Structural and electronic properties of oxidized graphene

Origin of Charge Trapping in TiO2/Reduced Graphene Oxide Photocatalytic Composites: Insights from Theory

Composites of titanium dioxide (TiO₂) and reduced graphene oxide (RGO) have proven to be much more effective photocatalysts than TiO₂ alone. However, little attention has been paid so far to the chemical structure of TiO₂/RGO interfaces and to the role that the unavoidable residual oxygen functional groups of RGO play in the photocatalytic mechanism. In this work, we develop models of TiO₂ rutile (110)/RGO interfaces by including a variety of oxygen functional groups known to be present in RGO. Using hybrid density functional theory calculations, we demonstrate that the presence of oxygen functional groups and the formation of interfacial cross-links (Ti-O-C covalent bonds and strong hydrogen bonds between TiO₂ and RGO) have a major effect on the electronic properties of RGO and RGO-based composites. The electronic structure changes from semimetallic to semiconducting with an indirect band gap, with the lowest unoccupied band positioned below the TiO₂ conduction band and largely localized on RGO oxygen and carbon orbitals, with some contributions of RGO-bonded Ti atoms. We suggest that this RGO-based lowest unoccupied band acts as a photoelectron trap and the indirect nature of the band gap hinders electron-hole recombination. These results can explain the experimentally observed extended lifetimes of photoexcited charge carriers in TiO₂/RGO composites and the enhancement of photocatalytic efficiency of these composites.

Structural, electronic, optical, and thermodynamic properties of hydrochlorinated Janus graphene: a first-principle study

The structural, electronic, optical, and thermodynamic properties of hydrochlorinated Janus graphene (J-GN) have been studied using first-principle DFT calculations. The band structure and density of states have been discussed. The values of 16 parameters have been calculated for the most stable chair (C) structure of hydrochlorinated J-GN. Out of sixteen, 12 parameters such as static dielectric constant ε(0), refractive index n(0), birefringence Δn(0), threshold conductivity σ(ω), plasmon energy (ћω_p), binding energy (E_b), cohesive energy (E_c), enthalpy (E), entropy (S), free energy (F), heat capacity (C_p), and Debye temperature (Θ_D) have been calculated for the first time. The structural and electronic properties have also been studied at 0-GPa, 25-GPa, 35-GPa, 50-GPa, 90-GPa, 100-GPa, 150-GPa, 200-GPa, and 220-GPa external pressures. The hydrochlorinated J-GN shows the direct band gap behavior up to 35 GPa and becomes indirect band gap after 35 GPa. Further, it shows a stable structure up to 90 GPa and becomes unstable at 100-GPa external pressure. The calculated values of all parameters agree well with the available reported values of some parameters at 0 GPa.

Solid-State Fluorescent Carbon Dots with Aggregation-Induced Yellow Emission for White Light-Emitting Diodes with High Luminous Efficiencies

For practical applications of carbon dots (CDs), a major challenge is to prevent the notorious aggregation-caused quenching (ACQ) effect. Herein, a new type of CDs (CD1) has been developed that can transform from ACQ to an enhancement of fluorescence by aggregation-induced emission (AIE). The blue fluorescence of the CDs is suppressed by ACQ. However, this is accompanied by the phenomenon of AIE at a longer wavelength, resulting in the emergence and gradual enhancement of yellow fluorescence. The obtained CD1 solid powder shows a bright yellow emission with a photoluminescence quantum yield (PLQY) of 65%. The photoluminescence (PL) spectra, absorption spectra, and time-resolved PL decay curves indicate that Förster resonant energy transfer from dispersed CD1 particles to large CD1 agglomerations leads to the enhancement of yellow fluorescence. To exploit its high PLQY and strong AIE, CD1 is applied as a color-converting layer on blue light-emitting diode (LED) chips to fabricate white LEDs (WLEDs). The obtained devices show white light coordinates of (0.29, 0.38) and (0.32, 0.42), which are close to pure white light (0.33, 0.33), and luminous efficiencies of 97.8 and 93.9 lm·W^-1 and show good stability. The low cost, easy fabrication, controllability, and favorable fluorescence properties signify that CD1 of AIE will have superior performance in a variety of applications.

Computational studies on the molecular insights of aptamer induced poly(N-isopropylacrylamide)-graft-graphene oxide for on/off- switchable whole-cell cancer diagnostics

This work deals with first-principles and in silico studies of graphene oxide-based whole-cell selective aptamers for cancer diagnostics utilising a tunable-surface strategy. Herein, graphene oxide (GO) was constructed as a surface-based model with poly(N-isopropylacrylamide) (PNIPAM) covalently grafted as an "on/off"-switch in triggering interactions with the cancer-cell protein around its lower critical solution temperature. The atomic building blocks of the aptamer and the PNIPAM adsorbed onto the GO was investigated at the density functional theory (DFT) level. The presence of the monomer of PNIPAM stabilised the system's π-π interaction between GO and its nucleobases as confirmed by higher bandgap energy, satisfying the eigenvalues of the single-point energy observed rather than the nucleobase and the GO complex independently. The unaltered geometrical structures of the surface emphasise the physisorption type interaction between the nucleobase and the GO/NIPAM surface. The docking result for the aptamer and the protein, highlighted the behavior of the PNIPAM-graft-GO  is exhibiting globular and extended conformations, further supported by molecular dynamics (MD) simulations. These studies enabled a better understanding of the thermal responsive behavior of the polymer-enhanced GO complex for whole-cell protein interactions through computational methods.

