Amount Of Graphene Quantum Dots Research Articles

Effect of the Graphene Quantum Dot Content on the Thermal, Dynamic-Mechanical, and Morphological Properties of Epoxy Resin.

Different amounts of graphene quantum dots (CQDs) (0, 1, 2.5, and 5 wt%) were incorporated into an epoxy matrix. The thermal conductivity, density, morphology, and dynamic mechanical thermal (DMTA) properties were reused from the study of Seibert et al.. The Pearson plot showed a high correlation between mass loading, thermal conductivity, and thermal diffusivity. A poorer correlation with density and heat capacity was observed. At lower CQD concentrations (0.1 wt%), the fracture surface showed to be more heterogeneous, while at higher amounts (2.5 and 5 wt%), a more homogeneous surface was observed. The storage modulus values did not change with the CQD amount. But the extension of the glassy plateau increased with higher CQD contents, with an increase of ~40 °C for the 5 wt% compared to the 2.5 wt% and almost twice compared to the neat epoxy. This result is attributed to the intrinsic characteristics of the filler. Additionally, lower energy dissipation and a higher glass transition temperature were observed with the CQD amount. The novelty and importance are related to the fact that for more rigid matrices (corroborated with the literature), the mechanical properties did not change, because the polymer bridging mechanism was not present, in spite of the excellent CQD dispersion as well as the filler amount. On the other hand, thermal conductivity is directly related to particle size and dispersion.

Hyperactivation of lipases by immobilization on superhydrophobic graphene quantum dots inorganic hybrid nanoflower

Various nanoflowers are synthesized for enzyme immobilization. In order to increase the activity of nanoflowers, in this study, 3D flower-like structured organic–inorganic hybrid nanoflowers (hNFs) with various lipases Rhizomucor miehei lipase (RML), Candida antarctica lipase B (CALB), Humicola insolens lipase (HIL), Thermomyces lanuginosus lipase (TLL), Eversa® Transform 2.0 (ET) a genetically modified enzyme derived of TLL and graphene quantum dots (GQDs) were prepared and characterized.Lipase hNFs [lipase-(Cu/Co)3(PO4)2] and lipase@GQDs hNFs [lipase@GQDs-(Cu/Co)3(PO4)2] were straightforwardly prepared through mixing with metal ion (Cu2+or Co2+) aqueous solutions with or without GQDs. The ET@GQDs-(Cu)3(PO4)2 hNFs demonstrated 687 % higher activity than ET-(Cu)3(PO4)2 hNFs and 650 % higher activity than the free ET. Similar results were also observed with other lipase hybrid nanoflowers. For example, TLL@GQDs-(Cu)3(PO4)2 hNFs exhibited a 557 % higher activity than TLL-(Cu)3(PO4)2 hNFs and a 463 % higher activity than free TLL. Additionally, TLL@GQDs-(Co)3(PO4)2 hNFs showed a 141 % higher activity than TLL-(Co)3(PO4)2 hNFs and a 304 % higher activity than free TLL. Upon examining pH and thermal stability, it was revealed that lipase@GQDs hNFs exhibited higher activity compared to free lipase and other hNFs without GQDs. The effect of metal ions, enzyme concentrations and amount of GQDs on the morphology and enzyme activity of the lipase-hNFs was examined.

A novel of WS2–MoCuO3 supported with graphene quantum dot as counter electrode for dye-sensitized solar cells application

A novel tungsten disulfide-molybdenum copper oxide composite supported with graphene quantum dots (WM@GQDs) has been synthesized as a counter electrode (CE) for dye-sensitized solar cells (DSSCs) using a simple and low-cost ultrasonication method. The unique structure of WM@GQDs exhibits excellent power conversion efficiency due to its high catalytic activity and charge transport properties. In addition, the graphene quantum dots (GQDs) provide more active sites in the zero-dimensional materials for an I/I3− redox reaction which can improve the electrical and optical properties of the composite. The results indicate that the amount of GQDs in the composite affect the effectiveness of solar devices. When 0.9%wt of GQDs was used, the WM@GQDs composite achieved an efficiency of 10.38%, which is higher than that of the expensive platinum CE under the same conditions. The mechanism behind the improved power conversion efficiency (PCE) of the composite sample is also discussed in detail. Therefore, WM@GQDs can be an efficient material to replace platinum in DSSCs as a CE.

