Optical Properties of Colloidal Indium Tin Oxide Suspended in a

thylsiloxane-copolymer allowed dispersion of the particles into a thermal fluid, Duratherm S, which is capable of with- standing ... lattice, and brea...
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Optical Properties of Colloidal Indium Tin Oxide Suspended in a Thermal Fluid Ebrima Tunkara,† Drew DeJarnette,‡ Mit Muni,† Todd Otanicar,‡ and Kenneth P. Roberts*,† †

Department of Chemistry & Biochemistry and ‡Department of Mechanical Engineering, The University of Tulsa, 800 South Tucker Drive, Tulsa, Oklahoma 74104, United States ABSTRACT: We investigated changes in the optical properties of indium tin oxide (ITO) nanocrystals as a function of temperature. ITO is a plasmonic, wide-band-gap semiconductor capable of absorbing UV and IR radiation. The nanoparticles were made using a standard two-pot synthesis, which resulted in the formation of ITO nanoparticles with myristic acid and oleylamine as surface passivation ligands. Ligand exchange with (6−7% aminopropylmethylsiloxane)−dimethylsiloxane copolymer allowed dispersion of the particles into a thermal fluid, Duratherm S, which is capable of withstanding working temperatures of 300 °C. Nanoparticles suspended in the fluid were heated over a range of temperatures, and their properties were investigated using IR spectrometry, UV−vis, transmission electron microscopy, and particle size analysis. Although heating showed no change in particle size, spectral changes were clearly observed in the form of a strong blue shift and increased absorbance. These temperature-dependent changes in the spectral properties of ITO were attributed to various factors such as: (i) distribution of tin dopant throughout the nanocrystal to increase the carrier concentration, (ii) removal of interstitial oxygen to reduce electron scavenging, (iii) increased order in the In2O3 structure to enhance carrier mobility, and (iv) reduction in electron traps of tin−oxygen complexes.

1. INTRODUCTION Colloidal plasmonic nanoparticles have been extensively used in medicine, sensors, light emitting diodes, solar cells, and many other applications.1−4 This wide field of applications has been inspired by the many attractive optoelectric properties of nanoparticles, which include tunable localized surface plasmon resonance (LSPR), a phenomenon referring to the collective oscillation of all electrons in nanosized crystals when irradiated with light. Unlike conventional plasmonic nanomaterials (Au and Ag) whose surface plasmon resonance absorption band is tuned with size, shape, and dielectric environment, the positions of LSPR absorption bands in semiconductor plasmonic nanocrystals can be additionally tuned with impurity (dopant) concentration and distribution throughout the host crystal.5−7 Therefore, the chemical and physical properties of indium tin oxide (ITO) nanoparticles can be tuned to suit a particular optical and/or electrical application. Indium tin oxide (ITO), an n-type wide-band-gap nanomaterial with high optical transparency, carrier density, and electrical conductivity, is a semiconductor nanomaterial with optical and electronic features of considerable interest.8−10 Although ITO is used in many applications, the relative contributing factors to the spectral line shape, position, and intensity of the LSPR peak are still under study. Research by Lounis et al. indicated that ITO nanocrystals with tin-rich surfaces led to reduced dopant activation as compared to nanocrystals with uniform dopant distribution throughout the In2O3 lattice.5 It was additionally shown that ITO could be © XXXX American Chemical Society

modeled such that ITO with tin-rich surfaces exhibits symmetric absorption peaks, whereas ITO nanoparticles with uniform tin distribution exhibit asymmetric absorption peaks. Although their studies successfully accounted for the optical properties of ITO, no factors besides alterations in chemical synthesis procedures were proposed to have an effect on the location and distribution of tin within ITO nanocrystals. Experimental investigations by Jansons et al.11,12 addressed this issue by monitoring the optical properties of ITO core nanocrystals with variations in In2O3 shell thicknesses. In agreement with the modeling results mentioned above, it was shown that the position of the LSPR band was dependent on the location of tin in ITO, such that the LSPR of ITO nanocrystals with thicker In2O3 shells was relatively red-shifted compared to that of ITO with much thinner In2O3 shells. This was further supported by XPS analysis, which showed continuous diminishing of the 3d peaks of tin with every increment of the shell.11 Other factors have also been shown to alter the spectral properties of ITO. For example, Gilstrap, Jr., et al. related room-temperature (RT) spectral changes in ITO nanocrystals to the formation of an equilibrium between interstitial oxygen and activated Sn4+ donor species.13 Moreover, spectral changes in ITO when heating to temperatures greater than 250 °C were Received: November 2, 2017 Revised: February 14, 2018 Published: February 15, 2018 A

