chemical society reviews volume 42 issue 7 [doi 10.1039%2fc2cs35289c]

8/19/2019 Chemical Society Reviews Volume 42 Issue 7 [Doi 10.1039%2FC2CS35289C]

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This journal is c The Royal Society of Chemistry 2013 Chem. Soc. Rev., 2013, 42, 2497--2511 2497

Cite this: Chem. Soc. Rev., 2013,

42, 2497

Controlled synthesis of colloidal silver nanoparticles inorganic 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

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

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 organicsolutions, which enables the synthesis of high-quality nanoparticles.

1. Introduction

Silver (Ag) is a ductile, malleable coinage metal that exhibits the

highest electrical and thermal conductivity among all metals

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 form-

aldehyde.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

Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass

Avenue, A rgonne, Ill inois 60439, USA. E-mail: [email protected]† Part of the chemistry of functional nanomaterials themed issue.

Yugang Sun

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

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.

Received 28th July 2012

DOI: 10.1039/c2cs35289c

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2498 Chem. Soc. Rev., 2013, 42, 2497--2511 This journal is c The Royal Society of Chemistry 2013

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 traditionalrigid doped metal oxide conductive films, such as the most

commonly used tin-doped indium oxide (ITO).17

In addition to these commercial applications, Ag nano-

particles 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 con-

sequence, 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’’ toenhance Raman scattering 19–23 and fluorescence24 of molecules or

emitters (such as quantum dots and upconversion nanocrystals)

adjacent to the Ag nanoparticle. The unique SPRs in Ag nano-

particles 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 transfer-

rable 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 rate-

limiting 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 rate-

limiting 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 influ-

ence its properties and thus its performance in applications.For example, unpromoted, Ag 3 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

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 principlesfor 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 nano-

crystals. 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 mani-

pulate the nucleation processes by tuning the chemistry of the

synthetic reactions. A brief conclusion and personal perspectivesare 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, zero- valence Ag (Ag 0) 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 Ag 0 in

the solution. As long as more Ag 0 are produced, the solution is

saturated with Ag 0 quickly. Even at the saturation concentration

(C s), the Ag 0 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 Ag 0 species reaches a critical

value, i.e., critical concentration (C crit ), the Ag 0 can condense to

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form nuclei. Once stable nuclei are formed, they can grow larger at

a lower concentration of Ag 0 that is slightly above C s 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 Ag 0 much higher than C s, 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 Ag 0, leading to a controlled synthesis

of Ag nanoparticles with appropriate parameters.

Intensive studies on the synthesis of colloidal nanoparticleshave 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., Ag 0 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

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 anincrease 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 forthe 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 pre-

exponential 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., Ag 0 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 tem-

perature 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. Asdiscussed 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 supersatu-

rated 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

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

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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 thenanoparticles’ surfaces prevent the Ag nanoparticles from

being oxidized by air, leading the dispersions of the Ag nano-

particles 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 nano-

particles 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 anideal 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 nano-

particles 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

associated with direct reduction of Ag + with OAm, leading to

a competition with the self-nucleation from Ag 0 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, inthe reaction system silver species nucleate through two differ-

ent 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 self-

nucleation 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.

Fig. 3 (a) Schematic drawings and (b–e) HRTEM images of the individual Agnanoparticles shown in Fig. 2 viewed along different rotational axes: (left) twofold,

(middle) threefold, and (right) fivefold ones, revealing their icosahedral morphol-ogy. The red dashed lines in (a) highlight the twin planes corresponding to thesesymmetries. 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 theimages presented in (b, c) are the fast Fourier transforms (FTTs) of the corres-ponding 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 theimages in the corresponding rows. Reproduced with permission from ref. 47.


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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

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 nano-

crystals very quickly once the AgNO3 solution is injected. Mean-

while 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 thenfacilitates 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 asingle 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 single-

crystal 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-ray-induced 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

Fig. 4 (a) Modified LaMer model including an additional nucleation process

besides the self-nucleation. (b) Schematic illustration of the major steps involvedin the formationof single-crystal Ag nanocubes. (c) TEM image of the synthesizedAg nanocubes.The inset in (c) is the convergent beam electron diffraction patternof an individual nanocube. (b, c) Adapted with permission from ref. 50.


