surface capping agents and their roles in shape‐controlled

Capping Agents

Surface Capping Agents and Their Roles in Shape-Controlled Synthesis of Colloidal Metal NanocrystalsTung-Han Yang+, Yifeng Shi+, Annemieke Janssen+, and Younan Xia*

AngewandteChemie

Keywords:capping agents · crystal growth ·metal nanocrystal · shape control ·structure–property relationship

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How to cite:International Edition: doi.org/10.1002/anie.201911135German Edition: doi.org/10.1002/ange.201911135

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1. Introduction

Colloidal metal nanocrystals have received ever-increas-ing interest because of their unique position in bridging bulksolids with atomic/molecular species.[1–6] In addition, nano-crystals made of Au, Ag, Cu, Pd, Pt, Rh, Ru, and Ir havedistinctive properties for applications in photonics,[7] elec-tronics,[8] catalysis,[9,10] energy conversion,[11,12] sensing,[13–16]

and biomedicine.[17,18] In general, the properties of a metalnanocrystal are determined by a set of physical parametersthat include composition, size, shape, and crystal structure.One can maneuver any one of these parameters to tailor theirproperties and thus optimize their performance in a specificapplication. Among these parameters, shape has been mostextensively explored due to its strong correlations with bothphotonic and catalytic properties.

A notable example is localized surface plasmon resonance(LSPR), which arises from the collective oscillation ofconduction electrons in response to the electromagneticfield of incident light.[14,19] In this case, the shape determineshow the conduction electrons in a nanocrystal are polarizedand how the charges are distributed on the surface, control-ling the number, wavelengths, and intensities of the LSPRpeaks, as well as the ratio between absorption and scattering.For Au spheres with a diameter of 50 nm, they have a strongLSPR absorption peak around 525 nm, resulting in a ruby redcolor for their aqueous suspension.[20] When switched totriangular plates with an edge length of 68 nm, two LSPRpeaks (a strong one at 700 nm and a shoulder around 550 nm)are observed, corresponding to the in-plane and out-of-planedipole resonances, respectively.[21] Their aqueous suspensiongives a blue color. The ability to tune the LSPR peak positionhas immediate implications for biomedical applications suchas optical contrast enhancement and photothermal therapybecause soft tissues have a transparent window in the near-infrared region (650–900 nm) to allow deep penetration.[22]

In addition to the optical properties, one can tailor(electro)catalytic activity and/or selectivity of metal nano-crystals by maneuvering their shape to control the arrange-ment of atoms on the surface.[23,24] Taking the oxygenreduction reaction (ORR) as an example, it was reportedthat the area-specific activity (i.e., current density normalizedto the electrochemically active surface area) of Pt concavecubes (1.77 mAcm�2) enclosed by {720} high-index facets was6.3 and 1.3 times greater than those of Pt cubes and octahedracovered by {100} and {111} facets, respectively.[25] Similarcorrelations between catalytic properties and surface struc-tures have also been observed for many other reactions suchas carbon dioxide reduction,[26] formic acid oxidation,[27]

hydrogenation,[28] epoxidation,[29] and isomerization.[30]

These and other examples demonstrate the importance ofshape control in tailoring the properties of metal nanocrystalsand optimizing their performance in catalytic applications.

With the development of nanochemistry, metal nano-crystals with a wide variety of shapes have been synthesized

Surface capping agents have been extensively used to control theevolution of seeds into nanocrystals with diverse but well-controlledshapes. Here we offer a comprehensive review of these agents, witha focus on the mechanistic understanding of their roles in guiding theshape evolution of metal nanocrystals. We begin with a brief intro-duction to the early history of capping agents in electroplating andbulk crystal growth, followed by discussion of how they affect thethermodynamics and kinetics involved in a synthesis of metal nano-crystals. We then present representative examples to highlight thevarious capping agents, including their binding selectivity, molecular-level interaction with a metal surface, and impacts on the growth ofmetal nanocrystals. We also showcase progress in leveraging cappingagents to generate nanocrystals with complex structures and/orenhance their catalytic properties. Finally, we discuss various strategiesfor the exchange or removal of capping agents, together withperspectives on future directions.

From the Contents

1. Introduction 3

2. From Macroscopic Systems toNanocrystals 4

3. Thermodynamics versusKinetics 7

4. Case Studies 9

5. Computational Investigation 14

6. Characterization Tools 16

7. Nanocrystals Free of CappingAgent 17

8. Concluding Remarks 21

[*] Dr. T.-H. Yang,[+] Prof. Dr. Y. XiaThe Wallace H. Coulter Department of Biomedical EngineeringGeorgia Institute of Technology and Emory UniversityAtlanta, GA 30332 (USA)E-mail: [email protected]

Y. Shi,[+] Prof. Dr. Y. XiaSchool of Chemical and Biomolecular EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332 (USA)

A. Janssen,[+] Prof. Dr. Y. XiaSchool of Chemistry and BiochemistryGeorgia Institute of TechnologyAtlanta, GA 30332 (USA)

[+] These authors contributed equally to this work.

Supporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.201911135.


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using either one-pot or seed-mediated method.[1,5,24] In a one-pot synthesis, the formation of nanocrystals can be separatedinto two stages: i) homogeneous nucleation–assembly ofatoms from precursor reduction to generate nuclei and thenseeds and ii) growth–addition of atoms onto the surfaces ofseeds for their evolution into nanocrystals with distinctshapes. As for seed-mediated growth, pre-synthesized nano-crystals with defined shapes and twin structures are addedinto a growth solution for the deposition of atoms from thesame or a different metal. In either case, the shape of thenanocrystals can be changed by manipulating experimentalparameters such as temperature, solvent, metal precursor,reducing agent, colloidal stabilizer, and most importantly,surface capping agent. In most cases, it is the capping agentthat plays the decisive role in dictating the shape evolution.

Here we aim to offer a comprehensive review of surfacecapping agents, with a focus on our current understanding oftheir working mechanisms according to experimental andtheoretical studies. We present a number of examples tohighlight the power of capping agents in nanocrystal synthe-ses, with a focus on both qualitative understanding andquantitative analysis. We also discuss recent progress inutilizing the blocking effect of capping agents to constructnanocrystals with anisotropic and other complex structures;removing the capping agent after a synthesis or developingmethods for capping agent-free syntheses; and promoting thecatalytic performance of nanocrystals through electroniccoupling with a capping agent.

2. From Macroscopic Systems to Nanocrystals

The concept of using a capping agent to modify a metalsurface during its formation can be traced back to electro-plating, in which a layer of metal is deposited on the surface ofa substrate (serving as a cathode) immersed in a plating bathcontaining a metal precursor.[31–33] The precursor is reducedby applying a current to generate metal atoms for theirdeposition onto the surface of the substrate. As shown inFigure 1A, the color of a Cr-Ni-Fe alloy spoon changesdramatically when its surface is electroplated with differentmetals, including Ag, Cu, Au, Pt, Pd, Rh, and Ni.[31] Theelectroplated coating can have a major impact on themorphology, stability, and function of the underlying sub-strate. The morphology of an electroplated object criticallydepends on the metal deposition rates across the surface. Ifthe rates are the same across the surface, a uniform layer willbe deposited, preserving the original morphology and rough-ness of the surface (Figure 1B,C). However, on a surface witha recessed region, the electric field and current density will beconcentrated at the corners and edges, resulting in higherrates of deposition and thus thicker layers at these sites. Asa result, the surface roughness will increase during electro-plating (Figure 1D). To address this issue, a “levelling agent”(e.g., thiourea or coumarin) is introduced to ensure more orless even deposition across the surface.[32,33] By selectivelypassivating the corners and edges of the recessed region, theadditive can slow down the electroplating rates at these sites(Figure 1E), enabling the formation of an evenly depositedlayer.

Additives known as “habit modifiers” were widely used tocontrol the growth behaviors and thus shapes of bulk crystals

Tung-Han Yang received his B.S. in chemicalengineering from National Cheng Kung Uni-versity, Taiwan, in 2009. He then studied atNational Tsing Hua University, Taiwan, andreceived his M.S. and Ph.D. in materialsscience and engineering in 2011 and 2017,respectively. In 2015–2017, he was a visitingstudent in the Xia group. Currently, he isa postdoctoral fellow at Georgia Tech underthe supervision of Prof. Dong Qin. Hisresearch interests include quantitative analy-sis of the shape-controlled synthesis of noble-metal nanocrystals.

Yifeng Shi received her B.S. degree in chem-ical engineering from Sichuan University,China, in 2017. She joined the Xia group in2017 as a graduate student. Her currentresearch focuses on the surface science ofnoble-metal nanocrystals and their applica-tions in heterogeneous catalysis.

Annemieke Janssen received her B.S. inchemistry from University College Utrecht in2016, and her M.S. in materials sciencefrom Imperial College, London in 2017. Shejoined the Xia group in 2018 as a graduatestudent. Her current research focuses on thedevelopment of new methods for the syn-thesis of noble-metal nanocrystals.

Younan Xia studied at the University ofScience and Technology of China (B.S.,1987) and University of Pennsylvania (M.S.,1993), and received his Ph.D. from HarvardUniversity in 1996 (with George M. White-sides). He started as an assistant professor ofchemistry at the University of Washington(Seattle) in 1997 and joined the departmentof biomedical engineering at WashingtonUniversity in St. Louis in 2007 as the JamesM. McKelvey Professor. Since 2012, he holdsthe position of Brock Family Chair and GRAEminent Scholar in Nanomedicine at Geor-gia Tech.


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long before nanocrystal synthesis became a subject ofresearch. During the growth, the habit and thus shape ofa crystal could be altered through the introduction of an ionicor molecular additive. Taking CuSO4·5H2O as an example,crystals grown from an aqueous solution often display a mixof different facets.[34] In the presence of Fe3+, however, thesurface is dominated by {100} and {110} facets only. On theother hand, the introduction of a detergent such as sodiumlauryl sulfate promotes the formation of {11̄0} facets. Again,these additives are believed to be able to adsorb ontodifferent facets through selective binding, altering the growthrates along different directions.

The capping agent is equivalent to the levelling agentemployed in an electroplating process or the habit modifierinvolved in the growth of a bulk crystal: they are all supposedto operate by the same principle despite the difference inlength scale. Owing to the selectivity of a capping agenttowards a specific type of facet on a growing seed, it cansignificantly alter the landscape of surface-free energy, growthhabit, and even growth mechanism of the seed, leading tonanocrystals with shapes deviated from the Wulff poly-hedron.[35,36] In early studies, Pt nanoparticles supported onamorphous SiO2 or Al2O3 were found to change their shapeupon annealing under different environments such as N2, H2,or H2S.