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Figure 1. (A) Low-magnification TEM image of ITO nanoparticles, (B) high-magnification TEM image showing hexagonal ITO particles, (C) particle size analysis of TEM results, and (D) dynamic light scattering of ITO dispersed in hexane.

by suspending ITO nanoparticles in a heat transfer fluid where the nanoparticles selectively absorb solar IR radiation, preventing it from reaching the PV cell. With such a filter, band-gap solar radiation would reach the PV cell, whereas nonband-gap radiation absorbed by the nanoparticles could be dispatched as heat for various processes, including the generation of electrical energy, a so-called photothermal hybrid system.26,27

related to the reduction in interstitial oxygen, increases in the order of the In2O3 crystal lattice, and breakup of the Sn−O complexes.5,14−16 Herein, we report on the spectral properties of ITO after heating to elevated temperatures in a thermal fluid. It was observed that the LSPR band of ITO experienced a blue shift and increased absorption with increasing temperature and time. An increase in absorbance at high temperatures is in contrast with other metallic and semiconductor nanocrystal systems, where an increase in temperature generally resulted in a decrease in the optical properties of nanoparticles.13,17,18 These changes were mainly attributed to alterations of particle shape, oxidation, and/or the dissociation of surface ligands,13 which eventually led to agglomeration and sedimentation of the nanoparticles. The same effect was also reported in quantum dots, whose luminescence quenches with increasing temperature due to the creation of surface states in the forbidden gap as a consequence of detachment of the surface ligand.19−21 To minimize the loss of optical properties of these and other nanocrystals at high temperature, researchers have used various encapsulation materials, such as silica,22,23 diblock copolymers,24,25 and so on, to attain various levels of thermal stability. The observed enhancement of the optical properties of ITO at high temperature reported in this work makes ITO a potential candidate for applications as an optical absorber at elevated temperatures. For example, photovoltaic (PV) and concentrated photovoltaic systems often suffer from PV degradation by nonband-gap IR heat. This could be reduced

2. EXPERIMENTAL SECTION 2.1. Synthesis of ITO Nanoparticles. Synthesis chemicals: indium(III) acetylacetonate(In(acac) 3 ), tin(IV) bis(acetylacetonate) dichloride (Sn(acac)2Cl2), myristic acid (MA), octadecylamine (ODE), chloroform, absolute ethanol, and hexane were obtained from Sigma-Aldrich (St. Louis, MO) and used without further purification. Oleylamine (OLAM) was purchased from Acros (Pittsburg, PA). The 6−7% aminopropylmethylsiloxane (AMS)−dimethylsiloxane ligand was purchased from Gelest Inc. (Morrisville, PA), whereas Duratherm S was purchased from Duratherm Heat Transfer Fluids (Lewiston, NY). Batches of 16% tin-doped ITO nanoparticles were synthesized using a standard two-pot synthesis protocol.28,29 In brief, 0.41 g of In(acac)3, 0.078 g of Sn(acac)2Cl2, and 0.69 g of MA were added to a three-neck flask with 10.0 mL of ODE; a high boiling point, clear, and hydrocarbon-based fluid functioned as the solvent. The mixture was heated to 110 °C under vacuum for 2 h to remove gases and water that could B

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magnification image shows an average ITO diameter of 9.8 ± 1.7 nm (see Figure 1B,C). In addition, dynamic light scattering showed a similar particle size distribution and indicated minimal aggregation in solution (see Figure 1D). The as-synthesized ITO nanoparticles precipitated when attempting to disperse in Duratherm S, thus requiring replacement of the surface ligands (OLAM and MA) with AMS to provide stable, dispersed solutions (see the Experimental Section). This can be visualized in the pictures in Figure 2; the initial synthesis product with MA acid and

