Controllable Synthesis of Tetragonal and Cubic Phase Cu2Se

Article pubs.acs.org/JPCC

Controllable Synthesis of Tetragonal and Cubic Phase Cu2Se Nanowires Assembled by Small Nanocubes and Their Electrocatalytic Performance for Oxygen Reduction Reaction Suli Liu,† Zengsong Zhang,† Jianchun Bao,† Yaqian Lan,† Wenwen Tu,† Min Han,*,†,‡ and Zhihui Dai*,† †

Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China ‡ State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: The rich phase structure of Cu2Se may provide good opportunities for modulating and optimizing their catalytic properties. However, chemical synthesis of Cu2Se nanostructures with different crystal phases still is a key challenge, and their catalytic application in energy fields has not been studied. In this paper, pure tetragonal and cubic phases Cu2Se nanowires (NWs) that assembled by small nanocubes have been controllably synthesized via a simple solid−liquid phase chemical transformation method, i.e., thermal treatment of presynthesized Cu NWs in Se precursor solution containing proper surfactant and 1-octadecene. Besides reaction temperature and ripening time, the functional groups and alkyl chain length of used surfactant greatly affect the kinetics of the transformation reaction and phase structure control of Cu2Se NWs. The trioctylphosphine is found to be the optimal surfactant, which not only accelerates the transformation reaction but also improves the stable temperature of tetragonal phase Cu2Se NWs about 80 °C compared with that of their bulk counterparts. Electrochemical tests reveal that both the obtained Cu2Se NWs can be used to catalyze oxygen reduction reaction (ORR) in alkaline media. But the catalytic performance of tetragonal phase Cu2Se NWs is much higher than that of cubic phase ones, which is even better than that of commercial Pd/C and some reported non-Pt electrocatalysts. The diverse catalytic performances of those Cu2Se NWs result from their distinct spatial arrangement means of Cu and Se atoms that lead to different adsorption and activation of O2 molecules approaches, as evidenced by electrocatalytic dynamic experiments. The ORR on tetragonal phase Cu2Se NWs abides by the direct 4e− mechanism, whereas that on cubic phase ones complies with dual-path mechanism comprising both 2e− and 4e− pathways.

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better than that of cubic phase ones.13 To the best of our knowledge, the catalytic properties of tetragonal and cubic phases Cu2Se nanostructures, especially their application in renewable energy and technology fields, have not been investigated until now. So, controllable synthesis of novel tetragonal and cubic phases Cu2Se nanostructures and exploitation of their catalytic application in energy fields attract our interest, which is not only important for deepening our understanding on phase structure−property relationship but also for broadening their functionalities and application range. Designing highly active and long durability electrocatalysts to improve the sluggish kinetics of cathodic oxygen reduction reaction (ORR) is vital to advance fuel cells technology.19 Over the past decades, substantial efforts have been paid on geometric structure optimization or support selection of

INTRODUCTION Transition metal chalcogenides (TMCs) have been considered as potential candidates for many green energy devices, such as solar cells,1−3 lithium batteries,4,5 supercapacitors,6 and thermoelectric devices.7 As one of the TMCs, Cu2Se has attracted much attention owing to its unique properties, such as the large thermal power, a direct band gap of ∼2.2 eV, and superionic conductivity.8,9 Many recent efforts have been devoted to synthesizing Cu2Se micro- and nanoscaled building blocks as well as advanced nanoarchitectures with different morphologies, including particles, flakes, tubes, cages, and dendrites.10−13 Except for size and shape, the phase structure also has a great impact on the performance of a desired material.14−17 There are five kinds of crystal phases for Cu2Se: a cubic phase, a tetragonal phase, an orthorhombic phase, a hexagonal phase, and a monoclinic phase.13,18 Among them, the cubic phase is thermodynamically stable while the tetragonal phase is metastable. Recent research has discovered that the photoelectron properties of tetragonal phase Cu2Se nanostructures is © 2013 American Chemical Society

