CoS2 Nanotube Arrays in

Apr 11, 2017 - We have designed and synthesized CoS2 and MoS2/CoS2 nanotube arrays by one-step hydrothermal method using Co(OH)2 nanorod arrays as the...
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Supercapacitor Performances of the MoS2/CoS2 Nanotube Arrays in Situ Grown on Ti Plate Lina Wang,†,§ Xia Zhang,‡ Ying Ma,†,§ Min Yang,† and Yanxing Qi*,† †

State Key Laboratory for Oxo Synthesis & Selective Oxidation and National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, China § University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: We have designed and synthesized CoS2 and MoS2/CoS2 nanotube arrays by one-step hydrothermal method using Co(OH)2 nanorod arrays as the template. The products were characterized by X-ray diffraction pattern, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, and transmission electron microscopy. The MoS2/CoS2 electrode is demonstrated to have a relatively high area capacitance of 142.5 mF cm−2 at 1 mA cm−2, which is higher than that of CoS2 or MoS2 electrode. Besides, the electrode also shows excellent cycle stability of 1000 cycles with 92.7% retention, which is also superior to that of CoS2 and MoS2 electrodes. These results indicate that MoS2/CoS2 nanotube arrays have potential as electrode materials of supercapacitors because of the synergistic reaction of MoS2 (which supplies the specific surface area and effective electrolyte accessibility) and CoS2 (which serves as a conductive channel and reduces the phenomenon of aggregation). The design of MoS2/CoS2 architecture may open up a new strategy for synthesizing promising electrode materials for supercapacitors.

1. INTRODUCTION With the increasing societal demands for consumption of energy, there are more and more requirements for sustainable energy sources1−6 in conjunction with efficient energy-storage devices.7−10 The supercapacitor is one of the most promising energy-storage devices because of its excellent recharge ability, high power density, good energy density, excellent cycle stability, low maintenance cost, and nonpollution.11−13 In general, on the basis of the charge-storage mechanism, electrode materials for supercapacitors can be categorized into electrical double-layer capacitors (EDLCs) and pseudocapacitors.14,15 As is known to all, the electrode material is important for the high-performance supercapacitor. Molybdenum disulfide (MoS2) has a structure of stacked covalently bonded S−Mo−S layers16,17 and is a promising material with respect to its specific capacitance and cost effectiveness for use in energy-storage applications. For example, Krishnamoorthy et al.18 reported that the MoS2 nanostructure with the specific capacitance of 92.85 Fg−1 was synthesized. Huang et al. 19 reported polyaniline/MoS 2 composites as supercapacitor electrode, but the specific capacitance of the pure MoS2 electrode was only 98 F g−1 (1 Ag−1). Recently, Kim et al’s study20 showed the highperformance supercapacitive properties of MoS2−Mo binderfree electrodes. Loh et al.21 reported that the MoS2 nanowall was prepared by thermal evaporation and the electrochemical tests had been carried out. Choudhary et al.22 reported on the fabrication of MoS2 film supercapacitor electrodes using a direct © 2017 American Chemical Society

magnetron sputtering technique, which presented a good specific capacitance. Nevertheless, practical application of MoS2 as electrode material was limited by the aggregation phenomenon and poor conductivity. Therefore, to improve the performance of MoS2, we should design and synthesize MoS2 with more active edge sites and good conductivity. CoS2, as an important member of the semiconductor family, is considered to be one of the most promising electrode materials for application in supercapacitors. As is known to all, the size, morphology, porosity, and pore-size distribution have an obvious effect on the electrochemical behaviors of the electrode materials. So far, single-phased CoS2 ellipsoids23 and hollow spheres24 were synthesized and employed as electrode material for supercapacitors. Zhu et al.25 reported that the as-fabricated CoS2 electrode showed typical pseudocapacitive properties and exhibited an excellent cycling stability, suggesting that the octahedron-shaped CoS2 is a promising electrode material for supercapacitors. Bie et al.26 have reported that the cobalt sulfide nanotube electrode exhibited excellent electrochemical performance as electrode material for supercapacitors. Chen et al.27 reported that the hierarchical structured NixCo1−xS1.097 electrode exhibited a remarkable maximum specific capacitance approximately five times higher than that of the CoS1.097 precursors. In addition, highly conductive NiCo2S4 singleReceived: December 28, 2016 Revised: April 8, 2017 Published: April 11, 2017 9089