Z‐type heterojunction of graphene quantum dots/ g‐C 3 N 4 / BiOCl with excellent photocatalytic performance for nitrogen fixation

In this study, a Z-type g-C3N4/BiOCl composite was synthesized by a one-step hydrothermal technique with varying loading amounts of graphene quantum dots (GQDs) to investigate their photocatalytic activity of N2 fixation to NH3 under simulated solar light irradiation. Experimental results showed that the photocatalytic activity of as-prepared GQDs/g-C3N4/BiOCl (GCNB) was significantly higher than those of pure g-C3N4 and BiOCl, and the rate of NH3 formation reached 1773.8 μmol/(h·gcat), which was 7.3 and 5.2 times greater than those of pure g-C3N4 and BiOCl, respectively. After five cycles of experiments, the N2 fixation ability of GCNB did not decrease obviously, indicating that GCNB had high photocatalytic stability. The excellent photocatalytic performance of GCNB was attributed to the broad light absorption range of BiOCl/ultrathin g-C3N4 binary materials and the unique photoelectronic properties of GQDs, which were loaded to form Z-type heterojunctions with a strong redox capability. A Z-type composite photocatalytic mechanism was proposed in this paper, which could significantly improve the charge separation efficiency and maintain a good redox capability. This study could provide an effective method for designing Z-type composite catalysts with high photocatalytic activity. Highlights g-C3N4 can enhance photocatalytic activity by slowing down the recombination of electron-hole pairs. GQDs is an excellent electron transport material sandwiched between g-C3N4 and BiOCl. GQDs/g-C3N4/BiOCl composite material had the best performance of photocatalytic ammonia synthesis The fixation mechanism of N2 in Z-type heterojunction photocatalytic system was investigated.

Polyimide composite films reinforced by graphene quantum dots

Polyimide composite films have been widely used in some advanced field of microelectronic devices, precision instruments and national defense applications. Graphene quantum dots have great application potential in polymer doping modification because of their good mechanical strength, high specific surface area and excellent tunability of physical and chemical properties. In this paper, Graphene quantum dots are prepared by pyrolysis at different temperatures. The results show that different reaction temperatures have a great influence on the size and size distribution of graphene quantum dots. When the pyrolysis temperature is 200 °C, the size of graphene quantum dots is uniform at 3–5 nm. Graphene quantum dots-polyimide nanocomposite films prepared by in-situ polymerization show excellent mechanical properties. When the doping amount of graphene quantum dots is 0.5 wt%, the tensile strength and elastic modulus of the composite films can reach 130.24 MPa and 2.93 GPa respectively, which are 41.3% and 108% higher than that of pure polyimide films, respectively. Moreover, the thermal stability of polyimide composite films increases with the increase of the doping amount of graphene quantum dots. This shows that graphene quantum dots have good dispersion in polyimide matrix and can effectively improve the interface of composites. These results provide a successful preparation method to help guide to fabricate graphene quantum dots-polyimide composite films with high performance for the advanced applications.

Synthesis of graphene oxide and graphene quantum dots from miscanthus via ultrasound-assisted mechano-chemical cracking method

Whilst graphene materials have become increasingly popular in recent years, the followed synthesis strategies face sustainability, environmental and quality challenges. This study proposes an effective, sustainable and scalable ultrasound-assisted mechano-chemical cracking method to produce graphene oxide (GO). A typical energy crop, miscanthus, was used as a carbon precursor and pyrolysed at 1200 °C before subjecting to edge-carboxylation via ball-milling in a CO2-induced environment. The resultant functionalised biochar was ultrasonically exfoliated in N-Methyl-2-pyrrolidone (NMP) and water to form GOs. The intermediate and end-products were characterised via X-ray diffraction (XRD), Raman, high-resolution transmission electron microscopy (HR-TEM) and atomic force microscopy (AFM) analyses. Results show that the proposed synthesis route can produce good quality and uniform GOs (8–10% monolayer), with up to 96% of GOs having three layers or lesser when NMP is used. Ultrasonication proved to be effective in propagating the self-repulsion of negatively-charged functional groups. Moreover, small amounts of graphene quantum dots were observed, illustrating the potential of producing various graphene materials via a single-step method. Whilst this study has only investigated utilising miscanthus, the current findings are promising and could expand the potential of producing good quality graphene materials from renewable sources via green synthesis routes.

