Metal–organic framework-derived nickel phosphides as efficient electrocatalysts toward sustainable hydrogen generation from water splitting

Abstract

Developing robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is an ongoing scientific challenge. The cheap and active metal phosphides have emerged as new candidates for electrochemical HER. Herein, we report on the scalable synthesis of nickel phosphides (Ni₂P and Ni₁₂P₅) via directly phosphatizing a Ni-based metal–organic framework (MOF) for the first time. The MOF-derived Ni₂P nanoparticles exhibited high-performance for electrochemical HER, as manifested by a low overpotential and a large cathodic current density.

---

Introduction

The extensive utilization of traditional fossil fuels causes severe environmental problems and energy crises. This major concern drives the increased demand for exploring new clean energy sources to meet the sustainability of human society.^1,2 Hydrogen appears to be one of the most ideal and cleanest energy sources and is considered as a principal energy carrier for the future,^3,4 due to its high gravimetric energy density, ideal combustion efficiency and non-toxicity. However, current production of hydrogen fuel mainly relies on steam reforming and partial oxidation of hydrocarbons, inevitably bringing about large amounts of carbon dioxide emissions. Water electrolysis is an alternative process and a desirable way to produce molecular hydrogen, but both half-reactions of water splitting, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), still remain technical challenges. Although Pt-based electrocatalysts are most active and highly stable for HER, their widespread applications are restricted by the inevitable disadvantages of high price and limited abundance.⁵ Therefore, it is of great significance to seek earth-abundant alternatives to Pt catalysts to gain a sustainable molecular hydrogen production from water splitting.

At present, several solid-state catalysts composed of earth-abundant elements (e.g. Fe, Co, Ni, and Mo) are selected as versatile candidates for replacement of precious catalysts to achieve high-performance HER activity.^6–10 For example, molybdenum sulfides with various structure and tuneable composition have been demonstrated to be one of most active electrocatalysts for the HER.^11–14 Very recently, transition-metal phosphides (TMPs), formed by alloying of metals and phosphorus, have attracted intense attention as a new family of HER electrocatalyst. The TMPs including MoP,^15–17 FeP,^18–20 CoP,^21–24 Co₂P,²⁵ Cu₃P,²⁶ Ni₂P^27–30 and WP³¹ have shown promise for HER in strong acidic media with high activity and excellent stability. Among these, the HER behaviour of Ni₂P is of especial interest, because Ni₂P possesses charged natures similar to those of the hydride acceptor and proton acceptor in [NiFe] hydrogenase and its analogues. Based on density functional theory (DFT) calculation, Rodriguez and Liu have predicted that Ni₂P would be an excellent catalyst for HER, because the presence of the proton-acceptor (P site) and hydride-acceptor (Ni site) centers in Ni₂P can facilitate catalysis of the HER.³² Such behaviour has been experimentally demonstrated by Popczun et al.²⁷ and Sun et al.²⁸ in various Ni₂P nanostructures. Despite these advances, important challenges remain, notably how to produce Ni₂P nanostructures in a facile, cost-effective and scalable manner, since most of available synthesis methods normally suffer from complicated operations, special apparatus, harsh conditions and low yield.

Herein, we report a new method to synthesize nickel phosphide nanoparticles with phase selectivity (Ni₂P or Ni₁₂P₅) by directly phosphating Ni–MOF (Ni–BTC) at a mild condition. This strategy is simple, tunable, inexpensive, and scalable, and is thus highly promising for large scale production. More importantly, we have experimentally demonstrated that the MOF-derived Ni₂P nanoparticles would be an excellent candidate of non-precious HER electrocatalyst, showing a low overpotential and high current density.

Experimental

Materials

Sodium hypophosphite and benzene-1,3,5-tricarboxylic acid (BTC) were purchased from Aladdin Ltd. (Shanghai, China). Nickel nitrate, methanol, and H₂SO₄ were purchased from Kelong Chemical Reagents Company (Chengdu, China). All chemicals used in this study were of commercially available analytical grade and used without further purification.

