Synthesis of Colloidal Metal Nanocrystals: A ... - Wiley Online Library

DOI: 10.1002/chem.201802194

Review

& Colloids

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants Thenner S. Rodrigues+,[a, b] Ming Zhao+,[c] Tung-Han Yang+,[a] Kyle D. Gilroy,[a] Anderson G. M. da Silva,[a, b] Pedro H. C. Camargo,[b] and Younan Xia*[a, c]

Chem. Eur. J. 2018, 24, 16944 – 16963

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T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Review Abstract: There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidation half-reactions and the corresponding oxida-

1. Introduction Metal nanocrystals occupy a prominent position in modern science and technology, showing an indispensable role in an array of applications related to catalysis, photonics, electronics, photography, sensing, and information storage, among others.[1–12] To this end, a vast number of methods have been developed for the production of metal nanocrystals, including (photo)chemical reduction, radioactive reduction, thermal decomposition, and physical evaporation.[13–17] Among them, chemical reduction in the solution phase stands out for its simplicity of reaction procedures and high throughput per batch of synthesis (typically more than 1017 nanocrystals per liter of solution).[18] In particular, by controlling the experimental parameters (e.g., precursor, reactant, capping agent, and temperature), one can easily engineer the sizes, shapes, compositions, and structures of metal nanocrystals to optimize their properties toward a variety of applications.[19–21] The general protocol for chemically synthesizing metal nanocrystals involves the reduction of a salt precursor by a reductant in the solution phase. By judicially choosing the right capping agents, metal nanocrystals with a variety of well-defined shapes or structures have been synthesized.[22–27] However, the explicit role(s) of reductants is still ambiguous and de-

tion potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.

serves special attention. Other than the function as a supply of electrons, a reductant can also play additional roles, including functions as a solvent, a capping agent, and/or a colloidal stabilizer. Moreover, the reductant, depending on the physicochemical properties and experimental conditions, may directly affect the outcome of a synthesis due to its major impact on the reaction kinetics and thus the nucleation and growth of nanocrystals. To this end, many studies have recently started to systematically evaluate different types of reductants in an effort to elucidate the correlations between their properties and the final shape/morphology of the nanocrystals.[28–35]

[a] Dr. T. S. Rodrigues,+ Dr. T.-H. Yang,+ Dr. K. D. Gilroy, Dr. A. G. M. da Silva, Prof. Y. Xia The Wallace H. Coulter Department of Biomedical Engineering Georgia Institute of Technology and Emory University Atlanta, Georgia 30332 (USA) E-mail: [email protected]

[b] Dr. T. S. Rodrigues,+ Dr. A. G. M. da Silva, Prof. P. H. C. Camargo Departamento de Qu&mica Fundamental, Instituto de Qu&mica Universidade de S¼o Paulo Av. Prof. Lineu Prestes, 748 05508-000, S¼o Paulo-SP (Brazil) [c] M. Zhao,+ Prof. Y. Xia School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, Georgia 30332 (USA) [+] These authors contributed equally to this work. Chem. Eur. J. 2018, 24, 16944 – 16963

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Figure 1. Structural formulas of chemical compounds commonly employed as reductants for the synthesis of colloidal metal nanocrystals.

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Review Figure 1 summaries a series of compounds that have been employed as reductants for the colloidal synthesis of metal nanocrystals through chemical reduction, differentiated by a large number of distinctive functional groups such as hydroxyl, carbonyl, and benzene, among others. This review covers the important aspects of reductants in the chemical synthesis of metal nanocrystals. Our intention is

Thenner S. Rodrigues received his B.Sc. and M.Sc. in Industrial Chemistry in 2010 and 2013, respectively, both from Federal University of Ouro Preto (Brazil). He obtained his Ph.D. in Chemistry from the University of S¼o Paulo (Brazil) in 2017, under the supervision of Prof. Pedro Camargo. He was a visiting student in Prof. Younan Xia’s group at the Georgia Institute of Technology (USA) from February 2015 to March 2016. His research interests are focused on the synthesis of metal nanostructures and metal oxides with controlled shapes for catalytic and photocatalytic applications. Ming Zhao received his B.Sc. in Materials Science & Engineering and M.Sc. in Materials Physics & Chemistry in 2012 and 2015, respectively, both from Nanjing University (China). His M.Sc. thesis work was focused on the durability test of proton-exchange membrane fuel cells. In August 2015, he joined the Xia group as a graduate student and is pursuing his Ph.D. degree in School of Chemistry & Biochemistry at the Georgia Institute of Technology (USA). His research interests include the synthesis of nanomaterials for energy-related applications. Tung-Han Yang received his B.Sc. in Chemical Engineering from National Cheng-Kung University (Taiwan) in 2009 and M.Sc. in Materials Science & Engineering from National Tsing-Hua University (Taiwan) in 2011. Afterwards, he pursued his Ph.D. in Materials Science & Engineering at National Tsing-Hua University. In 2015–2017, he joined the Xia group as a visiting student and his research interest includes the quantitative analysis of the shape-controlled synthesis of noble-metal nanocrystals.

Kyle D. Gilroy received his B.Sc. in biomedical physics from the College of New Jersey (USA) in 2011 and Ph.D. in engineering from Temple University (USA) in 2015 under the supervision of Prof. Svetlana Neretina. He joined the group of Prof. Y. Xia as a postdoctoral fellow in 2015. At the start of 2018, he began as an Engineer for Vision Research under the Materials Analysis Division of Ametek, where he specializes in ultrahigh speed imaging.

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to highlight the fundamentals regarding the reduction of salt precursors by reductants, including the thermodynamic and kinetic factors involved in the chemical reaction as well as the mechanistic pathways. In addition, we provide a detailed discussion on the most critical reductants employed in the synthesis of metal nanocrystals, together with the reduction mechanisms and the effects of their properties (e.g., chemical structure and oxidation potential) on the final products. To this end, we choose the most relevant and comprehensive articles that best represent the vital concepts involved in the reduction of a salt precursor to generate metal atoms, and the role of reaction kinetics in dictating the formation of colloidal metal nanocrystals.

Anderson G. M. da Silva received his B.Sc. in Industrial Chemistry in 2010 and M.Sc. in Materials Engineering in 2013, both from the Federal University of Ouro Preto (Brazil). He pursued his Ph.D. at the University of S¼o Paulo (Brazil) under the supervision of Dr. Pedro Camargo. During his Ph.D., he spent one year in Prof. Younan Xia’s group at the Georgia Institute of Technology (USA) as a visiting student. Currently, he is a postdoctoral researcher at the University of S¼o Paulo. His research interest includes synthesis of nanomaterials displaying controlled shapes, compositions and architectures for applications in catalysis, plasmonic, and electrochemical applications. Pedro H. C. Camargo obtained his B.Sc. and M.Sc. in Chemistry from Federal University of Paran# (Brazil) in 2003 and 2005, respectively. In 2005, he was a recipient of a Fulbright/ CAPES Fellowship to pursue his Ph.D. in the USA. He obtained his Ph.D. from Washington University in St. Louis in 2009, working in the group of Prof. Younan Xia. He became an Assistant Professor at the University of S¼o Paulo in 2011. He was promoted to Associate Professor in 2015 and to Full Professor in 2018. His research interest includes the synthesis of nanomaterials with controlled physicochemical features for nanocatalysis and plasmonics. Younan Xia studied at the University of Science and Technology of China (B.Sc., 1987) and University of Pennsylvania (USA, M.Sc., 1993) before receiving his Ph.D. from Harvard University (USA) in 1996 (with George M. Whitesides). He started as an assistant professor of chemistry at the University of Washington (Seattle, USA) in 1997 and was promoted to associate professor and professor in 2002 and 2004, respectively. He joined the department of biomedical engineering at Washington University in St. Louis in 2007 as the James M. McKelvey Professor. Since 2012, he holds the position of Brock Family Chair and GRA Eminent Scholar in Nanomedicine at Georgia Institute of Technology.

