Evidence of band bending and surface Fermi level pinning in graphite oxide

We present the electronic structure of graphite oxide in the vicinity of the Fermi level measured using ultraviolet photoemission and inverse photoemission spectroscopies and compare it with X-ray absorption spectra. The expected p-type behavior of graphite oxide is not observed at the surface and the presence of band bending is invoked. The observed electronic structure of graphite oxide exhibited an n-type semiconducting band structure with a band gap of 2.3 ± 0.4 eV. An oxygen related state, at 0.8 eV above Fermi level, and the suppression of the unoccupied carbon weighted states at the conduction band minimum suggests that the oxygen vacancies at the surface of graphite oxide contribute to the n-type semiconducting electronic structure of the surface.     

Preparation of AlN powder of low oxygen content via carbothermal reduction and nitridation by active gas exchange technique

By generating a periodic impulse-like pressure (2.0–4.4 kPa, 84 s) to actively exchange the gas in synthesis furnace, pure AlN powder of low oxygen content was synthesized via additive free carbothermal reduction and nitridation (CRN) of Al₂O₃ powder. Compared with the conventional CRN method, the proposed extra gas exhaust process can more effectively remove the side-produced CO from the reaction sites to accelerate nitridation process and decrease the residual oxygen content in the obtained AlN powder. For example, with 39 wt% activated carbon loaded in the raw material at 1650 °C for 4 h, the prepared AlN powder by the proposed synthesis scheme has only 0.68 wt% residual oxygen. The effects of carbon content, synthesis temperature and holding time on the residual oxygen content in AlN powder by the proposed synthesis scheme were also studied. The ball-milled as-prepared AlN powder was pressureless sintered at 1880 °C for 2.5 h to obtain a translucent AlN ceramics (37.6% at ~5700 nm), which demonstrates the excellent sinterability of the as-prepared AlN powder.     

A Novel Method of Synthesis of Nanostructured Aluminum Nitride Through Sol‐Gel Route by In Situ Generation of Nitrogen

Nanostructured Aluminum Nitride (AlN) has been prepared by carbothermal reduction followed by nitridation (CTRN) of alumina gel over a temperature range 1200°C–1350°C and time period of 30 min to 3 h. Before heat treatment the gel is repeatedly evacuated and purged with ammonia. The nanopores of the gel are filled with ammonia which acts as a source of in situ nitrogen at heat‐treatment temperature. Dextrose also decomposes at the reduction temperature and generates ultrafine carbon. The stability diagram of the carbon saturated Al–N–O system is constructed and it shows that extremely low partial pressure of oxygen is required for the stability of AlN. The ultrafine carbon as well as hydrogen from the cracking of ammonia is not sufficient to create the extremely low partial pressure of oxygen required for the stabilization of AlN. So the sample is heat treated in charcoal boat in nitrogen atmosphere to achieve an extremely low partial pressure of oxygen required for the formation of AlN. The material has been characterized through XRD, FESEM, and HRTEM analyses. The spherical particle size of AlN is obtained ～21 nm.     

Controlled growth of nanocrystalline aluminum nitride films for full color range

Color films are widely used for visual effect as well as for their functional properties. To date, however, synthesizing thin films with desired color remains challenging. In this work, AlN color films are deposited on Si wafers by precise control of the deposition time for different thickness during reactive magnetron sputtering from an Al target in Ar/N₂ atmosphere. The thickness, morphology, structure, composition and color index are carefully examined by field emission scanning electron microscopy, atomic force microscopy, grazing incidence X-ray diffraction, X-ray photoelectron spectrometry and colorimeter, respectively. As the film thickness changes from 57 nm to 165 nm, the film exhibits purple, indigo, blue, green, yellow, orange and red in color. These colors repeat in the same order when the thickness goes over 165 nm. Once the thickness exceeds 467 nm, overlapping of colors takes place. The mechanisms are elucidated.     

Fabrication and photoactivities of spherical-shaped BiVO4 photocatalysts through solution combustion synthesis method

Spherical-shaped BiVO₄ photocatalysts were prepared by the solution combustion synthesis method. The as-prepared photocatalysts were characterized by X-ray diffraction (XRD), nitrogen absorption for the BET specific surface area, field emission scanning electron microscopy (FE-SEM) and ultraviolet–visible diffraction reflection spectroscopy (UV–vis). The BiVO₄ crystallites show a monoclinic structure with diameter of about 400–600 nm. UV–vis diffusion absorption spectra indicate that the band gap absorption edge of pure BiVO₄ crystallites prepared by the SCS method and the SSR method are 523 nm and 540 nm, corresponding to the band gap energies of 2.45 eV and 2.40 eV, respectively. It is also found that the photocatalytic activity of degradation of methylene blue improves when the molar ratio of fuels to oxidizer is 5.     

