Concentration-Mediated Shape Evolution of Palladium Nanocrystals

Subscriber access provided by UNIV OF BARCELONA

Article

Concentration-Mediated Shape Evolution of Palladium Nanocrystals and Their Structure-Electrocatalytic Functionality Lu Wei, Yu-Jie Mao, Yong-Sheng Wei, Jian-Wei Li, XinMing Nie, Xin-Sheng Zhao, You-Jun Fan, and Shi-Gang Sun Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00892 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Concentration-Mediated Shape Evolution of Palladium Nanocrystals and Their Structure-Electrocatalytic Functionality

Lu Wei,1 Yu-Jie Mao,1,2 Yong-Sheng Wei,1 Jian-Wei Li,1 Xin-Ming Nie,1 Xin-Sheng Zhao,*1 You-Jun Fan,*3 Shi-Gang Sun*2

1

Hydrogen Research Lab for Energy Storage and Application, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China.

2

State Key Lab of PCOSS, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.

3

Guangxi Key Laboratory of Low Carbon Energy Materials, School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, China

* Corresponding authors: E-mail: [email protected]; [email protected]; [email protected]

Fax: +86-592-2180181 Tel: +86-516 83500485; +86-592-2180181

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

ABSTRACT: We reported a new insight into electrochemical shape evolution of Pd nanocrystals (NCs) by controlling the concentration of Pd precursor, while the lower (EL) and upper (EU) potential limits of square-wave potential were optimized at –0.8 and 0 V, respectively. It was found that the ratio (R) of the growth rate in the to that of the on a crystal increased with the concentration of PdCl2 getting higher in the reaction, leading to the shape transformation of Pd polyhedron from cubes to truncated octahedra, and finally to octahedra. The electro-oxidation performances of CO and formic acid over the as-synthesized Pd polyhedrons were in the order of octahedra < truncated octahedra < cubes, indicating that the electrocatalytic activities of Pd polyhedrons were highly sensitive to the surface structure.

KEYWORDS: Palladium, Electrochemical shape evolution, Electrocatalysts, Structure-catalytic functionality, Deep eutectic solvent


Page 3 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Palladium nanocrystals (Pd NCs) with well-controlled atomic arrangement on the surface have received an increasing scientific interest due to the shape-dependence of catalytic reactions in the currently important applications, including alkene hydrogenation,1 Suzuki coupling,2-4 small organic molecules (SOMs) (methanol, ethanol, formic acid, etc.) electro-oxidations and oxygen reduction reaction (ORR) involved in a fuel cell.5-10 Numerous efforts have been made, in the past decades, to study the shape-controlled synthesis of Pd NCs with well-defined shapes by using wet-chemical and electrochemical strategies. Various shapes of Pd NCs including tetrahedron,11 octahedron,11-13 decahedron,11 icosahedron,11 cube or concave cube,12-15 rhombic dodecahedron,13 tetrahexahedron (THH),16 etc., have been synthesized by manipulating the thermodynamic and kinetic growth conditions. Thermodynamically, the adsorption of surfactants,11,15,17 halides (I-, Br- and Cl-)13,18,19 and small molecules (CO, H2, amines, etc.)20-23 on the surface of a crystal can selectively stabilize certain crystallographic planes through adsorptions and prevent them from disappearing during the growth process. For instance, the bromide ions prefers to adsorb on the Pd{100} facets, which can stabilize these facets and hence lead to the formation of cubic Pd NCs.18 Dai and coworkers demonstrated that CO specifically binds to Pd{111} facets and thus promotes the formation of Pd tetrapod and tetrahedral nanocrystals enclosed by {111} facets.20 Moreover, the surface structures of Pd NCs can also be tailored by the kinetic control over the crystal growth. For example, concave cubic Pd NCs were synthesized through a kinetically controlled process by manipulating the concentrations of reagents, including Pd precursor, additive of KBr, and reductant of ascorbic acid.14 For a cubic seed, the reactivity of different sites is supposed to increase in the order of side faces < edges < corners. The newly formed Pd atoms would be preferentially added to the corners and edges of the Pd cubic seed, and resulting in the formation of a concave nanocube. Besides, the electrochemical route has been proven to be a powerful strategy to tune the shape of ACS Paragon Plus Environment