interfere with the nanoparticle formation. The reaction mixture was then heated to 295 °C under an argon atmosphere. Upon reaching 295 °C, 1.0 mL of previously dehydrated and argonpurged OLAM was rapidly injected to the reaction to initiate nucleation and growth of the ITO nanoparticles. The reaction temperature was quickly reduced to 280 °C for 1 h and finally to 240 °C for an additional hour. The solution was then allowed to cool to room temperature, after which the nanoparticles were purified using a polar/nonpolar extraction technique, in which hexane was used as the solvent and ethanol as the antisolvent. 2.2. ITO Surface Functionalization. Surfaces of the assynthesized ITO nanoparticles were passivated with OLAM and MA during synthesis, all of which contain long hydrophobic chains. Because of the extremely nonpolar nature of these surface passivating ligands, the ITO nanoparticles could be suspended in hexane, cyclohexane, and other nonpolar solvents but not in Duratherm S, which is a siloxane-based thermal fluid. Duratherm S was chosen for this investigation because of its high boiling point (340 °C), flash point, and thus its ability of resisting thermal degradation when heated to the proposed working temperature (300 °C). Additionally, it has high transparency in the visible region of the solar spectrum. However, promoting the stable suspension of ITO in Duratherm S required exchange of the as-synthesized surface ligand of ITO with AMS. To accomplish this, ITO in hexane solution was combined with AMS in a 1:5 ratio (v/v %) in a round-bottom flask and the mixture was stirred while heating at 70 °C for 72 h. After the 72 h heating period elapsed, the ligand-exchange product (ITO−AMS) was purified by centrifuging at 3000 rpm for 10 min. High-temperature tests were carried out by combining the ligand-exchange product with Duratherm S in a 1:4 ratio (v/v %) and placing 4.0 mL of this solution into a 10 mL highpressure cylindrical glass reaction tube. The mixture was degassed and purged with argon before being heated with an Apollo Dual Channel Temperature Controller heating block (JKEM Scientific, Inc., St. Louis, MO). UV−vis data were collected using a Shimadzu UV−visible Spectrometer (UV2600/2700; Columbia, MD). IR measurements were taken using a Spectrum 2 PerkinElmer spectrometer (Waltham, MA). Initial size analysis was performed using a Microtrac Zetatrac size analyzer (Montgomeryville, PA), followed by transmission electron microscopy (TEM) using Hitachi H7000 (Schaumburg, Illinois) operated at 75 kV. The X-ray diffraction (XRD) measurements were performed using a Rigaku SmartLab (Woodlands, TX) system with a Cu Kα X-ray source.

Figure 2. Ligand exchange with (6−7% aminopropylmethylsiloxane) (AMS)−dimethylsiloxane copolymer: as-synthesized ITO with OLAM and MA surface ligands formed a cloudy solution with Duratherm S (left), whereas AMS-coated ITO after ligand exchange formed a clear solution with Duratherm S (right).

OLAM surface ligands formed a cloudy solution when attempting to disperse into Duratherm S. However, after ligand exchange, the AMS functionalized ITO surface formed a transparent solution in the heat transfer fluid. AMS functionalized nanoparticles were suspended in Duratherm S in a 1:4 (v/v %) ratio following the ligand-exchange procedure for heat treatment of ITO. The solution was repeatedly degassed and purged with argon before heating. Degassing was particularly important because Duratherm S and ITO can oxidize when exposed to air, especially at high temperatures. Figure 3 shows two room-temperature spectra: pure Duratherm S and a spectrum of ITO added to Duratherm S before heat treatment. As discussed earlier, Duratherm S by itself is transparent (transmittance greater than 92%) in the visible region of the solar spectrum. It is also slightly transparent in the near-infrared and UV regions. However, the addition of ITO, shown in the black spectrum of Figure 3, led to a decrease in transmittance in the UV and IR regions. Such absorption is mainly attributed to interband and intraband transitions of carriers in ITO occurring in those regions. On the contrary, not much changes were observed in the visible region of the spectrum because of the absence of any optical transitions for ITO in this region. As shown in the pictures of Figure 4, upon heating the ITO in the heat transfer fluid, the color of the solution changed from almost colorless to dark green. This indicates a higher carrier concentration in the conduction band of the heated sample. A spectral shift of the LSPR band was also observed (Figure 4) when the Duratherm S contribution was subtracted from the ITO spectrum. The LSPR blue shift was on the order of ∼ 290 nm, from an initial 2015 (before heating) to 1725 nm after

