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j.cplett.2017.10.047

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Accepted Manuscript
Research paper
New insight into electrooxidation of graphene into graphene quantum dots
Peihui Luo, Xiangfeng Guan, Yunlong Yu, Xiaoyan Li
PII:
DOI:
Reference:
S0009-2614(17)30983-1
https://doi.org/10.1016/j.cplett.2017.10.047
CPLETT 35196
To appear in:
Chemical Physics Letters
Received Date:
Accepted Date:
10 September 2017
24 October 2017
Please cite this article as: P. Luo, X. Guan, Y. Yu, X. Li, New insight into electrooxidation of graphene into graphene
quantum dots, Chemical Physics Letters (2017), doi: https://doi.org/10.1016/j.cplett.2017.10.047
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New insight into electrooxidation of graphene into graphene
quantum dots
Peihui Luo*, Xiangfeng Guan, Yunlong Yu, Xiaoyan Li
Organic Optoelectronics Engineering Research Center of Fujian’s Universities, College of Electronics
and Information Science, Fujian Jiangxia University, Fuzhou 350108, People’s Republic of China
*Corresponding Author. Tel: 86-591-23537557; Fax: 86-591-23531375; e-mail:
luopeihui1986@aliyun.com.
Abstract
A previous study reported that graphene quantum dots (GQDs) generated by electrooxidation of
graphene had a uniform size of 3−5 nm and exhibit
excitation wavelength-dependent
photoluminescence (PL). Here, larger GQDs with an average diameter of ca. 52 nm are obtained via
using similar method. And their PL show excitation wavelength-independent properties.
Keywords: Graphene; Graphene quantum dots; Electrooxidation
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1. Introduction
Graphene quantum dots (GQDs) [1,2], with excellent properties such as great biocompatibility and
discrete bandgap, are a promising candidate for applications in bioimaging [3−5], nanoelectronic
devices [6,7] and other applications. Usually, GQDs are single or few-layer graphene with tiny sizes of
only several nanometers. A variety of methods have been developed for preparing GQDs, especially
from graphene and graphene oxide, including hydrothermal cutting [8], chemical/electrochemical
oxidation [9,10] and electro-beam lithography [11], etc. Among them, electrooxidation of graphene is
believed to be a promising approach for environmentally friendly production of GQDs materials,
avoiding high temperatures and hazardous chemical reagents. Previously, Qu et al reported an
electrochimical avenue to GQDs with a uniform diameter of 3−5 nm [10]. Freestanding graphene film
composed of layer-by-layer graphene sheets was used as working electrode, and cutted into several
nanometers GQDs by electrochimical oxidation. The obtained GQDs exhibited obvious excitation
wavelength-dependent photoluminescence (PL). Because of excellent optical and electrical properties,
GQDs applied as electron-acceptor materials in polymer solar cell, significantly improved device
performance. Up to now, various methods and precursors have been used to produce GQDs. However,
large scale and high quality preparation of GQDs is still difficult. And PL mechanism of prepared
GQDs is also indistinct. In order to further explore the PL of GQDs, electrooxidation of graphene film is
also carried out in this work. Unexpected, GQDs of an average diameter ca. 52 nm are obtained, and
their PL show excitation wavelength-independent feature, different from that reported previously [10].
And they possess blue emission under 365 nm irradiation, instead of green emission for smaller GQDs
obtained by Qu et al.
2. Experimental
2.1. Materials
Natural graphite powder (325) mesh was purchased from Qingdao Huatai Lubricant Sealing S & T
Co., Ltd (Qingdao, China). Sulphuric acid (98%), hydrochloric acid (36%), potassium permanganate
(99.5%), hydrogen peroxide (30%) and ammonia (28%) were obtained from Beijing Chemical Reagent
Co., Ltd (Beijing, China). Hydrazine monohydrate (98%) was purchased from Sigma-Aldrich.
Dipotassium hydrogen phosphate trihydrate (99%) and potassium dihydrogen phosphate (99.5%) were
obtained from Xilong Chem. Industry Co., Ltd (Shantou, China). All reagents described above were
used as received.
