Aqueous Exfoliation of Graphite into Graphene Assisted by Sulfonyl

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Aqueous Exfoliation of Graphite into Graphene Assisted by Sulfonyl Graphene Quantum Dots for Photonic Crystal Applications Minxiang Zeng,†,§ Smit A. Shah,†,§ Dali Huang,‡ Dorsa Parviz,† Yi-Hsien Yu,‡ Xuezhen Wang,† Micah J. Green,*,†,‡ and Zhengdong Cheng*,†,‡ †

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States Department of Material Science & Engineering, Texas A&M University, College Station, Texas 77843, United States

‡

S Supporting Information *

ABSTRACT: We investigate the π−π stacking of polyaromatic hydrocarbons (PAHs) with graphene surfaces, showing that such interactions are general across a wide range of PAH sizes and species, including graphene quantum dots. We synthesized a series of graphene quantum dots with sulfonyl, amino, and carboxylic functional groups and employed them to exfoliate and disperse pristine graphene in water. We observed that sulfonyl-functionalized graphene quantum dots were able to stabilize the highest concentration of graphene in comparison to other functional groups; this is consistent with prior findings by pyrene. The graphene nanosheets prepared showed excellent colloidal stability, indicating great potential for applications in electronics, solar cells, and photonic displays which was demonstrated in this work. KEYWORDS: polyaromatic hydrocarbons, graphene quantum dots, surface chemistry, graphene exfoliation, photonic crystal

1. INTRODUCTION In addition to its exceptional thermal and mechanical properties, graphene possesses a large delocalized π-electron system which results in a strong affinity for polyaromatic hydrocarbons (PAHs), which are widely present in dyes, pollutants, and biomolecules.1,2 Owing to their tendency to interact with PAHs, graphene and its derivatives have been studied extensively as sorbents for wastewater treatment applications.3,4 Functionalized pyrene derivatives have been reported as dispersants for producing graphene dispersions because of their ability to stabilize graphene at high concentration/dispersant ratios in comparison with traditional surfactants.5,6 By introducing repulsive solvation and/or electrostatic forces on graphene sheets, pyrene derivatives are able to prevent nanosheet aggregation and thus stabilize graphene in aqueous dispersion.7−9 Such dispersions with high colloidal stability may be used for direct inkjet printing of electronic structures.10 Additionally, the ability of PAHs to form charged graphene species may allow for additional assembly techniques such as layer-by-layer film growth.11 The type and number of functional groups can play an important role in determining the quantity and quality of © 2017 American Chemical Society

graphene dispersions prepared using pyrene derivatives. Theoretical simulations suggest that a higher dispersant polarity can facilitate exfoliation of graphite by accelerating “sliding” of the dispersant into the graphite interlayer.12 Experimentally, the exfoliation efficiencies of pyrene derivatives with amino groups (−NH2), carboxylic groups (−COOH), and sulfonic groups (−SO3H) were investigated under various pH values and processing conditions.7 It was shown that functional groups with greater electronegativity can enhance the interaction between stabilizers and graphene layers. In particular, sulfonyl functionalized pyrenes produced stable colloidal dispersions for a broad range of pH values. Herein, we aim to demonstrate that the concept of functionalizing pyrene as graphene dispersants can be generalized to larger polyaromatic particles (i.e., graphene quantum dots) acting as graphene dispersants. Graphene quantum dots (GQDs) and carbon dots (CDs) are mono- or few-layered graphene sheets with lateral dimensions no larger than 0.1 μm, though GQDs are of higher Received: May 17, 2017 Accepted: August 17, 2017 Published: August 17, 2017 30797

DOI: 10.1021/acsami.7b06980 ACS Appl. Mater. Interfaces 2017, 9, 30797−30804

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Figure 1. Fabrication of sulfonyl-grafted graphene quantum dots. (a) Schematic illustration of the formation of S-GQDs. (b) TEM image of S-GQDs with scale bar of 50 nm. (c) Survey XPS and (d) high-resolution S2p XPS of S-GQDs.

