Seeds and Potentials Mediated Synthesis of High-Index Faceted Gold

Jun 28, 2017 - Because high-index facets (HIFs) possess high surface energy, the metal nanoparticles enclosed with HIFs are eliminated during their gr...
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Seeds and Potentials Mediated Synthesis of High-Index Faceted Gold Nanocrystals with Enhanced Electrocatalytic Activities Lu Wei,*,†,‡ Tian Sheng,‡ Jin-Yu Ye,‡ Bang-An Lu,‡ Na Tian,‡ Zhi-You Zhou,‡ Xin-Sheng Zhao,† and Shi-Gang Sun*,‡ †

Department of Physics, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China State Key Lab of PCOSS, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

‡

S Supporting Information *

ABSTRACT: Because high-index facets (HIFs) possess high surface energy, the metal nanoparticles enclosed with HIFs are eliminated during their growth in a conventional shapecontrolled synthesis due to the thermodynamics that drives the particles minimizing their total surface energy. This study develops a double-step potential method to prepare an unprecedentedly stellated Au nanocrystals (NCs) bounded by high-index {711} and {331} facets in deep eutectic solvent (DES) medium. The formation of Au NCs bounded by HIFs was systematically studied. It has demonstrated that the shapes of Au NCs are strongly dependent on the size of seeds and the growth potentials as well as the urea adsorbates in the DES. By adjusting the size of seeds and the growth potentials, the stellated Au NCs can be transformed into concave hexoctahedra (HOH) with high-index {421} facets and concave trisoctahedra (TOH) with high-index {991} facets. The electrocatalytic activities of the as-prepared Au NCs are evaluated by glucose oxidation. Thanks to HIFs having high density of atomic steps and kinks, the stellated, TOH, and HOH Au NCs exhibit higher electrocatalytic activity than that of the polycrystalline Au electrode, demonstrating that the steps and kinks serve as the active sites and play an important role in glucose electro-oxidation.

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INTRODUCTION Gold nanocrystals (Au NCs) have been extensively utilized as catalysts in fuel cells,1−3 sensors,4−7 and surface-enhanced Raman spectroscopy (SERS)8−10 due to their unique properties. Considerable studies demonstrated that both catalytic activities and surface plasmon resonance (SPR) behaviors of Au NCs are closely dependent on their surface structures, which are defined by the shapes of the NCs.11−19 Among the synthesized Au NCs reported so far, high-index faceted Au NCs (HIF-Au NCs) generally exhibit higher activity than those with low-index facets (LIFs), such as {111} or {100}, because of high-index facets (HIFs) possessing a high density of low coordination number atoms with superior reactivity.11−19 However, the synthesis of NCs with HIFs is a big challenge because the inherent thermodynamic instability of the HIFs makes them disappearing during crystal growth. To date, the synthetic strategy for Au NCs enclosed by HIFs is still limited. There are only a few surfactant-based chemical approaches to synthesize the HIF-Au NCs, including concave cubes,20 tetrahexahedra (THH),21−23 trisoctahedra (TOH),24,25 hexoctahedra (HOH),17 truncated ditetragonal nanoprisms (TDPs),26,27 hexagram and star shapes,18,19 etc. In these synthetic strategies, the stabilizers or capping agents are essential, which play a key role in modulation of facets that are exposed on the surface of a nanocrystal. However, the surfaces of the as-synthesized NCs are always covered by © XXXX American Chemical Society

surfactant molecules, which deactivate the surfaces of NCs and consequently hinder their applications, especially in heterogeneous catalysis and SERS.28−32 We develop here an electrochemical method to synthesize HIF-Au NCs in deep eutectic solvent (DES) without any surfactants. The DES is a nontoxic, biodegradable, and environmentally friendly ionic liquid (IL) analogue, usually comprising of quaternary ammonium or phosphonium salt and hydrogen bond donor such as amides, carboxylic acids, and polyols.33−36 Moreover, the DES has remarkable physicochemical properties such as good conductivity and wide electrochemical potential window, in which the effect of hydrogen evolution can be effectively avoided during the metal electrodeposition. Recently, DES has been proven to be an effective medium in electrochemically shape-controlled synthesis of metal NCs with HIFs.37−39 Herein HIF-Au NCs with a novel stellated shape, as well as concave HOH and concave TOH, were successfully synthesized by a double-step potential method in DES via controlling the size of seeds and the growth potentials. The electrochemical measurements and density functional theory (DFT) calculations were performed to gain insight into the eletrocatalytic Received: March 22, 2017 Revised: June 27, 2017 Published: June 28, 2017 A