Page 4 of 23

NCs, in which both the thermodynamics and kinetics during the crystals nucleation and growth can be easily controlled through adjusting potential, current or current density, precursor concentration, etc. Tian and coworkers developed a electrochemical square-wave potential method to synthesize THH Pd NCs enclosed by {730} high-index facets through manipulating the lower (EL) and upper (EU) potential limits in aqueous systems.16 Recently, we have demonstrated that the non-aqueous systems, deep eutectic solvents (DESs), are alternative electrolytes in electrochemically shape-controlled synthesis of metal nanostructures without using any surfactant or stabilizer. For example, noble metals (Pt, Pd, Au) NCs with well-defined shapes including concave THH,24 triambic

icosahedron,25

concave

disdyakis

triacontahedron,26

concave

hexoctahedra

and

trisoctahedra27 were fabricated by electrochemical methods in DESs. Remarkably, the shape evolution of Au NCs successively from concave rhombic dodecahedra to concave cubes, octopods, cuboctahedral boxes, and finally to hollow octahedra was achieved by controlling the growth overpotentials in a choline chloride-urea (ChCl-U) based DES.28 DESs, a new class of eutectic-based ionic liquids (ILs), can be produced using quaternary ammonium salts mixed with hydrogen bond donors such as acids, amides and alcohols.29-31 These liquids are non-toxic, biodegradable, environmentally friendly, and particularly exhibit remarkable physicochemical properties like good conductivity and wide electrochemical potential window. In addition, DESs contain abundant electroactive adsorption species such as choline cations, chloride ion and urea, which preferentially adsorb on particular crystal faces through electrochemical adsorption, serving as structure mediator in shape-controlled synthesis of nanocrystals.26,28,32,33 Herein, tuning the surface structure of Pd polyhedra by adjusting the Pd precursor concentration was explored via an electrochemical square wave potential method in a ChCl-U based DES. It has demonstrated that the concentration of Pd precursor played a vital role in the shape evolution of Pd ACS Paragon Plus Environment

Page 5 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NCs from cubes to truncated octahedrons and finally to octahedrons, because the ratio of growth rate in the to that of the is growing along with the increase of Pd precursor concentration. The structure-catalytic functionality of as-synthesized Pd NCs was evaluated towards the oxidation of CO and formic acid, indicating that the electrocatalytic activities were in the order of octahedrons < truncated octahedrons < cubes.

EXPERIMENTAL SECTION Chemical. Palladium chloride (PdCl2, AR reagent), choline chloride (HOC2H4N(CH3)3Cl, 99%), urea (CO(NH2)2, >99%), formic acid (HCOOH, AR reagent), perchloric acid (HClO4, GR reagent) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Choline chloride and urea were recrystallized from absolute ethanol and ultrapure water (18.0 MΩ·cm), respectively. Other chemicals were used as received without further purification. Synthesis of DES. Choline chloride-urea (ChCl-U) based DES was synthesized by a typical method reported in our previous works.24-28 The DES was made by stirring a mixture of purified choline chloride and urea with a mole ratio of 1 to 2 at 80 °C until a homogeneous, colourless liquid was obtained. The as-prepared DES was kept in a vacuum at 80 °C prior to use. Electrodeposition of Pd NCs. Electrochemical preparation of Pd NCs was carried out in a standard three-electrode cell connected to a 263A potentiostat/galvanostat (EG&G), with a platinum wire counter electrode and a platinum quasi-reference electrode. The volume of the cell is 50 mL. The working electrode was a glassy carbon disk (GC, Ф = 6 mm), which was polished mechanically by using successively finer Al2O3 powders with sizes of 5, 1, and 0.3 μm and then cleaned ultrasonically in an ultrapure water bath. By varying the Pd precursor concentration in a range from 1 to 5 mM, a series of Pd polyhedra were electrodeposited directly on GC substrate in 20 mL PdCl2-DES solutions at 60 C by applying an optimized square-wave potential (f = 10 Hz) with the ACS Paragon Plus Environment


Page 6 of 23

lower (EL) and upper (EU) potential limits of –0.8 and 0 V, respectively. Structural and electrochemical characterizations. The morphology and structure of the as-synthesized Pd NCs was analyzed by scanning electron microscopy (SEM, S-4800) and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100). The amount of Pd on the electrodes was determined by an inductively coupled plasma-mass spectrometry (ICP-MS, Thermo ICAP-QC). The electrocatalytic performances of the Pd NCs were measured in 0.1 M formic acid + 0.1 M HClO4 solution at room temperature (25 C). CO stripping voltammograms were obtained by oxidizing pre-adsorbed CO (COad) in 0.1 M HClO4 solution at a scan rate of 50 mV s−1. CO was first bubbled into 0.1 M HClO4 for 15 min to allow saturated adsorption of CO onto the Pd NCs while maintaining the potential at –0.1 V, and then the dissolved CO in the electrolyte was removed by purging with N2 for 20 min. The solutions were deaerated by purging with pure N2 gas before experiment, and a flux of N2 was kept over the solution during measurements to prevent the interference of atmospheric oxygen. A saturated calomel electrode (SCE) was used as reference electrode, and all potentials in the electrochemical performance tests are quoted versus the SCE scale.