crystallinity.13 Such quantum dots possess unique properties, such as high photoluminescent quantum yield, cost effectiveness, low cytotoxicity, and biocompatibility.14−18 Therefore, they have been investigated extensively for applications such as bioimaging,14,19 solar cells,15,20 and surfactants.21−23 GQDs have been fabricated by chemical oxidation of carbon black,24 solvothermal cleaving of graphene oxides,25 electrochemical exfoliation of graphite,26 thermal degradation of organic molecules,27,28 ruthenium-catalyzed decomposition of C60,29 and stepwise organic synthesis.30,31 In fact, GQDs possess aromatic cores and, with proper functional groups, can be expected to prepare stable graphene dispersions. GQDs with carboxylic groups (−COOH) have been used to stabilize commercial graphene powder by He et al. in 2014,32 but a direct exfoliation of graphite into graphene by GQDs has not been discussed. Recently, an attempt was made to prepare graphene from graphite by using small-sized CDs (∼1.8 nm) with amino functional groups (−NH2) to produce graphene aqueous dispersion.33 Despite these prior studies, the influence of GQDs with different functional groups on producing graphene dispersions is still poorly understood, and a comprehensive study that provides a control of the functionalities of GQDs derivatives is missing. In this study, we design and characterize novel graphene quantum dots with sulfonyl (−SO3−) functionalization (SGQDs) and compare these against carboxylic functionalized GQDs (C-GQDs) and amine functionalized GQDs (N-GQDs) as dispersants for preparing graphene in aqueous solution. We show that the concentration of dispersed graphene depends considerably on polar functional groups present on the polyaromatic GQDs. We demonstrate that their functionalities affect the formation and stability of π−π stacking interactions that dictate the graphene exfoliation efficiency. The as-prepared aqueous graphene dispersion demonstrates colloidal stability, which can be used for enhancing the color contrast of photonic crystals by suppressing incoherent scattering and multiple scattering.

GQDs with sulfonyl functionalization in the present work. As shown in Figure 1a, S-GQDs were fabricated by a direct condensation and pyrolysis of citric acid and 4-styrenesulfonic acid sodium salt. At 200 °C in the presence of air, a polymerlike aromatic intermediate was first formed by condensation of citric acid, and then linked with CC double bonds of 4styrenesulfonic acid sodium salts via a thermal polymerization reaction.27,34,35 No products were observed if the reaction was performed in the presence of pure nitrogen, indicating a crucial role of oxygen in free-radical polymerization as an initiator and promoter.35 To remove excess starting materials and potential polymeric byproducts, the crude products were further purified by performing dialysis (2000 Da) for 4 days. This one-pot fabrication of sulfonyl graphene quantum dots is straightforward, low-cost, eco-friendly, and easily scalable. The fabrication conditions are fairly gentle without need of expensive catalysts or toxic organic solvents. The morphology of S-GQDs was studied by transmission electron microscopy (TEM). Figure 1b shows a typical TEM image of S-GQDs, showing the size of the as-prepared S-GQDs distributed in the range of 15−55 nm with an average diameter of 28.7 ± 5.4 nm, according to the diameter distribution plot in the Supporting Information (Figure S1). This size distribution is similar to that of other types of GQDs prepared by different methods reported in the literature.14,21 The surface chemistry of S-GQDs was next characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). According to the survey XPS spectrum of SGQDs (Figure 1c), S-GQDs are mainly composed of carbon, oxygen, and a small amount of sulfur with an atomic ratio of 26:10:1, which is further confirmed by elemental analysis (see the Supporting Information, Table S1). The existence of C−S bonding is further proved by the high-resolution C1S XPS spectrum (Figure S2) and S2P XPS spectrum (Figure 1d), which show the presence of sulfonyl functional groups (SO3−1, 167.5 eV). In the FTIR spectrum of S-GQDs, several characteristic vibrational bands at 1175, 1405, 1610, 1730, 2930, and 3460 cm−1 are found, which are assigned to the stretching vibrations of C−S/C−O, bending vibrations of C− H, vibrational bands of CC, stretching vibrations of −CO, stretching vibrations of C−H, and the stretching vibrations of C−OH, respectively (see the Supporting Information, Figure

2. RESULTS AND DISCUSSION 2.1. Synthesis of Functionalized GQDs. In light of the recent reports on improving the graphite exfoliation efficiency using polycyclic aromatic compounds, such as pyrene derivatives, as dispersant, we synthesized novel aromatic 30798

DOI: 10.1021/acsami.7b06980 ACS Appl. Mater. Interfaces 2017, 9, 30797−30804

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Figure 2. Spectroscopy of S-GQDs. (a) PL spectra of S-GQDs at various excitation wavelengths from 360 to 420 nm. (b) Photographs of the SGQDs in water under visible light (VL) and ultraviolet (UV, 365 nm) light before and after sonication. (c) UV−vis absorption spectra of S-GQDs before and after sonication.

