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.     

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.     

N-doping TiO2 thin film prepared by heat treatment in electric field

A visible-light-active TiO_2−xN_x photocatalyst was synthesized in electric field. The N-doping TiO₂ film had a thickness of about 70–80 nm. The results of the photocatalytic experiments showed that the vis-activity rose largely. XPS and XRD analysis implied that the doping N substituted the oxygen-deficient in TiO₂ film. The Einstein shift in optical absorption spectrum of the N-doped film was also observed by the UV–VIS with a cut wave length of about 550 nm. Based on the experiment, it could be concluded that the N doping is effective to narrow the gap and enhance the photocatalysis.     

Improvement of field emission performance on nitrogen ion implanted ultrananocrystalline diamond films through visualization of structure modifications

The relationship between the electron field emission properties and structure of ultra-nanocrystalline diamond (UNCD) films implanted by nitrogen ions or carbon ions was investigated. The electron field emission properties of nitrogen-implanted UNCD films and carbon-implanted UNCD films were pronouncedly improved with respect to those of as-grown UNCD films, that is, the turn-on field decreased from 23.2 V/μm to 12.5 V/μm and the electron field emission current density increased from 10E−5 mA/cm² to 1 × 10E−2 mA/cm². The formation of a graphitic phase in the nitrogen-implanted UNCD films was demonstrated by Raman microscopy and cross-sectional high-resolution transmission electron microscopy. The possible mechanism is presumed to be that the nitrogen ion irradiation induces the structure modification (converting sp³-bonded carbons into sp²-bonded ones) in UNCD films.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Synthesis of double-walled carbon nanotube films and their field emission properties

Continuous double-walled carbon nanotube (DWCNT) films were synthesized using an Fe-Mo catalyst by the arc discharge method. This new catalyst has dramatically improved the purity and selectivity of DWCNT product. High-resolution transmission electron microscopy indicates that the outer and inner diameter of DWCNT are 1.9-4.7 nm and 1.2-3.8 nm, respectively. The field emission properties of DWCNT films have been studied. The directly grown film was transferred onto quartz substrates and used as emission cathodes, and has demonstrated a quite good emission performance. Moreover, the emissions of DWCNT films have been further improved by heat treatment. The film after 400 °C oxidation shows excellent field emission property with a low turn-on (E_to = 0.6 V/μm) and threshold field (E_th = 0.9 V/μm) corresponding to the emission current density of 1 μA/cm² and 1 mA/cm², respectively.     

Improvement of field-emission-lamp characteristics using nitrogen-doped carbon nanocoils

Carbon nanocoils (CNCs) synthesized using thermal pyrolysis chemical vapor deposition on 304 stainless steel wire substrates were used as the cathodes of field emission lamps (FELs). The effects of the growth temperature on the FE performances were studied, and we observed that uniform and dense CNCs that are suitable for use as FE cathodes can be synthesized at 600 °C. We also found that a nitrogen doping post-treatment can significantly improve the FE efficiency of the CNCs. When doped at 200 °C with a nitrogen flow rate of 500 sccm for 30 min, the nitrogen content of the CNC surface could reach 4.9 wt.%. ESCA analysis indicates that the doped nitrogen atoms formed CN_x bonding and increased the sp² clusters in the CNCs. The turn-on voltage was reduced from 2.1 V/μm to 1.4 V/μm, and the β value increased considerably from 2465 to 3241 after N-doping post-treatment. The bulb-type FELs using our N-doped CNC cathodes showed a good luminous efficiency as high as 75 lm/W at 8 kV.     

Simultaneous synthesis and densification of α-Zr(N)/ZrB2 composites by self-propagating high-temperature combustion under high nitrogen pressure

Simultaneous synthesis and densification of α-Zr(N)/ZrB₂ composites from a 85 mol% Zr/15 mol% B mixed-powder compacts have been achieved by self-propagating high-temperature under a nitrogen pressure of 10 MPa. Composites consist of fine and short rodlike ZrB₂ grains (0.1 μm^?–0.5 μm^l) dispersed into α-Zr(N) matrix (3 μm). Dense composite materials (96.5% of theoretical) exhibit excellent mechanical properties, in which their bending strength and H_v are 560 MPa and 6.5 GPa, respectively. This bending strength is much superior to those (205 and 480 MPa) of dense equi-axial α-Zr(N) (10 μm) and dense ZrB₂ (6 μm). Fine and rodlike ZrB₂ grains greatly enhanced their mechanical properties.     