Synthesis of Ni–BTC metal–organic framework (Ni–MOF)

Ni–BTC metal–organic framework (Ni–MOF) was synthesized according to previous report using methanol as solvent.³³ Typically, 4.4 mmol of Ni(NO₃)₃·6H₂O and 2.4 mmol of BTC were dissolved in 70 mL absolute methanol. The mixture was stirred for 1 h at room temperature, and then transferred into a Teflon-lined stainless steel autoclave with a volume capacity of 100 mL and heated at 150 °C for 24 h. After the heat treatment, the autoclave is allowed to cool naturally to room temperature, and the products are collected by centrifugation at 10 000 rpm for 5 min and washed with absolute methanol several cycles, and then dried at 60 °C in vacuum 12 h.

Synthesis of nickel phosphides derived from Ni–BTC metal–organic framework

In a typical procedure, 0.1 g of as-prepared Ni–BTC and 0.3 g of NaH₂PO₂ were mixed together and loaded in a ceramic crucible with a cover and then heated to different temperatures (275 °C for Ni₂P nanoparticles and 325 °C for Ni₁₂P₅ nanoparticles) with a ramp rate of 5 °C for 1 h in a muffle furnace. After naturally cooling to room temperature, the resulting product was washed with water and ethanol several times and dried in vacuum at 60 °C for 6 h.

Characterization

The powder X-ray diffraction (XRD) measurements were recorded on a RigakuDmax/Ultima IV diffractometer with monochromatized Cu Kα radiation (λ = 0.15418 nm). The Fourier transform infrared (FTIR) spectroscopy was recorded on Nicolet 6700 FTIR Spectrometric Analyzer using KBr pellets. The morphology was observed with the Hitachi S4800 field emission scanning electron microscopes (FESEM) and transmission electron microscope (TEM, FEI Tecnai G20). The elemental composition of the samples were characterized by energy-dispersive X-ray spectroscopy (EDS, Oxford instruments X-Max). X-Ray photoelectron spectroscopy (XPS) measurements were recorded on a Perkin-Elmer PHI 5000C spectrometer using monochromatized Al Kα excitation. All binding energies were calibrated by using the contaminant carbon (C_1S = 284.6 eV) as a reference.

Electrochemical measurement

5 mg of nickel phosphide powders was dispersed in 1 mL mixture of distilled water and ethanol (3 : 1 v/v). Then, 10 μL 5 wt% Nafion was added to the above solution. The mixed solution was sonication at least 30 min to form a homogeneous ink. 5 μL of the mixed solution was drop-casted onto the glassy carbon electrode with the diameter of 3 mm for the electrochemical measurements. All the electrochemical measurements were performed on an electrochemical workstation (CHI 660E, CH Instruments Inc., Shanghai) using a typical three-electrode mode with an electrolyte solution of 0.5 M H₂SO₄, a Pt wire counter electrode, an Ag/AgCl electrode (saturated KCl) reference electrode, and a modified glassy carbon working electrode. All potentials measured were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: E_RHE = E_Ag/AgCl + 0.197 + 0.059 pH. The polarization curves were obtained by sweeping the potential from 0.2 to −0.8 V (versus the RHE) at room temperature with a scan rate of 5 mV s⁻¹. Chronoamperometric responses were obtained at −170 mV (versus the RHE) in a 0.5 M H₂SO₄ solution. The current density was calculated by the geometric area of the glassy carbon electrode which is 0.07 cm⁻².

