Electrochemically Shape-Controlled Synthesis of Pd Concave

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Electrochemically Shape-controlled Synthesis of Pd Concave-Disdyakis Triacontahedra in Deep Eutectic Solvent Lu Wei, Chang-Deng Xu, Long Huang, Zhi-You Zhou, Sheng-Pei Chen, and Shigang Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03580 • Publication Date (Web): 27 May 2015 Downloaded from http://pubs.acs.org on May 30, 2015

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The Journal of Physical Chemistry

Electrochemically Shape-Controlled Synthesis of Pd Concave-Disdyakis Triacontahedra in Deep Eutectic Solvent Lu Wei,†,‡ Chang-Deng Xu,† Long Huang,‡ Zhi-You Zhou,‡ Sheng-Pei Chen,‡ and Shi-Gang Sun*,†,‡ †

‡

College of Energy, Xiamen University, Xiamen 361005, China

State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

* Corresponding authors:

E-mail: [email protected]

Fax: +86-592-2180181

Tel: +86-592-2180181

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Abstract: Concave-disdyakis triacontahedral palladium nanocrystals (C-DTH Pd NCs) bound with 120 {631} high-index facets were prepared by electrochemically shape-controlled method in deep eutectic solvent (DES). It has found that both the adsorption of urea derived from EDS and the upper (EU) and lower (EL) limit potentials of the square-wave potential applied in the synthesis are contributed synergetically in controlling the shape of Pd NCs. The formation of C-DTH Pd NCs with well-defined shape was achieved by the dynamic interaction between urea adsorption at EU and growth at EL. In-situ FTIR spectroscopic studies revealed that the urea adsorbates at EU play a crucial role in shape evolution, especially in the formation of C-DTH Pd NCs. It has demonstrated that the as-synthesized C-DTH Pd NCs enclosed by {631} high-index facets exhibit higher electrocatalytic activity than Pd NCs with other shapes bound by {111} low-index facets (octahedral (OH) and icosahedral (IH)) towards ethanol electrooxidation in alkaline medium.

Keywords: electrochemically shape-controlled synthesis, palladium, disdyakis triacontahedron, deep eutectic solvent

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1. Introduction High-index faceted nanocrystals (HIF-NCs) of face centred cubic (fcc) precious metals (e.g., Pd, Pt, Au, etc.) exhibit markedly higher catalytic activity than those of NCs enclosed by low-index facets (LIFs), since the high-index facets (HIFs) display a high density of low-coordinated atoms such as steps, ledges and kinks, which constitute catalytic centers.1-4 However, the HIFs of fcc metals always possess a high surface energy, and they are disappearing during the growth of a nanoparticle in a conventional synthesis process, owing to that the thermodynamics requires minimization of total surface energy of NCs. As a consequence, only those NCs enclosed by LIFs and with low total surface energy could be obtained. Therefore, the synthesis of NCs bound with HIFs is a big challenge. The breakthrough in synthesizing HIF-NCs has been made firstly by Tian et al.5 They have successfully prepared tetrahexahedral (THH) Pt NCs enclosed by {730} HIFs through development of electrochemically shape-controlled method.5 Furthermore, they have prepared NCs of other platinum group metals and their alloys enclosed by HIFs, including THH Pd, Rh and PdPt alloy NCs,6-8 trapezohedral (TPH) Pt, Pd and PtRh alloy NCs,9-11 and concave hexoctahedral (HOH) Pt NCs12. They have also demonstrated that all of these HIF-NCs exhibited enhanced electrocatalytic performances. Besides the electrochemical route, several groups have also developed wet-chemistry methods to achieve the shape-controlled synthesis of noble metal and alloy NCs enclosed by HIFs, such as concave cubic Au, Pd and Pt NCs,13-18 trisoctahedral (TOH) Au, Ag and AuPd alloy NCs,19-24 Au truncated ditetragonal prisms (TDPs),25,26 HOH Au and AuPd alloy NCs,27-29 THH or elongated THH Au NCs,30-34 and so on. It is worthwhile to point out that most of these above mentioned HIF-NCs were synthesized in aqueous solutions. 3



