Monolayer Graphite Research Articles

First-Principles Investigation of Phosphorus-Doped Graphitic Carbon Nitride as Anchoring Material for the Lithium-Sulfur Battery.

The utilization of lithium-sulfur battery is hindered by various challenges, including the "shuttle effect", limited sulfur utilization, and the sluggish conversion kinetics of lithium polysulfides (LiPSs). In the present work, a theoretical design for the viability of graphitic carbon nitride (g-C3N4) and phosphorus-doping graphitic carbon nitride substrates (P-g-C3N4) as promising host materials in a Li-S battery was conducted utilizing first-principles calculations. The PDOS shows that when the P atom is introduced, the 2p of the N atom is affected by the 2p orbital of the P atom, which increases the energy band of phosphorus-doping substrates. The energy bands of PC and Pi are 0.12 eV and 0.20 eV, respectively. When the lithium polysulfides are adsorbed on four substrates, the overall adsorption energy of PC is 48-77% higher than that of graphitic carbon nitride, in which the charge transfer of long-chain lithium polysulfides increase by more than 1.5-fold. It is found that there are powerful Li-N bonds between lithium polysulfides and P-g-C3N4 substrates. Compared with the graphitic carbon nitride monolayer, the anchoring effect of the LiPSs@P-g-C3N4 substrate is enhanced, which is beneficial for inhibiting the shuttle of high-order lithium polysulfides. Furthermore, the catalytic performance of the P-g-C3N4 substrate is assessed in terms of the S8 reduction pathway and the decomposition of Li2S; the decomposition energy barrier of the P-g-C3N4 substrate decrease by 10% to 18%. The calculated results show that P-g-C3N4 can promote the reduction of S8 molecules and Li-S bond cleavage within Li2S, thus improving the utilization of sulfur-active substances and the ability of rapid reaction kinetics. Therefore, the P-g-C3N4 substrates are a promising high-performance lithium-sulfur battery anchoring material.

Theoretical inspection of high-efficiency single-atom catalysts based on π-π conjugated holey graphitic g-C7N3 monolayer: Marvelous water-splitting and oxygen reduction reactions activities

Hydrogen evolution reaction (HER) and oxygen evolution/reduction reaction (OER/ORR) relying on high-performance and low-cost single-atom catalysts (SACs) driven by renewable energy sources offer a sustainable route to carbon-neutral chemicals and fuels. Herein, first-principles calculations were performed to investigate the catalytic HER/OER/ORR activity of a novel graphitic carbon nitride monolayer (g-C7N3) supported single transition metal (TM@g-C7N3). High stability as well as positively charged active site (TM-site) and desirable electrical conductivity lay the foundation for TM@g-C7N3 acting as efficient HER/OER/ORR electrocatalysts. We screened out the non-noble-metal Rh@g-C7N3 SAC exhibiting great potential as the trifunctional electrocatalysts for water splitting (ηHER= 0.06 V and ηOER= 0.46 V) and a metal-air battery (ηORR= 0.28 V) on both kinetic and thermodynamic scales, whereas the Ni@g-C7N3 can be served as a bifunctional OER/ORR catalyst with a low overpotential of 0.33 V/0.31 V, for both of which the high thermodynamic stability and oxidation barrier guarantee their outstanding performances at ambient conditions. The mechanism analysis indicates the filling of d-orbital electrons of TM-atom can play an important role in determining the value of an energy descriptor (ΔGOH*), and the suitable ΔGOH* values make for the TM@g-C7N3 candidates to possess favorable OER/ORR overpotential. Particularly, the Rh-d orbital of Rh@g-C7N3 is evidently hybridized with the OH*-p orbital, resulting in the lone electrons initially distributed in the antibonding orbital pairing up and occupying the downward bonding orbital, ensuring OH* can be adsorbed on Rh@g-C7N3 appropriately. Moreover, multiple-level descriptors including d-band center, COHP, Nd, and φ are used to reveal the origin of the electrocatalytic activity.

