One-step synthesis of nickel phosphide nanowire array supported on nickel foam with enhanced electrocatalytic water splitting performance

Abstract

The design and facile synthesis of noble metal-free efficient catalysts to accelerate the sluggish kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is still a big challenge for electrolytic water splitting. Herein, we present a facile one-step approach for constructing a self-supported nickel phosphide nanowire array/Ni foam electrode (Ni–P NA/NF) by direct phosphorization treatment of commercial Ni foam at low temperature according to a vapor-solid growth mechanism. As a three-dimensional bifunctional water splitting catalyst, the Ni–P NA/NF exhibits outstanding electrocatalytic activity with a low cell voltage of 1.69 V to drive current density of 10 mA cm⁻². In addition, it maintains its high catalytic activity for at least 20 h in alkaline media. The presented synthesis method opens up exciting new avenues to explore the design of self-supported three-dimensional electrodes made of transition metal phosphides, ranging from water splitting to other applications.

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Introduction

Hydrogen (H₂) has been proposed as the next-generation energy resource to replace conventional fossil fuels.^1,2 Electrolytic water splitting is a promising technology to generate hydrogen fuel³ by using electricity produced from fitful renewable energy resources, e.g., wind and solar energy. The key to electrolytic water splitting is to develop high-efficiency and durable catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Although platinum-based material and noble-metal oxides (e.g. Ir oxides) are the state-of-the-art HER and OER catalysts, respectively,^4,5 their scarcity and high cost restrict their widespread application.⁶ Thus, the development of non-noble metal electrocatalysts for water splitting remains an urgent task. In the past decades, many significant research efforts have already been made to develop efficient water splitting catalysts based on oxides,^7,8 sulfides,⁹ selenides,¹⁰ phosphides,^11–15 and borides¹⁶ of non-noble metals, such as Fe, Co, and Ni, among which transition metal phosphides (TMPs), especially nickel phosphide, have emerged as a new kind of earth-abundant excellent HER catalyst.

To date, nickel phosphides with various morphologies have been synthesized using a number of methods. Nickel phosphide nanosheet array was synthesized by direct phosphorization of nickel using red phosphorus under high temperature without any post-processing steps.^17–19 Sun et al.²⁰ reported self-supported nickel phosphide nanosheet array (NiP₂/CF) can be obtained from phosphorization of Ni(OH)₂ nanosheet that was prepared via a hydrothermal method. Similar to NiP₂/CF electrode, porous Ni/Ni₈P₃,²¹ carbon coated porous nickel phosphides nanoplates,²² nickel phosphide nanoflakes,²³ urchin-like Ni₂P²⁴ and so forth were made as efficient catalysts for water splitting. Nanostructured nickel phosphide (Ni₂P) nanoparticles²⁵ were synthesized by heating nickel salt precursor and tri-n-octylphosphine in organic solvents and used as HER catalyst in acidic medium. Subsequently, Ni₂P nanowire²⁶ was reported by applying the analogous approach. Nickel phosphide nanoparticles film²⁷ was synthesized through an electrodeposition methodology and used as competent bifunctional catalysts for overall water splitting. Peapod-like Ni₂P/C nanocomposite²⁸ with excellent HER performance was prepared by using temperature programmed reduction of NH₄NiPO₄. Ni₂P-nanorods/Ni was fabricated by direct phosphorization of a Ni foam under solvothermal conditions using red phosphorus as the precursor.²⁹ Although many works have been done on it, a simple method to fabricate nickel phosphide with unique topology remains attractive.

Herein, we propose a straightforward and cost-effective method to prepare free-standing and binder-free nickel phosphide nanowire array electrode for overall water splitting. The vertically-aligned nickel phosphide nanowire array supported on Ni foam (Ni–P NA/NF) is simply synthesized by a direct phosphorization treatment of the commercial Ni foam with NaH₂PO₂·H₂O under a flowing argon atmosphere at a relative low temperature without any post-processing steps. When directly used as a robust integrated three-dimensional electrode, the Ni–P NA/NF is highly active for the HER with a very low overpotential of 73 mV and 148 mV to drive current densities of 10 mA cm⁻² in acidic media and alkaline media, respectively. When used for the OER in 1.0 M KOH, the overpotential needed to drive the current density of 20 mA cm⁻² is only 358 mV. Furthermore, the electrode can be used as a bifunctional catalyst for the overall water splitting and sustain for up to 20 h at potential of 1.8 V with a steady catalytic current density of 27 mA cm⁻² in alkaline water electrolyzer, showing high activity and good long-term stability toward water splitting.

