Synergistically coupling of graphene quantum dots with Zn-intercalated MnO2 cathode for high-performance aqueous Zn-ion batteries

Cost-effective,safe,and highly performing energy storage devices require rechargeable batteries,and among various options,aqueous zinc-ion batteries(ZIBs)have shown high promise in this regard.As a cathode material for the aqueous ZIBs,manganese dioxide(MnO_(2))has been found to be promising,but certain drawbacks of this cathode material are slow charge-transfer capability and poor cycling performance.Herein,a novel design of graphene quantum dots(GQDs)integrated with Zn-intercalated MnO_(2)nanosheets is put forward to construct a 3D nanoflower-like GQDs@ZnxMnO_(2)composite cathode for aqueous ZIBs.The synergistic coupling of GQDs modification with Zn intercalation provides abundant active sites and conductive medium to facilitate the ion/electron transmission,as well as ensure the GQDs@ZnxMnO_(2)composite cathode with enhanced charge-transfer capability and high electrochemical reversibility,which are elucidated by experiment results and in-situ Raman investigation.These impressive properties endow the GQDs@ZnxMnO_(2)composite cathode with superior aqueous Zn^(2+) storage capacity(~403.6 mAh·g^(−1)),excellent electrochemical kinetics,and good structural stability.For actual applications,the fabricated aqueous ZIBs can deliver a substantial energy density(226.8 W·h·kg^(−1)),a remarkable power density(650 W·kg^(−1)),and long-term cycle performance,further stimulating their potential application as efficient electrochemical storage devices for various energy-related fields.

Graphene quantum dots as sulfiphilic and lithiophilic mediator toward high stability and durable life lithium-sulfur batteries

The development of lithium-sulfur(Li-S) battery as one of the most attractive energy storage systems among lithium metal batteries is seriously hindered by low sulfur utilization, poor cycle stability and uneven redeposition of Li anode. It is necessary to propose strategies to address the problems as well as improve the electrochemical performance. One of the effective solutions is to improve the sulfiphilicity of sulfur cathode and the lithiophilicity of the Li anode. Herein, we reported that a synergistic functional separator(graphene quantum dots(GQDs)-polyacrylonitrile(PAN) @polypropylene(PP) separator)improved the electrochemical activity of sulfur cathode as well as the stability of Li anode. GQDs induced uniform Li^(+)nucleation and deposition, which slowed down the passivation of Li anode and avoided shortcircuit. Further, three-dimensional network constructed by electrospinning nanofibers and the polar functional groups of GQDs could both effectively inhibit the shuttle of LiPSs and improve the sulfur utilization. The stability of Li-S battery was improved by the synergistic effect. In addition, GQDs and electrospinning nanofibers protector increased lifetime of separators. Benefiting from the unique design strategy, Li//Li symmetric battery with GQDs-PAN@PP separators exhibited stably cycling for over 600 h. More importantly, the Li-S full batteries based GQDs-PAN@PP separators enabled high stability and desirable sulfur electrochemistry, including high reversibility of 558.09 mA h g^(-1)for 200 cycles and durable life with a low fading rate of 0.075% per cycle after 500 cycles at 0.5 C. Moreover, an impressive areal capacity of 3.23 mA h cm^(-2)was maintained under high sulfur loading of 5.10 mg cm^(-2). This work provides a new insight for modification separator to improve the electrochemical performance of Li-S/Li metal batteries.

Graphene quantum dots doped poly(vinyl alcohol)hybrid membranes for desalination via pervaporation

Pervaporation desalination by highly hydrophilic materials such as poly(vinyl alcohol)(PVA)based separation membrane is a burgeoning technology of late years.However,the improvement of membrane flux in pervaporation desalination has been a difficult task.Here,a novel hybrid membrane with doped graphene oxide quantum dots(GOQDs)which is rich in hydrophilic groups and small size into the matrix of PVA was prepared to improve the membrane flux.The membranes structures were described by field emission scanning electron microscopy(FESEM),atomic force microscopy(AFM),Fourier transform infrared(FT-IR),differential scanning calorimetry(DSC),thermogravimetric analysis(TGA)and X-ray diffraction(XRD).And more,Water contact angle,swelling degree,and pervaporation properties were carried out to explore the effect of GOQDs in PVA matrix.In addition,GOQDs content in the hybrid membrane,NaCl concentration,and feed temperature were investigated accordingly.Moreover,the hydrogen bonds between PVA chains were weakened by the interaction between GOQDs and PVA chains.Significantly,the hybrid membrane with optimized doped GOQDs content,200 mg·L^(-1),displays a high membrane flux of 17.09 kg·m^(-2)·h^(-1)and the salt rejection is consistently greater than 99.6%.

