Nanotube Heterojunctions Research Articles

Electrospinning direct preparation of SnO2/Fe2O3 heterojunction nanotubes as an efficient visible-light photocatalyst

Abstract Herein SnO2/Fe2O3 heterojunction nanotubes are prepared by a facile electrospinning technique. The heterojunction nanotubes with a diameter of about 200 nm uniformly distribute SnO2 and Fe2O3 nanocrystals and present the obvious interfaces between them, which form perfect SnO2/Fe2O3 nano-heterojunctions. A possible mechanism based on self-polymer-templates is proposed to explain the formation of SnO2/Fe2O3 heterojunction nanotubes. The heterojunction nanotubes show high photocatalytic activity for the degradation of RhB dye under visible light irradiation. The prepared SnO2/Fe2O3 heterojunction nanotubes can also be applied to other fields such as sensor, lithium-ion batteries.

Electronic Structure of Semiconducting and Metallic Tubes in TiO2/Carbon Nanotube Heterojunctions: Density Functional Theory Calculations.

The electronic structure of the TiO2(110) surface interfaced with both a semiconducting and metallic carbon nanotube (CNT) was investigated by density functional theory. Our simulations rationalized visible light photocatalytic activity of CNT/TiO2 hybrid materials higher than that under ultraviolent irradiation and showed that the photoactivity of a semiconducting CNT decorating TiO2 is better than that of the metallic CNT/TiO2 system due to efficient charge separation across the interface. This suggests that semiconducting CNT/TiO2 could be a potential photovoltaic material. In contrast, strong interaction between a metallic CNT and TiO2 leads to large charge transfer. Such charge transfer reduces the built-in potential, in turn resulting in inefficient charge separation. Functionalizing the metallic CNT with a small platinum cluster can increase the built-in potential and drive charge separation. These observations indicate that the CNT/TiO2 interface can be a potential photovoltaic material by a metal cluster decorating a CNT despite a real tube being composed of the mixture of metallic and semiconducting CNTs.

Exact solution for nonlocal axial buckling of linear carbon nanotube hetero-junctions

CNT hetero-junctions offer the possibility of being used in micro-electromechanical systems and nano-electromechanical systems. However, in the present work, nonlocal axial buckling analysis of linear CNT hetero-junctions based on Euler–Bernoulli beam (EBB) model is investigated. In order to mathematically model a linear CNT hetero-junction, a CNT with two segments is considered. The constitutive equations are derived based on the Eringen's theory for various boundary conditions, namely clamped–clamped (C–C), clamped–pinned (C–P) and pinned–pinned (P–P). An analytical approach is applied to obtain the dimensionless buckling load of the CNT hetero-junctions. A detailed parametric study is conducted to elucidate the influences of the small-scale coefficient, homogeneity parameter, boundary conditions and CNT length of each segment on the axial buckling of the linear CNT hetero-junctions. The results indicate that the length of each segment and homogeneity parameter have a significant effect on the buckling load of the CNT hetero-junctions and should therefore be considered in its optimum design. Furthermore, the results are in good agreement with the previous researches.

Investigation into the mechanical properties of single-walled carbon nanotube heterojunctions

The mechanical properties of finite-length (6,0)/(8,0) single-walled carbon nanotube (SWCNT) heterojunctions with respect to different kinds of connection segments, either coaxial or bias, are investigated using molecular dynamics simulation calculations. It is found that the resulting significant deformation of structure and significant drop of stress under yielding strain is due to the strain localization. Moreover, the deformation is occurred below the heptagon ring in the thinner segment of the heterojunctions under tension at different temperatures, whereas under compression it occurs on the heptagon ring. The computed atomic bond number distribution and radius distribution function are applied to determine the deformed atomic structure. Finally, with increasing temperature, the yielding stresses decrease for both coaxial and bias heterojunctions under tension and compression, while the dependence of temperature on the Young's modulus of the heterojunctions is only observed in the case of tension.

