Ultralong PtNi alloy nanowires enabled by the coordination effect with superior ORR durability

Abstract

One-dimensional (1D) nanostructure with high aspect ratio is expected and experimentally proven as a promising candidate of catalysts with simultaneous high durability and efficiency. Due to the isotropic growth tendency of a face-centered cubic (fcc) structure and the complicated redox-alloying kinetics/thermodynamics; however, the synthesis of Pt-based bimetallic nanowires with a high aspect ratio remains a big challenge. In this study, we report the synthesis of ultralong PtNi nanowires via a surfactant-free method by taking advantage of the coordination effect to delicately control the reduction and alloying kinetics. The ORR electrocatalytic activities of the PtNi nanowires present a volcano-like trend depending on the Ni content with the Pt₇₅Ni₂₅ nanowires at the peak position, which can be attributed to the optimized electronic structure on the basis of the XPS characterizations. The electrochemical catalytic comparison between the nanowires and nanopolyhedrons further highlights the superiority of the nanowires as a high durability catalyst. The successful application of the coordination effect method in the fabrication of PtCo nanowires indicates its universality for creating alloy nanowires.

---

Introduction

Platinum (Pt) shows an excellent catalytic performance towards both the anodic oxidation reaction and the cathodic oxygen reduction reaction (ORR) in fuel cells.^1–6 However, the scarce reserve and poor catalyst poisoning-resistance of pure Pt catalysts largely impede their wide commercialization. Researches indicate that alloying Pt with transition metals (such as Fe, Co and Ni)^7–10 exhibits great potential in addressing this challenge. The performance improvement is believed to originate from the contracted lattice and modified electronic structure.¹¹

This improved performance has stimulated extensive efforts devoted to the controlled synthesis of Pt-based bimetallic nanoalloys with specific structures (such as cubes, octahedrons, branches and cages)^12–17 and compositions (PtNi, PtCo, PtCu and PtAu).^18–23 Despite the great progress achieved on catalytic activity, the poor durability and stability caused by the degradation of the Pt-based bimetallic nanocatalysts under the corrosive electrochemical condition has become a major limitation.^24,25 Atom migration, particle aggregation as well as structure evolution were frequently observed during the electrochemical process, which result in performance loss.^26,27

One-dimensional (1D) nanostructures, with unique characteristics, such as high surface area, high conductivity and high flexibility, are expected and experimentally proved as promising candidates of catalysts with simultaneously high durability and efficiency.^28,29 As for face-centered cubic (fcc) structured Pt-based alloy, it prefers to assume an isotropic form. To date, the limited successful cases of Pt-based alloy 1D nanostructures have mainly involved the surfactant-directed strategy.^24,27 Unfortunately, the use of surfactant may block the surface active sites and lead to activity decrease.^30,31 In addition, the aspect ratio of the as-prepared Pt-based alloy is rather low via the surfactant directed method. Therefore, it is of great importance to explore a surfactant-free method for the synthesis of 1D Pt-based bimetallic nanowires with high aspect ratio.

It is known that the template-based method is an important strategy for the fabrication of 1D nanowires. Pre-formed 1D nanowires or nanotubes are required to template the growth of the target by taking advantage of the space confinement and/or various interactions. Several reports have also emerged for the successful preparation of Pt-based alloys (such as PtAu and PbPt)³² employing the template-based method. Template removing is a major drawback limiting its application.³³ In general, it is more possible to promote the anisotropic growth of a monometallic nanocrystal by kinetic control rather than a multi-metallic one because of the complicated reduction/alloying kinetics and thermodynamic of the latter. Thus, we hypothesized that if we could achieve a delicate balance between the nucleation and alloying rates for a bimetallic system, a pre-formed component 1D structure (such as Pt nanowires) could be employed as a template to direct the formation of 1D bimetallic nanohybrids, which would efficiently overcome the shortage of template-removing processes.

In this study, we reported the successful fabrication of Pt-based nanowires (including PtNi and PtCo) with ultrahigh aspect ratio via a coordination effect-facilitated self-template process. Coordination effect was employed to adjust the nucleation rate of the second composition and thereafter the alloying kinetics. The ORR electrocatalytic performance of the as-prepared PtNi nanowires was investigated and a composition-dependent activity was observed.

