Preparation of triangular and hexagonal silver nanoplates on the

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Thin Solid Films 516 (2008) 5004 – 5009 www.elsevier.com/locate/tsf

Preparation of triangular and hexagonal silver nanoplates on the surface of quartz substrate Huiying Jia, Jianbo Zeng, Jing An, Wei Song, Weiqing Xu, Bing Zhao ⁎ State Key Laboratory for Supramolecular Structure and Materials, Jilin University, Changchun 130023, PR China Received 8 February 2007; received in revised form 14 December 2007; accepted 24 January 2008 Available online 6 February 2008

Abstract In this paper, triangular and hexagonal silver nanoplates were prepared on the surface of quartz substrate using photoreduction of silver ions in the presence of silver seeds. The obtained silver nanoplates were characterized by atomic force microscopy and UV–vis spectroscopy. It was found that the silver seeds played an important role in the formation of triangular and hexagonal silver nanoplates. By varying the irradiation time, nanoplates with different sizes and shapes could be obtained. The growth mechanism for triangular and hexagonal nanoplates prepared on quartz substrate was discussed. © 2008 Elsevier B.V. All rights reserved. Keywords: Photoreduction; Silver nanoplates; Atomic force microscopy

1. Introduction Currently, there is a great deal of interest in silver nanodisks with triangular and hexagonal shapes due to their shapedependent optoelectronic properties. Appearance of a longitudinal plasmon resonance, strong Surface-enhanced Raman Scattering (SERS), fluorescence, and anisotropic chemical reactivity are important properties of nanodisks and which can be exploited for various applications. Several synthetic strategies have been developed for simple, wet chemical synthesis of silver nanoprisms and nanodisks which based on surfactantbased seed-mediated growth [1–3], thermal growth [4], photoreduction of metal nanoparticles [5–9] and chemical reduction [10–13] in solution. Recently, nanoparticles prepared directly on surface have attracted considerable attention. Au nanorods with different length and aspect ration have been synthesized in the presence of alkyltrimethylammounium bromide (CTAB) on glass, Si/SiOx, and mica surface [14–18].

⁎ Corresponding author. State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China. Tel.: +86 431 85168473; fax: +86 431 85193423. E-mail address: [email protected] (B. Zhao). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.01.024

Syntheses of nanoparticles on flat substrates provide several advantages for studying the seed-mediated growth mechanism. Firstly, they allow the reaction to proceed under more constant and controlled conditions than that with seed particles in solution. Secondly, the reaction can be quenched rapidly by rinsing the substrate with water to remove growth solution. Furthermore, growth process can be directly monitored. Finally, surface growth yields and shape distributions are determined quantitatively since microscopic analysis takes place with little perturbation of the particle ensemble. This is significant because transfer of a sample from a bulk solution to a transmission electron microscopy (TEM) grid can be a source of selectivity and systematic errors in yield and shape measurements [16]. Aslan et al. in the first instance reported the deposition of silver triangular on conventional glass substrate which is based on the seed-mediated CTAB-directed growth of silver triangles on glass surfaces [19]. The size of the silver triangular was controlled by sequential immersion of silver seed-coated glass substrates into a growth solution and by the duration time of immersion. Although most of the silver particles have converted into triangular particles (80%) by this method, it was also found that the silver triangles were irregularly shaped; both the silver triangles and the rodlike particles were agglomerated. For the better understanding of the growth mechanism

H. Jia et al. / Thin Solid Films 516 (2008) 5004–5009

and wider application of such kind of anisotropic metal nanoparticles, a much reliable method which could synthesize lower size dispersed and well-shaped nanodisks on the surface is necessary. In our previous study [18], a method for enlargement of colloidal silver seeds directly on glass substrate has been developed. Based on the colloidal Ag surface catalyzed reduction of Ag+ by citrate under light irradiation, the silver seeds can grow into bigger nanoparticles with different sizes and shapes by varying the reduction time. Such method can be also a good starting point for developing growth of triangular nanoparticles in solution [20]. In this paper, we prepared the truncated triangular and hexagonal shaped nanoparticles of relatively well separated and well-shaped on quartz substrate. The size and shape of the silver particles can be controlled by varying the experimental parameters such as immersion time (growth time). The growth mechanism of the nanoplates was also proposed. 2. Experiment 2.1. Substrate preparation Quartz slides were cleaned by ultrasonication in ultra-pure water, ethanol, acetone, chloroform, acetone, ethanol, ultra-pure water for 5 min, respectively. They were then immersed in a 0.5% poly(diallydimethylammonium) (PDDA) solution for about 30 min, and finally rinsed with triply distilled water. The purpose of the PDDA used here is to make the quartz slide covered with positive charges in order to adsorb the silver nanoparticles. 2.2. Synthesis of seed solution For the preparation of aqueous solutions, deionized water was deoxygenated by bubbling with nitrogen gas for 30 min before used. The 0.5 ml sodium citrate (30 mM) was added to an aqueous solution of 50 ml silver nitrate (0.1 mM) with rapid stirring. Then 0.5 ml aliquot of freshly prepared sodium borohydride (5 mM) was added by drop-wise addition to the mixture under vigorous stirring, the solution changed color to yellow immediately. Before using, the colloid solution was diluted 100 times [18,20] (The solution thus prepared, we call it seed solution). 2.3. Synthesis of triangular silver nanoparticles on quartz surface The slides with PDDA prepared by the above method were immersed into the silver seed solution, mentioned above, for 1 min. Then the substrates were rinsed with triply distilled water and treated under sonication for 10 min. For the growth of nanoparticles, the substrates with the seed particles on them were immersed in a solution containing 0.1 mM silver nitrate and 0.3 mM sodium citrate and then irradiated with the Sodium lamp (OSRAM NAV-T 70-W λ = 589 nm) for a variable duration. Finally the substrates were thoroughly rinsed with triply distilled water and dried under a stream of nitrogen.