The structural, mechanical and electronic properties of (4, 4) SiC/C nanotube heterojunction: A first-principles study

The mechanical and electronic properties of the heterojunction formed by (4,4) carbon nanotube (CNT) and (4,4) silicon carbide nanotube (SiCNT) have been studied by using first-principles density functional theory. It is found that the Young’s modulus of the heterojunction is mainly determined by that of SiCNT. The SiC/C nanotube heterojunction is broken near the junction interface with applying a relative large uniaxial strain along the axial direction, and then forming a SiC cage. Band structure shows that (4,4) SiC/C nanotube heterojunction is a semiconductor with direct band gap. Both valence band maximum and conduction band minimum originate from CNT segment. The effect of CNT and SiCNT length on the electronic structures of the nanotube heterojunction is investigated. The Schottky barrier of the nanotube heterojunction is also estimated.

Carbon/Silicon Heterojunction Formed by Inserting Carbon Nanotubes into Silicon Nanotubes: Molecular Dynamics Simulations

Using molecular dynamics (MD) simulations, we report a carbon/silicon (C/Si) heterojunction formed by inserting carbon nanotubes (CNTs) into silicon nanotubes (SiNTs). Due to the weak mechanical property of the SiNTs, insertion of CNTs into them can not only reinforce their mechanical stabilities but also form multiwalled C/Si nanotube heterojunctions. The driving force of the coaxial assembly is primarily the intertube van der Waals (vdW) interactions. The coaxial self-assembly process is strongly tube size dependent, and the intertube distance (Δd) for a successful assembly between the two type nanotubes is around 3.5 Å. Simulations suggest possible bottom-up self-assembly routes for fabrication of novel nanomachines and nanodevices in nanomechanical systems. This study also suggests that the possibility of synthesizing SiNTs with fewer walls, even single-walled SiNT in aid of CNTs.

Surface structure and field emission properties of cost effectively synthesized zinc oxide nanowire/multiwalled carbon nanotube heterojunction arrays

A cost-effective and efficient approach for the synthesis of single-crystalline zinc oxide nanowires on vertically aligned multiwalled carbon nanotube (CNT) array is presented. ZnO nanowires are grown on the base of individual CNT through the low-temperature wet-chemical batch deposition technique, while the size and interspacing of the nanowires can be controlled by precursor concentration, growth temperature and time duration. The scanning electron microscopy image showed that the ZnO nanostructures are successfully grown on the CNT's surface uniformly. The produced nanostructures are characterized by x-ray diffraction, x-ray photoelectron spectroscopy and Raman spectroscopy. Also, field emission characteristics of the fabricated double-stage ZnO nanowire/CNT array are investigated and compared with the emission behaviour of CNT and ZnO nanowire arrays. The ZnO nanowire/CNT heterojunction array resulted in a low turn-on field of 1.5 V µm−1 and a threshold field of 4.5 V µm−1, which were lower than both the vertical CNT and ZnO arrays. The field emission properties and stability of the fabricated nanostructures also demonstrated great potential for field emission applications.

Synthesis of LaVO4/TiO2 heterojunction nanotubes by sol–gel coupled with hydrothermal method for photocatalytic air purification

With the aim of improving the effective utilization of visible light, the LaVO4/TiO2 heterojunction nanotubes were fabricated by sol–gel coupled with hydrothermal method. The photocatalytic ability was demonstrated through catalytic removal of gaseous toluene species. The nanotube samples were characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), surface photovoltage (SPV), Raman spectra and N2 adsorption–desorption measurements. The characterization results showed that the samples with high specific surface areas were of typical nanotubular morphology, which would lead to the high separation and transfer efficiency of photo induced electron–hole pairs. The as-prepared nanotubes exhibited high photocatalytic activity in decomposing toluene species under visible light irradiation with fine photochemical stability. The enhanced photocatalytic performance of LaVO4/TiO2 nanotubes might be attributed to the matching band potentials, the interconnected heterojunction of LaVO4 versus TiO2, and the large specific surface areas of nanotubes.

Cu2O/Cu/TiO2 nanotube Ohmic heterojunction arrays with enhanced photocatalytic hydrogen production activity