Figure 3. Exfoliation of graphite with S-GQDs. (a) Schematic illustration of the formation of S-GQDs-stabilized graphene. (b) TEM image of a fewlayered graphene nanosheet stabilized by S-GQDs (red circles indicate S-GQDs); the inset shows colloidal stability comparison between aqueous graphite suspension (left) and aqueous graphene dispersion (right). (c) SEM image of expanded graphite (parent material).

S3).36,37 These results confirm the successful grafting of sulfonyl groups onto GQDs. 2.2. Spectroscopy and Sonication Stability of GQDs. We next investigate the photoluminescence (PL) spectra of this novel type of graphene quantum dots. As shown in Figure 2a, as excitation wavelength is increased from 360 to 420 nm, the PL peak position shifts to longer emission wavelength. This excitation-dependent PL behavior is common in fluorescent carbon materials and is used to obtain desired PL colors by controlling the wavelength of excitation light.16 The PL intensity distribution can be explained by the effect of GQD size distribution on the band gaps.38 In PL spectra, S-GQDs have optimal excitation and emission wavelengths at 380 and 458 nm, respectively. The optimal excitation peak is relatively red-shifted compared with that of reported small-sized GQDs due to the quantum confinement effect which governs the ultraviolet light that is absorbed.39−41 In order to explore the potential of S-GQDs as an exfoliation agent and dispersant for graphene, its stability was examined under tip sonication, as sonication is a commonly used method to facilitate liquid-phase exfoliation of graphite into graphene.7 Tip sonication uses high energy sound waves which are generated by a piezoelectric

actuator. To prove that the sonication process will not cause fragmentation of S-GQDs, we compared the UV−vis spectra of S-GQDs before and after sonication for 1 h. As shown in Figure 2b, a bright blue luminescence of the S-GQDs aqueous solution (0.2 mg·mL−1) under the illumination of a UV (365 nm) light is shown without any visible change before/after sonication. Moreover, as shown in Figure 2c, there is no observable difference in absorption peak positions, indicating that GQDs are not altered by sonication. These results show that GQDs can tolerate sonication conditions required for exfoliating graphite into graphene. 2.3. Direct Exfoliation of Graphite into Graphene. We then evaluated the capability of S-GQDs to stabilize pristine graphene exfoliated from graphite in aqueous solution. Tip sonication was used to achieve lab-scale liquid exfoliation of graphite to graphene, as shown in Figure 3a. For this study, we sonicated a slurry of S-GQDs and graphite dispersed in deionized (DI) water. As a result, graphite is exfoliated into a mixture of single- to few-layered graphene sheets (referred to as graphene nanosheets here onward) and multilayered (layers > 5) graphitic flakes.42 The resulting slurry was centrifuged at 500−3000 rpm, causing the heavy graphitic fractions to 30799

DOI: 10.1021/acsami.7b06980 ACS Appl. Mater. Interfaces 2017, 9, 30797−30804

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Figure 4. Exfoliation efficiencies with various GQDs. (a) Graphene concentrations prepared by N-GQDs, C-GQDs, and S-GQDs. (b) ζ potential values of exfoliation products by the aforementioned three types of GQDs.

mg/mL under sonication (45 W). It was found that N-GQDs were able to disperse only a small amount (0.003 mg/mL) of graphene nanosheets after 240 min centrifugation. The stabilized graphene concentration is considerably lower than that of reported CQD with amine functional groups, likely due to the higher sonication power (800 W) and lower centrifugation time (30 min).33 Moreover, when C-GQDs were used to prepare a graphene dispersion, the concentration was observed to be negligible. We believe that the reason behind their poor performance as dispersants in contrast to SGQDs is due to the lower electronegativity of the carboxyl groups in C-GQDs and the amine groups in N-GQDs in comparison to the sulfonyl groups in S-GQDs. We also studied the effect of dispersant and initial graphite concentration on the final graphene yield. Upon increasing the initial graphite concentration, no significant change in final graphene concentration was observed when using C-GQDs and NGQDs as dispersants. However, graphene concentration showed a clear upward trend with increasing graphite concentration. These results indicate that the S-GQDs are able to produce a more stable graphene dispersion in comparison with C-GQDs and N-GQDs. To understand the mechanism behind the different exfoliation efficiencies, we studied the colloidal stability of exfoliation products by measuring ζ potential values of exfoliation products. Generally, a suspension of nanoparticles exhibiting a higher absolute value of ζ potential would be more colloidally stable than suspensions with low ζ potential absolute values.44 As shown in Figure 4b, it is obvious that the ζ potential absolute values of exfoliation products by C-GQDs and N-GQDs are relatively low (60 mV). The relatively low ζ potential values explain poor stability of graphene produced by C-GQDs and N-GQDs, resulting in extremely low concentrations of graphene after centrifugation. This trend is consistent with prior studies, where pyrenes functionalized with either an amino group or a carboxylic group produce graphene dispersions with lower ζ potential values than those with a sulfonyl group.7,12 It is worth noting that N-GQDs-stabilized graphene shows a positive potential value, likely originating from the amino functional groups of N-GQDs. These results suggest that S-GQDsstabilized graphene nanosheets show excellent colloidal stability, making them suitable for the application of enhancing photonic devices’ performance. 2.4. Optimizing Structural Colors with Graphene. Structural colors obtained from ordered photonic crystals have attracted interest for applications such as paints,45

