Controlling the Inner Structure of Carbon Spheres via “Protective

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Controlling the Inner Structure of Carbon Spheres via “ProtectiveDissolution” Strategy for Supercapacitor Yixin Zhang,† Lei Liu,† Yifeng Yu,† Yue Zhang,† Senlin Hou,*,‡ and Aibing Chen*,† †

College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China The Second Hospital of Hebei Medical University, 215 Heping Road, Shijiazhuang 050000, China

‡

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S Supporting Information *

ABSTRACT: Hollow, yolk−shell, and core−shell carbon spheres have attracted substantial attention, because of their unique structure and potential application in energy storage/ conversion. Unfortunately, the structural adjustment of the carbon sphere mostly depends on the template method with a tedious process, and most of the methods are not conductive to large-scale production. Furthermore, one particular structure requires one specific strategy. Here, the carbon spheres with hollow, yolk−shell, and core−shell structure are prepared via a facile and feasible “protective-dissolution” strategy. A suitable amount of acetone is used to dissolve the core of resorcinol-formaldehyde (RF) resin as an effective solvent, and also to regulate the structure as a critical controlling factor. The dissolved resin oligomer assembled outside of the undissolved RF resin sphere under the action of cetyltrimethylammonium (CTAB), forming a strong polymer shell which prevents the RF resin sphere from further dissolution in acetone. The obtained carbon sphere with the especially hollow structure shows a large cavity, high specific surface area, and excellent electrochemical performance with high capacitance of 272 F g−1 at 1 A g−1, and the capacitance retention is 95.2% after 10 000 charge−discharge cycles at 5 A g−1. These outstanding properties and remarkable performance make them potential as supercapacitor electrode materials.

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electrode materials.13 Hence, the preparation of a carbon sphere with a unique internal structure is of great significance to the practical application of carbon spheres. Recently, many researchers have designed and fabricated carbon spheres with different inner structures. The template method, including hard and soft templates, was commonly applied. Hollow carbon spheres were normally prepared by using a SiO2 particle or polystyrene as a hard template after deposition of a suitable carbon precursor on the hard template.14,15 The intermediate core−shell structure was first formed, and then transformed, into a hollow carbon sphere after annealing and etching processes. The soft template method provides another route for the preparation of a hollow carbon sphere through the self-assembly of carbon precursors and soft templates such as ionic liquids, sodium dodecyl sulfate, or cetyltrimethylammonium (CTAB), etc.16,17 Unfortunately, this strategy did not work well for controlling the morphology and uniformity of the prepared hollow carbon spheres, limiting its wide application. The template method has also been used to fabricate yolk−shell and core−shell carbon spheres, combined with a coating technology, but it suffers from a complicated procedure and low yield. Another issue

INTRODUCTION Due to the rapid development of electronic equipment and electric and hybrid electric vehicles, the increasing demand for sustainable energy storage has inspired numerous efforts to explore new high-efficiency technologies.1−3 A supercapacitor, as a promising candidate, has a high energy density, fast charge/discharge rate, and stable cycle life, which plays a critical role in the various application fields, including smart power grids and portable electronic devices.4,5 Afterward, considerable efforts have been focused on the study of electrode materials which have a close relationship to the performance of the supercapacitor. Among various electrode materials, carbon spheres with high specific surface area, good thermal stability, and high porosity have usually been used as supercapacitor electrode materials.6−8 It is generally believed that the inner structure of carbon spheres has great influence on the properties of electrodes. In general, solid carbon spheres mostly have a microporous structure which suppresses fast mass diffusion and transport resulting in low electrochemical performance. Therefore, various carbon spheres with well-designed structure have been developed. For example, carbon spheres with a hollow or yolk−shell structure show improved properties as supercapacitor electrode materials.9−12 This offers a short diffusion distance to facilitate mass transportation, which provides an effective strategy for the development of highly efficient © XXXX American Chemical Society

Received: November 27, 2018 Revised: January 13, 2019 Published: January 15, 2019 A

DOI: 10.1021/acs.jpcc.8b11429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Illustration of the synthesis routes for CSCS, YSCS, and HCS.