[37,38] Although the exact mechanism of shape trans-formation was not clear at that time, these studies demon-strated the feasibility to control the shape of metal nano-

crystals using gas molecules. In 1996, it was reported thata polymeric capping agent such as sodium polyacrylate couldbe used to control the evolution of Pt nanocrystals intotetrahedral, cubic, cuboctahedral, icosahedral, and irregular-prismatic shapes in an aqueous synthesis.[39] In 2002, our groupdemonstrated that Ag nanocubes could be obtained in thepresence of poly(vinyl pyrrolidone) (PVP) as a capping agenttowards {100} facets and thus the elimination of {111} facets ina polyol synthesis.[40] In the same year, we further demon-strated the growth of multiply-twinned, Ag decahedral seedsinto one-dimensional (1D) nanostructures such as pentagonalrods and wires.[41–44] Again, the success of this synthesis wasattributed to the selective binding of PVP towards {100}facets, facilitating growth along the axial direction of a dec-ahedral seed.

The phenomenon of adsorption on a solid surface can bedescribed using a simple model known as the Langmuirisotherm (Figure 2).[45] Despite the involvement of multipleassumptions, the Langmuir isotherm can be easily used to fitthe adsorption data. At equilibrium, the fraction (q) of thesurface sites occupied by the adsorbate molecules can beexpressed as [Eq. (1)]:

q ¼ KCbulk

KCbulk þ 1ð1Þ

whereK stands for the equilibrium constant of adsorption andCbulk is the concentration of the adsorbate in the bulk solution.This equation indicates that the driving force for surfaceadsorption can come from the high concentration of theadsorbate in the bulk solution and/or a strong affinity of theadsorbate toward the surface. The surface adsorption ofa capping agent should also follow this simple model. Whatdifferentiates it from other types of adsorbates is the involve-ment of a stronger binding such as covalent bonding in mostcases and the difference in binding affinity toward differenttypes of facets. When a molecular species is capable ofbinding (chemically or physically) to one particular type offacet more strongly than the other types, it should be calleda capping agent.

Figure 1. Electroplating offers a simple and versatile method for coat-ing a metal substrate with a thin layer made of a different metal.A) Metal spoons electroplated with Ag, Cu, Au, Pt, Pd, Rh and Ni (leftto right). B) A substrate with a V-shaped recess on the surface. C–E)Outcomes of electroplating for this substrate: C) retention of rough-ness when the deposition rate is uniform across the surface; D) inreality, the higher current density at ridges leads to a thicker depositionat these sites than in the recessed region; and E) formation ofa uniform coating by adding a “leveling agent” to selectively reducethe deposition rate at the ridges. Figure (A) was Reprinted fromRef. [31] with permission. Copyright 2012 Elsevier.

Figure 2. Langmuir isotherms for adsorbates differing in adsorptionequilibrium constant K.





By leveraging the concept of surface capping, a variety ofshapes have been achieved for nanocrystals made of differentmetals and their alloys (Table 1).[40–44,46–103] These resultsestablish that surface capping offers an effective and versatilemeans for maneuvering the shape of nanocrystals by control-ling the relative surface areas of different types of facets. Inthis review, we aim to illustrate the power of surface capping

in controlling the shape and thus surface structure of metalnanocrystals. Most importantly, through case studies ofexperimental and theoretical origins, we seek to highlightthe correlations between the shape taken by a nanocrystal andthe binding selectivity of a capping agent while shedding lighton the growth mechanism.

Table 1: Summary of shapes that have been successfully obtained for noble-metal nanocrystals using facet-selective capping agents in the setting ofone-pot synthesis.

Composition Metal Capping agent Capped facet Shape of the nanocrystal References

monometallic system

Ag

Cl� {100} cube [46,47]Br� {100} cube, bar, penta-twinned rod/wire [48–52]PVP {100} cube, penta-twinned rod/wire [40–44,53–55]citrate {111} plate [56–58]

Au

citrate {111} plate [59,60]I� {111} plate [61,62]bovine serum albumin {111} plate [63,64]PVP {111} tetrahedron, octahedron, plate [65,55]CTA+ {221} trisoctahedron [67]

Pd

Br� {100} cube, bar, penta-twinned rod/wire [68–72]I� {100} penta-twinned rod/wire, right bipyramid [73–75]citric acid {111} tetrahedron, octahedron, decahedron, icosahedron, plate [76,77]formaldehyde {111} tetrahedron, octahedron, decahedron, icosahedron, plate [78]CO {111} tetrahedron, tetrapod, plate [79,80]

Pt

Br� {100} cube [81–83]CO {100} cube [84–86]PVP {100} cube [87]3-hydroxybutyric acid {100} cube [88]peptide T7 {100} cube [88–90]citrate {111} octahedron, tetrahedron [91,92]peptide S7 {111} tetrahedron [88,90]sodium polyacrylate {111} tetrahedron [39]methylamine {411} octapod [93]

Rh

Br� {100} cube [83]citric acid {111} tetrahedron [94]oleylamine {111} plate [95]PVP {111} plate [96]

alloy[a]

Pd–Pt

Br� {100} cube [97]Br� and trace I� {100} cube [98]I� {100} cube (Pt51Pd49) [9]oleic acid {100} cube (Pt3Pd) [100]Cl� {111} octahedron (Pt52Pd48) [99]C2O4

2� and HCHO {111} tetrahedron [98]

Pt–Cu CO {100} cube (Pt4Cu)[101]

Pt–CoCO {111} octahedron (Pt3Co)diphenyl ether {111} octahedron (Pt3Co)

[100]oleic acid {100} cube (Pt3Co)

Pt–Fe oleic acid {100} cube (Pt3Fe)

Pt–Rh CO {111} octahedron (PtRh) [101]

Pt–NiCO {111}

octahedron (Pt3Ni) [86,101,102]octahedron (Pt1.5Ni) [103]

diphenyl ether {111} octahedron (Pt3Ni, PtNi)[100]

oleic acid {100} cube (Pt3Ni, PtNi, PtNi3)

[a] Depending on the composition of the alloy, the same capping agent may have selectivity towards different facets.




3. Thermodynamics versus Kinetics

At a fixed number of atoms, the nanocrystal can takemany different forms, including those with distinctive shapesand internal structures. Which form will prevail in the finalproduct? The answer to this question depends on boththermodynamics and kinetics.[104,105] The essence of these twoconcepts can be explained using the energy diagram shown inFigure 3. As a form with the least free energy, the thermody-namic product is supposed to reside at the global minimum.Any form deviating from the global minimum is a kineticallycontrolled product, a metastable form trapped at a localminimum. All kinetic products are supposed to spontaneouslytransform into the thermodynamic product, but they can bestable for different periods of time depending on the thermalenergy available for crossing the activation energy barrier.This can be illustrated using an example involving diamondand graphite, two allotropes of carbon. Although diamondhas a higher total free energy relative to graphite, diamondcan be considered “forever” once it has been created becausethe kinetics is infinitely slow.

As for the shape evolution of nanocrystals, the cappingagent can modify both thermodynamic and kinetic factorsthrough selective adsorption.[106] By lowering the surfaceenergy of a certain type of facet, the capping agent can changethe shape of nanocrystals favored by thermodynamics. Alter-natively, by posing a physical barrier to atom deposition anddiffusion, the capping agent can affect the outcome of shapeevolution through kinetic means.

3.1. Thermodynamic Control

According to thermodynamics, the mostfavorable form of a nanocrystal can beattained by minimizing its total free energy.For a nanocrystal, the energy of formationcan be divided into two parts: bulk andsurface. Since our discussion on the moststable form of a nanocrystal involves the samenumber of atoms, we can ignore the bulk partand just focus on the surface part, which isdetermined by the energy required to createa new surface.[107] The surface area of a nano-crystal is not the only determinant of totalsurface energy because it also depends on thecrystallographic planes involved. Figure 4Ashows the three low-index planes of a face-centered cubic (fcc) crystal.[104] For a surface,the more dangling bonds it contains, the lessstable it will be and thus the higher thespecific surface energy. In this case, each atomon the (111) plane has three dangling bonds.When it comes to the (100) and (110) planes,the number of dangling bonds increases tofour and six, respectively. As a result, thespecific surface energy increases in the orderg(111) < g(100) < g(110). In general, the moreclosely packed a given crystallographic plane

is, the lower the specific surface energy will be and viceversa.[107]

The thermodynamically favored shape with the lowestsurface energy is known as a Wulff polyhedron.[35] Based ona few parameters, including the relative surface energies ofdifferent facets and the crystal structure, one can construct theWulff shape. However, the presence of capping agent will

Figure 3. Schematic illustration showing the thermodynamic andkinetic products in terms of total free energy, and the activation energybarrier to the transformation from a kinetic product to a thermody-namic product (indicated by the arrow).

Figure 4. A) Models of the (111), (100), and (110) planes of a fcc metal and thecorresponding numbers of dangling bonds per surface unit cell (NB). B) Schematicillustration of the role of a capping agent in directing the growth of a single-crystal seedinto nanocrystals with different shapes. Reprinted from Ref. [104] with permission.Copyright 2015 American Chemical Society.





anisotropically change the surface energies of differentcrystallographic planes. The capping agent selectively ad-sorbed onto a facet can stabilize the surface atoms and thusdecrease the surface energy. To this end, Wulff constructionneeds to be modified.[36] A simple illustration of this concept isshown in Figure 4B. During the growth of an fcc nanocrystal,the presence of a capping agent for {100} will passivate thesefacets to lower their surface energy.[104] Therefore, thethermodynamically favorable shape will change from a cuboc-tahedron to a cube enclosed by six {100} facets. The sameargument is also valid when the synthesis is conducted in thepresence of a capping agent for {111}. In this case, octahedraencased by eight {111} facets will be obtained to minimize thetotal surface energy.

A typical example can be found in the synthesis of Agnanocubes as mediated by Cl� ions.[47] Typically, Cl� ions areused with PVP to direct the evolution of Ag nanocrystals intoa cubic shape. A recent report analyzed the changes in surfaceenergy for both Ag(111) and Ag(100) as the coverage of Cl�

was increased. Experimentally, it was noted that Ag nano-crystals evolved from truncated octahedra to cuboctahedra,and finally to cubes with sharp corners and edges, as theconcentration of Cl� in the reaction solution was increased.The ab initio calculation showed that the strong binding of Cl�

to Ag(100) reversed the order of surface energies for (100)and (111). Altogether, it was concluded that the cubic shapewas a product of thermodynamic control enabled by thecapping agent.

