Tailoring the field emission property of nitrogen-doped carbon nanotubes by controlling the graphitic/pyridinic substitution

Nitrogen-doped carbon nanotubes (CNTs) are known to be better field emitters than the pristine ones. But the field emissions (FE) property closely depends on what kinds of nitrogen moieties are substituted in the CNT matrix. While graphitic N-substituents (gN) give rise to additional sub-levels in the unoccupied states near the Fermi level, pyridinic N-substituents (pN) destroy the existing sub-levels. Here, we show that the FE property of N-doped CNTs can be tailored by controlling the gN/pN ratio therein. Vertically aligned N-doped CNTs were grown by chemical vapor deposition of camphor in the presence of ferrocene catalyst, using dimethylformamide (DMF) as a nitrogen source. A step-wise increase of DMF concentration (0-45 wt.%) in camphor caused a systematic rise of N-doping (0.8-5.2 at.%) in the resulting CNTs. The gN- and pN-dopings were identified by XPS analysis, and gN/pN ratio was found to increase from 1.7 to 3.5 in a regular manner. The FE measurements of as-grown N-doped CNTs exhibited a corresponding decrease of turn-on field from 1.8 to 0.6 V/μm and threshold field from 4.2 to 2.0 V/μm. This study thus presents the first experimental demonstration of FE enhancement by controlling the gN/pN ratio.     

Nanoscopic observations of stress-induced formation of graphitic nanocrystallites at amorphous carbon surfaces

The formation of graphitic nanocrystallites at the surface of amorphous carbon under large mechanical stresses was examined by using micro-Raman spectrometry, transmission electron microscopy and in-situ compressions. In the Raman analyses of severely deformed (above a strain energy density criterion of 5.9 J/m²) surface regions of nanoscratched and nanoindented amorphous carbon films, two additional sharp and narrow peaks, D_Gr and G_Gr at 1330 and 1580 cm⁻¹, appeared from the main unchanged broad spectra, revealing the transformation of some small-range amorphous carbon to nanocrystalline graphite. Transmission electron microscopic images presented the formation of surface shear layer within which dispersed graphitic nanocrystallites (a size of about 3 nm) were formed in the remaining amorphous matrix. The in-situ nanoscopic observation of amorphous carbon nanopillars under compressions confirmed the formation of graphitic nanocrystallites at pillar edge surfaces. The formed graphite (0 0 1) and (1 0 0) lattices were well oriented along maximum resolved shear stresses, being an evidence of lattice reconstruction and suggesting a possibility of stress-induced graphitization of amorphous carbon in the absence of heat.     

Intrinsic wettability of graphitic carbon

The wettability of graphitic carbon can impact the behavior of many carbonaceous materials in a variety of systems involving water, for example, the carbon aerosols–cloud interaction that affects the climate and the penetration of micropores by aqueous solution in activated carbons. Previous studies on this topic have been limited to the basal surface of graphite. Apparent water-contact angle (WCA) values reported in the literature for the basal surface of graphite or graphene sheet vary from 35° to 126°, due to factors such as surface contamination and roughness. The intrinsic wettability of graphitic carbon remains uncertain. Here we report the first experimental attempt of quantifying the intrinsic wettability of the edge surface of graphite. We show that the basal surface is intrinsically hydrophilic with a WCA of about 61°; the edge surface is more hydrophilic than the basal surface and has an intrinsic WCA likely less than 40°. We also show that the apparent WCA is highly sensitive to sample preparation procedures. Annealing, a commonly used method for removing surface organic contaminants, could result in an underestimated WCA due to oxidation by the trace O₂ in the inert gases.     

石墨质阴极炭块焙烧裂纹产生机理的探讨

通过分析不同类型的裂纹形式,探讨了石墨质阴极炭块焙烧裂纹形成的机理,并提出了在不同成型方法下控制成型和焙烧工艺的建议.     

