Metal-Organic-Framework-Hosted ZIF-67@Se@MnO2 Composite

Dec 25, 2018 - Metal-Organic-Framework-Hosted ZIF-67@Se@MnO2 Composite Cathode for Lithium-Selenium Batteries With Superior Cycling Stability...
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ZIF-67@Se@MnO2: A Novel Co-MOF-Based Composite Cathode for Lithium−Selenium Batteries Wenkai Ye,† Weiyang Li,† Ke Wang, Weihao Yin, Wenwen Chai, Yi Qu, Yichuan Rui,* and Bohejin Tang* College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, P. R. China

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S Supporting Information *

ABSTRACT: In this study, metal organic frameworks (MOFs) are prepared as a support to tether active selenium through melt-diffused method, in which MnO2 nanoparticles are loaded on the surface of a Co-MOF (ZIF-67) through simple postmodification means. Selenium is uniformly dispersed on the surface of ZIF67, and MnO2 nanoparticles (∼5 nm in diameter) are coated on the surface of Se. As an analogue of sandwich, the MOF@Se@metal-oxide structure can inhibit the shuttle effect and dissolution of polyselenides in the electrolyte, which is confirmed by the cyclic voltammetry, galvanostatic discharge/charge, and electrochemical impedance tests. In addition, the cathode displays an excellent electrochemical performance. Especially, the discharge capacity of the ZIF-67@ Se@MnO2 cathode is 329 mA h g−1 after 100 cycles at 1C (C = 675 mA g−1) with the high capacity retention of 83.7%. Furthermore, the ZIF-67@Se@MnO2 cathode displays stable capacities of 273 and 232 mA h g−1 after 100 cycles at 2C and 5C, respectively.

1. INTRODUCTION In recent years, lithium-ion batteries (LIBs) have been considered one of the powerful energy-storage devices because of their portable and environment-friendly advantages.1−4 However, the low theoretical specific capacity of traditional commercial cathode materials, such as LiCoO2 (274 mA h g−1), LiMn2O4 (148 mA h g−1), and LiFePO4 (170 mA h g−1), cannot meet the increasing demand of the electronic equipment market.5−9 Therefore, development of the higherspecific-capacity anode and cathode materials is urgent for researchers. Lithium is one of the ideal anode materials with a specific capacity of 3860 mA h g−1.10 In addition, S is considered to be one of the suitable cathode materials because of its higher theoretical specific capacity of 1672 mA h g−1. Unfortunately, lithium−sulfur (Li−S) batteries still face major drawbacks for functional applications despite the numerous advances in these fields, for instance, the low electronic conductivity (5 × 10−28 S m−1) and the shuttle effect owing to the high solubility of the intermediates (Li2Sn, n > 2) in the electrolyte.11−17 Recently, Li−Se batteries have been widely researched and are expected as promising candidates. Selenium, as an analogue of S element, has a theoretical specific capacity of 675 mA h g−1, whereas its theoretical volumetric capacity (3260 mA h cm−3) is same as that of S (3467 mA h cm−3) because of the higher density of Se.18,19 Moreover, Se has a conspicuously higher electronic conductivity (1 × 10−3 S m−1), which means that it shows a higher utilization of the cathode material. Identically, the application of lithium−selenium (Li−Se) batteries is limited by the shuttle effect, which always leads to the disappointing cycling © XXXX American Chemical Society

performance, low Coulombic efficiency, and poor life span.20−24 Since the pioneering work of Abouimrane and coworkers in 2012,25 various strategies have been used to improve the electrochemical performance of the selenium cathode, including microporous or mesoporous carbon,26−29 carbon nanotubes,30 and three-dimensional porous carbon and graphene.31−33 Metal organic frameworks (MOFs) are a species of porous materials with unique physical and chemical properties, such as large specific surface area, high porosity, adjustable function, and high stability.34,35 Therefore, MOFs show great potential applications in the gas adsorption/separation, sensors, catalysis, and drug delivery.36−39 In terms of electrochemistry, MOFs are considered to be the promising candidates in supercapacitors and lithium-ion batteries.40−43 Moreover, MOFs can be prepared as porous carbon, which has been applied as a support in Li−Se batteries. For example, Li et al.44 reported a nitrogen-doped carbon support with meso- and micropores derived from an Al-MOF (Al(OH)(1,4-NDC)· 2H2O). Lai et al.45 prepared the mesoporous carbon by heating MOF-5 at 900 °C. He et al.46 obtained the threedimensional hierarchical C−Co−N material by calcining CoMOF. Recently, Jiang’s group used the ppy-S-in-MOF material in a Li−S battery, which confirmed that MOFs could act as a support to host active substances.47 Received: October 31, 2018 Revised: December 21, 2018 Published: December 25, 2018 A

