A SnO2QDs/GO/PPY ternary composite film as positive and graphene oxide/charcoal as negative electrodes assembled solid state asymmetric supercapacitor for high energy storage applications

The work demonstrates tin oxide quantum dots/graphene oxide/polypyrrole (SnO₂QDs/GO/PPY) ternary composite deposited on titanium foil as a positive electrode and graphene oxide (GO)/charcoal on titanium foil as negative electrode separated by polyvinyl alcohol/potassium hydroxide (PVA/KOH) gel-electrolyte as a solid-state asymmetric supercapacitor for high energy storage applications. Here, tin oxide quantum dots (SnO₂QDs) were successfully synthesized by a hydrothermal technique, and SnO₂QDs/GO/PPY ternary composite was synthesized by an in situ method with pyrrole monomer, SnO₂, and GO. A pH value controlled, which maintained the uniform size of SnO₂QDs dispersed on PPY, through GO ternary composite was used for fabricating the asymmetric supercapacitor electrode with the configuration (SnO₂QDs/GO/PPY)/GO/charcoal (85 : 10 : 5). The device achieved the highest specific capacitance of 1296 F g⁻¹, exhibited an energy density of 29.6 W h kg⁻¹ and the highest power density of 5310.26 W kg⁻¹ in the operating voltage from 0 to 1.2 V. The device also possessed excellent reliability and retained the capacitance of 90% after 11 000 GCD cycles. This ternary composite is a prominent material for potential applications in next-generation energy storage and portable electronic devices.

Representation of the synthesis steps of SnO₂QDs/GO/PPY ternary composites and SnO₂QDs/GO/PPY//GO/charcoal asymmetric supercapacitor device.     

Rice husk-derived nano-SiO2 assembled on reduced graphene oxide distributed on conductive flexible polyaniline frameworks towards high-performance lithium-ion batteries

By combining rice husk-derived nano-silica and reduced graphene oxide and then polymerizing PANI by in situ polymerization, we created polyaniline-coated rice husk-derived nano-silica@reduced graphene oxide (PANI-SiO₂@rGO) composites with excellent electrochemical performance. ATR-FTIR and XRD analyses confirm the formation of PANI-SiO₂@rGO, implying that SiO₂@rGO served as a template in the formation of composites. The morphology of PANI-SiO₂@rGO was characterized by SEM, HRTEM, and STEM, in which SiO₂ nanoparticles were homogeneously loaded on graphene sheets and the PANI fibrous network uniformly covers the SiO₂@rGO composites. The structure can withstand the large volume change as well as retain electronic conductivity during Li-ion insertion/extraction. Over 400 cycles, the assembled composite retains a high reversible specific capacity of 680 mA h g⁻¹ at a current density of 0.4 A g⁻¹, whereas the SiO₂@rGO retains only 414 mA h g⁻¹ at 0.4 A g⁻¹ after 215 cycles. The enhanced electrochemical performance of PANI-SiO₂@rGO was a result of the dual protection provided by the PANI flexible layer and graphene sheets. PANI-SiO₂@rGO composites may pave the way for the development of advanced anode materials for high-performance lithium-ion batteries.

By combining rice husk-derived nano-silica and reduced graphene oxide and then polymerizing PANI by in situ polymerization, we created polyaniline-coated rice husk-derived nano-silica@reduced graphene oxide composites with excellent electrochemical performance.     

Oxygen reduction reaction activity of an iron phthalocyanine/graphene oxide nanocomposite

Electrocatalysts with metal–nitrogen–carbon (M–N–C) sites have recently attracted much attention as potential catalysts for the oxygen reduction reaction (ORR), and a hybrid of iron phthalocyanine (FePc) and reduced graphene oxide (rGO) is one of the promising candidates. Herein, a FePc/GO nanocomposite was synthesized by electrostatic deposition on the electrode. The electrochemically reduced FePc/GO nanocomposite (ER(FePc/GO)) contained Fe²⁺ centers in well reduced graphene sites without agglomeration. The ER(FePc/GO) exhibited high ORR activity with an ORR onset (E_onset) and half-wave potential (E_1/2) of 0.97 and 0.86 V, respectively. Furthermore, the ORR activity successfully improved by adding an electrolyte such as KCl or KNO₃. The small H₂O₂ yield of 2%, superior tolerance to methanol addition and high-durability indicate that the ER(FePc/GO) is a promising electrocatalyst. Theoretical studies, indicating that the presence of Cl⁻ and NO₃⁻ ions lowered the conversion energy barrier, strongly supported the experimental results.

We demonstrate from both experimental and theoretical viewpoints that an iron phthalocyanine/graphene oxide nanocomposite directly synthesized on electrode exhibits high ORR activity.     

Influence of SiO2 or h-BN substrate on the room-temperature electronic transport in chemically derived single layer graphene

The substrate effect on the electronic transport of graphene with a density of defects of about 0.5% (^0.5%G) is studied. Devices composed of monolayer ^0.5%G, partially deposited on SiO₂ and h-BN were used for transport measurements. We find that the ^0.5%G on h-BN exhibits ambipolar transfer behaviours under ambient conditions, in comparison to unipolar p-type characters on SiO₂ for the same flake. While intrinsic defects in graphene cause scattering, the use of h-BN as a substrate reduces p-doping.

Defects in graphene cause scattering and basal plane interactions shift the Dirac-point.