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Review 2. How is a Salt Precursor Reduced?

ing to Equation (8) in which R is the ideal gas constant and T is the absolute temperature.[36]

As a typical redox process, the chemical reduction of a salt precursor by a reductant involves electron transfer between two chemical species (i.e., electron donors and acceptors), which is driven by the difference in their reduction potentials (DE).[36] In general, the electron transfer process can be analyzed by examining the corresponding oxidation and reduction half-reactions [Eq. (1) and (2), respectively] and the global redox reaction [Eq. (3)], in which Rred is a reductant species, Roxi is an oxidized species, Mn + is a metal ion, and M0 is a metal atom. Rred ! Roxi þ ne@

ð1Þ

Mnþ þ ne@ ! M0

ð2Þ

Rred þ Mnþ ! Roxi þ M0

ð3Þ

Specifically, Mn + is reduced to M0 upon receiving electrons from Rred, which is subsequently converted to Roxi. In this process, the molar ratio between Mn + and Rred in a balanced reaction is dictated by the number of electrons donated per molecule of the reductant, which depends primarily on its chemical makeup. For example, alcohols, aldehydes, carboxylic acids, and hydroquinones can donate two electrons per molecule, whereas amines typically donate one electron per molecule.[28, 29, 37, 38] In terms of thermodynamics, the hypothetical redox reaction is only favorable when the value of DE is positive, that is, when the oxidation potential of the reductant is higher than that of the metal ion. This argument originates from Equation (4) in terms of the “change in free energy” (DG) for an electrochemical cell with a potential drop of DE. In Equation (4) n is the number of electrons transferred in the balanced equation and F is the Faraday’s constant (ca. 96 485 C mol@1). DG ¼ @nFDE

ð4Þ

As an illustrative example based on the synthesis of Pt nanocrystals by reducing PtCl62@ with BH4@ under standard conditions, the redox process can be desrcribed by the following reactions: the oxidation reaction [Eq: (5)], the reduction reaction [Eq. (6)] and the overall reaction [Eq. (7)]: BH@4 þ 3 H2 O ! BðOHÞ3 þ 7 Hþ þ 8 e@ @ 6

@

0

2 PtCl þ 8e ! 2 Pt þ 12 Cl

@

E 0 oxi ¼ 0:48 V E

0

red

¼ 0:72 V

2 PtCl@6 þ BH@4 þ 3 H2 O ! 2 Pt0 þ BðOHÞ3 þ 7 HCl þ 5 Cl@

ð5Þ ð6Þ ð7Þ

As DE8cell = Eooxi + E8red = 0.48 + 0.72 = 1.2 V, we can derive DG8cell = @926 kJ mol@1. The negative change in free energy means that this redox reaction is thermodynamically favored, that is, the reduction of the PtIV precursor will occur spontaneously. Interestingly, DE also determines the yield of a redox reaction, in which the value of the equilibrium constant (K) of a redox reaction is dependent on the magnitude of DE accordChem. Eur. J. 2018, 24, 16944 – 16963

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lnðK Þ ¼

nF DE RT

ð8Þ

It has been shown that the generation rate of metal atoms plays a key role in controlling the nucleation and growth processes, and eventually the formation of metal nanocrystals.[28, 39] Therefore, understanding the fundamentals of this process is crucial to the selection of a proper pair of salt precursor and reductant. Among all the physicochemical properties of a reductant, the reduction potential deserves special attention owing to its significant influence on the reaction kinetics, despite that it is a thermodynamic parameter. This, in turn, determines the features of final products, including their sizes, shapes, structures, and compositions (for systems involving more than one types of metals).[40–42] Typically, the thermodynamic parameters of a reaction should not affect its kinetics, in other words, a thermodynamically spontaneous reaction can be either slow or fast. However, the solution-phase redox reaction, in which the involved species (i.e., electron donor and acceptor) are solvated during the electron exchange process, represents an exception to this general rule.[43, 44] In this case, the difference in oxidation potential not only determines the thermodynamic spontaneity of a reaction but also affects the kinetics and subsequently the formation of nanocrystals. To obtain a product with desired features, it is essential to pair a salt precursor with the right reductant. The Marcus theory developed by Rudolph A. Marcus in the 1950’s is a powerful tool for predicting the rate of electron transfer from one chemical species to another, or specifically, from reductant to metal ions.[44] The reduction rate constant, k, depends only on temperature (T) and the change in Gibbs free energy (DG), given by Equation (9),[44] in which kB is the Boltzmann constant. k ¼ A ? e@DG=kBT

ð9Þ

The pre-factor A depends on the type of the electron transfer process (e.g., bimolecular or intramolecular). The change in Gibbs free energy, DG, can be further defined by Equation (10),[44] in which l is the reorganization term and DG8 is the standard free energy of the reaction. DG ¼

+ * l DG0 2 1þ 4 l

ð10Þ

These terms depend on both the solvational and vibrational changes that occur during the molecular reorganization processes.[40] As a key take-away point, the reduction rate is directly related to the thermodynamic parameters of the reactants and products, as well as other chemical species involved in the reaction. With this fundamental knowledge, we are able to optimize the experimental conditions to modulate the generation rate of metal atoms, providing a means to rationally control the nucleation and growth of metal nanocrystals. In summary,

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Review Table 1. Oxidation reactions and corresponding standard electrochemical potentials of various reductants commonly used in the synthesis of colloidal metal nanocrystals.[45, 46]

Reductant

Half-reaction

E88 [V vs. SHE]

pyridine N,N-dimethylformamide cerium(III) ion 1-methylindole 3-amino-1-propanol 3-indolepropionic acid tryptophan sodium citrate sodium borohydride (basic) aniline hydrazine (basic) 1,4-phenylenediamine indole 4-bromoaniline glycine citric acid triethylamine 4-aminophenol tyrosine iron(II) ion hydroquinone hydrogen peroxide sodium borohydride (neutral) polyvinylpyrrolidone acetaldehyde hydropexoxyl hydrazine (acidic) formic acid ethanol methanol ascorbic acid hydrogen

C5H5N!C5H5NC + e HOCN(CH3)2 + H2O!HOOCN(CH3)2 + 2 H + + 2e@ Ce3 + !Ce4 + + e@ C9H9N!C9H9NC + + e@ HO(CH2)3NH2 !HO(CH2)3NH2C + + e@ C11H11O2N!C11H11O2NC + + e@ C11H12O2N2 !C11H12O2N22 + + 2e@ C6H5O73@ + 2 H2O!3 CH2O + 3 CO2 + 3 H + + 6 e@ BH4@ + 8 OH@ !B(OH)@4 + 4 H2O + 8e@ C6H5NH2 !C6H5NH2C + + e@ N2H4 + 4 OH@ !N2 + 4H2O + 4e@ C6H8N2 !C6H8N22 + + 2e@ C8H7N!C8H7NC + + e@ C6H6BrN!C6H6BrNC + + e@ C2H5O2N!C2H5O2NC + + e@ C6H8O7!CH3COCH3 + 3 CO2 + 2 H + + 2e@ (CH3CH2)3N!(CH3CH2)3NC + + e@ HOC6H4NH2 !HOC6H4NH2C + + e@ C9H11O3N!C9H11O3NC + + e@ Fe2 + !Fe3 + + e@ C6H4(OH)2 !C6H4O2 + 2 H + + 2e@ H2O2 !O2 + 2 H + + 2e@ BH4@ + 3 H2O!B(OH)3 + 7 H + + 8e@ HO(C6H9NO)nOH!HOO(C6H7NO)(C6H9NO)n-1OH + 4 H + + 4e@ CH3CHO + H2O!CH3COOH + 2 H + + 2e@ O2@ !O2 + e@ N2H5 + !N2 + 5 H + + 4e@ HCOOH!CO2 + 2 H + + 2e@ CH3CH2OH!CH3CHO + 2 H + + 2e@ CH3OH!CH2O + 2 H + + 2e@ C6H8O6 !C6H6O6 + 2 H + + 2e@ H2 !2 H + + 2e@ +

@

both the rate and yield of a redox reaction can be adjusted by employing reductants having appropriate oxidation potentials to best suit the reduction process. Table 1 summaries the oxidation potentials of various compounds that are commonly employed as reductants for the synthesis of colloidal metal nanocrystals.[45, 46]

Controlling the reduction kinetics of a salt precursor is a powerful means to control the shapes and structures of metal nanocrystals and thus optimizing their performance in an array of applications. Despite the incredible progress over the last decade, it remains a grand challenge to rationally design a protocol for synthesizing metal nanocrystals with various shapes and structures due to the lack of a quantitative knob for controlling the syntheses.[47, 48] In this section, we focus on the explicit role of reduction kinetics in the synthesis of nanocrystals, which allows us to precisely tune the nucleation and growth pathways in a predictable way. To this end, the reduction kinetics was systematically investigated with respect to a set of experimental parameters, such as reaction temperature, the types of precursor and reductant, as well as their concentrations.[47, 48] In these studies, the reduction kinetics was demonstrated to play a pivotal role in generating metal nanocrystals www.chemeurj.org

with desired and predictable shapes, structures, and compositions (for systems involving more than one types of metals) by quantitatively correlating the outcome of a synthesis with a set of experimental parameters.

3.1. Qualitative vs. quantitative description of the reduction kinetics

3. Measurement of Reduction Kinetics

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1.980 1.900 1.720 1.641 1.589 1.308 1.283 1.271 1.240 1.183 1.160 1.160 1.129 1.122 1.112 1.100 1.079 1.062 0.930 0.771 0.699 0.680 0.481 0.400 0.390 0.330 0.230 0.199 0.197 0.180 0.077 0.000

Although many synthetic methods have been developed for the generation of colloidal metal nanocrystals with diverse shapes and structures, the reduction kinetics remains elusive and needs further study. Most of the previous analyses were primarily based upon a qualitative description or understanding of the reduction kinetics, as well as its effect on the outcome of a synthesis. For example, the reduction kinetics was often described using vague terms such as “slow” or “fast” while people often took a trial-and-error approach in searching for the optimal experimental conditions. To this end, if we could derive the exact numerical values of the kinetic parameters such as the reduction rate constant and activation energy, we should be able to quantitatively understand the role of reaction kinetics and then utilize it to further control or even predict the outcome of a synthesis.