Results and discussion

The powder X-ray diffraction (XRD) pattern of the Ni–MOF precursor (Fig. S1, ESI†) indicates the amorphous crystalline characteristic, which matches well with previously reported XRD pattern of Ni–BTC.³³ The FTIR spectrum further confirm the molecular structure of Ni–BTC (Fig. S2, ESI†). SEM images (Fig. S3, ESI†) reveal that the Ni–MOF is composed of solid microsphere particles with a diameter of 4.5 μm. Interestingly, the Ni₂P nanoparticles can be obtained via a solid chemical transformation from the Ni–MOF precursor by a heat treatment at 275 °C for 1 h. Fig. 1a shows a typical XRD pattern of the MOF-derived Ni₂P product. The well-defined diffraction peaks at 40.7°, 44.6°, 47.4°, 54.2°, 54.9°, 66.4°, 72.7° and 74.7° can be perfectly indexed to (111), (201), (210), (300)/(002), (211), (400), (302) and (321) planes of hexagonal Ni₂P (JCPDS file no. 89-4864), indicating a good crystallinity and high purity of the product. Fig. S4a† displays a low-magnification scanning electron microscopy (SEM) image of the Ni₂P product, which reveals that the sample is composed of numerous nanoparticles. These nanoparticles tend to aggregate together and have an average diameter of about 25 nm (Fig. 1b). The compositional analysis carried out by energy dispersive X-ray (EDX) spectroscopy (Fig. 1c) confirms the presence of Ni, P elements and a trace amount of O element. The atomic ratio of Ni to P is estimated to be 1.95 : 1, which is quite consistent with that of stoichiometric Ni₂P (2 : 1). The corresponding SEM-EDX elemental mapping images (Fig. 1d) further suggest that both Ni and P elements are uniformly distributed in the Ni₂P nanoparticles. The detailed microstructure of the Ni₂P nanoparticles was further characterized by transmission electron microscopy (TEM). Fig. S4b and c† clearly show that the Ni₂P product consists of a great deal of aggregated particles with diameters of ∼25 nm. A HRTEM image given in Fig. 1e presents the distance between the lattice fringes is 0.507 nm (see the line profile in Fig. 1f), corresponding to the d-spacing of the (100) plane of the Ni₂P. The well-defined lattice fringes in the Ni₂P nanoparticles reflect their good crystallinity. The corresponding fast Fourier transform (FFT) pattern (Fig. 1g) shows that the diffraction spots are projected by the (100), (110) and (001) planes, further revealing a hexagonal phase of the Ni₂P nanoparticles with good crystallinity.

Fig. 1 XRD pattern (a), SEM images (b), EDS spectrum (c), SEM-mapping images (d), HRTEM (e) and corresponding line profile (f) and FFT pattern (g) of the Ni₂P nanoparticles.

When phosphidation treatment of Ni–MOF precursor was conducted at a calcination temperature of 325 °C for 1 h, the Ni₁₂P₅ nanoparticles with pure phase can be produced. Fig. 2a shows a typical XRD pattern of the MOF-derived Ni₁₂P₅ nanoparticles. All diffraction peaks can be indexed as the tetragonal Ni₁₂P₅ phase (JCPDS file no. 74-1381). The Ni₁₂P₅ nanoparticles exhibit a similar morphology to that of Ni₂P nanoparticles, except for the increased particle size up to 80 nm (Fig. 2b and c). A representative TEM image of the Ni₁₂P₅ nanoparticles is given in Fig. 2d, from which aggregated nanoparticles can be clearly observed. The corresponding HRTEM image is shown in Fig. 2e. The lattice fringes are ca. 0.253 and 0.305 nm, corresponding to the (002) and (220) crystal planes of high crystalline Ni₁₂P₅, respectively. The corresponding FFT pattern (Fig. 2f) shows that the diffraction spots are projected by the (002) and (220) planes, further revealing a tetragonal phase of the Ni₁₂P₅ nanoparticles.

Fig. 2 XRD pattern (a), SEM images (b and c), TEM image (d), HRTEM image (e) and corresponding FFT pattern (f) of the Ni₁₂P₅ nanoparticles.

