Electrochemical Method To Prepare Graphene Quantum Dots and Graphene Oxide Quantum Dots

Abstract

In this study, we present the preparation of graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs). GQDs/GOQDs are prepared by an easy electrochemical exfoliation method, in which two graphite rods are used as electrodes. The electrolyte used is a combination of citric acid and alkali hydroxide in water. Four types of quantum dots, GQD1-GQD4, are prepared by varying alkali hydroxide concentration in the electrolyte, while keeping the citric acid concentration fixed. Variation of alkali hydroxide concentration in the electrolyte results in the production of GOQDs. Balanced reaction of citric acid and alkali hydroxide results in the production of GQDs (GQD3). However, three variations in alkali hydroxide concentration result in GOQDs (GQD1, GQD2, and GQD4). GOQDs show tunable oxygen functional groups, which are confirmed by X-ray photoelectron spectroscopy. GQDs/GOQDs show absorption in the UV region and show excitation-dependent photoluminescence behavior. The obtained average size is 2-3 nm, as revealed by transmission electron microscopy. X-ray diffraction peak at around 10° and broad D band peak at 1350 cm^-1 in Raman spectra confirm the presence of oxygen-rich functional groups on the surface of GOQDs. These GQDs and GOQDs show blue to green luminescence under 365 nm UV irradiation.

Figure 1

Schematic illustration of electrochemical exfoliation of defect-induced graphite rod. Intercalation of OH^– ions, O₂ production, and exfoliation process result in the production of graphene quantum dots.

Figure 2

Images of GQD1–GQD4 before (a–d) and after (e–h) electrochemical exfoliation and daylight/under 365 nm UV irradiation images for GQD1–GQD4 (i–l).

Figure 3

TEM images showing size distribution of GQD1–GQD4 (a–d) (scale bar, 10 nm). The inset images show high-resolution images of GQD1–GQD4 (scale bar, 2 nm) and the corresponding size distribution curves.

Figure 4

XPS survey peaks for GQD1–GQD4 (a, c, e, g). High-resolution C_1s spectra peaks for GQD1–GQD4 (b, d, f, h).

Figure 5

X-ray diffraction patterns of GQD1–GQD4 (a–d).

Figure 6

Raman spectra of GQD1–GQD4 (a–d) measured with 514 nm argon laser at 10 mW power. D band and G band are observed at ca. 1350 and 1590 cm^–1, respectively.

Figure 7

UV–vis spectra (black line) and PL excitation spectra (red line) for GQD1–GQD4 (a–d). The inset images are daylight images and under 365 nm UV irradiation images.

Figure 8

Excitation-dependent PL behavior of GQD1–GQD4 (a–d). The insets show excitation wavelengths and their corresponding emission wavelengths.

Figure 9

Fluorescence decay curves for GQD1–GQD4 (a–d) at 450 and 480 nm measured by TCSPC, excited at 375 nm. The red line shows decay curve, and the blue line shows fitted curve.

Figure 10

Schematic representation of the proposed energy levels of GQD1–GQD4. The intrinsic state depends on size. The surface states determine the optical properties.