Ionic Liquid as Reaction Medium for Synthesis of Hierarchically

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Ionic Liquid as Reaction Medium for Synthesis of Hierarchically Structured One-Dimensional MoO2 for Efficient Hydrogen Evolution Baohua Zhang,† Yiguo Xue,‡ Anning Jiang,† Zhimin Xue,§ Zhonghao Li,*,† and Jingcheng Hao† †

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan 250100, China Geotechnical and Structural Engineering Research Center of Shandong University, Jinan 250061, China § Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China ‡

S Supporting Information *

ABSTRACT: Hierarchically structured one-dimensional (1D) MoO2 is synthesized for the first time in ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][Tf2N]). The synthesis system is very simple (single [BMIM][Tf2N] solvent plus MoO2(acac)2 reactant). [BMIM][Tf2N] itself works as both the reaction medium and the template for the formation of these interesting 1D MoO2 particles with ultrathin nanosheet subunits. The as-synthesized hierarchically 1D MoO2_40 particles exhibit remarkable electrocatalytic activity with good long-term cycle stability for the hydrogen evolution reaction (HER) in acidic media. The HER activity of present synthesized MoO2 is comparable to those of the most active Mo-based electrocatalysts in acid media reported up to now. Therefore, the ionic liquid route provides us with a newly powerful tool for the synthesis of interesting alternative to noble metal catalysts for efficient electrocatalytic production of hydrogen in acidic environment. KEYWORDS: ionic liquid, nanostructure, MoO2, electrocatalysis, hydrogen evolution

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INTRODUCTION In view of the increasingly serious energy crisis and environmental pollution, hydrogen, as a renewable and environment-friendly energy, has been regarded as one of the most promising new fuels to replace fossil fuels.1−4 Among the methods to produce hydrogen, electrocatalytic splitting of water by the hydrogen evolution reaction (HER) is thought as an efficient and environmentally benign method for hydrogen production.5−8 Currently, Pt is regarded as one of the most efficient and durable HER electrocatalyst with a near-zero overpotential in acidic media.9,10 However, the high cost and low natural abundance of Pt considerably hinder its widespread application in a future sustainable “hydrogen economy”.11 Therefore, much effort is devoted to the development of efficient alternative electrocatalysts that are geologically abundant for the HER. To date, many potential alternatives to Pt electrocatalysts for HER in acidic solutions have been exploited based on nonprecious transition metals.12−27 Although some reported alternative electrocatalysts exhibit promising electrocatalytic property for HER, the synthesis of these electrocatalysts usually needs a complicated process, and the components of the fabricated electrocatalysts are complicated, which results in additional risk of uncertainty. Therefore, a straightforward synthesis of an efficient HER electrocatalyst with a very simple system (a single solvent plus a reactant) is in high demand. © XXXX American Chemical Society

MoO2, as a unique metallic semiconductor, has recently attracted considerable attention in various application areas due to its high stability and high metallic-like electrical conductivity.28−30 Recent works indicate that MoO2 could be a potential candidate as a non-noble-metal electrocatalyst for HER.31−36 However, the achieved HER electrocatalysts based on MoO2 are still less satisfactory in acidic media in terms of the complexity of synthesis, activity, and stability. Specifically, the reported works are mainly concentrated on the improvement of the HER property by realizing suitable MoO2-based composite materials and not specific nanostructures.3 Therefore, there is still much room to improve the synthesis strategy and enhance their HER properties by regulating the specific nanostructures of MoO2. Specifically, it remains challenging to synthesize hierarchically structured one-dimensional (1D) MoO2 for HER application. Ionic liquids (ILs) have recently received much interest due to their unique attractive properties achieved by combining the advantages of ionic and liquid properties.37−39 The unique properties of ILs grant them great potential in applications in various fields including material synthesis.40 Many examples have clearly demonstrated that ILs could provide a useful Received: January 15, 2017 Accepted: February 10, 2017 Published: February 10, 2017 A