Received: May 4, 2013 Revised: June 24, 2013 Published: June 28, 2013 15164

dx.doi.org/10.1021/jp4044122 | J. Phys. Chem. C 2013, 117, 15164−15173

The Journal of Physical Chemistry C

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Pt20,21 and Pt-based bimetallic nanocrystals22−25 to meet those challenges. For reducing cost and promote application of fuel cells, exploring and synthesizing non-Pt electrocatalysts have attracted many researchers’ interests nowadays. For instance, some non-Pt bimetallic nanostructures,26 MxMn3−xO4 (M = divalent metals) nanocrystals,27 graphene-supported Co3O4 nanocrystals,28 N-, S-, or P-doped carbon nanomaterials,29−32 and macrocyclic complexes,33 have been found to possess good electrocatalytic activity toward ORR in acidic or alkaline media. Recently, late transition metal chalcogenides (e.g., CoSe2, NiSe) nanomaterials have been considered to be one class of ORR electrocatalysts with acceptable activity and durability.34 Also, by supporting ZnSe nanocrystals on N-doped graphene,35 the enhanced catalytic activity has been observed for ORR. These findings imply the feasibility to design and screen high efficient TMCs electrocatalysts toward ORR. Being a member of TMCs family, Cu2Se nanomaterials are expected to possess good electrocatalytic activity toward ORR since the Cu element exists in a middle state and some Cu-based nanostructures (e.g., Cu3N nancubes36) have been suggested as ORR catalysts in alkaline media. Moreover, the rich phase structures of Cu2Se may provide opportunities for modulating ORR activity and durability. However, up to now, there is no report on catalyzing ORR by Cu2Se nanomaterials. Here, we report the controllable synthesis of pure tetragonal and cubic phase Cu2Se nanowires (NWs) that assembled by small nanocubes as well as their electrocatalytic performance for ORR. The typical synthesis is based on a simple solid− liquid phase chemical transformation method, i.e., thermal treatment of presynthesized Cu NWs in Se precursor solution containing proper surfactant (e.g., trioctylphosphine (TOP), dodecylamine (DDA), etc.) and 1-octadecene (ODE). The Cu NWs are first prepared according to our previous report.37 Except for reaction temperature and ripening time, we find that the used surfactant has great impacts on the kinetics of the transformation reaction and phase structure control of the obtained Cu2Se NWs. By using TOP as a surfactant, the transformation reaction can be accelerated and pure tetragonal and cubic phase Cu2Se NWs are obtained at 220 and 280 °C, respectively. Importantly, the stable temperature of tetragonal phase Cu2Se NWs improves about 80 °C compared with that of their bulk counterparts, beneficial to catalytic application. Related electrochemical measurements exhibit that both of the two kinds of Cu2Se NWs can efficiently catalyze ORR in alkaline media, whereas the catalytic activity and stability of tetragonal phase Cu2Se NWs are much higher than that of cubic phase ones. Also, the catalytic performances of those Cu2Se NWs are compared with those of commercial Pd/C, polyhedron-like Cu2Se nanocrystals, and some reported non-Pt ORR electrocatalysts. Moreover, the origin of the diverse catalytic performances observed on those Cu2Se NWs is briefly discussed based on electrocatalytic dynamic experimental results. To the best of knowledge, this is first example to focus on phase structure and its influence on ORR performance.

