Growing carbon nanotubes on patterned submicron-size SiO2 spheres

Growing carbon nanotubes (CNTs) perpendicularly to the surface of submicron-size SiO₂ spheres by pyrolyzing iron(II) phthalocyanine (FePc) is reported for the first time in this paper. The large curvature isolates the nanotubes and forms unique structures. The density, lengths and morphology of CNTs on SiO₂ spheres can be controlled by varying the experimental conditions. A method of growing CNTs on patterned SiO₂ spheres on conducting surface by photolithography is further developed based on the selective growth of CNTs. This may offer an effective way to control the density of patterned, aligned CNTs on conducting substrates for various applications, particularly for field emission.     

Synthesis of radially aligned single-walled carbon nanotubes on a SiO2/Si substrate by introducing sodium chloride

Radially aligned single-walled carbon nanotubes (SWCNTs) were synthesized on a SiO₂/Si substrate with thermal chemical vapor deposition by introducing sodium chloride (NaCl) onto the substrate surface. The growth of such SWCNTs was sensitive to the thickness of the SiO₂ layer on the Si substrate and the speed of the reactive gas flow. Cristobalite crystals were found to be formed on the substrate after the SWCNT growth process and were significant for the growth of radially aligned SWCNTs. The SWCNTs were assumed to be directed by the cristobalite crystals along a certain crystal direction on the (1 0 1) crystal face.     

Computational study of B- or N-doped single-walled carbon nanotubes as NH3 and NO2 sensors

The adsorption of NH₃ and NO₂ in B- or N-doped (10, 0) single-walled carbon nanotubes (SWCNTs) was investigated by using density functional computations to exploit their potential applications as gas sensors. NH₃ can be chemisorbed only in B-doped SWCNTs with apparent charge transfer, so B-doped SWCNTs can be used as NH₃ sensors. Both B- and N-doping make NO₂ chemisorption feasible in SWCNTs, but the binding of NO₂ with B is too strong, indicating an impractical recovery time as gas sensors. Due to the medium (optimal) adsorption energy and the conductance reduction accompanied with the charge transfer between SWCNTs and gas molecules, N-doped SWCNTs are potentially good NO₂ sensors.     

Lattice-directed growth of single-walled carbon nanotubes with controlled geometries on surface

How to control the orientations of single-walled carbon nanotubes (SWCNTs) on surface is the key point to controlling their geometries. In this work, we chose quartz (0 0 1), MgO (0 0 1) and layered mica with 3-, 4- and 6-fold symmetry, respectively as substrates to grow SWCNTs using gas-flow and lattice-directed modes. The produced SWCNTs were aligned along the symmetrical directions and displayed the homologous angles of 120°, 90° and 60° during growth on quartz (0 0 1), MgO (0 0 1) and mica surfaces, respectively. The obtained SWCNTs with controlled geometries would have wide applications in nanoelectronic devices in the future.     

Protective properties of SiO2 with SiO2 and Al2O3 nanoparticles sol-gel coatings deposited on FeCrAl alloys

Five layer SiO₂ coatings containing SiO₂ or Al₂O₃ nanopowders were deposited on FeCrAl alloy support by sol-gel method. Studies of protective properties of the coatings were carried out during high temperature cyclic oxidation. Changes in surface topography, structure and chemical composition of the surface layer of FeCrAl alloy were investigated. It has been shown that the type of nanofillers present in the SiO₂ coating (about 2.5?wt%) affects morphology of Al₂O₃ growing scale and determines the heat resistance of FeCrAl alloy. The lowest relative mass change (approx. 1.3%) after 10 oxidation cycles in air at 900?°C (one cycle = 12?h) was measured for the samples with coatings containing hydrophilic nanosilica (Aerosil 380) as filler. The protective efficiency of the coatings in the process of high-temperature oxidation is from 66% to 85%. The thickness of the formed scale and the value of the parabolic rate constant depend on the type of nanopowder in the coating.     

Synthesis and characterization of single-walled carbon nanotubes filled with the superionic material SnF2

We report X-ray powder diffraction, high resolution transmission electron microscopy and Raman spectroscopy of the semiconducting single-walled carbon nanotubes filled with the superionic material SnF₂. We first have obtained the Raman spectra of filling SWCNTs using the near-infrared excitation (1064 nm). Our results show that SnF₂ behaves as an electron acceptor with small charge transfer from nanotubes to SnF₂. We interpret the large high-frequency shift of radial breathing modes as due to both charge transfer and the increasing interaction between filling material and nanotubes.     