DOI: 10.1021/acsami.7b00722 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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model 760E). Here, a glassy carbon electrode (GCE, 3 mm in diameter) was used as the working electrode, while a Pt plate worked as the counter electrode. In all the experiments, a saturated calomel electrode (SCE) was used as the reference electrode. To make the synthesized MoO2 with clean surface, the synthesized MoO2 products were annealed for 2 h at 450 °C in H2 before electrochemical measurements. MoO2 catalysts (5 mg), Vulcan XC-72 (2.5 mg), and a Nafion solution (5 wt %, 40 μL) were added in 1 mL of water and dispersed by sonication to result in a homogeneous ink. After this, 5 μL of the ink was dropped on the surface of GCE. The GCE was then dried naturally before using. The electrochemical measurements were characterized in 0.5 M H2SO4 (pH 0), 1 M KOH (pH 14), and 1 M phosphate buffer solution (pH 7) electrolytes at room temperature. The linear sweep voltammetry (LSV) was obtained by sweeping at a potential sweep rate of 5 mV s−1. The obtained polarization data were calibrated with respect to the reversible hydrogen electrode (RHE) by ERHE = ESCE + E°SCE + 0.059(pH).3 Before measurement, the resistance test was characterized and the iR compensation was applied by CHI software. The accelerated stability test was carried out in 0.5 M H2SO4 by potential cycling at 50 mV s−1 for 2000 cycles. The electrochemical impedance spectroscopy (EIS) measurements of MoO2 were carried out at overpotential η = 200 mV from 106 to 0.02 Hz with an amplitude of AC potential of 5 mV in 0.5 M H2SO4 solution. Cyclic voltammograms (CV) were carried out to evaluate the electrochemical active surface areas of the catalysts through measurement of the double-layer capacitance in the region from 0.1 to 0.3 V vs RHE with various scanning rates (20, 40, 60, 80, 100, and 120 mV s−1) in 0.5 M H2SO4. The current density−time (I−t) study for the MoO2_40 was performed at an overpotential of 170 mV in 0.5 M H2SO4. The theoretically calculated and experimentally measured amount of the evolved hydrogen versus time for MoO2_40 was performed at an overpotential of 190 mV in 0.5 M H2SO4. All the potentials reported here are versus RHE.

platform for fabricating a wide range of interesting materials.41−49 Specifically, materials synthesis in ILs sometimes can result in interesting particles which are not easy to be realized from other conventional fabrication routes.50 Inspired by this, exploring the synthesis of HER electrocatalysts in ILs might bring more opportunities for hydrogen economy. However, the synthesis of electrocatalysts in ILs for HER application is rarely reported in the literature. Therefore, it is highly desirable to explore the synthesis of HER electrocatalysts in ILs, providing efficient alternatives to noble metal electrocatalyst for HER. Herein, we report for the first time the synthesis of novel hierarchically structured 1D MoO2 in an IL, 1-butyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][Tf2N]). The synthesis system is very simple (single [BMIM][Tf2N] solvent plus MoO2(acac)2 reactant). Specifically, the as-synthesized hierarchically 1D MoO2 composed of ultrathin nanosheet subunits shows a superior electrocatalytic activity with good long-term stability for the HER in acidic media and is among the most active Mo-based electrocatalysts in acid media reported up to now.

