Clay-Mediated Synthesis of Silver Nanoparticles Exhibiting Low

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Clay-Mediated Synthesis of Silver Nanoparticles Exhibiting Low-Temperature Melting Chih-Wei Chiu,† Po-Da Hong,*,† and Jiang-Jen Lin*,‡ † ‡

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

bS Supporting Information ABSTRACT: Nanohybrids of silver nanoparticles (AgNPs) supported on mica clay were synthesized by in situ reduction of silver nitrate in an aqueous solution. The required mica platelets of high aspect ratio were previously prepared by the exfoliation of mica clay stacks in a multilayered structure through an ionic exchange reaction with poly(oxypropylene)-amine-salt. The exfoliated nanoscale mica platelets (Mica) are polydispersed such that each platelet is 300
1000 nm in width and 1 nm in thickness. These platelets possess ionic charges in the form of tSiO
Na+ at 120 mequiv/100 g and are suitable for supporting AgNPs in the process of in situ reduction of silver nitrate. Transmission electronic microscopy revealed the formation of AgNPs with a narrow size distribution of ca. 8 nm in diameter on the rim of individual Mica platelets. However, the pristine layered Mica structure without exfoliation failed to produce a fine AgNP distribution but instead generated particles larger than 30 nm and some precipitates. Characterization by differential scanning calorimetry and field emission scanning electron microscopy revealed that the fine AgNPs on Mica platelets exhibited a low melting temperature of 110 °C. The AgNP/Mica nanohybrid not containing an organic dispersant is considered to be a “naked” silver particle.

1. INTRODUCTION The synthesis of nanoscale metal particles and simultaneous self-patterning is an important bottom-up process for fabricating miniature devices for electromagnetic and optoelectronic applications. The preparations of Au,1,2 Ag,3,4 Cu,5,6 and Pd7,8 nanoscale particles have been widely reported because of their potential applications in the fields of electronics,9 catalysis,10 and optics.11 The reduction of a metal salt to metal particles can be achieved by both chemical and physical methods. Some physical methods include laser ablation12 and metal vapor synthesis,13 whereas chemical means employ a variety of reducing agents such as sodium borohydride,14,15 formaldehyde,16 DMF,17 ethylene glycol,18 hydrazine,19 and alcohol.20 Recent developments have demonstrated that the reduction may be carried out by electrochemical,21 sonochemical,22 and seed-mediated routes.23,24 The synthesis of fine metal particles of sizes smaller than 5 nm has been achieved by using an organic dendrimer.25 The advantage of synthesizing water-dispersible silver nanoparticles (AgNPs) in the presence of inorganic stabilizers such as Al2O3,26 Sn,27 Cu,28 CNT,29 and laponite clay30,31 is that the use of organic components can be avoided. The clay support for surface interaction with Ag+ ion and the generated Ag0 can increase the solution stability for the fine particles.32 In this organic-free system, the naked AgNPs have the unique property of low-temperature melting.27,28,33
35 However, the AgNPs exhibited low-temperature melting behavior ranging from 150 r 2011 American Chemical Society

to 340 °C. The plate-like clays and particularly the exfoliated platelets with a high aspect ratio may provide high surface mobility for interacting with AgNPs. The previously developed process for the clay exfoliation by a polyamine
salt ionic exchange reaction36,37 allowed the isolation of the nanoscale mica platelets (Mica) having widths of 300
1000 nm and thicknesses of 1 nm.38
40 These thin platelets, which possess the functionalities of surface charges represented by tSiO
Na+, have high affinity for polar organics and inorganic salts including AgNO3. Herein, we report the uses of the exfoliated Mica with a high aspect ratio and ionic character for stabilizing AgNPs in an aqueous solution. The existence of AgNPs supported by the periphery of silicate platelets was characterized by electronic microscopes. Solution coating was carried out to form films, and then the AgNP mobility and melting property were investigated by differential scanning calorimetry and field emission scanning electronic microscope.

