Modulation of Oxygen Content in Graphene Surfaces Using Temperature-Programmed Reductive Annealing: Electron Paramagnetic Resonance and Electrochemical Study

Engineering Thermally Reduced Graphene Oxide for Synchronously Enhancing Photocatalytic Activity and Photothermal Effect

The structural composition of reduced graphene oxide (rGO) can be modified and controlled by appropriate reduction methods to modulate its electronic structure, rendering it a versatile platform for tailoring optoelectronic and catalytic properties. Nevertheless, it is uncommon to concurrently amplify the photocatalytic and photothermal effects when regulating and utilizing pure rGO. Here, we investigate the impact of structural variations in thermally reduced graphene oxide (TGO) on its photocatalytic and photothermal properties. Various characterization results demonstrate that appropriate thermal reduction facilitates the preservation and transformation of oxygenated groups and structure defects, which in turn encourages the formation of reactive carbon radicals and discrete graphitic domains, thereby strengthening the activation of molecular oxygen and the plasmonic photothermal effect under near-infrared (NIR) light irradiation. Moreover, the optimized TGOs exhibit efficient sterilization with NIR irradiation due to enhanced photocatalytic activities and photothermal effects. This work highlights the potential for developing photocatalytic and photothermal rGO-based materials through structural engineering.

The Enhanced Photoluminescence Properties of Carbon Dots Derived from Glucose: The Effect of Natural Oxidation

The investigation of the fluorescence mechanism of carbon dots (CDs) has attracted significant attention, particularly the role of the oxygen-containing groups. Dual-CDs exhibiting blue and green emissions are synthesized from glucose via a simple ultrasonic treatment, and the oxidation degree of the CDs is softly modified through a slow natural oxidation approach, which is in stark contrast to that aggressively altering CDs' surface configurations through chemical oxidation methods. It is interesting to find that the intensity of the blue fluorescence gradually increases, eventually becoming the dominant emission after prolonging the oxidation periods, with the quantum yield (QY) of the CDs being enhanced from ~0.61% to ~4.26%. Combining the microstructure characterizations, optical measurements, and ultrafiltration experiments, we hypothesize that the blue emission could be ascribed to the surface states induced by the C-O and C=O groups, while the green luminescence may originate from the deep energy levels associated with the O-C=O groups. The distinct emission states and energy distributions could result in the blue and the green luminescence exhibiting distinct excitation and emission behaviors. Our findings could provide new insights into the fluorescence mechanism of CDs.

Unravelling the formation of carbyne nanocrystals from graphene nanoconstrictions through the hydrothermal treatment of agro-industrial waste molasses

The delicate synthesis of one-dimensional (1D) carbon nanostructures from two-dimensional (2D) graphene moiré layers holds tremendous interest in materials science owing to its unique physiochemical properties exhibited during the formation of hybrid configurations with sp-sp2 hybridization. However, the controlled synthesis of such hybrid sp-sp2 configurations remains highly challenging. Therefore, we employed a simple hydrothermal technique using agro-industrial waste as the carbon source to synthesize 1D carbyne nanocrystals from the nanoconstricted zones of 2D graphene moiré layers. By employing suite of characterization techniques, we delineated the mechanism of carbyne nanocrystal formation, wherein the origin of carbyne nanochains was deciphered from graphene intermediates due to the presence of a hydrothermally cut nanoconstriction regime engendered over well-oriented graphene moiré patterns. The autogenous hydrothermal pressurization of agro-industrial waste under controlled conditions led to the generation of epoxy-rich graphene intermediates, which concomitantly gave rise to carbyne nanocrystal formation in oriented moiré layers with nanogaps. The unique growth of carbyne nanocrystals over a few layers of holey graphene exhibits excellent paramagnetic properties, the predominant localization of electrons and interfacial polarization effects. Further, we extended the application of the as-synthesized carbyne product (Cp) for real-time electrochemical-based toxic metal (As3+) sensing in groundwater samples (from riverbanks), which depicted superior sensitivity (0.22 mA μM-1) even at extremely lower concentrations (0.0001 μM), corroborating the impedance spectroscopy analysis.