DOI: 10.1021/acs.jpcc.6b13026 J. Phys. Chem. C 2017, 121, 9089−9095

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The Journal of Physical Chemistry C crystalline nanotube arrays grown on a flexible carbon fiber paper (CFP) have been reported, which could serve not only as a good capacitive material but also as a 3D conductive scaffold for loading additional electroactive materials.28 Wang et al.29 fabricated NiCo2S4@Co(OH)2 nanotube arrays in situ grown on Ni foam through two-step hydrothermal reactions and electrodepositing, which delivered a remarkable electrochemical performance. Within this context, nanotube arrays integrated with a conducting substrate may possess the advantages of good charge-carrier transport and electrochemical properties.30−35 Over the past few years, various synthesis routes (soft template, hard template, and self-assembly template-free methods) have focused on the fabricating of nanotube structures.36−42 On the basis of the above considerations, CoS2 nanotube may be an ideal conductive channel. The rational design and synthesis of nanotube arrays of MoS2/CoS2 is one of the major strategies for enhancing the structural stability and electrochemical performance. The structure has multiple apparent advantages, as follows: (i) the material and conductive substrate are integrated into one unit, (ii) nanosheets are well wrapped on the surface of nanotubes, which will achieve the anisotropic structure, high surface-to-volume ratio, and heterogeneous interfaces, and (iii) nanotube arrays with good electrical conductivity serve as the electron “superhighway” and overcome the limited electrical conductivity of nanosheets.43−47 In this work, MoS2/CoS2 nanosheet tube arrays (NSTAs) supported on titanium plate (Ti plate) were designed and fabricated via one-step hydrothermal method, using Co(OH)2 nanorod arrays (RAs) as the template. The novel MoS2/CoS2 NSTAs with the ultrathin MoS2 nanosheets (NSs) warping on the CoS2 nanotube arrays (TAs) would reduce the phenomenon of aggregation, increase the number of active sites, and improve the whole electrochemical performance. Furthermore, CoS2 TAs could function as efficient mechanical support and electron-conducting pathway, which would promote the related electrochemical kinetics. With the smart combination of MoS2 NSs and CoS2 TAs, a synergistic effect could be observed. Impressively, as a binder-free electrode, the MoS2/CoS2 NSTAs exhibited a high capacitance (142.5 mF cm−2 at 1 mA cm−2) and excellent cycling ability (92.7% retention after 1000 cycles). In a word, MoS2/CoS2 NSTAs could be expected to be a kind of promising electrode material in supercapacitor application, which deserves for further study.

impedance spectroscopy (EIS) analyses were carried out in the frequency range of 100 kHz to 0.01 Hz.

3. RESULTS AND DISCUSSION The MoS2/CoS2 NSTAs grown on Ti plate were obtained by one-step hydrothermal process using Co(OH)2 RAs as the template, which was schematically illustrated in Scheme 1. Scheme 1. Schematic Illustration of the Synthesis Process of MoS2 NSs, CoS2 TAs, and MoS2/CoS2 NSTAs on Ti Plate

