Nano Res

1

Shape-Controlled Syntheses of Rhodium Nanocrystals

for the Enhancement of Their Catalytic Properties

Shuifen Xie,1 Xiang Yang Liu,1 and Younan Xia*,2

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0674-x

http://www.thenanoresearch.com on December 2 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0674-x

1

Table of Contents

This review article highlights recent progress in the syntheses of Rh nanocrystals with a number

of well-controlled shapes, together with their use in various catalytic reactions, where the activity

and/or selectivity could be enhanced through shape engineering.

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Revised ms# NARE-D-14-01208

Shape-Controlled Syntheses of Rhodium Nanocrystals

for the Enhancement of Their Catalytic Properties

Shuifen Xie,1 Xiang Yang Liu,1 and Younan Xia*,2

1Research Institute for Soft Matter and Biomimetics and Department of Physics, Xiamen

University, Xiamen, Fujian 361005, P. R. China

2The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of

Technology and Emory University, and School of Chemistry and Biochemistry and School of

Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia

30332, United States

*Address correspondence to younan.xia@bme.gatech.edu

3

ABSTRACT: Rhodium (Rh) is a critical component of many catalysts for a variety of chemical

transformation processes. Controlling the shape of Rh nanocrystals offers an effective route to

the optimization of their catalytic performance owing to a close correlation between the catalytic

activity/selectivity and the surface atomic structure. It also helps to substantially reduce the

loading amount and thus achieve a sustainable use of this scarce and precious metal. In this

review article, we focus on recent progress in the shape-controlled synthesis of Rh nanocrystals

with a goal to enhance their catalytic properties. Both traditional and newly-developed synthetic

strategies and growth mechanisms will be discussed, including those based on the use of surface

capping agents, manipulation of reduction kinetics, control of surface diffusion rate, management

of oxidation etching, and electrochemical alteration. We also use two examples to highlight the

unique opportunities offered by shape-controlled synthesis for enhancing the use of this metal in

catalytic applications. The strategies can also be extended to other precious metals in an effort to

advance the production of cost-effective catalysts.

Keywords: Rhodium nanocrystals, shape control, material synthesis, surface structure, catalysis

1. Introduction

As a member of the platinum group metals (PGMs), Rh is one of the rarest and most precious

metals [1]. Unlike other PGMs such as Au, Ag, Pd, and Pt that have been applied to a wide

variety of different applications [2-9], the usage of Rh has been mainly limited to catalysis. Its

indispensable role in catalysis has been well recognized through its marvelous performance in

diverse reactions or transformation processes, including hydrogenation [10, 11], ethanol steam

reforming [12, 13], CO oxidation [14, 15], and NOx reduction [16, 17], among others. In fact,

Rh-based heterogeneous catalysts have found widespread use in an array of industrial processes

such as petroleum refining [18] and fine chemical production [19]. They have also played a

critical role in the protection of our environment, as exemplified by automobile catalytic

converters [20]. Of 30,000 kg of Rh consumed worldwide in 2012, more than 80% went into this

particular application for its popular use in the three-way catalytic converters [1, 21]. Similar to

Pt, the extremely low abundance in the earth’s crust and thereby the ever increasing price of Rh

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is a major concern for all existing and emerging applications enabled by Rh-based catalysts [22,

23]. It is necessary to use Rh in a finely divided state to enhance its specific surface area and thus

substantially reduce the loading amount and help achieve a sustainable use of Rh. To this end,

the size and shape of the nanoparticles can be engineered in a controllable fashion to greatly

enhance their catalytic performance in terms of both activity and selectivity

Through computational simulations and experimental measurements on clean single-crystal

surfaces, it has been established that the activity and selectivity of a heterogeneous catalyst for a

structure-sensitive reaction can be manipulated by controlling the type of crystallographic plane

and thus the arrangement of atoms on the surface [24-29]. Coincidentally, most of the reactions

catalyzed by Rh are indeed structure sensitive [30-33]. For example, Mavrikakis and coworkers

have demonstrated that the barrier for CO dissociation was ~120 kJ/mol lower on the stepped

Rh(211) surface than on the close-packed Rh(111) surface through periodic self-consistent

density functional theory (DFT) calculations [33]. Also, the rate constant of CO oxidation on

Rh(100) surface is an order of magnitude higher than that on Rh(111) surface [32]. Despite these

exciting insights, however, the state-of-the-art Rh catalysts used in the industry are still based on

polydisperse particles with poorly defined morphologies and thus a mix of different facets on the

surface [22]. “There is plenty of room at the bottom” for improving the catalytic performance of

Rh-based catalysts by carving the nanocrystals into a specific shape solely with the most reactive

facet exposed on the surface.

