Light Induced Aggregation of Gold Nanoparticles and Photoswitching

Nov 15, 2017 - Hybrid nanomaterials having tunable properties that can be reversibly conducted by external stimuli, in particular light, are of great ...
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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Light-Induced Aggregation of Gold Nanoparticles and Photoswitching of Silicon Surface Potential Mohammed Ikbal,∥,†,‡ Dora Balogh,∥,†,‡ Evgeniy Mervinetsky,†,‡ Ruthy Sfez,§ and Shlomo Yitzchaik*,†,‡ †

Institute of Chemistry, The Hebrew University of Jerusalem, Safra Campus,Givat Ram, Jerusalem 91904, Israel Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel § Azrieli, College of Engineering, Jerusalem, Israel ‡

S Supporting Information *

ABSTRACT: Hybrid nanomaterials having tunable properties that can be reversibly conducted by external stimuli, in a particular light, are of great importance since they enable synergetic behavior between their components and enable the design of stimuli responsive “smart” materials and surfaces. Here we describe the formation of organic−inorganic hybrid nanoparticles that photochemically aggregate and their effect on the electronic properties of a semiconducting surface, as a function of external irradiation. The inorganic component consists of 3 nm gold nanoparticles while the organic component is a covalently attached, photochromic spiropyran derivative. Aggregation/deaggregation patterns in solution were obtained and analyzed by UV−vis spectroscopy and transmission electron microscopy upon photoswitching. The assembly of spiropyran-modified gold nanoparticles on an Si/SiO2 surface proved useful in phototuning the electronic properties of semiconductors measured by contact potential difference.

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sensing,15−17 programmable molecular logic devices,18,19 photoprogrammable organic light-emitting diodes (OLEDs),20 molecular switches17,21 and in the tuning of surface properties.22−24 Raymo and co-workers make use of spiropyran by its photonic manipulation exhibiting a wealth of potential applications, as the development of light-driven molecular machines,25 optical storage media,26 strategies to establish communication between independent molecules 27 and more.28,29 The control of the hydrophilic/hydrophobic properties of surfaces is one of the most challenging topics in material science. Not surprisingly, the chemical design of signal triggered hydrophilic/hydrophobic properties gains substantial research efforts, due to the possible applications of such systems in the control of fluid transport (microfluidics),30 self-cleaning surfaces,31 drug delivery,32−34 liquid crystals,35,36 nonlinear optics,37,38 and more.39−41 As an example for fluid transport, glass slides and capillaries were modified with a mixture of organosilanes linking the photochromic spiropyran to the free amino groups via an amide bond. This resulted in a dilute layer of spiropyran on a silanized surface exhibiting reversible wettability changes when irradiated with UV and visible light; and water in the photosensitive layer functionalized capillary tubes was observed to rise when the light source was switched from Vis to UV, changing the surface from hydrophobic to hydrophilic.40

INTRODUCTION Over the last few decades, much research interest has been focused on photochromic switches due to the possibility of an easy and fast read-out of the molecular state.1−4 Using light to control selected properties of interest in surfaces by turning them “on” and “off” applications in molecular electronics5−7 and functional surfaces.8−11 Molecular switches responding to light as external stimuli by undergoing reversible conformational changes resulting in different properties of the molecules are great candidates for these purposes. Spiropyran derivatives are well-known photoswitchable molecules that undergo a reversible isomerization between two forms, having different properties. The colorless spiropyran (SP) incorporates an indoline and a chromene moiety bound together via a spiro junction, perpendicular to one another gaining a three-dimensional orientation. This closed ring isomer absorbs at two wavelengths: at 270 nm the π−π* electronic transition in the indoline part responsible for the absorbance and at 336 nm the band correlates with the chromene part of the molecule. UV irradiation (λ = 365 nm) on the hydrophobic SP isomer induces the opening of the ring generating the planar, zwitterionic, and hydrophilic merocyanine (MC). Because of the π-conjugation between the indoline and the chromene moieties, MC possesses a delocalized single absorbance peak at the visible region (540 nm) giving MC a blue color and a strong dipole moment. By the photoisomerization of the molecule between these two states, a large and reversible change in the molecular properties can be achieved. This led to great interest in their use, not only for ophthalmic lenses,12 but also in cell imaging,13,14 molecular © XXXX American Chemical Society