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EXPERIMENTAL SECTION

Materials. Molybdenyl acetylacetonate (MoO2(acac)2), H2SO4, KOH, Na2HPO4·12H2O, and NaH2PO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][Tf2N]) was bought from Lanzhou Greenchem, ILS, LICP, CAS, China. Commercial carbon black (Vulcan XC-72) was purchased from Cabot Corporation. Nafion solution (5 wt %) was bought from Sigma-Aldrich. Commercial MoO2 was purchased from Strem Chemicals, Inc. Johnson-Matthey 20 wt % Pt/C was bought from Alfa Aesar. Ultrapure nitrogen was bought from Jinan Deyang Gas Co. Ltd. Synthesis of Hierarchically Structured 1D MoO2. In a typical synthesis, 40 mg of MoO2(acac)2 was added in 0.5 g of [BMIM][Tf2N] in a glass bottle. Then, the solution was sonicated in an ultrasonic bath for 5 min to result in a uniform mixture. Finally, the glass bottle was transferred to a tube furnace. After this, the furnace was then thoroughly flushed with N2 gas for 20 min and heated to 330 °C at 5 °C min−1 under N2 protection. The furnace was held at 330 °C for 2 h and then was naturally cooled to room temperature under N2 protection. The reaction process was accompanied by a change of color from yellowish-brown to black, indicating the formation of MoO2. Finally, the obtained products were recovered by centrifugation, and they were then repeatedly washed with ethanol and water. The recovered products were put in a vacuum drying oven for drying at 60 °C for 5 h. The synthesis procedure with other amount of MoO2(acac)2 (10, 20, 30, 50, and 100 mg) was the same as above. The resultant nanostructured MoO 2 with different amounts of MoO2(acac)2 was defined as MoO2_x (x is the weight of the MoO2(acac)2). For the control experiment, the synthesis was performed by directly heating 40 mg of MoO2(acac)2 powder in N2 protection at 330 °C for 2 h without the use of [BMIM][Tf2N] reaction medium. Material Characterization. X-ray diffraction (XRD) characterization was conducted on a Rigaku Dmax-rc X-ray diffractometer. Transmission electron microscopy (TEM) characterization was examined on a JEM 1400 TEM. High-resolution transmission electron microscopy (HRTEM) characterization was carried out on a JEM2100F. Scanning electron microscopy (SEM) was conducted on a Hitachi SU-70 FESEM. X-ray photoelectron spectroscopy (XPS) characterization was performed with a photoelectron spectrometer (ESCALAB 250). N2 adsorption−desorption isotherms were characterized using a Builder SSA-4200 unit instrument. Gas chromatography analysis was performed on a GC-7806 (Shiweipx) for the characterization of the evolved hydrogen. Electrochemical Measurements. The electrochemical measurements were performed using an electrochemical workstation (CHI

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RESULTS AND DISCUSSION The MoO2 particles were directly synthesized by heating the solution of MoO2(acac)2 in [BMIM][Tf2N] IL under N2 protection. Scheme 1 shows the typical schematic illustration Scheme 1. (a) Typical Schematic Illustration for the Synthesis of MoO2 and (b) the Molecular Structure of [BMIM][Tf2N] Ionic Liquid

for the synthesis of MoO2 and the molecular structure of [BMIM][Tf2N] IL. Figure 1a shows the XRD pattern of the synthesized MoO2 (MoO2_40) particles with 40 mg of MoO2(acac)2 in 0.5 g of [BMIM][Tf2N] IL. All of the characteristic diffraction peaks in Figure 1a can be assigned to monoclinic MoO2 (Powder Diffraction File no. 65−5787, Joint Committee on Powder Diffraction Standards, 1967). Therefore, the XRD result verifies the formation of highly crystalline MoO2. Figure 1b,c shows the typical low- and highmagnification SEM images of the as-prepared MoO2_40, manifesting the formation of 1D MoO2 nanostructure with B

DOI: 10.1021/acsami.7b00722 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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completely different from the as-synthesized hierarchical 1D MoO2_40 nanostructure. Therefore, the [BMIM][Tf2N] IL reaction medium plays an important role in synthesizing the hierarchical 1D MoO2_40 nanostructure. To understand the growth process of MoO2_40 particles, time-dependent experiments were carried out. Figure 3a shows

Figure 1. (a) XRD pattern, (b, c) low-and high-magnification SEM images, (d, e) low- and high-magnification TEM images, and (f) HRTEM image of MoO2_40.

the diameter size of 71 ± 10 nm. By close observation, it is found that the 1D MoO2 nanostructure is constructed by lots of ultrathin nanosheets, which results in porous structure. The microstructure of the hierarchical 1D MoO2_40 nanostructure was further characterized by TEM, as depicted in Figure 1d,e. It can be clearly seen that the MoO2_40 is constructed by ultrathin nanosheets, which is agreeable to the SEM results. The HRTEM image illustrated in Figure 1f shows a lattice spacing of 0.48 nm, which corresponds to the (100) plane of MoO2. Therefore, hierarchically structured 1D MoO2 was successfully synthesized by the present route. We have also performed the control experiment in which the synthesis was by directly heating 40 mg of MoO2(acac)2 powder in N2 protection without the use of [BMIM][Tf2N] reaction medium. The results are shown in Figure 2. The XRD result confirms the amorphous structure of the as-synthesized product (Figure 2a). The TEM image indicates that the product has an irregular nanostructure (Figure 2b), which is