Highly selective room temperature ammonia sensors based on ZnO nanostructures decorated with graphene quantum dots (GQDs)

This paper presents a highly selective ammonia sensor based on ZnO nanostructures combined with graphene quantum dots (GQDs). Novel graphene quantum dots (GQDs) have an average lateral size distribution of 2.6 nm. Various amounts of GQDs were combined with the ZnO nanostructure surfaces. Prior to the ZnO:GQDs investigation, field emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to intensively characterize the surface morphologies, existence of functional groups, and chemical compositions. ZnO:GQD heterojunctions are crucial candidate materials due to their highly selective response to ammonia (NH3) vapor. The ammonia sensing characteristics of bare ZnO and ZnO:GQDs sensors at room temperature were systematically investigated via exposure to acetone and ethanol vapor. The ammonia sensing results show that ZnO:GQDs sensors with a volume of 15 μL have optimum sensor responses at an ammonia concentration of 1000 ppm with a value of 6047. The ammonia sensing properties of ZnO:GQDs sensors are due to the GQDs’ carboxyl and hydroxyl groups, which produce more oxygen-containing groups leading to a high H+ molecule density. This further contributes to their highly responsive and selective performance for sensing ammonia at room temperature.

Core-Shell Nanostructures of Graphene-Wrapped CdS Nanoparticles and TiO2 (CdS@G@TiO2): The Role of Graphene in Enhanced Photocatalytic H2 Generation

Aiming to achieve enhanced photocatalytic activity and stability toward the generation of H2 from water, we have synthesized noble metal-free core-shell nanoparticles of graphene (G)-wrapped CdS and TiO2 (CdS@G@TiO2) by a facile hydrothermal method. The interlayer thickness of G between the CdS core and TiO2 shell is optimized by varying the amount of graphene quantum dots (GQD) during the synthesis procedure. The most optimized sample, i.e., CdS@50G@TiO2 generated 1510 µmole g−1 h−1 of H2 (apparent quantum efficiency (AQE) = 5.78%) from water under simulated solar light with air mass 1.5 global (AM 1.5G) condition which is ~2.7 times and ~2.2 time superior to pure TiO2 and pure CdS respectively, along with a stable generation of H2 during 40 h of continuous operation. The increased photocatalytic activity and stability of the CdS@50G@TiO2 sample are attributed to the enhanced visible light absorption and efficient charge separation and transfer between the CdS and TiO2 due to incorporation of graphene between the CdS core and TiO2 shell, which was also confirmed by UV-vis, photoelectrochemical and valence band XPS measurements.

Enhanced emission from CH3NH3PbBr3 perovskite films by graphene quantum dot modification

Organic-inorganic hybrid perovskites have emerged as promising emitters with the benefits of low cost and high color purity, but their low luminescence efficiency is a drawback for practical application on light emitting devices. Here we show that by incorporating proper amount of graphene quantum dots (GQDs) into perovskite precursor, dense CH3NH3PbBr3 films with reduced grain size and well passivated grain boundaries could be obtained. This gives rise to enhanced emission from GQD modified perovskite films. Our work thus provides a viable way to prepare highly luminescent perovskite films for optoelectronic applications.

Water-Soluble Graphene Quantum Dots as High-Performance Water-Based Lubricant Additive for Steel/Steel Contact

In this work, graphene quantum dots (GQDs) were firstly synthesized via a simple hydrothermal route and then its tribological performances as water-based lubricant additive were evaluated for the first time. The morphological observation by transmission electron microscope indicates that uniform and monodispersed GQDs with a mean diameter of about 2 nm can be obtained. Fourier transfer infrared spectroscopy and X-ray photoelectron spectroscopy characterizations demonstrate that there are abundant hydrophilic groups existed on the as-prepared GQDs including hydroxyl, epoxy, and carbonyl groups, endowing GQDs with good dispersibility in water. Moreover, the tribological performances are investigated by optimal-SRV-IV reciprocation friction tester, and the results indicated that both the friction coefficient and the wear rate of water can be largely reduced through adding the appropriate amount of GQDs. As a result, compared with graphene oxide, GQD was more effective to enhance the tribological performances of water at the relatively low concentration. When the concentration of GQDs aqueous dispersion is 4 mg/mL, the corresponding sample of GQDs-4 shows the excellent tribological performances with a 42.5% reduction of friction coefficient and a 58.5% decrease of wear rate compared with those of the water. These promising performances demonstrate that the prepared GQDs can be employed as a novel water-based lubricant additive to greatly enhance the tribological performances of the water.