RESULTS AND DISCUSSION In the case of electrodeposition, the preferred orientation has been studied as a function of bath composition, current density, potential, temperature and pH, etc.34-36 Herein, we mainly focus on how to control the preferred orientation of Pd crystals by varying the precursor concentration. Figure 1(a-e) exhibit a set of SEM images of the Pd NCs electrodeposited in 1, 2, 3, 4 and 5 mM PdCl2-DES solutions by employing an square-wave potential (f = 10 Hz) with EL (–0.8 V) and EU (0 V) (vs. Pt reference electrode), respectively. The settings of EL and EU potential limits refer to the electrodeposition behavior of Pd in DES (Figure S1, see the Supplementary Information (SI)). At the ACS Paragon Plus Environment

Page 7 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Increasing the Pd precursor concentration

1 mM

a

50 nm

2 mM

3 mM

4mM

5 mM

b

c

d

e

50 nm

50 nm

50 nm

50 nm

f1

f2

f3

i1

i2

l1

l2

20 nm

20 nm

20 nm

20 nm

20 nm

20 nm

20 nm

g1

g2

g3

j1

j2

m1

m2

h1

h2

h3

k1

k2

n1

n2

[001]

[011]

[111]

[011]

[001]

[011]

[111]

Figure 1. (a-e) SEM images of the Pd NCs electrodeposited in different concentration of PdCl2-DES solutions: (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5 mM. (f1-3, i1,2 and l1,2) TEM images, (g1-3, j1,2 and m1,2) geometric models and (h1-3, k1,2 and n1,2) SAED patterns of individual Pd NCs obtained in 2, 3 and 4 mM of PdCl2-DES solutions, respectively. Top is an illustration of the shape evolution of Pd NCs by controlling the precursor concentration. optimized EL and EU potential limits, the Pd nanocubes enclosed by {100} facets were formed in 1 and 2 mM PdCl2-DES solutions (Figure 1(a, b)). To confirm the cubic structure, the as-prepared Pd NCs were further characterized by TEM. Figure 1(f1-3) show the typical TEM images of individual Pd NCs oriented along [001], [011] and [111] directions, respectively, determined by the selected area electron diffraction (SAED) patterns (Figure 1(h1-3)). Clearly, the TEM projections of Pd NCs are well in agreement with the oriented geometric models (Figure 1(g1-3)), suggesting the structure of as-prepared Pd NCs is a cube. When the concentration of PdCl2-DES solution was increased to 3 mM, the shape of Pd NCs was transformed into a truncated octahedron bounded by {100} and {111} facets (Figure 1c). Figure 1(i1-2) present the TEM images of individual the truncated octahedral Pd ACS Paragon Plus Environment


Page 8 of 23

NCs viewed along the [011] and [001] directions, respectively. The TEM projected images match well with those of oriented geometric models along [011] and [001] directions (Figure 1(j1-2)), indicating the Pd NCs possessing truncated octahedral shape. Interestingly, as the concentration of PdCl2-DES solution further increased to 4 mM or higher (5 mM), the shape of as-synthesized Pd NCs was eventually evolved into an octahedron enclosed by {111} facets (Figure 1(d, e)). The octahedral shape was also further determined by tilted TEM images along [011] and [111] directions, respectively, which was well consistent with those of oriented geometric models of octahedron (Figure 1(l1-2, m1-2)). In addition, the SAED patterns demonstrated that the resultant Pd NCs possess single-crystalline structure (Figure 1(h1-3, k1-2, and n1-2)). More details of large-area SEM images are given in Figure S2 (see SI). These results demonstrated that the shape evolution of Pd NCs from cubes to truncated octahedrons and finally to octahedrons is highly dependent on the concentration of Pd precursor.

a v < v

θ

b {100}

{111}

{100}

{111}

R=0.58

R=0.70

R=0.58

R=0.70

R=0.87

R=1.0

R=1.15 R=1.73

R =R=0.87 v R=1.0 / vR=1.15 R=1.73

R = v / v Figure 2. (a) Illustration of crystal shape evolution growing along the and directions. (b) The shape conversion from a cube into an octahedron with increased the ratio, R, of the growth rate in the to that of the .


Page 9 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


It is well-known that the final shapes of crystals are closely related to the surface energies or the growth velocities of crystal faces. As for face-centered cubic (fcc) metals (Pt, Pd, Au, etc.), the surface energy (γ) of (111) plane is lower than that of (100) plane, i.e. γ111 < γ100. According to the Bravais law, the growth velocities of different crystal faces depend on the atomic population density (reticular density) of the faces. The higher the number of atoms per unit area of the face, the growth rate in the direction normal to the face is slower. In other words, the growth rate of the facets of high surface energy is faster than those of low surface energy. Therefore the growth velocity (v) of (111) plane is slower than that of (100) plane, v111 < v100. On the basis of conservation law of crystal plane angle,37 the dihedral angle (θ) of (111) plane and (100) plane keeps constant during the growth process of a crystal (Figure 2a). Because of v111 < v100, however, the proportion of {100} facets on the surface is eliminated during their growth; conversely, the proportion of {111} facets is gradually increasing and eventually the surface is exclusively enclosed by {111} facets (Figure 2a). Therefore, the final shapes of Pd NCs are highly dependent on the growth velocities of {100} and {111} facets on a crystal. Wang38 has reported the shapes of cubooctahedral NCs as the ratio, R, of the growth rate in the to that of the . As shown in Figure 2b, the shape conversion from a cube into an octahedron is achieved by increasing the R value. In detail, when R is less than or equal to 0.58 (R ≤ 0.58), a cube forms. While between 0.58 and 1.73, the polyhedrons including truncated cube (0.58 < R < 0.87), cuboctahedron (R = 0.87) and truncated octahedron (0.87 < R < 1.73) are obtained, respectively. When R is greater than or equal to 1.73 (R ≥ 1.73), the shape of crystal transforms absolutely into an octahedron. It was found that the R value (the ratio of the growth rate in the to that of the ) was associated with the concentration of Pd precursor. For instance, Xia and coworkers have reported that the shape evolution of Pd polyhedrons was manipulated by adjusting the ratio of Pd precursor to the seed. 12 When the 18 nm Pd nanocubes was used as seeds, the transformation of a Pd cube into an octahedron was obtained by increasing the concentration of ACS Paragon Plus Environment


