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Communications in Physics, Vol. 29, No. 3 (2019), pp. 285-292 DOI:10.15625/0868-3166/29/3/13763 SYNTHESIS AND CHARACTERIZATION OF SILVER NANOPLATES BY A SEED-MEDIATED METHOD MAI NGOC TUAN ANH1,2,† DO THANH SINH1 , NGO VO KE THANH1 , NGUYEN THI THU TRANG3 , NGUYEN THI PHUONG PHONG2 AND NGUYEN DAI HAI4 1 Nanotechnology Lab, Research Laboratories of Saigon Hi-Tech Park, Lot I3 , N2 Street, District 9, Ho Chi Minh City, Vietnam 2 Falculty of Chemistry, Univesity of Science Vietnam National University Ho Chi Minh City, 227 Nguyen Van Cu Street, District 5, Ho Chi Minh City, Vietnam 3 Falculty of Material Technology, Univesity of Technology National University Ho Chi Minh City, 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Vietnam 4 Institute of Applied Materials Science, Vietnam Academy of Science and Technology, 01 Mac Dinh Chi Street, District 1, Ho Chi Minh City, Vietnam † E-mail: anh.maingoctuan@shtplabs.org Received 17 April 2019 Accepted for publication 22 August 2019 Published 3 September 2019 Abstract. Silver nanoplates (SNPs) having different size were synthesized by a seed-mediated method. The seeds-silver nanoparticles with 4–6 nm diameters were synthesized first by reducing silver nitrate with sodium borohydride in the present of Trisodium Citrate and Hydrogen peroxide. Then these seeds were developed by continue reducing Ag+ ions with various amount of L-Ascorbic acid to form SNPs. Our analysis showed that the concentration of L-Ascorbic acid, a secondary reducing agent, played an important role to form SNPs. In addition, the size and in-plane dipole plasmon resonance wavelength of silver nanoplates were increased when the concentration of added silver nitrate increased. The characterization of SNPs were studied by UV-Vis, FE-SEM, EDS and TEM methods. Keywords: silver nanoplate; seed-mediated; plasmon resonance; L-Ascorbic acid. Classification numbers: 61.46.-w, 62.23.Kn. c 2019 Vietnam Academy of Science and Technology 286 SYNTHESIS AND CHARACTERIZATION OF SILVER NANOPLATES BY A SEED-MEDIATED METHOD I. INTRODUCTION Metallic (gold and silver) are [6]. interesting scientistsalso around the citrate innanomaterials the formation of silver nanoparticles Jason M.objects Haber of andmany his colleagues studiedofthe formation silver nanoparticles with as thisthe method butplasmon have not resonance yet investigated world because their specialofoptical properties such surface (SPR) [1,2] the effect of Ascorbic Acidscattering on the size(SERS) of the nanoplate or the Raman enhances surface [3]. The[7]. optical properties of silver nanopartiour shape research, are made by ananoparticles seed-mediated have method according to absorpcles depend onIntheir andsilver sizenanoplates [1,2]. Spherical silver only one SPR Jason M. Haber with some modifications. We investigated the effect of L-Ascorbic Acid tion at 390 - 410 nm whereas silver nanoplates have three SPR absorptions due to their anisotropic on including the formation of silver nanoplates and investigated their surface properties, out-of-plane quadrupole SPR at 330-350 nm, plasmonics. in-plane quadrupole SPR at 400-450 nm and in-plane dipole SPR from 450 to 900 nm [1–3], this special optical property EXPERIMENTS makes theII.silver nanoplates red, orange, blue, green while spherical nano silver are yellow. Silver nanoplates can be synthesized by single step reduction of Ag+ ions [4] or through 2 Silver nanoplates (SNPs) were synthesized setps: developed synthesized into smalla spherical steps with small silver nanoparticles played as seeds,inistwo further plate under the silver nanoparticles played “seeds” and developed those seeds to form SNPs. effect of secondary reducing agents such as Ascorbic Acid or the effect of light [5]. An advantage of the seed-mediated is that it controls the size of the silver nanoplates formed by the Fabrication ofmethod Silver nanoparticles size of the plate depending on the size of the germ. B. Ajitha and his colleagues made silver Firstly, 0.93 mL of 10 mM AgNO3 solution (Sigma), 10 mL of 75 mM Trisodium nanoplates by mediating with Ascorbic the secondary authors Citrate (TSC, Normapur) and 