Graphene quantum dots as a novel conductive additive to improve the capacitive performance for supercapacitors

Graphene quantum dots (GQDs) as a novel conductive agent with nanometer-scale instead of the common carbon black (CB) offer a promising round to construct an effective conductive network for the improvement of the supercapacitors' capacitive performance. Interestingly, the electrochemical performance of activated carbon (AC)-based composite electrodes with different amounts of GQDs in two composite ways was tested, which showed the extremely apparent correlation between the electrode structure and the capacitive characteristics. It was found that the specific capacitance (Cm) and rate performance were enhanced when GQDs at an appropriate weight ratio (1 wt%) in the electrode material, compared with 10 wt% CB, and the corresponding Cm values are 110 and 100 F g−1 at a current density of 0.1 A g−1 and 85 and 65 F g−1 at 1 A g−1, respectively. Furthermore, composite electrodes by heating treatment displayed the better electronic conductivity and cycling stability, which was caused by removing oxygen functional groups, at the expense of decline for the ionic conductivity and rate performance. This work demonstrates an effective method to replace the conventional conductive agent to improve the electrochemical performance of the supercapacitors.

Enhanced performance of porphyrin sensitized solar cell based on graphene quantum dots decorated photoanodes

Porphyrin sensitized solar cells (PSSC) has been successfully fabricated composed of graphene quantum dots (GQD) functionalized ZnO photoanodes, zinc tetrakis (4-carboxy phenyl)porphyrin (TCPPZn) as sensitizer and polypyrrole coated oxidised multiwalled carbon nanotubes (PPy/OMWCNT) as counter electrode. The effect of the concentration of GQD on the structural, morphological, photophysical and photovoltaic properties of GQD@ZnO, and GQD@ZnO/TCPPZn nanocomposites has also been investigated. Studies indicate that TCPPZn adsorbed on the surface on GQD@ZnO. Hot electron injection mechanism and multiple exciton generation from GQD to ZnO were mainly responsible for the photoexcitation response in PSSC. This study indicates that GQD can play role of sensitizer to some extent. The time decay measurements reveals the evidences of FRET mechanism, and synergistic interaction between GQD and TCPPZn. The Jsc, Voc, PCE of the corresponding PSSC devices enhanced initially and then decreased. Among all the devices fabricated, the PSSC with a 40% amount of GQD (GQD@ZnO/TCPPZn 40%) attains the best performance with the Jsc = 10.1 mA/cm2, Voc = 0.48, PCE = 2.45% FF = 0.507 higher than ZnO@TCPPZn device fabricated without GQD. Overall, this design provides a new concept for the development of photoanodes which derive better efficiency for dye sensitized solar cell (DSSC) and PSSC at economical low cost.

Graphene quantum dots enhance UV photoresponsivity and surface-related sensing speed of zinc oxide nanorod thin films

Thin films of almost vertically-grown zinc oxide nanorods (ZnO NR) were prepared via a solvothermal method, and graphene quantum dots (GQDs) were synthesized by pyrolyzing citric acid. Various amounts of GQDs were loaded onto ZnO NR thin films by applying a dip-coating process for different times. Field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray diffractometry (XRD) were employed to characterize the structures. UV–visible spectra of the thin films showed no significant change in the bandgap (~3.3eV) after incorporating GQDs, which is appropriate for visible-blind UV sensing applications. Photoluminescence (PL) spectra showed a significant decrease in the PL intensity of the ZnO NR thin film after incorporating GQDs, suggesting that the generated electrons can be better separated from their hole counterparts and hence contribute to the generated photocurrent. Finally, UV sensing analysis was performed, indicating that GQDs can enhance both the UV-induced photocurrent and the sensing speed of ZnO NR thin films; however, the improvement in the surface-related UV sensing was higher than in the bulk-related one, which could be attributed to the numerous active sites and edge effects of GQDs. The enhanced photocurrent was discussed based on the band diagram.

The acetic acid gas sensing properties of graphene quantum dots (GQDs)–ZnO nanocomposites prepared by hydrothermal method

Graphene quantum dots (GQDs) and GQDs–ZnO composites with different amounts of GQDs have been synthesized via a hydrothermal method. The GQDs were characterized HRTEM and size distribution analyzer. The GQDs–ZnO composites were studied by XRD, SEM, TG, FTIR, Raman and XPS, respectively. The gas sensing properties of GQDs–ZnO composites were investigated. It was found that the amount of GQDs in the composite has great influence on the gas response and gas sensing selectivity of the composite; the sensors based on GQDs–ZnO composites could be operated at room temperature and showed higher response to acetic acid gas than pure ZnO sensor; the sensor based on GQDs–ZnO (S-10) composite could detect 1 ppm acetic acid vapor at room temperature.