Experimental

Preparation of PtNi nanowires

In a typical synthesis, 0.1 mmol H₂PtCl₆·6H₂O, 0.5 g KOH and 0.15 mmol NiCl₂·6H₂O were dissolved in a solution consisting of 9 mL ethylene glycol (EG) and 3 mL N,N-dimethylformamide (DMF). After stirring in a water bath at 50 °C for 12 h, the transparent brilliant yellow solution was transferred into a Teflon-lined stainless autoclave and maintained at 170 °C for 5 h. It was then left to cooled down to room temperature. The mixture was firstly centrifuged at a speed of 5000 rpm to remove the large sized precipitate. Then, the nanowires were precipitated by introducing ethanol into the suspension and collected by centrifugation at a speed of 10 000 rpm. After washing with water and ethanol alternately for several times, the obtained PtNi nanowires were dispersed in ethanol for further characterization. The composition of the nanowires can be tuned by adjusting the amount of the added NiCl₂·6H₂O according to the target. To synthesize PtCo nanowires, a similar procedure was applied but substituting NiCl₂·6H₂O with CoCl₂·6H₂O.

Characterizations

The morphologies of the products were observed by environmental scanning electron microscopy (SEM; Quanta 250 FEG with an accelerating voltage of 30 kV) and transmission electron microscopy (TEM; JEOL JEM-2100F with an accelerating voltage of 200 kV). The composition of the products was analyzed using an energy-dispersive X-ray spectrometer (EDAX) equipped within the JEM-2100F TEM instrument. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Rotaflex Dmax2200 diffractometer with Cu-Kα radiation of λ = 1.5406 Å. The valence state of the products was tested by X-ray photoelectron spectroscopy (XPS) on an Axis Ultra spectrometer using a standard Al-Kα radiation.

Electrochemical activity tests

The electrochemical activities of the as-prepared catalysts were tested by cyclic voltammetry (CV) curves and linear sweep voltammetry (LSV) curves. The experiment was performed with a three-electrode test system on Pine Instruments using a glassy carbon (GC) electrode (5 mm in diameter) as the work electrode, an Ag/AgCl/KCl (3 mol L⁻¹) electrode as the reference electrode and a Pt wire as the counter electrode. Before preparing the work electrode, the GC electrode was polished using an alumina aqueous suspension. To prepare the work electrode, 2 mg catalysts were ultrasonically dispersed into 1 mL mixture solution comprising 590 μL water, 400 μL alcohol and 10 μL Nafion alcohol solution (5%) to form homogeneous ink. Then, 10 μL catalyst ink was coated on the GC electrode and it was dried naturally.

The CV measurements were performed in 0.1 mol L⁻¹ HClO₄ aqueous solution at a scanning speed of 0.05 V s⁻¹ in the range of 0.05–1.25 V. Before examination, the electrolyte was ventilated with nitrogen for 30 min to exclude oxygen. LSV curves were recorded in an O₂-saturated 0.1 mol L⁻¹ KOH aqueous solution at a sweeping rate of 0.01 V s⁻¹ and the rotating rate of 1600 rpm. The stability of the catalyst was tested before and after 1000 cycles ranging from 0.7 to 1.1 V at a scanning rate of 0.05 V s⁻¹.

Results and discussion

The PtNi nanowires with ultrahigh aspect ratio were synthesized via a surfactant-free method using the mixture of EG and DMF as the solvent in the presence of KOH. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) observation (Fig. 1a) indicates that nanowires with ultrahigh aspect ratio dominated the product. The nanowire was about 2.5 nm in diameter (Fig. 1a–c) with a length of up to several micrometres (inset in Fig. 1a), which intended to aggregate into bundles with diameters of about 15 nm. The frequently-bent structure reveals their highly flexibility. The molar ratio of Pt and Ni is determined as about 3 : 1 as confirmed both by inductively coupled plasma atomic emission spectrometry (ICP-AES, 78 : 22) and energy dispersive X-ray spectrometry (EDS, 75 : 25). This indicates that these two characterization methods were consistent with each other. For simplicity, the EDS data were used thereafter.