Over the past decade, significant progress has been made in the shape-controlled synthesis of

colloidal noble-metal nanocrystals, including those made of Rh [34-39]. In general, the final

shape of a nanocrystal is determined by the type of seed formed in the nucleation step and the

presence of a capping agent capable of selectively binding to a specific type of facet [24, 40].

The reaction kinetics may also play an important role in control the growth habit of a seed and

thus the shape taken by the nanocrystal [41-43]. By varying the experimental conditions, noble-

metal nanocrystals with a wide variety of shapes have been achieved. However, compared to

other PGMs such as Au, Ag, Pd, and Pt, it has been much more challenging to control the shape

of Rh nanocrystals due to the extraordinarily high surface free energy of Rh. It is well-

established that the surface free energy plays a pivotal role in the growth process and thus the

shape taken by a nanocrystal because of the thermodynamic driving force to minimize the total

surface free energy [24, 25]. Figure 1 shows the surface free energy data calculated using the

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embedded-atom method (EAM) for four different PGMs with the same face-centered cubic (fcc)

structure [44, 45]. No matter which shape is involved, the total surface free energy of the Rh

nanocrystal is more than three times higher than that of the Au counterpart, and about two times

of those made of Pd and Pt. As a result, there are only a limited number of publications on the

shape-controlled syntheses of Rh nanocrystals. In this review article, we intend to provide an

overview of the recent progress on the shape-controlled syntheses of Rh nanocrystals and their

enhanced catalytic properties. We discuss a number of synthetic strategies for controlling the

shapes of Rh nanocrystals, including the introduction of a surface capping agent to enable

thermodynamic control, the manipulation of reduction rate and/or surface diffusion rate to enable

kinetic control, management of oxidation etching, and electrochemical alternation. By the end,

we highlight the importance of controlling the shapes of Rh nanocrystals in catalysis through the

use of two examples involving hydrogenation of arenes and electrochemical oxidation of CO and

ethanol.

2. Shape control enabled by the use of a surface capping agent

Thermodynamically, a nanocrystal prefers to take a shape having the lowest total surface free

energy, which is a sum of the products of the area and specific surface free energy for all facets

on the surface of a nanocrystal. In the absence of a surface capping agent (like the case in a

vacuum), the nanocrystal of an fcc metal would take a cuboctahedral shape due to a compromise

between the ratio of surface area to volume and the ratio of specific free energies for {111} and

{100} facets [24, 46]. The introduction of a surface capping agent into a reaction solution will

alter the shape of colloidal nanocrystals due to its preferential chemisorption onto a specific type

of facet to lower the specific surface free energy of that facet [47, 48]. It can be considered as a

typical example of thermodynamic control as the facet selectively stabilized by the capping agent

will be enriched in proportion on the surface. For noble-metal nanocrystals, the capping agents

could be inorganic species, such as Cl－, Br

－, I

－, Cu2+, Ag+, and CO, as well as a wide variety of

organic molecules or macromolecules, including citrate, cetyltrimethylammonum bromide

(CTAB), peptides, and poly(vinyl pyrrolidone) (PVP) [47-50].

Despite the large number of choices, there are only a few reports on the use of a capping

agent to control the shapes of Rh nanocrystals. Due to the extremely high surface free energy of

Rh, it is more difficult to alter the order of specific surface free energies of various facets through