Removal of vapor-phase elemental mercury by N-doped CuCoO4 loaded on activated carbon

The adsorption ability for vapor-phase Hg° on commercially available granular activated carbon (AC) loaded with CuCoO₄ (AC–C), CuCoO₄ + NH₄Cl (AC–Cl), and CuCoO₄ + NH₄Br (AC–Br) were investigated in an attempt to produce more economic and effective sorbents for the control of Hg^o emission from combustion processes. According to the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) and mass balance analysis on mercury, we found that N-doped AC–C had bigger S_BET and the nitrogen doping greatly improved CuCoO₄ Hg^o oxidation ability. It was considered that nitrogen atoms in doped CuCoO₄ polycrystalline and anion Cl⁻/Br^- activated by AC and N-doped CuCoO₄ were responsible for the significant enhancement in Hg^o oxidation ability of AC–Cl and AC–Br. We explored the Hg^o oxidation ability of the three kinds of materials under various loading values and adsorption temperatures. We found that AC–Cl and AC–Br were more sensitive to loading value and had much higher Hg^o oxidation ability than AC–C over a wide range temperature. The longevities of AC–C, AC–Cl and AC–Br were all less than the corresponding Al₂O₃ carrier material due to CuCoO₄ decomposition effect on AC, which destroyed the porous structure of AC. The effect of 0.31 vol.% SO₂ on the Hg^o oxidation ability of AC–C, AC–Cl and AC–Br was insignificant which indicated that N-doping did not adversely affect Co³⁺ and Cu²⁺-octahedral structure.     

Effect of Eu doping on structure and electrical properties of lead-free (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

Eu-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (BNBT6-xEu, x=0.00–2.00 at%) lead-free piezoelectric ceramics have been synthesized by the solution combustion method. The effect of Eu doping concentration on the phase structure, microstructure and electrical properties of BNBT6 ceramics has been investigated. The XRD analysis confirms that the europium additive incorporates into the BNBT6 lattice and results in a phase transition from the coexistence of rhombohedral and tetragonal phases to a more symmetric pseudocubic phase. The SEM images indicate that the europium additive has little effect on the ceramic microstructure and the average grain size is about 2.0 μm. The electrical properties of BNBT6 ceramics can be improved by appropriate Eu doping. The 0.25 at% Eu doped BNBT6 ceramic presents excellent electrical properties: piezoelectric constant d₃₃=149 pC/N, remnant polarization P_r=40.27 μC/cm², coercive field E_c=2.95 kV/mm, dielectric constant ε_r=1658 and dissipation factor tan δ=0.0557 (10 kHz).     

One-step fabrication of high quality double-walled carbon nanotube thin films by a chemical vapor deposition process

High-quality double-walled carbon nanotubes (DWCNTs) thin-films have been fabricated in one-step by the catalytic chemical vapor deposition gas-flow reaction process with acetone as a carbon source in an argon flow. The DWCNTs film is formed through the self-assembly of the DWCNTs in the gas flow, which is achieved by controlling the gas rates in the synthesis reaction. The DWCNT film is self-supported and consists of preferentially aligned high-quality DWCNT bundles. Raman spectral analysis shows a low intensity ratio of the D band and the G band with I_D/I_G being 0.025 indicating a high-quality of DWCNTs at a macroscopic scale. Property measurements show that the DWCNT film is mechanically robust and highly electrically conductive. The formation of high-quality DWCNTs can be attributed to the reaction in the argon environment that is inert and does not attack the DWCNTs at the high synthesis temperature (1170 °C). This one-step fabrication process is feasible for large-scale productions of high-quality DWCNTs films with promising structural and functional applications.     

