Copper Nanoflower Assembled by Sub-2 nm Rough Nanowires for

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Copper Nanoflower Assembled by sub-2 nm Rough Nanowires for Efficient Oxygen Reduction Reaction: High Stability and Poison Resistance, and Density Functional Calculations Zhenxing Li, Zhengzheng Ma, Yangyang Wen, Yu Ren, Zhiting Wei, Xiaofei Xing, Hui Sun, Ya-Wen Zhang, and Weiyu Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06722 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Applied Materials & Interfaces

Copper Nanoflower Assembled by sub-2 nm Rough Nanowires for Efficient Oxygen Reduction Reaction: High Stability and Poison Resistance, and Density Functional Calculations Zhenxing Li,*,†, Zhengzheng Ma,† Yangyang Wen,† Yu Ren,† Zhiting Wei,† Xiaofei Xing,† Hui Sun,† Ya-Wen Zhang,‡ and Weiyu Song*,† †State Key Laboratory of Heavy Oil Processing, Institute of New Energy, China University of Petroleum (Beijing), Beijing 102249, China ‡Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applica-tions, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China KEYWORDS: copper nanoflower, sub-2 nm nanowires, oxygen reduction reaction, methanol tolerance and stability, density functional calculations

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ABSTRACT: The copper nanoflowers, assembled by sub-2 nm rough nanowires with high catalytic active (200) facets, are prepared by a prompt and simple method with cetyltrimethylammonium bromide (CTAB) as a capping agent. The CTAB plays a vital role in the synthesis process, while the copper nanorod arrays assembled by copper nanoparticles are obtained without CTAB. The copper nanoflowers are used as catalysts in oxygen reduction reactions (ORR) and exhibit excellent electrocatalytic activity, which shows nearly the same activity compared with the commercial Pt/C catalyst, attributing to the nanoflowers exposed higher catalytic active (200) facets. Furthermore, the nanoflowers can avoid methanol-poison effect and show better long-term operation stability. The density functional theory (DFT) was used to calculate the atom energy of Cu (100) facets and Cu (111) facets. Both of O2 dissociation and H2O activation on the facets are very easy. However, the difference between Cu (100) facets and Cu (111) facets is the adsorption and dissociation energy of O2, and the adsorption and activation of oxygen molecular is much easier on Cu (100) facets than Cu (111) facets due to the more open nature of (100) facets.

1. INTRODUCTION As a shining point of the modern science, nanotechnology provides possibility in many new applications, which can exhibit different chemical and physical characters in materials science. It can be widely applied in diverse fields, including energy conversion and storage,1 optoelectronic,2 magnetics,3 environmental protection and chemical manufacturing.4,5 Recently, nano-catalysis has aroused a great deal of interest, the noble metal, such as Pt,6 Au,7 and Pd,8 has already used in fuel cell electrocatalysts, electrochemical reduction of CO2,9 oxygen reduction reaction (ORR) and so on.10 More recently, the earth-abundant and inexpensive metals are


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attractive too due to their price and reserves. Though nano noble metals, such as Pt and Rh, is more stable,11 the inexpensive nano copper still has possibilities to replace it in many chemical applications. Copper (Cu) is a 3d transition metal with some special characters,12 for instances, multiple oxidation states, and the transition metal Cu have the localized surface plasmon resonance (LSPR), which is useful in the field of optics.13-16 In addition, Cu can be used in high temperature or pressure conditions because of its high boiling points.17 In view of the advantages of Cu and peculiarities of nanostructure, Cu nanoparticles (NPs) have great potential in the future. Considering the size of Cu NPs brings some difficulties to manipulate, and the assembly can effectively solve this problem. Formed a larger structure spontaneously by NPs makes the crystal have nano-properties, which can be used and operated easily. When the self-assembly had been reported in nanoparticles superlattices prepared in 1995,18 this research area has affected the development of metal nanostructure, and the surface structure and size of NPs also effect the property of assembled material.19 For now, a large number of approaches can make nanoparticles assemble, for instance, using a soft organic template for metal structure,20 solution-based structure directing methods, vapor-liquid-solid growth method.21 Chad A. Mirkin et al. reported the gold nanocubes and nanoprisms assembled by template of single crystalline colloidal which is based by capillary-force.22 Besides chemical method, physical way like electronics23,24 and magnetics25 also can get that goal. For instance, Naomi J. Halas et al. used electrolytic way making Au nanorice assembled into mesostars.26 To date, the reserves of fossil-fuel became exhausted,27 so fuel cell becomes popular since it is a good energy conversion or storage devices and benefits to the environment,28,29 and the traditional ORR catalyst carbon supported platinum nanoparticles (Pt/C) are considered the best catalysts for a long time,30,31 which are limited the large-scale applications for the noble metal


