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Oct 16, 2012 - Introduction. Silver (Ag) is a ductile, malleable coinage metal that exhibits the ... exhibit many unique properties that cannot be observed in bulk.
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TUTORIAL REVIEW

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Controlled synthesis of colloidal silver nanoparticles in organic solutions: empirical rules for nucleation engineering† Yugang Sun* Controlled synthesis of colloidal nanoparticles in organic solutions is among the most intensely studied topics in nanoscience because of the intrinsic advantages in terms of high yield and high uniformity in comparison with aqueous synthesis. However, systematic studies on the formation mechanism of

Received 28th July 2012

nanoparticles with precisely tailored physical parameters are barely reported. In this tutorial review, we

DOI: 10.1039/c2cs35289c

take the synthesis of different Ag nanoparticles as an example to rule out the general principles for controlling the nucleation process involved in the formation of colloidal Ag nanoparticles in organic solutions, which enables the synthesis of high-quality nanoparticles.

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1. Introduction Silver (Ag) is a ductile, malleable coinage metal that exhibits the highest electrical and thermal conductivity among all metals

Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA. E-mail: [email protected] † Part of the chemistry of functional nanomaterials themed issue.

Yugang Sun received his BS and PhD degrees in chemistry from the University of Science and Technology of China (USTC) in 1996 and 2001, respectively. He is currently a staff scientist for the Center for Nanoscale Materials at Argonne National Laboratory. He is the 2007 recipient of The Presidential Early Career Awards for Scientists and Engineers (PECASE) and the 2008 recipient of DOE’s Office of Yugang Sun Science Early Career Scientist and Engineer Award. His current research interests focus on the synthesis of a wide range of nanostructures, including metal nanoparticles with tailored properties, the development of in situ synchrotron X-ray techniques for real-time probing of nanoparticle growth, and the application of these nanomaterials in energy storage, photocatalysis, and sensing.

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and high optical reflectivities, resulting in Ag being a widely used material in many areas such as electric contacts and conductors, mirrors, and catalysis of chemical reactions. As sizes of Ag particles decrease down to the nanometer scale, they exhibit many unique properties that cannot be observed in bulk Ag. For example, the high ductility of Ag dramatically reduced in Ag nanowires with fivefold twinning structures.1 Synthesis of Ag nanoparticles boomed in the past decade and their corresponding properties and applications were extensively studied.2–5 This progress has advanced the commercialization of manmade Ag nanomaterials that represent the most widely used materials in nanotechnology consumer products (i.e., 313 Ag-based products as analyzed on March 10, 2011).6 For instance, Ag nanoparticles have been used as a class of broad-spectrum antimicrobial reagents in medical and consumer products such as household antiseptic sprays and antimicrobial coating for medical devices.7,8 Water filters incorporating Ag nanowires have been demonstrated to be very efficient for cleaning water that is polluted with bacteria.9 Due to the large surface-to-volume ratios of the Ag nanoparticles in comparison with their bulk counterparts, Ag nanoparticles have been used as classic catalysts for important industrial reactions including oxidation of ethylene to ethylene oxide, propylene to propylene oxide, and methanol to formaldehyde.10,11 Heterocyclizations, addition of nucleophiles to alkynes (or allenes, or olefins), cycloaddition reactions (e.g., enantioselective [2+3]-cycloaddition of azomethine and nitrilimine), [4+2]-cycloaddition of imines, and acetylenic Csp–H and Csp–Si bond transformations can also be achieved through Ag-catalyzed processes.12,13 The high electrical and thermal conductivities of Ag make Ag nanoparticles to be widely used in electronics

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Tutorial Review industry as conductive fillers in conductive adhesives14 and thermal interfacial materials.15 Most recently, two-dimensional (2D) random networks of Ag nanowires have been exploited to serve as transparent conductive films due to the fact that the low percolation threshold for Ag nanowires assures a high percentage of open areas in the conductive networks.16 In combination with the thin diameters of Ag nanowires that are responsible for the mechanical flexibility of the nanowires, such 2D networks are very promising to replace the traditional rigid doped metal oxide conductive films, such as the most commonly used tin-doped indium oxide (ITO).17 In addition to these commercial applications, Ag nanoparticles also represent an important class of optical materials related to a recent hot research field, i.e., plasmonics.18 Ag nanoparticles exhibit strong surface plasmon resonances (SPRs) under illumination of light due to strong coherent oscillation of free surface electrons in the nanoparticles, resulting in strong absorption and scattering of incident light. As a consequence, dispersions of Ag nanoparticles always exhibit a colorful appearance. The evanescent electrical fields near the surface of an Ag nanoparticle are usually very high, providing ‘‘hot spots’’ to enhance Raman scattering19–23 and fluorescence24 of molecules or emitters (such as quantum dots and upconversion nanocrystals) adjacent to the Ag nanoparticle. The unique SPRs in Ag nanoparticles can benefit their traditional use such as in catalytic oxidation reactions (e.g., ethylene epoxidation, CO oxidation, and NH2 oxidation) because excitation of SPRs on the surfaces of the Ag nanoparticles can form energetic electrons that are transferrable to chemical species adsorbed on the nanoparticle surfaces.25 For example, in the commercially important partial oxidation of ethylene to form ethylene oxide O2-dissociation represents the ratelimiting elementary step that requires a large thermal energy (corresponding to a high temperature) to drive this reaction. Illumination of the Ag nanoparticles can excite plasmons on the Ag surface to populate O2 antibonding orbitals and so form a transient negatively ionic state, which thereby facilitates the ratelimiting O2-dissociation reaction. As a result, the thermal energy and temperature can be lowered to drive this oxidation reaction, leading to an increase in energy efficiency and long-term stability of catalysts and product selectivity. These results imply that continuous study of the unique properties of Ag nanoparticles can help us exploit their novel applications. Intensive studies in the past decade clearly show that the physical parameters including size, shape, surface coating, and surrounding environment of an Ag nanoparticle strongly influence its properties and thus its performance in applications. For example, unpromoted, Ag3 clusters and B3.5 nm Ag nanoparticles on alumina supports can catalyze the direct propylene epoxidation by O2 to selectively form propylene oxide with high activity at low temperatures.26 In contrast, using commercial industrial catalysts containing non-selected Ag nanoparticles the reaction selectivity and activity at low temperatures dramatically decreased. In another example, cubic Ag nanoparticles bounded with {100} facets exhibit much higher catalytic capability toward oxidation of styrene with tert-butyl hydroperoxide than Ag nanoplates mainly bounded with {111} facets,27 indicating