Wet-chemically prepared graphene from graphite can be stabilized in solution by covalently bound oxo-groups using established oxidation protocols.^1–3 In general, the materials obtained are termed graphene oxide (GO). However, the chemical structure varies and the carbon lattice may even be amorphous due to the evolution of CO₂ during synthesis.⁴ Thus, in this study we use oxo-functionalized graphene (oxo-G), a type of GO with a more defined structure, as proven in our previous work.³ The oxygen-containing groups on the graphene basal plane and rims of flakes and holes make GO a p-type semiconductor with a typical resistance of 10¹⁰–10¹³ Ω sq^−1 5,6 and a band gap of about 2.2 eV.^7,8 The reductive defunctionalization of GO leads to a certain type of graphene (G), often named reduced GO (r-GO).^4,9 Removal of oxo-groups from the surface can be achieved by chemical reduction,^9,10 electrochemical methods,^11,12 electron beam treatment¹³ and was observed in situ by transmission electron microscopy.¹³ Thermal processing of GO instead leads to a disproportionation reaction forming carbon with additional vacancy defects and CO₂.¹⁴ In general, the reduction of GO turns r-GO from a semi-conductive material to a semi-metal. Mobility values were determined in field effect transistor (FET) devices.^15,16 Generally, the quality of graphene strongly depends on the integrity of the hexagonal carbon lattice. Thus, mobility values of 10⁻³ and up to 10³ cm² V⁻¹ s⁻¹ were reported,^3,17,18 with the resistance fluctuating between 10³ and 10⁶ Ω sq⁻¹.^19–21 We reported on the highest mobility values of chemically reduced oxo-G (with about 0.02% of lattice defects) of 1000 cm² V⁻¹ s⁻¹,³ determined by Hall-bar measurements at 1.6 K.Hexagonal boron nitride (h-BN) has been proved to be an excellent substrate for matching graphene-based materials owing to its atomic flatness, chemical inertness and electronic insulation due to a bandgap of ∼5.5 eV.²² Up to now, most studies with graphene deposited on h-BN were restricted to measurements with virtually defect-free graphene.²³ To the best of the authors knowledge, no studies reported transport measurements based on single layers of GO or oxo-G on h-BN substrates. No studies are reported with graphene derived from GO or oxo-G on single-layer level. Recently, we found that chemical reactions can be selectively conducted close to the rims of defects.²⁴ However, before functionalized devices can be studied, the lack of knowledge on the ambient environment device performances of graphene with defects and the influence of substrates must be addressed. Therefore, we fabricated the devices composed of ^0.5%G, partially deposited on SiO₂ (SiO₂/^0.5%G) and h-BN (h-BN/^0.5%G) (Fig. 1). Areas of the same flake on both materials are used to ensure reliable measurements and to prove that the results stem from the influence of the substrate rather than from the difference between devices. Thereby, the ^0.5%G exhibits an I_D/I_G ratio of about 3–4, corresponding to 0.5% of defects, according to the model introduced by Lucchese and Cançado.^25–28 Our results demonstrate that the h-BN layer is responsible for a downshift of the Dirac point and a more narrow hysteresis, resulting in ambipolar transfer behaviours in h-BN/^0.5%G.Open in a separate windowFig. 1(a) Optical image of the fabricated h-BN/^0.5%G heterostructure on SiO₂. (b) The h-BN/^0.5%G heterostructure device. Electrodes 1 and 2 define the SiO₂/^0.5%G FET device. Electrodes 1 and 3 define the ^0.5%G on overlapped SiO₂/h-BN hetero-substrate device. Electrodes 3 and 4 define the h-BN/^0.5%G FET device. Distance between the electrodes 1–2 and 3–4 is 1.5 μm and 3 μm, respectively. (c) 3D illustration of the h-BN/^0.5%G transistor device.     

Hypercrosslinked porous polymers hybridized with graphene oxide for water treatment: dye adsorption and degradation

Hypercrosslinked porous polymer hybridized graphene oxide with polymeric high internal phase emulsions (polyHIPEs/GO) were designed as versatile composites for water treatment. Morphologies, chemical composition and thermal stability of the composites were characterized by SEM, FTIR, XPS, XRD and TGA. Tunable adsorption properties and enhanced visible-light photocatalysis towards organic dyes were achieved by the manipulation of functional groups and the inclusion of Ag₃PO₄, respectively. The adsorption capacity of polyHIPEs/GO towards cationic methyl blue (MB) and rhodamine B (RB) is 1250.3 and 1054.1 μg g⁻¹, respectively. Aminated polyHIPEs/GO (polyHIPEs_(NH2)/GO) possesses an adsorption capacity of 1967.3 μg g⁻¹ to anionic eosin Y (EY). The tandem columns of polyHIPEs_(NH2)/GO and polyHIPEs/GO can successively and selectively remove the cationic and anionic dyes in a mixed dye solution. Furthermore, enhanced photodegradation ability was obtained after GO reduction and Ag₃PO₄ addition on polyHIPEs_(NH2)/GO. Results show that 3.5 × 10⁻⁵ M of MB, RB and EY can be completely photodegraded by 20 mg of the novel photocatalyst within 20, 40 and 35 min, respectively. This work demonstrates that polyHIPEs/GO exhibits tunable properties for multiply progressive applications in water treatment and catalysis.

We report the synthesis of graphene oxide hybridized polymeric high internal phase emulsions and their applications in adsorption and photocatalysis.     