Computational design of single-atom catalysts embedded on reduced graphitic carbon nitride monolayers

The design of efficient single-atom catalysts (SACs) with optimal activity and selectivity for sustainable energy and environmental applications remains a challenge. In this work, comprehensive first-principles calculations are performed to validate the feasibility of single TM atoms (3d, 4d, and 5d series) embedded in two different conformations of graphitic carbon nitride (g-C3N4) monolayers. Additionally, we investigate the effect of nitrogen vacancies in the g-C3N4 monolayers on the absorption of SACs considering three potential absorption scenarios that correspond to different experimental conditions. Our results point to the most stable configurations with the lowest formation energies and indicate that the absorption of single TM atoms on-vacancy and on-center sites are more favorable than via-substitution. In addition to the thermodynamic stability, electrochemical stability is also investigated through the calculation of the dissolution potential of the SACs. Within the scenarios considered in this study, we find that Pt, Pd, Rh, Au, Ru, Ir, Cu, Co, Fe, and Ni will produce the most robust SACs on both (edge and bridge) N vacancy site of reduced g-C3N4. Our findings provide guidance for the design and development of g-C3N4 sheets decorated with single TM atoms for technological applications such as pollutant degradation, CO2 reduction, N2 fixation, selective oxidation, water splitting, and metal ion-based batteries.

Flat-band based high-temperature ferromagnetic semiconducting state in the graphitic C4N3 monolayer

Half-filled isolated flat-band paves a new way to realize high-temperature ferromagnetic semiconductor for spintronics applications, but it is extremely rare in lattice models and lacking in realistic materials. Herein, the 2 × 2 super-cell of the honeycomb lattice with a single-hole defect is proposed as a new lattice model (HL-D-1/8) to realize nontrivial isolated flat-bands. We further demonstrate that C4N3 monolayer of the experimentally realized graphitic carbon nitride (Adv. Mater., 22, 1004, 2010; Nat. Commun., 9, 3366, 2018) is a perfect system holding such a lattice to host flat-bands. A new corrugated Pca21 configuration is proposed as the ground state for the free-standing C4N3 monolayer, which is dynamically stable and energetically more favorable than the widely-used flat one without dynamical stability. The Pca21 configuration is found to be an intrinsic ferromagnetic half-semiconductor (Tc ≈ 241 K) with one semiconducting spin-channel (1.75 eV) and one insulating spin-channel (3.64 eV), which is quite rare in two-dimensional systems. Its ferromagnetic semiconducting property originates from the isolated pz-state flat-band as the corrugation shifts the flat-band upward to the Fermi level. Interestingly, such a corrugated Pca21 C4N3 monolayer is found to be both piezoelectric and ferroelectric, which makes it an unusual metal-free two-dimensional multiferroic.

Labile graphitic monolayers as a partition phase for containment of organic chemicals by graphene-based nanomaterials

Graphene-based nanomaterials (GBNMs) (e.g., graphene oxides and carbon nanotubes) display superior electronic and thermal conductivities and varying abilities to contain organic substances. This study sheds light to the idea that GBNMs behave as a dual sorbent, rather than a sole adsorbent, to extract nonionic organic solutes from water by both (competitive) adsorption and (noncompetitive) partition because of the solute interactions with various GBNM nanostructures formed by atomically-thin graphitic monolayers. Essential solute-sorption data with three model GBNMs from this research and similar data from the literature lead to a coherent view that labile graphitic monolayers in GBNMs undergo a liquid-like motion at room temperature to retain nonionic organic solutes by partition while structurally rigid graphitic clusters behave as adsorbents. Because the partition is noncompetitive, the GBNMs possessing high levels of labile graphene layers, as reflected by high BET surface areas, are capable of sequestering vastly higher levels of multiple organic solutes (especially, those of liquids) than conventional adsorbents, e.g., activated carbon (AC). Moreover, the postulated dual functionality of GBNMs makes sense of many otherwise puzzling phenomena, such as the highly concentration-dependent solute competitive effect with certain GBNMs and highly variable “adsorbed capacities” per unit surface area for different organic solutes with a GBNM versus those by a conventional adsorbent (e.g., graphite or AC).Graphical

Implications of the Pore Size of Graphitic Carbon Nitride Monolayers on the Selectivity of Dual-Boron Atom Catalysts for the Reduction of N2 to Urea and Ammonia: A Computational Investigation.

