Zengyong Chu

Water-soluble ribbon-like graphitic carbon nitride (g-C3N4): green synthesis, self-assembly and unique optical properties

Ribbon-like g-C3N4 was obtained from dicyandiamide using NaCl crystals as the template. Modification of the texture led to an increase of the band gap to 3.0 eV, with much stronger photoluminescence. Hydrogen bonding produces an exceptionally stable dispersion in water. Upon adding different alcohols, various special assemblies were observed to precipitate from the dispersion.

Simple synthesis of g-C3N4/rGO hybrid catalyst for the photocatalytic degradation of rhodamine B

Abstract A hybrid catalyst of g-C3N4 (graphitic carbon nitride)/rGO (reduced graphene oxide) was prepared by directly heating a mixture of melamine and GO in air. g-C3N4 in the hybrid retained the structure of pristine g-C3N4, and the heterojunction between g-C3N4 and rGO was formed by π-π interaction. The highest photocatalytic efficiency for the degradation of rhodamine B (RhB) was with the melamine/GO mass ratio of 800/1, with a first order rate constant 2.6 times that of pristine g-C3N4. The enhanced photocatalytic activity was assigned to the rGO-promoted separation of photo-generated electron (e−)-hole (h+) pairs. In addition, the photocatalytic activity of g-C3N4/rGO was pH sensitive with a much increased photodegrading rate at low pH values. The first order rate constant was 8.6 times that of pristine g-C3N4 at pH = 1.98. The pH sensitive behavior resulted from the promoted oxidation of h+ with RhB by the consumption of e− with the reaction of proton (H+) in which rGO acted as a good platform for transferring e− through its atomic sheets.

Graphene oxide based BCNO hybrid nanostructures: tunable band gaps for full colour white emission

The emission of BCNO phosphors has been easily tuned from the violet to the near red regions by varying the carbon content. Here we report the optimal conversion of graphene oxide (GO) into BCNO hybrid nanostructures by one-step air oxidation with boric acid and urea. White lighting phosphor was obtained in which the doped porous graphene acts as an interconnecting framework generating and transferring electrons under excitation light. Various carbon-related levels in the BN band structures play an essential role in emitting full colour white light. The quantum confinement in the various kinds of GQDs and GO are also beneficial to widen the emission spectrum.

Development of a new Si—C—O—N—B ceramic fiber using a hybrid precursor of polycarbosilane and polyborosilazane