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the chemisorption of a capping agent. In an early study, Somorjai and coworkers reported the

highly selective (>85%) synthesis of catalytically active Rh nanocubes enclosed by {100} facets

with a size smaller than 10 nm by using trimethyl(tetradecyl)ammonium bromide (TTAB) as a

capping agent in a polyol synthesis (Figure 2a) [51]. If the synthesis was conducted in the

absence of TTAB, the products were dominated by polydisperse Rh nanoparticles, with only

about 10% of Rh nanocubes (Figure 2b). In this case, the real capping agent seems to be the Br－

ions from TTAB, which could effectively chemisorb onto and thus stabilize Rh(100) surface,

inducing the evolution of a cubic shape. Our results from a later study suggest that the Rh cubic

nanocrystals could be easily obtained by simply adding a certain amount of KBr into a polyol

synthesis (Figure 2c) [52]. Recently, Tanaka and coworkers systematically studied the

mechanism of formation for Rh nanocubes using RhCl3 as a precursor in ethylene glycol, in the

presence of TTAB and PVP [53]. By using the in situ X-ray absorption fine structure (XAFS)

technique, they found a four-stage scenario for the formation of Rh nanocubes (Figure 2d),

including i) exchange of the ligand for Rh3+ between Cl－ and Br

－, ii) formation of Rh cluster

nuclei, iii) evolution of nuclei into Rh nanocrystals, and iv) development of cubic shape. In all

these stages, the participation of Br− ions was found to be crucial.

In addition to cube, another common shape of Rh nanocrystals that has been synthesized at

the assistance of a capping agent is nanoplate (or nanosheet) enclosed mainly by {111} facets.

To this end, Son and coworkers have successfully synthesized Rh ultrathin nanoplates with an

average thickness of only 1.3±0.2 nm by using oleylamine as a coordination and capping agent

(Figure 3, a and b). They proposed that the formation of such a two-dimensional structure was

highly dependent on the van der Waals interaction between the coordinated oleylamine

molecules (Figure 3c) [54]. Very recently, Li and coworkers reported the successful synthesis of

Rh ultrathin nanosheets with a thickness of only one layer of Rh atoms using PVP as the capping

agent in a solvothermal reaction (Figure 4) [55]. Through density functional theory studies, they

found that the single-layered Rh nanosheets involved a δ-bonding framework, which stabilizes

the single-layered structure together with the PVP ligands. Importantly, the percentage of surface

Rh atom in the single-layered Rh nanosheets could reach 100%, making them excellent catalysts

for both hydrogenation and hydroformylation reactions. Apparently, the use of capping agents

has shown its power in generating Rh nanocrystals with a variety of different shapes.

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3. Shape control through the manipulation of reduction kinetics

Controlling the reaction kinetics is another effective approach, commonly referred to as the

kinetic control, to engineering the shape of noble-metal nanocrystals [41-43, 56]. The essence of

kinetic control is to manipulate the rates at which zero-valent metal atoms are generated and

added onto the surface of a growing seed. Typically, the kinetics can be precisely tuned by

varying the experimental parameters such as temperature, solvent, types of reducing agent and

precursor, as well as their concentrations. To this end, Son and coworkers demonstrated the

synthesis of tetrahedral and spherical Rh nanocrystals by using rhodium carbonyl chloride and

rhodium acetylacetonate as the precursors, respectively [36]. Depending on the disparate

chemical stability of different precursors, the onset decomposition temperatures and thus the

generation rates of Rh atoms could be facilely tuned to generate Rh nanocrystals with different

shapes (Figure 5). Schaak and coworkers also demonstrated that, through the proper selection of

a solvent to control the reducing kinetic of a polyol synthesis, Rh nanocrystals with a myriad of

shapes could be obtained, including cubes, octahedra, triangular plates, and icosahedra [39].

Importantly, the products obtained under a kinetic control could break the thermodynamic

confinement and thus allow for the formation of nanocrystals with unconventional shapes, such

as those with concave surfaces or asymmetrical shapes [56, 57]. In the presence of a capping

agent and at an appropriate reduction rate, atoms generated from a precursor can be specifically

added to the uncovered regions of a growing seed, leading to the formation of nanocrystals with

concave surfaces. Using this strategy, we have developed a polyol process for the synthesis of

Rh concave nanocubes by using a syringe pump to control the injection rate of a Na3RhCl6

solution and thus manipulating the reduction kinetics [58]. By injection the precursor into a

growth solution at a relatively slow rate (4.0 mL/h), Rh concave nanocubes of 15 nm in edge

length were obtained in a yield approaching 100%. Figure 6 shows the morphological and

structural characterizations of the samples, revealing the concave surfaces on the six side faces of

each Rh nanocrystal. The high-resolution TEM (HRTEM) images recorded from tiled samples

(Figure 6, c-e) suggested that the surface of the concave nanocube was bounded by a mix of both