Flexible solid-state supercapacitor based on tin oxide/reduced graphene oxide/bacterial nanocellulose

We demonstrate a flexible and light-weight supercapacitor based on bacterial nanocellulose (BNC) incorporated with tin oxide (SnO₂) nanoparticles, graphene oxide (GO) and poly(3,4-ethylenedioxyiophene)-poly(styrenesulfonate) (PEDOT:PSS). The SnO₂ and GO flakes are introduced into the fibrous nanocellulose matrix during bacteria-mediated synthesis. The flexible PEDOT:PSS/SnO₂/rGO/BNC electrodes exhibited excellent electrochemical performance with a capacitance of 445 F g⁻¹ at 2 A g⁻¹ and outstanding cycling stability with 84.1% capacitance retention over 2500 charge/discharge cycles. The flexible solid-state supercapacitors fabricated using PEDOT:PSS/SnO₂/rGO/BNC electrodes and poly(vinyl alcohol) (PVA)-H₂SO₄ coated BNC as a separator exhibited excellent energy storage performance. The fabrication method demonstrated here is highly scalable and opens up new opportunities for the fabrication of flexible cellulose-based energy storage devices.

A novel, simple and scalable method for the incorporation of tin oxide (SnO₂) and graphene oxide (GO) into bacterial nanocellulose during its growth for the fabrication of a flexible, scalable and environmental-friendly energy storage device was reported.     

Thermally reduced pillared GO with precisely defined slit pore size

Graphene oxide (GO) pillared with tetrakis(4-aminophenyl)methane (TKAM) molecules shows a narrow distribution of pore size, relatively high specific surface area, but it is hydrophilic and electrically not conductive. Analysis of XRD, N₂ sorption, XPS, TGA and FTIR data proved that the pillared structure and relatively high surface area (∼350 m² g⁻¹) are preserved even after thermal reduction of GO pillared with TKAM molecules. Unlike many other organic pillaring molecules, TKAM is stable at temperatures above the point of GO thermal reduction, as demonstrated by TGA. Therefore, gentle annealing results in the formation of reduced graphene oxide (rGO) pillared with TKAM molecules. The TKAM pillared reduced graphene oxide (PrGO/TKAM) is less hydrophilic as found using dynamic vapor sorption (DVS) and more electrically conductive compared to pillared GO, but preserves an increased interlayer-distance of about 12 Å (compared to ∼7.5 Å in pristine GO). Thus we provide one of the first examples of porous rGO pillared with organic molecules and well-defined size of hydrophobic slit pores. Analysis of pore size distribution using nitrogen sorption isotherms demonstrates a single peak for pore size of ∼7 Å, which makes PrGO/TKAM rather promising for membrane and molecular sieve applications.

The porous structure of tetrakis(4-aminophenyl)methane (TKAM)-pillared graphene oxide preserves after thermal reduction providing rare example of true pillared reduced GO material with precise slit pore size and sizable surface area.     

Three-dimensional graphene oxide cross-linked by benzidine as an efficient metal-free photocatalyst for hydrogen evolution

The use of low-cost photocatalysts to split water into H₂ fuel via solar energy is highly desirable for the production of clean energy and a sustainable society. Here three-dimensional graphene oxide (3DG) porous materials were prepared by cross-linking graphene oxide (GO) sheets using aromatic diamines (benzidine, 2,2′-dimethyl-4,4′-biphenyldiamine, 4,4′-diaminodiphenylmethane) that reacted with the carboxyl groups of the GO sheets at room temperature. The prepared 3DG porous materials were used as efficient metal-free photocatalysts for the production of H₂via water splitting under full-spectrum light, where the photocatalytic activity was highly dependent on the cross-linker and the 3DG reduction level. It was also found that the 3DG prepared with benzidine as the linker demonstrated a significantly higher H₂ evolution rate than the 3DGs prepared using 2,2′-dimethyl-4,4′-biphenyldiamine and 4,4′-diaminodiphenylmethane as the cross-linkers. The photoactivity was further tuned by varying the mass ratio of GO to benzidine. Among the prepared 3DG materials, 3DG-3, with an intermediate C/O ratio of 1.84, exhibited the highest H₂ production rate (690 μmol g⁻¹ h⁻¹), which was significantly higher than the two-dimensional GO (45 μmol g⁻¹ h⁻¹) and the noncovalent 3DG synthesized by a hydrothermal method (128 μmol g⁻¹ h⁻¹). Moreover, this study revealed that the 3DG photocatalytic performance was favored by effective charge separation, while it could be further tuned by changing the reduction level. In addition, these results could prompt the preparation of other 3D materials and the application of new types of photocatalysts for H₂ evolution.

Three-dimensional graphene oxide covalently linked by benzidine works as an efficient metal-free photocatalyst for H₂ evolution.     

Agricultural waste-derived activated carbon/graphene composites for high performance lithium-ion capacitors

Agricultural waste, corncob-derived activated carbon (AC) is prepared by pre-carbonization of the precursors and activation of KOH of the pyrolysis products. The AC oxidized by HNO₃ is called OAC. The OAC/reduced graphene oxide (rGO) composites are prepared by urea reduction (aqueous mixture of OAC and graphene oxide). The influence of the mass ratio of graphene oxide (GO) on the electrochemical properties of OAC/rGO composites as electrode materials for electrochemical capacitors is studied. It is found that the rGO sheets were used as a wrinkled carrier to support the OAC particles. The pore size distribution and surface area are dependent on the GO mass ratio. In addition, the rate capability of OAC is improved by introducing GO. For the OAC/rGO composites prepared from precursors with a GO mass ratio of 5%, the best rate performance was achieved. The lithium ion capacitor, based on OAC/rGO as cathode and Si/C as anode, exhibits a high energy density of 141 W h kg⁻¹ at 1391 W kg⁻¹. 78.98% capacity retention is achieved after 1000 cycles at 0.4 A g⁻¹.

