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Ag nanostructures with various shapes created through degradation of rods
Materials Chemistry and Physics 138 (2013) 767e772
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Ag nanostructures with various shapes created through degradation of rods Yulan Zhang a, Ping Yang a, *, Lipeng Zhang b a b
School of Material Science and Engineering, University of Jinan, 250022 Jinan, PR China Wan Jie Group Co. Ltd, 255213 Zibo, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< A polyol synthesis created uniform silver nanostructures with various shapes. < Twinned seeds play an important role for fabricating Ag rods. < The rods degraded via the etching process of O2/Cl. < Uniform Ag cubes could be created by adjusting NaCl concentrations. < The size of the cubes depended on PVP concentrations during preparation.
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Article history: Received 27 June 2012 Received in revised form 20 December 2012 Accepted 22 December 2012
We developed a facile method (a polyol synthesis) to prepare uniform silver nanostructures with various morphologies using ethylene glycol (EG) reduction of silver nitrate at 120 C in the presence of poly(vinyl pyrrolidone) (PVP) and NaCl. Ag rods were fabricated by simply aging the freshly prepared AgNO3 solution with PVP, EG, and NaCl under ambient atmosphere without changing other parameters. The formation and degradation of the rods were systematically investigated. The result indicates that twinned-crystal seeds play an important role for the growth of anisotropic Ag rods. The NaCl concentration in solutions affected the formation of the rod. The rod was degraded subsequently via an etching process of O2/Cl (NaCl), resulting in the formation of Ag nanostructures with various shapes. Twinned Ag seeds in solutions were etched by increasing the concentration of NaCl, yielding uniform Ag cubes. Regular Ag cubes were fabricated under optimal preparation conditions. Ag cubes with various sizes (150e600 nm) were created by adjusting PVP concentrations in solutions. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Metals Nanostructures Chemical synthesis Crystal growth
1. Introduction Recently, many efforts have been devoted to metallic nanostructures that have considerable potential in emerging technology fields and the potential applications in microelectronics and optoelectronic devices as well as sensors due to their unique electrical, optical, magnetic, and thermal properties [1]. Among the metals, silver nanostructures have received considerable attention because * Corresponding author. E-mail address: [email protected] (P. Yang). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.12.055
of the high electrical and thermal conductivities of bulk silver, which is an important material in many fields [2]. Because of unique surface plasmon features, silver nanostructures have exhibited application potential as optical labels, nonlinear optical devices, near-field optical probes, and active substrates for surfaceenhanced Raman scattering (SERS) [3e6]. In addition, Ag nanostructures have found interesting applications as optical polarizers, photonic crystals and catalysts, as well as biomedical and chemical sensors [7e9]. Metallic nanostructures with various morphologies have been a subject of intensive research in recent years because it provides
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an effective strategy for tuning the electronic, magnetic, optical, and catalytic properties. Especially, the controllable preparation of metal nanostructures with different shapes is of great interest because it allows one to fine-tune the properties with a greater versatility in many cases. For instance, hydrogen tends to be absorbed on cubic platinum nanostructures. The surface of which are bounded by {100} facets, while buckyball-shaped platinum nanostructures bounded by {210} faces prefer to be covered by carbon monoxide [10,11]. In addition, SERS depended on the exact morphology of Ag and Au nanostructures. For example, the theoretical studies and experimental works for Ag and Au nanostructures have suggested that the number and position of surface plasmon resonance, as well as the effective spectral range of SERS can be tuned by controlling the shape of metal nanoparticles [12,13]. The shape of a nanostructure determines the types of crystal planes exposed on its surface and thus greatly affects its properties and functions; particularly, the crystal plane plays an essential role in determining their catalytic and electro catalytic properties [14]. Therefore, the tailoring shape of nanostructures becomes an emerging strategy to innovate functional materials. It could be critical to develop an effective preparation method for particles with well-controlled morphologies and sizes. Since Jin et al. [15] developed the photoinduced method for converting large quantities of silver nanospheres into nanoprisms, many methods have been reported for the synthesis of anisotropic metallic nanostructures. The synthesis of noble metal nanostructures with various shapes, such as prisms, rods, coreeshells, cups, rings, disks, and cubes, have been continually reported by various synthesis methods, such as a modified polyol process, the photoinduced method, microwave rapid heating, and the seedmediated growth method. Mirkin’s group has reported a seeding methodology to prepare Ag and Au nanoprisms and control their edge lengths [16,17]. Xia’s group has developed a modified polyol process, in which ethylene glycol (EG) serves as both the solvent and the reducing agent, to synthesize Ag nanocubes with controllable corner truncation, right bipyramids, pentagonal nanowires [18]. However, the study on the morphological controlling and formation mechanism of noble metallic nanostructures is still expected. Especially, nanostructures with various morphologies created by a facile synthesis will be very desirable. Various of chemical methods have been developed to prepare metal particles with different morphologies. For example, ultraviolet irradiation-assisted Au reduction in the presence of poly(vinyl alcohol) was used to prepare small triangular nanoplates [19]. A seed-mediated growth method was found to form Au nanoplates on indium tin oxide surfaces [20]. Template-directed synthesis can be used to generate metallic nanowires [21]. However, the methods mentioned above often encountered many problems such as relatively low yields, irregular morphologies, polycrystallinity, and low aspect ratios. Especially for template-directed synthesis, removing unwanted materials from the nanoparticle surface requires harsh conditions or multiple washings. Solvothermal method [22] was also used to synthesize Ag nanoparticles, whereas, the reaction process was not easy to control and the reaction phenomenon was not convenient to observe. In contrast, the polyol reduction route is widely used to shape-controlled synthesis of metal particles with different morphologies. Polyol synthesis is a technique in which the reaction occurs in an oil-bath vessel, and the reaction phenomenon can be easily observed and adjusted. Xia and co-workers [23,24] have successfully synthesized Ag nanocubes and nanowires through the polyol process. The resulting samples with high quality, and a higher degree of crystallization can be obtained by polyol reduction method. A number of one-phase methods have been developed for the synthesis of metal nanoparticles. Growth mechanism was a key for