Ratiometric Fluorescent Hydrogel Test Kit for On-Spot Visual Detection of Nitrite

In this work, we proposed a new method based on carbon dots (named m-CDs) for selective and efficient detection of nitrite (NO₂^-), which was based on the interaction between the amine group of m-CDs and NO₂^- via a diazo reaction that produced diazonium salts and induced the fluorescence quenching of m-CDs. The concentration of NO₂^- shows a good linear relationship with a quenched fluorescence intensity from 0.063 to 2.0 μM ( R² = 0.996) with a detection limit of 0.018 μM. In addition, a ratiometric fluorescence probe ( m-CDs@[Ru(bpy)₃]²⁺) was constructed via electrostatic interaction by introducing Ru(bpy)₃Cl₂·6H₂O as an internal reference fluorescent reagent. Interestingly, a transition of the fluorescent color of the ratiometric probe from cyan to red could be visually observed upon increasing the concentration of NO₂^-. Based on these findings, a ratiometric fluorescent-based portable agarose hydrogel test kit was fabricated and applied for on-spot assessment of NO₂^- content within 10 min. As far as we know, this is the first ratiometric fluorescent sensor for visual detection of NO₂^-. It has broad application prospects in environmental monitoring and food safety assessment.

Recent Progress on Metal Sulfide Composite Nanomaterials for Photocatalytic Hydrogen Production

Metal sulfide-based photocatalysts have gained much attention due to their outstanding photocatalytic properties. This review paper discusses recent developments on metal sulfide-based nanomaterials for H2 production, acting as either photocatalysts or cocatalysts, especially in the last decade. Recent progress on key experimental parameters, in-situ characterization methods, and the performance of the metal sulfide photocatalysts are systematically discussed, including the forms of heterogeneous composite photocatalysts, immobilized photocatalysts, and magnetically separable photocatalysts. Some methods have been studied to solve the problem of rapid recombination of photoinduced carriers. The electronic density of photocatalysts can be investigated by in-situ C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra to study the mechanism of the photocatalytic process. The effects of crystal properties, nanostructure, cocatalyst, sacrificial agent, electrically conductive materials, doping, calcination, crystal size, and pH on the performance of composite photocatalysts are presented. Moreover, the facet effect and light trapping (or light harvesting) effect, which can improve the photocatalytic activity, are also discussed.

Interaction-Dependent Interfacial Charge-Transfer Behavior in Solar Water-Splitting Systems

Dual-band-gap systems are promising for solar water splitting due to their excellent light-harvesting capability and high charge-separation efficiency. However, a fundamental understanding of interfacial charge-transfer behavior in the dual-band-gap configuration is still incomplete. Taking CdS/reduced graphene oxide (CdS/RGO) nanoheterojunctions as a model solar water splitting system, we attempt here to highlight the interaction-dependent interfacial charge-transfer behavior based on both experimental observations and theoretical calculations. Experimental evidence points to charge transfer at the CdS-RGO interface playing a dominant role in the photocatalytic hydrogen production activity. By tuning the degree of reduction of RGO, the interfacial interaction, and, thereby, the charge transfer can be controlled at the CdS-RGO interface. This observation is supported by theoretical analysis, where we find that the interfacial charge transfer is a balance between the effective single-electron- and hole-transfer probability and the surface free electron and hole concentration, both of which are related to the surface potential and tailored by interfacial interaction. This mechanism is applicable to all systems for solar water splitting, providing a useful guidance for the design and study of heterointerfaces for high-efficiency energy conversion.

Two-dimensional honeycomb borophene oxide: strong anisotropy and nodal loop transformation

The search for topological semimetals is mainly focused on heavy-element compounds by following the footsteps of previous research on topological insulators, with less attention on light-element materials. However, the negligible spin orbit coupling with light elements may turn out to be beneficial for realizing topological band features. Here, using first-principles calculations, we propose a new two-dimensional light-element material-the honeycomb borophene oxide (h-B2O), which has nontrivial topological properties. The proposed structure is based on the recently synthesized honeycomb borophene on an Al (111) substrate [W. Li, L. Kong, C. Chen, J. Gou, S. Sheng, W. Zhang, H. Li, L. Chen, P. Cheng and K. Wu, Sci. Bull., 2018, 63, 282-286]. The h-B2O monolayer is completely flat, unlike the oxides of graphene or silicene. We systematically investigate the structural properties of h-B2O, and find that it has very good stability and exhibits significant mechanical anisotropy. Interestingly, the electronic band structure of h-B2O hosts a nodal loop centered around the Y point in the Brillouin zone, protected by the mirror symmetry. Furthermore, under moderate lattice strain, the single nodal loop can be transformed into two loops, each penetrating through the Brillouin zone. The loops before and after the transition are characterized by different [Doublestruck Z] × [Doublestruck Z] topological indices. Our work not only predicts a new two-dimensional material with interesting physical properties, but also offers an alternative approach to search for new topological phases in 2D light-element systems.