Effects of Coal Rank and High Organic Sulfur on the Structure and Optical Properties of Coal-based Graphene Quantum Dots

Coal-based graphene quantum dots(GQDs) were successfully produced via a one-step chemical synthesis from six different coal ranks, from which two superhigh organic sulfur(SHOS) coals were selected as natural S-doped carbon sources for the preparation of S-doped GQDs. The effects of coal properties on coal-based GQDs were analyzed by means of high-resolution transmission electron microscopy(HRTEM), X-ray diffraction(XRD), Fourier transform infrared(FTIR) spectroscopy, X-ray photoelectron spectroscopy(XPS), ultraviolet-visible(UV-Vis) absorption spectroscopy, and fluorescence emission spectra. It was shown that all coal samples can be used to prepare GQDs, which emit bluegreen and blue fluorescence under ultraviolet light. Anthracite-based GQDs have a hexagonal crystal structure without defects, the largest size, and densely arranged carbon rings in their lamellae; the highrank bituminous coal-based GQDs are relatively reduced in size, with their hexagonal crystal structure being only faintly visible; the low-rank bituminous coal-based GQDs are the smallest, with sparse lattice fringes and visible internal defects. As the metamorphism of raw coals increases, the yield decreases and the fluorescence quantum yield(QY) initially increases and then decreases. Additionally, the surface of GQDs that were prepared using high-rank SHOS coal(high-rank bituminous coal) preserves rich sulfur content even after strong oxidation, which effectively adjusts the bandgap and improves the fluorescence QY. Thus, high-rank bituminous coal with SHOS content can be used as a natural S-doped carbon source to prepare S-doped GQDs, extending the clean utilization of low-grade coal.

Toxicity of Graphene Quantum Dots in Zebrafish Embryo

Objective To evaluate the bio-safety of graphene quantum dots （GQDs）, we studied its effects on the embryonic development of zebrafish. Methods In vivo, biodistribution and the developmental toxicity of GQDs were investigated in embryonic zebrafish at exposure concentrations ranging from 12.5-200μg/mL for 4-96 h post-fertilization （hpf）. The mortality, hatch rate, malformation, heart rate, GQDs uptake, spontaneous movement, and larval behavior were examined. Results The fluorescence of GQDs was mainly localized in the intestines and heart. As the exposure concentration increased, the hatch and heart rate decreased, accompanied by an increase in mortality. Exposure to a high level of GQDs （200μg/mL） resulted in various embryonic malformations including pericardial edema, vitelline cyst, bent spine, and bent tail. The spontaneous movement significantly decreased after exposure to GQDs at concentrations of 50, 100, and 200μg/mL. The larval behavior testing （visible light test） showed that the total swimming distance and speed decreased dose-dependently. Embryos exposed to 12.5 μg/mL showed hyperactivity while exposure to higher concentrations （25, 50, 100, and 200μg/mL） caused remarkable hypoactivity in the light-dark test. Conclusion Low concentrations of GODs were relatively non-toxic. However, GQDs disrupt the progression of embryonic development at concentrations exceeding 50 μg/mL.

Construction of nitrogen and phosphorus co-doped graphene quantum dots/Bi5O7I composites for accelerated charge separation and enhanced photocatalytic degradation performance

Nitrogen and phosphorus co-doped graphene quantum dot-modified Bi5O7 I(NPG/Bi5O7 I)nanorods were fabricated via a simple solvothermal method.The morphology,structure,and optical properties of the as-prepared samples were investigated by X-ray diffraction,scanning electron microscopy,high-resolution transmission electron microscopy,X-ray photoelectron spectroscopy(XPS),and diffused reflectance spectroscopy.The photocatalytic performance was estimated by degrading the broad-spectrum antibiotics tetracycline and enrofloxacin under visible light irradiation.The photodegradation activity of Bi5O7 I improved after its surface was modified with NPGs,which was attributed to an increase in the photogenerated charge transport rate and a decrease in the electron-hole pair recombination efficiency.From the electron spin resonance spectra,XPS valence band data,and free radical trapping experiment results,the main active substances involved in the photocatalytic degradation process were determined to be photogenerated holes and superoxide radicals.A possible photocatalytic degradation mechanism for NPG/Bi5O7 I nanorods was proposed.