Effects of the addition of graphite oxide to the precursor of a nanoporous carbon on the electrochemical performance of the resulting carbonaceous composites

Composites of reduced graphite oxide and nanoporous sodium-salt-polymer-derived carbons were prepared with 5 or 20 weight% graphite oxide. The materials were characterized using the adsorption of nitrogen, SEM/EDX, elemental analysis and potentiometric titration. DC conductivity was also measured. The performance of the carbon composites in energy storage was linked to their surface features and the effects of the graphene phase addition on these features. Even though the graphene phase increases the electronic conductivity of all materials, the porosity, and especially pores smaller than 0.8 nm, is the main factor governing the capacitive behavior. Owing to the sodium in the carbon precursor and the presence of graphene layers, unique small pores, similar in size to the electrolyte ions are developed in the resulting composites. These pores increase the capacitance by an electrical double layer mechanism. The composites obtained exhibit noticeable redox reactions due to the presence of oxygen and sulfur in the carbon matrix. An increase in the heat treatment temperature increases the capacitance retention ratio at higher currents owing to an increase in the conductivity and chemical stability of the surface.     

Synthesis and photoluminescence characteristics of blue‐green luminescent aluminum oxynitride

Aluminum oxynitride (AlON), which can be regarded a nitrogen‐stabilized cubic γ‐Al₂O₃, has attracted attention in terms of its good mechanical, chemical, and optical stability. Because of its optical inertness, however, photoluminescence (PL) emission from nominally pure AlON has not been carefully investigated and evaluated. In this work, we prepared visibly luminescent AlON by nitridation of γ‐Al₂O₃ under N₂ atmosphere without adding aluminum nitride (AlN) using a high‐frequency induction heating unit. The resulting AlON exhibits a broad PL emission in the blue/green spectral region under excitation with light of ~260 nm. In the luminescent AlON sample, the excitation and emission events will occur at different sites; the electron transfer from the excitation site to the emission site is preceded by the radiative recombination process. It has also been found that the PL peak wavelength shows an anomalous blue shift by ~50 nm with increasing temperature from 78 to 500 K. The observed temperature dependent PL characteristics are governed by thermalization among multiple emitting levels. Aluminum vacancies and oxygen vacancies, both of which are introduced into the crystalline lattice during nitridation without the presence of AlN, are very likely candidates for the excitation and emission centers, respectively. Hence, the present direct nitridation method provides a simple and effective way to add an additional optical functionality to otherwise optically inactive AlON.     

Study on the reduction of commercial MoO3 with carbon black to prepare MoO2 and Mo2C nanoparticles

A simple method was developed to synthesize MoO₂ and Mo₂C nanoparticles via controlling nucleation and growth in carbothermic reduction of commercial MoO₃ with carbon black. It was found that the appropriate C/MoO₃ molar ratio for preparation of Mo₂C was 2.8, and the carbothermic reduction process followed the sequence: MoO₃ → transport phase (TP) → MoO₂ → Mo₂C. It was revealed that the most crucial issues for controlling number of produced particles of product were migration of Mo source and aid of nucleating agent, which can be achieved by using MoO₃ and carbon black as starting materials. MoO₂ nanosheets with the thickness of 12 nm and lateral size of 60 nm, as well as Mo₂C nanoparticles with particle size of 30 nm were prepared via reduction of MoO₃ with carbon black. However, MoO₂ and Mo₂C produced via reducing MoO₃ by other kinds of carbon sources (activated carbon, graphite) or gas reductants (10% CH₄/H₂, CO) had much larger particle sizes of a few micrometers, which were tens of times than those using MoO₃ and carbon black, due to the too small amount of formed nuclei. The effects of C/MoO₃ molar ratio (0.5-2.8), molybdenum sources and carbon sources on the reaction mechanisms were investigated in detail.     