Carbon quantum dots derived by direct carbonization of carbonaceous microcrystals in mesophase pitch

Aggregation of the central aromatic ring system of asphaltene molecules due to π-π interaction can lead to the formation of carbon quantum dots (CQDs). However, to date, such a roadmap has not been demonstrated. Here, we present a simple approach to the synthesis of CQDs by direct carbonization of dispersed carbonaceous microcrystals in mesophase pitch. The size of the as-prepared CQDs is modulated by adjusting the nucleation temperature for mesophase formation. Due to the oxygen-free character, the CQDs exhibit excitation-independent fluorescent behavior with a quantum yield up to 87%. The CQDs were successfully applied to fluorescent detection of Fe³⁺ ions with good specificity and sensitivity. Our results not only provide a scalable production of CQDs at low cost, but also give valuable clues to understand the solidification of asphaltene at nanoscale.

Prediction of high-temperature Chern insulator with half-metallic edge states in asymmetry-functionalized stanene

A great obstacle for the practical applications of the quantum anomalous Hall (QAH) effect is the lack of suitable two-dimensional (2D) materials with a sizable nontrivial band gap, high Curie temperature, and high carrier mobility. Based on first-principles calculations, here, we propose the realizations of these intriguing properties in asymmetry-functionalized 2D SnHN and SnOH lattices. Spin-polarized band structures reveal that SnOH monolayer exhibits a spin gapless semiconductor (SGS) feature, whereas SnNH is converted to SGS under compressive strain. The Curie temperature of SnOH reaches 266 K, as predicted by Monte Carlo simulation, and it is comparable to the room temperature. When the spin and orbital degrees of freedom are allowed to couple, both systems become large-gap QAH insulators with fully spin-polarized half-metallic edge states and higher Fermi velocity of 4.9 × 105 m s-1. These results pave a new way for designing topological field transistors in group-IV honeycomb lattices.

Undulation induced tuning of electron acceptance by edge-oxidized graphene oxide

Edge-oxidized graphene oxide (EOGO) nanosheets are good acceptors of electrons. We have employed a suitably designed pyrene-tailed fluorescent probe to establish that the electron acceptability of EOGO can be tuned by undulation of the GO sheet. Comparison between EOGO and single-walled carbon nanotubes (SWCNT) on electron acceptance from the probe molecule shows that the efficiency of π-π stacking between pyrene and the graphene sheet plays the key role.

Basic Concepts and Recent Advances of Crystallographic Orientation Determination of Graphene by Raman Spectroscopy

Graphene is a kind of typical two-dimensional material consisting of pure carbon element. The unique material shows many interesting properties which are dependent on crystallographic orientations. Therefore, it is critical to determine their crystallographic orientations when their orientation-dependent properties are investigated. Raman spectroscopy has been developed recently to determine crystallographic orientations of two-dimensional materials and has become one of the most powerful tools to characterize graphene nondestructively. This paper summarizes basic aspects of Raman spectroscopy in crystallographic orientation of graphene nanosheets, determination principles, the determination methods, and the latest achievements in the related studies.

Facile reduction of graphene oxide suspensions and films using glass wafers

This paper reports a facile and green method for conversion of graphene oxide (GO) into graphene by low-temperature heating (80 °C) in the presence of a glass wafer. Compared to conventional GO chemical reduction methods, the presented approach is easy-scalable, operationally simple, and based on the use of a non-toxic recyclable deoxygenation agent. The efficiency of the proposed method is further expanded by the fact that it can be applied for reducing both GO suspensions and large-scale thin films formed on various substrates prior to the reduction process. The quality of the obtained reduced graphene oxide (rGO) strongly depends on the type of the used glass wafer, and, particularly, magnesium silicate glass can provide rGO with the C/O ratio of 7.4 and conductivity of up to 33000 S*cm^-1. Based on the data obtained, we have suggested a mechanism of the observed reduction process in terms of the hydrolysis of the glass wafer with subsequent interaction of the leached alkali and alkali earth cations and silicate anions with graphene oxide, resulting in elimination of the oxygen-containing groups from the latter one. The proposed approach can be efficiently used for low-cost bulk-quantity production of graphene and graphene-based materials for a wide field of applications.

A Low-Cost Non-explosive Synthesis of Graphene Oxide for Scalable Applications

A low cost, non-explosive process for the synthesis of graphene oxide (GO) is demonstrated. Using suitable choice of reaction parameters including temperature and time, this recipe does not require expensive membranes for filtration of carbonaceous and metallic residues. A pre-cooling protocol is introduced to control the explosive nature of the highly exothermic reactions during the oxidation process. This alleviates the requirement for expensive membranes and completely eliminates the explosive nature of intermediate reaction steps when compared to existing methods. High quality of the synthesized GO is corroborated using a host of characterization techniques including X-ray diffraction, optical spectroscopy, X-ray photoemission spectroscopy and current-voltage characteristics. Simple reduction protocol using ultra-violet light is demonstrated for potential application in the area of photovoltaics. Using different reduction protocols together with the proposed inexpensive method, reduced GO samples with tunable conductance over a wide range of values is demonstrated. Density functional theory is employed to understand the structure of GO. We anticipate that this scalable approach will catalyze large scale applications of GO.

Enhancement of CO2 adsorption on oxygen-functionalized epitaxial graphene surface under near-ambient conditions

The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult under ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied under near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.

