Copper nanoparticle/carbon quantum dots hybrid as green photocatalyst for high-efficiency oxidation of cyclohexane

Abstract

To develop green catalysts for cyclohexane oxidation with high efficiency and high selectivity is a trend in nanotechnology and nanocatalysis. In this work, we demonstrate that copper nanoparticles/carbon quantum dots (Cu/CQDs) hybrid as photocatalyst exhibits excellent catalytic activity in the oxidation of cyclohexane (conversion based on cyclohexane of 50.2% and selectivity to cyclohexanone of about 78.3%) under a low temperature with tert-butyl hydroperoxide as oxidant. It is worth mentioning that the Cu/CQDs hybrid photocatalyst can be implemented in efficient catalytic oxidation of cyclohexane under a mild condition (60 °C), which may provide a cogent pathway for the development of high-performance catalysts for C–H oxidation.

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1. Introduction

The oxidation products of alkanes, such as cyclohexanol and cyclohexanone, are important chemical raw materials and intermediates in chemical industry. But, the direct and efficient oxidation of available alkanes (cyclohexane, etc.) remains an ambitious goal yet to be achieved.^1–8 In general, the chemical inertness of the hydrocarbons leads to a huge challenge for the activation of their C–H bonds, usually requiring harsh conditions, such as high temperature and pressure, which make the process very low energy efficiency.⁹ To overcome these difficulties, many investigations have been done to develop more efficient catalyst systems for cyclohexane catalytic oxidation. A zirconium complex bonded to modified carbamate silica gel gave a product distribution ratio of 6.6 : 1 of cyclohexanol–cyclohexanone mixture with 21% conversion at 200 °C.¹⁰ Chromium-containing complex CrCoAPO–5(CH₃COOH) gave 50% conversion with some 55%, 8%, 15% and 22% selectivity towards cyclohexanol, cyclohexanone, adipic acid and others at 115 °C and 1 MPa under oxygen.¹¹ Those reaction processes require high temperature or pressure,^12,13 and also still suffer from low yield, selectivity and high cost.¹⁴

Copper nanoparticles (CuNPs) exhibit high catalytic activity and are much cheaper than precious metals^15–17 (like Au or Pt), which can typically provide highly active centers, having a great promise in catalysis.^18–20 At the same time, the deposition of metal nanocrystals on carbon materials for catalytic reactions has been the subject of meaningful works.^21,22 Carbon quantum dots (CQDs) also display great potential for applications in nanocatalysis due to their distinctive photophysical and photoelectrochemical properties;^23,24 especially, they can act as photo-induced electron acceptors or donors. More important, CQDs can be used as a multipurpose component in the design of novel photo-catalyst.²⁵ So, CQDs may be used as a powerful energy-transfer component in the design of superior photocatalyst for the oxidation of cyclohexane. With respect to the oxidant, tert-butyl hydroperoxide (TBHP) is preferable, because of the simplicity of handling and efficient performance in the oxidation reaction. The use of TBHP could significant promote the catalytic oxidation reaction.

Herein, we present the design and preparation of Cu/CQDs hybrids as photocatalyst for oxidation of cyclohexane under visible light. The Cu/CQDs hybrids exhibit efficient photocatalytic activity with the conversion of cyclohexane up to 50.2%, while the selectivity of cyclohexanone can reach 78.3%. Compared with traditional catalytic oxidation systems of cyclohexane, the present Cu/CQDs nanoparticles are able to efficiently catalyze the alkane oxidation with TBHP as an efficient oxidant under a mild condition, demonstrating the great potential of such an approach for C–H oxidation.

2. Experimental section

2.1 Chemical reagents

Unless otherwise noted, all materials were obtained from commercial suppliers and used without further purification. All chemicals were purchased from Sigma-Aldrich.

2.2 Preparation of CQDs

CQDs were synthesized through a method of electrochemical ablation of graphite, which was reported by our group.²⁶ In the synthetic process, a graphite rod (99.99%, Alfa Aesar Co. Ltd, 13 cm in length and 0.6 cm in diameter) was inserted into ultrapure water (18.4 MΩ cm⁻¹, 600 mL), placed parallel to another graphite rod as the counter-electrode with a separation of 7.5 cm. Static potential of 30 V was applied to the two electrodes using a direct current power supply for 120 h with continuous stirring. Then, a dark-yellow solution was obtained in the reactor. The solution was filtered with slow-speed quantitative filter paper two times, and the resultant solution was centrifuged (22 000 rpm) for 30 min. After that, the CQDs can be obtained.

