《新型二维纳米材料》专辑序言——新型二维纳米材料:掀起未来技术革命

Two-dimensional(2D) nanomaterials possess sheet-like structures with the thickness of nanoscale, but the lateral size is infinite. In 2004, Andre Geim and co-workers at the University of Manchester successfully exfoliated a sheet of graphene from graphite by the micromechanical cleavage technique, which marked the beginning of 2D nanomaterials. Given the ultrahigh carrier mobility, excellent mechanical property, good thermal stability, superior thermal conductivities and huge specific surface area of graphene, it causes general exploration of other graphene-like 2D nanomaterials.===The 2D feature is unique to access unprecedented physical, chemical, electronic and optical properties. For example, the electron confinement in two dimensions makes them ideal candidates for the fundamental study in condensed matter physics and electronic/optoelectronic devices; the large lateral size endows them with huge surface area and high exposure of active sites. Due to their unique properties, 2D nanomaterials have promising applications in energy storage and conversion, electronic devices, catalytic reaction, sensing and biomedicine. By now, nearly 20 types of 2D nanomaterials have been studied, such as graphene, graphitic carbon nitride(g-C₃N₄), transition metal dichalcogenides(TMDs), transition metal carbides/nitrides(MXenes), layered double hydroxides(LDHs), transition metal oxides(TMOs), Ⅲ to Ⅵ layered semiconductor(MX₄), and perovskite-type hybrids(AMX₃).===In this special issue of the novel 2D nanomaterials, we selected 12 related articles in reviews, research papers and brief communications involving supercapacitor, electrochemical catalysis, sensing, battery, fluorescence, water treatment and antiflaming performance of 2D nanomaterials. We hope that readers will have a deep understanding of the current development of 2D nanomaterials, and find it beneficial to their future researches.===Toward this end, I greatly appreciate the outstanding contribution of all authors, as well as the strenuous efforts from the editorial staff members.     

Thermal stability of graphene edge structure and graphene nanoflakes

One of the most exciting recent developments in nanoscience was the discovery of graphene (single sheets of carbon atoms, a two-dimensional "(2D) crystal") and the subsequent discovery of the fascinating properties of this new material, e.g., electrons behaving as massless relativistic particles and an anomalous quantum Hall effect [A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183 (2007)]. It is also surprising that large sheets of graphene exist as it was widely believed that 2D crystals are unstable. Furthermore, because of the stability of folded graphene sheets, i.e., carbon nanotubes (CNTs), a fascinating question is why does not graphene spontaneously transform into CNTs? In this paper, we explore the thermal stability of small pieces of graphene, i.e., graphene nanoflakes by ab initio quantum mechanical techniques. We find that indeed nanoflakes are stable to being heated and do not under any conditions used here transform to CNTs. They do not, however, remain strictly 2D as at finite temperatures, they undergo extensive vibrational motion and remain buckled if annealed and then quenched to room temperature.     

Reversible Hydrogenation of Graphene on Ni(111)—Synthesis of “Graphone”

Understanding the adsorption and reaction between hydrogen and graphene is of fundamental importance for developing graphene‐based concepts for hydrogen storage and for the chemical functionalization of graphene by hydrogenation. Recently, theoretical studies of single‐sided hydrogenated graphene, so called graphone, predicted it to be a promising semiconductor for applications in graphene‐based electronics. Here, we report on the synthesis of graphone bound to a Ni(111) surface. We investigate the formation process by X‐ray photoelectron spectroscopy (XPS), temperature‐programmed desorption (TPD), and density‐functional theory calculations, showing that the hydrogenation of graphene with atomic hydrogen indeed leads to graphone, that is, a hydrogen coverage of 1 ML (4.2 wt %). The dehydrogenation of graphone reveals complex desorption processes that are attributed to coverage‐dependent changes in the activation energies for the associative desorption of hydrogen as molecular H₂.     

