Microstructural investigation of magnetic CoFe2O4 nanowires inside carbon nanotubes by electron tomography

Magnetic nanowires of CoFe 2O4 were casted inside the channel of multiwall carbon nanotubes by mild chemical synthesis. A detailed investigation of these nanowires was performed using mainly the electron tomography technique; this study provides a complete characterization of their microstructure in terms of the spatial organization and the size distribution of individual particles forming the nanowire as well as its residual porosity. In particular, we have shown that the size of the CoFe 2O4 monocrystalline particles is closely dependent on the location of the particle within the nanotube, i.e., small particles close to the tube tip (5 nm) and bigger particles inside the tube channel (15 nm). As the theoretical critical size for superparamagnetic relaxation in CoFe 2O4 is estimated within the range of 4-9 nm, the size distribution obtained by 3D-TEM agrees with the Mossbauer study that suggests the presence of two different magnetic components inside the nanowire. We have shown also that, by using this preparation method and for this internal diameter of nanotube, the CoFe 2O4 nanowire exhibits a continuous structure along the tube, has a residual porosity of 38%, and can fill the tube at only 50%, parameters which influence in a significant manner the magnetic behavior of this system.     

Filling of carbon nanotubes with selenium by vapor phase process

A vapor phase method was developed to fill different kinds of nanotubes (including multiwall and singlewall nanotubes), which inner diameters range from 200 down to 1.5 nm. Experiments were performed with selenium in a sealed reactor in order to control the selenium partial pressure. This process allows a full control of the nanotube filling (partial or complete), i.e., either a selective diameter filling or a full filling rate. The weight gain of all the samples was plotted for different reaction temperature as a function of selenium pressure. Experimental isotherms are characteristic of capillary condensation of selenium, which occurs at first in the smallest nanotube inner diameters. X-ray diffraction data show that selenium confined in nanotubes with diameters larger than 3 nm keeps its trigonal structure.     

阳极氧化时间对二氧化钛纳米管阵列形貌的影响

采用电化学阳极氧化法,以乙二醇体系作为电解液,在60V的直流电压下,在纯钛基底表面制备高度有序的TiO2纳米管阵列,通过改变氧化时间来探究其对TiO2纳米管阵列形貌的影响。运用XRD、SEM分别对TiO2纳米管阵列的结构、形貌特征进行表征。结果表明,氧化时间对TiO2纳米管的管壁厚度、管径以及管长均有影响;氧化时间为2～2.5h时,所得到的二氧化钛纳米管管壁约为10nm、管径约为200nm、管长为2～3μm。     

A liquid-Ga-filled carbon nanotube: a miniaturized temperature sensor and electrical switch

Temperature control on the nanometer scale is a challenging task in many physical, chemical, and material science applications where small experimental volumes with high temperature gradients are used. The crucial difficulty is reducing the size of temperature sensors while keeping their sensitivity, working temperature range, and, most importantly, their simplicity and accuracy of temperature reading. In this work, we demonstrate the ultimate miniaturization of the classic thermometer using an expanding column of liquid gallium inside a multi-walled C nanotube for precise temperature measurements. We report that electrical conductivity through unfilled nanotube regions is diffusive with a resistance per unit length of approximately 10 kOmega microm(-1), whereas Ga-filled segments of the nanotube show metallic behavior with a low resistance of approximately 100 Omega microm(-1). No noticeable Schottky barrier exists between the nanotube carbon shell and the inner Ga filling. Based on these findings, an individual carbon nanotube partially filled with liquid Ga is used as a temperature sensor and/or switch. The nanotube's electrical resistance decreases linearly with increasing temperature as the metallic Ga column expands inside the tube channel. In addition, the tube resistance drops sharply when two encapsulated Ga columns approaching each other meet inside the nanotube, producing a switching action that can occur at any predetermined temperature, as the Ga column position inside the nanotube can be effectively pre-adjusted by nanoindentation using an atomic force microscope.     

New insight for enhancing photocatalytic activity of MWCNT/TiO2 by decorating palladium nanoparticles as charge-transfer channel

A surface bond-grafted multi-walled carbon nanotube (MWCNT)/TiO₂ as supporter, palladium nanoparticles, approximately 3 nm in diameter, are uniformly deposited on the functional MWCNT surface in first, constructing a novel Pd-MWCNT/TiO₂ photocatalyst for photocatalytic solar conversion. The characterization of photocatalysts by a series of joint techniques, including BET surface area, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), Raman spectroscopy and ultraviolet/visible (UV/vis) diffuse reflectance spectra, discloses that palladium nanoparticles has a crucial role in enhancement of photocatalytic activity of MWCNT/TiO₂, that is to act as a charge transfer channel, which helps to trap electrons from MWCNT to TiO₂.     

