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Facile fabrication of ultrasmall and uniform copper nanoparticles
Materials Letters 65 (2011) 3005–3008
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Facile fabrication of ultrasmall and uniform copper nanoparticles Zhipeng Cheng ⁎, Hui Zhong, Jiming Xu, Xiaozhong Chu, Yuanzhi Song, Man Xu, Hui Huang Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry & Chemical Engineering, Huaiyin Normal University, Huaian 223300, China
a r t i c l e
i n f o
Article history: Received 17 April 2011 Accepted 8 June 2011 Available online 14 June 2011 Keywords: Nanoparticles X-ray techniques Copper Cu(OH)2
a b s t r a c t Ultrasmall and uniform copper nanoparticles were synthesized through a trace-level ethylenediaminetetraacetic acid (EDTA)-assisted wet chemical route in which Cu(OH)2 colloid, KBH4 and polyvinyl pyrrolidone (PVP) were used as the Cu source, the reducing agent, and the protective agents, respectively. The copper nanoparticles exhibit a spherical morphology with a narrow size distribution, a uniform shape, and the average diameter of ca. 4 nm. The presence of trace EDTA is indispensable for the preparation of ultrasmall and uniform copper nanoparticles. EDTA concentration directly influences the copper nanoparticle size and uniformity. As EDTA concentration decreases, the size of the copper nanoparticles decreases, whereas the uniformity increases. The possible formation mechanism of ultrasmall and uniform copper nanoparticles was determined according to experimental results. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, there has been increasing interest on metal nanoparticles due to multiple applications involving their physical and chemical properties [1–3]. Among various metal nanoparticles, copper nanoparticles are popular owing to their potential use in a wide variety of fields such as optical, catalytic, electronic, and antifouling applications [4–6]. To date, numerous methods have been developed for the preparation of copper nanoparticles including chemical reduction [7–11], thermal reduction [12], radiation methods [13], micro emulsion techniques [14], laser ablation [15], polyol method [16], and the DC arc discharge method [17]. Among these methods, chemical reduction in an aqueous solution exhibits the greatest feasibility for further applications due to its simplicity and low cost. However, the size of copper nanoparticles obtained via chemical reduction is mostly located in the range of 10–100 nm; the synthesis of uniform copper nanoparticles with a mean diameter below 10 nm is very difficult [18,19]. The chemical and physical properties of the metal nanoparticles, including catalytic activity and melting point are significantly influenced by particle size and uniformity [20]. Therefore, there is a need to synthesize ultrasmall and uniform copper nanoparticles through simple chemical reduction. This paper presents a trace-level EDTA-assisted chemical reduction route for producing ultrasmall and uniform copper nanoparticles with the average diameter of 4 nm. The whole synthesis was performed at room temperature. The size of the copper nanoparticles can be altered
according to EDTA concentration. The method proposed here has not been described previously. 2. Experimental procedure All reagents were of analytical grade and were used without further purification. In the typical synthetic process, 0.125 g of CuSO4·5H2O and 0.05 g of PVP were dissolved in 30 mL of water. The solution was stirred with a magnetic stirrer for 10 min to ensure that the CuSO4 and PVP completely dissolved. Under constant stirring, 20 mL of 0.15 M NaOH was poured into the CuSO4 and PVP solution at room temperature. A blue Cu(OH)2 precipitate was soon produced. After stirring for 10 min, 0.2 g KBH4 and 0.002 g of EDTA was added into the Cu(OH)2 suspension. Subsequently, the mixture was kept at 25 °C for 12 h under vigorous stirring. After the reaction, copper nanoparticles were obtained, followed by centrifugal separation, washing with absolute alcohol and drying in vacuum at 40 °C for 6 h. The product was characterized by powder X-ray diffraction (XRD) pattern, using a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation. Transmission electron microscopy (TEM) was performed on a Philips Tecnai 12 transmission electron microscope. X-ray photoelectron spectroscopy (XPS) pattern was recorded on an Escalab MkII X-ray photoelectron spectrometer using Mg Kα X-ray as the excitation source. The UV–vis absorbance was measured on a GBC UV/vis 916 spectrometer. The average particle size was measured using a DLS 7000 spectrometer. 2.1. Characterization of the product
⁎ Corresponding author. Tel./fax: + 86 517 83525085. E-mail address: [email protected] (Z. Cheng). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.06.037
Fig. 1 shows the XRD pattern of the copper nanoparticles. Most of the peaks correspond to those of cubic copper and can be matched with reported data (JCPDS 04-0836). The peaks indexed as the (111)
3006
Z. Cheng et al. / Materials Letters 65 (2011) 3005–3008
Intensity ( a.u.)
