Natural Chrysotile-Based Nanowires Decorated with Monodispersed

Aug 25, 2015 - In particular, the as-prepared Ag/SiO2NWs nanocomposites with 5 mL of AgNPs exhibited excellent catalytic activitiy. These AgNPs/SiO2NW...
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Natural Chrysotile-Based Nanowires Decorated with Monodispersed Ag Nanoparticles as a Highly Active and Reusable Hydrogenation Catalyst Hong Zhang, Tao Duan,* Wenkun Zhu, and Wei-Tang Yao* Laboratory of Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang 621010, China S Supporting Information *

ABSTRACT: Silver nanoparticles/silica nanowires (AgNPs/SiO2NWs) with the nearly uniform diameters of 35 ± 5 nm were successfully fabricated by two steps consisting of the preparation of the silica nanowires (SNWs) by chemical dispersing and acid leaching from the natural mineral chrysotile and an in situ reduction approach (for AgNPs). The highly dispersed AgNPs assembled on the surface of SiO2NWs through the in situ reduction of Ag+ by NaBH4 were confirmed by transmission electron microscopy (TEM) and UV−vis absorption spectra. The catalytic activities of the as-prepared Ag/SiO2NWs (silver nanoparticles/silica nanowires) nanocomposites with different concentrations were assessed via using a classical reaction based on the reduction process of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of NaBH4 as the reductant. The results demonstrated that all the nanocomposite catalysts exhibited high catalytic activities because the nearly monodispersed AgNPs were embedded on the surface of SiO2NWs, allowing effective active contact with the reactants and catalysis of the reaction. In particular, the asprepared Ag/SiO2NWs nanocomposites with 5 mL of AgNPs exhibited excellent catalytic activitiy. These AgNPs/SiO2NWs nanocomposites could be easily reused without a decline of the catalytic activities due to chrysotile natural mineral frameworks with large amounts of active sites.

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INTRODUCTION As one of the most promising functional materials, silver nanoparticles (Ag NPs) have attracted extensive attention for a long time due to their outstanding properties in the field of electronic, chemical, biological, and catalysis.1−3 With the development of nanoscience and nanotechnology, Ag NPs have made wonderful progress in their catalytic performance for various chemical reactions, such as oxidative conversion of methanol into formaldehyde, partial oxidation of benzyl alcohols, and oxidation of carbon monoxide.4−6 Thus, in the field of catalysis, there are two fundamental problems which have an important relationship with the application of Ag NPs. First, Ag NPs normally incline to grow and form big agglomerates owing to their excellent surface energy, thus resulting in reduction of their surface area and obtainable active sites efficient for catalytic reactions.7,8 Second, it is another major problem for small sized Ag NPs to be regenerated from the reaction system, so their reuse is not appropriate for large scale potential applications.9 To solve these problems, traditional strategies are to disperse Ag NPs on suitable holders (such as silica nanotubes, polymers, metal oxides, carbon nanofibers, etc.)10−15 to form hybrid catalysts, which should be the unique choice to prevent the accumulation of Ag NPs. In recent years, many endeavors have been dedicated to assemble AgNPs on the spherical silicon oxide structures; the methods include pretreatment-chemical plating,16 self-assembly chemical plating,17 and ultrasonication.18 Although various architectures with Ag/silica nanocomposites have been © 2015 American Chemical Society

observed in solution-based processes, synthesis of one-dimensional Ag/silica nanocomposites, especially Ag/natural silica nanostructures, is rarely reported.19−21 In particular, as an excellent natural catalyst support candidate, chrysotile (Mg6[Si4O10](OH)6) has attracted much attention because of its outstanding physicochemical properties, such as fantastic specific surface area, tunable pore structures, thermal stability, and special surface network topological structure.22−24 Moreover, there is a great amount of hydroxyl on the exterior surface of chrysotile, which causes easy grafting and decoration on the chrysotile surface by functional organic molecules.25 Herein, we report a novel preparation process of synthesizing uniform silver nanoparticles/silica nanowires (AgNPs/ SiO2NWs) nanocomposites with well-defined nanostructures in an aqueous solution. The silica nanowires were functionalized by electron-rich 3-aminopropyltriethoxysilane, and AgNPs were restored by sodium borohydride in the presence of electron-deficient trisodium citrate. Electrostatic adsorption between the two materials made it easy for AgNPs to adsorb on the SiO2NWs. The motivations of this work are to develop a facile solution strategy to prepare one-dimensional AgSiO2NWs nanocomposites, as well as to investigate physical and chemical properties of low dimensional nanostructures. Received: June 8, 2015 Revised: August 24, 2015 Published: August 25, 2015 21465