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Review 3.2. Quantitative analysis of the reduction kinetics To quantitatively understand the effect of reduction kinetics on the formation of colloidal metal nanocrystals, it is necessary to understand the kinetic parameters involving the reduction reaction. This can be achieved by monitoring the concentrations of a salt precursor remaining in the reaction solution at different time points. When the salt precursor is mixed with a reductant in a solution phase, the reduction kinetics can be assumed to follow a second-order rate law due to the electron transfer and collision between the precursor and the reductant, in which the reduction rate is proportional to the concentrations of both reagents. Recent studies have employed a pseudofirst-order rate law by supplying the reductant in great excess relative to the precursor to ensure that the concentration of the reductant would mainly remain as a constant throughout the reaction.[47, 48] Under such assumption, the rate law can be described by Equation (11), in which k is the combined rate constant; t is the reaction time; and [Mnþ ] and [reductant] are the instantaneous concentrations of the precursor and reductant, respectively. rate ¼ @

d½Mnþ A 0 ¼ k ½Mnþ A½reductantA ¼ k ½Mnþ A dt

ð11Þ

The pseudo-first-order order rate k can be obtained by integration [Eq. (12)]; ½Mnþ A0 and ½Mnþ At correspond to the concentrations of the precursor at the beginning and a specific time point t of the synthesis, respectively. ln½Mnþ At ¼ @kt þ ln½Mnþ A0

ð12Þ

In this case, a plot of ln½Mnþ At vs. t gives a straight line with a slope of @k. It should be pointed out that the assumption of pseudo-first-order rate is only valid when the reductant is excess.[49, 50] It is feasible to use either ultraviolet/visible spectroscopy (UV/Vis) or inductively-coupled plasma mass spectrometry (ICP-MS) to analyze the concentrations of salt precursors as a function of reaction time and further derive the kinetic parameters, including the rate constant, activation energy, and prefactor. In some cases, however, the presence of coordination ligands, capping agents, and pre-formed seeds would cause dif-

ficulties to derive the kinetic parameters. At the current stage of development, there is only a limited database available regarding the kinetic parameters involved in the chemical synthesis of metal nanocrystals. Table 2 summarizes the kinetic parameters, including rate constant and activation energy, derived from the quantitative analyses of reaction kinetics for typical reactions through UV/Vis or ICP-MS measurements.[51] Once a systematic database is available, one can argue that it is feasible to rationally synthesize metal nanocrystals by selecting a pair of precursor and reductant with desired kinetic properties. 3.3. Correlation between the structure of nanocrystals and the initial reduction rate Our previous studies clearly demonstrated that the structures (e.g., single-crystal vs. multiply twinned) of Pd nanocrystals showed a correlation with the initial reduction rate of PdII precursors involved in a one-pot synthesis, as shown in Figure 2 A.[47] In the work, we demonstrated a UV/Vis spectroscopic analysis to derive the kinetic parameters involved in the reduction of PdCl42@ by polyols, including the rate constant and activation energy, which were then correlated with the structures of the final products. By manipulating the initial reduction rate using different types of polyols, we were able to synthesize Pd nanocrystals with distinct structures from singlecrystal to multiply twinned, and stacking-fault-lined products. This quantitative correlation paves the way for rationally designing protocols capable of controlling the internal defect structure of nanocrystals rather than relies on the trial-anderror optimization of experimental parameters such as the types and concentrations of precursor and reactant, as well as the reaction temperature. Most recently, the quantitative approach was also successfully extended to a one-pot synthesis of Pd–Pt bimetallic nanocrystals with different structures, as illustrated in Figure 2 B.[48] Based on the quantitative analysis of reaction kinetics for PdII and PtII precursors using ICP-MS, we showed that Br@ ions in the reaction played a critical role in modulating the reduction rate of PdII and PtII precursors by altering their reduction potentials through ligand exchange.[48, 55] When the synthesis was conducted in the absence of Br@ ions, the initial reduction rates of PdCl42@ and PtCl42@ precursors indicated a 10-fold dif-

Table 2. Summary of kinetic parameters (i.e., rate constant and activation energy) derived from the quantitative analysis of reaction kinetics involved in the synthesis of colloidal metal nanocrystals.[a]

Precursor HAuCl4 AgNO3 Na2PdCl4 Na2PdCl4

Reductant

Temperature

formic acid ethylene glycol ethylene glycol diethylene glycol triethylene glycol

22 8C 140 8C 140 8C 140 8C 85 8C

diethylene glycol

103 8C

Additive HCl (0.1 m)

Na2SO4 (0.1 m)

Product Au particles Ag particles Pd truncated octahedra Pd icosahedra Pd plates Pd decahedra Pd icosahedra and decahedra

Rate constant 1

Activation energy @1

k = 2.26 V 10 min k = 7.98 V 10@3 min@1 k = 6.72 V 10@1 min@1 k = 3.34 V 10@2 min@1 k = 1.26 V 10@4 min@1 k = 3.57 V 10@3 min@1 k = 3.45 V 10@4 min@1

Ea = 20.3 kJ mol@1 Ea = 24.0 kJ mol@1 Ea = 103.2 kJ mol@1 Ea = 110.7 kJ mol@1

Ref. [52] [53] [47] [54]

[a] The rate constant k was obtained from the pseudo-first-order kinetic model, see Equation (12). The activation energy Ea was derived using the Arrhenius equation. Chem. Eur. J. 2018, 24, 16944 – 16963

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Review

Figure 2. Experimental data and schematics highlighting the importance of reduction kinetics in controlling the structures of noble-metal nanocrystals. (A) Plot displaying the percentage of Pd plate-like (green curve), icosahedral (black curve), and single-crystal (blue curve) nanocrystals formed as a function of the initial reduction rate. (B) Schematic illustration showing how PdPt core–shell octahedra and Pd-Pt alloy nanocubes are formed in the absence and presence of Br@ ions by manipulating of the reduction rates of PdII and PtII precursors in a one-pot synthesis. The image in (A) was adapted with permission.[47] Copyright 2015 American Chemical Society. The image in (B) was adapted with permission.[48] Copyright 2016 American Chemical Society.

ference. The large difference in reduction rate resulted in the fast formation of Pd seeds first, followed by the deposition of Pt atoms on the seeds, leading to the formation of Pd@Pt core–shell octahedral nanocrystals. While in the presence of Br@ ions, the products were switched to Pd–Pt alloy nanocubes because the ratio between the initial reduction rates of PdII and PtII precursors dropped from 10 to 2.4 due to the formation of PdBr42@ and PtBr42@ via ligand exchange. In this case, both PdII and PtII precursors were reduced at a relatively comparable rate, forming Pd–Pt alloy nanocubes due to the strong capping effect of Br@ ions on the {100} facets. Taken together, the shapes (cubic vs. octahedral) and structures (alloy vs. core– shell) of Pd–Pt bimetallic nanocrystals could be easily manipulated by controlling the ratio between the initial reduction rates of PdII and PtII precursors. These examples indicate that the initial reduction rates of salt precursors can serve as a quantitative knob for the synthesis of mono-, bi- and even multi-metallic nanocrystals with desired shapes, compositions, and structures.

4. Case Studies A wide variety of compounds have been reported as reductants for the chemical syntheses of colloidal metal nanocrysChem. Eur. J. 2018, 24, 16944 – 16963

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tals. In most cases, the involved mechanisms have been proposed with regard to how reductants affect the shapes and structures of the final products. Table 3 shows a summary of the metal nanocrystals prepared using reductants ranging from hydrogen gas to inorganic hydrides and organic compounds. A number of metals, including Ag, Au, Pd, Pt, and Rh, were included, together with a series of shapes such as spheres, cubes, octahedra, truncated octahedra, cuboctahedra, tetrahedra, plates, decahedra, icosahedra, rods, wires, multipods, and bipyramids. As illustrated by these examples, most reductants have the versatility to produce different metal nanocrystals with diversified shapes. For example, EG was found to be of great versatility in producing Ag cubes, Au octahedra, Pd decahedra, Pt nanorods, and Rh icosahedra. It should be pointed out that the reductant is often combined with different additives such as capping agent, stabilizer, and even a second reductant, to control the synthesis. In general, the formation of nanocrystals with diverse shapes should be attributed to the synergistic role of all the chemicals involved in a synthesis rather than the reductant alone, as well as the control over a series of other reaction parameters such as reaction temperature and the type of salt precursor. In this section, we provide a case-by-case discussion of the reductants commonly used in the syntheses, including the features of reductants, reaction mechanism, synthetic methods, and physicochemical characteristics of the products. 4.1. Gaseous reductants Gaseous reductants are highly desired for the synthesis of colloidal metal nanocrystals due to the easy separation from the final products. To this end, gaseous compounds such as hydrogen (H2) and carbon monoxide (CO) have been widely used as reductants for the synthesis of noble-metal nanocrystals.[20, 144, 177–179] The reactions involved in the reduction of metal ions by H2 are described in Equations (13)–(15). In the oxidation half-reaction [Eq. (13)], H2 is converted to H + and donates one pair of electrons per molecule while in the reduction half-reaction [Eq. (14)], metal ions accept the electrons and are reduced to metal atoms. The involved mechanism in the reaction is quite simple and can be described using an universal global reaction, shown in Equation (15).[49] H2 ! 2 Hþ þ 2 e@