A quick method to determine the capacitance characteristics of thin layer X5R multilayer capacitors

The effect of Y₂O₃ concentration on the dielectric properties of ceramic disc capacitors and multilayer capacitors containing 50 dielectric layers with an approximate thickness of 3 μm were investigated. The relative permittivity and temperature coefficient of capacity of multilayer capacitors at low and high applied field suggest that two types of microstructures formed, depending on yttrium doping concentration. Yttrium concentrations of 1.5-2.0 mol% yielded identical relative permittivities over a wide temperature range. The permittivities at Y concentrations of 2.6-3.0 mol% were also identical, but somewhat higher. The relative permittivity of ceramic disc capacitors of similar composition, determined from hysteresis loop measurements as function of applied field, was compared with field-dependent permittivity measurements on multilayer capacitors. The results indicate that polarization measurements on CDCs are a good indicator for the relative permittivity values of MLCCs.     

Enhancement of piezoelectric and ferroelectric properties of BaTiO3 ceramics by aluminum doping

Ferroelectric and piezoelectric properties of BaTiO₃ and Al-doped BaTiO₃ ceramics were investigated. The ferroelectric study demonstrated that, by doping Al³⁺ ions in the A-site of BaTiO₃, the polarization–electric field loop exhibited enhanced remnant polarization (from 12 to 17.5  μC/cm²), saturation and switching. In addition, the piezoelectric constant (d₃₃) increased with Al-doping for both static and dynamic strain values (from 75 to 135 and from 29.2 to 57.9 pC/N, respectively, at a maximum applied electric field of 16 kV/cm). Furthermore, the dielectric constant values increased and both the dielectric loss factor and leakage current decreased, even though the transition temperature shifted to lower temperature (from 121 to 113 °C) for the Al-doped sample. Therefore, the Al-doped BaTiO₃ has adjustable piezoelectric and ferroelectric properties.     

A simple method to synthesize continuous large area nitrogen-doped graphene

Large area nitrogen (N)-doped graphene films were grown on copper foil by chemical vapor deposition. The as-grown films consisted of a single atomic layer that was continuous across the copper surface steps and grain boundaries, and could be easily transferred to a variety of substrates. N-doping was confirmed by X-ray photoelectron spectroscopy, Raman spectroscopy, and elemental mapping. N atoms were suggested to mainly form a “pyrrolic” nitrogen structure, and the doping level of N reached up to 3.4 at.%. The N-doped graphene exhibited an n-type behavior, and nitrogen doping would open a band gap in the graphene. This study presents use of a new liquid precursor to obtain large area, continuous and mostly single atom layer N-doped graphene films.     

The effect of atmospheric pressure plasma treatment on the field emission characteristics of screen printed carbon nanotubes

The effects of pin-to-plate type atmospheric pressure plasma treatment on the field emission properties of screen printed carbon nanotubes(CNTs) were investigated using He(10 slm)/N₂(0.1 slm). The plasma treatment for 10 s decreased the turn-on field from 3.13 V/μm to 1.21 V/μm, increased the emission current, and increased the number of emission sites. When, the 10 s plasma treatment was also applied to the CNTs which were previously treated by a tape activation method, the number of emission sites were further increased, therefore, the emission uniformity was improved even though, the plasma treatment on the tape-activated CNTs increased the turn-on field slightly 0.76 V/μm to 1.25 V/μm due to the removal of long CNTs.     

Effects of nitrogen-doping on the microstructure,bonding and electrochemical activity of carbon nanotubes

Vertically aligned carbon nanotubes produced with in-situ doping of nitrogen (CN_x NTs) during chemical vapor deposition exhibit unique structural and electrochemical properties, which are strongly correlated with their nitrogen (N) doping level. In this work, the effects of N-doping on CN_x NTs have been systematically investigated via microstructure and bonding studies, electron-transfer (ET) behaviors, and subsequent electrochemical deposition of catalyst. The CN_x NTs doped with an optimal N level, while showing a nearly reversible ET behavior, in fact exhibit uniform and high density of surface defects. These surface defects are desirable for further modification and/or nucleation of catalytic particles on the surface of CN_x NTs to form a composite electrode for electrochemical energy device applications such as fuel cells and capacitors.     