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2.4. Characterization Atomic force microscopy (AFM) images were obtained from a Digital Instruments Nanoscope IIIA used in tapping-mode with Si cantilevers purchased from DI and Nanosensor Co. Ltd. E and J scanners were used with the Digital Instruments Multimode AFM head. Furthermore, the scan rate was 1.01 Hz. Scanning electron microscopy (SEM) experiments were performed on JEOL JSM-6700F field emission scanning electron microscope operated at a voltage of 10 keV. UV–vis spectra were recorded on a Shimadzu UV-3100 spectrophotometer. 3. Results and discussion Fig. 1(a) presents a typical AFM image of a quartz /PDDA slide after it was placed in the diluted seed solution for 1 min. The bright dots represent the silver seeds as confirmed by height analysis. From the image and our previous experiment [18], we can estimate the diameter of the confined silver seeds 5–15 nm, which is similar to our previous preparation of the original silver seeds according to the TEM measurements [20]. Fig. 1(b) shows the AFM images of the quartz/PDDA substrate containing silver seeds after it was placed in the growth solution for 2 h. The circles are used to indicate triangular nanoparticles after growth. It can be clearly seen that all particles have grown substantially and parts of the particles have grown into small truncated triangular particles. From the AFM image, we can suggest that the estimate of the thickness of the small nanoplates is 9 ± 1 nm. There are two possible routes for the formation of the Ag nanostructures on the surface: (1) the Ag+ was reduced to Ag nanoparticles by citrate in the solution and subsequent deposition on the surface. (2) The Ag+ was reduced to Ag nanoparticles directly onto surface-confined silver seeds. Since there is no deposition of Ag on the surface without Ag seeds present and we do not observe nucleation of Ag in the growth solution with or without Ag seeds on the surface, route 2 may contribute to the formation of the triangular silver nanostructures on the surface as it has been observed previously the synthesis of Au nanorods directly on glass surfaces using seed-mediated deposition of Au from AuCl4− onto surface-attached 3–5 nm diameter Au nanoparticles in the presence of CTAB [15]. As the reaction proceeded to 4 h, the amounts of the initial spherical particles decreased, meanwhile, an increasing number of larger silver nanoparticles with truncated triangular shape appeared, and it is also found that some of the nanoparticles on the substrates had grown into hexagonal shape as we can see in Fig. 1(c). Compared with the 2 h slide, the size of the triangular nanoplates changed significantly while the thickness increased a little (The thickness of the triangular nanoplates of 4 h is about 11 ± 3 nm.). This implies that nanoplates grow mainly along its edge side at this stage of the reaction. Fig. 2(a) is the topographical image of a single triangular nanoplate after growth for 4 h. The triangular particles exhibited the shape of equilateral triangles, like the nanoprisms prepared by wet chemical synthesis in solution. The line scans across the same nanoplates is shown in Fig. 2(a). It is found that the angle between the basal

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Fig. 1. AFM images of silver seed deposited onto quartz/PDDA substrate (a), after nanoparticles growth for 2 (b), 4 (c) and 5 h (d).

plane and the lateral faces is 72°. TEM experiments have revealed that the triangular and hexagonal nanoplates have a face-centered cubic (fcc) structure and the (111) planes are the basal planes of the nanoplates [4,5,7]. So, if the basal plane of the nanoplates in Fig. 2(a) is the (111) plane, the side plane of the plate should be bound by three other (111) planes. Fig. 3(a) illustrates the top view of a (111) oriented triangular nanoplate on the substrate. When the reaction time reached 5 h, we could see from the AFM image that the amounts of triangular shaped particles

deceased and many more or less regular hexagonal nanoparticles formed. It is also interesting to note that the size and thickness of the nanoplates did not change much but the shape was altered dramatically. Fig. 2(b) is the topographical image of a single hexagonal nanoplate after growth for 5 h and the line scans across the same nanoplates. The profile plots clearly show that the angles between the two lateral faces and the substrate are different and estimated to be 58 and 73°, respectively. We have mentioned above that hexagonal nanoplates have a fcc structure and the (111) planes are the basal planes of nanoplates, so the two lateral faces are consistent with (100) and (111) faces. From these data, we conclude that the nanoplates appear as flat crystals with two (111) faces at the top and the bottom, limited by three straight (111) faces at the edges and by three (100) faces at the corners. Hence, their outside shape appears in top view as hexagonal as drawn in Fig. 3(b).