ð13Þ

Mnþ þ ne@ ! M0

ð14Þ

nH2 þ 2Mnþ ! 2M0 þ 2nHþ

ð15Þ

Clearly, the use of different salt precursors, as well as different pH conditions, can modify the kinetics of the global reaction. El-Sayed and co-workers employed H2 as a reductant to produce Pt nanocrystals with a cubic shape at a pH value of 7. However, when the synthesis was conducted at a pH value of 9, the product took a spherical shape, demonstrating the effect of pH on the final shape of the Pt nanocrystals.[70]

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Review Table 3. Summary of various reductants employed in the shape-controlled syntheses of metal nanocrystals.[a]

Schematic drawing

Metals Ag

Au

Pd

Pt

Rh

EG,[23, 56, 57] DEG,[58] PD,[21] CitA,[59] DMF,[60] HQ,[61]

SCit,[62] EG,[63] PD,[64, 65] SBH,[66] DMF,[60] PrOH,[67]

H2,[68] EG,[63] P123,[69]

H2,[70] Pt,[71, 72] SBH,[73] EG,[72]

H2,[74] EG,[41] BD,[75] OA,[76]

H2,[86, 87] EG,[88] DEG,[89] AA,[90] FOR,[91]

EG,[92] H2,[93, 94] CO,[95] SBH,[96] AA,[97]

EG,[98, 99] DEG,[41] TrEG,[41] AA,[100] CTAB,[101]

EG,[77, 78] DEG,[79] PD,[80] AA,[23] EG,[81] PD,[64, 82] AA,[83–85]

EG,[78] PD,[80] AA,[80]

EG,[81, 102] PD,[64, 82] AA,[103] PEG,[104] PVP,[105] ID,[106]

TEG,[107] FOR,[91] EG,[72, 92] [108, 109] [113] [107] CitA, AA, EG, PVP[110]

EG,[111] TEG,[41] CTAB,[101]

EG,[78] PD,[80] CitA,[59]

PD,[64] AA,[83]

EG,[47, 112] FOR,[91] AA,[113]

H2,[114] FOR,[115]

CTAB,[101]

EG,[78, 116] PD,[80]

PD,[64, 82] AA,[83]

HDZ,[117] FOR,[91] AA,[113] EG,[107]

H2,[96, 118] EG,[92]

EG,[41, 111] CTAB,[101]

EG,[116, 119] DMF,[120, 121]

EG,[81]

TrEG,[107] EG,[107]

H2,[70, 93, 122] EG,[72] FOR,[115] AA,[97]

EG,[111] BD,[75] OA,[76]

EG,[123] PVP,[124] SBH,[125] DMF,[61] HDZ,[126] HQ,[29]

HDZ,[127] AA,[84] HT,[128] HP,[129] EG,[131] DEG,[47] PVP,[124] PVP,[105] SBH,[130] AA,[ 132] CO,[95]

H2,[87, 94]

EG,[41] TrEG,[41] TEG,[41] BD,[75]

SBH,[17] DMF,[120, 121]

DMF,[133] DEG,[134]

EG,[135] DEG,[136] CitA,[109]

H2,[94] OA,[137] EG,[138]

EG,[111] BD,[75]

DMF,[120, 121]

EG,[81] AA,[139] DEG,[134] PVP,[140]

EG,[141] DEG,[ 136] TEG,[142, 143] CitA,[109, 141]

OA,[137] EG,[138] CO,[144]

EG,[41, 111] DEG,[41] TrEG,[41] TEG,[41] PVP,[145]

EG,[146] AA,[147, 148] SBH,[149]

SBH,[150] AA,[151, 152] PVP,[105] HQ,[153]

EG,[89] AA,[154, 155]

HeOH,[156] EG,[157]

CTAB,[101]

EG,[158]

AA,[159] DMF,[160]

EG,[161]

H2,[87, 94] EG,[162] EG,[72] AA,[97] HDD,[163]

EG,[41, 98, 111, 164] TrEG,[41] TEG,[41]

EG,[119]

AA,[152]

EG,[161] AA,[154]

AA,[97]

–

H2,[165] EG,[166, 167] SCit,[168] AA,[148] CO,[95] SBH,[149]

H2,[165] AA,[85] HA,[169] PVP,[105] H2,[170] EG,[171] PVP,[172] SBH,[130] AA,[173] DEG,[171]

H2,[165, 174] EG,[175] SBH,[176]

CTAB,[101]

[a] AA: Ascorbic acid; BD: 1,4-butanediol, CitA: Citric acid; CO: carbon monoxide; CTAB: Cetyltrimethylammonium bromide; DMF: N,N-dimethylformamide; EG: Ethylene glycol; FOR: Formaldehyde; H2 : Hydrogen gas; HA: Hydroxylamine; HDD: 1,2-hexadecanediol; HDZ: hydrazine; HeOH: Hexanol; HP: Hydrogen peroxide; HQ: Hydroquinone; HT: 5-hydroxytryptamine; ID: Iodide; OA: Oleylamine; PD: 1,4-pentanediol, PEG: Polyethylene glycol; PrOH: 2propanol; PVP: Polyvinylpyrrolidone; P123: block copolymer Pluronic; SCit: Sodium citrate; SBH: Sodium bohydrite; TrEG: Triethyleneglycol; TEG: Tetraethyleneglycol.

With regard to CO, although it is widely used as a reductant in the synthesis of metal nanocrystals, its explicit roles are yet to be fully uncovered. In practice, it can act as not only a reductant but also a capping agent due to its strong binding to the surface of various metals, resulting in the formation of metal nanocrystals with well-defined shapes including cubes, spherical particles, and wires.[20, 95] Murray and co-workers reported the synthesis of Pt nanocubes using CO as a reductant, in which CO was also found to selectively cap the {100} facets for the formation of cubic nanocrystals.[95] Using the same approach, they also prepared Au nanowires of 2.5 nm and several micrometers in width and length, respectively. In addition, it is Chem. Eur. J. 2018, 24, 16944 – 16963

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believed that the use of CO not only provides the slow reduction kinetics necessary to support the generation of multiplytwinned nanocrystals but also potentially protects the structures from oxidative etching caused by the dissolved oxygen, with typical example of Pt icosahedra.[20] In essence, the use of CO extends far beyond the concept of a simple reductant, likely providing multiple roles or functions. 4.2. Hydrides Hydrides, including NaBH4, LiBH4, LiAlH4, and Li(C2H5)3BH, represent a class of well-known reductants. Introducing a hydride

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Review into a salt precursor solution leads to the formation of metal nanocrystals within a very short period of time, which limits the utility of hydrides in the shape-controlled synthesis because the fast formation of plenty tiny metal nanocrystals often leads to growth via attachment.[180–182] Among the hydrides, NaBH4 stands out as the most extensively explored reductant in the synthesis of noble-metal nanocrystals such as Ag,[183–185] Au,[184, 186, 187] Pd,[185, 188, 189] and Pt.[185, 190, 191] As expected from the fast reduction kinetics, these protocols involving NaBH4 often gives rise to nanocrystals with relatively small sizes, typically below 5 nm. It is well-known that the oxidation half-reaction and the corresponding oxidation potential of NaBH4 are strongly dependent on the pH of reaction solutions. More specifically, the oxidation potential is increased from 0.48 to 1.24 V when the pH is changed from neutral [Eq. (16)], to alkaline [Eq. (17)], causing a major boost to the reducing capability of NaBH4 toward the formation of metal nanocrystals. @ 4

þ

@

BH þ 3 H2 O ! BðOHÞ3 þ 7 H þ 8 e @ 4

@

@ 4

E @

BH þ 8 OH ! BðOHÞ þ 4 H2 O þ 8 e

E

2

2

oxi

¼ 0:48 V

ð16Þ

oxi

¼ 1:24 V

ð17Þ

For the synthesis of metal nanocrystals involving NaBH4, the first step can be described as a fast reduction of the salt precursor, followed by the supersaturation of free atoms to generate nuclei and then the formation of nanoparticles. As the precursor is consumed quickly within a short period, nanocrystal growth will be largely dominated by coalescence (or attachment), which depletes the particles quickly.[180, 192] However, the as-synthesized nanoparticles from a system only containing metal precursor and reductant span a broad range of sizes. To address this issue, we can add an effective stabilizer such as poly(vinyl pyrrolidone) (PVP), thiol, or amine, to achieve nanoparticles with a narrow size distribution, together with reasonable colloidal stability.[180, 192] In the shape-controlled synthesis of metal nanocrystals, NaBH4 is often employed as an initial reductant for the production of seeds through a fast reduction. The seeds then undergo growth to generate well-defined nanocrystals in the presence of a second reductant with relatively weaker reducing capability such as ascorbic acid, aldehyde, or hydroxylamine sulfate, as well as a capping agent and/or a colloidal stabilizer such as PVP, hexadecyltrimethylammonium bromide (CTAB), hexadecyltrimethylammonium chloride (CTAC), and sodium citrate. Based on this strategy, a number of protocols have been reported for the synthesis of nanocrystals with well-defined shapes, including spheres,[193] dendrites,[194] octahedra,[195] and rods.[195] 4.3. Hydrogen peroxide Hydrogen peroxide (H2O2) is a well-known oxidant for a variety of applications, including organic synthesis, environmental remediation, and nanocrystal synthesis.[196, 197] In the context of nanocrystal synthesis, H2O2 is often employed as an oxidant for the selective removal of Ag or Pd from bimetallic structures Chem. Eur. J. 2018, 24, 16944 – 16963