{100} and {110} facets. These concave nanocubes have great potential for catalytic applications

owning to the presence of high energy {110} facets on the surface [57, 58]. In a set of

experiments, it was found that the Rh concave nanocubes evolved through preferential

overgrowth at both the corner and edge sites of Rh nanocubes (formed in the initial stage of a

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synthesis) since the {100} side face were blocked by a layer of chemisorbed Br− ions. In

comparison, a faster injection rate (60 mL/h) resulted in the formation of Rh octapods due to a

confined overgrowth only at the corner sites of a cubic seed along the <111> direction. This

strategy has also be extended to the syntheses of Rh-based bimetallic nanocrystals with concave

surfaces, including Pd-Rh and Pt-Rh concave nanocubes, by using Pd and Pt nanocubes as the

seeds, respectively [58, 59].

4. Shape control through the manipulation of surface diffusion rate

Recently, we demonstrated that the surface diffusion rate could also be manipulated relative

to the atom deposition rate (as determined by the reaction kinetics) to maneuver the shape or

morphology taken by a metal nanocrystal [60]. Surface diffusion is a well-understood process in

surface science that involves the migration of adatoms, molecules, or clusters across the surface

of a solid substrate [61, 62]. At a relatively slow reaction rate, the facet-selectivity of a capping

agent can be used to confine the deposition of atoms to specific sites on the surface of a seed.

This allows us to track the surface diffusion of adatoms during the growth of a nanocrystal and

thus better understand the role of surface diffusion in a shape-controlled synthesis.

Based on the new mechanistic insights, we have recently demonstrated the synthesis of Rh

tetrahedra with concave side faces encased by a mix of {111} and {110} facets [63]. The success

of this synthesis can be attributed to our ability to collectively manipulate the facet selectivity of

a capping agent, the reduction kinetics of a precursor, and the surface diffusion rate of adatoms.

Figure 7 shows a schematic illustration of the growth mechanism and structural characterizations

of the as-obtained Rh concave tetrahedra. The Rh concave tetrahedra could only be generated at

a moderate reduction rate for the RhIII precursor at 145 oC through the use of a proper polyol,

together with the use of a right ligand to coordinate with the RhIII ions. Compared with other

polyols, tri-ethylene glycol was found to have the right reducing power to work with ascorbic

acid as a capping agent for Rh(111) surface. As schematically illustrated in Figure 7a, the newly

formed atoms were selectively deposited onto the corner sites of small Rh tetrahedral seeds

formed through self-nucleation in the initial stage of a synthesis. Subsequently, the deposited Rh

atoms could diffuse from the corner sites to the edges, while their diffusion to side faces was

inhibited by the capping effect of citric acid, to generate the concave side faces. The surface

diffusion rate of the adatoms was highly dependent to the reaction temperature. When the

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synthesis was conducted at 125 oC, both the reduction of RhIII and surface diffusion would be

slowed down, favoring the formation of Rh hierarchical tetrahedra. The suppression of surface

diffusion helped confine Rh atoms to the corner sites, leading to their eventual evolution into an

additional tetrahedral unit at the corner site of each tetrahedral seed. On the contrary, when the

temperature was increased from 145 to 165 oC, both the reduction rate of RhIII and the surface

diffusion of the Rh atoms selectively deposited at the corners were accelerated. The size of the

resultant Rh nanocrystals was slightly reduced due to the increase in number for the tetrahedral

seeds formed in the initial stage. Meanwhile, the concave structure became less significant

because more atoms deposited at the corner sites diffused to the edges and even side faces.

5. Shape control through the management of oxidation etching

For all noble metals, the zero-valent species, including atoms, clusters, and small crystallites,

can all be oxidized back to the ionic forms in the presence of an appropriate oxidative etchant.

We have demonstrated that the O2 from air, when coupled with a coordination ligand, especially

a halide such as Cl－, Br

－, or I

－, could serve as an effective etchant in a solution-phase synthesis

[64]. The etching prefers to selectively start from the defect sites rather than a region with perfect

crystallinity. As a result, in the synthesis of Rh nanocrystals, the use of a salt precursor with Cl−

in it (e.g., Na3RhCl6 or RhCl3) tended to result in the formation of Rh nanocrystals with a single-

crystal structure [34, 35, 38, 51, 58]. This is because the O2 dissolved form air in the solution,

even in a limited amount, can work with Cl− ions to serve as an etchant. As a result, to Rh

twinned nanocrystals in high yields, we have to choose a salt precursor that does not contain any

halide to completely eliminate the etching process. To this end, we have demonstrated that the

use of a halide-free precursor such as [(CF3COO)2Rh]2 could indeed lead to an increase in yield

for the starfish-like Rh nanocrystals with a five-fold twinned structure (Figure 8a) [65]. HRTEM

analysis indicated that these five-fold twinned Rh nanocrystals with five branched arms evolved

from small Rh decahedra formed in the initial stage of a synthesis (Figure 8b). The effect of

precursor type on the crystallinity of resultant Rh nanocrystals was also systematically

investigated to clarify the role of halide. It can be concluded that in the presence of Cl− (due to

the use of Na3RhCl6 or [Rh(CF3COO)2]2 plus HCl as the precursor), the products were

dominated by single-crystal Rh nanocrystals in the form of tripods and other irregular shapes

(Figure 8, c and d). Conversely, nanocrystals containing at least one twin defect were obtained

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when Cl−−free precursors such as [Rh(CH3COO)2]2 or [Rh(CF3COO)2]2 plus citric acid were

used (Figure 8, e and f).

In addition to the control of crystallinity, anisotropic or site-selective etching can also be

used to control the facets expressed on the surface of single-crystal nanoparticles, showing great

promise in the generation of nanocrystals with concave or frame structures [66]. For example,

Guo and coworkers reported the synthesis of Rh concave nanocubes with high-index facets on

the surface through a site-selective etching approach [67]. The concavity of Rh nanocubes could

be further manipulated by controlling the concentration of HCl that served as an etchant in the

synthesis. Recently, our group developed a template-directed route to generating Rh nanoframes

with a highly open structure [59]. Using Pd nanocubes as a template, we could selectively

deposit Rh atoms only at the corner and edge sites due to the blocking effect of the Br－ ions

chemisorbed on the Pd{100} side faces, generating Pd–Rh core-frame nanocubes (Figure 9, a-c).

Because there is a difference in resistance to corrosion between Pd and Rh, we were able to

selectively remove the Pd cores from the Pd-Rh core–frame nanocubes using FeIII/Br－ as the

etchant, generating Rh cubic nanoframes (Figure 9d). Interestingly, this strategy could be extend

to the syntheses of other types of Rh nanoframes with a variety of different morphologies, such

as cuboctahedral and octahedral Rh nanoframes by using Pd cuboctahedra and octahedra as the

templates, respectively [68]. These frame-like Rh nanocrystals possess large surface areas and a

unique hollow and open structure, showing great value in catalytic applications [69].

6. Shape control through electrochemical alternation

In recent years, noble-metal nanocrystals with high-index facets have received ever

increasing attention due to the presence of steps and kinks in high densities for catalytic

applications. In general, the specific surface free energies of a nanocrystal made of an fcc metal

increase in the order of γ111 <γ100 <γ110 < γhkl (with at least one of the h, k, l values being larger

than one), when no capping agent is involved [24, 25, 70]. For Rh nanocrystals, it is more

difficult than other noble metals to overturn the order of the surface free energies between low-

and high-index facets through preferential adsorption of capping agents because of the extremely

high specific surface free energy [45]. As a result, no convex Rh nanocrystal enclosed by high-

index facets has been observed in a conventional solution-phase synthesis. To overcome this

limitation placed by thermodynamics, Sun and coworkers have developed an electrochemical

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square-wave-potential (SWP) approach to the generation of convex noble-metal nanocrystals

with high-index facets, including Pt tetrahexahedra (THH), Pt hexoctahedra (HOH) and Pd THH

[71-73]. The formation of high-index facets on the surface of a nanocrystal during this approach

can be attributed to the dynamic oxygen adsorption/desorption process. Very recently, they

extended this method to the synthesis of convex nanocrystals of Rh THH enclosed by high-index

{830} facets [45]. Figure 10 shows the morphological and structural characterizations of the Rh

THH nanocrystals obtained by SWP (f=100 Hz) with an upper potential limit (EU) at 0.70 V and

a lower potential limit (EL) at −0.07 V (vs. the saturated calomel electrode or SCE) for a growth

time of 45 min. Detailed analysis suggested these Rh THH nanocrystals were enclosed by the

{830} facets, which were periodically consisted of two {310} sub-facets followed by a {210}

sub-facet, as illustrated in Figure 10e. The calculation result indicated the {830} high-index

facets have a high density (4.61×1014 cm-2) of step atoms with a low coordination number of six,

which can provide plenty of active sites for catalytic reactions. Another attractive feature of this

method is that no additional capping agent is introduced into the reaction system and therefore

the obtained nanocrystals are supposed to have a very clean surface for the catalytic reaction.

7. Catalytic applications of shape-controlled Rh nanocrystals

Similar to other noble metals, Rh nanocrystals exhibit extraordinary catalytic performance

and structure sensitivity for a myriad of chemical reactions, including hydrogenation, CO

oxidation, and hydroformylation, as well as various electrochemical oxidation reactions [14, 15,

74-82]. Although Rh-based catalysts have already been applied to many industrial processes,

study of the correlation between the catalytic performance of Rh nanocrystals and their shapes

just started to emerge in recent years. Shape-controlled synthesis of Rh nanocrystals has played

an important role in enabling such studies by selectively exposing a specific type of facet on the

surface. For example, octahedral and tetrahedral Rh nanocrystals are solely covered by {111}

facets whereas {100} facets are exclusively exposed on the surface of cubic Rh nanocrystals.

Son and coworkers conducted a systematic stud of the hydrogenation of arenes catalyzed by

tetrahedral and spherical Rh nanocrystals [36]. Prior to the catalytic tests, the Rh nanocrystals

were immobilized on activated charcoal. To evaluate both activity and selectivity, anthracene

was chosen as a model system because three main products could be formed after hydrogenation,

i.e., those derived from hydrogenation of the central ring (B), two side rings (C), and only one

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side ring (D). As shown in Table 1, Rh tetrahedra on charcoal showed excellent activity and

selectivity toward product (D) as compared with either spherical or commercial Rh/C catalysts.

Remarkably, the catalytic activity of Rh tetrahedra were six times and over 100-fold higher than

those of spherical particles and commercial Rh/C catalyst, respectively. The superior catalytic

performance of Rh tetrahedra can be attributed to the surface of a tetrahedral nanocrystal, which

is solely covered by the more active {111} facets. In comparison, the spherical particles were

enclosed by a mix of {111} and the less active {100} planes.

The dependence of catalytic activity on the shape of Rh nanocrystals can also be probed

through electrochemical measurements. To this end, Sun and coworkers recently compared the

electrochemical properties of Rh THH nanocrystals, Rh irregular nanoparticles (S1 and S2), and

commercial Rh black [45]. Figure 11a shows the typical cyclic voltammograms (CVs) in 0.1 M

aqueous H2SO4 solution. Apparently, THH Rh nanocrystals could promote the adsorption of O2

(the C1 region) in a lower potential region to give a higher electric charge density, supporting

fact that the Rh THH nanocrystals had a higher density of low-coordinated atoms on the surface.

The electro-oxidation of CO and ethanol was also conducted to evaluate the electrochemical

catalytic performance of Rh THH nanocrystals. As shown in Figure 11b, the onset potential and

peak potential of CO oxidation on the Rh THH nanocrystals were obviously more negative than

that on the Rh irregular nanoparticles or commercial Rh black. For ethanol oxidation reaction,

the peak current density on the linear sweep voltammograms was used to evaluate the catalytic

activity. As shown in Figure 11c, the Rh THH nanocrystals exhibited the highest electro-

catalytic activity of 2.69 mA/cm2. The long-term chronoampermetric measurements for ethanol

oxidation indicated that both the catalytic activity and durability of the Rh THH nanocrystals

were greatly improved during the entire period of time, compared to both the irregular Rh

nanoparticles and commercial Rh black (Figure 11d). These results directly demonstrate the

superiority of the Rh nanocrystals with high-index facets in an electrocatalytic application.

8. Conclusion and outlook

For most of the reactions catalyzed by Rh nanocrystals, there is a strong correlation between

the activity/selectivity and the atomic structure on the surface. Such a correlation has triggered a

large amount of research activities in recent years with regard to the synthesis of Rh nanocrystals

with well-controlled shapes. Most of these new developments have been briefly discussed in this

13

short review article, including those based on the use of a surface capping agent to selectively

enrich the expression of a specific type of facet; the control of reduction kinetics and the

manipulation of surface diffusion rate to achieve site-selective deposition; the management of

oxidative etching to generate nanocrystals with twin structures; and the development of high-

index facets through electrochemical modification. In choosing the examples, we have attempted

to single out the key factor that determines the final shape taken by the Rh nanocrystals in each

synthetic method. These recent advancements in methodology development have allowed people

to generate Rh nanocrystals with a number of unique shapes. Compared to other PGMs, however,

the scope of shape-controlled syntheses for Rh nanocrystals is rather limited due to the extremely

high surface free energy of Rh. In this respect, further effort is still needed in developing new

routes to Rh nanocrystals with many other well-defined shapes, as well as the demonstration of

scaling-up capability. The shape- and thus facet-controlled Rh nanocrystals would provide a

great opportunity to systematically investigate the relationship between the surface structure and

catalytic activity/selectivity, leading to the development of rules and guidelines for the rational

design of Rh catalysts with substantially enhanced performance or new catalytic properties.

Acknowledgement

This work was supported in part by the National Science Foundation (DMR-1215034) and

startup funds from the Georgia Institute of Technology. S.X. was also supported by the

Fundamental Research Funds for the Central Universities of China (Grant No. 20720140529)

and the National Natural Science Foundation of China (Grant No. 21401155).

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21

Table 1. A comparison of catalytic activity and selectivity for the Rh nanocrystals supported on

charcoal toward the hydrogenation of anthracene. (Reproduced with the permission of Ref. [36].

Copyright Wiley-VCH, 2007)

Entry Catalyst Pressure

(atom)

Time

(h)

Conv.

(%)

Selectivity [%]

B C D

1 Tetrahedral Rh/C 10 0.5 100 0.7 0.0 99.3

2 Tetrahedral Rh/C 1 2 100 2.0 0.0 98.0

3 Spherical Rh/C 1 2 99.5 6.7 76.3 17.0

4 Commercial Rh/C 10 0.5 57.3 6.4 81.0 12.6

5 Commercial Rh/C 1 2 4.8 35.4 45.8 18.8

22

Figure 1. A comparison of the specific surface energies of different types of facets and the

corresponding shapes for nanocrystals made of Rh, Pt, Pd, and Au. From left to right, the shapes

correspond to octahedron, trisoctahedron (TOH), trapezohedron (TPH), rhombic dodecahedron

(RD), tetrahexahedron (THH), and cube. (Reproduced with permission from Ref. [45]. Copyright

Wiley-VCH, 2014)

23

Figure 2. (a) A schematic illustration showing the polyol synthesis of Rh nanocubes. (b) Shape

distributions of Rh nanocrystals synthesized in the presence (left) and the absence (right) of

trimethyl(tetradecyl)ammonium bromide (TTAB). (Reproduced with permission from Ref. [51].

Copyright The American Chemical Society, 2008) (c) TEM images of the as-obtained Rh

nanocubes. The scale bar in the insert is 10 nm. (Reproduced with permission from Ref. [52].

Copyright The American Chemical Society, 2011) (d) A summary of the four-stage process

involved in the formation of Rh nanocubes from RhCl3. (Reproduced with permission from Ref.

[53]. Copyright The American Chemical Society, 2012)

24

Figure 3. (a) TEM image (top view) of the Rh nanoplates with four different profiles and their

distributions. (b) TEM and HRTEM images (cross-sectional view) of the Rh nanoplates and their

thickness distribution. (c) A plausible mechanism for the growth of Rh nanoplates under the

assistance of oleylamine. (Reproduced with permission from Ref. [54]. Copyright The American

Chemical Society, 2010)

25

Figure 4. (a) Low-magnification TEM image of the single-layered Rh nanosheets. (b) High-

magnification TEM image of a single-layered Rh nanosheet. (c) Aberration-corrected

microscopy image of a single-layered Rh nanosheet and (insert) the corresponding filtered image

using the crystallographic average method to improve signal-to-noise ratio. (d) AFM image and

the corresponding heights of a bare Rh nanosheets. (Reproduced with permission from Ref. [55].