Ultra-narrow deep-red emitting AlN:Sm2+phosphor with nano-branched construct for wide color gamut backlight display and high-pressure sensing applications

Rare-earth (RE) doped AlN are excellent candidate materials for electroluminescent devices, full color displays and white lighting technology. In this paper, a deep red Sm²⁺ doped AlN (AlN:Sm²⁺) phosphor was synthesized for the first time by a one-step direct nitridation method. Detailed XRD and EDS studies show the presence of samarium (Sm) ions the AlN, and XPS measurements indicate Sm ions are divalent. SEM and TEM studies show that the AlN:Sm²⁺ have a branched nanostructure, consisting of a primary stem and secondary short nano-branches. AlN:Sm²⁺ phosphor has a broad and strong excitation bands in the range of 300–600 nm, ultra-narrow deep red emission at 686 nm, near unity color purity, and good thermal stability (78.2% at 413 K). A blue-pumped warm white light emitting diode with high color rendering index (Ra∼87.5) and low correlated color temperature (CCT∼4875 K) was fabricated. Moreover, a super-wide color gamut (117.6% of the NTSC) can be achieved by using AlN:Sm²⁺ as the red component. Furthermore, photoluminescence (PL) and Raman spectra of AlN:Sm²⁺ were studied under hydrostatic pressure up to 25 GPa. The shift of the ⁵D₀→⁷F₀ emission band (dλ/dP≈0.13 nm/GPa) and the decrease of PL intensity ratio (⁵D₀→⁷F₀/⁵D₀→⁷F₁, dI_R(0/1)/dP≈−5.6%/GPa) with applied pressure can be used for optical pressure sensor. Raman spectroscopy revealed a phase transition of AlN:Sm²⁺ from wurtzite to rocksalt phase at 19.9 GPa. The large doping of Sm²⁺ ions and unique intrinsic geometry in branched nanostructure co-affect its compressibility and structural stability under high pressure. The results indicate that AlN:Sm²⁺ phosphor has promising applications in backlight displays and optical pressure sensors due to their excellent luminescent properties.     

Synthesis and characterization of magnesium–carbon compounds for hydrogen storage

Magnesium (Mg) and carbon (C) compounds were synthesized by ball-milling a mixture of Mg and different graphites with different crystallinities. The materials were characterized by X-ray diffraction, X-ray absorption spectroscopy, and X-ray total scattering techniques. Hydrogen storage properties were also investigated. In the case of the material using low-crystalline graphite, a Mg and C compound was formed as main phase, and its chemical bonding state was similar to that of magnesium carbide (Mg₂C₃). The hydrogen absorption reaction of the Mg–C compound occurred at around 400 °C under 3 MPa of hydrogen pressure to form magnesium hydride (MgH₂) and the C–H bonds in the carbon material. The hydrogenated Mg–C material desorbed about 3.7 mass% of hydrogen below 420 °C with two processes, which were the decomposition of MgH₂ and the subsequent reaction of the generated Mg and the C–H bonds. From the results, it is concluded that the Mg–C compound absorb and desorb about 3.7 mass% of hydrogen below 420 °C.     

Low-temperature sintering behavior of the nano-sized AlN powder achieved by super-fine grinding mill with Y2O3 and CaO additives

For low-temperature sintering, mixtures of AlN powder doped with 3.53 mass% Y₂O₃ and 0–2.0 mass% CaO as sintering additives were pulverized and dispersed in a vertical super-fine grinding mill with very small ZrO₂ beads. The particle sizes achieved ranged between 50 and 100 nm after grinding for 90 min. The mixtures were then fired at 1000–1500 °C for 0–6 h under nitrogen gas pressure of 0.1 MPa. All nano-sized powders showed pronounced densification from 1300 °C as revealed by shrinkage measurement. The larger amounts of sintering additives enhanced AlN sintering at lower temperatures. Densified AlN ceramics with very fine and uniform grains of 0.3–0.4 μm were obtained at a firing temperature of 1500 °C for 6 h.     

Clickable,versatile poly(2,5-dithienylpyrrole) derivatives

In this study a novel, clickable, azide containing conducting polymers based on 1-(2-azido-ethyl)-2,5-dithiophene-2-yl-1H-pyrrole (SNS-N₃) were synthesized and characterized. Optical and electronic properties of homopolymer (PSNS-N₃) were investigated and colorimetric studies were performed. The homopolymer has a band gap of 2.49 eV and it displays yellow to blue coloration upon doping. Electrochemically prepared copolymers of SNS-N₃ and 3,4-ethylenedioxythiophene (EDOT) formed multichromic, color tunable electrochromic materials with continuous color gradient from cinnamon, mustard, lime green, blue and dark blue. Spectroelectrochemical analyses revealed that the neutral copolymers possess two absorption maxima (～320 and 450 nm) where the relative intensity and position of the two depends on polymerization potential. Copolymer films could be fully switched between their neutral and oxidized forms in ～1.2 s with a percent transmittance of ～65% at 950 nm. Moreover, a PSNS-N₃ coated ITO electrode was subjected to click reaction using ethynylferrocene. CV and FTIR studies revealed that ferrocene could easily be attached onto the electrode surface without loss of electroactivity of both ferrocene and PSNS backbone. Our results suggest that electrochemically prepared PSNS-N₃ films offer a novel and multipurpose platform for simple, effective post-functionalization of poly(2,5-dithienylpyrrole)s under mild conditions.     