the preparation. Simultaneous nucleation is followed by particle growth without additional nucleation, allowing the growth histories of particles to be identical and thus enabling the control of the size distribution of the ensemble of particles as a whole during growth. The growth of a metal nanostructure in a solution synthesis consists of three stages: nucleation, seeding, and growth. Several approaches such as the selective oxidative etching to control the structure and population of seeds and the selective chemisorptions of capping agents to modify the surface free energy and growth rate of different crystal facets have been used to fabricate metal nanostructures. Uniform Ag nanostructures with various shapes were successfully prepared by employing the same strategy at the same reaction temperature, which nicely demonstrates the important role of seed structures and selective etching processes in the shape evolution of metal nanocrystals [25]. Despite its success in controlling the size and shape of many noble-metal nanostructures, it is still unclear why subtle changes to the reaction conditions cause metal atoms to nucleate and grow into nanostructures with different shapes. In this paper, we developed a facile strategy to create Ag nanostructures with various morphologies. A mechanism of rod degradation was proposed to explain the formation process of the nanostructure. Namely, two kinds of Ag rods were first fabricated by adjusting the concentration of AgNO3. Ag nanostructures with various shapes were then fabricated by creating Ag monomers through the degradation of the rods. In particularly, Ag cubes with a high yield were obtained through adjusting the concentration of sodium chloride (NaCl). These results indicated why the polyol synthesis has been so successful in generating Ag nanostructures with well-defined and controllable shapes. 2. Experimental section 2.1. Materials All chemicals were used as received without further purification. EG (99.8%), silver nitrate (AgNO3, 99%) and sodium chloride (NaCl) were taken from Shanghai Chemical Reagent Company. Poly (vinyl pyrrolidone) (PVP, K30) was supplied by Aldrich. 2.2. Preparation of Ag samples To investigate the formation process of Ag nanostructures with various morphologies using a mixture of AgNO3, PVP, NaCl, and EG, the molar ratio of AgNO3/PVP, the amount of NaCl, and reaction time were adjusted. The mole of PVP was calculated using average molecular weight expressed in terms of monomer. The preparation conditions of Ag samples are summarized in Table 1. Typically, 10 mL of EG was heated to 120 C in an oil bath. AgNO3 and Table 1 Preparation conditions and morphology of as-prepared Ag samples. Sample AgNO3 PVP (M)a (M)a
NaCl Temperature Reaction Morphology time (h) (mM)a ( C)
1 2
0.33 0.33
0.021 0.021
5.1 5.1
120 120
24 46
3 4 5 6 7 8 9 10
0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67
0.021 5.1 0.021 5.1 0.021 5.1 0.021 5.1 0.021 5.1 0.021 10.3 0.021 10.3 0.042 10.3
120 120 120 120 120 120 120 120
0.17 24 44 45 46 0.12 24 24
a
Rod Spherical-like, bony-rod Irregular particle Rod Irregular particle Spherical-like particle Spherical-like particle Irregular particle Cube Cube
Concentration of starting materials during preparation.
Y. Zhang et al. / Materials Chemistry and Physics 138 (2013) 767e772
a mixture of PVP and NaCl were dissolved in 6 mL of EG solvent, respectively. The AgNO3 solution was injected into the heat-treated EG solution at a rate of 1.2 mL min1, following the injection of the mixture of PVP and NaCl solutions at a same rate. After that, the mixture was heat-treated at 120 C and reacted at this temperature for several minutes to several tens hours. After being cooled to room temperature, resulting samples were separated and washed with ethanol for four times by centrifuging at 15,000 rpm for 8 min. Finally, the samples were re-dispersed in ethanol for further characterization. 2.3. Characterizations Scanning electron microscopy (SEM) images were taken using a field emission scanning electron microscope (QUANTA 250 FEG, FEI, America). Transmission electron microscopy (TEM) images were obtained using JEM-100CX and JEM-2100 electron microscope. The absorption spectra of samples were recorded on a Hitachi U-4100 spectrophotometer at room temperature using ethanol as the reference solution. 3. Results and discussion In a polyol synthesis process, the majority of larger silver particles were directly grown into nanorods with uniform diameters in the early stage of the ripening process. Furthermore, the EG solution of PVP and AgNO3 was heat-treated at 120 C to create metal colloids, where EG as a reducing agent and a solvent, AgNO3 as the precursor of Ag, and PVP as a polymeric capping agent to control the shape. The experimental conditions determine the growth process of Ag nanostructures. Table 1 summarizes the preparation conditions and morphology of as-prepared Ag samples. Oxidative etching has already been validated for a number of noble metal nanostructures including Ag, Pd, and Rh. Both ambient atmosphere and ligands are required for creating metal nanostructures with uniform shape. In contrast, Xia and co-workers have reported that pentagonal Ag nanowires are performed under argon [26]. These pentagonal nanowires are created through the growth of multiple twinned seeds forming in the early stage of the reaction. In addition, the multiple twinned seeds will be evolved into quasispherical particles quickly in the case of no ligand (such as Cl ions) addition. This means both ambient atmosphere and ligands play important roles to create single crystal seeds with high yields.
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Two kinds of mechanisms are proposed to fabricate Ag nanostructures. One is a nucleation and growth through an Ostwald ripening effect and the other is seed growth. The nucleation and growth of Ag particles were allowed when an AgNO3 solution was injected in a reaction solution. Because the surface energy of large particles is lower than that of small ones, the large ones are grown at the cost of the dissolution of small ones via an Ostwald ripening process. Because of multiple twinned boundaries existed inside, multiple twinned particles are etched into small particles that deposited on single twinned particles, in which a seed growth process occurs. The twinned particles are etched into small particles that deposited on single twinned particles, in which a seed growth process occurs. Since PVP as a polymer capping reagent, the subsequent anisotropic growth results in a portion of large Ag particles grew into Ag rods. In addition, nanoparticles will be aggregated during the reaction process in most cases because these particles are active and prone to coalesce in the solution due to van der Waals forces and high surface energy. Therefore, stabilizers or polymers (such as PVP) are used to prevent them from aggregation [5,6,27]. The long polyvinyl chain of PVP can affect the surface of silver particles and produce steric effect which was benefit for antiagglomeration, whereas the OeAg hydrogen bond formed between polymeric surfactant (PVP) and the surfaces of silver solid could also effectively prohibit the silver particles from gathering together. 3.1. Growth and degradation of Ag rods Ag rods were fabricated through a seed growth mechanism and subsequently degraded via an etching process by O2/Cl (NaCl). Fig. 1 shows the formation procedure of Ag rods (Sample 1 shown in Table 1). Fig. 2 shows the formation procedure of Sample 4 (Ag rods) shown in Table 1. Single Ag crystals created by reducing AgNO3 with PVP were as seeds. Agþ ions were produced into Ag atoms by EG. Multiple and single twinned as well as single crystal Ag nanoparticles created following a nucleation process. Because of multiple twinned particles with multiple twinned boundaries compared with single ones, O2/Cl preferred to etch the multiple twinned particles [26]. Because PVP molecules interacted strongly with {100} facets rather than {111} facets, the deposition of Ag atoms on {111} facets occurs. By etching multiply twinned decahedral seeds by O2/Cl (NaCl), Ag rods were synthesized with a high yield via the anisotropic deposition of small Ag nanoparticles on
Fig. 1. Formation procedure of Sample 1 (Ag rods) shown in Table 1. Single twinned Ag particles created by reducing AgNO3 with EG were as seeds.