catalyst (10% Pd supported on C, CAS No. 7440-05-3) were purchased from Alfa Aesar. All reagents were used as received without further purification. Synthesis of Pure Tetragonal Phase Cu2Se NWs. For synthesis of pure tetragonal phase Cu2Se NWs, the Cu NWs were first synthesized according to previous report,37 which were used as “self-template” to direct the growth of desired Cu2Se NWs. In a typical synthetic procedure, 32 mg of presynthesized Cu NWs (about 0.5 mmol) was mixed with 7 mL of 1-octadecene (ODE) in a 50 mL three-neck flask. Then, the reactor was heated to 220 °C and maintained at that temperature for 20 min. Subsequently, the Se precursor solution that obtained by dissolving 0.25 mmol of Se powder in 1 mL of TOP (trioctylphosphine) and 3 mL of ODE, was quickly injected into the reaction system. After reacted at 220 °C for 1 h, the reactor was naturally cooled down to room temperature. 10 mL of heptane and 5 mL of absolute ethanol were added, and the crude product was separated by centrifugation at 10 000 rpm for several minutes. The obtained crude product was washed by heptane and absolute ethanol for 3 times to remove the byproducts. Finally, the product was dried and stored in vacuum, which was used for characterization and analysis. Synthesis of Pure Cubic Phase Cu2Se NWs. The synthetic procedure and post-treatment steps were similar to that for synthesis of pure tetragonal phase Cu2Se NWs. The only difference lies on the reaction temperature. Keeping other conditions constant and only changing the reaction temperature to 280 °C, pure cubic phase Cu2Se NWs were obtained. Characterization. The X-ray energy dispersive spectra (EDS) were taken on a JSM-5610LV-Vantage typed energy spectrometer. The X-ray diffraction (XRD) patterns were recorded on a powder sample using a D/max 2500 VL/PC diffractometer (Japan) equipped with graphite monochromatized Cu Kα radiation (λ = 1.540 60 Å) in 2θ ranging from 10° to 70°. Corresponding work voltage and current is 40 kV and 100 mA, respectively. MDI Jade 5.0 software was used to deal with the acquired diffraction data. The XPS data were acquired on a scanning X-ray microprobe (PHI 5000 Verasa, ULACPHI, Inc.) using Al Kα radiation. Binding energies of Cu 2p and Se 3d were calibrated using the C 1S peak (BE = 284.6 eV) as standard. FE-SEM images were taken on an ultrahighresolution thermal emission scanning electron microscope (JSM-7600F, Japan), operating at an accelerating voltage of 20 kV. The transmission electron microscopy (TEM) images were taken on a JEM-200CX instrument (Japan), using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images were obtained on JEOL-2100F apparatus at an accelerating voltage of 200 kV, which is equipped with STEM and EDS detectors that can be used for elemental mapping analysis. Electrocatalytic Measurements. All the electrochemical experiments were carried out on a CHI 660D electrochemical workstation (Shanghai, Chenghua Co.). A standard threeelectrode system was used for all measurements, which consisted of a platinum wire as the auxiliary electrode, a saturated calomel reference electrode (SCE), and the Cu2Se NWs modified glassy carbon electrode as the working electrode. Rotating disk electrode test was performed on Gamry’s rotating disk electrode (RDE710) with a glassy carbon disk (5 mm diameter). All potentials in this study were reported with respect to the SCE. All the electrochemical measurements were carried out at room temperature.

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EXPERIMENTAL SECTION Material. CuCl (>99%), Se powder (>99.8%), heptane, and absolute alcohol were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai). The trioctylphosphine (>90%), tribenzylphosphine (>98%), oleic acid (>90%), dodecylamine (>98%), and 1-octadecene (Tech, >90%) were purchased from Aldrich. The Nafion solution (5.0 wt %) and commercial Pd/C 15165



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(200), (220), (311), and (400) planes of cubic Cu2Se (space group: Fm/3m) with the lattice parameters of a = 5.760 Å (JCPDS 65-2982). To further identify the elemental valence states and purity of those two samples, the XPS measurements are also performed. Their survey XPS spectra are shown in the Supporting Information Figure S1. As illustrated in Figure 1C, their Cu 2p core level spectra (Cu 2p3/2 and 2p1/2) are symmetric, narrow, and devoid of satellite peaks, indicating the valences of Cu elements in those two samples are +1.8,13 Corresponding binding energies (BEs) of Cu 2p3/2 and Cu 2p1/2 for tetragonal phase Cu2Se NWs are 931.1 and 950.8 eV, respectively, consistent with a recent literature report.13 While for cubic phase Cu2Se NWs, the BEs of Cu 2p3/2 and Cu 2p1/2 are 931.4 and 951.1 eV, respectively, which are slightly higher than those of tetragonal phase ones. As for Se 3d spectra, only the single broad peak at 52−56 eV is observed in those two samples (Figure 1D), which is the characteristic shape of Se2− in a consistent bonding environment.8,13 The broad peak for Se 3d can be fitted into two symmetric peaks that assigned to Se 3d5/2 and Se 3d3/2, respectively (Figure S2). Similar to Cu 2p spectra, the BEs of Se 3d5/2 and Se 3d3/2 for cubic phase Cu2Se NWs (54.1 and 54.9 eV, respectively) are slightly higher than that of tetragonal phase ones (53.9 and 54.8 eV, respectively). The slight differences on BEs of Cu 2p and Se 3d for tetragonal and cubic phases Cu2Se NWs confirm that their surface atoms arrangement means or electronic structure are distinct, which may lead to diverse catalytic performance. No other impuries are found in the EDS, XRD, and XPS analysis, revealing that the obtained samples are pure tetragonal and cubic phases Cu2Se, respectively. The size and microstructure of the obtained products are further examined by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). Figure 2A shows the high magnification FE-SEM image of the tetragonal phase Cu2Se NWs. By statistical analysis, the average diameter of those NWs is about 50−70 nm, and their length is about several micrometers. Moreover, the appearance of those NWs is different from that of the initial Cu NWs (Figure S3). From Figure 2A, we can see that those Cu2Se NWs are assembled by small cubic-like building blocks (marked with arrows) according to a certain means. Because of the nonuniform of the small nanocube units, the surfaces of those Cu2Se NWs are rough and some protuberances can be observed. These features can be further evidenced from the TEM image (Figure 2B). Corresponding HRTEM analysis (Figure 2C) at the middle section of an individual NW exhibits clear lattice fringes and the lattice spacing is measured to be about 3.50 Å, which can be assigned to interplanar separation of (311) plane of tetragonal phase Cu2Se. The fast Fourier transform (FFT) analysis at the middle and end sections of the NWs show clear spots (Figure 2D), indicating that the obtained tetragonal phase Cu2Se NWs are well crystallized. Related elemental mapping (area scanning and line scanning) measurements for an individual tetragonal phase Cu2Se NW are shown in Figure 2E,F. From the figures we can see that both Cu and Se elements are uniformly distributed along the whole NW. These results imply that the presynthesized Cu NWs are completely converted into tetragonal phase Cu2Se NWs. The size and morphology of the obtained cubic phase Cu2Se NWs are similar to that of tetragonal phase ones. Detailed FE-SEM and TEM images as well as elemental mapping patterns for cubic phase Cu2Se NWs are shown in Figure S4. The HRTEM image of an individual