2.3 Preparation of CuNPs

CuNPs were prepared by a chemical reduction method. A solution (30 mL) containing copper(II) nitrate pentahydrate (Cu(NO₃)₂·5H₂O) (0.01 M) and ascorbic acid (0.01 M) was prepared in a conical flask under continuous vigorous stirring. Then ice-cold fresh NaBH₄ solution (1 mL, 0.1 mol L⁻¹) was added into the above solution. The resultant solution turned pink immediately after adding NaBH₄, indicating the formation of CuNPs.

2.4 Preparation of Cu/CQDs hybrids

The Cu/CQDs hybrids were prepared through dropwise addition of Cu(NO₃)₂ solution (2 mL, 0.02 M) to CQDs solution (15 mL) with vigorous magnetic stirring at room temperature. After stirring at room temperature for about 1 h, a solution of NaBH₄ (0.4 mL, 0.05 M) in deionized water was added to the mixture under continuous strong stirring for about 60 min. The above solution turned gray-blue immediately on adding the NaBH₄ solution. The Cu/CQDs composites were separated and washed with deionized water by centrifugation. After that, the Cu/CQDs can be obtained.

2.5 Preparation of CuO/CQDs hybrids

The CuO/CQDs hybrids were prepared through dropwise addition of Cu(NO₃)₂ solution (3 mL, 0.02 M) into a conical flask containing 0.2 mL sodium dodecyl benzene sulfonate solution (0.1 mM) with vigorous magnetic stirring at room temperature. After 10 min, NH₄HCO₃ (4 mL, 0.04 M) solution was added into the flask at a rate of 0.5 mL min⁻¹. After 1 h, 5 mL CQDs solution was added to the above flask with continuous strong stirring and heating. After boiling the above solution for 6 h, the mixture was cooled to room temperature. The mixture was separated by centrifugation and washed with deionized water. After that, the products were dried at 100 °C for 30 min. Then, the CuO/CQDs hybrids can be obtained.

2.6 Photocatalytic oxidation of cyclohexane using Cu/CQDs hybrids

In the photocatalytic experiments, TBHP (10 mL) as oxidant was added dropwise to a mixture of Cu/CQDs hybrids (20 mg) and cyclohexane (10 mL) in a three-necked flask (50 mL) with a water condenser and continuous magnetic stirring under visible light (xenon lamp with visible-CUT filter to cut off light with a wavelength of λ < 420 nm) or in the dark. The oxidation products were analyzed by gas chromatography (GC) and gas chromatography-mass spectroscopy (GC-MS).

2.7 Reactive species trapping experiments

TBHP (10 mL) as oxidant was added dropwise to a mixture of Cu/CQDs composites (20 mg) and cyclohexane (10 mL) in a three-necked flask (50 mL) with visible light irradiation and continuous magnetic stirring. Then the tert-butanol (5 mL) as radical scavenger was added into the above flask. The other two radical scavengers (benzquinamide (BZQ, 5 mL) and disodium ethylenediaminetetraacetate (Na₂-EDTA, 5 mL)) replaced the tert-butanol to repeat the experiment process. The oxidation products were analyzed by GC and GC-MS.

2.8 Characterization

Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the samples were obtained with a FEI-Tecnai F20 (200 kV) transmission electron microscope (FEI). Raman spectra were obtained with an HR 800 Raman spectroscope equipped with a Synapse CCD detector. The spectrograph used 600 g mm⁻¹ gratings and a 633 nm He–Ne laser. UV-visible spectra were obtained with an Agilent 8453 UV-visible diode array spectrophotometer. The fluorescent spectrophotometry study was carried out with a Horiba Jobin Yvon (FluoroMax 4) luminescence spectrometer with a slit of 3 nm, and the power of the excitation light was 150 W (ozone-free Xe lamp). Powder X-ray diffraction (XRD) was performed by using an 8/X'Pert-ProMPD (Holland) D/max-γA X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). X-ray photoelectron spectra were recorded with a KRATOS Axis ultra DLD X-ray photoelectron spectrometer with monochromatised Al Kα X-rays (hν = 1486.7 eV). GC was conducted using a Varian 3400 GC column with a cross-linked 5% PhMe silicone column (25 m × 0.20 mm × 0.33 μm) and a FID detector. While the fluorescence lifetimes of the CQDs were measured by FluoroLog 3-211-TCSPC.