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2.2. Electrochemical preparation of GQDs
GQDs were synthesized via electrooxidation of graphene film according to Qu’s method [10]. In
order to obtain graphene film, GO and RGO were firstly prepared according to our method reported
previously [12]. After electrooxidation process, the pure GQDs aqueous solution were collected by
dialyzing electrolyte aqueous solution with a dialysis bag with different pore sizes. GQDs of an average
diameter of ca. 52 nm were obtained by dialyzing with a dialysis bag (8000−14000 molar mass cutoff).
And GQDs of mostly sizes about several nanometers were presented by dialyzing with a dialysis bag
(1000 molar mass cutoff).
2.3. Characterization
Transmission electron microscope (TEM) images were acquired with a H-7560B TEM at 80 kV
(Hitachi, Japan) . Atomic force microscope (AFM) images were measured by using a SPM-9600 AFM
(Shimadzu, Japan). UV-visible spectra were recorded on a U-3010 UV-visible spectrometer (Hitachi,
Japan). Fluorescence spectra were measured using a LS 55 fluorescence spectrometer (PerkinElmer).
3. Results and discussion
The morphologies of graphene and GQDs are shown in Fig. 1a and 1b, respectively. Graphene
possesses a wrinkled paper-like structure up to micro grade (Fig. 1a), but GQDs only reveal an average
size of 52 ± 13 nm (Fig. 1b). AFM measurement (Fig. 2a) indicates that the typical height of singlelayer graphene sheet is ca. 0.93 nm. The height of GQDs is distributed in the range of 0.6−2.8 nm with
an average value of ca. 1.4 nm (Fig. 2b), which indicates that the synthesized GQDs consist of of 1−3
layers of graphene.
Fig. 1 TEM images of graphene (a) and GQDs (b) obtained by dialyzing with a dialysis bag (8000−14000 molar mass
cutoff)
3
Fig. 2 AFM images of graphene (a) and GQDs (b) obtained by dialyzing with a dialysis bag (8000−14000 molar mass
cutoff)
UV-visible spectrum of GQDs are shown in Fig. 3a. The peak located between 300 and 400 nm is
not obvious, induced by n−π* absorption which is attributed to defect state [13]. The aqueous dispersion
of GQDs is yellowish under daylight, and it emits strong blue luminescence when excited by a 365 nm
UV light (Fig 3a, inset). PL peaks of GQDs excited by various irradiation nearly locate at 440 nm (Fig.
3b), indicating excitation wavelength-independent behaviour.
Fig. 3 Optical properties of GQDs aqueous dispersion obtained by dialyzing with a dialysis bag (8000−14000 molar
mass cutoff): (a) UV-visible absorption spectrum, insets are photographs excited by daylight (left) and 365 nm UV
lamp (right), respectively; (b) PL spectra upon excitation at the indicated wavelengths.
4
Fig. 4 FTIR (a) and C 1s XPS spectra (b) of GQDs obtained by dialyzing with a dialysis bag (8000−14000 molar mass
cutoff)
The structural characterization of GQDs is provided in Fig. 4. FTIR spectrum (Fig. 4a) of GQDs
shows several peaks located at 3435 (OH), 1711 (C=O), 1607 (C=C), 1258 (C-O-C) and 1140 (C-OH)
cm-1, respectively. It indicates that GQDs prepared via electrooxidation of graphene contain π
conjugated domains and oxygenated groups, similar to GQDs synthesized by other methods. The C 1s
XPS spectra of GQDs (Fig. 4b) show three peaks centered at 284.8, 286, and 288.8 eV that are
attributed to C=C/C-C, C-O and COOH, respectively, also indicating the prepared GQDs are rich in
oxygen functional groups on the surfaces. The presence of various oxygenated groups makes the GQDs
soluble in aqueous medium, and benefits for their further functionalization.
In our work, GQDs prepared via electrooxidation of graphene, have an average diameter of ca. 52
nm, and their PL exhibit excitation wavelength-independent feature. While GQDs of sizes ca. 3−5 nm
were obtained by similar method in previous study, and their PL showed excitation wavelengthdependent properties [10]. In order to clarify the confusion, we carry out preparing GQDs via
electrooxidation of graphene film again. The only difference is purifying GQDs using a dialysis bag
(1000 molar mass cutoff) with smaller pore size. As a result, obtained GQDs not only have large sizes
up to tens of nanometers, but also small sizes of several nanometers (Fig. 4a) similar to Qu’s report.