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through oxidative etching.[198, 199] This process is interesting as it can transform solid bimetallic nanocrystals (e.g., core–shell nanocrystals) into hollow nanocrystals. In addition, H2O2 can also serve as a reductant, for example, to reduce AuCl4@ and generate Au atoms in an aqueous system.[129, 201] The proposed mechanism for the reduction of metal ions by H2O2 is shown in Equations (18)–(20). The oxidation half-reaction [Eq. (18)] consists of the oxidation of H2O2 into O2 and H + . In this process, each oxygen atom in H2O2 loses one electron, implying that one pair of electrons is donated per H2O2 molecule. In the reduction half-reaction [Eq. (19)], metal ions accept the electrons and are converted to metal atoms. The global reaction is shown in Equation (20) and similar to that of H2, in which a change of pH could also show significant effect on the reaction. nH2 O2 ! nO2 þ 2nHþ þ 2ne@

ð18Þ

Mnþ þ ne@ ! M0

ð19Þ

nH2 O2 þ 2Mnþ ! 2M0 þ nO2 þ 2nHþ

ð20Þ

Because of the formation of only O2 and H + as the products, H2O2 is considered a promising green reagent for the synthesis of metal nanocrystals. In terms of reducing capability, H2O2 is relatively weaker compared to other reductants such as ascorbic acid and sodium citrate, due to the relatively low oxidation potential. Additionally, to stabilize the produced nanoparticles, an additional reagent such as sodium dodecyl sulfate (SDS), CTAC, or CTAB is also essential to prevent the particles from agglomeration.[129, 200] 4.4. Alcohols, polyalcohols (polyols), and phenols Organic compounds such as alcohols, polyalcohols (polyols), and phenols are attractive for the synthesis of colloidal metal nanocrystals as they can play multiple roles in a reaction, acting as a solvent, and/or stabilizer, in addition to a reductant.[24] Among various alcohols, methanol and ethanol are the most commonly used because of their abundance and relatively lower prices than other alcohols such as n-propanol, n-butanol, and 1-hexanol.[37, 201–215] In general, primary alcohols always work as reductants due to their superior reactivity relative to secondary and tertiary alcohols. Mechanistically, when a primary alcohol is used as the reductant for the reduction of metal ions, an aldehyde is generated as the product, together with H + ions, as illustrated in Equation (21): nR @ CH2 OH þ 2Mnþ ! 2M0 þ nR @ CHO þ 2nHþ

ð21Þ

In addition to primary alcohols, polyols (molecules with more than one hydroxyl groups in the structure) have also played a pivotal role in the synthesis of metal nanocrystals over the past decades.[24, 41, 112] By employing a variety of polyols such as ethylene glycol (EG),[23, 56, 57] diethylene glycol (DEG),[58] triethylene glycol (TREG),[41] tetraethylene glycol (TEG),[41] 1,2-hexadecanediol,[163] 1,4-butanediol,[75] and 1,4-pentadiol[64, 82] as reductants, a series of metal colloidal nanocrystals have been pro-

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Review duced with typical examples including Ag, Au, Ir, Pd, Pt, and Rh. For the polyol reduction, the mechanism is strongly dependent on the reaction temperature, in which different derivatives might be involved at different temperatures. Equations (22)–(26) show the influence of reaction temperature on EG which acts as a reductant. When the reaction is conducted at a temperature above 160 8C, EG is dehydrated to generate acetaldehyde [Eq. (22)],[216] after which the as-obtained acetaldehyde is responsible for the reduction of metal ions, accompanied by its own oxidation to diacetyl [Eq. (23)].

shape, and dispersity showed direct correlations with the oxidation potentials of the different polyols serving as the reductants, including EG, DEG, TREG, and TEG. According to Figure 3 A, the applied potential necessary for the onset of solvent oxidation was increased with the molecular size of the polyol, and therefore the oxidation potentials of the polyols at room temperature were decreased in the order of EG > DEG > TREG > TEG, likely due to enhanced electronic stability of the alcohols afforded by the greater intermolecular bonding provided by the ether functionalities.[41] In addition, as depicted in Figure 3 B, the oxidation potentials of all these polyols were increased accordingly as the temperature was elevated from HOCH2 CH2 OH ! CH3 CHO þ H2 O ð22Þ room temperature to 60 8C,[43, 44] which still shows the same decrease trend of oxidation potentials from EG to DEG, TREG and ð23Þ 2nCH3 CHO þ 2Mnþ ! 2M0 þ nCH3 COCOCH3 þ 2nHþ TEG, as summarized in Figure 3 C. Figure 3 D–G, shows TEM images of Rh nanocrystals prepared by employing RhCl3 as a 2 HOCH2 CH2 OH þ O2 ! 2 HOCH2 CHO þ 2 H2 O ð24Þ precursor, together with the use of EG, DEG, TREG, and TEG as reductants, respectively. The difference in reductant significantð25Þ nHOCH2 CHO þ 2Mnþ ! 2M0 þ nHOCCHO þ 2nHþ ly affected the shapes, sizes, and sample uniformity of the final products. For example, branched Rh particles were obtained ð26Þ nHOCH2 CH2 OH þ 2Mnþ ! 2M0 þ nHOCH2 CHO þ 2nHþ by employing EG as a reductant while triangular plates were predominantly observed when polyols with larger molecular However, if the reaction temperature is in a range from 140 to sizes (i.e., DEG, TREG and TEG) were used. This observation is 160 8C, glycolaldehyde becomes the primary reductant which consistent with the trends observed in the Pd system when a can be generated by heating EG in air [Eq. (24)], and then PdII precursor was reduced by different polyols at various temserves as a reductant for reducing metal ions while it is oxiperatures, where a slow reduction led to the formation of Pd dized to glyoxal [Eq. (25)]. When the reaction temperature is nanoplates.[47] below 140 8C, EG can act as a reductant by itself, leading to In addition to polyols, phenolic compounds characterized by the formation of glycolaldehyde as the oxidized product an aromatic ring with OH groups can also work as reductants, [Eq. (26)]. and the involved mechanisms and concepts in a reaction are Figure 3 shows an example based on the polyol synthesis of similar to those for alcohols and polyols. When a phenolic Rh nanocrystals.[41] In this work, the differences in particle size, compound is used as the reductant, the redox process leads to the formation of quinone as the oxidized products, together with the generation of H + ions. In this case, the number of electrons involved in the redox process is determined by the number of hydroxyl groups contained in the phenolic compound. In general, each hydroxyl group is able to donate one electron.[217] Among the wide variety of phenols, hydroquinone, catechol, and resorcinol are the most commonly employed reductants for the synthesis of metal nanocrystals.[217, 218] Especially, hydroquinone has attracted much attention recently due to its strong capability to reduce metal ions, making it possible to synthesize metal nanocrystals within a very short period of time (around 15 s) and under relatively mild conditions.[219–221] Moreover, as the reducing capability of hydroquinone strongly depends on pH, the reaction can be completely Figure 3. Effect of reduction kinetics on the size, shape, and structures of Rh nanocrysquenched by the addition of acid, which could benetals. Linear sweep voltammograms of the oxidation of EG (black curve), DEG (green fit the kinetic studies.[218] In all cases, metal ions are curve), TREG (Blue curve), and TEG (red curve) using a Pt working electrode at (A) room @1 reduced to metal atoms by a two-electron mechatemperature and (B) 60 8C in the presence of a 0.4 m NaNO3 at a sweep rate 10 mV s vs. Ag/AgCl reference electrode. (C) Chemical structure of EG, DEG, TREG, and TEG, and the nism when using hydroquinone as the reductant, totrend of their relative oxidation potentials and reduction rates when employed as reducgether with the formation of quinones and H + ions tants in the synthesis of metal nanocrystals. TEM images of Rh nanocrystals synthesized as byproducts. using (D) EG, (E) DEG, (F) TREG, and (G) TEG as reductants, respectively. Scale bar in Similar to those of other reductants with hydroxyl panel D is 20 nm and applied to panel (D-G). Adapted with permission.[41] Copyright 2011 American Chemical Society. groups, the oxidation potential of hydroquinones can Chem. Eur. J. 2018, 24, 16944 – 16963