132Acid μL ofas0.6% Hydrogen substance. peroxide (H2These O2) and Milli-Qfocus on the ratio water of germ and the protective agent, regardless of the effect of Ascorbic Acid [5].ofJie Zeng (Millipore) were added into a beaker and stirred for 5 minutes. The total volume and colleagues focus on thewas role of trisodium in mL the of formation of silver nanoparticles [6]. the reaction solution fixed at 200 mL. citrate Then 0.92 100 mM NaBH4 solution were to initiate reduction, color the of solution immediately to yellow, Jason M.rapidly Haberadded and his colleagues also the studied formation of silver turned nanoparticles with this demonstrating theinvestigated formation of the silver nanoparticles. Samples stored room method but have not yet effect of Ascorbic Acid were on the sizeinofdark the at nanoplate [7]. temperature for 24 hours. In our research, silver nanoplates are made by a seed-mediated method according to Jason M. Haber with some modifications. We investigated the effect of L-Ascorbic Acid on the of silver nanoplates formationFabrication of silver nanoplates and investigated their surface plasmonics. 20 mL of these seeds were poured in a 50 mL beaker, then 5 mM L-Ascorbic acid (C6H8O6, Sigma) were poured in the mixture with various volume from 0.475 mL to 7.6 II. EXPERIMENT mL. 1.9 mL of AgNO3 10 mM (Sigma) solution were droped into the mixtures at a Silver nanoplates (SNPs) synthesized two setps: controlled rate ranging 60 were μL/min. The color ofinsolution was synthesized changed fromsmall orangespherical  red silver nanoparticles played “seeds” and depending developedonthose seeds to SNPs.acid. The process was  purple  green  blue the amount of form L-Ascorbic shown in Fig. 1. Acid L-Ascorbic Dropwise AgNO3 NaBH4 Stirr 5 mins AgNO3 TSC/H2O2 30 mins Stirr Age in dark Spherical AgNPs (seeds) 24h Seeds Silver Nanoplate Fig.1. Schematic Illustration of Synthesis of SNPs. Fig. 1. (Color online) Schematic illustration of synthesis of SNPs. Characterisation The samples were analyzed by UV-Vis (Jacco V-670), FE-SEM in combination with EDSof (Hitachi-S4800, Japan), TEM (JEM-1400, Japan). Fabrication Silver nanoparticles analysis, samples were diluted 10 times, cuvette to Citrate Firstly, For 0.93UV-Vis mL of 10 mM AgNO (Sigma), 10 mLusing of 75quartz mM Trisodium 3 solution measure. For FE-SEM-EDS and TEM analysis, the samples were dripped on a 3 mm (TSC, Normapur) and 132 µL of 0.6% Hydrogen peroxide (H2 O2 ) and Milli-Q water (Millipore) MAI NGOC TUAN ANH et al. 287 were added into a beaker and stirred for 5 minutes. The total volume of the reaction solution was fixed at 200 mL. Then 0.92 mL of 100 mM NaBH4 solution were rapidly added to initiate reduction, the color of solution immediately turned to yellow, demonstrating the formation of silver nanoparticles. Samples were stored in dark at room temperature for 24 hours. Fabrication of silver nanoplates 20 mL of these seeds were poured in a 50 mL beaker, then 5 mM L-Ascorbic acid (C6 H8 O6 , Sigma) were poured in the mixture with various volume from 0.475 mL to 7.6 mL. 1.9 mL of AgNO3 10 mM (Sigma) solution were droped into the mixtures at a controlled rate ranging 60 µL/min. The color of solution was changed from orange → red → purple → green → blue depending on the amount of L-Ascorbic acid. The process was shown in Fig. 1. Characterisation The samples were analyzed by UV-Vis (Jacco V-670), FE-SEM in combination with EDS (Hitachi-S4800, Japan), TEM (JEM-1400, Japan). For UV-Vis analysis, samples were diluted 10 times, using quartz cuvette to measure. For FE-SEM-EDS and TEM analysis, the samples were dripped on a 3 mm diameter copper grid and dried at room temperature, placed on conductive carbon tape and analyzed at the voltage 10kV for FE-SEM, 100kV for TEM. III. RESULTS AND DISCUSSION Silver nanoparticles preparation The UV-Vis spectra, the TEM image and the size distribution of silver nanoparticles were shown in Fig. 2. The UV-Vis spectra of M1 had a plasmon resonance (SPR) peak at the 390 nm, demonstrating the presence of spherical silver nanoparticles [1–4]. The TEM image of this sample showed spherical silver nanoparticles with 4-5 nm diameters, average particle