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catalyst carrying high cost,32 lacking of persistence,33 and deactivated easily by methanol or CO.34 Therefore, finding a non-noble metal nanocatalyst provides a solution.35 The nanoscale copper brings mass of low-coordinated surface atoms which promote oxygen adsorption, and helps to enhance electrocatalytic performance, showing a good catalytic activity for the oxygen reduction reaction (ORR).36 Hao Zhang et al. had reported a nanocopper ribbon which had been assembled in colloidal solution and used as a catalyst for ORR successfully.37 However, ultrasmall copper nanoparticles had the high surface energy which is easily polymerized in electrochemical reaction,38,39 the assemble nanostructure can solve this problem, meantime maintained good catalytic activity and durability. Herein, we demonstrate that the copper nanoflowers assembled by sub-2 nm rough nanowires are prepared by a prompt and simple method with cetyltrimethylammonium bromide (CTAB) as a capping agent. The CTAB plays a vital role in the synthesis process, while the copper nanorod arrays assembled by copper NPs are obtained without CTAB. In addition, the nanoflowers exposed both (111) and (200) facets, while the nanorod arrays only exposed (111) facet. Moreover, both of the copper nanostructures are used as catalysts in ORR. The copper nanoflowers exhibited higher catalytical activity than the copper nanorod arrays in ORR, which is nearly same activity to the commercial Pt/C catalyst, attributing to the nanoflowers exposed higher catalytical active (200) facets. Furthermore, the nanoflowers can avoid methanol-poison effect and show better long-term operation stability. The density functional theory (DFT) was used to calculate the atom energy of Cu (100) facets and Cu (111) facets. Both of O2 dissociation and H2O activation on the facets are very easy. However, the difference between Cu (100) facets and Cu (111) facets is the adsorption and dissociation energy of O2, and the adsorption and


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activation of oxygen molecular is much easier on Cu (100) facets than Cu (111) facets, because of the more open nature of (100) facets, which agrees well with the electrocatalytic result. 2. EXPERIMENTAL SECTION 2.1. Synthesis of Cu Nanoflowers. Precursor mixture is prepared by dissolving copper acetylacetonate [Cu(acac)2] (97%, 0.04 mmol) in 1 mL oleylamine (OAm). Subsequently, 0.1g exadecyltrimethy bromide (CTAB) which act as a capping agent, were dissolved 15 mL of oleylamine (OAm) forming a uniformity solution and were added in a 100 mL three-necked flask, then were heated up to 140 ºC and vacuumed for 30 mins to remove the impurities. Then the solution was heated to 230 ºC, and then Cu(acac)2 oleylamine solution was injected, and kept this temperature for 20 mins by continuous stirring under Ar Then cooled rapidly by ice water bath till the solution down to room temperature. Finally, the dispersion was centrifuged and washed with ethanol at 11000 rpm for 10 mins to separate the solid product, and further dispersed in ethanol. 2.2. Synthesis of Cu Nanorod arrays. 15 mL of oleylamine (OAm) added in a 100 mL threenecked flask, then heated up to 140 ºC and vacuumed for 30 mins to remove the impurities. When the solution was heated to 230 ºC, added 0.4 mmol Cu(acac)2 to the three-necked flask, kept 230 ºC for 20 mins and realized the solution turn to red black and the reaction was stopped. The reaction solution was cooled down to room temperature in the air. The solution was centrifuged with ethanol at 11000 rpm for 10 mins with ethanol for removing the residual species. Finally the product was dispersed in ethanol just like the nanoflowers before further analysis. 2.3. Electrochemical Catalysts Preparation. 2 mg of Cu NCs and 2 mg of Ketjen Carbon (C) mixture (weigh ratio of 1:1) were dispersed in ethanol (5 ml), and sonicated for an hour making Cu NCs uniformly deposited on Carbon, and then the Cu-C catalysts dispersed in acetic acid (4