3.2. Kinetic Control

The capping agent can also affect the shape of nano-crystals from the kinetic perspective. Instead of lowering thesurface energy, the adsorption of capping agent on certaincrystal facets can pose a physical barrier to the surface.[5,54,108]

As such, the capping layer will slow down the deposition rateof atoms. Meanwhile, the presence of this passivating layerwill retard the surface diffusion of adatoms. By carefullycontrolling reaction conditions, a variety of shapes deviatingfrom the Wulff polyhedron can be obtained. Figure 5 showsthe effect of surface diffusion on the shape evolution of a two-dimensional (2D) nanocrystal. The square seed is originallycovered by a capping agent that prevents the side faces fromreceiving atoms. As a result, the atoms newly formed in thesolution are preferentially deposited at the corners. The finalmorphology will be determined by the relative rates respon-sible for deposition and diffusion. Fast diffusion of adatomson the surface will give a thermodynamic product further tothe right, whereas slow diffusion of adatoms will give kinetic-controlled products such as the multipod on the left side.

In essence, the competition between thermodynamics andkinetics is settled by the kinetics of surface diffusion.[108,109]

With enough energy to overcome the diffusion barrier,adatoms will spontaneously migrate across the surface toachieve the lowest total surface energy for the system andthus the formation of a thermodynamically controlled shape.If the diffusion barrier is too high to overcome, the productwill stay at the local minimum with a higher total surface

energy and thus the formation of a kinetic product. Althoughthe thermodynamic and kinetic controls seem to be distin-guishable, it is often hard to completely decouple one effectfrom the other in a real synthesis. When experimentalconditions are varied, the growth can switch from thermody-namic to kinetic control and vice versa. For most syntheses,both mechanisms tend to be involved in directing theevolution of shape.

3.3. The Dynamic Nature of Surface Capping

In order to grow into a specific size and shape, atoms haveto be deposited on the surface of a nanocrystal and allowed todiffuse around. This means the capping must be a dynamicprocess in order to allow new atoms to be continuouslydeposited. This dynamic process can be enabled through threedifferent scenarios. Firstly, the capping agent constantlydesorbs and adsorbs during the growth of the nanocrystalby breaking and forming the bonds between the cappingagent and metal surface. This allows new atoms to nucleate onthe instantaneously exposed regions of a surface, maintainingthe growth process. Secondly, the new atoms are deposited onthe region free of capping agent due to the facet selectivity ofa capping agent. The deposited atoms can then “slide in”between the capping layer and metal surface through surfacediffusion. To this end, the interaction between the cappingagent and metal needs to be kept relatively weak to facilitatethe sliding process. Thirdly, the capping layer only interactswith the metal surface weakly through van der Waals (vdW)forces to maintain some free space in between.[110] In this case,however, the capping agent may lack facet selectivity due tothe lack of relatively strong chemical bonds. There is a need todevelop methods capable of resolving this dynamic process bymonitoring the variation in the surface coverage of a cappingagent with the right temporal resolution. Techniques such assecond-harmonic generation (SHG) might be suitable for thispurpose.[111]

Figure 5. Schematic illustration of a 2D square seed whose side facesare covered by a capping agent and the products obtained underdifferent conditions. Reprinted from Ref. [5] with permission, copyright2017 Wiley-VCH.




4. Case Studies

Table 1 shows a list of capping agents commonly used forcontrolling the shapes of noble-metal nanocrystals.[40–44,46–102]

It is generally accepted that the capping agent works moreeffectively in the growth stage than in nucleation. However,some capping agents may coordinate with the precursor ionsto form new complexes during a synthesis.[112] Such ligandexchange can have a profound impact on the reductionkinetics and thus the outcome of a nucleation process. Herewe cover capping agents capable of generating nanocrystalswith controlled shapes through facet-selective binding viaboth one-pot and seed-mediated routes.

4.1. Poly(vinyl pyrrolidone) (PVP)

As the most commonly used stabilizer for the synthesis ofcolloidal particles, PVP is also an effective capping agent forthe synthesis of metal nanocrystals enclosed by {100} facets(Figure 6). It was first reported in 2002 for the synthesis of Ag

nanocubes through a polyol method based on ethylene glycol(EG).[40] It was proposed that PVP could preferentiallyadsorb onto Ag(100) surface to reduce its growth rate,facilitating the formation of nanocubes enclosed by {100}facets. In the original report, the edge lengths of the Agnanocubes could be tuned in the range of 80–175 nm byadjusting the reaction time, temperature, and AgNO3 con-centration. To produce smaller Ag nanocubes with sizesranging from 30–70 nm, the PVP-assisted polyol method wasmodified by introducing trace amounts of NaSH (for theformation of single-crystal seeds) and HCl (for the selectiveoxidative etching and dissolution of twinned seeds) and byswitching the precursor from AgNO3 to CF3COOAg.[113]

Furthermore, by replacing the most commonly used EGwith diethylene glycol (DEG), the size of the Ag nanocubeswas further reduced to 18–32 nm.[53] When the size dropsbelow 15 nm, however, the long-chain polymer may not beable to effectively stabilize the small {100} facets anymore,[49]

and halide ions have to be introduced (Section 4.2).In addition to nanocubes, it was shown that 1D nano-

structures such as Ag penta-twinned nanowires could beprepared using a modified polyol method(Figure 6B).[41–44] In this case, one neededto reduce the concentration of AgNO3 toslow down the reduction kinetics whilekeeping the molar ratio of PVP to AgNO3

the same as that used for nanocube syn-thesis. Specifically, it was feasible to attainthe right kinetics for the formation ofdecahedral seeds. In the growth process,the Ag atoms were preferentially depos-ited along the twin boundaries and thedecahedral seed was uniaxially elongatedinto a penta-twinned nanorod and thennanowire covered by {100} facets on theside faces. PVP could selectively bind tothe {100} facets, facilitating the longitudi-nal growth of nanorods into nanowireswith aspect ratios as high as ca. 1000. Inrecent studies, similar growth mechanismswere also observed in the syntheses of Cuand Pd penta-twinned rods/wires fromdecahedral seeds, albeit halide ions werealso typically involved (Sec-tion 4.2).[73,114,115] In contrast, PVP selec-tively passivated the Au(111) surface andultimately induced the formation of Aunanocrystals enclosed by {111} facets, suchas octahedra and icosahedra.[65] This argu-ment is also consistent with the result ofa recent density functional theory (DFT)study, in which the PVP-covered Au(111)surface was found to be thermodynami-cally more stable relative to Au(100).[116]

Figure 6. Use of PVP as a capping agent for the selective stabilization of {100} facets on Agnanocrystals. Schematic illustrations and TEM images of Ag nanocubes with different sizes(A, C–E) and penta-twinned nanowires (B). The images in (A–E) were reprinted fromRef. [40,44,113,53,49], respectively, with permission. Copyright 2002 AAAS, 2003 AmericanChemical Society, 2010 Wiley-VCH, 2013 American Chemical Society and 2016 AmericanChemical Society.





4.2. Halides

In general, PVP is too bulky to effectively cap the surfaceof metal nanocrystals with sizes below 15 nm. As analternative, halide ions (e.g., Cl� , Br� , and I�) can forma close-packed adlayer on the face of small nanocrystals viastrong covalent or coordination binding.[117] Most importantly,they are able to preferentially chemisorb onto the {100} facetsof nanocrystals made of Ag, Pd, Pt, Rh, and Pd-Pt alloys. Asshown in Figure 7A, Pd nanocubes encased by {100} facetscan be made by employing Br� as a capping agent.[70] Ina typical synthesis, aqueous Na2PdCl4 (precursor) wasinjected into an aqueous mixture containing l-ascorbic acid(reducing agent), PVP (in this case, colloidal stabilizer), andKBr (capping agent). Similarly, Ag, Pt, and Rh nanocubeswere also obtained in the presence of halide ions (Figure 7B–D).[49,81,82] In addition, it was shown that the presence of Br�

ions at an adequate concentration could promote theformation Pd-Pt alloy nanocrystals with a cubic shape and

an edge length of ca. 6 nm.[97] This is because the Br� ionscould alter the ratio between the initial reduction rates of thePdCl4

2� and PtCl42� precursors through a ligand exchange

process while capping the {100} facets of the alloy nano-crystals.

If the concentration of halide ions is below a critical value,the resultant nanocubes are likely covered by a submonolayerof halide ions.[48,68,72] Under this condition, oxidative etching(i.e., removal of metal atoms from the surface) may selec-tively occur on one of the side faces, making this face mostactive for the subsequent deposition of atoms. As a result,anisotropic growth is initiated to elongate a nanocube intoa nanobar whose surface is still bound by {100} facets. Thisphenomenon is often observed in the Pd and Ag systems(Figure 7E,F).[48] When switched to I� ions, Pd penta-twinnednanowires were obtained with an average diameter of 7.8 nmand aspect ratios up to 100 (Figure 7G).[73] In this case, I� ionsnot only tuned the reduction rate of the PdII precursor into theproper regime favorable for the formation of decahedral

Figure 7. Halide ions as capping agents for the selective stabilization of {100} facets on metal nanocrystals. A–D) TEM images of A) Pd, B) Ag,C) Pt, and D) Rh nanocubes that were obtained in the presence of Br� ions. E,F) TEM images of E) Pd and F) Ag nanobars that were obtained inthe presence of Br� ions. G,H) TEM images of G) Pd and H) Ag penta-twinned nanowires obtained by introducing I� and Br� ions, respectively.I) Time-lapsed in situ TEM images showing the evolution of a Au decahedral seed to truncated decahedron and finally to penta-twinned nanorodin the presence of Br� ions. The images in (A–I) were reprinted from Refs. [70,49,82,83,72,48,73,50,119], respectively, with permission.Copyright 2009 Springer, 2016 American Chemical Society, 2010 American Scientific Publishers, 2008 American Chemical Society, 2009 Wiley-VCH,2007 American Chemical Society, 2017 American Chemical Society, 2016 American Chemical Society, and 2019 American Chemical Society.




seeds,[118] but also acted as a capping agent for {100} facets.Using a similar strategy, Br� ions were used with PVP ascapping agents for the Ag(100) surface to generate Ag penta-twinned nanowires with diameters below 20 nm, togetherwith aspect ratios over 1000.[50] The nanowires could beformed rapidly (within 35 min) and in high (> 85%) mor-phology purity (Figure 7H). In situ liquid-cell transmissionelectron microscopy (TEM) was also used to analyze theevolution of Au decahedral seeds into penta-twinned nano-rods in the presence of Br� ions, as shown in Figure 7I.[119] Thesequential TEM images revealed that the formation of the Aupenta-twinned nanorod started from the growth of a regulardecahedron and proceeded to reentrant groove-inducedanisotropic growth of a truncated decahedron and, eventually,to penta-twinned nanorods with increasing aspect ratios.