Hydroxylated graphene quantum dots as fluorescent probes for sensitive detection of metal ions

Highly sensitive methods are important for monitoring the concentration of metal ions in industrial wastewater.Here,we developed a new probe for the determination of metal ions by fluorescence quenching.The probe consists of hydroxylated graphene quantum dots(H-GQDs),prepared from GQDs by electrochemical method followed by surface hydroxylation.It is a non-reactive indicator with high sensitivity and detection limits of 0.01μM for Cu2+,0.005μM for Al3+,0.04μM for Fe3+,and 0.02μM for Cr3+.In addition,the low biotoxicity and excellent solubility of H-GQDs make them promising for application in wastewater metal ion detection.

Recent Advances on Graphene Quantum Dots for Electrochemical Energy Storage Devices

Graphene quantum dots(GQDs)which are nanofragments of graphene with an average size between 2 and 50 nm have attracted much attention due to their outstanding properties such as high conductivity,high surface area,and good solubility in various solvents.GQDs combine the quantum confinement and edges effects and the properties of graphene.Therefore,GQDs offers a broad range of applications in various fields(medicine,energy conversion,and energy storage devices).This review will present the recent research based on the introduction of GQDs in batteries,supercapacitors,and microsupercapacitors as electrodes materials or mixed with an active material as an auxiliary agent.Tables,discussed on selected examples,summarize the electrochemical performances and finally,challenges and perspectives are recalled for the subsequent optimization strategy of electrode materials.This review is expected to appeal a broad interest on functional GQDs materials and promote the further development of high-performance energy storage device.

Ionic polymer metal composites actuators with enhanced driving performance by incorporating graphene quantum dots

In order to further improve the driving performance of ionic polymer metal composites(IPMCs),Nafion/graphene quantum dots(GQDs)hybrid membranes incorporating GQDs with various contents of 0,0.1 wt.%,0.5 wt.%,1.0 wt.%,2.0 wt.%and 4.0 wt.%were fabricated by solution casting,and then IPMCs were manufactured by electroless plating.The water contents and elastic moduli of the hybrid membranes were tested.The morphology characteristics of the hybrid membranes and the IPMCs were observed,and the current,AC impedance,blocking force and displacement of the IPMCs were measured.The results show that the elastic modulus of the hybrid membranes decreases,the water content increases,and the actuation performance of the IPMCs improves significantly after the addition of GQDs.IPMC with 1.0 wt.%GQDs exhibits the best driving property.Compared with the IPMC without GQDs,the working current,ion conductivity,blocking force,and tip displacement increase by 94.67%,311.11%,53.66%,and 66.07%,respectively.These results lay a solid foundation for the preparation of IPMCs with high performance,and further broaden their applications in biomedical devices and bionic robots.

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.

Graphene quantum dots for tumor optical diagnosis and treatment

Graphene quantum dots(GQDs)are two-dimensional carbon nano-material with excellent physicochemical properties,including biocompatibility,high photoluminescence,water-solubility,hypotoxicity and so on.Owing to bigπ-conjugated structure and oxygen-containing functional groups,GQDs have the ability to absorb various chemical drugs viaπ-πstacking and electrostatic interaction.On the other hand,owning to photoluminescence,GQDs also have potential to serve as fluorescence probe for bioimaging,especially for optical diagnostics.In addition,GQDs have the ability to generate reactive oxygen species(ROS)including singlet oxygen upon photoexcitation.Therefore,GQDs are likely to be used for photodynamic therapy.This article aims to review the frontier researches about GQDs in the field of fluorescence probe,photodynamic therapy and anti-tumor drug delivery system.

Graphene Quantum Dots Derived from Carbon Fibers for Oxidation of Dopamine

We demonstrated a facile method to prepare photoluminescent graphene quantum dots using commercial polyacrylonitrile（PAN） based carbon fibers（CFs） as the raw material by facile chemical oxidation and exfoliation method. The as-prepared GQDs with uniform size exhibit an excitation-independent photoluminescence behavior, which is similar to other semiconductor quantum dots. Moreover, when acting as catalyst the uniform GQDs have better activity for electrochemical oxidation of dopamine（DA） than graphene oxides（GOs）. The square wave voltammogram（SWV） peak values of GQDs are in good correspondence with DA concentrations and can act as a sensor of DA.

