Films of Graphene Nanomaterials Formed by Ultrasonic Spraying of Their Stable Suspensions

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Films of Graphene Nanomaterials Formed by Ultrasonic Spraying of Their Stable Suspensions

Luis B. Modesto-Lopez1,*,§, Mirella Miettinen1, Joakim Riikonen2, Tiina Torvela1, Carsten Pfüller1, Vesa-Pekka Lehto2, Anna Lähde1, and Jorma Jokiniemi1,3

1 Fine Particle and Aerosol Technology Laboratory, Department of Environmental Science, University of Eastern Finland, Kuopio, Finland. 2 Department of Applied Physics, University of Eastern Finland, Kuopio, Finland. 3 VTT Technical Research Centre of Finland, P.O. Box 1000, 02044 VTT, Finland

* Corresponding author, , tel: +358 (50) 369-6419

§ Current affiliation: Department of Aerospace Engineering and Fluid Mechanics, University of Seville, Spain. E-mail:

S1. Separation and dispersion of graphene nanomaterials in organic solvent

The general procedure for suspension preparation is summarized in Figure S1a. The graphene nanomaterial powder was suspended in ethanol, DMF, or NMP and dispersed with a bath ultrasonicator (VWR International, Model USC1200D, frequency output of 45 kHz/180 W) operated at maximum power for 3 h. Subsequently, the suspension was centrifuged at 1000 rpm for 30 min to remove large agglomerates and silicon carbide (SiC) crystals formed during synthesis in the induction furnace (Miettinenet al., 2014).

Figure S1. a) Ultrasonication (US) and centrifugation (CF) procedure to separate the graphene nanopowder into graphene nanoflowers (GNFs) and multi-layer graphene (MLG) flakes. b) TEM image of GNFs. Inset shows several GNFs, scale bar: 200 nm. c) TEM image of a MLG flake with GNF agglomerates. The inset shows a GNF connected to a corner of a MLG flake; inset scale bar: 50 nm.

The supernatant was removed by pipetting and the sediment was stored. Then, the supernatant was further centrifuged at 6000 rpm during 1 h to obtain a second supernatant consisting mainly of graphene nanoflowers (GNFs). Figure S1b depicts a TEM image of characteristic GNFs and the inset (right top) shows an image of a dried GNF suspension aliquot. Note also their relatively high size monodispersity after centrifugation. The mean diameter of the GNFs from quantitative image analyses of TEM micrographs is 48 nm ±9 nm. The corresponding sediment was re-suspended by adding fresh solvent followed by 10-min bath ultrasonication at maximum power. The suspension was then centrifuged at 6000 rpm for 30 min. Again, the supernatant was pipetted out and stored and its sediment, rich in MLG flakes, was re-suspended in fresh solvent by 10-min ultrasonication at maximum power to yield a MLG suspension. Figure 1c shows a TEM image of a MLG flake with individual nanoflowers and their agglomerates connected to it. Conversely to the GNF suspension, MLG flake suspensions consisted of a mixture of MLG and large agglomerates of GNFs that were attached on the surface of flakes, in some cases, entirely coating them. Most likely, covalent bonding formed during induction annealing and/or strong interparticle interactions between nanoflowers and flakes prevented their deagglomeration.

S2. Absorption coefficient of graphene nanomaterials

The value of 1488 L g-1 m-1 in DMF was obtained by measuring the absorbance as a function of concentration in the linear absorption regime (which follows the Lambert-Beer law). Samples were bath ultrasonicated for 10 min at maximum power before each measurement to ensure dispersability. The figure is within the range of reported values for SLG, although, literature data scatter from 931.9 L g-1 m-1 for exfoliated graphene in DMF (Quintana et al., 2013) up to 3620 L g-1 m-1 for exfoliated graphene in NMP (Khan et al., 2010). The value of 2460 L g-1 m-1 has also been reported by recording measurements in four organic solvents (Hernandez et al., 2008).

S3. Temperature distribution of experimental setup

Figure S2. Temperature of the hot plate surface and its surroundings.

The temperature of the hot plate surface and during spraying of pure ethanol (same spraying conditions as film deposition) was measured with an infrared camera (FLIR Systems, model FLIR I3). In the figure, the white dahsed line was drawn on to guide the eye.