Kass-controlled Hg transport from Crystal Violet Lactone to Fluoran

We have investigated the Hg²⁺ transport from Crystal Violet Lactone to Fluoran dye based on the association constant, K_ass. Upon addition of Hg²⁺, the Crystal Violet Lactone shows a new peak at around 603 nm, and the color of the solution changed from colorless to blue. With the addition of Fluoran dye in this solution containing Crystal Violet Lactone and Hg²⁺, the absorption intensity of Fluoran dye at 447 nm and 586 nm was all increased. So the color of solution gradually became black from blue color. From the changes of the ratio A₅₈₆/A₄₄₇, it is apparent that the Hg²⁺ in Crystal Violet Lactone-Hg²⁺ was transported to colored Fluoran. The Hg²⁺ transport from Crystal Violet Lactone to Fluoran dye was also carried out by the calculation of the association constant: the binding ability for the complex formation of Fluoran dye and Crystal Violet Lactone-Hg²⁺ is much greater in CH₃CN solution (K_ass = 3.0 × 10⁴ M⁻¹) than that of the Crystal Violet Lactone with Hg²⁺ (K_ass = 1.2 × 10³ M⁻¹).     

Diamonds of new alkaline carbonate–graphite HP syntheses: SEM morphology,CCL-SEM and CL spectroscopy studies

Colorless octahedral diamonds up to 150 μm in size were spontaneously crystallized from carbon solutions in alkaline–carbonate melts in the Na₂Mg(CO₃)₂–graphite and NaKMg(CO₃)₂–graphite systems at pressures of 8–10 GPa and temperatures of 1700–1800 °C. Seeded growth of carbonate–carbon (CC) diamond layers was realized on both octahedral {111} and cubic {100} faces of natural and synthetic “metal–carbon” (MC) diamond single crystals 0.5–0.7 mm in size. Scanning electron microscopy (SEM) morphology studies clearly demonstrate that a preferable mechanism of diamond growth from alkaline CC melts is the deposition of newly formed layers in parallel with octahedral faces, in much the same way as in the case of natural diamonds. A color cathodoluminescence (CL) SEM study shows that the specific feature of the CC diamonds is the lack of surface color CL as for natural diamonds of type II with lower nitrogen concentration. The CL spectra of the CC diamonds consist of three-band system H3, 575 nm, and a weak blue A-band. The structure of the H3 band closely resembles that of natural diamonds of type IIa.     

The synthesis,two-photon absorption and blue upconversion fluorescence of novel,nitrogen-containing heterocyclic chromophores

Novel, nitrogen-containing heterocyclic chromophores based on either 1,2,4-triazine or an imidazole core were synthesized using a three-component, one-pot reaction under microwave irradiation. Structures were verified by ¹H NMR, IR, MS and elemental analyses while crystal structure was determined using X-ray diffraction. The two-photon absorption and two-photon upconverted blue fluorescent emission characteristics were investigated experimentally; preliminary structure–photophysical property relationships were established. Chromophores that contained the imidazole moiety displayed more potent two-photon absorption than compounds based upon 1,2,4-triazine and also exhibited a strong two-photon upconverted blue fluorescent emission peak at around 443–476 nm. Significant enhancement of the two-photon absorption cross-section was achieved by fusing a benzoxazole moiety onto the phenanthro[9,10-d]imidazole ring.     

Tuning the electronic and optical properties of monatomic carbon chains

The electronic, mechanical and optical properties of the monatomic carbon chains recently fabricated are investigated by hybrid density functional calculations. It is shown that the chain owns a direct band gap of 2.21 eV and the ultimate strength of 12.2 nN, which are in good agreements with experiments. The light absorption shows only one sharp peak in a wide photonic energy range up to 10 eV and exhibits a strong anisotropic feature as the absorption of the visible light polarized along the chain axis is five orders of magnitude stronger than that of the light polarized perpendicularly to the chain. By applying elastic strains on the chain, the band gap can be continuously changed from 1.58 to 3.86 eV with the absorption peak shifting correspondingly. Then a realistic graphene-chain device is proposed and is verified to be capable of applying strains up to 9% on the chain, suggesting potential applications in nano-size electro-optical and polarization devices with tunable working wavelengths from 345 to 561 nm.     