Agricultural waste, corncob-derived activated carbon (AC) is prepared by pre-carbonization of the precursors and activation of KOH of the pyrolysis products.     

Sonochemistry-enabled uniform coupling of SnO2 nanocrystals with graphene sheets as anode materials for lithium-ion batteries

SnO₂/graphene nanocomposite was successfully synthesized by a facile sonochemical method from SnCl₂ and graphene oxide (GO) precursors. In the sonochemical process, the Sn²⁺ is firstly dispersed homogeneously on the GO surface, then in situ oxidized to SnO₂ nanoparticles on both sides of the graphene nanosheets (RGO) obtained by the reduction of GO under continuous ultrasonication. Graphene not only provides a mechanical support to alleviate the volume changes of the SnO₂ anode and prevent nanoparticle agglomeration, but also serves as a conductive network to facilitate charge transfer and Li⁺ diffusion. When used as a lithium ion battery (LIB) anode, the SnO₂/graphene nanocomposite exhibits significantly improved specific capacity (1610 mA h g⁻¹ at 100 mA g⁻¹), good cycling stability (retaining 87% after 100 cycles), and competitive rate performance (273 mA h g⁻¹ at 500 mA g⁻¹) compared to those of bare SnO₂. This sonochemical method can be also applied to the synthesis of other metal-oxide/graphene composites and this work provides a large-scale preparation route for the practical application of SnO₂ in lithium ion batteries.

SnO₂/graphene nanocomposite was successfully synthesized by a facile sonochemical method from SnCl₂ and graphene oxide (GO) precursors.     

Facile preparation of flexible binder-free graphene electrodes for high-performance supercapacitors

A facile two-step strategy to prepare flexible graphene electrodes has been developed for supercapacitors using thermal reduction of graphene oxide (GO) and thermally reduced graphene oxide (TRGO) composite films. The tunable porous structure of the GO/TRGO film provided channels to release the high pressure generated by CO₂ gas. The graphene electrode obtained from reduced-GO/TRGO (1 : 1 in mass ratio) film showed great flexibility and high film density (0.52 g cm⁻³). Using the EMI-BF₄ electrolyte with a working voltage of 3.7 V, the as-fabricated free-standing reduced-GO/TRGO (1 : 1) film achieved a great gravimetric capacitance of 180 F g⁻¹ (delivering a gravimetric energy density of 85.6 W h kg⁻¹), a volumetric capacitance of 94 F cm⁻³ (delivering a volumetric energy density of 44.7 W h L⁻¹), and a 92% retention after 10 000 charge/discharge cycles. In addition, the solid state flexible supercapacitor with the free-standing reduced-GO/TRGO (1 : 1) film as the electrodes and the EMI-BF₄/poly (vinylidene fluoride hexafluopropylene) (PVDF-HFP) gel as the electrolyte also demonstrated a high gravimetric capacitance of 146 F g⁻¹ with excellent mechanical flexibility, bending stability, and electrochemical stability. The strategy developed in this study provides great potentials for the synthesis of flexible graphene electrodes for supercapacitors.

A supercapacitor electrode is developed with a free-standing graphene film by a facile two-step strategy. The graphene electrode achieved a gravimetric capacitance of 180 F g⁻¹ and a volumetric capacitance of 94 F cm⁻³.     

A strategy for preparing non-fluorescent graphene oxide quantum dots as fluorescence quenchers in quantitative real-time PCR

In recent years, graphene oxide quantum dots (GOQDs) have emerged as novel nanomaterials for optical sensing, bioimaging, clinical testing, and environmental testing. However, GOQDs demonstrate unique photoluminescence properties, with GOQDs having quantum limitations and edge effects that often affect the accuracy of the test results in the sensory field. Herein, GOQDs with a large content of hydroxyl groups and low fluorescence intensity were first prepared via an improved Fenton reaction in this study, which introduces a large amount of epoxy groups to break the C–C bonds. The synthesized GOQDs show no significant variation in the fluorescence intensity upon ultraviolet and visible light excitations. We further utilized the GOQDs as fluorescence quenchers for different fluorescent dyes in real-time fluorescence quantitative polymerase chain reaction (qRT-PCR), and verified that the addition of GOQDs (5.3 μg ml⁻¹) into a qRT-PCR system could reduce the background fluorescence intensity of the reaction by fluorescence resonance energy transfer (FRET) during its initial stage and its non-specific amplification, and improve its specificity. In addition, the qRT-PCR method could detect two different lengths of DNA sequences with a high specificity in the 10⁴ to 10¹⁰ copies per μl range. It is of paramount importance to carry out further investigations to establish an efficient, sensitive, and specific RT-PCR method based on the use of GOQD nanomaterials as fluorescence quenchers.

Non-fluorescent GOQDs quench the fluorescence of TaqMan probes and increase the specificity of qRT-PCR by reducing non-specific amplification.     

Modification of graphene oxide film properties using KrF laser irradiation

Modification of various properties of graphene oxide (GO) films on SiO₂/Si substrate under KrF laser radiation was extensively studied. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and the electrical resistance measurements were employed to correlate the effects of laser irradiation on structural, chemical and electrical properties of GO films under different laser fluences. Raman spectroscopy shows reduced graphene oxide patterns with increased I_2D/I_G ratios in irradiated samples. X-ray photoelectron spectroscopy shows a high ratio of carbon to oxygen atoms in the reduced graphene oxide (rGO) films compared to the pristine GO films. X-ray diffraction patterns display a significant drop in the diffraction peak intensity after laser irradiation. Finally, the electrical resistance of irradiated GO films reduced by about four orders of magnitudes compared to the unirradiated GO films. Simultaneously, reduction and patterning of GO films display promising fabrication technique that can be useful for many graphene-based devices.