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Fig. 2. Formation procedure of Sample 4 (Ag rods) shown in Table 1. Single Ag crystals created by reducing AgNO3 with PVP were as seeds.
single twinned Ag seeds in the presence of PVP. With increasing time, the degradation of the rods occurs via a same etching process by O2/Cl (NaCl) in the case of starting materials almost over. The morphology observations of Ag samples supported our explanation as follows. Fig. 3a and b show the SEM images of Sample 1 shown in Table 1 with reacting time for 24 h. The result indicates Sample 1 revealed rod morphology in Fig. 3a and b. The inset in Fig. 3b shows the higher magnification of rod structure. The results indicate clearly that the rods created through the growth of the single twinned seeds according to their morphology. The average length of the rods is 11 mm and their average diameter is 1.8 mm. One possible role of PVP is to kinetically control the growth rate of different crystalline surfaces by interacting with these faces through adsorption and desorption. The surface plane of a single twinned particle tends to be enclosed by a mix of {111} and {100}. Because the {100} facets other than {111} facets of the singly twinned seeds were selectively covered by PVP, the anisotropic growth occurs. The same mechanism was proposed in literature. For example, Xia’s group has indicated that resulting samples were Ag nanowires with high aspect ratios if the solution of PVP was injected to the reaction system after adding an AgNO3 solution; otherwise, resulting samples were predominated by nanoparticles [21]. Because the surfaces of silver nanoparticles can be passivated at a high concentration of PVP resulting in anisotropic growth for all different
Fig. 3. SEM images of as-prepared Ag samples shown in Table 1. (a), (b), Sample 1 under different magnifications, the inset in b shows a rod with a high magnification; (c) and (d), Sample 2. The inset in d shows a sample with bony morphology.
faces, the morphology of silver nanostructures could be adjusted with a high yield in various shapes. To investigate the degradation of Ag rods via an etching process by O2/Cl (NaCl), Fig. 3c and d shows the SEM images of Sample 2 shown in Table 1 with reaction time for 46 h. A large amount of irregular Ag particles were observed as shown in Fig. 3c. In addition, a little amount of Ag nanostructures with bony rod morphology was observed as shown in Fig. 3d (the inset in Fig. 3d shows Ag samples with bony morphology), in which fractures are present along the length of the nanostructure. It is well-known that the defects in Ag crystals provide actively sites for oxidative dissolution which results in the degradation of the crystal. Because a NaCl-mediated reaction was performed under air, oxidative etching was at least partially responsible for the dissolution of singly twinned silver rods and the growth of spherical-like Ag particles. Because of the difference of defect concentration in different areas of rods, some areas are covered with a lower concentration of PVP and the others were covered completely by PVP. Therefore, a rod was evolved into a bony-rod (Fig. 3c) and sphericallike Ag particles (Fig. 3d) via O2/Cl (NaCl) etching process. PVP is a linear polymer that has a polyvinyl skeleton. Because of the NeC]O group in PVP with powerful binding ability, PVP can be easily bound to the surface of Ag particles due to the reaction between NeC]O group and Ag crystals, in which the growth speed of the crystal facet bound to PVP was retarded. The interaction strengths between PVP and the various facets of Ag crystals led to an anisotropic growth of Ag nanostructures. Several groups have reported that PVP serves as reducing and capping agent. The reducing properties of PVP depended strongly on the molar ratio of PVP/AgNO3 in solutions [18]. Therefore, PVP plays an important role in determining the final morphologies of Ag nanostructures. To further investigate the effect of PVP concentration on the formation process of Ag nanostructures, Fig. 4 shows the SEM (a) and TEM (bee) images and absorption spectra (f) of as-prepared Ag samples shown in Table 1: (a) Sample 3, (b) Sample 4, (c) Sample 5, (d) Sample 6, (e) Sample 7, and (f) the absorption spectra of Samples 4e7. In the case of reaction time for 0.17 h (Sample 3), the Ag sample revealed an irregular spherical morphology. After 24 h, the irregular spheres were transferred into rods with an average diameter of 70 nm shown in Fig. 4b (Sample 4). Irregular spherical Ag particle was observed again in Fig. 4c (Sample 5 with a reaction time for 44 h) following a degradation of rods and seed growth processes. Note that most of the irregular spherical Ag particles were twinned crystal (labeled as tw in Fig. 4c) which had been reported by Xia’s group [26]. These twinned Ag particles were grown into large particles with time as shown in Fig. 4d and e (Samples 6 and 7 shown in Table 1). The average diameters of Samples 5e7 are 83 nm, 87 nm, and 100 nm, respectively. These results indicate that the growth process and degradation mechanism of Sample 3 is same with that of Ag rods as mentioned before (Sample 1 and 2). Fig. 4f shows the UVevisible absorption spectra of Sample 1, 4e7 with different reaction time. A strong Plasmon peak appeared at 410 nm confirmed the formation of Ag rod nanostructures. This phenomenon is consistent with the previous conclusion that the rod is composed of particles. An obvious red shift of the absorption peak wavelength was observed with time for Samples 5e7. This means the particles growth occurs with time. The results shown in Fig. 4f illustrate this phenomenon very well. To investigate the amount of AgNO3 on the morphology of Ag samples, Sample 3 was prepared using a higher AgNO3 concentration compared with Samples 1 and 2. The concentration of Sample 3 is 2 times higher than that of Samples 1 and 2. Even though the rods morphology can also be observed in Fig. 4b, the morphology of Sample 3 differed from that of Sample 1. This means the formation mechanism of Sample 3 is different from that of
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Fig. 4. SEM (a) and TEM (bee) images and absorption spectra (f) of as-prepared Ag samples shown in Table 1: (a), Sample 3. (b), Sample 4. (c), Sample 5. (d), Sample 6. (e), Sample 7. (f), Absorption spectra of Samples 4e7.