The formation of urea by electrocatalytic means remains a great challenge due to the lack of a suitable catalyst that is capable of not only activating inert N2 and CO2 molecules but also circumventing the complexity associated with subsequent reaction steps leading to urea formation. Herein, by means of comprehensive density functional theory simulations, we investigate the catalytic activity of highly stable transition-metal-free dual-boron atom-doped graphitic carbon-nitride monolayers with different pore sizes toward urea production under ambient conditions. As per the results, dual boron atoms impregnated in g-C2N and g-C6N6 monolayers with large pore diameters can successfully activate the N2 molecule and lead to the spontaneous formation of the *NCO*N intermediate, which is the most crucial step for urea formation via direct coupling of N2 and CO2. Interestingly, the B2@g-C2N and B2@g-C6N6 favor urea production with low limiting potentials of -1.11 and -1.18 V compared to very high limiting potentials of -1.71 and -1.88 V, respectively, for ammonia synthesis, leading to an almost 100% Faradaic efficiency for urea formation over ammonia. The dual-boron doping in g-C3N4 with a smaller pore size depicts comparatively weaker N2 adsorption than g-C2N and g-C6N6 counterparts. Further, B2@g-C3N4 prefers ammonia formation at a very low limiting potential of -0.40 V compared to a very high limiting potential of -2.11 V for urea formation. Thus, our findings clearly highlight the critical role played by the pore size of carbon-nitride monolayers in tuning the reactivity and catalytic activity of dual-boron atom catalysts toward urea formation in a selective manner, thereby providing valuable guidance in exploring other highly efficient urea catalysts.

Lithium and phosphorus-functionalized graphitic carbon nitride monolayer for efficient hydrogen storage: A DFT study

We have explored the consequence of lithium and phosphorous functionalization on the graphitic carbon nitride (g-C3N4) monolayer for hydrogen storage using density functional theory. Both pristine and Li and P decorated g-C3N4 show a semiconductor nature. The substantial overlap between the s orbital of Li and the p orbital of nitrogen near the Fermi level shows the binding between Li and the g-C3N4. The repositioning of HOMO and LUMO is noticed in the Li and P decorated g-C3N4. The Bader charge analysis indicates the charge allocation from the Li and P atom to the g-C3N4, which results in the adsorption of H2 by electrostatic interaction. The hydrogen storage capacity of 5.78 wt% is obtained after functionalizing Li and P into the g-C3N4. The obtained adsorption energies for the H2 adsorption and the H2 desorption temperature confirm that Li and P functionalized g-C3N4 is a fascinating candidate for the reversible loading of H2 at ambient conditions.

Silk Fibroin Hybrids for Biological Scaffolds with Adhesive Surface and Adaptability to the Target Tissue Change