Minimising non-selective defects in ultrathin reduced graphene oxide membranes with graphene quantum dots for enhanced water and NaCl separation

Reduced graphene oxide(rGO)membranes have been intensively evaluated for desalination and ionic sieving applications,benefiting from their stable and well-confined interlayer channels.However,rGO membranes generally suffer from low permeability due to the high transport resistance resulting from the narrowed two-dimensional(2D)channels.Although high permeability can be realized by reducing membrane thickness,membrane selectivity normally declines because of the formation of nonselective defects,in particular pinholes.In this study,we demonstrate that the non-selective defects in ultrathin rGO membranes can be effectively minimised by a facile posttreatment via surfacedeposition of graphene quantum dots(GQDs).The resultant GQDs/rGO membranes obtained a good trade-off between water permeance(14 L·m^(-2)·h^(-1).MPa^(-1))and NaCl rejection(91%).This work provides new insights into the design of high quality ultrathin 2D laminar membranes for desalination,molecular/ionic sieving and other separation applications.

Facilitative effect of graphene quantum dots in MoS_2 growth process by chemical vapor deposition

The substrate treatment with seeding promoter can promote the two-dimensional material lateral growth in chemical vapor deposition （CVD） process. Herein, graphene quantum dots （GQDs） as a novel seeding promoter were used to obtain uniform large-area MoS2 monolayer. The obtained monolayer MoS2 films were confirmed by optical microscope, scanning electron microscope, Raman and photoluminescence spectra. Raman mapping revealed that the MoS2 monolayer was largely homogeneous.

Novel Formaldehyde Sensor based on Hydrogen Peroxide/Melamine Modulated Photoluminescence of Nitrogen-doped Graphene Quantum Dots

A modulated photoluminescence nanosensor was developed for the quantitative detection of formaldehyde with nitrogen-doped graphene quantum dots and melamine. The sensing system was based on the different activated effects of melamine and hydrogen peroxide on the photoluminescence intensity of nitrogendoped graphene quantum dots. Under the optimal conditions, the modulated photoluminescence sensing system can be used to detect formaldehyde with a good linear relationship between the nitrogen-doped graphene quantum dots photoluminescence difference and the concentration of formaldehyde. The novel sensing system provided new directions for the detection of formaldehyde with high selectivity and quick response.

Graphene quantum dots assisted photovoltage and efficiency enhancement in CdSe quantum dot sensitized solar cells

CdSe quantum dot sensitized solar cells （QDSCs） modified with graphene quantum dots （GQDs） have been successfully achieved in this work for the first time. Satisfactorily, the optimized photovoltage （Voc） of the modified QDSCs was approximately 0.04 V higher than that of plain CdSe QDSCs, consequently improving the photovoltaic performance of the resulting QDSCs. Served as a novel coating on the CdSe QD sensitized photoanode, GQDs played a vital role in improving Voc due to the suppressed charge recombination which has been confirmed by electron impedance spectroscopy as well as transient photovoltage decay measure- ments. Moreover, different adsorption sequences, concentration and deposition time of GQDs have also been systematically investigated to boost the power conversion efficiency （PCE） of CdSe QDSCs. After the coating of CdSe with GQDs, the resulting champion CdSe QDSCs exhibited an improved PCE of 6.59% under AM 1.5G full one sun illumination.

Dynamic Variation of Excitonic Coupling in Excited Bilayer Graphene Quantum Dots

The intermolecular interaction determines the photophysical properties of the organic aggregates,which are critical to the performance of organic photovoltaics.Here,excitonic coupling,an important intermolecular interaction in organic aggregates,between theπ-stacking graphene quantum dots is studied by using transient absorption spectroscopy.We find that the spectral evolution of the ground state bleach arises from the dynamic variation of the excitonic coupling in the excitedπ-stacks.According to the spectral simulations,we demonstrate that the kinetics of the vibronic peak can be exploited as a probe to measure the dynamics of excitonic coupling in the excitedπ-stacks.

Graphene quantum dots:preparations,properties,functionalizations and applications

Zero-dimensional graphene quantum dots(GQDs)exhibit many different properties,such as strong fluorescence,nonzero bandgap and solubility in solvents,compared to two-dimensional graphene.GQDs are biocompatible and have low toxicity;hence,they are widely used in the biomedical field.The edge effect of GQDs is of particular interest because edge modification can regulate the performance of nanomaterials.In this review,various preparation methods for GQDs,which can be divided into three main categories,namely top-down,bottom-up and chemical methods,are discussed.The unique optical,electrical,thermal and magnetic properties of GQDs are reviewed.The functionalization of GQDs by doping with heteroatoms and forming composites with other materials is studied,and the characteristics of these GQDs are also discussed.The applications of these GQDs in the fields of optics,electricity,optoelectronics,biomedicine,energy,agriculture and other emerging interdisciplinary fields are reviewed to highlight the enormous potential of nanomaterials.This review reports on the recent advancement in GQD research and suggests future directions for the development of GQDs.