Modification of various properties of graphene oxide (GO) films on SiO₂/Si substrate under KrF laser radiation was extensively studied.     

A new insight into the transfer and delivery of anti-SARS-CoV-2 drug Carmofur with the assistance of graphene oxide quantum dot as a highly efficient nanovector toward COVID-19 by molecular dynamics simulation

Currently, a preventive and curative treatment for COVID-19 is an urgent global issue. According to the fact that nanomaterial-based drug delivery systems as risk-free approaches for successful therapeutic strategies may led to immunization against COVID-19 pandemic, the delivery of Carmofur as a potential drug for the SARS-CoV-2 treatment via graphene oxide quantum dots (GOQDs) was investigated in silico using molecular dynamics (MD) simulation. MD simulation showed that π–π stacking together with hydrogen bonding played vital roles in the stability of the Carmofur–GOQD complex. Spontaneous attraction of GOQDs loaded with Carmofur toward the binding pocket of the main protease (M_pro) resulted in the penetration of Carmofur into the active catalytic region. It was found that the presence of GOQD as an effective carrier in the loading and delivery of Carmofur inhibitor affected the structural conformation of M_pro. Higher RMSF values of the key residues of the active site indicated their greater displacement to adopt Carmofur. These results suggested that the binding pocket of M_pro is not stable during the interaction with the Carmofur–GOQD complex. This study provided insights into the potential application of graphene oxide quantum dots as an effective Carmofur drug delivery system for the treatment of COVID-19.

Potential usage of graphene oxide quantum dot as a M_pro inhibitor as well as an effective strategy in delivery of Carmofur into the active site of the main protease to combat COVID-19.     

UCN–SiO2–GO: a core shell and conjugate system for controlling delivery of doxorubicin by 980 nm NIR pulse

Herein, graphene oxide (GO) has been attached with core–shell upconversion-silica (UCN–SiO₂) nanoparticles (NPs) to form a GO–UCN–SiO₂ hybrid nanocomposite and used for controlled drug delivery. The formation of the nanocomposite has been confirmed by various characterization techniques. To date, a number of reports are available on GO and its drug delivery applications, however, the synergic properties that arise due to the combination of GO, UCNPs and SiO₂ can be used for controlled drug delivery. New composite UCN@SiO₂–GO has been synthesized through a bio-conjugation approach and used for drug delivery applications to counter the lack of quantum efficiency of the upconversion process and control sustained release. A model anticancer drug (doxorubicin, DOX) has been loaded to UCNPs, UCN@SiO₂ NPs and the UCN@SiO₂–GO nanocomposite. The photosensitive release of DOX from the UCN@SiO₂–GO nanocomposite has been studied with 980 nm NIR laser excitation and the results obtained for UCNPs and UCN@SiO₂ NPs compared. It is revealed that the increase in the NIR laser irradiation time from 1 s to 30 s leads to an increase in the amount of DOX release in a controlled manner. In vitro studies using model cancer cell lines have been performed to check the effectiveness of our materials for controlled drug delivery and therapeutic applications. Obtained results showed that the designed UCN@SiO₂–GO nanocomposite can be used for controlled delivery based therapeutic applications and for cancer treatment.

A GO–UCN–SiO₂ hybrid nanocomposite for loading of doxorubicin and its use in in vitro efficiency for killing carcinoma cells.     

Laser wavelength modulated pulsed laser ablation for selective and efficient production of graphene quantum dots

Graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs) can be used in different applications such as optoelectronic and biomedical applications, respectively. Hence, the selective synthesis of GQDs and GOQDs is highly desirable but challenging. Here, we present GQDs and GOQDs selectively prepared by an easy and simple pulsed laser ablation in liquid (PLAL) method by controlling the laser wavelength. The obtained GQDs and GOQDs showed a significantly different optoelectronic nature mainly due to the existence of surface oxygen-rich functional groups (e.g. carboxyl or hydroxy groups). Also, we described a possible mechanism for the formation of oxygen functional groups during the PLAL process based on the Coulomb explosion model, which can give further insight for designing functional carbon materials.

Graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs) can be selectively produced by wavelength-modulated pulsed laser ablation in liquid (PLAL) method, which can used in different applications such as optoelectronic and biomedical applications, respectively.

Graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs) are zero-dimensional graphene nanomaterials composed of one or few-layer graphene sheets.^1,2 Compared to large size graphene, GQDs or GOQDs exhibit unique optoelectrical properties due to quantum confinement or edge effects related to oxygen-rich functional groups.^3–6 Especially, GQDs exhibit a high quantum yield with excellent photostability owing to the intrinsic band structure and physicochemical robustness, which is beneficial for optoelectronic applications.^7–10 In addition, although the optical properties of GOQDs are less fascinating compared to those of GQDs, GOQDs have a high potential for biomedical applications, such as drug delivery systems (DDSs) and bioimaging, thanks to excellent biocompatibility with acceptable optical efficiency.^11–14To date, GQDs and GOQDs have been prepared mainly via wet chemical cutting routes. However, chemical reactions in strong acid as well as long-term washing procedure are generally required during a wet chemical process, which limits the practical applications of GQDs and GOQDs.^15–19 As an alternative method, pulsed laser ablation in liquid (PLAL) has recently attracted great attention for the synthesis of GQDs and GOQDs. The PLAL method is facile, simple, and environmentally benign because it does not require strong acidic chemical and post-purification steps.^20–23Coulomb explosion model has been proposed to account for the ablation mechanism of carbon materials by PLAL.^21,22 Briefly, when the laser pulses inject into the carbon precursor, ionization occurs by multiphoton absorption, resulting in the formation of high temperature and pressure plasma plume. In the plasma plume, Coulomb explosion can take place and subsequently the carbon precursors are ablated to the quantum size leading to the formation of GQDs. Simultaneously, the Coulomb explosion can decompose the solvent during the formation of GQDs or GOQDs.^23,24 The decomposed small molecules in the solvent can functionalize the GQDs by inducing chemical bonds with the surface of the GQDs. This can significantly alter the final optoelectronic properties of GQDs. Importantly, laser wavelength has a strong relationship with the decomposition of the solvent given that reaction area and depth between light and liquid matter is mainly determined by the wavelength of laser. Hence, not only natures of solvent but also the laser wavelength can significantly influence the optoelectronic functionalities of GQDs prepared by PLAL method. In our previous report, the effects of chemical properties of solvent on the functionalization of GQDs prepared by PLAL has been studied.²⁵ However, to our best knowledge, relationships between laser wavelength and the functionalization of GQDs during PLAL still remain as unexplored scientific area.GQDs and GOQDs were fabricated by PLAL methods using MWCNTs as carbon source in high-purity ethanol. Briefly, 50 mg of MWCNTs was dispersed in 500 mL of ethanol by ultrasonication. A Q-switch ND:YAG laser system was employed for PLAL. The MWCNTs suspension, which forms a vertical water column, was ablated by using the horizontal pulsed laser with the wavelength of 355 nm and 532 nm, respectively, at a repetition rate of 10 Hz and ablation energy of 50 mJ. The pulsed laser beam was focused on the center of MWCNTs suspension (see the ESI^† for Experimental details) (Fig. 1).Open in a separate windowFig. 1Representative schematic for the possible mechanism of the transform MWCNTs to GQDs and GOQDs using PLAL process (oxygen-rich site are shown as red dots).The morphology of starting MWCNTs was characterized by transmission electron microscopy (TEM) (Fig. S1^†). The TEM images of MWCNTs shows that the wall of CNT has consisted of approximately 25–27 layers of graphene sheets (Fig. S1^†). An average diameter of the tube was about 25 nm. Ultrathin morphology of MWCNTs can effectively suppress the pyrolytic phenomenon during PLAL, which is desirable for the formation of homogeneous GQDs or GOQDs colloidal.²⁵ The MWCNTs was transformed to GQDs by PLAL using 532 nm laser source. GQDs with size less than 5 nm was obtained after 10 min of ablation (Fig. 2a). The size distribution of GQDs is fitted by Gaussian curves and shown in Fig. S2a.^† An average diameter of 1–5 nm was calculated by counting more than 30 numbers of GQDs. HR-TEM image and corresponding fast Fourier transform (FFT) pattern of GQDs show a highly crystalline structure with a lattice parameter of ∼0.24 nm (Fig. 2b). No crystalline features that correspond to graphite, such as [002] plane, were observed by HR-TEM.^26,27 The atomic force microscopy (AFM) data in Fig. 2c and d illustrate the topographic morphology and the height distribution of GQDs. The height line profile in Fig. 2d shows that the thickness of GQDs is less than 1.5 nm corresponding to 3–4 graphene layers. Statistical analysis reveals that more than 85% of the GQDs has a thickness between 0.5 and 1.5 nm, indicating that the produced GQDs has a mono or a few layered structures.Open in a separate windowFig. 2Morphology characterization of GQDs and GOQDs. (a, b) TEM and HR-TEM images of GQDs and corresponding FFT image. (c, d) AFM image and height profile, height distribution of GQDs, respectively. (e, f) TEM and HR-TEM images of GOQDs and corresponding FFT images. (g, h) AFM image and height profile, height distribution of GOQDs, respectively.To investigate the effect of laser wavelength on the functionalities of GQDs, we employed the 355 nm laser source (3.4 eV) for PLAL process, which has much higher photon energy than 532 nm laser source (2.33 eV). MWCNTs was ablated in ethanol using the laser source with a wavelength of 355 nm. All the experimental parameters except for laser wavelength were kept identical during PLAL. X-ray photoelectron spectroscopy (XPS) was firstly carried out for the compositional analysis of GQDs ablated by different laser wavelengths, 532 and 355 nm. For the comparison purpose, starting MWCNTs was also studied by XPS. XPS spectra of C 1s for MWCNTs only presents sp² carbon peak at the binding energy of 284.4 eV (Fig. S3^†). Similarly, the GQDs ablated by 532 nm shows a major peak at 284.4 eV, which can be attributed to sp² carbon peak. The deconvoluted peaks at 286.0 eV and 288.8 eV are associated with hydroxyl or carboxyl functional groups bonded in sp³ orbital structure, respectively (Fig. 3a). According to quantitative analysis of the XPS spectra, fractions of sp² and sp³ carbon peaks are estimated as 86.55% and 13.45%, respectively. In contrast, the GOQDs ablated by 355 nm show a significantly increased fraction of sp³ carbon peaks from oxygen-rich functional groups (Fig. 3b). Quantitative fractions are calculated as 55.91% and 44.09% for sp² and sp³ carbon peaks, respectively (Table S1^†), indicating that GOQDs ablated by shorter wavelength (355 nm) is literally GOQDs rather than GQDs. These results suggest that the surface functionalization of GQDs by oxygeneous species can be easily achieved by simply changing laser wavelength for PLAL process.Open in a separate windowFig. 