The catalyst dispersions were prepared by mixing a certain amount of Cu2Se NWs with appropriate amount of water. A designed amount of dispersion was drop-cast onto the surface of the pretreated glassy carbon electrode. After drying the electrode at room temperature, 2.5 μL of 5.0 wt % Nafion solution was spread on the surface of the catalyst layer and allowed to dry. The Cu2Se NWs that loaded on the electrode surface were about 0.020 mg. All modified electrodes were pretreated by cycling the potential between −0.7 and 0 V for 50 cycles in order to remove any surface contamination prior to electrochemical test. The rotating disk cyclic voltammetry (CV) measurements were conducted in N2-saturated 0.1 M KOH aqueous solutions. All oxygen reduction reaction (ORR) tests were conducted in O2-saturated 0.1 M KOH aqueous solutions. The polarization curves were obtained by sweeping the potential from −0.7 to 0 V at the scan rate of 5 mV s−1 under the rotating rate ranging from 600 to 2500 rpm.

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RESULTS AND DISCUSSION The component, phase structure, and purity of the assynthesized samples are examined by energy-dispersive X-ray spectrum (EDS), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Figure 1A shows the EDS

Figure 1. (A) EDS and (B) XRD patterns as well as the XPS spectra of (C) Cu 2p and (D) Se 3d for tetragonal and cubic phases Cu2Se NWs, respectively.

patterns of the tetragonal and cubic phases Cu2Se NWs. Except for the small C peak that originates from the adsorbed organic capping reagents, only Cu and Se elements are detected in those two samples. By integrated calculation, the atomic ratios of Cu and Se for those two samples are close to 2:1, implying that they may be stoichiometric Cu2Se. Further evidence comes from XRD analysis, shown in Figure 1B. In the 10° < 2θ < 70° range, seven obvious diffraction peaks associated with two small shoulder peaks are observed in the tetragonal phase sample. Compared with Joint Committee on Powder Diffraction Standard card (JCPDS 29-0575), those diffraction peaks can be indexed to (111), (311), (222), (323), (403), (510), (404), (533), and (703) planes of tetragonal phase Cu2Se (space group: P42/n) with the lattice parameters of a = 11.521 and c = 11.740 Å. As for cubic phase sample, only five obvious diffraction peaks are seen, which can be assigned to (111), 15166



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Figure 2. (A) FE-SEM, (B) TEM, and (C) HRTEM images of tetragonal phase Cu2Se NWs. (D) Corresponding FFT pattern taken at the middle section of a single tetragonal phase Cu2Se NW. (E, F) Element area and line scanning analysis of a single tetragonal phase Cu2Se NW. (G) HRTEM image of a single cubic phase Cu2Se NW. (H) Corresponding FFT pattern taken at the middle section of a single cubic phase Cu2Se NW.