Experimental

Preparation of Ni–P NA/NF

Before phosphorization, a piece of Ni foam (1 × 2.5 cm²) was cleaned in 3.0 M HCl for 10 min, subsequently washed by ethanol and deionized water several times and then dried at 50 deg C for 10 min. The as-treated Ni foam and ca. 4.0 g of NaH₂PO₂·H₂O were put at two separate positions in a quartz boat with NaH₂PO₂·H₂O at the upstream side of the furnace. Afterwards, the sample was heated at 300 deg C for 120 min (6 min, 15 min and 30 min for time-dependent experiments) in a static Ar atmosphere (100 sccm), and then cooled to ambient temperature naturally under Ar atmosphere. Finally, the resultant foam was washed with deionized water and ethanol, and dried in a N₂ flow. The actual loading of active Ni–P on Ni foam was determined to be 31.5 mg cm⁻² with the use of a high precision microbalance.

Characterization

The morphologies of the products were examined by field-emission scanning electron microscope (SEM) using Nova NanoSEM 450 at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) observations were carried out on a FEI-Tecnai G20 U-Twin operating both at 200 kV. X-ray powder diffraction (XRD) data was acquired on a diffractometer (X'Pert PRO, Panalytical B.V.) with a Cu K_α radiation source. X-ray photoelectron spectroscopy (XPS) measurements were taken on a VG ESCALAB 250 spectrometer with an Al K_α X-ray source (1486 eV), X-ray radiation (15 kV and 10 mA), and hemispherical electron energy analyzer.

Electrochemical measurements

All of the electrochemical measurements were performed with a CHI 760E electrochemical analyzer (CH Instruments, Inc., Shanghai) at room temperature (about 25 deg C) in a standard three-electrode system with a saturated calomel reference electrode (SCE) in a 0.5 M H₂SO₄ solution (the Hg/HgO electrode as the reference electrode in 1.0 M KOH solution). The Ni–P NA/NF materials as-prepared were used as the working electrodes. For comparison, the electrocatalytic performances of a bare Ni foam, commercial Pt/C (20 wt% Pt) deposited on glassy carbon electrode (1.41 mg cm⁻²) and commercial RuO₂ deposited on glassy carbon electrode (1.41 mg cm⁻²) were also measured. Linear sweep voltammetry (LSV) was performed at a scan rate of 5 mV s⁻¹. Unless otherwise specified, all potentials were iR-compensated to 95% with the built-in program. Electrochemical impedance spectroscopy (EIS) analysis was carried out at different overpotentials in the frequency range of 100 kHz to 50 mHz with a 10 mV ac amplitude. The accelerated degradation test was conducted using cyclic voltammetry at a scan rate of 50 mV s⁻¹ between −290 mV and 160 mV vs. RHE for 5000 continuous cycles. The long-term stability tests for the HER were performed using chronoamperometry measurements at an overpotential of 200 mV (at an overpotential of 400 mV for the OER, at a potential of 1.8 V for overall water splitting). All potentials measured were calibrated to reversible hydrogen electrode (RHE) using the following equations: E(RHE) = E(SCE) + 0.241 + 0.059 pH (in 0.5 M H₂SO₄), E(RHE) = E(Hg/HgO) + 0.098 + 0.059 pH (in 1.0 M KOH).