Cu2O/Cu/TiO2 nanotube heterojunction arrays were prepared by assembling Cu@Cu2O core-shell nanoparticles on TiO2 nanotube arrays (NTAs) using a facile impregnation-reduction method. SEM and TEM results show that Cu@Cu2O plate-like nanoparticles with tens of nanometers in size are confined inside TiO2 NTAs. Only the outmost several nanometers of the nanoparticles are Cu2O and the predominant inner of the nanoparticles are Cu metals. Cu L3VV Auger spectra of Cu2O/Cu/TiO2 NTAs suggest that Cu metals are enveloped by at least several nanometers Cu2O on the surface, which further confirms the Cu@Cu2O core shell structure of Cu nanoparticles. The ability of light absorption of Cu2O/Cu/TiO2 NTAs is enhanced. The range of absorption wavelengths changes from 400 to 700 nm due to the surface plasmon response of Cu metals core and Cu2O nanoparticles shell. The photocatalytic hydrogen production rate of Cu2O/Cu/TiO2 heterojunction arrays is enhanced when compared with those of Cu2O/TiO2 NTAs and TiO2 NTAs under UV light. Moreover, a stable H2 generation property was obtained under visible light (λ gt; 400 nm). The Cu metal core is believed to play a key role in the enhancement of photocatalytic properties of Cu2O/Cu/TiO2 nanotube heterojunction arrays.

Enhanced visible light response of CuO nanoparticle-modified sodium titanate nanotube heterojunction thin films

Sodium titanate nanotube thin films were synthesized on titanium plate by alkaline hydrothermal method. CuO nanoparticle-modified sodium titanate nanotube heterojunction thin films were prepared by a simple ion-exchange method. The p-type CuO nanoparticles with an average diameter of 3nm deposit on the surface of n-type sodium titanate nanotubes, which would form a p–n heterojunction. Ultraviolet–visible absorption spectra revealed that modification of CuO nanoparticles leads to a red shift of the optical absorption edge from 333nm to 422nm compared with pure sodium titanate nanotubes, showing an improvement in visible light response. Based on the energy band structure obtained from the valence band and ultraviolet–visible absorption spectra, we conclude that the red shift observed in the experiment derives from the up-shift of the top of the valence band, which results in the band gap narrowing.

Study on the interfacial structures of Tin oxide/multiwalled carbon nanotube heterojunctions

Tin oxide/multiwalled carbon nanotube (SnO2/MWCNT) heterojunctions were synthesized using the SnCl2 solution method. The interfacial structures, especially the interactions between the SnO2 and MWCNTs, were investigated by field emission scanning electron microscopy, transmission electron microscopy, ultrasonic destruction experiments, Raman spectroscopy, and thermal analysis. At the initial reaction stage, chemical bonds were the predominant interactions between the MWCNTs and SnO2. Physical interactions, including Coulomb interactions and van der Waals forces, play key roles under an increasing reaction time. Raman spectroscopy and thermal analysis proved to be two valid methods for the characterization of the interactions between SnO2 and MWCNTs. The strong interactions result in a low G/D strength ratio, as well as a low thermal stability of the MWCNTs in the SnO2/MWCNT heterojunctions. The binding energy of the MWCNTs and SnO2 was calculated based on differential thermal analysis and Hess's law and the results agreed with the interfacial morphological characteristics. This work presents a facile and low-cost approach to study the interfacial structures of carbon-based nanomaterials and opens a new possibility for investigating structural property relationships.

Self-filtering oscillations in carbon nanotube hetero-junctions

We evaluate the vibrational properties of single-wall carbon nanotube (SWCNT)hetero-junction (HJ) oscillators using a hybrid atomistic-continuum approach validated bymolecular mechanics/molecular dynamics simulations. The SWCNT-HJs show a brokensymmetry topology of their mode shapes, with striction effects caused on the bending andradial modes by the combined effect of the HJ and the tube with the thinner radius. Thesingle-wall nanotube HJs also show selective mass sensing properties based solely on thegeometry and type of the boundary conditions of the specific nanostructure. Thisunusual behaviour has not been observed so far in classical SWCNT systems.

Interfacing carbon nanotubes of arbitrary chiralities into linear heterojunctions

Motivated by recent advances in synthesis and characterization of carbon nanotube (CNT) heterojunctions, we introduce a systematic approach for obtaining atomic geometries that connect two carbon nanotubes of different chiralities. Using our approach, it is straightforward to construct atomic interface geometries between two single-walled CNT's of arbitrary chiralities arranged at different orientations and angles. Our method generalizes existing approaches and is readily applicable to joining domains of graphene nanoribbons as well. As an example, we focus on linear heterojunctions, and we postulate the minimum number of simple topological defects required at the interface, and the preferred spatial arrangements, to obtain maximally linear heterojunctions given any two arbitrary chiralities. We also provide a physical picture of the defect structure of the resultant interface geometries using the results of classical force-field simulations.