Initially, Co(OH)2 RAs were grown on Ti plate. Then, MoS2/ CoS2 NSTAs and CoS2 TAs were synthesized by using Co(OH)2 RAs as the template. MoS2 NSs were directly grown on Ti plate without adding the template of Co(OH)2 RAs. The details of the synthesis procedures are in the Experimental Section (Supporting Information (SI)). The growth mechanism of the MoS2/CoS2 NSTAs based on anion exchange reaction and Kirkendall effect.48 During the hydrothermal reaction, CH3CSNH2 solution reacted with Co(OH)2 nanorod arrays, and the inward diffusion of S2− from CH3CSNH2 was slower than the outward diffusion of Co2+ from Co(OH)2, which created voids at the center of the rod to form CoS2 TAs. Concurrently, Na2MoO4·2H2O reacted with CH3CSNH2 to form MoS2 NSs, which were deposited on the surface of CoS2 TAs. The morphology of pristine Co(OH)2 RAs on Ti plate was examined by FESEM and TEM measurements (Figure S1). The pristine Co(OH)2 nanorod (NR) was relatively uniform in diameter (300−400 nm) with a highly smooth surface (inset to Figure S1a), which was the template for the synthesis of MoS2/ CoS2 NSTAs. The MoS2/CoS2 NT with a diameter of 500 nm and a length of 3 to 5 um grew densely on Ti plate (shown in Figure 1a). The structure would promote the diffusion of the electrolyte, transporting of electron and conductivity of materials. The higher magnification SEM measurements showed a uniform coverage of interconnected MoS2 nanosheets on the surface of CoS2 TAs (Figure 1b), leading to a 3D network with a large surface area and more active sites. Figure 1c shows a typical TEM image of an individual CoS2 nanotube covered by thin MoS2 NSs. The MoS2 NSs were about 20−30 nm thick and grown on the surface of CoS2 NT. From the TEM image of Figure 1d, the MoS2 NSs could be clearly seen with an interlayer distance of 0.6 nm that was consistent with the MoS2 (002). We also could see well-defined lattice fringes of CoS2 with a spacing of 0.28 nm. Elemental mapping studies further confirmed the intimate contact between CoS2 and MoS2 in MoS2/CoS2 NSTAs with a distribution of the Mo, Co, and S

2. CHARACTERIZATION The crystallographic information on the as-synthesized samples was established by X-ray diffraction (XRD) patter on a PANalytical X’pert PRO instrument using Cu Ka radiation. The morphological investigations of the samples were taken using field-emission scanning electron microscopy (FESEM, model JEOL-JSM6701F) and transmission electron microscopy (TEM, JEOLJEM-2100). The X-ray photoelectron spectroscopy (XPS) measurement were performed using a VG ESCALAB 210 system. The electrochemical measurements of the samples were carried out in a three-electrode electrochemical cell in the electrolyte of 1 M Kcl. The MoS2/CoS2 NSTAs, MoS2 NSs, and CoS2 TAs grown on Ti plate were directly used as the working electrode. The saturated calomel electrode (SCE) and Pt foil were used as the reference electrode and counterelectrode, respectively. The cyclic voltammetry (CV) and galvanostatic charge−discharge (GCD) measurements of the samples were performed within the potential range at different scanning rates and different current densities. Electrochemical 9090

