As-prepared Graphene Quantum Dots Research Articles

Oxygen reduction reactions from boron-doped graphene quantum dot catalyst electrodes in acidic and alkaline electrolytes

Background: Boron- and nitrogen-codoped graphene quantum dots (GQDs) were synthesized and served as metal-free electrocatalyst for oxygen reduction reaction (ORR) in acidic and alkaline electrolytes. Methods: The GQD catalysts were prepared using an efficient solvothermal technique based on the pyrolysis of 1,3,6-trinitropyrene and boric acid at 180°C for 12 h. The as-prepared GQDs with an average particle size of ∼4 nm contain a high B/C and N/C level (B/C and N/C atomic ratio: 14.6 and 6.5 at.%) with a large amount of O-rich functionalities. Significant Findings: The B- and N-codoped GQD catalytic electrodes provide ultra-high electrochemically active surface area for ORR with exceptional durability. Indeed, the ORR catalytic activity in both acidic and alkaline electrolytes is significantly enhanced via B- and N-doping into the GQDs lattices. Highly improved ORR activity was majorly due to the synergistic effect of the B- and N-codoped GQD catalysts, where the N functionalities facilitate the formation of C*–O2(ads) sites, whereas the B dopants regenerate the occupied and poisoned sites for the subsequent catalytic cycle. The solvothermal synthesis technique developed in this study sets the stage for developing high-performance metal-free catalytic nanomaterials for ORR.

Synthesis of corn straw-based graphene quantum dots (GQDs) and their application in PO43- detection

Phosphate (PO43-) can cause eutrophication of water bodies due to soil erosion and agricultural fertilization, increasing toxic substances in water, therefore detecting PO43- is vital in water eutrophication. Graphene quantum dots (GQDs), zero-dimensional nanometer-sized graphene pieces, are made of a few graphene lattices and exhibit outstanding optical properties. The traditional synthesis precursors are limited by large carbon domains or toxic polycyclic aromatic molecules. Consequently, synthesis of GQDs from renewable biomass has attracted broad interests. Herein corn straw was used to directly prepare GQDs via a one-pot hydrothermal strategy. The as-prepared GQDs, with an average size of 2.67 nm, presented high quantum yield (15.67%), high crystallinity and dispersibility. The as-prepared non-doped GQDs had a special selectivity to Ce4+/Fe3+, causing the fluorescence (FL) quenching of GQDs and the formation of GQDs-Ce4+/GQDs-Fe3+ aggregation. In the presence of PO43-, a strong affinity between PO43- and Ce4+/Fe3+ can destroy GQDs-Ce4+/GQDs-Fe3+ aggregation, resulting in the FL recovery of GQDs. The GQDs-Ce4+ probe system can selectively detect PO43- with a detection limit of 0.06 μM in a linear range from 0.1 to 20.0 μM. Based on this, GQDs-Ce4+ paper-based sensors were also prepared for the visual detection of PO43-, which showed good responses to PO43-. This development is expected to be a promising strategy for convenient and rapid phosphate detection.

Green synthesis of graphene quantum dots and the dual application of graphene quantum dots-decorated flexible MSM p-type ZnO device as UV photodetector and piezotronic generator

A novel method to synthesize graphene sheets and graphene quantum dots (GQDs) from agricultural waste is reported in this paper. The as-prepared GQDs were decorated on the antimony (Sb)-doped zinc oxide (ZnO) nanostructures to improve the opto-electrical properties of the photodetector. Morphological studies, elemental analysis, absorption and Raman spectroscopy studies were performed on the as-prepared graphene sheets and GQDs. A GQDs-decorated, p-type Sb-doped ZnO metal–semiconductor–metal (MSM) interdigitated UV photodetector was fabricated on a flexible ITO/PET substrate. Structural, morphological and elemental analyses were performed on the GQDs-decorated nanostructures. The Hall measurements show that the device exhibits p-type conductivity with hole concentration of 3 × 1019 cm−3 and Hall mobility of 2067 cm2 V−1 s−1. It is observed from the I–V studies that the Sb-doped ZnO MSM device with GQDs exhibits 103 times higher dark and photocurrents as compared to the device without GQDs decoration. The dynamic response is measured, and the turn-on (τon) and turn-off (τoff) times are found to be 3.67 and 12.26 s, respectively. The device also exhibited piezotronic dark potential on compression at a convex arc angle. Repeated compressions and relaxations were performed on the device and it behaved as a piezotronic generator.