Bromination of graphitic pitch-based carbon fibers

Bromination by exposure to bromine vapor at room temperature was effective for inhibiting the oxidation of carbon-carbon composites in the form of PAN-based carbon fibers impregnated with resin, which was subsequently carbonized at 1000 or 2000°C. The oxidation rate was decreased by up to 43%. In addition, electrical resistivity was decreased by up to 39%. However, the tensile modulus was decreased by 16–17% and tensile strength was decreased by 17–20%; the ductility was not affected. The increased oxidation resistance may be due to bromine adsorption or the electron transfer from graphite to bromine. The bromine resided in the matrix of the composite, particularly in pores within the matrix. Bromination was also attempted by the electrochemical method, but it resulted in negligible weight uptake in the carbon-carbon composite. Carbon-carbon composites that had been graphitized at 2700°C were damaged by bromination.     

Synthesis of nitrogen-doped porous graphitic carbons using nano-CaCO3 as template,graphitization catalyst,and activating agent

Nitrogen-doped porous graphitic carbons (NPGCs) with controlled structures were synthesized using cheap nano-CaCO₃ as template, melamine-formaldehyde resin as carbon precursor, and dilute HCl as template removing agent. In addition to its use as a template, the nano-CaCO₃ acted as an internal activating agent to produce micro- and mesopores, as an adsorbent to remove the released hazardous gases (i.e. HCN, NH₃), and as a mild graphitization catalyst. The obtained NPGCs with hierarchical nanopores contained as high as 20.9 wt% of nitrogen, had surface areas of up to 834 m² g^–1, and also exhibited high thermal stability with respect to oxidation. Using carbohydrate or phenolic resin as the carbon precursor, this simple approach was also capable of producing hierarchical porous graphitic carbons with high surface area (up to 1683 m² g^–1) and extremely large pore volumes (>6 cm³ g^–1). X-ray diffraction and infrared spectroscopy suggested that the intermediate CaCN₂ or CaC₂ generated during the carbonization plays a critical role in the formation of the graphitic structure.     

A practical route to the production of carbon nanocages

Carbon-coated iron nanoparticles with diameters ranging from 5 to 50 nm and a few layers of graphitic shells have been synthesized in a mass scale by laser-induction complex heating evaporation. Through the studies on the dissolution behavior of the inner iron cores under the treatments of HCl and HNO₃ acids at different conditions, a practical route with the combined treatments of HNO₃ and HCl acids has been optimized to produce carbon nanocages. The nanocages thus obtained have some channels and are full of defects in the shells, as characterized by high-resolution transmission electron microscopy and Raman spectroscopy. Similar treatments should also be applicable to some other carbon-coated metal nanoparticles, e.g., Ni-C and Co-C, for the same purpose.     

Hydrogen adsorption on nitrogen-doped carbon xerogels

Nitrogen-doped (1.2-4.5 wt%) carbon xerogels were synthesized from carbonization of resorcinol-formaldehyde polymer in an ammonia atmosphere at various temperatures. The textural properties and the chemical nature of nitrogen in the nitrogen-doped carbon xerogels were analyzed by Ar adsorption/desorption isotherms and X-ray photoelectron spectroscopy, respectively. The maximum hydrogen uptakes were measured to be 3.2 wt% at −196 °C and 0.28 wt% at 35 °C. Hydrogen adsorption had a stronger correlation with specific surface area than nitrogen content at the low temperature of −196 °C. At the higher temperature of 35 °C, optimal nitrogen doping enhanced hydrogen adsorption by electronic modification of carbon in agreement with previous theoretical predictions.     

Efficient separation of nitrogen-doped carbon nanotube cups

Nitrogen-doped carbon nanotubes synthesized from a mixture of ferrocene, xylenes, and acetonitrile using chemical vapor deposition technique comprise compartmented hollow structures resembling stacked cups. Separating stacked nitrogen-doped carbon nanotube cups (NCNCs) into individual cups is an attractive way to further manipulate their morphology, providing additional opportunities for applications of NCNCs. Here we demonstrate an effective, simple, and low-cost separation method to obtain individual NCNCs around 180 nm in length, by sonicating as-synthesized stacked NCNCs in concentrated KCl solution. Another advantage of this technique is that it is highly selective to the separation of adjacent NCNCs and capable of preserving the surface structure and functionalities of stacked NCNCs. Here we demonstrate that the presence of potassium ions is vital for the separation and we hypothesize that the separation mechanism involves a partial intercalation that facilitates the separation of NCNCs.     