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ml) with vigorous stirring in 70 ºC for 8 hours to remove the organic compounds. After cooling down to room temperature, the Cu-C catalysts were centrifuged by ethanol twice at 11000 rpm for 10 mins for washing, after that, adding 1 ml ethanol to prepare the Cu-C catalyst solution (2 mgCu·ml-1). Dispense 10 µL catalyst solutions on the top of a 5mm diameter glassy carbon rotating disk electrode (RDE), add Nafion solution (0.5 wt%, 10 µL) on the top of catalyst and then dry the RDE in the air for 30 mins. All electrochemical measurements were carried out using a three-electrode system on an electrochemical working station (CHI760E, Shanghai Chenhua Instrument Factory, China). The RDE were performed as the working electrode, Ag/AgCl as reference electrode and a 6 mm diameter graphite electrode as counter electrode. The electrochemical properties of copper catalysts were tested in 0.1 M KOH solution at room temperature, and before the test, N2 or O2 were ventilated for 30 mins to remove the gas in the electrolyte solution. 2.4. Characterization. The scanning electron microscopy (SEM) was conducted using Hitachi SU8010 scanning electron microscopy (SEM, Japan) at 200 kV. Transmission electron microscopy (TEM) was performed on a JEM 2100 LaB6 with an accelerating voltage of 200 kV. The high-resolution transmission electron microscope (HRTEM) was showed on Tecnai F20 with an accelerating voltage at 200 kV. The small-angle and wide-angle X-ray diffraction (XRD) patterns of samples were recorded on a Burker D8-advance X-ray power diffractometer operated at 40 kV and current of 40 mA with Cu-Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectrometer (XPS) was carried out on an ion-pumped chamber (evacuated to 2 × 10-9 Torr) of an Escalad5 spectrometer, using Mg KR radiation (BE) 1253.6 eV. The UV-visible absorption spectrums were obtained using a on a Hitachi U-3010 spectrometer.


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2.5. Computational details. In this work, the Vienna ab initio simulation package (VASP)40,41 was used to calculate all states with the electron exchange correlation effect described by the Perdew-Burke-Ernzerhof functional within the generalized gradient approximation (GGAPBE).42 The spin-polarized calculations were performed. PAW pseudo potential was used to describe the core-valence electron interaction.43 Plane-wave basis set with an energy cutoff of 400 eV was used in this work. The climbing nudged elastic band method (CI-NEB)44,45 was employed to locate the transition states. We built a periodic slab with four layers for copper (111) and (100) facets, respectively. 5 × 5 surface unit cells were used. We fixed the bottom two layers of Cu. The vacuum gap thickness was set to be 15 Å. The surfaces are constructed from the theoretical lattice constant which is calculated to be 3.636 Å using a (11 × 11 × 11) k-point grid. A Monkhorst pack 3 × 3 × 1 k-point mesh was used for the Brillouin zone integration. During structural optimizations, all of the atoms except those in the bottom copper two layers of the slab were allowed to relax until atom forces were smaller than 0.05 eV/Å. Adsorption energy has been calculated using the following expression: Ead = Etot - Eslab - Ex Where Etot is the total energy of the combined system with the adsorbate X bound to the slab, Eslab is the energy of the slab alone, and Ex is the energy of the adsorbate in the gas phase. According to this definition, exothermic adsorption results in a negative value of Ead. 3. RESULTS AND DISCUSSION The colloidal dispersion was purified and dispersed by ethanol for further analyzing. The size and morphology of nanorod arrays were characterized though the transmission electron microscope (TEM) in Figure 1a. Figure 1a shows that the products without CTAB are self-