Enhanced Electronic Properties of SnO2 via Electron Transfer from Graphene Quantum Dots for Efficient Perovskite Solar Cells.

Tin dioxide (SnO2) has been demonstrated as an effective electron-transporting layer (ETL) for attaining high-performance perovskite solar cells (PSCs). However, the numerous trap states in low-temperature solution processed SnO2 will reduce the PSCs performance and result in serious hysteresis. Here, we report a strategy to improve the electronic properties in SnO2 through a facile treatment of the films with adding a small amount of graphene quantum dots (GQDs). We demonstrate that the photogenerated electrons in GQDs can transfer to the conduction band of SnO2. The transferred electrons from the GQDs will effectively fill the electron traps as well as improve the conductivity of SnO2, which is beneficial for improving the electron extraction efficiency and reducing the recombination at the ETLs/perovskite interface. The device fabricated with SnO2:GQDs could reach an average power conversion efficiency (PCE) of 19.2 ± 1.0% and a highest steady-state PCE of 20.23% with very little hysteresis. Our study provides an effective way to enhance the performance of perovskite solar cells through improving the electronic properties of SnO2.

Improved Solar-Driven Photocatalytic Activity of Hybrid Graphene Quantum Dots/ZnO Nanowires: A Direct Z-Scheme Mechanism

Herein, an electrochemical technique as a cost-effective and one-step approach was utilized to fabricate graphene quantum dots (GQDs). Different amounts of GQDs (0, 0.2, 0.4, 0.8, and 1.2 wt %) were decorated uniformly on the surface of anodized ZnO nanowires (NWs) forming GQD/ZnO NWs. Transmission electron microscopy and atomic force microscopy confirmed formation of GQDs on the ZnO NWs, 12–22 nm in width and 1–3 graphene layers thick. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were employed to verify the functional groups on the surface of GQDs, and the results indicated that GQDs readily anchored on the surface of ZnO NWs. The GQD/ZnO NWs exhibited a considerable improvement on the photocatalytic degradation of methylene blue under solar irradiation, due to efficient light absorption. In addition, the results indicated that the optimized GQD (0.4 wt %)/ZnO NWs showed the highest photoactivity with about 3-fold enhancement as compared to pure ZnO NWs. Finally, a mechani...

Graphene Quantum Dot-Modified Lipase for Synthesis of L-menthyl Acetate with Improved Activity, Stability and Thermostability

Title: Pseudomonas cepacia lipase modified with Graphene Quantum Dots (GQD) offers a higher activity, stability and thermostability compared to the lipase modified with graphene oxide. Background: GQD presents extraordinary properties attracting extensive attention from scientists in chemistry, physics, materials, biology and other interdisciplinary sciences. However, no research involves the application in biocatalysts in non-aqueous media up to now. In the study, we for the first time reported a promising GQDmodified lipase for the synthesis of l-menthyl acetate. Methods and findings: Pseudomonas capaci lipase was modified with GQD and then used as a biocatalyst for the synthesis of l-menthyl acetate in 1-isobutyl-3- methylimidazolium hexafluorophosphate medium. As contrasts, the reaction was also carried out using bare lipase and graphene oxide-modified lipase as the catalyst. Besides, the modification method of enzyme, the amounts of GQD, the reaction temperature, molar ratio of the two substrates, and reuse of the enzyme were investigated. The result shows that the GQD-modified lipase as a biocatalyst was best among the three enzymes. Under optimal reaction conditions, the reaction reaches the equilibrium within 8 h with a high conversion of lmenthol (97.3%). Its initial enzyme activity and halflifetime were more than 1.08 and 2.07-fold that of the bare lipase, and 1.04 and 1.66-fold that of the graphene oxide-modified lipase, respectively. The lipase was recycled 10 times without substantial diminution in activity. In addition, the GQD-modified lipase also offers a better thermostability compared with bare lipase. These improvements could be attributed to the unique small size and edge effect of GQD. Conclusion: GQD-modified lipase showed a higher activity, stability and thermostability compared to GO- modified lipase. GQD was widely used as a promising material for enzyme modification due to its good biocompatibility and small size.