Synthesis and characterization of nitrogen-doped carbon xerogels

Carbon xerogels were prepared from a nitrogen-containing polymer precursor, using melamine and urea as nitrogen sources incorporated into the polymer matrix using the sol-gel process. To investigate the effects of nitrogen on the texture, morphology and surface chemistry, the carbon xerogels were characterized by nitrogen adsorption, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and acid-base titrations. The results showed that nitrogen was incorporated onto the surface as pyridine, pyrrolic/pyridine, quaternary nitrogen, and pyridine-N-oxide. From the deconvolution of the XPS spectra, the pyrrolic/pyridine and quaternary nitrogen functionalities were found to dominate in the samples prepared from urea. All samples showed increased basicity after nitrogen incorporation.     

氮掺杂改性碳材料的研究进展

通过对碳材料进行氮掺杂改性,可强化碳材料的性能从而拓宽其应用领域。总结了近年来国内外对氮掺杂改性碳材料的研究进展,详细介绍了利用活化法、水热法、化学气相沉积法、模板法、溶胶-凝胶法和后处理法制备氮掺杂改性碳材料的方法,对其在催化剂、吸附材料、超级电容器、储氢等领域上的应用作了详细介绍,对氮掺杂改性碳材料的发展趋势进行了展望。     

Easy and controlled synthesis of nitrogen-doped carbon

A novel synthesis of N-doped carbon with not only a high surface area (∼1000 m² g⁻¹) but also with a controlled amount of N-doping is reported from the solvothermal reduction of hexachlorobenzene (HCB) and pentachloropyridine (PCP), without the emission of harmful byproducts. In the presence of metallic sodium as a reducing agent, the N-doping amount can be regulated up to 0.12 of N/C atomic ratio by simply altering the initial HCB and PCP ratios. The mechanism is proposed where the chlorine in the HCB and PCP is reacted with metallic sodium by producing NaCl, and the N-doped carbon is synthesized as the activated carbon edges of C₅N and C₆ rings being bonded together. The surfaces of the prepared N-doped carbons are modified through heat-treatment and this dramatically improves the mechanical and electrical properties. The dominant doping phases of N are pyridinic-N and amide or amine groups; however, the amide or amine groups are eliminated and graphitic-N is newly generated through heat-treatment.     

石墨相氮化碳光催化剂研究进展

石墨相氮化碳(g-C₃N₄)作为一种可见光响应的半导体聚合物光催化剂,具有廉价易得、化学稳定性好、无毒无害以及合适的禁带宽度和能带位置等优点,但也存在只能吸收波长小于475nm的光且光生载流子复合严重等问题,需要对其进行改性来提高光催化能力。本文在介绍g-C₃N₄的结构、特性和制备方法的基础上,着重评述了g-C₃N₄在形貌调控、半导体复合、元素掺杂、分子掺杂和染料敏化等改性手段方面的研究进展以及在降解有机污染物、分解水制氢、还原CO₂和有机合成等方面的应用。最后指出g-C₃N₄未来的研究方向在于用多种手段共同改性g-C₃N₄、拓展g-C₃N₄在光催化领域的应用和深入进行机理研究等方面。     

Plasma-enabled,catalyst-free growth of carbon nanotubes on mechanically-written Si features with arbitrary shape

Simple, rapid, catalyst-free synthesis of complex patterns of long, vertically aligned multiwalled carbon nanotubes, strictly confined within mechanically-written features on a Si(1 0 0) surface is reported. It is shown that dense arrays of the nanotubes can nucleate and fully fill the features when the low-temperature microwave plasma is in a direct contact with the surface. This eliminates additional nanofabrication steps and inevitable contact losses in applications associated with carbon nanotube patterns.     