Fig. 1 (a and c) TEM and (b) HRTEM image of the as-prepared Pt₇₅Ni₂₅ nanowires. The inset in (a) is the survey SEM image.

The crystalline structure was characterized by high-resolution TEM (HRTEM) and X-ray powder diffraction (XRD). The adjacent lattice distance was measured as 0.221 nm, which lies between the d-spacing of the (111) planes of face-centered cubic (fcc) Pt (0.226 nm) and Ni (0.203 nm), implying the formation of PtNi alloys. The alloy structure was further confirmed by the XRD pattern (Fig. 2a2). The XRD pattern exhibited two diffraction peaks in the range of 30–60°, which can be indexed to the fcc phase structure. The diffractions were located between those of fcc-Pt and fcc-Ni. Therefore, both the lattice spaces and diffraction peaks provide solid evidence for the alloy structure of the product.

The composition of the products can be easily tuned by adjusting the relative feed ratio of NiCl₂·6H₂O and H₂PtCl₆·6H₂O. The XRD diffractions shifted towards the higher degree region with Ni increase (Fig. 2a). According to Vegard's law, the lattice of a bimetallic alloy would contract with the increase of the smaller atomic radius composition. Because the radius of Ni (0.115 nm) is smaller relative to Pt (0.130 nm), the composition-dependent diffraction shift verified their alloy structure in turn. Simultaneously, the average diameter of the as-prepared nanowires increased with the Ni content. As shown in Fig. 2b, the diameters of the nanowire gradually increased from 2.2 through 2.5 to 3.1 and 3.5 nm as the ratio of Ni/Pt increased from 10 : 90 to 25 : 75, 45 : 55 and 60 : 40. Moreover, it is worthy to be noted that all the alloy nanowires were thicker than the pure Pt nanowires synthesized by a similar procedure but without the addition of Ni (Fig. S2b†).

Fig. 2 (a) The XRD pattern and (b) the TEM images of as-prepared PtNi nanowires with four different atom ratios of Pt/Ni as (1) 90/10, (2) 75/25, (3) 55/45 and (4) 40/60.

To identify more details over the growth process, the products at different reaction stages were collected and characterized. The sequential EDS data are shown in Fig. S2.† The nanowires were first identified when the reaction was carried out for about 40 min. The EDS results show that the nanowires at this stage were primarily composed by Pt with a small amount of Ni (11%). As the reaction proceeded, the content of Ni gradually increased. After 2 h of reaction, the Ni content reached 23%. The component and size evolvement with either duration or feed ratio suggests that the Pt nanowires might form first and then the subsequently reduced Ni atoms diffuse and gradually alloy with the pre-formed Pt nanowires. Considering the higher redox potential of PtCl₆²⁻/Pt (φ^θ = 0.705 eV vs. SHE) than that of Ni²⁺/Ni (φ^θ = −0.25 eV vs. SHE), it was reasonable to conclude that Pt species were reduced first. It was believed that the newly produced Pt nanoparticles would oriented-attach into nanowires under the direction of NH₃ released by DMF in the presence of KOH.²⁷ With the reaction proceeding, the newly generated Ni species would alloy with pre-formed Pt nanowires and the diameter was enlarged.

The usage of NiCl₂ was found to be crucial for the successfully fabrication of PtNi alloy nanowires. We performed the control experiments using different anionic nickel salts (such as NiSO₄ and Ni(NO₃)₂) as Ni sources, while keeping other conditions identical. The reactions were carried out at 170 °C for 5 h. TEM observation indicates that the products comprised mixtures of nanowire and nanoparticle aggregates (Fig. S3a and b†). The EDS data revealed that the irregular particle aggregates were dominated by Ni, whereas the Ni content was less than 5% in the nanowires, which suggests that Ni could not be effectively doped into the nanowires and that severe phase separation had occurred. Scheme 1 illustrates the formation process of PtNi nanowires and the impact of Cl⁻ ions.

Scheme 1 Illustration of the formation process of the PtNi nanowires.