Received: September 18, 2017 Revised: November 5, 2017 Published: November 15, 2017 A

DOI: 10.1021/acs.jpcc.7b09266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Chemicals. 3-Aminopropyltriethoxysilane (APTES, 99%, Aldrich) was distilled before use. Ethanol HPLC graded, H2SO4, and H2O2 were purchased from Merck. Acetone CP and dichloromethane (DCM) were purchased from BioLab. Synthesis of 2-(3′,3′-Dimethyl-6-nitrospiro[chromene-2,2′-indolin]-1′-yl)ethyl-5-(1,2-dithiolan-3-yl) Pentanoate. Condensation of phenyl hydrazine hydrochloride, (1), with isopropyl methyl ketone gives 2,3,3trimethyl-3H-indole, (2), which is a brown liquid. 2 was treated with 2-bromoethanol in acetonitrile under reflux for 24 h, resulting in compound 3 as a salt, which was used in the next step without further purification. A solution of 3 and KOH in water was stirred at room temperature for 10 min, and extracted with Et2O, to yield compound 4 as a yellow oil. A mixture of 4 and 5-nitrososalicylaldehyde in ethanol was heated under reflux for 3 h, on cooling a precipitate was formed, which on purification yielded compound 5 as a purple solid. DLthioactic acid, 5, and DMAP were dissolved in DCM at 0 °C under argon. A solution of N,N’-dicyclohexylcarbodiimide in DCM was added to the mixture. The reaction mixture was allowed to reach room temperature, and was stirred overnight. The solvent was removed in high vac. Purification of the crude product by column chromatography yielded compound 6 as a yellow solid (Scheme 1), and it was characterized by 1H NMR, 13 C NMR, IR and XPS analysis (Supporting Information).

The electrical properties of silicon-based WF, depending on the molecular dipole moments and the surface coverage of the active molecules.42 The channel conductance of these devices can also be reversibly modified under UV and visible-light irradiation, by chemically grafting spiropyran derivatives onto the channel region between the source and drain junctions.43 In our previously reported work, we showed that the electrical conductance of polyaniline nanowires films can be changed by the covalent grafting of spiropyran to the system, resulting in a reversibly tuned photochromic system. By UV irradiation of the system, the conductance of the polyaniline nanowires is increased due to the increased dipole that leads to the increase of the field-effect mobility. This was found to be fully reversible by the appropriate photoirradiation with no significant photofatigue.44 The optoelectronic properties of gold nanoparticles (AuNPs) can be tailored by functionalization with chromophores of specific properties and functions, resulting in light modulated organic−inorganic nanohybrid materials.45−47 During the past decade, light induced reversible self-assembly (LIRSA) of AuNPs have gained considerable interest due to their unique properties and prominent roles in the fields of materials science. As an example, the design of coumarin-functionalized AuNPs capable of reversible self-assembly was reported, based on the photolysis of coumarin in response to light irradiation. Irradiation at 365 nm leads to aggregated AuNPs, whereas the dimers can be disassembled by exposure to benign UV light.48 In this work, we demonstrate the light controlled aggregation and/or deaggregation of SP-modified AuNPs in addition to the WF tunability of Si/SiO2, via directed photoinduced molecular dipole changes. The light induced large molecular dipole changes on the Si/SiO2 substrate have shown fully reversible changes in the work function.