Sample 1. As mentioned before, PVP can be used as a reductant in spite of the reducing ability of PVP is weaker than that of EG. Once the solution of AgNO3 is injected into the reaction solution, one portion of the AgNO3 was reduced by EG, while the other portion was reduced by PVP following the process of the reaction. The reaction was easily observed through the color change of reaction solutions. After 0.17 h (Sample 3), the reaction solution was changed from yellow into red brown, and irregular spherical morphologies were observed in Fig. 4a. O2 and Cl played important roles in the dissolution of multiply and singly twinned crystals during the process when NaCl was injected to the reaction solution. It is worth to mention that the rods were attached to a lot of small particles as shown in Fig. 4b. Fig. 2 shows the formation process of these rods. In the case of EG as a reductant, the side surface of Ag rods can be passivated by PVP. The adsorption Ag precursor particles along the active surface resulted in the formation of Ag rods shown in Fig. 3a and b. However, in the case of PVP as a reductant instead of EG, the seeds were coated by linear PVP which led to Ag particles adsorbed. Therefore, rods attached to a lot of Ag particles is shown in Fig. 4b. In addition, most of Ag particles
shown in Fig. 4cee are the mixture of single crystals and singly twinned crystals, which can further demonstrate that multiply twinned crystals were degraded before the formation of rods. 3.2. Growth of Ag cubes In this section, a degradation and growth mechanism was proposed to fabricate Ag cubes by using PVP as a stabilizer and NaCl as ligands via a polyol synthesis method. As it is well-known, twinned Ag nanoparticles can be effectively dissolved in an EG solution with small amount of chloride. Multiply and singly twinned crystals must be removed from solutions in order to create Ag cubes because the cubes was grown using single Ag crystals as seeds. For this purpose, the concentration of NaCl was increased during synthesis compared with experiments mentioned above (Samples 1e7). Fig. 5 shows the SEM images of as-synthesized Ag samples (Samples 8 and 9 shown in Table 1) prepared using a high NaCl concentration. In the case of reaction time of 0.12 h, irregular spherical Ag particles were observed in Fig. 5a (Sample 8). This means the nucleation and growth occurs at the beginning of the
Fig. 5. SEM images of as-prepared Ag samples shown in Table 1. (a), Sample 8. (b) and (c), Sample 9.
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Y. Zhang et al. / Materials Chemistry and Physics 138 (2013) 767e772
Fig. 6. SEM images of as-prepared Ag sample (Sample 10 shown in Table 1) prepared using a high PVP and NaCl concentrations. (a), Image with a low magnification. (b), Image with a high magnification.
prolonged. One reason was that a low coverage concentration of PVP localized at the surface of the rods, and the other was that a higher proportion of defects exist along the singly twinned Ag rods at some regions. The degradation of Ag rods resulted in Ag nanostructures with various morphologies in the presence of PVP via controlling the interplay between the population of twinnedcrystal Ag seeds and the concentration of etchants. With increasing the concentration of NaCl, Ag cubes were fabricated. This is ascribed to the etching process of O2/Cl (NaCl) facilitated the degradation of multiple crystals and to the selective adsorption of PVP to {100}. Ag cubes with different sizes (150e600 nm) were prepared at a high Cl concentration. These results directly demonstrated the importance of seed crystal structures and etching processes for the shape-controlled solution synthesis of silver nanostructures. Acknowledgment
polyol synthesis. However, regular Ag cubes with an average edge length of 150 nm instead of those irregular Ag particles were observed after 24 h as shown in Fig. 5 (Sample 9). This is ascribed to the degradation of the irregular Ag particle and the subsequent deposition of Ag monomers on single crystal Ag seeds. A high NaCl concentration enables a quickly etching process of the irregular Ag particle. Therefore, no Ag rods were observed. In contrast, PVP resulted in the single crystal Ag seeds an anisotropic growth which led to the creation of Ag cubes. In the case of large amount of O2 and Cl in solutions, twinned Ag seeds were selectively etched away. Thus, single crystal Ag seeds may be favored for the formation of cubic Ag nanostructures in the presence of PVP. The Ag monomers have the tendency to absorb on {111} facets because PVP has the tendency to adsorb on {100} facets. Therefore, single-crystal seeds can be grown into different shapes once the added rate of atomic to different facets was controlled. Tao et al. reported on single-crystal Ag nanocubes formed by adding trace amount of CuCl2 in a 1,5-pentanediol-based polyol synthesis for a few minutes [28]. Xia et al. demonstrated a very rapid synthesized route to create Ag nanocubes by adding sulfide or hydrosulfide to an EG solution [29]. In current experiments, Ag cubes were fabricated by introducing a large amount of Cl ions. This is facile route compared with that in literature. To indicate the effect of PVP concentration on the properties of Ag cubes, Fig. 6 shows the SEM images of sample 10 prepared using a high PVP concentration: (a) image with a low magnification, (b) image with a high magnification. The average edge length of Ag cubes is 600 nm. This means PVP accelerated the growth of Ag cubes. This is ascribed that PVP effectively retard the growth of {100} facets. As a result, Ag monomers deposited on {111} facets, in which large Ag cubes were created. 4. Conclusion Ag nanostructures with various shapes such as rods, spheres, and cubes were synthesized using a PVP-mediated polyol process. Two kinds of rods were fabricated with changing the concentration of AgNO3. The formation of rods was ascribed to the selective covering of PVP on the facets of Ag particles. At low AgNO3 concentrations, EG served as a reductant and the seeds were supplied by reducing AgNO3 using EG; otherwise, PVP served as a reductant and the seeds were supplied by reducing AgNO3 using PVP. Ag rods were degraded into spherical-like Ag nanostructures via an etching process of O2/Cl (NaCl) when the reaction time was
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Ag nanostructures with various shapes created through degradation of rods Yulan Zhang a, Ping Yang a, *, Lipeng Zhang b a b
School of Material Science and Engineering, University of Jinan, 250022 Jinan, PR China Wan Jie Group Co. Ltd, 255213 Zibo, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< A polyol synthesis created uniform silver nanostructures with various shapes. < Twinned seeds play an important role for fabricating Ag rods. < The rods degraded via the etching process of O2/Cl. < Uniform Ag cubes could be created by adjusting NaCl concentrations. < The size of the cubes depended on PVP concentrations during preparation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 June 2012 Received in revised form 20 December 2012 Accepted 22 December 2012