Two-dimensional graphitic carbon nitride for membrane separation

Recent years, membrane separation technology has attracted significant research attention because of the efficient and environmentally friendly operation. The selection of suitable materials to improve the membrane selectivity, permeability and other properties has become a topic of vital research relevance. Two-dimensional (2D) materials, a novel family of multifunctional materials, are widely used in membrane separation due to their unique structure and properties. In this respect, as a novel 2D material, graphitic carbon nitride (g-C₃N₄) have found specific attention in membrane separation. This study reviews the application of carbon nitride in gas separation membranes, pervaporation membranes, nanofiltration membranes, reverse osmosis membranes, ion exchange membranes and catalytic membranes, along with describing the separation mechanisms.     

Efficient helium separation of graphitic carbon nitride membrane

An efficient membrane for helium separation from natural gas is quite crucial for cryogenic industries. However, most experimentally available membranes fail in separating helium from small molecules in natural gas, such as H₂, as well as in ³He/⁴He isotopes separation. Using first-principles calculations, we theoretically demonstrated that the already-synthesized graphitic carbon nitride (g-C₃N₄) has high efficiency in helium separation from the gas molecules (H₂, N₂, CO and CH₄) in natural gas and the noble gas molecules (Ne and Ar). The selectivity of He over H₂ molecule at room temperature is calculated to be as high as 10⁷. More interestingly, the g-C₃N₄ membrane can also serve as a quantum sieving membrane for ³He/⁴He separation with a predicted transmission ratio of 18 at 49 K, thus offers a combined means of both He and ³He isotope separation.     

Formation of graphitic rods in carbon/carbon composites reinforced with carbon nanotubes

The formation of graphitic rods with a carbon nanotube (CNT) in the center was observed in CNT-reinforced phenolic resin-based carbon/carbon composites heat treated at 2000 °C. TEM characterization indicated that the carbon surrounding the CNT has a much better degree of graphitization compared to the carbon in most of the matrix. The formation temperature (2000 °C) of the graphitic rod is lower than for stress graphitization and normal graphitization of phenolic resin.     

石墨相氮化碳纳米片膜研究进展

石墨相氮化碳（g-C₃N₄）纳米片由于具有本征孔、高孔密度、高稳定性、高力学强度、大比表面积、化学环境可调节等特性,在气体分离、渗透汽化、脱盐等膜分离工艺中具有独特的优势,从而引起了研究人员的广泛关注。本文介绍了g-C₃N₄纳米片的结构和性质,总结了g-C₃N₄纳米片的制备方法,阐述了不同形式的g-C₃N₄纳米片基分离膜,讨论了g-C₃N₄纳米片膜在分离中的应用,提出了g-C₃N₄纳米片膜的存在的问题和未来的发展趋势。     

Structure and electrical conductivity of nitrogen-doped carbon nanofibers

The effect of nitrogen concentration in carbon nanofibers (CNFs) on the structural and electrical properties of the carbon material was studied. CNFs with nitrogen concentration varied from 0 to 8.2 wt.% (N-CNFs) with “herringbone” structure were prepared by decomposition of ethylene and ethylene mixture with ammonia over 65Ni-25Cu-10Al₂O₃ (wt.%) catalyst at 823 K. Detailed investigation of the CNFs and N-CNFs by XPS, FTIR and Raman spectroscopy showed that the nitrogen introduction in carbon material distorts the graphite-like lattice and increases the structure defectiveness. Both effects become more significant as the nitrogen concentration in N-CNFs grows.The electrical conductivity of N-CNFs with different nitrogen concentrations is caused by the competition of the nanofiber graphite-like structure disordering after introduction of nitrogen atoms and doping of an additional electron into the delocalized π-system of the graphite-like material. As a result, the maximum electrical conductivity among the samples studied was observed at nitrogen concentration in N-CNFs equal to 3.1 wt.%.