Scheme 1. Synthesis of 2-(3′,3′-dimethyl-6nitrospiro[chromene-2,2′-indolin]-1'-yl)ethyl-5-(1,2dithiolan-3-yl) Pentanoate

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EXPERIMENTAL SECTION H NMR spectra were recorded on a BRUKER-AC 400 MHz spectrometer. Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance as the internal standard (deuterochloroform: 7.26 ppm). 13C NMR spectra were recorded on a BRUKER-AC 400 MHz Spectrometer with complete proton decoupling. Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance as the internal standard (deuterochloroform: 77.0 ppm). Chromatographic purification was done with 60−120 mesh silica gels (Merck). For reaction monitoring, precoated silica gel 60 F254 TLC sheets (Merck) were used. The synthesis and characterization of 2-(3′,3′-dimethyl-6-nitrospiro[chromene-2,2′-indolin]-1′-yl)ethyl-5-(1,2-dithiolan-3-yl) pentanoate are provided in the Supporting Information. All starting materials, reagents and solvents were obtained from commercial sources and used without further purification unless otherwise stated. All anhydrous reactions were performed under a dry nitrogen atmosphere. UV−vis absorption spectra were recorded on a Shimadzu UV-3101PC spectrophotometer using glass substrates. XPS spectra were collected at ultrahigh vacuum (2.5 × 10−10 Torr) on a 5600 Multi-Technique (AES/XPS) system (PHI) using an X-ray source of Al K (1486.6 eV). UV and visible light exposure was carried out by using a UV lamp (we used a Prizmatix mic-LED 365 nm light emitting diode; light intensity ca. 1.0 mW/cm2) and a visible light lamp (520 nm, light intensity ca. 0.7 mW/cm2), respectively. 1

AuNPs Synthesis. AuNPs were prepared by using the Brust−Shiffrin method, detailed in the Supporting Information. Briefly, to a stirred solution of toluene and tetraoctylammonium bromide, an aqueous solution of HAuCl4 was added dropwise. 6, diluted in toluene, was added to the above solution. After 30 min an aqueous solution of NaBH4 was added dropwise and stirred for further 3 h. The organic layer was collected and suspended in ethanol, and was kept in ice. The precipitate obtained was suspended in a mixture of toluene and methanol and was centrifuged at 6000 rpm for 40 min. The precipitate obtained was again suspended in toluene−methanol mixture and the process was repeated to remove any unbound ligand from the suspension. Surface Modification. Highly doped n-Si ⟨100⟩ (Si R < 0.003 Ω/cm) substrates were washed with triple distilled water (TDW), dipped in piranha solution (H2O2/H2SO4 concentrated, 30%:70% v/v) for 15 min. The substrates were then rinsed with TDW and sonicated in NH 3 /H 2 O/H 2 O 2 (10%:10%:80% v/v/v) solution for 30 min at 60 °C.49 The B

DOI: 10.1021/acs.jpcc.7b09266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C substrates were washed with TDW, acetone and then dried with nitrogen. Freshly cleaned and activated substrates of Si/ SiO2 were immersed in 0.2% (v/v) APTES/ethanol solution for 20 min. After this step, the Si/SiO2 surface was washed three times with ethanol and dried with nitrogen, followed by curing in an oven at 100 °C for 30 min. The surface was further dipped in a DCM solution of spiropyran-functionalized AuNPs until DCM evaporation. Surface Potential. CPD measurements were conducted with a commercial instrument, Kelvin probe S (DeltaPhi Besocke, Jülich, Germany), with a vibrating gold electrode (work function 5.1 eV) in a home-built Faraday cage under Ar (argon) atmosphere.

Figure 2. UV−vis spectral changes upon (A) UV irradiation of a 2 × 10−5 M SP (6) solution in methanol for 13 min and (B) visible-light irradiation of 2 × 10−5 M MC solution in methanol for 75 min.

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reversible SP ↔ MC isomerization by UV or visible-light irradiation, correspondingly. AuNPs were prepared by using the Brust−Shiffrin method, detailed in the Supporting Information. The resulting AuNPs were characterized by transmission electron microscopy (TEM) showing spherical NPs with an average diameter of 3 nm, spatially aligned with similar distances (2.5 nm), with highly faceted low-energy surfaces (Supporting Information, Figure S3). X-ray photoelectron spectroscopy (XPS) data (Supporting Information, Figure S4) supported the formation of Au−S bond between the AuNPs and 6, suggesting the covalent linkage of the thiol moiety of the spiropyran to the surface of the AuNPs. Figure 3A shows the schematic presentation of the photoinduced aggregation of the spiropyran-functionalized AuNPs. UV irradiation of the AuNPs results in the isomer-

RESULTS AND DISCUSSION The UV/vis absorption spectra of a degassed 2 × 10−5 M solution of 6 were recorded. The colorless closed form (SP) has no characteristic absorbance in the visible region, whereas after UV irradiation, the zwitterionic open form (MC) displays an intense electronic spectral band centered at 540 nm (Figure 1).