Scrolling reduced graphene oxides to induce room temperature magnetism via spatial coupling of defects

Due to its intriguing features and numerous applications, graphene has garnered a lot of interest in recent years. However, it is still very difficult to create graphene-based room-temperature magnets without transition metals or rare earth elements since pristine graphene is inherently diamagnetic due to the delocalized π bonding network. Herein, room-temperature ferromagnetism with a saturation magnetization of 0.93 emu g-1 (300 K) is achieved in defect-rich-reduced graphene oxide (DR-rGO) nanoscrolls by creating a spatial coupling of defects. The experiments and DFT calculations verify that spatial coupling of defects could enhance Rudermann-Kittel-Kasuya-Yosida interactions to induce magnetism in graphene. It displays high-efficiency electromagnetic wave absorption performance with a minimal reflection loss of -62.1 dB and an effective absorption bandwidth of 7.8 GHz (3.0 mm) thanks to greatly improved magnetism. This breakthrough serves as a building block for the creation of room-temperature magnetic carbon materials and expands their applications in many pertinent domains.

Crosslinking Multilayer Graphene by Gas Cluster Ion Bombardment

In this paper, we demonstrate a new, highly efficient method of crosslinking multilayer graphene, and create nanopores in it by its irradiation with low-energy argon cluster ions. Irradiation was performed by argon cluster ions with an acceleration energy E ≈ 30 keV, and total fluence of argon cluster ions ranging from 1 × 109 to 1 × 1014 ions/cm2. The results of the bombardment were observed by the direct examination of traces of argon-cluster penetration in multilayer graphene, using high-resolution transmission electron microscopy. Further image processing revealed an average pore diameter of approximately 3 nm, with the predominant size corresponding to 2 nm. We anticipate that a controlled cross-linking process in multilayer graphene can be achieved by appropriately varying irradiation energy, dose, and type of clusters. We believe that this method is very promising for modulating the properties of multilayer graphene, and opens new possibilities for creating three-dimensional nanomaterials.

Application of Molecular Catalysts for the Oxygen Reduction Reaction in Alkaline Fuel Cells

The development of precious group metal-free (PGM-free) catalysts for the oxygen reduction reaction is considered as the main thrust for the cost reduction of fuel cell technologies and their mass production. Within the PGM-free category, molecular catalysts offer an advantage over other heat-treated PGM-free catalysts owing to their well-defined structure, which enables further design of more active, selective, and durable catalysts. Even though non-heat-treated molecular catalysts with exceptional performance have been reported in the past, they were rarely tested in a fuel cell. Herein, we report on a molecular catalyst under alkaline conditions: fluorinated iron phthalocyanine (FeFPc) supported on cheap and commercially available high-surface area carbon─BP2000 (FeFPc@BP2000). It exhibits the highest activity ever reported for molecular catalysts under alkaline conditions in half-cells and fuel cells.

Graphene Oxide and Reduced Graphene Oxide Nanoflakes Coated with Glycol Chitosan, Propylene Glycol Alginate, and Polydopamine: Characterization and Cytotoxicity in Human Chondrocytes

Recently, graphene and its derivatives have been extensively investigated for their interesting properties in many biomedical fields, including tissue engineering and regenerative medicine. Nonetheless, graphene oxide (GO) and reduced GO (rGO) are still under investigation for improving their dispersibility in aqueous solutions and their safety in different cell types. This work explores the interaction of GO and rGO with different polymeric dispersants, such as glycol chitosan (GC), propylene glycol alginate (PGA), and polydopamine (PDA), and their effects on human chondrocytes. GO was synthesized using Hummer's method, followed by a sonication-assisted liquid-phase exfoliation (LPE) process, drying, and thermal reduction to obtain rGO. The flakes of GO and rGO exhibited an average lateral size of 8.8 ± 4.6 and 18.3 ± 8.5 µm, respectively. Their dispersibility and colloidal stability were investigated in the presence of the polymeric surfactants, resulting in an improvement in the suspension stability in terms of average size and polydispersity index over 1 h, in particular for PDA. Furthermore, cytotoxic effects induced by coated and uncoated GO and rGO on human chondrocytes at different concentrations (12.5, 25, 50 and 100 µg/mL) were assessed through LDH assay. Results showed a concentration-dependent response, and the presence of PGA contributed to statistically decreasing the difference in the LDH activity with respect to the control. These results open the way to a potentially safer use of these nanomaterials in the fields of cartilage tissue engineering and regenerative medicine.