Figure 3. XRD patterns of the products (a) and TEM images of the recovered products at 10 min (b, c), 30 min (d, e), and 1 h (f−h).

the XRD results for the products recovered at 10 and 30 min and 1 h, respectively. For the products recovered at 10 and 30 min, there are no obvious characteristic peaks in the XRD patterns, indicating their amorphous structures. However, when the reaction time increased to 1 h, all of the diffraction peaks can be readily indexed to the monoclinic MoO2, indicating successfully the formation of MoO2. Figure 3b−h depicts the TEM images of the products corresponding to the different reaction time. At 10 min, it was found that 1D nanostructures are formed, as shown in Figure 3b,c. However, the nanosheets on the 1D nanostructure like MoO2_40 (Figure 1) are absent. When the reaction time increased to 30 min (Figure 3d), the product morphology is similar to that obtained at 10 min. However, the higher TEM image (Figure 3e) indicates the 1D nanostructure is clearly composed of plenty of nanoparticles,

Figure 2. (a) XRD pattern and (b) TEM image of the syntheiszed product without using [BMIM][Tf2N] IL as the reaction medium. C

DOI: 10.1021/acsami.7b00722 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces which are still in amorphous structure as demonstrated by XRD result (Figure 3a). When the reaction time is extended to 1 h, we find that nanosheets start to form on 1D nanostructure (Figure 3f−h). However, this 1D nanostructured product is well-crystallized (Figure 3a). Upon further increasing the reaction time to 2 h, well-defined hierarchically structured 1D MoO2 is formed, as described above (Figure 1). Therefore, the formation process can be as follows. First, the 1D nanostructure forms, and the 1D nanostructure then evolves into nanosheetconstructed hierarchically 1D MoO2. According to the literature,42,51 1,3-dialkylimidazolium-based ILs are able to self-organize into polymeric supramolecular structure. Therefore, the organized supramolecular structure of the [BMIM][Tf2N] can work as the template for the formation of the 1D nanostructure at the initial stage. With the time prolonged, the metastable amorphous nanoparticles in the 1D nanostructure are consumed through the well-known Ostwald ripening process, which results in the final formation of nanosheetconstructed hierarchically 1D MoO2. During the Ostwald ripening process, the transformation of amorphous nanoparticles within the 1D nanostructure into the nanosheet structure could be ascribed to the template effect of IL since the template effect of IL has been previously demonstrated for the formation of 1D or 2D nanostructures.42 Therefore, the welldefined hierarchically structured 1D MoO2 could be successfully formed in [BMIM][Tf2N] reaction medium. XPS characterization was used to understand the chemical state of the elements in MoO2_40. Figure 4a shows the high-

Figure 5. XRD patterns (a) and TEM images of MoO2_10 (b), MoO2_20 (c), MoO2_30 (d), MoO2_50 (e), and MoO2_100 (f). The white circles in panel f indicate the nanotube-like morphologies.

the morphology of MoO2_100 is quite different from that of others (Figure 5f). Besides several irregular aggregates, part of the MoO2_100 product shows nanotube-like morphologies, as indicated by the white circles (Figure 5f). The average diameter size of MoO2 _ 10 , MoO 2 _ 20 , MoO 2 _ 30 , MoO 2 _ 50 , and MoO2_100 is 96 ± 18, 83 ± 9, 92 ± 11, 69 ± 6, and 100 ± 23 nm, respectively. The HER electrocatalytic activities of the synthesized catalysts were investigated in 0.5 M H2SO4 with a threeelectrode system. We chose MoO2_40 and MoO2_100 as the test catalysts since they represented the typical morphologies of the synthesized products. For comparison, the commercial MoO2 (Strem Chemicals) and 20 wt % Pt/C (Johnson-Matthey) were also studied. The polarization curves and Tafel plots are shown in Figure 6a,b. Based on Figure 6, the onset potential, Tafel slope, overpotential η values at a current density of 10 mA cm2 (η10), and the exchange current density (j0) can be obtained. Among all MoO2 catalysts, MoO2_40 exhibits very high HER activity only second to Pt/C. MoO2_40 possesses a lower onset potential of 115 mV compared to those of MoO2_100 (125 mV) and commercial MoO2 (231 mV). To obtain a current density of 10 mA cm2, MoO2_40 catalyst needs smaller overpotential of 169 mV compared to those required by MoO2_100 (216 mV) and commercial MoO2 (335 mV). MoO2_40 shows a small Tafel slope of 58 mV dec−1, lower than those of MoO2_100 (73 mV dec−1) and commercial MoO2 (93 mV dec−1). The Tafel results indicate that all the MoO2 catalysts follow the Volmer− Heyrovsky HER mechanism according to the classic theory on