Figure 1. UV−vis absorption spectra of a 2 × 10−5 M SP (6) solution in methanol (blue) and a 2 × 10−5 M MC solution in methanol (pink).

The optical switching property of spiropyran has been studied extensively and its photochromic reaction between states is shown in Figure 1. In the dark, 6 shows almost no absorption at 540 nm, and the spectrum does not change even when the sample is left for 12 h, indicating that 6 exists in the SP form. UV irradiation of the methanolic solution of compound 6 (2 × 10−5 M) with ≥360 nm band-pass filter resulted in the formation of a new absorption band at 540 nm (Figure 2A), which corresponds to the bond between the spiro carbon and the oxygen that undergoes scission, facilitating the structural change to the highly polar, zwitterionic merocyanine form (MC). Interestingly, we also noted an isosbestic point in the absorption spectra at 270 nm, indicating the presence of two distinct species in the equilibrium. The concentration of MC increases with increasing UV exposure time and reaches maxima, which is the photostationary state. In the reverse reaction MC can be isomerized to SP by thermal relaxation in the dark, or by visible-light irradiation. As shown in Figure 2B, the sample containing the MC, when irradiated with visible light shows a decrease in the MC band, along with the formation of the SP band. These data suggest that 6 undergoes

Figure 3. (A) Schematic presentation of the spiropyran-modified AuNPs undergoing aggregation upon UV irradiation due to the structural change and the formation of the zwitterionic merocyanine, or deaggregation upon visible-light irradiation of the sample. (B) UV− vis spectral changes of the spiropyran-modified-AuNPs sample, upon 20 min UV irradiation. (C) UV−vis spectral changes of the merocyanine-modified AuNPs sample upon 220 min visible-light irradiation. C

DOI: 10.1021/acs.jpcc.7b09266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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interactions between MC units on different NPs that decelerates the MC → SP back-isomerization. Further support for the existence of the MC form after UV irradiation was gained by IR spectral analysis. As presented in the Supporting Information, Figure S6, UV irradiation of 6 shows characteristic band at 1353 cm−1 due the formation of C−N+ starching vibration of the MC form. This band can also be observed after the UV irradiation of the AuNPs, proving the photoisomerization of the SP form to the open-ring MC. The SP-MC photoisomerization presented above can result changes in the electronic properties of a layer and can be used as a photoswitching mechanism. We fabricated an APTESmodified highly doped Si surface and attached the spiropyranmodified AuNPs (Figure 5).

ization of SP to the highly polar, zwitterionic MC isomer that triggers the rapid aggregation of the particles based on the electrostatic interactions between the MC moieties forming molecular dimers.50 NPs aggregation is accompanied by a red shift of the localized surface plasmon resonance (SPR) band, although it should be noted, that the SPR band overlaps with the absorbance of the MC, when the AuNPs are switched by the UV irradiation. Figure 3B shows the UV−vis spectral changes of the NPs with increasing times of UV irradiation. The shift in the absorbance band from 560 nm to longer wavelength (570 nm) reflects the aggregation of the AuNPs, whereas the increase in the absorbance band suggests the formation of the MC isomer. The absorbance band is centered at 560 nm before the photoirradiation due to the solvatochromic effect,51,52 as the AuNPs are dispersed in a methanol-toluene mixture (1:4). Increasing irradiation time results in the increasing aggregation of the NPs with the appearance of more MC isomers on the surface of the particles. The more MC isomers are generated the more electrostatic interactions between the zwitterionic MC isomers on different particles can be achieved, enabling the aggregation of the AuNPs. The aggregated MC-functionalized AuNPs can be redispersed with visible-light irradiation. Figure 3C shows the change in absorption spectra of the backward reaction. The SPR band for aggregated AuNPs blue-shifts with time (from 570 to 560 nm). This indicates the isomerization of MC → SP on the Au surface under visible-light irradiation, decreasing the electrostatic attractive force between the AuNPs and leads to their redispersion. This can also be achieved thermally by leaving the sample in the dark. AuNPs will disassemble after a time interval of 350 min (Supporting Information, Figure S5). For the visible-light irradiation, a low intensity light source was used (light intensity ca. 0.7 mW/cm2), explaining the long irradiation time required for the backward reaction. Further support that UV irradiation stimulates aggregation of the spiropyran-functionalized AuNPs was obtained from transmission electron microscopy measurements. Figure 4A