3. Results and discussion

3.1 Characterizations of Cu/CQDs hybrids

Cu/CQDs hybrids were synthesized by a simple and convenient method at room temperature. Fig. 1a shows the TEM image of the obtained Cu/CQDs hybrids. The resultant Cu/CQDs hybrids appear as spherical dots, indicating the hybrids have a similar morphology and are dispersed well with sizes in the range of 4–7 nm (inset in Fig. 1a). Fig. 1b displays the HRTEM image of Cu/CQDs hybrid, which exhibits a clear lattice spacing of around 0.212 nm for CQDs corresponding to the crystallographic planes of graphitic carbon.²⁷ While the lattice pitch of 0.208 nm corresponds to the (111) crystallographic plane of Cu. The above results confirmed the formation of Cu/CQDs hybrids. Considering the good catalytic activity of CuNPs and the excellent photophysical and photoelectrochemical properties of CQDs, we suspect that Cu/CQDs hybrids may be a promising candidate for a novel photocatalyst.

Fig. 1 (a and b) TEM and HRTEM images of Cu/CQDs; inset in (a) is the particle sizes histogram of Cu/CQDs. (c–f) Raman spectra, FTIR spectra, UV-vis absorption spectra and XRD patterns of CQDs (black line) and Cu/CQDs (red line), respectively.

In order to further confirm the formation of Cu/CQDs hybrids, a series of tests were carried out. The Raman spectra (λ_ex = 633 nm) of Cu/CQDs and CQDs were measured as shown in Fig. 1c. As shown, the CQDs (black trace) exhibit the typical D-band and G-band of carbon located at about 1341 and 1610 cm⁻¹, respectively. The D-band is disorder-induced because of the defects, dislocations and lattice distortions in carbon structures.^28,29 The G-band can be interpreted as the stretching vibration mode of graphite crystals.³⁰ The presence of CuNPs in the Cu/CQDs hybrids led to an enhanced Raman signal based on surface-enhanced Raman scattering.^31,32 So, the Raman signal of Cu/CQDs is higher than that of CQDs. Moreover, it can be seen that the ratio of I_D/I_G for Cu/CQDs is higher than that of CQDs, which indicated that the presence of CuNPs led to an increase of disorder degree in Cu/CQDs hybrids compared with CQDs. The FTIR spectrum of Cu/CQDs hybrids, shown in Fig. 1d (red line), displays four prominent peaks. The peak at about 3400 cm⁻¹ corresponds to the O–H stretching mode. The peaks at about 1700, 1440 and 1200 cm⁻¹ indicate the presences of carbonyl (C–O) groups, the C=C stretch of polycyclic aromatic hydrocarbons, and the epoxide/ether C–O–C, respectively.²⁶ These results show that the Cu/CQDs hybrids have abundant carboxyl and hydroxyl groups on their surfaces, which are beneficial for the synthesis of nanocomposite with CQDs as a multipurpose component.

Fig. 1e shows the UV-visible absorption spectra of CQDs (black line) and Cu/CQDs hybrids (red line) in aqueous solution at room temperature. As shown, the Cu/CQDs displays two strong absorption peaks located about 230 nm and around 520–640 nm, which represent the typical absorption of an aromatic π system and the surface plasmon resonance of CuNPs, respectively.^33,34 The UV-visible absorption spectra results demonstrate the Cu/CQDs composite can absorb significantly more visible light, which is more useful in photocatalysis. These evidences indicated that Cu/CQDs composites may have excellent catalytic activity for the target reactions. The XRD patterns of Cu/CQDs and CQDs are shown in Fig. 1f. The characteristic diffraction peaks marked by a black circle (●) can be readily indexed to (111), (200) and (220) planes of metallic Cu.³⁵ The broad diffraction peak at 26° marked by a black square (■) is the characteristic peak of carbon. The above results further confirm the Cu/CQDs hybrids have been successfully obtained.