S4. Probe Ultrasonication

Significant sedimentation was observed after a few hours of ultrasonication with the probe. The ultrasonic probe can deliver approximately 100 times higher ultrasonication intensity than the ultrasonic bath, and it was hypothesized that such high power may break up the flakes. However, as shown in Figure S3 a, the main peak at 90 nm shifts to 112 nm after 7-h probe ultrasonication and its intensity decreases by 18 % compared to the as-prepared condition. A secondary small peak appears at about 5 μm, although the origin of this peak is uncertain, it may arise from dust particles or air bubbles in the dispersion (Lotya et al., 2013). The polydispersity of the 90-nm peak also increases gradually after each ultrasonication period with an increase of standard deviation from 44 nm at 0 h to 73 nm at 7 h. Significant sedimentation was observed after a few hours of ultrasonication with the probe. Examination of dried samples of suspensions with SEM before (Figures S3 b-e) and after 7-h (Figures S3 f-j) probe ultrasonication did not show any significant size variation. Changes in flake morphology, if any, are subtle and their identification by visual inspection results complex, partially because of the flake shape. Note that in preliminary dispersion studies, graphene nanomaterial suspensions in ethanol, which initially were stable, sedimented after being probe ultrasonicated for 1 h. Although, the determinant factor is unclear, it seems as though the high-power ultrasonication modifies the colloidal stability of the dispersions.

Figure S3. a) Particle size distribution of a 0.005 wt% MLG suspension in DMF as a function of probe ultrasonication time. TEM images of MLG flakes before (b - e) and after 7-h (f - j) probe ultrasonication; scale bar inidicates 500 nm in all images. k) Absorption spectra of GNFs and MLG in DMF:ethanol and NMP:ethanol.

S5. Substrate treatment for film deposition

Prior to spraying experiments, glass substrates were treated with a piranha solution (H2SO4:H2O2 1:1 v/v) to clean their surfaces and give them hydrophilic character. Subsequently, substrates were rinsed with DI water and dried with compressed air.

To prepare MA-glass substrates the following procedure was used: after treatment with the piranha solution, glass slides were immersed in a methacrylic acid - methyl methacrylate (MA) copolymer (1:1) solution in ethanol (20 mg/mL), for less than a minute, and then dried with compressed air. After drying, a rough polymer coating, which was optically transparent to the naked eye, formed on the slides. The coating aimed at improving the adhesion of graphene nanomaterials with the substrate.

S6. Characterization of suspensions and films

Transmission electron microscopy images were obtained with a field emission transmission electron microscope (TEM, JEOL, 2100F) operated at an acceleration voltage of 200 kV. The morphology of nanomaterials and their films was investigated with a ZEISS scanning electron microscope (SEM, SIGMA HDVP), operated at an acceleration voltage of 3 kV and at a typical working distance of 3.5 mm. In some cases, an Au coating of ~ 15 nm was sputtered on the films to obtain highly resolved images with the SEM. Samples for TEM analyses were collected on copper grid-supported holey carbon films (S147-4, Agar Scientific) by dipping the grid into an aliquot of suspension followed by drying it at ambient conditions.

The percentage of substrate surface covered by the films was estimated from analyses of SEM images with the freeware image analysis software ImageJ (National Institute of Health, US). First, the contrast/brightness of the images was adjusted and then a threshold mask was applied to transform them into black and white images. The surface coverage percentage was obtained by counting the number of pixels in the threshold range divided by the total number of pixels.Optical microscopy images of the films were acquired with a ZEISS Axio Imager microscope using 10x and 50x magnification objectives.

The particle size and zeta potential of graphene nanomaterials in liquids was measured by dynamic light scattering (DLS) and electrophoretic light scattering (ELS), respectively (Malvern Instruments, Zetasizer Nano ZS) using a 1-cm path length quartz cuvette for DLS and disposable folded capillary cells for ELS measurements. During zeta potential measurements the instrument also records the electrical conductivity of the liquid. The ELS cell is made of polycarbonate and has limited resistance to high concentrations of DMF and NMP. Based on manufacturer's recommendation, nanomaterial suspensions were substantially diluted in ethanol (at a ratio of 1:19 v/v, dilution in ethanol is also advantageous for spraying). No degradation of the ELS cell was observed during the entire measurement period. Two sets of 5 measurements were performed on each sample and the average value is reported (a total of 10 measurements/sample). Between each set of measurements the cell was washed vigorously with DI water and DI water/ethanol mixtures. The DLS instrument uses a 633-nm He-Ne laser and was operated in backscatter mode at an angle of 173 °. Samples were equilibrated at 25 °C for 120 s. An automatic measurement positioning and attenuation was used to minimize multiple light scattering by highly concentrated samples since the sample is probed near the cuvette wall. Note that the calculation of particle size distribution assumes a spherical shape of particles and does not account for irregular shaped particles. It, however, provides a comparison between similar samples.