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assembled with uniform copper NPs, which constitutes the orderly nanorod arrays. The width of the nanorod array is about 10 nm, and the length is 79-382 nm. Figure 1b displays a part of the nanorod arrays that demonstrate the sample is formed by the copper NPs with an average diameter of 10 nm. Further, the high-resolution transmission electron microscope (HRTEM) reveals a clear lattice spacing of 0.208 nm for the copper NPs, which corresponds to the (111) facets of copper phase.

Figure 1. Representative images of nanorod arrays: (a) TEM, (b) high magnification TEM, (c) HRTEM. When the capping agent CTAB was added in the synthesis process, the morphology of copper NPs was changed significantly. Figure 2a shows the representative scanning electron microscopy (SEM) images of the final samples, which are nanoflowers with a diameter of 1.66 µm and a petal width of 195 nm. The corresponding TEM images (Figure 2b and Figure 2c) demonstrate that the nanoflowers are assembled by numerous rough nanowires (NWs), with 1.7 nm in width and about 720 nm in length for these nanowires. In addition, it can be seen that all the surface of these nanowires is rough. The HRTEM image in Figure 2d suggests that the nanowires are not


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monocrystalline but polycrystalline with two well-resolved lattice spacings of 0.212 nm and 0.178 nm, which correspond to the (111) and (200) facets of copper phase.

Figure 2. Representative images of nanoflowers: (a) SEM, (b) TEM, (c) high magnification TEM, and (d) HRTEM. The XRD patterns of two shapes of Cu nanostructures are shown in Figure 3. There is a dominant peak in the nanoflowers at 43º (Figure 3a), which identify the face centered cubic (111) reflection of Cu (JCPDS No. 04-0836), and two weak peaks at 51º and 75º, corresponding to the reflects of (200) and (220) facets, respectively. The small angle x-ray diffraction (SAXRD) pattern of nanoflowers is also presented in Figure 3a (inserted). A high diffraction peak at 1.76° and two weak peaks at 5.18° and 5.6° can be observed, indicating that the sample is highly


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ordered, and these peaks can be attributed to p6mm hexagonal symmetry. Combining with the TEM observation (Figure 2d), the d spacing of nanoflowers is 5 nm, which is consistent with the TEM results. The XRD pattern of nanorod arrays is shown in Figure 3b and besides the strong reflection peak (111), only exist a weak diffraction (200), which can be indexed to the facecentered cubic, in addition, the series of SAXRD peaks are assigned by “00L” (L=1,2…) and inset the Figure 3b. The full width at half maxima of nanoflowers is smaller than nanorod arrays because the nanowires which assembled the nanoflowers are much longer than the particles in nanorod arrays.

Figure 3. XRD and SAXRD of nano copper (a) nanoflowers, (b) nanorod arrays. In order to further confirm the results of synthesis. The Figure 4 shows X-ray photoelectron spectroscopy (XPS) of the two Cu nanostructure and the main peaks are Cu2p3/2 and Cu2p1/2 in 932.1 eV and 951.88 eV, which can confirm there is only Cu0 existed, and no feature of Cu+ or Cu2+.46 And the signal of Cu2p3/2 is stable during the test suggesting that the synthesis method avoids the Cu oxidation effectively.