To gain an in-depth understanding on the role of the Cl⁻, we varied the Cl⁻ concentration using KCl as the additive with Ni(NO₃)₂ as the precursor. EDS data reveal that the content of Ni in the products exhibits as a volcano curve depending on the Cl⁻/Ni²⁺ molar ratio (Fig. S4†). The Ni content initially increased with the Cl⁻ concentration and then decreased after it reached the maximum value (60%) at the Cl⁻/Ni²⁺ of 1/2.

It is believed that the complexation between Cl⁻ and Ni²⁺ controlled the reaction dynamics. According to the Pearson's hard and soft acid–base (HSAB) theory, soft Lewis acids bond more tightly with soft bases, whereas hard acids prefer to compact with hard bases. It is known that Cl⁻ is a borderline base, whereas SO₄²⁻ and NO₃⁻ are hard bases.³⁴ Therefore, Cl⁻ would exhibit a stronger coordination ability with Ni²⁺, a borderline acid, relative to SO₄²⁻ and NO₃⁻. The Ni(II)–Cl⁻ complexation lowered the redox potential of Ni(II)/Ni and thus slowed down the reduction rate of Ni, which guaranteed sufficient time for the newly generated Ni species to fuse into and alloy with pre-formed Pt nanowires instead of self-nucleation. Therefore, the Ni “doping” was promoted, which resulted in the increase in Ni content when the Cl⁻/Ni²⁺ increased from 1 to 2. However, if the stability of the complex is too high, the reduction would be greatly retarded and less Ni species were produced, which might give an explanation for the decreasing Ni with the further increasing of Cl⁻/Ni²⁺ from 2 to 3 and finally to 4.

In addition to PtNi nanowires, the coordination effect-facilitated method also allows for the synthesis of PtCo nanowires (Fig. S5†). The EDS data demonstrate that the presence of Cl⁻ anions is also helpful for the formation of PtCo bimetallic nanowires. The similar processes for the formation of PtCo and PtNi systems indicate the universality of the coordination effect-assisted method in creating the alloy nanowires.

The electrochemical performances of the as-prepared PtNi nanocatalysts with four different atomic radios of Pt/Ni (Pt₉₀Ni₁₀, Pt₇₅Ni₂₅, Pt₅₅Ni₄₅, and Pt₄₀Ni₆₀) towards ORR were explored by CV and LSV. The pure Pt nanowires synthesized by the same procedure but without the introduction of Ni (Fig. S1†) was also examined for comparison. Fig. 3a shows the CV curves of the five as-obtained electrocatalysts examined in 0.1 M HClO₄ aqueous solution from 0 to 1.2 V (RHE) with a sweep rate of 0.05 V s⁻¹. The PtNi nanocatalysts exhibit similar hydrogen adsorption–desorption peaks with the pure Pt nanocatalyst. The electrochemically active surface area (ECSA) was calculated according to the hydrogen adsorption/desorption region. The ECSA of Pt₉₀Ni₁₀, Pt₇₅Ni₂₅, Pt₅₅Ni₄₅ and Pt₄₀Ni₆₀ were 65, 81, 135 and 164 cm² mg_pt⁻¹, respectively, which were all higher than that of the pure Pt nanocatalyst (46 cm² mg_Pt⁻¹). The enhanced ECSA values indicate that the introduction of Ni optimized the utilization of Pt with more electrochemical active sites.

Fig. 3 (a) Cyclic voltammetry (CV) in 0.1 M HClO₄; (b) ORR polarization curves in O₂-saturated 0.1 M KOH solution at the rotating rate of 1600 rpm; polarization curves of (c) Pt₇₅Ni₂₅ nanowires and (d) Pt₇₅Ni₂₅ nanoparticles before and after 1000 cycles; comparison of (e) mass activity at 0.9 V and (f) durability at 0.85 V. The NWs and NPs represent nanowires and nanoparticles, respectively.