H2O, 25 wt %), acetone, and ethanol (EtOH) were purchased from Tianjin Yongda Chemical Corp. Deionized water was used in all experiments. Synthesis of CSCS, YSCS, and HCS. Resorcinol (0.2 g) was first dissolved in a solution containing absolute ethanol and deionized water with a total amount of 28 mL (the volume ratios of ethanol/water were 4/24), and ammonia aqueous solution (0.1 mL, 25 wt %). After the reaction mixture stirred for 0.5 h at 30 °C to form a uniform solution, the formaldehyde solution (0.28 mL) was then added and stirred for 2 h until it changed from a transparent solution to an emulsion. Subsequently, acetone (10, 20, 25 mL, respectively) was added, which selectively removes the interior part of the forming solid inhomogeneous spheres. Then, CTAB (0.38 g) was added and continually stirred for 24 h at 30 °C. The solid product was obtained by centrifugation and air-dried at room temperature. For carbonization, the obtained product was heated at 800 °C for 3 h under N2 atmosphere; then, the CSCS, YSCS, and HCS were obtained, respectively. Synthesis of Carbon Spheres (CS). The CS were derived from RF resin, which was prepared under the same experiment conditions as CSCS, YSCS, and HCS except for the addition of acetone and CTAB. Characterization. Transmission electron microscopy (TEM, JEOL JEM-2100) measurements were carried out to characterize the morphology properties of samples. The specific surface area and pore structure of samples were measured by N2 adsorption−desorption isotherm tests which were measured using a Micromeritics TriStar 3020 volumetric adsorption analyzer. The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) method. In addition, the pore size distribution and the total pore volumes were derived from the adsorption branches of the isotherms on the basis of the Barrett−Joyner−Halenda (BJH) model. Electrochemical Analysis. To prepare the working electrode, samples (80%), carbon black (10%), and polytetrafluoroethylene (10%) were mixed in ethanol, then were coated onto the Ni foam current collector, and were dried at 100 °C for 24 h. The mass of active material loaded on each working electrode was 4−5 mg cm−2. Cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrical impedance spectroscopy (EIS) were used to calculate the electrochemical performance of the samples via an electrochemical workstation (CHI 760E, Chenhua Instruments, China) with 6 M KOH electrolyte. A Pt wire and Hg/HgO electrode were used as the counter and reference electrodes,

preventing the in-depth investigation of the preparation methods is that the preparation strategies of carbon spheres with different structures are different. The carbon precursor is an important factor for the preparation, as this is a large-scale application of the physical and chemical properties of carbon materials. Resorcinolformaldehyde (RF) resin, because of its low cost, ease of preparation, high porosities, and thermal stability, has often been used as a carbon source to prepare carbon spheres.18−20 By using the extended Stöber method, monodisperse RF resinbased carbon spheres are easily obtained, and the particle size can be adjusted by varying the concentration of ammonia or alcohol/water ratio.21 Through nanoassembly engineering, RF resin and tetraethoxysilane (TEOS) are assembled to prepare hollow and yolk−shell carbon spheres under the action of CTAB. In this preparation, a silica additive is necessary which is actually a hard method with a complicated process.22 Recently, by engineering the compositional inhomogeneity inside the RF resin to allow selective dissolution by acetone, the hollow carbon sphere was prepared via a simple templatefree strategy.23 However, the applicability of this method to core−shell or yolk−shell structured carbon spheres was not investigated. Inspired by this work, herein, we use an easily prepared RF resin sphere as a carbon precursor to prepare a carbon sphere with adjustable internal structure by a “protective-dissolution” strategy. First, the RF resin sphere with gradient inner composition is dissolved by acetone to create a cavity and obtain the dissolved resin oligomer. Then, the dissolved RF oligomer assembles with CTAB to form a high-molecularweight polymer shell, which prevents the internal resin from being further dissolved. Notably, a suitable amount of acetone and CTAB play a critical role in the controllable synthesis of carbon spheres. By adjustment of the amount of acetone to controllably dissolve the resin oligomer, the internal structure of the carbon sphere from hollow to yolk−shell to core−shell can be achieved easily. In comparison with core−shell carbon spheres (CSCS) and yolk−shell carbon spheres (YSCS), hollow carbon spheres (HCS) as a supercapacitor electrode exhibit the highest electrochemical performance, ascribed to the high surface area and unique larger hollow structure.