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Figure 4. XPS of nano copper: (a) nanoflowers, (b) nanorod arrays. The UV-vis spectra can further measure the conclusion. The UV-vis of Cu nanoflowers (black line) is showed in Figure 5, which displays the surface plasmon resonance (SPR) band at ∼380 nm. The spectrum for the nanorod arrays (red line) is showed a rising feature at ∼385 nm. For adding the CTAB the red shift rising band increases 5 nm in the UV-vis spectrum. As the particles became larger, this band shifted to higher wavelength (from 380 nm to 385 nm).47 The UV-vis peaks of large size copper usual appear at 560-600 nm, the optical difference is become from the ultra-small size,48 which further confirm that the nanoflowers are composed by the ultra-small Cu nanowires.

Figure 5. UV-vis of nanoflowers (black line) and nanorod arrays (red line).


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The nanoflowers and nanorod arrays catalysts exhibit excellent electrochemical activity and durability in ORR. The electrocatalytic properties of Cu nanoflowers and nanorod arrays for ORR were evaluated by depositing the Cu on a glassy carbon (GC) electrode. Figure 6a shows the cyclic voltammograms (CVs) of the nanorod arrays and nanoflowers in 0.1 M KOH saturated with N2 or O2 at a scan rate of 50 mV·s-1. The representative ORR cathodic peaks appear in the O2- saturated KOH solution, which not displayed in N2-saturated solution, meaning that they all possess ORR electrocatalytic activity. Besides, the peak of nanoflowers is more pronounced than nanorod arrays, although the copper catalysts did not show better activity than Pt/C catalysts, but both of the Cu catalysts show good ORR activity and the more cost-effective than commercial Pt/C catalyst (Figure S1-S2).


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Figure 6. (a) Cyclic voltammograms of nanoflowers and nanorod arrays in O2- and N2-saturated, (b) linear sweep voltammetry and K-L plots (inserted) of nanoflowers, (c) linear sweep voltammetry and K-L plots (inserted) of nanorod arrays, (d) transferred electron number (n) from -0.305 V to -0.225 V (vs. RHE) of two copper catalysts, (e) i-t chronoamperometric response of


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nanoflowers, nanorod arrays and Pt/C at 1000 rpm, (f) i-t chronoamperometric response of 3.0 M methanol addition. To get some insight on the ORR kinetics, rotating-disk electrode (RDE) measurements were investigated. Figure 6b and 6c show the linear sweep voltammetry (LSV) of copper catalysts at different rotating speeds from 400 to 2500 rpm with a scan rate of 10 mV s−1 in a O2-saturated 0.1 M KOH electrolyte (the background was subtracted in N2-saturated). The ORR onset potential (at 1600rpm) of nanoflowers is at 0.821 V vs. RHE and the nanorod arrays is at 0.8 V vs. RHE, indicating the better performance of nanoflowers catalyst with a 0.021 V vs. RHE higher onset potential and the limiting current density of nanoflowers (-4.1 mAcm-2) is higher than nanorod arrays (-3.8 mAcm-2). The transferred electron number (n) per oxygen molecule is shown in Figure 6d, and can be calculated by Koutecky-Levich plots (K-L).49 The K-L plots at different potentials indicate a good linearity from -0.305 V to -0.225 V vs. RHE, and the corresponding number of transferred electrons (n) per oxygen molecule is calculated according equation.50 1 1 1.61 = + 2/3 j jk nFDO2 v-1/6 CO2 ω1/2 The j is the measured current density, jk expresses the kinetic-limiting current density, F the Faraday constant (F=96485 C·mol-1), DO2 is the diffusion coefficient of O2 (2 × 10-5 cm2s-1), v refers the kinetic viscosity (0.01 cm2s-1) and CO2 the bulk concentration of O2 (1.26×10-3 mol/L).51 The n value for nanoflowers is 3.93, which is better than the value for nanorod arrays (only 3.38). The n values for these two Cu catalysts are comparable with the commercial Pt/C catalyst (3.99), demonstrating an ideal 4e- oxygen reduction process (Figure S3). For further confirming the ORR process, both of the copper catalysts were tested by rotating disk-ring


