N, S and P-ternary doped carbon nano-pore/tube composites derived from natural chemicals in waste sweet osmanthus fruit with superior activity for oxygen reduction in acidic and alkaline media

Abstract

To promote the practical application of novel heteroatom-doped carbon electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells, in this work, low-cost nitrogen (N), sulfur (S) and phosphorus (P)-ternary doped carbon nano-pore/tube composites (NSP-CNPTCs) were synthesized by natural, cheap and environment-friendly N, S and P-containing chemicals extracted from waste sweet osmanthus fruit and a certain amount of dicyandiamide with ferric sulfate as a catalyst. Electrochemical tests demonstrated that the as-prepared NSP-CNPTCs exhibited superior ORR activity in both acidic and alkaline media, showing a new approach for utilizing natural heteroatom-containing chemicals in all heteroatom-rich biomasses to synthesize value-added heteroatom-doped carbon electrocatalysts in future fuel cells.

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1. Introduction

To avoid the drawbacks of noble metal platinum (Pt)-based cathode electrocatalysts and promote the large-scale application of fuel cells, strenuous efforts have been devoted to developing many highly active and durable non-Pt cathode electrocatalysts due to their critical impact on the total performance of fuel cells. Among the developed non-Pt cathode catalysts such as non-precious metal and non-metal catalysts, novel heteroatom-doped metal-free carbon materials as one of the most promising alternatives to noble metal Pt catalysts have attracted considerably increasing interest since nitrogen (N)-, phosphorus (P)- and boron (B)-doped carbon materials^1–3 were reported to show superior activities for the oxygen reduction reaction (ORR). Currently, the development of novel heteroatom-doped carbon electrocatalysts mainly focuses on the following two directions. One is to further improve the ORR activity through tailing the electronic properties and physical structures by developing new doping heteroatoms such as sulfur (S), selenium (Se), silicon (Si), arsenic (As) and haloid elements (F, Cl, Br and I)^4–9 or employing dual or multi-doping of these reported heteroatoms.^10–16 To reduce the excessive dependence on the high-price and environment-unfriendly synthetic heteroatom-containing precursors, the other is to exploit low-cost and environment-friendly heteroatom precursors for large-scale applications.^17–20 Thus, cost-effective and high-activity heteroatom-doped carbon materials derived from cheap and environment-friendly heteroatom-containing precursors is highly desirable for the commercialization of fuel cells.

Most recently, some researchers reported the successful fabrications of some low-cost heteroatom-doped carbon materials by directly pyrolyzing some environment-friendly heteroatom-containing biomasses.^18–20 However, almost 95% components such as cellulose, hemicellulose and lignin in biomasses do not contain any active heteroatoms such as nitrogen and sulfur, the direct carbonization of some selected biomasses often brought low heteroatom doping content and uncontrollable morphology, directly leading to low activity. This is the reason that most of the reported biomass-based heteroatom-doped carbon electrocatalysts exhibited much less activity for the ORR than the traditional noble metal Pt catalyst. Extracting heteroatom-containing chemicals from the biomass would be an effective strategy to overcome the negative influence of cellulose, hemicellulose and lignin in the biomass on the ORR activity of biomass-based carbon electrocatalysts. So, in this work, we tried to extract N, S and P-containing chemicals from the waste sweet osmanthus fruit and use them as cheap and environment-friendly N, S and P precursors to synthesize the morphology and doping contents-optimizable carbon materials (Fig. 1). Results indicated that the natural cheap and environment-friendly N, S and P containing chemicals could be extracted facilely from the waste sweet osmanthus fruit by water. And the extracted N, S and P-containing substances could be easily converted to N, S and P-ternary doped carbon nano-pore/tube composites (NSP-CNPTCs) by mixing with an amount of dicyandiamide with ferric sulfate as catalyst. Most interestingly, the electrochemical test results demonstrated that the as-prepared NSP-CNPTCs exhibited much comparable ORR activity to the Pt-based catalysts in both acidic and alkaline media, especially in alkaline medium, indicating that the natural heteroatom-containing chemicals could be used as cheap and environment-friendly precursors to synthesize value-added heteroatom-doped carbon catalysts in future fuel cells.