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Chem Soc Rev that enhanced catalytic performance can be achieved by carefully choosing nanoparticles with appropriate shapes as catalysts. Controlling the shape of Ag nanoparticles can also change their optical properties over a broader spectral range,28–30 thus their optoelectronic applications such as solar cells.31,32 From these examples, it is clear that controlled synthesis of colloidal Ag nanoparticles is critical to tailor their properties as well as optimize their performance in applications. Material scientists have witnessed great successes in the synthesis of various Ag nanoparticles in the past decade.3,5,28,33 For example, the shapes of the synthesized Ag nanoparticles include spheres, spheroids, cubes, cuboctahedrons, octahedrons, tetrahedrons, decahedrons, icosahedrons, thin plates, rods or wires. Although significant progress has been made and a number of very good reviews are available, there is still lack of review articles focusing on the controlled synthesis of Ag nanoparticles in organic solutions. The advantages for synthesizing Ag nanoparticles in organic solvents include high yield, narrow size distribution, and ease in assembly of the synthesized particles into superlattices in comparison with the nanoparticles synthesized in aqueous solutions.34,35 In this tutorial review, the empirical principles for controlling the synthesis of colloidal Ag nanoparticles in organic solvents are discussed by summarizing the work done by our group. The controllability relies on chemically engineering the nucleation processes involved in the formation of Ag nanocrystals. In Section 2 the classic nucleation theory and the corresponding classic LaMer model for the formation of colloidal nanoparticles are briefly discussed to highlight that engineering nucleation processes can be an efficient strategy for tuning the parameters of final Ag nanoparticles. Exemplar syntheses of Ag nanoparticles with different sizes, shapes, and composites are then discussed with details in Section 3 to demonstrate how to manipulate the nucleation processes by tuning the chemistry of the synthetic reactions. A brief conclusion and personal perspectives are provided in the final section to wrap up the review.

2. Classical nucleation theory In general, colloidal Ag nanoparticles are synthesized through either reduction of Ag+ ions with reducing reagents (or reductive solvents) or thermal decomposition of organometallic compounds in the presence of surfactant molecules that can attach to the nanoparticles’ surfaces to stabilize them. The basic model used to describe the formation of colloidal nanocrystals in a solution phase was presented by LaMer and Dinegar in 1950 and the model is summarized in Fig. 1a.36 According to this model, zerovalence Ag (Ag0) should be continuously provided to maintain a sustainable growth of Ag nanoparticles. As a result, an appropriate chemical reaction is first chosen to continuously generate Ag0 in the solution. As long as more Ag0 are produced, the solution is saturated with Ag0 quickly. Even at the saturation concentration (Cs), the Ag0 still cannot spontaneously condense into solid nuclei because forming a new solid phase in the homogeneous liquid environment is an energy-consuming process (Fig. 1b). As a result, only when the concentration of the Ag0 species reaches a critical value, i.e., critical concentration (Ccrit), the Ag0 can condense to

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Fig. 1 (a) LaMer model describing nucleation and growth of nanocrystals as a function of reaction time and concentration of precursor atoms. Adapted with permission from ref. 36. (b) Classical nucleation model showing the free energy diagram for nucleation. Adapted with permission from ref. 41.

form nuclei. Once stable nuclei are formed, they can grow larger at a lower concentration of Ag0 that is slightly above Cs because this process is a less energy-consuming process or an energy-saving process. As a result, the nucleation and growth steps are two relatively separated processes: formation of nuclei occurs only at a concentration of Ag0 much higher than Cs, otherwise growing the existing nuclei dominates. Therefore the two individual steps (i.e., nucleation and growth) can be reasonably engineered by tuning the concentration of Ag0, leading to a controlled synthesis of Ag nanoparticles with appropriate parameters. Intensive studies on the synthesis of colloidal nanoparticles have proven that nucleation is critical to determine the properties of the final nanoparticles. Crystal nucleation can be considered as a chemical reaction that takes solvated precursor atoms or molecules (e.g., Ag0 for the synthesis of Ag nanoparticles) into a solid-state crystalline product. As a chemical reaction, one can understand the nucleation process from both thermodynamic and kinetic aspects. In the classical nucleation theory (Fig. 1b), the driving force for spontaneous phase transition is the exothermicity of lattice formation. In this thermodynamic aspect, the free energy change required for the formation of nuclei (DG) is determined by

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Tutorial Review the sum of the free energy change for the phase transformation (DGv) and the free energy change for the formation of a solid surface (DGs). As the solid-state crystals are more stable than the solvated precursors, DGv is negative to decrease the total Gibbs free energy of the system. In contrast, the introduction of solid/liquid interfaces generally increases the free energy with the increase in the surface area of the nuclei. As a result, the evolution of nuclei depends on the competition between a decrease in DGv, which favors condensation of solvated precursors into nuclei, and an increase in DGs, which destabilizes the nuclei toward solvation in proportion to the crystals’ surface area. When the radii (R) of the nuclei are very small, the positive surface free energy DGs term dominates the total free energy change, leading the small nuclei to be dissolved. When the size of the nuclei increases, the total free energy change reaches a maximum (DG*) at a critical size (R*) and then turns over and continuously decreases to favor the stabilization and growth of the nuclei. From this thermodynamic aspect, one can change the pathway for the formation of nuclei by modulating the function of surface free energy and/or volume free energy to change the dependence of the total free energy on the size of the nuclei. As a result, controlling the nucleation process for the synthesis of colloidal Ag nanoparticles can be realized through the possible strategies: (i) varying surfactants that can change the surface free energy of Ag nuclei; (ii) forming nuclei of different materials that exhibit DGv and DGs different from Ag nuclei, followed by their chemical transformation to Ag nuclei; (iii) changing the reaction environment that can influence the stability of the nuclei, such as etching and dissolving the nuclei. According to the Arrhenius reaction rate equation, kinetics of the nucleation reaction can be described by the steady-state  rate of nucleation, J ¼ A exp DG kT , which equals the number of nuclei formed per unit time per unit volume. In this equation, k is the Boltzmann’s constant and A is the preexponential factor. The theoretical value of the pre-exponential factor is given as 1030 cm3 s1 although the value is very difficult to measure in practice.37 This kinetic factor depends on the mobility of precursor species (e.g., Ag0 for the synthesis of Ag nanoparticles) that can influence the rate of attachment of the precursor species to the critical nuclei. Since the mobility of precursor species varies rapidly with temperature, the temperature dependence of the pre-exponential factor can be quite significant. In addition, variation of temperature also changes the value of the exponential term. As a result, from the kinetic aspect we can change reaction temperature to influence the kinetics of the nuclei formation. The value of DG* also plays an important role in determining the nucleation kinetics. As discussed in the previous paragraph, this thermodynamic energy diagram can be tuned by controlling the chemical environment of the synthetic reactions. As a consequence, the nucleation kinetics can be tuned to control the synthesis of Ag nanoparticles. The classical nucleation theory indicates that supersaturated precursor species spontaneously condense into nuclei with critical sizes (this is called self-nucleation) followed by gradually enlarging the nuclei with continuous addition of precursor species (Fig. 1a). However, recent studies using the

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advanced in situ techniques reveal that the nucleation process is usually complex and involves multiple steps that are not reflected in the classical nucleation theory.38 For example, the nuclei with small sizes may coalesce to form larger nuclei that are more stable to support the gradual enlargement through continuous attachment of precursor species. Two models, i.e., the Lifshitz–Slyozov–Wagner (LSW) model39,40 and the two-step model,41 are proposed to describe the detailed nucleation process for the formation of solid crystals from reaction solutions. Even though these progresses are promising for comprehensively understanding the nucleation process, the principles for controlling the nucleation process revealed by the classical nucleation theory can still serve as the practical guidelines for design and synthesis of colloidal nanoparticles with tailored parameters. Typical examples for the synthesis of different Ag nanoparticles in organic solutions are discussed in the next section.