石墨烯膜质子传输巨大光效应的微观机理

Graphene monolayers are permeable to thermal protons and impermeable to other atoms and molecules, exhibiting their potential applications in fuel cell technologies and hydrogen isotope separation. Furthermore, the giant photoeffect in proton transport through catalytically activated graphene membranes was reported by Geim et al. Their experiment showed that the synergy between illumination and the catalytically active metal plays a key role in this photoeffect. Geim et al. suggested that the local photovoltage created between metal nanoparticles and graphene could funnel protons and electrons toward the metal nanoparticles for the production of hydrogen, while repelling holes away from them, causing the giant photoeffect. However, based on static electric field theory, this explanation is not convincing and the work lacks an analysis on the microscopic mechanism of this effect. Herein, we provide the exact microscopic mechanism behind this phenomenon. In semi-metal pristine graphene, most photon excited hot electrons relax to lower energy states within a timescale of 10⁻¹² s, while the typical timescale of a chemical reaction is 10⁻⁶ s. Thus, hot electrons excited by incident photons relax to lower energy states before reacting with protons through the graphene. When graphene is decorated with metal, electron transfer between the graphene and the metal, induced by different work functions, would result in the formation of interface dipoles. When using metals such as Pt, Pd, Ni, etc., which can strongly interact with graphene, local dipoles form. Protons are trapped around the negative poles of the local dipoles, while electrons are around the positive poles. Upon illumination, the electrons are excited to metastable excited states with higher energy levels. Due to the energy barriers around them, the free electrons in the metastable excited states will have a relatively longer lifetime, which facilitates the production of hydrogen through their effective reaction with protons that permeated through the graphene. The concentration of high-energy electrons under illumination was estimated, and the results showed that more electrons are energized to the excited state with strong illumination. According to the analysis, the giant photoeffect in proton transport through the catalytically activated graphene membrane is attributed to long-lived hot electrons and a fast proton transport rate. Since there is no change in the activation energy of the reaction, the metal catalyst increases the rate of the reaction by increasing the number of successful collisions between the reactants to produce the significant photoeffect. This might lead to a new microscopic mechanism that clarifies the role of the catalyst in improving the efficiency of photo(electro)catalytic reactions.     

Increasing the Sensitivity and Single‐Base Mismatch Selectivity of the Molecular Beacon Using Graphene Oxide as the “Nanoquencher”

Here, we report a novel, highly sensitive, selective and economical molecular beacon using graphene oxide as the “nanoquencher”. This novel molecular beacon system contains a hairpin‐structured fluorophore‐labeled oligonucleotide and a graphene oxide sheet. The strong interaction between hairpin‐structured oligonucleotide and graphene oxide keep them in close proximity, facilitating the fluorescence quenching of the fluorophore by graphene oxide. In the presence of a complementary target DNA, the binding between hairpin‐structured oligonucleotide and target DNA will disturb the interaction between hairpin‐structured oligonucleotide and graphene oxide, and release the oligonucleotide from graphene oxide, resulting in restoration of fluorophore fluorescence. In the present study, we show that this novel graphene oxide quenched molecular beacon can be used to detect target DNA with higher sensitivity and single‐base mismatch selectivity compared to the conventional molecular beacon.     

“Metal‐Free” Catalytic Oxygen Reduction Reaction on Heteroatom‐Doped Graphene is Caused by Trace Metal Impurities

The oxygen reduction reaction (ORR) is of high industrial importance. There is a large body of literature showing that metal‐based catalytic nanoparticles (e.g. Co, Mn, Fe or hybrid Mn/Co‐based nanoparticles) supported on graphene act as efficient catalysts for the ORR. A significant research effort is also directed to the so‐called “metal‐free” oxygen reduction reaction on heteroatom‐doped graphene surfaces. While such studies of the ORR on nonmetallic heteroatom‐doped graphene are advertised as “metal‐free” there is typically no sufficient effort to characterize the doped materials to verify that they are indeed free of any trace metal. Here we argue that the claimed “metal‐free” electrocatalysis of the oxygen reduction reaction on heteroatom‐doped graphene is caused by metallic impurities present within the graphene materials.     

