Distinct Chemical and Physical Properties of Janus Nanosheets Al C. de Leon,† Bradley J. Rodier,† Qinmo Luo,† Christina M. Hemmingsen,† Peiran Wei,† Kevin Abbasi,‡ Rigoberto Advincula,§ and Emily B. Pentzer*,† †
Department of Chemistry, ‡Swagelok Center for Surface Analysis of Materials, School of Engineering, and §Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States S Supporting Information *
ABSTRACT: Janus particles have recently garnered significant attention for their distinct properties compared to particles that are homogeneously functionalized. Moreover, high aspect ratio Janus particles that are rod-like or planar (i.e., nanosheets) are especially intriguing considering their interfacial properties as well as their ability to assemble into higher order and hybrid structures. To date, major challenges facing the exploration and utilization of 2D Janus particles are scalability of synthesis, characterization of tailored chemical functionalization, and ability to introduce a diverse set of functionalities. Herein, a facile method to access Janus 2D graphene oxide (GO) nanosheets by combining a Pickering-type emulsion and grafting-from polymerization via ATRP is reported. Janus GO nanosheets bearing PMMA on one face as well as the symmetrically functionalized analogue are prepared, and the chemical, thermal, structural, surface, and interfacial properties of these materials are characterized. Time-of-flight secondary ion mass spectrometry coupled with Langmuir−Blodgett films is shown to be an ideal route to conclusively establish asymmetric functionalization of 2D materials. This work not only provides a facile route for the preparation of Janus nanosheets but also demonstrates the direct visualization of polymer grown from the surface of GO. KEYWORDS: Janus particles, ToF-SIMS, asymmetric functionalization, grafting-from, graphene oxide
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reacting the exposed face with chlorine radicals.28 Transfer of these monofacially functionalized nanosheets using a PMMA film then allowed for modification of the second, previously protected face of the nanosheet with benzoyl peroxide radicals. A variety of other approaches have been used to isolate one face of a nanosheet while protecting the other, including wax particles and oil droplets. Moreover, various functionalization chemistries have been used, including organic reactions with small molecules and attachment of end-functionalized polymers.24,29−31 Challenges in studying Janus 2D materials include scalability of synthesis, characterization of the products, verification of the asymmetric functionalization, and ability to easily access different chemical functionalities on each face. In this paper, we report the preparation of Janus GO nanosheets using a grafting-from methodology and comprehensively characterize the chemical, morphological, and interfacial properties of the resulting material. GO nanosheets were assembled onto wax beads, then ATRP was used to graft poly(methyl methacrylate) (PMMA) selectively from the
anus particles are anisotropic materials composed of two halves of different structure, chemistry, or polarity.1−7 These particles have distinct properties compared to symmetrically functionalized analogues and therefore garner interest in applications as diverse as drug delivery, sensing, optics, actuators, and interfacial modification.8−13 While the most common Janus particles are spherical, one-dimensional (1D) Janus rods and two-dimensional (2D) Janus nanosheets are also possible, though typically more difficult to prepare and characterize.14−17 In specific, 2D Janus nanosheets have attracted much attention because of their high aspect ratio, large adsorption energy, and highly confined rotation at interfaces (e.g., fluid−fluid).18−20 Of the 2D nanosheets that can serve as substrates for Janus particles, graphene oxide (GO) is especially attractive due to its controllable aspect ratio, ability to be covalently modified, and multifunctional properties (antimicrobial, good gas barrier, and precursor to electrically and thermally conductive materials).21−25 Common approaches to prepare Janus nanosheets use an inert template to protect one face, thereby allowing chemical modification of only the exposed face of the nanosheet using traditional chemical reactions, plasma treatment, or sputtering.19,26,27 For example, Zhang et al. fabricated Janus graphene by protecting one face of the nanosheet with a silicon wafer and © 2017 American Chemical Society
Received: June 8, 2017 Accepted: July 11, 2017 Published: July 11, 2017 7485
DOI: 10.1021/acsnano.7b04020 ACS Nano 2017, 11, 7485−7493
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ACS Nano Scheme 1. Schematic Representation of the Preparation of PMMA-GO-PMMA and PMMA-GO-X
Figure 1. (A) TGA of PMMA-GO-PMMA and PMMA-GO-X. (B) XRD and (C) Raman spectra of GO, PMMA-GO-PMMA, and PMMA-GOX.