sediment, and the supernatant was extracted. In the extracted supernatant, the graphene nanosheet concentration was calculated to be 0.16 mg·mL−1 using UV−vis spectroscopy (see the Experimental Section section for more details). This graphene concentration of 0.16 mg·mL−1 is comparable to that reported for graphene sheets prepared by direct exfoliation with surfactants, polymers, and polyaromatic hydrocarbons at the similar concentrations of stabilizers.6,7,43 Such dispersions showed colloidal stability without any noticeable sedimentation and aggregation over a period of 30 days. The zeta potential (ζ) of this graphene dispersion was measured to be about −64.3 mV, indicating excellent colloidal stability. TEM was used to verify the presence of single- to few-layered graphene nanosheets in aqueous dispersions. As shown in Figure 3b, the S-GQDs-stabilized graphene nanosheets were estimated to be 2- to 5-layers thick by counting the layers at the graphene edges in TEM, which is common for sonicated graphene samples.7 Atomic force microscopy (AFM) was performed on these S-GQDs-stabilized graphene nanosheets (Figure S5). AFM height profiles were consistent with graphene nanosheets of 2−3 layers; these profiles also indicate the presence of the S-GQD dispersant. However, for starting material graphite, the SEM image shows a large lateral size around 5−20 μm with a thickness of 0.5−1 μm (Figure 3c), indicating that the exfoliated graphene sheets have much smaller lateral size with reduced thickness. Our prior work has shown that functional groups can influence the exfoliation efficiency of pyrene derivatives.7 It was shown that the pyrene molecules functionalized by sulfonyl groups (1-pyrenesulfonic acid sodium salt) yielded much higher graphene concentration (∼0.8 mg/mL) in comparison with carboxylic functionalized pyrene (1-pyrenecarboxylic acid) and amine functionalized pyrene (1-pyrenemethylamine hydrochloride), which yield graphene dispersions of about 0.4 and 0.1 mg/mL, respectively. This difference in dispersant performance among pyrene derivatives was shown to originate from the variation in their tendency for accepting π (pi) electrons from the sp2 carbon of graphene. It was found that, as the electronegativity of the functional groups increases, π electron density in the aromatic rings of pyrene derivative decreases. This in turn will increase pyrene derivative’s tendency of accepting π electrons from the sp2 carbon of graphene. In the current paper, we analyzed whether a similar functionalization-performance mechanism could also be applied for GQDs as a dispersant. Graphene quantum dots with amine functional groups (N-GQDs) and carboxylic acid groups (CGQDs) were synthesized and studied. As shown in Figure 4a, N-GQDs were first examined at a graphite concentration of 0.5 30800

DOI: 10.1021/acsami.7b06980 ACS Appl. Mater. Interfaces 2017, 9, 30797−30804

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Figure 5. Coloring of PS nanoparticles with graphene. (a) Photographs of PS nanoparticles (192, 224, 274 nm) at various graphene concentrations. Concentrations of graphene additive increases from left to right ((i) Pristine PS; (ii) 0.008 wt %; (iii) 0.012 wt %, (iv) 0.016 wt %). (b) SEM image of PS nanoparticles (224 nm). (c) SEM image of PS nanoparticles (224 nm) with addition of graphene. (d) Normalized UV−vis reflectance spectra of PS-graphene (224 nm) showing increase in reflected light intensity with graphene concentration.