a

Page 10 of 23

b

200 nm

200 nm

c

d

200 nm

200 nm

Figure 3. SEM images of Pd NCs electrodeposited in 2 mM PdCl2-DES solution by square-wave potential: EL = −0.8 V and EU = −0.1 V (a), −0.05 V (b), 0.05V (c), and 0.1 V (d), respectively, at f = 10 Hz for 30 min. Na2PdCl4 in the reaction. In this study, similarly, the ratio (R) of the growth rate in the to that of the on a crystal increased with the concentration of Pd precursor getting higher, and resulting in the shape evolution of Pd NCs from cubes to truncated octahedrons and finally to octahedrons. In our previous studies the EU and EL of square wave potential play an important role in tuning the nanocrystal surface structure.16,24-26 To comprehensively investigate the influence of the EU and EL on the shape evolution of Pd NCs, a series of EU and EL of square wave potential were applied in the synthesis of Pd NCs with a fixed Pd precursor concentration (2 mM) that was formed cubes. As shown in Figure 3, when the value of EU decreased from 0 V to –0.05 V, cubes and other shapes such as tetrahedrons and nanorods were obtained at the fixed EL of –0.8 V. Particularly, the purity of cube is very low. It is worth pointing out that further decreasing of the value of EU to –0.1 V in the typical synthesis did not advantageously form cubes. Likewise, when EU increased from 0 V to 0.05 and 0.1 V, only a few cubes were obtained. Similarly, the influence of the EL on the shape evolution ACS Paragon Plus Environment

Page 11 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

200 nm

c

200 nm

d

200 nm

e

200 nm

f

200 nm

200 nm

Figure 4. SEM images of Pd NCs electrodeposited in 2 mM PdCl2-DES solution by square-wave potential: EU = 0 V and EL = −1.1 V (a), −1.0 V (b), −0.9 V (c), −0.7 V (d), −0.6 V (e) and −0.5 V (f), respectively, at f = 10 Hz for 30 min. of Pd NCs was also investigated. As exhibited in Figure 4, when the values of EL decreased from –0.8 V to –0.9, –1.0 and –1.1 V or increased from –0.8 V to –0.7, –0.6 and –0.5 V, the purity of cube was all gradually decreasing in both variation tendencies. However, when only applying a constant potential at EL (–0.8 V), Pd chains were formed (Figure 5a). As seen from their TEM images, the chains were assembled by a certain number of Pd nanoparticles (Figure 5b-d). Both SAED and high-resolution TEM (HRTEM) characterizations on individual Pd nanoparticles in the chains clearly proved their polycrystalline nature, as shown in the insets in Figure 5b and Figure 5e. While a constant potential at E U (0 V) was employed, the electrodeposition of Pd particles were not obtained (Figure S3, see SI), suggesting the E U only play a role in the selective binding of the species from DES on the surfaces of Pd NCs during their growth. As demonstrated in previous ACS Paragon Plus Environment


Page 12 of 23

a

50 nm

100 100 nm nm

c

b

A

50 nm

B

10 nm

e

d 0.23 nm

0.23 nm

10 nm

5 nm

Figure 5. (a) SEM image of Pd nanochains electrodeposited in 2 mM PdCl2-DES solution by a potentiostatic method at E = –0.8 V. The inset is the high-magnification SEM image. (b) TEM image of Pd nanochains. The inset is the SAED pattern of Pd nanochains. (c and d) HRTEM images of the areas in the A and B white boxes in Figure 5b, respectively. (e) HRTEM image of the area in the white boxes in Figure 5c. reports, the interaction between the Pd nanoparticles and DES was observed at EU during the electrodeposition process,26 and anionic urea species form DES were more favorable binding with elemental metal forming an adsorption complex.39 On the basis of these observations, the shape evolution of Pd NCs from cube to octahedron is incapable to realize by varying the EU and EL potentials. Yet to some extent the surface structure of Pd NCs is controlled by the dynamic interplay between the growth at EL and surface adsorption of urea species at EU. To estimate the structure-activity relationship of the as-synthesized Pd polyhedra, CO and formic ACS Paragon Plus Environment