3. RESULTS AND DISCUSSION The synthesis of ITO nanoparticles was initially confirmed by a color change from slightly yellow to dark green within 10 min after oleylamine injection. Oleylamine (OLAM) initiates particle nucleation through nucleophilic attack (lone pair on the nitrogen atom in OLAM) on the carbonyl atom attached to indium and tin precursors or through a hydrolysis reaction using water molecules obtained from condensation reactions between free carboxylic molecules and OLAM.28 The assynthesized particles were passivated with OLAM and myristic acid (MA), both of which prevent aggregation and provide stable suspensions of nanoparticles in hexane. The TEM images show a narrow size distribution of the nanoparticles. In the lowmagnification image shown in Figure 1A, it can be seen that the particles are nearly identical in diameter. The higher C

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Figure 5. TEM images of ITO−AMS in Duratherm S showing no changes in particle size (A) before heating and (B) after heating.

distribution of Sn4+ throughout In2O3, reduction of interstitial oxygen, liberation of carriers from Sn−O traps, carrier mobility, and increased order of the In2O3 host matrix.5,7,14−16,28,33−37 Furthermore, studies on doping levels in ITO revealed that when doped beyond a certain concentration (mostly 10% Sn),7,37 the excess dopant settles on the surfaces of the crystals where they form nonreducible Sn2+ complexes by reacting with diffusing oxygen atoms. Such complexes are believed to form depletion regions (electron traps) on the surfaces of the crystals, which decreases the electron density and leads to attenuation of plasmon absorption.7 Therefore, because the tin concentration in this study exceeded 10%, which is believed to generate the maximum carriers, surfaces enriched in tin were likely formed. These exposed Sn atoms are susceptible to reaction with oxygen to form complexes that are believed to form depletion regions (electron traps) on the surfaces of the crystals, which decreases the electron density and leads to attenuation of plasmon absorption.7 Consequently, at the level of dopant concentration in this study, it is believed that the maximum carriers are activated when heating ITO−AMS solutions, allowing tin to migrate from the nanocrystal surface to the inner crystalline sites, thereby increasing the latticeincorporated-tin distribution.33 Such migrations have the ability of increasing the electron concentration to above the Mott acriterion,34 which is 1019 cm−3, thereby moving the Femi level to the base of the conduction band for the easy promotion of free electrons to the conduction band. As a result, the band gap widens and blue shifting of the LSPR occurs. This observation correlates to a previously observed decrease in resistivity, due to excess carrier generation, when ITO thin films were heated to different temperatures.15,33−36 In plasmonic nanocrystals, an increase in light absorption is most commonly caused by increased electron density due to defects, oxygen vacancies, interstitial atoms, or breakage of agglomerates, which leads to the generation of more independent electron clouds.36 The peak intensities of semiconductor nanocrystals were found to be affected by the same phenomena, in addition to effects from dopant concentration and distribution. The increase in absorption observed here was attributed to dopant distribution, increased carrier mobility created at high temperature,33 and the presence of oxygen defects. Although no experimental investigation was performed to confirm the presence or absence of oxygen defects in this work, TEM confirmed that the size of the particles did not change upon heating (please see Figure 5), and the excess electrons observed from spectroscopic data would be mainly from the redistribution of activated Sn4+. The lack of a decrease in particle size at these temperatures was not surprising because our maximum temperature (300 °C) was far below the melting point of ITO.

Figure 3. Spectra of Duratherm S (red spectrum) and ITO in Duratherm S (black spectrum) showing ITO and Duratherm S absorbance from UV to near IR.

Figure 4. Before and after heating ITO in Duratherm S for 72 h at 250 °C. Shown on the top left is a picture of ITO in Duratherm S before heating and the bottom left is ITO in Duratherm S after heating. Frame A: percent transmittance spectra showing the temperaturedependent spectral shifts of the LSPR of ITO in Duratherm S at room temperature (black spectrum) and after heating for 72 h at 250 °C (red spectrum). The Duratherm S absorbance was subtracted from each spectrum.