We developed a facile method (a polyol synthesis) to prepare uniform silver nanostructures with various morphologies using ethylene glycol (EG) reduction of silver nitrate at 120 C in the presence of poly(vinyl pyrrolidone) (PVP) and NaCl. Ag rods were fabricated by simply aging the freshly prepared AgNO3 solution with PVP, EG, and NaCl under ambient atmosphere without changing other parameters. The formation and degradation of the rods were systematically investigated. The result indicates that twinned-crystal seeds play an important role for the growth of anisotropic Ag rods. The NaCl concentration in solutions affected the formation of the rod. The rod was degraded subsequently via an etching process of O2/Cl (NaCl), resulting in the formation of Ag nanostructures with various shapes. Twinned Ag seeds in solutions were etched by increasing the concentration of NaCl, yielding uniform Ag cubes. Regular Ag cubes were fabricated under optimal preparation conditions. Ag cubes with various sizes (150e600 nm) were created by adjusting PVP concentrations in solutions. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Metals Nanostructures Chemical synthesis Crystal growth
1. Introduction Recently, many efforts have been devoted to metallic nanostructures that have considerable potential in emerging technology fields and the potential applications in microelectronics and optoelectronic devices as well as sensors due to their unique electrical, optical, magnetic, and thermal properties [1]. Among the metals, silver nanostructures have received considerable attention because * Corresponding author. E-mail address: [email protected] (P. Yang). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.12.055
of the high electrical and thermal conductivities of bulk silver, which is an important material in many fields [2]. Because of unique surface plasmon features, silver nanostructures have exhibited application potential as optical labels, nonlinear optical devices, near-field optical probes, and active substrates for surfaceenhanced Raman scattering (SERS) [3e6]. In addition, Ag nanostructures have found interesting applications as optical polarizers, photonic crystals and catalysts, as well as biomedical and chemical sensors [7e9]. Metallic nanostructures with various morphologies have been a subject of intensive research in recent years because it provides
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an effective strategy for tuning the electronic, magnetic, optical, and catalytic properties. Especially, the controllable preparation of metal nanostructures with different shapes is of great interest because it allows one to fine-tune the properties with a greater versatility in many cases. For instance, hydrogen tends to be absorbed on cubic platinum nanostructures. The surface of which are bounded by {100} facets, while buckyball-shaped platinum nanostructures bounded by {210} faces prefer to be covered by carbon monoxide [10,11]. In addition, SERS depended on the exact morphology of Ag and Au nanostructures. For example, the theoretical studies and experimental works for Ag and Au nanostructures have suggested that the number and position of surface plasmon resonance, as well as the effective spectral range of SERS can be tuned by controlling the shape of metal nanoparticles [12,13]. The shape of a nanostructure determines the types of crystal planes exposed on its surface and thus greatly affects its properties and functions; particularly, the crystal plane plays an essential role in determining their catalytic and electro catalytic properties [14]. Therefore, the tailoring shape of nanostructures becomes an emerging strategy to innovate functional materials. It could be critical to develop an effective preparation method for particles with well-controlled morphologies and sizes. Since Jin et al. [15] developed the photoinduced method for converting large quantities of silver nanospheres into nanoprisms, many methods have been reported for the synthesis of anisotropic metallic nanostructures. The synthesis of noble metal nanostructures with various shapes, such as prisms, rods, coreeshells, cups, rings, disks, and cubes, have been continually reported by various synthesis methods, such as a modified polyol process, the photoinduced method, microwave rapid heating, and the seedmediated growth method. Mirkin’s group has reported a seeding methodology to prepare Ag and Au nanoprisms and control their edge lengths [16,17]. Xia’s group has developed a modified polyol process, in which ethylene glycol (EG) serves as both the solvent and the reducing agent, to synthesize Ag nanocubes with controllable corner truncation, right bipyramids, pentagonal nanowires [18]. However, the study on the morphological controlling and formation mechanism of noble metallic nanostructures is still expected. Especially, nanostructures with various morphologies created by a facile synthesis will be very desirable. Various of chemical methods have been developed to prepare metal particles with different morphologies. For example, ultraviolet irradiation-assisted Au reduction in the presence of poly(vinyl alcohol) was used to prepare small triangular nanoplates [19]. A seed-mediated growth method was found to form Au nanoplates on indium tin oxide surfaces [20]. Template-directed synthesis can be used to generate metallic nanowires [21]. However, the methods mentioned above often encountered many problems such as relatively low yields, irregular morphologies, polycrystallinity, and low aspect ratios. Especially for template-directed synthesis, removing unwanted materials from the nanoparticle surface requires harsh conditions or multiple washings. Solvothermal method [22] was also used to synthesize Ag nanoparticles, whereas, the reaction process was not easy to control and the reaction phenomenon was not convenient to observe. In contrast, the polyol reduction route is widely used to shape-controlled synthesis of metal particles with different morphologies. Polyol synthesis is a technique in which the reaction occurs in an oil-bath vessel, and the reaction phenomenon can be easily observed and adjusted. Xia and co-workers [23,24] have successfully synthesized Ag nanocubes and nanowires through the polyol process. The resulting samples with high quality, and a higher degree of crystallization can be obtained by polyol reduction method. A number of one-phase methods have been developed for the synthesis of metal nanoparticles. Growth mechanism was a key for