Figure 5. Stepwise assembly of AuNPs on a silicon surface, where step I is the bare silicon surface, step II is the modification of the surface with APTES, and step III is the addition of the SP-functionalized AuNPs.

In order to investigate the photoswitching process of SPmodified AuNPs attached to the silicon surface, CPD was monitored as a function of UV/vis irradiation. As shown in Figure 6, the light irradiation of the SP-modified AuNPs on the

Figure 4. TEM images of (A) the spiropyran-modified Au NPs and (B) the aggregated, merocyanine-modified Au NPs, after 20 min UV irradiation

shows individual AuNPs, as well as chains of NPs due to drying effects. After UV irradiation of the NPs significant self-assembly occurs, resulting in spherical aggregates (Figure 4B). These results are consistent with the UV−vis spectra obtained, confirming the switching of SP to MC by UV irradiation and the consequent aggregation of the NPs. Interestingly, when merocyanine is attached to the AuNPs, its switch to spiropyran is slower than in solution (220 min vs 75 min, respectively). It can be explained by the attractive

Figure 6. Experimental changes in contact potential difference (CPD) of the modified Si surface as a function of UV and visible irradiation. D

DOI: 10.1021/acs.jpcc.7b09266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Author Contributions

Si surface promotes repeated CPD change. The reversible SP ↔ MC isomerization by UV/vis irradiation results in the changes of electronic properties of the semiconductor surface. The CPD value increased with UV irradiation and decreased with visible-light irradiation. This can be explained by the before mentioned vastly different physicochemical properties of the two isomers. Because of the charge separation in MC, after UV irradiation a large electric dipole moment is generated on the AuNPs that induces the development of localized dipole field on the Si surface, increasing the CPD value, which is due to the raising of the vacuum level. As the electron crosses the surface dipole on its way out of the solid, its potential energy is raised by an amount equal to the dipole energy.53 On the other hand, the CPD values decrease by visible-light irradiation of the sample, due to the generation of the less polar SP on the NPs. The presented behavior can be used as a photoswitching mechanism in electro-optical devices, for example in OFETs. In a previous publication, we already showed the photogated switching of the electrical conductance of polyaniline nanowires on OFETs, where the photochromic polyaniline layer was the organic semiconducting layer, tuning the transport properties of the nanowires reversibly.54 The conformational changes of spiropyran are accompanied by variation of the electronic properties of the surface therefore photochromism is applicable in future applications to optical memory, switching and sensing as well.

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M.I. and D.B. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project (RECORD-IT) has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No 664786; S.Y. is the Binjamin H. Birstein Chair in Chemistry.

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ABBREVIATIONS SP,spiropyran; MC,merocyanine; AuNPs,gold nanoparticles; SAM,self-assembled monolayer; OLEDs,organic light-emitting diodes; MOSFET,metal-oxide-semiconductor field-effect transistor; LIRSA,light induced reversible self-assembly; CPD,contact potential difference