Electronic structure manipulation of graphene dots for effective hydrogen evolution from photocatalytic water decomposition

This paper presents a heteroatom doping strategy to manipulate the structure of graphene-based photocatalysts for effective hydrogen production from aqueous solution. Oxygenation of graphene creates a bandgap to produce semiconducting graphene oxide, nitrogen doping extends the resonant π-conjugation to prolong the charge lifetime, and sulfur doping breaks the electron neutrality to facilitate charge transfer. Accordingly, ammonia-treated sulfur-nitrogen-co-doped graphene oxide dots (A-SNGODs) are synthesized by annealing graphene oxide sheets in sulfur-ammonia, oxidizing the sheets into dots, and then hydrothermally treating the dots in ammonia. The A-SNGODs exhibit a high nitrogen content in terms of quaternary and amide groups that are formed through sulfur-mediated reactions. The peripheral amide facilitates orbital conjugations to enhance the photocatalytic activity, whereas the quaternary nitrogen patches vacancy defects to improve stability. The simultaneous presence of electron-withdrawing S and electron-donating N atoms in the A-SNGODs facilitates charge separation and results in reactive electrons. When suspended in an aqueous triethanolamine solution, Pt-deposited A-SNGODs demonstrate a hydrogen-evolution quantum yield of 29% under monochromatic 420 nm irradiation. The A-SNGODs exhibit little activity decay under 6-day visible-light irradiation. This study demonstrates the excellence of the heteroatom-doping strategy in producing stable and active graphene-based materials for photoenergy conversion.

Uncovering the Design Principle of Amino Acid-Derived Photoluminescent Biodots with Tailor-Made Structure-Properties and Applications for Cellular Bioimaging

Natural amino acids possess side chains with different functional groups (R groups), which make them excellent precursors for programmable synthesis of biomolecule-derived nanodots (biodots) with desired properties. Herein, we report the first systematic study to uncover the material design rules of biodot synthesis from 20 natural α-amino acids via a green hydrothermal approach. The as-synthesized amino acid biodots (AA dots) are comprehensively characterized to establish a structure-property relationship between the amino acid precursors and the corresponding photoluminescent properties of AA dots. It was found that the amino acids with reactive R groups, including amine, hydroxyl, and carboxyl functional groups form unique C-O-C/C-OH and N-H bonds in the AA dots which stabilize the surface defects, giving rise to brightly luminescent AA dots. Furthermore, the AA dots were found to be amorphous and the length of the R group was observed to affect the final morphology (e.g., disclike nanostructure, nanowire, or nanomesh) of the AA dots, which in turn influence their photoluminescent properties. It is noteworthy to highlight that the hydroxyl-containing amino acids, that is, Ser and Thr, form the brightest AA dots with a quantum yield of 30.44% and 23.07%, respectively, and possess high photostability with negligible photobleaching upon continuous UV exposure for 3 h. Intriguingly, by selective mixing of Ser or Thr with another amino acid precursor, the resulting mixed AA dots could inherit unique properties such as improved photostability and significant red shift in their emission wavelength, producing enhanced green and red fluorescent intensity. Moreover, our cellular studies demonstrate that the as-synthesized AA dots display outstanding biocompatibility and excellent intracellular uptake, which are highly desirable for imaging applications. We envision that the material design rules discovered in this study will be broadly applicable for the rational selection of amino acid precursors in the tailored synthesis of biodots.

On the Solvation Behavior of Graphene Oxide in Ethylene Glycol/Water Mixtures

The self-association and solvation pattern of graphene oxide (GO) in water, ethylene glycol (EG), and their mixtures were analyzed by means of UV/Vis spectrophotometry. A careful analysis of the absorbance dependencies vs. the GO concentration shows that self-association of the GO sheets in EG occurs at higher concentration compared to that in water. It was established that depending on the mixed solvent composition, two different types of the GO solvates are formed. The results of quantum chemical calculations allow one to suggest that in the water-rich compositions, the GO oxygen-containing groups are in direct contact with water molecules while in the glycol-rich media the EG molecules fully substitute water in the GO's first solvation layer.

Revealing the linear relationship between electrical, thermal, mechanical and structural properties of carbon nanocoils

The special helical morphologies and polycrystalline-amorphous internal structures differ carbon nanocoils (CNCs) from carbon nanotubes or carbon nanofibers, but bring difficulties in illuminating the correlations between physical and structural properties. In this paper, we measure the electrical conductivity (σ), thermal diffusivity (α) and Young's modulus (E) of single CNCs at the same time, using a transient electrothermal technique and an electromechanical vibration technique. Based on the statistical results of 8 single CNC samples, a linear correlation between the three parameters is uncovered, expressed as σ = 0.052(α - 2.5) × 104 S m-1, E = (-10.38σ + 14.04) GPa and E = (-0.59α + 16.08) GPa, where the unit of α is 10-7 m2 s-1. Concise proportional relations between the three parameters and average graphite grain size (ld) are deduced, expressed as σ = Ald(C1 - T)-1, α = Bld(C2 + T)-1 and E = -Dld + E0. The proportional relation between physical parameters and ld demonstrates the confinement originated from the nano-grain system.

Unravelling Some of the Structure-Property Relationships in Graphene Oxide at Low Degree of Oxidation

Graphene oxide (GO) is a versatile 2D material whose properties can be tuned by changing the type and concentration of oxygen-containing functional groups attached to its surface. However, a detailed knowledge of the dependence of the chemo/physical features of this material on its chemical composition is largely unknown. We combine classical molecular dynamics and density functional theory simulations to predict the structural and electronic properties of GO at low degree of oxidation and suggest a revision of the Lerf-Klinowski model. We find that layer deformation is larger for samples containing high concentrations of epoxy groups and that correspondingly the band gap increases. Targeted chemical modification of the GO surface appears to be an effective route to tailor the electronic properties of the monolayer for given applications. Our simulations also show that the chemical shift of the C-1s XPS peak allows one to unambiguously characterize GO composition, resolving the peak attribution  uncertainty often encountered in experiments.