Recent advances in the use of graphene-family nanoadsorbents for removal of toxic pollutants from wastewater

Adsorption technology is widely considered as the most promising and robust method of purifying water at low cost and with high-efficiency. Carbon-based materials have been extensively explored for adsorption applications because of their good chemical stability, structural diversity, low density, and suitability for large scale production. Graphene – a single atomic layer of graphite – is the newest member in the family of carbon allotropes and has emerged as the “celeb” material of the 21st century. Since its discovery in 2004 by Novoselov, Geim and co-workers, graphene has attracted increased attention in a wide range of applications due to its unprecedented electrical, mechanical, thermal, optical and transport properties. Graphene's infinitely high surface-to-volume ratio has resulted in a large number of investigations to study its application as a potential adsorbent for water purification. More recently, other graphene related materials such as graphene oxide, reduced graphene oxide, and few-layered graphene oxide sheets, as well as nanocomposites of graphene materials have also emerged as a promising group of adsorbent for the removal of various environmental pollutants from waste effluents. In this review article, we present a synthesis of the current knowledge available on this broad and versatile family of graphene nanomaterials for removal of dyes, potentially toxic elements, phenolic compounds and other organic chemicals from aquatic systems. The challenges involved in the development of these novel nanoadsorbents for decontamination of wastewaters have also been examined to help identify future directions for this emerging field to continue to grow.     

The Nanoparticle Size Effect in Graphene Cutting: A “Pac‐Man” Mechanism

Metal‐nanoparticle‐catalyzed cutting is a promising way to produce graphene nanostructures with smooth and well‐aligned edges. Using a multiscale simulation approach, we unambiguously identified a “Pac‐Man” cutting mechanism, characterized by the metal nanoparticle “biting off” edge carbon atoms through a synergetic effect of multiple metal atoms. By comparing the reaction rates at different types of edge sites, we found that etching of an entire edge carbon row could be triggered by a single zigzag‐site etching event, which explains the puzzling linear dependence of the overall carbon‐atom etching rate on the nanoparticle surface area observed experimentally. With incorporation of the nanoparticle size effect, the mechanisms revealed herein open a new avenue to improve controllability in graphene cutting.     

A facile “graft from” method to prepare molecular‐level dispersed graphene–polymer composites

Graphene–polymer composites of positive‐charged poly(dimethyl aminoethyl acrylate), negative‐charged poly(acrylic acid), and neutral polystyrene were prepared by “graft from” methodology using reversible addition fragmentation chain transfer (RAFT) polymerization via a pyrene functional RAFT agent (PFRA) modified graphene precursor. Fluorescence spectroscopy and attenuated total reflection infrared (ATR‐IR) evidenced that the PFRA was attached on the graphene basal planes by π–π stacking interactions, which is strong enough to anti‐dissociation in the polymerization mixture up to 80°C. Atomic force microscopy (AFM) revealed that the thickness of a graphene–polymer sheet was about 4.0 nm. Graphene composites of different polymers with the same polymerization degree exhibited similar conductivity; however, when the polymer chain was designed as random copolymer the conductivity was significantly decreased. It was also observed that the longer the grafted polymer chains the lower the conductivity. ATR‐IR spectroscopy and thermogravimetric analysis were also performed to characterize the as‐prepared composites. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis of polystyrene nanoparticles “armoured” with nanodimensional graphene oxide sheets by miniemulsion polymerization

Polystyrene particles “armoured” with nanosized graphene oxide (GO) sheets have been prepared by aqueous miniemulsion polymerization of styrene, exploiting the amphiphilic properties of GO in the absence of conventional surfactants. The nanoscale GO sheets were prepared from graphite nanofibers of diameter approximately 100 nm based on a novel procedure, thus effectively ensuring the absence of larger sheets. Polymerization proceeded to high conversion with minor coagulation, with final number‐average particle diameters of approximately 500 nm, but relatively broad particle size distributions. Scanning electron microscopy analysis revealed particles with a textured surface, consistent with the expected morphology. Interestingly, analysis of GO sheets recovered from the polymerization revealed that the GO sheets are partially reduced during the polymerization—approximately 50% of the initial carboxyl groups of the GO were lost, consistent with some loss in colloidal stability at high conversion. The overall approach offers a convenient and attractive synthetic route to novel graphene‐based polymeric nanostructures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Carbocatalysis: The State of “Metal‐Free” Catalysis

The rise in global demand for crucial chemical compounds has driven immense research in the fundamental science of catalysis. Graphene and its derivatives (chemically modified graphene, CMGs) have recently emerged as a new class of heterogeneous catalyst that promises economically viable and greener routes to these compounds. Although CMGs possess unique catalytic properties, the actual active sites are often points of discussion. Current minimal understanding on the possible effects of metallic impurities on the electrocatalytic performances of these CMGs calls forth the need to raise awareness on possible metallic impurities misrepresenting the actual chemical catalytic performances of the CMGs. This Minireview highlights the latest advances in the application of CMGs as catalysts, with an emphasis on the possible effects of metallic impurities on CMG catalysis.     