3XPS spectra GQDs and GOQDs. (a) GQDs and (b) GOQDs.TEM images revealed that the GOQDs have a diameter less than 5 nm (Fig. 2e). The HR-TEM image and the corresponding FFT pattern showed that the crystal structure of the GOQDs has a similar lattice structure with that of GQDs (Fig. 2b). The size distribution of GOQDs was calculated by counting about 45 GOQDs, and the result was fitted by Gaussian curves (Fig. S2b^†). An average diameter of 1–5 nm was obtained for the GOQDs. In addition, the height profile for GOQDs was obtained by using AFM, indicating that more than 90% of the GOQDs have topographic height of ∼1.5 nm. Consequently, most of the exfoliated GOQDs mainly exist as a single or few-layered graphene sheets. The size and height of GOQDs were comparable with those of the GQDs produced by low-energy laser source (i.e., longer wavelength). Also, the yield of PLAL process was calculated by dividing the weight of dried GQDs or GOQDs product by the weight of the starting MWCNTs. Based on this method, the yield of GQDs and GOQDs were determined as about 10–12%.The wavelength of laser can induce multiple effects on PLAL process. Noted that multiphoton absorption is a predominant ablation mechanism when high-frequency pulses such as nanosecond laser is employed for PLAL. The electron density (n₀) induced by laser pulse depends on the photon energy E and the number of photons (N).^21,28 Laser pulse with high photon energy can lead to more significant multiphoton-ionization and Coulomb explosion, expediting decomposition of molecules.^28–30 Hence, the solvent (i.e., ethanol in this study) can be photo-thermally decomposed during PLAL which forms cavitation bubbles, consisted of carbon- or oxygen-based small molecules, on the surface of MWCNTs. The surface functionalization of GQDs by oxygen functional groups during PLAL in ethanol can be described as eqn (1).³¹1The shorter laser wavelength with higher photon energies can facilitate decomposition of ethanol, correspondingly an easy formation of OH functional groups or defects on the surface of the ablated GQDs or MWCNTs. The OH-enriched solvent can efficiently functionalize the defect-enriched surface of GQDs, and thus the transformation from GOQs to GOQDs likely occurs.The optical properties of GQDs and GOQDs were investigated using PL (photoluminescence) and PLE (photoluminescence excitation) measurements. Fig. 4a show the PL spectra of the colloidal solutions of GQDs and the GOQDs. The PL intensity of GQDs was about 1.5 times higher than that of GOQDs. The digital images of the GQDs show distinct blue emission (the inset in Fig. 4a, left digital image), while the GOQDs exhibits a mixed emission of blue and green (the inset in Fig. 4a, right digital image). Also, the GQDs shows the excitation independent PL properties at excitation wavelength with 300 to 400 nm, in contrast the GOQDs have excitation dependent PL properties (Fig. S4^†). The PLE spectra was investigated at various detection emission wavelengths of PL spectrum (Fig. 4b). Both GQDs and GOQDs exhibit the PLE peaks at about 260 and 360 nm. However, the GOQDs have much broader PLE peak than the GQDs with emission dependent PLE properties. The characteristics of PL and PLE spectra reflect that emissions from the GQDs and GOQDs may have a different mechanism.Open in a separate windowFig. 4(a) PL spectra of GQDs and GOQDs in ethanol. (b) PLE spectra of GQDs and GOQDs with various emission source.To investigate PL mechanisms of GQDs and GOQDs, we carried out UV-vis and time-resolved photoluminescence (TRPL) analysis (Fig. 5). The PLE spectra of GOQDs shows excitation dependent PL properties (Fig. S4b^†), and also the UV-vis absorbance of GOQDs shows a broad absorption spectrum with a gradual change up to 700 nm (Fig. 5a). These properties of GOQDs are similar to previously reported PLE and UV-vis absorbance results of GOQDs with oxygen-rich functional groups.^32,33 Compared to GOQDs, the absorption spectrum of GQDs presents broad absorption peaks with high intensity at about 245 and 310 nm, respectively (Fig. 5b). These absorption peaks correspond to PLE peaks at about 260 and 360 nm, respectively (Fig. 4b). The mixed blue and green emission from GOQDs is originated from intrinsic states and/or extrinsic defect states in the bandgap. Notably, GQDs exhibits distinct blue emission with the strong absorption peak, indicating the intrinsic states in the band structure dominantly affect PL emissions of GQDs.^2,7 To clarify possible recombination mechanisms of GQDs and GOQDs, we carried out TRPL analysis (Fig. 5c and d). eqn (2)), where fluorescence decay occurs through three different relaxation pathways.fit = A + B₁e^(−t/τ1) + B₂e^(−t/τ2) + B₃e^(−t/τ3)2where “τ” is the fluorescence lifetime and “B” represents amplitude of the corresponding lifetime. The obtained chi-square (χ²) ranges between 1.05 and 1.1. The χ² value in the range of 1.0 < χ² < 1.2 is generally assumed to be acceptable for fitting. Generally, the emission that originates from defect states shows a longer recombination lifetime than that from intrinsic states.^34–36 Also, among the three lifetimes, one is associated with an intrinsic state, while the other two are due to the existence of oxygen-rich functional groups on the surface of GQDs and GOQDs. Fluorescence lifetimes of GQDs and GOQDs are recorded at 450 nm, where the excitation wavelength of a diode laser was 370 nm. The lifetimes of GQDs are τ1 = 1.3 ns (78%), τ2 = 3.1 ns (17%) and τ3 = 11 ns (5%), whereas those of GOQDs are τ1 = 0.7 ns (53%), τ2 = 3 ns (35%) and 9 ns (11%) (19 Two different electronic transitions were observed at 360 nm (3.4 eV) and 260 nm (4.76 eV) in the PLE spectra (Fig. 4b). Thus, δE can be calculated as 1.36 eV for GQDs and GOQDs, which is below the critical value of 1.5 eV. Hence, the assignment of two respective transition states is reasonable for GQDs and GOQDs. The blue emissions of GQDs can be attributed to the intrinsic state. The GQDs has a longer lifetime (τ1) for intrinsic states than GOQDs, and therefore the green emission can be effectively suppressed (Fig. 4a). Compared to GQDs, the quantitative fraction of lifetimes for τ2 and τ3 was much higher for GOQDs (46%) probably due to a large amount of oxygen-rich functional groups on the surface of GOQDs (Fig. 3b), resulting in mixed green and blue emission. The produced GQDs and GOQDs exhibit distinctly different optoelectronic properties, which can be used for different applications.Open in a separate windowFig. 5(a, b) UV-vis spectra of GQDs and GOQDs, respectively. (c, d) TCSPC decay curves of the GQDs and GOQDs with excitation wavelength at 370 nm and emission wavelength at 450 nm, respectively. The green and blue line show the three exponential fits of GQDs and GOQDs, respectively.Excitation emission values, χ² value, excitation lifetimes, and their corresponding amplitudes for GQDs and GOQDs