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Figure 1. Illustration of the method for the synthesis of ZIF-67@Se@MnO2 composites.

solution under sonication. The mixture was heated in an autoclave at 90 °C for 3 days. The purple precipitate (ZIF-67) was collected by centrifugation, washing with DMF three times, and drying under vacuum at 60 °C overnight. Furthermore, the obtained ZIF-67 powder was put into a glass flask and dried at 200 °C under vacuum for 3 h to remove excess solvent molecules. Then, it was cooled to room temperature. 2.2. Synthesis of ZIF-67@Se. The processed ZIF-67 was mixed with Se powder in the ratio of ZIF-67/Se = 1:2. The mixture was ground in a mortar until it was uniform. Then, the mixture was poured into a glass tube and sealed under vacuum. The tube was heated at 260 °C in an oven and kept for 12 h. Finally, the crude ZIF-67@Se sample was heated at 260 °C for 12 h with a heating rate of 5 °C min−1 in a tubular furnace under N2 atmosphere to eliminate the excess Se. 2.3. Synthesis of ZIF-67@Se@MnO2. The processed ZIF67@Se sample (0.15 g) was dispersed in 20 mL of deionized water by sonication, and the mixture was placed in an ice bath. Then, 0.01 g of KMnO4 was dissolved in 20 mL of deionized water and added into the ZIF-67@Se solution dropwise. After vigorous stirring for 30 min, the mixture was centrifuged and washed several times with deionized water. ZIF-67@Se@ MnO2 samples could be obtained after being dried under vacuum at 60 °C overnight. 2.4. Characterization and Electrochemical Tests. The powder X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (D8 Advance, Bruker Corp., Germany) using Cu Kα radiation (λ = 0.15418 nm) at 40 kV. The samples were also examined by an X-ray photoelectron spectroscope (XPS, Thermo ESCALAB 250XI) using an Al Kα X-ray source. The micromorphologies and nanostructures of the ZIF-67@Se@MnO2 composite were analyzed by a scanning electron microscope (SEM, Hitachi S4800) and a transmission electron microscope (TEM, Hitachi H-800). The energy-dispersive X-ray spectroscopy elemental mapping was conducted by X-ray spectroscopy on a Hitachi H-800. The specific surface areas were determined by Brunauer−Emmett− Teller (BET) theory. The thermogravimetric analysis (TGA)

Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs, which combines the benefits of zeolites and MOFs.48 According to previous reports, the solvent molecules could not be directly released in the synthesis process, which would ultimately fill the pore space.49 Herein, we reported an advanced ZIF-67@Se@MnO2 composite cathode for lithium− selenium (Li−Se) batteries, in which the solvent molecules were removed and Co2+ ion exposed to more active sites to tether selenium.50 However, ZIF-67 has too small aperture pores to hold the whole selenium on its surface.48 To resolve this problem, we introduced MnO2 nanoparticles to form a MOF@Se@metal-oxide structure composite. The organic ligand (MeIm) can be postmodified for introducing the desired object to the framework. On the basis of the above points, we loaded selenium on the surface of ZIF-67 (after processing) by vacuum-heating in a glass tube to prepare ZIF67@Se. Then, we improved the cycle performance under high current by modifying ZIF-67 in a convenient method in which MnO2 was introduced on the surface of ZIF-67@Se to reduce the shuttle effect through the oxidation of a methyl group on MeIm by KMnO4.51 The reaction can be described as follows Co(MeIM)2 + KMnO4 → Co(MeIC)2 + MnO2

In comparison with the cathode materials of previous reports shown in Table S1, the ZIF-67@Se@MnO 2 cathode demonstrated a superior reversible capacity retention in cycle performance. Notably, compared with obtaining carbon materials derived from MOFs, our method is energy-efficient and safe. The strategy of preparing a ZIF-67@Se@MnO2 composite provided the new ideas of synthesizing the modified MOF material.