The oxygen–reduction reaction (ORR) polarization curves were recorded in an O₂-saturated 0.1 M KOH solution with a sweep rate of 0.01 V s⁻¹ at room temperature. Fig. 3b shows the ORR polarization curves (vs. RHE) for the as-prepared pure Pt and PtNi nanowires with different composition at a rotation speed of 1600 rpm. The polarization curves of the PtNi nanowires are similar to those of the pure Pt nanowires, exhibiting a limiting diffusion current in the potential region of 0.2 to 0.6 V and a mixed kinetic-limiting control region from 0.6 to 1.0 V. However, the onset reduction potential of PtNi nanowire catalysts appears at approximately 0.95 V, which is 0.05 V more positive than that of the pure Pt nanowire catalyst. Moreover, the half-wave potential of PtNi nanowires (Pt₉₀Ni₁₀, 0.85 V; Pt₇₅Ni₂₅, 0.86 V; Pt₅₅Ni₄₅, 0.84 V; Pt₄₀Ni₆₀, 0.84 V) is also positively shifted relative to the pure Pt catalyst (0.81 V). The positively shifted potential indicates that the fusion of Ni into Pt nanowires enhanced the catalytic performance.

To further study the kinetic processes of these ORR catalytic reactions, the ORR polarization plots of Pt₇₅Ni₂₅ nanowires was tested under different rotation rates (Fig. S6a†). The ORR polarization plots under different potentials were calculated according to the Koutecky–Levich (K–L) equation to obtain the plot slope (Fig. S6b†). The result clarifies the 4 electron reaction for the electrocatalytic ORR, which reveals a consistent kinetic process on the commercial Pt catalyst recorded in 0.1 M KOH solution. The mass activities at 0.9 V (Fig. 3e) normalized against the mass of Pt were summarized to quantify the ORR activities of different catalysts. As shown in Fig. 3e, the mass activities present a volcano-like trend depending on the Ni content. The Pt₇₅Ni₂₅ nanowires catalyst revealed the highest mass activity (8.5 mA mg_Pt⁻¹, about 3 fold of the pure Pt nanowire value). The distinguished limiting diffusion currents for the five catalysts also reveal a bidirectional consequence that the activity first increased and then decreased with the Ni amount.³⁵

To reveal the underlying mechanism, we carried out XPS characterizations to explore the electronic states on the surface of the PtNi nanowires with different composition. Fig. 4 shows the Pt-4f XPS spectra of the Pt₉₀Ni₁₀, Pt₇₅Ni₂₅ and Pt₅₅Ni₄₅ nanowires. All these three XPS spectra comprise two peaks, which can be allocated to the 4f_5/2 and 4f_7/2 of metallic Pt. No Pt(II)-related peaks were detected. In comparison with the pure Pt nanoparticles (71.32 eV, dashed line in Fig. 3a), the Pt-4f core-level peaks of the three as-prepared PtNi nanowires negatively shifted for about 0.07 eV, 0.20 eV, and 0.29 eV. The weakened binding energy could be attributed to the electron interaction between Pt and Ni atoms. The larger electronegativity of Pt than that of Ni induces the electron shift from Ni to Pt, which leads to the lower Pt binding energy. It is obvious that the sufficient surface contact would benefit the electron transfer. The atomic interaction increased with Ni and thus resulted in the lower binding energy.

Fig. 4 . The Pt-4f XPS spectra of Pt₅₅Ni₄₅, Pt₇₅Ni₂₅, and Pt₉₀Ni₁₀ nanowires, respectively. The dashed lines represent the peak positions of pure metallic Pt.

Previous studies demonstrated that the optimal level of Pt–O towards the ORR was calculated about 0.2 eV weaker than that on Pt (111) surface.²⁶ Apparently, the binding energy in Pt₇₅Ni₂₅ nanowires was almost equal, which optimized the absorption/desorption energy between Pt and oxygenated intermediates and thus promoted the kinetic process.

We also evaluated the electrochemical durability performance of the Pt₇₅Ni₂₅ nanowires using the accelerated durability test between 0.7 and 1.1 V at a scan rate of 0.05 V s⁻¹. For comparison, PtNi nanoparticle with the same composition (Pt₇₅Ni₂₅) was also examined. Fig. 3c and d show the corresponding cycling curves of the Pt₇₅Ni₂₅ nanowires and nanoparticles. After 1000 potential cycles, the activity of the nanowires declined slightly, showing only less than 5 mV shifts for its half-wave potential. In contrast, the polarization curve of the nanoparticles exhibited an over 50 mV negative shift after the test. Fig. 3f shows the comparisons of the activity loss of these two catalysts after 1000 cycles at 0.85 V, which were normalized against their respective initial value. Only less than 10% decrease was observed for Pt₇₅Ni₂₅ nanowires, whereas the decrease was more than 50% for Pt₇₅Ni₂₅ nanoparticles. We also examined the morphology and composition of the two catalysts before and after electrochemical tests (Fig. S7†). No obvious difference was identified for the nanowires. However, the nanoparticles experienced an obvious morphology variation. Moreover, relatively more Ni was dissolved during the electrochemical cycling (from 25% to 20%). The durability comparison further highlights the superiority of the nanowires as an electrocatalyst of ORR.