2. EXPERIMENTAL SECTION 2.1. Materials. The synthetic fluorinated mica clay (Mica, SOMASIF ME-100) used in this study was obtained from CO-OP Chemical Co. (Japan). The mica clay possesses a layered silicate structure with Received: July 12, 2011 Revised: August 12, 2011 Published: August 15, 2011 11690

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chemical composition of Si (26.5 wt %), Mg (15.6 wt %), Al (0.2 wt %), Na (4.1 wt %), Fe (0.1 wt %), and F (8.8 wt %). The synthetic mica has a generic structure of 2:1 layered silicate/aluminum oxide as two tetrahedron sheets sandwiching an edge-shared octahedral sheet. The aluminosilicate mica has exchangeable Na+ counterions with a cationic exchange capacity (CEC) of 120 mequiv/100 g. Silver nitrate (AgNO3, purity 99.9%) was purchased from Aldrich Chemical Co.

2.2. Exfoliation of Layered Mica into Random Platelets in Water Suspension. Direct exfoliation of synthetic mica to prepare the delaminated nanoscale mica platelets (Mica) using the poly(oxypropylene)-amine salt (POP-salt) has been reported previously.36,37 The exfoliated platelet was dispersible in water and was purified by a solvent extraction process until less than 10 wt % of the organic amine remained. The exfoliated platelets were characterized to be featureless in the Braggs pattern of X-ray powder diffraction (XRD), by thermal gravimetric analysis (TGA) for organic content and zeta potential analyzer for ionic character. The mica platelets of ionic character were hydrophilic, dispersible in water, and estimated to be approximately 300
1000 nm in diameter and 1 nm in thickness as demonstrated by transmission electron microscope (TEM) and atomic force microscope (AFM).

2.3. Synthesis of AgNPs in the Presence of Exfoliated Nanoscale Mica Platelets (Mica). Typical experimental procedures for preparing the AgNPs on Mica surface are described below. The Mica slurry (0.04 g of Mica in 4 g water; 1 wt %) was first dispersed well by agitation and soaking in deionized water at room temperature for several minutes, followed by adding a solution of AgNO3 in water (0.04 g of AgNO3 in 4 g water; 1 wt %) at the designed weight ratios of silver nitrate/ Mica, 1:1, 1:5, 1:10, 1:20, 1:30, 1:40, 1:50, 1:60, and 1:70. The mixture was stirred vigorously (stirring speed 300
600 rpm) at 80 °C for 3 h. The reaction progress was monitored by the color change of the solution from yellow to dark yellow, indicating the reduction of Ag+ to Ag0 had occurred. The control experiment using the layered mica without the exfoliation treatment only generated inhomogeneous precipitates rather than a welldispersed colloid solution. The ionic exchange reaction of Mica with a POPsalt was performed according to procedures reported previously.36 The organic layered Mica contained 10 wt % of organics and 90 wt % of silicate Mica (characterized by TGA). Experiments using organic layered Mica with different weight ratios of Ag+ were performed for comparison. UV absorption was measured at a concentration of 0.1 g sample in 3.0 g distilled water. The products were further analyzed by TEM, DSC, and FE-SEM. 2.4. Characterization and Instruments. The AgNP solution was characterized by UV
vis absorption using a Shimadzu UV-2450 spectrophotometer. Transmission electron microscopy (TEM) was performed on a Zeiss EM 902A, operated at 80 kV. The samples (1 wt % in deionized water) were deposited onto a carbon-coated copper grid. Thermal analysis for the AgNP melting was carried out with a differential scanning calorimeter (DSC, Perkin-Elmer Pyris 6). Typical sample sizes of 3
8 mg were placed on a sealed aluminum pan and heating at 10 °C/min rate from 0 to 250 °C under a nitrogen flow of 20 mL/min. The melting point (Tm) was determined from the thermogram and the enthalpy of crystallization (ΔH) by integrating the peak area. Field emission scanning electronic microscope (FE-SEM) was performed on a Zeiss EM 902A, operated at 80 kV. The samples were prepared by dropping a small amount of the AgNP suspension on a clean glass surface, followed by a dehydration oven step at 60 °C for 2 h. The samples were then fixed on a FE-SEM holder with conductive carbon paste and coated with a thin layer of Au prior to measuring. Surface element analysis was carried out by using energy dispersive X-ray spectroscope (EDS, Oxford Inca Energy 400).

3. RESULTS AND DISCUSSION 3.1. Synthesis of AgNPs by In Situ Reaction of Ag+ to Ag0.

The clay exfoliation agent was synthesized by reacting poly(oxypropylene)-amine (POP-amine) with 4,40 -isopropylidenediphenol

Figure 1. To identify POP-amine’s ability for reduction, the synthesis of AgNPs was performed by mixing aqueous solutions of AgNO3 and POPamine at 1:1 weight ratio and monitored by the characteristic UV
vis absorption at 421 nm. UV
vis absorption of AgNP formation under POP-amine reduction. Inset: (a) the solution of AgNO3 and POP-amine at ambient temperature, (b) golden yellow for AgNP formation at 80 °C for 1 h, (c) 2 h, (d) 3 h, and (e) precipitation after 48 h.

diglycidyl ether followed by acidification.36 The POP-amine salt was originally designed for the ionic exchanging intercalation of the layered silicate clays and further exfoliation into silicate platelets. During the process of preparing Mica platelets, the POP-amine was removed by several repeated extractions until 10 wt % organics remained in order to facilitate the silicate platelet dispersion. For the synthesis of AgNPs supported on Mica, it was found that conventional reducing agents such as sodium borohydride may be avoided since the POP-amine served as an instant reducing agent for AgNO3. To identify its capacity for reduction at 80 °C, the synthesis of AgNPs was performed by mixing aqueous solutions of AgNO3 and POP-amine at 1:1 weight ratio and monitored by the characteristic UV
vis absorption at 421 nm (Figure 1). It was shown that, in the absence of conventional reducing agents, the POP-amine was effective for converting AgNO3 to silver metal particles, while the solution changed to pale yellow in color after 3 h stirring. After 48 h, the solution became transparent and contained solid precipitates. The mechanism for the reduction may involve initial binding of Ag+ ions to the POP-amine possessing amine and hydroxyl functionalities followed by the redox reaction with POPamine being oxidized as described in eqs 1 and 2 below: Agþ þ NO3
þ POP-amine f ðAgþ þ POP-amineÞ þ NO3
ð1Þ where (Ag+ + POP-amine) represents Ag ions bound to the POPamine through chelation with oxypropylene segments, amine, and hydroxyl functionalities. The resulting Ag0 species is free to migrate and aggregate into AgNPs. ðAgþ þ POP-amineÞ þ NO3
f Ag0 þ OxidativePOP-amineNO3


ð2Þ

The coalescence of metal atoms leads to the formation of Ag0 clusters and ultimately the formation of nanosized metal particles. Previous literature has shown that polymers possessing amine41,42 and hydroxyl43,44 functional groups can first form complexes with silver ions and subsequently serve as stabilizers for the generated AgNPs. It has been demonstrated that the 11691

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Figure 2. Conceptual illustration of well-dispersed AgNPs on a Mica surface.

Table 1. Reduction of Silver Nitrate to AgNPs in the Presence of Different Amounts of Mica Platelets average AgNO3/Mica Ag+/CEC platelets

(molar

(weight ratio)a

ratio)b

particle size UV absorption (by TEM) color

(nm)

(nm)c

AgNO3/Mica 1/1

7.72

dark brown

none

—

1/5

1.54

dark brown

none

—

1/10

0.77

dark yellow

421

1/20

0.38

dark yellow

413

9.1

1/30

0.25

yellowish-brown

409

8.8

1/40 1/50

0.19 0.15

yellow gold

407 406

8.3 8.1

1/60

0.12

yellowish

1/70

0.11

white

none

—

0/100

0

white

none

—

1/10

0.77

dark brown

none

29.4

1/30

0.25

dark brown

none

15.4

412

11.7

19.1

AgNO3/Layered Mica

a

Addition of AgNO3 to Mica platelets solution; calculated on the basis of weight ratios of 1:1, 1:5, 1:10, 1:20, 1:30, 1:40, 1:50, 1:60, 1:70, and 0:100 AgNO3/Mica platelets. b Molar ratio of Ag+/CEC of Mica platelets. c Average particle sizes of AgNPs were measured by TEM.

presence of POP-amine could effectively bind the silver species and control the growth in size of the particles. 3.2. Reduction of Silver Nitrate to AgNPs by the Exfoliated Platelets with 10 wt % of POP-salt. Mica platelets containing

10 wt % POP-salt were prepared using a two-step process involving exfoliation of layered Mica and extraction.36,37 The isolated platelets were characterized by TGA (Supporting Information Figure S1), showing levels of organic component to be 10 wt %. As illustrated in Figure 2, the conceptual diagram illustrates the AgNPs associated with the platelet surface through the POP
brushes interaction. The presence of oxypropylene segment, amine, and hydroxyl functionalities may contribute to the AgNP formation through complexation with silver ions and subsequent reduction to Ag0. During the mixing of Mica platelet with AgNO3, the solution was observed to undergo a color change and UV absorption, as shown in Table 1. At 80 °C, the aqueous solution of silver nitrate and Mica at weight ratios of 1:1 and 1:5 gave no peak corresponding to the AgNP absorption around 400 nm but appeared a dark brown color and contained aggregates. The formation of homogeneous AgNPs occurred when the AgNO3/Mica weight ratios were raised to 1:10 and 1:50. In the presence of Mica as the stabilizer, the solution appeared to be yellow to dark yellow in color and homogeneous. The UV
vis spectra indicated the presence of nanometersized Ag0 particles by the characteristic peak absorption of 406
421 nm. By further increasing the Mica platelets component (i.e., 1:60 AgNO3/Mica), the intensity of the absorption peak (412 nm) subsided, implying that the generation of AgNPs had leveled off. 3.3. TEM Observations of AgNPs. The nanohybrids of AgNPs on Mica can be finely dispersed in water and detected by TEM. TEM analysis indicated the location of the AgNPs on the platelet rim having particle diameters of ca. 8 nm (Figure 3). During the reduction of AgNO3, AgNPs are simultaneously generated and grow in size while adhering to the silicate surface. As summarized in Table 1, the average particle sizes of AgNPs 11692

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Figure 3. TEM micrographs and the corresponding particle size histograms of AgNO3/exfoliated Mica reduction at different weight fractions of (a) 1:10, (b) 1:20, (c) 1:30, (d) 1:40, (e) 1:50, and (f) 1:60.

Figure 4. TEM micrographs and the corresponding particle size histograms of AgNPs/layered Mica hybrids prepared with different weight fractions (a and b) 1:10 and (c and d) 1:30. 11693

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Figure 5. (a) Particle size and composition of AgNPs on Mica from AgNO3 reduction in the presence of Ag+/CEC molar ratios at layered Mica and exfoliated platelets. (b) Conceptual illustration of AgNP-supported Mica platelet.

Figure 6. DSC analyses of the AgNPs/Mica hybrids prepared with different weight fractions of (a) 1:10, (b) 1:20, (c) 1:30, (d) 1:40, and (e) 1:50 (the first heating scan).

appeared to be dependent on the Mica amount. It is clearly demonstrated that the average size of AgNPs decreased from 11.7 to 8.1 nm and the size distribution became narrower when

increasing the addition of Mica platelets. In a control experiment, the use of the pristine Mica without exfoliation failed to produce AgNP from AgNO3. When the AgNO3/layered Mica weight ratios were kept between 1:10 and 1:30, no absorbance due to the AgNPs (ca. 400 nm) was observed and the solution was dark brown in color, again indicating aggregation of the AgNPs (Supporting Information Figure S2). However, under TEM analysis, the AgNPs were observed to have a broad polydispersity of ca. 30 nm and some particles were aggregated into larger sizes of ca. 100 nm (Figure 4). The immobilization of AgNPs on the Mica surface was considerably stable within the range of 1:10 to 1:50 with respect to the Mica platelets (Figure 5a). The exfoliated platelets stabilized the AgNPs more efficient than the layered Mica clay without exfoliation. The stability of AgNP dispersion is attributed to the high-aspect-ratio surface of the exfoliate platelets, as described in Figure 5b. 3.4. Melting of the AgNPs by DSC Analysis. DSC analysis indicated that the nanohybrid AgNO3/Mica weight ratios between 1:10 and 1:50 had a melting peak in the range 118
135 °C and a melting onset temperature at 105
110 °C (Figure 6). The melting point of the AgNPs (size 8.1 nm, AgNO3:Mica of 1:50) was determined to be approximately 118 °C, about 17 °C lower than the melting point of larger Ag particles (11.7 nm, 135 °C), confirming the size-dependent effect. The melting point can be 11694

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Figure 7. FE-SEM micrographs of AgNPs/Mica hybrid films after heat treatments at 80 °C for 20 min and 110 °C for 20 min on glass substrate, prepared with different weight fractions (a) 1:10, (b) 1:20, (c) 1:30, (d) 1:40, (e) 1:50, and (f) 1:60.

Figure 8. (a) Conceptual diagram of AgNP migration to surface. FE-SEM micrographs of AgNPs/Mica hybrid films with a weight ratio of 1:30, following different heat treatments: (b and c) 80 °C for 20 min, (d and e) 80 °C for 20 min and 110 °C for 20 min, (f and g) 80 °C for 20 min, 110 °C for 20 min, and 200 °C for 20 min, and (h and i) 80 °C for 20 min, 110 °C for 20 min, 200 °C for 20 min, and 300 °C for 20 min, at different magnification.

highly decreased when the size of the particles is reduced to nanometer size.45 However, the size-dependent melting behavior has been found both experimentally and theoretically.46,47 The high surface of nanoparticles has been known as one of the driving forces for the size-dependent melting point depression. Furthermore, the ΔH value of AgNPs (size 8.1 nm, 246 J/g) was smaller than those of the larger Ag particles (size 11.7 nm, 640 J/g). The normalized heat of fusion of AgNPs correlated well to particle amount. 3.5. Direct Observation of AgNP Melting by FE-SEM. FESEM images and the corresponding DSC studies of the AgNP/ Mica of different weight fractions (between 1:10 and 1:60) that were heated at 80 °C for 20 min and 110 °C for 20 min on glass substrates are summarized in Figure 7. It was observed that heat

treatment resulted in the melting of AgNPs into larger aggregates. The AgNPs on Mica were homogeneous in size distribution and exhibited an extremely low surface energy. Specifically, AgNPs/Mica of 1:30 weight ratio was subjected to a different heating profile and monitored by FE-SEM (Figure 8). When the solution was evaporated on a glass substrate and continuously heated at stages of 80 °C, 110 °C, 200 °C, and 300 °C for periods of 20 min each, the morphologies indicated the movement of AgNPs to the Mica surface and subsequent aggregation into lumps. Further investigation of the cross section of hybrid films demonstrated the grain size of the grown silver melt to be 50
500 nm, as observed by FE-SEM (Supporting Information Figure S3). Elemental analyses with EDS revealed a C/Ag content ratio of 14.44:1.67 (Supporting Information Figure S4). 11695

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4. CONCLUSION AgNPs supported on Mica platelet surfaces were synthesized from the in situ reduction of AgNO3 in the presence of exfoliated Mica platelets with tethered poly(oxypropylene)-amine salt. The AgNPs were characterized by UV
vis absorption and TEM, which revealed a narrow distribution of spherical AgNPs with a diameter of ca. 8 nm. The POP-amine containing amine and hydroxyl functional groups simultaneously functioned as an organic reducing agent and as a stabilizer. When the AgNPs were cast in the form of a film on a glass slide, the AgNPs exhibited an unusually low melting temperature of 110 °C, as evidenced by the DSC endothermic peak and FE-SEM observation. The mechanism of the melting process involves the migration of AgNPs from the matrix to the film surface through platelet surface interaction. The surface effect of ionic silicate platelets is essential for stabilizing and transporting the embedded AgNPs for fabricating Ag melt. ’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental descriptions of thermal gravimetric analyses of nanoscale mica platelets and solution color, UV
vis spectra, FE-SEM, and EDS analysis of the AgNPs/Mica nanohybrid. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +886-2-3366-5312; Fax: +886-2-8369-1384; E-mail: jianglin@ ntu.edu.tw (J.-J. Lin). Tel: +886-2-2737-6539; Fax: +886-2-27376544; E-mail: [email protected] (P.-D. Hong).

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