In order to analyze the elemental composition of Cu/CQDs hybrids, X-ray photoelectron spectroscopy (XPS) was carried out. Fig. S1a† shows the XPS survey full spectrum of Cu/CQDs hybrids, which indicates the as-obtained sample contains carbon, copper and oxygen elements without other impurities. The high-resolution C 1s XPS spectrum of Cu/CQDs hybrids is displayed in Fig. S1b,† in which the peaks at 288.7, 286.4 and 285.1 eV respectively are attributed to C=O, C–OH and C–H species.^36–38 The high-resolution O 1s XPS spectrum of Cu/CQDs hybrids, shown in Fig. S1c,† can be mainly divided into three peaks located at about 534.6, 533.9 and 531.8 eV, which may be ascribed to C–H, C=O and C–O, respectively.³⁹

3.2 Catalytic oxidation of cyclohexane using Cu/CQDs hybrids

To investigate the photocatalytic activity of Cu/CQDs hybrids, cyclohexane oxidation with Cu/CQDs hybrids as photocatalyst was performed under visible light and in the dark. Fig. 2 shows the relationships between conversion of cyclohexane/selectivity to cyclohexanone and reaction time using Cu/CQDs hybrids as photocatalyst under visible light and in the dark. The details of the conversion of cyclohexane and selectivity to cyclohexanone are shown in Tables S1 and S2.† As shown in Fig. 2 and Table S1,† after 48 h under visible light, the conversion of cyclohexane and the selectivity to cyclohexanone reach 50.2% and 78.3%, respectively. While in the dark, the conversion of cyclohexane and the selectivity to cyclohexanone reach 4.9% and 73.3%, respectively. These results indicate the Cu/CQDs hybrids are efficient photocatalyst under visible light.

Fig. 2 Reaction time dependence of conversion of cyclohexane/selectivity to cyclohexanone with Cu/CQDs as photocatalyst under visible light irradiation (●) and in the dark (■).

To further confirm the photocatalytic activity of Cu/CQDs hybrids, a series of control experiments for cyclohexane oxidation were carried out with different catalysts. Table 1 details the oxidation of cyclohexane with different catalysts (no catalyst, CQDs, CuNPs, Cu/CQDs hybrids and a physical mixture of CuNPs and CQDs (Cu&CQDs)) under visible light. The blank reactions (without catalyst and nitrogen saturation treatment) would not produce any oxidation products, which confirmed the necessity of adding catalyst for the oxidation of cyclohexane. With CQDs and CuNPs as photocatalysts, the conversions of cyclohexane reach 8.1% and 6.5%, respectively, while the selectivity to cyclohexanone is 72.7% and 70.2%, respectively. In remarkable contrast, when Cu/CQDs hybrids were used as catalysts, the conversion of cyclohexane (50.2%) and the selectivity of cyclohexanone (78.3%) are obviously higher than those when using CQDs and CuNPs. To further compare, a physical mixture of CuNPs and CQDs (Cu & CQDs) was used as photocatalyst. As shown in Table 1, the Cu&CQDs show higher conversion and selectivity than CQDs and CuNPs, but much lower than Cu/CQDs hybrids. The catalytic activity of Cu/CQDs was significant higher than CQDs, CuNPs and Cu & CQDs. The turn-over frequency (TOF) was further calculated by determining the moles of product formed (or reactant consumed) per unit time per mole of available active sites (based on Cu) on the catalyst.⁴⁰ As shown in Table 1, the TOF of Cu/CQDs is higher than that of other catalysts, which indicates the catalytic activity of Cu/CQDs is higher than that of other catalysts. The above results further indicate the Cu/CQDs hybrids show efficient photocatalytic activity. More important, the photocatalytic activity of CQDs is higher than that of CuNPs, which demonstrated that CQDs play a crucial role in the catalytic reaction.

Table 1 Photocatalytic oxidation of cyclohexane with different catalysts under visible light

Catalysts	Conversion (%)	Selectivity (%)		TOF (h^−1)
		Cyclohexanone	∑selC6
No catalyst	—	—	—	—
CQDs	8.1	72.7	94.1	—
CuNPs	6.5	70.2	92.4	36.4
Cu/CQDs	50.2	78.3	95.7	246.1
Cu & CQDs	10.1	73.2	96.3	56.3

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The Cu/CQDs photocatalyst driven by different wavelengths of light was further investigated with cyclohexane oxidation (Fig. 3a). The highest conversion for oxidation of cyclohexane was obtained when irradiated by red light (620 nm), whose wavelength matches the surface plasma resonance zone of CuNPs (540–640 nm).²⁶ At the same time, the oxidation of cyclohexane was carried out under different light source intensities (Fig. 3b). It was found that the conversion and source intensity have a linear relationship when irradiated with weak light (below 30 mW cm⁻²). While with intense light (above 60 mW cm⁻²), the conversion are almost constant. The selectivity almost remained the same. These results could be because the catalyst active sites were fully activated when irradiated by intense light.