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Figure 1. (a) SEM image of stellated Au NCs. Inset is the particle size histogram. (b) High-resolution SEM (c−e) TEM images and corresponding SAED patterns of stellated Au NCs oriented along the [001], [111], and [011] directions, respectively. (f, g) High-resolution TEM images of the A and B areas in the white boxes in (e1), respectively. (h) Atomic models of Au(771) and (331) planes. Insets are the corresponding geometric model images of stellated Au NCs oriented along the different directions. bath. All the samples were prepared by double-step potential in a ChCl-urea based DES solution containing 24.28 mM HAuCl4 at 60 °C. In a typical synthesis, the stellated Au NCs were prepared as follows: The GC working electrode was first subjected at nucleation potential (EN) of −0.97 V (vs Pt) for 0.25 s to generate Au nuclei. Then the growth of the Au nuclei into stellated Au NCs was achieved at growth potential (Egrowth) of −0.50 V for 500 s, as shown in Scheme S1. Under otherwise identical experimental conditions, the concave TOH Au NCs were obtained at Egrowth of −0.45 V, while the concave HOH Au NCs were prepared with the presence of the large size Au seeds that were deposited at −0.97 V of EN for 1.50 s. Structure Characterization. Scanning electron microscopy (SEM) characterizations of the as-prepared Au NCs were performed on a Hitachi S4800 scanning electron microscope with a field emission electron gun. Transmission electron microscopy (TEM) images were taken using a FEI Tecnai-F30 high-resolution transmission electron microscope operated at 300 kV. Electrochemical Measurements. The electrochemical measurements were carried out in a three-electrode cell with Pt black sheet as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The electrochemically active surface area (EAS) of Au catalysts were estimated from the electric charge of the reduction of a full monolayer of Au oxides in cyclic voltammetry performed at 50 mV s−1 in 0.5 M H2SO4 solution at 25 °C. The electrocatalytic properties of the as-prepared Au NCs were evaluated by linear sweep voltammetry at 50 mV s−1 in 0.1 M NaOH solution containing 10 mM D-glucose at 25 °C. Prior to each electrochemical experiment, the test solutions were deaerated by purging high-purity N2 gas (>99.999%, Linde Industrial Gases, China); then a flux of N2

properties of the HIF-Au NCs. In comparison with polycrystalline Au electrode, the as-synthesized HIF-Au NCs demonstrate superior catalytic activity toward D-glucose electro-oxidation in basic media, ascribing to the HIFs that contain a high density of low-coordinated step and kink atoms with excellent reactivity.

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EXPERIMENTAL SECTION

Materials. Choline chloride (ChCl) (HOC2H4N(CH3)3Cl, 99%), urea (CO(NH2)2, >99%), hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, 99.9%), D-glucose (C6H12O6, AR reagent), sodium hydroxide (NaOH, GR reagent), and sulfuric acid (H2SO4, GR reagent) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). ChCl and urea were recrystallized from absolute ethanol and ultrapure water (18.0 MΩ cm), respectively. Other chemicals were used without further purification prior to use. Synthesis of DES. The DES was prepared as described previously.37−40 In brief, a mixture of purified ChCl and urea with a stated proportion (ChCl:urea = 1:2, mole ratio) was added into a beaker and stirred at 80 °C until a homogeneous and colorless liquid formed. Then the as-synthesized DES was kept in a vacuum at 80 °C prior to use. Preparation of Au NCs with High-Index Faces. The high-index faceted Au NCs were electrodeposited on GC substrate in a threeelectrode cell connecting a 263A potentiostat/galvanostat (EG&G), with a Pt quasi-reference electrode as reference and a Pt wire as counter electrode. Prior to the electrodeposition, the GC electrode (ϕ = 6 mm, Takai Carbon Co., Ltd., Tokyo, Japan) was polished mechanically by successively with alumina powders with sizes of 5.0, 1.0, and 0.3 μm and then cleaned ultrasonically in an ultrapure water B