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Deep eutectic solvent (DES) is an ionic liquids (ILs) analogues, usually formed by a quaternary ammonium or phosphonium salt and a hydrogen bond donor such as amides, carboxylic acids and polyols.35-38 Unlike conventional ILs, the DESs possess advantages of non-toxic, biodegradable and environmental friendship, which make them more suitable as “greener solvents”. Besides, the DESs are stable in both water and air, and much cheaper in comparison with ILs.39 In the past ten years, DESs have attracted increasing interests and have been widely used as a new-style solvent in both fundamental research and applications, including in catalysis or biocatalysis,40-44 and as (co-)solvents in organic and inorganic syntheses.45-49 Especially, DESs have been proven to be effective media for electrodeposition of various metals and alloys,50-55 thanks to their remarkable physicochemical properties such as good conductivity and wide electrochemical potential window. Abbott and co-workers56 have reported that the double-layer capacitance can affect the final morphology of Zn deposited in different DESs. Furthermore, Xing et al.57 have determined that the pulse current also affect the electrodeposition of copper in DES. Very recently, we have found that the DES is a versatile medium in electrochemically shape-controlled synthesis of Pt NCs bounded by HIFs without using any surfactant or stabilizer. We have successively prepared, via electrochemical method in DES, concave THH Pt NCs enclosed by {910} HIFs

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triambic icosahedral (TIH) Pt NCs bound with {771} HIFs.59 Although different nanostructures were synthesized in DES, understanding the role of DES in electrochemically shape-controlled synthesis has been less reported so far. Herein, we report the synthesis of concave-disdyakis triacontahedral Pd nanocrystals (C-DTH Pd NCs) in a ChCl-urea based DES by using electrochemically shape-controlled method. The emphasis is put upon investigation of the role of DES in the shape-controlled synthesis. In-situ FTIR 4


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spectroscopy was employed to trace the adsorbates from DES on a Pd electrode surface to elucidate the interaction of DES with Pd nanoparticles during their growth. Interestingly, a shape evolution of Pd NCs from octahedra (OH) to icosahedra (IH), and finally to C-DTH has been realized by systematically adjusting the upper (EU) and lower (EL) limit potentials of square wave potential used in the electrochemically shape-controlled method. It has found that the EU plays a pivotal role in tuning the shape of Pd NCs due to specific adsorption of urea related species from DES at EU. We have also demonstrated that the as-synthesized C-DTH Pd NCs exhibit higher catalytic activity towards ethanol electrooxidation than that of the OH and IH Pd NCs in alkaline medium.

2. Experimental Section Preparation of DES. Choline chloride and urea were purchased from Shanghai Chemical Reagent Ltd., China. Choline chloride (ChCl) was purified by recrystallization in absolute ethanol, then the solvent was removed by filtering. The purified ChCl crystals were dried under vacuum prior to use. Urea was purified by recrystallization in Millipore water (18.2 MΩ cm) supplied by a Milli-Q Lab apparatus (Nihon Millipore Ltd.), further filtered and dried under vacuum to remove residual water. The preparation of ChCl-urea based DES was carried out by mixing the above two as-purified components with the stated proportion (ChCl/urea=1/2, mole ratio), under stirring at 80 °C until a homogeneous and colorless liquid was formed.58,59 The as-synthesized DES were stored in a vacuum at 80 °C prior to use. Preparation of C-DTH Pd NCs. Electrochemical preparation of C-DTH Pd NCs from DES was carried out in a standard three-electrode cell. A Pt wire counter electrode and a Pt quasi-reference electrode were used. Glassy carbon electrode (GCE, Ф = 6 mm) was chosen as working electrode, which was polished mechanically by successively with alumina powders of size of 5.0, 1.0 and 0.3 5