DOI: 10.1021/acs.jpcc.6b13026 J. Phys. Chem. C 2017, 121, 9089−9095

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out to further confirm the chemical composition. The survey XPS spectrum in Figure S5a confirmed the presence of Co, Mo, S, and Ti in MoS2/CoS2 NSTAs on Ti plate. From the highresolution scan in Figure S5b, the Mo 3d spectrum might be deconvoluted into two peaks at 233.05 and 229.9 eV, respectively.49 The peak at ∼226 eV corresponded to the S 2s. For the Co 2p electron (Figure S5c), two distinct peaks could be identified at 779.9 and 793.5 eV, which could be assigned to Co 2p3/2 and Co 2p1/2, respectively.39 The S 2p electron was manifested by the two peaks at 162.2 and 163.3 eV, corresponding to S 2p3/2 and S 2p1/2 in Figure S5d, respectively.49,51,52 The above material characterization indicated that the MoS2/ CoS2 NSTAs on Ti plate were an ideal candidate for electrode material of supercapacitors. Accordingly, CV and GCD tests were carried out to study the electrochemical properties in a three-electrode electrochemical cell. Interestingly, the MoS2/ CoS2 NSTAs on Ti plate exhibited apparent good specific capability in 1 M KCl. Experimentally, the material supported on Ti plate was used directly as electrode, without the binding agent and conducting additive. The CV curves at 40 mV s−1 in Figure S6a shows that the MoS2/CoS2 NSTAs exhibited a much better performance than that of MoS2 NSs and CoS2 TAs, which resulted from the area of the CV curves. The area surrounded by the CV curve was dramatically enlarged by the syncretizing MoS2 NSs and CoS2 TAs, suggesting a larger area specific capability of MoS2/CoS2 NSTAs. The specific capacitance values for the MoS2/CoS2 NSTAs, MoS2 NSs, and CoS2 TAs electrode were 122.3, 90.4, and 2.35 mF cm−2 at a scan rate of 40 mV s−1, respectively. Figure S6b depicts the comparison of GCD curves for the MoS2 NSs, CoS2 TAs, and MoS2/CoS2 NSTAs at a current density of 1 mA cm−2. As expected, MoS2/CoS2 NSTA electrode delivered a much longer charge−discharge time than the bare MoS2 NSs or CoS2 NTs, indicating its higher capacitances. Afterward, the areal capacitance of MoS2/CoS2 NSTAs electrode was calculated to be 142.5 mF cm−2 at 1 mA cm−2, which was evidently higher than the CoS2 NTs with 3.13 mF cm−2 and the MoS2 NS with 84 mF cm−2. Therefore, we could see that the MoS2/CoS2 NSTAs electrode showed server times higher capacitance than that of bare MoS2 or CoS2 electrode at 1 mA cm−2 (almost two times as high as that of bare MoS2 electrode and 50 times as high as that of CoS2 electrode). Figure 3a shows the CV curves of MoS2/CoS2 NSTAs electrode at different scan rates of 10, 20, 40, and 60 mv s−1, which presented a perfectly rectangular shape without any obvious redox peaks, indicating a typical electrical double-layer capacitance. With the scan rate increasing, the CV curves had a tendency to further augment of the approximate rectangle shape, suggesting the good ion responses. Figure 3b further illustrates the special areal capacitance of MoS2/CoS2 NSTAs at different scan rates. As the scan rates increased, the specific capacitance of electrode decreased. This observation resulted from the fact that the ions accessed the pores of the electrodes with more difficulty with increasing scan rate. When the scan rates of MoS2/CoS2 NSTAs increased from 10 to 60 mV s−1, the specific capacitance of it retained 84.6%, suggesting excellent reaction kinetics and good rate capability. Figure 3c shows different GCD curves of MoS2/CoS2 NSTAs electrode, which were dependent on current densities. Figure 3d further shows the specific capacitances of MoS2/CoS2 NSTAs electrode from the GCD curves at different current densities. In evidence, with current density increasing, the ions had less chance of

Figure 1. (a,b) FESEM of MoS2/CoS2 NSTAs, (c) TEM of MoS2/ CoS2 NSTAs, and (d) high-resolution TEM image of MoS2/CoS2 NSTAs.

elements (Figure 2). Inductively coupled plasma mass spectroscopy analysis reveals that the ratio of Mo/Co was 3:2. The

Figure 2. Elemental mapping images of MoS2/CoS2 NSTAs.

unique hollow structure of MoS2 /CoS 2 NSTAs could effectively mitigate the volume variation during the charge− discharge process. The TAs were grown on conductive substrate, which created an open and conductive network for electrolyte diffusion and electron transport, and the MoS2 NSs also provided sufficient active sites, which allowed the penetration of the electrolyte for fast transport of ions. By contrast, Figure S2a,b exhibits the SEM of the CoS2 nanotube with the diameter of 300−400 nm. Figure S2c shows a typical TEM image of an individual CoS2 NT. It should be noted that the CoS2 TAs possessed a rough structure during the reaction between CH3CSNH2 and Co(OH)2. They were, with the distinct morphology, different from Co(OH)2 NR (Figure S1b), which possessed relatively smooth surface, and the SEM images of bare MoS2 nanosheets grown on the Ti plate are shown in Figure S3. The MoS2/CoS2 NSTAs were further confirmed by XRD and XPS analysis. Figure S4 depicts the XRD patterns of MoS2/ CoS2 NSTAs on Ti plate. We could see sharp peaks at 13.4, 32.6, and 57.2°, which correspond to the MoS2 (JCPDS no. 371492).49 The broad diffraction peaks at 38.4, 40.2, and 53.0° arose from the Ti plate (JCPDS no. 65-9622).50 The diffraction peaks at 28.7, 32.9, and 35.2° could be identified as CoS2 (JCPDS no. 41-1471).51 XPS measurement was then carried 9091

DOI: 10.1021/acs.jpcc.6b13026 J. Phys. Chem. C 2017, 121, 9089−9095

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The Nyquist spectra of MoS2/CoS2 NSTAs, MoS2 NSs, and CoS2 TAs at a frequency of 0.01 Hz to 100 kHz are shown in Figure 4a. The Nyquist spectra made a good characterization of

Figure 3. (a) CV curves of the MoS2/CoS2 NSTAs electrode at different scan rates. (b) Specific capacitances of the MoS2/CoS2 NSTAs electrode at different scan rates. (c) GCD curves of the MoS2/CoS2 NSTAs electrode at different current densities. (d)Specific capacitances of the MoS2/CoS2 NSTAs electrode at different current densities.