DOI: 10.1021/acs.jpcc.5b05450 J. Phys. Chem. C 2015, 119, 21465−21472

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EXPERIMENTAL SECTION I. Materials and Methods. Silicon dioxide nanowires (SiO2NWs),26 3-aminopropyltriethoxysilane (γ-APS, 98%), silver nitrate (AgNO3, 99.8%), sodium citrate (C6H5Na3O7· 2H2O, 99%), and ethanol (EtOH, 99.7%) were supplied by Kelong Chemical Factory (Chengdu, China), sodium borohydride (NaBH4, 97%), 4-nitrophenol (C6H5NO3, 99.7%) were supplied by Aladin Ltd. (Shanghai, China). All chemicals were used without further purification. The water used was purified through a Youpu system. Procedure for amination of SiO2NWs follows: 1 mL of γ-APS was dissolved into 60 mL of water with magnetic stirring, and pH value was adjusted to 3.5 by adding acetic acid dripped into the solution. The solution was stirred at normal atmospheric temperature for 30 min to ensure the hydrolysis of γ-APS. After that, 1 g of SiO2NWs was mixed and stirred under the reflux of solvent at 353 K for 12 h. The SiO2NWs were filtered off after cooling the dispersion, washed with water for 5 times, and dried at 353 K for 12 h to produce modified SiO2NWs. The preparation scheme of the Ag-SiO2NWs nanocomposite was shown in Chart 1.

200FE, Zeiss Libra, Germany). Catalytic activity of Ag-SiO2 for 4-nitrophenol reduction was examined by using NaBH4. A 1.5 mL portion of 4-nitrophenol solution (4-NP, 0.25 mmol L−1) and 1.5 mL of sodium borohydride (NaBH4, 15 mmol L−1) were dropped into quartz cells. Consequently, a 100 μL portion of of Ag-SiO2NWs nanocomposite solution was dropped into the mixture solution, and reaction was maintained at an appropriate time. The reaction was measured by using an UV−vis spectrophotometer (UV-3150 UV−vis spectrophotometer).

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RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the products obtained with different volumes of Ag NPs. The broader diffraction peak

Chart 1. Scheme To Illustrate Preparation of Ag-SiO2NWs Nanocomposite

Figure 1. XRD spectrum of Ag-SiO2NWs nanocomposite with different volumes of AgNPs: (a) 0, (b) 1, (c) 2, (d) 5, (e) 10 mL. Marked cyan peaks are attributed to standard card (JCPDF 04-0783).

located at 2θ value of 15°−30° was attributed to silicon oxide which indicated that the SiO2NWs that were derived from chrysotile were low ordered (Figure 1). It could be concluded that the peak of the SiO2NWs displayed almost no intensity change before and after chemical treatment, which suggested the silica nanowires were well-preserved. With the increase of AgNP content, the intensities of the diffraction peaks with 2θ value of 38.7°, 65.1°, and 77.9° were enhanced; those diffraction peaks corresponded to the (111), (220), and (311) crystal faces of the face-centered cubic (fcc) crystalline silver, which were in accord with the values in standard card (JCPDF 04-0783) (Figure 1e). No diffraction peaks from any other impurities were detected. Figure 2 shows the SEM images of the as-synthesized SiO2NWs and Ag-SiO2NWs nanocomposite. It was clearly revealed that the SiO2 nanowires are relatively uniform in diameter and very flexible. As prepared SiO2 nanowires have a diameter of 35 ± 5 nm, and a length of 1−4 μm (Figure 2a). Compared with the SiO2NWs, the nanostructure of AgNPs decorated SiO2NWs was not collapsed (Figure 2b). Interestingly, Ag-SiO2NWs nanocomposites show that the AgNPs are well-distributed on the surface of SiO2NWs without aggregation (Figure 2b). The results indicated that the functionalized silica with amino groups was helpful for unanimous growth and good distribution of silver nanoparticles on the surfaces of SiO2NWs, which due to the SiO2NWs with large functional groups (−OH) and 3-aminopropyltriethoxysilane interact with each other lead to the amino groups modification on the surface of SiO2NWs. Electrostatic adsorption between the

