Acid-less direct gram scale exfoliation of graphite to partially oxidized graphene

Abstract

All methods to synthesize graphene oxide on a large scale utilize a strongly acidic medium. Graphene oxide (GO) to reduced graphene oxide (rGO) conversion, i.e., obtaining widespread sp²-sp² hybridized carbon allotropes is a laborious and tedious process. Herein, we report an innovative one-step method, through which we fabricate 'partially oxidized graphene' on a gram scale directly from graphite powder, without using any acids. The emergence of the (002) XRD peak, band gap (E_g ~ 1.42 ± 0.01 eV), and carbon-to-oxygen (C/O) ratio of 3.67 validate the direct POG phase formation without any reduction step. Raman (I_D/I_G ~ 0.80 ± 0.06) and AFM studies reveal the less defective and bi- to few-layer character of POG sheets of thickness ca. 1.59 ± 0.14 nm. The measured conductivity of the POG material (9.22 ± 0.04 S/cm) is found to be higher than the conductivity of the rGO using the standard method (0.30 ± 0.03 S/cm). The zeta potential study suggests a good stability of the POG aqueous dispersion. Furthermore, the proposed method is efficient (2-3 h), safe, and renders a higher yield (77%) than many conventional methods (40-70%) for GO preparation. This offers a promising source for conductive electrodes, carbon paste, and more.

Keywords: Acid-less; Aqueous medium; Exfoliation; Graphene; Partially oxidized graphene.

Fig. 1

Schematic step-by-step view of the process for the exfoliation of POG material. NaNO₃ aqueous solution is prepared under constant magnetic stirring, step A—addition of graphite under ultrasonication, step B—addition of KMnO₄, step C—addition of DI water, step D—providing some heat of 70 °C, step E—supernatant collection, step F—centrifugation.

Fig. 2

(a) Digital photograph of POG material, rGO (standard reduction method), and GO (modified Hummers method) water dispersion; all pictures were taken on the same day. (b) zeta potential values for the fresh samples and after 3 months of storage time-duration. The POG material shows a good dispersion behavior, similar to GO in water and exhibits long-term stability than rGO and GO. Few additional digital photographs of the three suspensions are added in SI as Figure S1 (a, b).

Fig. 3

UV–visible spectra (a–c) and their corresponding Tauc plots (d–f) for the POG, rGO, and GO samples, respectively. The as-synthesized POG has a lower band gap (1.42 ± 0.01 eV) than the rGO (1.66 ± 0.03 eV). The GO has an optical band gap of 2.91 ± 0.02 eV.

Fig. 4

XRD patterns of graphite powder, POG, rGO, and GO. A minimal lower 2θ shift (from 26.33 to 21.79°) suggests the mild oxidation of the POG material.

Fig. 5

AFM images of the POG sample at a scan size of (a) 3 μm and (b) 1 μm. (c) and (d) are sectional or height profile analysis at different sites of the same bi/multilayered POG. The as-synthesized POG material contains bi to few-layer graphene sheets.

Fig. 6

TEM images (a–d) and SAED pattern (inset of d) of the POG material, illustrate proper exfoliation of graphite to single-few layers graphene with good crystallinity behavior.

Fig. 7

Raman spectra of POG, rGO, and GO samples. The lower I_D/I_G value of ~ 0.80 suggests that the POG material has lesser defects in the graphene lattice.

Fig. 8

FESEM images of the POG sample at different magnifications; illustrate the homogeneous wide area distribution with aggregation and crumpling character of the few layer POG flakes.

Fig. 9

High-resolution X-ray photoelectron spectra (a) C 1s, (b) O 1s of the POG sample. The sample exhibits higher carbon-to-oxygen (C/O) ratio (3.67), which indicates the generation of partially oxidized graphene.

Fig. 10

FTIR spectra of graphite powder, POG material, and GO. The weak absorption bands in the POG spectrum illustrate mild oxidation character of the POG material.

Fig. 11

Two-probe I-V measurements of POG, rGO, and GO drop casted thin films. The POG material has higher conductivity (9.22 ± 0.04 S/cm) than the rGO (0.30 ± 0.03 S/cm), while GO conductivity is found to be S/cm.