size was 4.6 nm. On the other hand, H2 O2 was an agent that promotes development of planar twinned seeds which the anisotropic development necessary for nanoplates formation [7, 8]. Fabrication of silver nanoplates The UV-Vis absorption spectra of SNPs preparared with different amounts of C6 H8 O6 from 0.475 mL to 7.6 mLwere shown in Fig. 3. The absorption peaks of the samples were given in Table 1. Our results showed that the SNPs prepared with 0.475 mL of C6 H8 O6 gived two unclear peaks at 337 nm and 522 nm (Fig. 3a). It suguested that small silver nanoplates had begun to appear in this sample. With increasing amounts of C6 H8 O6 from 0.95 mL to 7.6 mL, the UV-Vis spectra of samples showed three SPR peaks, which was also the characteristic absorption spectrum for the SNPs [2–8]. All samples showed out-of-plane quadrupole peaks at 337 nm, characterized for the thickness of plates [8]. When the volume of C6 H8 O6 increased from 0.475 mL to 1.9 mL, the in-plane dipole peaks were shifted to the red zone, from 522 nm to 600 nm, respectively (Table 1). We found that when when the amount of C6 H8 O6 increased, Ag+ was reduced to Ag0 faster so it would be arranged to create larger plates, leading to make red-shift of dipole peak of SNPs. It was suitable with other studies that in-plane dipole peak of the SNPs depends on their size [1, 2, 5, 7]. This was also consistent with our previous research [4] when silver nanoplates manufactured by diameter copper grid and dried at room temperature, placed on conductive carbon tape and analyzed at the voltage 10kV for FE-SEM, 100kV for TEM. III. 288 RESULTS AND DISCUSSION SYNTHESIS AND CHARACTERIZATION OF SILVER NANOPLATES BY A SEED-MEDIATED METHOD Silver nanoparticles preparation Fig.2. UV-Vis spectra (a), TEM image (b) and size distribution diagram (c) of seeds. Fig. 2. UV-Vis spectra (a), TEM image (b) and size distribution diagram (c) of seeds. The UV-Vis spectra, the TEM image and the size distribution of silver nanoparticles were shown in Fig.2. The UV-Vis spectra of M1 had a plasmon resonance (SPR) peak at the 390 nm, demonstrating the presence of spherical silver nanoparticles [1-4]. The TEM image of this sample showed spherical silver nanoparticles with 4-5 nm diameters, average particle size was 4.6 nm. On the other hand, H2O2 was an agent that promotes development of planar twinned seeds which the anisotropic development necessary for nanoplates formation [7-8]. Fabrication of silver nanoplates The UV-Vis absorption spectra of SNPs preparared with different amounts of C6H8O6 from 0.475 mL to 7.6 mLwere shown in Fig. 3. The absorption peaks of the samples were given in Table 1. Our results showed that the SNPs prepared with 0.475 mL of C6H8O6 gived two unclear peaks at 337 nm and 522 nm (Fig.3a). It suguested that small silver nanoplates Fig.3.inUV-Vis spectra of SNPs with different L-Ascorbic acid volumes had begun to appear this sample. Fig. 3. UV-Vis spectra of SNPs with from different ranging 0.475L-Ascorbic to 7.6 mL. acid volumes ranging from 0.475 to 7.6 mL. Table 1. Absorption peaks of SNPs with different L-Ascorbic acid volumes (left) andmethod. SNPs with C6 H different reactionfortimes (right). a single step reduction On 1.9 the mL other hand, TSC is necessary making SNPs because it 8O6 at not only serves as a selective for (111) facets, butPeak also 1actsPeak as a strong coordination Peak 1capping Peakagents 2 Peak 3 2 Peak 3 Samples Samples ligand with Ag+ ions [6]. (nm) (nm) (nm) (nm) (nm) (nm) 0.475 mL 337 - 522 5 min 337 410 510 0.95 mL 337 410 544 10 min 337 418 542 1.9 mL 337 441 600 15 min 337 420 555 3.8 mL 337 421 558 20 min 337 423 567 5.7 mL 337 420 539 25 min 337 428 578 MAI NGOC TUAN ANH et al. 289 However, by continuing to increase the volume of C6 H8 O6 from 3.8 mL to 7.6 mL the dipole peak wavelengths decreased from 558nm to 535nm, respectively (see Table 1). Therefore, we found that the amount of C6 H8 O6 increased as Ag+ was reduced to Ag0 faster, the new Ag0 atoms did not completely cling to the seeds to form a plate, that aggregateded together to form spherical silver nanoparticles without joining and developing SNPs. So, in our study, the amount developing SNPs. So, in our study, the amount of 5 mM C6H8O6 is suitable to form largest of 5 mM C6 H8 O6 is suitable to form largest SNPs was 1.9 mL. SNPs was 1.9 mL. Fig.4. 