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µm, and washed ultrasonically in a Millipore water bath. The square-wave potential applied in the synthesis was generated and controlled by a 263A potentiostat/galvanostat (EG&G). In particular, the preparation of C-DTH Pd NCs was achieved in 1 mM PdCl2-DES solution at 60 °C by using a square-wave potential (f = 10 Hz) with the lower (EL) and upper (EU) potentials of –0.40 and 0.05 V, respectively, and with a synthesis time of 45min (as illustrated in Figure S1 in the Supporting Information). SEM and TEM Measurements. Scanning electron microscopy (SEM) images of the samples were obtained using a Hitachi S-4800 electron microscope. Briefly, after finish of each electrodeposition the working electrode was immediately taken out from solution and adequately washed with Millipore water to remove residual solution on the electrode surface. The as-prepared samples, Pd NCs/GCE, were dried under air and then directly characterized by SEM. The SEM size distribution histograms were counted statistically 200 particles for each sample. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) measurements were carried out on a FEI Tecnai-F30 electron microscope with an operating voltage of 300 kV. In-situ FTIR spectroscopy. Electrochemical in-situ FTIR measurements were performed on a Nicolet Nexus 8700 FTIR spectrometer with a liquid-nitrogen-cooled MCT-A detector. The spectroelectrochemical cell consists of a prismatic CaF2 window beveled at 60°, a Pt foil counter electrode and a Pt quasi-reference electrode. Before each FTIR measurement, the working electrode was pushing against the IR window as a result that a thin-layer solution about 10 µm was formed. In this configuration, the IR radiation sequentially passed through the CaF2 window and thin-layer then reflected by working electrode. Unless otherwise stated, the spectra were the average of four hundred interferograms, with a spectral resolution of 8 cm-1. The sample spectra were acquired in the potential region between –0.05 and 0.10 V, and the reference spectrum was collected at –0.32 V. The 6


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spectra are presented as the relative change in reflectivity and calculated as follows, ∆R/R = (R(ES)-R(ER))/R(ER)

(1)

where R(ES) and R(ER) are the single-beam spectra collected at the sample potential ES and reference potential ER, respectively. Electrocatalytic tests. The measurements of electrocatalytic activity of as-synthesized C-DTH Pd NCs were carried out in a three-electrode cell with Pt black sheet counter electrode. A saturated calomel electrode (SCE) was employed as reference electrode, which was separated from bulk electrolyte in the cell by a solution bridge made of a stopcock of two ends. All potentials in the electrochemical activity experiments were quoted versus the SCE scale. In this study the electrocatalytic properties of the as-prepared C-DTH Pd NCs were evaluated in 0.1 M ethanol + 0.1 M NaOH solution at 25 °C and a sweep rate of 50 mV s-1. The test solutions were deaerated by purging with pure N2 gas (Linde Industrial Gases, China >99.999%) before each experiment, then a flux of N2 was kept over the solution during measurements to prevent the interference of atmospheric oxygen.

3. Results and discussion Electrochemical square wave potential has been proven as an efficient and universal method to prepare NCs of platinum group metals and their alloys with well-defined shape in aqueous solutions,5-12 as well as in DES.58,59 In the present work, a novel concave-disdyakis triacontahedral Pd nanocrystals (C-DTH Pd NCs) with highly symmetrical structure was directly electrodeposited on GCE by applying square wave potential in DES, as illustrated in Figure S1. The as-prepared C-DTH Pd CNs were systematically characterized by SEM and TEM. Figure 1a displays a typical SEM image of the C-DTH Pd CNs. The monodispersed star shaped polyhedral Pd NCs of size 7



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223±19 nm are clearly observed. The yield of the C-DTH Pd NCs was measured to be over 80%, and the other shapes, imperfect C-DTH and irregular polyhedral, were concomitantly formed in this synthetic conditions. To further examine the structure of Pd particles, high-resolution SEM was carefully performed. As shown in the inset to Figure 1a, the shape of as-prepared Pd NC matches well with the geometry model of concave DTH60 oriented along a five-fold symmetry (Figure 1d). Additional high-resolution SEM images of the C-DTH Pd NCs with different orientations and the corresponding models are shown in the Figure S2. The geometrical relationship of icosahedron (IH), triambic icosahedron (TIH) and C-DTH is illustrated in detail in Figure S3. It can be revealed that the shape of C-DTH Pd NCs consists of epitaxial growth concave hexangular pyramids along each vertical direction of the exposed (111) facets of IH. To some extent, the concave DTH structure can also be transformed from TIH (Figure S3), namely, one {hhl} facet will be divide into two {hkl} facets when a TIH is transformed to concave DTH. It is similar to the transformation from a TOH to concave HOH.10,28 Therefore, the surface of an as-synthesized C-DTH Pd NC expose 120 {hkl} facets. Figure 1b exhibits a TEM image of a single C-DTH Pd NC viewed along a five-fold symmetry axis, as identified by the corresponding selected area electron diffraction (SAED) pattern (Figure 1c), which illustrates a typical five-fold symmetry SAED pattern of C-DTH Pd NC, by superimposing five [110] direction electron diffraction patterns rotated with respect to each other.61 Obviously, the projection of the C-DTH presents a profile with ten peaks, consistent with the corresponding geometry model (Figure 1d). It indicates that the as-synthesized C-DTH Pd NCs possess a high symmetry. Figure 1e displays a HRTEM image marked with the boxed area in Figure 1b. The distinct continuous fringe pattern confirms the single crystalline property for each pyramid. The lattice spacing of 0.20 and 0.23 nm consist with the distances between two {100} planes and two {111} planes of Pd, respectively. It is worth noting that the edge atomic arrangement, indicated 8