3. Controlled synthesis of Ag nanoparticles Two classic reaction systems, i.e., reduction of AgNO3 with hot oleylamine (cis-1-amino-9-octadecene, OAm) and reduction of AgNO3 with hot ethylene glycol (EG), are used as the model systems to highlight the importance of nucleation engineering in determining the final Ag nanoparticles. In the synthesis, both OAm and EG are reaction media and are also used as the solvents for dissolving AgNO3 and the possible additional surfactant. Dissolution of AgNO3 in the solvents releases Ag+ ions that might coordinate with the solvent molecules or surfactant molecules.42 In the following content, ‘‘Ag+ ions’’ is used for simplicity regardless of the coordination states. Since the reducing ability of OAm and EG highly depends on the reaction temperature, the generation rate of the precursor Ag0 species and thus the following nucleation kinetics for the formation of Ag nanocrystals can be tuned by controlling the reaction temperature. However simply controlling the reaction temperature is not enough to synthesize high-quality Ag nanoparticles. As highlighted in Section 2, controlling the nucleation kinetics can also be realized by tuning the reaction thermodynamics. This review focuses on the aspect: controlled synthesis of colloidal Ag nanoparticles in organic solutions through manipulation of chemistries of the synthetic reactions that influence the nucleation thermodynamics and kinetics. 3.1. Synthesis of icosahedral Ag nanoparticles through fast reduction of Ag+ ions Reactions in OAm have been widely used to synthesize colloidal nanoparticles made of a broad range of materials including magnetic materials,43 metals,44 and oxides.45 Fig. 2 shows transmission electron microscopy (TEM) images of the Ag nanoparticles with varying sizes that have been synthesized through a fast reduction of Ag+ ions in hot OAm.46,47 In a typical synthesis of 10 nm Ag nanoparticles (Fig. 2f), addition of 1 mmol AgNO3 to 20 ml OAm at room temperature forms a suspension that is then heated up to 60 1C. The temperature is maintained until the granular AgNO3 crystals are completely dissolved. The solution is colorless, indicating that the OAm

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Fig. 2 TEM images of Ag nanoparticles with different diameters. The number in front of the  sign in each frame is the average diameter of the nanoparticles shown in the same frame and the number following the  sign represents the three standard deviations (3s). Adapted with permission from ref. 46.

molecules are not active enough to reduce Ag+ ions. Quickly heating the solution with a ramp Z10 1C min1 to 180 1C dramatically enhances the reducing ability of OAm to reduce Ag+ ions at a very high rate, which is confirmed by the observation of quick appearance of a dark color corresponding to the SPRs of Ag nanoparticles formed in the solution. Ag+ ions are completely reduced within a couple of minutes although the nanoparticles formed at this stage exhibit a broad size distribution. By taking advantage of the Ostwald ripening process, continuous incubation of the nanoparticles under the reaction conditions for a longer time, such as 1 hour, significantly decreases their size distribution, resulting in the formation of Ag nanoparticles with spherical morphologies and a very narrow size distribution. Post size-selection based on simple centrifugation can further narrow their size distribution. As shown in Fig. 2f, the size distribution of the 10 nm nanoparticles is only 5%. By controlling the reaction conditions including temperature, growth time, and additives (e.g., oleic acid), size of the Ag nanoparticles can be tuned in the range of 2–20 nm and the typical size distributions are controlled to be lower than 10% (Fig. 2). In the synthesis, OAm serves as both reducing reagent and surfactant that helps stabilize the synthesized Ag nanoparticles. The surfaces of the nanoparticles are coated with OAm molecules through interactions between the amine groups (–NH2) and surface Ag atoms in the nanoparticles. Fourier transform infrared (FTIR) spectroscopy of the synthesized Ag nanoparticles shows major peaks essentially similar to the characteristic peaks of pure OAm molecules except a new peak at 1540 cm1. The appearance of this new peak indicates the formation of Ag–N bonds. Meanwhile the peak at about 1070 cm1 corresponding to the vibration mode

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Chem Soc Rev of C–N slightly shifts to the position with a lower wave number. Such differences in FTIR spectra imply that the surfaces of the Ag nanoparticles are primarily coated with OAm molecules through the formation of chemical bonds between the surface Ag atoms in the nanoparticles and the nitrogen atoms in the OAm molecules. The long hydrocarbon chains of the OAm molecules assist the Ag nanoparticles to well disperse in non-polar and low-polar solvents, such as hexane, toluene, and chloroform. In addition, the dense OAm capping layers on the nanoparticles’ surfaces prevent the Ag nanoparticles from being oxidized by air, leading the dispersions of the Ag nanoparticles to exhibit an excellent stability in the ambient environment. Due to the high growth rate, the Ag nanoparticles always exhibit morphologies close to spheres that have the lowest surface energy when they are small.48 Their exact morphologies have been carefully studied by high-resolution TEM (HRTEM). As shown in Fig. 3, regardless of the particle size each Ag nanoparticle exhibits an icosahedral shape with the characteristic co-existence of twofold, threefold, and fivefold symmetries (Fig. 3a). Each icosahedral nanoparticle has twenty faces terminated with {111} crystalline facets of face-centered cubic (f.c.c.) Ag, thirty edges and twelve vertices. Formation of this unique morphology requires the existence of 30 fivefold twin planes that connect 20 tetrahedral subunits. Fig. 3b–e present the HRTEM images of differently sized Ag nanoparticles along different rotational axes, confirming their icosahedral morphology with multiply twinned crystallinity. The existence of twin planes is responsible for the inhomogeneous contrast that is reflected by the randomness of dark spots in the TEM image of individual Ag nanopartilces (Fig. 2). These characterizations indicate that the synthesized Ag nanoparticles with different sizes shown in Fig. 2 have the consistent morphology, surface coating, and narrow size distribution. Such consistency makes these Ag nanoparticles to be an ideal class of model materials for studying the size-dependent properties. For example, the dependence of the absorption peak position of the Ag nanoparticles on their particle size is very interesting: as the particle size decreases from B20 nm the absorption peak blue-shifts but then turns over near 12 nm and strongly red-shifts. This exceptional size dependence is quite different from large nanoparticles (with diameters >20 nm) for which the peak position constantly blue-shifts as particle size decreases.29 This turnover dependence is ascribed to the significant effect of surface chemistry between the capping molecules (i.e., OAm) and the surface Ag atoms in the nanoparticles that cannot be ignored for small nanoparticles. 3.2. Synthesis of Ag nanocubes mediated with the formation of AgCl nanocrystals Given the fact that the reduction of Ag+ ions with hot OAm is very fast, it is difficult to control and manipulate the nucleation process to grow nanoparticles with morphologies other than icosahedron. One possibility is to introduce another reaction that can also quickly form solid nanocrystals with different crystalline structures (or shapes). This additional nucleation process has a lower nucleation barrier (i.e., DG*) than the self-nucleation