Figure 4. (a) Nyquist plots of the MoS2 NSs, CoS2 TAs, and MoS2/ CoS2 NSTAs electrodes. (b) Cycling stability of the MoS2 NS, CoS2 TA, and MoS2/CoS2 NSTA electrodes at a current density of 1 mA cm−2.

the conductivity of materials. Noticeably, the three electrodes had quite small quasi-semicircles and a straight line. At high frequency, the x intercept on real axis represented a combined resistance (Rs) including intrinsic resistance of electrode materials, ionic resistance of electrolyte, and contact resistance. The diameter of the semicircle corresponded to the chargetransfer resistance (Rct) caused by Faradaic reactions or EDLC. The portion of low-frequency region, known as Warburg resistance (Zw), was a result of the frequency dependence of electrolyte diffusion/transport into the electrode. The Nyquist plots of the electrodes were analyzed by applying the equivalent circuit shown in Figure S8. The calculation showed that the equivalent internal resistance (Rs) is 0.08, 0.1, and 0.7 for MoS2/CoS2 NSNTs, MoS2 NSs, and CoS2 NTs. Therefore, ions could easily accumulate on the surface of the electrode and obtain access to the inner active site of MoS2/CoS2 NSNTs. As a result of electrical double-layer capacitance, the capacitance would be improved. Rct values of MoS2/CoS2 NSNTs, MoS2 NSs, and CoS2 NTs electrodes were 1.465, 1.83, and 1.56, respectively. MoS2/CoS2 NSNTs could enhance the diffusivity of the electrolyte in the electrode, lowering the charge-transfer resistance value, and the straight line of MoS2/CoS2 NSNTs at low frequency, which was nearly parallel to the imaginary axis, indicated typical EDLC behavior. The lower slope of MoS2/ CoS2 NSNTs electrode compared with that of MoS2 NSs and CoS2 NTs represents the smaller Warburg resistance and faster ion diffusion of electrolyte than that of MoS2 NSs and CoS2 NTs. The cycling performances of MoS2/CoS2 NSTAs, MoS2 NSs, and CoS2 TAs were also investigated at a current density of 1 mA cm−2 by a repeated charge−discharge process. As shown in Figure 4b, for all of the curves, the capacity retention slowly decreases with the cycle number. After 1000 cycles, the MoS2 NS and CoS2 TA electrodes retained only 83.3 or 60% of its initial capacity, while the retention of capacity of the MoS2/ CoS2 NSTAs electrode was still 92.7% after 1000 cycles, exhibiting good cycling stability. Even after 3000 cycles, the discharge capacity for the MoS2/CoS2 NSTAs electrode was measured to be 81.8% of initial discharge (Figure S9). In addition, to highlight the superior electrochemical performance, the Ragone plot shown in Figure 5 is used to display the power performance of the MoS2/CoS2 NSTAs. The energy density and power density were calculated by the following equations

penetrating into more active materials of the electrode. That was why the specific capacitance decreased gradually. Impressively, the specific capacitances of MoS2/CoS2 NSTAs were 149.1, 142.5, 137.5, and 125 mF cm−2, respectively (at current densities of 0.5, 1, 2.5, and 4 mA cm−2). In addition, when the current densities increased from 0.5 to 4 mA cm−2, the specific capacitance of MoS2/CoS2 NSTAs retained ∼83.8%. The result was similar to the foregoing CV results, demonstrating the excellent electrochemical performance of the MoS2/CoS2 NSTAs. In addition, the mass specific capacitance MoS2/CoS2 NSTAs was also calculated, which is shown in Figure S7. The area-specific capacitances associated with CV and GCD curves were calculated according to the following equation