heating to 250 °C. The spectral properties of Duratherm S were tested before and after heating, where there was shown to be no difference after 72 h of heating at 300 °C (data not shown). The blue shift of the LSPR peaks in semiconductor plasmonic nanoparticle has been known for many years to be mainly a function of size, morphology, compositional change, refractive indices of media, and so on.7,30−32 In terms of size, it is known that when the size of nanoparticles approaches the quantum regime, absorption bands shift blue owing to quantum confinement effects.28,27 Therefore, it could be erroneously assumed that the shifting observed here was due to a change in particle size. However, TEM measurements of the before and after heated samples show no changes in the sizes of the heated nanoparticles (Figure 5). Change in refractive index of the medium was also considered; however, because refractive index is a reversible phenomenon and all of the optical property measurements were taken at room temperature, it was clear that it has no effect on the shifting spectral properties. Therefore, the main factors contributing to the spectral changes of ITO were the D

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the RT sample. It can be clearly seen from the figure that there was no observed difference in terms of peak shape, intensity, or broadness, all of which would be expected if there was a change in particle size. Although the composition of the lattices might change due to the increased incorporation of tin atoms, such changes would be expected to have little effects on the crystalline structure of the nanoparticles because indium and tin are similar in size (0.81 and 0.71 Å, respectively) and their substitutions with one another have already been reported to only cause slight perturbation in the crystalline framework.30 In addition, no diffraction lines belonging to free Sn−O were observed in either the room-temperature or heated samples. Although the diffraction lines are relatively broad, making it hard to convincingly dispel the presence of tin oxide species, other researchers already reported that the presence of tin− oxygen complexes would result in decreased carrier concentration due to the carrier traps usually generated by such species.14−16,28,33 However, the lack of appreciable amounts of tin−oxygen complexes can be inferred by the spectral blue shift and absorbance increase after heating. On the contrary, others have discovered that heating ITO thin films to temperatures exceeding 300 °C led to increased crystallinity.15,33−38 As a result, diffraction lines increased with every 100 °C increment of the annealing temperature, indicating improved crystallinity. However, this was not observed here at 300 °C. Further temperature-dependent investigations on how heating at different temperatures affect the position of the LSPR of ITO were carried out by heating a 1:4 (v/v %) ratio of

XRD analysis was also performed to investigate whether heating the ITO solution to 300 °C resulted in changes in crystal size, improved crystallinity, or formation of free Sn−O complexes in solution. Figure 6 shows broad and relatively low-

Figure 6. Overlay of the XRD spectra for room-temperature (black spectrum) versus 300 °C (red spectrum) ITO samples.

intensity diffraction peaks. The observed low intensity is due to the low concentration of the nanoparticles in the heat transfer fluid, whereas the broadness of the lines is a common phenomenon in small-sized nanoparticles. Nonetheless, the main characteristic peaks of ITO assignable to the (222), (400), (440), and (622) planes are all visible in both the heated and

Figure 7. Temperature-dependent spectral shifts of ITO in Duratherm S: (A) spectra of the heated ITO−polydimethylsiloxane solution from RT to 300 °C in 50 °C increments; (B) 72 h of cooling of the heated ITO-Duratherm S under atmospheric conditions; (C) thermally cycled ITO from room temperature to 300 °C; (D) color of the heated solution after exposure to atmospheric oxygen for 24, 48, and 72 h. E