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Fig. 3 (a) Schematic drawings and (b–e) HRTEM images of the individual Ag nanoparticles shown in Fig. 2 viewed along different rotational axes: (left) twofold, (middle) threefold, and (right) fivefold ones, revealing their icosahedral morphology. The red dashed lines in (a) highlight the twin planes corresponding to these symmetries. The diameters of the nanoparticles shown in (b–e) are (b) 5.3 nm, (c) 7.3 nm, (d) 10.0 nm, and (e) 15.6 nm, respectively. Insets on the bottom left of the images presented in (b, c) are the fast Fourier transforms (FTTs) of the corresponding HRTEM images showing the nanoparticles’ symmetries. Scale bars in (b), (c), (d), and (e) represent 4, 5, 5, and 10 nm, respectively, and apply to all the images in the corresponding rows. Reproduced with permission from ref. 47.

associated with direct reduction of Ag+ with OAm, leading to a competition with the self-nucleation from Ag0 species. As shown in Fig. 4a, halide ions, such as chloride ions, can be added to the reaction system to quickly precipitate with Ag+ ions to form silver chloride (AgCl) nanoparticles. As a result, in the reaction system silver species nucleate through two different ways to form two different types of particles. The Ag particles derived from AgCl particles are usually polyhedral single crystals49 while the Ag particles formed through selfnucleation are multiply twinned crystals (similar to those shown in Fig. 2) with sizes smaller than the single-crystal particles. Continuously heating the reaction system facilitates an Ostwald ripening process to gradually dissolve the smaller multiply twinned particles and grow the single-crystal polyhedral particles into nanocubes.

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Fig. 4 (a) Modified LaMer model including an additional nucleation process besides the self-nucleation. (b) Schematic illustration of the major steps involved in the formation of single-crystal Ag nanocubes. (c) TEM image of the synthesized Ag nanocubes. The inset in (c) is the convergent beam electron diffraction pattern of an individual nanocube. (b, c) Adapted with permission from ref. 50.

Fig. 4b shows an example for the synthesis of Ag nanocubes with the assistance of dimethyl distearyl ammonium chloride (DDAC) in hot OAm mixed with octyl ether (OE).50 In a typical synthesis, 8.0 mL of OE and 1.0 mL of OAm are sequentially added to a 50 mL three-neck flask connected to a Schlenk line purged with nitrogen. OE is desirable for dissolving DDAC and OAm plays a role in reducing Ag+ ions and stabilizing the synthesized Ag nanocubes. To the binary solvent (OE–OAm) are added 0.3 mmol DDAC powders. Heating the solvent to 60 1C and maintaining the temperature for 10 min completely dissolves the DDAC powders. The resulting colorless solution is then quickly heated up to 260 1C at a ramp of B10 1C min1. To this hot DDAC solution is quickly injected 1.0 mL of OAm solution of AgNO3 with a concentration of 0.2 M. The reaction solution instantaneously turns milky yellowish, indicating the quick formation of both AgCl and Ag nanoparticles. Continuous reaction diminishes the milky color within 2 min, indicating the disappearance of AgCl nanoparticles. Maintaining the reaction

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Chem Soc Rev at 260 1C for 1 h completes the synthesis of pure Ag nanocubes as shown in Fig. 4c. At elevated temperatures DDAC can release free Cl ions to precipitate with Ag+ ions to form AgCl nanocrystals very quickly once the AgNO3 solution is injected. Meanwhile Ag+ ions are also reduced by OAm to form multiply twinned Ag nanoparticles similar to those shown in Fig. 2 through the self-nucleation process. Continuous reaction reduces the AgCl nanocrystals to single-crystal Ag particles with polyhedral morphologies. An Ostwald ripening process then facilitates the growth of the single-crystal polyhedrons to cubes with consumption of the smaller multiply twinned particles. Fig. 4c presents a typical TEM image of the synthesized Ag nanoparticles through this DDAC-mediation reaction, clearly showing their cubic morphology with slight truncation at the corners and uniform size with an average edge length of 34 nm. Each Ag nanocube exhibits a highly uniform contrast in the TEM images, indicating the nanoparticles are free of twin defects. The convergent beam electron diffraction pattern (inset, Fig. 4c) obtained by aligning the electron beam perpendicular to one of the six surfaces of an individual Ag nanocube exhibits a simple square symmetry, confirming that each nanocube is a single crystal with its surfaces bounded by {100} facets. In this synthesis, there are at least three different nucleation processes involved in the formation of AgCl nanocrystals, multiply twinned Ag nanocrystals, and solid phase transition from AgCl to singlecrystal Ag crystals. The complex nucleation and growth processes involved in the synthesis of Ag nanocubes have been probed in real time with the time-resolved high-energy X-ray diffraction (XRD).51 The use of high-energy synchrotron X-ray beam is advantageous because of the strong penetration of high-energy X-ray into liquid solutions and reaction vessels as well as weak absorption of the X-ray in the solvents and reaction precursors. The weak absorption of X-ray eliminates possible undesirable X-rayinduced reactions. Fig. 5a presents the 2D contour of the XRD patterns recorded at different times during the synthesis of Ag nanocubes. It clearly shows the appearance of AgCl and Ag crystals as well as the transformation of AgCl to Ag at different reaction stages: AgCl nanocrystals are formed first once the reaction is initiated; Ag nanocrystals are then formed through reduction of Ag+ ions with OAm; AgCl disappears due to the reduction with OAm when the time is long enough. More information on the reaction can be obtained from the variations in the XRD peak area, which is approximately proportional to the mass of crystalline materials, and peak width, which is related to the lateral dimensions of nanocrystals. Fig. 5b plots the integrated peak areas of the Ag(111) peak and the AgCl(200) peak that represent the major peaks of these two crystalline materials. According to the Scherrer equation, lateral dimensions of individual crystalline domains can be calculated from the peak width of XRD patterns. Fig. 5c compares the crystalline domain size in Ag nanoparticles along the (111) direction and in AgCl nanoparticles along the (200) direction. As shown in Fig. 5b, AgCl nanoparticles are formed within the first 3 s through the fast precipitation between Ag+ and Cl ions. Reducing Ag+ ions with OAm is initiated only after the complete formation of AgCl (i.e., at 3 s). The reduction of Ag+ ions is also very

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quick and most of the Ag+ ions are reduced within several seconds (period I). The peak areas of both Ag and AgCl exhibit plateaus in the period II while their particle sizes slightly increase, indicating that Ostwald ripening processes occur. Only when the reaction time is long enough, i.e., at B60 seconds, AgCl nanoparticles start to be reduced and transformed into single-crystal Ag nanoparticles in the period III. During this phase transition process, the reaction rate follows the Avrami phase-boundary based nuclei growth model in a 3D fashion.52,53 The Avrami exponent is determined to be B4, indicating the nucleation process with a constant nucleation rate. The size of the AgCl nanoparticles calculated from the XRD patterns remains essentially constant during this period, indicating that once the phase transition of an AgCl nanoparticle is initiated it can be quickly reduced to pure Ag before the phase transition of another AgCl nanoparticle starts. This chemical transformation process lasts B40 s. The increase in the crystalline size of Ag during period III is ascribed to the fact that the sizes of the single-crystal Ag nanoparticles derived from the AgCl nanoparticles are larger than the multiply twinned Ag nanoparticles formed during period II. As more and more Ag nanoparticles are formed through the chemical transformation of AgCl nanoparticles, the average crystalline size of Ag nanoparticles continuously increases until all of the AgCl nanoparticles are reduced. In period IV, the mixture of single-crystal and multiply twinned Ag particles in the reaction solution undergoes an Ostwald ripening process. Because the multiply twinned Ag particles exhibit smaller sizes than the single-crystal Ag particles and contain twinning defects, continuous incubation of the nanoparticles gradually dissolves the multiply twinned Ag particles and drives the single-crystal Ag particles to grow into uniform Ag cubes as shown in Fig. 4c. Apparently the time-resolved highenergy XRD studies provide more information on the complex nucleation and growth processes involved in the synthesis of Ag nanocubes than that obtained through the traditional sampling strategy. Techniques with higher temporal resolutions are expected in the future for better understanding the synthesis. 3.3. Controlled synthesis of Ag nanoparticles through selectively etching defective nuclei