Conclusions

1D PtNi nanowires with ultrahigh aspect ratio were synthesized via a self-template method. The coordination between Cl⁻ and Ni(II) is crucial for the bimetallic nanowire construction, which efficiently tuned the reduction rate of Ni and thus its nucleation-alloy balance. The composition of the nanowires can be facially adjusted in a wide region by simply varying the Cl⁻ concentration. The electrochemical catalytic investigation reveals that all the as-prepared PtNi nanowires exhibited higher activities relative to pure Pt nanowires, which follows a volcano-like tendency with the Ni increasing. The Pt₇₅Ni₂₅ nanowires expressed the best performance, which was believed to be due to its optimized electronic state on the basis of the XPS characterization. The comparison of the durability between the Pt₇₅Ni₂₅ nanowires and Pt₇₅Ni₂₅ nanoparticles further highlighted the superiority of the 1D nanostructure as the electrochemical nanocatalyst towards ORR.

Acknowledgements

The study was supported by the Beijing Natural Science Foundation (2162021), the National Natural Science Foundation of China (21173015) and Fundamental Research Funds for the Central Universities.

Notes and references

1. L. Bu, J. Ding, S. Guo, X. Zhang, D. Su, X. Zhu, J. Yao, J. Guo, G. Lu and X. Huang, Adv. Mater., 2015, 27, 7204–7212.
2. Y. Bing, H. Liu, L. Zhang, D. Ghosh and J. Zhang, Chem. Soc. Rev., 2010, 39, 2184–2202.
3. Y. Li, B. P. Bastakoti, V. Malgras, C. Li, J. Tang, J. H. Kim and Y. Yamauchi, Angew. Chem., Int. Ed., 2015, 54, 11073–11077.
4. R. Xie, Y. Pan and H. Gu, RSC Adv., 2015, 5, 16497–16500.
5. W. Hong, C. Shang, J. Wang and E. Wang, Nanoscale, 2015, 7, 9985–9989.
6. V. R. Stamenkovic, B. Fowler, B. S. Mun, G. Wang, P. N. Ross, C. A. Lucas and N. M. Marković, Science, 2007, 315, 493–497.
7. Y. Nie, L. Li and Z. D. Wei, Chem. Soc. Rev., 2015, 44, 2168–2201.
8. Y. Jiao, Y. Zheng, M. Jaroniec and S. Z. Qiao, Chem. Soc. Rev., 2015, 44, 2060–2086.
9. T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto and T. Yamaguchi, Energy Environ. Sci., 2015, 8, 3545–3549.
10. L. Liu, R. Scholz, E. Pippel and U. Gösele, J. Mater. Chem., 2010, 20, 5621–5627.
11. J. Ryu, J. Choi, D. H. Lim, H. L. Seo, S. Y. Lee, Y. Sohn, J. H. Park, J. H. Jang, H. J. Kim, S. A. Hong, P. Kim and S. J. Yoo, Appl. Catal., B, 2015, 174, 526–532.
12. R. M. Aran-Ais, F. Dionigi, T. Merzdorf, M. Gocyla, M. Heggen, R. E. Dunin-Borkowski, M. Gliech, J. Solla-Gullon, E. Herrero, J. M. Feliu and P. Strasser, Nano Lett., 2015, 15, 7473–7480.
13. Z. Niu, D. Wang, R. Yu, Q. Peng and Y. Li, Chem. Sci., 2012, 3, 1925–1929.