bromide (BIBB). PMMA was grown from both faces of this BIBB-GO-BIBB to yield PMMA-GO-PMMA, symmetrically functionalized, non-Janus GO.29,30 Alternatively, Janus PMMAGO-X was prepared by assembling GO on the surface of wax particles via a Pickering-type emulsion (Figure S1).19 These GO-covered wax particles (Figure S2) were dispersed in DMF, then the exposed face of GO was modified with BIBB to give BIBB-GO-X. Subsequent dissolution of the wax substrate and suspension of BIBB-GO-X in DMF then allowed for PMMA to be grown from only the face of the nanosheet modified with BIBB to give the asymmetrically functionalized, Janus PMMAGO-X. The presence of polymer in both PMMA-GO-PMMA and PMMA-GO-X was confirmed by Fourier transform infrared spectroscopy (FTIR, Figure S3) and X-ray photoelectron spectroscopy (XPS, Figure S4).32,33 Briefly, in the FTIR spectrum, an ester CO peak at 1725 cm−1 is observed for PMMA-GO-X and PMMA-GO-PMMA, but is absent for GO.34 Similarly, in the XPS spectrum of the materials, bromide (∼70 eV) and the CO of PMMA (∼289 eV) are only present in PMMA-GO-PMMA and PMMA-GO-X, and not in GO (the spectrum of BIBB-GO-BIBB shows the presence of Br as well).35 Moreover, PMMA-GO-X and PMMA-GO-PMMA can be suspended in good solvents for PMMA (Figure S5), unlike GO itself, and the products of control experiments performed without initiator or monomer (Table S1, Figure S6).36,37 Of note, the color change of the material from orange to brown-
exposed face (to give Janus PMMA-GO-X). These Janus nanosheets were then compared to the precursor GO as well as symmetrically functionalized GO (non-Janus PMMA-GOPMMA) using a number of techniques. We then overcame one of the greatest challenges in studying Janus GO by confirming the two faces of the nanosheet are distinct from each other, using Langmuir−Blodgett (LB) techniques and time-of-flight secondary ion mass spectrometry (ToF-SIMS). This work not only provides a facile methodology for preparing a number of Janus GO structures but also reports the direct chemical analysis of the two faces of Janus nanosheets, proving the asymmetric nature of functionalization and providing a foundation for researchers to prepare and characterize this class of materials.
RESULTS AND DISCUSSION To access both Janus and non-Janus 2D materials, GO nanosheets were modified with an initiator for atom-transfer radical polymerization (ATRP), and then polymer was grafted from the surface using methyl methacrylate as monomer (Scheme 1). PMMA-GO-PMMA was prepared by dispersing GO in DMF, sonication, and centrifugation; this ensured all nanosheets were exfoliated, high aspect ratio of the nanosheets was retained, and aggregated GO was removed. An ATRP initiator was then introduced onto both faces of GO by reacting the hydroxyl groups of the nanosheet with 2-bromoisobutyryl 7486
DOI: 10.1021/acsnano.7b04020 ACS Nano 2017, 11, 7485−7493
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Figure 2. Topography, phase, amplitude, and lateral force images of GO, PMMA-GO-PMMA, and PMMA-GO-X on mica.
shifted relative to GO by ∼10 cm−1, which could be attributed to noncovalent interactions between GO and PMMA.39 To evaluate the impact of functionalization on the morphological, viscoelastic, and adhesive properties of the nanosheets, different imaging modes of atomic force microscopy (AFM) were used to characterize the materials drop cast on mica (Figure 2). From topography images, both GO nanosheets and BIBB-GO-BIBB (Figure S10) are ∼1 nm thick, while symmetrically functionalized PMMA-GO-PMMA nanosheets are ∼20 nm thick.49 The thickness of GO nanosheets is consistent with previous reports of single sheets of exfoliated GO, and the thickness of PMMA-GO-PMMA indicates ∼10 nm of polymer is grafted from each face of the nanosheet, assuming the polymerization was controlled and the nanosheets were homogeneously functionalized with ATRP initiator. Gratuitously, the thickness of the asymmetrically functionalized Janus PMMA-GO-X is ∼10 nm, half the thickness of its symmetrically functionalized counterpart. This difference in nanosheet thickness supports that selective monofacial functionalization was achieved and that polymer was grafted from the nanosheet surface in a controlled fashion. Topography images of PMMA-GO-PMMA and PMMA-GO-X also show domain texture consistent with surface-grown PMMA brushes, suggesting that the PMMA face of the Janus nanosheet was exposed for characterization.50,51 Evaluation of phase and amplitude indicates differences in surface stiffness and adhesion/interaction between the AFM tip and surface of the samples.52−54 The weak contrast between GO and the mica substrate in the phase and amplitude images implies that these materials have comparable stiffness and that they interact similarly with the AFM tip. Conversely, phase and amplitude images of PMMA-GO-PMMA and PMMA-GO-X show distinct contrast between the nanosheet and substrate, indicating substantial differences in viscoelastic properties. Lateral force imaging was used to probe the interaction of the sample with the AFM tip by measuring the lateral deflection of the cantilever as it scans across the sample, thereby indicating adhesion between the tip and sample.55,56 For GO nanosheets,