Page 13 of 23

0.08

{111}

a

{100}

j / mA cm

-2

0.04 0.00 -0.04 Cube Truncated octahedron Octahedron

-0.08

{111}

{100}

-0.2

0.0

0.2

0.4

0.6

E / V (vs. SCE)

j / mA cm

-2

0.8

0.6

b

Cube Truncated octahedron Octahedron

0.4

0.2

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

E / V (vs. SCE) 30 25

j / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c

Cube Truncated octahedron Octahedron

20 15 10 5 0 -0.2

0.0

0.2

0.4

0.6

0.8

E / V (vs. SCE)

Figure 6. (a) The first CV profiles and (b) CO stripping voltammograms of Pd polyhedrons in 0.1 M HClO4 solution, (c) The first CV profiles on Pd polyhedrons in 0.1 M formic acid + 0.1 M HClO4 solution. acid were chosen as probe molecules, which involve in the operation of a direct formic acid fuel cell (DFAFC). Three representative Pd polyhedra (cubes, truncated octahedrons and octahedrons, as shown in Figure 1b, c and d, respectively) were selected for samples to study. The electrochemical behaviors of selected Pd polyhedra were investigated by cyclic voltammetry (CV) measurement, which was performed in 0.1 M HClO4 solution at a scan rate of 50 mV s−1. As shown in Figure 6a, there is different features of redox peaks in the hydrogen adsorption/desorption, which could be ACS Paragon Plus Environment


Page 14 of 23

assigned to different surface arrangements on these Pd polyhedrons. Two pairs of quasi-reversible peaks recorded at ca. 0.025 and –0.11 V in the positive-going scan (at ca. 0.075 and –0.14 V on the negative-going sweep) are related to hydrogen adsorption/desorption on {100} and {111} surfaces, respectively. In the case of cube, only a couple of strong peaks are observed at 0.025 and 0.075 V, indicating that the resulted Pd polyhedrons are principally composed of cubes bounded exclusively by {100} facets. For truncated octahedrons, the peaks of the hydrogen adsorption/desorption on {100} and {111} surfaces are simultaneously observed, but the {100} surfaces is dominant. It is worth noting that, except for a couple of dominant peaks for hydrogen adsorption/desorption on {111} surfaces, the peaks of the hydrogen adsorption/desorption on {100} surfaces are still observed on octahedrons. It suggests that the as-obtained Pd polyhedrons mainly consisted of octahedrons, as well as a few nanoparticles containing {100} surfaces like imperfect octahedrons (truncated octahedrons) and other irregular polyhedrons (see Figure S2 d). However, the peaks of hydrogen adsorption/desorption on {100} and {111} surfaces are going to fade away by long-time potential cycling between -0.21 and 0.6 V in 0.1 M HClO4 electrolyte. For instance, after 100 CV cycles, there is a significant change of CV profile for hydrogen adsorption/desorption can be noticed on the Pd cubes (Figure S4 a). Figure S4 b further shows that the cubic shape has transformed into a near-spherical morphology, which is related to the conjunction of intensive hydrogen insertion/desinsertion process and further electrooxidation of hydrogen inserted on Pd surface.40 Figure 6b shows the COad stripping voltammograms of these Pd polyhedrons. It can be seen that the onset potential of COad electrooxidation on Pd cube is 0.50 V, much lower than that on truncated octahedron (0.53 V) and octahedron (0.55 V). Meanwhile, the potentials of the primary COad stripping peaks have the same tendency and followed the order of cube (0.58 V) < truncated octahedron (0.63 V) < octahedron (0.65 V). The result suggests that the antitoxic of CO on Pd(100) was higher than that on Pd(111). Figure 6c compares the CV curves of formic acid oxidation on the ACS Paragon Plus Environment

Page 15 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


three different Pd polyhedrons. The oxidation current was normalized to the electroactive surface area, which was calculated from the electric charge of the CO stripping on Pd surfaces (seen the SI). Obviously, Pd cubes exhibit higher electrocatalytic activity than truncated octahedrons and octahedrons towards formic acid oxidation. The maximum current densities on the positive-going sweep increased in the order of octahedron (6.78 mA cm-2) < truncated octahedron (10.19 mA cm-2) < cube (23.52 mA cm-2), indicating that the oxidation of formic acid on Pd(100) was much faster than that on Pd(111). Meanwhile, the shape of Pd polyhedron can also affect the peak potential, which is in the order to cube (0.28 V) < truncated octahedron (0.30 V) < octahedron (0.35 V). The electrocatalytic performance of cubic, truncated octahedral and octahedral Pd NCs was also evaluated by the mass activity. As seen from Figure S5, the cubes also exhibit higher mass activity (11.13 mA µgPd-1) than truncated octahedrons (4.16 mA µgPd-1) and octahedrons (2.93 mA µgPd-1) towards formic acid oxidation. These results reveal that the electrocatalytic activities of CO and formic acid oxidations are highly sensitive to the surface structure of Pd polyhedrons. The structure-electrocatalytic functionality of CO and formic acid electro-oxidations increases in the order octahedron < truncated octahedron < cube. It suggests that the electrocatalytic activities of CO and formic acid on Pd(100) are higher than that on Pd(111). This result is well consistent with the previous single crystal studies.41