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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.52,53The 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 nano-

particles are larger than the multiply twinned Ag nanoparticles

formed during period II. As more and more Ag nanoparticlesare formed through the chemical transformation of AgCl nano-

particles, the average crystalline size of Ag nanoparticles con-

tinuously 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 high-

energy 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

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 environ-

ment to select nuclei (i.e., seeds) with desirable crystalline

structures that determine the morphology of final nano-

particles. 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 thermo-

dynamics to selectively dissolve twinned nuclei to obtain high

yield of single-crystal Ag nanoparticles (Fig. 6a). In contrast,

Fig. 5 Time-resolved XRD patterns recorded from the reaction solution for thesynthesis of Ag nanocubes shown in Fig. 4. (a) 2D contour plot of the XRDpatterns at different reaction times. The black and red sticks represent the peakpositions 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 AgNO3solution was injected to initiate the reaction. The wavelength of X-ray was0.1771 Å. Data were collected on the X-ray Operations and Research beamline1-ID at the Advanced Photon Source, Argonne National Laboratory. (b) Variationin the integrated peak areas of the Ag(111) peak and the AgCl(200) peak as afunction of reaction time. (c) Dependence of the lateral dimensions of thecrystalline domains in the Ag nanoparticles along the {111} crystalline directionand in the AgCl nanoparticles along the {200} direction as a function of thereaction time. The dotted lines highlightthe time periods (I, II, III, and IV) assignedaccording to the important processes discussed in the text. Adapted withpermission from ref. 51.


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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

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 eachnanoparticle 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 thesynthesized 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

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


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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

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 micro-

structured 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 areessentially free of defects except the {111} twin planes (high-

lighted 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 micro-

structured 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 impor-

tance 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 twinnednuclei due to the higher reactivity of the twinning defects

towards oxygen than the defect-free surfaces of the single-

crystal 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

Fig. 7 Synthesis and characterization of Ag nanowires with fivefold twin planes.(a) Illustration of the possible mechanism showing how the oxygen speciesadsorbed on the Ag surfaces can be removed by Fe( II). Depletion of adsorbedoxygen on the Ag surfaces is responsible for blocking the oxidative etching of thetwinned Ag nuclei with fivefold twinning structure that can grow to form Ag

nanowires. Reproduced with permission from ref. 57. (b) SEM image of the Agnanowires synthesized through a reduction of AgNO3 in hot EG in the presenceof PVP, NaCl, and Fe(acac)3. The inset is a TEM image of the cross section of ananowire that was viewed along the longitudinal axis of the nanowire. (c, d)HRTEM images of a cross-sectioned Ag nanowire obtained by cutting it againstthe planes that are perpendicular to the longitudinal axis of the nanowire. (e)Schematic drawing of an Ag nanowire with a highly strained core includinglattice defects and a less strained sheath. The core–shell structure exposesstrained/defective lattices (highlighted by the random lines) only at the ends ofthe nanowire to the surrounding environment. Due to the high reactivity of thestrained/defective surfaces and stability of the less strained side surfaces, theshort nanowires formed at the early stage tend to grow longer by preferentiallyadding more Ag atoms to the strained/defective end surfaces. (c, d) Reproducedwith permission from ref. 58.


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nuclei coexist. As shown in Fig. 6b, decahedron and icosahe-

dron represent the two typical morphologies of nanoparticles

with fivefold twinning structures. Experimental observations

and theoretical predictions indicate that icosahedral nano-

particles 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 nano-

particles, 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 nano-

particles.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 decomposi-

tion 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

and yield,60 Liz-Marzán and co-workers have demonstrated the

preparation of Ag nanoplates in boiled DMF containing AgNO3and 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 adegree 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 nano-

plates 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-2-

pyrrolidone) containing PVP when a microwave oven has been

used to drive the reaction. Ag nanoplates are formed only in

DMF while pseudospherical nanoparticles and irregular nano-

particles 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 molarratio of PVP/AgNO3 to 5, monodisperse triangular Ag nano-

plates 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 nano-

plates. 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 inkinetics 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 nano-

plates 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 Ag 0

in an homogeneousliquid environment, one can preload nanoparticles to a reac-

tion 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 free-

standing 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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nanoparticles decreases, resulting in that a relatively low

concentration of Ag 0 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

different from Ag and have different properties, the synthesized

hybrid structures can exhibit multiple functionalities.