Fig. 2. AFM topographic images and corresponding line scans of triangular (a) and hexagonal nanoplates (b) after 4 and 5 h of growth, respectively.

Fig. 3. Schematic representation of triangular and hexagonal crystals (111) oriented on the substrate.


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Fig. 1 also suggests that there is a seed diameter range for the growth of triangular nanoplates. Since the thickness of the initially formed small triangular nanoplates ranges from 8 to 10 nm, the critical seed size, therefore, could never be greater than 10 nm. For the spherical seeds larger than 10 nm, photoreduction on the surface only lead to the increase in the diameter which finally grew into bigger spherical particles as we can see in Fig. 1(a). Xia et al. found that a few of triangular nanoplates with relatively small sizes (5–15 nm) had been formed when the dispersion of silver nanospheres (3.5 nm in diameter) were exposed to the natural light for several days [7].

These plates served as the seeds in directing the transformation of spherical colloids into triangular nanoplates at elevated temperatures via a process similar to Oswald ripening. But when the seed is immobilized onto the surface, Ostwald ripening is prevented. So the yield of nanodisks prepared on the surface is lower than that in solution. SEM was also used to characterize the growth process of the nanoplates. Fig. 4 presents the SEM images of quartz/PDDA after nanoparticles growth for 3 and 5 h from which the size of the triangular and the hexagonal nanoplates are estimated to be about 116 ± 8 and 160 ± 60 nm, respectively. The growth process was also monitored by UV–vis spectroscopy, it exhibited surface plasmon resonance (SPR) band at different frequencies, if the Ag nanostructure was varied in size or shape. Fig. 5 presents a statistical analysis of Ag nanoplates' formation as a function of the irradiation time and Fig. 6 shows the UV–vis spectra at different reaction time. No adsorption peak was seen from the 0 hour surface (seed surface); this is likely because of the very low density of seeds on the surface. After that, the absorbance increases with increasing time in growth solution. This indicates that the amount of reduced Ag(0) increases as time in growth solution increases, which is consistent with what is shown in the AFM images. The spectrum of the two hour surface exhibits a broad peak at 611 nm, characteristic of inplane dipole plasmon resonance for triangular nanoplates [5]. This peak is very sensitive to the size and aspect ratio and the fact that it shifts towards the red as the reaction proceeds indicates that the nanoplate size increases with time. When the reaction proceeds for 7 h, this peak shifts to 703 nm, indicating the formation of larger nanoplates. But from the UV–vis spectra, we can not ascertain the time when the triangular nanoparticles change to hexagonal nanoparticles. It can also be seen from Fig. 6 that a peak at ~ 400 nm appears. This peak is normally attributed to the out-of-plane dipole resonance of nanoplates. However there are still many spherical particles on the surface and their size increases with time, as we can see in the AFM images, and these may also have their absorption band in the 400 nm range. Unfortunately, we do not observe an outof-plane quadrupole resonance of nanoplates at around 350 nm. This is most likely because of the low yield of nanoplates on the surface.

Fig. 5. Statistical analysis of Ag nanoplates formation versus irradiation time.

Fig. 6. UV–vis spectra of quartz/PDDA/silver nanoparticles after growth for different time.

Fig. 4. SEM images of quartz/PDDA after nanoparticles growth for 3 (a) and 5 h (b).