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Langmuir was kept over the solution to prevent the interference of atmospheric oxygen during measurements. Computational Methods. All the calculations were realized with the Perdew−Burke−Ernzerh (PBE) generalized gradient approximation (GGA) exchange-correlation functional using the Vienna Abinitio Simulation Package.41−47 The projector-augmented-wave (PAW) pseudopotentials were used to describe the core electron interaction. The cutoff energy was set as 400 eV. The vacuum region layers were built more than 12 Å to ensure the slab interaction was eliminated. A p(4 × 4) supercell was used for modeling the Au(111) surface with 64 atoms. The stepped Au(331), Au(511), and Au(421) surfaces were modeled by 96 atoms. The surface models are shown in Figure S1, and k-point sampling for Brillouin zones is listed in Table S1 (see the Supporting Information). During the geometry optimization processes, the bottom half atoms were fixed in their bulk position and the top half atoms were allowed to remove. The transition states were located with a constrained optimization approach with the force converge criteria below 0.05 eV/Å in modified VASP.48−50

three typical projection images agree well with the profiles of the corresponding geometric models (see the insets to Figure 1c−e). The SAED patterns demonstrated that the assynthesized Au NCs possess single-crystalline structure (Figure 1c2−e2). To further determine the surface structure that is defined by Miller indices of the facets on the stellated Au NCs, the HRTEM was carefully performed. Figure 1f,g shows the HRTEM images recorded from the boxed areas of A and B marked in Figure 1e1. It can be clearly seen that the surface atomic arrangement of Au NCs fits well with the features of atomic models of {711} and {331} planes (Figure 1h), respectively, indicating that the surfaces of the stellated Au NCs consist of high-index {711} and {331} facets. The formation of crystals usually involves two processes: nucleation and growth.51 In this study, the former is used for adjusting the size and structure of seeds by controlling the parameters of EN and tN, and the latter is employed to determine the crystal growth rate by mediating the Egrowth and the crystal size by manipulating the tgrowth. Therefore, the shape and size of crystals are strongly dependent on the parameters of nucleation and growth. Figure 2 displays SEM images of the fresh GC surface (Figure 2a), the Au seeds formed at −0.97 V for 0.25 (Figure 2c) and 1.50 s (Figure 2e), and the Au NCs

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RESULTS AND DISCUSSION In a typical synthesis, a double-step potential strategy was developed to synthesize novel stellated Au NCs with HIFs by adjusting the parameters of nucleation and growth, that is, nucleating potential (EN), nucleating time (tN), growth potential (Egrowth), and growth time (tgrowth). As illustrated in Scheme S1 (see Supporting Information), the first step potential controls the formation of Au seeds. Subsequently, the growth of the seeds into stellated Au NCs is governed by the second step potential. When the four parameters of EN, tN, Egrowth, and tgrowth were set at −0.97 V, 0.25 s, −0.50 V, and 500 s, respectively, the stellated Au NCs were successfully electrodeposited on the GC substrate. Figure 1a,b shows the representative SEM images of the as-synthesized Au NCs. As seen from the low-magnification SEM image (Figure 1a), the star-shaped Au particles are the dominant products with a high yield of about 90%. The size of the stellated Au particles is measured to be 280 ± 12 nm (see the inset to Figure 1a), which was determined by statistics of over 200 Au nanoparticles. To better confirm the shape of the as-synthesized Au NCs, the high-resolution SEM characterization of stellated Au NCs with four different orientations were carefully examined (Figure 1b). It can be clearly seen that the as-synthesized Au NCs present an unprecedentedly stellated structure, which possess a highly symmetrical geometry. The geometric structure of the stellated Au crystal can be viewed as a concave TOH with eight apexes by overgrowth of ⟨111⟩ crystal axis directions. It can be also regarded as a concave cube with the centers of each face formed concave quartet pyramids along the ⟨100⟩ crystal axis directions. The surface structures of the stellated Au NCs were further investigated by using TEM and high-resolution TEM (HRTEM). Figure 1c−e depicts the typical TEM images, corresponding selected-area electron diffraction (SAED) patterns, and oriented geometric models of stellated Au NCs viewed along the [001], [111], and [011] directions, respectively. In the case of [001] direction, the projection image of Au NC presents an eight-pointed star-like by alternating with two types of apex angles of 45° and 100° (Figure 1c1). When the view was tilted to the [111] direction, the projection image of Au NC is a hexagonal star-shape with apex angles of 70° (Figure 1d1). As for the sample oriented along [011] direction, another hexagram-like projection image consisting of four slender angles of 60° and two stubby angles of 113° was observed, as shown in Figure 1e1. Obviously, these

Figure 2. (a) SEM image of GC electrode before electrodeposition. (b) SEM image of concave rhombic dodecahedron Au NCs with excavated {110} facets electrodeposited at −0.50 V for 500 s, growth without Au seeds. (c) SEM image of Au seeds on GC electrode electrodeposited at −0.97 V for 0.25 s; then the Au nanoseeds were grown into stellated Au NCs (d) by electrodeposited at −0.50 V for 500 s. (e) SEM image of Au seeds on GC electrode electrodeposited at −0.97 V for 1.50 s; then the Au seeds were grown into concave HOH Au NCs (f) by electrodeposited at −0.50 V for 500 s. (g) TEM image, (h) geometric model, and (i) SAED pattern of concave HOH Au NC oriented along the [011] direction. Bottom: a table showing the calculated values for the angles α, β, and γ when the HOH is bounded by {421} crystallographic facets. C

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Langmuir grown at −0.50 V for 500 s on fresh GC (Figure 2b) and started with seeds of different size (Figure 2d,f). The corresponding low-magnification SEM images of large viewing area are shown in Figure S2 (see Supporting Information). It is obvious that in the synthesis of the stellated Au NCs the presence of seeds with size of several nanometers is essential (Figure 2c,d). When the nucleation step was skipped, a concave structure of Au rhombic dodecahedron (RD) NCs was obtained (Figure 2b and Figure S2b). The as-prepared concave RD NCs are viewed as a RD with the centers of each {110} face excavated to form rhombic pyramid-shaped depressions. When replacing the small Au seeds with large ones of about 67 nm (Figure S2e), the concave HOH Au NCs with yield of about 60% were obtained (Figure 2f and Figure S2f). To identify the Miller indices of the facets exposed on the as-prepared HOH Au NCs, the TEM was carefully carried out by tuning the viewing angle to the [011] direction.17 Figure 2g displays a typical TEM image of single HOH Au NC oriented along [011] direction that was determined by the SAED patterns (Figure 2i). The projection angles, α, β, and γ, denoted in the geometric model (Figure 2h), have been measured and indicated in Figure 2g. In comparison to the measured angles and the calculated ones which listed in the table below Figure 2, it has demonstrated that the as-prepared HOH Au NCs are mainly enclosed by high-index {421} facets. More details are given in Figure S3 and Table S2. These results indicate that the presence or absence of Au seeds, as well as the size of Au seeds, plays a key role in the shape of Au NCs. The growth potentials (Egrowth) were also investigated by using the same Au seeds under otherwise identical growth conditions. In this study, the Au seeds with size of several nanometers were prepared by the first step potential, as shown in Figure 3a. When the Egrowth is −0.45 V, the products consisted of uniform concave TOH shape with a high yield of about 90%, as illustrated in Figure 3b and Figure S4a. A few flower-like particles were also obtained as the byproduct. Similar to HOH, the Miller indices of the facets exposed on the as-prepared TOH Au NCs can also be determined by measuring the projection angle.16,24 Figure 4a−c exhibits a TEM image of a single TOH NC and the corresponding SAED pattern and geometric model oriented along the [011] direction, respectively. Compared with the measured angles and the calculated ones (listed in the table below Figure 4), it has demonstrated that the as-synthesized concave TOH Au NCs are mainly enclosed by {991} HIFs. More details are given in Figure S5 and Table S3. As seen from the Figure 3b−e, it appears that the overgrowth takes place on the ⟨111⟩ crystal directions based on the concave TOH and which become more and more developed with the increase in Egrowth. At the same time, the growth rate of the ⟨100⟩ crystal directions decreases with increasing the Egrowth. For example, when the Egrowth increase to −0.50 V, the stellated Au NCs that is viewed as a concave TOH with eight apexes by overgrowth of ⟨111⟩ crystal axis directions were obtained (Figure 3c and Figure S4b). When further augmenting the growth potential to −0.55 V, the overgrowth of ⟨111⟩ crystal axis directions become dominant and that of ⟨100⟩ crystal axis directions is subordinate, resulting in the formation of concave cube with each face capped by a small concave quartet pyramids (Figure 3d and Figure S4c). With further increasing Egrowth to −0.60 V, the shape of Au NCs completely transforms into concave cubes, as illustrated in Figure 3e and Figure S4d. On the basis of the above observations, we suggest that the growth rates along different

Figure 3. Illustration of the electrodeposition procedure and SEM images of Au NCs deposited at different growth potentials: (a) Au seeds; (b−e) Au NCs deposited at −0.45, −0.50, −0.55, and −0.60 V, respectively. Scale bars are 100 nm.

Figure 4. (a) TEM image, (b) geometric model, and (c) SAED pattern of concave TOH Au NC oriented along the [011] direction. Bottom: a table showing the calculated values for the angles α, β, and γ when the TOH is bounded by {991} crystallographic facets.

crystallographic directions depend on the growth potentials, which can be understood from the two-dimensional nuclei theory.51,52 That is the rate of formation of two-dimensional nuclei of the type {hkl} is dependent on the overpotential. Therefore, the shapes of the Au NCs are highly related to the growth potentials. Except for the seeds and growth potentials, the DES medium is also important in the formation of HIF-Au NCs, including stellated, HOH, and TOH. The HIFs are usually eliminated during the crystal growth because the thermodynamics requires minimization of total surface energy of NCs. We consider that the presence of the specific adsorption of adsorbent such as urea in the DES may play an important role in the formation of HIFs. It has demonstrated that urea adsorbates play an D

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containing 10 mM D-glucose at 25 °C. To eliminate influence of GC substrate, the linear sweep voltammograms of the GC electrode was also performed at the same condition (Figure S7). Clearly, there is not any oxidation current observed on the GC electrode, indicating that the oxidation currents produced on stellated, TOH, and HOH Au NCs samples are attributed to glucose oxidation on such Au CNs. The oxidation current was normalized to the electrochemically active surface area (EAS), which was measured from the electric charge of the reduction of a full monolayer of Au oxides (Figure S6).55 The current densities of peak I taking place at nearby −0.40 V (vs SCE) were measured to be 3.33, 3.06, 1.48, and 0.25 mA cm−2 on TOH, stellated, HOH, and polycrystalline Au electrode, respectively. This peak is attributed to dehydrogenation of anomeric carbon, transforming D-glucose to δ-gluconolactone. Soon afterward, δ-gluconolactone is further transformed to Dgluconate.56 This oxidative process takes place in the potential range from −0.15 to 0.4 V, recording on peak II. The peak current density is in the order to HOH (3.16 mA cm−2) > stellated (2.49 mA cm−2) > TOH (2.21 mA cm−2) > polycrystalline Au electrode (1.31 mA cm−2). These results demonstrate that the as-prepared stellated, TOH, and HOH Au NCs enclosed by HIFs exhibit higher electrocatalytic activity toward D-glucose oxidation than that on the polycrystalline Au electrode, attributed to the HIFs that contain a high density of low-coordinated atoms with excellent reactivity. When the asprepared Au NCs, such as HOH Au NCs, were examined by 100 potential cycles in 0.1 M NaOH solution containing 10 mM D-glucose, the shape of Au NCs still maintains a perfect HOH structure (Figure S7) because the high-index surfaces hardly change after potential cycling.57,58 Because of D-glucose electro-oxidation under heterocatalysis, the dehydrogenation of anomeric carbon under adsorption control is important to this reaction. To quantitatively understand the D-glucose electro-oxidation reactions on these low-coordinated Au atoms, we thus performed density functional theory (DFT) calculations on a series of Au surfaces. The initial D-glucose oxidation is the dehydrogenation of anomeric carbon forming carbonyl group with two couples of proton−electron transfer. This reaction is calculated to be endothermic of 0.43 eV, indicating the equilibrium potential is 0.22 V (vs SHE). It is a two-step reaction: (i) the formation of C6H11O6* from the deprotonation of hydroxyl group on the anomeric carbon in glucose, in which the small barrier could be negligible; (ii) and then the C−H bond breaking occurs. The energy profiles of the anomeric carbon oxidation in glucose into carbonyl group, at the electrode potential of 0 V (vs SHE), on these Au surfaces are presented in Figure 6, and the calculated data are listed in Table 1. From the position of the transition states in Figure 6, it can be seen that Au(111) is the most inactive toward D-glucose electro-oxidation, while on the other surfaces the oxidation is much facilitated. Especially, Au(511) could provide the highest catalytic activity, which is close to that on Au(331). The catalytic activity is in the order to Au(511) > Au(331) > Au(421) > Au(111).