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EXPERIMENTAL SECTION Chemicals and Materials. Resorcinol was purchased from Macklin Corp. Cetyltrimethylammonium bromide (CTAB), formaldehyde (HCHO, 37 wt % solution), ammonia (NH3· B

DOI: 10.1021/acs.jpcc.8b11429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C respectively. In the two-electrode system, the specific capacitance (C, F g−1), energy density (E, Wh kg−1), and power density (P, W kg−1) were calculated by the following equations: C = 4IΔt/ΔVm, E = C(ΔV)2/(2 × 3.6), and P = 3600E/Δt. In the three-electrode system, the specific gravimetric capacitance is assessed according to the GCD measurements: C = IΔt/ΔVm, where I (A) is GCD current, Δt (s) is discharge time, ΔV (V) is voltage window, and m (g) is the mass of active material.24

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RESULTS AND DISCUSSION The fabrication procedure of the carbon sphere with different internal structure was depicted in Figure 1. The solid RF resin sphere was prepared by the extended Stöber method, and fast polymerization of the resin precursor in a short time resulted in compositional inhomogeneity inside the RF resin. The outside RF resin shell has a higher polymerization degree than that of the inside on the basis of its step-growth nature.23 From addition of the acetone, the inside low-molecular-weight resin oligomer would be dissolved, forming an interspace between the outside shell and core. It was worth noting that the innermost species of the RF resin sphere remained the radial type and results in a core with higher polymerization degree relative to interlayer species between the shell and core, so dissolution started from the interlayer. Then, in the presence of cationic surfactant CTAB, the dispersed RF oligomer would reassemble and coat on the outside of the RF resin sphere by electrostatic forces and hydrophilic groups which prevent the further dissolution by acetone. The different internal structure of carbon spheres was controlled by adjusting the acetone amount. As shown in route A, after the addition of 10 mL of acetone, a small amount of RF resin oligomer was dissolved and dispersed again into the reaction solution. Then, the dispersed RF oligomer reassembled with CTAB to form a thin shell outside the remaining resin sphere, and the RF resin sphere with core−shell structure will be obtained (CSCS). When the amount of acetone increased to 20 mL (route B), more inner RF oligomer was dissolved and a smaller core was left in the center of RF resin sphere. Subsequently, after the CTAB-assisted reassembly process, the yolk−shell structured carbon sphere was obtained, and its outer shell was thicker than the CSCS shell. With 25 mL of acetone added (route C), the core completely disappeared, because much more resin oligomer inside was dissolved, forming the hollow structure. Then, the dissolved RF reacted with CTAB forming a much thicker polymer layer. On the basis of the gradient inner composition of RF resin and controllable solution by acetone, RF resin spheres with core− shell, yolk−shell, and hollow structure were prepared via a reassembly process with the assistance of CTAB, and the diameter of the corresponding carbon sphere was also increased. The morphology and internal structure of these samples were revealed by TEM images. Upon addition of 10 mL of acetone, the CSCS was obtained after reassembly with assistance of CTAB and the annealing process. Figure 2a showed that the CSCS had retained the regular spherical morphology well without obvious collapse or deformation. From a higher-magnification TEM image of CSCS in Figure 2b, the CSCS had a distinct core−shell structure, of which the outer diameter of the carbon sphere was ca. 580 nm and the inner core diameter was ca. 320 nm. In addition, the unclear

Figure 2. TEM images of CSCS (a, b), YSCS (c, d), and HCS (e, f).