CONCLUSIONS In summary, the electrochemical shape evolution of Pd polyhedrons from a cube into an octahedron was obtained by controlling the concentration of Pd precursor with optimized the EL and EU of square-wave potential at –0.8 V and 0 V, respectively. It was found that the ratio (R) of the growth rate in the to that of the on a crystal increased with the concentration of PdCl2 getting higher in the reaction. When the concentration was varied from 1 mM to 5 mM, Pd ACS Paragon Plus Environment


Page 16 of 23

polyhedrons with different proportions of {100} to {111} facets on the surface were formed, including cubes, truncated octahedrons and octahedrons. The CO and formic acid oxidations on the as-synthesized Pd NCs were systematically evaluated to insight into the structure-catalytic functionality. The electro-oxidation performances of these reaction over the as-prepared Pd NCs were in the order of octahedron < truncated octahedron < cube, indicating that the electrocatalytic activities of CO and formic acid oxidations are highly sensitive to the surface structure. That is, the catalytic performances for CO and formic acid oxidations over the Pd(100) are higher than that of Pd(111). This study provides a new insight into an electrochemical shape-controlled synthesis of Pd or other noble metal NCs with different surface structure by controlling the precursor concentration in DES.

ASSOCIATED CONTENT Supporting Information Available. Details of electrodeposition behavior of Pd in DES (Figure S1), large-area SEM images of the Pd NCs (Figure S2), SEM image of surface of GC electrode was subjected to a potential at E = 0 V for 30 min in 2 mM PdCl2-DES solution (Figure S3), the long-time potential cycling test (Figure S4), the measure of electrochemical active surface area and the mass activity test (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.


Page 17 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel: +86-516 83500485 * E-mail: [email protected]; Fax: +86-592-2180181; Tel: +86-592-2180181 * E-mail: [email protected]

ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China (21703088, 21776119), Natural Science Fund Project in Jiangsu Province (BK20160210), Science and Technology Innovation Project of Xuzhou (KC18007), Opening Foundation Project of the Guangxi Key Laboratory of Low Carbon Energy Materials (Guangxi Normal University), and Natural Science Foundation of the Jiangsu Higher Education Institutions of China (16KJB510009).

REFERENCES (1) Schauermann, S.; Nilius, N.; Shaikhutdinov, S.; Freund, H.-J. Nanoparticles for heterogeneous catalysis: new mechanistic insights. Acc. Chem. Res. 2013, 46, 1673-1681. (2) Collins, G.; Schmidt, M.; O’Dwyer, C.; McGlacken, G.; Holmes, J. D. Enhanced catalytic activity of high-index faceted palladium nanoparticles in Suzuki–Miyaura coupling due to efficient leaching mechanism. ACS Catal. 2014, 4, 3105-3111. (3) Sreedhala, S.; Sudheeshkumar, V.; Vinod, C. P. Structure sensitive chemical reactivity by palladium concave nanocubes and nanoflowers synthesised by a seed mediated procedure in aqueous medium. Nanoscale 2014, 6, 7496-7502. ACS Paragon Plus Environment