6 at different reaction times. Fig. 4. UV-Vis UV-Vis spectra spectra of of SNPs SNPs with with 1.9 1.9 mL mLof ofCC66HH8O 8 O6 at different reaction times. We chose a sample of 1.9 mL of C6H8O6 to study a quantitative investigation of the amount AgNO reaction times. The UV-Vis results 3 added, We of chose a sample of corresponding 1.9 mL of C6 H8toOthe study a quantitative investigation of the amount 6 todifferent were shown in Fig.4 and Table 1. of AgNO3 added, corresponding to the different reaction times. The UV-Vis results were shown in Fig. 4After and Table 1. 5 minutes, the UV-Vis spectra at 5 min showed that there were three unclearly SPR peaks. From 10 minutes onwards, the shape of the peaks was clearer. The SPR of 1. Absorption peaks of SNPs with different L-Ascorbic acid volumes (left) and SNPs Table showed three peaks corresponding to the out-of-plane quadrupole (OPQ), in-plane SNPs with 1.9 mL C6 H8 O6 at different reaction times (right). quadrupole (IPQ) and in-plane dipole (IPD) resonance [10]. All samples had the first peak corresponding to the out-of-plane quadrupole SPR at 337 nm, which was characterized by Samples Peak 1 Peak 2 Peak 3 Samples Peak 1 Peak 2 Peak 3 the thickness of the plate [4-7] and did not change significant with the reaction time. (nm) (nm) (nm) (nm) (nm) (nm) + On the other hand, as the reaction time increased corresponding to the amount 0.475 mL 337 522 5 min 337 410 510 of Ag added, dipole SPR peak 3) was544 shifted to (510 0.95 mL 337 (peak410 10 the minred zone: 337 5 minutes 418 542 nm), 10 minutes (542 nm), 15337 minutes441 (555 nm), (567 nm), (578 1.9 mL 60020 minutes 15 min 337 25 minutes 420 555 nm) and the value 3.8 at 30 minutes was 600 nm, demonstrated plate developed in size and more in mL 337 421 558 20 min 337 423 567 number, resulting in absorption intensity also increased. 5.7 mL 337 420 539 25 min 337 428 578 7.6 mLthere337 420 535 peak30atmin 441 to 600 However, was a unclearly weak 400 nm337 corresponding the in-plane dipole resonance of silver spherical nanoparticles. Because the absorbance of dipole peak of SNPs extremely silver nanoparticles, we suggested that the Afterwas 5 minutes, thestronger UV-Vis than spectra at 5spherical min showed that there were three unclearly SPR SNPs were formed by almost in this sample. peaks. From 10 minutes onwards, the shape of the peaks was clearer. The SPR of SNPs showed three peaks corresponding to the out-of-plane quadrupole (OPQ), in-plane quadrupole (IPQ) and in-plane dipole (IPD) resonance [10]. All samples had the first peak corresponding to the out-ofplane quadrupole SPR at 337 nm, which was characterized by the thickness of the plate [4–7] and did not change significant with the reaction time. 290 SYNTHESIS AND CHARACTERIZATION OF SILVER NANOPLATES BY A SEED-MEDIATED METHOD On the other hand, as the reaction time increased corresponding to the amount of Ag+ added, dipole SPR peak (peak 3) was shifted to the red zone: 5 minutes (510 nm), 10 minutes (542 nm), 15 minutes (555 nm), 20 minutes (567 nm), 25 minutes (578 nm) and the value at 30 minutes was 600 nm, demonstrated plate developed in size and more in number, resulting in absorption intensity also increased. However, there was a unclearly weak peak at 400 nm corresponding to the in-plane dipole resonance of silver spherical nanoparticles. Because the absorbance of dipole peak of SNPs was extremely stronger than silver spherical nanoparticles, we suggested that the SNPs were formed by almost in this sample. Fig.5.Fig. FE-SEM images andand the the sizesize distribution differentAscorbic Ascorbic acid 5. FE-SEM images distributiongraphs graphsof ofSNPs SNPs with with different acid concentration. concentration. The imagesand andthethe size distribution of SNPs various 8O6 content The FE-SEM FE-SEM images size distribution of SNPs with with various C6 H8 OC66H content were were presented The with sample C H O showed an existence 6 8 6 presented in (Fig.in 5).