The electrocatalytic activities of the Ni₂P and Ni₁₂P₅ nanoparticles towards HER were tested by using a typical three-electrode setup in a 0.5 M H₂SO₄ solution. The working electrode was prepared through loading the catalyst onto a surface (0.07 cm²) of glassy carbon electrode (GCE) with a same mass loading (∼0.35 mg cm⁻²). Fig. 3a shows the polarization curves of Ni₂P and Ni₁₂P₅ nanoparticles modified GCE in acid medium. As a comparison, the Ni–MOF, commercial Pt/C catalyst (20 wt%), and bare GCE were also performed under the identical measurements. Pt/C catalyst exhibits the expected HER activity with a near zero overpotential. The bare GCE is totally inactive toward hydrogen generation, while the Ni–MOF shows negligible electrocatalytic performance as well. As for the Ni₁₂P₅ nanoparticles, they are able to electrocatalytically produce molecular hydrogen, but a poor HER activity with a high onset overpotential (about 380 mV) and a low cathodic current density is distinctly found. In a sharp contrast, the Ni₂P nanoparticles are highly active toward the HER, as evidenced by its small onset overpotential (∼75 mV), large current density and the quickly formed bubbles at the electrode surface under cathodic bias (Fig. S5, ESI†). Moreover, above onset overpotential, the cathodic current increases sharply under more negative potentials. Generally, the potential required for the current density of 10 mA cm⁻² is a matric relevant to solar fuel synthesis, commonly used to evaluate the HER activity. Of note, the Ni₂P nanoparticles can afford such current density at a small η of ∼172 mV, which is lower than that of Ni₂P synthesized by using NiCl₂ as a Ni-precursor under the same condition (Fig. S6, ESI†) and even compare to that of the reported metal phosphides, such as Ni₂P nanoparticles,^27,29 CoP nanoparticles,²² MoP nanoparticles,¹⁶ FeP nanosheets.³⁴

Fig. 3 (a) Polarization curves of the Ni₂P nanoparticles, Ni₁₂P₅ nanoparticles, Ni–MOF, commercial Pt/C catalysts and bare GCE with a scan rate of 5 mV s⁻¹ in 0.5 M H₂SO₄ solution. (b) Tafel plots of the Ni₂P nanoparticles, Ni₁₂P₅ nanoparticles and commercial Pt/C catalysts. (c) Capacitive currents at 0.13 V as a function of scan rate for Ni₂P and Ni₁₂P₅ nanoparticles (Δj₀ = j_a − j_c). (d) Nyquist plots of the Ni₂P and Ni₁₂P₅ nanoparticles. (e) Durability test for the Ni₂P nanoparticles by CV scanning at different cycles in 0.5 M H₂SO₄ solution. (f) Continuous hydrogen evolution over Ni₂P/GEC electrode at an overpotential of ∼170 mV.

The catalytic kinetics of the Ni₂P and Ni₁₂P₅ nanoparticles was examined by using Tafel plots (log j − η) derived from polarization curves. The linear regions of the Tafel plots are fitted to the Tafel equation (η = b log j + a, where η is overpotential, j is the current density, and b is the Tafel slope). As shown in Fig. 3b, the Tafel slope of 30 mV per decade for Pt/C catalyst agrees with the reported value.²⁷ The Tafel slope of the Ni₂P nanoparticles is as low as 62 mV per decade, which is much smaller than that of the Ni₁₂P₅ nanoparticles (∼270 mV per decade). The small Tafel slope of the Ni₂P nanoparticles benefits them for electrocatalysis of HER, as a smaller Tafel slope would give rise to faster incrementation of the HER rate with increasing overpotential.^35–37 The exchange current densities (j₀) for the Ni₂P and Ni₁₂P₅ nanoparticles are further calculated by extrapolating the Tafel plots (Fig. S7, ESI†). The Ni₂P nanoparticles afford a much larger exchange current density of 0.071 mA cm⁻² than that of Ni₁₂P₅ nanoparticles (0.045 mA cm⁻²). The exchange current density is usually expressed in terms of projected or geometric surface area and relative to the surface roughness.²⁶ Therefore, electrochemical surface area of the Ni₂P and Ni₁₂P₅ nanoparticles was estimated from cyclic voltammetry (CV) (Fig. S8, ESI†) at a region without electrochemical reactions by measuring the electrochemical double layer. As shown in Fig. 3c, the capacitance of the Ni₂P and Ni₁₂P₅ nanoparticles is 2.44 and 1.84 mF cm⁻², respectively, which reveals that the Ni₂P nanoparticles possess a higher active surface area than Ni₁₂P₅ nanoparticles. Also, the electrochemical impedance spectroscopy (EIS) analysis of the Ni₂P and Ni₁₂P₅ nanoparticles was further performed to understand the electrode kinetics in HER processes. The Nyquist plots given in Fig. 3d reveal a smaller charge transfer resistance of the Ni₂P nanoparticles, suggesting the much faster electron transfer process of the Ni₂P nanoparticles. This result is consistent with the HER polarization curves of catalysts (Fig. 3a).