Graphene Oxide-Doped Gellan Gum-PEGDA Bilayered Hydrogel Mimicking the Mechanical and Lubrication Properties of Articular Cartilage

Articular cartilage (AC) is a specialized connective tissue able to provide a low-friction gliding surface supporting shock-absorption, reducing stresses, and guaranteeing wear-resistance thanks to its structure and mechanical and lubrication properties. Being an avascular tissue, AC has a limited ability to heal defects. Nowadays, conventional strategies show several limitations, which results in ineffective restoration of chondral defects. Several tissue engineering approaches have been proposed to restore the AC's native properties without reproducing its mechanical and lubrication properties yet. This work reports the fabrication of a bilayered structure made of gellan gum (GG) and poly (ethylene glycol) diacrylate (PEGDA), able to mimic the mechanical and lubrication features of both AC superficial and deep zones. Through appropriate combinations of GG and PEGDA, cartilage Young's modulus is effectively mimicked for both zones. Graphene oxide is used as a dopant agent for the superficial hydrogel layer, demonstrating a lower friction than the nondoped counterpart. The bilayered hydrogel's antiwear properties are confirmed by using a knee simulator, following ISO 14243. Finally, in vitro tests with human chondrocytes confirm the absence of cytotoxicity effects. The results shown in this paper open the way to a multilayered synthetic injectable or surgically implantable filler for restoring AC defects.

Wear Behavior Characterization of Hydrogels Constructs for Cartilage Tissue Replacement

This paper aims to characterize the wear behavior of hydrogel constructs designed for human articular cartilage replacement. To this purpose, poly (ethylene glycol) diacrylate (PEGDA) 10% w/v and gellan gum (GG) 1.5% w/v were used to reproduce the superior (SUP) cartilage layer and PEGDA 15% w/v and GG 1.5% w/v were used to reproduce the deep (DEEP) cartilage layer, with or without graphene oxide (GO). These materials (SUP and DEEP) were analyzed alone and in combination to mimic the zonal architecture of human articular cartilage. The developed constructs were tested using a four-station displacement control knee joint simulator under bovine calf serum. Roughness and micro-computer tomography (µ-CT) measurements evidenced that the hydrogels with 10% w/v of PEGDA showed a worse behavior both in terms of roughness increase and loss of uniformly distributed density than 15% w/v of PEGDA. The simultaneous presence of GO and 15% w/v PEGDA contributed to keeping the hydrogel construct's characteristics. The Raman spectra of the control samples showed the presence of unreacted C=C bonds in all the hydrogels. The degree of crosslinking increased along the series SUP < DEEP + SUP < DEEP without GO. The Raman spectra of the tested hydrogels showed the loss of diacrylate groups in all the samples, due to the washout of unreacted PEGDA in bovine calf serum aqueous environment. The loss decreased along the series SUP > DEEP + SUP > DEEP, further confirming that the degree of photo-crosslinking of the starting materials plays a key role in determining their wear behavior. μ-CT and Raman spectroscopy proved to be suitable techniques to characterize the structure and composition of hydrogels.

2D Sandwiched Nano Heterostructures Endow MoSe2 /TiO2- x /Graphene with High Rate and Durability for Sodium Ion Capacitor and Its Solid Electrolyte Interphase Dependent Sodiation/Desodiation Mechanism

Nano heterostructures relying on their versatile construction and the breadth of combined functionality have shown great potential in energy storage fields. Herein, 2D sandwiched MoSe₂ /TiO_{2- x} /graphene nano heterostructures are designed by integrating structural and functional effects of each component, aiming to address the rate capability and cyclic stability of MoSe₂ for sodium ion capacitors (SICs). These 2D nano heterostructures based on graphene platform can facilitate the interfacial electron transport, giving rise to fast reaction kinetics. Meanwhile, the 2D open structure induces a large extent of surface capacitive contribution, eventually leading to a high rate capability (415.2 mAh g^-1 @ 5 A g^-1 ). An ultrathin oxygen deficient TiO_{2- x} layer sandwiched in these nano heterostructures provides a strong chemical-anchoring regarding the products generated during the sodiation/desodiation process, securing the entire cyclic stability. The associated sodiation/desodiation mechanism is revealed by operando and ex situ characterizations, which exhibits a strong solid electrolyte interphase (SEI) dependence. The simulations verify the dependent sodiation products and enhanced heterostructural chemical-anchoring. Assembled SICs based on these nano heterostructures anode exhibit high initial Coulombic efficiency, energy/power densities, and long cycle life, shedding new light on the design of nano heterostructure electrodes for high performance energy storage application.