Abstract Background Regenerative Medicine (RM) is a branch of medicine that aims to regenerate tissues and organs to overcome the problems transplants entail (poor availability, risk of rejection and intense immunosuppression). To do this, RM makes use of tissue engineering (TE). This fundamental branch deals with creating biological scaffolds capable of performing the role that physiologically belongs to the extracellular matrix (ECM). In this review, we report how specific characteristics of the scaffolds (bio-compatibility, biodegradability and mechanical and conformal properties) can be obtained using 3D printing, which facilitates the emulation of physiological tissues and organs. Purpose and scope This review reports recent advances in the fabrication method of bioactive scaffolds that can be used clinically, providing support for cell seeding and proliferation. To this end, silk fibroin, tannin and graphene were used to improve the scaffold’s electro-bio-mechanical properties. These materials in different compositions are studied to demonstrate their potential use as bio-ink in bioadhesives and cellularized and implantable 3D-printed scaffolds. Summary of new synthesis and conclusions reached in the review Silk fibroin is a natural biopolymer; tannin, on the other hand, is a biological polyphenol, highly reactive with other molecules by nature and with promising antioxidant capabilities. Finally, graphene is nothing more than a monolayer of graphite that has been shown to implement the mechanics and electrical conductivity of the compounds in which it is inserted; it also has excellent biocompatibility and surface area, qualities that promote cell adhesion and growth. Conclusion Polyphenols and graphene have been shown to work in synergy in improving the electro-mechanical properties of silk fibroin scaffolds. We reported optimal and potentially market-competitive bioadhesives, but above all, the proliferation of neuronal precursor cells in vitro was successfully demonstrated.

Surface ionization of CsCl molecules in the learning process

The paper presents the results of studying the catalytic properties of “metal-carbon” systems using the method of surface ionization of CsCl molecules. Using the Ir-C system as an example, it was shown that the poisoning effect of the same poison (carbon) strongly depends on the structure of the layer formed on the catalyst. In metals that do not form bulk carbides, the carbon monolayer completely poisons the catalytic properties of these metals. It is shown that individual atoms and clusters of samarium deposited on iridium coated with a monolayer of graphite lead to efficient catalytic dissociation of CsCl molecules. The mechanism of formation of a carbon film on the surface of a metal obtained by diffusion from the volume, which differs from the mechanism of formation of a carbon film obtained on the surface of a metal by adsorption, has been elucidated.

Positive and negative effects of graphite flake and monolayer graphene oxide templates on protein crystallization

Heterogeneous template-induced nucleation is a promising way to regulate protein crystallization events and could be employed for purification processes and crystallographic studies. Protein crystallization process with graphite and graphene oxide, as heterogeneous templates, were investigated. More than 640 hanging drops with different concentrations of Lysozyme (30, 50, 70, 100 mg/mL) and NaCl (0.7, 0.9, 1.1, 1.3, 1.5 M) were crystallised at 4 °C with or without graphite/graphene oxide templates. The induction times and crystallization process were observed under the microscope. The lysozyme in the solutions with graphite flakes nucleated faster under all the conditions than the lysozyme with equal experimental conditions without templates. The crystals preferred to grow around the edge of graphite flakes than on the flat surfaces. In the droplets with monolayer graphene oxide, more crystals appeared around graphene oxide particles, and the faster or slower nucleation processes with templates were dependent on the lysozyme and NaCl concentrations. Graphene oxide templates strongly inhibited nucleation at high lysozyme concentrations but promoted nucleation at low lysozyme concentrations. Both heterogeneous templates changed the crystal morphology and the crystallization kinetics. More crystals were observed in the solution with graphite templatesthan with graphene oxide templates and without any template.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids.

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

Accelerated Ultrafast Magnetization Dynamics at Graphene/CoGd Interfaces.