4.3. Citric Acid and Citrate

Citric acid and citrate are commonly used as reducingagents for the syntheses of metal colloids. They can also serveas capping agents for some metals. For example, citric acidpossesses a hydroxyl and three carboxylic acid groups thatmatch well with the hexagonal symmetry of the (111) surfaceof fcc metals.[76,77,94] As a result, citric acid can selectivelypassivate {111} facets on Pd or Rh nanocrystals throughchemisorption. For the Pd system, the ratio of PdCl4

2� (1.2–17.4 mm) to citric acid (10–84 mm) could be manipulated forthe production of Pd polyhedral nanocrystals (Figure 8A–D),including octahedra, icosahedra, decahedra, and plates, that

are all dominated by {111} facets.[76,77] In this protocol, citricacid acts as a mild reducing agent and a capping agent for thePd(111) surface. Taking the synthesis of Pd octahedra as anexample, the product contained cuboctahedral particlesenclosed by a mix of {100} and {111} facets at t= 1 h. Theproduct became octahedra bound by {111} facets at t= 20 h.Such shape evolution suggests that citric acid favors theformation of octahedra due to its strong binding towards {111}instead of {100} facets during the growth stage. Using a similarstrategy, Rh tetrahedra enclosed by four {111} facets were alsosynthesized.[94]

Like citric acid, citrate was found to preferentially bind toAg(111).[56–58] A possible explanation can be attributed to thematch in symmetry and dimensions between this compoundand Ag(111) surface. In one study, it was shown that citratewas critical to the formation of Ag nanoplates due to itsability to passivate the two {111} basal planes and thus limitthe increase in plate thickness.[56] By adjusting the ratio of AgI

precursor to citrate, Ag nanoplates with controllable lateraldimensions and thickness were obtained.[57,58] A similarprotocol was also reported for the synthesis of Au nano-plates.[59,60]

4.4. Carbon Monoxide (CO)

Recently, CO from a variety of sources, including thedirect use of CO gas or decomposition of W(CO)6 orFe(CO)5, has been adopted to control the shape of Pd, Pt,or Pt–M (M=Pd, Co, Cu, Ni, and Rh) alloy nanocrystals

owing to its facet-selective of CO bind-ing.[79,80,84–86,101–103] The CO adsorbed onmetal nanocrystals could be effectivelyremoved by exposing the sample to air,offering a major advantage for catalyticapplications. An early study demonstratedthe synthesis of ultrathin, hexagonal Pdnanoplates using CO as a capping agentfor Pd(111) surface (Figure 9A).[79] In thesynthesis, Pd(acac)2, cetyl trimethyl am-monium bromide (CTAB), and PVP weredissolved in dimethylformamide (DMF) toinitiate homogeneous nucleation, and thesolution was then transferred into a glasspressure vessel under 1 bar of CO for theformation of nanoplates. Due to the strongadsorption of CO molecules on the basal(111) planes, the resulting Pd nanoplateswere only 1.8 nm in thickness and 96% ofthe exposed surface was terminated in{111} facets. By introducing H2 to vary theadsorption strength and coverage of COon Pd(111) planes, single-crystal Pd tetra-pods and tetrahedra enclosed by {111}facets were obtained (Figure 9B,C).[80]

In contrast to Pd, CO molecules forma much stronger chemical bond with Pt-(100) rather than Pt(111), and this selec-tivity led to Pt nanocrystals with a cubic

Figure 8. Citric acid (CA) as a capping agent for the selective stabilization of {111} facets onPd nanocrystals. Schematic illustrations and TEM images of A) octahedra, B) icosahedra,C) decahedra, and D) plates obtained in the presence of citric acid. The images in (A–C) werereprinted from Ref. [76] with permission. Copyright 2007 Wiley-VCH. The image in D wasreprinted from Ref. [77] with permission. Copyright 2018 Royal Society of Chemistry.





shape (Figure 9D).[84–86] A number of research groups alsofound that the introduction of a secondmetal such Ni, Co, andRh could result in the formation of octahedral Pt–Ni (Fig-ure 9E,F), Pt–Co, and Pt–Rh alloy nanocrystals covered by{111} facets.[86,101–103] When the second metal was switched toCu, however, the Pt–Cu alloy nanocrystals took a cubic shapesimilar to that of Pt.[101] The forma-tion of cubic or octahedral shapecan be attributed to the facet selec-tivity of CO. The binding betweenCO and Cu is much weaker thanthat between CO and Pt and thusthe inclusion of Cu had essentiallyno impact on the growth behavior ofthe nanocrystals. In contrast, CObinds strongly to Co, Ni, and Rhatoms on the {111} facets and theinclusion of such a metal will com-pete with that of Pt, leading to theformation of octahedral nanocrys-tals when its content is high enoughto dominate the adsorption of CO.This speculation was supported bythe experimentally observed shapetransformation from cubic Pt tocuboctahedral Pt6Ni and finally tooctahedral Pt3Ni when the Ni con-tent was gradually increased.[101]

Computational modeling will helpus understand the binding selectiv-ity of CO towards an alloy surface,and the results are expected toenable rationally designed synthesisof bimetallic nanocrystal with con-trolled shapes.

4.5. Peptides

Peptides with various sequences and conformations havealso been explored as capping agents to control the shapes ofmetal nanocrystals.[88–90] Figure 10A summarizes a studybased on Pt.[89] The authors examined two peptides: Acyl-Thr-Leu-Thr-Thr-Leu-Thr-Asn-CONH2 (sequence:TLTTLTA; termed peptide T7) for {100} facets and Acyl-Ser-Ser-Phe-Pro-Gln-Pro-Asn-CONH2 (sequence:SSFPQPN; termed peptide S7) for {111} facets. Thesepeptides could guide the formation of Pt cubic and tetrahedralnanocrystals, respectively, with their sizes controlled below10 nm. In the presence of peptide T7, the Pt nanocrystalsevolved from cuboctahedra covered by a mix of {100} and{111} facets to cubes encased by {100} facets, suggesting theselective stabilization of {100} facets by peptide T7. Whenswitching to peptide S7, the nanocrystals showed a cubocta-hedral shape in the early stage and then evolved intotetrahedral shape with {111} facets on the surface, indicatingthe selectivity of peptide S7 towards Pt{111}. The correlationsbetween the conformational structures of peptides T7 and S7and the different atomic arrangements on (100) and (111)planes might be responsible for the facet selectivity. Thehighly hydroxylated (-OH) “TLT” end of peptide T7 makes itpossible for oxygen atoms to bind to the Pt atoms on (100),generating a monolayer to stabilize this surface. In compar-ison, peptide S7 has the phenyl group (-C6H5) in the “F” unitand it prefers (111) surface due to the hexagonal array ofatoms.

Figure 9. CO as a capping agent for the selective stabilization of {111}or {100} facets on metal nanocrystals. A–C) TEM images of A) Pdplates, B) Pd tetrahedra, and C) Pd tetrapods enclosed by {111} facets.D) TEM image of Pt nanocubes enclosed by {100} facets. E) TEM andF) high-resolution TEM (HRTEM) images of Pt–Ni alloy octahedraenclosed by {111} facets. The image in (A) was reprinted from Ref. [79]with permission. Copyright 2010 Nature Publishing Group. The imagesin (B) and (C) were reprinted from Ref. [80] with permission. Copyright2012 American Chemical Society. The images in (D–F) were reprintedfrom Ref. [86] with permission. Copyright 2013 American ChemicalSociety.

Figure 10. Peptides serving as capping agents for the selective stabilization of {100} or {111} facetson Pt nanocrystals. A) Schematics and TEM images of nanocubes and octahedra that were obtainedby introducing peptides T7 and S7, respectively. B) Schematics and TEM images of T7-capped Ptnanocrystals at different concentrations of T7. The images in (A) were reprinted from Ref. [89] withpermission. Copyright 2011 Nature Publishing Group. The images in (B) were reprinted fromRef. [90] with permission. Copyright 2015 Wiley-VCH.




It is worth noting that the peptide concentration also hasa profound impact on the nanocrystal synthesis. For example,the synthesis of Pt cubes in the presence of peptide T7 couldonly be achieved at intermediate peptide concentrations (10–20 mgmL�1), which can be attributed to the attraction andrepulsion resulting from the competition between the peptideand water molecules on the Pt(100) surface at differentconcentrations of peptide T7.[90] As shown in Figure 10B, theauthors were able to apply the principle to the design of smallorganic molecules with the phenyl ring (e.g., 3-hydroxy-2-phenylpropanoic acid) for the synthesis of tetrahedral andoctahedral Pt or Rh nanocrystals covered by {111} facets byleveraging the epitaxial coordination between phenyl ring and{111} facets.[88]

4.6. One-Pot versus Seed-Mediated Growth

The examples shown in Sections 4.1 to 4.5 all involve theone-pot approach. In recent years, capping agents have alsobeen combined with seed-mediated growth to obtain nano-crystals with a variety of complex shapes or structures that areotherwise difficult to achieve by homogeneous nucleation orone-pot synthesis. By choosing the right capping agent topassivate a specific type of facet on the preformed seeds,different growth rates for different sites on the seeds caninduce specific growth pattern and eventually result in theevolution of seeds into predictable shapes. In one study, it wasdemonstrated that single-crystal, spherical Ag seeds could bedirected to grow into nanocrystals with octahedral and cubicshapes by using citric acid and PVP as the capping agents,respectively.[120] This method has also been applied to inves-tigate growth patterns of Ag nanoplates with differentcapping agents.[121] The nanoplate seeds are enclosed by two{111} basal planes as the top and bottom faces and containtwin planes and stacking faults on the side faces along thevertical direction. With the use of citric acid as a cappingagent, lateral growth was more favorable, leading to increasein edge length for the nanoplates. This growth pattern can beattributed to the selective adsorption of citric acid on theAg{111} facets. In contrast, when citric acid was substitutedwith PVP, thicker plates were obtained due to the verticalgrowth caused by the relatively stronger binding of PVP to{100} than {111}. It is worth pointing out that the overallgrowth rate was significantly faster for the case of citric acidthan PVP. Such difference can be explained by the fact thattwin planes and stacking faults on the side faces should havehigher surface energies than those located on both the top andbottom faces. Similar results were also observed in the growthof Pd nanoplates into enlarged plates during seed-mediatedgrowth in the presence of a low concentration of Br� asa capping agent for Pd{100}. When Br� ions were used ata higher concentration, all three of the {100} side faces of thetriangular plates would be completely blocked by Br� ions,forcing the Pd nanoplate to grow from three of the corners forthe generation of tripods.[122]

