A Low-Cost Non-explosive Synthesis of Graphene Oxide for Scalable Applications

Abstract

A low cost, non-explosive process for the synthesis of graphene oxide (GO) is demonstrated. Using suitable choice of reaction parameters including temperature and time, this recipe does not require expensive membranes for filtration of carbonaceous and metallic residues. A pre-cooling protocol is introduced to control the explosive nature of the highly exothermic reactions during the oxidation process. This alleviates the requirement for expensive membranes and completely eliminates the explosive nature of intermediate reaction steps when compared to existing methods. High quality of the synthesized GO is corroborated using a host of characterization techniques including X-ray diffraction, optical spectroscopy, X-ray photoemission spectroscopy and current-voltage characteristics. Simple reduction protocol using ultra-violet light is demonstrated for potential application in the area of photovoltaics. Using different reduction protocols together with the proposed inexpensive method, reduced GO samples with tunable conductance over a wide range of values is demonstrated. Density functional theory is employed to understand the structure of GO. We anticipate that this scalable approach will catalyze large scale applications of GO.

Figure 1

XRD spectra of (a) graphite (C) and potassium permanganate (KMnO₄) mixture after washing, (b) Samples in (a) mixed with sulphuric acid and hydrogen peroxide (without heating) after washing and with zero soaking time, (c) Sample in (b) with heating and washing, (d) ICDD card number of carbon, graphite and various Mn oxides.

Figure 2

(a) Raman spectra for the drop casted film of the as grown GO sample. The D and G peaks are marked. The slight hump in the wavenumber range 2550–2780 cm⁻¹ corresponds to the 2D peak. The inset in panel (a) shows the powder x-ray diffraction data for the dried as grown sample. It shows the characteristic peak corresponding to GO. The bottle marked P shows the dispersed GO sample whereas the bottle marked Q contains the reaction mixture just prior to adding H₂O₂ to terminate the reaction used in our process. (b) FTIR spectra for the dried as grown GO sample obtained in the transmittance mode. The bands and dips corresponding to various functional groups are marked. The inset in panel (b) shows the UV-Vis spectra for the as grown GO sample dispersed in de-ionised water. The region enclosed by the square (dotted lines) is characteristic feature corresponding to GO.

Figure 3

(a) SEM image of a spin coated GO film. (b) TEM image of a drop casted GO film on the TEM grid. The regions F and C are marked corresponding to the region with and without the wrinkles/folds. (c) TEM image of a portion of the drop casted film containing a fold/wrinkle. (d) SAED pattern obtained from a region containing wrinkles/folds (similar to F marked in panel (b)). (e) SAED pattern obtained from a region which does not contain any wrinkles/folds (similar to C marked in panel (b)).

Figure 4

(a) AFM image of a drop coated GO film. (b), (c) and (d) height profile data of scan2, scan1 and scan3 respectively in (a). (e) AFM image of drop casted GO film with corrugation and wrinkles. (f) Height profile of the GO film along the wrinkles obtained in (e).

Figure 5

(a) The UV exposure dependent current-voltage (I-V) characteristics of a GO film drop-casted on a glass substrate. These I-V characteristics were obtained after different durations of exposure (ranging from 0–2 hour) to a 4 Watt, UV lamp emitting at 365 nm. Inset: I-V characteristics of CVD grown graphene, (b) Current (I) as a function of UV exposure time keeping the voltage (V) across the GO film fixed at 1.0 Volt and 2.0 Volt, respectively. (c) C 1 s XPS obtained for the GO and the rGO (reduced via exposure to a 4 Watt, 365 nm UV lamp for 2 hours) samples and that of CVD grown graphene. The peaks corresponding to the various functional groups are marked. (d) The comparison of Raman spectra for the GO film and the rGO film obtained after a 2 hour exposure to a 4 Watt, 365 nm UV lamp and Raman spectra of 1 L, 2 L and multilayer CVD grown graphene. The D, G and 2D peaks are appropriately labeled.

Figure 6

(a) Intensity (I) versus binding energy (B. E.) in the full scan range for the XPS of the rGO film obtained after a 2 hour exposure to a 4 Watt, 365 nm UV lamp. (b) A multiple peak deconvolution of the GO C1s XPS data (corresponding to –C–C–, –C=C–, –C–O, –O–C–O–, –OH and –C=O respectively). The inset in panel (a) shows the XPS data corresponding to Mn 2p binding energy region for the rGO sample. (c) rGO C1s XPS data for the sample along with the deconvoluted peak structure corresponding to –C–C–, –C=C–, –C–O, –O–C–O–, and –C=O are marked.

Figure 7

(a) Schematic diagram summarizing the typical applications demonstrated using the as synthesized GO material (dried GO foam). The actual optical image of a piece of dried GO foam obtained after vacuum desiccation of the as grown GO sample is also shown within the schematic. The GO films deposited on AZO is used for studying diode like behavior while the photovoltaic property is explored in the GO-rGO bilayer. Ultra-violet, microwave, thermal and chemical reduction protocols are used for making rGO samples. (b) Current-Voltage (I-V) characteristics of the GO-AZO bilayer. (c) Current-Voltage (I-V) characteristics of the GO-rGO bilayer in dark and upon exposure to 1.5AM Global sunlight. (d) Current-Voltage (I-V) characteristics of drop-casted films of rGO samples made using different reduction techniques. For a comparison, the I-V characteristics of a drop-casted film of commercially available graphene nanoplatelets is also shown. (e) Raman spectra for graphene nanoplatelet sample along with rGO samples made using different protocols.

Figure 8

Optimized geometries for a 7 × 7 nanoflakes of graphene (top view (a); side view (b)), GO with comparable number of functional groups along the edges as well as in the basal plane (top view (c); side view (d)), GO with functional groups primarily in the basal plane and edge passivation using H (top view (e); side view (f)), rGO with a number of functional groups removed from the GO nanoflake with functional groups along the edges as well as in the basal plane (top view (g); side view (h)). Various distances are marked to provide an idea regarding the wrinkles.