The durability of the Ni₂P nanoparticles toward the HER was further evaluated by sweeping catalyst for different cycles in 0.5 M H₂SO₄ solution at a scan rate of 50 mV s⁻¹ (Fig. 3e). The HER activity of the Ni₂P nanoparticles shows the slight degradation after each cycle test compared to that of the initial cycle. Additionally, chronoamperometric durability test (Fig. 3f) also presented that the current density is reduced to ca. 75% of initial value in 6 hours. This result is consistent with the previously reported Ni₂P nanoparticles.²⁷

Considering that heat treatment is crucial for the transformation of Ni–MOF into nickel phosphides, we further studied how the calcined temperature affected on the HER activity of Ni₂P nanoparticles. XRD patterns shown in Fig. S9† suggest that all the samples obtained at the calcined temperatures of 250, 275 and 300 °C are the hexagonal Ni₂P. SEM observations indicate that these Ni₂P nanoparticles display similar morphology (Fig. S10, ESI†). Fig. S11a† shows the corresponding polarization curves of these Ni₂P nanoparticles toward HER. The HER activity follows the order Ni₂P-275 > Ni₂P-300 > Ni₂P-250. Tafel plots also demonstrate a similar trend, which are calculated to be 231, 62, and 119 mV per decade, for Ni₂P-250 Ni₂P-275 and Ni₂P-300, respectively (Fig. S11b, ESI†). Such observations reveal that the calcined temperature controlling could optimize the HER activity of the Ni₂P nanoparticles.

Based on previous DFT calculation, the excellent activity of the Ni₂P for HER is ascribed to the presence of the proton-acceptor (P site) and hydride-acceptor (Ni site) centers on the surface of Ni₂P, which work cooperatively to facilitate the HER.³² As demonstrated by Sun's recent works, the charged natures of Ni and P in Ni₂P are analogous to those of proton acceptors and hydride acceptors in [NiFe] hydrogenase and its analogues.^28,38 To clarify the origin of the present HER activity, the charged characteristic of the Ni₂P nanoparticles was further analysed by the X-ray photoelectron spectroscopy (XPS). As shown in Fig. 4a, three peaks around 852.5, 855.8 and 861.0 eV in the energy level of Ni 2p_3/2 correspond to Ni^δ+ in Ni₂P,³⁹ oxidized Ni species and the satellite peak of Ni 2p_3/2. There are two peaks in the P 2p XPS spectrum (Fig. 4b). One peak at 129.6 eV can be assigned to P in Ni₂P,⁴⁰ the other peak at 133.5 eV is attributed to oxidized P species.⁴¹ Meanwhile, SEM-EDX spectrum (Fig. 1c) also confirms the existence of oxygen in the Ni₂P nanoparticles due to superficial oxidation of Ni₂P, consistent with previous reported Ni₂P.²⁸ Of note, the binding energy (852.5 eV) of Ni in Ni₂P is very close to that of zero valence state Ni,^41,42 while the binding energy (129.6 eV) of P is negatively shifted from elemental P (130.0 eV),^28,38 suggesting that the related Ni and P species in Ni₂P have a partial positive charge (δ⁺) and a negative charge (δ⁻), respectively. This result implies that the HER activity of the Ni₂P nanoparticles might be correlated with the intrinsically charged natures of Ni and P. Moreover, the higher δ⁺ value^30,42 and electrical conductivity (Fig. 3d) of Ni₂P is critical to the better HER activity compared to the Ni₁₂P₅.