In addition to the monometallic system, seed-mediatedgrowth has been used to generate a number of bimetallicnanocrystals. In general, the products are limited to a core–

shell structure due to a thermodynamic control that forces thedeposition of the second metal to occur in a conformalfashion.[24,108] Compared to the core–shell structure, site-selected deposition and thus formation of an incomplete shellis highly desired for catalytic applications.[123] In 2012, ourgroup reported the synthesis of Pd–Rh bimetallic nanocubeswith a “core-frame” structure and concave faces throughconfined deposition of Rh atoms onto the corners and edgesof Pd cubic seeds, as shown in Figure 11A.[124] Througha combination of selective capping of Pd{100} facets by Br�

and kinetic control, site-selective overgrowth could be readilyachieved (Figure 11B,C). This strategy has been successfullyapplied to achieve site-selected deposition of Pt and Ru on Pdcubic seeds in recent years, as well as to the synthesis of star-like Au–Cu nanocrystals referred to as pentacles.[108,125,126] It isworth noting that site-selected growth also allows one tofabricate a “nanoframe” with a highly open structure whenthe deposited metal was more resistant to oxidative etchingthan the core metal.[124,125,127–129] As shown in Figure 11D,E,the Pd–Rh core-frame nanocubes could be readily convertedto Rh cubic nanoframes with ridges as thin as 2 nm byselectively etching away the Pd cores.[124,127–129] Using a similarapproach, Rh octahedral, Au triangular, and Au decahedralnanoframes were also fabricated (Figure 11F–H).[127–129] Webelieve site-selected growth enabled by surface capping canbe combined with post-synthesis etching to generate nano-frames with tunable compositions, morphologies, and proper-ties for a variety of applications.

Figure 11. Influence of capping agent on the growth pattern of nano-crystals. A) Schematic illustration of the synthesis of Rh cubic nano-frames. B) TEM image and C) HRTEM image and energy-dispersive X-ray (EDX) spectroscopy mapping of the Pd–Rh core-frame nanocubes.D) TEM image and the E) 3D atomic model of Rh cubic nanoframes.F–H) TEM images of F) Rh octahedral nanoframes, G) Au triangularnanoframes, and H) Au decahedral nanoframes. The plots and imagesin (A–E) were reprinted from Ref. [124] with permission. Copyright2012 Wiley-VCH. The image in (F) was reprinted from Ref. [127] withpermission. Copyright 2013 American Chemical Society. The image in(G) was reprinted from Ref. [129] with permission. Copyright 2013Wiley-VCH. The image in (H) was reprinted from Ref. [128] withpermission. Copyright 2011 American Chemical Society.





5. Computational Investigation

It is still difficult to gather information about the bindingenergy or strength of a capping agent, as well its adsorption/desorption dynamics, through experimental analyses. Alter-natively, DFT calculations and other advanced theoreticalstudies can offer insightful and quantitative trends withregard to surface capping.[90,130] It has been shown that theefficacy of surface capping is dependent on the structuralconformations and chemical functional groups of the cappingligand, as well as the crystallographic plane and electronicstructures of the metal.[130] Here we focus on the studies thatuse computational methods to understand the interactionbetween a capping agent and a metal surface in terms of facetselectivity and shape evolution.

5.1. PVP on Ag(100)

For the synthesis of Ag nanocrystals encased by {100}facets, PVP with different molecular weights are commonlyused as a capping agent. To resolve the role of PVP in thesynthesis while understanding the differences between smallmolecules and macromolecules, DFT calculations were con-ducted with an aim to elucidate the complex interactionsbetween PVP and Ag surface.[131] The repeating unit of PVPwas represented by two-component sub-units: ethane and 2-pyrrolidone (2P). Because of the inert and nonpolar characterof ethane, it was expected that the interaction between ethaneand Ag surface would be dominated by vdW attraction.Different from ethane, the oxygen and possibly nitrogenatoms in the 2P ring could bind to an Ag surface throughdirect chemical bond formation. The results suggest that, inthe case of macromolecular capping agent, both vdW andcovalent bonding should be considered when analyzing thebinding configurations of PVP on Ag and calculating the totalbinding energy, different from the case of small cappingmolecules.

When the authors simulated the binding conformations ofethane and 2P on Ag(100) and Ag(111) surfaces with no vdWinteractions, there was a slight preference for 2P to bind toAg(111), inconsistent with experimental observations. Incontrast, when the vdW forces were included in the bindinginteractions between PVP and Ag, the angles between the 2Prings and the Ag surface were in the ranges of 10–208,indicating that the rings of PVP were slightly tilted relative tothe surface when binding to (100) and (111) planes. Theresults suggest that the vdW interactions between PVP andAg can dramatically affect and “flatten” the binding con-formations on the surface. To understand the facet selectivityof PVP binding, the authors divided the total binding energy(Ebind) into three components: long-range vdW attraction(EvdW), short-range direct bonding (Edirect), and Pauli repul-sion (EPauli, i.e., the repulsive interaction between theelectrons of metal atoms and the adsorbed molecules)[Eq. (2)]:

Ebind ¼ EvdW þ Edirect þ EPauli ð2Þ

The results indicate that the total binding energy of PVPon Ag(100) is larger than that on Ag(111), along witha significant contribution from the long-range vdW interac-tions. Comparing the binding energies of PVPonAg(100) andAg(111), there was 80 meV preference for PVP to bind toAg(100). Importantly, the binding energy (Ebind) derived fromthe simulation can be used to calculate the density of PVP ona given surface. Based on the Boltzmann–Gibbs statistics ata finite temperature T, the ratio of the densities on Ag(100)and Ag(111) can be expressed as [Eq. (3)]:[131]

N 100ð Þ

N 111ð Þ¼ Exp½E 100ð Þ

bind =kBT�Exp½E 111ð Þ

bind =kBT�ð3Þ

where kB is the Boltzmann constant. The ca. 80 meV differ-ence in binding energy on these two Ag planes leads to a 10times higher density of PVP on Ag(100) than on Ag(111),consistent with experimental observation. Take together, it isnecessary to consider the surface-sensitive balance betweendirect binding and vdW interactions, which determines theenergetic preference of capping towards a particular facet,especially in the case of macromolecules.

5.2. Bromide on Pd(100)

Small inorganic species such as halides (Cl� , Br� , and I�)can be advantageous over the commonly used macromolec-ular capping agents whose long chains may induce a sterichindrance effect. Halide ions tend to form strong covalent orcoordination bonds with the metal atoms on the surface.Importantly, they can form a close-packed adlayer structureon the metal surface because of their simple monatomicstructure. In an adlayer, two competing interactions (halide–metal and halide–halide) should be taken into account whendiscussing the role of halides in shape-controlled synthesis ofnanocrystals. One of the notable cases is the selectiveadsorption of Br� on Pd{100} facets, leading to the formationof nanocubes. Recently, an ab initio study based on DFTsimulation was conducted to examine the energetics involvedin the morphological transformation of Pd nanocrystals underdifferent surface coverages of Br atoms.[132] They calculatedthe binding energies of Br atoms at the most stable sites onvarious types of Pd surfaces (including low- and high-indexplanes) as a function of Br coverage, as shown in Figure 12A.At a coverage of 0.25 monolayer (ML), the binding energy(�2.33 eV) of Br to the (100) plane was the lowest among allcrystallographic planes. With the increase of Br coveragebeyond 0.25 ML, the binding energies on all planes quicklybecame less favorable and Br atom bound most strongly tothe most open Pd(110). This trend can be attributed to therepulsive, adsorbate–adsorbate interaction on the moreclosely packed Pd (100) surface.

The calculation results are, however, not consistent withthe experimental observations. This could be due to i) theadsorbate they chose was neutral Br atoms, rather than Br�

ions and ii) the simulation environment is vacuum instead ofaqueous solution. Both of these factors deviate from theconditions of a real synthesis. It still requires additional efforts




to achieve a good understanding of the interaction betweenthe halide ions and a clean metal surface.

5.3. Citric Acid on Ag(111)

Citric acid, containing one hydroxyl and three carboxylgroups, is widely used as a capping agent to block {111} facetswhile allowing {100} facets to grow for Ag, Au, and Pdnanocrystals. To understand the origin of this facet selectivity,ab initio DFT calculation was applied to establish the bindingmodel and evaluate the binding energy of citric acid on a Agsurface.[133] As an fccmetal, the (100) plane of Ag has square-packing symmetry, while the (111) plane shows hexagonalsymmetry. On the other hand, the approximate symmetry ofa citric acid molecule has three- and six-fold rotation axes thatinterchange the three carboxyl groups, as well as thereflection through a plane that relates themethylene-carboxylgroups. The authors hypothesized that the optimal bindingstructures of citric acid on Ag(100) and Ag(111) weredetermined by their matching geometry, as shown in Fig-ure 12B. The binding of citric acid to these two Ag surfaces

involves differentnumbers of surface-ligand bonds (i.e.,Ag�O bonds). Forthe citric acid ona square-packed(100) surface, the mis-match between thesurface and moleculeand the interatomicdistances only resultin two Ag�O bondswith the (100) surface.On the other hand,the hexagonal symme-try of the (111) sur-face matches theapproximate three-fold symmetry ofcitric acid, and thusfour Ag�O bonds areformed. The fourbonds between citricacid and Ag(111)induce a large bindingenergy of 13.84 kcalmol�1, which is signifi-cantly greater thanthat of Ag(100)(3.69 kcalmol�1).According to theBoltzmann distribu-tion [Eq. (3)], thislarge differenceresults in a higherdensity of citric acidon Ag(111) than on