Results and discussion

The sodium hypophosphite (NaH₂PO₂) has been proven to be an effective phosphorus source to prepare specific phosphides.^17–21 Here, a certain amount of NaH₂PO₂·H₂O is put in a tube furnace with a piece of Ni foam (1 × 2.5 cm²) at the downstream side and the phosphorization temperature is 300 deg C (Scheme S1†). The method used here can be completed at a low temperature with one-step and no toxic solvent is needed, which is more advantage than other methods.^11,15,22 Following the phosphorization reaction, the slivery gray bare Ni foam turns to black (Fig. S1†) but maintains its three-dimensional structure. Thus, it can be directly used as an electrode for HER and OER without any post-processing steps. The morphologies of the as-obtained sample together with Ni foam were firstly characterized by SEM. As shown in Fig. 1 and S2,† the original smooth Ni foam (Fig. S2†) is fully covered with high-density arrays of vertically aligned nanowires after phosphorization treatment (Fig. 1a). The high-resolution SEM images (Fig. 1b and c) reveal that the quadrangular-like nickel phosphide nanowires are ranges from 50 to 200 nm in width and several microns in length. The corresponding energy dispersive X-ray (EDX) spectrum verifies that except a small amount of C and O elements that caused by adsorbate, the sample only contains Ni and P elements, and the atom ratio of Ni to P is 2.29 : 1 (Fig. 1d). Moreover, the SEM-EDX mapping shows that the Ni and P are uniformly distributed over the ligament surface (Fig. 1e). These results uncover that the PH₃ gas, which generated from the thermal decomposition of the NaH₂PO₂·H₂O, can easily access the Ni foam surface for its three-dimensional porous structure and then react with all surface sites to generate nickel phosphide. TEM images show such nickel phosphide nanowires have a smooth surface (Fig. S3a†) and the selected-area electron-diffraction (SAED) pattern (Fig. S3b†) shows discrete spots indexed to the (200) and (111) planes of the Ni₂P phase.

Fig. 1 (a–c) SEM images, (d) EDX spectra and (e) elemental mapping of Ni–P NA/NF electrode.

The XRD pattern of the as-prepared electrode in Fig. 2a exhibits that the phosphorized samples are composed of a mixture of Ni₁₂P₅ (JCPDS no. 00-022-1190) and Ni₂P (JCPDS no. 01-074-1385). The expected atomic ratio of Ni to P lies between 2 : 1 and 2.4 : 1, which is consistent with the result of EDX. The diffraction peaks observed at 44.5°, 51.8° and 76.4° are attributed to the unreacted Ni foam. To further elucidate the chemical composition of the phosphorized sample electrode material, XPS measurement was performed. The survey spectrum of Ni–P NA/NF is presented in Fig. 2b, evidencing the existence of P, Ni and O, which again confirms that P was included on the sample and nickel phosphide was successfully formed on the surface of the Ni foam. The C 1s signal is mainly due to adsorbate on the sample owing to exposure to air.³⁰ The peaks located at 853.0 eV and 870.2 eV (Fig. 2c) correspond to the characteristic of Ni 2p3/2 and peaks of Ni^δ+ in Ni₁₂P₅–Ni₂P. And the peaks situated at 856.2 eV and 873.9 eV are assigned to be oxidized Ni species. The peaks centered at 861.7 eV and 880.1 eV are ascribed to the satellites of Ni 2p3/2 and Ni 2p1/2, respectively.²³ Meanwhile, the P 2p3/2 and P 2p1/2 peaks of the sample are located at 129.5 eV and 130.4 eV, respectively (Fig. 2d), are in line with the typical peaks of P^δ−, revealing the presence of P^δ− in the sample. Additionally, the peak at 133.3 eV can be ascribed to oxidized phosphate species,³¹ which is further confirmed by the peaks of O 1s line centered at 531.3 eV and 532.6 eV (Fig. S4†) that is due to the lattice oxygen (O²⁻) and O–P, respectively.³² All of these results clearly confirm the successful growth of nickel phosphide nanowire array on Ni foam after the phosphorization treatment.

Fig. 2 (a) Wide-angle XRD pattern of the prepared phosphides. The standard XRD patterns of Ni₁₂P₅ and Ni₂P are also given for reference. (b) XPS survey for a Ni–P NA/NF electrode, XPS in the region of Ni 2p (c), P 2p (d) orbits for Ni–P NA/NF electrode.