Fig. 1 The schematic illustration of the fabrication of NSP-CNPTCs by N, S and P-containing chemicals extracted from the waste sweet osmanthus fruit and dicyandiamide (C₂H₄N₄).

2. Experiment al section

2.1 Preparation of materials

The commercially available Pt–C (47.6 wt% on Vulcan XC-72) catalyst were purchased from BASF Fuel Cell, Inc., USA. Other chemicals were purchased and used without any further purification.

NSP-CNPTCs were synthesized by the fast pyrolysis of the mixture of natural N and S precursors from the sweet osmanthus fruit and dicyandiamide with ferric sulfate as catalyst. In a typical experiment, 5 g of pulverized sweet osmanthus fruit (<0.425 mm) and 60 ml of deionized water were added into a 100 ml round-bottom flask. The flask was heated to 85 °C and kept at 85 °C for 8 h. Then, the extraction liquid was filtered by a micropore filtration membrane (0.22 μm). The filtered liquor was evaporated to dryness and 0.5 g of the extracted chemicals was mixed with 3 g of dicyandiamide and 0.1 g of ferric sulfate and put in a quartz boat. The mixture was carbonized according to ref. 17 The resultant sample was collected from the quartz tube and denoted as NSP-CNPTCs1. NSP-CNPTCs2 and NSP-CNPTCs3 were synthesized according to the mass ratio of 1 : 10 and 1 : 14 of the extracted chemicals and dicyandiamide with 0.1 g of ferric sulfate as catalyst, respectively. For clear comparison, NSP-NPC and NS-CNPTCs were synthesized by pyrolyzing the extracted chemicals and dicyandiamide with ferric sulfate as catalyst under the same conditions, respectively.

2.2 Electrode preparation and electrochemical experiments

The preparation of glassy carbon (GC) electrodes (5.0 mm in diameter) and electrochemical experiments were according to ref. 16. And 0.31 mg cm⁻² of each example was loaded onto the surface of bare glassy carbon electrode. The Koutecky–Levich plots were obtained by I⁻¹ = I_k⁻¹ + (0.62nFCD^2/3v⁻¹/6ω^1/2)⁻¹, where I_k⁻¹ is the kinetic limiting current density, ω is the rotational speed, n is the number of electron transferred, F is the Faraday constant (F = 96 485 C mol⁻¹), C is the bulk concentration of O₂ (C = 1.2 × 10⁻³ mol L⁻¹ for 0.1 M KOH; C = 1.6 × 10⁻³ mol L⁻¹ for 1.0 M HClO₄), D is the diffusion coefficient of O₂ (D = 1.9 × 10⁻⁵ cm² s⁻¹ for 0.1 M KOH; D = 1.1 × 10⁻⁵ cm² s⁻¹ for 1.0 M HClO₄), v is the kinetic viscosity of the electrolyte (0.01 cm² s⁻¹ for both 0.1 M KOH and 1.0 M HClO₄), ω is the angular velocity of the disk (ω = 2πN, N is the linear rotation speed).

2.3 Characterizations

The morphologies of the samples and elemental compositions were characterized by scanning electron microscopy (SEM, ZEISS Merlin) and transmission electron microscopy (TEM, JEOL 2100F) and energy dispersive spectrometer (EDS, ZEISS Merlin). The surface areas, pore volume and pore size distributions of the samples were measured by the Brunauer–Emmett–Teller (BET, Autosorb IQ). X-ray photoelectron spectroscopic (XPS) measurements were performed on a Thermo Scientific ESCALAB 250XI using Al Kα radiation, and the C1s peak at 284.8 eV was taken as internal standard.

3. Results and discussion

SEM and TEM images illustrated that the sample (NSP-NPC) synthesized by the extracted chemicals from the waste sweet osmanthus fruit consisted of nano-porous carbon (Fig. 2A and B). The samples prepared by dicyandiamide (NS-CNPTCs) or the mixture of extracted chemicals and dicyandiamide (NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3) were mainly composed of carbon nano-pore and tube composites (Fig. 2C–J).

