around 10 nm and good monodispersity is achieved clearly. The uniform size distribution can be attributed to the fast reduction induced instantaneous nucleation, that is, for a short time interval relative to the duration of particle growth. Fig. 3b displays the gelatin-capped copper nanoparticles with larger diameter about 30 nm. This is corresponded with the result of XRD. Furthermore, the copper nanoparticles are polydispersity. This is because of the triple-helical structure of gelatin (Scheme 1), copper nanoparticles are so big that can fill the matrix of gelatin to the full.
Cu2þ þ 2e;Cu þ 0:7795 So we can conclude that Ag+ was reduced first, and then Cu2+. The two occurred reactions are shown below:
4Agþ þ N2 H4 þ 4OH ! 4Ag þ N2 þ 4H2 O
ð5Þ
2Cu2þ þ N2 H4 þ 4OH ! 2Cu þ N2 þ 4H2 O
ð6Þ
Due to the above reason, there is a little difference between the lines (a) and (b) in Fig. 5. We can see from Fig. 5, the surface plasmon resonance (SPR) band of silver has some shift, however, the SPR band of copper keep the same at these conditions. This is because that the silver was reduced by two reducing agents, on the contrary, the copper was only reduced by N2H4. The strong or weak reducing agents also influence the micrograph of nanoparticles.
Fig. 3. TEM of silver (a) and copper (b) nanoparticles.
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Scheme 1. The structure of gelatin.
Fig. 4. The XRD pattern of gelatin-encapsulated Ag–Cu bimetallic nanoparticles.
3.2.3. The TEM images of gelatin-encapsulated Ag–Cu bimetallic nanoparticles Fig. 6 shows the TEM images of gelatin-encapsulated Ag–Cu bimetallic nanoparticles synthesized under different conditions. In Fig. 6a, we can see that the Ag–Cu bimetallic are polydispersed with particle size of about 20 nm. Fig. 6b shows that the nanoparticles are monodispersed with particles size of 10–30 nm. The difference between the (a) and (b) is because of the different reducing progress. The results also correspond with the UV–vis spectra. 3.3. Surface enhanced Raman scattering effect The normal Raman spectrum of methylene blue water solution, the Raman spectrum of methylene blue adsorbed on Cu nanoparticles and Cu/Ag bimetallic nanoparticles are shown in Fig. 7. The Raman signals of methylene blue adsorbed on the copper and Cu/Ag bimetallic nanoparticles exhibit enhancements, as can be seen from Fig. 7b and c.
Fig. 5. UV–vis spectra of gelatin-encapsulated Ag–Cu bimetallic nanoparticles synthesized under different conditions. (a) Cu(NO3)2 was reduced firstly and then AgNO3 was added in. (b) AgNO3 and Cu(NO3)2 were added in at the same time.
To quantitatively evaluate the magnitude of the enhancement factor (EF) for silver, we compare the measured SERS intensities to the intensity of non-enhanced Raman scattering by using the following equation: [23,24]
EF ¼ ðISERS =C SERS Þ=ðInormal =C bulk Þ
ð7Þ
where Cbulk is the concentration of the molecules in the bulk samples, CSERS is the concentration of the adsorbed molecules on the silver surface; Inormal and ISERS are the intensity of a certain vibration in normal and SERS Raman spectra, respectively. The enhancement factor is obtained by comparing the SERS spectrum of methylene blue adsorbed on Cu nanoparticles or Cu/Ag bimetallic nanoparticles with normal Raman spectrum of methylene blue in water solution. Using the UV–vis spectra, we can likewise estimate the concentration of a monolayer of methylene blue adsorbed on the silver surface. We calculated the enhancement factor for 105 M
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Fig. 6. TEM images of gelatin-encapsulated Ag–Cu bimetallic nanoparticles synthesized under different conditions. (a) Cu(NO3)2 was reduced firstly and then AgNO3 was added in. (b) AgNO3 and Cu(NO3)2 were added in at the same time.