In a conventional process, 25 mL of AgNO3 (2 mmol L−1) and 25 mL of sodium citrate (4 mmol L−1) solution were mixed and stirred at 60 °C for about 20 min in 150 mL conical flasks. After adding 0.6 mL of NaBH4 (10 mmol L−1), the mixed solution changed from colorless to yellow. Then, 0.01 g of modified SiO2NWs was placed in a conical flask containing 50 mL of water. After ultrasonication for more than 1 h, the suspensions were mixed with different volumes of AgNPs. Then, it was stirred for more than 12 h. Subsequently, the mixture was centrifuged at 1000 r/min for 1 min, and the precipitates were redispersed in 1 mL of water. For characterization, all the centrifuged and washing water was collected carefully and set to the marked volume of 100 mL, which was used to confirm the residual Ag nanoparticle concentration in solution by AAS. Actually, the mole ratio of Ag and SiO2 is 0.300. II. Characterization. X-ray diffraction measurements of products were detected using an X-ray diffractometer (X’Pert PRO, The Netherlands) with Cu Kα radiation (λ = 0.033 43). The product morphologies were tested by scanning electron microscope (SEM, ULTRA55 Carl Zeiss, Germany), and AgNP size was tested by transmission electron microscope (TEM: 21466

DOI: 10.1021/acs.jpcc.5b05450 J. Phys. Chem. C 2015, 119, 21465−21472

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The Journal of Physical Chemistry C

Figure 2. SEM images of samples of (a) SiO2NWs and (b) Ag-SiO2NWs nanocomposite.

Figure 3. TEM images of samples: (a) SiO2 NWs, (b and c) Ag-SiO2NWs nanocomposite, and (d) HRTEM images of Ag nanoparticles, and insets showing the growth orientations, ring axis of ⟨111⟩.

electron-rich amino groups functionalized SiO2NWs and the electron-deficient silver nanoparticles (AgNPs) were restored by sodium citrate. In order to further explore the microstructure of AgSiO2NWs nanocomposite, transmission electron microscopy (TEM) and high transmission electron microscopy (HRTEM)

observations were carried out. It could be easily seen that the SiO2NWs silica nanowires inherited the structure of the natural mineral chrysotile. The average diameter of silica nanowires was 35 ± 5 nm (Figure 3a). The TEM image of as-prepared AgSiO2NWs nanocomposite showed that the nanostructure of the SiO2NWs was not collapsed after a series of chemical 21467