Fig. 2 The SEM and TEM images of the NSP-NPC (A and B), NS-CNPTCs (C and D), NSP-CNPTCs1 (E and F), NSP-CNPTCs2 (G and H) and NSP-CNPTCs3 (I and J).

The hysteresis loops in N₂ adsorption–desorption isotherms (Fig. 3A) indicated that all prepared samples were mesoporous carbon. However, micro-, meso- and total-volume data (Table S1†) of each sample indicated that the synthetic samples also contained an amount of micro-and macro-porous carbon. Among these samples, micro-volume proportion in NSP-NPC total volume even reached ca. 48.4%, which was much higher than those (8.2, 10.5 and 18.9%) in NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 and led to much larger BET surface area (431.6 m² g⁻¹) than those (289.1, 295.6 and 288.3 m² g⁻¹) of three NSP-CNPTCs. For NS-CNPTCs, the small total volume, smaller micro-volume proportion and larger macro-volume proportion led to its much small BET surface area (24.7 m² g⁻¹). And the small surface area showed that a small amount of carbon nanotubes existed in NS-CNPTCs. Although the micropore and mesopore size distributions (Fig. S1† and 3B) of each sample mainly centered at ca. 0.5 and 3.8 nm, their average pore size differed large, showing a wide size distribution for each sample. The average pore size (29.7 nm) of NS-CNPTCs was even 9 times that (3.3 nm) of NSP-NPC and the average pore diameters of three NSP-CNPTCs increased with the increase of dicyandiamide contents in the mixture, indicating that dicyandiamide could bring the increase of pore diameter. The main elemental compositions in synthesized samples analyzed by EDS were presented in Table S1.† From Table S1,† it could be found that the NSP-NPC synthesized by the extraction of waste sweet osmanthus fruit contained a certain amount of N (0.32 wt%), S (ca. 6.0 wt%) and P (0.54 wt%), demonstrating that N, S and P-containing chemicals in plants could be used as cheap and environment-friendly precursors to fabricate the heteroatom-doped carbon materials. Compared to N content in NSP-NPC, N contents in NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 increased greatly, indicating dicyandiamide as N precursor could effectively improved N doping content in NSP-CNPTCs. For sulfur, interestingly, 0.29 wt% of sulfur in NS-CNPTCs indicated that sulfur in ferric sulfate was doped into the carbon matrix since ferric sulfate had decomposed at 480 °C. However, unexpectedly, the sulfur contents in NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 decreased drastically when compared with that in NSP-NPC. Relative to the great change of N and S contents, P contents in NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 changed little. The successful doping of N, S and P into waste osmanthus fruit-based CNPTCs was further confirmed by the XPS measurements of NSP-NPC and NSP-CNPTCs2. In Fig. 3C, the high C1s peak at 284.8 eV and O1s peak at 531.8 eV were observed in the two XPS spectra. N1s peaks at ca. 398, 399 and 400 eV (Fig. 3D) were assigned to pyridinic-N, pyrrolic-N and graphite-N,²¹ respectively. S2p peaks (Fig. 3E) were attributed to sulfide (ca. 160 eV), C–S (ca. 161 and 163 eV) and C–SOx–C (ca. 168.5 and 169.5 eV).²² And P2p peaks at ca. 130 and 133 eV (Fig. 3F) were assigned to C–P and C–P–O bonds.² Compared to NSP-NPC, in NSP-CNPTCs2, C–S and C–P bonds increased and C–P–O bond decreased obviously. And sulfide 2p peak at 160.2 eV in NSP-CNPTCs2 disappeared completely, indicating that the addition of dicyandiamide could cause the decrease of sulfide and this was most probable reason that S contents decreased drastically in three NSP-CNPTCs. Meanwhile, pyridinic-N and pyrrolic-N proportions (41.8 and 20.8%) increased and graphite-N proportion decreased (37.4%) in NSP-CNPTCs2 when compared with those (34.1, 4.8 and 61.1%) in NSP-NPC.