Page 18 of 23

(4) Graham, L.; Collins, G.; Holmes, J. D.; Tilley, R. D. Synthesis and catalytic properties of highly branched palladium nanostructures using seeded growth. Nanoscale 2016, 8, 2867-2874. (5) Sheng, T.; Xu, Y.-F.; Jiang, Y.-X.; Huang, L.; Tian, N.; Zhou, Z.-Y.; Broadwell, I.; Sun, S.-G. Structure design and performance tuning of nanomaterials for electrochemical energy conversion and storage. Acc. Chem. Res. 2016, 49, 2569-2577. (6) Li, J.-T.; Zhou, Z.-Y.; Broadwell, I.; Sun, S.-G. In-situ infrared spectroscopic studies of electrochemical energy conversion and storage. Acc. Chem. Res. 2012, 45, 485-494. (7) Zhou, Z.-Y.; Tian, N.; Li, J.-T.; Broadwell, I.; Sun, S.-G. Nanomaterials of high surface energy with exceptional properties in catalysis and energy storage. Chem. Soc. Rev. 2011, 40, 4167-4185. (8) Wang, A.-J.; Li, F.-F.; Bai, Z.; Feng, J.-J. Large-scale electrosynthesis of Pd nanodendrites and their improved electrocatalytic properties for methanol oxidation. Electrochim. Acta 2012, 85, 685-692. (9) Zheng, J.-N.; Zhang, M.; Li, F.-F.; Li, S.-S.; Wang, A.-J.; Feng, J.-J. Facile synthesis of Pd nanochains with enhanced electrocatalytic performance for formic acid oxidation. Electrochim. Acta 2014, 130, 446-452. (10) Zhang, X.-F.; Chen, Y.; Zhang, L.; Wang, A.-J.; Wu, L.-J.; Wang, Z.-G.; Feng, J.-J. Poly-l-lysine mediated synthesis of palladium nanochain networks and nanodendrites as highly efficient electrocatalysts for formic acid oxidation and hydrogen evolution. J. Colloid Inter. Sci. 2018, 516, 325-331. (11) Niu, Z.; Peng, Q.; Gong, M.; Rong, H.; Li, Y. Oleylamine-Mediated Shape Evolution of Palladium Nanocrystals. Angew. Chem. Int. Ed. 2011, 50, 6315-6319. (12) Jin, M.; Zhang, H.; Xie, Z.; Xia, Y. Palladium nanocrystals enclosed by {100} and {111} facets in controlled proportions and their catalytic activities for formic acid oxidation. Energy Environ. Sci. 2012, 5, 6352-6357. ACS Paragon Plus Environment

Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Niu, W.; Zhang, L.; Xu, G. Shape-controlled synthesis of single-crystalline palladium nanocrystals. ACS Nano 2010, 4, 1987-1996. (14) Jin, M.; Zhang, H.; Xie, Z.; Xia, Y. Palladium concave nanocubes with high-index facets and their enhanced catalytic properties. Angew. Chem. Int. Ed. 2011, 50, 7850-7854. (15) Zhang, J.; Zhang, L.; Xie, S.; Kuang, Q.; Han, X.; Xie, Z.; Zheng, L. Synthesis of concave palladium nanocubes with high-index surfaces and high electrocatalytic activities. Chem. Eur. J. 2011, 17, 9915-9919. (16) Tian, N.; Zhou, Z.-Y.; Yu, N.-F.; Wang, L.-Y.; Sun, S.-G. Direct electrodeposition of tetrahexahedral Pd nanocrystals with high-index facets and high catalytic activity for ethanol electrooxidation. J. Am. Chem. Soc. 2010, 132, 7580-7581. (17) Sreedhala, S.; Sudheeshkumar, V.; Vinod, C. P. CO oxidation on large high-index faceted Pd nanostructures. J. Catal. 2016, 337, 138-144. (18) Peng, H.-C.; Xie, S.; Park, J.; Xia, X.; Xia, Y. Quantitative Analysis of the Coverage Density of Br– Ions on Pd{100} Facets and Its Role in Controlling the Shape of Pd Nanocrystals. J. Am. Chem. Soc. 2013, 135, 3780-3783. (19) Huang, X.; Zhang, H.; Guo, C.; Zhou, Z.; Zheng, N. Simplifying the creation of hollow metallic nanostructures: one ‐ pot synthesis of hollow palladium/platinum single-crystalline nanocubes. Angew. Chem. Int. Ed. 2009, 48, 4808-4812. (20) Dai, Y.; Mu, X.; Tan, Y.; Lin, K.; Yang, Z.; Zheng, N.; Fu, G. Carbon monoxide-assisted synthesis of single-crystalline Pd tetrapod nanocrystals through hydride formation. J. Am. Chem. Soc. 2012, 134, 7073-7080. (21) Huang, X.; Tang, S.; Yang, J.; Tan, Y.; Zheng, N. Etching growth under surface confinement: an effective strategy to prepare mesocrystalline Pd nanocorolla. J. Am. Chem. Soc. 2011, 133, 15946-15949. ACS Paragon Plus Environment