Ag(100), explaining why citric acid is able to retard thegrowth of Ag(111).[133] This study offers a quantitativedescription of the facet selectivity of citric acid towardsAg{111}. A similar trend was also observed for citric acid andPd nanocrystals.[134]

5.4. CO on Pt(100)

The triple bond (C�O) in CO is made of two normalcovalent bonds and one dative covalent bond in which the twoelectrons come fromO. As such, the C atom carries a negativefractional charge although it is less electronegative than Oatom. Owing to this electronic feature and thus a dipolemoment, the CO molecules chemisorbed on a metal tend tobe oriented perpendicular to the surface, with the C atompointing down. This feature has been proven by boththeoretical and experimental studies. In addition, the adsorp-tion energy of CO on a metal surface strongly depends on thecrystallographic plane and electronic structure of the metal,as well as the repulsion among the adsorbed molecules. Basedon a DFT study, the adsorption energy of CO at the most

Figure 12. Computational simulation of capping agent on the surface of metal nanocrystals. A) The calculatedbinding energies of Br atoms on the various Pd facets as a function of surface coverage density. B) Bindingconfigurations (top and side views) of citric acid on Ag(100) and Ag(111) surfaces. C) Binding configurations (topand side views) of peptide S7 on Pt(100) and Pt(111) surfaces, and their corresponding phenyl rings highlighted inyellow. D) Correlation of Pt tetrahedra yields with calculated peptide adsorption energies for Pt(100) and Pt(111)surfaces. The plots in (A) were reprinted from Ref. [132] with permission. Copyright 2014 Royal Society of Chemistry.The plots in (B) was reprinted from Ref. [133] with permission. Copyright 2008 Elsevier. The plots in (C) and (D)were reprinted from Ref. [88] with permission. Copyright 2013 American Chemical Society.





stable “top” site on a Pt(100) plane (1.95 eV) is larger thanthat on a (111) plane (1.72 eV), suggesting the preferentialbinding of CO towards the Pt(100) surface.[135]

It is worth pointing out that the preferable adsorptionstructure for CO on a surface depends on its coverage.[136,137]

For example, with the increase of CO coverage from 0.25 to0.5 ML, the most stable structure of CO on Pt(100) switchesfrom the top to the bridge configuration.[137] In another study,when the coverage of CO increased from 0.06 to 1.0 ML onPt(111), four stable ordered structures, including (

p3 �p

3)R308-CO, c(4 � 2)-4CO, c(p3� 5)rect-6CO, and c(

p3 �

3)rect-4CO, with various bridge to top ratios were identi-fied.[136] Interestingly, the trend in the bridge to top ratio firstincreased and then decreased with the increase in coverage,which can be ascribed to the change in Pt surface charge uponCO adsorption at the top and bridge sites. These predictionsindicate that the site preference and the stable structure forCO on various types of Pt surfaces at low, intermediate, andhigh coverages are totally different, which is expected tocreate different surface free energies for different Pt facets.

5.5. Peptides S7 on Pt(111)

The discovery of facet-specific biomolecules (e.g., pep-tides) has enriched the pool of capping agents for the shape-controlled synthesis of metal nanocrystals. As shown inFigure 12C,D, experimental and simulation studies havebeen integrated to understand the preferential and non-preferential adsorptions on Pt{111} when the compositionsand sequences of peptide S7 were altered (Section 4.5).[88] Theauthors designed a set of S7 variants to identify the key factorin controlling peptide capping. The variables they investi-gated including the sequence of peptide S7, the peptideconformation, the individual functional motifs, and thedetailed molecular effect of the phenyl ring in the aminoacid. The experimental results suggest amino acid F withphenyl ring (-C6H5) plays the key role in inducing theformation of Pt{111}.

Surprisingly, based on the computational results, allpeptides have large negative adsorption energies (Eads) onthe Pt(111) surface, suggesting effective binding interactionsbetween them but there was no direct correlation between theyield of tetrahedra and Eads(111) value, as shown in Fig-ure 12D.[88] On the other hand, the adsorption energies of thepeptides on Pt(100) surface strongly correlates with the yieldof tetrahedra. The F-containing (S7, S7�1, S7�2, SSF, andFPQPN) and F-free (S7-G, PQPN, S7-Y, and SSY) peptidesgave positive and negative Eads(100), respectively. Thus, it wasconcluded that the binding contrast between (111) and (100)surfaces was the origin of selectivity. Based on the molecularsimulations, the “lie-flat” configuration was observed for F-containing peptides on Pt(111) due to the hexagonal epitaxialcoordination between the phenyl group and the surface,which contributed to stronger attraction among the cappingagents (Figure 12C). In contrast, because of the mismatchbetween the phenyl ring and the square packing of (100)surface, the “stand-up” configuration dominated the bindinglandscape of F-containing peptides on the (100) surface. Such

a configuration creates much less steric hindrance, reducingthe overall peptide binding affinity towards Pt(100) whencompared to the lie-flat configuration. These results revealthat the phenyl group in a peptide is responsible for the highbinding contrast between Pt(111) and Pt(100), suggesting thatthe binding contrast enabled by the conformation of a cappingagent is critical to facet specificity.

Taken together, computation provides a powerful meansto reveal the role played by a capping agent in carving out theshape of a nanocrystal at an atomic/molecular level, as well asto identify their binding selectivity towards various types ofsurfaces. Although the computational studies have reachedsome agreement with the experimental observations made inthe shape-controlled synthesis of metal nanocrystals, they allinvolved assumptions with regard to the experimental con-ditions.[130] These assumptions may produce biased simulationresults. As such, there is still an urgent need to developcomputational methods that take into account the realconditions of a synthesis to examine the roles of variousexperimental parameters.

6. Characterization Tools

Despite their successful use in shape-controlled synthesis,a quantitative measure of most of the capping agents at thenanocrystal/solution interface is still elusive. As a result, theexplicit roles played by most capping agents are still underdebate. In addressing this issue, more and more analyticaltools are being applied to quantify the adsorption of cappingagent. For instance, zeta potential[138] measurements canindicate the overall charge of the particle, giving informationon the presence of positively/negatively charged cappingagent adsorbed at the interface. Ultraviolet-visible (UV-vis)spectroscopy[139] can provide information about the electronicstructure of the nanocrystals and the chemical environment atthe interface, both of which can change depending on thecapping agent. Fourier transform infrared spectroscopy(FTIR)[138,140] can be used to track the characteristic absorp-tion peaks of different functional groups associated witha specific capping agent. To gain more knowledge on thedistribution of elements in the capping agent, EDX[47] hasbeen implemented, while X-ray photoelectron spectroscopy(XPS)[141–143] has been used to investigate the presence ofcertain elements, as well as their chemical states. Also, nuclearmagnetic resonance spectroscopy (NMR)[144] has been used tostudy the binding mode of a capping agent on the metalsurface. Thermogravimetric analysis (TGA)[145,146] can detectthe presence of a capping agent based on its thermostability.The high sensitivity of inductively coupled plasma massspectrometry (ICP-MS)[141,147] makes it a powerful techniqueto quantify even a trace amount of a capping agent adsorbedon the surface of nanocrystals. Similarly, high performanceliquid chromatography (HPLC)[54] has recently beenemployed to derive the adsorption equilibrium constant ofa capping agent and determine the facet-binding preference.Finally, isothermal titration calorimetry (ITC)[148] has beenused to investigate the thermodynamics involved in thebinding of an adsorbent to the surface of nanoparticles.




Figure 13 shows a brief summary of the characterizationtechniques mentioned above, and more detailed discussioncan be found in the Supporting Information.

Quantification of surface capping can also be accom-plished by simply monitoring the shape of nanocrystals duringgrowth in the presence of a certain concentration of cappingagent in the reaction solution. As shown in Figure 14, thisapproach was used to quantitatively analyze the coveragedensity of PVP on Ag nanocubes.[149] With Ag nanocubesenclosed by {100} facets serving as seeds for growth, two setsof experiments were carried out: i) with a fixed initial PVPconcentration (C1) to determine the critical size at whichAg{111} facets started to appear; and ii) with the initialconcentration of PVP reduced to a critical point (C2) soAg{111} facets started to appear at the very beginning of seed-mediated growth. The coverage density (f) of PVP onAg{100} can be derived from the difference in concentration,C1�C2, and the total surface area of Ag cubes betweensamples 3 and 5 in Figure 14A using Equation (4):

� ¼ C1 � C2ð Þ � V �NA=DSAg ð4Þ

where V is the total volume of the solution, NA stands for theAvogadro�s number, and DSAg is the increase in surface area

between samples 3 and 5. It wasfound that the coverage densitieswere 140 and 30 repeating unitsper nm2 for PVP of 55000 and10000 gmol�1, respectively,when 40 nm Ag cubes wereused as the seeds. The valuesdropped slightly to 100 and 20repeating units per nm2, respec-tively, for cubic seeds of 100 nmin size. These results suggestedthat PVP with a relatively lowermolecular weight was moreeffective in reducing the freeenergy of Ag(100) via surfaceadsorption than PVP witha higher molecular weight. Ona quantitative basis, thisapproach is able to determinethe coverage density of a cappingagent and thus elucidate its effecton the shape evolution andgrowth habit of metal nanocrys-tals.

7. Nanocrystals Free ofCapping Agent

The capping agent remainingon the surface of metal nano-crystals after a synthesis mightcompromise their performancein heterogeneous catalysis andother applications. For example,

it was reported that the electrochemically active surface areaof the Pt–Ni octahedral catalyst without PVP capping layerwas much greater than that of the same catalyst but withPVP.[150] This issue can be attributed to the blocking effect ofPVP in restricting the free access of reactants to catalytic siteson a Pt–Ni octahedral nanocrystal. As such, nanocrystals freeof capping agent are generally more favored in heterogeneouscatalysis. As discussed in Section 2 using the Langmuirisotherm, the adsorption of a capping agent is determinedby both its concentration in solution and the equilibriumconstantK. In principle, repeated washing of the nanocrystalswith a good solvent for the capping agent should be able toremove all the adsorbed molecules. However, for cappingagents that bind to the surface with strong affinity, thedesorption kinetics could be very slow, making the washingprocess highly inefficient. In this section, we summarizerecent progress in the development of metal nanocrystals witha clean surface by either removing the capping agent after thesynthesis or conducting a synthesis without involving anycapping agent.