Effect of friction on oxidative graphite intercalation and high-quality graphene formation

Oxidative wet-chemical delamination of graphene from graphite is expected to become a scalable production method. However, the formation process of the intermediate stage-1 graphite sulfate by sulfuric acid intercalation and its subsequent oxidation are poorly understood and lattice defect formation must be avoided. Here, we demonstrate film formation of micrometer-sized graphene flakes with lattice defects down to 0.02% and visualize the carbon lattice by transmission electron microscopy at atomic resolution. Interestingly, we find that only well-ordered, highly crystalline graphite delaminates into oxo-functionalized graphene, whereas other graphite grades do not form a proper stage-1 intercalate and revert back to graphite upon hydrolysis. Ab initio molecular dynamics simulations show that ideal stacking and electronic oxidation of the graphite layers significantly reduce the friction of the moving sulfuric acid molecules, thereby facilitating intercalation. Furthermore, the evaluation of the stability of oxo-species in graphite sulfate supports an oxidation mechanism that obviates intercalation of the oxidant.

Scalable graphene production from graphite via an intercalation-oxidation-reduction process is still hampered by low reproducibility and many lattice defects. Here, the authors show that reducing molecular friction by using highly crystalline graphite and mild oxidizing conditions is the key to high quality graphene.

Reduced graphene oxide (rGO) based wideband optical sensor and the role of Temperature, Defect States and Quantum Efficiency

We report a facile and cost-effective approach to develop self-standing reduced Graphene Oxide (rGO) film based optical sensor and its low-temperature performance analysis where midgap defect states play a key role in tuning the crucial sensor parameters. Graphite oxide (GO) is produced by modified Hummers’ method and reduced thermally at 250 °C for 1 h in Argon atmosphere to obtain rGO. Self-standing rGO film is prepared via vacuum filtration. The developed film is characterized by HRTEM, FESEM, Raman, and XRD techniques. The developed sensor exhibits highest sensitivity towards 635 nm illumination wavelength, irrespective of the operating temperature. For a given excitation wavelength, photoresponse study at low temperature (123K–303K) reveals inverse relationship between sensitivity and operating temperature. Highest sensitivity of 49.2% is obtained at 123 K for 635 nm laser at power density of 1.4 mW/mm². Unlike sensitivity, response- and recovery-time demonstrate directly proportional dependence with operating temperature. Power dependent studies establish linear relation between power-density and sensitivity, and a safe limit beyond which sample heating prolongs the recovery time. Wavelength-dependent studies shows that proposed sensor can efficiently operate from visible to near NIR region. To the best of our knowledge such rGO based optical sensor performance at low temperature had not been reported earlier.

The influence of the concentration and adsorption sites of different chemical groups on graphene through first principles simulations

Carbon nanomaterials are one of the most promising nanostructures for adsorption of chemical species due to their high superficial area and possible interesting applications. A systematic study of chemical groups attached on graphene surfaces is necessary in order to evaluate the influence of the type and number of functionalizations on the resulting properties of a derived system. In this work, first principles simulations were used to evaluate the physical effects of different concentrations of chemical groups -COOH, -COH, -OH, -O- or -NH₂ adsorbed on the graphene surface. The functionalizations occur from one up to three chemical groups and either in the same or different carbon rings. It is observed that significant changes occur in the adsorption and electronic properties due to the hybridization and symmetry points of interaction of the chemical groups. Then, the results indicate that it is possible to control the properties of the desired system through the type, concentration and binding site of the functional groups attached to the graphene monolayer.

Tuning the activity of the inert MoS2 surface via graphene oxide support doping towards chemical functionalization and hydrogen evolution: a density functional study

The basal plane of MoS₂ provides a promising platform for chemical functionalization and the hydrogen evolution reaction (HER); however, its practical utilization remains challenging due to the lack of active sites and its low conductivity. Herein, using first principles simulations, we first proposed a novel and effective strategy for significantly enhancing the activity of the inert MoS₂ surface using a graphene oxide (GO) support (MoS₂/GOs). The chemical bonding of the functional groups (CH₃ and NH₂) on the MoS₂-GO hybrid is stronger than that in freestanding MoS₂ or MoS₂-graphene. Upon increasing the oxygen group concentration or introducing N heteroatoms into the GO support, the stability of the chemically functionalized MoS₂ is improved. Furthermore, use of GOs to support pristine and defective MoS₂ with a S vacancy (S-MoS₂) can greatly promote the HER activity of the basal plane. The catalytic activity of S-MoS₂ is further enhanced by doping N into GOs; this results in a hydrogen adsorption free energy of almost zero (ΔG_H = ∼-0.014 eV). The coupling interaction with the GO substrate reduces the p-type Schottky barrier heights (SBH) of S-MoS₂ and modifies its electronic properties, which facilitate charge transfer between them. Our calculated results are consistent with the experimental observations. Thus, the present results open new avenues for the chemical functionalization of MoS₂-based nanosheets and HER catalysts.