Fig. 5 Time-resolved XRD patterns recorded from the reaction solution for the synthesis of Ag nanocubes shown in Fig. 4. (a) 2D contour plot of the XRD patterns at different reaction times. The black and red sticks represent the peak positions and relative intensities of the standard powder XRD patterns for f.c.c. Ag and f.c.c. AgCl, respectively. Blue arrows highlight the time when the AgNO3 solution was injected to initiate the reaction. The wavelength of X-ray was 0.1771 Å. Data were collected on the X-ray Operations and Research beamline 1-ID at the Advanced Photon Source, Argonne National Laboratory. (b) Variation in the integrated peak areas of the Ag(111) peak and the AgCl(200) peak as a function of reaction time. (c) Dependence of the lateral dimensions of the crystalline domains in the Ag nanoparticles along the {111} crystalline direction and in the AgCl nanoparticles along the {200} direction as a function of the reaction time. The dotted lines highlight the time periods (I, II, III, and IV) assigned according to the important processes discussed in the text. Adapted with permission from ref. 51.

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Another possible means to control the nucleation pathway is to slow down the reaction rate for reducing Ag+ ions. In this case one can have enough time to manipulate the reaction environment to select nuclei (i.e., seeds) with desirable crystalline structures that determine the morphology of final nanoparticles. In a system with slow reaction rate, Ag atoms usually self-nucleate into nuclei with crystalline structures that fluctuate between single crystals and twined crystals. Such structural fluctuation is consistent with the TEM observation of small (o5 nm) metal particles made of Ag and Au showing that a mild heating induced by the electron beam could force fluctuations between single-crystal and twinned morphologies.54 The rate of such fluctuations decreases with the increase in crystal size. As a result, one can find a way to defeat thermodynamics to selectively dissolve twinned nuclei to obtain high yield of single-crystal Ag nanoparticles (Fig. 6a). In contrast,

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Fig. 6 (a) Modified LaMer model describing the inclusion of an extra step for selecting nuclei with appropriated crystal structures. (b) Schematic illustration of the possible mechanism for the selective growth of single-crystal Ag nanoparticles (truncated cubes and tetrahedrons) through reduction of AgNO3 in hot EG in the presence of PVP, NaCl, and oxygen. (c) TEM image of the Ag nanoparticles formed at 44 h 10 min, showing the absence of twin planes in the nanoparticles. (d) SEM image of the nanoparticles formed at 45 h containing exclusively truncated cubes (indicated by a white octagon) and truncated tetrahedrons (indicated by a white hexagon). (b–d) Adapted with permission from ref. 55.

twined particles dominate the product if the growth of single crystal nuclei is prevented. Fig. 6b shows an example that shaped Ag nanoparticles with controlled crystalline structures can be synthesized through the reduction of Ag+ ions with hot EG at a lower reduction rate in comparison with the reaction systems with hot OAm. Silver atoms formulate structures of nuclei with sizes less than 2 nm at early stage of this reaction and these structures fluctuate between twinned crystals and single crystals. As these nuclei grow in size, the structural fluctuations slow down until the

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Chem Soc Rev crystallites are locked in a specific morphology. The nuclei with stable crystalline structures then serve as seeds to guide their further growth into nanoparticles with appropriate shapes and crystalline structures. In this reaction system, poly(vinyl pyrrolidone) (PVP) is used as a surfactant and the EG solutions are heated at 148 1C.55 When there are oxygen and trace amounts of Cl ions in the reaction system, the initially formed twinned structures could be dissolved because the twinning defects provide active sites for the oxidation reaction between Ag and oxygen. In this etching process, Cl ions may serve as coordinate ligands to promote the oxidation reaction by stabilizing the resultant smaller nanoclusters because no AgCl crystals are observed during the synthesis. On the other hand, when the structures are single crystalline, the nanoparticles continue their growth with the assistance of PVP. As a result, products consisting of pure single crystals are obtained by adding a small amount of sodium chloride (0.06 mM NaCl) to the reaction system to air. Fig. 6c and d show the electron microscopic images of the single-crystal Ag nanoparticles formed at different reaction times. The TEM image of the Ag nanoparticles formed at 44 h 10 min (Fig. 6c) shows that each nanoparticle has a quasi-spherical morphology free of apparent facets. All the Ag nanoparticles are without twinning defects. Growth of these quasi-spherical nanoparticles leads to the development of well-defined {100} and {111} facets, resulting in the formation of truncated cubes (highlighted by an white octagon) and truncated tetrahedrons (highlighted by an white hexagon) (Fig. 6d for the sample formed at 45 h). The insets in Fig. 6d present the convergent beam electron diffraction patterns recorded by directing the electron beam perpendicular to a (100) facet of a truncated cube (upper right) and a (111) facet of a truncated tetrahedron (lower left), respectively. The diffraction patterns exhibit the standard symmetries of single-crystal f.c.c. Ag, confirming the single crystallinity of the synthesized Ag nanoparticles. As the reaction continues, the size of Ag nanoparticles increases accordingly while their single crystallinity remains. When the reaction shown in Fig. 6b occurs in the absence of oxygen, the oxidation reaction of Ag cannot be initiated to selectively dissolve the twinned nuclei. For example, the polyol reaction system including 0.06 mM NaCl produces uniform Ag nanowires that are grown from the twinned particles formed at the early stage when the reaction is performed under argon.55 Alternatively the concentration of oxygen can be controlled by adding either Fe(II) or Fe(III) species to the reaction solution, thus to select the crystallinity of the final Ag nanoparticles. Due to the increased reducing activity of EG at the elevated temperatures, the stable iron species is Fe(II) in hot EG. As shown in Fig. 7a, in the reaction system molecular oxygen (O2) dissolved in EG adsorbs on the Ag surfaces and dissociates to atomic oxygen (Oa) for catalyzing oxidation reaction on the Ag surfaces.56 Since Fe(II) species are more active than Ag atoms to be oxidized, the adsorbed oxygen on the Ag surfaces can be consumed by the Fe(II) species. The resulting Fe(III) species are reduced back to Fe(II) by hot EG, leading to a continuous removal of the adsorbed oxygen from the Ag surfaces. As a result, the