Structural and morphological control of aligned nitrogen-doped carbon nanotubes

Nitrogen-doped carbon nanotubes (CN_x-NTs) were prepared using a floating catalyst chemical vapor deposition method. Melamine precursor was employed to effectively control nitrogen content within the CN_x-NTs and modulate their structure. X-ray photoelectron spectroscopy (XPS) analysis of the nitrogen bonding demonstrates the nitrogen-incorporation profile according to the precursor amount, which indicates the correlation between the nitrogen concentration and morphology of nanotubes. With the increase of melamine amount, the growth rate of nanotubes increases significantly, and the inner structure of CN_x-NTs displayed a regular morphology transition from straight and smooth walls (0 at.% nitrogen) to cone-stacked shapes or bamboo-like structure (1.5%), then to corrugated structures (3.1% and above). Both XPS and CHN group results indicate that the nitrogen concentration of CN_x-NTs remained almost constant even after exposing them to air for 5 months, revealing superior nitrogen stability in CNTs. Raman analysis shows that the intensity ratio of D to G bands (I_D/I_G) of nanotubes increases with the melamine amount and position of G-band undergoes a down-shift due to increasing nitrogen doping. The aligned CN_x-NTs with modulated morphology, controlled nitrogen concentration and superior stability may find potential applications in developing various nanodevices such as fuel cells and nanoenergetic functional components.     

Y doping and grain size co-effects on the electrical energy storage performance of (Pb0.87Ba0.1La0.02) (Zr0.65Sn0.3Ti0.05)O3 anti-ferroelectric ceramics

(Pb_0.87Ba_0.1La_0.02) (Zr_0.65Sn_0.3Ti_0.05)O₃+x mol% Y (PBLZST-x, x=0–1.25) anti-ferroelectric (AFE) ceramics have been prepared by the solid-state reaction process, and the effect of Y-doping on the microstructure and electrical properties has been investigated. When the Y content increases from 0 mol% to 1.25 mol%, the average grain size of the PBLZST ceramics decreases by more than 3 times (from 4.7 μm to 1.5 μm). The doping and grain size co-effects lead to a significant increase in the AFE-to-FE and FE-to-AFE phase transition electric field (E_F and E_A), and result in a decrease in the width of the double hysteresis loops. As the Y content increases from 0 mol% to 0.75 mol%, the E_F increases from 53 KV/cm to 83 KV/cm and the E_A increases from 35 KV/cm to 72 KV/cm. The large recoverable energy density of 2.75 J/cm³ and the high energy efficiency of 71.5% can be achieved when 0.75 mol% Y is doped. The results indicate that Y-doping is an effective method to modulate the average grain size and improve the energy storage performance of the PBLZST anti-ferroelectric ceramics.     

Structural, electrical and dielectric properties of Bi1.5Zn0.92Nb1.5−xTaxO6.92 pyrochlore ceramics

The micro-structural, compositional, temperature dependent dielectric and electrical properties of the Bi_1.5Zn_0.92Nb_1.5−xTa_xO_6.92 solid solution has been investigated. The increasing Ta content from 0.2 to 1.5 caused; single phase formation, a pronounced grain size reduction from ∼7.0 to 2.5 μm, sharp decrease in the dielectric constant from 198 to 88 and an increase in the electrical conductivity from 3.16 × 10⁻¹⁰ to 5.0 × 10⁻⁹ (Ω cm)⁻¹, respectively. The temperature dependent dielectric constant which is found to be frequency invariant in the frequency range of (0.0-2.0 MHz) exhibited a sharp change in the temperature coefficient of dielectric constant at a (doping independent) critical temperature of 395 K. The analysis of the measured data reflects a promising future for this type of pyrochlore to be used in high voltage passive device applications.     

Field emission properties from the tip and side of multi-walled carbon nanotube yarns

Multi-walled carbon nanotube (MWCNT) yarns were prepared by spinning a bundle of MWCNTs from vertically super-aligned MWCNTs on a substrate, and field emission from the tip and side of the yarns was induced in a scanning electron microscope. We found that the field emission behavior from the tip of the yarn was better than the field emission from the side. The field emission turn-on voltages from the tip and side of MWCNT yarns were 1.6 and 1.7 V/μm, respectively, after the yarn was subjected to an aging process. Both the configuration of the tip end and the body of the yarn were changed remarkably during the field emission. Both the field emission of the tip and side exhibited a transition of the Fowler-Nordheim slope from the region of low voltage to the region of high voltage. From the Fowler-Nordheim plots, the field enhancement factors of field emission from the tip and side, γ_tip and γ_side, in the region of high voltage were determined to be 5430 ± 5 and 3940 ± 5, respectively, and their ratio was approximately 1.38:1.