Computational characterisation of dried and hydrated graphene oxide membranes

A multi-step molecular dynamics procedure was developed to construct fully flexible atomistic models of graphene oxide (GO) membranes. The method of preparation replicates the experimental synthesis of the material; i.e. the flow-directed self-assembly of individual flakes onto a substrate or filter. A total of 180 GO membrane models were prepared with water contents varying between 0 and 20%, providing an insight into changes in the membrane's interlayer distance with swelling. Membranes with 15% water content have an average interlayer distance (0.80 nm), bulk density (1.77 g cm^-3) and tensile modulus (18.1 GPa) in excellent agreement with the experimental literature, demonstrating that air-dried membranes have 15% water content. The models reveal the intrinsic structural heterogeneity and complex morphology of GO membranes. This feature has previously been unaccounted for in both experimental interpretations and GO nanopore models, which often use pre-defined and idealised 2D geometries. Completely dried membranes have considerable free pore volume. This observation explains the modest change in interlayer distance (0.02 nm) as the membrane's water content is increased from 0% to 10% compared to a much more significant change (0.12 nm) as the water content is increased from 10% to 20%. Combined with this new understanding of membrane swelling, the availability of such representative models opens the possibility of the molecular-level design of GO membranes for a variety of applications, such as gaseous and aqueous separations.

Study of Graphene Oxide Structural Features for Catalytic, Antibacterial, Gas Sensing, and Metals Decontamination Environmental Applications

This study represents a comprehensive review about the structural features of graphene oxide (GO) and its significance in environmental applications. Two dimensional (2D) GO is tremendously focused in advanced carbon-based nanomaterials for environmental applications due to its tunable physicochemical characteristics. Herein, we report foundational structural models of GO and explore the chemical bonding of oxygen moieties, with graphite basal plane using various characterization tools. Moreover, the impact of these oxygen moieties and the morphology of GO for environmental applications such as removal of metal ions and catalytic, antibacterial, and gas sensing abilities have here been critically reviewed for the first time. Environmental applications of GO are highly significant because, in the recent era, the fast progress of industries, even in the countryside, results in air and water pollution. GO has been widely investigated by researchers to eradicate such environmental issues and for potential industrial and clinical applications due to its 2D structural features, large surface area, presence of oxygen moieties, nonconductive nature, intense mechanical strength, excellent water dispersibility, and tunable optoelectronic properties. Thence, particular emphasis is directed toward the modification of GO by varying the number of its oxygen functional groups and by coupling it with other exotic nanomaterials to induce unique properties in GO for potential environmental remediation purposes.

Eco-friendly reduced graphene oxide for the determination of mycophenolate mofetil in pharmaceutical formulations

Graphene oxide (GO) was synthesized and characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). GO was then electrochemically reduced and used for electrochemical study of mycophenolate mofetil (MMF). The electrochemically reduced graphene oxide (ERGO) film on glassy carbon electrode (GCE) showed enhanced peak current for electrooxidation of MMF. MMF exhibited two irreversible oxidation peaks at 0.84 V (peak a₁) and 1.1 V (peak a₂). Effects of accumulation time, pH and scan rate were studied and various electrochemical parameters were calculated. A differential pulse voltammetric method was developed for the determination of MMF in bulk samples and pharmaceutical formulations. Linear relationship was observed between the peak current and concentration of MMF in the range of 40 nM-15 μM with a limit of detection of 11.3 nM. The proposed method is simple, sensitive and inexpensive and, hence, could be readily adopted in clinical and quality control laboratories.

Structure and Optical Features of Micro/Nanosized Carbon Forms Prepared by Electrochemical Exfoliation

Micro/nanosized carbon materials were prepared by electrochemical exfoliation method in the forms of the colloids and thin films. Scanning electronic microscopy, optical and luminescent microscopy, and Raman scattering and luminescent spectroscopy were applied for characterization of materials. The wide photoluminescence band in the visible spectral region was observed for each of the samples. The shape of the photoluminescence band depends on excitation wavelength and on the size of the particles. At least two components with maxima at ~580 and ~710 nm can be distinguished in the photoluminescence spectra. The relations between the photoluminescence properties and morphology of the samples have been described and discussed.

Preparation of an Efficient Ratiometric Fluorescent Nanoprobe (m-CDs@[Ru(bpy)3]2+) for Visual and Specific Detection of Hypochlorite on Site and in Living Cells

Hypochlorite (ClO^-) is one of the most important reactive oxygen species (ROS), which plays an important role in sustaining human innate immunity during microbial invasion. Moreover, ClO^- is a powerful oxidizer for water treatment. The safety of drinking water is closely related to its content. Herein, m-phenylenediamine (mPD) is used as a precursor to prepare carbon dots (named m-CDs) with highly fluorescent quantum yield (31.58% in water), and our investigation shows that the strong fluorescent emission of m-CDs can be effectively quenched by ClO^-. Based on these findings, we developed a novel fluorescent nanoprobe (m-CDs) for highly selective detection of ClO^-. The linear range was from 0.05 to 7 μM (R² = 0.998), and the limit of detection (S/N = 3) was as low as 0.012 μM. Moreover, a portable agarose hydrogel solid matrix-based ratiometric fluorescent nanoprobe (m-CDs@[Ru(bpy)₃]²⁺) sensor was subsequently developed for visual on-site detection of ClO^- with the naked eyes under a UV lamp, suggesting its potential in practical application with low cost and excellent performance in water quality monitoring. Additionally, intracellular detection of exogenous ClO^- was demonstrated via ratiometric imaging microscopy.