Fig. 3 (a) Wavelength dependence of the conversion with Cu/CQDs hybrids as photocatalyst under different lighting conditions: blue (400 nm), green (525 nm), red (620 nm), and IR (760 nm) light. A xenon lamp was used as the light source with CUT filters to get light of different wavelengths. (b) Light intensity dependence of the conversion (black line)/selectivity (red line) of the cyclohexane oxidation with Cu/CQDs as photocatalyst.

To test the repeatability of Cu/CQDs hybrids as photocatalyst, ten cycles of cyclohexane oxidation were performed with the recycled catalyst as shown in Fig. 4. As shown, after recycling 10 times, the Cu/CQDs still keep high catalytic activity with nearly constant conversion and selectivity. Moreover, the filtrate solution test (an additional 36 h of reaction after removing the catalyst from the reaction medium) showed no catalytic activity, indicating that leaching did not play an important role in the present system. The above results demonstrate Cu/CQDs are indeed an efficient photocatalyst in the present reaction system.

Fig. 4 Relationship between the conversion of cyclohexane/selectivity of cyclohexanone and cycle times with Cu/CQDs as photocatalyst over ten cycles.

3.3 Proposed mechanism

Based on the above experimental results, the efficient catalytic performance of Cu/CQDs might be due to their structural features. The surface plasmon resonance of CuNPs can enhance the visible light absorption. When irradiated, the hydroxyl radical (HO˙) group is quickly generated from TBHP,⁴¹ which serves as a strong oxidant for the conversion of cyclohexane to cyclohexanone under mild conditions. Simultaneously, the CQDs are excellent as both electron donors and acceptors, which can further enhance the photocatalytic activity. In this photocatalytic system, the CuNPs and CQDs as important components are involved in synergistic catalysis for the oxidation of cyclohexane. To confirm that HO˙ is the main active oxygen specie and responsible for the selective oxidative reaction, three different scavengers, tert-butanol (a HO˙ radical scavenger), BZQ (an O²⁻˙ radical scavenger) and Na₂-EDTA (a hole scavenger), were used in reactive species trapping experiments. Fig. 5 displays the photocatalytic results when the three different scavengers were added. The addition of tert-butanol significantly reduced the conversion after 48 h reaction, while with the introduction of BZQ and Na₂-EDTA, the conversion nearly remained the same, which confirm that HO˙ is the active oxygen species in this photocatalytic oxidation process.

Fig. 5 Photocatalysis activities of Cu/CQDs in reactive species trapping experiments with three types of reactive species scavengers.

To investigate the photoinduced electron transfer properties of CQDs, the photoluminescence (PL) properties of Cu/CQDs hybrids were studied, shown in Fig. S2.† As shown in Fig. S2a and b,† CQDs can exhibit a broad luminescence peak at about 550 nm with excitation at 485 nm, and the emission intensities were quenched by known electron donors (N,N-diethylaniline, DEA, 0.03 M) and electron acceptors (2,4-dinitrotoluene, 0.03 M). It was found that the PL of CQDs excited at 485 nm with emission located at 550 nm could be quenched by either electron acceptor or electron donor molecules in solution, confirming the CQDs are excellent as both electron donors and acceptors under visible light. Fig. S2c and d† show the luminescence decays of CQDs with Stern–Volmer quenching constants (insets of Fig. S2c and d,† K_sv = τ⁰_Fk_q) from linear regression of 22.7 and 37.2 M⁻¹, respectively. In following experiments, the interaction between CuNPs and CQDs was further investigated. Fig. S3† exhibits the PL spectra and luminescence decays of CQDs (black line) and Cu/CQDs (red line). It can be clearly seen that the PL of CQDs can obviously be quenched in the current system.