Fabrication, structural characterization and formation mechanism of multiferroic BiFeO3 nanotubes

Multiferroic BiFeO3 (BFO) nanotubes have been successfully fabricated by the modified sol-gel method within the nanochannels of porous anodic aluminum oxide (AAO) templates. The morphology, structure and composition of the nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM, (HRTEM) and energy-dispersive X-ray spectroscopy (EDX). Postannealed (650 degrees C for 1 h), BFO nanotubes were polycrystalline and X-ray diffraction study revealed that they are of the rhomohedrally distorted perovskite crystal structure. The results of SEM and TEM revealed that BFO nanotubes possessed a uniform length (up to 60 microm) and diameter (about 200 nm), which were controlled by the thickness and the pore diameter of the applied AAO template, respectively and the thickness of the wall of the BFO nanotube was about 15 nm. Y-junctions in the BFO nanotubes were observed. EDX analysis demonstrated that stoichiometric BiFeO3 was formed. HRTEM analysis confirmed that the obtained BFO nanotubes made up of nanoparticles (3-6 nm). The possible formation mechanism of BFO nanotubes was discussed.     

Capillarity-induced disassembly of virions in carbon nanotubes

Studying the transport and fate of viruses through nanochannels is of great importance. By?using the nanochannel of a carbon nanotube (CNT) as an ideal model, we evaluated the possibility of capillarity-induced viral transport through a closely fitting nanochannel and explored the mechanisms involved. It is shown both experimentally and theoretically that Cowpea mosaic virus can enter CNTs by capillarity. However, when introduced into a nanotube the protein capsid may disassemble. During the initial capillary filling stage, anomalous needle-shaped high pressure exists in the centre of the nanotube's entrance. This high pressure, combining with the significant negative pressure within the nanotube, may account for the disassembly of the virions.     

Preparation of ceria nanoparticles supported on carbon nanotubes

By chemical reaction of CeCl₃ and NaOH on carbon nanotube solution and subsequent heat treatment, ceria nanoparticles supported on carbon nanotubes were prepared. The processing parameters affecting the size of ceria particles were discussed. The particles were characterized by XRD and TEM. XRD patterns revealed that the particles exhibited CaF₂-type crystal structure. The TEM micrograph showed that the mean sizes of ceria particles were about 6 nm.     

Ce3+掺杂钛酸钡纳米管薄膜的制备与性能

以阳极氧化制备的TiO_2纳米管薄膜为模版,通过水热法制备了Ba_(1-x)Ce_xTiO_3(0≤x≤0.08)纳米管薄膜,研究了Ba_(1-x)Ce_xTiO_3的结构、表面形貌及其电性能。采用X射线衍射仪表征其晶体结构,采用扫描电子显微镜和透射电子显微镜观察其表面及断口形貌,采用宽频介电阻抗谱仪测试其介电性能。结果表明,在较为温和的条件下用水热法成功制备出立方相结构的Ba_(1-x)Ce_xTiO_3纳米管薄膜,纳米管孔径在80~95nm之间;将制备的Ba_(1-x)Ce_xTiO_3经退火后生成多晶的Ba1-xCexTiO3纳米管薄膜,且样品的管外径尺寸在90~100nm之间,管壁的厚度为25~30nm,介电常数在1kHz下最高可达472,介电损耗为0.41。     

原位合成TiO2纳米管阵列及其光催化性能研究

采用液相沉积法(LPD), 以阳极氧化铝(AAO)为模板, 原位合成高度有序的TiO₂纳米管薄膜材料. 实验结果表明, 经过400℃热处理后, 制备的TiO₂纳米管为锐钛矿相, 长度达5μm, 管外径为150nm左右, 管壁厚为25nm左右. 热处理后的TiO₂薄膜具有良好的光催化降解甲基蓝的性能, 即经过120min卤灯照射后, 甲基蓝被完全降解.     