Polyethylenimine-modified graphene quantum dots promote endothelial cell proliferation

Endothelial cell proliferation plays an important role in angiogenesis and treatment of related diseases.The aim of this study was to evaluate the effect of polyethylenimine(PEI)-modified graphene quantum dots(GQDs)gene vectors on endothelial cell prolifera-tion.The GQDs-cationic polymer gene vectors were synthesized by amidation reaction,and used to deliver pzNF580 gene to Human umbilical vein endothelial cells(HUVECs)for promoting their proliferation.The chemical modification of GQDs can ad-just gene vectors'surface properties and charge distribution,thereby enhancing their interaction with gene molecules,which could effectively compress the pZNF580 gene.The CCK-8 assay showed that the cell viability was higher than 80%at higher vector concentration(40μg/mL),demonstrating that the GQDs-cationic polymer gene vectors and their gene complex nanoparticles(NPs)having low cytotoxicity.The results of the live/dead cell double staining assay were consistent with those of the CCK-8 assay,in which the cell viability of the A-GQDs/pZNF580(94.38±6.39%),C-GQDs-PEI-polylactic acid-co-polyacetic acid(PLGA)/pZNF580(98.65±6.60%)and N-GQDs-PEI-PLGA/pZNF580(90.08±1.60%)groups was significantly higher than that of the Lipofectamine 2000/pzNF580(71.98±3.53%)positive treatment group.The results of transfection and western blot experiments showed that the vector significantly enhanced the delivery of plasmid to HUVECs and increased the expression of pZNF580 in HUVECs.In addition,the gene NPs better promote endothelial cell migration and proliferation.The cell migration rate and proliferation ability of C-GQDs-PEI-PLGA/pZNF580 and N-GQDs-PEI-PLGA/pZNF580 treatment groupS were higher than those of Lipofectamine 2000/pDNA treatment group.Modified GQDs possess the potential to serve as efficient gene carriers.They tightly bind gene molecules through charge and other non-covalent interactions,significantly improving the effciency of gene delivery and ensuring the smooth release of genes within the cell.This innovative strategy provides a powerful means to promote endothelial cell proliferation.

N-doped graphene quantum dot-decorated N-TiO2/P-doped porous hollow g-C_(3)N_(4) nanotube composite photocatalysts for antibiotic photodegradation and H2 production

Exclusive responsiveness to ultraviolet light (~3.2 eV) and high photogenerated charge recombination rate are the two primary drawbacks of pure TiO_(2). We combined N-doped graphene quantum dots (N-GQDs), morphology regulation, and heterojunction construction strategies to synthesize N-GQD/N-doped TiO_(2)/P-doped porous hollow g-C_(3)N_(4) nanotube (PCN) composite photocatalysts (denoted as G-TPCN). The optimal sample (G-TPCN doped with 0.1wt% N-GQD, denoted as 0.1% G-TPCN) exhibits significantly enhanced photoabsorption, which is attributed to the change in bandgap caused by elemental doping (P and N), the improved light-harvesting resulting from the tube structure, and the upconversion effect of N-GQDs. In addition, the internal charge separation and transfer capability of0.1% G-TPCN are dramatically boosted, and its carrier concentration is 3.7, 2.3, and 1.9 times that of N-TiO_(2), PCN, and N-TiO_(2)/PCN(TPCN-1), respectively. This phenomenon is attributed to the formation of Z-scheme heterojunction between N-TiO_(2) and PCNs, the excellent electron conduction ability of N-GQDs, and the short transfer distance caused by the porous nanotube structure. Compared with those of N-TiO_(2), PCNs, and TPCN-1, the H2 production activity of 0.1%G-TPCN under visible light is enhanced by 12.4, 2.3, and 1.4times, respectively, and its ciprofloxacin (CIP) degradation rate is increased by 7.9, 5.7, and 2.9 times, respectively. The optimized performance benefits from excellent photoresponsiveness and improved carrier separation and migration efficiencies. Finally, the photocatalytic mechanism of 0.1% G-TPCN and five possible degradation pathways of CIP are proposed. This study clarifies the mechanism of multiple modification strategies to synergistically improve the photocatalytic performance of 0.1% G-TPCN and provides a potential strategy for rationally designing novel photocatalysts for environmental remediation and solar energy conversion.