To gain insight into the formation mechanism of this novel structure, the detailed time-dependent experiments were conducted. As shown in Fig. S5,† a layer of film is firstly forming on the surface of Ni foam at the beginning of the phosphorization reaction (Fig. S5a†), with extension of the reaction time, the nickel phosphide nanowires gradually grow along the vertical direction of Ni foam (Fig. S5b†), and a nanowires forest can be observed on the surface of Ni foam when the reaction time is up to 30 min (Fig. S5c†). Two mechanisms, vapor-liquid-solid (VLS)³³ and vapor-solid (VS)^34,35 have been most commonly used to illuminate the growth of nanowires in the gas phase. Based on our SEM and TEM observations, there are no nanoparticles found at the end of nickel phosphide nanowires. So, the VLS growth mechanism can be excluded because the terminated particle is necessary at the growth front of the nanowire to act as the catalytic active site. As a result, the VS mechanism, which has been used to prepare a variety of metal oxides nanowires,^34,35 seems to be responsible for the growth of nickel phosphide nanowires observed in our case. In order to verify the growth mechanism, another experiment was conducted. The Ni foam was firstly oxidized at 600 deg C for 2 h in air atmosphere before phosphorization treatment. However, only a layer of film is covered on the surface of the Ni foam (Fig. S6a and b†). We speculate that the Ni atom is chained by oxygen atom and can't migrate in the growth process. This result further confirms that the proposed growth process is based on the VS mechanism.

The catalytic performance of Ni–P NA/NF for HER was assessed by using linear sweep voltammetry (LSV). LSV measurement was firstly carried out in 0.5 M H₂SO₄ at a scan rate of 5 mV s⁻¹ with the nickel phosphide nanowire array electrode as the working electrode. The analogous tests for bare Ni foam and commercial 20 wt% Pt/C deposited on glassy carbon electrode (1.41 mg cm⁻²) were also performed for comparison. Fig. 3a shows their polarization curves with iR-compensation for different electrodes on the reversible hydrogen electrode (RHE) scale (see Experimental sections for detail). As expected, Pt/C electrode shows excellent HER activity with negligible overpotential. However, the bare Ni foam exhibits very poor HER performance with an onset overpotential (the potential at which the current density is 1 mA cm⁻²) of 168 mV. On the contrary, the Ni–P NA/NF electrode exhibits a remarkably high activity for HER with a low onset overpotential of 17 mV, which is very close to that of the Pt/C electrode. Further increasing the cathodic potential causes a rapid rise of cathodic current, indicating that the Ni–P NA/NF act as a high performance three-dimensional cathode for HER in acidic media. In addition, this electrode can offer cathodic current densities of 10, 20 mA cm⁻² for HER at overpotentials of 73 mV (η₁₀) and 97 mV (η₂₀), respectively. In terms of η₁₀, The Ni–P NA/NF compares favorably with most of reported non-noble metal HER catalysts in acidic media, including Ni₅P₄ Film (58 mV),¹⁷ Ni-doped graphene (152 mV),³⁶ CoNi@NC (142 mV)³⁷ and core–shell MoO₃–MoS₂ (255 mV)³⁸ (see Table S1† for a detailed comparison). When tested in 1.0 M KOH, the Ni–P NA/NF also exhibits outstanding electrocatalytic activity with a overpotential of 148 mV to drive 10 mA cm⁻², which is also comparable to those of many reported non-noble metal HER catalysts in alkaline media (see Table S1†).

Fig. 3 (a) LSV curves and (b) Tafel plots of the bare Ni foam, Ni–P NA/NF electrode, and commercial Pt/C (20%) catalysts measured in 0.5 M H₂SO₄ and 1.0 M KOH (dotted line) with a scan rate of 5 mV s⁻¹. (c) LSV curves of the Ni–P NA/NF electrode before and after cyclic voltammetry for 1000 and 5000 cycles in 0.5 M H₂SO₄ at a scan rate of 50 mV s⁻¹. (d) Chronoamperometric curve of Ni–P NA/NF measured at an overpotential of 200 mV in 0.5 M H₂SO₄ and 1.0 M KOH.

To get more insight into the catalytic performance of the nickel phosphide nanowires, a Tafel plot was constructed. As shown in Fig. 3b, the Tafel slope of 30.1 mV dec⁻¹ for commercial Pt/C is consistent with the reported value.³⁹ The Ni–P NA/NF exhibits a Tafel slope of 70.8 mV dec⁻¹ in 0.5 M H₂SO₄ within the range of 40–120 mV dec⁻¹, which is comparable to some previously reported non-noble metal catalysts such as Ni₅P₄–Ni₂P-NS (79.1 mV dec⁻¹),¹⁸ Co/nitrogen-doped graphene (Co-NG) (82 mV dec⁻¹),⁴⁰ MoSe₂/RGO (101 mV dec⁻¹)⁴¹ and WN NA/CC (92 mV dec⁻¹),⁴² much smaller than that of bare Ni foam (104.3 mV dec⁻¹). The Tafel slope suggests that the HER take place on the nickel phosphide surface via a Volmer–Heyrovsky mechanism.⁴³ The Tafel slope is 115.2 mV dec⁻¹ for Ni–P NA/NF in alkaline media (1.0 M KOH). By extrapolating the Tafel plot to an overpotential of 0 V, the exchange current density of HER on the Ni–P NA/NF electrode is determined to be 0.85 and 0.51 mA cm⁻² in acidic and basic media, respectively, which is large and can be associated with its large ECSA (86.4 mF cm⁻², Fig. S7†).⁴⁴