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Fig. 7 Synthesis and characterization of Ag nanowires with fivefold twin planes. (a) Illustration of the possible mechanism showing how the oxygen species adsorbed on the Ag surfaces can be removed by Fe(II). Depletion of adsorbed oxygen on the Ag surfaces is responsible for blocking the oxidative etching of the twinned Ag nuclei with fivefold twinning structure that can grow to form Ag nanowires. Reproduced with permission from ref. 57. (b) SEM image of the Ag nanowires synthesized through a reduction of AgNO3 in hot EG in the presence of PVP, NaCl, and Fe(acac)3. The inset is a TEM image of the cross section of a nanowire that was viewed along the longitudinal axis of the nanowire. (c, d) HRTEM images of a cross-sectioned Ag nanowire obtained by cutting it against the planes that are perpendicular to the longitudinal axis of the nanowire. (e) Schematic drawing of an Ag nanowire with a highly strained core including lattice defects and a less strained sheath. The core–shell structure exposes strained/defective lattices (highlighted by the random lines) only at the ends of the nanowire to the surrounding environment. Due to the high reactivity of the strained/defective surfaces and stability of the less strained side surfaces, the short nanowires formed at the early stage tend to grow longer by preferentially adding more Ag atoms to the strained/defective end surfaces. (c, d) Reproduced with permission from ref. 58.

oxidative dissolution of twinned Ag nuclei can be efficiently prevented, leading to a preferential growth of twinned nuclei to uniform Ag nanowires because the twinned particles are more thermodynamically stable than the single-crystal particles.57 Fig. 7b shows an SEM image of the Ag nanowires synthesized from the reaction solution containing 2.2 mM tris(acetylacetonato)iron(III) (Fe(acac)3), clearly highlighting their high aspect ratios. The cross section of each Ag nanowire exhibits a pentagonal symmetry due to the existence of five {111} twin planes that crossed along a line in the center of the nanowire (inset, Fig. 7b). Generally, each nanowire

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Tutorial Review can be considered to be composed of five single crystalline f.c.c. subunits sharing their {111} crystallographic facets. However, the five subunits cannot completely fill spaces as predicted by the simple solid geometry model, leading to the formation of a solid-angle deficiency. This angular deficiency leads to lattice strains and/or defects in the nanowires to fill the 7.351 gap. The high-resolution XRD patterns indicate that the lattice strains in the Ag nanowires induce tetragonal distortions in the f.c.c. lattices.58 Studies on the cross-sectional samples of individual Ag nanowires with electron microscopy and electron diffraction reveal that the lattice strains distributed non-uniformly, i.e., the lattice strains are concentrated in the central region of each nanowire. The HRTEM images of a thick cross-sectional sample that essentially retains the internal lattice strains and microstructured defects are presented in Fig. 7c and d. The image of the central region (Fig. 7c) shows that the solid-angle gaps induce lattice defects including stacking faults, associated partial dislocations, slips, and possible additional small crystal domains. These defects are responsible for partially releasing the strong internal strains to stabilize the tetragonally distorted nanowires. In contrast, the crystalline lattices near surfaces are essentially free of defects except the {111} twin planes (highlighted by the red arrow), indicating much less strains in the surface regions (Fig. 7d). Cross-sectional samples of different nanowires exhibit the similar morphology and microstructures, implying that each Ag nanowire is a core–shell structure with a highly strained core that is responsible for the tetragonal distortion and a thin less-strained sheath that protects the strained core. The core–shell structure is responsible for the enhanced stability of the strained Ag nanowires and might provide the strong driving force for their anisotropic growth. Because the fivefold twin planes do not twist or bend during the growth of nanowires, the core–shell geometry and microstructured defects exist throughout the entire nanowires along their longitudinal axes. As a result, the defects that represent the most active sites for the addition of Ag atoms during nanowire growth can be exposed only at the ends of the nanowires (Fig. 7e). In contrast, the less-strained side surfaces of the nanowires have lower reactivity towards the attachment of Ag atoms for growing them thicker. The different reactivity between the end surfaces and side surfaces of the nanowires may be responsible for anisotropic growth of the nanowires. The examples shown in Fig. 6 and 7 highlight the importance of trace amounts of additives, e.g., NaCl and Fe(acac)3, in the selection of stable nuclei and the final nanoparticles. The presence of Cl ions prompts the oxidative etching of twinned nuclei due to the higher reactivity of the twinning defects towards oxygen than the defect-free surfaces of the singlecrystal nuclei. In the reaction systems including iron species, the reaction between Fe(II) and oxygen species adsorbed on Ag surfaces can effectively prevent the oxidative etching of twinned nuclei of Ag. Similar to the high reactivity towards oxidation, the twinning defects and other crystalline lattice defects also exhibit higher activity for the deposition of Ag atoms during nanoparticle growth, leading to a preferential enlargement of twinned nuclei when both twinned nuclei and single-crystal

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Tutorial Review nuclei coexist. As shown in Fig. 6b, decahedron and icosahedron represent the two typical morphologies of nanoparticles with fivefold twinning structures. Experimental observations and theoretical predictions indicate that icosahedral nanoparticles with large sizes (>30 nm) barely exist due to strong three-dimensional (3D) constraints. In contrast, decahedral Ag nanoparticles that exhibit tetragonal lattice distortions and core–shell strain distributions58 are easily elongated along their fivefold axes to form nanowires. The selective growth of thermodynamically stable Ag nanowires exhibits a much faster kinetics than the selective growth of single-crystal Ag nanoparticles, which is reflected from the difference in reaction times for the formation of the fivefold twinned nanowires shown in Fig. 7b (40 min) and the single-crystal nanoparticles shown in Fig. 6d (45 h). 3.4.

Synthesis of Ag nanoplates in N,N-dimethylformamide

Reduction of Ag+ ions with polyol solvents (e.g., ethylene glycol) in the presence of PVP has been extensively explored for the synthesis of Ag nanoparticles with single crystallinity and/or fivefold twinning. When the polyol solvents are replaced with N,N-dimethylformamide (DMF), the Ag+ ions can be reduced to form Ag nanoplates with multiple twin planes parallel to the basal surfaces of the nanoplates. Previous studies have proven that DMF represents an organic solvent with powerful reducing ability against metal ions in the synthesis of metal nanoparticles.59 The reduction can take place at room temperature, but an increase in temperature can remarkably increase the reaction rate. In addition, DMF slightly decomposes to a more easily oxidized amine upon aging or upon catalytic decomposition with a solid base. The resulting amine can accelerate the reduction of metal ions in particular during the formation of metal nanoparticles that can provide the solid base to catalyze the decomposition of DMF. Shortly after the first report of photochemically synthesized Ag nanoplates with high quality ´n and co-workers have demonstrated the and yield,60 Liz-Marza preparation of Ag nanoplates in boiled DMF containing AgNO3 and PVP.61 Control experiments indicate that increasing the concentration of Ag+ ions relative to the concentration of PVP changes the synthesized particles from isotropic spheres to anisotropic nanowires and nanoplates. At a concentration of AgNO3 that is higher than a critical value (i.e., 0.02 M), higher concentration of PVP is beneficial for improving the yield of Ag nanoplates. Since the co-existing Ag nanospheres are much smaller than the nanoplates, the Ag nanoplates can be easily purified by centrifugation. Time-dependent analysis reveals a degree of size control based on the reaction time: longer reaction time leads to larger nanoplates. In addition to refluxing the reaction solutions with a heating mantle, the thermal energy can also be delivered to the reaction solutions with ultrasonication62 and microwave.63 Ag nanoplates have been observed as the major products in both syntheses. He et al. have compared the reduction of AgNO3 in different solvents (e.g., pyridine, ethanol, DMF, and N-methyl-2pyrrolidone) containing PVP when a microwave oven has been used to drive the reaction. Ag nanoplates are formed only in