In‐Situ “molecular welding” preparation of graphene/polyimide hybrid film with superior thermal conductivity and flexibility

With the rapid development of electronic industry, thermal management has become a critical issue that severely restricts the application of portable devices. In this work, we fabricate a flexible and free‐standing graphitized‐graphene/polyimide (I‐g‐GO/PI) film via an in‐situ “molecular welding” strategy. With the help of in‐situ polymerization, PI can be well‐dispersed with GO and serves as a solder to enlarge the grain size of GO, resulting in an enhanced thermal conductivity of the film. The 7 wt % addition of PI into GO (I‐g‐GO/PI‐7%) leads to an in‐plane thermal conductivity as high as 1269.700 ± 1.498 W/m/K, which is 81.8% higher than that of the pristine graphene and also superior to that fabricated via solution blending method by 58.3%. Simultaneously, the hybrid film exhibits an excellent flexibility and survives from a 2000 cycles bending test. The large‐area hybrid film prepared by such an in‐situ “molecular welding” method provides a promising way to fabricate graphene‐based film for highly efficient thermal management. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1215–1223     

Goethes Experimente mit „chemischen Farben”︁

In connection with the chemical aspects of his work „Farbenlehre”︁, which involved him almost his whole life, Goethe, too, performed chemicals experiments which he recorded so carefully that his notes can still he reproduced today. It deals with tests series on the reactions of metal salts solutions with yellow prussiate of potash (potassium ferrocyanide), at that time called „Berlinerblau Lauge”︁, acid-base reactions of numerous blossom dyes (today noted as anthocyanes), silver pigments and the coloured ions of manganese in its several oxidation states, called as mineral chameleon.     

“Click”‐Triggered Self‐Healing Graphene Nanocomposites

Strategies to compensate material fatigue are among the most challenging issues, being most prominently addressed by the use of nano‐ and microscaled fillers, or via new chemical concepts such as self‐healing materials. A capsule‐based self‐healing material is reported, where the adverse effect of reduced tensile strength due to the embedded capsules is counterbalanced by a graphene‐based filler, the latter additionally acting as a catalyst for the self‐healing reaction. The concept is based on “click”‐based chemistry, a universal methodology to efficiently link components at ambient reaction conditions, thus generating a “reactive glue” at the cracked site. A capsule‐based healing system via a graphene‐based Cu₂O (TRGO‐Cu₂O‐filler) is used, acting as both the catalytic species for crosslinking and the required reinforcement agent within the material, in turn compensating the reduction in tensile strength exerted by the embedded capsules. Room‐temperature self‐healing within 48 h is achieved, with the investigated specimen containing TRGO‐Cu₂O demonstrating significantly faster self‐healing compared to homogeneous (Cu(PPh₃)₃F, Cu(PPh₃)₃Br), and heterogeneous (Cu/C) copper(I) catalysts.

     

Graphene Quantum Dots Assembled with Metalloporphyrins for “Turn on” Sensing of Hydrogen Peroxide and Glucose

Noncovalent and multifunctional hybrids have been generated via π–π stacking and electrostatic interactions by combining the nanometer‐scale graphene structure of graphene quantum dots (GQDs) with Fe^III 5,10,15,20‐tetrakis(1‐methyl‐4‐pyridyl)porphine (FeTMPyP). The inner filter effect (IFE) of FeTMPyP on the GQDs results in substantial PL quenching of the GQDs. The quenched PL of GQDs by the FeTMPyP can be switched back “on” in response to the reaction between FeTMPyP and H₂O₂, which causes rupture of the cyclic tetrapyrrolic nucleus with consequential loss of iron from FeTMPyP, and then proceeds further to produce colorless dipyrroles and monopyrroles. This “turn on” system can be applied for simple and convenient H₂O₂ sensing and can be further extended to the detection of glucose in combination with the specific catalytic effect of glucose oxidase (GOx) through the oxidation of glucose and formation of H₂O₂. Because of the inherent synthetic control available for the design of metalloporphyrins, the GQDs‐based optical sensing approach described here has the potential to be highly versatile for other target analytes.     