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by the blue dotted line boxed area in Figure 1e, possesses the edge feature of a high-index {631} plane oriented along the [110] direction as illustrated in Figure 1f, which may suggest the presence of high-index {631} facets on the as-prepared C-DTH Pd NCs. The {631} plane shown in Figure 1f was further tilted an angle in order to illustrate the atomic arrangement on the {hkl} facets, which presents a high density of step and kink atoms (Figure 1g). In our previous works,5-12,58,59 the upper limit potential (EU) of the square wave potential played a pivotal role generally in tuning the shape of NCs. Figure 2 shows SEM images of Pd polyhedra electrodeposited on GCE at various values of EU while fixing EL at -0.40 V, under otherwise identical experimental conditions. When the EU was –0.05 V (vs. Pt reference electrode), the resulted nanoparticles were composed of mixed morphologies including perfect OH and IH Pd NCs bounded exclusively by low-index {111}-facets (Figure 2a). When EU increased to 0.00 V, the shape of Pd NCs evolved into malformed IH (Figure 2b). Further increasing EU to 0.025 V, crude stellated polyhedra were obtained (Figure 2c). Perfect C-DTH Pd NCs were achieved with EU at 0.05 V (Figure 2d). However, the perfect shape of Pd NCs further transformed into imperfect C-DTH with further augmenting EU to 0.10 V (Figure 2e). These results demonstrated that the evolution of Pd NCs’ shape from OH, IH to C-DTH is closely dependent on EU, as summarized in Figure 2f. We consider that the dependence of EU in shape evolution of Pd NCs may associate with adsorption of some specific species on Pd surface, presumably urea, which blocks the growth of certain crystal facets. To confirm our assumption, in-situ FTIR reflection spectroscopy was skillfully performed, which could provide information at molecular level to understand the bonding mode and the relative coverage of adsorbed species.62-65 Figure 3a illustrates a set of in-situ FTIR spectra for a polycrystalline Pd electrode immersed in ChCl-urea based DES recorded with both s- and 9



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p-polarized light, respectively. The sample potential (ES) was 0.05 V (vs. Pt reference electrode), and the reference potential (ER) was –0.32 V, at which urea adsorption was not observed. It is well known that the s-polarized spectra contain only information about solution species, while the p-polarized spectra include information of both adsorbates and solution species.62-67 Comparing with the spectrum of s-polarized light, four strong negative-going bands between 1450-1650 cm-1, which corresponding to IR absorption at ES, are clearly observed in the spectrum recorded with p-polarized light (Figure 3a). Two bands, near 1649 and 1571 cm-1, could be assigned to IR absorption of C-O stretching of two bonding geometries of adsorbed urea molecules, i.e. N-bonded via only one of the N atoms and O-bonded via the oxygen of carbonyl group, respectively.62-65 The IR bands near 1493 and 1462 cm-1 could be attributed to the asymmetric C-N stretching of adsorbed urea molecules in these two modes of bonding geometry.62-65 According to surface selection rule of reflection infrared spectroscopy,66,67 the s-polarized IR light could not be absorbed by surface adsorbate species. As a consequence, the four negative-going bands appeared exclusively in the spectrum of p-polarized IR light evidence the adsorption of urea species from DES at 0.05 V, which plays a key role in the formation of the C-DTH Pd NCs. Furthermore, a set of in-situ FTIR spectra of p-polarized IR light acquired at sample potentials range from -0.05 to 0.10 V are illustrated in Fig. 3b. The corresponding spectra of s-polarized IR light are also provided in the Supporting Information (Figure S4). The absence of IR bands corresponding to urea adsorbed species in the spectrum acquired at ES = –0.05 V indicates that the OH Pd NCs and IH Pd NCs are formed without urea adsorbed species. At potentials equal or above 0.00 V, however, the four strong negative-going absorption bands between 1450 and 1650 cm-1 appear in the spectra, and their intensities reach a maximum at 0.05 V. It is worth noting that the absorption band around 1593 cm-1 for C-O stretching in the spectrum acquired at 0.00 V is red-shifted to 1571 cm-1, when the sample potentials are increased to 0.05 and 10