or 180 °C and ripening for 1 h, the Cu nanotubes disappear and the composite NWs composed of tetragonal phase Cu2Se and cubic phase Cu are formed (Figure 3D). Related EDS and XRD patterns are given in Figure S5A,B. It is necessary to point out that the initial Cu NWs can be completely converted at 150 and 180 °C by prolonging the ripening time to 4−8 h, while only tetragonal and cubic mixed phase Cu2Se product are generated (Figure S5C) and no wire-like nanostructures are observed under this case (Figure S5D). For complete conversion of Cu NWs and formation of pure tetragonal phase Cu2Se NWs (see Figure 2A,B), the optimal reaction temperature is 220 °C and the ripening time is 1 h. At 220 °C, when the ripening time is longer than 1 h, partial metastable tetragonal phase Cu2Se NWs turn into thermodynamic stable cubic phase Cu2Se. However, even if the ripening time is extended to 4 h, no pure cubic phase Cu2Se NWs are formed. So, for obtaining pure cubic phase Cu2Se NWs, we further increase the reaction temperature to 250 and 280 °C and keep the ripening time to be 1 h. At 250 °C, the collected product is still the tetragonal and cubic mixed phase Cu2Se NWs (Figure S6). At 280 °C, pure cubic phase Cu2Se NWs are obtained (Figure 2G,H and Figure S4). Besides the reaction temperature and ripening time, we found that the used TOP plays very important roles for controllable synthesis and stability of pure tetragonal phase Cu2Se NWs. If there is no TOP, i.e., dissolving Se powder in ODE39 and using it as Se resource, only the mixed NWs composed of tetragonal phase Cu2Se and residual Cu (Figure

cubic phase Cu2Se NW is given in Figure 2G. Clear lattice fringes can be observed and the lattice spacing is measured to be about 2.1 Å, corresponding to the interplanar separation of (220) plane of cubic phase Cu2Se. Additionally, the FFT analysis (Figure 2H) at the middle and end sections of an individual NW gives clear spots, confirming that the prepared cubic phase Cu2Se NWs are also well crystallized. On the basis of the above analysis, we can learn that the presynthesized Cu NWs can be transformed into well-crystallized Cu2Se NWs that assembled by small nanocube building blocks. By adjusting experimental parameters, the phase structure of the Cu2Se NWs can be well-controlled while their size and morphology are nearly unchanged. Many factors, such as reaction temperature, ripening time, surfactant, and copper resource, are found to affect reaction kinetics and phase structure control of the final products. In order to get some insight on the growth mechanism of tetragonal and cubic phases Cu2Se NWs, a series of temperature- and time-dependent experiments are performed. When the reaction temperature is set at 120 °C, only a part of Cu NWs are converted into tetragonal phase Cu2Se after ripening for 1h, which is confirmed by XRD (Figure 3A). Corresponding TEM and HRTEM analyses (Figure 3B,C) reveal that the original Cu NWs turn into tube-like nanostructures, and many small Cu2Se nanocrystals with tetragonal phase structure are covered on outer surfaces of the formed Cu nanotubes. This may be attributed to the Kirkendall effect.38 Increasing the reaction temperature to 150 15167



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Figure 3. (A) XRD pattern, (B) TEM image, and (C) HRTEM image of the sample synthesized at 120 °C for 1 h. (D) TEM image of the sample synthesized at 150 °C for 1 h.

Figure 4. (A) Rotating disk voltammetry curves (RDVCs) for cubic phase Cu2Se NWs in N2-saturated 0.1 M KOH solution (plot a), and bare GCE (plot b), cubic and tetragonal phases Cu2Se NWs (plots c and e) as well as commercial Pd/C catalyst (plot d) in O2-saturated 0.1 M KOH solution at the potential ranging from −0.7 to 0 V with the scan rate of 5 mV s−1. The electrode rotation rate is 1600 rpm. (B) ORR mass activities comparison of cubic and tetragonal phases Cu2Se NWs and commercial Pd/C catalyst at −0.45 and −0.55 V. (C, D) ORR durability tests for tetragonal and cubic phases Cu2Se NWs before (plot a) and after 1000 cycles (plot b).