Adsorption of methylene blue and zinc ions on raw and acid-activated bentonite from Morocco

Adsorption of methylene blue (BM) and zinc ions on raw and acid-activated Moroccan bentonite composed of montmorillonite (88 mass%), a mixture of quartz and K-feldspar (9 mass%), calcite (3 mass%) and insignificant amounts of organic matter was evaluated at 25 °C using UV–visible and atomic absorption spectrometry. The adsorption capacity of MB and Zn ions by raw bentonite were about 2.2 and 1.1 mmol/(g of bentonite) and the best-fit isotherm models were those of Harkins–Jura and Langmuir. Acid-activation of the bentonite reduced the maximum uptake of MB and Zn ions by 30 and 95% and the best-fit models of the isotherms were Freundlich and Dubinin–Radushkevich for MB and Zn ions respectively. The reduced adsorption was associated with partial collapse of the montmorillonite particles and the formation of amorphous silica.     

Synthesis and characterization of multiwalled carbon nanotubes-protoporphyrin IX composites using acid functionalized or nitrogen doped carbon nanotubes

In this research we describe the synthesis and characterization of composite materials based on multiwalled carbon nanotubes and protoporphyrin IX. We compare the results of using three types of carbon nanotubes: pristine (diameter < 10 nm), acid functionalized (diameter < 10 nm), and nitrogen doped carbon nanotubes (diameter ≈ 20 nm). Carbon nanotubes were mixed with protoporphyrin IX via two simple and straightforward methods using sonication, or heating-stirring. The characterization of the composites was done by Raman spectroscopy, field emission scanning electron microscopy, thermogravimetric analysis, ultraviolet-visible and fluorescence spectroscopy and infrared spectroscopy. A diversity of coatings of the nanotubes by protoporphyrin were obtained depending on the type of nanotube used or the method of synthesis. Some carbon nanotubes increased their diameter up to 40% after the reaction with protoporphyrin. Percentages by weight up to 20% of protoporphyrin were measured by thermogravimetric analysis. We obtained experimental evidences by different techniques of the electronic interaction and the formation of covalent bonds between both constituents, above all for the composites using nanotubes < 10 nm in diameter. Some of these evidences were ~ 98% of fluorescence quenching, reduction in the intensity of the absorption bands in ultraviolet visible spectroscopy, strong reduction in the intensity of some bands in Raman spectroscopy, red and blue shifts, as well as the presence of new absorption bands in infrared spectroscopy. Nitrogen doped carbon nanotubes showed low chemical reactivity to protoporphyrin IX, perhaps due to their lower acceptor character as they could have charge transfer from nitrogen dopants to the nanotube network, or because of their metallic character.     

导模法生长蓝宝石晶体的退火工艺

采用导模法生长了片状蓝宝石单晶。由于石墨发热体的高温挥发,使晶体尾部产生黑色絮状包裹体,晶体内部生成色心。为了消除片状蓝宝石晶体内的包裹体和色心,在不同气氛下对生长的晶体样品进行了退火处理。退火实验表明,含有包裹体的尾部样品在1500℃空气中退火20h并以50℃／h的速率降温,可消除晶体内的碳包裹体,晶体变为无色、透明。在氢气中1600℃退火37h后,F色心引起的205mm的吸收峰和Fe^3＋所引起的200～230的吸收峰均被消除。表明高温氢气中退火是消除导模法生长蓝宝石晶体内部F色心和Fe^3＋吸收的最佳退火方法。     

Preparation and characterization of pillared carbons obtained by pyrolysis of silylated graphite oxides

Pillared carbons were prepared by pyrolyzing various graphite oxides silylated by 3-aminopropylmethyldiethoxysilane. They were formed when silylated graphite oxides with silicon contents of 12.6% or higher were pyrolyzed in vacuo at 500-600 °C. Their interlayer spacings were 1.23-1.31 nm. When silylated graphite oxide was prepared at 90 °C, the reductive decomposition of graphite oxide by amino groups of 3-aminopropylmethyldiethoxysilane was suppressed and pillared carbon with higher crystallinity was obtained. At higher temperatures of pyrolysis, silylated graphite oxide decomposed to residual carbon without pillars. The pillars between the carbon layers contained methyl groups originating from the 3-aminopropylmethyldiethoxysilane. Based on the interlayer spacing and elemental analysis data, a structure model for the pillar is proposed. Pillared carbons showed type IV nitrogen adsorption isotherms and they contained both mesopores and a small volume of micropores. The BET surface area of the pillared carbon reached a maximum value of 236 m²/g, when it was prepared from graphite oxide silylated at 105 °C for 20 days.