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0.10 V. The above in-situ FTIR spectroscopic results suggest that the change in the bonding mode and the relative coverage of urea adsorbed species depend on ES,65 which is well correlated with the effect of EU on shape evolution of Pd polyhedra (Figure 2). In addition, the presence of urea adsorbed species on the surface of nanoparticles plays an essential role in the formation of the C-DTH Pd NCs with HIFs. The effect of the lower limit potential (EL) of square wave potential on the morphology of Pd NCs was also carefully studied (Figure 4). We found that the polyhedra of DTH shape were formed at the potential region of EL between –0.60 and –0.35 V (EU was kept constant at 0.05 V) under otherwise identical experimental conditions. The perfect C-DTH Pd NCs were obtained at EL between –0.40 and –0.50 V, as shown in Figure 1 and Figure 4b. When the EL was beyond this potential region, i.e. EL < –0.60 V and EL > –0.35 V, the Pd NCs appear in crude DTH shape (Figure 4a and c), indicating that the deposition rate is faster at EL < –0.60 V or slower at EL > –0.35 V than the deposition rate for the formation of perfect C-DTH. However, when a constant low potential (such as –0.40 V) was applied, OH Pd NCs and flower-like aggregates of Pd nanoparticles of pyramid sharp with different size were obtained (Figure 5a). Applying a constant high potential of 0.05 V, moreover, there were not any Pd particles formed on the surface of GCE (Figure 5b). These results imply that the shape evolution is manipulated by the dynamic interaction between the adsorbed urea formed at EU and the growth at EL. To better understand growth mechanism of the C-DTH Pd NCs, the synthetic process along with increasing growth time was carefully monitored by using SEM, as illustrated in Figure 6a and Figure S5. Quasi-spherical Pd nanoparticles about 45 nm in size were obtained at 10 min, which served as seeds in the formation of C-DTH Pd NCs. Along with increasing the growth time, a rudiment DTH was formed at 20 min, and then further grown into a truncated C-DTH at 30 min. The perfect 11



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C-DTH shape was formed at growth time of 45 min or longer. From the above SEM and in-situ FTIR data, the growth mechanism of C-DTH Pd NCs could be preliminarily suggested in Figure 6b. The quasi-spherical Pd seeds are formed in the initial phase of growth. Then the seeds further grow into the DTH rudiments, truncated C-DTH, and finally to perfect C-DTH. In the crystal growth process, the urea adsorbed species formed at EU are preferentially bonded on the side facets of pyramid on the surface of NCs, which could protect these side facets. As a result, Pd atoms derived from Pd2+ reduction at the EL are preferentially added to the truncated section sites of the truncated C-DTH, and finally grow into a perfect C-DTH (Figure 6b). To estimate the electrocatalytic activity of the C-DTH Pd NCs, electrooxidation of ethanol in alkaline media is chosen as probe reaction.1,6,68 For comparison, the as-synthesized OH Pd NCs and IH Pd NCs enclosed by {111} facets are used as the reference samples. The current densities were normalized to the electrochemical active surface areas (ECSAs), which was determined from the electric charge of hydrogen adsorption/desorption on Pd surfaces (Figure 7a). The normalized current density per ECSA is used to compare the electrocatalytic activities of different samples towards ethanol electrooxidation. Figure 7b compares cyclic voltammetric (CV) curves recorded on the C-DTH Pd NCs and the mixed morphologies of OH Pd NCs and IH Pd NCs (as shown in Figure 1a) in 0.1 M ethanol + 0.1 M NaOH solution. In the forward (positive) potential sweep, the peak current densities of ethanol electrooxidation on C-DTH Pd NCs and that on the mixed morphologies of OH Pd NCs and IH Pd NCs were 2.55 and 1.55 mA cm-2, respectively; and the corresponding values were 3.78 and 2.51 mA cm-2, respectively, in the negative-going potential sweep. These results clearly demonstrated that the electrocatalytic activity of the C-DTH Pt NCs towards ethanol electrooxidation is higher than that of the OH Pd NCs and IH Pd NCs. The outstanding electrocatalytic performance of the C-DTH Pd NCs is mainly attributed to the presence of 12