The light absorption patterns of suspensions were recorded with a Perkin-Elmer UV-vis spectrophotometer (Lambda 25 UV/vis System), using a quartz cuvette with a path length of 1 cm, in the 200 nm - 1000 nm spectral range. The instrument has a resolution of 1 nm.

S7. Graphene nanoflower film on a MA-glass

Figure S4. Optical image of a typical GNF film on MA-coated glass, ts = 14 min.

S8. Solubility parameters of graphene and solvents used

The long-term stability of MLG flakes in NMP or DMF compared to ethanol may be associated with the Hansen and Hildebrand solubility parameters of the solvents and graphene (O’Neill et al., 2011; see Table S1). The solubility parameters assess the capacity of a liquid to exfoliate graphene layers (typically from graphite). Stable dispersions with relatively high concentrations can be achieved if the parameters of the solvent match those of graphene. The data in Table S1 indicate that NMP is the most suitable solvent, among the ones used in this work, to exfoliate and disperse graphene. Our observations suggest that such principle is still valid for the new type of graphene nanomaterials

Table S1. Hansen and Hildebrand of solubility parameters and boiling points of solvents used in this work. The bracketed figures are the approximate range of acceptable values for graphene exfoliating solvents. *1Lide (2012) and *2O’Neill et al. (2011). Viscosity values are at room temperature.

S9. Raman spectra of the films

The Raman fingerprint of the nanomaterials was acquired with a BrukerSenterra 200LX micro-Raman setup equipped with a 532-nm wavelength laser. An output power of 20 mW was focused by a 100x microscope objective to a spot with a diameter of ~ 1 μm. The setup provides a spectral resolution of about 3 - 5 cm-1. The laser power at the sample surface was reduced by neutral density filters such that no heating effects are observed in the Raman spectra. The total integration times varied from 30 s to 15 min. No physical degradation from the laser was observed by reflected-light optical microscopy. The observed spectra were deconvolved using Lorentzian band shapes by a peak analyzing software (Origin).

Figure S5. Raman spectra of MLG flakes and GNFs sprayed onto glass substrates

The Raman spectra show the characteristic Raman bands of graphene at 1351 cm-1 (D band), 1584 cm-1 (G band), 1620 cm-1 (shoulder, D’ band), and 2697 cm-1 (2D band) and agree with reported data (Ferrari, 2007). The D band is typically associated with structural disorders in graphene layers. The data suggests that a smaller number of structural defects are present in MLG films compared to GNF films. However, the laser spot is about 1µm in diameter (or ~ 0.8 µm2), which most probably would include contribution from GNFs present in the films as shown in Figures 1c, 7, and 8. Furthermore, the width of the 2D band is generally interpreted as a fingerprint of the number of graphene layers. In our samples, the 2D band shows a full-width at height maximum (FWHM) of ~ 59 cm-1 for the GNFs and ~ 50 cm-1 for the MLG, and it is fitted with a single Lorentzian line. The Raman spectra of the GNF and MLG films show the characteristic features of so-called turbostratic graphene, that is a single Lorentzian 2D band as in SLG but with a broader FWHM. In turbostratic MLG the stacking of the graphene layers is rotationally random with respect to one another along the c axis (Malard et al., 2009). Hass et al. (2008) reported that MLG grown on the carbon-terminated face of SiC films does not grow as a simple AB stacked graphite film, but rather, graphene grows with highly dense rotational faults where adjacent sheets are rotated relative to each other. They also showed that these faults decouple adjacent graphene sheets, thus making the MLG behave like SLG because their band structure is nearly identical to isolated graphene (Hass et al., 2008). Note that the graphene nanomaterial powder used in this work was synthesized from Si-C nanoparticles in which there is an excess of carbon (Miettinen et al., 2011, 2014). Although, the graphene growth mechanism differs from that of Hass et al. (2008) similarities in the Raman spectra exist.

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