Fig. 4 XPS spectra of the Ni₂P nanoparticles: (a) Ni 2p and (b) P 2p.

Conclusions

In summary, we have synthesized nickel phosphide nanoparticles by solid chemical transformation of Ni–BTC MOF precursor under mild condition. Such economical, earth abundant MOF-derived Ni₂P nanoparticles exhibit excellent electrocatalytic activity for the HER with an onset overpotential as low as 75 mV and large cathodic current density, which compare favorably with the HER performance of reported metal phosphides, showing great potential as a low cost alternative to precious Pt catalyst in practical applications. More importantly, our synthetic strategy is simple, tunable, inexpensive, and scalable, and is thus highly promising for large scale production, which can be extended for the general synthesis of other TMPs from MOFs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21103141 and 21207108), the Sichuan Youth Science and Technology Foundation (2013JQ0012), and the Research Foundation of CWNU (12B018, 14E016).

Notes and references

1. S. Zinoviev, F. Muller-Langer, P. Das, N. Bertero, P. Fornasiero, M. Kaltschmitt, G. Centi and S. Miertus, ChemSusChem, 2010, 3, 1106–1133.
2. N. Armaroli and V. Balzani, ChemSusChem, 2011, 4, 21–36.
3. Z. Y. Lu, H. T. Wang, D. S. Kong, K. Yan, P. C. Hsu, G. Y. Zheng, H. B. Yao, Z. Liang, X. M. Sun and Y. Cui, Nat. Commun., 2014, 5, 4345–4351.
4. J. A. Turner, Science, 2004, 305, 972–974.
5. C. G. Morales-Guio, L.-A. Stern and X. Hu, Science, 2014, 43, 6555–6569.
6. D. Merki, H. Vrubel, L. Rovelli, S. Fierro and X. Hu, Chem. Sci., 2012, 3, 2515–2525.
7. Y.-F. Xu, M.-R. Gao, Y.-R. Zheng, J. Jiang and S.-H. Yu, Angew. Chem., Int. Ed., 2013, 52, 8546–8550.
8. J. Kibsgaard, Z. Chen, B. N. Reinecke and T. F. Jaramillo, Nat. Mater., 2012, 11, 963–969.
9. D. S. Kong, J. J. Cha, H. T. Wang, H. R. Lee and Y. Cui, Energy Environ. Sci., 2013, 6, 3553–3558.
10. M. S. Faber, R. Dziedzic, M. A. Lukowski, N. S. Kaiser, Q. Ding and S. Jin, J. Am. Chem. Soc., 2014, 136, 10053–10061.
11. D. Merki and X. Hu, Energy Environ. Sci., 2011, 4, 3878–3888.
12. A. B. Laursen, S. Kegnæs, S. Dahl and I. Chorkendorff, Energy Environ. Sci., 2012, 5, 5577–5591.
13. J. D. Benck, T. R. Hellstern, J. Kibsgaard, P. Chakthranont and T. F. Jaramillo, ACS Catal., 2014, 4, 3957–3971.
14. Y. Yan, B. Xia, Z. Xu and X. Wang, ACS Catal., 2014, 4, 1693–1705.
15. P. Xiao, M. A. Sk, L. Thia, X. Ge, R. J. Lim, J.-Y. Wang, K. H. Lim and X. Wang, Energy Environ. Sci., 2014, 7, 2624–2629.
16. Z. Xing, Q. Liu, A. M. Asiri and X. Sun, Adv. Mater., 2014, 26, 5702–5707.
17. X. Chen, D. Wang, Z. Wang, P. Zhou, Z. Wu and F. Jiang, Chem. Commun., 2014, 50, 11683–11685.