(Fig.5). The sample 0.475with mL 0.475 C6 H8 OmL showed an existence of particles withof 6 particles with a plate shape but it was unclear, the average size was 16.37 nm, the size a plate shape but it was unclear, the average size was 16.37 nm, the size distribution was stable in the distribution was in the of 15 With 1.9inmL C6H8O 6, we in the range of 15stable - 20 nm. Withrange 1.9 mL of C- 620 H8nm. O6 , we found theofsample that the found SNPs were sample that the SNPs were clear with the average size of 40.06 nm relatively uniform. When increasing to 5.7 mL of C6H8O6, the size of SNPs was reduced by almost half, from 40.06 nm to 21.61 nm and their shapes were unclear and less uniform. This result was consistent with the findings from the UV-Vis results presented above. The suitable amount of C6H8O6 to form largest SNPs in our study was 1.9 mL. Fig.5. FE-SEM images and the size distribution graphs of SNPs with different Ascorbic acid concentration. The FE-SEM images and the size distribution of SNPs with various C6H8O6 content were presented in (Fig.5). The sample with 0.475 mL C6H8O6 showed an existence of MAI NGOC TUAN ANH et al. 291 particles with a plate shape but it was unclear, the average size was 16.37 nm, the size distribution was stable in the range of 15 - 20 nm. With 1.9 mL of C6H8O6, we found in the clear with that the average sizewere of 40.06 relatively uniform. When increasing to 5.7 mL of C6 H8 O6 , sample the SNPs clearnmwith the average size of 40.06 nm relatively uniform. theWhen size of SNPs was reduced by almost half, from 40.06 nm to 21.61 nm and their shapes were increasing to 5.7 mL of C6H8O6, the size of SNPs was reduced by almost half, from unclear and less uniform. This result was consistent with the findings from the UV-Vis results 40.06 nm to 21.61 nm and their shapes were unclear and less uniform. This result was presented above. suitable amount C6 H8 O6results to form largest SNPs in The our study wasamount 1.9 mL. consistent with The the findings from theofUV-Vis presented above. suitable of C6H8O6 to form largest SNPs in our study was 1.9 mL. Fig. 6. TEM images with two different orientations (a,b) and EDS result (c) of SNPs with 1.9 mL of C6 H8 O6 . The TEM images (Fig.6a,b) showed SNPs with a clear plate shape and their sizes were about 40 nm, a uniform thickness of approximately 7-8 nm. The rod-like shape were SNPs that seen from the edge (Fig. 6a). Besides, there were some spherical silver nanoparticles with sizes of 10-15 nm. This result was consistent with UV-Vis and FE-SEM results. On the other hand, the EDS spectra (Fig.6c) were studied by placing the sample on a copper grid and recording the scan signal, so that the Cu signal was relatively strong. The results of the EDS showed that the peak of silver was clear and that the peak of carbon element and oxygen element were in TSC. IV. CONCLUSION Successfully fabricated silver nanoplates by a seed-mediated method in which L-Ascorbic acid plays an important role in the formation of silver nanoplates. The UV-Vis chart shows three surface plasmon resonance peaks of silver nanoplates, including the out-of-plane quadrupole at 337 nm, which characterize plate thickness, in-plane quadrupole and in-plane dipole according to the size of silver nanoplates. The results of UV-Vis, FE-SEM and TEM analysis showed that the size of silver nanoplates depended on the concentration of L-Ascorbic acid and the suitable amount of L-Ascorbic acid to form largest silver nanoplates was 1.9 mL. ACKNOWLEDGMENTS This study was funded by the People’s Committee of Ho Chi Minh City, performed with the support of the Board of Management of Saigon High-Tech Park (SHTP) and Research Laboratory of Saigon Hi-Tech Park (SHTPLABs) and Institute of Applied Materials Science, Vietnam Academy of Science and Technology (IAMS). 292 SYNTHESIS AND CHARACTERIZATION OF SILVER NANOPLATES BY A SEED-MEDIATED METHOD REFERENCES [1] Leif J. Sherry, Rongchao Jin, Chad A. Mirkin, George C. Schatz, and Richard P. Van Duyne, Nano Letters 6 (2006) 2060. [2] Matthew Rycenga, Claire M. Cobley, Jie Zeng, Weiyang Li, Christine H. Moran, Qiang Zhang, Dong Qin, and Younan Xia, Chemical Review 111 (2011) 3669. 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