Figure 4. (a) XPS spectra of Mo 3d and (b) N2 adsorption− desorption isotherms of MoO2_40. The insert in panel b is the corresponding pore size distribution.

resolution XPS spectrum of Mo 3d, which can be divided into four peaks. The peaks at 232.9 and 229.1 eV are attributed to Mo(IV) 3d3/2 and Mo(IV) 3d5/2 of MoO2, respectively.3,26 The peaks at 235.5 and 231.7 eV are assigned to the Mo(VI) 3d3/2 and Mo(VI) 3d5/2 of MoO3, respectively, resulting from the slight surface oxidation of MoO2 exposed to air.3,26,31 Figure 4b shows the N2 adsorption−desorption isotherms of the synthesized MoO2_40. The Brunauer−Emmett−Teller (BET) specific surface area and the pore size were determined to be 84.5 m2 g−1 and 3.6 nm, respectively. Such high BET specific surface area could be attributed to the unique ultrathin nanosheet-constructed hierarchically 1D nanostructure. We also synthesized the MoO2 products with different amounts of MoO2(acac)2 in 0.5 g of [BMIM][Tf2N]. The amount of MoO2(acac)2 ranges from 10 to 100 mg. From the XRD results (Figure 5a), we can see that all the as-synthesized products are pure monoclinic MoO2 with good crystallization. Figure 5b−f shows the TEM images of the synthesized products. The morphologies of synthesized MoO 2 _ 10 , MoO2_20, MoO2_30, and MoO2_50 have no noticeable differences relative to that of MoO2_40 in Figure 1. However, D

DOI: 10.1021/acsami.7b00722 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Thus, MoO2_40 has a much higher ECSA than others, indicating more exposed active sites in MoO2_40. The high HER electrocatalytic activity of our synthesized MoO2_40 can be explained as follows. On one hand, the ultrathin nanosheet subunits in 1D MoO2_40 nanostructure can provide many active sites such as atomic steps, kinks, and edges, which largely enhance the electrocatalytic activity. On the other hand, the 1D nanostructure with ultrathin nanosheet subunits can result in a high surface area (as demonstrated in Figure 4b); therefore, the interfacial contact area between the catalyst and the electrolyte is largely enhanced. In this case, the ion diffusion paths are shortened; therefore, the charge transfer is accelerated for hydrogen evolution. We also investigated the HER electrocatalytic activities of the synthesized catalysts in alkali solution and neutral solution. Here we used 1 M KOH (pH 14) as the alkali solution and 1 M phosphate buffer solution (pH 7) as the neutral solution. The HER performance study in alkali solution and neutral solution indicated that our catalysts could also catalyze hydrogen production in alkali solution and neutral solution (Figure S2). However, the catalytic properties are much less active than those in 0.5 M H2SO4 solution. For example, the overpotential for achieving 10 mA cm2 is 233, 337, and 546 mV for MoO2_40, MoO2_100, and commercial MoO2, respectively, in 1 M KOH solution. For the HER electrocatalytic performance in 1 M phosphate buffer solution, the overpotential for 10 mA cm2 is 523, 577, and 766 mV for MoO2_40, MoO2_100, and commercial MoO2, respectively. Therefore, our synthesized catalysts are not suitable for practical HER application in alkali solution and neutral solution based on their corresponding HER electrocatalytic activities. However, the excellent HER electrocatalytic activities of our MoO2_40 catalyst in 0.5 M H2SO4 demonstrate its promising application as efficient HER catalyst in acid solution.