edges of the core indicated that the interlayer RF resin oligomer was preferentially dissolved. When 20 mL of acetone was added, through the same process, the YSCS was obtained. As shown in Figure 2c, the YSCS showed uniform yolk−shell architecture with an obvious undissolved core in the center. Figure 2d shows a highermagnification TEM image of YSCS. The diameter of the core was ca. 270 nm which was smaller than that of CSCS, demonstrating that more low-molecular-weight RF oligomer was dissolved by acetone. Moreover, the diameter of YSCS was ca. 600 nm which was larger than that of CSCS, indicating that the dissolved RF resin oligomer successfully reassembled with CTAB on the outside of the RF resin sphere. More acetone was applied, and more RF oligomer was dissolved. When the amount of acetone increased to 25 mL, the core was dissolved completely and the hollow structure was obtained. As displayed in Figure 2e, the hollow sphere was observed. Moreover, the high-magnification TEM images of HCS (Figure 2f) showed that the sphere size was about 700 nm and the cavity size was about 560 nm, which were larger than the diameter and cavity size of YSCS. It demonstrated that more dissolved RF resin was reassembled with CTAB, forming a thicker shell than that of YSCS. To achieve adjustment of the internal structure of the carbon sphere, the amount of CTAB and acetone is very critical. We conducted a series of comparative tests to investigate their effect on the final structure (Figure 3). First, the resorcinol and formaldehyde solution rapidly polymerized to form an RF sphere with a different internal polymerization degree using ammonia catalyst. According to the previous report, if CTAB was not applied in the reaction system, the dissolved RF resin oligomer will not be reassembled after adding more or less acetone, and a hollow carbon sphere will be always generated (route 1).23 Furthermore, as the amount of acetone increased, the thickness of the shell gradually C

DOI: 10.1021/acs.jpcc.8b11429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Nitrogen adsorption−desorption isotherms of CSCS, YSCS, and HCS.

relative pressure (P/P0 < 0.1) indicates the presence of a large number of micropores in all samples. The hysteresis loop of the hybrid H3−H4 type with a P/P0 range of 0.4−0.9 is the most unique in the CSCS sample and implies the presence of slit-like pores in its structure, which is consistent with the TEM image, as shown in Figure 2a,b. Moreover, the hysteresis loops in adsorption−desorption isotherms from CSCS to YSCS to HCS gradually disappeared at the relative pressure range of 0.9−1, demonstrating an enlarged cavity as seen from TEM images. The corresponding pore size distribution curves of CSCS, YSCS, and HCS derived from the adsorption branch were shown in Figure S2. The detailed textural parameters of the CSCS, YSCS, and HCS were tabulated in Table 1. It was

Figure 3. Schematic illustration of comparative experiments.

decreased. However, when CTAB was added to the reaction solution, carbon spheres with different internal structure can be prepared with a suitable amount of acetone. The internal structure could be tuned by the amount of acetone (route 2). As mentioned above, the appropriate amount of acetone is another important factor which can greatly affect the internal structure of the carbon sphere. We used 40 mL of acetone in three comparative experiments. For sample a, 40 mL acetone was added in the reaction solution totally at one time. A clear solution was obtained, indicating that RF resin was dissolved completely. The reaction continued for 24 h, and the solution was still clear demonstrating that the dissolved RF was not reassembled (Figure S1a). In sample b, 5 mL of acetone was first added to the reaction system. In this case, a very small amount of dissolved oligomer dispersed into solution, which was not enough to form a dense shell with CTAB to prevent the resin sphere from being dissolved. After 3 h, an additional 35 mL of acetone was added. Then, the resin was completely dissolved in total of 40 mL of acetone, forming a slightly cloudy solution (Figure S1b). In sample c, as shown in Figure S1c, when 20 mL of acetone was first added, the turbidity of the solution decreased, indicating a certain amount of RF oligomer was dissolved. However, when CTAB was added, white precipitate formed, indicating that the dissolved oligomers assembled on the RF sphere. Subsequently, another 20 mL of acetone was added and stirred for 24 h, and then a solution with pink precipitate was observed, different from that shown in Figure S1a,b under the same conditions, demonstrating that the newly formed polymer shell prevented further dissolution of RF resin. On the basis of these comparative experiments, we can conclude that the amount of acetone not only determines formation of the carbon sphere but also regulates the inner structure of the carbon sphere. Carbon spheres with adjustable inner structure can be controllably prepared by using a proper amount of acetone. Nitrogen isothermal adsorption−desorption measurements were performed to analyze the textural properties of the assynthesized CSCS, YSCS, and HCS. As depicted in Figure 4, all the samples exhibited similar type IV adsorption− desorption isotherms. In addition, a sharp increase in low