One-Step Synthesis of Diamine-Functionalized Graphene Quantum Dots from Graphene Oxide and Their Chelating and Antioxidant Activities.

2,2’-(Ethylenedioxy)bis(ethylamine)-functionalized graphene quantum dots (GQDs) were prepared under mild conditions from graphene oxide (GO) via oxidative fragmentation. The as-prepared GQDs have an average diameter of ca. 4 nm, possess good colloidal stability, and emit strong green-yellow light with a photoluminescence (PL) quantum yield of 22% upon excitation at 375 nm. We also demonstrated that the GQDs exhibit high photostability and the PL intensity is poorly affected while tuning the pH from 1 to 8. Finally, GQDs can be used to chelate Fe(II) and Cu(II) cations, scavenge radicals, and reduce Fe(III) into Fe(II). These chelating and reducing properties that associate to the low cytotoxicity of GQDs show that these nanoparticles are of high interest as antioxidants for health applications.

Graphene quantum dot-decorated carbon electrodes for energy storage in vanadium redox flow batteries.

Nitrogen-doped graphene quantum dots (GQDs) and graphitic carbon nitride (g-C3N4) quantum dots are synthesized via a solid-phase microwave-assisted (SPMA) technique. The resulting GQDs are deposited on graphite felt (GF) and are employed as high-performance electrodes for all-vanadium redox flow batteries (VRFBs). The SPMA method is capable of synthesizing highly oxidized and amidized GQDs using citric acid and urea as the precursor. The as-prepared GQDs contain an ultrahigh O/C (56-61%) and N/C (34-66%) atomic ratio, much higher than the values reported for other carbon-based nano-materials (e.g. oxidized activated carbon, carbon nanotubes, and graphene oxide). Three types of quantum dots, having an average particle size of 2.8-4.2 nm, are homogeneously dispersed onto GF electrodes, forming GQD/GF composite electrodes. Through deposition of GQDs onto the electrode structure, the catalytic activity, equivalent series resistance, durability, and voltage efficiency are improved. The capacity utilization using GQD/GF electrode is substantially enhanced (∼69% increase within 40 cycles). The improved performance is attributed to the synergistic effect of GQDs containing oxygen functionalities (epoxy, phenolic and carboxylic groups) and lattice N atoms (quaternary, pyrrolic and pyridinic N) which result in enhanced wettability and increased electrochemical surface area providing increased reaction sites.

Facile and Highly Effective Synthesis of Controllable Lattice Sulfur-Doped Graphene Quantum Dots via Hydrothermal Treatment of Durian.

Recently, the biomass "bottom-up" approach for the synthesis of graphene quantum dots (GQDs) has attracted broad interest because of the outstanding features, including low-cost, rapid, and environmentally friendly nature. However, the low crystalline quality of products, substitutional doping with heteroatoms in lattice, and ambiguous reaction mechanism strongly challenge the further development of this technique. Herein, we proposed a facile and effective strategy to prepare controllable sulfur (S) doping in GQDs, occurring in a lattice substitution manner, by hydrothermal treatment of durian with platinum catalyst. S atoms in GQDs are demonstrated to exist in the thiophene structure, resulting in good optical and chemical stabilities, as well as ultrahigh quantum yield. Detailed mechanism of the hydrothermal reaction progress was investigated. High-efficiency reforming cyclization provided by platinum was evidenced by the coexistence of diversified sp2-fused heterocyclic compounds and thiophene derivatives. Moreover, we also demonstrated that saccharides in durian with small molecular weight (<1000 Da) is the main carbon source for the forming GQDs. Because of the desulfurizing process, controllable photoluminescence properties could be achieved in the as-prepared GQDs via tuning doping concentrations.