Solution-processed graphene oxide electrode for supercapacitors fabricated using low temperature thermal reduction

We present a low temperature and solution-based fabrication process for reduced graphene oxide (rGO) electrodes for electric double layer capacitors (EDLCs). Through the heat treatment at 180 °C between the spin coatings of graphene oxide (GO) solution, an electrode with loosely stacked GO sheets could be obtained, and the GO base coating was partially reduced. The thickness of the electrodes could be freely controlled as these electrodes were prepared without an additive as a spacer. The GO coating layers were then fully reduced to rGO at a relatively low temperature of 300 °C under ambient atmospheric conditions, not in any chemically reducing environment. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that the changes in oxygen functional groups of GO occurred through the heat treatments at 180 and 300 °C, which clearly confirmed the reduction from GO to rGO in the proposed fabrication process at the low thermal reduction temperatures. The structural changes before and after the thermal reduction of GO to rGO analyzed using Molecular Dynamic (MD) simulation showed the same trends as those characterized using Raman spectroscopy and XPS. An EDLC composed of the low temperature reduced rGO-based electrodes and poly(vinyl alcohol)/phosphoric acid (PVA/H₃PO₄) electrolyte gel was shown to have high specific capacitance of about 240 F g⁻¹ together with excellent energy and power densities of about 33.3 W h kg⁻¹ and 833.3 W kg⁻¹, respectively. Furthermore, a series of multiple rGO-based EDLCs was shown to have fast charging and slow discharging properties that allowed them to light up a white light emitting diode (LED) for 30 min.

We present a low temperature and solution-based fabrication process for reduced graphene oxide (rGO) electrodes for electric double layer capacitors (EDLCs).     

Building with graphene oxide: effect of graphite nature and oxidation methods on the graphene assembly

During nearly 2 centuries of history in graphene researches, numerous researches were reported to synthesize graphene oxide (GO) and build a proper graphene assembly. However, tons of research prevail without verifying the reproducibility of GO that can be sensitively attributed by the graphite nature, and chemical processes. Here, the structure and chemistry of GO products were analyzed by considering parent graphite sources, and three different oxidation methods based on Hummer''s method and the addition of H₃PO₄. The oxidation level of GO was characterized by monitoring the C/O and sp² carbon ratio from X-ray photoelectroscopy (XPS) spectra. It was observed that the oxidant intercalation behavior was dependent on the morphological differences of graphite; synthetic and natural flake graphite were compared based on their origins in shape and size from different suppliers. Thermal reduction and exfoliation were applied to GO powders to prepare thermally expanded graphene oxide (TEGO) as a graphene assembly. Gas releases from the reduction of oxygen functional groups split layered GO structure and build a porous structure that varied specific surface area regarding oxidation degrees of GO.

In this study, the article investigated the effect of starting graphite sources, and oxidation methods on graphene oxide (GO) synthesis and the porous structure of building assembly into thermally expanded graphene oxide (TEGO).     

Graphene-oxide loading on natural zeolite particles for enhancement of adsorption properties

Multiple methods of grafting graphene oxide (GO) nanosheets to natural clinoptilolite-rich zeolite particles were developed in our laboratory. In this study, we have systematically characterized the GO coated particles prepared by various methods to select the most promising method for further research efforts. This study revealed that the most promising coating method was the clean-acid-treated zeolite particles followed by deposition of GO nanosheets onto the zeolite surface and mild thermal treatment of the particles. GO and its synergistic interaction in zeolite was attributed to electrostatic interactions, hydrophobic interactions and hydrogen bonds. Hydrophobic interactions are enhanced both due to dealumination of zeolite caused by the cleaning method followed by acid treatment and due to partial thermal deoxygenation of GO. This method provided a ten times larger surface area (from 10.55 m² g⁻¹ to 117.96 m² g⁻¹) and three times smaller pore diameter (from 81.91 Å to 30.68 Å), providing great particles for a variety of applications as adsorbents or catalysts.

Multiple methods of grafting graphene oxide (GO) nanosheets to natural clinoptilolite-rich zeolite particles were developed in our laboratory.