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high-index {hkl}-facets on the surface, which possess a high density of low-coordinate atomic steps and kinks.

4. Conclusion Concave DTH Pd NCs bounded by 120 {631} HIFs were synthesized in ChCl-urea based DES via an electrochemically shape-controlled method. The role of the DES in shape-controlled synthesis was investigated by using in-situ FTIR spectroscopy, and the shape evolution of Pd NCs was illustrated by SEM and studied systematically by adjusting the EU and EL of square wave potential that applied in the synthesis process. It has demonstrated that urea adsorbed species derived from DES at the EU played a pivotal role in the formation of C-DTH Pd NCs, by fixing EL at -0.40 V and increasing progressively EU from -0.05 V to 0.05 V the Pd NCs changes their shape from mixed morphologies of perfect OH and IH enclosed by {111} LIFs to perfect C-DTH bound with {631} HIFs. When the EU was set at 0.05V, perfect C-DTH Pd NCs could be obtained at EL between –0.40 and –0.50 V, while crude DTH shape were yielded in potential region of EL < –0.60 V and EL > –0.35 V. The results illustrated that the shape evolution is manipulated by the dynamic interaction between the adsorbed urea formed at EU and the growth at EL. We have also demonstrated that the as-prepared C-DTH Pd NCs exhibit a high catalytic activity towards ethanol electrooxidation, which was attributed to the HIFs enclosed the C-DTH Pd NCs. The current study presents a new strategy for controlling surface structure and growth of Pd NCs in DES.

Acknowledgments. This study was supported financially by NSFC (21361140374, 21229301, 21321062).

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Supporting Information Available. Figures S1–S5. This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes (1) Tian, N.; Zhou, Z. Y.; Sun, S. G. Platinum Metal Catalysts of High-Index Surfaces: From Single-Crystal Planes to Electrochemically Shape-Controlled Nanoparticles. J. Phys. Chem. C 2008, 112, 19801-19817. (2) Zhang, H.; Jin, M.; Xia, Y. Noble-Metal Nanocrystals with Concave Surfaces: Synthesis and Applications. Angew. Chem., Int. Ed. 2012, 51, 7656-7673. (3) Zhang, L.; Niu, W.; Xu, G. Synthesis and Applications of Noble Metal Nanocrystals with High-Energy Facets. Nano Today 2012, 7, 586-605. (4) Quan, Z.; Wang, Y.; Fang, J. High-Index Faceted Noble Metal Nanocrystals. Acc. Chem. Res. 2013, 46, 191-202. (5) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity. Science 2007, 316, 732-735. (6) 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. (7) Yu, N. F.; Tian, N.; Zhou, Z. Y.; Huang, L.; Xiao, J.; Wen, Y. H.; Sun, S. G. Electrochemical Synthesis of Tetrahexahedral Rhodium Nanocrystals with Extraordinarily High Surface Energy and High Electrocatalytic Activity. Angew. Chem., Int. Ed. 2014, 53, 5097-5101.

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of

an

Open-Framework

Zirconium

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CO2/CH4 Adsorption Ratio. Angew. Chem., Int. Ed. 2011, 50, 8139-8142.

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Palladium Nanocubes with High-Index Surfaces and High Electrocatalytic Activities. Chem. Eur. J. 2011, 17, 9915-9919.

Figure Captions Figure 1 (a) SEM image of C-DTH Pd NCs. The insets are size histogram and high-resolution SEM image of C-DTH Pd NC, respectively. TEM image (b), SAED pattern (c) and (d) Geometry model of C-DTH Pd NC oriented along a five-fold symmetry, respectively. (e) High-resolution TEM image of the area in the white box marked in Figure 1b. (f) and (g) Atomic model of the Pd{631} plane.