To explore the HER kinetics on Ni–P NA/NF electrode, electrochemical impedance spectroscopy experiments were carried out at different applied overpotentials and the experimental Nyquist plots shown in Fig. S8a† display a semicircle. While the semicircle decreases with increasing negative overpotential. Experimental data is fitted with the 2CPE model (Fig. S8c†) proposed by Chen and Lasia,⁴⁵ in which the semicircle (CPE – R_ct; CPE, constant phase element; R_ct, charge transfer resistance) is related to the kinetics of the HER and a small R_ct indicates the fast charge transfer kinetics. Variations of R_ct of the Ni–P NA/NF electrode with the overpotential are shown in Fig. S8b,† it can be concluded that the charge transfer kinetics of the HER is remarkably promoted by increasing the overpotential. The Tafel slope can also be obtained from the plot of overpotential versus log(1/R_ct) (Fig. S8b,† inset), which is 73.6 mV dec⁻¹, close to that obtained from the polarization curve (70.8 mV dec⁻¹).

Durability and stability are important parameters for evaluating HER catalyst. Hence, continuous cyclic voltammetry (CV) of Ni–P NA/NF was conducted from −0.29 V to 0.16 V (vs. RHE, without IR-compensation) for 5000 cycles at 50 mV s⁻¹ in 0.5 M H₂SO₄. The polarization curves before and after CV tests are shown in Fig. 3c. It can be seen that the catalyst shows only a slight degradation after first 1000 cycles and then maintains the same catalytic activity. Moreover, the electrode is able to maintain a steady cathodic current density at ∼120 mA cm⁻² in 0.5 M H₂SO₄ and ∼26 mA cm⁻² in 1.0 M KOH for 20 h, respectively, at a constant overpotential of 200 mV (Fig. 3d). In short, the as-prepared Ni–P NA/NF electrode exhibits good durability and the excellent HER activity can be perfectly retained in long-term practical applications.

The electrocatalytic activity of the Ni–P NA/NF for OER was tested in 1.0 M KOH solution. Fig. 4a shows the typical LSV curves of the bare NF, commercial RuO₂ and as-prepared Ni–P NA/NF. It can be seen that the bare Ni foam exhibits negligible catalytic activity. RuO₂ (1.41 mg cm⁻², deposited on glassy carbon electrode) displays high OER activity with a low overpotential of 311 mV for a current density of 20 mA cm⁻². As expected, the OER catalytic current density of the Ni–P NA/NF reaches 20 mA cm⁻² at a markedly small overpotential of 357 mV, and then rises fast by applying a little more potential (from 20 mA cm⁻² at 1.587 V to 50 mA cm⁻² at 1.688 V). Although this overpotential is higher than that for commercial RuO₂, it's still lower than that obtained from many other non-noble metal-based catalysts, including Co–P film (375 mV),⁴⁶ N-CG–CoO (373 mV),⁴⁷ N-doped graphene NiCo₂O₄ (526 mV),⁴⁸ PCN-CFP (430 mV),⁴⁹ Co@Co₃O₄/NC (420 mV)⁵⁰ and so on (Table S2†). The catalytic kinetics for OER of the above catalysts were examined by the corresponding Tafel plots. As shown in Fig. 4b, the Tafel slope value of Ni–P NA/NF (76.0 mV dec⁻¹) is lower than that of Ni foam (98.5 mV dec⁻¹), implying the superior OER kinetics of the Ni–P NA/NF electrode. Fig. 4c displays a multi-step chronopotentiometric curve for Ni–P NA/NF in 1.0 M KOH, the current density is increased from 20 to 160 mA cm⁻² with an increment of 20 mA cm⁻² per 500 s. The potential for every step maintains constant in the corresponding 500 s, implying the excellent mass transport performance, conductivity and mechanical robustness of the Ni–P NA/NF electrode. We probed the long-term electrochemical stability of Ni–P NA/NF electrode by a 20 h chronoamperometric test in 1.0 M KOH (Fig. 4d). As observed, the current density reaches about 33 mA cm⁻² at a potential of 1.63 V versus RHE, and it then stabilizes around this value during the 20 h reaction session.

Fig. 4 (a) LSV curves and (b) Tafel plots of the bare Ni foam, Ni–P NA/NF electrode, and commercial RuO₂ catalysts measured in 1.0 M KOH with a scan rate of 5 mV s⁻¹. (c) Multi-current process of Ni–P NA/NF. The current density started at 20 mA cm⁻² and ended at 160 mA cm⁻², with an increment of 20 mA cm⁻² per 500 s without iR-compensation. (d) Chronoamperometric curve of Ni–P NA/NF measured at an potential of 1.63 V in 1.0 M KOH.

Given that Ni–P NA/NF is active and stable catalyst for two half reactions of water splitting in 1.0 M KOH, we believed that it could act as an electrocatalyst for overall water splitting. Therefore, an alkaline electrolyzer in a two-electrode setup using Ni–P NA/NF as both anode and cathode catalysts was constructed (Fig. 5a, inset). As shown in Fig. 5a, the alkaline water electrolyzer can afford 10 mA cm⁻² water splitting current density at a cell voltage of 1.69 V in 1.0 M KOH. Such a voltage is smaller than that of the majority of the state-of-the-art bifunctional catalysts (Ni₃S₂/NF: 1.71 V,⁹ Co₂B: 1.81 V,¹⁶ Ni₅P₄ films: 1.70 V,¹⁷ NiFe layered double hydroxide: 1.70 V,⁵¹ Ni(OH)₂: 1.82 V,⁵¹ carbon paper/carbon tubes/cobalt–sulfide sheets (CP/CTs/Co–S): 1.74 V (ref. 52)) at the same catalytic current density. Meanwhile, Ni–P NA/NF electrodes can undergo continuous electrolysis for at least 20 h at a cell voltage of 1.8 V in 1.0 M KOH with negligible degradation of water splitting current (Fig. 5b), suggesting their promise to replace noble metal catalysts for the production of clean hydrogen. It is worth mentioning that the three-dimensional vertically-aligned nanowire array morphology of the Ni–P NA/NF remains unchanged after long-term stability tests for HER and OER (Fig. S9a–d†).

Fig. 5 (a) LSV curves of water electrolysis for Ni–P NA/NF in two-electrode setup with a scan rate of 5 mV s⁻¹ (inset: the photograph of two-electrode setup). (b) Chronoamperometric curve of Ni–P NA/NF measured at a cell voltage of 1.8 V in 1.0 M KOH.

The excellent catalytic performance of the Ni–P NA/NF electrode can be attributed to the following factors: (1) the active material directly grown on three-dimensional microporous conductive skeleton ensures the effective mass-transfer and charge-transfer; (2) the unique vertically-aligned nanowire array structure would remarkably enlarge the electrochemically active surface area; (3) the stability of nickel phosphide nanowires promises long-term use of the water splitting devices.

Conclusions

In summary, the nickel phosphide nanowire array on Ni foam is synthesized via a one-step route through phosphating treatment of the commercial Ni foam with NaH₂PO₂·H₂O at low temperature according to the vapor-solid growth mechanism. The remarkable three-dimensional structure together with the features of the obtained Ni–P NA/NF electrode, showing an excellent electrocatalytic HER and OER activity and long-term stability, promises its practical use in water splitting catalysts. In addition, the whole fabrication process is low cost and simple. It is easy to extend to other technological devices. Hence, this study would open up exciting new avenue to explore the design of self-supported three-dimensional electrodes made of transition metal phosphides, ranging from water splitting catalysts to other applications.

Acknowledgements

This work is supported by the National Natural Science Foundation (Project No. 51173055, 51572094, 21401060) and China Postdoctoral Science Foundation (Project No. 2015M572135). We thank the Analytical and Testing Center of Huazhong University of Science and Technology, the Wuhan National Laboratory for Optoelectronics.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20737e

‡ These authors contributed equally to this work.

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