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Chem Soc Rev DMF while pseudospherical nanoparticles and irregular nanoparticles are produced in other solvents, indicating that DMF plays an important role in the formation of Ag nanoplates. Yang et al. have used the solvothermal method to reduce AgNO3 in DMF containing PVP to synthesize Ag nanoparticles.64 The morphologies of the resulting Ag nanoparticles highly depend on the molar ratio of PVP/AgNO3. Spherical Ag nanoparticles and a small fraction of Ag nanorods with an aspect ratio of B2 are formed at PVP/AgNO3 = 0.9. Upon increasing the molar ratio of PVP/AgNO3 to 5, monodisperse triangular Ag nanoplates are formed in a very high yield and uniformity that are superior to the Ag nanoplates in the previously reported work. Larger triangular Ag nanoplates are obtained by continuously increasing the molar ratio of PVP/AgNO3. The authors argue that the higher pressure in the solvothermal process is helpful for the formation and growth of triangular Ag nanoplates. Compared to the products formed without PVP in DMF, the authors also argue that PVP plays an important role in the formation of Ag nanoplates due to its reducing power in kinetically controlling the nucleation and growth of Ag nanoplates. Although the reducing ability of the end hydroxyl (–OH) groups of PVP has been extensively studied by Xia et al. to synthesize metal nanoplates in aqueous solutions,65 a similar role in the formation of Ag nanoplates in organic solutions has not been confirmed. For example, Hupp and Schatz groups have demonstrated the successful synthesis of Ag nanoplates by using carboxylate-functionalized polystyrene (PS) spheres instead of PVP in DMF solutions.66 As a result, in the synthesis of Ag nanoplates PVP molecules mainly play similar roles as a stabilizer of the nanoparticles and a coordination reagent towards Ag+ ions to the reactions for synthesizing Ag nanocubes and nanowires discussed in Section 3.3. With higher PVP/ AgNO3 ratios more Ag+ ions can be coordinated and the Ag nanoparticles can be deeply passivated, leading to a change in kinetics of nucleation and growth. In all these examples, DMF is used as the solvent that also serves as the reducing reagent regardless of other reaction conditions, indicating the importance of DMF in determining the anisotropic plate morphologies of Ag nanoparticles. However, the exact mechanism for the formation of Ag nanoplates is not well understood yet unless the morphological and structural evolutions can be in situ observed. 3.5. Synthesis of Ag/iron oxide hybrid nanoparticles through hetero-nucleation In addition to self-nucleation from Ag0 in an homogeneous liquid environment, one can preload nanoparticles to a reaction system to provide nucleation sites for condensation of Ag atoms (Fig. 8a). Due to the existence of foreign nanoparticles, Ag atoms can more easily condense on the surfaces of the nanoparticles in comparison with self-nucleation into freestanding Ag nuclei because of the thermodynamic energy benefit. Nucleation on the existing nanoparticles can decrease the Ag/solution interfacial surface areas thus lowering the surface free energy (DGs) (Fig. 1b). The corresponding overall energy barrier for nucleation of Ag on the preloaded

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Fig. 8 (a) Modified LaMer model describing the formation of hybrid structures through hetero-nucleation. (b) Schematic illustration showing the major steps involved in the synthesis of hybrid nanostructures made of Ag nanodomains and Fe/FexOy nanodomains. (c) Summary of the TEM images obtained from the products formed through the synthetic reaction shown in (b) at different times that was adjusted against the time when the AgNO3 solution was injected into the dispersion of Fe/FexOy nanoparticles. From the left top to the bottom left following the arrow direction, the reaction times were 0, 2, 180, and 300 s, respectively. The sample shown in the center was the same as that shown in the bottom left arc. The images were false colored and the scale bar applies to all the images. (b, c) Adapted with permission from ref. 67.

nanoparticles decreases, resulting in that a relatively low concentration of Ag0 species can drive the nucleation process. This strategy is always called hetero-nucleation. With this method nanoparticles decorated with Ag nanodomains can be synthesized. If the original nanoparticles are made of materials

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Tutorial Review different from Ag and have different properties, the synthesized hybrid structures can exhibit multiple functionalities. Fig. 8b shows an example for the synthesis of magnetoplasmonic bi-functional nanoparticles consisting of magnetic iron oxide (FexOy) nanodomains and plasmonic Ag nanodomains by using amorphous iron nanoparticles (as the preloaded foreign nanoparticles) to mediate the nucleation and growth of Ag nanodomains on their surfaces.67 In a typical synthesis, amorphous Fe nanoparticles with uniform sizes are first synthesized through a thermal decomposition of Fe(CO)5 in 1-octadecene (ODE) containing OAm.68 Separating the synthesized Fe nanoparticles from the reaction solution followed by washing them with hexane leads to a partial oxidation of the nanoparticles’ surfaces forming thin iron oxide layers that are also amorphous. Such oxidation is ascribed to the high reactivity of metallic Fe with the trace amount of oxygen dissolved in hexane. Formation of the thin FexOy shells passivates the Fe nanoparticles and significantly prevents the inner Fe cores from quick oxidation.69 Once an OAm solution of AgNO3 is injected into a hot ODE–OAm solvent containing the amorphous Fe/FexOy nanoparticles, Ag nanodomains quickly deposit on the surfaces of the Fe/FexOy nanoparticles because the amorphous FexOy surfaces provide the nucleation sites for Ag. Due to the fast reduction of Ag+ with hot OAm and the high density of nucleation sites on the amorphous FexOy surfaces, this heterogeneous nucleation leads to the formation of multiple Ag domains (as many as eight) on the surface of each Fe/FexOy nanoparticle. Continuously heating the reaction system initiates the ripening process of the Ag nanodomains because of the high mobility of Ag atoms on the FexOy surfaces at high temperatures, resulting in a gradual decrease in the average number of the Ag domains on each Fe/FexOy nanoparticle. The ripening process enlarges the most stable Ag nanodomain on a single Fe/FexOy nanoparticle by consuming the others until a dimer is formed. During the ripening process, the iron nanoparticles are converted to hollow iron oxide nanoshells through a complete oxidation of the iron with nitrate ions dissociated from AgNO3. Fig. 8c presents a series of typical TEM images of samples formed at different reaction stages, agreeing well with the growth mechanism highlighted in Fig. 8b. These samples are obtained by injecting AgNO3 solution (0.05 M in OAm, 2.0 mL) into hot (180 1C) ODE–OAm (10 mL/0.5 mL) in the presence of Fe/FexOy core–shell nanoparticles with an average diameter of 14 nm (top left, Fig. 8c), followed by a continuous heating for different times. Mixing the AgNO3 solution with the hot dispersion of Fe/FexOy nanoparticles leads to an instantaneous appearance of intense yellow color within 1 s due to the formation of Ag nanoparticles that exhibit strong SPRs. In contrast, it takes a much longer time (>60 s) to develop a light yellow color from a hot ODE–OAm solvent without Fe/FexOy nanoparticles after the AgNO3 solution is injected. The significant difference in the reaction rate for the formation of Ag nanoparticles highlights the role of the amorphous Fe/FexOy nanoparticles in facilitating the nucleation and growth of Ag nanocrystals from solutions. TEM images of the sample formed