DOI: 10.1021/acs.jpcc.5b05450 J. Phys. Chem. C 2015, 119, 21465−21472

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However, after decoration with AgNPs, each spectrum represented a localized surface plasmon resonance (LSPR) band with a maximum centered at ca. 400 nm. The presence of a minimum at ca. 320 nm can be also observed, characteristic of the interband transition in the metal that damps the plasmon oscillation in this spectral region. With the decreased silver size, the location of the absorption peak is tiny shifted to lower wavelengths. This behavior is due to two different factors: first, the lower excitation cross-section of the metal nanoparticle in each growth step, and second, the decrease of the particle volume. The 5 mL Ag-SiO2NWs exhibited the maximum blueshift. It is well-known that catalytic reduction of 4-nitrophenol (4NP) has become one of the model reactions for appraising the catalytic activity of noble nanoparticles.28,29 To demonstrate the potential applicability of as-prepared Ag-SiO2NWs nanocomposite in this application, we investigated their photocatalytic activity by choosing photocatalytic reduction of a nitroaromatic as a test reaction. As a well-known fact, the absorption peak of 4-NP solutions was at 317 nm under nonalkaline conditions (Figure 5b). The peak was red-shifted to 400 nm after being treated by NaBH4, which was due to the formation of 4-nitrophenolate ion (Figure 4b), and the color also changed from light-yellow to yellow-green. Figure 6 displayed the UV−vis spectra of reduction of 4-NP catalyzed by different concentrations of Ag-SiO2NWs nanocomposites with 1, 2, 5, and 10 mL, respectively. With addition of the Ag-SiO2NWs nanocomposite catalyst, a new peak at 295 nm appeared, and the BH4− adhered to the surface of the AgSiO2NWs nanocomposites, accompanied by the electrons transferring from the BH4− donor to the AgNPs acceptor. The hydrogen atom formed from the hydride that attacked the 4-NP to form 4-aminophenol (4-AP).16,17 It can be concluded that the absorption peak at 400 nm decreased as the reaction continued; however, the peak located at 295 nm increased, until the peak of 400 nm disappeared which indicated the nitration had been successful for degradation (Figure 6a−d). Among the different concentrations of AgNPs, the catalytic efficiency of 5 mL AgNPs/SiO2NWs was the best, with the absorption peaks corresponding to the 4-aminophenol, such as 400 nm, almost diminishing smoothly as the exposure time increases, and completely disappearing after about 25 min. No new absorption bands appear in either the visible or ultraviolet regions, suggesting the complete photodegradation of 4-NP. When the

treatments, such as functionalization with 3-aminopropyltriethoxysilane, or silver nanoparticle growth on the surface of nanowires: it appears that the microstructure of silica nanowires is very steady. Also, the AgNPs uniformly distribute on the surface of SiO2NWs (Figure 3b,c). The average diameter of silver nanoparticles on the silica was 20 nm (inset in Figure 3c). Figure 3d shows a high-resolution TEM (HRTEM) image of Ag nanoparticles. The lattice fringe spacing is calculated to 2.35 Å, corresponding to the (111) plane of Ag (d = 0.235 nm).27 In order to further demonstrate whether silver nanoparticles were modified to SiO2NWs, electrochemical impedance spectroscopy was recorded in a mixture of the same volume solution with two different concentrations of 1 mol L−1 K3Fe(CN)6 and 0.1 mol L−1 KCl (Figure 4). As shown in

Figure 4. Nyquist plot of AgNPs and Ag-SiO2NWs nanocomposites with different volumes of AgNPs.

Figure 4, the Nyquist plots contain a semicircle in the high frequency region and a straight sloping line in the low frequency range. Evidently, the semicircle diameter of the high frequency region became big with Ag-SiO2 NWs concentrations increasing, while in the low frequency range, these nanocomposites exhibit straight and nearly vertical lines, which indicate that, with Ag-SiO2NWs increasing concentration, electrochemical impedance of Ag-SiO 2NWs composites enhanced greatly. It also indicates that silver nanoparticles grafted on SiO2NWs tightly and homogeneously. Figure 5a shows the UV−vis spectra of the four different AgSiO2NWs composites in solution, and also the same as the pristine SiO2 nanowires at a concentration of 2 mg mL−1.

Figure 5. UV−vis absorption spectra of (a) Ag-SiO2NWs nanocomposite with different volumes of AgNPs and (b) 4-nitrophenol and 4-nitrophenol with NaBH4 formation of 4-nitrophenol ion. 21468

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Figure 6. UV−vis absorption spectra during the catalytic reduction of 4-NP of Ag-SiO2NWs nanocomposites: (a) 1, (b) 2, (c) 5, (d) 10 mL.