Figure 13. Spectroscopy analysis of capping agent on the surface of metal nanocrystals. A) UV-visanalysis of the as-prepared Au nanospheres and after modification with different capping agents.B) FTIR analysis of the species on the surface of Au nanospheres and reference spectra of thecorresponding compounds. C) XPS analysis (N 1s spectra) of the PVP on Pd nanocubes and concavenanocubes. D) ICP-MS analysis of the coverage density (f) of Br� ions on the surface of Pd nanocubeswith different edge lengths (L). The plot in (A) was reprinted from Ref. [139] with permission. Copyright2004 American Chemical Society. The plot in (B) was reprinted from Ref. [138] with permission.Copyright 2018 American Chemical Society. The plot in (C) was reprinted from Ref. [142] withpermission. Copyright 2014 American Chemical Society. The data in (D) was reprinted from Ref. [141]with permission. Copyright 2013 American Chemical Society.





7.1. Post-Synthesis Removal of Capping Agent

The interaction between a capping agent and a metalsurface can involve either physical or chemical adsorp-tion.[130,151–154] For the physisorbed capping agent, it can beremoved from the surface through simple washing witha proper solvent. It is more difficult to remove a chemisorbedcapping agent due to the involvement of chemical bonding.Although the chemisorbed capping agent can be removed

through post-synthesis treatment such as thermal annealing, itoften induces changes to the shape, in addition to irreversibleaggregation or fusion of the nanocrystals.[151] Here we focuson the strategies for removing the capping agent withoutaltering the original shape.

Halides are commonly used as capping agents for thesynthesis of metal nanocubes, nanobars, and penta-twinnedrods/wires mainly enclosed by {100} facets since they prefer-entially adsorb on the (100) surface. It was shown that Br�

Figure 14. A) Schematic illustrations of the growth of a Ag cubic seed with an edge length of a in the presence of PVP at a high concentration C1

and a critical concentration C2 (C1 > C2). B–G) SEM images of Ag polyhedra grown from 40-nm cubic seeds in the presence of A–C) 1.0 mm andD–F) 0.1 mm PVP (55000 gmol�1), respectively. The samples were collected from the synthesis at different time points: B,E) 5 min, C,F) 10 min,and D,G) 20 min. Reprinted from Ref. [149] with permission. Copyright 2012 American Chemical Society.




could be removed from Pd surface via desorption at 100 8Cunder a reductive environment.[141] Due to the involvement ofchemisorption, the Pd atoms on the surface should bepartially oxidized. If the oxidized atoms are reduced to thezero oxidation state, the adsorbed Br� ions can be easilyremoved according to the half-reaction in Equation (5):

PdII � Br� þ 2e� ! Pd0 þ Br� ð5Þ

This concept was proven by conducting a set of experi-ments in which aqueous or EG suspensions containing citricacid (reducing agent), PVP (colloidal stabilizer), and Br�-capped Pd nanocubes were aged at different temperatures. Asshown in Figure 15A, the Pd 3d XPS spectrum of Br�-cappedPd nanocubes displayed two peaks at 340.4 and 335.1 eV,corresponding to elemental Pd 3d3/2 and Pd 3d5/2, respectively,while shoulders at higher energies were also observed,

implying the existence of PdII on the sur-face. The shoulders gradually decreased inintensity and disappeared for the EGsample aged at 100 8C, revealing the reduc-tion of PdII to Pd0 and thus the desorptionof Br� ions. This result was also consistentwith the quantitative analysis based onICP-MS (Figure 15B). The amount of Br�

ions dropped to 12% of the original valuefor the EG sample aged at 100 8C. Alto-gether, the chemisorbed Br� ions can beremoved without changing the shape of thePd nanocrystals in the presence of a mildreducing agent.[141]

Other methods such as UVO (UV-ozone) irradiation have also been adoptedto remove the capping agent on nanocrys-tals.[155–157] Unlike thermal treatment, UVOirradiation at room temperature can yielda clean surface while preserving the shapeof the nanocrystals. It involves the simulta-neous action of UV light and ozone, whichare responsible for the oxidation of anorganic capping agent into CO2 andH2O. Inone study, UVO treatment was used toremove organic capping agents such astetradecyl tributylammonium bromide(TTAB) from Pt nanocubes.[156] Figure 15Cshows FTIR spectra of the TTAB-coated Ptnanocubes as a function of time for UVOtreatment, revealing the complete disap-pearance of the alkyl chain, while thequaternary ammonium moiety remainedafter 120 min of treatment. In another case,PVP-capped Pd nanocubes were subjectedto UVO cleaning, as shown in Fig-ure 15D.[155] The removal of PVP wasanalyzed by ATR-FTIR spectra. After 3 hof treatment, PVP was removed and onlya few bands (1900 and 1650 cm�1) were stillvisible. The broad band at 1650 cm�1 can beassigned to the H2O bending mode, whilethe two new bands at 1930 and 2045 cm�1

suggested that a small amount of CO wasformed during the decomposition of PVP.

For some applications, post-synthesisligand exchange is used to replace theoriginal capping agent with a new onehaving the desired functional group. Forexample, it is necessary to replace the toxiccetyltrimethylammonium bromide (CTAB)

Figure 15. Removal of capping agent and ligand exchange without altering the shape of thenanocrystals. A) Pd 3d XPS spectra and B) concentration of Br� ions on Pd nanocubessubjected to aging in aqueous solution or EG containing citric acid at different temperaturesfor 18 h. C) FTIR spectra of TTAB-capped Pt nanocubes as a function of duration for UVOtreatment. D) ATR-FTIR spectra of PVP-capped Pd nanocubes after UVO treatment (3 h) andthe other controlled samples without UVO treatment. E) Schematic illustration and TEMimages of indirect ligand exchange on a Au nanosphere as assisted by the deposition andetching of Ag. The spectra in (A) and plot in (B) were reprinted from Ref. [141] withpermission. Copyright 2013 American Chemical Society. The spectra in (C) was reprintedfrom Ref. [156] with permission. Copyright 2009 American Chemical Society. The spectra (D)were reprinted from Ref. [155] with permission. Copyright 2011 American Chemical Society.The images in (E) were reprinted from Ref. [138] with permission. Copyright 2018 AmericanChemical Society.





or cetyltrimethylammonium chloride (CTAC) on Au nano-crystals with biocompatible ligands for biomedical applica-tions. To this end, an indirect method was reported foreffectively replacing the ligand on Au nanocrystals, as shownin Figure 15E.[138] In this method, an ultrathin layer (ca.1.5 nm) of Ag was deposited on Au nanocrystals to helpremove the original, toxic capping agent such as CTAC,followed by etching of the Ag layer in the presence of thesecond ligand such as biocompatible citrate ions. This strategyoffers a facile and robust means to completely replacea strong ligand such as CTAC with an oppositely charged,

weak ligand such as citrate, without inducing changes to theshape of the nanocrystals.

7.2. Synthesis without Capping Agent

Compared to the conventional synthesis that needs addi-tional cleaning procedures to remove the capping agent,synthesis conducted in the absence of any capping agent ismore straightforward in obtaining nanocrystals with a cleansurface. Without the stabilization by a capping agent (or

a colloidal stabilizer), itwill be difficult to keepthe nanocrystals dispersedin the reaction medium.This issue can beaddressed by directlydepositing the nanocrys-tals on a solid sup-port.[150,158,159] The controlof shape is typically ach-ieved through the use ofa template.[24] For exam-ple, a simple method wasreported for the prepara-tion of Pd@Pt3-4L core–shell nanocrystals withwell-defined {111} facets,as shown in Fig-ure 16A.[159] In the firststep, 21-nm Pd octahedralseeds were synthesizedand then loaded ontocarbon black for use asa template. Upon optimi-zation of the reaction con-ditions, ultrathin shells ofPt could be conformallydeposited on the octahe-dral seeds in a layer-by-layer fashion withoutinvolving self-nucleation(Figure 16B,C). XPS anal-ysis was used to examinethe surface species for thesamples prepared in theabsence and presence ofPVP (a colloidal stabilizerin this case), as shown inFigure 16D. Even after thesample had been washedeight times with water, thesample prepared in thepresence of PVP stillexhibited a noticeableN1s peak at ca. 398 eVassociated with PVP. Incontrast, the N1s signal ofPVP was negligible in the

Figure 16. Capping-free synthesis of nanocrystals. A–D) Synthesis of capping-free Pd@Pt3-4L core–shelloctahedral nanocrystals. A) Schematic of the procedure that includes loading of Pd octahedral nanocrystalsonto a carbon support, followed by the conformal deposition of Pt ultrathin shells to produce the Pd@Pt3-4Lnanocrystals. B) TEM and C) HRTEM images of the Pd-Pt3-4L nanocrystals. D) N1s XPS spectra of the Pd@Pt3-4Lnanocrystals synthesized with and without PVP. E–J) TEM images of capping-free E) Ag icosahedral, G) Agdecahedral, and I) Ag cuboctahedral nanocrystals supported on SiO2 substrates. F–J) HRTEM images of anindividual F) icosahedral, H) decahedral, and J) cuboctahedral particle. Red lines indicate the twin planes. Thespectra and image in (A–D) were reprinted from Ref. [159] with permission. Copyright 2017 Wiley-VCH. Theimages in (E–J) were reprinted from Ref. [162] with permission. Copyright 2017 Wiley-VCH.




XPS spectrum for the sample prepared in the absence of PVP,implying that the sample was free of PVP.[159] It is worthnoting that some synthetic systems involve the use of organicreducing agents such as l-ascorbic acid and formaldehyde,which could be oxidized to generate CO during a synthesis.[160]

The CO would strongly adsorb onto the surface of noblemetals, blocking the active sites and thus negatively impactingthe catalytic activity.