Comparative Investigation of Graphene Quantum Dots and Graphitic-phase C3N4 Nanosheets in Terms of Photoluminescence Properties and Biomedical Imaging

In this work, highly water-dispersible graphene quantum dots (GQDs) and graphitic-phase C3N4 (g-C3N4) nanosheets were prepared by a similar top-down process, chemically oxidizing thermally exfoliated graphite oxide and bulk g-C3N4 with concentrated nitric acid, respectively. UV-Vis absorption and photoluminescence (PL) spectral analyses were used to compare their optical properties. Compared to GQDs, g-C3N4 nanosheets demonstrate higher PL quantum yield. Cytotoxicity measurements indicate that both GQDs and g-C3N4 nanosheets are of low cytotoxicity with favorable biocompatibility. The remarkable performances of GQDs and g-C3N4 nanosheets in biomedical imaging are demonstrated by a simple incubation study with HepG2 cells. The HepG2 cells incubated with these two materials for 12 h do not weaken the cell activity and maintain their normal morphology. GQDs can effectively distribute in not only the cell membrane and cytoplasm, but also the nucleus, while the g-C3N4 nanosheets mainly exist at the cell’s membrane and cytoplasm. All these studies suggest that both as-prepared GQDs and g-C3N4 nanosheets are greatly promising for biomedical imaging application.

Solvothermal method to prepare graphene quantum dots by hydrogen peroxide

Graphene quantum dots (GQDs) have been synthesized by different chemical methods in recent years. For conventional chemical methods, it is inevitable to introduce a large amount of impurities in the preparation process. Long time of dialysis process increases the time cost extremely. Herein, we report a one-step solvothermal method for synthesizing GQDs with the application of hydrogen peroxide in N, N-Dimethylformamide (DMF) environment, which completely avoids the use of concentrated sulphuric acid and nitric acid to treat raw material and introduces no impurity in whole preparation process simultaneously for the first time. Pure GQDs can be obtained after evaporation/redissolution and filtration process with a strong blue emission at 15% quantum yield. This solvothermal method, not requiring dialysis process and complicated equipments, exhibits simple, eco-friendly and low time-cost properties. Besides high quantum yields, the as-prepared GQDs also show good photoluminescence stability in different pH conditions. The optical properties, morphology and structure of GQDs were studied by various equipments, implying potential application in biomedical fields and electronic device.

Synthesis of Luminescent Graphene Quantum Dots with High Quantum Yield and Their Toxicity Study.

High fluorescence quantum yield graphene quantum dots (GQDs) have showed up as a new generation for bioimaging. In this work, luminescent GQDs were prepared by an ameliorative photo-Fenton reaction and a subsequent hydrothermal process using graphene oxide sheets as the precursor. The as-prepared GQDs were nanomaterials with size ranging from 2.3 to 6.4 nm and emitted intense green luminescence in water. The fluorescence quantum yield was as high as 24.6% (excited at 340 nm) and the fluorescence was strongest at pH 7. Moreover, the influences of low-concentration (12.5, 25 μg/mL) GQDs on the morphology, viability, membrane integrity, internal cellular reactive oxygen species level and mortality of HeLa cells were relatively weak, and the in vitro imaging demonstrated GQDs were mainly in the cytoplasm region. More strikingly, zebrafish embryos were co-cultured with GQDs for in vivo imaging, and the results of heart rate test showed the intake of small amounts of GQDs brought little harm to the cardiovascular of zebrafish. GQDs with high quantum yield and strong photoluminescence show good biocompatibility, thus they show good promising for cell imaging, biolabeling and other biomedical applications.

Solid‐phase synthesis of graphene quantum dots from the food additive citric acid under microwave irradiation and their use in live‐cell imaging

The work demonstrated that solid citric acid, one of the most common food additives, can be converted to graphene quantum dots (GQDs) under microwave heating. The as-prepared GQDs were further characterized by various analytical techniques like transmission electron microscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, fluorescence and UV-visible spectroscopy. Cytotoxicity of the GQDs was evaluated using HeLa cells. The result showed that the GQDs almost did not exhibit cytotoxicity at concentrations as high as 1000 µg mL(-1). In addition, it was found that the GQDs showed good solubility, excellent photostability, and excitation-dependent multicolor photoluminescence. Subsequently, the multicolor GQDs were successfully used as a fluorescence light-up probe for live-cell imaging.