And PL of GQDs exhibits excitation wavelength-dependent behaviour (Fig. 4b). Obviously,
electrooxidation of graphene film produce two different sizes of GQDs, with diameters of several tens
of nanometers and several nanometers, respectively. Previous reports indicate that PL mechanism of
GQDs may derive from intrinsic emission and defect emission [13]. Typically, quantum size effect and
zig-zag sites can be classified as intrinsic state emission, while oxygen functional groups are ranged as
defect sate emission. In general, the intrinsic emission observed from low-oxidation GQDs is closely
associated with the highly populated sp2 domains in GQDs plane which are called subdomains, and the
defect emission from intensively oxidized GQDs stems from surrounding functional groups. Blue
emission (near 440 nm) arsing from larger GQDs of sizes ca. 52 nm when excited near 320 nm should
5
2
be ascribed to intrinsic emission. Because of limited tunability of size of sp domains in these larger
GQDs, their PL shows excitation wavelength-independent behaviour [14]. In contrast, smaller GQDs
with sizes of several nanometers exhibit maximum emission around 520 nm when excited at 440 nm,
and their PL mechanism is decided by defect state, ascribed to higher oxidation. Because of the different
level of oxidation for these GQDs, their PL have excitation wavelength-dependent properties [15],
which can also be inferred from PL spectra in Fig. 5b via deducting attribution from larger GQDs at ca.
440 nm.
Fig. 5 TEM image (a) and PL spectra (b) of GQDs obtained via dialyzing the electrolyte aqueous solution using a
dialysis bag (1000 molar mass cutoff)
4. Conclusion
By electrooxidation of graphene film, the prepared GQDs have two different sizes with an average
diameter of ca. 52 nm and several nanometers, respectively. GQDs with an average diameter of ca. 52
nm can be separated and purified via dialyzing electrolyte solution using a dialysis bag (8000−14000
molar mass cutoff) with large pore size. PL of these GQDs exhibit excitation wavelength-independent
behaviour. And they emit strong blue luminescence upon excitation at 365 nm UV light.
Acknowledgments
This work was supported by National Natural Science Foundation of China (51503036), Natural
Science Foundation of Fujian Province (2015J05091), Middle and Young Teacher Education and
Scientific Research Project of Department of Education of Fujian Province (JA15536), Training
Program for Distinguished Young Scholars in Fujian Province University (Minjiaoke[2016] No. 23),
and Program for New Century Excellent Talents in Fujian Province University (Minjiaoke[2017] No.
52).
References
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Figure captions
8
Fig. 1 TEM images of graphene (a) and GQDs (b) obtained by dialyzing with a dialysis bag
(8000−14000 molar mass cutoff)
Fig. 2 AFM images of graphene (a) and GQDs (b) obtained by dialyzing with a dialysis bag
(8000−14000 molar mass cutoff)
Fig. 3 Optical properties of GQDs aqueous dispersion obtained by dialyzing with a dialysis bag
(8000−14000 molar mass cutoff): (a) UV-visible absorption spectrum, insets are photographs excited by
daylight (left) and 365 nm UV lamp (right), respectively; (b) PL spectra upon excitation at the indicated
wavelengths.
Fig. 4 FTIR (a) and C 1s XPS spectra (b) of GQDs obtained by dialyzing with a dialysis
bag
(8000−14000 molar mass cutoff)
Fig. 5 TEM image (a) and PL spectra (b) of GQDs obtained via dialyzing the electrolyte aqueous
solution using a dialysis bag (1000 molar mass cutoff)
9
Graphical abstract
10
Highlights

Graphene quantum dots (GQDs) with an average diameter of ca. 52 nm are prepared via
electrooxidation of graphene.

GQDs emit strong blue luminescence upon excitation at 365 nm UV light.

GQDs exhibit excitation wavelength-independent photoluminescence behaviour.
11
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