displays,46,47 and sensors.48−50 Structural colors of photonic crystals are typically pale due to strong incoherent light scattering. Black additives have been incorporated in photonic crystals for absorbing the incoherently scattered light and improving the contrast.51 Insects such as Morpho butterflies use black melanin deposited below the multilayered ridges on the wings to efficiently absorb transmitted light, resulting in a desaturating background signal and enhanced color visibility.52 Synthetic black materials, such as carbon black,53 Fe3O4 nanoparticles,54 and broad-absorption polymers,51,55 have also been incorporated into photonic crystals to absorb light across the entire visible region. However, despite the great potential of graphene as a black additive, graphene platelets have seldom been used for enhancing optical performance of photonic crystals, likely due to the poor colloidal stability in aqueous dispersion. To the best of our knowledge, this is the first demonstration of the color enhancement of colloidal photonic crystals by adding graphene nanosheets. The photonic crystals were fabricated by drop-casting a mixture of polystyrene (PS) nanoparticles and S-GQDsstabilized graphene dispersion. Figure 5a shows the iridescent PS nanoparticles with the addition of different concentrations of graphene. With increasing graphene concentration, the visual appearance of PS photonic crystals changes from comparatively faint colors to brilliant structural colors. Additionally, it is interesting that the color enhancement by graphene additives can apply to various sizes of PS nanoparticles (d = 192, 224, 274 nm), leading to a variety of structural colors. As shown in Figure 5b, the SEM image of pure polystyrene crystals reveals a close-packed face-centered cubic structure (FCC(111)), which is similar to previous reports of PS crystals.46,51 Figure 5c presents the SEM image of PS with graphene additives, which shows an almost identical FCC structure to that of PS; this suggests that adding graphene into PS does not introduce significant change in the lattice structure of the photonic crystal. UV−vis reflective spectra allowed quantitative evaluation of the optical changes induced by introducing graphene, as shown in Figure 5d. By normalizing the background of UV−vis spectra, a dramatic increase (∼12 times) in color contrast is evident after introducing 0.016 wt % graphene into PS (further discussion in the Supporting Information). It is interesting to see that the UV−vis peak positions remain almost unchanged at 528 nm, further confirming that the lattice structure of PS is not

changed by the addition of graphene platelets. These results suggest great potential of graphene for enhancing the color contrast by suppressing incoherent diffused scattering. The demonstrated strategy of incorporating graphene into the photonic crystal lattice offers a new path to prepare highcontrast films for a wide range of applications, including displays, pigments, and sensing.

3. CONCLUSION In summary, we showed that π−π stacking interactions with graphene are not limited to pyrene derivatives and perylenediimide,56 but are also applicable to larger polyaromatic structures such as graphene quantum dots. The exfoliation efficiencies of GQDs with different functional groups were evaluated, demonstrating that only strong electron-withdrawing groups allow for effective stabilization of graphene dispersions. The mechanism could be explained by the polarization of the aromatic core of graphene quantum dots that facilitates πelectron sharing between graphene and GQDs. Finally, the asprepared graphene dispersion demonstrated excellent stability and was used for improving the visual appearance of photonic crystals even at minimal graphene concentration. 4. EXPERIMENTAL SECTION 4.1. Materials. Citric acid monohydrate (99.5%) and 4styrenesulfonic acid sodium salt (97%) were purchased from SigmaAldrich, USA. Expanded graphite was obtained from Asbury Carbons (CAS no. 7782-42-5, grade 3805). All chemicals and solvents were used as received without further purification unless otherwise stated. 4.2. Synthesis of S-GQDs, N-GQDs, and C-GQDs. S-GQDs were fabricated by direct condensation and pyrolysis of citric acid and 4-styrenesulfonic acid sodium salt. In a typical procedure, 1.4 g of citric acid and 0.6 g of 4-styrenesulfonic acid sodium salt were mixed by a homogenizer. The white solid mixture was then transferred into a 20 mL glass vial and calcinated in air at 200 °C for 80 min. The dark solid residue was further purified by a dialysis tube (2000 Da) to remove possible unreacted starting materials. The final product S-GQDs were further dried by a freeze-drying machine. GQDs with amine functional groups (N-GQDs) and carboxylic acid groups (C-GQDs) were prepared based on previous works (see the Supporting Information, Figure S4).27,57 4.3. Preparing Aqueous Dispersions of Graphene Nanosheets. Functionalized graphene quantum dots were utilized as dispersants. Specifically, 100 mg of dispersant was stirred into 50 mL of DI water at room temperature to obtain a uniform colloidal 30801