C60 encapsulation inside single-walled carbon nanotubes using alkali-fullerene plasma method

It is reported that alkali-fullerene plasmas consisting of positive alkali-metal ions, negative fullerene ions, and residual electrons are effective in encapsulating fullerenes inside single-walled carbon nanotubes (SWNTs). When positive or negative bias-voltages are applied to SWNTs in plasmas, accelerated negative fullerene or positive alkali-metal ions are irradiated to the SWNTs through the plasma sheath, respectively. Field emission gun transmission electron microscopy (FEG-TEM) clearly shows that drastic structural modifications such as severe bending of SWNT bundles, tube dislocation, and tube tip termination take place after the ion irradiation. Energy dispersive X-ray spectrometry (EDS) confirms the existence of the alkali-metal elements in the sample after the alkali-metal irradiation. In addition to this, the SWNTs encapsulating fullerene molecules are directly observed after only 1 h fullerene-ion irradiation. These results suggest that our experimental system could permit us to intercalate not only fullerenes but also other elements inside the SWNTs by the applied-bias control. Raman scattering spectroscopy is also adopted for the purpose of evaluating pure SWNTs and fullerene encapsulated SWNTs.     

SiO2载体对Cu/SiO2催化剂的影响

分别以硅溶胶、气相SiO2、介孔SBA-15分子筛为载体,采用共沉淀法制备了Cu/SiO2催化剂,含活性组分Cu质量分数为25%。运用BET、XRD、H2-TPR、NH3-TPD等对催化剂进行表征。结果表明:以硅溶胶和气相SiO2为载体制得催化剂的Cu物种分散性较差,以SBA-15为载体制得催化剂的Cu物种以高度分散的形式存在,且具有较大的比表面积,较低的还原温度和较大的酸量。     

Improvement of electrochemical capacitor electrodes using SiO2 nanoparticles

For applications that require power densities greater than 500 W/kg, electrochemical capacitors are better suited than batteries because of their high power and long cycle life, but improvement in their energy density is still desired. Creating sophisticated surface morphologies and utilizing faradaic materials have been successful in improving the energy density of electrochemical capacitors. However, to our knowledge, there have been no studies exploring coating carbon material with non-transition metal oxides as a means to improve energy storage of electrochemical capacitors. By utilizing SiO₂ nanoparticles coated on a variety of carbon substrates, we can increase the energy density of electrochemical capacitors between 10% and 29% at 500 W/kg and between 30% and 85% at 1000 W/kg in organic electrolytes. We have also shown that these coatings can increase the capacitance normalized to specific surface area between 39% and 47%.     

Combined TiO2/SiO2 mesoporous photocatalysts with location and phase controllable TiO2 nanoparticles

Combined TiO₂/SiO₂ mesoporous materials were prepared by deposition of TiO₂ nanoparticles synthesised via the acid-catalysed sol–gel method. In the first synthesis step a titania solution is prepared, by dissolving titaniumtetraisopropoxide in nitric acid. The influences of the initial titaniumtetraisopropoxide concentration and the temperature of dissolving on the final structural properties were investigated. In the second step of the synthesis, the titania nanoparticles were deposited on a silica support. Here, the influence of the temperature during deposition was studied. The depositions were carried out on two different mesoporous silica supports, SBA-15 and MCF, leading to substantial differences in the catalytic and structural properties. The samples were analysed with N₂-sorption, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) to obtain structural information, determining the amount of titania, the crystal phase and the location of the titania particles on the mesoporous material (inside or outside the mesoporous channels). The structural differences of the support strongly determine the location of the nanoparticles and the subsequent photocatalytic activity towards the degradation of rhodamine 6G in aqueous solution under UV irradiation.     

Role of SiO2 in synthesis of SiO2-supported CeO2 composites

Pure and SiO₂-supported CeO₂ samples were prepared by Ce(NO₃)₃ decomposition, precipitation, and sol–gel methods in an attempt to study the role of SiO₂ in the synthesis of these materials. During synthesis process, SiO₂ support uniformly adsorbed cerium ions in aqueous solution, preventing nucleation and crystal growth of CeO₂ during the subsequent water evaporation and calcination steps. Uniform adsorption and inhibition were enhanced by NH₄⁺ and, to a larger extent, C₅H₇O₅COO^-. Despite the dispersion of cerium ions on SiO₂ reduced the temperature at which CeO₂ was formed, crystal size and crystallinity of CeO₂ in composites were significantly lower than that of pure CeO₂ sample prepared by the same synthesis method and at the same temperature. Composites were quite stable upon increasing the temperature from 400 to 800?°C. Visible light absorption, reduction, and photocatalytic activity characteristics of CeO₂ were improved upon dispersion on SiO₂. This work can help synthetize supported oxides with high activity and thermal stability.     

Synthesis and magnetic properties of monodisperse CoFe2O4 nanoparticles coated by SiO2

The monodisperse CoFe₂O₄ nanoparticles were synthesized by a modified chemical coprecipitation method. Coating SiO₂ on the surface of the CoFe₂O₄ nanoparticles was carried out to keep single domain particles non-interacting with cubic magnetocrystalline anisotropy. The Curie temperatures (T_c) of the monodisperse CoFe₂O₄ nanoparticles can be accurately measured because the SiO₂ shells prevented the aggregation and growth of nanoparticles at high temperature. The magnetic properties of the CoFe₂O₄@SiO₂ nanoparticles with core-shell structure in a wide temperature range (300～950?K) were investigated. It is remarkable that the coercive field (H_c) of CoFe₂O₄ nanoparticles increased from about 760?Oe to 1806?Oe after being coated with SiO₂, which increased by 137.6% compared to the uncoated samples at 300?K. The saturation magnetization (M_s) of the CoFe₂O₄@SiO₂ nanoparticles is 34.59?emu/g, which is about 52% of the naked CoFe₂O₄ nanoparticles value (66.51?emu/g) at 300?K. The hysteresis loops of the CoFe₂O₄@SiO₂ nanoparticles showed an orderly magnetic behavior at high temperature, such as the M_s, remanence magnetization (M_r) and H_c decreased as temperature increasing, being equal to zero near T_c. This is a good indication that the CoFe₂O₄@SiO₂ nanoparticles are suitable for a wide variety of technological applications at high temperature.     

纳米二氧化硅/凹土复合材料的制备以及吸附性能

以硅酸钠为主要原料,用硫酸调节pH,通过液相沉淀法在凹土单晶表面原位生成纳米粒状二氧化硅,制备纳米二氧化硅/凹土复合粉体。利用XRD、TEM、TG-DTG、氮气吸附脱附曲线等方法对复合粉体进行表征。XRD表明,二氧化硅以非晶态的形式包覆在凹土表面,防止了凹土的团聚。TEM照片显示,包覆后的纳米二氧化硅粒径减小,粒度分布均匀,表面光滑。     

Preparation of MEMO silane-coated SiO2 nanoparticles under high pressure of carbon dioxide and ethanol

The objective of this study is to investigate and compare methods of nanosilica coating with γ-methacryloxypropyltrimethoxy (MEMO) silane using supercritical carbon dioxide and carbon dioxide-ethanol mixture. Characterization of grafted silane coupling agent on the nanosilica surface was performed by the infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The d₅₀ value and particle size distribution were determined by laser particle size analyzer (PSA). The operating parameters of silanization process at 40 °C, such as the silica/silane weight ratio, the presence of ethanol, and the pressure, were found to be important for the successful coating of silica particles with minimum agglomeration. The results indicate that presence of ethanol in high-pressure carbon dioxide plays an important role in achieving successful deagglomeration of coated nanoparticles. Dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM) revealed that dispersion of the silica particles in the PMMA matrix and interfacial adhesion between silica particles and polymer matrix were enhanced, when silica nanoparticles treated with silane under high pressure of carbon dioxide and ethanol were used for the nanocomposite preparation.     

In situ EPR spectroelectrochemistry of single-walled carbon nanotubes and C60 fullerene peapods

The first in situ electron paramagnetic resonance (EPR) spectroelectrochemical study of C₆₀ fullerene peapods (C₆₀@SWCNT) as well as that of single-walled carbon nanotubes (SWCNTs) in different electrolyte solutions describes the formation of spin states by charge transfer reactions. Electrochemical reduction of peapods at high negative potentials causes the production of spins at the SWCNT site, while the intratubular fullerene is unchanged.Slightly anisotropic EPR signals were detected during electrochemical reduction of single-walled carbon nanotubes and fullerene peapods in the potential region from −1.75 to −2.15 V vs. decamethylferrocene/decamethylferrocinium couple. They are centered at g = 2.0038 and exhibit a hyperfine structure indicating the presence of functional groups containing N, O, H atoms in neighborhood. They differ from the EPR signals of chemically (potassium) doped SWCNT and C₆₀@SWCNT. As the EPR signal is influenced by the electrolyte counter ions a reaction with electrolysis products of tetraalkylammonium cations is taken into consideration. No EPR lines of fullerene anions were found in electrochemically treated peapods, but these anions are detectable, if a free C₆₀ in solution is cathodically reduced on a SWCNT electrode.     

Importance of Cr2O3 layer for growth of carbon nanotubes on superalloys

We present a detailed investigation of the formation of stable catalyst particles on Inconel substrates, an important step in the growth mechanism of carbon nanotubes on these substrates. The chemical nature of the interaction of catalyst and substrate was investigated using XPS, high-resolution cross-sectional TEM, and EDS studies. The results indicate that Cr₂O₃, an electrically conductive oxide, plays an important role in stabilizing nanoclusters of Fe catalyst under typical growth conditions. The nature of growth on a number of other superalloys is also presented and is examined with respect to their chemical composition.     

CeO2 nanoparticles decorated multi-walled carbon nanotubes for electrochemical determination of guanine and adenine

Sub-10 nm CeO₂ nanoparticles decorated multi-walled carbon nanotubes has been constructed for electrochemial determination of guanine and adenine. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the nanoparticles CeO₂/MWCNTs. Electrochemical impedance spectroscopy (EIS) was used to characterize the electrode modifying process. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to study the electrocatalytic activity toward the electrochemical oxidation of guanine and adenine. The detection limit (S/N = 3) for adenine and guanine was found to be 20 and 10 nM, respectively. The obtained sensitivity toward guanine and adenine was 1.26 and 1.13 μA/μM in the linear concentration range 5–50 μM and 5–35 μM, respectively. These results demonstrate that the carbon nanotubes could provide huge locations and facilitate the adsorptive accumulation of the guanine and adenine, and the CeO₂ nanoparticles are promising substrates for the development of high-performance electrocatalysts for biosensing.