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The Journal of Physical Chemistry C ITO in Duratherm S solution to 150, 200, 250, and 300 °C. As shown in Figure 7A, the LSPR peak intensity increased by almost 50% when the temperature was raised to 150 °C. Additionally, the peak began to shift blue from increased order of the In2O3 host crystal and activation of extra carriers. This trend of increasing absorbance and increasing shifting to higher energy continued for each 50 °C increment tested. At 300 °C, the LSPR peak made a 400 nm blue shift when compared to that of the RT samples and the intensity also increased by over a 100%. The intensity enhancement as well as intensity of the color of the heated sample is mainly dependent on the heating temperature and to a lesser extent on the duration of heat treatment. For example, a sample of ITO in the heat transfer fluid at 250 °C was not expected to make a 400 nm blue shift even at heating periods exceeding 100 h. The peak intensity at 300 °C was not much different from that at 250 °C, which could mean that the maximum carrier density was generated at 250 °C. However, a significant blue shift was still observed at 300 °C, indicating further homogenization of the tin dopant within the crystal lattice. Although the observed spectral changes are believed to primarily result from tin redistribution, other factors were considered because such other defects can also lead to increased carrier concentration and cause a blue spectral shift of the LSPR. For example, because the heating was done in an oxygen-free environment, it is likely that more oxygen vacancies were created during the heating process, resulting in a higher carrier density that could be reversed when exposed to oxygen. This was tested by heating a sample to 300 °C for 72 h and cooled to room temperature under atmospheric conditions. Spectra were taken at 0, 24, 48, and 72 h following heating. As can be seen in Figure 7B, an ∼70 nm red shift (as compared to the original 400 nm blue shift) was observed after 24 h with no further red shift at 48 and 72 h. On the contrary, the LSPR intensity decreased throughout the test period. This indicates that the increase in In2O3 crystallinity is mostly conserved upon cooling, but the number of activated free carriers continues to decrease from the reintroduction of oxygen into the In2O3 lattice and the formation of electron traps. The decreased intensity of the LSPR could be visualized as a decrease in green color from the ITO solution over time, as shown in Figure 7D. Similar results were observed by Gross et al.,38 who detected lower conductivity in ITO films exposed to air and heated to 250 °C. Under such conditions, oxygen diffuses into the crystalline framework of ITO and reduces oxygen vacancies and defects, which reduces the number of activated carriers. Thermal cycling was also conducted to test whether such enhancement in the optical property occurs only in a single heating experiment. This investigation was carried out by heating a sample to 300 °C for 72 h, taking measurements of the optical properties of the cooled sample, and then reheating to 300 °C for another 72 h. Each heating at 300 °C for 72 h represents a cycle, and a total of four cycles were investigated. Figure 7C clearly shows a 400 nm blue shift of the LSPR and an increase in peak intensity from RT to the first cycle. Additionally, Duratherm S peaks (1689, 1703, 1746, and 1786 nm) were more visible in the RT sample. After the 2nd cycle, a 22 nm blue shift and a slightly lower peak intensity as compared to those of the first cycle were observed. This indicates that tin redistribution and electron generation were stable at 300 °C and that both the phenomena were independent of the number of cycles. Furthermore, the peak positions and intensities of the 2nd through the 4th cycle

almost overlap, further confirming that the number of carriers generated at each given cycle was almost constant. A series of time-dependents were also conducted to test the minimum heating period at which the observed blue shift and increased absorbance occur. In Figure 8, an ITO solution was

Figure 8. Time-dependent spectra of ITO in Duratherm S at 300 °C.

heated at 300 °C for 6, 12, 24, and 72 h. Results show an initial 327 nm blue shift (from 2015 to 1688 nm) during the first 6 h, with a slower shifting throughout the remaining 72 h period. The LSPR was observed at 1678, 1616, and 1600 nm, representing the 12, 24, and 72 h of heating, respectively. The increased absorbance followed a similar time dependence, with the maximum observed at the 72 h period. This demonstrates that the prolonged exposures to 300 °C cause a spectral shift and increased absorbance.

4. CONCLUSIONS In monitoring the spectral profile of ITO suspended in a heat transfer fluid, it was found that elevated temperatures cause a redistribution of tin throughout the crystal matrix, displace interstitial oxygen, break up Sn−O complexes, and refine the host In2O3 crystallinity. The result is a 400 nm blue shift of the LSPR peak and an increase in absorption intensity. The process was found to be partially reversible with a 70 nm red shift and decreased intensity of the LSPR when the samples were exposed to air after heating. It is believed that the spectral position is mainly attributed to the increased order of the In2O3 matrix, whereas the spectral intensity is attributed to the amount of free carriers versus interstitial oxygen and tin− oxygen complexes. The ability of controlling the LSPR position of doped semiconductors at high temperature has tremendous importance in a variety of applications. For example, ITO nanoparticles could be used as part of a spectral filter to remove nonband-gap radiation in various types of PV cell architectures. The use of ITO in such a system will not just help PV systems resist thermal degradation, but the improved optical properties observed herein are expected to improve the efficiencies of such systems. In the future, we intend to test other doped metal oxide nanoparticles to determine whether a similar temperature-dependent effect is present.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (918) 631-3090. F

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Kenneth P. Roberts: 0000-0001-5424-0745 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the ARPA-E FOCUS program for this research (grant number DE-AR0000463). We also thank Aaron Saunders (NanoComposix, San Diego, CA) for his assistance in TEM analyses.

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DOI: 10.1021/acs.jpcc.7b10855 J. Phys. Chem. C XXXX, XXX, XXX−XXX