Atomically thin graphene layers can act as a spin-sink material when adjacent to a nanoscale magnetic surface. The enhancement in the extrinsic spin-orbit coupling (SOC) strength of graphene plays an important role in absorbing the spin angular momentum injected from the magnetic surface after perturbation with an external stimulus. As a result, the dynamics of the excited spin system is modified within the magnetic layer. In this paper, we demonstrate the modulation of ultrafast magnetization dynamics at graphene/ferrimagnet interfaces using the time-resolved magneto-optical Kerr effect (TRMOKE) technique. Magnetically modified interfaces with a systematic increase in the number of graphene layers coupled with the 10 nm-thick Co74Gd26 layer are studied. We find that the variation in the dynamical parameters, i.e., ultrafast demagnetization time, remagnetization times, decay time, effective damping, precessional frequency, etc., observed at different time scales is interconnected. The demagnetization time and decay time for the ferrimagnet become approximately two times faster than the corresponding intrinsic values. We found a possible correlation between the demagnetization time and damping. The effect is more pronounced for the interfaces with monolayer graphene and graphite. The spin-mixing conductance is found to be approximately 0.8 × 1015 cm-2. The effect of SOC, pure spin current, the appearance of structural defects, and thermal properties at the graphene/ferrimagnet interface are responsible for the modifications of several dynamical parameters. This work demonstrates some important properties of the graphene/ferrimagnet interface which may unravel the possibilities of designing spintronic devices with elevated performance in the future.

G‐C3N4‐Supported Metal‐Pair Catalysts toward Efficient Electrocatalytic Nitrogen Reduction: A Computational Evaluation

AbstractAmbient electrocatalytic nitrogen reduction reaction (eNRR) is a low‐cost and clean method for large‐scale ammonia production. In this work, the authors investigate systematically the potential of 17 3d–5d double‐atom metal‐pairs catalysts supported on graphitic carbon nitride monolayer (M2@g‐C3N4) toward efficient eNRR. Two thermodynamically stable catalysts, V2@g‐C3N4 and Ni2@g‐C3N4, are identified as excellent candidates with low limiting potentials, small ammonia desorption free energies, and a high catalytic selectivity over the competing hydrogen evolution reaction (HER). The good performances toward eNRR of the two catalysts are both attributed to the cooperation effect of double active sites on nitrogen adsorption and on the early hydrogenation steps; it breaks the intrinsic linear scaling relationships between the adsorption energies of N‐containing intermediates and balances the competing needs of a good eNRR catalysts under the help of additional N2 adsorption: a low limiting potential, a small ammonia desorption energy, and a superior ability to suppress the HER. The other considered M2@g‐C3N4 behave more like a tuned single‐atom catalyst in the eNRR process, and thereby do not exhibit the expected eNRR activity.

Two-dimensional porous graphitic carbon nitride C6N7 monolayer: First-principles calculations

The fabrication of the C6N7 monolayer [Zhao et al., Sci. Bull. 66, 1764 (2021)] motivated us to discover the optical, structural, mechanical, and electronic properties of the C6N7 monolayer by employing the density functional theory (DFT) method. We find that the shear modulus and Young's modulus of the C6N7 monolayer are smaller than the relevant values of graphene. However, Poisson's ratio is more significant than that of graphene. Applying the PBE (HSE06) functional bandgap of the C6N7 monolayer is 1.2 (1.97) eV, and the electronic dispersion is almost isotropic around the Γ point. C6N7 is more active in the ultraviolet region as compared to the visible light region. This study provides outstanding results, highlighting the bright viewpoints for the applications of the C6N7 monolayer in electronic and optical systems.

Design of Colorimetric Sensor Employing CdSe QDs Decorated Graphene Xerogel for Onsite Visual Detection of TNT