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Review

Figure 4. Schematic illustration of three different modes derived from the variation of pH values involved in the deposition of Ag atoms on the surface of an Au seed using hydroquinone as a reductant: i) no reduction of Ag + ions at pH < 7.0, ii) self-nucleation of Ag nanoparticles on Au seeds in a pH range of 7.0– 9.2, and iii) Au@Ag core–shell nanocrystals at pH > 9.2, respectively. The image was reproduced with permission.[222] Copyright 2016 American Chemical Society.

be modulated by adjusting the reaction conditions such as pH and temperature. For hydroquinones, a simple adjustment of pH can cause drastic changes to the reducing capability, and thus significantly affect the nucleation and growth of nanocrystals. As shown in Figure 4,[222] the authors demonstrated that the variation of pH in the reduction of Ag + ions by a hydroquinone derivative (denoted as H2Q) could completely change the growth patterns of Ag atoms on Au seeds. Specifically, when the reaction was carried out under acidic conditions (pH < 7.0), the lone electron pair of H2Q became protonated [Eq. (27)] and, consequently, unavailable for electron donation. In this case, the reducing capability of H2Q was largely suppressed and no Ag + reduction was observed. While under relatively mild alkaline conditions (7.0 < pH < 9.2), the H2Q is deprotonated [Eq. (28)], leading to the formation of an anionic species displaying two additional electron pairs. H2 Q þ 2 Hþ ! H4 Q2þ

ð27Þ

H2 Q þ 2 OH@ ! Q2@ þ 2 H2 O

ð28Þ

In contrast to the acidic condition, the reducing capability of H2Q was significantly increased in an alkaline condition, and consequently, the reduction of Ag + ions was accelerated, leading to the fast nucleation of Ag atoms on the surface of Au nanocrystals. However, when the pH value is above 9.2, although the hydroquinone was fully deprotonated and in the most active form, the conversion of Ag + ions to Ag2O [Eq. (29)] resulted in a significant decrease in the reduction potential: Ag + /Ag (0.799 V vs. SHE) and Ag2O/Ag (0.342 V vs. SHE). Therefore, the reduction kinetics was decelerated to a level that favors a layer-by-layer growth mode of Ag atoms, forming Au@Ag core–shell nanocrystals. 2 Agþ þ 2 OH@ ! Ag2 O þ H2 O

Chem. Eur. J. 2018, 24, 16944 – 16963

ð29Þ

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4.5. Aldehydes In addition to the reductants discussed above, simple aldehydes, as well as complex aldehyde-based compounds such as sugars, also show great versatility as reductants for the synthesis of metal nanocrystals.[223–228] This class of compounds has received special attention for shape-controlled synthesis due to their relatively mild reducing capability. Our group has reported the synthesis of Pd nanocrystals growing from cubes to cuboctahedra and further to octahedra using formaldehyde as a reductant.[91] Another example is the synthesis of concave Pd tetrahedra using formaldehyde or benzaldehyde as the reductant,[229] where the aldehyde group in the reductant was proven to be directly related to the formation of a concave structure, and the degree of concavity could be controlled simply by varying the concentration of the aldehyde.[229] Mechanistically, when aldehydes are employed as reductants for the reduction of metal ions, there are two types of involved mechanisms depending on the presence or absence of water in the system, as shown in Equations (30) and (31). In a synthesis containing water [Eq. (30)], the aldehyde is oxidized to the corresponding carboxylic acid, together with the generation of H + ions. In a water-free system, the reduction process leads to the formation of diketone and H + ions as the oxidized byproducts [Eq. (31)]. nR @ CHO þ 2Mnþ þ n H2 O ! 2M0 þ nR @ COOH þ 2nHþ ð30Þ 2nR @ CHO þ 2Mnþ ! 2M0 þ nR @ COCO @ R þ 4nHþ

ð31Þ

Interestingly, aldehyde-based reductants can be either directly introduced into a reaction solution or generated during a synthesis. As discussed in the prior section, the oxidation of alcohols and polyols would lead to the generation of aldehydes. For examples, ethanol and EG could be oxidized to form acetaldehyde and glycolaldehyde, respectively. Another route is the hydrolysis of hexamethylenetetramine (HMTA) at elevated

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Review temperatures, which results in the slow generation of formaldehyde.[230] 4.6. Alkoxides In the last several sections, we have discussed the use of alcohols, polyols, and aldehydes in the synthesis of noble-metal nanocrystals. However, some reports showed that the use of these compounds under alkaline conditions could lead to the formation of alkoxides, which were demonstrated to work as powerful reductants capable of reducing metal ions such Ag + ions at room temperature.[231] Although there are some reports related to the use of alkaline media for the synthesis of metal nanocrystals, the role of hydroxyl ions (OH@) in the reaction has not been well elucidated. For a number of cases, OH@ is considered to work as a mere accelerator to facilitate the reduction process (i.e., the reaction only occurs in its presence). In addition, OH@ may also coordinate with the metal ions and thereby affect the reduction kinetics.[231–233] Mechanistically, the reaction involves the alkoxides and metal ions can be described as a two-step process. The first step refers to the nucleophilic addition of OH@ from the alkaline media to either the carbon atom bounded to the hydroxyl group of an alcohol/ polyol, or to the carbonyl group of an aldehyde/ ketone, leading to the formation of alkoxides. In the second step, metal ions are reduced by alkoxides to generate metal atoms for the nucleation and growth processes.[231] Figure 5. Influence of pH on the reduction of metal ions using ascorbic acid as a reductant. (A) Graph showing three types of derivatives from ascorbic acid as a function of pH value. Mechanisms of metal ion reduction by (B) ascorbic acid, (C) ascorbate, and (D) diascorbate. When pH is increased, ascorbic acid is progressively deprotonated to ascorbate and further to diascorbate, showing an increase in reduction power and the electron donation rate.

4.7. Organic acids

Organic acids and their derivatives, including formic, acetic, oxalic, malic, ascorbic, and citric acids, have been shown to be versatile for the synthesis of metal nanocrystals. Among them, ascorbic, citric, and formic acid are the most popular ones and will be the focus of our discussion here. Interestingly, when these acids are employed as reductants, due to their weak acidity relative to inorganic acids, the pH at which the synthesis is performed shows significant effects. More specifically, the variation of pH affects both the redox potentials of the organic molecules and the surface charge density on the nanocrystals, making it possible to precisely control the reduction kinetics and thus the shapes and/or structures of the as-obtained nanocrystals. 4.7.1. Ascorbic acid Ascorbic acid, also known as Vitamin C and commonly abbreviated as AA or HAsc, is widely used as a reductant in the synthesis of noble-metal nanocrystals and can be found in extracts of various plants, vegetables, and fruits.[234–240] As illustrated in Figure 5 A, three different species can be observed when the pH is varied from 0 to 14: ascorbic acid, ascorbate, and diaChem. Eur. J. 2018, 24, 16944 – 16963

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scorbate. For all three forms, the mechanism involved in the synthesis of noble-metal nanocrystals is relatively similar to each other in terms of electron transfer and the oxidized product formed in the reaction (Figure 5 B–D). Each molecule donates one pair of electrons during the reduction of metal ions, leading to the formation of the same oxidized product: dehydroascorbate (Figure 5 B–D). Figure 6 A,B, shows TEM images of Au nanocrystals prepared in the presence of AA as a reductant at different pH values. When the reaction was conducted under an acidic condition with pH = 3, the Au nanocrystals took a branched morphology. In contrast, if the pH value was increased to 7, Au nanospheres were obtained. The difference could be attributed to a faster nucleation and growth process promoted by the increase in pH. In terms of reducing capability, AA is considered a relatively weak reductant under non-alkaline conditions, which could benefit the synthesis of metal nanocrystals via seed-mediated growth by preventing the self-nucleation of newly formed

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Review high concentration of ascorbate ions).[151, 228] When combined with a variety of capping agents and/or colloidal stabilizers, noble-metal nanocrystals with various shapes could be well prepared using ascorbic acid/ascorbate as the reductant. 4.7.2. Citric acid and sodium citrate Citric acid and sodium citrate are two types of well-established reductants for the synthesis of noble-metal nanocrystals, due to their abundance and versatility to work as both the reductant and stabilizer. Taking the preparation of Au colloids as an example, it is mostly based on the Turkevich method or citrate method, in which the reduction of Au3 + ions is usually performed at around 100 8C in an aqueous solution.[243–246] By modifying the synthetic conditions (e.g., reductant concentration, pH, reaction time, and temperature), Au nanocrystals with different sizes and shapes can be readily obtained. Similar to ascorbic acid, citric acid can be found in different forms as a function of pH shown in Figure 7 A.[246] As expected, increasing