In addition to wet chemical synthesis carried out in thepresence of solid support, physical vapor deposition (PVD)has been explored to produce metal nanocrystals free ofcapping agent or colloidal stabilizer.[161–164] As demonstratedin a series of studies, both monometallic and bimetallicnanocrystals made of Au, Ag, Pd, and Pt were fabricated onamorphous SiO2 or (0001)-oriented sapphire substratesthrough the PVD-based method. In brief, a thin metal filmwas first deposited on the substrate by sputtering orevaporation techniques, followed by the formation of nano-crystals with desired twin structures through careful heatingand then cooling. In one example, Ag films with differentthicknesses were deposited on SiO2 and then subjected toheating/cooling to obtain capping-free Ag nanocrystals withicosahedral, decahedral, or single-crystal structures underthermodynamic control, as shown in Figure 16E–J.[162]

Another study showed that a periodic array of PVD-derivedAu nanocrystals on (0001)-sapphire could serve as seeds forthe kinetically controlled growth of Ag to generate Au–Agnanocrystals with either a Janus or a core–shell structure.[163]

Capping agent-free synthesis could also be achieved in the gasphase by incorporating a trace amount of foreign metalpowders as the shape-directing agent, followed by evapora-tive dealloying.[165] A recent report shows that tetrahexahe-dral (THH) nanocrystals with a range of sizes and composi-tions could be synthesized using this method. The solid metalprecursor or metal particles, together with a foreign metal(Sb, Te, Pb, or Bi) were heated together under an Ar/H2

atmosphere. The {210} facets exposed on the surface of THHnanocrystals could be stabilized in the presence of the foreignmetal atoms before they were removed by dealloying. Thesedemonstrations suggest that gas-phase synthesis offers anattractive platform for the preparation of metal nanocrystalswith a clean surface.

8. Concluding Remarks

Maneuvering the shape of metal nanocrystals and thus thetypes of facets exposed on their surface offers a powerfulmeans for tailoring their physicochemical properties torealize various applications. Under the assistance of cappingagents, significant progress has been made in recent yearswith regard to the colloidal synthesis of metal nanocrystalswith controlled shapes. It is worth pointing out that, inaddition to their role in directing the shape evolution ofnanocrystals, the capping ligands can also serve as a colloidalstabilizer to prevent the suspended nanocrystals from aggre-gation during synthesis and/or storage.[166] Despite the incred-ible accomplishments, many scientific problems or techno-logical challenges still need to be addressed before we are

able to use the as-obtained nanocrystals for a broad spectrumof applications.

One of the challenges is a quantitative analysis andunderstanding of the roles played by a capping agent ina typical synthesis of metal nanocrystals in the context ofreduction, deposition, and diffusion. In the seed-mediatedgrowth, it was shown that the deposition rate of atoms relativeto the surface diffusion rate of adatoms dominates theevolution of a seed into nanocrystals with diverse shapes.[108]

However, we still have no quantitative measure of howa capping agent and its coverage density would affect thesekinetics processes due to the lack of experimental tools. Forthe deposition of atoms from precursor reduction in thesolution phase, they will be prohibited if the atoms strikea region on the seed capped and thus passivated by a cappingagent. However, if the coverage density of the capping agentis not high enough to passivate the entire surface, atomdeposition can still occur on the spared regions to evolve intospecific shapes depending on the coverage density.[54,141,149] Inaddition, recent studies also suggested that the precursor canfirst adsorb onto the surface of a growing nanocrystal,followed by chemical reduction to atoms.[167–169] In this case,the interaction between the precursor and the capping agenton the surface will play a key role in determining thereduction and thus deposition processes. For instance, thedeposition is expected to preferentially take place on thecapped region if there is an attractive interaction between theprecursor and capping agent, giving a growth patterncomplementary to the one arising from surface passivationby the capping agent. On the other hand, the adsorbedcapping agent is also believed to play a crucial role ininfluencing surface diffusion of adatoms on the surface of theseed.[170] The capping layer with a specific binding strengthwould present a kinetic barrier, weakening the ability ofadatoms to diffuse to more energetically favored sites. Takentogether, it is highly desirable to have a quantitative measureand understanding of the capping agent involved in a syn-thesis. This can be achieved through the development of insitu tracking methods such as synchrotron X-ray diffrac-tion[171,172] and surface-enhanced Raman scattering (SERS)while including computational modeling with an emphasis onthe dynamic aspects.[173]

Another major challenge is how to screen and identifynew capping agents for the syntheses of nanocrystals coveredby various types of facets, in particular, the high-index onessuch as {221}, {310}, {411}, and {720}, that contain highdensities of low-coordination surface atoms. As shown ina one-pot synthesis, CTACmolecules could serve as a cappingagent in the formation of Au trisoctahedrons enclosed by{221} facets because the long alkyl (CTA+) chains have a sizecomparable to the atomic spacing on the high-index facetsand can thus adsorb onto the less closely packed, high-indexfacets rather than the low-index counterparts.[67] In anotherstudy, concave Pt nanocrystals with high-index {411} facetswere prepared in the presence of amine compounds as thecapping agent.[93] Overall, these capping agents were discov-ered randomly. Before rational design becomes available, itwould be more productive to systematically screen a library ofcompounds for their selectivity towards various combinations





of metals and facets. This can be achieved by conductinga synthesis with the pre-synthesized nanocrystals asseeds.[88,120] Upon the introduction of additional metal pre-cursor and reducing agent, together with a specific compoundof interest, the growth can be initiated and continued at anelevated temperature to facilitate surface diffusion for theachievement of thermodynamic control. By monitoring theshape evolution of the seeds either ex situ or in situ, oneshould be able to elucidate the capping ability and facetselectivity of the compound. This rapid and simple approachcan be easily extended to a broad range of compounds toevaluate their potential for the preparation of a variety ofmetal nanocrystals bound by diverse facets.

As for the application in heterogeneous catalysis, it isgenerally accepted that the capping molecules on a metalnanocrystal tend to block the active sites and compromise itscatalytic activity. However, several groups demonstrated anunexpected benefit of capping agent in serving as an activitypromoter and/or selectivity modifier towards a specific cata-lytic reaction.[153,174,175] This benefit can be attributed to thecreation of a metal–ligand interface, through which geometric(steric) and electronic factors can be leveraged to generatea favorable change to the local environment. The influence ofthese factors on the catalytic reaction was found to bedependent on the composition, coverage density, spatialarrangement, and interaction strength between the cappingagent and other chemical species such as reactants andsolvents. In one study, it was reported that the Au nano-crystals stabilized by PVP at a proper coverage densityshowed higher activity towards aerobic oxidation of alcoholthan that of Au nanocrystals stabilized by poly(allyla-mine).[174] As illustrated in Figure 17A, the Au nanocrystalswere negatively charged due to the donation of electrons fromPVP to Au and thus alteration to the electronic structure.During the oxidation catalysis, the electrons flew from theanionic Au on PVP-capped Au nanocrystals into the lowestunoccupied molecular orbital (LUMO) (p*) of O2, generatingsuperoxo (O2

�)- or peroxo (O22�)-like species. As a result, the

catalytic activity of the PVP-capped Au nanocrystals wassignificantly enhanced. In another study, UVO-treatment(Section 7.1) was used to control the degree of removal andthus coverage density of PVP on the surface of Au nano-crystals to assess the impact of PVP on the hydrogenation ofp-chloronitrobenzene (p-CNB) and cinnamaldehyde (CAL),as shown in Figure 17B.[175] The presence of PVP was found tobe beneficial to the activity of Au towards the hydrogenationof p-CNB while compromising the hydrogenation of CAL. Inthe case of p-CNB, p-chloroaniline (p-CAN) was the soleproduct, and the corresponding turnover frequency (TOFp-

CNB, the number of moles of p-CNB transformed into p-CANby one mole of active sites per hour) decreased rapidly from178 to 100 h�1 within the first hour of UVO-treatment. Thisresult was attributed to the difference in orientation for theactivated complex that interacted differently with the metalsurface depending on the presence or absence of the cappingagent. The benzene ring of p-CNB in the activated complexon the PVP-capped Au nanocrystals was oriented perpendic-ular to the surface, and its interaction with the catalyst wouldbe remarkably weaker than its flat-on orientation parallel to

the surface of the clean Au nanocrystals. The mobility of theactivated complex would be facilitated by a weakenedinteraction of the activated complex with the Au in thepresence of PVP, leading to a higher activity towards thehydrogenation of p-CNB. In contrast, the significantly lowerTOFCAL of the as-prepared, PVP-capped Au catalyst than thatof the UVO-cleaned Au-catalyst indicated an adverse impactof PVP on the catalytic reaction, which can be attributed tothe partial blockage of catalytic sites by the organic residues,either through direct interaction with or steric hindering ofactive sites from being accessed by the reactant. Takentogether, these experimental results demonstrated that thecapping agent could serve as a “poison” to limit theaccessibility of active sites, or a “promoter” to improve theactivity and selectivity. Owing to the advantage of diversecapping agents with different functionalities, there is a lot ofroom for further development in this field, where thesynergetic effect between metal nanocrystals and cappingagents can be used to manipulate the activity and selectivity.To this end, it is an urgent challenge to integrate ex situ and insitu characterization techniques for probing catalytic process-es at the metal–ligand interface under reaction conditions todistinguish the electronic and geometric changes induced bycapping molecules on the catalytic performances of metalnanocrystals.

When these challenges are fully addressed, it will becomepossible to rationally synthesize and engineer nanocrystalswith desired properties by choosing the suitable cappingagents together with other experimental parameters. It is

Figure 17. Influence of capping agent on the catalytic activity andselectivity of metal nanocrystals. A) The effect of PVP on the catalyticactivity of Au nanocrystals towards an aerobic oxidation reaction.B) Product selectivity of Au nanocrystals with different degrees of PVPremoval towards the hydrogenation reactions of p-CNB and CAL. Theschemes in (A) was reprinted from Ref. [174] with permission. Copy-right 2009 American Chemical Society. The plot and schemes in (B)were reprinted from Ref. [175] with permission. Copyright 2014 Amer-ican Chemical Society.




hoped that the concepts and case studies presented in thisarticle will not only assist researchers in understanding theroles played by surface capping but also offer the insights andguidelines for designing their own syntheses.

Acknowledgements

This work was supported in part by research grants from theNSF (DMR-1505400, CHE-1505441, and CHE-1804970) andstartup funds from the Georgia Institute of Technology.

Conflict of interest

The authors declare no conflict of interest.

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Manuscript received: August 30, 2019Accepted manuscript online: October 9, 2019Version of record online: && &&, &&&&




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Reviews

Capping Agents

T.-H. Yang, Y. Shi, A. Janssen,Y. Xia* &&&&—&&&&

Surface Capping Agents and Their Rolesin Shape-Controlled Synthesis of ColloidalMetal Nanocrystals

A pivotal role in directing the growth ofcolloidal metal nanocrystals into diversebut well-controlled shapes is played bysurface capping agents. This article offersa comprehensive review of cappingagents as well as their use in engineeringthe surface structures and catalytic prop-erties of metal nanocrystals.




surface capping agents and their roles in shape‐controlled

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