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A mechanism for the growth of silver triangular nanoplates in solution has been proposed previously [20]. The photoinduced conversion process can afford the energy of electron transfer and promote anisotropic growth. Mirkin's group reported that triangular seeds with small sizes (5–10 nm) were formed in the early stage of their photoinduced conversion process [5]. The application of a continuous irradiation helped to reduce the spherical colloids into smaller clusters, which then grew on the tiny triangular seeds, leading to the formation of larger nanoplates. Recently, the same group of authors has also proposed that a bimodal growth process occurs caused by an edgeselective particle fusion mechanism [6], with four small nanoprisms coming together in step-wise fashion to form a bigger nanoprism. It is important to note however that the growth of nanoprisms directly on surfaces has several differences compared to the solution growth. Many processes, which can occur in solution, such as seed particle aggregation, Ostwald ripening and fragmentation, are prevented when the seed is confined and immobilized on the surface. Edge-selective particle fusion leads to the forming of bigger nanoprisms; however, this process likely does not occur with the seeds attached to the surface. Previous studies have indicated that the particle shapes are closely related to the crystallographic surfaces that enclose the particles [21]. Surface energies associated with different crystallographic planes are usually different. For a spherical single crystalline particle, its surface must contain high index crystallography planes, which possibly results in a higher surface energy. Facets tend to form on the particle surface to increase the portion of the low-index planes. Therefore, for those small particles, the surface is a polyhedron. With the influence of other factors, the particles could grow toward some selected directions to get the final shapes. Murphy and co-workers have also explained the influence of the surfactant properties on the nanorod aspect ratio [22]. In that model, the alkyltrimethylammounium headgroup selectively binds different gold facets on the seed particle to create growth anisotropy and the surfactant chains stabilize the bilayers through van der Waals interactions along the nanorod sides. Our observations support the mechanism proposed by them. In our whole reaction procedure, citrate molecules played a key role. Citrate acted as a capping ligand for the silver particles as well as a photoreducing agent for the silver ions. The reaction we have observed is likely to be the reaction described by the following equation. h

Citrate þ Agn Y Acetone 1; 3 dicarboxylate þ CO2 þ Ag n

Fig. 7. The formation process of silver nanoplates.

(d b 10 nm), citrate might bind more strongly to the (111) side planes than the (100) side plane, resulting that the relative growth rate of the (100) was significantly larger than that of the (111) side planes. Thus, the (100) faces vanished and larger triangular nanoparticles with (111) side planes formed. As the nanoprism grows in size, the areas of the (111) side planes increase and more citrate molecules are needed to stabilize the crystal surface. At the same time, the concentration of citrate in growth solution is a little lower than that at the beginning of reaction. So the inhibition of citrate on the nanoparticle will be weakened so that Ag(0) tends to deposit onto the (111) side planes. Hence, the difference between the growth rate of (111) and (100) is not as significant as that at the beginning of the reaction. Therefore the nanoparticles change from triangular nanoplates to hexagonal nanoplates. In contrast, the height of each particle increases slowly, which indicates that the (111) basal plane is effectively blocked from further growth compared with other side planes. Wang has also found that the relatively low growth rate on the (111) basal planes results in the formation of triangular nanoplates [21]. If the seed has a diameter above 10 nm, citrate cannot promote anisotropic growth and photoreduction on the surface but only leads to an increase in diameter as we have seen in the AFM images in which many large spherical particles are formed. A detailed mechanism for the growth process of nanoplates needs to be established by more experiments and quantitative calculations. The growth process of the nanoplates that we propose is illustrated in Fig. 7.

þ Ag n þ Ag YAgnþ1

4. Conclusions

When the immobilized silver seeds were put into the growth solution, they would form a strong complex with citrate anions. Under light irradiation, citrates were likely selectively adsorbed on the different crystal planes of the silver seeds and silver ions in the growth solution could easily be reduced by the citrate on the nanoparticle surface. It is known that for the fcc metal, (111) faces are more stable than (100) faces. The growth rates of the two faces are much more different. For the small seeds

We have described a simple method for the synthesis of triangular and hexagonal silver nanoplates on surface and proposed a growth mechanism. The formation process of nanoplates was studied by AFM and UV–vis spectra. We found that only the seed particles whose diameter smaller than 10 nm could transform into nanoplates. At early stages of reaction, the stronger adsorption of citrate molecules on the (111) side plane than that of the (100) side plane of the silver seeds may account


for the growth of triangular nanoparticles. As the reaction proceeds, the inhibition of citrate on (111) side plane will be weakened and the difference between the growth rate of (111) and (100), which lead to the formation of hexagonal nanoplates, is not as prominent as that at the beginning of the reaction. By this method, we can get triangular and hexagonal shaped nanoplates with different sizes by varying the reaction time. There are still unresolved questions, but the results represent a significant step to gain a better understanding of growth mechanism of silver nanoplates that may lead to a refinement of the synthesis method, achieve the control of nanoplates' properties, and lead to future applications of triangular and hexagonal nanoplates. Acknowledgment The Research was supported by the National Natural Science Foundation (Grant Nos. 20473029, 20573041, 20773044) of P. R. China; by the Program for Changjiang Scholars and Innovative Research Team in University (IRT0422), Program for New Century Excellent Talents in University (NCET); by the 111 project (B06009). References [1] S.H. Chen, Z.Y. Fan, D.L. Carroll, J. Phys. Chem., B 106 (2002) 10777. [2] S.H. Chen, D.L. Carroll, Nano Lett. 2 (2002) 1003. [3] S. Chen, D.L. Carroll, J. Phys. Chem., B 108 (2004) 5500.

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