Fig. 6 demonstrates the high-resolution Cu 2p XPS spectra before (Fig. 6a) and after (Fig. 6b) the oxidation of cyclohexane with Cu/CQDs as photocatalyst. As shown in Fig. 6a, the Cu 2p spectrum has two peaks located at 952.7 eV and 931.4 eV. After several times of photocatalytic oxidation reaction, the Cu/CQDs hybrids show an impurity (CuO). As shown in Fig. 6b, two new peaks (962.6 eV and 943.1 eV) of CuO appeared. In spite of this, the photocatalytic activity of Cu/CQDs hybrids exhibits no obvious decrease after several catalytic cycles (Fig. 4). In following experiments, CuO/CQDs hybrids were synthesized to further investigate the photocatalytic activity for cyclohexane oxidation. According to our experiments, the conversion based on cyclohexane reaches 23.9% and selectivity to cyclohexanone up to 70.6%, which are lower than those of Cu/CQDs hybrids (50.2% and 78.3%). In fact, CuO is also an efficient catalyst in some reports.^42,43 The reasons for the catalytic activity of CuO being lower than Cu are still a matter of research.

Fig. 6 High-resolution Cu 2p XPS spectra before (a) and after (b) the oxidation of cyclohexane with Cu/CQDs as photocatalyst.

For further comparison between carbon dots and graphene as substrates, we prepared Cu/graphene hybrid as photocatalyst. The catalytic activity of Cu/graphene hybrid for oxidation of cyclohexane was investigated under visible light (20 mg catalyst, 10 mL TBHP, and 10 mL cyclohexane with 48 h). The hybrid was synthesized by adding NaBH₄ (0.4 mL, 0.05 M) into a solution containing Cu(NO₃)₂ (2 mL, 0.02 M) and graphene (1 mg) under stirring for 1 h. However, we found that the photocatalytic activity of the Cu/graphene hybrid was lower compared with Cu/CQDs. The results are shown in Table S3.† As shown, after 48 h, the conversion of cyclohexane and the selectivity to cyclohexanone reached 32.3% and 69.5%, respectively. After recycling 3 times, the catalytic activity of the Cu/graphene significantly decreased (conversion down from 32.3% to 13.2%). The above results indicated the Cu/CQDs hybrid as photocatalyst exhibits efficient photocatalytic activity and stability.

For the present catalysis system, compared with some other catalytic reactions which demand harsh reaction conditions or use precious metals,¹⁸ our experiments were conducted at atmospheric pressure and a low temperature (60 °C). Secondly, we did not utilize any complicated equipment to carry out these experiments. Thirdly, there are no organic solvents (like acetonitrile, acetone, etc.) added to improve the selectivity of cyclohexanone. Therefore, the Cu/CQDs photocatalyst is a green and efficient catalysis material for the selective oxidation of cyclohexane. Although conversion into cyclohexanone at this stage of research is not very high, we believe that this is a proof of concept that copper-based catalysts could effectively improve the oxidation of C–H bonds. How to further improve the conversion and selectivity of the selective oxidation of cyclohexane still needs more exploration.

4. Conclusions

We demonstrate that Cu/CQDs hybrids as photocatalyst exhibit excellent catalytic activity in the oxidation of cyclohexane (conversion based on cyclohexane of 50.2% and selectivity to cyclohexanone of about 78.3%) under a low temperature (60 °C) with TBHP as oxidant. In comparison with previous catalytic systems for cyclohexane oxidation, there are three advantages to the Cu/CQDs catalyst: (1) the synthesis pathway of Cu/CQDs hybrids is fairly simple, requiring neither complicated devices nor complex preparation processes; (2) the approach and raw materials for manufacture of Cu/CQDs hybrids are mild, cheap and have potential for large-scale production; and (3) the reaction process of cyclohexane oxidation requires neither high temperature nor high pressure. The as-prepared Cu/CQDs hybrids as a cheap and easily accessible photocatalyst provide a good choice for green and high-performance oxidation of cyclohexane.

Acknowledgements

This work was supported by Collaborative Innovation Center of Suzhou Nano Science and Technology, the National Basic Research Program of China (973 Program) (2012CB825803, 2013CB932702), the National Natural Science Foundation of China (51422207, 51132006, 21471106), the Specialized Research Fund for the Doctoral Program of Higher Education (20123201110018), a Suzhou Planning Project of Science and Technology (ZXG2012028), the Natural Science Foundation of Jiangsu Province of China (BK20140310), China Postdoctoral Science Foundation (2014M560445), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04753f

‡ The authors contributed equally to this work.

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