confirmed. According to the surface-enhanced Raman scattering effects, copper and copper/silver nanoparticles can be used as nice SERS-active substrates to obtain the information of adsorbed molecule. But the SERS of copper/silver nanoparticles is little higher. Acknowledgements This work was financially supported by Linyi science and technology development plan, the National Natural Science Foundation of China (No. 50772048) and also by the R&D Foundation of Jiangsu Province of China (JHB 06-04). References Fig. 7. The SERS spectra of methylene blue (0.5 105 M) only (a), methylene blue adsorbed on the copper substrates (b) and crystal violet adsorbed on Cu/Ag bimetallic nanoparticles (c).
of methylene blue adsorbed on the copper and Cu/Ag bimetallic nanoparticles, to be 1.03 103 and 5.8 103, respectively. Therefore, the copper or Cu/Ag bimetallic nanoparticles can be used as nice SERS-active substrates to obtain the information of adsorbed molecules such as the interaction and orientation of molecules with the surface. 4. Conclusions The spherical-shaped Cu and Cu/Ag bimetallic nanoparticles were synthesized using hydrazine and gelatin as reducing and stabilizing agents, respectively. The advantages of the synthetic method are that water dispersible copper and copper/silver nanoparticles are produced at room temperature under no inert atmosphere. From the UV–vis spectra, we know that the surface plasmon absorbance of Ag and Cu nanoparticles at 420 and 572 nm, respectively. Transmission electron microscopy showed the formation of nanoparticles in the range of 10 nm (silver), and 30 nm (copper). Due to the standard electrode potential of silver is lower than the copper, the reducing order of Cu2+/Ag+ is
[1] F. Douglas, R. Yañez, J. Ros, S. Marín, A. Escosura-Muniz, S.A. Arben Merkoc, J. Nanopart. Res. 10 (2008) 97. [2] G. Schmid, Chem. Rev. 921 (1992) 709. [3] G.A. Ozin, Adv. Mater. 4 (1992) 612. [4] W.H. Wang, X.L. Tian, K. Chen, G.Y. Cao, Colloids Surf. A: Physicochem. Eng. Aspects 273 (2006) 35. [5] N. Toshima, J. Macromol. Sci. Chem. A 27 (1990) 1225. [6] P. Biloen, J.N. Helle, H. Verbeek, F.M. Dautzenberg, W.M.H. Sachtler, J. Catal. 63 (1980) 112. [7] C.W. Chen, T. Takezako, K. Yamamoto, T. Serizawa, M. Akashi, Colloids Surf. A: Physicochem. Eng. Aspects 169 (2000) 107. [8] M. Wu, J. Long, A. Huang, Y. Luo, S. Feng, R. Xu, Langmuir 15 (1999) 8822. [9] M.M. Mdleleni, T. Hyeon, K.S. Suslick, J. Am. Chem. Soc. 120 (1998) 6189. [10] D. Tsamouras, E. Dalas, S. Sakkopulos, P.G. Koutsoukos, Langmuir 15 (1999) 7940. [11] D.H. Chen, S.H. Wu, Chem. Mater. 12 (2000) 1354. [12] C.C. Wang, D.H. Chen, T.C. Huang, Colloids Surf. A: Physicochem. Eng. Aspects 189 (2001) 145. [13] M.L. Wu, D.H. Chen, T.C. Huang, Chem. Mater. 13 (2001) 599. [14] M.L. Wu, D.H. Chen, T.C. Huang, Langmuir 17 (2001) 3877. [15] C. Liu, B. Zou, A.J. Rondinonoue, Z.J. Zhang, J. Phys. Chem. B 104 (2000) 1141. [16] P. Duwez, R.H. Willens, J.W. Klement, J. Appl. Phys. 311 (1960) 136. [17] O. Kozlova, R. Voytovych, P. Protsenko, N. Eustathopoulos, J. Mater. Sci. 45 (2010) 2099. [18] Z.Q. Li, T.B. Chen, Mater. Character. 49 (2002) 67. [19] N. Li, J.Z. Xu, H. Yao, J.J. Zhu, H.Y. Chen, J. Phys. Chem. B 110 (2006) 11561. [20] J.J. Zhang, M.M. Gu, T.T. Zheng, J.J. Zhu, Anal. Chem. 81 (2009) 6641. [21] P.K. Khanna, S. Gaikwad, P.V. Adhyapak, N. Singh, R. Marimuthu, Mater. Lett. 61 (2007) 4711. [22] L. Huang, M.L. Zhai, J. Nanopart. Res. 10 (2008) 1193. [23] R. Sanci, M. Volkan, Sensors Actuators B 139 (2009) 150. [24] M. Culha, M. Kahraman, N. Tokman, J. Phys. Chem. C 112 (2008) 10338.