2. EXPERIMENTAL SECTION All chemicals were of analytical grade and were used without purification. 2.1. Synthesis of ZIF-67. According to Yaghi’s report,48 1.45 g (0.005 mol) of Co(NO3)2·6H2O was dissolved in 20 mL of dimethylformamide (DMF) solution. Then, 4.1 g (0.05 mol) of 2-methylimidazole was added into the cobalt nitrate B

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The Journal of Physical Chemistry C curves were recorded in a NETZSCH-STA-409 CD thermal analyzer with a programmed temperature rise from 50 to 700 °C (10 °C min−1) under a flow of air (50 mL min−1). The cathode was prepared according to a conventional slurry coating method: (1) ZIF-67@Se@MnO2, super P, and poly(vinylidene fluoride) in a weight ratio of 70:20:10 were mixed to form a homogeneous slurry in N-methyl-2pyrrolidone; (2) the slurry was spread onto an aluminum foil and dried at 60 °C under vacuum overnight; and (3) the active cathode material mass loading in the electrode was about 1 mg cm−2. Coin cells (CR2032) were assembled in an argon-filled glovebox (H2O, O2 < 0.1 ppm). LiTFSI (1.0 M) in dioxolane/ DME (1:1 by volume) with 1 wt % LiNO3 additive was used as an electrolyte (about 0.16−0.20 mL for each cell), pure Li was used as an anode, and Celgard 2400 was used as a separator. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out by a CHI660D electrochemical work station. Galvanostatic discharge/charge curves were recorded by a NEWARE CT-3008 battery testing instrument in the fixed voltage range of 1.65−2.6 V (vs Li+/ Li) at room temperature.

3. RESULTS AND DISCUSSION Figure 1 shows the synthesis process of ZIF-67@Se@MnO2 composites. To increase the porosity of ZIF-67, the asprepared ZIF-67 sample was dried under vacuum to remove the solvent molecules (DMF), which produced unsaturated Co2+ sites to tether selenium.49,50 ZIF-67@Se@MnO2 was obtained through a redox reaction in which KMnO4 oxidizes a methyl group on MeIm and MnO2 was loaded on the surface of the ZIF-67@Se composite. The XRD patterns of ZIF-67 and ZIF-67@Se@MnO2 composites are shown in Figure 2.

Figure 2. XRD patterns of the ZIF-67 and ZIF-67@Se@MnO2 composites. The yellow line is the PDF card (73-0465) of Se, and the blue line is the PDF card (72-1982) of MnO2.

The sharp characteristic peaks of the as-prepared ZIF-67 reflected the high crystallinity of the matrix. Besides, both composites showed the diffraction peak of ZIF-67 clearly, which confirmed that ZIF-67 crystals still remained in the composites. ZIF-67@Se@MnO2 showed distinct peaks at 23.4 and 29.6°, which were ascribed to Se (PDF #73-0465). It is worth noting that the standard MnO2 (PDF #72-1982) was not apparent in the pattern but was obvious in XPS spectra (Figure 3b). This is mainly due to the fact that MnO2 nanoparticles were synthesized with poor crystallinity.

Figure 3. XPS spectra of (a) survey scan, (b) Co 2p, (c) Mn 2p, and (d) Se 3d in ZIF-67@Se@MnO2 composites.

To acquire the information of chemical composition and electronic structure, the XPS measurements were performed. C