Fig. 3 The N₂ sorption isotherms of the NSP-NPC, NS-CNPTCs, NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 (A) and their pore size distributions (B). XPS surveys of NSP-NPC and NSP-CNPTCs2 (C), and N1s (D), S2p (E) and P2p (F) spectra.

To investigate the electrocatalytic activity of the synthesized samples for the ORR, the CV measurements were carried out in an aqueous solution of O₂ saturated 0.1 M KOH or 1.0 M HClO₄ solution with a flow rate of 25 ml min⁻¹. In alkaline medium, Fig. 4A showed that the ORR peak current density (1.27 mA cm⁻²) of NSP-NPC at −0.49 V was much larger than that (0.38 mA cm⁻²) of the bare GC at −0.52 V, indicating that natural extracted N, S and P-containing chemicals from the waste sweet osmanthus fruit could be expected to be used as heteroatom precursors to prepare novel heteroatom-doped carbon electrocatalysts. For three NSP-CNPTCs, the peak potentials (ca. −0.26, −0.25 and −0.27 V) of NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 were much more positive than those (−0.49 and −0.30 V) of NSP-NPC and NS-CNPTCs. And their corresponding peak current densities (1.95, 2.55 and 2.30 mA cm⁻²) all were also much larger than those (1.27 and 1.39 mA cm⁻²) of NSP-NPC and NS-CNPTCs. Similarly, in acidic medium, the ORR peak potential (+0.08 V) and current density (1.02 mA cm⁻²) of NSP-NPC (Fig. 4B) illustrated that NSP-NPC possessed much better ORR activity than the bare GC. The peak potentials (ca. +0.24, +0.28 and +0.25 V) of NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 were much higher than that (+0.08 V) of NSP-NPC and similar to that (+0.26 V) of NS-CNPTCs. However, their peak current densities (2.70, 5.82 and 2.90 mA cm⁻²) all were much larger than those (1.02 and 1.98 mA cm⁻²) of NSP-NPC and NS-CNPTCs. These results indicated that the combination of the extracted chemicals with dicyandiamide could bring the drastic improvement of ORR activity of NSP-CNPTCs. Among three NSP-CNPTCs, CNSP-NPTCs2 exhibited most superior ORR activity in both acidic and alkaline media, showing that the activity of NSP-CNPTCs did not enhance proportionally with the increase of dicyandiamide in the mixture.

Fig. 4 The typical CVs of the GC, NSP-NPC, NS-CNPTCs, NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 in O₂-saturated 0.1 M KOH (A) or 1.0 M HClO₄ (B) solution. The LSV curves of the GC, NSP-NPC, NS-CNPTCs, NSP-CNPTCs1, NSP-CNPTCs2, NSP-CNPTCs3 and commercial Pt–C in O₂-saturated 0.1 M KOH (C) or 1.0 M HClO₄ (D) solution at a rotation speed of 1600 rpm.