DOI: 10.1021/acsami.7b06980 ACS Appl. Mater. Interfaces 2017, 9, 30797−30804

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solution. We then introduced expanded graphite into the above solution, followed by 1 h of tip sonication (output wattage ∼ 45 W) using a Qsonica sonicator (Q-700). To prevent the temperature of the sonication slurry from rising during exfoliation, we employed a water bath at ambient temperature. As-prepared sonicated slurry was allowed to settle for a day, and then centrifuged (Sorvall ST-16, Fisher Scientific) at 500−3300 rpm for 30−240 min to remove larger aggregates, and the supernatant was collected. This stable dispersion was used for further characterizations. 4.4. Fabrication of Graphene-Incorporated Polystyrene. Monodispersed polystyrene nanoparticles with different sizes (Figure S6) were first synthesized by emulsion polymerization (for details, see the Supporting Information). Then, graphene-incorporated PS photonic crystals were fabricated through mechanical mixing, followed by a drop-casting strategy. Specifically, 200 μL of graphene aqueous dispersions with various concentrations (0.008, 0.012, 0.016 wt %) was incorporated into the prepared polystyrene latex dispersions (400 μL, 5.0 wt %) of each of three sizes. After homogenizing in a vortex mixer and ultrasonicating, uniform PS-graphene dispersions were acquired (Figure S7). A 50 μL aliquot of the composite dispersion was spread onto a substrate using drop-casting, followed by drying at 50 °C. UV− vis reflectance spectra of graphene-incorporated polystyrene crystals were collected and were compared with calculations (Figure S8). 4.5. Characterization. X-ray photoelectron spectroscopy (XPS) was performed on an Omicron DAR 400, and Mg Kα radiation was used as the excitation source. Surface functional groups of GQDs were evaluated by using Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet 380 FTIR spectrometer). TEM images were taken by a high-resolution transmission electron microscopy (HR-TEM, JEM 2010, JEOL). The concentrations of dispersions were measured using the Beer−Lambert law which states that the concentration of a dispersion is proportional to its absorbance. A UV−vis spectrophotometer (Shimadzu 2550) was used to perform UV−vis spectroscopy on aqueous dispersions for measuring their absorbance. Dispersant solution was used as a reference sample while measuring the absorbance of graphene dispersions to eliminate the contribution of stabilizer solution to the absorbance. To calculate the extinction coefficient of the graphene dispersion in water, a concentration versus absorbance calibration was performed at a wavelength of 660 nm. For calibration, the unknown concentrations of graphene dispersions were measured using a vacuum filtration assembly with a hydrophilic polytetrafluoroethylene filter paper (20 nm pore size). The differences in mass of the filter papers before and after filtration were used to calculate the nanosheet concentrations. A ZetaSizer ZS90 (Malvern Instruments Limited) was used to conduct measurements of ζ potential at room temperature. Scanning electron microscopy (SEM) micrographs of expanded graphite, polystyrene, and grapheneincorporated polystyrene nanospheres were collected by using a scanning electron microscope (JEOL JSM-7500F, Japan). For SEM characterization, nonconducting samples were sputter-coated with platinum/palladium (4:1) layer of 5 nm in thickness to prevent charging.

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Micah J. Green: 0000-0001-5691-0861 Zhengdong Cheng: 0000-0003-0929-4749 Author Contributions §

These authors have contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

The authors are thankful for the financial support from the NASA (NASA-NNX13AQ60G), the DuPont Young Faculty Award, and by the U.S. National Science Foundation (NSF) under CAREER Award No. CMMI-1253085. We would also like to thank Dr. Mustafa Akbulut’s group for assistance in ζ potential measurement. The authors acknowledge use of the Microscopy and Imaging Center (MIC) and Materials Characterization Facility (MCF) at TAMU. We thank Dr. Wilson Serem for his assistance with AFM characterization.

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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/acsami.7b06980. Additional experimental procedures, material characterization methods, and further discussion on photonic crystals (PDF)

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Corresponding Authors

*E-mail: [email protected] (M.J.G.). *E-mail: [email protected]. Tel.: 979.845.3413. Fax: 979.845.6446 (Z.C.). 30802

DOI: 10.1021/acsami.7b06980 ACS Appl. Mater. Interfaces 2017, 9, 30797−30804

Research Article

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DOI: 10.1021/acsami.7b06980 ACS Appl. Mater. Interfaces 2017, 9, 30797−30804