Fig. 8b shows an example for the synthesis of magneto-

plasmonic bi-functional nanoparticles consisting of magnetic

iron oxide (FexO y) nanodomains and plasmonic Ag nano-

domains by using amorphous iron nanoparticles (as the pre-

loaded 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 arefirst synthesized through a thermal decomposition of Fe(CO)5in 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 FexO y shells passi-

vates 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/FexO y nanoparticles, Ag nanodomains quickly deposit on the surfaces of the Fe/FexO y nanoparticles because

the amorphous FexO y 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 FexO y surfaces,

this heterogeneous nucleation leads to the formation of multi-

ple Ag domains (as many as eight) on the surface of each

Fe/FexO y nanoparticle. Continuously heating the reaction

system initiates the ripening process of the Ag nanodomains

because of the high mobility of Ag atoms on the FexO y surfaces

at high temperatures, resulting in a gradual decrease in the

average number of the Ag domains on each Fe/FexO y nano-

particle. The ripening process enlarges the most stable Ag

nanodomain on a single Fe/FexO y 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/FexO y 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/FexO y 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/FexO ynanoparticles after the AgNO3 solution is injected. The signifi-

cant difference in the reaction rate for the formation of Ag

nanoparticles highlights the role of the amorphous Fe/FexO ynanoparticles in facilitating the nucleation and growth of Ag

nanocrystals from solutions. TEM images of the sample formed

Fig. 8 (a) Modified LaMer model describing the formation of hybrid structuresthrough hetero-nucleation. (b) Schematic illustration showing the major stepsinvolved in the synthesis of hybrid nanostructures made of Ag nanodomains andFe/Fe x O y nanodomains. (c) Summary of the TEM images obtained from theproducts formed through the synthetic reaction shown in (b) at different timesthat was adjusted against the time when the AgNO3 solution was injected into

the dispersion of Fe/Fe x O

y nanoparticles. From the left top to the bottom leftfollowing 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 thebottom left arc. The images were false colored and the scale bar applies to all theimages. (b, c) Adapted with permission from ref. 67.


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at 2 s reveal that each Fe/FexO y 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/FexO y 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 formedat 300 s (bottom left and center, Fig. 8c). Each dimer is

consisted of a single Ag domain and a hollow FexO y shell.

During the reaction, the dimensions of the Ag domains and the

morphology of the Fe/FexO y seed nanoparticles also undergo

significant changes such as those highlighted in Fig. 8b.

The success in selective deposition of Ag nanodomains on

the Fe/FexO y nanoparticles is ascribed to that the amorphous

FexO y 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

amorphous FexO y layers followed by decoration with Ag nano-

domains through the same strategy shown in Fig. 8b. For

instance as shown in Fig. 9a, one can first form a thin layer

of amorphous FexO y 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 contain-

ing these nanoparticles followed by controlled post-oxidation.In the next step, the Ag nanodomains can be grown on the

FexO y 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 FexO ylayers.

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

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


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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. Twodifferent strategies have been demonstrated to control the nano-

particles’ 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 nano-

particles 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 nano-

particles 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 non-

invasively probing the complex nucleation process in real time is

highly demanded to help better understand the nucleation process.

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 time-

resolved synchrotron X-ray techniques (e.g., transmission X-ray

microscopy,71 wide-angle X-ray scattering,72 small-angle X-ray scat-

tering,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 para-

meters can be used as a class of physical templates to direct thedeposition of other materials on the surfaces of the Ag nano-

particles to form core–shell nanoparticles with multiple com-

positions 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 nano-

particles and precursors of more noble metals (e.g., Au, Pt, Pd)

result in the formation of hollow metal nanoparticles.74 Reac-

tion with appropriate oxidizing reagents (e.g., S, FeCl3, etc.) can

transform the Ag nanoparticles into semiconductor nano-particles (e.g., Ag 2S, AgCl, etc.).75 Assembly of the synthesized

Ag nanoparticles and the derived nanoparticles through tem-

plated transformations into complex superlattices represents

another interesting direction for developing functional materi-

als 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-AC02-

06CH11357. 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/FexO y hybrid nanoparticles are greatly appreciated.

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chemical society reviews volume 42 issue 7 [doi 10.1039%2fc2cs35289c]

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