Development of Tailored Graphene Nanoparticles: Preparation, Sorting and Structure Assessment by Complementary Techniques

Abstract

We present a thorough structural characterization of Graphene Nano Particles (GNPs) prepared by means of physical procedures, i.e., ball milling and ultra-sonication of high-purity synthetic graphite. UV-vis absorption/extinction spectroscopy, Dynamic Light Scattering, Transmission Electron Microscopy, IR and Raman spectroscopies were performed. Particles with small size were obtained, with an average lateral size <L> = 70−120 nm, formed by few <N> = 1−10 stacked layers, and with a small number of carboxylic groups on the edges. GNPs relatively more functionalized were separated by centrifugation, which formed stable water dispersions without the need for any surfactant. A critical reading and unified interpretation of a wide set of spectroscopic data was provided, which demonstrated the potential of Specular Reflectance Infrared Spectroscopy for the diagnosis and quantification of chemical functionalization of GNPs. Raman parameters commonly adopted for the characterization of graphitic materials do not always follow a monotonic trend, e.g., with the particle size and shape, thus unveiling some limitations of the available spectroscopic metrics. This issue was overcome thanks to a comparative spectra analysis, including spectra deconvolution by means of curve fitting procedures, experiments on reference materials and the exploitation of complementary characterization techniques.

Keywords: Raman spectroscopy; chemical functionalization; drug delivery platform; graphene-nanoparticles; infrared spectroscopy; transmission electron microscopy.

Figure 1

GNP samples, at the different steps of production: (A) Nanoparticles samples after milling, sonication, and first centrifugation steps (First Top and First Bottom). (B) Nanoparticles after second centrifugation (TOP60 and BOTTOM60). BOTTOM60 and TOP60 dispersion are stable in water.

Figure 2

Dynamic light scattering (DLS) diameter (d) distribution curve of TOP60 and BOTTOM60.

Figure 3

Representative transmission electron microscopy images of (A) TOP60 and (B) BOTTOM60 GNPs. Both nanoparticles show a similar irregular morphology while exhibiting a different lateral size. TOP60 GNPs have an average lateral dimension of 70 nm, while BOTTOM60 GNPs of 120 nm. Furthermore, BOTTOM60 nanoparticles exhibit a pronounced tendency to entangle with each other in a random manner, producing multilayer aggregates more frequently than TOP60 ones.

Figure 4

UV-vis normalized extinction/absorption spectra of the GNPs samples of TOP60, BOTTOM60, First BOTTOM and Graphene Oxide (GO).

Figure 5

Comparison among normalized UV-vis absorption/extinction spectra of water dispersions containing commercial graphene nano-sheets (3L/SC, 5–7L/SC) and TOP60, BOTTOM60 GNPs.

Figure 6

Normalized FTIR spectra of TOP60 and BOTTOM60 obtained in double transmission: (a) Spectra, as recorded; and (b) Spectra after baseline correction.

Figure 7

FTIR spectra of HOPG, ROD, GO, and GNPs samples at different preparation steps: (a) Specular reflection (SR) spectra; (b) absorption spectra from SR after Kramers–Kronig transformation; and (c) absorption spectra after baseline correction in the region 1950–750 cm⁻¹.

Figure 8

Result of the curve fitting (deconvolution) of the Raman spectra (λ_exc = 633 nm) of ROD(i) sample and of TOP60 sample.

Figure 9

Raman (λ_exc = 633 nm) spectra of the pristine synthetic graphite sample (ROD) and of the commercial 3L GNPs (3L/P: the pristine sample; 3L/S: the sample after sonication in water; 3L/SC: the sample after sonication and centrifugation).

Figure 10

Raman spectra (λ_exc = 633 nm) of GNP samples at different preparation steps.

Figure 11

From the top to the bottom: Raman spectra of 3L/SC, TOP60, and subtraction spectra (3L/SC—TOP60) and (TOP60—3L/SC). Raman spectra were recorded with λ_exc = 633 nm, with 0.5 mW laser power.

Figure 12

Multiwavelength Raman spectra (λ_exc = 633, 532, 405 nm) of: TOP60, BOTTOM60, 3L/SC, ROD, HOPG samples.

Figure 13

Raman spectra of the GNPs samples of TOP60 and BOTTOM60 excited by blue (λ_exc = 405 nm) laser at different power values at the same point.

Scheme 1

Diagram of the GNPs preparation process.