S7) are obtained at 220 °C after ripening for 1 h. This result implies that the TOP may be served as a phase-transfer catalyst

for the transformation reaction, leading to the Se resource more easily approach the Cu NWs surface and facilitating the 15168



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Figure 5. (A, B) Rotation rate dependent ORR polarization plots for cubic and tetragonal phases Cu2Se NWs, respectively. (C, D) Koutecky−Levich plots of J−1 vs ω−1/2 for cubic and tetragonal phases Cu2Se NWs at different potential obtained from (A, B), respectively. (E, F) Plots of transferred electron number and kinetic limiting current density (Jk) versus potential for cubic and tetragonal phases Cu2Se NWs, respectively.

coauthors in controlling the phase structure of CdSe nanocrystals.40 It should be mentioned that bulk tetragonal phase Cu2Se is only stable at the temperature below 141 °C (414 K).41 As the size of tetragonal phase Cu2Se is reduced to nanometer scale, its stability temperature should be lower than that of bulk counterparts. However, in the present case, the stability temperature of our tetragonal phase Cu2Se NWs (∼220 °C) is much higher than that of their bulk counterparts. Such abnormal phenomena may be attributed to the used TOP, which possesses strong coordination ability and benefits to reduce the surface energy of metastable tetragonal phase Cu2Se. Furthermore, in order to understand the effect of presynthesized Cu NWs in our reaction system, the control experiment is performed by using solid CuCl instead of Cu NWs. Under this circumstance, only large-sized Cu2Se nanocrystals with cubic phase structure are generated (Figure S10). This indicates that the presynthesized Cu NWs may be served as a template to

diffusion of Cu atoms from the interior of Cu NWs to outer surface. Thus, the chemical transformation reaction can be accelerated in the presence of TOP. Further evidence comes from another experiment by using DDA (dodecylamine) instead of TOP and keeping other conditions constant. In this case, the main product is tetragonal phase Cu2Se NWs, but there is still a small amount of unreacted Cu, as evidenced by XRD analysis (Figure S8). Additionally, by keeping other conditions constant and only using TBP (tribenzylphosphine) instead of TOP, pure cubic phase Cu2Se nanocrystals are formed at 220 °C (Figure S9), but their shape is irregular polyhedron-like, not wire-like. These results demonstrate that not only the functional groups but also the alky chain length of used surfactant affect the reaction kinetics and phase structure control of desired product. The longer alkyl chain facilitates to stabilize metastable tetragonal phase Cu2Se and spontaneously organize in-situ formed small Cu2Se nanobuilding blocks into NWs. A similar phenomenon has been observed by Talapin and 15169



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some recently reported non-Pt electrocatalysts, such as Agriched AgPd nanoalloy,26 Cu3N nanocubes,36 carbon-supported Co1.67Te2 nanoparticles,43 graphene-Ni-α-MnO2 and -Cu-αMnO 2 nanowire blends, 44 disordered brownmillerite Ba2InCeO5+δ,45 polypyrrole,46 N-, S-, or P-doped carbon nanomaterials,31−33,47 and functionalized graphene48 or graphene nanoplates.49 For comparison, the detailed ORR data for some above-mentioned electrocatalysts are summarized in Table S1. Moreover, the catalytic performance of tetragonal phase Cu2Se NWs is also better than that of polyhedron-like Cu2Se nanocrystals (Figure S13), confirming that wire-like nanostructures or assemblies can enhance ORR performance of a desired catalyst.50−53 In view of their low cost and high abundance, the tetragonal phase Cu2Se NWs may be served as a promising non-Pt ORR electrocatalyst to apply in alkaline fuel cells after further structure or component optimization. Furthermore, in order to tentatively understand the origin of distinct catalytic performances observed on pure cubic and tetragonal phases Cu2Se NWs, the electrocatalytic dynamic experiments are also carried out to study their ORR mechanisms. Figures 5A and 5B show the polarization plots for those two kinds of Cu2Se NWs catalysts measured at different rotating speeds (600, 1000, 1600, and 2500 rpm). Based on those polarization plots, the number of transferred electrons (n) involved in ORR process can be calculated according to the Koutecky−Levich equation