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electrode (RRDE). The disk current density and the ring current density at 1600 rpm at a scan rate of 10 mV s−1 in O2-saturated 0.1 M KOH electrolyte of the copper are shown in Figure S4. The Pt ring electrode corresponding currents of two copper catalysts were negligible like commercial Pt/C catalyst in Figure S5, which represented a four-electron pathway, and the transfer electrons (n) can be calculated by according equation. n=4ID /(ID + IR ⁄N ) Where the IR is the ring current and the ID is disk current, and the N represents collection efficiency (37%).52 The n of nanoflowers is 3.68-3.81 and 3.34-3.41 for nanorod arrays electrode (at the potential from –0.3 V to 0.4 vs. RHE), so close to commercial Pt/C (3.95-3.97, at the potential from 0.2 V to 0.8 vs. RHE). Meantime, peroxide yield (H2O2) can be calculated by x (%) = 200(1-n/4), the n value and H2O2 yield of copper and Pt/C catalysts all showed in Figure S6-7.53 The durability of copper catalysts was performed using the chronoamperometric technique under 0.275 V vs. RHE at a rotating speed of 1000 rpm and a scan rate of 50 mVs-1 over 20000 seconds (Figure 6e). Impressively, as shown in Figure 6e, both two copper catalysts exhibit excellent stability, with only 5 % and 9 % loss for nanoflowers and nanorod arrays, respectively, both better than the 17 % loss for the commercial Pt/C catalyst. We also conducted the possible poison tests of the Cu electrodes by adding 3.0 M methanol at 1000s. We used the corresponding current-time (i-t) curve at 0.275 V vs. RHE for 4000s to compare the toxicity of the Pt/C and nanocopper electrodes. As shown in Figure 6f, the relative current of the commercial Pt/C decreases rapidly to 79 % after adding the methanol. The nanorod arrays decrease to 94 %, while the nanoflowers give a 95 % retention after the addition, which


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demonstrates a good methanol tolerance than the nanorod arrays. The results above suggest the excellent ORR performances of the nanostructured copper catalysts in this work, which may pave a new way to provide the promising non-noble metal catalyst candidatures in alkaline solution for fuel cells. We speculate that the different ORR activity of these two morphology catalysts may be related with the different sizes of copper particles. The Cu NPs in the nanoflowers were assembled by 1.7 nm rough nanowires (Figure 2c), which are much smaller than the particle size in nanorod arrays. Further, the rough nanowires could provide more active surface area, and the nanowires in nanoflowers expose some (200) facets, which have higher activity than (111) facets, whereas the nanorod arrays only provide (111) facets.

Figure 7. Potential energy diagram of oxygen activation and water dissociation on Cu (111). All the intermediate structures and transition states (TS) are shown in top view (color scheme: bronze - Cu; red - O; white - H). The surface structures of the catalyst can influent the electrochemical activity remarkably.54 For instance, the low index facets of Pt (111) had been demonstrated which provide higher ORR activity than Pt (100) in 1994,55 because Pt (111) facets step atom density is higher than Pt (100)


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facets, and the active site of the ORR is located between the (111) terrace edge and the (111) terrace atomic rows neighboring to the edge on Pt electrodes.56,57 For investigating the cause of the difference electrochemical activity and redox catalysis mechanism of two copper catalysts, density functional theory (DFT)43 was used to calculate the energy of copper facet. Calculated the entire ORR process can found there is little in the energy barrier when formation water, which expressed that the oxygen adsorption is a key step (Figure S8). On Cu (111) facets (Figure 7), oxygen molecular first adsorbs on the hollow site with the adsorption energy of 0.69 eV. The dissociation of adsorbing O2 needs to overcome a barrier of 0.08 eV and reaction energy is -2.05 eV. The small energy barrier and large exothermic reaction energy indicates a facile process for O2 dissociation. Then the water molecular adsorbs with an energy release of 0.59 eV. One of the hydrogen points to one dissociated O atom with the hydrogen-interaction. It takes 0.13 eV to dissociate this H to O forming two hydroxyl groups. The reaction energy is exothermic by 0.21 eV. The dissociated oxygen atoms have a strong ability to capture the hydrogen atom from water.


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Figure 8. Potential energy diagram of oxygen activation and water dissociation on Cu (100). All the intermediate structures and transition states (TS) are shown in top view (color scheme: bronze - Cu; red - O; white - H). Same process has been explored on Cu (100) facets (Figure 8). The adsorption energy of oxygen molecular on Cu (100) increases to 1.63 eV, much higher than on Cu (111) (0.69 eV). This is due to the less coordinated Cu atoms on the (100) than (111) facets. Oxygen is dissociated on Cu (100) with exothermic reaction energy of 2.15 eV and without any barrier. Then the water molecular adsorbs with adsorption energy of 0.24 eV. Finally, the dissociated oxygen atom reacts with the hydrogen atom of water with a barrier of 0.23 eV and reaction energy of -0.04 eV. It shows the reaction of oxygen atoms with water molecular is also very easy. To figure out if the OOH or HOOH dissociation mechanisms exist on the studied materials, we calculated OOH adsorption and the result is showed in Figure S9, it is easy to find OOH cannot adsorb on Cu(100) stable, OOH will spontaneously split into OH and H. To summarize, the dissociation of O2 and the reaction of H2O with dissociated O atom are easy to happen on two facets. However, due to the more open nature of (100) facet, the adsorption and activation of oxygen molecular is much facile on Cu (100) than Cu (111), which is different from


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Pt. The adsorption energy of oxygen on the Cu(100) is 1.63eV, and on Pt(111) is 2.59eV, Pt(100) is 1.3eV56, so the results of calculation verify the ORR electrocatalytic data. 4. CONCLUSIONS We have successfully synthesized copper nanorod arrays by high-temperature solution-phase synthesis with oleylamine as solution and reducing agent, don’t need any complex method. In the synthesis process, the morphology of copper nanostructures is easily manipulated by using the CTAB as capping agent. The copper nanoflowers are prepared by adding the CTAB in the synthesize process, which are assembled by rough sub-2 nm nanowires. Both of the copper nanostructures show better maneuverability in catalysis because of their self-assembled prosperity. As catalysts in ORR, the nanoflowers is better than nanorod arrays due to the smaller sizes of particles, besides that the nanoflowers were exposed higher active (200) facets. Furthermore, those copper nanostructures showed good electrochemical performance in catalytically activity and durability. The copper catalysts showed superior methanol tolerance and stability than the commercial Pt/C, which suggests that the nanoflowers are potential catalysts for fuel cells. The DFT provides evidence that the adsorption and activation of oxygen molecular is much easier on Cu (100) facets than Cu (111) facets, because of the more open nature of (100) facets, which is corresponded to the experimental results. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The CV, LSV, K-L plots and transferred electron number (n) images of commercial Pt/C catalyst. The additional RRDE electrochemical measurements curves of the Cu and commercial Pt/C catalysts, and the supplement of DFT. (PDF)


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AUTHOR INFORMATION Corresponding Author *Email for Z.L.: [email protected] * Email for W.S.: [email protected] ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC) (Grant Nos. 21501197) and Beijing Natural Science Foundation (Grant Nos. 2182061). REFERENCES (1) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Vander Elst, L.; Muller, R. N. Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications. Chem. Rev. 2008, 108, 2064-2110. (2) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies. Annu. Rev. Mater. Sci. 2000, 30, 545–610. (3) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices. Science 2000, 287, 1989-1992. (4) Gawande, M. B.; Branco, P. S.; Parghi, K.; Shrikhande, J. J.;Pandey, R. K.; Ghumman, C. A. A.; Bundaleski, N.; Teodorod, O. M. N. D.; Jayaram, R. V. Synthesis and Characterization of Versatile MgO-ZrO2 Mixed Metal Oxide Nanoparticles and Their Applications. Catal. Sci. Technol. 2011, 1, 1653-1664.


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Copper Nanoflower Assembled by Sub-2 nm Rough Nanowires for

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