the preparation. Simultaneous nucleation is followed by particle growth without additional nucleation, allowing the growth histories of particles to be identical and thus enabling the control of the size distribution of the ensemble of particles as a whole during growth. The growth of a metal nanostructure in a solution synthesis consists of three stages: nucleation, seeding, and growth. Several approaches such as the selective oxidative etching to control the structure and population of seeds and the selective chemisorptions of capping agents to modify the surface free energy and growth rate of different crystal facets have been used to fabricate metal nanostructures. Uniform Ag nanostructures with various shapes were successfully prepared by employing the same strategy at the same reaction temperature, which nicely demonstrates the important role of seed structures and selective etching processes in the shape evolution of metal nanocrystals [25]. Despite its success in controlling the size and shape of many noble-metal nanostructures, it is still unclear why subtle changes to the reaction conditions cause metal atoms to nucleate and grow into nanostructures with different shapes. In this paper, we developed a facile strategy to create Ag nanostructures with various morphologies. A mechanism of rod degradation was proposed to explain the formation process of the nanostructure. Namely, two kinds of Ag rods were first fabricated by adjusting the concentration of AgNO3. Ag nanostructures with various shapes were then fabricated by creating Ag monomers through the degradation of the rods. In particularly, Ag cubes with a high yield were obtained through adjusting the concentration of sodium chloride (NaCl). These results indicated why the polyol synthesis has been so successful in generating Ag nanostructures with well-defined and controllable shapes. 2. Experimental section 2.1. Materials All chemicals were used as received without further purification. EG (99.8%), silver nitrate (AgNO3, 99%) and sodium chloride (NaCl) were taken from Shanghai Chemical Reagent Company. Poly (vinyl pyrrolidone) (PVP, K30) was supplied by Aldrich. 2.2. Preparation of Ag samples To investigate the formation process of Ag nanostructures with various morphologies using a mixture of AgNO3, PVP, NaCl, and EG, the molar ratio of AgNO3/PVP, the amount of NaCl, and reaction time were adjusted. The mole of PVP was calculated using average molecular weight expressed in terms of monomer. The preparation conditions of Ag samples are summarized in Table 1. Typically, 10 mL of EG was heated to 120 C in an oil bath. AgNO3 and Table 1 Preparation conditions and morphology of as-prepared Ag samples. Sample AgNO3 PVP (M)a (M)a
NaCl Temperature Reaction Morphology time (h) (mM)a ( C)
1 2
0.33 0.33
0.021 0.021
5.1 5.1
120 120
24 46
3 4 5 6 7 8 9 10
0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67
0.021 5.1 0.021 5.1 0.021 5.1 0.021 5.1 0.021 5.1 0.021 10.3 0.021 10.3 0.042 10.3
120 120 120 120 120 120 120 120
0.17 24 44 45 46 0.12 24 24
a
Rod Spherical-like, bony-rod Irregular particle Rod Irregular particle Spherical-like particle Spherical-like particle Irregular particle Cube Cube
Concentration of starting materials during preparation.
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a mixture of PVP and NaCl were dissolved in 6 mL of EG solvent, respectively. The AgNO3 solution was injected into the heat-treated EG solution at a rate of 1.2 mL min1, following the injection of the mixture of PVP and NaCl solutions at a same rate. After that, the mixture was heat-treated at 120 C and reacted at this temperature for several minutes to several tens hours. After being cooled to room temperature, resulting samples were separated and washed with ethanol for four times by centrifuging at 15,000 rpm for 8 min. Finally, the samples were re-dispersed in ethanol for further characterization. 2.3. Characterizations Scanning electron microscopy (SEM) images were taken using a field emission scanning electron microscope (QUANTA 250 FEG, FEI, America). Transmission electron microscopy (TEM) images were obtained using JEM-100CX and JEM-2100 electron microscope. The absorption spectra of samples were recorded on a Hitachi U-4100 spectrophotometer at room temperature using ethanol as the reference solution. 3. Results and discussion In a polyol synthesis process, the majority of larger silver particles were directly grown into nanorods with uniform diameters in the early stage of the ripening process. Furthermore, the EG solution of PVP and AgNO3 was heat-treated at 120 C to create metal colloids, where EG as a reducing agent and a solvent, AgNO3 as the precursor of Ag, and PVP as a polymeric capping agent to control the shape. The experimental conditions determine the growth process of Ag nanostructures. Table 1 summarizes the preparation conditions and morphology of as-prepared Ag samples. Oxidative etching has already been validated for a number of noble metal nanostructures including Ag, Pd, and Rh. Both ambient atmosphere and ligands are required for creating metal nanostructures with uniform shape. In contrast, Xia and co-workers have reported that pentagonal Ag nanowires are performed under argon [26]. These pentagonal nanowires are created through the growth of multiple twinned seeds forming in the early stage of the reaction. In addition, the multiple twinned seeds will be evolved into quasispherical particles quickly in the case of no ligand (such as Cl ions) addition. This means both ambient atmosphere and ligands play important roles to create single crystal seeds with high yields.
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Two kinds of mechanisms are proposed to fabricate Ag nanostructures. One is a nucleation and growth through an Ostwald ripening effect and the other is seed growth. The nucleation and growth of Ag particles were allowed when an AgNO3 solution was injected in a reaction solution. Because the surface energy of large particles is lower than that of small ones, the large ones are grown at the cost of the dissolution of small ones via an Ostwald ripening process. Because of multiple twinned boundaries existed inside, multiple twinned particles are etched into small particles that deposited on single twinned particles, in which a seed growth process occurs. The twinned particles are etched into small particles that deposited on single twinned particles, in which a seed growth process occurs. Since PVP as a polymer capping reagent, the subsequent anisotropic growth results in a portion of large Ag particles grew into Ag rods. In addition, nanoparticles will be aggregated during the reaction process in most cases because these particles are active and prone to coalesce in the solution due to van der Waals forces and high surface energy. Therefore, stabilizers or polymers (such as PVP) are used to prevent them from aggregation [5,6,27]. The long polyvinyl chain of PVP can affect the surface of silver particles and produce steric effect which was benefit for antiagglomeration, whereas the OeAg hydrogen bond formed between polymeric surfactant (PVP) and the surfaces of silver solid could also effectively prohibit the silver particles from gathering together. 3.1. Growth and degradation of Ag rods Ag rods were fabricated through a seed growth mechanism and subsequently degraded via an etching process by O2/Cl (NaCl). Fig. 1 shows the formation procedure of Ag rods (Sample 1 shown in Table 1). Fig. 2 shows the formation procedure of Sample 4 (Ag rods) shown in Table 1. Single Ag crystals created by reducing AgNO3 with PVP were as seeds. Agþ ions were produced into Ag atoms by EG. Multiple and single twinned as well as single crystal Ag nanoparticles created following a nucleation process. Because of multiple twinned particles with multiple twinned boundaries compared with single ones, O2/Cl preferred to etch the multiple twinned particles [26]. Because PVP molecules interacted strongly with {100} facets rather than {111} facets, the deposition of Ag atoms on {111} facets occurs. By etching multiply twinned decahedral seeds by O2/Cl (NaCl), Ag rods were synthesized with a high yield via the anisotropic deposition of small Ag nanoparticles on
Fig. 1. Formation procedure of Sample 1 (Ag rods) shown in Table 1. Single twinned Ag particles created by reducing AgNO3 with EG were as seeds.
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Fig. 2. Formation procedure of Sample 4 (Ag rods) shown in Table 1. Single Ag crystals created by reducing AgNO3 with PVP were as seeds.
single twinned Ag seeds in the presence of PVP. With increasing time, the degradation of the rods occurs via a same etching process by O2/Cl (NaCl) in the case of starting materials almost over. The morphology observations of Ag samples supported our explanation as follows. Fig. 3a and b show the SEM images of Sample 1 shown in Table 1 with reacting time for 24 h. The result indicates Sample 1 revealed rod morphology in Fig. 3a and b. The inset in Fig. 3b shows the higher magnification of rod structure. The results indicate clearly that the rods created through the growth of the single twinned seeds according to their morphology. The average length of the rods is 11 mm and their average diameter is 1.8 mm. One possible role of PVP is to kinetically control the growth rate of different crystalline surfaces by interacting with these faces through adsorption and desorption. The surface plane of a single twinned particle tends to be enclosed by a mix of {111} and {100}. Because the {100} facets other than {111} facets of the singly twinned seeds were selectively covered by PVP, the anisotropic growth occurs. The same mechanism was proposed in literature. For example, Xia’s group has indicated that resulting samples were Ag nanowires with high aspect ratios if the solution of PVP was injected to the reaction system after adding an AgNO3 solution; otherwise, resulting samples were predominated by nanoparticles [21]. Because the surfaces of silver nanoparticles can be passivated at a high concentration of PVP resulting in anisotropic growth for all different
Fig. 3. SEM images of as-prepared Ag samples shown in Table 1. (a), (b), Sample 1 under different magnifications, the inset in b shows a rod with a high magnification; (c) and (d), Sample 2. The inset in d shows a sample with bony morphology.
faces, the morphology of silver nanostructures could be adjusted with a high yield in various shapes. To investigate the degradation of Ag rods via an etching process by O2/Cl (NaCl), Fig. 3c and d shows the SEM images of Sample 2 shown in Table 1 with reaction time for 46 h. A large amount of irregular Ag particles were observed as shown in Fig. 3c. In addition, a little amount of Ag nanostructures with bony rod morphology was observed as shown in Fig. 3d (the inset in Fig. 3d shows Ag samples with bony morphology), in which fractures are present along the length of the nanostructure. It is well-known that the defects in Ag crystals provide actively sites for oxidative dissolution which results in the degradation of the crystal. Because a NaCl-mediated reaction was performed under air, oxidative etching was at least partially responsible for the dissolution of singly twinned silver rods and the growth of spherical-like Ag particles. Because of the difference of defect concentration in different areas of rods, some areas are covered with a lower concentration of PVP and the others were covered completely by PVP. Therefore, a rod was evolved into a bony-rod (Fig. 3c) and sphericallike Ag particles (Fig. 3d) via O2/Cl (NaCl) etching process. PVP is a linear polymer that has a polyvinyl skeleton. Because of the NeC]O group in PVP with powerful binding ability, PVP can be easily bound to the surface of Ag particles due to the reaction between NeC]O group and Ag crystals, in which the growth speed of the crystal facet bound to PVP was retarded. The interaction strengths between PVP and the various facets of Ag crystals led to an anisotropic growth of Ag nanostructures. Several groups have reported that PVP serves as reducing and capping agent. The reducing properties of PVP depended strongly on the molar ratio of PVP/AgNO3 in solutions [18]. Therefore, PVP plays an important role in determining the final morphologies of Ag nanostructures. To further investigate the effect of PVP concentration on the formation process of Ag nanostructures, Fig. 4 shows the SEM (a) and TEM (bee) images and absorption spectra (f) of as-prepared Ag samples shown in Table 1: (a) Sample 3, (b) Sample 4, (c) Sample 5, (d) Sample 6, (e) Sample 7, and (f) the absorption spectra of Samples 4e7. In the case of reaction time for 0.17 h (Sample 3), the Ag sample revealed an irregular spherical morphology. After 24 h, the irregular spheres were transferred into rods with an average diameter of 70 nm shown in Fig. 4b (Sample 4). Irregular spherical Ag particle was observed again in Fig. 4c (Sample 5 with a reaction time for 44 h) following a degradation of rods and seed growth processes. Note that most of the irregular spherical Ag particles were twinned crystal (labeled as tw in Fig. 4c) which had been reported by Xia’s group [26]. These twinned Ag particles were grown into large particles with time as shown in Fig. 4d and e (Samples 6 and 7 shown in Table 1). The average diameters of Samples 5e7 are 83 nm, 87 nm, and 100 nm, respectively. These results indicate that the growth process and degradation mechanism of Sample 3 is same with that of Ag rods as mentioned before (Sample 1 and 2). Fig. 4f shows the UVevisible absorption spectra of Sample 1, 4e7 with different reaction time. A strong Plasmon peak appeared at 410 nm confirmed the formation of Ag rod nanostructures. This phenomenon is consistent with the previous conclusion that the rod is composed of particles. An obvious red shift of the absorption peak wavelength was observed with time for Samples 5e7. This means the particles growth occurs with time. The results shown in Fig. 4f illustrate this phenomenon very well. To investigate the amount of AgNO3 on the morphology of Ag samples, Sample 3 was prepared using a higher AgNO3 concentration compared with Samples 1 and 2. The concentration of Sample 3 is 2 times higher than that of Samples 1 and 2. Even though the rods morphology can also be observed in Fig. 4b, the morphology of Sample 3 differed from that of Sample 1. This means the formation mechanism of Sample 3 is different from that of
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Fig. 4. SEM (a) and TEM (bee) images and absorption spectra (f) of as-prepared Ag samples shown in Table 1: (a), Sample 3. (b), Sample 4. (c), Sample 5. (d), Sample 6. (e), Sample 7. (f), Absorption spectra of Samples 4e7.
Sample 1. As mentioned before, PVP can be used as a reductant in spite of the reducing ability of PVP is weaker than that of EG. Once the solution of AgNO3 is injected into the reaction solution, one portion of the AgNO3 was reduced by EG, while the other portion was reduced by PVP following the process of the reaction. The reaction was easily observed through the color change of reaction solutions. After 0.17 h (Sample 3), the reaction solution was changed from yellow into red brown, and irregular spherical morphologies were observed in Fig. 4a. O2 and Cl played important roles in the dissolution of multiply and singly twinned crystals during the process when NaCl was injected to the reaction solution. It is worth to mention that the rods were attached to a lot of small particles as shown in Fig. 4b. Fig. 2 shows the formation process of these rods. In the case of EG as a reductant, the side surface of Ag rods can be passivated by PVP. The adsorption Ag precursor particles along the active surface resulted in the formation of Ag rods shown in Fig. 3a and b. However, in the case of PVP as a reductant instead of EG, the seeds were coated by linear PVP which led to Ag particles adsorbed. Therefore, rods attached to a lot of Ag particles is shown in Fig. 4b. In addition, most of Ag particles
shown in Fig. 4cee are the mixture of single crystals and singly twinned crystals, which can further demonstrate that multiply twinned crystals were degraded before the formation of rods. 3.2. Growth of Ag cubes In this section, a degradation and growth mechanism was proposed to fabricate Ag cubes by using PVP as a stabilizer and NaCl as ligands via a polyol synthesis method. As it is well-known, twinned Ag nanoparticles can be effectively dissolved in an EG solution with small amount of chloride. Multiply and singly twinned crystals must be removed from solutions in order to create Ag cubes because the cubes was grown using single Ag crystals as seeds. For this purpose, the concentration of NaCl was increased during synthesis compared with experiments mentioned above (Samples 1e7). Fig. 5 shows the SEM images of as-synthesized Ag samples (Samples 8 and 9 shown in Table 1) prepared using a high NaCl concentration. In the case of reaction time of 0.12 h, irregular spherical Ag particles were observed in Fig. 5a (Sample 8). This means the nucleation and growth occurs at the beginning of the
Fig. 5. SEM images of as-prepared Ag samples shown in Table 1. (a), Sample 8. (b) and (c), Sample 9.
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Fig. 6. SEM images of as-prepared Ag sample (Sample 10 shown in Table 1) prepared using a high PVP and NaCl concentrations. (a), Image with a low magnification. (b), Image with a high magnification.
prolonged. One reason was that a low coverage concentration of PVP localized at the surface of the rods, and the other was that a higher proportion of defects exist along the singly twinned Ag rods at some regions. The degradation of Ag rods resulted in Ag nanostructures with various morphologies in the presence of PVP via controlling the interplay between the population of twinnedcrystal Ag seeds and the concentration of etchants. With increasing the concentration of NaCl, Ag cubes were fabricated. This is ascribed to the etching process of O2/Cl (NaCl) facilitated the degradation of multiple crystals and to the selective adsorption of PVP to {100}. Ag cubes with different sizes (150e600 nm) were prepared at a high Cl concentration. These results directly demonstrated the importance of seed crystal structures and etching processes for the shape-controlled solution synthesis of silver nanostructures. Acknowledgment
polyol synthesis. However, regular Ag cubes with an average edge length of 150 nm instead of those irregular Ag particles were observed after 24 h as shown in Fig. 5 (Sample 9). This is ascribed to the degradation of the irregular Ag particle and the subsequent deposition of Ag monomers on single crystal Ag seeds. A high NaCl concentration enables a quickly etching process of the irregular Ag particle. Therefore, no Ag rods were observed. In contrast, PVP resulted in the single crystal Ag seeds an anisotropic growth which led to the creation of Ag cubes. In the case of large amount of O2 and Cl in solutions, twinned Ag seeds were selectively etched away. Thus, single crystal Ag seeds may be favored for the formation of cubic Ag nanostructures in the presence of PVP. The Ag monomers have the tendency to absorb on {111} facets because PVP has the tendency to adsorb on {100} facets. Therefore, single-crystal seeds can be grown into different shapes once the added rate of atomic to different facets was controlled. Tao et al. reported on single-crystal Ag nanocubes formed by adding trace amount of CuCl2 in a 1,5-pentanediol-based polyol synthesis for a few minutes [28]. Xia et al. demonstrated a very rapid synthesized route to create Ag nanocubes by adding sulfide or hydrosulfide to an EG solution [29]. In current experiments, Ag cubes were fabricated by introducing a large amount of Cl ions. This is facile route compared with that in literature. To indicate the effect of PVP concentration on the properties of Ag cubes, Fig. 6 shows the SEM images of sample 10 prepared using a high PVP concentration: (a) image with a low magnification, (b) image with a high magnification. The average edge length of Ag cubes is 600 nm. This means PVP accelerated the growth of Ag cubes. This is ascribed that PVP effectively retard the growth of {100} facets. As a result, Ag monomers deposited on {111} facets, in which large Ag cubes were created. 4. Conclusion Ag nanostructures with various shapes such as rods, spheres, and cubes were synthesized using a PVP-mediated polyol process. Two kinds of rods were fabricated with changing the concentration of AgNO3. The formation of rods was ascribed to the selective covering of PVP on the facets of Ag particles. At low AgNO3 concentrations, EG served as a reductant and the seeds were supplied by reducing AgNO3 using EG; otherwise, PVP served as a reductant and the seeds were supplied by reducing AgNO3 using PVP. Ag rods were degraded into spherical-like Ag nanostructures via an etching process of O2/Cl (NaCl) when the reaction time was
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