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The Journal of Physical Chemistry C The XPS survey scan (Figure 3a) exhibited that ZIF-67@Se@ MnO2 contained Co, Mn, C, O, N, and Se. The highresolution spectrum of Co 2p (Figure 3b) showed that two main peaks at 780.6 and 796.9 eV were assigned to Co 2p3/2 and Co 2p1/2, respectively. Meanwhile, two distinguishable satellite peaks of the Co 2p3/2 main peaks were situated at about 783.1 eV (Sat.a) and 787.0 eV (Sat.b). It was reported by Qin et al.52 that the energy gap (ca. 6.0 eV) between the Co 2p main peaks and the satellite peaks can serve as an important standard for calculating the oxidation states of Co cations. As a result, Co2+ was the main form existing in the ZIF-67 matrix. As shown in Figure 3c, the Mn 2p spectrum displayed two distinct peaks with a spin-energy separation of 11.7 eV, which were assigned to Mn 2p3/2 (642.7 eV) and Mn 2p1/2 (654.4 eV). According to the fitting curves, the peaks at 653.27 and 641.71 eV were attributed to Mn3+, and another group of peaks was attributed to Mn4+ at 654.65 and 643.03 eV.53 Both XPS and XRD pattern (Figure 2) demonstrated that the MnO2 nanoparticles were successfully synthesized. Figure 3d shows the Se 3d spectrum, in which two major peaks at 55.1 and 55.9 eV were assigned to the Se 3d5/2 and Se 3d3/2 states, respectively. Then, an emission scanning electron microscope (SEM) and a transmission electron microscope (TEM) were used to record the morphologies and microstructures of samples. Figure 4a shows the SEM image of the as-prepared ZIF-67 sample and Figure 4b shows that of ZIF-67@Se, both of which presented the rhombic dodecahedral nanocrystals with a diameter of ca. 800 nm. After loading MnO2, the surface of ZIF-67 became rough and the structure of each crystal was slightly collapsed, whereas the whole frame was still maintained (see Figure 4c). According to Figure 4d, the red lines corresponded to MnO2. As shown in Figure 4e, the highresolution TEM (HRTEM) image of ZIF-67@Se@MnO2 indicated that selenium was uniformly combined with ZIF-67 and the MnO2 nanoparticles with the size of about 5 nm were distributed on these MOFs. The 0.31 nm lattice spacing corresponded to the (310) plane of MnO2 nanoparticles. Besides, the HRTEM image also displayed a lattice spacing of 0.218 nm, which corresponded to the (110) plane of Se. Furthermore, Figure 4f shows the spatial distribution of Se in the ZIF-67@Se@MnO2 composite. According to the TEM image (Figure 4g), Figure 4h−k shows the typical elemental mapping of C, Se, Co, and Mn in the ZIF-67@Se@MnO2 composite. The Se mapping image (Figure 4h) fitted well with the C map (Figure 4g) and Co map (Figure 4i), which also confirmed that selenium was homogeneously and densely dispersed throughout the surface of ZIF-67. Obviously, MnO2 was also dispersed uniformly throughout the composite. Figure 5a shows the TGA curves of ZIF-67@Se@MnO2 and ZIF-67 (Figure 5b). Both the samples were heated at a constant rate of 10 °C min−1 under an air flow. A weight loss of 7 wt % below 300 °C is attributed to the evaporation of moisture in ZIF-67 (black curve). ZIF-67 decomposed rapidly at 410 °C. The major weight losses occurred in the range of 300−375 °C in the ZIF-67@Se@MnO2 composite, which were mainly due to the calcination of ZIF-67. The content of Se was calculated as 38 wt %. The nitrogen sorption isotherm was adopted to obtain the specific surface area and the poresize distribution of the ZIF-67@Se@MnO2 composite. As shown in Figure 5c, the Brunauer−Emmett−Teller (BET) specific surface area of ZIF-67@Se@MnO2 was 33.8 m2 g−1, which was smaller than that of ZIF-67@Se (272.2 m2 g−1).

Figure 4. SEM images of (a) ZIF-67, (b) ZIF-67@Se, and (c) ZIF67@Se@MnO2. (d, e) TEM images of ZIF-67@Se@MnO2 with (f) EDX analysis and elemental mapping based on the area in (g) for (h) carbon, (i) selenium, (j) cobalt, and (k) manganese.