To better understand the ORR activities of each prepared sample, the LSV measurements were also performed in an oxygen-saturated 0.1 M KOH or 1.0 M HClO₄ solution with a flow rate of 25 ml min⁻¹ at a rotation speed of 1600 rpm. As shown in Fig. 4C, in alkaline medium, the current densities of NSP-NPC at the potential range from −0.30 to −1.0 V were much larger than those of GC, confirming that the N, S and P-containing water extraction from the waste sweet osmanthus fruit could be used to prepared novel heteroatom-doped electrocatalysts. The ORR onset potentials (ca. +0.02, −0.08 and −0.09 V) of NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 all were much more positive than that (ca. −0.30 V) of NSP-NPC and similar to that (ca. −0.05 V) of NS-CNPTCs. And their diffusion current densities were much larger than those of NSP-NPC at the potential range of −0.09 to −1.0 V and those of NS-CNPTCs at the potential range of −0.15 to −0.55 V. Surprisingly, the diffusion current densities at the potential range from −0.32 to −1.0 V were even exceeded those of the commercial Pt C⁻¹ catalyst (47.6 wt%). In acidic medium, Fig. 4D clearly showed that the current densities at potential range of −0.2 to +0.37 V were much larger than those of GC. Meanwhile, the onset potentials (at +0.44, +0.43 and +0.44 V) of NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 all were higher than those (+0.37 and +0.42 V) of NSP-NPC and NS-CNPTCs. And their diffusion current densities at the potential range of +0.43 to −0.20 V all were also obviously larger than those of NSP-NPC and NS-CNPTCs and similar to those of the Pt-based catalysts (1.0 wt%).^23,24 Similar to the CV results, NSP-CNPTCs2 also exhibited highest ORR activity in the LSV measurements both in acidic and alkaline media. These results confirmed that the extracted natural N, S and P-containing chemicals could be used as cheap N, S and P precursors to fabricate the value-added heteroatom-doped carbon electrocatalysts. It is worth noting that NS-CNPTCs with much smaller surface area exhibited much better ORR activity than NSP-NPC with much larger BET surface area, probably caused by the much higher N content in NS-CNPTCs than that in NSP-NPC. Meanwhile, the large surface area of NSP-NPC contributed by the large number of micropores centering at about 0.5 nm did not provide more effective active sites for the ORR because it was quite difficult for O₂ with about 0.34 nm of diameter to diffuse to the micropore surface of NSP-NPC. This may be also the reason why the NSP-CNPTCs1 with high N doping content and large BET surface area showed relatively lower ORR activity than NSP-CNPTCs3. Meanwhile, from XPS analyses, it could be found that the much higher ORR activity of NSP-CNPTCs2 than that of NSP-NPC was also further attributed to the changes of N species and the increase of C–S and C–P bonds except its higher N doping content.^2,17 Since ferric sulfate was employed as catalyst in this work, the possible influence of Fe on the ORR activity of NSP-CNPTCs was also investigated. Firstly, from Fe2p spectra of NSP-CNPTCs1, NSP-CNPTCs2, NSP-CNPTCs3 and NS-CNPTCs in Fig. S2,† it could be found that obvious Fe2p peaks at 706.6 and 720.0 eV for NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 could be attributed to Fe^o2p_3/2 and Fe^o2p_1/2.²² And Fe2p peaks at 712.0 and 724.0 eV for NS-CNPTCs could be assigned to Fe²⁺2p_3/2 and Fe²⁺2p_1/2 bound by N.²⁵ These results indicated that main Fe^o in three NSP-CNPTCs were quite different from the C–N–Fe in NS-CNPTCs. The C–N–Fe was believed to be active site for the ORR²⁶ while Fe^o was thought to contribute little to the improvement of ORR.²⁷ Secondly, to further estimate the real influence of Fe^o in NSP-CNPTCs on the ORR activity, Fe was removed from NSP-CNPTCs2 by hydrochloric acid and added in three new synthesized NSP-CNPTCs by increasing the amount of ferric sulfate catalyst (0.2, 0.4 and 0.6 g). As can be seen from Fig. S3 and 4,† the decrease of Fe did not bring the obvious decrease of the ORR activity of NSP-CNPTCs2 and the increase of Fe in three new NSP-CNPTCs did not enhance obviously the ORR activity of NSP-CNPTCs as well in both acidic and alkaline media, further confirming the superior ORR activity mainly arising from the dopings of N, S and P and the high N doping content.

To gain an in-depth understanding of the electrochemical behavior of prepared samples, Koutecky–Levich plots (I⁻¹ vs. ω^−1/2) at −0.70 and +0.10 V were calculated in an oxygen-saturated 0.1 M KOH or 1.0 M HClO₄ solution on the basis of the LSVs at different rotation speeds (Fig. 5A and B). And based on the slopes in Koutecky–Levich plots (I⁻¹ vs. ω^−1/2), the transferred electron number (n) per oxygen molecule during the ORR course of each sample was calculated (Fig. 5C). As shown in Fig. 5C, the n values (3.5, 4.0 and 3.7 in alkaline medium and 2.9, 3.1 and 3.6 in acidic medium) of NSP-CNPTCs1, NSP-CNPTCs2 and NSP-CNPTCs3 all were obviously higher than those (3.3 in alkaline medium and 2.8, 2.6 in acidic medium) of NSP-NPC and NS-CNPTCs. The n value of NSP-CNPTCs2 in alkaline medium was even equal to that of the commercial Pt–C catalyst. And its n value in acidic medium was just a bit smaller than that of the Pt–C. These results meant that the efficient 4-electron reduction dominated the electrocatalytic process on the surface of NSP-CNPTCs in both alkaline and acidic media, especially for NSP-CNPTCs2.

Fig. 5 The Koutecky–Levich plots (I⁻¹ vs. ω^−1/2) of the NSP-NPC, NS-CNPTCs, NSP-CNPTCs1, NSP-CNPTCs2, NSP-CNPTCs3 and commercial Pt–C at −0.7 or +0.1 V in an O₂-saturated 0.1 M KOH (A) or 1.0 M HClO₄ (B) solution and the electron transferred numbers of the NSP-NPC (a), NS-CNPTCs (b), NSP-CNPTCs1 (c), NSP-CNPTCs2 (d), NSP-CNPTCs3 (e) and commercial Pt–C catalyst (f) in alkaline and acidic media (C).

To investigate the potential practical application of NSP-CNPTCs2 with superior ORR activity, the stability and methanol tolerance tests were performed by chronoamperometry at a constant voltage of −0.30 and +0.4 V for 25 000 and 2000 s in an oxygen-saturated 0.1 M KOH or 1.0 M HClO₄ solution, respectively. Fig. 6A showed that the initial current of NSP-CNPTCs2 only experienced a loss of 3.6 or 9.8% while that of the commercial Pt–C catalyst lost 13.0 or 20.4% in alkaline or acidic medium,¹⁷ indicating a longer-term stability of NSP-CNPTCs2. Meanwhile, NSP-CNPTCs2 also exhibited remarkably excellent methanol tolerance. As can be seen from Fig. 6B, the current density of NSP-CNPTCs2 almost remained unchanged after the addition of 3 M methanol into the oxygen-saturated 0.1 M KOH or 1.0 M HClO₄ solution, which was much better than the that of commercial Pt–C catalyst.¹⁷ The outstanding stability and excellent methanol tolerance of NSP-CNPTCs2 showed a great promise for the practical application in the future fuel cells.

Fig. 6 (A) The chronoamperometric response of the NSP-CNPTCs2 in an oxygen-saturated 0.1 M KOH or 1.0 M HClO₄ solution with a flow rate of 25 ml min⁻¹ and graphite as the counter electrode at −0.3 or +0.4 V for 30 000 s, and (B) I–t chronoamperometric response of the NSP-CNPTCs2 upon the addition of 3.0 M methanol into an oxygen-saturated 0.1 M KOH or 1.0 M HClO₄ solution at −0.3 V or +0.4 V for 2000 s. The arrow indicates the addition of methanol.

4. Conclusions

In summary, we have successfully synthesized a series of NSP-CNPTCs in this work by fast pyrolyzing the mixture of the extracted N, S and P-containing chemicals from the waste sweet osmanthus fruit and a certain amount of dicyandiamide with ferric sulfate as catalyst. Results demonstrated that N, S and P-containing chemicals could be extracted from the waste sweet osmanthus fruit under mild condition and be converted to NSP-CNPTCs easily through mixing with a certain amount of dicyandiamide. Most important of all, electrochemical tests demonstrated the as-prepared NSP-CNPTCs2 exhibited comparable ORR activities to Pt-based catalysts and much better stability and methanol tolerance than the commercial Pt–C catalyst in both acidic and alkaline media. This method of extracting natural cheap and environment-friendly heteroatom-containing chemicals to prepare value-added heteroatom-doped carbon electrocatalysts could apply to all heteroatom-rich biomasses, showing a great potential application for developing low-cost and much more efficient cathode catalysts in future fuel cells.

Acknowledgements

We acknowledge the financial support from the Fundamental Research Funds for the Central Universities of China (No. 2015XKZD08). The authors also thank Dr Shiheng Yin and Dongxiao Wu at the Analysis and Test Center, South China University of Technology, Guangzhou, for SEM and TEM tests.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08371d

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