direct the formation of desired Cu2Se NWs, which may not be replaced by other copper resource. The electrocatalytic performances of the obtained pure tetragonal and cubic phases Cu2Se NWs are evaluated by using ORR as the probe reaction. Figure 4A shows the rotating disk cyclic voltammographs (RDCVs) representing the ORR in 0.1 M KOH solution by depositing those Cu2Se NWs on glassy carbon disk electrodes (GCEs), which are rotated at 1600 rpm during the experiments. As the control experiments, in N2saturated 0.1 M KOH solution, those Cu2Se NWs modified GCEs display no noticeable reduction features between −0.7 and 0 V. In order to show clearly, we only give the RDCVs under N2-saturated condition for pure cubic phase Cu2Se NWs in Figure 4A (plot a). As that for pure tetragonal phase Cu2Se NWs under N2-saturated atmosphere, it has been combined with its ORR plot and given in Figure S11A. While in an O2saturated 0.1 M KOH system, the use of a bare GCE (the substrate onto which the examined catalysts are deposited) leads to an onset of reduction current at about −0.13 V (plot b in Figure 4A), which increases gradually in the potential window studied (up to −0.6 V). Nevertheless, the observed current is negligible with respect to the reduction current observed when cubic and tetragonal phases Cu2Se NWs are deposited on GCEs (plots c and e in Figure 4A), indicating that the two kinds of Cu2Se NWs modified GCEs contribute catalytic effects toward enhanced ORR. The onset reduction potential on pure tetragonal phase Cu2Se NWs modified GCE is found to be about −0.039 V vs SCE (for observing clearly, please see Figure S11A), which is slightly positive than that on cubic phase ones (−0.053 V vs SCE, Figure S11B). Additionally, the apparent current density on the former modified electrode is much higher than that on the latter one at the potential ranging from −0.2 to −0.7 V. These results indicate that the electrocatalytic activity of tetragonal phase Cu2Se NWs is better than that of cubic phase ones. Compared with commercial Pd/C catalyst (plot d and Figure S11C), the onset reduction potential of our Cu2Se NWs is more positive. Moreover, the mass specific activities of pure tetragonal phase Cu2Se NWs catalyst at −0.45 and −0.55 V are higher than those of commercial Pd/C catalyst (Figure 4B), implying that the catalytic activity of tetragonal phase Cu2Se NWs is better than that of commercial Pd/C catalyst. Except for catalytic activity, the durability is another important parameter for assessing the performance of ORR catalysts. Corresponding durability data for pure tetragonal and cubic phase Cu2Se NWs catalysts are given in Figures 4C and 4D, respectively. After 1000 cycles, the apparent current density of pure tetragonal phase Cu2Se NWs retains 97% of their initial value at each potential while that for cubic phase ones only reserve 90% of their initial value, revealing that the durability of tetragonal phase Cu2Se NWs is also better than that of cubic phase ones. By using TEM to analyze tetragonal phase Cu2Se NWs sample after durability tests, we find that their size becomes larger and their morphology or structure only undergoes a slight change (Figure S12). A similar phenomenon has been observed by Sun and coauthors in Pd/FePt core/shell nanoparticles ORR electrocatalyst after durability tests.42 These results indicate that both the catalytic activity and stability of the tetragonal phase Cu2Se NWs are higher than that of cubic phase Cu2Se NWs. It should be mentioned that the catalytic performance of our tetragonal phase Cu2Se NWs toward ORR is still lower than that of conventional Pt-based catalysts, but it is comparable to or better than that of commercial Pd/C and

1 1 1 = + J Jk Bω0.5 B = 0.62nF(DO2)2/3 v−1/6CO2

where J is the measured current density, Jk and Jd (= Bω1/2) are the kinetic- and diffusion-limiting current densities, ω is the angular velocity of the disk electrode (ω = 2πN, N is the linear rotation speed), n is the overall number of electrons transferred in the O2 reduction process, F is the Faraday constant (96485 C mol−1), DO2 is the diffusion coefficient of O2 (1.8 × 10−5 cm2 s−1), CO2 is the saturated concentration of O2 in 0.1 M KOH at 1 atm O2 pressure (1.2 × 10−6 mol cm−3), and ν is the kinematical viscosity of the electrolyte (1.0 × 10−2 cm2 s−1). The Koutecky−Levich plots of J−1 vs ω−1/2 at a potential of −0.4, −0.45, −0.50, −0.55, −0.6, and −0.65 V on both pure cubic (Figure 5C) and tetragonal (Figure 5D) phases Cu2Se NWs catalysts exhibit good linearity, respectively. From the slope values (1/B) of Koutecky−Levich plots, the n values for pure cubic and tetragonal phases Cu2Se NWs catalysts at −0.4, −0.45, −0.50, −0.55, −0.60, and −0.65 V are calculated according to the above-mentioned formula. Related plots of n values versus potential for those two kinds of catalysts are given in Figure 5E. For cubic phase Cu2Se NWs catalyst, the calculated n value is 2 at −0.4 V. With the decrease of potential from −0.4 to −0.6 V, the n value is gradually increased. At −0.6 V, its n value reaches maximum, ∼3.32, which is close to 4.0. Further decreased the potential to −0.65 V, the n value begins to reduce. Such a result demonstrates that the ORR mechanism on cubic phase Cu2Se NWs is neither pure “4e−” nor “2e−” pathway. It may take place via the recently proposed dual-path ORR mechanisms,54 which comprises both direct “4e−” and series “2e− + 2e−” pathways. A similar mechanism has been suggested on carbon-supported Pt nanoparticles.54 Different from that on cubic phase Cu2Se NWs, the n value for tetragonal phase Cu2Se NWs under all the studied potential is close to 4.0, 15170