The result indicated that MnO2 was successfully loaded on ZIF-67. Notably, the loaded samples showed a typical IV isotherm with a hysteresis loop due to mesoporous in the composite. Figure 5d presents that the ZIF-67@Se@MnO2 composite had a pore-size distribution centered at ca. 4 nm. The mesoporous structure allowed the diffusion of Li+ and electrolyte during the discharge/charge cycling.54 The oxidation/reduction curves of the cells were recorded by cyclic voltammetry (CV). Figure 6a shows the primary three CV curves of the cells (ZIF-67@Se@MnO2 cathode) measured at a scan rate of 0.1 mV s−1 between 1.65 and 2.6 V (vs Li+/Li). The cathode peaks were ascribed to the oxidation of Se to Li2Se, whereas the anode peak was attributed to the reduction of Li2Se to Se. Three main cathode peaks can be seen in the initial cycle, which appeared at 2.29, 2.17, and 1.94 V during the discharge process, respectively. Two anode peaks appeared at 2.16 and 2.27 V during the charge process, respectively.55 However, no additional redox peaks were found in the CV curves for the composite cathode, indicating that the contribution to the measured capacity of MnO2 nanoparticles (9%, analyzed by inductively coupled plasma atomic emission D

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spectroscopy) can be ignored. The main peaks remained unchanged during the first three cycles, indicating excellent cycling stability. Galvanostatic discharge/charge curves of ZIF-67@Se and ZIF-67@Se@MnO2 composite cathodes were measured at the rate of 1C in the voltage range of 1.65−2.6 V (Figure 6b). Both discharge and charge voltage plateaus were in good agreement with the corresponding CV curves. Capacity values were calculated on the basis of the amount of Se present in the cathode material. The initial capacity of 746 mA h g−1 of ZIF67@Se@MnO2 was greater than the theoretical capacity of Se (675 mA h g−1) because of the solid electrolyte interphase (SEI) formation and decomposition of the electrolyte on the surface of ZIF-67@Se@MnO2. Furthermore, the curve exhibited a capacity of 454 mA h g−1 at the fifth cycle. The separation between the first discharge/charge curve for the ZIF-67@Se@MnO2 composite cathode was much smaller than that for ZIF-67@Se, confirming that the introduced MnO2 could reduce the shuttle effect (see Figure 6c). The results affirmed superior reversibility of the cell with the ZIF-67@Se@ MnO2 cathode. Figure 6d shows that the cathode was cycled at different C rates to assess its rate capability. When the C rate was increased from 1C to 2C, 5C, 10C, 20C, and 50C, the specific capacities reached 397, 347, 305, 256, and 147 mA h g−1, respectively. Notably, when the current density returned to 1C from 50C, the specific capacity still remained as high as 393 mA h g−1. This was due to the synergistic effect of MOFs and MnO2, so that selenium can be effectively protected under the impact of super current density. The cycling performances of the ZIF-67@Se@MnO2-based cell at higher current rates (1C, 2C, and 5C) are shown in Figure 6e. The ZIF-67@Se@ MnO2 composite delivered the initial discharge capacity of 306 mA h g−1 at 1C rate, which then reached 393 mA h g−1 at the third cycle. Finally, the ZIF-67@Se@MnO2-based cell retained a capacity of 329 mA h g−1 after 100 cycles with the high capacity retention of 83.7% (compared with the third cycle). Figure 6e also shows the comparison of the cycling stability of the two cathodes at 1C rate. The discharge capacities of the ZIF-67@Se composite continued to decline from 300 mA h g−1 (the third cycle) to 162 mA h g−1 (the 100th cycle) with the low capacity retention of 54%. Both the initial discharge capacity and durability of ZIF-67@Se@MnO2 were obviously superior to those of ZIF-67@Se, which confirmed that the MnO2 coating on the surface could increase specific capacity and cycle performance by reducing the shuttle effect. In addition, the ZIF-67@Se@MnO2 cathode displayed reversible capacities of 273 and 232 mA h g−1 with capacity retentions of 86.3 and 90.3% at 2C and 5C, respectively. The Coulombic efficiency was evaluated in Figure 6e, which exceeds 110% in first seven cycles because of the slight shuttle effect. Furthermore, the Coulombic efficiency was unstable at around 100% in the subsequent cycles. The composite cathode exhibits superior cycling stability at high current owing to the fact that MOF@Se@metal-oxide structure is conducive to encapsulating selenium. This structure could prevent the diffusion of polyselenides into the electrolyte and reduce the shuttle effect. Electrochemical impedance spectroscopy (EIS) analysis was carried out to obtain further insight into the improved electrochemical performance with the MnO2 loading. As shown in Figure 6f,g, both impedance responses consisted of one semicircle and a pitched line. The inside equivalent circuit model was constructed to fit the data. In addition, a pitched line at low frequency according to the