Figure 6. Electrochemical measurements of the catalysts: (a) Polarization curves and (b) Tafel slope of MoO2_40, MoO2_100, commercial MoO2, and 20% Pt/C. (c) Electrochemical impedance spectra at 200 mV vs RHE. (d) Polarization curves of the MoO2_40 before and after 2000 cycles. (e) Current density−time (I−t) curve for the MoO2_40 at an overpotential of 170 mV. (f) Theoretically calculated (black line) and experimentally measured (red line) amount of hydrogen evolved versus time for MoO2_40 at an overpotential of 190 mV.

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HER in acidic media.52,53 The exchange current density of MoO2_40 is 14 μA cm−2, which is higher than those of MoO2_100 (11 μA cm−2) and commercial MoO2 (4 μA cm−2). A comparison with literature data (Table S1) demonstrates that our synthesized MoO2_40 is among the most active Mo-based electrocatalysts up to date, verifying its promising application in HER. The electrochemical impedance spectra (EIS) were further used to understand the electrode kinetics of the catalysts (Figure 6c). The Nyquist plots show that the chargetransfer resistance (Rct) of MoO2_40 is 62 Ω, which was much lower than those of MoO2_100 (276 Ω) and commercial MoO2 (773 Ω), indicating its highly efficient Faradaic process and better HER kinetics. In addition to excellent electrocatalytic activity, MoO2_40 electrode also exhibits excellent stability for HER. Based on the polarization curve with 2000 cycles (Figure 6d) and the I−t curve (Figure 6e), only an ignorable decay is observed, indicating its excellent durability of the MoO2_40 electrode for HER in 0.5 M H2SO4. The evolved hydrogen amount was further determined by gas chromatography analysis, and the result was shown in Figure 6f. It shows that the Faradaic efficiency based on the amount of theoretically calculated and experimentally measured hydrogen is nearly 100%, indicating the excellent hydrogen production efficiency. To better understand the enhanced HER electrocatalytic activity of MoO2_40, we compared the electrochemically active surface areas (ECSA) of the studied catalysts by measuring the double-layer capacitance (Cdl), which is in proportion to the ECSA (Figure S1). The Cdl’s of MoO2_40, MoO2_100, and commercial MoO2 are 201, 150, and 85 mF cm−2, respectively.

CONCLUSIONS In summary, we have successfully synthesized hierarchically structured 1D MoO2 in an IL [BMIM][Tf2N] for the first time. The synthesis system is very simple (single [BMIM][Tf2N] solvent plus MoO2(acac)2 reactant). The as-synthesized hierarchically 1D MoO2 comprises plenty of ultrathin nanosheet subunits. The formation of such unique hierarchically 1D MoO2 is ascribed to the unique role of the IL [BMIM][Tf2N] environment. The synthesized hierarchically 1D MoO2_40 with well-defined morphology shows an excellent electrocatalytic activity with good long-term stability for the HER in acidic media, comparable to those of state-of-the-art well-developed nonprecious metal catalysts for the HER. Our strategy for synthesis of nonprecious metal HER electrocatalyst in an IL reaction medium opens new opportunities in achieving interesting alternative to noble metal catalysts for efficient electrocatalytic production of hydrogen.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00722. Comparison of HER performance for the as-synthesized MoO2 with the reported nonprecious metal HER eletrocatalysts including Mo-based electrocatalysts in acid electrolyte, electrochemically active surface areas (ECSA) of the studied catalysts by measuring the E

DOI: 10.1021/acsami.7b00722 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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double-layer capacitance (Cdl), electrochemical measurements of the catalysts in 1 M KOH (pH 14) and 1 M phosphate buffer solution (pH 7) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 531-88564750. Tel.: (+86) 531-88363821. E-mail: [email protected]. ORCID

Zhimin Xue: 0000-0001-6554-8788 Zhonghao Li: 0000-0003-0699-300X Jingcheng Hao: 0000-0002-9760-9677 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant No. 21673128), and the Fundamental Research Funds of Shandong University (Grant No. 2015JC003).

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REFERENCES

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DOI: 10.1021/acsami.7b00722 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX