Role of Temperature in the Growth of Silver Nanoparticles Through a Synergetic Reduction Approach
© Jiang et al. 2010
Received: 25 June 2010
Accepted: 9 September 2010
Published: 23 September 2010
This study presents the role of reaction temperature in the formation and growth of silver nanoparticles through a synergetic reduction approach using two or three reducing agents simultaneously. By this approach, the shape-/size-controlled silver nanoparticles (plates and spheres) can be generated under mild conditions. It was found that the reaction temperature could play a key role in particle growth and shape/size control, especially for silver nanoplates. These nanoplates could exhibit an intensive surface plasmon resonance in the wavelength range of 700–1,400 nm in the UV–vis spectrum depending upon their shapes and sizes, which make them useful for optical applications, such as optical probes, ionic sensing, and biochemical sensors. A detailed analysis conducted in this study clearly shows that the reaction temperature can greatly influence reaction rate, and hence the particle characteristics. The findings would be useful for optimization of experimental parameters for shape-controlled synthesis of other metallic nanoparticles (e.g., Au, Cu, Pt, and Pd) with desirable functional properties.
Precious metallic nanoparticles have become more attractive because of their fascinating functional properties such as optical, electronic, and physicochemical properties due to their high surface-to-volume effect [1–7]. In most cases, the properties are heavily affected by the morphology, size, and size distribution of nanoparticles. The shape/size control of metal nanocrystals is critical and has increasingly attracted attention in the past [8–27]. Of the achieved nanoparticles such far, low-dimensional (LD) silver nanostructures (e.g., plates, discs, rods, and wires) have been extensively investigated because of an extreme degree of anisotropic geometry together with corners and/or edges (or ends) for generating maximum electromagnetic field enhancement. These have inspired not only the development of synthesis, growth, and mechanistic understanding but also the exploration of functional applications in many areas, such as near-field optical probes, optical sensors, surface-enhanced Raman spectroscopy (SERS), and biomedical labelling [8–27].
Many efforts have been made to control the formation and growth of silver nanoparticles with two-dimensional (2D) morphologies for unique functional properties and potential applications through chemical methods, such as photoinduced method [28–31], electrochemical method , ultrasonic-assistant method , solvothermal method [34–38], and templating method (e.g., 'soft' reverse micelles and 'hard' polystyrene spheres) [39–43]. Many methods have shown that the controlled growth of silver nanocrystals in solution requires specific reaction conditions (e.g., photo illumination with specially selected light wavelength or laser, and γ-ray radiation with a unique Co60 source). Among the experimental parameters, reaction temperature has been considered of great importance [34–55]. A variety of studies investigated the synthesis process at a fixed temperature, although the optimization process is probably performed. Unfortunately, little literature systematically studied the effect of the reaction temperature on the formation of silver nanoplates. For instance, Mirkin and co-workers [28, 29] demonstrated a photochemical synthesis method for generating silver nanoprisms at room temperature. By extending this technique, both Maillard  and Callegari groups  have reported the formation of Ag nanoprisms through photoinduced conversion in which the shape and size can be directly influenced by illumination wavelengths at room temperature, while the formation of silver nanoprisms usually took a long time (a few days).
Other methods in literature also paid less attention to systematically discuss the function of the reaction temperature, although a certain temperature was applied. Henglein and Giersig  supposed a γ-irradiation method to synthesize colloidal silver sols at room temperature. Electrochemical reaction for the synthesis of anisotropic gold or silver nanoparticles in an aqueous electrolyte solution could happen at room temperature or higher temperatures [45, 46]. For example, the surface condition and composition of the nanowires varied with electro-deposition time at 60°C under an applied potential of 1.6 V . By applying a cathodic voltage to the microelectrode, silver nanowires could be deposited at ~150°C . In addition, Chen and Carroll reported a surfactant-assisted method to generate silver nanodiscs (e.g., triangle, truncated, and spherical particles) in water by ageing at 40°C for a few hours . Zhang et al.  used a water/PVP/n-pentanol ternary system to synthesize silver nanoprisms by heating at 95°C for 48 h, followed by obtaining a mixture of platelets with different morphologies/sizes. The polyol-mediated synthesis, as a solvothermal reduction method, has been widely studied by Xia and his colleagues who demonstrated the shape control of silver nanoplates through heating ethylene glycol at a relatively high temperature (e.g., 160°C), which could lead to the kinetics or thermodynamics growth depending on experimental parameters [32, 34–36].
Recently, a synergetic reducing approach has been developed by our laboratory for the synthesis of silver nanoplates using two or three reducing agents including citric acid, L-ascorbic acid, and NaBH4 at room temperature [49–55]. The role of various components, such as reducing agents (citric acid, L-ascorbic acid, and NaBH4), pH, and concentration, has been investigated. Moreover, the stability, electrochemical property, and sensing application of silver plates in aqueous system have also been discussed in detail. However, the role of reaction temperature in the growth rate of silver nanoparticles is still not properly understood. This would impede the progress in the exploitation of shape-controlled synthesis and functional applications of silver nanoplates. A systematical study of the thermodynamics (temperature) effect on particle formation and growth is needed.
In this study, we investigate the role of the reaction temperature in the shape-controlled synthesis of silver nanoplates through a synergetic reduction approach. This is a part of our series studies in this area. The function of temperature in the formation and growth of silver nanoparticles will be identified by transmission electron microscopy (TEM) and UV–vis spectrometer techniques. The possible mechanisms on particle growth are finally discussed.
Silver nitrate (AgNO3, 99.9%), citric acid (99%), sodium citrate (99%), L-ascorbic acid (≥99.0%), sodium borohydride (NaBH4, 99%), and sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT, 99%) are all purchased from Sigma–Aldrich and used as received without further treatment. All the solutions were freshly made for the synthesis of silver nanoparticles, especially the freshly made NaBH4 aqueous solution that was ice bathed before use. Ultra-pure water was used in all the synthesis processes.
Synthesis of Anisotropic Silver Nanoparticles
The experimental procedure was performed according to our recent work with some modifications [49–55]. In a typical procedure, three steps were involved as followings. Firstly, 1.25 mL aqueous AgNO3 (0.02 M) and 2.5 mL NaAOT (0.02 M) solution were added into a 200-mL conical flask with ultra-pure water and then stirred for 10 min to ensure homogeneity. The final volume of mixture was then fixed at 100 mL, and the concentrations of AgNO3 and NaAOT were adjusted at 2.5 × 10-4 M and 5.0 × 10-4 M, respectively. Secondly, 1.20 mL citric acid (1.0 M) (citric acid/Ag+ = 40) and 0.30 mL L-ascorbic acid (Laa, 0.10 M) (Laa/Ag+ = 1.2) aqueous solution was added to the 100 mL solution and then stirred vigorously to obtain a homogeneous solution. The molar ratio of citric acid to silver ions was selected as high to 40 based on our recent study on the role of citric acid. At lower ratios of citric acid to silver (<12), the reaction will be very fast especially at a temperature over 30°C, where the data for TEM analysis and the UV–vis spectra are difficult to collect and record. Finally, 0.02 mL NaBH4 (0.002 M) aqueous solution was rapidly added into the above mixed solution and stirred for ~30 s. The colour of the reaction solution changed gradually from light yellow to purple, then pink, green, and finally blue.
The effect of the reaction temperature was systematically investigated by adjusting from 0 to 55°C. A higher temperature over 60°C was not considered in this work because it could result in reaction too quick to track using UV–vis and TEM techniques. For example, in a boiling solution, citric acid will become more active in reducing silver ions than a lower temperature (e.g., <60°C) [53, 56, 57], and the reaction can finish within a minute as observed. Other experimental parameters, such as the molar ratio of silver to other reactants, concentration of silver ions, L-ascorbic acid, and sodium borohydride, remain constant. To minimize the volume effect of the reactants, the total volume of the proposed system was basically kept at 100 mL by adjusting the concentration of reducing agent.
Transmission electron microscope (TEM) patterns were conducted under JEOM 1400 and operated at 100 kV. The specimen was prepared by dropping the solution onto the copper grids covered with amorphous carbon and air dried naturally. UV–vis absorption spectrum was obtained on a CARY 5G UV–visible Spectrophotometer (Varian) with a 1-cm quartz cell.
Results and Discussion
Two possible reasons can be used for understanding the phenomena: first, at the initial stage, two or more kinds of nuclei or silver clusters with different shapes (e.g., plates, spheres, hexagonal, tetragonal, or octagonal) co-exist in the reaction system, which may lead to the formation of plate-like particles and/or spherical particles. The ratio of different shapes is dependent on the amount of the silver clusters with different geometry at the initial stage. Such a description has been reported in different synthesis method, such as ultrasonic-assistant method , solvothermal method [34–38], templating method (e.g., 'soft' reverse micelles and 'hard' polystyrene spheres) [39–43], and hydrochemical method [49–55]. The assumption can be proved by existence of and (citrate) clusters identified by electrospray ionization mass spectrometry (ESI–MS) measurements, as demonstrated in our recent work . The existence of other kinds of silver clusters (e.g., , , or Ag3) has been investigated in the past. Henglein et al.  has shed some light on the nucleation process by controlling the generation of zero-valent atoms and thus their agglomeration into small clusters in a gamma-radiation-based synthesis. Silver clusters consisting of a few atoms, which do not possess metallic properties. The investigators found that silver ions were radiolytically reduced in AgClO4 solutions containing sodium poly(phosphate). In the initial stages of reduction, small clusters of silver are formed as well as colloidal particles of silver metal. In the early stages of the silver reduction, the clusters are the main products (e.g., , , or Ag3). The clusters survive for a short time (~1 h). They absorb at 275, 300, and 325 nm, while the metallic particles absorb at 380 nm even longer.
Both UV/Vis spectroscopic and scanning tunnelling microscopic studies of these clusters suggested that and were the most abundant species involved in the nucleation stage . Growth of these clusters into nanocrystals likely occurred through a combination of aggregation and atomic addition. Xia et al.  demonstrated that there exists a smaller cluster, or Ag3, in the nucleation stage of a solution-phase synthesis that employs AgNO3 as a precursor to silver. These trimeric clusters can serve as nuclei for the addition of newly formed silver atoms and eventually lead to the formation of triangular nanoplates, while the , , and might benefit for the formation of spheres. These authors also demonstrated that mass spectrometry provides a tool for simple identification and characterization of silver clusters possibly contained in aqueous AgNO3 solution. Since a mass spectrometer can separate and detect ions of different masses, it allows the different isotopes of a given element to be easily distinguished. The positively charged cluster is a trimer whose ground state has an equilateral-triangular structure (1A1) and D 3h symmetry. The linear 1Σg state is predicted to lie approximately 1 eV above the 1A1 state. Thus, the cluster should exist as a triangle, in the lowest-energy state. Similarly, the ground state of the neutral Ag3 cluster is a 2E' state with an equilateral-triangular structure (D 3h symmetry) . The triangular configuration of the trimeric clusters may naturally result in a triangular shape for the nuclei and thus for the final products.
Second, the co-existence of different morphologies may be caused by the essential crystalline silver structure, face-centre-cubic (fcc) structure, which usually shows single-, twin-, or multiple-plane structures. This could play an important role in the formation of nanoparticles with different geometries. The ratios of plates or rods to spheres/near-spheres could be mainly determined by the amount of single or twin structures formed at the initial nucleation stage. Some approaches using surface control like surfactants or polymers molecules or assisted by templates will benefit for the formation and growth of non-spheres [28–31, 34–43, 49–55]. After a carefully comparison, with temperature increasing, the spheres seem to reduce in quantity. For example, at 17°C, the ratio of plates to spheres is around 1:1, while at 55°C, it reduces to 1:3. This indicates that the yield of silver nanoplates could be tuned by changing temperature under the reported conditions.
Similar scenario was observed for the growth of spheres, and the size also shows a growth jump, i.e., the average size increases from 25 to 48 nm when temperature was raised from 17 to 32°C. While it is over 32°C, the average size of spheres decreases to ~45 nm (43°C) and ~30 nm (55°C), respectively. The jumped size for particles is probably caused by a fusion growth process. Tang et al.  reported the formation of CdTe nanorods/wires fused by spherical nanoparticles after the removal of surface ligands. Similar examples involving spherical particle fusion have also been reported by Penn et al.  who presented a mechanism for dislocation formation that may operate during early growth that involves attachment between two or more nanoparticles.
We have demonstrated the role of temperature played in the synthesis of silver nanoparticles in a synergetic reduction approach. The temperature can significantly affect the formation and growth, the shape, size, and size distribution of particles. This can be confirmed by TEM observations, UV–vis spectra measurements, and kinetics analysis. At the temperature range of 0–55°C, we found that (1) a low temperature (~0°C) could significantly slow down the formation and growth reaction, which usually takes tens of hours to complete the reducing reaction; (2) from 17 to 55°C, the reaction rate increases, and the particle size increases as well. There is a size jump at around 32°C for this reaction system, i.e., from ~90 to ~180 nm for the edge length of silver nanoplates and from 25 to 48 nm for the diameter of spheres; and (3) a high temperature (>60°C) was not investigated because at a higher temperature, for example, close to 100°C, citric acid will become more active in reducing silver ions [56, 57]. This will lead to reaction too faster to collect data for TEM and/or UV–vis analysis.
In general, the heating or cooling of the reaction system will heavily affect the reaction capability of components in reduction, surfactant adsorption/desorption, and complexing stability, the formation and growth rate, and hence the shape, size, and size distributions. Much work should be performed in this area. The thermodynamical parameters, including Gibbs free energy (ΔG), enthalpy (ΔH), entropy (ΔS), and heat capacity (Cp), will be estimated quantitatively according to the TEM observations and UV–vis spectrum analysis and reported in our upcoming work. Nonetheless, the findings would be helpful to understand the effect of temperature on the formation and growth of silver nanoparticles. This approach could be extended into other systems associated with heating or cooling for shape/size and functional property control.
We gratefully acknowledge the financial support of the Australia Research Council (ARC) through the ARC Centres of Excellence for Functional Nanomaterials.
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