Cu(111)
Cu2 O (111) Cu(200)
Cu(220) Cu2 O (220)
20
30
40
50
60
70
80
Fig. 1. XRD pattern of the copper nanoparticles.
and (220) diffraction of cubic Cu2O (JCPDS 05-0667) is associated with slow oxidation of metallic copper in air to form Cu2O [21]. Fig. 2 shows the UV–vis absorption spectrum of the copper nanoparticles. A band centered at ca. 560 nm appears, characteristic of surface plasmon absorption on the copper nanoparticles. Compared to the previously reported Cu nanoparticles, the as-prepared Cu nanoparticles exhibit an absorption peak blue-shift of ca. 10 nm [20], suggestive of much smaller particles. Fig. 3a shows the TEM image of the copper nanoparticles. The copper nanoparticles are uniform and exhibit a spherical morphology with an average diameter of ca. 4 nm. Further structural information was obtained from the high-resolution HRTEM image shown in Fig. 3b. The lattice fringes in the image are consistent with the (111) planes of the fcc phase of metallic Cu, with a ~ 2.08 Å periodicity.
2.2. Effect of EDTA concentration on the size and uniformity of copper nanoparticles
Absorbance (a.u.)
Experiments were done to investigate the influence of EDTA concentration on the size and uniformity of copper nanoparticles while other experimental conditions remained unchanged. Fig. 4 shows the TEM images and corresponding histograms of the copper
560
Fig. 3. TEM (a) and HRTEM (b) images of the copper nanoparticles.
nanoparticles prepared at different EDTA concentrations. Obviously, Lower EDTA concentration caused the smaller size and higher dispersion of the copper nanoparticles obtained. 2.3. Formation mechanism of ultrasmall and uniform copper nanoparticles In the traditional concept, EDTA acts only as a complexing agent for metal ions to prevent the formation of hydroxide in the alkaline solution [22]. However, trace EDTA in the present system contributes to the formation of ultrasmall and uniform copper nanoparticles as well. To understand the role of the trace levels of EDTA, it should first be pointed out that KBH4 cannot reduce Cu(OH)2 to Cu or Cu2O at room temperature. This is proven by the absence of a color change after mixing Cu(OH)2 and KBH4. After trace levels of EDTA were added into the solution, it rapidly attacks the surface of Cu(OH)2, leading to the formation of a free trace Cu-EDTA complex ion. The complexing reaction of Cu(OH)2 and EDTA is shown in Eq. (1). −
CuðOHÞ2 þ EDTA→Cu EDTA þ 2OH
−
−
ð1Þ
−
Cu EDTA þ 2BH4 þ 2OH þ 2H2 O→Cu þ 2BO2 þ 7H2 þ EDTA:
400
500
600
700
Wavelength (nm) Fig. 2. UV–vis absorption spectrum of the copper nanoparticles.
800
ð2Þ
Free trace Cu-EDTA is promptly reduced into metallic Cu atom upon the release of trace EDTA once Cu-EDTA and KBH4 make contact. The corresponding reduction is depicted in Eq. (2) and is supported by an early report [23]. Afterwards, regenerated trace levels of EDTA dissolves the surface of Cu(OH)2 again, and the process repeats. The reaction can therefore be completed even with trace levels of EDTA as long as an adequate amount of reducing agent is present in the
Z. Cheng et al. / Materials Letters 65 (2011) 3005–3008
3007
a a’ Frequency ( % )
40 30 20 10 0 5
10
15
20
25
30
35
40
45
Diameter ( nm )
b
40
Frequency ( % )
b’ 30
20
10
0 5
10
15
Diameter ( nm )
c c’ Frequency ( % )
40 30 20 10 0 0
1
2
3
4
5
6
7
8
Diameter ( nm )
Fig. 4. TEM images and corresponding histograms of the copper nanoparticles prepared at different concentrations of EDTA. (a, a′) 0.05 g, (b, b′) 0.01 g, (c, c′) 0.005 g.