Graphene quantum dots directly generated from graphite via magnetron sputtering and the application in thin-film transistors

This work presents a novel method to prepare graphene quantum dots (GQDs) directly from graphite. A composite film of GQDs and ZnO was first prepared using the composite target of graphite and ZnO via magnetron sputtering, followed with hydrochloric acid treatment and dialysis. Morphology and optical properties of the GQDs were investigated using a number of techniques. The as-prepared GQDs are 4–12nm in size and 1–2nm in thickness. They also exhibited typical excitation-dependent properties as expected in carbon-based quantum dots. To demonstrate the potential applications of GQDs in electronic devices, pure ZnO and GQD–ZnO thin-film transistors (TFTs) using ZrOx dielectric were fabricated and examined. The ZnO TFT incorporating the GQDs exhibited enhanced performance: an on/off current ratio of 1.7×107, a field-effect mobility of 17.7cm2/Vs, a subthreshold swing voltage of 90mV/decade. This paper provides an efficient, reproducible and eco-friendly approach for the preparation of monodisperse GQDs directly from graphite. Our results suggest that GQDs fabricated using magnetron sputtering method may envision promising applications in electronic devices.

An approach to controlling the fluorescence of graphene quantum dots: From surface oxidation to fluorescent mechanism

We report a facile method of synthesizing graphene quantum dots (GQDs) with tunable emission. The as-prepared GQDs each with a uniform lateral dimension of ca. 6 nm have fine solubility and high stability. The photoluminescence mechanism is further investigated based on the surfacestructure and the photoluminescence behaviors. Based on our discussion, the green fluorescence emission can be attributed to the oxygen functional groups, which could possess broad emission bands within the π–π* gap. This work is helpful to explain the vague fluorescent mechanism of GQDs, and the reported synthetic method is useful to prepare GQDs with controllable fluorescent colors.

Facile synthesis of soluble graphene quantum dots and its improved property in detecting heavy metal ions

An effective approach to produce graphene quantum dots (GQDs) has been developed, which based on the cutting of graphene oxide (GO) powder into smaller pieces and being reduced by a green approach, using sodium polystyrene sulfonate (PSS) as a dispersant and l-ascorbic acid (l-AA) as the reducing agent, which is environmentally friendly. Then the as-prepared GQDs were further used for the detection of heavy metal ions Pb2+. This kind of GQDs has greater solubility in water and is more biocompatible than GO that has been reduced by hydrazine hydrate. The few-layers of GQDs with defects and residual OH groups were shown to be particularly well suited for the determination of metal ions in the liquid phase using an electrochemical method, in which a remarkably low detection limit of 7×10−9M for Pb2+ was achieved.

Field effect transport properties of chemically treated graphene quantum dots

We report the field effect properties of lithographically fabricated FET with as-prepared graphene quantum dots (GQDs) and hydrazine treated GQDs as channel material. GQDs of 4.5 ± 0.55 nm average diameter are synthesised via an electrochemical approach using multiwalled carbon nanotubes (MWCNTs) as precursor. After treatment in hydrazine vapour for 24 h, the field effect measurements yield hole mobility of 0.01 cm 2 V –1 s –1 and I on / I off ratio of about 45. Hydrazine treated channel shows a significant decrease in resistance in comparison to the channel with as-prepared GQDs and is p-type under ambient conditions.

Blue and green photoluminescence graphene quantum dots synthesized from carbon fibers

A facile chemical oxidation and exfoliation method has been developed to prepare luminescent graphene quantum dots (GQDs) using commercial carbon fibers as the raw material, which were synthesized from pitch through an annealing process at 1500°C. The as-prepared GQDs, in the thickness range of 1–2 graphene layers, exhibit an excitation-independent photoluminescence (PL) behavior similar to that of semiconductor quantum dots, and can be dissolved in water and common polar solvents. This approach has enabled a simple and effective way for production of the GQDs solution, which could be separated into blue and green photoluminescence (PL) GQDs with different dialysis bags.