High Catalytic Activity of Nitrogen and Sulfur Co‐Doped Nanoporous Graphene in the Hydrogen Evolution Reaction

Chemical doping has been demonstrated to be an effective way to realize new functions of graphene as metal‐free catalyst in energy‐related electrochemical reactions. Although efficient catalysis for the oxygen reduction reaction (ORR) has been achieved with doped graphene, its performance in the hydrogen evolution reaction (HER) is rather poor. In this study we report that nitrogen and sulfur co‐doping leads to high catalytic activity of nanoporous graphene in HER at low operating potential, comparable to the best Pt‐free HER catalyst, 2D MoS₂. The interplay between the chemical dopants and geometric lattice defects of the nanoporous graphene plays the fundamental role in the superior HER catalysis.     

Application of Si‐doped graphene as a metal‐free catalyst for decomposition of formic acid: A theoretical study

Using density functional theory calculations, the adsorption and catalytic decomposition of formic acid (HCOOH) over Si‐doped graphene are investigated. For the stable adsorption geometries of HCOOH over Si‐doped graphene, the electronic structure properties are analyzed by adsorption energy, density of states, and charge density difference. A comparison of the reaction pathways reveals that both dehydration and dehydrogenation of HCOOH can occur over Si‐doped graphene. The estimated reaction energies and the activation barriers suggest that for the dehydration of HCOOH on the Si‐doped graphene, the rate‐controlling step is H + OH → H₂O reaction. For the dehydrogenation of HCOOH, the rate‐determining step is the breaking of the C? H bond of the HCOO group to form the CO₂ molecule and the atomic H. Our results reveal that the low cost Si‐doped graphene can be used as an efficient nonmetal catalyst for O? H bond cleavage of HCOOH. © 2015 Wiley Periodicals, Inc.     

“Titration” polymerization of monovinylacetylene

A polymer consisting of a saturated carbon backbone with pendent acetylenic groups was prepared from monovinylacetylene. A titration was performed between the monomer and tertiary butyllithium, its lithiating agent. The charge transfer complex formed between the solvent THF and the tertiary butyllithium was used as an indicator of the unreacted butyllithium. Hence, a stoichiometric quantity of tertiary butyllithium was added dropwise to a solution of monovinylacetylene in THF to form lithiovinylacetylene. The addition of a slight excess of butyllithium led to the polymerization of the lithiated monomer. The obtained polymer was reprotonated and characterized. This polymerization was evaluated as a possible route to synthesize poly(vinylacetylene) with processable molecular weights, for its application as a potential carbon fiber precursor. © 1996 John Wiley & Sons, Inc.     

One‐Step Electrodeposition to Layer‐by‐Layer Graphene–Conducting‐Polymer Hybrid Films

One‐step fabrication of graphene–polyaniline (graphene–PANI) hybrid film was facilely achieved by cyclic voltammetric electrolysis of a bath containing both graphene oxide (GO) and aniline, where graphene is obtained by electrochemical reduction of GO and PANI is simultaneously obtained by aniline electropolymerization. As there is no strong attraction between aniline and GO under the electrodeposition conditions, the independent depositions of PANI and reduced GO nanosheets at their greatly differed potentials led to alternate layered graphene–PANI films, with the topmost layer being PANI particles or graphene sheets just by changing the initial scan directions. The two kinds of graphene–PANI hybrid films present excellent but different electrical and electrochemical behaviors.     

Polymer nanocomposites with “ultralow” refractive index

Nanocomposites of gelatine and gold were prepared by reduction of AuCl₄H in a gelatine solution, followed by a spin coating process and a brief washing. The thickness of the films thus obtained was typically 200–400 nm. The surface of the nanocomposites was investigated by scanning electron microscopy (SEM) and surface profilometry. The size of the gold particles is in the range of ca. 2–10 nm, as elucidated by transmission electron microscopy (TEM). Refractive indices were measured by ellipsometry at wavelengths of 632.8 and 1300 nm. The refractive index depends linearly on the volume fraction of gold, at least at 1300 nm. The lowest refractive indices measured are 1.008±0.060 at 1300 nm and 0.963±0.062 at 632.8 nm (limits are 95% confidence interval). These values are, to our best knowledge, the lowest reported for a polymer or polymer composite.