Figure 6. (A, B) TEM images of Au nanocrystals prepared using ascorbic acid as a reductant at pH values of (A) 3 and (B) 7, respectively. (C–F) TEM images of Ag nanocrystals prepared using citric acid as a reductant at pH values of (C) 5.7, (D) 6.1, (E) 8.3, and (F) 11.1, respectively. (G, H) TEM images of Cu nanocrystals prepared using hydrazine as a reductant except in the (G) absence and (H) presence of 12.5 mm of NaOH. The images in (A and B) were adapted with permission.[237] Copyright 2014 Royal Society of Chemistry. The images in (C–F) were adapted with permission.[246] Copyright 2009 American Chemical Society. The images in (G and H) were adapted with permission.[255] Copyright 2014 Royal Society of Chemistry.

metal atoms.[159, 191] This concept is best illustrated by the preparation of Au nanorods with high aspect-ratios through depositing Au atoms onto Au seeds. In the reaction, the weak reducing capability of AA, together with the stabilization role of ascorbate anion, are considered the crucial keys to the formation of the elongated nanostructures.[159, 193] Besides, the addition of OH@ ions is a well-established strategy to increase the rate of metal ion reduction, which is also known as forced reduction because of the increase in reducing capability of AA and the better stabilization role of deprotonated AA formed under higher pH conditions.[241] For example, the addition of OH@ is imperative to enable the reduction of ions for relatively reactive metals such as Ag, in which a basic pH condition contributes to the initiation of the nucleation, growth, and the formation of branched nanocrystals (only formed under relatively Chem. Eur. J. 2018, 24, 16944 – 16963

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Figure 7. (A) The speciation diagram of citric acid as a function of pH value, in which an increase in pH leads to the progressive deprotonation of citric acid, forming species that display one, two, and three negative charges, respectively. (B) Influence of the pH on the reduction kinetics of Ag + ions using citrate as a reductant, indicating that an increased pH leads to a higher rate constant k. The image was reproduced with permission.[246] Copyright 2009 American Chemical Society.

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Review the pH value leads to the progressive deprotonation of citric acid, forming species that display one, two, and three negative charges, respectively. The increased electron density available for donation during a redox process leads to a stronger reducing capability. As depicted in Figure 7 B, a significant increase in the reduction of Ag + ions to atoms was observed when the pH was increased from 5.7 to 11.1. Specifically, when the pH value was 5.7, 6.1, 8.3, and 11.1, the measured k corresponded to 0.02, 0.04, 1.11, and 3.34, respectively. As the pH value was increased from 5.7 to 11.1, the rate constant showed a 167-fold enhancement. Figure 6 C–F, shows the Ag nanocrystals obtained at pH values of 5.7, 6.1, 8.3, and 11.1, respectively. When the pH value was 5.7, the prod- Figure 8. Schematic showing three main steps involving the reduction of metal ions by employing sodium citrate as a reductant: (i) metal ions are reduced to metal atoms while ucts were mainly composed of triangular and polygocitrate is oxidized to acetone dicarboxylate; (ii) acetone dicarboxylate is then hydrolyzed, nal nanocrystals, with an average size of 96 nm (Fig- leading to the formation of acetone; (iii) acetone reduces metal ions to metal atoms and ure 6 C). As the pH value was increased to 6.2 and is oxidized to formaldehyde. further to 8.3, the size of the resultant Ag nanocrystals was decreased to 69 and 59 nm, respectively, verifying a faster nucleation process, in agreement with the kinetfor the additional reduction of metal ions, generating formalics study shown in Figure 7 B. Especially, when the synthesis dehyde and H + ions as the oxidized byproducts.[62, 247] was conducted at a pH value of 11.1, the reducing power of citric acid was substantially enhanced for the production of Ag 4.7.3. Formic acid nanospheres and nanorods, with a size range of 25–35 nm. The changes in both the particle size and shapes could be atDue to its extensive use in the synthesis of metal nanocrystals, tributed to the different reducing powers of citric acid at differformic acid also deserves special attention. Similar to some of ent pH values. An increase in pH promoted the reduction of the compounds we have discussed above, formic acid is not Ag + ions and thus accelerated the nucleation and growth proconsidered a reductant that directly involves in a redox reaccesses. tion, but rather a precursor to their derivatives which act as Mechanistically, the synthesis of noble-metal nanocrystals inthe real reductants in the reactions. Formic acid has been volving the use of sodium citrate can be attributed to the conshown to serves as a source of gaseous H2 and CO under lowtributions from two reductants: a) sodium citrate by itself and pressure environments, both of which show considerable reb) acetone generated during the reaction. As shown in ducing capabilities (as discussed in section 4.1). In addition, Figure 8, the global process can be described by three succesformic acid also creates an environment that prevents the assive steps. In the first step, metal ions are reduced to atoms by obtained nanocrystals from oxidation as it is more preferentialcitrate ion, which is oxidized to acetone dicarboxylate, a wellly to be oxidized rather than the nanocrystals.[95, 248–250] known ligand capable of coordinating with metal ions and faFigure 9 illustrates the general process for the reduction of cilitating the growth of nanocrystals. In this case, CO2 and H + metal ions involving formic acid, in which both H2 and CO can are also formed as byproducts. Afterward, acetone dicarboxybe generated by two simultaneous processes: i) dehydration, late is quickly hydrolyzed, leading to the formation of CO2, leading to the formation of H2O/CO; ii) decomposition, correOH@ and acetone. Finally, the acetone also acts as a reductant sponding to the formation of H2O/CO and CO2/H2. Although

Figure 9. Schematic illustrations of two main steps involving the reduction of metal ions using formic acid as a reductant: i) the formation reductive species from formic acid through two different pathways: dehydration and decomposition, leading to the generation of CO and CO + H2, respectively; ii) the reduction of metal ions to metal atoms by the generated CO and H2. Chem. Eur. J. 2018, 24, 16944 – 16963

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Review the oxidized product for H2 is well known to be H + , the oxidized product for CO remains unclear and needs further investigation.[95, 251] Interestingly, some reports have demonstrated that the presence of metals nanoparticles or seeds in a reaction could facilitate the decomposition of formic acid, leading to the formation of H2 and CO under much milder conditions. Therefore, after the generation of the nuclei or when seeds are introduced in a chemical reaction, the decomposition process becomes autocatalytic and would generate more reducing species, which could significantly affect the reaction kinetics and thus the shape-controlled synthesis of metal nanocrystals.[95] 4.8. Nitrogen compounds 4.8.1. Hydrazine The reducing capability of hydrazine originates from the two lone electron pairs on the nitrogen atoms. As all four electrons can be donated, hydrazine has been shown to be a strong reductant.[252, 253] Similar to ascorbic acid, hydrazine is more powerful when employed under alkaline conditions due to its higher oxidation potential relative to that under acidic conditions.[36, 252–255] Equations (32) and (33) show the half-reactions for hydrazine oxidation and their corresponding oxidation potentials in alkaline and acidic media, respectively:[36, 254] @

N2 H4 þ 4 OH ! N2 þ 4 H2 O N2 H

þ 5

! N2 þ 5 H

þ

E E

2

2

oxi

¼ 1:160 V

ð32Þ

oxi

¼ 0:230 V

ð33Þ

As shown in Figure 10, the oxidation potential and corresponding reduction mechanism of hydrazine is sensitive to the pH of the reaction system.[36] A small variation in pH can cause significant changes to the half-reaction of hydrazine oxidation and the oxidation potential, which would further affect the reaction

kinetics for the nucleation and growth of metal nanocrystals as well as their final properties (size, shape, structure and compositions for bimetallic systems).[36] A typical example is the synthesis of Cu nanocrystals, in which the addition of NaOH was critical to the formation of Cu nanowires.[255] As shown in Figure 6 G, Cu nanoparticles with irregular shapes were obtained if the synthesis was conducted in the absence of NaOH. When NaOH was present, however, Cu nanowires with high quality were obtained (Figure 6 H). 4.8.2. Hydroxylamine Hydroxylamine, mostly under the form of its derivatives by staying with hydrochloride or sulfate, is commonly used as a reductant in the synthesis of Ag, Au, Pd, and Cu nanocrystals.[28, 256, 257] Among all the forms of hydroxylamine, hydroxylamine hydrochloride (NH2OH·HCl) is the most popular derivative, which presents a slightly acidic behavior in aqueous solutions. Under acidic conditions, hydroxylamine hydrochloride is considered to be a weak reductant due to its low oxidation potential, as described in Equation (34).[258, 259] In contrast, under alkaline media, its reducing capability can be significantly increased, as shown in Equation (35), making hydroxylamine a strong reductant.[258, 259] 2 NH3 OHþ ! N2 O þ H2 O þ 6 Hþ þ 4e@

E 2 oxi ¼ 0:05 V

ð34Þ

2 NH2 OH þ 4 OH@ ! N2 O þ 5 H2 O þ 4e@

E 2 oxi ¼ 1:05 V

ð35Þ

Therefore, by varying the pH of the reaction systems, one is able to adjust the reducing capability of hydroxylamine, which offers a variety of relatively simple and robust protocols for the synthesis of metal nanocrystals.[256, 260–262] Concerning the reduction mechanism of hydroxylamine, an interesting dependence on the molar ratio between hydroxylamine and metal ions has been reported.[263] When the molar ratio of hydroxylamine to metal ions was high (i.e., in the excess of hydroxylamine), a two-electron process with the formation of N2 was observed, as described in Equation (36). While in the excess of metal ions, a four-electron process would be favored and N2O was observed as the main product, as shown in Equation (37). 2 NH2 OH ! N2 þ 2 H2 O þ 2 Hþ þ 2 e@