In today's world, extensive use of various organic explosives such as nitroaromatics, nitramines and peroxide can be seen in both military and terrorism. Among the different explosives, 2,4,6-Trinitrotoluene (TNT) and its degraded compound 2,4-Dinitrotoluene (DNT) are the most used energetic material in different legitimately produced explosives and improvised explosive devices (IEDs). The detonation of explosives usually causes damages to the environment, which in turn affects humans’ health and safety. Owing to the importance of explosive detection for the security of land and the environment, the exploration of new methodologies for sensing electron-deficient nitroaromatics explosives (NAEs) is urgently imperative. [1,2] In recent years, a number of fluorescent chemosensors/probes have been developed and widely used in chemical sensing of DNT and TNT, especially quantum dots (QDs) and nanoparticles, because of their comprehensive investigation in various applications such as optical sensor, bio-imaging, bio-labeling, biomolecular and NAEs detection. [3] Here, our research efforts are to promote fluorescent Cysteamine capped-CdSe QDs for decorating porous graphene xerogel (GSXS) to design a robust sensor with a new and elegant methodology. Consequently, this sensor can provide good opportunities for visual TNT detection in wearable applications. Graphene oxide (GO), a single monolayer of graphite, has recently attached great scientific attention; besides, the 3D-porous structure of GO has shown impulsive properties, such as large surface areas, high compressibility, ultralow density and adsorption potential. On the other hand, chitosan (CS), alkaline deacetylation of chitin, owned many fascinating properties such as biocompatibility, biodegradability and antimicrobial ability. The 3D-network of GO and CS blend scaffold is expected to have high porosity, surface area, electrochemical properties and high adsorption potential because of hydrogen-bond linking of epoxy groups of GO with the amino group of CS. This nanocomposite could effectively adsorb the QDs and NPs via covalent and non-covalent interactions. Inspired by this top-notch property of GO-CS nanocomposite and the excellent fluorescence property of CdSe QDs, a simple and elegant approach is introduced to design a colorimetric sensor based on QDs decorated graphene xerogel (QD-GSXS), also called GO-CS nanocomposite. Its dual benefit can provide a novel path of sensitive and selective sensing for chemical and biological applications. The preparation of this fluorescent QD-GSXS nanosensor would be a challenge of relevance because of the fluorescent material is usually quenched by graphene. [4] So, this work aims to successfully synthesis fluorescent QD-GSXS nanosensor and exhibits its application for on-site and visual detection of TNT via the formation of the Meisenheimer complex. Herein firstly, we synthesized porous graphene xerogels wrapped with an optimized mass ratio of chitosan. This xerogel surface is further decorated with highly fluorescent Cysteamine capped-CdSe QDs. These composite were characterized using XRD, FE-SEM, FTIR, Raman, BET, UV-Vis and fluorescence spectroscopy. This spongy fluorescent QD-GSXS nanosensor probe was employed for the sensitive and selective detection of TNT. The sensing mechanism is further stabilized via Time-correlated Single Photon Counting (TCSPC) measurement and with the frontier-molecular energy level analysis, calculated by Density-Functional-Theory (DFT). More importantly, we further demonstrated the utility of this designed nanosensor probe as on-site visual TNT sensor by observing a rapid fluorescence change in optical images captured under the illumination of Green Fluorescent Protein (GFP). The design chemosensor showed several advantages, including good selectivity, excellent stability and having linearity with the concentration of TNT. Such a simple colorimetric sensor allows the visual detection of TNT at µM level without any sophisticated instrument. In addition, CdSe-NH2 QDs were also exhibited sensing with DNT and TNT in solution with LOD 18.2 and 9.7 µM, respectively. Considering their structure versatility and design flexibility, we anticipate that our efforts were to combine the electron-donor unit, cystamine capped-CdSe QDs, with an intrinsic porosity of GSXS and make it functional for NAEs detection. This novel and elegant approach gives a new direction to make a prototype chemosensor for visual detection of different NAEs with the advantages of simplicity, ease of operation and high sensitivity. However, there is a wide scope to increase the sensitivity further. This approach will also benefit future research on developing fluorescent hybrid nanosensors based on fluorescent nanoparticles and quantum dots for various chemical and biological applications.