AgNP concentration addition was less than 5 mL, due to insufficient Ag reacting with SiO2NWs and the rare dosage of the cross-section AgNPs decorated on SiO2NWs, the ion absorbability with BH4− was reduced, leading to the decrease of the catalytic activity (Figure 6a,b). The final results demonstrated that the photocatalytic activity of 5 mL AgNPs/SiO2NWs is outstanding among all kinds of AgNPs/ SiO2NWs nanocomposites prepared in the current reaction system. It can also correspond to the rapid color change (see the Supporting Information, Figure S1). Furthermore, the nitrogen adsorption−desorption isotherm of SiO2NWs was also explored. The BET surface area is 210 m2/g, confirming that AgNPs/SiO2NWs nanocomposites have excellent catalytic activities (see the Supporting Information, Figure S2). Figure 7 showed the photographs of catalysts after catalytic reaction which formed film via filtration process and could be easily recycled, and the results indicated that AgNPs/SiO2NWs nanocomposites would greatly promote their industrial potential application. Especially, gold nanoparticles (Au NPs) have also applied this kind of catalytic reaction (see the Supporting Information, Figure S3).The results indicated that the as-prepared Au/SiO2NWs nanocomposites with 10 mL AgNPs exhibited excellent catalytic activity on the reduction of 4-nitrophenol (4−NP) into 4-aminophenol (4−AP) via using NaBH4 as the reductant (see the Supporting Information, Figures S4−S6). To understand the potential advantage of the 5 mL AgSiO2NWs catalytic effect, pseudo-first-order kinetics can be applied to evaluate the rate constants for 4-NP reduction. The decomposition kinetics was understood according to physical chemistry principles: the results displayed that the previous catalytic reduction reactions followed the Langmuir−Hinshelwood apparent first order kinetics model because of the superfluous NaBH4 used to protect the 4-AP from aerial oxidation compared with 4-NP and catalyst. The concentration of 4-NP at time t is denoted as C, and the initial concentration at t = 0 is denoted as C0. Ct/C0 is measured from the relative intensity of the absorbance (A/A0). The linear relationship of

Figure 7. Photographs of Ag/SiO2NWs nanocomposite films after catalytic reaction with different concentrations.

ln(A/A0) versus t indicates that the reaction of 4-NP in the presence of silver nanoparticle or Ag-SiO2NWs nanocomposite follows pseudo-first-order kinetics. The first order model is described as following: ln A /A 0 = −kKt = −kappt

(1)

It is clear that the rate constant of Ag-SiO2NWs nanocomposite, 2.535 × 10−3 s−1, for 4-NP reduction is bigger than that of Ag nanoparticles (Figure 8a,b). This indicates that the Ag-SiO2NWs nanocomposites are superior nanocatalysts to Ag nanoparticles, which can prompt the catalytic efficiency for reduction when trace amounts of catalysts are used. 21469

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Figure 8. ln(A/A0) and A/A0 versus reaction time for the reduction of 4-NP (a) Ag nanoparticles, with the slope of a straight line −0.143, (b) the reduction of 4-NP over 5 mL Ag-SiO2NWs nanocomposite, with the slope of a straight line −0.1521.

Table 1. Comparison with Different Catalysts catalyst support

size of Ag NPs (nm)

support surface

K (×10−3 s‑1)

TOF (h‑1)

ref

polymer PZS nanotubes PVA iron oxide carbon sphere SNTs CNFs OMS OMC FDU-15 chrysotile

2−10 5−20 5 2−5 10 ± 2 3−25 5−100 7 13.2 5.6 ± 0.5 20 ± 5

hydrophilic not defined hydrophilic not defined hydrophilic hydrophilic hydrophilic hydrophilic hydrophilic hydrophilic hydrophilic

0.12 11.1 5.3 2.38 1.69 16.2 4.6 127.4 5.32 13.08 2.5

122.4 101.4 457 12.9 0.07 104 34.7 24.3 22 444 59.79

11 30 31 12 15 32 33 34 35 36 this work

leaching route which provided a large amount of active sites which were beneficial to relative uniform growth and distribution of silver nanoparticles (Scheme 1a).26 This resulted in the large surface areas for decorated AgNPs and a number of Ag/SiO2NWs interfaces which were in charge of the high catalytic activity exhibited by 5 mL Ag concentration. In addition, the Si−OH groups are abundant on the surface of silica nanowires (Scheme 1a); these active functional groups will play an important role to capture and absorb 4-NP molecules easily to the reaction region (Scheme 1b). Gaining further insight into the detailed mechanism, although we know how difficult this is, thus absolutely needs further investigation in our future work.