The development of Nicalon Si C O fibers (Nippon Carbon, Japan) by polymer pyrolysis represents a considerable breakthrough in the field of ceramic matrix composites (CMC) [1, 2]. However, this kind of SiCbased fiber is metastable at elevated temperatures due to an amorphous SiCx Oy phase present in the bulk and much effort has been devoted to improve their thermal stabilities [3–8]. One approach is to replace the oxygen-curing step with other methods such as electron beam irradiation in anaerobic conditions and thus obtained fibers, for example, Hi-Nicalon or Hi-Nicalon type S from Nippon Carbon, could stable up to ∼1600 ◦C due to a much reduced oxygen content (<0.5 wt%) [3, 4]. The other approach, however, is to introduce sintering aids into the fibers, which act as densification effects at elevated temperatures. Boron and aluminum are usually adopted and boron-doped nearly pure SiC fiber (Sylramic, Dow Corning, USA) [5] and aluminum-containing alkaliresistant sintered SiC fiber (Tyranno SA, Ube Industries Ltd, Japan) [6] have become commercially available. They could survive as high as 1800 ◦C with much strength retention. In addition to the above-mentioned polycrystalline SiC fibers, a new generation amorphous SiBN3C fiber (Siboramic, Bayer, Germany) has been prepared from a novel polyborosilazane (PBSN) precursor [7]. It marks another great progress in the field of SiC-based fibers due to its excellent mechanical properties at elevated temperatures [7]. Then, one is ready to think of incorporating both N and B into Nicalon type fibers to make an amorphous Si C O N B fiber. For example, Fan et al. [8] demonstrated an incorporation process in which NH and NH2 groups were introduced first by nitridation of polycarbosilane (PCS) in ammonia and B N bond was formed successively by exposure to boron tricholoride (BCl3). But that process is relatively long and time-consuming. In this letter, however, we are going to present a new approach to Si C O N B fiber by simply blending PCS with PBSN, with other techniques same as those of Nicalon fiber. PBSN was synthesized by polymerizing silazane oligomers with BCl3 [9] and a liquid PBSN with a number average molecular weight of 800 Da was used as the boron-containing precursor in this study. The main precursor, PCS, obtained from polydimethylsilane at ambient pressure [10], had a number average molecular weight of 1400 Da with a melting point of 205 ± 4 ◦C. Typically, 10 wt% of PBSN was blended into PCS by a dissolving-distilling technique. Then the hybrid precursor was melt spun into green fiber at about 285 ◦C and heated in air up to 190 ◦C with a mass gain of 11.0 wt%. After that, the air-cured fiber was pyrolyzed in a quartz tube up to 1250 ◦C for 30 min under a high purity nitrogen gas flow and thus, a new kind of Si C O N B fiber was obtained. To investigate its thermal stability, the Si C O N B fiber was annealed at various elevated temperatures in a graphite furnace under an argon gas flow, with a uniform retaining time of 30 min. Fourier transform infrared spectroscopy (FT-IR) was performed using a Perkin Elmer System 2000 spectrometer. Chemical analyses of bulk ceramic fibers were done for silicon, carbon, oxygen, nitrogen and boron at the Central South Institute of Metallurgy of China. Hydrogen was not measured in this study. Tensile strength was tested using a universal testing machine, at a constant displacement rate of 2 mm/min until fracture occurred. The gauge length was 25 mm. Scanning electron microscopy (SEM, Jeol JSM-6300 microscope, Japan.) was used to observe the fibers’ surface and cross-section morphologies. The X-ray diffraction (XRD, Siemens D-500 diffractometer, Germany) spectra (Cu Kα , λ = 0.154 nm) were obtained from powdered fiber. The apparent mean grain size, L , of the β-SiC crystalline phase present in the samples was calculated from the width, D, of the (111) diffraction peak at mid-height, according to the Scherrer equation [11]. Fig. 1 illustrates FT-IR spectra of the precursors and the as-fabricated and annealed ceramic fibers. Their corresponding chemical compositions as well as associated properties are shown in Table I. The main peaks were assigned to N H (3450 cm−1, 1170 cm−1), OH (3450 cm−1, 1600 cm−1), B OH (3150 cm−1, 3050 cm−1), C H (2958 cm−1, 2896 cm−1, 1420 cm−1, 1350 cm−1), Si H (2100 cm−1), B N (1380 cm−1), Si CH3 (1250 cm−1), Si O Si (1080 cm−1), Si N (1000–800 cm−1), Si C (1020 cm−1, 771 cm−1), respectively [2, 5, 9]. It can be seen that B N peak in the PBSN precursor was quite distinct although boron content was only about 0.6 wt%. B OH group present in the PBSN precursor was due to the hydrolysis of B Cl bond during preparation [9], which is in agreement with the chemical analysis showing oxygen content of 0.15 wt%. After being blended into the hybrid precursor shown in Fig.1c, however, the characteristics of PBSN were almost completely covered by those of the PCS precursor due to its small fraction (i.e., 10 wt%). At the same

Improved Mechanical Properties of SiC Fibers Pyrolyzed from Polycarbosilane under Tension

Factors that influence the mechanical properties of polymer-derived SiC fibers vary from the polymer structure to firing conditions, among which firing tension was studied in this paper. Tensile strength, Youngs modulus, density, shrinkage and diameter were recorded as characteristics of SiC fibers obtained under different tensions and a great improvement of tensile strength, from 1.70GPa to 1.90GPa, could be achieved by using an optimal two-step tension schedule. From the SEM and XRD analysis, we believe that firing tension has two ways to improve the mechanical properties of SiC fibers; one is to eliminate bend portions and the other, to slightly induce the crystallization.

Initiation and propagation behaviors of interior microflaws in the pyrolysis of an air-cured polycarbosilane precursor fiber

In the preparation of Nicalon-type Si—C—O fibers using a normal-pressure synthesized polycarbosilane precursor, a network of microflaws were detected on the cross-section of ceramic fibers obtained. Cross-section morphologies of the interior microflaws were investigated via SEM technique after certain temperature pyrolysis and their initiation and propagation behaviors were studied accordingly. The results showed that no microflaws came into being at the temperature up to 600°C due to the organic nature of the material. As a result of high weight loss and composition change, however, microflaws initiated over 600°C, formed at ∼700°C and propagated along the “core-edge” direction from 700–;900°C. Up to 1100°C, a microflaw network was formed because new microflaws generated across the oriented ones and interconnected them to form a network.

Gyrification-Inspired Highly Convoluted Graphene Oxide Patterns for Ultralarge Deforming Actuators

Gyrification in the human brain is driven by the compressive stress induced by the tangential expansion of the cortical layer, while similar topographies can also be induced by the tangential shrinkage of the spherical substrate. Herein we introduce a simple three-dimensional (3D) shrinking method to generate the cortex-like patterns using two-dimensional (2D) graphene oxide (GO) as the building blocks. By rotation-dip-coating a GO film on an air-charged latex balloon and then releasing the air slowly, a highly folded hydrophobic GO surface can be induced. Wrinkling-to-folding transition was observed and the folding state can be easily regulated by varying the prestrain of the substrate and the thickness of the GO film. Driven by the residue stresses stored in the system, sheet-to-tube actuating occurs rapidly once the bilayer system is cut into slices. In response to some organic solvents, however, the square bilayer actuator exhibits excellent reversible, bidirectional, large-deformational curling properties on wetting and drying. An ultralarge curvature of 2.75 mm-1 was observed within 18 s from the original negative bending to the final positive bending in response to tetrahydrofuran (THF). In addition to a mechanical hand, a swimming worm, a smart package, a bionic mimosa, and two bionic flowers, a crude oil collector has been designed and demonstrated, aided by the superhydrophobic and superoleophilic modified GO surface and the solvent-responsive bilayer system.

Microwave Absorbing Property of Pre-oxidized PAN Fibers Carbonized in BCl 3 : Microwave Absorbing Property of Pre-oxidized PAN Fibers Carbonized in BCl 3

PAN预氧丝在BCl 3 气氛中进行碳化处理, 研究了处理温度与时间对纤维B元素含量的影响. 采用X射线光电子能谱(XPS)、傅立叶红外分析(FTIR)、扫描电镜(SEM)对碳化后纤维的形貌与结构进行表征, 并对其吸波性能进行分析. 结果表明: PAN预氧丝在BCl 3 气氛下碳化后, 纤维表面生成了BCN包状附着物, 纤维中有B?N键的生成. 与氮气中碳化的碳纤维相比, 纤维的介电常数有所降低, 介电常数虚部降低得较快, 有利于改善纤维的吸波性能.PAN预氧丝在BCl 3 气氛中进行碳化处理, 研究了处理温度与时间对纤维B元素含量的影响. 采用X射线光电子能谱(XPS)、傅立叶红外分析(FTIR)、扫描电镜(SEM)对碳化后纤维的形貌与结构进行表征, 并对其吸波性能进行分析. 结果表明: PAN预氧丝在BCl 3 气氛下碳化后, 纤维表面生成了BCN包状附着物, 纤维中有B?N键的生成. 与氮气中碳化的碳纤维相比, 纤维的介电常数有所降低, 介电常数虚部降低得较快, 有利于改善纤维的吸波性能.

Chapter 7 – Synthesis and Properties of Silicon Carbide Nanopapers

Abstract It has been generally accepted that the band gap width of crystallized silicon carbide (SiC) is about two to three times more than silicon. This feature of SiC results in higher activation energy demand during the electron transition, which is vital for maintaining the semiconducting electron transfer. Although this feature used to be considered as a drawback of SiC electronic devices under mild circumstances, now the SiC-based electronics are expected to serve in harsh environments of higher temperature, higher power, and higher frequency conditions. The theoretical design seems practical since SiC nanopapers were discovered. This chapter starts with the review of the preparation of one-dimensional SiC nanostructures (1D SiC) since the 1D SiC are the most ideal building blocks of SiC nanopaper. Especially to the centimeters-long SiC nanowires, the as-prepared SiC nanopapers could be a strong “bridge” between microworld and macroworld due to their unique fabric morphology. Based on practical and potential strategies of organizing 1D SiC into SiC nanopaper, the novel nanopaper has been proved could be practically applied. SiC nanopapers exhibit electrical resistance that linearly increases with increasing environmental relative humidity in a very short time, and high photoelectrocatalytic activity under UV irradiation, which are potentially applied to high performance sensors and new energy transfer devices.