Epitaxy of boron nitride monolayers for graphene-based lateral heterostructures

Monolayers of hexagonal boron nitride (hBN) are grown on graphite substrates using high-temperature molecular beam epitaxy (HT-MBE). The hBN monolayers are observed to grow predominantly from step edges on the graphite surface and exhibit a strong dependence of the morphology, including the dominant crystallographic edge, of the hBN monolayers, on the growth temperature, as well as systematic variations in growth rate and coverage, and significant differences in the growth at monolayer and multilayer graphite steps. At graphite monolayer steps hBN grows laterally across the surface on the lower terrace, but hBN growth on the upper side of the graphite step is more limited and is nucleated by three-dimensional clusters. Multilayer graphite steps exhibit a much higher density of non-planar hBN aggregates and growth on both the upper and lower terraces occurs. The results show that the hBN monolayer growth edge type, hBN island shape and the presence of hBN aggregates can be controlled in HT-MBE, with the highest quality layers grown at a substrate temperature of about 1390 °C. Sequential HT-MBE growth of hBN, graphene (G) and a second cycle of hBN growth results in the formation of monolayer thick lateral hBN–G–hBN heterostructures, in which a strip of G is embedded between monolayers of hBN.

First-Principles Calculations of Exciton Radiative Lifetimes in Monolayer Graphitic Carbon Nitride Nanosheets: Implications for Photocatalysis

In this work, we report on the exciton radiative lifetimes of graphitic carbon nitride monolayers in the triazine- (gC$_3$N$_4$-t) and heptazine-based (gC$_3$N$_4$-h) forms, as obtained by means of ground- plus excited-state ab initio calculations. By analysing the exciton fine structure, we highlight the presence of dark states and show that the photo-generated electron-hole pairs in gC$_3$N$_4$-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC$_3$N$_4$-h in photocatalysis and makes it attractive for the emerging field of exciton devices. Although very long intrinsic radiative lifetimes are an important prerequisite for several applications, pristine carbon nitride nanosheets show very low quantum photo-conversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC$_3$N$_4$-t and gC$_3$N$_4$-h layers which shows a type-II band alignment and looks promising for achieving net charge separation.

Graphene Oxides Derivatives Prepared by an Electrochemical Approach: Correlation between Structure and Properties.

Graphene oxide (GO) can be defined as a single monolayer of graphite with oxygen-containing functionalities such as epoxides, alcohols, and carboxylic acids. It is an interesting alternative to graphene for many applications due to its exceptional properties and feasibility of functionalization. In this study, electrochemically exfoliated graphene oxides (EGOs) with different amounts of surface groups, hence level of oxidation, were prepared by an electrochemical two-stage approach using graphite as raw material. A complete characterization of the EGOs was carried out in order to correlate their surface topography, interlayer spacing, defect content, and specific surface area (SSA) with their electrical, thermal, and mechanical properties. It has been found that the SSA has a direct relationship with the d-spacing. The EGOs electrical resistance decreases with increasing SSA while rises with increasing the D/G band intensity ratio in the Raman spectra, hence the defect content. Their thermal stability under both nitrogen and dry air atmospheres depends on both their oxidation level and defect content. Their macroscopic mechanical properties, namely the Young’s modulus and tensile strength, are influenced by the defect content, while no correlation was found with their SSA or interlayer spacing. Young moduli values as high as 54 GPa have been measured, which corroborates that the developed method preserves the integrity of the graphene flakes. Understanding the structure-property relationships in these materials is useful for the design of modified GOs with controllable morphologies and properties for a wide range of applications in electrical/electronic devices.

Holey Reduced Graphene Oxide/Carbon Nanotube/LiMn0.7Fe0.3PO4 Composite Cathode for High-Performance Lithium Batteries