Cs =

∫ I dV vSΔV

where I (A) is the response current, V (V s−1) is the scan rate, ΔV (V) is the potential window, and S (cm−2) is the area of active electrode material, respectively. It Cm = SΔV where I (A) is the current, ΔV (V) is the potential window, t (s) is the discharge time, and S (cm−2) is the area of active electrode material, respectively. The smart designed MoS2/CoS2 NSTAs present excellent electrochemical performance for supercapacitors, owing to the following reasons: (i) CoS2 nanotubes were uniformly decorated with interconnected MoS2 nanosheets, forming unique forest-like nanotube arrays. Obviously, the nanosheets are ultrathin and wrinkled, which further increases the surface area of the electrode and provides more active sites, performing a superior capacitance. (ii) The nanosheets are interconnected with each other, thus forming a highly porous surface morphology. Besides the space between each nanotube constitutes ordered channels on a large scale. In such a particular structure, the opened channels were served as diffusion pathways, making the active materials more accessible to electrolyte ions, resulting in a sufficient connection between the electrolyte and the electrode, which can significantly promote full utilization of electrode. 9092

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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/acs.jpcc.6b13026. Experimental sections, the SEM images of Co(OH)2 RAs, CoS2 TAs, and MoS2 NSs, the TEM images of Co(OH)2 RAs and CoS2 TAs, the XRD and XPS of MoS2/CoS2 NSTAs, CV curves and GCD curves of the CoS2 TAs, MoS2 NSs, MoS2/CoS2 and NSTAs, specific capacitances of the MoS2/CoS2 NSTAs electrode at different scan rates and different current densities, equivalent circuit of the electrodes, and cycling stability of the MoS2/CoS2 NSTAs electrode after 3000 cycles. (PDF)

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Figure 5. Ragone plots of energy density and power density of MoS2/ CoS2 NSTAs.

Corresponding Author

*E-mail: [email protected].

C ΔV 2 E= 2 P=

AUTHOR INFORMATION

ORCID

Yanxing Qi: 0000-0003-3770-5656 Notes

E Δt

The authors declare no competing financial interest.

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where C (F g−1) is mass specific capacitance, V (V) is the voltage window, E (Wh kg−1) is the energy density, and P (kW kg−1) is the power density. For the MoS2/CoS2 NSTAs, the specific energy density was 11.11 Wh kg−1 at a power density of 0.48 kW kg−1, implying its feasible and promising applications for energy storage (Figure 5). The excellent supercapacitive performance of MoS2/CoS2 NSTAs was mainly attributed to its structure and component: MoS2/CoS2 NSTAs directly grew on Ti plate, avoiding interferences from binding materials; CoS2 TAs were designed as a hollow tubular structure, resulting in easy electrolyte diffusion, good conductivity, and accommodation of change of volume; and the thin MoS2 NSs wrapped CoS2 TAs contributed to increase the specific surface area and effective electrolyte accessibility. As a result, the MoS2/CoS2 NSTAs provided numerous active sites, endowed with excellent performance in the application of electrode material of the supercapacitors.

ACKNOWLEDGMENTS This work is supported by the Chinese Academy of Sciences and Technology Project (XBLZ-2011-013) and the Technologies R&D Program of Gansu Province (1104FKCA156).

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REFERENCES

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4. CONCLUSIONS We have directly grown MoS2/CoS2 NSTAs on Ti plate using a facile and commendable one-step hydrothermal reaction by using Co(OH)2 RAs as template, which can simplify the electrode preparation process, reduce the electrode weight/ volume and increase the electrical conductivity, and boost the supercapacitor performance. The unique architecture provides many advantages, such as more active sites, short diffusion pathway of ions, and large expandable spaces for volume changes. When evaluated as an electrode material for supercapacitors, the MoS2/CoS2 NSTAs exhibit superior performances with an area capacitance of 142.5 mF cm−2 at 1 mA cm−2, which is almost two times the MoS2 NSs and 50 times the CoS2 TAs electrode. It also exhibits excellent cycle stability of 1000 cycles with only 7.3% loss. Therefore, MoS2/ CoS2 NSTAs reported in this paper are proved to be a promising supercapacitor electrode material, and the method will offer a way for rational design and synthesis of other materials. 9093

DOI: 10.1021/acs.jpcc.6b13026 J. Phys. Chem. C 2017, 121, 9089−9095

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DOI: 10.1021/acs.jpcc.6b13026 J. Phys. Chem. C 2017, 121, 9089−9095