Page 20 of 23

(22) Wu, B.; Zheng, N.; Fu, G. Small molecules control the formation of Pt nanocrystals: a key role of carbon monoxide in the synthesis of Pt nanocubes. Chem. Commun. 2011, 47, 1039-1041. (23) Lu, B.-A.; Du, J.-H.; Sheng, T.; Tian, N.; Xiao, J.; Liu, L.; Xu, B.-B.; Zhou, Z.-Y.; Sun, S.-G. Hydrogen adsorption-mediated synthesis of concave Pt nanocubes and their enhanced electrocatalytic activity. Nanoscale 2016, 8, 11559-11564. (24) Wei, L.; Fan, Y.-J.; Tian, N.; Zhou, Z.-Y.; Zhao, X.-Q.; Mao, B.-W.; Sun, S.-G. Electrochemically shape-controlled synthesis in deep eutectic solvents—A new route to prepare Pt nanocrystals enclosed by high-index facets with high catalytic activity. J. Phys. Chem. C 2012, 116, 2040-2044. (25) Wei, L.; Zhou, Z.-Y.; Chen, S.-P.; Xu, C.-D.; Su, D.; Schuster, M. E.; Sun, S.-G. Electrochemically shape-controlled synthesis in deep eutectic solvents: triambic icosahedral platinum nanocrystals with high-index facets and their enhanced catalytic activity. Chem. Commun. 2013, 49, 11152-11153. (26) Wei, L.; Xu, C.-D.; Huang, L.; Zhou, Z.-Y.; Chen, S.-P.; Sun, S.-G. Electrochemically shape-controlled synthesis of Pd concave-disdyakis triacontahedra in deep eutectic solvent. J. Phys. Chem. C 2016, 120, 15569-15577. (27) Wei, L.; Sheng, T.; Ye, J.-Y.; Lu, B.-A.; Tian, N.; Zhou, Z.-Y.; Zhao, X.-S.; Sun, S.-G. Seeds and potentials mediated synthesis of high-index faceted gold nanocrystals with enhanced electrocatalytic activities. Langmuir 2017, 33, 6991-6998. (28) Wei, L.; Lu, B.; Sun, M.; Tian, N.; Zhou, Z.; Xu, B.; Zhao, X.; Sun, S. Overpotential-dependent shape evolution of gold nanocrystals grown in a deep eutectic solvent. Nano Res. 2016, 9, 3547-3557. (29) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel solvent properties of choline chloride/urea mixtures. Chem. Commun. 2003, 70-71. ACS Paragon Plus Environment

Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic liquids. J. Am. Chem. Soc. 2004, 126, 9142-9147. (31) Abbott, A. P.; Harris, R. C.; Ryder, K. S. Application of hole theory to define ionic liquids by their transport properties. J. Phys. Chem. B 2007, 111, 4910-4913. (32) Hammons, J. A.; Muselle, T.; Ustarroz, J.; Tzedaki, M.; Raes, M.; Hubin, A.; Terryn, H. Application of hole theory to define ionic liquids by their transport properties. J. Phys. Chem. C 2013, 117, 14381-14389. (33) Abbott, A. P.; Barron, J. C.; Frisch, G.; Gurman, S.; Ryde, K. S.; Silva, A. F. Double layer effects on metal nucleation in deep eutectic solvents. Phys. Chem. Chem. Phys. 2011, 13, 10224-10231. (34) Nishimura, T.; Nakade, T.; Morikawa, T.; Inoue, H. Effect of current density on electrochemical shape control of Pt nanoparticles. Electrochim. Acta 2014, 129, 152-159. (35) Ohta, N.; Inokuma, K.; Miyabayashi, K.; Miyake, M.; Yagi, I. Electrochemical Fabrication of Cubic-Shaped Pt Nanoparticles onto Carbon Fiber Electrodes. Electrochemistry 2010, 78, 132-135. (36) Luo, B. B.; Li, X. M.; Li, X. L.; Xue, L. P.; Li, S. Y.; Li, X. L. Copper nanocubes and nanostructured cuprous oxide prepared by surfactant-assisted electrochemical deposition. CrstEngComm 2013, 15, 5654-5659. (37) Mullin, J. W. Crystallization. Third Edition. Oxford: Butterworth 1993, 172. (38) Wang, Z. L. Transmission electron microscopy of shape-controlled nanocrystals and their assemblies. J. Phys. Chem. B 2000, 104, 1153-1175. (39) Rimsza, J. M. ; Corrales, L. R. Adsorption complexes of copper and copper oxide in the deep eutectic solvent 2: 1 urea–choline chloride. Comput. Theor. Chem. 2012, 987, 57-61. (40) Zadick, A.; Dubau, L.; Zalineeva, A.; Coutanceau, C.; Chatenet, Marian. When cubic ACS Paragon Plus Environment


Page 22 of 23

nanoparticles get spherical: An Identical Location Transmission Electron Microscopy case study with Pd in alkaline media. Electrochem. Commun. 2014, 48, 1-4. (41) Hoshi, N.; Kida, K.; Nakamura, M.; Nakada, M.; Osada, K. Structural effects of electrochemical oxidation of formic acid on single crystal electrodes of palladium. J. Phys. Chem. B 2006, 110, 12480-12484.


Page 23 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Use Only

Title：Concentration-Mediated Shape Evolution of Palladium Nanocrystals and Their Structure-Electrocatalytic Functionality

Author(s): Lu Wei, Yu-Jie Mao, Yong-Sheng Wei, Jian-Wei Li, Xin-Ming Nie, Xin-Sheng Zhao, You-Jun Fan, Shi-Gang Sun

This study provides a new insight into electrochemical shape evolution of Pd nanocrystals by controlling the precursor concentration. It was found that the ratio of the growth rate in the to that of the on a crystal increased with the precursor concentration getting higher in the reaction, resulting in the shape transformation from cubes to octahedrons.


Concentration-Mediated Shape Evolution of Palladium Nanocrystals

Recommend Documents