important role in the shape evolution of Au and Pd NCs, especially in the formation of concave disdyakis triacontahedral Pd NCs with HIFs.39,40 In the crystal growth process, urea adsorbates prefers to adsorb on the HIFs through electrochemical adsorption at the growth potentials, which can effectively decrease their surface energy and promote the formation of Au NCs with HIFs. In addition, the change in the bonding mode and the relative coverage of urea adsorbed species depend strongly on the applied potentials,39 resulting in shape evolution of Au NCs with increasing the Egrowth (see Figure 3). The as-prepared stellated Au NCs, as well as TOH and HOH Au NCs, were further characterized by electrochemical cyclic voltammetry, which is commonly used to study the surface structure of polycrystalline and nanostructured Au.18,20,53,54 Figure S6 shows the cyclic voltammograms recorded on the stellated, TOH, HOH Au NCs, and polycrystalline Au electrode in 0.5 M H2SO4 solution at 25 °C. Because the amount of stellated and TOH Au NCs on GC electrode is small, the large background current at high potential was mainly due to the high-area glass carbon substance. The current increase above 1.3 V was partially caused by the oxidation of the glass carbon substance (Figure S6a,b). As a consequence, the oxygen absorption current peaks are inconspicuous, which is disadvantageous to identify the surface structure of Au NCs. In the case of HOH Au NCs sample, the feature of oxygen absorption current peaks is obvious (Figure S6c) and is clearly different from that of the polycrystalline Au electrode (Figure S6d) as well as different from those of low-index faceted Au single crystal electrodes reported.53 Compared with the previous literatures, we have found that the features of oxygen absorption current peaks are similar to those of Au single crystal electrodes of high-index planes,54 suggesting that the facets of HOH Au NCs are mainly HIFs. Electrochemical oxidation of glucose in alkaline medium is chosen as probe reaction to evaluate the electrocatalytic activity of the as-prepared Au NCs with HIFs. To understanding the role of steps and kinks in glucose electro-oxidation, the polycrystalline Au electrode was used as a reference sample because it mainly consists of low-index facets such as {111} and {100} facets. Figure 5 demonstrates the linear sweep voltammograms of the stellated, TOH, HOH Au NCs, and polycrystalline Au electrode in 0.1 M NaOH solution

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CONCLUSIONS In summary, the HIF-Au NCs including stellated, HOH, and TOH shapes were synthesized by a double-step potential method in a DES medium. The results indicated that the stellated Au NCs are enclosed by high-index {711} and {331} facets, and the TOH and HOH Au NCs are mainly with highindex {991} and {421} facets, respectively. A series of studies of

Figure 5. Linear sweep voltammograms recorded on stellated, concave TOH, concave HOH Au NCs, and polycrystalline Au electrode in 10 mM D-glucose + 0.1 M NaOH solution. Scan rate: 50 mV s−1; temperature: 25 °C. E

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*E-mail [email protected]; Fax +86-592-2180181; Tel +86592-2180181. ORCID

Lu Wei: 0000-0003-1136-6916 Zhi-You Zhou: 0000-0001-5181-0642 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (21621091, 21376113), Natural Science Fund project in Jiangsu Province (BK20160210), and Natural Science Fund project of Jiangsu Normal University (15XLR022).