An ab initio study of hydroxylated graphane

Graphene-based derivatives with covalent functionalization and well-defined stoichiometry are highly desirable in view of their application as functional surfaces. Here, we have evaluated by ab initio calculations the energy of formation and the phase diagram of hydroxylated graphane structures, i.e., fully functionalized graphene derivatives coordinated with -H and -OH groups. We compared these structures to different hydrogenated and non-hydrogenated graphene oxide derivatives, with high level of epoxide and hydroxyl groups functionalization. Based on our calculations, stable phases of hydroxylated graphane with low and high contents of hydrogen are demonstrated for high oxygen and hydrogen partial pressure, respectively. Stable phases of graphene oxide with a mixed carbon hybridization are also found. Notably, the synthesis of hydroxylated graphane has been recently reported in the literature.

A Broadband Fluorographene Photodetector

High-performance photodetectors operating over a broad wavelength range from ultraviolet, visible, to infrared are of scientific and technological importance for a wide range of applications. Here, a photodetector based on van der Waals heterostructures of graphene and its fluorine-functionalized derivative is presented. It consistently shows broadband photoresponse from the ultraviolet (255 nm) to the mid-infrared (4.3 µm) wavelengths, with three orders of magnitude enhanced responsivity compared to pristine graphene photodetectors. The broadband photodetection is attributed to the synergistic effects of the spatial nonuniform collective quantum confinement of sp² domains, and the trapping of photoexcited charge carriers in the localized states in sp³ domains. Tunable photoresponse is achieved by controlling the nature of sp³ sites and the size and fraction of sp³ /sp² domains. In addition, the photoresponse due to the different photoexcited-charge-carrier trapping times in sp² and sp³ nanodomains is determined. The proposed scheme paves the way toward implementing high-performance broadband graphene-based photodetectors.

Stability, transport and ecosystem effects of graphene in water and soil environments

Graphene nanomaterials (GMs), such as graphene oxide (GO) and reduced graphene oxide (rGO), have been widely applied in various fields. Due to the rapid increase in production and application, the inevitable release of GMs into water and soil environments poses potential health and ecosystem risks. Upon exposure, the behavior, transport, and fate of GMs may be altered after interacting with the relevant environmental conditions. GMs can affect the microbial communities as well. Thus, it is imperative to understand the interaction between the GMs and the environmental systems for predicting their risks. For this purpose, this review highlights the influence of the most relevant environmental factors on the stability, aggregation, and transformation of GMs in aquatic environments. Moreover, the transport of GMs and microbial communities changes have also been presented based on the recent findings. To the best of our knowledge, this review covered most of the recent related studies and will allow for accurate predictions of the fate and risks associated with GMs. In consideration of the diversity of GMs and the complexity of environmental factors, further studies should be focused on their inherent properties and amicable development.

Photoluminescence enhancement of amino-functionalized graphene quantum dots in two-dimensional optical resonators

This paper reports on the emission characteristics of amino-functionalized graphene quantum dots (af-GQDs). We employed the variable stripe length method to measure the net optical gain of af-GQDs. Photoluminescence emission was enhanced through the efficient confinement of photons using an optical resonator. The two-dimensional resonator is made up of a cholesteric liquid crystal (CLC) reflector to enable the redistribution of spontaneous emission from the af-GQDs. The proposed method was shown to increase the intensity of peak emission to more than three times that of the reference sample without a CLC reflector. The peak emission intensity of af-GQDs in the optical resonator grows exponentially with an increase in excitation energy. These results demonstrate the feasibility of two-dimensional optical amplifiers based on CLC reflectors.

Diels–Alder reactions of graphene oxides: greatly enhanced chemical reactivity by oxygen-containing groups

Oxygen-containing groups of graphene oxides greatly enhanced the Diels–Alder (DA) reactivity of pristine graphene.

Contrasting diffusion behaviors of O and F atoms on graphene and within bilayer graphene

Our DFT calculations reveal the origin of the contrasting diffusion behaviors of O and F atoms within bilayer graphene.

Semi-Dirac semimetal in silicene oxide

Silicene upon covalent addition of group-VI elements and strain engineering could exhibit semi-Dirac cones at the Brillouin zone center.

Band gap opening of bilayer graphene by graphene oxide support doping

In contrast to the metallic monolayer graphene by graphene oxides (GOs) doping, the sizable band gap of bilayer graphene is opened by GOs.

Distribution of oxygen functional groups of graphene oxide obtained from low-temperature atomic layer deposition of titanium oxide

The island-like distribution of the oxygen functional groups of graphene oxide was identified by deposition of TiO₂ on the graphene oxide surface using low-temperature atomic layer deposition.

Photo-Response of Functionalized Self-Assembled Graphene Oxide on Zinc Oxide Heterostructure to UV Illumination

Convective assembly technique which is a simple and scalable method was used for coating uniform graphene oxide (GO) nanosheets on zinc oxide (ZnO) thin films. Upon UV irradiation, an enhancement in the on-off ratio was observed after functionalizing ZnO films by GO nanosheets. The calculations of on-off ratio, the device responsivity, and the external quantum efficiency were investigated and implied that the GO layer provides a stable pathway for electron transport. Structural investigations of the assembled GO and the heterostructure of GO on ZnO were performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The covered GO layer has a wide continuous area, with wrinkles and folds at the edges. In addition, the phonon lattice vibrations were investigated by Raman analysis. For GO and the heterostructure, a little change in the ratio between the D-band and G-band was found which means that no additional defects were formed within the heterostructure.