solution. Reactions depicted in Eqs. (1) and (2) proceed alternately until Cu(OH)2 is completely reduced to Cu. In the whole process, EDTA acts as the triggering agent and is recycled constantly. Therefore, controlling EDTA concentration regulates the concentration of free Cu-EDTA. A lower concentration of EDTA produces lower Cu-EDTA concentration, indicating that only trace amounts of copper atoms are achieved in a certain period of time. Therefore, trace EDTA contributes to the formation of ultrasmall copper nanoparticle. Moreover, Cu-EDTA concentration is approximately constant due to the trace amount of EDTA used, equal amounts of EDTA are consumed in the complexing reaction [Eq. (1)] and released in the reduction reaction [Eq. (2)]; the two reactions occur within a short interval. As a result, the effects of changing the Cu-EDTA concentration on product size are minimized, thereby resulting in uniform copper nanoparticle sizes.
3. Conclusion Ultrasmall and uniform copper nanoparticles with an average diameter of ca. 4 nm were successfully synthesized via a trace-level EDTA-assisted wet chemical route. The presence of trace EDTA is necessary for the preparation of ultrasmall and uniform copper nanoparticles. The size of copper nanoparticles can be tuned by varying the concentration of EDTA. The method can be extended to prepare other metallic nanoparticles such as Ni, Co, and Ag. Acknowledgments The authors are grateful for the financial support of the Natural Science Foundation of Jiangsu Education Committee (10KJB430003), Technological Research Foundation of Huai'an City (HAG2010073),
3008
Z. Cheng et al. / Materials Letters 65 (2011) 3005–3008
the National Natural Science Foundation of China (20975043), Jiangsu Higher Institutions Key Basic Research Projects of Natural Science (07KJA15012), and S&T support program of Huai'an City (HAS2010003). References [1] Fu BS, Missaghi MN, Downing CM, Kung MC, Kung HH, Xiao GM. Chem Mater 2010;22:2181–3. [2] Wang H, Wang JG, Shen ZR, Liu YP, Ding DT, Chen TH. J Cata 2010;275:140–8. [3] Eliyahu S, Vaskevich A, Rubinstein I. Thin Solid Films 2010;519:1661–6. [4] Tang XF, Zhen-Guo Yang ZG, Wang WJ. Colloid Surf A 2010;360:99–104. [5] Vaseem M, Lee KM, Kim DY, Hahn YB. Mater Chem Phys 2011;125:334–41. [6] Ryu J, Kim HS, Hahn HT. J Electronic Mater 2010;40:42–50. [7] Biçer M, Şişman İ. Powder Technol 2010;198:279–84. [8] Khanna PK, Gaikwad S, Adhyapak PV, Singh N, Marimuthu R. Mater Lett 2007;61: 4711–4. [9] Abdulla-Al-Mamun M, Kusumoto Y, Muruganandham M. Mater Lett 2009;63: 2007–9.
[10] Guajardo-Pacheco MJ, Morales-Sánchez JE, González-Hernández J, Ruiz F. Mater Lett 2010;64:1361–4. [11] Yu W, Xie HQ, Chen LF, Li Y, Zhang C. Nanoscale Res Lett 2009;4:465–70. [12] Salavati-Niasari M, Davar F. Mater Lett 2009;63:441–3. [13] Zhou F, Zhou RM, Hao XF, Wu XF, Rao H, Chen YK, et al. Radiat Phys Chem 2008;77: 169–73. [14] Solanki JN, Sengupta R, Murthy ZVP. Solid State Sci 2010;12:1560–6. [15] Saito M, Yasukawa K, Umeda T, Aoi Y. Opt Mater 2008;30:1201–4. [16] Park BK, Jeong S, Kim D, Moon J, Lim S, Kim JS. J Colloid Interf Sci 2007;311:417–24. [17] Charinpanitkul T, Soottitantawat A, Tonanon N, Tanthapanichakoon W. Mater Chem Phys 2009;116:125–8. [18] Brege JJ, Hamilton CE, Crouse CA, Barron AR. Nano Lett 2009;9:2239–42. [19] Dadgostar N, Ferdous S, Henneke D. Mater Lett 2010;64:45–8. [20] Zhang HX, Siegert U, Liu R, Cai WB. Nanoscale Res Lett 2009;4:705–8. [21] Cho YS, Huh YD. Mater Lett 2009;63:227–9. [22] Zhang QL, Yang ZM, Ding BJ, Lan XZ, Guo YJ. Trans Nonferrous Met Soc China 2010;20:240–4. [23] Lin RH, Fang L, Xi YX, Shao YX. Acta Chimica Sinica 2004;62:2365–8.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Facile fabrication of ultrasmall and uniform copper nanoparticles Zhipeng Cheng ⁎, Hui Zhong, Jiming Xu, Xiaozhong Chu, Yuanzhi Song, Man Xu, Hui Huang Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry & Chemical Engineering, Huaiyin Normal University, Huaian 223300, China
a r t i c l e
i n f o
Article history: Received 17 April 2011 Accepted 8 June 2011 Available online 14 June 2011 Keywords: Nanoparticles X-ray techniques Copper Cu(OH)2