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Fig. 9 (a) Schematic illustration describing the synthesis of dumbbell nanostructure made of two different nanoparticles linked with amorphous FexOy layers. (b–j) TEM images of Au@FexOy core–shell nanoparticles (c, f, i) and Au@FexOy–Ag dumbbell nanoparticles (d, g, j) that were synthesized from Au nanoparticles with different sizes (b, e, h). The scale bar shown in (h) applies to all the images.

at 2 s reveal that each Fe/FexOy nanoparticle is decorated with multiple Ag nanodomains with an average number of 3.6 (top right, Fig. 8c). As the reaction proceeds, the average number of Ag domains on each Fe/FexOy particle continuously decreases, for example, the average number lowers to 1.25 at 180 s (bottom right, Fig. 8c). When the reaction time is sufficiently long, the product is dominated by dumbbell-like dimers that are formed at 300 s (bottom left and center, Fig. 8c). Each dimer is consisted of a single Ag domain and a hollow FexOy shell. During the reaction, the dimensions of the Ag domains and the morphology of the Fe/FexOy seed nanoparticles also undergo significant changes such as those highlighted in Fig. 8b. The success in selective deposition of Ag nanodomains on the Fe/FexOy nanoparticles is ascribed to that the amorphous FexOy surfaces provide active sites to facilitate the nucleation and growth of Ag. As a result, more complicated hybrid structures can be synthesized by coating nanoparticles with

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amorphous FexOy layers followed by decoration with Ag nanodomains through the same strategy shown in Fig. 8b. For instance as shown in Fig. 9a, one can first form a thin layer of amorphous FexOy around nanoparticles made of varying materials (e.g., metal, semiconductor, oxide, etc.) through a decomposition of Fe(CO)5 in a hot ODE–OAm solution containing these nanoparticles followed by controlled post-oxidation. In the next step, the Ag nanodomains can be grown on the FexOy surfaces, leading to the formation of structures more complex than those shown in Fig. 8c. Fig. 9b–j show the formation of hybrid structures containing both Au and Ag nanodomains that are separated by the amorphous FexOy layers. 3.6.

Summary of the synthesis of Ag nanoparticles

The examples presented in Sections 3.1–3.5 clearly demonstrate that the synthesis of colloidal Ag nanoparticles in organic

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Chem Soc Rev solvents can be controlled by appropriately controlling the reaction solution chemistries that influence the thermodynamic energy diagrams involved in the nucleation processes. In summary, fast reduction of Ag+ ions with hot OAm results in a burst nucleation and formation of Ag nanoparticles with icosahedral morphology that represents the morphology with the lowest surface energy for f.c.c. metal nanoparticles with small sizes. The reaction rate of this reaction is too fast to be conveniently tuned for synthesizing Ag nanoparticles with morphologies other than icosahedron. Two different strategies have been demonstrated to control the nanoparticles’ morphologies. First, to the fast reaction system are added high-concentration Cl ions that can quickly precipitate with Ag+ ions to form single-crystal AgCl nanocrystals to compete with the formation of multiply twinned Ag nanoparticles formed from the direct reduction of Ag+ ions with hot OAm. The single-crystal AgCl nanoparticles are then chemically converted to single-crystal Ag nanoparticles with polyhedral morphologies, which can grow into Ag nanocubes with consumption of the multiply twinned Ag nanoparticles through an Ostwald ripening process. Second, the reaction for reducing Ag+ ions can be slowed down to enable the selection of nuclei with desirable crystalline structures by adding appropriate chemical additives. Single-crystal Ag nanoparticles can be achieved through reduction of Ag+ ions with hot EG by selectively dissolving the nuclei with twinning defects while the product is mainly composed of fivefold twinned Ag nanowires if the growth of single-crystal nuclei is not prompted. In addition to chemical species (e.g., DDAC, Cl, Fe(acac)3, etc.), foreign nanoparticles can also be preloaded to the reaction solution to provide nucleation sites for condensation of Ag atoms, resulting in hybrid structures with multiple functionalities. Such hetero-nucleation is preferential in comparison with the self-nucleation through which freestanding Ag nanoparticles are formed because the formation of interfaces between the Ag nuclei and the preloaded nanoparticles can lower the free energy barrier for nucleation. By applying these rules, the nucleation process can be engineered to synthesize high-quality Ag nanoparticles shown in Fig. 2–9 that exhibit the well-controlled sizes, shapes, and compositions of hybrids.

4. Conclusions and remarks The examples discussed in this review clearly demonstrate that chemically engineering the synthetic reactions can effectively influence the thermodynamic energy diagram of the nucleation process to kinetically control the formation of Ag nanocrystals with tailored parameters including size, shape, crystallinity, and composites. These strategies, in principle, can be extended for controlled synthesis of nanoparticles made of materials other than Ag. As discussed in Section 2, the nucleation process for the formation of colloidal nanocrystals is usually complicated with involvement of a number of chemical and physical events (e.g., formation of non-crystalline clusters with a magic number of atoms, coalescence of clusters, crystallization of nuclei, ripening of nuclei, etc.). Development of in situ techniques that are capable of noninvasively probing the complex nucleation process in real time is highly demanded to help better understand the nucleation process.

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Tutorial Review The understanding will in turn help us better design and synthesize high-quality nanoparticles. Environmental transmission electron microscopy with specially designed thin liquid cells70 and timeresolved synchrotron X-ray techniques (e.g., transmission X-ray microscopy,71 wide-angle X-ray scattering,72 small-angle X-ray scattering,38 X-ray absorption fine structure,73 etc.) represent the major advances emerged in the past several years. The synthesized Ag nanoparticles with well-controlled parameters can be used as a class of physical templates to direct the deposition of other materials on the surfaces of the Ag nanoparticles to form core–shell nanoparticles with multiple compositions and functionalities. The Ag nanoparticles can also serve as chemical templates to react with appropriate reagents to transform the Ag nanoparticles into nanoparticles made of different materials while the resulting nanoparticles can inherit the morphology and/or crystallinity of the Ag nanoparticles. For example, galvanic replacement reactions between the Ag nanoparticles and precursors of more noble metals (e.g., Au, Pt, Pd) result in the formation of hollow metal nanoparticles.74 Reaction with appropriate oxidizing reagents (e.g., S, FeCl3, etc.) can transform the Ag nanoparticles into semiconductor nanoparticles (e.g., Ag2S, AgCl, etc.).75 Assembly of the synthesized Ag nanoparticles and the derived nanoparticles through templated transformations into complex superlattices represents another interesting direction for developing functional materials because coupling between neighboring nanoparticles may lead to novel properties and applications.76

Acknowledgements This work was performed at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC0206CH11357. Data discussed in this review were partially obtained with the use of Advanced Photon Source and Electron Microscopy Center for Materials Research at Argonne National Laboratory that are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Dr Sheng Peng’s efforts on the synthesis of Ag icosahedral nanoparticles, Ag nanocubes, and Ag/FexOy hybrid nanoparticles are greatly appreciated.

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