Copyright Nature Publishing Group, 2014)

26

Figure 5. Molecular structures of (a) rhodium carbonyl chloride and (b) rhodium acetylacetonate.

TEM and HRTEM images (insets) of (c) tetrahedral and (d) spherical Rh nanoparticles prepared

from rhodium carbonyl chloride and rhodium acetylacetonate, respectively. (Reproduced with

permission from Refs [36]. Copyright Wiley-VCH, 2007)

27

Figure 6. Morphological and structural characterizations of Rh concave nanocubes prepared at

140 oC with an injection rate of 4.0 mL/h. (a, b) TEM images of the as-prepared samples, and (c-

e) HRTEM images of individual concave nanocubes recorded along the [100], [110], and [111]

zone axes. The inset in (a) and (b) show a typical SEM image of the concave nanocubes and the

3D model, respectively. (Reproduced with permission from Ref. [58]. Copyright The American

Chemical Society, 2011)

28

Figure 7. (a) A schematic illustration showing the major steps involved in the formation of an

Rh concave tetrahedron: (1) corner-selected deposition, (2) diffusion from corners to edges, and

(3) diffusion from corners/edges to side faces. (b-e) Structural characterizations of the Rh

concave tetrahedrons synthesized in tri-ethylene glycol at 145 oC: (b) low-magnification TEM

image, (c) tilted TEM images of two Rh concave tetrahedra, (d, e) high-resolution high-angle

annular dark-field scanning TEM (HAADF STEM) images of a concave tetrahedron recorded

along [111] and [211] zone axes, respectively, together with the corresponding atomic models in

the insets. The scale bars in (c) are 10 nm. (Reproduced with permission from Ref.

[63].Copyright The American Chemical Society, 2011)

29

Figure 8. (a) TEM image of a typical sample of Rh nanocrystals with a penta-twinned starfish

structure. (b) TEM image of Rh nanocrystals obtained 1 min after the injection of precursor. The

inset shows a HR-TEM image taken from the sample showing its penta-twinned crystal structure.

(c-f) TEM images of Rh nanocrystals synthesized using (c) Na3RhCl6, (d) [Rh(CF3COO)2]2 +

HCl, (e) [Rh(CH3COO)2]2, and (f) [Rh(CF3COO)2]2 + citric acid, as precursor, respectively.

Arrows in (f) indicated the formation of Rh nanoplates in the presence of citric acid, which

should be attributed to their strong binding to the {111} facets. (Reproduced with the permission

of Ref [65]. Copyright Wiley-VCH, 2011.)

30

Figure 9. Synthesis of bimetallic Pd-Rh core-frame nanocubes and Rh cubic nanoframes via

site-selected overgrowth and selectively etching of the Pd cores. (a) A schematic illustration of

the proposed mechanism. (b) SEM image of the Pd-Rh core-frame nanocubes. (c) HAADF-

STEM image together with the energy dispersive X-ray (EDX) mapping of an individual Pd-Rh

core-frame nanocube. (d) TEM image of the resultant Rh cubic nanoframes. (Reproduced with

permission from Ref. [59], copyright Wiley-VCH, 2012)

31

Figure 10. Tetrahexahedral (THH) nanocrystals of Rh synthesized using the electrochemical

square-wave-potential (SWP) route. (a) Low- and (b) high-magnification SEM images of the as-

obtained nanocrystals. (c) TEM image of a THH Rh nanocrystal and (d) selected area electron

diffraction (SAED) pattern recorded along the [001] direction. (e) Atomic model of the {830}

plane. (Reproduced with permission from Ref. [45]. Copyright Wiley-VCH, 2014.)

32

Figure 11. Electrochemical characterizations of three different types of Rh catalysts:

tetrahexahedral (THH) nanocrystals, irregular nanoparticles (S1 and S2), and commercial Rh

black. (a) Cyclic voltammograms (CVs) in 0.1 M H2SO4 solution; (b) linear sweep

voltammograms (LSV) of CO oxidation in 0.1 M H2SO4 solution; (c) LSVs of ethanol oxidation

in a mixture of 1.0 M ethanol and 1.0 M aqueous NaOH (scan rate: 50 mV/s); (d) current–time

curves for ethanol oxidation at −0.45 V (vs. SCE). (Reproduced with permission from Ref. [45].

Copyright Wiley-VCH, 2014)