18. R. Liu, S. Gu, H. Du and C. M. Li, J. Mater. Chem. A, 2014, 2, 17263–17267.
19. Y. Liang, Q. Liu, A. M. Asiri, X. Sun and Y. Luo, ACS Catal., 2014, 4, 4065–4069.
20. P. Jiang, Q. Liu, Y. Liang, J. Tian, A. M. Asiri and X. Sun, Angew. Chem., Int. Ed., 2014, 53, 12855–12859.
21. P. Jiang, Q. Liu, C. Ge, W. Cui, Z. Pu, A. M. Asiri and X. Sun, J. Mater. Chem. A, 2014, 2, 14634–14640.
22. Q. Liu, J. Tian, W. Cui, P. Jiang, N. Cheng, A. M. Asiri and X. Sun, Angew. Chem., Int. Ed., 2014, 53, 6710–6714.
23. J. Tian, Q. Liu, A. M. Asiri and X. Sun, J. Am. Chem. Soc., 2014, 136, 7587–7590.
24. E. J. Popczun, C. G. Read, C. W. Roske, N. S. Lewis and R. E. Schaak, Angew. Chem., Int. Ed., 2014, 53, 5427–5430.
25. Z. Huang, Z. Chen, Z. Chen, C. Lv, M. G. Humphrey and C. Zhang, Nano Energy, 2014, 9, 373–382.
26. J. Tian, Q. Liu, N. Cheng, A. M. Asiri and X. Sun, Angew. Chem., Int. Ed., 2014, 53, 9577–9581.
27. E. J. Popczun, J. R. McKone, C. G. Read, A. J. Biacchi, A. M. Wiltrout, N. S. Lewis and R. E. Schaak, J. Am. Chem. Soc., 2013, 135, 9267–9270.
28. Z. Pu, Q. Liu, C. Tang, A. M. Asiri and X. Sun, Nanoscale, 2014, 6, 11031–11034.
29. Z. Jin, P. Li, X. Huang, G. Zeng, Y. Jin, B. Zheng and D. Xiao, J. Mater. Chem. A, 2014, 2, 18593–18599.
30. A. R. J. Kucernak and V. N. N. Sundaram, J. Mater. Chem. A, 2014, 2, 17435–17445.
31. J. M. McEnaney, J. C. Crompton, J. F. Callejas, E. J. Popczun, C. G. Read, N. S. Lewis and R. E. Schaak, Chem. Commun., 2014, 50, 11026–11028.
32. P. Liu and J. A. Rodriguez, J. Am. Chem. Soc., 2005, 127, 14871–14878.
33. L. Liu, H. Guo, J. Liu, F. Qian, C. Zhang, T. Li, W. Chen, X. Yang and Y. Guo, Chem. Commun., 2014, 50, 9485–9488.
34. Y. Xu, R. Wu, J. Zhang, Y. Shi and B. Zhang, Chem. Commun., 2013, 49, 6656–6658.
35. J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. W. Lou and Y. Xie, Adv. Mater., 2013, 25, 5807–5813.
36. J. Xie, S. Li, X. Zhang, J. Zhang, R. Wang, H. Zhang, B. Pan and Y. Xie, Chem. Sci., 2014, 5, 4615–4620.
37. J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan and Y. Xie, J. Am. Chem. Soc., 2013, 135, 17881–17888.
38. P. Jiang, Q. Liu and X. Sun, Nanoscale, 2014, 6, 13440–13445.
39. P. E. R. Blanchard, A. P. Grosvenor, R. G. Cavell and A. Mar, Chem. Mater., 2008, 20, 7081–7088.
40. Q. Guan, X. Cheng, R. Li and W. Li, J. Catal., 2013, 299, 1–9.
41. Z. Huang, Z. Chen, Z. Chen, C. Lv, H. Meng and C. Zhang, ACS Nano, 2014, 8, 8121–8129.
42. Y. Pan, Y. Liu, J. Zhao, K. Yang, J. Liang, D. Liu, W. Hu, D. Liu, Y. Liu and C. Liu, J. Mater. Chem. A, 2015, 3, 1656–1665.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15680c

---