Negative differential resistance in an (8, 0) carbon/boron nitride nanotube heterojunction

Using the method combined non-equilibrium Green's function with density functional theory, the electronic transport properties of an (8, 0) carbon/boron nitride nanotube heterojunction coupled to Au electrodes were investigated. In the current voltage characteristic of the heterojunction, negative differential resistance was found under positive and negative bias, which is the variation of the localization for corresponding molecular orbital caused by the applied bias voltage. These results are meaningful to modeling and simulating on related electronic devices.

Achieving High Efficiency Silicon-Carbon Nanotube Heterojunction Solar Cells by Acid Doping

Various approaches to improve the efficiency of solar cells have followed the integration of nanomaterials into Si-based photovoltaic devices. Here, we achieve 13.8% efficiency solar cells by combining carbon nanotubes and Si and doping with dilute HNO(3). Acid infiltration of nanotube networks significantly boost the cell efficiency by reducing the internal resistance that improves fill factor and by forming photoelectrochemical units that enhance charge separation and transport. Compared to conventional Si cells, the fabrication process is greatly simplified, simply involving the transfer of a porous semiconductor-rich nanotube film onto an n-type crystalline Si wafer followed by acid infiltration.

A novel catalyst-free synthesis of vertically aligned silicon nanowire–carbon nanotube heterojunction arrays for high performance electron field emitters

A novel, catalyst-free strategy for the direct synthesis of vertically aligned silicon nanowire-carbon nanotube (SiNW-CNT) heterojunction arrays is presented. Such a heterojunction with the junction area in the nanoscale displays enhanced field emission characteristics at low turn-on field, with a nearly three times increase in the field enhancement factor.

Unusual deformation mechanisms in carbon nanotube heterojunctions (5,5)–(10,10) under tensile loading

AbstractThe Poisson's ratio effect of single wall carbon nanotube heterojunctions (HJ) of the type (5,5)–(10,10) is investigated using an hybrid finite element atomistic‐continuum approach to simulate the mechanical behaviour of nanostructures. The nanotube HJs provide a peculiar deformation pattern, with combined bending and axial stretching of carbon nanotubes (CNTs), and a broad agreement of their axial stiffness with spring series continuum mechanics and existing molecular dynamics (MD) simulations. A possible auxetic behaviour is observed for the pure HJ with no tubes when subjected to tensile loading.

Axial vibration of carbon nanotube heterojunctions using nonlocal elasticity

In the present study, axial vibration of carbon nanotube heterojunctions is studied using nonlocal rod theory. The nonlocal constitutive equations of Eringen are used in the formulations. The carbon nanotubes with different lengths, chirality and diameters are considered in the heterojunctions. Effect of nonlocality, length of the carbon nanotubes and lengths of each segment are investigated in detail for each considered problem. It is obtained that, by joining carbon nanotubes good vibrational properties are obtained by suitable selection of parameters.

Electronic transport properties of an (8, 0) carbon/boron nitride nanotube heterojunction

The structure of a heterojunction made up of an (8, 0) carbon nanotube and an (8, 0) boron nitride nanotube is achieved through geometry optimization implemented in the CASTEP package. Based on the optimized geometry, the model of the heterojunction is established. Its transport properties are investigated by combining the nonequilibrium Green's function with density functional theory. Results show that both the lowest unoccupied molecular orbital and the highest occupied molecular orbital mainly locate on the carbon nanotube section. In the current–voltage characteristic of the heterojunction, a rectification feature is revealed.

Theoretical study of heat conduction in carbon nanotube hetero-junctions

Thermal conductivity of single-wall carbon nanotubes with linear, “Y” and “X” hetero-junctions is studied using nonequilibrium molecular dynamics method. The three types of junctions with almost equal number of point defects show a similar decrease in thermal conductivity compared to that of the pristine carbon nanotube. Thermal conductivity of the nanotubes with different number of point defects is also calculated for a qualitative comparison. The thermal conductivity decreases with the defect number increasing. When the defects form an interface, they can cause a larger decrease in the thermal conductivity of CNT than when they are distributed dispersedly. The number of defects contained in the nanojunction determines the thermal conductivity to a great extent more than the geometrical forms of the nanojunctions. Phonon spectra of these junctions are also analyzed to explore the mechanism of degradation in thermal conductivity of these junctions.