Effect of the geometry of the anodized titania nanotube array on the performance of dye-sensitized solar cells

Highly ordered TiO2 nanotube arrays are superior photoanodes for dye-sensitized solar cells (DSSCs) due to reduced intertube connections, vectorial electron transport, suppressed electron recombination, and enhanced light scattering. Performance of the cells is greatly affected by tube geometry, such as wall thickness, length, inner diameter and intertube spacing. In this paper, effect of geometry on the photovoltaic characteristics of DSSCs is reviewed. The nanotube wall has to be thick enough for a space charge layer to form for faster electron transportation and reduced recombination. When the tube wall is too thin to support the space charge layer, electron transport in the nanotubes will be hindered and reduced to that similar in a typical nanoparticle photoanode, and recombination will easily take place. Length of the nanotubes also plays a role: longer tube length is desired because of more dye loading, however, tube length longer than the electron diffusion length results in low collecting efficiency, which in turn, results in low short-circuit current density and thus low overall conversion efficiency. The tube inner diameter (pore size) affects the conversion efficiency through effective surface area, i.e., larger pore size gives rise to smaller surface area for dye adsorption, which results in low short-circuit current density under the same light soaking. Another issue that may seriously affect the conversion efficiency is whether each of the tube stands alone (free from connecting to the neighboring tubes) to facilitate infiltration of dye and fully use the outer surface area.     

Molecular dynamics study of effects of intertube spacing on sliding behaviors of multi-walled carbon nanotube

The effects of intertube spacing, tube length and wall numbers on the sliding behaviors of multi-walled carbon nanotubes (MWCNTs) are investigated using molecular dynamics (MD) simulation method. The interaction between carbon atoms is modeled using the second-generation reactive empirical bond-order potential coupled with the Lennard-Jones potential. The simulations indicate that, regardless of tube length and wall numbers, small intertube spacing of MWCNT can provides an effective channel for load transfer between tubes, and permits mechanical participation of walls. The results also show that the sliding behaviors of MWCNT are strongly dependent of tube length and wall numbers, especially MWCNT with small intertube spacing. It is observed that small intertube spacing of triple-walled carbon nanotube (TWCNT) result in the formation of intertube sp3 bonds during pullout process. The general conclusions derived from this work may be of importance in devising high-performance carbon nanotube (CNT) composites.     

壳聚糖接枝丙烯酸负载钯纳米纤维催化剂的制备及催化氢化

制备了壳聚糖接枝丙烯酸负载纳米钯催化剂并对其催化氢化性能进行了研究。首先对分子量为30万的商品壳聚糖进行降解,得到了黏均分子量为10万的壳聚糖,用后者制备壳聚糖接枝丙烯酸共聚物并负载氯化钯,最后通过电纺丝技术得到了纳米纤维状催化剂。对制备的催化剂进行了透射电镜（TEM）,X射线光电子能谱（XPS）和红外光谱（IR）的表...     

Filling double-walled carbon nanotubes with WO3 and W nanowires via confined chemical reactions

Carbon nanotubes filled with metals and semiconductors have been regarded as one of the most promising materials for nanodevices. Here, we demonstrate a simple and effective method to produce tungsten trioxide (WO3) and tungsten (W) nanowires with diameters of below 4 nm inside double-walled carbon nanotubes (DWCNTs). First, the precursors, i.e., phosphotungstic acid (HPW, H3PW12O40) molecules, are successfully introduced into DWCNTs. Subsequent decomposition and reduction lead to the formation of WO3 and W nanowires inside DWCNTs. The products were carefully characterized by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. FTIR spectra provide a direct proof that the HPW molecules enter the DWCNTs as an ionic state, i.e., PW12O40(3-) and H+, instead of the molecular state. HRTEM analysis shows that the diameter of the WO3 nanowires inside DWCNTs is 1.1-2.4 nm with the average length of 16-18 nm, and that for W nanowires is 1.2-3.4 nm with the average length of 15-17 nm. Meanwhile, DWCNTs are doped by the encapsulated WO3 and W nanowires. Tangential band shift in Raman spectra revealed the charge transfer between the nanowires and carbon nanotubes.     

Highly porous metal oxide networks of interconnected nanotubes by atomic layer deposition

Mesoporous metal oxide networks composed of interconnected nanotubes with ultrathin tube walls down to 3 nm and high porosity up to 90% were fabricated by atomic layer deposition (ALD) of alumina or titania onto templates of swelling-induced porous block copolymers. The nanotube networks possessed dual sets of interconnected pores separated by the tube wall whose thickness could be finely tuned by altering ALD cycles. Because of the excellent pore interconnectivity and high porosity, the alumina nanotube networks showed superior humidity-sensing performances.     