Figure 5. TG−differential scanning calorimeter curves of (a) ZIF67@Se@MnO2 and (b) ZIF-67. (c) Nitrogen adsorption/desorption isotherms and (d) pore-size distributions of ZIF-67@Se and ZIF-67@ Se@MnO2. E

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Figure 6. (a) CV curves of the ZIF-67@Se@MnO2 cathode at 0.1 mV s−1 in the voltage range of 1.65−2.6 V vs Li+/Li. (b) Galvanostatic discharge/charge profiles of the ZIF-67@Se@MnO2 composite at 1C for the early five cycles. (c) First cycle of discharging/charging profile of the ZIF-67@Se and ZIF-67@Se@MnO2 composite cathodes at 1C. (d) Rate performances of the ZIF-67@Se@MnO2 cathodes. (e) Cycling performances of the ZIF-67@Se@MnO2 cathodes at 1C, 2C, and 5C for 100 cycles. Nyquist plots of the (f) ZIF-67@Se and (g) ZIF-67@Se@ MnO2 composite cathodes. Both the red points in two images are fitted to the inside equivalent circuit. F

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Warburg impedance (W) reflects the Li+ diffusion into the active material. The CPE1 and CPE2 were the constant-phase elements of the charge transfer and SEI resistance, respectively. As Figure 6f,g shows, the Rct values of the ZIF-67@Se and ZIF67@Se@MnO2 cathodes were determined as 470 and 112 Ω, respectively, which decreased about 76% after loading MnO2 on ZIF-67@Se. The result confirmed that a rapid decrease in the cathode resistance was caused by the improvement of the conductivity of the materials from the MnO2 nanoparticles.

ACKNOWLEDGMENTS This project was supported by the Shanghai University of Engineering Science Innovation Fund for Graduate Students (18KY0409).

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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.8b10598. Surface resistance (Rs) pattern of cathode materials; images of the cycled electrode morphology, separator, and lithium foil; and a table of a survey of Li−Se battery cathode materials; “surface resistances (Rs) of four materials” (Figure S1); “SEM images of (a) ZIF-67@Se after 100 cycles at 1C and (b) ZIF-67@Se@MnO2 after 100 cycles at 1C, 2C, and 5C” (Figure S2); “(a−c) images of the cycled separator and lithium foil of ZIF67@Se; (d, e) images of the cycled separator and lithium foil of ZIF-67@Se@MnO2” (Figure S3); “survey of Li− Se battery cathode materials” (Table S1) (PDF)

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4. CONCLUSIONS In this work, a ZIF-67@Se@MnO2 composite was successfully prepared through melt-diffusion and postmodification method. Selenium and MnO2 nanoparticles were uniformly coated on the surface of ZIF-67, which were confirmed by HRTEM examination and elemental mapping. The ZIF-67@Se@MnO2 cathode showed an initial specific discharge capacity of 306 mA h g−1 at 1C, which remained 83.7% (compared with the third cycle: 393 mA h g−1) after 100 cycles at 1C. Moreover, ZIF-67@Se@MnO2 displayed excellent stabilities of 273 mA h g−1 (at 2C) and 232 mA h g−1 (at 5C) at the 100th cycle, and the specific capacities were decreased by 0.139 and 0.097% per cycle, respectively. Notably, the Coulombic efficiency under both conditions showed high stability of nearly 100%. Because the ZIF-67@Se@MnO2 composite provided more active sites for tethering selenium and constructing the MOF@Se@metaloxide structure to encapsulate selenium, we obtained the new MOF-based cathodes with superior electrochemical performance and reduced shuttle effect.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.R.). *E-mail: [email protected] (B.T.). ORCID

Yi Qu: 0000-0003-4243-5785 Bohejin Tang: 0000-0002-1144-3355 Author Contributions †

W.Y. and W.L. contributed equally to this work.

Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.jpcc.8b10598 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b10598 J. Phys. Chem. C XXXX, XXX, XXX−XXX