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis of polymer-stabilized monometallic Cu and bimetallic Cu/Ag nanoparticles and their surface-enhanced Raman scattering properties Danhui Zhang a,b,⇑, Xiaoheng Liu b a b
School of Mechanical Engineering, Linyi University, Linyi 276005, China Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China
h i g h l i g h t s " Advantages of the synthetic method include production of copper nanoparticles at room temperature under no inert atmosphere. " The Ag/Cu bimetallic nanoparticles were also synthesized successfully. " The SERS of copper and silver/copper bimetallic nanoparticles were also discussed.
a r t i c l e
i n f o
Article history: Received 9 October 2012 Accepted 12 December 2012 Available online 21 December 2012 Keywords: Gelatin Copper nanoparticles Bimetallic nanoparticles SERS
a b s t r a c t The present study demonstrates a facile process for the production of spherical-shaped Cu and Ag nanoparticles synthesized and stabilized by hydrazine and gelatin, respectively. Advantages of the synthetic method include its production of water dispersible copper and copper/silver nanoparticles at room temperature under no inert atmosphere. The resulting nanoparticles (copper or copper/silver) are investigated by X-ray diffraction (XRD), UV–vis spectroscopy, and transmission electron microscopy (TEM). The nanometallic dispersions were characterized by surface plasmon absorbance measuring at 420 and 572 nm for Ag and Cu nanoparticles, respectively. Transmission electron microscopy showed the formation of nanoparticles in the range of 10 nm (silver), and 30 nm (copper). The results also demonstrate that the reducing order of Cu2+/Ag+ is important for the formation of the bimetallic nanoparticles. The surface-enhanced Raman scattering effects of copper and copper/silver nanoparticles were also displayed. It was found that the enhancement ability of copper/silver nanoparticles was little higher than the copper nanoparticles. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction There is currently immense interest in the synthesis and characterization of mono- and bimetallic nanoparticles [1]. This field presents a broad scope of research, as nanoparticles (NPs) can exhibit unusual chemical, physical and electrical properties that are not apparent in bulk materials [2], as well as possessing size-dependent properties and the potential for constructing nanoand micro assemblies [3]. Recently, considerable efforts have been devoted to bimetallic nanoparticles since they are of great interest from both scientific and technological perspective for the modification of physical and chemical properties of metal nanoparticles [4,5]. Bimetallic colloids, in which two kinds of metals are assembled in one entity, have well different catalytic, electronic and optical properties distinct from those of the corresponding monometallic nanoparticles ⇑ Corresponding author at: School of Mechanical Engineering, Linyi University, Linyi 276005, China. Tel./fax: +86 539 8766260. E-mail address: [email protected] (D. Zhang). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.12.023
[6]. Bimetallic colloids can be prepared by simultaneous co-reduction of two kinds of metal ions with or without protective agent (usually polymer or surfactant) or by successive reduction of one metal over the nuclei of another. Many methods for the preparation of bimetallic nanoparticles have been reported, such as alcohol reduction [7], citrate reduction, hydrothermal [8], sonochemical method [9], co-precipitation [10] and reverse micelles [11–15]. In general, their stability and sizes are controlled by the addition of protective agents, such as soluble polymers, surfactants, organic ligands. Moreover, the size distribution, structure and composition of the bimetallic nanoparticles have also been affected by the preparation conditions. Ag–Cu alloys were first produced by rapid quenching in 1960 [16,17]. Since then, a lot of methods have been used to prepare Ag/Cu bimetallic nanoparticles due to their bactericida, catalytic activities and their well-studied structure [18]. Gelatin is the thermally and hydrolytically denatured product of collagen, which has been extensively applied as the immobilization matrix for the preparation of biosensors [19]. It has a triple-helical structure and offers distinctive advantages such as good
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biocompatibility, nontoxicity, remarkable affinity to proteins, and excellent gel-forming ability [20]. To the best of our knowledge, there is no report on the use of gelatin as stabilizing agent to prepare Cu or Cu/Ag NPs-embedded gelatin nanocomposites. In this study, Cu or Cu/Ag NPs were prepared in the gelatin solution without using any other additional stabilizing agents. The major advantage of gelatin as a stabilizing agent is that it can be used to tailor the nanocomposite properties and also provide long-term stability of the nanoparticles by preventing particle agglomeration. Herein, we report a simple method to synthesize colloidal Cu or bimetal Cu/Ag nanoparticles by reducing copper and silver ions with hydrazine using gelatin as a stabilizing agent. The advantages of this method are that the use of atmospheric air (or no use of inert atmosphere) for the synthesis of nanoparticles. The reducing order of Cu2+/Ag+ is important for the formation of the bimetallic nanoparticles. The surface-enhanced Raman scattering effects of copper and copper/silver nanoparticles were also displayed. 2. Experimental section 2.1. Reagents Copper (II) nitrate trihydrate was obtained from Guangdong Chem. Co.; hydrazine hydrate solution (80%) and gelatin were gotten from Kermel; Silver nitrate (AgNO3) and methylene blue were obtained from Nanjing Chem. Co. All the reagents were used as received, without further purification, and the water was deionized.
on the metallic nanoparticles, the metallic substrates were mixed with 105 M of methyl blue in 10 mL water solution. After 24 h of adsorption, the sample was measured. 3. Results and discussion 3.1. The formation of monometallic nanoparticles 3.1.1. The UV–vis spectra of monometallic nanoparticles After addition of hydrazine hydrate to the AgNO3 and Cu(NO3)2 solutions led to the appearance of yellowish brown and dark red color in solutions after 10 h of reaction, indicating the formation of silver and copper nanoparticles, respectively. These colors arise due to light absorption by surface plasmons in the metal nanoparticles. Fig. 1a and b shows the UV–vis spectra recorded from the aqueous silver nitrate-gelatin and copper (II) nitrate-gelatin reaction medium, as a function of time of reaction. The silver surface plasmon resonance (SPR) band occurred at 414 nm and steadily increased in intensity as a function of time of reaction without any shift in the peak wavelength. In copper ion reduction, the SPR band occurred at 572 nm [21]. The gelatin mediated syntheses of metal nanoparticles (silver and copper) were observed to be stable in solution even 2 months after their synthesis. 3.1.2. The XRD patterns The formation of Ag and Cu nanoparticles were confirmed by using XRD patterns, respectively. Fig. 2a showed diffraction peaks
2.2. Synthesis of metal nanoparticles In a typical synthesis of copper or copper/silver nanoparticles, about 0.5 g of gelatin were completely dissolved in H2O (30 mL) under magnetic stirring at 60 °C for about 30 min, and then cool to room temperature. At this time, copper (II) nitrate trihydrate or the mixture of copper (II) nitrate trihydrate and silver nitrate were added in. After stirring for 30 min, 8 mL of hydrazine hydrate solution was dropped into the above solution under constant stirring. The reactor was kept at room temperature without needing any inert atmosphere. It should be pointed out that hydrazine hydrate can also increase the pH of the solution. The reaction was monitored by UV–vis spectroscopy until no change of the absorbance spectrum was observed. The metallic nanoparticles in this work were obtained from the redox reaction between metal and hydrazine in the presence of gelatin as capping agent. Another method is similar to the above description, the difference is that copper (II) nitrate trihydrate was first added in, and reduced by hydrazine hydrate. And then the silver nitrate was injected. 2.3. Characterization UV–vis spectra were recorded on a Shimadzu UV-2500 spectrophotometer in a 1 cm optical path quartz cuvette over a 300–800 nm range at room temperature. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advanced X-ray diffractometer using Cu Ka radiation (k = 0.1542 nm). Nanoparticle size was analyzed by TEM using a JEOL 2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Raman spectra were recorded on a Renishaw Invia Raman microscope excited by an argon ion laser beam (514.5 nm, 20 mW). The laser power at the sample position was 10 mW with a spot size of ca. 1–2 lm. The data acquisition time used in the measurement was 25 s. Replicate measurements on different areas were made three times to verify that the spectra were a true representation of each experiment. To record the Raman spectrum of methylene blue adsorbed
Fig. 1. UV–vis spectra recorded as a function of time of reaction of aqueous solution of silver nitrate (a) and copper (II) nitrate (b) with hydrazine hydrate.
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3.2. The formation of Cu/Ag bimetallic nanoparticles 3.2.1. The XRD pattern of gelatin-encapsulated Ag–Cu bimetallic nanoparticles The formed Ag–Cu bimetallic nanoparticles are also characterized by the XRD. In Fig. 4, the Bragg diffraction peaks of silver and copper can be seen at the same time, which confirms the formation of Ag–Cu bimetallic nanoparticles.
Fig. 2. The XRD patterns of silver (a) and copper (b) nanoparticles.
for Ag nanoparticles, we can see four Bragg diffraction peaks at 38.2°, 44.3°, 64.5° and 77.6° which pertains to (1 1 1), (2 0 0), (2 2 0), (3 1 1) planes of a face centered cubic (fcc) lattice of silver (JCPDS File No. 04–0783), respectively [22]. In Fig. 2b, all Bragg’s reflections due to the metallic copper are observed at 43.6°, 50.7° and 74.4° representing (1 1 1), (2 0 0) and (2 2 0) planes of fcc crystal structure of copper. An estimation of particle measurement by use of Scherrer’s equation reveals a crystallite size of about 30 nm in such power.
3.2.2. The influence of reducing order of Cu2+/Ag+ Fig. 5 shows the UV–vis spectra of gelatin-encapsulated Ag–Cu bimetallic nanoparticles synthesized under different conditions. The surface plasmon resonance (SPR) bands of silver and copper are also displayed under different conditions. In the first condition, Cu(NO3)2 was added in and reduced at the beginning. Then AgNO3 was added. The reactions occurring in this condition are shown in the following:
2Cu2þ þ N2 H4 þ 4OH ! 2Cu þ N2 þ 4H2 O
ð1Þ
Cu þ Agþ ! Cu2þ þ Ag
ð2Þ
4Agþ þ N2 H4 þ 4OH ! 4Ag þ N2 þ 4H2 O
ð3Þ
2Cu2þ þ N2 H4 þ 4OH ! 2Cu þ N2 þ 4H2 O
ð4Þ
However, under the second condition, due to the standard electrode potentials of Ag and Cu which are shown below:
Agþ þ e;Ag þ 0:337 3.1.3. TEM studies Fig. 3 shows the TEM images of Ag and Cu colloidal samples taken from a typical synthesis, in which AgNO3 and Cu(NO3)2 were reduced respectively, in the presence of gelatin as a capping agent. Fig. 3a shows Ag colloid samples taken after 10 h of reaction. It can be seen from the Fig. 3a that silver nanoparticles with mean size are around 10 nm and good monodispersity is achieved clearly. The uniform size distribution can be attributed to the fast reduction induced instantaneous nucleation, that is, for a short time interval relative to the duration of particle growth. Fig. 3b displays the gelatin-capped copper nanoparticles with larger diameter about 30 nm. This is corresponded with the result of XRD. Furthermore, the copper nanoparticles are polydispersity. This is because of the triple-helical structure of gelatin (Scheme 1), copper nanoparticles are so big that can fill the matrix of gelatin to the full.
Cu2þ þ 2e;Cu þ 0:7795 So we can conclude that Ag+ was reduced first, and then Cu2+. The two occurred reactions are shown below:
4Agþ þ N2 H4 þ 4OH ! 4Ag þ N2 þ 4H2 O
ð5Þ
2Cu2þ þ N2 H4 þ 4OH ! 2Cu þ N2 þ 4H2 O
ð6Þ
Due to the above reason, there is a little difference between the lines (a) and (b) in Fig. 5. We can see from Fig. 5, the surface plasmon resonance (SPR) band of silver has some shift, however, the SPR band of copper keep the same at these conditions. This is because that the silver was reduced by two reducing agents, on the contrary, the copper was only reduced by N2H4. The strong or weak reducing agents also influence the micrograph of nanoparticles.
Fig. 3. TEM of silver (a) and copper (b) nanoparticles.
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Scheme 1. The structure of gelatin.
Fig. 4. The XRD pattern of gelatin-encapsulated Ag–Cu bimetallic nanoparticles.
3.2.3. The TEM images of gelatin-encapsulated Ag–Cu bimetallic nanoparticles Fig. 6 shows the TEM images of gelatin-encapsulated Ag–Cu bimetallic nanoparticles synthesized under different conditions. In Fig. 6a, we can see that the Ag–Cu bimetallic are polydispersed with particle size of about 20 nm. Fig. 6b shows that the nanoparticles are monodispersed with particles size of 10–30 nm. The difference between the (a) and (b) is because of the different reducing progress. The results also correspond with the UV–vis spectra. 3.3. Surface enhanced Raman scattering effect The normal Raman spectrum of methylene blue water solution, the Raman spectrum of methylene blue adsorbed on Cu nanoparticles and Cu/Ag bimetallic nanoparticles are shown in Fig. 7. The Raman signals of methylene blue adsorbed on the copper and Cu/Ag bimetallic nanoparticles exhibit enhancements, as can be seen from Fig. 7b and c.
Fig. 5. UV–vis spectra of gelatin-encapsulated Ag–Cu bimetallic nanoparticles synthesized under different conditions. (a) Cu(NO3)2 was reduced firstly and then AgNO3 was added in. (b) AgNO3 and Cu(NO3)2 were added in at the same time.
To quantitatively evaluate the magnitude of the enhancement factor (EF) for silver, we compare the measured SERS intensities to the intensity of non-enhanced Raman scattering by using the following equation: [23,24]
EF ¼ ðISERS =C SERS Þ=ðInormal =C bulk Þ
ð7Þ
where Cbulk is the concentration of the molecules in the bulk samples, CSERS is the concentration of the adsorbed molecules on the silver surface; Inormal and ISERS are the intensity of a certain vibration in normal and SERS Raman spectra, respectively. The enhancement factor is obtained by comparing the SERS spectrum of methylene blue adsorbed on Cu nanoparticles or Cu/Ag bimetallic nanoparticles with normal Raman spectrum of methylene blue in water solution. Using the UV–vis spectra, we can likewise estimate the concentration of a monolayer of methylene blue adsorbed on the silver surface. We calculated the enhancement factor for 105 M
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Fig. 6. TEM images of gelatin-encapsulated Ag–Cu bimetallic nanoparticles synthesized under different conditions. (a) Cu(NO3)2 was reduced firstly and then AgNO3 was added in. (b) AgNO3 and Cu(NO3)2 were added in at the same time.
confirmed. According to the surface-enhanced Raman scattering effects, copper and copper/silver nanoparticles can be used as nice SERS-active substrates to obtain the information of adsorbed molecule. But the SERS of copper/silver nanoparticles is little higher. Acknowledgements This work was financially supported by Linyi science and technology development plan, the National Natural Science Foundation of China (No. 50772048) and also by the R&D Foundation of Jiangsu Province of China (JHB 06-04). References Fig. 7. The SERS spectra of methylene blue (0.5 105 M) only (a), methylene blue adsorbed on the copper substrates (b) and crystal violet adsorbed on Cu/Ag bimetallic nanoparticles (c).
of methylene blue adsorbed on the copper and Cu/Ag bimetallic nanoparticles, to be 1.03 103 and 5.8 103, respectively. Therefore, the copper or Cu/Ag bimetallic nanoparticles can be used as nice SERS-active substrates to obtain the information of adsorbed molecules such as the interaction and orientation of molecules with the surface. 4. Conclusions The spherical-shaped Cu and Cu/Ag bimetallic nanoparticles were synthesized using hydrazine and gelatin as reducing and stabilizing agents, respectively. The advantages of the synthetic method are that water dispersible copper and copper/silver nanoparticles are produced at room temperature under no inert atmosphere. From the UV–vis spectra, we know that the surface plasmon absorbance of Ag and Cu nanoparticles at 420 and 572 nm, respectively. Transmission electron microscopy showed the formation of nanoparticles in the range of 10 nm (silver), and 30 nm (copper). Due to the standard electrode potential of silver is lower than the copper, the reducing order of Cu2+/Ag+ is
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