14. H. Yang, J. Zhang, K. Sun, S. Zou and J. Fang, Angew. Chem., Int. Ed., 2010, 49, 6848–6851.
15. L. Zhang, L. T. Roling, X. Wang, M. Vara, M. Chi, J. Liu, S.-I. Choi, J. Park, J. A. Herron, Z. Xie, M. Mavrikakis and Y. Xia, Science, 2015, 349, 412–416.
16. C. Wang, M. Chi, D. Li, D. Strmcnik, D. van der Vliett, G. Wang, V. Komanicky, K.-C. Chang, A. P. Paulikas, D. Tripkovic, J. Pearson, K. L. More, N. M. Markovic and V. R. Stamenkovic, J. Am. Chem. Soc., 2011, 133, 14396–14403.
17. X. Yu, D. Wang, Q. Peng and Y. Li, Chem. Commun., 2011, 47, 8094–8906.
18. D. Wang, Y. Yu, H. L. Xin, R. Hovden, P. Ercius, J. A. Mundy, H. Chen, J. H. Richard, D. A. Muller, F. J. DiSalvo and H. D. Abruna, Nano Lett., 2012, 12, 5230–5238.
19. C. Cui, L. Gan, M. Neumann, M. Heggen, B. R. Cuenya and P. Strasser, J. Am. Chem. Soc., 2014, 136, 4813–4816.
20. S. H. Kim, H. Jeong, J. Kim and I. S. Lee, Small, 2015, 11, 4884–4893.
21. D. F. Zhang, J. Li, J. X. Kang, T. W. Chen, Y. Zhang, L. L. Wang and L. Guo, CrystEngComm, 2014, 16, 5331–5337.
22. Y. Jia, Y. Jiang, J. Zhang, L. Zhang, Q. Chen, Z. Xie and L. Zheng, J. Am. Chem. Soc., 2014, 136, 3748–3751.
23. B. Y. Xia, H. B. Wu, N. Li, Y. Yan, X. W. D. Lou and X. Wang, Angew. Chem., Int. Ed., 2015, 54, 3797–3801.
24. H. Zhu, S. Zhang, S. Guo, D. Su and S. Sun, J. Am. Chem. Soc., 2013, 135, 7130–7133.
25. M. Escudero-Escribano, P. Malacrida, M. H. Hansen, U. G. Vej-Hansen, A. Velázquez-Palenzuela, V. Tripkovic, J. Schiøtz, J. Rossmeisl, I. E. L. Stephens and I. Chorkendorff, Science, 2016, 352, 73–76.
26. X. Huang, Z. Zhao, L. Cao, Y. Chen, E. Zhu, Z. Lin, M. Li, A. Yan, A. Zettl, Y. M. Wang, X. Duan, T. Mueller and Y. Huang, Science, 2015, 348, 1230–1234.
27. B. Y. Xia, H. B. Wu, Y. Yan, X. W. Lou and X. Wang, J. Am. Chem. Soc., 2013, 135, 9480–9485.
28. X. Huang, E. Zhu, Y. Chen, Y. Li, C.-Y. Chiu, Y. Xu, Z. Lin, X. Duan and Y. Huang, Adv. Mater., 2013, 25, 2974–2979.
29. M. Cao, D. Wu and R. Cao, ChemCatChem, 2014, 6, 26–45.
30. H. Chen, D. Wang, Y. Yu, K. A. Newton, D. A. Muller, H. Abruna and F. J. DiSalvo, J. Am. Chem. Soc., 2012, 134, 18453–18459.
31. C. Cui, L. Gan, H. H. Li, S. H. Yu, M. Heggen and P. Strasser, Nano Lett., 2012, 12, 5885–5889.
32. C. Zhu, S. Guo and S. Dong, Adv. Mater., 2012, 24, 2326–2331.
33. P. Wu, H. Zhang, Y. Qian, Y. Hu, H. Zhang and C. Cai, J. Phys. Chem. C, 2013, 117, 19091–19100.
34. R. G. Pearson, Chem. Educ., 1968, 45, 581–587.
35. S. Liu, Q. Zhang, Y. Li, M. Han, L. Gu, C. Nan, J. Bao and Z. Dai, J. Am. Chem. Soc., 2015, 137, 2820–2823.

---

Footnote

† Electronic supplementary information (ESI) available: Morphologies and components, electrochemical data. See DOI: 10.1039/c6ra14192g

---