black upon functionalization also suggests that GO nanosheets of PMMA-GO-PMMA and PMMA-GO-X are partially reduced compared to the starting material.38−40 Although this data supports the use of grafting-from polymerization to functionalize GO nanosheets, the morphological differences between the materials must be established using other techniques. In compliment to the chemical characterization of the materials by XPS and FTIR, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Raman spectroscopy can be used to evaluate thermal and structural characteristics. Thermograms of PMMA-GO-PMMA and PMMA-GO-X showed weight loss at 350 °C and similar thermal degradation profiles (Figure 1A), consistent with the presence of PMMA.41,42 As bulk PMMA degrades completely when heated above 500 °C, residual mass observed after heating the samples to this temperature can be attributed to GO nanosheet (Figure S8), and the higher amount of residual mass for PMMA-GO-X compared to PMMA-GO-PMMA implies that PMMA-GO-X has higher GO content (and less PMMA, as expected). Comparison of the XRD spectra of GO and the symmetrically and asymmetrically functionalized nanosheets reveals that GO nanosheets stack, as indicated by the peak centered at 10°, corresponding to 1.26 nm interlayer distance (Figure 1B).43,44 Both PMMA-GOPMMA and PMMA-GO-X show only a broad feature spanning from 10° to 27° which is typical for amorphous PMMA.45 The amorphous nature of PMMA was further confirmed by 2D XRD in which sharp crystalline peaks were absent for all samples (Figure S9). Raman spectra of GO, PMMA-GOPMMA, and PMMA-GO-X are shown Figure 1C and show typical D and G bands of GO nanosheets at ∼1350 cm−1 and ∼1590 cm−1, respectively.46,47 The relative disorder of the basal plane of the GO nanosheets can be evaluated by the intensity ratio of these two bands (Table S2); this ratio decreases upon conversion of GO to PMMA-GO-PMMA and PMMA-GO-X (ID/IG of 0.969 for GO, 0.839 for PMMA-GO-PMMA and 0.885 for PMMA-GO-X), indicating a slight reduction of the nanosheet upon functionalization.48 Furthermore, the position of the G band of PMMA-GO-PMMA and PMMA-GO-X is red7487
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Figure 3. Surface wettability of different phases of PMMA-GO-X and PMMA-GO-PMMA by measuring the water contact angle of samples deposited from LB films.
Figure 4. Secondary-ion images via ToF-SIMS of GO, PMMA-GO-PMMA, and each face of PMMA-GO-X.
hydrophilic (i.e., “X” = GO) face is expected to be exposed to water, and the more hydrophobic PMMA face is expected to be exposed to air; the symmetric nature of functionalization of PMMA-GO-PMMA ensures that polymer is exposed to both water and air. Assembly of the nanosheets at an air/water interface allows for isolation of each face of the nanosheet. If a sample is collected by LB technique (upstroke), then the airoriented surface is exposed for characterization, and if the Langmuir−Schaeffer technique is used, then the water-oriented surface is exposed for characterization.57,58 As expected, the water contact angle for the symmetrically functionalized PMMA-GO-PMMA was similar regardless of deposition technique, ∼62° (Figure 3A). However, the Janus nature of PMMA-GO-X led to distinctly different wettabilities for the two faces of the material (Figure 3B). The water-exposed face of
no significant contrast for lateral force was observed, again indicating similar properties of GO and the mica substrate. However, for PMMA-GO-PMMA and PMMA-GO-X, sharp contrast was seen between the substrate and nanosheets; moreover, similar interactions between the AFM tip and sample (i.e., PMMA) were observed for both materials. Taken together, these AFM images indicate the functionalization of GO in symmetric and asymmetric manners and that PMMA was grown from the surface of the nanosheets, further confirming the chemical characterization discussed above. Surface properties of both faces of PMMA-GO-X and PMMA-GO-PMMA were characterized by wettability measurements using LB films and water contact angle.57,58 First, each sample was deposited from a dilute chloroform solution onto a water surface. For the Janus PMMA-GO-X, the more 7488
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ACS Nano PMMA-GO-X had a contact angle of ∼16°, while the airexposed face had a contact angle of ∼61°. Not only does this data support the Janus nature of PMMA-GO-X, but it also supports that PMMA is grafted from one face of the nanosheet. Of note, the contact angle of the water-exposed face (i.e., GO) of PMMA-GO-X is consistent with partial reduction of the nanosheet, in agreement with color change and Raman and UV−vis spectroscopy (Figure S11) and in contrast to as prepared GO (water contact angle of ∼0°, Figure S12).59−61 Although contact angle measurements suggest that each face of PMMA-GO-X has a different chemical functionality, a more direct characterization method is required to understand the Janus nature of the nanosheets. One attractive technique to differentiate the chemical composition of materials is time-offlight secondary ion mass spectrometry (ToF-SIMS), a highly sensitive imaging technique that acquires the mass spectrum of the fragment from the topmost surface of a sample (