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(1) Fihey, A.; Perrier, A.; Browne, W. R.; Jacquemin, D. Multiphotochromic Molecular Systems. Chem. Soc. Rev. 2015, 44 (11), 3719−3759. (2) Tian, H.; Feng, Y. Next Step of Photochromic Switches? J. Mater. Chem. 2008, 18, 1617−1622. (3) Bléger, D.; Hecht, S. Visible-Light-Activated Molecular Switches. Angew. Chem., Int. Ed. 2015, 54 (39), 11338−11349. (4) Belser, P.; De Cola, L.; Hartl, F.; Adamo, V.; Bozic, B.; Chriqui, Y.; Iyer, V. M.; Jukes, R. T. F.; Kühni, J.; Querol, M.; et al. Photochromic Switches Incorporated in Bridging Ligands: A New Tool to Modulate Energy-Transfer Processes. Adv. Funct. Mater. 2006, 16 (2), 195−208. (5) Orgiu, E.; Samorì, P. 25th Anniversary Article: Organic Electronics Marries Photochromism: Generation of Multifunctional Interfaces, Materials, and Devices. Adv. Mater. 2014, 26 (12), 1827− 1845. (6) Tsujioka, T.; Irie, M. Electrical Functions of Photochromic Molecules. J. Photochem. Photobiol., C 2010, 11 (1), 1−14. (7) Seibold, M.; Handschuh, M.; Port, H.; Wolf, H. C. Photochromic Fulgides: Towards Their Application in Molecular Electronics. J. Lumin. 1997, 72−74, 454−456. (8) Stumpel, J. E.; Broer, D. J.; Schenning, A. P. H. J. StimuliResponsive Photonic Polymer Coatings. Chem. Commun. 2014, 50 (100), 15839−15848. (9) Bley, K.; Sinatra, N.; Vogel, N.; Landfester, K.; Weiss, C. K.; et al. Switching Light with Light − Advanced Functional Colloidal Monolayers. Nanoscale 2014, 6 (1), 492−502. (10) Meng, Q.; Wang, G.; Jiang, H.; Wang, Y.; Xie, S. Preparation of a Fast Photochromic Ormosil Matrix Coating for Smart Windows. J. Mater. Sci. 2013, 48 (17), 5862−5870. (11) Evans, R. A.; Hanley, T. L.; Skidmore, M. A.; Davis, T. P.; Such, G. K.; Yee, L. H.; Ball, G. E.; Lewis, D. A. The Generic Enhancement of Photochromic Dye Switching Speeds in a Rigid Polymer Matrix. Nat. Mater. 2005, 4 (3), 249−253. (12) Crano, J. C.; Flood, T.; Knowles, D.; Kumar, A.; Van Gemert, B. Van. Photochromic Compounds: Chemistry and Application in Ophthalmic Lenses. Pure Appl. Pure Appl. Chem. 1996, 68 (7), 1395−1398. (13) Xiong, Y.; Rivera-Fuentes, P.; Sezgin, E.; Vargas Jentzsch, A.; Eggeling, C.; Anderson, H. L. Photoswitchable Spiropyran Dyads for Biological Imaging. Org. Lett. 2016, 18 (15), 3666−3669. (14) Putri, R. M.; Fredy, J. W.; Cornelissen, J. J. L. M.; Koay, M. S. T.; Katsonis, N. Labelling Bacterial Nanocages with Photo-Switchable Fluorophores. ChemPhysChem 2016, 17 (12), 1815−1818.

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CONCLUSIONS The present study has demonstrated the photochemically induced aggregation and/or deaggregation of AuNPs using electrostatic interactions as driving force for the aggregation process by the formation of the molecular dimers between the zwitterionic MC isomers. The successful synthesis of the spiropyran-functionalized AuNPs suggests that other photoactive molecules could also be used to stimulate the photoinduced aggregation processes. Moreover, spiropyranfunctionalized AuNPs modified smart silica surfaces were fabricated, where the concept of photoswitching is adapted to the surface by altering the contact potential with light irradiation.

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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.7b09266. Synthesis and NMR data of 2-(3′,3′-dimethyl-6nitrospiro[chromene-2,2′-indolin]-1′-yl)ethyl-5-(1,2-dithiolan-3-yl) pentanoate, synthesis of SP-functionalized AuNPs, characterization of AuNPs, UV−vis spectral changes of the MC-modified AuNPs in dark, IR spectra of the photoactive species, and XPS of the different steps of the surface modification (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(S.Y.) E-mail: [email protected]. ORCID

Shlomo Yitzchaik: 0000-0001-5021-5139 E

DOI: 10.1021/acs.jpcc.7b09266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b09266 J. Phys. Chem. C XXXX, XXX, XXX−XXX