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indicating the ORR mechanism on tetragonal phase Cu2Se NWs mainly abides by direct “4e−” pathway. Moreover, as shown in Figure 5F, the Jk values for the two kinds of Cu2Se NWs catalysts first increase with decreasing potential from −0.4 to −0.55 V and then reach the maximum value at about −0.55 V (0.44 and 1.70 mA cm−2 for cubic and tetragonal phases Cu2Se NWs, respectively). On the whole, the Jk value of tetragonal phase Cu2Se NWs is about 4 times higher than that of cubic phase ones at each potential, further confirming that the catalytic performance of tetragonal phase Cu2Se NWs is superior to cubic phase ones. Based on the above analysis and XPS results (Figure 1C,D), the distinct catalytic performance of tetragonal and cubic phases Cu2Se NWs may result from their different spatial arrangement means of Cu and Se atoms that causes to different adsorption and activation of O2 molecules avenues or mechanisms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.H.), [email protected] (Z.D.); Tel +86-25-85891051. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China for the project (Nos. 21271105, 21175069, 21171096, 20901041, and 21205061), the Program for New Century Excellent Talents in University (NCET-090159), Foundation of the Jiangsu Education Committee (11KJA150003), and Research Fund for the Doctoral Program of Higher Education of China (20113207110005). We greatly appreciate the financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions and the opening research foundations of State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Solid State Microstructures, Nanjing University.

CONCLUSIONS

In summary, pure tetragonal and cubic phases Cu2Se NWs have been successfully synthesized via a simple solid−liquid phase chemical transformation method. Such NWs are composed of small Cu2Se nanocube building blocks and well crystallized. Except for reaction temperature and ripening time, the functional groups and alkyl chain length of used surfactants affect the formation and phase structure control of those Cu2Se NWs. Among various examined surfactants, the TOP is found to be the best one, which not only accelerates chemical transformation reaction but also improves the stable temperature of tetragonal phase Cu2Se NWs about 80 °C compared with that of their bulk counterparts. Electrochemical study demonstrates that both the obtained Cu2Se NWs can be used to catalyze ORR in alkaline media. However, the catalytic activity and stability of the tetragonal phase Cu2Se NWs are much higher than that of cubic phase ones. Moreover, the catalytic performance of tetragonal phase Cu2Se NWs are comparable or better than that of commercial Pd/C and some recently reported non-Pt electrocatalysts. Such novel phase structure modulated electrocatalytic ORR performance observed on those Cu2Se NWs are attributed to their different spatial arrangement means of Cu and Se atoms that causes to different adsorption and activation of O2 molecules avenues. Related electrocatalytic dynamic experiments provide strong evidence for this. The ORR on pure tetragonal phase Cu2Se NWs is identified to abide by direct “4e−” mechanism while that on cubic phase ones is confirmed to comply with dual-path mode that comprises both “2e−” and “4e−” approaches. This work not only provides a simple method for control the phase structure of desired TMCs nanostructures but also explores their electrocatalytic application in alkaline fuel cells and identifies that phase structure greatly influence ORR performance of a specific catalyst, which needs to be considered in future design of advanced electrocatalysts for clean energy fields.

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ASSOCIATED CONTENT

S Supporting Information *

Survey XPS spectra and splitting peaks of Se 3d spectra for pure tetragonal and cubic phases Cu2Se NWs, detailed structure characterization of pure cubic phase Cu2Se NWs and presynthesized Cu NWs, additional experimental data for phase structure control and ORR test results. This material is available free of charge via the Internet at http://pubs.acs.org. 15171



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Controllable Synthesis of Tetragonal and Cubic Phase Cu2Se

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