Graphene-based carbocatalysts for carbon-carbon bond formation

Organic transformations are usually catalyzed by metal-based catalysts. In contrast, metal-free catalysts have attracted considerable attention from the viewpoint of sustainability and safety. Among the studies in metal-free catalysis, graphene-based materials have been introduced in the reactions that are usually catalyzed by transition metal catalysts. This review covers the literature (up to the beginning of April 2020) on the use of graphene and its derivatives as carbocatalysts for C-C bond-forming reactions, which are one of the fundamental reactions in organic syntheses. Besides, mechanistic studies are included for the rational understanding of the catalysis. Graphene has significant potential in the field of metal-free catalysis because of the fine-tunable potential of the structure, high stability and durability, and no metal contamination, making it a next-generation candidate material in catalysis.

Photoactivated Graphene Oxide to Enhance Photocatalytic Reduction of CO2

Graphene oxide (GO) is commonly used in photocatalytic reactions but mainly as a cocatalyst. Limited information is available on the intrinsic ability of GO to photocatalytically reduce CO₂ as a sole photocatalyst and the activation of light irradiation. In this study, simulated sunlight (SS) and UV-irradiated GO (GO_SS and GO_UV, respectively) showed enhanced efficiency of photocatalytic reduction of CO₂ compared to pristine GO, with a CO yield (4 h) ratio of GO_SS/GO_UV/GO of 2.7:2.1:1. Here, irradiation plays two important roles: (1) irradiating GO to eliminate CO released under photolysis from photocatalytic reactions and (2) activating GO to create defects and restore the large π-conjugated network, obtaining photolysis-saturated and photoactivated GO for photocatalytic reduction of CO₂. The increasing defect density and π conjugation of irradiated GO, as supported by X-ray photoelectron and Raman spectroscopy, improve the yield of photoelectrons and prolong the lifetime of photogenerated charge carriers, as supported by electron spin resonance and transient absorption spectroscopy. This results in an enhanced photocatalytic efficiency of irradiated GO. The higher CO yield of GO_SS compared to GO_UV indicates that simulated sunlight irradiation is more favorable for GO activation. Our results show that activating GO under irradiation enhances the photocatalytic reduction of CO₂.

Functionalized Carbon Dots on Graphene as Outstanding Non-Metal Bifunctional Oxygen Electrocatalyst

Carbon-based bifunctional electrocatalysts for both oxygen reduction and evolution reactions are potentially cost-effective to replace noble metals in energy devices such as fuel cells, metal-air batteries, and photoelectrochemical converters, but enrichment of active sites holds the key to efficiency. Here, graphene frameworks with heteroatom-doped carbon dots (CDs) are developed via a hydrothermal route followed by pyrolysis. The CDs are rationally prepared with careful selection of heteroatoms, embedded on the substrate to provide enriched active sites. Structural characterizations (e.g., transmission electron microscopy and X-ray photoelectron spectroscopy) reveal the successful addition of CDs with nitrogen and sulfur species. Especially, a heat-treated N,S codoped sample, NS-CD@gf_a900, exhibits the optimum oxygen electrocatalysis, even closer to noble-metal counterparts, as a result of the effect of active sites of the CDs and the synergistic behavior of N and S. Considering the importance of size and dopants of the material, this approach not only suggests a straightforward preparation route of nanocarbons, but also appoints the utilization of a new class of non-metal species as efficient oxygen electrocatalysts.

Metal organic frameworks as a catalyst for oxygen reduction: an unexpected outcome of a highly active Mn-MOF-based catalyst incorporated in activated carbon

Owing to their unique chemistry and physical properties, metal-organic frameworks (MOFs) are an interesting class of materials which can be utilized for a wide array of applications. MOFs have been proposed to be used as catalysts for fuel cells, but their low intrinsic electronic conductivity hampered their utilization as is. In this work, we present the synthesis and application of MOF-based precious-metal-group-free (PGM-free) catalysts for oxygen reduction based on a unique metal-organic framework-carbon composite material. Benzene tricarboxylic acid-based MOFs were synthesized inside activated carbon (AC) with four different, first row transition metals: Mn, Fe, Co, and Cu. The MOFs@AC were analyzed electrochemically to measure their catalytic activity. Further physical and chemical characterization studies are performed to measure the material properties. The MOFs@AC are found to be conductive and active catalysts for the oxygen reduction reaction in an alkaline environment. Surprisingly, the Mn-MOF-based@AC exhibits the best performance with an onset potential of 0.9 V vs. RHE and the almost four-electron mechanism, as opposed to most other known PGM-free catalysts, which show Fe and Co as the most active metals.

Paramagnetic Properties of Fullerene-Derived Nanomaterials and Their Polymer Composites: Drastic Pumping Out Effect

The evolution of paramagnetic properties of the fullerene soot (FS), fullerene black (FB), and their polymer composites Phenylon C-2/FS, FB has been studied using the electron paramagnetic resonance (EPR) method. For the first time, a drastic growth of the EPR signals in the FB, FS, and composite samples was observed under pumping out at temperatures T = 20 ÷ 300 °C, which is attributed to the interaction between carbon defects and adsorbed gas molecules, mainly oxygen.It is shown that the ensemble of paramagnetic centers in the FB, FS, and the composite is heterogeneous. This ensemble consists of three spin subsystems 1, 2, and 3 related with different structural elements. The subsystems give three corresponding contributions, L 1, L 2 and L 3, into the overall contour of the EPR signal. The most intensive and broad signal L 3 is caused by 2D electrons from the surface of carbon flakes. Theoretical calculations of the L 3 signal line shape were carried out, and the decay rate of the integral intensity has been obtained for each component L 1, L 2, and L 3 after the contact of the sample with the ambient air. The signal decay process in the bulk composite samples is much slower due to their low gas permeability at room temperature (RT).

Effect of the GO Reduction Method on the Dielectric Properties, Electrical Conductivity and Crystalline Behavior of PEO/rGO Nanocomposites

The effect of the reduction method to prepare reduced graphene oxide (rGO) on the melt linear viscoelastic properties, electrical conductivity, polymer matrix crystalline behavior and dielectric properties of PEO-rGO nanocomposites was investigated. Reduction was performed chemically with either sodium borohydride (NaBH₄) or hydrazine monohydrate (N₂H₄·H₂O) or both reduction agents consecutively as well as thermally at 1000 °C. The different reduction methods resulted in exfoliated rGO sheets with different types and amounts of remaining functional groups, as indicated by FT-IR, Raman, TGA and XRD characterization. Moreover, their electrical conductivity ranged between 10⁻⁴ and 10⁻¹ S/cm, with the consecutive use of both chemical reduction agents being far superior. PEO nanocomposites with filler loadings of 0.5 wt %, 1 wt % and 2 wt % were prepared by solvent mixing. The rGO fillers affected the melt linear viscoelastic and crystalline behavior of the PEO matrix and resulted in nanocomposites with a substantially increased electrical conductivity. Despite the wide variability in filler conductivity, the effects on the polymer nanocomposite properties were less distinctive. A correlation was obtained between the reduction of the mobility of the polymer chains (evaluated by the glass transition temperature) and the dielectric strength of the interfacial polarisation originating from the effective entrapment of GO/rGO filler charges at the interface with the less conductive PEO. Thus, favorable interactions of the polar PEO with the filler led to reduced mobility of the PEO chains and thereby a more effective entrapment of the filler charges at the PEO interface.

Light-Enhanced Antibacterial Activity of Graphene Oxide, Mainly via Accelerated Electron Transfer

Before graphene derivatives can be exploited as next-generation antimicrobials, we must understand their behavior under environmental conditions. Here, we demonstrate how exposure to simulated sunlight significantly enhances the antibacterial activity of graphene oxide (GO) and reveal the underlying mechanism. Our measurements of reactive oxygen species (ROS) showed that only singlet oxygen (¹O₂) is generated by GO exposed to simulated sunlight, which contributes only slightly to the oxidation of antioxidant biomolecules. Unexpectedly, we find the main cause of oxidation is light-induced electron-hole pairs generated on the surface of GO. These light-induced electrons promote the reduction of GO, introducing additional carbon-centered free radicals that may also enhance the antibacterial activities of GO. We conclude that GO-mediated oxidative stress mainly is ROS-independent; simulated sunlight accelerates the transfer of electrons from antioxidant biomolecules to GO, thereby destroying bacterial antioxidant systems and causing the reduction of GO. Our insights will help support the development of graphene for antibacterial applications.