Table 1. Textural Properties of CSCS, YSCS, and HCS sample

SBET (m2 g−1)

Smicroa (m2 g−1)

Vtb (cm3 g−1)

Vmicroc (cm3 g−1)

Vn (%)

CSCS YSCS HCS

820 1034 1399

583 600 649

0.50 0.61 0.87

0.29 0.30 0.33

58.0 49.1 37.9

a Micropore surface area determined by the t-plot. bTotal pore volume at P/P0 ∼ 0.99. cMicropore volume calculated by the t-plot method. d Vn= Vmicro/Vt.

clear that the microporous surface area increases and the fraction of microporous volumes reduced with the increase in acetone amount. Therefore, as the amount of acetone increases, the contribution rate of micropores to the sample pore volume decreases gradually; that is, the corresponding mesoporous volume increases gradually. The electrochemical performance of carbon materials strongly depends on their surface area and internal structure. Moreover, the adjustable internal structure, large cavity, and high surface area make the samples promising candidates for supercapacitor electrode materials. The large surface area provides abundant sites for ion accumulation, and the large cavity has more advantages for mass diffusion of electrolyte and transmission of ions and electrons, thus promoting the electrochemical properties.24 In order to evaluate the electrochemical behaviors of CS, CSCS, YSCS, and HCS samples, the CV and GCD curves were evaluated in a three-electrode system. As illustrated in Figure 5a, the CV curves at the 5 mV s−1 scan rate over the range −1 to 0 V showed a nearly rectangular shape, indicating a favorable EDLC behavior with a good reversibility.25 Obviously, the CV curve area of HCS was larger than those of CSCS, YSCS, and CS, indicating much higher specific capacitance of HCS than CSCS, YSCS, and CS. Moreover, as shown in the GCD curves of Figure 5b, the discharging time of the HCS was significantly longer compared D

DOI: 10.1021/acs.jpcc.8b11429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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calculated at the current density of 1 A g−1, which was much higher than that of many other spherical carbon materials and other carbon materials.26−33 Discharge capacitances at various current densities were calculated from the GCD curves from 1 to 20 A g−1 in Figure 5e. The capacitor retention of HCS is 80.9%, which was obviously higher than those of YSCS (70.7%) and CSCS (76.1%), indicating better rate performances, rapid ion diffusion, and practical electrochemical application. As shown in Figure 5f, the EIS test of HCS was carried out in the frequency range from 10−2 to 105 Hz. The Nyquist plots show an approximately vertical line in the low-frequency region because of Warburg impedance, which is the result of the frequency dependence of ions diffusing to the electrode interface in the electrolyte.34 In addition, the equivalent series resistance (ESR) was 0.48 Ω estimated from the intersection of the plot at the x-axis in the high-frequency range. The low resistance of HCS revealed a good ion response in the highfrequency ranges, which was crucial for the performance of electrode materials. In Figure 5g, the electrode material of HCS exhibits a very stable capacitance (95.2% of original capacitance) after 10 000 cycles at 5 A g−1. The GCD curves of the 10 000th cycles are almost similar to those of the first cycles (Figure 5g, inset), which are linear and symmetrical. Furthermore, as shown in Table S1, compared to other carbonbased electrode materials, this shows long-term electrochemical stability.35−39 To measure the practical applications of the prepared HCS, the electrochemical properties in a two-electrode system were also performed in a potential range 0−0.8 V. All CV curves exhibited a rectangular shape and unapparent current peak at high potential that are probably caused by active oxygencontaining groups, indicating a good capacitive behavior (Figure 6a). From Figure 6b, the GCD curves of HCS from 1 to 20 A g−1 were quasilinear with a small IR drop, reflecting a good EDLC performance and a relatively low internal resistance. The specific capacitance of the HCS was measured to be 224 F g−1 at 1 A g−1. In Figure 6c, the energy density and power density of CSCS, YSCS, and HCS were calculated from the GCD discharging curves based on a two-electrode cell.

Figure 5. Electrochemical performances of the samples were measured in a three-electrode system with 6 M KOH electrolyte: (a) CV curves at 5 mV s−1 and (b) GCD curves at 1 A g−1 of CSCS, YSCS, HCS and CS. (c) CV curves of HCS at different scan rates of 5−200 mV s−1. (d) GCD curves of HCS at 1−20 A g−1. (e) The specific capacitance of CSCS, YSCS, and HCS electrodes with different current and the specific capacitor of other carbon materials. (f) Nyquist plot of HCS. (g) Cycle life of HCS at a current density 5 A g−1.

with those of CSCS, YSCS, and CS at a low current density of 1 A g−1, which was in good agreement with the CV results. In addition, the specific capacitance of CS, CSCS, YSCS and HCS samples are 137, 174, 213, and 272 F g−1, respectively. Due to the higher capacitance value, the electrochemical performance of HCS was further investigated. CV experiments were performed in a scan rate ranging from 5 to 200 mV s−1 (Figure 5c). When the scan rate gradually increased from 5 to 100 mV s−1, a regular rectangular shape was retained, indicating a good capacitance performance at a high scan rate and an ideal EDLC. However, the CV curve showed a little tilt in the rectangular-like shape when the current density increased to 200 mV s−1; this implied a fast charge/discharge process with high power capability and low equivalent series resistance. The GCD curves at varied current densities were shown in Figure 5d. All the curves at a wide range of current densities from 1 to 20 A g−1 were closely linear and showed isosceles triangle shapes, indicating typical EDLC behavior and superior charge−discharge reversibility. In addition, the HCS exhibited excellent electrochemical capacitance of 272 F g−1

Figure 6. Electrochemical performances of the samples were measured in a two-electrode system with 6 M KOH electrolyte: (a) CV curves at 5−200 mV s−1, (b) GCD curves at 1−20 A g−1 of HCS, and (c) Ragone plots of CSCS, YSCS, and HCS at different GCD. E

DOI: 10.1021/acs.jpcc.8b11429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Because HCS has the highest ratio of volume of meso-/ micropores which facilitates the ion transport and accumulation, the energy density and power density of HCS are higher than those of CSCS and YSCS. Moreover, the energy densities of CSCS, YSCS, and HCS are 13.3, 16.8, and 25.2 Wh kg−1 at a specific power density of 1.596, 1.608, and 2.000 kW kg−1, respectively. Meanwhile, the energy density of HCS is higher than those of other reported carbon materials, as shown in detail in Table S2.40−44 The results demonstrated that HCS had a remarkable energy-power characteristic and is promising as a supercapacitor electrode material.

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CONCLUSIONS

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ASSOCIATED CONTENT

REFERENCES

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In conclusion, a facile and effective “protective-dissolution” strategy is developed to prepare uniform carbon spheres with an adjustable internal structure by using RF resin as the carbon precursor. Acetone can selectively dissolve the low-molecularweight RF resin to form the cavity. The dissolved resin oligomer then reassembled with CTAB on the outside of the resin forming a dense shell which can protect the RF resin sphere from being dissolved again. The core−shell, yolk−shell, and hollow structured carbon sphere can be controllably prepared by changing the amount of acetone. Moreover, with the increase of acetone amount, the volume ratio of meso-/ micropores and the cavity increases gradually, which endows the hollow carbon sphere with outstanding performance, including a high capacitance value and excellent energy density. This strategy provides a new idea for realizing controllable synthesis of resin-based carbon spheres with subtle structures and improves its practical application performance.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b11429. Optical photographs of comparative tests and comparison of cycle stability (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone/fax: +86 0311 8863 2183. E-mail: [email protected]. (A.C.) *Phone/fax: +86 0311 8704 6901. E-mail: housenlin2006@ 126.com. (S.H.) ORCID

Aibing Chen: 0000-0002-2764-5234 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21676070), Hebei Natural Science Foundation (B2015208109), Hebei Training Program for Talent Project (A201500117), Hebei One Hundred-Excellent Innovative Talent Program (III) (SLRC2017034), Hebei Science and Technology Project (17214304D, 16214510D), and Beijing National Laboratory for Molecular Sciences. F

DOI: 10.1021/acs.jpcc.8b11429 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.8b11429 J. Phys. Chem. C XXXX, XXX, XXX−XXX