1/f Noise in Carbon Nanotube Devices—On the Impact of Contacts and Device Geometry

We report on the 1/f noise in various ballistic carbon nanotube devices. A common means to characterize the quality of a transistor in terms of noise is to evaluate the ratio of the noise amplitude A and the sample resistance R. By contacting semiconducting tubes with different metal electrodes we are able to show that a small A/R value by itself is no indication of a suitable metal/tube combination for logic applications. We discuss how current in a nanotube transistor is determined by the injection of carriers at the electrode/nanotube interface, while at the same time excess noise is related to the number of carriers inside the nanotube channel. In addition, we demonstrate a substantial reduction in noise amplitude for a tube transistor with multiple carbon nanotubes in parallel     

Preparation and modification of carbon nanotubes

Carbon nanotubes (CNTs) were prepared by the catalytic decomposition of methane at 680 °C for 120 min, using nickel oxide–silica binary aerogels as the catalyst. The morphological structure of CNTs was investigated by transmission electron microscopy (TEM), X-ray Diffraction (XRD) and Raman spectroscopy. The results revealed that CNTs with diameter 40–60 nm showed high quality, uniform diameter and high length/diameter ratio, the wall structure of CNTs was similar with that of highly oriented pyrolytic graphite (HOPG), and some metal catalyst particles were encapsulated at the tip of CNTs. Different methods were compared to modify CNTs. Investigated by TEM, XRD, Raman spectroscopy and nitrogen adsorption/desorption for modified CNTs, it was confirmed that after modification treatment by immersion in diluted HNO₃ solution with ultrasonic and then milling by ball at a high velocity, the metal catalyst particles at the tip of CNTs disappeared, the unique cylinder wall structure remained, the CNT length became short, the cap at the tip of nanotube was opened, and thus the internal surface area could be effectively used, leading to the increase of the specific surface area and pore volume. This technique is relatively simple and effective for modifying CNTs which can be scaled up for industrial applications.     

TEM-based metrology for HfO2 layers and nanotubes formed in anodic aluminum oxide nanopore structures

Nanotubes are fabricated by atomic layer deposition (ALD) into nanopore arrays created by anodic aluminum oxide (AAO). A transmission electron microscopy (TEM) methodology is developed and applied to quantify the ALD conformality in the nanopores (thickness as a function of depth), and the results are compared to existing models for ALD conformality. ALD HfO2 nanotubes formed in AAO templates are released by dissolution of the Al2O3, transferred to a grid, and imaged by TEM. An algorithm is devised to automate the quantification of nanotube wall thickness as a function of position along the central axis of the nanotube, by using a cylindrical model for the nanotube. Diffusion-limited depletion occurs in the lower portion of the nanotubes and is characterized by a linear slope of decreasing thickness. Experimentally recorded slopes match well with two simple models of ALD within nanopores presented in the literature. The TEM analysis technique provides a method for the rapid analysis of such nanostructures in general, and is also a means to efficiently quantify ALD profiles in nanostructures for a variety of nanodevice applications.     

Photon‐Pair Generation with a 100 nm Thick Carbon Nanotube Film

Nonlinear optics based on bulk materials is the current technique of choice for quantum‐state generation and information processing. Scaling of nonlinear optical quantum devices is of significant interest to enable quantum devices with high performance. However, it is challenging to scale the nonlinear optical devices down to the nanoscale dimension due to relatively small nonlinear optical response of traditional bulk materials. Here, correlated photon pairs are generated in the nanometer scale using a nonlinear optical device for the first time. The approach uses spontaneous four‐wave mixing in a carbon nanotube film with extremely large Kerr‐nonlinearity (≈100 000 times larger than that of the widely used silica), which is achieved through careful control of the tube diameter during the carbon nanotube growth. Photon pairs with a coincidence to accidental ratio of 18 at the telecom wavelength of 1.5 µm are generated at room temperature in a ≈100 nm thick carbon nanotube film device, i.e., 1000 times thinner than the smallest existing devices. These results are promising for future integrated nonlinear quantum devices (e.g., quantum emission and processing devices).     

Carbon nanotube growth at the tip of SiO2 nanocone

A well aligned growth of carbon nanotube (CNT) at the tip of SiO₂ nanocone using chemical vapor deposition (CVD) method is described. Fe particle at the tip of a nanocone has been observed to work as the catalyst for CNT growth. Initially, a number of self organized SiO₂ nanocones were grown via thermal annealing of MnCl₂ on Si substrate in the presence of H₂ gas. The average diameters of the tip and base of the nanocones were nearly 50 nm and 1 μm, respectively, with length up to 2.4 μm. At the tip of the nanocone a CNT was grown successfully. The CNT grows from the tip of the nanocone where Fe particles accumulate after the reduction of FeCl₃ at 950 °C. The accumulation point of Fe particles depends on the orientation of the nanocone tip inside the reaction tube during CVD process. Therefore, the alignment of nanotube at the tip of SiO₂ nanocone can be controlled by orientation of the nanocone in the reaction tube.