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(1) Qiu, H.-J.; Xu, H.-T.; Liu, L.; Wang, Y. Correlation of the Structure and Applications of Dealloyed Nanoporous Metals in Catalysis and Energy Conversion/Storage. Nanoscale 2015, 7, 386− 400. (2) Kuttiyiel, K. A.; Sasaki, K.; Su, D.; Wu, L.; Zhu, Y.; Adzic, R. R. Gold-Promoted Structurally Ordered Intermetallic Palladium Cobalt Nanoparticles for the Oxygen Reduction Reaction. Nat. Commun. 2014, 5, 5185. (3) Pedireddy, S.; Lee, H. K.; Tjiu, W. W.; Phang, I. Y.; Tan, H. R.; Chua, S. Q.; Troadec, C.; Ling, X. Y. One-Step Synthesis of ZeroDimensional Hollow Nanoporous Gold Nanoparticles with Enhanced Methanol Electrooxidation Performance. Nat. Commun. 2014, 5, 4947. (4) Kong, D.; Liu, L.; Song, S.; Suryoprabowo, S.; Li, A.; Kuang, H.; Wang, L.; Xu, C. A. A Gold Nanoparticle-Based Semi-Quantitative and Quantitative Ultrasensitive Paper Sensor for the Detection of Twenty Mycotoxins. Nanoscale 2016, 8, 5245−5253. (5) Su, S.; Zou, M.; Zhao, H.; Yuan, C.; Xu, Y.; Zhang, C.; Wang, L.; Fan, C.; Wang, L. Shape-Controlled Gold Nanoparticles Supported on MoS2 Nanosheets: Synergistic Effect of Thionine and MoS2 and Their Application for Electrochemical Label-Free Immunosensing. Nanoscale 2015, 7, 19129−19135. (6) Han, X.; Liu, Y.; Yin, Y. Colorimetric Stress Memory Sensor Based on Disassembly of Gold Nanoparticle Chains. Nano Lett. 2014, 14, 2466−2470. (7) Russell, C.; Welch, K.; Jarvius, J.; Cai, Y.; Brucas, R.; Nikolajeff, F.; Svedlindh, P.; Nilsson, M. Gold Nanowire Based Electrical DNA Detection Using Rolling Circle Amplification. ACS Nano 2014, 8, 1147−1153. (8) Jubb, A. M.; Jiao, Y.; Eres, G.; Retterer, S. T.; Gu, B. Elevated Gold Ellipse Nanoantenna Dimers as Sensitive and Tunable Surface Enhanced Raman Spectroscopy Substrates. Nanoscale 2016, 8, 5641− 5648. (9) Zheng, Y.; Soeriyadi, A. H.; Rosa, L.; Ng, S. H.; Bach, U.; Gooding, J. J. Reversible Gating of Smart Plasmonic Molecular Traps Using Thermoresponsive Polymers for Single-Molecule Detection. Nat. Commun. 2015, 6, 8797. (10) Leem, J.; Wang, M. C.; Kang, P.; Nam, S. W. Mechanically SelfAssembled, Three-Dimensional Graphene−Gold Hybrid Nanostructures for Advanced Nanoplasmonic Sensors. Nano Lett. 2015, 15, 7684−7690. (11) Zhang, J.; Xi, C.; Feng, C.; Xia, H.; Wang, D.; Tao, X. High Yield Seedless Synthesis of High-Quality Gold Nanocrystals with Various Shapes. Langmuir 2014, 30, 2480−2489. (12) Zhang, L.-F.; Zhang, C.-Y. Controlled Growth of Concave Gold Nanobars with High Surface-Enhanced Raman-Scattering and Excellent Catalytic Activities. Nanoscale 2013, 5, 5794−5800. (13) Ke, F.-S.; Solomon, B.; Ding, Y.; Xu, G.-L.; Sun, S.-G.; Wang, Z. L.; Zhou, X.-D. Enhanced Electrocatalytic Activity on Gold Nanocrystals Enclosed by High-Index Facets for Oxygen Reduction. Nano Energy 2014, 7, 179−188.

Figure 6. (a) Energy profiles for the anomeric carbon oxidation in glucose on Au(111) (in blue), Au(511) (in red), Au(331) (in yellow), and Au(421) (in purple) with the molecular structures. (b) Optimized intermediates and transition states for C−H bond breaking on different gold surfaces.

Table 1. Calculated Energies (in eV) of the Elementary Steps on Different Surfaces at 0 V (vs SHE) reactions

Au(111)

C6H12O6 → C6H11O6* + H+ + e− C6H11O6* → TS C6H11O6* → C6H10O6 + H+ + e−

Au(511)

Au(331)

Au(421)

1.02

0.88

0.84

0.91

0.71 −0.59

0.43 −0.45

0.53 −0.41

0.62 −0.48

crystal formation demonstrate that the shape of Au NCs is strongly dependent on the size of seeds and the growth potentials. Thanks to their HIFs that contain a high density of low-coordinated step and kink atoms, the as-prepared stellated, TOH, and HOH Au NCs exhibit higher electrocatalytic activity toward D-glucose oxidation than that of the polycrystalline Au electrode. This study provides a new approach to the shapecontrolled synthesis of Au and other metal NCs bounded with HIFs by combining the DES with electrochemical technique, and it is also of significance in potential applications of electrocatalysis, electrochemical sensors, and directly glucose fuel cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00964. Details of SEM and TEM characterizations (Figures S1− S8 and Tables S1−S3) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +86-516 83500485. F

DOI: 10.1021/acs.langmuir.7b00964 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.7b00964 Langmuir XXXX, XXX, XXX−XXX