The turnover frequency (TOF) of Ag-SiO2NWs nanocomposites can reach 59.79 h−1, which is better than that of most of the other substrate-supported Ag nanocatalysts reported in most previous studies (see Table 1). As shown in Table 1, we found that those Ag NPs with larger size and wider size distribution have poorer catalytic activities; meanwhile, it is noticeable that the Ag NPs supported on the hydrophilic supports possess higher catalytic activities than those on the hydrophobic supports. As we know, the reduction of 4-NP to 4AP by NaBH4 happened on the surface of Ag NPs.37 It is indicated that the hydrophilic supports are more beneficial to the catalytic reaction than hydrophobic supports because they could grasp the reactants (borohydride ions and 4-NP molecules) more easily on the surface of Ag NPs in the solution system.38 Thus, we predicted that the outstanding catalytic activity of Ag/SiO2NWs nanocomposites may originate from the monodispersed and small sized Ag NPs, as well as the greater number of −OH groups attributed with a relatively hydrophilic surface of silica nanowires, which promote the effective contact between the reactants and Ag NPs in the reaction. Although the turnover frequency (TOF) of AgSiO2NWs nanocomposites is not the best, compared with other reported catalysts, the concentrations of Ag-SiO2NWs nanocomposites are relative low (Table 1). Due to the above experimental results and the theoretical analysis, a possible catalytic mechanism is illuminated schematically in Scheme 1. According to basic theory about the catalytic reduction of 4-NP by AgNPs, electron transfer occurs from BH4− to 4-NP through adsorption of the reactant molecules onto the Ag catalyst surface; the catalytic efficiency is highly dependent on the large surface areas of AgNPs (Scheme 1b).33,39 In our work, high purity silica nanowires were obtained through a simple chemical dispersing and acid

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CONCLUSION

We have presented a simple in situ autoreduction approach for the preparation of Ag-SiO2NWs nanocomposites, in which the small sized Ag NPs are nearly monodisperse in the surfaces of SiO2NWs which were derived from the natural mineral chrysotile. The results demonstrated that Ag-SiO2NWs nanocomposites were prepared by electrostatic adsorption between the electron-rich amino groups functionalized on silica nanowires and the electron-deficient AgNPs, which were reduced by sodium borohydride in the presence of sodium citrate. The as-prepared 5 mL Ag-SiO2NWs nanocomposites exhibited excellent catalytic activity and reusability in the reduction of 4-NP, which might be owing to the high surface areas of zero-dimensional (0D) AgNPs affixed to the onedimensional (1D) SiO2 nanowires. The simplicity of this catalyst synthetic route makes it fascinating which can provide a facile strategy to fabricate silica-supported catalysts from natural minerals. 21470

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Scheme 1. Possible Mechanism of the Catalytic Reduction of 4-NP with the Ag/SiO2NWs

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org/. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05450. Time-dependent color change images, XRD spectra, UV−vis absorption spectra, and photographs of AuSiO2NWs nanocomposite films after catalytic reaction with different concentration (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 816 6089881. *E-mail: [email protected]. Phone: +86 816 6089471. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Foundation of Laboratory of Extreme Conditions Matter Properties (13ZXJK03) from Southwest University of Science and Technology, and the Transverse Project Foundation from Southwest University of Science and Technology (11ZH0163). 21471

DOI: 10.1021/acs.jpcc.5b05450 J. Phys. Chem. C 2015, 119, 21465−21472

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DOI: 10.1021/acs.jpcc.5b05450 J. Phys. Chem. C 2015, 119, 21465−21472