ð36Þ

2 NH2 OH ! N2 O þ H2 O þ 4 Hþ þ 4 e@

ð37Þ

4.8.3. Amines

Figure 10. (A) Different reaction mechanisms involved in the reduction of hydrazine under i) acidic and ii) alkaline conditions, respectively. (B) Plot showing the oxidation potential of hydrazine vs. pH, in which a higher pH leads to an increase in oxidation potential. The image in (B) was reproduced with permission.[36] Copyright 1998 Royal Society of Chemistry. Chem. Eur. J. 2018, 24, 16944 – 16963

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Amines are a class of compounds extensively employed as reductants for the synthesis of metal nanocrystals due to their relative high oxidation potentials and the affinity of nitrogen atoms with metal species.[28, 32] In this context, a variety of amino compounds will be discussed, with typical examples including amino acids, proteins, polymeric amines such as poly(ethyleneoxide diamine), as well as aliphatic and aromatic amines.[264] Some amines, such as aniline and pyridine, have re-

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Review ceived great interest due to their strong reducing capability, making it possible to produce metal nanocrystals under relatively mild conditions (e.g., at room temperature).[28, 265–270] Interestingly, amines can play multiple roles in the synthesis of metal nanocrystals, by acting as a solvent, a stabilizer, and a capping agent, in addition to a reductant. The stabilization provided by amines is due to the coordination of nitrogen atoms with the surface of the nanocrystals.[267] To this end, Li and co-workers reported the oleylamine-mediated synthesis of Pd nanocrystals with various shapes ranging from triangular plates to tetrahedra, octahedra, decahedra, and icosahedra. These shapes could be selectively achieved by simply varying the volume of oleylamine involved.[268] Besides, the employment of amines in the synthesis of metal nanocrystals offers a promising strategy for surface modification, which opens a door to incorporate specific functional groups (e.g., targeting ligands) for a variety of applications. In terms of the reduction mechanism, the final structure of the oxidized product is determined by the chemical structure of the employed amine. In this context, a variety of studies have reported the reduction of metal ions to atoms through a one-electron process, in which one electron is transferred from the nitrogen electron lone pair to the metal ion, leading to the formation of metal atoms and cationic radical as the oxidized product.[28, 249] According to this proposition, a monomeric radical is expected to be the oxidized product. However, some studies have reported the polymerization of generated radical cations, resulting in the formation of oligomeric and polymeric species.[271] Besides, imines and nitriles have also been shown to be the oxidized products from amines by a sequential process, in which only the metal ion is responsible for the oxidation of the organic compounds.[254] As depicted in Figure 11, the first step involves the reduction of Ag + ions to Ag atoms and the oxidation of amine to a radical cation. Afterward, the as-obtained radical cation is sequentially deprotonated and oxidized to an imine, which is further deprotonated and oxidized to a nitrile. Interestingly, all the nitrogen compounds (amines, imines, and nitriles) can act as stabilizers for the as-

obtained metal nanocrystals through the coordination of the lone pair of electrons on the nitrogen atom (amines), or the psystem of the double (imines) or triple bond (nitriles) with the nanocrystal surface.[254] 4.8.4. N,N-Dimethylformamide N,N-Dimethylformamide (DMF) is one of the most powerful organic compounds worked as a reductant in the synthesis of metal nanocrystals, which also acts as a solvent during the synthesis. Due to its high oxidation potential (1.9 V vs. SHE), some studies have reported the capability of DMF to reduce metal ions at room temperature.[60, 273–275] However, in order to promote its role in the synthesis of noble metals with uniform size distribution and shapes, the reaction parameters are needed to be optimized, including the reaction kinetics, the addition of stabilizers as well as capping agents.[60, 61, 273–275] Mechanistically, the redox reaction involved in the use of DMF as a reductant is very similar to the case of aldehyde. This is due to the fact that both aldehyde and DMF have a terminal carbonyl group, which is responsible for metal ion reduction. As shown in Equation (38), the terminal carbonyl group is oxidized to a carboxylic group by metal ions while the metal ions are reduced to atoms in the presence of water, together with the generation of carbamic acid and H + ions.[273] nðCH3 Þ2 NCHO þ 2Mnþ þnH2 O ! 2M0 þ nðCH3 Þ2 NCOOH þ 2nHþ

ð38Þ

Equation (38) precisely describes a typical synthesis performed at room temperature. However, most studies employing DMF as a reductant were conducted at relatively high temperatures (reflux) for the synthesis of metal nanocrystals. In this case, the carbamic acid will further decomposed to dimethylamine and carbon dioxide, as shown in Equation (39):[255] ðCH3 Þ2 NCOOH ! ðCH3 Þ2 NH þ CO2

ð39Þ

In exploring the capability of amide-based organic compounds as reductants, some studies have reported the use of N,N-dimethylacetamide (DMAc) in the synthesis of highly dispersed Ag spheroidal nanocrystals in a surfactant-free system, in which DMAc served as a solvent, and a stabilizer, in addition to a reductant.[30, 276, 277] However, in contrast to DMF, the exact mechanism involved in the oxidation of DMAc is yet to be explored.

5. Concluding Remarks

Figure 11. Schematic of three consecutive steps involved in the reduction of Ag + ions in the presence of amine as a reductant: i) reduction of Ag + ions to Ag atoms by amine and the generation of radical cation; ii) reduction of Ag + ions to Ag atoms by radical cation and the generation of imine; iii) reduction of Ag + ions to Ag atoms by imine and the generation of nitrile. Chem. Eur. J. 2018, 24, 16944 – 16963

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In this review, we have discussed a series of reductants and addressed their versatile roles in the synthesis of colloidal metal nanocrystals. Based on a set of case studies, we have demonstrated that these compounds could not only serve as a source of electrons, but also significantly affect the reduction kinetics in a chemical synthesis. Since the nucleation and growth processes of metal nanocrystals are highly sensitive to the reduction kinetics of the salt precursors, it is essential to understand

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Review the correlation between the reductants and reaction kinetics in order to achieve experimental controls over the synthesis of nanocrystals. Moreover, depending on the chemical structure, a reductant could also play additional roles in a reaction, including working as a solvent, a stabilizer, or a capping agent, which deserves future exploration because of its promise in simplifying a reaction system. Based on the case studies of various typical reductants, we have also presented a variety of approaches for controlling the synthesis of metal nanocrystals with various sizes, shapes and structures by selecting proper reductants, together with optimizing the reaction conditions such as pH and reaction temperature. In addition, a collection of half-reactions and corresponding oxidation potentials of diverse reductants under standard conditions have also been summarized, from which we could well recognize the reducing capabilities of different reductants and the number of electrons able to be donated per molecule. We believe this collection will provide the researchers with a comprehensive understanding of the reductants from the fundamental perspective as well as the guidance in selecting suitable reductants for various chemical syntheses. Although many groups have discussed the role of reductants in the chemical synthesis of metal nanocrystals, unfortunately, most aspects remain poorly investigated or understood. One reason could be attributed to the complexity of the reaction systems which generally involves not only the metal precursor and reductant, but also the stabilizer, capping agent and other additives. For the synthesis of bimetallic or ternary nanocrystals, the complexity of a reaction system could be further increased. The explicit role(s) of each chemical is hard to be elucidated due to the synergistic effect between different chemicals. Besides, there are a variety of derivatives that are generated during a chemical synthesis and some of them may further evolve into secondary derivatives, making the reaction system far beyond complication. In the future, more advanced characterization techniques, especially the in situ characterizations, are needed to analyze each species separately throughout a chemical synthesis. In addition to the qualitative characterizations, a quantitative analysis of the reduction kinetics involved in a chemical reaction has recently emerged as a powerful tool to understand the role of different reaction parameters in determining the shapes or structures of final products. By correlating the kinetic parameters such as the rate constant and activation energy to the final products, one could realize quantitative controls over the synthesis of metal nanocrystals and even predict the outcome of a synthesis. Although we have discussed a few achievements made by the quantitative approach,[48, 50, 51, 54] in the future, more efforts are worth devoting to building a systematic correlation between the experimental parameters and the reaction kinetics. Furthermore, computational simulations based on molecular dynamics could also be implemented to validate the empirical trends established in the vast number of experimental studies or even help explain the ambiguous mechanism from a computational perspective, which could be a good supplement for comprehensively understanding the involved chemistry in various syntheses. Chem. Eur. J. 2018, 24, 16944 – 16963

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Acknowledgements This work was supported in part by NSF (DMR, 1215034 and 1505400) and startup funds from the Georgia Institute of Technology. T.S.R and A.G.M.S. thank CNPq for the Science without Borders fellowships.

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Manuscript received: May 2, 2018 Revised manuscript received: June 13, 2018 Accepted manuscript online: June 19, 2018 Version of record online: August 24, 2018

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