Figure 2 (a-e) SEM images of Pd NCs electrodeposited on GCE in 1 mM PdCl2-DES solution at 60 °C by square-wave potential: EL = −0.40 V and EU = −0.05, 0, 0.025, 0.05, 0.10 V, respectively , at f = 10 Hz for 45 min. (f) Illustration of shape evolution of polyhedral Pd NCs by adjusting EU.

Figure 3 (a) In-situ FTIR spectra obtained with a polycrystalline Pd electrode immersed in a ChCl-urea based DES: reference potential, −0.32 V. 400 interferograms were collected at each potential with either p- or s-polarized light, spectral resolution: 8 cm-1. (b) The spectra of p-polarized light at different sample potentials.

Figure 4 SEM images of Pd NCs electrodeposited on GCE in 1 mM PdCl2-DES solution at 60 °C by square-wave potential: EL = (a) −0.60, (b) −0.50, (c) −0.35 V and EU = 0.05 V at f = 10 Hz for 45 min, respectively. 22


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Figure 5. SEM images electrodeposited Pd NCs on GCE in 1 mM PdCl2-DES solution at 60 °C by applying a constant potential at E = -0.40 V (a) and 0.05 V (b) for 45 min, respectively.

Figure 6 (a) SEM images of Pd NPs electrodeposited on GCE for different time, showing the growth process of C-DTH Pd NCs. Scale bars are 100 nm. (b) Schematic illustration of the proposed growth mechanism of C-DTH Pd NCs.

Figure 7. Cyclic voltammograms recorded with C-DTH Pd NCs (red line), mixture of OH and IH Pd NCs (black line) in 0.1 M HClO4 solution (a) and 0.1 M ethanol + 0.1 M NaOH solution (b), respectively. The cyclic voltammograms were recorded at room temperature and at a scan rate of 50 mV s−1.

23



a

d = 223±19 nm

Frequency / %

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

20 15 10 5 0 180

200

220

240

260

500 nm

d / nm

c

b

d

{220} {111} {200}

100 nm

e

f

g

{631}

0.23 nm

0.23 nm 0.20 nm

1 nm

[110]

Figure 1

24


{631}

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a

b

500 nm

500 nm

c

d

500 nm

500 nm

e

f EU Pd2+

SWP

500 nm

Pd polyhedrons

Figure 2

25



a

Es / V

△R / R=1.0×10-3

0.05 (s)

0.05 (p)

O H2N

1800

1700

1600

NH2

1500

1400

1300

~ ν / (cm-1)

b

Es / V 0.10 V

0.05 V

△R / R=1.0×10-3

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

0 .00 V

-0.05 V

1800

1700

1600

1500

~ ν / (cm-1)

1400

1300

Figure 3

26


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a

500 nm

b

500 nm

c

200 nm

Figure 4

27



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

500 nm

b

500 nm

Figure 5

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a

10 min

20 min

30 min

45 min

60 min

80 min

Growth time O

b

Pd

2+

C

Pd atom

Pd NP

H2N

NH2

Truncated C-DTH Pd NC

Figure 6

29


C-DTH Pd NC


C-DTH Pd NCs OH and IH Pd NCs

a

4 -2

0.2

j / mA cm

-2

0.4

j / mA cm

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

0.0 -0.2

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C-DTH Pd NCs OH and IH Pd NCs

b

3 2 1 0

-0.4

-1 -1.0

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

E / V (vs. SCE)

-0.8

-0.6

-0.4

-0.2

E / V (vs. SCE) Figure 7

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0.0

0.2

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SYNOPSIS TOC Concave-disdyakis triacontahedral Pd nanocrystals (C-DTH Pd NCs) have been successively prepared by electrochemically shape-controlled synthesis method in deep eutectic solvent (DES). The formation of the C-DTH Pd NCs with well-defined shape is controlled by the dynamic interplay between surface adsorption of urea species derived from DES at upper limit potential (EU) and growth at lower limit potential (EL) of the square-wave potential.

EU Pd2+

SWP

Pd polyhedrons

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Electrochemically Shape-Controlled Synthesis of Pd Concave

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