Introduction Graphene is a single atomic monolayer of graphite and has been found a variety of applications in energy conversion and storage devices, due to the excellent properties such as high carrier mobility (~10,000 cm2 V−1 s−1 at room temperature), structural flexibility, chemical and thermal stability, mechanical strength and ultrahigh theoretical specific surface area (2630 m2g−1) [1]. However, when the graphene sheet has large area, the wrapping of active materials may hinder the transportation of Li+ between active particles and electrolyte, especially at high charge/discharge rates.Herein, holey graphene oxide (h-GO) [2], which is made by a green wet ball-milling of GO in one step without using any catalysts or chemicals, is combined with carbon nanotubes (CNTs) and LiMn0.7Fe0.3PO4 (LMFP) to make a composite cathode for lithium batteries [3]. Results show that the LMFP cathode with h-GO/CNT shows a remarkedly improved electrochemical performances due to the facilitated Li+ transport pathway, compared to that with conventional GO/CNT. This study provides a new approach for fabricating holey graphene and can open up new possibilities for applications on power sources. Results and discussion The synthesis of h-GO and LMFP composite material is shown in Fig. 1. Graphite oxide suspension (~0.6 mg/mL) was obtained by mixing with purified water and after the ultrasonic treatment for 3 hours. Subsequently, the ball-milling of 50 mL GO suspension was performed in a planetary ball-mill machine with 30–120 g of 100 mm size zirconia balls for 20 min at 400 rpm (the corresponding weight ratio of balls to GO aqueous suspension is 0.6−2.4). The balls were fully removed by sieve afterward. Finally, the mixture was freeze-dried for 12 hours. LMFP, h-GO, CNTs and binder were mixed at a mass ratio of 95.6%: 2%: 0.4%: 2% and coated onto Al foil to obtain a cathode electrode. Electrochemical characteristics were evaluated by preparing a 2032 type coin cell using Li foil as the counter electrode. The electrolyte was 1 mol dm-3 LiPF6/EC-DEC (1: 2).SEM image of the synthesized h-GO is shown in Fig. 2. It is found that irregular micropores can be generated on the surface of GO after the ball-milling of GO aqueous suspension. At the conference, we will show that the more balls used, the more and bigger holes could be observed. That is, the hole size and surface density could be controlled by the adjustment of the weight ratio of balls to GO aqueous suspension. We will also show that LMFP electrode with h-GO/CNT can deliver more than half of discharge capacities than that with conventional GO/CNT at a higher discharge rate (>=20 C), also superior to that with h-GO or CNT only. The use of h-GO made by the other method, such as the chemical etching method, the performances will also be discussed. The scalable and robust synthesis method of h-GO and the novel properties of h-GO/CNT reported here greatly advance the practicality of using it as electrodes in lithium batteries as well as other energy storage technologies. Acknowledgment We thank Taiheiyo Cement Co., Ltd for the providing of LiMn0.7Fe0.3PO4 powders and NIMS for the TEM measurement. Reference 1. Han, et.al., Small. 9 (2013). 2. Y. Xu, et al., Nat. Commun. 5 (2014). 3. D. Ding, et.al., J. Power Sources. 449 (2020). Figure 1

Titanium dioxide based self-cleaning smart surfaces: A short review

Abstract The bio-inspired nanotechnology of self-cleaning surfaces and coatings are nowadays being much attracted in the field of energy and environment because of the ever-increasing demand of uncontaminated, self-disinfected and hygienic surfaces. Such surfaces/coatings can be adopted on glass windows, automobile windshields, anti-fouling membranes, textiles, building construction materials, paints; on optoelectronic devices like solar panels, photochromic glasses, etc; in the food industry and for medical aids. This review gives a special emphasis to superhydrophilic self-cleaning surfaces. The mechanism of photocatalytic self-cleaning and superhydrophilicity of nano TiO2 based systems is carefully investigated. The photogenerated reactive oxygen species generated during photoexcitation of electrons from the valence band to the conduction band of TiO2 is responsible for its photocatalytic behaviour, anti-bacterial property and superhydrophilicity. Emphasis is given on various systems and methods that could enhance the self-cleaning behaviour of TiO2 in the solar spectrum since pristine TiO2 is only ultraviolet responsive. TiO2-semiconductor heterojunctions, hybrids of TiO2 with graphene and graphitic monolayers, tailoring the exposed crystal facets in TiO2, metal and non-metal doping and dye sensitization, attributing to visible light photocatalytic self-cleaning activity are focused in detail. Various techniques adopted for the synthesis and fabrication of superhydrophilic self-cleaning materials and coatings are discussed.