a b s t r a c t Ultrasmall and uniform copper nanoparticles were synthesized through a trace-level ethylenediaminetetraacetic acid (EDTA)-assisted wet chemical route in which Cu(OH)2 colloid, KBH4 and polyvinyl pyrrolidone (PVP) were used as the Cu source, the reducing agent, and the protective agents, respectively. The copper nanoparticles exhibit a spherical morphology with a narrow size distribution, a uniform shape, and the average diameter of ca. 4 nm. The presence of trace EDTA is indispensable for the preparation of ultrasmall and uniform copper nanoparticles. EDTA concentration directly influences the copper nanoparticle size and uniformity. As EDTA concentration decreases, the size of the copper nanoparticles decreases, whereas the uniformity increases. The possible formation mechanism of ultrasmall and uniform copper nanoparticles was determined according to experimental results. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, there has been increasing interest on metal nanoparticles due to multiple applications involving their physical and chemical properties [1–3]. Among various metal nanoparticles, copper nanoparticles are popular owing to their potential use in a wide variety of fields such as optical, catalytic, electronic, and antifouling applications [4–6]. To date, numerous methods have been developed for the preparation of copper nanoparticles including chemical reduction [7–11], thermal reduction [12], radiation methods [13], micro emulsion techniques [14], laser ablation [15], polyol method [16], and the DC arc discharge method [17]. Among these methods, chemical reduction in an aqueous solution exhibits the greatest feasibility for further applications due to its simplicity and low cost. However, the size of copper nanoparticles obtained via chemical reduction is mostly located in the range of 10–100 nm; the synthesis of uniform copper nanoparticles with a mean diameter below 10 nm is very difficult [18,19]. The chemical and physical properties of the metal nanoparticles, including catalytic activity and melting point are significantly influenced by particle size and uniformity [20]. Therefore, there is a need to synthesize ultrasmall and uniform copper nanoparticles through simple chemical reduction. This paper presents a trace-level EDTA-assisted chemical reduction route for producing ultrasmall and uniform copper nanoparticles with the average diameter of 4 nm. The whole synthesis was performed at room temperature. The size of the copper nanoparticles can be altered
according to EDTA concentration. The method proposed here has not been described previously. 2. Experimental procedure All reagents were of analytical grade and were used without further purification. In the typical synthetic process, 0.125 g of CuSO4·5H2O and 0.05 g of PVP were dissolved in 30 mL of water. The solution was stirred with a magnetic stirrer for 10 min to ensure that the CuSO4 and PVP completely dissolved. Under constant stirring, 20 mL of 0.15 M NaOH was poured into the CuSO4 and PVP solution at room temperature. A blue Cu(OH)2 precipitate was soon produced. After stirring for 10 min, 0.2 g KBH4 and 0.002 g of EDTA was added into the Cu(OH)2 suspension. Subsequently, the mixture was kept at 25 °C for 12 h under vigorous stirring. After the reaction, copper nanoparticles were obtained, followed by centrifugal separation, washing with absolute alcohol and drying in vacuum at 40 °C for 6 h. The product was characterized by powder X-ray diffraction (XRD) pattern, using a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation. Transmission electron microscopy (TEM) was performed on a Philips Tecnai 12 transmission electron microscope. X-ray photoelectron spectroscopy (XPS) pattern was recorded on an Escalab MkII X-ray photoelectron spectrometer using Mg Kα X-ray as the excitation source. The UV–vis absorbance was measured on a GBC UV/vis 916 spectrometer. The average particle size was measured using a DLS 7000 spectrometer. 2.1. Characterization of the product
⁎ Corresponding author. Tel./fax: + 86 517 83525085. E-mail address: [email protected] (Z. Cheng). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.06.037
Fig. 1 shows the XRD pattern of the copper nanoparticles. Most of the peaks correspond to those of cubic copper and can be matched with reported data (JCPDS 04-0836). The peaks indexed as the (111)
3006
Z. Cheng et al. / Materials Letters 65 (2011) 3005–3008
Intensity ( a.u.)
Cu(111)
Cu2 O (111) Cu(200)
Cu(220) Cu2 O (220)
20
30
40
50
60
70
80
Fig. 1. XRD pattern of the copper nanoparticles.
and (220) diffraction of cubic Cu2O (JCPDS 05-0667) is associated with slow oxidation of metallic copper in air to form Cu2O [21]. Fig. 2 shows the UV–vis absorption spectrum of the copper nanoparticles. A band centered at ca. 560 nm appears, characteristic of surface plasmon absorption on the copper nanoparticles. Compared to the previously reported Cu nanoparticles, the as-prepared Cu nanoparticles exhibit an absorption peak blue-shift of ca. 10 nm [20], suggestive of much smaller particles. Fig. 3a shows the TEM image of the copper nanoparticles. The copper nanoparticles are uniform and exhibit a spherical morphology with an average diameter of ca. 4 nm. Further structural information was obtained from the high-resolution HRTEM image shown in Fig. 3b. The lattice fringes in the image are consistent with the (111) planes of the fcc phase of metallic Cu, with a ~ 2.08 Å periodicity.
2.2. Effect of EDTA concentration on the size and uniformity of copper nanoparticles
Absorbance (a.u.)
Experiments were done to investigate the influence of EDTA concentration on the size and uniformity of copper nanoparticles while other experimental conditions remained unchanged. Fig. 4 shows the TEM images and corresponding histograms of the copper
560
Fig. 3. TEM (a) and HRTEM (b) images of the copper nanoparticles.
nanoparticles prepared at different EDTA concentrations. Obviously, Lower EDTA concentration caused the smaller size and higher dispersion of the copper nanoparticles obtained. 2.3. Formation mechanism of ultrasmall and uniform copper nanoparticles In the traditional concept, EDTA acts only as a complexing agent for metal ions to prevent the formation of hydroxide in the alkaline solution [22]. However, trace EDTA in the present system contributes to the formation of ultrasmall and uniform copper nanoparticles as well. To understand the role of the trace levels of EDTA, it should first be pointed out that KBH4 cannot reduce Cu(OH)2 to Cu or Cu2O at room temperature. This is proven by the absence of a color change after mixing Cu(OH)2 and KBH4. After trace levels of EDTA were added into the solution, it rapidly attacks the surface of Cu(OH)2, leading to the formation of a free trace Cu-EDTA complex ion. The complexing reaction of Cu(OH)2 and EDTA is shown in Eq. (1). −
CuðOHÞ2 þ EDTA→Cu EDTA þ 2OH
−
−
ð1Þ
−
Cu EDTA þ 2BH4 þ 2OH þ 2H2 O→Cu þ 2BO2 þ 7H2 þ EDTA:
400
500
600
700
Wavelength (nm) Fig. 2. UV–vis absorption spectrum of the copper nanoparticles.
800
ð2Þ
Free trace Cu-EDTA is promptly reduced into metallic Cu atom upon the release of trace EDTA once Cu-EDTA and KBH4 make contact. The corresponding reduction is depicted in Eq. (2) and is supported by an early report [23]. Afterwards, regenerated trace levels of EDTA dissolves the surface of Cu(OH)2 again, and the process repeats. The reaction can therefore be completed even with trace levels of EDTA as long as an adequate amount of reducing agent is present in the
Z. Cheng et al. / Materials Letters 65 (2011) 3005–3008
3007
a a’ Frequency ( % )
40 30 20 10 0 5
10
15
20
25
30
35
40
45
Diameter ( nm )
b
40
Frequency ( % )
b’ 30
20
10
0 5
10
15
Diameter ( nm )
c c’ Frequency ( % )
40 30 20 10 0 0
1
2
3
4
5
6
7
8
Diameter ( nm )
Fig. 4. TEM images and corresponding histograms of the copper nanoparticles prepared at different concentrations of EDTA. (a, a′) 0.05 g, (b, b′) 0.01 g, (c, c′) 0.005 g.
solution. Reactions depicted in Eqs. (1) and (2) proceed alternately until Cu(OH)2 is completely reduced to Cu. In the whole process, EDTA acts as the triggering agent and is recycled constantly. Therefore, controlling EDTA concentration regulates the concentration of free Cu-EDTA. A lower concentration of EDTA produces lower Cu-EDTA concentration, indicating that only trace amounts of copper atoms are achieved in a certain period of time. Therefore, trace EDTA contributes to the formation of ultrasmall copper nanoparticle. Moreover, Cu-EDTA concentration is approximately constant due to the trace amount of EDTA used, equal amounts of EDTA are consumed in the complexing reaction [Eq. (1)] and released in the reduction reaction [Eq. (2)]; the two reactions occur within a short interval. As a result, the effects of changing the Cu-EDTA concentration on product size are minimized, thereby resulting in uniform copper nanoparticle sizes.
3. Conclusion Ultrasmall and uniform copper nanoparticles with an average diameter of ca. 4 nm were successfully synthesized via a trace-level EDTA-assisted wet chemical route. The presence of trace EDTA is necessary for the preparation of ultrasmall and uniform copper nanoparticles. The size of copper nanoparticles can be tuned by varying the concentration of EDTA. The method can be extended to prepare other metallic nanoparticles such as Ni, Co, and Ag. Acknowledgments The authors are grateful for the financial support of the Natural Science Foundation of Jiangsu Education Committee (10KJB430003), Technological Research Foundation of Huai'an City (HAG2010073),
3008
Z. Cheng et al. / Materials Letters 65 (2011) 3005–3008
the National Natural Science Foundation of China (20975043), Jiangsu Higher Institutions Key Basic Research Projects of Natural Science (07KJA15012), and S&T support program of Huai'an City (HAS2010003). References [1] Fu BS, Missaghi MN, Downing CM, Kung MC, Kung HH, Xiao GM. Chem Mater 2010;22:2181–3. [2] Wang H, Wang JG, Shen ZR, Liu YP, Ding DT, Chen TH. J Cata 2010;275:140–8. [3] Eliyahu S, Vaskevich A, Rubinstein I. Thin Solid Films 2010;519:1661–6. [4] Tang XF, Zhen-Guo Yang ZG, Wang WJ. Colloid Surf A 2010;360:99–104. [5] Vaseem M, Lee KM, Kim DY, Hahn YB. Mater Chem Phys 2011;125:334–41. [6] Ryu J, Kim HS, Hahn HT. J Electronic Mater 2010;40:42–50. [7] Biçer M, Şişman İ. Powder Technol 2010;198:279–84. [8] Khanna PK, Gaikwad S, Adhyapak PV, Singh N, Marimuthu R. Mater Lett 2007;61: 4711–4. [9] Abdulla-Al-Mamun M, Kusumoto Y, Muruganandham M. Mater Lett 2009;63: 2007–9.
[10] Guajardo-Pacheco MJ, Morales-Sánchez JE, González-Hernández J, Ruiz F. Mater Lett 2010;64:1361–4. [11] Yu W, Xie HQ, Chen LF, Li Y, Zhang C. Nanoscale Res Lett 2009;4:465–70. [12] Salavati-Niasari M, Davar F. Mater Lett 2009;63:441–3. [13] Zhou F, Zhou RM, Hao XF, Wu XF, Rao H, Chen YK, et al. Radiat Phys Chem 2008;77: 169–73. [14] Solanki JN, Sengupta R, Murthy ZVP. Solid State Sci 2010;12:1560–6. [15] Saito M, Yasukawa K, Umeda T, Aoi Y. Opt Mater 2008;30:1201–4. [16] Park BK, Jeong S, Kim D, Moon J, Lim S, Kim JS. J Colloid Interf Sci 2007;311:417–24. [17] Charinpanitkul T, Soottitantawat A, Tonanon N, Tanthapanichakoon W. Mater Chem Phys 2009;116:125–8. [18] Brege JJ, Hamilton CE, Crouse CA, Barron AR. Nano Lett 2009;9:2239–42. [19] Dadgostar N, Ferdous S, Henneke D. Mater Lett 2010;64:45–8. [20] Zhang HX, Siegert U, Liu R, Cai WB. Nanoscale Res Lett 2009;4:705–8. [21] Cho YS, Huh YD. Mater Lett 2009;63:227–9. [22] Zhang QL, Yang ZM, Ding BJ, Lan XZ, Guo YJ. Trans Nonferrous Met Soc China 2010;20:240–4. [23] Lin RH, Fang L, Xi YX, Shao YX. Acta Chimica Sinica 2004;62:2365–8.