Continuous and ultrathin platinum films on graphene using atomic layer deposition: a combined computational and experimental study

Integrating metals and metal oxides with graphene is key in utilizing its extraordinary material properties that are ideal for nanoelectronic and catalyst applications. Atomic layer deposition (ALD) has become a key technique for depositing ultrathin, conformal metal(oxide) films. ALD of metal(oxide) films on graphene, however, remains a genuine challenge due to the chemical inertness of graphene. In this study we address this issue by combining first-principles density functional theory (DFT) simulations with ALD experiments. The focus is on the Pt ALD on graphene, as this hybrid system is very promising for solar and fuel cells, hydrogen technologies, microreactors, and sensors. Here we elucidate the surface reactions underpinning the nucleation stage of Pt ALD on pristine, defective and functionalized graphenes. The employed reaction mechanism clearly depends on (a) the available surface groups on graphene, and (b) the ligands accompanying the metal centre in the precursor. DFT calculations also indicate that graphene oxide (GO) can afford a stronger adsorption of MeCpPtMe₃, unlike Pt(acac)₂, as compared to bare (non-functionalized) graphene, suggesting that GO monolayers are effective Pt ALD seed layers. Confirming the latter, we evince that wafer-scale, continuous Pt films can indeed be grown on GO monolayers using a thermal ALD process with MeCpPtMe₃ and O₂ gas. Besides, the current in-depth atomistic insights are of practical use for understanding similar ALD processes of other metals and metal oxides on graphene.

Macrocycles inserted in graphene: from coordination chemistry on graphene to graphitic carbon oxide

Tuning electronic structures and properties through chemical modifications has become the focus of recent research on graphene. The adsorption of metal atoms on graphene showed strong potential but is limited due to weak binding. On the other hand, macrocyclic molecules are well known for their strong and selective binding with metal atoms in solutions through coordination bonds. The alliance of the two substances will largely benefit the two parallel fields: it will provide a scaffold for coordination chemistry as well as a controllable method for tuning the electronic structure of graphene through strong binding with metals. Here, using crown ether as an example, we demonstrate that the embedment of macrocyclic molecules into the graphene honeycomb lattice can be very thermochemically favored. The combination also leads to a family of new materials that has potential in many areas including photolysis and two-dimensional superconductivity.

Excitation Wavelength Independence: Toward Low-Threshold Amplified Spontaneous Emission from Carbon Nanodots

Carbon nanodots (CDs) are known to be a superior type of lasing material due to their low cost, low toxicity, high photostability, and photobleaching resistance. Significant attention has been paid to synthesizing CDs with high fluorescence quantum yields (FLQYs) to achieve higher optical gains. In this report, we reveal that excitation wavelength-independent (λex-independent) photoluminescence (PL) characteristics, rather than high FLQYs, should be given priority to realize CD-based light amplification. CDs with excitation wavelength-dependent (λex-dependent) PL characteristics and FLQYs as high as 99% and 96% were found not to exhibit amplified spontaneous emission (ASE), while those with λex-independent PL characteristics and FLQYs of only 38% and 82% realized ASE with low thresholds. The difficulty of achieving ASE using CDs with λex-dependent PL characteristics is likely attributable to their high contents of C-O-H or C-O-C groups. These groups can induce numerous localized electronic states within the n-π* gap, which could decentralize the excited electrons, thus increasing the difficulty of population inversion. In addition, the radiative transition rates and stimulated emission cross sections of CDs with λex-independent PL characteristics were found to be significantly higher than those of CDs with λex-dependent PL characteristics. ASE in a planar waveguide structure, which is a practical structure for solid-state lasing devices, was also demonstrated for the first time using CDs with λex-independent PL characteristics. These results provide simple and effective guidelines for synthesizing and selecting CDs for low-threshold lasing devices.

Electrical conductivity of oxidized-graphenic nanoplatelets obtained from bamboo: effect of the oxygen content

The large-scale production of graphene and reduced-graphene oxide (rGO) requires low-cost and eco-friendly synthesis methods. We employed a new, simple, cost-effective pyrolytic method to synthetize oxidized-graphenic nanoplatelets (OGNP) using bamboo pyroligneous acid (BPA) as a source. Thorough analyses via high-resolution transmission electron microscopy and electron energy-loss spectroscopy provides a complete structural and chemical description at the local scale of these samples. In particular, we found that at the highest carbonization temperature the OGNP-BPA are mainly in a sp(2) bonding configuration (sp(2) fraction of 87%). To determine the electrical properties of single nanoplatelets, these were contacted by Pt nanowires deposited through focused-ion-beam-induced deposition techniques. Increased conductivity by two orders of magnitude is observed as oxygen content decreases from 17% to 5%, reaching a value of 2.3 × 10(3) S m(-1) at the lowest oxygen content. Temperature-dependent conductivity reveals a semiconductor transport behavior, described by the Mott three-dimensional variable range hopping mechanism. From the localization length, we estimate a band-gap value of 0.22(2) eV for an oxygen content of 5%. This investigation demonstrates the great potential of the OGNP-BPA for technological applications, given that their structural and electrical behavior is similar to the highly reduced rGO sheets obtained by more sophisticated conventional synthesis methods.